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 b een 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 obs erved. This signal, at approximately 180 ppm, corresponds to approxima tely 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 m onths, 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 sp ecific proton of this type is unusually long for inorganic coordinatio n compounds in aqueous solution, almost-equal-to 0.1 s in 2.2 M Zr sol ution 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 molecul es is replaced completely upon addition of one nitrate per Zr. Additio n of acetone to an aqueous solution of the tetramer leads to coordinat ion of acetone to the tetramer.