Synthesis, Structure and Biological Activity of Novel 4,5-dihydro-1H-imidazol-2-yl-phthalazine Derivatives and Their Copper(II) Complexes ^†

As a continuation of our previous investigations aimed at the synthesis of novel nitrogen-containing heterocycles and their metal complexes, we have now prepared two series of compounds incorporating a phthalazine ring at the position C₂ of 4,5-dihydro-1H-imidazole. The starting phthalazine (I) in the reaction with 2-chloroimidazoline (II) gives rise to the formation of pseudobase III. Then, compound III upon treatment with HOSA yields betaine which under basic conditions gives 2-(4,5-dihydro-1H-imidazol-2-yl)phthalazin-1(2H)-imine (IV). In turn, the reactions of compound IV with a variety of acyl and sulfonyl chlorides lead to the formation of benzamides (V) and benzenesulfonamides (VI). Moreover, compounds V and VI can be transformed into corresponding 2-(4,5-dihydro-1H-imidazol-2-yl)phthalazin-1(2H)-one derivatives VII and VIII. Such ligands are susceptible to the reaction with CuCl₂ giving rise to the formation of corresponding copper(II) complexes: dichloro[2-(4,5-dihydro-1H-imidazol-2-yl)phthalazin-1(2H)-imine]copper(II) (1), dichloro[2-(1-benzoyl-4,5-dihydro-1H-imidazol-2-yl)phthalazin-1(2H)-one]copper(II) (2) and dichloro{bis-[2-(1-(phenylsulfonyl)-4,5-dihydro-1H-imidazol-2-yl)phthalazin-1(2H)-one]}copper(II) (3). The most promising results of biological studies were obtained for complex 1 towards the HeLa cell line (IC₅₀ = 2.13 μM) without a toxic effect against fibroblasts BALB/3T3 (IC₅₀ = 135.30 μM), which pointed towards its selectivity as a potential antitumor agent. It should be pointed out, that corresponding free ligand 2-(4,5-dihydro-1H-imidazol-2-yl)phthalazin-1(2H)-imine (IV) was less active than its metal complex (IC₅₀ = 87.74 μM).