Electronic Structure and Reactivity of One-Electron-Oxidized Copper(II) Bis(phenolate)–Dipyrrin Complexes

Abstract

The sterically hindered bis(phenol)-dipyrrin ligands ^HLH₃ and ^PhLH₃ were reacted with 1 equiv of copper(II) under ambient conditions to produce the copper radical complexes [Cu(^HL)] and [Cu(^PhL)]. Their X-ray crystal structures show relatively short C-O bond distances (mean bond distances of 1.287 and 1.291 Å), reminiscent of mixed pyrrolyl-phenoxyl radical species. Complexes [Cu(^HL)] and [Cu(^PhL)] exhibit rich electronic spectra, with an intense near-IR (NIR) band (ε > 6 mM^-1 cm^-1) at 1346 and 1321 nm, respectively, assigned to a ligand-to-ligand charger-transfer transition. Both show a reversible oxidation wave ( E_1/2^1,ox = 0.05 and 0.04 V), as well as a reversible reduction wave ( E_1/2^1,red = -0.40 and -0.56 V versus ferrocenium/ferrocene, respectively). The cations ([Cu(^HL)]⁺ and [Cu(^PhL)]⁺) and anions ([Cu(^HL)]^- and [Cu(^PhL)]^-) were generated. They all display an axial ( S = ¹/₂) signal with a copper hyperfine structure in their electron paramagnetic resonance spectra, consistent with ligand-centered redox processes in both reduction and oxidation. Complex [Cu(^HL)](SbF₆) was cocrystallized with [Cu(^HL)]. Oxidation is accompanied by a slight contraction of both the C-O bonds (mean bond distance of 1.280 Å) and the C-C bonds connecting the peripheral rings to the dipyrrin. The cations show vis-NIR bands of up to 1090 nm due to their quinoidal nature. The anions do not show a significant band above 700 nm, in agreement with their bis(phenolate)-dipyrrin character. The radical complexes efficiently catalyze the aerobic oxidation of benzyl alcohol, 1-phenylethanol, and unactivated 2-phenylethanol in basic conditions.