Scalar coupling constants across the intramolecular NHN hydrogen bond of symmetrically and non-symmetrically substituted 6-aminofulvene-1-aldimines

Abstract

Multinuclear liquid state magnetic resonance experiments have been performed on two seven-membered 15N-labeled H-chelates of the 6-aminofulvene-1-aldimines type in order to characterize the strong intramolecular NHN hydrogen bonds as a function of the molecular symmetry. In particular, the symmetrically substituted N,N′-diphenyl-6-aminopentafulvene-1-aldimine-15N2 (1) and its asymmetric analog N-phenyl-N′-(1,3,4-triazol)- 6-aminopentafulvene-1-aldimine-15N5 (2) have been studied. For 1, an NN coupling constant across the hydrogen bridge of 2hJ(15N,15N) = 10.6 Hz was determined indirectly by 13C NMR at two different Larmor frequencies, 125.76 and 67.93 MHz; this coupling constant is characteristically enhanced compared with the value of 8.6 Hz obtained previously for 2. Because of a fast degenerate proton tautomerism the hydrogen bond proton in 1 is coupled with both nitrogen atoms with a coupling constant of −40.8 Hz. {15N} tickling experiments were performed on 2 in order to determine the relative signs of the coupling constants of the NHN hydrogen bridge. We find that 2hJ(15N,15N) and 1hJ(1H,15N) = +4.4 Hz exhibit the same sign, i.e. the opposite sign compared with 1J(15N,1H) = −88.6 Hz. This finding proves that 1hJ(1H,15N) corresponds to an intrinsic coupling, which is not induced by a tautomerism absent in 2 because of the large difference in basicities of the aniline and the amino-1,3,4-triazole substituents. Therefore, these observations indicate a sign change of J(15N,1H) when the proton is transferred successively from one nitrogen to the other, as observed previously for FHF hydrogen bonds. The relation between the values of the coupling constants and the hydrogen bond geometries is discussed in terms of the valence bond order model, as are the implications for obtaining equilibrium constants of tautomerism from coupling constant data. Copyright © 2001 John Wiley & Sons, Ltd.