Marked Stabilization of Redox States and Enhanced Catalytic Activity in Galactose Oxidase Models Based on Transition Metal S‑Methylisothiosemicarbazonates with −SR Group in Ortho Position to the Phenolic Oxygen

Reactions of 5-tert-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde S-methylisothiosemicarbazone and 5-tert-butyl-2-hydroxy-3-phenylsulfanylbenzaldehyde S-methylisothiosemicarbazone with pentane-2,4-dione (Hacac) and triethyl orthoformate in the presence of M­(acac)₂ as template source at 107 °C afforded metal complexes of the type M^IIL¹ and M^IIL², where M = Ni and Cu, with a new Schiff base ligand with thiomethyl (H₂L¹) and/or thiophenyl (H₂L²) group in the ortho position of the phenolic moiety. Demetalation of NiL¹ in CHCl₃ with HCl­(g) afforded H₂L¹. The latter reacts with Zn­(OAc)₂·2H₂O with formation of ZnL¹. The effect of −SR groups and metal ion identity on stabilization of phenoxyl radicals generated electrochemically was studied in detail. A marked stabilization of phenoxyl radical was observed in one-electron-oxidized complexes [ML²]⁺ (M = Ni, Cu) at room temperature, as demonstrated by cyclic voltammetry, EPR spectroscopy, and UV–vis–NIR measurements. In solution, the oxidized CuL² and NiL² display intense low-energy NIR transitions consistent with their classification as metal-delocalized phenoxyl radical species. While the CuL² complex shows reversible reduction, reduction of NiL², CuL¹, and NiL¹ is irreversible. EPR measurements in conjunction with density functional theory calculations provided insights into the extent of electron delocalization as well as spin density in different redox states. The experimental room temperature spectroelectrochemical data can be reliably interpreted with the ³[CuL²]⁺ and ²[NiL²]⁺ oxidation ground states. The catalytic activity of synthesized complexes in the selective oxidations of alcohols has been studied as well. The remarkable efficiency is evident from the high yields of carbonyl products when employing both the CuL²/air/TEMPO and the CuL²/TBHP/MW­(microwave-assisted) oxidation systems.