M(C6H16N3)2(VO 3)4 as heterogeneous catalysts. Study of three new hybrid vanadates of cobalt(ii), nickel(ii) and copper(ii) with 1-(2-aminoethyl) piperazonium

Article abstract of DOI:10.1039/c1dt10971e

Three new hybrid vanadates have been synthesized under hydrothermal conditions with the formula M(C₆H₁₆N₃) ₂(VO₃)₄, where M = Co(ii), Ni(ii) and Cu(ii). The structural analyses show that the phases are isostructural and crystallize in the monoclinic space group P2₁/c. These compounds show a two-dimensional crystal structure, with sheets composed of [VO₃] _n^n- chains and metal centres octahedrally coordinated, chelated by two 1-(2-aminoethyl)piperazonium ligands. The thermal study reveals that the copper containing phase is less stable than the cobalt and nickel containing ones. The IR spectra of the three phases are very similar, with little differences in the inorganic bond region of the copper containing phase. The UV-visible spectra show that the cobalt(ii) and the nickel(ii) are in slightly distorted octahedral environments. The catalytic tests show that the phases act as heterogeneous catalysts for the selective oxidation of alkyl aryl sulfides, with both H₂O₂ and tert-butylhydroperoxide as oxidizing agents. The influence of the steric hindrance in the kinetic profile has been studied. The catalytic reactions induce the partial amorphization of the phases.

Full text of DOI:10.1039/c1dt10971e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 12690
www.rsc.org/dalton
PAPER
M(C6H16N3)2(VO3)4 as heterogeneous catalysts. Study of three new hybrid
vanadates of cobalt(II), nickel(II) and copper(II) with
1-(2-aminoethyl)piperazonium†
Edurne S. Larrea,a Jose´ L. Mesa,b Jose´ L. Pizarro,a Marta Iglesias,c Teo´filo Rojob and Mar´ıa I. Arriortua*a
Received 24th May 2011, Accepted 31st August 2011
DOI: 10.1039/c1dt10971e
Three new hybrid vanadates have been synthesized under hydrothermal conditions with the formula
M(C6H16N3)2(VO3)4, where M = Co(II), Ni(II) and Cu(II). The structural analyses show that the phases
are isostructural and crystallize in the monoclinic space group P21/c. These compounds show a
n-
two-dimensional crystal structure, with sheets composed of [VO3]n chains and metal centres
octahedrally coordinated, chelated by two 1-(2-aminoethyl)piperazonium ligands. The thermal study
reveals that the copper containing phase is less stable than the cobalt and nickel containing ones. The
IR spectra of the three phases are very similar, with little differences in the inorganic bond region of the
copper containing phase. The UV-visible spectra show that the cobalt(II) and the nickel(II) are in
slightly distorted octahedral environments. The catalytic tests show that the phases act as
heterogeneous catalysts for the selective oxidation of alkyl aryl sulfides, with both H2O2 and
tert-butylhydroperoxide as oxidizing agents. The influence of the steric hindrance in the kinetic profile
has been studied. The catalytic reactions induce the partial amorphization of the phases.
great ability to polymerize, giving rise to clusters, rings, chains,
Introduction
layers and three-dimensional frameworks.6 The polymer grade is
Vanadium is one of the most interesting and reactive ele-
closely related to the synthetic conditions and, in particular, to the
pH during the reaction.7
ments with a very rich chemistry in all of its oxidation states
from +5 to -1. Its good catalytic activity, as well as its interesting
sorption, optical, electrochemical and magnetic properties, are
well documented in the literature.1 Since the discovery of vanadate-
dependent enzymes from various algae2 and terrestrial fungi,3 the
coordination chemistry of this element has received increasing
interest.
The choice of the organic ligand is determinant for the structure
of the resulting vanadates. Depending on the number of donor
atoms, their relative positions to each other and the flexibility of
the ligand, it could act chelating a metal centre or bridging two
metal atoms. This fact conditions the structure and, hence, the
properties of the material.
Hybrid vanadates with first-row transition metals exhibit very
rich crystal chemistry, with several structural archetypes according
to the metal centre, the geometry of the ligand and the vanadium
oxide subunit.4 Each oxidation state of vanadium shows different
coordination environments. These polyhedra, range from the ideal
octahedron, for the oxidation state three, to the tetrahedron, for
the oxidation state five, going through the distorted octahedron,
the square-based pyramid and the trigonal bipyramid (oxidation
states four and five).5 In addition, vanadium polyhedra have a
In order to obtain open frameworks, there are two possible
synthetic strategies. First, using long and rigid bridging
ligands such as 4,4¢-bipyridine, 1,2-bis(4-pyridyl)ethylene or
1,2-bis(4-pyridyl)ethane, which could act as pillars between
inorganic sheets.8 The other strategy comprises the use
of chelating ligands, such as ethylenediamine, N,N¢-bis(3-
aminopropyl)ethylenediamine, tris(2-aminoethyl)amine, 2,2¢-
bipyridine, 1,10-phenanthroline, etc., combined with low
dimensionality vanadate subunits.9
The last strategy was used to synthesize the vanadates included
in this work, which were tested as heterogeneous catalysts for the
oxidation of alkyl aryl sulfides. Sulfoxide derivatives are known
to have interesting and useful biological and pharmacodynamic
properties.10 The sulfoxides are obtained by oxidation of thioethers
by peracids, peroxides and alkyl peroxides using transition
metal catalysts.11 Depending on the catalyst selectivity and the
method used, different proportions of sulfoxide and sulfone are
produced.12
aDpto. Mineralog´ıa y Petrolog´ıa, Universidad del Pa´ıs Vasco (UPV/EHU),
Apdo. 644, 48080, Bilbao, Spain. E-mail: maribel.arriortua@ehu.es;
Fax: +0034 946 013 500; Tel: +0034 946 012 534
bDpto. Qu´ımica Inorga´nica, Universidad del Pa´ıs Vasco (UPV/EHU), Apdo.
644, 48080, Bilbao, Spain
cInstituto de Ciencia de Materiales de Madrid, CSIC, c/Sor Juana Ine´s de
la Cruz 3, Cantoblanco, 28049, Madrid, Spain
† CCDC reference numbers 826549–826551. For crystallographic data in
CIF or other electronic format, see DOI: 10.1039/c1dt10971e
12690 | Dalton Trans., 2011, 40, 12690–12698
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
n-
Heterogeneous catalysis is a vital component of the chemical
industry and it is involved to some degree in 90% of the chemical
manufacturing processes currently in use.13 On the other hand,
catalytic oxidation in the liquid phase is becoming increasingly
important in the synthesis of fine chemicals, where traditional
processes employing stoichiometric inorganic oxidants are under
increasing environmental pressure.14 Catalytic oxidations in the
liquid phase generally use soluble metal salts or complexes in
combination with clean, inexpensive oxidants such as O2, H2O2,
or RO2H. However, heterogeneous catalysts have the advantage of
facile recovering and recycling.
As far as we are concerned, this is the first report of the
catalytic properties of hybrid vanadates. There are some studies
of related materials, such as vanadyl arsenates templated with
amine molecules15 or vanadyl phosphates,16 as well as of vanadium
containing metal–organic compounds.17 Recently, four silver(I)
vanadates have also been studied as photocatalysts.18 These
compounds show a synergistic effect between silver ions and
vanadate oxoanion that could improve their catalytic selectivity.
All of them decompose methylene blue, an organic dye, under
ultraviolet and/or visible light.
atoms of two [VO3]n chains in the axial positions and to four
nitrogen atoms of two HAep+ molecules, in the equatorial plane.
The organic molecule is chelating the metal centres via the nitrogen
1 of the piperazine ring and the nitrogen of the aminoethylic
radical. The other nitrogen atom of the ring is protonated.
The joint between the octahedra and the vanadate chains
through the O(1) atoms gives rise to the sheets, parallel to the
(100) plane (Fig. 2). These sheets are linked to each other via
hydrogen bonds between the hydrogen atom H(11), bonded to the
protonated nitrogen atom, N(3), of the HAep+, and the O(2) atom
of the [VO3]nn- chain, N(3)–H(11) ◊ ◊ ◊ O(2) (Fig. 1). There are, also,
hydrogen bonds into the (100) sheets.
During the course of our study, in order to obtain new
hybrid vanadates with catalytic properties, we have syn-
thesized three new vanadates of Co(II), Ni(II) and Cu(II)
with 1-(2-aminoethyl)piperazonium (HAep+), with the formula
M(HAep)2(VO3)4. In this work we report on the crystal structure,
thermal behaviour, and spectroscopic properties together with the
catalytic activity tests of these phases.
Results and discussion
Fig. 2 Polyhedral view of the (100) sheet.
Crystal structures
The M–N distances are very similar for the three compounds
The structural analyses show that Co(HAep)2(VO3)4, 1,
Ni(HAep)2(VO3)4, 2 and Cu(HAep)2(VO3)4, 3 are isostructural
and crystallize in the monoclinic space group P21/c, with very
similar cell parameters. These compounds show a two-dimensional
crystal structure (Fig. 1).
˚
and range from 1.992(1) to 2.271(1) A. However, the M–O
distances of phase 3 are higher than those of the phases 1 and 2.
For the cobalt(II) and nickel(II) compounds, the M–O distances are
˚
approximately 2.1 A, whereas for the copper(II) phase, this distance
˚
is 2.330(1) A. This difference is due to the Jahn–Teller effect of
the Cu(II) cation, whose octahedra are slightly elongated, and, as
a consequence, the c parameter of the phase 3 is unexpectedly
bigger than that of the nickel(II) containing one. The cis angles
range from 81.82(5)◦ to 98.18(5)◦ and the trans ones are 180◦,
because the metallic atoms are at special positions, over symmetry
centres.
The V–O bond distances observed in the vanadium(V) tetra-
hedra are in the normal range for this oxoanion.19 Depending
on the connectivity of the oxygen atom the V–O distances
˚
can be classified as short (approx. 1.6 A), when the oxygen is
˚
terminal, as medium (1.65–1.70 A), when the oxygen is linked
˚
to the metal, or as long (approx. 1.8 A), when the oxygen is
linked to another vanadium atom. In this structure, there are
two crystallographically independent vanadium(V) atoms, with
three terminal oxygen bonds, one oxygen shared with a divalent
metal and two shared by two vanadium(V) atoms. However,
Fig. 1 Polyhedral view of the two-dimensional crystal structure.
˚
Vanadium(V) atoms are in tetrahedral coordination environ-
ments. Each VO4 tetrahedra is linked to other two tetrahedra
through two of their vertex. This joint, gives rise to [VO3]nn- chains
in the [010] direction. On the other hand, the metallic ions, Co2+,
Ni2+ or Cu2+, are octahedrally coordinated, linked to two O(1)
only one short distance, of 1.62 A, is found, for the V(2)–O(6)
bond. The other two terminal oxygen atoms, O(2) and O(3), have
distances near 1.65 A, because they are part of hydrogen bonds.
The medium bond distances are formed by V(1)–O(1), and are
1.657(1) and 1.658(1) A for phases 1 and 2, whereas for phase
˚
˚
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12690–12698 | 12691
©

View Article Online
˚
Table 1 Selected bond distances (A) for 1, 2 and 3
V(1)O4 tetrahedra
V(2)O4 tetrahedra
1
2
3
1
2
3
V(1)–O(2)
V(1)–O(1)
V(1)–O(5)
V(1)–O(4)
1.648(1)
1.657(1)
1.776(1)
1.781(1)
1.653(1)
1.658(1)
1.785(1)
1.781(1)
1.653(1)
1.631(1)
1.787(1)
1.784(1)
V(2)–O(6)
V(2)–O(3)
V(2)–O(5)
V(2)–O(4)ii
1.648(1)
1.657(1)
1.776(1)
1.781(1)
1.629(1)
1.651(1)
1.807(1)
1.811(1)
1.624(1)
1.652(1)
1.804(1)
1.807(1)
MO2N4 octahedra
(C6H16N3) 1-(2-aminoethyl)-piperazonium
1
2
3
1
2
3
M–O(1)i
M–O(1)
M–N(2)i
M–N(2)
M–N(1)i
M–N(1)
2.104(1)
2.104(1)
2.119(1)
2.119(1)
2.271(1)
2.271(1)
2.093(1)
2.093(1)
2.075(1)
2.075(1)
2.227(1)
2.227(1)
2.330(1)
2.330(1)
1.992(1)
1.992(1)
2.182(1)
2.182(1)
N(2)–C(2)
C(2)–C(6)i
N(1)–C(6)
N(1)–C(1)
N(1)–C(4)
N(3)–C(3)
N(3)–C(5)
C(1)–C(5)
C(3–C(4)
1.474(2)
1.502(3)
1.495(2)
1.482(2)
1.482(2)
1.485(2)
1.490(2)
1.516(2)
1.517(2)
1.479(2)
1.502(2)
1.499(2)
1.477(2)
1.493(2)
1.489(2)
1.4929(2)
1.525(2)
1.523(2)
1.478(2)
1.504(2)
1.499(2)
1.484(2)
1.487(2)
1.476(2)
1.477(2)
1.519(2)
1.519(2)
Symmetry codes: i = -x, -y, -z; ii = -x, y + 1/2, -z + 1/2.
25
˚
˚
3 it is 1.631(1) A, smaller than expected. This fact is related to
the Jahn–Teller effect mentioned above: the O(1) is more distant
from the copper(II) atom, and, therefore, in the tetrahedra takes
a medium distance between the terminal oxygen atom and the
oxygen atom shared with a metallic centre. The bond distances V–
O(4) and V–O(5), where the oxygen atoms are shared between both
crystallographically independent vanadium(V) atoms, are around
3.557 A]. The residue of 2 consists of a mixture of NiV3O8 [a =
13.310 A, b = 8.395 A, c = 10.040 A], and V2O5.25 Finally, the
26
˚
˚
˚
˚
copper(II) phase, 3, decomposed into Cu2V2O7 [C2/c, a = 7.687 A,
◦
b = 8.007 A, c = 10.090 A, b = 1140.45 ] and V2O5.25
27
˚
˚
The thermal behaviour of the phases was also studied using
time-resolved X-ray diffractometry (Fig. 3). Phases 1 and 2 are
stable until 270 ◦C and 280 ◦C, respectively. The copper(II) phase
is less stable, and its structure collapses at 180 ◦C. The destruction
of the crystal structure is due to the calcination of the organic
ligand, according to the thermogravimetric data. Above these
temperatures, the formation of amorphous compounds takes
place. In the case of phase 1, the heating residues crystallise at
380 ◦C, for phase 2 the diffraction maxima start to appear at
430 ◦C, and in the case of 3, this crystallization process occurs
at 360 ◦C. At the end of the heating process two phases were
˚
1.8 A, as expected. The bond angles of the vanadium(V) tetrahedra
range from 107.4(1) to 112.3(1)◦. Bond distances of the described
polyhedra are included in Table 1.
To complete the structural description, the polyhedral distor-
tion study was carried out by the Continuous Symmetry Measure
(CSM),20 method based on the comparison between the real
polyhedron and the most similar ideal one. These calculations
were carried out with the Shape v1.1a program, developed by S.
Alvarez et al.21 The S(Oh) values for phases 1, 2 and 3 are 0.49,
0.34 and 0.59, respectively, which represent very slight distortions
with regard to the ideal octahedron.22 The S(Td) values calculated
for the vanadium(V) tetrahedra are around 0.12 for V(1) and 0.25
for V(2). These values indicate that the vanadium(V) environment
is very regular.23
determined for the cobalt(II) compound: Co2V2O7 and V2O5.25
24
˚
For phase 2 a mixture of◦Ni2V2O7 [P21/c, a = 6.515 A, b = 8.303
28
25
˚
˚
A, c = 9.350 A, b = 99.86 ] and V2O5 is obtained. The thermal
¯
decomposition of 3 gives rise to a mixture of C◦uV2O6 [P1, a ◦=
˚
˚
˚
9.170 A, b = 3.552 A, c = 6.480 A, a = 92.30 , b = 110.35 ,
g = 91.78◦]29 and V2O5.25 In the thermal stability range of the
phases, an angular variation on the diffraction maxima can be
Thermal study
The thermogravimetric decomposition curves of the three phases
show a sequence of overlapped processes of mass loss, from
approximately 250 ◦C to almost 500 ◦C for compounds 1 and
2. Whereas, for compound 3, these processes occur between 185
and 470 ◦C. The total weight loss is 38.9% for 1, 38.8% for 2
and 38.1% for 3. These mass percentages are very close to the
theoretical percentages of the organic cation in the phases, 37.2,
37.3 and 36.2%, respectively. The excess of mass that gets lost
during the heating process could correspond to the evaporation
of the sample at high temperature. The calcination products were
identified by X-ray powder diffraction analysis. The residue of 1
˚
˚
consists of Co2V2O7 [P21/c, a = 6.599 A, b = 8.388 A, c = 9.492
◦
24
˚
˚
˚
A, b = 100.17 ] and V2O5 [Pmn21, a = 11.503 A, b = 4.369 A, c =
Fig. 3 Thermodiffractogram of compound 2.
12692 | Dalton Trans., 2011, 40, 12690–12698
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
observed, due to the thermal expansion of the unit cell parameters.
Thermodiffractometry of phase 2 is shown in Fig. 3 as an example.
The temperature at which the crystal structure of 3 collapses is
significantly lower, comparing with the isostructural phases. This
lower stability temperature is due to the weaker bonds that Cu2+
establishes with the O(1), making the framework less stable.
IR and UV-visible (diffuse reflectance) spectroscopy
In the IR spectra of the three phases there are two characteristic
regions, the first one, between 3300 and 990 cm-1, where the bands
due to the vibration of the organic molecule bonds appeared, and
the later one, between 950 and 500 cm-1, where the vibrations of the
vanadate groups are detected. The three spectra are very similar
(Fig. 4). Only the spectrum of 3 has some differences, mainly in
the inorganic bond region, due to the differences observed in the
Cu–O bond distances.
Fig. 4 IR spectra of phases 1, 2 and 3.
The UV-visible spectra of phases 1 and 2 show that the cobalt(II)
and the nickel(II) are in a slightly distorted octahedral environment
(Fig. 5). In the spectrum of Co(II) phase there is a band at 8300
cm-1, with a shoulder at 10 400 cm-1. The first absorption is
4
due to the spin-allowed 4T1g(4F) → T2g(4F) electronic transition,
2
while the shoulder is caused by the forbidden 4T1g(4F) → Eg(2G)
transition. Charge transfer band appears at ca. 25 000 cm-1. At
17 900 cm-1 a shoulder on the charge transfer band appears,
which can be related with the spin-allowed T1g(4F) → T1g(4P)
electronic transition. The spectrum of 2 shows two absorptions
at 11 000 and 16 600 cm-1. These bands are due to the spin-
4
4
allowed A2g(3F) → T2g(3F) and 3A2g(3F) → T1g(3F) transitions.
The charge transfer band appears ca. at 28 000 cm-1. Taking into
account the absorption band positions it was possible to calculate
the crystal field splitting parameter, Dq, and the Racah parameters
B and C for 1, and only B for 2. For 1, Dq = 935 cm-1, B = 710
cm-1, 65% of the free ion (Bo(Co2+) = 1115 cm-1)30 and C = 4765
cm-1. For 2, Dq = 1100 cm-1 and B = 630 cm-1, approximately 60%
of the free ion (Bo(Ni2+) = 1030 cm-1).31 The spectrum of 3 shows
two absorption bands at 9950 and 15 500 cm-1. In this case it is
not possible to calculate the above mentioned parameters because
of the electronic configuration of the copper(II) ion, d9, and the
3
3
3
Fig. 5 UV-visible spectra of the three phases.
tetragonal distortion shown by the coordination polyhedron in this
structure. The bands of this spectrum are due to d–d transitions
between components of the same ionic term.32
Catalytic activity
Vanadium peroxides are known as very effective oxidants of
different organic and inorganic substrates such as sulfides, alkenes,
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12690–12698 | 12693
©

View Article Online
Table 2 Conversion (CT), selectivity towards the sulfoxides formation (SSO) and turnover frequency (TOF) data of oxidation of alkyl aryl sulfides
catalyzed by 1, 2 and 3
Catalyst
Substrate
Oxidizing agent
CT (%) (t, min)
SSO (%)
TOF (min-1)
1
MeSPh
MeSPh
MeSPh
MeSPh
EtBuSPh
MeSClPh
MeSp-tolyl
H2O2
TBHP
H2O2
H2O2
H2O2
H2O2
H2O2
82 (320)
89 (370)
83 (320)
64 (320)
93 (380)
99 (320)
100 (380)
95
90
94
95
90
84
96
0.54
1.40
0.48
0.26
0.44
1.01
1.37
2
3
alcohols, aromatic and aliphatic hydrocarbons, halides and sulfur
dioxide.33 These reactions, either stoichiometric or catalytic,
where the terminal oxidant is normally hydrogen peroxide or an
alkylhydroperoxide, are carried out, on average, in mild conditions
and associated with good product yield and selectivity.
The activity of the vanadium catalysts 1, 2 and 3 was probed
towards oxidation of sulfides. To note, this reaction is interesting
both for the preparation of sulfoxides and sulfones,33 as well
as for desulfurization of fuels.34 Various catalytic reactions were
performed with different oxidizing agents (H2O2 and TBHP) and
substrates (methyl phenyl sulfide, ethylbutyl phenyl sulfide, p-
chlorophenyl methyl sulfide and methyl p-tolyl sulfide) (Table 2).
First of all, the catalysts were tested for the oxidation of methyl
phenyl sulfide with H2O2 as the oxidizing agent. The kinetic profiles
of these reactions are shown in Fig. 6. The behaviour of all the
catalysts is very similar, reaching conversions between 83 and 64%,
with selectivity rates towards the sulfoxide over 94%.
Fig. 7 Kinetic profiles of the oxidation reaction of methyl phenyl sulfide
with H2O2 and TBHP as oxidizing agents over catalyst 1.
Fig. 6 Kinetic profiles of 1, 2 and 3 for the oxidation reaction of methyl
phenyl sulfide.
Fig. 8 Kinetic profiles of the oxidation reaction of methyl phenyl sulfide
(MeSPh), 4-chlorophenyl methyl sulfide (4-ClPhSMe), methyl p-tolyl
sulfide (MeSp-tolyl) and ethylbutyl phenyl sulfide (EtBuSPh) over the
catalyst 3 with H2O2 as the oxidizing agent.
The influence of the oxidizing agent was studied comparing the
kinetic profiles of the oxidation reactions of methyl phenyl sulfide
with H2O2 and TBHP over catalyst 1 (Fig. 7). The profiles are
very similar to each other, observing no loss in the conversion rate
when increasing the steric hindrance of the oxidizing agent.
On the other hand, when the steric hindrance of the substrate
is increased the kinetic profile and the conversion and selectivity
rates change (Fig. 8). The reactions were undertaken over the
catalyst 3, using H2O2 as the oxidizing agent, and with the
substrates ethylbutyl phenyl sulfide, p-chlorophenyl methyl sulfide
and methyl p-tolyl sulfide. The kinetic profiles of these reactions
are shown in Fig. 8. When a substituent is introduced in the
phenyl ring of the substrate (Cl, Me), an increase in the TOF
and conversion rate is observed. However, the kinetic profile for
ethylbutyl phenyl sulfide shows an important induction period due
12694 | Dalton Trans., 2011, 40, 12690–12698
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
to steric hindrance presented by this substrate, which makes the
access to the active site more difficult.
phenyl sulfide, p-chlorophenyl methyl sulfide and methyl p-tolyl
sulfide produce an important amorphization of the catalysts, with
almost only the first maxima of the diffraction pattern of 3 being
observed. The IR spectra of the catalysts 1, 2 and 3 before and
after the reaction with methyl phenyl sulfide are almost equal. On
the other hand, catalyst 3 after reacting with ethylbutyl phenyl
sulfide, p-chlorophenyl methyl sulfide and methyl p-tolyl sulfide
show IR spectra with very different features, although the bands
due to the organic cation and V–O bonds appear. It is very likely
that the amorphous phases obtained after the reactions keep the
connectivity of the original phase to some extent, even though the
order is not maintained.
When the reactions are completed, the catalysts were filtered,
washed, dried and characterized by powder X-ray diffraction and
IR spectroscopy. When the catalysts are used for the oxidation
of methyl phenyl sulfide with H2O2, the diffraction patterns show
the diffraction maxima of the corresponding crystal structure, but
the background is significantly higher, due to the amorphization
of the material during the reaction (Fig. 9). However, when
TBHP is used, the diffraction pattern of 1 does not show any
amorphization. Finally, the oxidation reactions of ethylbutyl
Experimental
Synthesis and characterization
Co(HAep)2(VO3)4, 1, Ni(HAep)2(VO3)4, 2 and Cu(HAep)2(VO3)4,
3, have been synthesized using mild solvothermal conditions em-
ploying PTFE (poly-tetrafluoroethylene) reactors. In a typical syn-
thesis of 1, a mixture of 250 mg (0.86 mmol) of Co(NO3)2·6H2O,
157 mg (1.28 mmol) of NaVO3, 225 ml (1.72 mmol) of 1-(2-
aminoethyl)piperazine (Aep) in 20 ml of methanol and 10 ml of
distilled water, pH = 10, was heated at 120 ◦C for three days.
This gave a mixture of orange prismatic crystals of 1 and an
unidentified yellow powder. Crystalline product was separated
from the powder, washed with distilled water and acetone, and
dried at ambient temperature.
The synthesis of 2 involved mixing 250 mg (0.86 mmol) of
Ni(NO3)2·6H2O, 104.82 mg (0.86 mmol) of NaVO3, 225 ml (1.72
mmol) of Aep in 20 ml of 1-butanol and 10 ml of distilled water,
giving a pH of 9. The reaction conditions were the same as those
used in the synthesis of 1. After slow cooling to room temper-
ature, green prismatic crystals appeared as the major product
together with a small amount of an unidentified brown yellowish
powder. Crystalline product was separated from the powder,
washed with distilled water and acetone, and dried at ambient
temperature.
In the synthesis of 3 350.00 mg (1.5 mmol) of Cu(NO3)·2.5H2O
salt was mixed with 227.50 mg (1.87 mmol) of NaVO3 and 175
ml (1.3 mmol) of Aep in 20 ml of methanol and 10 ml of distilled
water. The mixture, with a pH of 6, was heated up to 100 ◦C for
three days. After cooling the reaction system to room temperature,
a mixture of light blue prismatic crystal and a brown greenish
unidentified powder was obtained. The crystals were separated
from the powder, washed with water and acetone and dried.
The amounts of cobalt, nickel, copper and vanadium in
compounds 1, 2 and 3 were measured by inductively coupled
plasma - atomic emission spectrometry and the amounts of C,
N and H by elemental analysis. Calculated (%) for 1: Co, 8.2; V,
28.5; C, 20.2; N, 11.8; H, 4.5. Found (%) Co, 8.5(1); V, 29.2(2);
C, 20.0(1); N, 11.6(1); H, 4.4(1). Calculated (%) for 2: Ni 8.2; V,
28.5; C, 20.2; N, 11.8; H, 4.5. Found (%) Ni, 8.8(1); V, 28.8(2);
C, 19.9(1); N, 11.7(1); H, 4.5(1). Calculated (%) for 3: Cu 8.8; V,
28.3; C, 20.0; N, 11.7; H, 4.5. Found (%) Cu, 10.2(1); V, 27.4(2);
C, 19.9(3); N, 11.4(1); H, 4.5(1). The densities of the three phases
were measured by flotation method,35 using a mixture of CH2I2
and CH2Cl2, being 1.90(1) g cm-3 for 1, 1.90(1) g cm-3 for 2 and
1.93(1) g cm-3 for 3.
Fig. 9 Powder diffraction patterns of the catalysts before and after the
reactions.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12690–12698 | 12695
©

View Article Online
Table 3 Details of the crystal data, structural resolution and refinement procedure for 1, 2 and 3
Compound
1
2
3
Formula
C6H16Co0.5N3O6V2
357.56
Monoclinic
P21/c (14)
9.881(1)
9.400(1)
15.024(2)
116.54(1)
1248.4(2)
4
1.90(1), 1.902
722
293(2)
Stoe IPDS
0.71073
2.159
0.17 ¥ 0.17 ¥ 0.05
2.87–29.78
h = 13, k = 13, l 20
24 283
3531
0.0446
C6H16Ni0.5N3O6V2
357.45
Monoclinic
P21/c (14)
9.848(1)
9.442(1)
14.965(1)
116.52(1)
1245.2(2)
4
1.90(1), 1.907
724
293(2)
Stoe IPDS
0.71073
2.254
0.28 ¥ 0.25 ¥ 0.15
3.60–33.14
h = 15, -11 £ k £ 10, -19 £ l £ 22
16 043
4583
0.0314
4583/0/160
C6H16Cu0.5N3O6V2
359.86
Monoclinic
P21/c (14)
9.763(1)
Formula weight
Crystal system
Space group (no.)
˚
a/A
˚
b/A
9.295(1)
˚
c/A
15.158(2)
116.31(1)
1233.0(2)
4
1.93(1), 1.939
726
293(2)
O. D. Xcalibur2
0.71073
2.376
0.25 ¥ 0.20 ¥ 0.05
2.87–32.62
-14 £ h £ 13, -12 £ k £ 13, -21 £ l £ 22
12 128
4069
0.0255
b (◦)
3
˚
V/A
Z
r
obs, rcalc/g cm-3
F(000)
T/K
Diffractometer
˚
Radiation, l(Mo-Ka)/A
m/mm-1
Crystal size/mm
q range (◦)
Limiting indices, h, k, l
No. reflections measured
No. reflections unique
R(int)
Data/restraints/param.
Final R indices [I > 2s(I)]a
R indices [all data]a
3531/0/160
R1 = 0.0240, wR2 = 0.0379
R1 = 0.0412, wR2 = 0.0396
x = 0.015
4069/0/161
R1 = 0.0284, wR2 = 0.0677
R1 = 0.0393, wR2 = 0.0709
x = 0.0426
R1 = 0.0245, wR2 = 0.0484
R1 = 0.0378, wR2 = 0.0506
x = 0.0273
w = 1/[s |Fo|2 + (xp)2], p =
2
[max|Fo|2 + 2|Fc|2]/3
-3
˚
Largest diff. peak and hole/e A
G. O. F
0.409, -0.235
0.944
0.411, -0.359
0.921
0.712, -0.438
0.990
2
2
a
wR2 =
w F − F
w F
o
R1 = RꢀFo| - |Fcꢀ/R|Fo|;
.
(
)
o
c
∑
∑
◦
Single-crystal X-ray diffraction
heated at 5 C min-1 in the temperature range 30–500 ◦C. The
thermal behaviour was also studied using time-resolved X-ray
diffractometry. A Bruker D8 Advance Vantec diffractometer
(Cu-Ka radiation) equipped with a variable-temperature stage
(Anton Paar HTK2000) with a Pt sample holder was used in
the experiments. The powder patterns were recorded in 2q steps
of 0.01667◦ in the 8–45◦ range,◦counting for 0.1 s per step and
increasing the temperature at 10 C min-1 from room temperature
to 450 ◦C, in the case of compound 1, and to 500 ◦C for compounds
2 and 3.
The IR spectra (KBr pellets with a 1% of the sample) were
obtained with a Nicolet FT-IR 740 spectrophotometer in the
4000–400 cm-1 range. The UV-visible spectra by diffuse reflectance
of 1, 2 and 3 powder samples were registered at room temperature
on a Varian Cary 5000 spectrometer in the 200–1800 nm range.
Prismatic single crystals of compounds 1, 2 and 3 with dimensions
0.17 ¥ 0.17 ¥ 0.05 mm, 0.28 ¥ 0.25 ¥ 0.15 mm and 0.29 ¥
0.20 ¥ 0.05 mm, respectively, were carefully selected under a
polarizing microscope and glued on a glass fibre. Diffraction data
were collected on a Stoe IPSD XArea diffractometer for 1 and
2, and on an Xcalibur2 diffractometer for 3, using in all cases
graphite monochromated Mo-Ka radiation. Details of crystal
data, intensity collection, and some features of the structural
refinement are reported in Table 3.
The diffraction data were corrected for Lorentz and polarization
effects.36 Absorption effect37 was also corrected by using the
diffractometer software.38 The structures were solved by direct
methods (SHELXS 97)39 and then refined by the full-matrix least-
squares procedure based on F2, using the SHELXL 97 computer
program39 belonging to the WINGX software package.40 The
scattering factors were taken from ref. 41. Anisotropic thermal
parameters were assigned to the non-H atoms (Fig. 10). The
hydrogen atoms were generated geometrically. At convergence,
R1 = 0.0240, wR2 = 0.0379 for 1, R1 = 0.0245, wR2 = 0.0484 for 2
and R1 = 0.0284, wR2 = 0.0677 for 3, with I > 2s(I).
Catalytic activity
Oxidation of organic sulfides was carried out in a batch reactor
at atmospheric pressure, using acetonitrile or dichloromethane as
solvents (2 ml). Before the reactions the materials (0.007 mmol)
were activated by stirring them with the oxidizing agent, H2O2
or tert-butyl hydroperoxide (TBHP) in the corresponding solvent,
acetonitrile or dichloromethane, for 30 min at 70 ◦C or 50 ◦C,
respectively. After this activation stage, the catalysts were separated
from the liquid media. The reactor with the activated catalyst was
then charged with 3 mmol of the corresponding sulfide (methyl
phenyl sulfide, 4-clorophenyl methyl sulfide, methyl p-tolyl sulfide
and 1-ethylbutyl phenyl sulfide) (ratio V/substrate = 1 : 100) in 2 ml
Physicochemical characterization techniques
The thermal analyses were carried out using an STD 2960
Simultaneous DSC-TGA TA Instruments thermobalance for
phases 1 and 2, and with a NETZSCH STA 449 C for phase
3. A crucible containing approximately 10 mg of sample was
12696 | Dalton Trans., 2011, 40, 12690–12698
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Conclusions
We report the synthesis and characterization of three isostructural
vanadates of cobalt(II), nickel(II) and copper(II) with the cationic
ligand 1-(2-aminoethyl)piperazonium. These phases present a
two-dimensional crystal structure in which the organic molecule
is monoprotonated and linked to the metallic centres via nitrogen
atoms. There are interlayer hydrogen bonds between the proto-
nated amine and terminal oxygen atoms of the vanadate groups.
The thermal stability of the phases depends on the metal that
compose them, with the copper(II) containing phase being the
less stable. The catalytic tests show that the phases are active and
oxidize selectively alkyl aryl sulfides. The use of TBHP as oxidizing
agent, instead of H2O2, does not actuallyaffect the kinetic profile of
the reaction. However, the increase of steric hindrance in the alkyl
substituent of the sulfide causes an important change in the kinetic
profile, which has an activation stage not shown in the case of other
sulfides. When the oxidizing agent is H2O2, the catalytic reactions
induce the partial amorphization of the catalysts. Although the
long-range order is lost after the reactions, the connectivity
between the components of the phases is maintained.
Acknowledgements
This work has been financially supported by the “Ministerio de
Ciencia e Innovacio´n” (MAT2010-15375, MAT2006-14274-C02-
02), and the “Gobierno Vasco” (IT177-07), which we gratefully ac-
knowledge. We also thank the technicians of SGIker (UPV/EHU),
Drs. J. Sangu¨esa, I. Orue, A. Larran˜aga, J. C. Raposo, L.
Bartolome´ and P. Vitoria, financed by the National Program for
the Promotion of Human Resources within the National Plan of
Scientific Research, Development and Innovation (Ministerio de
Ciencia e Innovacio´n) and the European Social Fund (ESF), for
the X-ray diffraction and chemical, spectroscopic and magnetic
measurements. Edurne S. Larrea thanks the UPV/EHU for
funding.
Notes and references
1 R. A. Shiels, K. Venkatasubbaiah and C. W. Jones, Adv. Synth. Catal.,
2008, 350, 2823; A. Zhang, J. Zhang, N. Cui, X. Tie, Y. An and L. Li, J.
Mol. Catal. A: Chem., 2009, 304, 28; D. E. Katsoulis, Chem. Rev., 1998,
98, 359; G. Centi and F. Trifiro, Appl. Catal., A, 1996, 143, 3; Y. Idota,
Eur. Pat., 0 567 149 A1, 1993; M. S. Whittingham, J. Electrochem. Soc.:
Electrochem. Sci. Tech, 1976, 123, 315; S. Denis, E. Baudrin, F. Orsini,
G. Ouvrard, M. Touboul and J.-M. Tarascon, J. Power Sources, 1999,
81–82, 79; Y. Ueda, Chem. Mater., 1998, 10, 2653; L. Liu, X. Wang and
A. J. Jacobson, J. Mater. Res., 2009, 24, 1901.
2 M. Weyad, H. J. Hecht, M. Kiess, M. F. Liaud, H. Vilter and D.
Schomburg, J. Mol. Biol., 1999, 293, 595; J. N. Carter-Franklin, J. D.
Parrish, R. A. Tchirret-Guth, R. D. Little and A. Butler, J. Am. Chem.
Soc., 2003, 125, 3688; D. Rechder, G. Santoni, G. M. Licini, C. Schilzke
and B. Meier, Coord. Chem. Rev., 2003, 237, 53.
Fig. 10 Asymmetric unit and the atomic labelling scheme of phases 1, 2
and 3. Thermal ellipsoid plot (50%).
of solvent. The oxidizing agent was added dropwise and then the
suspension was heated to 70 ◦C, when using H2O2, and to 50 ◦C,
with TBHP. Experiments in the same reaction conditions without
catalyst have been also conducted and no appreciable amount
of product was detected. Reaction samples were taken at regular
times and analyzed by a Hewlett-Packard 5890 II GC-MS in order
to determine the conversion. After the reaction, the catalysts were
filtered and characterized by X-ray powder diffraction and IR
spectroscopy.
3 A. Butler, Coord. Chem. Rev., 1999, 187, 17.
4 R. Ferna´ndez de Luis, M. K. Urtiaga, J. L. Mesa, T. Rojo and M. I.
Arriortua, J. Alloys Compd., 2009, 480, 54.
5 M. Schindler, F. C. Hawthorne and W. H. Baur, Chem. Mater., 2000,
12, 1248.
6 P. Y. Zavalij and M. S. Whittingham, Acta Crystallogr., Sect. B, 1999,
55, 627.
7 M. Schindler, F. C. Hawthorne and W. H. Baur, Can. Mineral., 2000,
38, 1443; J. Livage, Coord. Chem. Rev., 1998, 178–180, 999.
8 R. Ferna´ndez de Luis, J. L. Mesa, M. K. Urtiaga, L. Lezama, M. I.
Arriortua and T. Rojo, New J. Chem., 2008, 32, 1582; R. Ferna´ndez de
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12690–12698 | 12697
©

View Article Online
Luis, M. K. Urtiaga, J. L. Mesa, K. Vidal, L. Lezama, T. Rojo and M.
I. Arriortua, Chem. Mater., 2010, 22, 5543.
9 E. S. Larrea, J. L. Mesa, J. L. Pizarro, J. Rodr´ıguez-Ferna´ndez, M. I.
Arriortua and T. Rojo, Eur. J. Inorg. Chem., 2009, 3607; J. Do and A. J.
Jacobson, Inorg. Chem., 2001, 40, 2468; P. J. Hagrman and J. Zubieta,
Inorg. Chem., 2001, 40, 2800.
22 S. Alvarez, D. Avnir, M. Llunel and M. Pinsky, New J. Chem., 2002,
26, 996.
23 J. Cicera, P. Alemany and S. Alvarez, Chem.–Eur. J., 2004, 10,
190.
24 “Powder Diffraction File-Inorganic and Organic”, ICCD, Pennsylva-
nia. File 38-0252, 2001.
10 A. G. Renwick, in, Sulfur-Containing Drugs and Related Organic
Compounds, ed. L. A. Damani, Horwood, Chichester, 1989, vol. 1,
part B, p. 133.
25 “Powder Diffraction File-Inorganic and Organic”, ICCD, Pennsylva-
nia. File 86-2248, 2001.
26 “Powder Diffraction File-Inorganic and Organic”, ICCD, Pennsylva-
nia. File 22-0454, 2001.
27 “Powder Diffraction File-Inorganic and Organic”, ICCD, Pennsylva-
nia. File 73-1032, 2001.
28 “Powder Diffraction File-Inorganic and Organic”, ICCD, Pennsylva-
nia. File 29-0945, 2001.
11 F. Bonadies, F. De Angelis, L. Locat and A. Scettri, Tetrahedron Lett.,
1996, 37, 7129.
12 D. Alonso, C. Na´jera and M. Varea, Tetrahedron Lett., 2002, 43, 3459.
13 P. M. Forster and A. K. Cheethanm, Top. Catal., 2003, 24, 79.
14 R. A. Sheldon and R. S. Downing, Appl. Catal., A, 1999, 189, 163.
15 T. Berrocal, J. L. Mesa, J. L. Pizarro, B. Bazan, M. Iglesias, A. T.
Aguayo, M. I. Arriortua and T. Rojo, Chem. Commun., 2008, 4738; T.
Berrocal, J. L. Mesa, J. L. Pizarro, B. Bazan, M. Iglesias, J. L. Vilas, T.
Rojo and M. I. Arriortua, Dalton Trans., 2010, 39(3), 834; T. Berrocal,
E. S. Larrea, M. Iglesias and M. I. Arriortua, J. Mol. Catal. A: Chem.,
2011, 335(1–2), 176.
29 “Powder Diffraction File-Inorganic and Organic”, ICCD, Pennsylva-
nia. File 45-1054, 2001.
30 J. S. Griffith, The Theory of Transition-Metal Ions, Cambridge Univer-
sity Press, London, 1964.
31 Y. Tanabe and S. Sugano, J. Phys. Soc. Jpn., 1954, 9, 753.
32 A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier Science
Publishers B. V., Amsterdam, Nederland, 1984.
33 V. Conte and O. Bortolini, in The Chemistry of Peroxides Transition
Metal Peroxides. Synthesis and Role in Oxidation Reactions, ed. Z.
Rappoport, Wiley Interscience, 2006 and references cited therein; V.
Conte and B. Floris, Inorg. Chim. Acta, 2010, 363, 1935.
34 C. Song, Catal. Today, 2003, 86, 211.
16 P. Amoro´s, D. Marcos and A. Beltra´n-Porter, Curr. Opin. Solid State
Mater. Sci., 1999, 4, 123; H. Imai, Y. Kamiya and T. Okuhara, J. Catal.,
2008, 255, 213; Y. H. Taufiq-Yap, A. A. Rownaghi, M. Z. Hussein and
M. Irmawati, Catal. Lett., 2007, 119(1–2), 64; R. Sen, R. Bera, A.
Bhattacharjee, P. Gu¨tlich, S. Ghosh, A. K. Murkerjee and S. Koner,
Langmuir, 2008, 24, 5970.
17 M. R. Mayura, M. Kumar and A. Arya, Catal. Commun., 2008, 10,
187; M. R. Maurya, A. Arya, A. Kumar and J. C. Pessoa, Dalton Trans.,
2009, 2185; M. R. Maurya, A. Arya, U. Kumar, A. Kumar, F. Avecilla
and J. C. Pessoa, Dalton Trans., 2009, 9555.
18 H. Lin and P. A. Maggard, Inorg. Chem., 2008, 47, 8044; Y. Hu, F. Luo
and F. Dong, Chem. Commun., 2011, 47, 761.
19 R. D. Shanon and C. Calvo, J. Solid State Chem., 1973, 6, 538.
20 H. Zadrodsky, S. Peles and D. Avnir, J. Am. Chem. Soc., 1992, 114,
7843; M. Pinsky and D. Avnir, Inorg. Chem., 1998, 37, 5575.
21 M. Llunel, D. Casanova, J. Cicera, J. M. Bofill, P. Alemany, S. Alvarez,
M. Pinsky and D. Yatunir, SHAPE v1.1a Program for Continuous Shape
Measure Calculations of Polyhedral Xn and MLn fragments, 2003.
35 P. Roma´n and J. M. Gutie´rrez-Zorrilla, J. Chem. Educ., 1985, 62, 167.
36 W. Yingua, J. Appl. Crystallogr., 1987, 20, 258.
37 A. C. T. North, D. C. Philips and F. S. Mathews, Acta Crystallogr.,
1968, A24, 351.
38 XAREA, Program for X-ray Crystal Data Collection, Stoe, Darmstadt,
Germany 2002; Crysalis RED, Oxford Diffraction Ltd. CRYSALIS
1.171.29 version, 2006.
39 G. M. Sheldrick, SHELX97, Programs for Crystal Structure Analysis
(Release 97-2), University of Go¨ttingen, Germany, 1997.
40 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
41 International Tables for X-ray Crystallography, ed. T. Hahn, Kynoch
Press, Birmingham, England, 1974, vol. IV, p. 99.
12698 | Dalton Trans., 2011, 40, 12690–12698
This journal is
The Royal Society of Chemistry 2011
©