Weak interactions in 1,3-dimethyl-5-arylmethyl-5-cytisylmethylbarbituric acids. Unusually steady intramolecular organic sandwich complexes

The structure of 1,3-dimethyl-2-thio-5-(9-anthrylmethyl)-5-cytisylmethylbarbituric acid (1m) has been investigated by the combination of single-crystal X-ray diffraction analysis and H-1 NMR spectroscopy. It has been revealed that 1m adopts unusually steady intramolecular sandwich-like conformation both in the solid state and in solution, which is stabilized by weak attractive interactions, such as pi-pi stacking and C-H center dot center dot center dot pi and C-H center dot center dot center dot n (lone pair) hydrogen bonds. The sandwich conformation observed indicates that the pi-pi stacking interactions between planar pi-systems act even at significant deviations from the strictly parallel arrangement. The nature of the attractive interactions in 5-arylmethyl-5-cytisylmethylbarbituric acids (1) is discussed. (c) 2006 Elsevier B.V. All rights reserved.