Interplay of strong and weak hydrogen bonding in molecular complexes of some 4,4'-disubstituted biphenyls with urea, thiourea and water

Abstract

The crystal chemistry and engineering of a new family of host-guest complexes is described. 4,4'-Dicyanobiphenyl (DCBP) forms a 1:1 complex, 1 with urea wherein the DCBP host forms large hexagonal channels via C-H···N hydrogen bonds and the urea guest molecules are arranged in N-H···O ribbons which fit completely within the host channels. By analogy, 4,4'-bipyridine N,N'-dioxide (BPNO) was selected as a molecule that can form a corresponding C-H…O based channel. BPNO forms complexes with urea (2), thiourea (3) and water (4). Structures 2 and 3 provide some points of comparison with the structure of 1 but are not fully equivalent to it. In structure 4, the smaller guest water is able to fit neatly within the smaller hexagonal channel of BPNO and in this sense, the degree of structural predictability is satisfactory. To obtain another structure similar to that of 1, 4,4'-dinitrobiphenyl (DNBP) was identified as an alternative host compound. This choice was justified by the structure of its 1:1 complex, 5 with urea. In all cases, the guest molecules interact with each other via strong hydrogen bonds and form an essential template for the weak hydrogen bonded assembly of the host network structure but the latter is still of some significance. One finds consequently, in complexes 1-5, a constructive interplay of strong and weak hydrogen bonds.