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

Abstract

Two stable nitronyl nitroxide free radicals {R1 = 4′-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and R2 = 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·(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, P1̅, 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 P21/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/KB) are derived using isotropic spin Hamiltonian H = −2J∑(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 {∼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.