Mechanisms of Amine-Catalyzed Organosilicate Hydrolysis at Circum-Neutral pH

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 29Si 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 SN2 reaction. For TMOS,
the activation energy of proton-transfer coupled with SN2 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.