Single Crystal X‑ray Diffraction Studies of Two Polymorphic Modifications of the Dicarbonyl‑o‑Semiquinonato Rhodium Complex at Different Temperatures. Destruction Stimulated by Cooling Versus Stability

It was found that the dicarbonyl-rhodium-o-semiquinonate complex (which thread-like crystals can bend reversibly under light/warm activation) can form two polymorphic modifications: isometric prisms (1) and sticks (2). Some thin sticks can bend as mentioned above. X-ray diffraction studies of polymorphic modifications at different temperatures were carried out. It was found that crystals 1 are destructed after cooling to 110 K as opposed to crystals 2. In turn, the reversible phase transition is detected in 2. In both polymorphic modifications, stack packaging motifs through the direct Rh–Rh bond are observed. The principal difference between packages of polymorphic modifications is that molecules 1 in the adjacent stacks are shifted relative to each other along the stack, in contrast to crystal 2. It was found that different packing of stacks leads to different anisotropic compression of crystals 1 and 2 during cooling, which is a key factor of their stability. Using the molecular invariom approach, the nature of the chemical bonds and charge distribution was investigated; the energy of the Rh–Rh bonds was estimated.研究发现，二羰基-邻半醌合铑配合物（dicarbonyl-rhodium-o-semiquinonate complex）所形成的线状晶体可在光照/热激活下发生可逆弯曲，该配合物存在两种多晶型变体：等轴棱柱形（1型）与棒状（2型）。部分细棒状晶体可如前文所述发生弯曲。我们针对这两种多晶型变体开展了不同温度下的X射线衍射研究。结果显示，冷却至110 K后1型晶体发生结构破坏，而2型晶体则未出现该现象；与之相对，2型晶体中可检测到可逆相变。两种多晶型变体中均存在通过直接Rh-Rh键构筑的堆叠堆积基元。二者堆积方式的核心差异在于：相较于2型晶体，相邻堆叠中的1型分子沿堆叠轴方向发生相对位移。研究发现，堆叠方式的差异使得1型与2型晶体在冷却过程中呈现出不同的各向异性压缩行为，这是二者稳定性差异的关键因素。本研究采用分子不变量（molecular invariom）方法，对两种晶体的化学键本质与电荷分布开展了研究，并估算了Rh-Rh键的键能。