Halide Effects in Reductive Splitting of Dinitrogen with Rhenium Pincer Complexes.

Transition metal halide complexes are used as precursors for reductive N2 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 N2 splitting with the rhenium(III) complexes [ReX2(PNP)] (PNP = N(CH2CH2PtBu2)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 N2-bridged complexes, [{ReX(PNP)}2(µ-N2)], were identified as key intermediates in all cases. However, while the chloride complex is exclusively formed via 2-electron reduction and ReIII/ReI comproportionation, the iodide system also reacts via an alternative ReII/ReII-dimerization mechanism at less negative potentials. This alternative pathway relies on the absence of the potential inversion after reduction and N2 activation that was observed for the chloride precursor. Computational analysis of the relevant ReIII/II and ReII/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.

(Electro-)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex.

The splitting of N2 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 N2 splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl2(L2)] {L2 = N(CHCHPtBu2)2 -} is reported. The electrochemical characterization of [ReCl2(L2)] and comparison with our previously reported platform [ReCl2(L1)] {L1 = N(CH2CH2PtBu2)2 -} provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N2 derived nitride complex [Re(N)Cl(L2)] with electrophiles is presented.

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

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 ReII, N2 binding, chloride dissociation, and further reduction to ReI before formation of the N2-bridged, dinuclear intermediate by comproportionation with the ReIII 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.