**Molecular Interactions of Some Free Base Porphyrins with - and -Acceptor Molecules**

Abedien Zabardasti *Lorestan University, Iran* 

#### **1. Introduction**

48 Molecular Interactions

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chromatographic retention indices by the use of radial basis function neural

The basic structure of porphyrin consists of four pyrrole units linked by four methine bridges Fig. 1. The porphyrin macrocycle is an aromatic system containing 22 electrons, but only 18 of them are involved in any one delocalization pathway. It obeys Hückel's rule of aromaticity (4n+2 pi electrons) and has been shown by X-ray crystallography to be planar.

Fig. 1. Structure of free base porphyrin

#### **1.1 <sup>1</sup> H NMR spectra of porphyrins**

The aromatic character of porphyrins can also be seen by NMR spectroscopy. Studies performed in the last decades demonstrated that 1H NMR spectra are very informative and adequately reflect the structural features of porphyrins.[1] The presence of the extended delocalized -electron system of the porphyrin macrocycle gives rise to a strong ring current in the molecules placed in the magnetic field. The ring current causes anisotropic shielding of the protons located in the field of its action and (together with the diamagnetic component of paired -electrons) leads to a substantial shift of their signals in the 1H NMR spectra. It can be stated that the ring current and the aromaticity of porphyrins change in a similar way in response to the analogous changes in the molecular structure of the porphyrin and the medium, which is most clearly seen on comparison of the spectra of porphyrins and their precursors. Due to the anisotropic effect from the porphyrin ring current, the NMR signals for the deshielded meso protons (protons on the bridging methine carbons) show up at low field, whereas the signals for the shielded protons on the inner nitrogen atoms show up at very high field. Theoretical analysis of the 1H NMR spectra shows that the positions of the signals for the protons of porphyrins are determined primarily by the strength of ring -electron currents (the macrocyclic current enclosing the molecule as a whole and local currents localized in the pyrrole nuclei (Fig. 2a,b). It should be noted that the macrocyclic current (Fig. 2a) possesses the major `bed' corresponding to the main 18-membered contour of conjugation and `arms' formed by the semi-isolated C()-C() bonds in the pyrrolenine nuclei.

Fig. 2. Schematic representation of the circuits of the ring -electron currents in the porphyrin molecule; (a) the macrocyclic current, (b) local currents.[1]

The protons at the and meso positions are exocyclic with respect to the macrocyclic and local pyrrole currents and experience the deshielding influence of the latter. The signals for these protons are observed at low field (at from 9.7 to 11.2 ppm for the meso-protons and at from 8.5 to 9.9 ppm for the -protons, Table 1). The protons of the NH groups are exocyclic with respect to the local ring currents and endocyclic with respect to the macrocyclic current. Undoubtedly, the shielding effect of the latter prevails and the signals for the protons of the NH groups are observed at very high field (at from -1.4 to -4.4 ppm). [1]


Table 1. 1H NMR chemical shift (/ppm) of the free base tetraarylporphyrins relative to CHCl3.

The -hydrogen atoms of the pyrrole and pyrrolenine fragments are in principle nonequivalent. However, rapid (within the NMR time scale) transformations occur at room temperature and the signals for the -protons are averaged. Two different peaks corresponding to resonance absorption of the energy by the protons of the -CH groups of the pyrrole and pyrrolenine fragments can be observed at low temperature, -80 ºC. [1,2]

current, the NMR signals for the deshielded meso protons (protons on the bridging methine carbons) show up at low field, whereas the signals for the shielded protons on the inner nitrogen atoms show up at very high field. Theoretical analysis of the 1H NMR spectra shows that the positions of the signals for the protons of porphyrins are determined primarily by the strength of ring -electron currents (the macrocyclic current enclosing the molecule as a whole and local currents localized in the pyrrole nuclei (Fig. 2a,b). It should be noted that the macrocyclic current (Fig. 2a) possesses the major `bed' corresponding to the main 18-membered contour of conjugation and `arms' formed by the semi-isolated C()-C()

a b

The protons at the and meso positions are exocyclic with respect to the macrocyclic and local pyrrole currents and experience the deshielding influence of the latter. The signals for these protons are observed at low field (at from 9.7 to 11.2 ppm for the meso-protons and at from 8.5 to 9.9 ppm for the -protons, Table 1). The protons of the NH groups are exocyclic with respect to the local ring currents and endocyclic with respect to the macrocyclic current. Undoubtedly, the shielding effect of the latter prevails and the signals for the protons of the NH groups are observed at very high field (at from -1.4 to

Compound N-H Hm,p Ho H OCH3 , CH3

8.26 8.14 8.11 8.07

7.81 7.75 7.56 7.23

Table 1. 1H NMR chemical shift (/ppm) of the free base tetraarylporphyrins relative to

The -hydrogen atoms of the pyrrole and pyrrolenine fragments are in principle nonequivalent. However, rapid (within the NMR time scale) transformations occur at room temperature and the signals for the -protons are averaged. Two different peaks corresponding to resonance absorption of the energy by the protons of the -CH groups of the pyrrole and pyrrolenine fragments can be observed at low temperature, -80 ºC. [1,2]

Fig. 2. Schematic representation of the circuits of the ring -electron currents in the

porphyrin molecule; (a) the macrocyclic current, (b) local currents.[1]


N

8.87 8.85 8.86 8.79 - - 2.71 3.95

N

bonds in the pyrrolenine nuclei.


H2TPP H2T(4-Cl)PP H2T(4-CH3)PP H2T(4-CH3O)PP

CHCl3.

NH NH

N

N

On going from H2P to H4P2+ it would expect that the signals for the protons of the NH groups in the 1H NMR spectrum would be shifted downfield upon protonation of porphin. However, the signals are shifted upfield by 0.7 ppm. Simultaneously, the signals for the and meso-protons are shifted downfield. This character of the spectral changes indicates that protonation of porphin leads to a substantial increase in the strength of the magnetic field induced by the macrocyclic ring current.

Protonation of porphin is accompanied by a change in the geometric structure of the molecule. The C()-NH-C() fragments in the initial porphin are planar and the nitrogen atoms are sp2-hybridized. In the protonated porphin, these fragments adopt a pyramidal structure (the nitrogen atoms have nearly sp3 hybridization) and the degree of involvement of the nitrogen atoms in conjugation with the -carbon atoms is substantially reduced. This fact is confirmed by an increase in the C()-N bond length observed upon protonation of porphin. The above-mentioned rearrangements lead to a change in the contour of macrocyclic conjugation. In the case of protonated porphin, this conjugation is realized primarily along the outer contour of the molecule. Therefore, protonation causes an increase in the diameter of the conjugation ring resulting in upfield shifts of the signals for the internal protons and downfield shifts of the signals for the external protons. In the case of alkyl substitution in porphin (on going to octaethylporphin and ethioporphyrin), the signals for the protons of the NH groups and the signals for the meso-protons are shifted downfield and upfield, respectively. Analogous, but more pronounced, changes are observed in the presence of a substituent in the meso position. It should be noted that the introduction of both electron-donating (meso-tetra-iso-butyl- and meso-tetra-n-pentylporphyrin) and electron-withdrawing (tetraphenylporphyrin and its derivatives) substituents gives the same results, viz., the signals for -protons and for the protons of the NH groups are shifted upfield and downfield, respectively. It seems likely that the introduction of substituents both at the - and meso positions leads to reduction in the strength of the aromatic macrocyclic current regardless of the electronic nature of the substituent.

The currents in the porphyrin molecule are substantially affected by complex formation.[1] The higher the degree of covalence of the N-H bond, the larger the decrease in the ring current due to the presence of the coordinated metal atom. Coordination to the mediumsized M2+ cations (M =Mg, Zn, Cd, Ni or Pd) is accompanied by upfield shifts of the signals for the meso-protons (the ring current is reduced). In the spectra of the complexes in which the M2+ metal ion deviates from the plane due to its large radius and weak coordination interactions (M =Sn or Pb) or in the spectra of the complexes with the M3+ or M4+ ions in which the metal atom deviates from the plane of the macrocycle under the action of the extra ligands, the signals for the meso-protons are shifted downfield relative to those in the spectra of the porphyrin ligand. These shifts may be due to an increase in the ring current. However, this effect is insignificant and depends on the nature of the ions serving as the extra ligands.[1]

#### **1.2 Electronic absorption spectra of porphyrins**

Electronic absorption spectra of porphyrins are very characteristic and contain one intense band in the near-ultraviolet region of the spectrum around 390-425 nm depending on whether the porphyrin is - or meso-substituted with > 2 × 105, the Soret band or B band, followed by four low-intensity absorption bands at higher wavelengths (480 to 650 nm) in the visible region (Q Bands), see Fig. 3 and Table 2. These absorptions giving rise to striking colours of porphyrins. Thus, free base porphyrins have four Q bands, denoted by increasing wavelength as IV, III, II, and I. The commonly accepted classification of these bands is as follows. The bands I and III in the visible region of the spectrum (Fig. 3) belong to quasiforbidden electron transitions, whereas the bands II and IV are of electronic-vibrational origin, i.e., are vibrational satellites of the bands I and III, respectively. Although possessing a number of common features, the spectra of porphyrins show substantial variations, which reflect the changes in the molecular structure and the effect of the solvent.

Fig. 3. Typical UV-vis absorption spectrum of a porphyrin.


Table 2. UV-vis absorptions (CHCl3/nm) of the free base tetraarylporphyrins.

According to the universally accepted concepts, light absorption is accompanied by excitation of the porphyrin molecule and the characteristic features of the absorption spectrum are determined by transitions of the -electrons between two higher occupied and two lower unoccupied orbitals, the four-orbital Platt - Gouterman model.[1]

The intensity ratio of the absorption bands in the spectra of porphyrins depends on their structures in a peculiar fashion. When the relative intensities of these bands are such that IV > III > II > I, then the spectrum is said to be ethio-type after the ethioporphyrins in which

the visible region (Q Bands), see Fig. 3 and Table 2. These absorptions giving rise to striking colours of porphyrins. Thus, free base porphyrins have four Q bands, denoted by increasing wavelength as IV, III, II, and I. The commonly accepted classification of these bands is as follows. The bands I and III in the visible region of the spectrum (Fig. 3) belong to quasiforbidden electron transitions, whereas the bands II and IV are of electronic-vibrational origin, i.e., are vibrational satellites of the bands I and III, respectively. Although possessing a number of common features, the spectra of porphyrins show substantial variations, which

reflect the changes in the molecular structure and the effect of the solvent.

Fig. 3. Typical UV-vis absorption spectrum of a porphyrin.

Table 2. UV-vis absorptions (CHCl3/nm) of the free base tetraarylporphyrins.

two lower unoccupied orbitals, the four-orbital Platt - Gouterman model.[1]

According to the universally accepted concepts, light absorption is accompanied by excitation of the porphyrin molecule and the characteristic features of the absorption spectrum are determined by transitions of the -electrons between two higher occupied and

The intensity ratio of the absorption bands in the spectra of porphyrins depends on their structures in a peculiar fashion. When the relative intensities of these bands are such that IV > III > II > I, then the spectrum is said to be ethio-type after the ethioporphyrins in which

Compound Wavelength (nm)

H2TPP H2T(4-Cl)PP H2T(4-CH3)PP H2T(4-CH3O)PP the -substituents are all alkyl groups, Fig. 4a. In practice, the ethio-type Q band spectrum is found in meso-tetraphenylporphyrin, ethio-porphyrin and all porphyrins in which six or more of the -positions are substituted with groups without -electrons, e.g. alkyl groups. For porphyrins characterized by the rhodo-type of the spectra (after rhodoporphyrin XV) the following sequence III > IV > II > I is realized, Fig. 4b. Among the latter compounds are rhodoporphyrin and other porphyrins containing an electron-withdrawing substituent (COOH, NO2, Cl, etc.) in the pyrrole fragment. The rhodo-type spectrum has a "rhodoflying" or "reddening" effect on the spectrum by shifting it to longer wavelength. Phylloporphyrin and other porphyrins containing one or two meso-substituents are characterized by the phyllo type of the spectrum (IV > II > III > I), Fig. 4c. Chlorin, tetrabenzoporphyrin and phthalocyanine give characteristic spectra of their own. In their spectra, the long-wavelength band I has the maximum intensity. Variations of the peripheral substituents on the porphyrin ring often cause minor changes to the intensity and wavelength of these absorptions. Protonation of two of the inner nitrogen atoms or insertion of a metal into the porphyrin cavity also changes the visible absorption spectrum. These absorptions can often be very helpful in determining certain features on a porphyrin.

Fig. 4. Basic types of electronic absorption spectra of ethio- (a), rhodo- (b) and phylloporphyrins (c).

The meso-tetrasubstituted porphyrins are characterized by a distorted ethio type of spectra with rather large (25 - 40 nm) red shifts of the bands. The introduction of various substituents at the para positions of the phenyl rings of meso-tetraphenyl-porphyrin causes red shifts of the absorption bands in the visible region of the spectra. A comparison of the spectra of tetraphenylporphyrin and its para-substituted derivatives demonstrated that the intensities of the electron transition bands (I and III) rise as the electron-donating properties of the substituents increase. Simultaneously, the intensities of the vibrational bands (II and IV) are diminished.[3] The ethio type of the spectrum of tetraphenylporphyrin is distorted because the band I becomes more intense than the band II.

The presence of electron-donating substituents at the meta positions also leads to red shifts of the bands, the intensities of the vibrational bands being increased, while the intensities of the electron transition bands remaining virtually unchanged. The spectra of orthosubstituted tetraphenylporphyrins are characterized by lower intensities of the electron transition bands compared to those observed in the electronic absorption spectrum of tetraphenylporphyrin. In some cases, this gives rise to the spectra of the phyllo type, for example, in the case of tetra(2-halogenophenyl)porphyrins and, particularly, in the case of tetrakis(2,6-dihalogenophenyl)porphyrins.

The absorption spectra are also dependent on the solvent. Thus, the bands in the electronic absorption spectra of substituted meso-tetraphenylporphyrins are shifted redally and the intensities of the electron transition bands grow on going from nonpolar hexane to polar pyridine.

On going from porphyrin to metalloporphyrin, the symmetry of the planar macrocyclic fragment (of the -electron cloud of the macrocycle) increases due to which the spectrum is simplified. The Soret band changes only slightly upon complex formation. The visible region of the spectra of metal complexes has two absorption bands, viz., the band I corresponding to the electron transition and the band II corresponding to the electronicvibrational transition.

#### **1.3 Sitting-Atop (SAT) complexes**

A particularly attractive idea in the kinetic and mechanism of metalation of free base porphyrins is that of the Sitting-Atop (SAT) complexes.[4,5] In 1960, Fleischer and Wang [4] first proposed the so-called Sitting-Atop complexes of the protoporphyrin dimethyl ester with metal ions in chloroform on the basis of visible spectra, infrared spectra and composition of the SAT complex. According to the SAT idea the metalation of free base porphyrin begins with a preequilibrium step involving partial bonding of the metal ion to two of pyrrolenine nitrogens to form an intermediate, so-called Sitting-Atop complex. In this intermediate two protons on the pyrrole nitrogens still remain, M(H2P)2+ (Eq. 1). Then the overall metalation reaction will consist of at least two steps, i.e., the coordination step of two pyrrolenine nitrogens to form an intermediate, so-called Sitting-Atop complex, and the deprotonation step of the SAT complex to form the metalloporphyrin, as shown by following Equations, respectively:

$$\text{M}^{n+} + \text{H}\_2\text{P} \xrightarrow[\text{O}^-]{} \text{M}(\text{H}\_2\text{P})^{2+} \tag{1}$$

$$\text{M(H}\_2\text{P)}^{2+} \xleftarrow{\text{V} \longrightarrow \text{M(P)}^{\text{(n-2)+}} + 2\text{H}^+} \tag{2}$$

The existence of the SAT complex could explain the experimental kinetics of the metalation for several porphyrins in N,N-dimethylformamide (DMF),[6-8] acetic acid,[9] dimethyl sulfoxide (DMSO),[10] and H2O.[11,12] However, the formation constant of the SAT complex was reported as being on the order of 102 -104 mol -1dm3 for Cu(II) and Zn(II) ions in DMF.[13,14]

S. Funahashi et al.[15,16] have been succeeded to direct detection of SAT complexes of some tetraarylporphyrin with the Cu(II) ion in the acetonitrile as a solvent. Structural characterization of the Cu(II)-SAT complex in AN by the 1H NMR and EXAFS methods clarified that two pyrrolenine nitrogens coordinate to the equatorial sites of the Cu(II) ion and H2TPP act as a bidentate ligand.

They proposed the following overall mechanism for the metalation reaction of H2tpp with an octahedrally solvated metal(II) ion (MS62+) in the conventional basic solvent (S); which resolved into Eqs 3-8: the deformation of the porphyrin ring (eq 3), the outer-sphere association (eq 4), the rate-determining exchange of a coordinated solvent molecule with a first pyrrolenine nitrogen (eq 5), the chelate ring closure to form the SAT complex (eq 6), the first deprotonation on the pyrrole nitrogen in the SAT complex by the basic solvent molecule (S) (eq 7), and the second deprotonation to form the metalloporphyrin (eq 8),

$$\begin{array}{cccc}\text{K}\_{\text{D}}\\\text{H}\_{2\text{tpp}} \xrightarrow{\text{K}\_{\text{D}}} & \text{H}\_{2\text{tpp}}\end{array} \tag{3}$$

$$\text{H}\_2tpp + \text{MS}\_6^{2+} \xleftarrow{\text{K}\_{\text{OS}}} \text{H}\_2tpp \text{\* MS}\_6^{2+} \tag{4}$$

$$\mathrm{H\_2tpp\*^\*MS\_6^{2+}} \xrightarrow[k\_{\cdot 1}]{} \mathrm{MS\_6}(\mathrm{H\_2tpp})^{2+} + \mathrm{S} \tag{5}$$

$$\text{MS}\_{\text{S}}(\text{H}\_{2\text{ppp}})^{2+} \xrightarrow[\bullet]{\text{K}\_{\text{RC}}} \text{MS}\_{p}(\text{H}\_{2\text{ppp}})^{2+} + (\text{S}\cdot p)\text{S} \tag{6}$$

$$\text{MS}\_p(\text{H}\_2\text{tpp})^{2+} + \text{S} \xrightarrow[\bullet]{\text{K}\_{\text{-III}}} \text{MS}\_q(\text{H}\_2\text{tpp})^+ + \text{HS}^+ + (p\text{-}q)\text{S} \tag{7}$$

$$\text{MS}\_q(\text{H}\_2\text{tpp})^+ + \text{S} \xrightarrow[\bullet]{\text{K}\_{\text{Al}2}} \text{MS}\_r(\text{tpp}) + \text{HS}^+ + (q\text{-}r)\text{S} \tag{8}$$

Where, p 4, q p - 1, and r 2. In the case of M = Cu, S = AN, and S = py, it was determined by fluorescent XAFS measurements that p = 4 and r = 0.

There are numerous reports on thermodynamic parameters of SAT complex of free base porphyrins with bivalent metal ions. M. Tanaka et al.[7] have studied the incorporation of Cu(II), Zn(II) and Cd(II) in the free base H2TPP in DMF. They suggested a preequilibrium between metal ions and H2TPP prior to metal insertion and proton release. Their values of K were 1.6×104 M-1 for Cu(II), 7.2×102 M-1 for Zn(II), and no kinetic evidence for complex formation was found for Cd(II). Pasternack's group[11] kinetically found a K 5 M-1 for Cu(II)-H2TPP reaction in DMSO. P. Hambright and L.R. Rabinson [8] have studied the kinetic of Zn(II) incorporation into free base porphyrins in DMF as solvent. They obtained the following values of K for Zn(II) SAT complexes with tetraphenylporphyrin, tetrakis (2,6-diflourophenyl)porphyrin, tetra(4-aminophenyl)porphyrin, tetrakis(4-N,Ndimethylaminophenyl) porphyrin, and tetra(4-hydroxyphenyl)porphyrin, 12.3×103, 2.8×103, 7.7×103, 6.0×103, and 9.5×103 M-1, respectively. This short review on Sitting-Atop complexes illustrated that these intermediates have been interest from 1960.

Concerning non-basicity and noncoordinating properties of some solvents such as chloroform and dichloromethane make them suitable solvents for obtaining SAT complex as a solid product. On the other hand, molecular Lewis acids such as organotin(IV) halides because of some special properties, i.e., a high solubility in noncoordinating solvents, a suitable stability in solution, a good tendency to adduct formation, and enough stability of their adducts for characterization are remarkable candidates for determination of SAT complexes.

#### **1.4 Molecular adducts**

54 Molecular Interactions

The absorption spectra are also dependent on the solvent. Thus, the bands in the electronic absorption spectra of substituted meso-tetraphenylporphyrins are shifted redally and the intensities of the electron transition bands grow on going from nonpolar hexane to polar

On going from porphyrin to metalloporphyrin, the symmetry of the planar macrocyclic fragment (of the -electron cloud of the macrocycle) increases due to which the spectrum is simplified. The Soret band changes only slightly upon complex formation. The visible region of the spectra of metal complexes has two absorption bands, viz., the band I corresponding to the electron transition and the band II corresponding to the electronic-

A particularly attractive idea in the kinetic and mechanism of metalation of free base porphyrins is that of the Sitting-Atop (SAT) complexes.[4,5] In 1960, Fleischer and Wang [4] first proposed the so-called Sitting-Atop complexes of the protoporphyrin dimethyl ester with metal ions in chloroform on the basis of visible spectra, infrared spectra and composition of the SAT complex. According to the SAT idea the metalation of free base porphyrin begins with a preequilibrium step involving partial bonding of the metal ion to two of pyrrolenine nitrogens to form an intermediate, so-called Sitting-Atop complex. In this intermediate two protons on the pyrrole nitrogens still remain, M(H2P)2+ (Eq. 1). Then the overall metalation reaction will consist of at least two steps, i.e., the coordination step of two pyrrolenine nitrogens to form an intermediate, so-called Sitting-Atop complex, and the deprotonation step of the SAT complex to form the metalloporphyrin, as shown by

The existence of the SAT complex could explain the experimental kinetics of the metalation for several porphyrins in N,N-dimethylformamide (DMF),[6-8] acetic acid,[9] dimethyl sulfoxide (DMSO),[10] and H2O.[11,12] However, the formation constant of the SAT complex was reported as being on the order of 102 -104 mol -1dm3 for Cu(II) and Zn(II) ions

S. Funahashi et al.[15,16] have been succeeded to direct detection of SAT complexes of some tetraarylporphyrin with the Cu(II) ion in the acetonitrile as a solvent. Structural characterization of the Cu(II)-SAT complex in AN by the 1H NMR and EXAFS methods clarified that two pyrrolenine nitrogens coordinate to the equatorial sites of the Cu(II) ion

They proposed the following overall mechanism for the metalation reaction of H2tpp with an octahedrally solvated metal(II) ion (MS62+) in the conventional basic solvent (S); which resolved into Eqs 3-8: the deformation of the porphyrin ring (eq 3), the outer-sphere association (eq 4), the rate-determining exchange of a coordinated solvent molecule with a first pyrrolenine nitrogen (eq 5), the chelate ring closure to form the SAT complex (eq 6), the first deprotonation on the pyrrole nitrogen in the SAT complex by the basic solvent molecule (S) (eq 7), and the second deprotonation to form the metalloporphyrin (eq 8),

M(H2P) Mn+ 2+ + H2P (1)

M(H2P)2+ M(P)(n-2)+ + 2H+ (2)

pyridine.

vibrational transition.

**1.3 Sitting-Atop (SAT) complexes** 

following Equations, respectively:

and H2TPP act as a bidentate ligand.

in DMF.[13,14]

#### **1.4.1 Molecular adducts of organotin(IV) halides with free base** *meso***tetraarylporphyrins [17-23]**

#### **General procedure**

On excess addition of organotin(IV) halides to a purple solution of free base tetraarylporphyrin in dry chloroform, its color changed to green. Evaporation of the solvent

at room temperature results to shiny green powdery product. Elemental analysis, electronic absorptions spectra and 1H NMR spectroscopy were used to characterization of product.

#### **1.4.1.1 Molecular adducts of dimethyltin(IV) dichloride with free base mesotetraarylporphyrins, [(Me2SnCl2)2H2T(4-X)PP] [22]**

Table 3 shows the elemental analyses for adducts obtained from interaction of some organotin(IV) halides with free base tetraphenylporphyrin derivatives. These data are in agreement with a composition 2:1 of adducts in the solid state. On the basis of spectroscopic measurements a Sitting-Atop structure was suggested for these adducts. Of course, stoichiometry of adduct might differ in the solution from the solid product.

#### **1.4.1.2 UV-Visible analysis**

Interaction of tetraphenylporphyrins with sigma- and pi-acceptor species lead to green products that their UV-Vis spectra are comparable with diacid form of corresponding porphyrin. In the electronic absorption spectra of the diacid form of free base H2TPP the intensity of band I is greater than band II. All the bands in the UV-visible spectra of H4TPP2+ are shifted 20-40 nm to the red.[24-26] This is interpreted to be evidence of increased resonance interaction of the phenyl rings with the porphyrin nucleus in going from the free base to it's diacid form, which would be allowed by the tilting of the pyrroles observed in the solid state. The phenyl ring angle with plane of porphyrin in H2TPP is within 60-85 º, which this angle reduces to 21 º in H4TPP2+. The red shift in H4TPP2+ (UV-Vis H4TPP2+ (in CH2Cl2), 439, 602, 655 nm) relative to H2TPP is consistent with the structural observation that the phenyl rings in H4TPP2+ were more coplanar with the porphyrin nucleus than free base H2TPP. With this coplanarity some resonance interaction of these rings with porphyrin nucleus is thus allowed, and may be the reason why the diacid form of H2TPP is green in solution. Similar situations were observed for interaction of free base H2T(4-X)PPs with organotin(IV) halide Lewis acids. During these interactions original peaks of free base H2T(4-X)PPs (Soret band and Q band) were slowly changed to two new peaks which their position show about 20-40 nm red shift relative to the band V (Soret band) and the band I (of Q band) of free base H2T(4-X)PPs, Table 4. It seems that deformation of porphyrin structure during its interaction with various acceptor species is similar to deviations of porphyrins skeleton in porphyrin diacids H4T(4-X)PP2+ (Table 4). In the thermodynamic studies section we will review the spectral variation of free base meso-tetraarylporphyrins upon formation of their adducts.

Investigating the effect of adduct formation on the electronic absorption spectra of the free base porphyrins, the red shifts of absorption bands were observed on going from free bases to adducts. This is due to an increased in the resonance interaction of the peripheral phenyl rings with the porphyrin nucleus. The amounts of shift vary from 28 to 36 nm, depending on *the kind of acceptor, for the Soret band. In (MeSnBr3)2H2TPP, Soret band (448 nm) and Q band I (666 nm) with 30 and 20 nm shift, respectively, we find the greatest red shift in the H2TPP adducts. Also (MeSnBr3)2H2T(4-Cl)PP with 34 and 25, in (MeSnBr3)2H2T(4-CH3)PP with 31 and 29 nm, in (MeSnBr3)2H2T(4-CH3O)PP with 35 and 46 nm red shift they have the greatest red shift in the corresponding adducts. It indicates that there is a direct relation between the acceptor property of the organotin(IV) halide Lewis acid and the red shift of the electronic absorption spectra of the coordinated porphyrin. Also, the red shifts of the absorption bands of the coordinated tetraarylporphyrins depend on the substituents at the para positions of their phenyl rings. By* 


at room temperature results to shiny green powdery product. Elemental analysis, electronic absorptions spectra and 1H NMR spectroscopy were used to characterization of product.

Table 3 shows the elemental analyses for adducts obtained from interaction of some organotin(IV) halides with free base tetraphenylporphyrin derivatives. These data are in agreement with a composition 2:1 of adducts in the solid state. On the basis of spectroscopic measurements a Sitting-Atop structure was suggested for these adducts. Of course,

Interaction of tetraphenylporphyrins with sigma- and pi-acceptor species lead to green products that their UV-Vis spectra are comparable with diacid form of corresponding porphyrin. In the electronic absorption spectra of the diacid form of free base H2TPP the intensity of band I is greater than band II. All the bands in the UV-visible spectra of H4TPP2+ are shifted 20-40 nm to the red.[24-26] This is interpreted to be evidence of increased resonance interaction of the phenyl rings with the porphyrin nucleus in going from the free base to it's diacid form, which would be allowed by the tilting of the pyrroles observed in the solid state. The phenyl ring angle with plane of porphyrin in H2TPP is within 60-85 º, which this angle reduces to 21 º in H4TPP2+. The red shift in H4TPP2+ (UV-Vis H4TPP2+ (in CH2Cl2), 439, 602, 655 nm) relative to H2TPP is consistent with the structural observation that the phenyl rings in H4TPP2+ were more coplanar with the porphyrin nucleus than free base H2TPP. With this coplanarity some resonance interaction of these rings with porphyrin nucleus is thus allowed, and may be the reason why the diacid form of H2TPP is green in solution. Similar situations were observed for interaction of free base H2T(4-X)PPs with organotin(IV) halide Lewis acids. During these interactions original peaks of free base H2T(4-X)PPs (Soret band and Q band) were slowly changed to two new peaks which their position show about 20-40 nm red shift relative to the band V (Soret band) and the band I (of Q band) of free base H2T(4-X)PPs, Table 4. It seems that deformation of porphyrin structure during its interaction with various acceptor species is similar to deviations of porphyrins skeleton in porphyrin diacids H4T(4-X)PP2+ (Table 4). In the thermodynamic studies section we will review the spectral variation of free base meso-tetraarylporphyrins upon formation

Investigating the effect of adduct formation on the electronic absorption spectra of the free base porphyrins, the red shifts of absorption bands were observed on going from free bases to adducts. This is due to an increased in the resonance interaction of the peripheral phenyl rings with the porphyrin nucleus. The amounts of shift vary from 28 to 36 nm, depending on *the kind of acceptor, for the Soret band. In (MeSnBr3)2H2TPP, Soret band (448 nm) and Q band I (666 nm) with 30 and 20 nm shift, respectively, we find the greatest red shift in the H2TPP adducts. Also (MeSnBr3)2H2T(4-Cl)PP with 34 and 25, in (MeSnBr3)2H2T(4-CH3)PP with 31 and 29 nm, in (MeSnBr3)2H2T(4-CH3O)PP with 35 and 46 nm red shift they have the greatest red shift in the corresponding adducts. It indicates that there is a direct relation between the acceptor property of the organotin(IV) halide Lewis acid and the red shift of the electronic absorption spectra of the coordinated porphyrin. Also, the red shifts of the absorption bands of the coordinated tetraarylporphyrins depend on the substituents at the para positions of their phenyl rings. By* 

**1.4.1.1 Molecular adducts of dimethyltin(IV) dichloride with free base meso-**

stoichiometry of adduct might differ in the solution from the solid product.

**tetraarylporphyrins, [(Me2SnCl2)2H2T(4-X)PP] [22]** 

**1.4.1.2 UV-Visible analysis** 

of their adducts.

increasing electron donation of the *para*-substituents on the phenyl rings greater red shift were observed for the absorption bands in the visible spectra of tetraarylporphyrin during adduct formation. Also the electronic absorption spectra of adducts are indicating that the intensity of the electron transition band I of the adducts rises as the electron donation properties of the substituents increase.

Considering max of the electronic absorption bands (Soret and band I) of adducts it might be pointed out that the kind of acceptor have a minor effect on the position of the adduct bands. It seems positions of these bands depend on distortion and tilting of the porphyrin plane, which occurred during formation of 1:1 adduct for different acceptors. Therefore the kind of acceptor and also entering the second acceptor species isn't accompanied with a significant replacement in the position of the adduct bands. The same statement can be discussed for position of the isosbestic points of free base porphyrins adducts with various acceptors. The effect of distortion of porphyrin structure on electronic absorption bands and 1H NMR chemical shift of various free base porphyrin protons will be better understood by comparison of electronic absorption spectra of 5,10,15,20-tetrabutylporphyrin as a planar free base porphyrin [UV-vis (CH2Cl2): Soret band (417 nm) and Q bands ( 520, 555, 600, 659 nm); 1H NMR: N-H (-2.61 ppm) and H(9.45 ppm)] with 5,10,15,20-tetrakis(tert-butyl)porphyrin as a severely ruffled (distorted) free base porphyrin [Soret band (446 nm) and Q bands (552, 596, 628, 691 nm); 1H NMR: N-H (1.52 ppm) and H(9.08 ppm)]. This example finely shows that the observed red shift mainly resulted upon distortion of porphyrin structure.

#### **1.4.2 Molecular adducts of methyltin(IV) tribromide with free base** *meso***tetraarylporphyrins, [(MeSnBr3)2H2T(4-X)PP]: [21]**

On addition of methyltin(IV) tribromide to a solution of free base tetraarylporphyrins in dry chloroform its purple color changed to green, the electronic absorption spectra of adducts are given in the Table 4.

Fig. 5. Typical electronic absorption spectra of porphyrin diacid (H4TPP2+) and porphyrin molecular adducts [(RnSnCl4-n)2H2TPP].


Table 4. UV-vis absorptions (CHCl3/nm) of [(Me2SnCl2)2H2T(4-X)PP] adducts.

#### **1.4.2.1 1H NMR analysis**

58 Molecular Interactions

increasing electron donation of the *para*-substituents on the phenyl rings greater red shift were observed for the absorption bands in the visible spectra of tetraarylporphyrin during adduct formation. Also the electronic absorption spectra of adducts are indicating that the intensity of the electron transition band I of the adducts rises as the electron donation

Considering max of the electronic absorption bands (Soret and band I) of adducts it might be pointed out that the kind of acceptor have a minor effect on the position of the adduct bands. It seems positions of these bands depend on distortion and tilting of the porphyrin plane, which occurred during formation of 1:1 adduct for different acceptors. Therefore the kind of acceptor and also entering the second acceptor species isn't accompanied with a significant replacement in the position of the adduct bands. The same statement can be discussed for position of the isosbestic points of free base porphyrins adducts with various acceptors. The effect of distortion of porphyrin structure on electronic absorption bands and 1H NMR chemical shift of various free base porphyrin protons will be better understood by comparison of electronic absorption spectra of 5,10,15,20-tetrabutylporphyrin as a planar free base porphyrin [UV-vis (CH2Cl2): Soret band (417 nm) and Q bands ( 520, 555, 600, 659 nm); 1H NMR: N-H (-2.61 ppm) and H(9.45 ppm)] with 5,10,15,20-tetrakis(tert-butyl)porphyrin as a severely ruffled (distorted) free base porphyrin [Soret band (446 nm) and Q bands (552, 596, 628, 691 nm); 1H NMR: N-H (1.52 ppm) and H(9.08 ppm)]. This example finely shows that the observed red

properties of the substituents increase.

shift mainly resulted upon distortion of porphyrin structure.

**tetraarylporphyrins, [(MeSnBr3)2H2T(4-X)PP]: [21]** 

**Soret (II)**

are given in the Table 4.

1.2

0

molecular adducts [(RnSnCl4-n)2H2TPP].

0.3

0.6

Absorbance

0.9

**1.4.2 Molecular adducts of methyltin(IV) tribromide with free base** *meso***-**

On addition of methyltin(IV) tribromide to a solution of free base tetraarylporphyrins in dry chloroform its purple color changed to green, the electronic absorption spectra of adducts

> 420 470 520 570 620 670 Wavelength

Fig. 5. Typical electronic absorption spectra of porphyrin diacid (H4TPP2+) and porphyrin

**I**

Up to getting a detailed description of 1H NMR spectra of adducts, an overview on 1H NMR of Cu(II)-SAT complex, S. Funahashi et al.,[16] will be informative. S. Funahashi et al. have studied the 1H NMR variation of H2TPP during a reaction between Cu(II) ion and H2TPP in acetonitrile for detecting the SAT complex by spectroscopic method.

Table 5 summarized the chemical shift values of SAT complex together with the reported values for H2TPP.[27, 28] Although the SAT complex contains the paramagnetic Cu(II) ion having *S* = 1/2, the -pyrrole protons have been clearly observed. In the 1H NMR spectra signal of the N-H protons in the SAT complex was clearly observed at -2.05 ppm relative to TMS strongly indicated that the N-H protons remain in the SAT complex (Cu(H2TPP)2+). In contrast to the case of H2TPP, two kinds of -protons for the SAT complex were observed with a ratio of 1:1, which had twice the area of the peak for N-H protons. The peaks, one a singlet and the other a doublet, were attributed to the protons in the pyrrolenine group coordinated and in the pyrrole group not coordinated to the Cu(II) ion, respectively. A similar splitting was reported for H2TPP at the low temperature of -80 °C, because the N-H


Table 5. Values of chemical shift (; relative to TMS/ppm) for H2TPP and it's SAT complex in CD3CN.

tautomerism at such a low temperature was frozen where the signal of the -pyrrole protons of the pyrrole groups with N-H protons was observed at a lower field relative to that of the pyrrolenine groups without the N-H protons. The opposite trend in chemical shift was, however, observed for the SAT complex, i.e., the doublet peak assigned to the -pyrrole protons appears at a higher field (see Fig. 6). This indicates that the two pyrrolenine nitrogens without an N-H proton bind to the paramagnetic Cu(II) ion, because the dipoledipole and scalar coupling interactions with the paramagnetic ion lead to their downfield shift.[15,16] Furthermore, the peak for N-H protons was shifted downfield relative to that of H2TPP. This is ascribed to both the distortion of the porphyrin ring and the dipole-dipole interaction with the paramagnetic ion. Because the N-H protons in H2TPP are highly shifted to the upper field due to the ring current of the planar porphyrin ring, the distortion of the porphyrin ring in the SAT complex then leads to the downfield shift. The 1H NMR spectrum determines a symmetrical structure with pyrrolenine nitrogens coordinating to the Cu(II) ion.

We used the low temperature 1H NMR (-30 ºC) to study the molecular interaction of free base porphyrins with organotin(IV) halides. 1H NMR spectra of free base porphyrins undergo considerable variations during these interactions.

Data given in Table 6 are indicating that by interaction of free bases H2T(4-X)PP with RnSnX4-n the original bands of N-H, H, Ho, Hm, HCH3 and HOCH3, were shifted and a new pattern were appeared for 1H NMR spectrum of porphyrin moiety of adducts.


Table 6. 1H NMR chemical shift (/ppm) of [(Me2SnCl2)2H2T(4-X)PP] adducts relative to CHCl3.

tautomerism at such a low temperature was frozen where the signal of the -pyrrole protons of the pyrrole groups with N-H protons was observed at a lower field relative to that of the pyrrolenine groups without the N-H protons. The opposite trend in chemical shift was, however, observed for the SAT complex, i.e., the doublet peak assigned to the -pyrrole protons appears at a higher field (see Fig. 6). This indicates that the two pyrrolenine nitrogens without an N-H proton bind to the paramagnetic Cu(II) ion, because the dipoledipole and scalar coupling interactions with the paramagnetic ion lead to their downfield shift.[15,16] Furthermore, the peak for N-H protons was shifted downfield relative to that of H2TPP. This is ascribed to both the distortion of the porphyrin ring and the dipole-dipole interaction with the paramagnetic ion. Because the N-H protons in H2TPP are highly shifted to the upper field due to the ring current of the planar porphyrin ring, the distortion of the porphyrin ring in the SAT complex then leads to the downfield shift. The 1H NMR spectrum determines a symmetrical structure with pyrrolenine nitrogens coordinating to the Cu(II)

We used the low temperature 1H NMR (-30 ºC) to study the molecular interaction of free base porphyrins with organotin(IV) halides. 1H NMR spectra of free base porphyrins

Data given in Table 6 are indicating that by interaction of free bases H2T(4-X)PP with RnSnX4-n the original bands of N-H, H, Ho, Hm, HCH3 and HOCH3, were shifted and a new

Compound N-H Hm,p Ho H OCH3 ,CH3

8.57 8.54 8.50 8.48

8.56-61 8.46-55 8.49-59 8.46-53

8.59-8.62 8.52-8.59 8.47-8.54 8.53-8.55

8.64 8.54-8.57 8.53-8.62 8.37 8.49-8.59 8.51-8.56 8.37-8.42 8.24-8.27 8.57 8.54 8.50 8.48

8.56-61 8.46-55 8.49-59 8.46-53

8.59-8.62 8.52-8.59 8.47-8.54 8.53-8.55

8.64 8.54-8.57 8.53-8.62 8.52 8.49-8.59 8.51-8.56 8.37-8.42 8.48-8.53 - - 2.89 4.11




7.95 8.00 7.94 7.50

8.01-04 8.09-12 7.81-85 7.49-56

7.99-8.02 8.01-8.05 7.80-7.85 7.55-7.58

8.05-8.07 7.92-8.00 8.04-8.07 8.18-8.21 7.83-7.86 7.83-7.86 7.41-7.56 7.94

Table 6. 1H NMR chemical shift (/ppm) of [(Me2SnCl2)2H2T(4-X)PP] adducts relative to

pattern were appeared for 1H NMR spectrum of porphyrin moiety of adducts.

undergo considerable variations during these interactions.

0.00 0.00 0.00 0.00

0.00 0.06 0.15 0.20


0.00 -0.45 0.00 0.00 0.00 -0.30 0.00 0.00

ion.

(Me2SnCl2)2H2TPP (Me2SnCl2)2H2T(4-Cl)PP (Me2SnCl2)2H2T(4-CH3)PP (Me2SnCl2)2H2T(4-CH3O)PP

(Et2SnCl2)2H2TPP (Et2SnCl2)2H2T(4-Cl)PP (Et2SnCl2)2H2T(4-CH3)PP (Et2SnCl2)2H2T(4-CH3O)PP

(MeSnCl3)2H2TPP (MeSnCl3)2H2T(4-Cl)PP (MeSnCl3)2H2T(4-CH3)PP (MeSnCl3)2H2T(4-CH3O)PP

(Ph2SnCl2)2H2TPP (Ph2SnBr2)2H2TPP (Ph2SnCl2)2H2T(4-Cl)PP (Ph2SnBr2)2H2T(4-Cl)PP (Ph2SnCl2)2H2T(4-CH3)PP (Ph2SnBr2)2H2T(4-CH3)PP (Ph2SnCl2)2H2T(4-CH3O)PP (Ph2SnBr2)2H2T(4-CH3O)PP

CHCl3.

Comparing the chemical shifts of different protons of the free base H2T(4-X)PPs with the same protons of H2T(4-X)PPs in its complexes some useful information can be induced. The internal N-H signal moves downfield (2.7-2.8 ppm) and H protons signal move upfield (0.25-0.31 ppm). Both changes are discontinuous and are in the directions to be expected if the aromatic ring current decreases with the interaction of free base porphyrins with organotin(IV) halides. The two aromatic proton doublets related to the Ho and Hm,p of the phenyl rings in H2TPP, also Ho and Hm of phenyl of the H2T(4-X)PPs, moves downfield (0.31-0.25 ppm) and HCH3(HOCH3) singlets of H2T(4-X)PPs (X= CH3 and CH3O) moves downfield (0.31-0.14 ppm). These changes can be described by decreasing the aromatic ring current because of the coordination of porphyrin to the organotin(IV) halide Lewis acids that led to deformation of porphyrin structure from planarity. Such changes were seen in the SAT complexes, let alone the paramagnetic property of Cu(II) ion which lead to some differences between 1H NMR spectra of SAT complexes with our adducts. Also the 1H NMR spectra of these adducts are comparable with related porphyrin dication.[25-27] Concerning the 1H NMR spectra of 22,24-dihydro-5,10,15,20-tetraphenylporphyrin diperchlorate [H4TPP2+][ClO4- ]2; N-H (-2.47 ppm), H*m,p*(8.04 ppm), Ho (8.61), H (8.77ppm) and 22,24 dihydro-5,10,15,20-tetraphenylporphyrin dihydrogen sulfate [H4TPP2+][HSO4-]2; N-H (-1.21 ppm), H*m,p*(7.82 ppm), Ho (8.45), H (8.51ppm) , ( relative to TMS in CDCl3) similar trends can be found between 1H NMR chmical shift of porphyrin dication and our adducts.

According to 1H NMR pattern these adducts have symmetrical structures, so that coordination to organotin(IV) halides couldn't differ between each class of free base porphyrin protons (N-H, H, Ho, and … ) upon adduct formation and these protons remained equivalent after complex formation. Referring to elemental analysis data these adducts have the mole ratio 2:1 of acceptor to donor, [(R2SnCl2)2(H2T(4-X)PP)].

In Fig. 6 (a), because of attachment of tin atoms to two pyrrolenine nitrogens, splitting of 1H NMR signal of pyrroles and pyrrolenine Hs and producing a doublet band for beta hydrogens is expected, a singlet for pyrroles Hs and a singlet for pyrrolenines Hs. On the other hand, in Fig. 6 (b) and (c) Hs have relatively identical environment, therefore we predict a singlet band for both pyrroles and pyrrolenine Hs, but in Fig. 6 (c) because of high steric congestion of N-H pyrrole hydrogens with organotin(IV) halide upon adduct formation, doesn't seem it has a significant apart in adduct structure. Experimentally, in the low-temperature 1H NMR spectra of adducts a singlet was appeared for Hs, referring to S. Funahashi results on slowness of the N-H tautomerism in the SAT complex with respect to the time scale of the 1H NMR at 21 °C,[16] it is confirming the Fig. 6 (b) structure for these adducts. On the basis of these results we suggest that free base porphyrin as a bidentate bridging ligand make a bridge between two molecules of the Lewis acid through it's nitrogenes lone pairs. It is probable that two neighbor nitrogen atoms (a pyrrole and a pyrrolenine nitrogen) of the porphyrin binded to one of the R2SnCl2 molecules which posited on the above of the porphyrin plane and the other two nitrogen atoms (a pyrrole and a pyrrolenine nitrogen) binded to the second R2SnCl2 molecule from below of this plane. Therefore we have a structure close to that suggested by K.M. Smith et al. for XHg-TPP-HgX, X= Cl- and CH3COO-.[29]

Interactions of organotin(IV) halides with free base H2T(4-X)PP are very sensitive to temperature and donor property of the solvent, *Fig.* 7. By increasing the temperature the green color of the adduct solution changes to brown and the color return to the primary color of free base porphyrin solution, eventually. Also these adduct formations were not observed in ligating solvents such as CH3CN and DMSO under these conditions.

Fig. 6. Proposed structures of [(R2SnCl2)2H2TPP] adducts. (a) H2TPP as a monodentate bridging ligand made adducts with five-coordinated trigonal bipyramidal structure for tin atoms; (b) and (c) H2TPP as a bidentate bridging ligand make adducts with six-coordinated octahedral structure around the tin atoms. Our 1H NMR data are in consistent with structure (b).

Fig. 7. The temperature dependence change of the absorption profile of the [(Me2SnCl2)2H2T(4-Cl)PP] adduct in chloroform: By addition of Me2SnCl2 (0.0125 M, 0.5 ml) to the solution of H2T(4-Cl)PP (5 × 10-6 M, 2.5 ml) in chloroform in an UV-vis cell at 5 C, the (Me2SnCl2)2H2T(4-Cl)PP adduct was formed. Then the composition of the cell was remained constant and the temperature was raised to 45 C, stepwisely.

#### **1.4.2.2 Basicity of the free base porphyrins:**

The basicity of the free base porphyrins toward proton is usually determined when possible by pK3 measurements for the dissociation of the mono-cation H3T(4-X)PP+ into the free base H2T(4-X)PP measured in detergent solutions:

Molecular Interactions of Some Free Base Porphyrins with - and -Acceptor Molecules 63

$$\rm H\_3T(4\cdot \lambda)\rm PP^\* = \rm H\_2T(4\cdot \lambda)\rm PP + \rm H^\* \qquad K\_3 \tag{9}$$

On the other hand, the basicity of the porphyrins have been related to the reduction potential (in volts) of the free base porphyrin to its radical anion form.[30] The value of E1/2(1) for some of the free base porphyrins in DMF is given in Table 7.

$$\text{H}\_2\text{T}(\text{4-X})\text{PP} + \text{e} \xleftarrow{\text{Au} \to \text{H}} \text{H}\_2\text{T}(\text{4-X})\text{PP} \quad \text{E}\_{1/2}(\text{1}) \tag{10}$$


Table 7. Reduction potentials (in volts) for H2T(4-X)PPs ( in DMF, 25 ºC).[30]

A linear relationship between pK3 and E1/2(1) has been reported for some of porphyrins:

$$\mathrm{pK}\_3 = 5.9 \to \mathrm{E\_{1/2}(1)} - 5.2$$

according to this Equation the most basic porphyrins are the more difficult to reduce. Therefore, more negative values of E1/2(1) are indicating stronger basic properties of free base porphyrin.

#### **1.4.2.3 The thermodynamic studies**

62 Molecular Interactions

green color of the adduct solution changes to brown and the color return to the primary color of free base porphyrin solution, eventually. Also these adduct formations were not

Sn

Cl Cl

R

N

Sn Sn H H

N

N

Cl

Cl

N

N

Ph

Ph

Ph

Cl

R

R

Cl

Ph

R R

Ph

Sn

Cl Cl

R

N

R

<sup>H</sup> <sup>H</sup>

N

Ph Ph

N

observed in ligating solvents such as CH3CN and DMSO under these conditions.

R

Ph

R R

Sn

Cl

N

Ph

structure (b).

Sn

Cl

Cl

N

Ph

R R

<sup>H</sup> <sup>H</sup>

N

Ph Ph

Cl

N

(a) (b) (c)

Fig. 6. Proposed structures of [(R2SnCl2)2H2TPP] adducts. (a) H2TPP as a monodentate bridging ligand made adducts with five-coordinated trigonal bipyramidal structure for tin atoms; (b) and (c) H2TPP as a bidentate bridging ligand make adducts with six-coordinated

octahedral structure around the tin atoms. Our 1H NMR data are in consistent with

Fig. 7. The temperature dependence change of the absorption profile of the

constant and the temperature was raised to 45 C, stepwisely.

**1.4.2.2 Basicity of the free base porphyrins:** 

H2T(4-X)PP measured in detergent solutions:

[(Me2SnCl2)2H2T(4-Cl)PP] adduct in chloroform: By addition of Me2SnCl2 (0.0125 M, 0.5 ml) to the solution of H2T(4-Cl)PP (5 × 10-6 M, 2.5 ml) in chloroform in an UV-vis cell at 5 C, the (Me2SnCl2)2H2T(4-Cl)PP adduct was formed. Then the composition of the cell was remained

The basicity of the free base porphyrins toward proton is usually determined when possible by pK3 measurements for the dissociation of the mono-cation H3T(4-X)PP+ into the free base The thermodynamic parameters are useful tools for studying the interactions between the donor and the acceptor molecules. The first step in these studies is the determination of formation constants, which make help to better understanding of relative stability of molecular adducts. In these studies we used the SQUAD program [31] for data refinement. This program is designed to calculate the best values for the equilibrium constants of the proposed equilibrium model by employing a non-linear least-squares approach and UV-vis data. The formation constants and thermodynamic parameters Hº, Sº and Gº values for different systems are listed in Tables 8 and 9, respectively.

According to results presented for molecular interactions of organotin(IV) halides with free base porphyrins the following order was obtained for acceptor properties of organotin(IV) Lewis acids:

MeSnBr3 > Me2SnCl2> Ph2SnCl2 > Ph2SnBr2 > Et2SnCl2 > Bu2SnCl2

This trend shows that interaction of organotin(IV) halides with free base porphyrins become weaker by increasing the electron-releasing and the steric hindrance of substituents on the tin atom.

On the other hand, results show the effect of X substituents of phenyl rings of free base porphyrins on the stability of adducts, explicitly. Considering data given in Tables 8 and 9 for the interaction of organotin(IV) Lewis acids with free base porpyrins, H2T(4-X)PP (X= H, Cl, CH3, CH3O, NO2), basicity of free base porphyrins for interaction with organotin(IV) halides decreases as follows:

H2T(4-CH3O)PP > H2T(4-CH3)PP > H2TPP > H2T(4-Cl)PP > H2T(4-NO2)PP


Of course, H2T(4-NO2)PP under our working conditions (concentration and temperature) did not show a measurable interaction with diorganotin(IV) dihalides.

Table 8. The formation constants lgK for H2T(4-X)PP adducts in CHCl3 solvent.

Of course, H2T(4-NO2)PP under our working conditions (concentration and temperature)

6.95± 0.01 5.66± 0.01 9.30± 0.01 9.43± 0.01

5.87 ± 0.01 4.76 ± 0.01 6.51 ± 0.01 7.34 ± 0.02

5.34 ± 0.02 4.32 ± 0.02 6.40 ± 0.02 7.15 ± 0.02

6.46±0.01 5.90±0.03 7.66±0.01 9.04±0.02

6.00±0.02 5.33±0.01 6.80±0.02 8.40±0.02

11.33± 0.03 10.56± 0.04 6.11 ± 0.03 12.49± 0.06 13.12± 0.03

5 C 10 C 15 C 20 C 25 C

6.46± 0.01 5.14± 0.01 8.46± 0.01 8.69± 0.01 5.81± 0.01 4.64± 0.01 7.81± 0.01 7.96± 0.01 5.05± 0.01 - 6.99± 0.01 7.20± 0.01 - -

5.11 ± 0.02 5.93 ± 0.01

4.04 ± 0.04 3.11 ± 0.02 5.15 ± 0.02 5.80 ± 0.01

4.24±0.01 4.10±0.01 5.10±0.02 6.63±0.04

4.07±0.08 4.05±0.08 4.81±0.05 6.13±0.05

8.45 ± 0.02 7.95 ± 0.02 4.41 ± 0.04 9.31 ± 0.05 9.76 ± 0.09

5.08 ± 0.04 4.04 ± 0.04 5.45 ± 0.05 6.38 ± 0.02

4.38 ± 0.02 3.54 ± 0.01 5.32 ± 0.02 6.20 ± 0.02

5.25±0.04 4.85±0.04 5.94±0.03 7.50±0.02

4.83±0.07 4.71±0.09 5.35±0.06 6.68±0.06

9.58 ± 0.03 9.15 ± 0.02 4.90 ± 0.08 10.45± 0.04 10.88± 0.02

5.53 ± 0.02 4.36 ± 0.02 6.04 ± 0.04 7.04 ± 0.04

4.88 ± 0.01 3.82 ± 0.01 5.99 ± 0.01 6.76 ± 0.01

5.57±0.09 5.40±0.09 6.69±0.09 8.20±0.09

5.36±0.09 5.07±0.09 5.71±0.09 7.52±0.09

10.46± 0.03 9.75 ± 0.02 5.58 ± 0.05 11.53± 0.04 11.96± 0.02

did not show a measurable interaction with diorganotin(IV) dihalides.

7.61± 0.02 6.10± 0.01 10.06±0.01 10.25± 0.01

6.38 ± 0.02 5.25 ± 0.02 6.96 ± 0.01 8.08 ± 0.03

5.69 ± 0.01 4.70 ± 0.01 6.89 ± 0.01 7.85 ± 0.01

7.12±0.01 6.32±0.02 8.02±0.01 9.90±0.02

6.45±0.01 5.91±0.01 7.50±0.02 9.20±0.02

12.51 ± 0.06 11.41 ± 0.03 6.68 ± 0.01 13.61 ± 0.06 14.28 ± 0.03

Table 8. The formation constants lgK for H2T(4-X)PP adducts in CHCl3 solvent.

Adduct lgK

[(Me2SnCl2)2H2TPP] [(Me2SnCl2)2H2T(4-Cl)PP] [(Me2SnCl2)2H2T(4-CH3)PP]

[(Me2SnCl2)2H2T (4-CH3O)PP] [(Et2SnCl2)2H2TPP] [(Et2SnCl2)2H2T(4-Cl)PP] [(Et2SnCl2)2H2T(4- CH3)PP]

[(Et2SnCl2)2H2T (4-CH3O)PP] [(Bu2SnCl2)2H2TPP] [(Bu2SnCl2)2H2T(4-Cl)PP] [(Bu2SnCl2)2H2T(4-CH3)PP]

[(Bu2SnCl2)2H2T (4-CH3O)PP] (Ph2SnCl2)2H2TPP (Ph2SnCl2)2H2T(4-Cl)PP (Ph2SnCl2)2H2T(4-CH3)PP

(Ph2SnCl2)2H2T (4-CH3O)PP (Ph2SnBr2)2H2TPP (Ph2SnBr2)2H2T(4-Cl)PP (Ph2SnBr2)2H2T(4-CH3)PP

(Ph2SnBr2)2H2T (4-CH3O)PP (MeSnBr3)2H2TPP (MeSnBr3)2H2T(4-Cl)PP (MeSnBr3)2H2T(4-NO2)PP (MeSnBr3)2H2T(4-CH3)PP (MeSnBr3)2H2T(4-CH3O)PP


Adducts have negative values of Hº , Sº, and Gº (Table 9) which correspond to exothermic adduct formations between organotin(IV) Lewis acids and free base porphyrins.

<sup>a</sup>Hº (kJ.mol-1), Sº (J.K-1. mol-1), and Gº at 10 C (kJ. mol-1).

Table 9. The thermodynamic parameters Hº, Sº and Gº for H2T(4-X)PP adducts in CHCl3.a

#### **2. Molecular interactions of organic -acceptors with H2T(4-X)PP (X= H, Cl, CH3, CH3O)**

Intermolecular charge-transfer **(CT)** complexes are formed when electron donors and electron acceptors interact, a general phenomenon in organic chemistry. Mulliken [32] considered such complexes to arise from a Lewis acid-Lewis base type of interaction, the bond between the components of the complex being postulated to arise from the partial transfer of an electron from the base to orbitals of the acid. TCNE, DDQ, TBBQ and TCBQ can form **CT** complexes when mixed with molecules possessing -electrons or groups having atoms with an unshared electron pair [33-35]. The thermodynamic of CT complexes of several free base tetraaryl- as well as tetraalkylporphyrins with organic -acceptors TCNE, DDQ, TBBQ and TCBQ have been investigated. It would be instructive to compare their thermodynamic parameters since it improve our insight about characteristics of their interactions and efficiency of their applications.


Adducts which have been prepared from mixing of organic -acceptors with a solution of free base porphyrin in dry chloroform, those 1H NMR and UV-vis data were been given in Tables 10-12.

Table 10. 1H NMR of free base tetraarylporphyrins and their adduct with DDQ and TCNE.

The schematic chemical equilibria for interaction of molecular -acceptors with free base porphyrins could be written according to Equation 11: [36-38]

2(-acceptor) + (free base porphyrin) (-acceptor)2(free base porphyrin) (11)

#### **2.1 <sup>1</sup> H NMR analysis**

1H NMR analysis: in the 1H NMR spectra of adducts the signals correspond to N-H, Ho, Hm,p, and CH3- or -OCH3 protons of tetraarylporphyrin moved downfield, while H has an upfield shift (Table 11). But tetraalkylporphyrins, the signals correspond to H has an upfield shift the N-H signal in 5 and 11 moved downfield while for 6 and 12 upfield shift observed for this proton with adduct formation (Table 12).


Table 11. 1H NMR of free base tetraalkylporphyrins and their adduct with DDQ and TCNE.

#### **2.2 Infra red spectra**

66 Molecular Interactions

Adducts which have been prepared from mixing of organic -acceptors with a solution of free base porphyrin in dry chloroform, those 1H NMR and UV-vis data were been given in

> 7.75 8.04 7.98 7.79 8.06 7.75 8.05 8.00 7.76 7.80 7.56 7.83 7.80 7.57 7.57 7.27 7.55 7.50 7.27 7.60

Table 10. 1H NMR of free base tetraarylporphyrins and their adduct with DDQ and TCNE.

The schematic chemical equilibria for interaction of molecular -acceptors with free base

2(-acceptor) + (free base porphyrin) (-acceptor)2(free base porphyrin) (11)

1H NMR analysis: in the 1H NMR spectra of adducts the signals correspond to N-H, Ho, Hm,p, and CH3- or -OCH3 protons of tetraarylporphyrin moved downfield, while H has an upfield shift (Table 11). But tetraalkylporphyrins, the signals correspond to H has an upfield shift the N-H signal in 5 and 11 moved downfield while for 6 and 12 upfield shift

Compound N-H CH3 CH2 CH2 CH2 H

Table 11. 1H NMR of free base tetraalkylporphyrins and their adduct with DDQ and TCNE.

1.73-1.91 1.93 1.90 - - -

2.41-2.58 2.54 2.55 - - -

4.88 4.83 4.81 - - -

9.45 9.03 9.02 9.08 8.14 8.16

1.10 1.22 1.20 2.01 2.13 2.10

porphyrins could be written according to Equation 11: [36-38]

observed for this proton with adduct formation (Table 12).


8.24 8.64 8.67 8.28 8.67 8.14 8.57 8.56 8.20 8.58 8.11 8.52 8.55 8.12 8.12 8.11 8.57 8.59 8.13 8.40

8.85 8.81 8.59 8.81 8.81 8.85 8.81 8.56 8.83 8.83 8.86 8.76 8.55 8.84 8.84 8.86 8.68 8.48 8.76 8.76



2.71 2.80 2.77 2.75 2.75 3.95 4.17 4.11 3.96 4.10

Compound N-H Hm,p Ho H CH3


Tables 10-12.

(TCNE)2H2TPP (DDQ)2H2TPP (TCBQ)2H2TPP (TBBQ)2H2TPP H2T(4-Cl)PP

(TCNE)2H2T(4-Cl)PP (DDQ)2H2T(4-Cl)PP (TCBQ)2H2T(4-Cl)PP (TBBQ)2H2T(4-Cl)PP H2T(4-CH3)PP

(TCNE)2H2T(4-CH3)PP (DDQ)2H2T(4-CH3)PP (TCBQ)2H2T(4-CH3)PP (TBBQ)2H2T(4-CH3) PP H2T(4-CH3O)PP (TCNE)2H2T(4-OCH3)PP (DDQ)2H2T(4-OCH3)PP (TCBQ)2H2T(4-OCH3)PP (TBBQ)2H2T(4-OCH3)PP

**H NMR analysis** 

H2TPP

**2.1 <sup>1</sup>**

H2TnBP

H2TtBP

(TCNE)2H2TnBP (DDQ)2H2TnPP

(TCNE)2H2TtBP (DDQ)2H2TtBP

The most significant difference which emerges from a comparison of the vibrational spectra of free base porphyrins and their molecular adducts with -acceptor molecules is disappearance of the band (N–H) as a consequence of the complex formation. It seams that N–H was contributed in intermolecular hydrogen bonds that formed between its proton and N or O atoms of -acceptor molecules. The IR spectra of free TCNE shows CN stretching frequencies at 2225 and 2270 cm-1. The significant shift of these CN vibrations toward lower frequencies (2214 cm-1) is indicating charge transferring from free base porphyrins to an orbital of CN group of TCNE. This charge transfer phenomenon leads to weakening of corresponding CN bond. Similarly, in the free DDQ the CO and the CN stretching frequencies were appeared at 1675 and 2245 cm-1, respectively. Upon complex formation the CO vibrations shift to 1651 and 1696 cm-1 and CN vibration shift to 2229 cm-1.


Table 12. UV-vis peaks (λ/nm in CHCl3) of FBPs and [(A)2D] adducts.

#### **2.3 UV-vis analysis**

By interaction of free base porphyrin with -acceptor molecules, their original peaks changed to a new pattern with two new peaks which their positions show about 20-40 nm red shift relative to the band V (Soret band) and the band I (of Q band) of free base H2T(4- X)PPs, Table 10.


On the other hand, for TCNE, TBBQ and TCBQ interactions are endothermic and at elevated temperatures the stability of adducts increased, so that a purple solution of reactant at 5 C turns to a green solution of adduct at 35 C. It means that, for these acceptors the equilibrium in Equation 11 is going to completion at higher temperatures.

#### **2.4 Thermodynamic studies**

The thermodynamic parameters were investigated for these interactions by UV-vis spectrometry method. The formation constants, K, were determined at several temperatures by analyzing the concentration and temperature dependence of UV-vis absorptions using the SQUAD program, Tables 13 and 14. Van't Hoff plots of these formation constants were used for obtaining the other thermodynamic parameters Δ*H*, Δ*S*, and Δ*G* (Table 15).

The data of Table 13 shows that stability of adducts for all acceptors undergo a regular increase from H2T(4-Cl)PP to H2TPP, H2T(4-CH3)PP, and H2T(4-OCH3)PP; also adducts of H2TtBP are more stable than H2TnBP. These sequences are in agreement with electron releasing property of free base porphyrins.


Table 13. The formation constants lgK for H2T(4-X)PP adducts in CHCl3 solvent.


Table 14. The formation constants lgK for H2T(4-X)PP adducts in CHCl3 solvent.

Table 13 shows that formation constants for interaction of DDQ with free base porphyrins decreased at higher temperatures. In contrast for other acceptors (TCNE, TBBQ and TCBQ)

The thermodynamic parameters were investigated for these interactions by UV-vis spectrometry method. The formation constants, K, were determined at several temperatures by analyzing the concentration and temperature dependence of UV-vis absorptions using the SQUAD program, Tables 13 and 14. Van't Hoff plots of these formation constants were used for obtaining the other thermodynamic parameters Δ*H*, Δ*S*, and Δ*G* (Table 15).

The data of Table 13 shows that stability of adducts for all acceptors undergo a regular increase from H2T(4-Cl)PP to H2TPP, H2T(4-CH3)PP, and H2T(4-OCH3)PP; also adducts of H2TtBP are more stable than H2TnBP. These sequences are in agreement with electron

> 6.47±0.02 5.01±0.02 6.65±0.03 6.87±0.03 6.33±0.03 7.86±0.03 9.41±0.01 9.24±0.02 9.85±0.01 10.12±0.02 9.30±0.02 10.01±0.02

5 ◦C 10 ◦C 15 ◦C 20 ◦C 25 ◦C

15 ◦C 20 ◦C 25 ◦C 30 ◦C 35 ◦C

4.51±0.08 4.64±0.07 5.01±0.04 5.70±0.08 6.42±0.08 7.69±0.09 7.85±0.08 7.93±0.07

6.79±0.01 5.42±0.01 7.06±0.01 7.30±0.01 6.82±0.01 8.29±0.01 9.24±0.01 9.09±0.03 9.64±0.03 9.93±0.04 9.13±0.04 9.81±0.04 7.33±0.03 6.20±0.03 7.88±0.02 8.00±0.03 7.06±0.03 8.59±0.02 9.06±0.03 8.92±0.01 9.45±0.01 9.69±0.01 8.95±0.01 9.62±0.01

4.70±0.07 4.98±0.09 5.32±0.09 6.05±0.03 6.67±0.08 8.20±0.08 8.67±0.09 8.93±0.07 8.03±0.02 6.45±0.02 8.36±0.01 8.62±0.01 7.30±0.01 8.80±0.01 8.83±0.01 8.70±0.02 9.24±0.02 9.42±0.03 8.81±0.03 9.48±0.03

5.00±0.09 5.20±0.06 5.58±0.04 6.34±0.03 7.00±0.09 8.50±0.08 9.00±0.08 9.18±0.08

**2.4 Thermodynamic studies** 

(TCNE)2H2TPP (TCNE)2H2T(4-Cl)PP (TCNE)2H2T(4-CH3)PP (TCNE)2H2T(4-OCH3)PP

(TCNE)2H2TnBP (TCNE)2H2TtBP (DDQ)2H2TPP (DDQ)2H2T(4-Cl)PP (DDQ)2H2T(4-CH3)PP (DDQ)2H2T(4-OCH3)PP

(DDQ)2H2TnPP (DDQ)2H2TtBP

(TCBQ)2H2T(4-Cl)PP (TCBQ)2H2TPP

(TCBQ)2H2T(4-CH3)PP (TCBQ)2H2T(4-OCH3)PP (TBBQ)2H2T(4-Cl)PP (TBBQ)2H2TPP

(TBBQ)2H2T(4-CH3)PP (TBBQ)2H2T(4-OCH3)PP

releasing property of free base porphyrins.

Adduct lg K

5.72±0.02 4.40±0.01 6.10±0.02 6.30±0.02 6.03±0.02 7.51±0.02 9.66±0.02 9.50±0.01 10.11±0.01 10.34±0.02 9.45±0.02 10.17±0.02

Table 13. The formation constants lgK for H2T(4-X)PP adducts in CHCl3 solvent.

Table 14. The formation constants lgK for H2T(4-X)PP adducts in CHCl3 solvent.

4.17±0.09 4.39±0.06 4.80±0.09 5.35±0.04 6.01±0.09 7.20±0.06 7.45±0.07 751±0.05

Table 13 shows that formation constants for interaction of DDQ with free base porphyrins decreased at higher temperatures. In contrast for other acceptors (TCNE, TBBQ and TCBQ)

Adduct lg K

3.89±0.03 4.07±0.06 4.41±0.09 4.97±0.04 5.66±0.06 6.70±0.06 6.86±0.07 7.00±0.07 formation constants increased at higher temperatures. This discrepancy might be due to presence of different mechanisms for caring out charge transfer between free bases and these -acceptors. Since these -acceptors are common oxidizing agents in organic chemistry, this dual influence of temperatures on their interactions with free base porphyrins is an interesting point that less has been attended previously. According to formation constants the following order was suggested for acceptor property of these molecules: DDQ TCNE TBBQ TCBQ

Table 15 gives the thermodynamic parameters (ΔH, ΔS, and ΔG) for charge transfer adducts of DDQ, TCNE, TBBQ and TCBQ with free base porphyrins in chloroform. These results show that meso-group of free base porphyrins significantly affected strength of their interactions with acceptor species. While electron releasing meso-groups improves interactions of free base porphyrins with -acceptors molecules, electron withdrawing meso-groups diminished these interactions.


<sup>a</sup> Δ*H*(kJ mol-1) and Δ*S* (J K-1 mol-1); Δ*G* at 25 C (kJ mol-1).

Table 15. The thermodynamic parameters for [(Acceptor)2(Free base porphyrin)] adducts in CHCl3.a

For [(DDQ)2FBP] adducts, Δ*H* and Δ*S* are negative. But other adducts ([(TCNE)2FBP], [(TBBQ)2FBP], [(TCBQ)2FBP]) have positive values for Δ*H* and Δ*S* parameters. Since interactions of free base porphyrins with various acceptors are exothermic, endothermic interactions for these -acceptors are interesting and unexpected. It seems presence of strong self -stacking between these -acceptor molecules make their dissolving endothermic, on the other hand dissociation of such -stacks make their dissolving, entropically a favorable phenomena.

The standard Gibbs free energies, Δ*G*, of interactions are negative in all cases. It seems that for TCNE, TBBQ and TCBQ term *S*º 0 is more effective than *H*º 0 in the following Equation: Δ*G* = Δ*H* - TΔ*S* so that negative values for Δ*G* were occurred. Negative values of Δ*G* show that interactions of these molecules with free bases are favorable with the same ordering as their electron releasing property.

The main goal in this work is "positive Δ*H*" for TCNE, TBBQ and TCBQ adducts. In view of the discrepancy, further studies are certainly necessary to clarify this essential issue. Our results are indicating that electron acceptor properties of these molecules depend on temperature and this might be important to consider it for facilitating their subsequent applications.

#### **3. Hydrogen bond complexes of 2,4-dichloro-; 2,4,6-trichloro- and 4-nitrophenol with free bases** *meso***-tetraarylporphyrins**

Hydrogen bonds play a crucial role in many chemical, physical, and biochemical processes, and they are also very important in crystal engineering [39-42]. Hydrogen bonds usually designated as X-H…Y in which there is an X-H proton donating bond and an acceptor of protons (Y-center). Interaction of phenol derivatives (PD) such as 4-nitrophenol (4NP), 2,4- Dichlorophenol (24DCP), and 2,4,6-Trichlorophenol (246)TCP with biological systems are of interest. Porphyrins with two N-H proton donating sites and numerous nitrogen atoms as proton acceptor centers can form HB complexes with some hydroxylated compounds. Last studies were demonstrated that hydrogen bonded complexes of phenol derivatives with free base porphyrins has a 2:1 mole ratio of phenol to porphyrin.

On addition of phenol derivatives (PD; 4NP, 24DCP and 246TCP) to a solution of H2T(4- X)PP in chloroform, PD form HB complex with free base porphyrin according to following equation: [43, 44]

$$\text{(2PD + H\_2T(4-X))PP} \xrightarrow[\bullet]{} \text{(PD)2H\_2T(4-X)PP} \tag{12}$$

Fig. 8. Titration spectra for interaction of 24DCP with H2T(4-CH3O)PP in chloroform. Bands appeared at 455 and 688 nm are related to adduct, isosbestic point at 428.

of Δ*G* show that interactions of these molecules with free bases are favorable with the same

The main goal in this work is "positive Δ*H*" for TCNE, TBBQ and TCBQ adducts. In view of the discrepancy, further studies are certainly necessary to clarify this essential issue. Our results are indicating that electron acceptor properties of these molecules depend on temperature and this might be important to consider it for facilitating their subsequent

Hydrogen bonds play a crucial role in many chemical, physical, and biochemical processes, and they are also very important in crystal engineering [39-42]. Hydrogen bonds usually designated as X-H…Y in which there is an X-H proton donating bond and an acceptor of protons (Y-center). Interaction of phenol derivatives (PD) such as 4-nitrophenol (4NP), 2,4- Dichlorophenol (24DCP), and 2,4,6-Trichlorophenol (246)TCP with biological systems are of interest. Porphyrins with two N-H proton donating sites and numerous nitrogen atoms as proton acceptor centers can form HB complexes with some hydroxylated compounds. Last studies were demonstrated that hydrogen bonded complexes of phenol derivatives with free

On addition of phenol derivatives (PD; 4NP, 24DCP and 246TCP) to a solution of H2T(4- X)PP in chloroform, PD form HB complex with free base porphyrin according to following

396 446 496 546 596 646 696 **Wavelength(nm)**

Fig. 8. Titration spectra for interaction of 24DCP with H2T(4-CH3O)PP in chloroform. Bands

appeared at 455 and 688 nm are related to adduct, isosbestic point at 428.

496 546 596 646 696

2PD + H2T(4-X)PP (PD)2H2T(4-X)PP (12)

**3. Hydrogen bond complexes of 2,4-dichloro-; 2,4,6-trichloro- and** 

**4-nitrophenol with free bases** *meso***-tetraarylporphyrins** 

base porphyrins has a 2:1 mole ratio of phenol to porphyrin.

ordering as their electron releasing property.

applications.

equation: [43, 44]

0

0.2

0.4

0.6

**Absorbance**

0.8

1

1.2

These interactions were studied by means of UV-vis spectrometry method and data refinement were carried out by SQUAD program. Interaction of PDs with free base porphyrins leads to a fundamental change in the porphyrins electronic absorption spectra, Fig.8 and Table 16. For example, when a solution of H2T(4-CH3O)PP in chloroform was interacted with 24*DCP*, the original peaks of H2T(4-CH3O)PP were vanished and adduct peaks' were appeared at 455 and 688 nm. A clear isosbestic point at 428 nm represents formation of hydrogen bond complex in a reversible reaction. For experimental illustration of an endothermic equilibrium adduct formation a solution of [(PD)2H2T(4-CH3)PP] adduct at 25 °C was cooled to 5 °C in a UV-vis cell. Observations show that upon cooling the adduct [(PD)2H2T(4-CH3)PP] with Soret band at 445 nm was dissociated to PD and free base porphyrin H2T(4-CH3)PP with Soret band at 419 nm. By heating of solution to 25 °C the adduct was formed again and Soret band was returned to 445 nm (Fig. 9).


Table 16. UV-vis peaks (λ/nm in CHCl3) of hydrogen bonded complexes of FBPs with PDs.

Fig. 9. In a UV-vis cell a solution of H2TPP (5 10-6 M, 2 ml) and 24DCP (3 M, 90 mic.lit.) at 25 °C (profile A) was cooled to 5 °C (profile B). Observations show that upon cooling the adduct [(24DCP)2H2TPP], Soret at 445 nm, has be dissociated to 24DCP and H2TPP, Soret at 419 nm. By heating of solution to 25 °C the adduct [(24DCP)2H2TPP] was formed again and Soret returned to 445 nm.

Stabilities of the adducts increase from H2T(4-Cl)PP, H2TPP, H2T(4-CH3)PP, to H2T(4- CH3O)PP for each PD, Table 17. For example, at 20 C we have the following order of the formation constants, *K*:

(24DCP)2H2T(4-Cl)PP (24DCP)2H2TPP (24DCP)2H2T(4-CH3)PP (24DCP)2H2T(4-CH3O)PP

lg K: 1.14 1.51 1.75 1.93

246*TCP* did not show a measurable interaction with H2TPP and H2T(4-Cl)PP under our experimental conditions also its interactions with H2T(4-CH3)PP and H2T(4-CH3O)PP are sizably weaker than corresponding interactions for 24DCP. It might be due to high steric hindrances which coincident to interaction of 246TCP with free base porphyrins. From obtained K for HB complexes the following order could be defined for HB formation ability of PDs: 4NP 24DCP 246TCP


Table 17. The lgK for HB complexes (PD)2H2T(4-X)PP in CHCl3 as solvent.

Thermodynamic parameters *H*º, *S*º and *G*º of [(PD)2H2T(4-CH3)PP] complexes are given in Table 18. The free energy changes (*G*<sup>º</sup> ) of HB complexes become more negative through the series H2T(4-Cl)PP, H2TPP, H2T(4-CH3)PP, to H2T(4-CH3O)PP which indicates stronger HB interaction along this sequence.


<sup>a</sup> Hº (kJmol-1), Sº (JK-1mol-1), Gº (kJmol-1) at 25º C

Table 18. The thermodynamic parameters for HB complexes (PD)2H2T(4-X)PP in CHCl3a.

Adducts have positive values of *H*º and *S*º but greater contribution of *S*º relative to *H*<sup>º</sup> leads to a negative value of *G*<sup>º</sup> .

The positive value of *S*<sup>º</sup> may return to association between PDs and free base porphyrin, which accompanied with releasing a greater number of chloroform molecules, that solvated the initial substances. The aggregation between PD and free bases in the selected range of temperatures is endothermic. The following sequence was obtained for interactions of H2T(4-X)PPs with phenol derivatives:

$$\text{H}\_2\text{T(4-CH}\_3\text{O)}\text{PP} > \text{H}\_2\text{T(4-CH}\_3\text{)}\text{PP} > \text{H}\_2\text{TPP} > \text{H}\_2\text{T(4-Cl)}\text{PP}$$

#### **4. Effect of nonplanarity of free base porphyrins on their interactions with dimethyl- and diethyltin(IV) dichlorides**

The nonplanarity of the free base porphyrin can be regulated by electronic and steric effects of the *meso-* and -pyrrole substitution and it has profound consequences on spectral, electrochemical, and other properties of porphyrins. [45-48] Chemical properties that are known to be modified by nonplanar distortion include oxidation potentials, basicity of the inner nitrogen atoms, and axial ligand binding affinity, all of which can influence the biological function of porphyrin cofactors in proteins. A good result of nonplanar distortion of porphyrins appears in porphyrin metalation. For example, the predeformed octabromotetramesityl-porphyrin incorporates Zn2+ about 4000 times faster than the planar tetramesityl-porphyrin in DMF with the same rate law. Since the rates of metal ion incorporation are attributed to the conformational change of the rigid planar porphyrin core, thus the metalation of a nonplanar porphyrin is distinct in reaction rate from that of planar porphyrins.

This fact has be pointed out that nonplanarity enhances the basicity of porphyrins. This part do to comparison of the acceptor property of nonplanar *meso*-tetrakis-tert-butylporphyrin (H2TtBP) with planar *meso*-tetra-n-propylporphyrin (H2TnPP), in their interactions with dimethyl- and diethyltin(IV) dichloride.

On mixing of the R2SnCl2 (R= Me, Et) with a solution of free bases H2TnPP and H2TtBP Lewis acid-base interactions were occurred between them.[49] Porphyrins as Lewis bases coordinated to organotin compounds as Lewis acids. Extent of these interactions depends on acid-base properties of interacting components.

#### **4.1 <sup>1</sup> H NMR analysis**

72 Molecular Interactions

Stabilities of the adducts increase from H2T(4-Cl)PP, H2TPP, H2T(4-CH3)PP, to H2T(4- CH3O)PP for each PD, Table 17. For example, at 20 C we have the following order of the

(24DCP)2H2T(4-Cl)PP (24DCP)2H2TPP (24DCP)2H2T(4-CH3)PP (24DCP)2H2T(4-CH3O)PP

lg K: 1.14 1.51 1.75 1.93

246*TCP* did not show a measurable interaction with H2TPP and H2T(4-Cl)PP under our experimental conditions also its interactions with H2T(4-CH3)PP and H2T(4-CH3O)PP are sizably weaker than corresponding interactions for 24DCP. It might be due to high steric hindrances which coincident to interaction of 246TCP with free base porphyrins. From obtained K for HB complexes the following order could be defined for HB formation ability

> 0.99±0.02 1.37±0.01 1.58±0.03 1.72±0.01 0.67±0.06 0.87±0.02

> 1.09±0.02 1.16±0.05 2.12±0.01 2.22±0.01

Thermodynamic parameters *H*º, *S*º and *G*º of [(PD)2H2T(4-CH3)PP] complexes are given in Table 18. The free energy changes (*G*º) of HB complexes become more negative through the series H2T(4-Cl)PP, H2TPP, H2T(4-CH3)PP, to H2T(4-CH3O)PP which indicates stronger

> 19.9±1.8 25.7±2.3 30.1±2.9 35.2±1.7 15.1±1.4 17.5±1.3 38±2 48±2 62±3 78±4

5 °C 10 °C 15 °C 20 °C 25 °C

1.08±0.02 1.43±0.03 1.69±0.04 1.81±0.03 0.70±0.05 0.92±0.01

1.24±0.02 1.29±0.05 2.35±0.02 2.40±0.01

> 89.8±6.4 116.7±7.9 136.4±9.9 156.8±5.8 65.9±4.7 78.4±4.6 156±5 193±7 260±9 316±13

1.14±0.02 1.51±0.03 1.75±0.04 1.93±0.05 0.76±0.05 0.99±0.02

1.35±0.02 1.47±0.05 2.48±0.02 2.71±0.02

1.20±0.02 1.60±0.03 1.86±0.05 2.03±0.02 0.80±0.05 1.02±0.02

1.44±0.02 1.63±0.08 2.70±0.03 2.89±0.02

6.9±0.1 9.1±0.1 10.5±0.1 11.5±0.1 4.5±0.1 5.9±0.1 8±2 9±2 15±3 17±4

formation constants, *K*:

of PDs: 4NP 24DCP 246TCP

(24DCP)2H2T(4-Cl)PP (24DCP)2H2TPP (24DCP)2H2T(4-CH3)PP (24DCP)2H2T(4-CH3O)PP (246TCP)2H2T(4-CH3)PP (246TCP)2H2T(4-CH3O)PP

(4NP)2H2T(4-Cl)PP (4NP)2H2TPP (4NP)2H2T(4-CH3)PP (4NP)2H2T(4-CH3O)PP

HB interaction along this sequence.

(JK-1mol-1), Gº

(24DCP)2H2T(4-Cl)PP (24DCP)2H2TPP (24DCP)2H2T(4-CH3)PP (24DCP)2H2T(4-CH3O)PP (246TCP)2H2T(4-CH3)PP (246TCP)2H2T(4-CH3O)PP (4NP)2H2T(4-Cl)PP (4NP)2H2TPP (4NP)2H2T(4-CH3)PP (4NP)2H2T(4-CH3O)PP

(kJmol-1), Sº

<sup>a</sup> Hº

Adduct lgK

0.97±0.02 1.02±0.07 1.90±0.02 1.91±0.01

Table 17. The lgK for HB complexes (PD)2H2T(4-X)PP in CHCl3 as solvent.

Complex Hº S <sup>º</sup> -Gº

(kJmol-1) at 25º

C Table 18. The thermodynamic parameters for HB complexes (PD)2H2T(4-X)PP in CHCl3a. The 1H NMR spectra of porphyrins moiety in these adducts show clear differences relative to the free bases reactants. Upon complexation of free base porphyrins with organotin(IV) halides the H of pyrrole rings (H2TnPP 9.5 and H2TtBP 9.08 ppm) were shifted up field (9.03 and 8.1 ppm, respectively) while the N-H pyrrolic protons (H2TnPP -2.6 and H2TtBP 1.52 ppm) show down field shift and collapsed with alkyls protons at chemical shifts 1-2.5 ppm. These variations in 1H NMR spectra result from enhancement of nonplanar distortion in porphyrin structure during adduct formation. Comparison of chemical shifts of 1H NMR of free bases H2TnBP and H2TtBP especially for NH protons clearly shows the effect of nonplanarity on the 1H NMR of free base porphyrins. In the 5,10,15,20-tetrabutylporphyrin (H2TnBP) as a planar porphyrin the N-H and H signals were appeared at -2.61 and 9.45 ppm, respectively, while in the 5,10,15,20-tetrakis(tert-butyl)porphyrin (H2TtBP) as a severely distorted porphyrin the N-H and H were appeared at 1.52 and 9.08 ppm.

#### **4.2 UV-vis analysis**

#### **4.2.1 Interactions of R2SnCl2 with H2TnPP**

By interaction of R2SnCl2 with H2TnPP Soret band of free base porphyrin (original peak at 417 nm) was shifted to 433 nm, see Table 19 and Fig.9. In addition other peaks of H2TnPP were weakened while a new peak was appeared at 643 nm. Presence of a clear isosbestic point at 425 nm can aroused from an equilibrium in solution.

Fig. 9. Titration spectra for titration of H2TnPP with Et2SnCl2 in chloroform. Band appeared at 433 nm is related to adduct, isosbestic points at 425 nm.


Table 19. UV-vis peaks of free base porphyrins and their adducts with R2SnCl2 in CHCl3.

#### **4.2.2 Interaction of R2SnCl2 with H2TtBP**

By interaction of R2SnCl2 with H2TtBP, the Soret band of free base porphyrin (original peak at 446 nm) was shifted to 453 nm, see Table 19 and Fig. 10. In addition, other peaks of H2TtBP were weakened while a new peak was appeared at 695 nm. Presence of a isosbestic point at 454 nm can be argued by existence of equilibrium in solution.

Amount of replacement of Soret band in H2TnPP (16 nm) is greater than H2TtBP (7 nm). This aroused from greater nonplanar distortion in H2TnPP skeleton comparing to H2TtBP during adduct formation. Since the observed red shift mainly depends on the distortion of porphyrins structures during adduct formations.

For practical illustration of this subject, if in a batched UV-vis cell at 5 °C, a solution of H2TnPP titrated with Me2SnCl2 the Soret band of free base (417 nm) will diminish and Soret band related to adduct will appear at 433 nm. By keeping the composition of solution unchanged and raising the temperature to 35 °C the Soret band will go back to 417 nm. This phenomenon which could be repeated successively shows that adduct dissociate to Me2SnCl2 and H2TnPP at elevated temperatures and forms on cooling in equilibrium.

Fig. 10. Titration spectra, for titration of H2TtBP with Me2SnCl2 in chloroform. Band appeared at 453 nm is related to adduct, isosbestic point at 454 nm.

About 20-30 nm red shift have been observed for Soret band of porphyrin moiety in the complexes of tetraarylporphyrins with organotin(IV) halides. A similar situation but with smaller red shifts (16 and 7 nm) were observed for interaction of H2TnPP and H2TtBP with R2SnCl2. Of course, with a considerable resonance that there is between peripheral aryl rings and porphyrin core in tetraarylporphyrins and lack of such resonance in tetraalkyl porphyrins this difference will be reasonable. Generally, porphyrin distortion is accompanied by bathochromic shift of its electronic absorption bands. [24,26] For example, in UV-vis spectra (in CH2Cl2) of planar porphyrin H2TnPP Soret band and Q bands were observed at (417 nm) and (520, 555, 600, 659 nm), respectively. But in distorted H2TtBP, Soret band and Q bands appeared at (446 nm) and (552, 596, 628, 691 nm). The effect of distortion of porphyrin plane on electronic absorption and 1H NMR spectra of the free base porphyrin clearly observed from these data. Thus in these adducts greater red shifts in the UV-vis spectra of H2TnPP may be due to more distortions in its structure during adduct formations.

#### **4.3 Thermodynamic studies**

74 Molecular Interactions

ppm, respectively, while in the 5,10,15,20-tetrakis(tert-butyl)porphyrin (H2TtBP) as a

By interaction of R2SnCl2 with H2TnPP Soret band of free base porphyrin (original peak at 417 nm) was shifted to 433 nm, see Table 19 and Fig.9. In addition other peaks of H2TnPP were weakened while a new peak was appeared at 643 nm. Presence of a clear isosbestic

> 390 400 410 420 430 440 450 **Wavelength/nm**

> > H2TtBP

(Me2SnCl2)2H2TtBP (Et2SnCl2)2H2TtBP

446 552 596 628 691 453 - - - 695 453 - - - 695

Fig. 9. Titration spectra for titration of H2TnPP with Et2SnCl2 in chloroform. Band appeared

Table 19. UV-vis peaks of free base porphyrins and their adducts with R2SnCl2 in CHCl3.

By interaction of R2SnCl2 with H2TtBP, the Soret band of free base porphyrin (original peak at 446 nm) was shifted to 453 nm, see Table 19 and Fig. 10. In addition, other peaks of H2TtBP were weakened while a new peak was appeared at 695 nm. Presence of a isosbestic

Amount of replacement of Soret band in H2TnPP (16 nm) is greater than H2TtBP (7 nm). This aroused from greater nonplanar distortion in H2TnPP skeleton comparing to H2TtBP during adduct formation. Since the observed red shift mainly depends on the distortion of

For practical illustration of this subject, if in a batched UV-vis cell at 5 °C, a solution of H2TnPP titrated with Me2SnCl2 the Soret band of free base (417 nm) will diminish and Soret band related to adduct will appear at 433 nm. By keeping the composition of solution unchanged and raising the temperature to 35 °C the Soret band will go back to 417 nm. This phenomenon which could be repeated successively shows that adduct dissociate to Me2SnCl2 and H2TnPP at elevated temperatures and forms on cooling in equilibrium.

Compound (nm) Compound (nm)

417 520 555 600 659 433 - - - 643 433 - - - 643

point at 454 nm can be argued by existence of equilibrium in solution.

severely distorted porphyrin the N-H and H were appeared at 1.52 and 9.08 ppm.

**4.2 UV-vis analysis** 

H2TnPP

(Me2SnCl2)2H2TnPP (Et2SnCl2)2H2TnPP

**4.2.1 Interactions of R2SnCl2 with H2TnPP** 

0

at 433 nm is related to adduct, isosbestic points at 425 nm.

0.3

0.6

**Absorbance**

**4.2.2 Interaction of R2SnCl2 with H2TtBP** 

porphyrins structures during adduct formations.

0.9

point at 425 nm can aroused from an equilibrium in solution.

The data given in Table 20 show that formation constants for adducts of H2TtBP with dimethyl- and diethyltin(IV) dichloride are greater than related values for H2TnPP. It shows that (R2SnCl2)2H2TtBP adducts are more stable than (R2SnCl2)2H2TnPP. For example at 5 C we have the following order of the formation constants:

$$\text{(Me2SnCl\_2)\_2H\_2TtBP} > \text{(Me2SnCl\_2)\_2H\_2TnPP}; \qquad \text{lg K} \ 9.70 > 6.58$$

$$\text{(Et}\_2\text{SnCl\_2)\_2H\_2TtBP} > \text{(Et}\_2\text{SnCl\_2)\_2H\_2TnPP}; \qquad \text{lg K} \ 8.35 > 5.69$$

Also comparison of stability constants show that Me2SnCl2 adducts are more stable than their Et2SnCl2 counterparts.

> (Me2SnCl2)2H2TnPP (Et2SnCl2)2H2TnPP; lg K 6.58 5.69 (Me2SnCl2)2H2TtBP (Et2SnCl2)2H2TtBP; lg K 9.70 8.35

Table 21 shows the thermodynamic parameters obtained for the interactions of R2SnCl2 with free base porphyrins in chloroform. Adducts have negative values of Hº, Sº , and Gº. The negative values of Sº refer to association between donor and acceptor molecules.


Table 20. The formation constants (lg K) of adducts in CHCl3.


<sup>a</sup>Hº (kJmol-1), Sº (JK-1mol-1), and Gº at 10 C (kJmol-1).

Table 21. The thermodynamic parameters of adducts in CHCl3.a

Both free energy and enthalpy of adducts of H2TtBP are more negative than corresponding values for H2TnPP. This indicates that interactions of H2TtBP with organotin compounds are stronger than H2TnPP. This trend is in according to the electron donation of the free base porphyrins. Since, in the pre-distorted H2TtBP lone pair electrons of nitrogen atoms are more ready for coordination to R2SnCl2 Lewis acid. Therefore, we found greater formation constants for complexes of R2SnCl2 with H2TtBP than H2TnPP.

#### **5. References**


6.17±0.01 5.33±0.03 9.15±0.01 7.87±0.02

Adduct -Hº -S <sup>º</sup> -Gº

134 ±1 121 ±4 170 ±6 123 ±2

Both free energy and enthalpy of adducts of H2TtBP are more negative than corresponding values for H2TnPP. This indicates that interactions of H2TtBP with organotin compounds are stronger than H2TnPP. This trend is in according to the electron donation of the free base porphyrins. Since, in the pre-distorted H2TtBP lone pair electrons of nitrogen atoms are more ready for coordination to R2SnCl2 Lewis acid. Therefore, we found greater formation

[1] N. Zh. Mamardashvili, O A Golubchikov, *Russian Chemical Reviews*, 70 (2001) 577 and

[5] F.R. Longo, E.M. Brown, W.G. Rau, and A.D. Alder in "*The Porphyrins*" Vol. V, D.

[11] Pasternack, R. F., Vogel, G. C., Skowronek, C. A., Harris, R. K., Miller, J. G., *Inorg. Chem.*,

[15] S. Funahashi, M. Inamo, N. Kamiya, Y. Inada, M. Nomura, *Inorg. Chem.,* 40 (2001)

[2] H Ogoshi, E-i Watanabe, N Koketsu, Z-i Yoshida, *Bull. Chem. Soc. Jpn.*, 49 (1976) 2529.

5 C 10 C 15 C 20 C 25 C

5.72±0.01 4.84±0.09 8.66±0.09 7.52±0.09

> 356 ±5 328 ±40 426 ±20 284 ±5

5.30±0.04 4.52±0.04 8.01±0.03 7.11±0.02

> 28 ±1 24 ±4 43 ±20 39 ±2

4.90±0.01 4.18±0.01 7.51±0.02 6.73±0.04

Adduct lgK

6.58±0.02 5.69±0.02 9.70±0.01 8.35±0.02

Table 20. The formation constants (lg K) of adducts in CHCl3.

 (JK-1mol-1), and Gº at 10 C (kJmol-1). Table 21. The thermodynamic parameters of adducts in CHCl3.a

constants for complexes of R2SnCl2 with H2TtBP than H2TnPP.

[3] N. Datta-Gupta, T. J. Bardos, *J. Heterocycl. Chem.*, 3 (1966) 495. [4] E.B. Fleischer and J.H. Wang, *J. Am. Chem. Soc*., 82 (1960) 3498.

[8] Robinson, L. R.; Hambright, P., *Inorg. Chim. Acta*, 185 (1991) 17. [9] Bain-Ackerman, M. J.; Lavallee, D. K., *Inorg. Chem.*, 18 (1979) 3358. [10] Funahashi, S.; Saito, K.; Tanaka, M., *Bull. Chem. Soc. Jpn.*, 54 (1981) 2695.

[12] Hambright, P. and Chock, P. B., *J. Am. Chem. Soc.*, *9*6 (1974) 3123.

[13] Turay, J., Hambright, P., *Inorg. Chem.*, 19 (1980) 562. [14] Fleischer, E. B., Dixon, F., *Bioinorg. Chem.*, 7 (1977) 129.

Dolphin, Ed., Academic Press, New York, 1979, pp 459. [6] Funahashi, S., Yamaguchi, Y., Tanaka, M., *Bull. Chem. Soc. Jpn.,* 57 (1984) 204. [7] Funahashi, S., Yamaguchi, Y., Tanaka, M., *Inorg. Chem.,* 23 (1984) 2249.

(Me2SnCl2)2H2TnPP (Et2SnCl2)2H2TnPP (Me2SnCl2)2H2TtBP (Et2SnCl2)2H2TtBP

(Me2SnCl2)2H2TnPP (Et2SnCl2)2H2TnPP (Me2SnCl2)2H2TtBP (Et2SnCl2)2H2TtBP

(kJmol-1), Sº

**5. References** 

references there in.

20 (1981) 3763.

5636.

<sup>a</sup>Hº

