A simple acyclic molecule binds sulfate strongly (K a>104 M−1) in a competitive 4 : 1 d6-acetone:D2O mixture. The molecule also reacts readily with BF4 − and BPh4 − to give zwitterions containing an anion tetrahedral boronate centre. These zwitterions were characterised crystallographically.
Abstract
A simple dihydroxy isoquinolinium molecule (3+ ) was prepared by a modification of a literature procedure. Interestingly, during optimisation of the synthesis a small amount of the natural product pseudopalmatine was isolated, and characterised for the first time by X-ray crystallography. Compound 3+ contains a catechol motif and positive charge on the same scaffold and was found to be a potent anion receptor, binding sulfate strongly in 8 : 2 d6-acetone:D2O and 7 : 3 d6-acetone:D2O (K a>104 and 2,100 M−1, respectively). Unsurprisingly, chloride binding was much weaker, even in the less polar solvent mixture 9 : 1 d6-acetone:D2O. The sulfate binding is remarkably strong for such a simple molecule, however anion binding studies were complicated by the tendency of the molecule to react with BPh4 − or BF4 − species during anion metathesis reactions. This gave two unusual zwitterions containing tetrahedral boronate centres, which were both characterised by X-ray crystallography.