Catalyst Speciation During -Zirconocene-Catalyzed Polymerization of 1-Hexene Studied by UV-vis Spectroscopy-Formation and Partial Re-Activation of Zr-Allyl Intermediates.

Catalyst speciation during polymerization of 1-hexene in benzene or toluene solutions of the catalyst precursor SBIZr(μ-Me)AlMe B(CF)(SBI = -dimethylsilyl-bis(1-indenyl)) at 23 °C is studied by following the accompanying UV-vis-spectral changes. These indicate that the onset of polymerization catalysis is associated with the concurrent formation of two distinct zirconocene species. One of these is proposed to consist of SBIZr-σ-polyhexenyl cations arising from SBIZr-Me (formed from SBIZr(μ-Me)AlMe by release of AlMe) by repeated olefin insertions, while the other one is proposed to consist of SBIZr-η-allyl cations of composition SBIZr-η-(1-R-CH) (R = npropyl), [.

Reversible 1,2-Alkyl Migration to Carbene and Ammonia Activation in an N-Heterocyclic Carbene-Zirconium Complex.

Addition of trimethylphosphine to a bis(phenolate)benzylimidazolylidene(dibenzyl)zirconium complex induces migration of a benzyl ligand from the metal center to the C(carbine) atom. This process may be reversed, resulting in Csp(3)-Csp(3) activation, by abstraction of the phosphine, an example of regulated, reversible alkyl migration. Addition of ammonia to the dibenzyl complex results in migration of one benzyl group and protonolysis of the other to generate a bis(NH2)-bridged dimer via an NMR-observable intermediate NH3 adduct.

Lewis acid promoted titanium alkylidene formation: off-cycle intermediates relevant to olefin trimerization catalysis.

Two new precatalysts for ethylene and α-olefin trimerization, (FI)Ti(CH2SiMe3)2Me and (FI)Ti(CH2CMe3)2Me (FI = phenoxy-imine), have been synthesized and structurally characterized by X-ray diffraction. (FI)Ti(CH2SiMe3)2Me can be activated with 1 equiv of B(C6F5)3 at room temperature to give the solvent-separated ion pair [(FI)Ti(CH2SiMe3)2][MeB(C6F5)3], which catalytically trimerizes ethylene or 1-pentene to produce 1-hexene or C15 olefins, respectively. The neopentyl analogue (FI)Ti(CH2CMe3)2Me is unstable toward activation with B(C6F5)3 at room temperature, giving no discernible diamagnetic titanium complexes, but at -30 °C the following can be observed by NMR spectroscopy: (i) formation of the bis-neopentyl cation [(FI)Ti(CH2CMe3)2](+), (ii) α-elimination of neopentane to give the neopentylidene complex [(FI)Ti(═CHCMe3)](+), and (iii) subsequent conversion to the imido-olefin complex [(MeOAr2N═)Ti(OArHC═CHCMe3)](+) via an intramolecular metathesis reaction with the imine fragment of the (FI) ligand.

Spectral studies of a Cr(PNP)-MAO system for selective ethylene trimerization catalysis: searching for the active species.

Variable temperature spectroscopic, kinetic, and chemical studies were performed on a soluble CrCl(PNP) (PNP = bis(diarylphosphino)alkylamine) ethylene trimerization precatalyst to map out its methylaluminoxane (MAO) activation sequence. These studies indicate that treatment of CrCl(PNP) with MAO leads to first replacement of chlorides with alkyl groups, followed by alkyl abstraction, and then reduction to lower-valent species. Reactivity studies demonstrate that the majority of the chromium species detected is not catalytically active.

Addition of a phosphine ligand switches an N-heterocyclic carbene-zirconium catalyst from oligomerization to polymerization of 1-hexene.

A catalyst for the oligomerization of 1-hexene, generated by the activation of a benzimidazolylidene zirconium dibenzyl complex, switches to a polymerization catalyst on addition of a trialkylphosphine.

Upgrading light hydrocarbons via tandem catalysis: a dual homogeneous Ta/Ir system for alkane/alkene coupling.

Light alkanes and alkenes are abundant but are underutilized as energy carriers because of their high volatility and low energy density. A tandem catalytic approach for the coupling of alkanes and alkenes has been developed in order to upgrade these light hydrocarbons into heavier fuel molecules. This process involves alkane dehydrogenation by a pincer-ligated iridium complex and alkene dimerization by a Cp*TaCl2(alkene) catalyst.

Formation of trivalent zirconocene complexes from ansa-zirconocene-based olefin-polymerization precatalysts: an EPR- and NMR-spectroscopic study.

Reduction of Zr(IV) metallocenium cations with sodium amalgam (NaHg) produces EPR signals assignable to Zr(III) metallocene complexes. The chloro-bridged heterodinuclear ansa-zirconocenium cation [(SBI)Zr(μ-Cl)2AlMe2](+) (SBI = rac-dimethylsilylbis(1-indenyl)), present in toluene solution as its B(C6F5)4(-) salt, thus gives rise to an EPR signal assignable to the complex (SBI)Zr(III)(μ-Cl)2AlMe2, while (SBI)Zr(III)-Me and (SBI)Zr(III)(μ-H)2Al(i)Bu2 are formed by reduction of [(SBI)Zr(μ-Me)2AlMe2](+) B(C6F5)4(-) and [(SBI)Zr(μ-H)3(Al(i)Bu2)2](+) B(C6F5)4(-), respectively. These products can also be accessed, along with (SBI)Zr(III)-(i)Bu and [(SBI)Zr(III)](+) AlR4(-), when (SBI)ZrMe2 is allowed to react with HAl(i)Bu2, eliminating isobutane en route to the Zr(III) complex.

Activator-free olefin oligomerization and isomerization reactions catalyzed by an air- and water-tolerant Wacker oxidation intermediate.

A bench-stable, hydroxy-bridged α-diimine-Pd dimer can self-activate to an olefin oligomerization and isomerization catalyst in the presence of substrate. A cationic Pd-hydride is generated principally through a Wacker oxidation of olefin to ketone, and with C(4+) olefins, lesser amounts of allylic C-H activation, β-H transfer, and release of diene products are observed.

Mechanistic Studies of Ethylene and α-Olefin Co-oligomerization Catalyzed by Chromium-PNP Complexes.

To explore the possibility of producing a narrow distribution of mid- to long-chain hydrocarbons from ethylene as a chemical feedstock, co-oligomerization of ethylene and linear α-olefins (LAOs) was investigated, using a previously reported chromium complex, [CrCl(3)(PNP(OMe))] (1, where PNP(OMe) = N,N-bis(bis(o-methoxyphenyl)phosphino)methylamine). Activation of 1 by treatment with modified methylaluminoxane (MMAO) in the presence of ethylene and 1-hexene afforded mostly C(6) and C(10) alkene products. The identities of the C(10) isomers, assigned by detailed gas chromatographic and mass spectrometric analyses, strongly support a mechanism that involves five- and seven-membered metallacyclic intermediates comprising of ethylene and LAO units.

Highly regioirregular polypropylene from asymmetric group 4 anilide(pyridine)phenoxide complexes.

Group 4 complexes containing an anilide(pyridine)phenoxide ligand and activated with methylaluminoxane (MAO) catalyze the formation of highly regioirregular polypropylene.

Groups 5 and 6 terminal hydrazido(2-) complexes: N(β) substituent effects on ligand-to-metal charge-transfer energies and oxidation states.

Brightly colored terminal hydrazido(2-) (dme)MCl(3)(NNR(2)) (dme = 1,2-dimethoxyethane; M = Nb, Ta; R = alkyl, aryl) or (MeCN)WCl(4)(NNR(2)) complexes have been synthesized and characterized. Perturbing the electronic environment of the β (NR(2)) nitrogen affects the energy of the lowest-energy charge-transfer (CT) transition in these complexes. For group 5 complexes, increasing the energy of the N(β) lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium dialkylhydrazides, which pyramidalize N(β) in order to reduce the overlap between the Nb═N(α) π bond and the N(β) lone pair.

Trialkylborane-Assisted CO(2) Reduction by Late Transition Metal Hydrides.

Trialkylborane additives promote reduction of CO(2) to formate by bis(diphosphine) Ni(II) and Rh(III) hydride complexes. The late transition metal hydrides, which can be formed from dihydrogen, transfer hydride to CO(2) to give a formate-borane adduct. The borane must be of appropriate Lewis acidity: weaker acids do not show significant hydride transfer enhancement, while stronger acids abstract hydride without CO(2) reduction.

Cationic alkylaluminum-complexed zirconocene hydrides: NMR-spectroscopic identification, crystallographic structure determination, and interconversion with other zirconocene cations.

The ansa-zirconocene complex rac-Me(2)Si(1-indenyl)(2)ZrCl(2) ((SBI)ZrCl(2)) reacts with diisobutylaluminum hydride and trityl tetrakis(perfluorophenyl)borate in hydrocarbon solutions to give the cation [(SBI)Zr(μ-H)(3)(Al(i)Bu(2))(2)](+), the identity of which is derived from NMR data and supported by a crystallographic structure determination. Analogous reactions proceed with many other zirconocene dichloride complexes. [(SBI)Zr(μ-H)(3)(Al(i)Bu(2))(2)](+) reacts reversibly with ClAl(i)Bu(2) to give the dichloro-bridged cation [(SBI)Zr(μ-Cl)(2)Al(i)Bu(2)](+).

Cationic alkylaluminum-complexed zirconocene hydrides as participants in olefin polymerization catalysis.

The alkylaluminum-complexed zirconocene trihydride cation [(SBI)Zr(μ-H)(3)(Al(i)Bu(2))(2)](+), which is obtained by reaction of (SBI)ZrCl(2) with [Ph(3)C][B(C(6)F(5))(4)] and excess HAl(i)Bu(2) in toluene solution, catalyzes the formation of isotactic polypropene when exposed to propene at -30 °C. This cation remains the sole observable species in catalyst systems free of AlMe compounds. In the presence of AlMe(3), however, exposure to propene causes the trihydride cation to be completely converted, under concurrent consumption of all hydride species by propene hydroalumination, to the doubly Me-bridged cation [(SBI)Zr(μ-Me)(2)AlMe(2)](+).

(dme)MCl3(NNPh2) (dme = dimethoxyethane; M = Nb, Ta): a versatile synthon for [Ta=NNPh2] hydrazido(2-) complexes.

Complexes (dme)TaCl(3)(NNPh(2)) (1) and (dme)NbCl(3)(NNPh(2)) (2) (dme =1,2-dimethoxyethane) were synthesized from MCl(5) and diphenylhydrazine via a Lewis-acid assisted dehydrohalogenation reaction. Monomeric 1 has been characterized by X-ray, IR, UV-vis, (1)H NMR, and (13)C NMR spectroscopy and contains a kappa(1)-bound hydrazido(2-) moiety. Unlike the corresponding imido derivatives, 1 is dark blue because of an LMCT that has been lowered in energy as a result of an N(alpha)-N(beta) antibonding interaction that raises the highest occupied molecular orbital (HOMO).

Thermodynamic studies of [H(2)Rh(diphosphine)(2)](+) and [HRh(diphosphine)(2)(CH(3)CN)](2+) complexes in acetonitrile.

Thermodynamic studies of a series of [H(2)Rh(PP)(2)](+) and [HRh(PP)(2)(CH(3)CN)](2+) complexes have been carried out in acetonitrile. Seven different diphosphine (PP) ligands were selected to allow variation of the electronic properties of the ligand substituents, the cone angles, and the natural bite angles (NBAs). Oxidative addition of H(2) to [Rh(PP)(2)](+) complexes is favored by diphosphine ligands with large NBAs, small cone angles, and electron donating substituents, with the NBA being the dominant factor.

Homogeneous CO hydrogenation: dihydrogen activation involves a frustrated Lewis pair instead of a platinum complex.

During a search for conditions appropriate for Pt-catalyzed CO reduction using dihydrogen directly, metal-free conditions were discovered instead. A bulky, strong phosphazene base forms a "frustrated" Lewis pair (FLP) with a trialkylborane in the secondary coordination sphere of a rhenium carbonyl. Treatment of the FLP with dihydrogen cleanly affords multiple hydride transfers and C-C bond formation.

Dehydrogenation of amine-boranes with a frustrated Lewis pair.

Bulky tertiary phosphine/borane Lewis pairs P(t)Bu(3)/B(C(6)F(5))(3) react with amine-boranes to afford dehydrocoupling products and phosphonium borohydride salts.

Electronic structures of Pd(II) dimers.

The Pd(II) dimers [(2-phenylpyridine)Pd(mu-X)](2) and [(2-p-tolylpyridine)Pd(mu-X)](2) (X = OAc or TFA) do not exhibit the expected planar geometry (of approximate D(2h) symmetry) but instead resemble an open "clamshell" in which the acetate ligands are perpendicular to the plane containing the Pd atoms and 2-arylpyridine ligands, with the Pd atoms brought quite close to one another (approximate distance 2.85 A). The molecules adopt this unusual geometry in part because of a d(8)-d(8) bonding interaction between the two Pd centers.

Activation of the C-N bond in nitromethane by palladium alpha-diimine complexes.

The reaction of glyoxal-derived alpha-diimines with palladium acetates in nitromethane leads to cleavage of the C-N bond in nitromethane, to give palladium nitro complexes in which the alpha-diimine ligand has been methylated.

Amine, amido, and imido complexes of tantalum supported by a pyridine-linked bis(phenolate) pincer ligand: Ta-N pi-bonding influences pincer ligand geometry.

A series of tantalum imido and amido complexes supported by a pyridine-linked bis(phenolate) ligand has been synthesized. Characterization of these complexes via X-ray crystallography reveals both C(s) and C(2) binding modes of the bis(phenolate)pyridine ligand, with complexes containing two or fewer strong pi-donor interactions from ancillary ligands giving C(s) symmetry, whereas three strong pi-donor interactions (e.g.