Relationship between phase transition temperature and accessible volume for substituent in Cu(hfac)₂ chain-polymer complexes with pyridine-based nitroxides

Reaction of Cu(hfac)₂ with pyridyl-substituted nitronyl nitroxides yielded chain-polymer complexes {[Cu(hfac)₂L^R]₂[Cu(hfac)₂]}_∞, where L^R – 2-(4-R-pyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, R = Me, MeO, Et, n-Pr, i-Pr, n-Bu, and CP (cyclopentyl). X-ray crystallography experiments proved that the solid state of the investigated compounds has centrosymmetric dimers [Cu(hfac)₂L^R]₂ bonded by the Cu(hfac)₂ fragments into chain polymers. The environment of the Cu atoms in the dimeric fragments undergoes structural rearrangement upon temperature variation. Such transformations cause anomalous temperature dependences of µ_eff. We established the relationship between the phase transition temperature and the amount of space accessible for pyridine substituent R. In the complexes containing L^Me, L^MeO, L^n-Bu, and L^CP substituents R occupy relatively small amount of space, which results in ordering of R and high transition temperatures of these complexes, whereas the presence of the accessible volume for the substituent disordering leads to increased entropy of the high temperature phase, and, therefore, decreased transition temperature of the complexes with L^Et, L^i-Pr, and L^n-Pr.