Tol 2 Zn (Tol is p -methylphenyl) and 18-crown-6 in benzene rapidly and almost completely form a 1:1 complex (peripheral) in which Zn is probably bonded to two oxygens. A rotaxane (threaded) 1:1 species forms more slowly; at equilibrium, [rotaxane]/[peripheral] = 0.7 at 23 °C. Several observations indicate that formation and dethreading of the rotaxane require free Tol 2 Zn (not incorporated into the peripheral complex). (1) Rotaxane formation is considerably more rapid when [Tol 2 Zn] 0 > [18-crown-6] 0 than when the crown ether is initially in excess. (2) Added tetramethylethylenediamine stops both formation and dethreading of the rotaxane. (3) Pure rotaxane is stable indefinitely in a benzene solution, but dethreading occurs when some of a diorganozinc compound is added. (4) The rate of dethreading is approximately proportional to the concentrations of rotaxane and free Tol 2 Zn.
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Chubb et al. (1998) studied this question.
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