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Access to a CuII–O–CuII Motif: Spectroscopic Properties, Solution Structure, and Reactivity

Haack, Peter, Kärgel, Anne, Greco, Claudio, Dokic, Jadranka, Braun, Beatrice, Pfaff, Florian F., Mebs, Stefan, Ray, Kallol, Limberg, Christian
Journal of the American Chemical Society 2013 v.135 no.43 pp. 16148-16160
ambient temperature, anions, catalysts, copper, cuprous oxide, dibenzofuran, ligands, mass spectrometry, methane, methanol, nuclear magnetic resonance spectroscopy, oxidation, oxygen, spectral analysis, superoxide anion, ultraviolet-visible spectroscopy
We report a complex with a rare Cuᴵᴵ–O–Cuᴵᴵ structural motif that is stable at room temperature, which allows its in-depth characterization by a variety of spectroscopic methods. Interest in such compounds is fueled by the recent discovery that a Cuᴵᴵ–O–Cuᴵᴵ species on the surface of Cu-ZSM-5 is capable of oxidizing methane to methanol, and this in turn ties into mechanistic discussions on the methane oxidation at the dicopper site within the particulate methane monooxygenase. For the synthesis of our Cu₂O complex we have developed a novel, neutral ligand system, FurNeu, exhibiting two N-(N′,N′-dimethylaminoethyl)(2-pyridylmethyl)amino binding pockets connected by a dibenzofuran spacer. The reaction of FurNeu with CuCl yielded [FurNeu](Cu₂(μ-Cl))(CuCl₂), 1, demonstrating the geometric potential of the ligand to stabilize Cu–X–Cu moieties. A Cuᴵ precursor with weakly coordinating anions was chosen in the next step, namely [Cu(NCCH₃)₄]OTf, which led to the formation of [FurNeu](Cu(NCCH₃))₂(OTf)₂, 3. Treatment of 3 with O₂ or PhIO led to identical green solutions, whose UV–vis spectra were markedly different from the one displayed by [FurNeu](Cu)₂(OTf)₄, 4, prepared independently from FurNeu and Cu(OTf)₂. Further investigations including PhIO consumption experiments, NMR and UV–vis spectroscopy, HR-ESI mass spectrometry, and protonation studies led to the identification of the green product as [FurNeu](Cu₂(μ-O))(OTf)₂, 5. DOSY NMR spectroscopy confirmed its monomeric character. Over longer periods of time 5 decomposes to give [Cu(picoloyl)₂], formed through an oxidative N-dealkylation reaction followed by further oxidation of the ligand. Due to its slow decomposition reaction, all attempts to crystallize 5 failed. However, its structure in solution could be determined by EXAFS analysis in combination with DFT calculations, which revealed a Cu–O–Cu angle that amounts to 105.17°. Moreover, TDDFT calculations helped to rationalize the UV–vis absorptions of 5. The reactivity of complex 5 with 2,4-di-tert-butylphenol, DTBP, was also investigated; the initially formed biphenol product, TBBP, was found to further react in the presence of excessive O₂ to yield 2,4,7,9-tetra-tert-butyloxepino[2,3-b]benzofuran, TBOBF, via an intermediate diphenoquinone. It turned out that 5, or its precursor 3, can even be employed as a catalyst for the oxidation of DTBP to TBBP or for the oxidation of TBBP to TBOBF.