Specific features of the spin density distribution and magnetic properties in a series of pyrazolyl-substituted nitronyl nitroxides: a magnetochemical and quantum chemical study

Abstract

A series of 2-(1-R-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl radicals (NN-4Pz^R) were synthesized and structurally and magnetochemically characterized. A comprehensive comparative crystallochemical analysis and quantum chemical calculations were carried out for a series of new and previously obtained radicals NN-4Pz^R and NN-5Pz^R [2-(1-R-pyrazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl]. It was shown that alteration of the spin label position drastically affects the molecular geometry and spin density distribution within the radicals. Planar molecules of NN-4Pz^H are conducive to π–π stacking interactions between pyrazole rings, which bring the radicals closer together and hence promote effective exchange interactions in the π stacks. In contrast, in NN-5Pz^R, substituent R of the pyrazole ring makes the formation of the planar structure infeasible due to steric effects, and exchange interactions arise mainly owing to short contacts between nitronyl nitroxide moieties. Thus, we proved that alteration of the spin label location is a powerful supramolecular tool and represents a mechanism of spin density variation in pyrazolyl-substituted NN-4Pz^R and NN-5Pz^R radicals.