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 O2-, whereas compounds 2 and 3 are composed of chains containing fully ordered F- and O2-. 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.