Why Do <i>C</i>₁-Symmetric <i>ansa</i>-Zirconocene Catalysts Produce Lower Molecular Weight Polymers for Ethylene/Propylene Copolymerization than for Ethylene/Propylene Homopolymerization?

We have carried out a combined QM/MM study to rationalize the factors that can affect the performance of C1-symmetric ansa-zirconocene catalysts that contain bridged cyclopentadienyl (Cp) and fluorenyl (Flu) ligands in olefin homo- and copolymerization. Two growing chains with different β-C (tertiary or secondary) and two olefins (propene and ethylene) have been used for this purpose. Our calculations indicate that chain transfer has a higher barrier than chain propagation in EE (ethylene homopolymerization), PP (propylene homopolymerization), and PE (propylene complexation to a metal with a propyl chain) systems. However, the two processes are competitive in EP (ethylene complexation to a metal with a 2-methylpropyl chain) system. Substituents on the carbon in a C−H link weaken the C−H bond. This in turn determines the order EP β atom of the growing chain as PP > PE ∼ EP > EE. The different propensity of the four systems for termination and propagation results in the higher barrier of termination for EE, PP, and PE, whereas the barriers are similar for EP. Our analysis explains why ethylene/propylene homopolymerization affords high molecular weight polymers, whereas ethylene/propylene copolymerization affords low molecular weight polymers

Group 4 Post-metallocene Complexes Incorporating Tridentate Silyl-Substituted Bis(naphthoxy)pyridine and Bis(naphthoxy)thiophene Ligands: Probing Systems for “Oscillating” Olefin Polymerization Catalysis

New bulky silyl ortho-substituted tridentate 2,6-bis(naphthol)pyridine ({ONOSiR3}H2, SiR3 = SiPh3, SiMe2tBu) and 2,5-bis(naphthol)thiophene ({OSOSiPh3}H2) pro-ligands were synthesized via a four-step approach. The solid-state structures of pro-ligands {ONOSiPh3}H2 (3a) and {OSOSiPh3}H2 (3b) were established by X-ray diffraction analysis. Both types of ligands were introduced onto group 4 metal centers (M = Ti, Zr, Hf) using straightforward one-step alkane, amine, or alcohol elimination protocols. Dibenzyl {ONOSiPh3}M(CH2Ph)2 (M = Ti, 4a; Zr, 5a; Hf, 6a) and {ONOSiMe2tBu}M(CH2Ph)2 (M = Ti, 4b; Zr, 5b), diamido {ONOSiPh3}Hf(NMe2)2 (7a and 7a·(NHMe2)), and di(isopropoxy) {ONOSiPh3}Ti(OiPr)2 (8a) complexes were authenticated using NMR spectroscopy and X-ray crystallography methods for some of them. In the solid state, complexes 4a, 4b, and 6a feature rac-like binding of the ligand, while ligands in complexes 5b and 7a·(NHMe2) are meso-like coordinated. The solution structures of 4b and 5b were investigated by VT NMR spectroscopy, which revealed that both complexes exist as rac and meso stereoisomers, which interconvert (activation parameters: 4b, ΔH⧧ = 12.9(7) kcal·mol−1 and ΔS⧧ = −3(1) cal·mol−1·K−1; 5b, ΔH⧧ = 13.4(8) kcal·mol−1 and ΔS⧧ = −7(1) cal·mol−1·K−1). A mechanism for this interconversion process, implying straightforward racemization, was proposed on the basis of DFT computations at the B3LYP (BP86) level, with computed activation barriers for Ti, Zr, and Hf complexes of 11.4 (10.1), 12.5 (11.2), and 12.2 (11.1) kcal·mol−1, respectively. The catalytic activity of dibenzyl and diamido precursors in homopolymerization of propylene and ethylene, upon activation with MAO, “dried-MAO”, and [Ph3C](B(C6F5)4]/Al(iBu)3, has been explored as well

Group 4 Post-metallocene Complexes Incorporating Tridentate Silyl-Substituted Bis(naphthoxy)pyridine and Bis(naphthoxy)thiophene Ligands: Probing Systems for “Oscillating” Olefin Polymerization Catalysis

New bulky silyl ortho-substituted tridentate 2,6-bis(naphthol)pyridine ({ONOSiR3}H2, SiR3 = SiPh3, SiMe2tBu) and 2,5-bis(naphthol)thiophene ({OSOSiPh3}H2) pro-ligands were synthesized via a four-step approach. The solid-state structures of pro-ligands {ONOSiPh3}H2 (3a) and {OSOSiPh3}H2 (3b) were established by X-ray diffraction analysis. Both types of ligands were introduced onto group 4 metal centers (M = Ti, Zr, Hf) using straightforward one-step alkane, amine, or alcohol elimination protocols. Dibenzyl {ONOSiPh3}M(CH2Ph)2 (M = Ti, 4a; Zr, 5a; Hf, 6a) and {ONOSiMe2tBu}M(CH2Ph)2 (M = Ti, 4b; Zr, 5b), diamido {ONOSiPh3}Hf(NMe2)2 (7a and 7a·(NHMe2)), and di(isopropoxy) {ONOSiPh3}Ti(OiPr)2 (8a) complexes were authenticated using NMR spectroscopy and X-ray crystallography methods for some of them. In the solid state, complexes 4a, 4b, and 6a feature rac-like binding of the ligand, while ligands in complexes 5b and 7a·(NHMe2) are meso-like coordinated. The solution structures of 4b and 5b were investigated by VT NMR spectroscopy, which revealed that both complexes exist as rac and meso stereoisomers, which interconvert (activation parameters: 4b, ΔH⧧ = 12.9(7) kcal·mol−1 and ΔS⧧ = −3(1) cal·mol−1·K−1; 5b, ΔH⧧ = 13.4(8) kcal·mol−1 and ΔS⧧ = −7(1) cal·mol−1·K−1). A mechanism for this interconversion process, implying straightforward racemization, was proposed on the basis of DFT computations at the B3LYP (BP86) level, with computed activation barriers for Ti, Zr, and Hf complexes of 11.4 (10.1), 12.5 (11.2), and 12.2 (11.1) kcal·mol−1, respectively. The catalytic activity of dibenzyl and diamido precursors in homopolymerization of propylene and ethylene, upon activation with MAO, “dried-MAO”, and [Ph3C](B(C6F5)4]/Al(iBu)3, has been explored as well

Chromium(III) Complexes of Sterically Crowded Bidentante {ON^R} and Tridentate {ONN^R} Naphthoxy-Imine Ligands: Syntheses, Structures, and Use in Ethylene Oligomerization

New bidentate {ONR}H (R = C6F5, 2c) and tridentate {ONNR}H (R = quinolyl, 2a; 2-pyridylmethyl, 2b} naphthol-imine and phenol-imine (R = quinolyl, 2d) pro-ligands sterically encumbered by an ortho-triphenylsilyl moiety have been prepared and converted to the corresponding naphthoxy-imino (3a−c) and phenoxy-imino (3d) CrBr2{ON(N)R}(CH3CN) complexes, respectively, via reaction with (p-tolyl)CrBr2(THF)3 and subsequent recrystallization from acetonitrile. The molecular structures of 2a, 2d, 3a, and 3b have been established by single-crystal X-ray diffraction studies. Upon activation with MAO, complexes 3a, 3b, and 3d, despite the presence of coordinated acetonitrile in those precursors, lead to highly active catalysts for the oligomerization of ethylene (activities up to 23 730 kg mol−1 h−1 at 25−100 °C, 6 bar), yielding selectively linear α-olefins (89−96% vinyl-end; Mn = 600−1450 g mol−1, Mw/Mn = 1.9−2.3)

Group 4 Post-metallocene Complexes Incorporating Tridentate Silyl-Substituted Bis(naphthoxy)pyridine and Bis(naphthoxy)thiophene Ligands: Probing Systems for “Oscillating” Olefin Polymerization Catalysis

New bulky silyl ortho-substituted tridentate 2,6-bis(naphthol)pyridine ({ONOSiR3}H2, SiR3 = SiPh3, SiMe2tBu) and 2,5-bis(naphthol)thiophene ({OSOSiPh3}H2) pro-ligands were synthesized via a four-step approach. The solid-state structures of pro-ligands {ONOSiPh3}H2 (3a) and {OSOSiPh3}H2 (3b) were established by X-ray diffraction analysis. Both types of ligands were introduced onto group 4 metal centers (M = Ti, Zr, Hf) using straightforward one-step alkane, amine, or alcohol elimination protocols. Dibenzyl {ONOSiPh3}M(CH2Ph)2 (M = Ti, 4a; Zr, 5a; Hf, 6a) and {ONOSiMe2tBu}M(CH2Ph)2 (M = Ti, 4b; Zr, 5b), diamido {ONOSiPh3}Hf(NMe2)2 (7a and 7a·(NHMe2)), and di(isopropoxy) {ONOSiPh3}Ti(OiPr)2 (8a) complexes were authenticated using NMR spectroscopy and X-ray crystallography methods for some of them. In the solid state, complexes 4a, 4b, and 6a feature rac-like binding of the ligand, while ligands in complexes 5b and 7a·(NHMe2) are meso-like coordinated. The solution structures of 4b and 5b were investigated by VT NMR spectroscopy, which revealed that both complexes exist as rac and meso stereoisomers, which interconvert (activation parameters: 4b, ΔH⧧ = 12.9(7) kcal·mol−1 and ΔS⧧ = −3(1) cal·mol−1·K−1; 5b, ΔH⧧ = 13.4(8) kcal·mol−1 and ΔS⧧ = −7(1) cal·mol−1·K−1). A mechanism for this interconversion process, implying straightforward racemization, was proposed on the basis of DFT computations at the B3LYP (BP86) level, with computed activation barriers for Ti, Zr, and Hf complexes of 11.4 (10.1), 12.5 (11.2), and 12.2 (11.1) kcal·mol−1, respectively. The catalytic activity of dibenzyl and diamido precursors in homopolymerization of propylene and ethylene, upon activation with MAO, “dried-MAO”, and [Ph3C](B(C6F5)4]/Al(iBu)3, has been explored as well

Computational Design of <i>C</i>₂-Symmetric Metallocene-Based Catalysts for the Synthesis of High Molecular Weight Polymers from Ethylene/Propylene Copolymerization

Indenyl-based C2-symmetric metallocenes have been used extensively as catalysts for the synthesis of high molecular weight polymers from ethylene or propylene homopolymerization. However, the same catalysts afford only low molecular weight polymers in ethylene/propylene copolymerization. We have in a recent study shown [Wang et al. Organometallics 2008, 27, 2861] that the poor performance of fluorenyl-based C1-symmetric zirconocenes in ethylene/propylene polymerization is a result of electronic effects. In the present computational study, we demonstrate how it is possible by substitutions in the 2- and 4-positions of the indenyl ligands to design catalysts that afford high molecular weight polymers from ethylene/propylene copolymerization

Chromium(III) Complexes of Sterically Crowded Bidentante {ON^R} and Tridentate {ONN^R} Naphthoxy-Imine Ligands: Syntheses, Structures, and Use in Ethylene Oligomerization

New bidentate {ONR}H (R = C6F5, 2c) and tridentate {ONNR}H (R = quinolyl, 2a; 2-pyridylmethyl, 2b} naphthol-imine and phenol-imine (R = quinolyl, 2d) pro-ligands sterically encumbered by an ortho-triphenylsilyl moiety have been prepared and converted to the corresponding naphthoxy-imino (3a−c) and phenoxy-imino (3d) CrBr2{ON(N)R}(CH3CN) complexes, respectively, via reaction with (p-tolyl)CrBr2(THF)3 and subsequent recrystallization from acetonitrile. The molecular structures of 2a, 2d, 3a, and 3b have been established by single-crystal X-ray diffraction studies. Upon activation with MAO, complexes 3a, 3b, and 3d, despite the presence of coordinated acetonitrile in those precursors, lead to highly active catalysts for the oligomerization of ethylene (activities up to 23 730 kg mol−1 h−1 at 25−100 °C, 6 bar), yielding selectively linear α-olefins (89−96% vinyl-end; Mn = 600−1450 g mol−1, Mw/Mn = 1.9−2.3)

Group 4 Post-metallocene Complexes Incorporating Tridentate Silyl-Substituted Bis(naphthoxy)pyridine and Bis(naphthoxy)thiophene Ligands: Probing Systems for “Oscillating” Olefin Polymerization Catalysis

New bulky silyl ortho-substituted tridentate 2,6-bis(naphthol)pyridine ({ONOSiR3}H2, SiR3 = SiPh3, SiMe2tBu) and 2,5-bis(naphthol)thiophene ({OSOSiPh3}H2) pro-ligands were synthesized via a four-step approach. The solid-state structures of pro-ligands {ONOSiPh3}H2 (3a) and {OSOSiPh3}H2 (3b) were established by X-ray diffraction analysis. Both types of ligands were introduced onto group 4 metal centers (M = Ti, Zr, Hf) using straightforward one-step alkane, amine, or alcohol elimination protocols. Dibenzyl {ONOSiPh3}M(CH2Ph)2 (M = Ti, 4a; Zr, 5a; Hf, 6a) and {ONOSiMe2tBu}M(CH2Ph)2 (M = Ti, 4b; Zr, 5b), diamido {ONOSiPh3}Hf(NMe2)2 (7a and 7a·(NHMe2)), and di(isopropoxy) {ONOSiPh3}Ti(OiPr)2 (8a) complexes were authenticated using NMR spectroscopy and X-ray crystallography methods for some of them. In the solid state, complexes 4a, 4b, and 6a feature rac-like binding of the ligand, while ligands in complexes 5b and 7a·(NHMe2) are meso-like coordinated. The solution structures of 4b and 5b were investigated by VT NMR spectroscopy, which revealed that both complexes exist as rac and meso stereoisomers, which interconvert (activation parameters: 4b, ΔH⧧ = 12.9(7) kcal·mol−1 and ΔS⧧ = −3(1) cal·mol−1·K−1; 5b, ΔH⧧ = 13.4(8) kcal·mol−1 and ΔS⧧ = −7(1) cal·mol−1·K−1). A mechanism for this interconversion process, implying straightforward racemization, was proposed on the basis of DFT computations at the B3LYP (BP86) level, with computed activation barriers for Ti, Zr, and Hf complexes of 11.4 (10.1), 12.5 (11.2), and 12.2 (11.1) kcal·mol−1, respectively. The catalytic activity of dibenzyl and diamido precursors in homopolymerization of propylene and ethylene, upon activation with MAO, “dried-MAO”, and [Ph3C](B(C6F5)4]/Al(iBu)3, has been explored as well

Synthesis, Structure, and Polymerization Activity of Neutral Halide, Alkyl, and Hydrido Yttrium Complexes of Isopropylidene-Bridged Cyclopentadienyl-Fluorenyl Ligands

Reactions of the anionic complex [(Cp-CMe2-Flu)YCl2]-[Li(ether)4]+ (1) (Cp = C5H4, Flu = 9-C13H8), prepared in situ from YCl3(THF)3.5 and 1 molar equiv of the dilithium salt [Cp-CMe2-Flu]Li2, with equimolar amounts of RLi give alkyl mono-THF complexes [(Cp-CMe2-Flu)]Y(R)(THF) (R = CH(SiMe3)2, 3; CH2SiMe3, 4) in high yields. The solid-state structure of 3 was established by X-ray diffraction, showing the fluorenyl moiety symmetrically coordinated to yttrium in an intermediary η3-η5 mode. Hydrogenolysis of 3 and 4 with H2 or PhSiH3 gives the hydride {[(μ:η5,η5-Cp-CMe2-Flu)]Y(μ-H)(THF)}2 (5). The solid-state structure of 5 was determined by X-ray diffraction, revealing a dimeric structure with both bridging Cp-CMe2-Flu and hydride ligands (Y−H = 1.99(4)−2.01(4) Å). Complex 5 is the first structurally characterized example of a group 3 metal hydride stabilized by a fluorenyl ligand. Reaction of 1 with PhCH2MgBr gives, instead of a benzyl derivative, the neutral base-free bromo complex {[(η5,η5-Cp-CMe2-Flu)]Y(μ-Br)}2 (6), which shows a dimeric structure in the solid state with chelating Cp-CMe2-Flu and bridging bromide ligands. Introduction of the bulky tert-butyl substituent on the Cp ring of the ligand system enabled the preparation of the neutral chloro complex [(3-tBuCp)-CMe2-Flu]YCl(THF) (7), using a salt elimination between the dilithium salt of the ligand and YCl3(THF)3.5. Reaction of 7 with LiCH(SiMe3)2 gives the alkyl complex {[(3-tBuCp)-CMe2-Flu)]Y(CH(SiMe3)2) (8), which contains no THF molecule in its coordination sphere in contrast to unsubstituted analogues 3 and 4. Preliminary studies of the catalytic activity of these new complexes for ethylene and MMA polymerization are reported

In Silico Design of <i>C</i>₁- and <i>C</i>_<i>s</i>-Symmetric Fluorenyl-Based Metallocene Catalysts for the Synthesis of High-Molecular-Weight Polymers from Ethylene/Propylene Copolymerization

C1- and Cs-symmetric fluorenyl-based metallocenes have been used extensively as catalysts for the synthesis of high-molecular-weight polymers from ethylene and propylene homopolymerization. However, these same catalysts produce only low-molecular-weight polymers in ethylene/propylene copolymerization. We have shown in a recent study that the poor performance of fluorenyl-based C1-symmetric zirconocenes in ethylene/propylene polymerization is a result of electronic effects. Furthermore, we have also shown in another investigation that incorporating sterically demanding substituents in the 2- and 4- positions of C2-symmetric zirconocenes can significantly increase molecular weight in copolymerization. In the present study we shall use the same approach (modifying substituents on the cyclopentadienyl and fluorenyl ligands) to design C1- and Cs-symmetric fluorenyl-based metallocenes that afford high-molecular-weight polymers from ethylene/propylene copolymerization