Controlling Chemical Dynamics of Molecular Assemblies through Nanoconfinement: o-Nitrosocumene@Pd Nanocage

The change in chemical dynamics of confined ortho-nitrosocumene (o-NC), as observed by ¹H NMR spectroscopy, illustrates the power of nanoconfinement to alter reaction mechanisms from those observed in isotropic solutions where molecules have more freedom. When o-NC molecules that exist as a dynamic mixture of monomer (M), covalent dimers (D), and supramolecular aggregates (A) are provided a choice of being free in water or confined within an organometallic cage (Pd nanocage, PdNC), they prefer the latter. The confinement significantly alters the molecular distribution such that A no longer exists, and new equilibria among M and Z/E isomers of D are established within the nanocage and with components in the aqueous exterior. The change in equilibrium distribution of guest components demonstrates how the o-NC molecular forms of A, M, and D depend not only on concentration and temperature but also on the microenvironment surrounding the molecules. Temperature-dependent NMR and exchange spectroscopy (EXSY) with various mixing times are used to explore the M ⇄ D equilibrium and the D_Z ⇄ D_E interchange within the PdNC host. Nanoconfinement in the cage influences the dynamic interplay of dissociation, rotation, translation, and recombination events in these processes. Strikingly, the previously unreported D_Z-to-D_E exchange bypassing splitting to free M is enabled by entropy control in the nanoreactor.