Water Splitting Processes on Mn₄O₄ and CaMn₃O₄ Model Cubane Systems

Abstract

Catalytic conversion of solar energy into chemical energy has been frequently investigated to develop clean energy sources in the last few decades. Metal oxide complexes show high potential for the catalytic conversion process, but the biochemical process in green plants has better efficiency than artificial photocatalysts consisting of metal oxides. In this work, the water splitting process is theoretically investigated using two synthetic model complexes whose structures are similar to the manganese-based oxygen evolving complex in photosystem II. Model A consists of four Mn atoms and Model B consists of three Mn atoms and a Ca atom in the core. Model A shows a better ability for water splitting than Model B when comparing the highest reaction energy. The highest reaction energies are 2.56 and 2.99 eV for Models A and B, respectively. In Model B, the first oxidation in the water splitting process is exothermic, which is different from Model A. In both models, the molecular oxygen generation step is endothermic by about 1.0 to 2.5 eV.

Structural models of the biological oxygen-evolving complex: achievements, insights, and challenges for biomimicry

“…212 Similar conclusions regarding the higher reactivity of the all-Mn cubane versus the Mn 3 CaO 4 one were reached from computational studies that investigated hypothetical water oxidation pathways. 213 The original Mn 3 CaO 4 compound shows quasireversible reduction to Mn(IV) 2 Mn(III)CaO 4 at −940 mV versus Fc/Fc + in DMA, whereas the all-Mn cubane shows a quasireversible reduction at −700 mV in similar conditions. These data suggest that the presence of redox-inactive Ca may facilitate the formation of species with higher oxidation states at lower for the calciumcontaining cubane.…”

Theoretical Investigation of Water Oxidation Catalysis by a Model Manganese Cubane Complex

“…The authors also used theoretical calculations to investigate the reaction pathways for oxygen atom transfer to phosphine from cubane cluster. In related work, Lee and Aikens 49 reported a theoretical study of water splitting process on model compounds of these synthetic clusters. The ligands were truncated with simpler ones to reduce the computational time.…”

Open-shell coupled-cluster valence-bond theory augmented with an independent amplitude approximation for three-pair correlations: Application to a model oxygen-evolving complex and single molecular magnet

“…The geometry shown in Figure 6 was taken from ref. 126, which is a simplified cubane subunit of the synthetic model reported by Agapie and co-workers. 122 The structure is very similar to what Agapie and co-workers used in their theoretical study 125 An interesting feature of this molecule is that the three Mn atoms form a nearly perfect triangle and thus 3P configurations ought to play a crucial role as we learned from the isoelectronic model systems above.…”

“…The cubane subunit, [CaMn 3 O 4 ], in the oxygen-evolving complex (OEC) has attracted a lot of interest in both experimental and theoretical chemistry. 7,[117][118][119][120][121][122][123][124][125][126] From a theoretical chemistry point of view, this is a very challenging system because it requires a balanced treatment of both static and dynamic correlations and there is no readily usable quantum chemistry model which can achieve this. Moreover, understanding electron correlations in this molecule may assist in the rational design of synthetic OECs, which is also of much interest.…”

“…The geometry shown in Figure 6 was taken from ref. 126, which is a simplified cubane subunit of the synthetic model reported by Agapie and co-workers. 122 The structure is very similar to what Agapie and co-workers used in their theoretical study 125 space is (9e, 9o) which includes all the d-electrons in 3 Mn IV (d 3 ).…”