Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

Despite their technological importance for water splitting, the reaction mechanisms of most water oxidation catalysts (WOCs) are poorly understood. This paper combines theoretical and experimental methods to reveal mechanistic insights into the reactivity of the highly active molecular manganese vanadium oxide WOC [Mn4V4O17(OAc)3]3− in aqueous acetonitrile solutions. Using density functional theory together with electrochemistry and IR-spectroscopy, we propose a sequential three-step activation mechanism including a one-electron oxidation of the catalyst from [Mn23+Mn24+] to [Mn3+Mn34+], acetate-to-water ligand exchange, and a second one-electron oxidation from [Mn3+Mn34+] to [Mn44+]. Analysis of several plausible ligand exchange pathways shows that nucleophilic attack of water molecules along the Jahn–Teller axis of the Mn3+ centers leads to significantly lower activation barriers compared with attack at Mn4+ centers. Deprotonation of one water ligand by the leaving acetate group leads to the formation of the activated species [Mn4V4O17(OAc)2(H2O)(OH)]− featuring one H2O and one OH ligand. Redox potentials based on the computed intermediates are in excellent agreement with electrochemical measurements at various solvent compositions. This intricate interplay between redox chemistry and ligand exchange controls the formation of the catalytically active species. These results provide key reactivity information essential to further study bio-inspired molecular WOCs and solid-state manganese oxide catalysts.


Square wave voltammetry
While cyclic voltammetry (CV) is a standard method to evaluate electron transfer kinetic data and reaction mechanisms in electrochemical experiments, square-wave voltammetry (SWV) offers significant advantages for the system studied here. In contrast to CV, where capacitive current may mask the faradaic signal for redox active species evolving at low concentration, SWV eliminates the capacitive current component as it is a differential method and thus achieves significantly higher sensitivity (up to at least 3 magnitudes, also see Figure S2). 1 In addition, recording the forward and backward scan and varying either the scan frequency f or the current amplitude E SW allows the effective study of electron transfer kinetics and reaction mechanisms. 2 Depending on the experimental conditions, SWV can differentiate two or more closely spaced redox processes by providing a better peak separation (resolution) in comparison to CV. Hence, SWV was chosen here as electrochemical method to study the electrochemistry of [Mn 4 V 4 O 17 (OAc) 3 ] 3-(abbreviated hereafter as {Mn 4 V 4 }). Figure S2, when comparing CV and SWV data for the first two oxidation redox processes (P1, P2) at different water concentrations, marked differences are observed. While the CV data is dominated by capacitive currents, the SWV data is more sensitive and allows observations of five distinguishable redox processes. Based on the CV data, the quasi-reversibility of P1 and P2 are observed. In addition, we note that the peak separation E p increases with increasing water content, see Table S1.   Figure S3. In sum, these observations and calculations suggest that two independent oxidation paths, i.e. P2 (2 nd oxidation step without ligand exchange) and P4 (ligand exchanged species that undergoes a second oxidation), are accessible starting from the non-ligand exchanged species [Mn 3+ Mn 4+ 3 ], see Figure S3. The correlation of the peaks P2 and P4 was further investigated by variation of the SWV frequency. This method (in analogy to scan rate variation in CV) allows frequency-dependent analyses of electron transfer processes and their kinetics, while retaining the advantages of SWV over CV. Here, we used frequency-variation SWV to gain further insights into the observed electrochemical processes P1 -P5. Variation of the SWV frequency between 5

As shown in
and 50 Hz showed that with increasing frequency (analogous to faster scan rate in CV), process P2 is partially recovered ( Figure S4a,b). This is in line with the interpretation that at higher frequency, the [Mn 3+ Mn 4+ 3 ]  [Mn 4+ 4 ] oxidation is preferred, as the rate constant for ligand exchange is lower than the electron transfer rate constant for this oxidation step. ( Figure S3, Step 2a). Thus, at high frequency, the oxidation can compete with the acetate-towater ligand exchange, see Figure S3, Step 2b.

S2. Calculation of ligand exchange pathways
In pathways Ia-Ic (Ox-LEx-Ox mechanism), the presence of a Mn 3+ ion in an octahedral environment gives rise to  All pathways Ia-Ic and IId-IIe were determined stepwise, by first optimizing the transition state (TS) of a ligand exchange reaction, followed by the optimization of the two intermediates connected by the TS using the two extrema of the oscillation associated with the imaginary frequency as guess structures for the reactant and the product. This process was repeated for the structures involved in the second ligand exchange reaction of each pathway. More specifically, for the TS determination we first performed a constrained geometry optimization, in which the atoms participating in the ligand substitution were frozen, followed by a relaxed TS optimization. The Gibbs free energies of all structures considered are collected in Table S2 and depicted in Figure

S3. Calculation of Redox Potentials
The redox processes considered were (also see step-labelling in Figure S3):

I.
Step 1 Figure S8. Thus, this configuration was used as an initial guess structure for the calculations. The standard reduction potentials resulting from our calculations are in