Geometrical Features of Hydrogen Bonded Complexes Involving Sterically Hindered Pyridines

Abstract

The ability of strongly sterically hindered pyridines to form hydrogen bonded complexes was inspected using low-temperature 1H and 15N NMR spectroscopy in a liquefied Freon mixture. The proton acceptors were 2,6-di(tert-butyl)-4-methyl- and 2,6-di(tert-butyl)-4-diethylaminopyridine; the proton donors were hydrogen tetrafluoroborate, hydrogen chloride, and hydrogen fluoride. The presence of the tert-butyl groups in the ortho positions dramatically perturbed the geometry of the forming hydrogen bonds. As revealed by experiment, the studied crowded pyridines could form hydrogen bonded complexes with proton donors exclusively through their protonation. Even the strongest small proton acceptor, anion F-, could not be received by the protonated base. Instead, the simplest hydrogen bonded complex involved the [FHF]- anion. This complex was characterized by the shortest possible N...F distance of about 2.8 A. Because the ortho tert-butyl groups did not prevent the hydrogen bond interaction between the protonated center and the anion completely, an increase of the pyridine basicity caused a further shortening of the N-H distance and a weakening of the hydrogen bond to the counterion.

Heteroatoms moving protons: Synthetic and mechanistic studies of bifunctional organometallic catalysis

“…Figure 7 shows that the 15 N shift is almost insensitive to coordination at P (20 to 21), or ionization of a chloride ligand or presence of a π-acidic ligand, as seen by the chemical shift for the nonchelating ligands in 12b and 22. On the other hand, coordination to a metal (chelating ligand in 12b) or N-protonation (model salt 25) lead to dramatic upfield shifts, as seen by other researchers for other metal-pyridine or pyridinium species (for examples, see [56][57][58][59]). With these background data in hand, the power of using 15 N NMR data to study hydrogen bonding or proton transfer is shown by considering species 4b and 23.…”

Structures, Mechanisms, and Results in Bifunctional Catalysis and Related Species Involving Proton Transfer

“…Figure 3 shows that the 15 N shift is almost insensitive to coordination at P (8-9), or ionization of a chloride ligand or presence of a p-acidic ligand, as seen by the chemical shift for the nonchelating ligands in 7b and 10. On the other hand, coordination to a metal (chelating ligand in 7b) or N-protonation (model salt 13) lead to dramatic upfield shifts, as seen by other researchers for other metal-pyridine or pyridinium species (for examples, see [51][52][53][54]). With these background data in hand, the power of using 15 N NMR data to study hydrogen bonding or proton transfer is shown by considering species 4b and 11.…”

19F NMR Spectroscopic Investigation of the Reaction of Bis(pentafluorophenyl)borinic Acid with a “Proton Sponge”: Deprotonation, Trimerization and Stepwise Dearylation

“…[28,29] Boroxinates containing five or four aryl groups had been previously obtained in very few cases [17,19,20] (and sometimes serendipitously), [17,19] but this is the first time that a clean stepwise dearylation process is described, leading from a hexaaryl (36 v.e.s) to a pentaaryl (34 v.e.s) and then to a tetraaryl (32 v.e.s) B 3 O 3 anionic cyclic oligomer, as summarized in Scheme 7.…”