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

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

Using the 1-(m-tolyl)-1H-1,2,3-triazole-4-(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) (TlTrzNIT) radical and metal β-diketonate complexes [M(hfac)₂(H₂O)₂], where hfac is hexafluoroacetylacetonato, three new 2p-3d heterospin complexes were synthesized. Their structures were solved using single crystal X-ray diffraction data, and magnetic investigation was performed by DC and AC measurements and multifrequency EPR spectroscopy. Compounds 1 and 2 are isostructural complexes with molecular formula [M₃(TlTrzNIT)₂(hfac)₆] (M^II = Mn or Cu) while compound 3 is the mononuclear [Co(TlTrzNIT)(hfac)₂] complex. In all complexes, the radical acts as a bidentate ligand through the oxygen atom of the nitroxide moiety and the nitrogen atom from the triazole group. Furthermore, in compounds 1 and 2, the TlTrzNIT is bridge-coordinated between two metal centers, leading to the formation of trinuclear complexes. The fitting of the static magnetic behavior reveals antiferromagnetic and ferromagnetic intramolecular interactions for complexes 1 and 2, respectively. The EPR spectra of 1 are well described by an isolated ferrimagnetic S = ¹³/₂ (= ⁵/₂ - ¹/₂ + ⁵/₂ - ¹/₂ + ⁵/₂) ground state with a biaxial zero-field splitting (ZFS) interaction characterized, respectively, by 2nd order axial and rhombic parameters, D and E, such that E/D is close to the maximum of 0.33. Meanwhile, EPR spectra for 2 are explained in terms of a ferromagnetic model with weakly anisotropic Cu-radical exchange interactions, giving rise to an isolated S = ⁵/₂ (= 5 × ¹/₂) ground state with both an anisotropic g tensor and a weak ZFS interaction. Complex 2 represents one of only a few examples of Cu-radical moieties with measurable exchange anisotropy.

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

Density functional theory (DFT) and ab initio calculations were performed to probe the origin of the magnetic relaxation barriers for two finite single-chain magnets (SCMs) featuring a one-dimension chain, Co(hfac)₂(R-NapNIT) (R-NapNIT = 2-(2'-(R-)naphthyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, R = MeO (1) or EtO (2)). Our calculations show that the strong intrachain Co^II-Co^II exchange coupling interactions transmitted by radicals can contribute much more than ionic anisotropy to the height of the reversal barrier of magnetization for the single-chain magnets (SCMs) with |2E| < |4J/3|. In addition, the anisotropic energy barrier Δ_A decreases with the decrease of |2E/J| ratio and finally vanishes in the limit of broad domain walls (|2E| < < |4 J/3|). Therefore, the total magnetic relaxation energy barriers of two SCMs mostly originate from the correlation energy barrier Δ_ξ deriving from the indirect ferromagnetic interaction between Co^II-Co^II transmitted by the strong Co^II-radical antiferromagnetic interactions.

Probing through-space and through-bond magnetic exchange couplings in a new benzotriazinyl radical and its metal complexes

A new Blatter radical and its Zn(ii) (1), Ni(ii) (2) and Co(ii) (3) complexes were isolated. Complex 1 exhibited radical⋯radical antiferromagnetic exchange coupling, whereas complexes 2 and 3 showed ferromagnetic metal–radical coupling.

A loop chain and a three-dimensional network assembled from a multi-dentate nitronyl nitroxide radical and M(hfac)2 (M = CoII, CuII)

A multi-dentate nitronyl nitroxide radical Nit-Ph-3,5-bIm (Nit-Ph-3,5-bIm = 2-[3,5-bis(1-imidazole)-phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) reacted with Co(hfac)2·2H2O or Cu(hfac)2 to yield two novel 3d-radical complexes {[Co(hfac)2]2(Nit-Ph-3,5-bIm)}n1 and {[Cu(hfac)2]7(Nit-Ph-3,5-bIm)3}n2 (hfac- = hexafluoroacetylacetonate). In both compounds, the Nit-Ph-3,5-bIm radical ligand behaves as a tetradentate ligand to link four M(hfac)2 units in a μ4-η1:η1:η1:η1-coordination mode via its two NO groups and two N atoms of imidazole, generating a unique loop chain for 1 and a three-dimensional framework for 2. The magnetic studies revealed a strong antiferromagnetic Co-ON exchange interaction in 1 while a ferromagnetic Cu-ON interaction is observed in 2. The dc magnetic behaviors of two complexes are analyzed by means of appropriate magnetic models. Furthermore, ac magnetic susceptibilities of the Co complex reveal slow relaxation of magnetization.