Theoretical study of oxoborate complexes with MO4n− tetraoxo anions in the inner and outer spheres of the B20O30 cluster

Abstract

The energies and structural and spectroscopic characteristics of endohedral (MO₄©B₂₀O₃₀ⁿ⁻) and exohedral (MO₄ · B₂₀O₃₀ⁿ⁻) isomers of oxoborate complexes with MO₄ⁿ⁻ tetraoxo anions with 32 valence electrons located in the inner and outer spheres of the B₂₀O₃₀ cluster have been calculated by the density functional theory method (B3LYP). It has been demonstrated that, among the endohedral MO₄©B₂₀O₃₀ⁿ⁻ clusters with strong multiply charged anions (VO₄³⁻, CrO₄²⁻, PO₄³⁻, SO₄²⁻, AsO₄³⁻, SeO₄²⁻, etc.), the isomer in which a “guest” tetrahedron MO₄ is located at the center of the B₂₀O₃₀ cage and bonded to it through internal oxygen bridges M-O*-B is the most favorable one. Among the exohedral analogues MO₄ · B₂₀O₃₀ⁿ⁻, two most favorable isomers contain the “capping” MO₄ tetrahedron bonded to the B₂₀O₃₀ cage through two and three external M-O-B bridges. For the complexes with doubly charged SO₄²⁻ and SeO₄²⁻ anions, the third exohedral isomer in which the sulfite or selenite group MO₃ is bidentately coordinated to the oxidized B₂₀O₂₉(OO) cage with one peroxide bridge turns out to be close in energy to the above two isomers. For the systems with high negative charge n, the exohedral isomers are much more favorable than the endohedral isomer; however, with decreasing charge, the difference in energy between them decreases to ~10–18 kcal/mol, so that the exo–endo transition between them can require moderate energy inputs. For the endohedral complexes with singly charged ClO₄⁻ and BrO₄⁻ anions, two isomers with close energies are preferable in which the central atoms of the guest tetrahedra are reduced to the state of singly charged ions, while the oxoborate cage is oxidized to B₂₀O₂₆(OO)₄ with four peroxide groups B-O-O-B and retains its closed (closo) structure. In the most favorable isomer of the complexes with multicharged ortho-anions BO₄⁵⁻, CO₄⁴⁻, and NO₄³⁻, the outersphere anion is reduced to, respectively, borate, carbonate, and nitrate bidentately coordinated to the oxidized B₂₀O₂₉(O)₂ cage with an open structure and two strongly elongated terminal B-O bonds. The results are compared with the data of previous calculations of endohedral and exohedral vanadate complexes MO₄©V₂₀O₅₀ⁿ⁻ and MO₄ · V₂₀O₅₀ⁿ⁻ with the same guest anions MO₄ⁿ⁻.