Quantitative comparison of Kinetic Stabilities of Metallomacrocycle-Based Rotaxanes

摘要

Four mononuclear metallo-macrocycles 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 ~1H NMR spectroscopic techniques, including two-dimensional exchange spectroscopy (2D-EXSY) experiments. The activation free energies (DELTAG) of the exchange between the rotaxnes 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 deg 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 DELTAH = 35 kcal mol~(-1) and the activation entropy DELTAS = 27 eu for the disassembly of the rotaxane 2-Re in Cl_2CDCDCl_2. These thermodynamic parameters gave the activation free energy DELTAG_(off) = 27.1 kcal mol~(-1) at 25 deg 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 temperature.