Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties.

Copper ion is a versatile and ubiquitous facilitator of redox chemical and biochemical processes. These include the binding of molecular oxygen to copper(I) complexes where it undergoes stepwise reduction-protonation. A detailed understanding of thermodynamic relationships between such reduced/protonated states is key to elucidate the fundamentals of the chemical/biochemical processes involved. The dicopper(I) complex [Cu(BPMPO)] {BPMPOH = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper(II) peroxo [Cu(BPMPO)(O)], hydroperoxo [Cu(BPMPO)(OOH)], and superoxo [Cu(BPMPO)(O)] species, characterized by UV-vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [Cu(BPMPO)(O)] is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [Cu(BPMPO)(O)] ⇄ [Cu(BPMPO)(O)], is established utilizing MeFc/Cr(η-CH), allowing for [Cu(BPMPO)(O)]/[Cu(BPMPO)(O)] reduction potential calculation, °' = -0.44 ± 0.01 V vs Fc, also confirmed by cryoelectrochemical measurements (°' = -0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [Cu(BPMPO)(O)] produces [Cu(BPMPO)(OOH)]; using a phosphazene base, an acid-base equilibrium was achieved, p = 22.3 ± 0.7 for [Cu(BPMPO)(OOH)]. The BDFE = 80.3 ± 1.2 kcal/mol, as calculated for [Cu(BPMPO)(OOH)]; this is further substantiated by H atom abstraction from O-H substrates by [Cu(BPMPO)(O)] forming [Cu(BPMPO)(OOH)]. In comparison to known analogues, the thermodynamic and spectroscopic properties of [Cu(BPMPO)] O-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO framework and the resulting enhanced Cu-ion Lewis acidity.