Alkali-metal ion coordination in uranyl(VI) poly-peroxo complexes in solution, inorganic analogues to crown-ethers. Part 2. Complex formation in the tetramethyl ammonium-, Li+-, Na+- and K+-uranyl(VI)-peroxide-carbonate systems

he constitution and equilibrium constants of ternary uranyl(VI) peroxide carbonate complexes [(UO2)(p)(O-2)(q)(CO3)(r)](2(p-q-r)) have been determined at 0 degrees C in 0.50 M MNO3, M = Li, K, and TMA (tetramethyl ammonium), ionic media using potentiometric and spectrophotometric data; O-17 NMR data were used to determine the number of complexes present. The formation of cyclic oligomers, "[(UO2)(O-2)(CO3)](n)", n = 4, 5, 6, with different stoichiometries depending on the ionic medium used, suggests that Li+, Na+, K+ and TMA ions act as templates for the formation of uranyl peroxide rings where the uranyl-units are linked by mu-eta(2)-eta(2) bridged peroxide-ions. The templating effect is due to the coordination of the M+-ions to the uranyl oxygen atoms, where the coordination of Li+ results in the formation of Li[(UO2)(O-2)(CO3)](4)(7-), Na+ and K+ in the formation of Na/K[(UO2)(O-2)(CO3)](5)(9-) complexes, while the large tetramethyl ammonium ion promotes the formation of two oligomers, TMA[(UO2)(O-2)(CO3)] 5 9-and TMA[(UO2)(O-2)(CO3)](6)(11-). The NMR spectra demonstrate that the coordination of Na+ in the five-and six-membered oligomers is significantly stronger than that of TMA(+); these observations suggest that the templating effect is similar to the one observed in the synthesis of crown-ethers. The NMR experiments also demonstrate that the exchange between TMA[(UO2)(O-2)(CO3)](5)(9-) and TMA[(UO2)(O-2)(CO3)](6)(11-) is slow on the O-17 chemical shift time-scale, while the exchange between TMA[(UO2)(O-2)(CO3)](6)(11-)and Na[(UO2)(O-2)(CO3)](6)(11-) is fast. There was no indication of the presence of large clusters of the type identified by Burns and Nyman (M. Nyman and P. C. Burns, Chem. Soc. Rev., 2012, 41, 7314-7367) and possible reasons for this and the implications for the synthesis of large clusters are briefly discussed.