Luminescent zinc and cadmium complexes incorporating 1,3,5-benzenetricarboxylate and a protonated kinked organodiimine: From a hydrogen-bonded layer motif to thermally robust two-dimensional coordination polymers

Article abstract of DOI:10.1016/j.jssc.2007.04.010

Hydrothermal treatment of zinc chloride, 1,3,5-benzenetricarboxylic acid (H₃BTC), and 4,4′-dipyridylamine (dpa) afforded two different complexes depending on reaction conditions, which were characterized by single-crystal X-ray diffraction, infrared spectroscopy, and elemental analysis. Under acidic conditions, a discrete neutral molecular species with formulation [Zn(HBTC)₂(Hdpa)₂] (1) was isolated, which aggregates into two-dimensional hydrogen-bonded layers. Under more basic conditions, the two-dimensional layered coordination polymer [Zn(BTC)(Hdpa)] (2) is obtained, which manifests covalent linkage of [Zn(BTC)(Hdpa)] serpentine chain motifs into 3-connected undulating 4.8² topology 2-D layers. Both 1 and 2 possess tetrahedral coordination at Zn. Use of cadmium nitrate in the synthesis resulted in [Cd(BTC)(H₂O)(Hdpa)] (3), which displays a similar layer topology as 2 but with significant adjustments imparted by octahedral coordination at Cd. In all cases, supramolecular hydrogen bonding promoted by Hdpa ligands provide an important assistive structure-directing role. All materials display blue luminescence upon excitation with ultraviolet light, ascribed to intraligand transitions. Crystallographic data: 1: monoclinic, C2/c, a=25.389(6) A, b=9.811(2) A, c=17.309(4) A, and β=128.957(3)°, 2: monoclinic, P2₁/c, a=13.212(17)c, b=17.15(2) A, c=7.506(10) A, and β=93.71(2)°, and 3: monoclinic, C2/c, a=14.241(6) A, b=15.218(6) A, c=17.976(7) A, and β=109.330(6)°.

Full text of DOI:10.1016/j.jssc.2007.04.010

ARTICLE IN PRESS
Journal of Solid State Chemistry 180 (2007) 1852–1862
www.elsevier.com/locate/jssc
Luminescent zinc and cadmium complexes incorporating
1,3,5-benzenetricarboxylate and a protonated kinked organodiimine:
From a hydrogen-bonded layer motif to thermally robust
two-dimensional coordination polymers
Ã
Maxwell A. Bravermana, Ronald M. Supkowskib, Robert L. LaDucaa,
aLyman Briggs School of Science and Department of Chemistry, Michigan State University, E-30 Holmes Hall, East Lansing, MI 48825, USA
bDepartment of Chemistry and Physics, King’s College, Wilkes-Barre, PA 18711, USA
Received 7 March 2007; received in revised form 12 April 2007; accepted 16 April 2007
Available online 25 April 2007
Abstract
Hydrothermal treatment of zinc chloride, 1,3,5-benzenetricarboxylic acid (H3BTC), and 4,40-dipyridylamine (dpa) afforded two
different complexes depending on reaction conditions, which were characterized by single-crystal X-ray diffraction, infrared
spectroscopy, and elemental analysis. Under acidic conditions,
a discrete neutral molecular species with formulation
[Zn(HBTC)2(Hdpa)2] (1) was isolated, which aggregates into two-dimensional hydrogen-bonded layers. Under more basic conditions,
the two-dimensional layered coordination polymer [Zn(BTC)(Hdpa)] (2) is obtained, which manifests covalent linkage of
[Zn(BTC)(Hdpa)] serpentine chain motifs into 3-connected undulating 4.82 topology 2-D layers. Both 1 and 2 possess tetrahedral
coordination at Zn. Use of cadmium nitrate in the synthesis resulted in [Cd(BTC)(H2O)(Hdpa)] (3), which displays a similar layer
topology as 2 but with significant adjustments imparted by octahedral coordination at Cd. In all cases, supramolecular hydrogen
bonding promoted by Hdpa ligands provide an important assistive structure-directing role. All materials display blue luminescence upon
˚
excitation with ultraviolet light, ascribed to intraligand transitions. Crystallographic data: 1: monoclinic, C2/c, a ¼ 25.389(6) A, b ¼
˚
˚
˚
˚
9.811(2) A, c ¼ 17.309(4) A, and b ¼ 128.957(3)1, 2: monoclinic, P21/c, a ¼ 13.212(17)c, b ¼ 17.15(2) A, c ¼ 7.506(10) A, and b ¼
˚
˚
˚
93.71(2)1, and 3: monoclinic, C2/c, a ¼ 14.241(6) A, b ¼ 15.218(6) A, c ¼ 17.976(7) A, and b ¼ 109.330(6)1.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Coordination polymer; Zinc; Cadmium; Benzenetricarboxylate; Dipyridylamine; Crystal structure; Thermogravimetric analysis
1. Introduction
functional-group selective adsorption [7,8], and photolumi-
nescence [9]. The tremendous structural diversity uncovered
in this MOF system hangs in part on the wide variety of
accessible carboxylate binding modes (from monodentate to
exohexadentate) and protonation levels, as well as numerous
possible supramolecular interaction pathways including
hydrogen bonding and p–p stacking [6–9]. In addition, the
coordination geometry flexibility at d10 divalent Zn or Cd
ions imparted by their lack of crystal field stabilization allows
the local steric environment of the carboxylate ligand to play
a very large role in structure direction. Tetrahedral, square
pyramidal, trigonal bipyramidal, and octahedral coordination
have all been observed in zinc BTC coordination polymers,
while six- to seven-coordinate complexes have been reported
for cadmium BTC MOF systems [6–9].
While their systematic study dates back approximately 15
years, metal-organic framework (MOF) materials remain an
active focus of research due to their significant potential in
diverse applications such as hydrogen storage [1], shape-
selective separations [2], ion exchange [3], catalysis [4], non-
linear optics [5], and luminescence [5]. Divalent d10 metal
1,3,5-benzenetricarboxylate (BTC) coordination polymers
have proven to be an especially fruitful MOF system,
displaying intriguing properties including microporosity [6],
reversible crystal-to-amorphous phase transitions [7],
Ã
Corresponding author.
E-mail address: laduca@msu.edu (R.L. LaDuca).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.04.010

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1853
More recently, the level of structural complexity in this
system has been elevated by the inclusion of organodiimine
co-ligands. Use of chelating imines such as 2,20-bipyridine
(2,20-bpy) [10–12] or exobidentate rigid or flexible tethers
such as 4,40-bipyridine (4,40-bpy) [13,14] or 1,3-di-4-
pyridylpropane (dpp) [15] has allowed access to MOFs
with previously unseen structure types. Many of these
materials manifest interesting properties such as pH-
dependent supramolecular isomerism [12,14] and non-
linear optical and photoluminescent behavior [15]. It is
clear that judicious variance of the organodiimine compo-
nent can foster continued structural elaboration within
zinc/BTC or cadmium/BTC coordination polymers.
lysis was performed on a TA Instruments TGA 2050
Thermogravimetric Analyzer with a heating rate of
10 1C minꢀ1. Elemental analysis was carried out using a
Perkin Elmer 2400 Series II CHNS/O Analyzer. IR spectra
were recorded on powdered samples on a Perkin Elmer
Spectrum One instrument. Luminescence spectra were
recorded with a Spex Fluorolog spectrometer, using
crystalline solid-state samples anchored to quartz slides
using Rexon RX-22P ultraviolet-transparent epoxy.
2.2. Preparation of [Zn(HBTC)2(Hdpa)2] (1)
ZnCl2 (75 mg, 0.55 mmol), dpa (96 mg, 0.56 mmol), and
H3BTC (156 mg, 0.74 mmol) were added to 10 mL of
distilled H2O in a 23 mL Teflon-lined Parr acid digestion
bomb. The pH level was adjusted to 4.7 with 0.6 mL of
1.0 M aqueous HCl. The bomb was sealed, heated to
120 1C for 48 h, and then gradually cooled to 23 1C. The
final reaction mixture had a pH of 4.3. Colorless blocks of
1 (132 mg, 57% yield based on dpa) were obtained after
filtration, washing with distilled water and acetone, and
drying in air. Crystals of 1 were stable indefinitely in air.
Synthetic experiments with ZnCl2, dpa, and BTC in a 1:2:2
molar ratio resulted in a diminished yield of product. Anal.
Calc. for C38H28N6O12Zn 1: C, 55.25; H, 3.42; N, 10.17%;
Found: C, 54.56; H, 3.49; N, 10.21%. IR (KBr, cmꢀ1):
3316 w, 3143 w, 3043 m br, 1716 s, 1654 m, 1616 s, 1583 m,
1532 s, 1490 m, 1358 s, 1251 m, 1215 m, 1190 m, 1080 m,
1031 m, 912 w, 824 m, 746 m, 724 m, 710 m, 680 w, 551 m,
480 w.
For some time we have aimed our research efforts
towards the characterization of coordination polymers
containing the organodiimine 4,40-dipyridylamine (dpa). In
contrast to 4,40-bpy, dpa possesses a kinked disposition of
its nitrogen donor atoms due to its central amine, which
also serves as a potential hydrogen bonding point of
contact. The dpa ligand therefore can cause coordination
polymer subunits to aggregate via supramolecular interac-
tions as well as covalent bonding. Taking full advantage of
the dual functionality of this ligand, we have been able to
prepare several carboxylate and oxide MOFs with diverse
structural motifs [16–18]. For example, {[Ni(dpa)2(succi-
nate)0.5]Cl} forms a unique 5-connected 3-D self-pene-
trated framework [16], [Mo4O13(Hdpa)2] possesses
unprecedented interdigitated 1-D molybdate ribbons that
can intercalate primary and secondary amines [17], and
[NiMoO4(dpa)2] has a ‘‘starburst’’ 3-D structure formed by
the linkage of cationic [Ni(dpa)2]2n+ layers through
molybdate tetrahedra [18]. Hanton has recently extended
the coordination chemistry of dpa into silver, copper, and
cadmium oxoanion systems, revealing the ability of dpa to
impart chirality by acting as a ‘‘double-bladed propeller’’
[19]. We report here the preparation and characterization
of the first benzenetricarboxylate complexes to incorporate
the dpa ligand. In the zinc case, the pH of the initial
reaction mixture causes toggling between a ‘‘zero-dimen-
sional’’ molecular species and an undulating 3-connected
two-dimensional slab coordination polymer displaying
high thermal stability. A different 3-connected two-dimen-
sional slab pattern is observed in the cadmium case due to
its altered coordination geometry. In all three materials,
significant supramolecular interactions promoted by the
dipyridylamine subunit serve an assistive role in structure
direction. Thermal and luminescent properties of these
materials are also described herein.
2.3. Preparation of [Zn(BTC)(Hdpa)] (2)
ZnCl2 (50 mg, 0.37 mmol), dpa (64 mg, 0.37 mmol), and
H3BTC (78 mg, 0.37 mmol) were added to 10 mL of
distilled H2O in a 23 mL Teflon-lined Parr acid digestion
bomb. The pH level was adjusted to 11.5 with 1.0 mL of
1.0 M aqueous NaOH. The bomb was sealed, heated to
120 1C for 24 h, and then gradually cooled to 23 1C. The
final reaction mixture had a pH of 5.3. Colorless blocks of
2 (112 mg, 67% yield) were obtained after filtration,
washing with distilled water and acetone, and drying in
air. Crystals of 2 were stable indefinitely in air. Anal. Calc.
for C19H13N3O6Zn 2: C, 51.31; H, 2.95; N, 9.45%; Found:
C, 50.85; H, 3.00; N, 9.43%. IR (KBr, cmꢀ1): 3400 w br,
3000 m br, 1624 s, 1550 m, 1540 m, 1517 s, 1439 s, 1358 s,
1229 m, 1185 m, 1100 w, 1082 w, 1037 w, 897 w, 872 w, 824
m, 795 m, 724 m, 647 w, 566 w, 536 m, 462 w.
2. Experimental section
2.4. Preparation of [Cd(BTC)(H2O)(Hdpa)] (3)
2.1. General considerations
Cd(NO3)2d4H2O (145 mg, 0.33 mmol), dpa (127 mg,
0.74 mmol), and H3BTC (78 mg, 0.33 mmol) were added
to 10 mL of distilled H2O in a 23 mL Teflon-lined Parr acid
digestion bomb. The pH level was adjusted to 11.5 with
1.0 mL of 1.0 M aqueous NaOH. The bomb was sealed,
heated to 120 1C for 26 h, and then gradually cooled to
ZnCl2, Cd(NO3)2d4H2O and 1,3,5-benzenetricarboxylic
acid (H3BTC) (Aldrich) were obtained commercially. dpa
was prepared via a published procedure [17]. Water was
deionized above 3 MO in-house. Thermogravimetric ana-

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1854
23 1C. The final reaction mixture had a pH value of 5.3.
Colorless blocks of 3 (128 mg, 76% yield based on Cd)
were obtained after filtration, washing with distilled water
and acetone, and drying in air. Crystals of 3 were stable
indefinitely in air. Anal. Calc. for C19H15CdN3O7 3: C,
44.77; H, 2.97; N, 8.24%; Found: C, 44.51; H, 2.84; N,
8.13%. IR (KBr, cmꢀ1): 3323 w br, 2900 w br, 1640 w, 1605
s, 1575 w, 1539 w, 1510 s, 1481 w, 1438 m, 1357 s, 1204 m,
1096 w, 1013 m, 931 w, 870 w, 846 w, 820 s, 758 s, 726 s,
710 s, 688 w.
The structures were solved using direct methods and
refined on F2 using SHELXTL [22]. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms bound
to carbon atoms were placed in calculated positions and
refined isotropically with a riding model. The hydrogen
atoms bound to the dpa moieties in 1–3, the protonated
carboxylate oxygen in 1 and the aquo ligand in 3 were
found via Fourier difference maps, restrained at fixed
positions, and then refined isotropically with 1.2U(eq) of
the N or O atoms to which they are bound. Relevant
crystallographic data for 1–3 are listed in Table 1.
3. X-ray crystallography
4. Results and discussion
A colorless block of 1 (0.25 mm ꢁ 0.20 mm ꢁ 0.20 mm),
a colorless block of 2 (with dimensions 0.50 mm ꢁ
0.40 mm ꢁ 0.10 mm), and a colorless block of 3 (with
dimensions 0.40 mm ꢁ 0.20 mm ꢁ 0.20 mm) were subjected
to single-crystal X-ray diffraction using a Bruker-AXS
SMART 1k CCD instrument at 293(2) K. Reflection data
were acquired using graphite-monochromated MoKa
4.1. Synthesis and spectral characterization
Compounds 1 and 2 were prepared cleanly under
hydrothermal conditions via combination of zinc chloride,
H3BTC, and dpa with appropriate pH adjustment. Use of
cadmium nitrate in a similar synthesis as 2 generated
compound 3 as single-phase colorless blocks. Phase purity
was judged as satisfactory by elemental analysis and
powder X-ray diffraction via comparison with spectra
predicted from the single-crystal structural determinations.
The infrared spectra of all complexes are consistent with
the features evident in their single-crystal X-ray structures.
Sharp, medium intensity bands in the range of
ꢂ1600–ꢂ1200 cmꢀ1 were ascribed to stretching modes of
the pyridyl rings of the dpa moieties [23] and the benzene
rings within HBTC or BTC ligands. Features correspond-
ing to pyridyl and benzene ring puckering mechanisms
˚
radiation (l ¼ 0.71073 A). The data were integrated via
SAINT [20]. Lorentz and polarization effect and empirical
absorption corrections were applied with SADABS [21].
Table 1
Crystal and structure refinement data for 1–3
Data
1
2
3
Empirical
formula
C38H28N6O12Zn
C19H13N3O6Zn
C19H15CdN3O7
were evident in the region between 820 and 600 cmꢀ1
.
Formula weight
Collection T (K)
826.04
293(2)
444.69
293(2)
509.74
293(2)
Asymmetric and symmetric C–O stretching modes of the
benzenetricarboxylate moieties were evidenced by very
strong, slightly broadened bands at ꢂ1600 and
ꢂ1360 cmꢀ1. The Dn values of ꢂ240 cmꢀ1 are consistent
with monodentate coordination of the carboxylate units
[24]. A band for the protonated uncoordinated carboxylate
group in 1 was observed at 1716 cmꢀ1. Spectral features in
the region of ꢂ3400 cmꢀ1 in all cases represent N–H
stretching modes within the dpa ligands in 1–3, and O–H
stretching modes within 3. The broadness of these latter
spectral features is caused by significant hydrogen bonding
pathways (vide infra).
˚
l (A)
Crystal system
0.71073
Monoclinic
C2/c
0.71073
Monoclinic
P21/c
0.71073
Monoclinic
C2/c
Space group
˚
a (A)
25.389(6)
9.811(2)
17.309(4)
128.957(3)
3353.0(4)
4
13.212(17)
17.15(2)
7.506(10)
93.71(2)
1697(4)
4
14.241(6)
15.218(6)
17.976(7)
109.330(6)
3676(3)
8
˚
b (A)
˚
c (A)
b (deg)
3
V (A )
˚
Z
Dcalc (g cmꢀ3
m (mmꢀ1
Min/max T
hkl ranges
)
1.636
0.814
0.962
1.740
1.494
0.792
1.842
1.239
0.787
)
ꢀ32php33,
ꢀ13pk p13,
ꢀ22plp22
18527
ꢀ17php17,
ꢀ22pk p22,
ꢀ9plp9
18102
ꢀ17php18,
ꢀ20pk p20,
ꢀ23plp22
18636
Total reflections
Unique
reflections
3911
3880
4178
4.2. Structural description of [Zn(HBTC)2(Hdpa)2] (1)
R(int)
Parameters/
restraints
0.0711
267/3
0.0392
268/2
0.0389
283/4
As revealed by single-crystal X-ray diffraction, the
asymmetric unit of 1 contains a single divalent Zn atom
on a crystallographic 2-fold axis, one singly protonated,
monodentate HTBC ligand, and one monodentate dpa
molecule protonated at its distal nitrogen locus. While
protonated monodentate dpa ligands are relatively un-
common, they have been previously observed in molydate
and sulfate complexes [17,19b]. The pH range for the
reaction mixture during the synthesis of 1 was between 4.3
and 4.7, allowing equilibrium concentrations of HBTC2ꢀ
R1 (all data)a
R1 (I42s(I))
wR2 (all data)b
wR2 (I42s(I))
Max/min
0.1005
0.0608
0.0427
0.0323
0.0467
0.0295
0.1568
0.1388
0.0817
0.0775
0.0841
0.0700
0.531/ꢀ0.361
0.434/ꢀ0.500
0.616/ꢀ0.840
ꢀ
3
˚
residual (e /A )
GOF
1.031
1.054
1.115
aR1 ¼ S||Fo|–|Fc||/S|Fo|.
P
P
1=2
bwR2 ¼ f½wðFo2 ꢀ F2c Þ ꢃ= ½wFo2ꢃ g
.
2
2

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1855
dianions and Hdpa+ cations as judged by the relative pKa
values for H3BTC (3.14, 3.85, 4.55) and pKb values for dpa
[Zn(HBTC)2(Hdpa)2] is shown with thermal ellipsoids in
Fig. 1. The coordination environment is a distorted
[ZnN2O2] tetrahedron, with the two N donors and two O
donors belonging to two separate Hdpa and HBTC
ligands, respectively. The bond lengths and angles about
Zn are standard for complexes of this type, and are listed in
Table 2. Within the HBTC ligand, the three carboxylate
moieties are twisted relative to the plane of the aromatic
ring by ꢂ171, ꢂ71, and ꢂ31. The torsion angle between the
pyridyl rings of the Hdpa ligand is ꢂ14.21. The hydrogen
atom bound to carboxylate oxygen atom O5 was found
during the structural determination. Protonation at this
site is further corroborated by the longer C18–O5 bond
length relative to C18–O4, indicating predominance of s-
bond characteristics.
(7.65, 8.27) [25].
A complete neutral molecule of
Neighboring molecules of 1 stack via interdigitation
along the c crystal direction and form pseudo 2-D slabs
coincident with the ac crystal plane through two distinct
supramolecular hydrogen bonding pathways (Fig. 2). The
dpa amine subunits (N2–H2N) donate hydrogen bonds to
unligated oxygen atoms of type O2, which belong to the
monodentate carboxylate moieties attached to Zn. Also,
the protonated carboxylate groups engage in hydrogen
bonding donation to oxygen atoms of type O4, which are
part of the unligated unprotonated carboxylate group. A
third type of hydrogen bonding pattern subsequently serves
to link juxtaposed pseudo 2-D slabs into the full 3-D
crystal structure of 1. Here, oxygen atoms of type O3,
which belong to unligated and unprotonated carboxylate
groups projecting above and below each 2-D slab, accept
charge-separated hydrogen bonds from the protonated
imine termini of the Hdpa ligands in neighboring slabs
(Fig. 3). Data for all classical supramolecular interactions
in 1 are given in Table 3.
Fig. 1. Molecular species 1, shown with thermal ellipsoids at 50%
probability. Most hydrogen atoms have been omitted.
Table 2
Selected bond distance (A) and angle (deg) data for 1
˚
Zn1–O1 ( ꢁ 2)
Zn1–N1 ( ꢁ 2)
O1–C19
1.933(3)
2.025(3)
1.283(5)
1.237(4)
1.264(5)
1.247(5)
1.321(5)
1.199(5)
O1#1–Zn1–O1
O1#1–Zn1–N1
O1–Zn1–N1
O1#1–Zn1–N1#1
O1–Zn1–N1#1
N1–Zn1–N1#1
116.80(18)
101.65(12)
114.78(12)
114.78(12)
101.65(12)
107.30(18)
4.3. Structural description of [Zn(BTC)(Hdpa)] (2)
O2–C19
O3–C17
O4–C17
O5–C18
O6–C18
The asymmetric unit of 2 consists of one divalent Zn
atom, one fully deprotonated BTC and one protonated dpa
ligand (Fig. 4). The elevated initial pH of the reaction
mixture as compared to 1 causes full deprotonation of
H3BTC, while the reduction in this value over the course of
Symmetry transformations to generate equivalent atoms: (#1) ꢀx+1, y,
ꢀz+1/2.
Fig. 2. Pseudo 2-D slab formed from the supramolecular interactions of neighboring molecules of 1. Hydrogen bonding is shown as dashed lines.

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1856
Fig. 3. Interaction of pseudo 2-D slabs in 1, showing hydrogen bonding as dashed lines.
Table 3
Hydrogen bonding distance (A) and angle (deg) data for 1–3
˚
D–H
d(H?A)
oDHA
d(D?A)
Symmetry transformation for A
1
N2–H2N?O2
N3–H3N?O3
O5–H5A?O4
1.91(5)
1.70(4)
1.85(4)
153(3)
163(6)
131(5)
2.790(3)
2.594(5)
2.543(5)
ꢀx+1, ꢀy, ꢀz+1
ꢀx+3/2, y+3/2, ꢀz+3/2
x, ꢀyꢀ1, zꢀ1/2
2
N2–H2N?O6
N3–H3N?O2
1.92(2)
1.86(2)
168(2)
143(2)
2.767(4)
2.634(4)
ꢀxꢀ1/2, ꢀy+1/2, ꢀz
ꢀx+1, ꢀy, ꢀzꢀ1
3
N2–H2N?O6
N3–H3N?O5
O7–H7A?O4
O7–H7B?O1
1.93(3)
1.74(3)
1.92(2)
1.92(3)
163(4)
157(4)
175(4)
161(3)
2.788(4)
2.593(4)
2.756(4)
2.724(4)
x, ꢀy+1, zꢀ1/2
x, y, zꢀ1
ꢀx+1, y, ꢀz+3/2
ꢀx+1, ꢀy, ꢀz+1
the reaction permits the presence of an equilibrium
concentration of the Hdpa+ cations required for charge
neutrality in 2. As in 1, the coordination environment
about Zn is best described as distorted tetrahedral,
however with a [ZnO3N] arrangement in this case.
Pertinent bond length and angle information for 2
is given in Table 4. The kinked monodentate Hdpa ligand
is twisted to a much greater extent (ꢂ36.51) than in 1
in order to maximize its supramolecular interactions
(vide infra).
Junction of neighboring coordination units through two
monodentate carboxylate units of a single BTC ligand gives
rise to 1-D [Zn(BTC)(Hdpa)] serpentine chains running
along the c crystal direction. The carboxylate groups
involved in the construction of this chain motif (O1/C17/
O2, group A; O3/C18/O4, group B) are rotated by ꢂ151

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and ꢂ91 relative to the plane of the BTC aromatic ring.
The Zn–Zn distance through the A and B carboxylate
˚
groups of a BTC ligand measures 9.034(2) A. Adjacent
chains then link covalently through a third type of
monodentate carboxylate moiety (O5/C19/O6, group C)
to build an undulating 2-D coordination polymeric slab
substructure (Fig. 5). In order to achieve this connectivity
pattern, carboxylate group C is required to contort
significantly (ꢂ321) from the plane of the BTC aromatic
ring. As
a result, the interchain Zn–Zn distance
˚
(6.715(2) A) is shorter than the intrachain contact. Within
the layer, each Zn atom is connected to five others through
three different triply monodentate BTC ligands, while each
BTC unit acts as an exotridentate connector to three
different Zn atoms. The Zn–ring centroid distances
through carboxylate groups A, B, and C measure 5.52,
˚
4.11, and 5.53 A, respectively. The coordination polymeric
nature of 2 can be viewed as 4.82 3-connected network 2-D
slabs constructed from the junction of 16-membered
[(ZnOC5O)2] rings with 32-membered [(ZnOC5O)4]
ellipsoids, evident from Fig. 5. As defined by the
Zn–Zn and H14–H14 through-space contacts, the distances
across the 16-membered ring patterns measure
Fig. 4. Asymmetric unit of 2, shown with thermal ellipsoids at 50%
probability. Most hydrogen atoms have been omitted.
Table 4
Selected bond distance (A) and angle (deg) data for 2
˚
˚
˚
6.715(2) A ꢁ 3.655(2) A. The smallest and largest through-
Zn1–O5#1
Zn1–O4#2
Zn1–O1
Zn1–N1
O1–C17
O2–C17
O3–C18
O4–C18
O5–C19
O6–C19
1.955(2)
1.967(2)
1.981(3)
2.031(3)
1.275(3)
1.242(3)
1.233(3)
1.285(3)
1.277(3)
1.249(3)
O5#1–Zn1–O4#2
O5#1–Zn1–O1
O4#2–Zn1–O1
O5#1–Zn1–N1
O4#2–Zn1–N1
O1–Zn1–N1
114.21(11)
100.42(9)
109.31(7)
113.41(9)
104.47(10)
115.34(9)
˚
space Zn–Zn distances of 18.082(2) and 4.431(1) A
demarcate the dimensions of the intralayer 32-membered
ellipsoids.
In addition to the information contained in the covalent
bonding in this system, structure direction is assisted by
intralayer hydrogen bonding between the central amine
units of the kinked Hdpa ligands and unligated oxygen
atoms (O6) belonging to carboxylate unit of type C.
Adjacent slabs are then connected into the pseudo 3-D
structure of 2 (Fig. 6) through the pendant protonated
Hdpa ligands which project into the interlamellar regions
Symmetry transformations to generate equivalent atoms: (#1) ꢀx+2, y–1/
2, ꢀz+1/2; (#2) ꢀx, ꢀy+1, ꢀz+1.
Fig. 5. A single 4.82 3-connected [Zn(BTC)(Hdpa)] layer in 2, formed from the junction of 1-D chain motifs through carboxylate group C. Only the N
atoms of the Hdpa ligands are shown for clarity.

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1858
Fig. 6. Interaction of 2-D layers in 2, with Hdpa-promoted intralayer and interlayer hydrogen bonding shown as dashed lines.
and engage in hydrogen bonding with unligated carbox-
ylate oxygen atoms denoted by O2, part of carboxylate
group A. The closest Zn–Zn distance between neighboring
˚
2-D slabs is 9.360(2) A. Supramolecular contact informa-
tion for 2 is given in Table 3.
4.4. Structural description of [Cd(BTC)(H2O)(Hdpa)]
(3)
The asymmetric unit of 3 (Fig. 7) consists of a single
cadmium atom, one fully deprotonated BTC and one
protonated pendant Hdpa ligand, as well as a single bound
water molecule. As is the case for 2, the initial basic reaction
conditions allow full deprotonation of H3BTC. Equilibrium
amounts of charge-compensating Hdpa+ cations could then
subsequently form as the pH of the reaction mixture
decreases. The coordination environment is a distorted
octahedral [CdO5N] pattern, with four of the oxygen atoms
belonging to carboxylate moieties of three distinct BTC
ligands. The coordination sphere is rounded out by a nitrogen
donor from the monodentate protonated dpa ligand and an
aquo ligand, which are disposed in a cis manner to each
other. The longer bond lengths in 3 and increased coordina-
tion number, as compared with 2, are consistent with the
larger ionic radius of Cd2+ relative to Zn2+ [26]. Selected
geometric parameters for 3 are given in Table 5.
Fig. 7. Asymmetric unit of 3 with thermal ellipsoids at 50% probability.
Hydrogen atoms have been omitted for clarity.
Within a single BTC unit, one of its carboxylates (group
A, marked by O1/C17/O2) binds to Cd in a chelating
fashion, while the other two (O3/C18/O4, group B; O5/
C19/O6, group C) display monodentate binding modes.
The bite angle at Cd of chelating carboxylate A is 55.21(8)1;
this group binds to Cd in a slightly asymmetric fashion,
˚
with the Cd–O1 distance 0.02 A shorter than Cd–O2. The
Cd-to-BTC ring centroid distances through carboxylate
˚
groups A, B, and C are ꢂ5.62, ꢂ5.81, and ꢂ4.52 A,
respectively.
Through the two monodentate carboxylate termini
(groups B and C) of the exotridentate BTC ligands,
adjacent Cd atoms are connected into rippled 1-D chain

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1859
Table 5
Selected bond distance (A) and angle (deg) data for 3
˚
Cd1–O3#1
Cd1–N1
Cd1–O6#2
Cd1–O7
Cd1–O1
Cd1–O2
O1–C17
O2–C17
O3–C18
O4–C18
O5–C19
O6–C19
2.210(2)
2.318(3)
2.350(3)
2.354(3)
2.371(2)
2.392(2)
1.269(4)
1.257(4)
1.262(4)
1.240(4)
1.249(4)
1.270(4)
N1–Cd1–O6#2
O3#1–Cd1–O7
N1–Cd1–O7
O6#2–Cd1–O7
O3#1–Cd1–O1
N1–Cd1–O1
O6#2–Cd1–O1
O7–Cd1–O1
O3#1–Cd1–O2
N1–Cd1–O2
O6#2–Cd1–O2
O7–Cd1–O2
O1–Cd1–O2
O2–C17–O1
O4–C18–O3
O5–C19–O6
94.37(9)
91.28(10)
94.52(10)
171.09(8)
130.18(9)
143.08(9)
85.24(9)
87.91(9)
172.18(9)
88.46(9)
97.66(9)
83.01(9)
55.21(8)
121.8(3)
125.1(3)
123.8(3)
O3#1–Cd1–N1
O3#1–Cd1–O6#2
86.66(10)
88.81(10)
Symmetry transformations to generate equivalent atoms: (#1) x, ꢀy, zꢀ1/
2; (#2) ꢀx+1/2, ꢀy+1/2, ꢀz+1.
Fig. 9. 4.82 3-connected networks in 2 (top) and 3 (bottom). In both cases
only the metal atoms and BTC ring centroids are shown.
Fig. 8. Undulating 2-D 4.82 3-connected network layer in 3.
largely within the confines of the layer motifs (Fig. 10), held
in place by supramolecular hydrogen bonding between
both N–H subunits and oxygen atoms belonging to
carboxylate group C in two different BTC ligands. The
Hdpa ligands exhibit an inter-ring torsion angle of 28.21 to
maximize these attractive interactions.
Because the aqua ligands in 3 are disposed cis to the
Hdpa units oriented within the layers, they project above
and below the layer plane and permit aggregation of
abutting 2-D coordination polymer units through hydro-
gen bonding to oxygen atoms belonging to BTC carbox-
ylate units of type A and B (see Table 3 for geometric
details). The confinement of the sterically bulky pendant
Hdpa ligands to the layer subunits allows close approach of
the 2-D coordination polymer subunits in 3 (Fig. 11), with
˚
motifs (Cd–Cd distances ¼ 9.220(1) and 9.371(1) A), which
in turn link to neighboring chains through the chelating
BTC carboxylate unit (group A) to construct undulating
2-D layers parallel to the bc crystal plane, in a 4.82
3-connected network with ring and ellipsoid structural
patterns similar to that observed in 2 (Fig. 8). However, the
chelating carboxylate binding mode and aquo ligand
necessitated by the preference for octahedral coordi-
nation in 3 acts to foster geometric alterations of
these patterns. The 16-membered [(CdOC5O)2] rings
measure 7.746(1) A ꢁ 3.833(1) A, as defined by through-
space Cd–Cd and H16–H16 distances. The 32-membered
[(CdOC5O)4] ellipsoids are substantially less oblate than
the related features in 2, with its cross-channel Cd–Cd
˚
˚
˚
an interlayer Cd–Cd contact distance of 5.052(1) A, almost
˚
4 A nearer than in 2.
˚
distances measuring 11.862(1) and 14.315(1) A. A side-by-
side framework perspective of the 4.82 networks in 2 and 3
(Fig. 9), where the metal atoms and ring centroids are
depicted as 3-connected nodes, highlights these coordina-
tion geometry-driven alterations. The extra width in the
ellipsoidal cavities allows the Hdpa ligands in 3 to lie
4.5. Thermal behavior
Thermogravimetric analysis was performed on ground
samples of 1–3 to ascertain their thermal robustness and

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1860
Fig. 10. Side view of the 2-D layer motif in 3, highlighting the in-plane orientation of the pendant Hdpa ligands.
4.6. Luminescent properties
Irradiation of complexes 1–3 with ultraviolet light
(l ¼ 300 nm) in the solid state resulted in blue–violet
visible light emission in all cases. Slightly different emission
maxima (1, lmaxꢂ375 nm; 2, lmaxꢂ400 nm; 3, lmaxꢂ390 nm)
were observed (Fig. 12). These emissions likely originate
from ligand-centered p–p* or p–n electronic transitions
within either the aromatic ring systems of the BTC or dpa
ligands. Ligand-centered blue light luminescence is a
commonly encountered feature within d10-metal coordina-
tion polymers [27]. The shifts in emission maxima likely
represent subtle differences in ligand-based molecular
orbital energy levels due to variances in carboxylate
binding mode, coordination geometry, and bond lengths
to the metal cations. While deliberate tailoring of emission
properties within d10 coordination polymer systems with
aromatic ligands remains an elusive goal, this class of
materials may provide functional luminescent solids for
light-emitting diode applications.
Fig. 11. Stacking of 2-D slab motifs in 3, shown in a framework
perspective. The aquo ligands engaged in interlayer hydrogen bonding can
be seen projecting above and below each individual layer.
decomposition behavior. Complex 1 underwent a slow
mass loss of 4.8% between 100 and 360 1C, corresponding
to possible decarboxylation of one of its coordinated BTC
ligands (5.3% calc’d). A precipitous mass loss of 86.6%
between 375 and 550 1C indicated complete decomposition
by elimination of all Hdpa and intact and decarboxylated
BTC ligands (86.5% predicted). The remnant of 9.7% of
the original mass corresponds well to the deposition of
ZnO (9.9% calc’d). Compound 2 manifested high thermal
stability, with virtually no mass loss occurring until 375 1C.
Between this temperature and 475 1C a mass loss of 50.2%
was observed, roughly consistent with the expulsion of
Hdpa (38.7% calc’d) and a single equivalent of CO2 due to
decarboxylation of BTC (9.9% calc’d). A slow mass loss of
24.4% took place between 475 and 900 1C, indicative of
continued decarboxylation and combustion of the BTC
ligands. The 25.4% remnant at the instrument temperature
limit of 900 1C indicates a likely admixture of ZnO (18.2%
calc’d) along with some uncombusted organic material.
The cadmium derivative 3 exhibited dehydration between
200 and 300 1C (4.6% mass loss, 3.5% predicted), reflective
of aqua ligand coordination. Loss of CO2 due to
decarboxylation then occurred between 300 and 350 1C
(15.5% mass loss observed, 17.2% calc’d for two equiva-
lents of CO2). Combustion of all organics was complete by
600 1C, with a mass loss of 54.5% consistent with expulsion
of Hdpa and a benzoate unit. The final remnant of 24.9%
of the original mass is consistent with a formulation of
CdO (25.2% theoretical). TGA traces for 1–3 are shown in
Figures S1–S3, respectively.
5. Conclusions
Hydrothermal synthesis has permitted the self-assembly
of three divalent d10-metal complexes incorporating both
BTC and dpa ligands. In the zinc case, the molecular
species [Zn(Hdpa)2(HBTC)2] (1) and the 2-D 4.82 layered
material [Zn(Hdpa)(BTC)] (2) were cleanly prepared
depending on the initial pH of the reaction mixture. In
both cases, the protonated pendant Hdpa ligand serves to
connect adjacent neutral units into the full crystal
structures through extensive supramolecular hydrogen
bonding interactions. Employment of a cadmium precursor
results in the 2-D 4.82 layered coordination polymer
[Cd(Hdpa)(BTC)(H2O)] (3). While the layered motifs in 2
and 3 display the same topological connectivity, the
expansion of the coordination geometry in 3 instigates a
dramatic shift in both the layer morphology and the
supramolecular interaction pathways, resulting in a denser
material. Both the covalent and hydrogen bonding path-
ways promote a high degree of thermal stability, even in the
case of the ‘‘molecular’’ species 1. All three materials
display blue–violet visible light luminescence upon excita-
tion with ultraviolet light, reinforcing the promise of zinc
and cadmium coordination polymeric complexes in light-
emitting device applications. Fruitful efforts to prepare
other dpa-containing coordination polymers continue in

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1861
Fig. 12. Emission spectra of 1–3.
our laboratory, taking advantage of this ligand’s unique
covalent bonding and supramolecular interaction patterns.
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Nature 404 (2000) 982;
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Acknowledgments
[3] (a) Q.-R. Fang, G.-S. Zhu, M. Xue, J.-Y. Sun, S.-L. Qiu, Dalton
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We thank Michigan State University for funding this
work. M.A.B. is grateful for the MSU Quality Fund
Undergraduate Research Program for financial support.
We are indebted to Amanda Smeigh for helpful discus-
sions, Troy Knight for acquisition of the emission spectra,
Dr. James McCusker for use of the fluorimeter, and Dr.
Rui Huang for elemental analysis. We thank the reviewers
for the kind suggestions to improve the manuscript.
(b) X.-M. Zhang, M.-L. Tong, H.K. Lee, X.-M. Chen, J. Solid State
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