Reversible C–C bond formation at a triply cyclometallated platinum(iv) centre† †Electronic supplementary information (ESI) available: Full experimental details of synthetic procedures, variable temperature NMR spectra for 7 and a comparison of those spectra with those of 5 are available. The data from which the analysis of the equilibrium between a-Me7 and b-Me7 was made is included. Full details and discussions of the X-ray structures are available. CCDC 1540378–1540384. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc01361b Click here for additional data file. Click here for additional data file.

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 ninemembered 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 monomethylated 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 P]O group.