Cyclopentadienide Research Articles

A computational study on azaazulenes

Abstract All possible aza analogs of azulene, containing from one to three nitrogen atoms in the five-membered ring or from one to five nitrogen atoms in the seven-membered ring, have been theoretically considered to obtain information about their stabilities and aromaticities. Total electronic energy and nucleus independent chemical shift (NICS) data have been used to evaluate stability and aromaticity, respectively. The stabilities of the structures are strongly affected by the positions of the nitrogen atoms. Calculations of azaazulenes show that stability is decreased with close proximity of the nitrogen atoms. When nitrogen in the five-membered ring is adjacent to a ring junction, aromaticity of the cyclopentadienyl anion is reduced and that of the tropylium cation is increased. The number of nitrogen atoms affects the aromaticity of the system.

Back Cover: In Silico Design of Heteroaromatic Half-Sandwich RhICatalysts for Acetylene [2+2+2] Cyclotrimerization: Evidence of a Reverse Indenyl Effect (Chem. Eur. J. 40/2013)

A reverse indenyl effect occurs in the acetylene [2+2+2] cyclotrimerization to benzene by RhI half-metallocene catalysts with heteroaromatic ligands. In their Full Paper on page 13337 ff., L. Orian, F. M. Bickelhaupt et al. have designed, in silico, heteroaromatic ligands that are isoelectronic to the classic hydrocarbon cyclopentadienyl and indenyl anions. Mechanistic explorations and activation strain analyses reveal and explain the reverse indenyl effect, that is, a decrease in reactivity associated with an increased metal slippage. On the cover, the initial bis- acetylene complexes of all ligands are shown: the different extent of Rh slippage controls the orientation of the alkynes, which resembles the pose of the wings of Siproeta stelenes, the Central American butterfly shown.

In Silico Design of Heteroaromatic Half‐Sandwich RhI Catalysts for Acetylene [2+2+2] Cyclotrimerization: Evidence of a Reverse Indenyl Effect

A mechanistic density functional theory study of acetylene [2+2+2] cyclotrimerization to benzene catalyzed by Rh(I) half metallocenes is presented. The catalyst fragment contains a heteroaromatic ligand, that is, the 1,2-azaborolyl (Ab) or the 3a,7a-azaborindenyl (Abi) anions, which are isostructural and isoelectronic to the hydrocarbon cyclopentadienyl (Cp) and indenyl (Ind) anions, respectively, but differ from the last ones on having two adjacent carbon atoms replaced with a boron and a nitrogen atom. The better performance of either the classic hydrocarbon or the heteroaromatic catalysts is found to depend on the different mechanistic paths that can be envisioned for the process. The present analyses uncover and explain general structure-reactivity relationships that may serve as rational design principles. In particular, we provide evidence of a reverse indenyl effect.

A comparative study of the aromaticity of pyrrole, furan, thiophene, and their aza-derivatives

The relative aromaticity of pyrrole, furan, thiophene, and their aza-derivatives has been examined using TRE (topological resonance energy), MRE (magnetic resonance energy), ring current (RC), and ring current diamagnetic susceptibility (χG) methods. The results obtained were compared with results obtained by others who used the energetic method ASE (aromatic stabilization energy), the geometric method HOMA (harmonic oscillator model of aromaticity), and the magnetic method NICS(1) (nucleus-independent chemical shift). The impact of nitrogen atoms on the aromaticity of the aza-derivatives of pyrrole, furan, and thiophene is discussed. An excellent correlation was found between the energetic (TRE, MRE) and magnetic (RC and χG) criteria of aromaticity for all compounds. It was expected that inclusion of a heteroatom would decrease the aromaticity relative to the cyclopentadienyl anion. Our results show that the type of the first heteroatom, which donates two electrons to the system, as well as the number of nitrogen atoms and their positions in the molecule have a strong effect on aromaticity. In general, aromaticity is enhanced when the nitrogen atom is adjacent to the first heteroatom. The magnitude of aromaticity is related closely with the uniformity of distribution of π-electrons in the molecule.

A six-carbon 10π-electron aromatic system supported by group 3 metals

Aromaticity is a fundamental concept with implications spanning all the chemical sciences. Hückel's (4n+2)π-electron rule is the standard criterion to determine aromaticity and it applies well to neutral arenes as well as to charged species such as the cyclopentadienyl anion, the cyclooctatetraene dianion and the cycloheptatrienyl cation (tropylium). In the series of all-carbon aromatic compounds, no example of a benzene tetraanion, which is theoretically a 6C, 10π-electron aromatic system, has been reported although heteroatom analogues of such a system, known as 'electron-rich aromatics', have been studied in detail for a long time. Here we present the isolation of the first tetraanionic-substituted benzene as a ligand coordinated to group 3 metals. The nature of the substituted benzene tetraanion and the aromaticity of the 6C, 10π-electron system were established by X-ray crystallographic studies, multi-nuclei nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy and density functional theory calculations.

Development of the isotropic site-site potential for exchange repulsion energy and combination with the isotropic site-site potential for electrostatic part

We extended the previously reported isotropic site-site potential for the exchange part [D. Yokogawa, Chem. Phys. Lett. 515, 179 (2011)] and combined it with isotropic site-site potential for the electrostatic part. To treat complex systems, such as excited-state molecules and metal complexes, multi-configuration self-consistent field method was employed. The method was applied to the calculation of intermolecular interactions between H(2)O and aromatic compounds, namely, p-nitoroaniline, imidazolium cation, and cyclopentadienyl anion. The potential thus obtained was combined with the extended reference interaction site model and the three-dimensional reference interaction site model for the calculation of the solvation structure. The present method gave reasonable intermolecular interactions and solvation structures at ground and excited states.

Comparative Study onortho-C–H vsortho-C–X (X = C, Cl, S) Bond Activation inortho-Caromatic–N Bond Fusion in Substituted Anilines Using Ruthenium(II) Mediators: Isolation and Characterization of Unusual Ru2Complexes

The chemical reactions of a selection of ortho-mono- and disubstituted anilines with two ruthenium polyene mediator complexes, CpRuIICl(PPh3)2 (Cp– = cyclopentadienyl anion) and (Bnz)2RuII2Cl4 (Bnz = benzene), have been undertaken with a primary aim to make a comparison between ortho-C–H and ortho-C–X (X = Cl, C, S) bond activation processes in ortho-C–N bond fusion reactions. The reaction of ortho-monosubstituted anilines, viz., 2-chloroaniline (HL1a), 2-methylaniline (HL1c), and 2-methylthioaniline (HL1b), with CpRuIICl(PPh3)2 yielded mononuclear complexes [CpRuIIL2a–cCl] (1, 3, and 5), containing in situ generated ligands N-(aryl)-ortho-quinonediimine, L2a–c, along with anilido-bridged RuIII2 complexes (2, [CpClRuIII{μ-η2-(L1a)−}]2; 4, [CpClRuIII{μ-η2-(L1c)–}]2; and [6]Cl2, [CpRuIII{μ-η2:η1-(L1b)–}]2), respectively. The new ligands, L2a–c are formed via ortho-C–H bond activation reactions, whereas ortho-C–X bonds remained unaffected. However, the ortho-C–Cl bond activation reaction is also noted in the...

Fullerene based endohedral metallocenes

In this study, the density functional theory (DFT)-based Becke’s three parameter hybrid exchange functional and Lee–Yang–Parr correlation functional (B3LYP) and dispersion corrected B3LYP (B3LYP-D) calculations have been performed to predict the structure and stability of fullerene based endohedral sandwich complexes. The role of confinement on the structure of half-sandwich complex (i.e. (η5-Cp)Fe+ (Cp=cyclopentadienyl anion)) has also been probed using energy decomposition analysis. The standard redox potential has been calculated to understand the electrochemical property of newly designed endohedral sandwich complexes. Results have been compared with exohedral sandwich complexes and ferrocene.

Spin-Coupled Theory for ‘N Electrons in M Orbitals’ Active Spaces

Spin-coupled (SC) theory, an ab initio valence bond (VB) approach which uses a compact and an easy-to-interpret single-orbital product wave function comparable in quality to a ‘N in N’ complete-active-space self-consistent field [CASSCF(N,N)] construction, is extended to ‘N in M’ (N ≠ M) active spaces. The SC(N,M) wave function retains the essential features of the original SC model: It involves just the products of nonorthogonal orbitals covering all distributions of N electrons between M orbitals in which as few orbitals as possible, |N – M|, are doubly occupied (for N > M) or missing (for N < M) and all other orbitals are singly occupied; each of these products is combined with a flexible spin function which allows any mode of coupling of the spins of the orbitals within the product. The SC(N,M) wave function remains much more compact than a CASSCF(N,M) construction; for example, the SC(6,7) wave function includes 35 configuration state functions (CSFs) as opposed to the 490 CSFs in the CASSCF case. The essential features of the SC(N,M) method are illustrated through a SC(6,5) calculation on the cyclopentadienyl anion, C5H5(–), and a SC(6,7) calculation on the tropylium cation, C7H7(+). The SC(6,5) and SC(6,7) wave functions for C5H5(–) and C7H7(+) are shown to provide remarkably clear modern VB models for the electronic structures of these aromatic cyclic ions which closely resemble the well-known SC model of benzene and yet recover almost all of the correlation energy included in the corresponding CASSCF(6,5) and CASSCF(6,7) wave functions: over 97% in the case of C5H5(–) and over 95% in the case of C7H7(+).

Borylation and Silylation of C–H Bonds: A Platform for Diverse C–H Bond Functionalizations

Methods that functionalize C-H bonds can lead to new approaches for the synthesis of organic molecules, but to achieve this goal, researchers must develop site-selective reactions that override the inherent reactivity of the substrates. Moreover, reactions are needed that occur with high turnover numbers and with high tolerance for functional groups if the C-H bond functionalization is to be applied to the synthesis of medicines or materials. This Account describes the discovery and development of the C-H bond functionalization of aliphatic and aromatic C-H bonds with borane and silane reagents. The fundamental principles that govern the reactivity of intermediates containing metal-boron bonds are emphasized and how an understanding of the effects of the ligands on this reactivity led us to broaden the scope of main group reagents that react under mild conditions to generate synthetically useful organosilanes is described. Complexes containing a covalent bond between a transition metal and a three-coordinate boron atom (boryl complexes) are unusually reactive toward the cleavage of typically unreactive C-H bonds. Moreover, this C-H bond cleavage leads to the formation of free, functionalized product by rapid coupling of the hydrocarbyl and boryl ligands. The initial observation of the borylation of arenes and alkanes in stoichiometric processes led to catalytic systems for the borylation of arenes and alkanes with diboron compounds (diborane(4) reagents) and boranes. In particular, complexes based on the Cp*Rh (in which Cp is the cyclopentadienyl anion) fragment catalyze the borylation of alkanes, arenes, amines, ethers, ketals, and haloalkanes. Although less reactive toward alkyl C-H bonds than the Cp*Rh systems, catalysts generated from the combination of bipyridines and iridium(I)-olefin complexes have proven to be the most reactive catalysts for the borylation of arenes. The reactions catalyzed by these complexes form arylboronates from arenes with site-selectivity for C-H bond cleavage that depends on the steric accessibility of the C-H bonds. These complexes also catalyze the borylation of heteroarenes, and the selectivity for these substrates is more dependent on electronic effects than the borylation of arenes. The products from the borylation of arenes and heteroarenes are suitable for a wide range of subsequent conversions to phenols, arylamines, aryl ethers, aryl nitriles, aryl halides, arylboronic acids, and aryl trifluoroborates. Studies of the electronic properties of the ancillary ligand on the rate of the reaction show that the flat structure and the strong electron-donating property of the bipyridine ligands, along with the strong electron-donating property of the boryl group and the presence of a p-orbital on the metal-bound atom, lead to the increased reactivity of the iridium catalysts. Based on this hypothesis, we studied catalysts containing substituted phenanthroline ligands for a series of additional transformations, including the silylation of C-H bonds. A sequence involving the silylation of benzylic alcohols, followed by the dehydrogenative silylation of aromatic C-H bonds, leads to an overall directed silylation of the C-H bond ortho to hydroxyl functionality.

Electrochemistry on Multi-walled Carbon Nanotubes in Organic Solutions

The oxidation of ferrocene (FeCp2) to ferrocenium cation (\(\mbox{FeCp}_{2}^{+}\)) (where Cp: cyclopentadienyl anion, \(\mbox{C}_{5}\mbox{H}_{5}^{-}\)) was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on either a glassy carbon (GC) electrode or a film consisted of multi-walled carbon nanotubes (MWCNT) in acetone (ACE), methanol (MeOH) and ACE/MeOH binary mixtures. The half-wave potentials (E1/2), the diffusion coefficients (D) and the heterogeneous electron transfer rate constants (ks) of the redox couple \(\mbox{FeCp}_{2}^{+/0}\) were determined. On both electrodes tested, the electron transfer process was found to be quasi-reversible and diffusion controlled in all investigated solvent mixtures. The E1/2 values were shifted toward less positive potentials with the enrichment of mixtures in MeOH, due probably to the larger Lewis basicity of the MeOH molecules. Furthermore, values of both D and ks diminish with the increase of MeOH content following the progressive increase of viscosity of the solvent medium. Among the electrodes probed, GC provides faster electrochemical process since it affords less charge-transfer resistance (Rct), and consequently, an insignificant barrier for interfacial electron transfer.

Application of Films Consisting of Carbon Nanoparticles for Electrochemical Detection of Redox Systems in Organic Solvent Media

In the present article, the electrochemical sensitivity and response of films consisting of carbon nanoparticles were examined to test their potential applications as electrode materials for the construction of electrochemical sensors. For this purpose, a film of vertically aligned multi-walled carbon nanotubes (MWCNTs) on silicon dioxide substrate was synthesized by means of catalytic chemical vapor deposition (CVD) at 900oC using ferrocene (FeCp2) as catalyst and acetonitrile (ACN) as a carbon source. Furthermore, bucky-paper (BP) film was produced upon ultrasonic treatment of aqueous solution of single-walled carbon nanotubes (SWCNTs) in the presence of sodium dodecyl sulfate (SDS) micelle. In order to examine the electrochemical activity of the fabricated films, electrochemical studies of the oxidation of ferrocene (FeCp2) to ferrocenium cation (FeCp2 +) (where Cp denotes cyclopentadienyl anion) in ACN containing n-tetrabutylammonium hexafluorophosphat (NBu4PF6) as supporting electrolyte were performed. For the electrochemical experiments the techniques of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed. The electrochemical parameters of the FeCp2 +/0 redox couple in ACN on the fabricated films were determined and compared with those obtained using conventional glassy carbon (GC) electrode. The results demonstrate that the FeCp2 +/0 redox couple exposes Nernstian response on either MWCNT or GC electrodes while its response on BP film can be recognized as near Nernstian. The findings verify that the synthesized MWCNT and BP films hold promising and important applications in electro-analytical chemistry.

C–H Activation of Cycloalkenes by Dimetallynes (M = Ge, Sn) under Ambient Conditions

Treatment of the dimetallynes Ar'EEAr' [E = Ge, Sn; Ar' = C(6)H(3)-2,6-(C(6)H(3)-2,6-iPr(2))(2)] with a cyclic olefin-cyclopentadiene (CpH), cyclopentene, 1,4-cyclohexadiene (CHD), or cyclohexene-showed that, with the exception of cyclohexene, they react readily, affording C-H activation at room temperature. Reaction of the digermyne and distannyne with CpH gave the cyclopentadienyl anion, which is bound in a π-fashion to a mononuclear group 14 element center, along with evolution of hydrogen gas. Unusually, the digermyne also reacted with cyclopentene to give the same dehydroaromatization product, formed from triple C-H activation/dehydrogenation. It also was found to react with CHD to give a mixture of (Ar'GeH)(2), benzene, and a new 7-germanorbornadiene species bound to a cyclohex-2-enyl fragment.

Allyl Radical Nature of a Phenylcyclopentadienyl Radical Annelated with Two Homoadamantene Frameworks

1-Phenylcyclopentadiene fused with two homoadamantene units at the 2,3- and 4,5-positions (4) was deprotonated with KH to lead to the quantitative formation of the corresponding cyclopentadienyl (Cp) anion 3(-). This anion was oxidized by AgCl to afford an orange crystalline solid consisting of Cp radical 3(•) and cyclopentadiene 4. The ESR spectrum in hexane exhibited approximately 15 lines, demonstrating that the two homoadamantene frameworks were equivalent and that the C1-C2(Ph)-C3 moiety of the five-membered ring formed a symmetrical allyl-like radical in agreement with the prediction by DFT calculation. The reaction of the Cp radical 3(•) with an oxygen molecule in the presence of Ag(+)SbF(6)(-) afforded the SbF(6)(-) salt of a phenylpyrylium ion annelated with two homoadamantene frameworks (8(+)SbF(6)(-)).

Electronic Structure of 2,2′-Bipyridine Organotransition-Metal Complexes. Establishing the Ligand Oxidation Level by Density Functional Theoretical Calculations

A density functional theoretical (DFT) study (B3LYP) has been carried out on 20 organometallic complexes containing η(5)- and/or η(3)-coordinated cyclopentadienyl anions (Cp(-)) and 2,2'-bipyridine (bpy) ligand(s) at varying oxidation levels, i.e., as the neutral ligand (bpy(0)), as the π-radical monoanion (bpy(•-))(-), or as the diamagnetic dianion (bpy(2-))(2-). The molecular and electronic structures of these species in their ground states and, in some cases, their first excited states have been calculated using broken-symmetry methodology. The results are compared with experimental structural and spectroscopic data (where available) in order to validate the DFT computational approach. The following electron-transfer series and complexes have been studied: [(Cp)(2)V(bpy)](0,+,2+) (1-3), [(Cp)(2)Ti(bpy)](-,0,+,2+) (4-7), [(Cp)(2)Ti(biquinoline)](0,+) (8 and 9), [(Cp*)(2)Ti(bpy)](0) (10) (Cp* = pentamethylcyclopentadienyl anion), [Cp*Co(bpy)](0,+) (11 and 12), [Cp*Co(bpy)Cl](+,0) (13 and 14), [Fe(toluene)(bpy)](0) (15), [Cp*Ru(bpy)](-) (16), [(Cp)(2)Zr(bpy)](0) (17), and [Mn(CO)(3)(bpy)](-) (18). In order to test the predictive power of our computations, we have also calculated the molecular and electronic structures of two complexes, A and B, namely, the diamagnetic dimer [Cp*Sc(bpy)(μ-Cl)](2) (A) and the paramagnetic (at 25 °C) mononuclear species [(η(5)-C(5)H(4)(CH(2))(2)N(CH(3))(2))Sc((m)bpy)(2)] (B). The crystallographically observed intramolecular π-π interaction of two N,N'-coordinated π-radical anions in A leading to an S = 0 ground state is reliably reproduced. Similarly, the small singlet-triplet gap of ~600 cm(-1) between two antiferromagnetically coupled (bpy(•-))(-) ligands in B, two ferromagnetically coupled radical anions in the triplet excited state of B, and the structures of A and B is reproduced. Therefore, we are confident that we can present computationally obtained, detailed electronic structures for complexes 1-18. We show that N,N'-coordinated neutral bpy(0) ligands behave as very weak π acceptors (if at all), whereas the (bpy(2-))(2-) dianions are strong π-donor ligands.

Multi-walled carbon nanotubes as electrode materials for electrochemical studies of organometallic compounds in organic solvent media

Films of vertically aligned multi-walled carbon nanotubes (MWCNT) were selectively synthesized on silicon dioxide substrate by catalytic chemical vapor deposition using either benzene or acetonitrile as carbon source and ferrocene (1% w/w) as catalyst. The MWCNT were extensively characterized by using scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and Raman spectroscopy. In order to examine the prospective application of the fabricated MWCNT films for the detection of electro-active compounds in organic solvent media, electrochemical studies of the oxidation of cobaltocene (CoCp2) to cobaltocenium cation (CoCp 2 + ) (Cp = cyclopentadienyl anion) in acetonitrile were performed on these films. For this purpose, cyclic voltammetry and electrochemical impedance spectroscopy were employed. The electrochemical parameters for the CoCp 2 +/0 couple in acetonitrile were derived and compared with those obtained using a conventional glassy carbon electrode. The results demonstrate that the synthesized MWCNT films are promising electrode materials for the electrochemical detection of electro-active species in organic solvents. The MWCNT film formed upon decay of benzene has higher capacitance, less Warburg impedance, and less charge transfer resistance, and consequently it provides faster electron transfer kinetics.

Addition of a Thallium Vertex to Empty and Centered Nine-Atom Deltahedral Zintl Ions of Germanium and Tin

Nickel atoms were inserted into nine-atom deltahedral Zintl ions of E(9)(4-) (E = Ge, Sn) via reactions with Ni(cod)(2) (cod = cyclooctadiene), and [Ni@Sn(9)](3-) was structurally characterized. Both the empty and the Ni-centered clusters react with TlCp (Cp = cyclopentadienyl anion) and add a thallium vertex to form the deltahedral ten-atom closo-species [E(9)Tl](3-) and [Ni@E(9)Tl](3-), respectively. The structures of [Ge(9)Tl](3-) and [Ni@Sn(9)Tl](3-) showed that, as expected, the geometry of the ten-atom clusters is that of a bicapped square antiprism where the Tl-atom occupies one of the two capping vertices. This illustrates that centering a nine-atom cluster with a nickel atom does not change its reactivity toward TlCp. All compounds were characterized by electrospray mass spectrometry.

The Reduction Chemistry of Ferrocenylborole

Time to B radical: One-electron reduction of 1-ferrocenyl-2,3,4,5-tetraphenylborole results in a radical anion with 5 π electrons in the borole ring. Both EPR spectroscopic investigations and spin density calculations confirm the formation of a borol radical (see picture). Further reduction stimulates an intramolecular [(C5H5)Fe] migration from the cyclopentadienyl anion to the borole dianion.

The Quadrupole Moment of Substituted Cyclopentadienyl Anions

Previous work in our group on the cation binding of substituted cyclopentadienyl anions (Cp) showed the curious result that Cp traceless electric quadrupole moments (Θ(zz)) are almost all positive. Probing this issue further here we show that substituted Cp Θ(zz) values are always significantly more positive than the analogous substituted benzenes. Given the nature of aromatic Θ(zz) values, this is the opposite of what would be predicted. Furthermore, we show that the quadrupole moments of Cp anions do not behave as one would expect based on Cp substitutions. Unlike the quadrupole moments of substituted benzenes, which generally become more negative with the addition of electron-donating groups and more positive with the addition of electron-withdrawing groups, Cp quadrupole moments become more positive when any substituent is added, regardless of the electron-donating/withdrawing nature of the substituent. To explain these results we propose a model where the anionic Cp π-electron density repels the substituent electron density toward the molecular periphery and AIM calculations support this view.

ChemInform Abstract: A Direct Route to Fluorinated Cyclopentadienes and Cyclopentadienyl Anions.

Fluoride-ion induced reactions of systems CH2XY with the novel diene [CF3CFC(CF3)]2 provides a route to derived cyclopentadienes and cyclopentadienyl anions via an unusual cyclisation step.