Cyclopentadienyl Ligands Research Articles

Boryloxy Titanium Complex-Enabled High Polar Monomer Contents in Catalytic Copolymerization of Olefins.

The preparation of polyolefins with high polar monomer contents (above 20 mol %) has long been a challenge. Half-titanocenes (Cp')[HCN(Ar)]2BOTiCl2 bearing bulky electron-donating N-heterocyclic boryloxy ligands have been designed and synthesized. The complexes (Cp*)[HCN(Ar)]2BOTiCl2 (2, Ar=2,6-iPr2C6H3; 5, Ar=2,4,6-Me3C6H2) supported by Cp* and the boryloxy ligands have been shown to efficiently catalyze the copolymerization of ethylene and long chain α-olefins. In particular, precatalyst 5 enabled the controlled synthesis of poly(ethylene-co-9-decen-1-ol) with unprecedented high polar monomer contents up to 32.1 mol % while maintaining the high catalytic activity. The structural analysis and DFT calculations disclosed that the bulky and strong electron-donating boryloxy ligands could effectively stabilize cationic active species. The mechanical studies on the hydroxyl-functionalized copolymers disclosed that they exhibited high strength and toughness because of the existence of hydrogen bonds in the polymer network.

Boryloxy Titanium Complex‐Enabled High Polar Monomer Contents in Catalytic Copolymerization of Olefins

AbstractThe preparation of polyolefins with high polar monomer contents (above 20 mol %) has long been a challenge. Half‐titanocenes (Cp′)[HCN(Ar)]2BOTiCl2 bearing bulky electron‐donating N‐heterocyclic boryloxy ligands have been designed and synthesized. The complexes (Cp*)[HCN(Ar)]2BOTiCl2 (2, Ar=2,6‐iPr2C6H3; 5, Ar=2,4,6‐Me3C6H2) supported by Cp* and the boryloxy ligands have been shown to efficiently catalyze the copolymerization of ethylene and long chain α‐olefins. In particular, precatalyst 5 enabled the controlled synthesis of poly(ethylene‐co‐9‐decen‐1‐ol) with unprecedented high polar monomer contents up to 32.1 mol % while maintaining the high catalytic activity. The structural analysis and DFT calculations disclosed that the bulky and strong electron‐donating boryloxy ligands could effectively stabilize cationic active species. The mechanical studies on the hydroxyl‐functionalized copolymers disclosed that they exhibited high strength and toughness because of the existence of hydrogen bonds in the polymer network.

Heteronuclear Fe-Mn Cyclopentadienyl Complexes Supported on Boehmite. Thermochemistry and Thermodynamics of their Decomposition

Boehmite samples with compounds of the composition (C5H5)2FeMnX2(μ-CO)n, where X=Cl, Br and n=1.2, precipitated at room temperature from tetrahydrofuran solutions and then heated in an air flow up to 873 K were obtained and characterized using X-ray diffractometry, infrared Fourier spectroscopy, electron magnetic resonance and temperature-programmed desorption methods. It was shown that the thermal decomposition of these compounds applied to the boehmite samples in the range from room temperature to 873 K occurs stepwise and consists of at least two stages. The first stage of thermal decomposition occurs in the range of 453–753 K, and the second – in the range of 813–843 K. The XRD data show that when calcining at 873 K the boehmite samples with the applied compounds of the above composition and containing these compounds less than 10 wt.%, the diffraction patterns show only reflections characteristic of poorly crystallized aluminum oxide. However, the electron paramagnetic resonance (EPR) spectra of these samples clearly show intense signals characteristic of superpara/ferromagnetic particles of iron and manganese oxides, as well as EPR signals from isolated Fe3+ substituting Al3+ ions in the aluminum oxide structure. EPR spectra most of the iron and manganese is stabilized on the surface of poorly crystallized aluminum oxide in the form of nanostructured iron and manganese oxides.

Attenuating Nucleophilicity of Titanocene Hydrides Beyond Steric Effects en Route to Fatty Alcohols.

Here, we introduce a new class of titanocene catalysts for epoxide hydrosilylation that frustrates their hydridicity and thereby emphasizes their electron transfer reactivity. This unique attenuation of hydridicity is accomplished by introducing Lewis acidic silicon centers to the cyclopentadienyl ligands for an intramolecular coordination of the titanium-bound hydride. The superiority of our rationally designed catalysts over classic titanocenes with alkyl-substituted cyclopentadienyl ligands is demonstrated in the dramatically improved regioselectivity of the hydrosilylation of monosubstituted epoxides to primary alcohols.

Lanthanide Complexes with 1,4,7-Trimethyl-1,4,7-triazacyclononane

The reaction of 1,4,7-trimethyl-1,4,7-triazacyclononane with samarium, gadolinium, and terbium chloride tetrahydrofuranates gives mononuclear complexes [LnCl3(Me3tacn)(THF)n] (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane; Ln = Sm (I), Gd (II), n = 1; Ln = Tb (III), n = 0). The treatment of complexes I or II with 1,2,4-triphenylcyclopentadienyl potassium affords mono(cyclopentadienyl) complexes [CpPh3LnCl2(Me3tacn)] (CpPh3 = = 1,2,4-triphenylcyclopentadienyl; Ln = Sm (IV), Gd (V)). Complexes IV and V are formed even when a twofold excess of CpPh3K is used. The molecular structure of complexes I–V was established by X-ray diffraction analysis (CCDC nos. 2299485 (I), 2299487 (II), 2299486 (III), 2305352 (IV), 2306051 (V)).

Semistable Half‐Sandwich Cp*Co(II)(P‐X) (X=N, O) Complexes: Synthesis, Characterization and Electrochemical Properties

AbstractThree Cp*Co(II)(P−X) (X=N, O) complexes were synthesized using cyclopentadienyl anionic and [P N]/[P O] bidentate ligands. Crystallographic analysis revealed five‐coordinate, 17‐electron unsaturated structures with low‐spin d⁷ Co(II) configurations. Electrochemical studies showed reversible Co(II)/Co(III) redox couples and an irreversible Co(II)/Co(I) process of Cp*Co(II)(P−O) complexes, while complex Cp*Co(II)(P−N) exhibited irreversible Co(II)/Co(III) process due to the pyridine imine ligand. Oxidation with AgBF4 produced stable Co(III) complexes, confirmed by crystallographic data and NMR data. These complexes exhibit significant electrochemical properties and stability, indicating potential for catalytic and organic molecule binding applications.

Chiral Bis(binaphthyl) Cyclopentadienyl Ligands for Rhodium-Catalyzed Desymmetrization of Diarylmethanes via Selective Arene Coordination.

Owing to substantial advances in the past several decades, transition-metal-catalyzed asymmetric reactions have garnered considerable attention as pivotal methods for constructing chiral molecules from abundant, readily available achiral counterparts. These advances are largely attributed to the development of chiral ligands that control stereochemistry through steric repulsion and other noncovalent interactions between the ligands and functional groups or prochiral centers on the substrates. However, stereocontrol weakens dramatically with increasing distance between the reaction site and the functional group or prochiral center. Herein, we report a symphonic strategy for remote stereocontrol of Rh(III)-catalyzed asymmetric benzylic C-H bond addition reactions of diarylmethanes in which the two aryl motifs differ at the meta and/or para position. Specifically, catalysts bearing a new type of chiral cyclopentadienyl (Cp) ligand differentiate between the two aromatic rings of the diarylmethane by arene-selective η6 coordination, setting up an opportunity for ligand-controlled stereoselective benzylic deprotonation and subsequent stereoselective addition to the 1,1-bis(arylsulfonyl)ethylene.

Silylation of N2 catalyzed by cubic [Mo3S4Ni] clusters bearing Mo-bound cyclopentadienyl ligands

Silylation of N2 catalyzed by cubic [Mo3S4Ni] clusters bearing Mo-bound cyclopentadienyl ligands

Poly(2,2'-Bibenzimidazole)-Supported Iridium Complex: A Recyclable Metal-Polymer Ligand Bifunctional Catalyst for the N-Methylation of Amines with Methanol.

The design and development of new types of catalysts is one of the most important topics for modern chemistry. Herein, a polymer-supported iridium complex Cp*Ir@Poly(2,2'-BiBzIm) was designed and synthesized by the coordinative immobilization of [Cp*IrCl2]2 on 2,2'-bibenzimidazoles. In the presence of the catalyst (0.5 mol % Ir) and Cs2CO3 (0.3 equiv), a variety of N-methylated amines were obtained in high yields with complete selectivity. More importantly, the catalyst could be recycled without an obvious loss of activity for six cycles. Apparently, the designed catalyst combines the advantages of both homogeneous and heterogeneous catalysis.

New Catalyst Systems for the Polymerization of Norbornene and Its Derivatives Based on Cationic Palladium Cyclopentadienyl Complexes

The catalytic properties of systems based on [Pd(Cp)(L)n]m[BF₄]m (where Cp = η5-C5H5; n = 2, m = 1: L = tris(orthomethoxyphenyl)phosphine, triphenylphosphine, tris(2-furyl)phosphine (TFP), n = 1, m = 1: L = 1,1'-bis(diphenylphosphino)ferrocene, 1,3-bis(diphenylphosphino)propane, 1,4-bis(diphenylphosphino)butane, 1,5-bis(diphenylphosphino)pentane; n = 1, m = 2 or 3: L = 1,6-bis(diphenylphosphino)hexane) in the addition homoand copolymerization of norbornene (NB) and its derivatives have been studied. These complexes can be activated with the Lewis-acids (BF₃ ∙ OEt2 or AlCl3). The productivity of the [Pd(Cp)(PPh3)2][BF₄]/BF₃ ∙ OEt2 catalyst system in the polymerization of norbornene is up to 188800 molNB molPd⁻¹. The homopolymerization of 5-methoxycarbonylnorbornene and copolymerization of NB with 5-methoxycarbonylnorbornene or 5-phenylnorbornene were studied in the presence of BF₃ ∙ OEt2 and [Pd(Cp)(L)2][BF₄] (L = PPh3 or TFP). A hypothesis for the catalyst’s formation via the intramolecular rearrangement of the η5-Cp ligand into the η1-Cp form upon interaction with a Lewis acid is suggested. The structure of the [Pd(Cp)(TFP)2]BF₄ (I) was determined by X-ray diffraction. In the crystal structure of I, the palladium coordination sphere is characterized by a slight distortion of the square planar geometry of the central atom, and the cyclopentadiyl fragment is in an eclipsed conformation. Based on the XRD data, the steric hindrance of the TFP ligand was estimated (the cone angle is 149°).

A Series of Cyclopentadienyl Lanthanum Complexes with Metalloid Germanium Clusters.

A series of cyclopentadienyllanthanum complexes with the disilylated metalloid germanium cluster [Ge9(Hyp)2]2- [Hyp = Si(SiMe3)3] has been prepared and fully characterized. The synthetic procedure is based on the salt metathesis reaction of two different cyclopentadienyllanthanum diiodides CpLaI2 (Cp: Cp*, pentamethylcyclopentadienyl; Cpttt, 1,2,4-tri-tert-butylcyclopentadienyl) with K2[Ge9(Hyp)2] in tetrahydrofuran (THF) with a subsequent extraction with n-hexane. The composition of the obtained compounds and the mode of coordination of the germanium cluster to the rare-earth metal are strongly influenced by the steric demand of the cyclopentadienyl ligands and the crystallization conditions. The centrosymmetric dimeric compounds with the common formula [CpLa(solv)(η2,3-Ge9(Hyp)2)]2 [1, Cp = Cp*, solv-THF; 2, Cp = Cpttt, solv-NCCH2C(Me)NSiMe3] have been isolated by the slow evaporation of a n-hexane solution, while a mononuclear complex [CptttLa(THF)2(η3-Ge9(Hyp)2)] (4) was found by crystallization from THF. The repeated recrystallization of 1 from n-hexane afforded the asymmetric dimer [Cp*La(THF)(η2,3-Ge9(Hyp)2)][Cp*La(η2,3-Ge9(Hyp)2)] (3) with only one coordinated THF molecule.

Iridium complex immobilized on covalent triazine framework derived from biomass as a recyclable catalyst for the dehydrogenation of formic acid

The first example of covalent triazine frameworks derived from biomass was designed and synthesized. Furthermore, an iridium complex Cp*Ir@CTF-biomass was prepared by the coordinative immobilization of [Cp*IrCl2]2 on the synthesized covalent triazine framework. In the presence of Cp*Ir@CTF-biomass (0.03 mol % Ir), the reaction was carried out for 24 min at 80 °C and an almost complete dehydrogenation of formic acid with an initial TOF of 7627 h−1 was observed. Further experiments confirmed that heterogeneous catalyst Cp*Ir@CTF-biomass and its homogeneous analog [Cp*Ir(pyridine)Cl2] have approximate catalytic activity. Moreover, catalytic activity of Cp*Ir@CTF-biomass is well maintained during six runs.©2024 Elsevier Inc. All rights reserved.

Ruthenium Phenoxo Complexes: An Isolobal Ligand to Cp with Improved Properties.

Catalytic π-arene activation is based on catalysts that allow for arene exchange. To date, cyclopentadiene (Cp)-derived catalysts are the most commonly used in π-arene activation despite their low arene exchange rates. Herein, we report the synthesis, analysis, and catalytic application of Ru(II) complexes supported by phenoxo ligands, which are isolobal alternatives to Cp. The phenoxo complexes exhibit arene exchange rates significantly faster than those of the corresponding Cp complexes. The rate can be further increased through the choice of appropriate counterions. The mechanism of the arene exchange process is elucidated by kinetic and computational analyses. We demonstrate the utility of the new catalysts through an SNAr reaction between fluorobenzene and alcohols, including secondary alcohols that could not be used previously in related reactions. Moreover, the catalytic thermal decarboxylation of phenylacetic acids is presented.

Insights into coordination and ligand trends of lanthanide complexes from the Cambridge Structural Database

Understanding lanthanide coordination chemistry can help develop new ligands for more efficient separation of lanthanides for critical materials needs. The Cambridge Structural Database (CSD) contains tens of thousands of single crystal structures of lanthanide complexes that can serve as a training ground for both fundamental chemical insights and future machine learning and generative artificial intelligence models. This work aims to understand the currently available structures of lanthanide complexes in CSD by analyzing the coordination shell, donor types, and ligand types, from the perspective of rare-earth element (REE) separations. We obtain four sets of lanthanide complexes from CSD: Subset 1, all Ln-containing complexes (49472 structures); Subset 2, mononuclear Ln complexes (27858 structures); Subset 3, mononuclear Ln complexes without cyclopentadienyl ligands (Cp) (26156 structures); Subset 4, Ln complexes with at least one 1,10-phenanthroline (phen) or its derivative as a coordinating ligand (2226 structures). The subsequent analysis of lanthanide complexes in these subsets examines the trends in coordination numbers and first shell distances as well as identifies and characterizes the ligands and donor groups. In addition, examples of Ln-complexes with commercially available complexants and phen-based ligands are interrogated in detail. This systematic investigation lays the groundwork for future data-driven ligand designs for REE separations based on the structural insights into the lanthanide coordination chemistry.

A lithium–aluminium heterobimetallic dimetallocene

Homobimetallic dimetallocenes exhibiting two identical metal atoms sandwiched between two η5 bonded cyclopentadienyl rings is a narrow class of compounds, with representative examples being dizincocene and diberyllocene. Here we report the synthesis and structural characterization of a heterobimetallic dimetallocene, accessible through heterocoupling of lithium and aluminylene fragments with pentaisopropylcyclopentadienyl ligands. The Al–Li bond features a high ionic character and profits from attractive dispersion interactions between the isopropyl groups of the cyclopentadienyl ligands. A key synthetic step is the isolation of a cyclopentadienylaluminylene monomer, which also enables the structural characterization of this species. In addition to their structural authentication by single-crystal X-ray diffraction analysis, both compounds were characterized by multinuclear NMR spectroscopy in solution and in the solid state. Furthermore, reactivity studies of the lithium–aluminium heterobimetallic dimetallocene with an N-heterocyclic carbene and different heteroallenes were performed and show that the Al–Li bond is easily cleaved.

Management and liming-induced changes in organo-Al/Fe complexes and amorphous mineral-associated organic carbon: Implications for carbon sequestration in volcanic soils

Amorphous mineral-associated organic carbon (AMAOC) and organo-Al/Fe complexes (Cp) play crucial roles in soil carbon (C) sequestration in volcanic soils. This study investigated the effects of land use management and liming on the C sequestration capacity of a young allophanic Andisol and old halloysitic-kaolinitic volcanic Ultisol sampled at 0–10 and 10–20 cm depth. This study was designed to test two hypotheses: (1) management and liming affect the C sequestration capacity of Andisol by reducing Cp and increasing AMAOC to offset Cp losses, and (2) they do not significantly affect the carbon sequestration capacity of Ultisol because of its more crystalline mineral forms and reduced amount of Cp. This study was conducted using a selective dissolution technique and density fractionation with agricultural soils and forests as control soils. After 43 years of intensive management and liming, the soil organic carbon (SOC) content decreased by 32% in the topsoil compared to the original native forest soil (178 ± 2 g kg−1 soil), mainly in the particulate organic matter (POM) fraction, which decreased by 74% compared to the native forest soil (61 ± 9 g kg−1 soil). However, Cp decreased only by 40% compared to the forest soil (62 ± 1 g kg−1 soil) from pHKCl 4.8–5.0, and AMAOC increased in 17% in the topsoil and 64% in the subsoil, supporting the first hypothesis of this study. In the cultivated Ultisol, SOC decreased by 35% due to the POM fraction, and mineral-associated organic C (MAOC) decreased by 37% compared to the forest soil (38 ± 7 g kg−1 soil). These results partially support the second hypothesis because the crystalline clay minerals from which MAOC originated remained unchanged and were strongly correlated with SOC. The mass balance of both soils indicated that the stable C losses from native forest soils were < 14%. These results highlight the importance of different C sequestration mechanisms to compensate for SOC losses in the young allophanic Andisol and old volcanic Ultisol. Our findings imply that both mechanisms can be used to estimate the C sequestration capcity in similar soil types.

Interfacial redox modulation of polysulfides with ferrocene functionalized separator in Al–S batteries

Despite the bright future of aluminum-sulfur (Al–S) batteries due to the ultrahigh energy-to-price ratios, the development of this new energy storage system is plagued by the shutting of polysulfides and sluggish kinetics during redox conversions. Herein, ferrocene as efficient polysulfide mediators and electrocatalysts is covalently grafted onto glass fiber (GF) membranes, which serves as separators in Al–S batteries. Attributed to the strong chemisorption and catalytic effect of metallocene toward polysulfide conversions, Al–S cells assembled with the grafted separators deliver an ultrahigh initial capacity of 1347.9 mAh g−1 (at 200 mA g−1) and desirable rate performance (328.1 mAh g−1 at 500 mA g−1). As validated by theoretical calculations, effective polysulfide binding is achieved through beneficial cation-π interactions between Al3+ in polysulfides and the negatively charged cyclopentadienyl ligands in ferrocene, providing novel insights into the exploration of metal-sulfur chemistries for stable energy storage.

IZCp and PZCp: Redox Non-innocent Cyclopentadienyl Ligands as Electron Reservoirs for Sandwich Complexes.

A long-sustained effort of systematic steric and electronic modification of cyclopentadienyl (Cp) ligands has enabled them to find wide-ranging, valuable applications. Herein, we present two novel Cp ligands: imidazolium- and pyrrolinium-substituted zwitterionic Cps (IZCp and PZCp), whose key utility is redox non-innocence─the ability to participate cooperatively with the metal center in redox reactions. Through the simple metalation of ZCps, the Cr(0) and Mo(0) half-sandwich complexes (IZCp)Cr(CO)3, (PZCp)Cr(CO)3, (IZCp)Mo(CO)3, and (PZCp)Mo(CO)3, respectively, as well as the Ru(II) sandwich complexes [(IZCp)RuCp]PF6 and [(PZCp)RuCp]PF6 were prepared. The sandwich complexes were fully characterized and showed by cyclic voltammetry reversible one-electron reduction at E1/2 potentials ranging from -1.7 to -2.7 V vs Fc/Fc+. These values are unusually low and have not been observed with other Cp ligands due to the instability of the reduced complexes. Density functional theory (DFT) calculations for the reduced sandwich derivatives with IZCp and PZCp showed their spin densities to be highly delocalized over their ZCp ligand moieties (70-90%). Electron paramagnetic resonance (EPR) analysis of the isolated K[(PZCp)Mo(CO)3] and (PZCp)RuCp also indicated a high degree of ligand-localized radical character. Thus, the IZCp and PZCp ligands act as electron reservoirs to sustain these sandwich complexes in highly reduced states. At the same time, the CO stretching frequencies of K[(PZCp)Mo(CO)3]: νCO 1871, 1748, and 1699 cm-1, rank the [PZCp]- ligand as the strongest electron-donating Cp ligand among the reported CpMo(CO)3 derivatives, whose νCO > 1746 cm-1. In addition, these redox non-innocent Cps were obtained in high yields and found to be practically air- and moisture-stable, unlike typical Cps.

A Metallocene Bis(phosphoranocarbene) of Uranium and a Probe of Its Reactivity with Alcohols.

The addition of 2 equiv of the phosphaylide H2C═PPh3 to the dimethyl uranium metallocene Cp*2UMe2 (Cp* = η5-C5Me5) in toluene with gentle heating at 40 °C generates the phosphorano-stabilized bis(carbene) Cp*2U[C(H)PPh3]2 (1) in good yield. Characterization of 1 by X-ray crystallographic analysis reveals two short uranium-carbon bonds, ranging from 2.301(5) to 2.322(5) Å, consistent with the presence of U═C carbene-type bonds. Monitoring the reaction by NMR spectroscopy suggests that it proceeds through the intermediate formation of the methyl carbene complex Cp*2U[C(H)PPh3](Me) (1Int); however, prolonged heating of these solutions leads to the ortho-cyclometalated carbene species Cp*2U{κ2-[C(H)PPh2(C6H4)]} (2) via intramolecular C-H activation. Rapid conversion from 1 to 2 occurs within hours upon heating its toluene solutions to 100 °C. Preliminary reactivity studies of 1 show that it readily reacts with alcohols, such as HODipp (Dipp = 2,6-diisopropylphenyl) and HOC(CF3)3, to give the mixed carbene alkoxide compounds Cp*2U[C(H)PPh3](OR) (R = Dipp (4Dipp), C(CF3)3 (5CF3)). In one case, the reaction of 1 with HODipp in the presence of adventitious water led to the formation of a few crystals of the terminal U(IV) oxo complex, [Ph3PCH3][Cp*2U(O)(ODipp)] (3oxo). The isolation of 1 marks the first instance of an unchelated, heteroatom-stabilized bis(carbene) complex of uranium that also provides an entryway to the synthesis of its monocarbene derivatives through protonolysis.

Exploiting Co(III)-Cyclopentadienyl Complexes To Develop Anticancer Agents.

In recent years, organometallic complexes have attracted much attention as anticancer therapeutics aiming at overcoming the limitations of platinum drugs that are currently marketed. Still, the development of half-sandwich organometallic cobalt complexes remains scarcely explored. Four new cobalt(III)-cyclopentadienyl complexes containing N,N-heteroaromatic bidentate, and phosphane ligands were synthesized and fully characterized by elemental analysis, spectroscopic techniques, and DFT methods. The cytotoxicity of all complexes was determined in vitro by the MTS assay in colorectal (HCT116), ovarian (A2780), and breast (MDA-MB-231 and MCF-7) human cancer cell lines and in a healthy human cell line (fibroblasts). The complexes showed high cytotoxicity in cancer cell lines, mostly due to ROS production, apoptosis, autophagy induction, and disruption of the mitochondrial membrane. Also, these complexes were shown to be nontoxic in vivo in an ex ovo chick embryo yolk sac membrane (YSM) assay.