Structural and electronic studies of substituted m-terphenyl lithium complexes

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)-4R-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 Li-7{H-1} NMR chemical shifts of the para-substituted complexes and their Hammett constants. The flanking methyl protons exhibit a similar electronic shift in the H-1 NMR spectra, which has been rationalised by the presence of through-space Li center dot center dot center dot H interactions, as evidenced by two-dimensional Li-7-H-1 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.

Automated summary

The effect of para-substitution on a series of m-terphenyl lithium complexes has been investigated. X-ray crystallography shows structural similarity, while in-depth NMR spectroscopic studies reveal notable electronic differences, correlated with the nature of the substituents.