Highly Selective Olefin Trimerization Catalysis by a Borane-Activated Titanium Trimethyl Complex

Reaction of a trimethyl titanium complex, (FI)­TiMe₃ (FI = phenoxy-imine), with 1 equiv of B­(C₆F₅)₃ gives [(FI)­TiMe₂]­[MeB­(C₆F₅)₃], an effective precatalyst for the selective trimerization of ethylene. Mechanistic studies indicate that catalyst initiation involves generation of an active Ti^II species by olefin insertion into a Ti–Me bond, followed by β-H elimination and reductive elimination of methane, and that initiation is slow relative to trimerization. (FI)­TiMe₃/B­(C₆F₅)₃ also leads to a competent catalyst for the oligomerization of α-olefins, displaying high selectivity for trimers (>95%), approximately 85% of which are one regioisomer. This catalyst system thus shows promise for selectively converting light α-olefins into transportation fuels and lubricants

Highly Selective Olefin Trimerization Catalysis by a Borane-Activated Titanium Trimethyl Complex

Reaction of a trimethyl titanium complex, (FI)­TiMe₃ (FI = phenoxy-imine), with 1 equiv of B­(C₆F₅)₃ gives [(FI)­TiMe₂]­[MeB­(C₆F₅)₃], an effective precatalyst for the selective trimerization of ethylene. Mechanistic studies indicate that catalyst initiation involves generation of an active Ti^II species by olefin insertion into a Ti–Me bond, followed by β-H elimination and reductive elimination of methane, and that initiation is slow relative to trimerization. (FI)­TiMe₃/B­(C₆F₅)₃ also leads to a competent catalyst for the oligomerization of α-olefins, displaying high selectivity for trimers (>95%), approximately 85% of which are one regioisomer. This catalyst system thus shows promise for selectively converting light α-olefins into transportation fuels and lubricants

Scope and Mechanism of Homogeneous Tantalum/Iridium Tandem Catalytic Alkane/Alkene Upgrading using Sacrificial Hydrogen Acceptors

An in-depth investigation of a dual homogeneous catalyst system for the coupling of alkanes and alkenes based on an early-/late-transition-metal pairing is reported. The system is composed of Cp*TaCl₂(alkene) for alkene dimerization and pincer-iridium hydrides for alkane/alkene transfer hydrogenation. Because there is no kinetically relevant interaction between the two catalysts, the tandem mechanism can be entirely described using the two independent catalytic cycles. The alkene dimerization mechanism is characterized by an entropically disfavored pre-equilibrium between Cp*TaCl₂(1-hexene) + 1-hexene and Cp*TaCl₂(metallacyclopentane) (Δ<i>H</i>° = −22(2) kcal/mol; Δ<i>S</i>° = −16(2) eu); thus, the overall rate of alkene dimerization is positive order in 1-hexene (exhibiting saturation kinetics), and increases only modestly with temperature. In contrast, the rate of 1-hexene/<i>n</i>-heptane transfer hydrogenation catalyzed by <i>t</i>-Bu­[PCP]­IrH₄ is inverse order in 1-hexene and increases substantially with temperature. Styrene has been investigated as an alternate sacrificial hydrogen acceptor. Styrene dimerization catalyzed by Cp*TaCl₂(alkene) is considerably slower than 1-hexene dimerization. The conversion of styrene/heptane mixtures by the Ta/Ir tandem system leads to three product types: styrene dimers, coupling of styrene and heptane, and heptene dimers (from heptane). Through careful control of reaction conditions, the production of heptene dimers can be favored, with up to 58% overall yield of heptane-derived products and cooperative TONs of up to 12 and 10 for Ta and Ir catalysts, respectively. There is only slight inhibition of Ir-catalyzed styrene/<i>n</i>-heptane transfer hydrogenation under the tandem catalysis conditions

Guanidine-Functionalized Rhenium Cyclopentadienyl Carbonyl Complexes: Synthesis and Cooperative Activation of H–H and O–H Bonds

Catalytic reactions utilizing carbon monoxide as a substrate are numerous, and they typically involve selective functionalization of a metal-bound CO. We have developed group 7 carbonyl complexes where secondary coordination sphere, Lewis acidic functionalities can assist in the activation of substrate molecules, mainly in the context of syngas conversion. This work describes a new class of cyclopentadienyl (Cp) rhenium carbonyl compounds of the type [Re­(η⁵-C₅H₄DMEG)­(CO)_3<i>–n</i>(NO)_<i>n</i>]^<i>n</i> (DMEG = dimethylethyleneguanidine, <i>n</i> = 0, 1), where a tethered guanidine base is appended to the Cp ring to participate in cooperative substrate activation with the electrophilic carbonyl. A reliable synthetic route for these complexes is presented, with crystallographic characterization of the free-base and protonated forms for both the carbonyl and mixed carbonyl-nitrosyl complexes. The latter are employed as platforms to study heterolytic H–H and O–H bond cleavage reactions that result in nucleophilic CO functionalization. The corresponding formyl complex is prepared by hydride transfer, and by measuring its hydricity (Δ<i><i>G</i>°</i>_H–) and p<i>K</i>_a of the protonated base, the free energy of H₂ cleavage is found to be +3.3(6) kcal/mol. The activation of methanol to form methoxycarbonyl complexes is found to be more favorable, with Δ<i><i>G</i>°</i> ≈ 0 for the intramolecular addition of methanol to the guanidine-appended carbonyl complex. A detailed thermodynamic study is described for both the intramolecular methanol activation reaction and related intermolecular reactions with external bases. The results highlight some tangible thermodynamic benefits of tethering the base in the secondary coordination sphere

Diverse C–C Bond-Forming Reactions of Bis(carbene)platinum(II) Complexes

The platinum(0) complex Pt­(PPh₃)₄ catalyzes coupling of the carbene ligands of (CO)₅Cr­{C­(OMe)­(<i>p-</i>MeOC₆H₄)} (<b>1</b>). The stable bis­(carbene)­platinum­(II) complexes Cl₂Pt­{C­(OMe)­(Me)}₂ (<b>3</b>), Br₂Pt­{C­(OMe)­(Me)}₂ (<b>4</b>), and Cl₂Pt­{C­(O^<i>i</i>Pr)­(Me)}₂ (<b>5</b>) can be induced to undergo C–C coupling reactions by several means. Reduction of <b>3</b>–<b>5</b> to platinum(0) with cobaltocene results in formation of internal olefins, (<i>E/Z</i>)-2,3-dimethoxybut-2-ene (<b>6</b>) or (<i>E/Z</i>)-2,3-diisopropoxybut-2-ene (<b>7</b>). Reaction of <b>3</b>–<b>5</b> with PPh₃ yields terminal olefins, 2,3-dimethoxybut-1-ene (<b>13</b>) or 2,3-diisopropoxybut-1-ene (<b>15</b>), along with Cl₂Pt­(PPh₃)₂ (<b>12</b>) or Br₂Pt­(PPh₃)₂ (<b>14</b>). In contrast, addition of pyridine to <b>3</b>–<b>5</b> does not effect C–C coupling; instead, the acyl complexes <i>cis</i>-Cl­(py)­Pt­(COMe)­{C­(OMe)­(Me)} (<b>8</b>), <i>cis</i>-Br­(py)­Pt­(COMe)­{C­(OMe)­(Me)} (<b>9</b>), and <i>cis</i>-Cl­(py)­Pt­(COMe)­{C­(O^<i>i</i>Pr)­(Me)} (<b>10</b>) are obtained, with concomitant formation of alkyl halide. Possible mechanistic pathways for C–C bond formation are discussed, as well as explanations for the different reactivities observed for pyridine and PPh₃

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, [CrCl₃(PNP^OMe)] (<b>1</b>, where PNP^OMe = <i>N</i>,<i>N</i>-bis­(bis­(<i>o</i>-methoxyphenyl)­phosphino)­methylamine). Activation of <b>1</b> by treatment with modified methylaluminoxane (MMAO) in the presence of ethylene and 1-hexene afforded mostly C₆ and C₁₀ alkene products. The identities of the C₁₀ isomers, assigned by detailed gas chromatographic and mass spectrometric analyses, strongly support a mechanism that involves five- and seven-membered metallacyclic intermediates comprised of ethylene and LAO units. Using 1-heptene as a mechanistic probe, it was established that 1-hexene formation from ethylene is competitive with formation of ethylene/LAO cotrimers and that cotrimers derived from one ethylene and two LAO molecules are also generated. Complex <b>1/</b>MMAO is also capable of converting 1-hexene to C₁₂ dimers and C₁₈ trimers, albeit with poor efficiency. The mechanistic implications of these studies are discussed and compared to previous reports of olefin cotrimerization

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 Cr^IIICl₃(PNP) (PNP = bis­(diarylphosphino)­alkylamine) ethylene trimerization precatalyst to map out its methylaluminoxane (MAO) activation sequence. These studies indicate that treatment of Cr^IIICl₃(PNP) with MAO leads first to 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 are not catalytically active

A Thermodynamic Analysis of Rhenium(I)–Formyl C–H Bond Formation via Base-Assisted Heterolytic H₂ Cleavage in the Secondary Coordination Sphere

Conversion of synthesis gas, a mixture of carbon monoxide and hydrogen, into value-added C_<i>n</i>≥2 products requires both C–H and C–C bond-forming events. Our group has developed a series of molecular complexes, based on group 7 (manganese and rhenium) carbonyl complexes, to interrogate the elementary steps involved in the homogeneous hydrogenative reductive coupling of CO. Here, we explore a new mode of H₂ activation, in which strong bases in the secondary coordination sphere are positioned to assist in the heterolytic cleavage of H₂ to form a formyl C–H bond at a rhenium-bound carbonyl. A series of cationic rhenium­(I) complexes of the type [Re^I(P∼B:-κ₁-P)­(CO)₅]^<i>n</i> (<i>n</i> = 0, +1), where P∼B: is a phosphine ligand with a tethered strong base, are prepared and characterized; measurement of their protonation equilibria demonstrates a pronounced attenuation of the basicity upon coordination. Formyl complexes supported by these ligands can be prepared in good yield by hydride delivery to the parent pentacarbonyl complexes, and several of the free-base formyl complexes can be protonated, generating observable [Re^I(P∼BH-κ₁-P)­(CHO)­(CO)₄]^<i>n</i> complexes. Intramolecular hydrogen bonding is evident for one of the complexes, providing additional stabilization to the protonated formyl complex. By measuring both the hydricity of the formyl, Δ<i>G</i>°_H–, and its p<i>K</i>_a, the overall free energy of H₂ cleavage is calculated from an appropriate cycle and found to be thermodynamically uphill in all cases (in the best case by only about 8 kcal/mol), although significantly dependent upon the properties of the supporting ligand

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

Kinetics and Mechanism of Indene C–H Bond Activation by [(COD)Ir(μ₂‑OH)]₂

The hydroxy-bridged dimer [(COD)­Ir­(μ₂-OH)]₂ (COD = 1,5-cyclooctadiene) cleanly cleaves C–H bonds in indene and cyclopentadiene to produce (COD)­Ir­(η³-indenyl) and (COD)­Ir­(η⁵-C₅H₅), respectively. The kinetics of the formation of (COD)­Ir­(η³-indenyl) are consistent with a mechanism that involves coordination of indene to [(COD)­Ir­(μ₂-OH)]₂ followed by rate-determining C–H activation from the iridium dimer–indene unit. Transition-state analysis of the Ir and Rh hydroxy dimers indicates that the C–H activation proceeds through a direct deprotonation of indene by the M–OH unit rather than a stepwise oxidative addition/reductive elimination mechanism. The crystal structure of [(COD)­Ir]₅(μ₄-O)­(μ₃-O)­(μ₂-OH), a dehydration product of [(COD)­Ir­(μ₂-OH)]₂, is presented