Synthesis, structure and mechanism ofpolyoxometalate self-assembly:towards designed nanoscale architectures

QD Chemistry

Wilson, Elizabeth Frances ; Cronin, Leroy

University:University of Glasgow

Department:School of Chemistry

关键词: polyoxometalates, mass spectrometry;   

Others  :  http://theses.gla.ac.uk/1149/1/2009Wilsonphd.pdf

来源: University of Glasgow

【 摘 要 】

Cryospray (CSI-) and electrospray mass spectrometry (ESI-MS) techniques have beenutilised to investigate the key features of the ‘in-solution’, self-assembly processes bywhich complex polyoxometalate systems, such as ((n-C4H9)4N)2n(Ag2Mo8O26)n and ((n-C4H9)4N)3[MnMo6O18((OCH2)3CNH2)2], are formed.CSI-MS monitoring of the rearrangement of molybdenum Lindqvist anions, [Mo6O19]2-, inthe presence of silver(I) ions, into a silver-linked β-octamolybdate structure, has allowedelucidation of the role of small isopolyoxomolybdate fragments and AgI ions in theassembly process. The observation of higher mass fragments, each with increasing organiccation contribution concomitant with their increasing metal nuclearity, has supported thepreviously proposed hypothesis that the organic cations have a structure-directing role inpromoting the mode of POM structure growth in solution. The combined use of UV/visspectroscopy and real-time CSI-MS monitoring of the reaction solution allowed correlationbetween the decreasing Lindqvist anion concentration and increasing β-octamolybdateanion concentration. Furthermore, UV/vis spectroscopy was used to show that the rate ofdecrease in Lindqvist anion concentration, and therefore, the inter-conversion of Lindqvistinto β-octamolybdate anions, decreases as the carbon chain length of the alkylammoniumcations in the system increases.This approach was extended to use ESI-MS monitoring in examining the formation of themore complex, organic-inorganic, Mn-Anderson polyoxomolybdate structure ((n-C4H9)4N)3[MnMo6O18((OCH2)3CNH2)2]. In this investigation, ESI-MS was used tomonitor the real-time, ‘in-solution’ rearrangements of α-octamolybdate anions, [α-Mo8O26]4-, and coordination of manganese(III) cations andtris(hydroxymethyl)aminomethane (TRIS) groups in the formation of the Mn-Anderson-TRIS structure. These investigations have led to the proposal that the rearrangement of [α-Mo8O26]4- anions occurs first through decomposition to [Mo4O13]2- cluster species, i.e. halffragmentsof the octamolybdate anion; followed by decomposition to smaller, stableisopolyoxomolybdate fragment ions such as dimolybdate and trimolybdate fragment ions.It has then been proposed these fragments subsequently coordinate with the tripodal TRIS ligands, manganese ions, and further molybdate anionic units to form the final, derivatizedMn-Anderson-TRIS cluster.Investigations into the encapsulation of the high oxidation state heteroanion templates{IVIIO6} and {TeVIO6} within polyoxomolybdate clusters, have led to the isolation andcharacterization of two new, molybdenum Anderson-based POM architectures, i.e.Cs4.67Na0.33[IMo6O24]·ca7H2O and Na4((HOCH2CH2)3NH)2[TeMo6O24]·ca10H2O. The useof coordinating caesium and sodium cations allowed the formation of a closely-packedstructure composed of the periodate-centred Anderson clusters arranged into two layers,which then form a repeating ABAB pattern through the lattice. In contrast, the mainbuilding-blocks of the tellurium-based cluster system features the [TeMo6O24]6- anions andtwo coordinated cation arrangements, each composed of a {Na2} dimer and coordinatedTEAH+ cation. The presence of this structural motif, and its inter-connection with adjacentclusters, has led to chain-like packing arrangements within the greater lattice structure.The introduction of three aromatic, phenanthridinium-based cations into polyoxometalatesystems has led to the isolation and characterization of three new POM architectures withemergent photoactivity. The polyoxometalate framework in each is composed of tungstenKeggin clusters, i.e. [PW12O40]3-, which are introduced into the systems as pre-formedbuilding-blocks. Two of the compounds use derivatives of Dihydro-Imidazo-Phenanthridinium (DIP) molecules as cations, i.e. (DIP-1)[PW12O40]·5DMSO·ca1H2O and(DIP-2)[PW12O40]·5DMSO·ca4H2O, whereas the final compound uses an Imidazo-Phenanthridinium (IP) molecule as the cationic unit, i.e. (IPblue)3[PW12O40]·4DMSO. Theuse of these cations, which have different steric bulk, geometry and charge states, has ledto the formation of interesting packing arrangements within the lattice structures of allthree compounds. Additionally, further characterization of these compounds has revealedthey all possess emergent photoactivity, in the form of intermolecular charge transferbands in the solid state. Some degree of intermolecular charge transfer in the solution statehas also been detected for the DIP-2-based structure.