Unmasking the Catalytic Activity of a Platinum Complex with a Lewis Acidic, Non-innocent Antimony Ligand.

With the view of developing self-activating electrophilic catalysts, we are now investigating complexes with a Lewis acidic moiety in the immediate vicinity of the transition metal center. Toward this end, we have synthesized a platinum complex in which the metal is connected to a Lewis acidic bis(triflato)stiboranyl ligand. This complex, ((o-(Ph2P)C6H4)2SbOTf2)PtCl (2), which was obtained by treatment of ((o-(Ph2P)C6H4)2SbCl2)PtCl (1) with 2 equiv of AgOTf, is surprisingly air stable. Yet, it promptly reacts with cyclohexylisocyanide to afford the dicationic chlorostibine complex [((o-(Ph2P)C6H4)2SbCl)PtCNCy]2+ ([3]2+) as a bis-triflate salt. Formation of [3]2+ occurs through abstraction of the platinum-bound chloride ligand by the adjacent Lewis acidic antimony center. This halide migration reaction leads to activation of the platinum center. In turn, 2 behaves as a self-activating catalyst in reactions involving alkynes and readily mediates both enyne cyclization and intramolecular hydroarylation reactions, at room temperature, without addition of a chloride abstracting reagent. These results demonstrate that the coordination non-innocence of antimony ligands can be exploited for the purpose of electrophilic catalysis.