Intramolecular hydrogen bonds in crystals of thiophosphorylbenzopyrane derivatives - X-ray and FT-IR studies

The crystal structures of two new benzopyrane derivatives are analyzed and compared with previous X-ray investigations on related compounds. A particular attention is focused on intramolecular interactions. For the chromone derivatives (1 and 3) only one kind of interaction is possible, i.e., N-H⋯O, whereas for the coumarine derivatives (2 and 4) two types of intramolecular hydrogen bonding are observed - N-H⋯O and O-H⋯N. Two types of H-bond for coumarine derivatives are the result of the existence of two tautomeric forms - enamine and iminoenol. Combined spectroscopic, NMR and IR measurements confirm such tautomeric equilibrium in solution. The influence of the additional intermolecular hydrogen bonds on the stabilization of tautomeric forms in crystals is also discussed here.