Single crystal structure feature of a new 0D Cr(III) complex, a catalyst for mild peroxidative oxidation of toluene
Abstract
A novel water-soluble inorganic complex of Cr(III), formulated as [Cr(dipic)(2,2'-bipyridine)center dot H2O](2)center dot 4H(2)O 2(NO3) (complex 1) where dipic(2-) = pyridine 2,6-dicarboxylate was synthesized. Complex 1 has been characterized by elemental analysis, Fourier transformed-infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), differential thermal analysis (DTA), and single-crystal X-Ray diffraction (SC-XRD).
SC-XRD results revealed a triclinic system with space group P (1) over bar for the expected composition. Each metal center is six-coordinates, and Cr(III) ions are in a distorted octahedral geometry. 1 is a zero-dimensional mononuclear complex that is expanded to a 3D supramolecular network by extensive hydrogen bonds between adjacent layers.
The catalytic activity of complex 1 was evaluated towards the peroxidative oxidation (with hydrogen peroxide as a green oxidant) of toluene to the corresponding product (benzyl alcohol and benzaldehyde) in a homogenous medium under mild conditions. The influence of various parameters such as temperature, time, amounts of nitric acid, acetonitrile, catalyst, and n(H2O2)/n(catalyst) was evaluated to optimize the catalytic system.
Complex 1 exhibited high efficiency in the homogeneous oxidation system without requiring the presence of an acid promoter. In this reaction, under optimized conditions, toluene was oxidized to benzyl alcohol and benzaldehyde with a maximum total yield of 43.4 %.
The study of the mechanism of reaction showed that this process is carried out through both carbon-centered and oxygen-centered radicals. Also, the reaction is promoted by the production of OH radicals that act as active oxidizing agents.