2025 Volume 8 Issues 1–3

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INEOS OPEN, 2025, 8 (1–3), 96–97 Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF Electronic supplementary information DOI: 10.32931/io2534a

New Catalysts for the Electrochemical Reduction of Proton

K. I. Utegenov,*^a D. A. Valyaev,^b T. T. Amatov,^c A. Sournia-Saquet,^b
O. V. Semeikin,^a and N. A. Ustynyuk^a

^a Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, str. 1, Moscow, 119334 Russia
^b LCC-CNRS, Université de Toulouse, CNRS, 205 route de Narbonne 31077 Toulouse cedex 4, France
^c New York University Abu Dhabi, PO Box 129188, Saadiyat Island, Abu Dhabi, United Arab Emirates
 

Corresponding author: K. I. Utegenov, e-mail: ukamil@ineos.ac.ru
Received 2 November 2024; accepted 9 February 2025

Abstract

  

Iron acetylide Ср(СО)(Ph₃P)Fe–C≡CPh (Fe1), rhenium vinylidene Cp*(CO)₂Re=С=С(H)Ph (Re1), and manganese isonitrile complexes Cp(CO)₂Mn=С=NR (Mn3, R = Me; Mn4 R = ^tBu) were tested for activity in the electrochemical reduction of proton (ERP). Complexes Fe1 and Re1 were found to be catalytically active in the ERP. Their protonated forms [Ср(СО)(Ph₃P)Fe=C=C(H)Ph]BF₄ (Fe1H⁺) and [Cp*(CO)₂Re≡С‒СH₂Ph]BF₄ (Re1H⁺) were reduced to corresponding 19e radicals (Fe1H^▪) and (Re1H^▪) followed by homolysis of C_β–H bonds to regenerate starting complexes Fe1 and Re1.

Key words: electrochemical reduction, cyclic voltammetry, vinylidene complexes, alkynyl complexes, carbyne complexes.

 

Introduction

Earlier we have shown [1] that manganese vinylidene Сp(CO)(Ph₃P)Mn=C=C(H)Ph (Mn1) and allenylidene Сp(CO)₂Mn=C=C=CPh₂ (Mn2) complexes catalyze the electrochemical reduction of proton in dichloromethane in the presence of HBF₄·OEt₂. The proposed scheme included the protonation of the complexes and the reduction of their protonated forms to a 19e state undergoing ready homolysis of the C_β–H bond. These results suggest that other transition metal η¹-σ,π-complexes with protonated forms containing a C–H or N–H bond conjugated with a multiple metal–carbon bond can also exhibit catalytic activity in the ERP.

In this work, the electrochemical behavior of iron acetylide Cp(CO)(Ph₃P)Fe–C≡CPh (Fe1), rhenium vinylidene Cp*(CO)₂Re=С=С(H)Ph (Re1), and manganese isonitrile complexes Cp(CO)₂Mn=C=NR (Mn3, R = Me; Mn4 R = ^tBu) in the presence of HBF₄ was studied by cyclic voltammetry.

Results and discussion

The cyclic voltammogram (CV) of complex Re1 in CH₂Cl₂ displays one irreversible oxidation peak A at +0.24 V (see Fig. S1 in the Electronic supplementary information (ESI)). The CV of its protonated form [Cp*(CO)₂Re≡C–CH₂Ph]BF₄ (Re1H⁺) displays a single-electron reduction peak B at –1.51 V (Fig. S1 in the ESI), which is irreversible even at the scan rate of 100 V·s^–1, and a hardly visible oxidation peak A at +0.24 V, identical to that of vinylidene complex Re1. The addition of HBF₄ (Fig. 1) afforded a significant increase in the intensity of the cathode peak B at –1.51 V (changes in the region of almost unobservable peak A occurs at a noise level). A significant increase in the cathode peak B current can be explained by the catalytic reduction of proton (catalytic current) similarly to the earlier observations for Mn1 and Mn2 [1].

Figure 1. CV for complex Re1H⁺ in the presence of different amounts of HBF₄
(GC electrode, CH₂Cl₂, 0.1 M Bu₄NPF₆, 1·10^–3 M, 200 mV·s^–1, potentials are given relative to Fc/Fc⁺).

We believe that the catalytic reduction of proton occurs according to Scheme 1.

Scheme 1. Catalytic cycle for the reduction of proton by complex Re1.

The cyclic voltammogram of Fe1 displays two irreversible oxidation peaks at +0.19 V (C) and +1.06 V (D) (Fig. S2 in the ESI). Upon reverse scan, there appeared a reduction peak at –1.07 V (E). The CV of protonated form Fe1H⁺ displayed a single-electron reduction peak F at –1.32 V (Fig. 2) totally irreversible even at the scan rate of 100 V·s^–1. With an increase in the acid amount, the reduction peaks of Fe1H⁺ appear to be catalytic and significantly increase and no oxidation peak of starting iron acetylide Fe1 is observed due to the fast protonation of Fe1 to Fe1H⁺.

Figure 2. CV for complex Fe1 in the presence of different amounts of HBF₄
(GC electrode, CH₂Cl₂, 0.1 M Bu₄NPF₆, 1·10^–3 M, 200 mV·s^–1, potentials are given relative to Fc/Fc⁺).

These observations fit into the catalytic cycle presented in Scheme 2.

Scheme 2. Catalytic cycle for the reduction of proton by complex Fe1.

We also studied the electrochemical behavior of manganese isonitrile complexes Mn3, Mn4. However, both these compounds were found to be inactive in the ERP, since the equilibrium in their protonation reactions is shifted towards the starting compounds and, therefore, no reduction peaks of their protonated forms are observed in the CVs.

The CV data obtained in this work and earlier allow us to compare complexes Mn1, Mn2 and Re1, Fe1 by the following parameters: 1) the reduction potential of the catalyst protonated form; 2) the irreversibility of reduction of the catalyst protonated form; and 3) the magnitude of catalytic current.

The solvated proton in dichloromethane is reduced at about –2.24 V relative to Fc/Fc⁺. The catalyst protonated form, rather than proton, is reduced during the ERP. The less negative is the reduction potential of the protonated form, the more favorable is the ERP process in terms of energy. In this regard, the above-mentioned complexes range as follows: Mn2 (–0.93 V) > Fe1 (–1.32 V) > Re1 (–1.51 V) > Mn1 (–1.78 V).

Regarding the irreversibility of reduction of protonated forms, the complexes arrange in the following order: Mn1 ≈ Fe1 > Re1 >> Mn2.

The data on the relative change in catalytic currents are given in Table 1.

Table 1. Relative change in the catalytic current vs. the amount of added acid

In this respect, the discussed complexes arrange as follows: Re1 > Mn1 > Fe1 > Mn2. The catalytic currents are higher for the vinylidene complexes, since their protonated (carbyne) forms have two C_β–H bonds instead of one as in the case of protonated allenylidene Mn2 and acetylide Fe1.

Considering all these parameters together, rhenium vinylidene complex Cp*(CO)₂Re=C=CHPh (Re1) seems to be the best compound among the above-mentioned complexes in terms of catalytic activity. This is not surprising, because the HOMO–LUMO gap increases on going downward the group.

Conclusions

Thus, a concept of catalytic reduction of proton to hydrogen through the activation of the C–H (rather than M–H [2]) bond is demonstrated by the example of rhenium complex Re1 and iron acetylide Fe1. The results obtained in this work and earlier for complexes Mn1, Mn2 indicate that, varying the ligand environment and the nature of a metal in the above-mentioned η¹-σ,π-complexes, one can change the catalyst efficiency and synthesize more active catalysts in a tailor-made manner.

Acknowledgements

This work was performed with financial support from the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-00276-25-00) using the equipment of the Center for Collective Use of INEOS RAS.

Electronic supplementary information

Electronic supplementary information (ESI) available online: the CV data, synthetic procedures, and a brief state-of-the-art review. For ESI, see DOI: 10.32931/io2534a.

References

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