Coexistence of an antiferromagnetically coupled dimer and isolated paramagnetic spin in 4-azaindol-2-yl nitronyl nitroxide crystal

A novel stable organic radical, 2-(4-azaindol-2-yl)-4,4,5,5- tetramethyl-4,5-dihydro-1H-imidazoline-1-oxyl-3-oxide (2) was synthesized and its magneto-structural correlation is discussed. 2 crystallizes in the space group C2/c with hydrogen bonding between two independent molecules (A and B) having different dihedral angles between the azaindole rings and nitronyl nitroxide units in the asymmetric unit. The molecules are further linked together with axisymmetrically related molecules (A* and B*) through hydrogen bonds (A-B* and A*-B) and π stacking (A-A*). The temperature dependence of the magnetic susceptibility reveals that half of the radicals are antiferromagnetically coupled as a dimer (singlet-triplet energy gap: 2J = -64 cm^-1) while the other half behaves as an isolated monomer. DFT calculations (UB3LYP/6-31G*) of the dimeric coordinates extracted from the X-ray analysis rationalize the fact that the π-stacked A-A* dimer contributes to the antiferromagnetic coupling by a close contact between nitronyl nitroxide units.