Elucidation of the Thermochemical Properties of Triphenyl- or Tributyl-Substituted Si-, Ge-, and Sn-Centered Radicals by Means of Electrochemical Approaches and Computations

Redox potentials of a number of triphenyl- or tributyl-substituted Si-, Ge-, or Sn-centered radicals, R3M•, have been measured in acetonitrile, tetrahydrofuran, or dimethyl sulfoxide by photomodulated voltammetry or through a study of the oxidation process of the corresponding anions in linear sweep voltammetry. For the results pertaining to the Ph3M• series (including literature data for M = C), the order of reduction potentials follows Sn > Ge > C > Si, while for the two oxidation potentials, it is C > Si. The effect of the R group on the redox properties of R3Sn• is pronounced in that the reduction potential is more negative by 490 mV in tetrahydrofuran (390 mV in dimethyl sulfoxide) when R is a butyl rather than a phenyl group. The experimental trends have been substantiated through quantum chemical calculations, and they can be explained qualitatively by considering a combination of effects, such as charge capacity being most pronounced for the heavier elements, resonance stabilization present for the planar Ph3C• and all R3M+, and finally a contribution from solvation. The solvation of R3M- is observed to be relatively strong because of a rather localized negative charge in the pyramidal geometry. However, there is no evidence in the calculations to support the existence of covalent interactions between solvent and anions. The solvation of R3M+ is relatively weak, which may be attributed to the planar geometry around the center atom, leading to more spread out charge than that for a pyramidal geometry. Although the calculated solvation energies based on the polarizable continuum model approach exhibit the expected trends, they are not able to reproduce the experimentally derived values on a detailed level for these types of ions. An evaluation of the general performance of the continuum model is provided on the basis of present and previous studies

Mechanistic Investigation of the Electrochemical Reduction of Cp₂TiX₂

The mechanism for the electrochemical reduction of titanocene dihalides, Cp2TiX2 (X = Cl, Br, I), in tetrahydrofuran has been described successfully using a common mesh scheme. On the basis of simulations of recorded cyclic voltammograms it has been possible to evaluate a number of thermodynamic and kinetic parameters for the species involved: i.e., Cp2TiX2 -, (Cp2TiX)2, Cp2TiX, and Cp2Ti+. In general, the standard potentials of the oxidized titanium-based species increase (i.e. become less negative) in the orders Cp2TiX2, (Cp2TiX)2 +, Cp2TiX+, CpTi2+ and X = Cl, Br, I. From the extracted data pertaining to electrochemically reduced solutions of Cp2TiX2, it becomes evident that while Cp2TiX2 - is the major constituent for X = Cl, Cp2TiX and (Cp2TiX)2 are the main species in the cases of X = Br, I. The presence of (Cp2TiX)2 is surprising, as the solvent tetrahydrofuran was believed to be capable of breaking the weak dimeric structure. Kinetic investigations of the reactions between electrochemically reduced solutions of Cp2TiX2 and benzyl chloride show that the reactive species are Cp2TiX and (Cp2TiX)2, with almost no contribution from Cp2TiX2 -, even in the case of X = Cl

General Approach for Monolayer Formation of Covalently Attached Aryl Groups Through Electrografting of Arylhydrazines

General Approach for Monolayer Formation of Covalently Attached Aryl Groups Through Electrografting of Arylhydrazine

Mechanistic Investigation of the Electrochemical Reduction of Cp₂TiX₂

The mechanism for the electrochemical reduction of titanocene dihalides, Cp2TiX2 (X = Cl, Br, I), in tetrahydrofuran has been described successfully using a common mesh scheme. On the basis of simulations of recorded cyclic voltammograms it has been possible to evaluate a number of thermodynamic and kinetic parameters for the species involved: i.e., Cp2TiX2 -, (Cp2TiX)2, Cp2TiX, and Cp2Ti+. In general, the standard potentials of the oxidized titanium-based species increase (i.e. become less negative) in the orders Cp2TiX2, (Cp2TiX)2 +, Cp2TiX+, CpTi2+ and X = Cl, Br, I. From the extracted data pertaining to electrochemically reduced solutions of Cp2TiX2, it becomes evident that while Cp2TiX2 - is the major constituent for X = Cl, Cp2TiX and (Cp2TiX)2 are the main species in the cases of X = Br, I. The presence of (Cp2TiX)2 is surprising, as the solvent tetrahydrofuran was believed to be capable of breaking the weak dimeric structure. Kinetic investigations of the reactions between electrochemically reduced solutions of Cp2TiX2 and benzyl chloride show that the reactive species are Cp2TiX and (Cp2TiX)2, with almost no contribution from Cp2TiX2 -, even in the case of X = Cl

Stoichiometric Studies on the Carbonylative Trifluoromethylation of Aryl Pd(II) Complexes using TMSCF₃ as the Trifluoromethyl Source

We have performed a series of stoichiometric studies in order to identify viable steps for a hypothetical catalytic cycle for the palladium-mediated carbonylative coupling of an aryl bromide with TMSCF3. Our work revealed that benzoyl Pd­(II) complexes bearing Xantphos or tBu3P as the phosphine ligands, which are generated from the corresponding PdII(Ph)Br complexes exposed to stoichiometric 13CO from 13COgen, were unable to undergo transmetalation and reductive elimination to trifluoroacetophenone. Instead, in the presence of base and additional CO, these organometallic complexes readily underwent reductive elimination to the acid fluoride. Attempts to determine whether the acid fluoride could represent an intermediate for acetophenone production were unrewarding. Only in the presence of a boronic ester did we observe some formation of the desired product, although the efficiency of transformation was still low. Finally, we investigated the reactivity of four phosphine-ligated PdII(Ph)­CF3 complexes (Xantphos, DtBPF, tBu3P, and triphenylphosphine) with carbon monoxide. With the exception of the tBu3P-ligated complex, all other metal complexes led to the facile formation of trifluoroacetophenone. We also determined in the case of triphenylphosphine that CO insertion occurred into the Pd–Ar bond, as trapping of this complex with n-hexylamine led to the formation of n-hexylbenzamide

Stepwise versus Concerted Electron Transfer-Bond Fragmentation in the Reduction of Phenyl Triphenylmethyl Sulfides

The link between stepwise and concerted electron transfer-bond fragmentation processes is described for the homogeneous reduction process of four para-substituted phenyl triphenylmethyl sulfides in N,N-dimethylformamide. The description is based on intrinsic barriers Δ and standard potentials Eo determined from free energy plots. For all reactions studied Δ is remarkable high and Eo is not far from what is expected for a concerted pathway, even if the mechanism is stepwise. This is attributed to a substantial elongation and weakening of the carbon−sulfur bond upon formation of the radical anion. At the same time, Δ increases going from electron-withdrawing to electron-donating substituents which points to a gradual transition between the two reaction pathways within the series of compounds

Revelation of the Nature of the Reducing Species in Titanocene Halide-Promoted Reductions

The fundamental nature of TiIII complexes generated in tetrahydrofuran by reduction of Cp2TiCl2 has been clarified by means of cyclic voltammetry and kinetic measurements. While the electrochemical reduction of Cp2TiCl2 leads to the formation of Cp2TiCl2-, the use of metals such as Zn, Al, or Mn as reductants affords Cp2TiCl and (Cp2TiCl)2 in a mixture having a dimerization equilibrium constant of 3 × 103 M-1, independent of the metal used. Thus, we find it unlikely that the trinuclear complexes or ionic clusters known from the solid phase should be present in solution as previously suggested. The standard potentials determined for the redox couples Cp2TiCl2/Cp2TiCl2-, (Cp2TiCl)2+/(Cp2TiCl)2, Cp2TiCl+/Cp2TiCl, and Cp2Ti2+/Cp2Ti+ increase in the order listed. However, the reactivity of the different TiIII complexes is assessed as (Cp2TiCl)2 ≳ Cp2TiCl ≈ Cp2Ti+ ≫ Cp2TiCl2- in their reactions with benzyl chloride and benzaldehyde. None of the reactions proceed by an outer-sphere electron transfer pathway, and clearly the inner-sphere character is much higher in the case of Cp2Ti+ than for (Cp2TiCl)2, Cp2TiCl, and in particular Cp2TiCl2-. As to the electron acceptor, the inner-sphere character increases, going from benzyl chloride to benzaldehyde, and it is suggested that the chlorine atom in benzyl chloride and the oxygen atom in benzaldehyde may function as bridges between the reactants in the transition state

Solvation of Carbanions in Organic Solvents: A Test of the Polarizable Continuum Model

The solvation of carbanions in the solvents N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) has been analyzed on the basis of experimental and theoretical data. Experimental solvation energies are obtained from present and previously reported electrochemical measurements of reduction potentials of the corresponding radicals. Theoretical solvation energies are obtained from quantum chemical calculations using the polarizable continuum model (PCM). It is found that the solvation energy is relatively independent of molecular size and structure for the saturated carbanions. This indicates that the negative charge is strongly localized to the anionic carbon. The conjugated carbanions have considerably lower absolute solvation energies ( |) than the saturated carbanions. This is a consequence of the strong delocalization of the negative charge in the former group. The propargyl anion is also found to have a surprisingly low absolute solvation energy. However, high-level quantum chemical calculations show that the electronic structure has large contributions from two different resonance structures, CH⋮CCH2- and -CHCCH2, which results in a significant charge delocalization. There is good agreement between calculated and experimental solvation energies for both the conjugated and nonconjugated primary anions. However, the PCM method consistently underestimates the absolute solvation energies of the secondary and tertiary carbanions. This is attributed to an insufficient treatment of first-layer solvation effects in the method. According to the experimental measurements, the absolute solvation energies are on average 2−3 kcal mol-1 lower in THF than in DMF. The theoretical data indicate a considerably larger solvent effect, 7−10 kcal mol-1. The discrepancy between theory and experiment may partly be attributed to the use of a supporting electrolyte in the measurements, but the main cause seems to be that the short-range interaction tendencies of the solvent cannot be fully characterized by its dielectric constant

Understanding the Enhanced Catalytic CO₂ Reduction upon Adhering Cobalt Porphyrin to Carbon Nanotubes and the Inverse Loading Effect

Adhering a cobalt porphyrin (Co­(TPP)) catalyst on a carbon nanotube (CNT) supporting material greatly enhances its reactivity and enables catalysis in water, which is otherwise impossible. However, the effect of solvent as well as supporting materials on catalysis is still elusive. On the basis of computational results we found that water as a reaction medium lowers the reductive potential required due to the stabilization of intermediates and transition states, and provides higher availability of protons. To understand the effect of the support materials, we combine computations and experiments and illustrate that the curvature of the nanotubes plays an essential role in aggregation through the competition between the π–π interactions between the porphyrin rings as well as between the Co­(TPP) and the nanotube, providing an insight into lessening the degree of aggregation