Martin Nieger

Facile Heterolytic H2 Activation by Amines and B(C6F5)3

In industrially important reactions, such as hydroformylation and hydrogenation, H2 gas serves as a reducing agent and/or a hydrogen-atom source. Even small improvements in the efficiency of these reactions translate into large monetary savings. The key step in these transformations is the activation of H2 at a transition metal. The nodal character of the energetically accessible d orbitals allows a transition-metal center to react directly with H2 in a concerted reaction with a low activation barrier. However, not only are transitionmetal complexes expensive, but the complete removal of metal impurities from the reaction product is generally required in the production of pharmaceutical intermediates owing to toxicity concerns. Although countless synthetic complexes and enzymes with transition metals at their reactive core are well known, there are significantly fewer examples of H H bond activation facilitated solely by a nonmetal. Several reactions of H2 with compounds containing main-group elements in lowtemperature matrices have been reported; however, H2 activation at nonmetals under mild conditions had only been observed by Power and co-workers in product mixtures of digermenes, digermanes, and primary germanes, until recently, when Stephan and co-workers reported the thermal liberation of H2 from a phosphonium borate salt. The resulting product, (C6H2Me3)2P(C6F4)B(C6F5)2, undergoes the addition of H2 at 25 8C to reform the original salt. [7] In an analogous fashion, mixtures of sterically demanding phosphanes and boranes (“frustrated Lewis pairs”) can also cleave H2 heterolytically to form phosphonium borates [R3PH][HBR’3]. [9] More recently, Bertrand and co-workers reported that selected organic carbenes are just nucleophilic enough to cleave H2 and NH3. [10] Herein we extend the family of “frustrated Lewis pairs” and demonstrate that not only bulky phosphanes and boranes or organic carbenes can cleave H2, but also inexpensive, stable amines in combination with B(C6F5)3. Solutions of stoichiometric mixtures of diisopropylethylamine, diisopropylamine, or 2,2,6,6-tetramethylpiperidine and B(C6F5)3 in toluene were investigated by H, B, and F NMR spectroscopy. The reactions of diisopropylethylamine and diisopropylamine with B(C6F5)3 gave mixtures of the salt 1a or 1b and the zwitterion 2a or 2b as expected (Scheme 1); however, no reaction was observed for 2,2,6,6tetramethylpiperidine, a bulky secondary amine with no a hydrogen atoms. Whereas the reaction between diisopropylamine and B(C6F5)3 is reversible at elevated temperature, the mixture of 1a and 2a from the reaction with diisopropylethylamine is thermally stable (Scheme 1).

Synthesis, structure, and characterization of dinuclear copper(I) halide complexes with P^N ligands featuring exciting photoluminescence properties.

A series of highly luminescent dinuclear copper(I) complexes has been synthesized in good yields using a modular ligand system of easily accessible diphenylphosphinopyridine-type P^N ligands. Characterization of these complexes via X-ray crystallographic studies and elemental analysis revealed a dinuclear complex structure with a butterfly-shaped metal-halide core. The complexes feature emission covering the visible spectrum from blue to red together with high quantum yields up to 96%. Density functional theory calculations show that the HOMO consists mainly of orbitals of both the metal core and the bridging halides, while the LUMO resides dominantly on the heterocyclic part of the P^N ligands. Therefore, modification of the heterocyclic moiety of the bridging ligand allows for systematic tuning of the luminescence wavelength. By increasing the aromatic system of the N-heterocycle or through functionalization of the pyridyl moiety, complexes with emission maxima from 481 to 713 nm are obtained. For a representative compound, it is shown that the ambient-temperature emission can be assigned as a thermally activated delayed fluorescence, featuring an attractively short emission decay time of only 6.5 μs at ϕPL = 0.8. It is proposed to apply these compounds for singlet harvesting in OLEDs.

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

Frustrated Lewis pairs are compounds containing both Lewis acidic and Lewis basic moieties, where the formation of an adduct is prevented by steric hindrance. They are therefore highly reactive, and have been shown to be capable of heterolysis of molecular hydrogen, a property that has led to their use in hydrogenation reactions of polarized multiple bonds. Here, we describe a general approach to the hydrogenation of alkynes to cis-alkenes under mild conditions using the unique ansa-aminohydroborane as a catalyst. Our approach combines several reactions as the elementary steps of the catalytic cycle: hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated-Lewis-pair reactivity) and facile intramolecular protodeborylation (product release). The mechanism is verified by experimental and computational studies. Frustrated Lewis pairs have been shown to be capable of heterolysis of strong covalent bonds such as those in molecular hydrogen, and have been used in the hydrogenation of polar multiple bonds. Here, a new type of ansa-aminohydroborane is shown to be active for the partial hydrogenation of alkynes under mild conditions.

Bright coppertunities: multinuclear Cu(I) complexes with N-P ligands and their applications.

Easy come, easy go: the great structural diversity of Cu(I) complexes is an ambivalent trait. Apart from the well-known catalytic properties of Cu(I), a great number of potent luminescent complexes have been found in the last ten years featuring a plethora of structural motifs. The downside of this variety is the undesired formation of other species upon processing. In here, strategies to avoid this behavior are presented: Only one favorable structural unit often exists for multinuclear Cu(I) complexes with bridging ligands. In addition, these complexes exhibit favorable photophysical properties due to cooperative effects of the metal halide core. Furthermore, we demonstrate the broad range of applications of emitting Cu(I) compounds.

Stability and Electrophilicity of Phosphorus Analogues of Arduengo Carbenes—An Experimental and Computational Study

A variety of differently substituted 1,3,2-diazaphospholenium salts and P-halogeno-1,3,2-diazaphospholenes (X = F, Cl, Br) were synthesized, and their molecular structures, bonding situation, and Lewis acid properties were characterized by experimental (single-crystal X-ray diffraction, NMR and IR/Raman spectroscopy, MS, conductometry, titrations with Lewis bases) and computational methods. Both experimental and computational investigations confirmed that the structure and bonding in the diazaphospholenium cations of OTf and BF4 salts resembles that of neutral Arduengo carbenes and that the cations should not be described as genuinely aromatic. P-Halogenodiazaphospholenes are, in contrast to earlier assumptions, molecular species with covalent P-X bonds whose bonding situation can be expressed in terms of hyperconjugation between the six pi electrons in the C2N2 unit and the sigma*(P-X) orbital. This interaction induces a weakening of the P-X bonds, whose extent depends subtly on substituent influences and contributes fundamentally to the amazing structural similarity of ionic and covalent diazaphospholene compounds. A further consequence of this effect is the unique polarizability of the P-Cl bonds in P-chlorodiazaphospholenes, which is documented in a considerable spread of P-X distances and bond orders. Measurement of the stability constants for complexes of diazaphospholene compounds with Lewis bases confirmed the lower Lewis acidities and higher stabilities of diazaphospholenium ions as compared with nonconjugated phosphenium ions; this had been inferred from computed energies of isodesmotic halide-transfer reactions, and permitted also to determine equilibrium constants for P-Cl bond dissociation reactions. The results suggest, in accord with conductance measurements, that P-chlorodiazaphospholenes dissociate in solution only to a small extent. On the basis of these findings, the unique solvatochromatic behavior of NMR chemical shifts of these compounds was attributed to solvent-dependent P-Cl bond polarization rather than to shifts in dissociation equilibria.

Intramolecular Diamination of Alkenes with Palladium(II)/Copper(II) Bromide and IPy2BF4: The Role of Halogenated Intermediates

The oxidative intramolecular diamination of alkenes with tethered ureas and related groups as the nitrogen source has been investigated both with the iodonium reagent IPy(2)BF(4) (Py=pyridine) and under palladium catalysis in the presence of copper(II) bromide as a reoxidant. For terminal alkenes, the two procedures enable selective and high-yielding transformations. Studies with deuterated material led to the conclusion that the reactions proceed through different stereochemical pathways. An advanced protocol for palladium-catalyzed diamination through six-membered-ring annulation was also developed, and the first examples of the intramolecular diamination of internal alkenes are described. In this case, the same stereochemical outcome was observed for the iodonium-promoted and palladium-catalyzed transformations. On this basis, it was possible to determine the importance of aminohalogenated intermediates in both diamination reactions. Overall, the disclosed procedures broaden significantly the synthetic applicability of the oxidative intramolecular diamination of alkenes.

ortho-Selective Phenol-Coupling Reaction by Anodic Treatment on Boron-Doped Diamond Electrode Using Fluorinated Alcohols

Enlarged scope by fluorinated mediators: Oxyl radicals are easily formed on boron-doped diamond (BDD) electrodes and can be exploited for the ortho-selective coupling to the corresponding biphenols (see scheme). At partial conversion, a clean transformation is achieved that can be applied to electron-rich as well as fluorinated phenols.

Valence Isomerization of a 1,3‐Diphosphacyclobutane‐2,4‐diyl: Photochemical Ring Closure to 2,4‐diphosphabicyclo[1.1.0]butane and Its Thermal Ring Opening to gauche‐1,4‐Diphosphabutadiene

The bond stretch isomer 1,3-diphosphacyclobutane-2,4-diyl 1 was transformed photochemically to give the previously unknown 2,4-diphosphabicyclo[1.1.0]butane 2, which itself can be converted thermally into gauche-1,4-diphosphabutadiene 3. The crystal structures of these three energy-rich valence isomers of 1,2-diphosphete have been determined. R=SiMe(3); Mes*=2,4,6-tBu(3)C(6)H(2).

Copper(I) Complexes Based on Five-Membered P∧N Heterocycles: Structural Diversity Linked to Exciting Luminescence Properties

Bridging P(^)N ligands bearing five-membered heterocyclic moieties such as tetrazoles, 1,2,4-triazoles, oxadiazoles, thiadiazoles, and oxazoles have been investigated regarding their complexation behavior with copper(I) iodide as metal salts. Different complex structures were found, depending either on the ligand itself or on the ligand-to-metal ratios used in the complexation reaction. Two different kinds of luminescent dinuclear complex structures and a kind of tetranuclear complex structure were revealed by X-ray single-crystal analyses and were further investigated for their photophysical properties. The emission maxima of these complexes are in the blue to yellow region of the visible spectrum for the dinuclear complexes and in the yellow to orange region for the tetranuclear complexes. Further investigations using density functional theory (DFT) show that the highest occupied molecular orbital (HOMO) is located mainly on the metal halide cores, while the lowest unoccupied molecular orbital (LUMO) resides mostly in the ligand sphere of the complexes. The emission properties were further examined in different environments such as neat powders, neat films, PMMA matrices, or dichloromethane solutions, revealing the high potential of these complexes for their application in organic light-emitting diodes. Especially complexes with 1,2,4-triazole moieties feature emission maxima in the blue region of the visible spectrum and quantum yields up to 95% together with short decay times of about 1-4 μs and are therefore promising candidates for blue-emitting materials in OLEDs.

Chiral Molecular Tweezers: Synthesis and Reactivity in Asymmetric Hydrogenation

We report the synthesis and reactivity of a chiral aminoborane displaying both rapid and reversible H2 activation. The catalyst shows exceptional reactivity in asymmetric hydrogenation of enamines and unhindered imines with stereoselectivities of up to 99% ee. DFT analysis of the reaction mechanism pointed to the importance of both repulsive steric and stabilizing intermolecular non-covalent forces in the stereodetermining hydride transfer step of the catalytic cycle.