In contrast to 8-10, tetraphenylborate 11 (Na + salt) is soluble in water, which allowed us to conveniently follow the T→C reaction by NMR spectroscopy. Figure shows the evolution of 1 H NMR spectra of T following the addition of 1.0 equiv of 11 at 60 ºC. Initially, 11 binds to T to form 11ÌT; however, the first spectrum recorded at 60 ºC already shows ~20% 11ÌC (Figure ). Over time, T's concentration steadily decreases until it becomes undetectable at t = 3 h (Figure ). Integrating the acidic imidazole signals of C vs. T allowed us to plot the fraction of T as a function of time -see Figure . By fitting C's decay to first-order kinetics, we obtained k = 0.85±0.02 h -1 . The rate constant could be determined independently by analyzing the signals of 11 bound within both cages; interestingly, integrating the guest signals resulted in a significantly smoother profile (Figure ) and a similar k = 0.92±0.01 h -1 . The first-order kinetics are consistent with fast encapsulation (11 + T → 11ÌT) being followed by a relatively slow isomerization step (11ÌT → 11ÌC). Finally, we found that tetraphenylmethane tetraphosphonic acid 12 also proved highly potent in converting T into C. This guest can be solubilized in basic water (the pH was raised to 8 using TMEDA); we found that in the presence of 1 equiv of 12, the reaction was completed within 2 hours at room temperature. Isolation and characterization of the metastable cage C. Having quantitatively converted cage T into C, we attempted to isolate and further characterize the metastable cage C. Encapsulation within T is primarily driven by the hydrophobic effect: we have previously reported that the addition of small volume fractions of organic solvents liberates the guest from the cage. Similarly, we hypothesized that treating aqueous solutions of stable GÌC complexes with organic solvents would result in the metastable C. Moreover, when a hydrophobic solvent is used, guest removal should be accompanied by its extraction to the organic phase, allowing for rapid access to C in a pure form.