Temperature Dependence of the Crystal Lattice Organization of Coordination Compounds Involving Nitronyl Nitroxide Radicals: A Magnetic and Structural Investigation

Four new mononuclear complexes of formula Cd(PN)4(NCS)2 (A), Cd(PNN)4(N3)2 (B), Zn(PNN)4(N3)2 (C), and
Zn(PNN)2(NCS)2 (D), where PNN stands for 2-(4-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and PN for
2-(4-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl, were synthesized and structurally and magnetically characterized.
The X-ray structures of compounds B and C were also determined at 90 K. Compounds A−C crystallize in the
triclinic space group P1̄ (No. 2), and D crystallizes in the monoclinic space group P21/m (No. 11). A−C adopt a
centrosymmetric distorted octahedral geometry in which the metal ions are bonded to four radical ligands through
the nitrogen atom of the pyridyl rings and the azido or thiocyanato ligands occupy the apical positions. Compound
D adopts a distorted tetrahedral geometry in which the zinc ion is bonded to two radicals and two thiocyanato
ligands. As suggested by their magnetic behavior, the low-temperature X-ray structures of B and C show that
these compounds undergo a clear structural change with respect to the room-temperature structures. The experimental
magnetic behaviors were perfectly reproduced by a dimer model for A−C and an alternating chain model for D
while the sudden breaks observed in the χMT versus T curves for B and C were well accounted for by the high-
and low-temperature X-ray structures. For all these complexes the crystal structures favor significant overlap between
molecular magnetic orbitals leading to rather strong intermolecular antiferromagnetic interactions.