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

Low-valent group 14 element hydride chemistry: towards catalysis

The chemistry of group 14 element(ii) hydride complexes has rapidly expanded since the first stable example of such a compound was reported in 2000. Since that time it has become apparent that these systems display remarkable reactivity patterns, in some cases mimicking those of late transition-metal (TM) hydride compounds. This is especially so for the hydroelementation of unsaturated organic substrates. Recently, this aspect of their reactivity has been extended to the use of group 14 element(ii) hydrides as efficient, "TM-like" catalysts in organic synthesis. This review will detail how the chemistry of these hydride compounds has advanced since their early development. Throughout, there is a focus on the importance of ligand effects in these systems, and how ligand design can greatly modify a coordinated complex's electronic structure, reactivity, and catalytic efficiency.

An arene-tethered silylene ligand enabling reversible dinitrogen binding to iron and catalytic silylation

A silylene–iron(0) dinitrogen complex enabled the catalytic silylation of N₂ with high activity.

Silicon chemistry in zero to three dimensions: from dichlorosilylene to silafullerane

As one of the simplest examples of functionalized Si(ii) species, the SiCl₂/[SiCl₃]⁻ system is not only fundamentally interesting, but also an important starting point for the assembly of oligosilane chains, rings, and clusters.

A new anthraquinoid ligand for the iron-catalyzed hydrosilylation of carbonyl compounds at room temperature: new insights and kinetics

The synthesis and characterization of a highly active Fe(ii) catalyst for the hydrosilylation of aldehydes and ketones have been described.

Constructing reactive Fe and Co complexes from isolated picolyl-functionalized N-heterocyclic carbenes

The Fe(ii) and Co(ii) complexes prepared from the isolated free carbene form of the picolyl-NHC ligands display excellent catalytic activity towards the hydrosilylation of ketones.

Synthesis and catalytic activity of N-heterocyclic silylene (NHSi) cobalt hydride for Kumada coupling reactions

The first silylene Co(iii) hydride6, an efficient catalyst for Kumada cross-coupling reactions, was preparedviaSi–H activation.

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

Hydrosilylation reactions, the (commonly) anti-Markovnikov additions of silanes to unsaturated bonds present in compounds such as alkenes and alkynes, offer numerous unique and advantageous properties for the preparation of polymeric materials, such as high yields and stereoselectivity. These reactions require to be catalyzed, for which platinum compounds were used in the initial stages. Celebrating the 50th anniversary of hydrosilylations in polymer science and, concomitantly, five decades of continuously growing research, hydrosilylation reactions have advanced to a level that renders them predestined for transfer into commercial products on the large scale. Facing this potential transfer, this review addresses and discusses selected current trends of the scientific research in the area, namely low-cost transition metal catalysts (focusing on iron, cobalt, and nickel complexes), metal-free catalysts, non-thermally triggered hydrosilylation reactions (highlighting stimuli such as (UV-)light), and (potential) industrial applications (highlighting the catalysts used and products manufactured). This review focuses on the hydrosilylation reactions involving alkene reactants.

First Insertions of Carbene Ligands into Ge-N and Si-N Bonds

The insertion of carbene ligands into Ge-N (three examples) and Si-N (one example) bonds has been achieved for the first time by treating Fischer carbene complexes (M=W, Cr) with bulky amidinatotetrylenes (E=Ge, Si). These reactions, which start with a nucleophilic attack of the amidinatotetrylene heavier group 14 atom to the carbene C atom, proceed through a stereoselective insertion of the carbene fragment into an E-N bond of the amidinatotetrylene ENCN four-membered ring, leading to [M(CO)₅ L] derivatives in which L belongs to a novel family of tetrylene ligands comprising an ECNCN five-membered ring.

Reactivity of an amidinato silylene and germylene toward germanium(ii), tin(ii) and lead(ii) halides

The coordination chemistry of an amidinato silylene and germylene toward group 14 element(ii) halides is described. The reaction of the amidinato silicon(ii) amide [LSiN(SiMe₃)₂] (1, L = PhC(NtBu)₂) with SnCl₂ and PbBr₂ afforded the amidinato silylene-dichlorostannylene and -dibromoplumbylene adducts [L{(Me₃Si)₂N}SiEX₂] (E = Sn, X = Cl (2); E = Pb, X = Br (3)), respectively, in which there is a lone pair of electrons on the Sn(ii) and Pb(ii) atoms. X-ray crystallography, NMR spectroscopy and theoretical studies show conclusively that the Si(ii)-E(ii) bonds are donor-acceptor interactions. Similar electronic structures were found in the amidinato germylene-dichlorogermylene and -dichlorostannylene adducts [L{(Me₃Si)₂N}GeECl₂] (E = Ge (5), Sn (6)), which were prepared by treatment of the amidinato germanium(ii) amide [LGeN(SiMe₃)₂] (4) with GeCl₂·dioxane and SnCl₂, respectively.

Unprecedented solvent induced inter-conversion between monomeric and dimeric silylene–zinc iodide adducts

The reaction of silylene [PhC(NtBu)₂SiN(SiMe₃)₂] with ZnI₂leads to both monomeric [PhC(NtBu)₂Si{N(SiMe₃)₂} → ZnI₂]·THF (1) and dimeric [PhC(NtBu)₂Si{N(SiMe₃)₂} → ZnI₂]₂(2) adducts which are inter-convertible.

Strikingly diverse reactivity of structurally identical silylene and stannylene

The divergent reactivity of structurally similar silylene and stannylene was observed with coinage metal halides. While the former tends to form an adduct, the latter undergoes ligand exchange reaction.

Bis(silylenyl)-substituted ferrocene-stabilized η6-arene iron(0) complexes: synthesis, structure and catalytic application

The synthesis of the first ferrocene bis(NHSi)-stabilized iron(arene) complexes and their application in the catalytic hydrogenation of ketones is reported.

The Si2H radical supported by two N-heterocyclic carbenes

Cyclic voltammetric studies of the hydridodisilicon(0,II) borate [(Idipp)(H)SiII[double bond, length as m-dash]Si0(Idipp)][B(ArF)4] (1H[B(ArF)4], Idipp = C[N(C6H3-2,6-iPr2)CH]2, ArF = C6H3-3,5-(CF3)2) reveal a reversible one-electron reduction at a low redox potential (E 1/2 = -2.15 V vs. Fc+/Fc). Chemical reduction of 1H[B(ArF)4] with KC8 affords selectively the green, room-temperature stable mixed-valent disilicon(0,I) hydride Si2(H)(Idipp)2 (1H), in which the highly reactive Si2H molecule is trapped between two N-heterocyclic carbenes (NHCs). The molecular and electronic structure of 1H was investigated by a combination of experimental and theoretical methods and reveals the presence of a π-type radical featuring a terminal bonded H atom at a flattened trigonal pyramidal coordinated Si center, that is connected via a Si-Si bond to a bent two-coordinated Si center carrying a lone pair of electrons. The unpaired electron occupies the Si[double bond, length as m-dash]Si π* orbital leading to a formal Si-Si bond order of 1.5. Extensive delocalization of the spin density occurs via conjugation with the coplanar arranged NHC rings with the higher spin density lying on the site of the two-coordinated silicon atom.

Stepwise isolation of an unprecedented silylene supported dinuclear gold(i) cation with aurophilic interaction

Isolation of the first Si(ii) supported dinuclear Au(i) cation with an intramolecular Au⋯Au interaction of 2.875(1) Å has been realized.

Peripheral mechanism of a carbonyl hydrosilylation catalysed by an SiNSi iron pincer complex

Combined experimental and theoretical analysis of the carbonyl hydrosilylation catalysed by an iron(0) pincer complex reveals an unprecedented mechanism of action. The iron(0) complex is in fact a precatalyst that is converted into an iron(ii) catalyst through oxidative addition of a hydrosilane. Neither the hydrogen atom nor the silicon atom bound to the iron(ii) centre are subsequently transferred onto the carbonyl acceptor, instead remaining at the sterically inaccessible iron(ii) atom throughout the catalytic cycle. A series of labelling, crossover and competition experiments as well as the use of a silicon-stereogenic hydrosilane as a stereochemical probe suggest that the iron(ii) site is not directly involved in the hydrosilylation. Strikingly, it is the silyl ligand attached to the iron(ii) atom that acts as a Lewis acid for carbonyl activation in this catalysis. The whole catalytic process occurs on the periphery of the transition metal. Computation of the new peripheral as well as plausible alternative inner and outer sphere mechanisms support the experimental findings.

Unprecedented silicon(ii)→calcium complexes with N-heterocyclic silylenes

The first N-heterocyclic silylene (NHSi) complexes of any s-block element to date are reported for calcium: [(η⁵-C₅Me₅)₂Ca←:Si(O-C₆H₄-2-^tBu){(N^tBu)₂CPh}] and [(η⁵-C₅Me₅)₂Ca←:Si(N^tBuCH)₂]. The synthesis, structure, bonding analysis by DFT methods and the reactivity towards oxygen containing substrates is discussed.