An Anionic, Tetragonal Copper(II) Superoxide Complex

Abstract

Insight into copper-oxygen species proposed as intermediates in oxidation catalysis is provided by the identification of a Cu(II)-superoxide complex supported by a sterically hindered, pyridinedicarboxamide ligand. A tetragonal, end-on superoxide structure is proposed based on DFT calculations and UV-vis, NMR, EPR, and resonance Raman spectroscopy. The complex yields a trans-1,2-peroxodicopper(II) species upon reaction with [(tmpa)Cu(CH 3 CN)]OTf, and, unlike other known Cu(II)-superoxide complexes, acts as a base rather than an electrophilic (Hatom abstracting) reagent in reactions with phenols.An important first step in copper-promoted aerobic oxidations in biology1 and catalysis2 is the formation of a 1:1 Cu/O 2 adduct, in which the O 2 molecule is activated for subsequent reactions, either with substrate or to form different copper-oxygen species. In one approach aimed at understanding such adducts, synthetic 1:1 Cu/O 2 complexes have been targeted for detailed structural, spectroscopic, and reactivity studies.3 To date, three types have been identified: (a) end-on, triplet Cu(II)-superoxos supported by tetradentate tripodal4 or, in one case, tridentate5 N-donor ligands, (b) side-on, singlet Cu(II)-superoxos supported by facially coordinating tris(pyrazolyl)hydroborates,6 and (c) side-on, singlet Cu(III)-peroxos supported by strongly electron-donating, bidentate β-diketiminates or anilido-imines.3 , 7 A key finding from reactivity studies of type (a) compounds is that they are electrophilic, with the ability to perform biologically relevant H-atom abstractions from phenols and weak C-H bonds; 4b , 5 investigations of the reactivity of type (b) compounds have not been reported, and type (c) compounds are relatively unreactive with external organic substrates. Herein we report that in seeking to expand the repertoire of available 1:1 Cu/O 2 structures for comparative evaluations, we have discovered an end-on Cu(II)-superoxide complex that displays unique characteristics, including a tetragonal geometry and non-electrophilic reactivity.Inspired by a recent report,8 we prepared complex 1 (Scheme 1) by treating N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide9 with NaOMe followed by CuCl 2 in the presence of CH 3 CN.10 In methanol or THF solution, 1 is green, whereas it is red-brown in the presence of CH 3 CN or pyridine. Consistent with this solvatochromism (Figure S2), attempts to obtain crystals of 1 suitable for X-ray crystallographic analysis were complicated by apparent CH 3 CN lability. The addition of 4-tBu-pyridine, however, yielded X-ray quality dark red crystals of 2 (Scheme 1). The complex is square planar with a geometry similar to other known 2,6-pyridinedicarboxamide Cu(II) complexes.11 wtolman@umn.edu. Supporting Information Available: Experimental procedures, spectra, computational details (PDF), and CIF. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public AccessAuthor Manuscript J Am Chem Soc. Author manuscript; available in PMC 2011 Nov...

Spectroscopic and computational insight into the activation of O 2 by the mononuclear Cu center in polysaccharide monooxygenases

“…Cu-superoxide species have been proposed as reactive intermediates in H-atom abstraction in the noncoupled binuclear Cu enzymes (1,29,30), although a Cu(II)-superoxide enzyme species has yet to be spectroscopically characterized. Alternatively, a variety of inorganic Cu(II)-superoxide model complexes have been trapped at low temperature (31)(32)(33)(34)(35). These include a diazacyclooctane supported complex (1) (32) (SI Appendix, Fig.…”

Structure of the Reduced Copper Active Site in Preprocessed Galactose Oxidase: Ligand Tuning for One-Electron O2 Activation in Cofactor Biogenesis

“…Dioxygen binds to mononuclear copper centers in models of metalloenzyme active sites, forming either side-on 41–44 (η 2 ) or end-on 45–58 (η 1 ) species, which lead to different electronic structures. Active site optimizations were performed with O 2 bound to the copper center in both η 1 and η 2 configurations and in both triplet and singlet spin states.…”

Transition Metal Complexes and the Activation of Dioxygen

“…Mechanistic experiments led to the proposed mechanism shown in Figure 42. In contrast to the generally electrophilic reactivity of 80-84, complex 85 ( Figure 39) acts as a base and was surmised to have nucleophilic character, presumably because of the overall negative charge of the complex and the strong electron donating properties of the pyridine(dicarboxamide) supporting ligand [290]. Thus, 85 is unreactive with alkyl-substituted phenols, appears to be protonated by p-nitrophenol, does not oxidize PPh 3 , and may be trapped by [(TMPA)-Cu(I)]OTf to yield a trans-η 1 :η 1 -peroxo-dicopper(II,II) complex [290].…”