Hydrosilylation of Aldehydes and Ketones Catalyzed by Half-Sandwich Manganese(I) N-Heterocyclic Carbene Complexes

Platinum(II) Phenylpyridyl Schiff Base Complexes as Latent, Photoactivated, Alkene Hydrosilylation Catalysts

Photoactivated catalysts for the hydrosilylation of alkenes with silanes offer temporal control in manufacturing processes that require silicone curing. We report the development of a range of air-stable Pt(II) (salicylaldimine)(phenylpyridyl), [Pt(sal)(ppy)], complexes as photoinitiated hydrosilylation catalysts. Some of these catalysts show appreciable latency in thermal catalysis and can also be rapidly (10 s) activated by a LED UV-light source (365 nm), to give systems that selectively couple trimethylvinylsilane and hexamethylsiloxymethylsilane to give the linear hydrosilylation product. Although an undetectable (by NMR spectroscopy) amount of precatalyst is converted to the active form under UV-irradiation in the timescale required to initiate hydrosilylation, clean and reliable kinetics can be measured for these systems that allow for a detailed mechanism to be developed for Pt(sal)(ppy)-based photoactivated hydrosilylation. The suggested mechanism is shown to have close parallels with, but also subtle differences from, those previously proposed for thermally-activated Karstedt-type Pt(0) systems.

Hydrosilylation of Terminal Alkynes Catalyzed by an Air-Stable Manganese-NHC Complex

In recent years, catalysis with base metal manganese has received a significant amount of interest. Catalysis with manganese complexes having N-heterocyclic carbenes (NHCs) is relatively underdeveloped in comparison to the extensively investigated manganese catalysts possessing pincer ligands (particularly phosphine-based ligands). Herein, we describe the synthesis of two imidazolium salts decorated with picolyl arms (L₁ and L₂) as NHC precursors. Facile coordination of L₁ and L₂ with MnBr(CO)₅ in the presence of a base resulted in the formation manganese(I)-NHC complexes (1 and 2) as an air-stable solid in good isolated yield. Single-crystal X-ray analysis revealed the structure of the cationic complexes [Mn(CO)₃(NHC)][PF₆] with tridentate N,C,N binding of the NHC ligand in a facile fashion. Along with a few known manganese(I) complexes, these Mn(I)-NHC complexes 1 and 2 were tested for the hydrosilylation of terminal alkynes. Complex 1 was proved to be an effective catalyst for the hydrosilylation of terminal alkynes with good selectivity toward the less thermodynamically stable β-(Z)-vinylsilanes. This method provided good regioselectivity (anti-Markovnikov addition) and stereoselectivity (β-(Z)-product). Experimental evidence suggested that the present hydrosilylation pathway involved an organometallic mechanism with manganese(I)-silyl species as a possible reactive intermediate.

Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones

Manganese has attracted significant recent attention due to its abundance, low toxicity, and versatility in catalysis. In the present study, a series of manganese (III) complexes supported by salan ligands have been synthesized and characterized, and their activity as catalysts in the hydrosilylation of carbonyl compounds was examined. While manganese (III) chloride complexes exhibited minimal catalytic efficacy without activation of silver perchlorate, manganese (III) azide complexes showed good activity in the hydrosilylation of carbonyl compounds. Under optimized reaction conditions, several types of aldehydes and ketones could be reduced with good yields and tolerance to a variety of functional groups. The possible mechanisms of silane activation and hydrosilylation were discussed in light of relevant experimental observations.

Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes

The coordination chemistry of the N-heterocyclic carbene ligand IMes^(NMe2)2, derived from the well-known IMes ligand by substitution of the carbenic heterocycle with two dimethylamino groups, was investigated with d⁶ [Mn(I), Fe(II)], d⁸ [Rh(I)], and d¹⁰ [Cu(I)] transition-metal centers. The redox behavior of the resulting organometallic complexes was studied through a combined experimental/theoretical study, involving electrochemistry, EPR spectroscopy, and DFT calculations. While the complexes [CuCl(IMes^(NMe2)2)], [RhCl(COD)(IMes^(NMe2)2)], and [FeCp(CO)₂ (IMes^(NMe2)2)](BF₄) exhibit two oxidation waves, the first oxidation wave is fully reversible but only for the first complex the second oxidation wave is reversible. The mono-oxidation event for these complexes occurs on the NHC ligand, with a spin density mainly located on the diaminoethylene NHC-backbone, and has a dramatic effect on the donating properties of the NHC ligand. Conversely, as the Mn(I) center in the complex [MnCp(CO)₂ ((IMes^(NMe2)2)] is easily oxidizable, the latter complex is first oxidized on the metal center to form the corresponding cationic Mn(II) complex, and the NHC ligand is oxidized in a second reversible oxidation wave.

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

The emerging field of organometallic catalysis has shifted towards research on Earth-abundant transition metals due to their ready availability, economic advantage, and novel properties. In this case, manganese, the third most abundant transition-metal in the Earth's crust, has emerged as one of the leading competitors. Accordingly, a large number of molecularly-defined Mn-complexes has been synthesized and employed for hydrogenation, dehydrogenation, and hydroelementation reactions. In this regard, catalyst design is based on three pillars, namely, metal-ligand bifunctionality, ligand hemilability, and redox activity. Indeed, the developed catalysts not only differ in the number of chelating atoms they possess but also their working principles, thereby leading to different turnover numbers for product molecules. Hence, the critical assessment of molecularly defined manganese catalysts in terms of chelating atoms, reaction conditions, mechanistic pathway, and product turnover number is significant. Herein, we analyze manganese complexes for their catalytic activity, versatility to allow multiple transformations and their routes to convert substrates to target molecules. This article will also be helpful to get significant insight into ligand design, thereby aiding catalysis design.

Hydrosilylation of Aldehydes and Ketones Catalyzed by a 2-Iminopyrrolyl Alkyl-Manganese(II) Complex

A well-defined and very active single-component manganese(II) catalyst system for the hydrosilylation of aldehydes and ketones is presented. First, the reaction of 5-(2,4,6-iPr₃C₆H₂)-2-[N-(2,6-iPr₂C₆H₃)formimino]pyrrolyl potassium (KL) and [MnCl₂(Py)₂] afforded the binuclear 2-iminopyrrolyl manganese(II) pyridine chloride complex [Mn₂{κ²N,N'-5-(2,4,6-iPr₃C₆H₂)-NC₄H₂-2-C(H)═N(2,6-iPr₂C₆H₃)}₂(Py)₂(μ-Cl)₂] 1. Subsequently, the alkylation reaction of complex 1 with LiCH₂SiMe₃ afforded the respective (trimethylsilyl)methyl-Mn(II) complex [Mn{κ²N,N'-5-(2,4,6-iPr₃C₆H₂)-NC₄H₂-2-C(H)═N(2,6-iPr₂C₆H₃)}(Py)CH₂SiMe₃] 2 in a good yield. Complexes 1 and 2 were characterized by elemental analysis, ¹H NMR spectroscopy, Evans' method, FTIR spectroscopy, and single-crystal X-ray diffraction. While the crystal structure of complex 1 has been identified as a binuclear entity, in which the Mn(II) centers present pentacoordinate coordination spheres, that of complex 2 corresponds to a monomer with a distorted tetrahedral coordination geometry. Complex 2 proved to be a very active precatalyst for the atom-economic hydrosilylation of several aldehydes and ketones under very mild conditions, with a maximum turnover frequency of 95 min^-1, via a silyl-Mn(II) mechanistic route, as asserted by a combination of experimental and theoretical efforts, the respective silanes were cleanly converted to the respective alcoholic products in high yields.

Half-sandwich manganese complexes Cp(CO)2Mn(NHC) as redox-active organometallic fragments

Oxidation of the half-sandwich Mn^I complexes Cp(CO)₂Mn(NHC) bearing dialkyl-, arylalkyl- and diarylsubstituted N-heterocyclic carbene ligands (NHC = IMe, IMeMes, IMes) affords the corresponding stable Mn^II radical cations [Cp(CO)₂Mn(NHC)](BF₄) isolated in 92-95% yield. Systematic X-ray diffraction studies of the series of Mn^I and Mn^II NHC complexes revealed the expected characteristic structural changes upon oxidation, namely the elongation of the Mn-CO and Mn-NHC bonds as well as the diminution of the OC-Mn-CO angle. ESR spectra of [Cp(CO)₂Mn(IMes)](BF₄) in frozen solution (CH₂Cl₂/toluene 1 : 1, 70 K) allowed the identification of two conformers for this complex and their structural assignment using DFT calculations. The stability of these NHC complexes in both metal oxidation states, moderate oxidation potentials and the ease of detection of Mn^II species by a variety of spectroscopic techniques (UV-Vis, IR, paramagnetic ¹H NMR, and ESR) make these compounds promising objects for applications as redox-active organometallic fragments.

Reduction of Carboxylic Acids to Alcohols via Manganese(I) Catalyzed Hydrosilylation

The reduction of carboxylic acids to the respective alcohols, in mild conditions, was achieved using [MnBr(CO)₅] as the catalyst and bench stable PhSiH₃ as the reducing agent. It was shown that the reaction with the earth-abundant metal catalyst could be performed either with a catalyst loading as low as 0.5 mol %, rare with the use of [MnBr(CO)₅], or on a gram scale employing only 1.5 equiv of PhSiH₃, the lowest amount of silane reported to date for this transformation. Kinetic data and control experiments have provided initial insight into the mechanism of the catalytic process, suggesting that it proceeds via the formation of silyl ester intermediates and ligand dissociation to generate a coordinatively unsaturated Mn(I) complex as the active species.

Hydrosilylation of Aldehydes by a Manganese α-Diimine Complex

This paper describes the catalytic activity of air stable and easy to handle manganese complexes towards the hydrosilylation of aldehydes. These catalysts incorporate a bulky diazabutadiene ligand and exhibit good functional group tolerance and chemoselectivity in the hydrosilylation of aldehydes, utilizing primary silanes as the reducing agent. The reactions proceed with turnover frequencies approaching 150 h−1 in some instances, similar to those observed for other manganese-based catalysts. The conversion of aromatic aldehydes to the corresponding alcohols was found to be more efficient than that for the analogous aliphatic systems.

Hydrosilylation of Esters Catalyzed by Bisphosphine Manganese(I) Complex: Selective Transformation of Esters to Alcohols

Selective and efficient hydrosilylations of esters to alcohols by a well-defined manganese(I) complex with a commercially available bisphosphine ligand are described. These reactions are easy alternatives for stoichiometric hydride reduction or hydrogenation, and employing cheap, abundant, and nonprecious metal is attractive. The hydrosilylations were performed at 100 °C under solvent-free conditions with low catalyst loading. A large variety of aromatic, aliphatic, and cyclic esters bearing different functional groups were selectively converted into the corresponding alcohols in good yields.

1-D manganese(ii)-terpyridine coordination polymers as precatalysts for hydrofunctionalisation of carbonyl compounds

Efficient hydroboration and hydrosilylation of ketones and aldehydes has been achieved using a Mn^II-coordination polymer as precatalyst under mild conditions_.

Well-defined nickel(II) tetrazole-saccharinate complex as homogeneous catalyst on the reduction of aldehydes: scope and reaction mechanism

A new (tetrazole-saccharin)nickel complex is shown to be a valuable catalyst for the hydrosilative reduction of aldehydes under microwave radiation at low temperatures. With typical 1 mol% content of the catalyst (microwave power range of 5–15 W) most reactions are complete within 30 min. The Ni(II)-catalyzed reduction of aldehydes, with a useful scope, was established for the first time by using this catalyst, and is competitive with the most effective transition-metal catalysts known for such transformation. The catalyst reveals tolerance to different functional groups, is air and moisture stable, and is readily prepared in straightforward synthetic steps. Supported by experimental data and DFT calculations, a plausible reaction mechanism involving the new catalytic system is outlined.

Rhenium-Catalyzed Reduction of Carboxylic Acids with Hydrosilanes

Re₂(CO)₁₀ efficiently catalyzes the direct reduction of various carboxylic acids under mild conditions (rt, irradiation 350 or 395 nm). While aliphatic carboxylic acids were readily converted to the corresponding disilylacetals with low catalyst loading (0.5 mol %) in the presence of Et₃SiH (2.2 equiv), aromatic analogues required more drastic conditions (Re₂(CO)₁₀ 5 mol %, Ph₂MeSiH 4.0 equiv) to afford the corresponding aldehydes after acid treatment.

Selective Hydroboration of Carboxylic Acids with a Homogeneous Manganese Catalyst

Catalytic reduction of carboxylic acid to the corresponding alcohol is a challenging task of great importance for the production of a variety of value-added chemicals. Herein, a manganese-catalyzed chemoselective hydroboration of carboxylic acids has been developed with a high turnover number (>99 000) and turnover frequency (>2000 h^-1) at 25 °C. This method displayed tolerance of electronically and sterically differentiated substrates with high chemoselectivity. Importantly, aliphatic long-chain fatty acids, including biomass-derived compounds, can efficiently be reduced. Mechanistic studies revealed that the reaction occurs through the formation of active manganese-hydride species via an insertion and bond metathesis type mechanism.

Manganese catalyzed α-methylation of ketones with methanol as a C1 source

The direct α-methylation of ketones with methanol under hydrogen borrowing conditions using a well-defined manganese PN³P pre-catalyst was, for the first time, achieved.

Hydrosilylation of carbonyl and carboxyl groups catalysed by Mn(i) complexes bearing triazole ligands

Manganese(i) complexes bearing readily accessible triazole ligands are effective catalysts for the hydrosilylation of carbonyl and carboxyl compounds.

Manganese-Catalyzed Hydrosilylation Reactions

Over the past few decades, manganese-catalyzed hydrosilylation of C=O or C=C/C≡C unsaturated bonds have undergone enormous developments. In this focus review, the hydrosilylation reactions of alkenes, alkynes, and carbonyl-containing substrates catalyzed by manganese complexes are summarized. Moreover, the mechanisms of the manganese-catalyzed hydrosilylation are briefly discussed.

Carbonyl and ester C–O bond hydrosilylation using κ4-diimine nickel catalysts

(^Ph2PPrDI)Ni chemoselectively catalyzes α-allyl ester C–O bond hydrosilylation to prepare silyl esters with turnover frequencies of up to 990 h⁻¹.

Manganese catalyzed reductive amination of aldehydes using hydrogen as a reductant

The first alkylation of amines via reductive amination of aldehydes catalyzed by a manganese bidentate pyridinyl–phosphine complex.

The Emergence of Manganese-Based Carbonyl Hydrosilylation Catalysts

In recent years, interest in homogeneous manganese catalyst development has intensified because of the earth-abundant and nontoxic nature of this metal. Although compounds of Mn have largely been utilized for epoxidation reactions, recent efforts have revealed that Mn catalysts can mediate a broad range of reductive transformations. Low-valent Mn compounds have proven to be particularly effective for the hydrosilylation of carbonyl- and carboxylate-containing substrates, and this Account aims to highlight my research group's contributions to this field. In our initial 2014 communication, we reported that the bis(imino)pyridine-supported compound (^Ph2PPrPDI)Mn mediates ketone hydrosilylation with exceptional activity under solvent-free conditions. Silanes including Ph₂SiH₂, (EtO)₃SiH, (EtO)₂MeSiH, and (EtO)Me₂SiH were found to partially reduce cyclohexanone in the presence of (^Ph2PPrPDI)Mn, while turnover frequencies of up to 1280 min^-1 were observed using PhSiH₃. This led us to evaluate the hydrosilylation of 11 additional ketones and allowed for the atom-efficient preparation of tertiary and quaternary silanes. At that time, it was also discovered that (^Ph2PPrPDI)Mn catalyzes the dihydrosilylation of esters (by way of acyl C-O bond hydrosilylation) to yield a mixture of silyl ethers with modest activity. Earlier this year, the scope of these transformations was extended to aldehydes and formates, and the observed hydrosilylation activities are among the highest obtained for any transition-metal catalyst. The effectiveness of three related catalysts has also been evaluated: (^Ph2PPrPDI)MnH, (^PyEtPDEA)Mn, and [(^Ph2PEtPDI)Mn]₂. To our surprise, (^Ph2PPrPDI)MnH was found to exhibit higher carboxylate dihydrosilylation activity than (^Ph2PPrPDI)Mn, while (^PyEtPDEA)Mn demonstrated remarkable carbonyl hydrosilylation activity considering that it lacks a redox-active supporting ligand. The evaluation of [(^Ph2PEtPDI)Mn]₂ revealed competitive aldehyde hydrosilylation and formate dihydrosilylation turnover frequencies; however, this catalyst is significantly inhibited by pyridine and alkene donor groups. In our efforts to fully understand how (^Ph2PPrPDI)Mn operates, a thorough electronic structure evaluation was conducted, and the ground-state doublet calculated for this compound was found to exhibit nonclassical features consistent with a low-spin Mn(II) center supported by a singlet PDI dianion and an intermediate-spin Mn(II) configuration featuring antiferromagnetic coupling to PDI diradical dianion. A comprehensive mechanistic investigation of (^Ph2PPrPDI)Mn- and (^Ph2PPrPDI)MnH-mediated hydrosilylation has revealed two operable pathways, a modified Ojima pathway that is more active for carbonyl hydrosilylation and an insertion pathway that is more effective for carboxylate reduction. Although these efforts represent a small fraction of the recent advances made in Mn catalysis, this work has proven to be influential for the development of Mn-based reduction catalysts and is likely to inform future efforts to develop Mn catalysts that can be used to prepare silicones.