Aromaticity in transition metal oxide structures.

The concept of aromaticity is useful for understanding the properties of some polyoxometalates containing transition metals such as vanadium, molybdenum, and tungsten having structures based on metal macropolygons and macropolyhedra with M-O-M edges. Thus, the aromatic macrocuboctahedral Keggin ions readily undergo one-electron reductions to highly colored mixed-valence "blues" (e.g., molybdenum blue), whereas the macroicosahedral Silverton ions, M(IV)Mo12O42(8-) (M(IV) = Ce, Th, U), which, like cyclohexane, do not have vertex valence orbitals available for delocalization, do not undergo analogous reduction reactions. A macrohexagon of d1 vanadium(IV) atoms as V-O-V units has been imbedded into an electronically inactive borate matrix in the ion [V6B20O50H8](8-). The small beta unit for the V-O-V interactions in this V6 macrohexagon leads to an unprecedented example of high spin aromaticity with a paramagnetism corresponding to four unpaired electrons per V6 unit in contrast to benzene, which is diamagnetic and hence exhibits low spin aromaticity. The M-O-M interactions in these aromatic metal oxides are closely related to the Cu-O-Cu interactions in the high critical temperature superconducting copper oxides which are essential to the electron transport in these systems.