Elucidating the Link between NMR Chemical Shifts and Electronic Structure in d0Olefin Metathesis Catalysts,Journal of the American Chemical Society

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Elucidating the Link between NMR Chemical Shifts and Electronic Structure in d0Olefin Metathesis Catalysts
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2016-02-09 , DOI: 10.1021/jacs.5b12597
Stéphanie Halbert ₁ , Christophe Copéret ₂ , Christophe Raynaud ₁ , Odile Eisenstein _{1,

3}

Affiliation

The nucleophilic carbon of d(0) 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).

中文翻译：

阐明 d0Olefin 复分解催化剂中 NMR 化学位移与电子结构之间的联系

d(0) Schrock 亚烷基复分解催化剂的亲核碳 [M] = CHR，显示出令人惊讶的低场化学位移 (δ(iso)) 和大的化学位移各向异性。化学位移张量的最先进的四分量相对论计算结合局部轨道的二分量分析，允许在分子水平上理解它们的方向，它们的主分量的大小 (δ11 > δ22 > δ33) 和相关的 δ(iso)。该分析揭示了产生高度去屏蔽亚烷基碳的顺磁贡献的主要影响。最大的顺磁贡献源于亚烷基σ(MC)和π*(MC)轨道在磁场作用下的耦合，类似于乙烯中σ(CC)和π*(CC)的耦合; 因此，δ11 在 MCH 平面内，垂直于 MC 核间方向。因此，相对于乙烯，亚烷基配合物中碳-13 δ(iso) 的较高值是由于 σ(MC) 和 π*(MC) 之间的能隙与 σ(CC) 和 π*(CC) 之间的能隙较小乙烯。这种效应也解释了为什么观察到 Mo 的 δ(iso) 的最高值和 Ta 的最低值，W 和 Re 的值介于两者之间。在存在积极相互作用的情况下，化学位移张量主成分方向（δ22 或 δ33 平行或垂直于 π（MX））受 MCH 角的影响，因为它决定了亚烷基 CHR 片段相对于 MC 核间轴的方向。轨道分析显示了通过局部键模型理解的顺磁项如何确定化学位移张量，从而确定 δ(iso)。

更新日期：2016-02-09

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