Electron-Rich N‑Heterocyclic Silylene (NHSi)–Iron Complexes: Synthesis, Structures, and Catalytic Ability of an Isolable Hydridosilylene–Iron Complex

The
first electron-rich N-heterocyclic silylene (NHSi)–iron(0)
complexes are reported. The synthesis of the starting complex is accomplished
by reaction of the electron-rich Fe0 precursor [(dmpe)2Fe­(PMe3)] 1 (dmpe =1,2-bis­(dimethylphosphino)­ethane)
with the N-heterocyclic chlorosilylene LSiCl (L =
PhC­(NtBu)2) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)2Fe­(←:Si­(Cl)­L)] 3. Reaction of in situ
generated 3 with MeLi afforded [(dmpe)2Fe­(←:Si­(Me)­L)] 4 under salt metathesis reaction, while its reaction with
Li­[BHEt3] yielded [(dmpe)2Fe­(←:Si­(H)­L)] 5, a rare example of an isolable SiII hydride complex
and the first such example for iron. All complexes were fully characterized
by spectroscopic means and by single-crystal X-ray diffraction analyses.
DFT calculations further characterizing the bonding situation between
the SiII and Fe0 centers were also carried out,
whereby multiple bonding character is detected in all cases (Wiberg
Bond Index >1). For the first time, the catalytic activity of a
SiII hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of
ketones in the presence of (EtO)3SiH as a hydridosilane
source. In most cases excellent conversions to the corresponding alcohols
were obtained after workup. The reaction pathway presumably involves
a ketone-assisted 1,2-hydride transfer from the SiII to
Fe0 center, as a key elementary step, resulting in a betaine-like
silyliumylidene intermediate. The appearance of the latter intermediate
is supported by DFT calculations, and a mechanistic proposal for the
catalytic process is presented.