Aggregation of Donor Base Stabilized 2-Thienyllithium in a Single Crystal and in Solution: Distances from X-ray Diffraction and the Nuclear Overhauser Effect
journal contributionposted on 2016-02-22, 08:27 authored by Markus Granitzka, Ann-Christin Pöppler, Eike K. Schwarze, Daniel Stern, Thomas Schulz, Michael John, Regine Herbst-Irmer, Sushil K. Pandey, Dietmar Stalke
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.
5 exchange2OsignalTHF moleculesbuildup curvesDMEcompounds 4Overhauser effectsnonlithiated thiophenetoluene moleculesaggregation degreetoluene solution2 D NMR experimentsPMDETATMEDAfirm groundhydrogen atomtetrameric coreheteronuclear Overhauser enhancement NMR spectroscopycrystal structuresSingle Crystalnondonating toluenestructure retentionAuxiliary exchange spectroscopy investigationsdistance relationnitrogen donor base