**3.4.4 Bis(imine) ligands – N^C^N pincer ligands**

Several crystallographic studies concerning the cycloplatinated complexes with bis(imine) ligands (where the N atom is not part of a ring) were reported (see Scheme 11). Complex **55** was prepared by reacting LiC≡CSiMe3 in THF with the first platinum(II) halide compounds containing the (N^C^N) isophtalaldimine ligands.

R=2,6-(i Pr)2Ph, X=-C≡CSiMe3 **55** R=2,6-(i Pr)2Ph, X=CH3 **56** R=-CH(CH3)Ph, X=Cl **57a** R=t Bu, X=Cl **57b** R=Ph, X=Cl **57c** 

Scheme 11.

270 Current Trends in X-Ray Crystallography

Table 7. Selected bond legths for cyclometalated Pt(II) complexes with terdentate imine

Torres & Mendiola, 2010. No emission properties were reported for such complexes.

purposes and no emission properties were investigated (Ryabov et al., 1995).

Me N

OH

R = R' = Me (Pt-C = 1.998(4) Å, Pt-N = 2.013(3) Å, Pt – S = 2.2677(11) Å, Pt – Cl = 2.4114(11) Å), (**54a**); R = Me, R' = Ph (Pt-C = 2.010(4) Å, Pt-N = 2.010(3) Å, Pt – S = 2.2192(12) Å, Pt – Cl = 2.3806(13) Å) , (**54b**)

Pt Cl <sup>S</sup> <sup>O</sup> <sup>R</sup> R'

There are several X-ray crystallographic structures reported for a series of cycloplatinated complexes with oxime ligands (see Scheme 10). They were prepared starting from [PtCl2(RR'SO)2] and the corresponding oxime ligand. Although they show an interesting structural feature, as it is the case of complex **54b** that reveals an extremely short Pt...Pt contact of 3.337 Å, these complexes were studied mostly for mechanistic and catalytic

**3.4.3 Pt(II) complexes bearing cyclometalated oxime ligands** 

On the other hand, complex **53b** was obtained by reacting the thiosemicarbazone ligand with the Pt dinuclear allyl complex [Pt(μ-Cl)(η3-C4H7)]2 in refluxing acetone, when, instead of the expected tetranuclear complex, a mononuclear cyclometalated Pt(II) complex containing two thiosemicarbazone molecules was formed. One of the molecule acts as a [C^N^S]- terdentate ligand while the second one is coordinated in a monodentate fashion through the S atom, thus completing the coordination sphere of Pt(II) metal. Bis(thiosemicarbazone) ligands were used for cycloplatination reaction and the first X-ray structure of a cyclometalated Pt(II) complex with such ligands was reported by Lopez-

ligands

(Ryabov et al., 2002)

Scheme 10.

On the other hand, complex **56** represents the first X-ray crystal structure of stable transarylplatinum methyl complexes [PtMeN^C^N] with imine-type ligands. It is well-known that due to the strong C(sp2)−C(sp3) bond, only very few transition-metal compounds having an aryl as well as a methyl group bonded to the same metal atom are reported. The reason is that in such cases reductive elimination occurs. For complex **56** this reaction is prevented due to the trans disposition of the methyl and aryl groups and the rigid coplanarity of the chelate rings. Another representative example is complex **57a** that is the first chiral bis-aldimine (N^C^N)–pincer complexes. Unfortunately, there was no emission data reported for such complexes.


Table 8. Selected bond lengths for cyclometalated Pt(II) complexes with bis(imine) N^C^N pincer ligands

#### **3.4.5 Cyclometalated Pt(II) complexes containing ferrocene based imine ligands**

Several crystal structures of cyclometalated Pt(II) complexes containing imine ligands with ferrocene fragment were reported (see Scheme 12).

R

Scheme 12. Cycloplatinated complexes bearing ferrocene imine ligands


X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 273

bright luminescent emitters (Eliseeva & Bünzli, 2010), but they have low quantum yield efficiency. Thus, by far, due to the possibility of harvesting both triplet and singlet states, as well as the emission in the red – NIR range, the late transition metallomesogens are the most promising candidates and could be employed for the preparation of highly effective phosphorescent OLEDs. In this respect, several studies have been reported recently including luminescent metallomesogens based on Pt(II), Ir(III) or Ru(II), Ag(I) or Au(I) (Binnemans, 2009). There are several studies dealing with light-emitting metallomesogens based on platinum(II) (Liao et al., 2011; Venkatesan et al., 2008; Mocanu et al., 2010) complexes, most of them containing the metal in a cyclometalating environment (Damm et al., 2006; Wang et al., 2011) with 2-arylpyridine or 2-thienylpyridine derivatives. Bruce et. al. reported phosphorescent liquid crystalline complexes of platinum(II) showing a stimulusdependant emission (Kozhevnikov et al., 2008) as well as highly luminescent (yields higher than 0.5) Pt(II) containing metallomesogens (Santoro et al., 2009). In terms of quantum yields, these later examples were exceeded only by the recently reported Pt(II) metallomesogens bearing pyridyl pyrazole chelates that show quantum yields nearly 1,

Fig. 6. Molecular structure of a mononuclear cycloplatinated metallomesogen containing an imine and an acetylacetonate ligand (**63**). The two independent molecules contained in the unit cell are shown (H atoms were omitted for clarity).(Pt-C = 1.983(10) and 1.976(11) Å, Pt-N = 1.986(8) and 2.008(7) Å, Pt-O1 = 2.101(6) and 2.080(6) Å, Pt-O2 = 1.992(7) and 2.004(6) Å

In most cases, luminescent Pt(II) complexes contain heterocyclic ligands, usually with one or two pyridine rings, while imine ligands were completely ignored from this point of view. Several examples of luminescent cycloplatinated (Scaffidi-Domianello et al., 2007; Pandya et al., 2010) or platinum(II) complexes with N,N-donor diimine ligands are known, though this class of ligands is widely spread in the design of Pt(II) metallomesogens (Cîrcu et al., 2009b; Buey et al., 1996; Diez, L. et al., 2002). Although several important results have been achieved in the design and the synthesis of luminescent liquid crystalline materials based on Pt(II) cyclometalated complexes, only few X-ray structural investigations have been made on such systems comprising either mononuclear (Cîrcu et al., 2007) or dinuclear species (Praefcke et al., 1994; Bilgin Eran et al., 2001). One reason could be that such molecules have long alkyl chains that make single crystals suitable for X-ray crystallography difficult to obtain. The only example of a crystal structure of a mononuclear cycloplatinated metallomesogen (**63**) is depicted in Figure 6. The Pt–C distance is similar to those found in

when recorded in degassed dichloromethane (Liao et al., 2011).

(Cîrcu et al., 2007)


Table 9. Selected bond lengths for cyclometalated Pt(II) complexes containing imine ligands based on ferrocene fragment

Most of these complexes were prepared by reacting the imine ligand with [PtCl2(DMSO)2] in refluxing toluene for a long period of time, with or without sodium acetate as a base agent. They can react further with phosphine ligands to replace the labile DMSO ligand (61b). Several interesting structural features are presented in Table 9. It is important to note that these complexes were studied more from mechanistic, electrochemical and catalytic properties point of view and less, or not at all in most of the cases, for emissive properties.

#### **3.5 Heteropolynuclear cycloplatinated complexes**

Due to the presence of metallophilic interactions that have serious consequences on luminescence properties, the study of different heteropolynuclear cycloplatinated complexes has dramatically increased in the last years and this topic has been very recently reviewed (Diez et al., 2011). It has been shown that the cycloplatinated complexes can be useful building blocks in the design of heteropolynuclear and/or multicomponent architectures. Although several interesting heteropolymetallic cycloplatinated systems have been reported, the photoluminescence properties have been studied only in few cases.

#### **3.6 Cyclometalated Pt(II) complexes as liquid crystals**

Liquid crystals are, by far, *the most important* molecular electronic materials of the present day. They were discovered more than 100 years ago and are often thought of as the high technology materials found in high content, low power, flat-panel displays known to the whole world as LCDs, their main application (O'Neill & Kelly, 2011). Stable inorganic phosphors showing liquid crystal properties are very promising multifunctional materials because, in contrast with pure organic materials, they are not subjected to photobleaching (loss of luminescence properties upon irradiation over the time), provide anisotropic long range order and thus polarised emission that should improve display performance parameters such as brightness, contrast, energy efficiency and, in some cases, the viewing angle. Potential new materials that fulfill these requirements are the metallomesogens (liquid crystalline materials incorporating metal ions). The first option, lanthanide mesogenic complexes (lanthanidomesogens) have been proposed as potential candidates for

Table 9. Selected bond lengths for cyclometalated Pt(II) complexes containing imine ligands

Most of these complexes were prepared by reacting the imine ligand with [PtCl2(DMSO)2] in refluxing toluene for a long period of time, with or without sodium acetate as a base agent. They can react further with phosphine ligands to replace the labile DMSO ligand (61b). Several interesting structural features are presented in Table 9. It is important to note that these complexes were studied more from mechanistic, electrochemical and catalytic properties point of view and less, or not at all in most of the cases, for emissive properties.

Due to the presence of metallophilic interactions that have serious consequences on luminescence properties, the study of different heteropolynuclear cycloplatinated complexes has dramatically increased in the last years and this topic has been very recently reviewed (Diez et al., 2011). It has been shown that the cycloplatinated complexes can be useful building blocks in the design of heteropolynuclear and/or multicomponent architectures. Although several interesting heteropolymetallic cycloplatinated systems have been

Liquid crystals are, by far, *the most important* molecular electronic materials of the present day. They were discovered more than 100 years ago and are often thought of as the high technology materials found in high content, low power, flat-panel displays known to the whole world as LCDs, their main application (O'Neill & Kelly, 2011). Stable inorganic phosphors showing liquid crystal properties are very promising multifunctional materials because, in contrast with pure organic materials, they are not subjected to photobleaching (loss of luminescence properties upon irradiation over the time), provide anisotropic long range order and thus polarised emission that should improve display performance parameters such as brightness, contrast, energy efficiency and, in some cases, the viewing angle. Potential new materials that fulfill these requirements are the metallomesogens (liquid crystalline materials incorporating metal ions). The first option, lanthanide mesogenic complexes (lanthanidomesogens) have been proposed as potential candidates for

reported, the photoluminescence properties have been studied only in few cases.

based on ferrocene fragment

**3.5 Heteropolynuclear cycloplatinated complexes** 

**3.6 Cyclometalated Pt(II) complexes as liquid crystals** 

bright luminescent emitters (Eliseeva & Bünzli, 2010), but they have low quantum yield efficiency. Thus, by far, due to the possibility of harvesting both triplet and singlet states, as well as the emission in the red – NIR range, the late transition metallomesogens are the most promising candidates and could be employed for the preparation of highly effective phosphorescent OLEDs. In this respect, several studies have been reported recently including luminescent metallomesogens based on Pt(II), Ir(III) or Ru(II), Ag(I) or Au(I) (Binnemans, 2009). There are several studies dealing with light-emitting metallomesogens based on platinum(II) (Liao et al., 2011; Venkatesan et al., 2008; Mocanu et al., 2010) complexes, most of them containing the metal in a cyclometalating environment (Damm et al., 2006; Wang et al., 2011) with 2-arylpyridine or 2-thienylpyridine derivatives. Bruce et. al. reported phosphorescent liquid crystalline complexes of platinum(II) showing a stimulusdependant emission (Kozhevnikov et al., 2008) as well as highly luminescent (yields higher than 0.5) Pt(II) containing metallomesogens (Santoro et al., 2009). In terms of quantum yields, these later examples were exceeded only by the recently reported Pt(II) metallomesogens bearing pyridyl pyrazole chelates that show quantum yields nearly 1, when recorded in degassed dichloromethane (Liao et al., 2011).

Fig. 6. Molecular structure of a mononuclear cycloplatinated metallomesogen containing an imine and an acetylacetonate ligand (**63**). The two independent molecules contained in the unit cell are shown (H atoms were omitted for clarity).(Pt-C = 1.983(10) and 1.976(11) Å, Pt-N = 1.986(8) and 2.008(7) Å, Pt-O1 = 2.101(6) and 2.080(6) Å, Pt-O2 = 1.992(7) and 2.004(6) Å (Cîrcu et al., 2007)

In most cases, luminescent Pt(II) complexes contain heterocyclic ligands, usually with one or two pyridine rings, while imine ligands were completely ignored from this point of view. Several examples of luminescent cycloplatinated (Scaffidi-Domianello et al., 2007; Pandya et al., 2010) or platinum(II) complexes with N,N-donor diimine ligands are known, though this class of ligands is widely spread in the design of Pt(II) metallomesogens (Cîrcu et al., 2009b; Buey et al., 1996; Diez, L. et al., 2002). Although several important results have been achieved in the design and the synthesis of luminescent liquid crystalline materials based on Pt(II) cyclometalated complexes, only few X-ray structural investigations have been made on such systems comprising either mononuclear (Cîrcu et al., 2007) or dinuclear species (Praefcke et al., 1994; Bilgin Eran et al., 2001). One reason could be that such molecules have long alkyl chains that make single crystals suitable for X-ray crystallography difficult to obtain. The only example of a crystal structure of a mononuclear cycloplatinated metallomesogen (**63**) is depicted in Figure 6. The Pt–C distance is similar to those found in

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dinuclear chloro- or thiocyanato-bridged, orthometalated platinum compounds with Schiff bases reported by Praefcke et al., 1994, whereas the Pt–N distances are slightly smaller (2.19 and 2.064 Å in Praefcke's work), which could be assigned to the difference in trans-effect of the atoms coordinated to platinum. It is interesting to note that an *anti* configuration (the perfluoroethyl group is trans to nitrogen atom of the imine group) is adopted by the two ligands (Schiff base and acetylacetonate derivative) around platinum centre. This complex **63** shows a monotropic nematic phase on cooling at 37°C.
