Photoproduction of Hydrogen by Decamethylruthenocene Combined with Electrochemical Recycling

Abstract

The photoinduced hydrogen evolution reaction (HER) by decamethylruthenocene,C p 2 *Ru II (Cp* = C 5 Me 5 ), is reported. The use of am etallocene to photoproduce hydrogen is presented as an alternative strategy to reduce protons without involving an additional photosensitizer.T he mechanism was investigated by (spectro)electrochemical and spectroscopic (UV/Vis and 1 HNMR) measurements.T he photoactivated hydride involved was characterized spectroscopically and the resulting [Cp 2 *Ru III ] + species was electrochemically regenerated in situ on af luorinated tin oxide electrode surface.Apromising internal quantum yield of 25 %w as obtained. Optimal experimental conditionsespecially the use of weakly coordinating solvent and counterions-are discussed.Thedevelopmentofsimpleandefficientmethodstoproduce molecular hydrogen (H 2 )i st he focus of intense research. Va rious state-of-the-art multicomponent artificial photosystems for H 2 generation are currently under heavy scrutiny and generally consist of ah ighly engineered catalyst, photosensitizer,electron mediator or relay combinations, [1] and are often fueled by sacrificial electron donors (for example, triethylamine, [2] triethanolamine, [2b] benzyl-dihydronicotinamide, [3] and so forth). Thel atter irreversibly oxidizes upon charge transfer and provides protons and electrons to the catalyst. Consequently,s acrificial systems consume af uel to produce H 2 while electrochemical systems only consume electricity (that is now being increasingly produced in as ustainable manner). Indeed, the electrode can both accept and donate electrons.N oi rreversible reactions take place at this step,a nd the protons are supplied from the solution.Metallocenes appear as an attractive class of molecules capable of achieving the complex photogeneration of H 2 by themselves.Indeed, they are able to both reduce protons and undergo photoactivation. Therefore,t hese "all-in-one" molecules would offer an interesting alternative to state-ofthe-art multicomponent photosystems as fewer electron transfer steps are involved. Moreover,they are simple,easily synthesized molecules,w ith ligands and metal centers that may be tuned to obtain certain desired properties,s uch as tailored solubility,a bsorbance wavelength, or redox potentials.Recently,w ed emonstrated the possibility to produce H 2 in the dark using decamethylferrocene (Cp 2 *Fe II ;C p* = C 5 Me 5 )asanelectron donor in abiphasic system.[4] Motivated by these early findings,weset out to explore the reactivity of other metallocenes as suitable electron donors.I nterestingly, both osmocene (Cp 2 Os II ;C p = C 5 H 5 ) [5] and decamethylosmocene (Cp 2 *Os II ) [6] demonstrated the capability to produce H 2 upon light irradiation. Other works have proposed the use of asingle molecule to achieve photogeneration of H 2 . Fore xample,C ole-Hamilton [7] reported ap latinum phosphine compound, while both Miller [8] and Gray [9] used iridium chloride complexes.Herein, we report Cp 2 *Ru II as the first metallocene capable of perfo...

SECM study of hydrogen photogeneration in a 1,2-dichloroethane | water biphasic system with decamethylruthenocene electron donor regeneration

“…The uses of ferrocene derivatives [1], decamethylferrocene [2], cyclopentadienylcobalt phosphine [3], cobaltocene [4], molybdocene [5], osmocene [6] or ruthenocene [7] as electron donors for hydrogen evolution reaction (HER) have been reported. In the case of osmocene and ruthenocene, the HER was carried out under light exposure.…”

“…Recently, it has been shown that light driven HER also occurs at a polarized ITIES in the presence of decamethylruthenocene (DMRc) [7]. DMRc due to its only moderately low redox potential is thermodynamically a weak electron donor for HER, however, in the presence of UV light it is capable to reduce protons dissolved in DCE [7][8][9].…”

“…DMRc due to its only moderately low redox potential is thermodynamically a weak electron donor for HER, however, in the presence of UV light it is capable to reduce protons dissolved in DCE [7][8][9]. Therefore, polarization of the liquid | liquid interface, ensuring transfer of protons from the acidic aqueous solution to DCE, is a key step of HER at ITIES.…”

“…H + can be transferred to DCE in the form of HTB, which then undergoes partial dissociation in the organic phase [13]. Inside the organic phase, the metallocene hydride [DMRc-H + ] [7,8] [8]:…”

“…Then amount of generated H 2 is not limited by amount of available molecular electron donor or its diffusion [21]. So far, electrochemical regeneration of DMRc + has been reported only in DCE single-phase system, where protons were initially present in the form of HTB acid [7]. Still, there is no report on DMRc present in organic phase, but are pumped across the chemically polarized ITIES.…”