Transformation of Dewar Metallabenzenes: Valence Bond Isomerization to Metallabenzenes and Beyond.

The study of valence isomerism in aromatic systems has long provided fundamental insights into molecular stability, reactivity, and electronic structure. While the mechanism and possibility of the interconversion between benzene and its Dewar isomer are well established in organic chemistry, analogous transformations in metallaaromatic systems remain largely unexplored. Here, we report the first experimental observations with well-defined Dewar metallabenzenes that are consistent with their rearrangement into metallabenzenes, a process previously considered only computationally. Structurally characterized Dewar rhenabenzenes can undergo a unique rearrangement to yield cyclopentadienyl complexes. DFT studies support a mechanism involving retro-4π electrocyclization of Dewar rhenabenzenes to give metallabenzene intermediates en route to cyclopentadienyl complexes, although the intermediates could not be detected in the present study. The computational results reveal that the retro-4π electrocyclization is both thermodynamically favorable and kinetically accessible and is the rate-determining event of the overall transformation.