113Cd 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 H(2)1 and Cd(OAc)(2)center dot 2 H2O or Cd-113(OAc)(2)center dot 2 H2O, respectively. The compounds form allosteric complexes with the positively charged guests N,N'-dimethylimidazolium hexafluorophosphate (DMI) and N,N'-dimethylviologen dihexafluorophosphate (Me2V), 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 similar to 31, while the presence of DMI or Me2V 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. Cd-113 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-Me2V revealed that the coordination of tbupy significantly slowed down the dissociation of the Me2V guest. Eyring plots of the dissociation process revealed that this kinetic allosteric effect is entropic in nature.

Automated summary

The cadmium porphyrin cage compounds Cd1 and (113)Cd1 have been synthesized and form allosteric complexes with various positively charged guests. The X-ray structures of the coordination complexes were solved and Cd-113 NMR was employed to quantify the binding of guests and ligands to (113)Cd1.