Catalysis of the silica sol-gel process by divalent transition metal bis(acetylacetonate) complexes

Transition metal bis(acetylacetonate) complexes of Co(II), Ni(II), Cu(II), and Zn(II) have been found to be active catalysts for the sol-gel process. The catalytic activity of these complexes decreases in going from Co(II) to Zn(II) and is highest for the acetylacetonate ligand system. 29Si NMR studies show that the complexes act primarily as condensation catalysts and are, in that regard, similar to Br?nsted bases such as hydroxide. Mechanistically, however, they appear to differ significantly from hydroxide in how they induce condensation. This is revealed in the catalyst concentration dependence, which is 1/2 order for the metal complexes and 1st order in hydroxide. Differences are also apparent in the thermochemical parameters that indicate that the metal complexes act to increase the entropy of the transition state leading to condensation. The catalytic activity is proportional to the degree of ligand dissociation of the metal complex, and experiments suggest that the active catalytic species is specifically the first dissociation product, MII(acac)+.