Evidence for unimolecular CO2 elimination 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.