Cyclopentadienyl Ligands Research Articles

Gelatin/chitosan nanofibres containing β‐cyclodextrin complex and corn poppy (Papaver rhoeasL.) for intelligent packaging

SummaryThis study aimed to produce an intelligent gelatin film containing chitosan nanofibre (CNF) and β‐cyclodextrin/corn poppy complex (β‐CD/CP) and monitor shrimp spoilage. The crystalline properties, surface morphology and chemical structure of the films were characterised. The results showed that the addition of CNF caused the appearance of a new peak at 2θ = 12.16° and improved the crystalline properties of the film. According to Fourier Transform Infrared (FT‐IR) results, new interactions were formed between CNF and the β‐CD/CP complex. Also, the addition of the β‐CD/CP complex significantly (P < 0.05) reduced the water solubility (WS) of the gelatin‐based film from 96% to 42%. The results of shrimp spoilage monitoring showed that after 7 days of storage, the colour of the intelligent film changed from coral to goldenrod due to the change in pH from 8.3 to 10.5 and the formation of ammonia vapours due to the decomposition of proteins, which shows the potential of the intelligent films containing the β‐CD/CP to be used for monitoring of seafood products.

Sulphur vs NH Group: Effects on the CO2 Electroreduction Capability of Phenylenediamine-Cp Cobalt Complexes.

The cobalt complex (I) with cyclopentadienyl and 2-aminothiophenolate ligands was investigated as a homogeneous catalyst for electrochemical CO2 reduction. By comparing its behavior with an analogous complex with the phenylenediamine (II), the effect of sulfur atom as a substituent has been evaluated. As a result, a positive shift of the reduction potential and the reversibility of the corresponding redox process have been observed, also suggesting a higher stability of the compound with sulfur. Under anhydrous conditions, complex I showed a higher current enhancement in the presence of CO2 (9.41) in comparison with II (4.12). Moreover, the presence of only one -NH group in I explained the difference in the observed increases on the catalytic activity toward CO2 due to the presence of water, with current enhancements of 22.73 and 24.40 for I and II, respectively. DFT calculations confirmed the effect of sulfur on the lowering of the energy of the frontier orbitals of I, highlighted by electrochemical measurements. Furthermore, the condensed Fukui function f - values agreed very well with the current enhancement observed in the absence of water.

Synthesis and Coordination Chemistry of Fluorinated Cyclopentadienyl Ligands

AbstractThe incorporation of fluorinated substituents in cyclopentadienyl anions leads to distinct changes in the properties of the corresponding metal complexes, e.g., with regards to electrochemical properties or metal–ligand bonding energies. This review summarizes the synthetic challenges of the preparation and coordination of fluorinated cyclopentadienyl anions.1 Introduction2 Fluorinated Cyclopentadienyls3 Cyclopentadienyls with One CF3 Group4 Cyclopentadienyls with Four CF3 Groups5 Cyclopentadienyls with Five CF3 Groups6 Cyclopentadienyls with ‘Fluorous Ponytails’7 Cyclopentadienyls with C6F5 Groups8 Trends in Interaction Energies9 Conclusion

Influence of the 1,2,4-Tri-tert-butylcyclopentadienyl Ligand on the Reactivity of the Uranium Bipyridyl Metallocene [η5-1,2,4-(Me3C)3C5H2]2U(bipy)

The uranium bipyridyl metallocene, [η5-1,2,4-(Me3C)3C5H2]2U(bipy) (2), prepared from [η5-1,2,4-(Me3C)3C5H2]2UCl2 (1) and 2,2′-bipyridine in the presence of potassium graphite (KC8) has been evaluated in small-molecule activation. In contact with AgF, Ph2E2 (E = S, Se), (PhNH)2, (PhCH═N)2, diazenes, pyridine N-oxide, organic azides, diazoalkanes, Ph2CS, and Ph2CO, it behaves as a synthon for the [η5-1,2,4-(Me3C)3C5H2]2U(II) fragment. In contrast, C–C bond coupling occurs when 2 is treated with (CH2)5CO, p-MePhCHO, and p-ClPhCHO to furnish [η5-1,2,4-(Me3C)3C5H2]2U[(bipy){(CH2)5CO}] (19), [η5-1,2,4-(Me3C)3C5H2]2U[(bipy)(p-MePhCHO)] (20), and [η5-1,2,4-(Me3C)3C5H2]2U[(bipy)(p-ClPhCHO)] (21), respectively. Moreover, a single-electron transfer (SET) process ensues after the addition of CuI to 2 to yield the uranium(III) iodide complex [η5-1,2,4-(Me3C)3C5H2]2UI (3). A comparison with the other uranium bipyridyl metallocene derivatives shows that minor variations in the coordinated cyclopentadienyl ligands changes the reactivity of these compounds.

Cobalt(III)-catalyzed asymmetric ring-opening of 7-oxabenzonorbornadienes via indole C–H functionalization

Asymmetric ring-opening of 7-oxabenzonorbornadienes is achieved via Co-catalyzed indole C–H functionalization. The utilization of chiral Co-catalyst consisting of a binaphthyl-derived trisubstituted cyclopentadienyl ligand resulted in high yields (up to 99%) and excellent enantioselectivity (>99% ee) for the target products with tolerance for diverse functional groups. Opposite diastereoselectivities are obtained with chiral Co-catalyst or Cp*CoI2CO. Combined experimental and computational studies suggest β-oxygen elimination being the selectivity-determining step of the reaction. Meanwhile, the reactions of 7-azabenzonorbornadiene could also be executed in a diastereodivergent manner.

Advances in the synthesis of chiral cyclopentadienyl ligands and metal complexes

Advances in the synthesis of chiral cyclopentadienyl ligands and metal complexes

Inducing Nucleophilic Reactivity at Beryllium with an Aluminyl Ligand.

The reactions of anionic aluminium or gallium nucleophiles {K[E(NON)]}2 (E = Al, 1; Ga, 2; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) with beryllocene (BeCp2) led to the displacement of one cyclopentadienyl ligand at beryllium and the formation of compounds containing Be-Al or Be-Ga bonds (NON)EBeCp (E = Al, 3; Ga, 4). The Be-Al bond in the beryllium-aluminyl complex [2.310(4) Å] is much shorter than that found in the small number of previous examples [2.368(2) to 2.432(6) Å], and quantum chemical calculations suggest the existence of a non-nuclear attractor (NNA) for the Be-Al interaction. This represents the first example of a NNA for a heteroatomic interaction in an isolated molecular complex. As a result of this unusual electronic structure and the similarity in the Pauling electronegativities of beryllium and aluminium, the charge at the beryllium center (+1.39) in 3 is calculated to be less positive than that of the aluminium center (+1.88). This calculated charge distribution suggests the possibility for nucleophilic behavior at beryllium and correlates with the observed reactivity of the beryllium-aluminyl complex with N,N'-diisopropylcarbodiimide─the electrophilic carbon center of the carbodiimide undergoes nucleophilic attack by beryllium, thereby yielding a beryllium-diaminocarbene complex.

(η5-Carb-oxy-cyclo-penta-dien-yl)(η7-cyclo-hepta-trien-yl)manganese(I) hexa-fluorido-phosphate.

The title compound, [Mn(C7H7)(C6H5O2)]PF6 or [(Cht)Mn(Cp'CO2H)]PF6, with Cht = cyclo-hepta-trienyl and Cp' = C5H4, is an air-stable, purple, heteroleptic, cationic sandwich complex with manganese in oxidation state +I and π-coordinating cyclo-hepta-trienyl and cyclo-penta-dienyl ligands. The latter ligand carries the carb-oxy-lic acid functionality. This 'tromancenium-8-carb-oxy-lic acid' with hexa-fluorido-phosphate as counter-ion represents a rare case of a cationic carb-oxy-lic acid. Structurally, this organometallic carb-oxy-lic acid displays the common motif of planar Osp 2⋯H-Osp 3/Osp 3-H⋯Osp 2 hydrogen-bonded carb-oxy-lic acid dimers with anti-oriented metallocenyl moieties, the cationic charge of which is balanced by octa-hedrally shaped hexa-fluorido-phosphate anions. Positional disorder is observed in the cyclo-hepta-trienyl ring and the PF6 - anion.

Synthesis and Characterization of Zr and Hf Triazenido Complexes with Rare κ1N-Coordination Built Directly in the Coordination Sphere of the Metal

The synthesis and characterization of triazenido complexes of the group IV metals zirconium and hafnium is reported. The reaction of the group IV pentafulvene complexes Cp*(π–η5:σ–η1-pentafulvene)-zirconium(IV) and -hafnium(IV)chlorides with the sterically demanding 2,6-dimesitylphenyl azide results in the insertion of the γ-nitrogen atom of the ligand precursor into the polarized M–Cexo (Cexo: exocyclic carbon atom) bond of the pentafulvene ligand, forming a formal Cp,N,N tridentate ligand system. The molecular structures of the furnished complexes reveal the rare κ1N coordination of the γ-nitrogen atom to the zirconium or hafnium center along with a localized Nα–Nβ double bond. Unusually, in one case, the ligand system shows additional coordination of the α-nitrogen atom to the central metal atom and well-balanced N–N distances, thus, forming the chelating κ2N,N coordination mode of the triazenido ligand. The potential use of the formed complexes as precursors for cationic group IV cationic complexes was investigated by reaction with methyl lithium.

Chiral Bicyclo[2.2.2]octane-Fused and Ferrocene-Derived Cyclopentadienyl Ligands for Asymmetric C–H Activation

AbstractTo fulfill increasing and diverse demands of cyclopentadienyl metal complexes (CpM) catalyzed asymmetric C–H activation, innovation in developing new types of chiral Cp ligands should be important. In this context, we found the structurally rigid chiral bicyclo[2.2.2]octane-fused Cp ligands originated by Vollhardt and ­Halterman were efficient to enable highly enantioselective C–H activation of N-methoxybenzamides with quinones. Besides, we also designed and synthesized a class of ferrocene-based chiral Cp ligand featuring a loose chiral pocket, whose rhodium complexes exhibited high reactivity and reasonable enantioselectivity in an asymmetric intramolecular aryl amination of alkene.1 Introduction2 Chiral Cp Ligands Applied in Asymmetric C–H Activation3 Chiral Bicyclo[2.2.2]octane-Fused Cp Ligands4 Chiral Ferrocene-Derived Cp Ligands5 Conclusion

Local and Global Spatial Information for Land Cover Semisupervised Classification of Complex Polarimetric SAR Data

Each of the three satellites constituting the RADARSAT Constellation Mission (RCM) provide compact polarimetric synthetic aperture radar (CP SAR) data. The complex CP data have similar properties to the complex quad polarimetric (QP) data provided by prior RADARSAT missions. In this paper, a land cover classification method using spatial information is designed based on the statistical characteristics of the complex CP and QP SAR data. First, the local spatial dependency among pixels is captured by superpixels. Second, a graph is constructed on the superpixels to model the global spatial dependency among superpixels. The land cover classification image with land cover type labels is then estimated by propagating labels from the few labeled superpixels to the unlabeled superpixels. Classification of two RCM complex CP and QP scenes demonstrates that the proposed method, with few labeled pixels, provides much higher classification accuracy than methods which do not exploit global spatial dependency.

Revisiting the origin of the bending in group 2 metallocenes AeCp2 (Ae = Be-Ba).

Metallocenes are well-established compounds in organometallic chemistry, and can exhibit either a coplanar structure or a bent structure according to the nature of the metal center (E) and the cyclopentadienyl ligands (Cp). Herein, we re-examine the chemical bonding to underline the origins of the geometry and stability observed experimentally. To this end, we have analysed a series of group 2 metallocenes [Ae(C5R5)2] (Ae = Be-Ba and R = H, Me, F, Cl, Br, and I) with a combination of computational methods, namely energy decomposition analysis (EDA), polarizability model (PM), and dispersion interaction densities (DIDs). Although the metal-ligand bonding nature is mainly an electrostatic interaction (65-78%), the covalent character is not negligible (33-22%). Notably, the heavier the metal center, the stronger the d-orbital interaction with a 50% contribution to the total covalent interaction. The dispersion interaction between the Cp ligands counts only for 1% of the interaction. Despite that orbital contributions become stronger for heavier metals, they never represent the energy main term. Instead, given the electrostatic nature of the metallocene bonds, we propose a model based on polarizability, which faithfully predicts the bending angle. Although dispersion interactions have a fair contribution to strengthen the bending angle, the polarizability plays a major role.

Influence of Ionization and Spin Transitions on Electron Delocalization in the Molecules of Transition Metal Sandwich Complexes

A quantum-chemical study of the structures of symmetric 3d metal sandwich complexes with benzene and cyclopentadienyl ligands has been carried out within the framework of the electron density of delocalized bonds (EDDB) model. Neutral and ionized molecules in various spin states were considered. It is shown that successive population of the d-electron shell by varying the metal atom in a series of similar complexes, as a rule, leads to a decrease in the degree of electron density delocalization. The detachment of an electron from neutral molecules also reduces the number of delocalized electrons in the sandwich system, butthe contribution of the metal atom to delocalization increases in most cases. Singlet-triplet transitions in metallocenes and bis-benzene complexes decrease the electron density of delocalized bonds, but to a lesser extent than in the free and C6H6 ligands.

Divalent metallocenes of the lanthanides - a guideline to properties and reactivity.

Since the discovery in the early 1980s, the soluble divalent metallocenes of lanthanides have become a steadily growing field in organometallic chemistry. The predominant part of the investigation has been performed with samarium, europium, and ytterbium, whereas only a few reports dealing with other rare earth elements were disclosed. Reactions of these metallocenes can be divided into two major categories: (1) formation of Lewis acid-base complexes, in which the oxidation state remains +II; and (2) single electron transfer (SET) reductions with the ultimate formation of Ln(III) complexes. Due to the increasing reducing character from Eu(II) over Yb(II) to Sm(II), the plethora of literature concerning redox reactions revolves around the metallocenes of Sm and Yb. In addition, a few reactivity studies on Nd(II), Dy(II) and mainly Tm(II) metallocenes were published. These compounds are even stronger reducing agents but significantly more difficult to handle. In most cases, the metals are ligated by the versatile pentamethylcyclopentadienyl ligand: (C5Me5). Other cyclopentadienyl ligands are fully covered but only discussed in detail, if the ligand causes differences in synthesis or reactivity. Thus, the focus lays on three compounds: [(C5Me5)2Sm], [(C5Me5)2Eu] and [(C5Me5)2Yb] and their solvates. We discuss the synthesis and physical properties of divalent lanthanide metallocenes first, followed by an overview of the reactivity rendering the full potential of these versatile reactants.

Rhodium complexes with planar-chiral cyclopentadienyl ligands: synthesis from tert-butylacetylene and catalytic performance in C-H activation of arylhydroxamates.

The rhodium complex [(C5H2tBu2CH2tBu)RhCl2]2 with an asymmetric cyclopentadienyl ligand was prepared in 95% yield by the reaction of [(cod)RhCl]2 with tert-butylacetylene in the presence of AlCl3. A similar reaction in the presence of InBr3 gave the cationic fulvene complex [(C5H2tBu2 = CHtBu)Rh(cod)]InBr4 (70%), which can add alcohols ROH and produce more bulky catalysts [(C5H2tBu2CH(OR)tBu)RhCl2]2. The enantiomers of these planar-chiral complexes were separated by thin-layer chromatography in the presence of L-phenylglycinol. The complexes catalyze the reactions of arylhydroxamates with alkenes giving dihydroisoquinolones in excellent yields (80-90%), but with moderate enantioselectivity (typically 20-50% ee).

The role of agostic interaction in the mechanism of ethylene polymerisation using Cr(III) half-sandwich complexes: What dictates the reactivity?

Chromium-based catalysts play a significant role in the production of ultra-high molecular weight polyethylene, and half-sandwich functionalised-metallocene complexes were proven to be one of the most suitable candidates as catalysts for generating such large polymeric-length olefins. Earlier experimental studies on olefin polymerisation using a series of catalysts such as [L1-2CrCl2] (where L1 = 1-((pyridin-2-yl)methyl)indenyl (1) and L2 = 2-methyl-1-{[4-(yridinene-1-yl)yridine-2-yl]methyl}-1H-indenyl (2)) reveal significant variation where peripheral substitution on the ligand was found to influence the reactivity significantly. However, the specific ligand position that affects the reactivity has not been established. As these reactions are fast and robust, it is challenging to establish reactive intermediates via experiments, and therefore, mechanistic clues for such reactions are elusive. Here we have undertaken a detailed computational study by employing an array of DFT (uB3LYP-D3/def2-TZVP, CASSCF/NEVPT2, and DLPNO-CCSD(T) methods to explore the substituted and non-substituted pyridine-cyclo-pentadienyl chromium complexes and their influence on the catalytic activity in ethylene polymerisation. Our study not only unravels the catalytic pathway for olefin polymerisation for such Cr(III)-half-sandwich complexes but also reveals that the energetics of the formation of pseudo-three-coordinate alkyl intermediates is key to the variation in the reactivity observed. A detailed examination using MO and NBO analysis unveils the presence of a C-H⋯Cr agostic interaction that is found to significantly stabilise this intermediate when the pyridine ligand has strong electron-donating groups at its para position. The other substitutions, such as on the cyclopentadienyl ligand, neither yield the desired stability nor the desired interaction. Further studies on models support this proposal. In order to improve the efficiency and selectivity of catalytic systems in olefin polymerisation, we strongly advocate for the integration of agostic interactions as a crucial criterion in the design of future catalysts. Considering the prevalence of electron-deficient metal centres in successful olefin polymerisation catalysts, this research prompts a broader mechanistic inquiry to propose a unified approach for this industrially crucial reaction.

Bis(tetrelocenes) - fusing tetrelocenes into close proximity.

We report the synthesis and structure of two bis(germanocenes) and a bis(stannocene), obtained by the reaction of unsymmetric ansa bis(cyclopentadienyl) ligands with germanium and tin dichloride. DFT calculations show that the formation of these bis(tetrelocenes) is energetically favoured over the formation of the corresponding [1]tetrelocenophanes. In the crystal structure authenticated structural motif, the two tetrel(II) centers are forced into close proximity to each other, resulting in weak donor-acceptor interactions, according to Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) analyses.

Photocatalytic dechlorination of unactivated chlorocarbons including PVC using organolanthanide complexes.

Simple lanthanide cyclopentadienyl (Cp) complexes can photochemically cleave the sp3 carbon-chlorine bond of unactivated chlorinated hydrocarbons including polyvinyl chloride (PVC). The excited state lifetimes of these simple complexes are among the longest observed for cerium complexes (175 ns for [(CpMe4)2Ce(μ-Cl)]2) and the light absorption by the Cp ligand is efficient, so photocatalytic reactivity is enhanced for cerium and now also made possible for neighboring, normally photoinactive, lanthanide congeners.

Dicarboxylate-containing and fully substituted ferrocene with rapid dissolvability, high solubility, good stability, and moderate formal potential for mediated electrochemical detection.

An electron mediator with rapid dissolvability and high solubility in aqueous electrolyte solutions is essential for point-of-care testing based on mediated electrochemical detection. However, most ferrocenyl (Fc) compounds have slow dissolvability and poor solubility owing to high hydrophobicity of the Fc backbone. Moreover, many Fc compounds have poor stability and nonoptimal formal potential (). Herein, we present an Fc compound, Fc8m2c, which exhibits rapid dissolvability, high solubility, good stability, and moderate along with its high electron-mediation rate. The of Fc8m2c (0.17 V vs. Ag/AgCl) is tuned by two electron-withdrawing acyl substituents and eight electron-donating methyl substituents. Two pendant carboxylate groups of Fc8m2c allow for rapid dissolvability and high solubility (0.63 M in water), whereas full substitution in its two cyclopentadienyl ligands facilitates good chemical stability against decomposition in the presence of dissolved O2 and ambient light. A moderate enables the application of a potential of 0.07 V at which electrochemical background currents are low and also contributes toward resisting the decomposition of both Fc8m2c and Fc8m2c+. Fc8m2c provides a high electron-mediation rate constant (2.4 × 106 M-1 s-1) in glucose detection using glucose dehydrogenase. When Fc8m2c is applied to a glucose sensor, the calculated detection limit is ∼0.1 mM with a measurement period of 5 s. Considering that the normal concentration of glucose in serum is between 3.9 and 6.6 mM, the detection limit is sufficiently low. These results show that Fc8m2c is an excellent electron-mediator candidate for sensitive and rapid glucose detection.

Cationic dialanes with fluxional π-bridged cyclopentadienyl ligands.

Unique π-cyclopentadienyl bridged dialanes are synthesized as complex salts with aluminate anions by comproportionation of aluminocenium cations [AlIII(Cp)(Cp*)]+/[AlIIICp2]+ with [(AlICp*)4]. Very short Al-Al bond lengths occur in positively charged Al24+ fragments. Intriguingly, the prepared asymmetric dialane shows a unique fluxional coordination of the cyclopentadienyl ligands.