MECHANISTIC STUDY OF HYDROGEN ACTIVATION BY CATIONIC DINUCLEAR (MU-SULFIDO)MOLYBDENUM COMPLEXES

The cationic derivatives [(MeCPMo)2(S2CH2)(mu-S)(mu-SR)]X (R = C4H3S, CH2CO2Me; X = Br, SO3CF3; MeCp = C5H4CH3) have been synthesized and characterized spectroscopically. A single crystal of [(MeCPMo)2(S2CH2)(mu-S)(mu-SC4H3S)]Br has been studied by X-ray diffraction. The complex crystallized in space group P2(1)/n with a = 7.030 (3) angstrom, b = 19.777 (8) angstrom, c = 15.106 (6) angstrom, beta = 90.84 (3)-degrees, and Z = 4. The structure showed that the mu-thiophenethiolate ligand was oriented away from the adjacent sulfido ligand. The reactions of the cationic derivatives with hydrogen have been studied. The bromide salts reacted with hydrogen to form RH, (MeCpMoS)2S2CH2, and 1 equiv of HBr. In the presence of a nitrogen base, the reactions of the cations with hydrogen proceeded to form the neutral complexes (MeCpMo)2S2CH2)(mu-SR)(mu-SH) and 1 equiv of protonated base. The kinetics of the latter hydrogen activation reaction have been studied over a range of temperatures, hydrogen pressures, and base concentrations. The kinetic data are consistent with a rate law which is first order in cation and first order in hydrogen. For the cation where R = CH2CO2Me, the deuterium isotope effect, k(H)/k(D), was found to be 2.5 at 50-degrees-C and the following activation parameters were determined: DELTA-H(double dagger) = 84 kJ/mol and DELTA-S(double dagger) = -9 J/(mol K). The data seem most consistent with a heterolytic mechanism for the activation of hydrogen. Several possible mechanisms for hydrogen addition to the cation are discussed. Attempts to model the initial hydrogen addition product have been made by reacting the related neutral complexes (CpMo)2(S2CH2)(mu-SMe)(mu-SR) with 1 equiv of strong anhydrous protic acid. The reactions with acid involved either protonolysis of thiolate ligands or oxidation of the dimer, depending on the nature of thiolate substituents. However, no protonated intermediates were detected.