NATURE OF THE INTERACTION AND PHOTOPHYSICAL PROPERTIES OF [MO6CL8I(SO3CF3)(6)(A)](2-) AND [MO6CL8ICL6A](2-) ON SILICA-GEL

The nature of the interaction between silica gel and [Mo6Cl8i(SO3CF3)( 6)(a)](2-) (1) and [(Mo6Cl8Cl6a)-Cl-i](2-) (2) in organic media is des cribed. Silica gel, which contains a high density of Si-OH groups, str ongly adsorbs clusters 1 and 2. Although 1 is adsorbed from CH2Cl2 and solvents of high dielectric constant, such as CH3OH, 2 is adsorbed on ly from CH2Cl2 solution. In CH2Cl2, basic silica gel, containing a sma ll percentage of anionic Si-O- sites, coordinates with Mo atoms of 1 b y replacement of labile triflate groups. In more polar solvents, such as CH3OH and CH3CN, the triflate ligands of 1 are replaced by solvent molecules to produce [Mo6Cl8i(solvent)(6)(a)](4+), which can then cova lently attach to the silica gel surface. Presumably, Si-OH ionization is promoted in these polar media to yield SiO- surface sites. Under th e same conditions, the less substitutionally labile cluster, [(Mo6Cl8C l6a)-Cl-i](2-), is not covalently attached to silica gel. The supporte d systems are stable to atmospheric moisture and O-2 but undergo decom position in the presence of liquid water. Clusters bound by either ele ctrostatic or covalent interaction display absorption and emission pro files virtually identical to those of solution species, indicating tha t the clusters are intact. Transient emission measurements reveal that supported 1 retains the characteristic long-lived excited state in 1 in solution: tau(gel) = 100 mu s and tau(soln) = 110 mu s. The photoch emical properties of the silica gel bound molybdenum cluster are retai ned, as indicated by luminescence quenching by O-2 and phenothiazine.