Self-Assembling Porphyrin [2]-Catenanes

Using the concepts of self-assembly developed for the construction of topologically complex molecules such as [2]-catenanes and rotaxanes, the syntheses of several prophyrin-catenanes are described. The successful insertion of the porphyrin subunit into [2]-catenanes has resulted from the adaptation of previously devised template-directed syntheses based primarily on π-π interactions and C-H…O and C-H…π hydrogen bonding. Several hydroquinol-containing ether chains were successfully synthesized and subsequently strapped across a porphyrin ring; the X-ray crystal structures of two such molecules which vary in the length of the ether chain are reported. These strapped porphyrins were found to complex paraquat with a parallel orientation of the hydroquinol, paraquat, and porphyrin rings. The complexation of paraquat (K_a = 1955 and 1640 M^-1, ΔG^°- -4.5 and -4.4 kcal mol^-1 for 9 and 11, respectively) by these strapped porphyrins led to their subsequent inclusion, using template-directed self-assembly, into structures which represent the first examples of porphyrin-containing [2] -catenanes. The catenanes were subsequently characterized by FAB-MS and thoroughly examined using dynamic ¹H NMR. In the case of the hydroquinol-containing catenanes, a dynamic process was observed in solution in which the bipyridinium subunits of the tetracation were exchanged between "inside" and "outside" environments (rotation around the hydroquinol ring axis). The diethylene glycol containing catenanes 14 and 15 were observed to maintain an orientation of the tetracation in which the bipyridinium subunits are parallel to the porphyrin ring. The length of the ether strap in these catenanes influenced the rotation rates of the tetracation macrocycles (50 and 80 times per second for 14 and 15 and 2500 and 1500 times per second for 16 and 17, respectively) at ambient temperature. However, in all the catenanes, the overall orientation of the tetracation, with bipyridinium and porphyrin rings coparallel, is retained in solution. As an extension of the original concepts, an analogous strapped porphyrin containing a naphthoquinol moiety was synthesized and found to complex paraquat with K_a = 21 000 M^-1 and ΔG^° = -5.9 kcal mol^-1. The general applicability of the template-directed synthetic procedure developed for porphyrin catenane synthesis was illustrated by the successful isolation of the naphthalene-containing porphyrin [2] -catenane 23 in 45% yield. Tetracation exchange processes similar to those for the hydroquinol catenanes were observed, along with an additional process which was found to be the decomplexation/complexation of the naphthalene ring in what is termed an "out, turn around, and in again" process similar to that occurring in nonporphynnic naphthalene [2]-catenanes.