The use of main-group elements to mimic catalytic behavior of transition metals I: reduction of dinitrogen to ammonia catalyzed by bis(Lewis base)borylenium diradicals

Abstract

The use of boron (B) atoms as transition metal mimics opens the door to new research in catalytic chemistry. An emerging class of compounds, bis(Lewis base)borylenes with an electron-rich B(I) center, are potential metal-free catalysts for dinitrogen bonding and reduction. Here, the molecular geometry, electronic structure, and possible reaction mechanism of a series of bis(Lewis base)borylene-dinitrogen compounds corresponding to the nitrogen reduction reaction have been investigated by using density functional theory (DFT) calculations. Our DFT calculations show that these free borylene compounds possess radical features and have the capability to activate N₂ molecules via an effective combination of π(B → N₂), π(N₂ → B), and σ(N₂ → B) electron transfer processes. The possible reaction mechanisms for direct conversion of N₂ into NH₃ for these bis(Lewis base)borylene-dinitrogen compounds have been systematically investigated along distal and alternating paths. The calculated free energy profiles indicate that the limiting potential of a bis(phosphine)borylene-dinitrogen compound is comparable to that of metal-based catalysts, which is the most promising candidate for the reduction of N₂ to NH₃via the alternating mechanism among all compounds studied here. The electronic structure analysis shows that the B center plays the role of an electron donor and acceptor alternatively in the consecutive six protonation and reduction processes, and thus acts as the electron transfer medium.