Unusual C-C bond cleavage in the formation of amine-bis(phenoxy) group 4 benzyl complexes: Mechanism of formation and application to stereospecific polymerization

Article abstract of DOI:10.1021/om500661y

Group 4 tetrabenzyl compounds MBn₄ (M = Zr, Ti), upon protonolysis with an equimolar amount of the tetradentate amine-tris(phenol) ligand N[(2,4-^tBu₂C₆H₂(CH ₂)OH]₃ in toluene from -30 to 25 °C, unexpectedly lead to amine-bis(phenoxy) dibenzyl complexes, BnCH₂N[(2,4- ^tBu₂C₆H₂(CH₂)O] ₂MBn₂ (M = Zr (1), Ti (2)) in 80% (1) and 75% (2) yields. This reaction involves an apparent cleavage of the >NCH₂-ArOH bond (loss of the phenol in the ligand) and formation of the >NCH ₂-CH₂Bn bond (gain of the benzyl group in the ligand). Structural characterization of 1 by X-ray diffraction analysis confirms that the complex formed is a bis(benzyl) complex of Zr coordinated by a newly derived tridentate amine-bis(phenoxy) ligand arranged in a mer configuration in the solid state. The abstractive activation of 1 and 2 with B(C₆F ₅)₃·THF in CD₂Cl₂ at room temperature generates the corresponding benzyl cations {BnCH₂N[(2,4- ^tBu₂C₆H₂(CH₂)O] ₂MBn(THF)}⁺[BnB(C₆F₅) ₃]^- (M = Zr (3), Ti, (4)). These cationic complexes, along with their analogues derived from (imino)phenoxy tri- and dibenzyl complexes, [(2,6-ⁱPr₂C₆H₃)N=C(3,5- ^tBu₂C₆H₂)O]ZrBn₃ (5) and [2,4-Br₂C₆H₂(O)(6-CH₂(NC ₅H₉))CH₂N=CH(2-adamantyl-4-MeC ₆H₂O)]ZrBn₂ (6), have been found to effectively polymerize the biomass-derived renewable β-methyl-α-methylene- γ-butyrolactone (_βMMBL) at room temperature into the highly stereoregular polymer P_βMMBL with an isotacticity up to 99% mm. A combined experimental and DFT study has yielded a mechanistic pathway for the observed unusual C-C bond cleavage in the present protonolysis reaction between ZrBn₄ and N[(2,4-^tBu₂C ₆H₂(CH₂)OH]₃ for the formation of complex 1, which involves the benzyl radical and the Zr(III) species, resulting from thermal and photochemical decomposition of ZrBn₄, followed by a series of reaction sequences consisting of protonolysis, tautomerization, H-transfer, oxidation, elimination, and radical coupling.

Full text of DOI:10.1021/om500661y

Article
pubs.acs.org/Organometallics
Unusual C−C Bond Cleavage in the Formation of Amine-
Bis(phenoxy) Group 4 Benzyl Complexes: Mechanism of Formation
and Application to Stereospecific Polymerization
Ravikumar R. Gowda,† Lucia Caporaso,*,‡ Luigi Cavallo,§ and Eugene Y.-X. Chen*,†
†Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States
‡Dipartimento di Chimica e Biologia, Universita
̀
di Salerno, Via Giovanni Paolo II, I-84084, Fisciano, Italy
§Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Kaust Catalysis Center,
Thuwal 23955-6900, Saudi Arabia
S
* Supporting Information
ABSTRACT: Group 4 tetrabenzyl compounds MBn4 (M =
Zr, Ti), upon protonolysis with an equimolar amount of the
t e t r a d e n t a t e a m i n e - t r i s ( p h e n o l ) l i g a n d N -
[(2,4-tBu2C6H2(CH2)OH]3 in toluene from −30 to 25 °C,
unexpectedly lead to amine-bis(phenoxy) dibenzyl complexes,
BnCH2N[(2,4-tBu2C6H2(CH2)O]2MBn2 (M = Zr (1), Ti (2))
in 80% (1) and 75% (2) yields. This reaction involves an
apparent cleavage of the >NCH2−ArOH bond (loss of the
phenol in the ligand) and formation of the >NCH2−CH2Bn bond (gain of the benzyl group in the ligand). Structural
characterization of 1 by X-ray diffraction analysis confirms that the complex formed is a bis(benzyl) complex of Zr coordinated
by a newly derived tridentate amine-bis(phenoxy) ligand arranged in a mer configuration in the solid state. The abstractive
activation of 1 and 2 with B(C6F5)3·THF in CD2Cl2 at room temperature generates the corresponding benzyl cations
{BnCH2N[(2,4-tBu2C6H2(CH2)O]2MBn(THF)}+[BnB(C6F5)3]− (M = Zr (3), Ti, (4)). These cationic complexes, along with
their analogues derived from (imino)phenoxy tri- and dibenzyl complexes, [(2,6-iPr2C6H3)NC(3,5-tBu2C6H2)O]ZrBn3 (5)
and [2,4-Br2C6H2(O)(6-CH2(NC5H9))CH2NCH(2-adamantyl-4-MeC6H2O)]ZrBn2 (6), have been found to effectively
polymerize the biomass-derived renewable β-methyl-α-methylene-γ-butyrolactone (βMMBL) at room temperature into the
highly stereoregular polymer PβMMBL with an isotacticity up to 99% mm. A combined experimental and DFT study has yielded
a mechanistic pathway for the observed unusual C−C bond cleavage in the present protonolysis reaction between ZrBn4 and
N[(2,4-tBu2C6H2(CH2)OH]3 for the formation of complex 1, which involves the benzyl radical and the Zr(III) species, resulting
from thermal and photochemical decomposition of ZrBn4, followed by a series of reaction sequences consisting of protonolysis,
tautomerization, H-transfer, oxidation, elimination, and radical coupling.
scratching.11 Two other monomers well applied in this facet
of study are γ-methyl and β-methyl derivatives of MBL, γ-
methyl-α-methylene-γ-butyrolactone (γMMBL) and β-methyl-
α-methylene-γ-butyrolactone (βMMBL). A two-step process
INTRODUCTION
■
Efforts in exploring naturally occurring or biomass-derived
renewable resources to replace dwindling petroleum-based raw
materials in large commodity markets, such as chemicals,
from the biomass-derived levulinic acid was developed in the
materials, and fuels, have been expedited in recent years.1−6 In
synthesis of γMMBL.12,13 βMMBL was traditionally prepared by
the context of developing sustainable polymeric materials based
on renewable monomers, a large number of compounds
a multistep process using petrochemicals,14−16 but a “sustain-
able” route from the biomass-derived itaconic acid has recently
comprising an α-methylene-γ-butyrolactone moiety, which have
been developed.17 Many polymerization methods have been
been found and extracted from multifarious plants,7 are of
utilized in the polymerization of MBL into low- to high-
potential interest. Tulipaline A, or α-methylene-γ-butyrolactone
(MBL), found naturally in tulips or produced chemically from
biomass feedstocks, is the simplest and the most explored
molecular-weight polymers, through radical,9,18−23 anionic,9
group-transfer,24 and coordination25 polymerization processes.
γMMBL has also been polymerized by free-radical emulsion
member of the naturally occurring sesquiterpene lactone
polymerization10,26 as well as radical, anionic, and group-
transfer polymerization methods.27 A number of highly active
catalyst systems have recently been developed for living and/or
family.8 Poly(α-methylene-γ-butyrolactone) (PMBL) exhibits
attractive physical properties, such as a high glass-transition
temperature (Tg) of 195 °C,9 good durability, and a high
refractive index of 1.540.10 The copolymers and blends
composed of MBL units render promising optical properties
as well as resistance to solvent, weathering, heat, and
Received: June 20, 2014
© XXXX American Chemical Society
A
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Article
high-speed polymerization of MMBL.28−34 Similarly, radical
to effectively polymerize βMMBL at room temperature into the
γ
highly stereoregular polymer PβMMBL.
polymerization of βMMBL led to formation of the correspond-
ing atactic polymer,14 but utilizing the protocol developed for
the metal-catalyzed coordination polymerization of polar vinyl
monomers,35 βMMBL has been converted into highly isotactic
or even stereodefect-free polymer PβMMBL.15,16,36
EXPERIMENTAL SECTION
■
Materials, Reagents, and Methods. All syntheses and
manipulations of air- and moisture-sensitive reagents and materials
were carried out in flamed Schlenk-type glassware on a dual-manifold
Schlenk line, on a high-vacuum line, or in an inert-gas-filled (Ar or N2)
glovebox. NMR-scale reactions were conducted in Teflon-valve-sealed
J. Young type NMR tubes. HPLC-grade organic solvents were first
sparged extensively with N2 during filling of 20 L solvent reservoirs
and then dried by passage through activated alumina (for Et2O, THF,
and CH2Cl2) followed by passage through Q-5 supported copper
catalyst (for toluene and hexanes) stainless steel columns. Benzene-d6
and toluene-d8 were dried over sodium/potassium alloy and vacuum-
distilled or filtered, whereas CD2Cl2 and CDCl3 were dried over
activated Davison 4 Å molecular sieves. NMR spectra were recorded
Group 4 nonmetallocene complexes, upon appropriate
activation, have been extensively utilized as catalysts for α-
olefin polymerization.37−43 Notable examples of such catalyst
systems include 2,2′-ethylenebis[N,N′-(triisopropylsilyl)-
anilinido]zirconium complexes (for living polymerization),44
an amine-bis(phenolate)titanium benzyl complex (for block
copolymerization),45 group 4 complexes supported by
[ONNO]-type bis(o-aminophenolato) ligands,46 bis-
(pentafluorophenylamido)zirconium benzyl complexes,47 steri-
cally hindered chelating phenoxy titanium and zirconium
complexes48 (for living polymerization of 1-hexene),49 a C2-
symmetric amine-bis(phenolate)zirconium benzyl complex (for
isospecific living polymerization of 1-hexene),50 highly active
[ONXO]-type zirconium and hafnium amine-bisphenolate (X
= N, O, S) dibenzyl complexes,51 amine-bis(phenolate)
zirconium dibenzyl complexes,52 titanium, zirconium, and
hafnium complexes of an amine-bis(phenolate) ligand contain-
ing a THF as well as furan side arm donors,53 and group 4
complexes of diamine-bis(phenolates).54 In 2005, Kol and co-
workers reported zirconium dibenzyl complexes of chiral salan
ligands for isospecific polymerization of 1-hexene and 4-methyl-
1-pentene together with cyclopolymerization of 1,5-hexa-
diene,55 group 4 dibenzyl complexes of robust salophan ligands
for 1-hexene polymerization,56 C1-symmetric [ONNO′]-type
salan zirconium complexes for 1-hexene polymerization,57 and
salan zirconium complexes for isospecific polymerization of
vinylcyclohexane.58 More recently, Kol et al. employed titanium
dibenzyl complexes incorporating salalen (i.e., half-salan/half-
salen) ligands for the synthesis of highly isotactic polypropy-
lene.59 Waymouth et al. recently reported group 4 bis-
(phenolate)ether dibenzyl complexes for stereospecific propy-
lene polymerization.60
1
on a Varian Inova 300 (300 MHz, H; 75 MHz, 13C; 282 MHz, 19F),
400, or 500 MHz spectrometer. Chemical shifts for 1H and 13C spectra
were referenced to internal solvent resonances and are reported as
parts per million relative to SiMe4, whereas 19F NMR spectra were
referenced to external CFCl3. Elemental analyses were performed by
Robertson Microlit Laboratories, Madison, NJ.
2,4-Di-tert-butylphenol, hexamethylenetetramine, triethylamine, 3,5-
dibromosalicylaldehyde, paraformaldehyde, and zirconium(IV) chlor-
ide were purchased from Alfa Aesar Chemical Co. Benzylmagnesium
chloride, 2-(aminomethyl)piperidine, 3,5-di-tert-butyl-2-hydroxyben-
zaldehyde, and titanium(IV) chloride were purchased from Sigma-
Aldrich Chemical Co. and used as received. Formaldehyde (36−38%
aqueous solution) was purchased from Mallinckrodt Chemicals.
Methyl methacrylate (MMA) was purchased from Sigma-Aldrich
Chemical Co., and α-methylene-γ-butyrolactone (MBL) and γ-methyl-
α-methylene-γ-butyrolactone (γMMBL) were purchased from TCI
America, while β-methyl-α-methylene-γ-butyrolactone (βMMBL) was
prepared according to literature procedures.17 These monomers were
first degassed and dried over CaH2 overnight, followed by vacuum
distillation; MMA was further purified by titration with neat tri-n-
octylaluminum to a yellow end point and distillation under reduced
pressure. The purified monomers were stored in brown bottles inside a
glovebox freezer at −30 °C. Butylated hydroxytoluene (BHT-H, 2,6-
di-tert-butyl-4-methylphenol) was purchased from Alfa Aesar Chemical
Co. BHT-H was recrystallized from hexanes prior to use. Tris-
(pentafluorophenyl)borane, B(C6F5)3, was obtained as research gift
from Boulder Scientific Co. and was further purified by sublimation.
B(C6F5)3·THF was prepared by addition of THF to a toluene solution
of the borane at ambient temperature, followed by removal of the
volatiles and drying under vacuum.
Tetrabenzyltitanium and tetrabenzylzirconium were prepared
according to published procedures62a and used shortly thereafter.
The ligands 6,6′,6″-nitrilotris(methylene)tris(2,4-di-tert-butylphe-
nol),63 3-adamantyl-2-hydroxy-5-methylbenzaldehyde,64 2-(bromo-
methyl)-4,6-dibromophenol,59 and 2,4-di-tert-butyl-6-((2,6-
diisopropylphenylimino)methyl)phenol ((2,6-iPr2C6H3)NC-
(3,5-tBu2C6H2)OH)65 and [(2,6-iPr2C6H3)NC(3,5-tBu2C6H2)O]-
ZrBn3 (5),61 [(2,6-iPr2C6H3)NC(3,5-tBu2C6H2)O]TiBn3 (6),61
and [2,4-Br2C6H2(O)(6-CH2(NC5H9))CH2NCH(2-adamantyl-4-
MeC6H2O)]ZrBn2 (7)61 were synthesized according to literature
procedures.
Recently, we extended our interest to the coordination
polymerization of renewable polar vinyl monomers such as
MBL and MMBL to include group 4 achiral (imino)phenoxy-
γ
tribenzyl and chiral amino(imino)bis(phenoxy) dibenzyl
complexes.61 The current work was originally intended to
synthesize tripodal amine-tris(phenoxy) group 4 monobenzyl
complexes through seemingly straightforward protonolysis of
group 4 tetrabenzyl complexes MBn4 (M = Zr, Ti) with the
tetradentate amine-tris(phenol) ligand N[(2,4-tBu2C6H2(CH2)-
OH]3 and subsequently investigate their catalytic properties in
the coordination polymerization of renewable methylene
butyrolactones, particularly βMMBL, as its coordination
polymerization by a non-metallocene catalyst has not been
reported prior to this work. However, through the course of
this study, we discovered an unusual C−C bond cleavage in the
proposed protonolysis reaction, producing the unexpected
amine-bis(phenoxy) dibenzyl complexes in good isolated yields
(75−80%) with concurrent liberation of the coproducts 2,4-di-
tert-butylphenol and toluene. Accordingly, the current study has
focused on the synthesis, characterization, and activation of
such dibenzyl complexes. A combined experimental and DFT
study has yielded a mechanistic pathway toward their formation
involving the benzyl radical and the Zr(III) species.
Furthermore, the generated cationic species have been found
Synthesis and Structural Characterization of BnCH2N-
[(2,4-tBu2C6H2(CH2)O]2ZrBn2 (1). The following reaction was
performed in a glass reactor wrapped with Al foil to minimize
exposure to light. A solution of N[(2,4-tBu2C6H2(CH2)OH]3 (0.50 g,
0.74 mmol) in toluene (10 mL) and a solution of ZrBn4 (0.34 g, 0.74
mmol) in toluene (10 mL) were precooled at −30 °C inside the
glovebox for 6 h. After the two solutions were mixed, a pale orange
color appeared instantly and then disappeared slowly to form a
colorless solution. Stirring continued for 24 h at room temperature,
after which the solvent was removed under reduced pressure to afford
a colorless powder. This powder was further purified by crystallization
B
dx.doi.org/10.1021/om500661y | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
from toluene at −30 °C to afford 1 as a colorless solid (0.49 g, 80%).
Anal. Calcd for C52H67NO2Zr·C7H8: C, 76.90; H, 8.20; N, 1.52.
Found: C, 77.18; H, 8.78; N, 2.11.
75%). Anal. Calcd for C52H67NO2Ti: C, 79.46; H, 8.59; N, 1.78.
Found: C, 79.09; H, 8.45; N, 1.74.
1H NMR (C6D6, 23 °C): δ 7.73 (s, 2H, Ar-H), 7.25 (d, J = 7.4 Hz,
3H, Ar-H), 7.13−7.01 (m, 7H, Ar-H, C7H8), 6.89−6.83 (m, 4H, Ar-
H), 6.74 (t, J = 7.5 Hz, 2H, Ar-H), 6.62−6.58 (m, 1H, Ar-H), 6.43−
6.41 (m, 2H, Ar-H), 6.33 (t, J = 7.7 Hz, 2H, Ar-H), 5.92 (t, J = 7.3 Hz,
1H, Ar-H), 3.79 (s, 2H, Ar-CH2), 3.63 (s, 2H, Ar-CH2), 2.90−2.74 (m,
6H, Ar-CH2-Ti, −CH2N), 2.12 (s, C7H8), 2.07 (s, 20H, Ar-C(CH3)3,
−CH2N), 1.43 (s, 18H, Ar-C(CH3)3). 13C NMR (C6D6, 23 °C): δ
160.1 (Ar-O-Ti), 143.6 (Ar-CH2-Ti), 143.4 (Ar-C(CH3)3), 141.8 (Ar-
C(CH3)3), 137.6 (C7H8), 135.6 (Ar-C), 130.1 (Ar-C), 129.3 (C7H8),
129.0 (Ar-C), 128.5 (C7H8), 128.4 (Ar-C), 128.3 (Ar-C), 127.7 (Ar-C),
126.3 (Ar-C), 125.7 (C7H8), 124.8 (Ar-C), 124.8 (Ar-C), 124.5 (Ar-C),
123.4 (Ar-C), 123.0 (Ar-C), 85.8 (1JC−H = 134.5 Hz, η2, Ar-CH2-Ti),
84.1 (1JC−H = 130.9 Hz, η1, Ar-CH2-Ti), 59.4 (−CH2N), 46.4
(−CH2N), 35.9 (Ar-C(CH3)3), 34.5 (Ar-C(CH3)3), 32.0 (Ar-
C(CH3)3), 31.9 (Ar-C(CH3)3), 26.5 (Ar-CH2CH2N), 21.5 (C7H8).
In Situ Generation of {BnCH2N[(2,4-tBu2C6H2(CH2)O]2ZrBn-
(THF)}+[BnB(C6F5)3]− (3). Cationic species 3 was generated by in situ
mixing of 1 and B(C6F5)3·THF in CD2Cl2 at ambient temperature,
following the procedure already established for the clean and
quantitative generation of the analogous cationic species
[((2,6-iPr2C6H3)NC(3,5-tBu2C6H2)O)ZrBn2(THF)]+[BnB-
(C6F5)3]−.61 In an argon-filled glovebox, a 4 mL glass vial was charged
with 5.00 mg (6.00 μmol) of 1 and 0.4 mL of CD2Cl2, while another
vial was charged with 3.50 mg of B(C6F5)3·THF (6.00 μmol) and 0.4
mL of CD2Cl2. The two vials were mixed via pipet at ambient
temperature to give instantaneously a pale yellow solution, and
subsequent NMR analysis showed the clean and quantitative
formation of ion pair 3. (Note that NMR analysis must be finished
within 15 min, as cation 3 starts to decompose at ambient
temperature, as indicated by a color change from pale yellow to
colorless after that period.)
1H NMR (C6D6, 23 °C): δ 7.64−7.61 (m, 5H, Ar-H), 7.16−6.65
(m, 17H, Ar-H, C7H8), 6.48−6.46 (m, 2H, Ar-H), 3.40 (d, J = 12 Hz,
2H, Ar-CH2), 3.07 (d, J = 12 Hz, 2H, Ar-CH2), 2.82 (s, 2H, CH2-N),
2.45−2.29 (m, 4H, Ar-CH2-Zr), 2.11 (s, C7H8), 2.02 (s, 2H,
NCH2CH2Ph), 1.75 (s, 18H, Ar-C(CH3)3), 1.43 (s, 18H, Ar-
C(CH3)3). 13C NMR (C6D6, 23 °C): δ 157.9 (Ar-O-Zr), 147.2 (Ar-
CH2-Zr), 141.4 (Ar-C(CH3)3), 137.9 (C7H8), 136.9 (Ar-C(CH3)3),
136.4 (Ar-C), 130.5 (Ar-C), 129.3 (C7H8), 129.0 (Ar-C), 128.9 (Ar-
C), 128.7 (Ar-C), 128.6 (Ar-C), 128.5 (C7H8), 126.7 (Ar-C), 125.7
(C7H8), 125.1 (Ar-C), 124.9 (Ar-C), 124.7 (Ar-C), 124.2 (Ar-C),
122.2 (Ar-C), 60.4 (1JC−H = 128.8 Hz, η1, Ar-CH2-Zr), 60.0 (1JC−H
=
137.7 Hz, η2, Ar-CH2-Zr), 59.0 (−CH2N), 45.7 (−CH2N), 35.5 (Ar-
C(CH3)3), 34.5 (Ar-C(CH3)3), 32.0 (Ar-C(CH3)3), 30.6 (Ar-
C(CH3)3), 27.2 (Ar-CH2CH2N), 21.4 (C7H8).
The molecular structure of 1 has been confirmed by single-crystal
X-ray diffraction analysis. A 20 mL glass vial was charged with 10 mg
(0.012 mmol) of 1 and 3 mL of toluene. The solution was carefully
layered with 4 mL of hexanes. The vial was cooled to −30 °C and
stored inside the glovebox freezer for 7 days to afford colorless single
crystals of 1. After the solvent was decanted, the crystals were quickly
coated with a layer of Paratone-N oil (Exxon, dried and degassed at
140 °C/10−6 Torr for 16 h) in the glovebox. A crystal was then
mounted on a thin glass fiber under a cold stream of dinitrogen gas.
Single-crystal X-ray diffraction data were acquired on a Bruker Kappa
APEX II CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å)
and a graphite monochromator. Initial lattice parameters were
obtained from a least-squares analysis of more than 100 reflections;
these parameters were later refined against all data. The crystal did not
show any significant decay during data collection. Data were integrated
and corrected for Lorentz and polarization effects using Bruker APEX2
software, and semiempirical absorption corrections were applied using
SCALE.66a Space group assignments were based on systematic
absences, E statistics, and successful refinement of the structure. The
structure was solved by the Patterson method and refined with the aid
of successive Fourier difference maps against all data using the
SHELXTL 6.14 software package.66b Thermal parameters for all non-
hydrogen atoms were refined anisotropically, while all hydrogen atoms
were assigned to ideal positions and refined using a riding model with
an isotropic thermal parameter 1.2 times that of the attached carbon
atom (1.5 times for methyl hydrogens). Selected bond distances and
angles for compound 1 are collected in the caption of Figure 2. All
other metric parameters can be found in the cif file included with the
Supporting Information. In the structure the disordered solvent
molecule had been found in Fourier difference maps to be disordered
over multiple sites. After numerous attempts to model the disorder
failed to improve agreement factors, SQUEEZE66c was implemented
to remove the electron density for the disordered solvent. Selected
1H NMR (CD2Cl2,, 23 °C): δ 7.71−7.67 (m, 1H, Ar-H), 7.57−7.50
(m, 3H, Ar-H), 7.46−7.33 (m, 4H, Ar-H), 7.27−7.11 (m, 6H, Ar-H,
C7H8), 7.07−7.01 (m, 3H, Ar-H), 6.88−6.84 (m, 2H, Ar-H), 6.79−
6.73 (m, 4H, Ar-H), 6.46−6.43 (m, 1H, Ar-H), 4.50−4.44 (m, 2H, Ar-
CH2NCH2-Ar), 4.04−3.97 (m, 2H, Ar-CH2NCH2-Ar), 3.73 (bs, 2H,
α-CH2 of THF), 3.54 (bs, 2H, α-CH2 of THF), 3.38 (s, 2H,
NCH2CH2Ph), 2.86−2.70 (bs, 4H, Ar-CH2-B, Ar-CH2-Zr), 2.34 (s,
3H, C7H8), 1.81 (bs, 4H, β-CH2 of THF), 1.56 (s, 9H, Ar-C(CH3)3),
1.49 (s, 2H, NCH2CH2Ph), 1.44 (s, 9H, Ar-C(CH3)3), 1.38 (s, 9H, Ar-
C(CH3)3), 1.35 (s, 9H, Ar-C(CH3)3). 19F NMR (CD2Cl2, 23 °C): δ
−131.4 (m, 6F, o-F), −164.9 (t, JF−F = 20.4 Hz, 3F, p-F), −167.7 (m,
6F, m-F).
In Situ Generation of {BnCH2N[(2,4-tBu2C6H2(CH2)O]2TiBn-
(THF)}+[BnB(C6F5)3]− (4). Cationic species 4 was generated by in situ
mixing of 2 and B(C6F5)3·THF in CD2Cl2 at ambient temperature,
following the procedure already established for the clean and
quantitative generation of the analogous cationic species
[((2,6-iPr2C6H3)NC(3,5-tBu2C6H2)O)ZrBn2(THF)]+[BnB-
(C6F5)3]−.61 In an argon-filled glovebox, a 4 mL glass vial was charged
with 10 mg (0.013 mmol) of 2 and 0.4 mL of CD2Cl2, while another
vial was charged with 7.60 mg of B(C6F5)3·THF (0.013 mmol) and 0.4
mL of CD2Cl2. The two vials were mixed via pipet at ambient
temperature to give instantaneously a dark red solution, and
subsequent NMR analysis showed the formation of ion pair 4,
accompanied by the decomposition products formed through the C6F5
ligand transfer from the borate anion to the Ti cation (see Results and
Discussion).
crystallographic data for 1: C H NO Zr, triclinic, space group P1, a =
̅
59 75
2
11.9094(12) Å, b = 16.5817(16) Å, c = 16.6031(16) Å, α =
108.638(4)°, β = 107.622(4)°, γ = 94.550(4)°, V = 2904.2(5) Å3, Z =
2, Dcalcd = 1.054 Mg/m3, GOF = 1.077, R1 = 0.0567 [I > 2σ(I)], wR2
= 0.1504. CCDC-999630 contains the supplementary crystallographic
data. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
Synthesis of BnCH2N[(2,4-tBu2C6H2(CH2)O]2TiBn2 (2). The
following reaction was performed in a glass reactor wrapped with Al
1H NMR (CD2Cl2, 23 °C): δ 7.58 (s, 1H, Ar-H), 7.54−7.44 (m,
4H, Ar-H), 7.37 (bs, 3H, Ar-H), 7.29−7.09 (m, 6H, Ar-H, C7H8),
7.06−6.95 (m, 3H, Ar-H), 6.88−6.85 (m, 2H, Ar-H), 6.79−6.73 (m,
4H, Ar-H), 6.46−6.38 (m, 1H, Ar-H), 4.25 (s, 1H, Ar-CH2NCH2-Ar),
3.95 (s, 1H, Ar-CH2NCH2-Ar), 3.79−3.64 (m, 4H, α-CH2 of THF),
3.45−3.39 (m, 1H, Ar-CH2NCH2-Ar), 3.18−3.11 (m, 1H, Ar-
CH2NCH2-Ar), 2.92 (NCH2CH2Ph), 2.85−2.75 (bs, 4H, Ar-CH2-B,
Ar-CH2-Ti), 2.34 (s, 3H, C7H8), 2.04−1.86 (bs, 4H, β-CH2 of THF),
1.78 (s, 9H, Ar-C(CH3)3), 1.64 (s, 9H, Ar-C(CH3)3), 1.56 (s, 2H,
NCH2CH2Ph), 1.39 (s, 9H, Ar-C(CH3)3), 1.37 (s, 9H, Ar-C(CH3)3).
foil to minimize exposure to light.
A solution of N-
[(2,4-tBu2C6H2(CH2)OH]3 (0.50 g, 0.74 mmol) in toluene (10 mL)
and a solution of TiBn4 (0.31 g, 0.74 mmol) in toluene (10 mL) were
precooled at −30 °C inside the glovebox for 6 h. After the solutions
were mixed, the red color appeared instantly. Stirring was continued
for 24 h at room temperature, after which the solvent was removed
under reduced pressure to afford a yellowish brown residue. This
residue was further purified by crystallization from toluene layered
with hexanes at −30 °C to afford 2 as a yellowish orange solid (0.44 g,
C
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Scheme 1. Protonolysis of MBn4 with N[(2,4-tBu2C6H2(CH2)OH]3 Leading to Formation of the Unexpected C−C Bond
Cleavage Product
19F NMR (CD2Cl2, 23 °C): δ −131.3 (m, 6F, o-F), −164.9 (t, JF−F
20.4 Hz, 3F, p-F), −167.7 (m, 6F, m-F).
=
with a polarization function of Ahlrichs and co-workers for H, C, N,
and O atoms (SVP keyword in Gaussian)71 while the quasi-relativistic
small-core Stuttgart ECP with the associated triple-ζ valence basis set
was used for zirconium (SDD keyword in Gaussian09).72 The
reported energies have been obtained via single-point energy
calculations with the BP86 and M06 functional with the triple-ζ
basis set of Ahlrichs for H, C, N, and O (TZVP keyword in
Gaussian09). Solvent (toluene) effects were included with the default
Gaussian PCM implementation.73 Thermal corrections from gas-phase
frequency analysis, performed with the SVP basis set on the optimized
geometries, were added to this in solvent energy to obtain the free
energies.
General Polymerization Procedures. Polymerizations were
performed in 30 mL glass reactors in the glovebox in toluene or
CH2Cl2 at ambient temperature, following the procedure established
for the polymerization of methacrylates and acrylamides by metal-
locene precatalysts.67 In a typical in-reactor activation polymerization
procedure, the activator B(C6F5)3 (13.2 μmol) and 200 equiv (2.64
mmol) of MMBL (or MMBL and MBL) were premixed in 3 mL of
β
γ
toluene or CH2Cl2 as indicated in the table. The polymerization was
timed immediately after addition of a precatalyst. The preactivation
method (i.e., premixing the neutral complex with an activator to
generate the corresponding cationic catalyst, followed by addition of
monomer to start the polymerization) was also adopted in some runs
for comparison. Similar procedures were followed in the case of MMA
polymerization. After the measured time interval, a 0.2 mL aliquot was
taken from the reaction mixture via syringe and quickly quenched into
a 4 mL vial containing 0.6 mL of undried “wet” CDCl3 stabilized by
250 ppm of BHT-H; the quenched aliquots were later analyzed by 1H
NMR to obtain monomer conversion data. The polymerization was
immediately quenched after the removal of the aliquot by adding 5 mL
of 5% HCl-acidified methanol. The quenched mixture was precipitated
into 100 mL of methanol, stirred for 3 h, filtered, and washed with
methanol. The polymer collected was dried in a vacuum oven at 50 °C
overnight to a constant weight.
Polymer Characterizations. Polymer number-average molecular
weights (Mn) and molecular weight distributions (MWD = Mw/Mn)
were measured by gel permeation chromatography (GPC) analyses
carried out at 40 °C and a flow rate of 1.0 mL/min, with DMF (for
PMBL and PγMMBL samples) as the eluent, on a Waters University
1500 GPC instrument coupled with a Waters RI detector and
equipped with four PLgel 5 μm mixed-C columns (Polymer
Laboratories; linear range of molecular weight 200−2000000). The
instrument was calibrated with 10 PMMA standards, and chromato-
grams were processed with Waters Empower software. 13C NMR
spectra for the analysis of PMBL,9,24,28 PγMMBL,27,28 and
PβMMBL16,15,36 microstructures were recorded and analyzed accord-
ing to the cited literature methods.
RESULTS AND DISCUSSION
■
C−C Bond Cleavage in Protonolysis of MBn4 with
N[(2,4-tBu2C6H2(CH2)OH]3. It is well established that
protonolysis of group 4 tetrakis(alkoxy) compounds M(OR)4
with equimolar tetradentate amine-tris(phenol) ligands, such as
N[(2,4-tBu2C6H2(CH2)OH]3, leads to the corresponding
tripodal amine-tris(phenoxy) group 4 alkoxides N-
[(2,4-tBu2C6H2(CH2)O]3M(OR), after concomitant elimina-
tion of 3 equiv of ROH.63,74 In sharp contrast, protonolysis of
group 4 tetrabenzyl complexes MBn4 (M = Zr, Ti) with such a
ligand in toluene from −30 to 25 °C unexpectedly afforded
amine-bis(phenoxy) dibenzyl complexes 1 (M = Zr) and 2 (M
= Ti) with concurrent liberation of the coproducts 2,4-di-tert-
butylphenol and toluene (Scheme 1). The dibenzyl Zr and Ti
complexes were both isolated in a pure state with good yields of
80% and 75%, respectively. Extended standing of Zr complex 1
at ambient temperature for longer than 48 h resulted in
decomposition, but the compound can be stored inside a
glovebox freezer at −35 °C without detectable decomposition
for 2 months. Ti complex 2 is more sensitive to light, heat, and
air/moisture; extended exposure to light and vacuum at
ambient temperature even in the glovebox showed gradual
decomposition.
Computational Methods. All of the density functional theory
(DFT) calculations were performed using the Gaussian09 package and
followed the procedures described in our prior publications.68 The
BP86 GGA functional of Becke and Perdew was used.69,70 Geometry
optimizations were performed with the standard split-valence basis set
The 1H NMR spectrum of 1 (Figure 1, C6D6) and 2 showed
that the benzylic CH2 protons are diastereotopic, appearing as
two sets of multiplets at δ 2.45−2.29 (in 1) and 2.90−2.74 (in
2). The chemical shifts revealed that the benzyl CH2 groups in
D
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Figure 1. 1 H NMR (C6 D6 ) spectrum of BnCH2 N-
[(2,4-tBu2C6H2(CH2)O]2ZrBn2 (1).
Figure 2. X-ray crystal structure of BnCH2N[(2,4-tBu2C6H2(CH2)-
O]2ZrBn2 (1) with thermal ellipsoids drawn at the 40% probability
level. Selected bond lengths (Å) and angles (deg): Zr(1)−O(1),
1.989(2); Zr(1)−O(2), 1.996(2); Zr(1)−N(1), 2.377(3); Zr(1)−
C(39), 2.288(4); Zr(1)−C(46), 2.272(3); Zr(1)−C(40), 2.680(3);
C(31)−C(32), 1.532(4), N(1)−Zr(1)−C(39), 124.64(11), C(39)−
Zr(1)−C(46), 118.94(13), C(46)−Zr(1)−N(1), 116.42(11), O(1)−
Zr(1)−C(39), 94.35(11), O(1)−Zr(1)−C(46), 97.31(11), O(1)−
Zr(1)−N(1), 79.17(9), O(2)−Zr(1)−C(39), 94.71(11), O(2)−
Zr(1)−C(46), 96.87(11), O(2)−Zr(1)−N(1), 78.11(9), Zr(1)−
C(39)−C(40), 88.2(2), Zr(1)−C(46)−C(47), 108.5(2); N(1)−
C(31)−C(32), 115.6(3).
2 are slightly deshielded relative to those in 1, which can be
attributed to the weaker Lewis acidic Zr center in comparison
to that of Ti. In addition, 13C NMR spectra of 1 and 2 also
revealed the benzyl −CH2 groups in 2 (δ 85.8 for η2-benzyl
coordination (1JC−H = 134.5 Hz), δ 84.1 for η1-benzyl (1JC−H
=
130.9 Hz) coordination) are considerably more deshielded than
the benzyl −CH2 groups in 1 (δ 60.4 for η1-benzyl coordination
(1JC−H = 128.8 Hz), δ 60.1 for η2-benzyl coordination (1JC−H
=
137.7 Hz))75 (cf. the crystal structure of 1 depicted in Figure
2). The four protons of the two benzyl −CH2 groups in both 1
1
and 2 coalesced as a singlet in variable-temperature H NMR
(toluene-d8) spectra at −20 °C (in 1) and −60 °C (in 2), thus
indicating rapid interconversion of η1 and η2 benzyl
coordination in solution at and above those coalescence
temperatures. The two Ar-CH2 protons in 1 appeared as two
doublets (J = 12 Hz) at δ 3.40 and 3.07 ppm, respectively, while
those in 2 showed two singlets at δ 3.79 and 3.63 ppm,
consistent with a mer mode of coordination76 between the
amine-bis(phenoxyl) ligand and the metal. The resonances for
the Ar-C(CH3)3) group, appearing as two singlets at δ 1.75 and
1.43 ppm in 1 and δ 2.07 and 1.43 ppm in 2, are also indicative
of the mer mode of coordination (four different resonances
would be anticipated for a fac mode of coordination). The rest
of the chemical shifts matched well with the related amine-
bis(phenoxy) zirconium dibenzyl compound.52
Single-crystal X-ray diffraction analysis of 1 reveals that the
Zr center exhibits a pentacoordinate, pseudo-trigonal-bipyr-
amidal geometry (Figure 2), with two axial O atoms and
equatorial [N,C,C] atomsfurnished with η1 binding of one
benzyl group, η2 binding of the other benzyl group, and the
datively bonded amido nitrogenoccupying all the coordina-
tion sites. The Zr−C(benzyl) distances, Zr(1)−C(39) =
2.288(4) Å and Zr(1)−C(46) = 2.272(3) Å, match well with
those reported in the analogous complex [(Pr)N-
(2,4-tBu2C6H2(CH2)O)2]ZrBn2 (2.292(2), 2.272(2) Å).52
The bond distance Zr(1)−N(1) = 2.377(3) Å represents a
classical dative bond between N and Zr. The nonclassical η2-
bonded benzyl group is identified by the distance Zr(1)−C(40)
= 2.680(3) Å and the acute bond angle Zr(1)−C(39)−C(40) =
88.2(2)°, presumably a result of electron deficiency at Zr and
lack of steric crowding around the Zr center. On the other
hand, the classical η1-bonded benzyl group is readily identified
by Zr(1)−C(47) = 3.08 Å and Zr(1)−C(46)−C(47) = 108.5°.
The acute bond angle Zr(1)−C(39)−C(40) and the obtuse
bond angle Zr(1)−C(46)−C(47) show that the phenyl rings of
both benzyl groups tilt toward the Zr center. Furthermore, the
bond angles O(1)−Zr(1)−O(2) = 156.82(9)°, O(1)−Zr(1)−
N(1) = 79.17(9)°, and O(2)−Zr(1)−N(1) = 78.11(9)°
indicate the O atoms in the two phenoxy groups are nearly
trans to each other. The equatorial coordination sites of the
pseudo-trigonal-bipyramidal geometry are characterized by the
obtuse bond angles N(1)−Zr(1)−C(39) = 124.64(11)°,
C(39)−Zr(1)−C(46) = 118.94(13)°, and C(46)−Zr(1)−
N(1) = 116.42(11)°, while the axial coordination sites are
characterized by the bond angles O(1)−Zr(1)−C(39) =
94.35(11)°, O(1)−Zr(1)−C(46) = 97.31(11)°, O(1)−
Zr(1)−N(1) = 79.17(9)°, O(2)−Zr(1)−C(39) = 94.71(11)°,
O(2)−Zr(1)−C(46) = 96.87(11)°, and O(2)−Zr(1)−N(1) =
78.11(9)°, overall confirming the pseudo-trigonal-bipyramidal
Zr center. Moreover, the bis(phenoxy)amine ligand is
coordinated to Zr in a mer fashion,76 with the amido N being
placed at the equatorial position and the two O atoms of the
phenoxy groups being placed at the axial positions, which is
characterized by the angles O(1)−Zr(1)−O(2) = 156.82(9)°,
O(1)−Zr(1)−N(1) = 79.17(9)°, and O(2)−Zr(1)−N(1) =
78.11(9)°.
Generation of Cationic Benzyl Complexes {BnCH2N-
[(2,4-tBu2C6H2(CH2)O]2MBn(THF)}+[BnB(C6F5)3]− (M = Zr
(3), Ti (4)). The stoichiometric reaction between dibenzyl 1 and
B(C6F5)3·THF in CD2Cl2 at room temperature led to the clean
and quantitative formation of the corresponding ion pair
{BnCH2N[(2,4-tBu2C6H2(CH2)O]2ZrBn(THF)}+[BnB-
(C6F5)3]− (3) through benzyl abstraction by the borane
(Scheme 2). The resulting free benzylborate anion [BnB-
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B(C6F5)3 and neutral benzyl complexes (precatalysts 1, 2, 5,
and 6) independently at room temperature in CH2Cl2 or
toluene resulted in no polymer formation up to 24 h. On the
other hand, their derived cationic complexes, upon appropriate
activation, were found effective for the polymerization of
βMMBL, the results of which are summarized in Table 1.
Scheme 2. Activation of the Dibenzyl Complexes To
Generate Cationic Monobenzyl Complexes
{BnCH2N[(2,4-tBu2C6H2(CH2)O]2MBn(THF)}+[BnB-
(C6F5)3]−
a
Table 1. Selected Results of MMBL Polymerization
β
b
b
b
run
isolated
[rr]
[mr]
[mm]
(%)
no. complex method solvent
yield (%) (%)
(%)
1
2
3
4
5
6
7
8
9
1
1
2
2
2
5
5
5
6
A
B
A
B
A
A
B
A
B
toluene
toluene
toluene
toluene
CH2Cl2
toluene
toluene
CH2Cl2
toluene
26
99
30
70
10
70
99
18
21
n.d.
0.5
0.3
1.6
n.d.
0.1
n.d.
1.1
3.5
0.1
n.d.
3.6
n.d.
98.1
96.2
98.1
n.d.
(C6F5)3]− is readily characterized by its 19F NMR resonances at
δ −131.4 (m, 6F, o-F), −164.9 (t, 3F, p-F), and −167.7 (m, 6F,
m-F), and the noncoordinating nature of the borate anion in 3
is established by 19F NMR, in which a small chemical shift
difference of <3 ppm (Δ(m,p-F) = 2.8 ppm in 3) between the
para and meta fluorines is diagnostic of the noncoordinating
borate anion.77 The freshly prepared ion pair solution is pale
yellow, which turned colorless within 15 min at solution due to
decomposition; standing of the CD2Cl2 solution for >1 h at
room temperature resulted in complete decomposition.
Attempts to generate the base-free cation using B(C6F5)3
only led to unidentifiable products due to the decomposition.
Thus, the Zr monobenzyl cation supported by the amine-
bis(phenolate) ligand is stabilized via THF coordination61 and
η2 coordination of the benzyl group to satisfy the octahedral
coordination sites of Zr. We assumed that the presence of one
η2-bonded benzyl group satisfied the octahedral coordination
sites of Zr, on the basis of the analogous cationic benzyl
complexes [((2,6-iPr2C6H3)NC(3,5-tBu2C6H2)O)-
MBn2(THF)]+[BnB(C6F5)3]− (M = Zr, Ti), which were
structurally characterized.61 Furthermore, on the basis of
literature precedents the group 4 cationic metal centers can
be stabilized by η6 coordination via π back-bonding of the
abstracted benzyl group of the corresponding anion [BnB-
(C6F5)3]−,78a,b which is a potential coordination mode when a
coordinating solvent such as THF is not accessible. 1H NMR of
3 exhibited one diffuse broad signal for PhCH2B and PhCH2Zr
at δ 2.70−2.86 ppm. The α-CH2 group of the coordinated THF
showed two broad signals at δ 3.54 and 3.73 ppm, while the
respective β-CH2 group showed one broad signal at δ 1.81
ppm. Four different singlets (δ 1.35, 1.38, 1.44, 1.56 ppm)
observed for Ar-C(CH3)3 indicated the overall C1 symmetry of
the ion pair.
96.2
99.0
n.d.
n.d.
0.9
n.d.
4.5
94.3
a
Conditions: [βMMBL]/[complex]/[B(C6F5)3], 200/1/1; solvent, 3
mL; temperature, 25 °C; time, 24 h; n.d., not determined. Method A:
preactivation (premixing the neutral complex with the activator to
generate the corresponding cationic catalyst, followed by addition of
monomer to start the polymerization). Method B: in-reactor activation
(premixing the monomer with the activator, followed by addition of
b
the neutral complex). Tacticity measured by 13C NMR spectroscopy
with TFA-d as a solvent at 70 °C.
Typically, the coordination−addition polymerization of acrylic
monomers using cationic group 4 metallocenium complexes are
performed in the relatively nonpolar hydrocarbon solvent
toluene or the polar noncoordinating solvent CH2Cl2, while
polar coordinating solvents such as THF and DMF usually
suppress the polymerization due to their strong coordination to
the highly electron deficient metal center.35 Accordingly,
toluene and CH2Cl2 were used for this polymerization study.
Using the preactivation protocol that pregenerates the
cationic catalyst (method A), the polymerization of MMBL
β
in toluene by the Zr complex 1 (0.5 mol %) and B(C6F5)3
achieved only 26% yield (run 1, Table 1). On the other hand,
the in-reactor activation protocol (method B) afforded
quantitative monomer conversion (99% isolated polymer
yield, run 2). The observed large yield difference between the
two activation methods reaffirmed the results of the activation
study described above that the cationic species is unstable in
the absence of monomer. Most significantly, solubility tests
showed that the resulting polymer PβMMBL is insoluble in
common organic solvents such as CHCl3, THF, DMF, DMSO,
acetonitrile, and dichlorobenzene, at room temperature or
under reflux conditions. This insolubility limited the ability to
measure the molecular weight of PβMMBL by GPC analysis,
but it strongly suggests that the resulting polymer is highly
stereoregular.16,15,36 Indeed, analysis of the resulting PβMMBL
by 13C NMR in TFA-d as a solvent at 70 °C showed the
polymer is highly isotactic with 98.1% mm (run 2, Figure 3).
Likewise, the polymerization of βMMBL in toluene by the Ti
complex 2 and B(C6F5)3 using method A achieved only 30%
polymer yield (run 3), while method B resulted in 70% isolated
polymer yield (run 4), although the resulting polymers by both
methods are highly isotactic with 96−98% mm. Owing to the
instability of the preformed cation in CH2Cl2 in the absence of
The cationic Ti species {BnCH2N[(2,4-tBu2C6H2(CH2)-
O]2TiBn(THF)}+[BnB(C6F5)3]− (4) was generated in the
same manner as for the generation of 3, and the resulting free
benzylborate anion [BnB(C6F5)3]− was readily characterized by
its 19F NMR resonances at δ −131.3 (m, 6F, o-F), −164.9 (t,
3F, p-F), and −167.8 (m, m-F) ppm. However, in the case of
the Ti cation 4, its generation was accompanied by formation of
two additional species, BnCH2N[(2,4-tBu2C6H2(CH2)O]2Ti-
(Bn)(C6F5)(THF) and PhCH2B(C6F5)2, due to decomposition
via facile C6F5 ligand transfer to the cation from the borate
anion.78a,c,d Thus, the Ti cation is even less thermally stable
than the Zr cation, and standing of its CD2Cl2 solution at room
temperature for 30 min resulted in complete decomposition.
Polymerization of βMMBL and γMMBL. Control runs for
the polymerization of βMMBL (200:1) with the activator
monomer, the polymerization of MMBL in this solvent using
method A gave only 10% isolated yield (run 5).
β
F
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Employing the superior in-reactor activation protocol with
B(C6F5)3, complexes 1 and 2 were also investigated for their
performance in the polymerization of γMMBL (200−400
equiv) at room temperature in toluene, the results of which
were summarized in Table 2. With a 0.5 mol % precatalyst
loading, near-quantitative monomer conversion (98% isolated
polymer yield) was achieved (run 1, Table 2). The resulting
PγMMBL showed a measured Mn value of 6.13 × 104 g/mol
and a molecular weight distribution of 1.97. Lowering the
precatalyst loading to 0.25 mol % (i.e., raising the [γMMBL]/
[1] ratio to 400) still afforded a high polymer yield of 93%, but
predictably, the resulting PγMMBL had a much higher
molecular weight of Mn = 9.26 × 104 (run 2, Table 2). The
polymerization by the Ti precatalyst 2 with a 0.5 mol % loading
performed comparably to that of the Zr precatalyst 1 (run 3 vs
run 1, Table 2), but the Ti catalyst achieved only a modest
polymer yield of 65% when the precatalyst loading was lowered
to 0.25 mol % (run 4, Table 2). However, regardless of the type
and loading of the precatalyst, all of the resulting polymers were
essentially atactic with [mr] values ranging from 40 to 47%.
The challenge to control the stereochemistry of γMMBL
polymerization is well-known in the literature. For example,
γMMBL polymerization by the catalyst derived from [rac-
C2H4(η5-indenyl)2]ZrMe[OC(OiPr)CMe2], which has
shown to be highly isospecific for MMA and βMMBL
polymerizations,36 produced only an atactic PγMMBL (42.4
mr).
Mechanism of Protonolysis Leading to C−C Bond
Cleavage Products 1 and 2. MBn4 (M = Zr, Ti) upon
protonolysis with equimolar N[(2,4-tBu2C6H2(CH2)OH]3 in
toluene from −30 to 25 °C afforded unexpectedly amine-
bis(phenoxy) dibenzyl complexes 1 (M = Zr) and 2 (M = Ti)
with concurrent liberation of the coproducts 2,4-di-tert-
butylphenol and toluene (Scheme 1), instead of the expected
tripodal amine-tris(phenoxy) monobenzyl complexes N-
[(2,4-tBu2C6H2(CH2)O]3MBn if the reaction involved only
protonolysis but no C−C bond cleavage. One of the possible
mechanisms for the formation of such C−C bond cleavage
products involves radical intermediates due to photochemical
Figure 3. 13C NMR (125 MHz, TFA-d, 70 °C) of the isotactic
PβMMBL (98% mm) produced by complex 1 (run 2, Table 1) in the
backbone quaternary carbon (rr, mr, mm) region.
To examine the generality of isospecific polymerization of
βMMBL by coordination polymerization catalysts, we extended
this investigation by including the two previously reported
(imino)phenoxy benzyl complexes 561 and 661 for polymer-
ization of MMBL. The polymerization results (runs 6−9)
β
showed a similar trend in the activity and isospecificity of the
βMMBL polymerization. In particular, the polymerization of
βMMBL by 5 using the in-reactor activation protocol gave a
99% isolated yield of PβMMBL that exhibited >99% mm (run
7). Overall, the polymerization by the precatalyst using the in-
reactor activation protocol gave better polymerization activity
in comparison to the preactivation protocol, which is especially
important for the cations having limited stability in the absence
of monomer. Regarding the high isotacticity of PβMMBL
achieved by the current catalyst system, the stereocontrol
mechanism should follow the path we already elucidated for the
coordination polymerization of MMBL.15,36 Specifically, the
β
predominant chain-end stereocontrol in the coordination−
addition polymerization of MMBL, which forms an isotactic
β
polymer, chiefly originates from steric interactions between the
methyl groups on the chiral β-C atom of the five-membered
ring of both the coordinated MMBL and the last inserted
β
βMMBL unit of the chain.
a
Table 2. Selected Results of MMBL Polymerization by Precatalysts 1 and 2
γ
b
b
c
c
c
run no.
complex
monomer/complex
isolated yield (%)
10−4Mn
PDI (Mw/Mn)
[rr] (%)
[mr] (%)
[mm] (%)
1
2
3
4
1
1
2
2
200
400
200
400
98
93
97
65
6.13
9.26
4.63
6.00
1.97
2.35
1.82
2.03
46.5
42.0
35.0
48.0
44.4
47.0
47.0
40.2
9.1
11.0
18.0
11.8
a
Conditions: [complex]/[B(C6F5)3], 1; solvent, 3 mL (toluene); temperature, 25 °C; time, 24 h; monomer was premixed with the activator
b
B(C6F5)3 followed by addition of complex (i.e., in-reactor activation). Number-average molecular weight (Mn) and polydispersity index (PDI)
c
determined by GPC relative to PMMA standards. Tacticity measured by 13C NMR spectroscopy with DMSO-d6 as solvent at 100 °C.
G
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a
Scheme 3. Comparison of Energetics (kcal/mol) Involved in the Reaction of I with [O3N] To Form II and III
a
Values in black and in green correspond to internal and free energies in toluene, obtained with the BP86 and M06 functionals.
a
Scheme 4. Reaction Pathway and Energetics (kcal/mol) Involved in the Formation of II
a
Values in black and in green correspond to internal and free energies in toluene, obtained with the BP86 and M06 functionals.
decomposition of the precursor MBn4 to the tribenzyl
metal(III) complex and the benzyl radicals,62 even though the
current reaction was performed in a glass reactor wrapped with
Al foil to minimize exposure to light. Photolysis of MBn4 was
reported to proceed more quickly in ethers and aromatic
hydrocarbons than in aliphatic hydrocarbons.62a These
precursors are unstable in toluene solution, and the
decomposition rate increases with the temperature (a typical
lifetime of TiBn4 in toluene solution is 8 h; the presence of light
changes the color of ZrBn4 in solution from yellow to brown
even at temperatures at which no thermal decomposition
occurs).62a We reasoned that, since the lifetime and
decomposition rate of such precursors are sensitive to solvent,
light, and temperature, the results from investigations into
effects of solvent, light, and temperature on the formation of
amine-bis(phenoxy) dibenzyl complex 1 should provide
evidence to argue for or against the hypothesis for the benzyl
radical and tribenzyl metal(III) complex involvement in the
above reaction.
Accordingly, we performed three additional sets of experi-
ments on protonolysis of ZrBn4 with N[(2,4-tBu2C6H2(CH2)-
OH]3 by varying factors such as solvent, light, and heat. The
first set of experiments involved the reaction in diethyl ether
that was performed in a glass reactor wrapped with Al foil to
minimize exposure to light; this condition increased the
selectivity for complex 1 (89% yield), as compared to that
performed in toluene (80% yield) under otherwise identical
conditions. In contrast, switching the solvent to hexanes
1
decreased the selectivity for complex 1 (55% yield; H NMR
analysis of the crude reaction mixture showed the presence of
the unreacted ligand and other unidentified byproducts). These
results from the above investigations into solvent effects
H
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seemed to support the role of photolysis of MBn4 under the
reaction conditions and thus the direct involvement of the
resulting benzyl radicals and trivalent tribenzyl Zr complex in
the formation of 1. Additional support for this hypothesis was
provided by a second set of experiments that performed the
protonolysis reaction in toluene in a reactor directly exposed to
light (i.e., no wrapping with Al foil); this condition led to an
increased selectivity for complex 1 formation (87% yield), in
comparison to that (80% yield) performed in toluene under
minimum light exposure using Al foil wrapping. Finally, when
the protonolysis reaction was performed in toluene at 80 °C
under light, unidentifiable compounds were formed, due to
thermal and photochemical decomposition of the precursor
ZrBn4 and the product 1. Overall, all of the above three sets of
the experiments investigating the effects of solvent, light, and
heat supported the hypothesis for the involvement of the
benzyl radicals and the trivalent Zr(III) complex resulting from
thermal and photochemical decomposition of the precursor
ZrBn4.
decomposition of tetrabenzylzirconium to tribenzylzirconium
with formation of benzyl radicals.62 As the first step, we
performed time-dependent DFT calculations (TDDFT) to
investigate the possible photochemical decomposition of ZrBn4.
The energetics involved in this reaction is reported in Figure 4.
DFT calculations were performed on the protonolysis
reaction of tetrabenzylzirconium complex (I) with tridentate
phenol N[(2,4-tBu2C6H2(CH2)OH)]3 ([O3N]) to rationalize
the formation of bis(phenoxy)-dibenzyl complex C6H5-CH2N-
(CH2-2,4-tBu2-C6H2-O)2Zr(CH2-C6H5)2 (III), instead of the
tris(phenoxy)-monobenzyl complex N(CH2-2,4-tBu2-C6H2-
O)3Zr(CH2-C6H5) (II), as expected on the basis of the
analogous reactions with a Zr tetraalkoxide precursor74
(Scheme 3). The internal energy (E) and the free energy (G)
reported herein were obtained with the BP86 and M06
functionals. In both cases the effects of solvent (toluene) were
considered.
As shown in Scheme 3, calculations revealed that formation
of the expected tris(phenoxy)-monobenzyl complex II is
favored by about 30−40 kcal/mol relative to the actually
obtained complex III, in terms of internal energy and free
energy as well, with both functionals. Subsequently, we focused
our attention on the possible mechanisms leading to complexes
II and III. Scheme 4 depicts the possible reaction pathway and
the energies involved in the formation of the tris(phenoxy)-
monobenzyl complex II.
Figure 4. Illustrative potential energy curves for the ZrBn4 ground state
and the first singlet excited state with the energetics (kcal/mol)
involved in the corresponding vertical transition and in the following
ZrBn3 plus benzyl radical formation. The values were calculated with
the BP86 (green) and M06 (purple) functionals in the gas phase.
As expected, the absorption band for the transition from the
ground-state singlet, 464 and 404 nm at the BP86 and M06
levels, is in the visible region. The optimized singlet excited
state, as well as the dissociated tribenzylzirconium plus benzyl
radical decomposition products, are almost at the same energy,
about 50 kcal/mol from the ground state.79 This indicates that
inclusion of entropy should facilitate decomposition of ZrBn4
after excitation of the complex to the first singlet excited state.
Accordingly, we decided to investigate reaction pathways
involving radical intermediates. The most reasonable mecha-
nism we found for explaining formation of complex III is
reported in Scheme 5.
The reaction involves three consecutive toluene elimination
steps leading to the mono(phenoxy)-tribenzyl complex RS_1A,
to the bis(phenoxy)-dibenzyl complex RS_2A, and finally to
the tris(phenoxy)-monobenzyl complex II, respectively. As
shown in Scheme 4, the rate-determining step is the first
elimination reaction, showing a transition state that, in terms of
internal energy, is 23.4 and 9.9 kcal/mol above the reactants at
the BP86 and M06 levels, respectively. In terms of free energy
the TS is 10 kcal/mol higher in energy, with both functionals
due to an unfavored entropic contribution, a consequence of
the bimolecular character of this reaction step. The first
elimination product is about −40 and −50 kcal/mol below the
reactants at the BP86 and M06 levels. Similar energy gains were
observed for the successive elimination products generated
through TSs that show energetic barriers not exceeding 10
kcal/mol in terms of both free energy and internal energy with
both functionals considered (Scheme 4).
The reaction starts with coordination of one phenol oxygen
of the tridentate ligand [O3N] to the tribenzyl-Zr(III) complex
to form coordination intermediate 1B, which undergoes
protonolysis with elimination of toluene and formation of the
mono(phenoxy)-dibenzyl intermediate RS_1B. The energy
barrier of this first step is about 15 kcal/mol with both of the
functionals considered. Intermediate RS_1B, which is remark-
ably lower in energy than the reactants, shows an interaction
between a second phenol oxygen of the ligand and the metal.
However, calculations suggest that, instead of elimination of a
second toluene molecule through the protonolysis transition
state TS_2B′, intermediate RS_1B collapses into the clearly
more stable intermediate RS_2B, where the two phenols of the
ligand interact with the Zr, one of them interacting through the
carbonyl oxygen of its tautomeric keto form (Scheme 5). The
geometries of RS_1B and RS_2B, depicted in Figure 5, indicate
that the sp3 Cα (RS_2B) determines the conformation of the
phenol ring that allows the coordination of the oxygen of a
third phenol to the Zr(III). Additional coordination of the third
phenol is prevented for steric reasons when both phenols of the
ligand are in the enol form, as in RS_1B.
To rationalize the formation of the bis(phenoxy)-dibenzyl
complex III, we examined several possible reaction pathways.
As we were not able to find an energetically feasible mechanism
mediated directly by Zr(IV) complexes, we focused on possible
reaction mechanisms where the active metal species is a Zr(III)
species, considering the well-known, facile photochemical
From intermediate RS_2B we hypothesized two TSs
involving a H transfer from one O atom to the ring of the
I
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a
Scheme 5. Reaction Pathways and Energetics (kcal/mol) Involved in the Proposed Mechanism for the Formation of III
a
Values in black and in green correspond to internal and free energies in toluene, obtained with the BP86 and M06 functionals.
Figure 6. Comparison between the geometries of transition states
Figure 5. Comparison between RS_1B and RS_2B geometries.
TS′_1C and TS_1C for H-transfer.
phenol which results in the oxidation of the Zr, in the formation
of a new Zr−O bond, and in delocalization of the radical onto
one of the phenol ring of the ligand RS_1C. Transition state
TS′_1C involves transfer of the α-hydrogen of the phenol in
the keto form to the Cα of the nearby phenol in the enol form
of the ligand (Figure 6), whereas in transition state TS_1C the
hydrogen of the hydroxyl group of the phenol in the enol form
is transferred to the carbonyl oxygen of the nearby phenol in
the keto form. While the H transfer in transition state TS′_1C
is direct, in transition state TS_1C it is mediated by the metal.
The H-transfer reaction can occur through both TSs, since
no meaningful energy difference between the two TSs was
found, independent of the functional considered. This H-
transfer reaction shows an energy barrier of about 15−20 kcal/
mol, depending on the specific functional considered.80
Moreover, since intermediate RS_1C is isoenergetic with
intermediate RS_2B with both functionals, the H-transfer
reaction is an equilibrium reaction. The driving force of the
reaction is the successive elimination of the experimentally
observed 2,4-tBu2C6H3-OH (Ar) which occurs through
transition state TS_1C with an energy barrier of about 18
kcal/mol and formation of intermediate RS_2C. This
intermediate shows an interaction of the methylenic carbon
bonded to the nitrogen atom of the ligand with the Zr center
and it is about 20 kcal/mol more stable than RS_1C at the
BP86 level.
Finally, we considered two pathways that start from the
radical RS_2C intermediate for the formation of the
experimentally observed complex III: one proceeds via
intermolecular radical coupling involving the radical RS_2C
and the benzyl radical formed in the decomposition reaction of
ZrBn4. The other is an intramolecular reaction involving one
benzyl group bonded to the Zr in RS_2C to form intermediate
RS_3C, which lies about 15 kcal/mol lower in energy. This
reaction proceeds through transition state TS_3C, with an
energy barrier of about 20 kcal/mol (with both functionals).
J
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Complex III is finally formed by coupling of RS_3C with the
benzyl radical (Scheme 5).
dibenzyl complex, BnCH2N[(2,4-tBu2C6H2(CH2)O]2ZrBn2, is
formed either through an intermolecular radical coupling
involving the benzyl radical present in the reaction medium
as result of the decomposition of ZrBn4, or through an
intramolecular reaction involving one benzyl group bonded to
the Zr, followed by radical coupling with the benzyl radical.
CONCLUSIONS
■
In summary, we have discovered an unusual C−C bond
cleavage in the protonolysis of group 4 tetrabenzyl complexes
MBn4 (M = Zr, Ti) with the tetradentate amine-tris(phenol)
ligand N[(2,4-tBu2C6H2(CH2)OH]3, resulting in the formation
of the unexpected amine-bis(phenoxy) dibenzyl complexes 1
(80% isolated yield) and 2 (75% isolated yield) with concurrent
liberation of the coproducts 2,4-di-tert-butylphenol and toluene.
The molecular structure of the Zr complex 1 has been
confirmed by X-ray diffraction analysis, featuring a pentacoor-
dinate, pseudo-trigonal-bipyramidal geometry at Zr, which is
coordinated by the amine-bis(phenoxy) ligand in a mer fashion,
with two axial O atoms and equatorial [N,C,C] atoms
furnished with η1 binding of one benzyl group, η2 binding of the
other benzyl group, and a datively bonded amido nitrogen.
Activation of the dibenzyl Zr complex 1 with B(C6F5)3·THF
in CD2Cl2 at room temperature generated cleanly and
quantitatively the corresponding cationic complex 3 through
benzyl abstraction by the borane. The analogous abstractive
reaction with the Ti dibenzyl complex 2 was less clean and was
accompanied by the two byproducts as a result of transfer of
the C6F5 ligand transfer from the anion to the cation due to the
instability of the Ti cation 4. The in situ generated Zr cation 3
through the in-reactor activation protocol promoted highly
stereospecific polymerization of the biomass-derived renewable
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF and xyz files giving single-crystal
X-ray diffraction data for 1 and computational details. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for L.C.: lcaporaso@unisa.it.
*E-mail for E.Y.-X.C.: eugene.chen@colostate.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(NSF-1300267) for the study carried out at Colorado State
University. L.C. thanks the HPC team of Enea (www.enea.it)
for using the ENEA-GRID and the HPC facilities CRESCO
(www.cresco.enea.it) in Portici, Italy. We thank Boulder
Scientific Co. for the research gift of B(C6F5)3 and Dr. Brian
Newell for assistance on X-ray structural analysis.
monomer MMBL into the highly isotactic polymer PβMMBL
β
with 98.1% mm and quantitative yield. The analogous Ti cation
4 was less effective in terms of polymer yield, but the tacticity of
the resulting polymer is similar. Both cationic complexes are
also effective for polymerization of γMMBL, leading to
essentially atactic PγMMBL.
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(79) Calculations indicated that decomposition of tetrabenzylzirco-
nium to dibenzylzirconium plus two benzyl radicals is unfavored by a
ΔG value of roughly 25−30 kcal/mol, relative to decomposition to
tribenzylzirconium plus benzyl radical. This is in qualitative agreement
with the experimental results reported in ref 62b.
(80) It is worth noting that the competitive toluene elimination
reaction shows an energetic barrier at least 10 kcal/mol higher than
that for the H transfer reaction, as shown in Scheme 5 (compare
TS′_1C and TS_1C with TS′_2B).
M
dx.doi.org/10.1021/om500661y | Organometallics XXXX, XXX, XXX−XXX