RESUMO
A series of six seven-coordinate pentagonal-bipyramidal (PBP) erbium complexes, with acyclic pentadentate [N3O2] Schiff-base ligands, 2,6-diacetylpyridine bis-(4-methoxybenzoylhydrazone) [H2DAPMBH], or 2,6-diacethylpyridine bis(salicylhydrazone) [H4DAPS], and various apical ligands in different charge states were synthesized: [Er(DAPMBH)(C2H5OH)Cl] (1); [Er(DAPMBH)(H2O)Cl]·2C2H5OH (2); [Er(DAPMBH)(CH3OH)Cl] (3); [Er(DAPMBH)(CH3OH)(N3)] (4); [(Et3H)N]+[Er(H2DAPS)Cl2]- (5); and [(Et3H)N]+[Y0.95Er0.05(H2DAPS)Cl2]- (6). The physicochemical properties, crystal structures, and the DC and AC magnetic properties of 1-6 were studied. The AC magnetic measurements revealed that most of Compounds 1-6 are field-induced single-molecule magnets, with estimated magnetization energy barriers, Ueff ≈ 16-28 K. The experimental study of the magnetic properties was complemented by theoretical analysis based on ab initio and crystal field calculations. An experimental and theoretical study of the magnetism of 1-6 shows the subtle impact of the type and charge state of the axial ligands on the SMM properties of these complexes.
RESUMO
A series of three mononuclear pentagonal-bipyramidal V(iii) complexes with the equatorial pentadentate N3O2 ligand (2,6-diacethylpyridinebis(benzoylhydrazone), H2DAPBH) in the different charge states (H2DAPBH0, HDAPBH1-, DAPBH2-) and various apical ligands (Cl-, CH3OH, SCN-) were synthesized and characterized structurally and magnetically: [V(H2DAPBH)Cl2]Cl·C2H5OH (1), [V(HDAPBH)(NCS)2]·0.5CH3CN·0.5CH3OH (2) and [V(DAPBH)(CH3OH)2]Cl·CH3OH (3). All three complexes reveal paramagnetic behavior, resulting from isolated S = 1 spins with positive zero-field splitting energy expected for the high-spin ground state of the V3+ (3d2) ion in a PBP coordination. Detailed high-field EPR measurements for compound 3 show that its magnetic properties are best described by using the spin Hamiltonian with the positive ZFS energy (D = +4.1 cm-1) and pronounced dimer-like antiferromagnetic spin coupling (J = -1.1 cm-1). Theoretical analysis based on superexchange calculations reveals that the long-range spin coupling between distant V3+ ions (8.65 Å) is mediated through π-stacking contacts between the planar DAPBH2- ligands of two neighboring [V(DAPBH)(CH3OH)2]+ complexes.
RESUMO
The syntheses, structure and magnetic properties are reported for five novel 1D polymeric azido-bridged lanthanide complexes with the general formula {[Ln(DAPMBH)(N3)C2H5OH]C2H5OH}n where H2DAPMBH = 2,6-diacetylpyridine bis(4-methoxybenzoylhydrazone)-a new pentadentate pyridine-base [N3O2] ligand and Ln = Dy (1), Y0.930Dy0.070 (2), Er (3), Y0.923Er0.077 (4), and Gd (5). X-ray diffraction analysis of 1-5 show that the central lanthanide atoms are eight-coordinated with the N5O3 donor set originating from the ligand DAPMBH, one coordinated ethanol molecule and two end-to-end type N3- bridges connecting the metal centers into infinite chain. The [LnN5O3] coordination polyhedron can be regarded as a distorted dodecahedron (D2d). AC magnetic measurements revealed that compounds 1-4 show field-induced single-molecule magnet behavior, with estimated energy barriers Ueff ≈ 47-17 K. The experimental study of magnetic properties was complemented by theoretical analysis based on crystal-field calculations. Direct current magnetic susceptibility studies revealed marginally weak intrachain exchange interaction between Ln3+ ions mediated by the end-to-end azide bridging groups (J ≈ -0.015 cm-1 for 5). Comparative analysis of static and dynamic magnetic properties of magnetically concentrated (1, 3) and diluted (2, 4) Dy and Er compounds showed that, despite fascinating 1D azido-bridged chain structure, compounds 1 and 3 are not single-chain magnets; their magnetic behavior is largely due to single-ion magnetic anisotropy of individual Ln3+ ions.