Acceleration and deceleration of chirality inversion speeds in a dynamic helical metallocryptand by alkali metal ion binding
Sk Asif Ikbal, Pei Zhao, Masahiro Ehara, Shigehisa Akine
Abstract
We report that the chirality inversion kinetics of a trinickel(II) cryptand can be controlled by guest recognition in the cryptand cavity. When the guest was absent, the nickel(II) cryptand underwent a dynamic interconversion between the P and M forms in solution, preferring the M form, with a half-life of t 1/2 = 4.99 min. The P / M equilibrium is reversed to P -favored by binding with an alkali metal ion in the cryptand cavity. The timescale of this M→P inversion kinetics was both notably accelerated and decelerated by the guest binding ( t 1/2 = 0.182 min for K + complex; 186 min for Cs + complex); thus, the equilibration rate constants differed by up to 1000-fold depending on the guest metal ions. This acceleration/deceleration can be explained in terms of the virtual binding constants at the transition state of the P / M chirality inversion; K + binding more stabilizes the transition state rather than the P and M forms to result in the acceleration.