Synthesis, structure, and preparative transamination of tetrazinc carbamato complexes having the basic zinc carboxylate structure

The series of tetranuclear zinc(II) carbamato complexes (Zn(4)O)(O(2)CR)(6), (1, R = diethylamino; 2, R = piperidino; 3, R = pyrrolidino) was prepared. Complexes 2 and 3 were crystallographically characterized and shown to have the same tetrahedral Zn(4)O(6+) core. Complex 2 crystallizes in the cubic space group I(-)43d, a = 24.0131(5) A, V = 13846.6(5) A(3), R(1976 observed reflections) = 0.0194, and GOF = 1.013. Complex 3 crystallizes in the triclinic space group P(-)1, a = 10.3178(6) A, b = 10.6962(6) A, c = 19.5130(11) A, alpha = 81.9070(10), beta = 75.4880(10), gamma = 81.6540(10), V = 2050.4(2) A(3), R(6141 observed reflections) = 0.0334, and GOF = 0.979. NMR spectroscopy was used to show that the (Zn(4)O)L(6) structure was maintained in nonpolar solvents. The complexes reacted with free amine in nonpolar solvents, which resulted in facile conversion of one member of the series to another. For example, reacting 1 with a stoichiometric amount of pyrrolidine in tetrahydrofuran followed by workup resulted in the quantitative formation of 3 with liberation of diethylamine. Formally, this is a transamination metathesis reaction between the diethylcarbamate ligand and pyrrolidine. The reaction is complete within 3 min at room temperature, in marked contrast to the extreme conditions required to effect transamination on organic carbamates. The complexes also undergo a facile transcarboxylation reaction with carbon dioxide which results in scrambling of the carboxyl group of the carbamate ligand with free CO(2), also complete in about 3 min. Both transamination and transcarboxylation reactions are consistent with the intermediacy of free CO(2). However, because of the propensity for the complexes to hydrolyze to liberate CO(2), the role of adventitious moisture in facilitating the reaction cannot presently be rejected.