Structural Insights from Co Solid-State NMR Experiments on Organocobalt(I) Catalysts.

A series of fumarate-based organocobalt(I) [CoCp(CO)(fumarate)] catalysts is synthesized and characterized by X-ray crystallography, multinuclear ( C and Co) solid-state NMR spectroscopy, and Co NQR spectroscopy. Given the dearth of Co solid-state NMR studies on Co compounds, the present work constitutes the first systematic characterization of the Co electric field gradient and chemical shift tensors for a series of cobalt complexes in this oxidation state. Using X-ray crystallography, the molecular geometry about the Co centre is found to be nearly identical in all compounds studied herein. Owing to the Co nucleus' large chemical shift range, solid-state NMR experiments are found to be able to detect small structural differences between the individual organocobalt(I) compounds. With the aid of density functional theory calculations on these complexes, it is shown that the Co chemical shift anisotropy and the Co quadrupolar coupling constant are both extremely sensitive gauges of the Fu-Co-Cp bond angle, providing a link between these Co NMR observables and the catalysts' structures.