structural and magnetic properties of one-dimensional squarate bridged coordination polymers...

Structural and Magnetic Properties of One-DimensionalSquarate Bridged Coordination Polymers Containing2-Aminomethylpyridine Ligand

Ahmet Bulut • _Ibrahim Ucar • Tolgay Kalyoncu •

Yusuf Yerli • Orhan Buyukgungor

Received: 12 July 2010 / Accepted: 23 August 2010 / Published online: 9 September 2010

� Springer Science+Business Media, LLC 2010

Abstract Three novel one-dimensional (1D) coordina-

tion polymers with general formula {[M(l-sq)2(amp)2]�H2O}n [M=Cu(II) (1), Zn(II) (2)] and [Cd(l-sq)2(amp)2]n

(3) [sq: Squarate; amp:2-aminomethylpyridine] have been

prepared and spectroscopically studied. Each metal (II) ion

in 1–3 is octahedrally coordinated by two sq and two amp

ligands. The amp ligands are N,N0 coordinated, while the sq

ligands bridge the metal centers forming 1D linear chain

structure. The individual chains are linked by O–H��Ohydrogen bonds involving the hydrogens of water mole-

cules (1–2), amino hydrogens and squarate O atoms (1–3).

The hydrogen bonded layers are further assembled into

three-dimensional supramolecular networks by weak aro-

matic p–p interactions. EPR results indicate that the ground

state of the paramagnetic electron in 1 and Cu2? doped

complexes 2–3 is dx2�y2 . Magnetic measurements reveal

that complex 1 shows paramagnetic behavior.

Keywords Squarate complexes � X-ray diffraction �Coordination polymer � EPR � Magnetic susceptibility

1 Introduction

Over recent years, the researchers in the crystal engineer-

ing field have found interest in studying metal–organic

polymers, with functional building block due to their

possible use in a catalysis, adsorption (gas storage), sepa-

ration, luminescence, magnetism and optical behaviors,

conductivity, drug delivery, etc. [1–7]. In this context,

some of the supramolecular structures generated by non-

covalent interactions such as coordination bonding,

hydrogen bonding, p–p stacking, etc. have been well-

defined on the concept of self assembly. Self-assembly of

metal-containing building blocks formed by non-covalent

interactions has been investigated and successfully

employed in the formation of supramolecular networks

[8–11].

Because of the presence of several potential donor

oxygen atoms, increasing attention has been devoted to the

coordination chemistry of the squarate ligand (C4O4)2-, by

both inorganic and bioinorganic chemists [12–14]. Squa-

rate acts as a bridge between two or more metal atoms

in mono- or polydentate coordination modes with first

row transition metal ions [15, 16]. In our ongoing research

on squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione,

C4H2O4), we have synthesized some mixed-ligand meta-

l(II) complexes of squaric acid, and their structures have

been reported [17–20]. In these compounds, squaric acid is

a monodentate ligand [21, 22] or both a monodentate and a

bidentate ligand [23], while in 1 and 2, reported in the

present study, it has l-1,3 bis(monodentate) coordination

between the M (II) ions [M=Cu(II), Zn(II), Cd(II)]. In this

paper, we report three new coordination polymers with a

general formula {[M(sq)2(amp)2]�H2O}n [M=Cu(II) (1),

Zn(II) (2)], [Cd(sq)2(amp)2]n (3) constructed from amp

ligand with the corresponding metal (II)-sq complexes.

The aim of this research is to report the new coordina-

tion polymers and investigation of their supramolecular

architecture together with spectroscopic and magnetic

properties.

A. Bulut � _I. Ucar (&) � T. Kalyoncu � O. Buyukgungor

Department of Physics, Faculty of Arts and Sciences,

Ondokuzmayıs University, 55139 Kurupelit, Samsun,

Turkey

e-mail: [email protected]

Y. Yerli

Department of Physics, Gebze Institute of Technology,

Gebze, 41400 Kocaeli, Turkey

123

J Inorg Organomet Polym (2010) 20:793–801

DOI 10.1007/s10904-010-9406-1

2 Experimental

2.1 General Method

All chemical reagents were analytical grade commercial

products. Solvents were purified by conventional methods.

The IR spectra were recorded on a Vertex 80v Bruker FT-

IR spectrometer using KBr pellets and operating in the

4000–400 cm-1 range. UV–vis spectra were obtained for

aqueous solutions of the complexes (10-3M) with a Uni-

cam UV2 spectrometer in the range 900–190 nm. The

15–300 K magnetization measurements were carried out

on a Quantum Design PPMS system. v-T graphs were

recorded under the constant magnetic field of 0.5 T.

Magnetic data were corrected for the diamagnetic contri-

bution of the sample holder. The EPR (Electron

Paramagnetic Resonance) powder spectrum was recorded

with a Bruker EMX X-band spectrometer (9.8 GHz) with

about 20 mW microwave power and 100 kHz magnetic

field modulation.

2.2 Synthesis of the Metal Complexes

Squaric acid (5 mmol) dissolved in 25 mL water, neutral-

ized with NaOH (0.40 g, 10 mmol), was added to a hot

solution of MCl2�2H2O [M=Cu(II), Zn(II), Cd(II), 5 mmol]

dissolved in 100 mL water. After stirring for 30 min,

precipitates were filtered and washed with acetone to yield

M(C4O4)�2H2O. An aqueous solution of 2-aminomethyl-

pyridine (2 mmol, 20 mL) was added into aqueous solu-

tions of these compounds (1 mmol, 20 mL), under stirring,

and the mixtures were left for slow evaporation at room

Table 1 Crystal data and

structure refinement for

complexes 1-3

Formula C16H20N4O6Cu C16H20N4O6Zn C16H16N4O4Cd

Formula weight 427.91 429.75 440.74

Temperature (K) 297(2) 297(2) 297(2)

Wavelength (Mo Ka) 0.71073 0.71073 0.71073

Crystal system Triclinic Triclinic Monoclinic

Space group P-1 P-1 P 21/c

Unit cell dimensions

a 7.6851(5) 7.7632(5) 8.0615(4)

b (A) 7.7598(5) 7.7739(6) 12.1869(7)

c 8.7365(6) 8.8324(6) 8.4905(4)

a 97.538(5) 96.754(6) 90.00

b (�) 103.163(5) 104.132(6) 96.147(4)

c 115.855(5) 117.524(5) 90.00

Volume (A3) 440.45(5) 441.64(5) 829.35(7)

Z 1 1 2

Calculated density (g cm-3) 1.613 1.615 1.765

l (mm-1) 1.283 1.433 1.346

F(000) 221.0 222.0 440.0

Crystal size (mm) 0.15 9 0.25 9 0.30 0.10 9 0.20 9 0.35 0.20 9 0.25 9 0.30

h range (�) 2.48–28.0 2.47–28.01 2.41–27.98

Index ranges -9 B h B 9 -9 B h B 9 -10 B h B 10

-9 B k B 9 -9 B k B 9 -15 B k B 15

-10 B l B 10 -11 B l B 11 -10 B l B 9

Reflections collected 16791 15609 21742

Independent reflections 1826 [Rint = 0.036] 1829 [Rint = 0.024] 1724 [Rint = 0.040]

Reflections observed [I C 2r(I)] 1820 1820 1624

Absorption correction Integration

Refinement method Full-matrix least-squares on F2

Data/restrains/parameters 1826/1/133 1829/0/133 1724/0/116

Goodness-of-fit on F2 1.057 1.024 1.053

Final R indices [I [ 2r(I)] 0.023 0.024 0.019

R indices (all data) 0.023 0.024 0.020

Largest diff. peak

and hole (A-3)

0.26, –0.30 0.41, –0.30 0.44, –0.49

794 J Inorg Organomet Polym (2010) 20:793–801

123

temperature. A few days later, well formed crystals were

selected for X-ray studies. For doping the complex 0.05%

CuCl2 was added to solution and the single crystals were

grown by slow evaporation of their saturated aqueous

solution.

2.3 X-ray Crystallography

Suitable single crystals were mounted on a glass fiber and

data collection were performed on a STOE IPDSII image

plate detector using Mo Ka radiation (k = 0.71019 A).

Details of the crystal structure are given in Table 1. Data

collection: Stoe X-AREA [24]. Cell refinement: Stoe

X-AREA [24]. Data reduction: Stoe X-RED [24]. The

structure was solved by direct-methods using SHELXS-97

[25] and anisotropic displacement parameters were applied

to non-hydrogen atoms in a full-matrix least-squares

refinement based on F2 using SHELXL-97 [25]. All

hydrogen atoms except the aqua hydrogens were positioned

geometrically and refined by a riding model with Uiso 1.2

times that of attached atoms (1–3) and remaining H atoms

were located from the Fourier difference map (1–2).

Molecular drawings were obtained using DIAMOND 3.0

(demonstrated version) [26].

3 Results and Discussion

3.1 Crystal Structures of Complexes 1–3

The title complexes 1–2 crystallize in the triclinic crystal

system with the space group P�1, while complex 3 crys-

tallizes in the monoclinic space group P 21/c. Since

complexes 1–2 have identical structures in terms of the

coordination geometry around the metals and intra- or

inter-molecular interactions, only the views of complexes 1

and 3 are presented in this paper. The fundamental building

units of 1 and 3 are shown in Figs. 1 and 2, respectively,

together with the atom labeling scheme. The selected bond

lengths and angles are listed in Table 2. The crystallo-

graphic analysis of the three complexes showed that their

structures consist of the [M(l-sq)2(amp)2] building block

units, in which each M (II) ion is octahedrally trans-

coordinated by two sq, two bidentate amp ligands consti-

tuting a MN4O2 chromophore. In the crystal structure of 1

and 2, there is also one crystal water molecule. The metal

ions are connected by the squarate dianions via l-O:O0

coordination, forming 1D polymeric chains in the direction

of the [110] for (1–2) (Fig. 3) and [001] for (3) (Fig. 4).

Only one squarate oxygen atom of each squarate dianions

is involved in metal coordination, and the mode of direct

coordination in which two neighboring oxygen atoms are

involved is not found. The M–Namp bond distances are in

the range 1.994(3)-2.353(1) A, and compare well with

those of the related amp complexes [27–30]. The M–Osq

(M: Zn, Cd) bond distances ranging from 2.205(1) to

2.350(1) A are similar to the corresponding values of

Fig. 2 The molecular structure of 3 showing the atom-numbering

scheme. Displacement ellipsoids are drawn at the 50% probability

level and H atoms are shown as small spheres of arbitrary radii

[symmetry codes (i): -x, -y, 1-z; (iii): 1-x, 1-y, 2-z]

Fig. 1 The molecular structure of 1 showing the atom-numbering

scheme. Displacement ellipsoids are drawn at the 50% probability

level and H atoms are shown as small spheres of arbitrary radii

[symmetry codes (i): -x, -y, 1-z; (iv): 1-x, 1-y, 1-z]

J Inorg Organomet Polym (2010) 20:793–801 795

123

reported metal-squarate complexes [31–33], whereas

the Cu1–Osq bond distances [2.421(1) A] are signifi-

cantly longer than other similar metal-squarate complexes

[34–36]. This axial elongation can easily be explained if

one considers the Jahn–Teller distortion observed in most

octahedral copper (II) complexes [37–39]. The dihedral

angles between the equatorial plane of the coordination

polyhedron and the squarate plane in complexes 1, 2 and 3

are 75.20(7)�, 52.86(6)� and 87.15(4)�, respectively.

The crystal packing analysis indicate that the crystal

packing of all three complexes is formed via intermolecular

hydrogen bonding (1–3) and weak p–p interactions (3).

The hydrogen bonding geometry in complexes 1–3 are

given in Table 3. One of the amino hydrogens involve in

intra-molecular hydrogen bonding with the squarate O

atom, while other amino hydrogen and squarate O atoms

form intermolecular hydrogen bonds between the individ-

ual chains in all complexes (Figs. 3, 4). In complex 3, 1D

linear chains running along the crystallographic axis c are

linked by strong hydrogen bonds into a two-dimensional

(2D) supramolecular array parallel to the bc–plane as

shown in Fig. 4. Additionally, in complexes 1–2 crystal

water molecule links 1D polymeric chains via hydrogen

bonds (Fig. 3) and one of the amino hydrogen atoms par-

ticipate in bifurcated hydrogen bonds. Intra-chain M-M

separations in complexes 1, 2 and 3 are 8.2014(7),

8.0575(7), and 8.4905(4) A, respectively. The nearest M-M

distances between the adjacent chains are 7.6851(5),

7.7632(5) and 7.42665(3) A for complexes 1, 2 and 3,

respectively. The 1D polymeric chains are also linked

Table 2 Selected bond lengths

(A) and angles (8)

Symmetry codes: (i) -x, -y,

1-z; (ii) 2-x, 2-y, 1-z; (iii)

1-x, 1-y, 2-z (iv) 1-x, 1 -y,

1-z

Complex-1 Complex-2 Complex-3

Bond lengths (A)

O1–Cu1: 2.4210(13) O1–Zn1: 2.2049(11) O1–Cd1: 2.3495(10)

N1–Cu1: 2.0369(14) N1–Zn1: 2.1497(14) N1–Cd1: 2.3525(14)

N2–Cu1: 1.9938(14) N2–Zn1: 2.0891(14) N2–Cd1: 2.2831(13)

C7–O1: 1.251(2) C7–O1: 1.258(2) C7–O1: 1.2485(18)

C7–C8: 1.462(2) C7–C8: 1.460(2) C8–O2: 1.2494(19)

C7–C8i: 1.466(2) C7–C8ii: 1.466(2) C7–C8iii: 1.468(2)

Bond angles (8)

O1–Cu1–N1: 96.51(5) O1–Zn1–N1: 94.37(5) O1–Cd1–N1: 96.51(5)

O1–Cu1–N2: 89.63(6) O1–Zn1–N2: 88.67(6) O1–Cd1–N2: 89.63(6)

N1–Cu1–N2: 81.53(6) N1–Zn1–N2: 79.48(6) N1–Cd1–N2: 81.53(6)

O1–Cu1–N1iv: 83.49(5) O1–Zn1–N1iv: 85.63(5) O1–Cd1–N1iv: 83.49(5)

N2–Cu1–N1iv: 98.47(6) N2–Zn1–N1iv: 100.52(6) N2–Cd1–N1iv: 98.47(6)

N2–Cu1–O1iv: 90.37(6) N2–Zn1–O1iv: 91.33(6) N2–Cd1–O1iv: 90.37(6)

Fig. 3 A stereoscopic packing

in the crystal of 1–2 complexes,

displaying the distorted

octahedral environment around

the cadmium ions. Intra- and

interchain hydrogen bonding

interactions are indicated by

dashed lines


123

together via offset face to face p–p interactions between

the amp ligands, which lead to a 2D network in complex 3

(see Table 3 and Fig. 5 for details).

3.2 Magnetic Properties

Figure 6 has shown the polycrystalline EPR spectrum of

complex 1 at 10 K and its computer simulation. EPR

spectra of complex 1 were recorded in the temperature

range 10–300 K, but there was not appreciable a change.

Only g components have been observed in this spectrum.

Hyperfine splitting could not be observed due to line

broadening originated from dipolar and exchange interac-

tions. The values of g// and g\ components were obtained

with computer simulation of the powder spectrum by using

WinEPR program. The principal values of these are the

following: g// = 2.198 and g\ = 2.075. The polycrystal-

line EPR spectra of Cu2? doped complexes (2–3) at 298 K

showed axial spectra with well-defined g// [2.364 for 2,

2.370 for 3] and g\ [2.047 for 2, 2.059 for 3) features

(Fig. 6). In Cu2? doped complexes 2 and 3 parallel com-

ponent of hyperfine (A//) [164 G (2), 152 G (3)] is clearly

observed, whereas perpendicular component of hyperfine

(A\) is not observed. The g parameters have indicated

that the local symmetry of paramagnetic center is axial.

From the order of g// [ g\[ ge (free electron g value,

ge = 2.0023), it can be concluded that Cu2? ions are

located in tetragonally distorted octahedral sites and

the ground state of the paramagnetic electron is dx2�y2

(2B1g state) in all three complexes [40–42].

The magnetic susceptibility of complex 1 was obtained

in the temperature range of 15–300 K. The temperature

dependence of the molar magnetic susceptibility (vm) and

1/vm are shown in Fig. 7. The temperature dependence vm

Fig. 4 A stereoscopic packing

in the crystal of 3, displaying

the distorted octahedral

environment around the

cadmium ions. Intra- and

interchain hydrogen bonding

interactions are indicated by

dashed lines

Table 3 Hydrogen bond data for complexes 1–3

D–H��A D–A H��A D��A D–H��A

Complex-1

N2–H2A��O3v 0.90 2.33 3.096(3) 143.6

N2–H2B ��O2vi 0.90 2.36 3.142(2) 146.0

N2–H2B��O2v 0.90 2.54 3.0762(19) 118.8

O3–H3A��O1 0.814(18) 1.968(18) 2.779(2) 175(3)

O3–H3B��O2v 0.83(4) 2.22(4) 3.052(3) 175(3)

Complex-2

N2–H1C ��O2vii 0.90 2.28 3.100(2) 150.6

N2–H1C��O2viii 0.90 2.66 3.144(2) 114.8

N2–H1D��O3viii 0.90 2.30 3.105(2) 148.4

O3–H3A��O2viii 0.82(4) 2.22(4) 3.039(2) 176(4)

O3–H3B��O1 0.76(3) 2.07(3) 2.831(2) 176(3)

Complex-3

N2–H2A ��O2ix 0.90 2.05 2.904(2) 157.9

N2–H2B ��O2x 0.90 2.13 3.010(2) 164.7

p–p interactions for 3

Cg(I) Cg(J) Cg–Cg (A) Cg(I)–Perp (A) Alpha (�)

Cg(1) Cg(1)vi 3.8072(13) 3.6216(9) 0

Symmetry codes: (v) -x, 1 -y, 1-z; (vi) 1 ? x, 1 ? y, z; (vii) x-1,

y-1, z; (viii) 2-x, 1-y, 1-z; (ix) x, y, z-1; (x) x, -y ? 1.5, z-0.5

Cg(I) Plane number I and J, Cg(1) Plane of the N1-C2-C3-C4-C5-C6

ring, Cg–Cg Distance between ring Centroids, Cg(I)–Perp Perpen-

dicular distance of Cg(I) on ring J, Alpha Dihedral Angle between

planes I and J


123

was fitted by the relation of C/T where C is the Curie con-

stant [43]. From this fitting process, C = 0.40 was deter-

mined. The effective magnetic moment, leff, was calculated

to be 1.70 at 300 K using the relation leff ¼ 2:83ðvTÞ1=2in

Bohr magneton (lB). The value of leff at room temperature

for complex 1 is consistent with the spin only value [44].

From these values, paramagnetic behavior of the structure is

concluded.

3.3 UV–Vis Spectra

The electronic spectra of aqueous solutions of the com-

plexes display strong absorption bands below 300 nm due

to intra-ligand transitions. The broad absorption band at

560 nm for complex 3 (e = 32 L mol-1 cm-1) is assigned

to the Eg ? T2g transition.

3.4 FT-IR Investigation

The infrared spectrum shows a very intense (Table 4) and

defined band at 1496 (1), 1495 (2) and 1517 cm-1 (3)

assigned to a coupled mode CO and CC stretching, which

is quite characteristic absorption band for squarate-con-

taining complexes. Since the quantities and the shape of the

bands depend on the symmetry of the chemical species,

vibrational band analysis around this region could be

related to an extended electronic delocalization over the

oxocarbon ring. The splitting of this stretching vibration in

these compounds compared to corresponding vibration in

K2C2O4 [45] strongly suggests the symmetry to be lower

than D4h, which is confirmed by the X-ray investigation.

The crystal structure data show that CC bonds of the anion

are approximately equal, as well as CO bonds, which

support the extended electronic delocalization. Weak

intensity IR bands observed above 1550 cm-1 were taken

as evidence for the presence of localized squarate C=O

[1730 (1–2), and 1722 cm-1 (3)] and C=C [1668 (1–2), and

1629 cm-1 (3)].

A strong and broad feature in the region 3650–

3000 cm-1 in the IR spectra of complexes 1 and 2 is

indicative of the presence of N–H��O and O–H��Ohydrogen bonds. The OH/NH frequency range implies that

neither the water molecules nor the amp NH groups par-

ticipate in very strong hydrogen bonds. Moreover, the

existence of m-(OH) bands close to 3600 cm-1 indicates

Fig. 5 The extended two-

dimensional structure of 3; the

interchain face to face p–p(between the pyridine rings) and

intra-chain hydrogen boning

(dashed lines) interactions


123

that some of the water OH groups are very weakly

hydrogen bonded. Considering the chemical structure of

amp, the vibration band information on metal coordination

of amp mainly originates from the symmetric and asym-

metric stretching vibrations of NH2. The vibration of the

NH2 is higher than free amp. This shift clearly indicates

that amp coordinates to M (II) through NH2.

4 Conclusion

Three novel coordination polymers formed by M (II)

[M=Cu(II), Zn(II) and Cd(II)] squarate with 2-amino-

methylpyridine have been prepared and characterized. The

crystal structure of all complexes consists of 1D chains,

which are linked by intra- and inter-molecular hydrogen

bonds and face to face p–p stacking interactions. The

crystallographic results indicate that the structures of

coordination polymers are controlled by squarate ligand,

but are not significantly influenced by the metal atoms,

while the supramolecular interactions in assemblies are

basically controlled by the amp and crystal water molecule.

Cu2? ions in 1 and Cu2? doped 2–3 are located in tetrag-

onally distorted octahedral sites and the ground state of the

paramagnetic electron is dx2�y2 .

5 Supplementary Material

Crystallographic data (excluding structure factors) for the

structure in this paper have been deposited with the Cam-

bridge Crystallographic Data Center as the supplementary

publication no CCDC 783245–783247 for complexes 1–3,

respectively. Copies of the data can be obtained, free of

charge, on application to CCDC, 12 Union Road, Cam-

bridge, CB12 1EZ, UK, fax:?44 1223 366 033, e-mail:

[email protected] or on the web http://www.ccdc.

cam.ac.uk.

Fig. 6 a The powder EPR spectrum of complex 1 at 10 K. b, c Cu2?

doped powder EPR spectrum of complexes 2–3 at room temperature

(DPPH: diphenylpicrylhydrazyl)

Fig. 7 The temperature dependence of the molar magnetic suscep-

tibility vm for complex 1. Solid line represents a fit by the Curie law.

Inset: The temperature dependence of 1/vm and fitting line


123

http://www.ccdc.cam.ac.uk

http://www.ccdc.cam.ac.uk

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Table 4 The most characteristic IR bands of squaric acid, amp and complexes (1–3)

Tentative

assignment (cm-1)

Squaric acid [45] amp [31] {[Cu(C4O4)2(amp)2]�H2O}n {[Zn(C4O4)2(amp)2]�H2O}n [Cd(C4O4)2(amp)2]n

m(OH)water – – 3649 w 3555 w –

m(OH)sq 3462 s – – – –

m(NH2) – 3366 m 3460 m 3462 vs 3231 s

m(NH2) – 3292 m 3346 m 3394 vs 3136 s

m(NH2) – 3188 m 3265 m 3290 vs 3080 s

m(CH) – 3051 m 3121 m 2954 s 3053 m

m(CH) – 3009 m 3080 m 2943 s 3018 m

m(CH2) – 2914 m 2953 m 2916 s 3007 m

m(CH2) – 2853 m 2924 m 2851 s 2961 m

m(C = O)sq 1818 vs – 1730 vw 1730 w 1722 vw

m(C = C)sq 1618 m – 1668 w 1668 b 1629 w

m(C = N) – 1609 s 1612 m 1606 vs 1598 vs

d(NH2) – 1592 vs 1595 vs 1573 vs 1571 s

m(CC) ? m(CO) 1530 s, 1516 sb – 1496 sb, 1485 sb 1495 sb, 1483 sb 1517 sb, 1487 sb

m(CC) – 1569 s 1447 s 1431 vs 1433 s

d(CH2) – 1474 s 1429 s 1294 vs 1381 s

d(CH2) – 1435 vs 1292 m 1252 s 1338 s

mMedium, s Strong, vs Very strong, w Weak, b Broad, sb Strong and broad, vw Very weak, d Bending


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