Functional Model for the [Fe] Hydrogenase Inspired by the Frustrated Lewis Pair Concept

Abstract

[Fe] hydrogenase (Hmd) catalyzes the heterolytic splitting of H2 by using, in its active site, a unique organometallic iron-guanylylpyridinol (FeGP) cofactor and, as a hydride acceptor, the substrate methenyltetrahydromethanopterin (methenyl-H4MPT(+)). The combination FeGP/methenyl-H4MPT(+) and its reactivity bear resemblance to the concept of frustrated Lewis pairs (FLPs), some of which have been shown to heterolytically activate H2. The present work exploits this interpretation of Hmd reactivity by using the combination of Lewis basic ruthenium metalates, namely K[CpRu(CO)2] (KRp) and a related polymeric Cp/Ru/CO compound (Rs), with the new imidazolinium salt 1,3-bis(2,6-difluorophenyl)-2-(4-tolyl)imidazolinium bromide ([(Tol)Im(F4)](+)Br(-)) that was designed to emulate the hydride acceptor properties of methenyl-H4MPT(+). Solid-state structures of [(Tol)Im(F4)](+)Br(-) and the corresponding imidazolidine H(Tol)Im(F4) reveal that the heterocycle undergoes similar structural changes as in the biological substrate. DFT calculations indicate that heterolytic splitting of dihydrogen by the FLP Rp(-)/[(Tol)Im(F4)](+) is exothermic, but the formation of the initial Lewis pair should be unfavorable in polar solvents. Consequently the combination Rp(-)/[(Tol)Im(F4)](+) does not react with H2 but leads instead to side products from nucleophilic substitution (k = 4 × 10(-2) L mol (-1) s(-1) at room temperature). In contrast, the heterogeneous combination Rs/[(Tol)Im(F4)](+) does split H2 heterolytically to give H(Tol)Im(F4) and HRuCp(CO)2 (HRp) or D(Tol)Im(F4) and DRp when using D2. The reaction has been followed by (1)H/(2)H and (19)F NMR spectroscopy as well as by IR spectroscopy and reaches 96% conversion after 1 d. Formation of H(Tol)Im(F4) under these conditions demonstrates that superelectrophilic activation by protonation, which has been proposed for methenyl-H4MPT(+) to increase its carbocationic character, is not necessarily required for an imidazolinium ion to serve as a hydride acceptor. This unprecedented functional model for the [Fe] hydrogenase, using a Lewis acidic imidazolinium salt as a biomimetic hydride acceptor in combination with an organometallic Lewis base, may provide new inspiration for biomimetic H2 activation.

Biomimetic Hydrogenation Catalyzed by a Manganese Model of [Fe]‐Hydrogenase

“…Notably these two models contained a carbamoyl donor, instead of the acyl donor found in the enzyme. Among them, the neutral derivative 3 (Figure d) mediated the hydrogenation of 2,6‐difluoro(phenyl)‐2‐(4‐ethyl)imidazolium cation, a model of methenyl‐H 4 MPT + . Despite this progress, none of the previous Fe models achieved turnovers in hydrogenation reactions.…”

“…In this context, we examined the hydrogenation of substrates similar to the native substrate of [Fe]‐hydrogenase, namely, methenyl‐H 4 MPT + . The hydrogenation of the 1,3‐bis(2,6‐difluorophenyl)‐2‐(4‐tolyl)imidazolinium cation (in its bromide salt 9 a ), a previously reported model of methenyl‐H 4 MPT + , was first tested. After optimizing conditions (see Table S2 in the Supporting Information), we obtained a yield of 92 % for the hydrogenation of 9 a to generate 10 a using 2.5 mol % of 5 as catalyst (Scheme ).…”

Reaction Coordinate Leading to H2 Production in [FeFe]-Hydrogenase Identified by Nuclear Resonance Vibrational Spectroscopy and Density Functional Theory

“…The Fe p and Fe d sites of [2Fe] H , respectively proximal and distal to [4Fe-4S] H , are bridged by a CO ligand and an azadithiolate (ADT) cofactor, the latter also providing the nitrogen base of the frustrated Lewis pair crucial to H 2 conversion processes. 4-6 Fe d , the site at which hydrogen binds, is the Lewis acid. 7,8 Many previous experimental and theoretical studies 9-12 implicate a Fe d -H h ⋯H A -N ADT species as the key intermediate leading to H-H bond formation (Figure 1A).…”

Solution Chemistry of N,N’‐Disubstituted Amidines: Identification of Isomers and Evidence for Linear Dimer Formation

“…[6,10,[32][33][34][35][36][37]41] Due to the large number of possible isomersa nd aggregates,i ti sg enerally difficult to predict the structures of amidines in solution. [13,43,44] As parto fo ur development of imidazoliniums alts that serve as surrogates for the methenyltetrahydromethanopterin substrate in [Fe] hydrogenase modelling, [45] we synthesized anumber of new amidines that are precursors to the imidazolinium salts. Historically,t his problem wasa ddressed by means of IR spectroscopy, [16,42] which turned out to be ap owerful tool to investigate molecular association through hydrogen bonding of amidines in solution, as well as tautomerism.…”