Polyoxometalate Chemistry : Hydrolysis Kinetics using 17O

Polyoxometalates (POMs) are a class of compounds with a wide variety of industrial and medicinal applications and are typically composed of group 5 and 6 metals from the periodic table. POMs are large anionic clusters where the metal centers are bridged by oxygens, a typical POM x- structure is the 'Lindqvist Ion', which has the general formula [M6O19] , where M = Mo, W, V, Nb or Ta and the value of x depends upon the metal substitutions (Figure 1). In my time at UC Davis 8- I conducted a detailed study of the reactivity of the hexaniobate, [Nb6O19] , Lindqvist Ion using 17O NMR as a technique for following hydrolysis reactions at specific oxygen sites of the ion. 17 8- Figure 1 shows the chemical shifts of O tagged oxygens of [Nb6O19] which is dissolved in isotopically normal water.

Figure 1: 17O Nuclear Magnetic Resonance spectrum of a 17O tagged hexaniobate compound 17 dissolved in isotopically normal water. Relative to the bulk water signal ( OH2) the signals for the tagged oxygens sites (6 terminal oxygens: η=O; 12 bridging oxygens: µ2-O(H); 1 central oxygen: µ6- O; highlighted in yellow) are shifted upfield (Black, Nyman and Casey, 2006).

Hydrolysis reactions, and subsequent oxygen exchange at specific sites of the ion, can be monitored as the 17O enriched compound equilibrates with the isotopically normal water. This results in an exponential decay in the 17O NMR signal of sites which are reacting, from which a characteristic time for isotopic exchange (τ, see inset in Figure 2) can be measured. In a detailed study of the hexaniobate's reactivity I measuring the characteristic time for isotopic exchange of bridging and terminal oxygen sites as a function of pH, temperature and solution composition (the central oxygen site was inert under the conditions of our study). Figure 2 shows the characteristic time for oxygen exchange at the bridging and terminal sites of the molecule at 24oC as a function of pH. Interestingly under certain pH regimes (pH < 11 and pH > 14) the bridging and terminal oxygen sites exchange at the same rate as one another, whereas, under other pH conditions (14 < pH < 11) the bridging oxygen sites react much faster than the terminal sites.

Figure 2: The characteristic time (τ) for oxygen exchange of bridging (µ2-O) and terminal (η=O) oxygens as a function of pH. Temperature = 24.4oC. The upper right inset shows how τ is derived from the exponential decay of a given 17O signal (integrated area) over time. (Black, Nyman and Casey, 2006)

These trends in reactivity can be quantitatively explained by the acid-base chemistry of the hexaniobate ion and fit using a rate law that defines the rate of oxygen exchange (via H2O) as a function of the protonation state of the molecule (pKa1 = 13.63; pKa2 = 9.92; pKa3 = 9.35). The mechanism by which oxygen exchange occurs is not as easy to interpret and probably takes place through a combination of intramolecular scrambling of µ2-O and η=O sites and direct exchange via H2O attack of an undersaturated Nb site in a hypothesised 'ring- opened' metastable intermediate structure (pictured right).