Evidence for unimolecular CO2elimination in C–N bond metathesis reactions of basic carbamatozinc complexes Zn4O(O2CAm)6(Am = N-diethylamino, N-piperidyl, N-pyrrolidyl)

Abstract

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.