The kinetics of ethylene trimerization by a chromium N-phosphinoamidine (Cr-(P,N)) precatalyst activated by modified methylaluminoxane (MMAO) has been investigated by high-pressure NMR techniques. An in-depth kinetic analysis of this metallacyclic mechanism has been conducted. It was found that an intermediate in the trimerization catalytic cycle, proposed in this study as the chromium alkenyl hydride species, degrades into a polymer active site where this degradation step is independent of ethylene concentration and is first order in catalyst. Additionally, we report that at least one of the first two ethylene coordination steps must be reversible in order to predict the features of the monomer consumption profiles. The reaction order in ethylene is dependent on the reversibility of the ethylene coordination steps. The observation of these details of the mechanism explains many of the challenges inherent in the examination of this and similar catalyst systems and emphasizes the usefulness of operando high-pressure NMR studies and a quantitative kinetic modeling approach in the study of such systems.

We report mechanistic studies on the insertion reactions of [(NHC)Cu(μ‐H)]2 complexes with carbonyl substrates by UV‐vis and 1H NMR spectroscopic kinetic studies, H/D isotopic labelling, and X‐ray crystallography. The results of these comprehensive studies show that the insertion of Cu‐H with an aldehyde, ketone, activated ester/amide, and unactivated amide consist of two different rate limiting steps: the formation of Cu‐H monomer from Cu‐H dimer for more electrophilic substrates, and hydride transfer from a transient Cu‐H monomer for less electrophilic substrates. We also report spectroscopic and crystallographic characterization of rare Cu‐hemiacetalate and Cu‐hemiaminalate moieties from the insertion of an ester or amide into the Cu−H bond.

We have demonstrated the ability of TEMPO to catalyze H· transfer from (C5Ph5)­Cr­(CO)3H to a trityl radical (tris­(p-tert-butylphenyl)­methyl radical). We have measured the rate constant and activation parameters for the direct reaction, and for each step in the catalytic process: H· transfer from (C5Ph5)­Cr­(CO)3H to TEMPO and H· transfer from TEMPO–H to the trityl radical. We have compared the measured rate constants with the differences in bond strength, and with the changes in the Global Electrophilicity Index determined with high accuracy for each radical using state of the art quantum chemical methods. We conclude that neither is a major factor in determining the rates of these H· transfer reactions and that the effectiveness of TEMPO as a catalyst is largely the result of its relative lack of steric congestion compared to the trityl radical.

Hydrogen atom (H•) donors generated from H2 facilitate the atom efficient reduction of small molecule substrates. However, generating H• donors with X–H bond dissociation free energies (BDFEs) below 52 kcal mol–1 is especially challenging because they thermodynamically favor the bimolecular evolution of H2. We have recently proposed that [CpV­(CO)3H]− catalyzes the conversion of H2 into a proton, an electron, and a hydrogen atom in the presence of a sacrificial base. In order to understand the driving force for H• transfer, the free energies of H+/H•/H–/e– transfer from [CpV­(CO)3H]− have been evaluated using solution phase techniques and state-of-the-art quantum chemical calculations. Thermochemical cycles have been constructed in order to anchor the computational values against experimental observations. This facilitates a quantitative comparison of the thermodynamic driving force for H+/H•/H–/e– transfer between isoelectronic anionic/neutral hydrides of the same row (the corresponding values are already available for CpCr­(CO)3H). The overall charge greatly influences the thermodynamics of transferring H+, H–, and e– (i.e., [CpV­(CO)3H]− is a much weaker acid, a stronger hydride donor, and a stronger reductant than CpCr­(CO)3H); there is almost no change in the thermodynamics of H• transfer (V–H BDFE 54.7 kcal mol, Cr–H BDFE 57.0 kcal mol–1). In MeCN, the one electron oxidation of [CpV­(CO)3H]− (−0.83 V vs Fc/Fc+) generates CpV­(CO)3H, which spontaneously evolves H2. The resulting CpV­(CO)3 is trapped as the solvent adduct CpV­(CO)3(MeCN). Because H• transfer is now coupled to metal–solvent binding, the V–H bond is substantially weakened for CpV­(CO)3H (V–H BDFE 36.1 kcal mol–1), amounting to a strategy for obtaining very reactive H atoms from H2.