María Hernández-Molina

Structural versatility of the malonate ligand as a tool for crystal engineering in the design of molecular magnets

The synthesis of ferro- and ferri-magnetic systems with a tunable Tc and three-dimensional (3-D) ordering from molecular precursors implying transition metal ions is one of the active branches of molecular inorganic chemistry. The nature of the interactions between the transition metal ions (or transition metal ions and radicals) is not so easy to grasp by synthetic chemists working in this field since it may be either electrostatic (orbital) or magnetic (mainly dipolar). Therefore, the systems fulfilling the necessary requirements to present the expected magnetic properties are not so easy to design on paper and realize in the beaker. In this work we show how the design of one-, two- and three-dimensional materials can strongly benefit from the use of crystal engineering techniques, which can give rise to structures of different shapes, and how these differences can give rise to different properties. We will focus on the networks constructed by assembling malonate ligands and metal centres. The idea of using malonate (dianion of propanedioic acid, H2mal) is that it can give rise to different coordination modes with the metal ions it binds. Extended magnetic networks of dimensionalities one (1-D), two (2-D) and three (3-D) can be chemically constructed from malonate-bridged metallic complexes. These coordination polymers behave as ferro-, ferri- or canted antiferro-magnets. We are currently trying to obtain analogous compounds using magnetically anisotropic ions, such as cobalt(II), in order to explore how structural differences influence the magnetic properties. In this case the control of the spatial arrangement of the magnetic building blocks is of paramount importance in determining the strength of the magnetic interaction. The possibility of controlling the shape of the networks depends on the coordination bond between the metal ion and the ligands and on supramolecular interactions such as stacking interactions or hydrogen bonding.

Malonate-based copper(II) coordination compounds: ferromagnetic coupling controlled by dicarboxylates

Studies on structural and magnetic properties of polynuclear transition metal complexes, aimed at understanding the structural and chemical factors governing electronic exchange coupling mediated by multiatom bridging ligands, are of continuing interest to design new molecular materials exhibiting unusual magnetic, optical and electrical properties, bound to their molecular nature. Looking at potentially flexible bridging ligands, the malonate group seems a suitable candidate. The occurrence of two carboxylate groups in the 1,3 positions allows this ligand to adopt simultaneously chelating bidentate and different carboxylato bridging modes (syn–syn, anti–anti and syn–anti trough one or two carboxylate groups) In the course of our research we have structurally and magnetically characterized several carboxylato bridged copper(II) complexes. In the present study we start describing briefly the structure and the magnetic behaviour of the compounds, subsequently we analyze the magneto-structural correlations concluding that the parameter that governs, in first order, the magnetic interaction between metal centres is the relative position of the carboxylato bridge of the malonate respect to the copper(II) ions: equatorial–equatorial (strong interaction), equatorial–apical (weak interaction) and apical–apical (negligible interaction). Inside this division another parameters become important such as β (angle between copper(II) basal planes) in the equatorial–equatorial, or the distortion t in the equatorial–apical.

Synthesis, crystal structure and magnetic properties of two-dimensional malonato-bridged cobalt(II) and nickel(II) compounds

Two isostructural malonato-bridged complexes of formula {[M(H2O)2][M(mal)2(H2O)2]}n [M = Co(II) (1), Ni(II) (2); H2mal = malonic acid] have been synthesised and characterized by X-ray diffraction. Their structure consists of corrugated layers of trans-diaquabismalonatemetalate(II) and trans-diaquametal(II) units bridged by carboxylate–malonate groups in the anti–syn conformation. Two crystallographycally independent metal atoms occur in 1 and 2. The malonate anion acts simultaneously as a bidentate and bis-monodentate ligand. Variable-temperature (1.9–295 K) magnetic susceptibility measurements indicate the occurrence of weak antiferro- (1) and ferromagnetic (2) interactions between the cobalt(II) (1) and nickel(II) ions (2) through the anti–syn caboxylate–malonate bridge. A brief discussion on the structural diversity and crystal engineering possibilities of the malonate complexes with divalent first-row transition metal ions other than copper(II) is carried out.

Malonic acid: a multi-modal bridging ligand for new architectures and properties on molecule-based magnets

Abstract In this work, we show how the design of one-, two- and three-dimensional materials can strongly benefit from the use of crystal engineering techniques, which can give rise to structures of different shapes, and how these differences can give rise to different properties. We will focus on the networks constructed by assembling malonate ligands and metal centres. The idea of using malonate (dianion of propanedioic acid, H 2 mal) is that they can give rise to different coordination modes with the metal ions bind. Extended magnetic networks of dimensionalities 1 (1D), 2 (2D) and 3 (3D) can be chemically constructed from malonato-bridged metallic complexes. These coordination polymers behave as ferro-, ferri- or canted antiferromagnets. The control of the spatial arrangement of the magnetic building blocks is of paramount importance in determining the strength of the magnetic interaction. It depends on the coordination bond between the metal ion and the ligands, and on supramolecular interactions such as stacking interactions or hydrogen bonds.

Synthesis, crystal structure and magnetic properties of the malonato-bridged bimetallic chain [Mn(II)Cu(II)(mal)2(H2O)4]·2H2O

Abstract The first malonato-bridged bimetallic chain of formula [MnCu(mal)2(H2O)4]·2H2O (1) (H2mal=malonic acid) was prepared and its structure determined by X-ray diffraction methods. Each copper atom in 1 is in a square planar environment formed by four malonate-oxygens from two malonate ligands. The manganese atom is six-coordinated with four water molecules and two cis-coordinated malonate-oxygens from two malonate groups building a distorted octahedral surrounding. The malonate group acts simultaneously as bidentate and monodentate ligand towards copper and manganese atoms respectively, leading to a bimetallic chain. Two structurally different carboxylato-bridges exhibiting an anti–syn conformation alternate regularly within the chain, the intrachain copper–manganese separations being 4.790(1) and 5.209(1) A. Its magnetic behaviour is typical of a ferrimagnetic Cu(II)Mn(II) bimetallic chain with intrachain antiferromagnetic coupling. The fully dehydrated phase of complex 1 exhibits an overall antiferromagnetic behaviour.

The flexibility of molecular components as a suitable tool in designing extended magnetic systems

In this work we show how the design of n-dimensional magnetic compounds (nD with n = 1–3) can strongly benefit from the use crystal engineering techniques, which can give rive to structures of different shapes with different properties. We focus on the networks built by assembling the malonato-bridged tetranuclear copper(II) units Cu4(mal)4 (mal2− is the dianion of propanedioic acid, H2mal) through the potentially bridging 2,4′-bipyridine (2,4′-bpy), 4,4′-bipyridine (4,4′-bpy) and pyrazine (pyz). The magneto-structural study of the complexes of formula [Cu4(mal)4(2,4′-bpy)4(H2O)4]·8H2O (1), [Cu4(mal)4(H2O)4(4,4′-bpy)2] (2) (this compound was the subject of a previous report but it is included here for comparison) and [Cu4(mal)4(pyz)2]·4H2O (3) reveals that the ferromagnetically coupled Cu4(mal)4 unit which occurs in 1–3 is propagated into two- (2) and three-dimensions (3) by using 4,4′-bpy and pyz as linkers, respectively. Whereas in the case of complex 1, this tetranuclear unit is magnetically isolated, significant antiferromagnetic interactions between these units mediated by the bridges 4,4′-bpy and pyz occur in 2 and 3.

Crystal structures and magnetic properties of two- and three-dimensional malonato-bridged manganese(II) complexes

Two new manganese(II) compounds of formula [Mn(mal)(H2O)(2,4′-bpy)]n (1) and [Mn2(mal)2(H2O)2(4,4′-bpy)]n (2) (2,4′-bpy = 2,4′-bipyridine, 4,4′-bpy = 4,4′-bipyridine and H2mal = malonic acid) have been prepared and structurally characterized by X-ray crystallography. Their structures are made up of two- (1) and three-dimensional (2) arrangements of manganese atoms linked by carboxylate-malonate groups in the anti–syn bridging mode (1 and 2) and bis(monodentate) 4,4′-bpy (2). The 2,4′-bpy group in 1 acts as a monodentate ligand. Each manganese atom in 1 and 2 is six-coordinated with four carboxylate-oxygens in the equatorial plane and a nitrogen atom and a water molecule in the axial positions. The magnetic properties of 1 and 2 and those of the related sheet-like polymer [Mn(mal)(H2O)2]n (3) (whose structure was published elsewhere, orthorhombic Pca21) have been investigated in the temperature range 2.0–290 K. The magnetic coupling between the manganese(II) ions in 1 and 2 through the single carboxylato bridge is weakly antiferromagnetic and practically identical (Jca. −0.2 cm−1). Weak antiferromagnetic coupling is also observed in complex 3 (occurrence of carboxylate bridges in the anti–anti and anti–syn bridging modes) together with spin canting behaviour at very low temperatures (Tc = 2.7 K). The antisymmetric exchange is most likely responsible for the small canting observed in 3.

Synthesis, crystal structure and magnetic properties of the three-dimensional compound [Na2Ni(mal)2(H2O)6]n (H2mal=malonic acid)

Abstract A new three-dimensional nickel–sodium compound of formula [Na 2 Ni(mal) 2 (H 2 O) 6 ] n (H 2 mal=malonic acid) was prepared and its structure was determined by X-ray diffraction methods. Four malonate–oxygen atoms and two trans -(Ni) or cis -(Na) coordinated water molecules build distorted octahedral surroundings around the metal atoms. The malonate group exhibits bidentate (Ni) and tetrakis-monodentate (Na) coordination modes. The structure can be described as corrugated sheets of malonato-containing Ni(II) and Na(I) cations which grow in the (101) plane, each sheet being linked to the adjacent one in the [101] direction through bis(μ-aqua)disodium(I) units. Within each corrugated sheet, the nickel atoms define a square lattice, the edge and the diagonal being 8.895(3) and 12.579(3) A, respectively. Variable-temperature magnetic susceptibility measurements (2.0–290 K) reveal the occurrence of a very weak antiferromagnetic interaction between the nickel(II) ions.

A phase transition in the novel three-dimensional compound [Eu2(mal)3(H2O)6](H2mal=malonic acid)

Slow diffusion of aqueous solutions of europium(III) chloride into gel of sodium metasilicate containing malonic acid (H2mal) yields single crystals of the three-dimensional compound of formula [Eu2(mal)3(H2O)6] whose structure was determined by X-ray diffraction methods at 293 and 173 K. It crystallizes in the monoclinic system but the spatial group changes from I2/a in the high temperature range (293 ≥ T ≥ 236 K) to Ia in the low temperature range (T < 236 K). In both cases, nine oxygen atoms forming a distorted monocapped square antiprism surround the Eu3+ ions. The structure at 293 K consists of a three-dimensional arrangement of triaquaeuropium(III) units bridged by malonate groups which result from cross-linking of the single chains running parallel to the c axis and the double zig-zag chains which grow in the ab plane. At low temperature the structure of the compound can be visualised as chains of europium(III) ions linked through two of the three crystallographically independent malonate ligands, whose chains run parallel to the b axis and a second family of chains (along the c axis) through the third independent malonate ligand forming a three-dimensional network. In both the crystal structure is stabilised through extensive hydrogen bonding involving carboxylate and water molecules. Studies of the magnetic behaviour, spectroscopic, thermogravimetric and calorimetric characteristics of [Eu2(mal)3(H2O)6] are reported. Laser-excited site selective spectroscopy shows a unique crystal-field site for EuIII ions in the crystal at room temperature and down to 236 K. However, below this temperature, two different sites are clearly identified, in agreement with a change in the crystal structure.

Anderson-Evans type molybdotellurates with diazolium cations: preparation, structure and properties of (H2imz)6 [TeMo6O24]·4H2O and (H2pyz)6 [TeMo6O24]·Te(OH)6

Abstract Two new Anderson-Evans molybdotellurates with imidazolium and pyrazolium cations have been prepared. X-ray crystallographic studies have been made from crystals of both compounds. The imidazolium salt (C3H5N2)6 [TeMo6O24]·4H2O (1) crystallizes in the monoclinic space group P2 1 /n with a = 14.372(3), b = 11.275(2), c = 13.019(3) A , β = 91.17(3)°, V = 2109.2(2) A 3 at 20°C and Z = 2; structural analysis is based on 6172 independent reflections (l ≥ 2σ(l)) with least-squares refinement of 297 parameters converged to R = 0.049. The crystal structure consists of a [TeMo6O24]6− with the TeO6 octahedron at the centre surrounded by six MoO6 sharing edges. The protonated organic bases bond with the anion in the crystal by hydrogen bonds. The pyrazolium salt (C3H5N2)6 [TeMo6O24]·Te(OH)6 (2) crystallizes in the triclinic space group P-1 with a = 10.920(2), b = 11.518(2), c = 10.056(2) A , α = 93.92(3), β = 103.97(3), γ = 109.80(3)°, V = 1139.1(4) A 3 at 20° C and Z = 2 . Structure analysis with least-squares refinement of 311 parameters for 6599 independent reflections (l ≥ 2σ(l)) converged to R = 0.0526. The crystal structure of this compound consists in discrete [TeMo6O24]6− anions and Te(OH)6 units bonded to pyrazolium cations by hydrogen bonds. The IR spectra of both compounds are discussed on the D3d symmetry of the [TeMo6O24]6 anion. The compounds have been studied by 1H, 95Mo and 125Te NMR spectroscopy, indicating in DMF the presence of octahedral species for both compounds, and a dissimilar behavior in H2O solution. While compound 1 undergoes a clear hydrolitic process forming tetrahedral MoO42− species, compound 2 is more stable in aqueous media. Thermogravimetry and differential scanning calorimetry of 1 show that this compound loses the four water molecules through an endothermic process ΔH = 137 kJ mol−1. The residual products were characterized by chemical analysis, IR and XPS spectroscopy. The best kinetic parameter values, corresponding to a solid state process involving the loss of the water molecules, were found on the basis of dynamic and isothermal methods. The best representation of a reaction mechanism is the random nucleation-one nucleus on each particle: −ln(1 − α) = kt. The values of activation energy (Ea = 93.3 kJmol−1), entropy of activation (ΔS≠ = −42.2 J K−1 mol−1), enthalpy of activation (ΔH≠ = 76.7 kJ mol−1) and free energy of activation (ΔG340.15 K≠ = 91.1 kJ mol−1) have been determined.