Multi-functional d10 metal–organic materials based on bis-pyrazole/pyridine ligands supported by a 2,6-di(3-pyrazolyl)pyridine with different spanning flexible dicarboxylate ligands: synthesis, structure, photoluminescent and catalytic properties

Abstract

A series of the novel zinc complexes, Zn₃(H₂L¹)₂(ox)₃(H₂O)₂·4H₂O (1), Zn(H₂L¹)(ad)·2H₂O (2), Zn₂(HL²)₂(suc)_0.5·(HCOO)·H₂O (3), Zn₂(HL²)₂(glu)·2H₂O (4), Zn₂(HL²)₂ (ad)_0.5·(OH)·0.5H₂O (5), Zn₂(HL²)₂(seb)(H₂O)·2.5H₂O (6), (H₂L¹ = 2,6-di(5-methyl-1H-pyrazol-3-yl)pyridine, H₂L² = 2,6-di(5-phenyl-1H-pyrazol-3-yl)pyridine, H₂ox = oxalic acid, H₂ad = adipic acid, H₂suc = succinic acid, H₂glu = glutaric acid, H₂seb = suberic acid) were synthesized hydrothermally from the self-assembly of a Zn(II) ion with different types of the new bi-pyrazolyl pyridine derivative ligands and different spanning flexible dicarboxylic compounds as an auxiliary ligand. They were characterized by elemental analysis, IR and UV–vis spectroscopy, powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. We found that they are all molecular compounds using the above detections. The structural analysis indicates that the zinc atom in complex 1 is a six-coordinate mode of ZnO₃N₃, forming a slightly distorted octahedral geometry, while zinc atoms in complexes 2–6 are all penta-coordination modes of ZnO₂N₃ and ZnON₄ in a distorted bipyramidal geometry. The H₂L¹ ligand is in the form of a μ₁–η¹–η¹–η¹ coordination with metallic Zn in 1 and 2; while the HL² ligand is in a μ₂–η¹–η¹–η¹–η¹ coordination mode in 3–6. For the oxalate ligand, there are two kinds of coordination modes, μ₂–η¹–η¹–η¹–η¹ bridging coordination mode and μ₁–η¹–η¹ terminal coordination mode; the coordination mode of glutaric acid ligand is in the form of μ₂–η¹–η¹; suc²⁻ ligand is in the form of a μ₄-bridging coordination fashion with each side carboxylate group in a μ₂–η¹–η¹-monodentate mode; the coordination of the adipic acid ligand presents two coordination modes of μ₂–η¹–η¹and μ₄–η²–η²; suberic acid ligand acts in a monodentate terminal coordination mode. In addition, thermal behaviors and photoluminescent properties for 1–6 were all performed and are discussed in detail. The catalytic activity of the complexes is evaluated firstly by the reaction of hydrogen peroxide promoted oxidation of cyclohexane and cyclopentane under mild conditions. It is found that the Zn complexes with five different spanning flexible dicarboxylic acids as an auxiliary ligand with oxalic acid, succinic acid, glutaric acid, adipate acid or sebacic acid, respectively, have potential catalytic activity in the oxidation reaction of cyclohexane.