4-Methoxy- and 4-cyano-substituted lithium aryloxides: electronic effects of substituents on aggregation

Abstract

The para-substituted lithium aryloxides [{4-NC–C₆H₄OLi·(Pyr)₂}₂·Pyr] 1a, [{4-NC–C₆H₄OLi·(THF)₂}₂] 1b, [{4-MeO–C₆H₄OLi·Pyr}₄] 2a, [4-MeO–C₆H₄OLi·(THF)_n] 2b, [{4-NC-2,6-(t-Bu)₂–C₆H₂OLi·(Pyr)₂}_∞] 3a, [{4-NC-2,6-(t-Bu)₂–C₆H₂OLi·(THF)₂}_∞] 3b, [{4-MeO-2,6-(t-Bu)₂–C₆H₂OLi·Pyr}₂·(Pyr)₂] 4a, and [4-MeO-2,6-(t-Bu)₂–C₆H₂OLi·(THF)_n] 4b were prepared by the direct deprotonation of the corresponding phenol with an alkyllithium base (BuLi or MeLi) in the appropriate solvent, either pyridine or THF. All compounds were characterized by ¹H and ¹³C NMR spectroscopy, and the crystal structures of 1a, 1b, 2a, 3a, 3b and 4a were elucidated. The cyano derivatives 1a and 1b adopt discrete tetrasolvated Li₂O₂ ring dimers whereas the methoxy analogue 2a crystallizes as a tetrasolvated molecular tetramer with a pseudo cubic Li₄O₄ core. The sterically encumbered cyano derivatives 3a and 3b form isostructural 1D polymeric chains of monomers via bridging of the phenolate ligands through Li⋯NC and Li–O contacts. In comparison, the crystal structure of the methoxy counterpart 4a is a disolvated molecular Li₂O₂ ring dimer. Solution NMR spectroscopic studies of 1–4 in d₅-pyridine and d₈-THF indicate that the methoxy complexes are more highly aggregated than the cyano derivatives, consistent with the solid-state studies. Ab initio molecular orbital calculations at the HF/6-31G* level of theory indicate that the origin of the aggregation state variations between the cyano and methoxy complexes is due to electronic effects.