Mixed Ca/Sr salt forms of salicylic acid: tuning structure and aqueous solubility

Ten isostructural single-crystal diffraction studies of mixed cation Ca/Sr salt forms of the salicylate anion are presented, namely catena-poly[[diaquacalcium(II)/strontium(II)]-bis(μ₂-2-hydroxybenzoato)], [Ca_1-xSr_x(C₇H₅O₃)₂(H₂O)₂]_n, where x = 0, 0.041, 0.083, 0.165, 0.306, 0.529, 0.632, 0.789, 0.835 and 1. The structure of an isostructural Sr/Ba species, namely catena-poly[[diaquastrontium(II)/barium(II)]-bis(μ₂-2-hydroxybenzoato)], [Sr_0.729Ba_0.271(C₇H₅O₃)₂(H₂O)₂], is also described. The Ca/Sr structures form a series where, with increasing Sr content, the unit cell expands in both the crystallographic a and c directions (by 1.80 and 3.18%, respectively), but contracts slightly in the b direction (-0.31%). The largest percentage structural expansion lies parallel to the direction of propagation of the one-dimensional coordination polymer that is the primary structural feature. This structural expansion is thus associated with increased M-O distances. Aqueous solubility measurements show that solubility generally increases with increasing Sr content. Thus, tuning the composition of these mixed counter-ion salt forms leads to systematic structural changes and allows solubility to be tuned to values between those for the pure Ca and Sr species.