Melamine-Bridged Bis(porphyrin-Zn(II)) Receptors: Molecular Recognition Properties

Dimeric metalloporphyrin hosts with tweezer-like structures have been synthesized by reacting the cyanuric chloride scaffold, CC, with 5-(4-aminophenyl)-10,15,20-triphenylporphyrin, P, and 5-(4- aminophenyl)-10,15,20-trimesitylporphyrin, M, to yield the homoconjugates free bases PP and MM and the heterodyad PM. Metalation with Zn(II), gives three structurally related ditopic receptors P(Zn)P(Zn), P(Zn)M(Zn), and M(Zn)M(Zn) with differential steric hindrance and conformational rigidity. The solution structure and supramolecular properties of these porphyrin dimers have been investigated as isolated molecules and in the presence of aliphatic R,?-diamines of general formula H2N-(CH2)n-NH2 (n=4-8) by spectroscopic and theoretical studies includingmultidimensionalNMR, UV-vis, molecular modeling, and computational NMRmethods. Binding constants in the range 4.2 106 to 3.4 107M-1 are observed in dichloromethane at 298K, with a 3 orders ofmagnitude increase as compared to monodentate nBuNH2, thus indicating the occurrence of a host-guest ditopic interaction. Linear correlation graphs are obtained by plotting the Soret band shift (??, cm-1) of the complex as a function of the diamine chain length. Combined NMRevidence and OPLS 2005 Force Field conformational analysis point to amutual adaption of both the binding partners in the host-guest complex,whose geometry is mainly dictated by the steric impact of the bulky substituents at the porphyrin periphery.