Proton reduction by phosphinidene-capped triiron clusters

Bis(phosphinidene)-capped triiron carbonyl clusters, including electron rich derivatives formed by substitution with chelating diphosphines, have been prepared and examined as proton reduction catalysts. Treatment of the known cluster [Fe-3(CO)(9)(mu(3)-PPh)(2)] (1) with various diphosphines in refluxing THF (for 5, refluxing toluene) afforded the new clusters [Fe-3(CO)(7)(mu(3)-PPh)(2)(kappa(2)-dppb)] (2), [Fe-3(CO)(7)(mu(3)-PPh) (2)(kappa(2)-dppv)] (3), [Fe-3(CO)(7)(mu(3)-PPh)(2)(kappa(2)-dppe)] (4) and [Fe-3(CO)(7)(mu(3)-PPh)(2)(mu-kappa(2)-dppf)] (5) in moderate yields, together with small amounts of the corresponding [Fe-3(CO)(8)(mu(3)-PPh)(2) (kappa(1)-Ph(2)PxPPh(2))] cluster (x = -C4H6-, -C2H2-, -C2H4-, -C3H6-, -C5H4FeC5H4-). The molecular structures of complexes 3 and 5 have been established by X-ray crystallography. Complexes 1-5 have been examined as proton reduction catalysts in the presence of p-toluenesulfonic acid (p-TsOH) in CH2Cl2. Cluster 1 exhibits two one-electron quasi-reversible reduction waves at -1.39 V (Delta E = 195 mV) and at -1.66 V (Delta E = 168 mV; potentials vs. Fc(+)/Fc). Upon addition of p-TsOH the unsubstituted cluster 1 shows a first catalytic wave at -1.57 V and two further proton reduction processes at -1.75 and -2.29 V, each with a good current response. The diphosphine-substituted derivatives of 1 are reduced at more negative potentials than the parent cluster 1. Clusters 2-4 each exhibit an oxidation at ca. +0.1 V and a reduction at ca. -1.6 V; for 4 conversion to a redox active successor species is seen upon both oxidation and reduction. Clusters 2-4 show catalytic waves in the presence of p-TsOH, with cluster 4 exhibiting the highest relative catalytic current (i(cat)/i(0) approximate to 57) in the presence of acid, albeit at a new third reduction process not observed for 2 and 3. Addition of the dppf ligand to the parent diphosphinidene cluster 1 gave cluster 5 which exhibited a single reduction process at -1.95 V and three oxidation processes, all at positive values as compared to 2-4. Cluster 5 showed only weak catalytic activity for proton reduction with p-TsOH. The bonding in 4 was investigated by DFT calculations, and the nature of the radical anion and dianion is discussed with respect to the electrochemical data. (C) 2021 The Author(s). Published by Elsevier B.V.