We thank Professor G. R. Desiraju for suggesting crystallization of pyridine-d¬5¬, and STFC for provision of neutron beam-time and a studentship for SC. RB gratefully acknowledges the support of the DFG FOR 618. [ † ] We dedicate this paper to Dr Frank Allen on the occasion of his retirement as Executive Director of the Cambridge Crystallographic Data Centre. Supporting information:Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Crystallographic information files can be obtained from the Cambridge Crystallographic Data Centre, quoting deposition numbers CCDC 695956 -695959. A summary table of crystal data is also given in the Supporting Information.

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Phenazine (1) and 5,10-dihydrophenazine (2) form not only a 1 : 1 complex (3), as expected from NÈHÉ É ÉN hydrogen bond requirements, but surprisingly also a 3 : 1 complex (4). Single crystal X-ray analyses of compound 2 and complexes 3 and 4 have been carried out. While the overall packing in complex 3 is as efficient as in pure 1 and 2, acidic CÈH groups, which contribute actively to the structural assembly in pure 2, are not involved in hydrogen bonding in 3. It is shown that the quest for hydrogen bonding by these CÈH groups steers the formation of complex 4 with an unprecedented NÈHÉ É ÉN and CÈHÉ É ÉN hydrogen bond mediated tape structure.

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Ethynylpyridines show straight and zigzag chain patterns formed by tC-H‚‚‚N hydrogen bonds; cross-stitching of zigzag chains by dC-H‚‚‚N hydrogen bonds leads to two-dimensional networks with predictable topology in 5-ethynylpyrimidine.