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Solvothermal syntheses and structural characterization of a series of metal–hydroxycarboxylate coordination polymers

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Solvothermal syntheses and structural characterization of a series

of metal–hydroxycarboxylate coordination polymers

Jian-Qiang Liu

a

, Yao-Yu Wang

a,*

, Lu-Fang Ma

a

, Wei-Hong Zhang

a

, Xi-Rui Zeng

b

,

Qi-Zhen Shi

a

, Shie-Ming Peng

c

aDepartment of Chemistry and Shaanxi Key Laboratory of Physico-inorganic Chemistry, Northwest University, Xi’an 710069, PR China bDepartment of Chemistry, JingGangShan College, Ji’an 343009, PR China

cDepartment of Chemistry, National Taiwan University, Taipei, Taiwan

Received 2 September 2007; received in revised form 28 November 2007; accepted 28 November 2007 Available online 14 December 2007

Abstract

The combination of transition metal ions with mixed ligands resulted in the formation of three new coordination polymers, {[Co(C4H4O5)(bpe)(H2O)2] (0.5bpe)(H2O)}n(1), {[Cu(C4H4O6)(bipy)] 5H2O}n(2) and {[Cu(C4H4O5)(bpa)] 2.5H2O}n(3) (C4H4O25

¼ malate dianion, C4H4O26 ¼ tartrate dianion, bpe = 1,2-bis(4-pyridyl)ethene, bipy = 2,20-bipyridine, bpa = 1,2-bis(4-pyridyl)ethane),

which were prepared under solvothermal conditions and characterized by single-crystal X-ray diffraction. 1 and 2 feature 1D chain struc-tures. Interestingly, each pair of chains recognizes each other through aromatic p–p stacking interactions, generating a zipper-like dou-ble-stranded chain in 2. Compound 3 shows 2D 63topology framework with a rectangle-like grid.

Ó 2007 Elsevier B.V. All rights reserved.

Keywords: Hydroxycarboxylate ligands; Crystal structures; Solvothermal syntheses; Magnetic behavior

1. Introduction

Crystal engineering of metal-organic networks via self-assembly of metal ions and multi-functional ligands has attracted considerable attention because of the structural diversity presented in such compounds which in turn facil-itates systematic evaluation of structure property relation-ships[1–9]. Carboxylate complexes have been investigated over the past years due to their interesting coordination chemistry, allowing for unusual structural features and leading to various physical and chemical properties and practical applications in some fields such as dyes, extract-ants, drug, pesticides and catalysts [10–12]. The aromatic multi-carboxylate ligands, such as benzenedi-, benzenetri-, benzenetetra- and nitrogen-containing heterocyclic dicar-boxylates, have been extensively introduced in the

prepara-tions of metal-organic supramolecular open frameworks

[13–17]. However, the hydroxyl polycarboxylates (HPCs),

such as malate, citrate and tartrate, have been less studied as building blocks in the construction of metal-organic frameworks [18,19]. We believe that the application of these bridging ligands with conformation freedoms is ben-eficial to the adjustment of structures and properties, although the structural control of the product is difficult. Xu et al. have synthesized successfully some coordination polymers [20,21]. 3D interpenetrating topology networks and supramolecular polymers with malate are reported recently by our group, which is a good example of 1D mag-netic systems with various magmag-netic exchange interactions [22,23].

Herein, we extend these studies to explore the interac-tion of two types of hydroxycarboxylate ligands with tran-sition-metal ions, and N-donor ligands to pursue the aim of designing new compounds exhibiting various architectures and magnetic ordering. This paper reports the syntheses and characterization of three new coordination polymers, 0020-1693/$ - see front matterÓ 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2007.11.036

*

Corresponding author. Tel./fax: +86 029 88303798. E-mail address:wyaoyu@nwu.edu.cn(Y.-Y. Wang).

www.elsevier.com/locate/ica Inorganica Chimica Acta 361 (2008) 2327–2334

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namely {[Co(C4H4O5)(bpe)(H2O)2] (0.5bpe)(H2O)}n (1),

{[Cu(C4H4O6)(bipy)] 5H2O}n (2) and {[Cu(C4H4O5

)(b-pa)] 2.5H2O}n(3) (C4H4 O2z5 ¼ malate dianion, C4H4O26

¼ tartrate dianion, bpe = 1,2-bis(4-pyridyl)ethene, bipy = 2,20-bipyridine, bpa = 1,2-bis(4-pyridyl)ethane). Single

crystal X-ray analyses show that functional ligands play an important role in the affecting the final structural motifs. In 1, with malate only acting as appending fashion and bpe acting as ditopic subunit, the self-assembly results in a 1D chain, while in 3, with malate acting as tetratopic subunit and bpa acting as ditopic subunit, an appealing 2D 63 topological framework is obtained. In 2, with bipy acting as terminal ligand, the self-assembly results in a zipper-like double-stranded chain. To the best of our knowledge, these are three new, previously unreported types of structural motifs.

2. Experimental 2.1. General procedures

All reagents were purchased from commercial sources and used as received. IR spectra were recorded with a Per-kin–Elmer Spectrum one spectrometer in the region 4000– 400 cm1using KBr pellets. The luminescent spectra of the solid samples were acquired at ambient temperature by using a JOBIN YVON/HORIBA SPEX Fluorolog t3 sys-tem (slit: 0.2 nm). TG analyses were carried out with a Metter–Toledo TA 50 in dry dinitrogen (60 mL min1) at a heating rate of 5°C min1. X-ray power diffraction (XRPD) data were recorded on a Rigaku RU200

diffrac-tometer at 60 kV, 300 mA for Cu Ka radiation

(k = 1.5406 A˚ ), with a scan speed of 2°/min and a step size of 0.04° in 2h. The magnetic susceptibility was obtained on crystalline sample using a Quantum Design MIPMS SQUID magnetometer. The experiment susceptibility was corrected for the sample holder and the diamagnetism con-tribution estimated from Pascal’s constant.

2.1.1. Synthesis of {[Co(C4H4O5)(bpe)(H2O)2]

(0.5 bpe)(H2O)}n(1)

A mixture of malic acid (0.210 g, 1.55 mmol), NaOH (0.080 g, 2 mmol), CoCl2 6H2O (0.152 g, 0.073 mmol),

bpe(0.028 g, 0.015 mmol), CH3OH (4 mL) and distilled

water (12 mL) was stirred for 5 min in air, then transferred and sealed in a 25 mL Teflon-lined autoclave, which was heated at 140°C for 72 h. The autoclave was cooled over a period of 12 h at a rate of 5°C h1, to yield a very fine pink crystalline product 1 in 50% yield. Anal. Calc. for C22H24CoN3O8(1): C, 51.35; H, 4.04; N, 7.49. Found: C,

51.88; H, 4.21; N, 7.33%. IR (KBr, cm1) for 1: 3462(m), 3102(m), 1692(vs), 1597(m), 1405(vs), 1273(vs), 1182(vs), 1064(m), 882(vs), 608(m),517(vs).

2.1.2. Synthesis of {[Cu(C5H4O6)(bipy)] 5H2O}n(2)

A mixture of tartaric acid (0.132 g, 0.89 mmol),

bipy(0.188 g, 0.52 mmol), NaOH (0.080 g, 2 mmol),

CuCl2 2H2O (0.112 g, 0.065 mmol), CH3OH (6 mL) and

distilled water (12 mL) was stirred for 5 min in air, then transferred and sealed in a 25 mL Teflon-lined autoclave, which was heated at 140°C for 72 h. The autoclave was cooled over a period of 10 h at a rate of 5°C h1, to yield a very fine blue crystalline product 2 in 38% yield. Anal. Calc. for C14H10CuN2O11 (2): C, 38.58; H, 4.10; N,

6.00. Found: C, 38.88; H, 4.01; N, 5.86%. IR (KBr, cm1) for 2: 3418(vs), 1616(vs), 1597(m), 1395(vs), 1297(m), 1222(m), 1078(vs), 1025(vs), 814(m), 611(m), 516(m).

2.1.3. Synthesis of {[Cu(C4H4O5)(bpa)] 2.5H2O}n(3)

A mixture of malic acid (0.132 g, 0.99 mmol), KOH (0.080 g, 2 mmol), CuCl2 2H2O (0.112 g, 0.065 mmol),

bpa (0.032 g, 0.0172 mmol), CH3OH (4 mL) and distilled

water (12 mL) was stirred for 5 min in air, then transferred and sealed in a 25 mL Teflon-lined autoclave, which was heated at 140°C for 72 h. The autoclave was cooled over a period of 10 h at a rate of 5°C h1, to yield a very fine blue crystalline product 3 in 38% yield. Anal. Calc. for C16H20CuN2O7.50(3): C, 33.45; H, 3.30; N, 4.59. Found:

C, 33.60; H, 3.26; N, 4.56%. IR (KBr, cm1) for 3: IR (KBr, cm1) 3414(s), 2919(w), 1639(vs), 1503(vs), 1426(s), 1220(s), 1030(m), 843(m), 628(w), 519(m).

3. X-ray crystallography

Single crystal X-ray data of 1–3 were collected at 298 K on a Bruker SMART APEX CCD diffractometer using graphite-monochromatic Mo Ka radiation (k = 0.71073 A˚ ). The crystal data and structure refinement of compounds 1–3 are summarized in Table 1. The linear absorption coefficients, scattering factors for the atoms, and the anomalous discussion corrections were taken from International Tables for X-ray Crystallography. The data integration and reduction were processed with

SAINT software [24]. The structures were solved by the

direct method usingSHELXTL and were refined on F2with SHELXL-97 program package[25]. All non-hydrogen atoms

present were anisotropically refined. All hydrogen atoms of water were located in successive different Fourier Maps and the other hydrogen atoms were treated as riding method.

4. Results and discussion 4.1. Syntheses of the complexes

All the complexes were obtained by solvothermal reac-tions of transition-metal salts with the hydroxyl polycarb-oxylates and assistant ligands in water at temperature of 140°C. Some variations of the starting materials that may have influenced on the assembly process including metal salts, assistant ligand and pH. No other kinds of crystals were obtained, however, suggesting the stability of the final structures in the hydrothermal conditions.

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4.2. {[Co(C4H4O5)(bpe)(H2O)2] (0.5bpe)(H2O)}n(1)

The asymmetric unit contains one Co2+ cation, one Hmal2anion (Hmal2= malate dianion), two coordina-tion water molecules and one lattice water molecule, and half exoteric bpe molecule and one bridging bpe ligand. As depicted inFig. 1. The bpe ligand serves as a bridge to con-nect two Co atoms, leading to a 1D chain running along the a direction. Each Co2+ion is located on a symmetry cen-ter and octahedral coordination formed by two oxygen atoms of one malate and two aqua ligands and two N atoms of two separated bpe ligands in trans-position. The Co–O bond distances fall in the region 2.053(3)–2.119(2) A˚ , and Co–N bond distance averaging 2.15 A˚ are practically identi-cal with the experimental limitation (Table 2). Thus, a zigzag Co–bpe–Co chain with Co  Co separation 13.646 A˚ is formed. The neighboring inter-chain Co  Co distance is 7.581(5) A˚ . The dihedral angle between the two pyridyl rings of the bpe is 9.2(2)°, dissimilar to those observed in other polymers[26–28].

It is interesting that the neighboring Co(II) atoms in 1 are bridged by bpe ligands rather than multifunctional malate dianion which coordinates to the cobalt center through

two oxygen atoms of alkoxy and a-carboxylate groups. The malate adopts the mode I fashion[20], while the b-car-boxyl group is not deprotonated and remains freedom, as confirmed by the strong absorption band at 1692 cm1 in the IR spectrum. This phenomenon has been found in three mononuclear structures [29–31]. These adjacent zigzag chains are further connected through hydrogen-bonding interactions [O1w  O6 = 2.789(5) A˚ O1w–H1w  O6 = 165(5)°; symmetry code: x + 1, y + 1, z; O2w  O8 = 2.712(4) A˚ O2w–H3w  O8 = 173(5)°; symmetry code: x 1, y, z] between the coordinated water molecules and uncoordinated oxygen atoms of carboxylate groups, leading to a 2D structural motif in the ac plane as illustrated inFig. 2. The free bpe molecules reside at the crossing positions of the square grid sheets. These 2D sheets are linked into a 3D net-work through another hydrogen bondings produced by lat-tice water with one non-coordinated bpe molecule, forming a large channel, as shown inFig. S1.

4.3. {[Cu(C4H4O6)(bipy)] 5H2O}n(2)

A single-crystal X-ray structural analysis shows that 2 crystallizes in orthorhombic chiral space group P212121,

with one copper(II) atom, one quadridentate tartrate ligand, one bipy ligand, and five lattice water molecules Table 1

Crystallographic data and structure refinement for 1–3

1 2 3

Formula C22H24CoN3O8 C14H10CuN2O11 C16H20CuN2O7.5

Molecular weight 517.37 445.78 407.88 Crystal system triclinic orthorhombic orthorhombic Space group P 1 P212121 Fdd2 a (A˚ ) 7.5814(8) 6.6556(8) 21.9944(13) b (A˚ ) 12.6006(13) 14.3048(16) 33.3369(19) c (A˚ ) 13.6456(14) 19.047(2) 10.3969(5) a(°) 67.3630(10) 90 90 b(°) 74.9070(10) 90 90 c(°) 76.6100(10) 90 90 V (A˚3) 1148.9(2) 1813.4(4) 7623.3(7) Z 2 4 16 Dcalc.(g/cm 3 ) 1.495 1.633 1.477 hRange (°) 1.65–26.00 1.78–24.50 2.22–27.50 l(mm1) 0.800 1.267 1.188 F(0 0 0) 536 900 3504 Parameters 335 223 260 Goodness-of-fit 1.016 1.093 1.030 R1[I > 2r(I)]a 0.0528 0.0851 0.0546 wR2(all data)b 0.1308 0.2469 0.1730 a R1¼PkFoj  jFck=PjFoj. b wR2¼ fP½wðF2o F2cÞ 2 =PðF2 oÞ 2 g1=2.

Fig. 1. Octahedral coordination spheres of Co ion in compound 1 with atom labeling schemes; hydrogen atoms are omitted for clarity.

Table 2

Selected bond distances (A˚ ) and angles (°) 1 Co(1)–O(2W) 2.052(4) Co(1)–O(4) 2.062(3) Co(1)–O(1W) 2.082(4) Co(1)–O(5) 2.119(2) Co(1)–N(1) 2.148(3) O(2W)–Co(1)–O(4) 172.14(12) O(1W)–Co(1)–O(5) 168.98(12) N(1)–Co(1)–N(2) 175.10(10) 2 Cu(1)–O(2) 1.978(10) Cu(1)–O(6)#1 1.995(10) Cu(1)–O(5)#1 2.277(11) Cu(1)–O(1) 2.295(10) Cu(1)–N(1) 1.977(6) Cu(1)–N(1) 1.968(5) N(1)–Cu(1)–O(6)#1 159.2(4) N(2)–Cu(1)–O(5)#1 111.0(4) N(2)–Cu(1)–O(2) 159.1(4) O(1)–Cu(1)–O(5)#1 154.5(3) 3 Cu(1)–O(4) 1.941(3) Cu(1)–O(5) 1.987(4) Cu(1)–N(2) 1.995(5) Cu(1)–N(1) 2.011(4) Cu(1)–O(2) 2.335(4) O(5)–Cu(1)–N(2) 175.37(18) O(4)–Cu(1)–N(1) 168.99(19) #1 x + 1, y, z.

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in each crystallographic unit, as shown inFig. 3. Each cop-per(II) ion is coordinated by four oxygen atoms from two different tartrate ligands, and two nitrogen atoms from a chelating bipy ligand to give [2 + 2 + 2] distorted octahe-dral geometry. The corresponding bond parameters in 2 are in close agreement with previous copper(II) complexes containing carboxylate ligands. In addition, the fact that the Cu–O(carboxyl) bonds [av. 1.987(10)] are significantly

shorter than the Cu–O(hydroxyl)bond [av. 2.86(10)], in which

is also found in other complexes[19,32–35].

It is worthwhile to note that the tartrate ligand bridges to the copper center to form a 1D polymeric chain running along the crystallographic a axis with adjacent Cu  Cu distance of 6.656 A˚ . The bipy ligands lie on one side of this chain in parallel fashion. Interestingly, a pair of 1D chains self-assembles to generate a molecular zipper-like double chain under the direction of strong aromatic p–p interac-tions between the bipy units with a face-to-face distance of ca. 3.122–3.356 A˚ (Fig. 4). Such zipper-like chain struc-tures are notable common, but so far, few examples of zip-per-like coordination polymers have been reported. To the

best of our knowledge, the zipper-like double chains repre-sent an example of perfect molecular double chains using tartrate ligand. The unique feature of compound 2 is the interesting arrangement of tartrate molecule and coordina-tion of copper atom forming helical structure, which is

con-structed by flexible O2C–C–C–CO2 groups bridging

between the Cu centers along a direction with a pitch of 6.8 A˚ . The formation of the helix in the structure may be attributed to the fact that the steric orientation of the car-boxyl groups is expected.

The most remarkable feature of 2 is that, the hydrogen bonding association of water molecules leading to the for-mation of a cyclic water pentamer consisting of one O11W, two symmetry-related O7W and O9W, whose conforma-tion is similar to another water cluster reported by Gao and his co-workers [36]. In addition, O8W and O10W, bond jointly to the water molecule (O7W) and form a dimer. The water molecules of the dimeric and pentameric water cluster are hydrogen bonded to carboxylate oxygen atoms (O3 and O5) and unprotonated hydroxyl groups (O1), resulting in an overall 3D framework that features 1D open channels viewed along the crystallographic a axis. Fig. 2. Two-dimensional sheet in compound 1 formed through hydrogen bonding between uncoordinated carboxylate group of malic acid and coordinated water molecules in the b axis. Symmetry codes: (1) Cu–O2: x, y, z; (2) Cu–O5: 1 + x, y, z.

Fig. 3. ORTEP diagram showing the coordination environments for metal atoms in 2 (the lattice water molecules are omitted for clarity).

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The approximate dimensions of these channels are 10.51 5.82 A˚2, which are large enough to encapsulate guest solvent molecules.

The water pentamer topology described here is in agree-ment with the puckered ring achieved from both experi-mental and theoretical studies by Liu, Saykally and their coworkers [37]. The bent angle of water pentamer in 2 is 45.8°, which is considerably larger than that of the discrete pentamer (ca. 20°), but is shorter than the water tape (52.2°)[36]. The differences may be attributed to the influ-ence of surrounding environments. The O  O distances within the pentamer range from 2.681(5) to 2.925(3) A˚ (Table S3). Adjacent pentamers are fused together by shar-ing one edge, formshar-ing a one-dimensional water tape along [0 1 0] direction (Fig. 5). According to Infantes’ classifica-tion [38], those water tapes in 2 have the symbol T5(2). The average O  O distance within the tape is 2.805 A˚ (298 K), which is closed to the corresponding value of 2.85 A˚ found in liquid water, indicating a great structural resemblance to liquid water and comparable to those in the ice(II) phase (2.77–2.84 A˚ )[39].

4.4. {[Cu(C4H4O5)(bpa)] 2.5H20}n(3)

Single crystal diffraction analysis reveals that 3contains one bpa molecule, one Hmal2anion (Hmal2= malate dianion), and two half water molecules in the asymmetric unit, as shown inFig. 6. Each Cu(II) ion is six-coordinated by two nitrogen atoms from two bpa molecules, four oxy-gen atoms from two Hmal2anions, forming an axially elongated [4 + 1 + 1] octahedral geometry. The adjacent Cu2+ ions are linked by two bpa ligands with gauche– gauche conformation, which result a square grid with a window of about 8.7 9.6 A˚ . Guest water molecules are found in this grid. These square grids are transversally joined by Hmal2anions and show Cu  Cu separation of 5.97(5) A˚ . The b-caboxylate group bridges neighboring metal centers, in which is similar to that found in [Cu(H-mal)(4pds) 6H2O] and {[Cu(Hmal)(bpp) 6H2O]}n

[23,40]. Therefore, the global structure should be described as 2D coordination polymer along the ac-plane, as shown in Fig. 7. There has a 36-membered [Cu6(bppHmal)4] ring

as its basic motif, each ring with 13.7 13.7 A˚ encloses water pentamer, as shown in Fig. S2. Due to the N–Cu– N bond angle of 93.4(8) A˚ and the flexibility of the bpa ligands, an interesting pipe-comb-like 2D layered structure with a channel of about 8.7 9.6 A˚ occurs in this net along the ab-plane (Fig. 8).

If, for reasons of classifying the net, we define chelated ring as a single point of connection to Cu (making each Hmal as effectively single linker), then Cu atoms are depicted as three-connected nodes, the framework can be represented simply by connecting the Cu nodes according to the connectivity defined by the bridging Hmal and bpa ligands (Scheme S1). This type of network is referred to a

common example of 63 topology [41], as shown in

Fig. 9a. Each Cu(II) is linked to six other radiative Cu(II)

Fig. 5. A view showing how the self-assembled chain of water molecules are bound to the MOF in 2.

Fig. 6. ORTEP diagram showing the coordination environments for metal atoms in 3. Hydrogen atoms are omitted for clarity.

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ions, and every three neighboring Cu(II) ions encircle a tri-angular hole. It should be noted that these hydrogen bonds and molecular forces result in adjacent 2D layer in 3 stacked in a slightly offset parallel fashion with ABAB sequence forming non-interpenetrating 3D array (Fig. 9b). 4.5. Thermogravimetric analysis and X-ray powder

diffraction

To investigate the thermal stability of polymers 2 and 3, the samples were analyzed by TGA under nitrogen. The TGA data of 2 and 3 suggest that there are three stages of weight loss in the temperature range (Fig. S3). The major weight losses occur above ca. 175°C in 2, corre-sponding to the lattice water molecules and the tartrate and bipy ligands, respectively. As expected from its struc-ture, compound 3 loses lattice water molecules in 43– 140°C (10.4% observed, 11.3% calcd). The next weigh loss occurs below 300°C to give a total weigh loss of ca. 74.4%, corresponding to the malate and bpa ligands. To confirm the TGA, the original sample of 3 was characterized by

X-ray Powder diffraction (XRPD) at a wide temperature range (25–450°C) (Fig. S4). At 300°C, the XRD pattern changes completely, revealing the collapse of the frame-work, which is in accordance with the result of the thermo-gravimetric analysis.

4.6. Magnetic study

The temperature dependence of the magnetic suscepti-bilities for complex 1 in the range 2–300 K, and shown as vMT and vM versus T plots in Fig. S5. The experimental

vMT curve exhibits a continuous decrease from 300 K to

2 K as the temperature is lowered for the compound. Such behavior is characteristic for Co(II) complex, essentially due to the single ion anisotropy, and may combine some contribution of anti-ferromagnetic exchange between the Co(II) centers.

Based on the structural information, the magnetic cou-pling mediated by Van der Waals interactions between the chains could be negligibly small. We have attempted to fit the experimental susceptibility using the classical spin Heisenberg model for a one-dimensional chain[42]. vM¼

Ng2b2SðS þ 1Þ 3KT

1þ u

1 u; ð1Þ

where u(K) = coth (K) 1/K. K¼ 2JSðS þ 1Þ=KTS ¼ 3=2:

An additional coupling parameter, zJ0, was added in Eq.

(2) to take into account the magnetic behavior through hydrogen bonding interaction between 1D chains[43].

The total magnetic susceptibility is vT¼

vM 1 2zJ0

Nb2g2vM

: ð2Þ

The least-squares analysis of magnetic susceptibilities data led to J =0.27 cm1, g = 2.05, zJ0=0.031 cm1

and R = 4.02 103 for 1. The agreement factor defined as R¼P½ðvMTÞobs ðvMTÞcalc

2

=P½ðvMTÞobs 2

. The J value indicates a weak anti-ferromagnetic between the nearest Co(II) ions bridged by bpe. The smaller negative zJ0value

can be assigned to a very weak anti-ferromagnetic interac-tion through hydrogen-bonding interacinterac-tions. Obviously, the result is not very satisfactory, especially in low temper-ature region. The inclusion of an interaction based on the molecular field approximation does not improve the theo-retical fitting distinctly. This may original from the fact that Fisher’s equation does not take into account the effects of the zero-field splitting and/or spin–orbit coupling which are significant for Co(II)[44,45].

4.7. Photoluminescence

The emissions spectra of complexes 1–3 in the solid state at room temperature are shown inFig. S6. While the 1–3 of analogous pyridine, it can be observed that the emission occurring in the same range for the two different metals Fig. 8. A view of net along the ab plane in 3 showing an interesting a

pipe-comb-like 2D layer structure: the hydrogen atoms are omitted for clarity.

Fig. 9. (a) Schematic illustration the (6, 3) topology of 2D network of 3; (b) two parallel off-set double-layer in 3.

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can be assigned to ligand-to-metal charge transfer (LMCT) bands. Similar to those observed for dipyridine other ligands[46,47] (Fig. S6a: kex= 300 nm, kem= 391 nm for

1; b: kex= 335 nm, kem= 365 nm for 2; c: kex= 330 nm,

kem= 383 nm for 3).

The UV–Vis spectra of complexes 1–3 are recorded in methanol and characterized by several spectral regions

(Fig. S7). The absorption bands in the range of 260–

285 nm for the bpe and 228–285 nm for the bipy ligand are ligand-centered (LC) due to p–p*transitions. The peaks

at 254 nm for complexes 1 and 2 are assigned to metal-to-ligand charge transfer transitions (MLCT)[48]. This great shift of absorption bands of the complexes relative to that of the bpe and bipy ligands are caused by the effects of metal-to-ligand charge transfer transitions. The absorption bands of the complex 3 and the bpa occur at the same region (256 nm) due to absence of conjugated system for bpa ligand.

5. Conclusion

In conclusion, the self-assembly of three polymeric com-plexes with chains or layer were constructed from two types of hydroxycarboxylic acids in the presence of multi-dentate

N-donor auxiliary ligands under solvothermal conditions. The auxiliary ligands play an important role in the synthe-sis of the complexes. In 1 and 2, the tartrate and malate in combination with terminated and rigid ligands, respec-tively, only exhibit 1D chain systems, while in 3, the sin-gle-connecting malate cooperates with the flexible bpa, forming a 63topological architecture.

We have concluded our investigations in this report and previous reports, which were coordination polymers of transition metal containing malic acid in combination with multi-dentate N-donor ligands (Scheme 1). The work shows that the conformations and the functions of ligands play an important role in affecting the final structural motifs. Further studies involving other hydroxycarboxylic acid in combination with multi-dentate N-donor ligands are in progress. Investigation of the different coordination modes of the ligands may also help in design of new struc-tural framework and fabrication of new functional material.

Acknowledgements

This work was supported by the National Natural Sci-ence Foundation of China (Nos. 20471048 and 20771090) Scheme 1.

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and TRAPOYT, and Specialized Research Found for the Doctoral Program of Higher Education (No. 20050697005). Appendix A. Supplementary material

CCDC 631996, 631997 and 631998 contains the supple-mentary crystallographic data for 1, 2 and 3. These data can be obtained free of charge from The Cambridge Crystal-lographic Data Centre via

www.ccdc.cam.ac.uk/data_re-quest/cif. Supplementary data associated with this article

can be found, in the online version, at doi:10.1016/ j.ica.2007.11.036.

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數據

Fig. 1. Octahedral coordination spheres of Co ion in compound 1 with atom labeling schemes; hydrogen atoms are omitted for clarity.
Fig. 2. Two-dimensional sheet in compound 1 formed through hydrogen bonding between uncoordinated carboxylate group of malic acid and coordinated water molecules in the b axis
Fig. 6. ORTEP diagram showing the coordination environments for metal atoms in 3. Hydrogen atoms are omitted for clarity.
Fig. 8. A view of net along the ab plane in 3 showing an interesting a pipe- pipe-comb-like 2D layer structure: the hydrogen atoms are omitted for clarity.

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