Monomeric peroxo titanate coordinated with cyclohexanediaminetetraacetate: Towards the active oxygen species of the Ti(IV) site hosted in the titanium silicalite catalyst TS-1

Article abstract of DOI:10.1016/j.poly.2011.12.037

Hexadentate chelated trans-1,2-cyclohexanediaminetetraacetato titanate(IV) was used as a model of the Ti(IV) species in the constrained environment of TS-1 (H₄cdta = trans-1,2-cyclohexanediaminetetraacetic acid, C ₁₄H₂₂O₈N₂; TS, titanium silicalite). Several complexes were isolated and well characterized, including [Ti(cdta)(H₂O)]·2H₂O (1), Na₂[Ti(O ₂)(cdta)]·2H₂O (2) and (NH₄) ₂[TiO(cdta)]·1.5H₂O (3). Octadentate cdta peroxo titanate(IV) (2) is obtained quantitatively from the reaction of the hydrated titanium cdta complex 1 with hydrogen peroxide. The bond distances between O-O and Ti-(O₂) are 1.464(3) and 1.908(2) , respectively. Transformation of the peroxo titanium(IV) complex with excess H₄cdta and heating results in the formation of an interesting oxotitanate in H₂O-H ₂O₂ solution. The catalytic activity of 2 was studied for phenol hydroxylation using 30% H₂O₂, and this was monitored by ¹H and ¹³C NMR techniques. The main product of the reaction is catechol.

Full text of DOI:10.1016/j.poly.2011.12.037

Polyhedron 35 (2012) 1–6
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Polyhedron
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Monomeric peroxo titanate coordinated with cyclohexanediaminetetraacetate:
Towards the active oxygen species of the Ti(IV) site hosted in the titanium
silicalite catalyst TS-1
⇑
Qiong-Xin Liu, Zhao-Hui Zhou
Department of Chemistry, College of Chemistry and Chemical Engineering and State Key Laboratory for Physical Chemistry of Solid Surfaces,
National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, 361005 Xiamen, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Hexadentate chelated trans-1,2-cyclohexanediaminetetraacetato titanate(IV) was used as a model of the
Ti(IV) species in the constrained environment of TS-1 (H4cdta = trans-1,2-cyclohexanediaminetetraacetic
acid, C14H22O8N2; TS, titanium silicalite). Several complexes were isolated and well characterized, includ-
ing [Ti(cdta)(H2O)]Á2H2O (1), Na2[Ti(O2)(cdta)]Á2H2O (2) and (NH4)2[TiO(cdta)]Á1.5H2O (3). Octadentate
cdta peroxo titanate(IV) (2) is obtained quantitatively from the reaction of the hydrated titanium cdta
complex 1 with hydrogen peroxide. The bond distances between O–O and Ti–(O2) are 1.464(3) and
1.908(2) Å, respectively. Transformation of the peroxo titanium(IV) complex with excess H4cdta and
heating results in the formation of an interesting oxotitanate in H2O–H2O2 solution. The catalytic activity
of 2 was studied for phenol hydroxylation using 30% H2O2, and this was monitored by 1H and 13C NMR
techniques. The main product of the reaction is catechol.
Received 20 November 2011
Accepted 15 December 2011
Available online 12 January 2012
Keywords:
Cyclohexanediaminetetraacetic acid
Peroxo titanate
Titanium silicalite
Hydroxylation
Hydrogen peroxide
Catalytic mechanism
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
It is widely recognized that titanium-containing molecular
sieves (TS-1) are among the best catalysts for selective oxidation
reactions, for example, alkane and alkene oxidation to alcohol or
epoxide, and phenol oxidation into hydroquinone and catechol
using hydrogen peroxides as the oxidant [1–4]. The later is an
industrially important process because the products are widely
used as starting materials for medicine, perfume and many fine
chemicals. In the past decade, considerable research effort was
dedicated to the development of mesoporous titanium-containing
materials capable of oxidizing large amounts of organic substrates.
The mechanism for the activation is considered to involve the
addition of a peroxo group and its transfer to the substrate [5].
However, not much evidence for the active species was obtained.
In this report, catalytic reactions by the restricted peroxo titanate
Na2[Ti(O2)(cdta)]Á2H2O (2) are studied for phenol hydroxylation,
in which the titanium atom is coordinated by the hexadentate cdta
ligand, and exists in an octa-coordinated environment. The conver-
sion of peroxo titanium complexes are studied, which might
provide structural information on titanium complexes that is
relevant to key intermediates in selective oxidation.
2.1. Preparations
2.1.1. Preparation of [Ti(cdta)(H2O)]Á2H2O (1)
Trans-cyclohexanediaminetetraacetic acid (1.09 g, 3.0 mmol)
was dissolved in 10 mL deionized water at pH 6.0 with ammonium
hydroxide (5.0 M). The solution was added to TiCl4 (0.57 g,
3.0 mmol) to give plenty of white powder, which was collected
and washed with cold water and 95% ethanol to give 1.26 g [Ti
(cdta)(H2O)]Á2H2O (1) in 95% yield. C, H and N elemental analyses
for C14H24N2O11Ti, Calc.: C, 37.9; H, 5.5; N, 6.3. Found: C, 38.0; H,
5.4; N, 6.2%. IR (KBr, cmÀ1):
mas(CO2) 1690vs, 1634w; ms(CO2)
1403m, 1354s, 1291m. 13C NMR dC (D2O) ppm: 181.2, 179.9 (–CO2),
70.6, 66.8 (–CH2CO2), 60.2 (@NCH), 28.0 (@NCHCH2), 26.1
(@NCHCH2CH2).
2.1.2. Preparation of Na2[Ti(O2)(cdta)]Á2H2O (2)
H4cdta (5.47 g, 15.0 mmol) was dissolved in 10 mL deionized
water at pH 6.0 with sodium hydroxide (5.0 M). The solution
was added to TiCl4 (2.85 g, 15.0 mmol) and a 30% solution of
H2O2 (5.0 mL). The pH value of the solution was adjusted to 5.5
with sodium hydroxide (5.0 M) to give a clear orange solution.
The mixture was stirred continuously for 5 h and kept in a refrig-
erator for 5 days to deposit red crystals of 2 (7.20 g, 95%). C, H
and N elemental analyses for C14H22N2Na2O12Ti, Calc.: C, 33.4;
⇑
Corresponding author. Fax: +86 592 2183047.
E-mail address: zhzhou@xmu.edu.cn (Z.-H. Zhou).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.12.037

2
Q.-X. Liu, Z.-H. Zhou / Polyhedron 35 (2012) 1–6
H, 4.4; N, 5.6. Found: C, 33.2; H, 4.5; N, 5.7%. IR (KBr, cmÀ1):
CO2) 1661s, 1630vs s(CO2) 1412s, 1367s, 1316m; (O–O) 861m;
[Ti–(O2)] 576m, 542m. 13C NMR dC (D2O) ppm: 181.2 (–CO2),
mas(-
3. Results and discussion
;
m
m
m
3.1. Synthesis and characterization
69.9, 66.5 (–CH2CO2), 59.3 (@NCH), 28.2 (@NCHCH2), 26.4
(@NCHCH2CH2).
The syntheses of cdta titanates were carried out in weakly
acidic aqueous solutions. The synthetic conditions are similar, ex-
cept for the molar ratios and reaction temperatures. Direct reaction
of TiCl4/H4cdta in an equimolar ratio results in the formation of an
aquo complex 1 quantitatively. When an excess of hydrogen per-
oxide is added to the solution of [Ti(cdta)(H2O)]Á2H2O (1), the per-
2.1.3. Preparation of (NH4)2[TiO(cdta)]Á1.5H2O (3)
H4cdta (10.93 g, 30.0 mmol) was dissolved in 10 mL deionized
water with ammonium hydroxide (5.0 M). The solution was added
to TiCl4 (1.91 g, 10.0 mmol) and 30% hydrogen peroxide (5.0 mL).
The pH value of the solution was adjusted to 6.0 with ammonium
hydroxide to give a clear orange solution. The mixture was stirred
at room temperature for 5 h and heated at 70 °C for 2 days to de-
posit colorless crystals of 3 (2.45 g, 52%). C, H and N elemental
analyses for C14H29N4O10.50Ti, Calc.: C, 35.8; H, 6.2; N, 12.0. Found:
oxo complex
2 is obtained in 95% yield, which could be
transformed to the oxotitanate 3 in warm solution with excess cdta
ligand, as shown in Scheme 1. Previously, an oxotitanate was a sus-
pected species in the reaction of edta titanate [8].
Figs. 1–3 show the neutral and anion structures of 1–3, respec-
tively. The cdta ligands in 1–3 coordinate hexadentately to the cen-
tral titanium cations through two nitrogen atoms and four oxygen
atoms of monodentate b-carboxy groups. In 1, the titanium(IV) cat-
ion is surrounded by the cdta ligand and a water molecule (O1W)
heptadentately, in a distorted pentagonal bipyramid environment.
This resembles to the coordinations in [Ti(edta)(H2O)] [9] and
Na[VL(H2O)]ÁnH2O (L = edta, n = 3; L = cdta, n = 5) [10,11]. The
coordinated water molecule is easily substituted with the addition
of hydrogen peroxide, forming a peroxo species 2 quantitatively. In
the distorted octa-coordinated geometry of 2, the peroxo
C, 35.9; H, 6.4; N, 12.3%. IR (KBr, cmÀ1):
mas(CO2) 1686m, 1626vs;
ms(CO2) 1448m, 1403s, 1353s;
m
(Ti@O) 941m. 13C NMR dC (D2O)
ppm: 182.3, 182.2 (–CO2), 67.0, 64.1 (–CH2CO2), 57.5 (@NCH),
27.2 (@NCH2), 26.6 (@NCH2CH2).
2.2. Physical methods and analyses
Data collections for 1–3 were performed on an Oxford Gemini S
Ultra system with graphite monochromated Mo Ka (k = 0.71073 Å)
2À
group O2 binds to the Ti(IV) cation with a strong Ti–O bond
[1.908(2) Å] in a side-on
g
2-fasion, occupying two coordination
radiation. Adsorption corrections were applied by using the CRYSALIS
program. Empirical absorption corrections were applied using the
SADABS program. Structures were solved by the WingGX package,
and refined by full-matrix least-squares procedures with
anisotropic thermal parameters for non-hydrogen atoms using
SHELXL-97 [6,7]. Hydrogen atoms were located from a difference
Fourier map and refined anisotropically. Crystallographic data are
summarized in Table S1. Selected bond distances and bond angles
of the complexes are listed in Tables S2–S4.
All chemicals used were analytical grade reagents. The pH val-
ues were measured using a potentiometric method with a digital
PHB-8 pH meter. Infrared spectra were recorded as Nujol mulls
between KBr plates on a Nicolet 360 FT-IR spectrometer. Elemen-
tal analyses were performed with an EA 1110 elemental analyzer.
1H and 13C NMR spectra were recorded in D2O on a Bruker AV
400 NMR spectrometer using DSS (sodium 2,2-dimethyl-2-sila-
pentane-5-sulfonate) as an internal reference. Confocal Raman
microspectroscopy was conducted at room temperature with a
Renishaw Inva Raman System equipped with a CCD detector
and a Leica DMLM microscope. The line at 532.5 nm of an Ar+ la-
ser was used for excitation. The laser power was reduced to
ꢀ1.5 mW to ensuring that no sample damage was caused by
the laser irradiation.
sites in the equational plane. This is different from the hepta-coor-
dinated edta peroxo titanium complex reported previously, which
could be degraded to edta titanate selectively [8,12]. It is interest-
ing to note that an oxotitanate is isolated in the H2O2/H2O system.
A terminal oxygen atom occupies the seventh coordination site
instead of coordinated water molecule and peroxo group, with a
strong short Ti–O bond [1.671 (2) Å].
As shown in Tables S2–S4, the Ti–Ocarboxy bond distances in the
cdta titanium complexes 1–3 vary systematically. The Ti–OW bond
distance [2.046(3) Å] in 1 is longer than those of the Ti–O bonds to
the coordinated carboxy groups; this is an expected result ascrib-
able to the ionic contribution from the charged glycinate oxygen
atom. The Ti–Operoxo bond distance of 2 is 1.908(2) Å, which is
longer than those found in the other seven-coordinated peroxo
titanium complexes, such as (NH4)2[Ti(O2)(edta)]Á2H2O [1.845(3),
1.858(3) Å] [8], (NH4)4[Ti(O2)(cit)]2Á2H2O [1.852(2), 1.890(2) Å]
[13]
and
Ba2(NH4)2[Ti4(O2)4(Hcit)2(cit)2]Á10H2O
[1.873(3),
1.884(3), 1.841(4), 1.855(4) Å] [14]. This should be related to the
steric hindrance of the octa-coordination. It is interesting to note
2.3. Hydroxylation of phenol
Reactions were performed in a two-necked round-bottom flask,
which was equipped with a magnetic stirrer, a reflux condenser
and a dropping funnel in a temperature controllable water-bath.
In a typical run, 1.89 g of phenol, 10 mL deionized water and
10 mL 30% hydrogen peroxide were charged into the flask. After
the mixture was heated to 90 °C under continuously stirring, a
solution of the catalyst Na2[Ti(O2)(cdta)]Á2H2O (2) (0.23 g,
0.46 mmol) was added drop-wise to the reactor. The pH value of
the solution was adjusted to 6.0, and the reaction proceeded for
a certain period of time. After the reaction was finished, the mix-
ture was cooled and analyzed. The conversion of phenol to catechol
in the presence of 2 with elapsed time was monitored by 1H and
13C NMR spectroscopies.
Scheme 1. Synthesis and transformation of the titanium(IV) cdta complexes.

Q.-X. Liu, Z.-H. Zhou / Polyhedron 35 (2012) 1–6
3
O9
O1w
O6
O5
O1
Ti1
O1
O3
O5
C5
O2
O2
Ti1
O7
N2
C1
O4
C5
C7
O7
C6
O6
C1
C3
O8
O3
C3
C7
O8
C8
C2
C2
O4
N1
C9
C8
N2
C14
C13
C4
C6
N1
C9
C4
C14
C10
C11
C13
C10
C12
C11
C12
Fig. 3. ORTEP plot of the anion structure in (NH4)2[TiO(cdta)]Á1.5H2O (3) with 30%
Fig. 1. ORTEP plot of the anion structure in [Ti(cdta)(H2O)]Á2H2O (1) with 30%
probability levels.
probability levels.
distances [2.504(2) and 2.447(2) Å] in 3 are the longest bond dis-
tances found in Ti-cdta species. The Ti–N bonds in 3 are weaker
than those of the other two titanium-cdta complexes. It is reason-
able that shorter Ti@O bonds cause longer Ti–N bonds, showing a
strong structural trans effect in the TiO complex.
O5
O1
O2
FT-IR spectra of 1–3 exhibit strong vibrations for the carboxy
groups of cdta, as shown in Fig. S1. Different types of Ti–O vibra-
tions are illustrated in Table S5. The sharp peaks near 1690 and
1350 cmÀ1 (1690 and 1354 cmÀ1 for 1, 1661 and 1412 cmÀ1 for
2, 1686 and 1448 cmÀ1 for 3) are assigned to the asymmetric and
symmetric stretching frequencies of the carboxy groups in the
equatorial glycinate ring, respectively, while the vibrations be-
tween 1634 and 1291 cmÀ1 (1634 and 1291 cmÀ1 for 1, 1630 and
1367 cmÀ1 for 2, 1626 and 1403 cmÀ1 for 3) are assigned to the
asymmetric and symmetric stretching frequencies of carboxy
groups that lie in the axial glycinate ring, respectively. They shift
to lower values with respect to the carboxy groups of the free cdta
ligand. This is in harmony with coordinated structures. The vibra-
tion of the peroxo group in 2 is found at 861 cmÀ1. There is also a
sharp peak at 576 cmÀ1, which is attributed to the vas[Ti–(O2)]
vibration. The corresponding frequency for vs[Ti–(O2)] is observed
around 542 cmÀ1, which is in agreement with the corresponding
assignments in the other peroxo titanium complexes [13,18]. The
obvious band at 941 cmÀ1, below the usual region of 972–
890 cmÀ1 [8], which is absent in 1 and 2, corresponds to the titanyl
moiety in 3. The lack of absorptions in the 1700–1750 cmÀ1 region
suggests that the carboxy groups in 1–3 are protonated.
Ti1
C1
C2
O3
O4
C3
C4
N1
C5
C6
C7
Fig. 2. ORTEP plot of the anion structure in Na2[Ti(O2)(cdta)]Á2H2O (2) with 30%
probability levels.
The peroxo vibrations in 2 are also in harmony with the corre-
sponding Raman spectrum. Fig. S2 demonstrates characteristic fea-
tures of the coordinated peroxo (O22À) species [14]. A sharp peak
near 550 cmÀ1 is assigned to the v(Ti–O2) symmetric stretching
vibration, while 856 cmÀ1 corresponds to the v(O–O) mode of the
peroxo group. The observed positions of v(O–O) and v(Ti–O2) are
that the Ti@O bond distance [1.671(2) Å] in (NH4)2[TiO(cdta)]
Á1.5H2O (3) is significantly shorter than any other Ti–O bond
distances, which are comparable to related Ti(IV) complexes,
such as [C(NH2)3]4[TiO(CO3)3]Á2H2O [1.680(2) Å] [15], TiO
(OEPMe2) (OEPMe2 =
a,c
-dimethyl-
a
,c
-dihydro octaethylporphi-
in accordance with a side-on g
2-peroxo coordination.
nate) [1.619(4) Å] [16] and [LTiO(NCS)2] (L = 1,4,7-trimethyl-
1,4,7-triazacyclononane) [1.638(3) Å] [17]. While the Ti–N bond
The solution 13C NMR spectra of 1–3 were measured in D2O as
shown in Figs. S3–S5. The titanium-cdta complexes 1–3 gave only

4
Q.-X. Liu, Z.-H. Zhou / Polyhedron 35 (2012) 1–6
ο
PhOH
ο
ο
ο
PhOH
after 2 h
o
o
PhOH
o
∗
∗
after 5.5 h
o
*
*
∗
#
160
140
120
100
δC (ppm)
160
140
120
100
Fig. 4. 13C NMR spectra of the reaction mixture after 2 and 5.5 h; phenol (o), 1,2-
catechol (Ã).
δC (ppm)
Fig. 5. 13C NMR spectrum of the reaction mixture after 7.5 h; phenol (o); catechol
(Ã); benzoquinone (#).
one set of resonances. The signals show obvious downfield shifts
compared with free ligand, which were attributed to the carboxy
and methylene groups of the cdta ligand. Due to the low solubility
of 1, its signals are weaker than those of the peroxo and oxotitan-
ates. No additional species of titanium-cdta complexes are
observed in the 13C NMR spectra, showing no decomposition of
the cdta complexes in solution.
PhOH
Due to the similarity of the 13C NMR spectra for 1–3, only com-
plex 2 is discussed. Its two resonances of 66.5 and 181.2 ppm are
assigned to the methylene and carboxy groups of the cdta ligand
from the equatorial glycinate ring, while the other two peaks
(69.9 and 181.2 ppm) are assigned to the methylene and carboxy
groups of the cdta ligand from the axial glycinate ring. The remain-
ing three peaks, appearing at 59.3, 28.2 and 26.4 ppm, were as-
signed to the methylene and methine groups of the cdta ligand.
Compared with the free cdta ligand [170.9 (CO2), 61.8 (–CH2
CO2), 52.0 (@NCH), 24.9 (@NCH2), 24.1(@NCH2CH2)], complex 2
DSS
shows large downfield shifts for the carboxy and
a-methylene
x
x
x
x
x
x
x
carbon resonances ( d 10.3 and 8.1 ppm) due to coordination. This
D
indicates that the carboxylic acid groups in cdta are fully deproto-
nated and coordinate to the titanium cation simultaneously.
200
150
100
50
0
δC (ppm)
3.2. Catalytic activity
Fig. 6. 13C NMR spectrum of the reaction mixture after 7.5 h. Na2[Ti(O2)(cd-
ta)]Á2H2O (x).
In order to find suitable conditions for the catalytic oxidation of
phenol, various parameters, such as solvent, amount of catalyst 2,
reaction time and temperature have been taken into consideration.
Water is used as a reaction medium. This is due to the solubility
of the reactant and catalyst. Na2[Ti(O2)(cdta)]Á2H2O (2) disperses
more easily in water than in organic solvents. The other reason is
the active species produced in organic solvents may give other
side-reactions. Moreover, water is safe, cheap and an environmen-
tally friendly solvent.
tion. After reacting for 5.5 h, the spectrum showed another two
peaks at 123.8 and 119.1 ppm, as illustrated in Fig. 5, which were
attributed to the 13C NMR signals of catechol. This indicated that
phenol was oxidized to catechol gradually.
It is noteworthy that the intensities of the catechol resonances
increase with time. When the solution reacted for 7.5 h, an obvious
peak at 146.7 ppm, assigned to the signal of catechol, was observed
in solution, which demonstrates that catechol is a main product
after a long period of time. Only a small amount of product was
further oxidized to benzoquinone, observed at 137.0 ppm, as
shown in Fig. 5. The peaks at 180.1, 179.9, 69.9, 65.0, 58.8, 28.2
and 26.3 ppm in Fig. 6 were attributed to the signals of Na2[-
Ti(O2)(cdta)]Á2H2O (2), which indicated that the peroxo titanium
complex 2 was used as a catalyst without change. The 1H NMR
spectrum of reaction solution in Fig. S6 is similar to the spectrum
of catechol, which confirms the oxidation of phenol to catechol.
No product was detected when the reaction was carried out
without the catalyst Na2[Ti(O2)(cdta)]Á2H2O (2). Complex
2
showed an obvious catalytic activity for hydroxylation of phenol
and resulted in a catechol selectivity of 99%.
Figs. 4 and 5 show the 13C NMR spectra of a reaction mixture re-
fluxed in a 90 °C water bath for 2, 5.5 and 7.5 h, respectively, which
demonstrate the influence of reaction time on the hydroxylation of
phenol at pH 6.0. As shown in Fig. 4, only four peaks at 155.0,
129.8, 121.1 and 115.5 ppm, assigned to the signals of phenol, were
observed after 2 h, revealing no hydroxylation of phenol in solu-

Q.-X. Liu, Z.-H. Zhou / Polyhedron 35 (2012) 1–6
5
Table 1
Catalytic conversion and selectivity in phenol hydroxylation by H2O2 over
Na2[Ti(O2)(cdta)]Á2H2O (2) with various reaction times.a
Time (h)
Phenol conv. (mol%)
Product selectivity (%)
Catechol
Benzoquinone
5.5
7.5
2
5
99.0
80.0
1.0
20.0
Scheme 2. Titanium peroxo species as a suggested intermediate for TS-1.
a
Reaction conditions: water as a solvent, reaction temperature 90 °C, H2O2/
phenol = 5 (molar ratio), catalyst/phenol = 1:50 (molar ratio).
It is clear that the yield of catechol and the conversion of phenol
increases with reaction time, as shown in Table 1. The phenol con-
version and selectivity to benzoquinone increased with reaction
time, while the selectivity of catechol increased to a maximum va-
lue, and then decreased with reaction time. A relatively short time
resulted in no conversion of phenol. In the region from 0 to 7.5 h, a
change in the reaction is notable. When the reaction time was 2 h,
no phenol hydroxylation reaction occurs. At 5.5 h, the catechol
selectivity is 99%, which is the maximal selectivity of the hydrox-
ylation products. When the reaction time was further prolonged,
the phenol conversion increased continuously, while catechol
selectivity decreased a little. A possible reason is that hydroqui-
none is consumed in the formation of the benzoquinone products,
whereas catechol, a thermodynamically favored isomer, is not eas-
ily oxidized [19].
The influence of the reaction temperature was also investigated.
When the reaction proceeded at 70 °C, no conversion of phenol was
observed. After the temperature was increased to 90 °C, signals for
catechol were found, which indicates that the hydroxylation of
phenol in the presence of catalyst Na2[Ti(O2)(cdta)]Á2H2O (2) could
only occur at a relative high reaction temperature.
Scheme 3. Catalytic cycle proposed for hydroxylation of phenol to catechol by the
peroxotitanium complex Na2[Ti(O2)(cdta)]Á2H2O (2).
3.3. Catalytically active species and suggested mechanism
As outlined in Scheme 1, [Ti(cdta)(H2O)]Á2H2O (1) in aqueous
solution could be transformed to the peroxo and oxotitanium(IV)
cdta complexes, respectively. The water soluble peroxo titanate
Na2[Ti(O2)(cdta)]Á2H2O (2) and the oxotitanate (NH4)2[TiO(cd-
ta)]Á1.5H2O (3) are accessible by the hydrolysis of TiCl4 in the pres-
ence of H2O2 and the cdta ligand, and are isolated as the sodium
and ammonium salts, respectively.
idly transforms into a relatively more stable peroxotitanium spe-
cies 2 on loss of a water molecule. The peroxotitanium complex
2 interacts with the substrate phenol in another pre-equilibrium
step to give a metallo-peroxy-arene intermediate species. Transfer
of oxygen by C–H bond activation takes place as a rate determining
step to give a titanium phenoxy intermediate species. Hemolytic
cleavage results in rapid dissociation to give catechol and regener-
ating the oxotitanate species 3 in the catalytic cycle.
Compared with the data for the other cdta complexes, 2 is the
best catalyst for the transformation of phenol. The following fac-
tors may be responsible for this. First of all, peroxo Ti(IV) has a rel-
atively high affinity for oxygen ligation compared to the other
transition metals, it prefers a seven or eight coordination number
rather than six [9], and the coordination ring size of cdta is large
enough to allow the flexible hexadentate cdta ligand to be
wrapped around the Ti(IV) ion, which leads to the formation of
the octa-coordinated complex 2 with a stable configuration, as
shown by structural analysis. Secondly, in the titanium silicalite
catalyst, Notari [20] and Huybrechts et al. [21] have suggested that
a titanium peroxo species is an active intermediate in Scheme 2.
Compared with the active oxidative species in TS-1, [Ti(O2)]2+ as
the possible catalytically active species is supported, which may
be related to the electric and geometric arrangement of the octad-
entate coordinated structure. This intermediate transfers the coor-
dinated oxygen atoms to the substrate for the product. Finally, the
oxo group persists during the oxidative cleavage reaction, indicat-
ing its participation in the process, presumably an oxo anion facil-
itates oxidative electron removal.
4. Conclusion
In conclusion, we have isolated and characterized three new
titanium-cdta complexes in aqueous solution, in which the peroxo
titanate(IV) 2 exists in an octadentate environment and an inter-
esting short Ti@O bond distance [1.671(2) Å] was found in the
oxo titanium complex 3. In this work, the catalytic reaction of phe-
nol conversion is tested, which shows obvious hydroxylation of
phenol to catechol in the presence of the peroxo titanium complex
2 at 90 °C after 5.5 h. The oxidative reaction is unique and is
believed to share some mechanistic commonality with the TS-1
catalyst. As a detailed mechanistic interpretation for each step in-
volves many possibilities, this shows direct evidence of
an oxo titanium species in the catalytic reaction.
g
2-O2 and
Acknowledgments
Here a catalytic mechanism for the hydroxylation of phenol is
proposed in Scheme 3, which involves the transformation of the
oxotitanate species 3 in the process, on interaction with hydrogen
peroxide in a pre-equilibrium step. The intermediate species rap-
Financial contributions from the National Science Foundation
of China (21073150), the National Basic Research Program of
China (2010CB732303) and PCSIRT (No.IRT1036) are gratefully
acknowledged.

6
Q.-X. Liu, Z.-H. Zhou / Polyhedron 35 (2012) 1–6
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Appendix A. Supplementary data
[7] G.M. Sheldrick, SHELXS-97 and SHELXL-97, University of Göttingen, Göttingen,
Germany, 1997.
IR, Raman and 13C NMR spectra of compounds 1–3, crystal data
and selected bond distances and angles for 1–3 can be found in the
online version. CCDC 851721–851723 contain the supplementary
crystallographic data for 1–3, respectively. These data can be ob-
tained free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at doi:10.1016/
j.poly.2011.12.037.
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