The insertion copolymerization of polar olefins and ethylene remains a significant challenge in part due to catalysts′ low activity and poor thermal stability. Herein we demonstrate a strategy toward ...addressing these obstacles through ligand design. Neutral nickel phosphine enolate catalysts with large phosphine substituents reaching the axial positions of Ni achieve activity of up to 7.7×103 kg mol−1 h−1 (efficiency >35×103 g copolymer/g Ni) at 110 °C, notable for ethylene/acrylate copolymerization. NMR analysis of resulting copolymers reveals highly linear microstructures with main‐chain ester functionality. Structure‐performance studies indicate a strong correlation between axial steric hindrance and catalyst performance.
Incorporation of polar functionalities into the polyethylene backbone via coordination copolymerization can provide value‐added polyolefins with improved properties. Nickel enolate catalysts with steric constraints in the axial positions were developed and applied to polyolefin synthesis. They are highly active (up to 7.7×103 kg mol−1 h−1) and thermally stable in ethylene/acrylate copolymerization, and their behavior correlates with the level of axial shielding.
Rapid, efficient development of homogeneous catalysts featuring desired performance is critical to numerous catalytic transformations but remains a key challenge. Typically, this task relies heavily ...on ligand design that is often based on trial and error. Herein, we demonstrate a "catalyst editing" strategy in Ni-catalyzed ethylene/acrylate copolymerization. Specifically, alkylation of a pendant phosphine followed by anion exchange provides a high yield strategy for a large number of cationic Ni phosphonium catalysts with varying electronic and steric profiles. These catalysts are highly active in ethylene/acrylate copolymerization, and their behaviors are correlated with the electrophile and the anion used in late-stage functionalization.Rapid, efficient development of homogeneous catalysts featuring desired performance is critical to numerous catalytic transformations but remains a key challenge. Typically, this task relies heavily on ligand design that is often based on trial and error. Herein, we demonstrate a "catalyst editing" strategy in Ni-catalyzed ethylene/acrylate copolymerization. Specifically, alkylation of a pendant phosphine followed by anion exchange provides a high yield strategy for a large number of cationic Ni phosphonium catalysts with varying electronic and steric profiles. These catalysts are highly active in ethylene/acrylate copolymerization, and their behaviors are correlated with the electrophile and the anion used in late-stage functionalization.
Polar monomer-induced β-H elimination is a key elementary step in polar polyolefin synthesis by coordination polymerization but remains underexplored. Herein, we show that a bulky neutral Ni ...catalyst,
, is not only a high-performance catalyst in ethylene/acrylate copolymerization (activity up to ∼37,000 kg/(mol·h) at 130 °C in a batch reactor, mol % tBA ∼ 0.3) but also a suitable platform for investigation of acrylate-induced β-H elimination.
, a novel Ni alkyl complex generated after acrylate-induced β-H elimination and subsequent acrylate insertion, was identified and characterized by crystallography. A combination of catalysis and mechanistic studies reveals effects of the acrylate monomer, bidentate ligand, and the labile ligand (e.g., pyridine) on the kinetics of β-H elimination, the role of β-H elimination in copolymerization catalysis as a chain-termination pathway, and its potential in controlling the polymer microstructure in polar polyolefin synthesis.
The neopentylidene−neopentyl complex (PNP)TiCHtBu(CH2 tBu) (2; PNP- = N2-P(CHMe2)2-4-methylphenyl2), prepared from the precursor (PNP)TiCHtBu(OTf) (1) and LiCH2 tBu, extrudes neopentane in neat ...benzene under mild conditions (25 °C) to generate the transient titanium alkylidyne, (PNP)Ti⋮CtBu (A), which subsequently undergoes 1,2-CH bond addition of benzene across the Ti⋮C linkage to generate (PNP)TiCHtBu(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the C−H activation process to obey pseudo-first-order in titanium, the α-hydrogen abstraction to be the rate-determining step (KIE for 2/2- d 3 conversion to 3/3- d 3 = 3.9(5) at 40 °C) with activation parameters ΔH ⧧ = 24(7) kcal/mol and ΔS ⧧ = −2(3) cal/mol·K, and the post-rate-determining step to be C−H bond activation of benzene (primary KIE = 1.03(7) at 25 °C for the intermolecular C−H activation reaction in C6H6 vs C6D6). A KIE of 1.33(3) at 25 °C arose when the intramolecular C−H activation reaction was monitored with 1,3,5-C6H3D3. For the activation of aromatic C−H bonds, however, the formation of the σ-complex becomes rate-determining via a hypothetical intermediate (PNP)Ti⋮CtBu(C6H5), and C−H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of 3 in C6D6 at 95 °C over 48 h generates 3- d 6 , thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) × 10-5 s-1, and with activation parameters ΔH ⧧ = 31(16) kcal/mol and ΔS ⧧ = 3(9) cal/mol·K. At 95 °C for one week, the EIE for the 2 → 3 reaction in 1,3,5-C6H3D3 was found to be 1.36(7). When 1 is alkylated with LiCH2SiMe3 and KCH2Ph, the complexes (PNP)TiCHtBu(CH2SiMe3) (4) and (PNP)TiCHtBu(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)TiCHSiMe3(CH2 tBu) (5) and (PNP)TiCHPh(CH2 tBu) (7). By means of similar alkylations of (PNP)TiCHSiMe3(OTf) (8), the degenerate complex (PNP)TiCHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene−alkyl complex (PNP)TiCHPh(CH2SiMe3) (11) can also be obtained, the latter of which results from a tautomerization process. Compounds 4/5 and 9, or 6/7 and 11, also activate benzene to afford (PNP)TiCHR(C6H5) (R = SiMe3 (10), Ph (12)). Substrates such as FC6H5, 1,2-F2C6H4, and 1,4-F2C6H4 react at the aryl C−H bond with intermediate A, in some cases regioselectively, to form the neopentylidene−aryl derivatives (PNP)TiCHtBu(aryl). Intermediate A can also perform stepwise alkylidene−alkyl metatheses with 1,3,5-Me3C6H3, SiMe4, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene−alkyls (PNP)TiCHR(R‘) (R = 3,5-Me2C6H2, R‘ = CH2-3,5-Me2C6H2; R = SiMe3, R‘ = CH2SiMe3; R = SiMe2C⋮CSiMe3, R‘ = CH2SiMe2C⋮CSiMe3; R = SiMe2OSiMe3, R‘ = CH2SiMe2OSiMe3).
Both the bisphosphine and bisphosphinite pincer complexes ( tBu4PCP)IrH2 and ( tBu4POCOP)IrH2 can cocatalyze alkane metathesis in tandem with olefin metathesis catalysts, but the two complexes have ...different resting states during catalysis, suggesting that different steps are turnover-limiting in each case. This led to the hypothesis that a complex with intermediate properties would be catalytically more active than either of these two species. Accordingly, “hybrid” phosphine–phosphinite pincer ligands (PCOP) and the corresponding iridium complexes were synthesized (3c–e). In tandem with olefin-metathesis catalyst MoF12, ( tBu4PCOP)IrH2 displays significantly higher activity for the metathesis of n-hexane than does ( tBu4PCP)IrH2 or ( tBu4POCOP)IrH2. ( tBu2PCOP iPr2)IrH4 (3d) is even more active (>30-fold more active than ( tBu4POCOP)IrH2) and affords nearly 4.6 M alkane products after 8 h at 125 °C.
The transient titanium alkylidyne complex (PNP)Ti⋮C t Bu (PNP = N-2-P(CHMe2)2-4-methylphenyl2 -), prepared from α-hydrogen abstraction of the corresponding alkylidene−alkyl species (PNP)TiCH t ...Bu(CH2 t Bu), can readily undergo intermolecular 1,2-addition of C−H bonds of benzene and SiMe4. Synthesis and reactivity, isotopic labeling, kinetics, and theoretical studies strongly favor an alkylidyne pathway and the α-H abstraction step to be the rate-determining step.
PNP pincer-type complexes of titanium(III) and -(IV) have been prepared, characterized, and proven to be remarkably stable, despite having terminal alkylidene, phosphinidene, and imide ...functionalities.
The insertion copolymerization of polar olefins and ethylene remains a significant challenge in part due to catalysts′ low activity and poor thermal stability. Herein we demonstrate a strategy toward ...addressing these obstacles through ligand design. Neutral nickel phosphine enolate catalysts with large phosphine substituents reaching the axial positions of Ni achieve activity of up to 7.7×103 kg mol−1 h−1 (efficiency >35×103 g copolymer/g Ni) at 110 °C, notable for ethylene/acrylate copolymerization. NMR analysis of resulting copolymers reveals highly linear microstructures with main‐chain ester functionality. Structure‐performance studies indicate a strong correlation between axial steric hindrance and catalyst performance.
Incorporation of polar functionalities into the polyethylene backbone via coordination copolymerization can provide value‐added polyolefins with improved properties. Nickel enolate catalysts with steric constraints in the axial positions were developed and applied to polyolefin synthesis. They are highly active (up to 7.7×103 kg mol−1 h−1) and thermally stable in ethylene/acrylate copolymerization, and their behavior correlates with the level of axial shielding.
Over 40 molybdenum and tungsten imido alkylidene mono(alkoxide) mono(pyrrolide) (MAP) or bis(alkoxide) olefin metathesis catalysts were examined in combination with Ir-based pincer-type catalysts for ...the metathesis of n-octane. The imido group, alkoxide, and metal in the metathesis catalysts were all found to be important variables. The best catalyst was W(NAr)(CHR)(OSiPh3)2 (Ar = 2,6-diisopropylphenyl), which performed about twice as well as the only previously employed catalyst, Mo(NAr)(CHR)OCMe(CF3)22. Product yields decreased at temperatures greater than 125 °C, most likely because of the instability of the metathesis catalysts at such temperatures. POCOP Ir catalysts gave higher yields than PCP Ir catalysts, although the latter exhibited some selectivity for formation of tetradecane. Eight catalysts were synthesized in situ through addition of alcohols to bis(2,5-dimethylpyrrolide) complexes; in situ catalysts were shown to perform approximately as well as the isolated complexes, which suggests that 2,5-dimethylpyrrole is not detrimental to the alkane metathesis process and that potential catalysts can be screened more conveniently in this way.
The neopentylidene-neopentyl complex (PNP)TiCHtBu(CH2 tBu) (1; (PNP− = N2-P(CHMe2)2-4-methylphenyl2) extrudes neopentane in neat fluorobenzene under mild conditions (25 °C) to generate the transient ...titanium alkylidyne (PNP)TiCtBu (A), which subsequently undergoes regioselective 1,2-CH bond addition of a fluorobenzene across the TiC linkage to generate (PNP)TiCHtBu(o-FC6H4) (2). Kinetic and mechanistic studies suggest that the C−H activation process is pseudo-first-order in titanium, with the α-hydrogen abstraction being the rate-determining step and the post-rate-determining step being the C−H bond activation of fluorobenzene. At 100 °C complex 2 does not equilibrate back to A and the preference for C−H activation in benzene versus fluorobenzene is 2:3, respectively. Compound 1 also reacts readily, and in most cases cleanly, with a series of hydrofluoroarenes (HArF), to form a family of alkylidene-arylfluoride derivatives of the type (PNP)TiCHtBu(ArF). Thermolysis of the latter compounds generates the titanium alkylidene-fluoride (PNP)TiCHtBu(F) (14) by a β-fluoride elimination, concurrent with formation of o-benzyne. β-Fluoride elimination to yield 14 occurs from 2 under elevated temperatures with k average = 4.96(16) × 10−5 s−1 and with activation parameters ΔH ⧧ = 29(1) kcal/mol and ΔS ⧧ = −3(4) cal/mol·K. It was found that β-fluoride elimination is accelerated when electron-rich groups are adjacent to the fluoride group, thus implying that a positive charge buildup at the arylfluoride ring occurs in the activated complex of 2. The alkylidene derivative (PNP)TiCHSiMe3(CH2SiMe3) (15) also undergoes α-hydrogen abstraction to form the putative (PNP)Ti’CSiMe3 (B) at higher temperatures (>70 °C) and dehydrofluorinates the same series of HArF when the reaction mixture is thermolyzed at >100 °C over 72 h to produce o-benzyne products and the fluoride analogue (PNP)TiCHSiMe3(F) (26). Only in the case of the substrate 1,2-F2C6H4 can the kinetic C−H activation product (PNP)TiCHSiMe3(o,m-F2C6H3) be isolated and crystallographically characterized. 1-Fluorohexane and fluorocyclohexane can also be dehydrofluorinated by intermediates A and B. No intermediates are observed, but in the case of 1-fluorohexane, the terminal olefin is spectroscopically identified. The dehydrofluorination of HArF and hydrofluoroalkanes (HAlF) can be made cyclic via the quantitative conversion of the alkylidene-fluorides to 1 and 15, by means of transmetalation with LiCH2XMe3 (X = C and Si), and the reactivity of 1 with halobenzenes is also presented and discussed.
