Iron Carbonyl Clusters as Proton Reduction Catalysts

Abstract: Abstract ? The mixed-valence triiron complexes [Fe₃(CO)_7-x(PPh₃)_x(µ-edt)₂] (x = 0, 1, 2; edt = SCH₂CH₂S) and [Fe₃(CO)₅(?²-diphosphine)(µ-edt)₂] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. In comparison to the diiron complex [Fe₂(CO)₆(µ-edt)], [Fe₃(CO)₇(µ-edt)₂] catalyzes proton reduction at 0.36 V less negative potentials, which is a significant energetic gain. In all complexes the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semi-bridging carbonyl, while the LUMO is highly delocalised in nature and is anti-bonding between both pairs of iron atoms but also contains an anti-bonding dithiolate interaction. The clusters [Fe₃(CO)₉(?₃-E)₂] (E = S, Se, Te), [Fe₃(CO)₇(?₃-E)₂(?- ?²-diphosphine)] (E = S, Se, Te), [Fe₃(CO)₇(?₃-CO)(?₃-E)(?-dppm)] (E = S, Se) and [Fe₃(CO)₈(µ₃-Te)₂(?²-diphosphine)] have been prepared and examined as proton reduction catalysts. The reduction potentials for the tellurium-capped clusters occur at lower potentials than for their sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues. The 52-electron clusters [Fe₃(CO)₈(µ₃-Te)₂(?²-diphosphine)] consist of Fe2(CO)6(µ-Te)2 ?butterfly? units that are capped by a Fe(CO)₂(?²-diphosphine) moiety. Cyclic voltammetry studies reveal that their redox behaviour and properties as proton reduction catalysts largely stem from the Fe₂(CO)₆(µ-Te)₂ entities, although computational modelling indicates that their LUMOs are centered on the briding tellurium ions and the unique ?capping? iron ion. The influence of the substitution, orientation and structure of the phosphido bridges on the electrochemical and electrocatalytic properties of [Fe₂(CO)₆(µ-phosphido)₂] clusters and bis(phosphinidene)-capped triiron carbonyl clusters, including electron-rich derivatives formed by substitution with chelating diphosphines, have been studied. The electrochemistry and electrocatalyses of the [Fe₂(CO)₆(µ-PR₂)₂] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms. The reduction potential of the phosphinidene- capped clusters shift negative way to increase the electron density on the iron centers. Abstract ? The mixed-valence triiron complexes [Fe₃(CO)_7-x(PPh₃)_x(µ-edt)₂] (x = 0, 1, 2; edt = SCH₂CH₂S) and [Fe₃(CO)₅(?²-diphosphine)(µ-edt)₂] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. In comparison to the diiron complex [Fe₂(CO)₆(µ-edt)], [Fe₃(CO)₇(µ-edt)₂] catalyzes proton reduction at 0.36 V less negative potentials, which is a significant energetic gain. In all complexes the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semi-bridging carbonyl, while the LUMO is highly delocalised in nature and is anti-bonding between both pairs of iron atoms but also contains an anti-bonding dithiolate interaction. The clusters [Fe₃(CO)₉(?₃-E)₂] (E = S, Se, Te), [Fe₃(CO)₇(?₃-E)₂(?- ?²-diphosphine)] (E = S, Se, Te), [Fe₃(CO)₇(?₃-CO)(?₃-E)(?-dppm)] (E = S, Se) and [Fe₃(CO)₈(µ₃-Te)₂(?²-diphosphine)] have been prepared and examined as proton reduction catalysts. The reduction potentials for the tellurium-capped clusters occur at lower potentials than for their sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues. The 52-electron clusters [Fe₃(CO)₈(µ₃-Te)₂(?²-diphosphine)] consist of Fe2(CO)6(µ-Te)2 ?butterfly? units that are capped by a Fe(CO)₂(?²-diphosphine) moiety. Cyclic voltammetry studies reveal that their redox behaviour and properties as proton reduction catalysts largely stem from the Fe₂(CO)₆(µ-Te)₂ entities, although computational modelling indicates that their LUMOs are centered on the briding tellurium ions and the unique ?capping? iron ion. The influence of the substitution, orientation and structure of the phosphido bridges on the electrochemical and electrocatalytic properties of [Fe₂(CO)₆(µ-phosphido)₂] clusters and bis(phosphinidene)-capped triiron carbonyl clusters, including electron-rich derivatives formed by substitution with chelating diphosphines, have been studied. The electrochemistry and electrocatalyses of the [Fe₂(CO)₆(µ-PR₂)₂] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms. The reduction potential of the phosphinidene- capped clusters shift negative way to increase the electron density on the iron centers.