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†
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.