Probing ionic association on metal oxide clusters by pulsed field gradient NMR spectroscopy:: The example of Sn12-oxo clusters

Pulsed field gradient H-1 NMR spectroscopy has been applied to investigate the association behavior of the Sn-12-oxo cluster macrocation [(BuSn)(12)O-14(OH)(6)](2+) with two different and smaller anions, p-toluenesulfonate (PTS-) and diphenylphosphinate (Ph2PO2-). By monitoring the translational diffusion coefficients of the various species involved, it is shown that the association depends on the anion involved and on the solvent used. Moreover, the possibility to individually monitor the diffusion characteristics of multiple anionic and cationic species in mixtures, by virtue of resolved H-1 resonances available from each species, allows us to evidence the occurrence of ion exchange in such systems. Thus when [(BuSn)(12)O-14(OH)(6)](PTS)(2) is mixed with two equivalents of Ph2PO2NMe4, PTS- is displaced by Ph2PO2-, highlighting the greater affinity of the organotin macrocation for the diphenylphosphinate. This example clearly illustrates the potential of pulsed field gradient H-1 NMR spectroscopy in inorganic/organometallic chemistry, to follow preferential ion pairing in multi-ion systems at the level of each individually charged species.