Structure of Solvation Sphere of Tris(acetylacetonato)chromium(III) in Acetonitrile

Two experimental techniques were used to elucidate how acetonitrile molecules are bound to the title complex, [Cr(acac)3]: (i) measurement of NMR longitudinal relaxation rates of the dipolar nuclei of the solvent (1H, methyl-13C, and cyano-13C of CH3CN) in the presence of [Cr(acac)3], which is a paramagnetic complex, and (ii) X-ray diffraction measurements on acetonitrile solutions of [Cr(acac)3] and [Ru(acac)3] using an isostructural substitution method. The results of both methods are in harmony with each other. Four acetonitrile molecules are found within about 6 × 10-10 m from the chromium atom. They are in the hollows on the peripheral surface of the three acetylacetonato ligands, one or two in the hollows along the C 3 axis of the complex, and two or three in the hollows along the C 2 axes. They are oriented with the methyl ends inward: the angle between the molecular axis of acetonitrile and the vector from the chromium atom to the methyl carbon of acetonitrile was about 100°.