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

Abstract

The present paper reports a study on the energetics of protonation of a hydrogenase biomimetic complex, [Fe 2 (μ-adt)(CO) 4 (PMe 3 ) 2 ] (adt = Nbenzylazadithiolate), 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 PPh 3 -bearing organometallic complex is highly favoured when compared to the analogous reaction involving the PMe 3 -containing species, but the trend is inverted

How general is the effect of the bulkiness of organic ligands on the basicity of metal–organic catalysts? H2-evolving Mo oxides/sulphides as case studies

“…Deviations of atomic charges turned out to be small (maximum deviation 0.03 e both for the oxo and the sulfide species). Such picture is analogous to what was found in the above-mentioned study on [FeFe]-hydrogenase biomimetic models 11 whose atomic charges varied only slightly as a function of the presence of phosphine ligands of different size and nature (PCH 3 , PPh 3 ).…”

“…The design of new metalorganic complexes for catalytic purposes requires careful consideration of electronic and steric effects in order to tune the activity of such species. In a previous theoretical study of organometallic diiron catalysts for proton reduction, 11 we showed that protonation of Fe ions in complexes bearing bulky and strongly electron-donating PPh 3 ligands is highly favoured compared to the analogous reaction involving the PMe 3containing species when studied in a vacuum. However, the trend inverted when the models instead are studied in a continuum solvent.…”

“…In fact, a comparison between protonation of the complexes considered in the present study and terminal protonation of dinuclear iron complexes in ref. 11 highlights a peculiar fact, namely that complexes with ligands featuring markedly different electronic properties (N-heterocycles/oxo vs. phosphines) and different protonation regiochemistry have, in vacuo, a similar variation in the protonation energies as a function of the increase in the bulkiness of ligands (the bulkier systems show a proton affinity larger than the small ones by about 12 kcal/mol in both cases). Despite the fact that in the previous work, the solvent had a dielectric constant smaller than that considered in the present work (acetonitrile vs water), for the iron complexes we even observed an inversion of the protonation energies going from vacuum to solvent 11 as also noticed above.…”

“…In a previous study, 11 we investigated the effects of phosphine ligand bulkiness on metal protonation in the case of biomimetic models of [FeFe]-hydrogenases. 12,13 By means of density functional theory (DFT) calculations, we demonstrated that differences in reactivity toward metal protonation between a biomimetic diiron catalyst [Fe 2 ((m-adt)(CO) 4 (PMe 3 ) 2 ] and its bulkier counterpart [Fe 2 (m-adt)(CO) 4 (PPh 3 ) 2 ] (adt = N-benzylazadithiolate) can be rationalized based on differences in the overall electrostatic properties of the complexes, which acquire key relevance upon changing the nature of the surrounding medium (vacuum vs. acetonitrile).…”

The Challenging in silico Description of Carbon Monoxide Oxidation as Catalyzed by Molybdenum-Copper CO Dehydrogenase

“…Such a study mainly regards the hydrogenase activity of the enzyme and includes a comparative analysis of the binding reactions of the physiologic substrate—i.e., CO—and of dihydrogen to the Cu ion. Similarly to previous studies on H 2 - and CO-binding enzyme models in which a pure functional was used in conjuction with triple-zeta bases (Greco et al, 2015; Rovaletti and Greco, 2018), geometry optimizations and energy calculations were carried out in vacuo at BP86/def2-TZVP level (Perdew, 1986; Becke, 1988; Weigend and Ahlrichs, 2005). As far as the energetics of CO binding is concerned, the computed ΔE was as negative as −64 kJ/mol.…”