Aggregation of Donor Base Stabilized 2-Thienyllithium in a Single Crystal and in Solution: Distances from X-ray Diffraction and the Nuclear Overhauser Effect

Various 2-thienyllithium derivatives were investigated in the solid state by X-ray diffraction and in solution by 2D NMR experiments. The determined structures of [(Et₂O)­Li­(C₄H₃S)]₄ (1), [(THF)₂Li­(C₄H₃S)]₂ (2), [(DME)­Li­(C₄H₃S)]₂ (3), [(TMEDA)­Li­(C₄H₃S)]₂ (4), and [(PMDETA)­Li­(C₄H₃S)] (5) (DME = 1,2-dimethoxyethane, TMEDA = N,N,N′,N′-tetramethylethylene-1,2-diamine, and PMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine) were solved in nondonating toluene and provide firm ground for diffusion-ordered NMR spectroscopy as well as heteronuclear Overhauser enhancement NMR spectroscopy. The distance relation of nuclear Overhauser effects with a factor of r^–6 is employed to gain further insight into the aggregation degree of 1–5 in solution. Comparison of the slope provided by the linear region of the buildup curves and of the ∑r^–6 calculated distances from the crystal structures offers a handle to judge the structure retention versus conversion in solution. The structures of 3–5 are maintained in toluene solution. The data of 2, however, indicate a partial dissociation or a rapid exchange between the vertices of a tetrameric core and free THF molecules. Auxiliary exchange spectroscopy investigations showed that the signals of the nitrogen donor base containing compounds 4 and 5 exchange with the signals of nonlithiated thiophene. This is explained by exchange of the deuterium by a hydrogen atom via lithiation of toluene molecules.