Journal of Physical Chemistry & Biophysics
Open Access
ISSN: 2161-0398
+44 1478 350008
ISSN: 2161-0398
+44 1478 350008
Ayi A. Ayi
Department of Pure and Applied Chemistry, Nigeria
Scientific Tracks Abstracts: J Phys Chem Biophys
Molybdophosphonates as building blocks or subunits in extended structures have been extensively studied, but reports of molybdenum diphosphonates in the absence of co-ligands are scanty (Armatas et al 2008; Jones et al., 2010). Herein is a report on three-dimensional molybdophosphonate frameworks formed by the combination of a dioxomolybdenum(VI) fragment with a diphosphonate (Ayi et al., 2013). The solvothermal reaction between sodium molybdate(VI) dihydrate (Na2MoO4•2H2O) and p-Xylylenediphosphonic acid (H4xdp) in the presence of concentrated hydrochloric acid led to colourless crystals of [(MoO2)2(xdp)(H2O)2]•2H2O 1, which were characterised by X-ray crystallography. The asymmetric unit of 1 consists of a molybdenum centre which is coordinated to two oxides, a half of a xdp4− anion (located at a crystallographic inversion centre) and a water molecule. One lattice water molecule is also present. The xdp ligand is fully deprotonated, adopting a transconformation, and each phosphonate group is coordinated to molybdenum centres through all three oxygen atoms. The molybdenum(VI) centre is six-coordinate with a distorted octahedral geometry, coordinated to two oxides, three phosphonate oxygen atoms, and a water molecule. The octahedral molybdenum units are linked by the tetrahedral phosphonate groups of the xdp ligand into sheets with the 4.82 topology (Fig. 1a). These sheets are pillared by the xdp linkers leading to a three-dimensional network (Fig. 1b).The structure contains channels of approximate dimensions 3.9 × 4.6 Å, based on the van der Waals radii of the surface atoms. In the crystal structure, however, these channels are occupied by the included water molecules, which form O–H...O hydrogen bonds with the oxygen atoms of the coordinated water molecules acting as acceptors. On heating under vacuum for 3 hours at 150 °C, 1 is converted into the fully dehydrated compound [(MoO2)2(xdp)], 2. This loss of water is accompanied by a colour change, from colourless to pale green. Significant differences in the Powder X-ray diffraction (PXRD) patterns of 1 and 2 (Fig. 2a and 2c) are indicative of a major structural change that occurs on removal of the water molecules. Interestingly, N2 sorption measurements revealed 2 to be non-porous, suggesting that the structural change occurs to minimize void space. The difference between the as-synthesized compound 1 and fully dehydrated compound 2 can also be observed from their infrared spectra. Compound 1 shows two ν(OH) bands and a corresponding deformation vibration δ(H2O). The spectrum for 2 shows the absence of these ν(OH) and δ(H2O) bands and a change in the bands characteristic of 2 the tetrahedral CPO groups. The compound shows reversible dehydration, which occurs with a structural change just like the isostructural anionic metal−organic frameworks (MOF): ZnBP-NH4 and ZnBP-CH3NH3 obtained from the hydrothermal synthesis using Zn(II) cations and 1, 3, 5-benzenetriphosphonic acid (BTP) (Kinnibrugh et al., 2013) Significance of the work : Molybdenum–diphosphonate coordination network have been prepared and structurally characterised and shown that it can undergo reversible dehydration, which occurs with a structural change. The dehydrated material shows size-selective adsorption of alcohols, adsorbing methanol but not ethanol. Thus, this material could be used to separate miscible liquids such as water, methanol and ethanol. Furthermore, the donor ability and/or hydrogen bonding potential of the oxo groups in the three-dimensional network structures could be exploited and the material could be investigated for their proton conducting properties. There is also commercial interest in their application in separation, storage, and heterogeneous catalysis. Keywords: Metal-organic frameworks, Solvothermal synthesis, Reversible dehydration, Coordination network.
Ayi Anyama Ayi received his B.Sc. and Ph.D. degrees in 1995 and 2002, respectively from the University of Calabar. He was a TWAS South-South Visiting Scholar in 2000, and a TWAS Postdoctoral Fellow (2003-2004) at Jawarhalal Nehru Centre for Advanced Scientific Research (JNCASR), Bangalore, India, under Prof. C. N. R. Rao. As a man of considerable drive and resourcefulness, he has received many Academic Fellowships/Awards