Suppression of β-hydride chain transfer in nickel(II)-catalyzed ethylene polymerization via weak fluorocarbon ligand-product interactions

Article

pubs.acs.org/Organometallics

Suppression of β-Hydride Chain Transfer in Nickel(II)-Catalyzed

Ethylene Polymerization via Weak Fluorocarbon Ligand−Product

Interactions

Michael P. Weberski, Jr.,^†Changle Chen,^†Massimiliano Delferro,*^,†Cristiano Zuccaccia,^‡

Alceo Macchioni,^‡and Tobin J. Marks*^,†

^†Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States

^‡Dipartimento di Chimica, Universita

̀

degli Studi di Perugia, Via Elce di Sotto, 8-06123 Perugia, Italy

S

* Supporting Information

ABSTRACT: The synthesis and characterization of two neutrally charged Ni(II) ethylene polymerization catalysts, [2-tert-butyl-

6-((2,6-(3,5-dimethylphenyl)phenylimino)methyl)phenolato]nickel(II) methyl trimethylphosphine ((CH₃)FI-Ni) and [2-tert-

butyl-6-((2,6-(3,5-bis(triﬂuoromethyl)phenyl)phenylimino)methyl)phenolato]nickel(II) methyl trimethylphosphine ((CF₃)FI-

Ni) are reported. In the presence of a Ni(COD)₂cocatalyst, these catalysts produce markedly diﬀerent polyethylenes: densely

branched oligomers with M_w= 1.4 × 10³g mol⁻¹for (CH₃)FI-Ni vs lightly branched polyethylenes with M_w= 92 × 10³g mol⁻¹for

(CF₃)FI-Ni and with ∼6.5× the polymerization activity and with much greater performance thermal stability. HOESY 2D ¹⁹F,¹H

NMR spectra of a model Ni-ethyl compound, [2-tert-butyl-6-((2,6-(3,5-bis(triﬂuoromethyl)phenyl)phenylimino)methyl)phenolato]-

nickel(II) ethyl 2,4-lutidine ((CF₃)FI-Ni-Et), indicate non-negligible C_β−H_β···F₃C through-space dipolar interactions, and molecular

modeling reveals that C_β−H_β···F(C) distances can be as small as ∼2.61 Å during the polymerization process. Furthermore, there is no

structural or spectroscopic evidence for ﬂuorocarbon inductive eﬀects on the structure, bonding, and reactivity of these complexes, and

a catalyst with CF₃introduced β to the imino N produces only low-M_woligomers with low activity. These results argue that weak

(ligand)C−F···H−C(polymer) interactions can signiﬁcantly inﬂuence the chain transfer characteristics of these catalysts.

recently demonstrated that (catalytic center)···(catalytic center)

cooperative eﬀects in bimetallic Ni(II) phenoxyiminato ethylene

polymerization catalysts (Chart 1, C), as well as bimetallic CGC-

based group 4 catalysts,⁸signiﬁcantly enhance polyethylene

branching and M_was well as polar comonomer enchainment via

very diﬀerent mechanisms.⁹Despite these signiﬁcant advances,

further enhancement of group 10 polymerization catalyst activity,

product molecular weight, and resistance to poorly understood

deactivation processes would be highly desirable.

With regard to modifying such catalysts, introducing ligand

ﬂuorine substituents in Ni(II) and Pd(II) α-diimine catalysts D

was reported to enhance polyethylene M_wand catalyst thermal

stability, albeit at the cost of lower polymerization activity

INTRODUCTION

■

Group 10 oleﬁn polymerization catalysis^1,2has advanced sub-

stantially³since the pioneering discovery by Brookhart and co-

workers⁴of highly active cationic Ni(II) and Pd(II) catalysts sup-

ported by bulky aryl-diimine ligands (Chart 1, A). These catalysts

eﬀect ethylene polymerization at substantial rates, as well as the co-

polymerization of ethylene with polar comonomers such as acry-

lates, vinyl ketones, silyl vinyl ethers, acrylonitrile, and vinyl

acetate.^3d,j,o,4d,5Ni(II) catalysts also produce highly branched poly-

ethylenes through sequences of rapid β-H elimination/reinsertion

termed “chain-walking”.^1i,6Additionally, highly active neutrally

charged Ni(II) phenoxyiminato ethylene polymerization catalysts

(Chart 1, B), activated with the phosphine scavenger Ni(COD)₂

or B(C₆F₅)₃, developed by Grubbs and co-workers,⁷are tolerant

to solvent polar functionality and are competent to coenchain

polar substituted norbornenes into polyethylene backbones. We

Received: April 4, 2012

A

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Chart 1

versus the corresponding ﬂuorine-free catalysts.¹⁰It is thought

that the F substituents stabilize 14-electron alkyl intermediates

via axial donation to the Pd(II) center (E), thereby suppressing

CH₃and OCH₃groups) but exhibit higher thermal stability

and yield polyethylenes with comparable M_wvalues.^11b,e

Furthermore, in Ni(II) phospho−keto−ylide catalytic sys-

tems^12,13or Ni(II) phenoxyiminato catalysts with an intra-

molecular hydrogen bond directed toward the active catalytic

site,^3cNi center electron deﬁciency signiﬁcantly enhances ethylene

polymerization rates but also enhances branch densities and

depresses product M_w, apparently reﬂecting an interplay of

lowered ethylene insertion barriers and enhanced β-agostic inter-

actions.¹⁴

The introduction of ﬂuorocarbon ligand substituents in

group 4 oleﬁn polymerization catalysts has also been associated

with changes in enchainment and/or chain transfer kinetics.¹⁵

Fujita reported a series of d⁰group 4 phenoxyiminato catalysts

with nonremote ﬂuoro-aryl substitution (Chart 3, e.g., I) that are

chain transfer processes involving β-H elimination. These

1

interactions were argued on the basis of H and ¹⁹F NMR

spectroscopic data and raise the intriguing question of whether

this type or other types of ﬂuorocarbon eﬀects might be more

general and might be used to modify chain transfer kinetics in

group 10 catalytic systems. In this regard, Mecking’s group

reported a series of single-component (i.e., no cocatalyst)

terphenylphenoxyiminato Ni(II) ethylene polymerization cata-

lysts bearing electron-withdrawing (EWG) or electron-donating

groups (EDG) at the terphenyl 3,5-positions (Chart 2, F−H).^3m,p,11

Chart 3. Proposed Ethylene Polymerization Intermediate I

and Model Compound J Involving C−H···F−C Interactions

Chart 2

active for the living polymerization of ethylene and propylene.

Polymerization activity and M_nvary widely with the exact aryl

substitution pattern, and polymerizations mediated by analogous

catalysts without ortho ﬂuorination are no longer living in

character and vary greatly in polymerization activity.¹⁶

A

The polymers produced by these catalysts under identical

reaction conditions range from high-M_wsemicrystalline poly-

ethylenes with low branch densities (EWG substituents) to

low-M_wamorphous polyethylenes with high branch densities

(EDG substituents). It was suggested that the remote 3′,5′-

terphenyl substituents impact the polyethylene microstructure

via through-bond inductive and/or steric eﬀects, despite their

remoteness to the Ni center.^11b,eIt was plausibly suggested that

the EWGs aﬀord higher M_wpolyethylenes with lower branch

densities by suppressing chain walking and chain transfer

relative to chain propagation. Note that catalyst variants bear-

ing a single NO₂EWG (Chart 2, H) are somewhat less poly-

merization active than the electron-rich analogues (Chart 2 F,

C−H···F−C interaction was proposed on the basis of DFT

calculations;¹⁶however, the wide reported variations in

activity and M_wsuggest other eﬀects are operative as well.

The DFT calculations indicate that the C−F···H−C

distances are 2.3−2.4 Å with unexpectedly large¹⁵⁻¹⁷com-

puted interaction energies of ∼7.2 kcal/mol. Experimental

evidence for C−H···F−C interactions in neutrally charged d⁰

1

Ti-benzyls is provided by H NMR and neutron diﬀraction data

(Chart 3, J).¹⁷Here C−H···F−C distances are reported to be

2.572(6) and 2.607(5) Å.

Traditionally, the role that ﬂuorocarbon substituents play in

transition-metal homogeneous catalysis has been in inductively

tuning catalyst¹⁸and cocatalyst¹⁹electronic properties, in stabilizing

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catalysts¹⁹and cocatalysts²⁰against electrophilic or free radical

attack, in imparting speciﬁc stabilization or separation proper-

ties to solvents,^18g,21or in coordinatively screening exposed

electrophilic sites, as in structure E. In Lewis acid catalysis,

other more subtle secondary interactions such as C−H···F−C

hydrogen bonding, arising from the substantial C−F dipole

moment,²²play a role in directing the stereochemical outcome

of reactions such as enantioselective aldehyde additions (K, L).²³

inductive eﬀects are unlikely to account for this ractivity pattern

and propose as an alternative, on the basis of 1D and 2D NMR

spectrocopy, X-ray diﬀraction studies of model compounds,

molecular modeling,²⁵and reactivity data for catalysts with

more proximate CF₃groups, that the origin of this remarkable

contrast in polymerization characteristics lies primarily in weak

secondary F···H interactions between the ligand CF₃

substituent and the propagating polymer chain, which signi-

ﬁcantly modify the chain transfer kinetics.

EXPERIMENTAL SECTION

■

Materials and Methods. All manipulations of air-sensitive

materials were performed with rigorous exclusion of O₂and moisture

in oven-dried Schlenk-type glassware on a dual-manifold Schlenk line,

interfaced to a high-vacuum line (10⁻⁶Torr), or in a N₂-ﬁlled Vacuum

Atmospheres glovebox with a high-capacity recirculator (<1 ppm of O₂).

Argon (Airgas, prepuriﬁed grade) was puriﬁed by passage through a

supported MnO oxygen-removal column and an activated Davison 4A

molecular sieve column. Ethylene (Airgas, prepuriﬁed grade) was

puriﬁed by passage through an oxygen/moisture trap (Matheson,

Model MTRP-0042-XX). Diethyl ether and tetrahydrofuran were distilled

over Na/benzophenone ketyl. Hydrocarbon solvents were vacuum-

transferred from Na/K alloy. Benzene-d₆(Cambridge Isotope Labo-

ratories, 99+ atom % D) was stored over Na/K alloy in vacuo and

vacuum-transferred immediately prior to use. All other deuterated solvents

were used as received (Cambridge Isotope Laboratories, 99+ atom % D).

The reagents trans-NiMeCl(PMe₃)₂,²⁵NiCl₂(2,4-lutidine)₂,²⁶1-tert-

butyl-2-methoxymethoxybenzene,²⁷and the terphenylamines²⁸were

prepared according to literature procedures. Ni(COD)₂(COD = 1,5-

cyclooctadiene) was purchased from Strem; all other reagents were

purchased from Sigma-Aldrich. The ligands (CH₃)HFI²⁹and (CF₃)HFI

were prepared according to literature procedures.^11b

Such interactions are also of great importance in biological

systems and play a major role in drug design related substrate−

receptor binding in enzymatic catalysis (e.g., Figure 1).²⁴These

Physical and Analytical Measurements. NMR spectra were

1

recorded on Varian INOVA 400 (400 MHz, H; 100 MHz, ¹³C; 162

1

MHz, ³¹P; 376 MHz, ¹⁹F), Varian INOVA 500 (500 MHz, H), and

Bruker AVANCE III 500 (500 MHz, ¹H; 125 MHz, ¹³C) NMR

spectrometers. Chemical shifts (δ) for H and ¹³C are referenced to

1

Figure 1. Importance of C−H···F−C dipolar interactions in enzymatic

substrate−receptor binding: crystal structure (PDB code: 1OYT)

showing dipolar C−H···F−C interaction between a ﬂuorinated-

tricyclic inhibitor and the enzyme thrombin: C, gray; O, red; N,

blue; F, green-yellow. Data are taken from ref 17.

TMS, internal solvent resonances are relative to TMS, and the

polymer CH₂backbone. Chemical shifts (δ) for ³¹P and ¹⁹F are

referenced to the external standards 85% H₃PO₄and CFCl₃dissolved

in CDCl₃, respectively. NMR spectra of air-sensitive samples were

acquired in airtight Teﬂon valve sealed J. Young NMR tubes. The 2D

¹⁹F,¹H HOESY NMR experiments were carried out on a Bruker

AVANCE DRX 400 equipped with a direct QNP probe by setting a

observations raise the question of whether, and to what extent,

such interactions might have importance in transition-metal homo-

geneous catalysis.

In this contribution we focus on the properties of

phenoxyiminato Ni(II) ethylene polymerization catalysts

(CF₃)FI-Ni and (CH₃)FI-Ni (Chart 4) activated with the

1

relaxation delay of 2 s and a mixing time of 0.8 s. The 2D H, H

NOESY NMR experiments were carried out on a Bruker AVANCE III

600 (600 MHz, ¹H; 150 MHz ¹³C) with a relaxation delay of 2 s and a

1

mixing time of 0.8 s. The 2D H,¹³C HSQC NMR experiments were

1

carried out on a Bruker AVANCE III 600 (600 MHz, H; 150 MHz,

¹³C) with a relaxation delay of 1 s. NMR analysis of polymers was

carried out in 1,1,2,2-tetrachloroethane-d₂at 120 °C with d1 = 10 s.

Polymer NMR spectra were assigned and polyethylene branch

numbers calculated according to standard literature procedures for

polyethylene.³⁰Gel permeation chromatography (GPC) was carried

out in 1,2,4-trichlorobenzene (stabilized with 125 ppm of BHT) at 150 °C

on a Polymer Laboratories 220 instrument equipped with a set of three

PLgel 10 μm mixed-B LS columns with diﬀerential refractive index and

viscosity detectors. Molecular weights were determined by Universal

Calibration relative to polystyrene standards. Elemental analyses were

conducted by Midwest Microlab, Indianapolis, IN.

Chart 4

Synthesis of N-(2,6-Diisopropylphenyl)-2,2,2-triﬂuoroaceti-

midoyl Chloride (1). The general procedure of Uneyama³¹was

followed with the following modiﬁcations. A 500 mL ﬂask was charged

with Ph₃P (34 g, 132 mmol), Et₃N (7.3 mL, 53 mmol), and CCl₄

(21.1 mL, 220 mmol). The mixture was stirred at 0 °C for 30 min.

Triﬂuoroacetic acid (5.0 mL, 65 mmol) was then added via syringe

and the reaction mixture stirred at 0 °C for another 30 min. A mixture

of 2,6-diisopropylaniline (1.0 mL, 53 mmol) and CCl₄(21.1 mL,

phosphine scavenger/cocatalyst Ni(cod)₂and report that

introducing remote ligand CF₃substituents leads to an ∼6.5×

increase in polymerization activity and 66× increase in product

M_wvs those of the ﬂuorine-free analogue. We argue that remote

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220 mmol) was then added, and the mixture was warmed to room

temperature and stirred for 30 min. The mixture was then reﬂuxed

with stirring for an additional 3 h. All the volatiles were removed under

vacuum, and the residue was diluted with hexanes and ﬁltered. The

volatiles were next removed from the ﬁltrate in vacuo, yielding a yellow

oil which was puriﬁed by column chromatography on silica gel with

15/1 hexane/ethyl acetate as the eluent to give 1 as a colorless oil

mixture was then ﬁltered, and the volatiles were removed from the

ﬁltrate under vacuum, aﬀording 0.695 g (1.14 mmol, 75% yield) of

orange solid. Crystals suitable for X-ray analysis were obtained by

slowly cooling an n-hexane/toluene solution of (CH₃)FI-Ni under N₂.

Anal. Calcd for C₃₇H₄₆NNiOP: C, 72.80; H, 7.60; N, 2.29. Found: C,

1

72.63; H, 7.74; N, 2.18. H NMR (C₆D₆, 500 MHz): δ 7.86 (s, 1H,

4-H), 7.29 (m, 5H, 1-, 8-, 9-, 11-, and 12-H), 7.18- 7.12 (m, 3H, 1-, 2-,

1

(11.5 g, 90% yield). The H NMR spectrum was in good agreement

and 3-H), 6.76 (s, 2H, 10- and 13-H), 6.69 (dd, ³J_HH= 7.8 Hz, ⁴J_HH

=

with that reported by Boere.³²

3

1.9 Hz, 1H, 3-H), 6.42 (t, J_HH= 7.5 Hz, 1H, 2-H), 2.13 (s, 12H, Ar

CH₃), 1.41 (s, 9H, ^tBu), 0.73 (d, ²J_PH= 9.5 Hz, 9H, P(CH₃)₃), −0.98

(s, 3H, Ni-CH₃) ppm. ¹³C NMR (C₆D₆, 125 MHz): δ 168.45, 166.67,

149.92, 141.02, 140.66, 137.56, 137.27, 133.29, 130.84, 130.47, 128.98,

128.79, 128.35, 125.71, 120.76, 113.02, 35.26, 29.74, 21.51, 13.58,

Synthesis of (2-Methoxymethoxy-3-tert-butylphenyl)-

lithium·TMEDA (2). A 500 mL ﬂask was charged with n-BuLi (36.0

mL, 90.0 mmol), TMEDA (13.4 mL, 90.0 mmol), and 100 mL of

diethyl ether at 0 °C. To this solution was added dropwise 1-tert-butyl-

2-methoxymethoxybenzene (17.0 g, 87.5 mmol) in diethyl ether at

0 °C over a period of 2 h. The resulting mixture was stirred at 0 °C for

2 h and then at room temperature overnight. All the volatiles were

then removed, and the resulting dark solid was washed with pentane

(250 mL × 3). The solid was then dried under vacuum to yield a white

solid (20.6 g, 75% yield). ¹H NMR (THF-d₈, 500 MHz): δ 7.47 (d, J =

7 Hz, 1H, Ar H), 6.72 (d, J = 7 Hz, 1H, Ar H), 6.55 (t, J = 7 Hz, 1H, Ar

H), 5.43 (s, 2H, OCH₂O), 3.42 (s, 3H, OCH₃), 2.31 (s, 4H, NCH₂), 2.16

(s, 12H, NCH₃), 1.36 (s, 9H, C(CH₃)₃) ppm. ¹³C NMR (THF-d₈, 125

MHz): δ 166.6, 141.8, 132.4, 121.8, 121.2, 92.7 (OCH₂O), 59.1 (OCH₃),

54.9 (NCH₂), 46.4 (NCH₃), 35.1 (C(CH₃)₃), 31.2 (C(CH₃)₃) ppm.

Synthesis of 2-tert-Butyl-6-(2,2,2-triﬂuoro-1-(2,6-

diisopropylphenyl)iminoethyl)(methoxymethoxy)benzene (3).

A 200 mL ﬂask was charged with 2 (2.29 g, 7.24 mmol) and 60 mL of

diethyl ether at 0 °C. Next, 1 (2.10 g, 7.24 mmol) in 30 mL of diethyl

ether was added by addition funnel over 30 min at 0 °C. The mixture

was stirred at 0 °C for 3 h and then at room temperature for 6 h.

Water (50 mL) was then added, the organic layer was separated, dried

over Na₂SO₄, and ﬁltered, and the ﬁltrate was evaporated to yield a

brown solid. The solid was recrystallized from methanol, aﬀording 2.66

1

2

13.48 (d, J_CP= 25.8 Hz, P(CH₃)₃), −13.68 (d, J_CP= 30.9 Hz,

Ni-CH₃) ppm. ³¹P NMR (C₆D₆, 162 MHz): δ −13.8 ppm.

Synthesis of [2-tert-Butyl-6-((2,6-(3,5-bis(triﬂuoromethyl)-

phenyl)phenylimino)methyl)phenolato]nickel(II) Methyl Tri-

methylphosphine ((CF₃)FI-Ni). A 100 mL Schlenk ﬂask was charged

with 0.489 g (0.699 mmol) of (CF₃)FI-Na, 0.183 g (0.700 mmol) of

trans-NiMeCl(PMe₃)₂, and 35 mL of dry benzene. The resulting red

reaction mixture was stirred at room temperature for 6 h. The mixture

was then ﬁltered, and the volatiles were removed from the ﬁltrate

under high vacuum, aﬀording 0.320 g (0.387 mmol, 55% yield) of a

red-orange solid. Crystals suitable for X-ray analysis were obtained by

cooling an n-hexane/toluene solution of (CF₃)FI-Ni under N₂. Anal.

Calcd for C₃₇H₃₄NF₁₂NiOP: C, 53.78; H, 4.15; N, 1.70. Found: C,

1

g of 3 (82% yield). H NMR (C₂D₂Cl₄, 120 °C, 400 MHz): δ 7.58

(d, J = 8 Hz, 1H, Ar-H), 7.44 (d, J = 8 Hz, 1H, Ar H), 7.00 (br, 4H, Ar H),

4.12 (s, 2H, OCH₂O), 3.05 (s, 3H, OCH₃), 2.88 (septa, J = 6 Hz, 2H,

CH(CH₃)₂), 1.05 (d, J = 6 Hz, 12H, CH(CH₃)₂), 1.04 (s, 9H,

C(CH₃)₃) ppm. GC-MS: m/z 449 (M⁺).

Synthesis of 2-tert-Butyl-6-(2,2,2-triﬂuoro-1-(2,6-diiso-

1

54.00; H, 4.08; N, 1.81. H NMR (C₆D₆, 500 MHz): δ 8.03 (s, 4H,

propylphenyl)iminoethyl)phenol ((CN_CF)HFI). A 100 mL

3

8-, 9-, 11-, and 12-H), 7.67 (s, 2H, 10- and 13-H), 7.21 (dd, J_HH

=

4

ﬂask was charged with 3 (2.66 g), CH₂Cl₂(30 mL), and concentrated

HCl (20 mL). The mixture was then stirred at room temperature for

6 h. Water (50 mL) was next added, the organic layer was separated,

7.3 Hz, J_HH= 1.8 Hz, 1H, 1-H), 7.18 (d, 1H, 4-H), 6.99−6.91 (m,

3H, 5-, 6-, and 7-H), 6.48 (dd, ³J_HH= 7.9 Hz, ⁴J_HH= 1.9 Hz, 1H, 3-H),

6.37 (t, ³J_HH= 7.4 Hz, 1H, 2-H), 1.33 (s, 9H, ^tBu), 0.70 (d, ²J_PH= 9.7

Hz, 9H, P(CH₃)₃), −1.33 (d, J_PH= 7.6 Hz, 3H, Ni−CH₃) ppm. ¹³C

3

dried over Na₂SO₄, and ﬁltered, and the ﬁltrate was evaporated to

1

dryness, aﬀording (CN )HFI as a red oil (2.06 g, 95% yield). H

CF₃

NMR (C₆D₆, 125 MHz): δ 168.18, 167.27, 150.46, 141.93, 140.76,

133.94, 132.95, 131.91, 131.63, 131.36, 131.10, 130.72, 127.72, 126.22,

123.82 (q, ¹J_CF= 274.2 Hz, CF₃), 120.86, 119.07, 113.87, 34.92, 29.44,

NMR (CDCl₃, 500 MHz): δ 14.10 (s, 1H, OH), 7.65 (d, J = 7 Hz, 1H,

Ar H), 7.52 (d, J = 7 Hz, 1H, Ar H), 7.21−7.16 (m, 3H, Ar H), 6.92

(t, J = 7 Hz, 1H, Ar H), 2.73 (septa, J = 7 Hz, 2H, CH(CH₃)₂), 1.48

(s, 9H, C(CH₃)₃), 1.20 (d, J = 7 Hz, 6H, CH(CH₃)₂), 1.17 (d, J =

7 Hz, 6H, CH(CH₃)₂) ppm. ¹³C NMR (CDCl₃, 125 MHz): δ 162.8,

1

2

13.24 (d, J_CP= 27.6 Hz, P(CH₃)₃), −13.48 (d, J_CP= 44.7 Hz, Ni-

CH₃) ppm. ³¹P NMR (C₆D₆, 162 MHz): δ −12.56 ppm. ¹⁹F NMR

(C₆D₆, 376 MHz): δ −62.92 ppm.

2

141.6, 139.0, 135.5 (d, J_CF= 30 Hz, CF₃CN), 131.8, 127.5 (d,

¹J_CF= 290.0 Hz, CF₃), 125.2, 123.2, 119.9, 117.9, 117.6, 113.6, 35.4

(C(CH₃)₃) 29.5 (C(CH₃)₃), 28.7 (CH(CH₃)₂), 23.8 (CH(CH₃)₂),

22.8 (CH(CH₃)₂) ppm. ¹⁹F NMR (CDCl₃, 376 MHz): δ −59.7

1

(d, J_C−F= 3 Hz) ppm. GC-MS: m/z 405 (M⁺).

Representative Procedure for the Synthesis of Sodium Salts

(CH₃)FI-Na, (CF₃)FI-Na, and (CN_CF)FI-Na. Under N₂, a 100 mL

3

Schlenk ﬂask was charged with 0.902 g (1.33 mmol) of (CF₃)HFI and

0.080 g (3.33 mmol) of NaH. To this was added 75 mL of dry THF via

syringe. This red-orange mixture was stirred for 24 h. The reaction mixture

was then ﬁltered, and the volatiles were removed in vacuo. The resulting

bright yellow solid was dried under high vacuum overnight and was used

without puriﬁcation in the synthesis of (CF₃)FI-Ni.

Synthesis of [2-tert-Butyl-6-((2,6-(3,5-dimethylphenyl)-

phenylimino)methyl)phenolato]nickel(II) Methyl Trimethyl-

phosphine ((CH₃)FI-Ni). A 100 mL Schlenk ﬂask was charged with

0.740 g (1.53 mmol) of (CH₃)FI-Na, 0.399 g (1.53 mmol) of trans-

NiMeCl(PMe₃)₂, and 45 mL of dry benzene. The resulting red-orange

reaction mixture was then stirred at room temperature for 6 h. The

Synthesis of [2-tert-Butyl-6-((2,6-(3,5-bis(triﬂuoromethyl)-

phenyl)phenylimino)methyl)phenolato]nickel(II) Chloro (2,4-

Lutidine ((CF₃)FI-Ni-Cl). A 100 mL Schlenk ﬂask was charged with

0.930 g (1.33 mmol) of (CF₃)FI-Na. THF (30 mL) was added, and

the resulting yellow solution was transferred slowly via cannula over

20 min to a ﬂask containing 0.458 g (1.33 mmol) of NiCl₂(2,4-lutidine)₂

and 15 mL of THF. An immediate color change to dark red was

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observed, and the reaction mixture was stirred for 90 min at room tem-

perature. The volatiles were then removed in vacuo, leaving a dark

orange oil. Dichloromethane (35 mL) was added, and the reaction

mixture was ﬁltered. The ﬁltrate was concentrated in vacuo, and

pentane (40 mL) was added. The reaction mixture was then cooled to

−40 °C overnight, upon which a yellow-brown solid precipitated from

solution. This solid was isolated via ﬁltration at −78 °C, and the

resulting microcrystalline solid was dried under high vacuum, aﬀording

0.632 g (0.720 mmol, 54%) of paramagnetic (CF₃)FI-Ni-Cl.³³

ﬁltration, and the resulting solid was dried under high vacuum, aﬀording

2.012 g (3.04 mmol, 70%) of paramagnetic (CH₃)FI-Ni-Cl.³³

Synthesis of [2-tert-Butyl-6-((2,6-(3,5-dimethylphenyl)-

phenylimino)methyl)phenolato]nickel(II) Ethyl 2,4-Lutidine

((CH₃)FI-Ni-Et). A 100 mL Schlenk ﬂask was charged with 0.749 g

(1.13 mmol) of (CH₃)FI-Ni-Cl and ether (50 mL). The reaction ﬂask

was cooled to −78 °C, and ethylmagnesium chloride (2.0 M, 0.57 mL,

1.14 mmol) was added dropwise. The reaction mixture was stirred at

−78 °C for 2 h and then stirred at −42 °C for 1 h. The orange-red

mixture was next warmed to room temperature and ﬁltered. The

orange-red reaction mixture was then warmed to room temperature

and ﬁltered, and the ﬁltrate was concentrated in vacuo. Dry n-pentane

(30 mL) was then added via syringe, precipitating an orange solid.

After ﬁltration and drying under high vacuum, 0.360 g (0.549 mmol,

Synthesis of [2-tert-Butyl-6-((2,6-(3,5-bis(triﬂuoromethyl)-

phenyl)phenylimino)methyl)phenolato]nickel(II) Ethyl 2,4-Lu-

tidine ((CF₃)FI-Ni-Et). A 50 mL Schlenk ﬂask was charged with 0.568 g

(0.647 mmol) of (CF₃)FI-Ni-Cl and ether (30 mL). The reaction ﬂask

was cooled to −78 °C, and ethylmagnesium chloride (2.0 M, 0.37 mL,

0.74 mmol) was added dropwise. The reaction mixture was stirred

at −78 °C for 2 h and then stirred at −42 °C for 1 h. The orange-red

mixture was next warmed to room temperature and ﬁltered. The

volatiles were removed from the ﬁltrate under high vacuum, and the

red solid was triturated with n-pentane, aﬀording 0.375 g (0.430 mmol,

67%) of (CF₃)FI-Ni-Et. X-ray-quality crystals of (CF₃)FI-Ni-Et were

obtained by slow evaporation of an n-hexane solution under N₂. Anal.

Calcd for C₄₂H₃₆F₁₂N₂NiO: C, 57.89; H, 4.16; N, 3.21. Found: C,

1

48%) of (CH₃)FI-Ni-Et was isolated. H NMR (C₆D₆, 500 MHz): δ

8.68 (d, J = 5.5 Hz, 1H), 7.66 (s, 1H), 7.59 (s, 2H), 7.47−7.36 (m,

4H), 7.15 (d, J = 8.2 Hz, 4H), 6.90 (s, 2H), 6.69 (d, J = 7.4 Hz, 1H),

6.31 (t, J = 7.4 Hz, 1H), 6.26 (s, 1H), 6.16 (d, J = 4.6 Hz, 1H), 3.14 (s,

3H), 2.29 (d, J = 16.6 Hz, 12H), 1.57 (s, 3H), 1.02 (s, 9H), 0.32−0.14

(m, 2H), 0.04 (t, J = 7.4 Hz, 3H) ppm. ¹³C NMR (C₆D₆, 125 MHz): δ

169.28, 166.58, 159.60, 150.86, 149.69, 146.78, 140.65, 140.55, 137.45,

137.16, 136.82, 132.65, 130.25, 130.15, 129.08, 128.82, 125.79, 124.76,

121.34, 120.40, 112.61, 34.62, 29.34, 25.31, 21.68, 21.61, 20.21, 16.07,

1.61 ppm.

1

57.13; H, 4.15; N, 3.14. H NMR (C₆D₆, 500 MHz): δ 8.74 (d, J = 5.7

Hz, 1H, 14-H), 8.50 (s, 2H, 11-, 12-H), 7.95 (s, 2H, 8-, 9-H), 7.93 (s, 1H,

13-H), 7.70 (s, 1H, 10-H/4-H), 7.09 (d, J = 6.8 Hz, 1H, 1-H), 6.96−6.88

(m, 4H, 5-, 6-, 7-, 16-H), 6.45 (d, J = 7.5 Hz, 1H, 3-H), 6.27 (t, J = 7.6

Hz, 1H, 2-H), 6.14 (s, 1H, 4-H/10-H), 6.10 (d, J = 5.5 Hz, 1H, 15-H),

2.90 (s, 3H, lut-CH₃), 1.54 (s, 3H, lut-CH₃), 0.93 (s, 9H, ^tBu), −0.08 (m,

4H, Ni-CH₂CH₃), −0.18 (m, 1H, Ni-CH₂CH₃) ppm. ¹³C NMR (C₆D₆,

125 MHz): δ 168.75, 167.33, 158.83, 151.01, 149.26, 147.47, 142.27,

141.76, 141.13, 134.01, 133.81, 132.02, 131.84, 131.71, 131.57, 131.37,

131.02, 130.89, 126.48, 125.29, 125.17, 125.09, 123.12, 123.00, 121.90,

121.17, 121.03, 119.36, 113.65, 34.44, 29.00, 24.92, 20.14, 15.48, 1.78

ppm. ¹⁹F NMR (C₆D₆, 376 MHz): δ −62.78, −62.95 ppm.

Synthesis of [2-tert-Butyl-6-(2,2,2-triﬂuoro-1-(2,6-

diisopropylphenyl)iminoethyl)phenolato]nickel(II) Methyl Tri-

methylphosphine ((CN_CF)FI-Ni). A 100 mL Schlenk ﬂask was

3

charged with 0.521 g of (CN_CF)FI-Na (1.04 mmol), 0.273 g of

3

trans-NiMeCl(PMe₃)₂(1.04 mmol), and 45 mL of dry benzene. The

resulting red-orange reaction mixture was stirred at room temperature

for 6 h. The mixture was then ﬁltered, and the volatiles were removed

from the ﬁltrate under high vacuum, aﬀording 0.528 g (91% yield) of a

red solid. Crystals suitable for X-ray analysis were obtained by slowly

Synthesis of [2-tert-Butyl-6-((2,6-(3,5-dimethylphenyl)-

phenylimino)methyl)phenolato]nickel(II) Chloro 2,4-Lutidine

((CH₃)FI-Ni-Cl). A 100 mL Schlenk ﬂask was charged with 2.10 g (4.35

mmol) of (CH₃)FI-Na. THF (50 mL) was added, and the resulting

yellow solution was transferred slowly via cannula over 20 min to a

ﬂask containing 1.50 g (4.36 mmol) of NiCl₂(2,4-lutidine)₂and 25 mL

of THF. An immediate color change to dark red was observed, and the

reaction mixture was stirred for 90 min at room temperature. The

volatiles were then removed in vacuo, leaving a dark orange oil.

Dichloromethane (50 mL) was added, and the reaction mixture was

ﬁltered. The ﬁltrate was concentrated in vacuo, and pentane (50 mL)

was added, precipitating a brown solid. This solid was isolated via

cooling an n-hexane/toluene solution of (CN_CF)FI-Ni under N₂.

3

Anal. Calcd for C₃₇H₄₆NNiOP: C, 60.67; H, 7.46; N, 2.53. Found: C,

1

61.62; H, 7.30; N, 2.30. H NMR (C₆D₆, 500 MHz): δ 7.75 (d, J =

7 Hz, 1H, Ar H), 7.38 (d, J = 7Hz, 1H, Ar H), 7.11−7.06 (m, 3H, Ar H),

6.58 (t, J = 7 Hz, 1H, Ar H), 4.10 (septa, J = 7 Hz, 2H, CH(CH₃)₂), 1.52

(d, J = 7 Hz, 6H, CH(CH₃)₂), 1.46 (s, 9H, C(CH₃)₃), 0.84 (d, J = 7 Hz,

6H, CH(CH₃)₂), 0.84 (d, ²J_PH= 10 Hz, 9H, P(CH₃)₃), −1.41 (d, J = 6 Hz,

3H, Ni-CH₃) ppm. ¹³C NMR (C₆D₆, 125 MHz): δ 169.0, 146.1, 142.0,

139.6, 131.2, 128.7, 126.6, 124.1, 123.9, 122.1, 120.7, 113.7, 36.0

(C(CH₃)₃) 31.0 (C(CH₃)₃), 29.6 (CH(CH₃)₂), 25.7 (CH(CH₃)₂),

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25.0 (CH(CH₃)₂), 14.9 (P(CH₃)₃), −11.5 (Ni-CH₃) ppm. ³¹P NMR

(C₆D₆, 162 MHz): δ −14.8 ppm. ¹⁹F NMR (C₆D₆, 376 MHz):

δ −53.0 ppm.

Figure 2. (A) Displacement ellipsoid plot (50%) of precatalyst

complex (CF₃)FI-Ni. H atoms are omitted for clarity. Selected bond

distances (Å) and angles (deg): Ni−C1, 1.953(2); Ni−P1, 2.1496(6);

Ni−N1, 1.936(2); Ni−O1, 1.914(1); Ni···F6, 4.299(2); ∠N1−Ni−

C1, 92.46(8); ∠C1−Ni−P1, 84.71(6); ∠P1−Ni−O1, 92.46(4);

∠O1−Ni−N1, 92.35(6); ∠N1−Ni−C1, 92.46(8); ∠N1−Ni−P1,

169.03(5); ∠O1−Ni−C1, 168.26(8). (B) Space-ﬁlling model of

(CF₃)FI-Ni: Ni, orange; F, green; C, gray; P, purple; O, red; H, white.

General Procedure for Ethylene Polymerization Experi-

ments. In a typical experiment, an oven-dried 350 mL glass thick-

walled pressure vessel was charged in the glovebox with 5 μmol of Ni

catalyst, 2.0 equiv of Ni(COD)₂, 50 mL of dry toluene, and a large

magnetic stir bar. The pressure vessel was then interfaced to a high-

pressure line, and the solution was degassed. The reactor was warmed

to the desired temperature using a water or oil bath and allowed to

equilibrate for 5 min. With rapid stirring, the reactor was then

pressurized to 8.0 atm of ethylene, which was maintained during

the course of the polymerization. After the desired run time, the reactor

was vented and the contents poured into acidic methanol (10%). The

precipitated polymer was collected by ﬁltration, washed with methanol,

and dried at 60 °C overnight under high vacuum.

Molecular Modeling Calculations. Molecular modeling was

performed with semiempirical and ab initio codes included in the

Spartan 06 software package.³⁴Preoptimization was performed at the

PM3 semiempirical level,³⁵and Hartree−Fock (HF) ab initio

calculations were carried out using 6-31G** basis sets to ﬁnd the

equilibrium geometry in the ground state for catalysts (CF₃)FI-Ni and

(CH₃)FI-Ni with an n-butyl group modeling the polyethylene chain.

Restriction to a square-planar geometry was applied at the Ni center.

In the semiempirical and ab initio calculations, the gradient norms

were 10⁻⁴hartree/Bohr and 0.05 kcal/Å, respectively.

X-ray Crystal Structure Determinations for (CH₃)FI-Ni,

(CF₃)FI-Ni, (CF₃)FI-Ni-Et, and (CN_CF)FI-Ni. Intensity data for

3

(CH₃)FI-Ni, (CF₃)FI-Ni, (CF₃)FI-Ni-Et, and (CN_CF)FI-Ni were

3

collected at −173 °C on a Bruker AXS APEXII diﬀractometer

equipped with a CCD area detector using graphite-monochromated

Mo Kα (λ = 0.7107 Å, (CF₃)FI-Ni-Et and (CN_CF)FI-Ni) or Cu

Figure 3. Displacement ellipsoid plot (50%) of precatalyst (CH₃)FI-

Ni. H atoms and solvent molecules are omitted for clarity. Selected

bond distances (Å) and angles (deg): Ni−C1, 1.937(3); Ni−P1,

2.1420(9); Ni−N1, 1.937(2); Ni−O1, 1.917(2); ∠N1−Ni−C1,

91.7(1); ∠C1−Ni−P1, 85.7(1); ∠P1−Ni−O1, 92.29(7); ∠O1−Ni−

N1, 92.70(9); ∠N1−Ni−C1, 91.7(1); ∠N1−Ni−P1, 168.05(7);

∠O1−Ni−C1, 167.5(1).

3

Kα (λ = 1.5418 Å, (CH₃)FI-Ni and (CF₃)FI-Ni) radiation. All data

were corrected for absorption using an empirical correction. Structure

solutions and reﬁnements were obtained by SIR-92,³⁶SHELXS-97,³⁷

or OLEX2³⁸using direct methods and were reﬁned on F²using full-

matrix least-squares techniques. All hydrogen atoms were reﬁned with

a riding model. All non-hydrogen atoms were reﬁned anisotropically.

Crystals of (CH₃)FI-Ni were found to be nonmerohedrally twinned.

The data were processed using both orientation matrices identiﬁed by

the program Cell_Now.³⁹The remaining structures were solved and

reﬁned routinely. In (CF₃)FI-Ni-Et, signiﬁcant disorder (70%/30%)

exists in the Ni-ethyl conformation and ﬂuorine atoms F10, F11, and

F12 (80/20%). Distance restraints for equivalent bonds were reﬁned

for the disordered ethyl and CF₃groups. Rigid bond restraints were

imposed on the displacement parameters as well as restraints on

similar amplitudes separated by less than 1.7 Å on disordered atoms.

Crystallographic results are summarized in Table S2 of the Supporting

Information, and displacement ellipsoid plots for (CF₃)FI-Ni,

that introducing remote CF₃substituents in these Ni(II)

catalysts has dramatic eﬀects on catalyst activity, thermal

stability, and product polyoleﬁn molecular weight character-

istics. These diﬀerences in polymerization properties are

attributed primarily to C−F···H−C interactions between the

remote CF₃substituent and a H atom on the β-carbon of the

growing polyethylene chain which conformationally impedes

chain transfer.

Catalyst Synthesis. The phenoxyiminato ligands employed

in this study were prepared following generalizable literature

procedures.^11bThe sodium salts of (CH₃)HFI and (CF₃)HFI

were prepared by stirring the free ligands with excess NaH in

THF (Scheme 1). The neutrally charged Ni(II) catalysts

(CH₃)FI-Ni and (CF₃)FI-Ni were then prepared by stirring

the sodium salts with 1 equiv of trans-NiClMe(PMe₃)₂in

benzene (Scheme 1). The constitutions and structures of

(CH₃)FI-Ni, (CF₃)FI-Ni-Et, and (CN_CF)FI-Ni are shown in

3

Figures 2−5, respectively.

RESULTS

■

A primary goal of this study is to investigate the eﬀects of

installing remote ﬂuorocarbon substituents on phenoxyiminato-

based Ni(II) ethylene polymerization catalysts. It will be seen

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under a variety of conditions either does not yield the ligand

(CN_CF)HFI or aﬀords a mixture of products with low yields of

3

(CN_CF)HFI which cannot be readily isolated (Scheme 2, eq 1).

3

Thus, a new approach was developed that aﬀords the ligand

(CN_CF)HFI in very high yield (Scheme 2, eq 2). Reaction of

3

N-(2,6-diisopropylphenyl)-2,2,2-triﬂuoroacetimidoyl chloride (1)

with 2-methoxymethoxy-3-tert-butylphenyl lithium·TMEDA (2)

aﬀords the MOM-protected ligand precursor 3. MOM depro-

tection with HCl in CH₂Cl₂then aﬀords the ligand (C

N_CF)HFI in 95% yield. (CN_CF)HFI was next treated with

3

NaH to produce the corresponding sodium salt, which was used

for an in situ reaction with trans-NiClMe(PMe₃)₂to aﬀord the

complex (CN_CF)FI-Ni in 91% yield (Scheme 2, eq 3).

3

To model a growing polyethylene chain, complexes (CF₃)FI-

Ni-Et and (CH₃)FI-Ni-Et were synthesized from the Ni-Cl

precursor complexes (CF₃)FI-Ni-Cl or (CH₃)FI-Ni-Cl by

reacting the ligand sodium salts with NiCl₂(2,4-lutidine)₂in

THF (Scheme 3); complexes (CF₃)FI-Ni-Et and (CH₃)FI-Ni-

Et were then prepared by alkylation with EtMgCl in diethyl

ether (Scheme 3). Complexes (CF₃)FI-Ni-Et and (CH₃)FI-Ni-

Et were isolated as red microcrystalline air-sensitive powders

that are stable for weeks at room temperature without

signiﬁcant decomposition. The ligand 2,4-lutidine was chosen

on the basis of literature precedent, showing that lutidine

suﬃciently stabilizes Ni-alkyl complexes with β-H's.^26,40

Figure 4. Displacement ellipsoid plot (50%) of the 70% Ni-Et

conformer of complex (CF₃)FI-Ni-Et. H atoms are omitted for clarity.

Selected distances (Å) and angles (deg): Ni−C1, 2.017(5); Ni−N1,

1.914(1); Ni−N2, 1.893(1); Ni−O1, 1.893(1); C2−H2···F9, 3.66(2);

Ni···F4, 4.113(2); Ni···F12, 4.412(2); ∠N2−Ni−C1, 96.57(19);

∠C1−Ni−N1, 88.17(19); ∠N1−Ni−O1, 82.74(6); ∠O1−Ni−N2,

93.04(6); ∠O1−Ni−C1, 166.97(18).

Molecular Structure of (CF₃)FI-Ni. Single crystals of the

precatalyst (CF₃)FI-Ni were grown by slowly cooling an n-

pentane/toluene solution under N₂. (CF₃)FI-Ni crystallizes in

triclinic space group P1 with two molecules in the asymmetric

̅

unit. Two intermolecular Ar-H···F−C hydrogen bonds⁴¹are

evident, with distances ranging from 2.52 to 2.66 Å.⁴²(CF₃)FI-

Ni displays a distorted-square-planar geometry around the Ni

atom with P1 and C1 displaced +0.363(3) and −0.368(2) Å

from the Ni/O1/N1 mean plane, respectively (Figure 2).

Despite the distorted-square-planar solid-state geometry,

(CF₃)FI-Ni and all other catalysts reported here exhibit

diamagnetic NMR chemical shifts in solution. The Ni-PMe₃

substituent is positioned trans to the imine functionality; this

solid-state geometry was conﬁrmed in solution by 1D ¹H NOE

experiments. A 50% displacement ellipsoid plot of (CF₃)FI-Ni

and a space-ﬁlling model showing the nonbonded interactions

in (CF₃)FI-Ni are shown in Figure 2. Interestingly, one

terphenyl ring exhibits an intramolecular Ni···arene centroid

distance of 3.864 Å. This Ni···centroid distance is much longer

than in typical Ni−arene complexes, where average crystallographic

distances from the 11 current structures in the Cambridge

Structural Database are much shorter (1.65(4) Å).⁴³Similar

Figure 5. Displacement ellipsoid plot (50%) of precatalyst complex (C

N

_CF)FI-Ni. H atoms are omitted for clarity. Selected bond distances (Å)

3

and angles (deg): Ni−C1, 1.931(2); Ni−P1, 2.1353(5); Ni−N1, 1.968(1);

Ni−O1, 1.900(1); ∠N1−Ni−C1, 94.04(7); ∠C1−Ni−P1, 85.42(6); ∠P1−

Ni−O1, 92.68(4); ∠O1−Ni−N1, 91.86(6); ∠N1−Ni−C1, 94.04(7);

∠N1−Ni−P1, 161.87(5); ∠O1−Ni−C1, 166.57(8).

(CH₃)FI-Ni and (CF₃)FI-Ni were conﬁrmed in detail by NMR

spectroscopy, elemental analysis, and X-ray diﬀraction. To further

probe the eﬀects that the electron-withdrawing ligand CF₃

substituents might have on these catalysts, a complex was targeted

with the CF₃substituent in closer proximity to the Ni center,

(CN_CF)FI-Ni. Attempted condensation of 2,6-diisopropylani-

3

line with ω,ω,ω-triﬂuoro-2-hydroxy-3-tert-butylacetophenone

Scheme 1. Synthesis of Catalysts (CH₃)FI-Ni and (CF₃)FI-Ni

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Scheme 2. Synthesis of Catalyst (CN_CF)FI-Ni

3

Scheme 3. Synthesis of Model Compounds (CH₃)FI-Ni-Et and (CF₃)FI-Ni-Et

intramolecular Ni···arene contacts are also seen in related

terphenyl phenoxyimine nickel complexes^11b,eand in the solid-

state structure of (CH₃)FI-Ni. No evidence of signiﬁcant Ni···F

interactions is observed in (CF₃)FI-Ni, with the closest Ni···F

distance being 4.299(2) Å (Ni···F6); the sum of the Ni and F

van der Waals radii is 3.31 Å.⁴⁴

Molecular Structure of (CH₃)FI-Ni. X-ray-quality crystals

of the precatalyst (CH₃)FI-Ni were grown by slowly cooling an

n-hexane/toluene solution under N₂. (CH₃)FI-Ni crystallizes in

(CF₃)FI-Ni-Et crystallizes in space group P2₁/c with four

molecules in the asymmetric unit and displays a distorted-

square-planar geometry around the Ni atom with N1 and C1

displaced +0.120(2) and −0.326(3) Å from the Ni/O1/N1

mean plane. The spatial conﬁguration around the Ni in

(CF₃)FI-Ni-Et indicates that the Ni−lutidine substituent is

positioned trans to the imine functionality; that this solid-state

1

geometry persists in solution is conﬁrmed by 1D H NOE

experiments. The 2,4-lutidine plane is oriented nearly

perpendicular to the Ni/C1/O1 plane, with a C1/Ni/N1/C7

torsion angle of 87.4°. In comparison to the Ni−ligand bond

distances in (CH₃)FI-Ni and (CF₃)FI-Ni, the Ni−N2 and Ni−

O bond distances are signiﬁcantly shorter in (CF₃)FI-Ni-Et

(e.g., Ni−N_imine= 1.936(2) and 1.893(1) Å for (CF₃)FI-Ni and

(CF₃)FI-Ni-Et, respectively). The Ni−N(lutidine) distance is

not unexpectedly shorter than the Ni−PMe₃contacts in

(CH₃)FI-Ni and (CF₃)FI-Ni. Relative to the PMe₃-coordi-

nated (CH₃)FI-Ni and (CF₃)FI-Ni, the shorter Ni−N and

Ni−O bond distances in 2,4-lutidine-coordinated (CF₃)FI-Ni-Et

likely reﬂect the more electron-deﬁcient metal center due to the

stronger π-acid characteristics of 2,4-lutidine.⁴⁵A 70%/30%

conformational disorder is found about the Ni−Et axis, and this

was modeled appropriately in the Ni−Et conformation as well

as for F atoms F10, F11, and F12 (80%/20%).⁴⁶The closer

F···H distance in the 70% conformer is consistent with an

energetically more favorable orientation. The derived

the triclinic space group P1. The coordination environment around

̅

Ni is similar to the structure of (CF₃)FI-Ni, with the PMe₃ligand

trans to the imine functionality (Figure 3). A distorted-square-

planar geometry around the Ni is observed with atoms P1 and C1

displaced +0.403(3) and −0.392(4) Å from the Ni/O1/N1

mean plane, respectively. Interestingly, despite the diﬀerent

electronic characteristics of (CF₃)FI-Ni and (CH₃)FI-Ni, the Ni−

phenoxyimine ligand bond distances in these complexes are

crystallographically identical (vide infra). (CH₃)FI-Ni exhibits a

shorter Ni−Me bond distance relative to (CF₃)FI-Ni (1.937(3)

and 1.953(2) Å, respectively). A displacement ellipsoid plot of

(CH₃)FI-Ni is shown in Figure 3. (CH₃)FI-Ni exhibits an

intramolecular Ni···arene centroid distance of 3.695 Å.

Molecular Structure of (CF₃)FI-Ni-Et. Single crystals of

(CF₃)FI-Ni-Et were grown by slow evaporation of an n-hexane

solution under N₂. A 50% displacement ellipsoid plot of the

70% conformer of (CF₃)FI-Ni-Et is shown in Figure 4.

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(C)F···H(C) distance in this precatalyst is greater than the sum

of the CH₃and F van der Waals radii (2.57 Å).⁴⁴Nevertheless,

molecular modeling³⁴reveals that, during polymerization,

(C_β)H_β···F(C) distances can be as small as ∼2.61 Å (vide

infra).⁴²No signiﬁcant Ni···F interactions are observed in

(CF₃)FI-Ni-Et, with the closest Ni···F contact being 4.113(2)

Å, longer than the sum of Ni and F van der Waals radii (3.31

Å).⁴⁴Similar to the case for (CF₃)FI-Ni, the crystal packing of

(CF₃)FI-Ni-Et features close intermolecular C−H···F hydro-

gen bond contacts with an H···F distance of 2.41 Å;⁴²the sum

of the H and F van der Waals radii is 2.57 Å.⁴⁴

Molecular Structure of (CN_CF)FI-Ni. Single crystals of

3

precatalyst (CN_CF)FI-Ni were grown by slowly cooling an n-

3

pentane solution under N₂. Complex (CN_CF)FI-Ni crystallizes

3

in space group P2₁/c. A 50% displacement ellipsoid plot of (C

N

_CF)FI-Ni is shown in Figure 5. The coordination environment

3

Figure 6. Stack plot showing ¹³C{¹H} NMR spectra (400 MHz, 120 °C,

1,1,2,2-tetrachloroethane-d₂) of polyethylenes produced in room-temper-

ature polymerizations under the same reaction conditions using catalysts

(CH₃)FI-Ni (top) and (CF₃)FI-Ni (bottom).

of Ni is similar to that in the structures of (CH₃)FI-Ni and

(CF₃)FI-Ni, with the Ni−PMe₃ligand trans to the imine

functionality. A distorted-square-planar geometry around the Ni is

observed, with atoms P1 and C1 displaced +0.403(3) and

−0.643(2) Å from the Ni/O1/N1 mean plane. Relative to

increased M_w’s and depressed branching, the ethylene polymer-

izations carried out at 50 °C reveal that the (CF₃)FI-Ni catalyst

has a thermal stability substantially greater than that derived

from (CH₃)FI-Ni, as judged by the marked fall in activity for

(CH₃)FI-Ni at 50 °C (Table 1).

(CH₃)FI-Ni and (CF₃)FI-Ni, (CN_CF)FI-Ni exhibits a

3

signiﬁcantly shorter Ni−O bond distance and a signiﬁcantly

longer Ni−N bond distance.

Ethylene Polymerization Experiments. (CH₃)FI-Ni,

(CF₃)FI-Ni, and (CN_CF3)FI-Ni are active ethylene polymer-

ization/oligomerization catalysts in the presence of Ni(COD)₂

as a phosphine scavenger/cocatalyst. In the absence of

cocatalyst, negligible polyethylene is produced by any of the

complexes. Typical experiments were conducted with 5.0 μmol

of catalyst in 50 mL of toluene under a continuous 8.0 atm

ethylene pressure for 10.0 min using conditions minimizing

mass transport and exotherm eﬀects.^8e,g−jRelevant data are

compiled in Table 1. The polymeric products were

characterized by ¹H and ¹³C NMR and gel permeation

chromatography (GPC) using refractive index and viscometry

The ethylene pressure dependence of the room-temperature

ethylene polymerization activity of (CF₃)FI-Ni was also

investigated (Table 2, Figure 7). First-order dependence of

the TOF on ethylene pressure is observed over an 8-fold

pressure range, with product branch densities progressively

falling with increasing ethylene pressure and molecular weights

initially increasing and then saturating with increasing ethylene

pressure. These data suggest competition between insertive

propagation and chain transfer/chain walking via β-hydride

elimination (vide infra). In the presence of Ni(COD)₂, (C

N

_CF)FI-Ni is a modestly active ethylene oligomerization

3

catalyst (7.9 kg of oligomer (mol of Ni)⁻¹h⁻¹atm⁻¹; Table 1).

1

detection. Polyethylene branch densities were assayed by H

NMR spectroscopy.³⁰The type of branching observed in all the

polyethylenes produced by the present catalysts is predom-

inantly methyl, with a small percentage of longer chains (≥C₆).

At 25 °C, CF₃-substituted (CF₃)FI-Ni is ∼6.5× more active

than the CH₃-substituted analogue (CH₃)FI-Ni (250 vs 38 kg

of polymer (mol of Ni)⁻¹h⁻¹atm⁻¹, respectively), with the

resulting polyethylene displaying dramatically greater M_w

(92 000 vs 1400 g mol⁻¹, respectively), and far lower alkyl

branch density (7 vs 88 branches/1000C, respectively). Figure

6 shows ¹³C NMR spectra illustrating the substantially greater

chain branching introduced in the polyethylenes produced by

(CH₃)FI-Ni vs those produced by (CF₃)FI-Ni. In addition to

The molecular weights of the oligomers produced by (C

N

_CF)FI-Ni were determined by GC-MS to range from C₄to C₂₆

3

in an approximate Schulz−Flory distribution.^42,47Oligomer end

1

group analysis by H NMR spectroscopy reveals predominantly

internal oleﬁnic groups (∼90%) with a small percentage (∼10%)

of vinyl end groups.⁴²

DISCUSSION

■

Propagation and Chain Transfer Pathways. Ethylene

polymerization experiments were conducted as a function of

ethylene pressure using (CF₃)FI-Ni to probe the operative

chain transfer and propagation pathways in this system. Over

a

Table 1. Ethylene Polymerization Data for Fluorocarbon-Substituted and Fluorine-Free Catalysts

d

e

f

entry

cat.

temp (°C)

polymer yield (g)

M_w(10³g mol⁻¹

)

PDI

total Me/1000C

activity

b

1

2

3

(CH₃)FI-Ni

23

0.253

1.661

0.419

1.4

1.1

3.0

88

7

38

250

7.9

b

(CF₃)FI-Ni

92.0

c

(CN_CF3)FI-Ni

C4−C26

b

4

5

(CH₃)FI-Ni

50

0.047

3.042

0.95

7.9

1.5

2.2

109

40

7.1

b

(CF₃)FI-Ni

460

a

Polymerizations carried out at 8.0 atm of ethylene pressure for 10.0 min using 2.0 equiv of Ni(COD)₂. Polymerizations were performed in

b

c

duplicate. Polymerizations with 5 μmol of catalyst for 10 min in 50 mL of toluene. Polymerizations with 10 μmol of catalyst for 40 min in 25 mL of

d

e

f

toluene. GPC vs polystyrene standards. ¹H NMR assay. In units of kg of polymer (mol of Ni)⁻¹h⁻¹atm⁻¹.

I

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the ethylene pressures studied, polymerizations are approx-

imately ﬁrst-order in [ethylene], the polymer M_winitially

increases then saturates with increasing [ethylene], and

polyethylene branch densities decrease with increasing [ethyl-

ene] (Table 2, Figure 7). These observations are in accord with

a scenario as in Scheme 4 (eq 1), where increasing M_wwith

increasing [ethylene] implicates chain transfer to the metal

(β-hydride elimination) as the predominant chain transfer

pathway,⁴⁸rather than chain transfer to monomer, which

should ideally yield a constant M_wwith increasing [ethylene]

(Scheme 4 (eq 3)) because the propagation/chain transfer

branch ratio is independent of [ethylene].⁴⁹Depending on the

details of the relative rate constants, TOF vs [ethylene] plots

may saturate at very high [ethylene], obeying essentially

Michaelis−Menten kinetics, as observed for related neutrally

charged Ni(II) catalysts at ethylene pressures >30 bar.^11b,40

This behavior and parallel saturation in product M_wmay also

reﬂect high-M_wpolyethylene precipitation, which is observed at

elevated ethylene pressures (>5.0 atm) and would inhibit

monomer diﬀusion to the catalyst. Additionally, at higher

ethylene pressures, polyethylene branching is expected to fall

due to ethylene interception of intermediate agostic species

such as III, thereby suppressing chain transfer and favoring

propagation via species such as II (Scheme 4, eqs 1 and 2).^4b,50

As noted above, installation of CF₃substituents in (CF₃)FI-

Ni results in ∼6.5× greater activity than for the CH₃-

substituted analogue (CH₃)FI-Ni (250 vs 38 kg of polymer

(mol of Ni)⁻¹h⁻¹atm⁻¹, respectively), with the resulting

polyethylene displaying dramatically greater M_w(92 000 vs

1400 g mol⁻¹, respectively), and far lower alkyl branch densities

(7 vs 88 branches/1000C, respectively). Since chain-walking

processes are typically accompanied by slow ethylene insertion

into secondary Ni−alkyl linkages,^6fsuppression of β-H

elimination/chain walking in (CF₃)FI-Ni would reasonably

enhance activity and M_wvs (CH₃)FI-Ni, which undergoes

signiﬁcantly greater competing chain walking. In addition to the

increased M_w’s and depressed branching, 50 °C ethylene

polymerization experiments reveal that CF₃-substituted

(CF₃)FI-Ni displays substantially greater thermal stability

than CH₃-substituted (CH₃)FI-Ni, judging from the marked

fall in activity for (CH₃)FI-Ni at 50 °C (Table 1). The deactivation

Table 2. Ethylene Polymerization Data as a Function of

a

Ethylene Pressure Utilizing (CF₃)FI-Ni

b

c

pressure

TOF

polymer

yield (g)

M_w

total Me/

c

entry (atm) (10³h⁻¹

)

(10³g mol⁻¹

)

PDI 1000C

1

2

3

4

1.0

3.0

5.0

8.0

4.6

24

0.107

0.572

1.247

1.632

40

72

81

92

2.6

3.5

3.0

27

13

11

7

53

70

a

Polymerizations carried out in 25 mL of toluene at 8.0 atm ethylene

pressure at 50 °C with 2.0 equiv of Ni(COD)₂. Polymerizations were

b

carried out with 5.0 μmol of catalyst. TOF = mol of C₂H₄(mol of

c

1

Ni)⁻¹h⁻¹. By H NMR assay.

Figure 7. Ethylene pressure dependence of (CF₃)FI-Ni-mediated

ethylene polymerization.

Scheme 4. Chain Transfer Scenarios and M_wDependence on [ethylene]

J

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pathways for these types of catalysts are known to be triggered

by β-H elimination to form unstable Ni hydrides.⁵¹In the

present case, ligand triﬂuoromethylation kinetically retards β-H

elimination, leading to enhanced thermal stability. To further

investigate the scope of CF₃substitution eﬀects on the present

in (CF₃)FI-Ni is signiﬁcantly longer than the corresponding

bond distances in (CH₃)FI-Ni and (CN_CF)FI-Ni, there is

3

no obvious trend in the Ni−CH₃bond lengths to support

major inductive eﬀects. It is also unlikely that steric eﬀects

alone are responsible for the marked polymerization

diﬀerences between (CH₃)FI-Ni and (CF₃)FI-Ni, since the

van der Waals radii of F and H are similar (1.47 and 1.10 Å,

respectively).⁴⁴

catalytic behavior, the complex (CN_CF)FI-Ni was designed

3

with a CF₃group located in closer proximity to the Ni center. In

direct contrast to the eﬀects observed in the relative properties of

(CH₃)FI-Ni and (CF₃)FI-Ni, (CN_CF)FI-Ni only catalyzes

C−H···F−C Interactions. Considering the lack of evidence

for CF₃-derived inductive eﬀects and the large Ni−terphenyl

substituent distances seen in the structural data, the possibility

that weak C−F···H−C interactions^16,17,41may disfavor chain

transfer in (CF₃)FI-Ni by destabilizing the syn-periplanar

conformation generally thought to facilitate β-H elimination⁵³

(e.g., Scheme 5) is considered as a reasonable alternative. For

catalyst (CH₃)FI-Ni, all other factors being equal, the absence

of such interactions would favor the correct conformation for

elimination. The pathway depicted in Scheme 5 is consistent

with observations (vide supra) that the predominant chain

transfer pathway in these catalysts is β-H elimination (chain

transfer to metal). To probe whether non-negligible C−

F···H_β−C_βinteractions might be operative in (CF₃)FI-Ni

catalysis, the model ethyl complex (CF₃)FI-Ni-Et was syn-

thesized from the Ni−Cl precursor (CF₃)FI-Ni-Cl (Scheme 3).

(CF₃)FI-Ni-Et was characterized by NMR, elemental

analysis, and X-ray diﬀraction (Figure 4).⁴²The closest C−

F···H−C(2) contact in the solid state (i.e., ground state) is

3.69 Å, longer than the sum of the H and F van der Waals radii

(2.57 Å).⁴⁴To probe the presence of such interactions in

3

ethylene oligomerization with low activity.⁵²If simple inductive

eﬀects were responsible for the substantial M_wand activity increases

in (CF₃)FI-Ni, these eﬀects might reasonably be observed or

enhanced in (CN_CF)FI-Ni.

3

Considering the dramatic eﬀects that the introduction of

electron-withdrawing CF₃groups have on the ethylene

polymerization properties of the present catalysts, the possible

contribution of electronic eﬀects was investigated. Non-

negligible distal CF₃ligand substituent eﬀects, among other

eﬀects, on group 10 mediated polymerizations and the resulting

polymer structures were noted previously and ascribed to

electronic/inductive eﬀects.^3k,11b,e,51While such eﬀects may be

operative here, the NMR spectral parameters of the present

complexes do not evidence clear substituent-related chemical

shift patterns (Table 3). Thus, NMR spectra exhibit no

Table 3. Electronic Structure Diagnostic NMR Spectral Data

(ppm) in Complexes (CH₃)FI-Ni, (CF₃)FI-Ni, and

(CN_CF)FI-Ni

3

¹H,

Ni−CH₃, δ Ni−CH₃, δ

¹³C,

³¹P,

¹³C,

54

solution, 2D ¹⁹F,¹H HOESY NMR and 2D H,¹H NOESY

NMR experiments were carried out on (CF₃)FI-Ni-Et (Figure 8)

and (CH₃)FI-Ni-Et (Figure S5 of the Supporting Information),

respectively. In the ¹⁹F,¹H HOESY NMR spectrum of (CF₃)FI-

Ni-Et, two ¹⁹F signals corresponding to CF₃groups on each

ring are observed at 25 °C, likely due to hindered rotation

around the N−aryl bond, with free rotation about the aryl C−C

bonds. In addition to the strong intra-ring ¹⁹F···¹H dipolar

contacts between the CF₃groups and ortho and para protons

(Figure 8, bottom), several ¹⁹F···¹H NOEs are observed (Figure

8, middle and top): CF₃(A) interacts with the o-methyl of the

2,4-lutidine ligand (medium), CF₃(B) with H14 of the 2,4-

lutidine ligand (medium-weak), both CF₃(A) and CF₃(B) with

the tert-butyl moiety (medium-weak), and both CF₃(A) and

CF₃(B) with the methyl group of the NiCH₂CH₃moiety. The

fact that CF₃(A) and CF₃(B) show selective interactions with

the o-methyl and H14 of the 2,4-lutidine ligand, respectively,

indicates that rotation of the lutidine about the Ni−N bond is

slow. However, this motion is not fully frozen, since exchange

cross-peaks are observed between H8/H9 and H11/H12 and

1

complex

PMe₃, δ

CN, δ

(CH₃)FI-Ni

(CF₃)FI-Ni

−0.98

−1.33

−1.41

−13.68

−13.48

−11.5

−13.8

−12.6

−14.8

168.45

168.20

169.0

(CN_CF)FI-Ni

3

signiﬁcant diﬀerences in the chemical shifts of the Ni−CH₃

or the imine C signals in (CH₃)FI-Ni and (CF₃)FI-Ni that

might be expected for through-bond electronic communica-

tion. Additionally, there is no discernible pattern in the

1

bound PMe₃³¹P NMR chemical shifts. In the H NMR, the

Ni-CH₃signal exhibits an unexpected upﬁeld shift as

triﬂuoromethylation is introduced. While these observations

do not rule out remote substituent inductive eﬀects on the

polymerization pathway, there is no NMR evidence to

support this. Furthermore, analysis of the diﬀraction-derived

metrical parameters in (CH₃)FI-Ni, (CF₃)FI-Ni, and (C

N

_CF)FI-Ni reveals no systematic evidence of electronic

3

eﬀects at the Ni center in the precatalysts. Selected bond

distances for complexes (CH₃)FI-Ni, (CF₃)FI-Ni, and (C

1

N

_CF)FI-Ni are compared in Table 4. The Ni−O and Ni−N

between H10 and H13 in the H NOESY NMR spectrum

3

recorded in the same solvent at the same temperature. In

contrast, the methyl of the NiCH₂CH₃moiety exhibits

comparable ¹⁹F···¹H NOEs with both CF₃(A) and CF₃(B)

and similar ¹H···¹H NOEs with both o-methyl and H14

hydrogen atoms of the lutidine ligand. This indicates that the

energy barrier associated with rotation of the ethyl group

Table 4. Electronic Structure Diagnostic Bond Distances in

Complexes (CH₃)FI-Ni, (CF₃)FI-Ni, and (CN_CF)FI-Ni

3

complex

Ni−CH₃, Å Ni−O, Å

Ni−N, Å

Ni−P, Å

(CH₃)FI-Ni

(CF₃)FI-Ni

1.937(3)

1.953(2)

1.931(2)

1.917(2)

1.914(1)

1.900(1)

1.937(2)

1.936(2)

1.968(1)

2.1420(9)

2.1496(6)

2.1353(5)

1

around the Ni−C bond is small. In the H,¹H NOESY NMR

(CN_CF)FI-Ni

3

spectrum of (CH₃)FI-Ni-Et, observed NOE interactions are

similar to those of (CF₃)FI-Ni-Et, suggesting the average 3D

structures of the two complexes in solution are similar.

The ¹H NMR chemical shifts in (CF₃)FI-Ni-Et and

(CH₃)FI-Ni-Et (with the exception of those belonging to the

bond distances are very similar in complexes (CH₃)FI-Ni and

(CH₃)FI-Ni and (CN_CF)FI-Ni also exhibit very similar

3

Ni−C bond distances. Although the Ni−CH₃bond distance

K

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Scheme 5. Newman Projections Illustrating How Signiﬁcant (ligand)F···H(alkyl-Ni) Interactions Would Disfavor the

Syn-Periplanar Conformation for β-H Elimination

Figure 9. ¹H NMR chemical shifts (δ) of model compounds (CF₃)FI-

Ni-Et and (CH₃)FI-Ni-Et.

Figure 8. Three sections of the ¹⁹F ¹H HOESY NMR spectrum

(CD₂Cl₂, 298 K) showing ¹⁹F···¹H dipolar interactions in the complex

(CF₃)FI-Ni-Et. The asterisk indicates decomposition product.

terphenyl moiety) are tabulated in Figure 9, and the Δδ values

are graphically compared in Figure 10. The CH₃CH₂nuclei

(“CH” in the table) are diastereotopic in both complexes and

appear as complex multiplets (see Figures S6 and S9 of the

Figure 10. Comparison of ¹H NMR chemical shifts in (CF₃)FI-Ni-Et

and (CH₃)FI-Ni-Et.

1

Supporting Information for 2D H−¹³C HSQC NMR spectra

of (CH₃)FI-Ni-Et and (CF₃)FI-Ni-Et, respectively). Note that

simulation of these second-order spectra (AA′X₃for (CH₃)FI-

Ni-Et and ABM₃for (CF₃)FI-Ni-Et, in which X₃and M₃

represent the three H nuclei of the CH₃group) indicates that it

is not necessary to include scalar coupling with the F nuclei to

reproduce the experimental multiplets (see Figures S3 and S7

of the Supporting Information), due to the ﬂuxional nature of

(CF₃)FI-Ni-Et in solution. In addition, the methylene ¹³C and

methyl ¹³C resonances appear as singlets in the ¹³C{¹H} NMR

spectrum (see Figure S8 of the Supporting Information), also

conﬁrming that is not necessary to include scalar coupling

L

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Chart 5

Figure 11. Spartan PM3/HF ab initio minimization of the polymerization-active catalysts derived from (CF₃)FI-Ni (A, C−F···H_β−C_β= 2.61 Å) and

(CH₃)FI-Ni (B, C−H···H_β−C_β= 4.11 Å) with an n-butyl group modeling the polyethylene chain. Selected distances (Å) between F/H and H_β−C_β:

H−a = 2.61, H−b = 3.11 Å, and H−c = 3.68 in (CF₃)FI-Ni; H−d = 4.11, H−e = 5.10, and H−f = 4.31 in (CH₃)FI-Ni.

between the F and H nuclei. In this regard, non-negligible

through-space ¹H−¹⁹F and ¹³C−¹⁹F scalar coupling is

frequently observed in molecules with rigid structures that

impose short H···F contacts, such as (η⁵-C₅H₅)M(PMe₃)-

Scheme 6. Qualitative Free Energy Proﬁle Showing How

(ligand)F···H(alkyl-Ni) Interactions Favor/Disfavor the Syn-

Periplanar Conformational Pathway for β-H Elimination

(C₆F₅)X (M = Rh, Ir; X = F, Cl, Br; M, Chart 5),⁵⁵

56

(η⁵-C₅H₅)Ir(C₂H₄)(η²-C₆F₆) (N, Chart 5), and Tp^{(CF )}

3

2

-

PtMe₃(O, Chart 5).⁵⁷

From Figures 9 and 10 it is evident that Δδ values are much

greater for some resonances than for others. In particular, the o-

Me and one of the two CH₃CH₂− resonances are signiﬁcantly

more shielded in (CF₃)FI-Ni-Et. The mean Δδ calculated

using all δ data except those of o-Me and one of the two

CH₃CH₂− resonances (green in Figure 10) is −0.008 ppm with

a standard deviation of 0.033 ppm. The Δδ values for o-Me and

one of the two CH₃CH₂− resonances are more than 2 standard

deviations away from the mean. This eﬀect may be due to the

close proximity of these groups to the electronic clouds of the F

atoms, indicating a through-space dipolar interaction.⁵⁸

Inductive eﬀects through the ligand scaﬀold are unlikely to be

responsible for the large o-Me and CH₂Δδ values because (1)

one of the two diastereotopic CH₂resonances is more aﬀected

than the other and (2) the o-Me H nuclei have a much greater

Δδ value than H4, which is much closer (through bond) to the

terphenyl substituents.

average F···H distances in (CF₃)FI-Ni-Et and do not represent

a measure of the shortest possible F···H distance. No signiﬁcant

CF₃interactions involving the NiCH₂CH₃α-hydrogen atoms

are observed in the 2D ¹⁹F,¹H HOESY experiments, and it is

unlikely on the basis of the diﬀraction data that there are

signiﬁcant CF₃···Ni interactions (Chart 1, C; Figure 1). Further

evidence supporting a F···H_βrather than a F···Ni interaction is

the far higher polymerization activity of (CF₃)FI-Ni vs that of

(CH₃)FI-Ni. In previous reports, such F···M interactions

(Chart 1, structure D) are accompanied by depressed

polymerization activity, likely reﬂecting signiﬁcant active site

steric/coordinative electronic constraints.¹⁰

An estimation of relevant average internuclear H···F

distances is obtained from quantitative analysis of the 2D

¹⁹F,¹H HOESY data. With the average H10/CF₃(B)

internuclear distance (3 Å) as a reference distance,⁵⁹the

following average distances can be calculated from the relative

cross-peak integrals, taking into account the number of

equivalent nuclei:⁶⁰⟨r⟩_{NiCH CH −CF (A)}= 5.2 Å; ⟨r⟩_{NiCH CH −CF (B)}

=

2

3

2

3

5.3 Å; ⟨r⟩

= 4.1 Å. Note that these distances are

o‑methyl−CF₃(A)

M

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Molecular modeling³⁴performed on the assumed polymer-

ization-active catalytic species derived from (CF₃)FI-Ni with an

n-butyl group to simulate the propagating polyethylene chain

reveals that C_β−H_β···F(C) distances in such species can be as

small as ∼2.61 Å (Figure 11A).⁴²The exact same calculation

performed on (CH₃)FI-Ni yields a much longer H···H distance

of 4.11 Å (Figure 11B).

AUTHOR INFORMATION

Corresponding Author

*Tel (+1) 847 491 5658 (T.J.M.). Fax (+1) 847 491 2990

(T.J.M.). E-mail: m-delferro@northwestern.edu (M.D.);

t-marks@nortwestern.edu (T.J.M.).

■

Notes

The authors declare no competing ﬁnancial interest.

Many experimental and theoretical studies have described

C−F···H−C interactions as a type of hydrogen bond.^41,61

A

ACKNOWLEDGMENTS

■

theoretical study of the hydrogen bonding in CH₃F···H₄C

computed an interaction energy of −1.62 kJ/mol in the gas

phase.^41cC−F···H−C hydrogen bonding has been shown to

stabilize the solid-state structures of ﬂuorobenzenes with

experimental F···H distances in the range 2.41−2.86 Å.⁶²Also,

rotational spectroscopy performed on CH₃F···HCF₃provided

detailed information on the chemical features of the C−F···H−C

Financial support by the DOE (Grant 86ER13511) is gratefully

acknowledged. We thank Dr. C. Weiss, Ms. J. McInnis, Mr.

S. Wobser, and Dr. A. Spokoyny for helpful discussions.

REFERENCES

■

(1) For recent reviews of group 10 oleﬁn polymerization catalysis,

see: (a) Takeuchi, D. Dalton Trans. 2010, 39, 311−328. (b) Nakamura,

A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109, 5215−5244. (c) Chen, E.

Y. X. Chem. Rev. 2009, 109, 5157−5214. (d) Domski, G. J.; Rose, J.

M.; Coates, G. W.; Bolig, A. D.; Brookhart, M. Prog. Polym. Sci. 2007,

32, 30−92. (e) Zhang, J.; Wang, X.; Jin, G. X. Coord. Chem. Rev. 2006,

250, 95−109. (f) Durand, J.; Milani, B. Coord. Chem. Rev. 2006, 250,

542−560. (g) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103,

283−316. (h) Mecking, S. Coord. Chem. Rev. 2000, 203, 325−351.

(i) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100,

1169−1204. (j) Boﬀa, L. S.; Novak, B. M. Chem. Rev. 2000, 100,

1479−1493. (k) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew.

Chem., Int. Ed. 1999, 38, 428−447.

hydrogen bond and the internal dynamics of the pair.⁶¹

b

Furthermore, as exempliﬁed by structures K and L above,

C−H···F−C interactions have been proposed to inﬂuence

enantioselectivity in reactions involving formyl groups, e.g. aldol

condensation,^23b−eand to stabilize ammonium salts in oleﬁn

epoxidation catalysts.^23aAdditionally, as noted above, in the

present crystal structures of (CF₃)FI-Ni and (CF₃)FI-Ni-Et, the

solid-state packing is dominated by short intermolecular

C−H···F−C hydrogen bonds with H···F distances of 2.52 and

2.41 Å, respectively.

(2) (a) Guironnet, D.; Friedberger, T.; Mecking, S. Dalton Trans.

2009, 41, 8929−8934. (b) Guironnet, D.; Goettker-Schnetmann, I.;

Mecking, S. Macromolecules 2009, 42, 8157−8165. (c) Camacho, D.

H.; Guan, Z. Macromolecules 2005, 38, 2544−2546. (d) Camacho, D.

H.; Salo, E. V.; Ziller, J. W.; Guan, Z. Angew. Chem., Int. Ed. 2004, 43,

1821−1825. (e) Guan, Z. Chem. Eur. J. 2002, 8, 3086−3092.

(f) Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 534−540.

CONCLUSIONS

■

Two phenoxyiminato Ni(II) catalysts where remote CF₃/CH₃

substituents exert dramatically diﬀerent eﬀects on ethylene

polymerization are reported. Remote CF₃substituents enhance

activity, thermal stability, and product M_wand depress

polyethylene branching vs the corresponding CH₃-substituted

catalyst. Rather than invoking inductive eﬀects, we propose that

such CF₃substituents participate in C−F···H−C interactions

involving the growing polyethylene chain, destabilizing the syn-

periplanar conformation which favors β-hydride elimination

(Scheme 5). Scheme 6 illustrates qualitatively how such a

C− F···H−C interaction might disfavor chain transfer via β-H

elimination.^40,49a,bIn the case of methyl-substituted (CH₃)FI-

Ni, the absence of such interactions leads to a slightly more

favorable ΔG^⧧for β-H elimination. The suppression of β-H

elimination also leads to signiﬁcantly enhanced catalyst

(CF₃)FI-Ni thermal stability. Heteronuclear 2D ¹⁹F,¹H

HOESY NMR spectroscopy and molecular modeling of a Ni-

CH₂CH₂CH₂CH₃complex provide evidence that the propagat-

ing polyethylene chain can be in close proximity to the remote

ligand CF₃substituents, thereby disfavoring chain transfer.

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Additionally, the ethylene insertion behavior of (CN_CF)FI-

3

Ni, where the electron-withdrawing CF₃moiety is in close

proximity to the metal center, argues that dominant inductive

eﬀects in (CF₃)FI-Ni would alter the propagation and chain

transfer kinetics in ways opposite to what is observed.

ASSOCIATED CONTENT

■

S

* Supporting Information

Figures, tables, and CIF ﬁles giving catalyst and polymer

characterization data and crystal data for (CH₃)HFI, (CF₃)FI-

Ni, (CF₃)FI-Ni-Et, and (CN_CF)FI-Ni. This material is

3

available free of charge via the Internet at http://pubs.acs.org.

N

dx.doi.org/10.1021/om3002735 | Organometallics XXXX, XXX, XXX−XXX

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Suppression of β-hydride chain transfer in nickel(II)-catalyzed ethylene polymerization via weak fluorocarbon ligand-product interactions

DOI: 10.1021/om3002735

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