Enhanced Catalytic Activity of Iridium(III) Complexes by Facile Modification of C,N-Bidentate Chelating Pyridylideneamide Ligands

Sterically and Electronically Flexible Pyridylidene Amine Dinitrogen Ligands at Palladium: Hemilabile cis/trans Coordination and Application in Dehydrogenation Catalysis

Ligand design is crucial for the development of new catalysts and materials with new properties. Herein, the synthesis and unique hemilabile coordination properties of new bis-pyridylidene amine (bis-PYE) ligands to palladium, and preliminary catalytic activity of these complexes in formic acid dehydrogenation are described. The synthetic pathway to form cationic complexes [Pd(bis-PYE)Cl(L)]X with a cis-coordinated N,N-bidentate bis-PYE ligand is flexible and provides access to a diversity of PdII complexes with different ancillary ligands (L=pyridine, DMAP, PPh3 , Cl, P(OMe)3 ). The 1 H NMR chemical shift of the trans-positioned PYE N-CH3 unit is identified as a convenient and diagnostic handle to probe the donor properties of these ancillary ligands and demonstrates the electronic flexibility of the PYE ligand sites. In the presence of a base, the originally cis-coordinated bis-PYE ligand adopts a N,N,N-tridentate coordination mode with the two PYE units in mutual trans position. This cis-trans isomerization is reverted in presence of an acid, demonstrating a unique structural and steric flexibility of the bis-PYE ligand at palladium in addition to its electronic adaptability. The palladium complexes are active in formic acid dehydrogenation to H2 and CO2 . The catalytic performance is directly dependent on the ligand bonding mode, the nature of the ancillary ligand, the counteranion, and additives. The most active system features a bidentate bis-PYE ligand, PPh3 as ancillary ligand and accomplishes turnover frequencies up to 525 h-1 in the first hour and turnover numbers of nearly 1000, which is the highest activity reported for palladium-based catalysts to date.

Iridium(i) complexes bearing hemilabile coumarin-functionalised N-heterocyclic carbene ligands with application as alkyne hydrosilylation catalysts

A set of iridium(i) complexes of formula IrCl(κC,η²-I^RCou^R')(cod) or IrCl(κC, η²-BzI^RCou^R')(cod) (cod = 1,5-cyclooctadiene; Cou = coumarin; I = imidazolin-2-carbene; BzI = benzimidazolin-2-carbene) have beeen prepared from the corresponding azolium salt and [Ir(μ-OMe)(cod)]₂ in THF at room temperature. The crystalline structures of 4b and 5b show a distorted trigonal bipyramidal configuration in the solid state with a coordinated coumarin moiety. In contrast, an equilibrium between this pentacoordinated structure and the related square planar isomer is observed in solution as a consequence of the hemilability of the pyrone ring. Characterization of both species by NMR was achieved at the low and high temperature limits, respectively. In addition, the thermodynamic parameters of the equilibrium, ΔH_R and ΔS_R, were obtained by VT ¹H NMR spectroscopy and fall in the range 22-33 kJ mol^-1 and 72-113 J mol^-1 K^-1, respectively. Carbonylation of IrCl(κC,η²-BzI^TolCou^7,8-Me₂)(cod) resulted in the formation of a bis-CO derivative showing no hemilabile behaviour. The newly synthesised complexes efficiently catalyze the hydrosilylation of alkynes at room temperature with a preference for the β-(Z) vinylsilane isomer.

Transition-Metal-Catalyzed Stereo- and Regioselective Hydrosilylation of Unsymmetrical Alkynes

Alkyne hydrosilylation is one of the most efficient methods for the synthesis of alkenyl silicon derivatives and has been a hot topic of research for decades. This short review summarizes the progress in transition-metal-catalyzed stereo- and regioselective hydrosilylation of unsymmetrical alkynes. Topics are discussed based on different types of alkynes and the selectivities.

1 Introduction

2 Terminal Alkyne Hydrosilylation

2.1 β-E Selectivity

2.2 β-Z Selectivity

2.3 α-selectivity

3 Internal Alkyne Hydrosilylation

3.1 Aryl–Alkyl Acetylenes

3.2 Alkyl–Alkyl Acetylenes

3.3 Internal Alkynes with Polarized Substituents

4 Summary and Outlook

Ruthenacycles and Iridacycles as Transfer Hydrogenation Catalysts

In this review, we describe the synthesis and use in hydrogen transfer reactions of ruthenacycles and iridacycles. The review limits itself to metallacycles where a ligand is bound in bidentate fashion to either ruthenium or iridium via a carbon–metal sigma bond, as well as a dative bond from a heteroatom or an N-heterocyclic carbene. Pincer complexes fall outside the scope. Described are applications in (asymmetric) transfer hydrogenation of aldehydes, ketones, and imines, as well as reductive aminations. Oxidation reactions, i.e., classical Oppenauer oxidation, which is the reverse of transfer hydrogenation, as well as dehydrogenations and oxidations with oxygen, are described. Racemizations of alcohols and secondary amines are also catalyzed by ruthenacycles and iridacycles.

Modulation of N^N'-bidentate chelating pyridyl-pyridylidene amide ligands offers mechanistic insights into Pd-catalysed ethylene/methyl acrylate copolymerisation

The efficient copolymerisation of functionalised olefins with alkenes continues to offer considerable challenges to catalyst design. Based on recent work using palladium complexes containing a dissymmetric N^N'-bidentate pyridyl-PYA ligand (PYA = pyridylidene amide), which showed a high propensity to insert methyl acrylate, we have here modified this catalyst structure by inserting shielding groups either into the pyridyl fragment, or the PYA unit, or both to avoid fast β-hydrogen elimination. While a phenyl substituent at the pyridyl side impedes catalytic activity completely and leads to an off-cycle cyclometallation, the introduction of an ortho-methyl group on the PYA side of the N^N'-ligand was more prolific and doubled the catalytic productivity. Mechanistic investigations with this ligand system indicated the stabilisation of a 4-membered metallacycle intermediate at room temperature, which has previously been postulated and detected only at 173 K, but never observed at ambient temperature so far. This intermediate was characterised by solution NMR spectroscopy and rationalises, in part, the formation of α,β-unsaturated esters under catalytic conditions, thus providing useful principles for optimised catalyst design.

Recent Development in the Synthesis and Catalytic Application of Iridacycles

Cyclometallated complexes are well-known and have found many applications. This article provides a short review on the progress made in the synthesis and application to catalysis of cyclometallated half-sandwich Cp*Ir(III) complexes (Cp*: pentamethylcyclopentadienyl) since 2017. Covered in the review are iridacycles featuring conventional C,N chelates and less common metallocene and carbene-derived C,N and C,C ligands. This is followed by an overview of the studies of their applications in catalysis ranging from asymmetric hydrogenation, transfer hydrogenation, hydrosilylation to dehydrogenation.

Resin-supported iridium complex for low-temperature vanillin hydrogenation using formic acid in water

Biorefinery seeks to utilize biomass waste streams as a source of chemical precursors with which to feed the chemical industry. This goal seeks to replace petroleum as the main feedstock, however this task requires the development of efficient catalysts capable of transforming substances derived from biomass into useful chemical products. In this study, we demonstrate that a highly-active iridium complex can be solid-supported and used as a low-temperature catalyst for both the decomposition of formic acid (FA) to produce hydrogen, and as a hydrogenation catalyst to produce vanillyl alcohol (VA) and 2-methoxy-4-methylphenol (MMP) from vanillin (V); a lignin-derived feedstock. These hydrogenation products are promising precursors for epoxy resins and thus demonstrate an approach for their production without the need for petroleum. In contrast to other catalysts that require temperatures exceeding 100 °C, here we accomplish this at a temperature of <50 °C in water under autogenous pressure. This approach provides an avenue towards biorefinery with lower energy demands, which is central to the decentralization and broad implementation. We found that the high activity of the iridium complex transfers to the solid-support and is capable of accelerating the rate determining step; the decomposition of FA into hydrogen and carbon dioxide. The yield of both VA and MMP can be independently tuned depending on the temperature. The simplicity of this approach expands the utility of molecular metal complexes and provides new catalyst opportunities in biorefinery.

Highly Efficient Transfer Hydrogenation Catalysis with Tailored Pyridylidene Amide Pincer Ruthenium Complexes

The rational optimization of homogeneous catalysts requires ligand platforms that are easily tailored to improve catalytic performance. Here, it is demonstrated that pyridylidene amides (PYAs) provide such a platform to custom-shape transfer hydrogenation catalysts with exceptional activity. Specifically, a series of meta-PYA pincer ligands with differently substituted PYA units has been synthezised and coordinated to ruthenium(II) centres to form bench-stable tris-acetonitrile complexes [Ru(R-PYA-pincer)(MeCN)₃ ](PF₆ )₂ (R=OMe, Me, H, Cl, CF₃ ). Analytic studies including ¹ H NMR spectroscopy, cyclic voltammetry, and X-ray crystallography reveal a direct influence of the substituents on the electronic properties of the ruthenium center. The complexes are active in the catalytic transfer hydrogenation of ketones, with activities directly encoded by the PYA substitution pattern. Their perfomance improves further upon exchange of an ancillary MeCN ligand with PPh₃ . While complexes [Ru(R-PYA-pincer)(PPh₃ )(MeCN)₂ ](PF₆ )₂ were only isolated for R=H, Me, an in situ protocol was developed to generate these complexes in situ for R=OMe, Cl, CF₃ by using a 1:2 ratio of the complexes and PPh₃ . This in situ protocol together with a short catalyst pre-activation provided highly active catalytic systems. The most active pre-catalyst featured the methoxy-substituted PYA ligand and reached turnover frenquencies of 210 000 h^-1 under an exceptionally low catalyst loading of 25 ppm for the benchmark substrate benzophenone, representing one of the most active transfer hydrogenation systems known to date.

Supported Ni nanoparticles with a phosphine ligand as an efficient heterogeneous non-noble metal catalytic system for regioselective hydrosilylation of alkynes

A convenient and effective heterogeneous non-noble metal catalytic system for regioselective hydrosilylation of alkynes was successfully developed by the combination of Ni/Al2O3 with a xantphos ligand. The resulting catalytic system displayed excellent catalytic performance in the heterogeneous hydrosilylation of PhSiH3 with a wide range of aromatic and aliphatic terminal alkynes, affording the corresponding (E)-vinylsilanes in good to excellent yields with high regioselectivity.

Effect of Ancillary Ligand in Cyclometalated Ru(II)-NHC-Catalyzed Transfer Hydrogenation of Unsaturated Compounds

In an effort to develop efficient Ru(II)-NHC-based catalyst considering their stereoelectronic effect for hydride-transfer reaction, we found that the ancillary NHC ligand can play a significant role in its catalytic performance. This effect is demonstrated by comparing the activity of two different types of orthometalated precatalysts of general formula [( p-cymene)(NHC)Ru^II(X)] (NHC = an imidazolylidene-based ImNHC, compound 2a-c, or a mesoionic triazolylidene-based tzNHC, compound 4) in transfer hydrogenation of carbonyl substrates. The electron-rich precatalyst, 2c, containing p-OMe-substituted NHC ligand performed significantly better than both unsubstituted complex 2a and p-CF₃ substituted electron-poor complex 2b in ketone reduction. Whereas bulky mesoionic triazolylidene ligand containing complex 4 was found to be superior catalyst for aldehyde reduction and the precatalyst 2a is more suitable for the selective transfer hydrogenation of a wide range of aromatic aldimines to amines. To the best of our knowledge, this is the first systematic study on the effect of stereoelectronic tuning of ancillary orthometalated NHC ligand in Ru(II)-catalyzed transfer hydrogenations of various types of unsaturated compounds with broad substrate scope.

Highly β( Z)-Selective Hydrosilylation of Terminal Alkynes Catalyzed by Thiolate-Bridged Dirhodium Complexes

A series of novel monothiolate-bridged dirhodium complexes, [Cp*Rh(μ-SR)(μ-Cl)₂RhCp*][BF₄] {Cp* = η⁵-C₅Me₅, R = tertiary butyl ( ^tBu), 1a; R = ferrocenyl (Fc), 1b; R = adamantyl (Ad), 1c} were designed and successfully synthesized, which can smoothly facilitate highly regioselective and stereoselective hydrosilylation of terminal alkynes to afford β( Z) vinylsilanes with good functional group compatibility. Furthermore, the hydride bridged dirhodium complex [Cp*Rh(μ-S ^tBu)(μ-Cl)(μ-H)RhCp*][BF₄] (5) as a potential intermediate was obtained by the reaction of 1a with excess HSiEt₃.

A mesoionic nitrogen-donor ligand: structure, iridium coordination, and catalytic effects

A first example of a mesoionic ligand with a nitrogen coordination site was prepared by a simple and robust synthesis and is shown to have excellent properties for promoting iridium-catalyzed water oxidation.