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Self Assembly of structurally diverse phosphomolybdates


Thesis paper by : Rev. Dr. Jisha Joseph CMC

Presented on : 25-08-2021

Polyoxometalates (POMs) are a prime class of oxo-bridged early transition metals in high oxidation states (such as MoV, MoVI, WVI, VV, TaV and NbV). These metal oxygen anion clusters have contributed a lot towards the rapid development of inorganic chemistry in the past decades on account of their structural diversity and potential applications in fields of research such as medicine, catalysis, magnetism, photochemistry and redox chemistry. The first POM, {PMo12O40} 3- was reported by Berzelius in 1826 and later Keggin solved the structure of related anion {PW12O40}3-.

Among the different heteropolyanion clusters, phosphorous and molybdenum containing clusters namely phosphomolybdates (PMOs) comprise of a distinguished family with versatile structural features and promising applications. Phosphomolybdates are further divided into different types. The four important types are (i) Strandberg type (ii) Keggin type (iii) Wells Dawson type and (iv) Fully reduced types.

The counter cations of phosphomolybdate anions can be (i) metal ions like Na+, K+ Ca2+ and Cu2+ (ii) metal complexes and (iii) organic cations. The formation of these types of crystalline solids is a self-assembly process. Being supramolecular materials, they can self-assemble into aggregates with multiple dimensionalities. There are a number of examples for 1-D, 2-D and 3-D networks of phosphomolybdates in the literature. Intermolecular interactions like hydrogen bonding, π-π stacking and electrostatic interactions play a vital role in stabilizing the supramolecular assembly. Although many researchers have been exploring these solids, crystal engineering of PMO cluster anion based solids faces challenges such as critical control of parameters such as pH and solubility of reactants. Based on the above details the present investigation is about the synthesis, characterization and properties of novel PMOs.

Two new Strandberg-type cluster based phosphomolybdates {H2a3mp}5[{PO3(OH)}{PO4}Mo5O15], and {H-2a4mp}5[{PO3(OH)}{PO4}Mo5O15].6H2O have been synthesised via solution evaporation technique using 2-amino-3-methylpyridine
(2a3mp) and 2-amino-4-methylpyridine (2a4mp) respectively. The electrochemical behaviour of synthesized solids was explored by means of three-electrode system using 1 mM K4[Fe(CN)6] in 0.1 M KCl as supporting electrolyte. In addition, the optical band gaps of the solids have also been calculated using UV-DRS data.
Self-assembly of molybdate and phosphate precursors in the presence of zinc ions and organic ligands viz. benzimidazole (bimi), 4-aminopyridine (4-ap) and pyrazole (pz) resulted in the crystallization of Strandberg cluster based solids (Hbimi)5{HP2Mo5O23}.5H2O, (Hbimi)6{P2Mo5O23}.H2O (4Hap)4{H2P2Mo5O23}.2H2O, (4-Hap)5{HP2Mo5O23}and (Hpz)6{Zn(pz)4(H2O)2}{Zn(pz)2P2Mo5O23}2.8H2O under hydrothermal condition. Scheme 1 describes the experimental procedure for the formation of these solids. This part highlights the structural differences in the supramolecular isomers; and the effect of supramolecular isomerism in {P2Mo5} cluster based solids and nature of ligands on the optical band gap energies (Eg) of the synthesized solids.

The present work also discusses the in situ self-assembly of phosphorous and molybdenum precursors in the presence of MCl2.xH2O (M = Co, Ni, Cu and Zn) with pyrazole to form PMO solids of varying dimensionality. The solids obtained were: (Hpz)6{Zn(pz)4(H2O)2} {Zn(pz)2P2Mo5O23}2.8H2O [{Cu(pz)4}2{H2P2Mo5O23}]H2O, {Ni(pz)4}[{Ni(pz)4}2 {H2P2Mo5O23}]2[{Ni(pz)4}{Ni(pz)4(H2O)}{HP2Mo5O23}]2.14H2O, [Ni(pz)4Cl2], (pz)2 [{Co(pz)4}5{P2Mo5O23}2].6H2O and [{Cu(pz)2}4{CuMo12O38(OH)2}].8H2O. Among which the last solid is a rare example of copper based Keggin cluster. The magnetic properties of the solids were investigated using Guoy Balance.

Synthesis, characterization and dye removal efficiency of ammonium phosphomolybdate (APM) which is a Keggin type solid have been discussed. It is concluded that APM can be effectively used as an ion exchanger to remove cationic dye stuffs from aqueous solution. The dyes used are methylene blue, malachite green, methyl red and eosin. The factors which affect dye removal like effect of light, effect of amount of APM, effect of contact time and effect of pH have been investigated.

Two composites of APM with polyaniline and poly (N-methylaniline), namely APM/PAni and APM/PNMAni have been synthesized and characterized successfully. The difference in band gap energy in APM upon composite formation has been investigated, and the electrochemical behaviour of APM and its composites were studied. The ability of APM, polymers and composites in the removal of hexavalent chromium from its solution has been studied. Polymers removed Cr(VI) from solution through electrostatic interactions, while APM/PNMAni could reduce Cr from harmful +6 oxidation state to environmentally benign +3 oxidation state.

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