Biomimics of [FeFe]-hydrogenases incorporating redox-active ligands: Ferrocene-bridged dithiolate complexes [Fe2(CO)6(μ-EC5H4FeC5H4E)] (E = S, Se)

Abstract

Ferrocene-bridged dithiolate complexes [Fe2(CO)6(μ-EC5H4FeC5H4E)] (E = S, Se) (1-2) have been studied as biomimics of [FeFe]-hydrogenases, ferrocene taking the role of an Fe4S4 surrogate. Both show a quasi-reversible electron reduction at E1/2 = -1.81 V and -1.56 V respectively with IR SEC showing that it is Fe2-based (ca. 50 cm−1 hypsochromic shift) and leads to a significant structural rearrangement with formation of a bridging carbonyl being at ca. 1715 cm−1. They also show reversible one-electron oxidation with a small (ca. 15 cm−1) hypsochromic ν(CO) shift, showing it is ferrocene-based, while oxidation of the Fe2 centre occurs outside of the potential window. DFT calculations on 1 show that the HOMO is ferrocene-based, and the LUMO Fe2-based, both in accord with the IR SEC results. However, the radical anion [1]− is calculated to have an elongated metal-metal vector rather than a bridging carbonyl, although a complex with an open structure [1(μ-CO)]− in which one thiolate group moves from a bridging to a terminal position is energetically similar. For 2, DFT shows the HOMO is Fe2-based, which is not supported experimentally. Complexes 1-2 are proton-reduction catalysts at their first reduction potential in the presence of trifluoroacetic acid. DFT suggests two competing pathways for H2 generation catalysed by 1, occurring upon protonation of either [1]− or [1(μ-CO)]− to give bridging and terminal hydride complexes respectively. For 2, DFT suggests that [2(μ-CO)]− is only slightly higher in energy than [2]−, probably a result of the weaker Fe-Se vs Fe-S bond, and thus proton-reduction likely proceeds (primarily) through the open-structure isomer.