Introduction

The growing interest in the design and assembly of silver-based coordination complexes is not only because of their fascinating molecular structures [1–4], but also because of their distinctive chemical and physical properties [5–16]. The geometrical flexibility of silver(I) results in intricate coordination architectures and also affords an opportunity to explore how the self-assembly of silver-based coordination compounds can be influenced by factors such as the structural characteristics of polydentate organic ligands, the metal–ligand ratio, solvents, and the counterions [1–4, 9–11]. It is noteworthy that counterions, especially anions, often play important roles in determining the structures of silver-based coordination compounds. The commonly used anions include BF4 −, ClO4 −, PF6 −, NO3 −, CF3SO3 −, and CF3CO2 −, which can participate in coordinated, uncoordinated, or mixed modes [17, 18]. The size, coordination ability, and supramolecular interactions of these anions exert different influences on the final Ag(I) coordination compounds. Compared to inorganic anions, the organic carboxylate anions are more numerous and versatile; we have, therefore, chosen to focus our efforts on the synthesis of silver complexes containing different organic carboxylate anions in order to explore how the self-assembly process can be influenced by these organic anions [2–4, 19].

Herein, we present six silver coordination compounds, namely [Ag3(bpy)3](su)·10H2O (1), [Ag2(bpy)2](tp)]·6H2O (2), [Ag2(dpe)2(H2O)2](dpadc)·H2O (3), [Ag6(dpe)6(H2O)4](tp)3·12H2O (4), [Ag(bpp)](naa) (5), and [Ag2(bpp)2](dpadc)·6H2O (6) constructed via self-assembly of silver(I) metal salts with N-donor ligands, namely 4,4′-bipyridine (bpy), 1,2-di(4-pyridyl)ethane (dpe) and 1,3-bis(4-pyridyl)propane (bpp), and organic carboxylate anions, like succinic acid (H2su), terephthalic acid (H2tp), 2,2′-diphenylaminedicarboxylic acid (H2dpadc), and naphthaleneacetic acid (Hnaa) (Scheme 1), in order to investigate the influence of rigid and flexible N-donor ligands and organic carboxylate anions on the crystal structures of the resulting silver-based coordination compounds.

Scheme 1
scheme 1

The structural formulae of bpy, dpe, bpp, H2su, H2tp, Hnaa, and H2bpadc

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Experimental

All chemicals were commercially available reagent grade and used without further purification. Elemental analysis for the title complexes was performed using an Elementar Vario EL-III instrument. FTIR spectra, in the region (400–4,000 cm−1), were recorded on a Perkin Elmer Spectrum 100 Fourier Transform infrared spectrophotometer.

Synthesis of [Ag3(bpy)3](su)·10H2O (1)

An ammonia solution (25 mL, 0.5 mol/L) containing AgNO3 (0.0085 g, 0.05 mmol) and H2su (0.006 g, 0.05 mmol) was added dropwise to an EtOH solution (25 mL) of bpy (0.0078 g, 0.05 mmol). The clear mixture was stirred for a few minutes and then allowed to evaporate slowly at room temperature. Block-like colorless crystals of [Ag3(bpy)3](su)·10H2O (1) were obtained after several weeks. Anal. Calcd. for C36H50Ag3N6O16 (%): C, 37.7; H, 4.4; N, 7.3. Found: C, 37.8; H, 4.5; N, 7.3. IR (KBr)/cm−1: 3405 m, 3030w, 2947w, 2925w, 2859w, 1933w, 1605s, 1558s, 1494s, 1414m, 1295m, 1218m, 1177w, 1076w, 1012w, 991w, 918m, 881m, 827m, 808m, 655m, 610w, 546m, 423m, 408m.

Synthesis of [Ag2(bpy)2](tp)]·6H2O (2)

Synthesis of block-like colorless crystals of [Ag2(bpy)2](tp)]·6H2O (2) followed the same procedure as for 1, except that H2su was replaced with H2tp. Anal. Calcd. for C28H30Ag2N4O10 (%): C, 42.1; H, 3.8; N, 7.0. Found: C, 42.2; H, 3.8; N, 7.1. IR (KBr)/cm−1: 3402m, 3051m, 3028m, 1953w, 1598s, 1573s, 1526m, 1496m, 1487w, 1399s, 1376s, 1313w, 1222m, 1089m, 1040w, 1071w, 1013w, 1001w, 993w, 962w, 883w, 846m, 824s, 805s, 743s, 621s, 564m, 508s, 451w.

Synthesis of [Ag2(dpe)2(H2O)2](dpadc)·H2O (3)

Synthesis of block-like colorless crystals of [Ag2(dpe)2(H2O)2](dpadc)·H2O (3) followed the same procedure as for 1, except that H2su and bpy were replaced with H2dpadc and dpe, respectively. Anal. Calcd. for C38H35Ag2N5O7 (%): C, 45.7; H, 4.2; N, 6.7. Found: C, 45.8; H, 4.2; N, 6.7. IR (KBr)/cm−1: 3433s, 1601s, 1571m, 1557s, 1498m, 1450w, 1384s, 1272m, 1204w, 1146w, 1073w, 1042w, 1009w, 997m, 972m, 848w, 826m, 735m, 697w, 548m, 405m.

Synthesis of [Ag6(dpe)6(H2O)4](tp)3·12H2O (4)

Synthesis of block-like colorless crystals of [Ag6(dpe)6(H2O)4](tp)3·12H2O (4) followed the same procedure as for 1, except that H2su and bpy were replaced with H2tp and dpe, respectively. Anal. Calcd. for C96H104Ag6N12O28 (%): C, 44.9; H, 2.8; N, 8.7. Found: C, 44.9; H, 2.9; N, 8.8. IR (KBr)/cm−1: 3272m, 2926w, 2861w, 1936w, 1604s, 1561s, 1519m, 1498m, 1455w, 1434m, 1407s, 1377s, 1296m, 1218m, 1100m, 1075w, 1012w, 1006w, 991m, 926m, 860m, 831m, 809m, 802m, 778m, 754w, 726s, 613m, 603w, 577m, 507m, 404m.

Synthesis of [Ag(bpp)](naa) (5)

Synthesis of block-like colorless crystals of [Ag(bpp)](naa) (5) followed the same procedure as for 1, except that H2su and bpy were replaced with H2naa and bpp, respectively. Anal. Calcd. for C25H23AgN2O2 (%): C, 61.1; H, 4.7; N, 5.7. Found: C, 61.2; H, 4.8; N, 5.7. IR (KBr)/cm−1: 3416m, 2926w, 2860w, 1604s, 1565s, 1497w, 1455w, 1421m, 1384s, 1257w, 1219w, 1075w, 1006w, 802m, 793m, 785m, 613w, 538w, 512m, 417w.

Synthesis of [Ag2(bpp)2](dpadc)·6H2O (6)

Synthesis of block-like colorless crystals of [Ag2(bpp)2](dpadc)·6H2O (6) followed the same procedure as for 1, except that H2su and bpy were replaced with H2naa and bpp, respectively. Anal. Calcd. for C40H49Ag2N5O10 (%): C, 49.3; H, 5.1; N, 7.2. Found: C, 49.3; H, 5.1; N, 7.2. IR (KBr)/cm−1: 3436m, 1604s, 1571s, 1558s, 1499s, 1455w, 1420m, 1393s, 1273m, 1219w, 1210w, 1148w, 1070w, 1041w, 1024w, 1006w, 849m, 809m, 802m, 753m, 734m, 698s, 681w, 613w, 512m, 405m.

X-ray crystallography

Diffraction intensities for all six complexes were recorded with a Bruker CCD area detector diffractometer with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) using φ-ω mode at 298(2) K. Semi-empirical absorption corrections were applied using the SADABS program [20]. The structures were solved by direct methods [21] and refined by full-matrix least-squares on F 2 using SHELXS 97 and SHELXL 97 programs, respectively [21, 22]. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in geometrically calculated positions. Crystallographic data and structural refinements for the complexes are summarized in Table 1. Selected bond lengths and angles are listed in Table 2.

Table 1 Details of X-ray data collection and refinement for the complexes
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Table 2 Selected bond lengths (Å) and angles (°) for compound (1), (2), (3), (4), (5), and (6)
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Results and discussion

Crystallographic analysis of complex (1)

The crystal structure reveals that [Ag3(bpy)3](su)·10H2O (1) is made up of infinite chains of [Ag3(bpy)3] 3n+ n cations, su2− anions, and H2O molecules, as illustrated in Fig. 1a. In the cationic chains of [Ag3(bpy)3] 3n+ n , both the Ag(1) and Ag(2) atoms have linear coordination geometry involving the nitrogen atoms from two different bpy ligands [Ag–N ranging from 2.153(3) to 2.157(3) Å; N–Ag–N from 159.34(12)° to 166.40(12)°], forming a simple topology of single-strand chain. The Ag(3) atoms, in slightly distorted T-shaped geometry, are coordinated by nitrogens from two different bpy ligands [Ag–N 2.184(3) and 2.193(3) Å; N–Ag–N 170.35(12)°] and by an oxygen atom from a COO− group [Ag–O 2.483(3) Å; N–Ag–O 99.09(11)° and 96.05(11)°], as illustrated in Fig. 1a and Table 2. The typical ranges of Ag–N and Ag–O bond distances are 2.11–2.63 Å and 2.3–2.6 Å [23–25], respectively. The oxygen atoms of the COO− groups of the su2− anions interact with the Ag centers through weak Ag…O interactions [Ag(1)…O(1) and Ag(1)…O(2) = 2.708(3) and 2.647(3) Å, respectively; Ag(2)…O(7) = 2.713(3) Å]. The Ag…O distances are thus shorter than their van der Waals contact distance of 3.24 Å [12]. 4,4-Bipyridine (bpy) acts as typical bidentate ligand, linking two Ag atoms via the nitrogens from two pyridyl rings, and the dihedral angles between the pyridyl rings of the same bpy are all ca. 32°. Deprotonated su2− acts as a counterion, balancing the cationic charge of the [Ag3(bpy)3] 3n+ n chains.

Fig. 1
figure 1

a Ortep view of the structure of [Ag3(bpy)3](su)]·10H2O (1) with atomic labeling of one asymmetric unit. Lattice water molecules and H atoms are omitted for clarity. b. Ag…Ag interactions between neighboring [Ag(bpy)] n+∞ chains in complex (1) (Ag…Ag ranging from 3.529(2) to 3.589(2) Å). c. Packing view of the sandwich-like framework built from anionic sheets (B) and cationic sheets (A) along the a-axis for 1

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The adjacent cationic chains of [Ag(1)(bpy)] n+n , [Ag(2)(bpy)] n+n , and [Ag(3)(bpy)] n+n are connected by ligand-unsupported Ag…Ag interactions (Ag(1)…Ag(2) and Ag(2)…Ag(3) = 3.3044(6) Å and 3.3188(6) Å, respectively) into 2D cationic sheets, as shown in Fig. 1b. The deprotonated su2− anions are linked into anionic sheets with the aid of lattice water molecules via intermolecular hydrogen-bonding interactions, as listed in Table 3. The neighboring cationic and anionic sheets are further joined into a 3D sandwich-like framework by weak Ag…O interactions, plus π–π stacking interactions with centroid–centriod distances ranging from 3.529(2) to 3.589(2) Å (Table 4) and electrostatic interactions. Viewed from the c-axis, it can be seen that the anionic sheet is inserted into the two cationic sheets to form a typical sandwich-like framework.

Table 3 Hydrogen bonds for compound (1), (2), (3), (4), and (6) [Å and °]
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Table 4 Defined ring and relative parameters of the π–π interactions in complexes (1) and (3)
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Crystallographic analysis of complex (2)

[Ag2(bpy)2](tp)·6H2O (2) is made up of infinite chains of [Ag2(bpy)2] 2n+n cations, tp2− anions, and H2O molecules, as illustrated in Fig. 2a. In the chains of [Ag2(bpy)2] 2n+n cations, the AgI atoms, in a linear coordination geometry, are coordinated by the two nitrogen atoms from two different bpy ligands [Ag–N bond distances of 2.173(6)–2.182(6) Å; N–Ag–N 176.1(2)°]. The two pyridyl rings of the same bpy ligand are coplanar, different to the bpy ligands in complex (1).

Fig. 2
figure 2

a Asymmetric unit of [Ag2(bpy)2](tp)·6H2O (2) and coordination environments around the AgI atoms. The position occupancy factor ratios of C(11)/C(11A), C(12)/C(12A), O(3)/O(3A), O(4)/O(4A), O(5)/O(5A), and O(6)/O(6A) are 0.50/0.50 (symmetry code: A −x + 3/2, y + 1/2, z). b Packing view of the sandwich-like framework built from anionic and cationic sheets along the c-axis for complex (2)

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The oxygen atoms of the waters interact with the AgI centers through weak Ag…O interactions [Ag(1)…O(1) and Ag(1)…O(2) = 2.633(8) and 2.897(10) Å, respectively]. The deprotonated tp2− anions balance the cationic charges of the [Ag(bpy)] n+n chains. The COO− groups are disordered over two positions, such that C(11), C(12), and their corresponding O atoms were split during refinement resulting in a site occupancy factor ratio of 0.50/0.50. The oxygen atoms of the lattice water molecules are also disordered over two positions, such that the site occupancy factor ratios of both O(1)/O(2) and O(7)/O(8) are 0.5/0.5.

Viewed from the c-axis, the anionic sheets built up of deprotonated tp2− anions and lattice water molecules via intermolecular hydrogen-bonding interactions (Table 3) are inserted between the two cationic sheets to form a typical sandwich-like framework. No apparent Ag…Ag, Ag…N, or π-π stacking interactions are found in complex 2, which is different from the previously reported similar complexes [2–4, 19].

Crystallographic analysis of complex (3)

As illustrated in Fig. 3a, in the complex [Ag2(dpe)2(H2O)2](dpadc)·H2O (3), the Ag(1) and Ag(2) atoms are coordinated in distorted T-shaped coordination geometries by the nitrogen atoms from two different dpe ligands [Ag(1)–N = 2.143(3) and 2.146(3) Å; and Ag(2)–N = 2.170(3) Å and 2.175(3) Å; N–Ag(1)–N = 161.24(12)°; N–Ag(2)–N = 157.45(13)°], comparable to the Ag–N distances in previously reported complexes [1–4, 19], and the oxygen atom of an aqua ligands [Ag(1)–O(1) = 2.553(3) and Ag(2)–O(3) = 2.467(3) Å]. The oxygen atoms of the aqua ligands also interact with the Ag(2) centers through weak Ag…O interactions [Ag(2)…O(2) = 2.667(3) Å]. The dihedral angles between the two pyridyl rings of the same dpe, which acts as typical bidentate linker, are 4.475(103)°, and the dihedral angle between the two benzene rings of the dpadc2−, which acts as counterion to balance the charge of the cationic [Ag2(dpe)2(H2O)2] 2n+n chains, is 44.925(112)°.

Fig. 3
figure 3

a Asymmetric unit of [Ag2(dpe)2(H2O)2](dpadc)·H2O (3) and coordination environments around the AgI atoms. b Packing view of the sandwich-like framework built from anionic and cationic sheets along the c-axis for (3)

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In the crystal structure of the complex (3), no apparent Ag…Ag or Ag…N interactions are found, which is different from similar complexes [2–4, 19]. The adjacent chains are interconnected by π–π stacking interactions with centroid–centriod distances of 3.665(2) Å (Table 4) and the dpadc2− counterions via electrostatic interactions to build up a 3D sandwich-like network. The lattice water molecules are held within the framework by hydrogen-bonding interactions (Table 3).

Crystallographic analysis of complex (4)

The crystal structure reveals that [Ag6(dpe)6(H2O)4](tp)3·12H2O (4) is made up of infinite cationic chains of [Ag6(dpe)6(H2O)4] 6n+n , tp2− anions, and H2O molecules, as illustrated in Fig. 4a. In the cationic chains of [Ag6(bpy)6(H2O)4] 6n+n , the Ag(1) and Ag(2) atoms, in slightly distorted T-shaped geometry, are coordinated by two nitrogen atoms from two different dpe ligands [Ag(1)–N = 2.126(5) and 2.135(5) Å; Ag(2)–N = 2.141(5) and 2.146(5) Å; N–Ag(1)–N = 165.5(2)° and N–Ag(2)–N = 162.3(2)] and by an oxygen atom from an aqua ligand [Ag(1)–O = 2.550(5) Å and Ag(2)–O = 2.465(5) Å], as illustrated in Fig. 1a and Table 2. Meanwhile, the Ag(3) atoms, in linear coordination geometry, are coordinated by two nitrogen atoms from two different dpe ligands [Ag–N = 2.128(5) and 2.131(5) Å; N–Ag1–N = 165.5(2)° and N–Ag2–N = 167.0(2)°]. The oxygen atom of the aqua ligand interacts weakly with Ag(3) ions [Ag(2)…O(2) = 2.609(6) Å]. In all the dpe ligands, the two pyridyl rings are nearly coplanar, the corresponding dihedral angles being 0.581(270)°, 6.734(280),° and 1.779(229)°.

Fig. 4
figure 4

a Asymmetric unit of [Ag6(dpe)6(H2O)4](tp)3·12H2O and coordination environments around the AgI atoms. b The ligand-unsupported Ag…N interactions between the adjacent cationic [Ag(dpe)] n+n chains. c Packing view of the sandwich-like framework built from anionic and cationic sheets along the a-axis for (4)

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The adjacent cationic [Ag(dpe)] n+n chains are connected by Ag…N interactions (Ag…N contacts ranging from 3.6528(56) to 3.9604(56) Å) into 2D cationic sheets, as shown in Fig. 4b. The deprotonated tp2− anions are joined into anionic sheets with the aid of lattice water molecules via intermolecular hydrogen-bonding interactions, as depicted in Table 3. The neighboring cationic and anionic sheets are further joined into a 3D sandwich-like framework by hydrogen-bonding and electrostatic interactions.

Crystallographic analysis of complex (5)

The crystal structure reveals that [Ag(bpp)](naa) (5) is made up of infinite sinusoidal cationic chains of [Ag(bpp)] n+n and naa− anions, as illustrated in Fig. 5a. In the cationic chains of [Ag(bpp)] n+n , the Ag atoms are disordered over two positions; hence, the Ag(1) and Ag(1)′ atoms were split during refinement resulting in a site occupancy factor ratio of 0.74(2)/0.26(2). Both Ag(1) and Ag(1)′, in linear coordination geometry, are coordinated by the nitrogen atoms from two different bent bpp ligands [Ag(1)–N = 2.173(8) and 2.207(8) Å; Ag(1)′−N = 2.214(12) and 2.116(11) Å; N–Ag(1)–N = 154.7(5)° and N–Ag(1)′–N = 161.4(6)°], as illustrated in Fig. 5a and Table 2. The oxygen atoms of the naa− anions interact with the AgI centers through weak Ag…O interactions [Ag…O = 2.681(17) and 2.771(15) Å], which are shorter than their van der Waals contacts distance of 3.24 Å. The bent bpp ligand acts as a flexible linker to join two AgI atoms, such that the dihedral angle between the two pyridyl rings is 70.649(224)°, while the naa− anions provide charge compensation.

Fig. 5
figure 5

a Asymmetric unit of [Ag(bpp)](naa) (5) and coordination environments around the AgI atoms. b The ligand-unsupported Ag…Ag interactions between adjacent cationic [Ag(bpp)] n+n chains. c Packing view of the sandwich-like framework built from anionic and cationic sheets along the c-axis for (5)

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The adjacent cationic [Ag(bpp)] n+n chains are connected by ligand-unsupported Ag…Ag interactions (Ag(1)–Ag(1)(−x + 1, −y + 1, −z + 1) = 3.040(9) Å and Ag(1)′–Ag(1) (−x + 1, −y + 1, −z + 1) = 2.854(19) Å) into 2D cationic sheets. The deprotonated naa− anions contact the cationic [Ag(bpp)] n+n chains via weak Ag…O interactions to build a 3D sandwich-like crystal structure, as depicted in Table 3.

Crystallographic analysis of complex (6)

In [Ag2(bpp)2](dpadc)·6H2O (6), the Ag(1) atoms have a linear coordination geometry involving the nitrogen atoms from two different bent bpp ligands [Ag–N bond distance being 2.131(7)–2.147(7) Å; N–Ag–N 178.3(3)°], forming simple sinusoidal cationic chains. The oxygen atoms of aqua ligands interact with the AgI centers through weak Ag…O interactions [Ag(1)…O(1) = 2.781(8) Å], while the Ag(2) atoms, in slightly distorted T-shape, are ligated by the nitrogen atoms from two different bpp ligands [Ag–N = 1.131(8) and 2.142(8) Å, N–Ag–N = 167.5(5)°] and the oxygen from an aqua ligand [Ag–O = 2.600(11) Å, N–Ag–O = 99.0(4) and 93.5(4)°]. The dihedral angle between the two pyridyl rings of bpp linkers is 73.41(29),° comparable to that in complex (5). The deprotonated dpadc2− anions balance the charge of the [Ag2(bpp)2] 2n+n cationic chains, as illustrated in Fig. 6. And the dihedral angle between the two benzene rings of dpadc2− is 45.152(34)°, similar to the angles in complex (3).

Fig. 6
figure 6

a Asymmetric unit of [Ag2(bpp)2](dpadc)·6H2O (6) and coordination environments around the AgI atoms. b Packing view of the sandwich-like framework built from anionic and cationic sheets along the c-axis for (6)

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In the complex (6), no apparent Ag…Ag, Ag…N, or π–π stacking interactions are found, which is different from similar complexes [2–4, 19]. The adjacent chains are interconnected by the dpadc2− counterions via electrostatic interactions to build up a 3D sandwich-like network. The lattice water molecules are held within the framework and stabilized by hydrogen-bonding interactions (Table 3).

Some complexes of silver(I) with bpy-like ligands but different counterions have been reported previously [1–4, 19]. The coordination modes of the ligands and the supramolecular interactions of the anions both help to determine the crystal structures of such complexes. Generally, the counterions can be present in coordinated, uncoordinated, or mixed modes. Coordinated anions normally increase the dimensionality of the crystal structures, while uncoordinated anions may help to extend the crystal structures via hydrogen bonding, π–π stacking, and/or ligand-unsupported Ag…Ag and Ag…N interactions. For example, in Ag(bpe)2(bpdc)2 (bpe = 1,2-bis(4-pyridyl)ethane, H2bpdc = 2,2′-bipyridine-3,3′-dicarboxylic acid), the bpdc2− acts as a coordinated counterion, linking the Ag(I) atoms into a 3D framework along with the bpe ligands [19]. In construct, in [Ag2(bpe)2](bdc)·8H2O (H2bdc = 1,3-benzenedicarboxylic acid), bdc2− only plays the role of an uncoordinated counterion to balance the charge of the 1D cationic [Ag2(bpe)2]2+ chains. Again, the 3D sandwich-like structure of [Ag2(bpe)2](bdc)·8H2O is constructed with the aid of Ag…Ag, Ag…N, and hydrogen-bonding interactions [19].

Conclusions

The six silver(I) complexes reported here all contain novel sandwich-like frameworks, showing that the different anions play an important role in determining the crystal structures. In these complexes, the rigid bpy and flexible dpe/bpp act as bidentate ligands to join the Ag(I) centers into 1D cationic chains, balanced by the different anions, like su2−, tp2−, dpadc2−, and naa−. The coordination numbers of silver are two and three, resulting in linear (complex (1–2), (4–6)) or T-shape (complexes (1), (3), (4), and (6)) geometries. All the organic carboxylate anions only act as counterions to balance the charge of the cationic [Ag(L)] n+n chains. In complexes (1), (4), and (5), the ligand-unsupported Ag…Ag and Ag…N interactions facilitate the formation of 3D sandwich-like structures, generally supported by rich hydrogen-bonding interactions. In complexes (1) and (3), besides the abundant hydrogen bonds, π–π stacking interactions also contribute to the formation of a 3D sandwich-like framework.

Supplementary material

CCDC 860827-860832 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.