High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L) 2 Br 2 ] (L = tert -Butyl 5-Phenyl-2-pyridyl Nitroxide).

Complexation of nickel(II) bromide with tert -butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) gave two morphs of doubly chelated [Ni(phpyNO) ₂ Br ₂ ] as a 2p-3d-2p heterospin triad. The α phase crystallizes in the orthorhombic space group Pbcn . An asymmetric unit involves a half-molecule. The torsion angle around Ni-O-N-C _2py 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 _{x ² - y ²} and 3d _{z ²} orbitals. Magnetic study revealed the high-spin ground state with the exchange coupling constant 2 J / k _B = +288(5) K, on the basis of a symmetrical spin Hamiltonian. The β phase crystallizes in the monoclinic space group P 2 ₁ / n . The whole molecule is an independent unit. The Ni-O-N-C _2py 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 S _total = 0 in the low-temperature structure while S _total = 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.