Ghosh Shishir, Hollingsworth Nathan, Warren Mark, Hrovat David A, Richmond Michael G, Hogarth Graeme
Department of Chemistry, King's College London, Britannia House, 7 Trinity Street, London SE1 1DB, UK.
Dalton Trans. 2019 May 7;48(18):6051-6060. doi: 10.1039/c8dt04906h.
[FeFe]-hydrogenases contain strongly electronically coupled diiron [2Fe]H and tetrairon [Fe4-S4]H clusters, and thus much recent effort has focused on the chemistry of diiron-dithiolate biomimics with appended redox-active ligands. Here we report on the synthesis and electrocatalytic activity of Fe2(CO)4(μ-edt)(κ2-bpcd) (2) in which the electron-acceptor 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) acts as a surrogate of the [Fe4-S4]H sub-cluster. The complex is prepared in low yield but has been fully characterised, including a crystallographic study which shows that the diphosphine adopts a basal-apical coordination geometry in the solid state. Cyclic voltammetry shows that 2 undergoes four reduction events with DFT studies confirming that the first reduction is localised on the low-lying π* system of the diphosphine ligand. The addition of the second electron furnishes a triplet dianion that exhibits spin density distributed over the diphosphine and diiron subunits. Protonation at the Fe-Fe bond of the triplet dianion furnishes the corresponding bridging hydride as the thermodynamically favoured species that contains a reduced bpcd ligand. Complex 2 functions as a catalyst for proton-reduction at its second reduction potential, in contrast to the related 2,3-bis(diphenylphosphino)maleic anhydride (bma) complex, Fe2(CO)4(μ-pdt)(κ2-bma) (1), which shows similar electrochemical behaviour but is not catalytically active. The difference in chemical behaviour is attributed to greater stability of the 4-cyclopenten-1,3-dione platform in 2 as compared to the maleic anhydride ring of the bma ligand in 1 following the uptake of the second electron. Thus protonation of the Fe-Fe bond in the 22- affords a species which is stable enough to undergo a further reduction-protonation event, unlike the bma ligand whose maleic anhydride ring undergoes deleterious C-O bond scission upon protonation or reaction with adventitious moisture. DFT studies, however, suggest that electron-transfer from the diphosphine to the diiron centre is not significant, probably due to their poor redox levelling. Thus, while the diphosphine is readily reduced, the added electron is apparently not utilised in proton-reduction and hence cannot truly be considered as an [Fe4-S4]H surrogate.
[铁铁]氢化酶含有电子耦合很强的二铁[2Fe]H和四铁[Fe4-S4]H簇，因此近期的很多研究都集中在带有氧化还原活性配体的二铁二硫醇盐生物模拟物的化学性质上。在此，我们报道了Fe2(CO)4(μ-edt)(κ2-bpcd) (2)的合成及其电催化活性，其中电子受体4,5-双（二苯基膦基）-4-环戊烯-1,3-二酮（bpcd）充当[Fe4-S4]H子簇的替代物。该配合物的产率较低，但已得到全面表征，包括晶体学研究，结果表明二膦在固态时采用底顶配位几何构型。循环伏安法表明2经历了四次还原过程，密度泛函理论（DFT）研究证实第一次还原定域在二膦配体的低能π*体系上。第二个电子的加入产生了一个三重态双负离子，其自旋密度分布在二膦和二铁亚基上。三重态双负离子在铁-铁键处质子化，生成相应的桥连氢化物，这是热力学上更有利的物种，其中包含一个还原的bpcd配体。与相关的2,3-双（二苯基膦基）马来酸酐（bma）配合物Fe2(CO)4(μ-pdt)(κ2-bma) (1)相比，配合物2在其第二个还原电位下可作为质子还原的催化剂，1表现出相似的电化学行为，但没有催化活性。化学行为的差异归因于2中的4-环戊烯-1,3-二酮平台在接受第二个电子后比1中的bma配体的马来酸酐环具有更高的稳定性。因此，22-中Fe-Fe键的质子化产生了一种足够稳定的物种，能够经历进一步的还原-质子化过程，这与bma配体不同，后者的马来酸酐环在质子化或与外界水分反应时会发生有害的C-O键断裂。然而，DFT研究表明，从二膦到二铁中心的电子转移并不显著，可能是由于它们的氧化还原能级较差。因此，虽然二膦很容易被还原，但添加的电子显然没有用于质子还原，因此不能真正被视为[Fe4-S4]H的替代物。
