Nuclear magnetic resonance and relaxation studies of simple cobalt complexes

Abstract

Nuclear magnetic resonance (NMR) studies have been performed on a series of diamagnetic cobalt(III) complexes in both solution and the solid state. Variable-temperature solution cobalt-59 nuclear magnetic relaxation studies revealed that the dominant relaxation mechanism of these complexes in solution was the quadrupolar mechanism at room temperature. At higher temperatures, three of the complexes studied were observed to experience non-linear relaxation as temperature increased indicating that the spin-rotation relaxation of the absolute chemical shielding scale for cobalt. For solid samples, the chemical shift and electric field gradient (efg) tensors of eleven complexes were elucidated via simulation of solid-state ^59Co NMR spectra. Symmetry arguments, Gaussian 94 calculations, or residual quadrupolar effects on the dipolar coupling with an adjacent spin-1/2 nuclaei allowed for the determination of the orientation of the efg tensor in the molecular frame. The relative orientation of the chemical shift tensor was known with respect to the efg tensor from the simulation of the solid-state ^59Co spectra, allowing the assignment of specific ligand planes responsible for observed values of chemical shift principal components. The possibility of using cobalt-59 NMR as a site-specific probe of molecular structure is proposed.