This lecture focuses on the application of solid-state quadrupolar NMR for the characterization of weak secondary interactions. 3-center-2-electron (3c-2e) bond interactions between a σ-C–H bond and an empty orbital on a metal are referred to as agostic C-H interactions (1). Agostic interactions are important in numerous reactions involving olefin insertion into a metal-carbyl bond. A C–H agostic interaction is characterized by short M…HC distances and acute M–C–C bond angles compared to sp3-hybridized carbon in the absence of such an interaction. In addition, the solution NMR spectra of agostic complexes show that the 1JCH becomes substantially lower than ca. 125 Hz expected for sp3 C-H bonds. Cp*2Sc–Et (2, Cp* = pentamethylcyclopentadienyl) was isolated by Bercaw and co-workers, and was proposed to contain an agostic interaction. However, the solid-state structure of 2 was severely disordered, and solution NMR studies showed that 2 is dynamic at low temperatures, prohibiting accurate measurement of 1JCH. We are interested in the solid-state NMR properties of organoscandium complexes. 45Sc is a high sensitivity nucleus with 100 % natural abundance and a spin of 7/2. We will discuss how the Sc–R bond in Cp*2Sc–R affects the quadrupolar coupling constant (CQ). We find that Cp*2Sc–R (R = Me, Ph, CH2tBu) have large CQ values of approximately 36 MHz, and that Cp*2Sc–Et has a smaller CQ value of 26.2 MHz. DFT analysis shows that the origin of the CQ in these complexes is related to valence bonding orbitals. In Cp*2Sc–R (R = Me, Ph, CH2tBu) the large CQ values are related to the σ-Sc–C bond. Cp*2Sc–Et has a small CQ value because the Sc–Et fragment also engages in π-bonding with a C–H bond, i.e. an agostic interaction.

Solid-state 45Sc NMR of Cp*2Sc–R: Evidence for an agostic interaction in Cp*2Sc–Et

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.