Carbonyl Diiron Azadithiolate Complexes with Azadiphosphine Chelates Related to the Active Site of [FeFe]-Hydrogenases: Protonation and Redox Behaviour

Bimetallic systems with several protonation sites are used for modelling the activity of polymetallic active sites such as the H-cluster of [FeFe]-hydrogenases or nitrogenases. In order to examine the behaviours of multi-proton sites dinuclear systems, the syntheses of the complexes [Fe2(CO)4(k2-PPh2NR2)(μ-adtBn)] (PPh2NR2 = {PhPCH2NRCH2}2, R = Bn (1), Ph (2); adtBn = {SCH2}2NCH2Ph) were carried out by reacting the hexacarbonyl precursor [Fe2(CO)6(µ-adtBn)] with cyclic diphosphines PPh2NR2. Investigation of the reactivity towards proton (CH3SO3H/CF3SO3H) and of the redox properties of these complexes featuring several amine functions in proximity of a diiron site was performed using spectroscopic and electrochemical methods. The experimental results were rationalized by DFT calculations. The protonation sites in such clusters are discussed on the basis of the strength of the acid and the R groups carried by the diphosphine PPh2NR2. The similarities and differences with analogues [Fe2(CO)4(k2-PPh2NR2)(μ-pdt)] and [Fe3(CO)5(k2-PPh2NR2)(μ-adtBn)(µ-pdt)] are highlighted. The main results show that protonations depend on the substituted group R of the PPh2NR2 chelate and of the strength of the acid used (CH3SO3H vs. CF3SO3H). With CH3SO3H, complexes are protonated once, at the PPh2N2Bn diphosphine in 1 and at the diiron site in 2. This latter observation suggests that the coordination of the amine-functionalized-diphosphine PPh2NPh2 assists a competitive pathway for the protonation of the diiron site versus NBn group of the adtBn group. With CF3SO3H, the protonation of two NBn sites is observed in 1, which shows that the protonation of the diiron core bearing an azadithiolate bridge is unfavoured. In 2, the protonation with CF3SO3H arises at the diiron site or the adtBn bridge. This shows the stabilization of the N-protonated forms when a fluorinated counter-anion, such as CF3SO3−, is used.