Electronic structure and reactivity studies of a nonsymmetric one-electron oxidized CuII bis-phenoxide complex

• Published in: Inorganica Chimica Acta 2018-09, Vol.481 (C), p.151-158
• Main Authors: Chiang, Linus, Wasinger, Erik C., Shimazaki, Yuichi, Young, Victor, Storr, Tim, Stack, T. Daniel P.
• Format: Article
• Language: English
• Subjects: INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2017.09.042

[Display omitted] •We characterize a localized ligand radical via oxidation of an asymmetric Cu salan.•The oxidized Cu salan complex decomposes slowly to a protonated phenoxide.•The oxidized complex oxidizes benzyl alcohol without a substrate-binding pre-equilibrium. The tetradentate mixed imino/amino phenoxide ligand (N-(3,5-di-tert-butylsalicylidene)-N′-(2-hydroxyl-3,5-di-tert-butylbenzyl))-trans-1,2-cyclohexanediamine (salalen) was complexed with CuII, and the resulting Cu complex (2) was characterized by a number of experimental techniques and theoretical calculations. Two quasi-reversible redox processes for 2, as observed by cyclic voltammetry, demonstrated the potential stability of oxidized forms, and also the increased electron-donating ability of the salalen ligand in comparison to the salen analogue. The electronic structure of the one-electron oxidized [2]+ was studied in detail, and Cu K-edge X-ray Absorption Spectroscopy (XAS) measurements confirmed a CuII-phenoxyl radical complex in solution. Resonance Raman (rR) and variable temperature 1H NMR studies, coupled with theoretical calculations, showed that [2]+ is a triplet (S=1) CuII-phenoxyl radical species, with localization of the radical on the more electron-rich aminophenoxide. Attempted isolation of X-ray quality crystals of [2]+ afforded [2H]+, with a protonated phenol bonded to CuII, and an additional H-bonding interaction with the SbF6− counterion. Stoichiometric reaction of dilute solutions of [2]+ with benzyl alcohol showed that the complex reacts in a similar manner as the oxidized CuII-salen analogue, and does not exhibit a substrate-binding pre-equilibrium as observed for the oxidized bis-aminophenoxide CuII-salan derivative.