Catalytic Activity of Ruthenium Complexes Containing Hydrotris(pyrazolyl)methane or Cyclopentadienyl Ligands in the Azide–Alkyne Click Cycloaddition Reaction

The Ru(II) complexes bearing the hydrotris(pyrazolyl)methane ligand (Tpm)─(Ru(κ3-N-Tpm)(NCMe)3(NO3)2 (1), RuCl(κ3-N-Tpm)(PPh3)(NCMe)Cl (2), Ru(κ3-N-Tpm)(PPh3)(NCMe)2(NO3)2 (3), Ru(κ3-N-Tpm)(κ2-N-C5H4NCH2NH2)(PPh3)(NO3)2 (4), Ru(κ3-N-Tpm)(κ2-N-NH2CH2CH2NH2)(PPh3)(NO3)2 (5), and RuCl(κ3-N-Tpm)(PPh3)(κ1-N-NH2(CH2)2OH)Cl (6))─were evaluated for the azide/alkyne cycloaddition reaction (“click” reaction) in aqueous and organic media. In parallel, and for comparative purposes, piano-stool–Ru(II) complexes featuring cyclopentadienyl and adamantane-like ligands, 1,3,5-triaza-7-phosphaadamantane (PTA) and its N-monomethylated derivative (mPTA = N-methyl-1,3,5-triaza-7-phosphaadamantane) (RuClCp(PTA)2 (7), RuCp(OH2)(PTA)2CF3SO3 (8), RuClCp(mPTA)2(CF3SO3)2 (9), and RuCp(OH2)(mPTA)2(CF3SO3)3 (10)), were also evaluated for azide–alkyne cycloaddition under similar conditions. Parameters, such as the solvent, substrate concentration, and temperature, were systematically varied to assess their influence on the reaction. The highest conversion rates and selectivity toward the 1,5-disubstituted triazole were achieved in water, acetonitrile/water, and dimethylformamide, while the formation of the 1,4-disubstituted product was slightly favored in methanol. Complexes 3 and 4 exhibited higher regioselectivity and conversion rates in DMF, favoring the formation of the respective 1,5-disubstituted triazole. Complex 3 also catalyzed the cycloaddition of benzyl azide to diphenylacetylene, a representative internal alkyne, reaching a TON of 75 (TOF = 12.5 h–1). DFT calculations and NMR and TOF-MS spectroscopic studies were conducted to elucidate the mechanism of catalytic cycloaddition between complex 3, phenyl azide, and phenylacetylene under the optimized conditions.