Solid-State Transformation (Stotal = 0, 1, and 2) in a Ni2+ Chelate with Two tert-Butyl 5-(p-Biphenylyl)-2-pyridyl Nitroxides

A novel <i>S</i> = 1/2 paramagnetic chelating ligand <i>tert</i>-butyl 5-(<i>p</i>-biphenylyl)-2-pyridyl nitroxide (bppyNO) and its <i>S</i> = 1 nickel(II) ion complex [Ni(bppyNO)₂Br₂] were synthesized. X-ray crystallography revealed a 2p-3d-2p heterospin triad, with half of the molecule being crystallographically independent. A relatively planar chelate geometry with the torsion angle <i>ϕ</i>(Ni-O-N-C_2py) = -10.6(5)° at 300 K becomes significantly out-of-plane distorted with <i>ϕ</i> = -46.9(8) and 26.1(11)° at 90 K accompanied by disorder at the oxygen site. The torsion angle changes, Δ<i>ϕ</i> = 36° and 37°, are among the largest reported for related compounds. Magnetic measurements indicate gradual and incomplete spin transition-like behavior around 143(2) K. A three-state model involving an intermediate-spin (<i>S</i>_total = 1) state is proposed to explain non-zero <i>χ</i>_m<i>T</i> plateau in a low-temperature region. Density functional theory calculations using the determined structures support the proposed mechanism. Furthermore, geometry optimizations assuming <i>S</i>_total = 0, 1, and 2 are in good agreement with the present model.