Evidence for Intramolecular N−H···O Resonance-Assisted Hydrogen Bonding in β-Enaminones and Related Heterodienes. A Combined Crystal-Structural, IR and NMR Spectroscopic, and Quantum-Mechanical Investigation

The resonance-assisted hydrogen bond (RAHB) is a model of synergistic interplay between π-delocalization and hydrogen-bond (H-bond) strengthening originally introduced (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023; Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1991, 113, 4917) for explaining the abnormally strong intramolecular O−H···O bonds formed by the ···OC−CC−OH··· β-enolone fragment I which are typical of β-diketone enols. The applicability of this model to the intramolecular N−H···O hydrogen bonds formed by a number of heteroconjugated systems (···OC−CC−NH···, β-enaminones II; ···OC−CN−NH···, ketohydrazones III; and ···ON−CC−NH···, nitrosoenamines IV) is investigated. The X-ray crystal structures of five molecules which close a six-membered ring by an intramolecular N−H···O bond through the resonant ···OX−CX−NH··· (X = C, N) fragments II−IV are compared to those of two other molecules closing the same ring through the nonresonant ···OC−C−C−NH··· β-aminone moiety V. Experimental findings are complemented by a CSD (Cambridge Structural Database) search of all compounds forming intramolecular N−H···O bonds through the molecular fragments II−V and by a comprehensive analysis of the IR νNH stretching frequencies and 1H NMR δNH chemical shifts available for compounds of these classes of known crystal structure. It is shown that all the descriptors of H-bond strength [d(N···O) shorthening, decrease of νNH, increase of δNH, and increase of π-delocalization within the heteroconjugated fragment] are mutually intercorrelated according to RAHB rules, which can then account for the strength of heteronuclear N−H···O bonds in II−IV as well as for that of the homonuclear O−H···O bonds in I. Heteronuclear N−H···O bonds appear, however, to have distinctive features. In particular, their strength turns out to be partially hampered by the proton affinity difference (ΔPA) between the N and O atoms, so that very strong H-bonds (2.65 ≥ d(N···O) ≥ 2.48 Å, 3200 ≥ νNH ≥ 2340 cm-1, 13 ≤ δNH ≤ 18 ppm) can occur only when the π-delocalization of the heterodienic moiety is associated with proper electron-attracting substituents which are able to decrease this ΔPA by increasing the NH acidity. Moreover, at variance with strong O−H···O RAHBs, whose protons are mostly found in nearly symmetrical positions, even the strongest N−H···O RAHBs are highly dissymmetric, despite the very similar changes undergone by both IR and 1H NMR spectra in O−H···O and N−H···O H-bonded systems. Specificities of heteronuclear H-bonds are shown to be interpretable by the electrostatic-covalent H-bond model (ECHBM) which was previously developed for the homonuclear case (Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1994, 116, 909). The conclusions drawn are corroborated by extended DFT quantum-mechanical calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory and by full geometry optimization carried out on 27 variously substituted heterodienes II−IV and nonresonant β-aminones V. Calculations allow the estimation of H-bond energies that are found to be approximately 2.75 kcal mol-1 for nonresonant V and 5.22, 6.12, and 7.03 kcal mol-1 for unsubstituted resonant II, III, and IV, respectively. Proper substitutions of β-enaminone II nearly double H-bond energies, making them comparable to those calculated for homonuclear O−H···O RAHB in β-diketone enols (9.51 and 13.08 kcal mol-1 for malondialdehyde and acetylacetone, respectively).