Alkyllithium Mixed Aggregates: Dynamic Behavior and Comprehensive Analysis of NMR²J_{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 13 C and 15 N and small values (<1 Hz) for 1 H. However, the homonuclear scalar coupling between the 7 Li 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 7 Li− 7 Li scalar couplings in unlabeled organolithium mixed aggregates based on 2D CT-COSY correlation spectroscopy. The obtained results on various alkyllithium mixed tetramers of n-BuLi/RLi (R = Me or n-BuO) show that in the absence of any exchange of the lithium nucleus a scalar coupling 2 J 7 Li− 7 Li = 0.586 Hz is expected between the 7 Li 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. ■ INTRODUCTIONOrganolithium compounds in solution most often afford dynamic mixtures of rapidly reorganizing species differing by their aggregation state and their solvation degree. Access to this information is crucial since it helps to understand and master their reactivity.NMR has become the favored method for the structural characterization of organolithium compounds. 1 The first structural studies were carried out on naturally abundant 7 Li (I = 3/2) and 6 Li (I = 1) nuclei. These two nuclei are quadrupolar, but the 6 Li nucleus has a quadrupole moment, which is significantly smaller than that of 7 Li. Therefore, 6 Li is a better probe for studying these compounds. 2 Studies performed with compounds enriched with this isotope have been rapidly developed, and numerous pulse sequences facilitating structural analysis were then developed. 2,3 In NMR, the scalar J-coupling is a structural probe employed for identifying internuclei connectivities via chemical bonds. This parameter is very sensitive to the length of the bonds, to the torsion angles, and to the overall geometry of a molecule. 4 In the case of organolithium compounds, at low temperature, the aggregation state is mainly determined by measuring the 6 Li−X coupling constant (X = 13 C or 15 N) and by the multiplicity of the X nucleus signals. 5 However, during the study of organolithium compounds, such as lithium amides, which are not labeled with 15 N, lithium halides, lithium alkoxides, or even lithium enolates, the X-nucleus signal multiplicity and the coupling constants are nonexis...

The interpretation of small molecule diffusion coefficients: Quantitative use of diffusion-ordered NMR spectroscopy

“…7 Li DOSY experiments have also been reported in studies of mixed-lithium-zinc complexes [147]. Combined with, for example, the use of 7 Li residual quadrupolar couplings [148] and analysis of the 2 J(Li-Li) coupling, and its variable temperature behaviour, between 7 Li atoms [149], there is plenty of scope for the development and wider use of 7 Li techniques in the study of small molecules and organolithium compounds.…”

On Atoms‐in‐Molecules Energies from Kohn–Sham Calculations

“…The second is the interacting quantum atoms (IQA) approach, pioneered by Pendás and co‐workers . In recent years, we have extensively made use of this decomposition to study intramolecular contacts between electronegative atoms and halogen bonds in intermolecular or intramolecular patterns, to rationalize mechanisms in organic chemistry or bonding in inorganic aggregates, and to decompose conceptual density functional theory descriptors in terms of atomic contributions …”

A Combined 1H/6Li NMR DOSY Strategy Finally Uncovers the Structure of Isopropyllithium in THF

“…We focused our attentiono n the isolated singlet at À2.25 ppm assigned to the [(iPrLi)(MeLi)] mixed-dimer M.I ts MW (288 gmol À1 )i ndicates that M is di-to tri-solvated (page 29S in SupportingI nformation). A 1 H/ 6 Li DOSY spectra superpositionw as then achieved ( Figure 5a nd page 28S in Supporting Information) [22][23][24] and the two 1 Ha nd 6 Li diffusion coefficients of M were matched (page 30S in Supporting Information). This led us to recalculate the diffusion coefficients of all internal references and tob uild a 6 Li diffusion calibration curve (page 30S in Supporting Information).…”

“…0a nd 1 m + n 4), was retained. [22] A1 :1 mixture of iPrLi and MeLi was first characterized( pages 19S to 27S in Supporting Information) throughas eries of standard NMR experiments at À103 8C. These led us to spot the following aggregates: [ MeLi]…”