Solid-state 17O NMR and computational studies of terminal transition metal oxo compounds

We report solid-state NMR characterization of the 17O (I = 5/2) chemical shift and quadrupole coupling tensors in two terminal oxo compounds, 17O≡Ti(IV)(TMP) and 17O≡Cr(IV)(TMP), in which TMP is 5,10,15,20-tetramesitylporphyrin and the oxo ligand is enriched by 17O (ca. 40%). This is the first time that 17O NMR tensors are determined for this important class of compounds. The 17O nuclei in the O≡Ti and O≡Cr triple bonds are found to exhibit very large chemical shift anisotropies but rather small 17O quadrupole coupling constants. Terminal oxo compounds represent one of the rare cases where the atomic nucleus under study simultaneously experiences a highly anisotropic magnetic shielding environment, but a highly symmetrical electric field distribution. The solid-state 17O NMR data are consistent with solid-state 13C, 15N and 31P NMR results on carbido, nitrido and phosphido compounds. The density functional theory (DFT) computations using the zeroth-order regular approximation (ZORA) to account for spin-orbital relativistic effects reproduce the experimental 17O NMR results reasonably well. DFT computations also reveal the origins at the molecular orbital level of the observed 17O NMR properties for terminal oxo compounds.