Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe₃(CO)₇(μ-edt)₂] and Phosphine Derivatives [Fe₃(CO)_7–x(PPh₃)_x(μ-edt)₂] (x = 1, 2) and [Fe₃(CO)₅(κ²-diphosphine)(μ-edt)₂] as Proton Reduction Catalysts

Abstract

The mixed-valence triiron complexes [Fe3(CO)7–x(PPh3)x(μ-edt)2] (x = 0–2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(μ-edt)2]+ and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2]+, species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)–Fe(II)–Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(μ-edt)2] is oxidized at −0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between −1.47 V for [Fe3(CO)7(μ-edt)2] and around −1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. 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. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron–iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.

New Systematic Route to Mixed-Valence Triiron Clusters Derived from Dinuclear Models of the Active Site of [Fe–Fe]-Hydrogenases

“…Theoretical calculations using density functional theory (level of theory B3LYP/TZVP; see Methods in the Supporting Information for details) were also performed. An analysis of the electronic structure reveals that the nature of the HOMO and the LUMO is similar to that reported previously for similar trinuclear compounds (Figure ). The HOMO can be described as a Fe–Fe bonding orbital involving the iron atoms not coordinated to the dppe ligand.…”

“…The structural determination of [Fe–Fe]-hydrogenases in the late 1990s has led during the last 15 years to a deep reinvestigation and development of the pioneering chemistry on iron carbonyl thiolate complexes developed in the 1980s . The use of well-known organometallic tools has allowed the reproduction of some chemical and structural precedents of the natural site which have contributed to a better understanding of the chemistry of the H cluster. , During the course of these studies very original side products featuring quasi-linear tri- and tetranuclear arrangements have been reported. − Even if these novel polynuclear species do not model directly the H cluster, their chemistry is related. For example, it has been shown that the reduced form of the mixed-valence tetrairon cluster [Fe 4 (CO) 8 {μ 3 -(SCH 2 ) 3 CMe} 2 ], featuring a Fe I Fe II Fe II Fe I core, may be a better catalyst than the related biomimetic hexacarbonyl diiron dithiolate complexes .…”

“…The main problem in studying such products, often obtained in low yields by serendipity, is to rationalize their synthesis. Very recent works reported by Hogarth and co-workers concerning the study of the complexes [Fe 3 (CO) 4 L(κ 2 -diphosphine)(μ-edt) 2 ] (edt = ethanedithiolate, L = PPh 3 ), obtained by using [Fe 3 (CO) 7 (μ-edt) 2 ] as a precursor and diphosphine, prompted us to report our results concerning the systematic synthesis of species of the general formula [Fe 3 (CO) 5 (κ 2 -dppe)(μ-pdt)(μ-xdt)] ( 1 , xdt = propanedithiolate (pdt); 2 , xdt = azadithiolate (adt Bn ) with adt Bn = {SCH 2 } 2 NBn ( 2 )). Preliminary results concerning the electrochemical behavior of [Fe 3 (CO) 5 (κ 2 -dppe)(μ-pdt) 2 ] ( 1 ), in the absence and the presence of acid, are also presented as well as Mössbauer measurements and calculations on 1 .…”

“…This might suggest that the semibridging carbonyl hinders the migration of one end of the diphosphine ligand from one iron to the other. It should be noted that the redox processes of complex 1 are appreciably more reversible and occur at potentials more negative than those of the edt-bridged analogue [Fe 3 (CO) 5 (κ 2 -dppe)(μ-edt) 2 ] …”

Chalcogenide-capped triiron clusters [Fe3(CO)9(μ3-E)2], [Fe3(CO)7(μ3-CO)(μ3-E)(μ-dppm)] and [Fe3(CO)7(μ3-E)2(μ-dppm)] (E = S, Se) as proton-reduction catalysts

“…3 formation via proton-reduction [14][15][16][17][18][19][20][21][22][23][24][25][26][27] . Two groups have independently studied protonreduction by the sulfide cluster [Fe 3 (CO) 9 (µ 3 -S) 2 ] (1S) [14,15] .…”