Investigation of metal cyclam complexes as potential catalysts for the production of dimethyl carbonate

While Zn cyclam complexes are known to readily react with CO2 and methanol to provide carbonato complexes, we were interested in trying to extend this chemistry into a possible catalytic cycle for the production of dimethyl carbonate. This route would require a cleavage of the Zn-carbonato complex with methanol. Towards this goal we modified the cyclam ligands by attaching various Lewis basic donors as pendant groups on the cyclam that could potentially act as hemilabile arms. The crystal structures of 1-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane 2, 1-(3-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane 3, and 1-propionitrile-1,4,8,11-tetraazacyclotetradecane 4 are presented. Ligand 2 was reacted with M[ClO4]2 (M = Zn, Cd) and the resulting complexes were structurally and spectroscopically characterized. Despite the modifications, again these complexes did not perform as catalysts for the formation of dimethyl carbonate. Ligand 4 was found to react with methanol in the presence of Zn[ClO4]2·6H2O to convert the nitrile group into a methyl propionimidate, forming the methyl propionimidate (1,4,8,11-tetraazacyclotetradecane) zinc perchlorate. It is likely that the donor arms used bind too strongly to Zn or Cd to allow complexation of CO2 or methanol.

Graphical abstractModified cyclam ligand complexes with Zn and Cd have been investigated as possible catalysts for the production of dimethyl carbonate. Cyclams containing pendant arms have been structurally characterized, as have their Zn and Cd complexes. Reactions with CO2/methanol under forcing conditions did not lead to the desired dimethyl carbonate, but one complex showed methanolysis of a nitrile using Zn.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Attempted synthesis of dimethylcarbonate from methanol and CO2. ► Crystal structures of pyridinyl- and propionitrile-substituted cyclams. ► Crystal structures of Zn- and Cd-cyclam complexes. ► Unexpected methanolysis of nitrile group using Zn.