Fig. 4: Metastable state in the redox cycle.
a, Top, graphical representation and structural formula of the metastable state with an optimized quantum mechanical model structure (M06-2X/6-31G* basis set, in tubular with superimposed space-filling representation). Bottom, partial 1H NMR (600 MHz, CD3CN, 298 K) spectra of [3]CMM•13PF6 measured over time (0–60 min) immediately after a cycle of reduction (Cp2Co) and reoxidation (NOPF6), with proton assignments labelled at the top and bottom of the spectra. The proton resonances attributable to the metastable state are labelled with an asterisk. b, Top, thermal relaxation associated with the co-conformational rearrangement from the metastable state to the reoxidized state. The activation energy barrier ΔG‡ of 21.6 kcal mol−1 was determined using the Eyring equation (\(k=\frac{{k}_{{\rm{B}}}T}{h}{{\rm{e}}}^{\frac{{-\Delta G}^{\ddagger }}{RT}}\)), in which k is the reaction rate constant, T is the absolute temperature, R is the gas constant, kB is the Boltzmann constant and h is the Planck constant. Bottom, plot of the changes in the normalized integral of protons on the BPM (H-12* and H-14) and IPP (H-22* and H-25) units with time at 298 K during the transformation from the metastable to the reoxidized state, as well as the fitting curves of these data according to the first-order kinetic model. ppm, parts per million.
