A 2-pyridyl-2,1-borazaronaphthalene derivative as forefather of a new class​ of bidentate ligands: synthesis and application in luminescent Ir(III) complexes

Borazaro compounds (or azaborines) are aromatic compounds in which a C=C unit is replaced by an isoelectronic B-N unit. The possibility to generate chemical diversity has led to an increasing interest in azaborines, especially in the fields of biomedical research and optoelectonics. In particular, Dewar’s synthesis of borazaronaphthalene is a common starting step to obtain different 1,2-azaborines via nucleophilic substitution on the boron atom. Here we present the synthesis of a novel 1,2-azaborine (i.e. 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene, named FAAH) via functionalization of 2-chloro-4-methyl-2,1-borazaronaphthalene with a 2-pyridyl unit. FAAH can be used as an anionic bidentate ligand for transition metal complexes, since it can chelate the metal center with both the pyridine and the azaborine nitrogen atoms. FAAH was used for the synthesis of a series of neutral luminescent Ir(III) complexes (named FAV, FAB and FAR) of general formula [Ir(C^N )2(FAA)], where C^N indicates three different cyclometalating ligands: i.e. 2-phenylpyridine in the case of FAV; 2-(2,4-difluorophenyl)pyridine in the case of FAB; 2-methyl-3-phenylquinoxaline in the case of FAR. The reaction yields are quite low, however it was always possible to characterize all the compounds by means of NMR spectroscopy. A complete photophysical and theoretical characterization is also presented. FAAH displays a good chemical stability and a high photoluminescence quantum yield (up to 28 % in solution). On the contrary, the Iridium complexes undergo degradation over time in solution. Despite this stability problem, it was possible to get a good understanding of the photophysics of the three complexes: the emission of both FAV and FAB is observed around 500 nm and arises from a 3LC state centered on the azaborine ligand. In the case of FAR, the emitting state is basically 3MLCT/3LLCT in nature and the resulting broad and unstructured emission band is centered around 700 nm.