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

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 A), reminiscent of mixed pyrrolyl–phenoxyl radical species. Complexes [Cu(HL)] and [Cu(PhL)] exhibit rich electronic spectra, with an intense near-IR (NIR) band (e > 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 (E1/21,ox = 0.05 and 0.04 V), as well as a reversible reduction wave (E1/21,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 re...