Olefin Ligands Research Articles

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Olefins are easily accessible and have diverse reacitivities. Meanwhile, they also can act as ligands in asymmetric catalysis. Acyclic chiral diene ligands as well as hybrid olefin ligands were developed for transition metal catalyzed asymmetric reactions. Chiral dienes can further be applied for acquiring chiral FLP catalysts via the in situ hydroboration. These FLP catalysts proved to be highly effective for asymmetric metal‐free hydrogenations and hydrosilylations.. More details are given in the article by Du et al. on page 1109—1116.image

Hybrid cycloolefin ligands for palladium–olefin cooperative catalysis

The engineering of molecular catalysts in homogeneous catalysis enables new catalytic modes, which leads to efficient synthetic transformations. In this regard, the design of ligands for transition metal catalysis has played a major role. In transition metal catalysed reactions, olefins can serve as steering ligands by tuning the electronic and steric nature of the metal centre. Here we report unstrained olefin ligands that bear a P or S coordination site for use in the Pd-catalysed Catellani reaction. This olefin ligand shows a covalent catalytic function and enables an efficient ipso,ortho-difunctionalization of iodoarenes. Mechanistic analysis reveals a reversible covalent bonding between the substrate and the cycloolefin unit of the ligand, which forms key organopalladium intermediates to enable a new reactivity. Our results demonstrate a design concept that employs hybrid cycloolefin ligands as a covalent catalytic module, which opens up possibilities for further cooperative catalysis with other transition metal–olefin hybrids. Taking inspiration from palladium–norbornene cooperative catalysis, Catellani-type reactions are now performed using a hybrid olefin ligand with a P or S coordination site. This olefin ligand enables efficient ipso,ortho-difunctionalization of iodoarenes. Mechanistic studies show the formation of organopalladium intermediates that comprise both the substrate and the hybrid olefin ligand.

Asymmetric addition of an N-methyl C(sp3)-H bond to cyclic alkenes enabled by an iridium/phosphine-olefin catalyst.

Iridium-catalyzed asymmetric C-H addition of an N-methyl group on 3-trifluoromethyl-2-(N-methylamino)pyridine to cyclic alkenes was realized by using a phosphine-olefin ligand. Indene and its 7-substituted derivatives, acenaphthylene, and some bicyclic alkenes underwent the addition to give the corresponding products in good yields with high enantioselectivity.

Rhodium Indenyl NHC and Fluorenyl-Tethered NHC Half-Sandwich Complexes: Synthesis, Structures and Applications in the Catalytic C-H Borylation of Arenes and Alkanes.

Indenyl (Ind) rhodium N‐heterocyclic carbene (NHC) complexes [Rh(η 5‐Ind)(NHC)(L)] were synthesised for 1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIPr) with L=C2H4 (1), CO (2 a) and cyclooctene (COE; 3), for 1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C2H4)2] did not give the desired SIPr complex, thus demonstrating the “indenyl effect” in the synthesis of 1. Oxidative addition of HSi(OEt)3 to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)3}(H)(SIPr)] (6) with loss of COE. Tethered‐fluorenyl NHC rhodium complexes [Rh{(η 5‐C13H8)C2H4N(C)C2H x NR}(L)] (x=4, R=Dipp, L=C2H4: 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5–31 %) in comparison to good yields for the monodentate complexes (49–79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B2pin2 (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl‐tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B2pin2 at 120 and 140 °C, respectively, was monitored by 11B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4–7 days, thus demonstrating catalysed sp3 C−H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C2H4 and COE ligands, whereas CO complexes were inert. Evidence for C−H bond activation in the alkyl groups of the NHC ligands was obtained.

Enantioselective Cobalt-Catalyzed Reductive Cross-Coupling for the Synthesis of Axially Chiral Phosphine–Olefin Ligands

Enantioselective Cobalt-Catalyzed Reductive Cross-Coupling for the Synthesis of Axially Chiral Phosphine–Olefin Ligands

Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)-Platinum(II) Olefin Complexes as Catalysts.

The nitrogen oxides NO2, NO, and N2O are among the most potent air pollutants of the 21st century. A bimetallic RhI–PtII complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3–4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N2O) to water and N2 was recorded, making these RhI–PtII complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON=38) or nitrogen dioxide (NO2, TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NOx were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO2 proceeds stepwise, to first give NO and H2O, followed by the generation of N2O and H2O, which is then further converted to N2 and H2O. The nitrogen−nitrogen bond‐forming step takes place in the conversion from NO to N2O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N2O2 2−) complex, which was detected as an intermediate.

Rhodium and Palladium Complexes of N-Heterocyclic Olefin (NHO) Ligand Fused with the 9,10-Dihydro-9,10-ethanoanthracene Framework

Rhodium and Palladium Complexes of N-Heterocyclic Olefin (NHO) Ligand Fused with the 9,10-Dihydro-9,10-ethanoanthracene Framework

Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)–Platinum(II) Olefin Complexes as Catalysts

AbstractThe nitrogen oxides NO2, NO, and N2O are among the most potent air pollutants of the 21st century. A bimetallic RhI–PtII complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3–4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N2O) to water and N2 was recorded, making these RhI–PtII complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON=38) or nitrogen dioxide (NO2, TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NOx were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO2 proceeds stepwise, to first give NO and H2O, followed by the generation of N2O and H2O, which is then further converted to N2 and H2O. The nitrogen−nitrogen bond‐forming step takes place in the conversion from NO to N2O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N2O22−) complex, which was detected as an intermediate.

Organometallic single-chain polymer nanoparticles via intra-chain cross-linking with dinuclear μ-halo(diene)Rh(I) complexes

An approach towards the expansion of Rh(I)-containing single-chain polymer nanoparticles (SCNPs) using linear polymers and μ-halo(diene)Rh(I) dimers as cross-linkers in high dilution was explored. Initially, the effects of utilizing different anionic ligands, i.e., halides, together with various labile alkene ligands, such as ethylene, 1,5-cyclooctadiene and 1,5-hexadiene, on the stability and activity of the resulting Rh(I) cross-linking complexes were examined. While the ethylene complexes for bromide and iodide bridges resulted too unstable and the 1,5-cyclooctadiene ligand produced inert complexes, the 1,5-hexadiene ligands provided the proper equilibrium of stability and lability to achieve efficient polymer folding. Thus, a range of new organometallic nanoparticles could be obtained via a direct exchange between the 1,5-hexadiene labile ligands of the Rh(I) complexes, and the corresponding 1,5-hexadiene elements present in ROMP derived 1,5-polycyclooctadiene (PCOD) and 1,4-polybutadiene (PBD). Notably, SEC analysis of the single-chain nanoparticles both for PCOD and PBD revealed the expected decrease in the hydrodynamic radius (Rh) of the parent polymer, indicating that other anions can also be used as elements in the cross-linking moieties to form these organometallic nanoparticles. Moreover, all of the Rh-SCNPs displayed intrinsic semi-conductivities in the range of similar undoped organic conjugated polymers, expanding the possible uses of this methodology for many applications.

Synthesis and characterization of Some transition metal complexes with bis(o-aminophenyl)disulfide alkane ligands

A series of new complexes of the type [M(L)Cl2] and [M(L)]Cl2 where M= Co(II),Ni(II) and Cu(II), L=L1 or L2 {L1= 1,2-Bis(o-aminophenyl)disulfide propane, L2=1,2-Bis(o- aminophenyl)disulfide butane, have been prepared in 1:1 (L:M) and characterized by molar conductance , magnetic moment,FT-IR,1H-NMR , Uv-visible spectra studies and metal content analysis. Conductivity data in DMF and DMSO solution showed that some complexes are non-electrolyte and others are electrolyte . Magnetic moment and electronic spectra indicate that some of the complexes have a tetrahedral and the others have an square planar environment.

Mechanical Bending and Modulation of Photoactuation Properties in a One-Dimensional Pb(II) Coordination Polymer

With emergent research on stimuli responsive materials, dynamic crystals are at the forefront of investigation. However, research on the mechanical properties of coordination polymers (CPs) is still in its infancy. Elastic deformation induced by pressure and photoactuation are rare occurrences in CPs, let alone their combination in a single CP. Here, we report a one-dimensional (1D) CP comprising PbBr2 chains with 3-fluoro-4’-styrylpyridine arms showing excellent elasticity and photomechanical properties. A slender crystal can be bent to make a circle and write different shapes with restoration of original shape upon removal of the applied force. In addition, photomechanical properties triggered by [2+2] cycloaddition of the olefinic ligand can be modulated easily by variation of the crystal sizes. Crystals with bigger width show destructive photosalient effects while the smaller ones show plastic deformation like bending, twisting, curling etc. upon UV irradiation. This example provides avenue for designing CPs for multi-stimuli responsive actuating properties.

Ni(COD)(DMFU): A Heteroleptic 16-Electron Precatalyst for 1,2-Diarylation of Alkenes

AbstractElectron-deficient olefin (EDO) ligands are known to promote a variety of nickel-catalyzed cross-coupling reactions, presumably by accelerating the reductive elimination step and preventing undesired β-hydride elimination. While there is a growing body of experimental and computational evidence elucidating the beneficial effects of EDO ligands, significant gaps remain in our understanding of the underlying coordination chemistry of the Ni–EDO species involved. In particular, most procedures rely on in situ assembly of the active catalyst, and there is a paucity of preligated Ni–EDO precatalysts. Herein, we investigate the 16-electron, heteroleptic nickel complex, Ni(COD)(DMFU), and examine the performance of this complex as a precatalyst in 1,2-diarylation of alkenes.

Tailoring charge transfer in perovskite quantum dots/black phosphorus nanosheets photocatalyst via aromatic molecules

Typical ligands like long-chain alkane or siloxane are not in favor of charge transfer due to their insulating nature, which hinders perovskites’ applications in photovoltaic devices or photocatalysis. Herein, short-chain conductive aromatic molecules benzylamine (BZA) and benzoic acid (BA) were employed to passivate CsPbBr3 QDs and to serve as charge mediators for the first time in CsPbBr3 QDs/black phosphorus nanosheets (BP NSs) for photocatalytic CO2 reduction. The influences of various ligands on interfacial charge transport were further investigated via several means of analyses like electrochemical measurement and photoluminescence (PL) decay. And it’s verified that short-chain aromatic molecules show much more potential as charge mediators than long-chain alkane or siloxane compounds. The electron consumption rate of composite photocatalyst bridged by aromatic mediators is 76.6 µmol g−1 h−1, which is almost 1-fold and 4-fold compared with that of long-chain alkane ligands group and siloxane ligands group, respectively.

Selectivity of Rh⋅⋅⋅H-C Binding in a σ-Alkane Complex Controlled by the Secondary Microenvironment in the Solid State.

Single‐crystal to single‐crystal solid‐state molecular organometallic (SMOM) techniques are used for the synthesis and structural characterization of the σ‐alkane complex [Rh(tBu2PCH2CH2CH2PtBu2)(η2,η2‐C7H12)][BArF 4] (ArF=3,5‐(CF3)2C6H3), in which the alkane (norbornane) binds through two exo‐C−H⋅⋅⋅Rh interactions. In contrast, the bis‐cyclohexyl phosphine analogue shows endo‐alkane binding. A comparison of the two systems, supported by periodic DFT calculations, NCI plots and Hirshfeld surface analyses, traces this different regioselectivity to subtle changes in the local microenvironment surrounding the alkane ligand. A tertiary periodic structure supporting a secondary microenvironment that controls binding at the metal site has parallels with enzymes. The new σ‐alkane complex is also a catalyst for solid/gas 1‐butene isomerization, and catalyst resting states are identified for this.

Cinnamoyl amide type chiral P,olefin ligands for Pd-catalyzed reactions.

We synthesized cinnamoyl amide type chiral P,olefin ligand (S)-4. We successfully obtained separable diastereomers of 4d and demonstrated Pd-catalyzed asymmetric allylic substitution reactions of indoles using (S,aS)-4d as a chiral ligand with high enantioselectivities (up to 98% ee).

Interfacial study of clathrates confined in reversed silica pores.

Storing methane in clathrates is one of the most promising alternatives for transporting natural gas (NG) as it offers similar gas densities to liquefied and compressed NG while offering lower safety risks. However, the practical use of clathrates is limited given the extremely low temperatures and high pressures necessary to form these structures. Therefore, it has been suggested to confine clathrates in nanoporous materials, as this can facilitate clathrate's formation conditions while preserving its CH4 volumetric storage. Yet, the choice of nanoporous materials to be employed as the clathrate growing platform is still rather arbitrary. Herein, we tackle this challenge in a systematic way by computationally exploring the stability of clathrates confined in alkyl-grafted silica materials with different pore sizes, ligand densities and ligand types. Based on our findings, we are able to propose key design criteria for nanoporous materials favoring the stability of a neighbouring clathrate phase, namely large pore sizes, high ligand densities, and smooth pore walls. We hope that the atomistic insight provided in this work will guide and facilitate the development of new nanomaterials designed to promote the formation of clathrates.

Single-Crystal-to-Single-Crystal [2 + 2] Photocycloaddition Reaction in a Photosalient One-Dimensional Coordination Polymer of Pb(II).

In a remarkable example, we report a one-dimensional coordination polymer (CP) of Pb(II) showing photosalient (PS) properties triggered by [2 + 2] cycloaddition of olefinic ligands, which is seldom observed in CPs. Macroscopic rod-shaped crystals show various photomechanical effects such as jumping, splitting, rolling, and breaking upon UV illumination. In this rare example, we could determine the solid-state structure of the 100% dimerized product and three intermediate structures, even after the shattering of crystals into small pieces. Detailed mechanistic investigation from the single-crystal data indicates that the strain generated in the unit cell due to anisotropic expansion played a bigger role for the PS effects. Nucleated growth of the photoproduct crystal created different domains inside the single crystal, which multiplied the already developed stress leading to the photomechanical movements. This example falls in the gray area of a clean single-crystal-to-single-crystal (SCSC) transformation and violent PS effect. Such photochemical behavior has never been reported before.

Solid-state spectroscopic properties of dinuclear cyclometalated Pt(II) complexes with different bridging ligands and anions

Crystal structures of dinuclear cyclometalated Pt(II) complexes with different bridging bis(diphenylphosphino)alkane ligands and counter anions have been compared, and solid-state emissions of crystallite powders and PMMA films doped with luminescent Pt(II) complexes have been investigated. Interestingly, mechanochromic and vapochromic properties are exhibited for solids of dinuclear cyclometalated Pt(II) complex (−)-1 bridged by bis(diphenylphosphino)methane (dppm), and multiple cycles have been repeated without any detectable degradation. However, no distinct stimuli-induced chromism can be observed with the naked eyes for solids of (−)-1 with other counter anions and PMMA films. In addition, with increasing the length of the bridging diphosphine ligand, green-solids without effective intramolecular Pt···Pt contacts have been obtained, and mechanochromic and vapochromic phenomena could not be detected.

Large vs. Small NHC Ligands in Nickel(0) Complexes: The Coordination of Olefins, Ketones and Aldehydes at [Ni(NHC)2

Investigations concerning the reactivity of Ni(0) complexes [Ni(NHC)2] of NHCs (N‐heterocyclic carbene) of different steric demand, Mes2Im (= 1,3‐dimesitylimidazoline‐2‐ylidene) and iPr2Im (= 1,3‐diisopropyl‐imidazoline‐2‐ylidene), with olefins, ketones and aldehydes are reported. The reaction of [Ni(Mes2Im)2] 1 with ethylene or methyl acrylate afforded the complexes [Ni(Mes2Im)2(η2‐C2H4)] 3 and [Ni(Mes2Im)2(η2‐(C,C)‐H2C=CHCOOMe)] 4, as it was previously reported for [Ni2(iPr2Im)4(µ‐(η2:η2)‐COD)] 2 as a source for [Ni(iPr2Im)2]. In contrast to 2, complex 1 does not react with sterically more demanding olefins such as tetramethylethylene, 1,1‐diphenylethylene and cyclohexene. The reaction of [Ni(NHC)2] with more π‐acidic ketones or aldehydes led to formation of complexes with side‐on η2‐(C,O)‐coordinating ligands: [Ni(iPr2Im)2(η2‐O=CHtBu)] 5, [Ni(iPr2Im)2(η2‐O=CHPh)] 6, [Ni(iPr2Im)2(η2‐O=CMePh)] 7, [Ni(iPr2Im)2(η2‐O=CPh2)] 8, [Ni(iPr2Im)2(η2‐O=C(4‐F‐C6H4)2)] 9, [Ni(iPr2Im)2(η2‐O=C(OMe)(CF3))] 10 and [Ni(Mes2Im)2(η2‐O=CHPh)] 11, [Ni(Mes2Im)2(η2‐O=CH(CH(CH3)2))] 12, [Ni(Mes2Im)2(η2‐O=CH(4‐NMe2‐C6H4))] 13, [Ni(Mes2Im)2(η2‐O=CH(4‐OMe‐C6H4))] 14, [Ni(Mes2Im)2(η2‐O=CPh2)] 15 and [Ni(Mes2Im)2(η2‐O=C(4‐F‐C6H4)2)] 16. The reaction of 1 and 2 with these simple aldehydes and ketones does not lead to a significantly different outcome, but NHC ligand rotation is hindered for the Mes2Im complexes 3, 4 and 11–16 according to NMR spectroscopy. The solid‐state structures of 3, 4, 11 and 12 reveal significantly larger CNHC‐Ni‐CNHC angles in the Mes2Im complexes compared to the iPr2Im complexes. As electron transfer in d8‐ (or d10‐) ML2 complexes to π‐acidic ligands depends on the L–M–L bite angle, the different NHCs lead thus to a different degree of electron transfer and activation of the olefin, aldehyde or ketone ligand, i.e., [Ni(iPr2Im)2] is the better donor to these π‐acidic ligands. Furthermore, we identified two different side products from the reaction of 1 with benzaldehyde, trans‐[Ni(Mes2Im)2H(OOCPh)] 17 and [Ni2(Mes2Im)2(µ2‐CO)(µ2‐η2‐C,O‐PhCOCOPh)] 18, which indicate that radical intermediates and electron transfer processes might be of importance in the reaction of 1 with aldehydes and ketones.

Remarkable Ligand Effect on Rh-Catalyzed C-H-Active [3 + 2] Annulation of Ketimines and Alkynes.

Externally added ligands were first found to have a significant impact on the Rh-catalyzed C-H-active [3 + 2] annulation of ketimines and alkynes. Olefin ligands have shown remarkable promotion effect for this reaction. The olefin promoted the reaction by increasing both the turnover rate and conversion of [Cp*RhCl2]2 in the formation of rhodacycle in the C-H activation step.