Novel coordination networks of phthalate/nicotinamide mixed-ligand metal-organic architectures of transition metals - a comparative IR-TGA study and crystal structure of the Zn(II) complex

AbstractIn this research, mixed-ligand Co(II), Ni(II), Cu(II), and Zn(II) complexes containing phthalic acid (phthalate, O-donor) and nicotinamide (N-donor) were synthesized and characterized in detail by FT-IR, TGA/DTA, and for the Zn(II) complex single-crystal X-ray diffraction (XRD). It was observed that the Δν values obtained from the asymmetric and symmetric vibration bands of carboxylate groups in FT-IR spectra changed depending on the metal center, generally indicating a binding regime between monodentate and bridging. The largely preserved position of the Amide I band of nicotinamide indicates that the amide carbonyl oxygen does not participate in coordination, while the ν(M–N) signals in the low-frequency region support N-donor bonding through the pyridine nitrogen. The appearance of distinct and sharp bands in the high-frequency region (N–H and O–H) in the Zn(II) complex indicated the presence of multiple/non-equivalent hydrogen bonding environments in the crystal; this finding was consistent with the dense H–bonding network observed in the X-ray structure. Single-crystal XRD revealed that phthalate acts as a μ₂-η¹:η¹ bridge for Zn(II), with each Zn center forming a four-coordinate, significantly distorted tetrahedral environment with a pyridine–N and three phthalate–O. TGA/DTA data were found to be consistent with multi-step decomposition profiles across all complexes (water loss at low temperatures, organic component decomposition at intermediate temperatures, and metal oxide formation at high temperatures); the resulting residues were confirmed to be the corresponding metal oxides. When all the data were evaluated together, it was concluded that thermally stable coordination architectures were obtained, guided by the bridging tendency of phthalate and the N–donor binding of nicotinamide, and that the IR-XRD-TGA results were consistent with each other. © 2026 Published by Elsevier B.V.