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

Brian M Lindley,Christian Würtele,Sven Schneider,Alexander J M Miller,Inke Siewert,Richt S Van Alten,Florian Schendzielorz,Markus Finger

doi:10.1021/jacs.8b03755

Brian M Lindley, Christian Würtele + Show 6 more

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https://doi.org/10.1021/jacs.8b03755

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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(CH2CH2PtBu2)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.

Highlights

Industrial ammonia synthesis produces more than 150 Mt/a of fixed nitrogen, securing the nutrition of about half of Earth’s population with synthetic fertilizers, and serving as the feedstock for most nitrogen-containing organic chemicals.[1]
We present a comprehensive mechanistic picture that evolved from a combined synthetic, spectroscopic, crystallographic, kinetic, electrochemical, and computational study
We report the first example of electrochemically driven N2 splitting into well-defined molecular nitrides

Summary

Introduction

Industrial ammonia synthesis produces more than 150 Mt/a of fixed nitrogen, securing the nutrition of about half of Earth’s population with synthetic fertilizers, and serving as the feedstock for most nitrogen-containing organic chemicals.[1] The Haber−Bosch process fixes N2 using the chemical reductant H2, and relies on energy-intensive steam reforming of fossil fuels, leading the overall process to consume about 1−2% of the global energy supply. Heterogeneous electrode materials capable of the nitrogen reduction reaction (NRR) to ammonia have been reported, with recent advances tending toward ambient temperature and pressure conditions.[3−5] Faradaic yields of the challenging 6e−/6H+ NRR are low, usually due to the competitive hydrogen evolution reaction (HER), a 2e−/ 2H+ process with a comparable standard reduction potential.[6] Molecular catalysts typically exhibit much higher ammonia selectivities,[7] but mainly rely on chemical reductants. Despite some early examples over 30 years ago, electrochemical NRR with molecular (pre)catalysts is still in its infancy.[8]

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