Guanidinium Para-Substituted Benzenesulfonates: Competitive Hydrogen Bonding in Layered Structures and the Design of Nonlinear Optical Materials

X-ray crystal structures of six guanidinium para-substituted benzenesulfonates, [C(NH₂)₃]+(p-XC₆H₄SO₃]⁻ (X = −CH₃, −NH₂, −OCH₃, −NO₂, −OH, and −CO₂H), are reported. Molecular packing in these salts is directed predominantly by hydrogen-bonding and Coulombic interactions. The planar guanidinium-sulfonate hydrogen-bonded sheet motif that is generally observed for these compounds is disrupted when the substituent X has hydrogen-bonding character. Consequently, the sheets “pucker” about guanidinium-sulfonate hydrogen-bonded ribbons, the puckering increasing with increasing hydrogen-bonding ability of X. For X = −CH₃, −NH₂, and −OCH₃ the typical nearly planar sheet motif is observed. For X = −NO₂ and −OH, the puckering of the hydrogen-bonded sheets is severe, and the ribbons assemble into noncentrosymmetric crystalline phases (Ama2 and Ima2, respectively) which exhibit second harmonic generation. For X = −CO₂H, the sheet motif is completely disrupted due to competitive hydrogen bonding of X for guanidinium and sulfonate hydrogen-bonding sites. However, guanidinium-sulfonate hydrogen-bonded ribbons are observed in all salts. These studies suggest that the highly directing hydrogen-bonding and ionic character of the guanidinium-sulfonate networks provide a sound strategy for materials design, particularly for low-dimensional electronic properties and second harmonic generation.