Magnetic Exchange Interaction in Nitronyl Nitroxide Radical-Based Single Crystals of 3d Metal Complexes: A Combined Experimental and Theoretical Study

Two stable nitronyl nitroxide free radicals {R-1 = 4'-methoxy-phenyl-4,4,5,5,- tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and R-2 = 2-(2'-thienyl)4,4,5,5- tetramethylimidazoline 3-oxide 1-oxyl (NNT)} are successfully synthesized using Ullmann condensation. The reactions of these two radicals with 3d transition metal ions, in the form of M(hfac)(2) (where M = Co or Mn, hfac: hexafluoroacetylacetone), result in four metal-organic complexes Co( hfac)(2)(NNPhOMe)(2), 1; Co(hfac)(2) (NNT)(2)center dot(H2O), 2; Mn(hfac)(2)(NNPhOMe). x(C7H16), 3; and Mn(hfac)(2)(NNT)(2), 4. The crystal structure and magnetic properties of these complexes are investigated by single-crystal X-ray diffraction, dc magnetization, infrared, and electron paramagnetic resonance spectroscopies. The compounds 1 and 4 crystallize in the triclinic, P (1) over bar, space group, whereas complex 3 crystallizes in the monoclinic structure with the C2/c space group and forms chain-like structure along the c direction. The complex 2 crystallizes in the monoclinic symmetry with the P2(1)/c space group in which the N-O unit of the radical coordinates with the Co ion through hydrogen bonding of a water molecule. All compounds exhibit antiferromagnetic interactions between the transition metal ions and nitronyl nitroxide radicals. The magnetic exchange interactions (J/K-B) are derived using isotropic spin Hamiltonian H = -2J Sigma(SmetalSradical) for the model fitting to the magnetic susceptibility data for 1, 2, 3, and 4. The exchange interaction strengths are found to be -328, -1.25, -248, and -256 K, for the 1, 2, 3, and 4 metal-organic complexes, respectively. Quantum chemical density functional theory (DFT) computations are carried out on several models of the metal-radical complexes to elucidate the magnetic interactions at the molecular level. The calculations show that a small part of the inorganic spins are delocalized over the oxygens from hfac {similar to 0.03 for Co(II) and similar to 0.015 for Mn(II)}, whereas a more significant fraction {similar to 0.24 for Mn(II) and similar to 0.13 for Co(II)} of delocalized spins from the metal ion is transferred to the coordinated oxygen atom(s) of nitronyl nitroxide.