Dinuclear, tricyclometallated platinum(II) derivatives - Substitution reactions and reactivity of the platinum-carbon bonds

In the recently reported dinuclear cyclometallated species [Pt-2(R)(L)(DMSO)(2)] [H3L = 6-phenyl-2,2'-bipyridine; R = Me (1a), Ph (1b)] the threefold deprotonated ligand L links the two platinum atoms acting as an N',C(3) and a C,N,C donor. Aspects of their reactivity have been studied first with regards to the substitution of the two DMSO ligands, which are coordinated to the platinum atom trans to a nitrogen and a carbon atom, respectively. Substitution of DMSO with neutral two-electron donors affords several new species, all having the same bridging ligand L: [Pt-2(R)(L)(L')(2)] [L' = PPh3 (2a, 2b), CO (3a, 3b)] and [Pt-2(R)(L)(L')(DMSO)] [L' = 3,5-Me-2-py (4a)]. The potentially bidentate diphosphane Ph2P(CH2)(3)PPh2 (dppp) gives a more complex species having. both chelating and bridging dppp ligands [{Pt(Me)(L)(dppp)}(2)(mu-dppp)] (5a). The reactivity of the platinum-carbon bonds with HCl has been studied in complexes 1a, 1b and 2a, 2b and found to be strongly dependent on the nature of the ancillary ligands. To discern the most labile of the four metal-carbon bonds, the reaction has been carried out with 1 equiv. of HCl under mild conditions. Although the reaction is not selective and minor species are formed, cleavage of the Pt-Me (1a) and Pt-Ph (1b) bonds is favoured - [Pt-2(L)Cl(DMSO)(2)] (6) can be isolated and spectroscopically characterised from the reaction with either la or 1b. Substitution of PPh3 for DMSO in 6 gives 1Pt(2)(L)Cl(PPh3)(2)] (7) as the thermodynamically stable isomer having a trans Cl-Pt-C arrangement. The structure of complex 7 has been solved by X-ray crystallography. In contrast, reaction of 2a or 2b with HCl proceeds by another route: even with excess HCl the Pt-Me and Pt-Ph bonds are unaffected whereas one of the Pt-C bonds of the pincer-like C,N,C system is cleaved to selectively give [Pt-2(R)(HL)Cl(PPh3)(2)] [R = Me (8a), Ph (8b)]. The same bond is cleaved also in 1a and 1b by excess HCl to give [Pt-2(HL)(Cl)(2)(DMSO)(2)] (9).