Supramolecular assemblies involving metal organic ring interactions: Heterometallic Cu(II)-Ln(III) two dimensional coordination polymers

Three isostructural two-dimensional coordination polymers of the general formula [Ln2(CuL)3(H2O)9]$5.5H2O, where Ln is La (1), Nd (2), and Gd (3), have been synthesized and isolated from aqueous solutions and their single-crystal structures determined by X-ray diffraction. The supramolecular interaction between the non-aromatic metallorings plays an important role in stabilizing the structure of these compounds. The thermal stability, reversible solvent uptake, electronic properties and magnetic studies of these compounds are also reported

A rational design by hydrothermal methods of a tetrazolate-bridged bimetallic spin-canted antiferromagnet. Synthesis, X-ray structure and magnetic properties of [CoNi(pmtz)4] (Hpmtz 12 = 5-(pyrimidyl)tetrazole)

The bimetallic complex [CoNi(pmzt)4] (pmtz 12 = 5-(pyrimidyl)tetrazolate ligand) has been prepared by hydrothermal treatment of an equimolecular mixture of the mononuclear complexes[Co(pmtz)2(H2O)2] and [Ni(pmtz)2(H2O)2]. The structure of [CoNi(pmzt)4] consists of (4,4) square-grid-like sheets, in which Co(II) and Ni(II) metal ions are connected by pmtz 12 bridging ligands. This compound is a spin-canted antiferromagnet (weak ferromagnet) with Tc = 18 K. Although Co(II) and Ni(II) ions have different spin values, the spin-canted structure can be formed because of the accidental compensation of their magnetic moments. Both magnetic anisotropy and antisymmetric exchange interaction promote the spin canting of the compensated magnetic moments in the 3D antiferromagnetic phase. On passing from the homometallic complex, [Co2(pmzt)4], which is also a spin-canted antiferromagnet with Tc = 15 K, to the bimetallic complex, [CoNi(pmzt)4], Tc increases and the hysteresis parameters, remnant magnetization (Mr) and critical field (Hc), decrease. The increase in Tc may be a consequence of the increase of the JCoNi, whereas the decrease in Mr and Hc is probably due to the presence in [CoNi(pmzt)4] of a lower content of the highly anisotropic Co(II) io

Design of a Family of Ln(3) Triangles with the HAT Ligand (1,4,5,8,9,12-Hexaazatriphenylene): Single-Molecule Magnetism

A series of trinuclear Ln(3) complexes (Ln(III) = Yb (1), Er (2), Dy (3) and Gd (4)) were prepared from the trischelate bidentate ligand 1,4,5,8,9,12-hexaazatriphenylene (HAT). 1 and 2, exhibited field-induced single-molecule-magnet (SMM) behavior with estimated U-eff values of 21.30 and 13.86 K, respectively. Complex 3 behaved as a SMM even at zero field, and two different thermally assisted relaxation processes were detected with U-eff values of 29.6 K (fast relaxation process, FR) and 69 K (slow relaxation process, SR) due to the existence of two magnetically different D-III centers in the molecule. Ab initio studies reveal that all the Dy3+ centers have almost an Ising ground state. The local anisotropy axes are not coplanar but form angles with the Dy3 plane in the range 58-78 degrees. The magnetic interaction between the anisotropic Dy3+ ions is antiferromagnetic in nature and very weak in magnitude. However, due to the extreme feebleness of the magnetic interaction with regard to the local excitation energies, the magnetization blockade is most probably of single -ion origin. Calculations support the existence of two relaxation processes, which take place through the first excited state following an Orbach/Raman mechanism. Finally, for complex 4, the magnetocaloric effect was simulated using the magnetic parameters extracted from the fit of the magnetization and susceptibility data and demonstrated that the simulated -Delta S-m values were almost coincident with those extracted from the integration of the field dependence of the magnetization. The simulated MCE value at 2 K and 5 T (20.46 J kg(-1) K-1) makes complex 4 an attractive candidate for cryogenic magnetization

Magneto-Structural Properties and Theoretical Studies of a Family of Simple Heterodinuclear Phenoxide/Alkoxide Bridged Mn(III)Ln(III) Complexes: On the Nature of the Magnetic Exchange and Magnetic Anisotropy

A family of Mn(III)Ln(III) strictly dinuclear complexes of general formula [Mn-III(mu-L)(mu-OMe)(NO3)Ln(III)(NO3)(2)(MeOH)] (Ln(III) = Gd, Dy, Er, Ho) has been assembled in a one pot synthesis from a polydentate, multipocket aminobis(phenol)ligand [6,6'-{(2-(1-morpholyl)ethylazanediyl)bis(methylene)}bis(2-methoxy-4-methylphenol)], Mn(NO3)(2)center dot 4H(2)O, Ln(NO3)(3)center dot nH(2)O, and NEt3 in MeOH. These compounds represent the first examples of fully structurally and magnetically characterized dinuclear Mn(III)Ln(III) complexes. Single X-ray diffraction studies reveal that all complexes are isostructural, consisting of neutral dinuclear molecules where the Mn-III and Ln(III) metal ions, which exhibit distorted octahedral MnN2O4 and distorted LnO(9) coordination spheres, are linked by phenoxide/methoxide double bridging groups. Static magnetic studies show that the (MnGdIII)-Gd-III derivative exhibits a weak antiferromagnetic interaction between the metal ions, with a negative axial zero-field splitting D parameter. The (MnGdIII)-Gd-III complex shows a notable magnetocaloric effect with magnetic entropy change at 5 T and 3 K of -Delta S-m = 16.8 J kg(-1) K-1. Theoretical studies were performed to support the sign and magnitude of the magnetic anisotropy of the Mn-III ion (ab initio), to predict the value and nature of J(MnGd), to disclose the mechanism of magnetic coupling, and to establish magneto-structural correlations (DFT calculations). The results of these calculations are corroborated by quantum theory of atoms in molecule analysis (QTAIM). Finally, Mn-III-Dy-III and Mn-III-Er-III complexes show field-induced slow relaxation of the magnetization but without reaching a maximum above 2 K in the out-of-phase ac susceptibility. Ab initio calculations were also performed on Mn-III-Dy-III/Ho-III systems to unravel the origin behind the weak SMM characteristics of the molecules possessing two strongly anisotropic ions. The mechanism of magnetic relaxation was developed, revealing a large QTM/tunnel splitting at the single-ion level. Furthermore, the anisotropy axes of the Mn-III and Ln(III) ions were calculated to be noncollinear, leading to reduction of the overall anisotropy in the molecules. Hence, the herein reported complexes demonstrate that a combination of two anisotropic metal ions does not guarantee SMM behavior

Effect of Ligand Substitution around the Dy-III on the SMM Properties of Dual-Luminescent Zn-Dy and Zn-Dy-Zn Complexes with Large Anisotropy Energy Barriers: A Combined Theoretical and Experimental Magnetostructural Study

The new dinuclear (ZnDyIII)-Dy-II and trinuclear (ZnDyIII)-Dy-II-Zn-II complexes of formula [(LZnBrDy(ovan) (NO3)(H2O)](H2O)center dot 0.5(MeOH) (1) and [((LZnBr)-Zn-1)(2)Dy(MeOH)(2)](ClO4) (3) (L and L-1 are the dideprotonated forms of the N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato and 2-{(E)-[(3-{[(2E,3E)-3-(hydroxyimino)butan-2-ylidene ]amino}-2,2-dimethylpropyl)imino]methyl}-6-methoxyphenol Schiff base compartmental ligands, respectively) have been prepared and magnetostructurally characterized. The X-ray structure of 1 indicates that the Dy-III ion exhibits a DyO9 coordination sphere, which is made from four O atoms coming from the compartmental ligand (two methoxy terminal groups and two phenoxido bridging groups connecting Zn-II and Dy-III ions), other four atoms belonging to the chelating nitrato and ovanillin ligands, and the last one coming to the coordinated water molecule. The structure of 3 shows the central Dy-III ion surrounded by two L1Zn units, so that the Dy-III and Zn-II ions are linked by phenoxido/oximato bridging groups. The Dy ion is eight-coordinated by the six O atoms afforded by two L1 ligands and two O atoms coming from two methanol molecules. Alternating current (AC) dynamic magnetic measurements of 1, 3, and the previously reported dinuclear [LZnClDy(thd)(2)] (2) complex (where thd = 2,2,6,6-tetramethyl-3,5-heptanedionato ligand) indicate single molecule magnet (SMM) behavior for all these complexes with large thermal energy barriers for the reversal of the magnetization and butterfly-shaped hysteresis loops at 2 K. Ab initio calculations on 1-3 show a pure Ising ground state for all of them, which induces almost completely suppressed quantum tunnelling magnetization (QTM), and thermally assisted quantum tunnelling magnetization (TA-QTM) relaxations via the first excited Kramers doublet, leading to large energy barriers, thus supporting the observation of SMM behavior. The comparison between the experimental and theoretical magnetostructural data for 13 has allowed us to draw some conclusions about the influence of ligand substitution around the Dy-III on the SMM properties. Finally, these SMMs exhibit metal- and ligand-centered dual emissions in the visible region, and, therefore, they can be considered as magnetoluminescent bifunctional molecular materials

Designing a Dy-2 Single-Molecule Magnet with Two Well Differentiated Relaxation Processes by Using a Nonsymmetric Bisbidentate Bipyrimidine-N-Oxide Ligand: A Comparison with Mononuclear Counterparts

Herein we report a dinuclear [(N-mbpymNO)-{(tmh)(3)Dy}(2)] (1) single-molecule magnet (SMM) showing two nonequivalent Dy-III centers, which was rationally prepared from the reaction of Dy(tmh)(3) moieties (tmh = 2,2,6,6-tetramethyl-3,5-heptanedionate) and the asymmetric bis-bidentate bridging ligand 4-methylbipyrimidine (mbpymNO). Depending on whether the Dym ions coordinate to the NAO or NAN bidentate donor sets, the Dym sites present a NO7 (D2d geometry) or N206 (D4d) coordination sphere. As a consequence, two different thermally activated magnetic relaxation processes are observed with anisotropy barriers of 47.8 and 54.7 K. Ab initio calculations confirm the existence of two different relaxation phenomena and allow one to assign the 47.8 and 54.7 K energy barriers to the Dy(N2O6) and Dy(NO7) sites, respectively. Two mononuclear complexes, [Dy(tta)3(mbpymNO)] (2) and [Dy(tmh)3(phenNO)] (3), have also been prepared for comparative purposes. In both cases, the Dym center shows a NO7 coordination sphere and SMM behavior is observed with [Jeff values of 71.S K (2) and 120.7 K (3). In all three cases, ab initio calculations indicate that relaxation of the magnetization takes place mainly via the first excited-state Kramers doublet through Orbach, Raman, and thermally assisted quantum-tunnelling mechanisms. Pulse magnetization measurements reveal that the dinuclear and mononuclear complexes exhibit hysteresis loops with double- and single-step structures, respectively, thus supporting their SMM behavior