Electrophilic lithium carbazolide as an efficient trap for organolithium species: complexes containing monomeric n-BuLi, t-BuLi and Me3SiCH2Li units

Abstract

The lithiation reactions of superbulky 3,6-di-tert-butyl-1,8-bis-(2,4,6-triisopropylphenyl)-9H-carbazole (1) with a two-fold molar excess of n-BuLi, t-BuLi, or Me₃SiCH₂Li in heptane afford the binuclear lithium complexes [Carb^2,4,6-iPrLi(μ²-n-Bu)Li] (2), [Carb^2,4,6-iPrLi(μ²-Me₃SiCH₂)Li] (3), and [Carb^2,4,6-iPrLi(μ²-t-Bu)Li] (4) in 80, 90, and 56% yields. The single-crystal X-ray diffraction studies revealed that 2–4 are binuclear ionic complexes comprising the alkyl group μ²-bridging two lithium centers, one of which is also η⁶-coordinated with the phenyl fragment of the carbazolyl ligand. For complexes 2 and 3, the binuclear structures are retained in solution as evidenced by ¹H, ¹³C, ⁷Li, and ¹H DOSY NMR spectroscopy. QTAIM and NCI data reveal a difference in the strength of Li–C interactions between 2–4, which are weaker for 4, and rationalised the tBu-substituted 4 prone to dissociation.