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 [CuI 2(BPMPO–)]1+ {BPMPOH = 2,6-bis­{[(bis­(2-pyridylmethyl)­amino]­methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper­(II) peroxo [CuII 2(BPMPO–)­(O2 2–)]1+, hydroperoxo [CuII 2(BPMPO–)­(−OOH)]2+, and superoxo [CuII 2(BPMPO–)­(O2 •–)]2+ species, characterized by UV–vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [CuII 2(BPMPO–)­(O2 •–)]2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [CuII 2(BPMPO–)­(O2 2–)]1+ ⇄ [CuII 2(BPMPO–)­(O2 •–)]2+, is established utilizing Me8Fc+/Cr­(η6-C6H6)2, allowing for [CuII 2(BPMPO–)­(O2 •–)]2+/[CuII 2(BPMPO–)­(O2 2–)]1+ reduction potential calculation, E°′ = −0.44 ± 0.01 V vs Fc+/0, also confirmed by cryoelectrochemical measurements (E°′ = −0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [CuII 2(BPMPO–)­(O2 2–)]1+ produces [CuII 2(BPMPO–)­(−OOH)]2+; using a phosphazene base, an acid–base equilibrium was achieved, pK a = 22.3 ± 0.7 for [CuII 2(BPMPO–)­(−OOH)]2+. The BDFEOO–H = 80.3 ± 1.2 kcal/mol, as calculated for [CuII 2(BPMPO–)­(−OOH)]2+; this is further substantiated by H atom abstraction from O–H substrates by [CuII 2(BPMPO–)­(O2 •–)]2+ forming [CuII 2(BPMPO–)­(−OOH)]2+. In comparison to known analogues, the thermodynamic and spectroscopic properties of [CuII 2(BPMPO–)] O2-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO– framework and the resulting enhanced CuII-ion Lewis acidity.