Cobalt-59 Nuclear Magnetic Resonance Studies of Chloro-, Bromo-, and Iodo-Substituted Tris(acetylacetonato)cobalt(III) Complexes

摘要

The cobalt-59 nuclear magnetic resonance spectra and longitudinal relaxation times have been studied for tris(acetylacetonato)cobalt(III) complexes with chloride, bromide, and iodide substitution at the methine carbon atom of the chelate ring. The cobalt-59 chemical shift is sufficiently sensitive to this remote substitution that cobalt-59 resonances are well resolved for each complex in a series of monohalo-, dihalo-, and trihalo-substituted complexes. The chemical shift is not linearly dependent on substitution. For the more electronegative halides, chlorine and bromine, static electric field gradient effects can be observed in the mixed-ligand complexes, but these effects amount to less than 10% of the observed relaxation rate. Through comparison of carbon-13 and cobalt-59 relaxation times it appears that in the solvents benzene, diglyme, and acetone, the cobalt-59 relaxation is a consequence of rotational reorientation, but for the solvent chloroform the cobalt-59 relaxation is different. Several possible explanations for this observation are considered. (Copyright (c) 1989 by the American Chemical Society and reprinted by permission of the copyright owner.)