Metal-Controlled Assembly of Coordination Polymers with the Flexible

Publication Date (Web): September 21, 2005 ... Self-assembly of a flexible building unit 4-pyridylacetic acid (Hpya) with inorganic metal salts Cu(ClO...
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CRYSTAL GROWTH & DESIGN

Metal-Controlled Assembly of Coordination Polymers with the Flexible Building Block 4-Pyridylacetic Acid (Hpya)

2006 VOL. 6, NO. 1 335-341

Miao Du,* Cheng-Peng Li, and Xiao-Jun Zhao College of Chemistry and Life Science, Tianjin Normal UniVersity, Tianjin 300074, P. R. China ReceiVed June 7, 2005; ReVised Manuscript ReceiVed August 17, 2005

ABSTRACT: Self-assembly of a flexible building unit 4-pyridylacetic acid (Hpya) with inorganic metal salts Cu(ClO4)2‚6H2O, Co(OAc)2‚4H2O, Ni(ClO4)2‚4H2O, AgNO3, and HgCl2 affords five new metal-organic coordination frameworks [M(pya)2(H2O)2]n (pya ) 4-pyridylacetate; M ) Cu for 1; Co for 2; and Ni for 3), [Ag(pya)]n (4), and [Hg(pya)Cl]n (5) under similar reaction conditions. Single-crystal X-ray diffraction analyses of these polymeric complexes suggest that 1-3 are isostructural and exhibit a two-dimensional (2D) wavelike layered architecture constructed by alternate left- and right-hand helical chains, in which pya takes the bidentate (Npy, OCOO-) bridging mode. However, polymer 4 possesses a three-dimensional (3D) brickwall network generated from the expansion of 2D layers with dinuclear [Ag2(RCOO)2] nodes via further interlayer Ag-OCOO- interactions, which is also stabilized by argentophilic interactions (Ag‚‚‚Ag ) 2.983(3) Å). The structure of 5 seems more complicated, in which the [Hg2(pya)2] boxlike dimeric subunits are extended to a unique 3D framework via multiple Hg-OCOO- coordination forces. In the structures of 4 and 5, the carboxylate groups display unusual µ-O, O′syn-µ-O, O and µ-O, O-η-O, O′-µ-O′, O′ binding modes, respectively. The significant differences of these metal-organic frameworks indicate that the flexible pya ligand may adjust its coordination fashion to meet the requirement of the coordination preference of the metal center. The fluorescent and thermal properties of these new materials have also been studied. Introduction Assembly of metal-organic frameworks (MOFs), also known as coordination polymers, is currently of great interest and importance since it may provide a new strategy for achieving solid functional materials with potential applications in molecular absorption/separation, magnetism, ion exchange, electric conductivity and catalysis, etc.1,2 One of the outstanding challenges is the rational and controlled preparation of such MOFs, and undoubtedly, from the viewpoint of crystal engineering, the most effective and facile approach is the appropriate choice of the well-designed organic bridging ligands (building blocks) containing modifiable backbones and connectivity information, together with the metal centers (nodes) with different coordination preferences.3 The most commonly used organic linkers are those bound to the metal centers via pyridyl, cyano, or carboxylate groups,4 including linear connectors (e.g., pyrazine,5 4,4′-bipyridine,6 1,4-benzenedicarboxylate,7 and isonicotinate3b,8), one-dimensional (1D) angular connectors (e.g., dicyanamide,9 2,5-dipyridyl-1,3,4-oxadiazole,10 and 1,3-benzenedicarboxylate11), two-dimensional (2D) connectors (e.g., trimesic acid,12 tricyanomethanide9), and three-dimensional (3D) connectors (e.g., hexamethylenetetramine3a and 1,3,5,6-adamantane-tetracarboxylate13). Among them, the series of isonicotinic acid derivatives, as predesigned and versatile building blocks with adjustable lengths and binding modes, attract considerable interest for the modular assembly of acentric coordination solids as potential nonlinear optical (NLO) materials.3b Significantly, far less common has been the investigation of related hybrid pyridyl/carboxylate linkers with a flexible backbone in construction of metal-organic frameworks, which do not readily fit into a predictable pattern due to their structural flexibility and conformational freedom.2d,14,15 Recently, we have focused on manipulating the length of the spacer between the pyridyl and the carboxylate groups as well * Corresponding author. Fax: 86-22-23540315. Tel: 86-22-23538221. E-mail: [email protected].

Chart 1

as the spatial tropism of such building blocks, and a series of novel inorganic-organic supramolecular networks have been generated from a flexible versatile 4-pyridylthioacetate anion (pyta) with adjustable configurations and binding modes and different metal centers such as CuII, CoII, MnII, ZnII, AgI, and PbII.14 In this regard, for our synthetic strategy, another analogue linker 4-pyridylacetate (pya, see Chart 1) has attracted our attention in which the existence of the methylene group between the pyridyl and the carboxylate terminals will regulate the rigid/ flexible nature of the ligand backbone. Recent research suggests that Hpya has a strong capability of affording new coordination polymers.15 However, it is interesting that in all pya-containing structures (metal ions: ZnII, MnII, CdII, and PtII) only the bidentate (Npy, OCOO-) bridging mode is detected,15,16a although a variety of binding fashions are anticipated upon metal complexation under appropriate conditions. For the sake of enriching such interesting systems and as a continuation of our work, we chose typical CuII, CoII, NiII ions (usually with an octahedral coordination, similar to that of ZnII, MnII, CdII in ref 15) and larger AgI, HgII centers with distinct coordination preferences to assemble with pya. In this contribution, five new coordination polymers, [M(Hpya)2(H2O)2]n (M ) Cu for 1; Co for 2; and Ni for 3), [Ag(pya)]n (4), and [Hg(pya)Cl]n (5), were prepared and investigated to determine the influence of the metal ions on the creation of such metal-organic frameworks. Interestingly, by careful choice of the metal nodes, this series of polymeric networks exhibit varied structural dimensionality (from 2D to 3D), as well as the binding sites of pya (from simple monodentate to complicated bridging and/or chelating fashion of the carboxylate group).

10.1021/cg0502542 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2005

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Table 1. Crystal Data and Structure Refinement Summary for Complexes 1-5

empirical formula M crystal size/mm crystal system space group a/Å b/Å c/Å β/deg V/Å3 Z Fcalcd/g cm-3 µ/mm-1 F(000) range of h, k, l total/independent reflections parameters Rint Ra, Rwb GOFc residuals/e Å-3 a

1

2

3

4

5

C14H16CuN2O6 371.83 0.28 × 0.16 × 0.06 monoclinic P21/n 10.0813(16) 5.9868(9) 11.6978(18) 91.770(2) 705.68(19) 2 1.750 1.583 382 -11/11, -5/7, -13/13 3616/1239

C14H16CoN2O6 367.22 0.20 × 0.16 × 0.10 monoclinic P21/n 10.532(6) 5.665(4) 11.752(7) 90.669(7) 701.2(7) 2 1.739 1.259 378 -12/12, -6/6, -10/13 3830/1242

C14H16NiN2O6 367.00 0.28 × 0.22 × 0.10 monoclinic P21/n 10.436(3) 5.6662(15) 11.668(3) 90.865(4) 689.8(3) 2 1.767 1.442 380 -12/12, -6/6, -13/12 3537/1221

C7H6AgNO2 244.00 0.21 × 0.19 × 0.09 monoclinic P21/c 5.3992(8) 16.576(3) 7.8082(12) 95.739(2) 695.29(18) 4 2.331 2.836 472 -6/6, -19/18, -9/6 3678/1233

C7H6ClHgNO2 372.17 0.37 × 0.15 × 0.08 monoclinic P21/c 10.3306(18) 9.3398(11) 8.8441(8) 105.588(8) 821.94(19) 4 3.008 19.002 672 -12/10, -9/11, -7/10 4245/1448

106 0.0185 0.0235, 0.0643 1.054 0.274, -0.225

106 0.0216 0.0283, 0.0815 1.068 0.312, -0.211

106 0.0359 0.0329, 0.0771 1.055 0.333, -0.231

100 0.0131 0.0178, 0.0487 0.994 0.437, -0.391

106 0.0362 0.0325, 0.0816 1.070 2.437, -2.009

R ) ∑|Fo| - |Fc|/∑|Fo|. b Rw ) [∑[w(Fo2 - Fc2)2]/∑w(Fo2)2]1/2. c GOF ) {∑[w(Fo2 - Fc2)2]/(n - p)}1/2.

Experimental Section General Materials and Methods. All the reagents were commercially available and employed without further purification prior to use. Distilled water was used throughout. Fourier transform (FT) IR spectra (KBr pellets) were taken on an AVATAR-370 (Nicolet) spectrometer and elemental (C, H, and N) analyses on a CE-440 (Leemanlabs) analyzer. Thermogravimetric analysis (TGA) experiments were carried out on a Dupont thermal analyzer from room temperature to 800 °C under nitrogen atmosphere at a heating rate of 10 °C/min. All fluorescence measurements were performed on a Cary Eclipse spectrofluorimeter (Varian) equipped with a xenon lamp and quartz carrier at room temperature. Syntheses of the Complexes. [Cu(pya)2(H2O)2]n (1). The pH value of an aqueous solution (5 mL) of Hpya‚HCl (17 mg, 0.1 mmol) was adjusted to ca. 6 by dropwise addition of an aqueous solution of KOH (0.1 M). A solution of Cu(ClO4)2‚6H2O (37 mg, 0.1 mmol) in MeOH (5 mL) was then carefully layered onto a buffer of MeOH/H2O (1: 1, 5 mL), below which the above Hpya solution was placed in a straight glass tube. Upon slow evaporation of the solvents, blue prism single crystals suitable for X-ray analysis were obtained after 1 week in 58% yield (based on Hpya‚HCl). Anal. Calcd for C14H16CuN2O6: C, 45.22; H, 4.34; N, 7.53%. Found: C, 44.98; H, 4.22; N, 7.24%. IR (cm-1): 3416m, 3134w, 2318w, 1594vs, 1501w, 1427s, 1361vs, 1224m, 1139w, 1096w, 1069w, 1030w, 935w, 878w, 815m, 720m, 650m, 574m, 508w. [Co(pya)2(H2O)2]n (2). A solution of Co(OAc)2‚4H2O (25 mg, 0.1 mmol) in MeOH (5 mL) was carefully layered onto a buffer of MeOH/ H2O (1: 1, 5 mL), below which an aqueous solution (5 mL) of Hpya‚ HCl (17 mg, 0.1 mmol) was placed in a straight glass tube. Upon slow evaporation of the solvents, pale red block single crystals suitable for X-ray analysis were generated after a period of 2 weeks in 47% yield (based on Hpya‚HCl). Anal. Calcd for C14H16CoN2O6: C, 45.79; H, 4.39; N, 7.63%. Found: C, 45.82; H, 4.54; N, 7.34%. IR (cm-1): 3270b, 2872b, 1576vs, 1424s, 1365vs, 1222m, 1136m, 1069w, 1020m, 944w, 815s, 765m, 650vs, 543m, 500m. [Ni(pya)2(H2O)2]n (3). The same synthetic procedure as for 1 was applied except that the MeOH solution of Cu(ClO4)2‚6H2O was replaced by a MeOH/EtOH (1:1, 5 mL) solution of Ni(ClO4)2‚6H2O (36 mg, 0.1 mmol), affording pale green block crystals by slow evaporation of the solvents after 3 days in 63% yield (based on Hpya‚HCl). Anal. Calcd for C14H16CoN2O6: C, 45.82; H, 4.39; N, 7.63%. Found: C, 45.60; H, 4.33; N, 7.37%. IR (cm-1): 3332b, 2858b, 1616vs, 1575vs, 1425s, 1369vs, 1223m, 1137w, 1092w, 1069w, 1024m, 947w, 850s, 813s, 769s, 721m, 654vs, 546m, 505m. [Ag(pya)]n (4). The same synthetic procedure as for 1 was used except that Cu(ClO4)2‚6H2O was replaced by AgNO3 (16.9 mg, 0.1 mmol). The solution was left in darkness at room temperature and colorless block crystals were obtained after a period of 1 week in 60%

yield. Anal. Calcd for C7H6AgNO2: C, 34.46; H, 2.48; N, 5.74%. Found: C, 34.81; H, 2.24; N, 5.58%. IR (cm-1): 1592vs, 1423s, 1371vs, 1289m, 1210w, 1070w, 1011w, 918m, 810m, 751w, 685m, 624m, 547w, 492w. [Hg(pya)Cl]n (5). The same synthetic procedure as for 1 was used except that the metal salt was replaced by HgCl2 (27 mg, 0.1 mmol), producing colorless block single crystals by slow evaporation of the solvents after a period of 3 weeks in 48% yield. Anal. Calcd for C7H6ClHgNO2: C, 22.59; H, 1.62; N, 3.76%. Found: C, 22.77; H, 1.35; N, 3.84%. IR (cm-1): 1727s, 1609vs, 1588s, 1498m, 1426s, 1372s, 1285w, 1221m, 1070m, 1013s, 944w, 894w, 811m, 745w, 671m, 634m, 611w, 540m, 489s, 420w. CAUTION! Perchlorate complexes of metal ions in the presence of organic ligands are potentially explosive. Only a small amount of material should be handled with care. Single-Crystal X-ray Diffraction Determination and Refinement. X-ray single-crystal diffraction data for coordination polymers 1-5 were collected on a Bruker Apex II CCD diffractometer at ambient temperature with Mo KR radiation (λ ) 0.71073 Å) by ω scan mode. There was no evidence of crystal decay during data collection for all complexes. A semiempirical absorption correction was applied using the SADABS program, and the program SAINT was used for integration of the diffraction profiles.17 All structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined with SHELXL.18 The final refinement was performed by full-matrix least-squares methods on F2 with anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen atoms bonded to carbon were placed geometrically and allowed to ride during subsequent refinement with an isotropic displacement parameter fixed at 1.2 times Ueq of the parent atoms. Hydrogen atoms of the aqua ligands in complexes 1-3 were first located in difference Fourier maps and then placed in the calculated sites and included in the final refinement. Notably, in the structure of 5, the pyridyl and methene groups of the pya moiety were treated using a disorder model (divided into three equivalent parts). Further crystallographic data and structural refinement parameters of complexes 1-5 are summarized in Table 1.

Results and Discussion Preparation of Complexes 1-5. Coordination polymers 1-5 were prepared through solution assemblies of Hpya and different metal ions as neutral molecular complexes, which are stable under ambient conditions and insoluble in common organic solvents and water, being consistent with their polymeric nature. For the preparation of 4 and 5, the immediate combination of Hpya and inorganic salt led to a mass of white microcrystalline

Metal-Controlled Assembly of Coordination Polymers

Crystal Growth & Design, Vol. 6, No. 1, 2006 337

Figure 1. (a) A portion view of complexes 1-3 with atom labeling of the asymmetric unit and the coordination sphere of the metal center (see Table 2 for symmetry codes). (b) (Left) 2D layered architecture consisting of [M4(pya)4] repeating units; (right) a perspective view highlighting the left- and right-handed helical chains. (c) A side view along the [010] direction of the parallel stacking fashion of the 2D layers.

precipitation, and thus, they were studied using a test tube to facilitate the slow growth of larger single crystals suitable for X-ray diffraction. In the cases of 1-3, direct combination of the reaction reagents followed by solvent evaporation gave no evidence of solid formation. Crystalline products were achieved using the slow diffusion method. The self-assembly processes are usually influenced by many factors such as the metal-ligand ratio and counteranion; therefore, the following attempts were carried out. Complexes 1-3 could be isolated by the same procedure, using MCl2, M(ClO4)2, M(OAc)2, and M(NO3)2 as the source of metal centers (confirmed by IR spectra and elemental analyses), suggesting the final products are independent of the counteranions. Complexes 1-3 have a 1:2 metalligand composition and for 4 and 5, 1:1 metal-organic frameworks are achieved. As a matter of fact, this ratio is independent of the ratio of reactants used.

Structural Analysis of Complexes 1-5. 2D Coordination Polymers 1-3. X-ray diffraction results reveal that complexes 1-3 are isostructural with the general formula [M(pya)2(H2O)2]n (M ) Cu for 1, Co for 2, and Ni for 3). The local structure is depicted in Figure 1a, and selected bond parameters are listed in Table 2. Each metal ion is six-coordinated by four pya anionic ligands, using their pyridyl nitrogen atoms and carboxylate oxygen donors, and two water molecules. The coordination geometry of CoII or NiII could be described as a nearly ideal octahedron. As expected, the CuII center is in an elongated octahedral environment and shows considerable Jahn-Teller distortion, with two Cu-Oaqua bonds being significantly longer than the other four Cu-O/Cu-N distances as listed in Table 2. For each carboxylate group of pya in 1-3, the coordination interaction makes the C(7)-O(2) bond length significantly

338 Crystal Growth & Design, Vol. 6, No. 1, 2006

Du et al.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes 1-3a

M(1)-O(3) M(1)-O(2B) M(1)-N(1) C(7)-O(1) C(7)-O(2) O(3)-M(1)-O(2B) O(3)-M(1)-O(2C) O(2B)-M(1)-N(1A) O(2C)-M(1)-N(1A) O(3A)-M(1)-N(1) O(3)-M(1)-N(1) C(5)-C(6)-C(7)

1

2

3

2.3822(16) 2.0081(14) 2.0324(17) 1.227(3) 1.274(2) 83.97(5) 96.03(5) 89.49(6) 90.51(6) 88.65(6) 91.35(6) 110.38(16)

2.104(2) 2.1193(19) 2.152(2) 1.238(3) 1.270(3) 87.47(8) 92.53(8) 89.63(8) 90.37(8) 86.82(8) 93.18(8) 110.0(2)

2.082(2) 2.085(2) 2.097(2) 1.236(4) 1.279(3) 86.79(9) 93.21(9) 89.17(9) 90.83(9) 86.73(9) 93.27(9) 110.6(2)

a Symmetry transformations used to generate equivalent atoms for (1) A -x + 1, -y + 1, -z + 1; B x + 1/2, -y + 1/2, z - 1/2; C -x + 1/2, y + 1/2, -z + 3/2; for (2) A -x + 2, -y + 1, -z + 1; B x + 1/2, -y + 1/2, z - 1/2; C -x + 3/2, y + 1/2, -z + 3/2; for (3) A -x + 1, -y + 1, -z + 1; B x - 1/2, -y + 1/2, z + 1/2; C -x + 3/2, y + 1/2, -z + 1/2.

Table 3. Hydrogen Bond Geometries in the Crystal Structures of 1-3 complex

D-H‚‚‚Aa

D‚‚‚A (Å)

H‚‚‚A (Å)

D-H‚‚‚A (deg)

1

O(3)-H(3A)‚‚‚O(2)a O(3)-H(3B)‚‚‚O(1)b O(3)-H(3A)‚‚‚O(1)c O(3)-H(3B)‚‚‚O(2)d O(3)-H(3A)‚‚‚O(2)e O(3)-H(3B)‚‚‚O(1)f

2.893 2.683 2.625 2.813 2.843 2.614

2.05 1.85 1.80 1.98 2.05 1.78

173 168 163 165 156 168

2 3

a Symmetry codes: a, -x + 1/2, y - 1/2, -z + 3/2; b, -x + 1/2, y + 1/2, -z + 3/2; c, -x + 3/2, y + 1/2, -z + 3/2; d, -x + 3/2, y - 1/2, -z + 3/2; e, -x + 3/2, y - 1/2, -z + 1/2; f, -x + 3/2, y + 1/2, -z +1/2.

longer than that of free C(7)-O(1), being consistent with its structural feature (monodentate binding to the metal center). As shown in Figure 1b, left, each pya ligand utilizes its pyridyl nitrogen and one carboxylate oxygen to bridge two metal centers [with the separation of 8.390(7) Å for 1, 8.426(3) Å for 2, and 8.379(2) Å for 3], generating a 2D layered metal-organic framework consisting of the [M4(pya)4] repeating unit (the diagonal M‚‚‚M distances are 15.677(2) and 5.987(1) Å for 1; 15.872(6) and 5.665(4) Å for 2; 15.771(3) and 5.666(2) Å for 3). The intralayer hydrogen bonding interactions between the adjacent aqua ligands and the carboxylates are observed (see Table 3 for details). Significantly, with the presence of the -CH2- spacer between the pyridyl and carboxylate groups, the flexible pya moieties exhibit a twisted fashion to link the metal ions. As a consequence, a 2D helical network is afforded in which the left and right helical chains appear alternatively by sharing the metal centers, as demonstrated in Figure 1b, right. These helical chains are extended along the crystallographic [010] direction; the repeating periods are ca. 5.99, 5.67, and 5.67 Å, and the sizes of the helical channels (see Figure 1c) are ca. 6.3 × 4.3, 6.5 × 4.4, and 6.4 × 4.4 Å2, respectively, for 1-3. Analysis of the crystal packing reveals that the adjacent sheets align in a parallel manner, as shown in Figure 1c, and the shortest separations between them are ca. 3.95, 3.74, and 3.73 Å, respectively, for 1-3, suggesting the presence of weak van der Waals forces. 3D Coordination Polymers [Ag(pya)]n (4) and [Hg(pya)Cl]n (5). The crystal structure of 4 shows a 3D polymeric coordination framework along with significant Ag-Ag interactions, which may induce and stabilize this 3D net. As shown in Figure 2a, each silver ion is tetrahedrally coordinated by one pyridyl nitrogen and three carboxylate oxygens from four distinct pya ligands, with the Ag-N distance being 2.289(2) Å

Figure 2. (a) A portion view of 4 with atom labeling of the asymmetric unit, the coordination sphere of silver, and the binding mode of the carboxylate group (see Table 4 for symmetry codes). (b) 2D layered network with dimeric silver nodes along the (011) plane. (c) A perspective view of the 3D brickwall coordination framework of 4 along the [100] direction.

and Ag-O lengths ranging from 2.240(2) to 2.489(2) Å (see Table 4 for details). Interestingly, the carboxylate group acts as a µ3-bridge to link three silver ions, adopting the µ-O, O′synµ-O, O coordination fashion, and two C-O bond lengths are equivalent. Additionally, in the dinuclear [Ag2(pya)2] species bridged by a pair of carboxylate groups, the distance of two silver ions is 2.983(3) Å (see Figure 2a, indicated by dashed lines), significantly shorter than the van der Waals Ag-Ag

Metal-Controlled Assembly of Coordination Polymers

Crystal Growth & Design, Vol. 6, No. 1, 2006 339 Table 4. Selected Bond Lengths (Å) and Angles (deg) for Complexes 4 and 5a Ag(1)-O(2A) Ag(1)-O(1) C(7)-O(2) O(2A)-Ag(1)-N(1C) O(2A)-Ag(1)-O(1B) N(1C)-Ag(1)-O(1B) C(5)-C(6)-C(7)

Complex 4 2.2400(17) Ag(1)-N(1C) 2.452(2) Ag(1)-O(1B) 1.251(3) C(7)-O(1) 131.74(7) O(2A)-Ag(1)-O(1) 109.32(7) N(1C)-Ag(1)-O(1) 91.15(7) O(1)-Ag(1)-O(1B) 112.2(2)

Hg(1)-O(2B) Hg(1)-O(2A) Hg(1)-O(1C) O(1)-C(7) O(2B)-Hg(1)-O(1A) O(2A)-Hg(1)-Cl(1) O(2A)-Hg(1)-O(2B) O(2B)-Hg(1)-Cl(1) O(1A)-Hg(1)-O(1C) O(1A)-Hg(1)-Cl(1) O(1C)-Hg(1)-Cl(1) N(1)-Hg(1)-Cl(1)

Complex 5 2.747(6) Hg(1)-N(1) 2.683(7) Hg(1)-O(1A) 2.821(7) Hg(1)-Cl(1) 1.262(9) O(2)-C(7) 151.5(2) O(1C)-Hg(1)-N(1) 128.73(12) O(2B)-Hg(1)-O(1C) 144.23(15) O(2B)-Hg(1)-N(1) 72.94(11) O(1A)-Hg(1)-N(1) 70.55(16) O(1A)-Hg(1)-O(2A) 81.82(12) O(1C)-Hg(1)-O(2A) 77.70(11) N(1)-Hg(1)-O(2A) 83.3(2) C(5)-C(6)-C(7)

2.289(2) 2.489(2) 1.249(3) 128.61(7) 94.21(7) 87.72(7)

2.532(5) 2.514(5) 2.791(2) 1.272(8) 147.9(2) 115.3(2) 82.7(2) 81.4(3) 50.2(2) 98.4(2) 73.8(2) 105.5(7)

a Symmetry transformations used to generate equivalent atoms for (4) A -x + 1, -y + 1, -z; B -x, -y + 1, -z; C x, -y + 3/2, -z; for (5) A -x, -y + 2, -z; B -x, y - 1/2,-z + 1; C x - 1, y, z.

Figure 3. (a) A portion view of 5 with atom labeling of the asymmetric unit, the coordination sphere of mercury, and the binding mode of the carboxylate group (symmetry codes A-C are listed in Table 4, D: -x, y + 1/2, -z + 1/2; E: x + 1, y, z). (b) 2D coordination network of 5 with boxlike dimeric subunits. (c) 3D open framework of 5 viewing along the [100] direction.

contact distance (3.40 Å) and only slightly longer than that in metallic silver (2.89 Å).19 This short contact indicates a considerable argentophilic interaction, the significance of which in the formation of polymeric complexes has attracted much attention nowadays.20 Considering each dimeric silver subunit as a node (N), thus, these nodes are linked via Ag-N coordination interactions to form a 2D sheet with [N(pya)]4 repeating units, along the crystallographic (011) plane. Further-

more, these 2D layers are combined together through additional interlayer Ag-Ocarboxylate bonds to construct a 3D brickwall coordination framework, as demonstrated in Figure 2c. The 3D polymeric structure of 5 was revealed by X-ray single-crystal determination, in which the HgII center is in a highly distorted octahedral coordination environment, consisting of one pyridyl nitrogen atom, four carboxylate oxygen atoms, and one chloride ion (see Table 4 for detailed bond parameters), as shown in Figure 3a. An intramolecular C-H‚‚‚Cl interaction between the 2-position C-H group of the pyridyl ring and the chloride ligand is detected (H‚‚‚Cl and C‚‚‚Cl distances: 2.711 and 3.386 Å; C-H‚‚‚Cl angle: 130°), forming an S(5) hydrogen-bonding pattern and stabilizing the coordination sphere. Remarkably, the carboxylate group binds to the HgII ions also as a µ3-bridge; however, it takes the unusual µ-O, O-η-O, O′-µ-O′, O′ mode, and two C-O distances are equivalent as expected. Analysis of this polymeric network suggests a fundamental dimeric boxlike unit, consisting of two HgI centers (Hg1 and Hg1A in Figure 3a) that are connected by a pair of pya ligands, with the dimensions of 6.83(2) × 4.48(1) Å2. Within each box, the center-to-center and center-to-plane separations of two antiparallel pyridyl rings are 4.262(6) and 4.251(5) Å, respectively, revealing no significant aromatic stacking interactions. Through a pair of Hg-O1 coordinative interactions (2.821(7) Å), these boxlike units are bridged to form 1D chain arrays along the [100] direction, as depicted in Figure 3b. Within each chain, the short Hg‚‚‚Hg contact between the corners of the neighboring boxes is 4.360(6) Å. The adjacent 1D chains (such as A and A′ in Figure 3b) are almost perpendicular, inclined to each other with a dihedral angle of 83.5(5)°. Self-assembled by further Hg-O2 bonds (2.747(6) Å), these A and A′ chains are alternatively arranged to generate a 2D layered architecture. Analysis of the crystal packing indicates that these 2D layers are closely stacking in an SS′SS′ sequence, with an approximate offset distance of 0.5(b2 + c2)1/2 and furthermore combined into a 3D framework via additional HgO2 strengths, as depicted in Figure 3c. Examination of the structure with PLATON program reveals that there are no solvent accessible voids in this open network. This work utilizes the fact that Hpya has a great ability to construct polymeric architectures with different metal ions, and

340 Crystal Growth & Design, Vol. 6, No. 1, 2006

Du et al. Chart 2

Table 5. Comparison of the Structural Character of pya and Metal Ions in the Coordination Complexes compound

metal coordination

pyaa

M-Npy (Å)

M-Ocarboxyl (Å)

network

refs

[Cu(pya)2(H2O)2]n [Co(pya)2(H2O)2]n [Ni(pya)2(H2O)2]n {[Zn(pya)2]‚7H2O}n trans-[Zn(pya)2(H2O)2]n trans-[Cd(pya)2(H2O)2]n cis-[Cd(pya)2(H2O)2]n trans-[Mn(pya)2(H2O)2]n [Pt(dppf)(pya)]2(OTf)2b [Ag(pya)]n [Hg(pya)Cl]n

octahedron (MN2O4) octahedron (MN2O4) octahedron (MN2O4) tetrahedron (MN2O2) octahedron (MN2O4) octahedron (MN2O4) octahedron (MN2O4) octahedron (MN2O4) square (MNOP2) tetrahedron (MNO3) octahedron (MNO4Cl)

I I I I I I I I I II III

2.0324(17) 2.152(2) 2.097(2) 2.046(3) 2.158(4) 2.339(7) 2.325(14) 2.281(2) 2.079(4) 2.289(2) 2.532(5)

2.0081(14) 2.1193(19) 2.085(2) 1.966(3) 2.126(3) 2.289(6) 2.276(15) 2.185(2) 2.082(3) 2.394c 2.691c

2D 2D 2D 2D 2D 2D 2D 2D dimeric 3D 3D

this work this work this work 15 15 15 15 15 16a this work this work

a

The binding modes (I, II, III) of pya are illustrated in Chart 2. b dppf ) 1,1′-bis(diphenylphosphino)ferrocene. c Average bond lengths.

of further importance, from the structural descriptions above, we can conclude that the coordination mode of pya is changeable when assembling to a given metal center, and also vital for the framework formation. In all M-pya coordination polymers, the pyridyl nitrogen always coordinates to the metal center, and the anionic nature of the carboxylate group leads to a neutral complex. However, the carboxylate group exhibits different binding fashions (see Chart 2). For 2D isostructural polymers 1-3 and all other related complexes reported previously,15,16 only the simple unidentate mode is observed. For 3D coordination polymers [Ag(pya)]n (4) and [Hg(pya)Cl]n (5), the complicated µ-O, O′syn-µ-O, O bridging and µ-O, O-η-O, O′-µ-O′, O′ bridging/chelating fashions of the carboxylate groups are detected, expanding the metal nodes to generate higher dimensional topologies. Additionally, significant argentophilic interactions exist in the coordination framework of 4 and further consolidate this structure. These results clearly reveal that the nature of the metal center plays a key role in adjusting the coordination fashions of pya and further topology/dimensionality of these coordination polymers. A comparison of the structural character of pya and metal ions in all known complexes is summarized in Table 5. Thermogravimetric Analysis. Coordination polymers 1-5 are stable at ambient conditions, and thermogravimetric experiments were performed to explore their thermal stabilities. The TGA curve of 1 shows the first weight loss of 9.54% from 40 to 120 °C (peaking at 92 °C), corresponding to the loss of the coordinated water molecules (calculated: 9.69%). The starting decomposition of the residuary [Cu(pya)2] section occurs at 160 °C, and further heating to 800 °C suggests a continuous weight loss. The TGA curves of 2 and 3 are similar, probably due to their structural similarity. They remain intact until heating to ca. 120 °C, and then there are four steps of weight losses ending at 580 °C for 2 and 430 °C for 3. Further heating to 800 °C reveals no weight loss. The loss of coordinated aqua moieties is not observed separately, maybe due to their strong coordination to CoII or NiII ion. As for 4 and 5, the TGA measurements indicate that the 3D frameworks remain stable upon heating until a series of complicated weight losses starting from 130 °C for 4 and 155 °C for 5, and not ending until 800 °C. Infrared and Luminescent Studies. In the IR spectra of Hpya and complexes 1-5, the absorption bands resulting from the skeletal vibrations of the pyridyl rings appear in the 1400-

1600 cm-1 region. For Hpya and 1-3, the broad band centered at ca. 3300-3400 cm-1 indicates the O-H stretching of the carboxylic group or water ligands. The IR spectrum of Hpya suggests that the characteristic absorption bands of the -COOH group occur in 1711 and 1450 cm-1. For complexes 1-5, the peaks at 1594, 1576, 1575, 1592, and 1609 cm-1, respectively, are attributed to the antisymmetric stretching vibrations, and 1361, 1365, 1369, 1371, and 1372 cm-1 to the symmetric vibrations of the carboxylate groups. Furthermore, the ∆ values (νas - νsym) of the carboxylate groups are 233, 211, 206, 221, and 237 cm-1 for 1-5, respectively, agreeing with their solid structural features.21 Inorganic-organic hybrid coordination polymers, especially for those that comprise the d10 metal center and aromaticcontaining system, have now been a powerful tool for achieving new luminescent materials.22 In this work, the luminescent properties of the ligand and its AgI, HgII coordination complexes (4 and 5) were investigated in the solid state at ambient temperature (λex ) 220 nm). The free Hpya moiety exhibits the emission maximum at 535 nm, together with a weak emission peak at 487 nm. Complexes 4 and 5 display similar emission peaks with almost equal intensity. These results clearly indicate that the luminescence of 4 and 5 should be assigned to the intraligand transitions, being independent to metal-ligand interactions, although which usually affect the emission wavelength of the organic component. Conclusions and Perspectives Synthesis, crystal structures, and properties of five new metal-organic coordination polymers with different metal ions are described, keeping in mind that changing the distance between the pyridyl and the carboxylic moieties by the insertion of a -CH2-spacer can give a twisting flexible ligand Hpya. Three types of coordination frameworks/topologies have been observed according to the selection of the metal centers with dissimilar coordination preferences, as well as the variation of the binding fashions of the carboxylate groups. With regard to complexes 1-3, they are topologically isostructural and constructed by alternate left and right helical chains to produce a 2D network. As for 4, it possesses a 3D brickwall framework, which may be induced and stabilized by the existence of AgAg interactions. The 3D open network of 5 is composed of the

Metal-Controlled Assembly of Coordination Polymers

boxlike dimeric subunits, which are extended via multiple Hg-O coordinative interactions. To summarize, the metal center plays a dominating and steering role in increasing the binding sites of the ligand and further framework dimensionality of these coordination polymers. Further studies are ongoing with different building blocks of this type (e.g., altering the spacer length or introducing other functional groups) and/or crystalline conditions (e.g., hydrothermal preparation) to enrich such inorganicorganic hybrid materials with interesting network topologies and properties. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20401012), the Key Project of Tianjin Natural Science Foundation (043804111), and Tianjin Normal University (to M.D.). Supporting Information Available: Crystallographic information files (CIF) of complexes 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

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