Solid-state NMR of a paramagnetic DIAD-Fe^II catalyst: Sensitivity, resolution enhancement, and structure-based assignments

A general protocol for the structural characterization of paramagnetic molecular solids using solid-state NMR is provided and illustrated by the characterization-of a high-spin Fe^II catalyst precursor. We show how good NMR performance can be obtained on a molecular powder sample at natural abundance by using very fast (> 30 kHz) magic angle spinning (MAS), even though the individual NMR resonances have highly anisotropic shifts and very short relaxation times. The results include the optimization of broadband heteronuclear (proton-carbon) recoupling sequences for polarization transfer; the observation of single or multiple quantum correlation spectra between coupled spins as a tool for removing the inhomogeneous bulk magnetic susceptibility (BMS) broadening; and the combination of NMR experiments and density functional theory calculations, to yield assignments.