Synthesis, Crystal Structure, and Magnetic Properties of μ-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. The acidity of the
hydroxo bridge in 1 is very high with an acid dissociation constant Ka ≫ 1 M.