Structural, Electronic, and Magnetic Properties of Quasi-1D Quantum Magnets [Ni(HF₂)(pyz)₂]X (pyz = pyrazine; X = PF₆^–, SbF₆^–) Exhibiting Ni-FHF-Ni and Ni-pyz-Ni Spin Interactions

Abstract

[Ni(HF₂)(pyz)₂]X {pyz = pyrazine; X = PF₆^– (<b>1</b>), SbF₆^– (<b>2</b>)} were structurally characterized by synchrotron X-ray powder diffraction and found to possess axially compressed NiN₄F₂ octahedra. At 298 K, <b>1</b> is monoclinic (<i>C</i>2/<i>c</i>) with unit cell parameters, <i>a</i> = 9.9481(3), <i>b</i> = 9.9421(3), <i>c</i> = 12.5953(4) Å, and β = 81.610(3)° while <b>2</b> is tetragonal (<i>P</i>4/<i>nmm</i>) with <i>a</i> = <i>b</i> = 9.9359(3) and <i>c</i> = 6.4471(2) Å and is isomorphic with the Cu-analogue. Infinite one-dimensional (1D) Ni-FHF-Ni chains propagate along the <i>c</i>-axis which are linked via μ-pyz bridges in the <i>ab</i>-plane to afford three-dimensional polymeric frameworks with PF₆^– and SbF₆^– counterions occupying the interior sites. A major difference between <b>1</b> and <b>2</b> is that the Ni–F–H bonds are bent (∼157°) in <b>1</b> but are linear in <b>2</b>. Ligand field calculations (LFT) based on an angular overlap model (AOM), with comparison to the electronic absorption spectra, indicate greater π-donation of the HF₂^– ligand in <b>1</b> owing to the bent Ni–F–H bonds. Magnetic susceptibility data for <b>1</b> and <b>2</b> exhibit broad maxima at 7.4 and 15 K, respectively, and λ-like peaks in dχ<i>T</i>/d<i>T</i> at 6.2 and 12.2 K that are ascribed to transitions to long-range antiferromagnetic order (<i>T</i>_N). Muon-spin relaxation and specific heat studies confirm these <i>T</i>_N’s. A comparative analysis of χ vs <i>T</i> to various 1D Heisenberg/Ising models suggests moderate antiferromagnetic interactions, with the primary interaction strength determined to be 3.05/3.42 K (<b>1</b>) and 5.65/6.37 K (<b>2</b>). However, high critical fields of 19 and 37.4 T obtained from low temperature pulsed-field magnetization data indicate that a single exchange constant (<i>J</i>_1D) alone is insufficient to explain the data and that residual terms in the spin Hamiltonian, which could include interchain magnetic couplings (<i>J</i>_⊥), as mediated by Ni-pyz-Ni, and single-ion anisotropy (<i>D</i>), must be considered. While it is difficult to draw absolute conclusions regarding the magnitude (and sign) of <i>J</i>_⊥ and <i>D</i> based solely on powder data, further support offered by related Ni(II)-pyz compounds and our LFT and density-functional theory (DFT) results lead us to a consistent quasi-1D magnetic description for <b>1</b> and <b>2</b>.