Cd as a Probe in NMR Studies of Allosteric Host-Guest-Ligand Complexes of Porphyrin Cage Compounds.

Cadmium porphyrin cage compounds and have been synthesized from the free base porphyrin cage derivative and Cd(OAc) ⋅ 2 HO or Cd(OAc) ⋅ 2 HO, respectively. The compounds form allosteric complexes with the positively charged guests '-dimethylimidazolium hexafluorophosphate () and '-dimethylviologen dihexafluorophosphate (), which bind in the cavity of the cage, and , which coordinates as an axial ligand to the outside of the cage. In the presence of , the binding of in is enhanced by a factor of ∼31, while the presence of or in the cavity of enhances the binding of by factors of 55 and 85, respectively. The X-ray structures of the coordination complexes of with acetone, acetonitrile, and pyridine, the host-guest complex of with a bound viologen guest, and the ternary allosteric complex of with a bound guest and a coordinated ligand, were solved. These structures revealed relocations of the cadmium center in and out of the porphyrin plane, depending on whether a guest or a ligand is present. Cd NMR could be employed as a tool to quantify the binding of guests and ligands to . 1D EXSY experiments on the ternary allosteric system revealed that the coordination of significantly slowed down the dissociation of the guest. Eyring plots of the dissociation process revealed that this kinetic allosteric effect is entropic in nature.