Mechanism of the cooperative Si-H bond activation at Ru-S bonds

Hydrogermylation of alkynes via metal-ligand cooperative catalysis

E-H bond activation (E = B, Ge, Sn) with a (PNS)Pd complex has been investigated theoretically and experimentally. Et3GeH readily adds across the Pd/S bond. Subsequent transfer to CC bonds enables catalytic hydrogermylation. The reaction is most regio and stereoselective with terminal alkynes. Downstream derivatization of silyl-functionalized vinyl germanes is exemplified.

An Isolable Base-Stabilized Diazosilenyl Cation

In contrast to the emerging chemistry of stable diazoalkenes, there are not yet any studies devoted to heavier silicon analogues, diazosilenes. Here, we report the synthesis of a base-stabilized diazosilenyl cation 2 by the reaction of base-stabilized C-phosphonio-silyne 1 with N2O. This silicon analog of diazoalkenes 2 exhibits a remarkable stability thanks to the coordination of phosphine and phosphine oxide ligands at the cationic silicon center. In addition, DFT calculations predict that, due to a particular stabilizing effect of the electropositive silicon atom, for diazosilenes (R2Si=C=N2), the presence of π-donor substituents is not essential to prevent N2 dissociation, contrary to carbon analogues. Interestingly, diazosilenyl cation 2 tends to isomerize into (silylene)(phosphonio)diazomethane 7 via a 1,2-phosphine ligand migration and thus exhibits dual reactivity as a diazoalkene and as a diazo-substituted silylene.

Redox Cycling with Tellurium. Si-H Bond Activation by a Lewis Superacidic Tellurenyl Cation

The C,N-chelated aryltellurenyl triflate [2-(tBuNCH)C6H4Te][OTf] (1) activates the Si-H bonds in the tertiary silanes R3SiH via umpolung of H- to H+ to give rise to the iminium salts (tBuN(H)CH)C6H4TeSiR3][OTf] (2R, R=Et, Ph (elusive) and R=Si(CH3)3 isolated; OTf=O3SCF3) comprising Te-Si bonds, which are capable of generating silyl triflates, R3SiOTf, under attack of a second equivalent of 1. The unprecedented Si-H activation was utilized in main group redox catalysis using p-quinones, which were converted into (silylated) hydroquinones.

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.

Mechanism Insights into the Iridium(III)- and B(C6F5)3-Catalyzed Reduction of CO2 to the Formaldehyde Level with Tertiary Silanes

The catalytic system [Ir(CF3CO2)(κ2-NSiMe)2] [1; NSiMe = (4-methylpyridin-2-yloxy)dimethylsilyl]/B(C6F5)3 promotes the selective reduction of CO2 with tertiary silanes to the corresponding bis(silyl)acetal. Stoichiometric and catalytic studies evidenced that species [Ir(CF3COO-B(C6F5)3)(κ2-NSiMe)2] (3), [Ir(κ2-NSiMe)2][HB(C6F5)3] (4), and [Ir(HCOO-B(C6F5)3)(κ2-NSiMe)2] (5) are intermediates of the catalytic process. The structure of 3 has been determined by X-ray diffraction methods. Theoretical calculations show that the rate-limiting step for the 1/B(C6F5)3-catalyzed hydrosilylation of CO2 to bis(silyl)acetal is a boron-promoted Si-H bond cleavage via an iridium silylacetal borane adduct.

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.

Activation of Si-H and B-H bonds by Lewis acidic transition metals and p-block elements: same, but different

In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si-H and B-H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E-H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η1 or η2) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.

Discovery of Electronic Structure and Interfacial Interaction Features in Catalytic Activity

The selective transformation of inert bonds (C-H, C-O, C-C, C-F, etc.) via various catalysts is one of the most challenging areas, with applications in organic synthesis, materials science, and biological and pharmaceutical chemistry. The catalytic performance of homogeneous and heterogeneous catalysts can be rationally controlled in two ways: (i) electronic structure modulation of the active site, such as the metal center, ligands, and coordination modes, to improve the catalytic activity and stability and (ii) tuning intermolecular or interfacial interactions to promoting the reaction kinetics by accelerating the transmission of electrons between the catalyst and solvents or support. The rational design of catalysts based on adjustable features, such as metal (monometallic or bimetallic) active sites, crystal phase, ligands, solvents, and supports for inert bond activation under mild conditions remains a challenge. This Perspective summarizes the features of electronic structures, interfacial interactions, and their effects on molecular catalysis, metal-organic frameworks (MOFs), and natural mineral catalysis. The discovery of efficient catalysts could be promoted using machine-learning methods with high-performance descriptors. More attention should be paid to high-throughput quantum-chemical computations and experiments, automatic searches of chemical reaction pathways, and efficient machine-learning or deep-learning methods to accelerate catalyst design and synthesis in the future.

Norbornene based-sulfide-stabilized silylium ions: synthesis, structure and application in catalysis

A norbornene-based sulfide stabilized silylium ion 4 has been synthesized. The S-Si interaction was studied in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis as well as DFT calculations. Unlike the previously reported phosphine-stabilized silylium ion VII, behaving as a Lewis pair, calculations predict that 4 should behave as a Lewis acid toward acrylate derivatives. Indeed, the base-stabilized silylium ion 4 has emerged as an easy-to-handle silylium ion-based Lewis acid catalyst, particularly for the Diels-Alder cycloaddition, with poorly reactive dienes, and hydrodefluorination reactions.

Comparative DFT Study on Dehydrogenative C(sp)-H Elementation (E = Si, Ge, and Sn) of Terminal Alkynes Catalyzed by a Cationic Ruthenium(II) Thiolate Complex

Described herein is a comparative theoretical study of dehydrogenative C(sp)-H functionalizations of a terminal alkyne with group-14-based hydrides (HEEt₃; E = Si, Ge, Sn) catalyzed by an Ohki-Tatsumi complex-a cationic Ru(II) complex with a tethered thiolate ligand ([Ru-S] = [(DmpS)Ru(PiPr₃)][BAr₄^F]; Dmp = 2,6-(dimesityl)₂C₆H₃; Ar^F = 3,5-(CF₃)₂C₆H₃). The calculations indicate that the energy barriers for heterolytic cleavage of the H-EEt₃ bonds at the Ru-S sites of the Ohki-Tatsumi complex highly vary depending on the group 14 elements from 3.8 kcal/mol (E = Sn) to 10.5 kcal/mol (E = Ge) and 18.5 kcal/mol (E = Si), where Ru and S elements cooperatively serve as the Lewis acid and base, respectively. Likewise, the transfer of the group 14 cation (Et₃E⁺) to the C-C triple bond to generate the β-element-stabilized vinyl cations-the rate-determining step (RDS) of the overall reaction-is predicted to be susceptible to the element's identity [E_a = 36.8 for Sn < 42.9 and Ge < 50.7 for Si (kcal/mol)]. The key transition states involved in the RDS are compared in terms of energy and structure within each system of the group 14 hydrides. The distortion/interaction-activation strain (DIAS) model analysis of the transition states responsible for dehydrogenative stannylation and hydrostannation of a terminal alkyne sheds light on the origin of the experimentally observed kinetic preference toward dehydrogenative C-H stannylation over hydrostannation.

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Cooperative B-H and Si-H Bond Activations by κ2-N,S-Chelated Ruthenium Borate Complexes

Cooperative E-H (E = B, Si) bond activations employing κ²-N,S-chelated ruthenium borate species, [PPh₃{κ²-N,S-(NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(NC₇H₄S₂)₂}], (1) are established. Treatment of 1 with BH₃·SMe₂ yielded the six-membered ruthenaheterocycle [PPh₃{κ²-S,H-(BH₃NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (2) formed by a hemilabile ring opening of a Ru-N bond and capturing of a BH₃ unit coordinated in an "end-on" fashion. On the other hand, the bulky borane H₂BMes shows different reactivity with 1 that led to the formation of the two dihydroborate complexes [{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (3) and [PPh₃{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru(κ²-N,S-C₇H₄NS₂)] (4), in which H₂BMes has been inserted into the Ru-N bond of the initial κ²-N,S-chelated ligand. In an attempt to directly activate hydrosilanes by 1, reactions were carried out with H₂SiPh₂ that yielded two isomeric five-membered ruthenium silyl complexes, namely [PPh₃{κ²-S,Si-(NSiPh₂)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a,b), and the hydridotrisilyl complex [Ru(H){κ²-S,Si-(SiPh₂NC₇H₄S₂}₃] (6). These complexes were generated by Si-H bond activation with the release of H₂ and the formation of N-Si and Ru-Si bonds. When the reaction of 1 was carried out in the presence of PhSiH_3, the reaction only produced the analogous complexes [PPh₃{κ²-S,Si-(NSiPhH)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a',b'). Density functional theory (DFT) calculations have been used to probe the bonding modes of boranes/silane with the ruthenium center.

Isolating reactive metal-based species in Metal-Organic Frameworks - viable strategies and opportunities

Structural insight into reactive species can be achieved via strategies such as matrix isolation in frozen glasses, whereby species are kinetically trapped, or by confinement within the cavities of host molecules. More recently, Metal-Organic Frameworks (MOFs) have been used as molecular scaffolds to isolate reactive metal-based species within their ordered pore networks. These studies have uncovered new reactivity, allowed observation of novel metal-based complexes and clusters, and elucidated the nature of metal-centred reactions responsible for catalysis. This perspective considers strategies by which metal species can be introduced into MOFs and highlights some of the advantages and limitations of each approach. Furthermore, the growing body of work whereby reactive species can be isolated and structurally characterised within a MOF matrix will be reviewed, including discussion of salient examples and the provision of useful guidelines for the design of new systems. Novel approaches that facilitate detailed structural analysis of reactive chemical moieties are of considerable interest as the knowledge garnered underpins our understanding of reactivity and thus guides the synthesis of materials with unprecedented functionality.

Half-sandwich ruthenium(ii) complexes with tethered arene-phosphinite ligands: synthesis, structure and application in catalytic cross dehydrogenative coupling reactions of silanes and alcohols

The preparation of the tethered arene-ruthenium(ii) complexes [RuCl₂{η⁶:κ¹(P)-C₆H₅(CH₂)_nOPR₂}] (R = Ph, n = 1 (9a), 2 (9b), 3 (9c); R = ⁱPr, n = 1 (10a), 2 (10b), 3 (10c)) from the corresponding phosphinite ligands R₂PO(CH₂)_nPh (R = Ph, n = 1 (1a), 2 (1b), 3 (1c); R = ⁱPr, n = 1 (2a), 2 (2b), 3 (2c)) is presented. Thus, in a first step, the treatment at room temperature of tetrahydrofuran solutions of dimers [{RuCl(μ-Cl)(η⁶-arene)}₂] (arene = p-cymene (3), benzene (4)) with 1-2a-c led to the clean formation of the corresponding mononuclear derivatives [RuCl₂(η⁶-p-cymene){R₂PO(CH₂)_nPh}] (5-6a-c) and [RuCl₂(η⁶-benzene){R₂PO(CH₂)_nPh}] (7-8a-c), which were isolated in 66-99% yield. The subsequent heating of 1,2-dichloroethane solutions of these compounds at 120 °C allowed the exchange of the coordinated arene. The substitution process proceeded faster with the benzene derivatives 7-8a-c, from which complexes 9-10a-c were generated in 61-82% yield after 0.5-10 h of heating. The molecular structures of [RuCl₂(η⁶-p-cymene){ⁱPr₂PO(CH₂)₃Ph}] (6c) and [RuCl₂{η⁶:κ¹(P)-C₆H₅(CH₂)_nOPⁱPr₂}] (n = 1 (10a), 2 (10b), 3 (10c)) were unequivocally confirmed by X-ray diffraction methods. In addition, complexes [RuCl₂{η⁶:κ¹(P)-C₆H₅(CH₂)_nOPR₂}] (9-10a-c) proved to be active catalysts for the dehydrogenative coupling of hydrosilanes and alcohols under mild conditions (r.t.). The best results were obtained with [RuCl₂{η⁶:κ¹(P)-C₆H₅(CH₂)₃OPⁱPr₂}] (10c), which reached TOF and TON values up to 117 600 h^-1 and 57 000, respectively.

Cleavage of Si-H, B-H, and C-H Bonds by Metal-Ligand Cooperation

Metal-ligand cooperation, in which metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. In contrast to the classical bond cleavage by active metal centers with spectator ligands, metal-ligand cooperation has enabled unprecedented reactivities. Especially, metal-ligand cooperative H-H bond cleavage has been extensively studied and applied to various catalysts. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H-H bond. This review summarizes the recent progress in the metal-ligand cooperative cleavages of Si-H, B-H, and C-H bonds and their catalytic applications.

Comparative DFT study of metal-free Lewis acid-catalyzed C–H and N–H silylation of (hetero)arenes: mechanistic studies and expansion of catalyst and substrate scope

Direct selective dehydrogenative silylation of thiophenes, pyridines, indoles and anilines to synthesize silyl-substituted aromatic compounds catalyzed by metal-free Lewis acids was achieved recently. However, there is still insufficient mechanistic data for these transformations. Using density functional theory calculations, we conducted a detailed investigation of the mechanism of the B(C₆F₅)₃-catalyzed dehydrogenative silylation of N-methylindole, N,N-dimethylaniline and N-methylaniline. We successfully located the most favourable reaction pathways that can explain the experimental observations notably well. The most favourable pathway for B(C₆F₅)₃-catalyzed C–H silylation of N-methylindole includes nucleophilic attack, proton abstraction and hydride migration. The C–H silylation of N,N-dimethylaniline follows a similar pathway to N-methylindole rather than that proposed by Hou's group. Our mechanism successfully explains that the transformations of N-methylindoline to N-methylindole produce different products at different temperatures. For N-methylaniline bearing both N–H and para-phenyl C–H bonds, the N–H silylation reaction is more facile than the C–H silylation reaction. Our proposed mechanism of N–H silylation of N-methylaniline is different from that proposed by the groups of Paradies and Stephan. Lewis acids Al(C₆F₅)₃, Ga(C₆F₅)₃ and B(2,6-Cl₂C₆H₃)(p-HC₆F₄)₂ can also catalyze the C–H silylation of N-methylindole like B(C₆F₅)₃, but the most favourable pathways are those promoted by N-methylindoline. Furthermore, we also found several other types of substrates that would undergo C–H or N–H silylation reactions under moderate conditions. These findings may facilitate the design of new catalysts for the dehydrogenative silylation of inactivated (hetero)arenes.

We investigated the mechanism of the dehydrosilylation of (hetero)arenes and extended the scope of the silylation catalysts and substrates.

The electrophilic aromatic substitution approach to C–H silylation and C–H borylation

Several approaches toward electrophilic C–H silylation of electron-rich arenes are discussed, comprising transition-metal-catalyzed processes as well as Lewis-acid- and Brønsted-acid-induced protocols. These methods differ in the catalytic generation of the silicon electrophile but share proton removal in form of dihydrogen. With slight modifications, these methods are often also applicable to the related electrophilic C–H borylation.

Preparation and Structural Study of a Thermally Stable Bis(Disilanyl) Platinum(II) Complex

The reaction of (SiH3)(SiMeBu t2) with [Pt(PEt3)4] and dcpe gave [Pt{(SiH2)(SiMeBu t2)}2(dcpe)] (dcpe=Cy2PCH2CH2PCy2), which has been structurally characterised by single-crystal X-ray analysis.

Transition-Metal-Free Ring-Opening Silylation of Indoles and Benzofurans with (Diphenyl-tert-butylsilyl)lithium

A practical method is presented for ring opening various indoles and benzofurans with concomitant stereoselective silylation using readily generated (diphenyl-tert-butylsilyl)lithium to afford ortho-β-silylvinylanilines or -phenols. Dearomatization of the heteroarene core proceeds in the absence of any transition-metal catalyst through addition of a silyl anion and a subsequent stereoselective β-elimination. DFT calculations provide insight into the mechanism. Functionalizing C-X bond cleavage of heteroarenes is rare and generally requires transition-metal catalysts.

Insights into trans-Ligand and Spin-Orbit Effects on Electronic Structure and Ligand NMR Shifts in Transition-Metal Complexes

Surprisingly general effects of trans ligands L on the ligand NMR shifts in third-row transition-metal complexes have been found by quasi-relativistic computations, encompassing 5d¹⁰ , 5d⁸ , and to some extent even 5d⁶ situations. Closer analysis, with emphasis on ¹ H shieldings in a series of linear HAu^I L^q complexes, reveals a dominance of spin-orbit (SO) effects, which can change sign from appreciably shielding for weak trans ligands to appreciably deshielding for ligands with strong trans influence. This may be traced back to increasing destabilization of a σ-type MO at scalar relativistic level, which translates into very different σ-/π-mixing if SO coupling is included. For the strongest trans ligands, the σ-MO may move above the highest occupied π-type MOs, thereby dramatically reducing strongly shielding contributions from predominantly π-type spinors. The effects of SO-mixing are in turn related to angular momentum admixture from atomic spinors at the metal center. These SO-induced trends hold for other nuclei and may also be used to qualitatively predict shifts in unknown complexes.

Cooperative Catalysis at Metal-Sulfur Bonds

Cooperative catalysis has attracted tremendous attention in recent years, emerging as a key strategy for the development of novel atom-economic and environmentally more benign catalytic processes. In particular, Noyori-type complexes with metal-nitrogen bonds have been extensively studied and evolved as privileged catalysts in hydrogenation chemistry. In contrast, catalysts containing metal-sulfur bonds as the reactive site are out of the ordinary, despite their abundance in living systems, where they are assumed to play a key role in biologically relevant processes. For instance, the heterolysis of dihydrogen catalyzed by [NiFe] hydrogenase is likely to proceed through cooperative H-H bond splitting at a polar nickel-sulfur bond. This Account provides an overview of reported metal-sulfur complexes that allow for cooperative E-H bond (E = H, Si, and B) activation and highlights the potential of this motif in catalytic applications. In recent years, our contributions to this research field have led to the development of a broad spectrum of synthetically useful transformations catalyzed by cationic ruthenium(II) thiolate complexes of type [(DmpS)Ru(PR₃)]⁺BAr^F₄^- (DmpS = 2,6-dimesitylphenyl thiolate, Ar^F = 3,5-bis(trifluoromethyl)phenyl). The tethered coordination mode of the bulky 2,6-dimesitylphenyl thiolate ligand is crucial, stabilizing the coordinatively unsaturated ruthenium atom and also preventing formation of binuclear sulfur-bridged complexes. The ruthenium-sulfur bond of these complexes combines Lewis acidity at the metal center and Lewis basicity at the adjacent sulfur atom. This structural motif allows for reversible heterolytic splitting of E-H bonds (E = H, Si, and B) across the polar ruthenium-sulfur bond, generating a metal hydride and a sulfur-stabilized E⁺ cation. Hence, this activation mode provides a new strategy to catalytically generate silicon and boron electrophiles. After transfer of the electrophile to a Lewis-basic substrate, the resulting neutral ruthenium(II) hydride can either act as a hydride donor (reductant) or as a proton acceptor (Brønsted base); the latter scenario is followed by dihydrogen release. On the basis of this concept, the tethered ruthenium(II) thiolate complexes emerged as widely applicable catalysts for various transformations, which can be categorized into (i) dehydrogenative couplings [Si-C(sp²), Si-O, Si-N, and B-C(sp²)], (ii) chemoselective reductions (hydrogenation and hydrosilylation), and (iii) hydrodefluorination reactions. All reactions are promoted by a single catalyst motif through synergistic metal-sulfur interplay. The most prominent examples of these transformations are the first catalytic protocols for the regioselective C-H silylation and borylation of electron-rich heterocycles following a Friedel-Crafts mechanism.

A Tethered Ru-S Complex with an Axial Chiral Thiolate Ligand for Cooperative Si-H Bond Activation: Application to Enantioselective Imine Reduction

An axial chiral version of the 2,6-dimesitylphenyl group attached to sulfur is reported. Its multistep preparation starts from (S)-binol, and the thiol group is established by a racemization-free thermal Newman-Kwart rearrangement. The new chiral thiolate ligand decorated with one mesityl group is used in the synthesis of a tethered ruthenium chloride complex. Its spectroscopic characterization revealed solvent-dependent epimerization at the ruthenium center. The major diastereomer is crystallographically characterized. Chloride abstraction with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBAr^F₄ ) yields the corresponding coordinatively unsaturated ruthenium complex with the Ru-S bond exposed. Si-H bond activation at this Ru-S bond proceeds in syn fashion but with moderate facial selectivity (d.r.=70:30), generating diastereomeric chiral-at-ruthenium hydrosilane adducts. Their application to catalytic imine hydrosilylation led to promising enantioinduction (40 % ee), thereby providing proof of concept for asymmetric catalysis involving cooperative Si-H bond activation.