Two-dimensional hydrogen-bonded assemblies: the influence of sterics and competitive hydrogen bonding on the structures of guanidinium arenesulfonate networks

Abstract

Guanidinium and organosulfonate ions self-assemble into crystalline lattices described by robust two-dimensional hydrogen-bonded networks with the general formula [C(NH ₂ ) ₃ ] ⁺ RSO ₃ ^- . These networks, which typically have quasihexagonal symmetry due to favourable hydrogen bonding between six guanidinium proton donors and six sulfonate electron lone pair acceptors, assemble in the third dimension by stacking in a manner which maximizes van der Waals interactions between R groups. The steric requirements of the R groups dictate whether this assembly results in interdigitated bilayer stacking in which all the R groups are orientated to one side of a given sheet or interdigitated single layer stacking in which R groups are orientated to both sides of a given hydrogen-bonded sheet. The two-dimensional network tolerates very different steric requirements of the R groups due to the ability to form either of these stacking motifs and to the inherent flexibility of the hydrogen-bonded network about one-dimensional hydrogen-bonding ‘hinges’. This flexibility allows the sheets to pucker in order to accommodate steric strain between R groups within the layers. We describe here the influence of substituents on the R groups whose steric and hydrogen bonding capacity influence the puckering of the two-dimensional guanidinium sulfonate network. In particular, we examine the X-ray crystal structures of the guanidinium salts of ferrocenesulfonate and methyl- and nitro-substituted benzenesulfonates. The retention of the hydrogen-bonding motif in spite of steric and hydrogen bonding interference by the R group substituents illustrates the robustness of the guanidinium sulfonate network. However, additional competing hydrogen bonding and sterics influence the crystal packing, and in the case of multiple substituents on the R groups, these factors may disrupt the guanidinium sulfonate network. Overall, this work demonstrates that the use of robust two-dimensional supramolecular modules can reduce the crystal engineering problem to the last remaining dimension, which can simplify the design of functional molecular materials.