Cocrystallization of bent dipyridyl type compounds with aromatic dicarboxylic acids: Effect of the geometries of building blocks on hydrogen-bonding supramolecular patterns

The high-yielding preparation and X-ray structures of six 1:1 molecular cocrystals consisting of bent dipyridyl bases and aromatic dicarboxylic acids, including [phthalic acid](.)[3-bpol (1), [phthalic acid](.)[4-bpol (2), [isophthalic acid]-[3-bpol (3), [isophthalic acid]-[4-bpol (4), [terephthalic acid]-[3-bpol (5), and [terephthalic acid][4-bpol (6), are described, in which 3-bpo refers to 2,5-bis(3-pyridyl)-1,3,4-oxadiazole and 4-bpo its 4-N-donor analogue 2,5-bis(4-pyridyl)-1,3,4-oxadiazole. A supramolecular synthon I [R-2(2) (7)] containing classical O-(HN)-N-... and weak C-(HO)-O-... interactions, usually observed in organic cocrystals of carboxylic acids with other heterocyclic bases, is again shown to be involved in constructing most of these hydrogen-bonding networks. In compounds 3-5, tapes of acid/base components are formed via synthon 1, which are further extended to result in two-dimensional (2-D) supramolecular sheets via additional C-(HO)-O-... interactions. Compound 2 contains similar but helical tapes with synthon 1, and the neighboring twisted chains are cross-linked via weak C-(HO)-O-... hydrogen bonds to show a 2-D corrugated network. Compound 6 contains a similar 2-D layered structure to that of 3-5, although synthon I was not found due to the stereochemistry effect. Unexpectedly, adjacent layers are further combined into a three-dimensional (3-D) architecture through acid-base C-(HO)-O-... hydrogen bonds. However, in compound 1, through intermolecular O-(HN)-N-... and C-(HO)-O-... bonds, a novel 3-D structure was formed, in which acid and base subunits arrange alternately to exhibit a helical motif along the crystallographic [1001 direction. For all these cocrystals, the common head-to-tail hydrogen-bonded ring motif [synthon II R-2(2) (8)] between carboxylic acids is absent. Melting points of cocrystals 1-6 are mainly correlated with the nature of the acid component and crystal packing. Thermogravimetric analysis of mass loss for six compounds has been shown to correlate with the strength of hydrogen bonds in the packing fraction.