Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry.

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Density Functional Theory Calculations for Multiple Conformers Explaining the Regio- and Stereoselectivity of Ti-Catalyzed Hydroaminoalkylation Reactions.

Hybrid Density Functional Theory (DFT) calculations for multiple conformers of the insertion reactions of a methylenecyclopropane into the Ti-C bond of two differently α-substituted titanaaziridines explain the experimentally observed differences in regioselectivity between catalytic hydroaminoalkylation reactions of methylenecyclopropanes with α-phenyl-substituted secondary amines and corresponding stoichiometric reactions of a methylenecyclopropane with titanaaziridines, which can only be achieved with α-unsubstituted titanaaziridines. In addition, the lack of reactivity of α-phenyl-substituted titanaaziridines as well as the diastereoselectivity of the catalytic and stoichiometric reactions can be understood.

Theoretical investigation of CH-bond activation by photocatalytic excited SO2 and the effects of C-, N-, S-, and Se-doped TiO2.

The photocatalytic sulfoxidation on TiO2 discovered by Parrino et al. represents a new, interesting and lower energy route for the synthesis of sulfonic acids. Sulfonic acids are important precursors for dyes, detergents and drugs. In the commonly known industrial process, SO2 and a specific hydrocarbon are converted into sulfonic acids using high-energy UV light. In this reaction, SO2 is excited into a metastable triplet state (3SO2), which has the potential to activate a CH-bond of hydrocarbons and start a radical reaction cycle. By introducing TiO2 as a photocatalyst, it has been shown that visible light can be used for the synthesis. This offers the potential to be a cost-effective reaction approach for industrial use. However, experimental studies indicate that the initial excitation mechanism of SO2 on TiO2 is significantly different from the catalyst-free mechanism. Parrino et al. were able to reveal first evidence for the existence of a charge-transfer process from SO2 to the TiO2 surface by means of electrochemical experiments. First theoretical investigations from first principles were able to further substantiate the existence of a charge-transfer. However, to fully understand this mechanism, more accurate methods such as Time Dependent Density Functional Theory (TD-DFT) or ab initio multireference methods such as the Complete Active Space Self Consistent Field (CASSCF) method are required. Furthermore, after understanding the charge-transfer mechanism, the introduction of dopants into TiO2 can be investigated in order to possibly redshift the excitation energy. This might open the route to using lower energy light for the sulfoxidation of hydrocarbons on TiO2 as a new potential industrial reaction for the synthesis of sulfonic acids. In this work, we will study the initial step of the photocatalytic sulfoxidation of hydrocarbons using the TD-DFT and CASSCF methods by using a combined approach consisting of calculations with periodic boundary conditions and a newly constructed embedded cluster model. Furthermore, we will explore the effects of doping by introducing four heteroatoms (C, N, S, and Se) into the TiO2 surfaces anatase[101] and rutile[110] to find a possible enhancement of the photocatalytic reactivity by lowering the electronic excitation energy.

CD Stretching Modes are Sensitive to the Microenvironment in Ionic Liquids.

Knowledge of the structure of the electrical double layer in ionic liquids (IL) is crucial for their applications in electrochemical technologies. We report the synthesis and applicability of an imidazolium-based amphiphilic ionic liquid with a perdeuterated alkyl chain for studies of electric potential-dependent rearrangements, and changes in the microenvironment in a monolayer on a Au(111) surface. Electrochemical measurements show two states of the organization of ions on the electrode surface. In situ IR spectroscopy shows that the alkyl chains in imidazolium cations change their orientation depending on the adsorption state. The methylene-d2 stretching modes in the perdeuterated IL display a reversible, potential-dependent appearance of a new band. The presence of this mode also depends on the anion in the IL. Supported by quantum chemical calculations, this new mode is assigned to a second νas (CD2 ) band in alkyl-chain fragments embedded in a polar environment of the anions/solvent present in the vicinity of the imidazolium cation and electrode. It is a measure of the potential-dependent segregation between polar and nonpolar environments in the layers of an IL closest to the electrode.

Electronic Transitions in Different Redox States of Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene-Bridged Titanium Complexes: Spectroelectrochemistry and Quantum Chemistry.

Multinuclear transition metal complexes bridged by ligands with extended π-electronic systems show a variety of complex electronic transitions and electron transfer reactions. While a systematic understanding of the photochemistry and electrochemistry has been attained for binuclear complexes, much less is known about trinuclear complexes such as hexaphenyl-5,6,11,12,17,18-hexaazatrinaphthylene-tristitanocene [(Cp2 Ti)3 HATN(Ph)6 ]. The voltammogram of [(Cp2 Ti)3 HATN(Ph)6 ] shows six oxidation and three reduction waves. Solution spectra of [(Cp2 Ti)3 HATN(Ph)6 ] and of the electrochemically formed oxidation products show electronic transitions in the UV, visible and the NIR ranges. Density functional theory (DFT) and linear response time-dependent DFT show that the three formally titanium(II) centers transfer an electron to the HATN ligand in the ground state. The optically excited transitions occur exclusively between ligand-centered orbitals. The charged titanium centers only provide an electrostatic frame to the extended π-electronic system. Complete active self-consistent field (CASSCF) calculation on a structurally simplified model compound, which considers the multi-reference character imposed by the three titanium centers, can provide an interpretation of the experimentally observed temperature-dependent magnetic behavior of the different redox states of the title compound in full consistency with the interpretation of the electronic spectra.

Photodesorption mechanism of water on WO3(001) - a combined embedded cluster, computational intelligence and wave packet approach.

In this work we investigate the mechanism of photodesorption of water from a WO3(001) surface by theoretical calculations, applying an embedded cluster model. Using the CASSCF method, we have calculated both the ground state as well as the energetically preferred charge-transfer state in three degrees of freedom of the water molecule on the surface. The calculated potential energy surfaces were afterwards fitted with a neural network optimized by a genetic algorithm. A final quantum dynamic wave packet study provided insight into the photodesorption mechanism.

Flexible NO2 -Functionalized N-Heterocyclic Carbene Monolayers on Au(111) Surface.

Invited for the cover of this issue are Elad Gross, F. Dean Toste, and co-workers at The Hebrew University and UC Berkeley. The image depicts the flexible anchoring geometry of addressable carbene molecules on Au surface, which upon exposure to reducing conditions changed their orientation from a standing into a flat-lying position. Read the full text of the article at 10.1002/chem.201903434.

Flexible NO2 -Functionalized N-Heterocyclic Carbene Monolayers on Au (111) Surface.

The formation of flexible self-assembled monolayers (SAMs) in which an external trigger modifies the geometry of surface-anchored molecules is essential for the development of functional materials with tunable properties. In this work, it is demonstrated that NO2 -functionalized N-heterocyclic carbene molecules (NHCs), which were anchored on Au (111) surface, change their orientation from tilted into flat-lying position following trigger-induced reduction of their nitro groups. DFT calculations identified that the energetic driving force for reorientation was the lower steric hindrance and stronger interactions between the chemically reduced NHCs and the Au surface. The trigger-induced changes in the NHCs' anchoring geometry and chemical functionality modified the work function and the hydrophobicity of the NHC-decorated Au surface, demonstrating the impact of a chemically tunable NHC-based SAM on the properties of the metal surface.

Methanol oxidation on the Pt(321) surface: a theoretical approach on the role of surface morphology and surface coverage effects.

We investigated methanol oxidation, decomposition and carbonylation reactions on a high indexed Pt(321) surface. Adsorption studies of the reactants and reaction products were analyzed, where the favourable binding was found on the low coordinated atoms which are located on the step edges of this surface. The dependence of the reaction mechanism on the surface structure could be observed by analysis of methanol decomposition via O-H and C-H bond cleavage, where the latter showed a significant preference in energy. Compared to flat surfaces, the stepped and kinked Pt(321) surface showed a much stronger adsorption behavior towards methanol decomposition exhibiting small activation barriers. The consideration of higher coverage of molecules, e.g. water, during the decomposition reaction showed significant reduction of the corresponding activation barriers. Finally, the influence of a second metal such as Au or Pd in the Pt(321) surface reduced the CO poisoning of the reactive steps and kinks by the formation of carbon dioxide resulting in the accessibility of the reactive surface atoms for next/further reaction steps.

Theoretical Studies on the Hydroaminoalkylation of Alkenes with Primary and Secondary Amines.

The α-alkylation of amines with alkenes catalyzed by early transition-metal complexes represents an efficient and atom economic method for the synthesis of functionalized amines from simple and easily available starting materials. While the successful use of secondary amines, such as dimethylamine, strongly underlines the enormous industrial potential of this reaction, the analogous intermolecular α-alkylation of primary amines, especially methylamine, remains an unsolved synthetic task to this day. Based on calculated thermodynamic data, these experimental findings can now be explained for the first time, whereby several competing reactions, which are explained in detail, are of crucial importance for the different behavior of primary and secondary amines.

CO adsorption and oxygen activation on group 11 nanoparticles - a combined DFT and high level CCSD(T) study about size effects and activation processes.

The focus of this study lies in the activation of molecular oxygen and reaction with CO within density functional theory (DFT) and high level CCSD(T) calculations. Therefore, we use M13 and M55 nanoparticles (NPs) and periodic M(321) surfaces as model systems and compare the catalytic activity of gold substrates to Ag and Cu based NP catalysts. In the first step, the adsorption energies of CO were compared for nanoparticles of different sizes for Au, Ag and Cu. The adsorption energies on M(321) and M55 NPs (M = Au, Ag, Cu) are virtually identical. For smaller M13 NPs the adsorption energies differ by ∼0.2 eV for Ag, ∼0.4 eV for Au, and ∼0.6 eV for Cu at the PBE level of theory. This can be explained by size effects, as the M13 NPs show a more molecule-like character. Presumably, CO binds more strongly to these very small NPs at the PBE level of theory. However, a benchmark calculation in the framework of CCSD(T)-theory reveals an adsorption energy of CO on Au13 of -0.88 eV, comparable to the adsorption energies calculated at the PBE level for Au55 and Au(321). For Au55, the adsorption energy calculated at the CCSD(T) level is -0.85 eV. This is in perfect agreement with the PBE result. In addition to adsorption energies, dissociation barriers have been calculated on M(321) surfaces. The dissociation energies of O2 on coinage metal catalysts are high, so that direct CO-oxidation reactions with molecular oxygen should be the dominant reaction mechanism compared to the dissociation and reaction of CO and atomic oxygen at least for silver and copper catalysts.

Explicitly Correlated Orbital Optimized Contracted Pair Correlation Methods: A Short Overview.

Designing efficient algorithms to apply highly accurate pair correlation methods to large molecules of scientific interest has remained an important field of research for a long period of time. We present a new approach toward fast algorithms, which represents an interesting alternative and extension to currently existing methods. The presented new contraction scheme saves a significant amount of memory and can be easily combined with efficient linear scaling algorithms. Additionally, the extension to orbital optimization and explicitly correlated f12-theory is demonstrated to further improve accuracy and applicablility.

Formation of Binuclear Zigzag Hexapentaene Titanium Complexes via a Titanacumulene [Ti=C=C=CH2 ] Intermediate.

The reaction of bis(η5 :η1 -pentafulvene)titanium complexes with an allylidenephosphorylide Ph3 P=C(H)- C(H)=CH2 leads to binuclear zigzag hexapentaene titanium complexes (Ti2a, Ti2b). The formation of the central C6 H4 unit can be described as a spontaneous double C-H bond activation process, leading to an R3 P=C=C=CH2 intermediate, as a synthon for a titanabutatriene fragment [(CpR )2 Ti=C=C=CH2 ] (R: 2-adamantyl, CH(p-tol)2 ). In a subsequent dimerization Ti2a and Ti2b are formed, proofed by single-crystal X-ray diffraction and NMR measurements. The reaction sequence is confirmed by DFT calculations.

Towards Polysulfuric Acids: The Hydrogentrisulfate Anion [HS3 O10 ]- in A[HS3 O10 ] (A=Na, K, Rb).

The reaction of Na2 SO4 and K2 SO4 with fuming sulfuric acid (65 % SO3 ) yielded colorless extremely sensitive crystals of Na[HS3 O10 ] (monoclinic; P21 /n (No. 14); Z=4; a=707.36(2), b=1378.56(4), c=848.10(3) pm; ß=94.817(1)°; V=824.09(4)⋅106  pm3 ) and K[HS3 O10 ] (orthorhombic; Pccn (No. 56); Z=4; a=1057.16(3), b=807.81(2), c=897.57(2) pm; V=766.51(3)⋅106  pm3 ). The analogous rubidium compound Rb[HS3 O10 ] (orthorhombic; Pnma (No. 62); Z=4; a=891.43(3), b=1095.34(4), c=839.37(3) pm; V=819.58(5)⋅106  pm3 ) originates from the reaction of Rb2 CO3 and SO3 . All of the different structures contain the hitherto unknown anion [HS3 O10 ]- and are stamped by strong hydrogen bonds linking the anions either to dimers or chains. Theoretical investigations by DFT methods give further insight in the structural characteristics of [HS3 O10 ]- . The preparation of the [HS3 O10 ]- anion can be seen as an important milestone on our way to the still elusive polysulfuric acids.

Palladium(IV) in an Oxoanionic Environment: The XeF2 Assisted Synthesis of [Pd(S2 O7 )3 ](2.).

For the first time tetravalent palladium is detected, coordinated exclusively by simple oxoanions. Stabilization was achieved by formation of the [Pd(S2 O7 )3 ](2-) complex in which three chelating disulfate groups surround the metal atom. The salt K2 [Pd(S2 O7 )3 ] could only be obtained if the reaction of K2 [PdCl6 ] and neat SO3 was performed in the presence of XeF2 .

Polysulfates [Sn O3n+1 ]2- : With the [S6 O19 ]2- Anion, has the End been Reached?

The S6 O192- ion was obtained both as rubidium and ammonium salt from the reaction of the respective sulfate with SO3 . It is the largest polysulfate ion known to date and exhibits a chain of six vertex-connected [SO4 ] tetrahedra. The unique compound was comprehensively characterized and the bonding within the anion was elucidated by theoretical investigations.

Oxoanionic noble metal compounds from fuming nitric acid: the palladium examples Pd(NO3)2 and Pd(CH3SO3)2.

The oxidation of elemental palladium at 100 °C in a mixture of fuming nitric acid and a pyridine-SO3 complex leads to the anhydrous nitrate Pd(NO3)2 (monoclinic, P2(1)/n, Z=2, a=469.12(3) pm, b=593.89(3) pm, c=805.72(4) pm, ß=105.989(3)°, V=215.79(2) Å(3)). The Pd(2+) ions are in square-planar coordination with four monodentate nitrate groups which are connected to further palladium atoms, leading to a layer structure. The reaction of elemental palladium with a mixture of fuming nitric acid and methanesulfonic acid at 120 °C leads to single crystals of Pd(CH3SO3)2 (monoclinic, P2(1)/n, Z=2, a=480.44(1) pm, b=1085.53(3) pm, c=739.78(2) pm, ß=102.785(1)°, V=376.254(17) Å(3)). Also in this structure the Pd(2+) ions are in square-planar coordination with four monodentate anions; however, the connection to adjacent palladium atoms leads to a chain-type structure. The thermal decomposition of the compounds has been investigated by means of DSC/TG measurements. Furthermore, IR and Raman spectra have been recorded, and an assignment of the observed vibrational frequencies has been carried out based on theoretical investigations.

A Highly Triflated Rare-Earth Ion in [Eu(O3SCF3)8](5-).

The reaction of Eu2O3 with fuming nitric acid, trifluormethanesulfonic acid, and its anhydride in torch-sealed glass ampoules at 120 °C gave the europium compound (NO)5[Eu(O3SCF3)8] (orthorhombic, Fddd, Z = 16, a = 1932.69(4), b = 2878.44(7), c = 2955.12(7) pm, V = 16439.7(7) Å(3)). The compound exhibits the [Eu(O3SCF3)8](5-) anion showing for the first time a lanthanide ion that is exclusively coordinated by eight triflate anions. The anion has been further investigated by DFT calculations, which also allowed clear assignment of the vibrational spectra. Moreover, magnetochemical and luminescence measurements gave additional insight into the properties of this complex. The luminescence spectra revealed that the Eu(3+) ions are in a pseudo D4d symmetric environment.

Photodesorption of water from rutile(110): ab initio calculation of five-dimensional potential energy surfaces of ground and excited electronic states and wave packet studies.

In this paper, we report on our results concerning the interaction of water with titanium dioxide in its rutile modification. The (110) surface is modelled by an embedded Ti9O18Mg7(14+) cluster. We present up to five-dimensional potential energy surfaces for the water molecule on this surface and include the dissociation of one hydrogen atom. The electronic ground state as well as one electronically excited state is included. To deal with the multi-configurational character of the wave function, we use the complete active space self-consistent field (CASSCF) approach. The resulting potential energy surfaces are fitted by means of an artificial neural network. As a first example of quantum dynamical studies based on our potential surfaces, we present results on the photodesorption of water from rutile(110).

Oxidizing elemental platinum with oleum under harsh conditions: the unique tris(disulfato)platinate(IV) [Pt(S2O7)3]2- anion.

For the first time, direct oxidation of elemental platinum by a mineral acid to its tetravalent state was observed in course of the reaction of platinum with oleum (65 % SO3) in the presence of barium carbonate. The reaction has been carried out in torch-sealed glass ampoules at 160 °C and gave yellow single crystals of Ba[Pt(S2O7)3](H2SO4)0.5(H2S O7)0.5 (triclinic, P1, Z=2, a=992.05(2), b=1069.07(3), c=1114.22(3) pm, α=69.49(7), ß=72.96(2), γ=72.93(1)°, V=1033.95(5) Å(3)). The structure of Ba[Pt(S2O7)3](H2SO4)0.5(H2S2O7)0.5 exhibits the unique tris-(disulfato)-platinate anion [Pt(S2O7)3](2-) with three chelating disulfate groups coordinated to the platinum atom. Charge balance is achieved by the Ba(2+) ions, which are coordinated by (S2O7)(2-) groups from the platinate complex and by disordered sulfuric acids and disulfuric acid molecules. Thermal decomposition of the bulk material revealed elemental platinum and barium sulfate as decomposition residual.