Synthesis, Structure, and Preparative Transamination of Tetrazinc Carbamato Complexes Having the Basic Zinc Carboxylate Structure

The series of tetranuclear zinc(II) carbamato complexes (Zn4O)(O2CR)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 Zn4O6+ core. Complex 2 crystallizes in the cubic space group I4̄3d, a = 24.0131(5) Å, V =
13846.6(5) Å3, R(1976 observed reflections) = 0.0194, and GOF = 1.013. Complex 3 crystallizes in the triclinic
space group P1̄, a = 10.3178(6) Å, b = 10.6962(6) Å, c = 19.5130(11) Å, α = 81.9070(10)°, β = 75.4880(10)°,
γ = 81.6540(10)°, V = 2050.4(2) Å3, R(6141 observed reflections) = 0.0334, and GOF = 0.979. NMR spectroscopy
was used to show that the (Zn4O)L6 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 CO2, also complete in about 3 min. Both transamination and transcarboxylation
reactions are consistent with the intermediacy of free CO2. However, because of the propensity for the complexes
to hydrolyze to liberate CO2, the role of adventitious moisture in facilitating the reaction cannot presently be rejected.