Using a set of pyrazolate-based dinucleating ligands with thioether sidearms and a set of different carboxylates, seven tetranuclear nickel(II) complexes of types [L2Ni4(N3)3(O2CR)2](ClO4) (1) and [L2Ni4(N3)(O2CR)4](ClO4) (2) featuring an unprecedented central mu4-1,1,3,3-azide could be isolated and fully characterized. X-ray crystal structures are discussed for 1a,b,e and 2b. The mu4-1,1,3,3-azide is symmetric in all cases except 1a but exhibits distinct binding modes with significantly different Ni-N(azide)-Ni angles and Ni-NNN-Ni torsions in type 1 and 2 complexes, which indicates high structural flexibility of this novel bridging unit. Also, IR-spectroscopic signatures and magnetic properties are distinct for type 1 and 2 complexes. Magnetic data for 1a,b,d,e and 2a,b were investigated and analyzed in a three-J approach. The only model that gave a satisfactory fit for all type 1 complexes includes one dominant antiferromagnetic coupling and two ferromagnetic interactions (one large and one smaller), indicating some degree of frustration. On the basis of magneto-structural correlations for end-on and end-to-end azide linkages, it is reasonable to assign the antiferromagnetic interaction to the intradimer exchange along the pyrazolate and the end-to-end linkage of the mu4-azide. Overall, the magnitude of the coupling constants differs significantly for the two distinct types of compounds, 1 or 2, and depends on the individual geometric details of the Ni4 array and the mu4-1,1,3,3-azide.

Pyrazolate-based dinucleating ligands with thioether-containing chelate arms have been used for the synthesis of a family of novel tetranuclear nickel(II) complexes [L2Ni4(N3)3(O2CR)](ClO4)2 that incorporate three azido bridges and one carboxylate (R = Me, Ph). Molecular structures have been elucidated by X-ray crystallography in four cases, revealing Ni4 cores with a unique topology in which two of the azido ligands adopt an unusual mu3-1,1,3 bridging mode. The compounds were further characterized by mass spectrometry, IR spectroscopy, and variable-temperature magnetic susceptibility measurements. Magnetic data analyses indicate a combination of significant intramolecular ferromagnetic and antiferromagnetic exchange interactions that give rise to an overall S(T) = 0 ground state. The sign and the magnitude of the individual couplings have been rationalized in the framework of the common magnetostructural correlations for end-to-end and end-on azido linkages, suggesting that these correlations also remain valid for the respective fragments of multiply bridging mu3-1,1,3 azido ligands.

Highly preorganized pyrazolate-based dinickel(II) systems are shown to constitute suitable building blocks for the targeted assembly of azido-bridged Ni4 complexes with rectangular arrangement of the metal ions. A set of such complexes has been prepared and structurally characterized. mu-1,1-Azide binding within the bimetallic sub-units is controlled by the chosen topology of the pyrazolate ligand scaffold and gives rise to the anticipated ferromagnetic intradimer coupling. Overall magnetic properties of the Ni4 array, however, are mainly determined by the Ni-NNN-Ni torsion of the interdimer mu-1,3-azido linkages. According to the crystallographic results, these torsion angles vary over a wide range, and partial disorder of the mu-1,3-azide bridge in one of the compounds indicates high structural flexibility even in the solid state. Two of the compounds represent rare examples of molecular complexes with a Ni-NNN-Ni torsion angle of almost exactly 90 degrees . The resulting magnetic ground state (neglecting zero-field splitting) is either S = 0 or S = 4 depending on the Ni-NNN-Ni torsion, and in one case a drastic change is observed upon extrusion of lattice solvent.

Various dinucleating pyrazole ligands with chelating side arms in the 3-and 5-positions of the heterocycle have been shown to form Ni II /azido complexes, where the metal ions are spanned by the pyrazolate, and an azido bridge. Four new complexes have been characterized by X-ray crystallography and variable-temperature magnetic susceptibility studies. The Ni···Ni distance, and hence the intra-dimer coordination mode of the azide (µ-1,1 or µ-1,3), is determined by the chelate arm length, such that the magnetic properties of the bimetallic units can be controlled. The intramolecular coupling between the Ni II (S = 1) ions was found to be ferromagnetic (J = +4.0 ± 0.5 cm

Electron spin resonance and magnetization data in magnetic fields up to 55 T of a novel multicenter paramagnetic molecular complex [L2Ni4(N3)(O2C Ada)4](Cl O4) are reported. In this compound, four Ni centers each having a spin S = 1 are coupled in a single molecule via bridging ligands (including a µ4-azide) which provide paths for magnetic exchange. Analysis of the frequency and temperature dependence of the ESR signals yields the relevant parameters of the spin Hamiltonian, in particular the single ion anisotropy gap and the g factor, which enables the calculation of the complex energy spectrum of the spin states in a magnetic field. The experimental results give compelling evidence for tuning the ground state of the molecule by magnetic field from a nonmagnetic state at small fields to a magnetic one in strong fields owing to the spin level crossing at a field of ∼ 25 T.

The first successful attempt to construct 3D supramolecular frameworks with high-nuclear 3d-4f heterometallic clusters as a node is reported. The self-assembly of Ln3+, Cu2+ and amino acid in solution leads to the formation of two polymers, 35-nuclear complex [Sm6Cu29] 1 with a primitive cubic net-like structure and 36-nuclear complex [Nd6Cu30] 2 with a face-centred cubic network type structure. Glycine and L-proline, respectively, were used as ligands. It should be noted that 2 has a chiral framework. X-ray structure analyses show that 1 crystallizes in the triclinic P1 space group (a=19.6451(8), b=20.4682(8), c=20.7046(8) A, alpha=89.453(1), beta=66.290(1), gamma=68.572(1) degrees, V=7003.0(5) A3 and Z=1) and 2 belongs to the cubic P2(1)3 space group (a=b=c=32.4341(3) A, V=34 119.7(5) A3 and Z=4). Both complexes utilize Ln6Cu24 octahedral clusters as nodes and trans-Cu(amino acid)2 groups as bridges. Electrical conductivity measurements reveal that both polymers behave as semiconductors.

By using the compartmental dinucleating pyrazolate ligand HL, dinickel(II) complexes [LNi2(micro-N3)(acetone)2]X2 (1: X = CIO4; 2: X = BPh4) and tetranickel(II) complex [{LNi2(micro-N3)(MeOH)2](CI04)4 (3) have been prepared and structurally characterized. Complexes 1 and 2 differ in the torsion along the bridging micro-1,3-azide moiety, while the azido ligands in 3 adopt an unusual micro-1,1,3 bridging mode to connect the two subunits. All three complexes show overall antiferromagnetic coupling and an S = 0 ground state, but the torsion along the azide moiety is a determining factor for the coupling strength. Compounds 1 and 2 serve as preorganized building blocks for the controlled synthesis of alternating 1D polymeric structures 4-6 by replacement of their labile acetone ligands by additional azido ligands. Due to the modular synthetic approach, 4-6 can be described as Heisenberg antiferromagnetic systems with inherent bond alternation (HABA), whereby the organic ligand framework ensures that the individual nickel/azido chains are well isolated in the crystal lattice. Like their precursors, 4-6 are mainly distinguished by torsion along the micro-1,3-azido bridges, both within and between the bimetallic constituents. Magnetic measurements confirm an overall 5 = 0 ground state for 4-6, and coupling parameters have been deduced from quantum Monte Carlo simulations. The two J values for the alternating 1D chains can be clearly assigned on the basis of the magnetostructural correlations established for the bimetallic building blocks. The alternation ratio gamma = J2J1(-1) places the three new systems in the HABA regime for a singlet-dimer ground state.

Dedicated to Professor Gottfried Huttner on the occasion of his 68th birthdayMaterials that exhibit hysteretic bistability at temperatures not too far from ambient conditions are expected to have great potential for future memory or sensing applications. [1] The two relatively stable states should be clearly distinguished by their physical properties, such as their magnetic or optical characteristics. Thermal magnetic hysteresis with an abrupt and rapid property change around room temperature is particularly rare for molecule-based materials, [2] with spincrossover compounds providing the most prominent examples.[3] More recently, magnetic hysteresis has also been observed for some molecular organic radicals [4] or transition metal coordination compounds, [5,6] where switching between two states occurs through a thermal phase transition with orientational changes in the 3D crystal lattice. Herein, we report thermal hysteresis of the magnetic susceptibility for single crystals of the molecular dinickel(ii) complex [LNi 2 (N 3 ) 3 ] (1), in which a m 1,3 -azido ligand functions as an on/off switch for the intramolecular antiferromagnetic coupling between the two metal ions.Pyrazolate-based compartmental ligands such as L À , which bears thioether side-arms, have previously proven suitable as dinucleating scaffolds for the synthesis of preorganized azidonickel(ii) complexes that can serve as modules for the assembly of oligonuclear species or 1D extended chain compounds; [7] the bimetallic complex 1 was prepared as a potential building block in this context. Single crystals of 1 were grown from acetone/hexane, and an initial X-ray crystallographic analysis was carried out at 133 K (Figure 1).[8] As anticipated, both nickel ions in the C 2 -symmetric array are nested within their respective ligand compartment (with two sulfur and one nitrogen donors from the ligand side-arm) and are spanned by the bridging pyrazolate. A m 1,3 -azido bridge is found within the bimetallic pocket, and an additional terminal azide fills the remaining site at each of the six-coordinate metal ions.Since 1 did not exhibit any unusual structural features at this stage, the temperature dependence of its magnetic susceptibility was highly unexpected. The product c m T for a polycrystalline sample of 1 is depicted in Figure 2 for both decreasing and increasing temperature. The value of 1.82 cm 3 K mol À1 at 300 K is somewhat lower then the spinonly value expected for two uncoupled high-spin nickel(ii) ions (2.14 cm 3 K mol À1 for g = 2.07), and decreases only slightly upon lowering the temperature to 220 K. At 215 K, however, an abrupt drop of c m T occurs (centered at T fl = 212 K), followed by a much more rapid decline at lower temperatures; below 50 K c m T tends towards zero. The lowtemperature data are indicative of strong antiferromagnetic coupling and an S = 0 ground state, while coupling is apparently only weak above 220 K. c m T shows an analogous behavior when measured as a function of increasing temperature, but exhibits therma...