Models of the iron-only hydrogenase: a comparison of chelate and bridge isomers of Fe2(CO)4{Ph2PN(R)PPh2}(μ-pdt) as proton-reduction catalysts

Abstract

Reactions of Fe₂(CO)₆(μ-pdt) (pdt = SCH₂CH₂CH₂S) with aminodiphosphines Ph₂PN(R)PPh₂ (R = allyl, ⁱPr, ⁱBu, p-tolyl, H) have been carried out under different conditions. At room temperature in MeCN with added Me₃NO·2H₂O, dibasal chelate complexes Fe₂(CO)₄{κ²-Ph₂PN(R)PPh₂}(μ-pdt) are formed, while in refluxing toluene bridge isomers Fe₂(CO)₄{μ-Ph₂PN(R)PPh₂}(μ-pdt) are the major products. Separate studies have shown that chelate complexes convert to the bridge isomers at higher temperatures. Two pairs of bridge and chelate isomers (R = allyl, ⁱPr) have been crystallographically characterised together with Fe₂(CO)₄{μ-Ph₂PN(H)PPh₂}(μ-pdt). Chelate complexes adopt the dibasal diphosphine arrangement in the solid state and exhibit very small P–Fe–P bite-angles, while the bridge complexes adopt the expected cisoid dibasal geometry. Density functional calculations have been carried out on the chelate and bridge isomers of the model compound Fe₂(CO)₄{Ph₂PN(Me)PPh₂}(μ-pdt) and reveal that the bridge isomer is thermodynamically favourable relative to the chelate isomers that are isoenergetic. The HOMO in each of the three isomers exhibits significant metal–metal bonding character, supporting a site-specific protonation of the iron–iron bond upon treatment with acid. Addition of HBF₄·Et₂O to the Fe₂(CO)₄{κ²-Ph₂PN(allyl)PPh₂}(μ-pdt) results in the clean formation of the corresponding dibasal hydride complex [Fe₂(CO)₄{κ²-Ph₂PN(allyl)PPh₂}(μ-H)(μ-pdt)][BF₄], with spectroscopic measurements revealing the intermediate formation of a basal–apical isomer. A crystallographic study reveals that there are only very small metric changes upon protonation. In contrast, the bridge isomers react more slowly to form unstable species that cannot be isolated. Electrochemical and electrocatalysis studies have been carried out on the isomers of Fe₂(CO)₄{Ph₂PN(allyl)PPh₂}(μ-pdt). Electron accession is predicted to occur at an orbital that is anti-bonding with respect to the two metal centres based on the DFT calculations. The LUMO in the isomeric model compounds is similar in nature and is best described as an antibonding Fe–Fe interaction that contains differing amounts of aryl π* contributions from the ancillary PNP ligand. The proton reduction catalysis observed under electrochemical conditions at ca. −1.55 V is discussed as a function of the initial isomer and a mechanism that involves an initial protonation step involving the iron–iron bond. The measured CV currents were higher at this potential for the chelating complex, indicating faster turnover. Digital simulations showed that the faster rate of catalysis of the chelating complex can be traced to its greater propensity for protonation. This supports the theory that asymmetric distribution of electron density along the iron–iron bond leads to faster catalysis for models of the Fe–Fe hydrogenase active site.