Crystalline Neutral Diboron Analogues of Cyclopropanes

Although Schleyer's computations in 1979 predicted that the ground state of the parent diborirane features a planar-tetracoordinate carbon atom (anti van't Hoff–Le Bel geometry), this work demonstrates that substitution of C coupled with N-heterocyclic carbene (NHC) coordination provides access to isolable diborirane derivatives 3 and 4 with van't Hoff–Le Bel geometry. Species 3 and 4 are isoelectronic with cyclopropane and the highly strained BC rings feature 2c–2e bent σ bonds. Consequently, the B−B and B−C bonds in 3 are cleaved in the presence of hydride, proton, and chalcogens. The former two reactions gave NHC-coordinated fluorenyldihydridoborane 5 and dichlorofluorenylborane 6, respectively, whereas the latter transformations afforded novel thiaborirane 8 and selenaborirane 9. In addition, transfer hydrogenation of 3 with ammonia borane (HN⋅BH) led to the formation of (μ-hydrido)diborane 7 via selective cleavage of the B−B bond. These reactivities show potential for their future application in organic synthesis.