Recognition through Self-Assembly. A Quadruply-Hydrogen-Bonded, Strapped Porphyrin Cleft That Binds Dipyridyl Molecules and a [2]Rotaxane

Abstract

Quadruply-hydrogen-bonded porphyrin homodimer Zn1·Zn1 has been designed, assembled, and evaluated as a supramolecular cleft-featured receptor for its ability to bind dipyridyl guests in chloroform-d. Monomer Zn1 consists of a 2-ureidopyrimidin-4(1H)-one unit, which was initially reported by Meijer et al., and a zinc porphyrin unit. The zinc porphyrin is strapped with an additional aliphatic chain for controlling the atropisomerization of porphyrin. The 2-ureidopyrimidin-4(1H)-one unit dimerizes exclusively in chloroform even at the dilute concentration of 10^-4 M, while the two “strapped” zinc porphyrin units of the homodimer provide additional binding sites for selective guest recognition. ¹H NMR studies indicate that the new homodimer Zn1·Zn1 adopts an S-type conformation due to strong donor−acceptor interaction between the electron-rich porphyrin units and the electron-deficient 2-ureidopyrimidin-4(1H)-one unit. ¹H NMR, UV−vis, and vapor pressure osmometry investigations reveal that Zn1·Zn1 could function as a new generation of assembled supramolecular cleft, to be able to not only efficiently bind linear dipyridyl molecules 14 − 17, resulting in the formation of stable termolecular complexes, with K_aasoc values ranging from 3.8 × 10⁶ to 8.9 × 10⁷ M^-1, but also strongly complex a hydrogen-bond-assembled [2]rotaxane, 18, which consists of a rigid fumaramide thread and a pyridine-incorporated tetraamide cyclophane, with K_aasoc = 1.2 × 10⁴ M^-1. ¹H NMR competition experiments reveal that complexation to the dipyriyl guests also promotes the stability of the quadruply-hydrogen-bonded dimeric receptor.