Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex

Abstract

A comprehensive mechanistic study of N₂ activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl₂(PNP)] is presented (PNP^- = N(CH₂CH₂P tBu₂)₂^-). Low-temperature studies using chemical reductants enabled full characterization of the N₂-bridged intermediate [{(PNP)ClRe}₂(N₂)] and kinetic analysis of the N-N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re(N)Cl(PNP)]. This first example of molecular electrochemical N₂ splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N₂ activation to form the N₂-bridged intermediate. CV data was acquired under Ar and N₂, and with varying chloride concentration, rhenium concentration, and N₂ pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where "E" is an electrochemical step and "C" is a chemical step) for N₂ activation that proceeds via initial reduction to Re^II, N₂ binding, chloride dissociation, and further reduction to Re^I before formation of the N₂-bridged, dinuclear intermediate by comproportionation with the Re^III precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N₂ splitting in the tetragonal frameworks enforced by rigid pincer ligands.