Chiral metal-complexes .24. Diastereoisomerism in ternary complexes of the type [(aminoacidate) (3R-methyl-1,6-di(2-pyridyl)-2,5-diazahexane)cobalt(III)]

A series of complexes of the type β-[Co(R-picpn) (R- or S-aa)]n+, where R-picpn is 3R-methyl-6-di(2-pyridyl)-2,5-diazahexane and aa is an α-aminoacidate, have been synthesised by reaction of Δ-[Co(R-picpn)Cl2]+ with the appropriate aminoacid in aqueous solution. The aminoacids used were valine, phenylalanine, tryptophane and proline, all of which have relatively bulky side-chains. The number of diastereoisomers produced with each chiral aminoacid, with the exception of proline, shows that variation of the size of the aminoacid side-chain has little effect in enforcing any particular geometry on the tetradentate in these ternary complexes. It is evident that more extensive substitution on the central chelate ring of the tetradentate is necessary to enforce its stereoselective coordination to Co(III).

The crystal and molecular structure of Δ-β1,-exo-[Co(R-picpn) (S-pro)](ClO4)2, where S-pro is the S-prolinate anion, has been determined. The crystals are orthorhombic, space group P212121, with a = 10.305(1), b = 10.857(1) and c = 22.650(2) Å · and Z = 4. The structure was refined by full-matrix least-squares methods to R = 0.051 for 2161 non-zero reflexions. This structural analysis proves the geometry of the complex, as had been deduced previously by nmr methods, to be correct in every detail, with both R-picpn amine nitrogen atoms having an S absolute configuration. The correlation of the spectroscopic characteristics of other related species, especially those derived from CD and nmr measurements, is made possible by the determination of the molecular structure of this S-pro complex.