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.