Quantum chemical models of reaction pathways can provide deep insight into the inner workings of transition metal complexes. Often, these simulations have relied on atomistic models where a single or a few conformational isomers of the complex are investigated. This Article will show that, for bisphosphine Ni complexes used to forge C−C bonds, a large number of conformers must be studied to provide confidence that the overall model is meaningful. Not only do conformer effects modify particular reaction barriers, but often the lowest barrier reaction pathway proceeds from a conformer that is not the lowest energy conformer. This finding suggests that errors on the order of more than a few kcal/mol could be present in single-conformer studies. The particular reaction pathway and conformer preferences for a series of eight common Ni bisphosphine complexes will provide some guidance as to when the effects of the conformer will be large or small.
Initial
catalyst dormancy has been mitigated for the enantioselective
polymerization of propylene oxide using a tethered bimetallic chromium(III)
salen complex. A detailed mechanistic study provided insight into
the species responsible for this induction period and guided efforts
to remove them. High-resolution electrospray ionization–mass
spectrometry and density functional theory computations revealed that
a μ-hydroxide and a bridged 1,2-hydroxypropanolate complex are
present during the induction period. Kinetic studies and additional
computation indicated that the μ-hydroxide complex is a short-lived
catalyst arrest state, where hydroxide dissociation from one metal
allows for epoxide enchainment to form the 1,2-hydroxypropanolate
arrest state. While investigating anion dependence on the induction
period, it became apparent that catalyst activation was the main contributor
for dormancy. Using a 1,2-diol or water as chain transfer agents (CTAs)
led to longer induction periods as a result of increased 1,2-hydroxyalkanolate
complex formation. With a minor catalyst modification, rigorous drying
conditions, and avoiding 1,2-diols as CTAs, the induction period was
essentially removed.
Microplastic pollution is omnipresenthaving been found in our land, air, food, and water. Over the last two decades, both identifying microplastics and sleuthing their sources has been a major research focus. Moving forward, the next goal should be remediation. Although removing microplastics from the environment is impractical, developing methods that prevent their release into the environment is essential. Herein, we report an approach for removing microplastics from water using a pressuresensitive adhesive. Specifically, we demonstrate that shaking zirconium silicate beads coated with poly(2-ethylhexyl acrylate) in aqueous suspensions containing polystyrene microplastics (10 μm) can remove up to 99% of the microplastics within 5 min. We show that the adhesive molar mass (ranging from 93−950 kg/ mol) is invariant with respect to removal efficiency at 5 min, as quantified by flow cytometry. Preliminary results suggest these adhesives can bind other microplastics as well, including nonpolar polymers (e.g., polyethylene, micronized rubber) and polar polymers (e.g., nylon, polyethylene terephthalate). Overall, this proof-of-concept study demonstrates a promising approach for remediating microplastics from aqueous suspensions using adhesives.
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