Alkyllithium Mixed Aggregates: Dynamic Behavior and Comprehensive Analysis of NMR ²<i>J</i>_{⁷Li–⁷Li} Spin–Spin Coupling

Abstract

The measurement and assignment of scalar couplings in NMR spectra have, of course, been of interest to chemists for a long time because of the useful structural and dynamic information that can be provided. In the case of organolithium compounds, the magnitude of spin–spin coupling involving lithium strongly depends on the coupling partner, with fairly large values (>2 Hz) for ¹³C and ¹⁵N and small values (<1 Hz) for ¹H. However, the homonuclear scalar coupling between the ⁷Li nuclei interacting in mixed aggregates has never been measured experimentally. Several methods have been proposed to measure small scalar coupling constants in high-resolution NMR spectra, but all of them are not well suited for quadrupolar nuclei such as lithium 7. In this paper, we introduce a new tool to measure ⁷Li–⁷Li scalar couplings in unlabeled organolithium mixed aggregates based on 2D CT-COSY correlation spectroscopy. The obtained results on various alkyllithium mixed tetramers of <i>n</i>-BuLi/RLi (R = Me or <i>n</i>-BuO) show that in the absence of any exchange of the lithium nucleus a scalar coupling ²<i>J</i>_{⁷Li<i>–</i>⁷Li} = 0.586 Hz is expected between the ⁷Li nuclei, which decreases as the rate of lithium exchange increases. These experimental observations were theoretically rationalized by density functional theory calculations, with a particular emphasis on the elucidation of the physical nature of the involved interatomic interactions and on the accurate calculation of Li–Li coupling constants