Access to a Cu^II–O–Cu^II Motif: Spectroscopic Properties, Solution Structure, and Reactivity

Abstract

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, TBDQ. 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.

Transition Metal Complexes and the Activation of Dioxygen

“…As described in a recent review [358], synthetic complexes with this core are relatively rare, in part because of their tendency to react readily with water to yield bis(μ-hydroxo)dicopper(II) complexes [359][360][361][362]. In a recent resurgence of interest in these species, ligands 110 and 111 ( Figure 52) have been used to prepare and characterize species proposed to contain (μ-oxo)dicopper(II) cores [363,364]. In both cases, these species formed upon treatment of dicopper(I) complexes with O 2 or PhIO.…”

“…For the system comprising 110, the (μ-oxo)dicopper(II) product was identified on the basis of UV-vis absorption spectroscopy and resonance Raman spectroscopy, but more complete characterization was hindered by its thermal instability [363]. Ligand 111 led to a more stable complex 112, which was more fully characterized, including by EXAFS (Cu-Cu ¼ 2.91 Å), NMR spectroscopy, high-resolution electrospray ionization mass spectrometry, and DFT calculations [364]. In initial reactivity studies, 112 induced coupling of 2,4-di-tert-butylphenol, a process reminiscent of the H-atom abstraction reactivity known for bis(μ-oxo)-dicopper(III) complexes [262].…”

“…In initial reactivity studies, 112 induced coupling of 2,4-di-tert-butylphenol, a process reminiscent of the H-atom abstraction reactivity known for bis(μ-oxo)-dicopper(III) complexes [262]. Figure 52 Ligands 110 and 111 used to prepare (μ-oxo)dicopper(II) species, the calculated structure of the one derived from 111 (reproduced with permission from [364]; copyright 2013, American Chemical Society), and the X-ray crystal structure of a tricopper cluster (113) featuring the (μ-oxo)dicopper(II) unit (reproduced with permission from [365]; copyright 2007, National Academy of Sciences of the USA).…”

Molekulare CuII‐O‐CuII‐Komplexe: Stille Wasser sind tief

“…[62] Ausgehend von [FurNeu{Cu I (NCCH 3 )} 2 ](OTf) 2 (34) führten die Reaktionen mit O 2 oder PhIO zur Bildung des raumtemperaturstabilen Produktes 33 (Schema 8). [62] Ausgehend von [FurNeu{Cu I (NCCH 3 )} 2 ](OTf) 2 (34) führten die Reaktionen mit O 2 oder PhIO zur Bildung des raumtemperaturstabilen Produktes 33 (Schema 8).…”

Oxidative dehydrogenation of 5-(pyridine-2-yl-methyl)-2-thioxo-4-imidazolidinones in complexation reaction with copper(II) chloride

“…It is known, that copper complexes can mediate activation of dioxygen, i.e., binding of O 2 to copper(I) ion and generate species capable of substrate oxidation/oxygenation reactions [30]. Copper hydroperoxides (Cu n -OOH, n = 1, 2), μ-peroxo dicopper(II) adducts, or a bis(μ-oxo)dicopper(III) complexes with a fully cleaved O-O bond [31], may be possible intermediates forming on this stage. Thus, the overall reaction can be written as follows:…”