Tautomeric equilibrium between penta- and hexacoordinate silicon chelates. A chloride bridge between two pentacoordinate silicons

I. Kalikhman, O. Girshberg, L. Lameyer, D. Stalke, D. Kost

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44 Scopus citations


The reaction of O-trimethylsilyl-1,1-dimethyl-2-trifluoroacetylhydrazine (1a) with chloromethyl(methyl)dichlorosilane affords an unexpected equilibrium mixture, 10a ⇄ 11a, between a neutral hexacoordinate silicon chelate with a covalent chloro ligand (10a) and an ionic pentacoordinate silicon complex (11a). The equilibrium reaction consists formally of a migration of the covalent chloro ligand from silicon to an adjacent ammonium nitrogen, as a chloride anion, and thus constitutes a novel type of tautomeric reaction. Crystallographic and NMR data provide evidence for the reaction. Temperature, solvent, substituent, and counterion effects on the tautomeric equilibrium are discussed; when the temperature of the mixture is raised, the equilibrium ratio 10a/11a increases. Formation of the mixture in toluene, a nonionizing solvent, shifts the equilibrium completely toward the neutral 10a. When the initial hydrazide has a phenyl (11c) or a hydrogen (11b) group as substituent, rather than CF3, the equilibrium is shifted to the ionic side. Replacement of the chloride counterion by triflate, using trimethylsilyl triflate, shifts the 10a/11a mixture to the ionic side. Low temperature NMR monitoring of the stepwise formation of 10/11 was carried out and provided insight into the reaction mechanism. In an attempt to grow crystals of 11c, the pentacoordinate tautomer analogue, an unprecedented chloride-bridged disiloxane complex, with two pentacoordinate silicons sharing a common axial chloro ligand, crystallized and was characterized and described.

Original languageEnglish
Pages (from-to)4709-4716
Number of pages8
JournalJournal of the American Chemical Society
Issue number20
StatePublished - 6 Oct 2001

ASJC Scopus subject areas

  • Catalysis
  • Chemistry (all)
  • Biochemistry
  • Colloid and Surface Chemistry


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