Chiral metal complexes 34. Stereospecific cis-α coordination to cobalt(III) by the new tetradentate ligand N,N′-dimethyl-N,N′-di(2-picolyl)-1,2-diaminocyclohexane

The new tetradentate N,N′-dimethyl-N,N′-di(2-picolyl)-1,2-diaminocyclohexane (picchxnMe₂) has been synthesised, in both optically active enantiomeric forms, by N,N′-dimethylation of the previously described stereospecific cis-β tetradentate N,N′-di(2-picolyl)-1,2-diaminocyclohexane (picchxn). The ligand has been shown to adopt stereospecific cis-α topology when binding to Co(III). R,R-PicchxnMe₂ coordinates to Co(III) to give the Δ-α-[Co(R,R-picchxnMe₂)Cl₂]⁺ cation stereospecifically. The nature of the intramolecular interactions which induce the change of coordination mode are commented on. The chloride donors in this cation may undergo substitution by nitrite, oxalate or aminoacidate ligands with full retention of Δ-α topology as demonstrated by the chiroptical properties of the products. The S,S antipode of the ligand was used to generate the corresponding enantiomeric chelate forms including the alanine complex Λ-α-[Co(S,S-picchxnMe₂) (R-ala)](ClO₄)₂ for which the crystal structure has been determined. Crystal data: C₂₃H₃₄N₅O₁₀Cl₂Co, M_r=670.4, orthorhombic, space group P2₁2₁2₁, with a=10.525(5), b=14.697(12), c=18.594(8) Å and Z=4. The structure was refined by least-squares methods to R=0.047 for 2270 non-zero reflexions and confirms unambiguously the absolute configuration at the metal centre. The tetradentate ligand completes an octahedral coordination sphere on the cobalt ion, with the central chelate ring adopting a δ conformation with a NCCN torsion angle of 50.9(5)°. The average CoN distance was found to be 1.97(1) Å and the average NCoN bite angle is 84.0(3)°.