³¹P NMR Chemical Shift Tensors: Windows into Ruthenium Phosphinidene Complex Electronic Structures

A series of octamethylcalix[4]pyrrole/ruthenium phosphinidene complexes (Na₂[1=PR]) can be accessed by phosphinidene transfer from the corresponding RPA (A = C₁₄H₁₀, anthracene) compounds (R = Bu, Pr, OEt, NH₂, NMe₂, NEt₂, NPr₂, NA, dimethylpiperidino). Isolation of the tert-butyl and dimethylamino derivatives allowed comparative studies of their ³¹P nuclear shielding tensors by magic-angle-spinning solid-state nuclear magnetic resonance spectroscopy. Density functional theory and natural chemical shielding analyses reveal the relationship between the ³¹P chemical shift tensor and the local ruthenium/phosphorus electronic structure. The general trend observed in the ³¹P isotropic chemical shifts for the ruthenium phosphinidene complexes was controlled by the degree of deshielding in the δ₁₁ principal tensor component, which can be linked to the σ_RuP/π_RuP* energy gap. A "δ₂₂-δ₃₃ crossover" effect for R = Bu was also observed, which was caused by different degrees of deshielding associated with polarizations of the σ_PR and σ_PR* natural bond orbitals.