catena-Poly[[[triaqua­copper(II)]-μ2-pyrazine-2,3-dicarboxyl­ato] monohydrate]

The Cu atom in the title complex, {[Cu(C6H2N2O4)(H2O)3]·H2O}n or {[Cu(L)(H2O)3]·H2O}n (L is pyrazine-2,3-dicarbox­yl­ate), displays octa­hedral coordination formed by the ligand L and three coordinated water mol­ecules. The ligand L is tridentate, with one N atom of the pyrazine ring and one O atom of one carboxyl­ate group forming a chelate ring, whereas one O atom from the second carboxyl­ate group is coordinated to another Cu atom. The ligand L links mol­ecules to form an infinite chain parallel to the [101] direction. The chains are further linked through O—H⋯O and O—H⋯N hydrogen bonds involving the water mol­ecules to build up a three-dimensional network

Poly[diaqua(μ2-3-carboxypyrazine-2-carboxylato)(μ2-pyrazine-2,3-dicarboxylic acid)potassium(I)]

The structural unit of the title compound, [K(C6H3N2O4)(C6H4N2O4)(H2O)2]n, consists of one potassium cation, one hydrogen pyrazine-2,3-dicarboxyl­ate anion, one pyrazine-2,3-dicarboxylic acid mol­ecule and two water mol­ecules; this is twice the asymmetric unit, since the potassium cation lies on an inversion centre. Each anion or acid mol­ecule is linked to two potassium cations, while the potassium cation has contacts to four symmetry-equivalent organic ligands, with two different coordination modes towards this cation. In addition, each potassium cation is coordinated by two water O atoms, raising the coordination number to eight. One of the carboxyl groups of the acid retains its H atom, which forms a hydrogen bond to a coordinated water mol­ecule. The other carboxyl group is deprotonated in half of the ligands and protonated in the other half, taking part in a strong O—H⋯O hydrogen bond disordered over an inversion centre. The stabilization of the crystal structure is further assisted by O—H⋯O and O—H⋯N hydrogen bonds in which water acts as the donor

Diacridinium trans-diaqua­bis(pyrazine-2,3-dicarboxyl­ato)cobaltate(II) hexa­hydrate

The title compound, (C13H10N)2[Co(C6H2N2O4)2(H2O)2]·6H2O, consists of mononuclear trans-[Co(pz-2,3-dc)2(H2O)2]2− complex anions, (acrH)+ cations and uncoordinated water mol­ecules (acr is acridine and pz-2,3-dcH2 is pyrazine-2,3-dicarboxylic acid). The CoII atom, which lies on a crystallographic center of symmetry, has a slightly distorted octa­hedral coordination environment, with two N and two O atoms from the (pz-2,3-dc)2− ligands in the equatorial plane and with two water mol­ecules in axial positions. In the crystal, the components are held together by two distinct N—H⋯O and C—H⋯O hydrogen bonds with R 2 2(8) graph-sets. The coordinated and uncoordinated water mol­ecules are also involved in O—H⋯O hydrogen bonds, which lead to the formation of layers with R 3 3(12) graph-set motifs. Extensive π–π stacking inter­actions between parallel aromatic rings of the acridinium ions, with distances ranging from 3.533 (1) to 3.613 (1) Å, occur in the structure

catena-Poly[[diaqua­rubidium(I)](μ2-3-carboxy­pyrazine-2-carboxyl­ato)(μ2-pyrazine-2,3-dicarboxylic acid)]

The structural unit of the title compound, [Rb(C6H3N2O4)(C6H4N2O4)(H2O)2]n, consists of one rubidium cation, one hydrogen pyrazine-2,3-dicarboxyl­ate anion, one pyrazine-2,3-dicarboxylic acid mol­ecule and two water mol­ecules. This formulation is repeated twice in the asymmetric unit as the rubidium cation lies on an inversion centre. Each anion or acid mol­ecule is linked to two rubidium cations, while the rubidium cation has close contacts to four symmetry-equivalent organic ligands, with two different coordination modes towards this cation. In addition, each rubidium cation is coordinated by two water O atoms, raising the coordination number to eight. One of the carboxyl groups of the acid holds its H atom, which forms a hydrogen bond to a coordinated water mol­ecule. The other carboxyl group is deprotonated in half of the ligands and protonated in the other half, taking part in a strong O—H⋯O hydrogen bond disordered over an inversion centre. The stabil­ization of the crystal structure is further assisted by O—H⋯O and O—H⋯N hydrogen-bonding inter­actions involving the water mol­ecules and carboxyl­ate O atoms

Bis(3-amino­pyrazine-2-carboxyl­ato-κ2 N 1,O)diaqua­manganese(II) mono­hydrate

In the title compound, [Mn(C5H4N3O2)2(H2O)2]·H2O, the MnII cation, located on a twofold rotation axis, is N,O-chelated by two 3-amino­pyrazine-2-carboxyl­ate anions and coordin­ated by two water mol­ecules in a distorted octa­hedral geometry. The uncoordinated water mol­ecules lies on a twofold rotation axis. Adjacent mol­ecules are linked by O—H⋯O and N—H⋯O hydrogen bonds into a three-dimensional network motif

A novel double-chain silver(I) coordination polymer: catena-poly[[[μ-aqua-aqua­disilver(I)]-bis­(μ3-5-methyl­pyrazine-2-carboxyl­ato)] dihydrate]

In the title silver(I) coordination polymer, {[Ag2(C6H5N2O2)2(H2O)2]·2H2O}n, the [Ag2(μ2-H2O)(H2O)] cores are extended by anti­parallel 5-methyl­pyrazine-2-carboxyl­ate (L) ligands, forming a novel double-chain structure. Both Ag+ cations show a distorted square-pyramidal coordination. Ag1 is bonded to two water molecules, one L N atom, one N atom and one carboxylate O atom from a neighbouring L, whereas Ag2 is surrounded by two L N atoms, two L carboxylate O atoms and one bridging water molecule. O—H⋯O hydrogen-bonding inter­actions involving water clusters and carboxyl­ate O atoms link the mol­ecules into a three-dimensional supra­molecular architecture, which is further consolidated by weak C—H⋯O inter­actions and π–π stacking inter­actions [centroid–centroid distance 3.643 (5) Å]

Diaqua­bis­(5-methyl­pyrazine-2-carboxyl­ato-κ2 N 1,O)cobalt(II) dihydrate

In the title complex, [Co(C6H5N2O2)2(H2O)2]·2H2O, the coordination geometry of the Co2+ cation is distorted octa­hedral, with two N atoms and two O atoms from two 5-methyl­pyrazine-2-carboxyl­ate ligands in the equatorial plane. The two remaining coordination sites are occupied by two water mol­ecules. In addition, there are two uncoordinated water mol­ecules in the asymmetric unit. The crystal structure is stabilized by a network of O—H⋯O and O—H⋯N hydrogen-bonding inter­actions, forming a three-dimensional structure

Hexaaqua­magnesium(II) bis­(pyridinium-2,6-dicarboxyl­ate)

In the title compound, [Mg(H2O)6](C7H4NO4)2, a single six-coordinate Mg2+ cation (site symmetry 2/m) is bonded to six water mol­ecules in a distorted octa­hedral geometry. The crystal packing between the complex cation and the zwitterionic organic cation (m symmetry) is stabilized by inter­molecular O—H⋯O hydrogen bonds and weak inter­molecular C—H⋯O inter­actions

Disorder-to-order transition in the magnetic and electronic properties of URh_2Ge_2

We present a study of annealing effects on the physical properties of tetragonal single--crystalline URh_2Ge_2. This system, which in as-grown form was recently established as the first metallic 3D random-bond heavy-fermion spin glass, is transformed by an annealing treatment into a long-range antiferromagnetically (AFM) ordered heavy-fermion compound. The transport properties, which in the as-grown material were dominated by the structural disorder, exhibit in the annealed material signs of typical metallic behavior along the crystallographic a axis. From our study URh_2Ge_2 emerges as exemplary material highlighting the role and relevance of structural disorder for the properties of strongly correlated electron systems. We discuss the link between the magnetic and electronic behavior and how they are affected by the structural disorder.Comment: Phys. Rev. B, in print (scheduled 1 Mar 2000

Spin glass behavior in URh_2Ge_2

URh_2Ge_2 occupies an extraordinary position among the heavy-electron 122-compounds, by exhibiting a previously unidentified form of magnetic correlations at low temperatures, instead of the usual antiferromagnetism. Here we present new results of ac and dc susceptibilities, specific heat and neutron diffraction on single-crystalline as-grown URh_2Ge_2. These data clearly indicate that crystallographic disorder on a local scale produces spin glass behavior in the sample. We therefore conclude that URh_2Ge_2 is a 3D Ising-like, random-bond, heavy-fermion spin glass.Comment: 10 pages, RevTeX, with 4 postscript figures, accepted by Physical Review Letters Nov 15, 199