Search Results (13)

Solid solutions of the metallocenes ferrocene (Cp₂Fe), nickelocene (Cp₂Ni), and cobaltocene (Cp₂Co) have been prepared by manually grinding the components together, or by co-crystallizing them from solution. In the solid solutions Cp₂Fe/Cp₂Ni and Cp₂Co/Cp₂Ni, the cyclopentadienyl (Cp) protons relax via dipolar electron–proton interactions, which represent the dominant relaxation mechanism. The ¹H T₁ relaxation times of the molecules Cp₂Ni and Cp₂Co, dissolved in CDCl₃, and in the solid solutions, show that the relaxation takes place intramolecularly. The relaxation of the protons is propagated exclusively via the unpaired electrons of the metal centers to which their Cp rings are coordinated, due to the large intermolecular distances that are greater than 3.91 Å. In contrast, the intramolecular distances between the electrons of the metal atoms and the protons of their coordinated Cp rings are merely 2.70 Å. Using these intramolecular distances and the ¹H T₁ relaxation times, the electron relaxation times T_1e have been determined as 17 × 10⁻¹³ s in CDCl₃ solutions and 45 × 10⁻¹³ s in the solid state for Cp₂Ni. The corresponding T_1e times for Cp₂Co are calculated as ca. 5 × 10⁻¹³ s and 20 × 10⁻¹³ s. Grinding Cp₂Fe and Cp₂Ni together leads to two different ¹H T₁ relaxation times for the protons of Cp₂Fe. The longer T₁ relaxation time indicates domains that consist mostly of Cp₂Fe molecules. The short T₁ times show a close contact of Cp₂Fe and Cp₂Ni molecules. An analysis of the short ¹H T₁ times reveals the presence of at least two to three short distances of 3.91 Å between Cp₂Fe and Cp₂Ni molecules. These results support the hypothesis that dry grinding of the metallocenes Cp₂Fe and Cp₂Ni in ratios that were changed in 10% increments from 90%/10% to 30%/70% leads to domains that mostly consist of Cp₂Fe molecules, and additionally to domains that contain a mixture of the components on the molecular level. Full article

The metallocenes ferrocene (Cp₂Fe, 1), nickelocene (Cp₂Ni, 2), and cobaltocene (Cp₂Co, 3) crystallize in the same space group (P2₁/a) and they have the same shape and similar size. Therefore, they form solid solutions with random distribution of the different molecules when crystallized from solution. Alternatively, the solid metallocenes can be ground together manually, and the solid solutions form at any molar ratio within minutes. The metallocenes 2 and 3 are paramagnetic. Solid solutions of 1/3 and 2/3 have been studied by paramagnetic solution and solid-state NMR spectroscopy. The effect of the paramagnetic species on the other components in the solid solutions has been investigated. The impact on the chemical shifts is limited. However, the halfwidths and the signal shapes, as defined by the rotational sideband intensities, change with increasing amounts of paramagnetic components. The ¹H T₁ relaxation times are shortened for diamagnetic protons in the presence of paramagnetic metallocenes in the solid solutions. It has been demonstrated that all metallocenes mix at the molecular level within the polycrystalline samples. The EPR spectra of the solid solutions are dominated by the most intensive signal of any paramagnetic metallocene in the solid samples. Full article

In this work, the single-walled carbon nanotubes (SWCNTs) were filled with cobaltocene. The growth properties of individual chirality nanotubes were studied with Raman spectroscopy. It was shown that the larger nanotubes grow slower. The growth of inner nanotubes becomes faster with increasing annealing temperature. These results are of high importance as they stimulate research on carbon nanotubes, and bring ideas from laboratories into factories. Full article

Metallocenes are toxic chemicals that are used for the growth of carbon nanotubes (CNTs). The study of the toxicity of metallocenes on plants is very important. It governs the issues of genetics. Toxicity studies should consider (I) the growth kinetics of carbon nanotubes, (II) the chemical reaction of metallocenes inside carbon nanotubes, and (III) investigations into the electronic properties of filled carbon nanotubes. Toxicity studies are influenced by the investigation of the growth processes of metallocene-filled carbon nanotubes, the evaporation of metals, and the formation of multiple-walled carbon nanotubes. Investigations into the modification of the Fermi level of filled carbon nanotubes also play a role in toxicity studies. Metallocenes are filled into carbon nanotubes through the various methods discussed here, including solution methods and gas-phase methods, each differing in methodology and requiring optimization. Many authors have published different methods of filling carbon nanotubes with metallocenes. They lead to high-filling ratios and allow future modifications of the electronic properties of carbon nanotubes. The kinetics of carbon nanotube growth are investigated with different methods. Environmental transmission electron microscopy is applied to studies with a time resolution of several nanoseconds. Here, we use Raman spectroscopy to study the growth process, which is revealed to have activation energies, growth rates, and temperature dependence. The dependence of growth temperature on the tube diameter and metallocene-type is revealed. It is shown that the growth temperature increases with the larger diameter of inner CNTs. The growth temperature increases for ferrocene compared to nickelocene and cobaltocene. With X-ray photoelectron spectroscopy, we show that the heating of metallocene-filled SWCNTs at 360–600 °C causes the n-doping of SWCNTs. Heating at higher temperatures results in p-doping. The obtained data are needed to tailor the toxicity issues of metallocene-filled carbon nanotubes on plants. Full article

In this work, I filled single-walled carbon nanotubes (SWCNTs) with nickelocene and cobaltocene molecules. I investigated the inner growth of carbon nanotubes on metallic and metal carbide catalysts. Raman spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy proved the metallic/metal carbide state of the catalysts, as well as the formation of inner SWCNTs. This is needed for applications of SWCNTs in buildings. Full article

In the present paper, the photophysical properties of metallocene-4-amino-1,8-naphthalimide-piperazine molecules (1-M²⁺), as well as their oxidized and protonated derivatives (1−M³⁺, 1-M²⁺-H⁺, and 1-M³⁺-H⁺), where M = Fe, Co, and Ni, were studied via DFT and TD-DFT, employing three functionals, i.e., PBE0, TPSSh, and wB97XD. The effect of the substitution of the transition metal M on their oxidation state, and/or the protonation of the molecules, was investigated. The present calculated systems have not been investigated before and, except for the data regarding their photophysical properties, the present study provides important information regarding the effect of geometry and of DFT methodology on absorption spectra. It was found that small differences in geometry, specifically in the geometry of N atoms, reflect significant differences in absorption spectra. The common differences in spectra due to the use of different functionals can be significantly increased when the functionals predict minima even with small geometry differences. For most of the calculated molecules, the main absorption peaks in visible and near-UV areas correspond mainly to charge transfer excitations. The Fe complexes present larger oxidation energies at 5.4 eV, whereas Co and Ni complexes have smaller ones, at about 3.5 eV. There are many intense UV absorption peaks with excitation energies similar to their oxidation energies, showing that the emission from these excited states can be antagonistic to their oxidation. Regarding the use of functionals, the inclusion of dispersion corrections does not affect the geometry, and consequently the absorption spectra, of the present calculated molecular systems. For certain applications, where there is a need for a redox molecular system including metallocene, the oxidation energies could be lowered significantly, to about 40%, with the replacement of the iron with cobalt or nickel. Finally, the present molecular system, using cobalt as the transition metal, has the potential to be used as a sensor. Full article

The paper is dedicated to the discussion of kinetics of growth, and electronic properties of filled carbon nanotubes investigated by spectroscopy for applications. The paper starts with discussion of growth of carbon nanotubes inside metallocene-filled carbon nanotubes. Nickelocene, cobaltocene are considered for growth of carbon nanotubes. Then, the investigations of filled carbon nanotubes by four spectroscopic techniques are discussed. Among them are Raman spectroscopy, near edge X-ray absorption fine-structure spectroscopy, photoemission spectroscopy, optical absorption spectroscopy. It is discussed that metal halogenides, metal chalcogenides, metals lead to changes in electronic structure of nanotubes with n- or p-doping. The filling of carbon nanotubes with different organic and inorganic substances results in many promising applications. This review adds significant contribution to understanding of the kinetics and electronic properties of filled SWCNTs with considering new results of recent investigations. Challenges in various fields are analyzed and summarized, which shows the author’s viewpoint of progress in the spectroscopy of filled SWCNTs. This is a valuable step toward applications of filled SWCNTs and transfer of existing ideas from lab to industrial scale. Full article

Conversion from superoxide (O₂⁻) to hydroperoxide (OOH⁻) on the metal center of oxygenases and oxidases is recognized to be a key step to generating an active species for substrate oxidation. In this study, reactivity of cobalt(III)-superoxido complexes supported by facially-capping tridentate tris(3,5-dimethyl-4-X-pyrazolyl)hydroborate ([HB(pz^Me2,X)₃]⁻; Tp^Me2,X) and bidentate bis(1-methyl-imidazolyl)methylborate ([B(Im^N-Me)₂Me(Y)]⁻; L^Y) ligands toward H-atom donating reagent (2-hydroxy-2-azaadamantane; AZADOL) has been explored. The oxygenation of the cobalt(II) precursors give the corresponding cobalt(III)-superoxido complexes, and the following reaction with AZADOL yield the hydroperoxido species as has been characterized by spectroscopy (UV-vis, resonance Raman, EPR). The reaction of the cobalt(III)-superoxido species and a reducing reagent ([Co^II(C₅H₅)₂]; cobaltocene) with proton (trifluoroacetic acid; TFA) also yields the corresponding cobalt(III)-hydroperoxido species. Kinetic analyses of the formation rates of the cobalt(III)-hydroperoxido complexes reveal that second-order rate constants depend on the structural and electronic properties of the cobalt-supporting chelating ligands. An electron-withdrawing ligand opposite to the superoxide accelerates the hydrogen atom transfer (HAT) reaction from AZADOL due to an increase in the electrophilicity of the superoxide ligand. Shielding the cobalt center by the alkyl group on the boron center of bis(imidazolyl)borate ligands hinders the approaching of AZADOL to the superoxide, although the steric effect is insignificant. Full article

This work studies synthesis of carbon nanotube (CNT) sheet using the high temperature (1400 °C) floating catalyst chemical vapor deposition (FC-CVD) method. Three metallocenes—ferrocene, nickelocene, cobaltocene—and their combinations are used as precursors for metal catalysts in the synthesis process. For the carbon source, an alcohol fuel, a combination of methanol and n-hexane (9:1), is used. First, the metallocenes were dissolved in the alcohol fuel. Then, the fuel mixture was injected into a tube furnace using an ultrasonic atomizer with Ar/H₂ carrier gas in a ratio of about 12/1. The synthesis of CNTs from a combination of two or three metallocenes reduces the percentage of metal catalyst impurity in the CNT sheet. However, there is an increase in structural defects in the CNTs when using mixtures of two or three metallocenes as catalysts. Furthermore, the specific electrical conductivity of the CNT sheet was highest when using a mixture of ferrocene and cobaltocene as the catalyst. Overall, the multi-catalyst method described enables tailoring certain properties of the CNT sheet. However, the standard ferrocene catalyst seems most appropriate for large-scale manufacturing at the lowest cost. Full article

Aerosol-assisted catalytic chemical vapor deposition (AACCVD) is a powerful one-step process to produce vertically aligned carbon nanotubes (VACNTs), characterized by the continuous supply of the catalyst precursor (metallocene). The behavior of catalyst species all along the synthesis is essential for the continuous growth of VACNTs. It is there investigated through detailed observations and elemental analyses at scales of VACNT carpets and of individual CNTs. Our approach is based on two complementary experiments: quenching of the sample cooling, and sequential injection of two distinct metallocenes. Metal-based nanoparticles nucleated in the gas-phase during the whole synthesis duration are shown to diffuse in between the growing VACNTs from the top of the CNT carpet towards the substrate. They are much smaller than the catalyst particles formed on the substrate in the initial steps of the process and evidences are given that they continuously feed these catalyst particles at the VACNT roots. Particularly, the electron energy-loss spectroscopy (EELS) analyses of metal-based segments found into a single CNT show that the second injected metal is very gradually incorporated in the particle initially formed from the metal firstly injected. The feeding of the catalyst particles by the nanoparticles continuously nucleated in the gas-phase is therefore an essential feature of the base-growth of CNTs by AACCVD. Full article

We have investigated the effects of temperature, diameter and metal catalyst type on the growth of inner nanotubes inside metallocene-filled single-walled carbon nanotubes (SWCNTs). The effects on the yield of different chiralities of inner nanotubes were scrutinized by multifrequency Raman spectroscopy. The investigated diameters range from ~0.7 to 1.3 nm and comprise 36 distinct chiralities. For all three investigated metals (Ni, Co, Fe), there is a linear correlation of growth temperature with nanotube diameter. The common slope for these metals is found to be 40.5 °C/Å. The temperature difference between the largest and the smallest diameter tubes amounts to ~230 °C for all three precursors. The growth temperatures are offset by 34 °C from Ni to Co and another 28 °C from Co to Fe. The quantified correlations of temperature, diameter and metal catalyst type provide the basis for engineering the diameter-specific growth of nanotubes. Full article

High-temperature tolerant enzymes offer multiple advantages over enzymes from mesophilic organisms for the industrial production of sustainable chemicals due to high specific activities and stabilities towards fluctuations in pH, heat, and organic solvents. The production of molecular hydrogen (H₂) is of particular interest because of the multiple uses of hydrogen in energy and chemicals applications, and the ability of hydrogenase enzymes to reduce protons to H₂ at a cathode. We examined the activity of Hydrogen-Dependent CO₂ Reductase (HDCR) from the thermophilic bacterium Thermoanaerobacter kivui when immobilized in a redox polymer, cobaltocene-functionalized polyallylamine (Cc-PAA), on a cathode for enzyme-mediated H₂ formation from electricity. The presence of Cc-PAA increased reductive current density 340-fold when used on an electrode with HDCR at 40 °C, reaching unprecedented current densities of up to 3 mA·cm⁻² with minimal overpotential and high faradaic efficiency. In contrast to other hydrogenases, T. kivui HDCR showed substantial reversibility of CO-dependent inactivation, revealing an opportunity for usage in gas mixtures containing CO, such as syngas. This study highlights the important potential of combining redox polymers with novel enzymes from thermophiles for enhanced electrosynthesis. Full article

Recently we have shown the importance of hypergolic reactions in carbon materials synthesis. However, hypergolic reactions could be certainly expanded beyond carbon synthesis, offering a general preparative pathway towards a larger variety of materials. Cyclopentadienyls are one of the most common ligands in organometallic chemistry that react hypergolicly on contact with strong oxidizers. By also considering the plethora of cyclopentadienyl compounds existing today, herein we demonstrate the potential of such compounds in hypergolic materials synthesis in general (carbon or inorganic). In a first example, we show that cyclopentadienyllithium reacts hypergolicly with fuming nitric acid to produce carbon. In a second one, we show that ferrocene and cobaltocene also react hypergolicly with the concentrated acid to afford magnetic inorganic materials, such as γ-Fe₂O₃ and metallic Co, respectively. The present results further emphasize the importance and universal character of hypergolic reactions in materials science synthesis, as an interesting new alternative to other existing and well-established preparative methods. Full article