Key questions in geo-and environmental chemistry concern interactions between water and metal-oxide/mineral surfaces as these are responsible for weathering and for the elimination of pollutants. Large oxide ions could be enormously useful to geochemists in testing hypotheses about reaction pathways at mineral surfaces, but most dissociate rapidly, exchange oxygen atoms too quickly, or have such complicated acid-base chemistry that they are not helpful. Simultaneously, information about reaction pathways in polyoxometalate (POM) ions is needed to understand the degradation of catalysts and the structural evolutions among different POMs. The nanometer-size decaniobate [1] ion ([H x Nb 10 O 28 ] (6Àx)À ) is unique in aqueous niobate chemistry as it does not strongly protonate when dissolved in water and is stable at nearneutral pH. We report here the rates of steady isotope exchange at all seven different oxygen sites in this ion (labeled A-G in Figure 1 a) as a function of solution composition. Separately, we follow the pathways for dissociation and identify the reaction products. Our results indicate that the entire structure is involved in the reaction dynamics. For example, rates of steady oxygen-isotope exchanges also increase with pH, even when these processes are much more rapid than dissociation. Furthermore, base-induced dissociation leaves much of the molecule intact, illustrating pathways for interconversion of all isopolyniobate types known to occur in aqueous media.We prepared We compared the 17 O NMR spectra with electrosprayionization mass spectra (ESI-MS) to identify dissociation pathways. The reactivity trends for steady isotope exchanges at all seven structural oxygen sites are surprising: 1) The rates span a range of approximately 10 4 and are not predictable from simple structural considerations (see the (6Àx)À at pH 6.6 and 308.5 K. Times range from 25 min to 15.5 h. C) Rates of steady oxygen isotopic exchange at 308.5 K as a function of pH, with k = 1/t, the characteristic time. The rate of exchange at the m 6 -oxo site A is proportional to the rate of dissociation of the molecule.

Mn2+ has five unpaired d-electrons, a long electronic relaxation time, and labile water exchange, all of which make it an attractive candidate for contrast agent application in medical magnetic resonance imaging. In the quest for stable and nonlabile Mn2+ complexes, we explored a novel dimeric triazacyclononane-based ligand bearing carboxylate functional groups, H4ENOTA. The protonation constants of the ligand and the stability constants of the complexes formed with some endogenously important metals (Ca2+, Cu2+, Zn2+), as well as with Mn2+ and Ce3+, have been assessed by NMR methods, potentiometry, and UV-vis spectrophotometry. Overall, the thermodynamic stability of the complexes is lower as compared to that of the corresponding NOTA analogues (H3NOTA, 1,4,7-triaazacyclononane-1,4,7-triacetic acid). The crystal structure of Mn2(ENOTA)(H2O) x 5H2O contains two six-coordinated Mn2+, in addition to the three amine nitrogens and the two oxygens from the pendent monodentate carboxylate groups, and one water (Mn2) or one bridging carboxylate oxygen (Mn1) completes the coordination sphere of the metal ion. In an aqueous solution, this bridging carboxylate is replaced by a water molecule, as evidenced by the 17O chemical shifts and proton relaxivity data that point to monohydration for both metal ions in the dinuclear complex. A variable-temperature and -pressure 17O NMR study has been performed on [Mn2(ENOTA)(H2O)2] to assess the rate and, for the first time on a Mn2+ chelate, also the mechanism of the water exchange. The inner sphere water is slightly more labile in [Mn2(ENOTA)(H2O)2] (k298ex = 5.5 x 107 s-1) than in the aqua ion (2.1 x 107 s-1, Merbach, A. E.; et al. Inorg. Chem. 1980, 19, 3696). The water exchange proceeds via an almost limiting associative mechanism, as evidenced by the large negative activation volume (deltaV = -10.7 cm3 mol-1). The proton relaxivities measured on [Mn2(ENOTA)(H2O)2] show a low-field dispersion at approximately 0.1 MHz arising from a contact interaction between the MnII electron spin and the water proton nuclear spins.