Rotational Correlation Times of Tris(acetylacetonato)cobalt(III) in Various Organic Solvents by NMR Relaxation Measurements.

NMR relaxation times of the dual-specific nuclei of metal cobalt-59 and ligand methine carbon-13 for tris(2,4-pentanedionato)cobalt(III), which is commonly named as tris(acetylacetonato)cobalt(III), [Co(acac)3], in four organic solvents, acetonitrile, dichloromethane, chloroform, and benzene, have been measured at several temperatures. The rotational correlation times of complex molecules in different solvents, obtained from the NMR relaxation times, can be semi-quantitatively described by the hydrodynamic Debye–Einstein–Stokes (DES) model. However, the slight deviations from the DES model were observed dependent on the kind of solvents, where the effective rotational volume increased in the order of C6H6 ~ CH3CN < CH2Cl2 < CDCl3. In chloroform solution, where two solvent molecules are specifically bound to [Co(acac)3] in the ligands' hollows, the complex molecule can rotate accompanying the chloroform molecules solvated and/or the friction in the outer coordination sphere can be increased by the specific interactions in view of the larger effective rotational volume. The present results indicate that the rotational dynamics of [Co(acac)3] in solution is quite sensitive to the solute–solvent interactions in the outer coordination sphere as well as the viscosity of solution. [ABSTRACT FROM AUTHOR]