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)2LR]2[Cu(hfac)2]}infinity, where LR - 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)2LR]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 de-pendences of mu eff. We established the relationship between the phase transition temperature and the amount of space accessible for pyridine substituent R. In the complexes containing LMe, LMeO, Ln-Bu, and LCP 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 LEt, Li-Pr, and Ln-Pr.

Automated summary

Cu(hfac)2 reacted with pyridyl-substituted nitronyl nitroxides to form chain-polymer complexes {[Cu (hfac)2LR]2[Cu(hfac)2]}infinity, where LR represents 2-(4-R-pyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, and R can be Me, MeO, Et, n-Pr, i-Pr, n-Bu, or CP (cyclopentyl). X-ray crystallography experiments demonstrated the presence of centrosymmetric dimers [Cu(hfac)2LR]2 bonded by the Cu (hfac)2 fragments in the solid state of the complexes, forming chain polymers. The Cu atoms in the dimeric fragments undergo structural rearrangement upon temperature variation, resulting in anomalous temperature dependence of the magnetic moment (mu eff).