Direct Interconversion between Bis(borylenes) and Diborenes

Over the past two decades, the chemistry of borylenes and their formal dimer, diborenes, has witnessed remarkable advances. However, direct evidence for a Wanzlick-type equilibrium between borylenes and diborenes has remained elusive. Here, we report CAAC-stabilized (CAAC = cyclic (alkyl)(amino)carbene) mono- and disubstituted ferrocene-based borylene systems. The monosubstituted ferrocene-based borylene undergoes intramolecular C-H, CO, and H₂ activation, with the C-H activation products existing in equilibrium. Most notably, the disubstituted ferrocene-based bis(borylene) undergoes direct coupling to form a diborene featuring a unique delocalized four-center-two-electron (4c-2e) π bond. Remarkably, this diborene reversibly dissociates back into the bis(borylene), which can be trapped with CO or via intramolecular C-H activation. The electronic and rotational flexibility of the ferrocene scaffold plays a pivotal role in facilitating the reversible coupling and decoupling between these bis(borylene) and diborene species. Furthermore, the ferrocenyl diborene species exhibits distinctive photophysical properties, redox behavior and reactivities.