Layer Substructures in

DOI: 10.1021/cg9006839

Supramolecular Isomerism and Various Chain/Layer Substructures in Silver(I) Compounds: Syntheses, Structures, and Luminescent Properties

2009, Vol. 9 4884–4896

Pei-Xiu Yin,†,‡ Jian Zhang,† Zhao-Ji Li,† Ye-Yan Qin,† Jian-Kai Cheng,† Lei Zhang,†,‡ Qi-Pu Lin,†,‡ and Yuan-Gen Yao*,† †

The State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and ‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China Received June 18, 2009; Revised Manuscript Received July 26, 2009

ABSTRACT: A systematic investigation on the Ag/bpy/(H)dpa family represents seven Ag(I) supramolecular compounds with distinct structural features, Ag2(bpy)0.5(dpa) (1), Ag2(bpy)1.5(dpa)(H2O) (2), Ag(bpy)0.5(Hdpa) (3), R-[Ag(bpy)] 3 (Hdpa) (4), β-[Ag(bpy)] 3 (Hdpa) (5), [Ag(bpy)]2 3 (dpa) 3 6(H2O) (6), and [Ag(bpy)]4 3 (dpa) 3 2(NO3) 3 6(H2O) (7) (H2dpa = 1,10 -biphenyl-2,20 dicarboxylic acid, bpy = 4,40 -bipyridine). Of particular interest about this work is that supramolecular isomerism is observed between compounds 4 and 5. Single-crystal X-ray structural analysis reveals the structural diversity of the formed products. Compounds 1-5 are extended from various one-dimensional (1D) chain substructures. Three distinct 1D chains exist in 1-3: one 1D chain of {Ag2(dpa)} exists in compound 1, one supramolecular chain of tetranuclear {Ag2(dpa)(H2O)}2 subunits linked by Ag 3 3 3 Ag interactions (argentophilic interactions) and hydrogen bonding interactions is observed in 2, and one 1D alternate chain of eight-membered {Ag2(Hdpa)2} ring bridged by bpy is present in 3. However, two different 1D supramolecular double chains of [Ag(bpy)(Hdpa)] are found in 4 and 5, which are sustained by the combination of weak Ag 3 3 3 O interactions, π-π stacking interactions, and argentophilic interactions in 4 while only by π-π stacking interactions in 5. These 1D chain substructures are further threaded into a 2D covalent layer bridged by bpy (1) or 2D (3 and 4)/3D (2 and 5) supramolecular architectures with the help of supramolecular driving forces in different ways. In contrast to compounds 1-5, compounds 6 and 7 are based upon 2D supramolecular substructures of [Ag(bpy)]2 3 (dpa) and {[Ag(bpy)]4 3 (dpa) 3 (H2O)}n2nþ, respectively. The most striking feature of 6 is that a quasi-1D water aggregate is confined in the 1D channel of the 3D porous interdigitated host framework. Greatly different from compound 6, compound 7 is a nitrate-templated 3D supramolecular framework constructed from a positively charged layer substructure through versatile hydrogen bonds. The diversity of the product structures illustrates that controlled synthesis by adjusting the molar ratio of precursors or the pH value of the solution is a facile and effective approach to further design and construct novel supramolecular assemblies with distinct structural features. Moreover, the thermal dynamic properties and fluorescent properties of all compounds except 1 are also investigated.

Introduction In the past few decades, the study of supramolecular chemistry and crystal engineering based upon the spontaneous self-assembly of metal ions and bridging organic ligands has drawn pronounced interest. Against this background, spectacular examples of supramolecular architectures organized by means of covalent bonds, hydrogen bonds, π-π interactions, or their combination in different ways have been reported.1 However, the influencing information on supramolecular assemblies is not yet well understood even though it has been documented that the crystallization process of these architectures is heavily influenced by many factors such as the pH value of the reaction,2 molar ratio of the molecular components,3 solvent,4 steric requirement of the counterions,5 reaction temperature6 together with the coordination nature of metal ions7 and organic ligands.8 For example, the pH value of the reaction has been shown to play a crucial role in controlling the topological structures of three keggin-based supramolecular architectures.2a Meng and his co-workers have demonstrated that two bilayered metal-organic frameworks can be synthesized at different ligand-to-metal ratios.3a Therefore, the exploitation of appropriate synthetic strategies *To whom correspondence should be addressed. E-mail: [email protected]. cn; fax: þ86-591-83714946; tel: þ86-591-83711523. pubs.acs.org/crystal

Published on Web 08/27/2009

and routines leading to the desired supramolecular species represents a considerable challenge. On the other hand, supramolecular isomerism introduced by Zaworotko and co-workers9 is of great importance and occupies a prominent position in the field of crystal engineering. Much attention has been directed toward this area, mostly driven by its structural diversity.10-12 Even though true supramolecular isomers lies at the very heart of supramolecular isomerism, it has not gained widespread recognition in this realm. According to the literature, most reported examples about supramolecular isomerism have different solvent included in their structures,11 and they can be categorized as a group of solvent-induced (guest-induced) pseudopolymorphism.12a Only very recently has sporadic true supramolecular isomers12 with fixed stoichiometry for all components been known. These facts are presumably because many factors can influence the composition of supramolecular isomers. So, the rational design and synthesis of true supramolecular isomers is still a formidable task and a great deal of work is required to extend the knowledge in understanding supramolecular isomerism and crystal growth. To continue our ongoing project on the chemistry of fluorescent materials from 1,10 -biphenyl-2,20 -dicarboxylic acid (H2dpa) and Zn(II) cation,13 we extend our investigations to Ag(I) compounds. The important shift is driven by the following thematic issues in Ag(I) chemistry: (i) the Ag(I) cation, similar to Zn(II) cation, has a d10 electronic configuration and is r 2009 American Chemical Society

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not expected to significantly alter the emission color of luminescent ligand, barring Ag-Ag interactions; (ii) in contrast to Zn(II) cation, Ag(I) cation is much larger and has a tendency to adopt more bonding modes and coordination geometries than Zn(II) cation, which may further increase the diversity of the structures of self-assembled Ag(I) compounds; (iii) close-shell d10 Ag 3 3 3 Ag interactions (termed as argentophilicity, Ag(I) 3 3 3 Ag(I) < 3.4 A˚14) may give rise to intriguing supramolecular motifs, crystal packing, and enhanced properties.15 On the basis of these considerations, we incorporate the extensively studied neutral nitrogen-donor ligand 4,40 -bipyridine (bpy) into the parent compound, [Ag2(dpa)]n (8),16 to determine the sensitivity of the structures of the resultant compounds to external stimuli. As a result, seven silver(I) supramolecular compounds, namely, [Ag2(bpy)0.5(dpa)] (1), [Ag2(bpy)1.5(dpa)(H2O)] (2), [Ag(bpy)0.5(Hdpa)] (3), R-[Ag(bpy)] 3 (Hdpa) (4), β-[Ag(bpy)] 3 (Hdpa) (5), [Ag(bpy)]2 3 (dpa) 3 6(H2O) (6), and [Ag(bpy)]4 3 (dpa) 3 2(NO3) 3 6(H2O) (7), were successfully synthesized through careful control of molar ratio of precursors or pH value of the reaction. Of particular interest about this work is that compounds 4 and 5 are two true supramolecular isomers. Comparison of their structures reveals that argentophilic interactions, hydrogen-bonding, and π 3 3 3 π stacking interactions play important roles in the fabrication of the two supramolecular architectures. Another outstanding feature about this work is that rich supramolecular chemistry of H2dpa ligand and great structural diversity of Ag/bpy/(H)dpa family based on various chain/layer substructures have been observed in this work. All crystal structures in this work have been determined by single-crystal X-ray diffraction, and most of them have also been characterized by IR, thermogravimetric analysis (TGA), elemental and luminescent analyses. Experimental Section General Considerations. All chemicals commercially purchased and used as received without further purification. Elemental analyses were determined on an Elemental vario EL III analyzer. The FT-IR spectra were measured as KBr pellets on a Nicolet Magna 750 FT-IR spectrometer in the range of 4000-400 cm-1. Thermogravimetric analyses of compounds 2, 3, 5, and 7 were performed with a NETZSCH STA 449C thermal analyzer under nitrogen at a heating rate of 10 °C min-1, while those of compounds 4 and 6 were analyzed using NETSCHZ STA-449C thermal analyzer coupled with a NETSCH QMS403C mass spectrometer under nitrogen with a heating rate of 10 °C/min. The fluorescence measurements were performed on an Edinbergh Analytical instrument FLS920. Synthesis of a Mixture of Ag2(bpy)0.5(dpa) (1) and Ag2(bpy)1.5(dpa)(H2O) (2). Five drops of aqueous ammonia solution (25%, about 0.25 mL) was added to an aqueous solution (15.0 mL) containing AgNO3 (0.090 g, 0.530 mmol), H2dpa(0.088 g, 0.363 mmol), bpy (0.039 g 0.250 mmol) while stirring, then 15 drops of aqueous nitric acid (v/v =1, about 0.75 mL) was added to the above mixtures. The pH value of the solution is about 6.0 after stirring for about half an hour in the air at room temperature, which was then placed in a 23 mL capacity Teflon lined autoclave and heated at 160 °C for 3 days, cooled to room temperature to give compound 2 as the main product (yield, 56% based on Ag) and compound 1 as byproduct (yield, 13% based on Ag). IR data (KBr pellet) for (1): 3199 (m, br), 3048 (m), 1599 (s), 1581 (s), 1485 (w), 1435 (w), 1410 (w), 1387 (s), 1218 (m), 1151 (w), 1108(w), 1070 (w), 1041 (w), 1003 (w), 847 (w), 805 (m), 781 (w), 759 (m), 716 (w), 626 (m), 532 (w). Anal. calcd for C29H22Ag2N3O5 (708.24) (2): C, 49.18; H, 3.13; N, 5.93%. Found: C, 49.12; H, 3.18; N, 5.96%. IR data (KBr pellet) for (2): 3199 (s, br), 3048 (s), 1600 (vs), 1582 (vs), 1485 (w), 1435 (w), 1411(m),1387 (vs), 1219 (m), 1151 (w), 1108 (w), 1070 (w), 1041 (w), 1004 (w), 847 (w), 806 (s), 781 (w), 759 (m), 716 (w), 636 (m), 532 (w).

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The synthesis can be optimized through varying mole ratios of the starting materials to gain pure phase of 2, which is described as follows: to an aqueous solution (15.0 mL) of AgNO3 (0.112 g, 0.659 mmol), H2dpa (0.106 g, 0.438 mmol), 4,40 -bpy (0.022 g, 0.141 mmol) was added 0.50 mL of aqueous ammonia solution (25%) and 50 drops of dilute aqueous solution nitric acid (v/v = 1, about 2.50 mL) to adjust the initial pH value to be about 6.0. The heterogeneous mixture was then processed under the same conditions as described above and compound 2 was obtained exclusively. Synthesis of Ag(bpy)0.5(Hdpa) (3). To an aqueous solution (15.0 mL) of AgNO3 (0.088 g, 0.518 mmol), H2dpa (0.085 g, 0.351 mmol), and 4,40 -bpy (0.040 g, 0.256 mmol) was added an aqueous solution of NaOH (0.40 mL, 1 M). The resulting mixture with an initial pH value of 4.6 was then processed as described above to give orange prism crystals of 3 (yield, 61% based on Ag), which was washed with water for several times and dried in air. Anal. calcd for C19H13AgNO4 (427.17): C, 53.42; H, 3.07; N, 3.28%. Found: C, 53.38; H, 3.22; N, 3.31%. IR data (KBr pellet): 3400 (vs, br), 3153 (vs, br), 2975 (s), 1627 (m), 1402 (vs), 1133 (m), 1093 (m), 1050 (m), 881 (w), 805 (w), 615 (w). Synthesis of a Mixture of r-[Ag(bpy)] 3 (Hdpa) (4) and Ag2(bpy)1.5(dpa)(H2O) (2). The procedures for the syntheses of the above compounds were applied to the synthesis of a mixture of 2 and 4 except that AgNO3 (0.091 g, 0.536 mmol), H2dpa (0.088 g, 0.363 mmol), 4,40 -bpy (0.040 g, 0.256 mmol) in 15.0 mL of water was used in this case. The initial pH value of the heterogeneous solution was adjusted to about 7.0. After the reaction, small amount of pale-yellow prism crystals of 4 (yield, 21% based on Ag) together with a large amount of 2 (yield, 47% based on Ag) was isolated. Anal. calcd for C24H17AgN2O4 (505.27): C, 57.05; H, 3.39; N, 5.54%. Found: C, 57.10; H, 3.32; N, 5.60%. IR data (KBr pellet): 3428 (s, br), 1708 (s), 1600 (s), 1418 (s), 1070 (m), 848 (w), 808 (m), 763 (m), 720 (w), 635(w). Synthesis of β-[Ag(bpy)] 3 (Hdpa) (5). To an aqueous solution (15.5 mL) of AgNO3 (0.085 g, 0.500 mmol), H2dpa (0.094 g, 0.388 mmol), 4,40 -bpy (0.039 g, 0.250 mmol) was added an aqueous solution of NaOH (0.70 mL, 1 M). The heterogeneous mixture with an initial pH of 5.8 was processed under the same conditions as described above to give colorless or pale yellow column crystals of 5 (yield, 54% based on Ag), which was washed with water for several times and dried in air. Anal. calcd for C24H17AgN2O4 (505.27): C, 48.34; H, 3.38; N, 4.70%. Found: C, 48.23; H, 3.35; N, 4.78%. IR data (KBr pellet): 3421 (m), 3129 (m), 1604 (m), 1401 (s), 1163 (m), 854 (w), 810 (w), 728 (w), 615 (w). Synthesis of [Ag2(bpy)]2 3 (dpa) 3 6(H2O) (6). To an aqueous solution (15.0 mL) containing Ag2O (0.061 g, 0.263 mmol), H2dpa (0.107 g, 0.442 mmol) and 4,40 -bpy (0.033 g, 0.211 mmol) was added 5 drops of aqueous ammonia solution (25%, about 0.25 mL). The final mixture was treated as described for other compounds in this work. After reaction, some uncharacterized yellow amorphous solids were filtered off and the yellow filtrate was left undisturbed to evaporate. About 3 weeks later, yellow block-like crystals of 6 were grown from the filtrate (yield, 63% based on Ag). Anal. calcd for C38H66Ag4N8O17 (1698.77): C, 48.08; H, 3.92; N, 6.60%. Found: C, 47.95; H, 3.87; N, 6.72%. IR data (KBr pellet): 3672 (br, m), 3447 (w), 3122 (br, s), 1661 (w), 1601 (m), 1554 (br, m), 1453 (w), 1399 (s), 1218 (w), 1161 (w), 1068 (w), 1004 (w), 806 (m), 764 (w), 723 (w), 627 (w). Synthesis of [Ag(bpy)]4 3 (dpa) 3 2(NO3) 3 6(H2O) (7). To an aqueous solution (15.0 mL) of Ag2O (0.102 g, 0.440 mmol), H2dpa (0.104 g, 0.429 mmol), 4,40 -bpy (0.028 g, 0.179 mmol) was first added aqueous ammonia solution (25%, 0.50 mL) then added 43 drops of dilute aqueous solution nitric acid (v/v = 1, about 2.15 mL). A large amount of crystals of 7 (yield, 68% based on Ag) was obtained after reaction, which were washed with water for several times and dried in air. Anal. calcd for C54H52Ag4N10O16 (1528.54): C, 42.43; H, 3.43; N, 9.16%. Found: C, 42.48; H, 3.36; N, 9.07%. IR data (KBr pellet): 3388 (br, s), 1599 (m), 1558 (m), 1486 (w), 1385(s), 1219 (s), 1071 (w), 1041 (w), 1004 (w), 847 (w), 806 (m), 759 (w), 626 (w), 508 (w). X-ray Crystallography Study. Suitable single crystals of compounds 1-7 were carefully selected under an optical microscope and glued to thin glass fibers. The intensity data were collected on a Rigaku SCXmini CCD diffractometer for compounds 1, 2 and 6, 7

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Table 1. Crystal Data and Structure Refinements for Compounds 1-7 formula Mr cryst syst space group a (A˚) b (A˚) c (A˚) R (°) β (°) γ (°) V (A˚3) Z Dc (g cm-3) μ (mm-1) F(000) GOF measured reflns unique reflns (Rint) observed reflnsa refined params R1b [I > 2σ(I)] wR2 (all data) max/min, e A˚-3

formula Mr cryst syst space group a (A˚) b (A˚) c (A˚) R (°) β (°) γ (°) V (A˚3) Z Dc (g cm-3) μ (mm-1) F(000) GOF measured reflns unique reflns (Rint) observed reflnsa refined params R1b [I > 2σ(I)] wR2 (all data) max/min, e A˚-3

1

2

3

4

C19H12Ag2NO4 534.04 orthorhombic Pbca 7.376(3) 18.396(7) 24.237(8) 90 90 90 3289(2) 8 2.157 2.407 2072 1.075 23929 3763 (0.0303) 3270 236 0.0370 0.1021 0.702/-0.784

C29H22Ag2N3O5 708.24 monoclinic P2(1)/n 11.562(10) 18.771(15) 11.794(9) 90 101.390(12) 90 2509(4) 4 1.875 1.608 1404 1.051 18538 5743 (0.0465) 4761 353 0.0681 0.1895 1.722/-1.480

C19H13AgNO4 427.17 monoclinic C2/c 29.487(3) 14.8474(12) 7.5179(6) 90 99.635(3) 90 3245.0(5) 8 1.749 1.266 1704 1.009 12036 3728(0.0209) 3161 229 0.0288 0.0670 0.330/-0.309

C24H17AgN2O4 505.27 triclinic P1 8.904(3) 9.342(3) 13.430(4) 73.67(2) 84.83(2) 77.49(3) 1046.1(6) 2 1.604 0.997 508 1.034 8103 4801(0.0164) 3704 283 0.0447 0.1090 0.496/-0.592

5

6

7

C24H17AgN2O4 505.27 monoclinic P2(1)/n 10.589(2) 11.409(3) 17.569(4) 90 100.172(5) 90 2089.2(8) 4 1.606 0.999 1016 1.076 16118 4799(0.0263) 3864 280 0.0352 0.0883 0.706/-0.690

C68H66Ag4N8O17 1698.77 monoclinic C2/c 21.783(11) 20.047(10) 14.820(8) 90 101.225(8) 90 6348(6) 4 1.778 1.296 3416 1.102 24776 7420(0.0466) 5200 465 0.0555 0.1429 0.953/-0.969

C54H52Ag4N10O16 1528.54 monoclinic P2(1)/c 14.999(6) 17.698(6) 23.385(7) 90 114.447(19) 90 5651(4) 4 1.782 1.444 3000 1.070 43367 12874(0.0301) 9622 757 0.0559 0.1666 1.561/-1.332

)

)

a Observation criterion: [I > 2σ(I)]. b R1 = Σ Fo| - |Fc /Σ|Fo| and wR2 = {Σ[w(F02 - Fc2)2]/Σ[w(F02)2]}1/2, w = 1/[σ2(F02) þ (aP)2 þ bP], where P = [max(Fo2,0) þ 2Fc2]/3 for all data.

and on a Rigaku Mercury CCD diffractometer for compounds 3-5. The CrystalClear software was used for data reduction and empirical absorption correction.17 All the structures were solved by the direct methods and refined by the full-matrix least-squares method on F2 using SHELXS-97 and SHELXL-97 programs, respectively.18 All non-hydrogen atoms were refined anisotropically. The H atoms attached to their parent atoms of organic ligands were geometrically placed and refined using a riding model. However, oxygen-bounded hydrogen atoms were located from difference peaks and kept in that position with isotropic thermal parameters fixed at 1.5 times that of the respective oxygen atoms. The hydrogen atoms of the lattice water molecules in compound 7 were not included in the final refinement. Crystal data as well as details of data collection and refinement for compounds 1-7 are summarized in Table 1.

Results and Discussion Synthesis and Characterization. The initial attempts to isolate crystals of Ag/bpy/(H)dpa family from solutions under ambient conditions gave no satisfying results. To

deprotonate H2dpa, an aqueous solution of NaOH was first selected, but an unidentified insoluble deep-brown powder immediately formed upon the addition of NaOH into the aqueous solution of AgNO3 and H2dpa, which is presumably due to the formation of “silver(I) complexes” via a rapid reaction between carboxylic groups and silver ions.19 It has been documented by Sun et al. that the hydrothermal method may effectively sidestep the formation of unknown precipitates formed by silver salts and carboxylate ligands.1b At the same time, a variety of hydrothermal parameters, such as solvent, counterions, pH value of the reaction, reaction temperature, source and molar ratio of the reactant components, have been demonstrated to have a profound influence on the final reaction outcome.2-8 Therefore, many interesting results can be obtained from limited starting materials. Taking the above factors into consideration, our synthetic strategy for Ag/bpy/(H)dpa supramolecular compounds is based on hydrothermal reactions with control on the molar ratio of precursors and the pH value of heterogeneous

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Figure 1. (a) View of the coordination environments of Ag(I) cations in 1. Symmetry codes: a, 1 - x, -y, 1 - z; b, 0.5 þ x, y, 1.5 - z; c, 1 þ x, y, z. (b) The 1D [Ag2(dpa)] chain substructure of 1. (c) View of the overall 2D architecture of 1.

solution together with replacement of NaOH with NH3 3 H2O and AgNO3 with Ag2O. Compounds 3 and 5 were synthesized hydrothermally with an aqueous solution of NaOH as the base. Throughout the preparation of them, a large amount of powder was often observed to copolymerize with the crystals and causing great difficulty to isolate them, presumably because the initially generated precipitate could not be well resolved during the assembly process and the hydrothermal method alone cannot work well in this case. To more effectively solve this problem, some possibly effective measures must be taken through controlling the release rate of Ag(I) cations to slow down the formation of “silver(I) complexes” and the growth rate of single crystals. A similar method has already been used by Michaelide et al. in the synthesis of a succinatodisilver complex via a gel permeation method.20 In this work, aqueous ammonia solution is thus used considering that it not only acts as base but can react with Ag(I) ions forming [Ag(NH3)2]þ species which in turn can slowly release free Ag(I) cations based on the chemical equilibrium between them. At the same time, the Ag(I) source was changed from AgNO3 to Ag2O based on the difference in their solubility. Expectedly, the use of NH3 3 H2O and Ag2O effectively solves the problems associated with the precipitation of powder and facilitates the formation of well-formed crystals. During close examination of the synthesis conditions, we found that hydrothermal parameters such as the molar ratio of precursors and the pH value of heterogeneous solution have an important effect on the resulting outcome. Compounds 3 and 5 with unique structural features were synthesized under different pH values (4.6 for 3 and 5.8 for 5) by keeping a similar molar ratio for AgNO3/H2dpa/bpy, illustrating that they are pH-dependent. So does the syntheses of a mixture of compounds 1 and 2 (initial pH value is 6.0) and a mixture of compounds 2 and 4 (initial pH value is 7.0). It is interesting to note that clusters of black powder were generally formed during stirring at room temperature when Ag2O was used as the Ag(I) source instead of AgNO3 in the syntheses of compounds 6 and 7. So, the measurement of the pH value of the heterogeneous solution may be meaningless owing to different degrees of dispersion. On the other hand, the molar ratio of precursors, another

external stimulus, can also play an important role in the assembly process. A pure phase of compound 2 and a mixture of compounds 1 and 2 were obtained from AgNO3/H2dpa/bpy with molar ratios of 4.7:3.1:1.0 for the former and 2.1:1.5:1.0 for the latter by keeping the same pH value. This fact shows that the outcome is molar-ratiodependent. In addition, it is notable that the pH value of heterogeneous solution and the molar ratio for AgNO3/ H2dpa/bpy were similar for the syntheses of compound 5 and a mixture of compounds 1 and 2, but the employed bases were different in these two cases (NaOH for the former and NH3 3 H2O for the latter), and the results differs greatly from each other. We tentatively ascribed this difference to different “silver(I) complexes” formed by addition of NaOH and NH3 3 H2O, respectively. In short, these compounds in this work are molar-ratio- or pH-dependent, illustrating that controlled syntheses through varying the molar ratio of reactant components or pH value of heterogeneous solution are a facile and effective way to realize the aesthetic diversity of supramolecular assembly chemistry. Crystal Structures 1D Chain of {Ag2(dpa)} Observed in 1. The structure of 1 is determined by X-ray single-crystal diffraction. It has a twodimensional (2D) periodic layered structure comprising a one-dimensional (1D) {Ag2(CO2)2} chain substructure. As shown in Figure 1a, two crystallographically nonequivalent Ag(I) cations are present in the asymmetric unit of 1. They all feature highly disordered T-shaped geometries (neglecting the weak Ag 3 3 3 O and Ag 3 3 3 Ag interactions) with different coordination spheres: Ag(1) is ligated by two carboxylate oxygen atoms from two different dpa2- (Ag(1)-O(1) = 2.483 A˚, Ag(1)-O(2b) = 2.275 A˚) and one nitrogen atom (Ag(1)-N(1) = 2.203 A˚), while Ag(2) is coordinated exclusively by carboxylate oxygen atoms (Ag(2)-O, 2.2602.414 A˚). The Ag-O bond distances are compatible with the values for its parent compound, [Ag2(dpa)]n (8).16 However, the distances of Ag(1)-O(1b) (2.693 A˚), Ag(1)-O(3) (2.927 A˚) and Ag(2)-O(2b) (3.132) are beyond the range of 2.32-2.52 A˚ for silver(I) carboxylate21 but still shorter than the sum of van der Waals radii (3.24 A˚) of the Ag(I)

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Scheme 1. Syntheses of Compounds 1-7

cation and oxygen atom,22 suggesting non-negligible Ag-O interactions and explaining the deviation of Ag(I) centers from perfect T-shaped geometry. There are also two types of carboxylate groups in this structure which are embedded in the same dpa2- and endows dpa2- a μ5-O,O,O0 ,O00 ,O00 coordination mode. Taking weak Ag-O interactions into account, the coordination mode of dpa2- can be described as pseudo μ5-(η2-O,O0 ),(η2-O0 ,O00 ),(η2-O0 ,O00 ),(η2-O00 ,O000 ),O, O0 ,O0 (mode I in Scheme 2) which has never been reported in dpa-containing compounds. The adjacent Ag(I) ions are connected by dpa2- to give 1D {Ag2(dpa)} covalent chain along the a axis (Figure 1b). The intrachain argentophilic interactions between Ag(1) and Ag(2) (The Ag(1) 3 3 3 Ag(2) distance of 2.948 A˚ is slightly longer than the Ag 3 3 3 Ag separation of 2.88 A˚ in the metallic state but greatly shorter than the van der Waals contact distance for Ag 3 3 3 Ag of 3.40 A˚, showing the existence of argentophilic interactions between them.14) further consolidate the 1D chain substructure. Then, each 1D chain substructure is further linked to two neighboring ones by bpy ligands, resulting in the overall 2D wavy grid layer, in which the diphenyl groups of dpa2protrude into the cavity to eliminate the inclusion of any guest molecules (Figure 1c). There is offset face-to-face

Scheme 2. Versatile Interaction Modes between Hdpa-/dpa2and Ag(I) Observed in 1-7

π 3 3 3 π stacking interactions between one benzenyl ring of dpa2- and one pyridyl ring of bpy, which is confirmed by the centroid-to-centroid distance of 3.799 A˚ and the dihedral angle of 12.15° between two corresponding planes defined by them. As a result, the strain deriving from the Ag-O connectivity and the π 3 3 3 π stacking interactions between dpa2- and bpy endows a torsion angle of 64.20° between two phenyl rings of dpa2-, revealing that dpa2- is a chiral ligand in this complex. However, the adjacent chains are related by

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Figure 2. (a) View of the coordination environments of Ag(I) cations in 2. Symmetry codes: a, 2 - x, -y, 1 - z; b, 1 - x, -y, -z. (b) 1D supramolecular chain substructure of 2. (c) The 2D supramolecular framework of 2. The diphenyl group of dpa2- has been omitted for clarity. (d) View of the interdigitated 3D supramolecular framework of 2.

an inversion center, so compound 1 crystallizes in the centrosymmetric Pbca space group. Its parent compound, [Ag2(dpa)]n (8),16 was studied previously by Wang et al. The overall structure was maintained as a 2D solid, but the polymeric layer is formed exclusively by Ag-COO interactions. As could be expected, the presence of auxiliary bpy ligand does have a great influence on the structures. This phenomenon has been observed in previously reported documents.23 The Ag-O linkages show a lower dimensionality in compound 1 (1D) than that in its parent compound (2D). Although the coordination number for dpa2- increases from 3 in its parent compound to 5 in compound 1, bpy has less coordination sites compared to dpa2- and compound 1 still shows an overall 2D framework. Were the substitution not taken place, dpa2- in this coordination mode may connect Ag(I) cations to give other structural motifs, often surprising ones with 2D or threedimensional (3D) dimensionality. Work on this speculation is in progress. 1D Supramolecular Chain of {Ag2(dpa)(H2O)}2 observed in 2. Although two crystallographically independent Ag(I) cations and one dpa2- are also present in the asymmetric unit of 2, as those observed in 1, they represent different coordination modes as depicted in Figure 2a. Ag(1) and Ag(2) also show T-shaped coordination geometries, but the coordination environments are quite different. For Ag(1), it is coordinated by two nitrogen atoms and one carboxylate oxygen atom, while Ag(2) is bound to one nitrogen atom, one carboxylate oxygen atom, and one terminal water molecule. It is obvious that the coordination sites for carboxylate

oxygen atom around Ag(I) cation in 1 are partially replaced by a nitrogen atom or water molecule. The distance of Ag(1)-O(1) (2.447 A˚) compares well with the Ag-O distances in 1, while the distance of 2.181 A˚ for Ag(2)-O(3) is shorter than that observed in 1. Similar to 1, there are weak Ag-O interactions which are confirmed by Ag-O distances of 2.685 A˚ for Ag(1)-O(3) and 3.197 A˚ for Ag(1)-O(1a).22 Thus, dpa2- features a pseudo μ3-(η2-O,O0 ),O,O0 coordination mode (mode II in Scheme 2) and connects three Ag(I) cations simultaneously. The nearest symmetry-related O(1) and O(3) link symmetry-related Ag(1) and Ag(2) into a centrosymmetric tetranuclear building unit with an inversion center located at the middle of Ag(1) and Ag(1a). Within this unit, the Ag(1) 3 3 3 Ag(1a) and Ag(1) 3 3 3 Ag(2) distances spanned by O(1) and O(3) are 3.161 and 3.164 A˚, implying the existence of ligand-supported argentophilic interactions.17 Two terminal Ag(2) atoms of adjacent tetranuclear units are connected with each other through ligand-unsupported argentophilic interactions (2.88 A˚)14 to extend these building units into 1D supramolecular chain substructure of 2 (Figure 2b). The intrachain O-H 3 3 3 O hydrogen bonds between coordinated water molecules and the uncoordinated carboxylate oxygen atoms (O(1W) 3 3 3 O(4a), 2.790(7) A˚, O(1W) 3 3 3 O(2b), 2.814(8) A˚, Table S1, Supporting Information) further strengthen the 1D substructure. It is interesting to note that great structural changes of 1D chain substructure occur compared to that found in 1 owing to partial replacement of carboxylate oxygen atoms in 1 by a nitrogen atom of bpy or water molecule in 2. Another prominent feature about the 1D chain substructure of 2 is

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Figure 3. (a) View of part of the 1D chain substructure of 3 with atom labeling. Symmetry codes: a, -x, y, 0.5 - z. (b) 2D supramolecular layer of 3 with some carbon atoms of Hdpa- are omitted for clarity.

the coexistence of ligand-supported and ligand-unsupported argentophilic interactions, which has never been found previously to the best of our knowledge. Each 1D chain of 2 is further connected to two symmetry equivalents by bpy ligands into a 2D layer perpendicular to b axis (Figure 2c). Within each layer, adjacent bpy ligands are aligned parallel to give two types of π 3 3 3 π stacking interactions: (A) bpy containing N(1) and N(2) are aligned in antiparallel offset face-to-face stacking mode. The centroid-to-centroid distance and the dihedral angle between the corresponding pyridyl rings are 3.937 A˚ and 16.2°, indicating there are weak π 3 3 3 π stacking interactions; (B) medium strong π 3 3 3 π stacking interactions exist between adjacent N(1)-containing pyridyl ring and N(3)-containing ring, and the corresponding values for centroid-to-centroid distance and dihedral angle are 3.675 A˚ and 10.9°. Undoubtedly, the two types of π 3 3 3 π stacking interactions contribute to the construction of the 2D supramolecular architecture of 2. Then, adjacent layers stack in an ABAB fashion with dpa2- of one layer protruding into the cavity of adjacent ones giving rise to the resulting 3D interdigitated framework (Figure 2d). As observed in 1, dpa2- shows a chiral configuration with two phenyl rings twisted by 50.49°, but it is only caused by constraint from Ag-O covalent bonds and contact interactions, which is different from that observed in 1. Similar to 1, the introduction of bpy has a great influence on the structure of 2. But more coordination sites around Ag(I) cation for each dpa2- are replaced by bpy than that found in 1. In addition, water molecules participate in coordination by taking up one coordination site for the carboxylate oxygen atom. Therefore, structural changes occur with a dimensionality of Ag-COO linkage further decreasing from 1 in 1 to zero in 2. It has been documented that argentophilic interactions as a design element can lead to metal-organic frameworks (MOFs) with aesthetically beautiful structures and unique properties.14,15 However, almost all of the argentophilic interactions in reported Ag(I) complexes are typically main-

tained by bridging ligands, which can be categorized as ligand-supported argentophilic interactions. Examples of ligand-unsupported Ag 3 3 3 Ag interactions based on structure determination have been studied only sparsely.15 Therefore, the successful synthesis of this compound provides another prototype for the study of ligand-unsupported argentophilic interactions. 1D Alternate Chain of [Ag(bpy)0.5] 3 (Hdpa) Observed in 3. X-ray structure analysis reveals compound 3 is a 2D supramolecular assembly based on a 1D alternate polymeric chain substructure of {Ag(Hdpa)}2 bridged by μ-bpy. As shown in Figure 3a, two crystallographically independent Ag(I) cations are bridged by a pair of symmetry-related carboxylate groups in syn-syn bridging mode (mode III in Scheme 2) to form an eight-membered {Ag2C2O4} ring (ring A) with a crystallographic mirror plane passing through Ag(1) and Ag(2). The Ag 3 3 3 Ag separation across μ-COO is 2.959 A˚, indicating the existence of ligand-supported argentophilic interactions.14 H2dpa is monodeprotonated to give Hdpa-1 to meet the requirement of charge balance requirement. The carboxyl group donates an H atom to the coordinated carboxylate oxygen atom (O(3) 3 3 3 O(1), 2.654(2) A˚, Table S1, Supporting Information) affording another ring (ring B), which may well explain the longer Ag-O distances for Ag(1) than those for Ag(2). It is noteworthy that the synergetic effect of Ag-O bonds and the O-H 3 3 3 O hydrogen bonds make Hdpa- racemic. The dihedral angle of two phenyl rings of Hdpa- are 83.87°, which is different from those found in compounds 1 and 2. Each neutral {Ag2C2O4} unit is linked by two μ-bpy bridges to two symmetry equivalent ones resulting in a 1D alternate chain extending in the b axis where the Ag 3 3 3 Ag distance spanned by μ-bpy is 11.88 A˚. This is a unique motif observed in this work and has never been reported before to the best of our knowledge. A close examination of the structure reveals that N(2)-containing pyridyl rings in adjacent chains are involved in medium strong π-π stacking interactions in a head-to-tail fashion with an offset centroid-to-centroid distance of 3.793 A˚,

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Figure 4. View of part of the 1D double chain substructures of 4 (a) and 5 (b) with atom labeling. (c) 2D supramolecular layer of 4. (d) The 3D supramolecular framework of 5.

which engage the 1D chains to 2D layered supramolecular network with the diphenyl rings of Hdpa-1 projecting away from each side of the layer (Figure 3b). Different 1D Supramolecular Double Chain of [Ag(bpy)]2 3 2(Hdpa) Observed in r-[Ag(bpy)] 3 (Hdpa) 4 and β-[Ag(bpy)] 3 (Hdpa) 5. In searching for diverse structural motifs of the Ag/ bpy/(H)dpa family, two true supramolecuar isomers are synthesized with serendipity which are both extended from 1D [Ag(bpy)]2 3 2(Hdpa) supramolecular double-chain substructures. Compound 4 is structurally ascribed to be a 2D supramolecular network. Similar to 3, H2dpa is half deprotonated in this case. However, Hdpa- does not coordinate with Ag(I) cations but only has coordinative interacions with Ag(I) cations with Ag-O distances being 2.608-2.934 A˚.22 Hdpa- features pseudo μ2-(η2-O,O0 ),O mode (mode IV in Scheme 2). Two symmetry-related carboxylate groups in pseudo chelating-bridging mode connect two crystallographically unique Ag(I) cations into a eight-membered {Ag2C2O4} ring, resmbling that in compound 3. Close examination about this ring reveals that the O-H 3 3 3 O hydrogen bonds (O(3) 3 3 3 O(2), 2.518(4) A˚, Table S1, Supporting Information) elongate the corresponding Ag-O distances. This phenomenon is also observed in 3. The Ag(1) 3 3 3 Ag(2) distance in this ring is 3.169 A˚, showing the presence of ligand-supported argentophilic interactions.14 However, this ring is rotated by 90° compared to that of 3 and is connected to two symmetry equivalents through four bpy ligands instead of two, and a 1D double chain substructure of 4 is thus formed (Figure 4a). Each double chain is further consolidated by strong intrachain π 3 3 3 π stacking interactions between antiparallel aligned pyridyl rings with centroid-to-centroid distances being 3.635 A˚. The adjacent double chains are displaced with each other about less than

half a bpy, resulting in offset π 3 3 3 π stacking interactions between perfectly parallel aligned N(2)-containing pyridyl rings and slipped π 3 3 3 π stacking interactions between N(1)containing pyridyl rings. The contact interactions are confirmed by the centroid-to-centroid distances (3.571 A˚ for pairs of N(2)-containing pyridyl rings) or the mean C-C and C-N distances (3.848 A˚ for pairs of N(1)-containing pyridyl rings), respectively. All these extensive interchain π 3 3 3 π stacking interactions engage these 1D double chains into 2D supramolecular aggregate parallel to ac with Hdpaprojecting away from each side of it (Figure 4c). Most interesting about this work is the observation of true supramolecular isomers. Although compounds 4 and 5 are all constructed from 1D double chain substructues and contain supramolecular frames of identical composition, their structural motifs have respective features. Different from that found in the isomer 4, Hdpa- in compound 5 adopts a semichelating mode (mode V in Scheme 2) with Ag 3 3 3 O distances being 2.858(2) A˚ and 2.713(2) A˚. The O(2) atom is involved in two aggressive hydrogen bonds: one (O(4) 3 3 3 O(2), 2.460(3) A˚, Table S1, Supporting Information) may elongate Ag(1)-O(2) bond while the other (C(19) 3 3 3 O(2), 3.196(3) A˚, Table S1, Supporting Information) may shorten it. Thus, the Ag-O distances only have a slight difference. Hdpa- acts as suspender dangling each side of 1D double chain and has been highly anchored via the mentioned Ag 3 3 3 O interactions and C(19) 3 3 3 O(2) hydrogen bonds. Another difference between these two isomers is the geometric conformations of 1D double chain substructure. Although the 1D double chain in 5 is also derived from a pair of alternate Ag-bpy chains (Figure 4b), they are displaced along the b axis about half a bpy resulting in the absence of Ag 3 3 3 Ag contact interactions. Moreover,

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Figure 5. (a) ORTEP drawing of 6 showing the coordination environment of Ag(I) cations. Symmetry codes: a, -0.5 - x, 0.5 - y, -1 - z; b, -0.5 þ x, 0.5 - y, -1.5 þ z. (b) 2D substructure of 6. (c) Projection of the overall 3D supramolecular framework of 6. (d) 1D water tape trapped in channels.

only one type of π 3 3 3 π stacking interactions between parallel aligned N(1)-containing pyridyl rings of bpy facilitates the formation of the 1D double chain. The centroid-to-centroid distance of 3.647 A˚ is close to those found in 4. Then, versatile interchain C-H 3 3 3 O hydrogen bonding interactions engage these 1D double chains into a 3D intricate supramolecular architecture with rectangular channel along the c axis (Figure 4d). It is obvious that the synergetic effect of weaker π 3 3 3 π stacking interactions and the absence of Ag 3 3 3 Ag contact interactions leads to the low thermal stability of 5 compared to that of 4 as concluded from TGA results, which distinguishes them greatly. Supramolecular isomerism (synonymous with polymorphism) is widely encountered in the realm of crystal engineering and supramolecular chemistry and has attracted increasing interest because the control over polymorphs or supramolecular isomers lies at the heart of the concept of crystal engineering.10-12 For given building synthons, different approaches of their arrangement can result in a series of supramolecular isomers and the designed construction of metal-organic supramolecular isomers thus remains undeveloped. A thorough comparison between compounds 4 and 5 reveals that the different interaction modes of Hdpa- with Ag(I) cation and the resulting different arrangement modes of double chain substructures may play a vital role in the assembly of these two supramolecular isomers. According to the classification of supramolecular isomer,12a these two isomers can be categorized as true supramolecular isomers with a fixed stoichiometry for all components which are scarcely encountered compared to (guest-induced) pseudopolymorphism.12 The successful synthesis of these two supramolecular isomers in this context enriches supramolecular chemistry and provides prototypes for the study of true supramolecular isomers with structure characterization. 2D Supramolecular Layer of [Ag2(bpy)2] 3 (dpa) Observed in 6. Different from the above-mentioned compounds, com-

pound 6 is constructed from a 2D supramolecular layer substructure with a 1D water tape trapped in 1D channels of the intricate 3D supramolecular host framework. Bpy ligands alternately bridge two crystallographically nonequivalent Ag(I) centers into a 1D Ag-bpy chain (Figure 5a,b). However, there are non-negligible Ag 3 3 3 O weak interactions22 between the Ag(I) cations and carboxylate oxygen atoms and one water molecule (Ag(1)-O(1) = 2.571 A˚, Ag(1)-O(4) = 2.614 A˚, Ag(2)-O(1a) = 2.785 A˚, Ag(1)-O(5Wa) = 2.870 A˚), which result in the deviation of each Ag(I) cation from perfect linear geometry (N(1)-Ag(1)-N(4) = 162.0°, N(1)-Ag(2)-N(3A) = 166.9°). Thus, the dpa2ligand in this compound shows a pseudo μ2-(η2-O,O0 ),O mode (mode VI in Scheme 2). Then, these versatile Ag 3 3 3 O weak interactions assemble these 1D Ag-bpy alternate chains into the 2D supramolecular substructure of 6 (Figure 5b). Within one layer, each pair of bpy ligands is aligned in a head-to-tail face-to-face stacking mode and four types of π 3 3 3 π stacking interactions are detected. The first two types exist between pairs of bpy containing N(1) and N(2) or N(3) and N(4), leading to the structural transformation from 1D chain to 1D double chain. The centroid-to-centroid distances and the dihedral angles defined by the corresponding pyridyl rings are 3.617 A˚ and 7.6° for bpy containing N(1) and N(2) and 3.729 A˚ and 13.4° for bpy containing N(3) and N(4), respectively. The adjacent double chains pack so effectively that another two types of medium strong interchain π 3 3 3 π interactions are also observed. The geometric parameters for centroid-to-centroid distances and dihedral angles are 3.732 A˚ and 3.5° for π 3 3 3 π interactions between pyridyl rings containing N(3) and N(2) or 3.779 A˚ and 7.7° for pyridyl rings containing N(4) and N(1). Therefore, the 2D substructure is highly stabilized by the above versatile π 3 3 3 π interactions. Then, the layers are further linked into the 3D supramolecular host framework of 6 through hydrogen bonds between water molecules and carboxylate oxygen

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atoms (Figure 5c, Table S2, Supporting Information). The formation of the 3D structure can also be viewed as follows: the phenyl groups of dpa2- of one layer protrude into the void space of two adjacent ones resulting in the 3D interdigitated porous host framework with a 1D hydrophilic channel propagating along the c axis where 1D water chains (Figure 5c,d) are confined and hydrogen-bonded to it. The offset π 3 3 3 π stacking interactions between phenyl rings of dpa2-, which are confirmed by closest C 3 3 3 C distances of 3.527-3.556 A˚, further enhance the stability of the resulting framework. Water, by virtue of playing important roles in human life and in biological and chemical processes, has been a hot research area of both theoretical and experimental study.24 Against this background, a variety of isolated water clusters with different configurations,25 as well as the recently reported 1D26 and 2D27 even 3D water morphologies28 found in the lattice of a crystal structure have been extensively studied, enhancing our understanding of the structures, properties, and functionalities of liquid water or ice. It has been demonstrated that the multifunctional synthons that are rich in hydrogen-bond donor/acceptor groups are helpful in accomplishing the incorporation of water clusters into polymeric architectures.29 The full-deprotonated dpa2- dianion contains two carboxylate groups that can be potentially exploited in hydrogen-bonding frameworks. However, water clusters in complexes of dpa2- have never been experimentally observed to the best of our knowledge. In this case, one uncoordinated carboxylate oxygen atom, four crystallographically lattice water molecules, and their symmetry-related ones link to each other forming a 1D water chain through hydrogen bonds (Figure 5c,d, Table S1, Supporting Information). Now we will describe the water chain stepwise. A centrosymmetric octameric core was constructed from a nonplanar eight-membered ring comprising pairs of O(1W), O(2W), O(3W) and uncoordinated O(3). The O 3 3 3 O distances in the ring range from 2.742 to 2.875 A˚, leading to an average O 3 3 3 O distance of 2.809 A˚ which is approximately the same as those observed in another analogous decameric water cluster26a and in the ice II phase (2.77-2.84 A˚),30 whereas the average O-O-O angle of 148° is much larger than that for a ideal tetrahedral coordination around the water molecule. A pair of O2W is linked to the eightmembered ring through weak O-H 3 3 3 O hydrogen bonding interactions (O(2W) 3 3 3 O(3), 2.938(7) A˚), resulting in the formation of decameric water cluster. Adjacent decamers share a dimer of O1W giving rise to a quasi 1D water aggregate which is trapped by the supramolecular host framework (Figure 5c). Notably, this quasi 1D water aggregate is constructed from both lattice water molecules and uncoordinated carboxylate oxygen atom. This observation indicates that this 1D water aggregate is stablished not only by hydrogen bonds but by coordinative interactions, which is similar to the reported infinite chains of decameric water cluster (H2O)10 with similar configuration.26a The existence of a 1D water aggregate in this structure may have an important effect on consolidating the overall supramolecular structure because of versatile hydrogen bonds between a 1D water aggregate and the host framework. From another point of view, water molecules can be viewed as space fillers in this structure where there are cavities with an appropriate size for hosting them. 2D Supramolecular Layer of [Ag4(bpy)4] 3 (dpa) 3 (H2O) Observed in 7. Compound 7 is a nitrate-templated intricate 3D


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supramolecular framework based on a 2D supramolecular layer substructure. Different from any compound in this context, there are four unique Ag(I) cations in the asymmetric unit of 7. For each Ag(I), two nitrogen atoms are incorporated into the coordination sphere (Figure 6a). The Ag-N bond distances compare well with those observed for the above compounds. Bpy alternately bridge Ag(I) centers into two similar 1D Ag-bpy chain containing Ag(1) and Ag(4) or Ag(2) and Ag(3). Taking weak Ag 3 3 3 O interactions into account, Ag(1), Ag(2), and Ag(4) adopt highly distorted tetrahedral geometry, their coordination spheres are further completed by a carboxylate oxygen atom, water molecule, or oxygen atom from a nitrate anion, while Ag(3) are in trigonal bipyramidal geometry which comprises two nitrogen atoms from two different bpy ligands and three oxygen atoms from carboxylate group, water molecule and nitrate anion. Dpa2in pseudo μ3-(η2-O,O0 ),O, O0 coordination mode (mode VII in Scheme 2), which resembles that found in compound 2, links Ag(1), Ag(2), and (3) into a linear trinuclear building unit. The Ag(1) 3 3 3 Ag(2) and Ag(1) 3 3 3 Ag(3) distances of 3.703 A˚ and 3.733 A˚ in this unit indicate the absence of Ag 3 3 3 Ag contact interaction. Ag(4) is separated from this cluster by a bpy ligand which coordinates with Ag(1) and Ag(4) simultaneously. The Ag(1) 3 3 3 Ag(4) separation is 11.386 A˚. All the bpy ligands together with dpa2- linkers connect Ag(I) cations into a 2D positively charged undulate supramolecular layer (Figure 6b). Very similar to that observed in compound 6, there are abundant intralayer π 3 3 3 π stacking interactions between the parallel arranged bpy ligands, which may enhance the stabilization of the 2D layer substructure. The centroid-to-centroid distances are in the range of (3.663-3.953) A˚, which are comparable to the values found in the above compounds, whereas the dihedral angle between the planes defined by the corresponding pyridyl rings range from 0.26° to 14.152°, indicating the π 3 3 3 π stacking interactions are medium strong. Notably, the lattice water molecules residing in the interlayer void space assembly the 2D sheets into the overall complex 3D supramolecular framework through hydrogen bonding to each other and to nitrate anions as well as to the carboxylate oxygen atoms (Figure 6c, Table S1, Supporting Information). Because the crystals are poor quality, the hydrogen atoms of water molecules are not included in the final refinement. Thus, the varied hydrogen bonds are only confirmed by O 3 3 3 O distances of 2.532-3.058 A˚. The dpa2anions are bridged by water clusters which are further hydrogen bonded to nitrate anions to form a 2D anionic supramolecular layer as shown in Figure 6d. Influence of Hydrothermal Parameters (pH Value and Molar Ratio of Precursors) and the Secondary bpy Ligand on the Structures of Compounds Compounds 1-7 were synthesized by introducing 4,40 -bpy into the parent compound [Ag2(dpa)]n (8).16 As expected, the introduction of bpy has a profound effect on the structures of the products, and the 2D covalent layer in the parent compound composed of exclusive silver(I) cations and dpa2anions are transformed into a 2D covalent layer (1) or 2D/ 3D supramolecular architectures (2-7) comprising the ternary system of silver(I) cation, Hdpa-/dpa2-, and bpy. As it is known that bpy has a stronger coordination ability for silver(I) cation than carboxylic acids, the coordination sites of Ag(I) cation for carboxylate group is thus partially even completely

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Figure 6. (a) ORTEP drawing showing the coordination environment of Ag(I) cations. Symmetry codes: a, 1 þ x, y, z; b, -1.5 þ x, 0.5 - y, -0.5 þ z. (b) 2D substructure of 7. (c) The overall 3D framework. (d) Hydrogen-bonding motifs self-assembled from dpa2-, water, and nitrate anions.

compound has a great influence on the structures of the resultant compounds, which once again demonstrate that secondary ligand plays an important role in structural transformation. More important about this work is to explore the possible influence of external physical stimuli on the structures of the resultant compounds. With this in mind, we performed systematic investigations and synthesized some molar-ratiodependent or pH-dependent products as described in the synthesis section. These results illustrate that external stimuli can sometimes direct the assembly of supramolecular aggregate as reported by other documents. Figure 7. TGA curves of compounds 2-7.

replaced by a bpy ligand when bpy is incorporated into the parent compound 8. Each Ag(I) cation in 1 still features T-shaped geometry resembling that observed in 8, but one coordination site around Ag(I) is replaced by nitrogen atom of bpy. In addition, versatile weak Ag-O interactions are also examined in this compound, which add more deviation from a perfect T-shaped coordination sphere for Ag(I) centers than that found in 8. The coordination numbers of silver(I) cation for carboxylate group further decreases from 3 in 8 to 2 in 3 or 1 and 2 in 2, while complete displacement occurs in compounds 4-7. Only weak Ag-OCOO interactions are found in 4-7. It has early been pointed out that -COO- can coordinate to more Ag(I) centers than 4,40 -bpy and the decrease in Ag-COO dimensionality occurs as a consequence of the introduction of bpy into the parent compound. The 2D covalent layer of Ag(I) cations and dpa2- dianions in 8 is transformed into 1D covalent chain in 1, supramolecular chain in 2, isolated eight-membered ring in 3 and ion pairs in 4-7. In a word, the introduction of bpy into the parent

Thermogravimetric Analyses Thermogravimetric analyses show that compounds 2-7 all have a low thermal stability as illustrated by Figure 7. There is no obvious weight loss before 100 °C in the TGA curve of compound 2. Beyond this temperature, the loss of water began and was completed at 190 °C with a weight loss of 2.35% (calcd. 2.54%). Closely followed by this water departure process, a second weight loss (observed 67.5%) was observed with two indiscerptible steps which can be attributed to the combustion of bpy (calcd. 33.08%) and dpa2- (calcd. 33.92%). As to the most stable compound 3 in this work, it can be stable up to 250 °C. Beyond this temperature, the combustion of Hdpa- with a sharp weight loss of 53.75% (calcd. 56.47%) occurred in the range 250-278 °C and that of bpy with a comparatively mitigable weight loss of 17.48% (calcd. 17.02%) occurred in the range 278-500 °C were observed. As to two supramolecular isomers, compounds 4 and 5, they show similar decomposition behavior but a small difference in stability temperature was observed, which is presumably

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absence of ligand-based emission suggests an energy transfer from the Ag(I) center to the ligands during photoluminescence. Therefore, the emissions may be assigned to a MLCT from the silver(I) to the ligand. This phenomenon has already been observed for a series of silver(I) complexes supported by 4,40 -bipyridine bridges incorporating different anionic sulfonate coligands.31b It is interesting to note that a great difference in luminescent emissions of 4 and 5 occurs which again provides evidence for distinguishing the two supramolecular isomers. Compounds 2-7 may be good candidates for photoactive materials due to their strong fluorescent emissions. Conclusions Figure 8. Fluorescent emission spectra of compounds 2-7 in the solid state at room temperature.

derived from the different degrees of π 3 3 3 π stacking interactions. The stability temperatures for compounds 4 and 5 are 200 and 230 °C, respectively. Over these temperatures, the organic ligands began to combust and the frameworks were thus collapsed. As there are more water molecules in the crystals of compounds 6 and 7 compared to the above compounds, they show lower thermal stability owing to the facile release of water molecules from the framework. The departure process of the lattice water molecule in compound 6 was completed under 190 °C, which can be divided into two steps corresponding to four water molecules below 92 °C (observed, 5.35% calcd. 5.31%) and five in the temperature range 92-190 °C (observed, 4.33%, calcd. 4.24%). This fact confirmed the different hydrogen bond strengths existing between water molecules and the carboxylate oxygen atoms of dpa2-, which is consistent with structural analysis results. Beyond 190 °C, a sharp weight loss was observed and the framework was collapsed. In contrast to 6, the release of six water molecules per formula of compound 7 was observed at higher temperature (222 °C) with a weight loss of 6.47%, which compare well with the calculated value of 7.47%. Beyond this temperature, two indiscerptible processes with a total weight loss of 55.19% corresponding to the combustion of bpy (calcd. 40.87%) and dpa2- (calcd. 15.72%) was observed, which resulted in the collapse of the supramolecular framework. Fluorescence Properties Ag(I) compounds usually emit weak photoluminescence at low temperatures, and only a few Ag(I) compounds exhibiting luminescent properties at room temperature have been reported. Interestingly, compounds 2-7 exhibit different fluorescent properties in the solid state at ambient temperature as shown in Figure 8. Almost similar emission shapes and close emission bands of 2 (540 nm with λex = 390 nm), 3 (530 nm with λex = 380 nm), and 5 (520 nm with λex = 380 nm) were observed except that the peak of 5 is much broader than the other two. Both compounds 6 and 7 give sharp green emission bands centered at 550 nm (λex = 390 nm) for 6 and 538 nm for 7 (λex = 370 nm), whose emission shapes differ greatly from those of 2, 3, and 5. It is notable that compound 4 gives two strong peaks at 520 and 590 nm upon excitation at 405 nm. The above results reveal that the fluorescent emissions arising from free H2dpa and bpy (intense emissions at 459 nm with λex = 310 nm for free H2dpa and at 436 nm with λex = 366 nm for free bpy, which are probably attributed to the π* f π or π* f n transitions.31) are not observable for these compounds. The

In conclusion, a series of Ag/bpy/(H)dpa supramolecular assemblies, 1-7, have been synthesized and characterized. All these compounds are synthesized by introducing bpy into their parent compound and through controlled syntheses, illustrating that the structures of these compounds may be greatly influenced by the secondary bpy ligand, the molar ratio of components, and the pH value of the reaction. The structures of these compounds are characterized either by 1D chain substructures (compounds 1-5) or by 2D layer substructures (compounds 6 and 7), illustrating the rich supramolecular chemistry of the Ag/bpy/(H)dpa family. And various interactive modes between Ag(I) cations and Hdpa/dpa also have been observed in this work, showing that the Ag(I) cation has a tendency to adopt various bonding modes and coordination geometries. Most interesting about this work is that two true supramolecular isomers 4 and 5 are also observed with serendipity. Luminescent measurements reveal that compounds 2-7 are good candidates for photoactive materials owing to their strong luminescent emissions. Therefore, new silver(I) compounds of the ternary Ag/L/(H)dpa system with intriguing structures and physical properties can be obtained using other polydentate N-donor ligands L and/or altering external stimulus. Work on research is underway in our laboratory. Acknowledgment. We thank the support of this work by the 973 Program of China (2007CB815302, 2009CB939803), The Chinese Academy of Sciences (KJCX2.YW.319), (KJCX2-YW-M05), the Knowledge Innovation Program of the Chinese Academy of Sciences, and the Fund of Fujian Key Laboratory of Nanomaterials (2006L2005). Supporting Information Available: Crystallographic data in CIF format for compounds 1-7; table of hydrogen-bonding geometric parameters for compounds 2-7. These materials are available free of charge via the Internet at http://pubs.acs.org.

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Layer Substructures in

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