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 a platform for 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 η -diborafulvene dianion. Salt metathesis of the latter with [(η -p-cymene)RuCl ] affords a mixed ruthenium sandwich complex of an η -borylborole dianion. Calculations highlight both the structural and electronic changes in the boron-substituted heterocyclic C B dianion upon switching coordination from magnesium (diborafulvene dianion) to ruthenium (borylborole dianion).