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

Abstract

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 Cdonor 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-, sulfurand 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