Biomimics of [FeFe]-hydrogenases with a pendant amine: Diphosphine complexes [Fe2(CO)4{?-S(CH2)nS}{?2-(Ph2PCH2)2NR}] (n=2, 3; R = Me, Bn) towards H 2 oxidation catalysts

We report the synthesis and molecular structures of [FeFe]-ase biomimics [Fe2(CO)4{mu-S(CH2)nS}{kappa 2- (Ph2PCH2)2NR}] ( 1-4 ) ( n = 2, 3; R = Me, Bn) and a comparative study of their protonation and re-dox chemistry, with the aim of assessing their activity as catalysts for H 2 oxidation. They are prepared in good yields upon heating the hexacarbonyls and PCNCP ligands in toluene, a minor product of one reaction ( n = 3, R = Bn) being pentacarbonyl [Fe2(CO)5(mu-pdt){Ph2PCH2N(H)Bn}] ( 5 ). Crystal structures show short Fe-Fe bonds (ca. 2.54 angstrom) with the diphosphine occupying basal-apical sites. Each undergoes a quasi-reversible one-electron oxidation and IR-SEC shows that this results in formation of a semi-bridging carbonyl. As has previously been observed, protonation products are solvent dependent, nitrogen being the favoured site of protonation site upon addition of one equivalent of HBF4.Et2O in d6-acetone, while hydride formation is favoured in CD2Cl2. However, the rate of N to Fe2 proton-transfer varies greatly with the nature of both the dithiolate-bridge and amine-substituent. Thus with NMe complexes ( 1-2 ) N-protonation is favoured in acetone affording a mixture of endo and exo isomers, while for NBn complexes ( 3-4 ) proton-transfer to afford the corresponding mu-hydride occurs in part (for 3 edt) or exclusively (for 4 pdt). In acetone, addition of a further equivalent of HBF4.Et2O generally does not lead to hydride for-mation, but in CD2Cl2 dications [Fe2(CO)4{mu-S(CH2)nS}(mu-H){kappa 2-(Ph2PCH2)2NHR}]2 + result, in which the diphosphine can adopt either dibasal or basal-apical positions. Proton-transfer from Fe2 to N has been previously identified as a required transformation for H 2 oxidation, as has the accessibility of the all-terminal carbonyl isomer of cations [Fe2(CO) 4{mu-S(CH2) nS}{kappa 2-(Ph2PCH2)2NR}] +. We have carried out a preliminary H 2 oxidation study of 3, oxidation by Fc[BF4] in the presence of excess P(o-tolyl)3 affording [HP(o-tol)3][BF4], with a turnover of ca. 2.3 +/- 0.1 mol of H 2 consumed per mole of 3 (c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )