15N NMR Spectroscopy, X-ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

摘要: Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}C5H4N (I) 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), [II-H2][BPh4]2 (3). Solution-state 1H 15N NMR spectroscopy shows symmetrical cation in 2, indicating facile proton-exchange process solution. Solid-state data differentiates between the groups, mixed guanidine/guanidinium. X-ray diffraction are consistent protonation at imine nitrogen, confirmed for 1a by single-crystal neutron diffraction. The crystal structure association [I-H]+ cations within cage [BPh4]− anions. Computational analysis performed gas phase MeCN solution that free energy barrier transfer proton imino centers [II-H]+ is 1 order magnitude lower than phase....