Nature of the Interaction and Photophysical Properties of [Mo₆Clⁱ₈(SO₃CF₃)^a₆]^2- and [Mo₆Clⁱ₈Cl_a6]^2- on Silica Gel

The nature of the interaction between silica gel and [Mo₆Clⁱ₈(SO₃CF₃)^a₆]^2- (1) and [Mo₆Clⁱ₈Cl_a6]^2- (2) in organic media is described. Silica gel, which contains a high density of Si-OH groups, strongly adsorbs clusters 1 and 2. Although 1 is adsorbed from CH₂Cl₂ and solvents of high dielectric constant, such as CH₃OH, 2 is adsorbed only from CH₂Cl₂ solution. In CH₂Cl₂, basic silica gel, containing a small percentage of anionic Si-O- sites, coordinates with Mo atoms of 1 by replacement of labile trillate groups. In more polar solvents, such as CH₃OH and CH₃CN, the triflate ligands of 1 are replaced by solvent molecules to produce [Mo₆Clⁱ₈(solvent)^a₆]⁴⁺, which can then covalently attach to the silica gel surface. Presumably, Si-OH ionization is promoted in these polar media to yield SiOsurface sites. Under the same conditions, the less substitutionally labile cluster, [Mo₆Clⁱ₈Cl^a₆]^2-, is not covalently attached to silica gel. The supported systems are stable to atmospheric moisture and O₂ but undergo decomposition in the presence of liquid water. Clusters bound by either electrostatic or covalent interaction display absorption and emission profiles virtually identical to those of solution species, indicating that the clusters are intact. Transient emission measurements reveal that supported 1 retains the characteristic longlived excited state in 1 in solution: τ_gel = 100 µs and τ_soin = 110 µs. The photochemical properties of the silica gel bound molybdenum cluster are retained, as indicated by luminescence quenching by O₂ and phenothiazine.