Allosteric binding of an Ag⁺ Ion to cerium(IV) bis-porphyrinates enhances the rotational activity of porphyrin ligands

Abstract: A series of cerium(IV) bis-porphyrinate double-deckers [Ce(bbpp)₂] (BBPP = 5,15-bis(4-butoxyphenyl)porphyrin dianion), [Ce(tmpp)₂] (TMPP = 5,10,15,20-tetrakis(4-methoxyphenyl)-porphyrin dianion), [Ce(tfpp)₂] (TFPP = 5,10,15,20-tetrakis(4-fluorophenyl)porphyrin dianion), [Ce(tmcpp)₂] (TMCPP = 5,10,15,20-tetrakis(4-methoxycarbonylphenyl)porphyrin dianion), and [Ce(tmpp)(tmcpp)] was prepared. They bind three Ag⁺ ions to their concave porphyrin π subunits (π-clefts) according to a positive homotropic allosteric mechanism with Hill coefficients (n_H) of 1.7-2.7. The rotation rates of the porphyrin ligands in [Ce(bbpp)₂] were evaluated to be 200s^-1 at 20°C (ΔG^†₂₉₃ = 14.1 kcalmol^-1) and 220 s^-1 at -40°C (ΔG^†₂₃₃ = 11.0 kcalmol^-1) without and with Ag⁺ ions, respectively. These results consistently support our unexpected finding that Ag⁺ binding can accelerate rotation of the porphyrin ligand. On the basis of UV-visible, ^IH NMR, and resonance Raman spectral measurements, the rate enhancement of the rotational speed of the porphyrin ligands is attributed to conformational changes of the porphyrin in cerium(IV) bis-porphyrinate induced by binding of Ag⁺ guest ions in the clefts. This novel concept of positive homotropic allosterism is applicable to the molecular design of various supramolecular and switch-functionalized systems.