Fe-Mediated HER vs N₂RR: Exploring Factors That Contribute to Selectivity in P₃^EFe(N₂) (E = B, Si, C) Catalyst Model Systems

Abstract

Mitigation of the hydrogen evolution reaction (HER) is a key challenge in selective small molecule reduction catalysis. This is especially true of catalytic nitrogen (N 2 ) and carbon dioxide (CO 2 ) reduction reactions (N 2 RR and CO 2 RR, respectively) using H + /e -currency. Here we explore, via DFT calculations, three iron model systems, P 3 E Fe (E = B, Si, C), known to mediate both N 2 RR and HER, but with different selectivity depending on the identity of the auxiliary ligand. It is suggested that the respective efficiencies of these systems for N 2 RR trend with the predicted N-H bonds strengths of two putative hydrazido intermediates of the proposed catalytic cycle, P 3 E Fe(NNH 2 ) + and P 3 E Fe(NNH 2 ). Further, a mechanism is presented for undesired HER consistent with DFT studies, and previously reported experimental data, for these systems; bimolecular proton-coupled-electrontransfer (PCET) from intermediates with weak N-H bonds is posited as an important source of H 2 , instead of more traditional scenarios that proceed via metal hydride intermediates and proton transfer/electron transfer (PT/ET) pathways. Wiberg bond indices provide additional insight into key factors related to the degree of stabilization of P 3 E Fe(NNH 2 ) species, factors that trend with overall product selectivity.

Selectivity of tungsten mediated dinitrogen splitting vs. proton reduction

“…The g-value (g av ¼ 1.90) indicates an orbital contribution in the typical range for W V complexes with multiply bound hard ligands, such as oxo or nitrido complexes. 25 Characterization of the redox series 6/7 + /8 2+ supports the electronic structure picture with {p 8 mutual coupling via the N 2 bridge (J(8 2+ ) ¼ À59 cm À1 ). In consequence, all three complexes of the redox series exhibit strong back bonding to the N 2 bridge with only weakly affected degrees of N 2 activation.…”

“…6,7 Computational evaluation of NR vs. HR selectivities for a series of Fe catalysts pointed at bimolecular H 2 loss from species with low N-H BDFEs. 8,9 Attempts to stabilize such Fe(N x H y ) species by hydrogen bonding with pendant bases so far resulted in shutdown of catalysis. 10 But, in fact, such secondary interactions might also be relevant for Nishibayashi's catalyst as indicated by selectivities obtained with 2,6-lutidinium acids ([LutH] + [X] À ) as the proton source.…”

Catalytic Nitrogen-to-Ammonia Conversion by Osmium and Ruthenium Complexes

“…Moreover, the N 2 ligand of [ Os ]-N 2 − can be protonated to afford a structurally characterized [ Os ] =NNH 2 + hydrazido complex, a likely intermediate by analogy to related [ Fe ] =NNH 2 + species that have been implicated as intermediates of N 2 RR by our lab. 15 Catalytically inactive Os-hydride species are characterized that appear to form as thermodynamic sinks of the spent catalyst system.…”

“…Tripodal tris(phosphine) hydrazido P 3 E Fe=NNH 2 + complexes have been spectroscopically (E = B) 15a and structurally characterized (E = Si), 15b generated via double protonation of P 3 E Fe-N 2 − at low temperature. Similarly, reaction of [ Os ]-N 2 − with 3 equiv of HOTf in thawing 2-MeTHF (−135 °C) produces an orange mixture, from which pale orange {[ Os ]=NNH 2 } {OTf} can be isolated upon precipitation (Scheme 1).…”

“…[ Os ]=NNH 2 + and its one-electron reduced state, [ Os ] =NNH 2 , should be active toward bimolecular proton-coupled-electron-transfer (PCET) steps to generate inactive hydrides (or competing H 2 ) owing to very weak N-H bonds. 2f,15c,20 Such reactivity likely explains the poor catalytic performance of isolated [ Os ]=NNH 2 + ; catalysis is likely to be more efficient when such hydrazido species are formed in lower concentration in situ during catalysis. Combined with the observation that Cp* 2 Co can reduce [ Os ]-N 2 to [ Os ]-N 2 − at low temperature, and that [ Os ]-N 2 − can be protonated to yield [ Os ]=NNH 2 +, the observation that [ Os ] =NNH 2 + can facilitate NH 3 formation lends support to an ET-PT N 2 RR pathway that proceeds, initially at least, via a distal pathway.…”