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. If the reaction is carried out at low temperature in the presence of ethylene, catalytic production of 1-hexene is observed, in addition to the titanacyclobutane complex [(FI)Ti(CH(CMe3)CH2CH2)](+), resulting from addition of ethylene to the neopentylidene [(FI)Ti(═CHCMe3)](+). None of the complexes observed spectroscopically subsequent to [(FI)Ti(CH2CMe3)2](+) is an intermediate or precursor for ethylene trimerization, but notwithstanding these off-cycle pathways, [(FI)Ti(CH2CMe3)2](+) is a precatalyst that undergoes rapid initiation to generate a catalyst for trimerizing ethylene or 1-pentene.

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. These two homogeneous catalysts operate with up to 60/30 cooperative turnovers (Ir/Ta) in the dimerization of 1-hexene/n-heptane, giving C13/C14 products in 40% yield. This dual system can also effect the catalytic dimerization of n-heptane (neohexene as the H2 acceptor) with cooperative turnover numbers of 22/3 (Ir/Ta).

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. Further studies concern interconversion reactions between these and other (SBI)Zr(III) complexes and reaction mechanisms involved in their formation.

Groups 5 and 6 terminal hydrazido(2-) complexes: N(ß) 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 ß (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(ß) lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium dialkylhydrazides, which pyramidalize N(ß) in order to reduce the overlap between the Nb═N(α) π bond and the N(ß) lone pair. For W complexes, increasing the energy of N(ß) eventually leads to reduction from formally [W(VI)≡N-NR(2)] with a hydrazido(2-) ligand to [W(IV)═N═NR(2)] with a neutral 1,1-diazene ligand. The photophysical properties of these complexes highlight the potential redox noninnocence of hydrazido ligands, which could lead to ligand- and/or metal-based redox chemistry in early transition metal derivatives.

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, ß-H transfer, and release of diene products are observed.

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)](+). Reaction with AlMe(3) first leads to mixed-alkyl species [(SBI)Zr(µ-H)(3)(AlMe(x)(i)Bu(2-x))(2)](+) by exchange of alkyl groups between aluminum centers. At higher AlMe(3)/Zr ratios, [(SBI)Zr(µ-Me)(2)AlMe(2)](+), a constituent of methylalumoxane-activated catalyst systems, is formed in an equilibrium, in which the hydride cation [(SBI)Zr(µ-H)(3)(AlR(2))(2)](+) strongly predominates at comparable HAl(i)Bu(2) and AlMe(3) concentrations, thus implicating the presence of this hydride cation in olefin polymerization catalyst systems.

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)](+). The latter then becomes the resting state for further propene polymerization, which produces, by chain transfer to Al, mainly AlMe(2)-capped isotactic polypropene.

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.

(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). Reaction of 1 with a variety of neutral, mono- and dianionic ligands generates the corresponding ligated complexes retaining the kappa(1)-bound [Ta-NNPh(2)] moiety.

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. The Pd-Pd dimers exhibit two successive one-electron oxidations: Pd(II)-Pd(II) to Pd(II)-Pd(III) to Pd(III)-Pd(III). Photophysical measurements reveal clear differences in the UV-visible and low-temperature fluorescence spectra between the clamshell dimers and related planar dimeric [(2-phenylpyridine)Pd(mu-Cl)](2) and monomeric [(2-phenylpyridine)Pd(en)][Cl] (en = ethylenediamine) complexes that do not have any close Pd-Pd contacts. Density functional theory and atoms in molecules analyses confirm the presence of a Pd-Pd bonding interaction in [(2-phenylpyridine)Pd(mu-X)](2) and show that the highest occupied molecular orbital is a d(z(2)) sigma* Pd-Pd antibonding orbital, while the lowest unoccupied molecular orbital and close-lying empty orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other Pd(II)-Pd(II) dimers that have short Pd-Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(mu-X)](2), but quite different from that found for d(8)-d(8) dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd and may explain why Pd d(8)-d(8) dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species.

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. Large pK(a) values for [HRh(PP)(2)(CH(3)CN)](2+) complexes are favored by small ligand cone angles, small NBAs, and electron donating substituents with the cone angles playing a major role. The hydride donor abilities of [H(2)Rh(PP)(2)](+) complexes increase as the NBAs decrease, the cone angles decrease, and the electron donor abilities of the substituents increase. These results indicate that if solvent coordination is involved in hydride transfer or proton transfer reactions, the observed trends can be understood in terms of a combination of two different steric effects, NBAs and cone angles, and electron-donor effects of the ligand substituents.

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.

Synthesis and characterization of iron derivatives having a pyridine-linked bis(anilide) pincer ligand.

A pyridine-linked bis(aniline) pincer ligand, [2]H(2) ([2]H(2) = (2,6-NC(5)H(3)(2-(2,4,6-Me(3)C(6)H(2))-NHC(6)H(4))(2)), has been synthesized in two steps. Deprotonation with Me(3)SiCH(2)Li followed by metalation with FeCl(2) yielded a LiCl adduct of [2]Fe. The complex is freed of LiCl with excess TlPF(6) or by crystallization from toluene/petroleum ether, giving [2]Fe(THF). [2]Fe(THF) reacts with I(2) and O(2) to generate [2]FeI and ([2]Fe)(2)O, respectively. The complexes have been characterized by (1)H NMR spectroscopy, elemental analysis, X-ray crystallography, and UV-vis spectroscopy. [2]Fe(THF) has been examined using cyclic voltammetry.

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., three amido ligands or one amido ligand and one imido ligand) give C(2)-symmetric binding of the bis(phenolate)pyridine ligand. DFT calculations and molecular orbital analyses of the complexes have revealed that the preference for C(s)-symmetric ligand binding is a result of tantalum-phenolate pi-bonding, whereas in cases where tantalum-phenolate pi-bonding is overridden by stronger Ta-N pi-bonding, C(2)-symmetric ligand binding is preferred, likely because conformationally this is the lowest-energy arrangement. This electronically driven change in geometry indicates that, unlike analogous metallocene systems, the bis(phenolate)pyridine pincer ligand is not a strong enough pi-donor to exert dominant control over the electronic and geometric properties of the complex.

Catalyst Speciation During ansa-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)2AlMe2+ B(C6F5)4-(SBI = rac-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)2AlMe2+ by release of AlMe3) by repeated olefin insertions, while the other one is proposed to consist of SBIZr-η3-allyl cations of composition SBIZr-η3-(1-R-C3H4)+ (R = npropyl), [...].

Alkylaluminum-complexed zirconocene hydrides: identification of hydride-bridged species by NMR spectroscopy.

Reactions of unbridged zirconocene dichlorides, (R(n)C(5)H(5-n))(2)ZrCl(2) (n = 0, 1, or 2), with diisobutylaluminum hydride (HAl(i)Bu(2)) result in the formation of tetranuclear trihydride clusters of the type (R(n)C(5)H(5-n))(2)Zr(mu-H)(3)(Al(i)Bu(2))(3)(mu-Cl)(2), which contain three [Al(i)Bu(2)] units. Ring-bridged ansa-zirconocene dichlorides, Me(2)E(R(n)C(5)H(4-n))(2)ZrCl(2) with E = C or Si, on the other hand, are found to form binuclear dihydride complexes of the type Me(2)E(R(n)C(5)H(4-n))(2)Zr(Cl)(mu-H)(2)Al(i)Bu(2) with only one [Al(i)Bu(2)] unit. The dichotomy between unbridged and bridged zirconocene derivatives with regard to tetranuclear versus binuclear product formation is proposed to be connected to different degrees of rotational freedom of their C(5)-ring ligands. Alkylaluminum-complexed zirconocene dihydrides, previously observed in zirconocene-based precatalyst systems activated by methylalumoxane (MAO) upon addition of HAl(i)Bu(2) or Al(i)Bu(3), are proposed to be species of the type Me(2)Si(ind)(2)Zr(Me)(mu-H)(2)Al(i)Bu(2), stabilized by interaction of their terminal Me group with a Lewis acidic site of MAO.

Reductive coupling of carbon monoxide in a rhenium carbonyl complex with pendant Lewis acids.

Phosphinoborane ligands impart unique reactivity to a rhenium carbonyl cation relative to simple phosphine complexes. Addition of either triethylborohydride or a platinum hydride (that can be formed from H2) forms a rhenium boroxycarbene. This carbene, which crystallizes as a dimer, disproportionates over a period of days to afford the starting cation and a structurally unprecedented boroxy(boroxymethyl)carbene, in which a new C-C bond has been formed between two reduced CO ligands. This product of C-C bond formation can be independently synthesized by addition of 2 equiv of hydride to the rhenium carbonyl cation.