Polar symmetry and intercalation of new multilayered hybrid molybdates: [M2(pzc)2(H2O)x][Mo 5O16] (M = Co, Ni)

Article abstract of DOI:10.1021/ic060253e

The layered molybdate [M₂(pzc)₂(H₂O) _x][Mo₅O₁₆] (I: M = Ni, x = 5.0; II: M = Co, x = 4.0; pzc = pyrazinecarboxylate) hybrid solids were synthesized via hydrothermal reactions at 160-165°C. The structures were determined by single-crystal X-ray diffraction data for I (Cc, Z = 4; a = 33.217(4) A, b = 5.6416(8) A, c = 13.982(2) A, β = 99.407(8)°, and V = 2585.0(6) A³) and powder X-ray diffraction data for II (C2/c, Z = 4; a = 35.42(6) A, b = 5.697(9) A, c = 14.28(2) A, β = 114.95(4)°, and V = 2614(12) A³). The polar structure of I contains new [Ni₂(pzc)₂(H₂O)₅] ²⁺ double layers that form an asymmetric pattern of hydrogen bonds and covalent bonds to stair-stepped [Mo₅O₁₆]^2- sheets, inducing a net dipole moment in the latter. In II, however, the [Co ₂(pzc)₂(H₂O)₄]²⁺ double layers have one less coordinated water and subsequently exhibit a symmetric pattern of covalent and hydrogen bonding to the [Mo₅O ₁₆]^2- sheets, leading to a centrosymmetric structure. Thermogravimetric analyses and powder X-ray diffraction data reveal that I can be dehydrated and rehydrated with from 0 to 6.5 water molecules per formula unit, which is coupled with a corresponding contraction/expansion of the interlayer distances. Also, the dehydrated form of I can be intercalated by ～4.3 H₂S molecules per formula unit, but the intercalation by pyridine or methanol is limited to less than one molecule per formula unit.

Full text of DOI:10.1021/ic060253e

Inorg. Chem. 2006, 45, 4721−4727
Polar Symmetry and Intercalation of New Multilayered Hybrid
Molybdates: [M2(pzc)2(H2O)x][Mo5O16] (M
) Co, Ni)
Bangbo Yan and Paul A. Maggard*
Department of Chemistry, 2620 Yarbrough DriVe, North Carolina State UniVersity,
Raleigh, North Carolina 27695-8204
Received February 14, 2006
The layered molybdate [M2(pzc)2(H2O)x][Mo5O16] (I:
pyrazinecarboxylate) hybrid solids were synthesized via hydrothermal reactions at 160
were determined by single-crystal X-ray diffraction data for I (Cc, Z 4; a 33.217(4) Å, b
13.982(2) Å, â ) 99.407(8) , and V
35.42(6) Å, b 5.697(9) Å, c
M
)
Ni, x
)
5.0; II:
M
)
Co, x
)
4.0; pzc
)
−165
°
C. The structures
)
)
)
5.6416(8) Å, c
4; a
)
)
°
)
2585.0(6) Å3) and powder X-ray diffraction data for II (C2/c, Z
)
)
)
14.28(2) Å, â ) 114.95(4)
°
, and V
)
2614(12) Å3). The polar structure of I
contains new [Ni2(pzc)2(H2O)5]2 double layers that form an asymmetric pattern of hydrogen bonds and co-
+
2-
valent bonds to stair-stepped [Mo5O16
]
sheets, inducing a net dipole moment in the latter. In II, however, the
+
[Co2(pzc)2(H2O)4]2 double layers have one less coordinated water and subsequently exhibit a symmetric pattern
of covalent and hydrogen bonding to the [Mo5O16]2- sheets, leading to a centrosymmetric structure. Thermogravimetric
analyses and powder X-ray diffraction data reveal that I can be dehydrated and rehydrated with from 0 to 6.5 water
molecules per formula unit, which is coupled with a corresponding contraction/expansion of the interlayer distances.
Also, the dehydrated form of I can be intercalated by
by pyridine or methanol is limited to less than one molecule per formula unit.
∼
4.3 H2S molecules per formula unit, but the intercalation
Introduction
distribution. In these structures, the organic ligands prefer-
entially coordinate to late transition metals to form metal-
organic layers or pillars, which then additionally bond to
the early transition metals of oxide layers. Examples include
Layered solids continue to be an intense focus of research,
owing to the diversity of applications which derive from their
intercalation chemistry, such as those found in catalysts,
battery electrodes, in separations, or in sensors.1-3 Alterna-
tive perspectives on layered structures emphasize investiga-
tions of their low-dimensional electronic or magnetic proper-
ties, such as typified by charge or spin-driven electronic
transitions.4,5 Our efforts in these fields have been directed
toward layered solids that can exhibit both intercalative and
desirable electronic/magnetic properties and that can be
targeted via the syntheses of multilayered or pillared metal-
oxides/organics featuring a segregated heterometallic atom
2+
the layered vanadium oxides interleaved with Cu(pyz)2
(pyz ) pyrazine) square nets or with M(pyz)2+ (M ) Co,
Ni, Zn) layers of chains,6,7 or the layered pillared perrhenates
involving simple organic or M(pzc)2(H2O)2-x (M ) Co, Ni,
Cu; pzc ) pyrazinecarboxylate) metal-coordinated pillars.8-10
These heterometallic layered hybrids enable novel combina-
tions of the intercalative/absorptive-type properties of layered
solids, such as porosity and open metal sites, together with
the expression of optical, electrical, and magnetic properties
of low-dimensional structures. Further, their organic com-
ponents allow for a wide range of functionality and possible
structural modifications.
* To whom correspondence should be addressed. E-mail:
Paul_Maggard@ncsu.edu.
(1) Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.;
Academic Press: New York, 1982.
(2) Pillared Layered Structures: Current Trends And Applications;
Mitchell, I. V., Ed.; Elsevier Applied Science, New York, 1990.
(3) Inorganic Materials; Bruce, D. W., O’Hare, D., Eds.; Wiley: Chich-
ester, 1996.
(4) New Horizons in Low-Dimensional Electron Systems; Aoki, H.,
Tsukada, M., Schlu¨ter, M., Le´vy, F. A., Eds.; Kluwer Academic
Publishers: Norwell, MA, 1991.
(6) Maggard, P. A.; Boyle, P. D. Inorg. Chem. 2003, 42, 4250.
(7) Yan, B.; Luo, J.; Dube, P.; Sefat, A. S.; Greedan, J.; Maggard, P. A.
Inorg. Chem. 2006, submitted for publication.
(8) Lin, H.; Yan, B.; Boyle, P. D.; Maggard, P. A. J. Solid State Chem.
2006, 179, 39.
(9) Yan, B.; Capracotta, M. D.; Maggard, P. A. Inorg. Chem. 2005, 44,
6509.
(5) Strong Interactions in Low Dimensions; Baeriswyl, D., Degiorgi, L.,
Eds.; Kluwer Academic Publishers: Norwell, MA, 1995.
(10) Maggard, P. A.; Yan, B.; Luo, J. Angew. Chem., Int. Ed. 2005, 44,
2553.
10.1021/ic060253e CCC: $33.50
Published on Web 05/09/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 12, 2006 4721

Yan and Maggard
amounts of in-house deionized water were also used in the
syntheses.
Hydrothermal synthetic techniques have contributed greatly
to the growth and number of known hybrid oxides/organics
in the vanadate, tungstate, and molybdate families.11-17
Metal-oxide layers found in these hybrid structures are often
related, or even isostructural, to those found in simpler
inorganic solids. For example, in the molybdate family, a
number of MoO3-derived layers is known, some of which
have been pillared by simple organics and investigated for
their intercalative properties.18-23 A representative system
is A2MonO3n+1 (A ) alkali metal; n ) positive integer) which
features ReO3-type molybdate layers with n number of
corner-shared MoO6 octahedra spanning the width of the
layer. The n ) 5 member contains a [Mo5O16]2- layer that
Synthesis. The hydrothermal synthesis of [Ni2(pzc)2(H2O)5]-
[Mo5O16] (I) was performed by combining nickel oxide (0.038 g,
0.52 mmol), pyrazinecarboxylic acid (0.064 g, 0.52 mmol), and
molybdenum trioxide (0.186 g, 1.29 mmol) in deionized water (3.7
mL) in a 1:1:2.5:400 molar ratio. The reactants were heat-sealed
inside FEP Teflon pouches. The Teflon pouches were then placed
inside a Teflon-lined stainless steel reaction vessel that was
backfilled with water (1/3 volume) before closing. This reaction
vessel was heated inside a convection oven to 160 °C for 72 h,
then to 165 °C for 48 h, and slowly cooled over 10 h to room
temperature. Green-colored platelets were obtained in ∼70% yield
based on the loaded stoichiometry, and these were filtered, washed,
and dried at 40 °C. The product was single phase according to the
powder X-ray diffraction data.
24
was first reported in Cs2Mo5O16 and also was recently
hydrothermally synthesized within both (C2H10N2)[Mo5O16]
and (C4H12N2)[Mo5O16].25 The latter structures result from
the replacement of interlayer Cs+ for organic cations. These
layered structures might be expected to exhibit microporosity
or intercalative-type properties, though none have been
reported. Thus, the hydrothermal synthesis of multilayered
molybdate/metal-organics was investigated in order to ex-
plore different and new types of interlamellar bonding
interactions in order to understand their effect on the
structures and properties of the molybdate layers.
The syntheses of new multilayered molybdates containing
[Mo5O16]2- layers and interlamellar [M2(pzc)2(H2O)x]2+ (I:
M ) Ni, x ) 5.0; II: M ) Co, x ) 4.0) double layers of
chains are reported herein. The interlayer metal-oxide/organic
interactions have been investigated for their role in inducing
a net polarization in the [Mo5O16]2- layers for I but which
are symmetric for II. Further, the activity of I toward small
molecule intercalation has been tested for H2O, H2S, meth-
anol, and pyridine. The employed characterization methods
included powder and single-crystal X-ray diffraction, ther-
mogravimetric analysis, and UV-vis diffuse reflectance.
The synthesis of [Co2(pzc)2(H2O)4][Mo5O16] (II) was performed
according to similar procedures as described for I above, with the
exception of replacing NiO with Co(OH)2. The reactants were
heated to 160 °C for 48 h and then slowly cooled to room
temperature over 24 h. The products exhibited very thin platelet
morphologies and were colored light orange/red. These platelets
were found to be too small for a single-crystal X-ray analysis, and
therefore, many different reaction conditions were tested to obtain
more suitable crystals. However, longer reaction times of up to
240 h or higher reaction temperatures of up to 180 °C result in the
conversion of these small platelets to CoMo4O13(H2O)2.26 In
addition, both shorter reaction times or lower temperatures yield
unreacted starting materials. Thus, as II slowly transforms over
time to an alternate phase, it appears to be a kinetic intermediate
that can only be obtained as very small crystallites. The purity of
II is >95% according to the powder X-ray diffraction data.
Single-Crystal Structure Determination. A single-crystal data
set for I was collected at 293K on a Bruker-Nonius Apex2 CCD
diffractometer using monochromatized Mo KR1 radiation (λ )
0.71073 Å). The crystal was fixed on a 20-µm nylon loop with a
small amount of immersion oil. The unit cell dimensions were
determined from a symmetry-constrained fit of 2878 reflections
for a 2θ range of 5.9-54.9°. The data collection employed a
combination of ω and æ scans to collect data up to 2θ ) 55.64°.
The integration of frames was performed using SAINTPLUS27
(12 410 total reflections, 5133 unique, Rint ) 0.0365), and the raw
data were corrected for absorption using SADABS.28 The structure
was solved and refined in the Bruker SHELXTL29 software package
in the monoclinic space group Cc, and the final space group
assignment was confirmed using the software program PLATON.30
Hydrogen atoms were introduced at idealized positions and were
allowed to ride on the parent carbon positions. The structure
refinement was performed using a full matrix least-squares based
on F2. The final least-squares cycle involved the anisotropic
Experimental Section
Materials. All starting materials were used as received from the
chemical supplier, including MoO3 (99.9995%), pyrazinecarboxylic
acid (99%), Co(OH)2 (99.9%+), and NiO (99.99%+). Reagent
(11) Zheng, L.-M.; Wang, X.; Wang, Y.; Jacobson, A. J. J. Mater. Chem.
2001, 11, 1100-1105.
(12) Devi, R. N.; Rabu, P.; Golub, V. O.; O’Connor, C. J.; Zubieta, J.
Solid State Sci. 2002, 4, 1095-1102.
(13) Khan, M. I.; Yohannes, E.; Nome, R. C.; Ayesh, S.; Golub, V. O.;
O’Connor, C. J.; Doedens, R. J. Chem. Mater. 2004, 16, 5273-5279.
(14) Shan, Y.; Huang, R. H.; Huang, S. D. Angew. Chem., Int. Ed. 1999,
38, 1751-1754.
(15) Liu, S.; Xie, L.; Gao, B.; Zhang, C.; Sun, C.; Li, D.; Su, Z. Chem.
Commun. 2005, 5023-5025.
(16) Rarig, R. S., Jr.; Lam, R.; Zavalij, P. Y.; Ngala, J. K.; LaDuca, R. L.
Jr.; Greedan, J. E.; Zubieta, J. Inorg. Chem. 2002, 41, 2124-2133.
(17) Clemente-Leo´n, M.; Coronado, E.; Gala´n-Mascaro´s, J.-R.; Gime´nez-
Saiz, C.; Go´mez-Garcia, C. J.; Ferna´ndez-Otero, T. J. Mater. Chem.
1998, 8(2), 309-312.
(24) Gatehouse, B. M.; Miskin, B. K. Acta Crystallogr. 1975, B31, 1293.
(25) Guillou, N.; Fe´rey, G. J. Solid State Chem. 1999, 147, 240.
(26) Eda, K.; Uno, Y.; Nagai, N.; Sotani, N.; Chen, C.; Whittingham, M.
S. J. Solid State Chem. 2006, published online.
(27) SAINT-Plus Ver. 6.22 Data Processing for SMART System; Bruker
Analytical X-ray Instruments, Inc.: Madison, WI.
(28) Sheldrick, G. M. SADABS Ver. 2.03, Software for Area Detector
Absorptions and Other Corrections; Bruker Analytical X-ray Instru-
ments, Inc.: Madison, WI.
(29) Sheldrick, G. M. SHELXTL NT Ver. 6.10, Software Package for the
Refinement of Crystal Structures; Bruker Analytical X-ray Instruments,
Inc.: Madison, WI.
(18) Haber, J.; Lalik, E. Catal. Today 1997, 33, 119-137.
(19) Hosono, K.; Matsubara, I.; Murayama, N.; Woosuck, S.; Izu, N. Chem.
Mater. 2005, 17, 349-354.
(20) Goward, G. R.; Kerr, T. A.; Power, W. P.; Nazar, L. F. AdV. Mater.
1998, 10, 449-452.
(21) Wei, X. M.; Zeng, H. C. Chem. Mater. 2003, 15, 433-442.
(22) Hagrman, P. J.; LaDuca, R. L., Jr.; Koo, H.-J.; Rarig, R., Jr.;
Haushalter, R. C.; Whangbo, M.-H.; Zubieta, J. Inorg. Chem. 2000,
39, 4311.
(30) Spek, A. L. PLATON-A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands. Spek, A. L. Acta Crystallogr.
A 1990, 46, C34.
(23) Xu, Y.; Lu, J.; Goh, N. K. J. Mater. Chem. 1999, 9, 1599.
4722 Inorganic Chemistry, Vol. 45, No. 12, 2006

New Multilayered Hybrid Molybdates
Table 1. Selected Crystal and Refinement Data for Ia
Table 2. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Parameters (Å2, ×103) for I
empirical formula
Ni2(C5N2O2)2(H2O)5Mo5O16
fw
1189.39
Cc, 4
atom
x
y
z
U(eq)a
space group, Z
a, Å
b, Å
c, Å
Mo1
Mo2
Mo3
Mo4
Mo5
Ni1
Ni2
O1
O2
O3
O4
O5
3117(1)
2187(1)
2512(1)
1921(1)
2861(1)
4137(1)
5777(1)
4674(2)
4421(2)
5509(1)
5235(2)
3589(2)
2837(2)
3060(2)
3237(2)
2224(2)
2110(2)
1710(2)
2469(2)
2363(2)
1955(2)
2657(2)
2564(2)
1798(2)
1478(2)
2928(2)
3347(2)
3876(2)
4703(2)
5198(2)
5998(1)
6347(2)
4153(2)
4101(2)
5786(2)
5753(2)
4312(2)
4348(2)
3925(2)
3902(2)
4490(2)
5587(2)
5568(2)
5954(2)
5971(2)
5427(2)
7033(1)
8279(1)
3585(1)
7201(1)
8354(1)
4786(2)
10397(2)
9917(9)
7880(8)
7306(8)
4021(1)
4346(1)
2544(1)
1082(1)
795(1)
10(1)
9(1)
33.217(4)
5.6416(8)
13.982(2)
99.407(8)
2585.0(6)
3.893
3.056
12 410
5133/2/421
0.997
0.030, 0.065
10(1)
11(1)
10(1)
13(1)
13(1)
23(1)
18(1)
16(1)
21(1)
17(1)
11(1)
12(1)
19(1)
12(1)
17(1)
15(1)
14(1)
14(1)
12(1)
11(1)
10(1)
18(1)
18(1)
15(1)
19(1)
22(1)
21(1)
19(1)
16(1)
17(1)
14(1)
15(1)
13(1)
15(1)
14(2)
21(2)
17(2)
19(2)
16(2)
11(1)
17(2)
19(2)
15(2)
15(2)
â, °
V, Å3
4977(1)
2779(1)
3584(4)
4713(4)
3053(3)
4170(4)
4752(4)
4915(4)
3118(4)
3341(4)
5235(4)
5120(4)
3727(4)
3502(4)
3396(4)
1975(4)
1721(4)
1622(4)
1794(4)
260(4)
µ (Mo KR), mm-1
d
calc, g cm-3
reflns (total)
data/ restraints/params
GOF
5308(9)
final R1, wR2 [I > 2σ(I)]b
6404(10)
8862(10)
4486(10)
9299(11)
10983(10)
6014(10)
8776(10)
6708(9)
1691(9)
4593(10)
1689(9)
6735(9)
9415(10)
6871(9)
O6
O7
O8
O9
a Refined cell parameters for II, in C2/c, were a ) 35.42(6) Å, b )
5.697(9) Å, c ) 14.28(2) Å, â ) 114.95(4)°, and V ) 2614(12) Å3. b R1
2
-2
) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo - Fc)2]/∑[wFo ]}1/2, w ) σF
.
O10
O11
O12
O13
O14
O15
O16
O17
O18
O19
O20
O1w
O2w
O3w
O4w
O5w
N1
N2
N3
N4
C1
C2
C3
C4
C5
refinement of all non-H atoms and which converged to R1/wR2 )
0.0304/0.0652 and a GOF ) 0.997 for 4393 reflections (I > 2σ(I))
and 421 parameters. The Flack parameter, which refined to
0.53(2), was indicative of racemic twinning. Some data collection
and refinement parameters are given in Tables 1 and 2, and selected
interatomic distances are compiled in Table 3.
Powder X-ray Diffraction. High-resolution powder X-ray
diffraction (PXRD) data were collected in transmission mode on
an INEL diffractometer using Cu KR1 (λ ) 1.54056 Å) radiation
from a sealed-tube X-ray generator (35 kV, 30 mA) and a curved
position-sensitive detector. All attempts at indexing the unit cell
and refining the structure of II were performed using the software
program GSAS.31 During the refinements, the structural models
that were tested included the noncentrosymmetric structure found
for I and the known centrosymmetric (C4H12N2)[Mo5O16] struc-
ture.25 The model with the best fit to the data, with wRp/Rp )
0.103/0.066, was found in the centrosymmetric space group C2/c
and yielded the formula [Co2(pzc)2(H2O)4][Mo5O16]. However, the
precise atomic positions and thermal ellipsoids could not be refined,
owing to the very high background and the asymmetric peak
profiles. The unit cell refinement is given in the footnotes to Table
1, and additional refinement and structural details are available in
the Supporting Information.
6153(10)
8817(11)
1565(9)
2(4)
1392(4)
5068(4)
5209(4)
2529(3)
2552(3)
3064(4)
1506(4)
3466(4)
4289(4)
6267(4)
3115(5)
2147(6)
1847(5)
2833(5)
3850(5)
4639(5)
5609(5)
5926(5)
4925(5)
3918(5)
3116(9)
12003(9)
13620(8)
8799(8)
4873(10)
4615(10)
10693(10)
10317(10)
6320(12)
6442(15)
3230(12)
3074(13)
8177(12)
8918(12)
8737(13)
12074(12)
12280(13)
7025(12)
C6
C7
C8
C9
C10
Bulk Characterization Techniques. Thermogravimetric analy-
ses of I and II were taken on a TA Instruments TGA Q50 under
flowing nitrogen at a heating rate of 5 °C/min. The UV-vis diffuse
reflectance spectra (DRS) were taken on powdered samples
mounted onto a fused-silica slide that was placed along the external
window of an integrating sphere inside a Cary 300 spectropho-
tometer. A pressed poly(tetrafluoroethylene) powder was used as
a reference, and the data were plotted as the remission function
F(R∞) ) (1 - R∞)2/(2R∞), where R is diffuse reflectance based on
the Kubelka-Monk theory of diffuse reflectance.
a U(eq) is defined as one-third of the trace of the orthogonalized Uij
tensor.
but the asymmetric and broad peak widths are indicative of
either structural disorder or smaller crystallite sizes. The
PXRD data of I and II are shown in Figure 1a for
comparison. As the water content of these layered structures
is highly variable (see below thermogravimetric analysis
(TGA) and PXRD data for I and II), the X-ray powder
refinement and TGA results were together used to assign
the chemical formula of II as [Co2(pzc)2(H2O)4][Mo5O16].
However, II appears to be a kinetic intermediate because it
slowly transforms over longer reaction times to CoMo4O13-
(H2O)2.26 Synthetic attempts in alternative organic solvents
were unsuccessful.
Structural Description. The structure of [Ni2(pzc)2-
(H2O)5][Mo5O16], shown in Figure 2 down the c axis, consists
of molybdate [Mo5O16]2- layers with double layers of
[Ni2(pzc)2(H2O)5]2+ chains in the interlamellar regions. The
[Mo5O16]2- layers stack down the a axis and are constructed
from vertex-shared MoO6 octahedra to give a ReO3-type slab
that is five octahedra wide and one octahedron thick. Each
Results and Discussion
Syntheses. The hydrothermal reaction of MoO3, NiO, and
pyrazinecarboxylate at 160-165 °C for 5 days yields
greenish-blue crystals with the composition [Ni2(pzc)2(H2O)5]-
[Mo5O16] (I). However, an analogous synthetic procedure,
except with the substitution of Co for Ni, results in light
orange/red colored crystals that are unsuitable for single-
crystal X-ray analysis. The PXRD data for the M ) Co phase
(II) show that it has a structure closely related to that of I,
(31) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System
(GSAS); Los Alamos National Laboratory Report LAUR 86-748, 2000.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4723

Yan and Maggard
Table 3. Selected Interatomic Distances (Å) and Bond Valence Sumsa
for I
Å), and two short (1.680(6)-1.764(6) Å) Mo-O contacts
and are consistent with prior known molybdate structures.24,25
The calculated bond valence sums are listed in Table 3 and
confirm the assignment of a Mo6+ oxidation state. Six
symmetry-unique Mo atoms are located within each molyb-
date layer, Figure 3. The number of vertex-shared oxygen
on each MoO6 ranges from all shared for Mo3 (i.e., none
are terminal), one terminal oxygen vertex for Mo2 and Mo5,
and two terminal oxygen vertexes for Mo1 and Mo4. The
terminal oxygen vertexes of the MoO6 octahedra are located
near the edges of the molybdate sheet. This layer is similar
in structure to that of Cs2Mo5O16 but with the addition of a
small structural distortion that destroys a previous inversion
relation between the Mo2/Mo5 and Mo1/Mo4 pairs.
distance,
Å
distance,
Å
distance,
atom atom Å
atom atom
atom atom
Mo1 O5
O6
1.764(6) Mo3 O7
1.934(6) Mo5 O6
1.989(5)
2.167(5)
1.967(5)
1.901(5)
1.680(6)
O12 2.231(5) O9
O7
O8
O13 1.731(5)
O14 1.973(5)
O15 1.698(5)
O16 2.219(5)
ΣSij ) 5.99
1.963(5)
O10 2.403(6)
O14 1.920(5)
O16 2.161(6)
O17 1.688(5)
O18 1.722(6)
ΣSij ) 6.06
O15 2.442(5)
O16 1.876(6)
O19 1.704(5)
O20 1.710(6)
ΣSij ) 6.06
O12 2.164(6)
O19 2.405(5)
ΣSij ) 5.96
Mo2 O6
O9
2.201(5) Mo4 O9
1.959(6)
O10 1.720(6)
O11 1.698(6)
O12 1.849(6)
O13 2.463(5)
ΣSij ) 6.14
The [Ni2(pzc)2(H2O)5]2+ double layers are a remarkable
structural feature of I that both lead to its polar symmetry
and also regulate its intercalation chemistry. Two types of
Ni-pzc chains occur, [Ni(pzc)(H2O)2]1+ and [Ni(pzc)-
(H2O)3]1+, and stack in layers oriented in opposite directions
between the molybdate sheets. Both types of chains can be
compared in either Figure 2 or Figure 4. Each Ni2+ is
chelated by a nitrogen and oxygen on one side of the pzc
ligand and is also coordinated (trans) to a lone nitrogen atom
on a second ligand, Figure 4. The orientation of the pzc
ligand is twisted by 67.4(2)° for Ni2 and 73.8(2)° for Ni1,
and the chains deviate from linearity owing to the chelation
of the ligand. The three remaining coordination sites on each
Ni2+ are in a mer configuration. In the Ni2 chain (Figure
4b), these sites are bonded to three water molecules at
2.104(5), 2.023(5), and 2.006(5) Å to give [Ni(pzc)(H2O)3]1+.
For the Ni1 chain (Figure 4a), two of the mer sites are bonded
to water molecules at 2.025(5) and 2.081(5) Å to give
[Ni(pzc)(H2O)2]1+. Both the Ni1 and Ni2 chains form
hydrogen bonds to neighboring chains within the bc plane
via the coordinated water and carboxylate groups at distances
of ∼2.8-2.9 Å for OH2‚‚‚O (between O1W-O4W and
O1-O4). These two layers are also hydrogen bonded to
each other and form the double-layer (O2W-H‚‚‚O4 and
O3W-H‚‚‚O1 at 2.758(8) and 2.726(8) Å, respectively),
drawn as thin lines in Figure 2. The remaining coordination
site on Ni1 is bonded to the [Mo5O16]2- layer, Ni1-O5 at
2.012(6) Å, while the equivalent site on Ni2 is coordinated
to a water molecule. Therefore, the [Ni2(pzc)2(H2O)5]2+
double layer is coordinated to the [Mo5O16]2- layer through
an asymmetric pattern of Ni-O and Ni-OH2 hydrogen
bonds for [Ni(pzc)(H2O)2]1+ (Ni1) or solely Ni-OH2 hy-
drogen bonds for [Ni(pzc)(H2O)3]1+ (Ni2). The asymmetric
Ni-O bonds of the [Ni2(pzc)2(H2O)5]2+ layers align down
the a axis and result in the polar space group Cc rather than
C2/c as found for Cs2Mo5O16.
Ni1 O2
O5
2.047(5) Ni2 O3
2.023(5)
O3w 2.104(5)
O4w 2.006(5)
O5w 2.074(5)
2.012(6)
O1w 2.025(5)
O2w 2.081(5)
N1
N2
2.139(6)
2.098(6)
N3
N4
2.113(6)
2.141(6)
a
Sij ) e[(R -R)/B], B ) 0.37, Ro ) 1.908 for Mo(VI).32,33
o
Figure 1. Powder X-ray diffraction patterns of [M2(pzc)2(H2O)x][Mo5O16]
for I (a, M ) Ni) and II (b, M ) Co).
Figure 2. ∼[010] view of the [Ni2(pzc)2(H2O)5][Mo5O16] layered structure
with the unit cell outlined. Red atoms ) O, light blue ) Ni, dark blue )
N, white ) C, and yellow polyhedra ) MoO6.
The tentative structure of II, Figure 5, consists of
[Mo5O16]2- molybdate layers separated by [Co2(pzc)2-
(H2O)4]2+ double layers of chains. In contrast to I, the Co
atoms in the double layers are now symmetry equivalent
because each is coordinated to two water molecules and to
the [Mo5O16]2- layer through one Co-O bond. Thus, the
structure of II may be viewed as arising from I with the
loss of one coordinated water molecule on the Ni2 chain.
5 × 1 ReO3-type slab is condensed via octahedral edges to
identical neighboring 5 × 1 slabs which are overlayed by
two and a half octahedra to form a stair-stepped sheet. The
bond lengths around each MoO6 octahedron, listed in Table
3, occur in a distinctive pattern and within the expected range
for two long (2.161-2.463 Å), two medium (1.849-1.989
4724 Inorganic Chemistry, Vol. 45, No. 12, 2006

New Multilayered Hybrid Molybdates
Figure 3. Structural view perpendicular to the [Mo5O16]2- layers in I. Red atoms ) O and green atoms ) Mo.
Figure 5. ∼[010] view of the structural model for the centrosymmetric
Figure 4. Structural view perpendicular to the two types of Ni-pzc layers,
(a) [Ni(pzc)(H2O)2]+ and (b) [Ni(pzc)(H2O)3]+. Red atoms ) O, light blue
) Ni, dark blue ) N, and white ) C.
structure of II, [Co2(pzc)2(H2O)4[Mo5O16]. The unit cell is outlined and
the red atoms ) O, light blue ) Ni, dark blue ) N, white ) C, and yellow
polyhedra ) MoO6.
This structural change leads to a symmetric Co-O bonding
pattern on both sides of the [Mo5O16]2- layer. Or alterna-
tively, the intercalation of an additional water molecule from
II to I occurs in an asymmetric fashion. These basic structural
features are generally confirmed in the model obtained from
powder X-ray data, but the precise interatomic distances and
thermal ellipsoids cannot be determined, owing to some
structural disorder and the variable water content (below).
While the [Ni2(pzc)2(H2O)5]2+ chains, described above, are
clearly asymmetric in their bonding to the [Mo5O16]2- layer,
it is not obvious if any net dipole moment is induced across
the [Mo5O16]2- layer itself. Local dipole moments of the
MoO6 octahedra were calculated for both Cs2Mo5O16 and I
using a combination of the well-known Debye equation (µ
) neR) and the charge assignments obtained from bond
valence sums.35 The results are compiled in Table 4. Absolute
Table 4. Calculated Local Dipole Moments for Each MoO6 Octahedra
in Cs2Mo5O16 and I, Together with the Hydrogen Bonding Interactions
for Eacha
Cs2Mo5O16
[Ni2(pzc)2(H2O)5][Mo5O16]
site MoO6 dipoleb equiv site MoO6 dipoleb H-bonds to MoO6, Å
Mo1
Mo2
6.63
7.77
Mo3
Mo1
Mo4
Mo2
Mo5
6.78
7.00
7.41
8.35
8.47
1.83
2.31, 1.76 (Ni-O)
1.88, 2.23, 2.13
1.90
Mo3
8.03
1.88
net moment: -5.16 (x axis), 1.29 (z axis)
a Calculated for each local MoO6 octahedron according to ref 35. b Dipole
moments are given in units of debye (10-18 esu cm).
values of the dipole moments should be viewed with caution
here, but their relative sizes show a clear relationship to the
number of interlayer hydrogen bonds and Ni-O bonding
(32) Brown, I. D.; Altermatt, D. Acta Crystallogr. B 1985, 41, 244-247;
Brese, N. E.; O’Keefe, M. Acta Crystallogr. B 1991, 47, 192-197.
(33) Hormillosa, C. Bond Valence Calculator, Version 2.00; McMaster
University: Hamilton, Canada, 1993.
(34) Wessels, A. L.; Jeitschko, W. J. Solid State Chem. 1997, 128, 205-
208.
(35) Maggard, P. A.; Nault, T. S.; Stern, C. L.; Poeppelmeier, K. R. J.
Solid State Chem. 2003, 175, 27.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4725

Yan and Maggard
interactions. The Mo1- and Mo4-centered MoO6 octahedra,
which are related by inversion symmetry in Cs2Mo5O16,
participate in the largest number of interlayer hydrogen bonds
and a Ni-O bond. Also, both Mo1 and Mo4 exhibit a
significant reduction in their dipole moments compared to
the equivalent site (Mo2) in Cs2Mo5O16 (i.e., 7.77 D in
Cs2Mo5O16 vs 7.00 and 7.41 D in I; D ) 10-18 esu cm). For
Mo2, Mo3, and Mo5, which are located more internally
within the layer and have a single hydrogen-bonding interac-
tion, a larger MoO6 dipole moment is observed (i.e., 6.78,
8.35, and 8.47 D for Mo3, Mo2, and Mo5 in I vs 6.63 and
8.03 D for Mo1 and Mo3 in Cs2Mo5O16). Note the relatively
small difference in dipole moments between Mo2 and Mo5
compared to much larger difference shown above between
Mo1 and Mo4, Table 4. The net dipole moment of the
[Mo5O16]2- layer in I is calculated to be -5.16 and 1.29 D
down the x axis and z axis, respectively, while in Cs2Mo5O16
there is no net dipole moment. Thus, the equivalent of
approximately one asymmetric MoO6 octahedron per formula
unit does not self-cancel. These results are consistent with
the space group symmetries of each, as the space group Cc
for I restricts the dipole moment to lie within the ac plane,
while the space group C2/c for Cs2Mo5O16 disallows a net
overall dipole moment.
Figure 6. The thermogravimetric analysis of I (a) and II (b) plotted as
wt % versus temperature.
Optical Properties. The UV-vis diffuse reflectances of
both I and II were measured to characterize their optical
absorption wavelengths and are provided in the Supporting
Information. The optical band gaps, extrapolated from the
linear rise in absorption, occur at ∼420 nm or 2.95 eV for
I and ∼430 nm or 2.88 eV for II. In previously reported
molybdates, this optical absorption originates from a ligand-
to-metal charge transfer between O2- and Mo6+ that is
typically around 2.8 eV.36 The absorptions by the d-d
transitions are weaker and are centered around 480 nm for
II, and a very weak and broad absorption is centered around
650 nm for I. The absorption wavelengths are confirmed by
their complementary red and blue crystal colors, respectively.
Powdered samples of both I and II were also screened for
second harmonic generation, but no activity was detected
down to ∼50% of the SiO2 signal. However, the tentative
structural model for II is centrosymmetric and the intense
sample colors suggests they might likely absorb much of
the frequency-doubled light.
Thermogravimetric Analyses. The structures of both I
and II contain relatively large amounts of water coordinated
to the late transition metals. The TGA plot of I, Figure 6,
shows a weight loss at ∼225-250 °C that corresponds with
the removal of five coordinated water molecules per formula
unit (obsd 7.8%, calcd 7.6%), in agreement with the
composition. The TGA product after this step is a dehydrated
layered structure of I, as described in the next section. A
second broad weight loss occurs at ∼350-500 °C, corre-
sponding to the decomposition of the organic ligand and
concomitant reduction of Ni2+ and [Mo5O16]2- to Ni and
MoO2 (obsd 27.4%, calcd 28.8%). The latter products were
identified via PXRD data of the TGA residues and given in
Figure 7. Powder X-ray diffraction patterns of I as synthesized (a), and
after dehydration (b), rehydration (c), and intercalation by H2S (d), methanol
(e), and pyridine (f).
the Supporting Information. The molybdate layer is likely
reduced by a reaction with the organic ligand, as investi-
gated previously using tandem MS/TGA techniques in
hybrid vanadate structures.7 In the temperature range of
∼225-250 °C, a weight loss of ∼6.8% occurs similarly for
II, but which is slightly higher than that expected for the
removal of approximately four water molecules per formula
unit (calcd 6.1%). However, II has a large possible range in
water content, as investigated for I (below). Thus, the
compositions of both I and II are dependent upon the sample
preparation and history.
Small Molecule Intercalation. The layered structure of
[Ni2(pzc)2(H2O)5][Mo5O16] suggested that the intercalation
of small molecules into the interlamellar regions might be
possible. Further, the complete dehydration of I can be
accomplished by heating under flowing N2 at 230 °C for 3
h, resulting in a weight loss of 7.5% (calcd 7.6%) and a
change in color from cyan to grass green. The PXRD patterns
of both the original and dehydrated forms of I, Figure 7a
and b, show a first intense diffraction peak for the interlayer
(200) d-spacing at 16.39 and 15.13 Å, respectively. Table 5
lists the interlayer d-spacings for the dehydrated structure
of I, as well as all intercalated products. Upon water removal,
the interlayer distance contracts by ∼1.2 Å and the higher-
angle diffraction peaks are both weaker and broader. The
latter likely arises from packing disorder between the layers.
When the dehydrated sample of I is soaked in water for a
(36) Zhang, H. J.; Boutinaud, P.; Garcia, A.; Chaminade, J. P.; Fouassier,
C. Solid State Comm. 1993, 85, 1031.
4726 Inorganic Chemistry, Vol. 45, No. 12, 2006

New Multilayered Hybrid Molybdates
Table 5. Interlayer (200) d-Spacings of I before and after the
of H2S in the interlayer region of I is likely similar to that
for H2O, as it can both coordinate to the Ni sites and (more
weakly) hydrogen bond to the [Mo5O16]2- layers. By
comparison, the intercalation of methanol into I was rela-
tively limited. The product exhibited a very small shift in
its interlayer spacing to 15.44 Å and showed a small
subsequent weight loss (∼2.7%) that was consistent with ∼1
intercalated methanol molecules per formula unit. Methanol
will not coordinate very well to Ni2+ under these aqueous
conditions, and as well, its limited ability to participate in
hydrogen bonds may prohibit its uptake into the interlayer
spaces. Pyridine also shows relatively limited intercalation
into the structure, though it has been found to coordinate
readily to Ni2+. The pyridine-intercalated product exhibited
two different interlayer d-spacings at 16.30 and 15.37 Å.
The splitting of this main diffraction peak indicates that the
intercalation by pyridine either is not complete or that a new
structure is forming. The TGA results on the products show
a small weight loss (∼4.2%) consistent with ∼0.6 pyridine
molecules being removed on average per formula unit. The
diffusion of pyridine into the interlayer spaces may be
restricted, owing to its larger size. Thus, the intercalation of
I shows a clear preference for smaller molecules that can
participate in hydrogen bonding, as well as coordinate to
Ni2+, and which is reflected in the types of available bonding
interactions within the interlayer spaces.
Intercalation of Guest Moleculesa
compound I
2θ, deg
5.389
5.836
5.124
d(200), Å
as synthesized
dehydrated
rehydration
H2S
16.39
15.13
17.23
19.14
4.612
methanol
pyridine
5.720
5.418, 5.744
15.44
16.30, 15.37
a The data are derived from the first intense reflections in Figure 7.
Figure 8. Thermogravimetric analysis at 230 °C for I (a), and after
intercalation of the dehydrated structure by H2O (c), H2S (d), methanol
(e), and pyridine (f). The labeling has been kept consistent with Figure 7.
few minutes, the sample color changes back to cyan to give
a rehydrated compound. The PXRD pattern, Figure 7c, shows
the (200) diffraction peak is shifted to a lower angle and
that the interlayer spacing has increased by ∼0.8 Å compared
to I. The TGA data of the rehydrated sample, Figure 8c,
exhibit a 10.1% weight loss that is consistent with the
removal of ∼6.5 water molecules per formula unit. The
resulting formula for the rehydrated sample is [Ni2(pzc)2-
(H2O)6.5][Mo5O16]. Further, I can be cycled between the fully
dehydrated and rehydrated states with from 0 to ∼6.5 water
molecules per formula unit. However, the structural orga-
nization and bonding of the interlayer water molecules must
be somewhat changed in order to accommodate the extra
∼1.5 water molecules compared to I. A possible mechanism
is the reversible insertion of H2O into either the Ni-O (to
MoO6) or OH2‚‚‚O (to carboxylate groups) bonds of I.
Similar dehydration/rehydration experiments for II were not
investigated, owing to the lack of a fully refined structure
for use in evaluating the data.
Alternative small molecule guests, including H2S, metha-
nol, and pyridine, were tested for their intercalation into the
dehydrated form of I. The dehydrated sample was exposed
to the guest molecules at room temperature by soaking for
∼20 h in either methanol or pyridine or by placing it in an
atmosphere of H2S for 17 h. The PXRD and TGA data of
the products are shown in Figure 7d-f and Figure 8d-f.
The calculated (200) d-spacings are listed in Table 5. After
the dehydrated sample was placed in an H2S atmosphere,
its color changed to gray-blue and the PXRD data revealed
a large increase in the interlayer d-spacing to 19.14 Å. The
TGA results on the intercalated product, Figure 8d, show a
weight loss (∼11.8%) consistent with the removal of ∼4.3
H2S molecules per formula unit. The structural organization
Conclusions
The new multilayered hybrid molybdates, [M2(pzc)2-
(H2O)x][Mo5O16] (I: M ) Ni; II: M ) Co) contain
[Mo5O16]2- layers that alternate in stacking with metal-
organic [M2(pzc)2(H2O)x]2+ double layers of chains. The
noncentrosymmetric structure of I is induced by an asym-
metric pattern of covalent and hydrogen bonding of the
[Ni2(pzc)2(H2O)5]2+ layers to the [Mo5O16]2- sheets. How-
ever, the structure of II is centrosymmetric, owing to the
loss of one water molecule per formula to yield [Co2(pzc)2-
(H2O)4]2+ layers. The structure of I can be fully dehydrated
and rehydrated up to a maximum of ∼6.5 water molecules
per formula unit. The dehydrated form of I also absorbs up
to ∼4.3 H2S molecules per formula unit. However, neither
methanol nor pyridine show appreciable intercalation into
the dehydrated structure, owing to their larger sizes and
weaker hydrogen bonding. The structures of these multilay-
ered hybrids enable a new and dynamic way to couple
intercalation selectivity with the physical properties that arise
from noncentrosymmetric solids. Further research is under-
way on the synthesis of new solids containing these interest-
ing [M2(pzc)2(H2O)x]2+ double layers.
Acknowledgment. Support from a Beckman Young
Investigator Award from the Beckman Foundation (P.A.M.),
an ACS-PRF grant (40963-G10), and assistance with the
single-crystal X-ray data (Paul Boyle) are acknowledged.
Supporting Information Available: Crystallographic data in
CIF format and PXRD data of the TGA products. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC060253E
Inorganic Chemistry, Vol. 45, No. 12, 2006 4727