Yan-Qin Wang

Solvent-modulated metamagnetism in a nickel(II) coordination polymer with mixed azide and carboxylate bridges

A novel 2D coordination polymer consisting of ferromagnetic Ni(ii) chains with alternating double EO-azide bridges and (EO-azide)bis(carboxylate) triple bridges exhibits solvent-modulated metamagnetism, and the reversible dehydration/hydration processes are accompanied by significant changes in critical temperature, critical field and hysteresis.

Complex Long-Range Magnetic Ordering Behaviors in Anisotropic Cobalt(II)–Azide Multilayer Systems

The crystal structures and magnetic properties of two new Co(II) molecular magnets, [Co(N(3))(2)(btzb)] (1) and [Co(N(3))(2)(btze)(2)] (2), are described and discussed (btzb=1,4-bis(tetrazol-1-yl)butane and btze=1,4-bis(tetrazol-1-yl)ethane). In the materials, (4,4) layers with mu-1,3-azide bridges are cross-linked by the monolayered btzb bridging ligands or spaced by bilayered btze terminal ligands to give a 3D (1) or 2D (2) coordination network with significantly different interlayer separations (10.6 vs. 15.2 A). The observation that the layers in 1 and 2 are almost identical have not only allowed us to determine how the interlayer separation imposes its influences on their magnetic behavior, but also helps us understand the complex magnetic behavior of each structure. In the high-temperature range (>25 K), almost identical magnetic behaviors, typical of 2D antiferromagnetic systems, are observed for 1 and 2. At low temperature they exhibit unusual and different behaviors that combine spin canting (weak ferromagnetism), metamagnetism, and stepped hysteresis. It has been found that the interlayer separation has little influence on the ordering temperature (23 vs. 22 K), but imposes very-strong influence on the metamagnetic critical field (6500 vs. 450 Oe), the coercivity (7500 vs. 650 Oe), and the hysteresis-step size. It may also play an adjusting role in determining the canting angle. Taking into account the strong anisotropy of the systems and the interlayer dipolar interactions, we have reasonably interpreted the unusual metamagnetic and hysteresis behaviors and the differences between 1 and 2. In particularly, the stepped hysteresis loops have been explained by two weak ferromagnetic states.

Magnetic Systems with Mixed Carboxylate and Azide Bridges: Slow Relaxation in Co(II) Metamagnet and Spin Frustration in Mn(II) Compound

Two coordination polymers formulated as [{[Co(2)(L)(N(3))(4)]·2DMF}(n) (1) and [Mn(2)(L)(H(2)O)(0.5)(N(3))(8)](n) (2) (L = 1,4-bis(4-carboxylatopyridinium-1-methyl)benzene) were synthesized and structurally and magnetically characterized. In compound 1, the anionic uniform Co(II) chains with mixed (μ-EO-N(3))(2)(μ-COO) triple bridges (EO = end-on) are cross-linked by the cationic bis(pyridinium) spacers to generate 2D coordination layers. It was demonstrated that the triple bridges mediate ferromagnetic coupling and that the compound represents a new example of the rare systems exhibiting the coexistence of antiferromagnetic ordering, metamagnetism, and slow magnetic dynamics. Compound 2 features the magnetic Δ-chain formed from isosceles triangular units with single μ-EE-N(3) and double (μ-EO-N(3))(μ-COO) bridges (EE = end-to-end). The Δ-chains are interlinked by long organic ligands into a 3D framework with novel net topology and 3-fold interpenetration. The magnetic properties of 2 indicate the presence of spin frustration characteristic of Δ-chains with antiferromagnetic interactions.

Synthesis, Structures, and Magnetism of Copper(II) and Manganese(II) Coordination Polymers with Azide and Pyridylbenzoates

Three transition-metal coordination polymers with azide and/or carboxylate bridges have been synthesized from 4-(3-pyridyl)benzoic acid (4,3-Hpybz) and 4-(4-pyridyl)benzoic acid (4,4-Hpybz) and characterized by X-ray crystallography and magnetic measurements. Compound 1, [Cu(4,3-pybz)(N(3))](n), consists of 2D coordination networks in which the uniform chains with (μ-EO-N(3))(μ-COO) double bridges are cross-linked by the 4,3-pybz ligands. Compound 2, [Cu(2)(4,4-pybz)(3)(N(3))](n)·3nH(2)O, consists of 2-fold interpenetrated 3D coordination networks with the α-Po topology, in which the six-connected dinuclear motifs with mixed (μ-EO-N(3))(μ-COO)(2) (EO = end-on) triple bridges are linked by the 4,4-pybz spacers. Compound 3, [Mn(4,4-pybz)(N(3))(H(2)O)(2)](n), contains 2D manganese(II) coordination networks in which the chains with single μ-EE-N(3) bridges (EE = end-to-end) are interlinked by the 4,4-pybz ligands, and the structure also features a 2D hydrogen-bonded network in which Mn(II) ions are linked by double triatomic bridges, (μ-EE-N(3))(O-H···N) and (O-H···O)(2). Magnetic studies indicated that the mixed azide and carboxylate bridges in 1 and 2 induce ferromagnetic coupling between Cu(II) ions and that 3 features antiferromagnetic coupling through the EE-azide bridge. In addition, compound 1 exhibits antiferromagnetic ordering below 6.2 K and behaves as a field-induced metamagnet. A magnetostructural survey indicates a general trend that the ferromagnetic coupling through the mixed bridges decreases as the Cu-N-Cu angle increases.

Diverse Manganese(II) Coordination Polymers with Mixed Azide and Zwitterionic Dicarboxylate Ligands: Structure and Magnetic Properties

The reactions of manganese(II) acetate or perchlorate, sodium azide, and the inner-salt-type dicarboxylate ligand 1,3-bis(4-carboxylato-1-pyridinium)propane (L) under different conditions yielded four different Mn(II) coordination polymers with mixed azide and carboxylate bridges: {[Mn(L)(N(3))]ClO(4).0.5H(2)O}(n) (1), {[Mn(2)(L)(2)(N(3))(2)][Mn(N(3))(4)(H(2)O)(2)].2H(2)O}(n) (2), {[Mn(2)(L)(2)(N(3))(2)(H(2)O)(2)]Br(N(3)).2H(2)O}(n) (3), and [Mn(4) (L)(2)(N(3))(8)](n) (4). The compounds exhibit great diversity in their structures and magnetic properties. Both 1 and 2 contain anionic chains featuring a mixed (OCO)(2)(EO-N(3)) triple bridge (EO = end-on) between adjacent Mn(II) ions. In 1, two independent sets of triple bridges with apparently different structural parameters alternate in the AABB sequence, and the resulting alternating chains are cross-linked into a cationic 3D framework by the cationic dipyridinium spacers. Differently, the chains in 2 have uniform bridges and are interlinked into a 2D coordination layer. An expression of the magnetic susceptibility for 1D systems with alternating J(1)J(1)J(2)J(2) interactions has been deduced and applied to 1. Magnetic studies on 1 and 2 reveal that the (OCO)(2)(EO-N(3)) triple bridges induce antiferromagnetic coupling between Mn(II) ions, and magnetostructural analyses suggest that the magnitude of the coupling can be correlated to the Mn-N and Mn-N-Mn parameters. Compound 3 contains 2D coordination layers in which the chains with alternating double EO-azide and double carboxylate bridges are interlinked by the dipyridinium spacers, and magnetic studies suggest alternating ferro- and antiferromagnetic interactions through the alternating bridges. The 3D framework of compound 4 is formed by the organic ligands interlinking the unique manganese-azide-carboxylate layers in which the [Mn(4)(mu(3)-N(3))(2)(mu(2)-N(3))(2)(mu(2)-COO)(4)] clusters are interlinked through EE-azide bridges (EE = end-to-end). The structure represents a novel type of self-catenated 8-connected 3D net. Magnetostructural analyses suggest that all of the short bridging moieties in 4, including (mu(3)-EO-N(3))(2), (OCO)(EO-N(3)), (OCO)(EO-N(3))(2), and single EE-N(3), propagate antiferromagnetic coupling.

Tricomponent Azide, Tetrazolate, and Carboxylate Cobridging Magnetic Systems: Ferromagnetic Coupling, Metamagnetism, and Single‐Chain Magnetism

Three novel coordination polymers with azide and a bifunctional zwitterionic ligand bearing carboxylate and tetrazolate as bridging groups, [M(L)(N(3))]·xH(2)O [L=1-(carboxylatomethyl)-4-(5-tetrazolato)pyridinium, M=Cu (1, x=2), Ni (2, x=1), and Co (3, x=1)], have been synthesized and characterized by X-ray crystallography and magnetic measurements. The compounds consist of two-dimensional coordination layers in which uniform anionic chains with the unprecedented tricomponent (μ-azide)(μ-tetrazolate)(μ-carboxylate) bridges are cross-linked by cationic 1-methylenepyridinium spacers. The tricomponent bridges induce ferromagnetic interactions in all the compounds. Furthermore, this isostructural series of ferromagnetic-chain-based compounds has allowed us to observe distinct bulk properties that are dependent upon the natures of the different spin carriers: with the isotropic Cu(II) ion, 1 exhibits a paramagnetic phase of the ferromagnetic chains without long-range magnetic order above 2 K; with the weakly anisotropic Ni(II) ions, 2 displays antiferromagnetic ordering and field-induced metamagnetism without slow dynamic relaxation; and with Co(II), which has strong magnetic anisotropy due to first-order spin-orbital coupling, 3 exhibits magnetic hysteresis and slow magnetization dynamics typical of single-chain magnets.

Manganese(II), Iron(II), and Mixed-Metal Metal–Organic Frameworks Based on Chains with Mixed Carboxylate and Azide Bridges: Magnetic Coupling and Slow Relaxation

Mn(II) and Fe(II) compounds derived from azide and the zwitterionic 1-carboxylatomethylpyridinium-4-carboxylate ligand are isomorphous three-dimensional metal-organic frameworks (MOFs) with the sra net, in which the metal ions are connected into anionic chains by mixed (μ-1,1-azide)bis(μ-carboxylate) triple bridges and the chains are cross-linked by the cationic backbones of the zwitterionic ligands. The Mn(II) MOFs display typical one-dimensional antiferromagnetic behavior. In contrast, with one more d electron per metal center, the Fe(II) counterpart shows intrachain ferromagnetic interactions and slow relaxation of magnetization attributable to the single-chain components. The activation energies for magnetization reversal in the infinite- and finite-chain regimes are Δτ1 = 154 K and Δτ2 = 124 K, respectively. Taking advantage of the isomorphism between the Mn(II) and Fe(II) MOFs, we have prepared a series of mixed-metal Mn(II)(1-x)Fe(II)(x) MOFs with x = 0.41, 0.63, and 0.76, which intrinsically feature random isotropic/anisotropic sites and competing antiferromagnetic-ferromagnetic interactions. The materials show a gradual antiferromagnetic-to-ferromagnetic evolution in overall behaviors as the Fe(II) content increases, and the Fe-rich materials show complex relaxation processes that may arise for mixed SCM and spin-glass mechanisms. A general trend is that the activation energy and the blocking temperature increase with the Fe(II) content, emphasizing the importance of anisotropy for slow relaxation of magnetization.

Unprecedented Self-Catenated Eight-Connected Network Based on Novel Azide-Bridged Tetramanganese(II) Clusters

The combination of the azide bridge and a neutral inner-salt-type dicarboxylate ligand leads to a three-dimensional coordination framework that contains unprecedented azide-bridged tetramanganese(II) clusters and defines a novel self-catenated, eight-connected net of 4(16) x 6(12) topology.

Magnetic Ordering in Three-Dimensional Metal–Organic Frameworks Based on Carboxylate Bridged Square-Grid Layers

Three isomorphous metal-organic frameworks of formula [M(ppdc)(H(2)O)(2)](n) [M = Mn(II), Fe(II), and Co(II)] were synthesized from sodium p-phenylenediacrylic (Na(2)ppdc). Crystallographic studies revealed that the compounds are layer-pillared 3D frameworks in which the square-grid M(II) layers with single carboxylate bridges are interlinked by long organic spacers with large interlayer separations of about 13 Å. Magnetic investigations indicated that they all display intralayer antiferromagnetic interactions through the carboxylate bridges in the unusual skew-skew coordination mode but the bulk behaviors are quite different. The Co(II) compound, like most compounds containing similar M-O-C-O-M layers, shows no 3D magnetic ordering down to 2 K, while the Mn(II) and Fe(II) compounds exhibit spin-canted ordering, behaving as a weak ferromagnet (T(C) = 3.8 K) and a metamagnet (T(N) = 3.8 K, H(c) = 650 Oe), respectively. Spin-canted ordering is still a rarity in this series of materials. Magnetostructural comparisons with analogous compounds indicate that the occurrence of spin-canted ordering can be related to the uncommon skew-skew and anti-anti coordination modes of carboxylate bridges, which induce stronger antiferromagnetic interactions than the common syn-anti mode.

Unusual composition dependence of magnetic relaxation for CoII1−xNiIIx chain-based metal–organic frameworks

A series of isomorphous 3D Co(II)(1-x)Ni(II)(x) MOFs based on ferromagnetic chains show SCM-type slow relaxation and the Co-rich system can exhibit a higher blocking temperature than both Co(II) and Ni(II) parent materials.