Photocatalytic H2 production using a hybrid assembly of an [FeFe]-hydrogenase model and CdSe quantum dot linked through a thiolato-functionalized cyclodextrin.

It is a great challenge to develop iron-based highly-efficient and durable catalytic systems for the hydrogen evolution reaction (HER) by understanding and learning from [FeFe]-hydrogenases. Here we report photocatalytic H2 production by a hybrid assembly of a sulfonate-functionalized [FeFe]-hydrogenase mimic (1) and CdSe quantum dot (QD), which is denoted as 1/ß-CD-6-S-CdSe (ß-CD-6-SH = 6-mercapto-ß-cyclodextrin). In this assembly, thiolato-functionalized ß-CD acts not only as a stabilizing reagent of CdSe QDs but also as a host compound for the diiron catalyst, so as to confine CdSe QDs to the space near the site of diiron catalyst. In addition, another two reference systems comprising MAA-CdSe QDs (HMAA = mercaptoacetic acid) and 1 in the presence and absence of ß-CD, denoted as 1/ß-CD/MAA-CdSe and 1/MAA-CdSe, were studied for photocatalytic H2 evolution. The influences of ß-CD and the stabilizing reagent ß-CD-6-S- on the stability of diiron catalyst, the fluorescence lifetime of CdSe QDs, the apparent electron transfer rate, and the photocatalytic H2-evolving efficiency were explored by comparative studies of the three hybrid systems. The 1/ß-CD-6-S-CdSe system displayed a faster apparent rate for electron transfer from CdSe QDs to the diiron catalyst compared to that observed for MAA-CdSe-based systems. The total TON for visible-light driven H2 evolution by the 1/ß-CD-6-S-CdSe QDs in water at pH 4.5 is about 2370, corresponding to a TOF of 150 h-1 in the initial 10 h of illumination, which is 2.7- and 6.6-fold more than the amount of H2 produced from the reference systems 1/ß-CD/MAA-CdSe and 1/MAA-CdSe. Additionally, 1/ß-CD-6-S-CdSe gave 2.4-5.1 fold enhancement in the apparent quantum yield and significantly improved the stability of the system for photocatalytic H2 evolution.

Effect of Bridgehead Steric Bulk on the Intramolecular C-H Heterolysis of [FeFe]-Hydrogenase Active Site Models Containing a P2N2 Pendant Amine Ligand.

A series of pendant amine-containing [FeFe]-hydrogenase models, [X(CH2S-µ)2{Fe(CO)3}{Fe(CO)(P2(Ph)N2(Bn))}] (1H, X = CH2; 2Me, C(CH3)2; 3Et, C(CH2CH3)2; and P2(Ph)N2(Bn) = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) with different groups at the bridgehead carbon of the S-to-S linker were synthesized. The oxidations of these complexes as well as the reverse reduction reaction were studied by cyclic voltammetry and in situ IR spectroscopy. Regardless of the bridgehead steric bulk, all three complexes demonstrate intramolecular iron-mediated C(sp(3))-H bond heterolytic cleavage with the assistance of the pendant amine base within the chelating diphosphine ligand in the two-electron oxidation process. X-ray crystallographic analysis shows that the doubly oxidized products, [1'H](+), [2'Me](+), and [3'Et](+), all have a rigid FeSC three-membered ring at the open apical site of the rotated iron center. The most noticeable difference in structures of the oxidized complexes is that the single CO ligand of the rotated Fe(P2(Ph)N2(Bn))(CO) unit in [1'H](+) and [2'Me](+) is found below the Fe···Fe vector, while in [3'Et](+) an unusually rotated Fe(P2(Ph)N2(Bn))(CO) moiety positions one of the P donors within the bidentate ligand under the Fe···Fe vector. The starting Fe(I)Fe(I) complexes can be recovered from their corresponding doubly oxidized complexes by reduction in the presence of Brönsted acid.

Effect of the S-to-S bridge on the redox properties and H2 activation performance of diiron complexes related to the [FeFe]-hydrogenase active site.

Three biomimetic models of the [FeFe]-hydrogenase active site, namely diiron dithiolates of [(µ-edt){Fe(CO)3}{Fe(CO)(κ2-PNP)}] (1, edt = ethane-1,2-dithiolate, PNP = Ph2PCH2N(nPr)CH2PPh2), [(µ-bdtMe){Fe(CO)3}{Fe(CO)(κ2-PNP)}] (2, bdtMe = 4-methylbenzene-1,2-dithiolate), and [(µ-adtBn){Fe(CO)3}{Fe(CO)(κ2-PNP)}] (3, adtBn = N-benzyl-2-azapropane-1,3-dithiolate), were prepared and structurally characterized. These complexes feature the same PNP ligand but different S-to-S bridges. Influence of the S-to-S bridge on the electrochemical properties and chemical oxidation reactivity of 1-3 was studied by cyclic voltammetry and by in situ IR spectroscopy. The results reveal that the S-to-S bridge has a considerable effect on the oxidation reactivity of 1-3 and on the stability of in situ generated single-electron oxidized complexes, [1]+, [2]+, and [3]+. The performances of [1]+ and [2]+ for H2 activation were explored in the presence of a mild chemical oxidant, while rapid decomposition of [3]+ thwarted the further study of this complex. Gratifyingly, 1 was found to be catalytically active, although in a low turnover number, for H2 oxidation in the presence of excess mild oxidant and a proton trapper under 1 atm H2 at room temperature.

The influence of a S-to-S bridge in diiron dithiolate models on the oxidation reaction: a mimic of the H(air)(ox) state of [FeFe]-hydrogenases.

Two-electron oxidation of a diiron complex (1) containing a bulky S-to-S bridge with an exocyclic carbonyl group affords [1(OH)](+), which replicates the coordination structure and electronic configuration of H(air)(ox), and the chemically reversible reaction between 1 and [1(OH)](+) mimics the bioprocess of interconversion of the inactive H(air)(ox) and the active Hred states of the [FeFe]-hydrogenases.