Reactions of Hydrosilanes with Transition-Metal Complexes: Formation of Stable Transition-Metal Silyl Compounds

Covalency-Driven Differences in the Hydrogenation Chemistry of Lanthanide- and Actinide-Based Frustrated Lewis Pairs

The electronic organization of Frustrated Lewis Pairs (FLPs) allows them to activate strong bonds in mechanisms that are usually free of redox events at the Lewis acidic site. The unique 6d/5f manifold of uranium could serve as an interesting FLP acceptor site, but to date FLP-like catalysis with actinide ions is unknown. In this paper, the catalytic, FLP-like hydrogenation reactivity of trivalent uranium complexes is explored in the presence of base-stabilized silylenes. Comparison to isoelectronic, isostructural lanthanide and thorium complexes lends insight into the electronic factors governing dihydrogen activation. Mechanistic studies of the uranium- and lanthanide-catalyzed hydrogenations are presented, including discussion of likely intermediates. Computational modeling of the f-element complexes, combined with experimental comparison to p-block Lewis acids, elucidates the relevance of steric hindrance to productive reactivity with dihydrogen. Consideration of the complete experimental and theoretical evidence provides a clear picture of the electronic and steric factors governing dihydrogen activation by these FLPs.

Cationic Bis(hydrosilane)-Coinage-Metal Complexes: Synthesis, Characterization, and Use as Catalysts for Outer-Sphere C=O Hydrosilylation Not Involving Metal Hydrides

The preparation of cationic bis(hydrosilane)-coinage-metal complexes by chloride abstraction from the neutral metal chloride precursors with Na[BArF 4] is described. Unlike previously reported hydrosilane-stabilized copper and silver complexes, the presented complexes are cationic and feature two bidentate (ortho-silylphenyl)phosphine ligands. These complexes were fully characterized by NMR spectroscopy and X-ray diffraction analysis, revealing that both Si-H bonds are activated by the Lewis acidic cationic metal center. The new complexes were found to be effective in catalytic carbonyl hydrosilylation, leading to the corresponding silyl ethers under mild conditions without the addition of an external base. Combined mechanistic control experiments and quantum chemical calculations support an ionic outer-sphere mechanism, in which a neutral metal alkoxide species instead of a metal hydride is the key intermediate that interacts with the silylcarboxonium ion to generate the silyl ether.

Recent Advances on the Chemistry of Transition Metal Complexes with Monoanionic Bidentate Silyl Ligands

The chemistry of transition-metal (TM) complexes with monoanionic bidentate (κ2-L,Si) silyl ligands has considerably grown in recent years. This work summarizes the advances in the chemistry of TM-(κ2-L,Si) complexes (L=N-heterocycle, phosphine, N-heterocyclic carbene, thioether, ester, silylether or tetrylene). The most common synthetic method has been the oxidative addition of the Si-H bond to the metal center assisted by the coordination of L. The metal silicon bond distances in TM-(κ2-L,Si) complexes are in the range of metal-silyl bond distances. TM-(κ2-L,Si) complexes have proven to be effective catalysts for hydrosilylation and/or hydrogenation of unsaturated molecules among other processes.

X-type silyl ligands for transition-metal catalysis

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.

Reductive Retrocyclization of a Mangana(II)cyclopentasilane to Form Manganese(0) Bis(η2-disilene) Complexes

Ligand-exchange reactions on a mangana(II)cyclopentasilane complex that contains two THF ligands with aryl isocyanides led to the formation of manganese(0) bis(η2-disilene) complexes via a retrocyclization. In stark contrast, ligand-exchange reactions with CNtBu, an N-heterocyclic carbene, or pyridine-based ligands furnished manganese(II) complexes wherein the manganacyclopentasilane framework remained intact. The thermolysis of the obtained bis(η2-disilene) complex in the presence of mesityl isocyanide led to the formation of a cyclotetrasilane via the formal dimerization of the two η2-disilene moieties. The insertion of a mesityl isocyanide into the Mn-Siβ bond results in the formation of a manganese(II) complex supported by a [SiCSi]-type tridentate ligand scaffold.

Planar three-coordinate iron(II) complexes supported by sterically demanding -Si(SiMe3)2(SiMe2tBu) ligands

Sterically demanding organosilyl ligands support the formation of coordinatively unsaturated complexes. In this study, we found that using the ligand -Si(SiMe3)2(SiMe2tBu) affords exclusively planar three-coordinate iron bis(silyl) complexes that show good catalytic performance in the hydrosilylation of acetophenone.

Catalytic Synthesis of Silanols by Hydroxylation of Hydrosilanes: From Chemoselectivity to Enantioselectivity

As a crucial class of functional molecules in organosilicon chemistry, silanols are found valuable applications in the fields of modern science and will be a potentially powerful framework for biologically active compounds or functional materials. It has witnessed an increasing demand for non-natural organosilanols, as well as the progress in the synthesis of these structural features. From the classic preparative methods to the catalytic selective oxidation of hydrosilanes, electrochemical hydrolysis of hydrosilanes, and then the construction of the most challenging silicon-stereogenic silanols. This review summarized the progress in the catalyzed synthesis of silanols via hydroxylation of hydrosilanes in the last decade, with a particular emphasis on the latest elegant developments in the desymmetrization strategy for the enantioselective synthesis of silicon-stereogenic silanols from dihydrosilanes.

Unexpected alkyl isomerization at the silicon ligand of an unsaturated Rh complex: combined experiment and theory

The formation of dimer [(μ-Cl)Rh-(κ3(P,Si,Si)PhP(o-C6H4CH2SiiPr2)(o-C6H4CH2SiiPrnPr))]2 (Rh-3) with an n-propyl group on one of the silicon atoms as a minor product was affected by the reaction of [RhCl(COD)]2 with proligand PhP(o-C6H4CH2SiHiPr2)2, L1. The major product of the reaction was monomeric 14-electron Rh(III) complex [ClRh(κ3(P,Si,Si)PhP(o-C6H4CH2SiiPr2)2)] (Rh-1). Computations revealed that the monomer-dimer equilibrium is shifted toward the monomer with four isopropyl substituents on the two Si atoms of the ligand as in Rh-1; conversely, the dimer is favored with only one n-propyl as in Rh-3, and with less bulky alkyl substituents such as in [ClRh(κ3(P,Si,Si)PhP(o-C6H4CH2SiMe2)2]2 (Rh-2). Computations on the mechanism of formation of Rh-3 directly from [RhCl(COD)]2 are in agreement with the experimental findings and it is found to be less energetic than if stemming from Rh-1. Additionally, a Si-O-Si complex, [μ-Cl-Rh{κ3(P,Si,C)PPh(o-C6H4CH2SiiPrO SiiPr2CH-o-C6H4)}]2, Rh-4, is generated from the reaction of Rh-1 with adventitious water as a result of intramolecular C-H activation.

Four- and three-coordinate planar iron(II) complexes supported by bulky organosilyl ligands

The ligand exchange reaction of (THF)2Fe[Si(SiMe3)3]2 with 2 equivalents of an N-heterocyclic carbene (NHC) led to the formation of a square-planar iron(II) complex with trans-oriented -Si(SiMe3)3 ligands. Conversely, the introduction of a cis-coordinate bidentate organosilyl ligand instead of -Si(SiMe3)3 resulted in the formation of a square planar iron(II) complex supported by a cis-coordinate bidentate organosilyl ligand. A three-coordinate planar iron(II) bis(silyl) complex was also synthesized using a cis-coordinate bidentate organosilyl ligand and a cyclic (alkyl)(amino)carbene auxiliary ligand. Investigation of the catalytic performance of these complexes in the hydrosilylation of acetophenone revealed that the square-planar iron(II) complex with trans-oriented -Si(SiMe3)3 ligands exhibits superior reactivity relative to its tetrahedral precursor.

Chiral PSiSi-Ligand Enabled Iridium-Catalyzed Atroposelective Intermolecular C-H Silylation

Catalytic enantioselective intermolecular C-H silylation offers an efficient approach for the rapid construction of chiral organosilicon compounds, but remains a significant challenge. Herein, a new type of chiral silyl ligand is developed, which enables the first iridium-catalyzed atroposelective intermolecular C-H silylation reaction of 2-arylisoquinolines. This protocol features mild reaction conditions, high atom economy, and remarkable yield with excellent stereoselectivity (up to 99 % yield, 99 % ee), delivering enantioenriched axially chiral silane platform molecules with facile convertibility. Key to the success of this unprecedented transformation relies on a novel chiral PSiSi-ligand, which facilitates the intermolecular C-H silylation process with perfect chem-, regio- and stereo-control via a multi-coordinated silyl iridium complex.

Synthesis of a homoleptic tris(silylene)-palladium(0) complex and a silylyne-bridged tetranuclear palladium cluster

We report the reactions of an iminophosphonamido-silylene (1) with different palladium complexes. The reaction of 1 with [Pd(PPh3)4] furnished a homoleptic tris(silylene)-palladium(0) complex. In contrast, treatment of 1 with [PdMe2(tmeda)] led to the unprecedented formation of a non-planar μ3-silylyne-bridged tetranuclear palladium cluster that contains palladium atoms in different oxidation states.

Dinuclear and tetranuclear group 10 metal complexes constructed from linear tetrasilane comprising both Si-H and Si-Si moieties

The activation of Si-H bonds and/or Si-Si bonds in organosilicon compounds by transition-metal species plays a crucial role for the production of functional organosilicon compounds. Although group-10-metal species are frequently used to activate Si-H and/or Si-Si bonds, so far, systematic investigation to clarify the preferences of these metal species with respect to the activation of Si-H and/or Si-Si bonds remain elusive. Here, we report that platinum(0) species that bear isocyanide or N-heterocyclic-carbene (NHC) ligands selectively activates the terminal Si-H bonds of the linear tetrasilane Ph₂(H)SiSiPh₂SiPh₂Si(H)Ph₂ in a stepwise manner, whereby the Si-Si bonds remain intact. In contrast, analogous palladium(0) species are preferably inserted into the Si-Si bonds of the same linear tetrasilane, whereby the terminal Si-H bonds remain intact. Substitution of the terminal hydride groups in Ph₂(H)SiSiPh₂SiPh₂Si(H)Ph₂ with chloride groups leads to the insertion of platinum(0) isocyanide into all Si-Si bonds to afford an unprecedented zig-zag Pt₄ cluster.

Si-H Bond Activation and Dehydrogenative Coupling of Silanes across the Iron-Amide Bond of a Bis(amido)bis(phosphine) Iron(II) Complex

Despite the utility of Si-Si bonds, there are relatively few examples of Si-Si bond formation by base metals. In this work, a four-coordinate iron complex, (PNNP)Fe^II, is shown to strongly activate the Si-H bonds in primary silanes across the Fe-amide bonds in a metal-ligand cooperative fashion. Upon treatment with excess silane, Si-Si dehydrogenative homocoupling is shown to occur across the Fe-N_amide bond without concomitant oxidation and spin state changes at the Fe center.

Enantioselective Hydrosilylation of β,β-Disubstituted Enamides to Construct α-Aminosilanes with Vicinal Stereocenters

Despite the advances in the area of catalytic alkene hydrosilylation, the enantioselective hydrosilylation of alkenes bearing a heteroatom substituent is scarce. Here we report a rhodium-catalyzed hydrosilylation of β,β-disubstituted enamides to directly afford valuable α-aminosilanes in a highly regio-, diastereo-, and enantioselective manner. Stereodivergent synthesis could be achieved by regulating substrate geometry and ligand configuration to generate all the possible stereoisomers in high enantio-purity.

Heterolytic cleavage of a Si-H bond by a metal-ligand cooperation of a cationic iridium amido complex and hydrosilylation of aldehydes

Heterolytic cleavage of a Si-H bond was achieved mediated by a metal-ligand cooperation of a cationic iridium amido complex. The reaction was applied to the catalytic hydrosilylation of benzaldehyde and its derivatives, affording the corresponding hydrosilylated products in moderate to good yields.

Synthesis of α-Aminosilanes by 1,2-Metalate Rearrangement Deoxygenative Silylation of Aromatic Amides

An efficient nickel-catalyzed deoxygenative silylation reaction of aromatic amides with silylboranes in the presence of a Sm/SmI₂ system for the construction of α-aminosilanes is described. This strategy provides a direct method for synthesizing α-aminosilanes with high efficiency and good functional group compatibility and includes readily accessible starting materials and valuable products.

Si-H···Se Chalcogen-Hydride Bond Quantified by Diffraction and Topological Analyses

The Si-H···Se contact in 1-mesitylselanyl-8-(dimethylsilyl)naphthalene (1), which exhibits the spatial arrangement of a δ-agostic interaction from geometric considerations, was investigated. Is this just enforced by close 1,8-proximity or is this a favorable interaction? Charge density studies are best suited to investigate the exact origin of the interaction and to quantify the properties. Hence, they are most elucidating. High-resolution X-ray diffraction data of 1 were collected, and a multipole refinement followed by a topological analysis using Bader's quantum theory of atoms in molecules was employed. The resulting bond properties were set in relation to high-level computational parameters. The comparison to Si-H···[M] agostics, hydride bonding, chalcogen bonds, and charge-inverted hydrogen bonds qualified the Si-H···Se noncovalent interaction to be best classified as a chalcogen-hydride bond.

Recent Progresses on Designable Hybrids with Stimuli-Responsive Optical Properties Originating from Molecular Assembly Concerning Polyhedral Oligomeric Silsesquioxane

In this review, we describe recent progresses on the stimuli-responsive hybrid materials based on polyhedral oligomeric silsesquioxane (POSS) and their applications as a chemical sensor. In particular, we explain the unique functions originating from molecular assembly concerning POSS-containing soft materials mainly from our studies. POSS has an inorganic cubic core composed of silicon-oxygen (Si-O) bonds and organic substituents at each vertex. Owing to intrinsic properties of POSS, such as high thermal stability, rigidity, and low chemical reactivity, various robust hybrid materials have been developed. From the numerous numbers of POSS hybrids, we herein focus on the environment-sensitive optical materials in which molecular assembly of POSS itself and functional units connected to POSS should be a key factor for expressing material properties. We also explain the mechanisms of chemical sensors originating from these stimuli-responsive optical properties. Stimuli-responsive excimer emission and pollutant detectors, nanoplastic sensors with the water-dispersive POSS networks, trans fatty acid sensors, turn-on luminescent sensors for aerobic condition and fluoride anion sensors are described. We also mention the mechanochromic polyurethane hybrids and the thermally-durable mechanochromic luminescent materials. The roles of the unique optical properties from soft materials composed of rigid POSS, which doesn't have significant light-absorption and emission properties in the visible region, are surveyed.

On the reactivity of Al-group 11 (Cu, Ag, Au) bonds

Reactions of the seven-membered heterocyclic potassium diamidoalumanyl, [K{Al(SiN^Dipp)}]₂ (SiN^Dipp = {CH₂SiMe₂NDipp}₂; Dipp = 2,6-di-isopropylphenyl), with a variety of Cu(I), Ag(I) and Au(I) chloride N-heterocyclic carbene (NHC) adducts are described. The resultant group 11-Al bonded derivatives have been characterised in solution by NMR spectroscopy and, in the case of [{SiN^Dipp}Al-Au(NHC^iPr)] (NHC^iPr = N,N'-di-isopropyl-4,5-dimethyl-2-ylidene), by single crystal X-ray diffraction. Although similar reactions of LAgCl and LAuCl, where L is a more basic cyclic alkyl amino carbene (CAAC), generally resulted in reduction of the group 11 cations to the base metals, X-ray analysis of [(^CyCAAC)AgAl(SiN^Dipp)] (^CyCAAC = 2-[2,6-bis(1-methylethyl)phenyl]-3,3-dimethyl-2-azaspiro[4.5]dec-1-ylidene) provides the first solid-state authentication of an Ag-Al σ bond. The reactivity of the NHC-supported Cu, Ag and Au alumanyl derivatives was assayed with the isoelectronic unsaturated small molecules, N,N'-di-isopropylcarbodiimide and CO₂. While these reactions generally provided products consistent with nucleophilic attack of the group 11 atom at the electrophilic heteroallene carbon centre, treatment of the NHC-supported copper and silver alumanyls with N,N'-di-isopropylcarbodiimide yielded less symmetric Cu-C and Ag-C-bonded isomers. In contrast to the previously described copper and silver alumanyl derivatives, [(NON)Al(O₂C)M(Pt-Bu₃)] (M = Cu or Ag; NON = 4,5-bis(2,6-di-isopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), which were prone to facile CO extrusion and formation of carbonate derivatives, the NHC-supported dioxocarbene species, [(NHC^iPr)M(CO₂)Al(SiN^Dipp)] (M = Cu, Ag, Au), are all stable at room and moderately elevated temperatures. The stabilising role of the NHC co-ligand was, thus, assessed by preparation of the t-Bu₃P adducted copper-alumanyl, [(t-Bu₃P)CuAl(SiN^Dipp)]. Treatment of this latter compound, which was also structurally characterised by X-ray analysis, with both N,N'-di-isopropylcarbodiimide and CO₂ again provided smooth heteroallene insertion and formation of the relevant Cu-C-bonded products. Although both compounds were quite stable at room temperature, heating of [(t-Bu₃P)Cu(CO₂)Al(SiN^Dipp)] at 60 °C induced elimination of CO and formation of the analogous carbonate, [(t-Bu₃P)Cu(OCO₂)Al(SiN^Dipp)], which was identified by ¹³C and ³¹P NMR spectroscopy. Reflective of the more reliable nucleophilic behaviour of the gold centres in these group 11 alumanyls, computational (QTAIM and NBO) analysis highlighted a lower level of covalency of the Al-Au linkage in comparison to the analogous Al-Cu and Al-Ag interactions. Although substitution of the co-ligand significantly perturbs the charge distribution across the Cu-Al bond of [LCuAl(SiN^Dipp)] (L = NHC^iPr or t-Bu₃P), only a negligible difference is observed between the phosphine-coordinated copper systems derived from either the [SiN^Dipp]- or (NON)-based alumanyl ligands. Computational mapping of the reaction profiles arising from treatment of the various group 11 alumanyls with N,N'-di-isopropylcarbodiimide indicates that the observed formation of the Cu-N and Ag-N bound isomers do not provide the thermodynamic reaction outcome. In contrast, examination of the CO₂-derived reactions, and their potential toward CO extrusion and subsequent carbonate formation, implies that the identity of the co-ligand exerts a greater influence on this aspect of reactivity than the architecture of the diamidoalumanyl anion.

Palladium-Catalyzed ortho-C(sp2)-H Silylation of Aromatic Ketones Using an Aminooxyamide Auxiliary

A palladium-catalyzed direct and selective ortho-C(sp²)-H silylation of aromatic ketones has been achieved using an aminooxyamide auxiliary. The reaction tolerates various orth-, meta-, and para- substituents on the aromatic ring and can be applied to thiophenyl and vinyl ketones. The ortho-C(sp²)-H bond was monosilylated selectively in comparison with other aromatic C-H bonds, benzyl or allylic C(sp³)-H bonds, and acidic α-C(sp³)-H bonds. The aminooxyamide auxiliary can be easily installed and readily removed after the silylation reaction. The resulting ortho-silyl aromatic ketone derivatives are potentially useful building blocks for organic synthesis.

A square planar silylene nickel four-membered ring

Herein the new nickel silylene [PhC(NtBu)₂SiNi(C₅Me₅)]₂, which features a square planar central ring motif consisting of two silicon and two nickel atoms is presented. The title compound was obtained by an insertion of the Ni(0) precursor [Ni(cod)₂] (cod = 1,5-cyclooctadiene) in the Si-C bond of the silylene [PhC(NtBu)₂Si(C₅Me₅)]. Analytic characterisation including mass spectrometry as well as IR and Raman spectroscopies was combined with quantum chemical calculations to get an insight on the bonding situation within the four-membered Si-Ni-ring.

The mechanism of oxidative addition of Pd(0) to Si-H bonds: electronic effects, reaction mechanism, and hydrosilylation

The oxidative addition of Pd to Si-H bonds is a crucial step in a variety of catalytic applications, and many aspects of this reaction are poorly understood. One important yet underexplored aspect is the electronic effect of silane substituents on reactivity. Herein we describe a systematic investigation of the formation of silyl palladium hydride complexes as a function of silane identity, focusing on electronic influence of the silanes. Using [(μ-dcpe)Pd]₂ (dcpe = dicyclohexyl(phosphino)ethane) and tertiary silanes, data show that equilibrium strongly favours products formed from electron-deficient silanes, and is fully dynamic with respect to both temperature and product distribution. A notable kinetic isotope effect (KIE) of 1.21 is observed with H/DSiPhMe₂ at 233 K, and the reaction is shown to be 0.5^th order in [(μ-dcpe)Pd]₂ and 1^st order in silane. Formed complexes exhibit temperature-dependent intramolecular H/Si ligand exchange on the NMR timescale, allowing determination of the energetic barrier to reversible oxidative addition. Taken together, these results give unique insight into the individual steps of oxidative addition and suggest the initial formation of a σ-complex intermediate to be rate-limiting. The insight gained from these mechanistic studies was applied to hydrosilylation of alkynes, which shows parallel trends in the effect of the silanes' substituents. Importantly, this work highlights the relevance of in-depth mechanistic studies of fundamental steps to catalysis.

Nuclearity expansion in Pd clusters triggered by the migration of a phenyl group in cyclooligosilanes

Heptanuclear palladium clusters with six palladium atoms in a planar arrangement were obtained from the reaction of [Pd(CNtBu)2]3 with Ph-substituted cyclotetrasilane or cyclopentasilane via the migration of a phenyl group. The molecular structures of these clusters as well as those of two possible intermediates were determined by single-crystal X-ray diffraction analyses.

Hydrosilane σ-Adduct Intermediates in an Adaptive Zinc-Catalyzed Cross-dehydrocoupling of Si-H and O-H Bonds

Three-coordinate Ph BOXMe 2 ZnR (Ph BOXMe 2 =phenyl-(4,4-dimethyl-oxazolinato; R=Me: 2 a, Et: 2 b) catalyzes the dehydrocoupling of primary or secondary silanes and alcohols to give silyl ethers and hydrogen, with high turnover numbers (TON; up to 107 ) under solvent-free conditions. Primary and secondary silanes react with small, medium, and large alcohols to give various degrees of substitution, from mono- to tri-alkoxylation, whereas tri-substituted silanes do not react with MeOH under these conditions. The effect of coordinative unsaturation on the behavior of the Zn catalyst is revealed through a dramatic variation of both rate law and experimental rate constants, which depend on the concentrations of both the alcohol and hydrosilane reactants. That is, the catalyst adapts its mechanism to access the most facile and efficient conversion. In particular, either alcohol or hydrosilane binds to the open coordination site on the Ph BOXMe 2 ZnOR catalyst to form a Ph BOXMe 2 ZnOR(HOR) complex under one set of conditions or an unprecedented σ-adduct Ph BOXMe 2 ZnOR(H-SiR'3 ) under other conditions. Saturation kinetics provide evidence for the latter species, in support of the hypothesis that σ-bond metathesis reactions involving four-centered electrocyclic 2σ-2σ transition states are preceded by σ-adducts.

Palladium-Catalyzed C-H Silylation of Aliphatic Ketones Using an Aminooxyamide Auxiliary

A palladium-catalyzed β-C(sp³)-H silylation of aliphatic ketones with disilanes to afford β-silyl ketones is reported. The aminooxyamide auxiliary is critical for the C-H activation and silylation. The reaction tolerates a number of functional groups and shows good selectivity in silylating β-C(sp³)-H bonds in the company of C(sp²)-H bonds and acidic α-C(sp³)-H bonds. The reaction is scalable, and the aminooxyamide auxiliary is readily removed to give β-silyl ketones, which could serve as useful building blocks for organic synthesis. Late-stage diversification using this protocol is demonstrated in the silylation of santonin with good yield.

Copper-Catalyzed Direct sp2 C-H Silylation of Arylamides Using Disilanes

A copper-catalyzed method for direct intermolecular ortho-silylation of benzamides has been developed that affords organosilane products in moderate to high yields. The key features include: (i) use of commercially available disilanes as a silicon source with 8-aminoquinoline as a bidentate directing group, (ii) use of earth-abundant first-row transition metal, (iii) operationally simple conditions without the need of an inert atmosphere, and (iv) tolerance of a wide range of functional groups. The practicality and effectiveness of this method have been demonstrated by a gram-scale experiment. This strategy, therefore, constitutes a convenient way of constructing C-Si bonds useful for synthetic organic chemistry.

Rare-Earth-Catalyzed Hydrosilylation and Dehydrogenative Coupling of Hydrosilanes

Activation of Si–H bonds with rare-earth complexes could generate the highly reactive rare-earth hydrides or silyl complexes, which are key intermediates for hydrosilylation and cross-coupling reactions. This Account summarizes the recent advances in the rare-earth-catalyzed hydrosilylation of unsaturated substrates and dehydrogenative coupling of hydrosilanes with amines in our laboratory. The results demonstrated that rare-earth catalysts are unique in their reactivity and selectivity, enabling some unprecedented reactions.

1 Introduction

2 Dehydrogenative Coupling of Hydrosilanes with Amines

3 Catalytic Dehydrogenative Coupling of Hydrosilanes with Amines

4 Catalytic Hydrosilylation of Terminal Alkenes and Polymerization of Styrene

5 Catalytic Hydrosilylation of Internal Alkenes

6 Catalytic Dihydrosilylation of Internal Alkynes

7 Conclusions and Outlook

Reversible C(sp3)-Si Oxidative Addition of Unsupported Organosilanes: Effects of Silicon Substituents on Kinetics and Thermodynamics

The intermolecular oxidative addition of unactivated C(sp³)-Si bonds is reported for a family of organosilanes at a cationic pincer-supported iridium complex. To our knowledge, no examples of oxidative addition to give analogous unsupported (alkyl)metal silyl complexes have been previously reported. The generality of this transformation is excellent, with successful examples demonstrated for tetraorganosilanes, mono- and poly alkoxysilanes, and two siloxysilanes. Oxidative addition is found to be completely reversible, with the product of reductive elimination being subject to trapping by triethylsilane. The successful isolation of these metal silyl complexes has allowed for an in-depth kinetic analysis of C(sp³)-Si reductive elimination, a process with strong implications in both catalytic C-H silylation and olefin hydrosilylation. The apparent order of reactivity is SiMe₃ > SiMe₂(CF₃) > SiMe₂OSiMe₃ > SiMe₂OSiMe₂OSiMe₃ > SiMe₂(OMe) > SiMe₂(OEt) > SiMe(OMe)₂. A DFT analysis of the oxidative addition products shows that the thermodynamic stability of the (alkyl)metal silyl complexes span a range of ca. 10 kcal·mol^-1, which relate closely with the experimentally determined rates of C(sp³)-Si reductive elimination and trapping, though a clear kinetic distinction exists between methoxy- and siloxysilyl complexes.

Nickel-Catalyzed Hydrosilylation of Terminal Alkenes with Primary Silanes via Electrophilic Silicon-Hydrogen Bond Activation

We report a simple and effective nickel-based catalytic system, NiCl₂·6H₂O/^tBuOK, for the electrophilically activated hydrosilylation of terminal alkenes with primary silanes. This protocol provides excellent performance under mild reaction conditions: exclusive anti-Markovnikov selectivity, broad functional group tolerance (36 examples), and good scalability (TON = 5500). However, the secondary and tertiary silanes are not suitable. Mechanistic studies revealed that this homogeneous catalytic hydrosilylation includes an electrophilically activated Si-H bond process without the generation of nickel hydrides.

2-Pyridone-stabilized iridium silylene/silyl complexes: structure and QTAIM analysis

Iridium(iii) complexes of the general formula [Ir(X)(κ2-NSiiPr2)2] (NSiiPr2 = (4-methyl-pyridine-2-yloxy)diisopropylsilyl; X = Cl, 3; CF3SO3, 5; CF3CO2, 6) have been prepared and fully characterized, including X-ray diffraction studies and theoretical calculations. The presence of isopropyl substituents at the silicon atom favours the monomeric structure found in complexes 3 and 5. The short Ir-Si bond distances (2.25-2.28 Å) indicate some degree of base-stabilized silylene character of the Ir-Si bond in 3, 5 and 6 assisted by the 2-pyridone moiety. However, the shortening of these Ir-Si bonds might be a consequence of the constrained 2-pyridone geometry, and consequently the silyl character of these bonds can not be excluded. A DFT theoretical study on the nature of the Ir-Si bonds has been performed for complex 3 as well as for four other iridium complexes finding representative examples of different bonding situations between Ir and Si atoms: silylene, base-assisted silylene (both with an anionic base and with a neutral base), and silyl bonds, using the topological properties of the electron charge density. The results of these studies show that the Ir-Si bonds in Ir-NSiiPr2 complexes can be considered as an intermediate between the base-stabilized silylene and silyl cases, and therefore they have been proposed as 2-pyridone-stabilized iridium silylene/silyl bonds.

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

Recent development in catalytic application of transition metal complexes having an M-E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized. Main group metal and metalloid supporting ligands furnish unusual electronic and steric environments and molecular functions to transition metals, which are not easily available with standard organic supporting ligands such as phosphines and amines. These characteristics often realize remarkable catalytic activity, unique product selectivity, and new molecular transformations. This perspective demonstrates the promising utility of main group metal and metalloid compounds as a new class of supporting ligands for transition metal catalysts in synthetic chemistry.

Dual reactivity of B(C6F5)3 enables the silylative cascade conversion of N-aryl piperidines to sila-N-heterocycles: DFT calculations

A theoretical study reveals that the dual reactivity of B(C₆F₅)₃ enables the unique silylative cascade conversion of N-aryl piperidines to bridged sila-N-heterocycles.

A Four Coordinated Iron(II)-Digermyl Complex as an Effective Precursor for the Catalytic Dehydrogenation of Ammonia Borane

A coordinatively unsaturated iron(II)-digermyl complex, Fe[Ge(SiMe3)3]2(THF)2 (1), was synthesized in one step by the reaction of FeBr2 with 2 equiv of KGe(SiMe3)3. Complex 1 shows catalytic activity comparable to that of its silicon analogue in reduction reactions. In addition, 1 acts as an effective precursor for the catalytic dehydrogenation of ammonia borane. Catalytically active species can also be generated in situ by simple mixing of the easy-to-handle precursors FeBr2, Ge(SiMe3)4, KOtBu, and phenanthroline.

Proximity Enforced Agostic Interactions Involving Closed-Shell Coinage Metal Ions

A proximity enforcing diarylsilane ligand is reported, which gives rise to unusual Si-H···M interactions with the d¹⁰ metal ions Cu⁺ and Ag⁺ upon complexation. These interactions are studied in detail both experimentally and computationally and can be classified to be weakly agostic in nature for the Si-H···Cu interaction. The Si-H···Ag interaction has more signatures of an electrostatic contact.