Coordination chemistry of lipoic acid and related compounds.

N-(1-Adamantyl)lipoamide (2) has been prepared from racemic lipoic acid and reacted with hydrogen chloride in chloroform to afford [H(2)2]Cl·xCHCl3 (3). 1H and 13C NMR spectra of 3 in CDCl3 show that 2 is protonated at the carbonyl oxygen atom. There are no indications of protonation at the amide N atom or the 1,2-dithiolane S atoms. X-Ray crystal structure analysis of 3 revealed the presence of the O-protonated dimer [H(2)2]+. The central part of this proton complex consists of a CO⋯H+⋯OC hydrogen bridge with a short O⋯O distance of 2.439(4) Å. The CO bonds [1.273(5) and 1.282(5) Å] are significantly lengthened and the (O)C–N bonds [1.313(5) and 1.311(5) Å] are shortened compared to the typical bond lengths in non-protonated secondary amides. The chloride ions interconnect the [H(2)2]+ complexes into infinite chains via N–H⋯Cl−⋯H–N bridges [N⋯Cl 3.222(3) and 3.229(3) Å] and bind the solvent molecules via Cl3CH⋯Cl− hydrogen bonds [C⋯Cl 3.361(6) and 3.386(7) Å]. Theoretical DFT and MP2 investigations on the model complex [H(4)2]+, where 4 is N-methylacetamide, indicate that the central proton is highly dynamic. It can almost freely shift within a ±0.1 Å range around the center of the O⋯O separation.