NMR spectrocopy of organolithium compounds, part XXXIV: Cyclopropyllithium and lithium bromide (1:1) in diethylether/tetrahydrofuran-Identification of a fluxional mixed tetramer.

Abstract

Cyclopropyllithium, C3 H5 Li (1), was studied in the presence of one equivalent lithium bromide (LiBr) in diethylether (DEE)/tetrahydrofuran (THF) mixtures and in THF as solvents. Increasing the THF concentration in DEE/THF leads in the 6 Li NMR spectrum to a main signal (S1) at δ0.85 (rel. to ext. LiBr/THF) and a second resonance (S2) at δ0.26 aside from a minor component at δ0.07. In pure THF, the ratio of these signals was 66: 28:6. 6 Li and 13 C NMR allowed to identify the main signal as belonging to a mixed dimer, 1•LiBr, and the signal at 0.26 ppm to a fluxional mixed tetramer, 12 •(LiBr)2 . 1 J(13 C,6 Li) coupling constants of 11.0 and 9.8 Hz were measured at 168 K for S1 and S2, respectively, and chemical exchange between both signals was detected by 2D 6 Li,6 Li exchange spectroscopy and analyzed by temperature-dependent 1D 6 Li line-shape calculations. These yielded the equilibrium constants Keq for the chemical exchange Li4 (C3 H5 )2 Br2 ⇌ 2 Li2 C3 H5 Br. Their temperature dependence leads to van't Hoff parameters of ΔH° = 4.6 kJ/mol, ΔS° = 41.4 J/mol K, and ΔG°298 = -7.8 kJ/mol. From the rate constants k, Eyring parameters of ΔH* = 42.0 kJ/mol, ΔS* = 33.0 J/mol K, and ΔG*298 = 32.2 kJ/mol were calculated for the forward reaction Li4 (C3 H5 )2 Br2 → 2 Li2 C3 H5 Br and ΔH* = 37.5 kJ/mol, ΔS* = -8.4 J/mol K, and ΔG*238 = 40.0 kJ/mol for the reverse reaction 2Li2 C3 H5 Br → Li4 (C3 H5 )2 Br2 .