Selective Catalytic Reduction of N₂ to N₂H₄ by a Simple Fe Complex

Abstract

The catalytic fixation of N by molecular Fe compounds is a rapidly developing field, yet thus far few complexes can effect this transformation, and none are selective for NH production. Herein we report that the simple Fe(0) complex Fe(EtPCHCHPEt)(N) (1) is an efficient catalyst for the selective conversion of N (>25 molecules N fixed) into NH, attendant with the production of ca. one molecule of NH. Notably, the reductant (CoCp*) and acid (PhNHOTf) used are considerably weaker than conventional chemical H and e sources used in previous demonstrations of N turnover by synthetic Fe compounds. These results show that the direct catalytic conversion of N to the hydrazine oxidation state on molecular Fe complexes is viable and that the mechanism of NH formation by such systems may proceed via Fe-NH intermediates.

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

“…2f,17 These reactions highlight the far superior N 2 RR activity of [ Os ]-N 2 − by comparison to either [ Ru ]-N 2 − or [Fe]-N 2 − . Thus, treatment of a suspension of [ Os ]-N 2 − in Et 2 O at −78 °C with 46 equiv of [H 2 NPh 2 ][OTf| and 50 equiv of Cp* 2 Co results in the generation of 7.1 ± 0.6 equiv of NH 3 per Os (Table 1, entry 6; 46% based on acid).…”

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

“…Such conditions additionally afford unusually high selectivity and catalytic turnover for NH 3 . 20 Moreover, we note that the use of milder reagents as reductant ( E 0 ; eq 1) and acid (p K a ; eq 1) engenders a higher effective bond dissociation enthalpy (BDE effective ; eq 1). 15,21 This may in turn afford access to proton-coupled electron transfer (PCET) pathways (e.g., FeN 2 + H • → FeN 2 H) in addition to electron transfer (ET)/proton transfer (PT) pathways, thus enhancing overall catalytic efficiency.…”

“…Theoretical considerations, including DFT calculations, and experimental details are discussed that suggest the viability of a decamethylcobaltocene-mediated PCET pathway in this system; by extension we suggest that metallocene-mediated (e.g., Cp* 2 Cr) PCET pathways may be operative in previously studied Mo and Fe N 2 -fixing systems that use metallocene reductants. 10−13,20 …”

Cationic silyldiazenido complexes of the Fe(diphosphine)2(N2) platform: structural and electronic models for an elusive first intermediate in N2fixation

“…1 Only a few Fe-containing compounds have been reported that are capable of mediating the catalytic reduction of N 2 using chemical H + and esources, 2,3 and we recently reported that the simple Fe(0) phosphine complex Fe(depe) 2 (N 2 ) [1; depe = Et 2 PCH 2 CH 2 PEt 2 ] mediates the efficient catalytic reduction of N 2 in conjunction with excess acid and reductant; this was the first system using a molecular TM compound which was selective for N 2 H 4 production over NH 3 . 4 Nevertheless, mechanistic details for these N 2 to N 2 H 4 /NH 3 conversions remain scarce, mostly due to rapid reaction rates and the high lability of the various FeN x H y intermediates. 5 In particular, examples which establish the structural and electronic changes upon mono-protonation of Fe-ligated N 2 to form diazenide species (Fe-NN-H) remain elusive to date.…”