Kinetic and Binding Studies Reveal Cooperativity and Off-Cycle Competition for H-Bonding Catalysis with Silsesquioxane Silanols.

Abstract

The catalytic activity, kinetics, and quantification of H-bonding ability of incompletely condensed polyhedral oligomeric silsesquioxane (POSS) silanols are reported. POSS-triols, a homogeneous model for vicinal silica surface sites, exhibit enhanced H-bonding compared with other silanols and alcohols as quantified using a 31 P NMR probe. Evaluation of a Friedel-Crafts addition reaction shows that phenyl-POSS-triol is active as an H-bond donor catalyst whereas other POSS silanols studied are not. An in-depth kinetic study (using RPKA and VTNA) highlights the concentration-dependent H-bonding behavior of POSS-triols, which is attributed to intermolecular association forming an off-cycle dimeric species. Binding constants provide additional support for reduced H-bond ability at higher concentrations, which is attributed to competitive association. POSS-triol self-association disrupts H-bond donor abilities relevant for catalysis by reducing the concentration of active monomeric catalyst.