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 (nH) 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.
|Number of pages||10|
|Journal||Chemistry - A European Journal|
|Publication status||Published - 2002 Dec 16|
- Cooperative phenomena
- Host-guest systems
- Pi interactions