Novel cyano-activated Cu(ii) complexes of arylhydrazones of active methylene nitriles and their catalytic application for azide-alkyne cycloaddition in water and glycerol

The reaction of 2-(2-(dicyanomethylene)hydrazineyl)benzoic acid (H₂L¹) with copper(ii) acetate in the absence or presence of imidazole (im), 4,4′-bipyridine (4,4′-bipy) or pyridine (py) leads to the new complexes [Cu₂(CH₃OH)₂(μ-L^1a)₂] (1), [Cu(L^1a)(im)] (2), [Cu(L^1a)(H₂O)(4,4′-bipy)]·H₂O (3) or [Cu(L^1a)(py)]_n (4), respectively, where (L^1a)²⁻ = (Z)-1-(2-carboxylatophenyl)-2-(1-cyano-2-imino-2-methoxyethylidene)hydrazin-1-ide. A one-pot activation of nitrile groups in the reaction of copper(ii) acetate monohydrate with 2-(2-(dicyanomethylene)hydrazineyl)terephthalic acid (H₃L²) in the presence of pyridine in methanol affords the trinuclear Cu(ii) complex [Cu(py)₂{Cu(py)(μ-L^2a)}₂] (5), where (L^2a)³⁻ = (Z)-2-((1-cyano-2-imino-2-methoxyethylidene)-1-(2,5-dicarboxylatophenyl)hydrazin-1-ide). Both arylhydrazone ligands and the auxiliary ligands were used to modulate the nuclearity and design of the supramolecular arrangements of the obtained Cu(ii) complexes. The complexes were characterized by elemental analyses, electrospray ionization mass spectrometry (ESI-MS), and FT-IR spectroscopy. Their molecular structures were established using single crystal X-ray diffraction (SCXRD) analysis. The catalytic activity of the complexes was investigated for the microwave assisted 1,3-dipolar azide-alkyne cycloaddition reaction using a mixture of water and glycerol as reaction medium. Pre-catalyst 1 was found to be the most efficient one by affording a quantitative conversion to 1,4-disubstituted-1,2,3-triazoles after 30 minutes at 125 °C. The catalytic system proceeds with a broad scope of substrates according to “Click” rules.