High-Level ab Initio Predictions for the Ionization Energy, Electron Affinity, and Heats of Formation of Cyclopentadienyl Radical, Cation, and Anion, C₅H₅/C₅H₅⁺/C₅H₅^–

The ionization energy (IE), electron affinity (EA), and heats of formation (Δ<i>H</i>°_f0/Δ<i>H</i>°_f298) for cyclopentadienyl radical, cation, and anion, C₅H₅/C₅H₅⁺/C₅H₅^–, have been calculated by wave function-based ab initio CCSDT/CBS approach, which involves approximation to complete basis set (CBS) limit at coupled-cluster level with up to full triple excitations (CCSDT). The zero-point vibrational energy correction, core–valence electronic correction, scalar relativistic effect, and higher-order corrections beyond the CCSD­(T) wave function are included in these calculations. The allylic [C₅H₅(²A₂)] and dienylic [C₅H₅(²B₁)] forms of cyclopentadienyl radical are considered: the ground state structure exists in the dienyl form and it is about 30 meV more stable than the allylic structure. Both structures are lying closely and are interconvertible along the normal mode of b₂ in-plane vibration. The CCSDT/CBS predictions (in eV) for IE­[C₅H₅⁺(³A₁′)←C₅H₅(²B₁)] = 8.443, IE­[C₅H₅⁺(¹A₁)←C₅H₅(²B₁)] = 8.634 and EA­[C₅H₅^–(¹A₁′)←C₅H₅(²B₁)] = 1.785 are consistent with the respective experimental values of 8.4268 ± 0.0005, 8.6170 ± 0.0005, and 1.808 ± 0.006, obtained from photoelectron spectroscopic measurements. The Δ<i>H</i>°_f0/Δ<i>H</i>°_f298’s (in kJ/mol) for C₅H₅/C₅H₅⁺/C₅H₅^– have also been predicted by the CCSDT/CBS method: Δ<i>H</i>°_f0/Δ<i>H</i>°_f298[C₅H₅(²B₁)] = 283.6/272.0, Δ<i>H</i>°_f0/Δ<i>H</i>°_f298[C₅H₅⁺(³A₁′)] = 1098.2/1086.9, Δ<i>H</i>°_f0/Δ<i>H</i>°_f298[C₅H₅⁺(¹A₁)] = 1116.6/1106.0, and Δ<i>H</i>°_f0/Δ<i>H</i>°_f298[C₅H₅^–(¹A₁′)] = 111.4/100.0. The comparisons between the CCSDT/CBS predictions and the experimental values suggest that the CCSDT/CBS procedure is capable of predicting reliable IE­(C₅H₅)’s and EA­(C₅H₅) with uncertainties of ±17 and ±23 meV, respectively

Olefin Polymerization Behavior of Titanium(IV) Pyridine-2-phenolate-6-(σ-aryl) Catalysts: Impact of “py-Adjacent” and Phenolate Substituents

A series of Ti­(IV) post-metallocene bis­(benzyl) precatalysts supported by tridentate pyridine-2-phenolate-6-(σ-aryl) [O,N,C] ligands, featuring various substituents on the σ-aryl (directly adjacent to the pyridine ring: fluoro, trifluoromethyl, benzo [C₄H₄]) and phenolate groups (<i>tert</i>-butyl, trifluoromethyl, cumyl, 1,1-diphenylethyl), have been prepared. Multinuclear (including ¹H, ¹³C, and ¹⁹F) NMR characterizations of the complexes have been performed. The principal purpose of this study was to investigate the impact of these substituents upon ethylene polymerization reactivity and polymer properties. The cumyl-phenolate σ-naphthyl Ti precatalyst, in conjunction with [Ph₃C]­[B­(C₆F₅)₄], displays good activity and produces polyethylene with exceptionally high MW (<i>M</i>_n = 4 × 10⁶) and an <i>M</i>_w/<i>M</i>_n value (2.5) approaching single-site character at 50 °C, but multisite behavior is apparent for other catalysts. DFT calculations have been performed to probe the polymerization behavior and the role of the py-adjacent substituent. These studies revealed the possibility of two distinct polymerization reactions, namely conventional and ethylene-assimilated (comprising initial ethylene insertion into the Ti–C­(σ-aryl) bond) chain propagation, and found that the latter is kinetically preferred. Furthermore, the viability of another kinetically competitive pathway, namely the isomerization of the ethylene-assimilated [Ti−CH₂CH₂−aryl] species via β-H elimination and 2,1-reinsertion, was also indicated

Proton-Coupled O‑Atom Transfer in the Oxidation of HSO₃^– by the Ruthenium Oxo Complex <i>trans</i>-[Ru^VI(TMC)(O)₂]²⁺ (TMC = 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane)

We have previously reported that the oxidation of SO₃^2– to SO₄^2– by a <i>trans</i>-dioxoruthenium­(VI) complex, [Ru^VI(TMC)­(O)₂)]²⁺ (<b>Ru</b>^<b>VI</b>; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazcyclotetradecane) in aqueous solutions occurs via an O-atom transfer mechanism. In this work, we have reinvestigated the effects of the pH on the oxidation of S^IV by <b>Ru</b>^<b>VI</b> in more detail in order to obtain kinetic data for the HSO₃^– pathway. The HSO₃^– pathway exhibits a deuterium isotope effect of 17.4, which indicates that O–H bond breaking occurs in the rate-limiting step. Density functional theory calculations have been performed that suggest that the oxidation of HSO₃^– by <b>Ru</b>^<b>VI</b> may occur via a concerted or stepwise proton-coupled O-atom transfer mechanism

Highly Selective Molecular Catalysts for the CO₂‑to-CO Electrochemical Conversion at Very Low Overpotential. Contrasting Fe vs Co Quaterpyridine Complexes upon Mechanistic Studies

[M^II(qpy)­(H₂O)₂]²⁺ (M = Fe, Co; qpy: 2,2′:6′,2″:6″,2‴-quaterpyridine) complexes efficiently catalyze the electrochemical CO₂-to-CO conversion in acetonitrile solution in the presence of weak Brönsted acids. Upon performing cyclic voltammetry studies, controlled-potential electrolysis, and spectroelectrochemistry (UV–visible and infrared) experiments together with DFT calculations, catalytic mechanisms were deciphered. Catalysis is characterized by high selectivity for CO production (selectivity >95%) in the presence of phenol as proton source. Overpotentials as low as 240 and 140 mV for the Fe and Co complexes, respectively, led to large CO production for several hours. In the former case, the one-electron-reduced species binds to CO₂, and CO evolution is observed after further reduction of the intermediate adduct. A deactivation pathway has been identified, which is due to the formation of a Fe⁰qpyCO species. With the Co catalyst, no such deactivation occurs, and the doubly reduced complex activates CO₂. High scan rate cyclic voltammetry allows reaching kinetic conditions, leading to scan-rate-independent plateau-shaped voltammograms from which catalytic rate constant was obtained. The molecular catalyst is very active for CO production (turnover a frequency of 3.3 × 10⁴ s^–1 at 0.3 V overpotential), as confirmed by catalytic a Tafel plot showing a comparison with previous catalysts