Synthesis, structure, fluorescence, and electrochemical properties of a binuclear Ag(I) complex with 1,4-bis(benzo[d]oxazol-2-yl)butane as a ligand

2.1 Materials and physical measurements

All chemicals and solvents (Kaitong Chemical Reagent Co., Ltd., Tianjin, China) were reagent grade and used without further purification. Electronic spectra were taken using a Lab-Tech UV Bluestar spectrophotometer (Beijing Labtech Co., Ltd., Beijing, China). Absorbance was measured using the Spectrumlab722sp spectrophotometer (Shanghai Lengguang Technology Co., Ltd., Shanghai, China) at room temperature. The C, H, and N elemental analyses were performed using a Carlo Erba 1106 elemental analyzer (Carlo Erba, Rome, Italy). ¹H spectra were obtained using a Mercury plus 400 MHz NMR spectrometer (Varian, Palo Alto, CA, USA) with trimethylsilane as the internal standard. The IR spectra were recorded in the 4000–400 cm⁻¹ region using a Nicolet FT-VERTEX 70 spectrometer (Bruker, Madison, WI, USA) using KBr pellets. Electrochemical measurements were performed using a LK2005A electrochemical analyzer (Tianjin Lanlike Chemical Electronics High Technology Co., Ltd., Tanjin, China) under nitrogen at T=283 K. A glassy carbon working electrode, a platinum wire auxiliary electrode, and an Ag/AgCl reference electrode (c(Cl⁻)=1.0 mol L⁻¹) were used in the three-electrode measurements. The electroactive component was at 1.0×10⁻³ mol L⁻¹ concentration with tetrabutylammonium perchlorate (0.1 mol L⁻¹) used as the supporting electrolyte in N,N-dimethylformamide (DMF) at room temperature. The fluorescence spectra were obtained using an F97 Pro fluorescence spectra fluorophotometer (Shanghai Lengguang Technology Co., Ltd., Shanghai, China).

2.2 Synthesis of BBO

The BBO ligand was synthesized according to the literature method [34]. Yield: 52%. m.p. 118–120°C. – ¹H NMR (CDCl₃, 400 MHz): δ=7.65–7.67 (m, 2H, Ph-H), 7.44–7.47 (m, 2H, Ph-H), 7.65–7.67 (m, 4H, Ph-H), 2.99–3.03 (m, 4H, CH₂), 2.04–2.07 (m, 4H, CH₂). – Elemental analysis for C₁₈H₁₆N₂O₂: calcd. C 73.95, H 5.52, N 9.58; found C 73.87, H 5.46, N 9.49%. – IR (KBr): ѵ (cm⁻¹)=3051 (w), 1572 (s), 1246 (s), 831 (w), 746 (s). – UV/Vis (in DMF): λ (nm)=271, 278.

2.3 Synthesis of [Ag₂(BBO)₂ (ClO₄)₂]

A solution of BBO (29.2 mg, 0.1 mmol) in 3 mL of methanol and a solution of [Ag(CH₃CN)₄(ClO₄)] (37.2 mg, 0.1 mmol) in 3 mL of acetonitrile were combined and stirred at room temperature for 8 h to give a light yellow solution. The solution was evaporated to dryness to obtain a brown yellow solid. Bright brown single crystals suitable for X-ray analysis were obtained by direct diffusion of Et₂O into the DMF solution of the resulting complex. Yield 47%. – Elemental analysis for C₃₆H₃₂Ag₂Cl₂N₄O₁₂: calcd. C, 43.27; H, 3.23; N, 5.61; found C, 43.25; H, 3.27; N, 5.55%. – ¹H [dimethyl sulfoxide (DMSO)-d₆, 400 MHz]: δ=7.89–7.91 (m, 2H, Ph-H), 7.65–7.60 (m, 2H, Ph-H), 7.33–7.40 (m, 4H, Ph-H), 3.05–3.09 (m, 4H, CH₂), 2.25–2.31 (m, 4H, CH₂). – IR (KBr): ѵ(cm⁻¹)=3053(w), 1564(s), 1454(s), 1248(m), 1092(s), 760(s), 621(s). – UV/Vis (in DMF): λ (nm)=273, 279.

2.4 X-ray crystallography

A suitable single crystal of the complex was mounted on a glass fiber, and the intensity data were collected using a Bruker APEX-II CCD diffractometer with graphite-monochromatized MoKα radiation (λ=0.71073 Å) at T=296(2) K. Data reduction and cell refinement were performed using the Saint suite of programs [35]. The absorption corrections were made using empirical methods. The structure was solved by Direct Methods and refined by full-matrix least-squares against F² using the Olex 2 software [36], [37]. All nonhydrogen atoms were refined by using anisotropic displacement parameters [38]. All hydrogen atoms were included in calculated positions and refined with isotropic displacement parameters taken as riding on the parent atoms. Crystal data and details of the data collection and refinement are shown in Table 1. Selected bond lengths and bond angles are listed in Table 2. Due to disorder of the atoms O5, O6 of the ClO₄⁻ anions, they were treated as split-atom model between two positions. In the refinement, several constrains were applied: ISOR for the disordered O5 and O5′ atoms; DFIX for O3, O4, O5, O5′, O6, O6′, Cl1 and Ag1 atoms; DELU for Cl1 and O3 atoms; SIMU for Cl1, O3, O4, O5, O5′, O6 and O6 atoms.

CCDC 1848100 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif.

3.1 Characterization of the Ag(I) complex

The Ag(I) complex was prepared by the reaction of [Ag(CH₃CN)₄(ClO₄)] with BBO in acetonitrile and methanol (v/v, 1: 1). The crystals were obtained by diffusion of diethyl ether into a DMF solution of the product in the dark to avoid photodecomposition. The Ag(I) complex is soluble in polar aprotic solvents such as DMF and DMSO, partially soluble in ethanol and methanol, but insoluble in Et₂O and petroleum ether. The results of the elemental analyses show that the composition of the complex is [Ag₂(BBO)₂(ClO₄)₂], which was confirmed by the crystal structure analysis.

The free BBO shows strong IR bands at 1246 and 1572 cm⁻¹ attributed to the ѵ(C–O) and ѵ(C=N) vibrations of the benzoxazole group, respectively [11]. In comparison with the IR spectra of the ligand BBO, the ѵ(C–O) and ѵ(C=N) frequencies (1248 and 1564 cm⁻¹) for the Ag(I) complex exhibit shifts of 2–8 cm⁻¹ indicating the participation of benzoxazole nitrogen in coordination with Ag(I) ions [39]. An absorption peak of the perchlorate anion appeared at 1092 cm⁻¹ [40], [41]. The electronic spectra of the ligand BBO and its silver complex were recorded in DMF solution at room temperature. The UV bands of BBO (271, 278 nm) are marginally shifted in the Ag(I) complex. These bands are assigned to π-π* (benzoxazole) and π-π* (imidazole) transitions, providing clear evidence of (C=N) coordination to the metal ion [42], [43].

3.2 Crystal and molecular structure of the Ag(I) complex

Single crystal X-ray diffraction analysis indicated that the Ag(I) complex crystallizes in the triclinic crystal system with the space group P1̅;. As shown in Fig. 1a, the asymmetric unit of the Ag(I) complex is half of a centrosymmetric array containing two silver(I) atoms, two BBO ligands, and two perchlorate anions. The two Ag atoms are both three coordinated by one oxygen atom from a perchlorate anion and two nitrogen atoms from two BBO ligands. The sum of the angles around the Ag(I) ion amounts to 354.8° (Fig. 1b), indicating a deviation from planarity. Two BBO ligands are bridging two Ag(I) cations in a head-to-tail mode through Ag–N coordination to generate a binuclear unit. The perchlorate anions fill the 18-atom bimetallic macrocyclic cavity and participate in coordination with the distance Ag–O(5′) of 2.278(7) Å (Fig. 1c). The Ag–N bond lengths are 2.163 Å and 2.165 Å.

Fig. 1:

(a) Molecular structure and atom numberings of the Ag(I) complex showing displacement ellipsoids at the 30% probability level (hydrogen atoms were omitted for clarity; only one set of atoms of the disordered ClO₄⁻ anions is shown). (b) Coordination of the silver ion in the complex. (c) Metal ring diagram of the Ag(I) complex.

As shown in Fig. 2, the distance of intermolecular π-π stacking between benzoxazole rings, measured by centroid-to-centroid distance, is estimated to be 3.716 Å. These intermolecular π-π interactions assemble the dimeric units to form a supramolecular structure.

Fig. 2:

A view of the packing of the Ag(I) complex in the crystal highlighting the π···π stacking between neighboring units.

3.3 Fluorescence properties

Coordination compounds of d¹⁰ metals have attractive fluorescence properties, which make them eligible as candidates for chemical sensors or optical devices [44], [45]. Hence, the emission spectra of BBO and the Ag(I) complex in the solid state at room temperature were investigated (Fig. 3). The free ligand BBO displays photoluminescence with emission maxima at 382 and 401 nm (λ_ex=350 nm), tentatively assigned to π-π* and n-π* transitions [46]. The Ag(I) complex exhibits emission maxima at 379 and 400 nm upon excitation at 352 nm, which are also due to π-π* and n-π* transitions, showing a blue shift of only 3 and 1 nm, respectively. The observed partial quenching of the fluorescence of the Ag(I) complex may be due to the heavy atom effect of silver [47], [48]. The fluorescence spectra of the uncoordinated BBO and the Ag(I) complex are very similar, indicating that the coordination of BBO to silver has little effect on the electronic structure of the ligand.

Fig. 3:

The solid-state fluorescence spectra of uncoordinated 1,4-bis(benzo[d]oxazol-2-yl)butane and its Ag(I) complex.

3.4 Electrochemical studies

The electrochemical properties of the BBO ligand and the Ag(I) complex were studied by cyclic voltammetry in DMF [49], [50]. The data are collected in Table 3 and a voltammogram is shown in Fig. 4. The neutral free BBO ligand is not electroactive over the range –1.5 to +1.5 V. The separation between the cathodic and anodic peak potentials ΔE_p (E_pa–E_pc) of the Ag complex is greater than 60 mV, and the current i_pa/i_pc>1, indicating an irreversible redox process assignable to the Ag⁺/Ag couple [51], [52].

Fig. 4:

Cyclic voltammogram of the Ag(I) complex recorded with a platinum electrode in dimethyl formamide solution containing (nBu)₄N(ClO₄) (0.1 m) (scan rate=0.10 V s⁻¹).