Tris 3-bromoacetylacetonato chromium III

The direct substitution of bromine into the chelate rings of metal acetylacetonates is an example of the reaction of a coordinated ligand. Tris(3-bromoacetylacetonato)chro-mium(III) has been prepared by the action of elementary bromine on chromium(III) acetylacetonate [2,4-pentane-dionatochromium(III)]. The procedure given here is 

To a solution of 5.0 g. (0.014 mol) of chromium(III) acetylacetonate in 75 ml. of chloroform is added 8.0 g. (0.044 mol) of iV-bromosuccinimide, t and the mixture is heated to boiling for 5 minutes in a hood. During this period, mechanical agitation is necessary to prevent bumping. The initially violet solution becomes deep green and a brown precipitate forms. The mixture is transferred to an evaporating dish and the solvent is removed under a stream of air in a hood. 

The brown solid is collected on a suction filter and washed successively with 15 ml. of 95% ethanol, two 15-ml. portions of 5% aqueous sodium hydrogen sulfite solution, 20 ml. of water, and two 20-ml. portions of hot 95% ethanol. Although the ethanol washings are green, little product is lost in this step. 

The air-dried brown powder is dissolved in 50 ml. of boiling benzene the solution is filtered through a fluted filter and combined with 100 ml. of boiling heptane. The mixture is allowed to cool to room temperature over at least a 4-hour period, then chilled in an ice bath and filtered. The brown crystals are washed with two 10-ml. portions of 95% ethanol and air-dried. The yield is 5.8 to 6.2 g. (70 to 75%). The melting point is 227 to 229°. 

Tris(3-bromoacetylacetonato)chromium(III) is a dark red-brown crystalline material, which dissolves in benzene to form a green solution. The infrared spectrum of this chelate exhibits a characteristic strong singlet at 1540 cm. i, whereas chromium(III) acetylacetonate exhibits two peaks in this region, at 1560 and 1520 cm. b The ultraviolet spectrum of the brominated chromium chelate in chloroform exhibits a Xmax at 358 m/i(e = 13,070). The brominated chelate is reported to form a stable clathrate complex with chloroform (m.p. 240 to 241°).  

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