Catalytic N₂-to-NH₃ Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

Abstract

We have recently reported on several Fe catalysts for N2-to-NH3 conversion that operate at low temperature (−78 °C) and atmospheric pressure while relying on a very strong reductant (KC8) and acid ([H(OEt2)2][BArF4]). Here we show that our original catalyst system, P3BFe, achieves both significantly improved efficiency for NH3 formation (up to 72% for e– delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH3 per Fe site), when employing a significantly weaker combination of reductant (Cp*2Co) and acid ([Ph2NH2][OTf] or [PhNH3][OTf]). Relative to the previously reported catalysis, freeze-quench Mössbauer spectroscopy under turnover conditions suggests a change in the rate of key elementary steps; formation of a previously characterized off-path borohydrido–hydrido resting state is also suppressed. Theoretical and experimental studies are presented that highlight the possibility of protonated metallocenes as discrete PCET reagents under the present (and related) catalytic conditions, offering a plausible rationale for the increased efficiency at reduced driving force of this Fe catalyst system.

Nitrogen Fixation via a Terminal Fe(IV) Nitride

“…8 Of particular interest has been to distinguish between an early Chatt-type cleavage pathway (a distal mechanism via an Fe≡N intermediate) and a late-stage cleavage pathway (an alternating mechanism via an Fe—N 2 H 4 intermediate). 9 While we have considered the possibility that both scenarios might be viable, 10 a key feature of the tri(phosphine)borane Fe system is its structurally and electronically flexible Fe—B interaction, which allows access to both reduced trigonal bipyramidal Fe(N 2 ) species as well as, in principle, pseudotetrahedral terminal Fe≡N.…”

“…8 In a separate synthetic study, it was shown that [(P 3 B )Fe(N 2 )] − reacts rapidly with excess HBAr F 4 at very low temperatures to form the cationic hydrazido(2−) complex [(P 3 B )Fe(NNH 2 )] + (Scheme 1C; potentials shown are measured in THF vs the ferrocenium/ferrocene couple [Fc + /Fc]). 12 To probe further steps in the catalytic N 2 fixation mechanism, we therefore sought access to the one-electron reduced hydrazido complex (P 3 B )Fe(NNH 2 ) to evaluate the viability of N—N bond cleavage via subsequent protonation.…”

“…For all subsequent studies, [(P 3 B )Fe(N 2 )] 2− was produced in situ and used immediately. The 57 Fe Mössbauer spectrum of [(P 3 B )Fe(N 2 )] 2− produced from the reduction of 57 Fe-enriched [Na(12-crown-4) 2 ][(P 3 B )Fe(N 2 )] (Figure 1A) reveals parameters consistent with its diamagnetic ground state (Figure 1E), 8b and which are nearly identical with those of the isoelectronic and isostructural silyl complex [(P 3 Si )Fe(N 2 )] − (P 3 Si = tris( o -diisopropylphosphinophenyl)silyl). 13 …”

“…The most reduced state of the catalyst accessible with Cp* 2 Co is the anion [(P 3 B )Fe(N 2 )] − , 8c and this state appears to be catalytically relevant under all conditions canvassed, 8b including with TfOH and Cp* 2 Co as the acid/reductant combination (Table S11). An alternative pathway to form [(P 3 B )Fe≡N] + under these milder conditions is shown in Scheme 1C, D via the known cationic hydrazido [(P 3 B )Fe(NNH 2 )] + .…”

“…Other routes to [(P 3 B )Fe≡N] + via metallocene-mediated proton-coupled electron transfer reactions are also conceivable, as are late-stage N-N cleavage pathways. 8b,c …”