Helical chain Ag(I) complexes with a tridentate N-donor ligand: syntheses, structural characterization, and properties

2.1 Structural description of {[Ag₅(L)₅](ClO₄)₅}_n (1)

Determination of the structures of 1 and 2 by X-ray crystallography has shown isotypic crystal structures. As a consequence, only 1 is described in detail here. Complexes 1 and 2 have been formulated as pentamer {[Ag₅(L)₅](ClO₄)₅}_n (1) and {[Ag₅(L)₅](PF₆)₅}_n (2) although an infinite chain is formed. The sequence Ag(1)–Ag(2)–Ag(3)–Ag(1)′–Ag(2)′, with Ag(3) residing on a twofold axis, is the repeating unit and has been chosen as a basis of the molecular formulas.

Structural analysis by single crystal X-ray diffraction shows that complexes 1 and 2 crystallize in the monoclinic system with space group P2/c and Z = 2 (Table 1). In 1 there are 2.5 Ag(I) cations, 2.5 ligands L, and 2.5 ClO₄^– anions in the asymmetric unit. As is shown in Fig. 1a, each Ag(I) is two-coordinated by two nitrogen atoms from two different L ligands to furnish a nearly linear coordination geometry [AgN₂]. The Ag–N bond distances range from 2.118(4) to 2.157(4) Å; the N–Ag–N bond angles are in the range of 171.68(15)–178.7(2)° (Table 2). The distances of adjacent Ag···Ag pairs are 3.291 and 3.372 Å, respectively, which are shorter than the sum of the van der Waals radii (4.06 Å) [12–14]. The ligand L contains three N atoms, but in 1 the pyridyl N atom is un-coordinated (but see below), and thus the L ligand just acts as a μ₂-bridge. Compared with the previously reported 1,3-di(2-oxazolyl)benzene, L has a pyridyl N atom which also weakly interacts with adjacent Ag(I) cations (Ag–N_pyridyl being in the range of 2.866–3.080 Å), leading to further consolidation of the molecular structure of 1 and different crystallographic data. The C–C single bonds between pyridine and oxazoline can rotate to different angles to satisfy the coordination requirements. The oxazolyl groups in L thus have more spatial freedom to adopt different orientations. In 1, two N atoms of the oxazolyl groups protrude from the pyridyl plane toward opposite sides. The interconnection of twisted L and Ag(I) leads to a helical chain structure (Fig. 1b). As implied by the centrosymmetric space group, in crystals of 1 (and 2) there exist left- and right-handed helical chains (Fig. 1c) in equal amounts. Thus, the assembly of 1 leads to an achiral meso crystal structure.

Fig. 1:

(a) Coordination environment of Ag(I) in complex 1 with displacement ellipsoids drawn at the 30 % probability level; hydrogen atoms and ClO₄^– anions are omitted for clarity; (b) the chain structure of 1; (c) the different helical directions of the chains in 1.

2.2 PXRD and TGA measurements

The phase purity of 1 and 2 could be proven by powder X-ray diffraction (PXRD) measurements. as shown in Fig. 2, each PXRD pattern of the as-synthesized sample is consistent with the simulated one.

Fig. 2:

The PXRD patterns of complexes 1 and 2.

Complex 1 is potentially explosive for the existence of perchlorate in its molecular structure. Thus, only complex 2 was subjected to thermogravimetric analysis (TGA) in N₂ atmosphere to ascertain its thermal stability, from 30 °C to 660 °C, and the results are shown in Fig. 3. No obvious weight loss can be observed before the decomposition of the framework at 300 °C, confirming inter alia the absence of solvent in its structure.

Fig. 3:

TGA curve of complex 2.

2.3 Luminescent properties

Previous studies have shown that coordination compounds containing d¹⁰ metal centers such as Ag(I) and Cd(II) may exhibit excellent luminescence properties and have potential applications as photoactive materials [15, 16]. Therefore, the luminescence of complexes 1, 2 and the L ligand has been investigated in the solid state at room temperature. As shown in Fig. 4, an intensive fluorescence can be observed with emission bands at 430 nm (λ_ex = 348 nm) for 1, at 463 nm (λ_ex = 375 nm) for 2, and at 445 nm (λ_ex = 360 nm) for the L ligand. This fluorescence may be tentatively assigned to intra-ligand transitions of the coordinated L ligands, since a similar emission was observed for the free L [13, 14]. The observed blue shift of the emission maximum for 1 vs. L may be considered to originate from the coordination interactions between the metal atom and the ligand, and the different shift of the emission maximum of 2 relative to L might be related to the different anions in their structures [17, 18].

Fig. 4:

Emission spectra of 1, 2 and L in the solid state at room temperature.

3.1 Preparation of {[Ag₅(L)₅](ClO₄)₅}_n (1)

A mixture of L (0.10 mmol; 21.7 mg) and AgClO₄ (0.10 mmol; 20.7 mg) in 10 mL methanol was stirred for 10 min and then filtered to give a clear filtrate. Several days later, colorless block crystals suitable for X-ray diffraction analysis were collected by slow diffusion of diethyl ether into the clear filtrate. Yield: 40 % based on L. – C₅₅H₅₅Ag₅Cl₅N₁₅O₃₀ (2122.74): calcd. C 31.12, H 2.61, N 9.90; found C 31.32, H 2.36, N 9.66 %. – IR (KBr pellet, cm^–1): ν = 3444 (m), 1655 (s), 1573 (s), 1478 (m), 1457 (m), 1379 (s), 1276 (m), 1259 (s), 1181 (m), 1148 (s), 1090 (s), 991 (m), 970 (m), 929 (m), 830 (m), 747 (m).

3.2 Preparation of {[Ag₅(L)₅](PF₆)₅}_n (2)

Complex 2 was synthesized by the same procedure as used for preparation of 1, except that AgPF₆ (0.10 mmol; 25.3 mg) was used instead of AgBF₄ as the starting material. After several days, colorless block crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of diethyl ether into the clear filtrate. Yield 36 % based on the L ligand. – C₅₅H₅₅Ag₅F₃₀N₁₅O₁₀P₅ (2350.34): calcd. C 28.11, H 2.36, N 8.94; found C 27.92, H 2.56, N 8.70 %. – IR (KBr pellet, cm^–1): ν = 3435 (m), 1660 (s), 1577 (s), 1486 (m), 1461 (m), 1383 (s), 1280 (s), 1259 (s), 1185 (m), 1148 (m), 974 (s), 928 (m), 842 (s), 739 (m), 677 (m).

3.3 X-ray structure determinations

The crystallographic data collections for complexes 1 and 2 were carried out on a Bruker Smart ApexII CCD area detector diffractometer using graphite-monochromatized MoK_α radiation (λ = 0.71073 Å) at 293(2) K. The diffraction data were integrated by using the program Saint [19], which was also used for the intensity corrections for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the program Sadabs [20]. The structures of 1 and 2 were solved by Direct Methods, and all non-hydrogen atoms were refined anisotropically on F² by the full-matrix least-squares techniques using the Shelxs/l-97 crystallographic software package [21, 22]. In 1 and 2, all hydrogen atoms at C atoms were generated geometrically. The details of crystal parameters, data collection, and refinements for the complexes are summarized in Table 1; selected bond lengths and angles are listed in Table 2.

CCDC 1063596 and 1063597 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.