Structure and magnetism of a binuclear Cu(II) pyrophosphate: transition to a 3D magnetic behaviour studied by single crystal EPR.

A binuclear Cu(II) compound [Cu2(bpa)2(P2O7)(H2O)2]·2.5H2O, 1, (bpa = 2,2'-bipyridylamine), with pairs of Cu(II) ions bridged by one pyrophosphate tetra-anion, was synthesized and crystallized. Its triclinic structure was determined by single-crystal X-ray diffraction. Electron paramagnetic resonance (EPR) spectra of single crystal samples of 1 were recorded for a fixed orientation of the magnetic field (B0) as a function of temperature (T) between 4.7 and 293 K, and at T = 4.7, 50 and 293 K, as a function of the orientation of B0. Below ∼8 K, the spectra are assigned to two types of mononuclear crystal defects hyperfine-coupled to one copper and two nitrogen nuclei. The g-matrices and hyperfine couplings at these T provide information about the structures of these defects. Above 10 K, the spectrum is dominated by the response of the bulk binuclear Cu(II) material, showing hyperfine interactions with two copper nuclei, collapsing to a single peak above 18 K when the units are magnetically connected, and the magnetic behaviour becomes 3D. We attribute the results above 10 K to the interplay of an AFM intrabinuclear exchange interaction J0 = -28(3) cm(-1) (defined as Hex = -J0S1·S2), and three orders of magnitude weaker exchange coupling with average magnitude |J1| ≥ 0.022 cm(-1) between Cu(II) ions in neighbouring binuclear units. The interplays between structure, exchange couplings, magnetic dimension and spin dynamics in the binuclear compound are discussed. A previously unreported situation, where the structure of the spectra arising from the anisotropic spin-spin interaction term (D) within the binuclear unit is averaged out, but the forbidden half field transition is not, is observed and explained.

Bis[(dihydrogen pyrophosphato-κ(2)O,O')(2,2':6',2''-terpyridine-κ(3)N,N',N'')copper(II)] 4.5-hydrate.

The title copper complex, [Cu(H(2)P(2)O(7))(C(15)H(11)N(3))](2)·4.5H(2)O, consists of two very similar independent Cu(Tpy)H(2)P(2)O(7) monomeric units (Tpy is 2,2':6',2''-terpyridine) plus four and a half water molecules of hydration, some of which are disordered. Tpy units bind through the usual triple bite via their N atoms, and the H(2)P(2)O(7)(2-) anions coordinate through two O atoms from two different phosphate units. Each independent CuN(3)O(2) chromophore can be described as a slightly deformed square pyramid, with one of them having a sixth, semicoordinated, O atom from a centrosymmetrically related CuN(3)O(2) unit in a weakly bound second apical position suggesting an octahedral environment for the cation and weak dimerization of the molecule. The two independent complex molecules are connected via two strong O-H···O interactions between the phosphate groups to form hydrogen-bonded dinuclear units, further linked into [111] columns, resulting in a very complex three-dimensional supramolecular structure through a variety of classical and nonclassical hydrogen bonds, as well as π-π interactions.

Single crystal EPR study of the dinuclear Cu(II) complex [Cu(tda)(phen)](2)·H(2)tda (tda = thiodiacetate, phen = phenanthroline): influence of weak interdimeric magnetic interactions.

We report powder and single crystal EPR measurements of [Cu(tda)(phen)](2)·H(2)tda (tda = thiodiacetate, phen = phenanthroline) at 9.7 GHz. This compound consists of centrosymmetric copper(II) ion dimers, weakly ferromagnetically exchange-coupled (J = +3.2 cm(-1)), in which the dimeric units are linked by hydrophobic chemical paths involving the phen molecules. EPR revealed that the triplet spectra are collapsed by interdimeric exchange interactions mediated by that chemical path. Analysis and simulation of the single crystal EPR spectra were performed using Anderson's exchange narrowing model, together with statistical arguments. This approach allowed us to interpret the spectra modulated by the interdimeric interactions in situations of weak, intermediate, and strong exchange. We evaluated an interdimeric exchange constant J' = 0.0070(3) cm(-1), indicating that hydrophobic paths can transmit weak exchange interactions between centers at relatively long distances of the order of ∼10 Å.

Aqua(oxydiacetato-κ3O,O',O'')(pyridine-3-carboxamide-κN1)copper(II) sesquihydrate.

In the monomeric title compound, [Cu(C(4)H(4)O(5))(C(6)H(6)N(2)O)(H(2)O)]·1.5H(2)O, the Cu(II) cation is bound in a square-pyramidal coordination to a tridentate oxydiacetate (ODA) ligand, a monodentate pyridine-3-carboxamide (p3ca) ligand and one aqua ligand, where the two organic ligands form the basal plane and the water O atom occupies the unique apical site. The ODA ligand presents a slight out-of-plane puckering in its central ether O atom, while the p3ca ligand is essentially planar. The availability of efficient donors and acceptors for hydrogen bonding results in the formation of strongly linked hydrogen-bonded bilayers parallel to (101), with an interplanar distance of 3.18 (1) Šand a stacking separation between the bilayers of 3.10 (1) Å, both of them governed by extended π-π interactions. The disordered nature of the solvent water molecules around inversion centres is discussed. The monoaqua compound is compared with the octahedral diaqua analogue, [Cu(C(4)H(4)O(5))(C(6)H(6)N(2)O)(H(2)O)(2)], reported recently [Perec & Baggio (2009). Acta Cryst. C65, m296-m298].

Pyrophosphate-bridged Cu(II) chain magnet: {[Na3Cu(P2O7)(NO3)].3H2O}n.

A Cu(II)...Cu(II) pyrophosphate-bridged compound of formula {[Na(3)Cu(P(2)O(7))(NO(3))].3H(2)O}(n) (1) has been characterized. X-ray diffraction measurements show that it crystallizes in the monoclinic space group P2(1)/m, with unit cell dimensions a = 7.2492(5) A, b = 8.2446(6) A, c = 9.9050(7) A, beta = 107.123(1) degrees, and Z = 2. The structure consists of chains of Cu(II) cations at inversion symmetry sites bound to four equatorial oxygen atoms provided by two pyrophosphate anions halved by a symmetry plane and two axial oxygen atoms of nitrate anions. The molar magnetic susceptibility chi(0) of a powdered sample was measured in the temperature range 2 K < T < 273 K, and an isothermal magnetization curve, M(B(0),T), was obtained at T = 30 K, with the magnetic field B(0) between 0 and 5 T. Fitting a spin-chain model to the susceptibility data, we evaluate an antiferromagnetic exchange coupling 2J = -24.3(1) cm(-1) (defined as H(ex) = -2JS(i)S(j)) between Cu(II) neighbors. For any orientation of B(0), single-crystal electron paramagnetic resonance (EPR) spectra obtained at 9.8 and 33.9 GHz at 300 K display a single signal having a g matrix with orthorhombic symmetry, arising from the merger produced by the exchange interaction of the resonances corresponding to the two rotated Cu(II) sites. The g matrices of the individual molecules calculated assuming axial symmetry yielded principal values g(parallel) = 2.367(1) and g(perpendicular) = 2.074(1) at both frequencies, indicating a d(x(2)-y(2)) ground-state orbital for the Cu(II) ions. The angular variation of the EPR line width suggests exchange narrowing in a system with one-dimensional spin dynamics, as expected from the structure and susceptibility data. The results, discussed in terms of the crystal and electronic structures and of the spin dynamics of the compound, are compared with those obtained in other materials.

Di-µ-acetato-bis-[(acetato-κO,O')bis-(iso-nicotinamide-κN)copper(II)].

The title centrosymmetric bimetallic complex, [Cu(2)(C(2)H(3)O(2))(4)(C(6)H(6)N(2)O)(4)], is composed of two copper(II) cations, four acetate anions and four isonicotinamide (INA) ligands. The asymmetric unit contains one copper cation to which two acetate units bind asymmetrically; one of the Cu-O distances is rather long [2.740 (2) Å], almost at the limit of coordination. These Cu-O bonds define an equatorial plane to which the Cu-N bonds to the INA ligands are almost perpendicular, the Cu-N vectors subtending angles of 2.4 (1) and 2.3 (1)° to the normal to the plane. The metal coordination geometry can be described as a slightly distorted trigonal bipyramid if the extremely weak Cu-O bond is disregarded, or as a highly distorted square bipyramid if it is not. The double acetate bridge between the copper ions is not coplanar with the CuO(4) equatorial planes, the dihedral angle between the (O-C-O)(2) and O-Cu-O groups being 34.3 (1)°, resulting in a sofa-like conformation for the 8-member bridging loop. In the crystal, N-H⋯O hydrogen bonds occur, some of which generate a head-to tail-linkage between INA units, giving raise to chains along [101]; the remaining ones make inter-chain contacts, defining a three-dimensional network. There are in addition a number of C-H⋯O bonds involving aromatic H atoms. Probably due to steric hindrance, the aromatic rings are not involved in significant π⋯π inter-actions.

Magnetism and structure in chains of copper dinuclear paddlewheel units.

An anhydrous copper carboxylate compound of formula [Cu(trans-2-butenoate)(2)](n) has been characterized. X-ray analysis reveals a structure built by paddlewheel units bridged by pairs of Cu...O axial bonds to give infinite chains arranged in a new topological motif. Susceptibility measurements in the 10-300 K temperature range, and isothermal magnetization curves at 2, 5, 10, and 50 K with fields up to 5 T, were obtained. Electron Paramagnetic Resonance (EPR) spectra of powder samples were measured at 33.9 GHz at 300 K, and at 9.60 GHz at temperatures in the range 90

Alternate Cu2 and Er2 spin carriers in a carboxylate-bridged chain: EPR study.

We report powder EPR measurements at 9.48 GHz and temperatures of 4 K < or = T < or = 300 K and at 33.86 GHz and T = 300 K for the polymeric compound {[Cu2Er2(L)10(H2O)4].3H2O}n (HL = trans-2-butenoic acid) having alternate Cu2 and Er2 dinuclear units bridged by carboxylates along a chain. Above 70 K, when the Er(III) resonance is unobservable and uncoupled from the Cu(II) ions, the spectrum arises from the excited triplet state of antiferromagnetic Cu2 units, decreasing in intensity as T decreases, and disappearing when these units condensate into the singlet ground state. Fit of a model to the spectra at 9.48 and 33.86 GHz and 300 K gives g(Cu)(parallel) = 2.379, g(Cu)(perpendicular) = 2.065, D(Cu) = -0.340 cm(-1), and E(Cu) approximately 0 for the g-factors and zero field splitting parameters. From the T dependence of the intensity of the spectrum above 70 K, we obtain J(Cu-Cu) = -336(11) cm(-1) for the intradinuclear exchange interaction. Below 50 K, a spectrum attributed to Er(2) units appears, narrows, and resolves as T decreases, due to the increase of the spin-lattice relaxation time T1. The spectrum at 4 K allows calculating g values g1 = 1.489, g2 = 2.163, and g3 = 5.587 and zero field splitting parameters D(Er) = -0.237 cm(-1) and E(Er) = 0.020 cm(-1). The results are discussed in terms of the properties of the Cu and Er ions, and the crystal structure of the compound.

Diaqua(oxydiacetato-kappa3O,O',O'')(pyridine-3-carboxamide-kappaN1)copper(II).

In the mononuclear title compound, [Cu(C4H4O5)(C6H6N2O)(H2O)2], the Cu(II) centre is bound to a chelating oxydiacetate ligand, a monodentate pyridine-3-carboxamide unit and two water molecules, defining an octahedral coordination where the first two ligands form the equatorial plane and the last two occupy the apical sites. The planar oxydiacetate ligand is slightly disordered at its central ether O atom. The availability of efficient donors and acceptors for hydrogen bonding results in a complex interaction scheme where each monomer links to six similar units to define a well connected three-dimensional structure. A comparison is made with related structures in the literature, and the reasons for their differences are discussed.

1-D polymers with alternate Cu2 and Ln2 units (Ln = Gd, Er, Y) and carboxylate linkages.

Three isostructural Cu 2Ln 2 1-D polymers [Cu 2Ln 2L 10(H 2O) 4.3H 2O] n where Ln = Gd ( 1), Er ( 2), and Y ( 3) and HL= trans-2-butenoic acid, were synthesized and characterized by X-ray crystallography, electron paramagnetic resonance, and magnetic measurements. Pairs of alternate Cu 2 and Ln 2 dinuclear units are combined into a linear array by a set of one covalent eta (2):eta (1):mu 2 carboxylate oxygen and two H bonds, at Cu...Ln distances of ca. 4.5 A. These units exhibit four eta (1):eta (1):mu 2 and two eta (2):eta (1):mu 2 carboxylate bridges, respectively. Magnetic measurements between 2 and 300 K, fields B 0 = mu 0 H between 0 and 9 T, and electron paramagnetic resonance (EPR) measurements at the X-band and room temperature are reported. The magnetic susceptibilities indicate bulk antiferromagnetic behavior of the three compounds at low temperatures. Magnetization and EPR data for 1 and 3 allowed evaluation of the exchange couplings between both Cu and Gd ions in their dinuclear units and between Cu and Gd neighbor ions in the spin chains. The data for the isolated Cu 2 units in 3 yield g || = 2.350 and g [symbol: see text] = 2.054, J Cu-Cu = -338 (3) cm (-1) for the exchange coupling [ H ex(1,2) = - J 1-2 S1 x S2], and D 0 = -0.342 (0.003) cm (-1) and E 0 = 0.003 (0.001) cm (-1) for the zero-field-splitting parameters of the triplet state arising from anisotropic spin-spin interactions. Considering tetranuclear blocks Gd-Cu-Cu-Gd in 1, with the parameters for the Cu 2 unit obtained for 3, we evaluated ferromagnetic interactions between Cu and Gd neighbors, J Cu-Gd = 13.0 (0.1) cm (-1), and between Gd ions in the Gd 2 units, J Gd-Gd = 0.25 (0.02) cm (-1), with g Gd = 1.991. The bulk antiferromagnetic behavior of 1 is a consequence of the antiferromagnetic coupling between Cu ions and of the magnitude, |J Cu-Gd|, of the Cu-Gd exchange coupling. Compound 2 displays a susceptibility peak at 15 K that may be interpreted as the combined result from antiferromagnetic couplings between Er (III) ions in Er 2 units and their coupling with the Cu 2 units.

catena-Poly[[[tetra-kis(µ-2-butenoato)dicopper(II)]-µ-2-butenoato-[diaqua-(2-butenoato)holmium(III)]-di-µ-2-butenoato-[diaqua-(2-butenoato)holmium(III)]-µ-2-butenoato] trihydrate].

The title compound {[Cu(2)Ho(2)(C(4)H(5)O(2))(10)(H(2)O)(4)]·3H(2)O}(n), is a one-dimensional 3d/4f organic-inorganic hybrid complex, the Ho(III) member of the isotypic lanthanoid series with Ln = Gd(III), Er(III) and Y(III). The structure shows an alternation of Cu(2) and Ho(2) dinuclear units bridged by the ligands and hydrogen bonds only. The chains are composed of Cu(2) classical dinuclear η(1):η(1):µ(2) fourfold bridges [Cu⋯Cu = 2.6417 (9) Å] and of Ho(2) units bridged by two η(2):η(1):µ(2) carboxyl-ate units. This results in distorted square-based pyramidal CuO(5) units and irregular HoO(9) units. The alternating Cu(2) and Ho(2) units are bridged into linear arrays along the a axis by a set of one η(2):η(1):µ(2) carboxyl-ate O atom and two hydrogen bonds with Cu⋯Ho separations of 4.4883 (10) and 4.5086 (10) Å. The distance between adjacent chains, as calculated by the closest and furthest distances between two chains, covers the range 10-14 Å. The H atoms of the water mol-ecules could not be located, but the O⋯O separations for these species suggest the presence of O-H⋯O hydrogen bonds.

Gadolinium and neodymium citrates: evidence for weak ferromagnetic exchange between gadolinium(III) cations.

A new lanthanide citrate motif of general formula [Ln(Hcit)(H2O)2.H2O]n, where Ln = Gd (1) and Nd (2) and Hcit3- = C(OH)(COO-)(CH2COO-)2, has been synthesized hydrothermally from Ln2O3 and citric acid at 100 degrees C and characterized by elemental analysis, IR, TG-DTA, single-crystal X-ray diffraction, and magnetic measurements. The structures can be seen as "ladder chains" along the a axis, with dinuclear Ln2O2 units serving as "steps" and R-COO groups as "uprights", which are connected by H bonds. The magnetic susceptibility between 2 and 300 K and the magnetization at 2 K, as a function of magnetic field between 0 and 5 T, were measured for both compounds. By modeling the magnetic behavior of the Gd compound with a dinuclear Hamiltonian [symbol: see text](S) = gmu(B)(S(A) + S(B))B(o) - J(o)S(A)S(B) (S(A) = S(B) = 7/2), a ferromagnetic exchange interaction J(o) = 0.039 cm(-1) was evaluated between Gd ions situated at d(o) = 4.321 angstroms in dinuclear units bridged by two symmetry-related tridentate carboxylate oxygens. The EPR spectrum of the Gd compound is discussed. The temperature dependence of the susceptibility of the Nd compound is caused by the depopulation of the excited crystal-field levels when the temperature decreases. The magnetic-field dependence of the magnetization of 2 is attributed to the ground-state Kramers' doublet populated at 2 K. The g factor of this ground-state doublet is calculated from the data and compared with values for other compounds reported in the literature.

Isolation and characterization of a polymeric lanthanum citrate.

The first lanthanide citrate coordination polymer with the formula [La(Hcit)(H(2)O)](n) (Hcit(3-) = C(OH)(COO(-))(CH(2)COO(-))(2)) was prepared from La(2)O(3) and citric acid at pH = 2.2-2.5 under hydrothermal conditions at 120 degrees C. The compound was characterized by elemental analysis, IR, TG-DTA, and X-ray crystallography. It is thermally stable up to 158 degrees C and insoluble in common solvents. The compound crystallizes in the monoclinic space group C2/c with a = 16.765(3) A, b = 8.822(2) A, c = 14.048(3) A, beta = 120.64(3) degrees , and Z = 8. The structure consists of chains of La(III) cations bridged by O--C--O groups with pendant Hcit anions forming a pillar structure. The Hcit ligand is involved in six La--O bonds to five different La centers in a very compact 3D structure.

Polymeric hexaaquahexakis(mu3-2,2'-oxydiacetato)trizinc(II)digadolinium(III) dodecahydrate.

A polymeric heterometallic compound, ([Gd(2)Zn(3)(C(4)H(4)O(5))(6)(H(2)O)(6)].12H(2)O)(n), comprising zinc(II) and gadolinium(III) cations bridged by carboxylate groups from oxydiacetate ligands, is presented. The Gd(III) cations lie at sites with crystallographic 32 symmetry and display a tricapped trigonal-prism arrangement, which is defined by six carboxyl and three ether O atoms. The Zn(II) cations lie at sites with imposed 2/m symmetry and are octahedrally coordinated by four carboxyl O atoms and two apical water ligands, which form strong intramolecular hydrogen bonds. Comparison is made with the previously reported isostructural homologous copper-gadolinium complex.

A new polymeric phase of zinc(II) oxydiacetate.

A new polymeric phase of zinc(II) oxydiacetate, catena-poly[[[diaquazinc(II)]-mu-oxydiacetato] hydrate], [[Zn(C(4)H(4)O(5))(H(2)O)(2)].H(2)O](n), isomorphous with the Co homologue [Hatfield, Helms, Rohrs, Singh, Wasson & Weller (1987). Proc. Indian Acad. Sci. Chem. Sci. 98, 23-31], is reported. It presents a chain-like structure, generated by ZnO(6) cores which are bridged by carboxylate groups in an anti-anti conformation along the unique crystallographic b axis. The chains are held together through hydrogen-bonding interactions with the three water molecules.

Carboxylate-bridged copper(II)-lanthanide(III) complexes [(Cu(3)Ln(2)(oda)(6)(H(2)O)(6)).12H(2)O](n)(Ln = Dy, Ho, Er, Y; oda = oxydiacetate).

The hydrothermal reaction of Ln(2)O(3) (Ln = Dy and Ho), Cu(OAc)(2).2H(2)O, and oxydiacetic acid in the approximate mole ratio of 1:3:8 resulted in the formation of two new members of the isostructural series of polymers formulated as [(Cu(3)Ln(2)(oda)(6)(H(2)O)(6)).12H(2)O](n), crystallizing in the hexagonal crystal system, space group P6/mcc (No. 192). Temperature-dependent magnetic susceptibilities and EPR spectra are reported for the heterometallic compounds Cu-Dy 1, Cu-Ho 2, Cu-Er 3, and Cu-Y 4. The results are discussed in terms of the structure of the compounds, the electronic properties of the lanthanide ions, and the exchange interactions between the magnetic ions.

Bis(mu-acetato-kappa3O,O':O')bis[bis(acetato-kappa2O,O')diaquadysprosium(III)] tetrahydrate.

The title compound, [Dy(2)(C(2)H(3)O(2))(6)(H(2)O)(4)].4H(2)O, crystallizes in the form of dimeric units related by an inversion centre. Each cation is nine-coordinate, binding to two water molecules and three acetate groups, two of which are bidentate and the third tridentate. This last acetate group acts as a bridge between neighbouring metal atoms, leading to an intradimer Dy...Dy separation of 4.170 (1) A.