Photochromism of an Organorhodium Dithionite Complex in the Crystalline-State: Molecular Motion of Pentamethylcyclopentadienyl Ligands Coupled to Atom Rearrangement in a Dithionite Ligand

In the crystalline state, the rhodium dinuclear complex [(RhCp*)2(μ-CH2)2(μ-O2SSO2)] (1) with a photoresponsive dithionite group (μ-O2SSO2) and two pentamethylcyclopentadienyl ligands (Cp* = η5-C5Me5) undergoes a 100% reversible unimolecular type T inverse photochromism upon interconversion to [(RhCp*)2(μ-CH2)2(μ-O2SOSO)] (2). The photochromism can be followed directly by using stepwise crystal structure analysis (Angew. Chem., Int. Ed. 2006, 45, 6473). In this study, we found that the photoreaction of 1 was triggered by absorption of the 510 nm light (charge transfer band from σ(S−S) to σ*(S−S) and σ*(Rh−Rh) orbitals assigned by DFT calculation) and included two important processes: kinetically controlled oxygen-atom transfer to produce four stereoisomers of 2 and thermodynamically controlled isomerization between the four stereoisomers of 2 to afford the most stable isomer. Although the formation rate of the four stereoisomer products was kinetically controlled and the population of the four stereoisomers produced in the system was thermodynamically controlled, both processes were regulated by the steric hindrance between the μ-O2SSO2 or μ-O2SOSO ligand and the reaction cavity formed by the Cp* ligands. The cooperation of both processes achieved an intriguing stereospecific oxygen-atom rearrangement to produce only one stereoisomer of 2 at the final stage of the photoreaction at room temperature. We also determined the effect of the oxygen-atom rearrangement on the rotational motion of the two crystallographically independent Cp* ligands (parallel and perpendicular arrangement). Using variable-temperature 13C CP/MAS NMR and quadrupolar echo solid-state 2H NMR spectroscopies, before photoirradiation, the activation energies for the rotation of the parallel and perpendicular Cp* ligands in 1 were determined to be 33 ± 3 and 7.8 ± 1 kJ/mol, respectively, and after photoirradiation, in 2, they were much lower than those in 1 (21 ± 2 and 4.7 ± 0.5 kJ/mol, respectively). The large decrease in the activation energy for the parallel Cp* in 2 is attributed to the relaxation of molecular stress via a stereospecific oxygen-atom rearrangement, which suggests that the rotational motion of the Cp* ligands is coupled to the photochromism.

在晶态下，携带光响应连二亚硫酸盐基团（μ-O₂SSO₂，photoresponsive dithionite group）与两个五甲基环戊二烯基配体（pentamethylcyclopentadienyl ligand，Cp* = η⁵-C₅Me₅）的铑双核配合物[(RhCp*)₂(μ-CH₂)₂(μ-O₂SSO₂)] (1)，在转化为配合物[(RhCp*)₂(μ-CH₂)₂(μ-O₂SOSO)] (2)时，会发生100%可逆的单分子反T型光致变色。该光致变色过程可通过逐步晶体结构分析直接追踪（Angew. Chem., Int. Ed. 2006, 45, 6473）。

本研究发现，配合物(1)的光反应由510 nm光的吸收触发（经密度泛函理论（Density Functional Theory, DFT）计算归属的σ(S−S)→σ*(S−S)与σ(Rh−Rh)→σ*(Rh−Rh)轨道间电荷转移带），且包含两个关键过程：动力学控制的氧原子转移过程，用于生成配合物(2)的四种立体异构体；以及热力学控制的异构化过程，在配合物(2)的四种立体异构体之间进行，最终得到最稳定的异构体。

尽管四种立体异构体产物的生成速率受动力学控制，体系中生成的四种立体异构体的占比受热力学控制，但两个过程均受μ-O₂SSO₂或μ-O₂SOSO配体与Cp*配体形成的反应空腔之间的位阻调控。两个过程的协同作用实现了引人关注的立体专一性氧原子重排，使得室温下光反应的最终阶段仅生成配合物(2)的一种立体异构体。

本研究还探明了氧原子重排对两种晶体学独立Cp*配体（平行与垂直排布）旋转运动的影响。采用变温¹³C交叉极化/魔角旋转核磁共振（13C Cross Polarization/Magic Angle Spinning NMR, ¹³C CP/MAS NMR）与四极回波固态²H核磁共振（quadrupolar echo solid-state ²H NMR）光谱技术，研究人员测得光辐照前，配合物(1)中平行与垂直排布的Cp*配体旋转活化能分别为33 ± 3 kJ/mol与7.8 ± 1 kJ/mol；光辐照后，配合物(2)中的活化能远低于配合物(1)（分别为21 ± 2 kJ/mol与4.7 ± 0.5 kJ/mol）。配合物(2)中平行排布Cp*配体的活化能大幅降低，归因于立体专一性氧原子重排带来的分子应力松弛，这表明Cp*配体的旋转运动与光致变色过程耦合相关。