[M(rac-N-benzyl Asp)(H2O)](M = Co, Ni): Noncentrosymmetric coordination polymeric chains with racemic chiral ligands

Article abstract of DOI:10.1021/ic202302x

The hydrothermal reaction of fumaric acid, benzylamine, and metal salts yielded M[(rac-N-benzyl-Asp)(H₂O)] (M = Co, Ni), 1 and 2, and Ni[(rac-N-benzyl-Asp)(H₂O)₃]?H₂O 3. Under mild hydrothermal conditions, Michael addition of benzylamine to fumaric acid led to the formation of a racemic mixture of N-benzyl aspartic acid enantiomers. The noncentrosymmetric structures of 1 and 2 consist of one-dimensional polymeric chains in which metal cations are bridged by d- and l-N-benzyl aspartate anions alternating along the chain. The centrosymmetric structure of 3 is composed of discrete Ni[(rac-N-benzyl-Asp)(H₂O)₃] units that are connected by hydrogen bonds into layers. The single layers are homochiral but are hydrogen bonded to similar homochiral layers that contain the N-benzyl aspartate with the opposite handedness. Compounds 1 and 2 showed second harmonic generation (SHG), and their magnetic and thermodynamic properties are described.

Full text of DOI:10.1021/ic202302x

Article
pubs.acs.org/IC
[M(rac-N-benzyl Asp)(H2O)] (M = Co, Ni): Noncentrosymmetric
Coordination Polymeric Chains with Racemic Chiral Ligands
Junghwan Do,*,† Yumi Lee,† Jaeun Kang,† Yong Sun Park,† Bernd Lorenz,‡,§ and Allan J. Jacobson*,⊥
†Department of Chemistry, Konkuk University, Seoul 143-701, Republic of Korea
‡Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5002, United States
§Department of Physics, University of Houston, Houston, Texas 77204-5005, United States
⊥Department of Chemistry, University of Houston, Houston, Texas 77204-5641, United States
S
* Supporting Information
ABSTRACT: The hydrothermal reaction of fumaric acid, benzyl-
amine, and metal salts yielded M[(rac-N-benzyl-Asp)(H2O)]
(M = Co, Ni), 1 and 2, and Ni[(rac-N-benzyl-Asp)(H2O)3]·
H2O 3. Under mild hydrothermal conditions, Michael addi-
tion of benzylamine to fumaric acid led to the formation of
a racemic mixture of N-benzyl aspartic acid enantiomers.
The noncentrosymmetric structures of 1 and 2 consist of
one-dimensional polymeric chains in which metal cations are
bridged by D- and L-N-benzyl aspartate anions alternating along
the chain. The centrosymmetric structure of 3 is composed of
discrete Ni[(rac-N-benzyl-Asp)(H2O)3] units that are connected by hydrogen bonds into layers. The single layers are homochiral
but are hydrogen bonded to similar homochiral layers that contain the N-benzyl aspartate with the opposite handedness.
Compounds 1 and 2 showed second harmonic generation (SHG), and their magnetic and thermodynamic properties are
described.
containing a racemic ligand with one chiral center (monochiral).
Racemic N-benzyl aspartic acid was formed by Michael addition of
benzylamine to fumaric acid under mild hydrothermal conditions.
A structurally related discrete molecule, Ni[(rac-N-benzyl-Asp)-
(H2O)3]·H2O, 3 is also described.
INTRODUCTION
■
Recently, metal−organic coordination polymers with non-
centrosymmetric (acentric) structures have received significant
attention due to their potential applications in second-order
nonlinear optics (NLO), ferroelectrics, piezoelectrics, and pyro-
electrics.1,2 In general, noncentrosymmetric materials can be
rationally designed by using preferred coordination geometries
of metal centers and/or carefully chosen bridging ligands.2
Several strategies have been developed to synthesize coordina-
tion polymers with noncentrosymmetric or chiral structures.
The use of optically pure chiral multidentate ligands has proved
EXPERIMENTAL SECTION
■
Materials and Measurements. All chemicals used during this
work were of reagent grade and used as received from commercial
sources without further purification. Powder X-ray diffraction analyses
were performed using a PANalytical X’Pert PRO diffractometer (Cu
Kα radiation). Thermogravimetric analyses (TGA) were carried out in
air at a heating rate of 2 °C/min, using a high-resolution TGA 2950
thermogravimetric analyzer (TA Instruments). Infrared spectra were
recorded on a Hartmann & Braun BOMEM FTIR spectrometer within
a range of 400−4000 cm−1 using the KBr pellet method. Qualitative
analyses of the compounds were performed with a JEOL JSM-5200
scanning electron microscope (SEM) equipped with an EDAX Genesis
energy-dispersive spectroscopy (EDS) detector. UV−visible reflectance
data were collected on a Varian Cary 500 scan UV−vis-NIR spectro-
photometer over the 200−1500 nm spectral range at room temperature.
The absorption spectrum of the sample was approximated using the
Kubelka−Munk function: α/S = (1 − R)2/2R, where R is the reflectance
at a given wavelength, α is the absorption coefficient, and S is the scatter-
ing coefficient. Poly(tetrafluoroethylene) was used as a reference material.
Powder SHG measurements were performed on a modified Kurtz-NLO
to be the most effective and straightforward approach.3−6
A
racemic mixture of chiral ligands often results in crystallization
of a true racemate with equal amounts of L- and D-enantiomers
ordered in a way that leads to a centrosymmetric space group.
In a few cases, however, crystals are formed that contain both
ligand enantiomers, but in a noncentrosymmetric space group.6−8
Examples of this class include Cd(L-N3)2(H2O)2 (L = trans-2,3-
dihydro-2-(4′-pyridyl)-3-(3″-cyanophenyl)benzo[e]indole and
[Cd(papa)(Hpapa)]ClO4·6H2O (Hpapa = 3-(3-pyridyl)-3-
aminopropionic acid).8 In these structures, many weak
interactions, such as H-bonds and π−π stacking, are responsible
for preventing the cancellation of the dipoles.
Here, we report the synthesis, structures, and magnetic and ther-
modynamic properties of M[(rac-N-benzyl-Asp)(H2O)] (M = Co,
Ni; rac = racemate), 1 and 2, which are, to our knowledge, the first
examples of a noncentrosymmetric coordination polymer structure
Received: October 24, 2011
Published: February 24, 2012
© 2012 American Chemical Society
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system using a pulsed Nd:YAG laser with a wavelength of 1064 nm.
Converse piezoelectric measurements were performed on samples
pressed into 12 mm diameter, ∼1 mm thick pellets. The pellets were
cold isostatically pressed at 3500 psi at room temperature and then
heated to 200 °C for 1 day. Silver paste was applied to both sides of
the pellets. The polarization was measured on a Radiant Technologies
RT66A Ferroelectric Test System with a TREK high-voltage amplifier
between room temperature and 165 °C in a Delta 9023 environmental
test chamber. Elemental analyses were performed at Galbraith
Laboratories, Knoxville, TN. The magnetic properties were measured
in a magnetic properties measurement system (MPMS, Quantum
Design) at temperatures between 2 K and ambient as well as in mag-
netic fields up to 50 kOe. The specific heat was studied using a
relaxation method in a commercial Physical Property Measurement
System (PPMS, Quantum Design).
Synthesis. Co[(rac-N-benzyl-Asp)(H2O)] (1). Hydrothermal re-
actions were carried out in 23 mL capacity Teflon-lined stain-
less steel Parr hydrothermal reaction vessels at 180 °C for 3 days.
Cobalt acetate (0.130 g, 1.0 mmol), fumaric acid (0.116 g,
1.0 mmol), benzylamine (0.33 mL, 3.0 mmol), and H2O (5 mL)
were allowed to react. The solution pH values before and after
the reaction were ∼8 and ∼5, respectively. The product was
filtered, and red platy crystals were found as a single phase. The
yield of the product, 1, was 56%, based on cobalt. The product
is stable in air and water and insoluble in common solvents,
such as ethanol, DMF, acetone, and THF. EDS analysis con-
firmed the presence of Co. The elemental analyses gave the
following results: obsd. (wt %) (Co, 19.4; C, 44.2; N, 4.58; H,
4.48), calcd (wt %) (Co, 19.7; C, 44.2; N, 4.68; H, 4.68). IR
(KBr): 3366 m(br), 3195 s, 2959 w, 2923 w, 1653 s, 1541 s,
1445 m, 1400 m, 1351 w, 1339 w, 1311 w, 1227 w, 1109 w,
1071 w, 1033 w, 997 w, 970 w, 893 w, 877 w, 813 w, 752 w,
736 w, 700 m, 637 w, 599 w, 560 w cm−1.
coordinated water molecules for 1 and 2 were found by difference
Fourier analysis and refined isotropically; the hydrogen atoms of
coordinated and lattice water molecules for 3 were not located. All
non-hydrogen atoms were refined anisotropically. The Flack param-
eter refined to 0.04(3) and 0.01(2) for 1 and 2, respectively. Both
compounds 1 and 2 are SHG-active (see below), confirming that they
are noncentrosymmetric. Crystal data for compounds 1, 2, and 3 are
summarized in Table 1. CCDC reference nos. 783740, 783741, and
Table 1. Crystallographic Data for 1−3
1
2
3
empirical formula
formula wt
temperature, K
cryst syst
space group
a, Å
C11H13CoNO5
298.15
C11H13NNiO5
297.9
C11H19NNiO8
351.99
296(2)
296(2)
296(2)
monoclinic
Pn
monoclinic
Pn
monoclinic
P21/n
5.9522(8)
13.3808(17)
7.9745(10)
110.252(2)
595.87(13)
2
5.9343(4)
13.2243(10)
7.9584(6)
110.410(1)
585.34(7)
2
8.238(2)
6.0097(15)
30.584(8)
92.298(4)
1513.0(7)
4
b, Å
c, Å
β, deg
V, Å3
Z
Dc, g cm−3
μ, mm−1
GOF on F2
R1 (I > 2σ(I))
wR2 (all data)
1.662
1.690
1.550
1.452
1.670
1.319
1.076
1.084
0.861
0.0339
0.0282
0.0489
0.0904
0.0742
0.1339
783742 are for 1, 2, and 3, respectively. The data for 1−3 can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of Ni[(rac-N-benzyl-Asp)(H2O)] (2). Hydrothermal re-
actions were carried out in 23 mL capacity Teflon-lined stainless steel
Parr hydrothermal reaction vessels at 200 °C for 3 days. Nickel chlo-
ride hexahydrate (0.130 g, 1.0 mmol), fumaric acid (0.116 g, 1.0 mmol),
benzylamine (0.33 mL, 3.0 mmol), and H2O (5 mL) were allowed to
react. The solution pH values before and after the reaction were ∼9.
The product was filtered, and greenish blue platy crystals were found as
a single phase. The yield of the product, 2, was 65%, based on nickel.
The product is stable in air and water and insoluble in common
solvents, such as ethanol, DMF, acetone, and THF. EDS analysis con-
firmed the presence of Ni. The elemental analyses gave the following
results: obsd. (wt %) (Ni, 20.2; C, 44.3; N, 4.71; H, 4.51), calcd (wt %)
(Ni, 19.6; C, 44.2; N, 4.69; H, 4.69). IR (KBr): 3376 m(br), 3194 s,
3028 w, 2929 w, 1704 m, 1655 s, 1552 s, 1494 w, 1452 w, 1439 w, 1400 m,
1354 w, 1311 w, 1227 w, 1167 w, 1124 w, 1099 w, 1076 w, 1029 w,
1004 w, 974 w, 894 w, 879 w, 815 w, 742 w, 700 m, 603 w, 562 w cm−1.
Synthesis of Ni[(rac-N-benzyl-Asp)(H2O)3]·H2O (3). Hydrothermal
reactions were carried out in 23 mL capacity Teflon-lined stainless
steel Parr hydrothermal reaction vessels at 120 °C for 3 days. Nickel
chloride hexahydrate (0.130 g, 1.0 mmol), fumaric acid (0.116 g,
1.0 mmol), benzylamine (0.11 mL, 1.0 mmol), and H2O (5 mL) were
allowed to react. The solution pH values before and after the reaction
were ∼7 and ∼4, respectively. The product was filtered, and pale blue,
thin rod-shaped crystals were found as a single phase. The yield of the
product, 3 was 40%, based on nickel. The product is stable in air and
water and insoluble in common solvents, such as ethanol, DMF,
acetone, and THF. EDS analysis confirmed the presence of Ni. IR
(KBr): 3406 s(br), 3199 s, 1635 s, 1562 s, 1495 w, 1429 m, 1403 m,
1333 w, 1309 w, 1220 w, 1122 w, 1097 w, 1069 w, 1009 w, 978 w,
890 w, 746 m, 696 m, 632 w, 600 w, 556 w cm−1.
RESULTS AND DISCUSSION
■
Crystal Structures. We found that the Michael addition of
benzylamine to fumaric acid takes place smoothly under mild
hydrothermal reaction conditions. The direct amination of the
unsaturated dicarboxylic acid produced N-benzyl aspartic acid
during the reaction with a metal salt (nickel chloride hexahy-
drate or cobalt acetate) and water. Although several synthetic
methods for amination of α,β-unsaturated dicarboxylic acid
derivatives, such as mono- or diester and monoamide, are well-
known, the direct amination of α,β-unsaturated dicarboxylic
acid has been rarely reported.10
Compounds 1 and 2 are isostructural, and only the structure
of 1 will be described. One crystallographically distinct cobalt
atom is coordinated by one nitrogen and two oxygen atoms
from an N-benzyl aspartate group. Two additional oxygen atoms
from a second N-benzyl aspartate unit and one water molecule
complete the distorted octahedral geometry with the Co−O
distances in the range of 2.056(3)−2.242(3) Å; the Co−N dis-
tance is 2.124(3) Å (Figure 1). The CoO4N(H2O) octahedron
is severely distorted from the regular 90° angles (59.8(1)−
112.8(1)°) and 180° angles (161.35(7)−169.5(1)°) due to the
steric requirement of the chelating carboxylate group (Figure 1).
One nitrogen and three out of four oxygen atoms (O1, O2, O3)
in N-benzyl aspartate groups are coordinated to cobalt atoms, and
one remaining oxygen atom (O4) in a carboxylate group is termi-
nal. Cobalt atoms are bridged by O2 atoms to form ···Co−O2−
Co−O2··· chains with the alternating distances of 2.081(3) and
2.242(3) Å in the [101] direction, forming the infinite neutral
chain, Co[(rac-N-benzyl-Asp)(H2O)] (Figure 2). A novel feature
of this asymmetric chain is the arrangement of D- and L-N-benzyl
Single-Crystal Structure Determination. X-ray single-crystal
analyses for 1−3 were performed on a Siemens SMART platform
diffractometer outfitted with an Apex II area detector and mono-
chromatized graphite Mo Kα radiation. The structures were solved by
direct methods and refined using SHELXTL.9 The hydrogen atoms of
the N-benzyl aspartate were generated geometrically and allowed to
ride on their respective parent atoms. The hydrogen atoms of the
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Figure 1. View of the local structure of 1. Carbon, oxygen, nitrogen,
hydrogen, oxygen (H2O), and Co atoms are shown as black, red, blue,
white, light blue, and pink spheres, respectively.
Figure 3. Displacement ellipsoids of non-hydrogen atoms in the N-
benzyl aspartate unit in 1 drawn at the 50% probability level. Carbon,
oxygen, and nitrogen atoms are gray, red, and blue ellipses, respec-
tively. Hydrogen atoms are represented by white spheres.
Figure 4. Layers formed in the ac plane in 1 by hydrogen bonding
(dotted blue lines) between the chains and the stacking of the layers
along b. Colors are as shown in Figure 2.
Figure 2. View of 1 showing the interchain packing on the bc plane.
Pink circles and polyhedra represent Co atoms, and the other labeling
scheme is the same as that in Figure 1, except that gold and gray bonds
are used to highlight the arrangement of the two enantiomers of the N-
benzyl aspartate ions.
aspartate groups that are coordinated to cobalt atoms on the left
and right side of each chain, respectively.
Adjacent chains are arranged so that the benzyl groups are
interdigitated. The interchain distance between the benzyl groups
is >3.76 Å, suggesting that van der Waals interactions predominate
with only weak π−π stacking along the chain direction. The weak
interactions along the c axis are reflected in the high thermal
displacement parameters of the carbon atoms in the benzyl
group (Figure 3). The structure is further stabilized by the pres-
ence of hydrogen bonds between the coordinated water molecules
and the carboxylate oxygen atoms [O5W···O3 = 2.752(1) and
2.856(1) Å] in adjacent chains, which results in the formation
of layers on the ac plane (Figures 4 and 5).
Figure 5. Layers formed in the ac plane in 1 by hydrogen bonding
(dotted blue lines) between the chains and the stacking of the layers
along b.
The structure of 3 is built up of discrete Ni[(N-benzyl-Asp)-
(H2O)3] molecules packed with lattice water molecules (Figure 6).
Compound 3 contains the same racemic N-benzyl aspartate anions
as are found in 1 and 2. One crystallographically unique nickel
atom is coordinated by one nitrogen atom and two oxygen
atoms from a single N-benzyl aspartate and three water mole-
cules to form an octahedron with Ni−O, Ni−N, and Ni−OH2
with distances of 2.036(3), 2.058(3), 2.088(3), 2.058(3), 2.078(3),
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related (Figure S3, Supporting Information). The lattice water
molecules are lost immediately in flowing air and completely by
80 °C, making it difficult to establish an accurate overall weight
loss. Two of the coordinated water molecules are lost in two
well-defined steps that are complete by 150 and 200 °C.
The compound then decomposes to form NiO, beginning at
∼280 °C and complete by 320 °C, as found for compound 2.
Between 200 and 280 °C, the composition of 3 is the same as
that of 2. High-temperature X-ray diffraction measurements in
this temperature range did not indicate the formation of
crystalline 2 from the thermal decomposition of 3.
The solid-state UV−vis spectra of 1 and 2 were determined
(Figures S4 and S5, Supporting Information). Compound 1
shows one weak absorption band at 8900 cm−1, assigned to
4
4T1g(F) → T2g(F) and the other strong multiplet band in the
region of 19 000−22 000 cm−1 assigned to 4T1g(F) → 4T1g(P).
Between the major two bands, a very weak band is observed at
16 100 cm−1, assigned to T1g(F) → A2g(F). The observed
d−d transitions are expected for octahedrally coordinated
Co(II). The red color of the crystal 1 is due to the absorption
of light at 526 nm (∼19 000 cm−1), corresponding to the
complementary color of green. Compound 1 is an insulating
material with a band gap at room temperature of 4.5 eV. The
major UV bands for 2 occur at 9950, 15 500, and 26 000 cm−1.
These are associated with the d−d spin-allowed transitions for
octahedral Ni2+ from the 3A2g ground state to the three excited
triplet terms. The spin-allowed transitions are assigned to
transitions to the 3T2g, 3T1g(F), and 3T1g(P) states, respectively.
The shoulder at ∼13 300 cm−1 is the result of the spin-
4
4
Figure 6. Structure of 3 viewed along b. Carbon, oxygen, nitrogen,
hydrogen, oxygen (H2O), and Ni atoms are shown as black, red, blue,
white, light blue, and green spheres, respectively. Hydrogen bonds are
shown as dotted blue lines.
and 2.085(3) Å, respectively. The NiO2N(H2O)3 octahedron is
slightly distorted from ideal 90° angles (81.8(1)−96.3(1)°) and
180° angles (172.3(2)−178.1(1)°). Unlike 1 and 2, the N-benzyl
aspartate carboxylate groups do not chelate, and consequently, the
NiO2N(H2O)3 octahedron is not severely distorted.
The structure of 3 contains an extended network of O−H···O
hydrogen bonds (Figure 6). The coordinated water molecules
are H-bonded to adjacent oxygen atoms in the carboxylate
groups. Lattice water molecules are also hydrogen bonded to
oxygen atoms in the carboxylate groups. Single layers of Ni[(N-
benzyl-Asp)(H2O)3] molecules are formed in the ab plane
through hydrogen bonds. The single layers are homochiral but
are hydrogen bonded to similar homochiral layers that contain
the N-benzyl asparate with the opposite handedness, forming
double layers (Figure 6). The double layers stack along the c
axis and interact through van der Waals forces between the
projecting benzyl groups.
Characterization. Thermogravimetric analysis in air shows
that 1 remains stable up to ∼250 °C (Figure S1, Supporting
Information). On further heating, a weight loss of 72.50%
occurs near 300 °C in a single step. The final product after
heating was identified as Co3O4 by powder X-ray diffraction.
The overall observed weight loss is in good agreement with that
calculated for the formation of Co3O4 (73.16%). The thermal
decomposition behavior of 2 (Figure S2, Supporting Informa-
tion) is very similar to that of 1. Thermogravimetric analysis in
air shows that 2 remains stable up to ∼250 °C. On further heat-
ing, a weight loss of 74.32% occurs near 300 °C at a single step.
The final product after heating treatment was identified as NiO
by powder X-ray diffraction, and the overall observed weight loss
is in good agreement with that calculated for the formation of
NiO (75.00%). The decomposition behavior of compound 3 was
investigated because it seemed possible that 2 might form from 3
on dehydration as the structure of the molecular units are closely
1
forbidden transition to the Eg term. The results of 2 are com-
parable to the values for aqueous solutions of Ni2+.11 The greenish
blue color of the crystal 2 is due to the absorption of light at
9950 cm−1 (645 nm), corresponding to the complementary
color of orange. Compound 2 is an insulating material with a
band gap at room temperature of 4.3 eV.
Compounds 1 and 2 are SHG (second harmonic generation)-
active, and powder SHG measurements for 1 and 2 showed a
comparable efficiency to ∼ α-SiO2. No piezoelectric or polariza-
tion properties could be observed for 1 using 12 mm diameter,
1 mm thick pellets.
Magnetic Properties. The magnetic susceptibility χ(T) of
1 measured at 1000 Oe is shown in Figure 7a. A magnetic
phase transition is observed at 5 K, and the sharp peak in χ(T)
suggests that the magnetic order is antiferromagnetic in 1. The
AFM nature of the transition is further supported by the field
dependence of the magnetization (Figure 8a). Below 5 K, in the
AFM state, a metamagnetic transition similar to a spin-flop
transition is observed with increasing external field, resulting in
a steplike increase of the magnetization (inset of Figure 8a). At
higher fields, M(H) increases smoothly, but it does not saturate
at 50 kOe, the maximum field of this investigation. The mag-
netization value of about 0.7 μB at 50 kOe is significantly
smaller than that expected for the Co2+ with a spin of 3/2.
In contrast, the magnetic order of 2 shows a weak ferro-
magnetic (FM) moment below TC = 18 K. The susceptibility of
2 measured at a low field of 10 Oe is shown in Figure 7b. The
difference between field cooling (FC) and zero field cooling
(ZFC) of χ(T) (Figure 7b) and the M(H) data shown in Figure 8b
with a hysteresis loop at lower fields (inset) provide evidence
for the existence of a weak FM moment. However, the FM
moment is very small and the coercive field at 2 K is only 1200
Oe. At higher magnetic fields, M(H) increases almost linearly
with H up to 50 kOe, with a magnetization value at 50 kOe,
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Figure 7. (a) Magnetic susceptibility of 1 showing the antiferro-
magnetic transition at TN = 5 K. The inset shows the inverse suscep-
tibility and the linear Curie−Weiss dependence at higher temper-
atures. (b) Magnetic susceptibility of 2 revealing the ferromagnetric
transition at TC = 18 K. Inset: inverse susceptibility and Curie−Weiss
extrapolation. The Curie−Weiss parameters are discussed in the text.
Figure 8. Magnetization vs field of the antiferromagnet 1 (a) and the
ferromagnet 2 (b) at selected temperatures. The inset of (a) shows the
magnetization details of 1 at low fields with a metamagnetic (spin-
flop) transition in the ordered state. The low-field data shown in the
inset of (b) confirm the weak ferromagnetic character (M−H hysteresis
loops) of the low-temperature phase of 2.
which is even an order of magnitude smaller than that of the Co
compound 1.
which is expected for S = 3/2. This can be explained by an
incomplete quenching of the orbital momentum as reported for
various compounds with high-spin Co2+ before.13 For 2, the
Curie−Weiss parameters are θCW = −184 K (J/kB ≈ 92 K) and
μeff = 3.9 μB. The AFM intrachain exchange coupling of the Ni
spins is about twice as strong as the coupling of the Co spins.
Magnetic long-range order, however, cannot be established in
purely one-dimensional systems. The interchain magnetic cou-
pling is needed to stabilize an ordered state. Those interchain
exchange interactions are very weak because of the large distance
between the magnetic ions of neighboring chains and the weak
interchain (van der Waals) forces. The interchain magnetic cou-
pling could arise from magnetic dipolar interactions; however,
the microscopic nature of the magnetic correlations has yet to be
explored. The low transition temperatures (TN = 5 K for 1, TC =
18 K for 2) are consistent with a weak coupling of neighboring
chains. The results of the magnetic measurements indicate that
the ordered state in 1 is antiferromagnetic and it can be under-
stood as an ordering of the AFM chains in three dimensions. The
small ferromagnetic moment of 2 could be due to a slight canting
of the Ni spins, giving rise to an FM ground state. Whether the
canting exists already at higher temperatures or it arises at the
ferromagnetic TC is not clear, and other explanations, such as a
small ferromagnetic moment, may be considered also. Local
probes of the magnetic structure may shed more light onto the
details of the magnetic order and fluctuations.
The M(H) data of Figure 8 and the low magnetic moments
at 50 kOe show that the magnetic exchange interactions be-
tween nearest-neighbor transition-metal ions in 1 and 2 are strong
and AFM in nature. According to the structure discussed above,
the nearest transition-metal ions form quasi one-dimensional
chains with an oxygen ion (O2) linking two transition metals.
The dominant magnetic interactions along the chain are, there-
fore, superexchange interactions. With an angle of M−O2−M
slightly less than 180°, those interactions are antiferromagnetic
according to the Goodenough−Kanamori rules.12 Therefore,
strong AFM correlations can be expected among Co or Ni ions
within the chains.
The AFM nature of the spin−spin correlations is further
supported by the high-temperature magnetization data of 1 and
2, shown in the insets to Figure 7. The inverse susceptibilities
of both samples follow the Curie−Weiss law at high tempera-
tures and the relevant parameters, Curie−Weiss temperature
θCW and effective magnetic moment μeff, are derived from the
linear part of 1/χ. For 1, we obtain θCW = −80 K and μeff
=
5.0 μB. The negative θCW proves the dominant AFM interac-
tions within the chains. The nearest-neighbor exchange interaction
constant J can be estimated from the mean field formula θCW
=
zJ/kB. With z = 2 in the chain structure, we get J/kB ≈ 40 K.
The large effective moment of 5 μB confirms the high-spin state
of the Co2+, exceeding even the spin-only value of 3.87 μB,
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Specific Heat. The conclusions from magnetic measure-
ments are supported by the results of a specific heat study
at low temperatures. The specific heat for 1 and 2 is shown as
Cp/T in Figure 9. The AFM transition of 1 is clearly indicated
by a peak of Cp/T at TN, which is shown in more detail on an
expanded scale in the inset of Figure 9. However, this peak is
From magnetization and specific heat measurements, we
conclude that the primary magnetic fluctuations are anti-
ferromagnetic along the spin chains and they persist up to high
temperatures. The long-range magnetic order for both
compounds is weak and is a result of the weak interactions
between neighboring chains. Compounds 1 and 2 are second
harmonic generation (SHG)-active with efficiencies comparable
to ∼ α-SiO2. The d−d transitions of Ni and Co lower the trans-
parency of the crystals, and so to improve the SHG properties,
we are currently investigating compounds formed by optically
transparent metals, such as Zn and Cd.14 Also, we are studying
various other ligands formed by Michael addition of donor−
acceptor substituted conjugated amines to fumaric acid utilizing
in situ hydrothermal reactions.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data (CIF) for 1−3, TGA data for 1−3, and
solid-state emission spectra for 1 and 2. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
Figure 9. Heat capacity of 1 (squares) and 2 (circles) below 100 K.
The inset shows the peak at the AFM transition of 1 at an enlarged
scale.
*E-mail: junghwan@konkuk.ac.kr (J.D.), ajjacob@uh.edu (A.J.J.).
Tel: +82-2-450-3416 (J.D.), 713-743-2785 (A.J.J.). Fax: 82-2-
3436-5382 (J.D.), 713-743-2787 (A.J.J.).
relatively small in size and the entropy change associated with
the AFM transition, as derived by integrating the peak area of
the Cp/T data, is only ΔS ≈ 0.06 J/(mol K). This is a tiny
fraction of the maximum entropy change for a spin 3/2 system
from an ordered to a completely disordered state, ΔSmax = 11.5
J/(mol K). The small entropy change across TN proves the high
degree of remaining spin order, supposedly along the chains,
that survives to much higher temperatures, in accordance with
the conclusions from magnetic measurements.
The ferromagnetic transition of 2 is even weaker, and no
peak-like anomaly of the specific heat is detected near the FM
transition temperature (Figure 9). The small change of the
magnetization, though clearly visible in the magnetic data of
Figure 8b, is apparently associated only with a minute change of
entropy beyond the resolution of the specific heat measure-
ments. These results lend further support to the conjecture that
the FM order of 2 possibly results from a small canting of spins
with an existing strong AFM short-range correlation.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research Foundation
of Korea Grant funded by the Korean Government (KRF-2008-
314-C00190), the Robert A. Welch Foundation (Grant No.
E0024), and the Texas Center for Superconductivity at the
University of Houston.
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Noncentrosymmetric coordination polymers, 1 and 2, with
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N-benzyl aspartic acid. The ligand is formed under mild
hydrothermal conditions in situ by Michael addition of benzyl-
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