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Transition metal halide complexes are used as precursors for reductive N 2 activation up to full splitting into nitride complexes. Distinct halide effects on the redox properties and yields are frequently observed yet not well understood. Here, an electrochemical and computational examination of reductive N 2 splitting with the rhenium(III) complexes [ReX 2 (PNP)] (PNP = N(CH 2 CH 2 P t Bu 2 ) 2 and X = Cl, Br, I) is presented. As previously reported for the chloride precursor ( J. Am. Chem. Soc. 2018, 140, 7922), the heavier halides give rhenium(V) nitrides upon (electro-)chemical reduction in good yields yet with significantly anodically shifted electrolysis potentials along the halide series. Dinuclear, end-on N 2 -bridged complexes, [{ReX(PNP)} 2 (μ-N 2 )], were identified as key intermediates in all cases. However, while the chloride complex is exclusively formed via 2-electron reduction and Re III /Re I comproportionation, the iodide system also reacts via an alternative Re II /Re II -dimerization mechanism at less negative potentials. This alternative pathway relies on the absence of the potential inversion after reduction and N 2 activation that was observed for the chloride precursor. Computational analysis of the relevant Re III/II and Re II/I redox couples by energy decomposition analysis attributes the halide-induced trends of the potentials to the dominating electrostatic Re-X bonding interactions over contributions from charge transfer.

The splitting of N 2 into well-defined terminal nitride complexes is a key reaction for nitrogen fixation at ambient conditions. In continuation of our previous work on rhenium pincer mediated N 2 splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl 2 ( L 2)] { L 2 = N(CHCHP t Bu 2 ) 2 - } is reported. The electrochemical characterization of [ReCl 2 ( L 2)] and comparison with our previously reported platform [ReCl 2 ( L 1)] { L 1 = N(CH 2 CH 2 P t Bu 2 ) 2 - } provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N 2 derived nitride complex [Re(N)Cl( L 2)] with electrophiles is presented., (© 2020 The Authors. Published by Wiley‐VCH Verlag GmbH & Co. KGaA.)

A comprehensive mechanistic study of N 2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl 2 (PNP)] is presented (PNP - = N(CH 2 CH 2 P tBu 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 N 2 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 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 N 2 splitting in the tetragonal frameworks enforced by rigid pincer ligands.