Structural and electronic studies of substituted m-terphenyl lithium complexes. Issue 2 (21st December 2020) - Valentine, Andrew J.
Abstract : Spectroscopic and computational investigation of the effects of para -substituted m -terphenyl lithium complexes reveals significant electronic differences at the metal centre. 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, SiMe3 2, H 3, Cl 4, CF3 5 ; where R–Ar # = 2, 6-{2, 6-Xyl}2 -4-R-C6 H2 and 2, 6-Xyl = 2, 6-Me2 C6 H3 ) 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 7 Li{ 1 H} NMR chemical shifts of the para -substituted complexes and their Hammett constants. The flanking methyl protons exhibit a similar electronic shift in the 1 H NMR spectra, which has been rationalised by the presence of through-space Li⋯H interactions, as evidenced by two-dimensional 7 Li– 1 H 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.