The first report of a tetra-azide bound mononuclear cobalt(iii) complex and its comparative biomimetic catalytic activity with tri-azide bound cobalt(iii) compounds

Abstract

Three new azide-bound cobalt(III) complexes, [Co(HL₁)(N₃)₄] (1), [Co(L₂)(N₃)₃] (2) and [Co(L₃)(N₃)₃] (3), where L₁, L₂ and L₃ are N,N-dimethyldipropylenetriamine, N¹-isopropyldiethylenetriamine and N,N-diethyldiethylenetriamine, respectively, were synthesized and structurally characterised. X-ray crystallographic studies reveal that the structures of both 2 and 3 are very similar in which three terminal azide ions together with a triamine coordinate the metal centre. Complex 1 on the other hand is significantly different from the other two as the metal centre in 1 is bonded with four terminal azide ions and two donor sites of triamine L₁, leaving the tertiary amine group protonated. All the complexes are stabilized by rich hydrogen bonding interactions, leading to hydrogen bonded supramolecular chain structures. It is worth noting that complex 1 is the first example in cobalt(III) coordination chemistry in which all four azide ions coordinate the metal centre terminally. All these compounds exhibited efficient catalytic activity towards the oxidative coupling of o-aminophenols to phenoxazinone chromophores under aerobic conditions and the role of the structural factors in the catalytic activity has been explored. A mass spectrometry study was carried out to identify the products and important reactive intermediates and to support the mechanistic proposal.