O-17 AND H-1-NMR STUDY OF THE TETRANUCLEAR HYDROXO ZIRCONIUM COMPLEX IN AQUEOUS-SOLUTION

The tetrameric hydroxo zirconium(IV) complex in aqueous solution has been studied by means of O-17 and H-1 NMR. For the first time, an O-17 NMR signal from oxygens coordinated to Zr in this complex has been observed. This signal, at approximately 180 ppm, corresponds to approximately 2 O/Zr and has been assigned to two strongly bound terminal water molecules. Exchange of these waters with the bulk water takes a few months, as found by addition of O-17-enriched water. In H-1 NMR spectra, besides the bulk water signal, a signal at 7.9 ppm has been observed at room temperature and assigned to slowly exchanging protons, 2H/Zr, of the terminal water molecules in the tetramer. The lifetime of a specific proton of this type is unusually long for inorganic coordination compounds in aqueous solution, almost-equal-to 0.1 s in 2.2 M Zr solution at room temperature. On this basis, the formula of the tetramer in aqueous solution should be written [Zr4(OH)8(H2O)8I(H2O)8II]8+, i.e. there are two inert and two labile water molecules per Zr. Up to two of the coordinated water protons are so easily dissociated that this species constitutes a very strong acid. One of the inert water molecules is replaced completely upon addition of one nitrate per Zr. Addition of acetone to an aqueous solution of the tetramer leads to coordination of acetone to the tetramer.