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

A comprehensive mechanistic study of N-2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl2(PNP)] is presented (PNP- = N((CHCHPtBu2)-C-2-Pt-2)(2)(-)). Low-temperature studies using chemical reductants enabled full characterization of the N-2-bridged intermediate [{(PNP)ClRe}(2)(N-2)] 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 N-2 splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N-2 activation to form the N-2-bridged intermediate. CV data was acquired under Ar and N-2, 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 N-2 activation that proceeds via initial reduction to Re-II, N-2 binding, chloride dissociation, and further reduction to Re-I before formation of the N-2-bridged, dinuclear intermediate by comproportionation with the Rem 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 N-2 splitting in the tetragonal frameworks enforced by rigid pincer ligands.