Antiferromagnetism in a family of S = 1 square lattice coordination polymers NiX2(pyz)2 (X = Cl, Br, I, NCS; pyz = Pyrazine).

The crystal structures of NiX2(pyz)2 (X = Cl (1), Br (2), I (3) and NCS (4)) were determined at 298 K by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN4X2 units that are bridged by pyz ligands. The 2D layered motifs displayed by 1-4 are relevant to bifluoride-bridged [Ni(HF2)(pyz)2]ZF6 (Z = P, Sb) which also possess the same 2D layers. In contrast, terminal X ligands occupy axial positions in 1-4 and cause a staggering of adjacent layers. Long-range antiferromagnetic order occurs below 1.5 (Cl), 1.9 (Br and NCS) and 2.5 K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and g factor of 2, 3 and 4 are measured by electron spin resonance where no zero–field splitting was found. The magnetism of 1-4 crosses a spectrum from quasi-two-dimensional to three-dimensional antiferromagnetism. An excellent agreement was found between the pulsedfield magnetization, magnetic susceptibility and TN of 2 and 4. Magnetization curves for 2 and 4 calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. 3 is characterized as a three-dimensional antiferromagnet with the interlayer interaction (J⊥) slightly stronger than the interaction within the two-dimensional [Ni(pyz)2]2+ square planes (Jpyz)