Pyridine-substituted nitronyl nitroxide biradicals: a triplet (S = 1) ground state lasting out N-methylation

Abstract

A 3,5-pyridine-substituted biradical of nitronyl nitroxide (1) has been designed, synthesized and the magnetic properties fully characterized. The ground-state spin multiplicity of 1 has been found to be triplet (S = 1) with a singlet–triplet energy gap of 2J/k_B = 40 K from magnetic susceptibility measurements on a magnetically diluted system dispersed in organic polymer films. The 3,5-substituted pyridine 1 has a hydrogen-accepting site which is more accessible to hydrogen donors than previously known biradicals with sterically hindered 2,6-pyridine frameworks. N-Methylation of 1 has yielded a stable cationic species in a trifluoromethanesulfonate salt (2⁺·TfO⁻). The ground state of the cation 2⁺ has been found to be triplet as well with 2J/k_B = 32 K from magnetic susceptibility measurements for magnetically diluted films. The magnetic susceptibility of neat crystalline solids of 1 and 2⁺·TfO⁻ has been explained by Heisenberg exchange coupling models based on their X-ray crystal structures. It is well known that the energy preference of a high-spin ground state for m-phenylene, or m-xylylene, coupling units is disturbed in such cases as the π-conjugation is affected by heteroatomic substitution, an ionic charge, or molecular conformation. The present experimental results show that the high-spin preference in 1 and 2⁺ is little influenced by the heterocycle or the ionic charge. Intermolecular noncovalent bonds such as hydrogen bonding and electrostatic interactions are a driving force for crystallization of open-shell molecules in a controllable manner. The ground-state triplet biradicals serve as building blocks for molecule-based magnets of S > 1/2 based on intermolecular noncovalent bonding architecture.