Thienyl-substituted diboranes(4): electronic stabilization of radicals versus increased reactivity towards bond activation

Low-valent main-group chemistry involves a balancing act between steric and electronic stabilization of the electron-rich low-oxidation-state main-group centers and their desired reactivity. Herein we show that the combination of sterically-shielding mesityl and rotationally flexible 2-thienyl groups, the latter having the potential to be either electronically stabilizing or activating, at a diborane(4) provides both radical-anion stabilization and unusual bond activation and rearrangement reactions. The addition of a Lewis base to a 1,2-dimesityl-1,2-dithienyldiborane(4) (1) results in direct and unprecedented C?H borylation of one thienyl substituent with cleavage of the B?B bond. The facile one-electron reduction of 1 yields a stable diboron radical anion through delocalization of its unpaired electron over the entire planar 1,2-dithienyldiboron framework, as evidenced by EPR spectroscopy and DFT calculations. The two-electron reduction of 1 with magnesium-anthracene under more forcing conditions results in B?B-bond cleavage and replacement of one thienyl sulfur atom by a mesitylboron moiety, leading to the formation of a magnesium complex of an ?5-diborafulvene dianion. Salt metathesis of the latter with [(?6-p-cymene)RuCl2] affords a mixed ruthenium sandwich complex of an ?5-borylborole dianion. Calculations highlight the structural and electronic changes in the boron-substituted heterocyclic C4B dianion upon switching coordination from magnesium (diborafulvene dianion) to ruthenium (borylborole dianion).