Excited State Properties of Diiron Dithiolate Hydrides: Implications in the Unsensitized Photocatalysis of H2 Evolution

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• 作者：Luca Bertini ; Piercarlo Fantucci ; Luca De Gioia ; Giuseppe Zampella
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：September 3, 2013
• 年：2013
• 卷：52
• 期：17
• 页码：9826-9841
• 全文大小：673K
• 年卷期：v.52,no.17(September 3, 2013)
• ISSN：1520-510X

文摘

Density functional theory (DFT) and time-dependent DFT (TDDFT) have been used to investigate how visible light photons can excite an asymmetrically substituted diiron hydride, [Fe₂(pdt)(渭-H)(CO)₄dppv]⁺ (1⁺, dppv = cis-1,2-C₂H₂(PPh₂)₂; pdt = 1,3-propanedithiolate), as well as the symmetric species [Fe₂(pdt)(渭-H)(CO)₄(PMe₃)₂]⁺ (2⁺), which are the first photocatalysts of proton reduction operating without employing sensitizers (Wang, W.; Rauchfuss, T. B.; Bertini, L.; Zampella, G.; J. Am. Chem. Soc., 2012, 134, 4525). Theoretical results illustrate that the peculiar reactivity associated to the excited states of 1⁺ and 2⁺ is compatible with three different scenarios: (i) it can arise from the movement of the hydride ligand from fully bridging to semibridging/terminal coordination, which is expected to be more reactive toward protons; (ii) reactivity could be related to cleavage of a Fe鈥揝 bond, which implies formation of a transient Fe penta-coordinate species that would trigger a facile turnstile hydride isomerization, if lifetime excitation is long enough; (iii) also in line with a Fe鈥揝 bond cleavage is the possibility that after excited state decay, a highly basic S center is protonated so that a species simultaneously containing S鈥揌^未+ and Fe鈥揌^{未鈭?/sup> moieties is formed and, once reduced by a suitable electron donor, it can readily afford H₂ plus an unprotonated form of the FeFe complex. This last possibility is consistent with 31P NMR and IR solution data. All the three possibilities are compatible with the capability of 1+ and 2+ to perform photocatalysis of hydrogen evolving reaction (HER) without sensitizer. Moreover, even though it turned out difficult to discriminate among the three scenarios, especially because of the lack of experimental excitation lifetimes, it is worth underscoring that all of the three pathways represent a novelty regarding diiron carbonyl photoreactivity, which is usually associated with CO loss. Results provide also a rationale to the experimental observations which showed that the simultaneous presence of donor ligands (dppv in the case of 1+) and a H ligand in the coordination environment of diiron complexes is a key factor to prevent CO photodissociation and catalyze HER. Finally, the comparison of photoexcitation behavior of 1+ and 2+ allows a sort of generalization about the functioning of such hydride species.}