Synthesis and variable coordination modes of a bis-thiophene-appended macrocycle in complex with cobalt(III)

The bis-thiophene appended cyclam derivative L³ (trans-6,13-dimethyl-6,13-bis(thiophene-3′-ylmethylamino)-1,4,8,11-tetraazacyclotetradecane) has been synthesised, characterised and complexed with Co^III. The crystal structure of the diprotonated ligand as its cyanoborohydride salt H₂L³]NCBH₃]₂ is reported. Variable coordination modes to Co^III have been identified that find the ligand binding either as a tetradentate (cyclam-like) macrocycle in the structurally characterised complexes trans-CoL³Cl₂]Cl and trans-CoL³(NCBH₃)(OH)]Cl or as a hexadentate in CoL³]Cl₃ where both functionalised exocyclic amines coordinate in trans coordination sites. In this case, we have found that the structure of the hexadentate coordinated complex CoL³]³⁺ is significantly tetragonally elongated due to steric effects of the thiophene rings and this also leads to a very large 500 mV anodic shift in the Co^III/II redox potential relative to the unsubstituted hexaamine complex of Co^III.