Metal Alkyl Compounds Research Articles

Partial Double Metal-Carbon Bonding Model in Transition Metal Methyl Compounds.

The chemical bond between a transition metal and a methyl group (M-CH3) is typically defined as a single covalent bond, which is of fundamental significance and general interest in understanding the structural properties and reactivity of transition metal alkyl compounds. Herein, we demonstrate that the M-CH3 bonding involves varying σ and π components and thus should be best described in terms of the partial double M═CH3 bond. The often-neglected π bonding stems from an occupied π-symmetric orbital of the methyl group comprising all three C-H σ bonds (but one C-H' contributes more than the other two) and a vacant low-lying metal d(π) orbital, and is associated with the intramolecular C-H'···M agostic effect (i.e., an acute M-C-H' angle and a short H'···M distance), whose origin is still controversial. We quantify the geometric and energetic impacts of the π interaction involved in the M-CH3 bond by explicitly computing the intramolecular πCH' → dM interaction with the ab initio valence bond (VB) theory. Our computations of the ligand-free [TiCH3]3+ and a series of metallocene catalysts provide a direct proof for the presence of the π bonding in M-CH3 bonds, which is the cause for the agostic effect. The partial double M═CH3 bonding model is not only validated by a range of bonding analyses including VB self-consistent field (VBSCF)-based energy decomposition and quantum theory of atoms in molecules (QTAIM) but also authenticated by the specific activity of double M═CH3 bonds in the C-H activation and olefin insertion. More importantly, the σ bond gradually switches from a classical covalent bond to a novel charge-shift bond with the π bonding becoming increasingly significant. We anticipate that the recognition of the π interaction between electrophilic metal centers and C-H bonds can benefit the understanding of the nature of metal-carbon bonds in transition metal ethyl, alkyl, and carbene compounds.

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.

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.

Pendant ancillary ligand switches off auto-oxidation of group 13 metal alkyl compounds bearing non-bulky alkyl groups.

The reaction of 3,5-di-2-pyridyl-1,2,4-triazole with excess Al(CH3)3 and Ga(CH3)3 afforded (3,5-di-2-pyridyl-1,2,4-triazolate)Al(CH3)2•3Al(CH3)3 (1) and (3,5-di-2-pyridyl-1,2,4-triazolate) Ga(CH3)2•3Ga(CH3)3 (2) respectively. 1 and 2 reacted with oxygen gas to produce (CH3)2M(µ-3,5-di-2-pyridyl-1,2,4-triazolate)(µ-OCH3)M(CH3)2 (M = Al, 3; M = Ga, 4). 3 and 4 contain the non-bulky dimethylalumino moiety, yet they are indefinitely stable in the presence of oxygen gas. This increased stability towards oxygen is due to ancillary 2-pyridyl groups bonding to the metal centers producing a pseudo-trigonal pyramidal Al and Ga environments. This environment blocks oxygen from further inserting into the M–C bond. The Al–N(pyridine) and Ga–N(pyridine) bonds reported herein are extremely elongated yet inactive towards dissociation due to the chelate effect.

Interaction of aluminum and cobalt chlorides and their complexes with propane: A quantum chemical study

Density functional calculations of binary and ternary propane complexes with aluminum and cobalt(II) chlorides were carried out. Interaction of metal chloride complexes with the methylene and methyl groups of propane is coupled with strong polarization of C-H bonds. The direction of polarization is opposite to that observed under interaction with the typical Lewis acids including aluminum halides. In the interaction of bimetallic (transition metal/aluminum) complexes with saturated C-H bonds, the key role is played by the transition metal ion rather than the stronger Lewis center (Al). The negative atomic charge on carbon increases, which may facilitate the formation of metal-alkyl compounds with a carbanion-type organic radical.

Neutral and Cationic Rare Earth Metal Alkyl and Benzyl Compounds with the 1,4,6-Trimethyl-6-pyrrolidin-1-yl-1,4-diazepane Ligand and Their Performance in the Catalytic Hydroamination/Cyclization of Aminoalkenes

A new neutral tridentate 1,4,6-trimethyl-6-pyrrolidin-1-yl-1,4-diazepane (L) was prepared. Reacting L with trialkyls M(CH(2)SiMC(3))(3)(THF)(2) (M = Sc, Y) and tribenzyls M(CH(2)Ph)(3)(THF)(3) (M = Sc, La) yielded trialkyl complexes (L)M(CH(2)SiMe(3))(3) (M = Sc, 1; M = Y, 2) and tribenzyl complexes (L)M(CH(2)Ph)(3) (M = Sc, 3; M = La, 4). Complexes I and 2 can be converted to their corresponding ionic compounds [(L)M(CH(2)SiMe(3))(2)(THF)][B(C(6)H(5))(4)] (M = Sc, 5; M = Y, 6) by reaction with [PhNMe(2)H] [B(C(6)H(5))(4)] in THE Complexes 3 and 4 can be converted to cationic species [(L)M(CH(2)Ph)(2)](+) by reaction with [PhNMe(2)H][B(C(6)F(5))(4)] in C(6)D(5)Br in the absence of THE The neutral complexes 1-4 and their cationic derivatives were studied as catalysts for the hydroamination/cyclization of 2,2-diphenylpent-4-en-1-amine and N-methylpent-4-en-1-amine reference substrates and compared with ligand-free Sc, Y, and La neutral and cationic catalysts. The most effective catalysts in the series were the cationic L-yttrium catalyst (for 2,2-diphenylpent-4-en-1-amine) and the cationic lanthanum systems (for N-methylpent-4-en-1-amine). For the La catalysts, evidence was obtained for release of L from the metal during catalysis.

A General Process for the Fabrication of Air‐Stable Metallic Particles (Cd, Zn and Al) by the Decomposition of Alkyl–Metal Compounds

AbstractThe thermolysis of alkyl–metal compounds under autogenic pressure, with the three metals Cd, Zn, and Al as examples, suggest that this might become a general method for preparing a composite where the metal is the core and carbon the shell. The air‐stability of these composites is also demonstrated. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Alkyl- and aryl-substituted main-group metal amides

Publisher Summary The main-group metal alkyls and aryls are valuable tools in preparative organic and inorganic chemistry, and many industrial applications are known for these reactive substances. This chapter describes the syntheses, structures, and physical properties of metal alkyls or aryls that have a further “chelating” amino ligand attached to the metallic center. The formal substitution of one of the alkyl or aryl groups at the metal by nonmetallic and electronegative groups leads to compounds that have both electrophilic and nucleophilic centers in the molecule. The existence of alkyl– or aryl–calcium amides has not as yet been unambiguously demonstrated. The chemistry of alkyl- and arylaluminum, gallium, indium, and thallium amides has been investigated much more than the comparable chemistry of Group II elements. Many of the structural principles found for organoaluminum amides also hold for the corresponding compounds of gallium, indium, and thallium. There are numerous examples of chelating ligands that have been used to obtain monometal-centered rings. Macrocycles have been used to chelate alkyl–metal compounds efficiently. The coordination number of the indium atom is increased by interaction with the nitrogen atom of a second molecule. The alkyl– and aryl–metal silazanes and poly(metal) silazanes described so far are representatives of an unusual class of compound containing simultaneously electron-deficient and electron-rich centers that can interact with one another. The very prominent bond fluctuations observed for these molecules are often due to competitive reactions between bases for the same acidic center.

Reactivity of the metal–cyclopentadienyl (indenyl, fluorenyl and cycloheptatrienyl) bonds

The focus of this review concerns the reactivity patterns of metal–cyclopentadienyl (indenyl, fluorenyl and cycloheptatrienyl) bonds. The advances in the metal–carbon(η 5) insertion chemistry have demonstrated that the π-bonding character provides additional reaction opportunities which are not shown by metal–alkyl compounds and these differences are highlighted.

Titanium–Alkaline Earth Molecular Oxides as Supports for Carbanions Derived from μ3‐Ethylidyne Groups (Eur. J. Inorg. Chem. 11/2006)

AbstractThe cover picture shows how the treatment of the oxide molecular model [{TiCp*(μ‐O)}3(μ3‐CMe)] (Cp* = η5‐C5Me5) with alkaline earth metal amide and alkyl compounds produces the deprotonation of the μ3‐ethylidyne fragment to give μ3‐CCH2−, μ3‐CHCH− or μ3‐CCH2− carbanions. Simultaneously, the alkaline earth cation is incorporated through the oxygen atoms to build new cage structures. Details of this chemistry are described by M. Mena et al. on p. 2137 ff.

B(C6F5)3 adducts of transition metal acyl compounds

The transition metal acyl compounds [Co(L)(CO)3(COMe)] (L = PMe3, PPhMe2, P(4-Me-C6H4)3, PPh3 and P(4-F-C6H4)3), [Mn(CO)5(COMe)] and [Mo(PPh3)(eta(5)-C5H5)(CO)2(COMe)] react with B(C6F5)3 to form the adducts [Co(L)(CO)3(C{OB(C6F5)3}Me)] (L = PMe3, 1, PPhMe2, 2, P(4-Me-C6H4)3, 3, PPh3, 4, P(4-F-C6H4)3), 5, [Mn(CO)5(C{OB(C6F5)3}Me)] 6 and [Mo(eta(5)-C5H5)(PPh3)(CO)2(C{OB(C6F5)3}Me)], 7. Addition of B(C6F5)3 to a cooled solution of [Mo(eta(5)-C5H5)(CO)3(Me)], under an atmosphere of CO gave [Mo(eta(5)-C5H5)(CO)3(C{OB(C6F5)3}Me)] 8. In the presence of adventitious water, the compound [Co{HOB(C6F5)3}2{OP(4-F-C6H4)3}2] 9, was formed from [Co(P(4-F-C6H4)3)(CO)3(C{OB(C6F5)3}Me)]. The compounds 4 and 9 have been structurally characterised. The use of B(C6F5)3 as a catalyst for the CO-induced migratory-insertion reaction in the transition metal alkyl compounds [Co(PPh3)(CO)3(Me)], [Mn(CO)5(Me)], [Mo(eta(5)-C5H5)(CO)3(Me)] and [Fe(eta(5)-C5H5)(CO)2(Me)] has been investigated.

Unprecedented Coordination Mode Variation of Group 13 Metal–Alkyl Compounds Derived from Methyl Thiosalicylate

AbstractThe X‐ray structure analysis of the alkylaluminum and‐indium compounds derived from methyl thiosalicylate,Me2Al(SC6H4‐2‐CO2Me) and [Me2In(μ‐SC6H4‐2‐CO2Me)]2, revealed that the intermolecular S···C(π) interaction between the Al–S thiolate units and the ester π‐surface can effectively compete with the hypercoordinate sulfur–metal dative bond. The former compound exists as the noncovalently bonded dimer involving the thiolate sulfur atom and the electrophilic carbon atom of the ester functionality, whereas the indium complex is a five‐coordinate dimer with the In2(μ‐S)2 central core. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Regioselectivity in the interaction of rhodium hidridotricarbonyl with propene: a theoretical study

Quantum Mechanical calculations at the MP4(SDQ)//MP2 level of theory were carried out to investigate the energies and reaction mechanism for the propene (CH 3–CHCH 2) insertion reaction into the Rh–H bond, using the HRh(CO) 3 as the catalytic species. Since the reaction may occur on the branched carbon or in the normal carbon, we investigated these two reaction pathways. The results show that the insertion in the branched carbon has an activation energy of 6.5 kcal/mol, and the activation energy for the reaction to take place at the normal carbon is 9.9 kcal/mol. These activation energies, together with the calculated relative energy of the metal-alkyl compounds formed after the insertion for these two pathways, shows that the branched carbon is the preferential site for the reaction to take place, generating branched metal-alkyl products, with a regioselectivity ratio ( n: iso) of (16:84). The nature of the bonds on the reactants and intermediates found in both pathways were analyzed, using the MP2 wavefunction and are also discussed and comparisons with the previous results obtained for the phosphine-modified HRh(CO)(PH 3) 2 catalyst are made in order to understand the energetic changes that take place when CO ligands are replaced by phosphines.

Single-Source Approach for The Growth of I-III-VI Thin Films

AbstractThe ternary chalcopyrite semiconductors I-III-VI are currently used for photovoltaic solar cell applications. In this study, AgIn5S8 thin films were prepared from a series of single-source bimetalorganic precursors, e.g. [(PPh3)2AgIn(SC{O}R)4] (R = alkyl) by aerosol assisted chemical vapour deposition (AA-CVD). The compounds can be used as single-source precursors for the deposition of ternary compounds (I-III-VI) by one-pot reaction using CVD process and they are found to be air stable, which is favourable in comparison with metal alkyl compounds, which are found to be pyrophoroic. The optimum growth temperature for the preparation of these films on glass and Si(100) substrates, was found to be above 350°C in terms of crystallinity, although deposition occurred at low temperatures. The films have been investigated using XRD, SEM and EDS. SEM analysis shows that all films are microcrystalline but have different morphologies depending on the growth temperatures. XRD results show evidence of the crystalline nature of these films. The results of this comprehensive study are presented and discussed.

New Approach Towards The Deposition of I-III-VI Thin Films

AbstractThe ternary chalcopyrite semiconductor Cu(In/Ga)(Se/S)2 is currently used as an absorber layer in high efficiency thin film solar cells. In this study, various types of I-III-VI (I = Cu, III = Ga or In, VI = S or Se) thin films (CuGaS2, CuInS2 and CuInSe2) were prepared from a series of organometallic precursors, M[(S/Se)2CNMeR]n (M = Cu, In, Ga; R = alkyl) by aerosol-assisted chemical vapour deposition (AACVD). In contrast to the metal alkyl compounds, MR3 (M = In and Ga; R = alkyl), which are pyrophoric, the precursors are easy to synthesize by one-pot reactions and are air stable. The optimum growth temperature for the preparation of these films on glass substrates using aerosol-assisted chemical vapour deposition (AACVD) was found to be above 400 °C in terms of crystallinity, although deposition does occur at lower temperatures. The films have been characterised using XRPD, SEM and EDS. SEM analyses show all films are microcrystalline. XRPD results show evidence of the crystalline nature of theses films. The results of this comprehensive study are presented and discussed.

Quantum chemical models and electronic structure of active centers of heterogeneous polymerization of olefins

The peculiarities of the catalytic activity in olefin polymerizations which can find explanation in terms of the concepts suggested in this work are as follows. First, the low catalytic activity of the individual organometal compounds of group IV-VI transition metals is indicative [53] of the important role of the coordination state of the transition metal in AC, which, according to Cosse's model, must be octahedral (tetrahedral for individual metal-alkyl compounds MRn). Second, the activity of a catalytic system depends essentially on the nature of the ligand environment of the metal in AC. The catalysts based on titanium halides display the highest activity. Third, the results of [19, 20] show that the highly active catalytic centers of homogeneous Ziegler-Natta's systems are “cation-like” Zr(IV) complexes Cp2Zr+-R. All these features find explanation in terms of the concept of the competitive contributions from the AC metal s and d orbitals to the active M-R bond. Thus a transition of AC environment from tetrahedral to octahedral may be compared with a change in transition metal AO hybridization:d 3s1 (tetrahedron) ⇒d 3s1 (octahedron).

Cationic Group 4 metal alkyl compounds containing aryloxide ligation: synthesis, structure, reactivity and polymerization studies

Abstract A series of bis(alkyl) derivatives of titanium and zirconium [(ArO)2MR2] (M=Ti, Zr; R=Me, CH2Ph; ArO=various 2,6-di-substituted phenoxides) has been synthesized and their reactivity towards the Lewis acid [B(C6F5)3] examined. The benzyl compounds generate stable zwitterionic species such as [M(OC6HPh2-2,6-R2-3,5)2(CH2Ph)][η6-C6H5CH2B(C6F5)3] (M=Ti, R=H, 12; Me, 13: M=Zr, R=Me, 15). Structural studies of 12 and 15 show the boron anion π-bound to the metal center through the original benzyl phenyl ring. In contrast, treatment of the benzyl compound [Zr(OC6H3But2-2,6)2(CH2Ph)2] with [B(C6F5)3] leads to the cyclometallated compound [Zr(OC6H3But-CMe2CH2)(OC6H3But2-2,6)][η6-C6H5CH2B(C6F5)3] 16 which was structurally characterized. In contrast to this behavior the bis(methyl) species react with [B(C6F5)3] to produce unstable methyl cationic intermediates which decompose to a mixture of [Ti(OAr)2(CH3)(C6F5)] and [CH3B(C6F5)2]. The titanium zwitterionic benzyl compounds will react with alkynes and α-olefins to produce mono-insertion products such as [Ti(OC6H3Ph2-2,6)2{C(CH3)C(Ph)CH2(η6-C6H5)}][PhCH2B(C6F5)3] 24. In these compounds 1,2-insertion of olefins occurs followed by chelation of the original benzyl group to the metal center. Spectroscopic studies show the boron anion is non-coordinated to the metal center. Despite their thermal instability, the methyl cations can be generated in situ in the presence of olefins to produce polymers (ethene and propene) and oligomers (1-hexene). Studies show that the molecular weight of the polymers or oligomers increases systematically with the bulk of the aryloxide ligand. Spectroscopic studies of the polypropylene indicate 1,2-insertion is occurring with β-hydrogen abstraction to produce vinylidene end groups as the termination step.

Reactivity of Group 4 Metal Alkyl and Metallacyclic Compounds Supported by Aryloxide Ligands Toward Organic Isocyanides

The reactivity of the dimethyl compound [Ti(OC6H3Ph2-2,6)2Me2] (1) and titanabicyclic compounds [(ArO)2Ti(CH2CH(C4H8)CHCH2)] (6, 7) (formed by trans-coupling of 1,7-octadiene) toward organic isocyanides has been investigated. Dimethyl 1 reacts with 1 equiv of 2,6-dimethylphenylisocyanide (xyNC) to produce a cyclometalated product [Ti(OC6H3Ph2-2,6)(OC6H3Ph-η1-C6H4){N(xy)CHMe2}] (2), which was structurally characterized. Compound 2 is believed to arise via CH bond activation within an intermediate η2-imine (azatitanacyclopropane) formed by transfer of both methyl groups to the isocyanide. With an excess of xyNC, 1 produces an intermediate bis(η2-iminoacyl) 4, which slowly converts to the enediamide [Ti(OC6H3Ph2-2,6)2{N(xy)CMeCMeN(xy)}] (5). The solid-state structure of 5 shows the presence of a folded diazatitanacyclopent-3-ene ring formed by intramolecular coupling of the two iminoacyl groups. Titanabicyclic compounds 6 and 7 react with ButNC and xyNC respectively to generate products 8 and 9, which both contain amide ligands derived by CH bond activation and an iminoacyl group formed by insertion into the new Ti−C bond. The solid-state structure of [Ti(OC6H3Ph2-2,6){OC6H3Ph-(η2-(C6H4)CNBut)}{ButNCH(C8H14)}] (8) shows a trans-fusion between the five- and six-membered carbon rings as found in the initial titanabicycle 6. The treatment of 1 with xyNC in the presence of excess 3-hexyne or styrene produces azatitanacyclopent-2-ene 10 and azatitanacyclopentane 11 derivatives, respectively. These arise via coupling of the alkyne or olefin with an intermediate η2-imine. The solid-state structure of [Ti(OC6H3Ph2-2,6)2{N(xy)C(Me2)C(Et)C(Et)}] (10) confirms its formulation.

ChemInform Abstract: Bimetallic and Organometallic Supported Catalysts Prepared by Reaction of Metal Alkyl Compounds with Supported Transition Metals. Part 1. Preparation and Characterization

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Surface reactions of tetraneopentyl zirconium on zirconium carbide thin films

Interest has arisen recently in the use of metal alkyl compounds as precursors in the chemical vapor deposition of metal carbides. This paper presents results of a study of the surface chemistry of one such precursor, tetraneopentyl zirconium (ZrNp4), on zirconium carbide thin films. Decomposition of the precursor leads to the desorption of both methane and a larger hydrocarbon species that is tentatively identified as neopentane. The desorption of large amounts of methane may help to explain the fact that ZrC films grown to date with this precursor are not stoichiometric but rather contain excess carbon.