Dianiline-Diphenol Molecular Complexes Based on Supraminol Recognition Venu R. Vangala, † Raju Mondal, ‡ Charlotte K. Broder, ‡ Judith A. K. Howard, ‡, * and Gautam R. Desiraju †, * School of Chemistry, University of Hyderabad, Hyderabad 500 046, India (desiraju@uohyd.ernet.in) and Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, U.K. Received January 16, 2004 ABSTRACT: Dianilines and diphenols form well-defined crystalline molecular complexes that arise from complementary O-H‚‚‚N and N-H‚‚‚O recognition. The crystal structures of four such 1:1 complexes 1, 2, 3 and 4 based on diphenylmethane frameworks are reported and discussed. Three supramolecular synthons are found, the N(H)O based square motif and infinite chain, already reported, and a new cyclohexane chair consisting of N(H)O, O-H‚‚‚O and N-H‚‚‚N hydrogen bridges. The replacement of a -CH 2 - group by an S-atom leads in two cases to little change in the crystal structure and in another to a complete change. It is possible that the dianiline component plays a more important role in molecular recognition in these complexes than does the diphenol component. Introduction Strength and directionality of noncovalent interac- tions contribute to structural predictability in crystal engineering. 1 In recent years, efforts have been made to identify robust supramolecular synthons in order that the variety inherent in crystal packing is appropriately classified and simplified in structural analysis. 2 Speci- ficity of recognition between amino and hydroxy func- tionalities to give supraminol structures has been studied extensively. 3 The Ermer 3a and Hanessian 3b,d,h,m groups have shown that hierarchic crystal structures that have a saturation of O-H‚‚‚N and N-H‚‚‚O [henceforth jointly called N(H)O] hydrogen bridges are obtained in many cases. Work in our laboratories in Hyderabad and Durham has shown that deviations from hierarchy are also not uncommon with N-H‚‚‚π bridges making their appearance on occasion. 3f,n In a molecular complex, there are specific noncovalent interactions between chemically distinct molecules, and molecular complexes may accordingly be distinguished from, say solid solutions or inclusion compounds. 4 According to Kitaigorodskii, 5 all these two-component systems fall into the overall category of mixed crystals. Kitaigorodskii also noted that the formation of a mo- lecular complex is one of the most sensitive ways of probing the significance of particular intermolecular interactions in a crystal structure. He stated that studies of binary crystals of organic substances are a key to the study of intermolecular interactions. Accord- ingly, molecular complexes are of great significance in crystal engineering. We have noted, many years ago, that the formation of a molecular complex A‚B is, in itself, an indication that interactions of the type A‚‚‚B are more significant than interactions of the type A‚‚‚A or B‚‚‚B. 6 With reference to the supraminols, the formation of 1:1 amine-alcohol (or phenol) molecular complexes is favored because the resulting N(H)O hydrogen bridges are better than the O-H‚‚‚O and N-H‚‚‚N bridges in the crystal structures of the respec- tive individual components. Ermer and Eling 3a isolated the 1:1 complex between p-phenylenediamine and hy- droquinone and between the phenylogous extensions of these compounds. The Hanessian group has reported many examples of helical diamine-diol complexes. 3b Loehlin 3c,g and Toda 3e and their co-workers have also observed the formation of 1:1 amine-alcohol molecular complexes. In all these cases, saturation of N(H)O hydrogen bond forming ability is invariably observed. In this work, we have examined the 1:1 molecular complexes 1-4 formed by all possible permutations of diamines 5 and 6 with diols 7 and 8. The prototype structures for these molecular complexes are aminophe- nols 9, 4-(4-aminobenzyl)phenol, and 10, 4-(4-aminophe- nylsulfamyl)phenol. In these latter structures, which are based on that of 3-aminophenol, 3f optimization of her- ringbone interactions is the primary structural effect and this leads to the formation of weak N-H‚‚‚π, N-H‚‚‚S and C-H‚‚‚O bridges 7 rather than to a satura- tion of N(H)O bonds (Figure 1). Both 9 and 10 have a square motif N(H)O synthon. Replacement of the -CH 2- group in 9 by an isosteric S-atom in 10 leads, however, to an exchange of the N-H‚‚‚π by an N-H‚‚‚S bridge. This prompted us to investigate complexes 1 through 4. What would be the modularity or how much struc- tural interference would one observe here? † University of Hyderabad. ‡ University of Durham. Scheme 1 CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 99 - 104 10.1021/cg049967v CCC: $30.25 © 2005 American Chemical Society Published on Web 08/11/2004