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

A series of water soluble copper(II) complexes, [Cu(kappa(OON)-O-1-N-2-H2L1)(H2O)(2)]center dot 2H(2)O (2), [Cu(kappa O-H3L1)(2)(H2O)(4)] (3), [Cu(kappa O-H4L2)(2)(H2O)(4)] (5) and [Cu(H2O)(6)]center dot 2H(2)L(3)center dot 2(CH3)(2)NCHO (7), were prepared by the reaction of Cu(NO3)(2)center dot 3H(2)O with sodium (Z)-2-(2-(1-amino-1,3-dioxobutan-2-ylidene)hydrazineyl)benzenesulfonate, [Na(mu(4)-1:2 kappa O-1,2 kappa O-2,3 kappa O-3,4 kappa O-4-H3L1)](n) (1; for 2 and 3), sodium (Z)-3-(2-(1-amino-1,3-dioxobutan-2-ylidene)hydrazineyl)-4-hydroxybenzene-sulfonate, [Na(mu-1 kappa O-1,2 kappa O-2-H4L2)](2) (4; for 5) or sodium (Z)-2-(2(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazineyl)naphthalene-1-sulfonate, [Na(mu-1 kappa(OO2)-O-1,2 kappa O-3-H2L3)(CH3OH)(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(NO3)(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 degrees C, in a water/acetonitrile mixture under low power (10 W) MW irradiation.