Anhydrous Chromium III Bromide

Yield 8.5 g (93%, based on chromium loss at the anode). Anal. Calcd. for CrBr3 Cr, 17.8 Br, 82.2. Found Cr (atomic absorption), 17.8 Br (AgNOs/KCNS titration), 81.8%. 

The preparation of salts containing the [Cr(en)3]3+ cation from anhydrous chromium sulfate has been described previously in Inorganic Syntheses,1 and the merits of this, and other, methods have been reviewed.9 A more rapid route to this cation involves refluxing CrCl3 6H20 in methanol with ethylenediamine and zinc metal, which allows the substitution to proceed by way of the kineti-cally labile chromium(II) species.10 All of these preparations yield hydrated salts the procedure described below leads to anhydrous [Cr(en)3] Br3. 

Approximately 0.5 g (1.7 mmole) of CrBr3, as the dark-green oil derived in Section A, is added dropwise to 15 mL of colorless ethylenediamine (reagent grade). An exothermic reaction occurs, following which the mixture is allowed to cool to room temperature, resulting in the formation of yellow crystals. These are collected, washed with diethyl ether (2 X 25 mL), and dried in vacuo. Yield 0.74 g (1.55 mmole, 90%). Anal. Calcd. for C6HMN6CrBr3 Cr, 11.1 Br, 51.2. Found Cr, 11.1 Br, 51.4%. 

Freshly prepared [Cr(en)3] Br3 is a yellow solid which slowly turns violet-blue, probably because of some photochemically induced reaction (cf. Reference 9). The compound is sparingly soluble in water, from which it can be recrystallized as the hydrate. The resolution of [Cr(en)3]3+ salts by means of tartrate has been described in a previous volume of Inorganic Syntheses.11 

HEXAKIS(DIMETHYL SULFOXIDE)CHROMIUM(III) BROMIDE, [Cr-(DMSO)6]Br3 

---