Manipulating electron transfer - the influence of substituents on novel copper guanidine quinolinyl complexes.

Copper guanidine quinolinyl complexes act as good entatic state models due to their distorted structures leading to a high similarity between Cu(i) and Cu(ii) complexes. For a better understanding of the entatic state principle regarding electron transfer a series of guanidine quinolinyl ligands with different substituents in the 2- and 4-position were synthesized to examine the influence on the electron transfer properties of the corresponding copper complexes. Substituents with different steric or electronic influences were chosen. The effects on the properties of the copper complexes were studied applying different experimental and theoretical methods. The molecular structures of the bis(chelate) copper complexes were examined in the solid state by single-crystal X-ray diffraction and in solution by X-ray absorption spectroscopy and density functional theory (DFT) calculations revealing a significant impact of the substituents on the complex structures. For a better insight natural bond orbital (NBO) calculations of the ligands and copper complexes were performed. The electron transfer was analysed by the determination of the electron self-exchange rates following Marcus theory. The obtained results were correlated with the results of the structural analysis of the complexes and of the NBO calculations. Nelsen's four-point method calculations give a deeper understanding of the thermodynamic properties of the electron transfer. These studies reveal a significant impact of the substituents on the properties of the copper complexes.

Influence of the amine donor on hybrid guanidine-stabilized Bis(µ-oxido) dicopper(III) complexes and their tyrosinase-like oxygenation activity towards polycyclic aromatic alcohols.

The tyrosinase-like activity of hybrid guanidine-stabilized bis(µ-oxido) dicopper(III) complexes [Cu2(µ-O)2(L)2](X)2 (L = 2-{2-((Diethylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (TMGbenzNEt2, L2) and 2-{2-((Di-isopropylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (TMGbenzNiPr2, L3); X = PF6-, BF4-, CF3SO3-) is described. New aromatic hybrid guanidine amine ligands were developed with varying amine donor function. Their copper(I) complexes were analyzed towards their ability to activate dioxygen in the presence of different weakly coordinating anions. The resulting bis(µ-oxido) species were characterized at low temperatures by UV/Vis and resonance Raman spectroscopy, cryo-ESI mass spectrometry and density functional theory calculations. Small structural changes in the ligand sphere were found to influence the characteristic ligand-to-metal charge transfer (LMCT) features of the bis(µ-oxido) species, correlating a redshift in the UV/Vis spectrum with weaker N-donor function of the ligand. DFT calculations elucidated the influence of the steric and electronic properties of the bis(µ-oxido) species leading to a higher twist of the Cu2O2 plane against the CuN2 plane and a stretching of the Cu2O2 core. Despite their moderate stability at -100 °C, the bis(µ-oxido) complexes exhibited a remarkable activity in catalytic oxygenation reactions of polycyclic aromatic alcohols. Further the selectivity of the catalyst in the hydroxylation reactions of challenging phenolic substrates is not changed despite an increasing shield of the reactive bis(µ-oxido) core. The generated quinones were found to form exclusively bent phenazines, providing a promising strategy to access tailored phenazine derivatives.

Photoluminescence of Fully Inorganic Colloidal Gold Nanocluster and Their Manipulation Using Surface Charge Effects.

Fully inorganic, colloidal gold nanoclusters (NCs) constitute a new class of nanomaterials that are clearly distinguishable from their commonly studied metal-organic ligand-capped counterparts. As their synthesis by chemical methods is challenging, details about their optical properties remain widely unknown. In this work, laser fragmentation in liquids is performed to produce fully inorganic and size-controlled colloidal gold NCs with monomodal particle size distributions and an fcc-like structure. Results reveal that these NCs exhibit highly pronounced photoluminescence with quantum yields of 2%. The emission behavior of small (2-2.5 nm) and ultrasmall (<1 nm) NCs is significantly different and dominated by either core- or surface-based emission states. It is further verified that emission intensities are a function of the surface charge density, which is easily controllable by the pH of the surrounding medium. This experimentally observed correlation between surface charge and photoluminescence emission intensity is confirmed by density functional theoretical simulations, demonstrating that fully inorganic NCs provide an appropriate material to bridge the gap between experimental and computational studies of NCs. The presented study deepens the understanding of electronic structures in fully inorganic colloidal gold NCs and how to systematically tune their optical properties via surface charge density and particle size.

Catalytic Oxygenation of Hydrocarbons by Mono-µ-oxo Dicopper(II) Species Resulting from O-O Cleavage of Tetranuclear CuI /CuII Peroxo Complexes.

One of the challenges of catalysis is the transformation of inert C-H bonds to useful products. Copper-containing monooxygenases play an important role in this regard. Here we show that low-temperature oxygenation of dinuclear copper(I) complexes leads to unusual tetranuclear, mixed-valent µ4 -peroxo [CuI /CuII ]2 complexes. These Cu4 O2 intermediates promote irreversible and thermally activated O-O bond homolysis, generating Cu2 O complexes that catalyze strongly exergonic H-atom abstraction from hydrocarbons, coupled to O-transfer. The Cu2 O species can also be produced with N2 O, demonstrating their capability for small-molecule activation. The binding and cleavage of O2 leading to the primary Cu4 O2 intermediate and the Cu2 O complexes, respectively, is elucidated with a range of solution spectroscopic methods and mass spectrometry. The unique reactivities of these species establish an unprecedented, 100 % atom-economic scenario for the catalytic, copper-mediated monooxygenation of organic substrates, employing both O-atoms of O2 .

Shot noise limited soft x-ray absorption spectroscopy in solution at a SASE-FEL using a transmission grating beam splitter.

X-ray absorption near-edge structure (XANES) spectroscopy provides element specificity and is a powerful experimental method to probe local unoccupied electronic structures. In the soft x-ray regime, it is especially well suited for the study of 3d-metals and light elements such as nitrogen. Recent developments in vacuum-compatible liquid flat jets have facilitated soft x-ray transmission spectroscopy on molecules in solution, providing information on valence charge distributions of heteroatoms and metal centers. Here, we demonstrate XANES spectroscopy of molecules in solution at the nitrogen K-edge, performed at FLASH, the Free-Electron Laser (FEL) in Hamburg. A split-beam referencing scheme optimally characterizes the strong shot-to-shot fluctuations intrinsic to the process of self-amplified spontaneous emission on which most FELs are based. Due to this normalization, a sensitivity of 1% relative transmission change is achieved, limited by fundamental photon shot noise. The effective FEL bandwidth is increased by streaking the electron energy over the FEL pulse train to measure a wider spectral window without changing FEL parameters. We propose modifications to the experimental setup with the potential of improving the instrument sensitivity by two orders of magnitude, thereby exploiting the high peak fluence of FELs to enable unprecedented sensitivity for femtosecond XANES spectroscopy on liquids in the soft x-ray spectral region.

Exceptional Substrate Diversity in Oxygenation Reactions Catalyzed by a Bis(µ-oxo) Copper Complex.

The enzyme tyrosinase contains a reactive side-on peroxo dicopper(II) center as catalytically active species in C-H oxygenation reactions. The tyrosinase activity of the isomeric bis(µ-oxo) dicopper(III) form has been discussed controversially. The synthesis of bis(µ-oxo) dicopper(III) species [Cu2 (µ-O)2 (L1)2 ](X)2 ([O1](X)2 , X=PF6 - , BF4 - , OTf- , ClO4 - ), stabilized by the new hybrid guanidine ligand 2-{2-((dimethylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (L1), and its characterization by UV/Vis, Raman, and XAS spectroscopy, as well as cryo-UHR-ESI mass spectrometry, is described. We highlight selective oxygenation of a plethora of phenolic substrates mediated by [O1](PF6 )2 , which results in mono- and bicyclic quinones and provides an attractive strategy for designing new phenazines. The selectivity is predicted by using the Fukui function, which is hereby introduced into tyrosinase model chemistry. Our bioinspired catalysis harnesses molecular dioxygen for organic transformations and achieves a substrate diversity reaching far beyond the scope of the enzyme.

Dual oxidase/oxygenase reactivity and resonance Raman spectra of {Cu3O2} moiety with perfluoro-t-butoxide ligands.

A Cu(i) fully fluorinated O-donor monodentate alkoxide complex, K[Cu(OC4F9)2], was previously shown to form a trinuclear copper-dioxygen species with a {Cu3(µ3-O)2} core, TOC4F9, upon reactivity with O2 at low temperature. Herein is reported a significantly expanded kinetic and mechanistic study of TOC4F9 formation using stopped-flow spectroscopy. The TOC4F9 complex performs catalytic oxidase conversion of hydroquinone (H2Q) to benzoquinone (Q). TOC4F9 also demonstrated hydroxylation of 2,4-di-tert-butylphenolate (DBP) to catecholate, making TOC4F9 the first trinuclear species to perform tyrosinase (both monooxygenase and oxidase) chemistry. Resonance Raman spectra were also obtained for TOC4F9, to our knowledge, the first such spectra for any T species. The mechanism and substrate reactivity of TOC4F9 are compared to those of its bidentate counterpart, TpinF, formed from K[Cu(pinF)(PR3)]. The monodentate derivative has both faster initial formation and more diverse substrate reactivity.

On the Metal Cooperativity in a Dinuclear Copper-Guanidine Complex for Aliphatic C-H Bond Cleavage by Dioxygen.

Selective oxidation reactions of organic compounds with dioxygen using molecular copper complexes are of relevance to synthetic chemistry as well as enzymatic reactivity. In the enzyme peptidylglycine α-hydroxylating monooxygenase (PHM), the hydroxylating activity towards aliphatic substrates arises from the cooperative effect between two copper atoms, but the detailed mechanism has yet to be fully clarified. Herein, we report on a model complex showing hydroxylation of an aliphatic ligand initiated by dioxygen. According to DFT calculations, the proton-coupled electron-transfer (PCET) process leading to ligand hydroxylation in this complex benefits from cooperative effects between the two copper atoms. While one copper atom is responsible for dioxygen binding and activation, the other stabilizes the product of intramolecular PCET by copper-ligand charge transfer. The results of this work might pave the way for the directed utilization of cooperative effects in oxidation reactions.