B istable solid-sate materials without any change in chemical composition, spin-crossover complexes for instance, are of increasing interest for future applications to memory, display, and other devices. 1−3 Transition between S = 0 and S = 2 states in discrete iron(II) 3d 6 complexes is typical. 3 Heterospin systems provide the wide diversity of the characters of paramagnetic centers including the symmetry of magnetic orbitals. 4 Direct copper(II)-radical coordination compounds are the best documented among the 3d-2p heterospin molecules, 5 and nickel(II)-radical compounds are the second but relatively scarce. 6 Recently nickel(II)-iminosemquinonate chelates have been reported to exhibit ferromagnetic coupling, after the pioneering work on the copper(II)-semiquinonate compounds reported by Kahn and co-workers. 7 The equatorial coordination in copper(II)-nitroxide radical complexes is well-known to favor strong antiferromagnetic coupling, 5 but ferromagnetic interaction actually is available as well. 5b,8 The magnitudes of the couplings often exceed the order of 300 K, whether ferro-or antiferromagnetic, being sensitive to the geometry. Ferromagnetic coupling can be realized when two magnetic orbitals (radical π* and metal dσ) are strictly orthogonal. On the other hand, severe twist around the Cu−O (or Ni−O) coordination bond would lead to an appreciable overlap between Cu (or Ni) 3d x2−y2 and 3d z2 and O 2p z magnetic orbitals, affording antiferromagnetic coupling. 8 Spin-transition-like behavior in three-or four-centered spin systems has been reported owing to the chelate-ring deformation 9,10 by using tert-butyl 2-pyridyl nitroxides as a paramagnetic ligand. We will report here the first example of nickel(II)-radical compounds showing spintransition-like behavior. An advantage of the present system resides in the feature where the drastic spin transition occurs between dia-and paramagnetic states.This report includes a mechanistic investigation of the spintransition-like behavior, and the ground high-and low-spin states are directly regulated with the switch of Ni 2+ -radical exchange couplings. The situation is completely different from those of the known materials having spin-crossover ions and additional exchange coupling. 11 A 2p-3d-2p heterospin triad [Ni(phpyNO) 2 Cl 2 ] (1) was prepared by complex formation of NiCl 2 •6H 2 O with a paramagnetic ligand, tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) ligand 8c (S rad = 1/2). The spectroscopic and analytic characterizations were satisfactory, and finally the molecular structure was confirmed by X-ray crystallographic study (Figure 1). On cooling, chelate ring distortion very gradually took place in a single-crystal-to-single-crystal manner, and we determined the molecular structures at any temperature between 85 and 400 K.

Complexation 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 3d x 2–y 2 and 3d z 2 orbitals. Magnetic study revealed the high-spin ground state with the exchange coupling constant 2J/k B = +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 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.

Heterospin systems have a great advantage in frontier orbital engineering since they utilize a wide diversity of paramagnetic chromophores and almost infinite combinations and mutual geometries. Strong exchange couplings are expected in 3d–2p heterospin compounds, where the nitroxide (aminoxyl) oxygen atom has a direct coordination bond with a nickel(II) ion. Complex formation of nickel(II) salts and tert-butyl 2-pyridyl nitroxides afforded a discrete 2p–3d–2p triad. Ferromagnetic coupling is favored when the magnetic orbitals, nickel(II) dσ and radical π*, are arranged in a strictly orthogonal fashion, namely, a planar coordination structure is characterized. In contrast, a severe twist around the coordination bond gives an orbital overlap, resulting in antiferromagnetic coupling. Non-chelatable nitroxide ligands are available for highly twisted and practically diamagnetic complexes. Here, the Ni–O–N–Csp2 torsion (dihedral) angle is supposed to be a useful metric to describe the nickel ion dislocated out of the radical π* nodal plane. Spin-transition complexes exhibited a planar coordination structure in a high-temperature phase and a nonplanar structure in a low-temperature phase. The gradual spin transition is described as a spin equilibrium obeying the van’t Hoff law. Density functional theory calculation indicates that the energy level crossing of the high- and low-spin states. The optimized structures of diamagnetic and high-spin states well agreed with the experimental large and small torsions, respectively. The novel mechanism of the present spin transition lies in the ferro-/antiferromagnetic coupling switch. The entropy-driven mechanism is plausible after combining the results of the related copper(II)-nitroxide compounds. Attention must be paid to the coupling parameter J as a variable of temperature in the magnetic analysis of such spin-transition materials. For future work, the exchange coupling may be tuned by chemical modification and external stimulus, because it has been clarified that the parameter is sensitive to the coordination structure and actually varies from 2J/kB = +400 K to −1400 K.

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