Reactivity of a Monomeric Aluminium Hydrazide towards Isocyanates and Isothiocyanates: Active Lewis Pair Behaviour versus Classical Insertion Reactions

Uhl W., Bruchhage J., Willeke M., Hepp A., Kösters J.

Abstract

Hydroalumination of H10C5N–N=C(Ad) with HAliBu2yielded the hydrazide H10C5N–N(AliBu2)Ad (4; Ad = adamantyl, NC5H10= piperidinyl). Compound 4 is monomeric and contains a highly strained AlN2heterocycle formed by a donor–acceptor interaction between the Al atom and the β-N atom of the hydrazine. Two equivalents of the hydride afforded an adduct with an HAliBu2molecule coordinated to 4 by an Al–N and a 3c–2e Al–H–Al bond (5). tBuN=C=O did not insert into the relatively weak Al←N donor–acceptor bond of 4; instead, insertion into the shorter Al–N(amide) bond yielded five-membered AlN3C heterocycle 6 with an exocyclic C=O group. PhN=C=O showed a similar reaction, but the product 7 was dimeric in the solid state with two Al–O bonds. Reaction in a 1:2 ratio afforded a unique linear dimer of phenyl isocyanate, which is stabilised by the aluminium hydrazide (8). Isothiocyanates gave thiosemicarbazides 9–11, which have Al atoms coordinated in a chelating manner by formation of AlSCN heterocycles and show fascinating differences in their bonding situation.