Organophosphorous ligands in hydrogenase-inspired iron-based catalysts: A DFT study on the energetics of metal protonation as a function of P-atom substitution

The present paper reports a study on the energetics of protonation of a hydrogenase biomimetic complex, [Fe2(μ-adt)(CO)4(PMe3)2] (adt = N-benzylazadithiolate), and of its homologue featuring triphenylphosphine ligands in place of trimethylphosphines. Formation of a terminal hydride on one of the Fe centres was considered first, given the key relevance of terminal hydride species in the enzymatic mechanism. Theoretical calculations highlight that, in a vacuum, terminal protonation of the selected Fe ion in the PPh3-bearing organometallic complex is highly favoured when compared to the analogous reaction involving the PMe3-containing species, but the trend is inverted in the case of models optimized in a continuum polarizable. An unexpected parallel is thus established between relative basicities of PPh3 and PMe3 in vacuum or in solution phase [C. A. Tolman, J. Am. Chem. Soc. 1970, 92, 2953; G. M. Bancroft, Inorg. Chem. 1986, 25, 3675], and the energetics of terminal hydride formation upon protonation of [FeFe]-hydrogenase biomimetic complexes bearing such organophosphorous ligands. Bridging hydride formation was also considered in the present study: calculations showed that protonation of the PMe3-bearing organometallic complex is again strongly favoured in vacuo, as compared to the case of the PPh3-containing model. However, protonation energies become significantly smaller when solvent effects are taken into account. Such differences between protonation reactions modelled in vacuo and in the polarizable continuum are rationalized in light of the different electrostatic properties of the diiron complexes here considered. Implications for the design and modelling of biomimetic catalysts are briefly discussed in light of recent literature