Polymerization of methyltrimethoxysliane (MTMS) results in methylsilsesquioxane (MSQ), which has found important applications in recent years including use as low-k dielectric materials in the semiconductor industry, superhydrophobic materials, monolithic columns, and hybrid matrixes for immobilizing proteins. For polycondensation of MTMS in ethanolic solutions, we report the sol-gel behavior under two different sets of conditions. First, we examined one-step polymerization over a wide range of pH and show that the initial pH is important in determining both the gelation behavior of MTMS-derived sols and the morphology of the resulting MSQ materials. In the one-step method, we obtained either transparent precipitates and/or macroscopically phase-separated resins when the pH was below the isoelectric point (IEP) of the silanols; either macroscopically phase-separated resins or macroporous monolithic gels with pH > IEP; and homogeneous solutions when the pH was close to the IEP. We also report on the use of a two-step catalysis method using an initial acid catalysis step followed by a base-catalyzed condensation step (denoted as B2), which is able to produce bimodal micro/meso or trimodal micro/meso/macroporous MSQ monoliths, depending on the specific conditions employed. The resulting materials are shown to be more resistant to exposure to base relative to macroporous silica. These results indicate that MSQ materials derived by the two-step processing method should be useful for the development of chromatographic stationary phases and as porous materials for protein entrapment.

Silica derived from biocompatible silane precursors and containing covalently bound sugar moieties has recently been reported to be a much more biocompatible matrix for protein entrapment than any previously synthesized materials. To better understand the nature of these new materials, the steady-state and time-resolved fluorescence of human serum albumin (HSA) was used to examine the conformation, dynamics, accessibility, thermal stability, and degree of ligand binding after entrapment of the protein into sol−gel-processed glasses derived from either tetraethyl orthosilicate (TEOS) or diglycerylsilane (DGS), which in some cases contained covalently bound gluconamidylsilane (GLS) moieties. It was observed that the initial conformation, accessibility to external analytes, thermal stability, long-term stability, and degree of ligand binding to HSA were best in DGS-derived materials that contained covalently tethered GLS relative to unmodified DGS-derived materials, TEOS, or TEOS/GLS-derived materials. Measurement of protein rotational dynamics showed that entrapment led to an immediate loss of global motion in all materials. However, the restriction of motion was most dramatic in GLS-doped materials, suggesting preferential interactions of the protein with the sugar-coated surfaces. As aging proceeded, both protein dynamics and the degree of ligand binding decreased, with a gradual loss of segmental motion and a significant increase in local motion in the vicinity of the probe, consistent with unfolding and surface adsorption of the protein, leading to loss of function. Overall, our findings suggest that the use of a biocompatible precursor (DGS) and the addition of a covalently bound sugar both contribute to improved protein performance. However, of these two the use of a biocompatible precursor is the most important factor, and in such cases addition of sugars further improves protein performance. In contrast, the use of the sugar-based additive with a nonbiocompatible precursor such as TEOS imparted essentially no benefit, demonstrating the importance of biocompatible processing conditions.

Indenylsilanes bearing organic and organometallic substituents have been prepared in order to probe the effect of substitution on the rate of [1,5]-silicon shifts in this class of compounds. In an attempt to prepare 1,1,3-tris(trimethylsilyl)indene ( 7), the hitherto unknown silicon-functionalized bis(trimethylsilyl)dibenzo[a,d]fulvalene (9) was unexpectedly generated; this species was characterized by use of both NMR spectroscopy and X-ray crystallography and was rationally prepared in 68% yield from 3,3′-bis(1-(trimethylsilyl))indene ( 16). The molecular dynamics of 1,3-dimethyl-1-(trimethylsilyl)indene ( 18) and the crystallographically characterized chromium complex (η 6 -1,3-dimethyl-1-exo-(trimethylsilyl)indene)tricarbonylchromium ( 22) were examined by use of 1D-selective inversion and 2D-EXSY NMR techniques; surprisingly, the presence of chromium and methyl substituents has a negligible effect on the rate of [1,5]-silicon shifts (∆G q ) 23-24 kcal mol -1 ) versus the parent compound 1-(trimethylsilyl)indene (3) (∆G q ≈ 24 kcal mol -1 ). In the case of 18, the intermediate isoindene 18-iso was intercepted with tetracyanoethylene as the crystallographically characterized [4 + 2] cycloadduct 5,6-benzo-2,2,3,3-tetracyano-1,4-dimethyl-7-(trimethylsilyl)bicyclo(2.2.1)hept-5-ene (19).

The stabilization afforded a vinyl cation by a β-(CH3)3Si substituent has been determined by measuring in a high-pressure mass spectrometer the thermodynamic data for the association of three alkynes (RCCR‘) with (CH3)3Si+ and with the proton. The measured −ΔH° (kcal mol-1) and −ΔS° (in parentheses, cal K-1 mol-1) values for the reaction (CH3)3Si+ + RCCR‘ ⇄ (CH3)3Si·C(R)CR‘+ are as follows:  1-hexyne (R = H, R‘ = n-C4H9) 25.9 ± 1.5 (19.1 ± 0.2), 2-hexyne (R = CH3, R‘ = n-C3H7) 28.8 ± 1.4 (25.5 ± 0.3), and phenylacetylene (R = H, R‘ = C6H5) 28.2 ± 2.8 (16.5 ± 0.4). By comparison the values for 1-hexene which forms an alkyl cation are 38.2 ± 0.5 kcal mol-1 (48.2 ± 0.1 cal K-1 mol-1). The deduced stabilizations (A) for all the substituents (R, R‘ and (CH3)3Si) obtained from the isodesmic reaction (CH3)3Si·C(R)CR‘+ + CH2CH2 → (CH3)3Si·C(R)C(H)R‘ + CH2CH+ are (kcal mol-1) as follows:  1-hexyne 55, 2-hexyne 58, and phenylacetylene 58. The deduced stabilization for the (CH3)3Si+ adduct of 1-hexene relative to the ethyl cation is 60 kcal mol-1. The measured proton affinities are (kcal mol-1) as follows:  1-hexyne 194.5 ± 0.5, 2-hexyne 195.8 ± 0.2, phenylacetylene 198.6 ± 0.2, and 1-hexene 194.0 ± 0.5. The stabilizations (B) due to R and R‘ in the vinyl cations RC(H)CR‘+ produced by protonaton are calculated from the isodesmic reactions RC(H)CR‘+ + CH2CH2 → RC(H)C(H)R‘ + CH2CH+ and are (kcal mol-1) as follows:  1-hexyne 44, 2-hexyne 46, and phenylacetylene 50. The comparable value for the alkyl cation from the protonation of 1-hexene is 34 kcal mol-1. The stabilizations of the vinyl cations RC(H)CR‘+ due to the presence of a β-(CH3)3Si (A − B) are (kcal mol-1) as follows:  1-hexyne 11, 2-hexyne 12, and phenylacetylene 9. For the alkyl cation formed from 1-hexene, the value is 26 kcal mol-1. The stabilization of a vinyl cation by an α-alkyl or α-aryl substituent is subtantially greater than that afforded by the same substituent in an alkyl cation. The total stabilization afforded by both an α-alkyl or α-aryl substituent and a β-(CH3)3Si substituent appears to be approximately the same in both alkyl and vinyl cations and hence the β-silicon effect is considerably smaller for the vinyl cation.