Influence of the reaction temperature and pH on the coordination modes of the 1,4-benzenedicarboxylate (BDC) ligand: A case study of the Ni II(BDC)/2,2′-bipyridine system

Article abstract of DOI:10.1021/ic050644d

Three new Ni(BDC)/2,2′-bipy compounds, Ni₂(BDC)(HBDC) ₂(2,2′-bipy)₂ (2), Ni₃(BDC) ₃(2,2′-bipy)₂ (3), and Ni(BDC)-(2,2-bipy) ₂·2H₂O (5), in addition to the previously reported Ni(BDC)(2,2′-bipy)·0.75H₂BDC (1) and Ni(BDC)(2,2′-bipy)(H₂O) (4) [BDC = 1,4-benzenedicarboxylate; 2,2′-bipy = 2,2′-bipyridine], have been synthesized by hydrothermal reactions. A systematic investigation of the effect of the reaction temperature and pH resulted in a series of compounds with different compositions and dimensionality. The diverse product slate illustrates the marked sensitivity of the structural chemistry of polycarboxylate aromatic ligands to synthesis conditions. Compound 1, which has a channel structure containing guest H ₂BDC molecules, is formed at the lowest pH. The guest H ₂BDC molecules are connected by hydrogen bonds and form extended chains. At a slightly higher pH, a dimeric molecular compound 2 is formed with a lower number of protonated carboxylate groups per nickel atom and per BDC ligand. Reactions at higher temperature and the same pH lead to the transformation of 1 and 2 into the two-dimensional, layered trinuclear compound 3. As the pH is increased, a one-dimensional polymer 4 is formed with a water molecule coordinated to Ni²⁺. Bis-monodentate and bischelating BDC ligands alternate along the chain to give a crankshaft rather than a regular zigzag arrangement. A further increase of the pH leads to the one-dimensional chain compound 5, which has two chelating 2,2′-bipy ligands. Crystal data: 2, triclinic, space group P1, a = 7.4896(9) A, b = 9.912(1) A, c = 13.508(2) A, α = 86.390(2)°, β = 75.825(2)°, γ = 79.612(2)°, Z = 2; 3, orthorhombic, space group Pbca, a = 9.626(2) A, b = 17.980(3) A, c = 25.131(5) A, Z = 4; 5, orthorhombic, space group Pbcn, a = 14.266(2) A, b = 10.692(2) A, c = 17.171(2) A, Z= 8.

Full text of DOI:10.1021/ic050644d

Inorg. Chem. 2005, 44, 8265−8271
Influence of the Reaction Temperature and pH on the Coordination
Modes of the 1,4-Benzenedicarboxylate (BDC) Ligand: A Case Study of
the NiII(BDC)/2,2
-Bipyridine System
′
Yong Bok Go, Xiqu Wang, Ekaterina V. Anokhina, and Allan J. Jacobson*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003
Received April 26, 2005
Three new Ni(BDC)/2,2
(2,2-bipy)2 2H2O (5), in addition to the previously reported Ni(BDC)(2,2
bipy)(H2O) (4) [BDC 1,4-benzenedicarboxylate; 2,2 -bipy 2,2 -bipyridine], have been synthesized by hydrothermal
′
-bipy compounds, Ni2(BDC)(HBDC)2(2,2
′
-bipy)2 (2), Ni3(BDC)3(2,2′-bipy)2 (3), and Ni(BDC)-
‚
′
-bipy) 0.75H2BDC (1) and Ni(BDC)(2,2
‚
′-
)
′
)
′
reactions. A systematic investigation of the effect of the reaction temperature and pH resulted in a series of compounds
with different compositions and dimensionality. The diverse product slate illustrates the marked sensitivity of the
structural chemistry of polycarboxylate aromatic ligands to synthesis conditions. Compound 1, which has a channel
structure containing guest H2BDC molecules, is formed at the lowest pH. The guest H2BDC molecules are connected
by hydrogen bonds and form extended chains. At a slightly higher pH, a dimeric molecular compound 2 is formed
with a lower number of protonated carboxylate groups per nickel atom and per BDC ligand. Reactions at higher
temperature and the same pH lead to the transformation of 1 and 2 into the two-dimensional, layered trinuclear
compound 3. As the pH is increased, a one-dimensional polymer 4 is formed with a water molecule coordinated
to Ni2+. Bis-monodentate and bischelating BDC ligands alternate along the chain to give a crankshaft rather than
a regular zigzag arrangement. A further increase of the pH leads to the one-dimensional chain compound 5, which
has two chelating 2,2
′
-bipy ligands. Crystal data: 2, triclinic, space group P1
13.508(2) Å, R ) 86.390(2) â ) 75.825(2) γ ) 79.612(2) , Z 2; 3, orthorhombic, space group Pbca,
9.626(2) Å, b 17.980(3) Å, c 25.131(5) Å, Z 4; 5, orthorhombic, space group Pbcn, a 14.266(2)
10.692(2) Å, c 17.171(2) Å, Z 8.
h, a ) 7.4896(9) Å, b ) 9.912(1) Å,
c
a
)
)
°
,
°
,
°
)
)
)
)
)
Å, b
)
)
)
Introduction
possible coordination modes (monodentate, bridging, chelat-
ing), resulting in a low degree of structure predictability
compared to aromatic nitrogen-donor ligands. Multiple
coordination modes are often observed within the same
structure, and several examples have been reported where
as many as three different coordination modes occur within
the same polycarboxylate ligand.3 Moreover, the multiple
coordination modes are conducive to structural isomerism,
as illustrated by the two polymorphs of Co(dcbp)(H2O)2
(dcbp ) 4,4′-dicarboxylato-2,2′-bipyridine).4 The two poly-
morphs have three-dimensional frameworks of remarkably
Aromatic polycarboxylates are widely used as bridging
ligands in the construction of inorganic/organic hybrid
nanoporous materials1 because they are sterically rigid and
chemically robust, leading to frameworks of high thermal
stability approaching that of purely inorganic zeolites.2 The
presence of two potentially coordinating oxygen atoms in
the carboxylic acid group, on the one hand, is beneficial for
thermal stability and, on the other hand, leads to several
* To whom correspondence should be addressed. E-mail: ajjacob@uh.edu.
Tel: (713) 743-2785. Fax: (713) 743-2787.
(1) (a) Chae, H. K.; Siberio, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.;
Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523.
(b) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim,
K. Nature 2000, 404, 982. (c) Kahn, O.; Martinez, C. Science 1998,
279, 44. (d) Janiak, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 1431.
(2) (a) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999,
402, 276. (b) Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle,
F.; Henry, M.; Bataille, T.; Ferey, G. Chem. Eur. J. 2004, 10, 1373.
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D. B.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2001, 123,
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Zhang, D.; Xue, M.; Qiu, S. Eur. J. Inorg. Chem. 2004, 185. (c) Shi,
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Growth Des. 2004, 4, 25.
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10.1021/ic050644d CCC: $30.25
Published on Web 10/01/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 23, 2005 8265

Go et al.
different topologies as a result of different coordination
modes of the dcbp ligand: in the first polymorph, both
carboxylic groups are monodentate, while in the second
polymorph, one is bridging and the other is noncoordinating.
Another feature that distinguishes polycarboxylic ligands
from aromatic nitrogen-donor ligands is a much wider
variation in the degree of protonation and deprotonation,
which affects not only the ligand coordination ability but
also its charge and, therefore, the metal-to-ligand ratio and,
consequently, the resulting topology. Furthermore, carbox-
ylate groups are much more actively involved in hydrogen
bonding, as both acceptors (especially noncoordinating
oxygens of monodentate carboxylates) and donors (in the
case of protonated carboxylic groups). The optimization of
hydrogen-bonding interactions can become one of the
principal structure-determining factors. Many supramolecular
hydrogen-bonding patterns characteristic of carboxylate
anions and carboxylic acids are often encountered in
coordination polymers. For example, in purely organic
crystals, a ubiquitous “dicarboxylic dimer” connects coor-
dination polymer chains in a nickel 1,2,3-benzenetricarbox-
ylate.5 Other examples of these synthons are found in several
1,4-benzenedicarboxylates (BDCs) containing neutral H2BDC
as guest molecules: these molecules are located in one-
dimensional channels and form linear flat hydrogen-bonded
chains, which effectively become extended one-dimensional
templates.6
performed, and previously reported information has not been
critically systematized.
We have conducted a systematic study on the Ni(BDC)/
2,2′-bipyridine system. The introduction of 2,2′-bipyridine,
which has predictable coordination properties and almost
exclusively acts as a “terminal” chelating bidentate ligand,
confines the propagation of M/BDC/M linkages to one or
two dimensions and limits the extent of structural changes
that occur upon movement by a given increment in temper-
ature or pH, thus facilitating a more detailed study of the
effect of a particular synthesis parameter. The choice of a
terminal diamine ligand, for example, between 2,2′-bipyridine
or 1,10-phenanthroline, has additional structural conse-
quences because of the influence on aromatic/aromatic
interactions6 and was not varied in this study. The nature of
the transition metal in these systems is very important;9
therefore, all of our experiments in this work were limited
to nickel, which, compared to other divalent transition metals
under these conditions, has the least likely probability of
variations in its coordination geometry. In the present work,
the pH and temperature effects on the Ni(BDC)/2,2′-bipy
system have been further studied, and here we report the
syntheses and structures of three new Ni2+ coordination
compounds: one dimer, Ni2(BDC)(HBDC)2(2,2′-bipy)2 (2),
and two polymers, Ni3(BDC)3(2,2′-bipy)2 (3) and Ni(BDC)-
(2,2′-bipy)2‚2H2O (5).
Experimental Section
These diverse aspects of the structural chemistry of
polycarboxylate aromatic ligands are markedly sensitive to
synthesis conditions requiring a thorough systematic study
of the influence of each individual factor, such as the pH,
temperature, and solvent, on coordination modes and pro-
tonation states of these ligands. This information is especially
valuable because, once the conditions for the formation of a
specific building block are identified, it occasionally becomes
possible to expand the structure while preserving the
framework topology by increasing the ligand length and
generate “isoreticular” materials with much larger pores.7
Over the years of intensive studies of transition-metal mono-
and polycarboxylates, a vast amount of data have been
acquired, but systematic investigations focusing on the
influence of a particular synthesis parameter, while keeping
numerous other factors constant, are still very rare. For
example, two recent systematic studies of the effect of
temperature on the structure of cobalt succinates and divalent
transition-metal diglycolates and iminodiacetates clearly
illustrated the trend of an increase of the degree of condensa-
tion of metal polyhedra and an increase of the density of
carboxylic groups as a result of temperature increase.8 To
our knowledge, in the case of aromatic polycarboxylates,
however, no new studies of comparable scope have been
Materials and Methods. All of the reactants were reagent grade
and were used as purchased without further purification. The IR
spectra were measured on a Galaxy series FTIR 5000 spectrometer
with pressed KBr pellets. Thermal analyses were performed on a
thermogravimetric analysis V5.1A Du Pont 2100 instrument from
room temperature to 600 °C with a heating rate of 3 °C/min in air.
Synthesis. All compounds were prepared by a hydrothermal
reaction. A mixture of NiCl2‚6H2O (100 mg, 0.42 mmol), 1,4-
benzenedicarboxylic acid (69.9 mg, 0.42 mmol), KOH (0-118 mg,
0-2.1 mmol), 2,2′-bipyridine (65.7 mg, 0.42 mmol), and H2O (0.5
mL) was heated in a 23-mL stainless steel reactor with a Teflon
liner at 150-210 °C for 48 h. The crystals obtained were filtered
and washed with water and acetone (compounds 1-4) or with
ethanol and acetone (dark purple block-shaped compound 5).
Compound 5 decomposes slowly in water. The results from a series
of experiments are summarized in Figure 1. The diagram shows
the appearance of compounds 1-5 as a function of temperature
and the amount of KOH added. Optimized syntheses of compounds
2, 3, and 5 are given below.
Ni2(BDC)(HBDC)2(2,2′-bipy)2 (2). A mixture of NiCl2‚6H2O
(0.42 mmol, 100 mg), 1,4-benzenedicarboxylic acid (0.42 mmol,
69.9 mg), KOH (0.84 mmol, 47.1 mg), 2,2′-bipyridine (0.42 mmol,
65.7 mg), and H2O (0.5 mL) was heated in a 23-mL stainless steel
reactor with a Teflon liner at 150 °C for 48 h. The green block-
shaped crystals were filtered and washed with water and acetone.
Yield: 45% based on Ni. Anal. Calcd for C44H15N4Ni2O12: C, 56.94;
H, 3.69; N, 6.04. Found: C, 56.88; H, 3.33; N, 5.99. IR (KBr):
3100.99m, 3070.13w, 1710.56s, 1606.56w, 1598.70w, 1585.20m,
1563.99w, 1515.78s, 1471.42w, 1407.78m (multiple), 1349.33w,
1313.29w, 1297.86w, 1226.51m, 1170.58w, 1153.28w, 1143.58w,
(5) Prior, T. J.; Rosseinsky, M. J. Chem. Commun. 2001, 1222.
(6) Go, Y.; Wang, X.; Anokhina, E. V.; Jacobson, A. J. Inorg. Chem.
2004, 43, 5360.
(7) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi,
M.; Kim, J. Nature 2003, 423, 705.
(8) (a) Forster, P. M.; Burbank, A. R.; Livage, C.; Ferey, G.; Cheetham,
A. K. Chem. Commun. 2004, 368. (b) Forster, P. M.; Cheetham, A.
K. Microporous Mesoporous Mater. 2004, 73, 57.
(9) Sun, D.; Cao, R.; Liang, Y.; Shi, Q.; Su, W.; Hong, M. J. Chem.
Soc., Dalton Trans. 2001, 2335.
8266 Inorganic Chemistry, Vol. 44, No. 23, 2005

Study of the NiII(BDC)/2,2′-Bipyridine System
Table 1. Crystallographic Data for Compounds 2, 3, and 5
2
3
5
chemical formula
fw
cryst syst
space group
a, Å
C22H15N2NiO6 C44H28N4Ni3O12 C28H24N4NiO6
462.07
triclinic
P1h
980.83
orthorhombic
Pbca
9.626(2)
17.980(3)
25.131(5)
571.22
orthorhombic
Pbcn
14.266(2)
10.692(2)
17.171(2)
7.4896(9)
9.912(1)
13.508(2)
86.390(2)
75.825(2)
79.612(2)
956.1(2)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
4349.1(4)
4
2619.0(6)
4
Z
temp, K
Fc, g cm-3
µ(Mo KR), mm-1
273(2)
1.605
1.060
293(2)
1.498
1.352
293(2)
1.449
0.791
0.0306, 0.0815
0.0402, 0.0883
R1, wR2 [I > 2σ(I)]a 0.0490, 0.1021 0.0353, 0.0716
Figure 1. Composition diagram for the NiCl2‚6H2O/H2BDC/KOH/2,2′-
bipy system. The scale on the x axis represents the molar ratio of KOH to
NiCl2‚6H2O. The symbols correspond to individual reactions and the colors
to specific compounds. Grey squares: 1. Yellow circles: 2. Green
hexagons: 3. Blue circles: 4. Red triangles: 5.
R1, wR2 (all data)a
0.0762, 0.1120 0.0590, 0.0786
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
structures were solved by direct methods and refined on F 2 by full-
matrix least squares using SHELXTL.11 All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were refined isotro-
pically with geometric constraints. Crystal data for compounds 2,
3, and 5 are summarized in Table 1.
1110.80w, 1052.94w, 970.02w, 892.88w, 865.88w, 856.24m,
823.46w, 802.24w, 786.82m, 769.46m, 748.24m, 539.97m, 520.68w,
503.33m cm-1
.
Ni3(BDC)3(2,2′-bipy)2 (3). This compound was produced as a
mixture with compound 3 at temperatures 155-180 °C from the
same reactant ratio as 3. At temperatures above 180 °C, the reaction
mixture gives a pure phase of this compound. The green hexagonal
crystals were filtered and washed with water and acetone. Yield:
63% based on Ni. Anal. Calcd for C44H28N4Ni3O12: C, 53.88; H,
2.87; N, 5.71. Found: C, 53.33; H, 3.00; N, 5.74. IR (KBr):
3396.23w (br), 3108.70w, 3077.85w, 1617.99m, 1604.49m,
1562.06m, 1500.35m, 1475.28w, 1442.50w, 1380.79s, 1315.22w,
1249.65w, 1155.25w, 1097.30w, 1054.87w, 883.24w, 846.60w,
Results and Discussion
Synthesis. The hydrothermal reactions of NiCl2‚6H2O and
H2BDC with 2,2′-bipy and KOH gave compounds 1-5,
which formed under similar reaction conditions but with
different amounts of KOH and at different temperatures
(Figure 1). The syntheses and structures of compounds 1
and 4 were reported previously.
823.46m, 769.46m, 759.82m, 744.39m, 538.04m cm-1
.
The initial molar ratio of the reactants, Ni2+/H2BDC/KOH/
2,2′-bipy, from which one-dimensional coordination polymer
1 with diprotonated H2BDC guest molecules was synthesized
was 1:1:1.5:1. A large amount of unreacted H2BDC (ca. 10%
of the original amount) was found in the raw product. In an
effort to minimize the amount of unreacted H2BDC, the
reactant ratio was varied to 1:1:2:1, providing more base for
the completion of deprotonation but still keeping the pH low.
A mixture of the dimeric molecular complex, 2, and the two-
dimensional coordination polymer, 3, was obtained at tem-
peratures below 180 °C. Compound 2 was the dominant
product, with a few crystals of 3 as an impurity when the
temperature was 160 °C but was obtained as a pure phase at
150 °C. Compound 3 could be isolated as a pure phase from
the same reactant mixture above 180 °C. Compound 3 also
began to form from the reactant ratio of 1:1:1.5:1, from which
compound 1 was synthesized when the synthesis temperature
exceeded 180 °C and became the only product above 190
°C.
Ni(BDC)(2,2′-bipy)2‚2H2O (5). To achieve the high yield and
purity, a mixture of NiCl2‚6H2O (0.42 mmol, 100 mg), 1,4-
benzenedicarboxylic acid (0.63 mmol, 104.7 mg), KOH (1.68 mmol,
94.3 mg), 2,2′-bipyridine (0.84 mmol, 130.8 mg), and H2O (0.5
mL) was heated in a 23-mL stainless steel reactor with a Teflon
liner at 160 °C for 48 h. Yield: 75% based on Ni. Anal. Calcd for
C56H48N8Ni2O12: C, 58.88; H, 4.23; N, 9.81. Found: C, 58.87; H,
4.28; N, 9.84. IR (KBr): 3316m (br), 3104.85m, 1604.01s,
1587.18s, 1573.15s, 1493.60w, 1473.35m, 1442.02s, 1377.41s,
1307.02w, 1283.40w, 1248.20w, 1223.13w, 1172.99w, 1152.26m,
1187.18w, 1056.32m, 1040.89w, 1022.09m, 890.95ww, 817.19m,
772.35m, 753.55m, 737.16m, 653.27w, 632.54w, 548.17w, 497.54w,
416.55w cm-1
.
Crystallographic Studies. Single crystals of suitable dimensions
for 2, 3, and 5 were used for the structure determinations. All
measurements were made with a Siemens SMART platform
diffractometer equipped with a 1K CCD area detector. A hemi-
sphere of data (1271 frames at 5-cm detector distance) was collected
for each phase using a narrow-frame method with scan widths of
0.3° in ω and an exposure time of 30-40 s/frame. The first 50
frames were remeasured at the end of data collection to monitor
instrument and crystal stability, and the maximum correction applied
to the intensities was <1%. The data were integrated using the
Siemens SAINT program,10 with the intensities corrected for the
Lorentz factor, polarization, air absorption, and absorption due to
variation in the path length through the detector faceplate. The
A pure phase of 3 could also be obtained by reaction of
NiCl2‚6H2O, 1,4-dicyanobenzene (DCB), and 2,2-bipy in
H2O (Ni2+:DCB:2,2′-bipy:H2O ) 1:1:1:132). This method
was previously used to synthesize one-dimensional polymers
of M(BDC)(1,10-phen)(H2O)x for Co2+, Cu2+, and Zn2+ and
three-dimensional polymers of M(BDC)(1,10-phen)Clx for
(10) SAINT, Program for Data Extraction and Reduction; Siemens Analyti-
(11) SHELXTL, Program for Refinement of Crystal Structures; Siemens
cal X-ray Instruments Inc.: Madison, WI, 1996.
Analytical X-ray Instruments Inc.; Madison, WI, 1994.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8267

Go et al.
Chart 1. Versatile Coordination Modes of BDC Ligands in
Compounds 1-5
Mn2+ and Cd2+. Compared to these cations, Ni2+ formed a
different one-dimensional coordination polymer with 1,10-
phen, Ni(BDC)(1,10-phen)(H2O)‚0.5H2BDC,6 under these
conditions.
An increase of KOH corresponding to the ratio 1:1:2.5:1
results in one-dimensional coordination polymer 4. A further
increase in the pH lowers the yield of 4, and pink plate-
shaped crystals, which were identified as [Ni(2,2′-bipy)3]Cl2
by X-ray crystallography, begin to appear.12 When the
reactant ratio is increased to 1:1:4:1, compound 5 forms with
a large amount of [Ni(2,2′-bipy)3]Cl2. A higher yield of 5
could be obtained when the reactant ratio was adjusted to
1:1.5:4:2, as described in the Experimental Section.
Crystal Structures. Compounds 1-5 all contain a com-
bination of BDC and 2,2′-bipy ligands but have a remarkable
structural and compositional diversity that is achieved in part
by the versatile connectivity of the BDC ligands as shown
in Chart 1.
The structures of the compounds 1, 4, and 5 are formed
by infinite one-dimensional chains where octahedrally co-
ordinated Ni(II) centers are linked by BDC bridges. On the
other hand, compound 2 is based on a discrete dimeric
molecule where the two nickel centers are bridged by one
BDC ligand (Figure 3), and 3 is a two-dimensional polymer
formed by trinuclear Ni(II) units linked by BDC bridges.
These compounds were synthesized from the same reactants
and solvent but different reactant ratios. The crystal structures
of compounds 1 and 4 (Figure 2) were described in our
previous work.6
Compound 2. The environment of the Ni centers in 2 is
identical with that in 1 (Figure 3). The Ni-O bond distances
range from 2.056(10) to 2.160(7) Å, and the Ni-N bond
distances are 2.004(11) and 2.076(11) Å. However, com-
pound 2 has a molecular rather than a polymeric structure
because only one of the two BDC ligands coordinating to
each nickel atom bridges two nickel centers in bischelating
coordination mode while the other BDC has monochelating
and one-protonated, noncoordinating carboxylic groups,
resulting in the formation of discrete dimeric molecules. The
dimers form layers by face-to-face π-π interactions between
2,2′-bipy ligands, and these layers are stacked through CH-π
interactions [dH8-centroid ) 3.418(4) Å] between the HBDC
ligands. The hydrogen atom from a noncoordinating car-
Figure 2. One-dimensional chain structures of compounds 1 (a) and 4
(b).
boxylic group is hydrogen-bonded to the chelating oxygen
atom of a neighboring molecule [dO6‚‚‚O2 ) 2.682(5) Å]. A
similar molecular compound, [Ni2(BDC)(2,2′-bipy)4][ClO4]2,
has been synthesized at room temperature. This ionic
compound has only one BDC ligand, which connects two
Ni centers, and the remaining octahedral sites around each
Ni atom are occupied by two chelating 2,2′-bipy ligands.13
Compound 3. As shown in Figure 4, in compound 3, three
Ni2+ atoms are clustered by BDC ligands in bridging
coordination mode. The coordination environment around
Ni1, which is on the inversion center, is octahedral. The
ligands coordinated to the Ni2 atom form a distorted
octahedron, frequently found in compounds with chelating
BDC and 2,2′-bipy ligands. The Ni1-Ni2 distance is
3.443(1) Å, and the Ni2-Ni1-Ni2 angle is 180°. Each
Ni1O6 octahedron shares its opposite O1 corners with two
Ni2O4N2 octahedra. Each trinuclear Ni3(BDC)3(2,2′-bipy)2
unit is connected to six other trinuclear units by sharing each
of its six BDC ligands, resulting in the formation of a two-
dimensional (6,3) net with a thickness of the trinuclear unit.
The chelating 2,2′-bipy ligands are located on both sides of
the layer. The layers are stacked by π-π interactions of each
2,2′-bipy ligand with one ring from each of its two neighbor-
ing 2,2′-bipy ligands from an adjacent layer.
A trimeric building unit similar to that in 3 is also found
in several compounds14 including MOF-3, Zn3(BDC)3(CH3-
OH)4‚2(CH3OH)15 (Figures 4 and 5) which is a three-
dimensional porous network instead of a two-dimensional
(13) Deng, Z. L.; Shi, J.; Jiang, Z. H.; Liao, D. Z.; Yan, S. P.; Wang, G.
L.; Wang, H. G.; Wang, R. J. Polyhedron 1992, 11, 885.
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P. A. Chem. Eur. J. 2001, 7, 5168. (b) Kongshaug, K. O.; Fjellvag,
H. Solid State Sci. 2003, 5, 303. (c) Pan, L.; Liu, H.; Lei, X.; Huang,
X.; Olson, D. H.; Turro, N. J.; Li, J. Angew. Chem., Int. Ed. 2003, 42,
542.
(12) Ruiz-Perez, C.; Lorenzo-Luis, P. A.; Lloret, F.; Julve, M. Inorg. Chim.
Acta 2002, 336, 131.
8268 Inorganic Chemistry, Vol. 44, No. 23, 2005

Study of the NiII(BDC)/2,2′-Bipyridine System
Figure 3. (a) Perspective view of the dimeric molecule of 2. Hydrogen
atoms except for the carboxylic acid proton are omitted for clarity, and
thermal ellipsoids are at the 30% probability level. (b) Packing diagram of
2 showing π-π stacking of the 2,2′-bipy ligands. One discrete molecule is
highlighted by a green line. (c) CH-π interaction between two HBDC
ligands (blue dotted line) and hydrogen bonding (green dotted line).
sheet. As in 3, each trimer in MOF-3 is linked to six adjacent
trimers, but unlike 3, where the centers of all six neighbors
are located in the same plane leading to a layered structure,
in MOF-3, four of the neighbors are displaced (two above
and two below), resulting in a three-dimensional framework.
Figure 4. (a) Perspective view of the main building unit of 3. Hydrogen
atoms are omitted for clarity, and thermal ellipsoids are at the 50%
probability level. (b) Top view of a two-dimensional layer in 3 showing
connectivity of the trinuclear Ni(II) building units. A chelating 2,2′-bipy
ligand coordinates to each trimer on each side of the plane. (c) Interlayer
π-π interactions through sandwiched 2,2′-bipy ligands. Each 2,2′-bipy
shown in purple is sandwiched between two 2,2′-bipy ligands shown in
yellow and vice versa.
(15) Li, H.; Davis, C. E.; Groy, T. L.; Kelley, D. G.; Yaghi, O. M. J. Am.
Chem. Soc. 1998, 120, 2186.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8269

Go et al.
Figure 5. Schematic views of connectivity between trimeric M(II) units
in two-dimensional compound 3 (top) and three-dimensional compound
MOF-3 (bottom). The BDC linkers are schematically shown in blue. The
connectivity between trimers in the direction of the view is identical in the
two compounds.
Compound 5. Compound 5 (Figure 6) is based on infinite
zigzag chains where nickel centers chelated by two 2,2-bipy
ligands are bridged by bis-monodentate BDC ligands. The
monodentate oxygen atoms, O2, are in cis positions, as found
in all Ni(2,2′-bipy)2L2 octahedral complexes leading to the
idealized Ni-Ni-Ni angle of 90°. The observed Ni-Ni-
Ni angle in 5 is 98.24(6)°, while the angle in compound 4 is
87 or 88°. The Ni-Ni distance, 11.355(1) Å, is similar to
that found in compound 4 for nickel atoms bridged by bis-
monodenate BDC ligands [11.305(4) and 11.151(1) Å]. This
distance is longer than that in other compounds where two
nickel atoms are bridged by bischelating BDC ligands:
10.535(2) and 10.576(2) Å in 1, 10.652(10) Å in 2, and
10.572(1) Å in 3. The guest water molecule (O1w) in
compound 5 is hydrogen-bonded to a noncoordinating
carbonyl oxygen (O1) atom from the bis-monodentate BDC
ligand [dO1‚‚‚O1w ) 2.745(3) Å].
Figure 6. (a) Perspective view of the Ni2+ coordination environment in
5 showing hydrogen bonds with guest H2O molecules (dotted line).
Hydrogen atoms are omitted for clarity, and thermal ellipsoids are at the
30% probability level. (b) Fragment of the one-dimensional single-chain
structure. (c) Chain-packing diagram of 5 with a red line highlighting a
single chain.
of 3 is relatively weak compared to that of most compounds
containing BDC ligands. In the IR spectrum of 5, the
νasym(CO2) and νsym(CO2) stretching vibrations at 1587 and
1377 cm-1 show that all BDC ligands adopt monodentate
coordination modes. The splitting of the antisymmetric
stretching band around 1587(vs) cm-1 is due to the hydrogen
bonding between the carbonyl oxygen of the BDC ligand
and the guest water molecule, and a similar pattern was
observed in compound 4.
Characterization. (i) Vibrational Spectra. In the IR
spectrum of 2, the strong band at 1710 cm-1 indicates the
presence of a protonated carboxylic group, which confirms
the X-ray crystal structure. The νasym(CO2) and νsym(CO2)
stretching vibrations at 1515 and 1407 cm-1, respectively,
show that all other carboxylic groups are chelating. The
(ii) Thermal Analysis. Compound 3 is stable nearly up
to 400 °C and decomposes to NiO, as identified by X-ray
powder diffraction, in one step (onset at 400 °C, completion
at 450 °C; obs 76.80%, calcd 77.15%). Compound 2 shows
ν
asym(CO2) stretching band at 1604 cm-1 in the IR spectrum
8270 Inorganic Chemistry, Vol. 44, No. 23, 2005

Study of the NiII(BDC)/2,2′-Bipyridine System
an unexpected two-step decomposition to NiO from 310 to
400 °C (total weight loss: obs 85.55%, calcd 83.83%). The
intermediate product was identified as compound 3 by X-ray
powder diffraction and IR spectroscopy, and the first weight
loss was in agreement with the corresponding loss of
1/2H2BDC and 1/3(2,2′-bipy) molecules per nickel (obs
30.12%, calcd 29.25%). The second weight loss was
consistent with decomposition of 3 to NiO (obs 55.43%,
calcd 54.58%). This, at first sight surprising, condensation
of the dimeric molecules 3 to a polymer containing trinuclear
clusters, 3, was in agreement with the temperature effect in
the synthesis of these compounds: the reactant ratio that
gives a mixture of 2 and 3 below 180 °C results in a pure
phase 3 when heated above 180 °C. In a similar way,
compound 1 showed an unexpected decomposition profile
in our previous report [calculated weight losses for 0.75H2BDC
per Ni2+ and 1.0(BDC + 2,2′-bipy) per Ni2+ were 27.75%
and 60. 42%]. The new information on the structural
properties and thermal behavior of Ni-BDC/2,2′-bipy coor-
dination polymers allowed the identification of the previously
unknown product of the partial decomposition of the
framework of 1 as compound 3, which was confirmed by
X-ray power diffraction and FTIR spectroscopy. Conse-
quently, the first observed weight loss of 34.56% cor-
molecules were connected by hydrogen bonds and formed
extended chains. A ratio 1:1:2:1 corresponding a slightly
higher pH of 4.75 led to a molecular compound 2 that has,
overall, a lower amount of protonated carboxylic groups per
nickel and per BDC ligand. The same reactant ratio, but
higher synthesis temperature, led to the transformation of
compound 2 to the layered compound 3, where all BDC
ligands were deprotonated and bridging, resulting in the
formation of trinuclear Ni3(BDC)6(2,2′-bipy)2 units, was built
by corner-sharing nickel octahedra. Similarly, compound 3
formed from a mononuclear coordination polymer 1 at higher
synthesis temperature. The increase of the condensation of
metal polyhedra as a result of a synthesis temperature
increase is quite common in coordination polymers and has
been especially well-documented in the case of cobalt
succinates.8a The transformations of 1 and 2 to 3 also occur
as solid-state reactions at 300-310 °C, as shown in ther-
mogravimetric analysis experiments.
A further increase of the pH to 5.25 (reactant ratio 1:1:
2.5:1) led to the previously reported one-dimensional polymer
4, which forms at a wide range of temperatures, 150-210
°C. Increasing either the pH to 8.5 (reactant ratio 1:1:4:1)
or the 2,2′-bipy content in this mixture led to incorporation
of the second chelating 2,2′-bipy into the chain and the
formation of a one-dimensional polymer 5. In contrast, in
the case of a zinc coordination polymer with an overall
M/BDC/2,2′-bipy ratio of 1:1:2, Zn(BDC)(2,2′-bipy)‚(2,2′-
bipy),16 half of the 2,2′-bipy ligands are present as guest
molecules. The examples above further illustrate the struc-
tural and compositional diversity in metal/organic hybrid
materials that can be achieved by adjustment of the reactant
ratio as well as by the versatile connectivity of the BDC
ligands.
1
responded to the loss of 0.75 of guest H2BDC and /3 of
2,2′-bipy (calcd 35.08%) to form the trinuclear compound
3, and the second weight loss was due to the decomposition
of the framework to NiO (obs 50.50%, calcd 50.76%).
Similar to the transformation of 2 to 3, the thermal
transformation of 1 to 3 also paralleled the effect of
increasing the synthesis temperature. Compound 5 lost its
guest water molecules slowly until 150 °C (obs 5.24%, calcd
6.31%) followed by a two-step decomposition to NiO (obs
28.23% and 53.79%). The second weight loss starting at 250
°C was consistent with the loss of one of the two 2,2′-bipy
ligands (obs 28.23%, calcd 27.34%). The final weight loss
of 53.79% was in agreement with the decomposition of
Ni(BDC)(2,2′-bipy) to NiO (calcd 53.27%).
Acknowledgment. We thank the National Science Foun-
dation (Grant DMR-0120463) and the R. A. Welch Founda-
tion.
Supporting Information Available: X-ray crystallographic
data, in CIF format, for the structure determination of 2, 3, and 5.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Conclusion
Our investigation of the NiCl2/BDC/KOH/2,2′-bipy system
resulted in a series of compounds ranging from a dimeric
molecular compound to a two-dimensional polymer. At 160
°C, the reactant ratio of 1:1:1.5:1 (pH 4) led to a one-
dimensional coordination polymer 1. The guest H2BDC
IC050644D
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Inorganic Chemistry, Vol. 44, No. 23, 2005 8271