Molecular Cu(II)-O-Cu(II) complexes: still waters run deep.

Research on O2 activation at ligated Cu(I) is fueled by its biological relevance and the quest for efficient oxidation catalysts. A rarely observed reaction is the formation of a Cu(II) -O-Cu(II) species, which is more special than it appears at first sight: a single oxo ligand between two Cu(II) centers experiences considerable electron density, and this makes the corresponding complexes reactive and difficult to access. Hence, only a small number of these compounds have been synthesized and characterized unequivocally to date, and as biological relevance was not apparent, they remained unappreciated. However, recently they moved into the spotlight, when Cu(II) -O-Cu(II) cores were proposed as the active species in the challenging oxidation of methane to methanol at the surface of a Cu-grafted zeolite and in the active center of the copper enzyme particulate methane monooxygenase. This Minireview provides an overview of these systems with a special focus on their reactivity and spectroscopic features.

Access to a Cu(II)-O-Cu(II) motif: spectroscopic properties, solution structure, and reactivity.

We report a complex with a rare Cu(II)-O-Cu(II) 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 Cu(II)-O-Cu(II) 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 Cu(I) 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.

Unprecedented binding and activation of CS2 in a dinuclear copper(I) complex.

The first structural characterisation of a copper-carbondisulfide complex revealed a hitherto unknown binding mode for CS(2): it interacts with two metal centres (Cu(I)) simultaneously via both C=S π bonds. DFT calculations showed that complex formation occurs mainly due to a donation of electron density from the copper centres into the C=S π* orbitals.

Dinuclear copper complexes based on parallel ß-diiminato binding sites and their reactions with O2: evidence for a Cu-O-Cu entity.

Investigations concerning the system ß-diketiminato-Cu(I)/O(2) have revealed valuable insights that may be discussed in terms of the behavior of mononuclear oxygenases containing copper. On the other hand nature also employs dinuclear Cu enzymes for the activation of O(2). With this background the ligand system [(Me(2))(C(6)H(3))Xanthdim](2-) containing two parallel ß-diiminato binding sites linked by a xanthene backbone with 2,3-dimethylphenyl residues at the diiminato units was investigated with respect to its copper coordination chemistry. The diimine [(Me(2))(C(6)H(3))Xanthdim]H(2) was treated with CuOtBu in the presence of acetonitrile, PPh(3), and PMe(3) to yield the corresponding complexes [(Me(2))(C(6)H(3))Xanthdim](Cu(L))(2) (L = CH(3)CN, 1, PPh(3), 2, and PMe(3), 3) that proved to be stable and were fully characterized. Single crystal X-ray diffraction analyses performed for the three complexes showed that considerable steric crowding within the binding pockets of 2 leads to a very long Cu-Cu distance while the structures of 1 and 3 are relaxed. Compounds 2 and 3 are relatively robust toward air, whereas 1 is very sensitive and quantitatively reacts with O(2) at room temperature (r.t.) within less than 2 min to give intractable compounds. At low temperatures the formation of a green intermediate was observed that was identified as a Cu(II)-O-Cu(II) species spectroscopically and chemically. This finding is of relevance also in the context of the results obtained testing 1 as a catalyst for phenol oxidation using O(2): 1 efficiently catalyzes phenol coupling, while there was no evidence for any oxygenation reactions occurring.

Palladium complexes of 6-aminofulvene-2-aldiminate (AFA) ligands.

Bis(N,N'-2,6-diisopropylphenyl)-6-aminofulvene-2-aldimine (4) has been synthesised and characterised. The synthesis and characterisation of two zwitterionic Pd(ii) complexes [(Ph(2)AFA)Pd(Me)DMAP] (1) and [(Ph(2)AFA)Pd(N,N-dimethylbenzylamine-2-C,N)] (2) are reported. Activation of 1 and 2 for ethene polymerisation with Lewis acids such as BF(3) and B(C(6)F(5))(3) were not successful. Attempted synthesis of halide-bridged dimers of the form [(Ph(2)AFA)Pd(mu-X)](2) resulted in formation of bis-chelated complexes [(Cy(2)AFA)(2)Pd] (3) and [((t)Bu(2)AFA)(2)Pd] (5).

Peculiar binding modes of a ligand with two adjacent beta-diiminato binding sites in alkali and alkaline-earth metal chemistry.

Reaction of the diprotic ligand [Xanthdim]H(2) (a ligand system where two adjacent beta-dialdimine units are linked by a xanthyl backbone) with 2 equiv of potassium hydride or benzylcesium gave access to bimetallic alkali metal complexes. These complexes were structurally characterized by X-ray diffraction, which showed that the beta-diiminato units are orientated in a W-conformation. Treatment with 2 equiv of Mg[N(SiMe(3))(2)](2)(THF)(2) led to the formation of the heteroleptic complex [Xanthdim][MgN(SiMe(3))(2)(THF)](2), that crystallized as a highly strained monomer. The heteroleptic Mg complex is remarkably stable against ligand exchange but is not active in CO(2)/cyclohexene oxide copolymerization. Reaction with Ca[N(SiMe(3))(2)](2)(THF)(2) gave the homoleptic complex [Xanthdim][Ca(THF)]. Both alkaline-earth metal complexes display considerable distortions in their solid state structure.