Arylhydrazone ligands as Cu-protectors and -catalysis promoters in the azide-alkyne cycloaddition reaction

A series of water soluble copper( ii ) complexes, [Cu(κ O 1 O 2 N -H 2 L 1 )(H 2 O) 2 ]·2H 2 O ( 2 ), [Cu(κ O -H 3 L 1 ) 2 (H 2 O) 4 ] ( 3 ), [Cu(κ O -H 4 L 2 ) 2 (H 2 O) 4 ] ( 5 ) and [Cu(H 2 O) 6 ]·2H 2 L 3 ·2(CH 3 ) 2 NCHO ( 7 ), were prepared by the reaction of Cu(NO 3 ) 2 ·3H 2 O with sodium ( Z )-2-(2-(1-amino-1,3-dioxobutan-2-ylidene)hydrazineyl)benzenesulfonate, [Na(μ 4 -1:2κ O 1 ,2κ O 2 ,3κ O 3 ,4κ O 4 -H 3 L 1 )] n ( 1 ; for 2 and 3 ), sodium ( Z )-3-(2-(1-amino-1,3-dioxobutan-2-ylidene)hydrazineyl)-4-hydroxybenzene-sulfonate, [Na(μ-1κ O 1 ,2κ O 2 -H 4 L 2 )] 2 ( 4 ; for 5 ) or sodium ( Z )-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazineyl)naphthalene-1-sulfonate, [Na(μ-1κ O 1 O 2 ,2κ O 3 -H 2 L 3 )(CH 3 OH) 2 ] 2 ( 6 ; for 7 ). Compounds 1-7 were fully characterized, also by single-crystal X-ray diffraction analysis, and applied as homogeneous catalysts for the azide-alkyne cycloaddition (AAC) reaction to afford 1,4-disubstituted 1,2,3-triazoles. A structure-catalytic activity relationship has been recognized for the first time on the basis of the occurrence of resonance- and charge-assisted hydrogen bond interactions (RAHB and CAHB), in charge and ligand binding modes, enabling the catalytic activity of the compounds to be ordered as follows: Cu(NO 3 ) 2 < 7 (complex salt with RAHB and CAHB) < 3 (with RAHB and CAHB) < 5 (with RAHB) < 2 (neither RAHB nor CAHB). Complex 2 , without such non-covalent interactions, was found to be the most efficient catalyst for the AAC reaction, affording up to 98% product yield after being placed for 15 min, at 125 °C, in a water/acetonitrile mixture under low power (10 W) MW irradiation. Water-soluble copper( ii )-arylhydrazone complexes were synthesized and applied as catalysts in azide-alkyne cycloaddition, with the most active one lacking intramolecular hydrogen bond contacts.