Purpose and Scope of Research

1. Polyoxometalate-cationInteractionsTerry M. Hall, Kevin D. Reynolds, and Jason D. Powell†School of Natural Sciences and MathematicsFerrum College, Ferrum, VA 24088April 7, 2013: 245th American Chemical Society National Meeting and Exposition, New Orleans, LA Purpose and Scope of Research The purpose of this research is to determine the nature of interactions between three Kegginpolyoxometalateswith ten different metal cations in the context of coordination chemistry and solubility. Coordination chemistry is a well understood branch of inorganic chemistry in which Lewis bases (with lone pairs of electrons on a donor atom) coordinate to Lewis acids (with unoccupied orbitals on an acceptor atom). Oxygen atoms on the polyoxometalate act as the Lewis base while the metal cations act as the Lewis acid. The cations and the polyoxometalates were chosen because they represent a range of charges, charge densities, sizes, and hardness/softness. Polyoxometalates are well understood as being highly redox active and useful in homogeneous and heterogeneous catalysis, corrosion prevention, antimicrobial activity, and a variety of other applications. Our specific interest in this study stemmed from Dr. Powell’s earlier research where he examined the reaction of these compounds with silver and gold surfaces. There are still questions of the precise mode of interaction between the compounds and the surface, and this study has the potential to lend significant insight into that work. As an initial screening study, this work included only commercially-available Kegginpolyoxometalates. Published syntheses are readily available for a wide variety of other polyoxometalates for future work. Experimental Design In this experiment , three different quantities of 1.00 M solutions of Fe(NO3)3, Ca(NO3)2, LiNO3, Mg(NO3)2, Pb(NO3)2, Ni(NO3)2, Cd(NO3)2, AgNO3, Al(NO3)3, and Zn(NO3)2were combined with 0.100 M solutions of dodecamolybdophosphoric acid (H3PMo12O40), dodecatungstosilicicacid (H4SiW12O40), and dodecatungstophosphoric acid (H3PW12O40). The three different concentrations were designed to attempt to determine at what concentrations the polyoxometalates and the metal ions will react to form coordination compounds and/or precipitates. Solutions were screened for precipitates and crystal formation. In the absence of these, UV-visible spectra were obtained in a 1.0-cm quartz cuvette to look for shifts in absorbance peaks. If precipitates or crystals formed, they were isolated and dried for future analysis by FT-IR spectroscopy in KBr pellets. In addition, crystals were screened to determine whether any were of x-ray diffraction quality. Discussion (continued) It is significant to note that most of the interactions observed were for dodecatungstosilicic acid rather than dodecatungstophosphoric or dodecatungstomolybdic acid. This may be a direct result of the greater charge on the SiW12, so it may be fruitful to synthesize Kegginpolyoxometalates of greater charge as this study is expanded. Furthermore, most of the metal cations investigated have only limited coordination chemistry of their own. More strongly coordinating cations with a greater variety of electronic properties would be likely to generate more interesting and significant results. Results Of the combinations tested, only ? And ? produced crystals that will be examined by x-ray crystallography. ?, ?, ?, and ? yielded precipitates; lower concentrations will be investigated in an attempt to grow x-ray quality crystals from those components. All of the pure solutions of metal nitrates have similar absorbance spectra with a symmetric peak at λmax = 220 nm due to nitrate ion absorption. Each of the polyoxometalate solutions has an intense absorption peak extending into the ultraviolet and a second peak at longer wavelength. For the tungstates, the peak has λmax = 270 nm. Dodecamolybdophosphate has λmax = 315 nm. For the mixtures, we looked for shifts in λmaxrather than the changes in intensity that we would expect due to Beer’s law and the dilution of the polyoxometalate solution by the addition of the metal nitrate solution. Future Work The next step in this experiment is to perform x-ray crystallography on the samples in which crystals form. This will allow the structure and composition of the crystals to be determined. A key component of interest is whether the terminal oxygen atoms or the bridging oxygen atoms in the polyoxometalates are acting as the electron pair donors. Other techniques which will prove useful in the characterization of the interactions between the polyoxometalates and the metal cations are cyclic voltammetry, infrared spectroscopy, scanning electron microscopy, and electron impedance spectroscopy. KegginPolyoxometalates as Ligands Keggin ions are highly symmetrical. The general formula is XM12O40n–, where X is the heteroatom, M is Mo or W, and n is the net charge on the anion. It can be represented as (M12O36) ⊂ (XO4n–), where M12O36 is a neutral, Oh-symmetric metal oxide cage containing the tetrahedral XO4n– ion. The tetrahedral ion lowers the overall symmetry of the ion to Td. Most of the charge density remains on the four atoms of the central, tetrahedral ion. Since this is contained within the clathrate cage, it is unaccessible to cations for coordination. The only atoms available as Lewis base donors are the terminal oxygen atoms (12, one for each M atom) and the bridging oxygen atoms (24). The bridging oxygen atoms are very similar in their electronic structure to the oxygen atoms in crown ether compounds. The terminal oxygen atoms are very similar to carbonyl oxygen atoms. We would expect the bridging oxygen atoms to behave as simple sigma (σ) donors and the terminal oxygen atoms to behave as sigma donors and pi (π) acceptors.We would expect different Lewis acid cations to demonstrate preference for one type of donor atom over another depending on their electron configurations. In any case, these interactions are likely to be very weak and the conditions for complex formation will require precise ratios of metal and polyoxometalate concentration as well as solution ionic strength. Acknowledgements This project was supported by the School of Natural Sciences and Mathematics at Ferrum College. References 1Keggin, J. F., Nature1933, 908-909. 2Keggin, J. F., Nature1933, 351. 3Keggin, J. F., Proc. Roy. Soc. A1934, 144, 75-100. 4Pope, M. T. Heteropoly and IsopolyOxometalates, Springer-Verlag: Berlin, 1983. 5“Heteropolyanions: Molecular Building Blocks for Ultrathin Oxide Films” Powell, J. D.; Gewirth, A. A.; Klemperer, W. G. In Polyoxometalates: From Topology to Industrial Applications; Pope, M. T. and Müller, A., Eds.; Kluwer Academic: Dordrecht, 2000. Discussion The results of the UV-visible study are summarized in the table below. * Only dilution effects observed †Shift to higher wavelength observed for one or more concentration ‡Shift to lower wavelength observed for one or more concentration At first glance, the results are quite disappointing. Absorbance peaks for only four solutions shift to higher wavelength due to some sort of interaction between metal and polyoxometalate, while one solution shifts to lower wavelength. For More Information Contact Dr. Powell at jpowell@ferrum.edu or 540-365-4376 To Recruit Great Students Contact Terry at thall@ferrum.eduor Kevin at kreynolds@ferrum.eduu