X-Ray structure, absolute configuration, and circular dichroism spectra of cobalt and chromium malonate complexes as their diastereoisomers, Δ[Co{(–)1,2-diaminopropane}3]Λ[M(malonate)3],3H2O

Abstract

The complexes K₃[M(malonate)₃](M = Co or Cr) have been resolved via their diastereoisomers with (–)₅₈₉[Co{(–)1,2-(diaminopropane)}₃]Br₃. The only diastereoisomers precipitated are shown to have the absolute configurations Δ[Co{(–)1,2-diaminopropane}₃]Λ[M(malonate)₃],3H₂O where M is (+)₅₈₉Cr or (–)₅₈₉Co. Crystals of the chromium diastereoisomer are rhombohedral, space group R₃₂, with hexagonal lattice parameters, a= 16.12(2), c= 10.07(2)Å, and Z= 3. The structure was solved by Patterson and Fourier methods, and refined by full-matrix least-squares calculations to R 0.073 for 435 independent reflexions collected photographically. Both complex ions are disordered on sites of symmetry 32, and are linked in columns by three N–H ⋯ O hydrogen bonds. The columns of complexes are connected by hydrogen bonds involving both anion and cation. The cation adopts the expected Δ(λλλ) conformation with the methyl groups equatorial on the gauche rings. The malonate chelate ring skeleton is planar except for the methylene carbon atom. The absolute configuration of the (+)₅₈₉[Cr(malonate)]₃^3– ion is opposite to an earlier assignment based on spectroscopic data. The circular dichroism spectra of the diastereoisomers as KBr discs show one less peak than is observed in the solution spectra of both Δ[M(malonate)₃]³⁺ complexes in the region of their T_1g transitions. The remaining peak has the same sign for both cobalt and chromium complexes. In these spectra and in those of the diastereoisomers Λ[M(oxalate)₃]Λ[Co(ethylenediamine)₃](M = Co or Cr), the region of the T_1g transition of the tris(diamine) cations resemble the c.d. spectra of these cations in solution with added phosphate ion. The structure of the tris(malonate) diastereoisomer shows a similar hydrogen bonding scheme to that of a phosphate salt of the cation. In these diastereoisomers the complex cations have a similar effect in influencing the transition moments of the oxalato- and malonato-complex anions and it is therefore not readily possible to interpret solid to solution spectral changes in stereochemical terms.