Abstract
Silicon complexes with a new chelating donor group, the isopropylideneimino-acylimidato( N, O) moiety [OC(R)NNCMe 2], have been prepared ( 9– 12, 15), and their properties are compared with previously described silicon chelates ( 1, 2, and 16, respectively). The new ligand acts as a more powerful donor than the NNMe 2 ligand, based on three criteria: (a) the new complexes form directly as pentacoordinate siliconium salts 9– 12, i.e., the expected neutral hexacoordinate precursors ionize spontaneously; (b) comparison of crystallographic bond lengths with those of the NNMe 2 complexes shows consistently shorter NSi coordination bonds and longer Sihalogen bonds in equally substituted new relative to the previously studied complexes; (c) while in the previous series 1, the dihalo complexes 20– 22, 25, resisted ionization at any temperature or solvent, the dibromo-isopropylideneimino complexes 19a and 19b ionize reversibly upon decrease of temperature in chloroform solution. Steric congestion forces the trans-dihalo configuration on the dichloro and bibromo complexes 18, 19. The ionization-resistant complexes 17, 18 and the partly ionized 19 form stable ionic siliconium salts when their counterions are replaced by I −, BPh 4 −, or by reaction with strong Lewis acids, AlCl 3 and AlBr 3.
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