Unsaturated, Mixed‐Valence Diiron Dithiolate Model for the H_ox State of the [FeFe] Hydrogenase

Abstract

The [FeFe] hydrogenase enzymes are the most efficient catalysts known for the reduction of protons to H 2 . [1] The active site exists in two functional states (Scheme 1), H red , which is S =0, and H ox , which is S = 1/2. [2] Research in this area is aimed at elucidating the mechanism of the enzymatic catalysis and at using this information to develop protein-free bioinspired synthetic catalysts. [3] A specific research goal is the preparation of molecules that resemble the functional states of the active site with the expectation that function will follow form. Most studies on diiron dithiolato carbonyl complexes rely on organic ligands (e.g. phosphanes) in place of the naturally occurring cyanide and μ-SR[Fe 4 S 4 ] ligands, [4] which have complicated acid-base behavior that is often difficult to control outside of the protein. Another barrier to modeling has been the rarity of mixed-valence diiron dithiolate compounds with the appropriate structures, stability, and reactivity.The first evidence for mixed valency in diiron dithiolate models was obtained in the oneelectron oxidation of [Fe 2 {(SCH 2 ) 2 CMeCH 2 SMe}(CN) 2 (CO) 4 ] 2− , which afforded a thermally sensitive mixed-valence derivative with IR and EPR spectroscopy signatures resembling those for the CO-inhibited enzyme. [5] In very recent work, the oxidation of [Fe 2 (S 2 C 3 H 6 )(CO) 4 (PMe 3 )L 1 ] (L 1 =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) was shown to give a mixed-valence derivative with structural and spectroscopic features resembling H ox . [6] Our recently reported species [Fe 2 (S 2 C 2 H 4 )(CO) 3 (PMe 3 )-(dppv)] (1, dppv =cis-1,2-C 2 H 2 (PPh 2 ) 2 ) is attractive, because, like the active site, this diiron framework bears three donor ligands and three CO ligands. [7] We previously showed that oxidation of 1 in MeCN gives [1 (NCMe)] 2+ . In MeCN solution, one equivalent of oxidizing agent gives an approximately 1:1 mixture of unreacted starting material and [1(NCMe)] 2+ . Oxidation of 1 in the weakly coordinating solvent CH 2 Cl 2 , however, proceeds quite differently.Addition of one equivalent of FcBF 4 (Fc + =[Fe(C 5 H 5 ) 2 ] + ) to a CH 2 Cl 2 solution of 1 at −45 °C resulted in complete consumption of the diiron complex, as indicated by in situ IR spectroscopy. We probed the oxidation of 1 by cyclic voltammetry on a solution of 1 in 0.1m

Computational definition of a mixed valent Fe(II)Fe(I) model of the [FeFe]hydrogenase active site resting state

“…Preliminary studies find rapid and regioselective incorporation of 13 CO with 1 + , even at À78°C, in contrast to the inertness of analogous reactions with 1. The rapid irreversible binding of NO to [(PMe 3 )(CO) 2 Fe(l-edt)(l-CO)(dppv)] + (dppv = cis-1,2-C 2 H 2 (PPh 2 ) 2 , edt = 1,2-ethanedithiolate) reported by Rauchfuss and coworkers is consistent with this prediction, if we assume a similar oxidation state assignment for this phosphorus donor-rich mixed valent species [13]. Preliminary calculations examining the binding of substrates to the vacant coordination site in 1 + indicate that reactivity with radicals, particularly H atoms, is reasonable.…”

“…This so-called ''entatic'' structure appears to create an open site on the rotated iron, while achieving minimal octahedral coordination through a semi-bridging carbonyl ligand on the second, unrotated iron. Rauchfuss and coworkers have skillfully managed to produce the semibridging carbonyl ligand in Fe II Fe II and Fe I Fe I complexes, however until recently mixed valent Fe II Fe I species sufficiently stable for full characterization have eluded synthesis [9][10][11][12][13]. In our laboratory, diiron complexes with N-heterocyclic carbene (NHC) ligands have yielded to this goal; the NHC derivatives are shown in Scheme 1 [12].…”

Sulfoxygenation of Active Site Models of [NiFe] and [FeFe] Hydrogenases – A Commentary on Possible Chemical Models of Hydrogenase Enzyme Oxygen Sensitivity

“…[44] Only a handful of [FeFe]H 2 ase active site models have achieved the "rotated" or "inverted" structure and the mixed Fe I Fe II valency of the as-isolated enzyme active site ( Figure 9). [45][46][47][48][49] These are found to be very fragile com- pounds that are both thermally sensitive and highly susceptible to O 2 degradation; thus far, products from exposure to O 2 have not been identified. Nevertheless, as found for the nickel dithiolates described above, products of oxygen atom addition are found for S-and Se-containing analogues of the (μ-SRS)Fe I Fe I -CO complexes.…”

Synthesis of a [3Fe2S] Cluster with Low Redox Potential from [2Fe2S] Hydrogenase Models: Electrochemical and Photochemical Generation of Hydrogen

“…[7] A trinuclear tris(triphenylphosphane) complex has also recently been prepared in approximately 30 % yield by the treatment of disulfide 6c with LiEt 3 BH in the presence of triphenylphosphane and CuCl·PPh 3 or AgNO 3 ·PPh 3 . [8] On protonation [9][10][11][12][13] with excess triflic acid in CH 2 Cl 2 solution, complex 2 gave the bridged hydride species 3. Complexes 2 and 3 were characterized by IR and NMR spectroscopy and CV studies.…”