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 {<b>R</b> ^<b>1</b> = 4'-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and <b>R</b> ^<b>2</b> = 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, <b>1</b>; Co(hfac)₂(NNT)₂·(H₂O), <b>2</b>; Mn(hfac)₂(NNPhOMe)·<i>x</i>(C₇H₁₆), <b>3</b>; and Mn(hfac)₂(NNT)₂, <b>4</b>. 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 <b>1</b> and <b>4</b> crystallize in the triclinic, <i>P</i>1̅, space group, whereas complex <b>3</b> crystallizes in the monoclinic structure with the <i>C</i>2/<i>c</i> space group and forms chain-like structure along the <i>c</i> direction. The complex <b>2</b> crystallizes in the monoclinic symmetry with the <i>P</i>2₁/<i>c</i> 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 (<i>J</i>/<i>K</i> _B) are derived using isotropic spin Hamiltonian <i>H</i> = -2<i>J</i>∑(<i>S</i> _metal <i>S</i> _radical) for the model fitting to the magnetic susceptibility data for <b>1</b>, <b>2</b>, <b>3</b>, and <b>4</b>. The exchange interaction strengths are found to be -328, -1.25, -248, and -256 K, for the <b>1</b>, <b>2</b>, <b>3</b>, and <b>4</b> 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.