RESUMO
Group 10 metal catalysts have shown much promise for the copolymerization of nonpolar with polar alkenes to directly generate functional materials, but access to high copolymer molecular weights nevertheless remains a key challenge toward practical applications in this field. In the context of identifying new strategies for molecular weight control, we report a series of highly polarized P(V)-P(III) chelating ligands that manifest unique space filling and electrostatic effects within the coordination sphere of single component Pd polymerization catalysts and exert important influences on (co)polymer molecular weights. Single component, cationic phosphonic diamide-phosphine (PDAP) Pd catalysts are competent to generate linear, functional polyethylenes with Mw up to ca. 2 × 105 g mol-1, significantly higher than prototypical catalysts in this field, and with polar content up to ca. 9 mol %. Functional groups are positioned by these catalysts almost exclusively along the main chain, not at chain ends or ends of branches, which mimics the microstructures of commercial linear low-density polyethylenes. Spectroscopic, X-ray crystallographic, and computational data indicate PDAP coordination to Pd manifests cationic yet electron-rich active species, which may correlate to their complementary catalytic properties versus privileged catalysts such as electrophilic α-diimine (Brookhart-type) or neutral phosphine-sulfonato (Drent-type) complexes. Though steric blocking within the catalyst coordination sphere has long been a reliable strategy for catalyst molecular weight control, data from this study suggest electronic control should be considered as a complementary concept less prone to suppression of comonomer enchainment that can occur with highly sterically congested catalysts.
RESUMO
The title compound, [Mo(C(5)H(5))(C(2)H(3)O)(C(13)H(13)P)(CO)(2)], was prepared by reaction of [Mo(CH(3))(C(5)H(5))(CO)(3)] with methyl-diphenyl-phosphane. The Mo(II) atom exhibits a four-legged piano-stool coordination geometry with the acetyl and phosphane ligands trans to each other. There are several inter-molecular C-Hâ¯O hydrogen-bonding inter-actions involving carbonyl and acetyl O atoms as acceptors. A close nearly parallel π-π inter-action between the cyclo-penta-dienyl plane and the phenyl ring of the phosphane ligand is present, with an angle of 6.4â (1)° between the two least-squares planes. The centroid-to-centroid distance between these groups is 3.772â (3)â Å, and the closest distance between two atoms of these groups is 3.449â (4)â Å. Since each Mo complex is engaged in two of these inter-actions, the complexes form an infinite π-stack coincident with the a axis.
RESUMO
Intramolecular radical cation Diels-Alder reactions can be conducted under photocatalytic conditions using visible light irradiation. The photocatalyst system involves the use of a Ru(bpy)(3) (2+) chromophore and methyl viologen as a co-oxidant. These reactions enable the cycloaddition of substrates whose thermal Diels-Alder cycloadditions are electronically mismatched and thus require forcing conditions. Nevertheless, the radical cation cycloadditions can be conducted on gram scale using ambient sunlight as the only source of irradiation.
