Open Access
Thermostructural behavior in a series of lanthanide-containing polyoxotungstate hybrids with copper(II) complexes of the tetraazamacrocycle cyclam: a single-crystal-to-single-crystal transformation study
(American Chemical Society, 2019) Martín Caballero, Jagoba; Artetxe, Beñat; Reinoso, Santiago; San Felices, Leire; Vitoria, Pablo; Larrañaga, Aitor; Vilas, José Luis; Gutiérrez Zorrilla, Juan M.; Institute for Advanced Materials and Mathematics - INAMAT2; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
A series of 14 isostructural [Cu(cyclam)](2)[{Cu(cyclam)}4{(a-GeW11O39)Ln(H2O)-(OAc)(12)].18H(2)O (1-Ln, where Ln = La Lu; cyclam = 1,4,8,11-tetraazacyclotetradecane) polyoxometalate-based hybrids reported herein represent (i) the first example of a twodimensional covalent hybrid lattice involving the [{(aXWI 1039)Ln(H2O)(OAc)}2in- archetype and (ii) the first structural characterization of such a dimeric polyoxotungste for Ln = La and Pr as well as for the combination of X = Ge and Ln = Ce, Nd, Sm, or Lu. Compounds 1-Ln have been characterized by elemental analyses, infrared spectroscopy, and thermogravimetric analysis, and their thermostructural behavior has been monitored by powder and single-crystal Xray diffraction. The title compounds undergo two single-crystal-to-single-crystal transformations triggered dehydration leading to the [{Cu(cydam)}6{(alpha-GeW11O39)Ln(H2O)(OAc)}2]-4H(2)O intermediate (2-Ln, where Ln = Eu or Er) and [Cu(cyclam)]0.5[{Cu(cyclam)}55{(alpha-GeW11O39)Ln(0Ac)}2] (3-Ln, where Ln = Ce or Eu) final anhydrous phases, the latter evidencing a coordinatively unsaturated derivative of the dimeric archetype for the first time. These transitions involve formation and disruption of Cu Opom bonds that result in different {Cu(cydam)}(2+) moieties grafting onto and being released from Keggin surfaces, which reduces the dimensionality of 1-Ln to one-dimensional covalent assemblies for 2-Ln and 3-Ln. While all 3-Ln phases rehydrate fully upon exposure to air for 24 h, the kinetics governing the crystal transitions back toward 1Ln through 2-Ln depend on the nature of Ln. Under ambient moisture, the anhydrous structures fully revert back to the parent framework for Ln = La Sm, while the samples containing Eu to Lu afford mixtures of 1-Ln and 2-Ln and require immersion in water for the structural reversion to reach completion. Single-crystal X-ray diffraction analyses of the rehydrated 1R-Ln samples (Ln = Ce, Eu, and Er) support these observations.
Open Access
Thermally induced structural transitions between single-crystalline states in the first hybrid compound combining Keggin-type clusters with metal-cyclam complexes: from two-dimensional covalent assemblies to discrete molecular species
(American Chemical Society, 2020) Fernández Navarro, Leticia; Iturrospe, Amaia; Reinoso, Santiago; Artetxe, Beñat; Ruiz Bilbao, Estíbaliz; San Felices, Leire; Gutiérrez Zorrilla, Juan M.; Zientziak; Institute for Advanced Materials and Mathematics - INAMAT2; Ciencias
A hydrothermal reaction between the monolacunary polyoxometalate [SiW11O39]8-, a copper(II) salt, and 1,4,8,11-tetraazacyclotetradecane (cyclam) affords the first example of a hybrid compound combining metal complexes of such an N4-tetradentate macrocyclic ligand and Keggin-type clusters, namely, [Cu(cyclam)(H2O)][{Cu(cyclam)}2SiW11O39Cu(H2O)]·5H2O (1). This compound has been characterized by infrared spectroscopy and thermal and elemental analyses. Single-crystal X-ray diffraction (scXRD) reveals a layered crystal packing made of corrugated two-dimensional covalent grids in which four octahedral {Cu(cyclam)}2+ bridging moieties connect copper(II)-monosubstituted {SiW11O39Cu(H2O)}6- polyanions to four adjacent clusters, and additional [Cu(cyclam)(H2O)]2+ square-pyramidal counterions are embedded into square-like grid voids. Thermostructural analyses confirm the presence in the 130-250 °C temperature range of a stable and crystalline anhydrous phase, which displays a diffraction pattern different from that of 1. This thermally triggered phase transition proceeds through a single-crystal to single-crystal transformation pathway, which according to scXRD, involves not only the release of water molecules, but also the cleavage and formation of Cu-O bonds induced by the rotation of Keggin-type anions. These modifications fully dismantle the parent two-dimensional covalent assembly to result in the neutral, discrete [{Cu(cyclam)}3SiW11O39Cu] hybrid species (2), in which the cluster exhibits three square-pyramidal {Cu(cyclam)}2+ decorating moieties grafted at its surface. This species must display a five-coordinated copper(II) center in the Keggin skeleton, and therefore, it constitutes one of the scarce examples of such type of coordinatively unsaturated substituted cluster observed in the solid state. The irreversibility of the phase transition has been confirmed by combined thermal and diffractometric analyses, which evidenced also the great flexibility of the supramolecular framework of 2, as this anhydrous phase is able to adsorb up to six water molecules per cluster to lead to the hydrated derivative [{Cu(cyclam)}3SiW11O39Cu(H2O)]·5H2O (2h) without any significant alteration in its cell parameters, nor in its crystalline structure.

Reinoso, Santiago

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