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 { 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 m...

High-Frequency EPR Studies of New 2p–3d Complexes Based on a Triazolyl-Substituted Nitronyl Nitroxide Radical: The Role of Exchange Anisotropy in a Cu–Radical System

“…Previous studies of similar Mn-NIT radicals usually show moderate antiferromagnetic interactions (−77 cm –1 , and −76.8 cm –1 , with the same Hamiltonian used in this work). However, when coordinated by two NIT radicals, the antiferromagnetic interaction appears to be strengthened (−172 and −178 cm –1 ) . Compound 1 represents a confirmation of this magneto-structural correlation.…”

“…However, when coordinated by two NIT radicals, the antiferromagnetic interaction appears to be strengthened (−172 and −178 cm −1 ). 51 Compound 1 represents a confirmation of this magneto-structural correlation.…”

Origin of Magnetic Relaxation Barriers in a Family of Cobalt(II)–Radical Single-Chain Magnets: Density Functional Theory and Ab Initio Calculations

“…Bhatt et al used the hybrid functional, B3LYP, to calculate the Co II −radical interactions and the Mulliken spin density distributions for a series of nitronyl nitroxide radical-based Co II metal complexes and achieved good results in agreement with the experiment. 19 We also gave the calculated spin density maps of 1_a, 2_a, and 3 in HS and BS states, respectively, by B3LYP in Figure 4, where the blue and red regions indicate positive and negative spin populations, respectively. The spin populations on a given atomic orbital deriving from spin polarization are roughly proportional to the number of unpaired electrons on the metal atom.…”

“…In addition, strong metal–radical antiferromagnetic exchange coupling was found in Co­(hfac) 2 (NITPhOMe), about 44 meV (354.9 cm –1 ) for the Co II site, which is characterized by noncollinear uniaxial anisotropies. In 2018, density functional theory (DFT) calculations of the metal–organic complexes Co-(hfac) 2 (NNPhOMe) 2 and Co­(hfac) 2 (NNT) 2 demonstrate that antiferromagnetic spin–spin interactions exist, −328 K (−229.37 cm –1 ) and –1.25 K (−0.87 cm –1 ), respectively, because of the bonding overlap between Co II and the nitroxide contributions in the singly occupied molecular orbitals (SOMOs) . In terms of SCMs, the discovery of a novel light-induced domain-wall kickoff mechanism via absorbing a photon at one side of the chain achieved the extension of the characteristic time of spin-flips linked to the exciton lifetime and opened up new prospects for the field of molecular magnetism …”

Two Cu(II)-Nitronyl Nitroxide Complexes Including Cyclic Dimer: Synthesis, Structures and Magnetic Interactions

“…Among these metal-NITR complexes, copper (II) compounds are often used as unique objects for investigating the novel structures and the peculiarities of magnetic exchange pathways, for instance, Cu(II)-NITR complexes with different R groups can form mononuclear and polynuclear complexes, as well as one-dimensional and higher dimensional architectures, which result in different magnetic interactions between Cu(II) and NITR radicals [12] [13]. Investigations reveal that if ONCNO of the nitronyl nitroxide group links with Cu(II) ion at axial position, the magnetic property tends to be a ferromagnetic interaction, whereas equatorial coordinates to Cu(II) ion generally prefers to an antiferromagnetic interaction [14] [15] [16]. To obtain novel structural complexes and gain more information on magnetic interactions between transition metal ions and NITR radical ligands, we concentrate our study on designing and synthesizing new functional radical ligand and M-NITR compounds.…”