Inversed stability order in Keggin polyoxothiometalate isomers: a DFT study of 12-electron reduced alpha, beta, gamma, delta, and epsilon [(MoO4)Mo12O12S12(OH)12]2- anions

Density functional theory calculations have been carried out to investigate 12-electron reduced alpha, beta, gamma, delta, and epsilon Keggin-like [(MoO4)Mo12O12S12(OH)12]2- polyoxothiometalates (POTMs), which show that the stability order is alpha < beta < gamma < delta < epsilon that is perfectly inverse to the well-known trend of the classical Keggin polyoxometalates. Energy decomposition analysis reveals that the enhanced stabilities of gamma, delta, and epsilon isomers originate the favorable arrangements of their Mo12O12S12(OH)12 shell, in which the edge-sharing [MoV2(mu-S)2O2] fragment plays a fundamental role in stabilizing the overall structure. Both frontier orbital analysis and Mayer indexes exhibit that a Mo-Mo single bond is formed inside the [MoV2(mu-S)2O2] fragment, which leads to the localization of the two reduced electrons. As compared with experimentally discovered cyclic [(C9H3O6)@Mo12O12S12(OH)12]3-, all Keggin POTM structures are less stable due to their disfavored cage framework and the disadvantageous host-guest interaction. However, the epsilon-type Keggin POTM that has the largest similarity to the cyclic species is possibly available in the presence of appropriate templates.