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

Abstract

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.