Co(ii/iii), Ni(ii) and Cu(ii) complexes with a pyrazole-functionalized 1,3,5-triazopentadiene: synthesis, structure and application in the oxidation of styrene to benzaldehyde

Abstract

A series of coordination compounds [Co^III₂{(Co^IICl(H₄L)(H₂O)₃}₃{(Co^II(CH₃COO)(H₄L)(H₂O)₂}₃]⁶⁺(Cl⁻)₆·H₂O (1), [Ni(H₄L)₂] (2), [Cu(H₄L)₂] (3) and [(H₆L)CuCl₃]·H₂O (4) were synthesized by the reaction of (E)-1-(amino(1H-pyrazol-1-yl)methylene)guanidinium chloride (H₅L·HCl) with Co(CH₃COO)₂·4H₂O, Ni(CH₃COO)₂·4H₂O (in the presence of triethylamine), Cu(CH₃COO)₂·H₂O (in the presence of triethylamine) and CuCl₂·2H₂O in methanol, respectively. 1–4 were fully characterized by infrared spectroscopy, electrospray ionization mass spectrometry, single-crystal X-ray diffraction and elemental analyses. H₅L·HCl is potentially a multidentate N-donor ligand, and coordinates to cobalt as a N₄ donor in 1, and to nickel or copper as a N₂ donor in 2–4. In the structure of 1, the coordination geometry around the Co^III or Co^II atoms is distorted octahedral. In 2 and 3, the Ni and Cu atoms have a distorted square planar coordination geometry, whereas Cu in 4 exhibits a square-based pyramidal coordination geometry. In the packing diagrams of 1–4, the N atom 1H-pyrazole moiety and H–N of the 1,3,5-triazopentadienyl ligands act as a H-bond acceptor and donor towards neighbouring molecules, leading to 3D supramolecular networks. All complexes were tested as catalysts for the peroxidative oxidation of styrene to benzaldehyde. The effects of the oxidant type, styrene to catalyst molar ratio, and reaction temperature and time on the catalytic activity were investigated. Under optimized conditions in the presence of 4, high yields (up to 80%) of benzaldehyde can be achieved.