Novel CuII and CrIII Triazole–Pyrazole Complexes: Synthesis, Structural Characterization, and Antibacterial Evaluation

A bidentate ligand 1-methyl-3-(5-methyl-1H-pyrazol-3-yl)-1H-1,2,4-triazole (L21), and transition-metal complexes, [Cu(L21)₂(NO₃)₂] (1) and [Cr₂(L21)₄(OH)₂](NO₃)₄ (2), were synthesized and characterized by single-crystal X-ray diffraction, FT-IR, UV–vis spectroscopy, and high-resolution mass spectrometry. Complex 1 contains a Cu^II center located on an inversion center with chelating L21 ligands in the equatorial plane and coordinated nitrate anions, while 2 is a dinuclear Cr^III complex with a Cr···Cr separation of 2.998 Å, stabilized by bridging hydroxide ligands and counterbalanced by lattice nitrate anions. While L21 showed negligible antibacterial activity, its coordination with metal centers led to a marked improvement. In assays against E. coli, P. aeruginosa, S. aureus, and B. subtilis, 2 exhibited inhibition zones ranging from 9.75 ± 0.92 to 10.25 ± 0.31 mm and a minimum inhibitory concentration of 2.5 mmol/L. Frontier orbital analysis revealed a higher electrophilicity index for 2 (ω = 50.19/48.95 eV) compared with 1 (ω =  2.92/7.26 eV), indicating a greater tendency to accept electron density, while Hirshfeld and noncovalent interaction analyses revealed a denser interaction network in 2. Molecular docking against aquaporins, S. aureus phosphofructokinase, and human serum albumin revealed stronger predicted binding affinities for 2. L21 is a tunable scaffold for developing biologically active transition-metal complexes with antibacterial properties.