Efficient stereochemical regulation of octahedral cobalt(III) complexes by a chiral bidentate ligand. Part 2. Remarkable effect of the chelate-ring size in the stereoselective formation of sym-cis-(ethylenediamine-N,N′-diacetato)(pentane-2,4-diamine)cobalt(III)

Abstract

The chiral diamine (2R), (4R)-pentane-2,4-diamine (2R,4R-ptn) has been found efficiently to regulate the stereochemistry of the cobalt(III) complex [Co(edda)(2R,4R-ptn)]⁺(edda = ethylenediamine-N,N′-diacetate). The product ratio Λ-sym-cis:Δ-sym-cis is 3:97. The stereoselectivity is surprisingly high in comparison with that for the analogous complex involving (1R),(2R)-cyclohexane-1,2-diamine (R,R-chxn), Λ-sym-cis:Δ-sym-cis= 60:40. It is discussed on the basis of crystal structure determinations carried out for both diastereoisomers of sym-cis-[Co(edda)(2R,4R-ptn)]⁺. A notable difference was observed in the six-membered chelate ring of 2R,4R-ptn which adopts the chair conformation in Λ-sym-cis-[Co(edda)(2R,4R-ptn)]ClO₄·H₂O 1a but the λ skew-boat conformation in Δ-sym-cis-[Co(edda)(2R,4R-ptn)]ClO₄1b. The chair conformation in 1a is due to steric repulsions between the chelates, which is the important factor for the preferential formation of 1b over 1a. Such steric repulsions between the chelates are remarkably efficient for the chiral six-membered chelate compared with those for the five-membered one. N-Alkyl substitutions on the edda ligand caused preferential formation of the Λ-sym-cis form, as is the case for the [Co(R₂edda)(R,R-chxn)]⁺ system.