Homonuclear scalar ⁶Li⁶Li spin–spin coupling: First direct observation and detection via the ⁶Li,⁶Li INADEQUATE experiment,

Abstract

A homonuclear 6Li,6Li spin-spin coupling has been directly measured in the case of the dimer of (E)-2-lithio-1-phenyl-1-o-lithiophenylpent-1-ene. This constitutes the first direct observation of such a parameter. The well known INADEQUATE experiment in its one-and two-dimensional versions is applied for the first time to a spin system of spin-1 nuclei. It is demonstrated that the experiment is very sensitive for the detection of 6Li,6Li spinspin interactions, and thus provides a promising alternative to the COSY experiment and an additional tool for structural studies on organolithium compounds in solution.KEY WORDS 6Li,6Li NMR 6Li,6Li coupling INADEQUATE Recent research in the field of organolithium compounds has revealed a number of aggregates with two or even more chemically non-equivalent lithium Consequently, in the limit of slow inter-and intra-aggregate exchange, the 6Li NMR spectra of these systems display several distinct 6Li resonances. Direct Li-Li contacts or Li-C-Li structural units are then expected to yield observable 6Li,6Li spin-spin coupling transmitted over one or two bonds, but line splittings due to homonuclear 6Li,6Li interactions have so far, to the best of our knowledge, not been resolved. Attempts to detect heteronuclear 6Li,7Li coupling in alkyllithium compounds were also unsuccessful.6The first unequivocal evidence for the existence of scalar 6Li,6Li interactions came from the observation of cross-peaks in a 2D 6Li,6Li COSY spectrum.' The COSY pulse sequence, especially in its version optimized for long-range couplings of small magnitude'[COSY-LR, Eqn (l)], is a valuable tool when spin-spin coupling is hidden in the line width of 1D spectra, which is often the case for quadrupolar nuclei. Using COSY spectroscopy, in addition to 6Li,6Li interactions,' "B,11B8 and s1V,51V9 a s well as 'H,'H coupling constants" have been detected. 9Oo-t,-, A, 90°, A, FID(1)

NMR studies of dilithiostyrenes: aggregation, NMR parameters, and DFT calculations for (E)‐1‐Lithio‐2‐(o‐lithiophenyl)‐1‐trimethylsilylethene

“…0.1 M LiBr/THF) (Fig. a) and a 1D 6 Li, 6 Li INADEQUATE experiment showed a line splitting of ∆Δν = 0.50 ± 0.07 Hz that indicated homonuclear coupling between both lithium resonances (Fig. b) and proved that both lithium cations belong to the same cluster 5 2 .…”

Alkyllithium Mixed Aggregates: Dynamic Behavior and Comprehensive Analysis of NMR2J7Li–7LiSpin–Spin Coupling

“…The assignment of the 7 Li signals has been based on the works of F. Patéet al, 13 performed on a 6 Li-labeled solution, and confirmed by the 2D 7 Li− 7 Li COSY NMR spectrum ( Figure 1). On this spectrum, we have observed correlations between signals at δ = 2.41 ppm and δ = 2.21 ppm, between signals at δ = 2.15 ppm and δ = 1.98 ppm, and between signals at δ = 1.92 ppm and δ = 1.78 ppm, which are characteristic of the homonuclear couplings between the lithium signals belonging to the same Li 2 /Li 3 ((n-BuLi) 1 (MeLi) 3 ), Li 4 /Li 5 ((n-BuLi) 2 (MeLi) 2 ), and Li 6 /Li 7 ((n-BuLi) 3 (MeLi) 1 ) aggregate, respectively.…”

“…8a, 9 The latter has the advantage of being more sensitive than the 2D 6 Li− 6 Li COSYLr and requires a shorter acquisition time. In the case of a system containing two coupled nuclei, the lithium signals appear in antiphase with a deviation of 2J.…”

“…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 nonexistent, and the information provided by all of the standard NMR experiments remains insufficient to draw any conclusion regarding the structure of these compounds in solution.…”

The Directed Lithiation of Benzenoid Aromatic Systems

“…(R ϭ H 81, [97] SiMe 3 82, tBu 83, 4-toloyl 84 [98] ). At low temperature, a 6 Li-enriched sample of 81 reveals two signals, with 6 Li, 6 Li INAD-EQUATE spectroscopy [99] affording a homonuclear scalar coupling and suggesting retention of the tetranuclear cocomplex noted in the solid state (see below).…”