Chiral metal complexes 44. Enantiomeric discrimination in ternary cobalt(III) complexes of N,N′-dimethyl-N,N′-di(2-picolyl)-1S,2S-diaminocyclohexane and α-amino acids; including the crystal structure of the S-prolinato complex

When the C₂-symmetrical complex Λ-α[Co(S,S-picchxcMe₂Cl₂]⁺ (S,S-picchxnMe₂ is N,N′-dimethyl-N,N′-di(2-picolyl)-1S,2S-diaminocyclohexane) is reacted with the α-amino acids proline (proH), alanine (alaH) or 2-amino-2-methylpropandioic acid (AMMAH₂), the two chloride ions are replaced by the O,N-bidentates with retention of configuration at the metal centre. The steric environment generated by the metal complex is shown to enantiospecifically discriminate in favour of S-proline from a racemic mixture due to the steric requirements of this amino acid. With the less bulky alanine and AMMAH₂ ligands there is no enantiomeric preference observed upon their coordination. When the mixed diastereoisomeric Λ-α-[Co(S,S-piccchxnMe₂) (AMMAH)]²⁺ precursors are decarboxylated under warm acidic conditions, neither hand of alanine is found to predominate in the product. The crystal structure of the Λ-α-[Co(S,S-picchxnMe₂) (S-pro)] (ClO₄)₂· H₂O salt of the sole S-prolinate product was determined by X-ray diffraction methods. Crystal data: C₂₅H₂₈N₅O₁₁Cl₂Co is orthorhombic, space group P2₁2₁2₁, with a = 11.082(3), b = 13.838(4), c = 19.387(5) A ̊ and Z = 4. The structure was refined by least-squares methods to R = 0.033 for 2672 diffractometer data. The Co atom is six-coordinate with the N₄-tetradentate and the O,N-bidentate amino acidate comprising the octahedral coordination sphere. The average CoN(pyridine) and Coz.sbnd;N(tert-amine) distance are 1.96 and 2.00 Å, respectively.