Thermodynamically self-assembling porphyrin-stoppered rotaxanes

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Abstract
A variety of porphyrin-stoppered rotaxanes has been assembled under conditions of thermodynamic reversibility by judicious choice of components and temperatures. An admixture of a thread unit comprising a central naphthodiimide with terminal pyridines (2a), a ring unit dinaphtho-38-crown-10 (1) and either ZnII (3a), RuII (CO) (3b) or RhIII I (3c) as stoppers was shown to form an equilibrating mixture of pseudorotaxanes and the porphyrin-stoppered rotaxanes. The intact rotaxane could be crystallised from solution and chromatographed at low temperatures; at higher temperatures only a mixture of the components separated on chromatography. 1H NMR revealed NOE correlations between all three components for the rotaxane, and exchange peaks for related free and complexed species. The temperature and concentration dependence of the equilibria were studied by NMR methods, and van't Hoff plots for the Ru(CO) stoppered system enabled an estimation of ΔH° and ΔS°, −41.4 kJ mol−1, and −95 J K−1 mol−1, respectively, and a Ka at 273 K of 790 M−1, rising to 5.39 × 104 M−1 at 223 K. For the unstoppered pseudorotaxane systems 1–2a and 1–2b, the R ln K s. 1/T plots were not linear, implying a temperature-dependent ΔH and a non-zero ΔCp, indicative of a folding/unfolding of the extended thread unit on complexation. Nevertheless, although the thermodynamic stability of the overall rotaxane is expected to be comparable to that of the pseudorotaxane, there is clearly an enhanced kinetic barrier for formation of the metalloporphyrin-stoppered rotaxanes, but not for the more labile zinc analogue. While mixing of all three components of the rotaxanes at room temperature resulted in rapid rotaxane assembly irrespective of the order of addition (thermodyamic control), it was shown that at low temperatures it was possible to “lock out” or “lock on” the central thread unit under conditions of kinetic control. These concepts were further extended to the assembly of more complex multi-porphyrin arrays, where the central ring unit is a naphthocrown-strapped zinc porphyrin. It is therefore possible to use the kinetics of the remote event (metal ligation/coordination) to control the overall kinetics of rotaxane formation.

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