Bonding and Reactivity of a Diborabutadiene Fe Complex Synthesized by Boron–Boron Bond Activation

Abstract

Though precious-metal multiboryl complexes feature prominently in catalytic C–H borylation schemes, the corresponding first-row transition multiboryl species have been comparatively underexplored. We set out to explore the iron coordination chemistry of phenylene-bridged diboryl chelates by targeting oxidative addition of strained cyclic diboron reagents. Under UV photolysis, Fe(CO)5 reacts with a four-membered 1,2-diboracycle via B–B bond cleavage to afford an Fe complex that is most consistent with a 1,4-dibora-1,3-butadiene Fe(0) tricarbonyl complex rather than the target diboryl Fe(II) tetracarbonyl complex. The unsaturated phenylene linker is essential for favoring this bonding arrangement. The Fe tricarbonyl product readily undergoes ligand substitution of CO by PMe3 and PMe2Ph to form diboryl iron dicarbonyl phosphine complexes. All three Fe complexes were characterized by IR and NMR spectroscopy, single-crystal X-ray diffraction, high-resolution mass spectrometry, and cyclic voltammetry, which allow for comparisons to be drawn between foundational Fe tricarbonyl diene complexes and their diboradiene analogues.