A Saddle-Shaped Expanded Porphyrinoid Fitting C60.

We present the synthesis of a new type of an expanded porphyrinoid macrocycle with a saddle-shaped morphology and its complexation of C60 guest molecules. The new macrocycle contains four carbazole and four triazole moieties and can be readily synthesized via a copper-catalyzed click reaction. It shows specific photo-physical properties including fluorescence with a high quantum yield of 60 %. The combination of the saddle-shaped geometry with the expanded π-system allows for host-guest interactions with C60 in a stacked polymer fashion. Evidence for the presence of a host-guest complex is provided both in solution by NMR spectroscopy and in the solid state by X-ray structure analysis.

The Fascinating Flexibility and Coordination Modes of a Pentamethylene Connected Macrocyclic CNC Pincer Ligand.

The coordination chemistry of an electron-rich macrocyclic CNC pincer-ligand consisting of two pentamethylene tethered N-heterocyclic carbene moieties on a carbazole backbone (bimcaC5) is investigated by mainly NMR spectroscopy and X-ray crystal structure analysis. A bridging coordination mode is found for the lithium complex. With the larger and softer potassium ion, the ligand adopts a facial coordination mode and a polymeric structure by intermolecular potassium nitrogen interactions. The facial coordination is also confirmed at a Cp*Ru fragment, while C-H activation under dehydrogenation at the alkyl chain is observed upon reaction with [Ru(PPh3)3Cl2]. In contrast, Pd(OAc)2 reacts under C-H activation at the central carbon atom of the pentamethylene tether to an alkyl-pincer macrocycle.

Carbenaporphyrins: No Longer Missing Ligands in N-Heterocyclic Carbene Chemistry.

The synthesis of an NHC-containing porphyrinoid ligand is presented. The formally antiaromatic 20 πe- macrocyclic framework can be obtained via a 1,3-dipolar cycloaddition ("click-reaction") to form two triazole moieties which were alkylated to the respective triazolium macrocycle. Deprotonation of the ligand precursor with lithium bases to the respective dilithio carbenaporphyrin complex and transmetallation to scandium lead to complexes that exhibit orange fluorescence. Optical property combined with TD-DFT studies verify an aromatic character for each heterocyclic moiety rather than an antiaromatic macrocycle in the ligand precursor as well as in the complexes. While the geometric features of the carbenaporphyrin ligand strongly resemble those of porphyrin, DFT calculations reveal a stronger electron-donating ability of the new ligand.

Electron-Deficient Imidazolium Substituted Cp Ligands and their Ru Complexes.

The synthesis of electron-poor mono-, di- and tri(imidazolium)-substituted Cp-ylides is presented and their electronic properties are discussed based on NMR spectroscopy, X-ray structure analyses, electrochemical investigations and DFT calculations as well as by their reactivity toward [Ru(CH3 CN)3 Cp*](PF6 ). With mono- and di(imidazolium)-substituted cyclopentadienides the respective monocationic and dicationic ruthenocences are formed (X-ray), whereas tri(imidazolium) cyclopentadienides are too electron-poor to form the ruthenocenes. Cyclic voltammetric analysis of the ruthenocenes shows reversible oxidation at a potential that increases with every additional electron-withdrawing imidazolium substituent at the Cp ligand by 0.53-0.55 V in an electrolyte based on a weakly coordinating anion. A reversible oxidation can be observed for the free 1,3-disubstituted ligand as well.

Deoxygenation of Epoxides with Carbon Monoxide.

The use of carbon monoxide as a direct reducing agent for the deoxygenation of terminal and internal epoxides to the respective olefins is presented. This reaction is homogeneously catalyzed by a carbonyl pincer-iridium(I) complex in combination with a Lewis acid co-catalyst to achieve a pre-activation of the epoxide substrate, as well as the elimination of CO2 from a γ-2-iridabutyrolactone intermediate. Especially terminal alkyl epoxides react smoothly and without significant isomerization to the internal olefins under CO atmosphere in benzene or toluene at 80-120 °C. Detailed investigations reveal a substrate-dependent change in the mechanism for the epoxide C-O bond activation between an oxidative addition under retention of the configuration and an SN 2 reaction that leads to an inversion of the configuration.

Regio- and chemoselective rearrangement of terminal epoxides into methyl alkyl and aryl ketones.

The development of the highly active pincer-type rhodium catalyst 2 for the nucleophilic Meinwald rearrangement of functionalised terminal epoxides into methyl ketones under mild conditions is presented. An excellent regio- and chemoselectivity is obtained for the first time for aryl oxiranes.

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes.

Rh(CO)2Cl(NHC) complexes of dipyrido-annelated N-heterocyclic carbenes were prepared. From the C-H coupling constant of the respective imidazolium salts and the N-C-N angle of the N-heterocyclic carbene (NHC), a weaker σ-donor character than that of typical unsaturated NHCs is expected. However, the IR stretching frequencies of their Rh(CO)2Cl complexes suggest an electron-donor character even stronger than that of saturated NHCs. We ascribe this to the extremely weak π-acceptor character of the dipyrido-annelated NHCs caused by the conjugated 14 πe- system that thus allows for an enhanced Rh-CO backbonding. This extremely low π-acceptor ability is also corroborated by the 77Se NMR chemical shift of -55.8 ppm for the respective selenourea, the lowest value ever measured for imidazole derived selenoureas. DFT-calculations of the free carbene confirm the low σ-donor character by the fact that the σ-orbital of the carbene is the HOMO-1 that lies 0.58 eV below the HOMO which is located at the π-system. Natural population analysis reveals the lowest occupation of the pπ-orbital for the saturated carbene carbon atom and the highest for the pyrido-annelated carbene. Going from the free carbene to the Rh(CO)2Cl(NHC) complexes, the increase in occupancy of the complete π-system of the carbene ligand upon coordination is lowest for the pyrido-annelated carbene and highest for the saturated carbene.

Synthesis of a lithium-cyclopentadienide complex by addition of LiNTf2 to a zwitterionic fulvalene.

The synthesis of a η5-coordinated LiCp complex by simple addition of a Li-salt in benzene is presented. A strongly zwitterionic fulvalene serves as the Cp-precursor. Evidence for the coordination of Li+ was obtained by the characterisitic 7Li NMR chemical shifts, variable temperature experiments in solution and by X-ray structure analysis in the solid state.

Selective rearrangement of terminal epoxides into methylketones catalysed by a nucleophilic rhodium-NHC-pincer complex.

An efficient Rh(I)-NHC-pincer catalyst for the highly regioselective Meinwald rearrangement of monoalkylated epoxides into methylketones under mild conditions is presented. The nucleophilic epoxide opening is assisted by Lewis acids.

Electrosynthesis of imidazolium carboxylates.

Synthesis of imidazolium carboxylate compounds was efficiently achieved by electrochemical reduction of imidazolium precursors under very mild conditions.

Consequences of the one-electron reduction and photoexcitation of unsymmetric bis-imidazolium salts.

Coupling of uronium salts with in situ generated N-heterocyclic carbenes provides straightforward access to symmetrical [4](2+) and unsymmetrical bis-imidazolium salts [6](2+) and [9](2+) . As indicated by cyclic and square-wave voltammetry, [6](2+) and [9](2+) can be (irreversibly) reduced by one electron. The initially formed radicals [6](.+) and [9](.+) undergo further reactions, which were probed by EPR spectroscopy and density functional calculations. The final products of the two-electron reduction are the two carbenes. Upon irradiation with UV light both [6](2+) and [9](2+) emit at room temperature in solution but with dramatically different characteristics. The different fluorescence behavior is analyzed by emission spectroscopy and interpreted by using time-dependent density functional calculations as largely due to different excited-state dynamics of [6](2+) and [9](2+) . The geometries of both radicals [6](.+) and [9](.+) and excited states {[6](2+) }* and {[9](2+) }* are substantially different from those of the parent ground-state molecules.

First X-ray structure analyses of rhodium(III) eta1-allyl complexes and a mechanism for allylic isomerization reactions.

Metal-to-metal allyl transfer: Using the first structurally characterized rhodium eta(1)-allyl complexes it is shown that the sigma-bound allyl substituent can be transferred from the Rh(III) complex to a Rh(I) complex in a fast equilibrium. This process may account for the decrease in regioselectivity observed in allylic alkylation reactions in which complex 1 is used as a catalyst.

Controlling the coordination modes of a new and highly flexible ligand bearing two N-heterocyclic carbene moieties at a bipyridine backbone.

A new and highly flexible biscarbene ligand with two imidazolin-2-ylidene moieties in 6,6'-position of a 2,2'-bipyridine was prepared and structurally characterised by X-ray analysis. Various silver complexes were prepared from the imidazolium salts. Depending on the counterion, the coordination mode of the ligand can be mono- or bidentate. Pd complexes are feasible directly from the imidazolium salt using palladium(II)-acetate or via transmetallation from silver carbene complexes with [PdCl(2)(CH(3)CN)(2)] or [PdCl(2)(COD)]. Controlled by the structure of the silver complexes the biscarbenebipyridine ligand is a di- or tetracoordinating ligand forming mono- or dinuclear complexes. The high flexibility of this ligand is confirmed by transmetallation from silver to copper, which results in formation of a dinuclear copper complex with two bridging biscarbene ligands.

X-ray crystal structures of 10pi- and 14pi-electron pyrido-annelated N-heterocyclic carbenes.

The first X-ray crystal structures of mono- and dipyridoimidazolinylidenes are presented, and the size of the carbene N-C-N angles discussed and correlated with the 13C NMR chemical shifts.

Experimental and computational evidence for a boron-assisted, sigma-bond metathesis pathway for alkane borylation.

Photoejection of one CO ligand from isolated CpM(CO)n+1BR2 (n = 1: M = Fe, Ru; n = 2: M = Mo,W; R2 = catecholate or pinacolate) compounds produces a coordinatively unsaturated 16 e- intermediate, a cyclic dioxaboryl transition metal complex, that can efficiently and selectively initiate regioselective C-H bond activation and can be used in the functionalization of alkanes. This chemistry appears distinct from that reported previously for related CpM(CO)n complexes of alkyl and aryl ligands. We show here by a combination of experimental and theoretical studies that the "unoccupied" p orbital of dioxaboryl ligands are intimately involved in the C-H bond activation step and that this hydrogen transfer to boron occurs by a boron-assisted, metal-mediated sigma-bond metathesis. The "unoccupied" p orbital of boron lowers the energy of the transition state and the intermediates by accepting electron density from the metal. The metal-bound borane then rotates, transfers back through a sigma-bond metathesis to capture the alkyl, and leaves the metal hydride.