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.

本研究设计并合成了一系列在环戊二烯基（Cp）配体上带有大位阻取代基的新型钢琴凳型钌配合物，旨在探究取代基结构对Cp-Ru键周围旋转行为的影响。研究引入间二甲苯基、均三甲苯基及9-蒽基等取代基，通过与三脚架型配体产生空间位阻以提升旋转能垒。核磁共振波谱分析显示，带有间二甲苯基的配合物中Cp-Ru键的旋转速率快于核磁共振时间尺度，而带有均三甲苯基和9-蒽基的配合物则表现出更慢的旋转行为。变温核磁共振测量与谱线线型拟合分析表明，为克服取代基与三脚架型配体间的空间位阻以实现Cp配体旋转，所需的活化自由能（ΔG⧧）在均三甲苯基配合物（69.5 kJ mol–1）和9-蒽基配合物（67.8 kJ mol–1）中，显著高于此前报道的五苯基Cp配合物（18.9 kJ mol–1）。研究结果表明，活化焓是总活化能的主要贡献来源，提示大位阻取代基通过占据三脚架型配体吡唑环之间的空间间隙，提升了旋转能垒。这一结论通过理论计算以及对各物种的最低能量路径和过渡态的表征得到了验证。本研究发现可为可在室温或接近室温条件下运行的扫描隧道显微镜（STM）用分子马达的分子设计提供宝贵指导——这类马达无需像传统分子马达那样依赖极低温度来抑制热激发引发的随机分子运动。