Theoretical Study of Vanadate Complexes with MO 4 Tetraoxo Anions in the Inner Sphere of the Mixed-Valence V 20 O 46 Cluster

Calculations of high-spin isomers of MO 4 V 20 O_46^n- anions ( S = 9) with MO 4 tetraoxo anions located in the inner and outer spheres of the mixed-valence V 20 O 46 cage have been performed by the density functional theory method. It has been shown that for all systems the endohedral isomers are considerably more favorable than the exohedral isomers. For the complexes with transition metal atoms M (M = Ti, V, Cr), the centered ortho isomer MO 4 @V 20 O_46^n- is clearly more preferable. For the complexes with main-group metal atoms (M = Si, Ge, P, As, S, Se), the meta isomer MO 3 @V 20 O_47^n- has been localized in addition to ortho-MO 4 @V 20 O_46^n- . The meta isomer has a pyramidal МО 3 anion and the fourth oxygen atom bound to a trivalent vanadium atom of the cage through an inner О=V III bond. For complexes with SO_4^2- and SeO_4^2- dianions, both ortho and meta structures do not correspond to stationary points of the potential energy surface: upon optimization, they convert without a barrier to the MO 2 @V 20 O_48^2- isomer with two М−О−V III bridges and two inner О=V III bonds. The character of geometric distortions, normal mode shifts, and electron and spin density redistribution between the inserted MO 4 guest and the cage as compared with isolated MO_4^n- and V 20 O 46 systems has been considered. The results are compared with previous calculations of MO 4 @V 20 O_50^n- complexes with the same anions into the valence-saturated V 20 O 50 cluster. The stability and structure of the “nested” V 4 O 6 @V 20 O 50 ( S = 9) cluster with the more complex tetrahedral guest V 4 O 6 inside the V 20 O 50 cage are predicted.