Mechanisms of Amine-Catalyzed Organosilicate Hydrolysis at Circum-Neutral pH Katya M. Delak* ,†, ‡ and Nita Sahai †,§ Department of Chemistry, 1101 UniVersity AVenue, UniVersity of Wisconsin, Madison, Wisconsin 53706, and Department of Geology & Geophysics, 1215 West Dayton Street, UniVersity of Wisconsin, Madison, Wisconsin 53706 ReceiVed: April 3, 2006; In Final Form: June 22, 2006 Mono- and polyamines can catalyze the hydrolysis and condensation of organosilicate starting materials in biomimetic silica synthesis pathways at circum-neutral pHs and room temperature. Our study is focused on understanding the mechanistic role of amines in catalyzing the hydrolysis process that precedes condensation. We have conducted 29 Si NMR experimental studies over a range of temperature and pHs for the hydrolysis rates of trimethylethoxysilane (TMES), a model compound with only one hydrolyzable bond, combined with quantum mechanical hybrid density functional theory calculations of putative intermediate and transition- state structures for TMES and tetramethyl orthosilicate (TMOS). Comparison of calculated energies with experimentally determined activation energies indicates that amine catalysis of TMES is primarily a consequence of the amine’s acidity at neutral pH. The proton released by the amine is transferred to the organosilicate, producing a protonated ethoxy leaving group that can be displaced by water in an S N 2 reaction. For TMOS, the activation energy of proton-transfer coupled with S N 2 substitution is comparable to that for Corriu’s nucleophile-activated nucleophilic displacement, such that the mechanism of amine-catalyzed hydrolysis is dependent mostly on the ambient pH conditions as well as the type of amine. The relevance of our results to biological silica precipitation is discussed. Introduction Current sol-gel routes for synthesis of mesoporous silicates, such as MCM-41, utilize organosilicate precursors, involve templating by self-assembled amphiphilic molecules, and require one or more steps at extreme pH values and high temperatures. 1-4 The kinetics of organosilicate hydrolysis by strong acids and bases and the morphological effects of hydrolysis and condensa- tion rates on the silicas produced have been investigated thoroughly by 29 Si NMR 5-16 and Raman/FTIR vibrational spectroscopy. 17-19 On the basis of these studies, chemical mechanisms for the reactions have been proposed that have been addressed further by ab initio computational methods. 4,20-25 There is very little information in the literature, however, on organosilicate hydrolysis mechanisms at circum-neutral pHs. This gap is now being addressed by numerous investigations into biomimetic silica formation as catalyzed by nucleophiles such as amines, alcohols, and thiols, 26-36 inspired by the observation that organisms such as diatoms and sponges, capable of producing mesoporous silica at circum-neutral pH and room temperature, contain silica-precipitating proteins where nucleo- philic functional groups such as amines and alcohols are central to the catalytic function of the protein. 37-40 The biomimetic approach is based on using molecules that mimic protein activity and complements potential, new, genomic approaches where mesoporous silica could be harvested directly from genetically engineered organisms. 41,42 Using the biomi- metic approach, it has been demonstrated recently that mono- and polyamines have the ability to catalyze silica precipitation at near-neutral pH. 26-36,43-46 The effectiveness of the amine as a catalyst appears to be dependent on the isoelectric point of the amine 28,35,43 and, therefore, possibly the concentration of the amine conjugate base. Although it is unknown how diatoms and sponges store silica intracellularly prior to precipitation at concentrations exceeding silica solubility, small silicate oligo- mers and organosilicate compounds have been hypothesized as precursors. 42,47 In the present study, we use 29 Si NMR spec- troscopy and ab initio computational methods to investigate the role of amines in the chemical mechanisms of organosilicate hydrolysis at circum-neutral pH. Previous computational studies have demonstrated that or- ganosilicate hydrolysis can occur through a mechanism involv- ing a pentacoordinate intermediate, similar to those found in the S N 2 mechanism, under acidic or basic conditions. 20,23-25 In the presence of strongly nucleophilic amines, however, an alternative mechanism for hydrolysis was proposed by Corriu. This mechanism (Figure 1), termed “nucleophile-activated (N a ) nucleophilic (N b ) substitution” 48,49 (hereafter called nucleophile- activated substitution) was invoked to account for rapid pre- cipitation/gelation of silica in the presence of weakly basic nucleophiles such as amines. 48 In this mechanism, hydrolysis is initiated by coordination of the nucleophile (N a ) to silicon to form a pentacoordinate intermediate. A hexacoordinate transition structure is then formed as water (the substituting nucleophile, N b ) attacks the silicon center. Hydrolysis is completed once the leaving group and the nucleophile (N a ) break away from silicon, the nucleophile (N a ) abstracting a proton from water (N b ) as it * Corresponding author. E-mail: kd33@nyu.edu. Tel: (608)265-5192. Fax: (608)262-0693. † Department of Chemistry, University of Wisconsin, Madison. ‡ Current Address: New York University, Department of Chemistry, 100 Washington Square East, Room 1016, New York, NY 10003. Tel: (212)988-8442. Fax: (212)260-7905. § Department of Geology & Geophysics, University of Wisconsin, Madison. 17819 J. Phys. Chem. B 2006, 110, 17819-17829 10.1021/jp062054m CCC: $33.50 © 2006 American Chemical Society Published on Web 08/22/2006