Synthesis of a 14-electron iridium(III) complex with a xanthene-based bis(silyl) chelate ligand (xantsil): A distorted seesaw-shaped fourcoordinate geometry and reactions leading to 16-electron complexes

Synthesis, structure determination, and reactions of a 14-electron four-coordinate iridium(III) complex bearing a xanthene-based bis(silyl) chelate ligand, i.e., Ir{κ²(Si,Si)-xantsil}(PCy3)Cl (1a, xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)), are reported. A precursor of 1a, the 16-electron (dihydrido)iridium(V) complex Ir{κ² (Si,Si)-xantsil}(H)₂(PCy₃)Cl (2), was prepared by the reaction of [IrCl-(coe)₂]₂ (coe = cyclooctene) with 4,5-bis(dimethylsilyl)-9,9-dimethylxanthene (xantsilH₂) and PCy ₃. Dehydrogenation reaction of 2 with 3,3-dimethylbut-1-ene, a hydrogen acceptor, afforded a mixture of 1a and its isomer 1b, abbreviated as 1a + 1b, together with 2,2-dimethylbutane. Single crystals obtained from a CH ₂Cl₂ solution of 1a + 1b contained only isomer 1a. X-ray crystal structure analysis of one of the crystals revealed that 1a adopts a distorted seesaw-shaped four-coordinate geometry where the coordinatively unsaturated metal center is stabilized by weak agostic interaction of two γ-C-H bonds of the PCy₃ ligand. On the other hand, NMR spectroscopic analysis of 1a + 1b demonstrated that 1a is in fast equilibrium with a minor amount of 1b in solution. Replacement of the chloro ligand in complexes 1a + 1b by a triflato ligand with AgOTf (OTf = OSO₂CF ₃) afforded quantitatively a single product, i.e., the 16-electron iridiumetriflato complex Ir{κ³(Si,O,Si)-xantsil}(PCy ₃)(OTf) (3). 1a + 1b and 16-electron complex 2 are interconvertible via oxidative addition of dihydrogen (from 1a + 1b to 2) and alkene hydrogenation (from 2 to 1a + 1b). Complexes 1a + 1b and 2 were found to catalyze hydrogenation of 3,3-dimethylbut-1-ene.