The reaction of oxorhenium complexes that incorporate diamidopyridine (DAP) ligands with B(C6F5)3 results in the formation of classical Lewis acid-base adducts. The adducts effectively catalyze the hydrogenation of a variety of unactivated olefins at 100 °C. Control reactions with these complexes or B(C6F5)3 alone did not yield any hydrogenated products under these conditions. Mechanistic studies suggest a frustrated Lewis pair is generated between the oxorhenium DAP complexes and B(C6F5)3, which is effective at olefin hydrogenation. Thus, we demonstrate for the first time that the incorporation of a transition-metal oxo in a frustrated Lewis pair can have a synergistic effect and results in enhanced catalytic activity.

High melt strength (HMS), shear thinning, and extensional strain hardening (SH) are highly desirable properties in commercial polypropylene, which are typically achieved by the incorporation of long-chain branching (LCB). The current state-of-the-art approach to produce LCB involves post-reactor modification steps, which are not only costly but also generate undesirable side products as a result of polymer chain scission. We report a novel one-pot synthetic route to produce HMS isotactic polypropylene (iPP) ionomers bearing aluminum carboxylate groups. The synthesis of iPP ionomers is achieved by the direct copolymerization of an alkenyl aluminum comonomer and is facilitated by a novel C 1 -symmetric metallocene catalyst, producing highly isospecific iPP ionomers (T m > 157 °C) with high activity (>200 000 g-polymer mmol-Zr −1 h −1 ). X-ray scattering experiments conducted in the solid and melt states confirm the presence of ion clusters as independent entities from the crystalline lamellae. The ion content in the iPP ionomers is very low (<0.1 mol %), which results in insignificant effects on the crystallinity, melting point, and mechanical properties when compared to the iPP homopolymer. Remarkably, such a low level of ion content is sufficient to drastically improve the processability of the ionomers, as indicated by the increase in melt strength, shear thinning, and extensional SH.

Tertiary and quaternary phosphonium borane catalysts are employed as catalysts for CO2/epoxide copolymerization. Catalyst structures are strategically modified to gain insights into the intricate structure–activity relationship. To quantitatively and rigorously compare these catalysts, the copolymerization reactions were monitored by in situ Raman spectroscopy, allowing the determination of polymerization rate constants. The polymerization rates are very sensitive to perturbations in phosphonium/borane substituents as well as the tether length. To further evaluate catalysts, a nonisothermal kinetic technique has been developed, enabling direct mapping of polymerization rate constant (k p) as a function of polymerization temperatures. By applying this method, key intrinsic attributes governing catalyst performance, such as activation enthalpy (ΔH ‡), entropy (ΔS ‡), and optimal polymerization temperature (T opt), can be extracted in a single continuous temperature sweep experiment. In-depth analyses reveal intricate trends between ΔH ‡, ΔS ‡, and Lewis acidity (as determined using the Gutmann–Beckett method) with respect to structural variations. Collectively, these results are more consistent with the mechanistic proposal in which the resting state is a carbonate species, and the rate-determining step is the ring-opening of epoxide. In agreement with the experimental results, DFT calculations indicate the important contributions of noncovalent stabilizations exerted by the phosphonium moieties. Excitingly, these efforts identify tertiary phosphonium borane analogues, featuring an acidic phosphonium proton, as leading catalysts on the basis of k p and T opt. Mediated by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide (VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity. The tertiary phosphonium catalysts maintain their high activity in the presence of large excess of di-alcohols as chain-transferring agents, affording well-defined telechelic polyols. The results presented herein shed light on the cooperative catalysis between phosphonium and borane.

Combined experimental and computational studies have revealed factors that influence the nondirected C−H activation in Cp*Ir complexes that contain carboxylate ligands. A two-step acetate-assisted pathway was shown to be operational where the first step involves substrate binding and the second step involves cleavage of the C−H bond of the substrate. A nonlinear Hammett plot was obtained to examine substituted arenes where a strong electronic dependence (ρ = 1.67) was observed for electron-donating groups, whereas no electronic dependence was observed for electron-withdrawing groups. Electron-donating substituents in the para position were shown to have a bigger impact on the C−H bond cleavage step, whereas electron-withdrawing substituents influenced the substrate-binding step. Although cleavage of the C−H bond was predicted to be more facile with arenes that contain substituents in the para position by DFT calculations, the cyclometalations of anisole and benzonitrile were observed experimentally. This suggests that these substituents, even though they are weakly directing, still result in cyclometalation because the barriers for activation at the ortho and para positions of arenes are comparable (24.3 and 26.5 kcal/mol, respectively). Incorporation of a weakly bound ligand was found to be necessary for facile reactivity. It is predicted by DFT calculations that the replacement of an oxygen atom with a nitrogen atom in the carboxylate ligand would lead to a dramatic reduction in the barrier for C−H activation, as the incorporation of formimidate and N-methylformimidate ligands leads to barriers of 23.4 and 21.7 kcal/mol, respectively. These values are significantly lower than the barrier calculated for the analogous acetate ligand (28.2 kcal/mol).