Reductive Elimination at an Ortho-Metalated Iridium(III) Hydride Bearing a Tripodal Tetraphosphorus Ligand

The synthesis of the novel C₃-symmetric tripodal, tetradentate ligand 1, bearing only phosphorus atoms as donor groups, is described, starting from commercially available o-tolyldiphenylphosphine, and its molecular structure has been determined by X-ray crystallographic analysis. Coordination to the cationic Ir^I precursor [Ir­(COE)₂(acetone)₂]­PF₆ led to a highly unsymmetrical species (90% yield) with four inequivalent phosphorus atoms, as evidenced by ³¹P NMR spectroscopy. The corresponding ¹H NMR spectrum exhibited a pseudo doublet of quartets at δ −5.9 ppm with one large trans P–H coupling (²J_P–H = 115.4 Hz) and a much smaller cis coupling (²J_P–H = 10.8 Hz). X-ray crystallography confirmed the formation of complex 2, [Ir­(H)­(κ⁵P,P,P,P,C-1)]­PF₆, which is a rare example of a structurally characterized mononuclear Ir hydride species bearing an ortho-metalated phosphine ligand. This species does not react with hydride sources, but addition of 1 equiv of CF₃COOH resulted in facile overall formal protonation of the Ir–C bond. DFT calculations support a pathway involving initial reductive elimination, forming the highly distorted four-coordinate Ir^I species 2′, followed by protonation at iridium to give the dicationic monohydride species 3, with an activation barrier ΔG^† of 28.2 kcal mol^–1. Deuteration experiments support this mechanism. Reductive elimination can also be induced by reaction of 2 with carbon monoxide, yielding the monocationic carbonyl complex [Ir^I(CO)­(1)]­PF₆ as the sole product.