Structural Homology and Dynamic Variation in a Series of Porphyrin Bipyridinium Receptors and Their [2]Catenanes

Abstract

We describe the successful synthesis of the porphyrin [2]catenanes, which completes a set of six structures where the length of the polyethylene glycol strap over the porphyrin is regularly increased from diethylene to triethylene to tetraethylene, and the central electron-donor component is either hydroquinol- or naphthoquinol-based. In the synthetic route to the strapped porphyrin precursors, co-formation of "straight" and "twisted" atropisomers in the longer strapped derivatives, and dimeric and higher oligomeric structures for the shorter straps, has been observed. Catenation results in only a single [2]catenane in each case. X-ray crystal structures of two representative strapped porphyrins are described. For each of the untwisted singly-strapped porphyrins, there is significant binding of paraquat within the cavity formed by the strap, and the binding constants range over about two orders of magnitude. For the catenanes, in each case the overall structural motif is similar to that previously observed for related members of the series. The temperature-dependent dynamic properties of the catenanes have been examined by ¹H NMR methods, and the rates of rotation increase from 50 to 25,000 to 340,000 s-¹ at room temperature as the length of the strap increases by one ethylene glycol unit on each side. For the similarly sized naphthoquinol derivatives, the rotation rates are reduced by many orders of magnitude, ranging from essentially zero to 10 to 100 times per second at 25°C through the same increase in chain length as for the hydroquinol analogues. The rates for a second process described as out, turn around and in again, range from 2 to 400 to 110,000 s-¹ at 25°C in this naphthoquinol porphyrin series. The electronic spectra of the catenanes themselves do not show any significantly enhanced charge-transfer bands over the hydroquinol counterparts.