Efficient stereochemical regulation of octahedral cobalt(III) complexes by a chiral bidentate ligand. Part 1. Effect of N-alkyl substitutions

Abstract

A series of octahedral cobalt(III) complexes involving a tetradentate diaminedicarboxylate and a chiral diamine was prepared: [CoL(am)]⁺ where L = ethylenediamine-N,N′-diacetate (edda^2–), or its N,N′-dimethyl (dmedda^2–) and N,N′-diethyl (deedda^2–) derivatives; am =(1R),(2R)-cyclohexane-1,2-diamine (R,R-chxn), or its N,N′-dimethyl (R,R′-dmchxn) and N,N′-diethyl (R,R-dechxn) derivatives. The complexes were separated into their diastereoisomers using cation-exchange column chromatographies, and characterized by elemental analyses, electronic absorption, circular dichroism and ¹³C NMR spectroscopies The crystal structure of a dominant product in the [Co(edda)(R,R-dmchxn)]⁺ system, (–)₅₅₀[Co(edda)(R,R-dmchxn)]ClO₄·H₂O, has been established, and the stereochemistry determined to be Δ-sym-cis; orthorhombic, space group P2₁2₁2₁, a= 13.469(2), b= 19.184(3), c= 7.917(1)Å, Z= 4, R= 0.067, R′= 0.055, N-Alkyl substitutions on the edda ligand and those on the R,R-chxn ligand influence the stereochemistry of the cobalt(III) complexes in different ways. The former caused stereoselective formation of ∧-sym-cis forms, the latter preferential formation of Δ-sym-cis forms. High stereoselectivity was observed in the formation of [Co(deedda)(R,R-chxn)]⁺, yielding a final product distribution of 96%∧-sym-cis and 4%Δ-sym-cis forms, whereas [Co(edda)(R,R-dechxn)]⁺ yielded 2%∧-sym-cis and 98%Δ-sym-cis forms. The latter is due to steric interaction between N-alkyl substituents in the R,R-chxn chelate and acetate rings in the edda chelate, while the former is due to steric interaction between N-alkyl substituents in the edda chelate and NH in the R,R-chxn chelate. Such interactions are efficient in steric regulation of octahedral cobalt(III) complexes.