Spin-Transition-Like Behavior in [Cu(pyNO)X2] (pyNO = tert-Butyl 2-Pyridyl Nitroxide): 113 K Hysteresis for X = Cl and Incomplete Crossover for X = Br

The complexation of CuX₂ with tert-butyl 2-pyridyl nitroxide (pyNO) resulted in the formation of a linear array of discrete 2p-3d dyads, [Cu(pyNO)X₂] (X = Br, Cl). Powder X-ray diffraction studies suggest that the two compounds are isomorphous. Magnetic investigations of [Cu(pyNO)Br₂] revealed a gradual spin transition, behaving as a spin equilibrium that obeys the van't Hoff law. Structural analysis indicated a relatively planar chelate structure with a torsion angle of ϕ(Cu-O-N-C_2py) = 9(6)° at 400 K, ensuring an almost orthogonal alignment between the copper 3dσ and nitroxide π* orbitals. In contrast, a nonplanar coordination geometry with ϕ = 33.4(12)° was observed at 100 K, leading to orbital overlap between the two magnetic orbitals. A significant thermal hysteresis with a width of 113 K was recorded for [Cu(pyNO)Cl₂]. The DFT geometry optimizations on [Cu(pyNO)X₂] suggest that the singlet and triplet states have ϕ = 35.9° and 5.5°, respectively, for X = Br. Likewise, the singlet and triplet states have ϕ = 38.0° and 1.4°, respectively, for X = Cl. These experimental results can be explained as follows: for X = Cl, an energy level crossing occurs between the S_total = 1 and S_total = 0 states, whereas for X = Br, the two states approach each other but do not cross.