Structural and electronic studies of substituted m-terphenyl lithium complexes

Abstract

The effect of para-substitution upon the structural and electronic properties of a series of m-terphenyl lithium complexes [R–Ar^#–Li]₂ (R = t-Bu 1, SiMe₃2, H 3, Cl 4, CF₃5; where R–Ar^# = 2,6-{2,6-Xyl}₂-4-R-C₆H₂ and 2,6-Xyl = 2,6-Me₂C₆H₃) has been investigated. X-ray crystallography reveals the complexes to be structurally similar, with little variation in C–M–C bond lengths and angles across the series. However, in-depth NMR spectroscopic studies reveal notable electronic differences, showing a linear correlation between the ⁷Li{¹H} NMR chemical shifts of the para-substituted complexes and their Hammett constants. The flanking methyl protons exhibit a similar electronic shift in the ¹H NMR spectra, which has been rationalised by the presence of through-space Li⋯H interactions, as evidenced by two-dimensional ⁷Li–¹H heteronuclear Overhauser spectroscopy (HOESY). In both cases, electron-withdrawing substituents are found to cause an upfield peak shift. A computational analysis is employed to account for these trends.