Jahn‐Teller Effects in a Vanadate‐Stabilized Manganese‐Oxo Cubane Water Oxidation Catalyst

Abstract

Heuristic rules that allow identifying the preferred mixed‐valence isomers and Jahn‐Teller axis arrangements in the water oxidation catalyst [(Mn4O4)(V4O13)(OAc)3]n− and its activated form [(Mn4O4)(V4O13)(OAc)2(H2O)(OH)]n− are derived. These rules are based on computing all combinatorially possible mixed‐valence isomers and Jahn‐Teller axis arrangements of the MnIII atoms, and associate energetic costs with some structural features, like crossings of multiple Jahn‐Teller axes, the location of these axes, or the involved ligands. It is found that the different oxidation states localize on different Mn centers, giving rise to clear Jahn‐Teller distortions, unlike in previous crystallographic findings where an apparent valence delocalization was found. The low barriers that connect different Jahn‐Teller axis arrangements suggest that the system quickly interconverts between them, leading to the observation of averaged bond lengths in the crystal structure. We conclude that the combination of cubane‐vanadate bonds that are chemically inert, cubane‐acetate/water bonds that can be activated through a Jahn‐Teller axis, and low activation barriers for intramolecular rearrangement of the Jahn‐Teller axes plays an essential role in the reactivity of this and probably related compounds.

Flexibility Enhances Reactivity: Redox Isomerism and Jahn–Teller Effects in a Bioinspired Mn4O4 Cubane Water Oxidation Catalyst

“…As redox and JT isomerism were previously shown to play an important role in the reactivity of [Mn 4 V 4 O 17 (OAc) 3 ] 3– , 45 , 46 we decided to target the most important isomers and conformers of each intermediate of interest. To this end, we extended the sampling procedure of ref ( 46 ) by also targeting multiple ligand arrangements for each redox and JT isomer.…”

“…As redox and JT isomerism were previously shown to play an important role in the reactivity of [Mn 4 V 4 O 17 (OAc) 3 ] 3– , 45 , 46 we decided to target the most important isomers and conformers of each intermediate of interest. To this end, we extended the sampling procedure of ref ( 46 ) by also targeting multiple ligand arrangements for each redox and JT isomer. Accordingly, we next investigated all four arrangements of reaction steps (PCET-PCET-PCET-ET, PCET-PCET-ET-PCET, PCET-ET-PCET-PCET, and ET-PCET-PCET-PCET) as well as both inter- and intramolecular oxidations.…”

“…Final JT configurations of the intermediates were determined by comparing the lengths of Mn 3+ –O bonds, with the two longest coaxial bonds indicating the x -, y -, or z -orientation of the JT axis on a given Mn 3+ center. As in the precatalyst, 46 not every conceivable redox and JT isomer of an individual intermediate corresponds to a stable minimum on that intermediate’s potential energy surface. In the following, only the most stable isomers for each intermediate in the proposed water oxidation cycle shall be discussed (see Table S1 for a list of all stable isomers and conformers).…”

“…Furthermore, isomers featuring JT axes oriented toward the vanadate ligand were not specifically targeted, as these had been previously found to be energetically unfavorable. 46 …”

“… 46 Further, interconversion between redox isomers appears to be associated with low barriers (between 1.5 and 7.6 kcal/mol), while interconversion between JT isomers is almost barrierless (between 0.6 and 1.6 kcal/mol), showing that various redox and JT isomers can be easily accessed in the course of reactions on [Mn 4 V 4 O 17 (OAc) 3 ] 3– . 46 …”