Syntheses and crystal structures of two new silver–organic frameworks based on N-pyrazinesulfonyl-glycine: weak Ag···O/N interaction affecting the coordination geometry

2.1 Description of the crystal and molecular structure of [Ag₂(L)]_n (1)

The single-crystal X-ray diffraction structural analysis has revealed that 1 is a 3D coordination polymer, whose asymmetric unit comprises two crystallographically nonequivalent Ag(I) cations and a doubly deprotonated chelating L²⁻ ligand. As is shown in Fig. 1a, two unique Ag(I) ions display two types of coordination modes (neglecting the weak Ag···O and Ag···Ag interactions). Ag1 is coordinated by two nitrogen atoms (Ag1–N1=2.222(5) Å, Ag1–N2ⁱ=2.249(5) Å, symmetry code: i: x–1/2, –y+1/2, z+1/2) from two different L²⁻ anions with an N1–Ag1–N2ⁱ bond angle of 168.67(19)°, whereas Ag2 is ligated by two oxygen donors (Ag2–O2ⁱⁱ=2.310(5) Å, Ag2–O2^iv=2.363(4) Å) and two nitrogen atoms (Ag2–N1=2.399(5) Å, Ag2–N3ⁱⁱⁱ=2.327(6) Å) (symmetry codes: ii: x–1, y, z; iii: x–1/2, –y+1/2, z–1/2; iv: –x+2, –y, –z) from four discrete L²⁻ ligands (Table 1). Ag2 adopts distorted tetrahedral coordination geometry with the bond angles spanning the range from 87.08(16)° to 137.9(2)°, with τ₄=0.4 for Ag2. The distortion of the tetrahedron can be indicated by the calculated value of the τ₄ parameter introduced by Houser [26] to describe the geometry of a four-coordinated transition metal system (for perfect tetrahedral geometry, τ₄=1). The Ag–O/N bond lengths are comparable to those observed in related silver(I) compounds [22], [23], [24], [25]. However, the distances Ag1–O1 (2.570(5) Å), Ag1–O3ⁱ (2.660(7) Å), and Ag1–O4^vii (2.741(5) Å) (symmetry code: vii: x+1/2, –y+1/2, z+1/2) are beyond the range of 2.32–2.52 Å for silver(I) carboxylates but still shorter than the sum of van der Waals radii (3.24 Å) of the Ag and oxygen atoms, suggesting the existence of significant Ag···O interactions and explaining the deviation of the Ag(I) center from perfect linear-shaped geometry. Within this unit, the Ag1···Ag2 distance is 2.964(8) Å, implying the existence of ligand-supported argentophilic interactions. Taking weak Ag–O interactions into account, the coordination mode of L²⁻ can be described as μ₇-(η₂-O,N), (η₂-O′,N′), O,O″,O″′,N,N″ in 1, which has never been reported in Ag-containing compounds. A better insight into the nature of this intricate heptanuclear silver cluster can be achieved by removing the organic L²⁻ ligand. Four Ag ions (Ag1ⁱⁱⁱ/Ag1^vi/Ag2/Ag2^v) are co-planar and form an Ag₄ plane with the Ag1ⁱⁱⁱ/Ag2=Ag1^vi/Ag2^v and Ag1ⁱⁱⁱ/Ag1^vi=Ag2/Ag2^v (for symmetry codes see Fig. 1a) separations of 5.19(7) and 5.92(1) Å, respectively. The adjacent Ag1 ions are connected by one L²⁻ to give an [Ag(L)] covalent chain along the c axis (Fig. 1b), which is further extended by the bonds Ag2–N1 and Ag2–N3 leading to a layer. As shown in Fig. 1c, an overall 3D supramolecular framework results from the linkage of neighboring layers through Ag2–O2 bonds. From the topological point of view, each Ag₇ cluster links three L²⁻ units as a three-connected node, and each L²⁻ ligand bridges two Ag₇ clusters acting as a linker. The 3D MOF topology of 1 can be classified as a three-connected (6, 3) net (Fig. 1d). There are no remarkable hydrogen bonds or π stacking interactions in 1, but a weak non-classical C–H···O contact (Table 2).

Fig. 1:

(a) The coordination environment for Ag⁺ in 1 (symmetry codes: i: x–1/2, –y+1/2, z+1/2; ii: x–1, y, z; iii: x–1/2, –y+1/2, z–1/2; iv: –x+2, –y, –z; v: x+1, y, z; vi: x+1/2, –y+1/2, z–1/2; vii: x+1/2, –y+1/2, z+1/2). (b) The 1D coordination framework of Ag1 in 1. (c) View of the 3D coordination grid framework along the a axis (symmetry codes: i: x–1/2, –y+1/2, z+1/2; iv: –x+2, –y, –z; v: x+1, y, z; viii: –x+5/2, y+1/2, –z+1/2; ix: x+3/2, –y+1/2, z–1/2). (d) The simplified topology (6, 3) considering the Ag₇ cluster as a node and the linkers representing L²⁻.

2.2 Description of the crystal and molecular structure of {[Ag_2.5(L)(4,4′-bipy)₂]· (NO₃)_0.5·(H₂O)_4.5}_n (2)

Single-crystal X-ray diffraction analysis has revealed that 2 crystallizes in the triclinic space group P1̅ as a 2D supramolecular network generated from a combination of coordinative bonds, ligand-supported Ag···Ag interactions, and weak Ag···O/N coordinative interactions. The asymmetric unit of 2 consists of two and a half crystallographically unique Ag(I) ions, one L²⁻ dianion, a half of an NO₃⁻ ion, two 4,4′-bipy ligands, as well as four and a half solvent water molecules. As shown in Fig. 2a, the Ag1 and Ag2 atoms (neglecting the weak Ag···O, Ag···N, and Ag···Ag interactions) adopt a distorted linear geometry with two nitrogen atoms from one L²⁻ and one bpy for Ag1, and two different bpy for Ag2, whereas Ag3 is located at a center of symmetry, connecting two symmetry-related bpy with a bond N7–Ag3–N7ⁱⁱ angle of 180° (symmetry code: ii: –x+2, –y+4, –z+2). The preferred coordination of the silver ions to the pyridyl sites is manifested by the relatively short Ag–N bond distances. In 2, these are Ag1–N 2.224(4) and 2.281(4) Å, Ag2–N 2.164(4) and 2.170(4) Å, and Ag3–N 2.147(4) Å, with additional binding of Ag1 and Ag2 to the carboxylate, sulfonyl, and imine function of the bridging ligand L²⁻ (Fig. 1a). The inversion-related Ag1 pair is coordinated from both sides in a μ₂-η¹:η¹ bridging mode by carboxylate and imine groups at relatively long Ag–O and Ag–N distances of 2.566(5) and 2.626(4) Å, with an Ag···Ag separation of 2.8777(9) Å, implying the existence of ligand-supported argentophilic interactions. Ag2 is coordinated in a monodentate fashion to one of the sulfonyl O atoms at Ag2–O=2.66(8) Å, forming [Ag₄L₂] secondary building units (SBUs between adjacent [Ag-bpy] polymeric chains). Ag3 lies at an inversion center, coordinated by two symmetry-related bpy in a μ₂-η¹:η¹ bridging mode. Taking weak Ag–O/N interactions into account, the coordination mode of L²⁻ can be denoted as μ₃-(η₂-O,N),N,O′ in 2. There are open voids within the 2D array (Fig. 2b), as there is an alternating connection with bpy between the Ag1, Ag2, and Ag3 metal nodes of neighboring chains in the layer. These voids contain solvent molecules and NO₃⁻ anions. From the topological point of view, each [Ag₄L₂] SBU links four bpy and two Ag(bpy)₂ as a six-connected node, and each bpy or Ag(bpy)₂ bridges two [Ag₄L₂] acting as a linker. The 2D MOF structure of 2 can be simplified as a six-connected (3, 6) grid layer topology (Fig. 1c). On the other hand, each [(Ag₄L₂)(bpy)]₂ unit serves as a four-connected node, and each [Ag(bpy)₂]⁺ acts as a linker. The 2D MOF structure of 2 can be simplified as a four-connected (4, 4) grid layer topology.

Fig. 2:

(a) The coordination environment for Ag⁺ in 2 (symmetry codes: i: –x+1, –y, –z+1; ii: –x+2, –y+4, –z+2; iv: –x+1, –y+1, –z+1). (b) The Ag-pby chain in 2 (symmetry codes: i: –x+1, –y, –z+1; ii: –x+2, –y+4, –z+2; iv: –x+1, –y+1, –z+1; viii: x, y–1, z). (c) The simplified representation of the 2D (3, 6) grid network (green lines represent bpy ligand, brown lines represent Ag(bpy)₂) chains. (d) View of O–H···O hydrogen bonded rings in 2.

The stacking between the 2D polymeric arrays in structure 2 is further stabilized by numerous hydrogen bonding interactions, involving the water molecules, the NO₃⁻ anions, and the carboxylate groups. Intermolecular O–H···O hydrogen bonds constitute R816,R48,R34, and R24 rings [27] with the solvate H₂O (O8, donor and acceptor; O9, O10, O11, and O12, donors), the nitrate O atoms (O5 and O7, acceptors), and the carboxylate moiety (O1 and O2, acceptors). O12···O1 and H12A···O1 distances of 4.39(8) and 3.84(3) Å, respectively, are too long to form an effective hydrogen bond, resulting in an open-loop R410. These rings alternate by translation along the c axis (Fig. 2d and Table 2) and link the grid layers in reversely alternating parallel arrangements, defining a 3D hydrogen-bonded network supporting the supramolecular architecture. Furthermore, between adjacent layers, there exists a pair of π stacking interactions between the pyridine rings (N4–C7–C8–C9–C10–C11, N5–C12–C13–C14–C15–C16, N6–C17–C18–C19–C20–C21, and N7–C22–C23–C24–C25–C26) (face-to-face). The dihedral angles between the two planes are 4.5(3) and 26.6(3)°, respectively, with centroid-to-centroid distances of 3.826(3) and 3.937(3) Å. Thus, these layers are extended into an interwoven 3D supramolecular architecture through O–H···O and π···π interactions.

2.3 Infrared spectra of 1 and 2

Complexes 1 and 2 exhibit infrared bands in the range 4000–450 cm⁻¹, which are different from those of the free ligand. The band ν(NH)=3278 cm⁻¹ of the H₂L molecule is absent in the spectra of 1 and 2 indicating the deprotonation of the imine N atom upon coordination to the metal ion. Bands assigned to ν_as(COO⁻) and ν_s(COO⁻), which are observed for the free ligand at 1730 and 1406 cm⁻¹, respectively, are shifted to 1586 for 1, 1598 for 2 and 1398 for 1, 1404 cm⁻¹ for 2, indicating that deprotonation of the carboxylate groups occurred upon coordination. The ν_as(–SO₂–) (1330 cm⁻¹ for H₂L, 1223 cm⁻¹ for 1, and 1218 cm⁻¹ for 2) and ν_s(–SO₂–) (1128 cm⁻¹ for H₂L, 1045 cm⁻¹ for 1, and 1086 cm⁻¹ for 2) bands show differences from that of H₂L, suggesting that this group does also interact with the metal ion. Bands centered at 3406, 1384, and 853 cm⁻¹ ascribed to H₂O and ν_as(NO₃⁻), ν_s(NO₃⁻) for 2, are absent in the spectra of 1. All these spectroscopic features of H₂L and 1, 2 are consistent with the crystal structure determinations.

4.1 Materials and physical methods

H₂L was synthesized by the literature methods [28], [29]. All other chemicals were commercially available and used as received without further purification. The elemental analyses (C, H, N, and S) were performed on a Perkin-Elmer 240C apparatus. FT-IR spectra were recorded from KBr pellets in the range of 4000–450 cm⁻¹ on a Varian FT-IR 640 spectrometer.

4.2 Synthesis of [Ag₂(L)]_n (1)

H₂L (0.043 g, 0.2 mmol) dissolved in distilled water (5 mL) was added dropwise to a stirred solution of excessive AgNO₃ (0.085 g, 0.5 mmol) in ethanol (10 mL). The pH value was adjusted to about 7.0 with 0.1 mol L⁻¹ NaOH solution, and the resulting mixture was stirred at 333 K for 2 h, cooled to room temperature, and filtered. The filtrate was allowed to slowly concentrate by evaporation at room temperature. Two weeks later, colorless block-shaped crystals suitable for X-ray structure analysis were obtained in a yield of 30% (based on Ag). – C₆H₅Ag₂N₃O₄S (430.93): calcd. C 16.71, H 1.16, N 9.75, S 7.43; found C 17.69, H 1.18, N 9.76, S 7.45. – IR (KBr): ν=1586 (s), 1456 (w), 1398 (s), 1336 (w), 1305 (m), 1223 (m), 1178 (m), 1090 (m), 1045 (w), 1017 (w), 921 (m), 605 (m) cm⁻¹.

4.3 Synthesis of {[Ag_2.5(L)(4,4′-bipy)₂]· (NO₃)_0.5·(H₂O)_4.5}_n (2)

H₂L (0.043 g, 0.2 mmol) dissolved in distilled water (5 mL) was added dropwise to a stirred solution of AgNO₃ (0.085 g, 0.5 mmol) in ethanol (10 mL). The pH value was adjusted to about 7.0 with 0.1 mol L⁻¹ NaOH solution, and the resulting mixture was stirred at 333 K for 2 h. Then 2 mL of an ethanol solution of 4,4′-bipyridine (0.064 g, 0.4 mmol) was added slowly, and the stirring continued for 2 h. The mixture was cooled to room temperature, and filtered. The filtrate was allowed to slowly concentrate by evaporation at room temperature. Two weeks later, colorless block-shaped crystals suitable for X-ray structure analysis were obtained in a yield of 25% (based on Ag). – C₂₆H₃₀Ag_2.5N_7.5O₁₀S (909.31): calcd. C 34.31, H 3.30, N 11.55, S 3.52; found C 34.32, H 3.28, N 11.57, S 3.50. – IR (KBr): ν=3406 (m), 1598 (s), 1530 (w), 1484 (w), 1404 (s), 1384 (s), 1218 (m), 1174 (m), 1086 (m), 1042 (w), 853 (m), 620 (m) cm⁻¹.

4.4 Crystal structure determinations

Single-crystal data collections were performed on a Bruker Smart Apex II CCD diffractometer with graphite-monochromatized MoK_α radiation (λ=0.71073 Å) at 296(2) K. The structures were solved with Direct Methods using Shelxs-97 [30], [31], and structure refinements were performed against F² using Shelxl-97 [32], [33]. All non-hydrogen atoms were refined with anisotropic displacement parameters. Carbon-bound H atoms were placed in calculated positions (d_C–H=0.93–0.97 Å) and were included in the refinement in the riding model approximation, with U_iso(H) set to 1.2U_eq(C). The H atoms of coordinating water in 2 were located in difference Fourier maps, and were refined with distance restraints of d_O–H=0.85±0.01 Å and H···H 1.387±0.02 Å. Their displacement parameters were tied to those of the parent atoms by a factor of 1.5. The maximum residual electron density peaks of 1.61 e Å⁻³ for 1 and 1.29 e Å⁻³ for 2 were located 1.31 Å from the Ag1 atom for 1 and 0.60 Å from the N8 atom for 2. DFIX, SADI, and DANG instructions from Shelxl have been applied to constrain some O–H, N–O and H···H distances in 2. Further details of the structure determinations are summarized in Table 3.

CCDC 1481728 (1) and CCDC 1481729 (2) 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.