Cobalt-59 NMR relaxation times of symmetric tris (-substituted-acetylacetonato)cobalt(III) complexes in benzene

Cobalt-59 NMR relaxation times, T-1(Co-59), and chemical shifts, delta(Co-59), of symmetric tris(gamma-substituted-acetylacetonato)cobalt(III) complexes, [Co(acac-X)(3)], which have chloride, bromide, nitro, and phenyl groups at the methine carbon atoms in the chelate ligands, have been measured in benzene at four temperatures between 284 and 319 K. In view of the viscosity dependence of 1/T-1(Co-59), the Co-59 relaxation is caused by the reorientation of the static electric field gradient at the nucleus in every complex. Assuming that the rotational volume is replaced by the van der Waals volume, the nuclear quadrupolar coupling products, {(1 + eta(2)(q)/3)(eQq/h)(2)}(1/2), were obtained for the series of [Co(acac-X)(3)] complexes in benzene. The negative correlation between {(1 + eta(2)(q)/3)(eQq/h)(2)}(1/2) and delta(Co-59) was observed for the four complexes [Co(acac)(3)], [Co(acac-Cl)(3)], [Co(acac-Br)(3)], and [Co(acac-NO2)(3)] except for [Co(acac-Ph)(3)]. This discrepancy strongly suggests that the rotational volume of the propeller-like [Co(acac-Ph)(3)] complex with the protruding phenyl groups cannot be simply treated as the van der Waals volume, different from the other almost spherical complexes. Actually, the estimation of the rotational volumes from the distances between the central cobalt and g-substituents gave much better correlation between {(1 + eta(2)(q)/3)(eQq/h)(2)}(1/2) and delta(Co-59) for all the complexes including [Co(acac-Ph)(3)].