Tetrahydroxy-p-benzoquinone as a source of polydentate O-donor ligands. Synthesis, crystal structure, and magnetic properties of the [Cu(bpy)(dhmal)]2 dimer and the two-dimensional [{SiW 12O40}{Cu2(bpy)2(H 2O)(ox)}2]·16H2O inorganic-metalorganic hybrid

Article abstract of DOI:10.1021/ic061671m

The use of tetrahydroxy-p-benzoquinone as a slow source of dihydroxymalonato and oxalato ligands led to the isolation under open-air mild reaction conditions of five different compounds, two of them prepared for the first time: [Cu(bpy)(dhmal)]₂ (1) and [{SiW₁₂O ₄₀}{Cu₂(bpy)₂(H₂O)(ox)} ₂]·16H₂O (5) (bpy, 2,2′-bipyridine; dhmal, dihydroxymalonate; ox, oxalate). A possible mechanism for the oxidation of the benzoquinone to give the croconate dianion, which undergoes further ring-opening oxidation to decompose into dihydroxymalonate and oxalate, is proposed. All compounds have been characterized by elemental analysis, thermogravimetry, infrared spectroscopy, and powder X-ray diffraction. Single-crystal X-ray diffraction, electron paramagnetic resonance, and magnetic susceptibility measurements have been performed for compounds 1 and 5. A complete band assignment of the experimental FT-IR spectra is given through comparison with the ones calculated using density functional theory (DFT). The neutral dimer 1 constitutes the first structurally characterized example of a transition metal-dhmal complex, and it contains two copper atoms bridged by two dihydroxymalonato ligands acting in a μ²-κ³O, O′,O″: κ¹O coordination fashion, so that an equatorial-axial Cu₂(μ²-O)₂ rhomboid core is formed. On the other hand, the inorganic-metalorganic hybrid compound 5 shows a two-dimensional arrangement of Keggin polyanions linked by one of the Cu atoms of the oxalate cationic dimers to give layers parallel to the (101) plane, the remaining oxCu-bpy fragments acting as interlamellar separators. In both cases, magnetic and EPR results are discussed with respect to the crystal structure of the compounds and, for compound 1, also with respect to DFT calculations of the exchange coupling constant.

Full text of DOI:10.1021/ic061671m

Inorg. Chem. 2007, 46, 1237−1249
Tetrahydroxy-p-benzoquinone as a Source of Polydentate O-Donor
Ligands. Synthesis, Crystal Structure, and Magnetic Properties of the
[Cu(bpy)(dhmal)]2 Dimer and the Two-Dimensional
[{SiW12O40}{Cu2(bpy)2(H2O)(ox)} ]‚16H2O Inorganic−Metalorganic Hybrid
2
Santiago Reinoso,† Pablo Vitoria, Leire San Felices, Ainara Montero, Luis Lezama,* and
Juan M. Gutie´rrez-Zorrilla*
Departamento de Qu´ımica Inorga´nica, Facultad de Ciencia y Tecnolog´ıa, UniVersidad del Pa´ıs
Vasco, P.O. Box 644, E-48080 Bilbao, Spain
Received September 4, 2006
The use of tetrahydroxy-p-benzoquinone as a slow source of dihydroxymalonato and oxalato ligands led to the
isolation under open-air mild reaction conditions of five different compounds, two of them prepared for the first
time: [Cu(bpy)(dhmal)]2 (1) and [{SiW12O40}{Cu2(bpy)2(H2O)(ox)} ]‚16H2O (5) (bpy, 2,2′-bipyridine; dhmal,
2
dihydroxymalonate; ox, oxalate). A possible mechanism for the oxidation of the benzoquinone to give the croconate
dianion, which undergoes further ring-opening oxidation to decompose into dihydroxymalonate and oxalate, is
proposed. All compounds have been characterized by elemental analysis, thermogravimetry, infrared spectroscopy,
and powder X-ray diffraction. Single-crystal X-ray diffraction, electron paramagnetic resonance, and magnetic
susceptibility measurements have been performed for compounds 1 and 5. A complete band assignment of the
experimental FT-IR spectra is given through comparison with the ones calculated using density functional theory
(DFT). The neutral dimer 1 constitutes the first structurally characterized example of a transition metal
complex, and it contains two copper atoms bridged by two dihydroxymalonato ligands acting in a
2 κ3O,O
1O coordination fashion, so that an equatorial 2-O)2 rhomboid core is formed. On the other hand, the
axial Cu2(
inorganic metalorganic hybrid compound 5 shows a two-dimensional arrangement of Keggin polyanions linked by
one of the Cu atoms of the oxalate cationic dimers to give layers parallel to the (101) plane, the remaining ox
Cu bpy fragments acting as interlamellar separators. In both cases, magnetic and EPR results are discussed with
−dhmal
µ
-
′
,O′′
:
κ
−
µ
−
h
−
−
respect to the crystal structure of the compounds and, for compound 1, also with respect to DFT calculations of
the exchange coupling constant.
Introduction
alization of POMs with rare-earth or transition-metal (TM)
coordination complexes to give hybrid inorganic-metalor-
ganic compounds constitutes an emerging area of interest in
recent years.7 The inclusion of the metal-organic complex
moiety can be achieved either to provide charge compensa-
tion (ionic-type compounds), to modify the inorganic POM
surface or framework with organic ligands (discrete decorated
polyanions), or to link the clusters into infinite ordered arrays
of variable dimensionality (extended materials). The motiva-
tion for this functionalization lies on the possibility of
combining the different features of the components to lead
to novel structural architectures; to a rational design of new
catalytic systems; to tailored electronic, optic, and magnetic
properties; or to unexpected and more selective applications
Polyoxometalates1 (POMs) constitute a large family of
metal-oxygen cluster compounds of surprising composi-
tional variability, electronic versatility, and topological
diversity. These features endow them with applications in a
wide range of fields, such as catalysis,2 material science,3
medicine,4 or photochemistry,5 in such a way that there are
already more commercial applications of polyoxometalates
than any other class of cluster compounds.6 The function-
* To whom correspondence should be addressed. E-mail: luis.lezama@
ehu.es (L.L.), juanma.zorrilla@ehu.es (J.M.G.-Z.). Fax: +34946013500.
† Current address: School of Engineering and Science, International
University Bremen (Jacobs University Bremen as of Spring 2007), P.O.
Box 750 561, 28725 Bremen, Germany.
10.1021/ic061671m CCC: $37.00
Published on Web 01/23/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 4, 2007 1237

Reinoso et al.
that arise from synergistic effects and, therefore, that are not
associated with simple materials.
materials can be found in the literature. They mainly show
a one-dimensional nature, solids of higher dimensionalities
being even rarer and, in most cases, based on highly reduced
multicapped clusters.12
Currently, we are exploring the applicability of Keggin-
POMs and TM-carboxylate cationic complexes in the
preparation of new magnetically attractive hybrid com-
pounds. Using a building-block self-assembly synthetic
procedure under open-air mild reaction conditions, we have
been able to prepare different hybrid compounds on the basis
To date, several of these hybrid compounds based on
vanadium8 and molybdenum9 isopolyanions have been
reported; in contrast, the number of structurally characterized
examples incorporating Keggin heteropolyanions and their
derivatives as the inorganic component is still comparatively
lower. Most of them are hydrothermally synthesized and
consist of ionic-type compounds10 or discrete decorated
polyanions,11 so that only a few examples of extended hybrid
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1238 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tetrahydroxy-p-benzoquinone as O-Donor Ligand Source
of the copper-monosubstituted [SiW11O39Cu(H2O)]6- Keg-
gin heteropolyanion as the inorganic building block: on one
hand, ionic compounds13 and discrete polyanions14 in com-
bination with C-shaped µ-diacetatobis(phenanthrolinecopper)
complexes; and, on the other hand, one-dimensional com-
pounds with planar µ-oxalatobis(bipyridinecopper) metal-
organic blocks.15 Following with this strategy, we are
extending this study to other TM cationic dimers with
5352B microwave frequency counter; maintenance of the crystal
structures in the powder samples was confirmed by powder X-ray
diffraction; computer simulation, WINEPR-Simfonia, version 1.5,
Bruker Analytische Messtechnik GmbH).
Synthesis of K6[SiW11O39Cu(H2O)]‚nH2O Precursor. A solu-
tion of K8[SiW11O39]‚13H2O (6.44 g, 2 mmol) in a 1 M acetic acid/
potassium acetate buffer (80 mL) was added to a solution of
Cu(NO3)2‚3H2O (0.5 g, 2.1 mmol) in water (20 mL). The reaction
mixture was heated to 100 °C for 45 min and, after cooling to room
temperature, an excess of solid KCl (19.5 g) was added. The
K6[SiW11O39Cu(H2O)]‚nH2O precursor is obtained as a pale green
powder after stirring for 18 h. Yield: 3.92 g (60% based on W).
FT-IR (cm-1). Main bands obsd (DFT-calcd): νas(Si-Oc) + νas-
(W-Ot) (asymmetric coupling), 1011w (992); νas(W-Ot), 958s
(915); νas(Si-Oc) + νas(W-Ot), (symmetric coupling) 903vs (888);
νas(W-Oe-W), 798vs and 742s (812 and 744); νas(Cu-O), 687
m (694); δ(W-Oe-W), 540w and 527w (535 and 517); δ(W-
Ov-W), 491sh (481).
2-
aromatic dianions belonging to the C6O4X2 family as
bridging ligands between metal centers (X ) O, Cl, Br, I,
CN). In the course of our investigations, we found that when
tetrahydroxy-p-benzoquinone (H2thq, X ) OH) is reacted
with CuII and 2,2′-bipyridine (bpy), instead of acting as a
bridging ligand, it oxidizes to give the croconate dianion
C5O52- (crc), which undergoes further ring-opening oxidation
to decompose into dihydroxymalonate C3(OH)2O42- (dhmal)
2-
and oxalate C2O4 (ox) dianions.
Synthesis of [Cu(bpy)(dhmal)]2 (1), [Cu(bpy)(H2O)(crc)] (2),
and [{Cu(bpy)(PF6)}2(ox)] (3). (a) Method A (Single Crystal).
A solution of tetrahydroxy-p-benzoquinone (34 mg, 0.2 mmol) in
water (20 mL) was dropwise added to a solution of CuCl2‚2H2O
(68 mg, 0.4 mmol) and 2,2′-bipyridine (62 mg, 0.4 mmol) in a
water:methanol mixture (20:10 mL). The resulting dark-red solution
was left to slowly evaporate at room temperature, and, after 2
days, greenish-brown crystals of compound 2 were removed by
filtration. The resulting green mother liquors were left again to
slowly evaporate, and compound 1 was obtained after 1 month as
prismatic blue crystals suitable for single-crystal X-ray diffraction.
If excess KPF6 is added to the green mother liquors, a dark-blue
crystalline powder of compound 3 is obtained after 2 weeks instead
of complex 1.
(b) Method B (Powder). A solution of disodium dihydroxy-
malonate (72 mg, 0.4 mmol) in water (20 mL) was added to a
solution of CuCl2‚2H2O (68 mg, 0.4 mmol) and 2,2′-bipyridine (62
mg, 0.4 mmol) in a water:methanol mixture (40:20 mL). Compound
1 was obtained as a blue powder after stirring for 18 h. Yield: 92
mg (65% based on Cu).
(c) Compound 1. Anal. Calcd (found) for C26H20Cu2N4O12: C,
44.14 (42.91); H, 2.85 (3.01); N, 7.92 (7.84). FT-IR (cm-1). Main
bands obsd (DFT-calcd): ν(O4-H), 3415s (3726m); ν(O5-H),
3202m (3564m); ν(C-H), 3116m, 3063m and 3040w (3264w,
3237w and 3211w); νas(O2-C-O3), 1663vs (1771vs); νas(O1-
C-O6), 1642vs (1734s); νas(C-C)bpy + νas(C-N), 1616s and
1574m (1655w and 1632w); ν(bridge)bpy, 1499w (1532w); νas(C-
C)bpy + νs(C-N), 1478w (1515w); νs(C-C)bpy + νs(C-N), 1448m
(1492w); δip(C-O5-H) + ν(C-C)dhmal, 1398s (1468w and 1379vs);
ν(ring)bpy, 1325w and 1253w (1355w and 1307w); δip(C-O4-H),
1294m (1322m); ν(C-O5), 1182s (1192s); δip(C-C-H), 1125w
(1136w); δip(C-C-C)dhmal + ν(C-O4), 1049m (1055m); δip-
(ring)bpy, 1034m (1033w); wag(HO-C-OH), 978w (978w); δip-
(O-C-O) + ν(Cu-O), 856w (844w); δip(C-C-C)dhmal + δoop(C-
C-O), 822m (813w); δoop(C-C-O) + ν(Cu-O), 809w (789w);
Here we report the synthesis, chemical, and spectroscopic
characterization, X-ray crystal structure, and magnetic prop-
erties of [Cu(bpy)(dhmal)]2 (1), the first structurally char-
acterized TM-complex containing the dihydroxymalonate
dianion as ligand, and [{SiW12O40}{Cu2(bpy)2(H2O)(ox)}2]‚
16H2O (5), a two-dimensional inorganic-metalorganic hy-
brid compound based on [SiW12O40]4- R-Keggin polyanions
and µ-oxalatobis(bipyridinecopper) cationic complexes. Al-
though both compounds can be directly synthesized starting
from the corresponding carboxylic acids, they could only
be isolated as single crystals suitable for X-ray diffraction
studies by using tetrahydroxy-p-benzoquinone as a slow
source of dihydroxymalonato and oxalato ligands.
Experimental Section
Materials and Methods. The K4[R-SiW12O40]‚17H2O and K8-
[R-SiW11O39]‚13H2O POM precursors were synthesized according
to the procedure described in the literature.16 All other chemicals
were obtained from commercial sources and used without further
purification. Carbon, hydrogen, and nitrogen were determined by
organic microanalysis on a LECO CHNS 932 analyzer. Copper
was determined on a Perkin-Elmer 4110ZL atomic absorption
analyzer. Infrared spectra (FT-IR) for solid samples were obtained
as KBr pellets on a Mattson 1000 infrared spectrometer. Thermo-
gravimetric analysis (TGA) and differential thermal analysis (DTA)
were carried out on a TA Instruments SDT 2960 thermobalance
under a 100 mL/min flow of synthetic air; the temperature was
ramped from 20 to 800 °C at a rate of 5 °C/min. Magnetic
susceptibility was measured on a Quantum Design MPMS-7 SQUID
magnetometer (T range, 2-300 K; applied field, 0.1 T; diamagnetic
corrections estimated from Pascal’s constants). Electron paramag-
netic resonance (EPR) powder spectra were recorded on a Bruker
ESP300 spectrometer (X- and Q-bands) equipped with Oxford low-
temperature devices (magnetic field calibration, NMR probe;
determination of the frequency inside the cavity, Hewlett-Packard
δoop(C-C-H), 783s (771m); δoop(ring)bpy, 733w (740w); δip(ring)bpy
+ ν(Cu-N), 662m (644w and 644w); δ(OH-C-OH), 563w
(554w); δoop(C-O5-H), 544w (502w); δip(OH-C-OH) + ν(Cu-
O4), 460w (446w). TGA/DTA. The thermal analysis shows that
compound 1 is anhydrous and stable up tp 135 °C, after which it
undergoes three overlapped exothermic mass losses to lead to CuO
as the final product at 295 °C [residue calcd (found): 22.5% (20.8)].
(d) Compound 2. Anal. Calcd (found) for C15H10CuN2O6: C,
47.69 (47.35); H, 2.67 (2.63); N, 7.41 (7.35). FT-IR (cm-1): 1678
m, 1604s, 1509vs, 1448s, 781m. Single-crystal XRD. Triclinic
(13) Reinoso, S.; Vitoria, P.; San Felices, L.; Lezama, L.; Gutie´rrez-Zorrilla,
J. M. Chem. Eur. J. 2005, 11, 1538.
(14) Reinoso, S.; Vitoria, P.; San Felices, L.; Lezama, L.; Gutie´rrez-Zorrilla,
J. M. Inorg. Chem. 2006, 45, 108.
(15) (a) Reinoso, S.; Vitoria, P.; Lezama, L.; Luque, A.; Gutie´rrez-Zorrilla,
J. M. Inorg. Chem. 2003, 42, 3709. (b) Reinoso, S.; Vitoria, P.;
Gutie´rrez-Zorrilla, J. M.; Lezama, L.; San Felices, L.; Beitia, J. I. Inorg.
Chem. 2005, 44, 9731.
(16) Te´ze´, A.; Herve´, G. Inorg. Synth. 1990, 27, 85.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1239

Reinoso et al.
Table 1. Crystallographic Data for Compounds 1 and 5
crystal system, P1h space group, a ) 7.163(3) Å, b ) 9.294(4) Å,
c ) 11.901(4) Å, R ) 69.20(3)°, â ) 84.98(3)°, γ ) 69.12(4)°, V
) 691(1) Å3.
(e) Compound 3. Anal. Calcd (found) for C22H16-
Cu2F12N4O4P2: C, 32.33 (32.38); H, 1.97 (2.57); N, 6.85 (6.98).
FT-IR (cm-1): 1655vs, 1450s, 858vs, 816s, 773m, 731m, 556s,
477m. Powder XRD. Monoclinic crystal system, C2/m space group,
a ) 14.493(2) Å, b ) 14.021(1) Å, c ) 7.944(1) Å, â ) 121.829-
(5)°, V ) 1371.5(2) Å.3
Synthesis of K2[{SiW11O39Cu(H2O)}{Cu2(bpy)2(H2O)2(ox)}2]‚
14H2O (4) and [{SiW12O40}{Cu2(bpy)2(H2O)(ox)}2]‚16H2O (5).
(a) Method A (Single Crystal). A solution of CuCl2·2H2O (68
mg, 0.4 mmol), 2,2′-bipyridine (62 mg, 0.4 mmol), and tetrahy-
droxy-p-benzoquinone (34 mg, 0.2 mmol) in a water:methanol
mixture (40:10 mL) was dropwise added to a solution of
K6[SiW11O39Cu(H2O)]‚nH2O (670 mg) in water (50 mL), and a
brown precipitate was formed, which was removed by filtration.
The resulting dark orange solution was left to slowly evaporate at
room temperature, and a blue crystalline powder of compound 4
appeared after 1 week. This powder was also removed, and the
resulting pale yellow mother liquors were left again to slowly
evaporate. Compound 5 was obtained after 1 month as prismatic
blue crystals suitable for single-crystal X-ray diffraction.
1
5
formula
C26H20Cu2N4O12
707.6
triclinic
P1h
8.4965(6)
9.3315(11)
9.7891(10)
60.807(11)
68.977(8)
88.397(7)
622.1(1)
1
C44H68Cu4N8O66SiW12
4253.5
monoclinic
P21/n
14.440(1)
13.423(1)
fw (g·mol-1
Cryst syst
)
space group
a (Å)
b (Å)
c (Å)
23.497(3)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
104.44(1)
4410.5(7)
2
F(000)
358
1.889(1)
1.791
4170
2401 (0.032)
1545
3868
D
calc (g·cm-3
)
3.176(1)
16.645
28 094
8646 (0.077)
3829
µ (mm-1
)
collctd reflcns
unique reflcns (Rint
obsd reflcns [I > 2σ(I)]
refined params
)
204
323
goodness of fit (S)
R(Fo)a (obsd reflcns)
0.875
0.0393
0.0754
0.977
0.0736
0.1323
2
Rw(Fo )a (all reflcns)
a R(Fo) ) ∑||Fo| - |Fc||/∑|Fo|; Rw(Fo2) ) {∑[w(Fo - Fc )2]/
2
2
∑[w(Fo )2]}1/2
.
2
(b) Method B (Powder). Compound 5 was obtained as a blue
powder by adding a solution of CuCl2·2H2O (68 mg, 0.4 mmol),
2,2′-bipyridine (62 mg, 0.4 mmol) and oxalic acid (25 mg, 0.2
mmol) in a water:methanol mixture (30:10 mL) to a solution of
K4[SiW12O40]‚17H2O (670 mg, 0.2 mmol) in water (50 mL).
Yield: 740 mg (87% based on W).
Table 1. Data collection for the single-crystal X-ray diffraction
(XRD) studies of 1 and 5 was performed at room temperature on
an Xcalibur single-crystal diffractometer (Mo KR radiation, λ )
0.710 73 Å) fitted with a Sapphire CCD detector. A total of 1120
(1148) frames of data was collected with exposure time of 70 s
(25 s) per frame, using the ω-scan technique with frame width of
∆ω ) 0.50° (0.40°), for compound 1 (5). 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.19
Neutral atom scattering factors and anomalous dispersion factors
were taken from the literature.20 The structures were solved using
direct methods (DIRDIF99)21 and refined by full-matrix least-
squares analysis with the SHELXL-97 program.22 The positions
of the aromatic H atoms were calculated geometrically and
refined as riding atoms using default SHELXL parameters.
The H atoms of the dihydroxymalonate ligand in compound 1
were located in a Fourier difference map. Thermal vibrations were
treated anisotropically for all non-hydrogen atoms in compound 1
and only for W, Cu, and Si atoms in compound 5. All calculations
were performed using the WinGX crystallographic software
package.23
(c) Compound 5. Anal. Calcd (found) for C44H36Cu4N8O50-
SiW12‚16H2O: C, 12.42 (12.51); H, 1.61 (1.39); N, 2.63 (2.62);
Cu, 5.98 (5.89). FT-IR (cm-1). Main bands obsd (DFT-calcd) for
the [Cu2(bpy)2(H2O)(ox)]2+ block: δ(H-O-H) and υas(O-C-
O)+υas(C-C)bpy, 1653s and 1612sh (1662vs and 1619m); υas(C-
C)bpy+υas(C-N), 1570w (1599sh); υ(bridge)bpy, 1498w (1504m);
υas(C-C)bpy+υs(C-N), 1471w (1485 m); υs(C-C)bpy+υs(C-N),
1446m (1463m); υs(O-C-O), 1350w (1369m); υ(ring)bpy, 1315w
and 1252w (1333m and 1283s); δip(C-C-H), 1159w and 1107w
(1186w, 1174w and 1107m); δip(ring)bpy, 1059w and 1038w (1031m
and 1018sh); δoop(C-C-H), 764s (772s); δip(ring)bpy+δoop(C-C-
H), 727m (728w); δip(ring)bpy+υ(Cu-N), 667w and 648w (665w
and 639w); δip(C-C-O), 415w (415m); main bands obsd (DFT-
calcd) for the [SiW12O40]4- block: νas(Si-Oc) + νas(W-Ot),
(asymmetric coupling) 1012m (1005); νas(W-Ot), 968s (961); νas-
(Si-Oc) + νas(W-Ot), (symmetric coupling) 920vs (929); νas(W-
Ov-W), 881m (899); νas(W-Oe-W), 796vs (828); δ(W-Oe-W),
534m (532); δ(W-Ov-W), 478w (505). TGA/DTA. The thermal
decomposition of compound 5 takes place through four highly
overlapped steps: it starts at room temperature with an endothermic
dehydration process involving the release of at least 14 water
molecules below 170 °C, after which it undergoes three exothermic
mass losses to lead to the final residue at 510 °C. The thermo-
gravimetric final residue is composed of monoclinic WO3 (PDF
No. 83-0951),17 triclinic CuWO4 (PDF No. 88-0269),18 and most
probably, amorphous SiO2, as shown by powder X-ray diffraction
[residuecalcd(found)for8WO3 +4CuWO4 +1SiO2: 75.1%(74.3)].
Powder X-ray diffraction (XRD) studies of compounds 1, 3, and
5 were performed at room temperature on a Phillips PW-1470
X’Pert diffractometer (Cu KR1 radiation, λ ) 1.540 59 Å) with
Bragg-Brentano geometry. Data were collected by scanning in the
2θ range of 5-50° with increments of 0.02° and an exposure time
of 10 s/increment. The powder XRD patterns of the thermogravi-
metric final residues of compounds 1 and 5 were registered at room
(19) CrysAlis CCD and RED, version 1.70; Oxford Diffraction: Oxford,
U.K., 2003.
(20) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, U.K., 1974.
(21) Beurkens, P. T.; Beurkens, G.; de Gelder, R.; Garc´ıa-Granda, S.; Gould,
R. O.; Israel, R.; Smits, J. M. M. The DIRDIF Program System;
Crystallography Laboratory, University of Nijmegen: Nijmegen, The
Netherlands, 1999.
(22) Sheldrick, G. M. SHELXL97; University of Go¨ttingen: Go¨ttingen,
Germany, 1999.
X-ray Data Collection and Structure Determination. Experi-
mental details and crystal data for compounds 1 and 5 are given in
(17) Woodward, P. M.; Sleight, A. W.; Vogt, T. J. Phys. Chem. Solids
1995, 56, 1305.
(18) Schoefield, P. F.; Knight, K. S.; Redfern, S. A. T.; Cressey, G. Acta
Crystallogr. 1997, B53, 102.
(23) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
1240 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tetrahydroxy-p-benzoquinone as O-Donor Ligand Source
temperature on a Phillips PW-1700 diffractometer (Cu KR1
radiation, λ ) 1.540 59 Å) with Bragg-Brentano geometry. Data
were collected by scanning in the 2θ range of 5-40° with
increments of 0.02° and an exposure time of 5 s/increment. In all
cases, the pattern matching was performed by using the FULLPROF
program.24
configuration (EBS) are obtained, whereupon the coupling constant
is approximately:
JD ) EBS - EHS
Several combinations of the recently developed double- to
quadruple-ú valence basis set of Weigend and Ahlrichs,31 with
polarization functions for all the atoms with the exception of
hydrogens, was used in order to calculate the coupling constant.
Computational Details. All the quantum calculations have been
carried out using the Gaussian03 program25 running on computers
with GNU/Linux operating systems. Density functional theory
and specifically Becke’s hybrid method with three parameters26
based on nonlocal exchange and correlation funtionals, as imple-
mented in Gaussian03 (B3LYP), has been used in all calculations.
The optimization procedure for the [SiW11O39Cu(H2O)]6- and
[SiW12O40]4- polyanions13 and the [{Cu(bpy)(H2O)2}2(µ-ox)]2+
cationic complex15b has been previously reported. Experimental data
were used as the starting point in the global optimization of the
binuclear [{Cu(bpy)(dhmal)]2 neutral complex (Ci, S ) 1). The
standard 6-31G(d)27 basis has been chosen for all atoms. This basis
contains polarization functions28 in all atoms, except hydrogen.
Infrared and Raman spectra were calculated on the optimized
geometry of the four mentioned species. The infrared active
vibrational frequencies were identified using the GaussviewW 3.0
program,29 which provides a visual representation of the vibrational
modes. In all cases, the DFT-calculated infrared spectra were in
fair agreement with the experimental ones, so that a complete band
assignment of the latter could be given by comparison with the
formers.
Results and Discussion
Synthesis. In an attempt to synthesize a cationic CuII-
bpy dimer with the thq dianion as bridging ligand, we reacted
a water:methanol mixture (solution A, pH ∼ 3) of CuCl2,
bipyridine, and tetrahydroxy-p-benzoquinone in a 2:2:1 ratio.
However, instead of acting as a bridging ligand, we observed
that the benzoquinone was oxidized to give the croconate
dianion, which underwent further ring-opening oxidation to
decompose into dihydroxymalonate and oxalate dianions.
Thus, no CuII complex containing the thq ligand was obtained
from this experiment. As can be seen in Scheme 1, in a first
stage greenish-brown crystals of the CuII-crc complex [Cu-
(bpy)(H2O)(crc)] (2) were isolated from the dark-red solution
A after 2 days. The identification of this compound, which
was previously reported by Castro et al.,32 was acomplished
by means of elemental analysis, FT-IR spectroscopy, and
single-crystal XRD. If the CuII-crc complex is removed, in
a second stage the CuII-dhmal neutral dimer 1 is obtained
as blue single crystals from the resulting green mother liquors
(solution B) after 1 month. The complete evaporation of
solution B leads to a green precipitate, whose infrared
spectrum shows characteristic bands of a CuII-bpy complex
with a bridging ox ligand in a bis(bidentate) coordination
fashion and whose elemental analysis is in good agreement
with that calculated for the formula [{Cu(bpy)}2(ox)]Cl2‚
H2O.33 This CuII-ox cationic dimer can be isolated by using
bulky inorganic anions as crystallizing agents, such as
hexafluorophosphate. Thus, when an excess of KPF6 is added
to solution B, a dark-blue crystalline powder of [{Cu(bpy)-
(PF6)}2(ox)] (3) is obtained after 2 weeks instead of complex
1. Compound 3, which was previously reported by Thomas
et al.,34 was identified by means of elemental analysis, FT-
IR spectroscopy, and powder XRD.
The exchange coupling constant Jij (defined through the phe-
nomenological Heisenberg Hamiltonian: Hij ) -JijSiSj) of the
binuclear [{Cu(bpy)(dhmal)]2 neutral complex has been calculated
using the broken-symmetry computational strategy of Ruiz et al.,30
which has been shown to provide good results as compared to
experimental data. This calculation has used the experimental
structure, since the calculated coupling constants are very sensitive
to small deviations in the geometrical parameters. For the evaluation
of the coupling constant of the copper(II) dimer complex, two
separate DFT calculations have been carried out, from which the
energies of the triplet state (EHS) and a broken-symmetry singlet
(24) Rodr´ıguez-Carvajal, J. Physica 1992, 192, 55.
(25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.;
Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann,
R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski,
J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A.
D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko,
A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.;
Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford,
CT, 2004.
According to this observations, the tetrahydroxy-p-ben-
zoquinone is a convenient slow source of oxalate dianions
to in situ generate [{Cu(bpy)}2(ox)]2+ building blocks for
the preparation under open-air mild reaction conditions of
inorganic-metalorganic hybrid compounds containing
bulky inorganic anions, such as Keggin-type POMs. In
this way, the addition of solution A to an aqueous
solution of the potassium salt of the CuII-monosub-
stituted [SiW11O39Cu(H2O)]6- Keggin-type POM leads to an
(26) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(27) (a) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (b) Hay, P. J.
J. Chem. Phys. 1977, 66, 4377. (c) Raghavachari, K.; Trucks, G. W.
J. Chem. Phys. 1989, 91, 1062.
(31) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297.
(32) Castro, I.; Sletten, J.; Faus, J.; Julve, M. J. Chem. Soc., Dalton Trans.
1992, 2271.
(28) Frisch, M. J.; Pople, J. A.; Winkley, J. S. J. Chem. Phys. 1984, 80,
3265.
(29) GaussViewW, version 3.0; Gaussian, Inc.: Wallingford, CT, 2004.
(30) Ruiz, E.; Alemany, P.; AÄ lvarez, S.; Cano, J. J. Am. Chem. Soc. 1997,
119, 1297.
(33) Anal. Calcd (found) for C22H18Cl2Cu2N4O5: C, 42.87 (42.19); H, 2.94
(3.34); N, 9.09 (9.10). FT-IR (cm-1): 1649vs, 1449m, 784s, 730m,
480m.
(34) Thomas, A. M.; Mandal, G. C.; Tiwary, S. K.; Rath, R. K.;
Chakravarty, A. R. J. Chem. Soc., Dalton Trans. 2000, 1395.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1241

Reinoso et al.
Scheme 1. Synthetical Routes for Compounds 1-5
orange reaction mixture from which the discrete hybrid
polyanion [{SiW11O39Cu(H2O)}{Cu2(bpy)2(H2O)2(ox)}2]2-
(4), recently reported by our group,15b is isolated as a blue
crystalline powder after 1 week. After removal of this
compound, in 1 month the resulting pale yellow mother
liquors yield the two-dimensional inorganic-metalorganic
hybrid compound 5 as blue single crystals. The formation
of this compound involves, besides the oxidation of tetrahy-
droxy-p-benzoquinone into oxalate, the transformation of the
CuII-monosubstituted Keggin POM into its parent “complete”
cluster [SiW12O40]4-, which is not surprising if the acidity
of the reaction medium (pH ∼ 3) and the pH stability range
of the monolacunary keggin tungstosilicate anions (pH 4-6)
is considered.35
As shown by powder XRD analysis36 (see Supporting
Information), both compounds 1 and 5 can be directly
synthesized starting from the corresponding carboxylic
acids: in the former case, from a water:methanol solution
containing CuCl2, bipyridine, and dihydroxymalonic acid
disodium salt in a 1:1:1 ratio; and in the latter, by addition
of a water:methanol solution containing CuCl2, bipyridine,
and oxalic acid in a 2:2:1 ratio to an aqueous solution of the
K4[SiW12O40] POM precursor. However, in both cases they
were isolated as precipitates which could not be recrystal-
lized. Since other crystallization techniques did not give any
successful result, the use of tetrahydroxy-p-benzoquinone as
a slow source of dihydroxymalonato and oxalato ligands was
the only method that allowed us to isolate compounds 1 and
5 as single crystals suitable for X-ray diffraction analysis.
Reaction Mechanism. A possible mechanism for the
oxidation of H2thq into croconate anion, followed by
subsequent ring-opening oxidation of the latter into dihy-
droxymalonate and oxalate dianions, can be proposed on the
basis of the literature (Scheme 2). The pKa values of the
benzoquinone (4.8 and 6.8)37 indicate that it will behave in
solution as a dianion with negative charges in an 1,4-
arrangement. Thus, in the presence of 2 equiv of CuII and
bipyridine it could act as a bridging ligand to form a cationic
2-
dimer analogous to those reported with related C6O4X2
ligands,38 which would be expected to display an equilibrium
with its corresponding monomeric neutral complex (CuII-
thq), as in the case of the CuII-ox dimers.39
The CuII complexes with N-donor ligands are known to
show catecholase activity and catalyze the oxidation of
o-diphenols into o-quinones.40 The proposed intermediate for
this reaction consists of a CuI complex with a radicalary
semiquinone anion originated from an intramolecular elec-
tronic transfer from the ligand to the metal. This type of
intermediate has been detected by EPR spectroscopy, and it
is in equilibrium with its parent CuII complex, the formation
constant being very low. Taking into account that the thq
ligand displays a catechol substructure, an analogous in-
tramolecular electronic transfer for the CuII-thq monomer
(37) Preisler, P. W.; Berger, L.; Hill, E. S. J. Am. Chem. Soc. 1947, 69,
326.
(38) Fujii, C.; Mitsumi, M.; Kodera, M.; Motoda, K.; Ohba, M.; Matsumoto,
N.; Okawa, H. Polyhedron 1994, 13, 933.
(39) (a) Castro, I.; Faus, J.; Julve, M.; Gleizes, A. J. Chem. Soc., Dalton
Trans. 1991, 1937. (b) Castro, I.; Faus, J.; Julve, M.; Mun˜oz, M. C.;
D´ıaz, W.; Solans, X. Inorg. Chim. Acta 1991, 179, 59. (c) Castro, I.;
Faus, J.; Julve, M.; Mollar, M.; Monge, A.; Gutie´rrez-Puebla, E. Inorg.
Chim. Acta 1989, 161, 97.
(40) (a) Kaizer, J.; Pap, J.; Speier, G.; Parakanyi, L.; Korecz, L.;
Rockenbauer, A. J. Inorg. Biochem. 2002, 91, 190. (b) Selmeczi, K.;
Reglier, M.; Giorgi, M.; Speier, G. Coord. Chem. ReV. 2003, 245,
191.
(35) Te´ze´, A.; Herve´, G. J. Inorg. Nucl. Chem. 1977, 39, 2151.
(36) Powder XRD. Compound 1: triclinic, P1h, a ) 8.486(2) Å, b ) 9.413-
(1) Å, c ) 9.886(1) Å, R ) 60.24(1)°, â ) 69.56(1)°, γ ) 88.75(1)°,
V ) 631.2(3) Å3. Compound 5: monoclinic, P21/n, a ) 14.452(1) Å,
b ) 13.431(1) Å, c ) 23.344(2) Å, â ) 104.76(1)°, V ) 4352.0(7)
Å3.
1242 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tetrahydroxy-p-benzoquinone as O-Donor Ligand Source
Scheme 2. Proposed Mechanism for the Oxidation of Tetrahydroxy-p-benzoquinone into Croconate in the Presence of CuII and 2,2′-Bipyridine
could be a priori expected, which would lead to the formation
of a CuI-thq• species. According to the model for the
oxidation of the TM-semiquinone complexes,41 this species
can uptake an O2 molecule, which would bind the metal
center and the radicalary position to generate a CuII-peroxo-
semiquinone intermediate (CuII-O2-thq). The formation of
the peroxo-carbon bond resembles the hydroxyl addition
to rhodizonate dianion C6O62-, which promotes the formation
of croconate dianion, CO2, and water after ring contraction.42
Thus, the CuII-O2-thq intermediate could undergo a similar
contraction process to give the corresponding five-membered
ring substituted with a peroxocarboxilic group, which could
be eliminated as CO2 and water to lead to the croconate
dianion and subsequent formation of compound 2. This type
of ring contraction of a peroxo-semiquinone intermediate has
been previously proposed by Speier et al. to explain the
isolation of [{Cu(tmen)}2(µ-ox)(crc)] (tmen, tetramethyleth-
ylenediamine) from a mixture of Cu0 and C6O6.43
According to the studies by Fabre et al.,44 in an acidic
aqueous medium (pH < 4) the croconate dianion is in
equilibrium with its monohydrated form, which under aerobic
conditions and the exposure to light has been shown to be
unstable and to oxidize to give oxalic and dihydroxymalonic
acids, the intermediate for this process being a cyclic diene
over which 1,4-additions of O2 molecules take place.
Therefore, in this situation compound 2 would be expected
to slowly evolve and to give CuII-dhmal and CuII-ox
complexes, as the formation of the neutral dimer 1 and the
isolation by a Keggin POM crystallizing agent of a CuII-
ox cationic dimer in the hybrid compound 5 indicates.
Crystal Structure of Compound 1. To date, there is a
reduced number of compounds related to the dihydroxyma-
lonate dianion whose crystal structures have been reported
in the literature, namely, the dihydroxymalonic acid,45 the
benzylethylammonium salt of dihydroxymalonate,45 and the
manganese(II)46 and zinc(II)47 complexes of the 2-hydroxy-
2-hydroxylatemalonate trianion. Thus, to our knowledge
compound 1 constitutes the first structurally characterized
example of a TM complex containing the dihydroxymalonate
dianion as ligand.
Figure 1 shows the ORTEP view of the centrosymmetric
neutral dimer [Cu(bpy)(dhmal)]2, together with the atom
labeling scheme. Selected bond lengths and angles are given
in Table 2. It is made of two dihydroxymalonate anions
acting as µ2-κ3O,O′,O′′:κ1O bridges between two copper
atoms which are separated by 3.49 Å. Each copper atom is
involved in tetragonally elongated octahedral CuN2O2O′2
chromophores with two bipyridine N atoms and two O atoms
belonging to the carboxylate groups of one dihydroxyma-
lonate ligand in the equatorial plane, the Cu-N and Cu-O
bond lengths ranging from 1.93 to 1.99 Å. Due to this, the
carboxylate C-O bonds are not symmetrical, those involving
(41) Barbaro, P.; Bianchini, C.; Linn, K.; Mealli, C.; Meli, A.; Vizza, F.;
Laschi, F.; Zannello, P. Inorg. Chim. Acta 1992, 198-200, 31.
(42) West, R.; Niu, J. In The Chemistry of the Carbonyl Group; Zabicky,
J., Ed.; Wiley: New York, 1970; p 241.
(43) Speier, G.; Speier, E.; Noll, B.; Pierpoint, C. G. Inorg. Chem. 1997,
36, 1520.
(45) Aakero¨y, C. B.; Nieuwenhuyzen, M.; Robinson, P. J. Chem. Crys-
tallogr. 1998, 28, 111.
(46) Shi, J.-M.; Zhu, S.-C.; Liu, W.-D. Transition Met. Chem. 2004, 29,
358.
(44) Fabre, P. L.; Castan, P.; Deguenon, D.; Paillous, N. Can. J. Chem.
1995, 73, 1298.
(47) Shi, J.-M.; Zhu, S.-C.; Wu, C.-J. Jiegou Huaxue 2004, 23, 1027.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1243

Reinoso et al.
Figure 1. ORTEP view of the centrosymmetric neutral dimer [Cu(bpy)-
(dhmal)]2 in compound 1, together with the atom labeling scheme. Hydrogen
atoms have been omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Compound 1a
Cu1 Coordination Sphere
Cu1-O1
Cu1-O2
Cu1-N1
Cu1-N12
Cu1-O1i
Cu1-O4
1.967(2)
1.933(2)
1.987(3)
1.989(2)
2.494(3)
2.604(3)
O1-Cu1-O2
O1-Cu1-O1i
O1-Cu1-O4
O2-Cu1-O4
N1-Cu1-N12
N1-Cu1-O4
N12-Cu1-O4
Cu1-O1-Cu1i
91.8(1)
77.6(1)
71.6(1)
73.3(1)
81.3(1)
108.4(1)
110.8(1)
102.4(1)
Cu1‚‚‚Cu1i
3.494(1)
dhmal Ligand
C22-O1
C20-O2
C20-O3
C22-O6
C21-O4
C21-O5
1.285(5)
1.272(5)
1.228(5)
1.229(4)
1.430(4)
1.374(5)
Distortion Measuresb
9.0(1)
89.5(1)
O2-C20-O3
O1-C22-O6
O4-C21-O5
C20-C21-C22
125.6(3)
124.7(4)
112.2(3)
108.6(3)
Figure 2. (a) Projection on the (100) plane of a layer of hydrogen-bonded
[Cu(bpy)(dhmal)]2 complexes. Bipyridine ligands have been omitted for
clarity. (b) View of the crystal packing of compound 1 along the
crystallographic c axis.
The crystal packing shows layers of dimeric complexes
parallel to the yz plane (Figure 2a). The dimers are held
together by means of an extended network of strong O-H‚
‚‚O hydrogen bonds involving both hydroxyl groups, and
those carboxylate O atoms which are not coordinated to the
copper atoms. The layers are packed along the [100] direction
with the bipyridine ligands pointing to the interlamellar space
(Figure 2b). They are connected through two types of weak
interactions, C-H‚‚‚O hydrogen bonds and π-interactions.
The C-H‚‚‚O hydrogen bond network involves the N1/C6
bipyridine aromatic ring, the two O atoms of the gem-diol
functionality, and the non-coordinated O atoms of the
carboxylate groups. On the other hand, the π-interactions
are established between pairs of bipyridine ligands, but they
lack the usual π-stacking bond-over-ring arrangement with
partial overlap of the aromatic rings. This fact is due to the
pronounced off-set displacement of one ligand in a direction
nearly parallel to the 4-1-1′-4′ axis of the other ligand, in
such a way that the C-C bridges are placed over the center
of the aromatic C7/N12 rings. Table 3 summarizes the
geometrical parameters of the O-H‚‚‚O, C-H‚‚‚O, and
π-interactions.
Cu1-bpy1
Cu1-Cu2O2
equatorial distortion
Cu1-Cu2 line
0.4(1)
44.6(1)
a Symmetry code: (i) -x, 1 - y, 1 - z. b Planes: for Cu1, O1, O2, N1,
N12; for bpy1, N1, N12, C2-C11; for Cu2O2, Cu1, O1, Cu1i, O1i. Lines:
for Cu2, Cu1-Cu1i. Equatorial distortion: O2-N12-N1-O1 torsion angle.
the O atoms coordinated to the Cu center being significantly
longer (1.27 and 1.29 Å) than those involving the non-
coordinated O atoms (1.23 Å). The copper-dihydroxyma-
lonate six-membered chelating ring displays a boat confor-
mation, in such a way that one hydroxyl group of the gem-
diol functionality coordinates to the copper atom through
an axial position, the Cu-OH bond length being 2.60 Å.
Two structural features have their origin in the axial
coordination of the hydroxyl group: on one hand, the gem-
diol functionality shows substantially different C-OH bond
lengths (1.43 and 1.38 Å for the coordinated and non-
coordinated hydroxyl groups, respectively), and on the other
hand, the copper coordination sphere becomes highly dis-
torted, with an Oax-Cu-Oax angle of 144°. The remaining
axial position is occupied by a carboxylate O atom belonging
to the centrosymmetrically related dihydroxymalonate ligand,
with a Cu-O bond length of 2.49 Å. This arrangement gives
rise to a dimer with an equatorial-axial Cu2(µ2-O)2 rhomboid
core, which shows O-Cu-O and Cu-O-Cu angles of 78
and 102°, respectively.
Crystal Structure of Compound 5. To date, only one
two-dimensional inorganic-metalorganic compound based on
[SiW12O40]4- R-Keggin polyanions and TM-complexes can
be found in the literature, namely, [{SiW12O40}{Cu(bpy)2}2]‚
10H2O.12i Thus, compound 5 constitutes the second structur-
1244 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tetrahydroxy-p-benzoquinone as O-Donor Ligand Source
Table 3. Hydrogen-Bonding and π-Stacking Geometry in Compound 1
a water molecule occupies the apical position at 2.14 Å. The
µ-oxalatodicopper complex displays a chair conformation,
the copper equatorial/basal planes forming dihedral angles
of 6 and 11° with the bipyridine and oxalato ligands,
respectively. In addition to this structural distortion, the dimer
is also twisted with respect to the Cu1-Cu2 axis, in such a
way that the dihedral angle between the two bipyridine planes
is 18°.
(Å, deg)a
Hydrogen Bonding
D-H‚‚‚A
D-H
H···A
D···A
D-H···A
O4-H14‚‚‚O3i
O5-H15‚‚‚O6ii
C3-H3‚‚‚O4iii
C4-H4‚‚‚O6iv
C5-H5‚‚‚O3iv
C11-H11‚‚‚O5i
0.79(4)
0.81(3)
0.93
0.93
0.93
2.00(4)
2.11(4)
2.46
2.54
2.38
2.750(4)
2.870(4)
3.299(5)
3.447(5)
3.310(5)
3.197(6)
158(3)
157(2)
150
164
173
0.93
2.36
150
The crystal structure shows R-Keggin subunits placed at
the center and at the vertices of the unit cell. Each polyanion
is linked to the four nearest neighbors in the [111] and [1h11h]
directions through four µ-oxalatodicopper complexes and,
more specifically, through axial coordination of the Cu1
atoms to the terminal oxygen atoms of adjacent W1 and W4
octahedra. This type of linking between building blocks gives
rise to hybrid layers parallel to the (101h) plane, which are
based on rhombi of 13.4 × 24.3 Å diagonals as basic
structural motif (Figure 4a). The layers are packed along the
c axis with the metal-organic blocks pointing to the
interlamellar space. In this way, the bpy2-Cu2-ox frag-
ments act as interlamellar separators, leading to an open-
framework crystal packing with trapezoidal tunnels of 7.5
× 6 Å (R ∼ 109°) dimensions along the [010] direction
(Figure 4b). These tunnels host a cascade of hydration water
molecules which connect adjacent layers through an extended
and intrincate network of strong O-H‚‚‚O hydrogen bonds
involving oxygen atoms of both inorganic and metal-organic
building blocks. The hybrid layers are further connected
by means of Cu‚‚‚π interactions established between
pairs of bpy2-Cu2-ox fragments. These fragments are
arranged in such a way that the Cu2 atoms are placed over
the C27/N32 aromatic rings at 3.60 Å from the ring-plane
and 3.71 Å from the ring-centroid, the Cu2‚‚‚Cu2 distance
being 4.66 Å.
EPR Spectroscopy. Figure 5 shows the room-temperature
X- and Q-band EPR powder spectra for compound 1. At
lower temperatures there is a large increase of the intensity
of all the signals but their position, line shape and resolution
are essentially the same. The room-temperature X-band
spectrum is rather complex with four signals centered at
about 2500, 2950, 3220, and 3600 G. In addition, a low-
field resonance corresponding to a ∆Ms ) (2 forbidden
transition is clearly observed. At a first glance the Q-band
spectrum appears to be the sum of an axial signal plus a
weaker contribution. However, the extrapolation of the
positions of the Q-band signals to the X-band spectrum
reveals the presence of a first-order independent field
phenomenon, as dipolar or pseudo-dipolar interactions. In
fact, both spectra are characteristic of a triplet spin state with
a small and anisotropic zero-field splitting tensor. The
observation of the ∆Ms ) (2 line indicates that the Cu(II)
dimeric entities are well-isolated from the magnetic point
of view. We have tried to fit the spectra using the reported
formulas for the transition fields along the principal axes,50
but no reasonable fitting was obtained, suggesting the
existence of noncollinear g and D tensors. This misalignment
π-Stackingb
DC
DZ
ANG
DXY
Cg(1)-Cg(2)v
Cg(1)-Cg(1)v
4.119(3)
4.078(3)
3.523
3.588
10.02
0.00
2.133
1.939
a Symmetry codes: (i) -x, -y, 1 - z; (ii) -x, -y, 2 - z; (iii) 1 - x, 1
- y, 1 - z; (iv) 1 + x, 1 + y, -1 + z; (v) 1 - x, 1 - y, - z. b Cg(i):
centroid of the aromatic ring i (i ) 1: C7, C8, C9, C10, C11, N12; i ) 2:
N1, C2, C3, C4, C5, C6). DC: Distance between ring centroids. DZ:
Perpendicular distance of Cg(i) on ring j. ANG: dihedral angle between
planes i and j. DXY: Distance between the projection of Cg(i) on ring j
and Cg(j).
ally characterized example of this type of hybrid compounds.
It crystallizes in the monoclinic space group P21/n with half
a centrosymmetric [SiW12O40]4- Keggin polyanion, one [Cu2-
(bpy)2(H2O)(ox)]2+ cationic dimer and eight hydration water
molecules in the asymmetric unit. Figure 3 shows the ORTEP
view of a fragment of the two-dimensional hybrid network,
containing one polyanion linked to four dimers, together with
the atom labeling scheme.
The inorganic building block [SiW12O40]4- shows the
characteristic structure of the R-Keggin isomer, which
consists of a central SiO4 tetrahedron surrounded by four
vertex-sharing W3O13 trimers that result from the association
of three edge-sharing MO6 octahedra, in such a way that
the ideal polyanion has Td symmetry. Since it lies on a center
of inversion, the central SiO4 tetrahedron is disordered over
two crystallographic positions leading to a distorted cube.
The W-O bond distances are concordant with those
observed in the barium48 or the tetramethylammonium49 salts.
They fall within the ranges 2.34-2.49 Å for central oxygen
atoms coordinated to Si and W centers, 1.84-1.94 Å for
bridging oxygen atoms between WO6 octahedra, and 1.66-
1.68 Å for terminal oxygen atoms.
Selected bond lengths and angles for the metal-organic
building block [Cu2(bpy)2(H2O)(ox)]2+ are given in Table
4. It is made of two copper atoms bridged by an oxalate
anion in the usual bis(bidentate) fashion with a Cu‚‚‚Cu
distance of 5.16 Å. The coordination geometries around each
copper atom are significantly different, tetragonally elongated
ocathedral CuN2O2O′2 for Cu1 and square-pyramidal
CuN2O2O′ for Cu2. In both cases, the equatorial/basal planes
are formed by two oxalate O atoms and two bipyridine N
atoms, the Cu-O and Cu-N bond lengths ranging from 1.94
to 2.01 Å. For Cu1, the axial positions are accupied by
terminal oxygen atoms of two Keggin subunits, the Cu-Ot
bond lengths and Cu-Ot-W angles being almost identical
(around 2.55 Å and 135.5°, respectively). In the case of Cu2,
(48) Kobayashi, A.; Sasaki, Y. Bull. Chem. Soc. Jpn. 1975, 48, 885.
(49) Fuchs, J.; Thiele, A.; Palm, R. Z. Naturforsch. 1981, 36B, 161.
(50) Wasserman, E.; Snyder, L. C.; Yager, W. A. J. Chem. Phys. 1964,
41, 1763.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1245

Reinoso et al.
Figure 3. ORTEP view of a fragment of the two-dimensional hybrid
network in compound 5, containing one [SiW12O40]4- polyanion linked to
four [Cu2(bpy)2(H2O)(ox)]2+ dimers, together with the atom labeling scheme.
Hydrogen atoms have been omitted for clarity.
Figure 4. (a) Polyhedral projection of a hybrid layer of compound 5 on
the (101h) plane. Cu2 atoms, oxalato bridges, and bipyridine ligands have
been intentionally supressed to clearly illustrate the linkage of the Keggin
polyanions through the coordination sphere of the Cu1 atoms. (b) View of
the crystal packing of compound 5 along the crystallographic b axis. Keggin
polyanions are represented as cubooctahedra for clarity.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for the
Metal-Organic Block of Compound 5a,b
Cu1 coordination
sphere
Cu2 coordination
sphere
Cu1-O50
Cu1-O51
Cu1-N1
Cu1-N12
Cu1-O1
Cu1-O4i
Cu1‚‚‚Cu2
1.96(2)
2.01(1)
1.96(2)
2.00(2)
2.56(2)
2.54(2)
5.158(2)
Cu2-O52
Cu2-O53
Cu2-N21
Cu2-N32
Cu2-O54
1.96(2)
1.99(2)
2.00(2)
1.94(2)
2.14(2)
Figure 6 shows the room- and low-temperature X- and
Q-band EPR powder spectra for compound 5. At room
temperature, it gives rise to a single anisotropic broad
resonance centered at ca. 3250 G in the X-band spectrum
and at ca. 11 700 G in the Q-band one. These resonances
can be simulated as a rhombic signal with the best fit
parameters g1 ) 2.218, g2 ) 2.120, and g3 ) 2.057 ( g )
2.132) and g1 ) 2.245, g2 ) 2.130, and g3 ) 2.060 ( g )
2.145) for the X- and Q-band experiments, respectively. The
absence of transitions associated with a well-isolated triplet
spin state clearly indicates that a nonnegligible magnetic
interaction must take place between the copper-oxalate
dimers, so that the rhombic signal can be attributed to an
exchange g tensor. According to our previous studies in
analogous one-dimensional systems,52 Keggin polyanions are
effective in transmitting intermolecular magnetic interactions
between copper-oxalate dimers that are supported on the
terminal oxygen atoms of adjacent WO6 octahedra. There-
fore, an exchange pathway between the Cu1 atoms through
the [SiW12O40]4- subunits can be proposed, although the
participation in the magnetic exchange of the Cu‚‚‚π interac-
tions involving the Cu2 atoms cannot be a priori disregarded
(Figure S6 of the Supporting Information). The intensity of
the signal diminishes as the temperature is lowered, in line
O50-Cu1-O51
N1-Cu1-N12
85.4(7)
82.3(8)
168.5(6)
9.4(8)
O52-Cu2-O53
O52-Cu2-N32
O53-Cu2-N21
N21-Cu2-N32
84.0(8)
168.3(8)
161.6(7)
80.8(8)
O1-Cu1-O4i
O1-Cu1-Cu2-O54
Cu1-bpy1
5.4(8)
11.1(8)
14.9(8)
4.6(8)
Cu2-bpy2
6.0(8)
10.6(8)
16.3(8)
5.5(8)
Cu1-ox
Cu2-ox
bpy1-ox
equatorial distortion
bpy2-ox
equatorial distortion
Cu1-Cu2
8.2(8)
bpy1-bpy2 17.8(8)
1
1
1
a Symmetry code: (i) /2 - x, /2 - y, /2 - z. b Planes: for Cu1, O50,
O51, N1, N12; for Cu2, O52, O53, N21, N32; for ox, O50, O51, C13,
C14, O52, O53; for bpy1, N1, N12, C2-C11; for bpy2, N21, N32, C22-
C31. Equatorial distortion: N1-N12-O51-O50 (Cu1) and N21-N32-
O53-O52 (Cu2) torsion angles.
between g and D is not surprising taking into account the
fact that the copper-copper intradimer direction deviates
strongly from the normal to the equatorial planes of the Cu-
(II) cromophores.51
(51) Bencini, A.; Gatteschi, D. Electron Paramagnetic Resonance of
Exchange Coupled Systems; Springer-Verlag: Berlin, Germany, 1990.
(52) Reinoso, S. Ph.D. thesis, Universidad del Pa´ıs Vasco, Spain, 2005.
1246 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tetrahydroxy-p-benzoquinone as O-Donor Ligand Source
Figure 6. High- and low-temperature X-band (top) and Q-band (bottom)
EPR powder spectra of compound 5.
0.85 cm3‚K‚mol-1 and a Weiss temperature of +1.2 K. This
positive temperature intercept suggests the predominance of
ferromagnetic interactions in this compound, and it is
consistent with the overall appearance of the ømT vs T curve.
At room temperature, the ømT product is equal to 0.85
cm3‚K‚mol-1, which is the expected value for two uncoupled
Cu(II) ions with a g value of 2.13. It slightly increases upon
cooling below 30 K and reaches a value of 1.10 cm3‚K‚mol-1
at 2 K. The rise in magnetic moment is small but unambigu-
ous, and it is indicative of ferromagnetic couplings. To
confirm the ferromagnetic nature of the dominant interac-
tions, magnetization versus field measurements have been
carried out at our lowest available temperature (2 K). Under
a magnetic field of 70 kG, the magnetization is practically
saturated with a maximum value of M(Nâ)-1 ) 2.09, which
corresponds to the theoretical saturation value for two
unpaired electrons with an average g value of 2.12. The
resulting curve (included as Supporting Information) has been
compared with the Brillouin function showing a more rapid
saturation than that expected for a strictly paramagnetic
system.54 Therefore the ferromagnetic coupling is predomi-
nant in compound 1. As expected for a system without
appreciable long-range interactions, no remanent magnetiza-
tion neither coercitive fields have been observed. The
observed behavior is in good agreement with the parallel
orbital topology of the dimer. The two singly occupied
Figure 5. Room-temperature X-band (top) and Q-band (bottom) EPR
powder spectra of compound 1.
with the strong intramolecular antiferromagnetic coupling
expected for the copper-oxalate dimer considering its
coplanar orbital topology. This fact allows the observation
of a magnetically isolated monomeric impurity in the low-
temperature X-band spectrum. Its contribution shows a
poorly resolved hyperfine structure on the parallel region
(ca. 2800 G), and it can be simulated by using an axial g
tensor with the values of its principal components analogous
to those reported for the [SiW11O39Cu(H2O)]6- polyanion.15,53
This fact indicates that, when the [SiW11O39Cu(H2O)]6-
Keggin-type polyanion is used as the POM precursor, it is
not possible to completely separate compound 5 from the
potassiumsaltofthe[{SiW11O39Cu(H2O)}{Cu2(bpy)2(H2O)2(ox)}2]2-
hybrid polyanion (compound 4) by using fractional crystal-
lization techniques.
Magnetic Properties. Figure 7 shows the thermal evolu-
tion of the magnetic molar susceptibility and the ømT product
(ømT ) µeff2/8) for compounds 1 and 5. For compound 1,
the magnetic susceptibility experiments a continuous increase
with decreasing temperature and the øm vs T curve shows
no remarkable feature. Above 30 K, the data are well-
described by a Curie-Weiss law with a Curie constant of
(53) (a) Scholz, G.; Lu¨ck, R.; Sto¨âer, R.; Lunk, H.-J.; Ritschl, F. J. Chem.
Soc., Faraday Trans. 1991, 87, 717. (b) Gamelas, J. A.; Santos, I. C.
M. S.; Freire, C.; de Castro, B.; Cavaleiro, A. M. V. Polyhedron 1999,
18, 1163.
(54) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, Germany,
1986.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1247

Reinoso et al.
Figure 8. Singly occupied molecular orbitals (SOMOs) of compound 1,
calculated with the B3LYP functional and a triple-ú valence basis set in all
atoms.
dimer in compound 5, which shows CuII-equatorial planes
Figure 7. Thermal evolution of the magnetic susceptibility and the ømT
product for compounds 1 (top) and 5 (bottom). Continuous lines represent
the least-squares fit to eq 1.
almost coplanar with the oxalato ligand (coplanar orbital
II
2
2
topology). In this situation, the Cu -equatorial dx -y magnetic
orbital is directed toward the oxalate frontier orbitals, so that
the antiferromagnetic contribution is maximized and ex-
change coupling constants ranging from -300 to -400 cm-1
are usually observed.
2
2
molecular orbitals (SOMOs) of the Cu(II) ions (dx -y ) are
parallel between them and perpendicular to the exchange
pathway through the dihydroxymalonate bridge, as can be
clearly seen in the DFT calculated magnetic orbitals, obtained
as a linear combination of the SOMOs (Figure 8). In this
situation, the overlap between both magnetic orbitals are
close to zero and only weak couplings, ferro- or antiferro-
magnetic, can be expected.
The experimental curves of compounds 1 and 5 have been
compared with those calculated with the following general
expression
N(g′)2â2
ANg2â2
øm ) (1 - F)
+ F
(1)
4kT
kT[3 + exp(-J/kT)]
In the case of compound 5, the room temperature øm value
is 2.78 × 10-3 cm3‚mol-1 and it rapidly decreases with
decreasing temperature, in such a way that neither a
maximum nor a Curie-Weiss behavior is observed. The øm
curve reaches a minimum at around 70 K (0.82 × 10-3
cm3‚mol-1) from which it strongly increases to a value of
9.37 × 10-3 cm3‚mol-1 at 5 K. On the other hand, the room-
temperature ømT value (0.83 cm3‚K‚mol-1) is substantially
lower than that expected for four CuII ions per formula unit,
considering them to be uncoupled ions with g ) 2 (1.50
cm3‚K‚mol-1). When the system is cooled down, the ømT
product rapidly decreases from 300 to ca. 70 K and it remains
approximately constant below this temperature, reaching a
value of 0.05 cm3‚K‚mol-1 at 5 K. This behavior indicates
the presence of very strong antiferromagnetic interactions,
together with the contribution of a small fraction of
paramagnetic impurities, so that the local maximum in the
øm curve is expected to be masked. This observation is in
agreement with the structural features of the copper-oxalate
where N, â, and k have their usual meanings and g is the
average g-factor of the copper-oxalate or copper-dihy-
droxymalonate dimers. The first term in the above expression
is the classical Bleaney-Bowers55 equation for a dinuclear
CuII complex, expressed per copper atom and modified to
take into account the presence of noncoupled impurities. The
singlet-triplet energy gap (J) is defined by the Hamiltonian
H ) -JSASB (SA ) SB ) 1/2), whereas the A factor represent
the number of CuII ions per formula unit (A ) 2 and A ) 4
for compounds 1 and 5, respectively). The second term
corresponds to the paramagnetic contribution of the non-
coupled impurities, F being their fraction and g′ being their
local g-factor. According to the EPR spectra, compound 1
is almost free of impurities, and therefore F was fixed to
zero during the fitting process. In the case of compound 5,
(55) Bleaney, B.; Bowers, K. D. Proc. R. Soc. London, Ser. A 1952, 214,
451.
1248 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tetrahydroxy-p-benzoquinone as O-Donor Ligand Source
EPR spectroscopy shows that the noncoupled impurities
mainlyconsistofcopper-monosubstituted[SiW11O39Cu(H2O)]6-
Keggin polyanions, so that g′ was fixed to the value reported
in the literature (2.205).15 Although the EPR spectra also
indicate that the copper-oxalate dimers in compound 5 are
magnetically coupled, the magnitude of the interdimeric
coupling through the Keggin polyanions is expected to be
negligible in comparison with the intradimeric one, and
therefore interdimeric interactions have been intentionally
suppressed from eq 1 to avoid an excessive number of
variables.
Conclusions
In the presence of CuII and 2,2′-bipyridine in an aqueous
acidic medium, tetrahydroxy-p-benzoquinone oxidizes to give
croconate, which undergoes further ring-opening oxidation
to decompose into dihydroxymalonate and oxalate dianions.
On the basis of the literature, a possible mechanism for the
oxidation of the benzoquinone into croconate is proposed.
We have made use of this “slow source” behavior to isolate
under open-air mild reaction conditions the neutral dimer
[Cu(bpy)(dhmal)]2 (1) and the inorganic-metalorganic hy-
brid compound [{SiW12O40}{Cu2(bpy)2(H2O)(ox)}2]‚16H2O
(5) as single crystals suitable for X-ray diffraction. Com-
pound 1 constitutes the first structurally characterized
example of a transition metal-dihydroxymalonate complex,
and it contains two copper atoms bridged by two dihydroxy-
malonato ligands acting in a µ2-κ3O,O′,O′′:κ1O coordination
fashion, so that an equatorial-axial Cu2(µ2-O)2 rhomboid
core is formed. On the other hand, compound 5 shows a
two-dimensional arrangement of Keggin polyanions linked
by one of the Cu atoms of the oxalate cationic dimers to
give hybrid layers parallel to the (101h) plane, the remaining
oxalato-Cu-bipyridine fragments acting as interlamellar
separators. EPR and magnetic susceptibility studies shows
a relatively weak ferromagnetic coupling for the Cu2(µ2-O)2
rhomboid core of compound 1 and a strong intradimeric
antiferromagnetic coupling, together with interdimeric mag-
netic interactions, in the case of the copper-oxalate dimers
of compound 5. DFT calculations have offered invaluable
help since they have been successfully applied, on one hand
to confirm the ferromagnetism in compound 1 and on the
other hand to obtain a complete band assignment of the
experimental FT-IR spectra of both compounds.
Least-squares fits of eq 1 to the experimental data were
performed by minimizing the following function
NP
R ) { [ø (exp) - ø (cal) ]2/(NP - NV)}1/2
(2)
∑
m
i
m
i
i)1
where NP is the number of data points and NV is the number
of variable parameters. The best fit results are J ) +3.4
cm-1, g ) 2.13, and R ) 1.1 × 10-3 for compound 1 and
J ) -331 cm-1, g ) 2.134, F ) 10.6%, and R ) 3.8 ×
10-4 for compound 5.
The exchange coupling constant of compound 1 is in very
good agreement with the DFT calculated ones, which are
weakly ferromagnetic and in the range +3.2 to +3.5 cm-1,
depending on the size of basis set used, and is +3.4 cm-1
with the best quality basis set: QZVPP on the copper atoms
and TZVPP on the rest.
In the case of compound 5, the observed exchange
coupling constant is in good agreement with those observed
for several compounds containing [{Cu(bpy)}2(µ-ox)]2+
complexes of coplanar orbital topoly,34,56 which range from
-330 to -390 cm-1. However, the J value is 20 cm-1 less
antiferromagnetic than those calculated for analogous one-
dimensional systems based on Keggin-type polyanions and
centrosymmetric copper-bypiridine-oxalate dimers with an
almost planar conformation.15b This weakening of the anti-
ferromagnetic coupling in compound 5 with respect to the
one-dimensional systems can be attributed to two different
types of structural factors. On one hand, the copper-oxalate
dimer is composed of two copper atoms with different
coordination geometries (octahedral and square-pyramidal),
a fact that weakens the magnetic coupling according to
Kahn’s equation.57 On the other hand, the dimer shows a
chairlike conformation and it is twisted with respect to the
Cu-Cu axis (with an angle of 8° between the equatorial/
basal planes), both structural distortions decreasing the
overlapping of the magnetic orbitals with the frontier orbitals
of the oxalato bridge, as shown by computational calcula-
tions.58
Acknowledgment. This work was supported by Univer-
sidad del Pa´ıs Vasco (Grant 9/UPV 00169.310-15329/2003)
and Ministerio de Ciencia y Tecnolog´ıa (Grant MAT2005-
03047). S.R. thanks Gobierno Vasco for his Doctoral
Fellowship. The SGI/IZO-SGIker UPV/EHU (supported by
the National Program for the Promotion of Human Resources
within the National Plan of Scientific Research, Development
and Innovation-Fondo Social Europeo and MCyT) is grate-
fully acknowledged for generous allocation of computational
resources.
Supporting Information Available: Magnetization vs field
measurements of compound 1 and comparison with the Brillouin
function, thermogravimetric curves of compounds 1 and 5, powder
X-ray diffractograms of compounds 1, 3, and 5, possible exchange
pathways between [Cu2(bpy)2(H2O)(ox)]2+ dimers proposed for
compound 5, and X-ray crystallographic files of compounds 1 and
5 in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(56) (a) Julve, M.; Faus, J.; Verdaguer, M.; Gleizes, A. J. Am. Chem. Soc.
1984, 106, 8306. (b) Gleizes, A.; Julve, M.; Verdaguer, M.; Real, J.
A.; Faus, J.; Solans, X. J. Chem. Soc., Dalton Trans. 1992, 3209. (c)
Castillo, O.; Muga, I.; Luque, A.; Gutie´rrez-Zorrilla, J. M.; Sertucha,
J.; Vitoria, P.; Roma´n, P. Polyhedron 1999, 18, 1235.
(57) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(58) (a) Cano, J.; Alemany, P.; Alvarez, S.; Verdaguer, M.; Ruiz, E. Chem.
Eur. J. 1998, 4, 476. (b) Alvarez, S.; Julve, M.; Verdaguer, M. Inorg.
Chem. 1990, 29, 4500.
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