Solid-State NMR and Density Functional Investigation of Carbon-13 Shielding Tensors in Metal−Olefin Complexes

Abstract

We have determined the principal elements of the chemical shift tensors for a series of metal−olefin complexes: [Ag(cod)2]BF4 (cod = cis,cis-cycloocta-1,5-diene), [CuCl(cod)]2, PtCl2(cod), [RhCl(cod)]2, and K[PtCl3(C2H4)] using magic-angle sample spinning and a Bayesian probability method to deduce μ, ρ in the Herzfeld-Berger equations. These principal elements have also been computed by using density functional methods with two different types of functionals and partial geometry optimization. The overall slope and R2 values between the theoretical and experimental tensor elements are good, ranging from 1.06 to 1.16 for the slope (versus the ideal value of 1) and 0.98−0.99 for the goodness of fit parameter R2 (versus the ideal value of 1). The use of a hybrid functional results in a slightly worse slope, an effect which is largest for the compounds with the largest paramagnetic shifts. There are no particularly good correlations between C−C bond lengths, isotropic/anisotropic shift tensor elements or computed bond orders; however, the correlation between shielding and (Mulliken) charge of ∼ −120 ppm/electron is consistent with previous experimental estimates on olefins and aromatic compounds. The orientations of the shielding tensor elements in the cod complexes change in a relatively continuous manner with increases in shielding (from d10 to d8 metals), with δ33 becoming rotated (37.5°) from the normal to the CC bond axis in [RhCl(cod)]2. Overall, these results indicate that density functional methods permit the relatively accurate reproduction of metal−ligand shielding patterns in systems whose structures are known, which should facilitate their use in probing metal−ligand geometries in systems whose structures are less certain, such as in metalloproteins.