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

The nucleophilic carbon of d(0) Schrock alkylidene metathesis catalysts, [M] = CHR, display surprisingly low downfield chemical shift (delta(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 (delta(11) > delta(22) > delta(33)) and associated delta(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 sigma(MC) and pi(MC)* orbitals under the action of the magnetic field, is analogous to that resulting from coupling sigma(cc) and pi(CC)* in ethylene; thus, delta(11) is in the MCH plane and is perpendicular to the MC internuclear direction. The higher value of carbon-13 delta(iso) in alkylidene complexes relative to ethylene is thus due to the smaller energy gap between sigma(MC) and pi(MC)* vs this between sigma(CC) and pi(CC)* in ethylene. This effect also explains why the highest value of delta(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 (delta(22) or delta(33) parallel or perpendicular to pi(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 delta(iso).