Zeolite-encapsulated copper(II) complexes of pyridoxal-based tetradentate ligands for the oxidation of styrene, cyclohexene and methyl phenyl sulfide

Article abstract of DOI:10.1002/ejic.200700797

Interaction of copper(II)-exchanged zeolite-Y with N,N′- ethylenebis(pyridoxyliminato) (H₂pydx-en, I) and N,N′- propylenebis(pyridoxyliminato) (H₂pydx-1,3-pn, II) ligands in refluxing methanol leads to the formation of the corresponding complexes, abbreviated herein as [Cu(pydx-en)]-Y (3) and [Cu(pydx-1,3-pn)]-Y (4), in the supercages of zeolite-Y. The neat complexes, [Cu(pydx-en)] (1) and [Cu(pydx-1,3-pn)] (2), have also been prepared with these ligands. Spectroscopic studies (IR, UV/Vis and EPR), elemental analyses, thermal studies, field emission scanning electron micrographs (FESEM) and X-ray diffraction patterns have been used to characterise these complexes. The crystal and molecular structures of 1 and of 2·CH₃OH, have been determined, confirming the ONNO binding mode of the ligands. The geometry around the metal ion is very slightly distorted square-planar in 1 and distorted square-pyramidal in 2. The encapsulated complexes catalyse the oxidation, by H₂O ₂ and tert-butyl hydroperoxide, of styrene, cyclohexene and thioanisole efficiently. Under optimised reaction conditions, the oxidation of styrene catalysed by 3 and 4 gave 23.6% and 28.0% conversion, respectively, using tert-butyl hydroperoxide as oxidant, where styrene oxide, benzaldehyde, benzoic acid and phenylacetaldehyde are the major products. Better conversions have been obtained using H₂O₂ as oxidant. Oxidation of cyclohexene catalysed by these complexes gave cyclohexene oxide, 2-cyclohexen-1-ol, cyclohexane-1,2-diol and 2-cyclohexen-1-one as the major products. A maximum of 90.1% conversion of cyclohexene with 3 and 83.0% with 4 was obtained under optimised conditions. Similarly, a maximum of 80.3% conversion of methyl phenyl sulfide with 3 and 81.0% with 4 was observed, where the selectivity of the major product methyl phenyl sulfoxide was found to be about 60%. Tests for the recyclability and heterogeneity of the reactions have also been carried out, and the results indicate their recyclability. A possible reaction mechanism has been proposed by titrating a methanol solution of 1 and 2 with H₂O₂ to identify the possible intermediates. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700797

FULL PAPER
DOI: 10.1002/ejic.200700797
Zeolite-Encapsulated Copper(II) Complexes of Pyridoxal-Based Tetradentate
Ligands for the Oxidation of Styrene, Cyclohexene and Methyl Phenyl Sulfide
Mannar R. Maurya,*[a] Baljit Singh,[a] Pedro Adão,[b] Fernando Avecilla,[c] and
João Costa Pessoa[b]
Keywords: Zeolites / Copper(II) complexes / Heterogeneous catalysis / Oxidation reactions
Interaction of copper(II)-exchanged zeolite-Y with N,NЈ-eth-
version, respectively, using tert-butyl hydroperoxide as oxi-
ylenebis(pyridoxyliminato) (H2pydx-en, I) and N,NЈ-propyl- dant, where styrene oxide, benzaldehyde, benzoic acid and
enebis(pyridoxyliminato) (H2pydx-1,3-pn, II) ligands in re-
fluxing methanol leads to the formation of the corresponding
complexes, abbreviated herein as [Cu(pydx-en)]-Y (3) and
[Cu(pydx-1,3-pn)]-Y (4), in the supercages of zeolite-Y. The
neat complexes, [Cu(pydx-en)] (1) and [Cu(pydx-1,3-pn)] (2),
have also been prepared with these ligands. Spectroscopic
studies (IR, UV/Vis and EPR), elemental analyses, thermal
studies, field emission scanning electron micrographs (FE-
SEM) and X-ray diffraction patterns have been used to char-
acterise these complexes. The crystal and molecular struc-
tures of 1 and of 2·CH3OH, have been determined, confirm-
ing the ONNO binding mode of the ligands. The geometry
around the metal ion is very slightly distorted square-planar
in 1 and distorted square-pyramidal in 2. The encapsulated
complexes catalyse the oxidation, by H2O2 and tert-butyl hy-
droperoxide, of styrene, cyclohexene and thioanisole ef-
ficiently. Under optimised reaction conditions, the oxidation
of styrene catalysed by 3 and 4 gave 23.6% and 28.0% con-
phenylacetaldehyde are the major products. Better conver-
sions have been obtained using H2O2 as oxidant. Oxidation
of cyclohexene catalysed by these complexes gave cyclohex-
ene oxide, 2-cyclohexen-1-ol, cyclohexane-1,2-diol and 2-cy-
clohexen-1-one as the major products. A maximum of 90.1%
conversion of cyclohexene with 3 and 83.0% with 4 was ob-
tained under optimised conditions. Similarly, a maximum of
80.3% conversion of methyl phenyl sulfide with 3 and 81.0%
with 4 was observed, where the selectivity of the major prod-
uct methyl phenyl sulfoxide was found to be about 60%.
Tests for the recyclability and heterogeneity of the reactions
have also been carried out, and the results indicate their re-
cyclability. A possible reaction mechanism has been pro-
posed by titrating a methanol solution of 1 and 2 with H2O2
to identify the possible intermediates.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
heterogeneous catalysts and share many of the advan-
tageous features of homogeneous catalysts.
Introduction
Zeolites find applications in many scientific disciplines as
catalysts and in all types of chemical engineering process
technologies. The exchangeable property of extra-frame-
work cations and suitable cavity size of the zeolites allow
their modification by inclusion of chemically interesting
molecules. A homogeneous catalyst, for example, a metal
complex, may be considered one such interesting molecule
and hence the term zeolite-encapsulated metal complexes.
The large size of the encapsulated homogeneous catalysts
and their rigidity make it difficult for them to escape the
zeolite cages. Zeolite-encapsulated metal complexes having
good catalytic activities possess the advantages of solid
The flexible nature of salen-type [H2salen = N,NЈ-bis-
(salicylidene)ethane-1,2-diamine] ligands has provided op-
portunities to design various transition-metal complexes in
the nanocavity of zeolites and to develop catalytic processes
for various reactions. Poltowicz et al. have encapsulated a
whole range of metallosalen complexes {e.g., [Fe(salen)],
[Mn(salen)], [Cu(salen)] and Co(salen)]} in zeolite-X to
study their catalytic activity for the oxidation of cyclooc-
tane.[1] Ratnasamy and co-workers have used copper(II)
and manganese(III) complexes of salen derivatives encapsu-
lated in the cavity of zeolite-X and zeolite-Y for the oxi-
dation of styrene under aerobic conditions using tert-butyl
hydroperoxide as an oxidant. The catalytic efficiency of
these encapsulated complexes was much higher than that of
the neat complexes. Electron-withdrawing substituents such
as Cl, Br and NO2 on the aromatic ring enhance the rate
of oxidation.[2,3] These complexes also catalyse the oxi-
dation of phenol and p-xylene.[4,5] Epoxidations of cyclo-
hexene, cyclooctene, 1-hexene and other various types of
alkenes, arenes and cycloalkenes, catalysed by different
[a] Department of Chemistry, Indian Institute of Technology
Roorkee,
Roorkee 247667, India
E-mail: rkmanfcy@iitr.ernet.in
[b] Centro Química Estrutural, Instituto Superior Técnico – TU
Lisbon,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal
[c] Departamento de Química Fundamental, Facultad de Ciencias,
Universidade da Coruña,
Zapateira s/n, 15071 A Coruña, Spain
5720
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5720–5734

Zeolite-Encapsulated Copper Complexes of Tetradentate Ligands
complexes of salen-type ligands, have also been carried
Reaction of cupric acetate with ligands H2pydx-en (I)
out.[6–9] [Co(salophen)]-Y [H2salophen = N,NЈ-bis(salicylid- and H2pydx-1,3-pn (II) in refluxing methanol yielded [Cu-
ene)benzene-1,2-diamine] and related derivatives catalysed (pydx-en)] (1) and [Cu(pydx-1,3-pn)(MeOH)] (2), respec-
the oxidation of β-isophorone (βIP) to ketoisophorone tively. Complex 1 has previously been isolated by treating I
(KIP) (along with other minor products) in the presence with cupric acetate in aqueous solution.[16] Encapsulation
of an oxidant at ambient conditions of temperature and of these complexes in the nanocavities of zeolite-Y involved
pressure.[10] Liquid-phase oxidation of phenol with H2O2 the exchange of copper(II) ion with Na-Y in water followed
has been reported using copper(II) and oxidovanadium(IV) by reaction of the metal-exchanged zeolite-Y with I or II in
complexes of N,NЈ-bis(salicylidene)propane-1,3-diamine methanol. This method is referred to as the flexible ligand
(H2sal-1,3-pn) and N,NЈ-bis(salicylidene)diethylenetriamine method. Here, the ligands enter into the cavity of zeolite-Y
(H2saldien).[11–13] Copper(II) and oxidovanadium(IV) com- because of their flexible nature and interact with the metal
plexes of N,NЈ-bis(salicylidene)cyclohexane-1,2-diamine ion to give [Cu(pydx-en)]-Y (3) and [Cu(pydx-1,3-pn)]-Y
(H2sal-dach) catalyse the oxidation of styrene, cyclohexene (4). Finally, extraction of impure samples with methanol
and cyclohexane efficiently.[14]
using a Soxhlet extractor removed excess free ligand, and
The catalytic potential of metal complexes encapsulated stirring in DMF removed neat metal complex formed on
in the nanocavity of zeolites prompted us to design new the surface of the zeolite, if any. As the crude mass was well
zeolite-Y-encapsulated copper(II) complexes of tetradentate extracted, the metal content found after encapsulation is
ligands I and II (Scheme 1). Catalytic potentials of these only due to the presence of copper(II) complexes in the su-
complexes have been demonstrated by studying the oxi- percages of the zeolite-Y. The diagonal dimension of the
dation of styrene, cyclohexene and methyl phenyl sulfide. complexes is about 12 Å {in [Cu(pydx-en)]} or about 11 Å
Copper(II), nickel(II) and oxidovanadium(IV) complexes of {in [Cu(pydx-1,3-pn)(MeOH)]}, which suggests that they
I have been reported previously.[15,16] The structure of [Ni- will fit nicely into the supercages of zeolite-Y. The molecu-
(pydx-en)] has been confirmed by single-crystal X-ray dif- lar formulae of the encapsulated complexes are based on
fraction,[17] while other complexes have been characterised the respective neat complex. The Si/Al ratio of 9.67 of zeo-
by elemental analysis and spectroscopic techniques. We lite-Y taken here corresponds to a unit cell formula
have also prepared neat copper(II) complexes of I and II to Na18(AlO2)18(SiO2)174. The estimated number of copper
evaluate their catalytic potential and characterised them by atoms per unit cell is 2.5 for [Cu(pydx-en)]-Y (3) and 2.4
single-crystal X-ray diffraction.
for [Cu(pydx-1,3-pn)]-Y (4).
Structure Description of [Cu(pydx-en)] (1) and [Cu(pydx-
1,3-pn)(MeOH)] (2)
Figures 1 and 2 include ORTEP representations of the
molecular structure of [Cu(pydx-en)] (1) and [Cu(pydx-1,3-
pn)(MeOH)] (2), respectively, and Table 1 provides selected
bond lengths and angles of these compounds. The mole-
cules are neutral, as both phenol groups are deprotonated,
and the ligands coordinate to the Cu atom through two
phenolate oxygen (O–phenolate) atoms and two imine nitro-
gen (Nimine) atoms. The geometry around the metal ion is
slightly distorted square-planar in 1 and distorted square-
pyramidal in 2. A methanol molecule occupies the fifth co-
ordination position in 2.
Scheme 1.
Results and Discussion
Synthesis and Characterisation of Catalysts
Scheme 2 presents the complexes prepared in this study.
Their structures are based on elemental analysis and vari-
ous spectroscopic studies, as well as on X-ray structure de-
terminations. The presence of very strong peaks corre-
sponding to CuLH+ and CuLNa+ in the ESI-MS spectra
(positive mode) of DMSO solutions of 1 and 2, with the
correct isotope ratios, also confirms the integrity of the
complexes in solution.
Figure 1. ORTEP diagram of [Cu(pydx-en)] (1) showing the atom-
labelling scheme. The thermal ellipsoids were drawn at the 30%
probability level.
Scheme 2.
Eur. J. Inorg. Chem. 2007, 5720–5734
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5721

M. R. Maurya, B. Singh, P. Adão, F. Avecilla, J. Costa Pessoa
FULL PAPER
Figure 2. ORTEP diagram of [Cu(pydx-1,3-pn)(MeOH)] (2) showing the atom-labelling scheme. The thermal ellipsoids were drawn at the
30% probability level.
Table 1. Selected bond lengths [Å] and angles [°] for [Cu(pydx-en)] (1) and [Cu(pydx-1,3-pn)(MeOH)] (2).
Bond lengths [Å] for [Cu(pydx3-en)] (1)
Bond lengths [Å] for [Cu(pydx-1,3-pn)(MeOH)] (2)
Cu(1)–O(2)
Cu(1)–N(3)
Cu(1)–O(3)
Cu(1)–N(2)
O(3)–C(13)
O(2)–C(6)
N(2)–C(8)
N(2)–C(9)
N(3)–C(11)
N(3)–C(10)
C(6)–C(7)
1.742(6)
1.796(7)
2.341(7)
2.373(9)
1.333(11)
1.380(12)
1.256(12)
1.377(11)
1.434(13)
1.690(9)
1.609(15)
1.338(13)
1.695(15)
1.330(13)
Cu(1)–O(2)
Cu(1)–O(3)
Cu(1)–N(3)
Cu(1)–N(2)
Cu(1)–O(1S)
O(2)–C(6)
N(2)–C(8)
N(2)–C(9)
O(3)–C(14)
N(3)–C(12)
N(3)–C(11)
C(6)–C(7)
1.8965(9)
1.9546(9)
1.9932(11)
1.9935(11)
2.3525(11)
1.2934(15)
1.2962(16)
1.4775(16)
1.2998(15)
1.2905(16)
1.4670(16)
1.4096(17)
1.4443(17)
1.5149(18)
1.5198(19)
1.4494(17)
1.4082(17)
C(7)–C(8)
C(11)–C(12)
C(12)–C(13)
C(7)–C(8)
C(9)–C(10)
C(10)–C(11)
C(12)–C(13)
C(13)–C(14)
Angles [°] for [Cu(pydx-1,3-en)] (1)
Angles [°] for [Cu(pydx-1,3-pn)(MeOH)] (2)
O(2)–Cu(1)–N(3)
O(2)–Cu(1)–O(3)
N(3)–Cu(1)–O(3)
O(2)–Cu(1)–N(2)
N(3)–Cu(1)–N(2)
O(3)–Cu(1)–N(2)
175.2(3)
90.0(3)
94.8(3)
91.2(3)
84.1(3)
178.8(2)
O(2)–Cu(1)–N(3)
O(2)–Cu(1)–O(3)
O(3)–Cu(1)–N(3)
O(2)–Cu(1)–N(2)
N(3)–Cu(1)–N(2)
O(3)–Cu(1)–N(2)
O(2)–Cu(1)–O(1S)
O(3)–Cu(1)–O(1S)
N(3)–Cu(1)–O(1S)
N(2)–Cu(1)–O(1S)
169.92(4)
82.08(4)
87.94(4)
92.29(4)
97.76(4)
166.61(4)
90.18(4)
94.46(5)
89.23(4)
97.71(5)
Lengths [Å] and angles [°] of hydrogen bonds in complexes
O(1)···H(1), 0.82; O(1)···O(3), 2.563(8)
O(3)···H(1), 2.00; O(1)–H(1)–O(3), 125.5
O(4)···H(4), 0.82; O(4)···N(1), 2.582(11)
N(1)···H(4), 2.01; O(4)–H(4)–N(1), 126.5
O(1)···H(1O), 0.69(2); O(1)···O(3), 2.7954(14)
O(3)···H(1O), 2.11(2); O(1)–H(1O)–O(3), 171(2)
O(1S)···H(1S), 0.64(3); O(1S)···N(1), 2.8037(16)
N(1)···H(1S), 2.16(3); O(1S)–H(1S)–N(1), 175(3)
O(4)···H(4O), 0.69(2); O(4)···N(4), 2.7959(15)
N(4)···H(4O), 2.12(2); O(4)–H(4O)–N(4), 166(2)
Deviations from the planarity of the rings
Ring: Cu(1)–O(2)–C(6)–C(7)–C(8)–N(2)
Dev.: 0.0799(52) Å
Ring: Cu(1)–N(2)–C(9)–C(10)–N(3)
Dev.: 0.0976(48) Å
Ring: Cu(1)–O(2)–C(6)–C(7)–C(8)–N(2)
Dev.: 0.0516(07) Å
Ring: Cu(1)–N(2)–C(9)–C(10)–C(11)–N(3)
Dev.: 0.2239(08) Å
Ring: Cu(1)–N(3)–C(11)–C(12)–C(13)–O(3)
Dev.: 0.0334(50) Å
Pyridoxal rings:
Ring: Cu(1)–N(3)–C(12)–C(13)–C(14)–O(3)
Dev.: 0.1928(07) Å
Pyridoxal rings:
Planar, rms 0.0244(63) and 0.0172(60) Å
Torsion angle between rings: 9.58 (0.57)°
Planar, rms 0.0037(08) and 0.0083(09) Å
Torsion angle between rings: 34.27(0.05)°
5722
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5720–5734

Zeolite-Encapsulated Copper Complexes of Tetradentate Ligands
The planar arrangement Cu(ONNO) is distorted, the
cis/trans angles not being 90° and 180°. The cis angles are
90.0(3), 94.8(3), 91.2(3) and 84.1(3)° in 1, and 82.08(4),
87.94(4), 92.29(4) and 97.76(4)° in 2, and the trans angles
are 175.2(3) and 178.8(2)° in 1, and 169.92(4) and
166.61(4)° in 2. The angles with the apical position of the
O atom of the methanol molecule are 90.18(4), 94.46(5),
89.23(4) and 97.71(5)° in 2.
In complex 1 the ligand forms a set of (6+5+6)-mem-
bered chelate rings, and the deviations from the planarity
are listed in Table 1. The plane formed by the donor atoms
N(2)–N(3)–O(2)–O(3) has a deviation from planarity of
0.0015(30) Å, and if the CuII ion is included, 0.0037(30) Å.
In complex 2 the ligand forms a set of (6+6+6)-membered
chelate rings, and the deviations from planarity are also
listed in Table 1. The basal plane of the square pyramid
formed by N(2)–N(3)–O(2)–O(3) atoms has a deviation
from planarity of 0.1063(05) Å, and the CuII ion is 0.1020 Å
above the plane. The Cu–O and Cu–N bond lengths
(Table 1) are similar and in the usual range found for these
compounds, 1.9–2.1 Å. The C–O distances are also typical
for this type of ligands,[15–21] and the C=N bonds are
double bonds, with distances of about 1.3 Å.
In complex 1 the HO(CH2) groups of the pyridoxal rings
are in a syn position to each other, but in complex 2 the
HO(CH2) groups of the pyridoxal rings are in an anti con-
formation, similar to other complexes with this type of li-
gand.[16] The pyridine nitrogen atoms and alcohol group of
HO(CH2) of the pyridoxal rings are involved in intermo-
lecular H bonds. In complex 2, the OH groups of the coor-
dinated methanol molecule are also involved in H bonds.
The bond lengths and angles of the hydrogen bonds are
also included in Table 1.
Figure 3. Field emission scanning electron micrograph of [Cu-
(pydx-en)]-Y (top) and [Cu(pydx-1,3-pn)]-Y (bottom).
Field Emission Scanning Electron Micrograph and Energy
Dispersive X-ray Analysis Studies
Powder X-ray Diffraction Studies
Figure 3 presents the field emission scanning electron
micrographs of [Cu(pydx-en)]-Y and [Cu(pydx-1,3-pn)]-Y.
It is clear from the micrographs that zeolite-entrapped cop-
per complexes have well-defined crystals, and there is no
indication of the presence of any metal ions or complexes
on the surface. Energy-dispersive X-ray analysis plots sup-
port this conclusion as no copper or nitrogen contents were
noted on the spotted surface in plots for both catalysts. The
average silicon and aluminium percentage obtained on the
spotted surfaces were about 30% and 10%, respectively. An
amount of about 3% sodium suggests the exchange of re-
maining free copper ions by sodium ions during the re-ex-
change process (see Experimental Section). Only a small
amount of carbon (about 5%) but no nitrogen suggests the
presence of a trace amount of solvent (methanol) from
which it was finally washed after Soxhlet extraction. No
morphological changes on the surface upon encapsulation
of the complexes in the cavity are seen because of their poor
loading.
The powder X-ray diffraction patterns of Na-Y, Cu-Y
and encapsulated copper(II) complexes were recorded at 2θ
values between 5 and 50°, and some representative patterns
are presented in Figure 4. Essentially similar diffraction
Figure 4. XRD patterns of Na-Y, Cu-Y and [Cu(pydx-en)]-Y (3).
Eur. J. Inorg. Chem. 2007, 5720–5734
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5723

M. R. Maurya, B. Singh, P. Adão, F. Avecilla, J. Costa Pessoa
FULL PAPER
patterns were noticed in encapsulated complexes Cu-Y and hydrogen bond followed by the coordination of the phenol
Na-Y, except zeolite with encapsulated metal complexes has oxygen atom after deprotonation. Existence of this band,
a slightly weaker intensity. These observations indicate that however, in neat complexes hints towards the presence of
the framework of the zeolites do not suffer any significant hydrogen bonding. A sharp band appearing at 1626 cm–1
structural changes during encapsulation, that is, crystal- (in I) or at 1632 cm–1 (in II) due to the ν(C=N) (azome-
linity of the zeolite-Y is preserved.
thine) shifts to higher wavenumbers in complexes, thereby
suggesting the coordination of the azomethine nitrogen
atom. Several multiple bands of medium intensity covering
the region 2850–3000 cm–1 are observed due to C–H bands.
The spectral patterns of these complexes are similar to
[Cu(pydx-en)] reported in the literature.[16]
Thermogravimetric Analysis Studies
The decomposition of complexes 3 and 4 occurs in three
overlapping steps. An endothermic loss of trapped water
and methanol occurs between 80 and 200 °C, while exother-
mic removal of intra-zeolite water occurs between 300 and
350 °C. This is followed by endothermic loss of the ligand’s
residue, which continues until the removal of all organic
UV/Vis Spectral Studies
UV/Vis spectral studies of I and its copper(II) complex
mass at about 700 °C has occurred. The low percentage of 1 have been discussed in detail in the literature.[16] Other
weight loss (about 10%) for this last step indicates the pres- complexes reported here exhibit very similar spectral pat-
ence of an only small amount of metal complexes in the terns. The well-defined band at 590 nm in 1 and a broad
cavities of the zeolite. This is in agreement with the low band covering the region 570–650 nm in 2 have been as-
copper content estimated for these complexes. A weight loss signed to a d–d transition. Such a d–d band in zeolite-
of 6.9%, equivalent to methanol, in 2 occurs up to about encapsulated complexes could not be obtained because of
130 °C. The anhydrous complex is stable up to 260 °C, and their poor loading, while ligand bands appearing in the UV
then it decomposes in two major steps that are complete at region are similar to those observed for neat complexes.
375 °C. The remaining residue of 18.1% slowly stabilises This data is collected in Table 2 and the spectra of encapsu-
with the formation of a final residue of 17.5% (about 17.1% lated complexes are presented in Figure 5.
for CuO) at about 800 °C. A slightly different decomposi-
tion pattern has been obtained for 1, where the decomposi-
tion of the stable complex starts at about 290 °C and occurs
in two overlapping steps. The decomposition is complete at
about 360 °C. The remaining residue of 20.6% further loses
weight up to about 800 °C and stabilises at 18.3%. The cal-
culated residue for CuO of 18.9% agrees with the value ob-
served.
IR Spectral Studies
A partial list of IR spectroscopic data is presented in
Table 2. The intensity of the peaks in encapsulated com-
Figure 5. UV/Vis spectra of [Cu(pydx-en)]-Y and [Cu(pydx-1,3-
plexes is, however, weak because of their low concentration pn)]-Y dispersed in nujol.
in zeolite matrix; the spectra of the encapsulated as well as
neat complexes showed essentially similar bands. Compari-
son of the spectra of these complexes with the respective
ligand provides evidence for the coordinating mode of li-
EPR Studies
gands in complexes. The ligand H2pydx-en exhibits a me-
The EPR spectra of the neat CuII complexes 1 and 2 in
dium-intensity band around 2700 cm–1 due to intramolecu- the polycrystalline state are broad because of dipolar and
lar hydrogen bonding. Absence of this band in the spectra spin–spin exchange interactions. It is clear that they are
of encapsulated complexes indicates the destruction of the characterised by an axial g tensor, but the hyperfine features
Table 2. IR and UV/Vis spectroscopic data of ligand, pure and encapsulated complexes.
Compound
IR [cm–1]
UV/Vis
ν(OH)
ν(C=N)
Solvent
λmax [nm]
H2pydx-en (I)
H2pydx-1,3-pn (II)
[Cu(pydx-en)] (1)
[Cu(pydx-1,3-pn)(MeOH)] (2)
[Cu(pydx-en)]-Y (3)
[Cu(pydx-1,3-pn)]-Y (4)
3280–3450
2846–3480
ca. 3400
ca. 3450
ca. 3400
ca. 3450
1626
1632
1634
1635
1633
1638
CH3OH
CH3OH
CH3OH
CH3OH
nujol
216, 252, 335
215, 252, 334
235, 277, 378
233, 273, 377
207, 283, 374
217, 246, 402
nujol
5724
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5720–5734

Zeolite-Encapsulated Copper Complexes of Tetradentate Ligands
were not resolved, neither at room temperature nor at 77 K, ilar gʈ and Aʈ parameters and gʈ/Aʈ ratios were found for
and not much structural information can be obtained. The [CuII(salen)] (2.212, 203.7ϫ10–4 cm–1, 109, respectively)
EPR spectra of the neat CuII complexes 1 and 2 in DMF and [CuII(sal-1,3-pn)] (2.254, 172.5ϫ10–4 cm–1, 131, respec-
solutions, and of the catalysts 3 and 4 are given in Figures 6 tively),[25] in DMF solutions, indicating very similar geome-
and 7.
tries and balance of charge to those of 1 and 2. No super-
hyperfine structure is distinguished.
The EPR spectra of the encapsulated complexes were re-
corded at room temperature and are broader than those for
frozen solutions, suggesting that the complexes located in
the supercages may be present in more than one geometri-
cal conformation. However, the gʈ and Aʈ parameters and
gʈ/Aʈ ratios (gʈ/Aʈ = 113 for 3 and 126 for 4) almost coincide
with those obtained in DMF solutions, indicating similar
geometries and balance of charge for the complexes in the
encapsulated and dissolved forms.
Figure 6. EPR spectra of a frozen DMF solution of [Cu(pydx-en)]
(1) and of [Cu(pydx-en)]-Y (3) at room temperature.
Catalytic Activity Studies
The catalytic activities of the zeolite-encapsulated and
neat complexes have been demonstrated by studying the
oxidation of styrene, cyclohexene and methyl phenyl sulfide.
Oxidation of Styrene
The oxidation of styrene, catalysed by [Cu(pydx-en)]-Y
and [Cu(pydx-1,3-pn)]-Y, was carried out using H2O2 and
tert-butyl hydroperoxide as oxidants to give styrene oxide,
benzaldehyde, benzoic acid and phenylacetaldehyde
(Scheme 3) along with minor amounts of unidentified prod-
ucts. Hulea and Dumitriu[26] have reported some of these
products while using catalysts TS-1 and MCM-41. All oxi-
dation products as observed here were identified recently
using the polymer-anchored catalyst PS-[VO(sal-ohyba)-
(DMF)] (H2sal-ohyba = Schiff base derived from salicylal-
dehyde and o-hydroxybenzylamine)[27] and the zeolite-Y-en-
capsulated complex [VO(sal-dach)]-Y.[14]
Figure 7. EPR spectra of a frozen DMF solution of [Cu(pydx-1,3-
pn)(MeOH)] (2) and of [Cu(pydx-1,3-pn)]-Y (4) at room tempera-
ture.
The spectra of the encapsulated complexes 3 and 4 are
reasonably resolved, indicating the encapsulation of individ-
ual molecules in the supercages of Na-Y. The inter-
molecular interactions are avoided because of isolation of
the zeolite cavities, indicating that the CuII sites are well
separated in the catalyst, and thereby the spin Hamiltonian
parameters can be estimated with reasonable accuracy;
these are listed in Table 3.
Both neat complexes 1 and 2 have an N2O2 binding mode
and are monomeric in the solid state. Complex 1 has a
square-planar configuration, and 2 has square-planar con-
figuration with a pyramidal distortion. In DMF solutions
the representation of the Aʈ, gʈ values fit well with those of
other [CuII(salen)] complexes.[5,22–25] The gʈ/Aʈ ratio is 112.7
for 1, indicating a remarkable planarity of the square-
planar geometry, as found in the solid state. For 2 this ratio
is 126, but this is mainly due to a decrease in Aʈ. Very sim- Scheme 3.
Table 3. EPR data for [Cu(pydx-en)] and [Cu(pydx-1,3-pn)(MeOH)] complexes as neat and encapsulated in zeolite-Y.
Complex
gxx
gyy
gzz(ʈ)
Axx
Ayy
Azz(ʈ)
(ϫ104 cm–1)
(ϫ104 cm–1)
(ϫ104 cm–1)
[Cu(pydx-en)] (1)
2.07
2.04
2.03
2.06
2.22
2.24
2.32
2.34
17.39
21.85
28.43
16.88
196.94
178.53
156
[Cu(pydx-1,3-pn)(MeOH)] (2)
[Cu(pydx-en)]-Y (3)
2.08
2.09
–
–
[Cu(pydx-1,3-pn)]-Y (4)
138
Eur. J. Inorg. Chem. 2007, 5720–5734
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5725

M. R. Maurya, B. Singh, P. Adão, F. Avecilla, J. Costa Pessoa
FULL PAPER
In order to achieve the maximum oxidation of styrene,
catalyst 3 was taken as a representative, and the effect of
four different reaction parameters – viz. the amount of
H2O2 (mol of H2O2 per mol of styrene), amount of catalyst,
volume of solvent and temperature of the reaction – was
studied.
Three different aqueous 30% H2O2/styrene molar ratios
of 1:1, 2:1 and 3:1 were considered while keeping the fixed
amount of styrene (1.04 g, 10 mmol) and catalyst (0.025 g)
in CH3CN (20 mL) at 80 °C. The periodic analyses of re-
sults up to 6 h are illustrated in Figure 8. Increasing the
H2O2/styrene ratio from 1:1 to 2:1 improved the conversion
from 9.1% to 14.8% while a 3:1 ratio showed a further in-
crement of conversion of hardly 1%. Therefore, it is clear
that the 2:1 molar ratio is quite suitable to obtain the opti-
mum styrene conversion of 14.8% in the 6 h reaction time.
Figure 9. Effect of the amount of catalyst on the oxidation of sty-
rene. Reaction conditions: styrene (1.04 g, 10 mmol), 30% H2O2
(2.27 g, 20 mmol), CH3CN (20 mL) and temp. (80 °C).
Figure 10. Effect of the temperature on the oxidation of styrene.
Reaction conditions: styrene (1.04 g, 10 mmol), [Cu(pydx-en)]-Y
(0.025 g), 30% H2O2 (2.27 g, 20 mmol) and CH3CN (20 mL).
Figure 8. Effect of H2O2 on the oxidation of styrene. Reaction con-
ditions: styrene (1.04 g, 10 mmol), [Cu(pydx-en)]-Y (3) (0.025 g),
CH3CN (20 mL) and temp. (80 °C).
Thus, for the maximum oxidation of 10 mmol of styrene,
the other required conditions as concluded were: [Cu(pydx-
Similarly, for three different amounts – viz. 0.015, 0.025 en)]-Y (0.025 g), H2O2 (2.27 g, 20 mmol), CH3CN (10 mL)
and 0.035 g – of [Cu(pydx-en)]-Y at a styrene/aqueous 30% and temperature (80 °C). Under these optimised reaction
H2O2 ratio of 1:2 under the above reaction conditions, conditions, the catalytic activities of 1–4 were further tested,
0.015 g of catalyst, achieved only 7.8% conversion, whereas and the results obtained are shown in Figure 11. Table 4
0.025 g of catalyst gave a maximum conversion of 14.8% in compares the selectivity of the various products obtained
6 h of reaction time. A further increment of catalyst after 6 h of reaction time, along with the percent conversion
(0.035 g) has shown no improvement (Figure 9). The reason of styrene and turnover frequency. It is clear from Table 4
for reduced activity at higher catalyst dose may be adsorp- and Figure 11 that [Cu(pydx-1,3-pn)]-Y exhibits 37.4%
tion/chemisorption of the two reactants on separate catalyst conversion, which is better than that exhibited by [Cu(pydx-
particles, thereby reducing the chance to interact.
en)]-Y (33.3%). Both catalysts gave mainly four products:
Figure 10 illustrates the oxidation of styrene at three dif- styrene oxide, benzaldehyde, phenylacetaldehyde and ben-
ferent temperatures – viz. 60, 70 and 80 °C –, while keeping zoic acid. The selectivity for benzaldehyde is much higher,
the optimised conditions of 10 mmol of styrene, 20 mmol while the other products are obtained in small amounts.
of H2O2 and 0.025 g of catalyst in 20 mL of acetonitrile. It
We have also tested the catalytic activity of the neat com-
is evident from the figure that the performance of the cata- plexes 1 and 2 for the oxidation of styrene using an equi-
lyst is poor at 60 °C wherein only 3.9% conversion was ob- molar concentration of metal ion as in their respective zeo-
served. Increasing the reaction temperatures to 70 and lite-encapsulated metal complex. Figure 11 also provides
80 °C increases the conversion to 7.0% and 14.8%, respec- the conversion as a function of time, while Table 4 lists all
tively. Thus, conducting the catalytic reaction at 80 °C is other details. Thus, under the above reaction conditions, the
most suitable to maximise the oxidation of styrene. We have conversion obtained by the neat complexes (18.7% for 1
also observed that reducing the volume of CH3CN to and 22.3% for 2) is significantly lower than that shown by
10 mL under the above optimised reaction conditions im- the respective encapsulated complexes. Again, all four prod-
proved the conversion considerably, and two more compo- ucts have been obtained with the selectivity order: benzalde-
nents, though in poor yield, have also been obtained.
hyde Ͼ styrene oxide Ͼ phenylacetaldehyde Ͼ benzoic acid.
5726
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5720–5734

Zeolite-Encapsulated Copper Complexes of Tetradentate Ligands
Figure 12. Catalytic comparison of zeolite-encapsulated metal
complexes and neat complexes for the oxidation of styrene in the
presence of TBHP as oxidant. Reaction conditions: styrene (1.04 g,
10 mmol), catalyst (0.025 g for encapsulated and 0.00133 g in case
of neat complexes), TBHP (2.57 g, 20 mmol), CH3CN (20 mL) and
temp. (80 °C).
Figure 11. Catalytic comparison of zeolite-encapsulated and neat
complexes for the oxidation of styrene in the presence of H2O2 as
oxidant. Reaction conditions: styrene (1.04 g, 10 mmol), catalyst
(0.025 g for encapsulated and 0.00133 g in the case of neat com-
plexes), H2O2 (2.27 g, 20 mmol), CH3CN (10 mL) and temp.
(80 °C).
Table 5. Product selectivity and percent conversion of styrene with
TBHP after 6 h of reaction time.
Table 4. Product selectivity and percent conversion of styrene after
6 h of reaction time with H2O2 as oxidant.
[a] TOF [h–1] (turnover frequency): mol of substrate converted per
mol of metal (in the solid catalyst) per hour.
tion of other products such as phenylacetaldehyde is pos-
sible through isomerisation of styrene oxide, while the for-
mation of benzoic acid from benzaldehyde is straightfor-
ward.
Under the above optimised reaction conditions, the cata-
lytic action of neat as well encapsulated complexes has also
been tested using tert-butyl hydroperoxide (TBHP) as an
oxidant: 70% TBHP (2.57 g, 20 mmol), catalyst (0.025 g for
encapsulated and 0.00133 g, for neat complexes) and sty-
Oxidation of Cyclohexene
The oxidation of cyclohexene catalysed by encapsulated
rene (1.04 g, 10 mmol) were used in CH3CN (20 mL), and complexes gave cyclohexene oxide, 2-cyclohexen-1-ol, cyclo-
the reaction was carried out at 80 °C. Figure 12 shows the hexane-1,2-diol and 2-cyclohexen-1-one, as shown in
conversion as a function of time, and Table 5 compares the Scheme 4. Recently, we observed all these products while
selectivity data along with the conversion and turnover fre- using catalyst [VO(sal-dach)] encapsulated in zeolite-Y.[14]
quency after 6 h of reaction time. It is clear that the ob-
The reaction conditions have been optimised considering
tained percentage conversion varies in the range 13.6– 3 as a representative catalyst and varying the amount of
28.0% with the order: [Cu(pydx-1,3-pn)]-Y (28.0%) Ͼ [Cu- catalyst, oxidant, solvent and temperature of the reaction
(pydx-en)]-Y (23.6%) Ͼ [Cu(pydx-1,3-pn)] (16.2%) Ͼ [Cu- mixture. The effect of H2O2 concentration on the oxidation
(pydx-en)] (13.6%). Thus, the overall conversion of styrene of cyclohexene is shown in Figure 13. Three different H2O2/
with TBHP is always lower than with H2O2. The selectivity cyclohexene molar ratios of 1:1, 2:1 and 3:1 were used at
of the formation of an important component – styrene ox- a fixed amount of cyclohexene (0.82 g, 10 mmol), catalyst
ide – is better (43.4–70.5%), but two more products, namely (0.025 g), acetonitrile (20 mL) and temperature (75 °C).
benzoic acid and phenylacetaldehyde, have also been ob- From Figure 13 it is clear that a 3:1 molar ratio is the best
tained.
The highest yield of benzaldehyde using H2O2 as oxidant
of those tested, to obtain a conversion of 38.8% in 6 h.
The amount of catalyst has shown considerable affect on
is possibly due to further oxidation of styrene oxide formed the oxidation of cyclohexene. For a cyclohexene/H2O2 ratio
in the first step by a nucleophilic attack of H2O2 on styrene of 1:3, five different amounts of catalyst – viz. 0.015, 0.025,
oxide followed by cleavage of the intermediate hydroper- 0.035, 0.050 and 0.065 g – were used under the above reac-
oxystyrene.[28] The formation of benzaldehyde may also be tion conditions, and the results obtained are summarised in
facilitated by the direct oxidative cleavage of the styrene Figure 14. The conversion increases with an increase in the
side-chain double bond by a radical mechanism.[27] Forma- amount of catalyst from 0.015 to 0.050 g (28 to 56.2%), and
Eur. J. Inorg. Chem. 2007, 5720–5734
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5727

M. R. Maurya, B. Singh, P. Adão, F. Avecilla, J. Costa Pessoa
FULL PAPER
Scheme 4.
Figure 15. Effect of the temperature on the oxidation of cyclohex-
ene. Reaction conditions: cyclohexene (0.82 g, 10 mmol), [Cu(pydx-
en)]-Y (3) (0.050 g), 30% H2O2 (3.40 g, 30 mmol) and CH3CN
(10 mL).
Figure 13. Effect of H2O2 on the oxidation of cyclohexene. Reac-
tion conditions: cyclohexene (0.82 g, 10 mmol), [Cu(pydx-en)]-Y
(0.025 g), CH3CN (20 mL) and temp. (75 °C).
Thus, suitable reaction conditions for the maximum oxi-
dation of cyclohexene were optimised as follows: cyclohex-
ene (0.82 g, 10 mmol), H2O2 (3.40 g, 30 mmol), catalyst
(0.050 g), acetonitrile (10 mL) and a temperature of 75 °C.
Catalyst 4 was also tested under the above reaction condi-
tions, and results along with the selectivity of various reac-
tion products are summarised in Table 6. A maximum of
83.0% conversion has been achieved with 4, which is less
than that obtained by 3 (90.1%) in 6 h of contact time.
However, both catalysts show the highest selectivity
towards the formation of 2-cyclohexen-1-ol, which is fol-
lowed by cyclohexene oxide. The selectivity of the two other
products, 2-cyclohexen-1-one and cyclohexane-1,2-diol, has
no definite trend.
then assumes a lower trend with 0.065 g of catalyst (52.4%).
Thus, an amount of 0.050 g of catalyst is considered ade-
quate to carry out the oxidation of cyclohexene. It was also
noted that by reducing the volume of acetonitrile to 10 mL,
this conversion reached 90.1% with 0.050 g of catalyst un-
der the above reaction conditions.
We have also tested the catalytic activities of encapsu-
lated complexes using tert-butyl hydroperoxide as an oxi-
dant. Thus, catalyst (0.050 g), tert-butyl hydroperoxide
(3.855 g, 30 mmol) and cyclohexene (0.82 g, 10 mmol) were
used in acetonitrile (10 mL), and the reaction was carried
out at 75 °C. The data, also presented in Table 6, show a
poor conversion of cyclohexene (19.5% for 3 and 17.8% for
4) with tert-butyl hydroperoxide in 6 h of contact time.
Here, the major product obtained is cyclohexane-1,2-diol,
while amounts of the other three products are very poor.
The neat complexes gave 45.5% (with 0.0028 g of 1) or
Figure 14. Effect of the amount catalyst on the oxidation of cyclo-
hexene. Reaction conditions: cyclohexene (0.82 g, 10 mmol), 30%
H2O2 (3.40 g, 30 mmol), CH3CN (20 mL) and temp.(75 °C).
Figure 15 illustrates the effect of temperature of the reac- 50.7% (with 0.0027 g of 2) conversion using H2O2 as an
tion medium on the conversion of cyclohexene as a function oxidant (Table 6). Here, the selectivity of cyclohexene oxide
of time. Amongst the three different temperatures (55, 65 is better (33 to 38.7%) in comparison with the encapsulated
and 75 °C) for the fixed amount of cyclohexene (0.82 g, complexes, although the overall selectivity orders of all
10 mmol), aqueous 30% H2O2 (3.40 g, 30 mmol) and cata- products are the same. However, the turnover rates for en-
lyst (0.050 g) in acetonitrile (10 mL), running the reaction capsulated complexes, their stability towards decomposition
at 75 °C has shown the best result, where 90.1% conversion and easy recovery from the reaction mixture make encapsu-
has been obtained.
lated complexes better catalysts than neat ones.
5728
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5720–5734

Zeolite-Encapsulated Copper Complexes of Tetradentate Ligands
Table 6. Oxidation of cyclohexene and product selectivity using various catalysts.
The conversion of cyclohexene and selectivity of different
reaction products using [Cu(pydx-1,3-pn)]-Y as catalyst un-
der the optimised reaction conditions have been analysed
as a function of time and are presented in Figure 16. It is
Scheme 5.
clear from the plot that a selectivity of 20.7% of cyclohex-
ene oxide and 35.5% of cyclohexen-1-ol has been obtained
The catalytic potential of these catalysts has also been
at a conversion of 20.2% of cyclohexene in 1 h of reaction
time. The selectivity for these products increases with time
and reaches 37.1% and 51.1%, respectively, with the in-
crease of the conversion of cyclohexene to 83.0% in 6 h.
The other two products, 2-cyclohexen-1-one and cyclohex-
ane-1,2-diol, have 26.8% and 14.7% selectivity in the first
hour, but their selectivities decrease with time and reach 3.5
and 8.2%, respectively. Thus, to obtain a good selectivity
for the important product cyclohexene oxide, it is essential
to run the reaction for at least 6 h under the optimised reac-
tion conditions. After 6 h, only minor changes have been
noted in the selectivities of the different reaction products.
optimised considering the amount of oxidant, solvent and
catalyst. Thus, catalyst 4 was taken as a representative, and
the amount of oxidant was varied at a fixed amount of
methyl phenyl sulfide (1.242 g, 10 mmol) and catalyst
(0.025 g) in acetonitrile (10 mL), and the reaction was car-
ried out at ambient temperature. At an aqueous H2O2/sub-
strate ratio of 2:1, a maximum of 69.3% conversion was
achieved in 3 h of reaction time. Decreasing this ratio to 1:1
decreases the conversion of methyl phenyl sulfide consider-
ably, while increasing this ratio to 3:1 has a marginal effect
on the conversion (Figure 17). The only significant influ-
ence when using a 3:1 ratio is the completion of the reaction
in less time. Increasing the volume of solvent to 20 mL un-
der the above reaction conditions decreases the conversion
considerably (to 64.3%).
Figure 16. Conversion of cyclohexene and variation in the selectiv-
ity of various products as a function of time: (a) conversion of
cyclohexene, (b) cyclohexene oxide, (c) 2-cyclohexen-1-ol, (d) 2-cy-
clohexen-1-one and (e) cyclohexane-1,2-diol.
Figure 17. Effect of the amount of H2O2 on the oxidation of methyl
phenyl sulfide as a function of time. Reaction conditions: methyl
phenyl sulfide (1.242 g, 10 mmol), [Cu(pydx-1,3-pn)]-Y (0.025 g)
and acetonitrile (10 mL).
Oxidation of Methyl Phenyl Sulfide (Thioanisol)
Enzyme sulfoxidases[29] facilitate sulfide peroxidase ac-
Similarly, four different amounts of catalyst – viz. 0.015,
tivity where the electron-rich sulfur atom of the sulfide un- 0.025, 0.035 and 0.050 g – were considered, while keeping
dergoes electrophilic oxidation by H2O2 to give the sulfox- methyl phenyl sulfide (1.242 g, 10 mmol) and 30% H2O2
ide and, further, sulfone. Such oxidation of methyl phenyl (2.27 g, 20 mmol) in acetonitrile (10 mL). As mentioned in
sulfide was tested with the copper complexes prepared in Figure 18, increasing the catalyst amount from 0.015 to
this work. Oxidation of methyl phenyl sulfide in the pres- 0.025 g increases the conversion from 46.8 to 69.3%. In-
ence of H2O2 gave two products, namely methyl phenyl sulf- creasing it to 0.035 g improves the conversion to 81.0%,
oxide and methyl phenyl sulfone, as shown in Scheme 5.
while 0.050 g shows no improvement. Therefore, it is clear
Eur. J. Inorg. Chem. 2007, 5720–5734
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5729

M. R. Maurya, B. Singh, P. Adão, F. Avecilla, J. Costa Pessoa
FULL PAPER
that 0.035 g of catalyst is adequate to obtain an optimum
methyl phenyl sulfide conversion of 81.0% in 3 h of contact
time.
Figure 19. Oxidation of methyl phenyl sulfide by various catalysts
using H2O2 as an oxidant.
Table 7. Conversion percentage of methyl phenyl sulfide in 3 h
using H2O2 as an oxidant and selectivity of sulfoxide and sulfone.
Figure 18. Effect of the amount of catalyst on the oxidation of
methyl phenyl sulfide. Reaction conditions: methyl phenyl sulfide
(1.242 g, 10 mmol), H2O2 (2.27 g, 20 mmol) and acetonitrile
(10 mL).
Thus, under the optimised reaction conditions, that is,
catalyst (0.035 g), methyl phenyl sulfide (1.242 g, 10 mmol),
30% H2O2 (2.27, 20 mmol) and acetonitrile (10 mL), cata-
lyst 3 was also tested, and the results of both catalysts are
compared in Figure 19, while Table 7 provides selectivity
details. Under the optimised reaction conditions, 3 and 4
gave 80.3% and 81.0% conversion, respectively. In terms
of the product selectivity, the selectivities of methyl phenyl
sulfoxide and methyl phenyl sulfone are 63.3 and 36.7%,
respectively, for 3 and 53.6 and 46.4%, respectively, for 4.
A blank reaction under similar conditions, that is, methyl
phenyl sulfide (1.242 g, 10 mmol), aqueous 30% H2O2
(2.27 g, 20 mmol) and acetonitrile (10 mL), resulted in
35.2% conversion with 68.3% selectivity towards sulfoxide
and 31.7% sulfone. Thus, catalysts 3 and 4 only enhanced
the percent conversion of methyl phenyl sulfide and played
no positive role in enhancing the selectivity of sulfoxide. In
contrast, a positive role in enhancing the selectivity towards
the formation of sulfoxide as well as conversion of methyl
phenyl sulfide has been observed by oxidovanadium(IV)
and copper(II) complexes encapsulated in the cavities of ze-
olite-Y.[30] The neat complexes also exhibited a good cata-
lytic activity (68.4–69.9% conversion) when 0.0019 g
Figure 20. Oxidation of methyl phenyl sulfide by various catalysts
using tert-butyl hydroperoxide as an oxidant.
Test for Recyclability and Heterogeneity of the Reactions
The recyclability of both the encapsulated complexes has
amounts were used under the above optimised conditions. been tested in all three catalytic reactions. In a typical ex-
However, the selectivity for sulfone was better than for sulf- periment, for example, for styrene, after a contact time of
oxide.
6 h, the reaction mixture was filtered, and after activating
Catalytic activities of these complexes have also been the catalyst by washing with acetonitrile and drying at
tested using TBHP as an oxidant under similar conditions. 120 °C, it was subjected to a further catalytic reaction under
Again, 0.035 g of catalyst has shown the best results at a similar conditions. No appreciable loss in catalytic activity
fixed amount of methyl phenyl sulfide (10 mmol) and suggests that the complex is still present in the cavity of
TBHP (20 mmol) in acetonitrile (10 mL). A maximum of zeolite-Y. The filtrate collected after separating the used cat-
47.1% conversion has been obtained with 3, while 4 gave alyst was placed in the reaction flask and the reaction was
only 18.1% conversion in 3 h of reaction time (Figure 20). continued after adding fresh oxidant for another 4 h. The
Neat complexes 1 and 2 exhibited very poor conversion of gas chromatographic analysis showed no improvement in
4.7 and 4.2%, respectively. Independently of the catalytic conversion, and this confirms that the reaction did not pro-
systems and conversions, the sulfoxide has been obtained ceed upon removal of the solid catalyst. The reaction was,
selectively with TBHP.
therefore, heterogeneous in nature.
5730
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5720–5734

Zeolite-Encapsulated Copper Complexes of Tetradentate Ligands
Possible Reaction Pathway of the Catalysts
[Cu(pydx-1,3-pn)] are monomeric in the cavity of zeolite,
the facile formation of intermediates [(HOO)-Cu(pydx-en)]
and [(HOO)-Cu(pydx-1,3-pn)], respectively, are expected
because of the vacant available site in them (for example
complex 3 was characterised in the solid state with a coordi-
nated methanol molecule). This intermediate is expected to
transfer the oxygen atoms to the substrates to obtain the
products. Thus, the catalytic performance of the supported
catalyst could be attributed to the facile formation of this
Cu hydroperoxide intermediate species. The broadening of
the band at about 590 nm indicates the merging of the d–d
band with this charge-transfer band because of Cu hydro-
peroxide complex formation. In fact characteristic charge-
transfer bands because of Cu hydroperoxide coordination
are known to appear around 590 nm.[33]
Attempts to detect Cu hydroperoxide intermediates by
ESI-MS (in the positive and negative modes) failed. The
addition of H2O2 only increased the intensities of the peaks
corresponding to CuLH+ and CuLNa+ (and CuL·2H2+ for
larger amounts of H2O2) in the positive mode, but no peaks
could be found in the negative mode that we could assign
to Cu hydroperoxide intermediate species.
In order to better understand the reaction mechanism,
the neat complexes were treated with H2O2 and the progress
of the reaction was monitored by electronic absorption
spectroscopy. Thus, titration of a solution of 1 (about
10–4 ) in methanol with one-drop portions of 30% H2O2
dissolved in methanol resulted in the spectral changes
shown in Figure 21. The intensity of the band at 378 nm
changes marginally, but the band at 277 nm increases con-
siderably, while the band at 235 nm (not shown) continu-
ously gains intensity. With a more concentrated solution
(about 10–3 ) now dissolving complex 1 in DMSO, upon
equivalent H2O2 additions the band appearing at 590 nm
broadens with an increase in the intensity (Figure 21, inset).
A very similar spectral pattern has also been observed for
2 (Figure 22). The data suggest the interaction of copper(II)
complexes with H2O2.
Conclusions
Zeolite-Y-encapsulated copper(II) complexes [Cu(pydx-
en)]-Y and [Cu(pydx-1,3-pn)]-Y with N,NЈ-ethylenebis(pyri-
doxyliminato) (H2pydx-en) and N,NЈ-propylenebis(pyridox-
yliminato) (H2pydx-1,3-pn) ligands have been prepared.
Neat complexes, [Cu(pydx-en)] and [Cu(pydx-1,3-
pn)(MeOH)], have also been prepared with these ligands
and characterised, and their crystal and molecular struc-
tures were determined, confirming the ONNO binding
mode of the ligands. The encapsulated complexes catalyse
the oxidation of styrene, cyclohexene and thioanisole ef-
ficiently, either using H2O2 or tert-butyl hydroperoxide as
oxidants. Oxidation of styrene catalysed by [Cu(pydx-en)]-
Y and [Cu(pydx-1,3-pn)]-Y gave 23.6% and 28.0% conver-
sion, respectively, using TBHP as an oxidant. Using H2O2
as oxidant the major products were styrene oxide and benz-
aldehyde, while TBHP gave styrene oxide, benzaldehyde,
benzoic acid and phenylacetaldehyde. Under the optimised
conditions the maximum conversions obtained were 90.1%
of cyclohexene, using [Cu(pydx-en)]-Y, and 83.0% using
[Cu(pydx-1,3-pn)]-Y as catalyst; the major products iden-
tified were cyclohexene oxide, 2-cyclohexen-1-ol, cyclohex-
ane-1,2-diol and 2-cyclohexen-1-one. Similarly, maximums
of 80.3% conversion of methyl phenyl sulfide with [Cu-
(pydx-en)]-Y and of 81.0% with [Cu(pydx-1,3-pn)]-Y were
Figure 21. Titration of a methanol solution (about 10–4 ) of
[Cu(pydx-en)] (1) with aqueous 30% H2O2 dissolved in methanol.
The inset displays equivalent titrations, but with higher concentra-
tions of 1 (about 10–3 ) dissolved in DMSO.
Figure 22. Titration of a methanol solution (about 10–4 ) of
[Cu(pydx-1,3-pn)MeOH] (2) with aqueous 30% H2O2 dissolved in
methanol. The inset displays equivalent titrations, but with higher observed, where the selectivity for the major product methyl
concentrations of 2 (about 10–3 ) dissolved in DMSO.
phenyl sulfoxide was found to be about 60%. Tests for re-
cyclability and heterogeneity of the reactions indicated their
At least three types of intermediates having a copper– good recyclability. Of the alternative suggested mechanisms
oxygen interaction – side-on CuIII–(µ-η2-peroxido)–CuIII
,
for catalysis, namely regarding the intermediates specifying
bis(µ-oxido–CuIII) and CuIII–O–O–H (copper hydroperox- the copper–oxygen interaction, our data suggest the in-
ide) – have been discussed in the literature during the cata- volvement of CuIII–O–O–H (copper hydroperoxide) inter-
lytic action.[31,32] As copper complexes [Cu(pydx-en)] and mediates.
Eur. J. Inorg. Chem. 2007, 5720–5734
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5731

M. R. Maurya, B. Singh, P. Adão, F. Avecilla, J. Costa Pessoa
FULL PAPER
X-ray Crystal Structure Determination: Three-dimensional X-ray
data were collected with a Bruker Kappa Apex CCD diffractometer
by the φ-ω scan method. Data were collected at room temperature.
Reflections were measured from a hemisphere of data collected
from frames each covering 0.3° in ω. Of the 13687 reflections mea-
sured for 1 and 20715 for 2, all of which were corrected for Lorentz
and polarization effects, and for absorption by semiempirical meth-
ods based on symmetry-equivalent and repeated reflections, 2139
independent reflections for 1 and 8116 for 2 exceeded the signifi-
cance level |F|/σ(|F|) Ͼ 4.0. Complex scattering factors were taken
from the program package SHELXTL.[35] The structures were
solved by direct methods and refined by full-matrix least-squares
methods on F2. The hydrogen atoms were included in calculated
positions and refined by using a riding mode for 1 and were left to
refine freely for 2. Refinement converged with allowance for ther-
mal anisotropy of all non-hydrogen atoms. Minimum and maxi-
mum final electron density: 2.494 and –2.167°eÅ–3 for 1, 0.997 and
–0.673°eÅ–3 for 2. Crystal data and details on data collection and
refinement are summarised in Table 8. CCDC-655305 (for 1) and
-655304 (for 2) 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.
Experimental Section
Materials: Analytical reagent-grade pyridoxal hydrochloride
(Hpydx·HCl, Himedia, Mumbai, India), methyl phenyl sulfide
(Alfa Aeaser, USA), styrene, cyclohexene (Acros Organics, NJ,
USA), cupric nitrate (E. Merck, Mumbai, India) and 30% aqueous
H2O2 (Qualigens, Mumbai, India) were used as obtained. Y-zeolite
(Si/Al = 10) was obtained from Indian Oil Corporation (R&D),
Faridabad, India. All other chemicals and solvents used were of
AR grade. Ligands H2pydx-en (I) and H2pydx-1,2-pn (II) were pre-
pared as described in the literature.[15]
Physical Methods and Analysis: The copper content was measured
using an inductively coupled plasma spectrometer (ICP; Labtam
8440 Plasmalab) after leaching the metal ions with conc. nitric acid
and diluting with double-distilled water to specific volumes. IR
spectra were recorded as KBr pellets with a Nicolet NEXUS Alig-
ent 1100 series FTIR spectrophotometer, after grinding the sample
with KBr. UV/Vis spectra of zeolite-encapsulated metal complexes
were recorded in Nujol using a Shimadzu 1601 UV/Vis spectropho-
tometer by layering a mull sample to the inside of one of the cu-
vettes and keeping the other one layered with Nujol as reference.
UV/Vis spectra of other ligands and neat complexes were recorded
in methanol. X-ray powder diffractograms of solid catalysts were
recorded using a Bruker AXS D8 advance X-ray powder dif-
fractometer with a Cu-Kα target. Thermogravimetric analyses of
pure as well as encapsulated complexes were carried out using a
TG Stanton Redcroft STA 780 instrument. Scanning electron
micrographs (SEM) of zeolite-containing samples were recorded
with a Leo instrument, model 435VP. The samples were dusted on
alumina and coated with a thin film of gold to prevent surface
changing and to protect the surface material from thermal damage
by the electron beam. In all analyses, a uniform thickness of about
0.1 mm was maintained. The energy-dispersive X-ray analysis
(EDAX) plots of encapsulated complexes were recorded with an
FEI Quanta 200 FEG instrument. The ESI-MS data were obtained
with a Varian 500-MS LC ion-trap mass spectrometer operating
either in the positive or in the negative mode. The EPR spectra
were recorded with a Bruker ESP 300E X-band spectrometer. The
spectra were measured at room temperature and at 77 K (with
glasses, made by freezing solutions in liquid nitrogen, or just using
the frozen samples). For the neat complexes we recorded spectra in
DMF and in DMSO. In DMSO apparently there was precipitation
of solids upon freezing, as only broad signals were obtained. The
spin Hamiltonian parameters were obtained by simulation of the
spectra with the computer program of Rockenbauer and Korecz.[34]
All catalysed reaction products were analysed using a Thermoelec-
tron gas chromatograph fitted with an HP-1 capillary column
(30 mϫ0.25 mmϫ0.25 µm) and FID detector. Styrene, cyclohex-
ene and methyl phenyl sulfide were used as external standards to
quantify their conversion upon catalytic action. To quantify the
conversion of styrene, for example, the calibration of the GC was
carried out using styrene as external standard by injecting three
different but known concentrations of styrene into the GC under
the reaction conditions at which the analyses of catalytic products
were performed and noting the peak area in each case. Finally, the
peak area obtained for the remaining amount of styrene during
catalytic oxidation was correlated with the peak area of the stan-
dard sample. This ultimately gave the percent conversion of sub-
strate. The selectivity of products was then obtained directly from
the peak areas of the respective products. Other external standards
were used similarly. The identities of the products were confirmed
by GC-MS with a Perkin–Elmer Clarus 500 model.
Table 8. Crystal data collection and refinement.
1
2
Empirical formula
C18H20CuN4O4
419.92
C20H26CuN4O5
465.99
Mr [gmol–1]
Crystal system
Space group
triclinic
P1
triclinic
P1
¯
¯
T [K]
a [Å]
b [Å]
c [Å]
α [°]
β [°]
293(2)
293(2)
9.6664(17)
10.0904(19)
10.150(3)
101.418(14)
93.432(14)
116.898(9)
852.7(4)
434
8.0870(2)
9.0492(2)
14.2599(3)
105.8640(10)
90.9030(10)
95.5120(10)
998.20(4)
486
γ [°]
V [Å3]
F(000)
Z
2
2
Dcalcd. [gcm–3]
1.636
1.315
1.550
1.135
µ [mm–1]
Rint
0.0918
13687
2139
0.0334
20715
8116
Measured reflections
Observed reflections
Data/restraints/parameters 2975/3/248
10700/0/375
1.029
0.0415
Goodness-of-fit on F2
1.093
0.0918
0.2713
[a]
R1
[b]
wR2 (all data)
0.1029
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(||Fo|2 – |Fc|2|)2]|/
Σ[w(Fo4)]}1/2
.
Preparations
Preparation of [Cu(pydx-en)] (1): A stirred solution of H2pydx-en
(0.717 g, 2 mmol) dissolved in methanol (20 mL) was added to cu-
pric acetate (0.40 g, 2 mmol) dissolved in methanol (30 mL) with
stirring, and the reaction mixture was refluxed in a water bath for
5 h. On reducing the solvent volume to about 20 mL and keeping
the mixture at ambient temperature, a brown solid of [Cu(pydx-
en)] slowly separated within a few hours. This was filtered off,
washed with methanol and dried in vacuo over silica gel. Yield:
78%. Green crystals suitable for X-ray diffraction study were ob-
5732
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5720–5734

Zeolite-Encapsulated Copper Complexes of Tetradentate Ligands
tained from the filtrate on slow concentration at room temperature.
C18H20CuN4O4 [Cu(pydx-en)] (419.93): calcd. C 51.48, H 4.80, Cu
15.13, N 13.34; found C 52.0, H 4.6, Cu 15.1, N 13.1.
the UV spectra measured. A solution of H2O2 was prepared by
dissolving five drops of 30% H2O2 in methanol (5 mL). One drop
of this H2O2 solution was added and the UV spectra recorded
again. Several similar additions were made, and after each addition
the spectral changes were recorded similarly. As the solubility of
complexes 1 or 2 is low in methanol, only the UV region could be
covered in this way. For the visible region, the complexes were dis-
solved in DMSO (concentration about 10–3 , 10 mL), and ad-
ditions of the H2O2 solution were made similarly, now recording
the visible region of the spectra (see insets of Figures 21 and 22).
Preparation of [Cu(pydx-1,3-pn)(MeOH)] (2): [Cu(pydx-1,3-
pn)(MeOH)] was prepared according to essentially the same pro-
cedure as for 1. A green solid was obtained. Green crystals suitable
for X-ray diffraction study were obtained from the filtrate on keep-
ing it at room temperature for slow concentration. Yield: 70%.
C20H26CuN4O5 [Cu(pydx-1,3-pn)(MeOH)] (466.0): calcd. C 51.55,
H 5.62, Cu 13.64, N 12.02; found C 51.8, H 5.8, Cu 13.8, N 12.2.
Preparation of Cu-Y: Na-Y zeolite (5.0 g) was suspended in distilled
water (300 mL) and after addition of cupric nitrate (2.26 g,
12 mmol), the reaction mixture was heated at 90 °C with stirring
for 24 h. The pale bluish solid was filtered, washed with hot dis-
tilled water until the filtrate was free from any copper(II) ion con-
tent and dried at 150 °C in an air oven for 24 h. Found: Cu 7.60.
Acknowledgments
The authors are thankful to the Council of Scientific and Industrial
Research, New Delhi for financial assistance. The authors also wish
to thank FEDER, Fundação para a Ciência e a Tecnologia and
POCI 2010 (ref. POCI/QUI/55985/2004) for financial support, and
Kamila Koci for the ESI-MS experiments.
Preparation of [Cu(pydx-en)]-Y (3): Cu-Y (3.0 g) and H2pydx-en
(3.0 g) were mixed in methanol (50 mL), and the reaction mixture
was heated under reflux in an oil bath with stirring for 14 h. The
resulting material was filtered and then Soxhlet-extracted with
methanol to remove unreacted ligand. It was finally treated with
hot DMF while stirring for 1 h, filtered and washed with DMF
followed by hot methanol. The uncomplexed metal ions present in
the zeolite were removed by stirring with double-distilled aqueous
NaCl (0.0.1 , 150 mL) for 10 h. The resulting pale yellow solid
was filtered, and washed with hot distilled water until no precipitate
of AgCl was observed in the filtrate on treating with AgNO3. Fi-
nally it was dried at 120 °C for several hours. Found: Cu 0.79.
[1] J. Poltowicz, K. Pamin, E. Tabor, J. Haber, A. Adamski, Z.
Sojka, Appl. Catal. A 2006, 299, 235–242.
[2] S. P. Verkey, C. Ratnasamy, P. Ratnasamy, J. Mol. Catal. A
1998, 135, 295–306.
[3] S. P. Verkey, C. Ratnasamy, P. Ratnasamy, Microporous Meso-
porous Mater. 1998, 22, 465–474.
[4] S. P. Verkey, C. Ratnasamy, P. Ratnasamy, Appl. Catal. A 1999,
182, 91–96.
[5] S. Deshpande, D. Srinivas, P. Ratnasamy, J. Catal. 1999, 188,
261–269.
[6] D. Chatterjee, A. Mitra, J. Mol. Catal. A 1999, 144, 363–367.
[7] C. Bowers, P. K. Dutta, J. Catal. 1990, 122, 271–279.
[8] M. S. Niassary, F. Farzaneh, M. Ghandi, L. Turkian, J. Mol.
Catal. A 2000, 157, 183–188.
[9] M. Silva, C. Freire, B. de Castro, J. L. Figueiredo, J. Mol. Ca-
tal. A 2006, 258, 327–333.
[10] T. Joseph, S. B. Halligudi, C. Satyanarayan, D. P. Sawant, S.
Gopinathan, J. Mol. Catal. A 2001, 168, 87–97.
[11] M. R. Maurya, S. J. J. Titinchi, S. Chand, Appl. Catal. A 2002,
228, 177–187.
[12] M. R. Maurya, S. J. J. Titinchi, S. Chand, J. Mol. Catal. A
2003, 201, 119–130.
[13] M. R. Maurya, M. Kumar, S. J. J. Titinchi, H. S. Abbo, S.
Chand, Catal. Lett. 2003, 86, 97–105.
[14] M. R. Maurya, A. K. Chandrakar, S. Chand, J. Mol. Catal. A:
Chem. 2007, 270, 225–235.
[15] I. Correia, J. Costa Pessoa, M. T. Duarte, R. T. Henriques,
M. F. M. Piedade, L. F. Veiros, T. Jackusch, A. Dornyei, T.
Kiss, M. M. C. A. Castro, C. F. G. C. Geraldes, F. Avecilla,
Chem. Eur. J. 2004, 10, 2301–2317.
Preparation of [Cu(pydx-1,3-pn)]-Y (4): The pale yellow [Cu(pydx-
1,3-pn)]-Y was prepared according to essentially the same pro-
cedure as outlined for 3. Found: Cu 0.74.
Catalytic Activity
Oxidation of Styrene: The catalytic oxidation of styrene was carried
out in 50-mL flasks fitted with a water-circulated condenser. In a
typical reaction, an aqueous solution of 30% H2O2 or 70% tert-
butyl hydroperoxide (TBHP) (20 mmol) and styrene (1.04 g,
10 mmol) was mixed in acetonitrile (20 mL). After the temperature
of the oil bath reached 80 °C, the zeolite-Y-encapsulated catalyst
(0.025 g) was added to the reaction mixture which was continuously
stirred. During the reaction, the products were periodically ana-
lysed using a gas chromatograph by withdrawing small aliquots,
and their identities were confirmed by GC-MS. Various param-
eters, such as amounts of oxidant and catalyst as well as the tem-
perature of the reaction mixture were studied in order to under-
stand their effect on the conversion and patterns of the reaction
products.
[16] I. Correia, A. Dornyei, T. Jakusch, F. Avecilla, T. Kiss, J. Co-
sta Pessoa, Eur. J. Inorg. Chem. 2006, 2819–2830.
[17] I. Correia, A. Dornyei, T. Jakusch, F. Avecilla, T. Kiss, J. Co-
sta Pessoa, Eur. J. Inorg. Chem. 2006, 656–662.
[18] M. Valko, Inorg. Chem. 1993, 32, 4131–4138.
[19] H. Elias, Angew. Chem. Int. Ed. Engl. 1992, 31, 623–625.
[20] D. F. Evans, J. Chem. Soc. 1959, 2003–2005.
[21] E. Sinn, A. B. Blake, W. Haussain, R. Paschke, J. R. Chip-
perfield, Inorg. Chem. 1995, 34, 1125–1129.
Oxidation of Cyclohexene: The catalytic oxidation of cyclohexene
was also carried out using [Cu(pydx-en)]-Y and [Cu(pydx-1,3-pn)]-
Y. Aqueous 30% H2O2 (2.28 g, 20 mmol), cyclohexene (0.82 g,
10 mmol) and catalyst (0.025 g) were mixed in acetonitrile (20 mL),
and the reaction mixture was heated at 75 °C with continuous stir-
ring in an oil bath. The progress of the reaction was monitored as
mentioned above. The identification of the various products was
confirmed by GC-MS.
[22] J. Peisach, W. E. Blumberg, Arch. Biochem. Biophys. 1974, 165,
Oxidation of Methyl Phenyl Sulfide: Aqueous 30% H2O2 (2.28 g,
20 mmol) and catalyst (0.025 g) were added to a solution of methyl
phenyl sulfide (1.24 g, 10 mmol) dissolved in acetonitrile (20 mL),
and the reaction mixture was stirred at ambient temperature for
3 h. The formation of the reaction products was monitored by GC.
691–708.
[23] U. Sakaguchi, A. W. Addison, J. Chem. Soc. Dalton Trans.
1979, 600–608.
[24] V. T. Kasumov, F. Koksal, Spectrochim. Acta Part A 2005, 61,
225–231.
[25] K. M. Ananth, M. Kanthimathi, B. U. Nair, Transition Met.
Chem. 2001, 26, 333–338.
[26] V. Hulea, E. Dumitriu, Appl. Catal. A 2004, 277, 99–106.
Measurement of UV/Vis Spectra by Adding H2O2: A solution of
complex 1 or 2 in methanol (ca. 10–4 , 10 mL) was prepared and
Eur. J. Inorg. Chem. 2007, 5720–5734
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5733

M. R. Maurya, B. Singh, P. Adão, F. Avecilla, J. Costa Pessoa
FULL PAPER
[27] M. R. Maurya, U. Kumar, P. Manikandan, Dalton Trans. 2006,
3561–3575.
[28] M. R. Maurya, U. Kumar, P. Manikandan, Eur. J. Inorg. Chem.
2007, 2303–2314.
[29] A. Butler, M. J. Clague, G. E. Meister, Chem. Rev. 1994, 94,
625–638.
[32] J. P. Klinman, Chem. Rev. 1996, 96, 2541–2561.
[33] K. D. Karlin, J. C. Hayes, Y. Gultneh, R. W. Cruse, J. W.
McKown, J. P. Hutchinson, J. Zubieta, J. Am. Chem. Soc. 1984,
106, 2121–2128.
[34] A. Rockenbauer, L. Korecz, Appl. Magn. Reson. 1996, 10, 29–
43.
[30] M. R. Maurya, A. K. Chandrakar, S. Chand, J. Mol. Catal. A:
Chem. 2007, 263, 227–237.
[31] E. I. Solomon, P. Chen, M. Metz, S.-K. Lee, A. E. Palner, An-
gew. Chem. Int. Ed. 2001, 40, 4570–4590.
[35] G. M. Sheldrick, SHELXTL, release 5.1, Bruker Analytical X-
ray Systems, Madison, WI, 1997.
Received: July 27, 2007
Published Online: November 8, 2007
5734
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5720–5734