Electronic structure of mono(Lewis base)-stabilized borylenes

This work demonstrates that the mono(Lewis base)-stabilized three-center-six-electron (3c-6e) or LB → B-R type borylenes may have a ground state singlet (σ²π⁰ and σ⁰π²) or triplet (σ¹π¹) electronic structure, depending on the substitution group, Lewis base, and molecule topology (cyclic or acyclic). The singlet-triplet energy gap of a borylene increases with electron-donating substitution groups and decreases with Lewis bases of strong σ-donating and π-accepting feature (e.g., carbenes and pyridine). The gap can be widely varied (ranging up to about 300 kJ mol^-1) and even inversed by the proper combination of substitution groups and Lewis bases. These borylenes may even have singlet-and-triplet coexisting ground state electronic structures, which imply their complex reactivity.