A series of three-dimensional lanthanide coordination polymers with rutile and unprecedented rutile-related topologies

Article abstract of DOI:10.1021/ic050906b

The complexes of formulas Ln(pydc)(Hpydc) (Ln = Sm (1), Eu (2), Gd (3); H₂pydc = pyridine-2,5-dicarboxylic acid) and Ln(pydc)(bc)(H ₂O) (Ln = Sm (4), Gd (5); Hbc = benzenecarboxylic acid) have been synthesized under hydrothermal conditions and characterized by elemental analysis, IR, TG analysis, and single-crystal X-ray diffraction. Compounds 1-3 are isomorphous and crystallize in the orthorhombic system, space group Pbcn. Their final three-dimensional racemic frameworks can be considered as being constructed by helix-linked scalelike sheets. Compounds 4 and 5 are isostructural and crystallize in the monoclinic system, space group P2 ₁/c. pydc ligands bridge dinuclear lanthanide centers to form the three-dimensional frameworks featuring hexagonal channels along the a-axis that are occupied by one-end-coordinated be ligands. From the topological point of view, the five three-dimensional nets are binodal with six- and three-connected nodes, the former of which exhibit a rutile-related (4.6²) ₂(4²·6⁹·8⁴) topology that is unprecedented within coordination frames, and the latter two species display a distorted rutile (4.6²)₂(4²· 6¹⁰·8³) topology. Furthermore, the luminescent properties of 2 were studied.

Full text of DOI:10.1021/ic050906b

Inorg. Chem. 2005, 44, 7122−7129
A Series of Three-Dimensional Lanthanide Coordination Polymers with
Rutile and Unprecedented Rutile-Related Topologies
Chao Qin, Xin-Long Wang, En-Bo Wang,* and Zhong-Min Su*
Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal UniVersity,
Changchun 130024, P. R. China
Received June 5, 2005
The complexes of formulas Ln(pydc)(Hpydc) (Ln
acid) and Ln(pydc)(bc)(H2O) (Ln Sm (4), Gd (5); Hbc
hydrothermal conditions and characterized by elemental analysis, IR, TG analysis, and single-crystal X-ray diffraction.
Compounds 1 3 are isomorphous and crystallize in the orthorhombic system, space group Pbcn. Their final three-
)
Sm (1), Eu (2), Gd (3); H2pydc
) pyridine-2,5-dicarboxylic
)
)
benzenecarboxylic acid) have been synthesized under
−
dimensional racemic frameworks can be considered as being constructed by helix-linked scalelike sheets. Compounds
4 and 5 are isostructural and crystallize in the monoclinic system, space group P21/c. pydc ligands bridge dinuclear
lanthanide centers to form the three-dimensional frameworks featuring hexagonal channels along the a-axis that
are occupied by one-end-coordinated bc ligands. From the topological point of view, the five three-dimensional
nets are binodal with six- and three-connected nodes, the former of which exhibit a rutile-related (4.62)2(42
topology that is unprecedented within coordination frames, and the latter two species display a distorted rutile
(4.62)2(42 610 83) topology. Furthermore, the luminescent properties of 2 were studied.
‚ ‚
69 84)
‚
‚
Introduction
contrast to the fruitful production of MOFs with d-block
transition metal ions, the design and control over high-
dimensional lanthanide-based frameworks is currently a
formidable task5 owing to coordination diversity of lanthanide
ions. Whereas, lanthanides, with their high and variable
coordination numbers and flexible coordination environ-
The crystal engineering of metal-organic frameworks
(MOFs) is becoming an increasing popular field of research
in view of the potential applications and unusual topologies
of these new materials.1 Much work has focused on the
rational design of multidimensional infinite architectures by
controlling the favored geometry of ligands and metals, in
which the construction of transition-metal-carboxylate
polymers is a successful paradigm.2-4 Unfortunately, in
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Li, Z. J.; Yao, Y. G. Inorg. Chem. 2005, 44, 3386.
* To whom correspondence should be addressed. E-mail:
wangenbo@public.cc.jl.cn (E.-B.W.).
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7122 Inorganic Chemistry, Vol. 44, No. 20, 2005
10.1021/ic050906b CCC: $30.25
© 2005 American Chemical Society
Published on Web 09/08/2005

Three-Dimensional Lanthanide Coordination Polymers
ments, provide unique opportunities for discovery of unusual
network topologies,6 thus leading us to this interesting and
challenging field.
fore, a further investigation on the self-assembly of MOFs
having mixed nodes, besides enriching the database of
coordination polymers, will contribute to discovery of new
and previously unrecognized topologies.
The “network approach” or topological approach is a
powerful tool for the analysis, comparison, and design of
network structures.7 By reducing multidimensional structures
to simple node-and-connection reference nets, it plays an
essential role in structural simplification and subsequent
systematization. As stated by Wells, the crystallographer
generally describes patterns of atoms in terms of their
symmetries; namely, the description is geometrically based
on lengths and angles, whereas the approach of topological
is concerned with the way in which the points are connected
and with the numbers of edges of the polygons.7a Generally,
topological analysis of a crystal structure needs a simplified
process; that is, remove all the unnecessary elements that
have no topological relevance, leaving only the essentials,
and thus the crystal structure was reduced to an irreducible
net represented by points and lines linking them together.8
The notation for 2D or 3D nets is based on the analysis of
the “circuits”, i.e. the shortest path which starts from a point
along one link and returns to the point by another link,
characterized by the number q of edges in a loop. A
p-connected node (p is the number of connections to
neighboring nodes that radiate from any node, the minimal
value of which is 3) can be identified by a Schla¨fli symbol
(or point symbol) of the type qa.qb..., where the number of
circuits of each kind is shown as a superscript.7a,9g Network
topologies in coordination polymers have already been
discussed in several detailed reviews.9 Ockwig, O’Keeffe,
Yaghi, and co-workers have recently systematically analyzed
the underlying topologies of all 1127 three-dimensional (3D)
metal-organic frameworks reported in the Cambridge
Structure Database (CSD), and the statistical results show
that only 353 (31.3%) MOFs out of the 1127 refcodes have
different connectivities (3,4; 3,5; 4,5; 3,6; 4,6; 5,6).8 There-
In this regard, we chose pyridine-2,5-dicarboxylate (pydc)
as an organic spacer since this rigid molecule has proven to
be able to establish bridges between metal centers.10 Here,
we report on five three-dimensional binodal nets resulting
from the combination of six- and three-connecting nodes that
are Ln(pydc)(Hpydc) (Ln ) Sm (1), Eu (2), Gd (3)) with an
unprecedented (4.62)2(42‚69‚84) topology and Ln(pydc)(bc)-
(H2O) (Ln ) Sm (4), Gd (5); Hbc ) benzenecarboxylic acid)
with a rutile (4.62)2(42‚610‚83) topology. A search in the
Cambridge Structure Database reveals that only a 2D MOF
assembled from Ln-only (Ln ) lanthanides) and pydc2- has
been reported;11 the rest contain 3d-4f mixed metals.12
Therefore, the five compounds reported herein represent the
first three-dimensional series of a {Ln/pydc} system. Fur-
thermore, the luminescent properties of 2 were studied.
Experimental Section
Materials. All chemicals purchased were of reagent grade and
used without further purification. All syntheses were carried out in
20 mL Teflon-lined autoclaves under autogenous pressure. The
reaction vessels were filled to approximately 60% volume capacity.
Water used in the reactions is distilled water.
Synthesis of Sm(pydc)(Hpydc) (1). A mixture of Sm2O3 (87
mg, 0.25 mmol), H2pydc (167 mg, 1 mmol), HNO3 (0.2 mL, 4 M),
and water (10 mL) was heated at 140 °C for 5 days; colorless
crystals of 1 were obtained when cooling to room temperature at
10 °C/h (yield: 173 mg, 72% based on Sm). Anal. Calcd for
C14H7N2SmO8: C, 34.91; H, 1.47; N, 5.81. Found: C, 34.53; H,
1.25; N, 6.14. IR data (KBr, cm-1): 3032 w, 1720 m, 1628 w,
1610 m, 1597 s, 1562 s, 1487 m, 1406 s, 1369 s, 1286 m, 1263 w,
1184 m, 1159 m, 1039 m, 904 w, 831 m, 792 m, 760 s, 693 s, 649
m, 570 m, 521 m, 436 w.
Synthesis of Eu(pydc)(Hpydc) (2). An identical procedure with
1 was followed to prepare 2 except Sm2O3 was replaced by Eu2O3
(88 mg, 0.25 mmol) (yield: 181 mg, 75% based on Eu). Anal.
Calcd for C14H7N2EuO8: C, 34.80; H, 1.46; N, 5.79. Found: C,
34.55; H, 1.62; N, 5.38. IR data (KBr, cm-1): 3029 w, 1718 m,
1630 w, 1592 w, 1570 s, 1485 s, 1406 s, 1372 s, 1300 m, 1280 w,
1185 m, 1045 m, 1036 m, 904 w, 835 m, 791 m, 767 s, 686 s, 650
m, 529 s, 428 w.
Synthesis of Gd(pydc)(Hpydc) (3). An identical procedure with
1 was followed to prepare 3 except Sm2O3 was replaced by Gd2O3
(91 mg, 0.25 mmol) (yield: 171 mg, 70% based on Gd). Anal.
Calcd for C14H7N2GdO8: C, 34.42; H, 1.44; N, 5.73. Found: C,
34.79; H, 1.26; N, 5.70. IR data (KBr, cm-1): 3037 w, 1715 m,
1632 w, 1610 m, 1560 s, 1490 s, 1400 s, 1369 s, 1284 m, 1243 w,
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S.; Lee, S. W. Inorg. Chim. Acta 2001, 324, 293. (b) Garc´ıa-Zarracino,
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L. C. Inorg. Chem. Commun. 2005, 8, 22.
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Inorganic Chemistry, Vol. 44, No. 20, 2005 7123

Qin et al.
Table 1. Crystal Data and Structure Refinement for 1-5
param
1
2
3
4
5
empirical formula
C14H7N2SmO8
481.57
293(2)
0.710 73
orthorhombic
Pbcn
9.939(2)
8.6219(17)
15.731(3)
90
90
90
1348.0(5)
4
4.411
C14H7N2EuO8
483.18
293(2)
0.710 73
orthorhombic
Pbcn
9.969(2)
8.6644(17)
15.736(3)
90
90
90
1359.3(5)
4
4.669
C14H7N2GdO8
488.47
293(2)
0.710 73
orthorhombic
Pbcn
9.932(2)
8.6161(17)
15.717(3)
90
90
90
1344.9(5)
4
4.986
C14H10NSmO7
454.58
293(2)
0.710 73
monoclinic
P21/c
9.2735(19)
14.244(3)
10.579(2)
90
95.65(3)
90
1390.6(5)
4
4.261
0.0272
0.0557
C14H10NGdO7
M
461.48
293(2)
0.710 73
monoclinic
P21/c
9.2498(18)
14.169(3)
10.514(2)
90
95.77(3)
90
1371.0(5)
4
4.877
T/K
λ/Å
cryst system
space group
a/Å
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
µ/mm-1
R1a [I > 2σ(I)]
0.0172
0.0375
0.0204
0.0407
0.0166
0.0338
0.0255
0.0478
b
wR2
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) Σ[w(Fo - Fc )2]/Σ[w(Fo )2]1/2
.
2
2
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1-5a
Compound 1
1178 m, 1138 m, 1038 m, 882 w, 845 m, 792 m, 760 s, 696 s, 652
m, 578 m, 525 s, 432 w.
Sm(1)-O(2A)
Sm(1)-O(2B)
Sm(1)-O(1C)
Sm(1)-O(1)
2.3503(17) Sm(1)-O(3D)
2.3503(17) Sm(1)-O(3E)
2.3754(17) Sm(1)-N(1C)
2.3755(17) Sm(1)-N(1)
2.4075(18)
2.4075(18)
2.5851(19)
2.5852(19)
Synthesis of Sm(pydc)(bc)(H2O) (4). A mixture of Sm2O3 (87
mg, 0.25 mmol), H2pydc (84 mg, 0.5 mmol), Hbc (61 mg, 0.5
mmol), HNO3 (0.10 mL, 4 M), and water (10 mL) was heated at
140 °C for 5 days; colorless crystals of 4 were obtained when
cooling to room temperature at 10 °C/h (yield: 177 mg, 78% based
on Sm). Anal. Calcd for C14H10NSmO7: C, 36.99; H, 2.21; N, 3.08.
Found: C, 36.52; H, 2.27; N, 3.41. IR data (KBr, cm-1): 3222 m,
br, 3073 w, 3047 w, 1665 s, 1602 m, 1592 s, 1579 s, 1482 m,
1432 s, 1414 w, 1394 s, 1363 s, 1283 m, 1172 m, 1034 s, 821 s,
771 s, 698 m, 680 w, 651 w, 507 m, 432 m.
O(1)-Sm(1)-O(3D) 109.40(6)
O(3E)-Sm(1)-N(1C) 155.35(6)
O(2B)-Sm(1)-O(1)
O(2B)-Sm(1)-O(3D) 76.45(6)
O(2A)-Sm(1)-O(1) 146.40(6)
74.45(7)
O(1)-Sm(1)-N(1) 64.95(6)
O(2A)-Sm(1)-N(1) 123.66(6)
O(2B)-Sm(1)-N(1)
Compound 2
2.350(2) Eu(1)-O(3D)
2.350(2) Eu(1)-O(3E)
2.384(2) Eu(1)-N(1)
2.384(2) Eu(1)-N(1C)
78.95(6)
Eu(1)-O(2A)
2.405(2)
2.405(2)
2.581(2)
2.581(2)
Eu(1)-O(2B)
Eu(1)-O(1C)
Eu(1)-O(1)
O(2B)-Eu(1)-O(1)
O(2A)-Eu(1)-O(1)
74.59(8)
146.55(7)
O(2A)-Eu(1)-N(1) 123.54(7)
Synthesis of Gd(pydc)(bc)(H2O) (5). An identical procedure
with 4 was followed to prepare 5 except Sm2O3 was replaced by
Gd2O3 (91 mg, 0.25 mmol) (yield: 189 mg, 82% based on Gd).
Anal. Calcd for C14H10NGdO7: C, 36.43; H, 2.18; N, 3.03.
Found: C, 36.74; H, 2.32; N, 2.75. IR data (KBr, cm-1): 3220 m,
br, 3070 w, 3044 w, 1660 s, 1603 m, 1594 s, 1586 s, 1480 m,
1432 s, 1416 w, 1397 s, 1363 s, 1280 m, 1198 m, 1048 s, 823 s,
770 s, 755 m, 724 m, 683 m, 654 w, 509 m, 434 m.
O(1C)-Eu(1)-N(1)
O(1)-Eu(1)-N(1)
O(3E)-Eu(1)-N(1)
70.43(7)
65.17(7)
75.35(7)
O(2A)-Eu(1)-O(3D) 75.19(8)
O(1)-Eu(1)-O(3D)
109.20(7)
Compound 3
Gd(1)-O(2A)
Gd(1)-O(2B)
Gd(1)-O(1)
2.3379(17) Gd(1)-O(3D)
2.3379(17) Gd(1)-O(3E)
2.3697(16) Gd(1)-N(1)
2.3697(16) Gd(1)-N(1C)
2.3877(17)
2.3877(17)
2.5693(18)
2.5693(18)
65.48(5)
Gd(1)-O(1C)
O(2A)-Gd(1)-O(1) 146.75(6)
O(1)-Gd(1)-N(1)
O(2B)-Gd(1)-O(1)
O(1)-Gd(1)-O(1C)
74.53(6)
80.37(8)
O(2A)-Gd(1)-N(1) 123.36(6)
O(3D)-Gd(1)-N(1) 155.34(6)
O(3D)-Gd(1)-N(1C) 75.08(6)
X-ray Crystallography. Single crystals of compounds 1-5 were
glued on a glass fiber. Data were collected on a Rigaku R-AXIS
RAPID IP diffractometer with Mo KR monochromated radiation
(λ ) 0.710 73 Å) at 293 K. Empirical absorption correction was
applied. The structures were solved by the direct method and refined
by the full-matrix least-squares methods on F2 using the SHELXTL
crystallographic software package.13 Anisotropic thermal parameters
were used to refine all non-hydrogen atoms. The hydrogen atoms
were included at idealized positions, and the H(4) atom of
compounds 1-3 is modeled with 50% occupancy. The crystal data
and structure refinement of compounds 1-5 are summarized in
Table 1. Selected bond lengths and angles for 1-5 are listed in
Table 2.
O(1)-Gd(1)-O(3D) 108.80(6)
Compound 4
2.330(3) Sm(1)-O(1W)
2.390(3) Sm(1)-O(1)
2.389(3) Sm(1)-O(2D)
2.413(3) Sm(1)-N(1)
Sm(1)-O(6A)
Sm(1)-O(5)
Sm(1)-O(3B)
Sm(1)-O(4C)
2.428(3)
2.467(3)
2.467(3)
2.628(3)
O(6A)-Sm(1)-O(5) 119.87(11) O(1)-Sm(1)-O(2D) 108.16(9)
O(6A)-Sm(1)-O(1) 149.23(10) O(5)-Sm(1)-N(1) 71.38(10)
O(5)-Sm(1)-O(4C) 72.80(11) O(6A)-Sm(1)-N(1) 146.89(10)
O(5)-Sm(1)-O(1W) 140.22(10) O(1W)-Sm(1)-N(1) 111.63(11)
62.58(9)
O(1W)-Sm(1)-O(2D) 72.14(9)
O(1)-Sm(1)-N(1)
O(1W)-Sm(1)-O(1)
77.14(10) O(2D)-Sm(1)-N(1) 71.54(9)
Compound 5
Gd(1)-O(5)
2.304(3) Gd(1)-O(1W)
2.358(3) Gd(1)-O(2D)
2.366(2) Gd(1)-O(1)
2.385(3) Gd(1)-N(1)
2.397(3)
2.442(2)
2.443(3)
2.601(3)
Gd(1)-O(6A)
Gd(1)-O(3B)
Gd(1)-O(4C)
Physical Measurements. Elemental analyses (C, H, and N) were
performed on a Perkin-Elmer 2400 CHN elemental analyzer. FTIR
spectra were recorded in the range 400-4000 cm-1 on an Alpha
Centaurt FTIR spectrophotometer using a KBr pellet. TG-DTA
measurements were performed on a Perkin-Elmer TGA7 instrument
in flowing N2 with a heating rate of 10 °C min-1. Excitation and
O(5)-Gd(1)-O(6A) 119.44(11) O(4C)-Gd(1)-O(1) 132.66(9)
O(6A)-Gd(1)-O(1)
O(1W)-Gd(1)-O(1)
O(5)-Gd(1)-O(1W)
O(5)-Gd(1)-O(1)
O(3B)-Gd(1)-O(1)
70.12(10) O(6A)-Gd(1)-N(1)
76.56(10) O(4C)-Gd(1)-N(1)
80.47(12) O(5)-Gd(1)-N(1)
72.01(10)
78.08(10)
146.64(10)
148.70(10) O(1W)-Gd(1)-N(1) 111.24(11)
76.32(9) O(1)-Gd(1)-N(1) 63.27(9)
a Symmetry codes follow. For 1-3: (A) x - 1/2, y - 1/2, -z + 1/2;
(B) -x + 1/2, y - 1/2, z; (C) -x, y, -z + 1/2; (D) x, -y, z - 1/2; (E) -x,
-y, -z + 1. For 4: (A) -x, -y, -z + 1; (B) x - 1, y, z; (C) -x + 1, -y,
-z + 1; (D) x, -y + 1/2, z - 1/2. For 5: (A) -x + 1, -y, -z; (B) x +
1, y, z; (C) -x, -y, -z; (D) x, -y - 1/2, z + 1/2.
(13) (a) Sheldrick. G. M. SHELXS97. A Program for the Solution of Crystal
Structures from X-ray Data; University of Go¨ttingen: Go¨ttingen,
Germany, 1997. (b) Sheldrick, G. M. SHELXL97. A Program for the
Refinement of Crystal Structures from X-ray Data; University of
Go¨ttingen: Go¨ttingen, Germany, 1997.
7124 Inorganic Chemistry, Vol. 44, No. 20, 2005

Three-Dimensional Lanthanide Coordination Polymers
Figure 1. ORTEP representation of atom numbering diagram for 1 (50% probability ellipsoids). Dashed lines illustrate the six-connected circumstance of
the Sm node.
Chart 1 Representations of the Observed Coordination Modes a-k of pydca
a Two new coordination modes in this paper are shown as modes l and m.
emission spectra were obtained on a SPEX FL-2T2 spectrofluo-
rometer equipped with a 450 W xenon lamp as the excitation source.
Results and Discussion
Synthesis and Characterization. We tried to isolate these
single crystals from a conventional solution method using
the reactions of lanthanide nitrate with pydc; unfortunately,
only uncharacterized white precipitates insoluble in most
common solvents were obtained. Hydrothermal reactions in
the presence of organic molecules have been established as
powerful methods for the isolation of new materials with
diverse structural architectures14 in that at the higher tem-
perature the reaction becomes faster thus leading to a higher
Figure 2. Intricate three-dimensional structure of 1 viewed down the c
axis.
(14) Feng, S. H.; Xu, R. R. Acc. Chem. Res. 2001, 34, 239.
Inorganic Chemistry, Vol. 44, No. 20, 2005 7125

Qin et al.
Figure 3. (a) Perspective view of the three-dimensional network of 1 constructed by the helix-linked scalelike sheets running along the b axis. (b) View
of a scalelike ‚‚‚Sm-O-C-O-Sm‚‚‚ sheet. (c) Helical chain formed by Sm atoms and pydc ligands.
degree of reversibility in the process of crystal growth that
is likely to encourage the generation of larger crystals. After
introducing hydrothermal methods, we obtained a small
quantity of microlite unsuitable for single-crystal X-ray
diffraction starting from the same reaction materials. Al-
though detailed studies are still needed, we sense that
changing lanthanide sources may be helpful for the crystal
growth. In view of this, we replaced lanthanide salts with
lanthanide oxides, accompanied with adding dilute HNO3,
hoping lanthanide ions could be slowly released during action
with HNO3 so as to lower the polymerization rate. As
expected, when the reactions of Ln2O3 and pydc were carried
out at 140 °C for 5 days under hydrothermal conditions, well-
formed single crystals of 1-5 were obtained in satisfying
Figure 4. Rutile (TiO2) framework. Six-connecting nodes (Ti) are
represented by large balls, and three-connecting nodes (O) are represented
by small balls. A square channel constructed by cross-linking TiO2 chains
yields.
The IR spectra show features attributable to the carboxylate
stretching vibrations of the complexes. For 1-3, the char-
acteristic bands of carboxylate groups are shown in the range
1560-1632 cm-1 for asymmetric stretching and 1370-1490
cm-1 for symmetric stretching. The characteristic bands
observed for 1-3 at around 1700 cm-1 attributed to the
protonated carboxylic groups indicate the incomplete depro-
tonation of H2pydc ligands. For 4 and 5, the signals in the
range 1580-1665 cm-1 can be assigned to the asymmetric
stretching vibrations for the carboxylate groups, and the
signals between 1360 and 1482 cm-1 correspond to the
symmetric stretching vibrations. The bands in the region ca.
650-1300 cm-1 for 1-5 can be assigned to the CH in-plane
or out-of-plane bend, ring breathing, and ring deformation
absorptions of pyridine ring. Weak absorptions observed at
3033-3075 cm-1 for 1-5 can be attributed to the νC-H of
the pyridyl. The broad bands at ca. 3200 cm-1 for 4 and 5
are attributed to the vibrations of water ligand.
Crystal Structures. Ln(pydc)(Hpydc) [1 (Ln ) Sm), 2
(Ln ) Eu), and 3 (Ln ) Gd)]. Since 2 and 3 are
isomorphous with 1, the structure of 1 is described repre-
sentatively. An atom numbering diagram of the fundamental
unit for 1 is shown in Figure 1. 1 crystallizes in the
orthorhombic system, space group Pbcn. There is a crystal-
lographically independent samarium ion in this structure. The
local coordination geometry for the eight-coordinate Sm(1)
center is close to a trigonal dodecahedron coordinated by
six oxygen atoms and two nitrogen atoms from six pydc
is highlighted in the center in which the three-connecting centers act as the
sides of the channels and the six-connecting centers occupy the corners.
ligands. This, therefore, defines a six-connecting node within
the structure. The Sm-O(carboxylate) bond distances range
from 2.3503(17) to 2.4075(18) Å, while the Sm-N band
length is slightly longer, 2.5851(19) Å. Similar trends are
observed for 2 and 3. The coordination modes of pydc in
structurally characterized coordination polymers observed up
to now are summarized in Chart 1. As can be seen, the
nitrogen and one of the 2-carboxylate oxygen atoms prefer
to chelate one metal atom, and the 5-carboxyl group tends
to ligate a metal in a bidentate or monodentate fashion or
even be free. pydc in 1 adopts a new coordination fashion,
shown in Chart 1. Besides linking three metal atoms,
deprotonation of H2pydc ligand is incomplete, which can be
verified by the presence of characteristic bands at about 1700
cm-1 in IR spectrum.15 And thus, each pydc ligand affords
a three-connecting node. The Sm centers are interconnected
through pydc ligands to generate an intricate three-
dimensional architecture with a wide range of C-O-Sm
angles (125.9-154.9°) and C-N-Sm angles (115.8-
126.3°), as shown in Figure 2. Viewed along the b axis, the
detailed links can be described stepwise: the Sm centers are
first linked by a nitrogen atom and 2-carboxylate groups to
(15) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Wiley:
New York, 1958.
7126 Inorganic Chemistry, Vol. 44, No. 20, 2005

Three-Dimensional Lanthanide Coordination Polymers
Figure 5. Three-connected subnets with the topology of 4.82 in a prototypical lattice of rutile (a) and 1 (b).
form a scalelike ‚‚‚Sm-O-C-O-Sm‚‚‚ sheet (see Figure
3b), and then these sheets are further cross-linked by C5-
NH3 spacers of pydc anions via 5-carboxlyate groups into
the three-dimensional structure (Figure 3a). Very intriguingly,
a careful examination shows that in the interlayer regions
there exist parallel helical chains, which are formed by pydc
ligands bridging samarium atoms along the crystallographic
21-axis with a pitch of 9.939(2) Å (see Figure 3c). The
rotation directions of helices between two adjacent sheets
are just opposite; therefore, the final three-dimensional
racemic framework can be considered as being constructed
by helix-linked scalelike sheets. Moreover, it is interesting
to note that two symmetry-related O4 atoms near each Sm
are very close to each other (2.401(3) Å). On this basis, the
missing H atom would appear to be disordered over two sites
between the two atoms. And thus with the Sm center between
O3E and O3D (see Figure 1) a hydrogen-bonded ring O3-
Sm-O3-C7-O4-H‚‚‚O4-C7-O3 is formed, which re-
sembles the hydrogen-bonded ring in the acetic acid dimer.
These intramolecular hydrogen-bonding interactions play
important roles in stabilizing the uncoordinated carboxyl
oxygen atoms and the whole 3D structure.
A better insight into the nature of this intricate framework
can be achieved by the application of topological approach,
i.e. reducing multidimensional structures to simple node-and-
connection nets. As discussed above, the structure of 1 is
binodal with six-connected (Sm atom) and three-connected
(pydc ligand) nodes. However, it does not exhibit the most
probable prototypical reference networksrutile (see Figure
4)sas expected to be with these types of centers, but adopts
an unprecedented (4.62)2(42‚69‚84) topology (the first symbol
for pydc ligand, the second one for Sm atom). To our
knowledge, it is completely new within coordination polymer
chemistry, and the finding of this new topology is useful at
the basic level in the crystal engineering of coordination
networks. The Schla¨fli notation, as we can seen, is different
but closely related to rutile (4.62)2(42‚610‚83). If we look at
an alternative view of the 3D network of 1 to imagine taking
it apart, the similarities and differences of the new network
to rutile become apparent. First, for a single 3D rutile
framework, it can be separated into sets of isolated segments
as shown in Figure 5a, each corresponding to a three-
connected subnet with the topology of 4.82. These subnets
Figure 6. Schematic illustrating the (4.62)2(42‚69‚84) topology of the 3D
network of 1. Alternatively intersecting subnets at an angle of ca. 46.8° are
highlighted in different colors.
are parallelly fused together in an AAAA stacking sequence
by sharing metal centers to form a single rutile net, and six-
membered rings are consequently formed. In the same way,
the complicated 3D network of 1 can also be considered as
being constructed by the similar independent subnets (see
Figure 5b). However, an essential difference is that these
subnets are alternately superposed in such a way that
adjoining subnets intersect at an incline angle of ca. 46.8°
in an ABAB sequence via sharing metal centers. As a result
of this orientation mode, the topology of the material changes
from (4.62)2(42‚610‚83) to (4.62)2(42‚69‚84) (see Figure 6).
From the topological symbol it can be seen that the number
of six-membered circuits around a six-connecting node
decreases while the number of eight-membered circuits
correspondingly increases in 1. Moreover, due to the unique
arrangement of these subnets, not only do the four-membered
rings in each chain not fall on a plane as in rutile (see Figure
S1) but also the interconnected four chains do not generate
square channels.
Ln(pydc)(bc)(H2O) [4 (Ln ) Sm) and 5 (Ln ) Gd)].
Atom numbering schemes for the isostructural 4 and 5 are
represented by that of 4 shown in Figure 7. The Sm(III) atom
exhibits a distorted triangle-dodecahedral configuration
coordinated by six oxygen atoms from four pydc and two
bc ligands (Sm-O 2.330(3)-2.468(3) Å), one nitrogen atom
from one pydc ligand (Sm-N 2.628(3) Å), and one aqua
Inorganic Chemistry, Vol. 44, No. 20, 2005 7127

Qin et al.
Figure 7. ORTEP representation of atom numbering diagram for 4 (50% probability ellipsoids). Dashed lines illustrate the six-connected circumstance of
bimetallic core.
Figure 8. Perspective view of the three-dimensional structure of 4 along
the a-axis, showing the one-dimensional hexagonal channels occupied by
one-end-coordinated bc ligands.
Figure 9. Schematic view of the deformed rutile topology of 4.
simplification principle, the resulting structure of compound
4 is an interesting mineralomimetic network containing
alternating six-connected (bimetallic core) and three-con-
nected nodes (pydc ligand) with the topology of (4.62)2(42‚
610‚83), which is a deformed version of the idealized rutile
structure (see Figure 9). As can be seen, the adjacent chains
are mutually inclined at an angle of ca. 45.8° rather than
perpendicular as shown in the rutile prototype (TiO2).
Recently, multicarboxylate ligands have been employed
to construct lanthanide coordination polymers. In general,
the greater the effective coordination sites of a carboxylate
ligand, the higher the dimensionality of the resulting Ln-
carboxylate network would be. To understand this, it is
instructive to compare compounds 1-5 with other related
compounds on the basis of lanthanides and carboxylic acids.
As evidenced in reported complexes [Ln(Hpdc)(H2pdc)-
(H2O)2]16 (Ln ) Er, Lu; H3pdc ) 3,5-pyrazoledicarboxylic
acid) and [{Nd2(pydc)2(Hpydc)2(H2O)4}‚2H2O]n,11 the result-
ing architectures are both 2D rather than 3D as reported
herein. The analysis for the coordination modes of carboxy-
late ligands reveals that this can be due to the fact that a
completely free carboxyl group exists in both cases that
neither is deprotonated nor takes part in coordination, thus
preventing spatial extension of skeleton to higher dimensions.
In addition, different from the previously reported 3D
ligand (Sm-Oaqua 2.428(3) Å). The O-Sm-O bond angles
range from 72.14(9) to 149.23(10)°, and O-Sm-N bond
angles range from 62.58(9) to 146.89(10)° (see Table 2).
The coordination mode of pydc in 4 is shown in Chart 1m
that has not been observed before. Different from that of 1,
the 5-carboxylate bridges two metal atoms in a carboxylate
O,O′ mode. The bc ligand adopts a bridging bidentate
coordination mode. These bridging bidentate ends of car-
boxylate ligands contribute to the formation of dinuclear
units, and thus, two crystallographically identical samarium
ions are bridged by four pairs of µ2-carboxylate ends into a
dinuclear unit with Sm‚‚‚Sm distance of 4.525 Å. Each
binuclear unit is further extended by the bridging pydc
ligands into a three-dimensional network. Interestingly, when
viewed along a-axis the three-dimensional network contains
one-dimensional hexagonal channels that are occupied by
one-end-coordinated bc ligands (see Figure 8 and Figure S2).
As shown in Figure 7, one dinuclear unit is surrounded
by 10 molecules, i.e. 6 pydc, 2 bc, and 2 aqua ligands. Being
end-capping units, two bc and two aqua ligands are disre-
garded from a topological perspective; this, therefore, defines
the bimetallic core as a six-connected node. Likewise,
although a pydc ligand ligates with four lanthanide ions, it
actually serves as a three-connected node because two metal
atoms bridged by 5-carboxlate constitute a bimetallic core
and should be considered as one. On the basis of the above
(16) Pan, L.; Huang, X. Y.; Li, J.; Wu, Y. G.; Zheng, N. W. Angew. Chem.,
Int. Ed. 2000, 39, 527.
7128 Inorganic Chemistry, Vol. 44, No. 20, 2005

Three-Dimensional Lanthanide Coordination Polymers
containing a europium ion was investigated. The emission
spectrum of 2 (Figure 10) at room temperature upon
excitation at 364 nm exhibits the characteristic transition of
5
7
Eu3+ ion. They are attributed to D0 f FJ (J ) 0, 1, 2, 3,
4) transitions, i.e. 580 nm (5D0 f 7F0), 587 and 595 nm (5D0
f 7F1), 612 and 616 nm (5D0 f 7F2), 650 and 652 nm (5D0
f 7F3), and 697 and 700 nm (5D0 f 7F4). The most intense
transition is 5D0 f 7F2, which implies red emission light of
2.
Conclusions
Reaction of lanthanide ions with pydc anions results in
the formation of five three-dimensional polymeric species
having different connectivities. The two new coordination
modes adopted by pydc anion further reveal the versatility
of pydc anions as bridging ligands. From the topological
point of view, though they all contain six-connecting metal
centers and three-connecting ligand centers, the former cases
(1-3), as already point out, do not exhibit the most probable
prototypical reference networksrutilesas expected to be
with these types of centers, but adopt an unprecedented
(4.62)2(42‚69‚84) topology, implying the diversification of
topological connectivity and highlighting the difficulty in
predicting crystal structures with certainty.
Figure 10. Solid-state emission spectrum for 2 at room temperature
(excited at 364 nm).
lanthanide-carboxylate coordination polymers, such as [Er-
(CTC)(H2O)]‚2.5H2O (CTC ) cis,cis-1,3,5-cyclohexanetri-
carboxylate),17 [Yb2(NDC)3(H2O)‚(H2O)2] (NDC ) 2,6-
naphthalenedicarboxylate),6d [Eu4(NDC)6(H2O)5]‚3H2O,6f and
Ln(bpdc)1.5(H2O)‚0.5DMF (bpdc ) 4,4′-biphenyldicarboxy-
late),5e no solvent molecule is directly connected to the
frameworks of compounds 1-3 due to the inherent steric
hindrance of six different pydc ligands coordinated to each
Ln center, which would be a useful feature to produce
functional luminescent materials considering the quenching
effect of -OH oscillators on luminescence.6d,18
Acknowledgment. We thank Prof. Chunshan Shi for
helpful discussions on luminescence. Special thanks are
offered to three reviewers for their constructive suggestions.
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 20371011).
Luminescent Properties. Taking into account the excel-
lent luminescent properties of Eu3+, the luminescence of 2
Supporting Information Available: Additional figures, TGA
and DTA curves, and X-ray crystallographic information files (CIF)
for 1-5. This material is available free of charge via the Internet
at http://pubs.acs.org.
(17) Pan, L.; Woodlock, E. B.; Wang, X. T. Inorg. Chem. 2000, 39, 4174.
(18) (a) Horrocks, W. D., Jr.; Albin, M. Prog. Inorg. Chem. 1984, 31, 1.
(b) Reineke, T. M.; Eddaoudi, M.; Fehr, M.; Kelley, D.; Yaghi, O.
M. J. Am. Chem. Soc. 1999, 121, 1651.
IC050906B
Inorganic Chemistry, Vol. 44, No. 20, 2005 7129