Rotational Behavior in Piano Stool Ru(II) Complexes with Bulky-Substituted Cyclopentadienyl Ligands

In this study, we designed and synthesized a series of novel piano stool ruthenium complexes featuring bulky substituents on the cyclopentadienyl (Cp) ligand to investigate how the substituent structure affects rotational behavior around the Cp-Ru bond. Substituents, including <i>m</i>-xylyl, mesityl, and 9-anthracenyl groups, were introduced to create steric hindrance with the tripodal ligand to increase the rotational barrier. NMR spectroscopy revealed that the Cp-Ru bond in the complex with the <i>m</i>-xylyl group rotated faster than the NMR time scale, whereas complexes bearing mesityl and 9-anthracenyl groups exhibited slower rotation. Variable-temperature NMR measurements and line shape fitting analysis showed that the activation free energy (Δ<i>G</i> ^⧧) required for the Cp ligand rotation by overcoming the steric hindrance between the substituent and tripodal ligand was significantly higher for the mesityl (69.5 kJ mol^-1) and 9-anthracenyl (67.8 kJ mol^-1) complexes compared to the previously reported pentaphenyl Cp complex (18.9 kJ mol^-1). The results indicate that the activation enthalpy is the primary contributor to the overall activation energy, suggesting that the bulky substituents increase the rotational barrier by occupying the spatial gap between the pyrazole rings of the tripodal ligand. This is confirmed by theoretical calculations and the characterization of the minimum energy paths and transition states for each species. These findings offer valuable guidance for the molecular design of STM-operable molecular motors that can function at or near ambient temperature instead of the typical extremely low-temperature conditions necessary to suppress random molecular motion caused by thermal excitation.