Structural Elucidation of the Illustrious Tebbe Reagent

Transition Metal-(μ-Cl)-Aluminum Bonding in α-Olefin and Diene Chemistry

Olefin and diene transformations, catalyzed by organoaluminum-activated metal complexes, are widely used in synthetic organic chemistry and form the basis of major petrochemical processes. However, the role of M−(μ-Cl)−Al bonding, being proven for certain >C=C< functionalization reactions, remains unclear and debated for essentially more important industrial processes such as oligomerization and polymerization of α-olefins and conjugated dienes. Numerous publications indirectly point at the significance of M−(μ-Cl)−Al bonding in Ziegler−Natta and related transformations, but only a few studies contain experimental or at least theoretical evidence of the involvement of M−(μ-Cl)−Al species into catalytic cycles. In the present review, we have compiled data on the formation of M−(μ-Cl)−Al complexes (M = Ti, Zr, V, Cr, Ni), their molecular structure, and reactivity towards olefins and dienes. The possible role of similar complexes in the functionalization, oligomerization and polymerization of α-olefins and dienes is discussed in the present review through the prism of the further development of Ziegler−Natta processes and beyond.

[(CH3 )Al(CH2 )]12 : Methylaluminomethylene (MAM-12)

The molecular structure of enigmatic "poly(aluminium-methyl-methylene)" (first reported in 1968) has been unraveled in a transmetalation reaction with gallium methylene [Ga8 (CH2 )12 ] and AlMe3 . The existence of cage-like methylaluminomethylene moieties was initially suggested by the reaction of rare-earth-metallocene complex [Cp*2 Lu{(μ-Me)2 AlMe2 }] with excess AlMe3 affording the deca-aluminium cluster [Cp*4 Lu2 (μ3 -CH2 )12 Al10 (CH3 )8 ] in low yield (Cp*=C5 Me5 ). Treatment of [Ga8 (CH2 )12 ] with excess AlMe3 reproducibly gave the crystalline dodeca-aluminium complex [(CH3 )12 Al12 (μ3 -CH2 )12 ] (MAM-12). Revisiting a previous approach to "poly(aluminium-methyl-methylene" by using a (C5 H5 )2 TiCl2 /AlMe3 (1 : 100) mixture led to amorphous solids displaying solubility behavior and spectroscopic features similar to those of crystalline MAM-12. The gallium methylene-derived MAM-12 was used as an efficient methylene transfer reagent for ketones.

Olefin Metathesis by First-Row Transition Metals

Catalytic olefin metathesis based on the second- and third-row transition metals has become one of the most powerful transformations in modern organic chemistry. The shift to first-row metals to produce fine and commodity chemicals would be an important achievement to complement existing methods with inexpensive and greener alternatives. In addition, those systems can offer unusual reactivity based on the unique electronic structure of the base metals. In this Minireview, we summarize the progress of the development of alkylidenes and metallacycles of first-row transition metals from scandium to nickel capable of performing cycloaddition and cycloreversion steps, crucial reactions in olefin metathesis. In addition, we will discuss systems capable of performing olefin metathesis; however, the nature of active species is not yet known.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

Structural elucidation of a methylenation reagent of esters: synthesis and reactivity of a dinuclear titanium(iii) methylene complex

Transmetallation of a zinc methylene complex [ZnI(tmeda)]₂(μ-CH₂) with a titanium(iii) chloride [TiCl₃(tmeda)(thf)] produced a titanium methylene complex. The X-ray diffraction study displayed a dinuclear methylene structure [TiCl(tmeda)]₂(μ-CH₂)(μ-Cl)₂. Treatment of an ester with the titanium methylene complex resulted in methylenation of the ester carbonyl to form a vinyl ether. The titanium methylene complex also reacted with a terminal olefin, resulting in olefin-metathesis and olefin-homologation. Cyclopropanation by methylene transfer from the titanium methylene proceeded by use of a 1,3-diene. The mechanistic study of the cyclopropanation reaction by the density functional theory calculations was also reported.

Transmetallation of a zinc methylene complex [ZnI(tmeda)]₂(μ-CH₂) with a titanium(iii) chloride [TiCl₃(tmeda)(thf)] produced a titanium methylene complex.

Lability of Ta–NHC adducts as a synthetic route towards heterobimetallic Ta/Rh complexes

This work highlights the reactivity of Ta–NHC adducts and the aptitude of the NHC motif to transfer from Ta to Rh which is used with profit as an efficient synthetic route to access early/late heterobimetallic complexes.

Aluminum Nanocubes Have Sharp Corners

Of the many plasmonic nanoparticle geometries that have been synthesized, nanocubes have been of particular interest for creating nanocavities, facilitating plasmon coupling, and enhancing phenomena dependent upon local electromagnetic fields. Here we report the straightforward colloidal synthesis of single-crystalline {100} terminated Al nanocubes by decomposing AlH₃ with Tebbe's reagent in tetrahydrofuran. The size and shape of the Al nanocubes is controlled by the reaction time and the ratio of AlH₃ to Tebbe's reagent, which, together with reaction temperature, establish kinetic control over Al nanocube growth. Al nanocubes possess strong localized field enhancements at their sharp corners and resonances highly amenable to coupling with metallic substrates. Their native oxide surface renders them extremely air stable. Chemically synthesized Al nanocubes provide an earth-abundant alternative to noble metal nanocubes for plasmonics and nanophotonics applications.

Gallium Methylene

Despite the eminent importance of metal alkylidene species for organic synthesis and industrial catalytic processes, molecular homoleptic metal methylene compounds [M(CH₂ )_n ] as the simplest representatives, have remained elusive. Reports on this topic date back to 1955 when polymeric [Li₂ (CH₂ )]_n and [Mg(CH₂ )]_n were accessed by pyrolysis of methyllithium and dimethylmagnesium, respectively. However, the insoluble salt-like composition of these compounds has impeded their application as valuable reagents. We report that rare-earth metallocene methyl complexes [(C₅ Me₅ )₂ Ln{(μ-Me)₂ GaMe₂ }] (Ln=Lu, Y) trigger the formation of homoleptic gallium methylene [Ga₈ (μ-CH₂ )₁₂ ] from trimethylgallium [GaMe₃ ] (Me=methyl) via a cascade C-H bond activation involving the dodecametallic clusters [(C₅ Me₅ )₆ Ln₃ (μ₃ -CH₂ )₆ Ga₉ (μ-CH₂ )₉ ] as crucial intermediates. Such gallium methylene compounds feature a reversible [Ga₈ (μ-CH₂ )₁₂ ]/[Ga₆ (μ-CH₂ )₉ ] oligomer switch in donor solvents and act as Schrock-type methylene-transfer reagents.

Titanium Carbene Complexes Stabilized by Alkali Metal Amides

Facile α-H elimination from tetrakis(trimethylsilylmethyl)titanium precursors to give adducts of (alkylidene)bis(alkyl)titanium complexes is induced by light alkali metal amides of the NNNN-type macrocyclic anionic ligand Me₃ TACD [(Me₃ TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane]. In the crystal, the alkali metal interacts with the carbene carbon atom or with the CH₂ group of the trimethylsilymethyl ligand. The nucleophilic character of the carbene carbon atom was shown by the reaction with benzophenone and terminal acetylenes.

Unveiling the Takai Olefination Reagent via Tris( tert-butoxy)siloxy Variants

The elusive Takai olefination reagent, namely, the iodo-methylidene Cr(III) complex [Cr₂Cl₄(CHI)(thf)₄], has been isolated by careful handling of the reaction between CrCl₂ and CHI₃ in THF at -35 °C. Alternatively, treatment of [Cr(OSi(O tBu)₃)₂] with CHI₃ gave the mixed-valent dihalido-methanide complex [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)], featuring a Cr(III)-CHI₂ moiety. In the presence of TMEDA nucleophilic attack at CHI₂ occurred generating the zwitterionic species [Cr^III(OSi(O tBu)₃)₂(tmeda-CHI)][I]. Complexes [Cr₂Cl₄(CHI)(thf)₄] and [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)] were screened for their ability to induce monohalido olefination of benzaldehyde. Remarkably, both complexes promote olefination, with [Cr₂Cl₄(CHI)(thf)₄] accomplishing the same E selectivity as Takai 's original mixture. Complex [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)], however, appeared to give preferentially Z isomer, corroborating the monoiodo-methylidene species Cr(III)-CHI-Cr(III) as the active olefination component of the original in situ generated Takai reagent mixture.

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.

Crystal structures of titanium-aluminium and -gallium complexes bearing two μ2 -CH3 units

The isotypic crystal structures of two titanocene complexes containing an EMe3 unit (E = Al, Ga; Me = meth-yl) with two μ2-coordinating methyl groups, namely [μ-1(η5)-(adamantan-1-yl-2κC1)cycylo-penta-dien-yl]di-μ2-methyl-methyl-2κC-[1(η5)-penta-methyl-cyclo-penta-dien-yl]aluminiumtitanium(III), [AlTi(CH3)3(C10H15)(C15H18)], and [μ-1(η5)-(adamantan-1-yl-2κC1)cycylo-penta-dien-yl]di-μ2-methyl-methyl-2κC-[1(η5)-penta-methyl-cyclo-penta-dien-yl]galliumtitanium(III), [GaTi(CH3)3(C10H15)(C15H18)], are reported. Reacting a dinuclear nitro-gen-bridged low-valent titanium(III) complex with the Lewis acids AlMe3 or GaMe3 results in the loss of mol-ecular di-nitro-gen and the formation of two monomeric titanocene(III) fragments bearing two μ2-bridging methyl groups. Single crystal X-ray diffraction reveals the formation of a new E-C bond involving the penta-fulvene ligand while the bridging and terminal methyl groups remain intact.

Structural elucidation of a mononuclear titanium methylidene

The first example of a structurally characterized titanium methylidene, (PN)₂TiCH₂, has been prepared via one-electron oxidation of (PN)₂Ti(CH₃) followed by deprotonation with an ylide or by H-atom abstraction using an aryloxyl radical.

A radical coupled pathway to a stable and terminally bound titanium methylidene

Radical coupling or oxidation of the titanium(iii)dimethyl precursor (PNP)Ti(CH₃)₂ produced the dimethyl compounds, (PNP)Ti(CH₃)₂(X) (X = TEMPO, OMes*, OTf), which then thermally extrude methane to cyclometallate (X = TEMPO) or form the methylidene (X = OMes* or OTf).

Rare-earth metal methylidene complexes with Ln3(μ3-CH2)(μ3-Me)(μ2-Me)3 core structure

Although protected from intermolecular deactivation by a picket-fence-type arrangement of bulky amido ligands the CH₂²⁻ moiety is readily converted into oxo species.