Proton-Coupled Electron Transfer. ChemRxiv. Preprint. The direct scission of the triple bond of dinitrogen (N2) by a metal complex is an alluring entry point into the transformation of N2 to ammonia (NH3) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N2 to NH3 via a well-defined reaction sequence involving reductive formation of a bridging N2 complex, photolytic N2 splitting, and proton-coupled electron transfer (PCET) reduction of the metal-nitride bond. The new complex (PONOP)ReCl3 (PONOP = 2,6-bis(diisopropylphosphinito)pyridine) is reduced under N2 to afford the trans,trans-isomer of the bimetallic complex [(PONOP)ReCl2]2(μ-N2) as an isolable kinetic product that isomerizes sequentially upon heating into the trans,cis and cis,cis isomers. All isomers are inert to thermal N2 scission, and thetrans,trans-isomer is also inert to photolytic N2 cleavage. In striking contrast, illumination of the trans,cisand cis,cis-isomers with blue light affords the octahedral nitride complex cis-(PONOP)Re(N)Cl2 in 47% spectroscopic yield and 11% quantum efficiency. The photon energy drives an N2 splitting reaction that is thermodynamically unfavorable under standard conditions, producing a nitrido complex that reacts with SmI2/H2O to produce a rhenium tetrahydride complex and furnish ammonia in 74% yield. File list (2) download file view on ChemRxiv Bruch_PONOPReN2NH3_ChemRxiv_Manuscript.pdf (1.06 MiB) download file view on ChemRxiv Bruch_PONOPReN2NH3_ChemRxiv_SI.pdf (15.70 MiB)

Despite advances in the development of molecular catalysts capable of reducing dinitrogen to ammonia using proton donors and chemical reductants, few molecular electrocatalysts have been discovered. This Perspective considers the prospects of electrocatalyst development based on a mechanism featuring the cleavage of N2 into metal nitride complexes. By understanding the factors that control the reactivity of individual steps along the electrochemical N2 cleavage path, opportunities for new advances are identified. Ligand design principles for facile electrochemical N2 binding, formation of bridging N2 complexes, thermal or photochemical N2 cleavage, and conversion of a nitride ligand into ammonia are described, featuring recent advances and the authors’ collaborative work on rhenium complexes.

A new family of low-coordinate Co complexes supported by three redox-noninnocent tridentate [OCO] pincer-type bis(phenolate) N-heterocyclic carbene (NHC) ligands are described. Combined experimental and computational data suggest that the charge-neutral four-coordinate complexes are best formulated as Co(II) centers bound to closed-shell [OCO] dianions, of the general formula [(OCO)CoL] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms of the [(OCO)CoL] complexes reveal three oxidations accessible at potentials below 1.2 V vs Fc/Fc, corresponding to generation of formally Co(V) species, but the true physical/spectroscopic oxidation states are much lower. Chemical oxidations afford the mono- and dications of the imidazoline NHC-derived complex, which were examined by computational and magnetic and spectroscopic methods, including single-crystal X-ray diffraction. The metal and ligand oxidation states of the monocationic complex are ambiguous; data are consistent with formulation as either [(OCO)Co(THF)] containing a closed-shell [OCO] diphenolate ligand bound to a S = 1 Co(III) center, or [(OCO)Co(THF)] with a low-spin Co(II) ion ferromagnetically coupled to monoanionic [OCO] containing a single unpaired electron distributed across the [OCO] framework. The dication is best described as [(OCO)Co(THF)], with a single unpaired electron localized on the d Co(II) center and a doubly oxidized, charge-neutral, closed-shell OCO ligand. The combined data provide for the first time unequivocal and structural evidence for [OCO] ligand redox activity. Notably, varying the degree of unsaturation in the NHC backbone shifts the ligand-based oxidation potentials by up to 400 mV. The possible chemical origins of this unexpected shift, along with the potential utility of the [OCO] pincer ligands for base-metal-mediated organometallic coupling catalysis, are discussed.

Hydride transfer catalysis is shown to be enabled by the nonspectator reactivity of a transition metal-bound low-symmetry tricoordinate phosphorus ligand. Complex 1·[Ru]+, comprising a nontrigonal phosphorus chelate (1, P­(N­(o-N­(2-pyridyl)­C6H4)2) and an inert metal fragment ([Ru] = (Me5C5)­Ru), reacts with NaBH4 to give a metallohydridophosphorane (1 H ·[Ru]) by P–H bond formation. Complex 1 H ·[Ru] is revealed to be a potent hydride donor (ΔG°H–,exp < 41 kcal/mol, ΔG°H–,calc = 38 ± 2 kcal/mol in MeCN). Taken together, the reactivity of the 1·[Ru]+/1 H ·[Ru] pair comprises a catalytic couple, enabling catalytic hydrodechlorination in which phosphorus is the sole reactive site of hydride transfer.

The conversion of metal nitride complexes to ammonia may be essential to dinitrogen fixation. We report a new reduction pathway that utilizes ligating acids and metal-ligand cooperation to effect this conversion without external reductants. Weak acids such as 4-methoxybenzoic acid and 2-pyridone react with nitride complex [(H-PNP)RuN] (H-PNP = HN(CHCHPBu)) to generate octahedral ammine complexes that are κ-chelated by the conjugate base. Experimental and computational mechanistic studies reveal the important role of Lewis basic sites proximal to the acidic proton in facilitating protonation of the nitride. The subsequent reduction to ammonia is enabled by intramolecular 2H/2e proton-coupled electron transfer from the saturated pincer ligand backbone.