Heterometallic Octanuclear NiII 4LnIII 4 (Ln = Y, Gd, Tb, Dy, Ho, Er) Complexes Containing NiII 2LnIII 2O4 Distorted Cubane Motifs: Synthesis, Structure, and Magnetic Properties.

The reaction of 2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}methyl] phenol (LH3) with lanthanide metal salts followed by the addition of nickel acetate allowed isolation of a family of octanuclear complexes, [Ni4Ln4(µ2-OH)2(µ3-OH)4(µ-OOCCH3)8(LH2)4]·(OH)2·xH2O. Single crystal X-ray diffraction studies of these complexes reveal that their central metallic core consists of two tetranuclear [Ni2Ln2O4] cubane subunits fused together by acetate and hydroxide bridges. The magnetic study of these complexes reveals a ferromagnetic interaction between the LnIII and the NiII center. The magnitude of exchange coupling between the NiII and LnIII center, parametrized from the magnetic data of the Gd analogue, gives J = +0.86 cm-1. The magneto caloric effect, studied for the NiII 4GdIII 4 complex, shows a maximum of magnetic entropy change, -ΔS m = 22.58 J kg-1 K-1 at 3 K for an applied external field of 5 T.

Hydroxide-free cubane-shaped tetranuclear [Ln4] complexes.

The reaction of the lanthanide(III) chloride salts [Gd(III), Tb(III), and Dy(III)] with a new chelating, flexible, and sterically unencumbered multisite coordinating compartmental Schiff-base ligand (E)-2-((6-(hydroxymethyl)pyridin-2-yl)methyleneamino)phenol (LH2) and pivalic acid (PivH) in the presence of triethylamine (Et3N) affords a series of tetranuclear Ln(III) coordination compounds, [Ln4(L)4(µ2-η(1)η(1)Piv)4]·xH2O·yCH3OH (1, Ln = Gd(III), x = 3, y = 6; 2, Ln = Tb(III), x = 6, y = 2; 3, Ln = Dy(III), x = 4, y = 6). X-ray diffraction studies reveal that the molecular structure contains a distorted cubane-like [Ln4(µ3-OR)4](+8) core, which is formed by the concerted coordination action of four dianionic L(2-) Schiff-base ligands. Each lanthanide ion is eight-coordinated (2N, 6O) to form a distorted-triangular dodecahedral geometry. Alternating current susceptibility measurements of complex 3 reveal frequency- and temperature-dependent two-step out-of-phase signals under zero direct current (dc) field, which is characteristic of single-molecule magnet behavior. Analysis of the dynamic magnetic data under an applied dc field of 1000 Oe to fully or partly suppress the quantum tunneling of magnetization relaxation process affords the anisotropic barriers and pre-exponential factors: Δ/kB = 73(2) K, τ0 = 4.4 × 10(-8) s; Δ/kB = 47.2(9) K, τ0 = 5.0 × 10(-7) s for the slow and fast relaxations, respectively.

Structural diversity due to amino alcohol ligands leading to rare µ4-hydroxo-bridged tetranuclear and "bicapped cubane" cores in copper(II) complexes: a theoretical and experimental magnetostructural study.

The µ4-hydroxo- and alkoxo-bridged tetranuclear copper(II) complexes, [Cu4(µ4-OH)(dmae)4][Ag(NO3)4] (1), [Cu4(µ4-OH)(dmae)4][Na(NO3)4] (2), [Cu4(µ4-OH)(dmae)4][K(NO3)4] (3), and hexanuclear alkoxo-bridged "bicapped cubane" copper(II) complex [Cu6(ae)8(ClO4)2](ClO4)2·MeOH (8) (dmae = N,N-dimethylaminoethanolato and ae = 2-aminoethanolato) were synthesized via self-assembly from chelating amino alcohols and copper(II), silver (1), sodium (2), and potassium (3) nitrates or copper(II) perchlorate (8). The complexes are characterized by elemental analyses, single-crystal X-ray diffraction, and variable temperature magnetic measurements. The crystal structures of complexes 1-3 consist of almost planar tetranuclear [Cu4(µ4-OH)(dmae)4](3+) units, in which Cu(II) ions are also weakly bonded to nitrate anions. The adjacent tetranuclear units of 1-3 are connected by ionic interactions between nitrate anions and sodium, potassium, or silver cations resulting in the formation of 1D polymers. The crystal structure of 8 consists of hexanuclear [Cu6(ae)8](4+) "bicapped cubane" units, in which the capping Cu(II) ions are weakly bonded to perchlorate anions. The adjacent hexanuclear units of 8 are connected by hydrogen bonds resulting in the formation of 3D hydrogen bonded networks. Also the results from the synthetic studies leading to the formation of new copper(II) and silver(I) complexes (4-7) using Hae, 3-aminopropanol (Hap), and N,N-dimethylaminopropanol (Hdmap) in similar reactions, which gave tetra- and hexanuclear complexes, are presented. Experimental magnetic studies showed that complexes 1 and 2 exhibit dominant antiferromagnetic coupling leading to an S = 0 ground state, whereas 8 exhibits a large dominant antiferromagnetic interaction between the capping copper atoms and the copper atoms of the top and bottom faces of the cubane unit that define two isosceles triangles. This interaction leads to an S = 1/2 ground state for each triangle. The weak ferromagnetic interaction between these doublet states through the cubane unit leads to a triplet S = 1 ground state for 8. The values of the magnetic exchange coupling constant were the following: J = +1.8 (1) and +2.9 cm(-1) (2) between adjacent copper atoms; J' = -29.2 (1) and -32.2 cm(-1) (2) between opposite copper atoms; J = -297.6 cm(-1) and zJ' = +0.07 cm(-1) (8). Magnetic coupling constants calculated for 2 and 8 by DFT methods are in general of the same nature and magnitude as the experimental ones.

Stepwise formation of a pentanuclear Ni4Cu heterometallic complex exhibiting a vertex-sharing defective double-cubane core and diphenoxo- and phenoxo/azide bridging groups: a magnetostructural and DFT theoretical study.

Sequential reaction of a N5O3 octadentate tripodal ligand with Ni(2+) and subsequently with Cu(2+) and azide ligand afforded the first example of a heterobridged (phenoxo/µ(1,1)-azido) pentanuclear heterometallic (Ni4Cu) compound, which exhibits a centrosymmetric vertex-sharing defective double-cubane structure. The study of the magnetic properties reveals that the compound shows ferromagnetic interaction interactions, leading to an S = 9/2 spin ground state. Density functional theory calculations on the X-ray structure and model compounds predict ferromagnetic interactions through the magnetic exchange pathways involving each couple of metal ions.

A novel 3D cyano-bridged mixed-valence CoII/CoIII canted antiferromagnet constructed from defective cubanes. Synthesis, X-ray structure and magnetic properties.

The hydrothermal reaction of 2-pyrimidine-carboxamide, CoCl2.6H(2)O and K3[Co(CN)6] affords a novel mixed-valence Co(II)/Co(III) 3D complex K[Co3(CN)6(ox)(H(2)O)2].H2O, which contains cyano-bridged Co(7) defective cubanes connected by oxalate and cyanide bridging groups and behaves as a canted antiferromagnet with Tc = 17.5 K.