Crystal structures and magnetic properties of m-phenylenebis(imidazole) derivatives having two nitronyl nitroxide or iminyl nitroxide radicals. The two kinds of antiferromagnetic interaction alternating along one-dimensional chains

1, 3-Bis{2-(1-oxyl-3-oxo-4, 4, 5, 5-tetramethylimidazolin-2-yl)imidazol-1-yl}benzene (1), 1, 3-bis{2-(1-oxyl-4, 4, 5, 5-tetramethylimidazolin-2-yl)imidazol-1-yl}benzene (2) and their chloro derivatives (3 and 4) were prepared. Both diradicals 1, 2 and 3, 4 crystallized in orthorhombic, space group Pbcn and in monoclinic, space group P2(1)/c, respectively. The molecular structures of diradicals 1 and 2 were similar to each other and had C-2 symmetry. Intra-and intermolecular average distances between the two NO groups of the nitronyl nitroxide (NN) moieties in 1 and those of the N-N and NO-NO of the iminyl nitroxide (IN) moieties in 2 were found to be 4.26 and 3.49 Angstrom for 1 and 4.38 and 3.54 Angstrom for 2, respectively, suggesting the formation of magnetically linear chain structures along the c axis. An alternating antiferromagnetic one-dimensional chain model (H = -2J Sigma{SA2iSA2i-1 + alpha SA2iSA2i+1}) was applied to the spin systems 1 and 2 and a theoretical equation derived therefrom was fitted to the observed gamma(mol)T vs T plots to give the exchange coupling parameters, J/k(B) = -158 +/- 2 and -239 +/- 8 K and alpha = 0.22 +/- 0.02 and 0.11 +/- 0.04, respectively. The molecular structures of 3 and 4 were also similar to each other, in which two NN and LN groups are far away from each other by the nearest intramolecular average distance between radical centers being 4.54 and 4.72 Angstrom for 3 and 4, respectively. One of the latter has a disorder with respect to the position of the oxygen atom in IN. One of the two NNs and INs in both diradicals had intermolecular short distances between the radical centers in the neighboring molecules; average distances 3.56 Angstrom for 3 and 3.82 Angstrom for 4, to form magnetic dimer structures. The exchange coupling parameters for 3 and 4 were determined to be J/k(B) = -4.85 +/- 0.15 and -37.3 +/- 0.3 K (one triplet species for the singlet-triplet model; H = -2JS(1)S(2), and isolated two doublets in a ratio of 1:1), respectively.