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

A series of water soluble copper(ii) complexes, [Cu(κO ¹ O ² N-H ₂ L ¹ )(H ₂ O) ₂ ]·2H ₂ O (2), [Cu(κO-H ₃ L ¹ ) ₂ (H ₂ O) ₄ ] (3), [Cu(κO-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 -1:2κO ¹ ,2κO ² ,3κO ³ ,4κO 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-1κO ¹ ,2κO 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-1κO ¹ O ² ,2κO 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.