Chiral and achiral (imino)phenoxy-based cationic group 4 non-metallocene complexes as catalysts for polymerization of renewable α-methylene- γ-butyrolactones

Article abstract of DOI:10.1039/c3dt50430a

Protonolysis of M(Bn)₄ (M = Zr, Ti; Bn = benzyl) with equimolar 2,4-di-tert-butyl-6-[(2,6-diisopropylphenylimino)methyl]phenol [(2,6- ⁱPr₂C₆H₃)NC(3,5-^tBu ₂C₆H₂)OH] in toluene at -30 °C to 25 °C cleanly affords the corresponding achiral (imino)phenoxy-tribenzyl complexes, [(2,6-ⁱPr₂C₆H₃)NC(3,5- ^tBu₂C₆H₂)O]Zr(Bn)₃ (1) and [(2,6-ⁱPr₂C₆H₃)NC(3,5- ^tBu₂C₆H₂)O]Ti(Bn)₃ (2). A chiral dibenzyl complex 3 incorporating the unsymmetric, tetradentate amino(imino)bis(phenoxy) ligand, [2,4-Br₂C₆H ₂(O)(6-CH₂(NC₅H₉))CH ₂NCH(2-adamantyl-4-MeC₆H₂O)]Zr(Bn)₂ (3), has also been prepared using the same protonolysis protocol. Abstractive activation of 1 with B(C₆F₅)₃·THF in CD₂Cl₂ at room temperature (RT) affords clean, quantitative formation of the corresponding zirconium cation [((2,6- ⁱPr₂C₆H₃)NC(3,5-^tBu ₂C₆H₂)O)Zr(Bn)₂(THF)] ⁺[BnB(C₆F₅)₃]^- (4). Likewise, benzyl abstraction of 2 with B(C₆F₅) ₃·THF in CD₂Cl₂ at RT generates the cationic titanium complex [((2,6-ⁱPr₂C₆H ₃)NC(3,5-^tBu₂C₆H₂)O) Ti(Bn)₂(THF)]⁺[BnB(C₆F₅) ₃]^- (5), accompanied by a small amount of decomposed species as a result of C₆F₅ transfer. The dibenzyl cations 4 and 5 have been characterized spectroscopically, and their structures have been confirmed by single crystal X-ray diffraction analysis. Characteristics of the coordination polymerization of renewable α-methylene-γ- butyrolactone monomers by the cationic catalysts derived from achiral complexes 1 and 2 as well as chiral complex 3 have been investigated, representing the first study of such polymerization by non-metallocene catalysts.

Full text of DOI:10.1039/c3dt50430a

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Chiral and achiral (imino)phenoxy-based cationic group
4 non-metallocene complexes as catalysts for
polymerization of renewable α-methylene-
γ-butyrolactones†
Cite this: Dalton Trans., 2013, 42, 9263
Ravikumar R. Gowda and Eugene Y.-X. Chen*
Protonolysis of M(Bn)4 (M = Zr, Ti; Bn = benzyl) with equimolar 2,4-di-tert-butyl-6-[(2,6-diisopropylphenyl-
imino)methyl]phenol [(2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)OH] in toluene at −30 °C to 25 °C cleanly affords
the corresponding achiral (imino)phenoxy-tribenzyl complexes, [(2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O]Zr-
(Bn)3 (1) and [(2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O]Ti(Bn)3 (2). A chiral dibenzyl complex 3 incorporating
the unsymmetric, tetradentate amino(imino)bis(phenoxy) ligand, [2,4-Br2C6H2(O)(6-CH2(NC5H9))-
CH2NvCH(2-adamantyl-4-MeC6H2O)]Zr(Bn)2 (3), has also been prepared using the same protonolysis
protocol. Abstractive activation of 1 with B(C6F5)3·THF in CD2Cl2 at room temperature (RT) affords clean,
quantitative formation of the corresponding zirconium cation [((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Zr-
(Bn)2(THF)]+[BnB(C6F5)3]− (4). Likewise, benzyl abstraction of 2 with B(C6F5)3·THF in CD2Cl2 at RT gener-
ates the cationic titanium complex [((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Ti(Bn)2(THF)]+[BnB(C6F5)3]− (5),
accompanied by a small amount of decomposed species as a result of C6F5 transfer. The dibenzyl cations
4 and 5 have been characterized spectroscopically, and their structures have been confirmed by single
crystal X-ray diffraction analysis. Characteristics of the coordination polymerization of renewable
α-methylene-γ-butyrolactone monomers by the cationic catalysts derived from achiral complexes 1 and 2
as well as chiral complex 3 have been investigated, representing the first study of such polymerization by
non-metallocene catalysts.
Received 15th February 2013,
Accepted 7th March 2013
DOI: 10.1039/c3dt50430a
www.rsc.org/dalton
and a high Tg of 195 °C (for atactic polymer).7 MBL units
present in various copolymers and blends have good optical
Introduction
The prospect of using naturally occurring or biomass-derived properties as well as resistance to heat, weathering, scratches,
renewable resources to substitute depleting petroleum-based and solvents.8 Another monomer considered in this area of
raw materials in large commodity markets, such as plastics, study is the γ-methyl derivative of MBL, γ-methyl-α-methylene-
fibers, and fuels, have been explored recently.1–3 In this γ-butyrolactone (MMBL), which can be readily prepared via a
context, several compounds containing an α-methylene- two-step process from the biomass-derived levulinic acid.9,10
γ-butyrolactone moiety were found and isolated from various MBL and MMBL are of special interest in exploring the pro-
plants.4 The best characterised tulipaline is tulipaline A, or spects of substituting the petroleum-based methacrylate
α-methylene-γ-butyrolactone (MBL), found in tulips,4,5 which monomers for specialty chemicals production.11 Various types
is the most studied monomer of this family. MBL consists of a of polymerization processes have been adopted to polymerize
five-membered lactone ring, exhibits structural features MBL to low to high molecular weight (MW) polymers, proceed-
similar to those of methyl methacrylate (MMA), and poly- ing through radical polymerization,7,12 anionic polymeriz-
merizes in a similar manner too (i.e., vinyl addition without ation,7 group-transfer polymerization,13 coordination poly-
ring-opening of the lactone). Poly(α-methylene-γ-butyrolactone) merization with metallocene complexes;14 MBL has been
(PMBL) has good durability, a high refractive index of 1.540,6 copolymerized with various comonomers12a such as MMA,15
styrene,12e,16 methoxystyrene,17 and vinyl thiophenes.18 The
polymerization of MMBL has not been studied extensively as
Department of Chemistry, Colorado State University, Fort Collins,
compared to MBL; nevertheless, it has also been polymerized
Colorado 80523-1872, USA. E-mail: eugene.chen@colostate.edu
by free-radical emulsion polymerization6,19 as well as radical,
†CCDC 924151 (4) and 924152 (5). For crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c3dt50430a
anionic, and group-transfer polymerization methods.20
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We have developed several catalyst systems for rapid, controlled, (ε-caprolactone, δ-valerolactone, β-butyrolactone, and lactides)
or stereoselective polymerization of such monomers, including as well as ethylene and propylene polymerization.46
bifunctional silicon propagators for living polymerization of
In view of the exciting benefits of MBL and MMBL mono-
MBL and MMBL,21 half-sandwich indenyl rare-earth metal di- mers and their corresponding polymers as well as the out-
alkyls with exceptional activity for the polymerization of MMBL standing performances of amine bis(phenolates) and salalen
in DMF,22 alane-based classical and frustrated Lewis pairs for complexes of group 4 dibenzyl complexes in the polymeriz-
rapid polymerization of MBL and MMBL into high MW poly- ation of 1-hexene and propylene, we were intrigued by the pro-
mers,23 a dinuclear silylium-enolate bifunctional catalyst with spect of such non-metallocene complexes being effective
high activity and stereoselectivity (at low temperature) for the catalysts for coordination polymerization of the renewable
polymerization of MMA and MMBL,24 and N-heterocyclic car- butyrolactone-based vinyl monomers such as MBL and MMBL.
benes for rapid organocatalytic polymerization of MMBL and However, to the best of our knowledge, there were no reports
MBL.25 Most recently, we have developed the first stereo- on the utilization of such catalysts for this polymerization cata-
selective polymerization of the β-methyl derivative, β-methyl- lysis. In the present contribution, we report syntheses and
α-methylene-γ-butyrolactone (βMMBL), into stereo-defect-free characterization of group 4 benzyl complexes supported by
polymers26 by single-site chiral metallocene catalysts that are imino(phenolate)47 and salalen43 ligands as well as the first
known to promote stereospecific coordination polymerization structural characterization of group 4 dibenzyl cationic com-
of polar vinyl monomers.27
Applications of group
plexes48 that incorporate the imino(phenolate) ligand and
non-metallocene catalysts in their polymerization activity towards MMBL, MBL and MMA.
4
α-olefin polymerization have been well documented.28 Selected
notable examples include: sterically hindered chelating
phenoxy titanium and zirconium complexes29 for living
polymerization of 1-hexene,30 2,2′-ethylenebis(N,N′-(triiso-
propylsilyl)anilinido)zirconium complexes for living polymeriz-
Experimental
Materials, reagents, and methods
ation of α-olefin,31 bis(pentafluorophenylamido)zirconium All syntheses and manipulations of air- and moisture-sensitive
benzyl complexes for ethylene polymerization,32 a C2-sym- materials were carried out in flamed Schlenk-type glassware
metric amine bis(phenolate)zirconium benzyl catalyst for iso- on a dual-manifold Schlenk line, on a high-vacuum line, or in
specific living polymerization of 1-hexene,33 an amine bis- an inert gas (Ar or N2)-filled glovebox. NMR-scale reactions
(phenolate)titanium benzyl catalyst for block copolymerization were conducted in Teflon-valve-sealed J. Young-type NMR
of α-olefins,34 amine bis(phenolate)zirconium dibenzyl com- tubes. HPLC-grade organic solvents were first sparged exten-
plexes,35 extremely active [ONXO]-type zirconium and hafnium sively with N2 during filling 20 L solvent reservoirs and then
dibenzyl complexes of amine bisphenolates (X = N, O, S),36 dried by passage through activated alumina (for Et2O, THF,
group 4 complexes of an amine bis(phenolate) ligand featur- and CH2Cl2) followed by passage through Q-5 supported
ing a THF molecule as well as furan sidearm donors,37 and copper catalyst (for toluene and hexanes) stainless steel
zirconium and titanium diamine bis(phenolate) catalysts38 columns. Benzene-d6 and toluene-d8 were dried over sodium/
utilized for 1-hexene polymerization. In 2005, Kol and co- potassium alloy and vacuum-distilled or filtered, whereas
workers employed zirconium dibenzyl complexes of chiral CD2Cl2 and CDCl3 were dried over activated Davison 4 Å mol-
salan ligands for iso-specific polymerization of 1-hexene and ecular sieves. NMR spectra were recorded on a Varian Inova
4-methyl-1-pentene as well as cyclopolymerization of 1,5-hexa- 300 (300 MHz, 1H; 75 MHz, 13C; 282 MHz, 19F) or 400 MHz
diene,39 titanium and zirconium dibenzyl complexes of robust spectrometer. Chemical shifts for 1H and 13C spectra were
salophan ligands in 1-hexene polymerization,40 salan zirco- referenced to internal solvent resonances and are reported as
nium complexes for production of highly isotactic poly(vinyl- parts per million relative to SiMe4, whereas 19F NMR spectra
cyclohexane),41 and C1-symmetric [ONNO′]-type salan zirconium were referenced to external CFCl3. Elemental analyses were
complexes in 1-hexene polymerization.42 More recently, Kol performed by Robertson Microlit Laboratories, Madison, NJ.
et al. have reported the use of titanium dibenzyl complexes
Triethylamine, 3,5-dibromosalicylaldehyde, paraformalde-
incorporating salalen (i.e., half-salan/half-salen) ligands for hyde, and zirconium(IV) chloride were purchased from Alfa
the synthesis of highly isotactic polypropylene with a melting Aesar Chemical Co. Benzyl magnesium chloride, 2-(amino-
transition temperature (Tm) of 169.9 °C, the highest Tm methyl)piperidine,
3,5-di-tert-butyl-2-hydroxybenzaldehyde,
reported for “as prepared” (not extracted or annealed) isotactic and titanium(IV) chloride were purchased from Sigma-Aldrich
polypropylene produced by either heterogeneous or homo- Chemical Co. and used as received. Formaldehyde (36–38%
geneous catalysts to date.43 Okuda and co-workers reported aqueous solution) was purchased from Mallinckrodt Chemi-
group 4 complexes supported by [ONNO]-type bis(o-amino- cals. Tetrabenzyltitanium and tetrabenzylzirconium were pre-
phenolato) ligands for α-olefin polymerization.44 Waymouth pared according to published procedures49 and used shortly
and co-workers developed hafnium and zirconium dibenzyl after. Ligands 3-adamantyl-2-hydroxy-5-methylbenzaldehyde,50
bis(phenolate)ether complexes for propylene polymerization.45 2-(bromomethyl)-4,6-dibromophenol,43 and 2,4-di-tert-butyl-
Aryloxy, imino and bis(imino)phenoxides of group 4 alkoxides 6-((2,6-diisopropylphenylimino)methyl)phenol [(2,6-iPr2C6H3)-
are also active for ring-opening polymerization of cyclic esters NvC(3,5-tBu2C6H2)OH]51 were synthesized according to
9264 | Dalton Trans., 2013, 42, 9263–9273
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published procedures. Methyl methacrylate (MMA) was pur- Synthesis of [(2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O]Zr(Bn)3 (1)
chased from Sigma-Aldrich Chemical Co., while α-methylene-
γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyro-
lactone (γ-MMBL) were purchased from TCI America. These
monomers were first degassed and dried over CaH2 overnight,
followed by vacuum distillation; MMA was further purified by
titration with neat tri(n-octyl)aluminum to a yellow end point
and distillation under reduced pressure. The purified mono-
mers 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(penta-
fluorophenyl)borane, B(C6F5)3, was obtained as a research gift
from Boulder Scientific Co. and was further purified by subli-
mation. B(C6F5)3·THF was prepared by addition of THF to a
toluene solution of the borane at ambient temperature, fol-
lowed by removal of the volatiles and drying under vacuum.
A solution of (2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)OH (1.00 g,
2.54 mmol) in toluene (15 mL) and Zr(Bn)4 (1.15 g,
2.54 mmol) in toluene (15 mL) was precooled at −30 °C inside
the glovebox for 12 h. After mixing the solutions, orange pre-
cipitates instantly appeared. Stirring continued for 24 h at
ambient temperature, the reaction mixture was filtered to
afford the crude product. This orange product was further
1
dried for 1 h under reduced pressure; yield: 1.50 g (78%). H
NMR (C6D6, 23 °C): δ 8.13 (s, 1H, CHvN), 7.76 (s, 1H, Ar-H),
7.13–6.87 (m, 21H, Ar-H, C7H8), 2.69–2.56 (m, 2H, Ar-CH-
(CH3)2), 2.26 (bs, 6H, Ar-CH2-Zr), 2.11 (s, 3H, C7H8), 1.52 (s,
9H, Ar-C(CH3)3), 1.24 (s, 9H, Ar-C(CH3)3), 1.22 (d, J = 6.8 Hz,
6H, Ar-CH(CH3)2), 0.90 (d, J = 6.7 Hz, 6H, Ar-CH(CH3)2). 13C
NMR (C6D6, 23 °C): δ 172.5 (Ar-CHvN), 160.2 (Ar-O-Zr), 152.1
(Ar-NvCH), 141.8 (Ar-C(CH3)3), 140.9 (Ar-C(CH3)3), 140.6 (Ar-
CH2-Zr), 138.3 (Ar-CH(CH3)2), 137.9 (C7H8), 132.2 (Ar-C), 130.0
(Ar-C), 129.4 (Ar-C), 129.3 (C7H8), 129.1 (Ar-C), 128.5 (C7H8),
127.4 (Ar-C), 125.7 (C7H8), 124.6 (Ar-C), 123.8 (Ar-C), 122.7 (Ar-
C), 72.6 (Ar-CH2-Zr), 35.4 (Ar-C(CH3)3), 34.3 (Ar-C(CH3)3), 31.4
(Ar-C(CH3)3), 30.2 (Ar-C(CH3)3), 29.3 (Ar-CH(CH3)2), 25.3
(Ar-CH(CH3)2), 23.2 (Ar-CH(CH3)2), 21.4 (C7H8). Anal. Calcd for
C48H59NOZr·2C7H8: C, 79.09; H, 8.03; N, 1.49. Found: C, 78.94;
H, 8.09; N, 1.85.
Synthesis of 2-[(aminomethyl)piperidine]-4-methyl-6-
adamantylphenol
2-(Aminomethyl)piperidine (0.84 g, 7.39 mmol) was added
to a solution of 3-adamantyl-2-hydroxy-5-methylbenzaldehyde
(2.00 g, 7.39 mmol) in benzene (20 mL). The resulting mixture
was refluxed for 8 h, after which the solvent was removed
under vacuum to give a yellow solid; yield: 2.41 g (88%). 1H
NMR (CDCl3, 23 °C): δ 13.62 (br, 1H, OH), 8.32 (s, 1H, CHvN),
7.36 (s, 1H, Ar-H), 7.07 (d, J = 2.0 Hz, 1H, Ar-H), 3.67 (m, 1H,
CH2N), 3.38 (m, 1H, CH2N), 3.05 (m, 1H, CHNH), 2.88 (m, 1H,
CH2NH), 2.65 (m, 1H, CH2NH), 2.28 (s, 3H, Ar-CH3), 2.23 (bs,
1H, NH), 2.17 (m, 6H, adamantyl), 2.08 (bs, 3H, adamantyl),
1.83 (m, 2H, CH2), 1.78 (bs, 6H, adamantyl), 1.71 (m, 1H, CH2),
1.64 (m, 1H, CH2), 1.40 (m, 1H, CH2), 1.26 (m, 1H, CH2).
Synthesis of [(2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O]Ti(Bn)3 (2)
A solution of [(2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)OH] (1.00 g,
2.54 mmol) in toluene (15 mL) and Ti(Bn)4 (1.04 g,
2.54 mmol) in toluene (15 mL) was precooled at −30 °C inside
the glovebox for 12 h. After mixing the solutions, a red solu-
tion appeared instantly. Stirring continued for 24 h at RT, after
which the solvent was removed under reduced pressure to
afford a red solid. This residue was further purified by crystalli-
zation from toluene at −30 °C; yield: 1.40 g (77%). 1H NMR
(C6D6, 23 °C): δ 8.21 (s, 1H, CHvN), 7.79 (s, 1H, Ar-H),
7.13–6.85 (m, 21H, Ar-H, C7H8), 3.51 (bs, 6H, Ar-CH2-Ti),
2.41–2.29 (m, 2H, Ar-CH(CH3)2), 2.11 (s, 3H, C7H8), 1.58 (s, 9H,
Ar-C(CH3)3), 1.24 (s, 9H, Ar-C(CH3)3), 1.11 (d, J = 6.7 Hz, 6H,
Ar-CH(CH3)2), 0.86 (d, J = 6.7 Hz, 6H, Ar-CH(CH3)2). 13C NMR
(C6D6, 23 °C): δ 171.6 (Ar-CHvN), 162.4 (Ar-O–Ti), 152.8
(Ar-NvCH), 142.0 (Ar-C(CH3)3), 141.5 (Ar-C(CH3)3), 140.6
(Ar-CH2–Ti), 138.4 (Ar-CH(CH3)2), 137.8 (C7H8), 132.2 (Ar-C),
131.7 (Ar-C), 129.7 (Ar-C), 129.3 (C7H8), 128.5 (Ar-C), 128.4
(C7H8), 127.5 (Ar-C), 125.7 (C7H8), 124.6 (Ar-C), 124.5 (Ar-C),
122.9 (Ar-C), 100.4 (Ar-CH2-Ti), 35.6 (Ar-C(CH3)3), 34.3 (Ar-C-
(CH3)3), 31.4 (Ar-C(CH3)3), 30.5 (Ar-C(CH3)3), 29.2 (Ar-CH-
(CH3)2), 25.2 (Ar-CH(CH3)2), 23.0 (Ar-CH(CH3)2), 21.4 (C7H8).
Synthesis of 2,4-Br2C6H2(OH)(6-CH2(NC5H9))CH2NvCH-
(2-adamantyl-4-MeC6H2OH)
A solution of 2-(bromomethyl)-4,6-dibromophenol (2.06 g,
5.97 mmol) in THF (30 mL) was added dropwise to a solution
of
2-(aminomethyl)piperidine)-4-methyl-6-adamantylphenol
(2.19 g, 5.97 mmol) and triethylamine (14 mL) in THF (30 mL)
and stirred for 5 h. The solid that formed was filtered off and
the solvent was removed under vacuum. The crude product
was re-crystallized from cold methanol yielding the titled
ligand as a yellow solid; yield: 3.76 g (93%).1H NMR (CDCl3,
23 °C): δ 8.23 (s, 1H, CHvN), 7.53 (d, J = 2.2 Hz, 1H, Ar-H),
7.08 (d, J = 1.7 Hz, 1H, Ar-H), 7.03 (d, J = 2.0 Hz, 1H, Ar-H),
6.86 (s, 1H, Ar-H), 4.23 (d, J = 14.7 Hz, 1H, Ar-CH2), 3.94 (m,
1H, Ar-CH2), 3.76–3.49 (m, 2H, CH2NvCH), 3.03–2.78 (m, 2H,
CH2N), 2.78–2.57 (m, 1H, CHN), 2.28 (s, 3H, Ar-CH3), 2.16 (m,
6H, adamantyl), 2.08 (bs, 3H, adamantyl), 1.93–1.82 (m, 2H,
Synthesis of [2,4-Br2C6H2(O)(6-CH2(NC5H9))CH2NvCH-
(2-adamantyl-4-MeC6H2O)]Zr(Bn)2 (3)
A
solution of 2,4-Br2C6H2(OH)(6-CH2(NC5H9))CH2NvCH-
CH2), 1.78 (bs, 6H, adamantyl), 1.69–1.62 (m, 1H, CH2), 1.43 (s, (2-adamantyl-4-MeC6H2OH) (0.50 g, 0.79 mmol) in toluene
1H, CH2), 1.21–1.05 (m, 2H, CH2). Anal. Calcd for (15 mL) and Zr(Bn)4 (0.36 g, 0.79 mmol) in toluene (15 mL)
C31H38Br2N2O2: C, 59.06; H, 6.08; N, 4.44. Found: C, 59.18; H, was precooled at −30 °C inside the glovebox for 12 h. After
6.19; N, 4.34.
mixing the solutions, an orange solution appeared instantly.
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Stirring continued for 30 h at RT, after which the solvent was instantaneously formed red solution was carefully layered with
removed under reduced pressure to afford the crude product. 4 mL of hexanes and 0.5 mL of CH2Cl2. The vial was cooled to
This crude product contained some unreacted Zr(Bn)4; it was −30 °C and stored inside the glovebox freezer for 6 days to give
washed with hexanes to remove Zr(Bn)4 and further purified by orange single crystals of 4. After decanting the solvent, the
crystallization from toluene at −30 °C to yield the pure product crystals were quickly coated with a layer of Paratone-N oil
1
as an orange solid; yield: 0.36 g (50%). H NMR (C6D6, 23 °C): (Exxon, dried and degassed at 140 °C/10−6 Torr for 16 h) in the
δ 7.79 (s, 1H, CHvN), 7.40–7.29 (m, 6H, Ar-H), 7.11–6.83 (m, glovebox. A crystal was then mounted on a thin glass fiber
8H, Ar-H, C7H8), 6.75–6.67 (m, 5H, Ar-H), 3.84–3.53 (br, 1H, under a cold stream of dinitrogen gas. Single crystal X-ray diffr-
Ar-CH2), 3.43 (d, J = 8.7 Hz, 1H, Ar-CH2), 3.35–3.21 (m, 1H, action data were acquired on a Bruker Kappa APEX II CCD
CH2NvCH), 3.09–2.92 (bm, 1H, CH2NvCH), 2.88–2.84 (bm, diffractometer with Mo Kα radiation (λ = 0.71073 Å) and a
1H, CH2N), 2.74–2.59 (bm, 2H, CH2N, CHN), 2.52 (bs, 6H, ada- graphite monochromator. Initial lattice parameters were
mantyl), 2.26 (bs, 4H, Ar-CH2-Zr), 2.24 (s, 3H, Ar-CH3), 2.11 (s, obtained from a least-squares analysis of more than 100 reflec-
3H, C7H8), 2.07 (bs, 3H, adamantyl), 1.90–1.86 (bm, 6H, ada- tions; these parameters were later refined against all data. The
mantyl), 1.31–1.19 (bm, 2H, CH2), 1.12 (t, J = 6.9 Hz, 2H, CH2), crystal did not show any significant decay during data collec-
0.97–0.91 (m, 1H, CH2), 0.59 (t, J = 14.8 Hz, 1H, CH2). 13C NMR tion. Data were integrated and corrected for Lorentz and
(C6D6, 23 °C): δ 170.4 (CHvN), 167.8 (Ar-O-Zr), 166.5 (Ar-O-Zr), polarization effects using Bruker APEX2 software, and
158.4 (Ar-C), 157.5 (Ar-C), 154.4 (Ar-C), 148.9 (Ar-C), 139.7 semiempirical absorption corrections were applied using
(Ar-C), 137.9 (C7H8), 135.9 (Ar-C), 134.7 (Ar-C), 132.0 (Ar-C), SCALE.54 Space group assignments were based on systematic
131.1 (Ar-C), 129.3 (C7H8), 129.2 (Ar-C), 128.7 (Ar-C), 128.5 absences, E statistics, and successful refinement of the struc-
(C7H8), 128.4 (Ar-C), 126.8 (Ar-C), 125.9 (Ar-C), 125.7 (C7H8), ture. The structure was solved by the Patterson method and
125.5 (Ar-C), 123.5 (Ar-C), 120.0 (Ar-C), 119.4 (Ar-C), 113.8 refined with the aid of successive Fourier difference maps
(Ar-CH2-Zr), 109.9 (Ar-CH2-Zr), 68.0 (CHN), 65.7 (CH2NvCH), against all data using the SHELXTL 6.14 software package.55
59.1 (CH2N), 58.7 (Ar-CH2), 48.03 (CH2), 45.3 (CH2), 40.9 (ada- Thermal parameters for all non-hydrogen atoms were refined
mantyl), 37.5 (adamantyl), 29.7 (adamantyl), 29.6 (adamantyl), anisotropically, while all hydrogen atoms were assigned to
23.0 (CH2), 21.4 (Ar-CH3), 20.9 (C7H8). This complex is highly ideal positions and refined using a riding model with an iso-
sensitive towards air, moisture, light and heat, rendering it tropic thermal parameter 1.2 times that of the attached carbon
unsuitable for elemental analysis.
atom (1.5 times for methyl hydrogens). Selected bond dis-
tances and angles for compound 4 were collected in the
caption of Fig. 2. All other metric parameters can be found in
the cif file included with the ESI.† In the structure, C42 and
C43 of the THF solvate molecule are disordered over two sites,
In situ generation, NMR data, and crystal structure of
[((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Zr(Bn)2(THF)]+-
[BnB(C6F5)3]− (4)
Cationic species 4 was generated by in situ mixing of 1 and with a site occupancy ratio refined to 40 : 60. In the structure
B(C6F5)3·THF in CD2Cl2 at ambient temperature, following the the disordered toluene solvate molecule was found in Fourier
procedure for the clean and quantitative generation of cationic difference maps to be disordered over multiple sites. After
ester enolate rac-C2H4(1-indenyl)Zr+(THF)[OC(OiPr)=CMe2]- numerous attempts to model the disorder failed to improve
[MeB(C6F5)3]−.52,53 Specifically, in an argon-filled glovebox, a agreement factors, SQUEEZE56 was used to remove the dis-
4 mL glass vial was charged with 10 mg (0.011 mmol) of 1 and ordered components. Selected crystallographic data for 4:
ˉ
0.4 mL of CD2Cl2, while another vial was charged with 6.87 mg C70H67BF15NO2Zr, triclinic, space group P1, a = 10.9985(4) Å,
of B(C6F5)3·THF (0.011 mmol) and 0.4 mL of CD2Cl2. The two b = 15.8569(6) Å, c = 20.4970(8) Å, α = 86.967(2)°, β =
vials were mixed via a pipette at ambient temperature to give 75.119(2)°, γ = 73.850(2)°, V = 3317.8(2) Å3, Z = 2, Dcalcd = 1.343
instantaneously a red solution, and subsequent NMR analysis Mg m−3, GOF = 1.056, R1 = 0.0580 [I > 2σ(I)], wR2 = 0.1282.
1
showed the clean and quantitative formation of ion pair 4. H CCDC 924151 contains the supplementary crystallographic
NMR (CD2Cl2, 23 °C): δ 8.41 (s, 1H, CHvN), 7.99 (s, 1H, Ar-H), data.
7.41–7.36 (m, 2H, Ar-H), 7.28–7.14 (m, 11H, Ar-H, C7H8),
In situ generation, NMR data and crystal structure of
6.90–6.85 (m, 2H, Ar-H), 6.80–6.71 (m, 6H, Ar-H), 3.52–3.47 (m,
4H, α-CH2 of THF), 2.80 (bs, 2H, Ar-CH(CH3)2), 2.34 (s, 3H,
C7H8), 2.20–2.16 (m, 2H, Ar-CH2-B), 2.09–2.02 (m, 4H, Ar-CH2-
[((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Ti(CH2Ph)2]+-
[CH2PhB(C6F5)3]− (5)
Zr), 1.71–1.66 (m, 4H, β-CH2 of THF), 1.60 (s, 9H, Ar-C(CH3)3), Cationic species 5 was generated by in situ mixing of 2 and
1.39 (s, 9H, Ar-C(CH3)3), 1.14 (d, J = 6.7 Hz, 6H, Ar-CH(CH3)2), B(C6F5)3·THF in CD2Cl2 at ambient temperature. In an argon-
0.89 (d, J = 6.6 Hz, 6H, Ar-CH(CH3)2). 19F NMR (CD2Cl2, 23 °C): filled glovebox, a 4 mL glass vial was charged with 10 mg
δ −131.2 (m, 6F, o-F), −164.9 (t, JF–F = 20.4 Hz, 3F, p-F), −167.7 (0.012 mmol) of 2 and 0.4 mL of CD2Cl2, while another vial
(m, 6F, m-F).
was charged with 7.23 mg of B(C6F5)3·THF (0.012 mmol) and
The molecular structure of 4 has been confirmed by single 0.4 mL of CD2Cl2. The two vials were mixed via a pipette at
crystal X-ray diffraction analysis. A 20 mL glass vial was ambient temperature to give instantaneously a dark red solu-
charged with 10 mg (0.011 mmol) of 1, 6.87 mg of tion, and subsequent NMR analysis showed the formation of
B(C6F5)3·THF (0.011 mmol), and 3 mL of toluene. The ion pair 5 (major) with some additional minor species
9266 | Dalton Trans., 2013, 42, 9263–9273
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((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Ti(CH2Ph)2(C6F5)(THF) polymerization) was also adopted in some runs for compari-
and PhCH2B(C6F5)2. 1H NMR (CD2Cl2, 23 °C) of the major son. Similar procedures were followed in the case of MMA
species: δ 8.53 (s, 1H, CHvN), 8.08 (s, 1H, Ar-H), 7.47–7.38 (m, polymerization, except for neat polymerizations where the acti-
2H, Ar-H), 7.28–7.11 (m, 11H, Ar-H, C7H8), 7.05–7.00 (m, 2H, vator B(C6F5)3 (39.6 μmol) and 400 equiv. of MMA (15.8 mmol)
Ar-H), 6.90–6.72 (m, 6H, Ar-H), 3.89 (bs, 4H, α-CH2 of THF), were premixed. After the measured time interval, a 0.2 mL
3.13–3.08 (m, 4H, Ar-CH2-Ti), 3.03–3.00 (m, 2H, Ar-CH2-B), 2.80 aliquot was taken from the reaction mixture via a syringe and
(bs, 2H, Ar-CH(CH3)2), 2.34 (s, 3H, C7H8), 1.72 (bs, 9H, Ar-C- quickly quenched into a 4 mL vial containing 0.6 mL of
(CH3)3), 1.68 (bs, 4H, β-CH2 of THF), 1.41 (s, 9H, Ar-C(CH3)3), undried “wet” CDCl3 stabilized by 250 ppm of BHT-H; the
1.08 (d, J = 6.7 Hz, 6H, Ar-CH(CH3)2), 0.82 (d, J = 6.5 Hz, 6H, quenched aliquots were later analyzed by 1H NMR to obtain
1
Ar-CH(CH3)2). H NMR (CD2Cl2, 23 °C) of the minor species: δ monomer conversion data. The polymerization was immedi-
8.36 (s, CHvN), 7.78 (s, Ar-H), 7.28–7.11 (m, Ar-H), 6.90–6.72 ately quenched after the removal of the aliquot by adding
(m, Ar-H), 4.08 (bs, α-CH2 of THF), 3.13–3.08 (m, Ar-CH2-Ti, 5 mL of 5% HCl-acidified methanol. The quenched mixture
Ar-CH2-B), 2.92 (bs, Ar-CH(CH3)2), 2.08 (bs, β-CH2 of THF), 1.53 was precipitated into 100 mL of methanol, stirred for 3 h, fil-
(bs, Ar-C(CH3)3), 1.37 (s, Ar-C(CH3)3), 1.28 (d, J = 7.2 Hz, Ar-CH- tered, and washed with methanol. The polymer collected was
(CH3)2), 1.16 (d, J = 6.7 Hz, Ar-CH(CH3)2), 1.02 (d, J = 4.1 Hz, dried in a vacuum oven at 50 °C overnight to a constant weight.
Ar-CH(CH3)2), 0.95 (d, J = 6.7 Hz, Ar-CH(CH3)2). 19F NMR
Polymer characterizations
(CD2Cl2, 23 °C) of the major species: δ −131.3 (m, 6F, o-F),
−164.9 (t, JF–F = 20.5 Hz, 3F, p-F), −167.8 (m, 6F, m-F). 19F NMR
Polymer number-average molecular weights (Mn) and mole-
(CD2Cl2, 23 °C) of the minor species: δ −136.0 (m, o-F), −140.5
(m, p-F), −160.3 (m, m-F).
cular 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−1, with DMF (for PMBL
and PMMBL samples) or CHCl3 (for PMMA) 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 = 200–2 000 000). The instrument was calibrated with
10 PMMA standards, and chromatograms were processed with
The molecular structure of 5 has been confirmed by single
crystal X-ray diffraction analysis. A 20 mL glass vial was
charged with 10 mg (0.012 mmol) of 2, 7.23 mg of
B(C6F5)3·THF (0.012 mmol), and 3 mL of toluene. The result-
ing solution was carefully layered with 4 mL of hexanes. The
vial was cooled to −30 °C and stored in the glovebox freezer for
12 days to afford single crystals of 5. After decanting the
solvent, the dark brownish 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 dinitro-
gen gas. Single crystal X-ray diffraction data were acquired
using the procedures already described for cation 4. Selected
bond distances and angles for compound 5 were collected in
the caption of Fig. 3. All other metric parameters can be found
1
Waters Empower software. H NMR and 13C NMR spectra for
the analysis of PMMA,52,53 PMBL21,58 and PMMBL11,59 micro-
structures were recorded and analyzed according to the litera-
ture methods.
Results and discussion
Synthesis of neutral benzyl complexes 1–3
in the cif file included in the ESI.† Selected crystallographic
ˉ
data for 5: C70H67BF15NO2Ti, triclinic, space group P1, a = Protonolysis of tetrabenzyl precursors M(Bn)4 (M = Zr, Ti) with
11.0260(4) Å, b = 15.5861(6) Å, c = 20.5604(8) Å, α = 84.504(2)°, equimolar
2,4-di-tert-butyl-6-((2,6-diisopropylphenylimino)-
[(2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)OH]
in
toluene at −30 °C to 25 °C for 24 h yields the corresponding
β = 74.448(2)°, γ = 73.522(2)°, V = 3263.4(2) Å3, Z = 2, Dcalcd
1.365 Mg m−3, GOF = 1.026, R1 = 0.0430 [I > 2σ(I)], wR2
=
=
methyl)phenol
0.0882. CCDC 924152 contains the supplementary crystallo- tribenzyl complexes 1 and 2 with concomitant liberation of the
graphic data.
co-product toluene (Scheme 1). The Zr compound 1 was iso-
lated in pure state as an orange solid by simple filtration of the
resulting reaction mixture, followed by drying under vacuum
General polymerization procedures
Polymerizations were performed in 30 mL glass reactors in the for 1 h. Drying or standing at ambient temperature longer
glovebox in toluene or DMF at ambient temperature, following than 1 h resulted in a noticeable colour change from orange to
the procedure established for the polymerization of methacry- yellow indicating decomposition, but the complex can be
lates and acrylamides by metallocene precatalysts.52,53,57 In a stored inside a −35 °C freezer without noticeable decompo-
typical in-reactor activation polymerization procedure, the acti- sition for an extended time period (up to a month). The Ti
vator B(C6F5)3 or B(C6F5)3·THF (13.2 μmol) and 200 equiv. compound 2, a red solid, was obtained by recrystallization of
(2.64 mmol) of MMBL or MBL were premixed in 3 mL of the crude product from toluene. Both complexes were isolated
toluene or DMF as indicated in the table. The polymerization in good yield (∼78%). Similarly, the 1 : 1 ratio reaction between
was timed immediately after addition of precatalyst 1 to 3. The Zr(Bn)4 and the neutral salalen ligand, 2,4-Br2C6H2(OH)-
pre-activation method (i.e., premixing the neutral complex (6-CH2(NC5H9))CH2NvCH(2-adamantyl-4-MeC6H2OH),
in
with an activator to generate the corresponding cationic toluene at −30 °C to 25 °C yielded dibenzyl 3 (Scheme 1). Iso-
catalyst, followed by addition of monomer to start the lation of the pure product (50% yield) was achieved through
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Scheme 2 Activation of the tribenzyl complexes to generate cationic dibenzyl
complexes
[((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)M(Bn)2(THF)]+[BnB(C6F5)3]−
(M = Zr, 4; M = Ti, 5).
ion pair [((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Zr(Bn)2(THF)]+-
[BnB(C6F5)3]− (4) (Scheme 2). Prolonged storage (>3 h) in
CD2Cl2 at room temperature resulted in complete decompo-
sition. Attempts to generate the base-free cation using B(C6F5)3
led to formation of unidentifiable decomposition products.
Hence, this zirconium dibenzyl cation supported by the mono-
imino(phenoxy) ligand is stabilized via THF coordination and
η2-coordination of one benzyl group to render the isolable
cation in that the Zr center adopts an octahedral coordination
geometry, as shown by the crystal structure of 4 (vide infra). On
the other hand, there are literature examples that group 4 cat-
ionic metal centers can be stabilized by back π-bonding
(η6-coordination) of the abstracted benzyl group in the result-
ing anion [BnB(C6F5)3]−.48a,b Non-metallocene group 4 alk-
oxide cationic species are also known.48c 1H NMR of 4 reveals
(Fig. 1) two individual sets of signals for Ar-CH2-B and Ar-CH2-
Zr in the expected ratio of 1 : 2 (δ 2.02–2.20). As expected,
α-CH2 and β-CH2 signals of the coordinated THF were
deshielded (δ, α-CH2, 3.52; β-CH2, 1.71) relative to those of the
free THF. Two different doublets (δ 1.14, 0.89) were observed
for Ar-CH(CH3)2, while the respective Ar-CH(CH3)2 signal
appeared as a broad singlet (δ 2.80). The instability of 4 in
CD2Cl2 limited the ability to obtain reliable 13C NMR data, but
19F NMR (Fig. 1) clearly shows clean formation of the
Scheme 1 Syntheses of Zr and Ti benzyl complexes 1–3.
evaporation of the solvent from the reaction mixture and
washing with hexanes to remove a small amount of the
unreacted residual precursors, followed by recrystallization
from toluene. Prolonged exposure to light and vacuum at room
temperature even in the glovebox resulted in decomposition of
benzyl complexes 1–3, with complex 3 being the most sensitive
towards air, moisture, light and heat.
1
The H NMR spectra of 1 and 2 reveal that the benzyl –CH2
in 2 (δ 3.51) was significantly more deshielded than the benzyl
–CH2 in 1 (δ 2.26), which can be attributed to the stronger
Lewis acidic Ti center as compared to that of Zr. Likewise, the
13C NMR spectra of 1 and 2 also indicate that the benzyl –CH2
in 2 (δ 100.4) was substantially more deshielded than the
benzyl –CH2 in 1 (δ 72.6). The six protons belonging to the
three benzyl –CH2 groups in both 1 and 2 appeared only as a
broad singlet in 1H NMR; consistently, the 13C NMR spectra of
both complexes showed only one type of benzyl –CH2, thus
suggesting rapid interconversion of η1 and η2 benzyl coordi-
nation in solution at room temperature (cf. the crystal structure
of the cationic complexes). Two sets of doublets (δ 1.22 and
0.90 in 1, 1.11 and 0.86 in 2) for Ar-CH(CH3)2 are indicative of
their chemical non-equivalency, due to restricted Ar–N bond
rotation, as compared to the free ligand (single doublet at δ
0.38); on the other hand, the respective Ar-CH(CH3)2 signals
appeared only as one multiplet (in both 1 and 2) and shielded
(δ 2.69 for 1, 2.41 for 2) relative to the free ligand (δ 3.00).
Other chemical shifts matched well with the analogous
(amidine)phenoxy zirconium tribenzyl complex.60 1H and 13C
NMR chemical shifts of the zirconium dibenzyl complex 3 also
closely matched with the analogous salalen titanium compound.43
Generation and characterization of cationic benzyl complexes
[((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)M(Bn)2(THF)]+-
[BnB(C6F5)3]− (M = Zr, 4; Ti, 5)
Fig. 1 1H and 19F NMR of
4
and 19F NMR spectra (CD2Cl2, 23 °C) of
The 1 : 1 ratio reaction between 1 and B(C6F5)3·THF in CD2Cl2
at ambient temperature generates cleanly and quantitatively B(C6F5)3·THF.
9268 | Dalton Trans., 2013, 42, 9263–9273
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O(1)–Zr(1)–O(2), 166.71(6)°, C(35)–Zr(1)–O(2), 90.68(10)°, O(1)–
Zr(1)–C(28), 92.08(8)°, C(35)–Zr(1)–C(28), 124.97(9)°, and O(2)–
Zr(1)–C(28), 93.62(8)°, collaborate with the distorted octahedral
Zr center.
Cationic species [((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Ti-
(Bn)2(THF)]+[BnB(C6F5)3]− (5) was generated in the same
manner as for generation of 4, but in the case of the Ti cation
5, its formation was accompanied by formation of two
additional species due to decomposition via facile C6F5 ligand
transfer48a,48e,48f (vide infra). Prolonged storage (>2 h) in
CD2Cl2 at room temperature resulted in complete decompo-
sition. Attempts to generate the base-free cation using B(C6F5)3
led to formation of unidentifiable decomposition products.
Two individual sets of signals were observed for Ar-CH2-B, Ar-
CH2-Ti methylene protons in the expected ratio of 1 : 2 (δ
3.03–3.13). As in the Zr cation 4, two different doublets (δ 1.08,
0.82) were also observed for Ar-CH(CH3)2 in the Ti cation 5. 19
F
NMR clearly shows formation of benzylborate anion [BnB-
(C6F5)3]−: δ −131.3 (m, 6F, o-F), −164.9 (t, 3F, p-F), −167.8 (m,
Fig. 2 X-ray crystal structure of [((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Zr-
(Bn)2(THF)]+[BnB(C6F5)3]− (4) with thermal ellipsoids drawn at the 40% prob- m-F). Two additional species were identified as ((2,6-iPr2C6H3)
NvC(3,5-tBu2C6H2)O)Ti(CH2Ph)2(C6F5)(THF) and PhCH2B-
(C6F5)2, decomposition products via transfer of the C6F5 ligand
ability. Selected bond lengths (Å) and angles (°): Zr(1)–O(1), 1.9655(17); Zr(1)–
O(2), 2.2529(19); Zr(1)–N(1), 2.3010(18); Zr(1)–C(28), 2.280(2); Zr(1)–C(35),
2.252(3); Zr(1)–C(36), 2.556(2); B(1)–C(64), 1.668(4); B(1)–C(58), 1.666(4); B(1)–
from the borate anion to the Ti cation.
C(46), 1.651(3); B(1)–C(52), 1.663(4); O(1)–Zr(1)–C(35), 95.87(10); O(1)–Zr(1)–
The solid-state structure of 5 (Fig. 3) was determined by
O(2), 166.71(6); C(35)–Zr(1)–O(2), 90.68(10); O(1)–Zr(1)–C(28), 92.08(8);
C(35)–Zr(1)–C(28), 124.97(9); O(2)–Zr(1)–C(28), 93.62(8); O(1)–Zr(1)–N(1), single crystal X-ray diffraction analysis. The structure reveals
78.74(6); C(35)–Zr(1)–N(1), 114.39(8); O(2)–Zr(1)–N(1), 88.03(7); C(28)–Zr(1)–
unassociated cation–anion pairs. As in the cation of 4, the tita-
nium center in the cation of 5 exhibits a distorted octahedral
N(1), 120.56(7); C(1)–O(1)–Zr(1), 142.60(14); C(15)–N(1)–Zr(1), 123.60(14);
C(36)–C(35)–Zr(1), 83.95(14).
benzylborate anion [BnB(C6F5)3]−: δ −131.2 (m, 6F, o-F), −164.9
(t, 3F, p-F), −167.7 (m, 6F, m-F).
The solid-state structure of 4 (Fig. 2) was determined by
single crystal X-ray diffraction analysis. The structure reveals
unassociated cation–anion pairs. The zirconium center in the
cation exhibits a distorted octahedral geometry furnished with
η2-coordination of one benzyl group, η1-coordination of the
other benzyl group, and the rest of the three coordination sites
occupied by the side-arm imino nitrogen, the coordinated THF
and the phenoxy oxygen. The Zr–C(benzyl) distances, Zr(1)–
C(28), 2.280(2) Å, Zr(1)–C(35), 2.252(3) Å, match well with Zr-
(OC6HPh2-2,6-Me2-3,5)2(CH2Ph)[η6-C6H5CH2B(C6F5)3] [2.230(4)
Å].48a,61 Bond distances of Zr(1)–N(1) = 2.3010(18) Å and Zr(1)–
O(2) = 2.2529(19) Å indicate typical datively bonded N and O
moieties, respectively. The η2-bonded benzyl group is charac-
terized by a distance of Zr(1)–C(36) = 2.556(2) Å and a bond
angle of C(36)–C(35)–Zr(1) = 83.95(14)°. The two coordinated
hetero atoms and the η2-bonded benzyl group play a key role
Fig. 3 X-ray crystal structure of [((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Ti-
to stabilize the octahedral cationic zirconium center
(Bn)2(THF)]+[BnB(C6F5)3]− (5) with thermal ellipsoids drawn at the 40% probability.
[((2,6-iPr2C6H3)NvC(3,5-tBu2C6H2)O)Zr(Bn)2(THF)]+,
which
Selected bond lengths (Å) and angles (°): Ti(1)–O(1), 1.8320(16); Ti(1)–O(2),
2.1114(17); Ti(1)–N(1), 2.1613(18); Ti(1)–C(28), 2.119(2); Ti(1)–C(29), 2.548(2);
Ti(1)–C(35), 2.156(2); B(1)–C(64), 1.656(4); B(1)–C(58), 1.671(4); B(1)–C(46),
1.672(3); B(1)–C(52), 1.649(4); O(1)–Ti(1)–C(35), 91.30(8); O(1)–Ti(1)–O(2),
171.13(6); C(35)–Ti(1)–O(2), 93.69(8); O(1)–Ti(1)–C(28), 93.93(9); C(35)–Ti(1)–
C(28), 121.33(9); O(2)–Ti(1)–C(28), 89.73(9); O(1)–Ti(1)–N(1), 83.47(7); C(35)–
Ti(1)–N(1), 119.38(8); O(2)–Ti(1)–N(1), 87.69(7); C(28)–Ti(1)–N(1), 119.27(8);
C(1)–O(1)–Ti(1), 142.14(14); C(7)–N(1)–Ti(1), 122.31(15); C(29)–C(28)–Ti(1),
88.68(15).
avoid back π-bonding (η6-coordination) of the abstracted
benzyl group ([BnB(C6F5)3]−) to the cationic Zr center. The
acute angles of C(29)–C(28)–Zr(1), 102.96(13)°, C(36)–C(35)–
Zr(1), 83.95(14)° indicate the phenyl rings of both benzyl
groups bent towards the zirconium center, but Zr(1)–C(28)
benzyl is best described as the η1-benzyl bond [Zr(1)–C(29) =
2.986(2) Å]. The bond angles, O(1)–Zr(1)–C(35), 95.87(10)°,
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geometry furnished with η2-coordination of one benzyl group, catalyst activity and polymerization control. Nevertheless, the
η1-coordination of the other benzyl group, and the rest of the polymerization of MMBL (200 equiv.) in toluene by 1 and B
three coordination sites occupied by the side-arm imino nitro- (C6F5)3 using the in-reactor activation protocol achieved quan-
gen, the coordinated THF, and the phenoxy oxygen. The Ti–C titative monomer conversion (99% isolated polymer yield) after
(benzyl) distances, Ti(1)–C(28), 2.119(2) Å, Ti(1)–C(35), 2.156(2) 24 h. The resulting atactic PMMBL (51.8% mr) exhibited a rela-
Å, match well with Ti(OC6H3Ph2-2,6)2(CH2Ph)[η6-C6H5CH2B- tively broad MWD with PDI = 2.19 and the measured Mn of
(C6F5)3] [2.159(9) Å].48a Bond distances of Ti(1)–N(1) = 2.1613(18) 5.06 × 104 g mol−1 was much higher than the calculated one,
Å and Ti(1)–O(2) = 2.1114(17) Å indicate typical datively thus giving rise to a low initiator efficiency of I* = 44% (run 1,
bonded N and O moieties, respectively. The η2-bonded benzyl Table 1). Increasing the [MMBL]/[1] ratio to 400 brought about
group is characterized by a distance of Ti(1)–C(29) = 2.548(2) much higher molecular weight of the resulting PMMBL (Mn
=
and a bond angle of C(29)–C(28)–Ti(1) = 88.68(15)°. Like the 8.59 × 104 g mol−1, run 2, Table 1) although the polymerization
cationic Zr complex 4, two coordinated hetero atoms and one achieved only 60% isolated polymer yield during the same
η2-bonded benzyl group in 5 play a key role to stabilize the time period (24 h). The polymerization in toluene using the
octahedral cationic titanium center, which avoid back pre-activation method (i.e., pre-mixing 1 with B(C6F5)3·THF to
π-bonding (η6-coordination) of the abstracted benzyl group generate the corresponding cationic catalyst before addition of
([BnB(C6F5)3]−) to the cationic titanium center. The acute monomer) gave similar polymerization results to those
angles of C(29)–C(28)–Ti(1), 88.68(15)°, C(36)–C(35)–Ti(1), obtained by the in-reactor activation procedure (run 4 vs. run
110.14(14)° indicate phenyl rings of both benzyl groups bent 1, Table 1). On the other hand, the polymerization in DMF by
towards the titanium center, but Ti(1)–C(35) benzyl is best the pre-activation method afforded only 26% polymer yield
described as the η1-benzyl bond [Ti(1)–C(36) = 3.003(2) Å]. The although the polymer molecular weight was still similar (Mn =
bond angles, O(1)–Ti(1)–C(35), 91.30(8)°, O(1)–Ti(1)–O(2), 5.43 × 104 g mol−1, PDI = 2.81, I* = 10%, run 3, Table 1). In
171.13(6)°, C(35)–Ti(1)–O(2), 93.69(8)°, O(1)–Ti(1)–C(28), 93.93(9)°, comparison, polymerizations by in-reactor activation of
C(35)–Ti(1)–C(28), 121.33(9)°, and O(2)–Ti(1)–C(28), 89.73(9)°, complex 2 in toluene achieved higher initiator efficiencies, up
collaborate with the distorted octahedral cationic titanium to 90% (runs 5–7, Table 1), and afforded syndio-biased poly-
center.
mers (47.8% rr to 57.0% rr). The MMBL polymerization by in-
reactor activation of the unsymmetric complex 3 with B(C6F5)3
achieved quantitative monomer conversion (99% isolated
yield) but the resulting polymer was still only syndio-biased
(61.2% rr, 30.6% mr, run 8, Table 1).
Polymerization of MMBL by catalysts derived from complexes 1–3
Control runs using the activators, B(C6F5)3 or B(C6F5)3·THF,
and precatalysts, neutral complexes 1–3, individually for
polymerization of MMBL (200 equiv.) at RT in toluene or DMF
yielded no polymer formation up to 24 h. Worth noting here is
Polymerization of MBL by catalysts derived from complexes 1–3
that coordination-addition polymerization of acrylic mono- Controlling the stereochemistry of MBL polymerization has
mers by cationic group 4 metallocenium catalysts was typically been a challenge. For example, MBL polymerization by a Cs-
carried out in nonpolar hydrocarbons such as toluene or polar ligated zirconocene catalyst, {[(p-Et3SiPh)2C(Cp)(2,7-tBu2-Flu)]-
non-coordinating solvents such as CH2Cl2, while polar coordi- Zr[OC(OiPr)=CMeCH2C(Me2)C(OiPr)=O]}+[B(C6F5)4]−,
which
nating solvents such as THF and DMF usually inhibit the has been shown to be a highly active and syndiospecific
polymerization.27 Owing to insolubility of PMMBL in toluene, polymerization catalyst for MMA polymerization,57a resulted in
polymerization of MMBL by group 4 catalysts in this solvent only a modest polymer yield of 40% and a syndio-biased
proceeds in a heterogeneous fashion, negatively impacting the atactic polymer (50.8% rr, 23.1% mr). On the other hand, near
Table 1 Selected results of MMBL polymerization by complexes 1–3a
MMBL/
complex
Isolated
yield (%)
10−4 Mn
PDIb
(Mw/Mn)
I*c
(%)
[rr]d
(%)
[mr]d
(%)
[mm]d
(%)
b
Run no.
Complex
Activator
Solvent
(g mol−1
)
1
2
3
4
5
6
7
8
1
1
1
1
2
2
2
3
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3·THF
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
200
400
200
200
200
400
200
200
Toluene
Toluene
DMF
Toluene
Toluene
Toluene
DMF
99
60
26
98
98
63
98
99
5.06
8.59
5.43
5.78
3.21
3.14
4.88
7.75
2.19
2.62
2.81
2.40
2.03
2.02
1.98
3.39
44
31
10
38
68
90
45
28
36.5
42.7
n.d.
n.d.
47.8
57.0
n.d.
61.2
51.8
47.3
n.d.
n.d.
39.7
33.2
n.d.
30.6
11.7
10.0
n.d.
n.d.
12.5
9.8
n.d.
8.2
Toluene
a Conditions: [complex]/[activator] = 1; solvent = 3 mL; temperature = 25 °C; time = 24 h; monomer was pre-mixed with the activator followed by
addition of complex (i.e., in-reactor activation), except for runs 3, 4 and 8 where complexes were pre-activated with an activator followed by
addition of monomer (i.e., pre-activation); n.d. = not determined. b Number-average molecular weight (Mn) and polydispersity index (PDI)
determined by GPC relative to PMMA standards. c Initiator efficiency (I*) = Mn(calcd)/Mn(exptl), where Mn(calcd) = MW(monomer) × [monomer]/
[catalyst] × conversion% + MW of chain-end groups. d Tacticity measured by 13C NMR spectroscopy with DMSO-d6 as a solvent at 100 °C.
9270 | Dalton Trans., 2013, 42, 9263–9273
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Table 2 Selected results of MBL polymerization by complexes 1–3a
Run
no.
MBL/
complex
Isolated
yield (%)
10−4 Mn
PDI
(Mw/Mn)
I*
(%)
[rr]
(%)
[mr]
(%)
[mm]
(%)
Complex
Activator
(g mol−1
)
1
2
3
4
1
1
2
3
B(C6F5)3
B(C6F5)3·THF
B(C6F5)3
200
200
200
200
96
96
87
99
4.85
2.01
2.35b
5.57
2.99
2.40
4.13b
3.11
39
94
73
35
49.2
n.d.
39.3
49.0
30.4
n.d.
31.7
33.1
20.4
n.d.
29.0
17.9
B(C6F5)3
a Conditions: [complex]/[activator] = 1; solvent (toluene) = 3 mL; temperature = 25 °C; time = 24 h; monomer was pre-mixed with the activator
followed by addition of complex (i.e., in-reactor activation), except for runs 2 and 4 where complexes were pre-activated with an activator followed
by addition of monomer (i.e., pre-activation). See footnotes under Table 1 for other abbreviations or explanations. b There was a minor (∼2%),
higher MW shoulder peak.
quantitative polymer yield (96%) can be achieved by the cata- cationic metal centers are stabilized by the side-arm (imino)-
lyst system based on complex 1 using either in-reactor acti- phenoxy nitrogen donor atom, coordinated THF, and the
vation with B(C6F5)3 (run 1, Table 2) or pre-activation with η2-bonded benzyl group. This mode of stabilization is different
B(C6F5)3·THF (run 2, Table 2), although the latter polymeriz- from the commonly observed back binding (η6-coordination)
ation approach exhibited a much higher initiator efficiency of the benzyl group abstracted to the anion.
(94%) than the former method (39%). The resulting PMBL is,
These three benzyl complexes, upon appropriate activation
however, also a syndio-biased atactic material (49.2% rr, 30.4% (in-reactor or pre-activation method, or both), have been inves-
mr, run 1, Table 2), so is the polymer produced by the catalyst tigated for their activity and stereoselectivity towards polymer-
based on the Ti complex 2 (39.3% rr, 31.7% mr, run 3, ization of renewable monomers MBL and MMBL. High to
Table 2). Again, the chiral catalyst based on the unsymmetric quantitative polymer yields can be achieved under certain con-
complex 3 also led to a syndio-biased atactic polymer (49.0% ditions, but the resulting polymers typically exhibit relatively
rr, 33.1% mr, run 4, Table 2).
broad molecular weight distributions and essentially atactic or
As controls, we also examined the performance of the cata- syndio-biased atactic microstructures. This trend holds true
lysts derived from benzyl complexes 1–3 for polymerization of even for the chiral catalyst derived from activation of the
MMA (200 equiv.) at RT in toluene, CH2Cl2, or neat. Under unsymmetric complex 3. These results, coupled with the
these conditions, the polymerizations by all three catalysts control run results obtained from MMA polymerization, indi-
were sluggish, achieving less than 20% monomer conversion cate that the herein investigated non-metallocene-based
even after 24 h. The PMMAs produced are syndio-rich atactic catalysts are considerably less active, stereoselective, and
materials, with the one produced by the catalyst based on the controlled in coordination polymerization of conjugated polar
Zr complex 1 in neat conditions exhibiting the highest syndio- alkenes such as MMA, MBL, and MMBL, than group 4 metallo-
tacticity of 83.4% rr, followed by the PMMA (71.5% rr) pro- cene or lanthanocene-based catalysts.
duced by the catalyst derived from the Ti complex 2. The
catalyst system based on the C1-symmetric 3 also led to syndio-
rich polymers, with syndiotacticity ranging from 68.5% to
73.2% rr, depending on the reaction medium (in toluene,
Acknowledgements
This work was supported by the National Science Foundation
(NSF-1012326). We thank Boulder Scientific Co. for the
research gift of B(C6F5)3, Dr Yuetao Zhang for GPC analysis of
PMMBL, PMBL and PMMA, and Dr Brian S. Newell for the
help on the X-ray structural analysis.
CH2Cl2, or neat). Overall, these non-metallocene-based cat-
ionic catalysts are considerably less active, stereoselective, and
controlled, as compared to metallocene-based cationic
catalysts.27
Conclusions
References
In summary, we have employed the protonolysis route to syn-
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ponding cationic dibenzyl complexes 4 (Zr) and 5 (Ti), which
were characterized by NMR and X-ray diffraction analysis. To
the best of our knowledge, complexes 4 and 5 are the first
examples of the structurally characterized (imino)phenoxy
group 4 dibenzyl cations to date. These hexa-coordinate
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