Solvent-Triggered Cis/Trans Isomerism in Cobalt Dioxolene Chemistry: Distinguishing Effects of Packing on Valence Tautomerism

In this article,
the synthesis and X-ray crystal structures of
two cis/trans isomers of valence tautomeric (VT) cobalt dioxolene
compounds are reported. The cis isomer (1) was isolated
from the polar protic methanol solvent as a kinetic product, whereas
the less polar nonprotic solvent acetone yielded the trans isomer
(2). It should be noted that, although some coordination
polymers involving cobalt bis­(dioxolene) with the cis disposition
are known for bridging ancillary ligands, such an arrangement is unprecedented
for mononuclear compounds. A careful study of intermocular interactions
revealed that the methanol solvent does not have much influence on
the crystal growth in 1, whereas acetone forms strong
halogen-bonding interactions that are crucial in the solid-state architecture
of 2. This behavior can likely be used in crystal engineering
to design new organic–inorganic hybrid materials. The energy
difference between the two isomers was examined using DFT calculations,
confirming that the trans form is in the thermodynamic state whereas
the cis isomer is a kinetic product that can be converted into the
trans isomer with time. Finally, both isomers exhibit solvent loss
at elevated temperatures that is accompanied by a change in magnetic
properties, associated with an irreversible valence tautomerism. Our
results highlight the crucial role of the solvents for the isolation
of cis/trans isomers in cobalt dioxolene chemistry, as well as the
distinguishing effects of intermolecular forces and the solid-state
packing on VT behavior.

本文报道了两种价互变异构（valence tautomeric, VT）二氧杂环戊烯合钴类化合物的顺/反异构体的合成与X射线晶体结构。顺式异构体（1）以动力学产物形式从极性质子性甲醇溶剂中分离得到，而低极性非质子性溶剂丙酮则得到反式异构体（2）。值得注意的是，尽管已有报道涉及采用桥联辅助配体的双(二氧杂环戊烯)合钴配位聚合物采取顺式排布，但单核化合物中出现此类排布尚属首次。对分子间相互作用的细致研究表明，甲醇溶剂对1的晶体生长影响甚微，而丙酮则形成了强卤键相互作用，这对2的固态结构至关重要。该现象有望应用于晶体工程，用于设计新型有机-无机杂化材料。通过密度泛函理论（DFT）计算对两种异构体的能量差进行了考察，结果证实反式异构体处于热力学稳定状态，而顺式异构体为动力学产物，可随时间转化为反式异构体。最后，两种异构体在升温条件下均会发生溶剂流失现象，同时伴随磁性质改变，该变化与不可逆价互变异构相关。本研究结果凸显了溶剂在二氧杂环戊烯合钴化学中分离顺/反异构体的关键作用，以及分子间作用力和固态堆积对价互变异构行为的显著影响。