Doreen Mollenhauer

Halogenated Benzene Cation Radicals

The halogenated benzenes C(6)HF(5), 2,4,6-C(6)H(3)F(3), 2,3,5,6-C(6)H(2)F(4), C(6)F(6), C(6)Cl(6), C(6)Br(6), and C(6)I(6) were converted into their corresponding cation radicals by using various strong oxidants. The cation-radical salts were isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy and by single-crystal X-ray diffraction. The thermal stability of the cation radicals increased with decreasing hydrogen content. As expected, the cation radicals [C(6)HF(5)](+) and 2,3,5,6-[C(6)H(2)F(4)](+) had structures with the same geometry as C(6)HF(5) and 2,3,5,6-[C(6)H(2)F(4)]. In contrast, the cation radicals [C(6)F(6)](+), [C(6)Cl(6)](+), and possibly also [C(6)Br(6)](+) exhibited Jahn-Teller-distorted geometries in the crystalline state. In the case of C(6)F(6)(+)Sb(2)F(11)(-), two low-symmetry geometries were observed in the same crystal. Interestingly, the structures of the cation radicals 2,4,6-[C(6)H(3)F(3)](+) and C(6)I(6)(+) did not exhibit Jahn-Teller distortions. DFT calculations showed that the explanation for the lack of distortion of these cations from the D(3h) or D(6h) symmetry of the neutral benzene precursor was different for 2,4,6-[C(6)H(3)F(3)](+) than for [C(6)I(6)](+).

Revealing the Position of the Substrate in Nickel Superoxide Dismutase: A Model Study

Reactive oxygen species (ROS) are a major factor in the development of several types of cancer, inflammation, and related diseases. These ROS are not only cytotoxic but also involved in cell signaling. [1] The protection from ROS is of vital importance for biological organisms. For aerobic organisms, superoxide dismutases (SODs) play the major role in protecting cells from ROS, which are generated by the reduction of molecular oxygen by reactive metabolites of the respiratory chain. [2] Because of their biological and medical importance, SODs are a subject of intense research, which yielded more than 2000 publications in the first six months of 2010. While this research has led to detailed knowledge about their biological function and enzyme kinetics, the precise mode of action of these enzymes is still not known and two different mechanisms were proposed. [3] A major reason for this lack of knowledge is the high catalytic rate constants of superoxide degradation (O2C ) by SODs. SODs destroy the superoxide anion radical by converting it into hydrogen peroxide and oxygen with a rate near the diffusion limit (kcat > 2�1 0 9 m 1 s 1 ). [4] Thus all transients involved in their action are too short lived to be amenable for a spectroscopic characterization. For this reason model systems of SODs were developed. Herein we show that the investigation of a model system of the nickel superoxide dismutase (NiSOD) is able to shed light into the mode of action of this enzyme and makes it possible to decide between the proposed mechanisms. In particular we are able to reveal not only the mode of binding of the substrate to the enzyme also the presence of functional water molecules in the active site of the enzyme. Three independent classes of SODs are known. They contain either a dinuclear (Cu, Zn) or a mononuclear (Fe, Mn, Ni) cofactor. [1b, 5] NiSOD, as a mononuclear nickel-containing metalloenzyme, cycles between Ni II and Ni III during catalysis. [3a, 4b, 6] NiSOD was first found in 1996 in Streptomyces. [5a] Crystallographic and spectroscopic studies give an impression of the structure of the whole enzyme and the geometry of its active site with a single covalently bound nickel ion. The nickel ion is embedded within the so-called nickel-hook formed by the first six amino acids of the N-terminus of the active form of S. coelicolor NiSOD (Scheme 1). [3a, 4b, 6, 7]

Imaging Successive Intermediate States of the On-Surface Ullmann Reaction on Cu(111): Role of the Metal Coordination

The in-depth knowledge about on-surface reaction mechanisms is crucial for the tailor-made design of covalently bonded organic frameworks, for applications such as nanoelectronic or -optical devices. Latest developments in atomic force microscopy, which rely on functionalizing the tip with single CO molecules at low temperatures, allow to image molecular systems with submolecular resolution. Here, we are using this technique to study the complete reaction pathway of the on-surface Ullmann-type coupling between bromotriphenylene molecules on a Cu(111) surface. All steps of the Ullmann reaction, i.e., bromotriphenylenes, triphenylene radicals, organometallic intermediates, and bistriphenylenes, were imaged with submolecular resolution. Together with density functional theory calculations with dispersion correction, our study allows to address the long-standing question of how the organometallic intermediates are coordinated via Cu surface or adatoms.

Adsorption Behavior of 4-Methoxypyridine on Gold Nanoparticles

We demonstrate a phase transfer method to create stable colloidal solutions of Au nanoparticles with 4-methoxypyridine ligands. We then investigate the adsorption behavior of 4-methoxypyridine onto gold surfaces by Raman spectroscopy, DFT calculations, and (1)H NMR. In contrast to unsubstituted pyridine and the frequently used (N,N-dimethylamino)pyridine (DMAP), a flat adsorption of 4-methoxypyridine on gold was found.

Accurate quantum‐chemical description of gold complexes with pyridine and its derivatives

Interaction of gold with pyridine and its derivatives was studied by means of different wavefunction‐based correlation methods and standar DFT functionals as well as accounting for dispersion correction. Comparison of the calculated binding energies with benchmark CCSD(T)results allows us to find an appropriate computational method, when considering the two structures reflecting the interaction of gold with the lone pair at nitrogen, on the one hand, and with the π‐system of pyridine, on the other hand. Additional binding sites were evaluated, when performing potential energy surface calculations and structure optimizations. The enhancement of the interaction energy due to donor substituents in the 4‐position of the pyridine molecule has been investigated.

Charge Spreading in Deep Eutectic Solvents

Ab initio molecular dynamic simulations reveal significantly reduced ion charges in several choline-based deep eutectic solvents, which are cheap and eco-friendly alternatives to ionic liquids. Increasing hydrogen bond strength between the anion and the organic compound enhances charge spreading from the anion to the organic compound while the positive charge is stronger located at the cation. Nonetheless, the negative charge transferred from chloride to urea in choline chloride urea mixtures is negligible. Thus, it seems questionable if charge delocalization occurring through hydrogen bonding between the halide anion and the organic compound is responsible for the deep eutectic melting point.

Phosphine passivated gold clusters: how charge transfer affects electronic structure and stability

A systematic evaluation of small phosphine ligand-protected gold clusters with six to nine gold atoms using density functional theory with dispersion correction has been performed in order to understand the major factors determining stability, including its size, shape, and charge dependence. We show that the charge per atom of the cluster is much more important for the interaction between the ligand shell and gold cluster than the system size. Thus, strong charge transfer effects determine the binding strength between the ligand shell and cluster. The clusters in this series are all non-spherical and exhibit large HOMO-LUMO gaps (above 2.7 eV). Analysis of the delocalized nature of the electronic states at the centre of the clusters demonstrates the presence of nascent superatomic states. However the number of delocalized electrons in these systems is significantly influenced by the charge transfer from the phosphine ligands, contrary to the usual accounting rule for superatom complex systems. Thus, not only electron withdrawing but also charge transfer effects should be considered to influence the superatomic structure of charged ligand surrounded clusters. In consequence in the phosphine gold cluster series under consideration the systems Au7(PPh3)7+ and Au8(PPh3)82+ exhibit nearly fully filled S and P states and the HOMO-LUMO gap increases by 0.2 eV and 0.9 eV, respectively. The interpretation for the stability of the gold phosphine systems is in agreement with experimental results and demonstrates the importance of the superatomic concept.

First principle investigation of the linker length effects on the thermodynamics of divalent pseudorotaxanes

Summary The Gibbs energies of association (Gibbs free (binding) energies) for divalent crown-8/ammonium pseudorotaxanes are determined by investigating the influence of different linkers onto the binding. Calculations are performed with density functional theory including dispersion corrections. The translational, rotational and vibrational contributions are taken into account and solvation effects including counter ions are investigated by applying the COSMO-RS method, which is based on a continuum solvation model. The calculated energies agree well with the experimentally determined ones. The shortest investigated linker shows an enhanced binding strength due to electronic effects, namely the dispersion interaction between the linkers from the guest and the host. For the longer linkers this ideal packing is not possible due to steric hindrance.

New insights into the mechanism of nickel superoxide degradation from studies of model peptides

A series of small, catalytically active metallopeptides, which were derived from the nickel superoxide dismutase (NiSOD) active site were employed to study the mechanism of superoxide degradation especially focusing on the role of the axial imidazole ligand. In the literature, there are contradicting propositions about the catalytic importance of the N-terminal histidine. Therefore, we studied the stability and activity of a set of eight NiSOD model peptides, which represent the major model systems discussed in the literature to date, yet differing in their length and their Ni-coordination. UV-Vis-coupled stopped-flow kinetic measurements and mass spectrometry analysis unveiled their high oxidation sensitivity in the presence of oxygen and superoxide resulting into a much faster Ni(II)-peptide degradation for the amine/amide Ni(II) coordination than for the catalytically inactive bis-amidate Ni(II) coordination. With respect to these results we determined the catalytic activities for all NiSOD mimics studied herein, which turned out to be in almost the same range of about 2 × 106 M−1 s−1. From these experiments, we concluded that the amine/amide Ni(II) coordination is clearly the key factor for catalytic activity. Finally, we were able to clarify the role of the N-terminal histidine and to resolve the contradictory literature propositions, reported in previous studies.

Sulfur versus Dioxygen: Dinuclear (Trisulfido)copper Complexes: Sulfur versus Dioxygen: Dinuclear (Trisulfido)copper Complexes

In contrast to dinuclear (disulfido)copper complexes related trisulfido compounds have not been reported so far. Here we describe two new complexes of this type, [Cu2(tmpa)2S3](SbF6)2 {tmpa = tris(2-pyridylmethyl)amine} and [Cu2(tet b)2S3](OTf)2 (tet b = rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), that have been investigated and could be structurally characterized. Furthermore, the reactivity of these complexes towards dioxygen showed that the corresponding (peroxido)copper complexes formed. Depending on the stoichiometric conditions (excess of sulfur) mononuclear end-on (sulfido)copper complexes were detected spectroscopically. DFT calculations were performed and supported the experimental results.