Intramolecular N-H···O and N-H···N hydrogen bonding patterns in N-benzyl and N-(pyridin-2-ylmethyl) benzamides

This Letter reports the evidences for intramolecular six-membered N-H···O hydrogen bonding in N-benzyl benzamides and five-membered N-H···N hydrogen bonding in N-(pyridin-2-ylmethyl) benzamide. Intramolecular six-membered N-H···X (X = O or F) hydrogen bonding in 2-methoxyl- or 2-fluorobenzamides is used to lock the amide proton from forming strong intermolecular N-H···OC hydrogen bonding. As a result, for the first time the new intramolecular hydrogen bonding patterns are observed in the crystal structures of nine amides, whereas the whole molecules give rise to a new class of three-center hydrogen bonding motif. ¹H NMR study in chloroform-d also supports that this weak intramolecular hydrogen bonding pattern exists in solution.