A novel sandwich polyoxometalate linked by mixed RE/TM interlayer: [Fe(phen)3]2[As2W18Fe2{Y(OH2)2}2O68]·6.5H2O

Article abstract of DOI:10.1016/j.molstruc.2007.03.003

A novel sandwich polyoxometalate, [Fe(phen)₃]₂[As₂W₁₈Fe₂{Y(H₂O)₂}₂O₆₈]·6. 5H₂O 1, (phen = 1,10-phenanthroline), has been hydrothermally synthesized and characterized by elemental analyses, IR(25 ～ 650 °C), single-crystal X-ray diffraction, ESR spectroscopy and thermal analyses (TG-DTA). The crystallographic analysis reveals that the dimeric polyanion, [As₂W₁₈Fe₂{Y(OH₂)₂}₂O₆₈]^6-, are constructed from two [B-AsW₉O₃₄]^9- fragments linked by two Y³⁺ ions and two Fe³⁺ ions. The ESR spectra exhibit that the high spin state and the low spin state of Fe(III) coexist in the title compound.

Full text of DOI:10.1016/j.molstruc.2007.03.003

Available online at www.sciencedirect.com
Journal of Molecular Structure 873 (2008) 29–34
www.elsevier.com/locate/molstruc
A novel sandwich polyoxometalate linked by mixed RE/TM
interlayer: [Fe(phen)3]2[As2W18Fe2{Y(OH2)2}2O68]Æ6.5H2O
*
Jingping Wang, Wei Wang, Jingyang Niu
Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University,
Minglun Street, Kaifeng 475001, PR China
Received 17 January 2007; received in revised form 26 February 2007; accepted 1 March 2007
Available online 12 March 2007
Abstract
A novel sandwich polyoxometalate, [Fe(phen)3]2[As2W18Fe2{Y(H2O)2}2O68]Æ6. 5H2O 1, (phen = 1,10-phenanthroline), has been
hydrothermally synthesized and characterized by elemental analyses, IR(25 ꢀ 650 ꢀC), single-crystal X-ray diffraction, ESR spectroscopy
and thermal analyses (TG-DTA). The crystallographic analysis reveals that the dimeric polyanion, [As2W18Fe2{Y(OH2)2}2O68]6ꢁ, are
constructed from two [B-AsW9O34]9ꢁ fragments linked by two Y3+ ions and two Fe3+ ions. The ESR spectra exhibit that the high spin
state and the low spin state of Fe(III) coexist in the title compound.
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: Polyoxometalates; Sandwich-type; Mixed interlayer; Hydrothermal synthesis
1. Introduction
generally display large and variable coordination num-
bers (CN = 8–12) and show minimal stereochemical pref-
erences due to the small energy differences associated
with the various coordination geometries [21], this makes
it difficult to control the coordination environment of the
RE3+ centers and therefore impedes fine-tuning of the
electronic, spectronic, and magnetic properties of such
clusters [22]. As a result, in contrast with the TMxOy
(x = 2–6) interlayer containing sandwich-type POMs that
have been intensively explored, a few works have been
carried out on the RExOy analogues [22–25] as in the
cases of [{Y4(H2O)8(l3-OH)4} (a-P2W15O62)2]16ꢁ [22]
and [{Ce3O3(OH2)2}2(PW9O34)2]12ꢁ [24]. Attentively, to
date all of the structurally characterized sandwich-type
POMs is built up from the trivacant moieties linked by
either TMxOy or RExOy interlayer, while no sandwich-
type POMs with mixed TM/RE linkage has been
observed. We thus describe in this paper the hydrother-
mal synthesis, characterization and crystal structure of
a novel sandwich polyoxometalate, [Fe(phen)3]2[As2W18-
Fe2{Y(OH2)2}2O68]Æ 6.5H2O (1), in which two Y3+ ions
Polyoxometalates (POMs) are a well-known class of
metal–oxygen clusters with an unmatched structural vari-
ety combined with a multitude of properties [1–3]. In
recent years, the sandwich-type POMs encapsulating
clusters of early transition metals have received much
attention both from applied and fundamental research
perspectives [4–7]. The great advantage of these com-
plexes is the possibility of varying either the type of
the metallic cluster (its topology and the nature of the
transition metals) or the hetero-atom [8]. Synthesis of
such compounds is usually accomplished by the reaction
of divalent or trivalent transition-metal ions (ab. TM;
e.g. Cu2+, Mn2+, Fe3+, Co2+) and the trivacant Keggin
or Dawson ligands (e.g. XW9O9ꢁ, X0 W15O12ꢁ X = P,
34
2
56
As, Ge, Si, Co, Zn et al.; X0 = P, As) [9]. The study
of this family has led to the discovery of numerous com-
pounds that have been investigated for their synthesis
and properties [9–20]. On the other hand, the RE3+ ions
and two Fe3+ ions are sandwiched by two AsW9O9ꢁ
*
Corresponding author. Tel.: +86 378 2192443; fax: +86 378 2853650.
34
units.
E-mail address: jyniu@henu.edu.cn (J. Niu).
0022-2860/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.03.003

30
J. Wang et al. / Journal of Molecular Structure 873 (2008) 29–34
Table 1
2. Experimental
Crystal data and structure refinement for 1
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Crystal system
Space group
C72H48As2Fe4N12O78.5W18Y2
6220.52
293(2)
0.71073
Triclinic
2.1. General information
YCl3ÆnH2O was prepared through adding Y2O3 to con-
centrated HCl, then the solution was evaporated to dry at
water bath (90 ꢀC). The other chemicals were commer-
cially purchased and used without further purification.
C, H and N elemental analyses were performed on a
Perkin-Elmer 2400 II elemental analyzer. IR spectra were
recorded on a Nicolet 170 FT-IR spectrometer using KBr
pellets in the range of 4000–400 cmꢁ1. TG-DTA curves
were obtained with EXSTAR 6000 instrument under air
atmosphere at a heating rate of 10 ꢀC/min. The ESR spec-
tra of powder were recorded on a Bruker ER-200-D-SRC
spectrometer at X-band at room temperature and 110 K.
˚
ꢀ
P1
˚
a (A)
12.8882(2)
13.4613(2)
17.7548(3)
95.0130(10)
104.1520(10)
97.8220(10)
2935.60(8)
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
3
˚
Volume (A )
Z
1
Crystal size (mm3)
Dcal (g/cm3)
0.18 · 0.16 · 0.14
3.555
19.676
Absorption coefficient (mmꢁ1
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
)
33509
9658 (Rint = 0.0599)
9658/12/875
1.032
R1 = 0.0473, wR2 = 0.1044
R1 = 0.0574, wR2 = 0.1089
2.2. Synthesis of the title compound
The title compound was prepared from hydrothermal
system in pretreated solution. A solution of Na2HA-
sO4Æ7H2O (0.52 g), Na2WO4Æ2H2O (6.1 g), FeSO4Æ7-
H2O(0.18 g), YCl3ÆnH2O (0.3 g) in water (15 ml) was
acidified with 65% nitric acid (pH = 4.2) and stirred for
0.5 h at 90 ꢀC, thereafter added to a solution of 1,10-phe-
nanthroline (0.08 g) in dehydrated alcohol (1 ml). The
resulting solution was stirred for 4 h and adjusted to
pH = 4.7 by 6 M NaOH, then the mixture was sealed in
a 30 ml Teflon-lined autoclave and heated at 165 ꢀC for
12 days. After cooling to room temperature slowly (65 h),
the red-black block crystals of 1 were collected by filtra-
tion, washed by distilled water and dried in air (21% yields
based on Fe). Anal. Calcd. for C72H48As2-
Fe4N12O78.5W18Y2(%): C, 13.90; H, 1.15; N, 2.70. Found
(%): C, 13.84; H, 1.21; N, 2.76.
2
wꢁ1 ¼ ½r2ðF 20Þ þ ð0:0000PÞ þ 760:0160Pꢂ,
P ¼ ðF 2o þ 2F 2cÞ=3,
R1 =
P
P
P
P
2
2
1=2
jjFoj ꢁ jFcjj/ jFojj, wR2 ¼ f ½wðF 2o ꢁ F c2Þ ꢂ= ½wðF o2Þ ꢂg
.
Table 2
Selected bond lengths (A) and angles (ꢀ) for the title compound
˚
Y(1)–O(26)
2.166(13)
W(4)–O(4)
1.686(10)
Y(1)–O(2w)
Y(1)–O(2w)
Y(1)–O(34)#1
O(34)–Y(1)#1
Fe(1)–O(28)#1
Fe(1)–O(34)#1
Fe(2)–N(1)
2.458(14)
2.428(14)
2.615(9)
2.615(9)
1.874(9)
2.296(9)
1.978(11)
2.009(13)
1.688(9)
1.662(9)
1.852(11)
2.496(9)
1.724(9)
W(5)–O(27)#1
W(8)–O(33)
W(9)–O(9)
W(9)–O(23)
1.758(15)
2.363(9)
1.724(9)
2.207(11)
1.840(9)
162.7(4)
164.4(4)
166.1(4)
174.0(5)
172.2(5)
176.4(5)
106.7(4)
112.9(5)
W(9)–O(24)
O(29)#1–Fe(1)–O(30)
O(25)–Fe(1)–O(34)#1
O(28)#1–Fe(1)–O(34)
N(1)–Fe(2)–N(4)
N(6)–Fe(2)–N(3)
N(2)–Fe(2)–N(5)
O(32)–As(1)–O(33)
O(34)–As(1)–O(31)
Fe(2)–N(5)
As(1)–O(33)
As(1)–O(34)
W(1)–O(14)
W(1)–O(31)
W(2)–O(24)
2.3. X-ray diffraction
The structure of compound 1 was determined by
single-crystal X-ray diffraction. Intensity data were
collected with a Rigaku-RAXIS-IV image plate area detec-
tor using graphite monochromatized Mo-Ka radiation
3. Results and discussion
3.1. Synthesis and the oxide state of Fe
˚
(k = 0.71073 A). The structure was solved by direct method
and refined by the Full-matrix least-squares method on F2
using SHELXL-97 [26] to R = 0.0599 for compound 1 for
9658 independent reflections [I > 2r(I)] of 33509 reflections
collected with ꢁ15 6 h 6 15, ꢁ16 6 k 6 16, ꢁ21 6 l 6 21
in the 1.82 6 h 6 25.00ꢀ range. Intensity data were cor-
rected for Lorentz and polarization effects as well as for
empirical absorption. All of the non- hydrogen atoms were
refined anisotropically. Hydrogen atoms were added
according to the theoretical models. Crystal data collection
parameters along with the values for the residuals after
final refinement are summarized in Table 1. Selected bond
lengths relevant to the structural discussion of the title
compound are listed in Table 2.
The crystal of the title compound was prepared from the
hydrothermal system without precursors (S1 in SI), this
situation is different from the most cases of the known
sandwich analogues that involve trivacant precursors,
XW9O9ꢁ or X0 W15O12ꢁ(S2 in SI). Other else, the title
compound was isolated as the composite complex, while
the most of known sandwich-type POMs exist as the salts
of K+ or Na+. The synthesis of 1 seemly indicates an
interesting approach to the synthesis of sandwich-type
polyoxometalates.
34
2
56
Even though the title compound was prepared from fer-
rous starting materials, three lines of evidence are consis-

J. Wang et al. / Journal of Molecular Structure 873 (2008) 29–34
31
tent with the FeIII oxidation state in 1: first, bond-length-
based valence sum calculations [27] from the crystal struc-
ture yield an average oxidation state for Fe of 3.28 0.27;
second, the lack of significant Jahn–Teller distortion is
more consistent with Fe(III) centers than with Fe(II) cen-
ters [12]; third, the ESR spectra exhibit obviously that there
are two kinds of Fe3+: high spin state and low spin state in
the title compound. The analogous case of Fe(II) fi Fe(III)
has been reported by Hill et al. [12].
3.2. Structural description
As shown in Fig. 1, the structure unit of the title
compound consists of one sandwich polyanion
Fig. 2. Ball-and-stick illustration of the structure of central
Y2Fe2O16(H2O)4 unit.
[As2W18Fe2{Y(OH2)2}2O68]6ꢁ
,
two discrete cations
[Fe(phen)3]3+, and 6.5 crystal water molecules. Although
the structure of the title polyanion shows the general modes
proposed earlier for sandwich analogues with 4 cores by
Hill et al and Wang et al. [12,16], interestingly, the two
equivalent moieties, [AsW9O34]9ꢁ, are held together by
two RE ions (Y3+) and two TM ions (Fe3+), this situation
has never been observed in sandwich POM chemistry. The
unprecedented interlayer, Fe2Y2(H2O)4O16(Fig. 2), consists
of two FeO6 octahedra and two YO8 bicapped trigonal
prisms via edge sharing, exhibiting the regular rhombic
arrangement (S3 in SI). For the FeO6 groups (S4 in SI),
each iron atom is defined by three oxygen atoms on each
[AsW9O34]9ꢁ polyanion (O34, O28, O29 and O30, O25,
O34), the Fe–O bond lengths very from 1.874(3) to
above data exhibit that the four polyhedra have been dis-
torted, which may be attributed to the effect of various
coordination atoms.
The trivacant Keggin unit, [B-AsW9O34]9ꢁ, linked by
the mixed Fe2Y2 unit is in a centrosymmetric arrangement
(C2h symmetry). The [B-AsW9O34]9ꢁ moiety derives from
the removal of one edge-sharing W3O13 triad from the par-
ent Keggin structure [AsW12O40]3ꢁ. Each [B-AsW9O34]9ꢁ
polyanion provides seven oxygen atoms (one from the
AsO4 group and one each from the six WO6 group) that
are capable of bonding to the central tetrameric metal unit.
The nine WO6 octahedra in [B-AsW9O34]9ꢁ moiety have
essentially similar distorted octahedral environments
defined by one terminal oxo-group with short W–O bond
˚
2.296(11) A while the opposed O–Fe–O angles are in the
˚
length (1.686–1.724 A), four doubly bridging oxo-groups
range of 162.69(40) ꢀ 166.12(39)ꢀ. For the yttrium groups
(S5 in SI), each Y atom is bonded to two aqua ligands
(O1W and O2W) and three oxygen atoms on each
[AsW9O34]9ꢁ polyanion (O27, O26, O24 and O23, O30,
O29) with distorted bicapped trigonal prismatic configura-
tion, the range of Y–O bond distances are in the range of
˚
with intermediate W–l–O bond lengths (1.852–2.008 A),
and one l4-oxygen, also bonded to As, with long W–l4–
˚
O distances (2.363–2.496 A). The As–O bond distances in
˚
the AsO4 polyhedra vary from 1.662(9) to 1.688(9) A,
and the O–As–O bond angles vary from 106.7(4)ꢀ to
112.9(5)ꢀ. The above data indicate that the AsO4 tetrahe-
dra have been distorted to some extent, which could be
attributed to the effect of rhombic interlayer, the analogous
˚
2.166(13) ꢀ 2.615(9) A, while the distance between Y atom
and two triangular bases a(O27, O30, O1w) and b(O26,
˚
O29, O2w), are 1.4766 and 1.4159 A, respectively. The
Fig. 1. Structure unit of the title compound, the free water molecules have been omitted for clarity, polyhedra represent the WO6 (red) and AsO4(yellow).
(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

32
J. Wang et al. / Journal of Molecular Structure 873 (2008) 29–34
situation were reported in the [As2W18M4(H2O)2O68]10ꢁ
examples [16].
Each Fe site in [Fe(phen)3]3+ discrete cations is defined
by six nitrogen atoms from three phen molecules with dis-
torted octahedral configuration. For Fe(2) group, the Fe–
˚
N bond lengths are in the range of 1.978(13)–2.009(13) A
˚
(av. 1.994 A), the opposed N–Fe–N angles vary from
172.19(56)ꢀ to 176.43(49)ꢀ, the small maximum difference
˚
in Fe–N bond distance (0.03 A) and the almost linear N–
Fe–N angles indicate the slightly distortion of FeN6 octa-
hedra. While the dihedral angles among three ideal phen
planes are 91.4ꢀ, 87.4ꢀ and 90.3ꢀ, which indicates that the
three ideal phen planes in each [Fe(phen)3]3+ fragment
are nearly vertical to each other. Besides, in Fe(2) group,
the mean deviations of three phen ideal planes, c(N1, N1,
C1 to C12), d(N3, N4, C13–C24) and e(N5, N6, C25,
˚
˚
C36) are 0.0249, 0.0373 and 0.5048 A, and the Fe(2) atom
deviates 0.0376, 0.0579, and 0.8484 A from the ideal planes
c, d and e, respectively. It is the evidence that the Fe(2)
atom is nearly in the plane c.
In addition, it is worth to note the distortion of FeO6
and FeN6 groups. On one hand, both FeO6 groups and
FeN6 groups exhibit the slight Jahn–Teller effect, indicated
by the small difference values between axial bonds and the
˚
˚
equatorial bonds, 0.067 A for FeO6 groups and 0.018 A for
FeN6 groups (Fig. 3). On the other hand, the various coor-
dination environments may play an important role in the
distortion of FeL6(L = N, O). For Fe(1) group, Fe(1) atom
is bonded to one l4-O atom (bonded to As atom), one l3-O
atom (shared by a W atom and two metal atoms) and one
l2-O atom (shared by a W atom and a metal atom) from
each AsW9O9ꢁ moiety. For Fe(2) group, the Fe atom is
34
coordinated to six N atoms from three phen molecules. It
is visible that the coordination environment in FeO6 group
are more complicated than those in FeN6 group, so it is
expectable that the FeO6 octahedra exhibit the stronger
distortion.
3.3. IR spectroscopy
In the IR spectrum of 1 at room temperature (S6 in SI),
the peaks at 960, 771, and 888 cmꢁ1 can be assigned to the
characteristic vibrations of m(W–Od), m(W–Oc) and m(As–
Oa), respectively [28]. Compared with those of
a ꢁ AsW12O34ꢁ0 , these peaks are shifted towards low fre-
quency region apparently and the last vibration overlaps
that of m(W–Ob), resulting from the increase of the negative
charges of the anion, this situation has been reported by
Wang et al. [16]. The bonds in the region of 1350–
1050 cmꢁ1 can be attributed to the characteristic peaks of
1,10-phenanthroline cyclobackbone. The coordination
between metal atoms and N donors are safely indicated
by the red-shifts of flexural oscillations d(C–H) outside
the 1,10-phenanthroline plane from 865, 742 cmꢁ1 to 839,
719 cmꢁ1 [29,30]. In addition, the strong peaks at
3445 cmꢁ1 can be assigned to the telescopic oscillations
of water molecules, then outside plane rocking vibrations
Fig. 3. View of the Jahn–Teller distortion of the FeO6(a) and FeN6(b),
da = mean axial distance, de = mean equatorial distance. (a) FeO6 case,
˚
˚
da ꢁ de = 0.067A; (b) FeN6 group, da ꢁ de = 0.018A.
of water appear 583 cmꢁ1, which exhibit the presence of
crystal water and coordination water molecules, respec-
tively [30]. As shown in S6b, the temperature variation of
the IR spectra exhibits the disappearances of characteristic
peaks for phen groups and polyanion framework in the
regions of 400 ꢀ 450 ꢀC and 550 ꢀ 600 ꢀC, respectively.
3.4. ESR spectrum
The ESR spectra of [Fe(phen)3]2[As2W18Fe2{Y(H2O)
2}2O68]Æ6 5H2O reveal obviously that there are two kinds
of Fe(III): high spin state and low spin state in compound

J. Wang et al. / Journal of Molecular Structure 873 (2008) 29–34
33
Fig. 5. TG-DTA curves of [Fe(phen)3]2[As2W18Fe2{Y(H2O)2}2O68]Æ6.5H2O.
Fig. 4. ESR spectra of the title compound: (a) 110 K, 9.4867 GHz; (b)
room temperature, 9.7763 GHz.
tively, which are accord with the results of the temperature
variation of the IR spectra.
1 (Fig. 4). At 110 K, the ESR spectrum display a broad sig-
nal centered at g = 1.44 and three narrow symmetric bands
g = 4.20, 3.44, 2.08. It has been reported that the Fe3+ with
high spin (S = 5/2) incorporated in silicotungstate and
phosphatomolydbate structures with the distorted octahe-
dral coordination give the isotropic ESR signal at
g = 4.12 [31] and 4.1–4.2, respectively [32,33]. Therefore,
the signal at g = 4.20 is assigned to the Fe sites in central
layer with FeO6 coordination. On the other hand, the
parameters g = 3.44 and 1.45 are consistent with the pres-
ence of low spin Fe3+ (S = 1/2) [34–37], which could be
4. Conclusions
In summary, we have successfully synthesized a novel
sandwich type polyoxometalate [Fe(phen)3]2[As2W18-
Fe2{Y(H2O) } O68] Æ 6 5H2O, exhibited by the incorpora-
2 2
tion of the mixed RE/TM interlayer. The presence of the
title compound enriches the field of sandwich POMs and
may promote the further development of this subclass.
Moreover, the synthesis of the title compound validates
the two-step hydrothermal synthetic protocol and the fea-
sibility of sandwich-type polyoxometalates.
attributed to the Fe sites in discrete cations [Fe(phen)3]3+
.
When the temperature increases to room temperature,
The ESR signal g = 1.45 disappears, which may be caused
by the zero-field splitting, and at the same time other sig-
nals g = 4.20, 3.44, 2.08 decrease. In addition, the noise sig-
nals become stronger owing to the increasing spin-lattice
relaxation time. The analogous situations have been
observed in our previous work for Co2+ with high spin
state and low spin state in same complex [38].
Acknowledgements
This work was supported by the Nature Science Foun-
dation of China, the Program for New Century Excellent
Talent in University of Henan province, the Foundation
of Educational Department of Henan Province, and the
Natural Science Foundation of Henan Province.
Appendix A. Supplementary data
3.5. Thermal analysis
Crystallographic data for structural analysis reported in
this paper have been deposited with the Cambridge crystal-
lographic Data center with the deposited numbers 604523
for the title compound. Copies of this information may
be obtained free of charge from the Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-
336033; e-mail: deposit@ccdc.cam.ac.uk). Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.molstruc.2007.03.003.
Thermal stability of the title compound has been studied
by the means of thermogravimetric and differential thermal
measurements (Fig. 5). TG curve of 1 exhibits two stages
for loss of weight. The first slow weight loss of 3.10%
occurs between room temperature and 300 ꢀC, correspond-
ing to the loss of crystal water molecules (cacl. 3.04%); the
second sharp weight loss of 17.39% occurs from 300 to
700 ꢀC, resulting from the loss of phen groups (cacl.
17.36%). A strong and wide endothermal peak is observed
at 251.5 ꢀC in DTA curve, this heating-absorbing process
should be related to the release of water molecules. Mean-
while, the two strong exothermal peaks appeared at 446.8
and 580.8 ꢀC show the combustion of phen groups and
the decomposition of the polyanion framework, respec-
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