Linkage Isomerism of Nitriles in Rhodium Half-Sandwich Metallacycles

A systematic analysis of the linkage isomerism of nitriles in half-sandwich five-membered rhodacycles is presented for 15 nitriles of the general formula RCN. The hitherto elusive transition states connecting the two isomers have been located using advanced DFT methods

Cyclotrimerization Reactions Catalyzed by Rhodium(I) Half-Sandwich Complexes: A Mechanistic Density Functional Study

We propose and examine a comprehensive mechanism of the [(η5-C5H5)Rh]-catalyzed [2+2+2] cycloadditions of acetylene to benzene and of acetylene and acetonitrile to 2-methylpyridine, based on an extensive and detailed exploration of the potential energy surfaces using density functional theory. Both processes involve the formation of a coordinatively unsaturated 16-electron metallacycle, occurring after the replacement of the ancillary ligands L of the catalyst precursor of general formula [(η5-C5H5)RhL2] (typically L = C2H4, CO, PH3 or L2 = 1,5-cyclooctadiene) by two acetylene molecules. The facile coordination of a third acetylene molecule, and its subsequent addition to the π electron system of the rhodacycle, leads to the formation of an intermediate, which is characterized by a six-membered arene ring coordinated to the metal in η4 fashion. The release of benzene occurs by stepwise addition of two acetylene molecules, which regenerates the catalyst. In the presence of acetonitrile, a nitrile molecule coordinates to the rhodacycle, and different stages are outlined for the process, leading to the eventual release of 2-methylpyridine. The steric and electronic effects of the π ligand coordinated to the metal are also included in our exploration by addressing the whole mechanism of the [(η5-C9H7)Rh]-catalyzed alkyne self-trimerization to benzene. The kinetic parameters, i.e., the energies in vacuum and in different solvents, and the geometries of the intermediates and of the transition states are analyzed in detail

Charge Transfer in Model Bioinspired Carotene–Porphyrin Dyads

We present a computational study based on accurate DFT and TD-DFT methods on model bioinspired donor–acceptor dyads, formed by a carotenoid covalently linked to a tetraphenylporphyrin (TPP) at the ortho position of one of the TPP phenyl rings. Dyadic systems can be used in the construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion, due to the unique advantages they offer in terms of synthetic feasibility. This study aims to describe the influence of chemical modifications on the absorption spectra, in particular on the lowest energy charge transfer bands. Effects of different metals of biological interest, i.e., Mg, Fe, Ni, and Zn, and of H₂O and histidine molecules coordinated to the metals in different axial positions are rationalized

Reactivity of Palladium(II)‑η³‑Allyl Complexes with Chalcogenolates: A Density Functional Study of Their Antitumor Implications

Palladium complexes are emerging as potential antitumor compounds. Recent experimental evidence suggests that the mechanism of action of some organopalladium derivatives might involve the target of proteins such as Thioredoxin Reductase (TrxR). In this work, we investigate different possible chemical mechanisms of interaction between selected palladium(II)-η3-allyl complexes and thiolates or selenolates, which are key functional groups present in the catalytic pocket of TrxR. Our investigation, focusing on both anionic and cationic complexes, suggests that in all cases, the interaction between the organopalladium species and chalcogenolates occurs via a ligand exchange reaction, which leads to the formation of a new Pd–S or Pd–Se bond. Allyl substitution, which takes place with the formation of a new C–S or C–Se bond, always appears to be the least favored reaction, from both the kinetic and thermodynamic points of view

Anti-Kasha’s Rule Fluorescence Emission in (2-Ferrocenyl)indene Generated by a Twisted Intramolecular Charge-Transfer (TICT) Process

A twisted intramolecular charge-transfer (TICT) process has been identified in (2-ferrocenyl)indene. This photochemical process explains the anti-Kasha’s rule fluorescence emission observed for this system. Experimental and model investigations on (2-ferrocenyl)tetramethylindene and (2-ferrocenyl)-hexamethylindene were also performed, in order to evaluate the effect of a steric hindrance on the TICT mechanism. The energy of the lowest main excited states was computed with a TD-DFT approach, as a function of the rotation of the dihedral angle between the indene and the cyclopentadienyl planes. To the best of our knowledge, this is the first example of TICT generated by metal-to-ligand charge transfer (MLCT) in a ferrocene-containing complex and, more generally, the first case of complexes in which a metal center is directly involved

Reactivity of Palladium(II)‑η³‑Allyl Complexes with Chalcogenolates: A Density Functional Study of Their Antitumor Implications

Palladium complexes are emerging as potential antitumor compounds. Recent experimental evidence suggests that the mechanism of action of some organopalladium derivatives might involve the target of proteins such as Thioredoxin Reductase (TrxR). In this work, we investigate different possible chemical mechanisms of interaction between selected palladium(II)-η3-allyl complexes and thiolates or selenolates, which are key functional groups present in the catalytic pocket of TrxR. Our investigation, focusing on both anionic and cationic complexes, suggests that in all cases, the interaction between the organopalladium species and chalcogenolates occurs via a ligand exchange reaction, which leads to the formation of a new Pd–S or Pd–Se bond. Allyl substitution, which takes place with the formation of a new C–S or C–Se bond, always appears to be the least favored reaction, from both the kinetic and thermodynamic points of view

Modelling of Ca²⁺-promoted structural effects in wild type and post-translationally modified Connexin26

Connexins (Cx) are a class of membrane proteins important for auditory function, intercellular signalling and skin biology. Although the presence of concentration of calcium ions is known to work as a trigger for the Cx functionality, the structural changes induced by calcium binding still need to be well elucidated. In this computational study, we have explored the structural effects promoted by Ca2+ on both the wild type (Cx26WT) and on two post-translationally modified Connexin 26 (Cx26): Cx26E42-47γ, which contains two glutamates (E42 and E47) that are γ-carboxylated and Cx26R75m, where a key arginine (R75) is N-monomethylated. These modified amino acids, whose forcefield parameters have been developed in this work, alter Cx26 structure around the Ca2+coordination site. Structural changes were assessed from the analysis of molecular dynamics (MD) simulations. We observed a strict relation between the chemical properties of the post-translational modifications and significantly different responses of Cx26 to Ca2+-binding, while charge-adding modifications have destabilising effects upon calcium coordination, the uncharged ones share the same structural properties of the wild-type counterpart. Overall, these findings suggest the critical role of the electrostatic network flanking the Ca2+ coordination site in maintaining the native tertiary and quaternary structures.</p

Role of Group 12 Metals in the Reduction of H₂O₂ by Santi’s Reagent: A Computational Mechanistic Investigation

PhSeZnCl, which is also known as Santi’s reagent, can catalyze the reduction of hydrogen peroxide by thiols with a GPx-like mechanism. In this work, the first step of this catalytic cycle, i.e., the reduction of H2O2 by PhSeZnCl, is investigated in silico using state-of-the-art density functional theory calculations. Then, the role of the metal is evaluated by replacing Zn with its group 12 siblings (Cd and Hg). The thermodynamic and kinetic factors favoring Zn are elucidated. Furthermore, the role of the halogen is considered by replacing Cl with Br in all three metal compounds, and this turns out to be negligible. Finally, the overall GPx-like mechanism of PhSeZnCl and PhSeZnBr is discussed by evaluating the energetics of the mechanistic path leading to the disulfide product

Charge Transfer Properties of Benzo[<i>b</i>]thiophene Ferrocenyl Complexes

The synthesis of 2-ferrocenylbenzo­[<i>b</i>]­thiophene, 3-ferrocenylbenzo­[<i>b</i>]­thiophene, 1,1′-bis­(2-indene)­ferrocene, and the two isomers of 1,1′-bis­(2-benzo­[<i>b</i>]­thiophene)­ferrocene was efficiently achieved by using the palladium-catalyzed Negishi C,C cross-coupling reaction of the appropriate bromobenzo­[<i>b</i>]­thiophene derivative with ferrocenylzinc chloride. The accessibility of differently substituted benzo­[<i>b</i>]­thiophenes and a comparison with indene analogues allowed an in-depth investigation on how the geometric modifications and the presence of sulfur affect their physical properties. The molecular structure of 3-ferrocenylbenzo­[<i>b</i>]­thiophene has been determined by X-ray diffraction. Electrochemistry and UV–vis–NIR spectroscopy, in particular the appearance upon oxidation of a charge transfer absorption in the NIR region, are rationalized through quantum chemistry calculations and in the framework of the Hush theory

Charge Transfer Properties of Benzo[<i>b</i>]thiophene Ferrocenyl Complexes

The synthesis of 2-ferrocenylbenzo­[<i>b</i>]­thiophene, 3-ferrocenylbenzo­[<i>b</i>]­thiophene, 1,1′-bis­(2-indene)­ferrocene, and the two isomers of 1,1′-bis­(2-benzo­[<i>b</i>]­thiophene)­ferrocene was efficiently achieved by using the palladium-catalyzed Negishi C,C cross-coupling reaction of the appropriate bromobenzo­[<i>b</i>]­thiophene derivative with ferrocenylzinc chloride. The accessibility of differently substituted benzo­[<i>b</i>]­thiophenes and a comparison with indene analogues allowed an in-depth investigation on how the geometric modifications and the presence of sulfur affect their physical properties. The molecular structure of 3-ferrocenylbenzo­[<i>b</i>]­thiophene has been determined by X-ray diffraction. Electrochemistry and UV–vis–NIR spectroscopy, in particular the appearance upon oxidation of a charge transfer absorption in the NIR region, are rationalized through quantum chemistry calculations and in the framework of the Hush theory