CoMOF₅(pyrazine)(H₂O)₂ (M = Nb, Ta): Two-Layered Cobalt Oxyfluoride Antiferromagnets with Spin Flop Transitions

Abstract

Two cobalt oxyfluoride antiferromagnets CoMOF5(pyz)­(H2O)2 (M = Nb 1, Ta 2; pyz = pyrazine) have been synthesized via conventional hydrothermal methods and characterized by thermogravimetric (TGA) analysis, FTIR spectroscopy, electron spin resonance (ESR), magnetic susceptibility, and magnetization measurements at both static low field and pulsed high field. The single-crystal X-ray diffraction indicates both compounds 1 and 2 are isostructural and crystallize in the monoclinic space group C2/m with a two-dimensional Co2+ triangular lattice in the ab plane, separated by the nonmagnetic MOF5 (M = Nb 1, Ta 2) octahedra along the c-axis with large intertriangular-lattice Co···Co distance. Because of low dimensionality together with frustrated triangular lattice, compounds 1 and 2 exhibit no long-range antiferromagnetic order until ∼3.7 K. Moreover, a spin flop transition is observed in the magnetization curves at 2 K for both compounds, which is further confirmed by ESR spectra. In addition, the ESR spectra suggest the presence of a zero-field spin gap in both compounds. The high field magnetization measured at 2 K saturates at ∼7 T with M s = 1.55 μB for 1 and 1.71 μB for 2, respectively, after subtracting the Van Vleck paramagnetic contribution, which is usually observed for Co2+ ions with pseudospin spin of 1/2 at low temperature. Powder-averaged magnetic anisotropy of g = 3.10 for 1 (3.42 for 2) and magnetic superexchange interaction J/k B = −3.2 K for 1 (−3.6 K for 2) are obtained.

MIO3F (M = Co and Ni): Magnetic Iodate Fluorides with Zigzag Chains

“…Meanwhile, when the magnetic field further increases, the magnetization of 1 steeply increases with an abrupt change of slope, indicating a field-induced transition. This metamagnetic transition corresponds to a spin-flop transition due to magnetic anisotropy, which is also commonly reported in other Co compounds, e.g., CoMOF 5 (pyrazine)­(H 2 O) 2 (M = Nb and Ta) but not widely observed for 2 and other Ni 2+ compounds. Note that such a field-induced transition like a spin-flop transition is reported in iodates in this work for the first time.…”

“…to a spin-flop transition due to magnetic anisotropy, which is also commonly reported in other Co compounds, e.g., CoMOF 5 (pyrazine)(H 2 O) 2 (M = Nb and Ta)72 but not widely observed for 2 and other Ni 2+ compounds. Note that such a field-induced transition like a spin-flop transition is reported in iodates in this work for the first time.…”

“…The P eff is larger than the spin-only value of 3.87 μ B for a high state of Co 2+ , suggesting a significant unquenched orbital contribution or magnetic anisotropy, which is widely observed in reported Co 2+ compounds. 71,72,83,84 The negative Weiss constant indicates that the dominant exchange interaction in 1 is antiferromagnetic. The χT value was 3.42 emu•K•mol −1 at 300 K, which is larger than the spin-only value due to the aforementioned significant orbital contribution or magnetic anisotropy.…”

“…These bond lengths agree well with those in reported compounds. 48,71,72 Owing to the opposite direction arrangements of the lone pairs on the IO 3 polyhedra, the structure of 1 exhibits nonpolarity. This is similar with those reported in Cd(IO 3 )X (X = Cl and OH), where CdO 4 Cl 2 octahedra are trans-edge-sharing in the [CdO 3 Cl] n chains compared to cis-edge-sharing CoO 4 F 2 octahedra in 1 in this work.…”

Ligand Vacancy Channels in Pillared Inorganic-Organic Hybrids for Electrocatalytic Organic Oxidation with Enzymatic Activities

“…Specifically, hybrid compounds with a formula of MX2(N-N) (M=Fe, Co, Ni; X=Cl, Br; N-N=bidentate organic linkers, such as pyrazine, 4,4'-bipyridine, trans-4,4'-azopyridine and etc.) are a class of inorganicorganic hybrids with excellent magnetic properties 31,35,36,37,38 . In the MX2(N-N) structures, the M centers are octahedrally coordinated by four X ligands and two N atoms from the bidentate ligand in a trans disposition.…”