The activation of element–hydrogen bonds by means of metal–ligand cooperation has received increasing attention as alternative to classical activation processes, which exclusively occur at the metal center. Carbene complexes derived from methandiide precursors have been applied in this chemistry enabling the activation of a series of EH bonds by addition reactions across the MC bond. However, no chiral carbene complexes have been applied to realize stereoselective transformations to date. Herein, we report the isolation and structure elucidation of an enantiomerically pure dilithiomethane, which could be prepared by direct double deprotonation. The obtained dilithium salt was used for the preparation of the first chiral methandiide-derived carbene complex, which was applied in stereoselective cooperative SH bond activation.

As a result of the increased polarity of the metal–carbon bond when going down the group of the periodic table, the heavier alkali metal organyl compounds are generally more reactive and less stable than their lithium congeners. We now report a reverse trend for alkali metal carbenoids. Simple substitution of lithium by the heavier metals (Na, K) results in a significant stabilization of these usually highly reactive compounds. This allows their isolation and handling at room temperature and the first structure elucidation of sodium and potassium carbenoids. The control of stability was used to control reactivity and selectivity. Hence, the Na and K carbenoids act as selective carbene-transfer reagents, whereas the more labile lithium systems give rise to product mixtures. Additional fine tuning of the M−C interaction by means of crown ether addition further allows for control of the stability and reactivity.

The synthesis, electronic structure, and reactivity of the first Group 9 carbene complex, [Cp*IrL] [L=C(Ph₂PS)(SO₂Ph)] (2), based on a dilithio methandiide are reported. Spectroscopic as well as computational studies have shown that, despite using a late transition-metal precursor, sufficient charge transfer occurred from the methandiide to the metal, resulting in a stable, nucleophilic carbene species with pronounced metal-carbon double-bond character. The potential of this iridium complex in the activation of a series of E−H bonds by means of metal-ligand cooperation has been tested. These studies have revealed distinct differences in the reactivity of 2 compared to a previously reported ruthenium analogue. Whereas attempts to activate the O−H bond in different phenol derivatives resulted in ligand cleavage, H−H and Si−H activation as well as dehydrogenation of isopropanol have been accomplished. These reactions are driven by the transformation of the carbene to an alkyl ligand. Contrary to a previously reported ruthenium carbene system, the dihydrogen activation has been found to proceed by a stepwise mechanism, with the activation first taking place solely at the metal. The activated products further reacted to afford a cyclometalated complex through liberation of the activated substrates. In the case of triphenylsilane, cyclometalation could thus be induced by a substoichiometric (i.e., catalytic) amount of silane.

As a result of the increased polarity of the metal–carbon bond when going down the group of the periodic table, the heavier alkali metal organyl compounds are generally more reactive and less stable than their lithium congeners. We now report a reverse trend for alkali metal carbenoids. Simple substitution of lithium by the heavier metals (Na, K) results in a significant stabilization of these usually highly reactive compounds. This allows their isolation and handling at room temperature and the first structure elucidation of sodium and potassium carbenoids. The control of stability was used to control reactivity and selectivity. Hence, the Na and K carbenoids act as selective carbene-transfer reagents, whereas the more labile lithium systems give rise to product mixtures. Additional fine tuning of the M−C interaction by means of crown ether addition further allows for control of the stability and reactivity.

In the last 15 years, methandiide ligands have given access to a new class of carbene complexes with unique electronic properties. The metal–carbon interactions in these complexes cover a range of bonding situations, from highly polar interactions to metal–carbon double bonds. This flexibility has allowed the isolation of complexes with metals covering the whole periodic table, including metals that had long been reluctant to form multiple bonds with carbon atoms. Thus, recent years have seen the revelation of many unusual carbene species with interesting reactivities. In this microreview, we focus on the latest developments in the chemistry of methandiides and their use as ligands in carbene complexes. We give an overview of the geminal dianionic compounds, their properties, and molecular and electronic structures, with special focus on those compounds that are applied in transition metal chemistry. The second part of the article deals with the preparation and electronic structures of methandiide-derived carbene complexes, and we highlight important examples that display the unique properties and efficiency of the ligands. The last section gives an overview of the reactivity and the non-innocent behavior of these methandiide ligands.

Invited for the cover of this issue is the group of Viktoria Gessner at the University of Würzburg, Germany. The cover image shows the versatility of methandiide-based carbene complexes with respect to bonding and the choice of metals.

The reactivity of the (carbene)ruthenium complex [(η⁶-p-cymene)Ru=CRR′] with the unsymmetrical methandiide ligand [CRR′=C(Ph₂PS)(SO₂Ph)] towards different unsaturated organic substrates was examined. While no transformation was observed with a series of different ketones (no Wittig-type reactivity), treatment with isocyanates as well as thioisocyanates resulted in selective [2+2] cycloaddition reactions across the Ru=C double bond. The products were isolated in high yields and characterized in solution as well as in the solid state. The addition reactions were found to proceed selectively according to the HSAB principle with the soft ruthenium center preferring the softer donor atoms. Hence, isocyanate addition occurred with the C=N bond, while the thioisocyanates added to the metal–carbon bond with the C=S bond. Density functional theory studies on the reaction mechanism confirmed the observed selectivity and the formed complexes as kinetically as well as thermodynamically favoured products.

Bisylides and methandiides are two unique families of carbon bases that have found a variety of applications in recent years. Metalated ylides (yldiides) are the link between these types of compounds. Yet, only little is known about their properties, reactivities, and particularly their electronic structure. Here, we report the preparation of the metalated ylide [Ph₃P-C-SO₂Tol]⁻ (1) with different alkali metal counterions. The compounds have been studied by X-ray diffraction analysis and NMR spectroscopy and the first structures of a sodium and potassium yldiide are presented. The electronic structure of 1 was explored by DFT calculations confirming its relation with other divalent carbon species. Reactivity studies demonstrate the strong nucleophilicity of the yldiide and its capability to act both as a σ- and π-donor.

Bisylide und Methandiide stellen aufgrund ihrer elektronischen Struktur zwei besondere Klassen von Kohlenstoffbasen dar, die in den letzten Jahren eine Vielzahl an Anwendungen gefunden haben. Metallierte Ylide (Yldiide) sind das Bindeglied zwischen beiden Verbindungsklassen. Über ihre Eigenschaften, Reaktivitäten und besonders über ihre elektronische Struktur ist jedoch bis heute nur wenig bekannt. Hier berichten wir über die Synthese des metallierten Ylids [Ph₃P-C-SO₂Tol]⁻ (1) mit verschiedenen Alkalimetall-Gegenionen. Die Verbindungen wurden mit Einkristall-Röntgenstrukturanalyse und NMR-Spektroskopie untersucht und die erste Struktur eines Natrium- sowie eines Kalium-Yldiids aufgeklärt. Die elektronische Struktur von 1 wurde mithilfe von DFT-Rechnungen untersucht, die die Verwandtschaft mit anderen divalenten Kohlenstoffverbindungen aufzeigen. Reaktivitätsstudien belegen die starke Nukleophilie des Yldiids, aber insbesondere auch sein ungewöhnliches Donorvermögen (σ- und π-Donor).

The transfer hydrogenation (TH) reaction of ketones with catalytic systems based on a methandiide-derived ruthenium carbene complex was investigated and optimised. The complex itself makes use of the noninnocent behaviour of the carbene ligand (MCR₂MHC(H)R₂), but showed only moderate activity, thus requiring long reaction times to achieve sufficient conversion. DFT studies on the reaction mechanism revealed high reaction barriers for both the dehydrogenation of iPrOH and the hydrogen transfer. A considerable improvement of the catalytic activity could be achieved by employing triphenylphosphine as additive. Mechanistic studies on the role of PPh₃ in the catalytic cycle revealed the formation of a cyclometalated complex upon phosphine coordination. This ruthenacycle was revealed to be the active species under the reaction conditions. The use of the isolated complex resulted in high catalytic activities in the TH of aromatic as well as aliphatic ketones. The complex was also found to be active under base-free conditions, suggesting that the cyclometalation is crucial for the enhanced activity.

The preparation of palladium thioketone and T-shaped carbene complexes by treatment of thiophosphoryl substituted Li/Cl carbenoids with a Pd⁰ precursor is reported. Depending on the steric demand, the anion-stabilizing ability of the silyl moiety (by negative hyperconjugation effects) and the remaining negative charge at the carbenic carbon atom, isolation of a three-coordinate, T-shaped palladium carbene complex is possible. In contrast, insufficient charge stabilization results in the transfer of the sulfur of the thiophosphoryl moiety and thus in the formation of a thioketone complex. While the thioketones are stable compounds the carbene complexes are revealed to be highly reactive and decompose under elimination of Pd metal. Computational studies revealed that both complexes are formed by a substitution mechanism. While the ketone turned out to be the thermodynamically favored product, the carbene is kinetically favored and thus preferentially formed at low reaction temperatures.

The synthesis of a ruthenium carbene complex based on a sulfonyl-substituted methandiide and its application in bond activation reactions and cooperative catalysis is reported. In the complex, the metal–carbon interaction can be tuned between a RuC single bond with additional electrostatic interactions and a RuC double bond, thus allowing the control of the stability and reactivity of the complex. Hence, activation of polar and non-polar bonds (OH, HH) as well as dehydrogenation reactions become possible. In these reactions the carbene acts as a non-innocent ligand supporting the bond activation as nucleophilic center in the 1,2-addition across the metal–carbon double bond. This metal–ligand cooperativity can be applied in the catalytic transfer hydrogenation for the reduction of ketones. This concept opens new ways for the application of carbene complexes in catalysis.

Herein, we report the preparation of a new unsymmetrical, bis(thiophosphinoyl)-substituted dilithio methandiide and its application for the synthesis of zirconium- and palladium-carbene complexes. These complexes were found to exhibit remarkably shielded ¹³C NMR shifts, which are much more highfield-shifted than those of “normal” carbene complexes. DFT calculations were performed to determine the origin of these observations and to distinguish the electronic structure of these and related carbene complexes compared with the classical Fischer and Schrock-type complexes. Various methods show that these systems are best described as highly polarized Schrock-type complexes, in which the metal–carbon bond possesses more electrostatic contributions than in the prototype Schrock systems, or even as “masked” methandiides. As such, geminal dianions represent a kind of “extreme” Schrock-type ligands favoring the ionic resonance structure M⁺CR₂⁻ as often used in textbooks to explain the nucleophilic nature of Schrock complexes.