Synthesis, Crystal Structure, and Magnetic Properties of (mu)-Hydroxo-bis[pentakis(acetonitrile)chromium(III)] Tetrafluoroborate: An Acetonitrile Analogue to "Acid Rhodo"

The reaction of [Cr(NCCH3)6]2+ with dioxygen in acetonitrile (MeCN) solution acidified with HBF4 gave red crystals of the binuclear complex [(CH3CN) 5Cr(OH)Cr(NCCH3)5](BF4)5 (1). From the X-ray crystal structure of 1, the Cr-O-Cr angle was found to be 147.5(2)°. Magnetic susceptibility measurements of 1 showed an antiferromagnetic coupling between the two chromium(III) centers with a triplet energy J = 35.9(1) cm^-1. On redissolution of 1 in MeCN, the hydroxo bridge was deprotonated, and a green solution of the complex [(CH3CN)5CrOCr(NCCH3)5]4+ formed. The electronic absorption spectrum of this solution is very similar to the spectrum of the classical complex [(H3N)5CrOCr(NH3)5]4+ with intense bands in the UV and near-UV region. From the temperature dependence of the absorption spectrum near 12900 cm^-1, the triplet energy J was found to be 1067(19) cm^-1.