Mechanisms of Amine-Catalyzed Organosilicate Hydrolysis at Circum-Neutral pH
2006-09-14T00:00:00Z (GMT) by
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 <sup>29</sup>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<sub>N</sub>2 reaction. For TMOS, the activation energy of proton-transfer coupled with S<sub>N</sub>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.