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

The synthesis of the novel <i>C</i>₃-symmetric tripodal, tetradentate ligand <b>1</b>, bearing only phosphorus atoms as donor groups, is described, starting from commercially available <i>o</i>-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 <i>trans</i> P–H coupling (²<i>J</i>_P–H = 115.4 Hz) and a much smaller <i>cis</i> coupling (²<i>J</i>_P–H = 10.8 Hz). X-ray crystallography confirmed the formation of complex <b>2</b>, [Ir­(H)­(κ⁵<i>P</i>,<i>P</i>,<i>P</i>,<i>P</i>,<i>C</i>-<b>1</b>)]­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 <b>2</b>′, followed by protonation at iridium to give the dicationic monohydride species <b>3</b>, with an activation barrier Δ<i>G</i>^† of 28.2 kcal mol^–1. Deuteration experiments support this mechanism. Reductive elimination can also be induced by reaction of <b>2</b> with carbon monoxide, yielding the monocationic carbonyl complex [Ir^I(CO)­(<b>1</b>)]­PF₆ as the sole product