Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties
Pradip Kumar Hota, Anex Jose, Sanjib Panda, Eleanor M. Dunietz, Austin E. Herzog, Laurianne Wojcik, Nicolas Le Poul, Catherine Belle, Edward I. Solomon, Kenneth D. Karlin
Abstract
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 I 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 [Cu II 2 (BPMPO – )(O 2 2– )] 1+, hydroperoxo [Cu II 2 (BPMPO – )( − OOH)] 2+, and superoxo [Cu II 2 (BPMPO – )(O 2 •– )] 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 [Cu II 2 (BPMPO – )(O 2 •– )] 2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [Cu II 2 (BPMPO – )(O 2 2– )] 1+ ⇄ [Cu II 2 (BPMPO – )(O 2 •– )] 2+, is established utilizing Me 8 Fc + /Cr(η 6 -C 6 H 6 ) 2, allowing for [Cu II 2 (BPMPO – )(O 2 •– )] 2+ /[Cu II 2 (BPMPO – )(O 2 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 [Cu II 2 (BPMPO – )(O 2 2– )] 1+ produces [Cu II 2 (BPMPO – )( − OOH)] 2+; using a phosphazene base, an acid–base equilibrium was achieved, p K a = 22.3 ± 0.7 for [Cu II 2 (BPMPO – )( − OOH)] 2+ . The BDFE OO–H = 80.3 ± 1.2 kcal/mol, as calculated for [Cu II 2 (BPMPO – )( − OOH)] 2+; this is further substantiated by H atom abstraction from O–H substrates by [Cu II 2 (BPMPO – )(O 2 •– )] 2+ forming [Cu II 2 (BPMPO – )( − OOH)] 2+ . In comparison to known analogues, the thermodynamic and spectroscopic properties of [Cu II 2 (BPMPO – )] O 2 -derived adducts can be accounted for based on chelate ring size variations built into the BPMPO – framework and the resulting enhanced Cu II -ion Lewis acidity.