Melamine-induced synthesis of a structurally perfect kagomé antiferromagnet

We report here a structurally perfect kagomé lattice {[Cu₃(bpy)₆](SiF₆)₃(melamine)₈}_n (1), where bpy is 4,4'-bipyridine and [SiF₆]^2- is a hexafluorosilicate anion. In comparison to general 1D linear, 2D layered and 3D cubic metal-organic frameworks, by using Cu²⁺ nodes and bpy ligands, a perfect kagomé lattice was synthesized by introducing C₃ symmetrical melamine molecules. Magnetic susceptibility and low-temperature heat capacity measurements indicated weak antiferromagnetic interactions between the spins and no long-range magnetic ordering to 0.7 K. Using C₃ symmetrical melamine molecules can be considered as a challenging synthetic strategy to afford new topological materials.

Construction of magnet-type coordination polymers using high-spin {Ni4}-citrate cubane as secondary building units

A robust ferromagnetic secondary building unit based on {Ni₄}-citrate cubane is identified to be able to construct diverse structures while retaining the original magnetism.

Poly[aqua-bis-(μ-formato-κO:O')(μ-pyrazine-κN:N')nickel(II)]

In the title compound, [Ni(CHO(2))(2)(C(4)H(4)N(2))(H(2)O)], the nickel(II) cations are coordinated by three O-bonded-formato anions, two N-bonded-pyrazine ligands and one water mol-ecule in an octa-hedral coordination mode. The nickel(II) cations are connected by μ-1,3-bridging formato anions and N,N'-bridging pyrazine ligands into a three dimensional coordination network. The asymmetric unit consists of one nickel(II) cation, one water mol-ecule and two crystallograph-ically independent formato anions in general positions as well as two crystallographically independent pyrazine ligands, which are located on centers of inversion.

A series of lanthanide-organic polymers incorporating nitrogen-heterocyclic and aliphatic carboxylate mixed-ligands: structures, luminescent and magnetic properties

Seven new lanthanide-organic coordination polymers incorporating both nitrogen heterocyclic dicarboxylate and various auxiliary ligands, {[Ln(3)(Hpimda)(4)(mu(2)-HCOO).5H(2)O].H(2)O}(n)} Ln = Sm (1), Ln = Eu (2), Ln = Gd (3), Ln = Dy (4), Ln = Ho (5), {[Ce(2)(Hpimda)(2)(mu(4)-C(2)O(4)).8H(2)O].2H(2)O}(n) (6), {[Yb(2)(pyda)(mu(4)-C(2)O(4))(2).4H(2)O].3H(2)O}(n) (7) (H(3)pimda = 1H-2-propyl-4,5-imidazoledicarboxylic acid, H(2)pyda = 2,6-pyridinedicarboxylic acid) have been fabricated successfully and characterized systematically. Complexes 1-5 are isomorphous and isostructural, and are built from two-dimensional (2-D) double-decker networks based on the tetranuclear basic carboxylate as a secondary building unit (SBU). Both polymers 6 and 7 feature a (3,4)-connected 3-D framework consisting of 2-D lanthanide-organic hexagonal grids, which are further interlinked via the mu(4)-oxalate ligand. The results of magnetic determination show the same end-to-end bridging fashion of formate group results in different magnetic properties occurring between lanthanide centers. The luminescence emission spectra of the complexes vary depending on the lanthanide ion present.

Formate-based magnetic metal-organic frameworks templated by protonated amines

A systematic study has been carried out on the 3d divalent metal formate 3D magnetic frameworks templated by protonated amines, and the achievements have revealed that metal formate frameworks are very malleable, and their structures depend on the size, shape, charge, and hydrogen bonding geometries of the templating cations. Six kinds of metal formate frameworks have been created. They are chiral frameworks with a (4(9) . 6(6)) topology, perovskite ones with a (4(12) . 6(3)) topology, bi-nodal frameworks of (4(12) . 6(3))(4(9) . 6(6))(n) (n = 1, 2, 3) topologies, and porous diamond frameworks with 6(6) topology. These materials display promising and abundant magnetic, dielectric, porous, and optical properties and the possible combination of them. Therefore, they are of great interest for the study of molecule-based materials. It has been demonstrated that formate, being the smallest and simplest carboxylate, cheap and with low toxicity, thus more biocompatible and environmentally friendly, and having been more or less ignored, will find an important role in the construction of molecule-based materials and provide new materials with interesting properties.

Dynamic combinatorial chemistry in a solvothermal process of Cu(I,II) and organosulfur ligands

An unprecedented microwave C(sp(2))-S and S-S bond activation of 2,2'-dipyridyldisulfide (2-dpds) and the formation of a new architecture of coordination networks obtained by reaction of Cu(HCO(2))(2).xH(2)O with 2-dpds under solvothermal microwave conditions are reported. This unusual reaction proceeds with the partial oxidation of 2,2'-dipyridyldisulfide to sulfate. The process generates: a Cu(I) dimetallic complex [Cu(2)(mu-Hpyt)(2)(Hpyt)(4)](SO(4)).approximately 5EtOH (1), a Cu(I,II) polycationic coordination polymer [Cu(H(2)O)(6)][Cu(6)(mu-Hpyt)(12)](SO(4))(4).4H(2)O (2) and a dimetallic Cu(II) complex [Cu(2-dps)(mu-SO(4))(H(2)O)](2).3H(2)O (3). It is worth mentioning the strong red luminescence shown by compounds 1 and 2.

Porous 2D coordination polymeric formate built up by Mn(II) linking of Fe3O units: influence of guest molecules on magnetic properties

A first representative of coordination polymers, built from polynuclear carboxylate bridged by mononuclear carboxylate, has been synthesised and structurally characterized. The compound [{Fe(3)O(HCOO)(6)}{Mn(HCOO)(3)(H(2)O)(3)}].3.5HCOOH crystallised in the hexagonal system (space group P6(3)), a = b = 12.369(5) A, c = 12.992(5) A, alpha = beta = 90 degrees , gamma = 120 degrees , V = 1721.4(12) A(3). The crystal structure is built from 2D layers, which are formed by the linking of trinuclear {Fe(3)O(HCOO)(6)}(+) units by mononuclear {Mn(HCOO)(3)(H(2)O)(3)}(-) bridges. These 2D honeycomb layers are connected by molecules of formic acid through a hydrogen bond network. The compound has the voids at the centre of the hexagons. These voids are filled by captured solvent molecules. Solvate molecules are also located between the 2D layers, linking them through a system of H-bonds. All these solvates can be easily removed and/or exchanged, which results in the formation of [{Fe(3)O(HCOO)(6)}{Mn(HCOO)(3)(H(2)O)(3)}].0.5HCOOH.H(2)O and [{Fe(3)O(HCOO)(6)}{Mn(HCOO)(3)(H(2)O)(3)}]. Magnetic properties of the compounds can be considered as the superposition of magnetism of Fe(3)O trinuclear units, for which magnetism is governed by the superexchange interaction between Fe(3+) spins (with one Fe-Fe superexchange parameter J) and the magnetism of spins of Mn(2+) ions, the interactions between these units can be taken into account by molecular field, zJ'. The destruction of the hydrogen bond network caused by evacuation of solvent molecules significantly modifies the overall magnetism. It was found that zJ' depends on the presence of solvent molecules in the structure.

Occurrence of a Rare 49·66 Structural Topology, Chirality, and Weak Ferromagnetism in the [NH4][MII(HCOO)3] (M = Mn, Co, Ni) Frameworks

We report the synthesis, crystal structures, thermal, IR, UV-vis, and magnetic properties of a series of divalent transition metal formates, [NH4][M(HCOO)3], where M = divalent Mn, Co, or Ni. They crystallize in the hexagonal chiral space group P6(3)22. The structure consists of octahedral metal centers connected by the anti-anti formate ligands, and the ammonium cations sit in the channels. The chiral structure is a framework with the rarely observed 49.66 topology, and the chirality is derived from the handedness imposed by the formate ligands around the metals and the presence of units with only one handedness. The thermal properties are characterized by a decomposition at ca. 200 degrees C. The three compounds exhibit an antiferromagnetic ground state at 8.4, 9.8, and 29.5 K for Mn, Co, and Ni, respectively. The last two display a weak spontaneous magnetization due to a small canting of the moments below the critical temperature, and the Co compound shows a further transition at lower temperatures. The isothermal magnetizations at 2 K show spin-flop fields of 600 Oe (Mn), 14 kOe (Co), and above 50 kOe (Ni) and a small hysteresis with a remnant magnetization of 25 cm3 G mol(-1) (Co) and 50 cm3 G mol(-1) (Ni) and coercive field of 400 Oe (Co) and 830 Oe (Ni).

Characterization of the Crystal and Magnetic Structures of the Mixed-Anion Coordination Polymer Cu(HCO2)(NO3)(pyz) {pyz = Pyrazine} by X-ray Diffraction, ac Magnetic Susceptibility, dc Magnetization, Muon-Spin Relaxation, and Spin Dimer Analysis

The mixed-anion coordination polymer Cu(HCO2)(NO3)(pyz) was synthesized, its crystal structure was determined by X-ray diffraction, and its magnetic structure was characterized by ac susceptibility, dc magnetization, muon-spin relaxation, and spin dimer analysis. The crystal structure consists of five-coordinate Cu2+ ions that are connected through syn-anti bridging mu-HCO2- and mu-pyz ligands to form a highly corrugated two-dimensional layered network. Bulk magnetic measurements show a broad maximum in chi(T) at 6.6 K. The HCO2- and pyz ligands mediate ferromagnetic and antiferromagnetic spin exchange interactions between adjacent Cu2+ ions with the spin exchange parameters J/kB = 8.17 and -5.4 K, respectively (H = -JSigmaSi x Sj). The muon-spin relaxation data show a transition to a long-range magnetic ordering below TN = 3.66(3) K. For T < TN, the M(H) and chi'ac measurements provide evidence for a field-induced spin-flop transition at 15.2 kOe. That Cu(HCO2)(NO3)(pyz) undergoes a long-range magnetic ordering is an unexpected result because the one-dimensional Cu(NO3)2(pyz) and three-dimensional Cu(HCO2)2(pyz) compounds display linear chain antiferromagnetism with no long-range magnetic ordering down to 2 K.

NaCl-Type Frameworks of [M(pyrazine)2NO2]ClO4 (M = Co, Cu), the First Examples Containing μ1,3-Nitrito Bridges Showing Antiferromagnetism

Two new 3D NaCl-type frameworks of [M(pyrazine)2NO2]ClO4 (M = Co, Cu), consist of (4, 4) sheets of metal-pyrazine connected by mu(1,3)-nitrito, representing the first examples of mu(1,3)-nitrito bridges and exhibiting antiferromagnetism.

One-, two- and three-dimensional Cu(ii) complexes built via new oligopyrazinediamine ligands: from antiferromagnetic to ferromagnetic coupling

Six new pyrazine-modulated N,N'-bis(alpha-pyridyl)-2,6-diaminopyridine ligands (PMN5) were synthesized and their complexes studied. Reaction of copper(II) with the ligand that contained one pyrazine ring in its terminal position led to formation of a one-dimensional zigzag complex whereas copper(II) reactions with ligands containing three pyrazine rings or one pyrazine ring in its middle position yielded straight one-dimensional complexes. A 2-D complex was produced from the ligand with two pyrazine rings at both terminals. When nickel(II) was introduced, a 3-D network was obtained from the three-pyrazine-modulated ligand. Researches on variable-temperature magnetic susceptibility measurements revealed excellent Heisenberg chains with weak antiferromagnetic interaction of J values from -2 to -3 cm(-1)viasigma and pi pathways in straight one-dimensional complexes between the Cu(II) centers separated by 6.8-6.9 A. The zigzag one-dimensional complex showed very poor magnetic coupling. The two-dimensional compound showed significant ferromagnetic interaction in spite of the Cu-Cu distance of 7.2 A. Ferromagnetic coupling was discussed and attributed to the unusual coordination mode of in-plane and out-of-plane linkage of bridging pyrazine rings. The three-dimensional heterometal Cu(II)-Ni(II) compound showed weak antiferromagnetic interaction, which was satisfactorily fitted with J=-2.4 cm(-1) following a one-dimensional theoretical model including MFA.

Cu(HCO2)2(pym) (pym = pyrimidine): Low-Dimensional Magnetic Behavior and Long-Range Ordering in a Quantum-Spin Lattice

We synthesized and structurally and magnetically characterized the novel 3D coordination polymer Cu(HCO2)2(pym) (pym = pyrimidine). The compound crystallizes in the monoclinic space group C2/c with a = 14.4639(8) A, b = 7.7209(4) A, c = 8.5172(5) A, beta = 126.076(2) degrees, and V= 768.76(7) A3. In the structure buckled layers of Cu(HCO2)2 are interconnected by pym ligands to afford 1D Cu-pym-Cu chains. Bulk magnetic susceptibility measurements show a broad maximum at 25 K that is indicative of short-range magnetic ordering. Between 12 and 300 K a least-squares fit of the chi(T) data to a mean-field-corrected antiferromagnetic chain model yielded excellent agreement for g = 2.224(3), J/kB = -26.9(2) K, and zJ'/kB = -1.1(3) K. Below approximately 3 K a transition to long-range magnetic ordering is observed, as suggested by a sharp and sudden decrease in chi(T). This result is corroborated by muon spin relaxation measurements that show oscillations in the muon asymmetry below T(N) = 2.802(1) K and rapidly fluctuating moments above T(N).

Anionic NaCl-type frameworks of [MnII(HCOO)3−], templated by alkylammonium, exhibit weak ferromagnetism

We present the synthesis, characterization by IR, TGA, single crystal X-ray structure and magnetic properties of a novel series of NaCl-type frameworks of [AmineH(+)][Mn(HCOO)(3)(-)], templated by alkylammonium. The anionic NaCl-framework of [Mn(HCOO)(3)(-)] is counter-balanced by the alkylammonium cations located in the cavities of the framework to which they are hydrogen-bonded. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. All the compounds exhibit long-range antiferromagnetism below 9 K with a slight non-collinear arrangement of the moments. The canting, likely due to second-order spin-orbit coupling, is via a Dzyaloshinski-Moriya antisymmetric exchange mechanism. A spin-flop is observed in each case at fairly low fields. An orthorhombic to monoclinic transformation was observed for the protonated cyclotrimethyleneamine that is accompanied by localization of the cations into two positions below 240 K from the rapid dynamic flipping of the ring observed at room temperature.