Copper(II) complexes of tetradentate pyridyl ligands supported on keggin polyoxometalates: Single-crystal to single-crystal transformations promoted by reversible dehydration processes

Article abstract of DOI:10.1021/ic302499f

Two new hybrid compounds constructed from Keggin type polyoxometalates and copper(II) complexes of tetradentate ligands containing amine and pyridyl groups, namely [Cu(bpmen)(H₂O)][SiW₁₂O ₄₀{Cu(bpmen)}] (1) and [SiW₁₂O₄₀{Cu(bpmpn) (H₂O)}₂]·3H₂O (2) (bpmen, N,N′-dimethyl-N,N′-bis-(pyridin-2-ylmethyl)-1,2-diaminoethane; bpmpn, N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-1,3- diaminopropane), have been synthesized under hydrothermal conditions and characterized by elemental analyses and infrared and Raman spectroscopy. Thermal stability of 1 and 2 has been studied by means of thermogravimetric analyses and variable temperature powder X-ray diffraction. Both compounds undergo single-crystal to single-crystal transformations promoted by reversible dehydration processes that have been followed by single-crystal X-ray diffraction. Structures of 1 and 2, and also of their corresponding anhydrous phases 1a and 2a, have been established. The layered structure of 1 shows rows of monodecorated polyanions with complex cations occupying intralamellar spaces, whereas trans-didecorated species in 2 lead to stacked honeycomb-like metal-organic layers forming channels where Keggin clusters are accommodated. Structural differences relate to changes in the complex geometry and ligand conformation when going from bpmen to bpmpn. Dehydration of 1 promotes coordination of the complex countercation and consequent formation of a cis-didecorated species in 1a, whereas changes in the structure of 2a are more subtle. Structural variations upon dehydration are reflected in the electron paramagnetic resonance spectra.

Full text of DOI:10.1021/ic302499f

Article
pubs.acs.org/IC
Copper(II) Complexes of Tetradentate Pyridyl Ligands Supported on
Keggin Polyoxometalates: Single-Crystal to Single-Crystal
Transformations Promoted by Reversible Dehydration Processes
Amaia Iturrospe, Benat Artetxe, Santiago Reinoso, Leire San Felices, Pablo Vitoria, Luis Lezama,
̃
and Juan M. Gutierrez-Zorrilla*
́
́
Departamento de Química Inorganica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, P.O. Box 644,
48080 Bilbao, Spain
S
* Supporting Information
ABSTRACT: Two new hybrid compounds constructed from
Keggin type polyoxometalates and copper(II) complexes of
tetradentate ligands containing amine and pyridyl groups, namely
[Cu(bpmen)(H2O)][SiW12O40{Cu(bpmen)}] (1) and
[SiW12O40{Cu(bpmpn)(H2O)}2]·3H2O (2) (bpmen, N,N′-di-
methyl-N,N′-bis-(pyridin-2-ylmethyl)-1,2-diaminoethane;
bpmpn, N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-1,3-diami-
nopropane), have been synthesized under hydrothermal con-
ditions and characterized by elemental analyses and infrared and
Raman spectroscopy. Thermal stability of 1 and 2 has been
studied by means of thermogravimetric analyses and variable
temperature powder X-ray diffraction. Both compounds undergo
single-crystal to single-crystal transformations promoted by
reversible dehydration processes that have been followed by single-crystal X-ray diffraction. Structures of 1 and 2, and also of
their corresponding anhydrous phases 1a and 2a, have been established. The layered structure of 1 shows rows of
monodecorated polyanions with complex cations occupying intralamellar spaces, whereas trans-didecorated species in 2 lead to
stacked honeycomb-like metal−organic layers forming channels where Keggin clusters are accommodated. Structural differences
relate to changes in the complex geometry and ligand conformation when going from bpmen to bpmpn. Dehydration of 1
promotes coordination of the complex countercation and consequent formation of a cis-didecorated species in 1a, whereas
changes in the structure of 2a are more subtle. Structural variations upon dehydration are reflected in the electron paramagnetic
resonance spectra.
the past years, our group reported several hybrid compounds
based on Keggin type POMs and CuII complexes with simple,
commercial bidentate ligands (acetate, oxalate, 2,2′-bipyridine,
1,10-phenanthroline).6 Besides some magnetic aspects, we
mainly focused our studies on the systematic analysis of the
different intermolecular POM−ligand interactions governing
the packing of such hybrids. In order to achieve further insight
into the role played by these weak interactions, we decided to
explore the structural influence of more elaborate organic
ligands and selected tetradentate species containing amine and
pyridil groups (N2Py2) for this purpose. Transition metal
complexes with N2Py2 ligands have been successfully applied in
catalytic and biological studies.7 To our knowledge, the
[Mo4O16{Cr(bispictn)}4](ClO4)4·3H2O compound represents
the only example where they have been combined with POM
clusters.8
INTRODUCTION
■
Polyoxometalates (POMs) constitute a large family of
transition-metal−oxide clusters. They have been widely
recognized to exhibit remarkable compositional and structural
diversity and properties that make them useful in different areas
including catalysis, magnetism, materials science, and medi-
cine.1 A significant advance in the field of POM chemistry is the
construction of hybrid compounds with transition-metal
coordination complexes bearing organic ligands.2 Functionali-
zation of POMs allows for new properties and applications to
be obtained because of the synergistic effect between the active
inorganic and metal−organic building blocks. Such types of
hybrids might bring POMs to a new area of advanced
multifunctional materials.3
Copper(II) complexes represent exceptional moieties for the
functionalization of POMs because of the geometrical flexibility
of the CuII coordination sphere.4 These complexes can add
remarkable properties to the POM system because they play
important roles in the active sites of cupro-proteins, as well as
in the homogeneous catalysis for different organic reactions.5 In
Received: November 20, 2012
Published: March 7, 2013
© 2013 American Chemical Society
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Table 1. Crystallographic Data for Compounds 1 and 2 and Their Anhydrous Phases 1a and 2a
1
1a
2
2a
formula
fw (g mol−1
cryst syst
space group
T (K)
C32H46Cu2N8O41SiW12
C32H44Cu2N8O40SiW12
C34H58Cu2N8O45SiW12
3660.11
monoclinic
P21/n
C34H48Cu2N8O40SiW12
3570.17
monoclinic
P21/n
)
3559.99
orthorhombic
Pbca
3538.09
orthorhombic
Pbca
100(2)
19.3953(6)
23.6891(7)
26.5521(10)
90
423(2)
19.2671(3)
23.9764(4)
26.6345(5)
90
100(2)
423(2)
a (Å)
10.9853(2)
12.0992(3)
24.7010(6)
90
11.0092(7)
12.0532(7)
24.1712(17)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
90
90
96.104(2)
90
98.403(6)
90
90
90
12199.6(7)
8
12304.0(4)
8
3264.48(13)
2
3173.0(3)
2
Dcalcd (g cm−3
μ (mm−1
)
3.875
3.820
3.713
3.737
)
23.329
74996
23.129
84001
21.805
22.423
collected reflns
23034
22073
unique reflns (Rint)
observed reflns [I > 2σ(I)]
params
18587 (0.056)
12734
13367 (0.046)
11442
8067 (0.033)
6449
6403 (0.048)
4964
577
573
320
293
a
R(F) [I > 2σ(I)]
0.059
0.068
0.041
0.060
a
wR(F2) (all data)
0.112
0.133
0.085
0.203
GoF
1.091
1.310
1.098
1.071
a
2
2
R(F) = ∑∥Fo − Fc∥/∑|Fo|. wR(F2) = {∑[w(Fo − Fc2)2]/∑[w(Fo )2]}1/2
.
Synthesis of bpmen and bpmpn Ligands. Bpmen and bpmpn
Here, we report the synthesis, spectroscopic characterization,
ligands were synthesized following the literature procedure for bpmen
and crystal structures of the first two hybrid compounds
obtained from the interaction between POMs and CuII/N2Py2
complexes, namely, [Cu(bpmen)(H2O)][SiW12O40{Cu-
(bpmen)}] (1, bpmen: N,N′-dimethyl-N,N′-bis-(pyridin-2-
ylmethyl)-1,2-diaminoethane) and [SiW12O40{Cu(bpmpn)-
(H2O)}2]·3H2O (2, bpmpn: N,N′-dimethyl-N,N′-bis(pyridin-
2-ylmethyl)-1,3-diaminopropane). Both compounds undergo
single-crystal to single-crystal transformations promoted by
reversible dehydration processes. The structures of the
anhydrous phases [SiW12O40{Cu(bpmen)}2] (1a) and
[SiW12O40{Cu(bpmpn)}2] (2a) have also been determined
by single-crystal X-ray diffraction, and structural modifications
upon dehydration have been detected by electron paramagnetic
resonance spectroscopy.
with slight modifications.10
N,N′-Dimethyl-N,N′-bis-(pyridin-2-ylmethyl)-1,2-diaminoethane
(bpmen). To a solution of 2-(chloromethyl)-pyridine hydrochloride
(3.0 g, 18.3 mmol) in water (10 mL) was dropwise added a solution of
K2CO3 (5.1 g, 37 mmol) in water (15 mL). After stirring at room
temperature for 30 min, the reaction mixture was extracted with
CH2Cl2 (3 × 20 mL). The organic layers were combined, dried over
Na2SO4, and concentrated in vacuo to afford an orange oil. This free-
base product (2.2 g, 17.5 mmol) was dissolved in CH2Cl2 (10 mL)
and dropwise added to N,N′-dimethylethylenediamine (0.942 mL,
8.75 mmol) dissolved in CH2Cl2 (25 mL). After the slow addition of
aqueous 1 M NaOH (20 mL, 20 mmol), the resulting mixture was
stirred for 60 h at room temperature, followed by the rapid addition of
a second fraction of aqueous 1 M NaOH (20 mL, 20 mmol). The
aqueous layer was then extracted with CH2Cl2 (3 × 50 mL), and the
combined organic layers were dried over Na2SO4 to afford the bpmen
ligand as an orange oil after concentration in vacuo (1.8 g, yield: 76%).
1H NMR (300 MHz, CD3OD): δ 2.20 (s, 6H, −N−CH3), 2.57 (s, 4H,
EXPERIMENTAL SECTION
■
Materials and Methods. The K8[α-SiW11O39]·13H2O POM
precursor was synthesized as described in the literature and identified
by infrared spectroscopy.9 All other chemicals were obtained from
commercial sources and used without further purification. Carbon,
hydrogen, and nitrogen were determined on a Perkin-Elmer 2400
−CH2−CH2−), 3.61 (s, 4H, N−CH2−Py), 7.04−8.46 (m, 8H, Py
ring) ppm.
N,N′-Dimethyl-N,N′-bis(pyridin-2-ylmethyl)-1,3-diaminopropane
(bpmpn). The above procedure was followed but using N,N′-
dimethyl-1,3-propanediamine (1.094 mL, 8.75 mmol) instead of
N,N′-dimethylethylenediamine. The bpmpn ligand was isolated as an
1
CHN analyzer. H Nuclear Magnetic Resonance (NMR) spectra of
the ligands (Figure S1 in the Supporting Information) were recorded
on a Bruker AC-300 spectrometer. Infrared (IR) spectra were obtained
as KBr pellets on a SHIMADZU FTIR-8400S spectrometer. Raman
spectra were recorded on single crystals by using a Renishaw InVia
Raman spectrometer, joined to a Leica DMLM microscope with a
Leica 50 × N Plan objective and excitation wavelength of 514 nm
(Modu-Laser). Thermogravimetric and Differential Thermal Analyses
(TGA/DTA) were carried out from room temperature to 750 °C at a
rate of 5 °C min−1 on a TA Instruments 2960 SDT thermobalance
under a 100 cm3·min−1 flow of synthetic air. Electron Paramagnetic
Resonance (EPR) spectra were recorded on Bruker ELEXSYS 500
(superhigh-Q resonator ER-4123-SHQ) and Bruker EMX (ER-510-
QT resonator) continuous wave spectrometers for Q- and X-bands,
respectively (magnetic calibration: NMR probe; frequency inside the
cavity determined with microwave counter).
1
orange oil (2.0 g, yield: 81%). H NMR (300 MHz, CD3OD): δ 1.73
(m, 2H, −CH2−(CH2N)2), 2.21 (s, 6H, −N−CH3), 2.45 (t, 4H,
−CH2N−), 3.61 (s, 4H, Py−CH2−N−), 7.08−8.50 (m, 8H, Py ring)
ppm.
Synthesis of Compounds 1 and 2. [Cu(bpmen)(H2O)]-
[SiW12O40{Cu(bpmen)}] (1). A mixture of K8[α-SiW11O39]·13H2O
(0.322 g, 0.1 mmol), Cu(CH3COO)2 (0.036 g, 0.3 mmol), bpmen
(0.054 g, 0.2 mmol), and 1 M KCH3COO/CH3COOH buffer (25
mL) was stirred for 1 h, transferred to a 50 mL Teflon-lined autoclave,
and kept at 140 °C for 3 days. After slowly cooling to room
temperature for 48 h, blue plates of 1 suitable for single-crystal X-ray
diffraction were obtained (90 mg, yield: 25%, based on W). Anal.
Calcd (found) for C32H46Cu2N8O41SiW12: C, 10.80 (10.45); H, 1.30
(1.56); N, 3.15 (3.08). IR (cm−1, KBr pellet): 3086(w), 2924(w),
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1613(m), 1450(m), 1304(w), 1258(w), 1018 (m), 964(s), 918(vs),
880(s), 802(vs), 756(sh), 525(m), 478(w). Raman (cm−1, single
crystal): 3062(w), 2930(w), 1614(m), 1554(m), 1460(m), 1305(w),
1140(w), 1047(w), 1025(w), 985(s), 956(m), 885(m), 550(w),
517(w), 225(w), 211(w), 148(w), 142(w).
spectroscopy, none of these precipitates correspond to pure 1
and 2 as shown by PXRD analyses (Figure S2 in the Supporting
Information). Isolation of these types of compounds also
depends on the choice of the transition metal. We could not
obtain analogues of 1 and 2 under the same conditions when
CuII was substituted by CoII or NiII but only amorphous
precipitates that could not be characterized adequately. The
flexible coordination geometries that CuII can adopt due to the
Jahn−Teller effect seem to play a key role in the formation of 1
and 2.
[[SiW12O40{Cu(bpmpn)(H2O)}2]·3H2O (2). The synthetic conditions
were similar to those of 1, but bpmpn (0.057 g, 0.2 mmol) was used
instead of bpmen. A mixture of an orange polycrystalline powder and
orange plates of 2 suitable for single-crystal X-ray diffraction was
isolated (75 mg, yield: 20% based on W). Crystals were manually
separated for structure determination. Powder X-ray diffraction shows
that the polycrystalline fraction is also compound 2 (Figure S2 in the
Supporting Information). Anal. Calcd (found) for
C34H58Cu2N8O45SiW12: C, 11.16 (11.75); H, 1.59 (1.53); N, 3.06
(2.96). IR (cm−1, KBr pellet): 3117(w), 2924(w), 1612(m), 1474(m),
1443(m), 1296(w), 1258(w), 1011(m), 964(s), 918(vs), 880 (s),
795(vs), 532(m), 478(w). Raman (cm−1, single crystal): 3064(w),
2937(w), 1614(m), 1561(m), 1311(w), 1141(w), 1044(w), 1014(w),
987(s), 954(m), 890(m), 540(w), 517(w), 227(w), 209(w), 143(w).
X-Ray Crystallography. Crystal data for 1 and 2 and their
anhydrous phases 1a and 2a are given in Table 1. Data for 1 and 2
were collected at 100(2) K on an Oxford Diffraction Xcalibur
diffractometer (graphite monochromated Mo Kα radiation, λ =
0.71073 Å, Sapphire CCD detector). In the case of 1a and 2a, data
were collected at 423(2) K on an Agilent Technologies SuperNova
diffractometer (mirror-monochromated Mo Kα radiation, λ = 0.71073
Å, Eos CCD detector) equipped with an Oxford Cryostream 700
PLUS temperature device. In all cases, data frames were processed
(unit cell determination, intensity data integration, correction for
Lorentz and polarization effects, and analytical absorption correction)
using the CrysAlis software package.11 The structures were solved
using OLEX12 and refined by full-matrix least-squares with SHELXL-
97.13 Final geometrical calculations were carried out with Mercury14
and PLATON15 as integrated in WinGX.16 Thermal vibrations were
treated anisotropically for heavy atoms (W, Cu, Si). Hydrogen atoms
of the organic ligands were placed in calculated positions and refined
using a riding model with standard SHELXL parameters. The bridging
O atoms of the Keggin clusters were disordered in two positions each,
the occupancies being 40/60 for 1, 30/70 for 1a, 50/50 for 2, and 45/
55 for 2a.
IR and Raman Spectroscopy. IR spectra of 1 and 2 are
differentiated in metal−organic and inorganic regions above
and below 1000 cm−1, respectively (Figure S4 in the
Supporting Information). The characteristic bands of the [α-
SiW12O40]4− Keggin cluster can be clearly identified in the
inorganic region,17 their positions being in agreement with
those calculated by DFT.18 The metal−organic region is
dominated by signals of medium intensity appearing in the
1440−1615 cm−1 range and associated to CC and CN
stretching vibrations. Compound 1 shows a single peak at 1450
cm−1, whereas splitting of this signal is observed for 2. The
group of overlapped peaks centered at 2925 cm−1 and the weak
resonances at ∼1260 and 1300 cm−1 originate from aliphatic
C−H bonds, whereas the multiplet above 3000 cm−1 is
indicative of aromatic C−H bonds. IR spectroscopy allows for
determining the metal−organic grafting on the POM surface
because such coordination produces red shifting of the bands
associated to W−Ot bonds compared to those of the naked
Keggin anion. This mainly affects to the peak originating from
the νas(W−Ot) mode, with displacements of ca. 15 cm−1.
Raman spectra of 1 and 2 (Figure S5 in the Supporting
Information) are dominated by a pair of narrow peaks in the
950−990 cm−1 range that are characteristic of the νas(W−Ot)
vibration of [SiW12O40]4−.17 They also exhibit additional signals
evidencing the presence of N2Py2 ligands: two broad bands at
3060−3065 and 2930−2940 cm−1 related to aromatic and
aliphatic C−H bonds and a group of peaks between 1000 and
1675 cm−1 originating from CC/CN stretching and C−H
bending.
Thermal Analyses. Thermal stability for 1 and 2 was
investigated by TGA/DTA and variable temperature PXRD.
For both compounds, thermal decomposition proceeds via
three steps with very similar profiles (Figures 1a and S6 in the
Supporting Information). The initial endothermic mass loss
below ∼160 °C for 1 and 145 °C for 2 corresponds to
dehydration and implies the release of one water molecule for
the former (calcd 0.5%; found 0.7%) and five water molecules
for the latter (calcd 2.5%; found 2.2%). The resulting
anhydrous phases 1a and 2a are thermally stable up to ∼290
and 255 °C, respectively. These thermal stability ranges are
followed by two overlapped exothermic mass losses corre-
sponding to organic pyrolysis and decomposition of the POM
framework. The overall mass loss is in good agreement with
two bpmen ligands in 1 (calcd 15.2%; found 15.3%) and two
bpmpn ligands in 2 (calcd 15.7%; found 16.0%). Final residues
are obtained above 630 °C for both 1 (calcd 84.3%; found
83.6%) and 2 (calcd 82.0%; found 81.4%)
Powder X-ray diffraction (PXRD) data were collected on a Bruker
D8 Advance diffractometer operating at 30 kV and 20 mA and
equipped with a Cu tube (λ = 1.5418 Å), a Vantec-1 PSD detector,
and an Anton Parr HTK2000 high-temperature furnace. The powder
patterns were recorded in 2θ steps of 0.033° in the 5 ≤ 2θ ≤ 39 range,
counting for 0.3 s per step using a Pt sample holder. Data sets were
recorded from 30 to 890 °C every 20 °C, with a 0.16 °C s−1 heating
rate between temperatures.
RESULTS AND DISCUSSION
■
Synthesis. The bpmen ligand was synthesized following the
reported method but with a slight modification in the final
purification step.10 This route was also used to prepare bpmpn
but employing N,N′-dimethyl-1,3-propane-diamine instead of
N,N′-dimethyl-ethylenediamine (Figure S3 in the Supporting
Information). Compounds 1 and 2 were obtained by treating a
mixture of K8[α-SiW11O39], Cu(CH3COO)2, and N2Py2 ligands
(1:3:2 ratio) in a potassium acetate buffer solution under
hydrothermal conditions.
Since 1 and 2 contain the plenary [α-SiW12O40]4− anion
instead of the monolacunary [α-SiW11O39]8− precursor, we
tried to prepare both compounds under the same conditions,
but using K4[α-SiW12O40] synthesized as reported.9 Reactions
were carried out with both the original 1:3:2 and the
stoichiometric 1:2:2 ratios, resulting in greenish blue
(bpmen) and brown (bpmpn) precipitates. Although contain-
ing [α-SiW12O40]4− clusters and N2Py2 ligands according to IR
PXRD experiments between room temperature and 890 °C
(Figure 1b) show that 1 and 2 retain crystallinity upon
dehydration. No substantial variations in the positions and
intensities of the diffraction maxima of the resulting anhydrous
phases 1a and 2a are observed compared to those of the initial
compounds, indicating that dehydration does not result in
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°C and they reach complete formation at 630 °C, which
corresponds to the end of the third TGA mass losses. These
final residues were both identified as mixtures of monoclinic
WO3 (PDF 88−269)19 and triclinic CuWO4 (PDF 43−1035)20
with Scheelite-type structure (Figure S7 in the Supporting
Information). This fact is fully consistent with our previous
observations in related systems.6g
Single-Crystal X-ray Diffraction Studies. Encouraged by
the TGA/DTA and PXRD results over polycrystalline samples,
we decided to carry out similar studies by single-crystal X-ray
diffraction. Single crystals of 1 and 2 for which complete
structural measurements were previously performed at 100(2)
K were selected. The temperature was raised from room
temperature to 215 °C at a rate of 1 °C min−1.
Both crystals preserved their integrity and crystallinity in the
complete temperature range, allowing for complete unit cell
determinations being performed at room temperature, 60, 105,
150, 185 and 215(2) °C (Table 2). Noticeable shortening of
the a parameter for 1 and the c parameter for 2 with
consequent contraction of the unit cell volumes is observed
when going from 105 to 150 °C (Figure 2). These variations
Figure 1. (a) TGA curves of 1 (left) and 2 (right). (b) Variable
temperature powder X-ray diffractograms of 1 (top) and 2 (bottom).
drastic modifications of the cell parameters. The crystalline
anhydrous phases are stable up to 290 °C for 1a and 270 °C for
2a in agreement with the above observations on the TGA
curves. Both transform into amorphous solids in the temper-
ature range corresponding to the ligand pyrolisis. New
crystalline phases start appearing at temperatures above 470
Figure 2. Thermal variation of the unit cell volumes of 1 and 2.
Table 2. Unit Cell Parameters of 1 and 2 at Different Temperatures
T (°C)
a (Å)
b (Å)
c (Å)
compound 1
α (deg)
β (deg)
90
γ (deg)
V (Å3)
r.t.
19.547(2)
19.552(2)
19.562(3)
19.333(2)
19.321(2)
19.325(2)
23.914(4)
23.925(3)
23.958(5)
23.995(4)
24.051(4)
24.074(4)
26.773(3)
26.778(3)
26.766(5)
26.707(3)
26.712(3)
26.727(3)
90
90
90
90
90
90
90
90
90
90
90
90
12515(3)
12528(3)
12544(4)
12389(3)
12413(3)
12434(3)
60
90
90
90
90
90
105
150
185
215
compound 2
r.t.
11.0914(17)
11.088(4)
11.046(3)
11.012(2)
11.044(4)
11.067(4)
12.2144(16)
12.222(4)
12.155(3)
12.059(3)
12.084(4)
12.105(4)
24.805(4)
24.785(11)
24.785(8)
24.202(7)
24.242(9)
24.248(9)
90
90
90
90
90
90
96.450(17)
96.68(5)
96.15(3)
98.55(3)
98.40(4)
98.43(4)
90
90
90
90
90
90
3339(1)
3336(2)
3308(2)
3178(2)
3201(2)
3213(2)
60
105
150
185
215
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indicate formation of the corresponding anhydrous phases 1a
and 2a in good agreement with TGA results. Thus, full data
collections for both crystals were performed at 423(2) K and
the structures of 1a and 2a were determined. These showed not
only the expected complete loss of water molecules, but also
other significant structural changes that will be commented in
detail below.
After cooling down the single crystals of both compounds to
room temperature, they were kept under a wet atmosphere for
approximately one month, and intensity data were collected at
100(2) K. The unit cells of the initial, hydrated 1 and 2 were
again obtained, and although collections were of poorer quality
because of partial cracking of the crystals during rehydration,
they diffracted acceptably enough for performing preliminary
structural solutions. X-ray crystallographic data in CIF format
for rehydrated 1 are provided in the Supporting Information. In
the case of rehydrated 2, the Keggin subunit and the CuN4O2
chromophore including the axial aquo ligand were located, but
unfortunately, the quality of the data did not allow complete
determination of the bpmpn ligand. These observations
demonstrate that both 1 and 2 undergo single-crystal to
single-crystal transformations at high temperatures promoted
by the previously described dehydration processes. Moreover,
these transformations implying substantial structural modifica-
tions beyond the loss of the coordination/hydration water
molecules are completely reversible upon rehydration with
retention of the crystallinity.
All four compounds contain discrete, hybrid species formed
by [SiW12O40]4− clusters onto which CuII/N2Py2 complexes are
grafted. In all cases, the [SiW12O40]4− anion shows the
characteristic structure of the α-Keggin isomer consisting of a
central SiO4 tetrahedron surrounded by four vertex-sharing
W3O13 trimers, each composed of three edge-sharing WO6
octahedra. Bond lengths and angles compare well with the
DFT-optimized ones18b (Table S1 in the Supporting
Information).
Figure 3. The {Cu(bpmen)} metal−organic block and the [Cu-
(bpmen)(H2O)]2+ cation in 1 (top) compared to the corresponding
complexes in the anhydrous 1a (bottom).
pyramidal geometry of Cu1A should be then described as
elongated octahedral (Figure 3, bottom).
In contrast to 1, compound 2 contains only one type of CuII/
bpmpn complex grafted at the POM. The Cu atom shows
elongated octahedral coordination geometry with four N atoms
in the equatorial plane and one water molecule and one
terminal O atom of the Keggin anion in axial positions (Figure
4, left). Selected bond lengths are shown in Table 3, and they
21d
are consistent with those reported for [Cu(bpmpn)](ClO4)2
and [Cu(NCS)(bpmpn)](ClO4).22 When 2 is heated to 150
°C, the axial water molecule is released and the octahedral
geometry of the Cu atom becomes square-pyramidal (Figure 4,
right).
There are two possible isomers for this type of CuII/N2Py2
complex: cis when the CH3 groups on the amine N atoms
(NCH3) are on the same side of the CuN4 equatorial/basal
plane and trans when they point to opposite directions (Figure
S8 in the Supporting Information). According to DFT
calculations,21d the cis isomer is favored for CuII/bpmen
complexes thanks to intramolecular hydrogen bonding between
the two NCH3 groups and the O atom occupying the apical
position, resulting in the stabilization of the square-pyramidal
geometry. This is exactly what we observed for both the
{Cu(bpmen)} metal−organic block and the [Cu(bpmen)]-
(H2O)]2+ cation in 1 (Figure 3). The cis form is maintained for
both complexes in the anhydrous 1a despite the fact that
coordination of the latter to the Keggin cluster upon
dehydration results in methyl substituents and apical OPOM
pointing to opposite directions and the consequent impossi-
bility of intramolecular hydrogen bonding.
CuII/bpmen and CuII/bpmpn Complexes. There are two
different CuII/bpmen complexes in the crystal structure of 1:
one supported {Cu(bpmen)} metal−organic block (Cu1A)
and one [Cu(bpmen)(H2O)]2+ complex acting as a counter-
cation (Cu1B). In both cases, the coordination geometry of the
CuII centers may be considered as tetragonally elongated
square-pyramidal with four N atoms in the basal plane and the
apical position occupied by one terminal O atom from the
[SiW12O40]4− cluster for Cu1A and one water molecule for
Cu1B (Figure 3, top). Coordination geometry and bond
lengths (Table 3) and angles (Table S2 in the Supporting
Information) are consistent with the scarce CuII/bpmen
complexes that have been structurally characterized to date.21
The sixth position is in both cases blocked by a terminal O
atom of a contiguous POM at Cu···OPOM distances slightly
above 3.0 Å.
In contrast to CuII/bpmen complexes, the trans isomer with
the chair conformation is favored for the CuII/bpmpn
fragments. As shown by DFT calculations, there is no hydrogen
bonding between NCH3 groups and apical O atoms, but the
larger chelating ring around the CuII center facilitates
accommodation of two O atoms in axial positions, in such a
way that octahedral geometries are stabilized. Again, this is the
situation observed for the {Cu(bpmpn)} metal−organic block
in 2 (Figure 4). However, this complex undergoes trans to cis
isomerization with boat conformation upon dehydration.
Although DFT calculations indicate that stabilization of the
When 1 is heated to 150 °C, a loss of the water molecule
coordinated to Cu1B takes place, in such a way that the
complex cation shifts and coordinates to the OPOM atom that
was blocking its sixth position. The square-pyramidal geometry
in this fragment is thereby maintained but with switching of the
apical coordination position and significant lengthening of the
Cu−Oap bond. In contrast, the Cu···OPOM distance to the O
atom blocking the sixth position of Cu1A is shortened by less
than 0.1 Å upon dehydration, resulting in the strengthening of
the interaction between adjacent hybrid POMs. If this long-
range contact is considered as semicoordination, the square-
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Table 3. Selected Bond Lengths (Å) for the CuII/bpmen and CuII/bpmpn Complexes
CuII/bpmen complexes
CuII/bpmpn complexes
Cu1A
Cu1B
Cu1
1
1a
1
1a
2
2a
Cu1A−N1A
Cu1A−N2A
Cu1A−N3A
Cu1A−N4A
Cu1A−O2A
Cu1A···O7A
2.001(11)
2.022(13)
2.015(12)
1.996(11)
2.357(9)
2.04(2)
Cu1B−N1B
Cu1B−N2B
Cu1B−N3B
Cu1B−N4B
Cu1B−O1W
Cu1B···O1A
1.993(12)
2.019(11)
2.041(12)
1.998(11)
2.242(10)
3.089(12)
1.98(2)
2.01(2)
2.02(3)
1.984(18)
Cu1−N1
Cu1−N2
Cu1−N3
Cu1−N4
Cu1−O1
Cu1−O1w
2.011(9)
2.025(8)
2.030(9)
1.979(8)
2.610(7)
2.789(11)
2.051(19)
2.05(2)
2.14(3)
1.95(3)
2.01(3)
2.008(19)
2.436(16)
2.985(17)
2.03(2)
2.333(18)
3.057(10)
2.752(18)
Figure 4. The {Cu(bpmpn)(H2O)} metal−organic block in 2 (left)
and its anhydrous form in 2a (right).
cis isomer for CuII/bpmpn complexes is not favored, the high
temperatures and the absence of a nearby donor atom for
potential hexa-coordination induce this form to be present in
the anhydrous 2a.
Crystal Packing of Compounds 1 and 1a. Compound 1
contains the [SiW12O40{Cu(bpmen)}]2− discrete POM com-
posed of one {Cu(bpmen)} metal−organic block supported on
a Keggin cluster via axial coordination to a terminal O atom
(Figure 5, top). The crystal packing can be viewed as
corrugated hybrid layers stacked along the c axis where the
POMs form rows along the [010] direction (Figure 6). The
N4A pyridyl ring is sandwiched between tetrameric {W4O18}
faces of two contiguous polyanions in a row, whereas the N1A
ring is placed over a {W3O13} trimer of an adjacent POM. The
distances of the ring centroids to the average planes of the
tetramers/trimers defined by their bridging and terminal O
atoms (Figure S9 in the Supporting Information) are
comparable to those found in related hybrid POMs with
dimeric CuII/ac/phen metal−organic blocks6e or in the diazine
salts of the [PMo12O40]3− anion,23 for which the existence of
aromatic interactions with POM surfaces have been demon-
strated on the basis of ab initio or Hirschfeld surface
calculations. The [Cu(bpmen)(H2O)]2+ counterions are
located in intralamellar spaces reinforcing the POM association.
The apical water molecules link contiguous POMs in a row via
Figure 5. Top: Two adjacent [SiW12O40{Cu(bpmen)}]2− POMs and
their associated [Cu(bpmen)](H2O)]2+ cations in 1 (Oap−H···OPOM
hydrogen bonds, red dashed lines; POM−aromatic interactions, blue
dotted lines). Bottom: Newly formed [SiW12O40{Cu(bpmen)}2]
neutral species in 1a, showing the structural modifications promoted
by dehydration (long-range Cu···OPOM contact: lilac dotted line).
strong hydrogen bonding with terminal O atoms (Oap···OPOM
=
contraction of the structure in the [100] direction. This is
reflected in a closer packing of the POM rows in the layers and
consequent shortening of the a cell parameter. The association
between adjacent POMs balances the loss of the Oap−H···OPOM
interactions with the strengthening of the remaining
intermolecular contacts, in such a way that while the distance
of the N1A ring to the W3O13 trimer is shortened, Cu1A
undergoes slight displacement toward the OPOM atom blocking
its sixth coordination position (Figure 6, bottom).
2.951(16), 2.724(15) Å), whereas an extended network of C−
H···OPOM interactions connects adjacent rows (Table S3 in the
Supporting Information).
Dehydration of 1 results in 1a with considerable variations in
the crystal structure. Loss of the apical water molecule in the
coordination sphere of Cu1B and consequent coordination to
the OPOM atom which was blocking its sixth position leads to
formation of the neutral [SiW12O40{Cu(bpmen)}2] hybrid
species showing two metal−organic blocks supported on two
edge-shared WO6 octahedra in relative cis arrangement (Figure
5, bottom). Displacement of the Cu1B complexes leads to
Crystal Packing of Compounds 2 and 2a. In contrast to
the hybrid polyanion in 1, the neutral species of 2 with Ci
symmetry shows two supported {Cu(bpmpn)(H2O)} metal−
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Figure 7. The [SiW12O40{Cu(bpmpn)(H2O)2}2] species in 2 (top)
and its anhydrous form 2a (bottom). Intramolecular POM−aromatic
interaction represented as blue dotted lines.
8, bottom). The Keggin subunits occupy interlamellar spaces
surrounded by six complexes for each metal−organic layer, and
they are hydrogen bonded by hydration water molecules
forming intercalated inorganic layers (Figure S10 in the
Supporting Information).
The crystal packing of 2 does not undergo drastic changes
upon dehydration as for 1. The architecture of the hybrid
species in the anhydrous phase 2a is preserved but for the
change in the complex geometry commented above (Figure 7,
bottom). The arrangement of six complexes around a central
Keggin cluster is also maintained in the metal−organic layers
despite the fact that release of the water molecules destroys the
C−H···Ow network (Figure S11 in the Supporting Informa-
tion). However, there is a compression of the structure that is
basically reflected in a significant shortening of the c cell
parameter. This compression arises from a closer packing of the
[SiW12O40]4− rows upon release of the interstitial hydration
water molecules, which is accompanied by an overall
contraction of the hexagonal-like metal−organic motifs, as
indicated by shortening of most of the Cu···Cu distances
(Figures S10 and S11 in the Supporting Information).
Moreover, the trans-to-cis ligand isomerization generates
additional sets of CNCH3−H···OPOM contacts (Table S4 in the
Supporting Information), so that reinforcement of these
interactions seems to play a key role in stabilizing the crystal
packing in absence of the C−H···Ow network.
EPR Spectroscopy. Although 1 displays two independent
CuII centers, the Q-band spectrum is characteristic of an
isolated CuII chromophore with axial g tensor and four-line
hyperfine splitting on the parallel region originating from a spin
doublet S = 1/2 interacting with a single I = 3/2 nucleus
(Figure 9, top). However, simulation of the spectrum with axial
symmetry did not result in a completely satisfactory fit. This
fact is probably due to overlapping of two individual signals
corresponding to the coexisting Cu1A and Cu1B centers with
virtually identical coordination geometries. These signals must
be very similar in the parallel region, and the average of the
calculated g∥ and A∥ values (2.223 and 170 × 10−4 cm−1) are
reasonable for a square-pyramidal CuII with four N atoms in the
basal plane and axial elongation. The fit in the perpendicular
Figure 6. Comparison between the hybrid layers in 1 (top) and its
anhydrous form 1a (bottom) highlighting the structural trans-
formation promoted by dehydration in the colored central row
(color code same as Figure 5; bpmen ligands omitted for clarity).
organic blocks in relative trans arrangement via axial
coordination to terminal O atoms. The complexes are anchored
in such a way that intramolecular {W4O18}−aromatic
interactions involving one of the pyridyl rings are established
(Figure 7, top).
The additional methylene unit in the N−(CH2)n−N bridge
with associated change in the ligand conformation and complex
geometry results not only in a different hybrid species in 2 but
also in a different type of crystal packing. This can be described
as honeycomb-like metal−organic layers parallel to the (10−1)
plane where an intricate network of C−H···Ow and Ow···Ow
interactions involving methyl groups and pyridyl rings and
coordination/hydration water molecules link the {Cu(bpmpn)-
(H2O)} fragments (Figure 8, top). The fragments form strings
along the [010] direction via hydrogen contacts involving
NCH3 groups and aquo ligands, whereas hydration water
molecules connect adjacent strings. Stacking of metal−organic
layers generates hexagonal channels running parallel to the a
axis where rows of Keggin clusters are accommodated (Figure
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Figure 9. Comparison between the room-temperature Q-band EPR
spectra of compounds 1 and 1a (top) and 2 and 2a (bottom).
Information for the two averaged CuII atoms. However, if A∥
is considered constant (170 × 10−4 cm−1) and the first and
fourth lines on the parallel region are assigned to different CuII
atoms, the fit results in different g∥ values for each atom (2.195
and 2.175). Taking into account the Cu−Oap bond lengths, the
latter g∥ values appear to better describe our system.
Considering the similarities of the CuII chromophores in
compounds 1 and 2 (both with axial geometries, four N atoms
of almost identical ligands in the equatorial/basal plane, Ow or
OPOM atoms in axial/apical positions, comparable Cu−N/Cu−
O bond lengths and angles), an EPR signal of axial symmetry
should also be a priori expected for 2 in close analogy to the
experimental observations in 1. However, the EPR spectrum of
2 (Figure 9, bottom) shows clear rhombic symmetry, and
furthermore, the hyperfine structure in this rhombic signal is
collapsed. A narrower, still collapsed rhombic signal was
observed at higher field for 2a. The g tensors originating these
rhombic signals appear to be cooperative tensors associated to a
certain phenomenon operating in our CuII system rather than
molecular tensors describing the individual geometries of our
CuIIN4O2 (2) or CuIIN4O (2a) chromophores. This is
supported by the fact that the calculated g values (Table S5
in the Supporting Information) do not undergo significant
variation with the change from the octahedral CuIIN4O2 to
square-pyramidal CuIIN4O chromophore and consequent
shortening of the Cu−Oap bond (Table 3), observed upon
dehydration. Fluxional behavior originating from a dynamic
Jahn−Teller effect or the presence of long-range magnetic
interactions appears to be the most plausible among all
phenomena that could lead to a rhombic signal with collapsed
hyperfine structure associated to a CuII system like ours. The
low g1 values obtained for both 2 and 2a would be in agreement
with this type of fluxional behavior, which should be reflected in
a shift of the spectrum with decreasing temperature. However,
Figure 8. Top: Projection of a metal−organic honeycomb-like layer of
2 on the bc plane with a detail of two adjacent Keggin clusters (C−
H···Ow hydrogen bonds: red dashed lines). Bottom: Crystal packing of
2 viewed along the b axis.
region requires the introduction of rhombic symmetry in g and
A tensors (Table S5 in the Supporting Information), most
likely because the values of their components are slightly
different for each CuII.
The Q-band spectrum of the anhydrous 1a is upfield shifted
compared to that of 1. This shift and consequent smaller g
values are consistent with the elongation of the apical Cu−O
bonds observed by X-ray diffraction for both Cu1A and Cu1B
upon dehydration (Table 3). The hyperfine structure on the
parallel region is maintained in 1a, but the spacing between the
four lines becomes irregular. This could be explained on the
basis of a more pronounced elongation for one of the CuII
atoms, which would lead to differences in the coordination
geometries significant enough for the EPR signals to start
differentiating in the parallel region. This is in agreement with
structural data, which shows that while apical elongation of
Cu1A is ∼0.1 Å, release of the water molecule for Cu1B and
consequent coordination to a POM results in a considerably
larger lengthening of the Cu1B−Oap bond (∼0.5 Å).
Simulation of the spectrum with axial symmetry affords the g
and A values displayed in Table S5 in the Supporting
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no significant change takes place at low temperatures (Figure
S12 in the Supporting Information); hence any dynamic Jahn−
Teller effect can be ruled out. Therefore, we believe that our
spectra can be attributed to g being an exchange cooperative
tensor (G = 2.59)24 instead of a molecular one, and therefore,
that long-range, extremely weak magnetic interactions are
present in both 2 and 2a. It is known that rhombic signals are
obtained when cooperative g tensors refer to interacting CuII
polyhedra with different orientations in the unit cell.25
Moreover, long-range, weak magnetic interactions between
distant CuII atoms have been found to have a clear effect in the
EPR spectra. For example, this is the case of [Cu(py)2Cl2],
where CuII atoms separated by ca. 9 Å and connected by
sequences of six consecutive bonds and interactions were found
to be coupled on the basis of EPR spectroscopy.26
CuII/N2Py2 complexes (Table S2); C−H···O interactions for
1/1a and 2/2a (Tables S3 and S4); and spin Hamiltonian
parameters (Table S5). Crystallographic data in CIF format for
1, 2, 1a, 2a, and rehydrated 1. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: juanma.zorrilla@ehu.es.
■
Funding
Eusko Jaurlaritza/Gobierno Vasco (EJ/GV) and Universidad
́
del Pais Vasco UPV/EHU.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CONCLUSIONS
■
■
This work has been funded by EJ/GV through the Program to
Support the Research Groups of the Basque University System
(grant IT477-10) and the SAIOTEK Program (grant S-
PE11UN062). The authors thank UPV/EHU for financial
support (grant UFI11/53). A.I. and B.A. are indebted to EJ/GV
for their predoctoral fellowships. Technical and human support
provided by SGIker (UPV/EHU) is gratefully acknowledged.
Two new hybrid compounds based on Keggin anions and CuII
complexes of N2Py2 tetradentate ligands have been hydro-
thermally prepared, [Cu(bpmen)(H2O)][SiW12O40{Cu-
(bpmen)}] (1) and [SiW12O40{Cu(bpmpn)(H2O)}2]·3H2O
(2) (bpmen, N,N′-dimethyl-N,N′-bis-(pyridin-2-ylmethyl)-1,2-
diaminoethane; bpmpn, N,N′-dimethyl-N,N′-bis(pyridin-2-yl-
methyl)-1,3-diaminopropane). The structure of 1 shows
complex cations enclosed between rows of monodecorated
POMs, whereas the trans-didecorated species of 2 forms
stacked honeycomb-like metal−organic layers with Keggin
clusters accommodated in channels. Structural differences relate
to changes in the complex geometry (square-pyramidal vs.
octahedral) and ligand conformation (cis vs. trans) when going
from bpmen to bpmpn.
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ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR spectra (Figure S1) and synthetic scheme (Figure S3)
for bpmen and bpmpn; PXRD diffractograms of 2, other
precipitates, and TGA residues (Figures S2 and S7); IR and
Raman spectra (Figures S4 and S5); TGA/DTA curves (Figure
S6); CuII/N2Py2 isomers (Figure S8); POM-aromatic inter-
actions in 1/1a (Figure S9); additional structural figures for 2/
2a (Figures S10 and S11); X-band EPR spectra of 2/2a
(Figures S12 and S14) and 1/1a (Figure S13); ranges of W−O
bond lengths (Table S1); selected bond lengths and angles for
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