SYNTHESIS AND CHARACTERIZATION OF NEW HYDRIDOIRIDIUM COMPLEXES CONTAINING CARBOXYLATE LIGANDS

The complexes IrH2(eta2-O2CR)(PPh3)2 (R = (S)-CH(NaphOMe)Me (2), (R)-CH(OMe)Ph (3), (R)-C(CF3)(OMe)Ph (4), (S)-CHOC(=O)CH2CH2 (5)) have been prepared by reaction of IrH5(PPh3)2 (1) with the corresponding carboxylic acid RCO2H. The reactivity of these compounds toward acetylenedicarboxylic dimethyl ester and HBF4 has been studied. 2-5 react with acetylenedicarboxylic dimethyl ester to afford the hydrido-vinyl complexes IrH-{C(CO2Me)-CH(CO2Me)}(eta2-O2CR)(PPh3)2 (R = (S)-CH(NaphOMe)Me (6), (R)-CH(OMe)Ph (7), (R)-C(CF3)(OMe)Ph (8), (S)-CHOC(=O)CH2(CH2 (9)) by insertion of the alkyne into one of the two Ir-H bonds of the starting complexes. Reactions of 2 and 3 with HBF4 in diethyl ether lead to the hydrido-bridged dinuclear complexes [Ir2H2(PPh3)4(mu-H)2 (mu-eta2-O2CR)]BF4 (R = (S)-CH(NaphOMe)Me (10), (R)-CH(OMe)Ph (11)). The molecular structure of 11 was determined by an X-ray investigation. Compound 11 crystallizes in the orthorhombic system, space group P2(1)2(1)2(1), with cell dimensions a = 14.082(1) angstrom, b = 23.169(2) angstrom, and c = 25.291(3) angstrom, and Z = 4. The structure was refined to the following R and R values: 0.0414 and 0.0389 for 8343 observed reflections. The cation of 11 can be described as a dinuclear species of 32 valence electrons. Electron counting, to satisfy the 18-electron rule, suggests the presence of an iridium-iridium double bond which is consistent with the observed iridium-iridium distance (2.6592(6) angstrom). EHT-MO calculations on the model [Ir2H2(PH3)4(mu-H)2(mu-eta2-O2CH)]+ suggests the existence of a ''partial'' double bond between the metal atoms. However, the interaction between metals is not based on the direct overlap of the iridium orbitals, but on the three center bonds formed by the bridging system.