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

Abstract

The oxidation of the tribenzylphosphine derivative of the doubly cylcometallated platinum(II) complex of diphenylpyridine, 1, with PhICl₂ 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 1, a reaction that could be supressed by the addition of water, and gave the neutral species 2(t). Octahedral complex 2(t) was induced to reductively couple, with two five-membered rings coupling to give square planar complex 5 containing a nine-membered ring. The crystal structure of 5 showed the nine-membered ring to span trans across the square planar metal accompanied by considerable distortion: the P–Pt–N bond angle is 155.48(5)°. Oxidation of 5 with PhICl₂ resulted in the addition of two chlorides and a change of the nine-membered ring ligand coordination to cis at an octahedral centre, still with considerable distortions: the P–Pt–N bond angle in the crystal structure of 6 is 99.46(5)°. Treatment of 2(t) with AgBF₄ also induced a coupling to give a nine-membered ring, and the fluxional three coordinate complex 7. A mono-methylated version of 1, Me-1, 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 (a-Me-7 and b-Me7) 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 7 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 PO group.