Isomerization Mechanism in Hydrazone-Based Rotary Switches: Lateral Shift, Rotation, or Tautomerization?

Two intramolecularly hydrogen-bonded arylhydrazone (aryl = phenyl or naphthyl) molecular switches have been synthesized, and their full and reversible switching between the E and Z configurations have been demonstrated. These chemically controlled configurational rotary switches exist primarily as the E isomer at equilibrium and can be switched to the protonated Z configuration (Z-H+) by the addition of trifluoroacetic acid. The protonation of the pyridine moiety in the switch induces a rotation around the hydrazone CN double bond, leading to isomerization. Treating Z-H+ with base (K2CO3) yields a mixture of E and “metastable” Z isomers. The latter thermally equilibrates to reinstate the initial isomer ratio. The rate of the Z → E isomerization process showed small changes as a function of solvent polarity, indicating that the isomerization might be going through the inversion mechanism (nonpolar transition state). However, the plot of the logarithm of the rate constant k vs the Dimroth parameter (ET) gave a linear fit, demonstrating the involvement of a polar transition state (rotation mechanism). These two seemingly contradicting kinetic data were not enough to determine whether the isomerization mechanism goes through the rotation or inversion pathways. The highly negative entropy values obtained for both the forward (E → Z-H+) and backward (Z → E) processes strongly suggest that the isomerization involves a polarized transition state that is highly organized (possibly involving a high degree of solvent organization), and hence it proceeds via a rotation mechanism as opposed to inversion. Computations of the Z ↔ E isomerization using density functional theory (DFT) at the M06/cc-pVTZ level and natural bond orbital (NBO) wave function analyses have shown that the favorable isomerization mechanism in these hydrogen-bonded systems is hydrazone–azo tautomerization followed by rotation around a C–N single bond, as opposed to the more common rotation mechanism around the CN double bond.

已合成两种芳基为苯基或萘基的分子内氢键键合芳基腙（arylhydrazone）分子开关，并证实其可在E型与Z型构型之间实现完全且可逆的切换。这类化学调控型构型旋转分子开关在平衡状态下主要以E型异构体形式存在；加入三氟乙酸（trifluoroacetic acid）后，可切换至质子化Z型构型（Z-H+）。开关中的吡啶基团（pyridine moiety）发生质子化，会诱导腙的C=N双键发生旋转，进而引发异构化反应。向Z-H+中加入碱（碳酸钾，K₂CO₃）处理后，会得到E型与“亚稳态”Z型异构体的混合物；该亚稳态Z型异构体可通过热平衡过程恢复至初始的异构体占比。

Z→E型异构化过程的速率随溶剂极性变化幅度极小，提示该异构化可能通过翻转机制进行，其过渡态为非极性。然而，以速率常数k的对数对迪姆罗参数（Dimroth parameter，ET）作图，得到了线性拟合曲线，证明其过渡态具有极性，对应旋转机制。这两组看似矛盾的动力学数据，尚无法明确该异构化是通过旋转还是翻转路径完成。正向（E→Z-H+）与逆向（Z→E）过程均得到高度负的熵变值，强烈表明该异构化涉及高度有序的极化过渡态（可能伴随溶剂分子的高度有序排列），因此相较于翻转机制，其更倾向于通过旋转机制进行。

采用密度泛函理论（DFT）在M06/cc-pVTZ基组水平下对Z↔E型异构化进行计算，并结合自然键轨道（NBO）波函数分析，结果显示：这类氢键键合体系中，最有利的异构化路径为腙-偶氮互变异构化，随后绕C-N单键旋转，而非更为常见的绕C=N双键旋转的机制。