Solid-state and solution structures of hetero-aggregates formed between nBuLi and NCN pincer aryl lithium

Abstract

The reaction of NCNLi pincers (NCN = [2,6-(R₂NCH₂)₂C₆H₃]⁻, R = Me (3), Et (4)) with various equivalents of nBuLi in non-polar solvent results in the generation of novel mixed alkyl–aryl organolithium hetero-aggregates. The identification (variable temperature ¹H, ¹³C, ⁷Li and 2D NMR spectroscopy and X-ray crystallography) of multiple, equilibrating mixed-aggregates that form in these reactions has been achieved. Fluxional processes in the parent [NCNLi]₂ dimeric homo-aggregates were re-evaluated and Li–N bond rupture was found to be in operation, a prerequisite towards further aggregation chemistry. The crystallized aggregates, with the formula 3₂·[nBuLi]₂ or 4₂·[nBuLi]₂, shows one amine arm from each NCNLi fragment stabilizing a [nBuLi]₂ dimer. The core of the aggregates exhibit a roughly cubic Li₄C₄ configuration with each aryl carbanion η³ coordinated to Li₃ triangular faces. Dissolution of microcrystalline powders of 3₂·[nBuLi]₂ or 4₂·[nBuLi]₂ regenerates the observed equilibria. Based on the NMR data, the remaining mixed aggregates are proposed to have the formula 3·[nBuLi]₃ and 4·[nBuLi]₃, respectively; the solution structure is again based on a Li₄C₄cluster. The relative concentration of the constituents in these equilibria was found to vary depending on the steric size of the amine groups. In the case of 3, the predominant species is the 3₂·[nBuLi]₂ aggregate while for 4, the dimer 4₂ is favoured.