Reversible C-C bond formation at a triply cyclometallated platinum(iv) centre.

The oxidation of the tribenzylphosphine derivative of the doubly cylcometallated platinum(ii) complex of diphenylpyridine, , with PhICl led, as a first step, to the formation of a highly electrophilic metal centre which attacked the benzyl phosphine to give a triply cyclometallated species as the arenium ion. The highly acidic arenium ion protonated unreacted starting , a reaction that could be supressed by the addition of water, and gave the neutral species . Octahedral complex was induced to reductively couple, with two five-membered rings coupling to give square planar complex containing a nine-membered ring. The crystal structure of showed the nine-membered ring to span across the square planar metal accompanied by considerable distortion: the P-Pt-N bond angle is 155.48(5)°. Oxidation of with PhICl resulted in the addition of two chlorides and a change of the nine-membered ring ligand coordination to at an octahedral centre, still with considerable distortions: the P-Pt-N bond angle in the crystal structure of is 99.46(5)°. Treatment of with AgBF also induced a coupling to give a nine-membered ring, and the fluxional three coordinate complex . A mono-methylated version of , , was prepared and similar reactions were observed. The presence of the methyl group allowed us to observe selectivity in the coupling reaction to give the nine-membered ring, with two products ( and ) being initially formed in the ratio 7 : 1. The concentrations of two products changed with time giving a final ratio of 1 : 8 at room temperature (half-life 48 hours), the equilibration being made possible by a reversible C-C bond forming reaction. Reaction of complexes with CO or hydrogen left the nine-membered ring intact, though oxidative degradation resulted in decomplexation of the phosphine donor, accompanied by formation of a P[double bond, length as m-dash]O group.