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{M 17 V 3 } Clusters

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{M 17 V 3 } Clusters

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  1. EPR Spectroscopy of Mixed-Metal Polyoxometalate Cages Encapsulating {VO4} MoietiesHaralampos N. Miras,[a] De-Liang Long,[a] Paul Kögerler,[b]Daniel Stone,[c] Eric McInnes,[c] Leroy Cronin[a][a]Department of Chemistry, University of Glasgow[b]AMES Laboratory, Department of Astronomy and Physics, Iowa State University[c]School of Chemistry, The University of Manchester Introduction {M17V3} Clusters • Polyoxometalates are metal-oxygen clusters built on a framework of transition metals surrounding one or more heteroanions • Potential applications in catalysis, magnetism, redox chemistry, medicine and materials science • Typical clusters based on Mo, W and V frameworks • Heteroanions generally main group, such as PO43- or SO42- • Clusters display a large number of structural types and physical properties • Significant number of clusters display Dawson type structure with general formula [M18O54(m9-XO4)2]n- where M = Mo, W or V and X = P, S • Ability to direct synthesis from smaller building blocks in a predetermined fashion remains a great challenge • Combination of synthetic approach with solution-based techniques to characterise new clusters enables rapid discovery of new clusters • Recent work has used cryospray mass spec to scan reaction mixtures before crystallisation to identify products warranting further analysis • Technique has led to discovery of several new cluster types • Dawson-type structures with unique {M17V3} composition recently isolated • (n-Bu4N)6[H2M17V3O62] where M = W, Mo • Clusters characterised by several techniques • Cryospray and electrospray mass spectrometry • IR and UV/Vis spectroscopy • Thermogravimetric analysis • Redox titration • Magnetic susceptibility (c ) • Single crystal X-ray diffraction (XRD) • Susceptibility indicates single uncoupled S = ½ VIVO vanadyl group per cluster • Two central {VVO4} vanadate templates located within {M17VIV} cage by XRD • VIVO vanadyl group disordered over cluster framework with other M17 centres • Tentative suggestion VIVO may be in ‘cap’ region due to bonding requirements • DFT also suggests ‘cap’ position for VIVO most energetically favourable • DE = -12.3 kJ mol-1 (Mo) and -10.7 kJ mol-1(W) compared to ‘belt’ regions • - Position of VIVO within cluster framework remains unclear Dawson-type structure for M17V3 cluster (cap region shown in green; belt region in blue) Polyoxometalate chemistry is a diverse field with many potential applications in catalysis, magnetism, redox chemistry, medicine and materials science Aims are to charactise M17V cages by EPR to elucidate the position of VIV within cluster and to determine electronic structures for potential applications in catalysis Synthesis 60 % Na2WO4.2H2O + NH2OH.HCl Na2MoO4.2H2O + TEA (TEAH)6[H2Mo17VO54(VO4)2] Na4(NH4)2[H2W17VO54(VO4)2] (n-Bu4N)6[H2W17VO54(VO4)2] (n-Bu4N)6[H2Mo17VO54(VO4)2] 75 % 1. H2O 1. H2O 1. H2O 1. H2O TEA=N(CH2CH2OH)3 2. (C4H9)4NBr 2. (C4H9)4NBr 2. NH4VO3 2. NH4VO3 {W17V3} {Mo17V3} • EPR investigation of (n-Bu4N)6[H2W17VO54(VO4)2] • EPR spectra for (n-Bu4N)6[H2W17VO54(VO4)2] in agreement with c data • Single S = ½ VIVO vanadyl per cluster • Multiple VIV sites evident in X-band powder spectrum • Powder simulation requires two sets of g-values • X-band powder, fluid and frozen solution spectra • K-band powder spectrum • - Spectra consistent with c data => single S = ½ VIVO vanadyl per cluster • - X- band powder spectrum displays no significant changes between 100 and 300 K • - Spectrum can be simulated with a single set of g-values, indicating a single cluster position for VIV • - Fluid and frozen solution spectra consistent with powder spectrum X-band powder spectrum X-band powder spectrum Parameters required for X-band powder simulation • Weighted average of parameters for X-band powder simulation gives • gav = 1.942 and Aav = 103 G • Frozen solution simulation gives gav = 1.943 and • Aav = 102 G • Simulation of fluid solution spectrum is consistent with powder and frozen solution spectra • giso = 1.96 • Aiso = 91 G • Fluid solution simulation gives giso = 1.945 and • Aiso = 97 G X-band fluid solution spectrum Parameters required for X-band powder and frozen solution simulations • Separation of g-values more apparent at K-band • VIV has more than one possible W17V cluster site X-band fluid solution spectrum • K-band powder spectrum shows distinct separation of peaks • VIV has more than one possible W17V cluster site • Simulation requires at least two sets of g-values Current best fit parameters for K-band powder simulation K-band powder spectrum • Determination of g-values and hyperfine coupling (A) for Mo17V3 could be further improved by measurement at higher frequencies • Experiments planned at Q-band Parameters required for K-band powder simulation K-band powder spectrum - Weighted average of parameters gives gav = 1.956 and Aav = 95 G Results show multiple VIVsites in both W17V3 and Mo17V3 and demonstrates that EPR can be used to reveal structural information unavailable by other methods Multifrequency EPR can be an important tool in structural analysis Acknowledgements References H.N. Haralampos, D-L. Long, P. Kögerler, L. Cronin, Dalton Trans., 2008, 214-221 Prof David Collison for advice, EPSRC for funding

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