Extending the motif of the [FeFe]-hydrogenase active site models: Protonation of Fe2(NR)2(CO)6−L species

Abstract

Studies on diiron dithiolato complexes have proven fruitful for modeling the active site of the [FeFe]hydrogenases. Here we present a departure from the classical Fe 2 S 2 motif by examining the viability of Fe 2 N 2 butterfly compounds as functional models for the diiron active site of [FeFe]-hydrogenases. Derivatization of Fe 2 (BC)(CO) 6 (1, BC = benzo-[c]-cinnoline) with PMe 3 affords Fe 2 (BC) (CO) 4 (PMe 3 ) 2 , which subsequently undergoes protonation at the Fe-Fe bond. The hydride [(μ-H) Fe 2 (BC)(CO) 4 (PMe 3 ) 2 ]PF 6 was characterized crystallographically as the C 2v isomer. It represents a rare example of a hydrido diiron complex that exists as observable isomers, depending on the location of the phosphine ligands -diapical and apical-basal. This hydride catalyzes the electrochemical reduction of protons. Keywords Fe complexes; Metal hydride; Hydrogenase; ElectrocatalysisIn recent years, many compounds of the type Fe 2 (S-R) 2 (CO) 4 L 2 have been prepared and examined as structural and functional models for the active site of [FeFe]-hydrogenases. Particular emphasis has been placed on variations of L, especially tertiary phosphines [1][2][3], but also isocyanides [4,5], N-heterocyclic carbenes [6][7][8][9], and cyanide [10][11][12][13]. While much effort has focused on functionalizing the dithiolate ligand [14][15][16][17][18][19][20], relatively little work has examined the possibility of replacing the thiolates with other bridging groups [21]. Best and coworkers demonstrated that the corresponding phosphides Fe 2 (PR 2 ) 2 (CO) 6 and [Fe 2 (PR 2 ) 2 (CO) 6 H] − are effective catalysts for hydrogen evolution [22].Incentives for broadening the range of bridging ligands include changing the steric profile of the catalysts, altering the susceptibility of the bridge toward electrophilic attack (important since these catalysts function in acidic media), incorporating functionality, and manipulating the electronic character (basicity, E 1/2 ) of the metals. Naturally, a fundamental motivation for investigating new bridging ligands is to elucidate the elementary design rules that will lead to greater mechanistic insight and, ultimately, a deeper understanding of hydrogenogenesis.Approximately 15 nitrogenous bridges are known to link pairs of iron centers, giving complexes of the general formula Fe 2 (NR 1,2 ) 2 (CO) 6 (Scheme 1). Many of the complexes were first reported in the 1960s and 1970s in an era where the interactions of nitrene-forming reagents and metal carbonyls were first systematized. We selected one representative of this series, Fe 2 (BC)(CO) 6 [23][24][25][26] properties would test most demandingly the scope of the hydrogenase behavior of the Fe 2 (μ-X) 2 L n systems (L = CO, donor ligands). This report summarizes these results, which indeed demonstrate the prospect that many nitrogenous bridging ligands could be applied to the modeling of hydrogenases.Compound 1 was prepared using the original method, using Fe 3 (CO) 12 in place of Fe(CO) 5 [27]. As for other donor ligands [26,...

Azo group(s) in selected macrocyclic compounds

“…The crystal structure of benzo[ c ]cinnoline complex with ytterbium Yb(BC) 3 (thf) 2 (BC = benzo[ c ]cinnoline) was described [ 108 ]. Fe 2 (BC)(CO) 6 complex was examined as a candidate for a new structural and functional model for [FeFe]-hydrogenases [ 109 , 110 ].…”

Bridging-hydride influence on the electronic structure of an [FeFe] hydrogenase active-site model complex revealed by XAES-DFT

“…[36][37][38][39][40][41][42][43][44][45][46] A substantial fraction of this work has been devoted to complexes, (a) which contain iron-bound hydrides in metal-bridging and/or terminal positions, (b) that focus for example on the reactivity and redox potential of the compounds, (c) that explore the role of the dithiolate bridge in proton transfer, and (d) that demonstrate the influence of ligand exchange and coordination geometry at the iron atoms. 44,[47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] Unfortunately, the stabilities, turnover rates, and numbers of synthetic [FeFe] catalysts are thus far too low for technical applications. Furthermore, detailed information on the electronic configuration is available only for a limited number of complexes.…”

Using polyethyleneimine (PEI) as a scaffold to construct mimicking systems of [FeFe]‐hydrogenase: preparation, characterization of PEI‐based materials, and their catalysis on proton reduction

“…PEI (branched, M w = 1800 and 600) and chlorodiphenylphosphine were purchased from Alfa Aesar and triphenylphosphine from Sinopharm Chemical Reagent Co., Ltd. General chemicals were purchased from local commercial suppliers and further purified when necessary. Complexes [Fe 2 (m-S) 2 (CO) 6 ] (1), [Fe 2 (m-S) 2 (CO) 5 PPh 3 ] (2), [Fe 2 (m-S 2 C 2 H 4 )(CO) 6 ] (3), and [Fe 2 (m-S 2 C 2 H 4 )(CO) 5 PPh 3 ] (4) (refer to ESI) were synthesized following published procedures. [41][42][43] Infrared (IR) spectra were recorded on a Scimitar 2000 (Varian).…”

“…In the past decade, a large number of diiron complexes of the core, [Fe 2 (CO) 6Àx ] (x = 1, 2), have been prepared. [5][6][7][8][9][10][11][12][13] Those diiron complexes, in one way or another, resemble the diiron center of the enzyme. In mimicking the natural systems, particular attention has been paid to the preparation of mimics possessing a basic group either directly coordinating to the diiron center or locating in the secondary coordination sphere because it is believed that such a group may play a role in proton transfer in the process of proton reduction and hence improve catalytic efficiency.…”