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High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)2Br2] (L = tert-Butyl 5‑Phenyl-2-pyridyl Nitroxide)
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posted on 2019-08-01, 12:34 authored by Yukiya Kyoden, Yuta Homma, Takayuki IshidaComplexation of nickel(II)
bromide with tert-butyl 5-phenyl-2-pyridyl nitroxide
(phpyNO) gave two morphs of doubly chelated [Ni(phpyNO)2Br2] 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–C2py 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
3dx2–y2 and 3dz2 orbitals.
Magnetic study revealed the high-spin ground state with the exchange
coupling constant 2J/kB = +288(5) K, on the basis of a symmetrical spin Hamiltonian. The
β phase crystallizes in the monoclinic space group P21/n. The whole molecule is an independent
unit. The Ni–O–N–C2py 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 Stotal = 0 in the
low-temperature structure while Stotal = 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.