Electronic spectroscopy of homo- and heterometallic binuclear coinage metal phosphine complexes in isolation.

Binuclear coinage metal phosphine complexes are examined under ion trap isolation in order to elucidate their noncovalent binding, structural properties and intrinsic electronic spectra. Our survey shows an intriguing order of electronic transitions obtained by in situ synthesis and mass-spectrometrically supported UV photodissociation spectroscopy on a series of six isolated homo- and heterobinuclear complexes of type [MM'(dcpm)2]2+ (M, M' = CuI, AgI, AuI; dcpm = bis(dicyclohexyl-phosphino)methane). This approach provides the unique opportunity to study all possible coinage metal interactions within a fixed ligand framework. A successive blue-shift (33 700-38 500 cm-1; 297-260 nm) of the lowest-energy bright electronic transition energy in gas phase was observed in the order of Cu2 < CuAu < CuAg < Au2 < AgAu < Ag2. This order was reproduced by quantum chemical calculations using a scalar-relativistic GW-Bethe-Salpeter-equation (GW-BSE) approach. Theory ascribes the electronic bands of all complexes to metal-centered 1MC(dσ*-pσ) transitions revealing a strengthening of metal-metal' (M-M') binding upon excitation, in agreement to mass spetrometric results. A test of the correlation of transition energies with M-M' distance by quantum chemical calculations of single point energies as a function of intermetallic distance indicates qualitative agreement with experimental results. However, the experimentally observed high sensitivity of spectroscopic shifts towards metal composition cannot be accounted for solely by M-M' distance variation. The differences in electronic transitions are qualitatively rationalized by the varying (n + 1)s (n = 3, 4, 5) orbital contributions (increase from Cu2via CuAu/CuAg to Au2/AgAu/Ag2) within the nd(n + 1)s/p-hybridization for the ground electronic state of the different complexes, whereas the excited state (of (n + 1)p orbital character) shows significantly less variation in energy. In particular, the observed spectroscopic and mass spectrometric sequence for the Ag/Au complexes is traced back to the interplay of Pauli repulsion and variation in metal-ligand bond strength within the orbital hybridization model.

Periodic Mesoporous Organosilica Nanomaterials with Unconventional Structures and Properties.

Periodic mesoporous organosilicas (PMOs) are high surface area organic-inorganic hybrid nanomaterials that have found broad applications in various fields of research such as in (bio)chemistry or material science. By choosing suitable organic groups in the framework of these materials, their surface properties such as polarity, optical/electrical characteristics and adsorption capacity can be tuned. This critical review provides an overview of the current state of the art in the developments and applications of some PMO nanomaterials in diverse fields of research. This is placed in the context of four leading areas of PMO nanomaterials, including chiral PMOs, plugged PMO nanomaterials, Janus PMOs and PMO-based nanomotors. The review briefly discusses the recent and key findings on these PMO nanomaterials as well as their potential applications for future developments.

Task-Specific Janus Materials in Heterogeneous Catalysis.

Janus materials are anisotropic nano- and microarchitectures with two different faces consisting of distinguishable or opposite physicochemical properties. In parallel with the discovery of new methods for the fabrication of these materials, decisive progress has been made in their application, for example, in biological science, catalysis, pharmaceuticals, and, more recently, in battery technology. This Minireview systematically covers recent and significant achievements in the application of task-specific Janus nanomaterials as heterogeneous catalysts in various types of chemical reactions, including reduction, oxidative desulfurization and dye degradation, asymmetric catalysis, biomass transformation, cascade reactions, oxidation, transition-metal-catalyzed cross-coupling reactions, electro- and photocatalytic reactions, as well as gas-phase reactions. Finally, an outlook on possible future applications is given.

Janus bifunctional periodic mesoporous organosilica.

Synthesis of a Janus periodic mesoporous organosilica material (JPMO) is presented here. In this strategy, the surface of the hollow silica material was selectively functionalized with two different bridged organic-inorganic hybrid groups. It was found that the resulting bifunctional material is able to form a stable Pickering emulsion. This new type of PMO material may be suitable for widespread applications in various fields related to material science and catalysis.

Fast and Selective Aqueous-Phase Oxidation of Styrene to Acetophenone Using a Mesoporous Janus-Type Palladium Catalyst.

A heterogeneous Janus-type palladium interphase catalyst was obtained by selective surface modification of a hollow mesoporous silica material. The catalyst comprises hydrophobic octyl groups on one side of the silica nanosheets and single-site bis-imidazoline dichlorido palladium(II) complexes on the other. The structure of this composite material has been analyzed by means of elemental analysis, atomic absorption spectroscopy, BET surface analysis, TGA, SEM and solid-state CP-MAS 13C and 29Si NMR spectroscopy. The catalyst showed extraordinary activity for the aqueous-phase oxidation of styrene to acetophenone using 30% hydrogen peroxide as the oxidant. An 88% yield of acetophenone could be achieved after 60 min.

A Cyclometalated NHC Iridium Complex Bearing a Cationic (η5 -Cyclopentadienyl)(η6 -phenyl)iron Backbone*.

Nucleophilic substitution of [(η5 -cyclopentadienyl)(η6 -chlorobenzene)iron(II)] hexafluorophosphate with sodium imidazolate resulted in the formation of [(η5 -cyclopentadienyl)(η6 -phenyl)iron(II)]imidazole hexafluorophosphate. The corresponding dicationic imidazolium salt, which was obtained by treating this imidazole precursor with methyl iodide, underwent cyclometallation with bis[dichlorido(η5 -1,2,3,4,5-pentamethylcyclopentadienyl]iridium(III) in the presence of triethyl amine. The resulting bimetallic iridium(III) complex is the first example of an NHC complex bearing a cationic and cyclometallated [(η5 -cyclopentadienyl)(η6 -phenyl)iron(II)]+ substituent. As its iron(II) precursors, the bimetallic iridium(III) complex was fully characterized by means of spectroscopy, elemental analysis and single crystal X-ray diffraction. In addition, it was investigated in a catalytic study, wherein it showed high activity in transfer hydrogenation compared to its neutral analogue having a simple phenyl instead of a cationic [(η5 -cyclopentadienyl)(η6 -phenyl)iron(II)]+ unit at the NHC ligand.

Dinuclear Copper Complex Immobilized on a Janus-Type Material as an Interfacial Heterogeneous Catalyst for Green Synthesis.

We herein describe a rational design of a heterogeneous catalyst composed of a dinuclear cuprate anion being immobilized electrostatically on one surface of Janus-type nanosheets while the other surface is decorated with highly hydrophobic octyl groups. The catalyst was found to be well dispersible in the organic phase of a biphasic aqueous/organic mixture. It was characterized by means of elemental analysis, atomic absorption spectroscopy, mass spectrometry, N2 absorption-desorption analysis, thermogravimetric analysis, scanning electron microscopy (SEM), and solid-state 13C and 29Si cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy. The Janus nature of the catalyst was investigated by employing a selective surface labeling method and by means of SEM. The catalyst shows higher activity compared to a non-Janus analogue in a biphasic synthesis. It was successfully used for the azide-alkyne cycloaddition and the Chan-Lam C-N coupling reaction. In addition, new and simple ways have been established for the production of a coumarin-triazole derivative and for the synthesis of the biologically active compound Monastrol via a solvent-free Biginelli reaction. The role of the dinuclear copper centers is discussed mechanistically.

Triazolyl Conjugated (Oligo)Phenothiazines Building Blocks for Hybrid Materials-Synthesis and Electronic Properties.

The Cu-catalyzed alkyne-azide 1,3-dipolar cycloaddition variant provides a highly efficient entry to conjugated triazolyl-substituted (oligo)phenothiazine organosilicon derivatives with luminescence and reversible redox characteristics. Furthermore, by in-situ co-condensation synthesis several representative mesoporous MCM-41 type silica hybrid materials with embedded (oligo)phenothiazines are prepared and characterized with respect to their structural and electronic properties. The hybrid materials also can be oxidized to covalently bound embedded radical cations, which are identified by their UV/Vis absorption signature and EPR signals.

Quantification of Cooperativity between Metal Sites in Dinuclear Transition Metal Complexes Containing the (2-Dimethylamino)-4-(2-pyrimidinyl)pyrimidine Ligand.

A concept for the quantification of cooperative effects in transition-metal complexes is presented. It is demonstrated for a series of novel N,N- (mononuclear) and C,N-coordinated homo- and heterometallic binuclear complexes based on the (2-dimethylamino)-4-(2-pyrimidinyl)pyrimidine ligand, which are accessible by applying roll-over cyclometallation. These iridium-, platinum-, and palladium-containing compounds are investigated with respect to their absorption and fluorescence spectra. The cooperative effects in the electronic absorptions, i. e., the energetic shifts between mononuclear and dinuclear complexes, and free ligands are analyzed on the basis of the lowest energy π-π* transitions and compared to calculated data, obtained from TD-DFT calculations. Furthermore the corresponding fluorescence spectra are presented and analyzed with respect to the concept of cooperativity.

Photoinitiated Charge Transfer in a Triangular Silver(I) Hydride Complex and Its Oxophilicity.

The photoexcitation of a triangular silver(I) hydride complex, [Ag3 (µ3 -H)(µ2 -dcpm)3 ](PF6 )2 ([P](PF6 )2 , dcpm=bis(dicyclohexylphosphino)methane), designed with "UV-silent" bis-phosphine ligands, provokes hydride-to-Ag3 single and double electron transfer. The nature of the electronic transitions has been authenticated by absorption and photodissociation spectroscopy in parallel with high-level quantum-chemical computations utilizing the GW method and Bethe-Salpeter equation (GW-BSE). Specific photofragments of mass-selected [P]2+ ions testify to charge transfer and competing pathways resulting from the unique [Ag3 (µ3 -H)]2+ scaffold. This structural motif of [P](PF6 )2 has been unequivocally verified by 1 H NMR spectroscopy in concert with DFT and X-ray diffraction structural analysis, which revealed short equilateral Ag-Ag distances (dAgAg =3.08 Å) within the range of argentophilic interactions. The reduced radical cation [P]. + exhibits strong oxophilicity, forming [P+O2 ].+ ,which is a model intermediate for silver oxidation catalysis.

Cyclopalladation in the Periphery of a NHC Ligand as the Crucial Step in the Synthesis of Highly Active Suzuki-Miyaura Cross-Coupling Catalysts.

Starting from 2,4-dichloropyrimidine, 4-(2-dialkylamino)pyrimidinyl functionalized mesitylimidazolium chlorides are accessible in a five-step reaction sequence. Two routes leading to palladium NHC complexes derived from these ligands have been worked out: By transmetalation with the corresponding NHC-AgCl complexes, C,N-coordinated palladium(II) complexes can be obtained. Treatment of palladium dichloride with the imidazolium salts in pyridine and in the presence of K2 CO3 gives cyclometalated and thus C,C-coordinated compounds. The reactivities of all these compounds were investigated in detail as well as their performance in the catalytic Suzuki-Miyaura cross-coupling reaction. It turned out that the C,C-coordinated derivatives exhibit high catalytic activities in the coupling of arylboronic acids with aryl chlorides, which is consistent with the generally accepted mechanistic ideas on substrate activation.

[2.2]Paracyclophanes with N-Heterocycles as Ligands for Mono- and Dinuclear Ruthenium(II) Complexes.

[2.2]Paracyclophane, with its unique structure, allows the design of unusual 3D structures by functionalization of this rigid and stable hydrocarbon scaffold. Therefore different mono- and homodisubstituted [2.2]paracyclophanes with pyridyl, pyrimidyl and oxazolinyl substituents were developed in order to evaluate their ability as bridging ligands for two ruthenium centres. With the successfully synthesized [2.2]paracyclophane-based N-donor functions, the cycloruthenation reaction using [RuCl2 (p-cymene)]2 as precursor was explored. Compared to 2-phenylpyridine, the [2.2]paracyclophane derivative is clearly inferior in the cycloruthenation reaction, resulting in poor yields for the neutral complexes. By addition of KPF6 , the cationic complexes can be obtained in good yields and are formed diastereoselectively in case of a pyridyl substituent, resulting in only one diastereomer for dinuclear ruthenium complexes of bispyridyl-substituted [2.2]paracyclophanes as bridging ligands.

Exploring the Gas-Phase Activation and Reactivity of a Ruthenium Transfer Hydrogenation Catalyst by Experiment and Theory in Concert.

This study elucidates structures, activation barriers, and the gas-phase reactivity of cationic ruthenium transfer hydrogenation catalysts of the structural type [(η6-cym)RuX(pympyr)]+. In these complexes, the central ruthenium(+II) ion is coordinated to an η6-bound p-cymene (η6-cym), a bidentate 2-R-4-(2-pyridinyl)pyrimidine ligand (pympyr) with R = NH2 or N(CH3)2, and an anion X = I-, Br-, Cl-, or CF3SO3-. We present infrared multiple-photon dissociation (IR-MPD) spectra of precursors (before HCl loss) and of activated complexes (after HCl loss), which elucidates C-H activation as the key step in the activation mechanism. A resonant two-color IR-MPD scheme serves to record several otherwise "dark" bands and enhances the validity of spectral assignments. We show that collision-induced dissociation (CID)-derived activation energies of the [(η6-cym)RuX(pympyr)]+ (R = N(CH3)2) complexes depend crucially on the anion X. The obtained activation energies for the HX loss correlate well with quantum chemical activation barriers and are in line with the HSAB concept. We further elucidate the reaction of the activated complexes with D2 under single-collision conditions. Quantum mechanical simulations substantiate that the resulting species represent analogues for hydrido intermediates formed after abstraction of H+ and H- from isopropanol, as postulated for the catalytic cycle of transfer hydrogenation by us before.

Mechanistic Studies on Ruthenium(II)-Catalyzed Base-Free Transfer Hydrogenation Triggered by Roll-Over Cyclometalation.

The synthesis of 2-substituted pyridine-pyrimidine ligands and their complexation with arene ruthenium(II) chloride moieties is reported. Depending on the electronic and steric influences of the ligand, the catalysts undergo CH activation by roll-over cyclometalation. This process opens up the route to the catalytic transfer hydrogenation of ketones with isopropanol as the hydrogen source under base-free and mild conditions. Barriers related to the roll-over cyclometalation process can be determined experimentally by collision-induced dissociation ESI mass spectrometry. They are supported by DFT calculations and allow the classification of the ligands according to their electronic and steric properties, which is also in accordance with critical bond parameters derived from X-ray structure data. DFT calculations furthermore reveal that the formation of a ruthenium(II) hydrido species is plausible through ß-hydride elimination from isopropanol.

Intermetallic Competition in the Fragmentation of Trimetallic Au-Zn-Alkali Complexes.

Cationization is a valuable tool to enable mass spectrometric studies on neutral transition-metal complexes (e.g., homogenous catalysts). However, knowledge of potential impacts on the molecular structure and catalytic reactivity induced by the cationization is indispensable to extract information about the neutral complex. In this study, we cationize a bimetallic complex [AuZnCl3 ] with alkali metal ions (M(+) ) and investigate the charged adducts [AuZnCl3 M](+) by electrospray ionization mass spectrometry (ESI-MS). Infrared multiple photon dissociation (IR-MPD) in combination with density functional theory (DFT) calculations reveal a µ(3) binding motif of all alkali ions to the three chlorido ligands. The cationization induces a reorientation of the organic backbone. Collision-induced dissociation (CID) studies reveal switches of fragmentation channels by the alkali ion and by the CID amplitude. The Li(+) and Na(+) adducts prefer the sole loss of ZnCl2 , whereas the K(+) , Rb(+) , and Cs(+) adducts preferably split off MCl2 ZnCl. Calculated energetics along the fragmentation coordinate profiles allow us to interpret the experimental findings to a level of subtle details. The Zn(2+) cation wins the competition for the nitrogen coordination sites against K(+) , Rb(+) , and Cs(+) , but it loses against Li(+) and Na(+) in a remarkable deviation from a naive hard and soft acids and bases (HSAB) concept. The computations indicate expulsion of MCl2 ZnCl rather than of MCl and ZnCl2 .

Multistate-Mediated Rearrangements and FeCl2 Elimination in Dinuclear FePd Complexes.

Mass spectrometric, spectroscopic, and computational characterization of a novel bifunctional iron-palladium complex proves a change of coordination upon solvation. Collisional excitation reveals FeCl2 and HCl elimination in a solvent-modulated competition. Hereby, syn and anti isomers, identified by theoretical calculations, favor and disfavor FeCl2 elimination, respectively. The FeCl2 elimination likely proceeds by chlorido and Cp ligand exchange among the metallic centers in a concerted, ballet-like manner. A multitude of stationary points were identified along the computed multistep reaction coordinates of the three conceivable spin states. The quintet state shows a static Jahn-Teller type relaxation by a tilt away of the Cp ligand at the iron center. The direct singlet-quintet spin crossover is an unprecedented assumption, leaving behind the triplet state as a spectator without involvement. The FeCl2 elimination would decrease catalytic activity. It is kinetically hindered within a range of applicable temperatures in conceivable technical applications.

Photoemission Studies on N-Substituted Dithienylated Phenothiazines.

Dithienylated phenothiazines (DTPTs) with different functional groups attached to the central nitrogen atom are presented as a class of versatile metal-free chromophores for the design of dye-sensitized solar cells (DSSCs) and organic light-emitting diodes (OLEDs). The electronic characteristics of spin-coated thin films on polycrystalline gold were studied using photoelectron spectroscopy assisted by theoretical calculations, scanning force microscopy, and UV/Vis spectroscopy. Complementary fluorescence spectra show light emission in the blue region (465 nm). The absorption properties and good hole-transporting abilities make DTPTs feasible hole-transporting materials (HTM) and metal-free chromophores in UV-sensitive solar cell designs.

P,C-bond cleavage in the ligand sphere of a nickel(II) complex.

Reacting nickel(ii)perchlorate with a bidentate P,N-ligand in methanol leads to P,C-bond cleavage and gives a five-coordinate nickel complex wherein the nickel(ii) site is coordinated by a tridentate P,N,P-ligand and a bidentate N,C-ligand. The carbanion of the latter is the result of the P,C-bond cleaving process. The diamagnetic nickel(ii) complex was characterized by means of elemental analysis, NMR spectroscopy, cyclic voltammetry and X-ray structure analysis.

A rechargeable hydrogen battery based on Ru catalysis.

Apart from energy generation, the storage and liberation of energy are among the major problems in establishing a sustainable energy supply chain. Herein we report the development of a rechargeable H2 battery which is based on the principle of the Ru-catalyzed hydrogenation of CO2 to formic acid (charging process) and the Ru-catalyzed decomposition of formic acid to CO2 and H2 (discharging process). Both processes are driven by the same catalyst at elevated temperature either under pressure (charging process) or pressure-free conditions (discharging process). Up to five charging-discharging cycles were performed without decrease of storage capacity. The resulting CO2/H2 mixture is free of CO and can be employed directly in fuel-cell technology.

Gold(I) complexes with heteroaryl phosphine ligands.

Gold(I) complexes ligated by phosphines with N-heterocycles in the periphery were prepared. First the synthesis of the ligands N-(diphenylphosphino)-4-(pyridin-2-yl)pyrimidin-2-amine (Hpypya) and N-(diphenylphosphino)-4-phenylpyrimidin-2-amine (Hphpya) are reported. These two compounds together with the related but earlier published ligands 3-(2-(diphenylphosphino)phenyl)-1H-pyrazole (Hph3py) and 5-(4-(diphenylphosphino)phenyl)-1H-pyrazole (Hph5py) were reacted with [(tht)AuCl] and [Au(tht)2]ClO4 to give the heteroleptic complexes [(L)AuCl] and the homoleptic compounds [(L)2Au]ClO4 (L = Hpypya, Hphpya, Hph3py, and Hph5py). Single crystal X-ray diffraction studies revealed that the heteroleptic complexes form hydrogen bonds between two N-heterocycles of neighboring complexes resulting in dimeric structures. The homoleptic complexes show a different behavior.