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 ^(Ph)DPB^(Mes) ligands in the context of H_2 and silane activation and catalysis (DPB = bis(o-diisopropylphosphinophenyl)phenylborane, ^(Ph)DPB^(Mes) = bis(o-diphenylphosphinophenyl)mesitylborane), we now explore the reactivity of iron, [(DPB)Fe]_2(N_2), 1, and cobalt, (DPB)Co(N_2), 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 (^(Ph)DPB^(Mes))Ni. Indeed, 2 appears to be the most active homogeneous cobalt catalyst reported to date for the hydrosilylation of acetophenone under the conditions studied.
© 2015 American Chemical Society. Received: June 18, 2015; Publication Date (Web): August 28, 2015. The authors would like to acknowledge the financial support provided by the NSF Center for Chemical Innovation (CCI; CHE-1305124) and the Gordon and Betty Moore Foundation. The authors thank Dr. Michael Takase and Lawrence Henling at the Caltech X-ray crystallographic facility for assistance with XRD studies. The authors also thank Dr. Tzu-Pin Lin for editorial assistance and insightful suggestions. This contribution is dedicated to the fond memory of Professor Gregory L. Hillhouse. The authors declare no competing financial interest.
Supplemental Material - om5b00530_si_001.pdf
Supplemental Material - om5b00530_si_002.cif