**3. X-ray structural characterization of cyclometalated Pt(II) complexes**

#### **3.1 Pt(II) complexes with 2-phenylpyridine (ppy) and its derivatives as cyclometalating ligand**

The homoleptic Pt(II) complex with 2-phenylpyridine was prepared by reacting [PtCl2(Et2S)2] with the lithiated ligand as the *cis* isomer and its crystal structure was reported in 1984 (Chassot et al., 1984).

Scheme 2. The preparation of mononuclear cyclometalated Pt(II) complexes with 2 phenylpyridine derivatives

Generally, the heteroleptic complexes are prepared by the cyclometalation of the coresponding ligand using K2PtCl4 in ethoxyethanol to give, in the first step, as a major product, the chloro-bridged dinuclear complex and, in some cases, as a minor product, the mononuclear complex having two molecules of ppy ligand. In the next step, the bridgesplitting reaction, various monodentate (L) or bidentate (A^B) ligands can be used to yield the mononuclear neutral or ionic heteroleptic complexes (see Scheme 2). By far, the most studied Pt(II) cyclometalated complexes bearing ppy ligand for their emission properties are the ones containing acetylacetonate (acac) derivatives as ancillary ligands (Brooks et al, 2002). Various other R1-R3 substituents were attached on the ppy core with the aim of conferring different multifunctionalities to the resulting molecules. For instance, He et al.,

**3.1 Pt(II) complexes with 2-phenylpyridine (ppy) and its derivatives as cyclometalating** 

The homoleptic Pt(II) complex with 2-phenylpyridine was prepared by reacting [PtCl2(Et2S)2] with the lithiated ligand as the *cis* isomer and its crystal structure was reported

> N N Pt Cl

R3

R1

R2

acac Na2CO3

L

A B N Pt Cl L

R1

R3

R2

R3

R2

R3

R2

N Pt A B

R1

R1

N Pt O <sup>O</sup> <sup>Y</sup>

X H/W

**3. X-ray structural characterization of cyclometalated Pt(II) complexes** 

**ligand** 

in 1984 (Chassot et al., 1984).

<sup>N</sup> K2PtCl4 Ethoxyethanol/H2O

R3 R3

R2 R2

phenylpyridine derivatives

N Pt Cl

R R1 R1 <sup>1</sup>

2 <sup>+</sup> R2 R3

R1=NPh2, R2=R3=H, X=Y=Me **1a** R1=R2=H,R3=-OHex, R1=-oxadiazole-4-Ph-p-NPh2, R2=R3=H, X=Y=Me **1b** A^B=5-NO2-8-quinoline **4**

R1=R2=R3=H, X=Y=Ph **1d** A^B=Et2NC(S)NCOPh **5**a

R1=R2=R3=H, L=CO **2a** A^B=MeO-4-C6H4NHC(S)NCOPh **5b**

R1=F, R2=R3=H, L=DMSO **2c** A^B=2-pyridyl hexafluoropropoxide **6** R1=F, R2=R3=H, L=py-2-Ph-4'-F **2d** R1=R2=F, R3=H, A^B=(pz)2BEt2 **7** R1=R2=F, R3=H, L=py-2-Ph-2',4'-F2 **2e** R1=R2=R3=H, A^B=(3,5-Me2pz)BH2 **8** R1=R2=F, R3=H, A^B=3,5-di-*t*Bu-catechol **3** R1=R2=R3=H, A^B=dppm, PF6- **9**  Scheme 2. The preparation of mononuclear cyclometalated Pt(II) complexes with 2-

Generally, the heteroleptic complexes are prepared by the cyclometalation of the coresponding ligand using K2PtCl4 in ethoxyethanol to give, in the first step, as a major product, the chloro-bridged dinuclear complex and, in some cases, as a minor product, the mononuclear complex having two molecules of ppy ligand. In the next step, the bridgesplitting reaction, various monodentate (L) or bidentate (A^B) ligands can be used to yield the mononuclear neutral or ionic heteroleptic complexes (see Scheme 2). By far, the most studied Pt(II) cyclometalated complexes bearing ppy ligand for their emission properties are the ones containing acetylacetonate (acac) derivatives as ancillary ligands (Brooks et al, 2002). Various other R1-R3 substituents were attached on the ppy core with the aim of conferring different multifunctionalities to the resulting molecules. For instance, He et al.,

R1=-oxadiazole-4-Ph-p-NTol2, R2=R3=H, X=Y=Me **1c** R1=R2=R3=H,

R1=R2=F, R3=H, X=Y=Me, W=-(CH2)2-norbornene **1e** R1=R2=R3=H,

R1=R2=R3=H, L=DMSO **2b** R1=R2=R3=H,

2006, reported the synthesis and structural, photophysical, electrochemical, and electroluminescent properties of a novel class of trifunctional Pt(II) cyclometalated complexes incorporating the hole-transporting triarylamine, electron-transporting oxadiazole, and electroluminescent metal components into a single molecule (**1a**-**c**). Other studies focused on functionalizing the acac derivatives, as it is the example reported by Cho et al., 2007, in which the norbornene-functionalized derivative of acetylacetone has been used to synthesize a series of new polymerizable norbornene-derivatized phosphorescent platinum complexes (**1e**). For these acac Pt(II) complexes, it was found that the two Pt-O bonds are different, with the Pt-O1 trans to C atom bond longer than the Pt-O2 trans to N bond due to the strong trans influence of C-ppy donor atom (Table 1).

Several crystal structures of cyclometalated Pt(II) complexes bearing monodentate neutral ligands such as DMSO or pyridine derivatives were reported (**2a-e**). Another strategy to obtain mononuclear Pt(II) species is to use bidentate mononegative ligands and various such examples were reported so far (**3** – **8**).

By using sodium salts of N-benzoylthiourea derivatives, a series of luminescent Pt(II) ppy complexes were prepared and investigated by single-crystal X-ray diffraction (**5a**, **b**) (Figure 2).

Fig. 2. ORTEP view of complex **5a** (a); the emission spectrum of **5a** recorded in CH2Cl2 solution at room temperature (b)

An interesting feature of the structure of complex **5b** is the orientation adopted by the *p*anisyl ring of the *N*-benzoyl thiourea ligand with a twist of 65.1º with respect to the core plane, leading to the formation of weak intermolecular NH...Pt interactions (H...Pt 2.74 Å; N-H...Pt 156.8°) compared to those found in the salt (NnPr4)2[PtCl4][PtCl2(NH2Me)2]. In this case, the shortest Pt-Pt distance between two neighbouring molecules is 4.18 Å.

X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 261

and **11c** show several interesting features such as the intermolecular Pt···Pt, π...π and C-H···π(C≡C) interactions in an orthogonal configuration. On the other hand, the crystal structure of **11a** shows the formation of dimers in a head-to-tail fashion with an interplanar distance of approximately 3.4 Å and a shortest intermetallic separation of 4.886 Å, indicating the presence of π···π interaction, but not a Pt···Pt interaction. The presence of such intermetallic interactions has a marked impact upon the solid state emission properties.

> <sup>N</sup> <sup>N</sup> Pt Cl

R1=R2=H, X=Cl **10a 12 13**

Table 2. A selection of bond lengths for some N^C^N Pt(II) complexes

Pt-N1 Pt-N2

The room temperature structureless emissions at λmax 819 (**11b**) and 702 nm (**11c**), respectively, strongly red-shifted compared to solution emission (λmax = 486 and 480, respectively), can be assigned to triplet metal-metal-to-ligand charge-transfer (3MMLCT) excited states. The photophysical properties of chloride complexes (**10a**-**c**) and analogous with various other R1 substituents were systematically studied by Farley et al., 2005. It is interesting to note that these N^C^N Pt(II) complexes show relatively shorter Pt-C distances

The C^N^N type ligands were used in the preparation of both neutral or ionic complexes

An important feature of these complexes is that the central Pt-N1 bond is shorter than the marginal Pt-N2 bond (Table 3) due to the *trans* influence of the strong field cyclometalated ligand. Two crystal structures of **14a** were determined (yellow and red polymorphs). The red colour of the latter polymorph was attributed to the Pt...Pt intermolecular interactions

than those found in C^N^N Pt(II) complexes (see comparatively Tables 2 and 3).

<sup>N</sup> <sup>N</sup> Pt Cl

F F

<sup>N</sup> <sup>N</sup> Pt X

R2 R2

R1=H, R2=Me, X=Cl **10b** R1=-C(O)OCH3, R2=H, X=Cl **10c** R1=R2=H, X=-C≡C-Ph **11a** R1=CF3, X=-C≡C-Ph **11b** R1=CF3, X=-C≡C-Ph-4-NMe2 **11c**

**3.2.2 C^N^N ligands** 

(see Scheme 4).

(3.366 Å).

Scheme 3.

R1


Table 1. Selected bond lengths for compounds **1** - **9** 

#### **3.2 Cyclometalated Pt(II) complexes with "pincer" ligands**

The cyclometalated Pt(II) complexes with terdentate "pincer" type ligands were investigated intensively, in part due to the fact that such ligands may provide additional rigidity to the molecule that could be responsible for favouring the luminescence properties. Many types of pincer ligands have been used in the preparation of both mono or polynuclear organometallic Pt(II) complexes and they are divided according to the donor atom set involved in bond formation with the metal centre, as follows: N^C^N, C^N^N and C^C^N complexes.

#### **3.2.1 N^C^N ligands**

Several crystal structures of N^C^N Pt(II) complexes were reported and these are summarised in Scheme 3, while the selected bond lengths are included in Table 2. The chloride Pt(II) complexes (**10a**-**c**, **12**, **13**) can be easily obtained by orthometalation reaction of the corresponding N^C^N pincer ligand with K2PtCl4 in acetic acid with high yields, while the acetylide complexes **11a-c** were prepared in quantitative yields starting from the corresponding chloride precursor, with an excess of aryl acetylene in methanol in the presence of NaOH for one day at room temperature. The crystal structures of complexes **11b**

and **11c** show several interesting features such as the intermolecular Pt···Pt, π...π and C-H···π(C≡C) interactions in an orthogonal configuration. On the other hand, the crystal structure of **11a** shows the formation of dimers in a head-to-tail fashion with an interplanar distance of approximately 3.4 Å and a shortest intermetallic separation of 4.886 Å, indicating the presence of π···π interaction, but not a Pt···Pt interaction. The presence of such intermetallic interactions has a marked impact upon the solid state emission properties.

R1=R2=H, X=-C≡C-Ph **11a** R1=CF3, X=-C≡C-Ph **11b** R1=CF3, X=-C≡C-Ph-4-NMe2 **11c**

Scheme 3.

260 Current Trends in X-Ray Crystallography

Table 1. Selected bond lengths for compounds **1** - **9** 

complexes.

**3.2.1 N^C^N ligands** 

**3.2 Cyclometalated Pt(II) complexes with "pincer" ligands** 

The cyclometalated Pt(II) complexes with terdentate "pincer" type ligands were investigated intensively, in part due to the fact that such ligands may provide additional rigidity to the molecule that could be responsible for favouring the luminescence properties. Many types of pincer ligands have been used in the preparation of both mono or polynuclear organometallic Pt(II) complexes and they are divided according to the donor atom set involved in bond formation with the metal centre, as follows: N^C^N, C^N^N and C^C^N

Several crystal structures of N^C^N Pt(II) complexes were reported and these are summarised in Scheme 3, while the selected bond lengths are included in Table 2. The chloride Pt(II) complexes (**10a**-**c**, **12**, **13**) can be easily obtained by orthometalation reaction of the corresponding N^C^N pincer ligand with K2PtCl4 in acetic acid with high yields, while the acetylide complexes **11a-c** were prepared in quantitative yields starting from the corresponding chloride precursor, with an excess of aryl acetylene in methanol in the presence of NaOH for one day at room temperature. The crystal structures of complexes **11b**


Table 2. A selection of bond lengths for some N^C^N Pt(II) complexes

The room temperature structureless emissions at λmax 819 (**11b**) and 702 nm (**11c**), respectively, strongly red-shifted compared to solution emission (λmax = 486 and 480, respectively), can be assigned to triplet metal-metal-to-ligand charge-transfer (3MMLCT) excited states. The photophysical properties of chloride complexes (**10a**-**c**) and analogous with various other R1 substituents were systematically studied by Farley et al., 2005. It is interesting to note that these N^C^N Pt(II) complexes show relatively shorter Pt-C distances than those found in C^N^N Pt(II) complexes (see comparatively Tables 2 and 3).

#### **3.2.2 C^N^N ligands**

The C^N^N type ligands were used in the preparation of both neutral or ionic complexes (see Scheme 4).

An important feature of these complexes is that the central Pt-N1 bond is shorter than the marginal Pt-N2 bond (Table 3) due to the *trans* influence of the strong field cyclometalated ligand. Two crystal structures of **14a** were determined (yellow and red polymorphs). The red colour of the latter polymorph was attributed to the Pt...Pt intermolecular interactions (3.366 Å).

X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 263

528 to 558 nm in acetonitrile at 298 K, which were assigned to 3MLCT excited states. Also, dinuclear phosphorescent C^N^N Pt(II) cyclometalated were prepared (also investigated by X-ray crystallography) by using diacetylide – carbazole bridging ligand (Seneclauze et al.,

The C^N^C cyclometalated Pt(II) complexes are synthesised by using K2PtCl4 and the corresponding 2,6-diphenyl-pyridine derivative, in two consecutive cyclometalation reactions. In the first step, a dinuclear chloride-bridged C^N cyclometalated Pt(II) complex is obtained, followed by reaction with a monodentate neutral ligand to give mononuclear neutral C^N^C species. Several crystallographic studies were carried out on such systems

> N Pt

<sup>N</sup> <sup>N</sup> Pt X

Pt X

<sup>S</sup> <sup>O</sup> Me Me

Complexes **19a-c** were prepared by reacting K2PtCl4 with the terdentate 2,6-di-(2'-naphthyl) pyridine ligands in glacial acetic acid, followed by heating in DMSO. Their crystal structures reveal that the molecules are paired in a head-to-tail orientation with Pt···Pt separations longer than 6.3 Å, with extensive close C−H···π (*d* = 2.656−2.891 Å) and π···π (*d* = 3.322−3.399 Å) interactions. An interesting feature of such C^N^C Pt(II) complexes is that the Pt-C bonds are longer compared to the N^C^N or N^N^C Pt(II) complexes and this is due the *trans* influence of C atoms on each other. These complexes are emissive in both solid state, with the maxima in the range 600 – 650 nm (**19a-c**), and in solution at 77 K (**18a**). Additionally, several dinuclear C^N^C cyclometalated Pt(II) complexes, either by using a dicyclometalated ligand (Zucca et al., 2006) or a diphosphine ligand as a bridge (dppm, diphenylphosphinomethane, Kui et al., 2006), were investigated by X-ray crystallography.

X=py-4-NMe2, R=H **18b** R=Ph **19b** X=3,5-Me2-py **20**

R

2007).

Fig. 3. ORTEP view of complex **14b**

N Pt X R R

and these are summarised in Scheme 5 and Table 4.

X=pz (pyrazine), R=H **18a** R=H **19a**

R=OC6H13, X=CO **18c** R=Ph-4-Br **19c**

**3.2.3 C^N^C ligands** 

Scheme 5.

A-

R1=Tol, X=Cl 14**b** R1=H, X=N≡CXyl, A=PF6- **16** R1=H, X=-C≡C-Me2-fluorene **15c** R1=H, R2=Me, R3=Xyl, A=ClO4- **17d**

R1=H, X=Cl **14a** R1=H, X=-C≡C-Me-carbazole **15d** R1=Ph-4-COOMe, X=Cl **14c** R1=H, R2=Me, R3=*t*Bu, A=ClO4- **17a** R1=H, X=-C≡C-Ph **15a** R1=H, R2=NH2, R3=*t*Bu, A=ClO4- **17b** R1=Ph, X=-C≡C-*n*Pr **15b** R1=H, R2=CH2Ph, R3=*t*Bu, A=ClO4- **17c**

Scheme 4.


Table 3. Selected bond lengths for C^N^N Pt(II) complexes **14** - **17** 

The chloride ligand can be easily replaced by acetylide ligands to yield various neutral Pt(II) complexes that could be investigated by X-ray crystallography. Thus, such acetylide complexes show high efficient luminescence properties due to the combination of the strong field cyclometalated ligand with the strong field acetylide ligands and the rigidity of the terdentate ligands. Important, their emission properties could be tuned by varying either R1 substituent on the C^N^N ligand or the acetylide ligand and several studies were reported on this topic (Lu et al., 2004). A series of luminescent cyclometalated platinum(II) diaminocarbene complexes (**17a-d**) were prepared by nucleophilic attack of amines at the coordinated isocyanide ligands of [(C^N^N)PtC≡NR]+ (R = *t*Bu or Ar). Weak π...π stacking interactions between the cyclometalated ligands were observed in the crystal lattice (range 3.5−3.6 Å). These complexes, **17a-d,** show structureless emissions, with λmax ranging from 528 to 558 nm in acetonitrile at 298 K, which were assigned to 3MLCT excited states. Also, dinuclear phosphorescent C^N^N Pt(II) cyclometalated were prepared (also investigated by X-ray crystallography) by using diacetylide – carbazole bridging ligand (Seneclauze et al., 2007).

#### Fig. 3. ORTEP view of complex **14b**

#### **3.2.3 C^N^C ligands**

262 Current Trends in X-Ray Crystallography

N <sup>N</sup> Pt

HN N R2 R3 <sup>C</sup> <sup>H</sup>

R1

+

A-

N <sup>N</sup> Pt X

Scheme 4.

R1=H, X=Cl **14a** R1=H, X=-C≡C-Me-carbazole **15d** R1=Tol, X=Cl 14**b** R1=H, X=N≡CXyl, A=PF6- **16** R1=Ph-4-COOMe, X=Cl **14c** R1=H, R2=Me, R3=*t*Bu, A=ClO4- **17a** R1=H, X=-C≡C-Ph **15a** R1=H, R2=NH2, R3=*t*Bu, A=ClO4- **17b** R1=Ph, X=-C≡C-*n*Pr **15b** R1=H, R2=CH2Ph, R3=*t*Bu, A=ClO4- **17c** R1=H, X=-C≡C-Me2-fluorene **15c** R1=H, R2=Me, R3=Xyl, A=ClO4- **17d**

Table 3. Selected bond lengths for C^N^N Pt(II) complexes **14** - **17** 

The chloride ligand can be easily replaced by acetylide ligands to yield various neutral Pt(II) complexes that could be investigated by X-ray crystallography. Thus, such acetylide complexes show high efficient luminescence properties due to the combination of the strong field cyclometalated ligand with the strong field acetylide ligands and the rigidity of the terdentate ligands. Important, their emission properties could be tuned by varying either R1 substituent on the C^N^N ligand or the acetylide ligand and several studies were reported on this topic (Lu et al., 2004). A series of luminescent cyclometalated platinum(II) diaminocarbene complexes (**17a-d**) were prepared by nucleophilic attack of amines at the coordinated isocyanide ligands of [(C^N^N)PtC≡NR]+ (R = *t*Bu or Ar). Weak π...π stacking interactions between the cyclometalated ligands were observed in the crystal lattice (range 3.5−3.6 Å). These complexes, **17a-d,** show structureless emissions, with λmax ranging from

R1

The C^N^C cyclometalated Pt(II) complexes are synthesised by using K2PtCl4 and the corresponding 2,6-diphenyl-pyridine derivative, in two consecutive cyclometalation reactions. In the first step, a dinuclear chloride-bridged C^N cyclometalated Pt(II) complex is obtained, followed by reaction with a monodentate neutral ligand to give mononuclear neutral C^N^C species. Several crystallographic studies were carried out on such systems and these are summarised in Scheme 5 and Table 4.

Scheme 5.

Complexes **19a-c** were prepared by reacting K2PtCl4 with the terdentate 2,6-di-(2'-naphthyl) pyridine ligands in glacial acetic acid, followed by heating in DMSO. Their crystal structures reveal that the molecules are paired in a head-to-tail orientation with Pt···Pt separations longer than 6.3 Å, with extensive close C−H···π (*d* = 2.656−2.891 Å) and π···π (*d* = 3.322−3.399 Å) interactions. An interesting feature of such C^N^C Pt(II) complexes is that the Pt-C bonds are longer compared to the N^C^N or N^N^C Pt(II) complexes and this is due the *trans* influence of C atoms on each other. These complexes are emissive in both solid state, with the maxima in the range 600 – 650 nm (**19a-c**), and in solution at 77 K (**18a**). Additionally, several dinuclear C^N^C cyclometalated Pt(II) complexes, either by using a dicyclometalated ligand (Zucca et al., 2006) or a diphosphine ligand as a bridge (dppm, diphenylphosphinomethane, Kui et al., 2006), were investigated by X-ray crystallography.

X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 265

Another interesting group of cyclometalated Pt(II) complexes bearing benzoquinolinate ligand is represented by those complexes containing isocyanide ligands. They have been found to show interesting photophysical properties, including low-energy emissions in fluid solution, depending on either the counteranion, isocyanide (CNR) ligand or crystal packing. Diez et al. recently reported the X-ray crystal structures of the isocyanide benzoquinolate Pt(II)

> - , PF6

Bu 9, 2-Np 10), where Xyl=2,6-dimethyl-phenyl or xylyl and Np = naphtyl. It was

CHCl3]∞ (**27a)**, and a yellow form which consists of discrete dimers,

counteranion and concentration on their luminescent properties (Diez et al., 2009a). The crystal packing of the two complexes containing the cation [Pt(bzq)(CN-Xyl)2]+ and different counteranions (**25a** and **25b**) was found to show significant differences, although in both cases π...π intermolecular interactions are present. The same authors (Diez et al., 2010a) reported the crystal structures and the photophysical properties of the neutral complexes [Pt(bzq)Cl(CNR)]

found that compound **26** forms only yellow crystals in which two monomers are weakly contacting through π...π (bzq) interactions. The complex **27** with tert-butylisocyanide ligand shows solid-state pseudopolymorphic behaviour. Thus, two X-ray structures are reported: a red form, which exhibits an infinite 1-D chain network, including a molecule of chloroform as

0.5H2O]2 (**27b)**. The crystal packing of these two forms is stabilised by short Pt...Pt distances and π...π (bzq) interplanar bonding interactions. Indeed, the extended 1D-chain of **27a** exhibits equivalent Pt(II)-Pt(II) distances of 3.3547(2) Å and a nearly linear Pt...Pt...Pt angle [169.12(2)°],

When the 2-naphthylisocyanide ligand was employed, again, two isomeric species have been isolated, a neutral yellow derivative, which crystallizes as a Pt...Pt dimer,

Another category of ancillary ligands widely used in the preparation of cyclometalated Pt(II) benzoquinolinate complexes are the phosphine ligands, both in mono and bidentate form. The simplest one is the neutral complex **29** in which the diphenylphosphine (PHPh2) ligand is bound to the platinum center while the coordination sphere is completed with a chloride anion. It is interesting that when the [Pt(bzq)(μ-Cl)]2 was reacted with PHPh2 and excess of

[Pt(bzq)Cl(CN-2-Np)]2 (**28a**), and a double salt [Pt(bzq)(CN-2-Np)2]+[Pt(bzq)Cl2]-

NEt3, then a phosphide-bridge platinum dimer [Pt(bzq)(μ-PPh2)]2 (**30)** was obtained.


(**28b**).

complexes [Pt(bzq)(CNR)2]X (R = Xyl, X = ClO4

thus indicating some degree of Pt...Pt interactions along the chain.

(R= Xyl, 8, t

[**27**.

crystallizing solvent, [**27**.

Fig. 4. ORTEP view of complex **23**


Table 4. Selected bond lengths for C^N^C Pt(II) compounds **18** - **20** 

#### **3.3 Pt(II) complexes with benzo(h)quinoline (bzq) as cyclometalating ligand**

The homoleptic Pt(II) complex with benzo(h)quinoline was prepared by reacting [PtCl2(Et2S)2] with the lithiated ligand, its crystal structure being reported in 1996 (Jolliet et al., 1996). It was found that the crystal structure shows an important distortion from the square planar geometry towards a two-bladed helix with average Pt–C and Pt–N distances being 1.988 and 2.151 Å, respectively.

Scheme 6. Heteroleptic cyclometalated Pt(II) complexes containing the benzo(h)quinoline ligand

Various heteroleptic cyclometalated Pt(II) complexes either ionic or neutral were synthesized starting from the binuclear μ-chloro bridged complex, and analysed through single-crystal X-ray diffraction. They are summarised in Scheme 6 and their crystallographic data are included in Table 5.

An interesting example showing how the Pt...M interactions affect the photoluminescence properties for such complexes bearing the cyclometalated bzq together with acetylide ligands is represented by complex **23**, whose molecular structure is depicted in Figure 4, which has very short PtII...PbII (2.9758(5) and 2.9182(5) Å) and Pt...Pt distances (3.579 Å). For this complex, the emissive state in solid state (77 K) is attributed to a 3MLM′CT [Pt(1)π(C≡CPh)→Pt(2)/Pb(sp)π\*(C≡CPh)] state mixed with some ππ\* excimeric character.

Table 4. Selected bond lengths for C^N^C Pt(II) compounds **18** - **20** 

being 1.988 and 2.151 Å, respectively.

X=Y=-C≡C-Ph, Kat=NBu4+ **21** X=Y=PPh2 **30** X=Y=-C≡C-Py-2, Kat=NBu4+ **22** X=Y=dppm, PF6- **31a** X=Y=-C≡C-Ph, Kat=Pb2+ **23** X=Y=dppe, A=PF6- **31b** X=-C≡C-Ph, Y=PPh2(C≡CPh) **24** X=Y=dppp, A=PF6- **31c** X=Y=-C≡N-Xyl, A=ClO4- **25a** X=dppm, Y=4-Tol, Tol=tolyl **32**

X=Y=-C≡N-Xyl, A=PF6- **25b** X=DMSO, Y=Cl **33a** X=-C≡N-Xyl, Y=Cl **26** X=4-MeO-Py, Y=Cl **33b** X=-C≡N-*t*Bu, Y=Cl **27** X=SMe2, Y=4-Tol **34** X=-C≡N-2-Np, Y=Cl, Np=Naphtyl **28** X=Y=[9]aneS3,

X=PHPh2, Y=Cl **29** 1,4,7-trithiacyclononane, A=PF6- **35**

Scheme 6. Heteroleptic cyclometalated Pt(II) complexes containing the benzo(h)quinoline

Various heteroleptic cyclometalated Pt(II) complexes either ionic or neutral were synthesized starting from the binuclear μ-chloro bridged complex, and analysed through single-crystal X-ray diffraction. They are summarised in Scheme 6 and their crystallographic

An interesting example showing how the Pt...M interactions affect the photoluminescence properties for such complexes bearing the cyclometalated bzq together with acetylide ligands is represented by complex **23**, whose molecular structure is depicted in Figure 4, which has very short PtII...PbII (2.9758(5) and 2.9182(5) Å) and Pt...Pt distances (3.579 Å). For this complex, the emissive state in solid state (77 K) is attributed to a 3MLM′CT [Pt(1)π(C≡CPh)→Pt(2)/Pb(sp)π\*(C≡CPh)] state mixed with some ππ\* excimeric character.

N Pt X Y 

X=Y=2-pyridyl hexafluoropropoxide **36**

data are included in Table 5.

ligand

**3.3 Pt(II) complexes with benzo(h)quinoline (bzq) as cyclometalating ligand** 

The homoleptic Pt(II) complex with benzo(h)quinoline was prepared by reacting [PtCl2(Et2S)2] with the lithiated ligand, its crystal structure being reported in 1996 (Jolliet et al., 1996). It was found that the crystal structure shows an important distortion from the square planar geometry towards a two-bladed helix with average Pt–C and Pt–N distances

> N Pt X Y

\_

Kat+

N Pt X Y +

A-

Another interesting group of cyclometalated Pt(II) complexes bearing benzoquinolinate ligand is represented by those complexes containing isocyanide ligands. They have been found to show interesting photophysical properties, including low-energy emissions in fluid solution, depending on either the counteranion, isocyanide (CNR) ligand or crystal packing. Diez et al. recently reported the X-ray crystal structures of the isocyanide benzoquinolate Pt(II) complexes [Pt(bzq)(CNR)2]X (R = Xyl, X = ClO4 - , PF6 -, **25a,b**) as well as the influence of the counteranion and concentration on their luminescent properties (Diez et al., 2009a). The crystal packing of the two complexes containing the cation [Pt(bzq)(CN-Xyl)2]+ and different counteranions (**25a** and **25b**) was found to show significant differences, although in both cases π...π intermolecular interactions are present. The same authors (Diez et al., 2010a) reported the crystal structures and the photophysical properties of the neutral complexes [Pt(bzq)Cl(CNR)] (R= Xyl, 8, t Bu 9, 2-Np 10), where Xyl=2,6-dimethyl-phenyl or xylyl and Np = naphtyl. It was found that compound **26** forms only yellow crystals in which two monomers are weakly contacting through π...π (bzq) interactions. The complex **27** with tert-butylisocyanide ligand shows solid-state pseudopolymorphic behaviour. Thus, two X-ray structures are reported: a red form, which exhibits an infinite 1-D chain network, including a molecule of chloroform as crystallizing solvent, [**27**. CHCl3]∞ (**27a)**, and a yellow form which consists of discrete dimers, [**27**. 0.5H2O]2 (**27b)**. The crystal packing of these two forms is stabilised by short Pt...Pt distances and π...π (bzq) interplanar bonding interactions. Indeed, the extended 1D-chain of **27a** exhibits equivalent Pt(II)-Pt(II) distances of 3.3547(2) Å and a nearly linear Pt...Pt...Pt angle [169.12(2)°], thus indicating some degree of Pt...Pt interactions along the chain.

Fig. 4. ORTEP view of complex **23**

When the 2-naphthylisocyanide ligand was employed, again, two isomeric species have been isolated, a neutral yellow derivative, which crystallizes as a Pt...Pt dimer, [Pt(bzq)Cl(CN-2-Np)]2 (**28a**), and a double salt [Pt(bzq)(CN-2-Np)2]+[Pt(bzq)Cl2]- (**28b**). Another category of ancillary ligands widely used in the preparation of cyclometalated Pt(II) benzoquinolinate complexes are the phosphine ligands, both in mono and bidentate form. The simplest one is the neutral complex **29** in which the diphenylphosphine (PHPh2) ligand is bound to the platinum center while the coordination sphere is completed with a chloride anion. It is interesting that when the [Pt(bzq)(μ-Cl)]2 was reacted with PHPh2 and excess of NEt3, then a phosphide-bridge platinum dimer [Pt(bzq)(μ-PPh2)]2 (**30)** was obtained.

X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 267

the different *trans* influence of the C and N atoms of the cyclometalated ligands, with Pt-S

Interestingly, complex **36** bearing chelating 2-pyridyl hexafluoropropoxide ancillary ligand shows evidence for the occurrence of π...π stacking between the cyclometalated ligands, but a lack of intermolecular Pt...Pt interaction. The π...π stacking was also confirmed by the observation of additional large Stokes shifted emission attributed to the aggregated

Although various cyclometalated Pt(II) species are intensively studied for their luminescent properties, surprisingly, almost no attention was given so far to the luminescent cycloplatinated imino species. It is almost very recent that these types of complexes started to be investigated for their photophysical properties as well (Scaffidi-Domianello et al., 2007).

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

Scheme 7. Cyclometalated Pt(II) complex with benzophenone imine ligands. R = Me, (**37a**), (Pt-C = 2.006(9)Å, Pt-N = 2.019(6)Å, Pt – S = 2.215(2)Å, Pt – Cl = 2.392(2)Å); R = (CH2)4,

In **37a**, the asymmetric unit contains two independent Pt complexes, while in **37b**, it includes four Pt complexes linked by the intermolecular hydrogen-bonding network between the NH group and Cl atoms (Figure 5). The Pt – X distances for only one molecule are indicated in Scheme 7. There is no significant Pt...Pt interactions in the solid state. The two Pt(II) complexes show an emission band with the maximum located at 535 nm, along with another less intense emission at 565 nm. The solution quantum yields of the complexes

Fig. 5. The hydrogen bonding network connecting four independent molecules in the unit

cell of **37b** (this figure was generated by using the Mercury 2.2 software)

(**37b**), (Pt-C = 2.016(4)Å, Pt-N = 2.019(4)Å, Pt – S = 2.2063(13)Å, Pt – Cl =

2.3925(11)Å)(Scaffidi-Domianello et al., 2007)

are rather low, with values smaller than 0.0071.

trans to C atom longer than the Pt-S trans to N atom.

**3.4 Cyclometalated Pt(II) complexes with imine ligands** 

counterparts in solid thin film.


Table 5. Crystallographic data of the cyclometalated Pt(II) complexes containing benzoquinoline ligand

A series of cyclometalated Pt(II) complexes with several bidentate bis-diphenylphosphine ligands where the bridge length between the two phosphine units was varied (bisdiphenylphosphinomethane (dppm), bis-diphenylphosphinoethane (dppe) and bisdiphenylphosphinopropane (dppp), respectively, **31a-c**) were prepared and investigated structurally by DePriest et al., 1997. These complexes are a beautiful examples where the C and N atoms can be distinguished based on their different *trans* influence: the anionic C is a better σ-donor and thus, has a greater *trans* influence compared to N atom. This difference can be easily seen in the Pt-P bond lengths, with the Pt-P *trans* to C atom bond being longer than Pt-P trans to N atom bond. All these complexes show emission properties in solution (ethanol/methanol = 4/1, v/v) at low temperatures (77 – 180 K), assigned to ligand-centred (LC) transitions.

Several other cycloplatinated clusters containing bridged phosphine or phosphido ligands have been prepared. If the neutral binuclear phosphido complex [Pt(bzq)(μ-PPh2)]2 (**30**) reacts with cis-[Pt(C6F5)2(thf)2] in CH2Cl2 at 1:2 molar ratio, then a bent neutral triplatinum cluster [Pt3(bzq)(μ-PPh2)2(C6F5)3] can be isolated. Its crystal structure was reported by Diez et al, 2006. The crystal structure of complex **35** reveals the two different Pt-S bonds due to the different *trans* influence of the C and N atoms of the cyclometalated ligands, with Pt-S trans to C atom longer than the Pt-S trans to N atom.

Interestingly, complex **36** bearing chelating 2-pyridyl hexafluoropropoxide ancillary ligand shows evidence for the occurrence of π...π stacking between the cyclometalated ligands, but a lack of intermolecular Pt...Pt interaction. The π...π stacking was also confirmed by the observation of additional large Stokes shifted emission attributed to the aggregated counterparts in solid thin film.

#### **3.4 Cyclometalated Pt(II) complexes with imine ligands**

266 Current Trends in X-Ray Crystallography

Table 5. Crystallographic data of the cyclometalated Pt(II) complexes containing

A series of cyclometalated Pt(II) complexes with several bidentate bis-diphenylphosphine ligands where the bridge length between the two phosphine units was varied (bisdiphenylphosphinomethane (dppm), bis-diphenylphosphinoethane (dppe) and bisdiphenylphosphinopropane (dppp), respectively, **31a-c**) were prepared and investigated structurally by DePriest et al., 1997. These complexes are a beautiful examples where the C and N atoms can be distinguished based on their different *trans* influence: the anionic C is a better σ-donor and thus, has a greater *trans* influence compared to N atom. This difference can be easily seen in the Pt-P bond lengths, with the Pt-P *trans* to C atom bond being longer than Pt-P trans to N atom bond. All these complexes show emission properties in solution (ethanol/methanol = 4/1, v/v) at low temperatures (77 – 180 K), assigned to ligand-centred

Several other cycloplatinated clusters containing bridged phosphine or phosphido ligands have been prepared. If the neutral binuclear phosphido complex [Pt(bzq)(μ-PPh2)]2 (**30**) reacts with cis-[Pt(C6F5)2(thf)2] in CH2Cl2 at 1:2 molar ratio, then a bent neutral triplatinum cluster [Pt3(bzq)(μ-PPh2)2(C6F5)3] can be isolated. Its crystal structure was reported by Diez et al, 2006. The crystal structure of complex **35** reveals the two different Pt-S bonds due to

benzoquinoline ligand

(LC) transitions.

Although various cyclometalated Pt(II) species are intensively studied for their luminescent properties, surprisingly, almost no attention was given so far to the luminescent cycloplatinated imino species. It is almost very recent that these types of complexes started to be investigated for their photophysical properties as well (Scaffidi-Domianello et al., 2007).

Scheme 7. Cyclometalated Pt(II) complex with benzophenone imine ligands. R = Me, (**37a**), (Pt-C = 2.006(9)Å, Pt-N = 2.019(6)Å, Pt – S = 2.215(2)Å, Pt – Cl = 2.392(2)Å); R = (CH2)4, (**37b**), (Pt-C = 2.016(4)Å, Pt-N = 2.019(4)Å, Pt – S = 2.2063(13)Å, Pt – Cl = 2.3925(11)Å)(Scaffidi-Domianello et al., 2007)

In **37a**, the asymmetric unit contains two independent Pt complexes, while in **37b**, it includes four Pt complexes linked by the intermolecular hydrogen-bonding network between the NH group and Cl atoms (Figure 5). The Pt – X distances for only one molecule are indicated in Scheme 7. There is no significant Pt...Pt interactions in the solid state. The two Pt(II) complexes show an emission band with the maximum located at 535 nm, along with another less intense emission at 565 nm. The solution quantum yields of the complexes are rather low, with values smaller than 0.0071.

Fig. 5. The hydrogen bonding network connecting four independent molecules in the unit cell of **37b** (this figure was generated by using the Mercury 2.2 software)

X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 269

The relevant Pt(II) systems bearing terdentate imine ligands are shown in Scheme 9. The cyclometalated Pt(II) complexes with thiosemicarbazone ligands were included here. The crystal structures of such complexes with terdentate imines reveal a strong *trans* influence of the carbon donor ligands. For instance, in complex **47**, the Pt–NMe2 bond length of 2.162(14) Å is longer than the imine Pt–N bond length of 2.04(2) Å as a consequence of *trans* aryl group which has a high *trans* influence. On the other hand, for such complexes, the longer Pt–N bond is also consistent with the weaker ligating ability of tertiary amines for platinum. The Pt–C bond lengths are found to be in the expected range. It is interesting to note that the complex **52** represents the first luminescent platinum(II) compound with a [C^N^S] terdentate ligand. The long intermolecular S...S distance precludes the existence of any direct interaction between these atoms. Also, the Pt...Pt distance between neighbouring molecules is 4.216(3) Å which is larger, for instance, than the values reported for related platinacycles having [C(sp2)^N^N]- and [C(sp2)^N^S]- terdentate ligands from the "pincer" family. The emission spectrum of this compound recorded at 298K in CH2Cl2 solution shows a maximum at 578 nm when excited at 388 nm with no additional information regarding the quantum yield and lifetime provided. For the rest of complexes (**43** – **51**), no data regarding the luminescence properties were reported. Interestingly, the thiosemicarbazone ligands can be used in cycloplatination reaction to give orthometalated complexes in which the ligand acts as terdentate through the C, N and S atoms and two such examples are presented in Scheme 9 (**53a** and **53b**). **53a** was obtained by bridge-cleavage with PPh3 of the first Pt(II) cyclometalated tetranuclear complexes bearing thiosemicarbazone ligands which were

> N Pt L

R'

X=Y =W=Z=H, L=Ph **43** L=Cl, R=Et, R'=H, n = 1 **50** (Riera et al., 2000)

N Pt L

Me

X=NO2, R=Me, L=thiosemicarbazone **53b**

X

N S

NHR

X=Y=W=Z=H, L=Br **44** L=Cl, R=Me, R'=COOMe, n =2 **51**

(CH2)n SR

**3.4.2 Cyclometalated Pt(II) complexes with terdentate imine ligands** 

prepared using K2PtCl4 as starting material.

N Pt L X Y W z

X=Z=W=F, Y=H, L=Cl **45** X=Y=Z=F, W=H, L=Me **46** X=W=H, Y=Me, Z=OMe, L=Me **47** X=Ph, Y=W=Z=H, L=Me **48a** X=Ph, Y=W=Z=H, L=Cl **48b** X=Y=W=H, Z=Cl, L=4-Tol **49** 

NMe2 

> N Pt Cl S Me

Scheme 9. Pt(II) complexes with terdentate imine ligands

 **52** X=Me, R=H, L=PPh3 **53a**

The solid-state emission spectra for these complexes show emission maxima at 571 and 559 nm, respectively. These two complexes also exhibit some structure of the emission bands with high and low energy shoulders at 542 and 624 nm and at 525 and 610 nm, respectively.The excited-state emission lifetimes at 575 nm are in the range of 320-615 ns, consistent with the phosphorescence emissive mechanism.

#### **3.4.1 Cyclometalated Pt(II) complexes with N-Benzylidenebenzylamine ligands**

This class of cycloplatinated complexes was intensively investigated, but almost for mechanistic studies and not for emission properties. The most representative structural types are presented in Scheme 8.

Scheme 8. Pt(II) compexes with N-benzylidenebenzylamine ligands

These complexes were prepared by reacting [Pt2Me4(μ-SMe2)2] with the imine ligand in the presence of sodium acetate followed by the exchange reaction with PPh3 ligand. They were studied intesively from the cycloplatination reaction mechanism point of view and none of them were investigated for their potential luminescent properties.

The stacked structures are well-known for platinum complexes, and they become increasingly favoured with increasing aromatic core. Despite the presence of biphenyl unit in complexes **39a,b**, no significant π...π interactions have been found for these compounds.


Table 6. Selected bond lengths for cyclometalated Pt(II) complexes containing Nbenzylidenebenzylamine

#### **3.4.2 Cyclometalated Pt(II) complexes with terdentate imine ligands**

The relevant Pt(II) systems bearing terdentate imine ligands are shown in Scheme 9. The cyclometalated Pt(II) complexes with thiosemicarbazone ligands were included here. The crystal structures of such complexes with terdentate imines reveal a strong *trans* influence of the carbon donor ligands. For instance, in complex **47**, the Pt–NMe2 bond length of 2.162(14) Å is longer than the imine Pt–N bond length of 2.04(2) Å as a consequence of *trans* aryl group which has a high *trans* influence. On the other hand, for such complexes, the longer Pt–N bond is also consistent with the weaker ligating ability of tertiary amines for platinum. The Pt–C bond lengths are found to be in the expected range. It is interesting to note that the complex **52** represents the first luminescent platinum(II) compound with a [C^N^S] terdentate ligand. The long intermolecular S...S distance precludes the existence of any direct interaction between these atoms. Also, the Pt...Pt distance between neighbouring molecules is 4.216(3) Å which is larger, for instance, than the values reported for related platinacycles having [C(sp2)^N^N]- and [C(sp2)^N^S]- terdentate ligands from the "pincer" family. The emission spectrum of this compound recorded at 298K in CH2Cl2 solution shows a maximum at 578 nm when excited at 388 nm with no additional information regarding the quantum yield and lifetime provided. For the rest of complexes (**43** – **51**), no data regarding the luminescence properties were reported. Interestingly, the thiosemicarbazone ligands can be used in cycloplatination reaction to give orthometalated complexes in which the ligand acts as terdentate through the C, N and S atoms and two such examples are presented in Scheme 9 (**53a** and **53b**). **53a** was obtained by bridge-cleavage with PPh3 of the first Pt(II) cyclometalated tetranuclear complexes bearing thiosemicarbazone ligands which were prepared using K2PtCl4 as starting material.

268 Current Trends in X-Ray Crystallography

The solid-state emission spectra for these complexes show emission maxima at 571 and 559 nm, respectively. These two complexes also exhibit some structure of the emission bands with high and low energy shoulders at 542 and 624 nm and at 525 and 610 nm, respectively.The excited-state emission lifetimes at 575 nm are in the range of 320-615 ns,

This class of cycloplatinated complexes was intensively investigated, but almost for mechanistic studies and not for emission properties. The most representative structural

These complexes were prepared by reacting [Pt2Me4(μ-SMe2)2] with the imine ligand in the presence of sodium acetate followed by the exchange reaction with PPh3 ligand. They were studied intesively from the cycloplatination reaction mechanism point of view and none of

The stacked structures are well-known for platinum complexes, and they become increasingly favoured with increasing aromatic core. Despite the presence of biphenyl unit in complexes **39a,b**, no significant π...π interactions have been found for these compounds.

Table 6. Selected bond lengths for cyclometalated Pt(II) complexes containing N-

N Pt Me PPh3

R

Y

X

**3.4.1 Cyclometalated Pt(II) complexes with N-Benzylidenebenzylamine ligands** 

X=R=H, Y=W=Z=F, L=Me **38a** X=Y=R=H, Z=Me, L=Me **40a** X=Y=R=H, W=Z=F, L=Me **38b** X=Y=R=H, Z=CF3, L=Me **40b** X=Y=Z=F, W=R=H, L=Me **38c** X=Y=Z=H, R=Cl, W=-C6F5, L=Br **41**

X=W=Z=R=H, Y=Ph, L=Me **39a** R=Me, X=F, Y=H **42a** X=Y=Z=R=H, W=Ph, L=Me **39b** R=-COOMe, X=H, Y=Cl **42b** 

Scheme 8. Pt(II) compexes with N-benzylidenebenzylamine ligands

them were investigated for their potential luminescent properties.

consistent with the phosphorescence emissive mechanism.

types are presented in Scheme 8.

W

benzylidenebenzylamine

Y z X

> N Pt L PPh3

> > <sup>R</sup>

 X=Y=W=Z=H, L=Br **44** L=Cl, R=Me, R'=COOMe, n =2 **51** X=Z=W=F, Y=H, L=Cl **45** X=Y=Z=F, W=H, L=Me **46** X=W=H, Y=Me, Z=OMe, L=Me **47** X=Ph, Y=W=Z=H, L=Me **48a** X=Ph, Y=W=Z=H, L=Cl **48b** X=Y=W=H, Z=Cl, L=4-Tol **49** 

N Pt L (CH2)n SR R'

X=Y =W=Z=H, L=Ph **43** L=Cl, R=Et, R'=H, n = 1 **50** (Riera et al., 2000)

Scheme 9. Pt(II) complexes with terdentate imine ligands

X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 271

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

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

<sup>N</sup> <sup>N</sup> <sup>R</sup> <sup>R</sup> Pt X

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

<sup>N</sup> R' Pt Cl

N Pt S

(CH2)n NMe2 Me <sup>O</sup> Me

H

Fe

R

Ph3P Fe

**3.4.5 Cyclometalated Pt(II) complexes containing ferrocene based imine ligands**  Several crystal structures of cyclometalated Pt(II) complexes containing imine ligands with

Pt-N1 Pt-N2

R=Me, R'=Ph **58** R=H, R'=-CH2CH2OH **61b** n = 0 **62a** R=H, R'=-CHMeCH2OH **59a** n = 1 **62b**

Scheme 12. Cycloplatinated complexes bearing ferrocene imine ligands

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

containing the (N^C^N) isophtalaldimine ligands.

Pr)2Ph, X=-C≡CSiMe3 **55**

data reported for such complexes.

ferrocene fragment were reported (see Scheme 12).

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

Me Me

R

<sup>O</sup> Fe

R=H, R'=-CH2CH2OH **61a** 

R=H, R'=-CH(CHMe2)CH2OH **59b**

Pr)2Ph, X=CH3 **56** R=-CH(CH3)Ph, X=Cl **57a**

Bu, X=Cl **57b** R=Ph, X=Cl **57c**  Scheme 11.

pincer ligands

R=Me, R'=OH **60**

R=2,6-(i

R=2,6-(i

R=t


Table 7. Selected bond legths for cyclometalated Pt(II) complexes with terdentate imine 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-Torres & Mendiola, 2010. No emission properties were reported for such complexes.

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

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

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**) (Ryabov et al., 2002)

Scheme 10.
