C−H Activation and Olefin Insertion in d⁸ and d⁰ Complexes: Same Elementary Steps, Different Electronics

Abstract

Metallocene alkyl complexes with d0 electron configuration and d8‐configured square planar α‐diimine late transition metal alkyl complexes show activity in both C−H activation through σ‐bond metathesis and olefin insertion. Herein, we show by analysis of their M−CH3 13C chemical shift tensors that these reactions involve a π(M−C) interaction in the horizontal plane of the complex for both d0 and d8 systems. While in the case of d0 systems the interaction of an empty metal d‐orbital and a filled carbon p‐orbital causes partial alkylidene character of the M−C bond, the corresponding metal d‐orbital is filled in d8 systems, thus generating a filled π*(M−C) orbital that increases the anionic character of the methyl group. This entails fundamentally different reaction mechanisms for d0 and d8 systems, which are reflected in the structures of the transition states: While d0 olefin insertion can be viewed as a [2+2] cycloaddition reaction, d8 olefin insertion rather resembles methyl group migration onto a positively polarized olefin, thus explaining the observed differences in regioselectivity. These findings are translated to σ‐bond metathesis, a reaction which is isolobal to olefin insertion for both early and late transition metals.

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

“…Especially with regard to reactivity toward olefin insertion and σ-bond metathesis, which is observed both with d 0 and d 8 compounds, the underlying differing electronic structures entail differing detailed mechanisms. More precisely, the alkyl ligand in Pd-catalyzed olefin insertion transfers as an anionic ligand to the polarized olefin, whereas the mechanism with d 0 systems resembles a [2 + 2] cycloaddition …”

“…We will start by describing the NMR signatures of ligating atoms, moving from left to right in the periodic table to enable comparisons ( 13 C, 31 P and 15 N, 17 O, 77 Se, and 125 Te). We will then discuss the NMR signatures of metalloid and metal nuclei ( 29 Si, 27 Al, 89 Y, 45 Sc, and 195 Pt) and how ligands and coordination environment affect their NMR signatures.…”

“…Sc-45 NMR (I = 7/2, Natural Abundance = 100%). 45 Sc NMR was developed to study small molecules, crystalline porous materials, glassy solids, and organoscandium sites supported on oxides. 45 Sc benefits from being a high-sensitivity nucleus with a 100% natural abundance.…”

“…45 Sc NMR was developed to study small molecules, crystalline porous materials, glassy solids, and organoscandium sites supported on oxides. 45 Sc benefits from being a high-sensitivity nucleus with a 100% natural abundance. 45 Sc NMR line shapes are usually dominated by the quadrupolar coupling interaction.…”

Expedient Synthetic Identification of a P‐Stereogenic Ligand Motif for the Palladium‐Catalyzed Preparation of Isotactic Polar Polypropylenes

“…[28] An essential and more fundamental understanding of the migratory insertion process for late transition metals was recently reported by CopØret and co-workers:F or successful chain propagation an occupied antibonding orbital of p-symmetry induces anucleophilic character in the M-C carbon. [29] Therefore,t he migratory insertion during polymerization can be seen in as implified picture as an ucleophilic attack of an populated p*-orbital on the p*-orbital of an olefin, which perfectly describes the electronic origin of the observed inverse insertion selectivity for either electron rich or poor olefins.…”