**3. Ferrocene-appended TPA derivatives and their ruthenium complexes: Intramolecular magnetic interactions between ruthenium and iron centers induced by oxidations**

Ferrocene is a well-known redox-active molecule and has been introduced to many kinds of functional molecules. Among those, ferrocene-containing ligands have been reported on their multiple redox behavior and mixed-valence states toward a construction of redoxresponsive functional molecules. Thus, we have prepared ferrocene-appended TPA derivatives (*N*-(6-ferrocenoylamide-2-pyridylmethyl)-*N*,*N*-bis(2-pyridylmethyl)amine = Fc-TPA, and *N*,*N*-bis(6-ferrocenoylamide-2-pyridylmethyl)-*N*-(2-pyridylmethyl)amine = Fc2- TPA) and their Ru(II) complexes. We have focused on intramolecular magnetic interactions between the Fe(III) center in the ferrocenium moiety and the Ru(III) center upon oxidation. (Kojima *et al.*, 2008)

#### **3.1 Crystal structures of a ferrocene-appended TPA derivative and its ruthenium(II) complex**

In the crystal of Fc-TPA (Figure 7a), two adjacent molecules form intermolecular hydrogen bonds with each other between the amide N-H and one of unsubstituted pyridine rings (3.042(2) Å for N2N5', 160.5º for N2–H1'–N5'). An intermolecular C-HO hydrogen bond is also found between the amide oxygen and one of unsubstituted pyridine ring (3.334(2) Å for C21O1; Figure 8a). Intermolecular - interactions are observed between a pyridine ring and a substituted cyclopentadienyl (Cp) ring, a nonsubstituted Cp ring and adjacent pyridine rings, and a substitute pyridine ring and nonsubstituted pyridine ring (3.325(4) Å for C14C7, 3.638(3) Å for C19C1, 3.618(4) Å for C22C4, 3.257(3) Å for C12-C28; Figure 8b). The -planes of the substituted pyridine ring, the amide plane, and the amide-linked Cp

Ruthenium(II)-Pyridylamine Complexes Having Functional Groups via Amide Linkages 249

coordinated amide oxygen, cyclic voltammetry (CV) has been performed in CH3CN containing 0.1 M TBAP as an electrolyte at room temperature. Redox potentials were

The cyclic voltammograms of Fc-TPA and Fc2-TPA showed reversible redox waves at *E*1/2 =

The CV of **8** showed two redox waves at 0.23 V (*E* = 86 mV) and 0.77 V (*E* = 159 mV) at 100 mv/s. The first redox wave has been assigned to the Fc/Fc+ couple of the coordinated Fc-TPA based on the redox potential of the free Fc-TPA. The redox potential of the second one is similar to that of **4** (0.79 V for RuII/RuIII), therefore the second redox process has been assigned to the RuII/RuIII couple. The RuII/RuIII couple has exhibited a change of peak

Complex **9** shows reversible redox waves at 0.27 V and 0.46 V at the scan rate of 20 mV/s. The peak current of the first wave is twice as large as the second one. Thus, the first peak can be assigned to an overlapped wave derived from the Fc/Fc+ couple of the uncoordinated Fc-amide moiety and the RuII/RuIII couple based on the redox potential of Fc2-TPA and **3** (0.27 V for RuII/RuIII) and Ru(II)-bisamide-TPA (0.34 V for **1**, 0.30 V for **2**, and 0.27 V for **3**). The second redox couple was assigned to the Fc/Fc+ couple of the coordinated Fc-amide moiety. The redox potentials of the two Fc moieties have been separated due to the

**3.3 Magnetic interaction between the Ru(III) center and the Fe(III) center in ferrocene-**

We examined intramolecular magnetic interactions between the Ru center and the Fe center in **8** and **9** upon oxidation by [Ru(bpy)3]3+ in CH3CN. Variable-temperature EPR spectra of two-electron oxidized species of **8**, [RuIIICl(Fc+-TPA)+(DMSO)]3+, showed a signal derived from a forbidden transition (*m*s = 2) in a triplet state at *g* = 4.28 and a signal due to an allowed transition (*m*s = 1) at *g* = 2.0038. The obtained EPR spectrum is different from reported EPR signal of Fc+ with *g//* = 4.36 and *g* = 1.30 in acetone at 20 K (Prins & Reinders, 1969) and also the EPR signal of the one-electron oxidized species of Fc-TPA with an intense signal at *g* = 2.0186 and a weak signal at *g* = 4.22 in CH3CN at 2.6 K. The intensity of the signal at *g* = 4.28 of [RuIIICl(Fc+-TPA)+(DMSO)]3+ increased as lowering temperature as shown in Figure 9a. Therefore, the ground state of the two-electron oxidized species should be a triplet state (*S* = 1). A ferromagnetic coupling constant has been estimated to be *J* = 13.7 cm–1 based on the Bleaney-Bowers equation (Bleaney & Bowers, 1952) using the temperature-dependent change of the intensity of the signal at *g* = 4.28 (see Figure 9b)

In order to gain structural insights into the triplet state, we have applied DFT calculations to optimize the structure of the two-electron oxidized species of **8** in the *S* = 1 state. The optimized structure has longer interatomic distance between Ru(III) center and Fe(III) center (7.523 Å) than that observed in the crystal structure of **8** (6.656 Å). The dihedral angle between the amide plane and the Cp ring (42.8º) has enlarged compared to that in the crystal structure of **8**. The calculated spin densities at the Ru(III) center (+0.925) and Fe(III) center (+1.397) suggests the localization of unpaired electrons at those metal centers. We assumed that the ferromagnetic interaction between Ru(III) and Fe(III) centers was induced

by a superexchage interaction through the amide linkage (Kojima *et al.*, 2008).

current with altering the scan rate, suggesting that this process is quasi-reversible.

determined relative to the ferrocene/ferrocenium ion couple as 0 V.

coordination of the amide oxygen of one of the Fc-amide arms in **9**.

**appended ruthenium(II)-TPA complexes** 

(Kojima *et al.*, 2008).

0.21 V and 0.23 V assigned to the Fc/Fc+ couples, respectively.

ring show a high coplanarity with the dihedral angle of 18.1º between the pyridine plane and the amide plane and 7.4º between the amide plane and the Cp ring due to the extended conjugation of -systems.

The coordination geometry of [RuCl(Fc-TPA)(DMSO)]PF6 **(8)** has been revealed to be similar to that of **4** (Figure 7b). The six coordination sites of the Ru(II) center of **8** are occupied by Fc-TPA as a tetradentate ligand, one chloride ion, and one DMSO molecule coordinating via the S atom. The high coplanarity of the substituted pyridine ring–the amide plane-the linked Cp ring observed in the free ligand is retained after the formation of the Ru(II) complex: The dihedral angle between the substituted pyridine ring and the amide plane is 22.8º and that between the amide plane and the Cp ring is 13.7º. An intramolecular hydrogen bond is observed between the chloride and the amide N-H hydrogen. The separation between the Ru(II) center and the Fe(II) center has been determined to be 6.656Å.

Fig. 7. ORTEP drawings of (a) Fc-TPA and (b) the cationic moiety of **8** with 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Fig. 8. Intermolecular interactions found in the crystal structure of Fc-TPA: (a) Intermolecular hydrogen bonds and (b) Intermolecular -interactions.

#### **3.2 Redox behavior of ferrocene-appended ruthenium(II)-TPA complexes**

In order to examine the redox properties of Fc-TPA, Fc2-TPA, **8**, and [RuCl(Fc2-TPA)](PF6) **(9)** that has been proposed to possess the same geometry as those of **1–3** with one

ring show a high coplanarity with the dihedral angle of 18.1º between the pyridine plane and the amide plane and 7.4º between the amide plane and the Cp ring due to the extended

The coordination geometry of [RuCl(Fc-TPA)(DMSO)]PF6 **(8)** has been revealed to be similar to that of **4** (Figure 7b). The six coordination sites of the Ru(II) center of **8** are occupied by Fc-TPA as a tetradentate ligand, one chloride ion, and one DMSO molecule coordinating via the S atom. The high coplanarity of the substituted pyridine ring–the amide plane-the linked Cp ring observed in the free ligand is retained after the formation of the Ru(II) complex: The dihedral angle between the substituted pyridine ring and the amide plane is 22.8º and that between the amide plane and the Cp ring is 13.7º. An intramolecular hydrogen bond is observed between the chloride and the amide N-H hydrogen. The separation between the

Fig. 7. ORTEP drawings of (a) Fc-TPA and (b) the cationic moiety of **8** with 50% probability

Fig. 8. Intermolecular interactions found in the crystal structure of Fc-TPA: (a)

**3.2 Redox behavior of ferrocene-appended ruthenium(II)-TPA complexes** 

In order to examine the redox properties of Fc-TPA, Fc2-TPA, **8**, and [RuCl(Fc2-TPA)](PF6) **(9)** that has been proposed to possess the same geometry as those of **1–3** with one


interactions.

Ru(II) center and the Fe(II) center has been determined to be 6.656Å.

thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Intermolecular hydrogen bonds and (b) Intermolecular

conjugation of


coordinated amide oxygen, cyclic voltammetry (CV) has been performed in CH3CN containing 0.1 M TBAP as an electrolyte at room temperature. Redox potentials were determined relative to the ferrocene/ferrocenium ion couple as 0 V.

The cyclic voltammograms of Fc-TPA and Fc2-TPA showed reversible redox waves at *E*1/2 = 0.21 V and 0.23 V assigned to the Fc/Fc+ couples, respectively.

The CV of **8** showed two redox waves at 0.23 V (*E* = 86 mV) and 0.77 V (*E* = 159 mV) at 100 mv/s. The first redox wave has been assigned to the Fc/Fc+ couple of the coordinated Fc-TPA based on the redox potential of the free Fc-TPA. The redox potential of the second one is similar to that of **4** (0.79 V for RuII/RuIII), therefore the second redox process has been assigned to the RuII/RuIII couple. The RuII/RuIII couple has exhibited a change of peak current with altering the scan rate, suggesting that this process is quasi-reversible.

Complex **9** shows reversible redox waves at 0.27 V and 0.46 V at the scan rate of 20 mV/s. The peak current of the first wave is twice as large as the second one. Thus, the first peak can be assigned to an overlapped wave derived from the Fc/Fc+ couple of the uncoordinated Fc-amide moiety and the RuII/RuIII couple based on the redox potential of Fc2-TPA and **3** (0.27 V for RuII/RuIII) and Ru(II)-bisamide-TPA (0.34 V for **1**, 0.30 V for **2**, and 0.27 V for **3**). The second redox couple was assigned to the Fc/Fc+ couple of the coordinated Fc-amide moiety. The redox potentials of the two Fc moieties have been separated due to the coordination of the amide oxygen of one of the Fc-amide arms in **9**.

#### **3.3 Magnetic interaction between the Ru(III) center and the Fe(III) center in ferroceneappended ruthenium(II)-TPA complexes**

We examined intramolecular magnetic interactions between the Ru center and the Fe center in **8** and **9** upon oxidation by [Ru(bpy)3]3+ in CH3CN. Variable-temperature EPR spectra of two-electron oxidized species of **8**, [RuIIICl(Fc+-TPA)+(DMSO)]3+, showed a signal derived from a forbidden transition (*m*s = 2) in a triplet state at *g* = 4.28 and a signal due to an allowed transition (*m*s = 1) at *g* = 2.0038. The obtained EPR spectrum is different from reported EPR signal of Fc+ with *g//* = 4.36 and *g* = 1.30 in acetone at 20 K (Prins & Reinders, 1969) and also the EPR signal of the one-electron oxidized species of Fc-TPA with an intense signal at *g* = 2.0186 and a weak signal at *g* = 4.22 in CH3CN at 2.6 K. The intensity of the signal at *g* = 4.28 of [RuIIICl(Fc+-TPA)+(DMSO)]3+ increased as lowering temperature as shown in Figure 9a. Therefore, the ground state of the two-electron oxidized species should be a triplet state (*S* = 1). A ferromagnetic coupling constant has been estimated to be *J* = 13.7 cm–1 based on the Bleaney-Bowers equation (Bleaney & Bowers, 1952) using the temperature-dependent change of the intensity of the signal at *g* = 4.28 (see Figure 9b) (Kojima *et al.*, 2008).

In order to gain structural insights into the triplet state, we have applied DFT calculations to optimize the structure of the two-electron oxidized species of **8** in the *S* = 1 state. The optimized structure has longer interatomic distance between Ru(III) center and Fe(III) center (7.523 Å) than that observed in the crystal structure of **8** (6.656 Å). The dihedral angle between the amide plane and the Cp ring (42.8º) has enlarged compared to that in the crystal structure of **8**. The calculated spin densities at the Ru(III) center (+0.925) and Fe(III) center (+1.397) suggests the localization of unpaired electrons at those metal centers. We assumed that the ferromagnetic interaction between Ru(III) and Fe(III) centers was induced by a superexchage interaction through the amide linkage (Kojima *et al.*, 2008).

Ruthenium(II)-Pyridylamine Complexes Having Functional Groups via Amide Linkages 251

Fig. 10. ORTEP drawings of the cation moieties of two isomers A (a) and B (b) of **10**.

intermolecular hydrogen bond with the distance of 2.637 Å (Figure 11).

Fig. 11. Intermolecular hydrogen bonds between catechol moieties of form A of **10**.

**complex** 

**4.2 Cu(II) binding to the converged catechol moieties to form a unique bimetallic** 

As observed in the crystal structure of **10**, the two independent catechol moieties have been successfully converged into one direction as a metal-binding site. Prior to the examination of metal binding to the catechol moieties, we have shed some lights on the deprotonation processes of **10**. The UV-vis titration of **10** with tetramethylammonium hydroxide (TMAOH) in DMF shows three-step spectral change. We assigned the first-step spectral change observed in the course of the addition of 0-2 equiv of TMAOH to the deprotonation of the coordinated amide N-H as observed for other Ru(II)-bisamide-TPA complexes (see Scheme 1 in Section 2.1) and one of the catechol O-H having lowest p*K*a value as a consequence of convergence of catechol moieties (Scheme 2). The second and third deprotonation steps observed in the course

substituted pyridine ring to form intramolecular hydrogen bond (2.938 (7) Å for O1N6, 148.9(4)° for O1H-N6). Disorder of the O2 atom can be found in the catechol moiety connected to the coordinated amide arm, giving rise to two conformational (rotational) isomers in the crystal structure (Form A and B in Figure 10). The intramolecular

interactions between the two catechol moieties are observed in both isomers with the shortest distance of 3.36 Å. The O3 atom in the catechol rings forms a hydrogen bond with a water molecule of crystallization (2.90 Å for O3O8). One of the oxygen atoms of the catechol moiety (O2) in the form A binds with O3 of an adjacent molecule through an


Fig. 9. (a) Variable-temperature EPR spectra of the two-electron oxidized species of **8** in the range of 2.6 – 140 K in CH3CN. (b) Temperature dependence of *IT* (*I* reporesents the integration intensity of the signal at *g* = 4.28) of the two-electron oxidized species of **8**, together with the curve-fit of the data to the Bleaney-Bowers equation.

EPR measurements of **9** have been also made in CH3CN at –150°C. In the case of its twoelectron oxidation, the uncoordinated Fc and the Ru center of **9** can be oxidized according to the CV measurement, and the product ([RuIIICl(Fc2-TPA)+]3+) exhibited EPR signals at *g =* 2.01 (*m*s = 1) and *g* = 4.11 (*m*s = 2) as observed in [RuIIICl(Fc-TPA)+]3+, indicating the formation of a resemble triplet state to that of [RuIIICl(Fc-TPA)+]3+.

#### **4. Ruthenium(II)-TPA complex having catechol pendants via amide linkages**

Catechol and its derivatives have been known to form stable complexes with metal ions. Introduction of catechols to TPA enables us to create a ruthenium-TPA complex having a metal binding site formed by convergence of the catechol moiety. Such compound may allow us to readily create multinuclear metal complexes with multistep redox systems involving electronic interactions among those redox centers. Thus, we have synthesized a catechol-containing TPA (*N*,*N*-bis[6-{3,4-(dihydroxy)benzamide}-2-pyridylmethyl]-*N*-(2 pyridyl-methyl)amine = (H2Cat)2-TPA) and its ruthenium complex, [RuCl((H2Cat)2-TPA)]Cl (**10**) (Kojima *et al.*, 2010).

#### **4.1 Crystal structure of a ruthenium(II) complex with two catechol pendants via amide linkages**

The coordination environment of the Ru(II) center in the structure of **10** is similar to those of other related Ru(II)-bisamide-TPA complexes in terms of the coordination of Cat2-TPA as a pentadendate ligand involving the coordination of one amide oxygen, as depicted in Figure 10. The chloride ion binds to the Ru(II) center at the *trans* position of the unsubstituted pyridine ring as can be seen in other related complexes (see Figure 2). The coordinated amide oxygen is located at the *trans* site to the tertiary amine, and the pyridine ring linked to the coordinated amide moiety is located at the *trans* position to the other amide

Fig. 9. (a) Variable-temperature EPR spectra of the two-electron oxidized species of **8** in the range of 2.6 – 140 K in CH3CN. (b) Temperature dependence of *IT* (*I* reporesents the integration intensity of the signal at *g* = 4.28) of the two-electron oxidized species of **8**,

EPR measurements of **9** have been also made in CH3CN at –150°C. In the case of its twoelectron oxidation, the uncoordinated Fc and the Ru center of **9** can be oxidized according to the CV measurement, and the product ([RuIIICl(Fc2-TPA)+]3+) exhibited EPR signals at *g =* 2.01 (*m*s = 1) and *g* = 4.11 (*m*s = 2) as observed in [RuIIICl(Fc-TPA)+]3+, indicating the

**4. Ruthenium(II)-TPA complex having catechol pendants via amide linkages**  Catechol and its derivatives have been known to form stable complexes with metal ions. Introduction of catechols to TPA enables us to create a ruthenium-TPA complex having a metal binding site formed by convergence of the catechol moiety. Such compound may allow us to readily create multinuclear metal complexes with multistep redox systems involving electronic interactions among those redox centers. Thus, we have synthesized a catechol-containing TPA (*N*,*N*-bis[6-{3,4-(dihydroxy)benzamide}-2-pyridylmethyl]-*N*-(2 pyridyl-methyl)amine = (H2Cat)2-TPA) and its ruthenium complex, [RuCl((H2Cat)2-TPA)]Cl

**4.1 Crystal structure of a ruthenium(II) complex with two catechol pendants via amide** 

The coordination environment of the Ru(II) center in the structure of **10** is similar to those of other related Ru(II)-bisamide-TPA complexes in terms of the coordination of Cat2-TPA as a pentadendate ligand involving the coordination of one amide oxygen, as depicted in Figure 10. The chloride ion binds to the Ru(II) center at the *trans* position of the unsubstituted pyridine ring as can be seen in other related complexes (see Figure 2). The coordinated amide oxygen is located at the *trans* site to the tertiary amine, and the pyridine ring linked to the coordinated amide moiety is located at the *trans* position to the other amide

together with the curve-fit of the data to the Bleaney-Bowers equation.

formation of a resemble triplet state to that of [RuIIICl(Fc-TPA)+]3+.

(**10**) (Kojima *et al.*, 2010).

**linkages** 

a) b)

Fig. 10. ORTEP drawings of the cation moieties of two isomers A (a) and B (b) of **10**.

substituted pyridine ring to form intramolecular hydrogen bond (2.938 (7) Å for O1N6, 148.9(4)° for O1H-N6). Disorder of the O2 atom can be found in the catechol moiety connected to the coordinated amide arm, giving rise to two conformational (rotational) isomers in the crystal structure (Form A and B in Figure 10). The intramolecular - interactions between the two catechol moieties are observed in both isomers with the shortest distance of 3.36 Å. The O3 atom in the catechol rings forms a hydrogen bond with a water molecule of crystallization (2.90 Å for O3O8). One of the oxygen atoms of the catechol moiety (O2) in the form A binds with O3 of an adjacent molecule through an intermolecular hydrogen bond with the distance of 2.637 Å (Figure 11).

Fig. 11. Intermolecular hydrogen bonds between catechol moieties of form A of **10**.

#### **4.2 Cu(II) binding to the converged catechol moieties to form a unique bimetallic complex**

As observed in the crystal structure of **10**, the two independent catechol moieties have been successfully converged into one direction as a metal-binding site. Prior to the examination of metal binding to the catechol moieties, we have shed some lights on the deprotonation processes of **10**. The UV-vis titration of **10** with tetramethylammonium hydroxide (TMAOH) in DMF shows three-step spectral change. We assigned the first-step spectral change observed in the course of the addition of 0-2 equiv of TMAOH to the deprotonation of the coordinated amide N-H as observed for other Ru(II)-bisamide-TPA complexes (see Scheme 1 in Section 2.1) and one of the catechol O-H having lowest p*K*a value as a consequence of convergence of catechol moieties (Scheme 2). The second and third deprotonation steps observed in the course

Ruthenium(II)-Pyridylamine Complexes Having Functional Groups via Amide Linkages 253

In this chapter, we have presented an overview of ruthenium complexes having TPA derivatives with functional groups via amide linkages. The strategy has allowed us to construct unique second coordination spheres of ruthenium complexes gaining novel functionality. The ruthenium coordination to bisamide-TPA derivatives affords the convergence of functional groups attached by the amide linkage into one direction to form specific environments, such as a hydrophobic cavity. The hydrophobic cavity made of the two naphthyl groups forms noncovalent van der Waals interactions with inserted substituents. Under this situation, we can observe the uni-directional intramolecular

interactions. The introduction of ferrocene moiety to the TPA ligand makes possible intramolecular ferromagnetic interaction between Ru(III) and Fe(III) centers in the ruthenium complex with ferrocene units as pendants. A catechol-appended TPA ligand can provide a preorganized metal binding site consisting of the two catechol moieties on the ruthenium complex to accept one Cu(II) complex in a unique coordination mode. Recently, we have also prepared a TPA derivative having two 1,10-phenanthroline (phen) moieties via amide linkages and its Ru(II) complex: The complex can form two appended Ru(II)-phen complexes and perform cooperative transfer hydrogenation of ketones (Kojima *et al*., 2011). In those complexes, the intramolecular hydrogen bond formed between the amide moieties can facilitate convergence of the functional groups and stabilize the complexes. This can be a significant advantage for the development of transition-metal-based functional molecules. Thus, the design of Ru-TPA complexes having functional groups via amide linkage will provide a nice opportunity to create cooperative molecular functionality on the basis of characteristics of both the ruthenium center and functional groups to provide a unique second coordination sphere as well as other metal ions attached to the functional groups.

Bleaney, B., Bowers, K. D. (1952). Anomalous Paramagnetism of Copper Acetate. *Proc. R.* 

Harata, M., Jitsukawa, K., Masuda, H., Einaga, H. (1994). A Structurally Characterized

Harata, M., Jitsukawa, K., Masuda, H., Einaga, H. (1995). Synthesis and Structure of a New

Harata, M., Hasegawa, K.; Jitsukawa, K., Masuda, H., Einaga, H. (1998). Preparations, Structures,


Nitrogen-containing Ligands. *Inorg. Chem.*, Vol.49 No.8, pp. 3737-3745. Kodera, M., Kawata, T., Kano, K., Tachi, Y., Itoh, S., Kojo, S. (2003). Mechanism for Aerobic

Mononuclear Copper(II)-Superoxo Complex. *J. Am. Chem. Soc.*, Vol.116, No.23, pp.

Tripodal Polypyridine Copper(II) Complex That Enables to Recognize a Small

and Properties of Copper(II) Complexes with New Tripodal Tetradentate Ligand. *N*-(2- Pyridylmethyl)bis(6-pyvalamido-2-pyridylmethyl)amine, and Reactivities of the Cu(I) Complex with Dioxygen. *Bull. Chem. Soc. Jpn.*, Vol. 71, No.5, pp. 1031-1038.

Oxidation of 3,5-Di-*tert*-butylcatechol to 3,5-Di-*tert*-butyl-*o*-benzoquinone Catalyzed


Interactions. *J. Am. Chem. Soc.,* 

Stacking in Metal Complexes with Aromatic

interactions to π-π

rearrangement based on the theromodynamic preference of CH/

*Soc. London, Ser. A,* Vol.214, No.1119, pp. 451-465.

Molecule. *Chem. Lett.*, Vol.24, No.1, pp.61-62.

Hunter, C. A., Sanders, J. K. M. (1990). The Nature of

Vol.112, No.14, pp. 5525-5534. Janiak, C. (2000). A Critical Account on

**5. Summary** 

**6. References** 

10817-10818.

of the addition of 2-3 and 3-5 equiv of TMAOH have been assigned to deprotonation of one catechol O-H and remaining two catechol O-H protons, respectively, as shown in Scheme 2.

Scheme 2. Deprotonation processes of **10**.

As a precursor for the complexation of the Cu(II) ion to **10**, we have employed [CuII(NO3)2(TMEDA)] (TMEDA = *N*,*N*,*N*',*N*'-tetramethylethylenediamine) (Pavkovic *et al*. 1977), since the Cu(II) ion can form the most stable complex in light of the Irwing-Williams series. The UV-vis titration of **10** with [CuII(NO3)2(TMEDA)] in MeOH allows us to reveal that the catechol moieties of **10** is capable of binding to only one Cu(II)-TMEDA unit. The reaction between **10** and [CuII(NO3)2(TMEDA)] in MeOH proceeds in the presence of NEt3. The addition of TBAPF6 to a mixture of **10** and [CuII(NO3)2(TMEDA)] gives a powder of [RuCl((HCat)2-TPA){Cu(TMEDA)}]PF6 (**11**). ESI-MS measurements on **11** afforded a peak cluster at *m*/*Z* = 906.1 assigned to [RuCl((HCat)2-TPA){Cu(TMEDA)}]+, indicating that each catechol moiety releases one proton and the converged two catechol moieties can accept one Cu(II)-TMEDA unit as demonstrated by the spectroscopic titration.

Fig. 12. Putative structure of **11**.

The EPR spectrum of **11** in MeOH at 77 K shows an anisotropic signal at *g//* = 2.239 and *g* = 2.060, together with hyperfine coupling constant (hfc) of A*//* = 184 G due to Cu(II). These parameters are different from those of [Cu(catecholato)(TMEDA)] (*g//* = 2.241, *g* = 2.071, and A*//* = 184 G) (Kodera *et al.*, 2003), even though the signals indicate that both Cu(II) complexes are with a monomeric tetragonal geometry in the (dx2 -y 2)1 ground state (Wojciechowski *et al.*, 2009; Nielsen *et al*., 2008). Considering the deprotonation behavior of catechol moieties, the complex **11** is supposed to have a unique coordination environment of a Cu(II)-catecholate complex as described in Figure 12. DFT calculations at the B3LYP/LANL2DZ level of theory suggest that the optimized structure of **11** shown in Figure 12 is more stable in 3.5 kcal/mol than that of a bimetallic complex holding the Cu-TMEDA unit at one catecholate moiety in a bidentate fashion.

### **5. Summary**

252 Current Trends in X-Ray Crystallography

of the addition of 2-3 and 3-5 equiv of TMAOH have been assigned to deprotonation of one catechol O-H and remaining two catechol O-H protons, respectively, as shown in Scheme 2.

As a precursor for the complexation of the Cu(II) ion to **10**, we have employed [CuII(NO3)2(TMEDA)] (TMEDA = *N*,*N*,*N*',*N*'-tetramethylethylenediamine) (Pavkovic *et al*. 1977), since the Cu(II) ion can form the most stable complex in light of the Irwing-Williams series. The UV-vis titration of **10** with [CuII(NO3)2(TMEDA)] in MeOH allows us to reveal that the catechol moieties of **10** is capable of binding to only one Cu(II)-TMEDA unit. The reaction between **10** and [CuII(NO3)2(TMEDA)] in MeOH proceeds in the presence of NEt3. The addition of TBAPF6 to a mixture of **10** and [CuII(NO3)2(TMEDA)] gives a powder of [RuCl((HCat)2-TPA){Cu(TMEDA)}]PF6 (**11**). ESI-MS measurements on **11** afforded a peak cluster at *m*/*Z* = 906.1 assigned to [RuCl((HCat)2-TPA){Cu(TMEDA)}]+, indicating that each catechol moiety releases one proton and the converged two catechol moieties can accept one

The EPR spectrum of **11** in MeOH at 77 K shows an anisotropic signal at *g//* = 2.239 and *g* = 2.060, together with hyperfine coupling constant (hfc) of A*//* = 184 G due to Cu(II). These parameters are different from those of [Cu(catecholato)(TMEDA)] (*g//* = 2.241, *g* = 2.071, and A*//* = 184 G) (Kodera *et al.*, 2003), even though the signals indicate that both Cu(II) complexes

2009; Nielsen *et al*., 2008). Considering the deprotonation behavior of catechol moieties, the complex **11** is supposed to have a unique coordination environment of a Cu(II)-catecholate complex as described in Figure 12. DFT calculations at the B3LYP/LANL2DZ level of theory suggest that the optimized structure of **11** shown in Figure 12 is more stable in 3.5 kcal/mol than that of a bimetallic complex holding the Cu-TMEDA unit at one catecholate moiety in a


2)1 ground state (Wojciechowski *et al.*,

Cu(II)-TMEDA unit as demonstrated by the spectroscopic titration.

Scheme 2. Deprotonation processes of **10**.

Fig. 12. Putative structure of **11**.

bidentate fashion.

are with a monomeric tetragonal geometry in the (dx2

In this chapter, we have presented an overview of ruthenium complexes having TPA derivatives with functional groups via amide linkages. The strategy has allowed us to construct unique second coordination spheres of ruthenium complexes gaining novel functionality. The ruthenium coordination to bisamide-TPA derivatives affords the convergence of functional groups attached by the amide linkage into one direction to form specific environments, such as a hydrophobic cavity. The hydrophobic cavity made of the two naphthyl groups forms noncovalent van der Waals interactions with inserted substituents. Under this situation, we can observe the uni-directional intramolecular rearrangement based on the theromodynamic preference of CH/ interactions to π-π interactions. The introduction of ferrocene moiety to the TPA ligand makes possible intramolecular ferromagnetic interaction between Ru(III) and Fe(III) centers in the ruthenium complex with ferrocene units as pendants. A catechol-appended TPA ligand can provide a preorganized metal binding site consisting of the two catechol moieties on the ruthenium complex to accept one Cu(II) complex in a unique coordination mode. Recently, we have also prepared a TPA derivative having two 1,10-phenanthroline (phen) moieties via amide linkages and its Ru(II) complex: The complex can form two appended Ru(II)-phen complexes and perform cooperative transfer hydrogenation of ketones (Kojima *et al*., 2011). In those complexes, the intramolecular hydrogen bond formed between the amide moieties can facilitate convergence of the functional groups and stabilize the complexes. This can be a significant advantage for the development of transition-metal-based functional molecules. Thus, the design of Ru-TPA complexes having functional groups via amide linkage will provide a nice opportunity to create cooperative molecular functionality on the basis of characteristics of both the ruthenium center and functional groups to provide a unique second coordination sphere as well as other metal ions attached to the functional groups.

#### **6. References**


**11** 

*Romania* 

**X-Ray Structural Characterization of** 

Viorel Cîrcu and Marin Micutz

*University of Bucharest,* 

**Cyclometalated Luminescent Pt(II) Complexes** 

The cyclometalated complexes represent one of the most interesting and broadly studied class of organotransition metal complexes. Although there is a strong interest in studying the mechanism of this bond-activation process, cyclometalation is a highly attractive and versatile synthetic approach for generating organometallic systems, with very important application potential (Crabtree, 2005). There are both mononuclear and dinuclear species, but also polynuclear cyclometalated complexes are known (Diez et al., 2011). Many reviews and books have been dedicated to this topic over the past years and one of the most recent

The cyclometalation process consists of a transition metal-mediated activation of a C-R bond to form a metallacycle that contains a metal-carbon *σ* bond (Hill, 2002). On the other hand, cyclometalation can be regarded as a special case of oxidative addition, in which a C−R (in

Although many examples are described, by far most of cyclometalation reactions occur via C-H bond activation. The reaction product is a metallacycle in which the metal is bound by a chelate C- donor ligand. It is important to note that such chelation leads to organometallic compounds with increased stability. Altogether, the cyclometalation reaction has been widely studied because it represents probably one of the mildest route for activating strong C-H and C-R bonds. The tendency of transition metal salts to undergo cyclometalation reaction, and, in particular, ortho-metalation reaction, with heteroaromatic ligands (mostly including nitrogen donors, but oxygen-, sulfur- and phosphorus-containing ligands have also been cyclometalated) to give five-memberd metallocycles has been demonstrated with various metals, including, for instance, Re(I), Pt(II), Pd(II). This review will take into account

In comparison with Pd(II), which is by far the most used metal in cyclometalation reactions, the cycloplatination reaction is not so intensively studied and not very easy to accomplish (cycloplatination reactions which took about four weeks or required relatively forcing conditions, e.g., refluxing toluene, with poor yields, have been reported). However, it is possible to increase the yields and reduce the time of reaction by using different starting materials such as bis(η3-allyl)-di-μ-chlorodiplatinum(II) or PtCl2(DMSO)2, etc, although K2PtCl4 or [Pt2Me4(μ-SMe2)2)] are commonly used to yield cyclometalated species. In most of the cases, the reaction products are halo-bridged dimers, that can be used further to form

most cases, C-H) bond in a ligand oxidatively adds to a metal to give rise to a ring.

only the cyclometalated Pt(II) complexes with nitrogen-containing ligands.

**1. Introduction** 

can be found here (Albrecht, 2010).

by Di-*μ*-hydroxo-dicopper(II) Complexes of Peralkylated Ethylelnediamine Ligands. *Bull. Chem. Soc. Jpn.,* Vol.76, No.10, pp. 1957-1964.

