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
Graphical abstract
Introduction
Hydrogen is a potentially clean and efficient energy-carrier [[1], [2], [3]]; however, its current synthesis is energy-intensive and uses fossil-fuel resources, while the direct utilization of solar energy for hydrogen production through photocatalytic [4] or photo-electrochemical [5,6] water-splitting is poorly developed. Algae can produce hydrogen and oxygen from water [7] and hydrogenases then act as catalysts for the reversible oxidation of hydrogen to protons and electrons. In the late 1990s, crystal structures of [FeFe]-hydrogenases revealed that the active site consists of a diiron sub-unit with a bridging dithiolate ligand and ancillary CO/CN− ligands linked to an Fe4S4 ferredoxin subunit [[8], [9], [10], [11]]. This generated enormous interest in the development of synthetic analogues of the active site with a wide range of diiron biomimetics being tested as proton-reduction catalysts [12]. Key features of (electro)catalysts for proton reduction include the abilities to bind a proton(s) and undergo facile reduction, both being well-established properties of low-valent transition metal clusters [13]. Hence, clusters with nuclearities of three or greater are promising candidates as catalysts for clean hydrogen formation via proton-reduction [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]]. Two groups have independently studied proton-reduction by the sulfide cluster [Fe3(CO)9(μ3-S)2] (1S) [14,15]. In MeCN with acetic acid, H2 production takes place at the second reduction potential (−1.75 V), establishing the dianion [Fe3(CO)9(μ3-S)2]2- (1S2−) as the catalyst [14]. In the presence of the strong acid HBF4.Et2O, both mono and dianions are active proton-reduction catalysts at potentials of −1.03 V and −1.30 V, respectively [15]. Since [Fe3(CO)9(μ3-S)2] (1S) does not readily protonate even with strong acids, initial reduction is a prerequisite for proton-reduction. Sun, Åkermark and co-workers [16] have explored the proton-reduction activity of the diphosphine-substituted cluster [Fe3(CO)5(μ3-S)2(κ2-dppv)2] [dppv = cis-1,2-bis(diphenylphosphino)ethylene] that, in contrast, is readily protonated by trifluoromethanesulfonic (triflic) acid and catalyses proton-reduction at -0.98 V in CH2Cl2, the first reduction potential of [(μ-H)Fe3(CO)5(μ3-S)2(κ2-dppv)2]+. In seeking to extend and develop the proton-reduction chemistry of low-valent triiron clusters, we turned our attention to the chalcogenide-capped clusters [Fe3(CO)7(μ3-CO)(μ3-E)(μ-dppm)] (2S, 2Se) and [Fe3(CO)7(μ3-E)2(μ-dppm)] (3S, 3Se) with the expectation that introduction of the diphosphine bis(diphenylphosphino)methane (dppm) may serve to stabilise the relatively fragile triiron core, while also favouring protonation at the triiron centre. Further, the ability to vary the chalcogenide cap potentially allows redox-tuning. Herein we report the electrocatalytic proton-reduction properties of 2 and 3, as well as [Fe3(CO)9(μ3-Se)2] (1Se), the latter being studied in order to compare with previous studies of [Fe3(CO)9(μ3-S)2] (1S) [14,15].
Section snippets
General procedures
Unless otherwise stated, purification of solvents, reactions, and manipulation of compounds were carried out under a nitrogen atmosphere using standard Schlenk techniques. Reagent grade solvents were dried by standard procedures and were freshly distilled prior to use. All chromatographic separations and ensuing workup were carried out in air. Thin layer chromatography was carried out on glass plates pre-coated with Merck 60 0.25 mm silica gel. Dppm was purchased from Acros Organics Chemicals
Synthesis and characterisation
For [Fe3(CO)7(μ3-CO)(μ3-E)(μ-dppm)] (2) (E = S, Se) two pathways were considered; (i) addition of dppm to [Fe3(CO)9(μ3-CO)(μ3-E)] (1) [44,45] and (ii) addition of group 16 elements to [Fe3(CO)10(μ-dppm)] [28]. Sulfide-capped [Fe3(CO)9(μ3-CO)(μ3-S)] has previously been reported [41] but yields were low and separation (from other sulfur-containing species) difficult. A brief report of [Fe3(CO)9(μ3-CO)(μ3-Se)] has appeared; it was formed in low yield upon addition of BiCl3 to K2[Fe3(CO)9(μ3-Se)] [
Summary and conclusions
We have reported comparative electrochemical and catalytic proton-reduction activity of three types of chalcogenide-capped clusters; [Fe3(CO)9(μ3-E)2] (1), [Fe3(CO)7(μ3-CO)(μ3-E)(μ-dppm)] (2) and [Fe3(CO)7(μ3-E)2(μ-dppm)] (3) (E = S, Se). In general, the nature of the chalcogenide has little effect on the triiron core and consequent proton-reduction ability. Sulfide-capped clusters appear to be slightly more electron-rich than their selenide analogues, protonating to a greater extent in the
Acknowledgements
We thank the European Commission for the award of an Erasmus Mundus pre-doctoral fellowship (AR) and a postdoctoral fellowship (S.B-M) and the Commonwealth Scholarship Commission for the award of a scholarship to SG. GH and EN thank the Royal Society for an International Exchange Award. MGR thanks the Robert A. Welch Foundation (Grant B-1093-MGR) for financial support. Computational resources through the High-Performance Computing Services and CASCaM at the University of North Texas are
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Proton reduction by phosphinidene-capped triiron clusters
2021, Journal of Organometallic ChemistryCitation Excerpt :Dinuclear iron complexes with thiolate-, sulfide-, selenide- and telluride bridges have been investigated as electrocatalysts for proton reduction in organic solvents in the presence of suitable proton sources [13–17]. The chemistry of electron-poor diphosphido-bridged diiron carbonyl complexes has been reported [18–27], but only a few diiron diphosphido complexes have been investigated as proton reduction catalysts [28–32]. In a recent study, Colbran and co-workers investigated electrocatalytic proton reduction effected by diiron complexes of the general formula [Fe2(CO)6{μ-P((CH2-η5-C5H4)Fe(η5-C5H5))H}](η5-C5H4)Fe(η5-C5H5) = the ferrocenyl radical), derived from the corresponding primary phosphine P((CH2-η5-C5H4)Fe(η5-C5H5))H2 [32].