Intermolecular N-H•••O hydrogen bonding assisted by resonance.: II.: Self assembly of hydrogen-bonded secondary enaminones in supramolecular catemers

The crystal structures of 15 compounds containing the 2-en-3-amino-1-one heterodienic system and forming intermolecular N-H ... O hydrogen bonds assisted by resonance (RAHB) are reported: (1) 3-phenylamino-2-cyclohexen-1-one; (2) 3-(4-methoxyphenylamino)-2-cyclohexen-1-one; (3) 3-(4-chlorophenylamino)-2-cyclohexen-1-one; (4) 3-(4-methoxyphenylamino)-2-methyl-2-cyclohexen-1-one; (5) 3-(4-methoxyphenylamino)-5-methyl-2-cyclohexen-1-one; (6) 3-isopropylamino-5,5-dimethyl-2-cyclohexen-1-one; (7) 3-phenylamino-5,5-dimethyl-2-cyclohexen-1-one; (8) 3-(3-methoxyphenylamino)-5,5-dimethyl-2-cyclohexen-1-one; (9) N,N-3-azapentane-1,5-bis[1-(3-oxo-5,5-dimethyl-1-cyclohexenyl)]; (10) 3-phenylamino-6,6-dimethyl-2-cyclohexen-1-one; (11) 3-(2-methoxyphenylamino)-6,6-dimethyl-2-cyclohexen-1-one; (12) 3-(3-chlorophenylamino)-6,6-dimethyl-2-cyclohexen-1-one; (13) 3-(4-chlorophenylamino)-6,6-dimethyl-2-cyclohexen-1-one; (14) 1-(4-chlorophenyl)-4-(4-chlorophenylamino)-6-methyl-2-pyridone; (15) 3-(4-chlorophenylamino)-5-phenyl-2-cyclopenten-1,4-dione. All compounds form intermolecular N-H ... O=C hydrogen bonds assisted by resonance connecting the heteroconjugated enaminonic groups in infinite chains. Chain morphologies are analyzed to find out crystal engineering rules able to predict and interpret the crystal packing. Simple secondary enaminones [i.e. (1)-(13) together with a number of structures retrieved from the Cambridge Structural Database] are found to form hydrogen bonds having pi-delocalizations, as characterized by a C=O bond-length average of 1.239 +/- 0.004 Angstrom, and hydrogen-bond strengths, represented by the N ... O average distance of 2.86 +/- 0.05 Angstrom, very similar to those previously found for amides. Enaminones, however, can be easily substituted by chemical groups able to influence both pi-conjugations and N ... O hydrogen-bond distances. Some substituted enaminones, retrieved from the literature, display, in fact, N ... O hydrogen-bond distances as short as 2.627 Angstrom and large pi-delocalizations with C=O double-bond distances as long as 1.285 Angstrom. These effects appear to be associated with (a) the presence of further pi-conjugated systems involving the C=O and NH groups of the enaminone moiety or (b) the transformation of the enaminone carbonyl group in an amidic function.