| Abstract
| - Transition metal bis(acetylacetonate) complexes of Co(II), Ni(II), Cu(II), and Zn(II) have been found to be activecatalysts for the sol−gel process. The catalytic activity of these complexes decreases in going from Co(II) toZn(II) and is highest for the acetylacetonate ligand system. 29Si NMR studies show that the complexes act primarilyas condensation catalysts and are, in that regard, similar to Brønsted bases such as hydroxide. Mechanistically,however, they appear to differ significantly from hydroxide in how they induce condensation. This is revealed inthe 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 toincrease the entropy of the transition state leading to condensation. The catalytic activity is proportional to thedegree of ligand dissociation of the metal complex, and experiments suggest that the active catalytic species isspecifically the first dissociation product, MII(acac)+.
- Transition metal bis(acetylacetonate) complexes of Co(II), Ni(II), Cu(II), and Zn(II) have been found to be very active catalysts for the sol−gel process. 29Si NMR studies show that the complexes act primarily as condensation catalysts and are, in that regard, similar to Brønsted bases such as hydroxide. The catalyst concentration dependence of the reaction is 1/2 order, and thermochemical data indicates that the metal reduces the steric barriers to condensation. The activity decreases in going from Co(II) to Zn(II) and is proportional to the degree of ligand dissociation of the metal complex. Analysis of ligand dissociation data suggests that the active catalytic species is specifically the first dissociation product, MII(acac)+.
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