Relativistic Density Functional NMR Tensors Analyzed with Spin-free Localized Molecular Orbitals

The implementation of fast relativistic methods based on density functional theory, in conjunction with localized molecular orbital (LMO) based analysis, allows straightforward interpretations of NMR parameters in terms of contributions from core shells, lone pairs, and bonds, for compounds containing elements from across the periodic table. We present a conceptual review of a frequently used LMO analysis of NMR parameters calculated in the presence of spin-orbit interactions and other relativistic effects. An accompanying example focuses on the ¹⁵ N shielding in a heavy metal complex.

Advances in Catalytic C–F Bond Activation and Transformation of Aromatic Fluorides

The activation and transformation of C–F bonds in fluoro-aromatics is a highly desirable process in organic chemistry. It provides synthetic methods/protocols for the generation of organic compounds possessing single or multiple C–F bonds, and effective catalytic systems for further study of the activation mode of inert chemical bonds. Due to the high polarity of the C–F bond and it having the highest bond energy in organics, C–F activation often faces considerable academic challenges. In this mini-review, the important research achievements in the activation and transformation of aromatic C–F bond, catalyzed by transition metal and metal-free systems, are presented.

Nuclear Magnetic Resonance: A Spectroscopic Probe to Understand the Electronic Structure and Reactivity of Molecules and Materials

This Perspective focuses on the ability of chemical shift to identify and characterize the electronic structure and associated reactivity of molecules and materials. After a general introduction on NMR parameters, we will show selected examples where the chemical shift of various NMR active nuclei has been used to investigate and understand electronic properties, with a particular focus on organometallic compounds and inorganic materials with relevance to catalysis. We will demonstrate how the NMR parameter of probe molecules and ligands can be used to elucidate the nature of active sites and how they can help to understand and predict their reactivity. Lastly, we will give an overview over recent advances in deciphering metal NMR parameters. Overall, we show how chemical shift is a reactivity descriptor that can be analyzed and understood on a molecular level.

Toward the Copolymerization of Propylene with Polar Comonomers

Polyolefins are produced in vast amounts and are found in so many consumer products that the two most commonly produced forms, polyethylene (PE) and polypropylene (PP), fall into the rather sparse category of molecules that are likely to be known by people worldwide, regardless of their occupation. Although widespread, the further upgrading of their properties (mechanical, physical, aesthetic, etc.) through the formation of composites with other materials, such as polar polymers, fibers, or talc, is of huge interest to manufacturers. To improve the affinity of polyolefins toward these materials, the inclusion of polar functionalities into the polymer chain is essential. The incorporation of a functional group to trigger controlled polymer degradation is also an emerging area of interest. Currently practiced methods for the incorporation of polar functionalities, such as post-polymerization functionalization, are limited by the number of compatible polar monomers: for example, grafting maleic anhydride is currently the sole method for practical functionalization of PP. In contrast, the incorporation of fundamental polar comonomers into PE and PP chains via coordination insertion polymerization offers good control, making it a highly sought-after process. Early transition metal catalysts (which are commonly used for the production of PE and PP) display poor tolerance toward the functional groups within polar comonomers, limiting their use to less-practical derivatives. As late transition metal catalysts are less-oxophilic and thus more tolerant to polar functionalities, they are ideal candidates for these reactions. This Account focuses on the copolymerization of propylene with polar comonomers, which remains underdeveloped as compared to the corresponding reaction using ethylene. We begin with the challenges associated with the regio- and stereoselective insertion of propylene, which is a particular problem for late transition metal systems because of their propensity to undergo chain walking processes. To overcome this issue, we have investigated a range of metal/ligand combinations. We first discuss attempts with group 4 and 8 metal catalysts and their limitations as background, and then focus on the copolymerization of propylene with methyl acrylate (MA) using Pd/imidazolidine-quinolinolate (IzQO) and Pd/phosphine-sulfonate (PS) precatalysts. Each generated regioregular polymer, but while the system featuring an IzQO ligand did not display any stereocontrol, that using the chiral PS ligand did. A further difference was found in the insertion mode of MA: the Pd/IzQO system inserted in a 1,2 fashion, while in the Pd/PS system a 2,1 insertion was observed. We then move onto recent results from our lab using Pd/PS and Pd/bisphosphine monoxide (BPMO) precatalysts for the copolymerization of propylene with allyl comonomers. These P-stereogeneic precatalysts generated the highest isotacticity values reported to date using late transition metal catalysts. This section closes with our work using Earth-abundant nickel catalysts for the reaction, which would be especially desired for industrial applications: a Ni/phosphine phenolate (PO) precatalyst yielded regioregular polypropylene with the incorporation of some allyl monomers into the main polymer chain. The installation of a chiral menthyl substituent on the phosphine allowed for moderate stereoselectivity to be achieved, though the applicable polar monomers currently remain limited. The Account concludes with a discussion of the factors that affect the insertion mode of propylene and polar comonomers in copolymerization reactions, beginning with our recent computational study, and finishing with work from ourselves and others covering both comonomer and precatalyst steric and electronic profiles with reference to the observed regioselectivity.

Direct synthesis of various polar functionalized polypropylene materials with tunable molecular weights and high incorporation ratios

As compared with the classical α-diimine catalyst, iminopyridyl catalysts were observed to be highly efficient for the direct synthesis of polar functionalized polypropylene with tunable molecular weights and high incorporation ratios.