Coordination geometry induced control of channel morphology in divalent metal pyridinedicarboxylate coordination polymers containing a kinked tethering organodiimine

Article abstract of DOI:10.1016/j.ica.2007.07.035

Hydrothermal synthesis has afforded two divalent metal coordination polymers incorporating tridentate 2,6-pyridinedicarboxylate (PDC) ligands and the kinked dipodal organodiimine 4,4′-dipyridylamine (dpa), {[Ni(PDC)(dpa)(H₂O)] · 2H₂O} (1) and {[Zn(PDC)(dpa)] · 3H₂O} (2), which were characterized by single crystal X-ray diffraction and spectral and thermogravimetric analyses. Although both 1 and 2 display one-dimensional (1-D) polymeric chain motifs, the different coordination environments (octahedral in 1, distorted square pyramidal in 2) provoke divergence in the structures and aggregations of the chain subunits. Compound 1 manifests both polycatenation and interdigitation of its 1-D polymeric chains, while 2 exhibits only interdigitation, resulting in widely disparate morphologies for water molecule-bearing channels within the extended structures. Compound 1 possesses three distinct channel types occupied by isolated water molecules. Compound 2 presents only one type of channel, larger than those in 1, filled with D(5) discrete-chain water molecule aggregations. In both cases the co-crystallized water molecules are anchored to the coordination polymer matrix by hydrogen bonding involving PDC carboxylate oxygen atoms and the central amine unit of the dpa ligands. These supramolecular interactions are crucial for stability, as 1 and 2 both undergo irreversible loss of crystallinity upon dehydration.

Full text of DOI:10.1016/j.ica.2007.07.035

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 317–326
www.elsevier.com/locate/ica
Coordination geometry induced control of channel morphology
in divalent metal pyridinedicarboxylate coordination
polymers containing a kinked tethering organodiimine
a
b
a,
*
Stephen M. Saylor , Ronald M. Supkowski , Robert L. LaDuca
a
Lyman Briggs College and Department of Chemistry, E-30 Homes Hall, Michigan State University, East Lansing, MI 48825, USA
b
Department of Chemistry and Physics, King’s College, Wilkes-Barre, PA 18711, USA
Received 30 June 2007; accepted 27 July 2007
Available online 11 August 2007
Abstract
Hydrothermal synthesis has afforded two divalent metal coordination polymers incorporating tridentate 2,6-pyridinedicarboxylate
0
(
PDC) ligands and the kinked dipodal organodiimine 4,4 -dipyridylamine (dpa), {[Ni(PDC)(dpa)(H
2
O)] Æ 2H
2
O} (1) and
{
[Zn(PDC)(dpa)] Æ 3H O} (2), which were characterized by single crystal X-ray diffraction and spectral and thermogravimetric analyses.
2
Although both 1 and 2 display one-dimensional (1-D) polymeric chain motifs, the different coordination environments (octahedral in 1,
distorted square pyramidal in 2) provoke divergence in the structures and aggregations of the chain subunits. Compound 1 manifests
both polycatenation and interdigitation of its 1-D polymeric chains, while 2 exhibits only interdigitation, resulting in widely disparate
morphologies for water molecule-bearing channels within the extended structures. Compound 1 possesses three distinct channel types
occupied by isolated water molecules. Compound 2 presents only one type of channel, larger than those in 1, filled with D(5) dis-
crete-chain water molecule aggregations. In both cases the co-crystallized water molecules are anchored to the coordination polymer
matrix by hydrogen bonding involving PDC carboxylate oxygen atoms and the central amine unit of the dpa ligands. These supramo-
lecular interactions are crucial for stability, as 1 and 2 both undergo irreversible loss of crystallinity upon dehydration.
ꢀ
2007 Elsevier B.V. All rights reserved.
Keywords: Hydrothermal synthesis; Crystal structure; Coordination polymer; Dipyridylamine; Pyridinedicarboxylate; Water chain
1
. Introduction
anion, aquo, or carboxylate ligands. The very large scope
of coordination polymer morphologies has therefore
resulted in many unique ‘‘container’’ environments for
supramolecular water molecule tetramers [3a,3b], hexamers
[3c,3d,3e,3f,3g,3h,3i,3j], octamers [3k,3l,3m], decamers [3n],
helices [3o] and nanotubular pipes [3p,3q]. Subtle variances
within a system of related coordination polymers can insti-
gate very large structural differences in the water molecule
aggregations [4]. For instance, 1-D water molecule tapes
based on tetrameric units exist within channels in the struc-
There has been substantial recent interest in hydrogen-
bonded water molecule aggregations in the development
of a deeper understanding of the ‘‘anomalous’’ properties
of water [1]. This effort is driven in part by the important role
of these groupings in aqueous transport in vivo [2], for
instance through the pores in the cell membrane protein
aquaporin-1 [2b]. A highly effective approach to the isola-
tion of water molecule aggregations has involved their sta-
bilization within coordination polymer matrices [3], taking
advantage of hydrogen bonding pathways provided by oxo-
0
ture of the 1-D coordination polymer [Cu(2,4 -bpy) SO ] Æ
4
4
0
0
6H O (2,4 -bpy = 2,4 -bipyridine). Removal of a hydrogen
2
bonding acceptor atom from the pendant ligand causes for-
mation of discrete acyclic water molecule clusters within
incipient voids in the analogous 1-D chain coordination
*
Corresponding author. Tel.: +1 517 432 2268.
E-mail address: laduca@msu.edu (R.L. LaDuca).
0
020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.07.035

318
S.M. Saylor et al. / Inorganica Chimica Acta 361 (2008) 317–326
polymer [Cu(4-phpyr) SO ] Æ 5H O (4-phpyr = 4-phenyl-
pyridine) [4].
Analyses of 2 gave varying results due to differing levels
of dehydration. IR spectra were recorded on powdered
samples using a Perkin–Elmer Spectrum One instrument.
Powder X-ray diffraction experiments were performed on
a Rigaku Rotaflex instrument using h–2h scans.
4
4
2
For some time our research group has been investigating
the synthesis and characterization of coordination polymers
0
containing the kinked dipodal organodiimine 4,4 -dipyridyl-
0
amine (dpa). In contrast to the more commonly used 4,4 -
bipyridine, dpa possesses a hydrogen bonding point of
contact at its central amine functional group. Thus, in addi-
tion to serving as a tethering ligand, dpa can engage in strong
supramolecular interactions and potentially anchor uncoor-
dinated water molecules. Hydrothermal treatment of dpa
with transition metal salts and dicarboxylate anion or poly-
oxometallate sources has to date afforded several coordina-
tion polymers with diverse structural motifs [5–7]. For
example, {[Co (phthalate) (dpa) (H O) ] Æ H O} forms 1-
2.2. Preparation of {[Ni(PDC)(dpa)(H O)] Æ 2H O} (1)
2
2
NiCl Æ 6H O (131 mg, 0.55 mmol), dpa (96 mg,
2
2
0.55 mmol) and 2,6-pyridinedicarboxylic acid (92 mg,
0.55 mmol), were placed into 10 mL distilled H O in a
2
23 mL Teflon-lined Parr acid digestion bomb. The bomb
was sealed and heated at 150 ꢁC for 48 h, whereupon it
was cooled slowly to 25 ꢁC. Very large blue blocks of 1
(90 mg, 35% yield based on Ni) were isolated after washing
with distilled water, ethanol, and acetone and drying in air.
Anal. Calc. for C H N NiO 1: C, 45.47; H, 4.04; N,
2
2
2
2
4
2
D coordination polymer chains that encapsulate co-crystal-
lized water molecules [5], [Mo O (Hdpa) ] possesses
4
13
2
17 19
4
7
ꢀ
1
unprecedented interdigitated 1-D molybdate ribbons that
can intercalate primary and secondary amines [6], and
12.48. Found: C, 45.09; H, 4.08; N, 12.54%. IR (cm ):
3170 w br, 1622 m, 1587 s, 1518 s, 1486 m, 1431 w, 1375
m, 1350 s, 1276 w, 1208 s, 1179 w, 1072 w, 1057 w, 1023
m, 1016 m, 918 w, 906 w, 844 w, 820 m, 768 m, 731 m,
693 m, 675 m.
[
NiMoO (dpa) ] has a ‘‘starburst’’ 3-D structure formed
4 2
2
n+
by the linkage of cationic [Ni(dpa)2]
layers through
molybdate tetrahedra [7]. Hanton and co-workers have
extended the coordination chemistry of dpa into monovalent
silver oxoanion and divalent cadmium oxoanion and thiocy-
anate coordination polymer systems, revealing the ability of
dpa to impart chirality via torsion around its central kink [8].
We report here the hydrothermal synthesis and charac-
terization of the first pyridinedicarboxylate coordination
polymers to incorporate the dpa ligand, {[Ni(PDC)(dpa)-
2
.3. Preparation of {[Zn(PDC)(dpa)] Æ 3 H O} (2)
2
An identical preparative procedure to that for 1 was fol-
lowed, with the substitution of ZnCl (75 mg, 0.55 mmol)
2
for NiCl Æ 6H O. Straw-colored blocks of 2 (80 mg, 32%
2
2
yield based on Zn) were obtained. Anal. Calc. for
C H N O Zn (with loss of one H O) 2: C, 46.67; H,
(
H O)] Æ 2H O} (1) and {[Zn(PDC)(dpa)] Æ 3H O} (2), where
2 2 2
1
7
18
4
7
2
PDC = 2,6-pyridinecarboxylate. Both new compounds dis-
play 1-D coordination polymer chains, albeit with signifi-
cant differences in their morphologies and aggregations
dictated by the coordination environment at the metal,
resulting in differing 1-D channels occupied by uncoordi-
nated water molecules. Three distinct types of isolated
water molecule-bearing channels of different size course
through the structure of 1. On the other hand, 2 manifests
one type of larger channel morphology that contains dis-
crete water molecule chains. In both cases, the supramolec-
ular interactions provided by the dpa ligand to the
uncoordinated water molecules are critical to coordination
polymer stability.
3
.68; N, 12.80. Found: C, 46.26; H, 4.21; N, 13.31%. IR
ꢀ1
(cm ): 3200 w br, 1627 m, 1592 s, 1519 m, 1488 w, 1451
w, 1435 w, 1346 s, 1277 m, 1210 s, 1183 w, 1078 w, 1062
m, 1027 s, 911 w, 852 w, 821 s, 756 m, 729 s, 686 w, 673 w.
2.4. X-ray crystallography
A blue fragment of 1 (with dimensions 0.35 mm ·
.30 mm · 0.30 mm) cleaved from a larger crystal and a
0
straw-colored block of 2 (0.36 mm · 0.26 mm · 0.24 mm)
were subjected to single crystal X-ray diffraction using a Bru-
ker-AXS SMART 1k CCD instrument. Reflection data was
acquired using graphite–monochromated Mo Ka radiation
˚
(k = 0.71073 A). The data was integrated via SAINT [9]. Lor-
2
. Experimental
entz and polarization effect and empirical absorption correc-
tions were applied with SADABS [10]. The structures were
solved using direct methods and refined on F using SHELXTL
2
.1. General considerations
2
[
11]. All non-hydrogen atoms were refined anisotropically.
Metal chlorides were obtained from Fisher, and 2,6-
Hydrogen atoms bound to carbon atoms were placed in cal-
culated positions and refined isotropically with a riding
model. The hydrogen atoms bound to the central nitrogen
of the dpa moieties and any water molecules (except
for O3W in 1 and 2) were found via Fourier difference
maps, then restrained at fixed positions and refined isotrop-
ically. Relevant crystallographic data for 1 and 2 is listed in
Table 1.
pyridinedicarboxylic acid was purchased from Aldrich.
0
4
,4 -Dipyridylamine (dpa) was prepared via a published
procedure [6]. Water was deionized above 3 MX in-house.
Thermogravimetric analysis was performed on a TA
Instruments TGA 2050 Thermogravimetric Analyzer with
a heating rate of 10 ꢁC/min up to 900 ꢁC. Elemental Anal-
ysis was carried out using a Perkin–Elmer 2400 Series II
CHNS/O Analyzer.

S.M. Saylor et al. / Inorganica Chimica Acta 361 (2008) 317–326
319
0
Table 1
via combination of the appropriate metal chloride, 4,4 -
dipyridylamine, and 2,6-pyridinedicarboxylic acid. The
infrared spectra of 1 and 2 were fully consistent with their
formulations. Sharp, medium intensity bands in the range
Crystal and structure refinement data for 1 and 2
Data
1
2
Empirical formula
Formula weight
Collection T (K)
C
449.06
173(2)
0.71073
orthorhombic
Ccc2
17
H
18
N
4
NiO
7
C
455.74
173(2)
0.71073
triclinic
17 18 4 7
H N O Zn
ꢀ1
ꢀ1
of ꢁ1600 cm to ꢁ1200 cm correspond to stretching
modes of the pyridyl rings of the dpa and PDC moieties
˚
k (A)
[
12]. Features corresponding to ring puckering exist in
the region between 820 cm and 600 cm . Asymmetric
and symmetric C–O stretching modes of the fully deproto-
Crystal system
Space group
ꢀ1
ꢀ1
ꢀ
P1
˚
a (A)
15.849(5)
24.720(8)
9.905(3)
90
90
90
3881(2)
8
1.534
9.3476(15)
9.3569(15)
11.4693(18)
94.826(2)
98.196(2)
107.202(2)
940.0(3)
2
˚
b (A)
nated, ligated PDC molecules were evidenced by very
˚
ꢀ1
c (A)
strong, slightly broadened bands at ꢁ1590 cm
and
a (ꢁ)
b (ꢁ)
c (ꢁ)
ꢀ1
ꢀ1
ꢁ
1350 cm , respectively. The 240 cm difference between
these two bands is consistent with monodentate binding of
each carboxylate terminus within the PDC ligands [13].
˚
3
V (A )
ꢀ1
ꢀ1
Z
Broad bands in the region of ꢁ3400 cm to ꢁ3200 cm
ꢀ3
D
calc (g cm
)
1.610
1.356
in all cases represent N–H stretching modes within the
dpa ligands and O–H stretching modes within the water
molecules of crystallization in 1 and 2, and the aquo ligand
in 1. The broadness of these latter spectral features is
caused by significant hydrogen bonding pathways in both
cases, which is described in detail below.
ꢀ
1
l (mm
)
1.048
Minimum/maximum T
hkl ranges
0.7106/0.7440
ꢀ20 6 h 6 20,
0.6410/0.7367
ꢀ11 6 h 6 12,
ꢀ12 6 k 6 12,
ꢀ15 6 l 6 15
11359
ꢀ
32 6 k 6 32,
12 6 l 6 12
ꢀ
Total reflections
Unique reflections
21080
4473
4443
R
int
0.0372
285/9
0.0222
286/7
Parameters/restraints
3
{
.2. Structural description of
R
R
1
1
(all data)
(I > 2r(I))
0.0480
0.0342
0.0978
0.0909
0.0437
0.0429
0.1116
0.1112
[Ni(PDC)(dpa)(H O)] Æ 2H O} (1)
2
2
wR
wR
2
(all data)
X-ray diffraction revealed that 1 crystallized in the non-
2
(I > 2r(I))
ꢀ
˚
3
Maximum/minimum residual (e /A ) 0.510/ꢀ0.222
Goodness-of-fit 1.058
0.887/ꢀ0.508
centrosymmetric orthorhombic space group Ccc2, with a
Flack parameter [14] of 0.011 indicating high enantiopurity
within the crystal. The asymmetric unit of 1 (shown in
Fig. 1) consists of a divalent nickel atom with a distorted
octahedral [NiN O ] coordination sphere, one PDC ligand,
one dpa ligand, one aquo ligand and a sum total of two full
water molecules of crystallization, one of which (O1W) lies
on a crystallographic twofold rotation axis. The oxygen
atom with the uncoordinated water molecule represented
1.298
3
. Results and discussion
.1. Synthesis and spectral characterization
3
3
3
Compounds 1 and 2 were prepared cleanly as single
phase crystalline products under hydrothermal conditions
Fig. 1. Asymmetric unit of 1, with thermal ellipsoids at 50% probability and atom numbering scheme. Most hydrogen atoms and all water molecules of
crystallization have been omitted for clarity.

320
S.M. Saylor et al. / Inorganica Chimica Acta 361 (2008) 317–326
Table 2
Selected bond distance (A) and angle (ꢁ) data for 1
involves interaction between dpa amine subunits in two
separate chains and the water molecule of crystallization
lying on crystallographic twofold rotation axes (O1W,
maroon in Fig. 3). The second pathway involves hydrogen
bonding donation from the aquo ligand (at H5B) to unli-
gated water molecule (O2W, light green in Fig. 3), which
in turn donates a hydrogen bond to a ligated carboxylate
oxygen (O3). The supramolecular hydrogen bonding
within the pseudo layer is assisted by p–p stacking (cen-
˚
#
1
Ni1–N4
Ni1–N3
Ni1–O5
Ni1–N1
Ni1–O4
Ni1–O3
C16–O1
C17–O2
C17–O3
C16–O4
1.980(3)
2.055(3)
2.097(2)
2.101(3)
2.143(2)
2.151(2)
1.225(4)
1.234(5)
1.266(5)
1.271(4)
N3 –Ni1–N1
O5–Ni1–N1
N4–Ni1–O4
92.93(10)
174.06(11)
77.06(10)
100.71(10)
88.25(11)
92.41(9)
77.47(10)
104.35(10)
91.06(11)
90.87(10)
154.52(10)
126.0(3)
#
1
#1
N3 –Ni1–O4
O5–Ni1–O4
N1–Ni1–O4
N4–Ni1–O3
#1
N3 –Ni1–O3
O5–Ni1–O3
N1–Ni1–O3
O4–Ni1–O3
O4–C16–O1
O2–C17–O3
˚
troid-to-centroid distance = 3.827(2) A) between dpa pyr-
idyl rings in abutting chains. Additionally, unligated
water molecules (O3W, light blue in Fig. 3) are held to
the coordination polymer backbone by donation of hydro-
gen bonds to unligated carboxylate oxygen atoms (O1).
However, O3W does not bridge neighboring coordination
polymer chains. From the crystal structure solution, only
half of these positions are occupied, a point corroborated
by elemental and thermal analysis.
#
1
N4–Ni1– N3
N4–Ni1–O5
170.73(10)
89.76(10)
81.15(10)
96.14(10)
127.0(3)
#
1
N3 –Ni1–O5
N4–Ni1–N1
Symmetry transformation to generate equivalent atoms: #1 x ꢀ 1/2, ꢀy
1/2, z + 1/2.
+
Adjacent pseudo 2-D layers are then further aggregated
into the full three-dimensional (3-D) crystal structure of 1
through another supramolecular hydrogen bonding path-
way, between aquo ligands in one layer (via H5A) and unli-
gated carboxylate oxygen atoms (O2) in another (Fig. 4).
To maximize this interaction, the PDC ligands belonging
to neighboring layers interdigitate slightly. The Ni–Ni
through-space separation between chains in adjacent
by O3W is best refined with half occupancy. Two cis-dis-
posed nitrogen donors belong to two different dpa ligands,
while the third nitrogen donor and two of the three oxygen
donors are part of a single chelating, tridentate PDC
ligand. The coordination sphere is rounded out by an aquo
ligand. The disposition of the oxygen and nitrogen donors
lie in a mer orientation about the nickel atom. Bond lengths
and angles within 1 are standard for distorted octahedral
geometry about Ni, and are given in Table 2.
˚
pseudo 2-D layers is 7.695(1) A. Metrical parameters for
all supramolecular hydrogen bonding pathways in 1 are
given in Table 3.
By means of the tethering dpa ligands, neighboring Ni
atoms are linked into 1-D chain motifs, as seen in Fig. 2.
The cis orientation of the dpa nitrogen donors at each Ni
atom and the torsion (37.9ꢁ) between pyridyl rings of the
dpa ligands results in a zig-zag pattern within the 1-D coor-
dination polymer chains. Weak C–Hꢂ ꢂ ꢂO interactions
Viewing the structure down the c crystal axis (Fig. 4)
reveals the presence of three different types of 1-D water
bearing channels, occupied by unligated water molecules.
Channel A is occupied by water molecules of type O1W
(maroon) and lies solely within a single pseudo 2-D layer.
As calculated by PLATON [16], the oblong-shaped chan-
nel A is extremely small, contributing no measurable void
space to the overall structure. Channel B, demarcated by
unligated water molecules of type O2W (light green), repre-
sents 5.2% of the overall volume of 1. The rhomboidal
˚
˚
(
C1ꢂ ꢂ ꢂO4, 3.080(4) A; C6ꢂ ꢂ ꢂO4, 3.228(4) A) provide an
ancillary intrachain contact pathway. The chelating PDC
ligands can be seen to alternate above and below the chain
motif. To accommodate the undulations within the chains,
the two carboxylate moieties are flexed slightly away from
the plane of the PDC pyridyl ring by ꢁ6.2ꢁ and ꢁ6.8ꢁ. The
˚
˚
shape of channel B measures approximately 7.7 A ·
Ni–Ni separation through the dpa tether is 11.367(1) A.
˚
6
.0 A. Channel C, partially occupied with water molecules
We have recently observed similar zig-zag 1-D [NiL (dpa)]
4
of type O3W (light blue), is the largest of the 1-D channels
chain patterns in the triply interpenetrated PtS-structure
three-dimensional coordination polymer [Ni(adipate)-
within 1. It manifests an ‘‘hourglass’’ appearance with
˚
maximum and minimum dimensions of ꢁ10.6 A and
(
dpa)(H O)] [15].
2
˚
ꢁ
2.2 A and occupies 7.8% of the unit cell volume. Both
Interwoven through each set of parallel 1-D chains is an
channels B and C spread through the regions between
neighboring pseudo 2-D layers. Channel A is adjacent to
six other channels, two of each of the three types. Channel
B is bracketed by four channels of type A and four of type
C. In turn, channel C is surrounded by four channels of
type A, two of type B, and two of type C. The total solvent
accessible void space within 1 is 13.0% of the total volume.
It is illustrative to compare the salient structural features
of 1 with two previous Ni complexes incorporating
identical set, offset by ꢁ64ꢁ, which forms a pseudo 2-D
layer motif parallel to the ac crystal plane. Individual sets
of parallel 1-D chains are shown in Fig. 3 in blue and
1
orange. The closest Ni–Ni separation between interwoven
˚
chains is 6.783(1) A. The interweaving is accomplished
through two different supramolecular hydrogen bonding
pathways involving water molecules of crystallization
trapped within the interchain spaces. The first of these
both 2,6-pyridinedicarboxylate and an organodiimine.
0
1
{
[Ni (PDC) (H O) (4,4 -bpy)] Æ 2 H O} forms discrete
For interpretation of the references in color in this figure legend, the
2
2
2
4
2
reader is referred to the web version of this article.
molecular dimeric complexes, with two [Ni(PDC)(H O) ]
2 2

S.M. Saylor et al. / Inorganica Chimica Acta 361 (2008) 317–326
321
Fig. 2. A single 1-D chain motif within 1.
Fig. 3. Pseudo 2-D layer formed by offset interweaving chain motifs in 1. Hydrogen bonding interactions are shown as dashed lines.
0
units bridged through the rigid rod 4,4 -bpy tethering
ligand [17]. These then aggregate through hydrogen bond-
ing and p–p stacking to construct small channels occupied
by water molecule dimers. Use of the longer tethering
gen bonding point of contact, the dpa ligand in 1 dramat-
ically alters the water-bearing channel morphology, thus
forcing the entrained water molecules to remain isolated
from one another.
0
ligand E-4,4 -dipyridylethene (dpe) resulted in a salt-like
complex with formula {([H dpe][Ni(PDC) ]) Æ 15H O}
3.3. Structural description of {[Zn(PDC)(dpa)] Æ 3 H O}
2
2
3
2
2
[
18]. This material contained 1-D channels occluding dis-
(2)
crete acyclic water trimers and nonamers, in addition to
isolated water molecules. By providing access to a 1-D
coordination polymer chain motif and possessing a hydro-
Single crystal X-ray diffraction revealed that compound
2 crystallized in the centrosymmetric triclinic space group

322
S.M. Saylor et al. / Inorganica Chimica Acta 361 (2008) 317–326
Fig. 4. Interdigitation of pseudo 2-D layers in 1, viewed down c. Inter-layer hydrogen bonding interactions are shown as dashed lines. The three different
types of 1-D water-bearing channels are also evident. Channels A, B, and C are marked by water molecules in maroon, light green, and light blue,
respectively. (For interpretation of the references in color in this figure legend, the reader is referred to the web version of this article.)
with an asymmetric unit (Fig. 5) consisting of one five-
Table 3
˚
coordinated divalent Zn atom, one tridentate PDC ligand,
one dpa ligand, and three unligated water molecules, one of
which is disordered equally over two positions (O3WA/
O3WB). The geometrical factor s for the coordination
environment at Zn is 0.42, indicative of a very distorted
Hydrogen bonding distance (A) and angle (ꢁ) data for 1 and 2
D–Hꢂ ꢂ ꢂA
d(Hꢂ ꢂ ꢂA) \DHA
d(Dꢂ ꢂ ꢂA) Symmetry
transformation
for A
Compound 1
N2–H2Nꢂ ꢂ ꢂO1W
O1W–H1Aꢂ ꢂ ꢂO4
[
ZnN O ] square pyramidal geometry [19]. The basal plane
3 2
2.01(3)
1.91(3)
175(4)
160(3)
2.875(4)
2.728(3)
x, y, z ꢀ 1
ꢀx + 1/2,
consists of the three donor atoms from the PDC ligand and
one nitrogen atom belonging to a bridging dpa ligand, with
ꢀ
y + 1/2,
z + 1/2
x, ꢀy,
˚
the Zn atom located ꢁ0.61 A above best-fit plane defined
O2W–H2Aꢂ ꢂ ꢂO3
O5–H5Aꢂ ꢂ ꢂO2
2.05(4)
1.78(3)
1.95(3)
157(4)
177(4)
169(4)
2.853(4)
2.627(5)
by these four atoms. A third nitrogen donor, from a differ-
ent dpa ligand, occupies the apical position. In order to
accommodate the distorted trans orientation of the oxygen
donors, the carboxylate moieties of the PDC ligands twist
by ꢁ9ꢁ and ꢁ8.4ꢁ relative to the plane of the pyridine ring,
slightly greater than seen in 1. The N–Zn–N bond angles
fall in the range of 113.63(11)–126.25(11)ꢁ, and the O–
Zn–O bond angle subtends 151.91(9)ꢁ. The Zn–O bond
z + 1/2
ꢀx, y + 1,
z + 1/2
O5–H5Bꢂ ꢂ ꢂO2W
O3Wꢂ ꢂ ꢂO1
2.752(4)
2.78
x + 1/2,
y + 3/2,
z ꢀ 1
Compound 2
N2–H2Nꢂ ꢂ ꢂO1W
O1W–H1Aꢂ ꢂ ꢂO3
1.895
2.013
165.5
160.3
2.761
2.829
˚
distances are approximately 0.15–0.20 A longer than the
ꢀx + 1,
ꢀ
y + 2,
z + 2
Zn–N bond distances. Relevant bond lengths and angles
are given in Table 4.
ꢀ
O1W–H1Bꢂ ꢂ ꢂO3WB 1.866
O1W–H1Aꢂ ꢂ ꢂO3WA 2.190
169.5
141.7
2.702
2.903
Extension of the structure of 2 through the tethering dpa
ligands affords a 1-D coordination polymer motif (Fig. 6).
x, y ꢀ 1,
z + 1
˚
Neighboring Zn atoms within the chain are 11.099(1) A
O2W–H2Aꢂ ꢂ ꢂO2
1.907
171.92 2.730
142.9 2.479
˚
O2W–H2Bꢂ ꢂ ꢂO3WA 1.766
ꢀx + 2,
apart, a metal–metal contact distance ꢁ0.3 A shorter than
ꢀ
y + 2,
z + 1
in 1. This is attributable to the square pyramidal coordina-
ꢀ
tion about Zn, which presents less steric hindrance than in

S.M. Saylor et al. / Inorganica Chimica Acta 361 (2008) 317–326
323
Fig. 5. Asymmetric unit of 2 with thermal ellipsoids at 50% probability and atom numbering scheme. Uncoordinated water molecules and most hydrogen
atoms have been omitted for clarity.
1
. As a result, the dpa ligand can adopt a less twisted con-
p–p stacking interactions between PDC pyridine rings (cen-
troid-to-centroid distance = 3.545(2) A), as seen in Fig. 7.
˚
formation in 2, with a ꢁ32.8ꢁ inter-ring torsion angle. All
PDC moieties point in the same direction within the chain
motifs in 2, forming a wave-like pattern in marked contrast
with the zig-zag pattern in 1.
Adjacent sets of chains, with their PDC ligands pointing
in opposite directions, interdigitate and aggregate to form
pseudo 2-D slabs parallel to the ab crystal plane through
Chain motifs within a single slab are also held together
through extensive supramolecular hydrogen bonding path-
ways between carboxylate oxygen atoms and water mole-
cules of crystallization which comprise water molecule
oligomers (Fig. 8) encased in incipient cavities running
down the a crystal direction. The oligomers are comprised
of an acyclic five-molecule discrete chain (O1Wꢂ ꢂ ꢂ
O3WBꢂ ꢂ ꢂO3WAꢂ ꢂ ꢂO2Wꢂ ꢂ ꢂO2W) with an additional water
molecule (O1W) hydrogen bonded to the third molecule in
the chain; the classification symbol for this uncommon [1b]
water molecule oligomer is therefore D(5) [20], with a con-
nectivity pattern reminiscent of 3-methylpentane in a
gauche conformation.
Table 4
˚
Selected bond distance (A) and angle (ꢁ) data for 2
#
1
#1
N3 –Zn1–N1
Zn1–N3
Zn1–N1
Zn1–N4
Zn1–O3
Zn1–O1
C16–O1
C17–O2
C17–O3
C16–O4
2.001(3)
2.014(3)
2.021(3)
2.181(2)
2.193(2)
1.256(4)
1.241(4)
1.273(4)
1.240(4)
119.49(11)
126.25(11)
113.63(11)
100.29(10)
101.06(10)
76.43(10)
92.41(10)
94.11(10)
75.85(10)
151.91(9)
126.6(3)
#1
N3 –Zn1–N4
N1–Zn1–N4
#1
N3 –Zn1–O3
N1–Zn1–O3
N4–Zn1–O3
The Zn–Zn distances between interdigitated and non-
˚
interdigitated chains within a slab subunit are 8.162(1) A
˚
#1
and 9.348(1) A, respectively. The amine functional groups
N3 –Zn1–O1
N1–Zn1–O1
N4–Zn1–O1
O3–Zn1–O1
O2–C16–O1
O4–C17–O3
of the dpa ligands are directed towards the exterior of
the pseudo 2-D slabs, providing a hydrogen bonding point
of contact to an unligated water molecule within a neigh-
boring slab. The full pseudo 3-D crystal structure of 2 is
thereby constructed (Fig. 8) via water-cluster mediated
126.2(3)
Fig. 6. View of the 1-D wavelike chain motif in 2.

324
S.M. Saylor et al. / Inorganica Chimica Acta 361 (2008) 317–326
Fig. 7. A view of the interdigitation of 1-D chain motifs in 2. Discrete branched chain water molecule oligomers are shown in orange. Hydrogen bonding
is indicated with dashed lines. (For interpretation of the references in color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. View down a of the pseudo 3-D structure of 2, highlighting the water-oligomer bearing cavities. Hydrogen bonding between dpa amine units and
unligated water molecules is shown as dashed lines.
hydrogen bonding. Supramolecular contact distance and
angle information for 2 is given above in Table 3.
3.4. Thermogravimetric analysis and dehydration/
rehydration behavior
Channels containing the water oligomers can be seen
coursing down the a crystal direction in Fig. 8. Unlike 1,
which displayed three distinct types of channel, all of the
channels in 2 are identical. According to void space calcu-
lation with PLATON [16], the elliptical channels in 2
occupy 18.8% of the unit cell volume, substantially larger
than the combined volumes of all channels within 1.
Compounds 1 and 2 were subjected to thermogravimet-
ric analysis under flowing N to probe their dehydration
2
and decomposition behavior. TGA traces for these com-
pounds are shown in Figs. S1 and S2. The Ni coordination
polymer 1 underwent dehydration at the beginning of the
heating cycle, with elimination of all coordinated and

S.M. Saylor et al. / Inorganica Chimica Acta 361 (2008) 317–326
325
uncoordinated water molecules complete by ꢁ210 ꢁC
11.9% observed, 12.0% calculated for three water mole-
incorporating the underutilized organodiimine dpa are
underway in our laboratory.
(
cules). The mass then remained steady until ꢁ300 ꢁC.
Above this temperature a broad 71% mass loss feature
extending to ꢁ600 ꢁC was observed, consistent with elimi-
nation of PDC and dpa ligands (75% calculated). The
Acknowledgements
Funding for this work was provided by Michigan State
University. We thank Dr. Rui Huang for performing the
elemental analyses. The thermogravimetric analyzer at
King’s College was purchased with a grant from the Alden
Trust.
1
6.9% remaining mass at 800 ꢁC aligned well with the
deposition of NiO (16.6% predicted). The Zn derivative 2
began to dehydrate immediately upon heating, with loss
of all uncoordinated water molecules occurring by 100 ꢁC
(
11.6% observed mass loss, 11.8% predicted). No further
decomposition was evident until ꢁ225 ꢁC. A 35.6% mass
loss was observed between this temperature and ꢁ390 ꢁC,
consistent with expulsion of the dpa ligand (37.5% calcu-
lated). A further 34.2% mass loss occurred after this point,
which was complete by 525 ꢁC, marking elimination of the
PDC ligand (36.2% predicted). The final mass remnant of
Appendix A. Supplementary material
CCDC 637995 and 637996 contain the supplementary
crystallographic data for 1 and 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1
8.4% is indicative of deposition of ZnO (17.8% expected).
To investigate potential dehydration/rehydration
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
behavior, ground samples of 1 and 2 were held above
their dehydration temperatures for 24 h. While TGA stud-
ies revealed that loss of organic components does not
occur until higher temperatures, both dehydrated samples
were determined to be amorphous by powder XRD. Both
materials remained amorphous upon re-exposure to excess
distilled water for 1 h; therefore the loss of uncoordinated
water resulted in an irreversible loss of crystallinity. It is
thus likely that the hydrogen bonding interactions
between uncoordinated water molecules and the coordina-
tion polymer matrices play a crucial role in the stability of
these materials.
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.ica.2007.07.035.
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