The synthesis and carbon dioxide reactivity of alkylaminozinc complexes
A number of organometallic complexes utilize carbamate ligands to incorporate CO2 into new molecules. In order to better understand metal-carbamate chemistry, a series of CO2 reactive alkylaminozinc complexes were synthesized by the addition of dimethylzinc to secondary amines, resulting in the isolation of Zn(CH3)2(HNC4H8O)2, 1. The addition of one equivalent of CO2 to 1 resulted in the formation of the partially converted complex [Zn(CH3,)(HNC4H8O)(02CNC4H8O)]n, 2. Addition of CO2 in excess under anhydrous conditions formed the complex [Zn(02CNC4H8O)2]n, 3, and the addition of one quarter equivalent of H2O to 3 caused the formation of the Zn4(O2CNC4H8O)6 complex, 4. The kinetics of CO2 addition to 1 indicated the presence of a pentacoordinate zinc transition state that is responsible for alkyl elimination in 1. Additional reactions of secondary amines with dimethylzinc also results in the pyrrolidine complex Zn2(NC4H8)2(HNC4H8)2, 5, the piperidine complex [Zn(CH3)(HNC5HIO)]m, 6, and the diisopropylamine complex [Zn(CH3)2(HNC6H14)]n, 7. The investigation also lead to the characterization of the polymeric di alkylpiperazinozinc complex [Zn(CH3)2(HC4H8NH)]n, 9. This complex does not insert CO2, but does catalyze the cyclization of phenyl isocyanate into the trimer phenylisocyanurate.