Anion tuning of the rheology, morphology and gelation of a low molecular weight salt hydrogelator

1-(3-Methyl-1H-pyrazol-5-yl)-3-(3-nitrophenyl)urea (1) forms hydrogels in a range of acids at pH 1–2. The morphology and rheology of these gels are dependent on the identity of the anion and represent a way of tuning the gels' physical properties. The magnitude of the elastic storage modulus of the gels follows the trend according to the acid used to protonate the gelator H2SO4 > MePO3H2 > H3PO4 ≈ HBF4 ≈ HPF6 > EtPO3H2. Chloride salts do not form gels and the nitrate gels are unstable with respect to crystallisation. Salting out or addition of methanol results in the characterisation of five salts of 1 which reveals extensive solvation in the solid state and the absence of urea⋯urea hydrogen bonding in contrast to the well known bis(urea) gelators.