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.