Neutral Diboron-Containing Heterocumulenes

The development of boron-containing heterocumulenes has attracted considerable recent interest owing to their intriguing properties and potential for applications unavailable in all-carbon systems. While the chemistry of multiply bonded boron species has been well established, cumulene systems involving B-B multiple bonds remain extremely rare. Herein we report the syntheses, characterizations and reactivities of two neutral heterocumulenes, notable for their CNBB architectures built around unsymmetrical B(sp)═B(sp²) units. Remarkably, the geometries of these cumulenes can be tailored by changing the π-acceptor ability of the supported ligands. X-ray diffraction analysis revealed that the cumulene supported by an N-heterocyclic carbene (NHC) exhibits a bent structure, whereas the cumulene stabilized by the more π-acidic (cyclic)(alkyl)(amino)carbene (cAAC) displays a nearly linear configuration. Notably, these cumulenes exhibit divergent reactivity toward the cleavage of homoatomic element-element bonds in bis(catecholato)diboron (B₂cat₂) and diphenyl diselenide (PhSeSePh), affording benzodioxadiborane derivatives and selenoboranes, respectively. Furthermore, the reaction with bromopentafluorobenzene (C₆F₅Br) involves the formal addition of the C(sp²)-Br bond across the unsaturated B-B bond of the CNBB units. Their potential as supporting ligands was also demonstrated through the formation of stable complexes with CuBr and CuI, where the metal centers are ligated by the B₂ units.