Bis-ferrocenyl-hydrazide metal complexes: studying electronic functional groups as newly potent homogeneous photocatalysts for C(sp3)–H and C(sp2)–H bond oxidation utilizing visible light condition†

A crucial challenge in using organo-metal complexes for photocatalytic organic reactions is the need to develop applications of homogeneous photocatalysts that can effectively function under visible light conditions. For the first time, the use of binuclear complexes containing ferrocenyl-hydrazides as a ligand and nickel or copper as central metals as homogeneous photocatalysts in the oxidation of organic compounds is presented. The new organometal photocatalysts were prepared and identified using techniques, such as FT-IR spectroscopy, NMR spectroscopy, XRD, XRF, XPS, SEM, TGA, EDX, UV-visible, and photocurrent measurements. The oxidation of benzylic C(sp³)–H bonds to produce oxygenated molecules and the selective conversion of C–C double bonds to benzaldehyde can be achieved using bis-ferrocenyl hydrazide complexes with electron-withdrawing or electron-donating groups on the hydrazide moiety under visible-light irradiation in an air atmosphere, at ambient temperature and without the need for external oxidants. The synthesized complexes also can be used to oxygenate 1H-indole to 1H-indole-2,3-dione. The investigation of the role of donating and withdrawing functional groups in the synthesized complexes for selected oxidation reactions is a significant benefit of this report. It was found that only the [(FcHz)₂Ni] and [(FcHz)₂Cu] complexes without functional groups were able to provide a suitable response in the oxidation of the compounds. Additionally, the theoretical DFT and TD-DFT methodologies enabled us to describe the photocatalytic oxidation behavior of these metal complexes. The calculations showed conformational changes in the structure of metal complexes after oxidation. The molecular orbital and natural transition orbital analyses revealed the nature of electronic transitions in the UV-visible absorption bands.