Influence of HF2- geometry on magnetic interactions elucidated from polymorphs of the metal-organic framework [Ni(HF2)(pyz)2]PF6 (pyz = pyrazine).

A tetragonal polymorph of [Ni(HF2)(pyz)2]PF6 (designated β) is isomorphic to its SbF6-congener at 295 K and features linear Ni–FHF–Ni pillars. Enhancements in the spin exchange (JFHF = 7.7 K), Neel temperature (TN = 7 K), and critical field (Bc = 24 T) were found relative to monoclinic α-PF6. DFT reveals that the HF2− bridges are significantly better mediators of magnetic exchange than pyz (Jpyz), where JFHF ≈ 3Jpyz, thus leading to quasi-1D behavior. Spin density resides on all atoms of the HF2− bridge whereas N-donor atoms of the pyz ring bear most of the density.