Structural diversity of copper(II) complexes with three dimensional network: Crystal structure, Hirshfeld surface analysis, DFT calculations and catalytic activity

Exploring structural, magnetic, and catalytic diversity in tetranuclear {Cu4O4} cubane cores and a dinuclear complex derived from closely related ligand systems

The coordination compounds featuring a {Cu4O4} core, typically bridged by hydroxo or alkoxo groups, are particularly intriguing due to their notable magnetic properties and catalytic activity. In this study, we explored the synthesis and characterization of four new Schiff base ligands and their subsequent complexation with CuII salts, which resulted in the formation of three tetranuclear complexes: [Cu4(L1)4]·2H2O (1), [Cu4(L2)2(HL2)2](Cl)(NO3)·5H2O (2), and [Cu4(L3)4] (3), as well as one dinuclear complex: [Cu2(L4)2] (4). These tetranuclear complexes all feature a {Cu4O4} core, but with differing coordination environments around the CuII centers. For instance, complex 1 exhibits μ3-phenoxido bridges and a distorted octahedral geometry, while complex 3 features μ3-alkoxido bridges and a square pyramidal geometry. Complex 2 displays an open cubane core with mixed μ2-phenoxido and μ3-alkoxido bridges, with both square planar (CuNO4) and octahedral (CuNO5) geometries. In addition, dinuclear complex 4 was synthesized, featuring square planar CuII centers linked by μ2-alkoxido bridges. The magnetic studies revealed that 1 and 2 exhibit strong antiferromagnetic coupling, which is attributed to their larger Cu-O-Cu bond angles, while complex 3 demonstrates moderate ferromagnetic behavior, associated with smaller bond angles. Literature reports indicate that {Cu4O4} cubane cores generally show ferromagnetic interactions at bond angles near 104°, but in complex 3, we observed moderate ferromagnetic interactions with a Cu-O-Cu bridging angle of 108.41(9)°, making it one of the highest bond angles observed for ferromagnetic interactions in a {Cu4O4} cubane-like system. The planar dinuclear complex 4 exhibits extremely strong antiferromagnetic coupling, which is attributed to the ideal Cu-O-Cu bridging angles and the planar Cu-O-O-Cu core. Additionally, EPR measurements of complex 3 at 4 K reveal a well-isolated S = 2 ground state, separated by a gap of 65.8 cm-1 from the three closest degenerate spin levels (S = 0, 1, and 1). Finally, all four complexes were used as catalysts for the aerobic oxidation of 2-aminophenol, and the mechanism of the oxidation process was elucidated by EPR spectroscopy.

Tutton salt (NH4)2Zn(SO4)2(H2O)6: thermostructural, spectroscopic, Hirshfeld surface, and DFT investigations

CONTEXT

Ammonium Tutton salts have been widely studied in recent years due to their thermostructural properties, which make them promising compounds for application in thermochemical energy storage devices. In this work, a detailed experimental study of the Tutton salt with the formula (NH4)2Zn(SO4)2(H2O)6 is carried out. Its structural, vibrational, and thermal properties are analyzed and discussed. Powder X-ray diffraction (PXRD) studies confirm that the compound crystallizes in a structure of a Tutton salt, with monoclinic symmetry and P21/a space group. The Hirshfeld surface analysis results indicate that the main contacts stabilizing the material crystal lattice are H···O/O···H, H···H, and O···O. In addition, a typical behavior of an insulating material is confirmed based on the electronic bandgap calculated from the band structure and experimental absorption coefficient. The Raman and infrared spectra calculated using DFT are in a good agreement with the respective experimental spectroscopic results. Thermal analysis in the range from 300 to 773 K reveals one exothermic and several endothermic events that are investigated using PXRD measurements as a function of temperature. With increasing temperature, two new structural phases are identified, one of which is resolved using the Le Bail method. Our findings suggest that the salt (NH4)2Zn(SO4)2(H2O)6 is a promising thermochemical material suitable for the development of heat storage systems, due to its low dehydration temperature (≈ 330 K), high enthalpy of dehydration (122.43 kJ/mol of H2O), and hydration after 24 h.

METHODS

Computational studies using Hirshfeld surfaces and void analysis are conducted to identify and quantify the intermolecular contacts occurring in the crystal structure. Furthermore, geometry optimization calculations are performed based on density functional theory (DFT) using the PBE functional and norm-conserving pseudopotentials implemented in the Cambridge Serial Total Energy Package (CASTEP). The primitive unit cell optimization was conducted using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. The electronic properties of band structure and density of states, and vibrational modes of the optimized crystal lattice are calculated and analyzed.