Titanium(IV)-salan catalysts for asymmetric sulfoxidation with hydrogen peroxide

Article abstract of DOI:10.1002/ejic.201000792

We report the synthesis and the solution and solid-phase characterization of several titanium(IV)-salan complexes. The structures of the ligands H ₂sal(R,R-chan), H₂ovan(S,S-chan), and H ₆pyr(R,R-chan)⁴⁺·4Cl^-·H ₂O were determined by single-crystal X-ray diffraction and their main features are compared and discussed. Single crystals suitable for X-ray diffraction studies were also obtained for [Ti₄{sal(R,R-chan)} ₄(μ-O)₄] and showed a tetranuclear core bearing μ-oxo bridges between the titanium atoms and the ligand wrapped around the Ti in a fac-mer geometry. The Ti-salan complexes were tested as catalysts in the benchmark oxidation of thioanisole with H₂O₂ as oxidant in organic solvents. High H₂O₂ conversion and moderate enantioselectivities (up to 51 % ee) were obtained. In an effort to perform greener catalysis the complexes were tested in selected ionic liquids with several oxidants. [Ti{5-MeO-sal(R,R-chan)}] was also covalently bound to a polystyrene matrix. tert-Butyl hydroperoxide was found to be the most suitable oxidant for use in ionic liquids (100 % conversion vs. 80 % with H ₂O₂) and in general the Ti-salan complexes showed better activity in ILs at 25 °C although with reduced induced asymmetry (up to 18 % ee). The solution and solid-phase characterization of several titanium(IV)-salan complexes is reported. The Ti-salan complexes were tested as catalysts in the benchmark oxidation of thioanisole with several oxidants in organic solvents and ionic liquids. The best catalyst bears no substituents on the aromatic groups and tert-butyl hydroperoxide is the most suitable oxidant for use in ILs. Copyright

Full text of DOI:10.1002/ejic.201000792

FULL PAPER
DOI: 10.1002/ejic.201000792
Titanium(IV)–Salan Catalysts for Asymmetric Sulfoxidation with Hydrogen
Peroxide
Pedro Adão,[a] Fernando Avecilla,[b] Marcella Bonchio,[c] Mauro Carraro,[c]
João Costa Pessoa,*[a] and Isabel Correia*[a]
Keywords: Titanium / Oxidation / Ionic liquids / Heterogeneous catalysis / Asymmetric catalysis
We report the synthesis and the solution and solid-phase
characterization of several titanium(IV)–salan complexes.
The structures of the ligands H2sal(R,R-chan), H2ovan(S,S-
chan), and H6pyr(R,R-chan)4+·4Cl–·H2O were determined by
single-crystal X-ray diffraction and their main features are
compared and discussed. Single crystals suitable for X-ray
diffraction studies were also obtained for [Ti4{sal(R,R-
chan)}4(μ-O)4] and showed a tetranuclear core bearing μ-oxo
bridges between the titanium atoms and the ligand wrapped
around the Ti in a fac-mer geometry. The Ti–salan complexes
were tested as catalysts in the benchmark oxidation of
thioanisole with H2O2 as oxidant in organic solvents. High
H2O2 conversion and moderate enantioselectivities (up to
51% ee) were obtained. In an effort to perform greener catal-
ysis the complexes were tested in selected ionic liquids with
several oxidants. [Ti{5-MeO-sal(R,R-chan)}] was also co-
valently bound to a polystyrene matrix. tert-Butyl hydroper-
oxide was found to be the most suitable oxidant for use in
ionic liquids (100% conversion vs. 80% with H2O2) and in
general the Ti–salan complexes showed better activity in ILs
at 25 °C although with reduced induced asymmetry (up to
18% ee).
precursors or by the reaction of the selected diamine with
a 2,4-disubstituted phenol and formaldehyde in a modified
Mannich reaction.[9] This class of ligands and their metal
complexes present much higher resistance to hydrolysis
than their salen counterparts,[10–13] which is very important
when using a “green” solvent such as water or ionic liquids
and/or a “green” oxidant such as aqueous hydrogen perox-
ide, or when anchored to cross-linked polystyrene.
Salan ligands lead to well-defined complexes that allow
the tuning of the metal geometry and the electronic and
steric properties by simply varying the ligand structure and
substitution pattern. Salan-type ligands bind to early-tran-
sition metals often in a fac-fac (cis-α) geometry to form oc-
tahedral C2-symmetric complexes with the two labile
groups in a cis geometry.[4,5,7,8,14,15] When bound in this
fashion, the nitrogen atoms of the salan ligand and the
metal become stereogenic centers.
Titanium–salan complexes have been applied as catalysts
in polymerization reactions,[6,15,16] in the alkylation of alde-
hydes,[8,17,18] and in asymmetric oxidation reactions. Most
of the work with chiral Ti(salan) in epoxidation and sulfox-
idation reactions has been performed by Katsuki and co-
workers: Ti(salan) complexes were found to be efficient cat-
alysts for asymmetric epoxidation reactions[19,20] with ee
values higher than 90% when using aqueous H2O2 as the
terminal oxidant. In the presence of phosphate buffer, the
reactions of various conjugate olefins proceeded smoothly
to afford the corresponding epoxides in high yields and with
high enantioselectivities.[19,21] Bryliakov and Talsi[22]
showed that a simpler version of the Katsuki complexes is
Introduction
The controlled catalytic oxidation of organic sulfides to
sulfoxides remains a challenge because of the widespread
application of sulfoxides in industry. For example, esomep-
razole, which is produced industrially by a modified method
developed by Kagan[1] and Modena[2] and their co-workers
independently in 1984, was the third biggest-selling phar-
maceutical drug in the world in 2005 (ca. 5.7 billion dollars
in sales).[3] However, the reaction is performed in toxic and
environmentally hazardous solvents like dichloromethane.
Thus, our ultimate goal is to design and synthesize efficient
titanium-based sulfoxidation catalysts that promote enan-
tioselective oxygen transfer to prochiral sulfides when im-
mobilized in recyclable phases such as ionic liquids or an-
chored on polymeric scaffolds such as cross-linked poly-
styrene.
The tetradentate diamino diphenolate ligands, also
known as tetrahydrosalen or salan ligands, are attracting
increasing interest due to their rich coordination chemis-
try.[4–8] They can be obtained from the reduction of salen
[a] Centro Química Estrutural, Instituto Superior Técnico,
TU Lisbon,
Av. Rovísco Pais, 1049-001 Lisboa, Portugal
[b] Departamento de Química Fundamental, Universidade da
Coruña,
Campus de A Zapateira, 15071A Coruña, Spain
[c] TM-CNR sezione di Padova, Dipartimento di Scienze
Chimiche, Università di Padova,
Via Marzolo 1, 35131 Padova, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000792.
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Titanium(IV)–Salan Catalysts for Asymmetric Sulfoxidation
capable of catalyzing the asymmetric oxidation of sulfides
to sulfoxides with H2O2 with simultaneous kinetic resolu-
tion of the sulfoxides. Optically active sulfoxides have been
obtained with good-to-high enantioselectivities (up to 97%
ee), but the overall yields and enantioselectivities are in-
ferior to those obtained by the salen system of Katsuki and
co-workers.[23]
In this paper we report the results of single-crystal X-ray
diffraction analyses of a few salan ligands, the characteriza-
tion of Ti–salan complexes, and their application as asym-
metric catalysts in the sulfoxidation of sulfides in both or-
ganic solvents and in ionic liquids. Some of the ligands were
also bound to cross-linked polystyrene to prepare the corre-
sponding TiIV complexes and these heterogeneous com-
plexes were also tested in sulfoxidation reactions.
Results and Discussion
Synthesis and Characterization of the Ligands and
Complexes
Most of the ligands were prepared according to literature
procedures in a two-step reaction: Condensation of the chi-
ral diamine with 2 equiv. of aldehyde followed by reduction
with NaBH4. Their characterization has been presented in
previous reports.[24,25] Ligand 5a and complex 5 are new
and their characterizations are reported below. For most of
the ligands derived from cyclohexanediamine, crystals suit-
able for single-crystal X-ray diffraction studies were ob-
tained and their structures are discussed below. The titani-
um(IV) complexes were prepared by the reaction of TiIV-
(OiPr)4 with the ligands in methanol and their characteriza-
tion by spectroscopic techniques is presented.
Figure 1. Circular dichroism and electronic spectra of complexes
1–3 in dichloromethane (DCM).
known and can be formed by controlled hydrolysis of the
titanium species.[26–28] From solutions containing complex
1 we obtained an interesting tetranuclear structure bearing
μ-oxo bridges between the titanium atoms and the ligand
wrapped around the Ti also in a fac-mer geometry (see be-
low). This resulted from the hydrolysis of the parent mono-
mer.
All of the TiIV–salan complexes 1–5 present ν(Ti–O)
bands in the range 860–890 cm–1 (see Exp. Sect.). The ν(N–
H) bands lie under broader bands centered at around
3400 cm–1 that arise from the presence of moisture in the
solid samples. As expected for d0 compounds, the electronic
Molecular Structures from Single-Crystal X-ray Diffraction
Studies
The molecular structures of H2sal(R,R-chan) (1a),
absorption spectra of the complexes show only bands in the H2ovan(S,S-chan) (2a), and H6pyr(R,R-chan)·4Cl– (4a)
UV region, which correspond to the πǞπ* transitions of were determined by single-crystal X-ray diffraction. OR-
the aromatic groups and are redshifted when compared TEP representations are presented in Figure 2 and selected
with the spectra of the ligands.[24]
bond lengths and angles are given in the Supporting Infor-
The electronic and circular dichroism spectra of com- mation. Ligands 1a and 2a show neutral structures and in
plexes 1–3 are shown in Figure 1. Because TiIV has no d–d 2a one proton has migrated from one of the phenolate
bands, there are no CD signals in the visible range. The groups to one of the amine groups. Only half of the mole-
Cotton effects observed are due to intraligand and CT cule of 4a was found in the asymmetric unit, the inversion
bands and thus mostly show up in the UV range. The spec- center being located at the midpoint of the cyclohexane
tra of complexes 2 and 3 resemble mirror images of each ring. The molecule is tetracationic with all of the nitrogen
other as they are complexes with ligands of opposite config- atoms protonated and four chloride anions as counterions.
urations of the stereogenic carbon atoms.
In all of the structures the cyclohexane ring adopts the
Most of the Ti–salan complexes characterized by X-ray chair conformation and the NH–CH2 (1.45–1.50 Å) and C–
diffraction studies are monomeric and show the ligand O distances (1.32–1.38 Å) are consistent with single-bond
wrapped around the Ti in a fac-fac geometry. However, for character. Overall, 1a, 2a, 4a, and the previously reported
the bulky complex derived from diphenyl diamine and 3,5- salan ligands[24,25] are quite flexible molecules, which proba-
di-tert-butylsalicylaldehyde,[7] a dinuclear complex was ob- bly explains the rather moderate ee values obtained in the
tained with a μ-oxo bridge between the titanium atoms, sulfoxidation reaction with Ti–salan complexes (see below).
only one isopropoxo unit, and the ligand coordinated in The molecular structures are discussed in the Supporting
a fac-mer fashion. Titanoxanes containing Ti4O4 units are Information.
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Figure 2. ORTEP diagrams of H2sal(R,R-chan) (1a), H2ovan(S,S-chan) (2a), and H6pyr(R,R-chan)4+ (4a) with the thermal ellipsoids of
the non-hydrogen atoms drawn at the 30% probability level. In the case of 4a the solvent molecules and Cl– anions have been omitted
for clarity.
H2sal(R,R-chan) (1a)
H6pyr(R,R-chan)4+·4Cl–·H2O (4a)
The structure of 4a is quite interesting because the mole-
cule is tetraprotonated with the protons located on each of
the nitrogen atoms. There are also four Cl– ions and one
water molecule. As a result of the protonation of the amine
atoms there are no intramolecular hydrogen bonds between
the phenolate oxygen (Ophen) and the amine nitrogen
(Namine) atoms, which in pyran [N,NЈ-ethylenebis(pyridoxyl-
aminato)][10] conditioned the molecular structure. Overall,
the geometrical parameters of pyr(R,R-chan) compare well
with those obtained for pyran, although pyr(R,R-chan)
shows a more distorted structure.
The asymmetric unit contains one neutral molecule. The
structure is very distorted due to the large flexibility of the
ligand, the torsion angles C(6)–C(7)–N(1)–C(8) and C(15)–
C(14)–N(2)–C(13) being –68.91(32) and –54.94(21)°,
respectively. The molecular structure of the same ligand but
with two protons on each amine group and two Cl– ions as
counterions has been reported previously.[24]
H2ovan(S,S-chan) (2a)
Figure 2 shows a representation of H2ovan(S,S-chan).
The asymmetric unit contains one neutral molecule of 2a
in which one proton has migrated from one of the phenol-
ate oxygen groups to the closest amine atom. The molecule
has a much more planar character than 1a. The molecular
[Ti4{sal(R,R-chan)}4(μ-O)4] (1b)
The molecule is tetrameric with each monomer linked
structure of the compound is partly determined by intra- through a μ-oxo bridge and only half a molecule found in
and intermolecular interactions involving hydrogen bonds the asymmetric unit. An ORTEP diagram is depicted in
(see Supporting Information), namely, strong intramolecu- Figure 3, and Table 1 includes selected bond lengths and
lar hydrogen bonds between the phenolate and amine angles. All monomers show cis-β geometry with the μ-oxo
groups.
ligands occupying mutually cis coordination sites and the
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Titanium(IV)–Salan Catalysts for Asymmetric Sulfoxidation
Table 1. Selected bond lengths [Å] and angles [°] for [Ti4{sal(R,R-chan)}4(μ-O)4] (1b).
Bond lengths [Å]
Bond angles [°]
102.52(13) O(1)–Ti(1)–N(1)
Ti(1)–O(6) 1.777(3)
Ti(1)–O(1) 1.890(3)
Ti(1)–O(5)* 1.907(3)
Ti(1)–O(2) 1.962(3)
Ti(1)–N(2) 2.204(4)
Ti(1)–N(1) 2.286(3)
O(2)–C(20)
N(2)–C(13)
N(2)–C(14)
O(3)–C(21)
N(3)–C(27)
N(3)–C(28)
N(4)–C(34)
N(4)–C(33)
O(4)–C(40)
1.320(4)
1.480(5)
1.488(5)
1.344(5)
1.488(5)
1.500(5)
1.499(13)
1.500(5)
1.027(14)
O(6)–Ti(1)–O(1)
O(6)–Ti(1)–O(5)*
O(1)–Ti(1)–O(5)*
O(6)–Ti(1)–O(2)
O(1)–Ti(1)–O(2)
O(5)*–Ti(1)–O(2)
O(6)–Ti(1)–N(2)
O(1)–Ti(1)–N(2)
O(5)*–Ti(1)–N(2)
O(2)–Ti(1)–N(2)
O(6)–Ti(1)–N(1)
C(7)–N(1)–Ti(1)
85.64(13)
99.63(11)
95.03(13)
94.79(12)
94.02(12)
160.88(11)
94.96(13)
162.22(13)
85.14(13)
81.14(11)
171.57(14)
112.7(2)
C(20)–O(2)–Ti(1) 133.2(2)
C(13)–N(2)–Ti(1) 113.2(2)
C(14)–N(2)–Ti(1) 110.9(2)
Ti(2)–O(5)–Ti(1)* 147.27(18)
Ti(1)–O(6)–Ti(2)
O(5)*–Ti(1)–N(1) 81.54(12)
O(2)–Ti(1)–N(1)
N(2)–Ti(1)–N(1)
C(1)–O(1)–Ti(1)
C(8)–N(1)–Ti(1)
149.87(17)
O(1)–C(1)
N(1)–C(7)
N(1)–C(8)
1.327(5)
1.480(5)
1.488(5)
82.239(11)
76.78(13)
137.9(3)
108.0(2)
* Symmetry transformations used to generate equivalent atoms in [Ti4{sal(R,R-chan)}4(μ-O)4] (1b): –x + 0, –y + 1, z + 0.
phenolate groups also mutually cis. Two monomers show Δ late ring (amine, phenolate) roughly perpendicular to this
helicity and the other two Λ helicity. The titanium atoms [torsion angle C(15)–C(14)–N(2)–C(3) = 46.87(38)°]. Tetra-
adopt a very distorted octahedral geometry with each li- meric structures have also been obtained with the maltolato
gand acting as tetradentate through the two Ophen and the and guaiacolato ligands [Ti4(maltolato)8(μ-O)4][28] and
two Namine atoms; The two bridging μ-oxo atoms complete [Ti4(guaiacolato)8(μ-O)4].[26]
the coordination sphere. The cyclohexane rings adopt a
chair conformation with the asymmetric carbon atoms in R
configuration. The Ti–μ–O(6)–Ti angle is 149.87(17)°. One
ESI-MS
of the Ti–μ-oxo bonds is stronger than the other: The one
ESI-MS experiments were conducted to investigate the
trans to the secondary amine groups shows π-bonding char- nature of the titanium complexes in solution. The positive
acter with an average length of 1.777(3) Å, whereas the ion MS spectra collected for acidic samples (addition of 1%
other is longer [1.884(3) Å]. The two Ti–Ophen bonds are formic acid) of complexes 2 and 3 dissolved in CH3CN/
also quite different with the bonds trans to the Ti–μ-oxo DCM (DCM = dichloromethane) show several intense
donors showing longer distances. The Ti(2)–O(4) bond peaks. Molecular ion peaks corresponding to the mono-
length is abnormally long, 2.375(3) Å, possibly due to dis- meric complexes were not observed in the MS spectra of
order in this phenolate ring that was not refined. In the Ti– either compound, all ion peaks corresponding to polymeric
N bonds this difference is not so noteworthy.
species and adducts with the formate anion (FA). For com-
plex 2 the major peaks are assigned to [Ti2L2O2HK2]3+ (m/z
= 324.9), [Ti2L2O2H]+ (m/z = 897.4), and [Ti2L2O(FA)]+
(m/z = 925.4). For complex 3 they are assigned to [Ti2-
L2O2HK2]3+ (m/z = 324.9), [Ti2L2O2HNa2(FA)]3+ (m/z =
495.4), and [Ti2L2O2K3(2FA)]+ (m/z = 1013.5). These ex-
periments indicate the aggregation of the complexes into
structures containing at least two Ti–O units as a result of
the hydrolysis of the parent isopropoxide complexes.
TGA Study
Thermogravimetric analysis under oxygen showed the
high stability of [PS-PIP-Ti{5-MeO-sal(R,R-chan)}] (6d;
see Scheme 4) up to around 230 °C. The first weight loss
started above this temperature. Thereafter the complex de-
composes in two steps but quantitative measurement of the
weight loss in these steps was not possible due to their over-
lap. However, the stability of the final residue at a tempera-
ture greater than around 640 °C suggests the formation of
TiO2 and the metal loading was calculated from the mass
of the final residue to be around 1.1 mmolTi/g of resin.
Figure 3. ORTEP diagram of [Ti4{sal(R,R-chan)}4(μ-O)4] (1b) with
the atom-labeling scheme. The thermal ellipsoids of the non-hydro-
gen atoms are drawn at the 30% probability level. The hydrogen
atoms and solvent molecules have been omitted for clarity.
Catalytic Studies: Asymmetric Sulfoxidation
The ligand molecules are quite distorted presenting two
chelate rings (Ophen–Namine–Namine) in a plane [torsion an-
The Ti complexes were screened for their potential as
gle C(6)–C(7)–N(1)–C(8) = –179.89(31)°] and the other che- catalysts in the asymmetric sulfoxidation of thioanisole,
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used as a thioether model substrate (see Scheme 1). In all confirmed by the direct oxidation of racemic methyl phenyl
cases, the products were either (R)- or (S)-methyl phenyl sulfoxide. The time evolution of the overall yields of sulfox-
sulfoxide and the sulfone resulting from the oxidation of ide and sulfone and the ee values of the sulfoxide are shown
the sulfoxides. In all the results presented, the percentage of in Figure 5. After 1 h and with only 0.5 mmol of oxidant,
sulfone is specified on a molar basis for these three prod- the amount of sulfone was 47%. The S enantiomer of
ucts. A few control reactions were made to test the noncata- methyl phenyl sulfoxide is oxidized faster than the corre-
lytic oxidation of thioanisole by H2O2 in the absence of a sponding R enantiomer. These results are in agreement with
catalyst: The conversions were always very low after 24 h of those published by Bryliakov and Talsi,[22] who also ob-
reaction.
served a tandem process involving the oxidation of the sul-
fide and kinetic resolution of the sulfoxide mixtures. The
same process has also been observed in the asymmetric sul-
foxidation of thioanisole with V–salan complexes.[24]
Scheme 1.
Several parameters were optimized, including catalyst
concentration, temperature, and solvent. The results
achieved after 24 h in 1,2-dichloroethane (DCE), acetone,
and ethyl acetate are presented in Table 2. The best results
were obtained with complex 1, which bears no substituents
on the aromatic rings. The use of complexes 2 and 3 led to
lower sulfide conversion and enantioselectivity. With com-
plex 1, 0.25 mol-% catalyst loading was enough to obtain
conversions higher than 94%, the enantioselectivity being
50% and with 13% of sulfone being formed (entry 2). The
only effect observed upon decreasing the temperature to
–10 °C (entry 5) was an increase in the formation of sulfone,
but this has been ascribed to the higher concentration of
oxidant used in this assay. This also suggests that the sec-
ond oxidation step might affect the ee of the sulfoxide
through kinetic resolution (see below).
Figure 4. Profile of the sulfoxidation of thioanisole in DCE. Condi-
tions: 1 mol-% of complex (R,R)-1, 1 mmol of substrate, 1.2 mmol
of H2O2, and t = 0 °C; SO: sulfoxide; SO2: sulfone.
Table 2. Sulfoxidation of thioanisole with titanium catalysts.[a]
Entry Catalyst
ncat
Cat.
T
Conv.
ee [%]
Sulfone
[%]
[mmol] [mol-%] [°C] [%] (Config.)
1
2
3
4
(R,R)-1 1.5ϫ10–4 0.015
0
0
0
16
95
94
97
94
87
86
84
88
87
37
63
88
62
60
8 (R)
50 (R)
51 (R)
51 (R)
51 (S)
21 (S)
8 (S)
38 (R)
46 (R)
41 (R)
16 (R)
38 (R)
47 (R)
0
0
(R,R)-1 2.4ϫ10–3
(R,R)-1 1.0ϫ10–2
(R,R)-1 5.0ϫ10–2
(S,S)-1 1.0ϫ10–2
(R,R)-2 1.0ϫ10–2
(S,S)-3 1.0ϫ10–2
(R,R)-1 1.0ϫ10–2
(R,R)-1 1.0ϫ10–2
(R,R)-5 1.0ϫ10–2
(R,R)-5 1.0ϫ10–2
(R,R)-5 1.0ϫ10–2
(R,R)-5 1.0ϫ10–2
0.24
1.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
6.6
3.3
13
16
13
23
20
20
14
27
14
1
0
5[b]
6
–10
10
10
0
0
0
0
25
0
0
0
Figure 5. Kinetic resolution of racemic methyl phenyl sulfoxide in
DCE. Conditions: 1 mol-% of complex (R,R)-1, 1 mmol of thio-
anisole, 0.5 mmol of H2O2, and t = 0 °C; SO: sulfoxide; SO2:
sulfone.
7
8[c,d]
9[d,e]
10[d]
11[c,d]
12[c,d]
13[d,e]
Because DCE is a volatile toxic solvent we decided to
test the model catalytic reaction under other less hazardous
conditions (see Table 2). However, the use of acetone (en-
try 8) led to a decrease in the conversion and enantio-
selectivity; the use of ethyl acetate (entry 9) had the same,
but less pronounced effect, but the amount of sulfone
formed was higher. The use of complex 5 in DCE (en-
try 10), acetone (entries 11 and 12), and ethyl acetate (en-
7
23
24
23
14[c,d] (R,R)-6d 6.6ϫ10–2
15[d,e] (R,R)-6d 3.3ϫ10–2
0
[a] Unless otherwise stated: nS = 1 mmol (S = substrate), nH O /nS
2
= 1.2, and reaction time = 24 h. [b] nH O /nS = 1.5. [c] Solven2t was
2
acetone. [d] Slow addition of oxidant. [e]2Solvent was ethyl acetate.
The kinetic profile was obtained in an experiment per- try 13) gave conversions and ee values lower than those
formed with 1 mol-% of (R,R)-1, 1 mmol of substrate, and found for complex 1 under similar conditions. Finally, the
1.2 mmol of H2O2 (Figure 4). After around 2 h the reaction use of polystyrene-anchored complex 6d either in acetone
was almost complete and the enantioselectivity increased or in ethyl acetate gave moderate conversion, no enantio-
only slightly. Accumulation of the sulfone occurred at the selectivity, and moderate amounts of sulfone (entries 14 and
expense of the S-sulfoxide. Thus, the kinetic resolution was 15).
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Titanium(IV)–Salan Catalysts for Asymmetric Sulfoxidation
We also tested the model reaction in environmentally be- absence of catalyst, in different ILs, and under different
nign solvents such as ionic liquids. Several ionic liquids conditions. All of the ILs tested in the absence of catalyst
(ILs) were prepared by standard procedures and used in the showed an ability to catalyze the oxidation of the sulfide to
catalytic experiments. Although we tried to use a wide vari- different extents, with bmimNTf2 presenting a 45% yield in
ety of ionic liquids some complexes were not soluble in sulfoxide after around 24 h. Moreover, H2O2 seemed to be
some of the ionic liquids. Scheme 2 shows those in which more active under the control reactions than tert-butyl hy-
the reactions were carried out and Table 3 reports the con- droperoxide (TBHP). This result is very interesting and
ditions and results obtained in each assay. Several control should be further explored as it shows that there may be
reactions were also carried out at room temperature, in the potential in using neat chiral ionic liquids to carry out
asymmetric catalysis.
In a first screening we tested all of the complexes in ionic
liquids containing the cation bmim at room temperature
with 1 mol-% of catalyst and noxidant/nsubstrate (nOx/nS) = 1.2.
Because complex 4 is insoluble in all the ILs tested no other
reactions were carried out with this complex. After 28 h,
with bmimBF4 the yields obtained were between 80–90%
for the three complexes tested (see entries 5, 10, and 15).
Unfortunately, in this IL no enantioselectivity was ob-
tained.
Several oxidants were tested: Aqueous H2O2, TBHP and
cumyl hydroperoxide (cumylHP). Their effect on the reac-
tion outcome was tested with complexes 1–3 in bmimNTf2
(entries 6–8 and 16–18) and TBHP was the one that showed
the highest yields after 20 h at 0 °C and with complex 3 at
room temperature. This may be due to the presence of water
when using H2O2, which promotes the polymerization of
the complexes with detrimental effects on the reaction rate
and asymmetric induction. We can also observe that the
Scheme 2.
catalyst with the best performance in terms of both activity
Table 3. Sulfoxidation of thioanisole using titanium–salan complexes as catalysts in ionic liquids.[a,b,c]
Entry
Complex
IL
Oxidant
nOx/nS
T [°C]
t [h]
Conv. [%]
SO2 [%]
ee [%]
1
2
3
4
5
6
7
8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
–
–
–
–
2
2
2
2
3
3
3
3
3
1
1
1
1
1
1
1
1
1
1
1
1
bmimNTf2
bmpyNTf2
bmpyNTf2
bmimOTf
bmimBF4
bmimNTf2
bmimNTf2
bmimNTf2
bmimBF4
bmimNTf2
bmimOTf
bmimNTf2
bmimNTf2
bmimBF4
bmimNTf2
bmimNTf2
bmimNTf2
bbimNTf2
C3OmimNTf2
bm2imNTf2
bmimOTf
bmpyNTf2
bmpyNTf2
bmpyNTf2
bmpyNTf2
H2O2
TBHP
TBHP
H2O2
H2O2
H2O2
cumylHP
TBHP
H2O2
H2O2
H2O2
1.2
1.2
0.5
0.5
1.2
2
25
25
25
25
25
10
10
10
25
0
22
20
22
22
24
24
24
24
24
24
24
18
18
24
24
24
24
24
24
24
24
24
21
20
23
45
12
8
2
0
0
0
20
0
nd
nd
nd
nd
0
0
6
4
0
0
0
0
0
0
4
19
19
12
5
0
0
12
90
20
35
50
90
70
40
5
30
80
25
45
100
80
50
50
100
80
100
14
6
2
2
1
1
1.2
1.2
0.5
1.2
1.2
1.2
2
20
10
35
0
16
16
1
10
10
2
2
2
3
1
0
cumylHP
TBHP
H2O2
25
25
25
10
10
10
25
25
25
25
25
25
0
H2O2
cumylHP
TBHP
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
2
2
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
10
18
7
2
0
0
–10
nd
[a] Conditions: nS = 1 mmol, catalyst = 1 mol-%. [b] The main product was the sulfoxide but in most cases some sulfone (SO2) was
obtained. [c] Two assays were performed with methyl p-tolyl sulfide as substrate in bmimNTf2 (25 °C, nOx/nS = 1.2, 22 h) and although
high-to-moderate conversions of the sulfide was obtained (91% with TBHP and 54% with cumylHP), the ee values were low (18 and
3%, respectively) and high amounts of sulfone were also formed (7 and 18%, respectively).
Eur. J. Inorg. Chem. 2010, 5568–5578
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5573

J. Costa Pessoa, I. Correia et al.
FULL PAPER
and asymmetric ability is complex (R,R)-1 (see, e.g., en-
try 19). Therefore the catalytic parameters were optimized
with this complex.
The solvent effect was tested by using complex 1 in dif-
ferent ILs with H2O2. In terms of activity bmimOTf was
the best solvent (entry 22), but in terms of enantioselectivity
(and high activity also) bbimNTf2 showed the best perform-
ance although a moderate ee value was still obtained (en-
try 19).
In an attempt to increase the enantioselectivity we de-
creased the reaction temperature as often this improves the
selectivity. However, as can be seen in Table 3, lowering the
temperature led to a decrease in the ee values. A compari-
son of entries 23 and 24 once again confirms the better per-
formance of TBHP in bmpyNTf2 compared with H2O2. In
two experiments performed with another substrate, methyl
p-tolyl sulfide, a good yield and a moderate enantio-
selectivity were again obtained with TBHP, but this de-
pends on a different stoichiometry.
Figure 7. Time dependence of the conversion for the sulfoxidation
of thioanisole in bmpyNTf2 and DCM with complex (R,R)-1. Con-
ditions: 1 mol-% of substrate, 0.5 mmol of TBHP, and t = 25 °C.
Conclusions
Overall, from these preliminary catalytic experiments
Several chiral salan compounds and the corresponding
carried out with the ILs tested we can conclude that the Ti–salan complexes have been synthesized and charac-
best catalyst is complex 1, without substituents on the aro- terized. Namely, the crystal and molecular structures of
matic rings, and the best conditions involve the use of NTf2- H2sal(R,R-chan) (1a), H2ovan(S,S-chan) (2a), and
based ionic liquids with TBHP as oxidant.
Another of our objectives was to study the effect of cata- methods, which showed the high flexibility of these ligands.
lyst loading. However, owing to the low solubility of com- Upon dissolution in common organic solvents such as
H6pyr(R,R-chan)4+ (4a) were solved by X-ray diffraction
plex 1 in the IL, some experiments were carried out with DCM, the Ti–salan compounds form polynuclear Ti(μ-O)-
1:1 mixtures of bmpyNTf2 and DCE. The kinetic profile is Ti species with binding modes involving the four donor
shown in Figure 6 and at 0 °C we can observe that the more atoms of the ligands and a μ-oxo bridge. The crystal and
catalyst, the slower the reaction, likely due to aggregation molecular structures of [Ti4{sal(R,R-chan)}4(μ-O)4] (1b)
phenomena: DCE may not be enough to guarantee solubil- were also determined by single-crystal X-ray diffraction,
ity.
which showed all Ti units with cis-β geometry and the μ-
oxo ligands occupying mutually cis coordination sites.
The Ti–salan complexes were applied as catalysts in the
sulfoxidation of thioanisole with H2O2 in DCE and showed
high activities and moderate enantioselectivities for this
process. Various parameters were tested such as different
solvents, temperature, catalyst loading, catalyst concentra-
tion, rate of addition of oxidant, and phenol ring substitu-
ent effect on the overall conversion and enantioselectivity.
The best catalyst is complex 1, which bears no substituents
on the aromatic groups of the ligand. The thioether conver-
sion and selectivity of the formation of sulfoxide is good,
but the enantioselectivity is rather low. Note that kinetic
resolution occurs during the oxidation of the R- and S-sulf-
oxides formed. Namely, the methyl phenyl (S)-sulfoxide is
oxidized faster to sulfone by catalyst 1 than the correspond-
ing R enantiomer.
Figure 6. Time dependence of the yield of sulfoxide in the sulfoxid-
ation of thioanisole in bmpyNTf2/DCE (1:1) with complex (R,R)-
1. Conditions: 1 mmol of thioanisole, 1 mmol of TBHP, and t =
0 °C.
The polystyrene-anchored complex 6d, with a [Ti{5-
MeO-sal(R,R-chan)}] unit covalently bound to the polysty-
rene matrix through a piperazine matrix, showed good con-
By studying the reaction profile of the oxidation with version, a moderate amount of sulfone formed, but no
TBHP in DCM and bmpyNTf2 (Figure 7), it was found enantioselectivity. Further investigations will be aimed at
that the reaction is much faster in the IL than in the organic exploring the use of the supported catalyst either in solids
solvent. This is an interesting result because with H2O2 the or in ILs in order to obtain an easily recyclable catalytic
reaction is faster in organic solvents. Indeed, the addition system.
of water has been shown to reduce the catalytic oxidation
The use of ionic liquids showed some limitations due to
the rather low solubility of the catalysts. The conversions
in IL phases.[29]
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Eur. J. Inorg. Chem. 2010, 5568–5578

Titanium(IV)–Salan Catalysts for Asymmetric Sulfoxidation
Scheme 3. Structural formulas of the chiral ligands and Ti complexes prepared.
precipitated after 15 min. Water (100 mL) was added and the solid
were moderate and the enantioselectivities were rather low.
Interesting results were found with mixtures of DCE and
ILs and in the absence of catalyst that deserve further atten-
tion.
was filtered and washed with water and n-hexane. The resulting
yellow solid was then dissolved in CH2Cl2, filtered to remove any
traces of TiO2, and the solvent was evaporated. A yellow solid was
obtained; yield 19%. IR: ν = 3246 (N–H), 3064 (C–H) , 2931 and
˜
ar
2857 (C–H)aliph, 1270 (C–O), 887 (Ti–O) cm–1.C20H24N2O3Ti·
CH2Cl2 (473.24; monomer): calcd. C 53.3, H 5.5, N 5.9; found C
53.7, H 5.9, N 6.0.
Experimental Section
Materials: Hydrogen peroxide, cumyl hydroperoxide, and tert-butyl
hydroperoxide were obtained from Aldrich, thioanisole from Acros,
and chloromethylated polystyrene (1.4–1.7 mmol Cl/g, cross linked
with 1% divinylbenzene, 100–200 mesh) from Acros. All other
chemicals were from Sigma–Aldrich.
[Ti{ovan(R,R-chan)}] (2): The procedure was similar to the one used
for the synthesis of 1. A light-yellow solid was obtained; yield 21%.
IR: ν = 3227 (N–H), 3058 (C–H) , 2934 and 2859 (C–H)aliph, 1278
˜
ar
and 1246 (C–O), 865 (Ti–O) cm–1. C22H28N2O5Ti·1.5H2O (475.38;
monomer): calcd. C 55.6, H 6.6, N 5.9; found C 55.8, H 6.7, N
5.7.
Synthesis of Ligands: The ligands were synthesized following pre-
viously published procedures[24,25] (see Scheme 3). Single crystals
suitable for X-ray diffraction studies were obtained for some of the
ligands.
[Ti{ovan(S,S-dpan)}] (3): The procedure was similar to the one used
for the synthesis of 1. The resulting orange-yellow oil obtained was
“triturated” with THF, which was then removed and a yellow solid
was obtained; yield 13%. IR: ν = 3227 (N–H), 3060, 3029, and
˜
H2sal(R,R-chan) (1a): Suitable crystals for X-ray diffraction studies
were grown by slow evaporation of 2-propanol solutions: The li-
gand (ca. 0.1 g, free base or hydrochloride) was dissolved in 2-pro-
panol (20 mL). This solution was left to evaporate in an open flask
at room temperature for about four weeks. Colorless prisms were
obtained.
3004 (C–H)ar, 2931 and 2836 (C–H)aliph, 1248 (C–O), 866 (Ti–
O) cm–1. C30H30N2O5Ti·THF (618.57; monomer): calcd. C 66.0, H
6.2, N 4.5; found C 66.0, H 6.4, N 4.8.
[Ti{pyr(R,R-chan)}] (4): The procedure was similar to the one used
for the synthesis of 1, but performed with pyridoxal instead of sali-
H2ovan(S,S-chan) (2a): The procedure was similar to the one used
for the synthesis of 1a. Colorless prisms were also obtained.
H6pyr(R,R-chan)·4Cl– (4a): Orange crystals were obtained from the
slow evaporation (ca. 8 weeks) of an aqueous solution of the com-
pound. Brown crystalline aggregates formed from the oily brown
residue that remained.
cylaldehyde. A light-yellow solid was obtained; yield 57%. IR: ν =
˜
3216 (N–H), 2930 and 2860 (C–H)aliph, 1248 (C–O), 887 (Ti–
O) cm–1. C24H36N4O6Ti·2H2O (560.48; monomer): calcd. C 51.4,
H 7.2, N 10.0; found C 51.6, H 7.2, N 9.6.
[Ti{5-MeO-sal(R,R-chan)}] (5): The procedure was similar to the
one used for the synthesis of 1. A bright-yellow solid was obtained;
yield 68% (0.33 g). IR: ν = 3240 (N–H), 3028 (C–H) , 2933 and
2857 (C–H)aliph
C44H56N4O10Ti2·3.5H2O (959.71; di-μ-oxo bridged dimer): calcd.
C 55.1, H 6.6, N 5.8; found C 55.0, H 6.2, N 5.5.
˜
ar
H25-MeO-sal(R,R-chan) (5a): The procedure was similar to that
used for the synthesis of 1a. A pale-yellow solid was obtained; yield
,
1263 and 1226 (C–O), 887 (Ti–O) cm–1.
89% (1.64 g). IR: ν = 3040 (C–H) , 2947 and 2870 (C–H)aliph, 1265
˜
ar
(C–O) cm–1. 1H NMR (400 MHz, [D4]MeOH): δ = 1.45 (t, 3J =
9.3 Hz, 2 H, CH2), 1.74 (m, 2 H, CH2), 1.87 (m, 2 H, CH2), 2.44
(d, J = 13 Hz, 2 H, CH2), 3.63 (m, 2 H, CH2CHNH), 3.75 (s, 6
H, ArOCH3), 4.25 (s, 4 H, ArCH2NH), 6.87 (s, 4 H, Ar), 7.13 (s,
2 H, Ar) ppm. C22H32Cl2N2O4·MeOH (512.45): calcd. C 56.2, H
7.4, N 5.7; found C 56.1, H 7.5, N 5.3.
Synthesis of Polystyrene-Anchored Compounds: Several polymer-an-
chored compounds were synthesized according to the procedure
outlined in Scheme 4.
3
PS-Piperazine (PS-PIP) (6a): Chloromethylated polystyrene (3.2 g,
1.4 mmolCl/g, 1% DVB, 100–200 mesh) was swollen in acetone for
30 min. Piperazine (0.6 g, 6.9 mmol) and K2CO3 (0.89 g, 6.4 mmol)
Synthesis of the Ti Complexes
[Ti{sal(R,R-chan)}] (1): TiIV(OiPr)4 (1.1 g, 3.9 mmol) was added to were added to the polymer suspension. The mixture was stirred
a methanolic solution (25 mL) of sal(R,R-chan)·2HCl (1.7 g, and heated at reflux for 3 d. The polymer beads were filtered and
3.9 mmol) and the solution turned to orange. A 2 m aqueous solu- washed with water, acetone, and diethyl ether. The resin was dried
tion of KOH was added dropwise until pH 7–8. A yellow solid under vacuum. Recovered mass: 3.35 g. Nitrogen anal. (for 100%
Eur. J. Inorg. Chem. 2010, 5568–5578
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5575

J. Costa Pessoa, I. Correia et al.
FULL PAPER
Scheme 4. Procedure for the synthesis of the polystyrene-anchored compounds. The process starts with the binding of a piperazine
spacer and then the reaction with 5-(chloromethyl)salicylaldehyde. This was followed by the addition of the tartrate salt of (1R,2R)-
cyclohexanediamine, reduction of the Schiff base compound obtained, and finally the binding of Ti to the anchored ligand. Reagents
and conditions: a) piperazine, K2CO3, acetone, reflux, 72 h; b) 5-(chloromethyl)salicylaldehyde, K2CO3, THF, reflux, 72 h; c) (1R,2R)-
cyclohexanediamine, 5-methoxysalicylaldehyde, THF/MeOH (1:1); d) NaBH4, THF/MeOH/H2O; e) HCl (pH ≈ 2); f) TiIV(OiPr)4, H2O.
substitution: 2.8 mmolN/g) N, 3.92%; found C 87.2, H 8.1, N 2.1
(ca. 1.5 mmolN/g, ca. 54% substitution of Cl).
copious amounts of water. The resin was suspended in water and
was subjected to an ultrasound bath for 15 min and then filtered
again through a coarse filter until all TiO2 (small particles) had
been removed. The resin was washed with THF and diethyl ether
and dried under vacuum. Recovered mass: 0.74 g. Elemental analy-
PS-PIP-salicylaldehyde (6b): PS-Piperazine (PSPIP, 6a; 2 g,
1.5 mmol N/g) was swollen in THF for 30 min. 5-(Chloromethyl)
salicylaldehyde (0.7 g, 4.1 mmol) and K2CO3 (0.56 g, 4.1 mmol)
were added to the polymer suspension. The mixture was stirred
and heated at reflux for 3 d. The polymer beads were filtered,
washed with water, acetone, and diethyl ether, and dried under vac-
uum. Recovered mass: 2.2 g. Elemental analysis: found C 88.4, H
sis: found C 82.2, H 7.9, N 2.0. IR: ν = 1265 (C–O) cm–1. TGA
˜
indicates a titanium loading of 1 mmolTi/gcm–1.
Physical and Spectroscopic Studies: IR spectra were recorded with
a BioRad FTS 3000 MX FTIR spectrometer. UV/Vis spectra were
recorded with a Hitachi U-2000 spectrophotometer and the CD
8.1, N 1.9. IR: ν = 2845 (C–H)aldehyde, 1653 (C=O), 1260 (C–
˜
O) cm–1.
1
spectra with a Jasco J-715 spectropolarimeter. The H NMR spec-
tra were obtained with a Bruker Avance+ 400 Spectrometer. TGA
was performed with a Perkin–Elmer STA 6000 thermal analyzer.
PS-PIP-5-MeO-sal(R,R-chan) (6c): PS-PIP-salicylaldehyde (6b;
1 g) was swollen in a mixture of THF/MeOH (1:1, 125 mL) for
30 min. The tartrate salt of (1R,2R)-1,2-cyclohexanediamine
(0.37 g, 1.4 mmol) and K2CO3 (0.19 g, 1.4 mmol) in water (25 mL)
were added to the polymer suspension. The mixture was stirred
for around 24 h. Then 5-methoxysalicylaldehyde (0.21 g, 1.4 mmol,
0.17 mL) was added and the mixture was stirred for another 24 h.
The bright-yellow polymer beads were filtered and washed with
water, acetone, and diethyl ether. The resin was again suspended in
THF/MeOH/H2O (1:1:0.2) and NaBH4 was added in excess. The
mixture was stirred until the bright-yellow color had disappeared
completely. The pH was adjusted with concd. aqueous HCl to
around pH 2. The polymer beads were filtered and washed with
water, THF, and diethyl ether. The resin was dried under vacuum.
Recovered mass: 1.2 g. Elemental analysis: found C 82.2, H 7.9, N
X-ray Crystal Structure Determinations: Three-dimensional X-ray
data for compounds 1a and 2a were collected with a Bruker
SMART 1000 CCD diffractometer, and for compounds 4a and 1b
with a Bruker Kappa X8 Apex CCD diffractometer by using the
θ–ω scan method. Data for 1a and 2a were collected at room tem-
perature and for 4a and 1b at low temperature. Reflections were
measured from a hemisphere of data collected of frames each
covering 0.3° in ω. Of the 21185 reflections measured for 1a, 24365
for 2a, 6363 for 4a, and 35147 for 1b, all of which were corrected
for Lorentzian and polarization effects and for absorption by semi-
empirical methods based on symmetry-equivalent and repeated re-
flections, 2155, 3157, 2408, and 8408 independent reflections ex-
ceeded the significance level |F|/σ(|F|)Ͼ4.0, respectively. Complex
scattering factors were obtained by using the SHELXTL software
package.[30] The structures were solved by direct methods and re-
2.0 (ca. 1.4 mmolN/g, ca. 54% substitution). IR: ν = 1267 (C–
˜
O) cm–1.
[PS-PIP-Ti{5-MeO-sal(R,R-chan)}] (6d): PS-PIP-5-MeO-sal(R,R- fined by full-matrix least-squares methods on F2. Refinement con-
chan) (6c; 0.62 g) was swollen in THF (100 mL) for 30 min. TiIV(O-
iPr)4 (0.35 g, 0.37 mL) was added to the polymer suspension. The
mixture was stirred for 1 h. The polymer beads acquired a bright-
orange color. Water was added and a suspension of TiO2 formed
readily. At this point the resin changed to a yellow color. The poly-
mer beads were filtered through a coarse filter and washed with
verged with allowance for the thermal anisotropy of all the non-
hydrogen atoms, except for O4 in 1b. All the hydrogen atoms were
left to refine freely in 1a, 2a, and 4a, except for the hydrogen atoms
of C21 and C22 in 2a and of C11 in 4a, which were included in
calculated positions and refined by using a riding mode. For 1b, all
hydrogen atoms were included at calculated positions and refined
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Eur. J. Inorg. Chem. 2010, 5568–5578

Titanium(IV)–Salan Catalysts for Asymmetric Sulfoxidation
Table 4. Crystal data, data collection and refinement data for compounds H2sal(R,R-chan) (1a), H2ovan(S,S-chan) (2a), H6pyr(R,R-
chan)4+·4Cl–·H2O (4a), and [Ti4{sal(R,R-chan)}4(μ-O)4] (1b).
H2sal(R,R-chan)
(1a)
H2ovan(S,S-chan)
(2a)
H6pyr(R,R-chan)·4Cl·H2O
(4a)
[Ti4{sal(R,R-chan)}4(μ-O)4}
(1b)
Formula
Mr
C20H26N2O2
326.43
C22H30N2O4
386.48
C22H38Cl4N4O5
580.36
C80H96N8O12Ti4
1553.25
Temperature [K]
293(2)
293(2)
100(2)
100(2)
Wavelength [Å]
Crystal system
Space group
0.71073
orthorhombic
P2(1)2(1)2(1)
0.71073
orthorhombic
P2(1)2(1)2(1)
0.71073
monoclinic
C2
0.71073
tetragonal
I4(1)
Unit cell dimensions
a [Å]
b [Å]
c [Å]
α [°]
10.046(4)
10.380(4)
17.683(6)
90
8.2903(4)
11.2018(5)
21.7422(11)
90
12.7428(5)
9.7347(3)
11.4164(4)
90
24.0490(4)
24.0490(4)
12.4115(5)
90
β [°]
90
90
104
90
γ [°]
90
90
90
90
7178.2(3)
4
1.437
0.500
Volume [Å3]
1844.0(11)
4
1.176
0.076
704
0.50ϫ0.35ϫ0.35
2.33–28.33
–13ՅhՅ13
–13ՅkՅ13
–23ՅlՅ23
21185
2019.12(17)
4
1.271
0.087
832
0.50ϫ0.23ϫ0.23
1.87–28.31
–10ՅhՅ11
–14ՅkՅ14
–28ՅlՅ28
24365
1374.18(8)
2
1.403
0.470
612
0.42ϫ0.37ϫ0.30
1.84–28.29°
–16ՅhՅ16
–6ՅkՅ12
–15ՅlՅ15
6363
Z
Calcd. density [mgm–3]
Absorption coefficient [mm–1]
F(000)
3264
Crystal size [mm3]
θ range for data collection [°]
Index ranges
0.49ϫ0.32ϫ0.18
1.20–28.31
–32ՅhՅ26
–32ՅkՅ32
–16ՅlՅ16
35129
Reflections collected
Independent reflections
Completeness to θ = 28.33° [%]
Max. / min. transmission
Refinement method
4451 [R(int) = 0.0589]
97.70
0.9739 / 0.9630
4932 [R(int) = 0.0473]
99.30
0.9802 / 0.9576
2437 [R(int) = 0.0145]
99.10
0.8718 / 0.8270
8897 [R(int) = 0.0292]
99.7
0.9154 / 0.7911
full-matrix least squares on F2 full-matrix least squares on F2 full-matrix least squares on F2 full-matrix least squares on F2
Data/restraints/parameters
Goodness-of-fit on F2
4451/0/321
0.887
4932/0/352
1.088
2437/1/224
1.154
8897/31/465
1.073
Final R indices [IϾ2σ(I)]
R1 = 0.0376
wR2 = 0.0791
R1 = 0.1147
wR2 = 0.0988
R1 = 0.0443
wR2 = 0.0971
R1 = 0.0931
wR2 = 0.1207
0.4(13)
R1 = 0.0222
wR2 = 0.0614
R1 = 0.0229
wR2 = 0.0709
0.04(4)
R1 = 0.0571
wR2 = 0.1559
R1 = 0.0606
wR2 = 0.1592
0.05(3)
R indices (all data)
Absolute structure parameter
Extinction coefficient
Largest diff. peak and hole [eÅ–3]
0.0122(14)
0.292 and –0.234
0.098 and –0.110
0.610 and –0.224
2.199 and –0.696
by using a riding mode. The absolute configurations of 2a, 4a, and
1b were established by refinement of the enantiomorph polarity
parameter [χ = 0.4(13) for 2a, 0.04(4) for 4a, and 0.06(3) for
1b].[31,32] A final difference Fourier map showed no residual density
outside 0.098 and –0.110 eÅ–3 for 1a, 0.292 and –0.234 eÅ–3 for
2a, 0.610 and –0.224 eÅ–3 for 4a, except for 1b, 1.770 and
–0.933 eÅ–3, which presents disorder in one phenolate ring that
could not be refined. Crystal data and details on data collection
and refinement are summarized in Table 4.
MS Network (IST Node) and Dr. Maria da Conceição Oliveira for
the ESI measurements. We also thank the Programa Operacional
Ciência e Inovação (POCI) 2010, Fundo Europeu de Desenvolvi-
mento Regional (FEDER) and the Fundação para a Ciência e Tec-
nologia (FCT) (SFRH/BD/40279/2007, PTDC/QUI-QUI/112015/
2009 and PTDC/QUI-QUI/098516/2008) for financial support as
well as the Spanish-Portuguese Bilateral Programme (Acção Inte-
grada E-56/05, Acción Integrada HP2004-00).
CCDC-783626 (for 1a), -783627 (for 2a), -783628 (for 1b), and
-783629 (for 4a) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Received: July 21, 2010
Published Online: November 2, 2010
5578
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Eur. J. Inorg. Chem. 2010, 5568–5578