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

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.