We report that the polarity and dielectric constants of solvents used for grafting organosilanes on mesoporous materials strongly affect the concentration of grafted organic groups, the degree of their site-isolation, and the catalytic properties of the resulting materials. Polar and nonpolar organosilanes as well as polar-protic, dipolar-aprotic, and nonpolar solvents were investigated. Polar-protic solvents, which have high dielectric constants, resulted in smaller concentrations ( approximately 1-2 mmol/g) of polar organic groups such as 3-aminopropyl groups, higher surface area materials, site-isolated organic groups, and more efficient catalytic properties toward the Henry reaction of p-hydroxybenzaldehyde with nitromethane. On the other hand, dipolar-aprotic and nonpolar solvents resulted in larger concentrations ( approximately 2-3 mmol/g) of grafted polar functional groups, lower-to-higher surface area materials, more densely populated catalytic groups, and poor-to-efficient catalytic properties toward the Henry reaction. Both the polar-protic and dipolar-aprotic solvents resulted in significantly lower concentration of grafted groups for nonpolar organosilanes such as (3-mercaptopropyl)trimethoxysilane compared to corresponding grafting of the polar amino-organosilanes. The relationship between the solvent properties and the percentage and degree of site-isolation of the grafted functional groups was attributed to differences in solvation of the organosilanes and silanols in various solvents and possible hydrogen-bonding between the organsilanes and the solvents. The degree of site-isolation of the amine groups, which affect the material's catalytic properties, was elucidated by a new colorimetric method involving probing of the absorption maxima (lambdamax) on the d-d electronic spectrum of Cu2+ complexes with the amine-functionalized materials and the colors of the samples. The absorption lambdamax and the colors of the materials were found to be uniquely dependent on the type of solvents used for grafting the organoamines. For instance, the monoamine- and diamine-functionalized samples grafted in methanol resulted in pale blue and light purple colors with lambdamax at approximately 720 and 650 nm, respectively. These correspond to CuNO5 and CuN2O4 structures, respectively, which are indicative of the presence of site-isolated organoamines in samples grafted in methanol. The monoamine and diamine samples grafted in toluene resulted in purple and deep purple colors with lambdamax at approximately 590 and 630 nm, respectively. These correspond to CuN2O4 and CuN4O2, which are indicative of the presence of closely spaced organoamines in samples grafted in toluene. The samples grafted in isopropanol gave colors and lambdamax intermediate between those of samples grafted in toluene and methanol.

We report on the effects of the surface density and the spacing between grafted organoamines (and residual ungrafted silanols) of amine-functionalized mesoporous materials on their (cooperative) catalytic activity in the Henry reaction. The spacing between the bifunctional groups (amines and silanols), their site isolation, and their surface density were controlled by one-step or two-step grafting of a series of organosilanes containing linear alkylamine, alkyldiamine, alkyltriamine, and meta- and para-substituted aromatic amines onto mesoporous silica in ethanol and/or toluene. The grafting in ethanol produced site-isolated, flexible alkylamines, alkyldiamines, and alkyltriamines of different tether lengths and rigid meta- and para-substituted aromatic amines and high surface area materials, whereas the grafting in toluene resulted in closely spaced organoamines and materials with lower surface areas. The spacing between the organoamine groups was probed by complexing cupric ions with the amines and by measuring the electronic spectra of the complexes. The materials' catalytic activities were dependent not only on the degree of site isolation of the amine groups and the surface areas of the materials, but also on the relative spacing between the functional groups and their surface density. Samples grafted with monoamine groups in ethanol and samples grafted with diamine or triamine groups in toluene for 5 h gave approximately 100% conversion in 16 min of the Henry reaction between p-hydroxybenzaldehyde and nitromethane. However, the corresponding monoamine-grafted sample in toluene and diamine- and triamine-grafted samples in ethanol gave approximately 100% conversion after 1 h. On the basis of turnover number (TON) and TON per surface area, the samples containing optimum concentrations of approximately 0.8 - 1.5 mmol of grafted organoamines/g, which we dubbed as the critical density of organic grafted groups, gave the highest catalytic efficiencies. These samples have the most favorable amine-silanol cooperative catalytic activity. Furthermore, samples functionalized with rigid meta-substituted aminophenyl groups in ethanol showed higher catalytic efficiency than the corresponding sample containing the amine groups at the para-position, possibly due to the close proximity of the bifunctional groups in the former. The capping of ungrafted silanols with noncatalytic organosilanes in toluene resulted in reduction of catalytic activities, confirming the involvement of silanols.

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