Derivatization of Surfaces via Reaction of Strained Silicon-Carbon Bonds. Characterization by Photoacoustic Spectroscopy

Ferrocene-centered reagents can be bound to the surface of high surface area (400 m²/g) silica by reaction of the silica with alkane solutions of (1,1'-ferrocenediyl)dichlorosilane (Ia), (1,1'-ferrocenediyl)dimethylsilane (lb), or (1,1'-ferrocenediyl)diphenylsilane (Ic). Derivatization is not found when ferrocene itself is used as the derivatization reagent; ferrocene is easily washed from the silica powder by solvents which do not affect the ferrocene centers bound upon reaction of the silica with la-c. As determined by photoacoustic spectroscopy, derivatization with Ia-c results in ring opening of the strained C-Si-C linkage bridging the two cyclopentadienyl rings to yield a simple, monosubstituted ferrocene center attached to the surface. Surface -OH functionality apparently reacts in a manner analogous to homogeneous solution sources of -OH such as MeOH. Reaction of lb with MeOD yields [Fe(ƞ⁵-C⁵H⁴D)(ƞ⁵-C₅H₄Si(OMe)Me₂)]. Optical absorption spectral changes accompanying reaction of Ia-c with solution sources of -OH are consistent with the differences found in the photoacoustic spectra of the pure powders of Ia-c compared to the silica powders derivatized with these reagents. Reaction of silica powders derivatized with Ia-c with acidic solutions of benzoquinone results in the oxidation of the surface-confined ferrocene centers to ferricenium centers giving photoacoustic spectra nearly the same as that from an authentic sample of a pure ferricenium salt. The surface-confined ferricenium can be used to oxidize N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) to TMPD⁺. in solution; this reaction allows an estimate of the surface coverage at 10^-11 mol/cm².