Synthesis, characterisation and polynuclearisation reaction of trans(S)-[Co(aminothiolate-N,S)2(en)]-type cobalt(III) complexes with 2-aminoethanethiolate, L-cysteinate and D-penicillaminate

Abstract

The trans(S)-[Co(N)₄(S)₂]-type mononuclear complexes [Co(L)₂(en)]^+ or – (L = 2-aminoethanethiolate (aet) 1, L-cysteinate (L-cys) 3 or D-penicillaminate (3-sulfanyl-D-valinate) (D-pen) 5) were newly prepared by the reaction of trans-[CoCl₂(en)₂]⁺ with L at pH 8.5 and 25 °C. The crystal structure of 1 was determined by X-ray diffraction analysis. The geometry around the cobalt atom is approximately octahedral and two thiolate donor atoms in the aet ligands occupy trans positions to each other. Two Co–S bonds are lengthened by the double sulfur trans influence across the cobalt centre attributable to two thiolato sulfur donor atoms. The diastereomers of 3 and 5 were separated by the column chromatographic technique and characterised on the basis of their UV/Vis, CD and ¹³C NMR spectra. The Λ_DD isomer of 5 (5Λ_DD) is preferably formed by attractive (COO· · ·H–N–H) interactions. Though cis(S)-[Co(L)₂(en)]^+ or – (L = aet 2, L-cys 4 or D-pen 6) were also produced by the same preparative reaction, they could not be separated into the geometrical isomers because of very rapid isomerisation and/or polynuclearisation reaction during elution from the column. It is clear that all trans(S) complexes also show drastic and characteristic UV/Vis spectral changes with time in aqueous solutions and tend to isomerise to the cis(S) isomer, followed by the formation of S-bridged polynuclear complexes owing to both the double sulfur trans influence and high nucleophilicity of the thiolate donor atoms. The polynuclearisation reactions of the three trans(S) isomers proceed according to different processes depending on their ligands. In the polynuclearisation reaction of 3, especially, the novel dinuclear complex Λ_LLL-[Co{Co(L-cys-N,S)₃}(L-cys-N,O,S)]^2–7, which has not so far been identified in usual dinuclearisation reactions, was newly obtained.