NMR chemical shift analysis decodes olefin oligo- and polymerization activity of d⁰ group 4 metal complexes.

ABSTRACT: d⁰ metal-alkyl complexes (M = Ti, Zr, and Hf) show specific activity and selectivity in olefin polymerization and oligomerization depending on their ligand set and charge. Here, we show by a combined experimental and computational study that the ¹³C NMR chemical shift tensors of the ?-carbon of metal alkyls that undergo olefin insertion signal the presence of partial alkylidene character in the metal-carbon bond, which facilitates this reaction. The alkylidene character is traced back to the ?-donating interaction of a filled orbital on the alkyl group with an empty low-lying metal d-orbital of appropriate symmetry. This molecular orbital picture establishes a connection between olefin insertion into a metal-alkyl bond and olefin metathesis and a close link between the Cossee-Arlmann and Green-Rooney polymerization mechanisms. The ¹³C NMR chemical shifts, the ?-H agostic interaction, and the low activation barrier of ethylene insertion are, therefore, the results of the same orbital interactions, thus establishing chemical shift tensors as a descriptor for olefin insertion.

SUBMITTER: Gordon CP 

PROVIDER: S-EPMC6042142 | biostudies-literature | 2018 Jun

REPOSITORIES: biostudies-literature