Hydrogen Bonds Determining N,N-Diacylindigo Crystal Structure Conformations

The debate over C-H-0 hydrogen bonds lasted for many years before Taylor and Kennard surveyed the Cambridge Structural Database in 1982 and extracted strong statistical evidence in favor of such attractive interactions.' Further work by a number of other groups have concluded that although C-H*-0 bonds are much weaker than traditional hydrogen bonds (X-H-eX, where X = 0 or N) they are nonetheless ubiquitous in crystal 8tructures.a The current consensus is that C-H-*O bonds are weakly attractive, electrostatic interactions, with a moderate degree of directionality, that decrease with l l r and consequently extend well beyond the conventional van der Waals limit. Desiraju has also shown that the C.-O distance decreases with increased C-H acidity.5 At present there is no strong evidence correlating C-SO distance with 0 basicity; the general feeling is that, like traditional X-H-mX hydrogen bonds, the interaction is relatively insensitive to the basicity of the hydrogen bond acceptor.6 In general, any experimental study of the factors determining C-H-*O bond strengths is destined to be problematic due to the difficulty delineating such a weak interaction from the influence of stronger interactions. Correlations based on statistical surveys have proved to be an effective way to cancel out these environmental effects as "random noise". In this report, we compare three related crystal structures that provide insight into how a molecule with an excess of hydrogen bond acceptors attempts to counter these acceptors in the solid state by forming intramolecular C-H--O hydrogen bonds. Moreover, the relative order of the observed C-.O distances is rationalized in terms of the CH acidities and 0 basicities. We have recently characterized the structure and photochemistry of a number of N,N'-diacylindigo dye derivatives.7t8 This report focuses on the crystal structures of N,N'-diacetylindigo (11, N, N'-bis(chloroacety1) indigo (21, and N-acetyl-N'-chloroacetylindigo (3), which are depicted in Figure 1 and Scheme I. Steric crowding in these compounds is alleviated by two torsional mecha-