Synthesis, NMR, and X-ray Studies of Iridium Dihydride C,N and N,P Ligand Complexes

The iridium(III) dihydride complexes [Ir(H)(2)(L1)(6,6'-bi-2-picoline)]BArF (5; L1 = (S)-1-[2-(2-adamantan-2-yl-4,5-dihydrooxazol-4-yl)-ethyl]-3-(2,6-diisopropylphenyl)-1,2-dihydroimidazol-2-ylidene, BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) and [Ir(H)(2)(L2)(6,6'-bi-2-picoline)]BArF (6; L2 = (R)-2-((di-tert-butylphosphanyl)methyl)-4-phenyl-4,5-dihydrooxazole) were prepared from the corresponding [Ir(COD)(L)]BArF precursors by treatment with 6,6'-bi-2-picoline under H-2 and characterized by 2D NMR spectroscopy and X-ray diffraction. In solution, the N,P complex 6 exists as two isomeric dihydride species (6a and 6b) that are in rapid equilibrium at room temperature. Furthermore, the X-ray structures for [Ir(COD)(L1)]BArF (1) and [Ir(COD)(L2)]BArF (2) are reported. The structural comparison of the solid-state structures of the iridium(I) precursor 1 and the iridium(III) dihydride complex 5 revealed a significant change in the backbone geometry of the C,N ligand. The original U-shaped conformation of the ligand switches to an S-shaped conformation, and therefore, the substituent in the oxazoline ring occupies different quadrants in the iridium coordination sphere. Notable in this context is the finding that a similar switch in the ligand backbone was observed for the C,N iridium(III) dihydride olefin species 3 ([Ir(H)(2)[(E)-1-methyl-4-(1-phenylprop-1-en-2-yl)benzene-D-5](L1)]BArF), which represents a catalytically competent intermediate.