Structures and properties of coordination polymers with a rigid zwitterionic pyridinium-based tricarboxylate ligand

Article abstract of DOI:10.1016/j.ica.2016.11.001

Four coordination polymers of different transition metal ions with a new zwitterionic tricarboxylate ligand, 1-(3,5-dicarboxylatophenyl)-4-carboxylatopyridinium (L^2?) were synthesized, and characterized. They are formulated as [Cu₃L<

Full text of DOI:10.1016/j.ica.2016.11.001

Inorganica Chimica Acta xxx (2016) xxx–xxx
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Inorganica Chimica Acta
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Research paper
Structures and properties of coordination polymers with a rigid
zwitterionic pyridinium-based tricarboxylate ligand
⇑
Xiao Yang, Qi Sui, Teng Gong, En-Qing Gao
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, College of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four coordination polymers of different transition metal ions with a new zwitterionic tricarboxylate
ligand, 1-(3,5-dicarboxylatophenyl)-4-carboxylatopyridinium (L2ꢀ) were synthesized, and characterized.
They are formulated as [Cu3L2(OH)2] (1), [M(L)(H2O)] [M = Mn (2) and Cd (3)] and [Zn(L)(H2O)]ꢁ2H2O (4).
Compound 1 exhibits a 3D coordination framework containing carboxylate- and hydroxo-bridged chains.
Compounds 2 and 3 are isomorphic and both contain 2D coordination layers based on carboxylate-
bridged dinuclear units (Mn or Cd). Compound 4 shows a different 2D coordination layer, and extensive
Received 12 September 2016
Received in revised form 30 October 2016
Accepted 3 November 2016
Available online xxxx
Keywords:
Coordination polymers
Fluorescence
Magnetism
Zwitterionic ligands
hydrogen bonding and
p-p stacking interactions lead to a 3D architecture with 1D channels, in which
square tetraaqua clusters are captured through hydrogen bonds. The compound undergoes a structural
transformation upon dehydration and the structure can be switched back through reabsorbing water.
Magnetic studies demonstrated that the mixed l-OH and l-1,3-carboxylate bridges in 1 mediate ferro-
magnetic exchange owing to orbital countercomplementarity. The carboxylate bridges in 2 mediate weak
antiferromagnetic interactions. Compounds 3 and 4 show the fluorescence characteristic of the ligand.
Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction
convenient approach to positively charge the skeletons is to
introduce pyridinium [18] or imidazolium [19]. Such zwitterionic
The continuous study of metal-organic coordination polymers
(CPs) has disclosed the great versatility of metal-ligand coordina-
tion in creating new compounds with fascinating structures and
properties [1–9]. The choice of organic bridging ligands is of para-
mount importance for the construction of CPs through either
rational design or serendipitous synthesis. Multicarboxylate
ligands have been among the most popular ligands used to con-
struct functional CPs with potential applications in adsorption
[10,11], catalysis [12], magnetism [13], luminescence [14] and con-
ductance [15] for several advantages. For instance, the carboxylate
group can form strong coordination bonds in favor of the formation
of robust frameworks with permanent porosity; it can adopt vari-
ous bridging modes and induce magnetic exchange of different
nature and magnitude to the benefit of the study of molecular
magnetism [16,17]. The carboxylate ligands used are mainly those
with neutral aromatic or aliphatic skeletons. That is, the ligands are
anions with the charge depending solely upon the carboxylate
groups. Studies with multicarboxylate ligands bearing positive
charge on the skeletons have been much less common. The most
ligands have recently caused increasing attention for the synthesis
of functional CPs, such as photochromic materials taking
advantage of the photoredox properties of the bipyridinium
chromophore [20–22], and magnetic ones utilizing the charge
effects on the formation of simultaneous azide-carboxylate bridges
[23–25].
Up to now the zwitterionic ligands used for CPs are largely
limited to flexible dicarboxylates with alkyl groups between
pyridinium/imidazolium and carboxylate. We have explored the
coordination chemistry of
a rigid zwitterionic tricarboxylate
(L2ꢀ, Scheme 1). The rigid backbone and the tritopic connectivity,
together with the different charge compared with ordinary tricar-
boxylate ligands (when fully deprotonated), would bring in new
features in the coordination polymers. Here we report the synthe-
sis and structures of the first CPs derived from this ligand, includ-
ing a 3D frameworks with Cu(II), two isomorphous 2D networks
with Mn(II) and Cd(II), and a different 2D Zn(II) network. The last
compound shows 1D channels containing tetraaqua clusters and
exhibits reversible structural transformation switched by
release/absorption of water. Magnetic behaviors of the Cu(II) and
Mn(II) compounds and fluorescence properties are described.
⇑
Corresponding author.
E-mail address: eqgao@chem.ecnu.edu.cn (E.-Q. Gao).
http://dx.doi.org/10.1016/j.ica.2016.11.001
0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
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X. Yang et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
2.1.4. [Mn(L)(H2O)] (2)
COO
N
A solution of Mn(ClO4)2ꢁ4H2O (0.03 mmol, 5.48 mg) in water
(4 mL) was mixed under stirring with the methanol solution
(2 mL) containing [H3L]OTs (5 mg, 0.01 mmol) and NaN3 (13 mg,
0.2 mmol). The resulting solution was heated in a stainless steel
reactor with a Teflon liner (25 mL) at 120 °C for 3 d and cooled to
ambient temperature at a rate of ca.10 °C hꢀ1 to give yellow block
crystals (yield: 65%, based on [H3L]OTs). Elemental analysis calcd.
(%) for C14H9O7NMn: C 46.95, H 2.53, N 3.91; Found: C 46.53, H
2.60, N 3.98. Main IR (KBr, cmꢀ1): 3480s, 3069w, 1638s, 1610m,
1578w, 1557s, 1362s, 884w, 807w, 727w, 689w.
OOC
COO
Scheme 1. Structure of L2ꢀ
.
2.1.5. [Cd(L)(H2O)] (3)
A solution of [H3L]OTs (5 mg,0.01 mmol) in H2O/DMF (3 mL,
v/v = 2/1) was mixed under stirring with Cd(NO3)2ꢁ4H2O
(9.25 mg, 0.03 mmol). The resulting solution was heated in a stain-
less steel reactor with a Teflon liner (25 mL) at 120 °C for 1 d and
cooled to ambient temperature at a rate of ca.10 °C hꢀ1 to give col-
orless block crystals (yield: 69%, based on [H3L]OTs). Elemental
analysis calcd. (%) for C14H9O7NCd: C 40.46, H 2.18, N 3.37; Found:
C 40.21, H 2.21, N 3.28. Main IR (KBr, cmꢀ1): 3473br, 3056m,
1631s, 1608w, 1554s, 1579s, 1361s, 883w, 840w, 770s, 730s,
684w.
2. Experimental
2.1. Materials and synthesis
All the reagents and solvents employed were commercially
available and used without further purification. 2,4-Dinitrophenyl
toluenesulfonate was prepared according to literature procedures
[26].
2.1.6. [Zn(L)(H2O)]ꢁ2H2O (4)
2.1.1. [H3L]OTs
A solution of [H3L]OTs (5 mg,0.01 mmol) in H2O/DMF (3 mL,
v/v = 2/1) was mixed under stirring with Zn(NO3)2ꢁ6H2O
(8.91 mg, 0.03 mmol). The mixture was sealed in a tube and heated
at 80 °C for 1 d and cooled to the room temperature at a rate of
ca.10 °C hꢀ1 to give colorless block crystals (yield: 73% based on
To 2,4-dinitrophenyl toluenesulfonate (6.7 g, 12.59 mmol) dis-
solved in toluene (9 mL) was added ethyl isonicotinate (3 mL,
20.02 mmol), and the mixture was stirred for 18 h at 100 °C. The
resultant yellow precipitate was isolated by filtration and washed
with toluene. The reaction product (1.042 g, 2 mmol) and dimethyl
5-aminoisophthalate (0.84 g, 2 mmol) were dissolved in ethanol/
water (20 mL, v:v = 10:1), and stirred for 48 h at 100 °C. After filter-
ing off the insolvable solid, the filtrate was washed with Et2O three
times and evaporated to give a white product. The white product
was hydrolyzed by reacting it with 5 mL of 5 M HCl at reflux for
12 h at 100 °C. After the reaction mixture was cooled, it was con-
centrated with a rotary evaporator to give white powder. Yield:
0.76 g (73.4%). 1H NMR (400 MHz, DMSO): d = 9.58 (d, J = 6.4 Hz,
2H), 8.72 (s, 1H), 8.86 (d, J = 1.2 Hz, 2H), 8.58 (d, J = 6.8 Hz, 2H),
7.47 (d, J = 8 Hz, 2H), 7.11 (d, J = 7.6 Hz, 2H), 2.28 (s, 3H). Main IR
(KBr, cmꢀ1): 3440 m, 3074m, 1724s, 1436m, 1409m, 1228s,
912w, 809w, 767w, 680w.
[H3L]OTs). Elemental analysis calcd. (%) for C14H13O9NZn:
C
41.55, H 3.46, N 3.24; Found: C 41.61, H 3.28, N 3.44. Main IR
(KBr, cmꢀ1): 3523w, 3405w, 3303m, 3228m, 1633br, 1575s,
1363s, 912w, 835w, 771s, 730s, 688w.
2.2. Physical measurements
Elemental analyses were determined on an Elementar Vario
ELIII analyzer. The FT-IR spectra were recorded in the range
500–4000 cmꢀ1 using KBr pellets on a Nicolet NEXUS 670 spec-
trophotometer. NMR spectra were recorded on a Bruker Advance
400 MHz spectrometer. Powder X-ray diffraction (PXRD) was
recorded on a Rigaku Ultima IV X-ray diffractometer equipped with
a Cu-target tube at 35 kV, 25 mA, and a graphite monochromator.
Thermogravimetric (TG) analysis was performed using a Mettler
TGALSDTA851e/5FL1100 instrument. Solid fluorescence spectra
were recorded on a HitachiF-4500 spectrometer. Temperature-
dependent magnetic measurements were carried out on a Quan-
tum Design SQUID MPMS-5 magnetometer with an applied field
of 1 kOe, and diamagnetic corrections were made with Pascal’s
constants.
2.1.2. H2Lꢁ2H2O
A solution of [H3L]OTs (5 mg, 0.01 mmol) in 3 mL H2O was
mixed under stirring with Zn(NO3)2ꢁ6H2O (8.91 mg, 0.03 mmol).
The mixture was sealed in a tube and heated at 80 °C for 1 d and
cooled to the room temperature to give colorless flake crystals
(yield: 65% based on [H3L]OTs). Elemental analysis calcd. (%) for
C
14H13O8N: C 52.15, H 4.06, N 4.29; Found: C 52.02, H 4.05, N
4.33. Main IR (KBr, cmꢀ1): 3585s, 3480m, 1700s, 1633s, 1594m,
1548m, 1336s, 1226m, 896w, 836w, 765w, 688w.
2.3. X-ray crystallography
Diffraction data for 1–4 and H2Lꢁ2H2O were collected at 296 K
2.1.3. [Cu3L2(OH)2] (1)
on a Bruker Apex II CCD area detector equipped with graphite
[H3L]OTs (5 mg, 0.01 mmol) was dissolved in water (3 mL) and
the pH was adjusted to 6 with 0.1 mol Lꢀ1 NaOH solution. Then, Cu
(NO3)2ꢁ6H2O (7.25 mg, 0.03 mmol) was added to the solution. The
resulting solution was heated in a stainless steel reactor with a
Teflon liner (25 mL) at 120 °C for 3 d and cooled to ambient
temperature at a rate of ca.10 °C hꢀ1 to give green block crystals
(yield: 46%, based on [H3L]OTs). Elemental analysis calcd. (%) for
monochromated Mo Ka radiation (k = 0.71073 A°). Empirical
absorption corrections were applied using the SADABS program
[27]. The structures were solved by the direct method and refined
by the full-matrix least-squares method on F2 [28], with all non-
hydrogen atoms refined with anisotropic thermal parameters. All
the hydrogen atoms attached to carbon atoms were placed in cal-
culated positions and refined using the riding model. The water
hydrogen atoms were located from the difference maps and
refined isotropically. A summary of the crystallographic data, data
collection, and refinement parameters are provided in Table 1.
C28H16O14N2Cu3: C 42.30, H 2.03, N 3.52; Found: C 41.89, H 2.12,
N 3.39. Main IR (KBr, cmꢀ1): 3521m, 3457s, 1633s, 1604s, 1571s,
1558w, 1361s, 925w, 806w, 761s, 723s, 688w.
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X. Yang et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
3
Table 1
Crystal data and structure refinement for compound 1–4 and H2Lꢁ2H2O.
Compound
H2Lꢁ2H2O
14H13O8N
323.25
Monoclinic
C2/c
13.417(3)
14.954(3)
7.215(1)
90
103.01(8)
90
1
2
3
4
Formula
Mr
System
Space group
a [Å]
C
C28H16O14N2Cu3
795.05
Triclinic
C14H9O7NMn
358.16
Triclinic
C14H9O7NCd
415.63
Triclinic
C14H13O9NZn
404.66
Triclinic
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
P1
8.0306(6)
8.7010(6)
9.2968(7)
84.738(2)
84.462(2)
85.340(2)
642.13(8)
1
7.487(7)
10.057(10)
10.795(10)
62.29(3)
71.97(3)
72.49(3)
672.3(11)
2
7.5283
10.1140
10.7770
117.791
91.555
107.304
679.6(3)
2
6.8464(7)
9.7317(9)
12.1965(12)
103.580(2)
94.599(3)
104.781(3)
755.23(13)
2
b [Å]
c [Å]
a
[°]
b[°]
c
[°]
V (Å3)
1410.5(5)
4
1.522
Z
q
l
[gꢁcmꢀ3
]
2.056
2.549
1.769
1.022
2.031
1.646
1.779
1.679
[mmꢀ1
]
0.127
F(0 0 0)
672
397
362
408
412
Reflections collected
Unique reflections
GOF on F2
9567
1733
1.023
8104
2477
1.087
6805
2611
1.062
8295
2640
1.019
8854
2641
1.102
Rint
R1 [I > 2r(I)]
wR2 (all data)
0.0466
0.0498
0.1301
0.0232
0.0281
0.0804
0.0320
0.0364
0.0979
0.0961
0.0580
0.1404
0.0229
0.0254
0.0615
salt form (H2Lꢁ2H2O). No crystals were yielded when zinc nitrate
was absent or replaced by cadmium nitrate. Zn(II) nitrate could
play a subtle role in facilitating the crystallization of the inner-salt,
although the mechanism is unclear. The solvothermal reactions of
Cu(II) and Mn(II) salts with [H3L]OTs in water or aqueous ethanol
in the presence of a base (NaOH or NaN3) led to complexes 1 and
2, while the reactions of Cd(II) and Zn(II) salts with [H3L]OTs in
H2O/DMF gave complexes 3 and 4 in the absence of any base.
The [H3L]OTs ligands were completely deprotonated upon coor-
dination to the metal centers, which is evidenced by IR spectra
with the absence of the absorption bands in the range of 1760–
1680 cmꢀ1 typical of carboxylic acid groups. Multiple bands
observed in the ranges of 1645–1580 cmꢀ1 and 1410–1350 cmꢀ1
for the complexes are attributable to the asymmetric and symmet-
ric stretching modes of the deprotonated carboxylate groups. The
3. Results and discussion
3.1. Synthesis and general characterization
The targeted ligand was synthesized according to Scheme 2. The
reaction of ethyl isonicotinate with 2,4-dinitrophenyl toluenesul-
fonate derived from 2,4-dinitrobenzene and tosyl chloride gave
1-(2,4-dinitrophenyl)-4-ethoxycarbonylpyridinium tosilate (I)
[29]. The Zincke reaction of I with dimethyl 5-aminoisophthalate
gave the ester product (II), which was hydrolyzed in concentrated
HCl solution to afford [H3L]OTs in good yield. The organic salt is
insoluble in common solvents including water, but soluble in the
presence of bases. An initial attempt to synthesize metal com-
plexes by reacting Zn(II) nitrate with [H3L]OTs in water yielded
crystals of the ligand in the neutral, partially deprotonated inner-
NO2
N
NO2
NO2
TsCl
NO2
COOEt
NO2
a
N
NO2
b
OTs
OH
OTs
COOEt
I
COOEt
N
COOH
NH2
H3COOC
COOCH3
N
OTs
OTs
d
c
H3COOC
COOCH3
HOOC
COOH
[H3L]OTs
II
Scheme 2. Synthesis of 1-(3,5-dicarboxyphenyl)-4-carboxypyridinium tosilate [H3L]OTs. (a) CH2Cl2, triethylamine, 48 h; (b) toluene, 100 °C, 18 h; (c) ethanol/water, 100 °C,
48 h; (d) HCl, 100 °C, 24 h.
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X. Yang et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
results are well-consistent with the single crystal structures. Com-
pounds 2 and 3 show a broad peak at 3485–3470 cmꢀ1 due to
water molecules, while 1 shows a sharp band at 3450 cmꢀ1, which
can be attributed to the hydroxo group. Compound 4 displays
strong multiple bands in the 3530–3225 cmꢀ1 region, consistent
with the abundance of water molecules (either coordinated or free)
in the structure (see below). The phase purity of the complexes
was confirmed by PXRD measurements (Fig. S1).
another two carboxylate oxygens (O4C and O4E), with a long Cu–
O distance of 2.624(3) Å. If this ‘‘semi-coordination” is included,
the Cu1 geometry may be described as an elongated octahedron.
The Cu2 ion adopts a square pyramidal coordination geometry.
The axial positions are occupied by a carboxylate oxygen (O4C),
with a long Cu–O distance of 2.342(2) Å, and the equatorial plane
is completed by three carboxylate oxygens (O2, O3B and O5D)
and a hydroxo oxygen (O7), with shorter Cu–O distances of 1.916
(2)–1.971(2) Å.
The organic ligand is fully deprotonated with a formal charge of
-2, and each ligand binds six Cu ions through its three carboxylate
groups in different modes: the carboxylate attached to the pyri-
dinium ring is monodentate, while those attached to the benzene
3.2. Crystal structures
3.2.1. H2Lꢁ2H2O
The hydrated neutral zwitterionic compound exhibits a three-
ring adopt the
form a linear trinuclear unit, in which Cu1 and Cu2 are linked by a
-1,3-carboxylate group in the syn-syn mode, a -hydroxo group,
and a -O atom from the 3-1,1,3-carboxylate group, with a
Cuꢁ ꢁ ꢁCu distance of 3.1564(4) Å. The -1,3-carboxylate and -OH
bridges occupy equatorial positions of both Cu1 and Cu2 (i.e., an
eq–eq disposition is adopted). The 3-1,1,3-carboxylate group
serves as axial-axial-equatorial (ax-ax-eq) bridge, with -O axially
l-1,3 and l3-1,1,3 mode. An Cu1 and two Cu2 ions
dimensional assembly through hydrogen bonding and p-p stacking
interactions (Fig. 1). In the structure, the zwitterionic molecule is
bisected by the crystallographic 2-fold axis along the b direction
and takes a twisted conformation with a dihedral angle of 50.2
(6)° between the pyridinium and benzene rings. The carboxylic
group attached to the pyridinium ring is deprotonated (O1-C1-
O1A), while those attached to the benzene ring remain protonated
(O2-C8-O3-H3). The molecules in alternating up-down orienta-
tions are side by side aligned into a one-dimensional tape along
the (1 0 1) direction. The arrangement is sustained by strong
hydrogen bonds between the O3-H3 hydroxyl group and the O1
atom from neighboring molecules (For the relevant parameters,
see Table S1 in the supplementary information). The tapes are
arrayed along the (1 0 ꢀ1) plane to form 2D sheets through hydro-
gen bonds mediated by water molecules lying between the tapes.
The water molecule (O4) shares its hydrogen atoms with O1 from
one tape and with O2 from another tape.
l
l
l
l
l
l
l
l
coordinated to Cu1 and Cu2 in one trinuclear unit and the other O
atom equatorially bonded to another Cu2 ion in another trinuclear
unit. The inter-unit Cu2ꢁ ꢁ ꢁCu20 and Cu1ꢁ ꢁ ꢁCu20 distances spanned
by the
Through pairwise
units are connected into an infinite [Cu3(
3-COO)2(COO)2]n chain along the b direction.
The chains are cross-linked into a 3D coordination network,
l3-1,1,3 bridge are 4.0767(6) and 6.0167(5) Å, respectively.
l3-1,1,3 carboxylate bridges, the trinucluear
l
-OH)2( -COO)2-
l
(l
with each tricarboxylate ligand connected to three neighboring
chains. To gain a better understanding, the 3D structure may be
described as pillared layers: the above-mentioned chains are
linked into layers parallel to the bc plane by the 1,3-benzenedicar-
boxylate moiety of the ligand (Fig. 2b), and the layers are pillared
by the pyridinium carboxylate moiety (Fig. 2c).
The hydrogen-bonded sheets are packed in the ABAB fashion
with
ing sheets overlap in the offset face-to-face fashion, with an inter-
planar distance of 3.357(2) Å. The interactions align the
p-p stacking interactions. The benzene rings from neighbor-
p-p
molecules into infinite zipper-like arrays along the c direction.
3.2.2. Compound 1
3.2.3. Compounds 2 and 3
This compound is
a
3D coordination polymer containing
Single crystal X-ray analyses revealed that compounds 2 and 3
are isomorphic 2D coordination polymers based on carboxylate
bridged dinuclear units (Fig. 3). The structure of 2 is described here
in details. The asymmetric unit consists of one metal ion, one fully
deprotonated ligand (L2ꢀ) and one coordinated water molecule.
The metal ion assumes a distorted octahedral coordination geom-
etry defined by a chelating carboxylate group (O1, O2), a water
hydroxo- and carboxylate-bridged Cu(II) chains (Fig. 2). There are
two crystallographic independent copper ions. The Cu1 ion resides
in a centrosymmetric square planar environment defined by four
oxygen atoms from two carboxylate groups (O1, O1A, Cu–
O = 1.971(2) Å) and two hydroxo groups (O7 and O7A. Cu–
O = 1.933(2) Å). It is noted that Cu1 is weakly coordinated by
Fig. 1. (a) The 2D hydrogen bonding layer structure of H2Lꢁ2H2O. Symmetric code: A. 1 ꢀ x, y, 0.5 ꢀ z; B. 0.5 ꢀ x, 0.5 ꢀ y, z; C. x, 1 + y, z. (b) The 3D network through hydrogen
bonding and stacking interactions.
p-p
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X. Yang et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
5
Fig. 2. (a) The coordination environments of the metal ions in compound 1. Symmetric code: A. ꢀx, 1 ꢀ y, 2 ꢀ z; B. ꢀ1 + x, 1 + y, 1 + z; C. ꢀx, 1 ꢀ y, 1 ꢀ z; D. ꢀx, 2 ꢀ y, 2 ꢀ z; E.
x, y, 1 + z; F. ꢀx, 2 ꢀ y, 1 ꢀ z; G. x, y, ꢀ1 + z; H. 1 + x, ꢀ1 + y, ꢀ1 + z. (b) The 2D network in 1 formed by the ligand connecting the chains. (c) 3D architecture of 1.
hydrogen-bonded architecture can be represented by a 3D net with
point symbol (43)2(46.618.84) (Fig. 4d).
For 3, the coordination geometry around Cd ion is best
described as seven-coordinative pentagonal bipyramidal rather
than octahedral. The difference is related to the coordination mode
of the O3-C5-O4 carboxylate group. It adopts a chelating-bridging
mode (that is, it binds one metal ion through both oxygen atoms
and binds a second metal ion through O3) in 3, with Cd1-
O3 = 2.644(5) Å (the other Cd-O bonds range from 2.238(6) to
2.386(5) Å). The corresponding bond in 2 is 2.821(4) Å, which indi-
cates very weak coordination. The geometrical difference between
2 and 3 does not change the topology and hydrogen binding
pattern.
3.2.4. Compound 4
This compound is also a 2D coordination polymer. The asym-
Fig. 3. The coordination environments of the metal ions in compound 2. Symmetric
metric unit consists of one Zn ion, one L2ꢀ ligand and three water
code: A. ꢀ1 ꢀ x, ꢀy, ꢀz; B. x, ꢀ1 + y, z; C. ꢀ1 ꢀ x, 1 ꢀ y, ꢀz; D. ꢀx, ꢀy, 1 ꢀ z; E. ꢀ1
molecules (Fig. 5). The Zn ion resides in a distorted tetrahedral
coordination surrounding defined by four oxygen atoms (O2,
O4A, O6B and O7) from three independent carboxylate groups
and a water molecules. With the OAZnAO angles (100.5–101.0°)
involving the water molecule being all significantly smaller than
the ideal tetrahedral angles, the metal center is significantly dis-
placed towards the base defined by the three carboxylate oxygen
atoms. The three carboxylate groups of the ligand are all monoden-
tate, although the distances (2.814(2)–2.964(2) Å) between Zn and
the ‘‘uncoordinated” oxygen atoms of these carboxylates may sug-
gest weak interactions. The ligand and the metal ion are both
three-connected, generating a 2D coordination network with the
honeycomb 63 topology.
The honeycomb layers are packed in the (AB)(AB) mode with
some offset between A and B layers (Fig. 6). The packing mode is
different from the ordinary ABAB mode in that the layers are
arranged in pairs. In each layer, the ZnAOwater bonds are all orien-
tated toward one side. For each pair of neighboring layers, (AB), the
ZnAOwater bonds are antiparallel and deeply interdigitated with
each other to form a zipper-like bilayer (Fig. 6b). The bilayer is
reinforced by extensive interlayer interactions (Fig. 6c and
Table S1): strong OAHꢁꢁꢁO hydrogen bonds are formed between
coordinated water molecules (O7) from one layer and uncoordi-
nated carboxylate oxygen atoms (O5) from the other layer; there
are also relatively short interlayer CAHꢁꢁꢁO contacts, which involve
pyridinium C13-H13A groups and coordinated O6 atoms. For inter-
actions between bilayers, the coordinated water molecule (O7)
also forms a hydrogen bond with the uncoordinated carboxylate
+ x, y, ꢀ1 + z.
molecule (O7) and three oxygen atoms (O3B, O4C and O6D) from
different carboxylate groups. The distortion from octahedral is
related to the small bite angle (57.19(8)°) of the chelating carboxy-
late group, which leads to a trapezoidal shape for the equatorial
quadrilateral consisting of four carboxylate oxygen atoms.
The fully deprotonated organic ligand binds four Mn ions
through its three carboxylate groups in different modes: monoden-
tate (the carboxylate attached to the pyridinium ring), chelating,
and
l-1,3. The l-1,3-carboxylate bridges appear in pair and con-
nect two adjacent Mn ions into a centrosymmetric dinuclear unit,
with a Mnꢁ ꢁ ꢁMn distance of 3.878(3) Å. Each dimanganese unit is
connected to six identical ones through six ligands, generating a
2D layer parallel to the h1 0 ꢀ1i plane (Fig. 4a). Taking the diman-
ganese unit and the tricarboxylate ligand as six- and as three-con-
nected nodes, respectively, the layer can be reduced to a 2D net
with point symbol (43)2(46.66.83). The coordinated water molecule
(O7) affords OAHꢁ ꢁ ꢁO hydrogen bonds (Fig. 4c): one hydrogen
atom interacts with an uncoordinated carboxylate oxygen atom
(O5) within the same dinuclear unit as the water molecule; the
other hydrogen atom serves as a bifurcated donor to interact with
two coordinated carboxylate oxygen atoms (O4 and O1) from a
dinuclear unit arising from a neighboring layer. The layers are
stacked in the AA fashion with the interlayer hydrogen bonds
connecting the dinuclear units from adjacent layers (Fig. 4b). Con-
sequently, the dinuclear unit becomes 8-connected, and the 3D
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X. Yang et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
Fig. 4. (a) The 2D framework of 2; (b) the 3D framework of 2 formed by locking the 2D sheets by hydrogen bonding; (c) the 1D chain assembled by hydrogen bond along the a
direction; (d) the topology of the 3D framework.
face-to-face fashion to form an infinite
p-p stacking arrays along
the a direction, with alternate interplanar distances of 3.34 and
3.26 Å (Fig. 6d).
The offset stacking of the layers generates 1D rectangle-shaped
channels along the a directions, within which two independent and
well-defined water molecules (O8 and O9) are enclosed (Fig. 6a).
Interestingly, the water molecules form a square (H2O)4 cluster
(Fig. 6e), which contains an 8-membered (OAH)4 hydrogen bonded
ring with alternating O8-H and O9-H. The remaining H atoms of
the water cluster are bonded to uncoordinated carboxylate O1
atoms, which protrude toward the channels and act as bifurcated
acceptors. Two O8 water molecules, two O9-H fragments and
two O1 atoms generate a 12-membered hydrogen bonded ring,
which alternates with the (OAH)4 ring along the channel (Fig. 6).
3.3. Magnetic properties
Magnetic properties of compounds 1 and 2 were measured. The
v
T value of 1 at 300 K (1.45 cm3 molꢀ1 K per Cu3 unit) is typical of
Cu(II) systems with g > 2. Upon cooling, the
vT value increases to
2.11 cm3 molꢀ1 K at 2 K (Fig. 7a). The data above 70 K follow the
Curie–Weiss law with C = 1.37 cm3 K molꢀ1 and h = 17.6 K. The
temperature-dependent behavior clearly indicates ferromagnetic
coupling between the Cu(II) ions.
Fig. 5. A 2D layer assembled through the ligand. Symmetric code: A. x, y, ꢀ1 + z; B.
x, 1 + y, z.
The isothermal magnetization from 0 to 50 kOe was measured
at 2 K (Fig. 7b). The magnetization increases smoothly with
the applied field, and tends to saturate at high field. The
value (3.09 Nb) at 50 kOe is consistent with three Cu(II) ions. The
O3 atom from the next-closest layer (A-A or B-B interaction). Fur-
thermore, the benzene rings (together with the O5-C13-O6 car-
boxylate groups) from neighboring layers overlap in the offset
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X. Yang et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
7
Fig. 6. (a) View of the 2D frameworks of 4. Lattice guest water molecules are present in the small cavities. (b) The 3D framework of 4 formed by locking the 2D sheets by
hydrogen bonding. (c, d) the 1D chain are assembled hydrogen bond and
clusters in the channel.
p-p stacking arrays along the a direction. (e) A hydrogen-bonded tape formed from square (H2O)4
experiment M-H curve is in good agreement with that simulated
for a quartet state (assuming g = 2.1), which supports ferromag-
netic coupling between the Cu(II) ions.
terminal Cu(II) ions (Cu2 and Cu20) in the trinuclear unit is omitted
for simplicity. Accordingly, the susceptibility can be expressed as
follows [25,30].
According to the structural data of 1, there are a number of pos-
sible pathways for magnetic exchange between neighboring Cu(II)
vt ¼ ½Ng2b2=ð4kTÞꢂ½1 þ 10expð3J=2kTÞꢂ=½1 þ 2expð3J=2kTÞꢂ
ð1Þ
ions (see Fig. 2a): (i) the eq-eq
can induce magnetic exchange between Cu1 and Cu2 within the
trinuclear unit; (ii) the double ax-ax-eq 3-1,1,3-carboxylate
bridge not only constitutes a third intra-unit pathway (ax-ax
l-OH and l-1,3-carboxylate bridges
v
¼
v
t=½1 ꢀ ðzj
v
t=Ng2b2Þꢂ
ð2Þ
l
The experimental data of temperature-dependent susceptibility
l
-
of 1 can be well fitted by the above equations (Fig. 7a), with the
best-fit parameters being J = 88 cmꢀ1 and g = 2.10 with
zj = 0.02 cmꢀ1. The ferromagnetic interaction through the mixed
O) between Cu1 and Cu2, but also affords inter-unit pathways
(ax-eq 1,3-carboxylate) between Cu2 from one unit and Cu1/Cu2
from another unit; (iii) the Cu(II) ions from different units may also
interact through several pathways across the organic backbone of
the L ligand. The magnetic solution for this complicated system
may be simplified by considering the magneto-structural features
of these pathways. The dx2-y2-type magnetic orbital of Cu(II) in
the axially elongated square pyramidal or octahedral field is
mainly delocalized over the equatorial ligands, so the magnetic
exchange is predominately propagated through the eq-eq bridge.
The exchange through the eq-ax or ax-ax bridge should be much
weaker, if any. In addition, the interchain magnetic exchange
across the organic ligand should also be much weaker because of
the long distances between magnetic centers. Therefore, the mag-
netic system of 1 can be reduced to a trinuclear unit, with predom-
inating intra-unit magnetic interactions (represented by J) through
eq-eq
l-OH and l-1,3-carboxylate bridges is indicative of orbital
countercomplementarity [30]. In general, the overall magnetic
interaction between metal ions is the sum of ferromagnetic (F)
and antiferromagnetic (AF) contributions (J = JF + JAF), and the latter
contribution is governed by the energy gap between the two singly
occupied molecular orbitals (SOMOs) in the triplet state for a pair
of Cu(II) ions. The eq-eq
mode is a general antiferromagnetic coupler because of large gaps
between the SOMOs, and so is the eq-eq -OH bridge with a large
CuAOACu angle (>98°). The antiferromagnetic interaction through
-OH increases with the bridging angle. For 1, we have a bridging
l-1,3-carboxylate bridge in the syn-syn
l
l
angle of 110.18(9)°, which could induce strong antiferromagnetic
interactions. However, the incorporation of a carboxylate bridge
into the CuAOACu motif has a counterbalancing effect on the
energy gap. That is, the gap and hence the antiferromagnetic con-
tribution is reduced, leading to overall ferromagnetic coupling.
the eq-eq
l
-OH and
l-1,3-carboxylate bridges. The weak interac-
tions across the
l3-1,1,3-carboxylate bridge and the organic back-
bone can be accounted for by a gross inter-unit interaction
parameter (zj) in the molecular-field approximation. The
The
close to the value expected for a magnetically isolated Mn(II) ion
(4.375 cm3 molꢀ1 K). As the temperature is lowered, the
T value
decreases continuously (Fig. 8a). The data above 70 K follow the
v
T value for compound 2 at 300 K is about 4.37 cm3 molꢀ1 K,
ˆ
ˆ
ˆ
ˆ
ˆ
0
corresponding spin Hamiltonian is H = ꢀJ(SCu1ꢁSCu2 + SCu1ꢁSCu2 ) –
v
ˆ
zj < Sz > Sz, where the magnetic interaction between the two
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8
X. Yang et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
Fig. 8. (a) Temperature dependence of
v and vT for 2 under 1 kOe. (b) Isothermal
magnetization curve of 2 at 2 K. The solid lines represent the best fit.
Fig. 7. (a) Temperature dependence of
vT and (v
T)ꢀ1 for 1 under 1 kOe. (b) Field-
dependent isothermal magnetization curves for 1 at 2 K. The solid line represents
the best fit to the appropriate model.
be due to the release of water molecules (calcd. 2.3%, one water
molecule per Cu3(OH)2 unit). The quick weight loss between 270
and 380 °C indicates the decomposition of the organic ligand. Com-
pound 2 exhibits a weight loss of 4.8% in the 160–230 °C, attribu-
table to the loss of coordinated water molecules (calcd. 5.0%).
Decomposition of the organic component begins at about 360 °C.
Compound 3 shows a weight loss of 3.63% upon heating to
175 °C, which corresponds to the loss of coordinated water mole-
cules (calcd. 4.3%). Complex 4 releases both free and coordinated
water molecules (obsd. 13.0%; calcd. 13.3%) upon heating to
130 °C, and decomposition occurs after 380 °C.
According to X-ray crystallography, compound 4 exhibits a 3D
hydrogen-bonded framework with 1D channels occupied by water
clusters. The stability of the potential porous structure was exam-
ined. As shown in Fig. 9, the PXRD profile of a sample of 4 heated at
130 °C for 4 h is quite different from that for as-synthesized sam-
ples, which could suggest that the framework collapses and trans-
forms to a new crystalline phase. Nevertheless, the original
framework can be recovered by exposing the dehydrated sample
to water vapor, as indicated by PXRD measurements (Fig. 9). There-
fore, the framework can be switched between two crystalline
phases by release/absorption of water molecules. The minor differ-
ences between the PXRD profiles of the regenerated and the as-
synthesized samples are because preferential orientation is more
significant for the latter.
Curie–Weiss law with C = 4.42 cm3 K molꢀ1 and h = ꢀ3.37 K. The
behavior indicates overall antiferromagnetic interactions in this
compound.
According to the structural data, compound 2 can be magneti-
cally taken as a dimanganese(II) molecule in which the intramolec-
ular magnetic interaction (J) through double carboxylate bridges is
described by the isotropic Hamiltonian H = ꢀJS1ꢁS2. By applying the
Van Vleck equation [30], the temperature-dependent magnetic
data have been fitted to give J = ꢀ0.56 cmꢀ1 and g = 1.98. The J
parameter falls in the range (0–2 cmꢀ1) observed for previous
Mn(II) compounds with double carboxylate bridges [31–33].
The magnetization isotherm of 2 measured at 2 K (Fig. 8b) sug-
gests that magnetization increases slowly with applied field and
reached 4.71 Nb at 50 kOe. The curve is below that simulated for
two isolated S = 5/2 ions, which supports the presence of antiferro-
magnetic coupling.
ˆ
ˆ ˆ
3.4. Thermogravimetry and luminescence
The TG data of compounds 1–4 and H2Lꢁ2H2O are plotted in
Fig. S2. The hydrated zwitterionic compound (H2Lꢁ2H2O) shows a
weight loss of ꢃ10.3% upon heating up to 160 °C, which can be
assigned to the loss of the two water molecule (calcd 11.8%) per
formula unit. After this, it remains unchanged in weight up to
260 °C and undergoes gradual and complete decomposition upon
further heating. For compound 1, a gradual weight loss of 2.7%
was observed in the temperature range of 30–125 °C, which could
Photoluminescence properties of the Cd (3) and Zn (4) com-
plexes were measured. As shown in Fig. S3, the ligand [H3L]OTs
shows an emission maximum at 410 nm upon excitation at
260 nm. The emission profiles of 3 and 4 complexes resemble that
of the ligand without significant shift in the emission maximum.
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9
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This work is supported by the National Natural Science Founda-
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Research Fund for the Doctoral Program of Higher Education of
China.
Appendix A. Supplementary data
CCDC Nos. 1502012–1502016 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif. Supplementary data associated
with this article can be found, in the online version, at http://
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