Cyclometalation of 2,2'-Bipyridine, Mono- and Dinuclear C,N Platinum(II) Derivatives

Reaction of the electron-rich derivative cis-[Pt(Me)2(DMSO)2] with 2,2′-bipyridine (L) affords the mononuclear “rollover” N′,C(3) cyclometalate [Pt(L-H)(Me)(DMSO)] (1a) or the unusual dinuclear species [Pt2(μ-L-2H)(Me)2(DMSO)2] (2a), depending on the Pt/L molar ratio. The latter complex, 2a, can be obtained either directly from cis-[Pt(Me)2(DMSO)2] or by a two-step process through complex 1a. The synthesis of 2a implies a double C-H bond activation, with loss of two molecules of methane: the 2-fold deprotonated 2,2′-bipyridine links two [Pt(Me)(DMSO)] fragments, acting as a π-delocalized C,N∧N,C eight-electron donor. From 1a and 2a displacement of DMSO with two-electron-donor ligands such as PPh3, (b), CO (c), and 3,5-Me2py (d) gives two series of complexes, 1b-d and 2b-d, respectively. Reaction of 1a and 2a with HCl affords the corresponding chlorides [Pt(L-H)(Cl)(DMSO)] (3a) and [Pt2(μ-L-2H)(Cl)2(DMSO)2] (4a), from which complexes 3b-d and 4b,d can be obtained by substitution of DMSO. The crystal structures of the mono- and dinuclear complexes [Pt(L-H)(Cl)(PPh3)] (3b) and [Pt2(μ-L-2H)(Cl)2(PPh3)2] (4b) have been solved by X-ray diffraction. Protonation of the uncoordinated nitrogen atom in 1a can be achieved by reaction with 18-crown-6 · HBF4: [Pt(L*)(Me)(DMSO)][BF4] (5), where the zwitterionic ligand L* is still N′-C(3) bonded to platinum(II), can be isolated in the solid state. Taking advantage of the two-step approach used for the synthesis of complex 2a, unsymmetric species where the platinum atoms are in different environments, e.g. [(DMSO)(Cl)Pt(μ-L-2H)Pt- (Me)(DMSO)] (6), [(PPh3)(Me)Pt(μ-L-2H)Pt(Me)(DMSO)] (7), and [(CO)(Cl)Pt(μ-L-2H)Pt(Me)(DMSO)] (8), can be prepared in good yields. Attempts to build up a platinum(II)-gold(III) species by reaction of 3a with Na[AuCl4] failed: no platinum-gold derivative was obtained but a platinum(IV) complex, [Pt(LH)( Cl)3(DMSO)] (9), likely due to the strong oxidative capability of gold(III).