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 ·(H 2 O), 2 ; Mn(hfac) 2 (NNPhOMe)· x (C 7 H 16 ), 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 1̅, space group, whereas complex 3 crystallizes in the monoclinic structure with the C 2/ c space group and forms chain-like structure along the c direction. The complex 2 crystallizes in the monoclinic symmetry with the P 2 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 = -2 J ∑( S metal S radical ) 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 {∼0.03 for Co(II) and ∼0.015 for Mn(II)}, whereas a more significant fraction {∼0.24 for Mn(II) and ∼0.13 for Co(II)} of delocalized spins from the metal ion is transferred to the coordinated oxygen atom(s) of nitronyl nitroxide.