Molecular S = 2 High-Spin, S = 0 Low-Spin and S = 0 ⇄ 2 Spin-Transition/-Crossover Nickel(II)-Bis(nitroxide) Coordination Compounds

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