Crystallization behaviour of meso-Λ(δλ)-[Co{trans-(R,S)(meen)2}(C2O4)][Pb2Cl5]·2H2O, Λ(λλ)-[{[Co{trans-(R,R)(meen)2}(C2O4)]Br}2]·3H2O and meso-Δ(δλ)-[Co{cis-(R,S)(meen)2}(C2O4)]I (meen =N-methylethylenediamine)

Abstract

Proton NMR spectroscopy of a D₂O solution of the solid obtained by evaporating to dryness a reaction solution containing HCl, [Co(O₂CMe)₂]·4H₂O, PbO₂, H₂C₂O₄ and meen (meen =N-methylethylene-diamine in water revealed the presence of two different isomers of the cation [Co(meen)₂(C₂O₄)]⁺. Three isomers [Co(meen)₂(C₂O₄)][Pb₂Cl₅]·2H₂O 1, [{[Co(meen)₂(C₂O₄)]Br}₂]2 and [Co(meen)₂-(C₂O₄)]l 3 were isolated eventually by the selective use of appropriate counter anions and their crystal structures determined. The ¹H NMR spectra of D₂O solutions of pure crystalline material from the same batch as that from which the structural crystals were derived showed the presence of single species of each of the three isolated compounds. Given their structures it is evident that the isomer with both methyls in the equatorial plane (trans to the oxalato oxygens) was either not formed or it rearranged soon after being formed. Complexes 1 and 2 are conformational isomers differing only in the relative position of one of the two axial methyl groups; 1 is stable in solution for at least 1 week at 21 °C, as demonstrated by its ¹H NMR spectrum and rearrangement of cation 2 to 1 and 3 takes several days. Compound 3 contains the expected axial–equatorial isomer.