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Trichloroisocyanuric acid (TCCA) is an emerging reagent for various oxidation

As the free carbene is not stable, we thought of reacting trichloroisocyanuric acid (TCCA) as a chlorinating 
agent with the bismidazolium salt 1. We found TCCA already superior to other halogenation agents in benzylic chlorination of 3,6-dimethylpyridazine, a key step in the synthesis of the bis(imidazolium) salt. For a few years TCCA is an emerging reagent for various oxidation and chlorination reactions due to its mild reactivity and good selectivity as well as due to its good safety properties.Recently, we reported on the synthesis and reactivity of a bidentate pyridazine-annelated biscarbene ligand as an analog of 2,2′-bipyridine or 1,10-phenanthroline. To broaden the scope of methods to prepare metal complexes with this 
biscarbene ligand, we were interested in the synthesis of the respective di- and monochlorinated bis(imidazolium) salts 2 and 3. 
As all three chlorine substituents of trichloroisocyanuric acid (TCCA) can be transferred to the product, we used 0.66 equivalents TCCA for one equivalent of the bis(imidazolium) salt 1 in acetonitrile. In addition, the presence of a small 
amount of base is necessary to initiate or catalyze the reaction. We found that triethylamine works well, while – for reasons given further below  the use of acetate is not suitable. After 4 h at 40°C, the reaction mixture was worked up 
and the dichlorination product 2 isolated by precipitation with diethylether as a colorless crystalline solid in 85% yield. 
The trichloroisocyanuric acid (TCCA) anion could then act as a base to deprotonate either the triethylammonium salt to close the catalytic cycle or directly the imidazolium moiety, in which case NEt3 would act only as an initiator. On the other hand, it was suggested in the literature that tertiary amines are chlorinated by TCCA forming [ClNEt3]+ and the TCCA anion which, in our case (mechanism B), could deprotonate the imidazolium salt. The formed carbene would then reactwith [ClNEt3]PF6 under formation of the chloroimidazolium moiety and release of NEt3.