Pascual Román

Supramolecular architectures assembled by the interaction of purine nucleobases with metal-oxalato frameworks. Non-covalent stabilization of the 7H-adenine tautomer in the solid-state

The synthesis, crystal structure and variable-temperature magnetic measurements of the compounds [Mn(mu-ox)(H2O)(7H-pur-kappaN9)]n (1), {[Mn(mu-ox)(H2O)2].(7H-ade).(H2O)}n (2) and {[Cu(mu-ox)(H2O)(7H-ade-kappaN9)][Cu(mu-ox)(mu-H2O)(7H-ade-kappaN9)]. approximately 10/3H2O}n (3), (where ox: oxalato dianion, pur: purine, and ade: adenine) are reported. Compounds 1and 2 contain one-dimensional chains in which manganese(II) atoms are bridged by bis-bidentate oxalato ligands. The distorted octahedral geometry around each metal centre is completed in compound 1 by one water molecule and the imidazole N9 donor site of the purine ligand, which is a rare example of direct binding between the Mn(II) ion and the N donor site of an isolated nucleobase. Unlike 1, the adenine moiety in compound 2 is not bonded to manganese atoms and the metal coordination polyhedron is filled by two water molecules in a cis-arrangement. Its crystal building is constructed from pi-stacked layers of Watson-Crick hydrogen-bonded adenine...(H2O2)...adenine aggregates and zig-zag Mn(II)-oxalato chains held together by means of a strong network of hydrogen bonding interactions. The nucleobase exists in the lattice as the 7H-adenine tautomer which represents an unprecedented solid-state characterization of this minor tautomer as free molecule (without metal coordination) stabilized through non-covalent interactions. Compound consists of two slightly different [Cu(ox)(H2O)(7H-ade-kappaN9)] units in which the nucleobase coordinates through the imidazole N9 atom. The planar complex entities are parallel stacked and joined by means of long Cu-O bonds involving oxygen atoms from the oxalato and the aqua ligands, giving one-dimensional chains with a [4 + 1] square-planar pyramidal and a [4 + 2] octahedral coordination around the metal centre, respectively. Self-assembled process of compound 3 is further driven by an in-plane network of hydrogen bonding interactions to generate a porous 3D structure containing parallel channels filled by guest water molecules. Variable-temperature magnetic susceptibility measurements of all the complexes show the occurrence of antiferromagnetic interactions between the paramagnetic centres. DFT calculations have been performed to check the influence of packing in the stability of the 7H-amino tautomer of 2 and in the complex geometry of 3.

Synthesis, chemical characterization, X-ray crystal structure and magnetic properties of oxalato-bridged copper(II) binuclear complexes with 2,2′-bipyridine and diethylenetriamine as peripheral ligands

Abstract Two new μ-oxalato binuclear copper(II) complexes, [{Cu(NO3)(H2O)(bipy)}2(ox)] (1) and [{Cu(dien)}2(ox)](NO3)2 (2), with ox=oxalate, dien=diethylenetriamine and bipy=2,2′-bipyridine, have been synthesized and their crystal and molecular structures have been determined by single-crystal X-ray diffraction methods. The crystal structure of 1 consists of centrosymmetric neutral dimers where the copper atoms lie in a strongly elongated octahedral environment, surrounded by two nitrogen atoms of a bipy molecule and two oxygen atoms of the bridging oxalato group in the equatorial plane and oxygen atoms of water molecules and nitrate ions in the axial positions. Crystal structure of 2 is made up of non-coordinated nitrate anions and asymmetric binuclear cations in which copper atoms are in a distorted square–pyramidal coordination with three atoms of a diethylenetriamine ligand and an oxygen atom of the asymmetrically coordinated oxalato bridge building the basal plane and the other oxygen atom of the oxalato ligand filling the apical position. Both compounds have been also characterized by Fourier transform infrared (FT-IR) and electron spin resonance (ESR) spectroscopies, thermal analysis and variable temperature magnetic susceptibility measurements. The two compounds exhibit antiferromagnetic exchange with a singlet–triplet separation of −382 and −6.5 cm−1 for 1 and 2, respectively. Magnetic and ESR results are discussed with respect to the crystal structure of the compounds.

Lanthanide(III)/Pyrimidine-4,6-dicarboxylate/Oxalate Extended Frameworks: A Detailed Study Based on the Lanthanide Contraction and Temperature Effects

Detailed structural, magnetic, and luminescence studies of six different crystalline phases obtained in the lanthanide/pyrimidine-4,6-dicarboxylate/oxalate system have been afforded: {[Ln(μ-pmdc)(μ-ox)(0.5)(H(2)O)(2)]·3H(2)O}(n) (1-Ln), {[Ln(μ-pmdc)(μ-ox)(0.5)(H(2)O)(3)]·2H(2)O}(n) (2-Ln), {[Ln(μ(3)-pmdc)(μ-ox)(0.5)(H(2)O)(2)]·~2.33H(2)O}(n) (3-Ln), {[Ln(2)(μ(3)-pmdc)(μ(4)-pmdc)(μ-ox)(H(2)O)(3)]·5H(2)O}(n) (4-Ln), {[Ln(μ(3)-pmdc)(μ-ox)(0.5)(H(2)O)(2)]·H(2)O}(n) (5-Ln), and [Ln(pmdc)(1.5)(H(2)O)(2.5)] (6-Ln). The slow generation of the oxalate (ox) anion, obtained from the in situ partial hydrothermal decomposition of the pyrimidine-4,6-dicarboxylate (pmdc) ligand, allows us to obtain good shaped single crystals, while direct addition of potassium oxalate provides the same compounds but as polycrystalline samples. The crystal structures of all compounds are based on the double chelation established by the pmdc and ox ligands to provide distorted 2D honeycomb layers that, in some cases, are fused together, leading to 3D systems, by replacing some of the coordinated water molecules that complete the coordination sphere of the lanthanide by uncoordinated carboxylate oxygen atoms of the pmdc. The presence of channels occupied by crystallization water molecules is also a common feature with the exception of compounds 5-Ln. It is worth noting that compounds 3-Ln present a commensurate crystal structure related to the partial occupancy of the crystallization water molecules placed within the channels. Topological analyses have been carried out, showing a previously nonregistered topology for compounds 4-Ln, named as jcr1. The crystal structures are strongly dependent on the lanthanide ion size and the temperature employed during the hydrothermal synthesis. The lanthanide contraction favors crystal structures involving sterically less hindranced coordination environments for the final members of the lanthanide series. Additionally, reinforcement of the entropic effects at high temperatures directs the crystallization process toward less hydrated crystal structures. The magnetic data of these compounds indicate that the exchange coupling between the lanthanide atoms is almost negligible, so the magnetic behavior is dominated by the spin-orbit coupling and the ligand field perturbation. The luminescence properties that exhibit the compounds containing Nd(III), Eu(III), and Tb(III) have been also characterized.

One-dimensional oxalato-bridged copper(II) complexes with 3-hydroxypyridine and 2-amino-4-methylpyridine

Two new one-dimensional oxalato-bridged copper(II) compounds of formula [Cu(ox)L2]n (1) and {[Cu2(ox)2L%3]·L%}n (2) [ox oxalate dianion, L3-hydroxypyridine (pyOH) and L% 2-amino-4-methylpyridine (ampy)] have been synthesized and characterized by FT-IR spectroscopy, variable-temperature magnetic measurements and single-crystal X-ray diffraction. The crystal structure of 1 comprises chains of copper atoms in which cis-[Cu(pyOH)2] 2 units are sequentially bridged by asymmetric bis-bidentate oxalato ligands with an intrachain copper‐copper separation of 5.548(1) A, . Each copper atom is six-coordinated: four oxygen atoms belonging to two bridging oxalato ligands and two nitrogen atoms from two 3-hydroxypyridine ligands build a distorted octahedral environment around the metal atom. As in 1, the structure of compound 2 is made up of chains of copper atoms bridged sequentially by bis-bidentate oxalato ligands. Two types of copper(II) ion, one being six-coordinated (Cu(1)) and the other five-coordinated (Cu(2)), alternate regularly within the chain. The environment around Cu(1) is elongated octahedral with two cis-coordinated pyridine‐nitrogen and two oxygen atoms from two oxalate ligands building the equatorial plane, the apical positions being filled by two oxalate-oxygen atoms. The environment around Cu(2) is distorted square pyramidal with four oxalato-oxygen atoms in the basal plane, and the pyridine nitrogen atom from one aromatic base in the apical position. Magnetic susceptibility data in the temperature range 2.0‐300 K reveal regular ferromagnetic (J 1.3 cm 1 ) and alternating antiferromagnetic (J66.6 cm 1 , aJ58.6 cm 1 ) chain behaviours for 1 and 2. The nature and magnitude of the magnetic coupling through the oxalato bridge in 1 and 2 are analysed and discussed in the light of the available structural data.

Synthetic control to achieve lanthanide(III)/pyrimidine-4,6-dicarboxylate compounds by preventing oxalate formation: structural, magnetic, and luminescent properties.

Control over the synthetic conditions in many metal/diazinedicarboxylato systems is crucial to prevent oxalate formation, since dicarboxylato ligands easily undergo degradation in the presence of metal salts. We report here an efficient route to obtain oxalato-free compounds for the lanthanide/pyrimidine-4,6-dicarboxylato (pmdc) system on the basis of the reaction temperature and nonacidic pH or oxygen free atmosphere. Two different crystal architectures have been obtained: {[Ln(μ-pmdc)(1.5)(H(2)O)(3)]·xH(2)O}(n) (1-Ln) and {[Ln(2)(μ(4)-pmdc)(2)(μ-pmdc)(H(2)O)(2)]·H(2)O}(n) (2-Ln) with Ln(III) = La-Yb, except Pm. Both crystal structures are built from distorted two-dimensional honeycomb networks based on the recurrent double chelating mode established by the pmdc. In compounds 1-Ln, the tricapped trigonal prismatic coordination environment of the lanthanides is completed by three water molecules, precluding a further increase in the dimensionality. Crystallization water molecules are arranged in the interlamellar space, giving rise to highly flexible supramolecular clusters that are responsible for the modulation found in compound 1-Gd. Two of the coordinated water molecules are replaced by nonchelating carboxylate oxygen atoms of pmdc ligands in compounds 2-Ln, joining the metal-organic layers together and thus providing a compact three-dimensional network. The crystal structure of the compounds is governed by the competition between two opposing factors: the ionic size and the reaction temperature. The lanthanide contraction rejects the sterically hindered coordination geometries whereas high-temperature entropy driven desolvation pathway favors the release of solvent molecules leading to more compact frameworks. The characteristic luminescence of the Nd, Eu, and Tb centers is improved when moving from 1-Ln to 2-Ln compounds as a consequence of the decrease of the O-H oscillators. The magnetic properties of the compounds are dominated by the spin-orbit coupling and the ligand field perturbation, the exchange coupling being almost negligible.

Synthesis, characterisation, crystal structures, and magnetic properties of one-dimensional oxalato-bridged metal(II) complexes with 3-hydroxypyridine and isoquinoline

One-dimensional oxalato-bridged metal(II) compounds of formula [M(-ox)(L)2]n [L = 3-hydroxypyridine (pyOH) or isoquinoline (isq)] have been synthesised and characterised by FT-IR spectroscopy, TG-DTA techniques, variable-temperature magnetic measurements and X-ray diffraction methods. The complexes [M(-ox)(pyOH)2]n [M= Co (1), Ni (2)] are isomorphous and crystallise in the orthorhombic space group Pnab. The compounds [M(-ox)(isq)2]n [M= Co (3), Ni (4), Cu (5)] are also isomorphous and belong to the monoclinic space group C2/c. Crystal structures consist of zig-zag chains in which cis-[M(L)2] 2 + units are sequentially bridged by bis-bidentate oxalato ligands with intrachain M···M distances in the range 5.38–5.49 A . The divalent metal centres have a distorted octahedral coordination with four oxygen atoms from two symmetry-related bridging oxalato ligands and the pyridinic nitrogen atoms from two cis-oriented aromatic ligands. Magnetic susceptibility measurements in the temperature range 2.0–300 K show the occurrence of antiferromagnetic intrachain interactions for the compounds 1– 4 and a weak ferromagnetic coupling for 5.

Magneto-structural studies and thermal analysis of n-ethylpyridinium (n=2, 3, 4) tetrabromocuprate(II) complexes

Abstract Three n-ethylpyridinium salts of the tetrabromocuprate(II) anion [n=2 (1), 3 (2) and 4 (3)] were prepared by air evaporation of an ethanolic solution of the aromatic amine base, HBr and CuBr2 in a molar ratio 2:2:1. The compounds were characterized by elemental analysis, IR, UV–Vis and ESR spectroscopies, thermal analysis and variable-temperature magnetic susceptibility measurements. The crystal structures of 2 and 3 were determined by means of single-crystal X-ray diffraction methods. The [CuBr4]2− anion (with C2 symmetry for 2) is connected to two organic cations through N–H⋯Br hydrogen bonds to form cation–anion–cation motifs which are held together by means of offset face-to-face interactions between the aromatic rings to give zig–zag one-dimensional chains in 2 and a dimeric entity in 3. The dimeric units of 3 are joined by means of weak Br⋯Br contacts to give one-dimensional chains as well. The X-band ESR spectra show quasi-axial signals which are clearly resolved operating at Q-band, with g∣∣≫g⊥>2.0, characteristic of a flattened [CuBr4]2− tetrahedral complex. The magnetic susceptibility measurements of powdered samples show that compounds behave as low-dimensional magnetic systems with weak antiferromagnetic exchange interactions. Thermal decomposition of the compounds yield copper(II) oxide and metallic Cu as stable final residues in synthetic air and dinitrogen atmospheres, respectively.

A 2D polymer constructed through bridging oxalato and 4,4′-bipyridine ligands: crystal structure and magnetic behavior of [Cu3(μ-ox)3(μ-4,4′-bpy)2(4,4′-bpy)2]n

Abstract The crystal structure of the compound [Cu3(μ-ox)3(μ-4,4′-bpy)2(4,4′-bpy)2]n 1 (ox=oxalato, 4,4′-bpy=4,4′-bipyridine) is comprised of two-dimensional sheets in which copper(II)–oxalato chains are cross-linked by bridging bidentate 4,4′-bpy ligands. Metal centers show a tetragonally elongated octahedral environment formed by four oxygen atoms from two asymmetrically coordinated oxalato ligands and two nitrogen atoms from two trans-coordinated 4,4′-bpy molecules. The magnetic measurements show the occurrence of weak ferromagnetic couplings.

One-dimensional oxalato-bridged copper(II) complex possessing two structurally different metallic centres

Abstract The crystal structure of the oxalato-bridged copper(II) compound [Cu2(μ-ox)2(ampy)3]n 1 (ox=oxalate dianion, ampy=2-amino-3-methylpyridine) consists of infinite corrugated one-dimensional chains in which two types of copper(II) centres, five- and six-coordinated, are bridged sequentially by asymmetric bis-bidentate oxalato ligands. Magnetic susceptibility measurements show the occurrence of a significant intrachain antiferromagnetic coupling (J=−22.9 cm −1 ) .

Organoammonium diphosphopentamolybdates(VI): influence of organic cations and anion protonation on crystal packing and geometrical features of polyanion

Abstract Four organoammonium diphosphopentamolybdate(VI) with the formulae (C5H7N2)6[P2Mo5O23]·5H2O (1), (C2H10N2)3[P2Mo5O23]·6H2O (2), Al(C4H15N3)4[HP2Mo5O23]2Cl·10H2O (3) and (C4H12N)4[H2P2Mo5O23]·5H2O (4) have been synthesized. The crystal structures have been determined by means of single crystal X-ray diffraction data. The geometrical characteristics of the [HnP2Mo5O23](6−n)− heteropolyanions have been compared with those described in the literature for other salts. Several relationships between the protonated state and topological changes in the heteropolyanions have been found. The crystal structure in compound 1 is stabilized by electrostatic forces, an extensive network of hydrogen contacts involving anions, cations and water molecules and π–π interactions among 4-aminopyridinium cations. The anions of compound 2 are arranged in layers perpendicular to the [ 1 10] direction. The interactions between anions are established through hydrogen contacts which involve water molecules and ethylenediammonium cations. The monohydrogendiphosphopentamolybdate(VI) anions in the crystal structure of compound 3 are joined along the [010] direction by means of strong O–H⋯O interactions (d(O⋯O)=2.529(10) A) which lead to a polymeric structure of [HP2Mo5O23]5− polyanions. Likewise, a similar anion polymeric arrangement is found in compound 4 in which the diprotonated polyanions are held together by means of two strong hydrogen bonds (d(O⋯O)=2.591(6) and 2.596(6) A).