Syntheses and magnetic properties of a bis-bidentate nitronyl nitroxide radical based on triazolopyrimidine and its metal complexes

A novel bis-bidentate nitronyl nitroxide radical based on triazolopyrimidine, NIT-2-TrzPm (NIT-2-TrzPm = (2-(2'-triazolopyrimidine)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide)) and six new transition metal complexes of this ligand, namely [M(hfac)₂(NIT-2-TrzPm)]·CH₂Cl₂ (M = Mn (1Mn) and Co (2Co)), [M(hfac)₂]₂(NIT-2-TrzPm) (M = Mn (3Mn) and Co (4Co)), [Mn(NIT-2-TrzPm)₂(MeOH)₂](ClO₄)₂·MeOH (5Mn), and [Co(NIT-2-TrzPm)₂(MeOH)₂]₂(ClO₄)₄·4MeOH (6Co) were prepared and characterized structurally and magnetically. These complexes can be selectively synthesized by controlling the reaction ratio of M(hfac)₂·2H₂O to the radical ligand (for 1Mn to 4Co) or using metal perchlorates as the starting materials (for 5Mn and 6Co). Single crystal X-ray crystallographic analyses confirmed that 1Mn and 2Co are isostructural 3d-2p M^II-radical complexes, in which the NIT-2-TrzPm radical acts as a terminal bidentate ligand chelating to one 3d ion, while 3Mn and 4Co are isostructural 3d-2p-3d M^II-radical-M^II complexes with the NIT-2-TrzPm radical acting as a bridging ligand between two 3d ions. For complexes 5Mn and 6Co, two NIT-2-TrzPm ligands from the equatorial positions coordinate with the metal center to form the 2p-3d-2p structures with the axial positions occupied by two methanol molecules. Magnetic analysis on the Mn^II complexes revealed the existence of a strong antiferromagnetic interaction between the Mn^II and the NIT radical spin, while weak ferromagnetic coupling for Mn⋯Mn and Rad⋯Rad in the Mn-NIT-Mn and Rad-Mn-Rad spins was confirmed. Interestingly, although the NIT-bridged complexes 3Mn and 4Co possess significantly different magnetic anisotropy, field-induced slow magnetic relaxation can be observed in both complexes, which was assigned to the phonon bottleneck effect for 3Mn and field-induced SMM behavior for 4Co. To the best of our knowledge, 3Mn is the first example of the NIT-bridged binuclear Mn^II complex undergoing slow magnetic relaxation.

Cyclic [Cu-biRadical]2 Secondary Building Unit in 2p-3d and 2p-3d-4f Complexes: Crystal Structure and Magnetic Properties

Employing the new nitronyl nitroxide biradical ligand biNIT-3Py-5-Ph (2-(5-phenyl-3-pyridyl)-bis(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide)), a 16-spin Cu-radical complex, [Cu8(biNIT-3Py-5-Ph)4(hfac)16] 1, and three 2p-3d-4f chain complexes, {[Ln(hfac)3][Cu(hfac)2]2(biNIT-3Py-5-Ph)2}n (LnⅢ= Gd 2, Tb 3, Dy 4; hfac = hexafluoroacetylacetonate), have been prepared and characterized. X-ray crystallographic analysis revealed in all derivatives a common cyclic [Cu-biNIT]2 secondary building unit in which two bi-NIT-3Py-5-Ph biradical ligands and two CuII ions are associated via the pyridine N atoms and NO units. For complex 1, two such units assemble with four additional CuII ions to form a discrete complex involving 16 S = 1/2 spin centers. For complexes 2–4, the [Cu-biNIT]2 units are linked by LnIII ions via NO groups in a 1D coordination polymer. Magnetic studies show that the coordination of the aminoxyl groups with Cu or Ln ions results in behaviors combining ferromagnetic and antiferromagnetic interactions. No slow magnetic relaxation behavior was observed for Tb and Dy derivatives.

Copper(II)-N-hydroxy-N,N'-diarylformamidine complexes: Synthesis, crystal structures, antibacterial and molecular docking studies

A series of Cu(II) complexes were synthesized by using N-hydroxy-N,N'-diarylformamidine ligands: N-hydroxy-N,N'-(phenyl)formamidine (L1), N-hydroxy-N'-(4-methylphenyl)formamidine (L2), N-hydroxy-N,N'-(2,6-dimethylphenyl)formamidine (L3), N-hydroxy-N,N'-(2,6-diisopropylphenyl)formamidine (L4). Reaction of ligands L1-L4 with hydrated copper acetate furnished mononuclear Cu(II) complexes 1-4 with general formula [Cu-(L)₂]. The molecular structures of complexes 3 and 4, as determined by single crystal X-ray diffraction, showed both to have square planar geometry with a near C₂ symmetry. The antimicrobial potency of all four complexes was evaluated against three gram-(-) bacteria (S. typhimurium, P. aeruginosa, and E. coli) and two gram-(+) bacteria (Methicillin-resistant S. aureus (MRSA) and S. aureus), with ciprofloxacin as the reference drug. All tested complexes were inactive against gram-(+) bacteria strains except for complex 1, which displayed excellent activity when compared to the reference. Molecular docking studies showed that hydrogen bonding, pi-sigma and van der Waals interactions are prominent complex-protein connections, with complex 2 displaying good binding affinities with the studied biological targets.

Slow relaxation of magnetization for a Tb derivative in a biradical-based lanthanide chain

Four novel lanthanide-biradical chains have been obtained and the Tb derivative reveals field-induced slow magnetic relaxation behavior with an anisotropy barrier Δ/k_B = 22.08 K.

A series of CuII–LnIII complexes of an N2O3 donor asymmetric ligand and a possible CuII–TbIII SMM candidate in no bias field

Among four isostructural heterometallic Cu^II–Ln^III complexes (Ln = Tb, Dy, Ho or Er) of an N₂O₃ donor asymmetric ligand, only the Cu^II–Tb^III complex shows SMM behavior at zero bias field but at applied bias field both the Cu^II–Tb^III and Cu^II–Dy^III complexes show SMM behavior.

A designed room-temperature triplet ligand from pyridine-2,6-diyl bis(tert-butyl nitroxide)

The magnetic study on newly developed 4-mesitylpyridine-2,6-diyl bis(tert-butyl nitroxide) shows that almost the whole population has a ground triplet state at room temperature, and the ability of complex formation as a tridentate triplet ligand was proven with a diamagnetic yttrium(iii) ion.

3- and 4-(α-diazobenzyl)pyridine-N-oxides as photoresponsive magnetic couplers for 2p–4f heterospin systems: formation of carbene–TbIII and carbene–DyIII single-molecule magnets

Dinuclear Tb^III – and Dy^III – D1pyO complexes were photolyzed to afford heterospin SMMs with U_eff/k_B = 30 and 39 K, respectively.

2p–3d–4f hetero-tri-spin molecule-based magnetic compounds

The most up-to-date advances in the design and synthesis of molecular magnetic materials by means of a 2p–3d–4f strategy are reviewed.

Rare Earth Metal Complexes of Bidentate Nitroxide Ligands: Synthesis and Electrochemistry

We report rare earth metal complexes with tri- and bidentate ligands including strongly electron-donating nitroxide groups. The tridentate ligand 1,3,5-tris(2'-tert-butylhydroxylaminoaryl)benzene (H3arene-triNOx) was complexed to cerium(IV) in a 2:1 ligand-to-metal stoichiometry as Ce(Harene-triNOx)2 (1). Cyclic voltammetry of this compound showed stabilization of the tetravalent cerium cation with a Ce(IV/III) couple at E1/2 = -1.82 V versus Fc/Fc(+). On the basis of the uninvolvement of the third nitroxide group in the coordination chemistry with the cerium(IV) cation, the ligand system was redesigned toward a simpler bidentate mode, and a series of rare earth metal-arene-diNOx complexes were prepared with La(III), Ce(IV), Pr(III), Tb(III), and Y(III), [RE(arene-diNOx)2](-) ([2-RE](-), RE = La, Pr, Y, Tb) and Ce(IV)(arene-diNOx)2, where H2arene-diNOx = 1,3-bis(2'-tert-butylhydroxylaminoaryl)benzene. The core structures were isostructural throughout the series, with three nitroxide groups in η(2) binding modes and one κ(1) nitroxide group coordinated to the metal center in the solid state. In all cases except Ce(IV)(arene-diNOx)2, electrochemical analysis described two subsequent, ligand-based, quasi-reversible redox waves, indicating that a stable [N-O•] group was generated on the electrochemical time scale. Chemical oxidation of the terbium complex was performed, and isolation of the resulting complex, Tb(arene-diNOx)2·CH2Cl2 (3·CH2Cl2), confirmed the assignment of the cyclic voltammograms. Magnetic data showed no evidence of mixing between the Tb(III) states and the states of the open-shell ligand.

Unusual Gd-nitronyl nitroxide antiferromagnetic coupling and slow magnetic relaxation in the corresponding Tb analogue

The lanthanide-radical approach has been applied to construct three isomorphous mononuclear lanthanide complexes [Ln(hfac)3(NIT2-PyOCH3)] (Ln(III) = Gd , Tb , Er ) in which the nitronyl nitroxide radical NIT2-PyOCH3 (NIT2-PyOCH3 = 2-(3'-methoxy-2'-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) acts as a tridentate chelating ligand via the pyridyl nitrogen atom, the oxygen atom from the NO group and the oxygen atom from the methoxy group. The central lanthanide ions are nine-coordinated and their coordination geometry can be considered as an intermediate between muffin and spherical capped square antiprisms. Interestingly, an unusual antiferromagnetic interaction was observed between the Gd(iii) and the coordinated nitronyl nitroxide radical in complex . Moreover, ac magnetic susceptibilities display frequency-dependent out-of-phase signals in the case of the anisotropic Tb complex, indicating magnetic relaxation behavior.

Single-molecule magnet involving strong exchange coupling in terbium(iii) complex with 2,2′-bipyridin-6-yl tert-butyl nitroxide

Intramolecular antiferromagnetic interaction in Tb-6bpyNO was confirmed by comparison with the magnetic properties of Tb-6bpyCO and Y-6bpyNO. Tb-6bpyNO showed a hysteresis loop below 1.6 K, but Tb-6bpyCO did not behave as a single-molecule magnet.

Slow magnetic relaxation in two-dimensional 3d–4f complexes based on phenyl pyrimidyl substituted nitronyl nitroxide radicals

Three isostructural two-dimensional nitronyl nitroxide bridged 3d–4f complexes have been achieved. The Tb complex shows field-induced two-step magnetic relaxation behavior.

Doubly TEMPO-coordinated gadolinium(iii), lanthanum(iii), and yttrium(iii) complexes. Strong superexchange coupling across rare earth ions

The X-ray crystal structure of TEMPO-coordinated lanthanide compounds was determined for the first time. The magnetic study clarified that [Gd(hfac)₃(TEMPO)₂] behaved as a ground S_total = 7/2 species. The La and Y analogues showed the superexchange interactions across the diamagnetic ions.