Thienyl‐Substituted Diboranes(4): Electronic Stabilization of Radicals Versus Increased Reactivity towards Bond Activation

Abstract

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 η5‐diborafulvene dianion. Salt metathesis of the latter with [(η6‐p‐cymene)RuCl2] affords a mixed ruthenium sandwich complex of an η5‐borylborole dianion. Calculations highlight both the structural and electronic changes in the boron‐substituted heterocyclic C4B dianion upon switching coordination from magnesium (diborafulvene dianion) to ruthenium (borylborole dianion).