Close Amide NH···F Hydrogen Bonding Interactions in 1,8-Disubstituted Naphthalenes

In
this note, we present a series of N-(8-fluoronaphthalen-1-yl)­benzamide
derivatives designed to maximize amide-NH···F hydrogen
bond interactions therein. A combination of IR and NMR spectroscopy
indicates a linear correlation between the high energy shift in NH
stretching frequency and the electron withdrawing nature of the substituent,
consistent with the trend predicted by DFT calculations. Additionally,
a limiting case of hydrogen bonding is observed when the benzamide
derivatives are replaced with trifluoroacetamide, causing an additional
red shift of 44 cm–1 in the NH stretching frequency.
Most importantly, 1H–19F coupling constants
in this series are among the largest measured for amide-NH···F
interactions. X-ray crystallography reveals face-to-face alignment
of naphthalene rings in these derivatives resulting in part from the
NH···F hydrogen bonds. This motif also dictates the
formation of sheets composed of stacked naphthalene rings in the crystal
structure as opposed to unfluorinated analogues wherein NH···OC
hydrogen-bonding interactions force benzamide and naphthalene rings
to engage in T-shaped π–π interactions instead.
Additionally, the NH proton in the trifluoroacetamide derivative engages
in extended H-bond interactions in its crystal structure.