Iridium-Catalyzed Hydrogen Production from Hydrosilanes and Water

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.

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.

Tailor-Made Synthesis of Hydrosilanols, Hydrosiloxanes, and Silanediols Catalyzed by di-Silyl Rhodium(III) and Iridium(III) Complexes

Siloxanes and silanols containing Si-H units are important building blocks for the synthesis of functionalized siloxane materials, and their synthesis is a current challenge. Herein, we report the selective synthesis of hydrosilanols, hydrosiloxanes, and silanodiols depending on the nature of the catalysts and the silane used. Two neutral ({MCl[SiMe2(o-C6H4PPh2)]2}; M = Rh, Ir) and two cationic ({M[SiMe2(o-C6H4PPh2)]2(NCMe)}[BArF4]; M = Rh, Ir) have been synthesized and their catalytic behavior toward hydrolysis of secondary silanes has been described. Using the iridium complexes as precatalysts and diphenylsilane as a substrate, the product obtained is diphenylsilanediol. When rhodium complexes are used as precatalysts, it is possible to selectively obtain silanediol, hydrosilanol, and hydrosiloxane depending on the catalysts (neutral or cationic) and the silane substituents.

Selective oxidation of silanes into silanols with water using [MnBr(CO)5] as a precatalyst

The development of earth-abundant catalysts for the selective conversion of silanes to silanols with water as an oxidant generating valuable hydrogen as the only by-product continues to be a challenge. Here, we demonstrate that [MnBr(CO)₅] is a highly active precatalyst for this reaction, operating under neutral conditions and avoiding the undesired formation of siloxanes. As a result, a broad substrate scope, including primary and secondary silanes, could be converted to the desired products. The turnover performances of the catalyst were also examined, yielding a maximum TOF of 4088 h^-1. New light was shed on the debated mechanism of the interaction between [MnBr(CO)₅] and Si-H bonds based on the reaction kinetics (including KIEs of PhMe₂SiD and D₂O) and spectroscopic techniques (FT-IR, GC-TCD, ¹H-, ²⁹Si-, and ¹³C-NMR). The initial activation of [MnBr(CO)₅] was found to result from the formation of a manganese(i) hydride species and R₃SiBr, and the experimental data are most consistent with a catalytic cycle comprising a cationic tricarbonyl Mn(i) unit as the active framework.

Synthesis and Characterization of Ir-(κ2-NSi) Species Active toward the Solventless Hydrolysis of HSiMe(OSiMe3)2

The reaction of [IrH(Cl)(κ2-NSitBu2)(coe)] (1) with 1 equiv of PCy3 (or PHtBu2) gives the species [IrH(Cl)(κ2-NSitBu2)(L)] (L = PCy3, 2a; PHtBu2, 2b), which reacts with 1 equiv of AgOTf to afford [IrH(OTf)(κ2-NSitBu2)(L)] (L = PCy3, 3a and PHtBu2, 3b). Complexes 2a, 2b, 3a, and 3b have been characterized by means of NMR spectroscopy and HR-MS. The solid-state structures of complexes 2a, 2b, and 3a have been determined by X-ray diffraction studies. The reversible coordination of water to 3a, 3b, and 4 to afford the corresponding adduct [IrH(OTf)(κ2-NSitBu2)(L)(H2O)] (L = PCy3, 3a-H2O; PHtBu2, 3b-H2O; coe, 4-H2O) has been demonstrated spectroscopically by NMR studies. The structure of complexes 3b-H2O and 4-H2O have been determined by X-ray diffraction studies. Computational analyses of the interaction between neutral [NSitBu2]• and [Ir(H)L(X)]• fragments in Ir-NSitBu2 species confirm the electron-sharing nature of the Ir-Si bond and the significant role of electrostatics in the interaction between the transition metal fragment and the κ2-NSitBu2 ligand. The activity of Ir-(κ2-NSitBu2) species as catalysts for the hydrolysis of HSiMe(OSiMe3)2 depends on the nature of the ancillary ligands. Thus, while the triflate derivatives are active, the related chloride species show no activity. The best catalytic performance has been obtained when using complexes 3a, with triflate and PCy3 ligands, as a catalyst precursor, which allows the selective obtention of the corresponding silanol.

Hydrogen Tunneling in Catalytic Hydrolysis and Alcoholysis of Silanes

An unprecedented quantum tunneling effect has been observed in catalytic Si−H bond activations at room temperature. The cationic hydrido‐silyl‐iridium(III) complex, {Ir[SiMe(o‐C₆H₄SMe)₂](H)(PPh₃)(THF)}[BAr^F₄], has proven to be a highly efficient catalyst for the hydrolysis and the alcoholysis of organosilanes. When triethylsilane was used as a substrate, the system revealed the largest kinetic isotopic effect (KIE_{Si−H/Si−D}=346±4) ever reported for this type of reaction. This unexpectedly high KIE, measured at room temperature, together with the calculated Arrhenius preexponential factor ratio (A_H/A_D=0.0004) and difference in the observed activation energy [(EaD −EaH )=34.07 kJ mol⁻¹] are consistent with the participation of quantum tunneling in the catalytic process. DFT calculations have been used to unravel the reaction pathway and identify the rate‐determining step. Aditionally, isotopic effects were considered by different methods, and tunneling effects have been calculated to be crucial in the process.

Pd, Rh and Ru Nanohybrid-catalyzed Tetramethyldisiloxane Hydroysis for H2 Generation, Nitrophenol Reduction and Suzuki-Miyaura Cross-Coupling

The hydrolysis of tetramethyldisiloxane, which is a silicone industrial refuse, provides a convenient method to generates H2 on demand. Herein, the highly selective and efficient 2D graphene-like carbon nanosheets (GCN)...

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.

In this Perspective we compare the ability of transition metals and p-block Lewis acids to activate electrophilically hydrosilanes and hydroboranes. The mechanistic similarities and dissimilarities in different catalytic transformations are analyzed.

Photocatalyzed cross-dehydrogenative coupling of silanes with alcohols and water

An efficient method for the dehydrogenative coupling of silanes with alcohols under photocatalysis was developed. The reaction proceeded in the presence of Ru(bpy)₃Cl₂ (0.5 mol%) under visible light irradiation in acetonitrile at room temperature. The developed methodology was also applicable for the synthesis of silanols using water as a coupling partner.

Low-temperature catalytic hydrogenation of bio-based furfural and relevant aldehydes using cesium carbonate and hydrosiloxane

A benign catalytic system consisting of Cs₂CO₃ and PMHS can effectively reduce furfural to furfuryl alcohol with a high yield of 99.5% at 25–80 °C via siloxane, which is also applicable to other aromatic aldehydes.

Direct Dehydrogenative Coupling of Alcohols with Hydrosilanes Promoted by Sodium tri(sec-butyl)borohydride

Alkoxysilanes find application in many areas of chemistry ranging from research-scale organic synthesis to multi-ton production of materials. Classically, they are obtained in stoichiometric reaction of alcoholysis of chlorosilanes, however, recent years brought development in the field of direct dehydrogenative coupling of hydrosilanes with alcohols, which is a more atom-economic and benign alternative to the former process. In this paper, we report the use of sodium tri(sec-butyl)borohydride as a convenient promoter of this reaction. Exemplary syntheses carried out under mild conditions and without additional solvents, followed by very easy work-up procedure, show excellent potential for application of so devised catalytic system.

Revisiting the iridacycle-catalyzed hydrosilylation of enolizable imines

In situ¹H NMR spectroscopy reveals a cascade mechanism for the hydrosilylation of enolizable imines catalyzed by iridium(iii) metallacycles.

Dehydrogenative Coupling of Hydrosilanes and Alcohols by Alkali Metal Catalysts for Facile Synthesis of Silyl Ethers

Cross-dehydrogenative coupling (CDC) of hydrosilanes with hydroxyl groups, using alkali metal hexamethyldisilazide as a single-component catalyst for the formation of Si–O bonds under mild condition, is reported. The potassium salt [KN(SiMe3)2] is highly efficient and chemoselective for a wide range of functionalized alcohols (99 % conversion) under solvent-free conditions. The CDC reaction of alcohols with silanes exhibits first-order kinetics with respect to both catalyst and substrate concentrations. The most plausible mechanism for this reaction suggests that the initial step most likely involves the formation of an alkoxide followed by the formation of metal hydride as active species.

Silylation of O–H bonds by catalytic dehydrogenative and decarboxylative coupling of alcohols with silyl formates

Silylformates HCO₂SiR₃ are used for the first time as efficient silylating agents for alcohols, in the presence of a ruthenium molecular catalyst.

Tuning the activity and selectivity of iridium-NSiN catalyzed CO2 hydrosilylation processes

Catalyst design for iridium-catalyzed CO₂ hydrosilylation processes: improvement of the selectivity and reduction of the reaction time.

Catalytic hydrodechlorination of benzyl chloride promoted by Rh-N-heterocyclic carbene catalysts

The rhodium(I) complexes [Rh(Cl)(COD)(R-NHC-(CH2 )3 Si(OiPr)3 )] [COD=cyclooctadiene; R=2,6-diisopropylphenyl (1 a); n-butyl (1 b)] are effective catalyst precursors for the homogeneous hydrodechlorination of benzyl chloride using HSiEt3 as hydrogen source. This reaction is selective to the formation of toluene. However, in presence of a stoichiometric amount of potassium tert-butoxide (KtBuO) the formation of mixtures containing toluene together with 17-19 mol % of the C--C homocoupling product, namely PhCH2 CH2 Ph, is observed. A mechanism proposal based on experimental insights and theoretical calculations at the DFT level that allows explanation of the experimental findings is included. Moreover, the heterogeneous catalytic system based on catalyst 1 a supported on MCM-41 has been demonstrated to be effective for the solvent-free hydrodechlorination of benzyl chloride using HSiEt3 and HSiMe(OSiMe3 )2 .

Orthometallation of N-substituents at the NHC ligand of [Rh(Cl)(COD)(NHC)] complexes: its role in the catalytic hydrosilylation of ketones

Orthometallation of one N-substituent of the NHC ligand in Rh-NHC species affords hydrido-bridged binuclear rhodium(iii) complexes which have proven to be resting states in catalytic ketone hydrosilylation reactions.

Solvent-free iridium-catalyzed CO2 hydrosilylation: experiments and kinetic modeling

Solvent-free iridium(iii)-catalyzed CO₂ hydrosilylation to silylformate and kinetic modeling of such reaction are reported.