π-Complexes Derived from Non-classical Diboriranes: Side-on vs. End-on Carbonylative Ring Expansion

Unlike cyclopropanes, the analogous B₂C species (diboriranes) tend to adopt non-classical Hückel-aromatic structures with bridging moieties R between the boron atoms. The coordination of the thus generated cyclic 2e^- π-system to transition metals is completely unexplored. We here report that complexation of non-classical diboriranes cyclo-μ-RB₂Dur₂CPh (R=H, SnMe₃; Dur=2,3,5,6-tetramethylphenyl) to Fe(CO)₃ fragments allows for the carbonylative ring expansion of the B₂C ring to either four- or five-membered rings depending on the nature of the BRB 3-center-2-electron bond (3c2e): The H-bridged diborirane (R=H) initially reacts with Fe₂(CO)₉ to the allylic π-complex with an agostic BH/Fe interaction. Subsequent formal hydroboration of CO from excess Fe₂(CO)₉ results in the side-on ring expansion under formation of a five-membered B₂C₂O ring, coordinated to the Fe(CO)₃ moiety. In contrast, in case of the stannyl-bridged diborirane (R=SnMe₃) under the same conditions, CO is added end-on to the B-B bond with the carbon terminus formally inserting into the B₂Sn 3c2e-bond. The two carbonylative ring expansion products can also be described as nido and closo clusters, respectively, according to the Wade-Mingos rules.