Studies on indolyl- and pyrrolylsilanes as potential precursors for Si-centred ligands

Abstract

Reaction of tris(3-methylindol-2-yl)methane (tris(skatyl)methane, tsmH3) with HSiCl3 and SiCl4 was attempted, in order to eventually synthesise tris(3-methylindol-2- yl)methane silanide (tsmSi-). This is projected to be a very strong-field ligand. Unfortunately, synthesis of both precursors tsmSiH and tsmSiCl has failed, and it was suggested this is due to large strain energy and consequent preference for polymerisation instead of intermolecular reaction. In addition to these synthesis attempts, DFT calculations were done. These indicate similar strain energies between tsmSi- and tsmP, while both tsmSiH and tsmSiCl are approximately 15 kcal/mol higher, showing a higher preference for a more open structure, for example polymers. 1,1,2-tris(3-methylindol-2-yl)ethane is instead proposed as a scaffold for eventual silanide synthesis. Focus then switched to penta- and hexacoordinate silicon compounds with two related ligands: N-phenyl-2-formiminopyrrolyl (pfp) and N-(2,6-diisopropylphenyl)-2-formiminopyrrolyl (dipfp). The reaction of two equivalents of pfp with tetrachlorosilane afforded a hexacoordinate compound Si(pfp)2Cl2. Using dipfp instead of pfp also showed a reaction but it could not be ascertained whether one or two equivalents of dipfp were involved. The reaction of one equivalent of dipfp with trichlorosilane afforded first the pentacoordinate SiH(dipfp)Cl2, which undergoes hydrosilylation to form the tetracoordinate SiLCl2, where L is (2,6-diisopropylphenyl)(2- yrrolylmethyl)amide. Reduction of Si(pfp)2Cl2 has been unsuccessfully attempted. Here also DFT has been employed to gain insight into both synthesis properties and reduction properties. These show that instead of reducing to a silylene, instead a biradical with electrons in the ligands may be formed.