(μ-Pentafluorophenylthiorato)(o-bis(diphenylphosphino)benzene) Silver(I) Dimer

Abstract

This study reports the synthesis and characterization of a novel thiolate-bridged heteroleptic dinuclear silver(I) complex, [Ag(μ-C₆F₅S)(dppbz)]₂, incorporating o-bis(diphenylphosphino)benzene (dppbz). The complex was synthesized in high yield via the reaction of silver(I) oxide with pentafluorothiophenol and dppbz. Single crystal X-ray diffraction analysis revealed a distorted tetrahedral structure with an Ag₂S₂ core, where each Ag atom is coordinated by dppbz. The Ag…Ag distance (3.2652(3) Å) suggests weak argentophilic interaction. Notably, an uncommon intermolecular fluorine-fluorine contact is observed, likely stabilized by intermolecular hydrogen bonding stabilizing the structure. The complex shows weak luminescence under UV radiation with an emission maximum at 514 nm.

1. Introduction

Since some d¹⁰ metal complexes in which the metal is coordinated by diphosphine ligands exhibit highly efficient luminescence, the luminescent properties of the complexes have been studied over the past few decades [1]. Several reports have described that the coordination of o-bis(diphenylphosphino)benzene (dppbz), which is the diphosphine bridged by a phenylene ring, to silver(I) ions leads to the formation of mononuclear complexes exhibiting luminescence based on charge transfer to the dppbz moiety. These complexes have been explored for luminescent properties [2,3], an application of luminescent devices [4,5], and mechanochromic behavior [6,7].

In recent years, heteroleptic dinuclear Cu(I)- or Ag(I)-dppbz complexes bridged by anionic ligands have been actively investigated due to their potential applications, e.g., optical devices [8,9,10,11,12,13] and catalysis [14]. However, the reports on thiolato-bridged compounds remain scarce with the exception of the Ag(I)-phenylthiorato complex containing fluoride groups [15]. Since thiols are often oxidized to form disulfide bonding, it has been suggested that the introduction of the fluoro groups into phenylthiolate is necessary for the formation of such dinuclear complexes. Indeed, the utilization of pentafuluorothiol as a thiol derivative readily led to the formation of a heteroleptic dinuclear silver(I) complex bridged thiolate anion.

2. Results and Discussion

It is well-known that the treatment of the corresponding thiol with basic silver oxide leads to the formation of a thiolate ligand through neutralization, followed by the coordination to silver. In this system, the addition of the dppbz ligand resulted in the facile formation of a heteroleptic dinuclear complex [Ag(μ-C₆F₅S)(dppbz)]₂ (1) in good yield (Scheme 1). The elemental analysis results also showed good consistency with the theoretical values.

In ¹H NMR, the ratio of integrals of signals assigned to phenylene and phenyl protons is consistent with the theoretical values of dppbz (Figure S1). The ¹³C{¹H} NMR spectrum shows seven distinct signals of dppbz involving the splitting by coupling to ³¹P and possibly ¹⁰⁷Ag and/or ¹⁰⁹Ag (Figure S2). ³¹P{¹H} NMR shows a broad signal at δ-7.4 (Figure S3). The broadening is attributed to both the rapid ligand exchange of phosphine-silver(I) complexes [16] and the coupling to ¹⁰⁷Ag and/or ¹⁰⁹Ag. In addition, a trace singlet signal assigned to a free dppbz ligand at δ 31.4 suggests that the slight dissociation may occur in solution.

The formation of the dimer complex 1 was confirmed by a single-crystal X-ray diffraction study. Suitable crystals were obtained by the slow diffusion of diethyl ether into a dichloromethane solution of the complexes. The complex crystallized in the monoclinic space group P2₁/n.

Figure 1 shows that two Ag atoms are bridged by two sulfur atoms, with dppbz coordinated to each Ag atom. Since an inversion center is present at the center of the Ag₂S₂ core and the sum of the four bond angles in this core is 360°, the Ag and S atoms lie in the same plane, and the two pentafluorophenyl groups in the thiolato ligands adopt an anti-conformation. This is a similar structure to that of the corresponding complexes with 3,4-difluorophenylthiolato ligands [15], whereas the crystals adopting syn-conformation could not be obtained. The Ag-S bond lengths are 2.5597(7) and 2.6282(7) Å, while the Ag-P bond lengths are 2.4803(7) and 2.5171(7) Å. The P-Ag-P bite angle of dppbz is 82.69(2)°, indicating a highly distorted tetrahedral structure. The dihedral angle between a coordination plane (mean plane consisting of P1, Ag1, and P2) and a phenylene ring (mean plane consisting of P1, P2, C1, C2, C3, C4, C5, and C6) is 28.08°, suggesting that the phenylene ring is moderately bent from the coordination plane (Figure S4).

The distance between the silver centers is 3.2652(4) Å, which is longer than that in the reported thiolato complexes [15], yet shorter than the sum of van der Waals radii (3.44 Å). It suggests the presence of weak argentophilic interaction between silver centers [17,18]. Surprisingly, the distance between the F2 atom on the neighboring molecule is 2.828 Å, which is smaller than the sum of the van der Waals radii of fluorine atoms (2.94 Å) despite the expected repulsion due to the large electronegativity. The distance is compared with that of sp³-based C-F…F-C in trans-[PdCl₂(2-HCF₂(CF₂)₃CH₂OCH₂-py)₂] (2.828 Å) [19]. This phenomenon may result from the fixed positioning of the C₆F₅ moieties, stabilized by three C-H…F hydrogen bonds in F1…H3-C4(2.320 Å) and F5…H18-C23(2.355 Å) (Figure S5).

The photophysical properties of complex 1 are shown in Figure S6. No remarkable absorption is observed in the visible region. The tail absorption rising from ca. 390 nm is assigned to M+XLCT by comparison with other Ag-dppbz complexes. Higher energy absorption above 300 nm is attributed to ligand-centered ones of pentafluorophenyl groups and dppbz. Complex 1 exhibits only weak luminescence under UV irradiation (λ_ex = 350 nm). The emission maximum is at 514 nm, which corresponds to a lower energy emission than those of the reported difluorophenylthiolato complexes [15]. It may result from a weaker argentophilic interaction in complex 1 and/or a decrease in XLCT energy due to the electron deficiency of the sulfur atom in the presence of more electron-withdrawing fluorine atoms.

3. Materials and Methods

3.1. General Considerations

NMR spectra were recorded on a Bruker Avance III HD spectrometer (Bruker Japan Co., Ltd., Kanagawa, Japan) (operating at 500 MHz for ¹H, 125 MHz for ¹³C, and 203 MHz for ³¹P). The chemical shifts were referenced to tetramethylsilane (¹H and ¹³C; as internal) or 85% H₃PO₄ (³¹P{¹H}; as external). The UV/vis spectrum was measured and recorded on a Shimadzu UV-3100 instrument (Shimadzu Co., Ltd., Kyoto, Japan). The emission spectrum was measured with the combined instrument system of a JASCO-MV3600 spectrometer (JASCO, Tokyo, Japan) and a xenon lamp (λ_ex = 350 nm). The intensities for the X-ray determinations were collected on a RIGAKU Saturn 70 instrument (Rigaku Co., Ltd., Tokyo, Japan) with MoKα radiation. Absorption correction was performed using an empirical method based on spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm within CrysAlisPro 1.171.42.49 (Rigaku Oxford Diffraction, 2022) [20]. Structure solution and refinement were performed with the SHELX programs [21,22] included in Yadokari, version 1.5 [23]. Hydrogen atoms were calculated for idealized positions and treated with the ‘riding model’ option of SHELXL. The representation of molecular structure was realized using the program MERCURY, version 4.2.0. Elemental analysis was performed by A-Rabbit-Science Japan Co., Ltd., Kanagawa, Japan.

3.2. Synthetic Methods

Ag₂O 0.023 g (0.14 mmol) and dppbz 0.061 g (0.10 mmol) were stirred in 20 mL THF overnight. The mixture was filtered through celite. The obtained filtrate was concentrated under reduced pressure. From the residue, recrystallization from dichloromethane and diethyl ether leads to red-orange crystals. Yield: 0.097 g (94%). Elemental analysis: Calcd. for C₇₂H₄₈P₄S₂F₁₀Ag₂: C, 57.39; H, 3.20% Found: C, 57.43 H, 3.20%. ¹H NMR δ 7.42–7.48 (br, 1H), 7.34–7.42 (br, 1H), 7.24–7.30 (m, 2H), 7.08–7.14 (m, 8H): ¹³C{¹H} NMR, δ 140.3(t), 134.4, 133.4(t), 131.5(t), 130.6, 130.0, 128.7(t): ³¹P{¹H} NMR, δ -7.4 (br).