Cyclopentadienyl Ligands Research Articles

Cobalt(III)‐Catalyzed Enantioselective Intermolecular Carboamination by C−H Functionalization

AbstractHigh‐valent cyclopentadienyl cobalt catalysis is a versatile tool for sustainable C−H bond functionalizations. To harness the full potential of this strategy, control of the stereoselectivity of these processes is necessary. Herein, we report highly enantioselective intermolecular carboaminations of alkenes through C−H activation of N‐phenoxyamides catalyzed by CoIII‐complexes equipped with chiral cyclopentadienyl (Cpx) ligands. The method converts widely available acrylates as well as bicyclic olefins into attractive enantioenriched isotyrosine derivatives as well as elaborated amino‐substituted bicyclic scaffolds under very mild conditions. The outlined reactivity is unique to the CpxCoIII complexes and is complementary to the reactivity of 4d‐ and 5d‐ precious‐metal catalysts.

Cobalt(III)-Catalyzed Enantioselective Intermolecular Carboamination by C-H Functionalization.

High-valent cyclopentadienyl cobalt catalysis is a versatile tool for sustainable C-H bond functionalizations. To harness the full potential of this strategy, control of the stereoselectivity of these processes is necessary. Herein, we report highly enantioselective intermolecular carboaminations of alkenes through C-H activation of N-phenoxyamides catalyzed by CoIII -complexes equipped with chiral cyclopentadienyl (Cpx ) ligands. The method converts widely available acrylates as well as bicyclic olefins into attractive enantioenriched isotyrosine derivatives as well as elaborated amino-substituted bicyclic scaffolds under very mild conditions. The outlined reactivity is unique to the Cpx CoIII complexes and is complementary to the reactivity of 4d- and 5d- precious-metal catalysts.

Chiral Cyclopentadienyl Ligands: Design, Syntheses, and Applications in Asymmetric Catalysis.

The creation of new chiral ligands capable of providing high stereocontrol in metal-catalyzed reactions is crucial in modern organic synthesis. The production of bioactive molecules as single enantiomers is increasingly required, and asymmetric catalysis with metal complexes constitutes one of the most efficient synthetic strategies to access optically active compounds. Herein we offer a historical overview on the development of chiral derivatives of the ubiquitous cyclopentadienyl ligand (CpX ), and detail their successful application in a broad range of metal-catalyzed transformations. Those include the functionalization of challenging C-H bonds and beyond, giving access to an extensive catalogue of valuable chiral molecules. A critical comparison of the existing ligand families, their design, synthesis, and complexation to different metals is also provided. In addition, future research directions are discussed to further enhance the performance and application of CpX ligands in enantioselective catalysis.

Chiral Cyclopentadienyl Ligands: Design, Syntheses, and Applications in Asymmetric Catalysis

AbstractThe creation of new chiral ligands capable of providing high stereocontrol in metal‐catalyzed reactions is crucial in modern organic synthesis. The production of bioactive molecules as single enantiomers is increasingly required, and asymmetric catalysis with metal complexes constitutes one of the most efficient synthetic strategies to access optically active compounds. Herein we offer a historical overview on the development of chiral derivatives of the ubiquitous cyclopentadienyl ligand (CpX), and detail their successful application in a broad range of metal‐catalyzed transformations. Those include the functionalization of challenging C−H bonds and beyond, giving access to an extensive catalogue of valuable chiral molecules. A critical comparison of the existing ligand families, their design, synthesis, and complexation to different metals is also provided. In addition, future research directions are discussed to further enhance the performance and application of CpXligands in enantioselective catalysis.

A New Class of C2 -Symmetric Chiral Cyclopentadienyl Ligand Derived from Ferrocene Scaffold: Design, Synthesis and Application.

A new class of C2 -symmetric, chiral cyclopentadienyl ligand based on planar chiral ferrocene backbone was developed. A series of its corresponding rhodium(I), iridium(I), and ruthenium(II) complexes were prepared as well. In addition, the rhodium(I) complexes were evaluated in the asymmetric catalytic intramolecular amidoarylation of olefin-tethered benzamides via C-H activation.

Hexafluoroacetylacetonate (hfac) as ligand for pentamethylcyclopentadienyl (Cp*) rhodium and iridium complexes: Some surprising results, including an Ir3Na1O4 cubane structure

Attempts to prepare hexafluoroacetylacetonate (hfac) piano stool complexes of pentamethylcyclopentadienyl Cp* iridium and rhodium led to a surprising array of unexpected products. The desired complex, Cp*M(hfac)Cl, was obtained following the reaction of sodium hexafluoroacetylacetonate (hfacNa) with [Cp*MCl2]2 in dichloromethane. Variations of this synthetic method resulted in twelve unique crystallographically identified products, eleven of which contain the hfac ligand or a trifluoroacetylacetonate (TFA) ligand, either coordinated to the metal or as a non-coordinating anion. Five dinuclear Cp* iridium hydroxo-bridged products with various fluorinated non-coordinating anions were obtained. The most intriguing two complexes are trinuclear Cp*IrIII hydroxo-bridged clusters that have, at their core, an Ir3NaO4 cubane structure. Attempts to devise rational syntheses of the hydroxo-bridged cluster compounds were not successful. Generation of the TFA moieties likely occurred following degradation of Cp*Ir(hfac)Cl. The reaction between [Cp*MCl2]2 and 1,1,1-trifluoroacetylacetonate (tfac) proceeded as expected to give Cp*M(tfac)Cl with no observed side product formation, indicating that the metal-coordinated tfac moiety is significantly more stable than the metal-coordinated hfac moiety.

Study on Structural and Thermal Characteristics of Heteroleptic Yttrium Complexes as Potential Precursors for Vapor Phase Deposition

Yttrium oxide (Y2O3) thin films are implemented as a functional component in a broad field of applications such as optics, electronics or thermal barrier coatings. Atomic layer deposition (ALD) is a promising technique to fabricate high‐quality thin films with atomic level precision in which the precursor choice plays a crucial role in process development. The limited number of suitable yttrium precursors available for ALD of Y2O3 has triggered increasing research activity seeking new or modified precursors. In this study, heteroleptic compounds of yttrium bearing the cyclopentadienyl (Cp) ligand in combination with the chelating amidinate or guanidinate ligands were targeted as potential precursors for ALD. In this context, a systematic and comparative study of the structure and thermal characteristics of (bis‐cyclopentadienyl‐(N,N'‐diisopropyl‐2‐methyl‐amidinato)yttrium) [YCp2(dpamd)] 1 and (bis‐cyclopentadienyl‐(N,N'‐diisopropyl‐2‐dimethylamido‐guanidinato)yttrium) [YCp2(dpdmg)] 2 was performed. Complementary characterization tools such as 1H‐NMR, elemental analysis, electron‐impact mass spectrometry (EI‐MS) and single‐crystal X‐ray diffraction (XRD) confirmed the spectroscopic purity and the monomeric nature of the metalorganic compounds. Hirshfeld surface analysis revealed influence of the ligand choice on the intermolecular interactions of the compounds. The important figures of merit for a precursor, namely the thermal properties were investigated via thermogravimetric analysis. Thus, the volatility, transport behavior and thermal stability were examined and compared to their homoleptic counterparts [YCp3], [Y(dpamd)3] or [Y(dpdmg)3].

Rhodium Catalysts with a Chiral Cyclopentadienyl Ligand Derived from Natural R‐Myrtenal

A new chiral cyclopentadiene CpmyrH was synthesized from the natural terpene (R)‐myrtenal in 5 steps and about 40 % total yield. The key step was the reaction of vinyl‐dibromocyclopropane derivative with MeLi which provided the diene CpmyrH via Skattebøl rearrangement. The reaction of CpmyrH with [(cod)Rh(OAc)]2 gave the rhodium(I) complex (Cpmyr)Rh(cod) and subsequent oxidation with halogens X2 gave the rhodium(III) complexes [(Cpmyr)RhX2]2 (X = Br, I). Complex (Cpmyr)Rh(cod) in combination with benzoyl peroxide catalyzed the C–H activation of O‐pivaloyl‐phenylhydroxamic acid and its subsequent annulation with alkenes giving dihydroisoquinolones with high yield (70–90 %) but moderate enantioselectivity (16–36 % ee).

Crystal Structure of Decakis(μ-chloro)-tetrakis(1,2,4-triphenylcyclopentadienyl)-hexakis(tetrahydrofuran)-di-potassium-tetra-neodymium(III) Tetrahydrofuran Trisolvate

The crystal structure of decakis(μ-chloro)-tetrakis(1,2,4-triphenylcyclopentadienyl)-hexakis(tetrahydrofuran)-di-potassium-tetra-neodymium(III) tetrahydrofuran trisolvate, 1, is reported. The centrosymmetric complex 1 has a rare tetranuclear core [Ln4K2Cl10] and crystalizes in triclinic space group (P $$\bar{1}$$ ) with unit cell parameters: a = 11.9858(3) A, b = 14.0698(3) A, c = 19.8129(4) A, α = 74.1120(4)°, β = 81.2073(4)°, γ = 83.9521(4)° and Z = 1. The catalytic system based on 1 and nBuMg(O-2,6-tBu2-4-MeC6H2) exhibits moderate activity in coordinative chain transfer polymerization of ethylene. In case of the system 1/nBu2Mg, oligomerisation of ethylene was not detected. The mono(cyclopentadienyl) complex {[1,2-Ph2-4-(2-MeOC6H4)C5H2]NdCl5K(THF)2}2(THF)3 has been obtained from NdCl3(THF)2.5 and K[1,2-Ph2-4-(2-MeOC6H4)C5H2] (1:1) in THF, crystallized from a THF/hexane mixture, and structurally characterized.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach.

The use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non‐innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C−H activation at one methyl terminus of C5Me5 in [(η‐C5Me5)Rh(PMe3)2] to form a new Rh−H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C−H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E−H bonds (E=O, N), including the N−H bonds in ammonia, can be understood in terms of bimetallic frustration.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach

AbstractThe use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non‐innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C−H activation at one methyl terminus of C5Me5 in [(η‐C5Me5)Rh(PMe3)2] to form a new Rh−H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C−H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E−H bonds (E=O, N), including the N−H bonds in ammonia, can be understood in terms of bimetallic frustration.

Phenylvinylidene RePt Complexes Containing Diphenylphosphine Ligands at the Platinum Atom: Synthesis, IR, NMR Spectroscopic and Electrochemical Properties

The μ-phenylvinylidene complexes Cp(CO)2RePt(μ-C=CHPh)(PPh2H)(L) [L = PPh3 (1), P(OPri)3 (2)] were synthesized for the first time. Their IR, NMR spectroscopic and redox properties were studied. Based on IR, NMR spectroscopic data their molecular structures were proposed. The pathways of redox-reactions of the complexes were determined

Rhodium(III)-Catalyzed Oxidative ortho-Olefination of Phenyl Carbamates with Alkenes: Elucidation of Acceleration Mechanisms by Using an Unsubstituted Cyclopentadienyl Ligand.

It has been established that an unsubstituted cyclopentadienyl (Cp) Rh(III) complex is an effective catalyst for the oxidative ortho-olefination of phenyl carbamates with both acrylates and styrenes under mild conditions. In addition, diolefination of a protected BINOL (1,1'-binaphthalene-2,2'-diol) proceeded in high yields and disubstituted acrylates could participate in this catalysis. Experimental and theoretical mechanistic studies elucidated that an electron-deficient nature of the unsubstituted CpRh(III) complex accelerates both the electrophilic aryl C-H rhodation and the rate-limiting alkene insertion steps.

Cationic RuII Cyclopentadienyl Complexes with Antifungal Activity against Several Candida Species.

Fungal infections, including those caused by antifungal-resistant Candida, are a very challenging health problem worldwide. Whereas different ruthenium complexes were previously studied for their anti-Candida potential, Ru-cyclopentadienyl complexes were overlooked. Here, we report an antifungal activity assessment of three Ru-cyclopentadienyl complexes with some insights into their potential mode of action. Among these complexes, only the cationic species [Ru-ACN]+ and [Ru-ATZ]+ displayed a significant antifungal activity against different Candida strains, notably against the ones that did not respond to one of the most currently used antifungal drugs fluconazole (FCZ). However, no apparent activity was observed for the neutral species, Ru-Cl, thus indicating the important role of the cationic backbone of these complexes in their biological activity. We suggest that reactive oxygen species (ROS) generation might be involved in the mechanism of action of these complexes as, unlike neutral Ru-Cl, [Ru-ACN]+ and [Ru-ATZ]+ could generate intracellular concentration-dependent ROS. We also observed a correlation between the ruthenium cellular uptake, ROS generation and fungal growth inhibitory activity of the compounds. Furthermore, docking simulations showed that the CYP51 enzyme can form more energetically favorable complexes with [Ru-ATZ]+ than fluconazole (FCZ); this suggests that CYP51 inhibition could also be considered as a potential mode of action.

Aluminum Nanocrystals Grow into Distinct Branched Aluminum Nanowire Morphologies.

Plasmonic nanowires (NWs) have generated great interest in their applications in nanophotonics and nanotechnology. Here we report the synthesis of Al nanocrystals (NCs) with controlled morphologies that range from nanospheres to branched NW and NW bundles. This is accomplished by catalyzing the pyrolysis of triisobutyl aluminum (TIBA) with Tebbe's reagent, a titanium(III) catalyst with two cyclopentadienyl ligands. The ratio of TIBA to Tebbe's reagent is critical in determining the morphology of the resulting Al NC. The branched Al NWs grow in their ⟨100⟩ directions and are formed by oriented attachment of isotropic Al NCs on their {100} facets. Branched NWs are strongly absorptive from the UV to the mid-IR, with longitudinal dipolar, higher-order, and transverse plasmons, all contributing to their broadband response. This rapid Al NW synthesis enables the expanded use of Al for plasmonic and nanophotonic applications in the ultraviolet, visible, and infrared regions of the spectrum.

Cp and indenyl ruthenium complexes containing dithione derivatives: Synthesis, antibacterial and antifungal study

A series of cationic complexes [(Cp/Ind)Ru(к2(SS)-L)(PPh3)]PF6 (1–6) are obtained by the reaction of [CpRu(PPh3)2Cl] or [(Ind)Ru(PPh3)2Cl] (Cp = η5-C5H5, Ind = η5-C9H7) with respective dithione derivatives 1,2-di(piperidin-1-yl)ethane-1,2-dithione (L1), 1,2-dimorpholinoethane-1,2-dithione (L2) and 1,2-dithiomorpholinoethane-1,2-dithione (L3). All the compounds are characterized using spectroscopic techniques. The molecular structures of complexes 1, 2 and 4 are established by single-crystal X-ray diffraction studies. Antimicrobial studies were tested against three strains of bacterial microorganisms Staphylococcus aureus (gram + ve), Bacillus subtilis (gram + ve), Klebsiella pneumoniae (gram -ve) and one strain of fungal microorganism Candida albicans.

Synthesis of Dinuclear Mo-Fe Hydride Complexes and Catalytic Silylation of N2.

Two transition-metal atoms bridged by hydrides may represent a useful structural motif for N2 activation by molecular complexes and the enzyme active site. In this study, dinuclear MoIV -FeII complexes with bridging hydrides, CpR Mo(PMe3 )(H)(μ-H)3 FeCp* (2 a; CpR =Cp*=C5 Me5 , 2 b; CpR =C5 Me4 H), were synthesized via deprotonation of CpR Mo(PMe3 )H5 (1 a; CpR =Cp*, 1 b; CpR =C5 Me4 H) by Cp*FeN(SiMe3 )2 , and they were characterized by spectroscopy and crystallography. These Mo-Fe complexes reveal the shortest Mo-Fe distances ever reported (2.4005(3) Å for 2 a and 2.3952(3) Å for 2 b), and the Mo-Fe interactions were analyzed by computational studies. Removal of the terminal Mo-H hydride in 2 a-2 b by [Ph3 C]+ in THF led to the formation of cationic THF adducts [CpR Mo(PMe3 )(THF)(μ-H)3 FeCp*]+ (3 a; CpR =Cp*, 3 b; CpR =C5 Me4 H). Further reaction of 3 a with LiPPh2 gave rise to a phosphido-bridged complex Cp*Mo(PMe3 )(μ-H)(μ-PPh2 )FeCp* (4). A series of Mo-Fe complexes were subjected to catalytic silylation of N2 in the presence of Na and Me3 SiCl, furnishing up to 129±20 equiv of N(SiMe3 )3 per molecule of 2 b. Mechanism of the catalytic cycle was analyzed by DFT calculations.

Transition metal catalyzed Fe–C coupling reactions in synthesis of dicarbonyl(2-thienylethynyl)(η5-cyclopentadienyl)iron complex: Spectroscopic, structure and electrochemical study

The new σ-alkynyl iron(II) complex Cp(CO)2Fe-C≡C-(2-C4H3S) was synthesized with application of several known approaches based on the transition metal (Pd/Cu–, Au–, Cu– and Pd–) catalyzed Fe–C coupling reactions of 2-ethynylthiophene or 2-[(trimethylsilyl)ethynyl]thiophene with cyclopentadienyliron dicarbonyl iodide. The yield of the complex in these reactions was found to strongly depend on the catalyst used. The conditions, catalysts, and reagents that provide the highest yields of the desired 2-thienylethynyl iron complex were determined. The complex was characterized by IR, 1H and 13C NMR spectroscopy. The molecular structure of Cp(CO)2Fe–C≡C-(2-C4H3S) established by X-ray diffraction analysis exhibits a three-leg piano stool geometry. The redox properties of the new complex were studied.

Asymmetric CpxRh(III)-Catalyzed Acrylic Acid C–H Functionalization with Allenes Provides Chiral γ-Lactones

A chiral cyclopentadienyl (Cpx) ligand featuring the semi-saturated H8-binaphthyl backbone as the chiral element was developed. Its application potential was demonstrated by CpxRhIII-catalyzed C-H ...

The Emergence of Actinide Cyclobutadienyl Chemistry

Since its inception in the 1950s, the field of organoactinide chemistry has developed and is still burgeoning today. A plethora of molecular actinide cyclopentadienyl (C5), arene (C6), cycloheptatrienyl (C7), and cyclooctatetraenyl (C8) complexes are known. However, the first f‐element cyclobutadienyl complex, a uranium derivative, was only reported as recently as 2013, which contrasts to transition metal chemistry where the first cyclobutadienyl derivatives were realised in the 1950s. A small but growing number of uranium and thorium cyclobutadienyl complexes are now known, so now is an opportune time to review progress to date. This Minireview addresses bonding considerations for the cyclobutadienyl ligand, surveys synthetic routes to crystallographically authenticated actinide cyclobutadienyl complexes and their novel bonding features, and highlights future directions that merit development.