Coordination and lonic compounds of benzamide and related molecules and metal halides

Abstract

Organic-inorganic hybrid materials of divalent transition metal halides and simple organic molecules are known to exhibit interesting properties, with both the organic and the inorganic metal halide components contributing to the overall properties of the material (Aakeroy, Champness and Janaik, 2010) (Alexandre et al., 2007) (Aruta et al., 2004) (Criado et al., 1999) (Chondroudis and Mitzi, 1999) (Mitzi, Chondroudis and Kagan, 2001) (Robin and Fromm, 2006). The benzamide molecule was chosen as the primary organic component in this study, due to its potential to form aromatic interactions, and the fact that it possesses two potential sites for hydrogen bonding or protonation or coordination, via the amide group. In addition, the 4-aminobenzamide molecule was investigated due to its similarity to the benzamide molecule, except for an additional functional group on the opposite side of the amide group. A range of inorganic, divalent transition metal chlorides and bromides were selected as inorganic components, including CuX2, CdX2, CoX2, MnX2, HgX2 and ZnX2, where X = Cl or Br. Both ionic and neutral coordination type materials can be formed through the combination of the organic components and inorganic components described above, and the outcome of the reaction is controlled by the presence or absence of acid in the reaction medium. In acidic medium protonation of the organic component results in the formation of an ionic material, and in the absence of an acid a neutral coordination material results from the coordination of the organic component to the metal atom of the metal halide component. The stoichiometry and concentration of the acid will also have an influence on the structure as some of the organic ligand may remain unprotonated. In this study seven novel neutral coordination type structures and one ionic material were characterised by single crystal X-ray diffraction. Four isostructural, neutral, one-dimensional coordination polymer structures were observed for the combination of benzamide with CuCl2, CuBr2, CdCl2 and CdBr2 respectively. In this isostructural series the metal ions adopt an octahedral geometry in the case of the Cd analogues, and a tetragonal geometry, due to Jahn-Teller distorsion, for the Cu members of the series. The combination of ZnCl2 with benzamide yielded an isolated, zero-dimensional, neutral coordination compound in which the zinc(II) ion displays a tetrahedral geometry. An octahedral, paddle wheel-type, isolated, zero-dimensional coordination molecule formed from the combination of MnBr2 and benzamide. In all the coordination compounds of benzamide, coordination was found to occur through the oxygen atom of the amide group, while the NH2 group participates in intra- and intermolecular hydrogen bonding interactions. Due to the poor basicity of benzamide, only one ionic compound of this organic component was characterised structurally. In this compound, formed in combination with HgBr2, half of the benzamide molecules are protonated, and a unique, novel, one-dimensional perhalometallate anion was observed in this structure. The combination of 4-aminobenzamide with CoBr2 yielded an isolated, zero-dimensional, neutral coordination structure, in which the cobalt(II) ion adopts an octahedral geometry. The neutral, coordination compound formed between 4-aminobenzamide and CuBr2 has a trigonal bipyramidal geometry, and in addition to the organic and halogeno ligands, aqua ligands are also coordinated to the metal ion in both of the 4-aminobenzamide-containing molecules. Hydrogen bonding and aromatic interactions occur in all of the structures, and these interaction geometries were analysed in detail. It was found that these interactions play an important role in the cohesion of the units in the structure, with exception of the ionic compound of benzamide, which displays strong hydrogen bonding interactions but long aromatic centroid-to-centroid distances. Diffuse reflectance spectroscopy (DRS) was employed to measure the band gaps of the series of isostructural, one-dimensional coordination polymers. These measurements indicated that the Cd analogues in this series have band gaps that place them in the category of electronic insulators. The Cu members of this series were found to be two-band gap materials, with the lower of the two band gaps falling within the conductor range, while the higher band gap falls in the semi-conductor range for the structure containing CuBr2 and benzamide and the structure containing CuCl2 and benzamide. The suitability of Density Functional Theory (DFT) calculations to theoretically calculate the electronic structures and band gap values of the isostructural series of one-dimensional coordination polymers was tested in this study. The plane-wave cut off energy and k-point grid were optimised for the structures, and a range of functionals were tested. The best performing functional was used to calculate the band gap energies, the band structures and the density of states for the four isostructural materials. DFT calculations are known to underestimate band gap energies, and even though the calculated band gaps differ from the experimentally measured band gaps, this difference is smaller than expected. In addition, the DFT calculations were successful in predicting and providing insight into the electronic characteristics of the materials. Copyright