Electron-rich N-heterocyclic silylene (NHSi)-iron complexes: Synthesis, structures, and catalytic ability of an isolable hydridosilylene-iron complex

Article abstract of DOI:10.1021/ja402480v

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(PMe ₃)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N^tBu)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 Si^II 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 Si^II 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 Si^II 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 Si^II 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.