FeB Double Bonds: Synthetic, Structural, and Reaction Chemistry of Cationic Terminal Borylene Complexes

Application of halide abstraction chemistry to asymmetric haloboryl complexes (η5-C5Me5)Fe(CO)2B(ERn)X leads to the first synthetic route to cationic multiply bonded group 13
diyl species, [(η5-C5Me5)Fe(CO)2B(ERn)]+. The roles of steric bulk and π electron release within
the ERn substituent in generating tractable borylene complexes have been probed, as has
the nature of the counterion. A combination of spectroscopic, structural, and computational
techniques leads to the conclusion that the bonding in complexes such as [(η5-C5Me5)Fe(CO)2B(Mes)]+ is best described as an FeB double bond composed of B→Fe σ donor and
Fe→B π back-bonding components. An extended study of the fundamental reactivity of
cationic borylene systems reveals that this is dominated not only by nucleophilic addition
at boron but also by iron-centered substitution chemistry leading to overall displacement of
the borylene ligand.