Excellent chiralselectivity in sulfur-bridged CoIIIMCoIII (M = NiII and PdII) trinuclear complexes containing 1,2-cyclohexanediamine

Abstract

The reaction of [Ni(aet)₂] with [CoCl₂(R,R-chxn)₂]⁺ (aet = 2-aminoethanethiolate, R,R-chxn = 1R,2R-cyclohexanediamine) in water gave a Co^IIINi^IICo^III trinuclear complex, Δ_RRΔ_RR-[Ni{Co(aet)₂(R,R-chxn)}₂]⁴⁺ ([1a]⁴⁺), in which two cis(S)-[Co(aet)₂(R,R-chxn)]⁺ units are linked by a central Ni^II ion through sulfur bridges. The two Co^III units in [1a]⁴⁺ uniformly adopt the Δ configuration, which is induced by the chirality of the terminal R,R-chxn ligands. The central Ni^II ion in [1a]⁴⁺ was replaced by a Pd^II ion to produce an analogous Co^IIIPd^IICo^III trinuclear complex, Δ_RRΔ_RR-[Pd{Co(aet)₂(R,R-chxn)}₂]⁴⁺ ([2a]⁴⁺), with retention of the Δ configuration. When racemic R,R/S,S-chxn was employed instead of R,R-chxn, not only the chirality about two Co^III centers but also the chirality about two chxn ligands was unified in the S-bridged trinuclear structure, leading to the selective formation of a pair of enantiomers, Δ_RRΔ_RR/Λ_SSΛ_SS-[M{Co(aet)₂(chxn)}₂]⁴⁺ (M = Ni^II ([1b]⁴⁺) and Pd^II ([2b]⁴⁺)). The stereochemical and spectroscopic features of these complexes are discussed on the basis of the electronic absorption, CD, and NMR spectroscopies, along with the crystal structures of [1a]⁴⁺ and [2a]⁴⁺.