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

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• 作者：Stéphanie Halbert ; Christophe Copéret ; Christophe Raynaud ; Odile Eisenstein
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：February 24, 2016
• 年：2016
• 卷：138
• 期：7
• 页码：2261-2272
• 全文大小：568K
• ISSN：1520-5126

文摘

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