Triapine Analogues and Their Copper(II) Complexes: Synthesis, Characterization, Solution Speciation, Redox Activity, Cytotoxicity, and mR2 RNR Inhibition

Abstract

Three new thiosemicarbazones (TSCs) <b>HL</b>^<b>1</b>-<b>HL</b>^<b>3</b> as triapine analogues bearing a redox-active phenolic moiety at the terminal nitrogen atom were prepared. Reactions of <b>HL</b>^<b>1</b>-<b>HL</b>^<b>3</b> with CuCl₂·2H₂O in anoxic methanol afforded three copper(II) complexes, namely, <b>Cu(HL</b>^<b>1</b><b>)Cl</b>_<b>2</b> (<b>1</b>), [<b>Cu(L</b>^<b>2</b><b>)Cl]</b> (<b>2'</b>), and <b>Cu(HL</b>^<b>3</b><b>)Cl</b>_<b>2</b> (<b>3</b>), in good yields. Solution speciation studies revealed that the metal-free ligands are stable as <b>HL</b>^<b>1</b>-<b>HL</b>^<b>3</b> at pH 7.4, while being air-sensitive in the basic pH range. In dimethyl sulfoxide they exist as a mixture of <i>E</i> and <i>Z</i> isomers. A mechanism of the <i>E/Z</i> isomerization with an inversion at the nitrogen atom of the Schiff base imine bond is proposed. The monocationic complexes <b>[Cu(L</b>^{<b>1</b>-<b>3</b>}<b>)]</b>^<b>+</b> are the most abundant species in aqueous solutions at pH 7.4. Electrochemical and spectroelectrochemical studies of <b>1</b>, <b>2'</b>, and <b>3</b> confirmed their redox activity in both the cathodic and the anodic region of potentials. The one-electron reduction was identified as metal-centered by electron paramagnetic resonance spectroelectrochemistry. An electrochemical oxidation pointed out the ligand-centered oxidation, while chemical oxidations of <b>HL</b>^<b>1</b> and <b>HL</b>^<b>2</b> as well as <b>1</b> and <b>2'</b> afforded several two-electron and four-electron oxidation products, which were isolated and comprehensively characterized. Complexes <b>1</b> and <b>2'</b> showed an antiproliferative activity in Colo205 and Colo320 cancer cell lines with half-maximal inhibitory concentration values in the low micromolar concentration range, while <b>3</b> with the most closely related ligand to triapine displayed the best selectivity for cancer cells versus normal fibroblast cells (MRC-5). <b>HL</b>^<b>1</b> and <b>1</b> in the presence of 1,4-dithiothreitol are as potent inhibitors of mR2 ribonucleotide reductase as triapine.