Negatively charged Ir(iii) cyclometalated complexes containing a chelating bis-tetrazolato ligand: synthesis, photophysics and the study of reactivity with electrophiles

Abstract

The bis-tetrazolate dianion [1,2 BTB]²⁻, which is the deprotonated form of 1,2 bis-(1H-tetrazol-5-yl)benzene [1,2-H₂BTB], is for the first time exploited as an ancillary N^N ligand for negatively charged [Ir(C^N)₂(N^N)]⁻-type complexes, where C^N is represented by cyclometalated 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)pyridine (F₂ppy). The new Ir(III) complexes [Ir(ppy)₂(1,2 BTB)]⁻ and [Ir(F₂ppy)₂(1,2 BTB)]⁻ have been fully characterised and the analysis of the X-ray structure of [Ir(ppy)₂(1,2 BTB)]⁻ confirmed the coordination of the [1,2 BTB]²⁻ dianion in a bis chelated fashion through the N-atoms adjacent to each of the tetrazolic carbons. Both of the new anionic Ir(III) complexes displayed phosphorescence in the visible region, with intense sky-blue (λ_max = 460–490 nm) or aqua (λ_max = 490–520 nm) emissions originating from [Ir(F₂ppy)₂(1,2 BTB)]⁻ and [Ir(ppy)₂(1,2 BTB)]⁻, respectively. In comparison with our very recent examples of anionic Ir(III)tetrazolate cyclometalates, the new Ir(III) tris chelate complexes [Ir(F₂ppy)₂(1,2 BTB)]⁻ and [Ir(ppy)₂(1,2 BTB)]⁻, display an improved robustness, allowing the study of their reactivity toward the addition of electrophiles such as H⁺ and CH₃⁺. In all cases, the electrophilic attacks occurred at the coordinated tetrazolate rings, involving the reversible – by a protonation deprotonation mechanism – or permanent – upon addition of a methyl moiety – switching of their global net charge from negative to positive and, in particular, the concomitant variation of their photoluminescence output. The combination of the anionic complexes [Ir(F₂ppy)₂(1,2 BTB)]⁻ or [Ir(ppy)₂(1,2 BTB)]⁻ with a deep red emitting (λ_max = 686 nm) cationic Ir(III) tetrazole complex such as [IrTPYZ-Me]⁺, where TPYZ-Me is 2-(2-methyl-2H-tetrazol-5-yl)pyrazine, gave rise to two fully Ir(III)-based soft salts capable of displaying additive and O₂-sensitive emission colours, with an almost pure white light obtained by the appropriate choice of the ionic components.