Quantitative Comparison of Kinetic Stabilities of Metallomacrocycle‐Based Rotaxanes

Four mononuclear metallomacrocycles with identical cavities but different transition metals (Os VI , Pd II , Pt II , and Re I ) were prepared. With these metallomacrocycles, the corresponding rotaxanes 2‐Os, 2‐Pd, 2‐Pt , and 2‐Re were self‐assembled by hydrogen‐bonding interactions. The kinetic stabilities of the rotaxanes were determined quantitatively and compared with each other by 1 H NMR spectroscopic techniques, including two‐dimensional exchange spectroscopy (2D‐EXSY) experiments. The activation free energies (Δ G ≠ ) for the exchange between the rotaxanes 2‐Os, 2‐Pd and 2‐Pt and their free components were determined to be 15.5, 16.0, and 16.4 kcal mol −1 , respectively. These magnitudes imply that the rotaxanes 2‐Os, 2‐Pd and 2‐Pt are kinetically labile at room temperature and exist only as equilibrium mixtures with free components in solution. In contrast, the rotaxane 2‐Re is kinetically stable enough to be isolated in pure form by silica gel chromatography under ordinary laboratory conditions. However, at higher temperatures (>60 °C) 2‐Re was slowly disassembled into its components until the equilibrium was established. The rate constants were measured at three different temperatures, and the Eyring plot yielded the activation enthalpy Δ H ≠ =35 kcal mol −1 and the activation entropy Δ S ≠ =27 eu for the disassembly of the rotaxane 2‐Re in Cl 2 CDCDCl 2 . These thermodynamic parameters gave the activation free energy Δ G $\rm{^{\ne }_{off}}$ =27.1 kcal mol −1 at 25 °C. Consequently, 2‐Re is one example of a novel metallomacrocycle‐based rotaxane that contains a coordination bond with enough strength to allow both for isolation in pure form around room temperature and for self‐assembly at higher temperatures.