Synthesis and Electronic Structure of Carbene Complexes Based on a Sulfonyl-Substituted Dilithio Methandiide

Revisiting sp2 Dilithio Methandiides: From Geometric Curiosity to Simple Bonding Description

The reported tetracoordinate dilithio methandiide complex from Liddle and co-workers (1) is investigated from a coordination chemistry perspective, to probe the origin of its intriguing geometry. Through the application of a variety of computational techniques, non-covalent (steric, electrostatic) interactions are found to be dominant. Further, we arrive at a bonding description which emphasizes the tricoordinate sp2 -hybridized nature of the central methandiide carbon, differing somewhat from the original proposal. Thus, 1 is distinct from other dilithio methandiides since it contains only one C-Li σ-bond, and is found to be comparable to a simple aryllithium compound, phenyllithium.

Solvation Effects on the Structure and Stability of Alkali Metal Carbenoids

s‐Block metal carbenoids are carbene synthons and applied in a myriad of organic transformations. They exhibit a strong structure–activity relationship, but this is only poorly understood due to the challenging high reactivity and sensitivity of these reagents. Here, we report on systematic VT and DOSY NMR studies, XRD analyses as well as DFT calculations on a sulfoximinoyl‐substituted model system to explain the pronounced solvent dependency of the carbenoid stability. While the sodium and potassium chloride carbenoids showed high stabilities independent of the solvent, the lithium carbenoid was stable at room temperature in THF but decomposed at −10 °C in toluene. These divergent stabilities could be explained by the different structures formed in solution. In contrast to simple organolithium reagents, the monomeric THF‐solvate was found to be more stable than the dimer in toluene, since the latter more readily forms direct Li/Cl interactions which facilitate decomposition via α‐elimination.

Cyclic Bis-alkylidene Complexes of Titanium and Zirconium: Synthesis, Characterization, and Reaction

Transition-metal alkylidenes have exhibited wide applications in organometallic chemistry and synthetic organic chemistry, however, cyclic Schrock-carbene-like bis-alkylidenes of group 4 metals with a four-electron donor from an alkylidene have not been reported. Herein, the synthesis and characterization of five-membered cyclic bis-alkylidenes of titanium (4 a,b) and zirconium (5 a,b) are reported, as the first well-defined group 4 metallacyclopentatrienes, by two-electron reduction of their corresponding titana- and zirconacyclopentadienes. DFT analyses of 4 a show a four-electron donor (σ-donation and π-donation) from an alkylidene carbon to the metal center. The reaction of 4 a with N,N'-diisopropylcarbodiimide (DIC) leads to the [2+2]-cycloaddition product 6. Compound 4 a reacted with CO, affording the oxycyclopentadienyl titanium complex 7. These reactivities demonstrate the multiple metal-carbon bond character. The reactions of 4 a or 5 a with cyclooctatetraene (COT) or azobenzene afforded sandwich titanium complex 8 or diphenylhydrazine-coordinated zirconacyclopentadiene 9, respectively, which exhibit two-electron reductive ability.

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.

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 Chlorine and Bromine Carbenoids: Their Thermal Stability and Structural Properties

The synthesis and structures of a series of M/X carbenoids of the type [Ph₂ P(S)]₂ CMX with M=Li, Na, and K and X=Cl and Br are reported, amongst the first isolated Na/Br and K/Br carbenoids. NMR spectroscopic as well as crystallographic studies showed distinct differences between the lithium carbenoids and their heavier congeners. In the solid state, all carbenoids showed no direct metal-carbon interaction, but an interaction between the metal and the halogen atom. This contact is only very weak in the case of the Li/Br carbenoid, but much more pronounced in the corresponding potassium and sodium compounds. Nevertheless, these interactions did not significantly influence the stability of the carbenoids by weakening the C-X bond and facilitating the MX elimination. As such all compounds were found to be stable up to approximately 60 °C in solution. Hence, M-X interactions-albeit being an essential feature for the structure formation of carbenoids-are not the only criterion determining the stability of such compounds. In the present systems, the stabilization by the thiophosphinoyl moieties is more important than the metal/halogen combination.

Taming Metal/Fluorine Carbenoids

Although Li/Cl carbenoids are versatile reagents in organic synthesis, the controlled handling of the extremely reactive and labile M/F carbenoids remains a challenge. We now show that even these compounds can be stabilized and isolated in solid state, as well as in solution. Particularly the sodium and potassium compounds exhibit a remarkable stability, thus allowing the first isolation of a room-temperature-stable fluorine carbenoid. Spectroscopic, as well as DFT studies confirmed the pronounced carbenoid character, showing M-F-C interactions with elongated C-F bonds. The different stabilities of the carbenoids was found to originate from the different strength of the M-F interaction. Hence, the lithium compounds are considerably more reactive than their heavier congeners. Reactivity studies showed that the nature of the metal also influences the reactivity, resulting in different selectivity in the addition to thioketones.

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.

Redox-induced umpolung of transition metal carbenes

Metal carbene complexes have been at the forefront of organic and organometallic synthesis and are instrumental in guiding future sustainable chemistry efforts. While classical Fischer and Schrock type carbenes have been intensely studied, compounds that do not fall within one of these categories have attracted attention only recently. In addition, applications of carbene complexes rarely take advantage of redox processes, which could open up a new dimension for their use in practical processes. Herein, we report an umpolung of a nucleophilic palladium carbene complex, [{PC(sp²)P} ^tBuPd(PMe₃)] ({PC(sp²)P} ^tBu = bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methylene), realized by successive one-electron oxidations that generated a cationic carbene complex, [{PC(sp²)P} ^tBuPdI]⁺, via a carbene radical, [{PC˙(sp²)P} ^tBuPdI]. An EPR spectroscopic study of [{PC˙(sp²)P} ^tBuPdI] indicated the presence of a ligand-centered radical, also supported by the results of reactions with 9,10-dihydroanthracene and PhSSPh. The cationic carbene complex shows electrophilic behavior toward nucleophiles such as NaH, ^p TolNHLi, PhONa, and PMe₃, resulting from an inversion of the electronic character of the Pd-C_carbene bond in [{PC(sp²)P} ^tBuPd(PMe₃)]. The redox induced umpolung is reversible and unprecedented.

Frustrated Lewis pair-like reactions of nucleophilic palladium carbenes with B(C6F5)3

Nucleophilic palladium carbenes react with the strong Lewis acid B(C₆F₅)₃via remote nucleophilic attack at the para-carbon of the supporting ligand or C–F activation of B(C₆F₅)₃; these reactions indicate frustrated Lewis pair-like behaviour given the steric inaccessibility of the nucleophilic carbene center.

Si–H activation by means of metal ligand cooperation in a methandiide derived carbene complex

Si–H bond activation of a series of silanes by means of metal ligand cooperation is reported.