Salts of a copper and a silver carbene complex were prepared from dimesityl diazomethane, made possible by the steric shielding of the N-heterocyclic carbene (NHC) ancillary ligand IPr**. The mint-green complex [IPr**Ag=CMes₂]⁺[NTf₂]⁻ is the first isolated silver carbene complex without heteroatom donor substituents. Single-crystal X-ray diffraction provides evidence for a predominant carbenoid character, and supports the postulation of such reactive species as intermediates in silver-catalyzed CH activation reactions. The greenish yellow copper carbene complex [IPr**Cu=CMes₂]⁺[NTf₂]⁻ has spectroscopic properties in between the isostructural silver complex and the already reported emerald green gold carbene complex. A comparison in the Group 11 series indicates that relativistic effects are responsible for the strong σ bond and the significant π back-bonding in the gold carbene moiety.

Salze eines Kupfercarben- und eines Silbercarben-Komplexes wurden aus Dimesityldiazomethan hergestellt, ermöglicht durch die sterische Abschirmung des N-heterocyclischen Steuerliganden IPr**. Der minzgrüne Komplex [IPr**AgCMes₂]⁺[NTf₂]⁻ ist der erste isolierte Silbercarben-Komplex ohne Heteroatom-Donorsubstituenten. Seine durch Einkristall-Röntgenstrukturanalyse erhaltene Festkörperstruktur erbringt den Nachweis für einen vorherrschenden Carbenoid-Charakter und stützt die Annahme solch reaktiver Spezies als Intermediate in Silber-katalysierten CH-Aktivierungsreaktionen. Die spektroskopischen Eigenschaften des grünlich-currygelben Kupfercarben-Komplexes [IPr**CuCMes₂]⁺[NTf₂]⁻ liegen zwischen denen des isostrukturellen Silberkomplexes und denen des schon beschriebenen smaragdgrünen Goldcarben-Komplexes. Ein Vergleich der Elemente der Kupfertriade weist darauf hin, dass relativistische Effekte für die starke σ-Bindung und die signifikante π-Rückbindung in der Goldcarben-Einheit verantwortlich sind.

A molecularly defined copper acetylide cluster with ancillary N-heterocyclic carbene (NHC) ligands was prepared under acidic reaction conditions. This cluster is the first molecular copper acetylide complex that features high activity in copper-catalyzed azide–alkyne cycloadditions (CuAAC) with added acetic acid even at −5 °C. Ethyl propiolate protonates the acetate ligands of the dinuclear precursor complex to release acetic acid and replaces one out of four ancillary ligands. Two copper(I) ions are thereby liberated to form the core of a yellow dicationic C₂-symmetric hexa-NHC octacopper hexaacetylide cluster. Coalescence phenomena in low-temperature NMR experiments reveal fluxionality that leads to the facile interconversion of all of the NHC and acetylide positions. Kinetic investigations provide insight into the influence of copper acetylide coordination modes and the acetic acid on catalytic activity. The interdependence of “click” activity and copper acetylide aggregation beyond dinuclear intermediates adds a new dimension of complexity to our mechanistic understanding of the CuAAC reaction.

Ein molekular definierter Kupferacetylidcluster mit N-heterocyclischen Carben-Steuerliganden (NHC) wurde zum ersten Mal unter sauren Reaktionsbedingungen hergestellt. Dieser Cluster ist der erste molekulare Kupferacetylid-Komplex, der eine hohe Aktivität in der Kupfer-katalysierten Azid-Alkin-Cycloaddition aufweist, bei Essigsäurezusatz sogar bei −5 °C. Ethylpropiolat protoniert alle Acetatliganden des zweikernigen Vorläuferkomplexes zu Essigsäure und ersetzt jeden vierten Steuerliganden. Als Konsequenz werden zwei Kupfer(I)-Ionen freigesetzt, die den Kern des Clusters des in dieser Studie beschriebenen gelben C₂-symmetrischen Hexa-NHC-Octakupfer-Hexaacetylid-Dikations bilden. Koaleszenzphänomene in Tieftemperatur-NMR-Experimenten offenbaren das Fluktuieren des Clusters, was jeweils zu einem schnellen Austausch der jeweiligen NHC- und Acetylidpositionen führt. Kinetische Untersuchungen gewähren Einsichten in die Rolle verschiedener Kupferacetylid-Koordinationsmodi und der Essigsäurezugabe auf die katalytische Aktivität. Die wechselseitige Abhängigkeit der “Klick”-Aktivität und der Kupferacetylid-Aggregation über zweikernige Intermediate hinaus fügt dem mechanistischen Verständnis der CuAAC-Reaktion eine neue Komplexitätsebene hinzu.

Goldcarben-Komplexe sind essentielle Intermediate in vielen Gold-katalysierten synthetischen Transformationen. Während Goldcarbene mit direkter, vinyloger oder phenyloger Heteroatomstabilisierung schon synthetisiert und charakterisiert worden sind, waren elektronisch nichtstabilisierte Goldcarbene bislang schwer zu beobachten. Wir haben nun einen sterisch extrem abgeschirmten, smaragdgrünen Komplex [IPr**Au=CMes₂]⁺[NTf₂]⁻ synthetisiert, isoliert und vollständig charakterisiert. Sein Absorptionsmaximum bei 642 nm, im Vergleich zu 528 nm des rotpurpurnen Carbokations [Mes₂CH]⁺, zeigt klar, dass Gold mehr ist als nur ein “weiches Proton”.

Gold–carbene complexes are essential intermediates in many gold-catalyzed organic-synthetic transformations. While gold–carbene complexes with direct, vinylogous, or phenylogous heteroatom substitution have been synthesized and characterized, the observation in the condensed phase of electronically non-stabilized gold–carbenes has so far remained elusive. The sterically extremely shielded, emerald-green complex [IPr**Au=CMes₂]⁺[NTf₂]⁻ has now been synthesized, isolated, and fully characterized. Its absorption maximum at 642 nm, in contrast to 528 nm of the red-purple carbocation [Mes₂CH]⁺, clearly demonstrates that gold is more than just a “soft proton”.

An N-heterocyclic carbene gold complex IPr**AuNTf₂ has been synthesized, spectroscopically investigated, structurally characterized, and used as a highly active and stable catalyst in the Hashmi phenol synthesis (IPr**=1,3-di-p-tolylimidazol-2-ylidene with four di-tert-butylbenzhydryl ortho substituents, Tf=trifluoromethansulfonyl). A side reaction comprises an irreversible arene oxide ring opening with subsequent 1,2 methyl shift. The advantage of the steric demand of the ancillary ligand was explained by higher equilibrium concentrations of the cationic gold species, the circumvention of inactive dinuclear intermediates, and the inhibition of catalyst decomposition pathways. The methanol addition–hydration of alkynes featured a turnover-limiting proton transfer step of an alkenylgold catalyst resting state, indicated by a large primary kinetic isotope effect and an alkyne competition experiment.

The main feature of the herein presented class of molecularly defined catalysts for the copper-catalyzed azide–alkyne cycloaddition reaction is the presence of two copper centres in one catalyst molecule. We report the facile three-step synthesis of two representative bis-NHC-dicopper complexes as well as their catalytic performance in the azide–alkyne cycloaddition. A screening with one of these complexes has proved its wide applicability and excellent performance as homogeneous catalyst in various organic solvents and with different alkyne and azide substrates.

The silver chloride and gold chloride complex of the extremely sterically shielding N-heterocyclic carbene ligand IPr** have been synthesized and characterized. Formally, eight methyl groups of IPr have been replaced by 4-tert-butylphenyl substituents, and two para-methyl groups have been added. The world record for the ligand's buried volume (55.4 % for IPr**AuCl, and 56.7 % for IPr**AgCl) has been determined by single-crystal X-ray analyses. Reaction of IPr**AuCl with AgSbF₆ in toluene yields AgCl and a cationic gold complex with toluene coordination. The same reaction in 1,2-dichloroethane leads to precipitation of AgCl and an equilibrium of [IPr**Au]SbF₆ and [IPr**AuClAg]SbF₆. Single crystals of the latter were suitable for an X-ray structure analysis, which reveal an Au–Cl–Ag triangle with an almost undisturbed IPr**–Au–Cl fragment and the coordination of two IPr** arenes to a silver cation. The structural parameters emphasize the role of NHC-Au cations as highly electrophilic “soft protons”.

A sterically shielded 3-substituted zwitterionic N,N-dimethylisotryptammonium carboxylate has been synthesized by consecutive chemoselective double alkylation of indole. The carboxylate undergoes a quantitative and unusually facile decarboxylation in dimethyl sulfoxide (DMSO) or dimethyl formamide (DMF) at room temperature. The breaking of a nearly equidistant hydrogen bond by solvent molecules initiates heterolytic CC cleavage. The decarboxylation rate decreases with increasing CO₂ partial pressure, proving the competitiveness of protonation and re-carboxylation of the carbanionic intermediate. Corresponding spiro compounds containing silylene and stannylene moieties show high thermal stability. Addition of an excess of methyllithium to the sodium salt triggers a reaction sequence comprising a deprotonation, carboxylate transfer, and nucleophilic trapping of the rearranged carboxylate by another equivalent of methyllithium. Hydrolytic work-up of the geminal diolate leads to an acetyl product. The role of the sodium counterion and the mechanism of the rearrangement have been unraveled by deuteration experiments.

The “exocyclic” 1,3-benzyl shift observed in iminium salts derived from 1-benzyl-1,2,3,4-tetrahydroisoquinolines is related to the “endocyclic” Knabe rearrangement. A crossover experiment, isotopic labelling, the study of initiators and inhibitors as well as DFT calculations of gas-phase model structures provide evidence for a free-radical pathway under kinetic entropy control that is not affected by “slow” radical traps.

This communication presents the facile and cost-effective synthesis as well as properties of salts of a lipophilic aluminate anion. Its unprecedented solubility in pentane makes possible tetrabutylammonium salt extraction with pentane. Nevertheless, its symmetry leads to high melting points of its thermostable salts. Several single crystals for X-ray diffraction studies have been obtained. The anion is devoid ofhalogens, but is stable against sterically demanding electrophiles such as the trityl cation. However, it is unstable in protic media and hydrolyzes in the presence of water within days.

The cover picture shows a Madagascar day gecko (Phelsuma madagascariensis grandis) in a coconut with a locust on top. Gecko toes hold on to slithery and vertical surfaces due to van der Waals forces, which are also relevant for the lipophilic altebate anion (left space-filling model). Its sterically demanding substituents shield the nucleophilic center, making diethyl ether a better hydrogen bond acceptor in comparison (middle X-ray structure). The high symmetry and the positive (blue) partial charges on the molecular surface are reflected in the right fractal-like model. Details are discussed in the Short Communication by B. F. Straub et al. on p. 1907 ff. Photograph by Michael Wrede, artwork by Bernd F. Straub.