Electrochemical Insights into the Mechanisms of Proton Reduction by [Fe₂(CO)₆{μ‐SCH₂N(R)CH₂S}] Complexes Related to the [2Fe]_H Subsite of [FeFe]Hydrogenase

Abstract

Electrochemical investigations on a structural analogue of the [2Fe](H) subsite of [FeFe]H(2)ases, namely, [Fe(2)(CO)(6){micro-SCH(2)N(CH(2)CH(2)- OCH(3))CH(2)S}] (1), were conducted in MeCN/NBu(4)PF(6) in the presence of HBF(4)/Et(2)O or HOTs. Two different catalytic proton reduction processes operate, depending on the strength and the concentration of the acid used. The first process, which takes place around -1.2 V for both HBF(4)/Et(2)O and HOTs, is limited by the slow release of H(2) from the product of the {2 H(+)/2 e} pathway, 1-2H. The second catalytic process, which occurs at higher acid concentrations, takes place at different potentials depending on the acid present. We propose that this second mechanism is initiated by protonation of 1-2H when HBF(4)/Et(2)O is used, whereas the reduction of 1-2H is the initial step in the presence of the weaker acid HOTs. The potential of the second process, which occurs around -1.4 V (reduction potential of 1-3H(+)) or around -1.6 V (the reduction potential of 1-2H) is thus dependent on the strength of the available proton source.

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

“…This shift is even greater than that reported for diprotonated adt [2Fe2S] model complexes (usually approximately 690 mV). [14][15][16] The first reduction peak appears at -0.98 V, which is quite close to the recently reported value for the triiron all carbonyl cluster (-1.03 V) [17] and also very close to the reduction potentials observed for [FeFe] hydrogenases in nature (-0.8 to -1.1 V, -0.4 to -0.7 V vs. NHE). [18] At this potential, however, the CV does not indicate any electrochemical activity upon addition of HOTf.…”

XAFS and DFT Characterisation of Protonated Reduced Fe Hydrogenase Analogues and Their Implications for Electrocatalytic Proton Reduction

“…As has recently been noted, [26] a characteristic of electrochemical simulations is that for favourable equilibria the computed response is highly sensitive to the kinetics but insensitive to the magnitude of the equilibrium constants. For example, the K b values of 2S 1-and 2S-H 1-used for electrochemical modelling were 10 and 100, respectively, which are significantly lower than the calculated values (10 7 and 10 6 ).…”

“…In this context, the close agreement between the rates of dihydrogen elimination from 2S, 3S and 3P is surprising, a point amplified by modelling of strong acid electrocatalysis by [Fe(CO) 3 ] 2 -{μ-SCH 2 N(CH 2 CH 2 OCH 3 )CH 2 S} (3SN). [26] While the potential of the electrocatalytic reaction is affected by protonation of the azadithiolate bridge, the rate of dihydrogen elimination from 3SN-H 2 is rate-limiting and is estimated to be 3 s -1 , i.e. similar to 2S, 3S and 3P.…”

Electrochemical and Computational Study of the Reactivity of a Diiron Azadithiolate Complex towards Protons in the Presence of Coordinating Anions

“…The reduction of 1-NH at E red,3 = -1.50 V increases the pK a of the metal center, so that moderately strong acids are able to protonate 1-NH -at the Fe-Fe bond. This reaction probably produces a μ-hydride intermediate, namely 3-H-NH, as previously proposed for other structurally related diiron azadithiolate complexes, [8,18,30,31,38] but we lack spectroscopic data to definitely rule out the formation of a terminal hydride at least transiently. In any case, it is most likely that 3-H-NH is reducible at a potential less negative than 1-NH.…”

“…[29] Moreover, a voltammetric response similar to that observed in the presence of HCl (pK a ≈ 9.5) has been reported for the structurally related complex [Fe 2 (μ-adt R )(CO) 6 ] (R = C 2 H 4 OCH 3 ) in the presence of HBF 4 ·Et 2 O (pK a ≈ 3). [38] We are thus left with an effect of the coordinating anion, which would be Cl -here. However, as suggested by one of the referees, an effect of Et 2 O added in large amount with both HCl and HBF 4 cannot be excluded, because this molecule can engage in H-bonding with the adt group that might also impede the tautomerization reaction.…”