A series of oxyfluoride chains containing asymmetric basic building units of both early- and late-transition metal cations

Four new transition metal oxyfluorides revealing chain structures composed of asymmetric basic building units of [MO₂F₄]²⁻ (M = Mo and W) and [M’(pz/mpz)₄]²⁺ (M′ = Cu and Ni) polyhedra, i.e., [Cu(mpz)₄][MoO₂F₄] (1), [Cu(pz)₄]₂[WO₂F₄]₂ (2), [Ni(pz)₄]₂[MoO₂F₄]₂ (3), and [Ni(pz)₄][WO₂F₄] (4) (pz = pyrazole; mpz = 3-methyl pyrazole) have been synthesized via hydrothermal reactions. The linear chain structures found from the title compounds are attributable to the trans-directing [MO₂F₄]^2- octahedral moiety and hydrogen bonding interactions. Strong Jahn–Teller distortions are observed for Cu²⁺ cations with d⁹ configuration in compounds 1 and 2. All the compounds show both lower-energy and higher-energy absorption band gaps, which originate from the d–d transitions and the distortion of octahedral geometry of M²⁺ cations, respectively. The magnetism of [Cu(mpz)₄][MoO₂F₄] turns out to be described by noninteracting Cu²⁺ spins rather than a spin chain due to long exchange path mediated via the nonmagnetic Mo⁶⁺ ions. The title compounds have been also thoroughly characterized using spectroscopic and thermal analyses along with calculations of local dipole moments and the extent of out-of-center distortions.