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.

The tetranuclear basic zinc carbamates Zn4O(O2CAm)6(1, Am =N-diethylamino; 2, Am =N-piperidyl; 3, Am =N-pyrrolidyl) were shown by transient FTIR spectroscopy to undergo C-N bond metathesis reactions that result in exchange of the carboxyl group with bulk carbon dioxide and exchange of the amino group with bulk secondary amine (transamination). The net rate for CO2 exchange was measured by monitoring the uptake of 13CO2 and the concomitant release of 12CO2. This revealed a CO2-dependent and CO2-independent component to the CO2 exchange process. The CO2-dependent process was interpreted in terms of preassociation of the incoming CO2 with the complex prior to the N-CO2 bond cleavage process while the CO2-independent process was interpreted in terms of a unimolecular elimination of CO2 from the complex. The transamination reaction gave rates that are independent of the concentration of the incoming amine. Furthermore, the transamination rate for each complex was within a factor of four of the CO2-independent CO2 exchange rate for that complex. These data were interpreted in terms of a common rate-limiting process for CO2 exchange and transamination that involves unimolecular elimination of CO2. Rate-limiting unimolecular dissociation of intact carbamato ligands was eliminated as a possible pathway by showing that ligand exchange dynamics is rapid relative to the overall rate of C-N bond metathesis. The ability of these metal carbamate complexes to undergo facile C-N bond metathesis reactions of this type has implications in heterocumulene metathesis and CO2 fixation chemistry.

Key indicatorsSingle-crystal X-ray study T = 173 K Mean '(C±C) = 0.003 A Ê Disorder in main residue R factor = 0.023 wR factor = 0.056 Data-to-parameter ratio = 12.6For details of how these key indicators were automatically derived from the article, see