NbBN cluster anions reduce four carbon dioxide molecules: reactivity enhancement by ligands.

The thermal gas-phase reactions of Nb2BN2- cluster anions with carbon dioxide have been explored by using the art of time-of-flight mass spectrometry and density functional theory calculations. Four CO2 molecules can be consecutively reduced by Nb2BN2-, resulting in the formation of Nb2BN2O1-4- anions and the release of one CO molecule each time. To illustrate the role of ligands in Nb2BN2-, the reactivities of Nb2N2- and Nb2B- toward CO2 were also investigated; two and three CO2 molecules are activated, respectively, and the rate constants are slower than that of Nb2BN2-/CO2 systems. This comparison indicates that metal-metal multiple bonds and appropriate ligands, such as B, are important factors for CO2 reduction. The synergy between a transition metal atom (Nb) and a main-group atom (B) in CO2 reduction mediated by gas-phase clusters is revealed for the first time. To the best of our knowledge, Nb2BN2- anions are gas-phase clusters that reduce the largest number of CO2 molecules. A fundamental understanding of the efficient reduction of carbon dioxide molecules may shed light on the rational design of active sites on supported transition metal/boron nitride catalysts.