Synthesis,crystal structure,fluorescence and antioxidant properties of a binuclear Ag(I) complex based on the SPPh 3 ligand

Abstract

The binuclear silver(I) complex [Ag₂(η²-SPPh₃)₂(SPPh₃)₂](BF₄)₂ has been synthesized. Crystal analysis shows the coordination sphere around Ag(I) can be described as a three-coordinated slightly distorted planar triangular configuration with both monodentate and monoatomic bidentate bridging coordination modes for the SPPh₃ ligand. The fluorescence properties of free SPPh₃ and the Ag(I) complex have been studied in the solid state. The Ag(I) complex shows quasi-reversible Ag⁺/Ag pairs in its electrochemistry and has significant antioxidant activity against hydroxyl radicals.

Introduction

Transition metal complexes have attracted considerable attention because of their potential applications as functional materials as well as their structural diversity.^1–3 For the synthesis of coordination compounds, d¹⁰ metals are widely used because their flexible coordination sphere allows the generation of different kinds of structures with ligands containing nitrogen, sulfur or oxygen.^4–8 Ag(I) is among the most labile metal ion due to its d¹⁰ electronic configuration with versatile coordination number varying from 2 to 8.^9–11 The Ag(I) ion demonstrates linear, trigonal, square planar, square-pyramidal, trigonal-bipyramidal, tetrahedral, and octahedral coordination geometries with high affinity to both nitrogen and sulfur atoms.^12–14 Moreover, This ion is apt to form short Ag-Ag contacts as well as ligand unsupported interaction, which has been proved to be two of the most important factors contributing to the formation of such complexes and their special properties.^15–16 Furthermore, silver complexes show potential applications in photoluminescence, molecular recognition, and as antioxidant, antibacterial, and anticancer agents.^17–19

Sulfur is a soft base that easily forms stable compounds with a transition metal such as silver that is a soft acid. ²⁰ Triphenylphosphine sulphide and its derivatives are important chemical reagents and ligands for homogeneous catalysts, and have broad application prospects in flame retardant, antistatic and anti-corrosion agents.^21,22 Therefore, research work on sulfur-containing metal complexes not only has important theoretical significance for enriching the structural chemistry of these compounds, but also has potential application value in many fields of biomedicine and materials science.^23–25 In this work, we report the synthesis, structure, fluorescence, electrochemical properties and antioxidant activity of a binuclear Ag(I) complex containing triphenylphosphine sulphide ligand.

Results and discussion

Characterisation of the complexes

The Ag(I) complex prepared in this study is soluble in polar aprotic solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), and slightly soluble in acetonitrile, but insoluble in water. Elemental analysis indicates that the composition of complex is [Ag₂(η²-SPPh₃)₂(SPPh₃)₂](BF₄)₂ (1), which was confirmed by single-crystal X-ray diffraction studies. The infrared (IR) spectrum of 1 shows absorptions around 1437 cm^-1 that are assigned to C-C stretching vibrations of the phenyl rings and the absorptions around 695 cm^-1 are caused by P-C vibrations.^26–28 A strong band shown at 1057 cm^-1 coincided with the BF₄^- anions present in the Ag(I) complex. ²⁹ This fact is consistent with the results determined by X-ray diffraction. The electronic spectra of SPPh₃ and 1 were studied at 298 K in DMF solution. The UV band of SPPh₃ (266 nm) is marginally shifted in the Ag(I) complex (268 nm). This band is assigned to ligand-centred π−π* transitions of SPPh_3.^30,31

Crystal structure of complex 1

An ORTEP diagram of 1 is shown in Figure 1, and selected bond distances and angles are listed in Table 1. X-Ray structural analysis reveals that 1 is a discrete dinuclear molecule that crystallizes in the P-1 space group. The crystal structure of 1 consists of a binuclear dicationic [Ag₂(η²-SPPh₃)₂(SPPh₃)₂]²⁺ motif and two boron tetrafluoride anions. As illustrated in Figure 1, both the two Ag(I) atoms are three-coordinated (S atoms from two SPPh₃ ligands) with slightly distorted planar triangle configuration. The sum of the angles around silver ions [S(2)-Ag(1)-S(1) = 164.27(3)°, S(2)-Ag(1)-S(1)#1 = 100.18(3)°, S(1)-Ag(1)-S(1)#1 = 94.59(2)°] is 359°. It is worth noting that there are two coordination modes, monodentate and monoatomic bidentate bridging, for the triphenylphosphine sulphide ligands. The BF₄^- anions do not participate in the coordination and only act as counter anions for charge equilibrium. The Ag···Ag distance is 3.6489(6) Å, which is longer than the sum of the van der Waals radii for two silver atoms (3.44 Å), indicating a weak intermolecular Ag-Ag interaction. The Ag-S bond average lengths are in the range of 2.583 Å which is shorter than in [Ag₂(bbt)₂](pic)₂•2DMF (2.9455 Å, bbt = 1,3-bis(2-benzimidazyl)-2-thiapropane). ³² In addition, as displayed in Figure 2, the spacious binuclear cations are effectively connected via the strong C-H···π [C(7)-H(7)···π = 2.700 Å], to form one-dimensional stacking.

Figure 1.

Molecular structure and atom numberings of 1, showing displacement ellipsoids at 30% probability level. Hydrogen atoms were omitted for clarity. (Symmetry transformations used to generate equivalent atoms: #1: -x + 1,-y,-z + 1).

Table 1.

Selected bond lengths (Å) and bond angles (°) of 1.

Bond distances
Ag(1)-Ag(1)#1	3.6489(6)	Ag(1)-S(2)	2.3912(9)
Ag(1)-S(1)	2.4458(8)	Ag(1)-S(1)#1	2.9107(8)
Bond angles
S(2)-Ag(1)-S(1)	164.27(3)	S(2)-Ag(1)-S(1)#1	100.18(3)
S(1)-Ag(1)-S(1)#1	94.59(2)	S(2)-Ag(1)-Ag(1)#1	141.82(3)

Symmetry transformations used to generate equivalent atoms: #1: -x + 1,-y,-z + 1.

Figure 2.

A view of the packing in 1 showing the C-H⋯π stacking between neighbouring units.

Fluorescence properties

The d ¹⁰ metal coordination complexes have attractive fluorescent properties, which make them eligible candidates for chemical sensors or optical devices. ³³ Hence, the fluorescent emission spectra of SPPh₃ and 1 were studied in the solid state at room temperature (Figure 3). The colour and fluorescence changes for these compounds under ambient light and UV light irradiation are depicted in Figure 3. The free ligand SPPh₃ displays photoluminescence with emission maxima at 381 and 401 nm (λ_ex = 351 nm). It can be presumed that the peaks originate from π-π* transitions. ³⁴ The Ag(I) complex exhibits emission maxima at 387 and 403 nm upon excitation at 350 nm, which are assiged to ligand-to-ligand charge-transfer (LLCT). ³⁵ Compared with the emission of the free SPPh₃, complex 1 shows a red shift of 2-4 nm. Based on the previous literature, red shifts might be attributed to the different architectures featuring different intensities of supramolecular interactions (such as π–π interactions). ³⁶ Moreover, the partially quenched fluorescence of 1 may be due to the heavy atom effect of silver. ³⁷

Figure 3.

The solid-state fluorescent emission spectra of the SPPh₃ ligand and 1.

Electrochemical studies

The electrochemical properties of SPPh₃ and 1 were investigated by cyclic voltammetry (CV) in DMF.^38–40 The Ag(I) complex exhibits a pair of cathodic and anodic peaks. The data are presented in Table 2 and a voltammogram is shown in Figure 4. The separation between the cathodic and anodic peak potentials, ΔE_p and the current i_pa/i_pc indicate a quasi-reversible redox process assignable to the Ag+/Ag couple. The neutral, uncomplexed SPPh₃ ligands are not electroactive over the range -1.2 to +1.2 V.

Figure 4.

Cyclic voltammogram of 1 recorded with a platinum electrode in DMF solution containing (nBu)₄N-ClO₄ (0.1 M) (scan rate = 0.10 V∙S^-1).

Table 2.

Electrochemical data and electrochemical potentials and energy of 1..

Complex	Epa	Epc	ΔEp	E1/2	ipa	ipc	I
	0.465	0.226	0.239	0.346	4.765	4.886	0.975

△Ep = E_pa - E_pc; E_1/2 =  (E_pa + E_pc)/2; I = i_pa/_ipc.

Antioxidant properties

On the basis of relevant reports in the literature, some silver complexes may display good biological activity, such as antioxidant activity. ⁴¹ Therefore, we also investigated whether complex 1 showed hydroxyl radical scavenging properties (experimental). The IC₅₀ values of 1 is (13.62 ± 0.02) × 10^-6 M (Figure 5), and the free ligand SPPh₃ shows no activity in the experiments. Moreover, using the same method, we compared the abilities of the compound to scavenge hydroxyl radical (OH•) with those of the well-known natural antioxidants mannitol (IC₅₀ = 9.6 ×  10^-3 M) and vitamin C(IC₅₀ = 8.7 × 10^-3 M).^42,43 The results suggests that complex 1 has a higher ability to scavenge hydroxyl radical(OH•) than mannitol and vitamin C. The complex also displays higher hydroxyl radical scavenging activity than the complexes [Ag(bbbt)(crotonate)] • CH₃CH₂OH (bbbt = 1,3-bis(1-benzylbenzimidazol-2-yl)-2-thiapropane), IC₅₀ = 42.65 × 10^-6 M) and {[Ag₄(μ-4,4’-bipy)₃(μ-salicylate)₂(salicylate)H₂O]₂ • 2C₂H₅OH}_n (IC₅₀ = 6.02 × 10^-5 M) in the literature.^44,45 As a result, complex 1 can be considered as a new potential antioxidant.

Figure 5.

A plot of scavenging percentage (%) versus concentration of complex 1 on hydroxyl radicals.

Conclusion

In this work, a new binuclear silver(I) complex 1, based on the SPPh₃ ligand, has been synthesized and structurally characterized. Complex 1 has a three-coordinated slightly distorted planar triangular structure. Triphenylphosphine sulphide ligands participate in coordination in two different coordination modes. Compared with SPPh₃, 1 shows the partially quenched fluorescence due to the heavy atom effect of silver. It has good hydroxyl radical scavenging activity in vitro. These results indicate that the Ag(I) complex might have potential applications in the development of new antioxidants.

Experimental

Materials and methods

All chemicals and solvents were reagent grade and used without further purification. [Ag(CH₃CN)₄BF₄], CH₂Cl₂ and hexane are all commercially produced. The triphenylphosphine sulphide (SPPh₃) ligand was synthesized according to the reported method. ⁴⁶ Na₂HPO₄–NaH₂PO₄ buffer and ethylenediaminetetraacetic acid (EDTA)-Fe(II) solution were prepared using twice-distilled water. Electronic spectra were taken on a Lab-Tech UV Bluestar spectrophotometer. Absorbance was measured with a Spectrumlab 722 sp spectrophotometer at room temperature. The C, H and N elemental analyses were performed using a Carlo Erba 1106 elemental analyzer. The IR spectra were recorded in the 4000–400 cm^-1 region with a Nicolet FT-VERTEX 70 spectrometer using KBr pellets. Electrochemical measurements were performed on a LK2005A electrochemical analyser under N₂ at 283 K. The fluorescence spectra were obtained with an F97 Pro fluorescence spectra fluorophotometer.

Synthesis of [Ag₂(η²-SPPh₃)₂(SPPh₃)₂](BF₄)₂ (1)

A mixture of [Ag(CH₃CN)₄](BF₄) (0.0372 g, 0.1 mmol) and SPPh₃ (0.0441 g, 0.15 mmol) in CH₂Cl₂ solution (5 mL) was stirred at ambient temperature for 2 h to yield a colourless solution. The solution was filtered and hexane slowly diffused into the solution. Colourless crystals suitable for X-ray diffraction studies were obtained after 5 days. Yield: 63%; Anal. calcd for C₇₂ H₆₀Ag₂B₂F₈P₄S₄: C, 55.20; H, 3.86; found: C, 54.93; H, 3.81%. Selected IR data (KBr ν/cm^-1). 1593(w), 1463(m), 1437(s), 1310(m), 1101(s), 1057(s), 987(m), 749(m), 695(s), 596(m), 515(s). UV–Vis (in DMF), λ_max (nm): 268.

X-ray crystallography

A suitable single crystal of 1 was mounted on a glass fibre, and the intensity data were collected using a Bruker APEX-II CCD diffractometer with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. Data reduction and cell refinement were performed using the SAINT suite of programmes.^47,48 The absorption corrections were made using empirical methods. ⁴⁹ The structures were solved by direct methods and refined by full-matrix least-squares against F² using SHELXTL software. ⁵⁰ All non-hydrogen atoms were refined by using anisotropic thermal parameters. All hydrogen atoms were included in the calculated positions and refined with the isotropic thermal parameters riding on the parent atoms. Crystal parameters and details of the final refinement parameters are shown in Table 3. The selected bond lengths and bond angles for Ag(I) complex are listed in Table 1.

Table 3.

Crystal data and structure refinement for 1..

Complex	[Ag2(η2-SPPh3)2(SPPh3)2](BF4)2
Empirical formula	C72H60Ag2B2F8P4S4
Molecular weight	1566.72
Crystal system	Triclinic
Space group	P-1
a (Å)	11.0363(7)
b (Å)	12.6483(9)
c (Å)	13.9774(9)
α (°)	66.4870(10)
β (°)	78.6960(10)
γ (°)	77.5410(10)
V (Å3)	1733.8(2)
Z	1
Dcalcd (g/cm3)	1.501
Absorption coefficient (mm-1)	0.841
F(000)	792.0
Crystal size (mm)	0.41 x 0.39 x 0.33
θ range for data collection (°)	1.78 to 31.27
Reflections collected	9182
Independent reflections	11993 / 11214 [R(int) = 0.0210]
Index ranges	−11,15 / -12,8 / -17,20
Refinement method	Full-matrix least-squares on F2
Data/restraints/parameters	11213 / 67 / 434
Goodness-of-fit on F2	1.022
Finial R1 and wR2 [I > 2r(I)]	R1 = 0.0526, wR2 = 0.1521
R indices (all data)Largest diff. peak and hole(e Å-3)	R1 = 0.0648, wR2 = 0.16730.847 and -1.212

Hydroxyl radical scavenging activity

Hydroxyl radicals were generated in aqueous media through the Fenton-type reaction.^51,52 The reaction mixture (3 mL) contained 1 mL of 0.1 mmol aqueous safranin, 1 mL of 1.0 mmol aqueous EDTA-Fe(II), 1 mL of 3% aqueous H₂O₂, and a series of quantitative micro-additions of solutions of the test compound. A sample without the test compound was used as the control. The reaction mixtures were incubated at 37°C for 30 min in a water bath. The absorbance was then measured at 520 nm.^53–55 The scavenging effect for OH^• was calculated from the following expression: Scavenging ratio (%) =  ((A_sample - A_r) / (A_o - A_r)) × 100%, where A_sample is the absorbance of the sample in the presence of the test compound, A_r is the absorbance of the blank sample in the absence of the test compound, and A_o is the absorbance in the absence of the test compound, EDTA-Fe(II) and H₂O₂.