Inorganic Chemistry Communications 48 (2014) 26–29

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Inorganic Chemistry Communications

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Syntheses, structures and luminescent properties of two new zinccoordination polymers basedon 4′-(4-aminephenyl)-4,2′:6′,4″-terpyridine

Fei Yuan, Xiaofang Wang, Huai-Ming Hu ⁎, Sa-Sa Shen, Ran An, Gang-Lin XueKey Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi'an 710069, China

⁎ Corresponding author.E-mail address: ChemHu1@NWU.EDU.CN (H.-M. Hu).

http://dx.doi.org/10.1016/j.inoche.2014.08.0071387-7003/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 July 2014Received in revised form 5 August 2014Accepted 8 August 2014Available online 9 August 2014

Keywords:ZincCoordination polymerHelical structureInterpenetrationLuminescence

Two new coordination polymers, namely, [Zn(L)Cl2]n (1) and [Zn4(L)4(ox)4]n·7nH2O (2) (L = 4′-(4-aminephenyl)-4,2′:6′,4″-terpyridine, H2ox= oxalic acid), have been synthesized under hydrothermal conditionand characterized by elemental analysis, IR, and single crystal X-ray diffraction. Compound 1 exhibits an achiral2D supramolecular network constructed by left-handed single-helical chains and right-handed H-bonding heli-cal chains, which further generated to a 2D + 2D→ 2D parallel interpenetration framework. Compound 2 dis-plays an achiral 2D layer based on a 1D left- and right-handed helical chains with alternate arrangement,which is further extended into a 2D + 2D → 3D supramolecular network by hydrogen bonds. Additionally,their photoluminescence properties have been investigated.

© 2014 Elsevier B.V. All rights reserved.

Currently, increasing interest still has been evoked by transitionmetal coordination polymers with helical character [1] or well-regulated network in the fields of crystal engineering [2], which resultfrom not only their potential applications as new function materials inmolecular magnetism [3], molecular adsorption [4], molecular catalyst[5] and luminescence [6] but also their intriguing variety of architec-tures and topological network [7]. However, the rational design andsynthesis of novel helical and interpenetrating polymericmetal–organicframeworks (MOFs) remain a great challenge in coordination chemistry[8,9]. As far as we know, lengthy bridging flexible ligands usuallyfacilitate helical and interpenetrating MOFs because they can bothimprove the helicity of the polymeric chain and lead to the formationof nanosized cages, porous frameworks enclosing cavities, cham-bers and channels [10]. Meanwhile, we are interested in thehelical and interpenetrating structures constructed from the semi-rigid V-shaped ligands with N-donor (polypyridine derivatives),shortest O-donor polycarboxylate ligand and d10 transition metal ions.Therefore, a V-shaped bridging ligand, 4′-(4-aminephenyl)-4,2′:6′,4″-terpyridine (L, Scheme 1) is employed in the realm of crystal engineer-ing according to the following consideration: (i) L, as a semi-rigid bridg-ing V-shaped ligand, can link adjacent metallic centers via coordinationbonds to form different helical chains because of rotation of terminalpyridyl groups. (ii) It has a large π-conjugated nonlinear structurewith N, \NH2 donors that can offer additional hydrogen bonding and

π–π interactions to construct various and interesting supramolecularnetworks. The incoming oxalate auxiliary ligand has abundant coordi-nation modes and is convenient bridging units, which mayform much more complicated and higher dimensional helicalstructures. Herein, we present the hydrothermal synthesis, crystalstructure and luminescent properties of two new Zn(II) coordinationpolymers with helical character based on L, namely [Zn(L)Cl2]n (1)and [Zn4(L)4(ox)4]n·7nH2O (2).

Ligand L was prepared according to literatural methods [11,12]. Amixture of ZnCl2 (0.0272 g, 0.2 mmol) and L (0.0324 g, 0.1 mmol) indistilled water (10 mL) that adjusted the pH to 3.0 with 0.5 mol/LHNO3 aqueous solution, then sealed in a 25 mL Teflon-lined stainlesssteel container, which was heated to 160 °C for 72 h. After cooling toroom temperature at a rate of 5 °C/h, yellowblock crystals 1were obtain-ed in yield 72.0% based on L. Elemental anal. calcd for: C21H16Cl2N4Zn (%):C 54.11, H 3.86, N 12.51; found: C 54.75, H 3.50, N 12.16; IR (KBr, cm−1):3440 (w), 1621 (s), 1596 (s), 1525 (s), 1401 (vs), 1180 (w), 1060 (w),1019 (m), 831 (w), 693 (w), 643 (w), 562 (m), 521 (m). An identical pro-cedure with 1 was followed to prepare 2 except for adding H2ox(0.0252 g, 0.2 mmol) to the mixture and adjusting the pH to 5.0 with0.5 mol/L NaOH aqueous solution. Yellow block crystals were obtainedin 71.2% yield based on L. Elemental anal. calcd for: C92H64N16O23Zn4

(%): C, 54.31, H, 3.19, N, 11.08; found: C, 54.62, H, 3.32, N, 11.13; IR(KBr, cm−1): 3373 (w), 1614 (vs), 1522 (s), 1503 (w), 1404 (w),1361 (m), 1313 (m), 1242 (w), 1188 (m), 1065 (w), 1020 (w),834 (m), 799 (m), 680 (w), 648 (w), 566 (w), 522 (w). To confirm thatthe crystal structures are truly representative of the bulk materials,

N

N N

NH2

Scheme 1.Molecular structure of L ligand.

Fig. 1. (a) The coordination environment of Zn(II) ion in 1. (b) The 2D layer structure construcment. (c) Perspective (left) and the space filling mode (right) view of the parallel penetrati(For interpretation of the references to color in this figure legend, the reader is referred to the

27F. Yuan et al. / Inorganic Chemistry Communications 48 (2014) 26–29

PXRD experiments were carried out for 1 and 2. The PXRD experimentaland computer-simulated patterns of the corresponding compoundsshow that the synthesized bulkmaterials are the same as the single crys-tals (see Supporting information, Fig. S9).

X-ray single crystal structure analysis [13,14] reveals that compound1 is an achiral 2D supramolecular network constructed by left-handedsingle-helical chains and right-handed H-bonding helical chains withinter-alternated arrangement. The asymmetric unit of 1 contains oneZn(II) ion, one L ligand and two Cl− anions. As shown in Fig. 1a, Zn1 iscoordinated to two nitrogen atoms (N1, N3A) from two L ligands andtwo Cl− anions. The coordination geometry of Zn1 can be described asa distorted tetrahedron. The Zn\N bond lengths are in the range of2.049(3) Å to 2.069(3) Å, which are similar to previously reportedvalues [15]. In 1, three pyridyl and one phenyl rings of the L ligand arenon-coplanar, two outer pyridyl and one phenyl groups are twistedwith respect to the central pyridyl ring, and the dihedral angles between

ted by left-handed helical chains and right-handed helical chains in the alternate arrange-on of the di-layers constructed by the 2D layer in 1 (each color represents a network).web version of this article.)

28 F. Yuan et al. / Inorganic Chemistry Communications 48 (2014) 26–29

the central and outer pyridyl and phenyl rings are 5.02° (N1 pyridylring), 2.45° (N3 pyridyl ring) and 28.37° (phenyl ring), respectively.The rotation of phenyl ring with the central pyridyl ring is larger thanthose of others because it well meets to form hydrogen bonds.

Notably, each L ligand acts as a bidentate-bridging ligand links twoZn(II) centers with two of the terminal N-donors into a 1D left-handed helical chain with the helical pitch of 17.29 Å (Fig. S1). Theneighbor helical chains linked by hydrogen bonding interaction[N4\H4B…Cl1i, 2.59 Å, 148°, symmetry code i: (x+ 1, y, z + 1)] to ex-tend the 2D layer structure (Figs. S1 and 1b). Because of the coexistenceof left- and right-handed helices, the whole crystal is mesomeric anddoes not exhibit chirality [16]. To the best of our knowledge, this isthe first report about the achiral 2D layer structure constructed byleft-handed single-helical chains and right-handed helical chains. Inter-estingly, the adjacent 2D layers in same directions interpenetrated togenerate a fascinating two-fold interpenetrating 2D + 2D → 2D bi-layer structure (Figs. S2 and 1c) and the π…π stacking interactionbetween the central pyridyl ring and terminal pyridyl rings (allcentroid-to-centroid distance of 3.689 Å) in neighboring layers sustainsthe system of interpenetrated bi-layers (Fig. S3), which is extremelyrare in the architecture of coordinated polymers based on terpyridylderivatives. The two same bi-layer framework structures are furthersuperimposed by π…π stacking interaction between central and termi-nal pyridyl rings (all centroid-to-centroid distance of 3.689 Å) fromneighboring to form a 2D + 2D → 3D interdigitation architecture,which firmed the whole backbone (Fig. S3).

Compound 2 crystallizes in the monoclinic C2/c space group.The asymmetric unit of 2 contains two Zn(II) ions, two L ligands, two(ox)2− anions, and half and three lattice water molecules. The Zn1 ishexa-coordinated with two nitrogen atoms (N1, N7A) from two L li-gands and four oxygen atoms from two (ox)2− anions, forming a slight-ly distorted octahedron, the N1, N7A, O3 and O6B atoms comprise theequatorial plane, and the O8B and O1 atoms occupy the axial positions[O8B–Zn1–O1 = 170.74(9)°] (Fig. 2a). The Zn2 center is similar tothe coordination environment of Zn1. The Zn\O bond lengthsand O\Zn\O angles lie in the range of 2.062(2)–2.180(2) Å and78.27(9)–169.56(9)°, respectively, in good agreement with previousstudies [15]. The Zn\N bond lengths are in the range of 2.131(3)–2.155(3) Å, which are slightly longer than those reported [15]. In 2,the L ligand acts as a bidentate-bridging ligand to join two Zn(II) centersto form helical chains (Fig. S4), and the minimum intrachain Zn…Zn

Fig. 2. (a) The coordination environment of Zn(II) ions in 2. (b) The achiral 2D layerstructure.

distance is 12.352 Å. The pyridyl and phenyl rings from two different Lligands are all non-coplanar, two outer pyridyl and one phenyl groupsare twistedwith respect to the central pyridyl ring, and the dihedral an-gles between the central and outer pyridyl and phenyl rings are 5.02°(N1 pyridyl ring), 2.45° (N3 pyridyl ring), 28.37° (phenyl ring) and23.97° (N5 pyridyl ring), 35.15° (N7 pyridyl ring), 8.30° (phenyl ring),respectively.

The most conspicuous feature of the structure in 2 is the alternateleft- and right-handed helical chains. As shown in Fig. S4, the L ligandsrespectively bridge two Zn(II) (Zn1 and Zn2) centers through two ter-minal N-donors to form left- and right-handed single helical chainswith the pitch of 9.47 Å, the twist mode of L ligand coordinated toZn(II) centers may be a crucial factor of the formation for the helices.Left- and right-handed helical chains arrange alternately to extend toa 2D network (Figs. 2b and S6). These adjacent 2D layers are furtherpiled up by hydrogen bonding interactions between oxygen atomsfrom (ox)2− ligands and \NH2 groups from L ligands and (H2O)3(trinuclear water clusters) with (H2O)6 (hexnuclear water clusters)generating a 2D + 2D → 3D supramolecular structure (Fig. S7). Inter-estingly, the tri- and hexnuclear water clusters in compound 2 exhibita steady triangle and chair type configuration, respectively (Fig. S8).From the topological perspective, if each six-coordinated Zn(II)ion acts as a 4-connected node, the architecture of 2 is simplified to a4-connected sql topological network with the Schläfli symbol of(44·62) (Fig. S5).

It should be pointed out that in 1 and 2, although H2ox was used inthe formation of 2, they all show helical chain structures. However,some difference exists between these two compounds. The averageZn\N bond distance in 2 (2.131–2.155 Å) is longer than the corre-sponding value in 1 (2.049–2.069 Å). Additionally, the pitch of the heli-cal chain in 2 (9.47 Å) is much shorter than that of 1 (17.29 Å), thereason for this phenomenon is the different arrangements of L ligands.As shown in Fig. S1, the L ligands almost “stand” in the single helicalchain with the dihedral angle between the L ligand and level is 87.65°.Whereas, the L ligands “incline” in the helical structure of 2, and the di-hedral angle between the L ligand and level is only 53.01°.

The d10 transition metal compounds have attracted considerable at-tention in the field of coordination chemistry due to versatile lumines-cence properties and their various applications in chemical sensorsand electroluminescence (EL) displays and light-emitting diodes(LEDs) [17–20]. The solid-state emission spectra of 1 and 2 as wellas free ligand L are shown in Fig. S10 at room temperature. The L ligandexhibits two broad emission bands with maximum at 410 nmand 452 nm upon excitation at 300 nm, which may be assigned to theπ*→ π and π*→ n transitions. The emission spectra of 1 and 2 displaytwo broad emission peaks at 393 nm, 534 nm and 398 nm, 510 nm, re-spectively. Notably, two higher energy emissions for 1 and 2 show blue-shifts of 17 and 12 nm relative to the higher energy emission of L ligandand may be assigned to the π* → π transition of intraligand. Further-more, two lower energy emissions of 1 and 2 show red-shifts of 82and 58 nm relative to the emission at 452 nm of free L ligand, whichmay be attributed to the π* → n transition of intraligand.

In summary, compound 1 with an achiral 2D supramolecularnetwork constructed by left- and right-handed helical chains and com-pound 2with an interesting achiral 2D network and sql topological nethave been hydrothermally synthesized based on a semi-rigid V-shapedpolypyridyl ligand. Compound 2 indicates that incoming oxalate auxil-iary ligands can improve complexity of coordination polymers, whichmay open a new way to generate MOFs from 1D to 2D with fascinatingstructure and luminescent properties.

Acknowledgments

This work was supported by the National Natural Science Founda-tion of China (Grant Nos. 20873098 and 21173164).

29F. Yuan et al. / Inorganic Chemistry Communications 48 (2014) 26–29

Appendix A. Supplementary material

Crystallographic data for the structural analysis have been depositedwith the Cambridge Crystallographic Data Centre, CCDC Nos. 1010183(1) and 1010184 (2). Copies of the data can be obtained free ofcharge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,UK (fax: +44-1223-336-033; e-mail: data_request@ccdc.cam.ac.uk).Supplementary data associatedwith this article can be found, in the on-line version, at http://dx.doi.org/10.1016/j.inoche.2014.08.007.

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