E–H Bond Activations and Hydrosilylation Catalysis with Iron and Cobalt Metalloboranes

An
exciting challenge in transition metal catalyst design is to
explore whether earth-abundant base metals such as Fe, Co, and Ni
can mediate two-electron reductive transformations that their precious
metal counterparts (e.g., Ru, Rh, Ir, and Pd) are better known to
catalyze. Organometallic metalloboranes are an interesting design
concept in this regard because they can serve as organometallic frustrated
Lewis pairs. To build on prior studies with nickel metalloboranes
featuring the DPB and PhDPBMes ligands in the
context of H2 and silane activation and catalysis (DPB
= bis­(o-diisopropylphosphinophenyl)­phenylborane, PhDPBMes = bis­(o-diphenylphosphinophenyl)­mesitylborane),
we now explore the reactivity of iron, [(DPB)­Fe]2(N2), 1, and cobalt, (DPB)­Co­(N2), 2, metalloboranes toward a series of substrates with E–H
bonds (E = O, S, C, N) including phenol, thiophenol, benzo­[h]­quinoline, and 8-aminoquinoline. In addition to displaying
high stoichiometric E–H bond activation reactivity, complexes 1 and 2 prove to be more active catalysts for
the hydrosilylation of ketones and aldehydes with diphenylsilane relative
to (PhDPBMes)­Ni. Indeed, 2 appears
to be the most active homogeneous cobalt catalyst reported to date
for the hydrosilylation of acetophenone under the conditions studied.