Ligand control on the reactions of oxygen- and sulphur-bonded (sulphito)pentaminecobalt(III) complexes in solution

Abstract

The kinetics of formation, acid-catalysed aquation, ligand isomerisation (Co^IIIOSO₂⁺→Co^IIISO₃⁺), intramolecular electron-transfer, and base-catalysed hydrolysis and isomerisation of O-bonded sulphito complexes, cis-[Co(en)₂(B)(OSO₂-O)]⁺[B = benzimidazole (bzimH), N-methylimidazole (N-meim)] and the anation of cis-[Co(en)₂(B)OH)]²⁺ [B=bzimH, N-meim and imH (imidazole)] by SO₃^2- are reported. Steric acceleration is observed in the formation and acid-catalysed aquation of the O-sulphito complexes. The ligand isomerisation leads to loss of the monodentate amine with the formation of trans-[Co(en)₂(SO₃-S)²]^- (in an excess of sulphite). Steric acceleration is more pronounced in the isomerisation and base hydrolysis than in the redox process. The results indicate cis labilisation of the coordinated O-sulphite. The [(tetraethylenepentamine)Co(OSO₂-O)]⁺ cation undergoes base hydrolysis 103 times faster than the corresponding (en)2(B) complexes; base-catalysed ligand isomerisation for the former is not observed unlike in the latter. The anation of cis-[Co(en)2(B)OH]²⁺ (B=imH, bzimH, N-meim) by SO₂^-3 in a mild alkali pH range (pH=7.9-9.6) and in an excess of SO₂^-3, yields exclusively trans-[Co(en)2(SO₃-S)2]^- with no evidence for the formation of the cis-[Co(en)2(B)(SO₃-S)]⁺ or its O-sulphito analogue. The intramolecularly generated amido conjugate base of the sulphite ion-pair, {cis-[Co(en)₂(B)OH]²⁺,SO₃^2-}.⇋{cis-[Co(en)(en-H)(B)- OH₂]²⁺,SO₃^2-}, is believed to generate a five-coordinate intermediate (TBP) that captures the S-end of SO₂^-3 selectively from a site trans-to the amine B so that the amine is labilised by the trans effect of the sulphite. The NH-deprotonated coordinated imidazolate or benzimidazolate species, cis-[Co(en)₂(bzm/im)OH]⁺, do not undergo anation by SO₂^-3.