Annealing 1,2,4-Triazine to Iridium(III) Complexes Induces Luminogenic Behaviour in Bioorthogonal Reactions with Stain Alkynes.

Phenanthroline-type ligand containing annealed 1,2,4-triazine ring was used to prepare novel Ir(III) complexes 3 and 4. The complexes are non-luminescent but show luminogenic behaviour following inverse electron demand Diels-Alder (IEDDA) reaction with bicyclononyne (BCN) derivatives. It was observed that the complex 3 reacts with BCN-C10 three times faster than the corresponding free ligand 7. The magnitude of this accelerating metal-coordination effect, however, is less profound that in previously reported Ir(III) complexes of 1,2,4-triazines in which the triazine was directly coordinated to Ir(III) metal centre. Nevertheless, luminogenic behaviour opens prospects for the use of such complexes in bioimaging applications, which was demonstrated by developing a convenient methodology using “chemistry on the complex” concept for labelling antibodies with luminescent Ir(III) complexes. Bioorthogonal reactivity of the complex 4 was then evaluated by metabolically labelling live cells with BCN groups followed by luminogenic IEDDA reaction with the triazine iridium complex.