A Functional [NiFe]-Hydrogenase Model Compound That Undergoes Biologically Relevant Reversible Thiolate Protonation

Abstract

Two model compounds of the active site of [NiFe]-hydrogenases with an unusual {S(2)Ni(μ-S)(μ-CO)Fe(CO)(2)S}-coordination environment around the metals are reported. The neutral compound [Ni(xbsms)(μ-CO)(μ-S)Fe(CO)(2)('S')], (1) (H(2)xbsms = 1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene) is converted to [1H][BF(4)] by reversible protonation using HBF(4)·Et(2)O. The protonation takes place at the terminal thiolate sulfur atom that is coordinated to nickel. Catalytic intermediates with a protonated terminal cysteinate were suggested for the native protein but have not yet been confirmed experimentally. [1H][BF(4)] is the first dinuclear [NiFe] model compound for such a species. Both complexes have been synthesized and characterized by X-ray crystallography, NMR-, FTIR-, and (57)Fe-Mössbauer spectroscopy as well as by electronic absorption and resonance Raman spectroscopy. The experimental results clearly show that the protonation has a significant impact on the electronic structure of the iron center, although it takes place at the nickel site. DFT calculations support the interpretation of the spectroscopic data and indicate the presence of a bonding interaction between the metal ions, which is relevant for the enzyme as well. Electrochemical experiments show that both 1 and [1H][BF(4)] are active for electrocatalytic proton reduction in aprotic solvents.

Novel reactions of homodinuclear Ni2complexes [Ni(RNPyS4)]2with Fe3(CO)12to give heterotrinuclear NiFe2and mononuclear Fe complexes relevant to [NiFe]- and [Fe]-hydrogenases

“…Obviously, the TON and TOF values for 2a-2e are much higher than those previously reported for the H 2 production from TFA catalyzed by the [NiFe]-hydrogenase model complex NiFe 2 (MeC 6 H 3 S 2 ) 2 (CH 2 ) 3 (CO) 6 60 (TON = 6, TOF = 6 h −1 ) and (H 2 xbsms = 1,2-bis(4-mercapto-3,3′-dimethyl-2-thiobutyl) benzene) (TON = 20, TOF = 5 h −1 ). 61 In addition, the overpotentials for 2a-2e in MeCN with TFA were also obtained using Evans' relationship 61,62 (see Table 5 and Fig. S5 and S6 in the ESI †), which are slightly lower than that (540 mV) for the aforementioned Lubitz complex.…”

Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site

“…Replicating terminal thiolate ligation at nickel remains one of the central challenges in this area. 50 …”

“…16 The structural difference between the model complex and the protein may reflect the influence of terminal thiolate ligands on nickel. 50 …”

Proposed Ligand-Centered Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation at a Noninnocent Mononuclear Metal–Thiolate

“…[10][11][12][13][14][15] Despite these advances, few H 2 oxidation catalysts are known and even fewer complexes catalyze the reaction in both directions. 16,17 In this manuscript, we report electrocatalytic H 2 evolution and H 2 oxidation using the non-innocent mononuclear metal-thiolate complex ReL 3 (L = diphenylphosphinobenzenethiolate). ReL 3 catalytically oxidizes H 2 in the presence of base at potentials greater than 0.42 V, as well as catalytically evolving H 2 from acid at potentials less than -1.60 V, Scheme 1.…”