Elucidating the Link between NMR Chemical Shifts and Electronic Structure in d0 Olefin Metathesis Catalysts

• DOI: 10.1021/jacs.5b12597
• Publication Date: 2016-01-20T02:31:18Z
• Authors: Stéphanie Halbert; Christophe Copéret; Christophe Raynaud; Odile Eisenstein
• Keywords: Olefin Metathesis; Chemical shift

The nucleophilic carbon of d0 Schrock alkylidene metathesis catalysts, [M] = CHR, display surprisingly low downfield chemical shift (δiso) and large anisotropy. State-of-the-art four-component relativistic calculations the tensors combined with a two-component analysis in terms localized orbitals allow molecular-level understanding their orientations, magnitude principal components (δ11 > δ22 δ33) associated δiso. This reveals dominating influence paramagnetic contribution yielding highly deshielded carbon. largest contribution, which originates from coupling σMC π*MC under action magnetic field, is analogous to that resulting σCC π*CC ethylene; thus, δ11 MCH plane perpendicular MC internuclear direction. higher value carbon-13 δiso complexes relative ethylene thus due smaller energy gap between vs this ethylene. effect also explains why highest observed for Mo lowest Ta, values W Re being between. In presence agostic interaction, tensor orientation (δ22 or δ33 parallel πMX) influenced by angle because it determines CHR fragment axis. orbital shows how terms, understood bond model, determine thereby