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

Four intramolecularly F···H−N hydrogen-bonded aromatic amide and hydrazide derivatives have been prepared.
Their crystal structures are investigated and compared with those of their MeO···H−N hydrogen-bonded analogues. It is found that
all the F···H−N hydrogen-bonded molecules form intermolecular CO···H−N hydrogen bonding and, for two of them, weak
F···H−C interactions. In contrast, the MeO···H−N hydrogen-bonded molecules display only very weak intermolecular C−H···π
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···OC 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 CO···H−N hydrogen bonding may also play
a role.