Controlling the Guanidinium Cation Rotation by Cation-π Interactions

Guanidinium plays an essential role in many disciplines of biology and chemistry, particularly due to its unique role of being engaged in a variety of molecular-recognition events that are facilitated by the ability to participate in a broad range of noncovalent interactions. Guanidinium cations in salts or perovskite materials are known to rotate along their local symmetry axes rather fast in the order of ps, even in the solid state. We herein employ a π-container to trap a guanidinium cation inside the aromatic cavity by cation-π interactions. X-ray crystallography and solid-state nuclear magnetic resonance (NMR) spectroscopy at fast magic-angle spinning (MAS) frequencies have been utilized to probe the underlying host-guest interactions. The guanidinium motion has been fully characterized by temperature-dependent MAS-NMR experiments down to 100 K, as well as by a variety of further solid-state NMR experiments, and supplemented by quantum-chemical calculations and molecular dynamics (MD) simulations. Our data point to a restriction of the guanidinium cation rotation about the local C₃-axis with correlation times in the order of ns. Our study, therefore, showcases that using the calixarene framework as a bearing for trapping a guanidinium cation, we are getting closer to the chemist's dream of controlling molecular rotations.