Mixed Transition Metal (Oxy)fluoride Paramagnet Chains: Synthesis, Structure, and Characterization

Three mixed transition metal (oxy)fluorides, [Ni(pz)₄][VO₂F₃] (1), [Cu(pz)₄][TiF₆] (2), and [Cu(mpz)₄][ZrF₆] (3) (pz = pyrazole; mpz = 3-methylpyrazole) were synthesized by combining asymmetric polyhedra of early transition metal cations, [VO₂F₃]^2–/[TiF₆]^2–/[ZrF₆]^2– and basic building units (BBUs) of late transition metal cations, [Ni(pz)₄]²⁺/[Cu(pz/mpz)₄]²⁺ via hydrothermal reactions. Compound 1 consists of chains with disordered F^– and O^2–, whereas compounds 2 and 3 are composed of chains containing fully ordered F^– and O^2–. The ordered structures for compounds 2 and 3 are achieved by providing the cationic contacts for [MF₆]^2– (M = Ti, Zr) anions to [Cu(pz/mpz)₄]²⁺ cations and to pyrazole/3-methylpyrazole ligands through covalent bonding and hydrogen bonding interactions, respectively. Detailed structural analysis suggests that the distinctive structure-directing anionic groups ([VO₂F₃]^2–, [TiF₆]^2–, or [ZrF₆]^2–) and hydrogen bonding networks are responsible for the linear chain structures. The lower-energy and higher-energy absorption optical band gaps are generated from the d–d electronic transitions, and the octahedral distortions of M′²⁺ (M′ = Ni, Cu) cations for all the reported compounds. Both magnetic susceptibility and magnetization measurements of compounds 1, 2, and 3 show no Bonner–Fisher behavior expected for the linear spin chain. Rather, compounds 1, 2, and 3 behave like paramagnets without developing any magnetic order.