Transition metal hydrides: Deprotonation of a cationic molybdenum hydride and studies of nickel(II) hydrides.

摘要

The acid-base properties of transition metal hydrides are of import to the understanding the nature of the metal-hydride bond. Some neutral transition metal hydrides behave as Bronsted acids, and cationic-metal hydrides may certainly serve as proton sources.; Rates for deprotonation of (HM(CO){dollar}sb2{dollar}(P{dollar}frown{dollar}P){dollar}sb2{dollar}) (BF{dollar}sb4{dollar}) were found to be many orders of magnitude slower than what would be found for the deprotonation of oxygen or nitrogen acids. For neutral amine bases, the rates of deprotonation were primarily dependent on the size of the amine, suggesting that the outer coordination sphere must undergo some rearrangement. There was noted a marked enhancement of proton transfer to the amine base in the presence of X{dollar}sp-{dollar} carriers.; A complex dependence on the phosphine ligand, involving both steric and electronic factors, was found. In addition, the solvent which solvates the X{dollar}sp-{dollar} the least produced the fastest rate. The kinetic isotope effect was examined and interpreted to signify a small amount of Mo-H{dollar}sp+{dollar} bond cleavage in the transition state.; Studies of metal hydride reactivity were also examined for nickel(II) complexes, namely trans-HNiX(PCy{dollar}sb3)sb2{dollar}. Reactivity with various reagents, were probed in terms of relative Ni-H and Ni-X bond strength, as implied from spectroscopic parameters.; The spectral properties, such as the hydride chemical shift in the {dollar}sp1{dollar}H-nmr and the Ni-H stretching frequency, correlated with each other. It was also noted to vary with the ligand X according to the trans-influence series.; The reactivity of trans-HNiX(PCy{dollar}sb3)sb2{dollar} was probed with a variety of reagents. It was found that HX could be reductively eliminated in the presence of CO and (RS){dollar}sb2{dollar}. For the case X = Me, it was also found that oxidatively induced reductive elimination could be achieved upon addition of ferrocenium. The reaction of CO{dollar}sb2{dollar} with HNi(Me)(PCy{dollar}sb3)sb2{dollar} produced HNi(O{dollar}sb2{dollar}CH)(PCy{dollar}sb3)sb2{dollar}), thru a mechanism that is at the present unknown. The reaction of CO{dollar}sb2{dollar} with nickel hydrides, to form formate groups, could possibly be the initial step in the hydrogenase enzyme (which takes CO{dollar}sb2{dollar} and reduces it to CH{dollar}sb4{dollar}). Therefore, the HNi(Me)(PCy{dollar}sb3)sb2{dollar} and HNi(O{dollar}sb2{dollar}CH)(PCy{dollar}sb3)sb2{dollar} would be good models to examine in further detail for the above biological process.