Organic-acid effect on the structures of a series of lead(II) complexes

Article abstract of DOI:10.1021/ic700613c

An investigation into the dependence of coordination polymer architectures on organic-acid ligands is reported on the basis of the reaction of Pb(NO ₃)₂ and eight structurally related organic-acid ligands in the presence or absence of N-donor chelating ligands. Eight novel lead(II)-organic architectures, [Pb(adip)(dpdp)]₂ 1, [Pb(glu)(dpdp)] 2, [Pb-(suc)(dpdp)] 3, [Pb(fum)(dpdp)]·H₂O 4, [Pb ₂(oba)(dpdp)₂]·2(dpdp)-2(NO₃) ·2H₂O 5, [Pb₂(1,4-bdc)₂(dpdp) ₂]·H₂O 6, [Pb(dpdc)(dpdp)] 7, and [Pb(1,3-bdc)(dpdp)]·H₂O 8, where dpdp = dipyrido[3,2-a: 2′,3′-c]-phenazine, H₂adip = adipic acid, H ₂glu = glutaric acid, H₂suc = succinic acid, H ₂fum = fumaric acid, H₂oba = 4,4′-oxybis(benzoic acid), 1,4-H₂bdc = benzene-1,4-dicarboxylic acid, H₂dpdc = 2,2′-diphenyldicarboxylic acid, and 1,3-H₂bdc = benzene-1,3-dicarboxylic acid, were successfully synthesized under hydrothermal conditions through varying the organic-acid linkers and structurally characterized by X-ray crystallography. Compounds 1-8 crystallize in the presence of organic-acid linkers as well as secondary N-donor chelating ligands. Diverse structures were observed for these complexes. 1 and 5 have dinuclear structures, which are further stacked via strong π-π interactions to form 2D layers. 2-3 and 6-8 feature chain structures, which are connected by strong π-π interactions to result in 2D and 3D supramolecular architectures. Compound 4 contains 2D layers, which are further extended to a 3D structure by π-π interactions. A systematic structural comparison of these 8 complexes indicates that the organic-acid structures have essential roles in the framework formation of the Pb(II) complexes.

Full text of DOI:10.1021/ic700613c

Inorg. Chem. 2007, 46, 6542−6555
Organic-Acid Effect on the Structures of a Series of Lead(II) Complexes
Jin Yang,† Jian-Fang Ma,*,† Ying-Ying Liu,† Ji-Cheng Ma,† and Stuart R. Batten*,‡
Key Lab for Polyoxometalate Science, Department of Chemistry, Northeast Normal UniVersity,
Changchun 130024, People’s Republic of China, and School of Chemistry, Monash UniVersity,
Victoria 3800, Australia
Received March 31, 2007
An investigation into the dependence of coordination polymer architectures on organic-acid ligands is reported on
the basis of the reaction of Pb(NO3)2 and eight structurally related organic-acid ligands in the presence or absence
of N-donor chelating ligands. Eight novel lead(II)
(suc)(dpdp)] 3, [Pb(fum)(dpdp)] H2O 4, [Pb2(oba)(dpdp)2]
[Pb(dpdc)(dpdp)] 7, and [Pb(1,3-bdc)(dpdp)] H2O 8, where dpdp
adipic acid, H2glu glutaric acid, H2suc succinic acid, H2fum
acid), 1,4-H2bdc benzene-1,4-dicarboxylic acid, H2dpdc 2,2
1,3-dicarboxylic acid, were successfully synthesized under hydrothermal conditions through varying the organic-
acid linkers and structurally characterized by X-ray crystallography. Compounds 1 8 crystallize in the presence of
−
organic architectures, [Pb(adip)(dpdp)]2 1, [Pb(glu)(dpdp)] 2, [Pb-
2(dpdp) 2(NO3) 2H2O 5, [Pb2(1,4-bdc)2(dpdp)2] H2O 6,
dipyrido[3,2-a:2 ,3 -c]-phenazine, H2adip
fumaric acid, H2oba 4,4 -oxybis(benzoic
benzene-
‚
‚
‚
)
)
‚
‚
‚
′
′
)
)
)
)
′
)
)
′-diphenyldicarboxylic acid, and 1,3-H2bdc )
−
organic-acid linkers as well as secondary N-donor chelating ligands. Diverse structures were observed for these
complexes. 1 and 5 have dinuclear structures, which are further stacked via strong π−π interactions to form 2D
layers. 2−3 and 6−8 feature chain structures, which are connected by strong π−π interactions to result in 2D and
3D supramolecular architectures. Compound 4 contains 2D layers, which are further extended to a 3D structure by
π−π interactions. A systematic structural comparison of these 8 complexes indicates that the organic-acid structures
have essential roles in the framework formation of the Pb(II) complexes.
Introduction
polydentate ligands, solvents, templates, counterions, non-
covalent interactions such as aromatic-aromatic interactions
that stabilize certain architectures, and so on, may all play a
role in determining the network structure of a compound.3
The rational design and synthesis of novel discrete and
polymeric metal-organic complexes have attracted intense
interest owing to the realization of their potential for use as
functional materials in catalysis, molecular recognition,
separation, and nonlinear optics.1 In this aspect, considerable
progress has been made on the theoretical prediction and
network-based approaches for controlling the topology and
geometries of the networks to produce useful functional
materials.2 However, it is still a great challenge to predict
the exact structures through controlling the factors that affect
the framework formation, and therefore, systematic research
on this topic is still very necessary for understanding the
roles of the factors in the formation of metal coordination
frameworks. Several factors, including the coordination
environment of metal nodes, structural characteristics of the
Up to now, the design and control over coordination
compounds is mainly focused on the incorporation of s-, d-,
and even f-block metals as coordination centers, whereas less
attention has been paid to the p-block metals despite their
important applications in electroluminescent devices and
fluorescent sensors.4 Lead(II), as a heavy p-block metal ion,
possesses a large radius, a variable stereochemical activity,
and a flexible coordination environment, which provides
unique opportunities for the construction of novel network
of topologies and interesting properties. So far, the effect of
organic-acid ligands on the structure formation of s-, d-, and
even f-block metal complexes has been investigated, how-
ever, systematic elucidation of the dependence of such factors
on the formation of lead(II) framework complexes is still
comparatively rare, especially in the presence of a secondary
N-donor chelating ligand.5 So far, 1,10-phenanthroline (phen)
has been widely used to build supramolecular architectures
* To whom correspondence should be addressed. E-mail: jianfangma@
yahoo.com.cn, Fax: +86-431-8509-8620 (J.-F.M.), E-mail: stuart.batten@
sci.monash.edu.au, Fax: +61-3-9905-4597 (S.R.B.).
† Northeast Normal University.
‡ Monash University.
6542 Inorganic Chemistry, Vol. 46, No. 16, 2007
10.1021/ic700613c CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/07/2007

Organic-Acid Effect
Scheme 1. Structure of the dpdp Ligand
because of its excellent coordinating ability and large
conjugated system that can easily form π-π interactions.
However, far less attention has been given to their deriva-
tives. Dipyrido[3,2-a:2′,3′-c]-phenazine (dpdp) as an impor-
tant phen derivative possesses fruitful aromatic systems and
is a good candidate for the construction of metal-organic
supramolecular architectures (Scheme 1). In this work,
through precise control of the organic-acid linker structural
Scheme 2. Organic-Acid Ligands Used in the Construction of Pb(II)
Compounds
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features such as shape, size, and flexibility (Scheme 2), in
the presence of secondary N-donor chelating ligands, a
remarkable class of lead(II) complexes with rich architectures
has been obtained: [Pb(adip)(dpdp)]2 1, [Pb(glu)(dpdp)] 2,
[Pb(suc)(dpdp)] 3, [Pb(fum)(dpdp)]‚H2O 4, [Pb2(oba)(dpdp)2]‚
2(dpdp)‚2(NO3)‚2H2O 5, [Pb2(1,4-bdc)2(dpdp)2]‚H2O 6, [Pb-
(dpdc)(dpdp)] 7, and [Pb(1,3-bdc)(dpdp)]‚H2O 8, where
H2adip ) adipic acid, H2glu ) glutaric acid, H2suc )
succinic acid, H2fum ) fumaric acid, H2oba ) 4,4′-oxybis-
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Inorganic Chemistry, Vol. 46, No. 16, 2007 6543

Yang et al.
Synthesis of [Pb2(1,4-bdc)2(dpdp)2]‚H2O 6. Complex 6 was
synthesized by a method similar to that of complex 1, using 1,4-
H2bdc (0.5 mmol) instead of H2adip as the organic-acid ligand
source. Pale-yellow block crystals were obtained in a 39% yield
based on Pb(II). Elemental analysis and ICP results for C52H30N8O9-
Pb2: Calcd: C 47.13, H 2.28, N 8.46, Pb 31.27; Found: C 47.25,
H 2.41, N 8.32, Pb 31.11.
Synthesis of [Pb(dpdc)(dpdp)] 7. Complex 7 was synthesized
by a method similar to that of complex 1, using H2dpdc (0.5 mmol)
instead of H2adip as the organic-acid ligand source. Pale-yellow
block crystals were obtained in a 47% yield based on Pb(II).
Elemental analysis and ICP results for C32H18N4O4Pb: Calcd: C
52.67, H 2.49, N, 7.68, Pb 28.40; Found: C 52.44, H 2.66, N 7.39,
Pb 28.51.
Synthesis of [Pb(1,3-bdc)(dpdp)]‚H2O 8. Complex 8 was
synthesized by a method similar to that of complex 1, using 1,3-
H2bdc (0.5 mmol) instead of H2adip as the organic-acid ligand
source. Pale-yellow block crystals were obtained in a 41% yield
based on Pb(II). Elemental analysis and ICP results for C26H16N4O5-
Pb: Calcd: C 46.50, H 2.40, N, 8.34, Pb 30.85; Found: C 46.72,
H 2.28, N 8.52, Pb 30.71.
(benzoic acid), 1,4-H2bdc ) benzene-1,4-dicarboxylic acid,
H2dpdc ) 2,2′-diphenyldicarboxylic acid, and 1,3-H2bdc )
benzene-1,3-dicarboxylic acid. On the basis of synthesis and
structural characterization, the influence of organic-acid
linkers on the control of the final complex structures is
discussed in detail. Furthermore, the role of weak intermo-
lecular forces such as aromatic π-π stacking interactions
in the creation of molecular architectures has also been
investigated for the obtained compounds.
Experimental Section
Materials and Methods. All of the reagents of analytical grade
were purchased and used without further purification. The dpdp
ligand was synthesized by following the procedures described
previously.6 A Perkin-Elmer 240 elemental analyzer was used to
collect microanalytical data. The inductively coupled plasma (ICP)
analysis was performed on a Perkin-Elmer Optima 3300DV ICP
spectrometer. The luminescent spectra were measured on a Perkin-
Elmer LS55 spectrometer.
Synthesis of [Pb(adip)(dpdp)]2 1. The pH value of a mixture
of Pb(NO3)2 (0.5 mmol), dpdp (0.5 mmol), and H2adip (0.5 mmol)
in 8 mL distilled water was adjusted to between 5 and 6 by the
addition of triethylamine. The resultant solution was heated at 170
°C in a Teflon-lined stainless steel autoclave for 4 days. The reaction
system was then slowly cooled to room temperature. Pale-yellow
crystals of 1 suitable for single-crystal X-ray diffraction analysis
were collected from the final reaction system by filtration, washed
several times with distilled water, and dried in air at ambient
temperature. Yield: 36% based on Pb(II). Elemental analysis and
ICP results for C24H18N4O4Pb: Calcd: C 45.50, H 2.86, N, 8.84,
Pb 32.70; Found: C 45.77, H 2.69, N 8.90, Pb 32.51.
Crystal Structure Determinations. Single-crystal X-ray dif-
fraction data for complexes 5 and 6 were recorded on a Bruker-
AXS Smart CCD diffractometer, using a ω scan technique with
Mo KR radiation (λ ) 0.71073 Å). Crystallographic data for
compounds 1-4, 7, and 8 were collected on a Rigaku RAXIS-
RAPID single-crystal diffractometer equipped with a narrow-focus,
5.4 kW sealed-tube X-ray source (graphite-monochromated Mo KR
radiation, λ ) 0.71073 Å) at a temperature of 20 ( 2 °C. Data
processing was accomplished with the PROCESS-AUTO processing
program. All of the structures were solved by the Direct Method
of SHELXS-977 and refined by full-matrix least-squares techniques
using the SHELXL-978 program. Non-hydrogen atoms were refined
with anisotropic temperature parameters, and hydrogen atoms of
the ligands were refined as rigid groups. Further details for structural
analysis are summarized in Tables 1 and 2.
Synthesis of [Pb(glu)(dpdp)] 2. Complex 2 was synthesized by
a method similar to that of complex 1, using H2glu (0.5 mmol)
instead of H2adip as the organic-acid ligand source. Pale-yellow
block crystals were obtained in a 44% yield based on Pb(II).
Elemental analysis and ICP results for C23H16N4O4Pb: Calcd: C
44.59, H 2.60, N, 9.04, Pb 33.44; Found: C 44.38, H 2.71, N 9.16,
Pb 33.31.
Results and Discussion
Syntheses and Single-Crystal X-ray Structures of 1-8.
Hydrothermal syntheses favor crystallization of compounds
1-8.9,10 In our first attempts, we only obtained a small
quantity of microcrystals unsuitable for single-crystal X-ray
diffraction under the hydrothermal conditions. Then, we
sensed that changing the pH value of the reaction mixture
may be helpful for crystal growth. In view of this, we
adjusted the pH values of the reaction system. As expected,
at pH 5-6 well-formed single crystals of 1-8 that are
Synthesis of [Pb(suc)(dpdp)] 3. Complex 3 was synthesized
by a method similar to that of complex 1, using H2suc (0.5 mmol)
instead of H2adip as the organic-acid ligand source. Pale-yellow
block crystals were obtained in a 42% yield based on Pb(II).
Elemental analysis and ICP results for C22H14N4O4Pb: Calcd: C
43.64, H 2.33, N, 9.25, Pb 34.22; Found: C 43.53, H 2.19, N 9.34,
Pb 34.36.
Synthesis of [Pb(fum)(dpdp)]‚H2O 4. Complex 4 was synthe-
sized by a method similar to that of complex 1, using H2fum(0.5
mmol) instead of H2adip as the organic-acid ligand source. Pale-
yellow block crystals were obtained in a 37% yield based on Pb-
(II). Elemental analysis and ICP results for C22H14N4O5Pb: Calcd:
C 42.51, H 2.27, N, 9.01, Pb 33.33; Found: C 42.40, H 2.35, N
9.21, Pb 33.14.
(7) Sheldrick, G. M. SHELXS-97, Programs for X-ray Crystal Structure
Solution; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
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Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
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C.; Huang, W.; Lin, B.; Gou, S. H.; Xu, Y. Inorg. Chem. Commun.
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Synthesis of [Pb2(oba)(dpdp)2]‚2(dpdp)‚2(NO3)‚2H2O 5. Com-
plex 5 was synthesized by a method similar to that of complex 1,
using H2oba (0.5 mmol) instead of H2adip as the organic-acid ligand
source. Pale-yellow block crystals were obtained in a 61% yield
based on Pb(II). Elemental analysis and ICP results for C86H52N18O13-
Pb2: Calcd: C 52.71, H 2.67, N, 12.86, Pb 21.15; Found: C 53.23,
H 2.55, N 12.66, Pb 21.47.
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6544 Inorganic Chemistry, Vol. 46, No. 16, 2007

Organic-Acid Effect
Table 1. Crystallographic Data for Compounds 1-4
compound
formula
1
2
3
4
C24H18N4O4Pb
C23H16N4O4Pb
C22H14N4O4Pb
C22H14N4O5Pb
621.58
fw
633.61
619.59
605.56
space group
a (Å)
b (Å)
P-1
P21/c
15.737(3)
7.8514(16)
16.347(3)
90
99.06(3)
90
1994.5(7)
P21/c
15.106(5)
7.885(5)
15.741(5)
90
93.945(5)
90
1870.5(15)
4
2.1506
9.061
P-1
8.770(5)
6.8124(14)
8.1738(16)
18.070(4)
90.88(3)
93.82(3)
102.61(3)
979.3(3)
2
9.733(5)
12.857(5)
81.219(5)
81.435(5)
74.993(5)
1040.7(9)
2
2.022
8.148
1.064
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
4
F (g cm-3
)
2.063
8.500
1.039
2.108
8.660
1.077
µ (mm-1
)
GOF on F 2
1.058
R [I > 2σ(I)]a,b
R1 ) 0.0254, wR2 ) 0.0454
R1 ) 0.0395, wR2 ) 0.0727
R1 ) 0.0398, wR2 ) 0.0903
R1 ) 0.0371, wr2 ) 0.0768
a R1 ) ||Fo| - |Fc||/|Fo|. b wR2 ) {[w(Fo - Fc )2] / [w(Fo )]2}1/2
.
2
2
2
Table 2. Crystallographic Data for Compounds 5-8
compound
formula
5
6
7
8
C86H52N18O13Pb2
C52H30N8O9Pb2
1325.22
C32H18N4O4Pb
C26H16N4O5Pb
671.62
fw
1959.84
729.69
space group
a (Å)
b (Å)
Pccn
22.067(3)
18.186(2)
19.244(3)
90
90
90
7722.6(18)
4
1.686
4.434
P-1
P21/c
7.7814(16)
24.643(5))
13.928(3)
90
103.83(3)
90
2593.2(9)
P-1
9.677(3)
6.1464(12)
9.902(2)
11.884(4)
11.988(4)
100.375(5)
112.273(4)
95.158(5)
1235.8(7)
1
1.781
6.867
1.012
c (Å)
19.151(4)
104.74(3)
90.03(3)
104.59(3)
1088.3(4)
2
2.049
7.801
1.072
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
4
F (g cm-3
)
1.869
6.554
1.063
µ (mm-1
)
GOF on F 2
1.177
R [I > 2σ(I)]a,b
R1 ) 0.0725, wR2 ) 0.1585
R1 ) 0.0519, wR2 ) 0.1254
R1 ) 0.0397, wR2 ) 0.0733
R1 ) 0.0374, wR2 ) 0.0702
2
2
2
a R1 ) ||Fo| - |Fc||/|Fo|. wR2 ) {[w(Fo - Fc )2]/[w(Fo )]2}1/2
.
suitable for single-crystal X-ray diffraction were obtained
in satisfying yields under hydrothermal conditions.
Notably, the dpdp ligands are arranged in a parallel fashion
at both sides of a dimer (Figure 1b), leading to a structure
suitable to form π-π interactions. Indeed, the lateral dpdp
ligands from adjacent dimers are paired to furnish strong
π-π interactions (ca. 3.54 and 3.49 Å) resulting in an
interesting 2D supramolecular layer in the bc plane (Figure
S1 in Supporting Information).
Selected bond distances and angles for compounds 1-8
are listed in Tables S1-S8 (Supporting Information). The
crystal structure of 1 consists of a discrete dinuclear [Pb-
(adip)(dpdp)]2. The [Pb(adip)(dpdp)]2 dimer comprises two
adip, two dpdp ligands, and two Pb(II) atoms and possesses
a crystallographic center of symmetry located at the midpoint
of two Pb(II) centers (Figure 1a). Each Pb(II) atom in 1 is
primarily coordinated by four oxygen atoms from two bis-
chelating adip ligands (Pb1-O1 ) 2.556(3), Pb1-O2 )
2.455(3), Pb1-O3 ) 2.350(3), Pb1-O4 ) 2.607(3) Å) and
two nitrogen atoms from a chelating dpdp ligand (Pb1-N1
) 2.800(3), Pb1-N2 ) 2.730(3) Å) to furnish a distorted
[:PbN2O4] pentagonal bipyramidal geometry.5g The O1, O2,
O4, N1, and N2 atoms comprise the equatorial plane, whereas
the O3 atom and the lone pair of electrons are located in the
axial positions. The Pb(II) center deviates from the coordina-
tion plane by ca. 0.59 Å. The N2 atom from the chelating
dpdp ligand has a weak bonding interaction with the Pb(II)
atom (Pb1-N1 ) 2.800(3) Å), which is longer than ones
found in [Pb(endc)2(phen)]n and [Pb(endc)(phen)‚3H2O]2
complexes (from 2.491(3) to 2.614(4) Å) (endc ) endonor-
bornene-cis-5,6-dicarboxylate),4d indicative of weak interac-
tions between the nitrogen atom and the lead(II) center. The
adjacent Pb(II) atoms are bridged by two adip ligands to
form a dimer with a long Pb-Pb distance of 9.019 Å.
To investigate the influence of organic-acid spacer length
on the complex framework, two short fatty acid ligands,
glutaric acid (H2glu) and succinic acid (H2suc), were used
to react with Pb(NO3)2 and dpdp under similar reaction
conditions as those used for preparing 1, and two new,
complexes [Pb(glu)(dpdp)] 2 and [Pb(suc)(dpdp)] 3, were
formed. When H2glu and H2suc were utilized, respectively,
two similar helical chain structures of 2 and 3 were obtained.
In 2, the Pb(II) center is located in a distorted pentagonal
bipyramidal coordination sphere formed by four oxygen
atoms from two distinct glu ligands, two nitrogen atoms from
one chelating dpdp ligand, and a lone electron pair (Figure
2a). The carboxylate oxygen atoms (O1, O2, and O4A) from
the glu ligands and two nitrogen atoms (N1 and N2) from
the same dpdp ligand make up the basal plane, whereas the
axial positions are occupied by one oxygen atom (O3A) and
the lone pair of electrons. A notable feature for 2 is the
presence of a helical chain (Figure 2b). The glu ligands
bridge each pair of adjacent Pb(II) atoms into a single-
stranded helical chain with a long pitch of 7.851 Å. The dpdp
Inorganic Chemistry, Vol. 46, No. 16, 2007 6545

Yang et al.
Figure 1. (a) View of the dinuclear compound 1 (all of the hydrogen atoms are omitted for clarity) and (b) the 2D supramolecular structure of 1 formed
through π-π interactions.
ligands are alternately attached to both sides of the helical
chain. Another interesting point is the presence of supramo-
lecular interactions between such dpdp ligands (Figure S2
in Supporting Information). The approximately parallel
orientation of the dpdp ligands allows pairing of neighboring
related single-stranded helical chains to generate a supramo-
lecular layer under the direction of aromatic π-π stacking
interactions among the dpdp pairs (Figure 2c). Two adjacent
helical chains of the same supramolecular layer show
opposite chiralities. The face-to-face distance between the
paired dpdp rings is about 3.40 Å, indicating a strong
aromatic π-π stacking interaction. In fact, π-π stacking
interactions play an important role in the formation and
stabilization of the supramolecular structure.
It should be pointed out that in 2 and 3, although H2glu
and H2suc differ in the number of -CH2- groups in their
alkyl chains, they show quite similar helical chain structures
(Figure 3). However, a small difference exists between these
two compounds. As shown in Tables S2 and S3 (Supporting
Information), the average Pb-O bond distance in 2 (2.488
Å) is slightly shorter than the corresponding one in 3 (2.514
Å), whereas the average Pb-N bond distance in 2 (2.734
Å) is longer than the corresponding one in 3 (2.682 Å).
Additionally, the pitch of the helical chain in 2 is slightly
shorter than that in 3 (7.885 Å), because of the influence of
fatty acid spacer length.
To examine the influence of the flexibility of the fatty
acid on the assembly of supramolecular entities, a rigid fatty
acid, H2fum, was used instead of flexible H2glu or H2suc.
Consequently, compound [Pb(fum)(dpdp)]‚H2O 4, which
contains the H2fum ligand and features a new layer structure,
was obtained. An ORTEP view of 4 is shown in Figure 4a.
Each asymmetric unit consists of one Pb(II) atom, one fum,
one dpdp, and one lattice water molecule. The Pb(II) center
is six-coordinated by four carboxylate oxygen atoms from
four different fum anions and by two nitrogen atoms from
the same chelating dpdp ligand in a severely distorted
pentagonal bipyramidal geometry. The distances of the Pb-O
bonds range from 2.582(5) to 2.619(5) Å, which are close
to those of {[Pb(Lb)(H2O)]‚(H2O)0.25}n (from 2.450(4) to
2.662(5) Å) (H2Lb ) 2,4-dinitro-5-hydroxybenzoic acid).4e
In the structure of 4, each fum ligand bridges four Pb(II)
centers in a tetradentate mode, generating a novel layer
structure (Figure 4b). These layers are decorated with dpdp
ligands alternately at two sides of each layer. It is worth
noting that the 2D layers repeat in an -ABAB- stacking
sequence instead of the stacking fashion observed in other
layer compounds.11 Additionally, the interplanar distance
between two neighboring dpdp ligands of adjacent layers
are ca. 3.54 and 3.50 Å (Figure S4 in Supporting Informa-
(11) Chen, C.-L.; Goforth, A. M.; Smith, M. D.; Su, C.-Y.; zur Loye, H.-
C. Inorg. Chem. 2005, 44, 8762.
6546 Inorganic Chemistry, Vol. 46, No. 16, 2007

Organic-Acid Effect
Figure 2. (a) Coordination environment of the Pb(II) center in compound 2 (all of the hydrogen atoms are omitted for clarity), (b) the 1D helical chain
structure of 2, and (c) the layer structure of 2 formed through π-π stacking interactions.
tion), respectively, indicating the presence of face-to-face
π-π interactions that further stabilize the crystal structure
and extend the layers to a unique 3D supramolecular structure
(Figure 4c). Notably, significant O-H‚‚‚O hydrogen bonds
involving the free water molecule (O1W) and weakly
coordinated carboxylate oxygen atoms are also present within
the supramolecular structure.
complexes, four structurally related ligands, H2oba, 1,4-
H2bdc, H2dpdc, and 1,3-H2bdc, were utilized to react with
Pb(NO3)2 using a preparation procedure similar to that for
1-4. Complexes 5-8 were obtained. In 5, each Pb(II) center
coordinates to two carboxylate oxygen atoms and two dpdp
nitrogen atoms (Figure 5a). The overall geometry is best
described as a distorted tetragonal pyramid. The O1, O2,
N1, and N2 atoms lie in equatorial positions, and a lone
electron pair lies in the apical position. It is noteworthy that
To evaluate the effects of bulky phenyl groups of the
dicarboxylate ligands within the framework formation of their
Inorganic Chemistry, Vol. 46, No. 16, 2007 6547

Yang et al.
Figure 3. (a) Coordination environment of the Pb(II) center in compound 3 (all of the hydrogen atoms are omitted for clarity), (b) the 1D helical structure
of 3, and (c) the layer structure of 3 formed through π-π interactions.
the lower coordination numbers of the Pb(II) center are
clearly different from those in 1-4, probably as a result of
steric hindrance from the aromatic dicarboxylate groups. It
should be also noted that for the lattice nitrate group there
is also a weak coordination interaction between the O4 atom
and the center Pb(II) atom, with the Pb1-O4 distance being
2.799 Å. Thus, if the weak Pb1-O4 bond is considered, the
coordination geometry of the central Pb(II) atom can also
be described as a very distorted octahedron. The oba ligand
links two Pb(II) atoms to form a dinuclear structure. In the
dimer, the Pb-O (carboxylate) bond distances are 2.495(9)
and 2.574(8) Å, respectively, being comparable to those of
other Pb(II) complexes in this report. A striking feature of
compound 5 is the presence of free lattice dpdp ligands. To
the best of our knowledge, such an aromatic system is the
first example observed in Pb(II) complexes.5 In contrast with
compounds 1-4, compound 5 has a Pb(II)/dpdp ratio of 1:2.
Although we changed the various molar ratios in the reaction
6548 Inorganic Chemistry, Vol. 46, No. 16, 2007

Organic-Acid Effect
Figure 4. (a) Coordination environment of the Pb(II) center in compound 4 (all of the hydrogen atoms are omitted for clarity), (b) the layer structure of
4, and (c) the 3D supramolecular structure of 4 formed through interlayer π-π interactions.
mixtures, the same compound 5 was always obtained.
Furthermore, the free and coordinating dpdp ligands are held
face-to-face through π-π interactions to generate an inter-
esting two-stranded helical motif (Figure 5b). There are also
strong π-π interactions, which are both intrachain (ca. 3.57
Å) and interchain (ca: 3.49 Å, Figure S5 in Supporting
Information) in nature and form columnar aromatic stackings
as shown in Figure 5c. The overall structure thus formed is
a 2D polymeric framework via the columnar aromatic
stacking involving two-stranded helical motifs (Figure 5c).
To investigate the influence of the length of a ligand spacer
on the structure of a complex, a short 1,4-H2bdc ligand
instead of a H2oba ligand was used, and a new complex [Pb2-
(1,4-bdc)2(dpdp)2]‚H2O 6 was obtained and structurally
determined. In the asymmetric unit of 6, there exists one
Pb(II) atom, two 1,4-bdc ligands, one dpdp ligand, and half
of a water molecule (Figure 6a). The Pb(II) atom is six-
coordinated by two nitrogen atoms of one chelating dpdp
ligand and four oxygen atoms of two 1,4-bdc ligands in a
distorted [:PbN2O4] pentagonal bipyramidal geometry. The
Inorganic Chemistry, Vol. 46, No. 16, 2007 6549

Yang et al.
Figure 5. (a) Coordination environment of the Pb(II) center in compound 5 (all of the hydrogen atoms are omitted for clarity), (b) the two-stranded helical
structure of 5, and (c) the layer structure of 5 formed through π-π interactions.
O1, O3, O4, N1, and N2 atoms comprise the basal plane,
whereas the O2 atom and the lone pair of electrons occupy
the axial positions. The average Pb-O bond length is 2.550
Å, with the O-Pb-O bond angles varying from 49.73(18)
to 119.8(2)°, which compares well with those in 5. In 6,
each Pb(II) atom is linked to two other Pb(II) atoms through
the bis-chelating 1,4-bdc ligands, forming a unique zigzag
chain structure with a pitch of 15.29 Å, where the Pb-Pb-
Pb angle, defined by the orientation of the 1,4-bdc ligands
in the chain, is 83.2°. The dpdp ligands in 6 are extended
on both sides of the chain in a parallel fashion (Figure 6b).
It is noteworthy that there are two types of π-π interactions
in 6. First, the neighboring zigzag chains interact through
π-π stackings between the dpdp and 1,4-bdc ligands (ca.
3.39 Å, Figure S6 in Supporting Information), leading to
interesting 2D supramolecular layers (Figure 6c). Second,
the 2D layers repeat in an -ABAB- stacking sequence as
observed in 4 through another type of π-π interactions
between the dpdp ligands (ca. 3.55 Å, Figure S6 in
Supporting Information), which extends the 2D supramo-
lecular arrays into an interesting 3D supramolecular structure
(Figure 6d).
6550 Inorganic Chemistry, Vol. 46, No. 16, 2007

Organic-Acid Effect
Figure 6. (a) Coordination environment of the Pb(II) center in compound 6 (all of the hydrogen atoms are omitted for clarity), (b) the zigzag chain
structure of 6, (c) the 2D supramolecular structure of 6 formed through interchain π-π interactions, and (d) the 3D structure of 6 formed through π-π
interactions between different supramolecular layers.
To investigate the effects of carboxylate position on the
structure of a compound, the two ligands H2dpdc and 1,3-
H2bdc were used. Single-crystal X-ray structural analysis
revealed that the structure of 7 contained one crystallographi-
Inorganic Chemistry, Vol. 46, No. 16, 2007 6551

Yang et al.
Figure 7. (a) Coordination environment of the Pb(II) center in compound 7 (all of the hydrogen atoms are omitted for clarity), (b) the helical chain
structure of 7, (c) the unusual 1D supramolecular dpdp arrays through two types of π-π interactions, and (d) the 3D supramolecular structure of 7 formed
through the 1D supramolecular dpdp arrays.
cally unique Pb(II) atom, one dpdc ligand, and one dpdp
ligand. An ORTEP view of the local coordination geometry
around the Pb(II) atom is shown in Figure 7a. The Pb(II)
atom is located in the center of a distorted pentagonal
bipyramidal coordination geometry, which is defined by four
oxygen atoms of two dpdc ligands, two nitrogen atoms from
one chelating dpdp ligand, and the lone pair of electrons.
The Pb-O bond lengths are in the range of 2.289(4)-2.643-
(4) Å, and the O-Pb-O angles vary from 51.12(13) to
120.63(14)°. The Pb-O bond lengths correspond to those
observed in [Pb6(dpdc)4O2]n (from 2.248(7) to 2.741(9) Å).4f
Each pair of adjacent Pb(II) atoms are bridged by one dpdc
ligand to form a helical chain with a pitch of 13.928 Å. The
dpdp ligands are extended in a parallel fashion at both sides
of a single-stranded helical chain (Figure 7b). Interestingly,
the aromatic stacking of dpdp ligands forms a 1D supramo-
lecular dpdp array through two types of π-π interactions
(Figure 7c). The neighboring helical chains interact through
these π-π stackings (ca. 3.29 Å, Figure S7 in Supporting
Information) to form 2D supramolecular layers. Furthermore,
another type of π-π interaction between the dpdp ligands
(ca. 3.28 Å, Figure S7 in Supporting Information) in
neighboring layers extend the 2D supramolecular arrays into
an interesting 3D supramolecular structure, which is the
striking features of complex 7 (Figure 7d).
Unexpectedly, when 1,3-H2bdc instead of H2dpdc was
used, structurally different chains are formed in 8 under a
similar hydrothermal reaction for the preparation of 7. As
6552 Inorganic Chemistry, Vol. 46, No. 16, 2007

Organic-Acid Effect
Figure 8. (a) Coordination environment of the Pb(II) center in compound 8 (all of the hydrogen atoms are omitted for clarity), (b) the chain structure of
8, (c) the layer structure of 8 formed through interchain π-π interactions, and (d) the double-layer structure of 8 formed through π-π interactions from
different supramolecular layers.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6553

Yang et al.
shown in Figure 8a, the Pb(II) atom in 8 is coordinated by
four oxygen atoms from two bis-chelating 1,3-bdc ligands
and two nitrogen atoms from a chelating dpdp ligand to
furnish a highly distorted pentagonal bipyramidal coordina-
tion sphere. The Pb-O distances in 8 vary from 2.423(4) to
2.700(4) Å, which are close to those found in Pb2(PMIDA)‚
1.5H2O (from 2.331(9) to 2.876(9) Å) (H4PMIDA ) H2O3-
PCH2N(CH2CO2H)2)).4g Each pair of adjacent Pb(II) atoms
is bridged by the 1,3-bdc bridges to give rise to a zigzag
chain. Unlike the structures of 5-7, the dpdp ligands in 8
are almost extended on one side of the zigzag chain in a
slightly slanted fashion (Figure 8b). The chains are extended
into 2D layers through π-π intercalations between the dpdp
ligands from adjacent chains with a face-to-face distance of
about 3.32 Å (Figure 8c and Figure S8 in Supporting
Information). These 2D layers are further extended through
off-set aromatic π-π stacking interactions of dpdp groups
(ca. 3.41 Å, Figure S8 in Supporting Information) into the
final unusual supramolecular double-layer structure (Figure
8d). It is worth noting that double-layer coordination
polymers have been reported; however, lead(II) complexes
with a supramolecular double-layer feature constructed by
π-π interactions have rarely been documented.12 From the
results presented above, it is concluded that the variation of
organic-acid ligands result in structural changes, and
sometimes it is the dominant factor determining the su-
pramolecular structural features of the coordination com-
pounds.
relatively rigid coordination properties due to the presence
of a -CHdCH- spacer between the two carboxylate groups.
These geometrical differences in the fatty acids result in the
formation of 1-4 with quite distinct supramolecular archi-
tectures under similar synthetic conditions. In 1, two twisted
adip ligands bridge two Pb(II) centers to form a very stable
dinuclear structure. Clearly, the longer adip ligand helps the
formation of the discrete complex 1. 2 and 3 exhibit similar
helical structures, whereas compound 4 possesses a layer
structure. The structural discrepancy between 1 relative to 2
and 3 highlights the effects of ligand spacers upon the
supramolecular framework formation of the complexes.
However, the differences between 3 and 4 show the
importance of ligand flexibility on the complex constructions.
Simultaneously, the influence of aromatic acid ligands such
as H2oba, 1,4-H2bdc, H2dpdc, and 1,3-H2bdc on the complex
structures is also revealed by the structural differences of
5-8. The common feature of the four ligands is that each
contains a large aromatic group. The long oba ligand is a
benzoic acid-based ligand, where the two phenyl rings can
freely twist around the -O- group to meet the requirements
of the coordination geometries of metal ions in the assembly
process. H2dpdc as a flexible ligand allows the two carboxyl-
ate groups to bridge the metals from different directions. In
contrast, the short 1,4-bdc and 1,3-bdc ligands are very rigid
and not twisted. Like the structure of 1, compound 5 also
adopts a dinuclear structure, where a 2D supramolecular
structure constructed through π-π interactions is observed.
Thus, it is clear that the long and flexible nature of the oba
ligand is an important factor during the formation of 5.
Compounds 6 and 8 show similar zigzag chain structures,
and the chains recognize each other under the direction of
π-π interactions to give the 3D and 2D supramolecular
structures, respectively. The structural differences among 5,
6, and 8 can be attributed to the differences in the length
and flexibility in their organic-acid ligands. In comparison
with 5, 6, and 8, the striking feature of 7 is the presence of
single-stranded helices. The key factor for the formation of
helical chains is presumed to be the geometry and flexibility
of the dpdc ligand. Obviously, as far as compounds 1-8
are concerned, the length and flexibility of the organic-acid
ligands are the most critical factors in determining the
final supramolecular structures of the coordination com-
pounds.
Among compounds 1-8, only compound 5 shows lumi-
nescent property, which is described in the Supporting
Information.
In addition to compounds 1-8, two related compounds,
[Pb6(1,4-bdc)6(DMF)7] 9 and [Pb2(1,4-bdc)2(DMF)2(H2O)]
10, have also been synthesized under solvothermal conditions
in the absence of dpdp. The syntheses and structures of 9
and 10 are described in the Supporting Information.
Effect of Organic Acid. The effects of the organic-acid
ligands on the supramolecular structures of compounds 1-8
have been clearly demonstrated. Through varying the organic-
acid ligands under similar synthesis conditions, eight related
Pb(II) compounds were successfully isolated and they exhibit
significant differences in their supramolecular architecture
due to the presence of the different organic-acid ligands.
Generally, the role of organic-acid ligands can be explained
in terms of their differences in shape, size, and flexibility.
In this work, to investigate the influence of the spacer length
(-CH2-) and flexibility of fatty acid ligands on the complex
structures, typical fatty acids such as H2adip, H2glu, H2suc,
and H2fum were used under similar synthetic conditions. The
H2adip, H2glu, and H2suc ligands possess flexibility owing
to the presence of -CH2- spacers between the two car-
boxylate groups. The only structural difference in the three
flexible ligands is the length of their alkyl chains (ligand
spacer). In contrast with H2suc, the H2fum ligand shows
Conclusions
A series of Pb(II) coordination complexes with various
organic-acid ligands have been successfully prepared under
hydro(solvo)thermal conditions and structurally character-
ized, giving lead(II)-organic architectures from dinuclear
to 3D structures. The structural differences among 1-8
shows the important role played by organic acids. The crystal
structures of compounds 1-4 indicate that the flexibility and
spacer length of the fatty acid are essential in determining
the final structures of their compounds. The structural
discrimination among 5-8 indicates the influence of flex-
ibility, length, and carboxylate position of aromatic acid
ligands on structure formation of their complexes. This work
(12) (a) Zhang, J.; Li, Z. J.; Kang, Y.; Cheng, J. K.; Yao, Y. G. Inorg.
Chem. 2004, 43, 8085. (b) Willett, R. D.; Twamley, B. Inorg. Chem.
2004, 43, 954. (c) Mao, J. G.; Clearfield, A. Inorg. Chem. 2002, 41,
2319.
6554 Inorganic Chemistry, Vol. 46, No. 16, 2007

Organic-Acid Effect
greatly enriches the chemistry of self-assembly of Pb(II)-
based coordination compounds. The results also present a
feasible strategy for controlling the synthesis of framework
architectures by organic-acid ligand design. Further studies
are now under way in our laboratory.
east Normal University, and Science Foundation for Young
Teachers of Northeast Normal University (grant 20060304).
Supporting Information Available: Ten X-ray crystallographic
files (CIF), selected bond distances and angles, the syntheses and
structures for compounds 9 and 10, intermolecular π-π interactions
for 1-8, as well as the luminescent property for compound 5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. We thank the Program for New
Century Excellent Talents in Chinese University (grant
NCET-05-0320), National Natural Science Foundation of
China (grant 20471014), the Fok Ying Tung Education
Foundation, the Analysis and Testing Foundation of North-
IC700613C
Inorganic Chemistry, Vol. 46, No. 16, 2007 6555