Kyoden, Yukiya Homma, Yuta Ishida, Takayuki High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)<sub>2</sub>Br<sub>2</sub>] (L = <i>tert</i>-Butyl 5‑Phenyl-2-pyridyl Nitroxide) Complexation of nickel­(II) bromide with <i>tert</i>-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) gave two morphs of doubly chelated [Ni­(phpyNO)<sub>2</sub>Br<sub>2</sub>] as a 2p–3d–2p heterospin triad. The α phase crystallizes in the orthorhombic space group <i>Pbcn</i>. An asymmetric unit involves a half-molecule. The torsion angle around Ni–O–N–C<sub>2py</sub> is as small as 6.5(3)° at 100 K and 7.0(6)° at 400 K, guaranteeing an orthogonal arrangement between the magnetic radical π* and metal 3d<sub><i>x</i><sup>2</sup>–<i>y</i><sup>2</sup></sub> and 3d<sub><i>z</i><sup>2</sup></sub> orbitals. Magnetic study revealed the high-spin ground state with the exchange coupling constant 2<i>J</i>/<i>k</i><sub>B</sub> = +288(5) K, on the basis of a symmetrical spin Hamiltonian. The β phase crystallizes in the monoclinic space group <i>P</i>2<sub>1</sub>/<i>n</i>. The whole molecule is an independent unit. The Ni–O–N–C<sub>2py</sub> torsion angles are 24.2(6) and 37.2(5)° at 100 K and 10.4(7) and 25.9(6)° at 400 K. A magnetic study revealed a very gradual and nonhysteretic spin transition. An analysis based on the van’t Hoff equation gave a successful fit with the spin-crossover temperature of 134(1) K, although the susceptibility did not reach the theoretical high-spin value at 400 K. Density functional theory calculation on the β phase showed ground <i>S</i><sub>total</sub> = 0 in the low-temperature structure while <i>S</i><sub>total</sub> = 2 in the high-temperature structure, supporting the synchronized exchange coupling switch on both sides. Consequently, the β phase can be recognized as an “incomplete spin crossover” material, as a result of conflicting thermal depopulation effects in a high-temperature region. Br;space group Pbcn;400 K;tert;β phase crystallizes;high-spin ground state;Ni;butyl 5- phenyl -2-pyridyl nitroxide;α phase crystallizes;torsion;Incomplete Spin-Crossover Polymorphs;z 2 orbitals;100 K;β phase 2019-08-01
    https://acs.figshare.com/articles/media/High-Spin_and_Incomplete_Spin-Crossover_Polymorphs_in_Doubly_Chelated_Ni_L_sub_2_sub_Br_sub_2_sub_L_i_tert_i_-Butyl_5_Phenyl-2-pyridyl_Nitroxide_/9205652
10.1021/acs.inorgchem.9b00885.s002