Synthesis, properties and reactivity of BCl2 aza-BODIPY complexes and salts of the aza-dipyrrinato scaffold

The synthesis and characterisation of the BCl2-chelated complexes of both archetypal aza-dipyrrin sub-types are presented. A stepwise halogen exchange, leading to a mixed-halide Cl-B-F intermediate, is implicated in the conversion of F-aza-BODIPYs to Cl-aza-BODIPYs upon treatment with BCl3. The utility of the Cl-aza-BODIPY scaffold to facilitate substitutions at boron is demonstrated under mild conditions through treatment with aryl Grignard reagents. Additionally, the lability of the B-Cl bond enables facile removal of the BCl2 group, i.e. deprotection of F-aza-BODIPYs, under aqueous conditions. Three aza-dipyrrin HX salts were also synthesised and characterised. The pK(a) of the protonated aza-dipyrrin was determined to be 4, thereby providing insight regarding the storage and stability of such species.