The Isomers of the Bis(Diethylenetriamine)Chromium(III) and Bis[Di(3-Aminopropyl)Amine]Chromium(III) Cations, [Cr(Dien)2]3+ and [Cr(Dpt)2]3+: Preparation and Characterization

Abstract

Addition of dien in slight excess to CrCl₃.6H₂O in Me₂SO (which has been dehydrated at 190ºC, then cooled to 170º) produces an isomeric mixture of orange-yellow [Cr( dien )₂] Cl₃.Me₂SO of   approximate composition mer 70%; s- fac 30%; u- fac <2%. The isomers are partially separated by dissolving the mixture in 3M HCl and adding ZnCl₂; pure lemon-yellow s- fac -[Cr( dien )₂] (ZnCl₅) then deposits. Addition of NaClO₄ to the mother liquor results in the crystallizationof an orange-yellow mer/u-fac -[Cr( dien )₂] (ClO₄)₃ (>98% mer ) mixture. All three isomers can be obtained as (Hg2Cl₇)^3- salts, suitable for single-crystal X-ray analysis, and the cation geometries have been established from these X-ray data. The mer and s- fac isomers are distinguished by their different labilities in acidic solution ( mer is relatively inert, s- fac decomposes rapidly), their different infrared and visible absorption spectra, and their different chromatographic behaviour on SP- Sephadex C-25. However, this latter technique failed to detect the u- fac form and was complicated by the facile decomposition of the s- fac isomer. The mer and s- fac forms isomerize rapidly in Me₂SO and the equilibrium is largely towards the mer. [Cr( dpt )₂]Cl₃ [ dpt = di(3- aminopropyl )amine] could be obtained only by direct reaction between anhydrous CrCl₃ and dpt , and was subsequently precipitated as the iodide and perchlorate salts. These consisted entirely of the mer isomer, from their isomorphism with the corresponding mer -[Cr( dpt )₂ ]³⁺ salts. The six-membered rings in -[Cr( dien )₂ ]³⁺ are considerably more resistant to hydrolytic decomposition than the rings in mer -[ cr ( dien )₂]³⁺ and mer -[Co( dpt )₂]³⁺.