Bis(phenoxy-azo)titanium(IV) complexes: Synthesis, structure, and catalytic activity in styrene polymerization

Article abstract of DOI:10.1021/om3001636

A new class of titanium(IV) complexes bearing two phenoxy-azo ligands was synthesized. The Ti-N bonds from X-ray structures were longer than those of corresponding bis(phenoxy-imine)titanium(IV) complexes, which indicates that the coordination of phenoxy-azo ligands becomes weaker on replacing the imine carbon by nitrogen. These titanium complexes were applied for the polymerization of styrene using DMAO as cocatalyst, and syndiotactic polystyrene was obtained. Premixing DMAO with the titanium complex resulted in higher activity, similar to the related bis(phenoxy-imine)titanium(IV) complexes. The coordinating azobenzene subunits did not undergo E → Z photoisomerization upon UV irradiation.

Full text of DOI:10.1021/om3001636

Article
pubs.acs.org/Organometallics
Bis(phenoxy-azo)titanium(IV) Complexes: Synthesis, Structure, and
Catalytic Activity in Styrene Polymerization
Ryo Tanaka, Philipp Viehmann, and Stefan Hecht*
Department of Chemistry, Humboldt-Universitat zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: A new class of titanium(IV) complexes bearing two
phenoxy-azo ligands was synthesized. The Ti−N bonds from X-ray
structures were longer than those of corresponding bis(phenoxy-
imine)titanium(IV) complexes, which indicates that the coordination of
phenoxy-azo ligands becomes weaker on replacing the imine carbon by
nitrogen. These titanium complexes were applied for the polymerization
of styrene using DMAO as cocatalyst, and syndiotactic polystyrene was
obtained. Premixing DMAO with the titanium complex resulted in higher
activity, similar to the related bis(phenoxy-imine)titanium(IV) com-
plexes. The coordinating azobenzene subunits did not undergo E → Z photoisomerization upon UV irradiation.
and allows for unprecedented control over wavelength/
INTRODUCTION
■
intensity and exposure time. Several polymerization catalysts
triggered by light have been reported.5 For example, metathesis
polymerizations of alkynes and norbornenes can be initiated by
Stereoblock homopolymers often show different characteristics
as compared to stereoregular polymers. For example, isotactic
polypropylene containing atactic interior blocks has been
reported by Coates and Waymouth to show elastic properties.1
This particular polymer was synthesized by a titanium complex
bearing freely rotating bis-indenyl ligands, which causes a
change of stereoselectivity during the propagation of the
polymer chain. Moreover, Po and Spera utilized a combination
of a nickel and a titanium catalyst to synthesize syndiotactic
polystyrene capped by isotactic polystyrene blocks.2 In this
case, the polymer chain was transferred between the two
complexes, which are associated with different stereoselectivity.
The two strategies outlined above do not allow for complete
control over the change of stereoselectivity and, hence, exact
adjustment of the ratio and average length of the individual
blocks. However, synthesis of stereoblock polypropylene in a
completely controlled manner was achieved by changing the
ratio of activator and zirconium catalyst during the polymer-
ization.3 Three different polypropylenes with the same
molecular weight and isotactic content yet distributed in blocks
of different average length have been synthesized using this
catalyst system. Furthermore, Shiono and co-workers suc-
ceeded in controlling the syndiospecificity of living propylene
polymerization using an ansa-metallocene titanium complex by
changing the reaction temperature,4a monomer pressure,4b and
polarity of the solvent.4c,d
6
photolysis of W(CO)6 and ruthenium catalysts,7 respectively.
In all of these examples, photoexcitation was used to promote
the dissociation of labile ligands and therefore to generate
unsaturated, catalytically active transition-metal fragments.
However, to the best of our knowledge, there has been no
report of reversible control of catalyst activity and/or selectivity
by light.5 Thus, we were motivated to design and establish
photoswitching polymerization catalyst systems by introducing
photochromic azobenzene units to catalytically active tran-
sition-metal complexes. Bis(phenoxy-imine)titanium com-
plexes, such as 1a,b, are well-known for their activity in the
coordination polymerization of various olefins.8 In our first
attempt to render these catalysts photoswitchable, we replaced
the phenoxy-imine moiety by various phenoxy-azo units and
used these new photochromic ligands to prepare the novel
titanium complexes 3a,b (Figure 1). Note that o-hydroxyazo-
benzene-derived complexes of group 4 metals have not been
described in the literature.9 Herein, we report the synthesis of a
new class of titanium complexes, which bear phenoxy-azo
ligands and their application for the polymerization of styrene.
RESULTS AND DISCUSSION
■
Synthesis of Bis(phenoxyazo) Titanium(IV) Com-
plexes 3a,b. o-Hydroxyazobenzene ligands 2a,b were
synthesized by azo coupling of 2,6-di-tert-butylphenol and the
corresponding diazonium salts in 83% and 48% yields,
respectively. The reaction of 0.5 equiv of TiCl4 with the
sodium salts of 2a,b, which were prepared by the reaction of
Stimulus-responsive catalysts are another promising way to
modulate stereoselectivity during the propagation. Ideally, a
stimulus, which can be precisely controlled with regard to its
energy/intensity with sufficient temporal resolution, will allow
the external “programming” of the catalyst system: i.e., when
the catalyst is generating the type of stereoblock. Among
several potential stimuli, light offers perhaps the most
advantages, since it is a truly noninvasive external stimulus
Received: February 27, 2012
© XXXX American Chemical Society
A
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Figure 1. Design of bis(phenoxy-azo)titanium(IV) complexes.
2a,b with an excess amount of sodium hydride, gave
bis(phenoxy-azo)titanium(IV) complexes 3a,b in 72% yield,
both as dark brown solids (Scheme 1). The 1H NMR spectra of
3a,b showed a single set of broad signals in the aryl region,
indicating that the structures of 3a,b are C2 symmetric.
Figure 2. Crystal structure of 3a (50% thermal ellipsoids). The minor
part of the disordered moiety, CH2Cl2 in the crystal, and hydrogen
atoms are omitted for clarity.
X-ray Structures of 3a,b. Suitable single crystals of 3a,b
were grown from CH2Cl2 solution by slow evaporation. Both
crystals (Figure 2 and 3) showed that the o-hydroxyazobenzene
ligand coordinates to the titanium center in such a fashion that
the distant β-N atom engages in a Ti−N bonding interaction,
thereby forming a six-membered ring. This is rather typical, as
o-hydroxyazobenzenes are known to coordinate to metal
centers by forming six-membered rings10 and only one example
of a five-membered ring has been reported.11 The two phenoxy
O atoms adopt a trans configuration, most likely to avoid steric
repulsion between their o-tert-butyl groups, while both N
donors as well as the two chlorido ligands each adopt a cis
configuration. This coordination behavior is similar to that of
the corresponding bis(phenoxy-imine)titanium complex 1a.12
The average lengths of Ti−O bonds (Table 1) in 3a (1.851
Å), 3b (1.850 Å), and 1a (1.850 Å) are almost the same,
indicating that changing from an imine group to an azo group
does not significantly influence the electronic properties of the
phenoxide. The average lengths of the two Ti−N bonds of 3a,b
are 2.264 and 2.269 Å, respectively, which is considerably
longer than that reported for 1a (2.231 Å),12 probably because
the N atom of the azo NN unit is more electron deficient as
compared to that of an imine CN bond. The N−Ti−N bond
angles of 3a (71.63°) and 3b (72.76°) are significantly smaller
than that of 1a (88.33°), whereas the Cl−Ti−Cl bond angles of
3a (107.77°) and 3b (106.58°) are larger than that of 1a
(97.80°). The latter means that the new complexes 3a,b exhibit
rather open, i.e. sterically less congested, coordination sites
under polymerization conditions.
Figure 3. Crystal structure of 3b (50% thermal ellipsoids). The minor
part of the disordered moiety and hydrogen atoms are omitted for
clarity. Half of the entire structure constitutes an asymmetric unit.
the related phenoxy-imine complex 1b.13 However, a small
1
amount of isotactic sequence was also observed by H NMR
when using 3b (entries 2 and 3), which was not obtained from
1
1b and 3a under the premix conditions (entries 1 and 5). H
NMR analysis of the mixture of 3b and 50 equiv of DMAO in
toluene-d8 showed broad signals with a single set of downfield-
shifted ligand signals, indicating that both phenoxyazo
aluminum and low-oxidation-state titanium complexes were
generated.14 The observed activity was lower than that of 1b
(entries 2 and 5). The number of formed polymer chains,
estimated from the Mn value of the obtained polystyrene, was
smaller than the amount of the catalyst, indicating that only a
fraction of the catalyst initiates chain growth and propagates
Polymerization of Styrene using 3a,b. Styrene polymer-
ization using either 3a or 3b as catalyst was investigated to
compare their catalytic activity with that of the known
bis(phenoxy-imine)titanium complexes. When dry methylalu-
minoxane (DMAO) was mixed with 3b before the addition of
styrene, the polymerization activity was greatly increased
(Table 2, entries 2−4). The produced polystyrene was mainly
syndiotactic, showing the same stereoselectivity for 3a,b as for
Scheme 1. Synthesis of Bis(phenoxy-azo)titanium(IV) Complexes 3a,b
B
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Table 1. Selected Bond Lengths (Å) and Angles (deg) of
3a,b and 1a
a
3a
3b
1a
Ti−O
Ti−N
Ti−Cl
1.8506(16)
1.8498(18)
1.8507(19)
2.248(2)
1.844(3)
1.855(3)
2.2635(18)
2.2873(7)
2.216(4)
2.289(2)
2.245(4)
2.2639(8)
2.2838(8)
169.59(9)
72.75(8)
2.2744(14)
2.2808(14)
165.29(14)
88.33(13)
97.79(5)
O−Ti−O
N−Ti−N
Cl−Ti−Cl
172.83(11)
71.62(9)
107.77(4)
106.58(3)
a
Data taken from ref 12.
Figure 4. UV/vis absorption spectra of 3a (5.8 × 10−5 M in CH2Cl2)
and 3b (4.6 × 10−5 M in CH2Cl2). Both spectra were taken under an
Ar atmosphere in sealed cuvettes at 25 °C.
throughout the polymerization. The achieved molecular weight
distributions under the premixed conditions (entries 1 and 2)
are comparable with that for the bis(phenoxy-imine)titanium-
(IV) reference compound 1b (entry 5).
Photochromism. After having demonstrated that com-
plexes 3a,b display activity for the polymerization of styrene, we
investigated their ability to undergo photoinduced E → Z
isomerization that should lead to potential photocontrol over
the polymerization outcome. First, the UV/vis absorption of
3a,b in dichloromethane was recorded (Figure 4). 3a,b both
showed an intense UV absorption band at 312 nm,
corresponding to the characteristic π → π* transition of the
azobenzene moiety. In addition, a broad shoulder with two
discernible peaks was observed in the range 370−470 nm,
which are assigned to the n → π* transitions of the azo groups
and quinoid tautomers.15
Irradiation of 3a,b with UV light (313 nm) led to no
observable changes in the UV/vis spectra, indicating the
inhibition of E → Z photoisomerization by the presence of the
metal center. In the literature, there is an example of an ortho-
palladated azobenzene in which the five-membered chelate
complex locks the azobenzene in its E configuration and shuts
off photoisomerization.16 Although in our case more flexible
six-membered-ring structures are being adopted, the stability of
the phenoxy-azo titanium complexes seems still too high to
allow for successful photoisomerization, which requires break-
ing of either the Ti−N or Ti−O bonds, respectively.
titanium(IV) complexes. Due to its poorer σ-donating
character, the N atom of the azo group forms longer Ti−N
bonds, which in turn lead to a more open coordination sphere
around the Ti center. The observed polymerization activity and
selectivity for producing syndiotactic polystyrene are both
somewhat reduced as compared to those of the imine-based
systems. Unfortunately, the bis(phenoxy-azo)titanium com-
plexes do not undergo photoinduced E → Z isomerization in
their chelating azobenzene ligand frameworks. We attribute this
behavior to the relatively strong Ti−N and Ti−O bonds,
respectively, which need to yet cannot be broken in the course
of the photoisomerization reaction. This finding suggests that
photoisomerization of the azobenzene moiety will only be
successful if the azo group is not coordinating, i.e. chelating, to
the metal center directly but rather influences the vicinity of the
active site. Although not successful, the study presented herein
constitutes an important lesson on the way to design
photoswitchable transition-metal catalyst systems5 bearing
photochromic azobenzene groups.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations involving the air- and
moisture-sensitive compounds were carried out by using standard
Schlenk techniques under argon. All the solvents, including deuterated
solvents, were distilled under argon and stored in the presence of 4A
molecular sieves. The toluene solution of MAO was purchased from
Sigma-Aldrich and dried under reduced pressure at 70 °C overnight.
The obtained dry MAO (DMAO) was redissolved into toluene prior
to use. Other materials were used as received. Column chromatog-
raphy was carried out with 130−400 mesh silica gel. NMR spectra
were recorded on a 300 MHz (75.6 MHz for 13C) Bruker DPX 300
CONCLUSION AND OUTLOOK
■
A new class of bis(phenoxy-azo)titanium(IV) complexes has
been synthesized and structurally characterized with the aim of
investigating their ability to catalyze the polymerization of
styrene. Their structures and polymerization behavior were
compared to that of the known related bis(phenoxy-imine)-
a
Table 2. Polymerization of Styrene using 3a,b/DMAO as Catalysts
b
c
c
d
entry
complex
premix (min)
temp (°C)
time (h)
activity (g/((mmol of Ti) h))
Mn
PDI
tacticity (syn/iso)
1
2
3
4
3a
3b
3b
3b
1b
10
10
0
60
60
60
20
60
5
1
1
1
1
0.24
2.1
7 800
5 700
8 400
3.1
3.6
6.1
100/0
96/4
94/6
1.0
0
trace
16.04
e
5
13
101 000
2.9
100/0
a
b
Conditions: 1.5 mL of toluene, 5 mL of styrene, 10 μmol of 3a,b, 2.3 mmol of DMAO (as Al). Calculated from the yield of polymer, which is
c
d
insoluble in 2-butanone. Determined by GPC calibrated with narrow polystyrene standards. Ratio of syndiotactic and isotactic polymer determined
1
by the integral ratio of methane signals of H NMR spectra in 1,1,2,2-tetrachloroethane-d2, referenced by isotactic polystyrene and syndiotactic
polystyrene.13 Data taken from ref 13.
e
C
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Article
spectrometer at 27 °C using residual protonated solvent signals as
internal standard (1H NMR, δ(CDCl3) 7.26 ppm; 13C NMR,
δ(CDCl3) 77.0 ppm). UV/vis absorption spectra were recorded
using quartz cuvettes of 1 cm path length on a Cary 50
spectrophotometer, equipped with Peltier thermostated cell holders
(ΔT = 0.05 °C). High-resolution mass spectrometry was performed
on a Thermo LTQ FT instrument (ESI) and MSI concept 1H
spectrometer (EI, 70 eV ionization) as well as on a QSTARXL Applied
Q-TOF instrument with a ISV of 950 V. Irradiation was performed
using a LOT-Oriel 1000 W medium-pressure Xe lamp, equipped with
a special cutoff filter (λmax > 310 nm). GPC measurements in THF as
eluent were performed on PSS columns in a WGE Dr. Bures TAU
2010 column oven, equipped with a WGE Dr. Bures Q-2010 HPLC
pump, a Knauer Smartline 3800 autosampler, and WGE ETA-2020 RI-
visco-detector. Calibration was done with a polystyrene calibration kit
(S-L-10 LOT 79).
Preparation of Phenoxyazo Ligand 2a. Aniline (1.82 mL, 20
mmol) was dissolved in 20 mL of 1 M aqueous HCl. To this solution
was slowly added NaNO2 (1.38 g, 20 mmol) in water (20 mL) at 0 °C,
and the mixture was stirred for 1 h. To the resulting yellow suspension
was added a solution of 2,4-di-tert-butylphenol (4.12 g, 20 mmol) and
NaOH (800 mg, 20 mmol) in MeOH (20 mL) and water (20 mL)
dropwise at 0 °C, and the mixture was stirred for 15 h. The red
precipitate was filtered, washed with 100 mL of water and 100 mL of
MeOH, and dried under vacuum. A 5.13 g amount (83%, 16.6 mmol)
of brown solid 2a was obtained. 1H NMR (300 MHz, CDCl3): δ 13.70
(br s, 1H), 7.91−7.85 (m, 2H), 7.80 (d, J = 3 Hz, 1H), 7.56−7.42 (m,
4H), 1.48 (s, 9H), 1.38 (s, 9H). 13C NMR (75 MHz, CDCl3): δ 150.7,
150.5, 141.5, 138.0, 137.1, 130.8 (CH), 129.5 (CH), 128.6 (CH),
127.7 (CH), 122.1 (CH), 35.5, 34.4, 31.6 (CH3), 29.6 (CH3). HRMS
(ESI+): 311.2130, calcd for C20H26N2O (M + H) 311.2123.
mmol) in small portions. The resulting purple solution was stirred at
room temperature for 15 min and then added dropwise at −30 °C to a
solution of TiCl4 (0.11 mL, 1 mmol) in hexanes (5 mL), and the
resulting brown solution was stirred at room temperature for 15 h. The
solvent was evaporated in vacuo, and the remaining brown solid was
dissolved in 10 mL of CH2Cl2. The solution was stirred for 10 min,
filtered, and evaporated. The remaining solid was washed three times
with 2 mL of cold pentane. The solid was dried in vacuo, and 584 mg
(72%, 0.72 mmol) of brown powder was obtained. Crystals for X-ray
structure analysis were grown by slow evaporation of a CH2Cl2
1
solution. H NMR (300 MHz, CDCl3): δ 7.69 (br, 1H), 7.63 (br,
1H), 6.80 (br, 2H), 6.56 (br, 1H), 1.56 (s, 9H), 1.36 (s, 9H). 13C
3
NMR (75 MHz, CDCl3): δ 162.2 (1JCF = 13 Hz, JCF = 249 Hz),
156.0, 148.1, 146.0, 143.2, 138.6, 135.8 (CH), 131.1 (CH), 107.6
(CH, 2JCF = 24 Hz), 104.2 (CH, 2JCF = 24 Hz), 35.7, 34.7, 31.1 (CH3),
30.0 (CH3). 19F NMR (282 MHz, CDCl3): δ −107.1 (br) HRMS
(ESI−) in MeOH: 839.2615, calcd for C41H49Cl2F4N4O3Ti ([M +
MeO]−) 839.2597.
Polymerization of Styrene Catalyzed by 3/DMAO. To a
solution of 3a,b (10 μmol) in toluene (1.5 mL) was added DMAO in
toluene (0.76 mL, 2.3 mmol as Al) dropwise, and the mixture was
stirred for 10 min at room temperature. To the reddish purple solution
was added styrene (5.0 mL, 43.5 mmol), and this mixture was stirred
at 60 °C for the appropriate time. The reaction mixture was poured
into MeOH (50 mL) containing concentrated HCl (0.5 mL). The
precipitated polymer was filtered, washed with MeOH, and dried. The
obtained colorless solid was extracted with boiling methyl ethyl ketone
(5 mL) to get rid of atactic polystyrene. The solid was filtered and
dried overnight to a constant weight. The activity of 3a,b was
calculated from the yield of remaining polymer (12 and 21 mg,
respectively). The isotactic/syndiotactic polymer ratio was analyzed by
1H NMR.
Preparation of Phenoxyazo Ligand 2b. 3,5-Difluoroaniline
(1.29 g, 10 mmol) was dissolved in 10 mL of 1 M aqueous HCl. To
this solution was slowly added NaNO2 (690 mg, 10 mmol) in water
(10 mL) at 0 °C, and the mixture was stirred for 1 h. To the resulting
yellow suspension was added a solution of 2,4-di-tert-butylphenol
(2.06 g, 10 mmol) and NaOH (400 mg, 10 mmol) in MeOH (10 mL)
and water (10 mL) dropwise at 0 °C, and the mixture was stirred at
room temperature for 15 h. The red precipitate was filtered, washed
with 100 mL of water and 100 mL of MeOH, and dried under vacuum.
A 1.63 g amount (48%, 4.8 mmol) of brown solid 2b was obtained. 1H
NMR (300 MHz, CDCl3): δ 13.29 (br s, 1H), 7.77 (d, J = 3 Hz, 1H),
7.50 (d, J = 2 Hz, 1H), 7.49−7.41 (m, 2H), 6.92 (tt, J = 9 Hz, 2 Hz,
1H), 1.46 (s, 9H), 1.38 (s, 9H). 13C NMR (75 MHz, CDCl3): δ 163.6
X-ray Crystallography. Crystals were transferred to the respective
diffractometers and cooled to 100 K by a stream of cold N2 gas. In
order to increase the signal to noise ratio for the diffracted intensities, a
data collection strategy involving ψ scans and multiple ω scans at
different ψ values and constant φ was employed. The resulting high-
redundancy intensity data were averaged using the programs SADABS
(Sheldrick/Bruker-AXS, 2005) and SCALEPACK (Otwinowski,
2
2
2
1997). Rint = ∑|Fo − Fo (mean)|/∑[Fo ], R1 = ∑||Fo| − |Fc||/∑|
Fo| and wR2 = {∑[w(Fo − Fc2)2]/∑[w(Fo )2]}1/2. Structures were
solved using direct methods (SHELXS-97) and refined by least-
squares (SHELXL-97) on Fo (both programs from G. Sheldrick,
2
2
2
University of Gottingen, 1997).
̈
1
3
3
(dd, JCF = 248 Hz, JCF = 14 Hz), 152.8 (t, JCF = 10 Hz), 150.8,
142.1, 138.3, 137.1, 130.0 (CH), 128.0 (CH), 105.5 (m, CH), 35.5,
34.5, 31.5 (CH3), 29.6 (CH3). 19F NMR (282 MHz, CDCl3): δ 163.6
(t, JHF = 8 Hz). HRMS (ESI−): 345.1776, calcd for C20H23F2N2O (M
− H) 345.1779.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files, tables, and figures giving details of the X-ray
structures and characterization data of new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Preparation of Titanium Complex 3a. To a solution of ligand
2a (620 mg, 2 mmol) in THF (10 mL) was added NaH (96 mg, 4
mmol) in small portions. The resulting purple solution was stirred at
room temperature for 15 min and added dropwise at −30 °C to a
solution of TiCl4 (0.11 mL, 1 mmol) in hexanes (5 mL). The resulting
brown solution was stirred at room temperature for 15 h. The solvent
was evaporated in vacuo, and the remaining brown solid was dissolved
in 10 mL of CH2Cl2. The solution was stirred for 10 min, filtered, and
evaporated. The remaining solid was washed three times with 2 mL of
cold pentane. The solid was dried in vacuo, and 533 mg (72%, 0.72
mmol) of brown powder was obtained. Crystals for X-ray structure
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sh@chemie.hu-berlin.de.
Notes
1
analysis were grown by slow evaporation of a CH2Cl2 solution. H
The authors declare no competing financial interest.
NMR (300 MHz, CDCl3): δ 7.61 (br, 2H), 7.24 (br, 2H), 7.12 (br,
3H), 1.53 (s, 9H), 1.36 (s, 9H). 13C NMR (75 MHz, CDCl3): δ 154.5,
147.8, 144.9, 143.0, 138.1, 134.2 (CH), 131.0 (CH), 129.0 (CH),
128.4 (CH), 123.2 (CH), 35.7, 34.6, 31.3 (CH3), 30.0 (CH3). HRMS
(ESI+): 701.3102, calcd for C40H50ClN4O2Ti (M − Cl) 701.3102.
Anal. Calcd for C40H50Cl2N4O2Ti: C, 65.13; H, 6.83; N, 7.60. Found:
C, 64.76; H, 6.79; N, 7.53.
ACKNOWLEDGMENTS
■
Generous support by the German Research Foundation (DFG
via SPP 1179 and SFB 658) and JSPS research fellowships for
young scientists are gratefully acknowledged. BASF AG, Bayer
Industry Services, and Sasol Germany are thanked for generous
donations of chemicals.
Preparation of Titanium Complex 3b. To a solution of ligand
2b (693 mg, 2 mmol) in THF (10 mL) was added NaH (96 mg, 4
D
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dx.doi.org/10.1021/om3001636 | Organometallics XXXX, XXX, XXX−XXX