Influence of structure-directing polyhedra and heterocyclic ligands on the chain structures and O/F ordering in a series of zinc vanadium oxyfluorides

Abstract

Three transition metal oxyfluorides consisting of distinctive structure-directing vanadium oxyfluoride polyhedra and highly polarizable Zn²⁺ cations, [Zn(pz)₂][Zn(pz)₃][V₂O₂F₈] (1, pz = pyrazole), [Zn(mpz)₃][V₂O₂F₆(H₂O)(mpz)] (2, mpz = 3-methylpyrazole), and [Zn(mpz)₃]₃[VOF₄(mpz)][VOF₄]₂ (3), were synthesized by mild hydrothermal reactions at 150 °C. Compound 1 reveals a layered structure with zigzag chains, in which [Zn(pz)_2/3]²⁺ cations and dimeric [V₂O₂F₈]⁴⁻ anions are combined in both cis- and trans-coordinating modes. The trans-directing dimeric [V₂O₂F₆(H₂O)(mpz)] and monomeric [VOF₄(mpz)] and [VOF₄] units direct the linear chain structures of compounds 2 and 3. The void space found in compound 2 is attributable to the interchain hydrogen-bonding networks. The observed absorption band gaps of 3.21–4.25 eV for the reported compounds originate from the octahedral distortion of Zn²⁺ cations and the interorbital electronic transitions. The reduction of V⁵⁺ to V⁴⁺ in acidic media might occur via a free-radical mechanism, where the dioxovanadium(V) undergoes a one-electron reduction by pyrazole and 3-methylpyrazole. Complete characterization including thermogravimetric analysis, spectroscopy, and dipole moment calculations is provided.