Synthesis, crystal structure and magnetic properties of a novel nitroxide biradical. Theoretical investigation of the exchange mechanisms

The novel nitroxide biradical 1,8-bis(3-ethinyl-2,2,5,5-tetramethyl-3-pyrroline-1-oxyl)-naphthalene was synthesized and its structure and magnetic properties were investigated. Using SQUID measurements an antiferromagnetic exchange coupling of 2J=−3.5 K was evaluated. Temperature-dependent measurements of the half-field EPR signal intensity and broken symmetry DFT computations confirm this result. To unravel the mechanisms of the intramolecular exchange interaction calculations on model systems were performed. These revealed a strong ferromagnetic through-bond interaction via the 1,8-substituted naphthalene bridge and a competing strong antiferromagnetic through-space interaction via the acetylene groups. Both interactions are of the same order of magnitude leading to the weak overall coupling observed.