Activation of Carbon Dioxide by Divalent Tin Alkoxides Complexes

A series of terminal tin(II) alkoxides have been synthesized utilizing the bulky β-diketiminate ligand [{N(2,6-iPr2C6H3)-C(Me)}2CH] (BDI). The nucleophilicities of these alkoxides have been examined, and unexpected trends were observed. For instance, (BDI)SnOR only reacts with highly activated aliphatic electrophiles such as methyl triflate, but reacts reversibly with carbon dioxide. Both the rate of reaction and the degree of reversibility is dependent upon minor changes in the alkoxide ligand, with the bulkier tert-butoxide ligand displaying slower reactivity than the corresponding isopropyl ligand, although the latter system is a more exergonic reaction. Density Function Theory (DFT) calculations show that the differences in the reversibility of carbon dioxide insertion can be attributed to the ground-state energy differences of tin alkoxides while the rate of reaction is attributed to relative bond strengths of the Sn−O bonds. The mechanism of carbon dioxide insertion is discussed.