Cobalt-59 nuclear magnetic resonance studies of chloro-, bromo-, and iodo-substituted tris(acetylacetonato)cobalt(III) complexes

Abstract

The cobalt-59 nuclear magnetic resonance spectra and longitudinal relaxation times have been studied for tris(acetyl-acetonato)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 explanation for this observation are considered. 23 refs., 5 figs., 5 tabs.