Construction of a 2D Co(II) Coordination Polymer with (4,4)-Connected Topology: Synthesis, Crystal Structure, and Surface Photo-electric Property

Abstract

A 2D grid-like (4, 4)-connected topology coordination polymer, $[Co(BTA)_2(H_2O)_2]_n$ (1), where HBTA = 2-(1H-benzotriazol-1-yl)acetic acid, has been synthesized by hydrothermal method and characterized by single crystal X-ray diffraction, IR spectroscopy, elemental analysis and surface photovoltage spectroscopy (SPS). X-ray diffraction analyses indicated that 1 displays octahedral metal centers with secondary building units (SBUs) [$Co(BTA)_2(H_2O)_2$] bridged by the $BTA^-$ ligands. In the crystal, the 2D supramolecular architecture is further supported by $O-H{\cdots}O$, $O-H{\cdots}N$, $C-H{\cdots}O$ hydrogen bonds and ${\pi}{\cdots}{\pi}$ stacking interactions. The SPS of polymer 1 indicates that there are positive response bands in the range of 300-600 nm showing photo-electric conversion properties. There are good relationships between SPS and UV-Vis spectra.

Introduction

In the last decades, one-, two-, and three-dimensional infinite coordination polymers have been found to have extensive applications due to their versatile properties such as reversible guest-exchange, shape selectivity, catalysis, gas storage, molecular recognition, photoluminescence, unusual magnetic, nonlinear, and semiconducting properties as well as chirality and clathration.1-8 However, such self-assembly processes involving metal ions and well-designed organic ligands couldn't be controlled by scientists so the crystal engineering of coordination frameworks with desired topo-logies and specific properties still remains highly challeng-ing since it depends on a variety of factors that can influence the self-assembly process.9-11 Series of ligands were intro-duced as “building blocks”, together with the coordination preferences of the metal atoms as “nodes”, to construct metal–organic frameworks. For instance, many neutral bridging ligands, such as pyridine-based ligands, quinoline-based ligands, tetrazole, and triazole were used in the construction of fascinating metal-organic networks.12-14

It is well-known that azole heterocycle and carboxylate groups play a crucial role in the catalytic activity of many metalloenzymes.15-19 Thus, a combination of such pairs of fatty acids and N-heterocyclic moieties (imidazole- and triazole-type ligands) has begun to attract attention recent years.20-25 However, there are still only rare reports of ligands based on N-heterocyclic and carboxylate groups as linking ligands for the construction of coordination polymers.26,27 In addition, the aromatic skeleton of these ligands may take part in the formation of π–π and C–H⋯π attractions whilst the oxygen and nitrogen atoms may function as the accep-tors or donors to form the hydrogen bonds. These weak intermolecular attractions have significantly influenced further assemblies of the polymers in solid structures.28

2-(1H-Benzotriazol-1-yl)acetic acid (HBTA, Scheme 1) with both a flexible and a rigid moieties, has multifunctional coordination sites with chelating and bridging ability through N and O atoms of the triazole and carboxylate groups. Re-cently, Hu’s and Zheng’s groups were interested in triazole-type ligands, specifically HBTA. They found that the HBTA has manifold coordination modes and may bridge metal ions to form the 0D,29-31 1D26,27 chain, 2D32-34 layer network, and the 3D35 frameworks.

Scheme 1.The synthesis for 1 and coordination mode of the BTA− in 1.

Surface photovoltage spectroscopy (SPS), as a very sensi-tive characterization tool, has been used to detect the charge transitions of functional semiconductors.36,37 SPS not only relates to the electron transition process caused by light absorption, but reflects the properties of separation and transition of photogenerated charges directly, so it is utilized to study the electron behaviors of the solid surface and interface.38,39

Following the above consideration and our ongoing work in this field,33,34,40,41 we herein present the synthesis and structure of the compound [Co(BTA)2(H2O)2]n (1) (Scheme 1), which has been structurally characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis and its surface photovoltage spectroscopy (SPS) property.

 

Experimental

Chemicals and Measurements. 2-(1H-Benzotriazol-1-yl)acetic acid (HBTA) was prepared in accordance with the procedure in literature.26 Other chemicals were of reagent grade obtained from commercial sources and used as received without further purification. The C, H, N, elemental analysis was performed on an Elenentar Vario EL elemental analyzer. The IR spectrum was recorded with a Shimadzu IR-408 spectrophotometer using the KBr pellet in the range of 4000-400 cm−1. The crystal structure was determined by a Bruker CCD area detector. The SPS was recorded with a home-built surface photovoltage spectrophotometer.

Synthesis of [Co(BTA)2(H2O)2]n (1). Compound 1 was synthesized from a reaction mixture of Co(NO3)2·6H2O(0.146 g, 0.5 mmol), HBTA (0.177 g, 1 mmol), NaOH (0.040 g, 1 mmol), and distilled H2O (13 mL) in a 25 mL Teflon reactor, under autogenous pressure at 130 °C for 3 days and then cooled to room temperature at a rate of 5 °C h−1. Light-red block crystals of compound 1 suitable for X-rays diffraction analysis were obtained (0.076 g, yield: 34% based on the metal). Anal. Calcd for C16H16CoN6O6 (%): C, 42.93; H, 3.58; N, 18.78. Found: C, 42.81; H, 3.62; N, 18.72. IR (cm−1): 3301 (s), 2918 (m), 1612 (s), 1453 (s), 1308 (m), 1245 (m), 1116 (m), 786 (s).

Table 1.aw = 1/[σ2(Fo)2 + (0.0997P)2 + 0.0988P], P = (Fo2 + 2Fc2)/3 for 1.

X-ray Crystallographic Studies. Single-crystal data col-lections were carried out on a Bruker Smart Apex II CCD diffractometer with graphite monochromatized MoKα radia-tion (λ = 0.71073 Å) at 296(2) K. The structures were solved with direct methods using SHELXS-97,42 and structure refinements were performed against F2 using SHELXL-97.43 All non-hydrogen atoms were refined with anisotropic dis-placement parameters. All H atoms were placed in calcu-lated positions (dC–H = 0.93–0.97 and dO–H = 0.85 Å) and were refined in the riding model approximation, with Uiso(H) set to 1.2Ueq(C) or 1.5Ueq(C). The crystal data are summariz-ed in Table 1. Selected bond lengths and angles are given in Table 2. Hydrogen bonds are listed in Table 3.

 

Results and Discussion

Synthesis. Comound 1 was synthesized from Co(NO3)2·6H2O, HBTA, NaOH, and distilled H2O under hydrothermal condi-tions (Eq. 1). This compound was characterized by single crystal X-ray diffraction, IR spectroscopy, elemental analysis, and surface photovoltage spectroscopy (SPS).

Structure Description of [Co(BTA)2(H2O)2]n (1). X-ray analysis reveals that compound 1 is a 2D coordination polymer and the asymmetric unit is comprised of one half Co2+ ion, one deprotonated doubly bridging syn,anti-ƞ1:ƞ1:μ2 BTA− ligand, one coordinated water molecule (Figure 1(a)). Each Co2+ ion sits at a symmetry center and has an octa-hedral coordination geometry, with the equatorial positions defined by four oxygen atoms from four symmetry-related carboxylate groups of BTA− ligands with the Co–O lengths of 2.068(4) and 2.084(4) Å and two axial positions by two symmetrically related water molecules with the Co–O length of 2.096(3) Å (Table 1). Each deprotonated HBTA ligand links two Co(II) atoms through carboxylate groups in a synskew coordination mode, and each Co(II) atom connects four BTA− ligands along the [010] and [001] directions, respectively, to form a 2-D grid-like open-framework (Figure 1(b)), in which the Co(II) atoms are arranged in an ideal layer and the BTA− ligands lie on two sides of the layer (Figure 1(c)). The 2-D layered structure of 1 shows a (4, 4) sql topology when the Co(II) atoms are regarded as con-nected nodes and carboxylate groups of BTA− as linkers. As show in Figure 1(c), from the topological point of view, this Co(II) atom can be defined as a 4-connected node. Thus, the overall structure of 1 is a 2-D layered structure with the short Schläfli symbol of 4462 (TD10 = 221).44,45 On the other hand, if we consider the hydrogen bonds involved in the uncoordinated triazole N and coordinating water in 3D supramolecular frameworks structure of 1, the Co(II) atom and BTA− bridging ligands can be defined as a 6- and a 3- connected nodes (Figures 1(d) and 1(e)), respectively. Thus, the 3D supramolecular framework structure of 1 has the short Schläfli symbol of (43)2·466683 (TD10 = 251).44,45

Figure 1.(a) The coordination environment of the CoII ion in 1. Hydrogen atoms have been omitted for clarity [symmetry code: i = −x + 1, y − 1/2, −z + 1/2; ii = x, −y + 1/2, z − 1/2; iii = −x + 1, −y, −z. (b) The 2-D plot structure for 1, showing the monomers are linked to form the coordination polymer by bridging carboxylate oxygens (C, N, H atoms and coordinated water molecule have been omitted, except for bridging carboxylate carbons). (c) The 2-D layered structure of 1 linked by BTA− ligands with a (4, 4) topology indicated by red color. Hydrogen atoms have been omitted for clarity. (d) The H-bonding plot structure for 1, showing hydrogen bonds as dashed lines. (H atoms have been omitted, except for O−H⋯O and O−H⋯N hydrogen bonds). (e) The 3D supramolecular frameworks structure of 1 linked by BTA− ligands and O−H⋯N2 hydrogen bonds with a (3, 6) topology.

The network is based on a (CoBTA)4 rhombus, a 16-membered metal–organic ring formed by four BTA− ligands and four quadruply connected Co2+ ions. The (CoBTA)4-grids are joined together by sharing the Co apices to give the final 2D layer structure. The edge Co⋯Co distance of the rhombus grid is 5.17(7) Å, and the Co⋯Co separations through the diagonal of the rhombus are 6.89(6) and 7.70(8) Å (Fig. 1(c)).

In the crystal structure, it is noteworthy that this 2D layer structure is further connect with pairs of intermolecular O–H⋯O, O–H⋯N, and non-classical C–H⋯O hydrogen bonds involving the coordinating carboxylate O, the uncoordinated triazole N and coordinating water, and help to stabilize the coordination polymer (Figure 1(d) and Table 2). These intermolecular contacts may be regarded as weak hydrogen bonds, but their contribution to the overall lattice energy cannot be ignored. In addition, the flanking phenyl rings of BTA− of neighboring chains are oriented in a face-to-face manner, and centroid-centroid distances are 3.955(4) Å, indicating a significant interaction. This type of stacking seems to govern the process of recognition and self-assemb-ly of the special linear chains in a parallel fashion resulting in the formation of a complementary aromatic stacking 2D grid, which lies in the bc crystallographic plane. Molecules from neighboring stacks interdigitate with each other in the c-axis direction, thus leading to an interwoven two-dimensional network held together by O–H⋯O, O–H⋯N, C–H⋯O and π−π interactions.

Table 2.Symmetry codes: i = −x + 1, y − 1/2, −z + 1/2; ii = x, −y + 1/2, z − 1/2; iii = −x + 1, −y, −z.

Table 3.Symmetry codes: i = −x + 1, y − 1/2, −z + 1/2; ii = x, −y + 1/2, z − 1/2; iii = x, y − 1, z.

Figure 2.The UV-vis of 1.

The Analysis of SPS and the Study of Photo-Electric Properties. The UV-vis absorption spectra of compound 1 (Figure 2) shows a strong absorption band in the UV-vis region, and its energy is within 3-5 EV that falls in the typical range of band-gap energy of semiconductors, so it can be seen as a broad semiconductor. In this paper, the energy–band theory of semiconductor and crystal field theory were combined to analyze and assign the SPS spectra. We think that the 2s2p orbitals of the coordinated oxygen atom form the valence band and the 4s4p orbitals of the metal ions form the conduction band; otherwise, the d orbits of metal ions are impurities of the semiconductor. Then, we would discuss the surface electron behaviors under light-induced and photo-electric conversion properties of the compound according to the results of the SPS.

In Figure 3, there is a wide photovoltage response band in the range of 300–600 nm, which reveals that the compound has a certain photoelectric conversion property, and four photovoltage response bands were obtained on treated with Origin 7.0: λmax = 342, 380, 446, and 513 nm. The response bands at λmax = 342 and 380 nm can be attributed to a charge transition between ligand and metal, O→Co (LMCT). The overlapped response band at λmax = 446 and 513 nm is assigned to dd transitions of Co(II) (d7) ion (4T1g → 4A2g, 4T1g → 4T1g).

Figure 3.The SPS of 1.

It is obvious that the UV-vis spectra (Figure 2) of compound 1 agree with the SPS. Because of (i) the existence of two types of O groups which are connected with the Co(II) ions and (ii) the higher sensitivity of SPS relative to the UV-vis, the number of response bands (O→Co (LMCT)) are two (λmax = 342 and 380 nm) in SPS and one (λmax = 322) in UV-vis. The other difference is the π→π* transition (λmax = 253 nm) from the ligand, and another d→d transition band at about 1122 nm from Co(II) (d7) (4T1g 4T2g) in UV-vis are not present in the SPS because of the limiting region of our SPS (300–800 nm).

 

Conclusion

In summary, we have described here the hydrothermal synthesis and structure of a CoII coordination polymer, in which the BTA− ligand exhibits a doubly bridging coordi-nation mode syn, anti-ƞ1:ƞ1:μ2. The surface electron behavior and photoelectric properties of 1 were studied by SPS. The SPS results of compound 1 indicated that it possesses clear photovoltaic response bands in UV or UV-vis region (300–600 nm) and therefore has certain photo-electric conversion properties. There is a good agreement between SPS and UV-Vis absorption spectra.