Metal Alkyl Research Articles

Metal Alkyls with Alkylidynic Metal‐Carbon Bond Character: Key Electronic Structures in Alkane Metathesis Precatalysts

The homologation of alkanes via alkane metathesis is catalyzed at low temperatures (150 °C) by the silica-supported species (≡SiO)WMe5 and (≡SiO)TaMe4 , while (≡SiO)TaMe3 Cp* is inactive. The contrasting reactivity is paralleled by differences in the 13 C NMR signature; the former display significantly more deshielded isotropic chemical shifts (δiso ) and almost axially symmetric chemical shift tensors, similar to what is observed in their molecular precursors TaMe5 and WMe6 . Analysis of the chemical shift tensors reveals the presence of a triple-bond character in their metal-carbon (formally single) bond. This electronic structure is reflected in their propensity to generate alkylidynes and to participate in alkane metathesis, further supporting the role of alkylidynes as key reaction intermediates. This study establishes chemical shift as a descriptor to identify potential alkane metathesis catalysts.

Metal Alkyls with Alkylidynic Metal‐Carbon Bond Character: Key Electronic Structures in Alkane Metathesis Precatalysts

AbstractThe homologation of alkanes via alkane metathesis is catalyzed at low temperatures (150 °C) by the silica‐supported species (≡SiO)WMe5 and (≡SiO)TaMe4, while (≡SiO)TaMe3Cp* is inactive. The contrasting reactivity is paralleled by differences in the 13C NMR signature; the former display significantly more deshielded isotropic chemical shifts (δiso) and almost axially symmetric chemical shift tensors, similar to what is observed in their molecular precursors TaMe5 and WMe6. Analysis of the chemical shift tensors reveals the presence of a triple‐bond character in their metal‐carbon (formally single) bond. This electronic structure is reflected in their propensity to generate alkylidynes and to participate in alkane metathesis, further supporting the role of alkylidynes as key reaction intermediates. This study establishes chemical shift as a descriptor to identify potential alkane metathesis catalysts.

Interception of Elusive Cationic Hf-Al and Hf-Zn Heterobimetallic Adducts with Mixed Alkyl Bridges Featuring Multiple Agostic Interactions.

Heterobimetallic complexes with inequivalent bridging alkyl chains are very often invoked as key intermediates in many catalytic processes, yet their interception and structural characterization are lacking. Such complexes have been prepared from reactions of the cationic cyclometalated hafnocene [CpPr Cp Hf][B(C6 F5 )4 ] (1) with main group metal alkyls to afford the corresponding hetero-bridged cationic products, [CpPr Cp Hf(μ-R)E(R)n ][B(C6 F5 )4 ] (E=Al or Zn; R=Me, Et, or iBu). NMR and DFT studies demonstrate that both bridging alkyls establish agostic interactions with Hf, which are appreciably stronger for ethyl rather than methyl groups. Hf-Al and Hf-Zn distances are surprisingly short and only slightly longer than computed Hf-Al or Hf-Zn single bond lengths (2.80 Å). Finally, a reaction of [CpPr Cp Hf(μ-Me)Zn(Me)][B(C6 F5 )4 ] with excess ZnMe2 yields an unprecedented heterotrimetallic species, [(CpPr )2 Hf(μ-Me)(ZnMe)(μ3 -CH2 )ZnMe][B(C6 F5 )4 ], the detailed structure of which is elucidated by a combination of NMR spectroscopic methods and molecular calculations.

C−H Activation and Olefin Insertion in d8 and d0 Complexes: Same Elementary Steps, Different Electronics

AbstractMetallocene alkyl complexes with d0 electron configuration and d8‐configured square planar α‐diimine late transition metal alkyl complexes show activity in both C−H activation through σ‐bond metathesis and olefin insertion. Herein, we show by analysis of their M−CH3 13C chemical shift tensors that these reactions involve a π(M−C) interaction in the horizontal plane of the complex for both d0 and d8 systems. While in the case of d0 systems the interaction of an empty metal d‐orbital and a filled carbon p‐orbital causes partial alkylidene character of the M−C bond, the corresponding metal d‐orbital is filled in d8 systems, thus generating a filled π*(M−C) orbital that increases the anionic character of the methyl group. This entails fundamentally different reaction mechanisms for d0 and d8 systems, which are reflected in the structures of the transition states: While d0 olefin insertion can be viewed as a [2+2] cycloaddition reaction, d8 olefin insertion rather resembles methyl group migration onto a positively polarized olefin, thus explaining the observed differences in regioselectivity. These findings are translated to σ‐bond metathesis, a reaction which is isolobal to olefin insertion for both early and late transition metals.

Synthesis and Characterisation of Two Lithium Tetramethylberyllate Salts and a Series of β-Diketiminato Beryllium Alkyl Complexes

A safer route than that previously published for the synthesis of [BeI2(OEt2)2] has been developed. Reactions of beryllium dihalide complexes [BeX2(OEt2)2] (X=Br or I) with either LiMe or LiBun lead to mixtures of products, which have been shown to act as sources of ‘BeR2’ (R=Me or Bun) in previous synthetic studies. Here, a titration method has been developed to determine the quantity of metal alkyl residues in these ‘BeR2’ mixtures that are available for subsequent alkane elimination reactions. Treating ‘BeMe2’ mixtures with N,N,N′,N′-tetramethylethylenediamine (tmeda) gave two lithium tetramethylberyllate compounds, [{(tmeda)Li}2(μ-BeMe4)] and [{[(tmeda)Li](BeMe4)(μ-Li)}∞], which were crystallographically characterised. Treatment of insitu-generated ‘BeR2’ solutions with several β-diketimines (HC{C(Me)=NR}{=C(Me)N(H)R}, NacnacH, R=aryl or alkyl) yielded a series of β-diketiminato beryllium alkyl complexes, [(Nacnac)BeR], including the first chiral example of such a compound. Crystallographic studies of these reveal them to be monomeric, with planar three-coordinate beryllium centres.

Iron-catalyzed α-C-H functionalization of π-bonds: cross-dehydrogenative coupling and mechanistic insights.

The deprotonation of propargylic C–H bonds for subsequent functionalization typically requires stoichiometric metal alkyl or amide reagents. In addition to the undesirable generation of stoichiometric metallic waste, these conditions limit the functional group compatibility and versatility of this functionalization strategy and often result in regioisomeric mixtures. In this article, we report the use of dicarbonyl cyclopentadienyliron(ii) complexes for the generation of propargylic anion equivalents toward the direct electrophilic functionalization of propargylic C–H bonds under mild, catalytic conditions. This technology was applied to the direct conversion of C–H bonds to C–C bonds for the synthesis of several functionalized scaffolds through a one-pot cross dehydrogenative coupling reaction with tetrahydroisoquinoline and related privileged heterocyclic scaffolds. A series of NMR studies and deuterium-labelling experiments indicated that the deprotonation of the propargylic C–H bond was the rate-determining step when a Cp*Fe(CO)2-based catalyst system was employed.

Dinuclear Metal-ProPhenol Catalysts: Development and Synthetic Applications.

The ProPhenol ligand is a member of the chiral aza-crown family that spontaneously forms a bimetallic complex upon treatment with alkyl metal reagents, such as Et2 Zn and Bu2 Mg. The resulting complex features Lewis acidic and Brønsted basic sites, enabling simultaneous activation of both nucleophile and electrophile in the same chiral environment. Since the initial report in 2000, metal-ProPhenol catalysts have been used to facilitate a broad range of asymmetric transformations, including aldol, Mannich, and Henry reactions, as well as alkynylations and conjugation additions. By promoting such a diverse array of reactions, these complexes provide rapid and atom-economical access to valuable complex building blocks. In this Review, we describe in detail the development and synthetic applications of these versatile catalysts with a special focus on recent efforts to improve reactivity and selectivity through ligand design and structural modification.

Metal Free Benzylation and Alkylation of Quinoxalin‐2(1H)‐ones with Alkenes Triggered by Sulfonyl Radical Generated from Sulfinic Acids

AbstractA metal‐free domino multicomponent reaction for the direct C−H benzylation and alkylation of quinoxalin‐2(1H)‐ones using alkenes is described. Triggered by the sulfonyl radical generated from sulfinic acid, the alkenes are transformed to alkyl radicals that react exclusively at the C‐3 position of quinoxalin‐2(1H)‐ones. Importantly, the method not only functionalizes medicinally important quinoxalin‐2(1H)‐one scaffold, but also furnishes diverse medicinally relevant sulfones in good to excellent yields under mild conditions.magnified image

Dispersion Effects in Stabilizing Organometallic Compounds: Tetra-1-norbornyl Derivatives of the First-Row Transition Metals as Exceptional Examples.

In 1972 Bower and Tennett first synthesized a series of tetra-1-norbornyl derivatives, (nor)4M, of the first-row transition metals from titanium to cobalt. These were found to be exceptionally stable for homoleptic metal alkyls containing only metal-carbon σ-bonds. The theoretical energies for the dissociation of 1-norbornyl ligands from these unusually high oxidation state organometallics through the reactions (nor)4M → (nor)3M + nor• and (nor)4M → (nor)2M + nor-nor indicate that dispersion effects play an important role in determining their exceptional stability. Thus, all of the (nor)4M (M = Ti to Cu) derivatives are viable with respect to 1-norbornyl radical dissociation when the London dispersion effect is considered. However, (nor)4Cu becomes disfavored if the dispersion correction is ignored. Thus, the stability of the (nor)4M molecules is seen to arise from the favorable combination of steric and dispersion force effects of the four 1-norbornyl groups tetrahedrally disposed around the metal atom and maximizing the dispersion attraction between them in a spherical hydrocarbon structure with a central metal atom. The tri-1-norbornyl derivatives (nor)3M appear be disfavored with respect to disproportionation into (nor)4M + (nor)2M. This is consistent with the experimental syntheses of the (nor)4M (M = Cr to Co) derivatives with the metal in the +4 oxidation by reactions with 1-norbornyllithium with metal halides in the +2 or +3 metal oxidation states. Both the OPBE method and the BPW91 method predict high-spin states for the d2 and d3 complexes (nor)4Cr and (nor)4Mn but low-spin states for (nor)4Fe and (nor)4Co, consistent with experiment.

(Benz)Imidazolin-2-iminato Aluminum, Zinc, and Magnesium Complexes and Their Applications in Ring Opening Polymerization of ε-Caprolactone.

Synthesis and characterization of a new family of aluminum, magnesium, and zinc complexes supported by (benz)imidazolin-2-iminato ligands are reported herein. Under different reaction conditions, (benz)imidazolin-2-imine ligands reacted with metal alkyl precursors to afford the corresponding complexes, displaying mononuclear, binuclear, and trinuclear structures. Reaction in 1:1 stoichiometry at room temperature afforded the simple Lewis acid-base adducts, which exhibited distorted tetrahedral geometries around the metal centers, whereas high temperature reactions gave rise to dimeric complexes, with each metal bridging two (benz)imidazolin-2-imine ligands to form a four-membered metallacycle ring at the center of the complexes. The catalytic ring opening polymerization (ROP) of ε-caprolactone was studied using these complexes as precatalysts in the presence of isopropanol, and very high activities were observed, affording high/medium molecular weight poly(ε-caprolactone) products with low polydispersity indices. Detailed kinetic studies revealed that the polymerizations proceeded in a living manner and molecular weights of the polymers could be precisely controlled.

Contra-thermodynamic Olefin Isomerization by Chain-Walking Hydrofunctionalization and Formal Retro-hydrofunctionalization.

We report a contra-thermodynamic isomerization of internal olefins to terminal olefins driven by redox reactions and formation of Si-F bonds. This process involves chain-walking hydrosilylation of internal olefins and subsequent formal retro-hydrosilylation. The process rests upon the high activities of platinum hydrosilylation catalysts for isomerization of metal alkyl intermediates and a new, metal-free process for the conversion of alkylsilanes to alkenes. By this approach, 1,2-disubstituted and trisubstituted olefins are converted to terminal olefins.

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Carbon-13 NMR Chemical Shift: A Descriptor for Electronic Structure and Reactivity of Organometallic Compounds.

Metal-bonded carbon atoms in metal-alkyl, metal-carbene/alkylidene, and metal-carbyne/alkylidyne species often show significantly more deshielded isotropic chemical shifts than their organic counterparts (alkanes, alkenes, and alkynes). While isotropic chemical shift is universally used to characterize a chemical compound in solution, it is an average value of the three principal components of the chemical shift tensor (δ11 > δ22 > δ33). The tensor components, which are accessible by solid-state NMR spectroscopy, can provide detailed information about the electronic structure (frontier molecular orbitals) at the observed nuclei. This information can be accessed in detail by quantum chemical calculations, most notably by an analysis of the paramagnetic contribution to the NMR shielding tensor. The paramagnetic term mainly results from the coupling of occupied and empty molecular orbitals close in energy-the frontier molecular orbitals-under the effect of the external magnetic field (B0). In organometallic compounds, a large deshielding of the isotropic carbon-13 chemical shift of the metal-bonded carbon atom is commonly related to the coupling between the occupied σM-C orbital and low-lying vacant orbitals of πM═C* character. The deshielding at the α-carbon hence probes the extent of σM-C and πM═C* interactions. This molecular orbital view readily explains the strong deshielding and large anisotropy (evidenced by the span Ω = δ11 - δ33) observed in metal alkylidenes and alkylidynes (200 < δiso < 400 ppm). Fischer carbenes are generally more deshielded than Schrock or Grubbs alkylidenes due to their low-lying πM═C* orbital. Chemical shift hence shows their higher electrophilic character, connecting NMR spectroscopy to reactivity patterns. Similarly, the α-carbon of metal-alkyls display deshielded chemical shifts in specific coordination environments. This deshielding, which is often prominently pronounced for cationic species, indicates the presence of partial π-bond character in the metal-carbon bond, making these bonds topologically equivalent to alkylidene π-bonds. The π-character in metal-alkyl bonds favors (i) α-H abstraction processes in metal bis-alkyl compounds yielding metal alkylidenes, (ii) [2 + 2]-retrocyclization of metallacyclobutanes that participate in olefin metathesis, (iii) olefin insertion in cationic metal alkyls thus explaining polymerization activity trends and the importance of α-H agostic interactions, and (iv) C-H bond activation on metal-alkyls via σ-bond metathesis. The presence of π-character in the metal-carbon bonds involved in these processes rationalizes the parallel reactivity patterns of metal-alkyls toward olefin insertion and σ-bond metathesis and the fact that σ-bond metathesis, olefin insertion, and olefin metathesis are commonly observed with metal atoms in the same ligand field. Because of the similarities in the frontier molecular orbitals involved in these processes, these reactions can be viewed as isolobal. This explains why certain fragments, such as bent metallocenes (d0 Cp2M) or T-shaped L3M, are ubiquitous in these reactions.

Sequential Infiltration Synthesis of Electronic Materials: Group 13 Oxides via Metal Alkyl Precursors

The sequential infiltration synthesis (SIS) of group 13 indium and gallium oxides (In2O3 and Ga2O3) into poly(methyl methacrylate) (PMMA) thin films is demonstrated using trimethylindium (TMIn) and trimethylgallium (TMGa), respectively, with water. In situ Fourier transform infrared (FTIR) spectroscopy reveals that these metal alkyl precursors reversibly associate with the carbonyl groups of PMMA in analogy to trimethylaluminum (TMAl), however, with significantly lower affinity. This is demonstrated to have important kinetic consequences that dramatically alter the synthetic parameters required to achieve material growth. Ab initio density functional theory simulations of the methyl methacrylate monomer with group 13 metal alkyls corroborate association energy that is 3× greater for TMAl than for either TMIn or TMGa. As a consequence, the kinetics of activated diffusion within the film is observed to be far more rapid for TMIn and TMGa than for TMAl. Spectroscopic ellipsometry and scanning electron micros...

An In‐Depth Look at the Reactivity of Non‐Redox‐Metal Alkylperoxides

AbstractOver the past 150 years, a certain mythology has arisen around the mechanistic pathways of the oxygenation of organometallics with non‐redox‐active metal centers as well as the character of products formed. Notably, there is a widespread perception that the formation of commonly encountered metal alkoxide species results from the auto‐oxidation reaction, in which a parent metal alkyl compound is oxidized by the metal alkylperoxide via oxygen transfer reaction. Now, harnessing a well‐defined zinc ethylperoxide incorporating a β‐diketiminate ligand, the investigated alkylperoxide compounds do not react with the parent metal alkyl complex as well as Et2Zn to form a zinc alkoxide. Upon treatment of the zinc ethylperoxide with Et2Zn, a previously unobserved ligand exchange process is favored. Isolation of a zinc hydroxide carboxylate as a product of decomposition of the parent zinc ethylperoxide demonstrates the susceptibility of the latter to O−O bond homolysis.

An In-Depth Look at the Reactivity of Non-Redox-Metal Alkylperoxides.

Over the past 150 years, a certain mythology has arisen around the mechanistic pathways of the oxygenation of organometallics with non-redox-active metal centers as well as the character of products formed. Notably, there is a widespread perception that the formation of commonly encountered metal alkoxide species results from the auto-oxidation reaction, in which a parent metal alkyl compound is oxidized by the metal alkylperoxide via oxygen transfer reaction. Now, harnessing a well-defined zinc ethylperoxide incorporating a β-diketiminate ligand, the investigated alkylperoxide compounds do not react with the parent metal alkyl complex as well as Et2 Zn to form a zinc alkoxide. Upon treatment of the zinc ethylperoxide with Et2 Zn, a previously unobserved ligand exchange process is favored. Isolation of a zinc hydroxide carboxylate as a product of decomposition of the parent zinc ethylperoxide demonstrates the susceptibility of the latter to O-O bond homolysis.

Reactivity of the carbodiphosphorane, (Ph3P)2C, towards main group metal alkyl compounds: coordination and cyclometalation.

The carbodiphosphorane, (Ph3P)2C, reacts with Me3Al and Me3Ga to afford the adducts, [(Ph3P)2C]AlMe3 and [(Ph3P)2C]GaMe3, which have been structurally characterized by X-ray diffraction. (Ph3P)2C also reacts with Me2Zn and Me2Cd to generate an adduct but the formation is reversible on the NMR time scale. At elevated temperatures, however, elimination of methane and cyclometalation occurs to afford [κ2-Ph3PC{PPh2(C6H4)}]ZnMe and [κ2-Ph3PC{PPh2(C6H4)}]CdMe. Analogous cyclometalated products, [κ2-Ph3P{CPPh2(C6H4)}]ZnN(SiMe3)2 and [κ2-Ph3P{CPPh2(C6H4)}]CdN(SiMe3)2, are also obtained upon reaction of (Ph3P)2C with Zn[N(SiMe3)2]2 and Cd[N(SiMe3)2]2. The magnesium compounds, Me2Mg and {Mg[N(SiMe3)2]2}2, likewise react with (Ph3P)2C to afford cyclometalated derivatives, namely [κ2-Ph3PC{PPh2(C6H4)}]MgN(SiMe3)2 and {[κ2-Ph3PC{PPh2(C6H4)}]MgMe}2. While this reactivity is similar to the zinc system, the magnesium methyl complex is a dimer with bridging methyl groups, whereas the zinc complex is a monomer. The greater tendency of the methyl groups to bridge magnesium centers rather than zinc centers is supported by density functional theory calculations.

Rare-Earth Metal Diimide Complexes via Alkylaluminate Templating, Including a Ceric Derivative.

Protonolysis of lanthanide tris(tetramethylaluminate)s with two equivalents of 2,6-diisopropylaniline affords LaIII and CeIII diimide compounds Ln[(μ-NC6 H3 iPr2 -2,6)2 AlMe2 ](thf)4 featuring a bidentate AlMe2 -linked diimido ligand. As revealed for the corresponding Ce(GaMe4 )3 -reaction, formation of the diimide complexes proceeds via tetrametallic complexes of the type [Ce{(μ-NC6 H3 iPr2 -2,6)(HNC6 H3 iPr2 -2,6)(MMe3 )}]2 (Me=Al, Ga). Oxidation of the cerium(III) complex with hexachloroethane leads to a neutral CeIV diimide species. Partial protonolysis with phenylacetylene and hydrogenolysis via H3 SiPh give conclusive insights into the reactive coordination sites of such diimide complexes.

π-Bond Character in Metal-Alkyl Compounds for C-H Activation: How, When, and Why?

C-H bond activation via σ-bond metathesis is typically observed with transition-metal alkyl compounds in d0 or d0fn electron configurations, e.g., biscyclopentadienyl metal alkyls. Related C-H activation processes are also observed for transition-metal alkyls with higher d-electron counts, such as W(II), Fe(II), or Ir(III). A σ-bond metathesis mechanism has been proposed in all cases with a preference for an oxidative addition-reductive elimination pathway for Ir(III). Herein we show that, regardless of the exact mechanism, C-H activation with all of these compounds is associated with π-character of the M-C bond, according to a detailed analysis of the 13C NMR chemical shift tensor of the α-carbon. π-Character is also a requirement for olefin insertion, indicating its similarity to σ-bond metathesis. This observation explains the H2 response observed in d0 olefin polymerization catalysts and underlines that σ-bond metathesis, olefin insertion, and olefin metathesis are in fact isolobal reactions.

Synthesis of 1,2-phenylenedimethanols by base-promoted reduction of isobenzofuran-1(3H)-ones with silane

An efficient method for preparation of substituted 1,2-phenylenedimethanols and aliphatic 1,4-diols that are valuable intermediates in organic synthesis, has been developed by the base-promoted reduction of isobenzofuran-1(3H)-ones and γ-lactones with silane under mild conditions. Compared with traditional procedures using stoichiometric amounts of metal hydrides and alkyl reductants, the present method avoids the use of sensitive reagents and is operationally simple and a broad variety of functional groups are tolerated.