OCO and NCO chelated derivatives of heavier group 15 elements. Study on possibility of cyclization reaction via intramolecular ether bond cleavage

Abstract

A set of four pincer ligands, either the OCO type ligands L^1–3 [2,6-(ROCH₂)₂C₆H₃]⁻, where R = Me (L¹), mesityl (L²), t-Bu (L³) or novel NCO ligand [2-(Me₂NCH₂)-6-(t-BuOCH₂)C₆H₃]⁻ was studied. The reaction of L⁴Li with PCl₃ resulted in isolation of [2-(OCH₂)-6-(Me₂NCH₂)C₆H₃]PCl (1) as a result of intramolecular ether bond cleavage and elimination of t-BuCl. The conversion between the organolithium compounds L^1,2,4Li and AsCl₃ led to the desired chlorides, i.e.(L¹)₂AsCl (2), L²AsCl₂ (3), L⁴AsCl₂ (5), but an analogous reaction using the L³Li compound gave [2-(OCH₂)-6-(t-BuOCH₂)C₆H₃]AsCl (4) as a result of intramolecular cyclization. The organoantimony chloride L³SbCl₂ was shown to undergo very slow cyclization in CDCl₃ again via elimination of t-BuCl giving [2-(OCH₂)-6-(t-BuOCH₂)C₆H₃]SbCl (6) and it was demonstrated that this reaction may be accelerated by preparation of L³Sb(Cl)(OTf) (7) with more Lewis acidic central atom. On the contrary, both antimony derivatives of the NCO ligand L⁴, not only the chloride L⁴SbCl₂ (8) but also the ionic pair containing highly Lewis acidic cation [L⁴SbCl]⁺[CB₁₁H₁₂]⁻ (9), are stable without any indication for etheral bond cleavage. The situation is rather similar in the case of organobismuth derivatives of L⁴, which allowed isolation of compounds L⁴BiCl₂ (10), L⁴Bi(Cl)(OTf) (11) and [L⁴BiCl]⁺[CB₁₁H₁₂]⁻ (12). All studied compounds were characterized by the help of ¹H and ¹³C NMR spectroscopy, ESI mass spectrometry, elemental analysis and (except 1) by single-crystal X-ray diffraction.