Magnetic Exchange Interaction in Nitronyl NitroxideRadical-Based Single Crystalsof 3d Metal Complexes: A Combined Experimental and Theoretical Study

摘要

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·(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$\bar{1}$, 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 susceptibilitydata 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–organiccomplexes, respectively. Quantum chemical density functional theory(DFT) computations are carried out on several models of the metal–radicalcomplexes to elucidate the magnetic interactions at the molecularlevel. The calculations show that a small part of the inorganic spinsare 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 delocalizedspins from the metal ion is transferred to the coordinated oxygenatom(s) of nitronyl nitroxide.