**1. Introduction**

254 Current Trends in X-Ray Crystallography

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Interaction in a Hydrophobic Cavity of a Ruthenium

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 can be found here (Albrecht, 2010).

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

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

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

X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 257

Fig. 1. Schematic representation of the radiative and non-radiative pathways

2+

quantum yield nearly 0.02 at 298K (Kvam, Jongstad, 1995; Kvam et al., 1995).

[Pt(bpy)2]2+ [Pt(bpy)(ppy)]+

The luminescence of Pt(II) complexes is assigned to either ligand-centred (LC) or metal-toligand charge transfer (MLCT) states or a mixed of both. The most useful strategy to promote luminescence in platinum(II) complexes is to employ ligands with a very strong ligand field in order to raise the dd states, so that they are not easily accessible for radiationless deactivation process, and this can be achieved by using cyclometalating ligands, mostly with 2-arylpyridine or 2-thienylpyridine derivatives, resulting both homoleptic (metal complexes with identical ligands) or heteroleptic (metal complexes with different ligands) complexes. The influence of cyclometalated ligands on the photophysical properties of square-planar Pt(II) complexes is clearly seen when the Pt(II) complex with 2,2'-bipyridine (bpy) is compared to the cyclometalated Pt(II) complex with 2 phenylpyridine (ppy) (Scheme 1). While the [Pt(bpy)2]2+ is almost non-emissive at room temperature, the Pt(II) cyclometalated complex [Pt(bpy)(ppy)]+ shows emission with a

N N N Pt

+

N N N N Pt

orbital.

Scheme 1.

Thus, in the case of transition metal complexes, there are four types of electronic states or transitions responsible for their luminescent properties, as follows: *1)* dd states (metalcentred, MC, transition). By coordination of the ligands to the metal centre, the d orbitals are split according to the symmetry of the complex. These excited dd states arise from promotion of an electron within d orbitals; *2)* dπ\* states (metal-to-ligand-charge-transfer, MLCT). These involve excitation of a metal-centred electron to a π\* anti-bonding orbital located on the ligand system; *3)* ππ\* or nπ\* states (intraligand, IL, transition). Promotion of an electron from a π-bonding or non-bonding orbital to a higher energy anti-bonding orbital gives rise to these states; *4)* πd states (ligand-to-metal-charge-transfer, LMCT). These states arise from the transfer of electronic charge from the ligand π system to a metal-centred

mononuclear cyclometalates of the type [MX(C^N)L1] or [M(C^N)L2] (M = Pt, Pd; C^N = orthometalated ligand; L1 = neutral monodentate ligand such as pyridine and phosphines; L2 = bidentate uninegative ligand such as acetylacetonate derivatives; X = halide).

The Pt(II) ions adopt a square-planar geometry, being part of the major group of exceptions to the otherwise very successful model of Kepert. They show coordinative unsaturation which can allow different interactions such as: excimer formation, chemical quenching, interactions with Lewis bases, and oxidative addition. The single-crystal X-ray diffraction is a very powerful technique for characterization of cyclometalated platinum(II) complexes and strong correlations between the structure and luminescence properties can be made as, for example, the solid-state emission is greatly influenced by the crystal packing and the presence of Pt...Pt or π...π interactions is favoured by the coordinative unsaturation. This review is intended to cover the most important structural types of cyclometalated Pt(II) complexes that have been investigated by X-ray diffraction with an emphasis on their luminescence properties.

#### **2. Photophysical properties of cyclometalated platinum(II) complexes**

Cyclometalated Pt(II) complexes have been extensively investigated in the past years because of their interesting photophysical properties and several reviews are dealing with this topic (Evans et al., 2006; Williams et al., 2008; Balashev et al., 1997; Ma et al., 2005; Williams, 2007; McGuire Jr. et al., 2010). Although there is a particular fundamental interest in studying their intrinsic emissive states, their luminescence properties find applications in optoelectronic devices (such as OLED, Williams et al., 2008), luminescent probes for biomolecules (cell imaging, biochemistry, Siu et al., 2005) and chemical sensors (for a recent example see Li et al., 2011). Figure 1 shows a simplified schematic representation (Perrin-Jablonski diagram) of the most important processes that take place through the interaction of matter with light: photon absorption, internal conversion, fluorescence, intersystem crossing and phosphorescence (Valeur, 2001). The emission of photons that accompanies the S1→ S0 relaxation is called *fluorescence*. Another process visible on this diagram is the *internal conversion* that is a non-radiative transition between two electronic states of the same spin multiplicity. Also, still a non-radiative process is the *intersystem crossing* that represents a transition between two isoenergetic vibrational levels belonging to electronic states of different multiplicities. Crossing between states of different multiplicity is in principle forbidden, but spin–orbit coupling (i.e. coupling between the orbital magnetic moment and the spin magnetic moment) can be large enough to make it possible. The probability of intersystem crossing depends on the singlet and triplet states involved. If the transition S0→S1 is of n→π\* type, for instance, intersystem crossing is often efficient. It should also be noted that the presence of heavy atom (i.e. whose atomic number is large, as it is the case of Pt) increases spin–orbit coupling and thus favors intersystem crossing. It was found that in solution, at room temperature, non-radiative de-excitation from the triplet state, T1, is preferred rather than the radiative de-excitation called *phosphorescence*. This happens because the transition T1→S0 is forbidden (but it can be observed because of spin–orbit coupling), and the radiative rate constant is thus very low. On the contrary, at low temperatures and/or in a rigid medium, phosphorescence can be observed. The lifetime of the triplet state may, under these conditions, be long enough to observe phosphorescence on a time-scale up to seconds, even minutes or more. Fluorescence and phosphorescence are particular cases of *luminescence* (emission of light from an electronically excited species).

Fig. 1. Schematic representation of the radiative and non-radiative pathways

Thus, in the case of transition metal complexes, there are four types of electronic states or transitions responsible for their luminescent properties, as follows: *1)* dd states (metalcentred, MC, transition). By coordination of the ligands to the metal centre, the d orbitals are split according to the symmetry of the complex. These excited dd states arise from promotion of an electron within d orbitals; *2)* dπ\* states (metal-to-ligand-charge-transfer, MLCT). These involve excitation of a metal-centred electron to a π\* anti-bonding orbital located on the ligand system; *3)* ππ\* or nπ\* states (intraligand, IL, transition). Promotion of an electron from a π-bonding or non-bonding orbital to a higher energy anti-bonding orbital gives rise to these states; *4)* πd states (ligand-to-metal-charge-transfer, LMCT). These states arise from the transfer of electronic charge from the ligand π system to a metal-centred orbital.

Scheme 1.

256 Current Trends in X-Ray Crystallography

mononuclear cyclometalates of the type [MX(C^N)L1] or [M(C^N)L2] (M = Pt, Pd; C^N = orthometalated ligand; L1 = neutral monodentate ligand such as pyridine and phosphines;

The Pt(II) ions adopt a square-planar geometry, being part of the major group of exceptions to the otherwise very successful model of Kepert. They show coordinative unsaturation which can allow different interactions such as: excimer formation, chemical quenching, interactions with Lewis bases, and oxidative addition. The single-crystal X-ray diffraction is a very powerful technique for characterization of cyclometalated platinum(II) complexes and strong correlations between the structure and luminescence properties can be made as, for example, the solid-state emission is greatly influenced by the crystal packing and the presence of Pt...Pt or π...π interactions is favoured by the coordinative unsaturation. This review is intended to cover the most important structural types of cyclometalated Pt(II) complexes that have been investigated by X-ray diffraction with an emphasis on their

L2 = bidentate uninegative ligand such as acetylacetonate derivatives; X = halide).

**2. Photophysical properties of cyclometalated platinum(II) complexes** 

Cyclometalated Pt(II) complexes have been extensively investigated in the past years because of their interesting photophysical properties and several reviews are dealing with this topic (Evans et al., 2006; Williams et al., 2008; Balashev et al., 1997; Ma et al., 2005; Williams, 2007; McGuire Jr. et al., 2010). Although there is a particular fundamental interest in studying their intrinsic emissive states, their luminescence properties find applications in optoelectronic devices (such as OLED, Williams et al., 2008), luminescent probes for biomolecules (cell imaging, biochemistry, Siu et al., 2005) and chemical sensors (for a recent example see Li et al., 2011). Figure 1 shows a simplified schematic representation (Perrin-Jablonski diagram) of the most important processes that take place through the interaction of matter with light: photon absorption, internal conversion, fluorescence, intersystem crossing and phosphorescence (Valeur, 2001). The emission of photons that accompanies the S1→ S0 relaxation is called *fluorescence*. Another process visible on this diagram is the *internal conversion* that is a non-radiative transition between two electronic states of the same spin multiplicity. Also, still a non-radiative process is the *intersystem crossing* that represents a transition between two isoenergetic vibrational levels belonging to electronic states of different multiplicities. Crossing between states of different multiplicity is in principle forbidden, but spin–orbit coupling (i.e. coupling between the orbital magnetic moment and the spin magnetic moment) can be large enough to make it possible. The probability of intersystem crossing depends on the singlet and triplet states involved. If the transition S0→S1 is of n→π\* type, for instance, intersystem crossing is often efficient. It should also be noted that the presence of heavy atom (i.e. whose atomic number is large, as it is the case of Pt) increases spin–orbit coupling and thus favors intersystem crossing. It was found that in solution, at room temperature, non-radiative de-excitation from the triplet state, T1, is preferred rather than the radiative de-excitation called *phosphorescence*. This happens because the transition T1→S0 is forbidden (but it can be observed because of spin–orbit coupling), and the radiative rate constant is thus very low. On the contrary, at low temperatures and/or in a rigid medium, phosphorescence can be observed. The lifetime of the triplet state may, under these conditions, be long enough to observe phosphorescence on a time-scale up to seconds, even minutes or more. Fluorescence and phosphorescence are particular cases of *luminescence* (emission of light from an electronically excited species).

luminescence properties.

The luminescence of Pt(II) complexes is assigned to either ligand-centred (LC) or metal-toligand charge transfer (MLCT) states or a mixed of both. The most useful strategy to promote luminescence in platinum(II) complexes is to employ ligands with a very strong ligand field in order to raise the dd states, so that they are not easily accessible for radiationless deactivation process, and this can be achieved by using cyclometalating ligands, mostly with 2-arylpyridine or 2-thienylpyridine derivatives, resulting both homoleptic (metal complexes with identical ligands) or heteroleptic (metal complexes with different ligands) complexes. The influence of cyclometalated ligands on the photophysical properties of square-planar Pt(II) complexes is clearly seen when the Pt(II) complex with 2,2'-bipyridine (bpy) is compared to the cyclometalated Pt(II) complex with 2 phenylpyridine (ppy) (Scheme 1). While the [Pt(bpy)2]2+ is almost non-emissive at room temperature, the Pt(II) cyclometalated complex [Pt(bpy)(ppy)]+ shows emission with a quantum yield nearly 0.02 at 298K (Kvam, Jongstad, 1995; Kvam et al., 1995).

X-Ray Structural Characterization of Cyclometalated Luminescent Pt(II) Complexes 259

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

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

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**)

Fig. 2. ORTEP view of complex **5a** (a); the emission spectrum of **5a** recorded in CH2Cl2

case, the shortest Pt-Pt distance between two neighbouring molecules is 4.18 Å.

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

bond due to the strong trans influence of C-ppy donor atom (Table 1).

**(a) (b)** 

such examples were reported so far (**3** – **8**).

solution at room temperature (b)

(Figure 2).
