The chemistry of zinc hydrides.

The thesis reviews structural and chemical properties of complex zinc, magnesium and beryllium hydrides. Least is known about complex zinc hydrides and this thesis records the results of a range of reactions between zinc dihydride and substrates containing acidic protons or unsaturated bonds. Zinc dihydride reacts with compounds containing N-H,O-H,S-H and P-H bonds. The hydride products ranged from dimers, in cases where intramolecular steric effects allowed this, to polymers where intramolecular coordination bonds could not be formed e.g. [m-Me[2]NC[6]H[4]OZnH][n]. Several unusual complex zinc hydrides were obtained, where the reacting substrate appeared to undergo condensation before reacting with the zinc dihydride e.g. a product with a stoichiometry of (C[9]H[17]O)[2]ZnH was obtained from the reaction of zinc dihyaride with camphor.Zinc dihydride was found to react unexpectedly with diethylamine. The expected product, (Et[2]NZnH)[x], was not obtained. Instead, a coordination polymer, [Et[2]NHZnH[2]] , was obtained. Attempts at trapping a monomeric Species from the reaction, using N,N,N;N'-tetramethylethylene-diamine failed. Zinc dihydride was found to undergo exchange with zinc chloride in the presence of donor solvents to give chlorozinc hydride complexes. In general, the structural features exhibited by complex zinc hydrides were, as expected, terminal zinc hydrogen bonds and four coordinate zinc in a tetrahedral environment. The reactivity of complex zinc hydrides towards ketones was intermediate to that of lithium tetrahydridoaluminate and zinc dihydride. This was explained on the basis of steric hindrance and electronic repulsion in the transition state. Addition of hydride took place at the carbonyl group in trans-cinnamaldehyde rather than at the alkene, while with phenylacetylene, matters were complicated by competing elimination reactions. Product distribution was controlled by steric effects in the complex zinc hydrides. A possible mechanism for the addition of [Me[2]NC[2]H[4]OZnH][2] to trans-2-pentene was advanced on the basis of kinetic experiments.