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Pentanuclear iron catalysts for water oxidation: substituents provide two routes to control onset potentials.

ABSTRACT: The development of robust and efficient molecular catalysts based on earth-abundant transition metals for water oxidation reactions is a challenging research target. Our group recently demonstrated the high activity and stability of a pentairon-based water oxidation electrocatalyst (M. Okamura, M. Kondo, R. Kuga, Y. Kurashige, T. Yanai, S. Hayami, V. K. K. Praneeth, M. Yoshida, K. Yoneda, S. Kawata and S. Masaoka, Nature, 2016, 530, 465-468). However, the development of strategies to decrease onset potentials for catalysis remains challenging. In this article, we report the construction of a series of pentanuclear iron complexes by introducing electron-donating (methyl) and electron-withdrawing (bromo) substituents on the ligand. Two newly synthesized complexes exhibited five reversible redox processes, similar to what is seen with the parent complex. These complexes can also serve as homogeneous catalysts for water oxidation reactions, and the faradaic efficiencies of the reactions were high. Additionally, the onset potentials of the newly developed complexes were lower than that of the parent complex. Mechanistic insights revealed that there are two methods for decreasing onset potentials: control of the redox potentials of the pentairon complex and control of the reaction mechanism.

SUBMITTER: Praneeth VKK

PROVIDER: S-EPMC6495723 | biostudies-literature | 2019 May

REPOSITORIES: biostudies-literature

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Pentanuclear iron catalysts for water oxidation: substituents provide two routes to control onset potentials.

Praneeth Vijayendran K K VKK Kondo Mio M Okamura Masaya M Akai Takuya T Izu Hitoshi H Masaoka Shigeyuki S

Chemical science 20190319 17

The development of robust and efficient molecular catalysts based on earth-abundant transition metals for water oxidation reactions is a challenging research target. Our group recently demonstrated the high activity and stability of a pentairon-based water oxidation electrocatalyst (M. Okamura, M. Kondo, R. Kuga, Y. Kurashige, T. Yanai, S. Hayami, V. K. K. Praneeth, M. Yoshida, K. Yoneda, S. Kawata and S. Masaoka, <i>Nature</i>, 2016, <b>530</b>, 465-468). However, the development of strategies ...[more]

PMID: 31123573

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Project description:Cobalt polyoxometalates (Co-POMs) have emerged as promising water oxidation catalysts (WOCs), with the added advantage of their molecular nature despite being metal oxide fragments. In comparison with metal oxides, that do not offer well-defined active surfaces, POMs have a controlled, discrete structure that allows for precise correlations between experiment and computational analyses. Thus, beyond highly active WOCs, POMs are also model systems to gain deeper mechanistic understanding on the oxygen evolution reaction (OER). The tetracobalt Weakley sandwich [CoII 4(H2O)2(B-α-PW9O34)2]10- (Co4-WS) has been one of the most extensively studied. We have compared its activity with that of the iron analog [FeIII 4(H2O)2(B-α-PW9O34)2]6- (Fe4-WS) looking for the electronic effects determining their activity. Furthermore, the effect of POM nuclearity was also investigated by comparison with the iron- and cobalt-monosubstituted Keggin clusters. Electrocatalytic experiments employing solid state electrodes containing the POMs and the corresponding computational calculations demonstrate that CoII-POMs display better WOC activity than the FeIII derivatives. Moreover, the activity of POMs is less influenced by their nuclearity, thus Weakley sandwich moieties show slightly improved WOC characteristics than Keggin clusters. In good agreement with the experimental data, computational methods, including pK a values, confirm that the resting state for Fe-POMs in neutral media corresponds to the S1 (FeIII-OH) species. Overall, the proposed reaction mechanism for Fe4-WS is analogous to that found for Co4-WS, despite their electronic differences. The potential limiting step is a proton-coupled electron transfer event yielding the active S2 (FeIV[double bond, length as m-dash]O) species, which receives a water nucleophilic attack to form the O-O bond. The latter has activation energies slightly higher than those computed for the Co-POMs, in good agreement with experimental observations. These results provide new insights for the accurate understanding of the structure-reactivity relationships of polyoxometalates in particular, and or metal oxides in general, which are of utmost importance for the development of new bottom-up synthetic approaches to design efficient, robust and non-expensive earth-abundant water oxidation catalysts.

Project description:Four heterotrinuclear complexes containing the ligands 3,5-bis(2-pyridyl)pyrazolate (bpp-) and 2,2':6',2''-terpyridine (trpy) of the general formula {[RuII(trpy)]2(?-[M(X)2(bpp)2])}(PF6)2, where M = CoII, MnII and X = Cl-, AcO- (M = CoII, X = Cl-: Ru2Co-Cl2 ; M = MnII, X = Cl-: Ru2Mn-Cl2 ; M = CoII, X = AcO-: Ru2Co-OAc2 ; M = MnII, X = AcO-: Ru2Mn-OAc2 ), have been prepared for the first time. The complexes have been characterized using different spectroscopic techniques such as UV-vis, IR, and mass spectrometry. X-Ray diffraction analyses have been used to characterize the Ru2Mn-Cl2 and Ru2Mn-OAc2 complexes. The cyclic voltammograms (CV) for all four complexes in organic solvent (CH3CN or CH2Cl2) display three successive reversible oxidative waves corresponding to one-electron oxidations of each of the three metal centers. The oxidized forms of the complexes Ru2Co-OAc2 and Ru2Mn-OAc2 are further characterized by EPR and UV-vis spectroscopy. The magnetic susceptibility measurements of all complexes in the temperature range of 2-300 K reveal paramagnetic properties due to the presence of high spin Co(ii) and Mn(ii) centers. The complexes Ru2Co-OAc2 and Ru2Mn-OAc2 act as precatalysts for the water oxidation reaction, since the acetato groups are easily replaced by water at pH = 7 generating the active catalysts, {[Ru(H2O)(trpy)]2(?-[M(H2O)2(bpp)2])}4+ (M = CoII: Ru2Co-(H2O)4 ; M = MnII: Ru2Mn-(H2O)4 ). The photochemical water oxidation reaction is studied using [Ru(bpy)3]2+ as the photosensitizer and Na2S2O8 as a sacrificial electron acceptor at pH = 7. The Co containing complex generates a TON of 50 in about 10 minutes (TOFi = 0.21 s-1), whereas the Mn containing complex only generates a TON of 8. The water oxidation reaction of Ru2Co-(H2O)4 is further investigated using oxone as a sacrificial chemical oxidant at pH = 7. Labelled water oxidation experiments suggest that a nucleophilic attack mechanism is occurring at the Co site of the trinuclear complex with cooperative involvement of the two Ru sites, via electronic coupling through the bpp- bridging ligand and via neighboring hydrogen bonding.

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Pentanuclear iron catalysts for water oxidation: substituents provide two routes to control onset potentials.

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Pentanuclear iron catalysts for water oxidation: substituents provide two routes to control onset potentials.

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