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

Cadmium porphyrin cage compounds Cd1 and 113 Cd1 have been synthesized from the free base porphyrin cage derivative H2 1 and Cd(OAc)2  ⋅ 2 H2 O or 113 Cd(OAc)2  ⋅ 2 H2 O, respectively. The compounds form allosteric complexes with the positively charged guests N,N '-dimethylimidazolium hexafluorophosphate ( DMI ) and N,N '-dimethylviologen dihexafluorophosphate ( Me2 V ), which bind in the cavity of the cage, and tbupy , which coordinates as an axial ligand to the outside of the cage. In the presence of tbupy , the binding of DMI in Cd1 is enhanced by a factor of ∼31, while the presence of DMI or Me2 V in the cavity of Cd1 enhances the binding of tbupy by factors of 55 and 85, respectively. The X-ray structures of the coordination complexes of Cd1 with acetone, acetonitrile, and pyridine, the host-guest complex of Cd1 with a bound viologen guest, and the ternary allosteric complex of Cd1 with a bound DMI guest and a coordinated tbupy 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. 113 Cd NMR could be employed as a tool to quantify the binding of guests and ligands to 113 Cd1 . 1D EXSY experiments on the ternary allosteric system Cd1-tbupy-Me2 V revealed that the coordination of tbupy significantly slowed down the dissociation of the Me2 V guest. Eyring plots of the dissociation process revealed that this kinetic allosteric effect is entropic in nature.