F···H−N and MeO···H−N Hydrogen-Bonding in the Solid States of Aromatic Amides and Hydrazides: A Comparison Study

Abstract

Four intramolecularly F center dot center dot center dot H-N hydrogen-bonded aromatic amide and hydrazide derivatives have been prepared. Their crystal structures are investigated and compared with those of their MeO center dot center dot center dot H-N hydrogen-bonded analogues. It is found that all the F center dot center dot center dot H-N hydrogen-bonded molecules form intermolecular CO center dot center dot center dot H-N hydrogen bonding and, for two of them, weak F center dot center dot center dot H-C interactions. In contrast, the MeO center dot center dot center dot H-N hydrogen-bonded molecules display only very weak intermolecular C-H center dot center dot center dot pi interactions. The hydrogen-bonded amide units in the fluorine-bearing molecules exhibit large torsion from the connecting benzene units. This has been attributed to the weaker ability of fluorine as a hydrogen-bonding acceptor, its smaller size (relative to the MeO group), and consequently the strengthened intermolecular NH center dot center dot center dot OC hydrogen bonding. The results also suggest that, although the weakness of fluorine as a hydrogen-bonding acceptor has been mainly attributed to its low polarizability and tightly contracted lone pairs, the great tendency of fluorine-bearing aromatic amides to form intermolecular CO center dot center dot center dot H-N hydrogen bonding may also play a role.