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

Abstract

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 Fe(0) precursor [(dmpe)2Fe(PMe3)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N(t)Bu)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 Fe(0) 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 Fe(0) 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.

Fifty Years of Hydrosilylation in Polymer Science: A Review of Current Trends of Low-Cost Transition-Metal and Metal-Free Catalysts, Non-Thermally Triggered Hydrosilylation Reactions, and Industrial Applications

“…To the best of our knowledge, this is the broadest range of substrates for one type of iron-based catalysts. By combined hydrosilylation experiments and density functional theory DFT calculations, Metaänen, Gallego and coworkers [22] investigated the hydrosilylation of a silylene/iron complex that was developed by Driess and coworkers [23]. According to these combined studies, a peripheral mechanism that does not directly involve the metal core was proposed for the hydrosilylation of ketones ( Figure 8).…”

Ring Opening and Bidentate Coordination of Amidinate Germylenes and Silylenes on Carbonyl Dicobalt Complexes: The Importance of a Slight Difference in Ligand Volume

“…In the last few years, new generations of cyclic heavier tetrylene molecules, particularly silylenes and germylenes stabilized by amidinate, β‐diketiminate, and other chelating fragments,4a have allowed a significant advance of the tetrylene–TM chemistry. Among them, the most studied ones have been those derived from amidinates, which form isolatable tetrylene–TM complexes with almost all of the groups of the TM series,9 some of which have already been successfully tested as catalyst precursors for useful reactions,10 such as Sonogashira cross‐couplings,9a ketone hydrosilylations,9b [2+2+2] cycloadditions,9c arene CH borylations,9d and cross‐coupling reactions of aryl halides with organometallic zinc and Grignard reagents 9w…”

“…[8] In the last few years, new generations of cyclic heavier tetrylene molecules, particularly silylenes and germylenes stabilized by amidinate, b-diketiminate, and other chelating fragments, [4a] have allowed a significant advance of the tetrylene-TM chemistry. Among them, the most studied ones have been those derived from amidinates, which form isolatable tetrylene-TM complexes with almost all of the groups of the TM series, [9] some of which have already been successfully tested as catalyst precursors for useful reactions, [10] such as Sonogashira cross-couplings, [9a] ketone hydrosilylations, [9b] [2+2+2] cycloadditions, [9c] arene CÀH borylations, [9d] and cross-coupling reactions of aryl halides with organometallic zinc and Grignard reagents. [9w] As part of our work on tetrylene-TM chemistry, [11] we have preliminary communicated that the amidinate-germylene [Ge-(iPr 2 bzam)(HMDS)] (1; iPr 2 bzam = N,N'-bis(isopropyl)benzamidinate), which is equipped with a very bulky hexamethyldisilazane group (HMDS) and armed with just one accessible lone pair of electrons on the Ge atom, can be transformed into a bridging 4-electron-donor k 2 Ge,N-ligand when treated with [Co 2 (CO) 8 ].…”

Chelating N-Heterocyclic Silylenes as Steering Ligands in Catalysis

“…22 This study suggested a cooperative relationship between the hydrido-silylene ligand and the Fe(0) center, which mediates the catalytic transformation in a very unique way. We were guided by our earlier work where we demonstrated that the complex [(dmpe) 2 Fe( :Si(H)L)] (27) (dmpe ¼ bis(dimethylphosphino)ethane; L ¼ N,N 0 -di-tertbutyl(phenylamidinato)) is very active in the hydrosilylation of a range of ketones to alcohols in the presence of a suitable hydride source.…”