Methandiide Complexes (R2CM2) of the Heavier Alkali Metals (M=Potassium, Rubidium, Cesium): Reaching the Limit?

THF-solvated Heavy Alkali Metal Benzyl Compounds (Na, Rb, Cs): Defined Deprotonation Reagents for Alkali Metal Mediation Chemistry

The incorporation of heavy alkali metals into substrates is both challenging and essential for many reactions. Here, we report the formation of THF‐solvated alkali metal benzyl compounds [PhCH₂M ⋅ (thf)_n] (M=Na, Rb, Cs). The synthesis was carried out by deprotonation of toluene with the bimetallic mixture n‐butyllithium/alkali metal tert‐butoxide and selective crystallization from THF of the defined benzyl compounds. Insights into the molecular structure in the solid as well as in solution state are gained by single crystal X‐ray experiments and NMR spectroscopic studies. The compounds could be successfully used as alkali metal mediating reagents. The example of caesium showed the convenient use by deprotonating acidic C−H as well as N−H compounds to gain insight into the aminometalation using these reagents.

Alkali Metal Complexes of a Bis(diphenylphosphino)methane Functionalized Amidinate Ligand: Synthesis and Luminescence

A novel bis(diphenylphosphino)methane (DPPM) functionalized amidine ligand (DPPM-C(N-Dipp)2 H) (Dipp=2,6-diisopropylphenyl) was synthesized. Subsequent deprotonation with suitable alkali metal bases resulted in the corresponding complexes [M{DPPM-C(N-Dipp)2 }(Ln )] (M=Li, Na, K, Rb, Cs; L=thf, Et2 O). The alkali metal complexes form monomeric species in the solid state, exhibiting intramolecular metal-π-interactions. In addition, a caesium derivative [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 was obtained by cleavage of a diphenylphosphino moiety, forming an unusual six-membered ring structure in the solid state. All complexes were fully characterized by single crystal X-ray diffraction, NMR spectroscopy, IR spectroscopy as well as elemental analysis. Furthermore, the photoluminescent properties of the complexes were thoroughly investigated, revealing differences in emission with regards to the respective alkali metal. Interestingly, the hexanuclear [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 metallocycle exhibits a blue emission in the solid state, which is significantly red-shifted at low temperatures. The bifunctional design of the ligand, featuring orthogonal donor atoms (N vs. P) and a high steric demand, is highly promising for the construction of advanced metal and main group complexes.

Synthesis, Crystal and Electronic Structures of a Thiophosphinoyl- and Amino-Substituted Metallated Ylide

α-Metallated ylides have revealed themselves to be versatile reagents for the introduction of ylide groups. Herein, we report the synthesis of the thiophosphinoyl and piperidyl (Pip) substituted α-metallated ylide [Ph2 (Pip)P=C-P(S)Ph2 ]M (M=Li, Na, K) through a four-step synthetic procedure starting from diphenylmethylphosphine sulfide. Metallation of the ylide intermediate was successfully accomplished with different alkali metal bases delivering the lithium, sodium and potassium salts, the latter isolable in high yields. Structure analyses of the lithium and potassium compounds in the solid state with and without crown ether revealed different aggregates (monomer, dimer and hexamer) with the metals coordinated by the thiophosphoryl moiety and ylidic carbon atom. Although the piperidyl group does not coordinate to the metal, it significantly contributes to the stability of the yldiide by charge delocalization through negative hyperconjugation.

Structural Motifs of Alkali Metal Superbases in Non-coordinating Solvents

Lochmann-Schlosser superbases (LSB) are a standard reagent in synthetic chemistry to achieve an exchange of a proton on an organic framework with an alkali metal cation, which in turn can be replaced by a wide range of electrophilic groups. In standard examples, the deprotonating reagent consists of an equimolar mixture of n-butyllithium and potassium t-butoxide. However, the nature of the reactive species could not be pinned down either for this composition or for similar mixtures with comparable high reactivity. Despite the poor solubility and the fierce reactivity, some insights into this mixture were achieved by some indirect results, comparison with chemically related systems, or skillful deductions. Recent results, mainly based on new soluble compounds, delivered structural evidence. These new insights lead to advanced and more detailed conclusions about the interplay of the involved components.

Geminal Dianions Stabilized by Main Group Elements

This review is dedicated to the chemistry of stable and isolable species that bear two lone pairs at the same C center, i.e., geminal dianions, stabilized by main group elements. Three cases can thus be considered: the geminal-dilithio derivative, for which the two substituents at C are neutral, the yldiide derivatives, for which one substituent is neutral while the other is charged, and finally the geminal bisylides, for which the two substituents are positively charged. In this review, the syntheses and electronic structures of the geminal dianions are presented, followed by the studies dedicated to their reactivity toward organic substrates and finally to their coordination chemistry and applications.

The hetero-cubane structures of the heavy alkali metal tert-amyloxides [MOCMe2Et]4 (M = K, Rb, Cs)

Hetero-cubane structures were revealed for the tert-amyloxide compounds of potassium, rubidium, and caesium.

Cooperative bond activation reactions with carbene complexes

This review highlights the recent advances in the application of carbene complexes in bond activation reactions via metal–ligand cooperation.

Alkali Metal Carbenoids: A Case of Higher Stability of the Heavier Congeners

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.

50 years of superbases made from organolithium compounds and heavier alkali metal alkoxides

A review of reactions of organolithium compounds (RLi) with alkali metal alkoxides is presented. On the one hand, simple lithium alkoxides form adducts with RLi the reactivity of which differs only slightly from that of RLi. On the other hand, after mixing heavier alkali metal alkoxides (R’OM, M = Na, K, Rb, Cs) with RLi, a new system is formed, which has reactivity that dramatically exceeds that of the parent RLi. A metal interchange, according to the equation RLi + R’OM = RM + R’OLi, occurs in this system, giving rise to a superbase. This reaction is frequently used for the preparation of heavier alkali metal organometallic compounds. Similar metal interchange takes place between R’OM and compounds such as lithium amides and lithium enolates of ketones or esters, thus demonstrating the general nature of this procedure. Superbases react easily with many types of organic compounds (substrates), resulting in the formation of a heavier alkali metal derivative of the substrate (metalation). The metalated substrate can react in situ with an electrophile to yield the substituted substrate, a procedure that is frequently used in synthetic and polymer chemistry. An improved mechanism of metal interchange and reaction of superbases with substrates is proposed.

Substitution effects on the formation of T-shaped palladium carbene and thioketone complexes from Li/Cl carbenoids

The preparation of palladium thioketone and T-shaped carbene complexes by treatment of thiophosphoryl substituted Li/Cl carbenoids with a Pd(0) 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.

Syntheses, structures and flexible coordination of sterically demanding di- and “tri”-lithiated methandiides

Sequential lithiation of a sterically demanding bis(iminophosphoranyl)methane afforded di- and trianionic methandiide lithium complexes and their structures and coordination behaviour have been studied with solid state and solution methods.

Synthesis and stability of Li/Cl carbenoids based on bis(iminophosphoryl)methanes

Bis(iminophosphoryl) substituted Li/Cl carbenoids – accessable via different preparation methods – show high thermal stabilities, which however depend on the N-substituent.

Synthesis and bonding in carbene complexes of an unsymmetrical dilithio methandiide: a combined experimental and theoretical study

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 (13)C 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.

Early metal bis(phosphorus-stabilised)carbene chemistry

Since the discovery of covalently-bound mid- and late-transition metal carbenes there has been a spectacular explosion of interest in their chemistry, but their early metal counterparts have lagged behind. In recent years, bis(phosphorus-stabilised)carbenes have emerged as valuable ligands for metals across the periodic table, and their use has in particular greatly expanded covalently-bound early metal carbene chemistry. In this tutorial review we introduce the reader to bis(phosphorus-stabilised)carbenes, and cover general preparative methods, structure and bonding features, and emerging reactivity studies of early metal derivatives (groups 1-4 and the f-elements).