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 m-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 m-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 (ΔG⧧) 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.