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

The sterically hindered bis(phenol)-dipyrrin ligands (LH3)-L-H and (LH3)-L-Ph were reacted with 1 equiv of copper(II) under ambient conditions to produce the copper radical complexes [Cu(L-H)] and [Cu(L-Ph)]. Their X-ray crystal structures show relatively short C-O bond distances (mean bond distances of 1.287 and 1.291 angstrom), reminiscent of mixed pyrrolyl-phenoxyl radical species. Complexes [Cu(L-H)] and [Cu(L-Ph)] exhibit rich electronic spectra, with an intense near-IR (NIR) band (epsilon > 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(L-H)](+) and [Cu(L-Ph)](+)) and anions ([Cu(L-H)](-) and [Cu(L-Ph)])(-) 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(L-H)](SbF6) was cocrystallized with [Cu(L-H)]. Oxidation is accompanied by a slight contraction of both the C-O bonds (mean bond distance of 1.280 angstrom) 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.