Synthesis of urea picket porphyrins and their use in the elucidation of the role buried solvent plays in the selectivity and stoichiometry of anion binding receptors.

The synthesis of alpha,alpha-5,10-diurea and alpha,alpha,alpha-5,10,15-triurea picket porphyrins are detailed in this report. In previous reports, these porphyrins, along with alpha,alpha,alpha,alpha-5,10,15,20-tetraurea picket porphyrin, were used to demonstrate the important role one buried solvent molecule plays in the selectivity and stoichiometry of binding inorganic anions. Building on prior work, this report discusses the results of acetate anion binding studies between tetra- and diurea picket porphyrins (the latter does not contain a buried solvent molecule in the anion-receptor complex), compares differences in thermodynamic data obtained from van't Hoff plots of a porphyrin anion receptor able to utilize buried solvent in its binding motif with one that does not, and compares the crystal structure of a tetraurea porphyrin 1-chloride anion complex that contains buried solvent with new X-ray crystal structures of tetraurea porphyrin 1-dichloride or bisdihydrogenphosphate anion complexes that contain no buried solvent. Data from our previous work, and the work described herein, demonstrates that one buried solvent molecule provides stability to the receptor-anion complex that is similar in energy to a moderately strong hydrogen bond.