Intramolecular interligand charge transfer and a red hexacoordinate Si-complex with salen-type ligand vs. colorless tetracoordinate salen-Si-complexes with similar substituents

The reactions between the tetradentate ligand o-HO-p-OMe–C6H3–C(Ph)N–(CH2)2–NC(Ph)–C6H3-o-OH-p-OMe (1) and the dichlorosilacycles [PhN-(CH2)2–NPh]SiCl2 (2) and [Me3SiN-(o-C6H4)-NSiMe3]SiCl2 (4) afforded the red colored complexes [o-O-p-OMe–C6H3–C(Ph)N–(CH2)2–NC(Ph)–C6H3-o-O-p-OMe]Si{2N}, {2N} = [PhN–(CH2)2–NPh] (3), {2N} = [Me3SiN-(o-C6H4)-NSiMe3] (5) with hexacoordinate central silicon atom, respectively. The intense red color of these compounds originates from interligand charge transfer [(amide) → Si → (salen)]. Compound 3a, a cyclic dimer of 3 with tetracoordinate Si atoms, was isolated from the reaction mixture. It is colorless, thus demonstrating that the interligand charge transfer from the amide ligand to the salen-type ligand requires the connection of the imine N atoms to the Si atom and is not possible via an Si–O-connection to the salen-type ligand only. The reaction between ligand 1 and the dichlorosilacycle 7 [Ph2MeSiN–(CH2)2–NSiMePh2]SiCl2 with bulky substituents at the amide N atoms gave rise to the monomeric salen-Si-compound 8 [o-O-p-OMe–C6H3–C(Ph)N–(CH2)2–NC(Ph)–C6H3-o-O-p-OMe]Si[Ph2MeSiN–(CH2)2–NSiMePh2] bearing only tetracoordinate Si atoms. Compound 8 is also colorless. The formation of 8 from 1 and 7 was proven to occur via a hexacoordinate silicon complex 8a, which, however, could not be isolated so far.

Tetradentate salen-type ligands are shown to form charge transfer complexes with silanes bearing electron releasing amide type substituents. This intramolecular interligand CT via Si-centre strongly depends on the hypercoordination of the respective Si-atom by imine-N donor action.Figure optionsDownload as PowerPoint slide