Catalysts For Hydrosilylation Research Articles

Synthesis of hexenyl aryl tellurides and the catalytic activity of their platinum(II) complexes as polysiloxanesupported catalysts in hydrosilylation of olefins

5-hexenyl 1-naphthyl telluride (1) and 5-hexenyl (1,1’-binaphthyl-2-yl) telluride (2) were prepared from the reaction of 1-bromonaphthalene and 2-bromobinaphthalene with di(5-hexenyl) ditelluride in presence of lithium metal. Reaction of 1 and 2 with K2PtCl4 afforded the platinum(II) complexes 3 and 4, respectively in good yields. Silica-bound telluride-platinum complexes were directly synthesized from fumed silica, triethoxysilane and platinum complexes (i.e. 3 and 4). The catalytic activities of polysiloxane-supported platinum complexes were examined with 1-hexene, 1-decene and 1-dodecene at different temperature. It was found that these catalysts are efficient for hydrosilylation of olefins with triethoxysilane.

The Effect of Substituents on the Formation of Silyl [PSiP] Pincer Cobalt(I) Complexes and Catalytic Application in Both Nitrogen Silylation and Alkene Hydrosilylation.

Four different [PSiP]-pincer ligands L1-L4 ((2-Ph2PC6H4)2SiHR (R = H (L1) and Ph (L2)) and (2-iPr2PC6H4)2SiHR' (R' = Ph (L3) and H (L4)) were used to investigate the effect of substituents at P and/or Si atom of the [PSiP] pincer ligands on the formation of silyl cobalt(I) complexes by the reactions with CoMe(PMe3)4 via Si-H cleavage. Two penta-coordinated silyl cobalt(I) complexes, (2-Ph2PC6H4)2HSiCo(PMe3)2 (1) and (2-Ph2PC6H4)2PhSiCo(PMe3)2 (2), were obtained from the reactions of L1 and L2 with CoMe(PMe3)4, respectively. Under similar reaction conditions, a tetra-coordinated cobalt(I) complex (2-iPr2PC6H4)2PhSiCo(PMe3) (3) was isolated from the interaction of L3 with CoMe(PMe3)4. It was found that, only in the case of ligand L4, silyl dinitrogen cobalt(I) complex 4, [(2-iPr2PC6H4)2HSiCo(N2)(PMe3)], was formed. Our results indicate that the increasing of electron cloud density at the Co center is beneficial for the formation of a dinitrogen cobalt complex because the large electron density at Co center leads to the enhancement of the π-backbonding from cobalt to the coordinated N2. It was found that silyl dinitrogen cobalt(I) complex 4 is an effective catalyst for catalytic transformation of dinitrogen into silylamine. Among these four silyl cobalt(I) complexes, complex 1 is the best catalyst for hydrosilylation of alkenes with excellent regioselectivity. For aromatic alkenes, catalyst 1 provided Markovnikov products, while for aliphatic alkenes, anti-Markovnikov products could be obtained. Both catalytic reaction mechanisms were proposed and discussed. The molecular structures of complexes 1-4 were confirmed by single-crystal X-ray diffraction.

Rhodium Nanoparticles Stabilized by PEG-Tagged Imidazolium Salts as Recyclable Catalysts for the Hydrosilylation of Internal Alkynes and the Reduction of Nitroarenes

PEGylated imidazolium (bromide and tetrafluoroborate) and tris-imidazolium (bromide) salts containing triazole linkers have been used as stabilizers for the preparation of water-soluble rhodium(0) nanoparticles by reduction of rhodium trichloride with sodium borohydride in water at room temperature. The nanomaterials have been characterized (Transmission Electron Microscopy, Electron Diffraction, X-ray Photoelectron Spectroscopy, Inductively Coupled Plasma-Optical Emission Spectroscopy). They proved to be efficient and recyclable catalysts for the stereoselective hydrosilylation of internal alkynes, in the presence or absence of solvent, and in the reduction of nitroarenes to anilines with ammonia-borane as hydrogen donor in aqueous medium (1:4 tetrahydrofuran/water).

Platinum-Catalyzed Hydrosilylation in Polymer Chemistry.

This paper addresses a review of platinum-based hydrosilylation catalysts. The main field of application of these catalysts is the curing of silicone polymers. Since the 1960s, this area has developed rapidly in connection with the emergence of new polymer compositions and new areas of application. Here we describe general mechanisms of the catalyst activity and the structural effects of the ligands on activity and stability of the catalysts together with the methods for their synthesis.

Sc and Y Heteroalkyl Complexes with a NCsp3N Pincer‐Type Diphenylmethanido Ligand: Synthesis, Structure, and Reactivity

New diphenylmethane [4‐tBu‐2‐(C3H2N2Me‐1)C6H3]2CH2 (1) bearing pendant imidazolyl groups in ortho‐positions of aromatic rings of diphenylmethane skeleton was synthesized using Stille cross‐coupling reaction between bis(4‐(tert‐butyl)‐2‐iodophenyl)methane and Bu3Sn(C3H2N2Me) in the presence of Pd(PPh3)4. Potassium diphenylmethanide [{[4‐tBu‐2‐(C3H2N2Me‐1)C6H3]2CH}K(OEt2)]2 (2K) was prepared by treatment of 1 with an equimolar amount of Lochmann‐Schlosser super base. Rare‐earth metal heteroalkyl complexes {[4‐tBu‐2‐(C3H2N2Me‐1)C6H3]2CH}Ln(CH2SiMe3)2(THF)n (Ln = Sc, n = 0 (2Sc); Y, n = 1 (2Y)) with NCsp3N pincer type diphenylmethanido ligand were synthesized by the selective sp3 CH‐bond activation of the central CH2 group in 1 by Ln(CH2SiMe3)3(THF)2. According to the X‐ray data the diphenylmethanido ligands in 2K and 2Sc perform a tridentate pincer‐type κ3‐N,C,N coordination. Complexes 2Ln were evaluated as catalysts for isoprene polymerization and hydrosilylation of alkenes and alkynes.

Developing a Highly Active Catalytic System Based on Cobalt Nanoparticles for Terminal and Internal Alkene Hydrosilylation.

This work describes the development of easy-to-prepare cobalt nanoparticles (NPs) in solution as promising alternative catalysts for alkene hydrosilylation with the industrially relevant tertiary silane 1,1,1,3,5,5,5-heptamethyltrisiloxane (MDHM). The Co NPs demonstrated high activity when used at 30 °C for 3.5-7 h in toluene, with catalyst loadings 0.05-0.2 mol %, without additives. Under these mild conditions, a set of terminal alkenes were found to react with MDHM, yielding exclusively the anti-Markovnikov product in up to 99% yields. Additionally, we demonstrated the possibility of using UV irradiation to further activate these cobalt NPs not only to enhance their catalytic performances but also to promote tandem isomerization-hydrosilylation reactions using internal alkenes, among them unsaturated fatty ester (methyl oleate), to produce linear products in up to quantitative yields.

Piperidinium and Pyrrolidinium Ionic Liquids as Precursors in the Synthesis of New Platinum Catalysts for Hydrosilylation

Six new air-stable anionic platinum complexes were synthesized in simple reactions of piperidinium [BMPip]Cl or pyrrolidinium [BMPyrr]Cl ionic liquids with platinum compounds ([Pt(cod)Cl2] or K2[PtCl6]). All these compounds were subjected to isolation and spectrometric characterization using NMR and ESI-MS techniques. Furthermore, the determination of melting points and thermal stability of the above derivatives was performed with the use of thermogravimetric analysis. The catalytic performance of the synthesized complexes was tested in hydrosilylation of 1-octene and allyl glycidyl ether with 1,1,1,3,5,5,5-heptamethyltrisiloxane. The study has shown that they have high catalytic activity and are insoluble in the reaction medium which enabled them to isolate and reuse them in consecutive catalytic cycles. The most active complex [BMPip]2[PtCl6] makes it possible to conduct at least 10 catalytic runs without losing activity which makes it an attractive alternative not only to commonly used homogeneous catalysts, but also to heterogeneous catalysts for hydrosilylation processes. The activity of the studied catalysts is also affected by the kind of anion and, to some extent, the kind of cation.

N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO2 Functionalizations.

The first acceptor-free heavier germanium analogue of an acylium ion, [RGe(O)(NHC)2]X (R = MesTer = 2,6-(2,4,6-Me3C6H2)2C6H3; NHC = IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; X = (Cl or BArF = {(3,5-(CF3)2C6H5)4B}), was isolated by reacting [RGe(NHC)2]X with N2O. Conversion of the germa-acylium ion to the first solely donor-stabilized germanium ester [(NHC)RGe(O)(OSiPh3)] and corresponding heavier analogues ([RGe(S)(NHC)2]X and [RGe(Se)(NHC)2]X) demonstrated its classical acylium-like behavior. The polarized terminal GeO bond in the germa-acylium ion was utilized to activate CO2 and silane, with the former found to be an example of reversible activation of CO2, thus mimicking the behavior of transition metal oxides. Furthermore, its transition-metal-like nature is demonstrated as it was found to be an active catalyst in both CO2 hydrosilylation and reductive N-functionalization of amines using CO2 as the C1 source. Mechanistic studies were undertaken both experimentally and computationally, which revealed that the reaction proceeds via an N-heterocyclic carbene (NHC) siloxygermylene [(NHC)RGe(OSiHPh2)].

Dichloro(ethylenediamine)platinum(II), a water-soluble analog of the antitumor cisplatin, as a heterogeneous catalyst for a stereoselective hydrosilylation of alkynes under neat conditions

A stereoselective method for the hydrosilylation of internal and terminal alkynes under heterogeneous catalysis by dichloro(ethylenediamine)platinum(II) is discussed. This commercially available platinum complex operates under neat conditions at 90 °C, producing exclusively the (trans) Z-isomer with symmetrical internal alkynes, while terminal alkynes produce a mixture of α- and β(E)-hydrosilylated products. No β(Z)-hydrosilylated product was observed during this study, and the selectivity between the α- and β(E)-hydrosilylated products appears to be influenced to a certain degree by the nature of the hydrosilanes. The catalyst was recyclable up to five runs under gram scale conditions without any loss of the catalytic activity.

O-Functionalised NHC Ligands for Efficient Nickel-catalysed C-O Hydrosilylation.

A series of C,O-bidentate chelating mesoionic carbene nickel(ii) complexes [Ni(NHC^PhO)₂] (NHC = imidazolylidene or triazolylidene) were applied for hydrosilylation of carbonyl groups. The catalytic system is selective towards aldehyde reduction and tolerant to electron-donating and -withdrawing group substituents. Stoichiometric experiments in the presence of different silanes lends support to a metal-ligand cooperative activation of the Si-H bond. Catalytic performance of the nickel complexes is dependent on the triazolylidene substituents. Butyl-substituted triazolylidene ligands impart turnover numbers up to 7,400 and turnover frequencies of almost 30,000 h-1, identifying this complex as one of the best-performing nickel catalysts for hydrosilylation and demonstrating the outstanding potential of O-functionalised NHC ligands in combination with first-row transition metals.

CNC]-Pincer Cobalt Hydride Catalyzed Distinct Selective Hydrosilylation of Aryl Alkene and Alkyl Alkene

The reactions of unsymmetrical N-heterocyclic carbene (NHC) [CNC]-pincer preligands with CoMe(PMe3)4 gave rise to NHC [CNC]-pincer cobalt(III) hydrides, [(CcarbeneNaminoCnaphthyl)Co(H)(PMe3)2] (3a) and (3b), via Csp2–H activation and the unexpected trans-bischelate [Ccarbene, Namino] cobalt(II) complexes 4a and 4b via a disproportionation reaction, respectively. It was found that both 3a and 3b are efficient catalysts for hydrosilylation of alkenes. With aryl alkenes as substrates, 3a has high Markovnikov selectivity in excellent yields, while 3a is an efficient anti-Markovnikov catalyst in good yields with alkyl alkenes as substrates. The catalytic process could be promoted with pyridine N-oxide as an initiator. The catalytic mechanisms for the two different selectivities were proposed. Complexes 3a, 3b, 4a, and 4b were characterized by spectroscopic methods, and the molecular structures of 3b, 4a, and 4b were determined by single crystal X-ray diffraction.

NNN‐Cobalt(II) Pincer Complexes: Paramagnetic NMR Spectroscopy in Solution and Application as Hydrosilylation Catalysts

The tridentate, monoanionic bis(oxazolinylmethylidene)‐isoindolate pincer ligand was utilized for the synthesis of various cobalt(II) high‐spin complexes. Employing standard complexation procedures, the corresponding acetato and chlorido complexes were isolated in good yields and thoroughly characterized in the solid‐state and in solution. In particular, a density functional theory (DFT) supported paramagnetic NMR analysis revealed a strong correlation between observed 13C NMR shifts and calculated spin‐density distributions, allowing for an unambiguous signal assignment in all 1H and 13C NMR spectra. An acetato‐bridged trinuclear cobalt species as well as the hexacoordinate “homoleptic” complex [(Rboxmi)2Co] were further identified as side products generated in the complex synthesis. The chlorido complexes were applied as catalysts for the cobalt‐catalyzed hydrosilylation of acetophenone, producing the chiral secondary alcohol with moderate enantioselectivities.

A Platinum Molecular Complex Immobilised on the Surface of Graphene as Active Catalyst in Alkyne Hydrosilylation

A platinum complex bearing a N‐heterocyclic carbene (NHC) ligand functionalised with a pyrene‐tag is immobilised onto the surface of reduced graphene oxide (rGO). The hybrid material composed of an organometallic complex and a graphene derivative is ready available in a single‐step process under mild reaction conditions. This methodology preserves the inherent properties of the active catalytic centre and the support. The platinum hybrid material is an efficient catalyst in hydrosilylation of alkynes and can be recycled and reused without significant loss of activity due to its high stability. Interestingly, the catalytic activity of the platinum complex is not affected by diffusion problems after immobilisation. The influence of graphene in hydrosilylation of alkynes is discussed in terms of activity and selectivity.

Highly Efficient and Reusable Alkyne Hydrosilylation Catalysts Based on Rhodium Complexes Ligated by Imidazolium-Substituted Phosphine

Rhodium complexes ligated by imidazolium-substituted phosphine were used as catalysts in the hydrosilylation of alkynes (1-heptyne, 1-octyne, and phenylacetylene) with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) or triethylsilane (TES). In all cases, the above complexes showed higher activity and selectivity compared to their precursors ([Rh(PPh3)3Cl] and [{Rh(µ-Cl)(cod)}2]). In the reactions with aliphatic alkynes (both when HMTS and TES were used as hydrosilylating agents), β(Z) isomer was mainly formed, but, in the reaction of phenylacetylene with TES, the β(E) product was formed. The catalysts are very durable, stable in air and first and foremost insoluble in the reactants which facilitated their isolation and permitted their multiple use in subsequent catalytic runs. They make a very good alternative to the commonly used homogeneous catalysts.

The effect of Schiff base ligands on the structure and catalytic activity of cobalt complexes in hydrosilylation of olefins

Environmental and economic aspects render the search for new nonprecious metal hydrosilylation catalysts an up-to-date challenge. Cobalt stands as an interesting alternative to the benchmark platinum-based species, nonetheless its relative infancy necessitates further studies, particularly regarding the structure of organic ligands that decorate the catalytically active metallic centre. As a continuation of our previous communication, we synthesized and characterized a series of new cobalt(II) chloride bench-stable precatalysts coordinated to a small library of structurally similar Schiff base ligands. Thus the synthesized species were evaluated for their ability to act as olefin hydrosilylation catalysts in the presence of alkali metal triethylborohydrides. From the crystal engineering point of view, it was observed that the number and arrangement of the Schiff base non-coordinating hydrogen bond donors affects the composition of formed complexes, resulting in predominant formation of either [CoLCl2] ‘open’ or [CoL2]2+ ‘closed’ species or a mixture of these. This affects their catalytic properties, with the benzimidazole/2H-imidazole ‘open’ system being the most efficient in terms of hydrosilylation selectivity and lowest catalyst loading. All in all, our work shows that seemingly similar coordination motifs do not necessarily lead to the isostructural group of catalytically open cobalt(II) coordination compounds, which is an important factor to consider in the design of new catalysts in general.

Bidentate N‐based Ligands for Highly Reusable, Ligand‐coordinated, Supported Pt Hydrosilylation Catalysts

AbstractA significant challenge in designing supported metal‐ligand catalysts for solution‐phase reactions is the stabilization of the metal active sites against leaching into solution. Here, we examine alkene hydrosilylation reactions as model systems to improve the stability of highly dispersed Pt using a metal‐ligand coordination strategy on high surface area oxide supports. By evaluating a series of bidentate N‐based ligands, we demonstrate several design strategies to improve stability of the highly dispersed Pt2+ centers against leaching, while maintaining a high level of catalytic activity, selectivity, and recyclability for alkene hydrosilylation batch reactions under mild conditions. These involve a bi‐functional approach to ligand design, which considers interaction to the support and a well‐defined coordination environment for the metal active site. Three strategies are reported: modifying ligands for stronger interaction with oxide surfaces, mixing ligands, and pre‐depositing an “anchoring ligand” to the support before loading the metal‐ligand catalyst. Each of these is successful in enhancing Pt recyclability. Particularly, two Pt‐phenanthroline catalysts exhibit excellent reusability for multiple batch reaction cycles, due to high stability of the active Pt species. Addressing the active site leaching problem significantly enhances the utility of ligand‐coordinated supported metal catalysts as highly stable and selective catalysts for solution‐phase reactions.

Hydrosilylation of Carbonyls Catalyzed by Hydridoborenium Borate Salts: Lewis Acid Activation and Anion Mediated Pathways.

The electronically unsaturated three-coordinated hydridoborenium cations [LBH]+[HB(C6F5)3]- (1) and [LBH]+[B(C6F5)4]- (2), supported by a bis(phosphinimino)amide ligand, were found to be excellent catalysts for hydrosilylation of a range of aliphatic and aromatic aldehydes and ketones under mild reaction conditions (L = [{(2,4,6-Me3C6H2N)P(Ph2)}2N]). The key steps of the catalytic cycle for hydrosilylation of PhCHO were monitored via in situ multinuclear NMR measurements for catalysts 1 and 2. The combined effect of carbonyl activation via the Lewis acidic hydridoborenium cation and the hydridic nature of the borate counteranion in 1 makes it a more efficient catalyst in comparison to that of carbonyl activation via the predominant Lewis acid activation pathway operating with catalyst 2. The catalytic cycle of 1 showed hydride transfer from the borate moiety [HB(C6F5)3]- to PhCHO in the first step, forming [PhCH2-O-B(C6F5)3]-, which subsequently underwent σ-bond metathesis with Et3SiH to form the product, PhCH2-O-SiEt3. Quantum chemical calculations also support the borate anion mediated mechanism with 1. In contrast, the reaction catalyzed by 2 proceeds predominantly via the Lewis acid activation of the carbonyl group involving [LB(H)←OC(H)Ph]+[B(C6F5)4]- as the transition state and [LBOCH2Ph]+[B(C6F5)4]- as the intermediate.

Partially charged platinum on aminated and carboxylated SBA-15 as a catalyst for alkene hydrosilylation.

In this paper, we report the successful preparation of a novel bifunctional heterogeneous catalyst Ptδ+/SBA-APTE-SA with a partial positively charged Ptδ+ electronic structure via post-synthesis modification of (3-aminopropyl)triethoxysilane (APTE), succinic anhydride (SA) and platinum precursors. The resulting catalyst showed superior catalytic performance for the hydrosilylation of 1,1,1,3,5,5,5-heptamethyltrisiloxane (MDHM) with allyloxy polyethylene glycol (APEG) compared to a heterogeneous platinum catalyst. In addition, our catalyst was suitable for the hydrosilylation of other alkenes. Furthermore, the catalyst displayed sufficient stability after being reused five times without noticeable inactivation. In terms of cycle number and atomic utilization efficiency, it has potential applications as a green hydrosilylation method for industry.

A general protocol for the synthesis of Pt-NHC (NHC = N-heterocyclic carbene) hydrosilylation catalysts.

A general, user-friendly synthetic route to [Pt(NHC)(L)Cl2] and [Pt(NHC)(dvtms)] (L = DMS, Py; DMS = dimethyl sulfide, dvtms = divinyltetramethylsiloxane, Py = pyridine) complexes has been developed. The procedure is applicable to a wide range of ligands and enables facile synthetic access to key Pt(0)- and Pt(ii)-NHC complexes used in hydrosilylation catalysis.

Recent Advances on Heterogeneous Metal Catalysts for Hydrosilylation of Olefins

Recent Advances on Heterogeneous Metal Catalysts for Hydrosilylation of Olefins