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

Abstract

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.

Anion recognition by hydrogen bonding: urea-based receptors

“…57 Pt II , as a d 8 cation, has a defined preference for square coordination and . 56 Structures redrawn from data deposited at the Cambridge Crystallographic Data Centre (VAVCUQ, TISSIY, TISSOE). SO 4 2À (added as a Na + salt) indicated the formation of 1 : 1 complexes, with K > 10 5 for both anions.…”

“…Tetrabutylammonium counter-cations and the hydrogen atoms of C-H fragments of each receptor have been omitted for clarity; (a) receptor 34, whose cavity includes a Cl À ion and a DMSO molecule; 54 (b) receptor 33, including two Cl À ions;56 (c) receptor 33 including the H-bond dimer [H 2 PO 4 ] 2 2À…”

Porphyrins and Expanded Porphyrins as Receptors

“…These solid state differences are consistent with the disparities in binding stoichiometry seen from Job plot analysis when anion-binding studies are carried out in different solvents. 63 As noted above, sapphyrins were first appreciated for their anion-binding ability in the early 1990s. Several X-ray crystal structures have been elucidated and revealed the ability of the protonated macrocycle to bind anions; Figure 32 shows a sapphyrin derivative, 32, in its diprotonated form, which is bound to two bromides.…”

“…This results in an anion-bound complex of greater stability than that produced by the corresponding porphyrin functionalized with just two urea pickets in the 5 and 10 positions (ortho). 63 On the basis of detailed studies of the thermodynamics of complexation, it was inferred that one buried solvent molecule adds the equivalent of an "additional" hydrogen bond in terms of anion affinity. These researchers also concluded that a buried solvent molecule can affect the receptor: anion-binding stoichiometry.…”

Adamantane bisurea derivatives: anion binding in the solution and in the solid state

“…29,35 Later, it was shown that stoichiometry 1:2 can be due to the formation of two 36 or 3 intermolecular H-bonds between two H2PO4 -anions. 37 This cooperative effect additionally stabilizes 1:2 stoichiometries rendering them more stable than the …”