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

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, pK 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.

Copper–oxygen adducts: new trends in characterization and properties towards C–H activation

“…Similarly, O-O stretching frequencies obtained from rR (O-O) are between 798 and 815 cm -1 for all peroxido species (Table 1). The rR data on superoxido species range from 1070 cm -1 (Cu II 2-O2 • based on Pyr-TACN2,2 ligand) to More in details, from various phenolato (Karlin group) 168,172 and pyrazolato (Meyer group) 161,167,171 bridging ligands, the particular binucleating ligand design was shown to considerably influence/control the reduction potential of the superoxido-dicopper(II) complex, the OO−H pKa and BDFE (see part 1.2, Table 1 and Scheme 4). The phenoxido-bridged complexes appeared as more basic compared to the pyrazolato/TACN2,2 based complex (Pyr/TACN2,2 ligand, Chart 2 and Table 1).…”

“…[169][170][171] Very recently Karlin and co-workers have completed the studies using a phenolato bridged ligand (BPMPO(H), Chart 2 and Table 1) analogous to XYLO(H). 172 A peroxido-dicopper(II) complex was characterized and shown to undergo reversible oxidation to a superoxido-dicopper(II) species. The various Cu 2 II (O 2 ) complexes (in the nature of the dinucleating ligand) were compared and discussed with respect to physical properties and reactivity.…”

“…The various Cu 2 II ( ) complexes (in the nature of the dinucleating ligand) were compared and discussed with respect to physical properties and reactivity. 172 Based on spectroscopic, titration and electrochemical data, thermodynamic parameters ( E 1/2 , p K a ) were obtained, allowing a comparative analysis ( Table 1 ). For each species described (peroxido, superoxido and hydroperoxido) despite various bridging ligands and various experimental conditions (temperature and solvent), the UV-Vis absorptions for each form are in a similar energy range.…”

“…Comparison of the main physical and thermodynamic properties determined for Cu II 2(O2 2− ), Cu II 2(O2 •− ) and Cu II 2(OOH) from Meyer162,167,171 and Karlin168,172 …”