[μ-1,2-Bis(dipheylphosphino)ethane-κ²P,P’]bis(3-mercapto-1,2-propanediolato-κS-gold(I))

Abstract

A new dinuclear gold(I) complex, possessing a bridging diphosphine ligand (1,2-bis(diphenylphosphino)ethane) and two terminal thiol ligands (1-thioglycerol), was synthesized and fully characterized by IR, ¹H and ³¹P NMR, fluorescence, ESI-mass, and diffuse reflection spectroscopy, together with X-ray diffraction and elemental analyses. The compound formed a 1D chain supramolecular structure through intermolecular aurophilic interactions in the crystal structure, leading to photoluminescence in the solid state.

1. Introduction

1-Thioglycerol (3-mercapto-1,2-diol; H₃tg) is one of the simplest mercapto-diols with one -SH and two -OH groups. H₃tg is highly water soluble and has low volatility compared to other low-molecular-weight thiols, so it has been utilized as a reducing agent for S-S bonds in biomolecules [1] or a potential drug [2]. H₃tg is also available as a protecting reagent for the aqueous synthesis of metal or metal chalcogenide nanoclusters, e.g., CdS [3], CdSe [4,5], CdTe [6], ZnO [7], ZnS [8], HgTe [9], TiO₂ [10], Bi₂O₃ [11], and Au [12,13,14,15]. In coordination chemistry, the reactivity of H₃tg toward most transition metal ions was investigated based on spectroscopic characterization [16,17,18], but the accurate coordination mode of H₃tg was uncertain until the late 20th century because of the lack of single-crystal X-ray diffraction analysis. In 1995, Weller and coworkers reported that the deprotonated H₂tg⁻ ligand readily binds with Cd²⁺ ions in a μ₂-κS mode to form an infinite 3D coordination polymer in [Cd(μ-H₂tg-S)₂]_∞ [19] (Figure S1a). This was the first crystallographic study on the coordination behavior of H₃tg. After Weller’s pioneering research, four metal complexes of H₃tg have been structurally characterized. A similar μ₂-κS bridging coordination mode of H₂tg⁻ was also detected in a dinuclear gold(I) complex, [Au₂(PPh₃)₃(μ-H₂tg-S)] [20], in which H₂tg⁻ ligand bridges linear {Au(PPh₃)}⁺ and trigonal {Au(PPh₃)₂}⁺ moieties through the S atom (Figure S1b). Tekeste and Vahrenkamp designed a Zn^II complex having a tripodal ligand and H₂tg⁻, [Zn(H₂tg-S,O)(Tp^PhMe)] (Tp^PhMe = tris(3-phenyl-5-methylpyrazolyl)borate) [21], as a model compound of zinc enzyme inhibition by H₃tg. In the Zn^II complex, H₂tg⁻ bound to the Zn^II center in a bidentate-S,O mode (Figure S1c). Tasiopoulos and coworkers reported that the single-crystal-to-single-crystal incorporation of H₃tg into lanthanide MOFs gave H₃tg-decorated MOFs, [Ln₂(CIP)₂(H₃tg-O)₂(H₂O)₂] (Ln = Nd, Eu; H₃CIP = 5-(4-carboxybenzylideneamino)isophthalic acid) [22,23], in which H₃tg adopted the monodentate-κO coordination mode for Ln³⁺ and one -OH, and one -SH groups remained uncoordinated (Figure S1d). These structural studies indicated that both -SH and -OH groups of H₃tg are potential metal binding sites. However, the non-bridging monodentate-κS, one of the most ubiquitous thiolate coordination modes [24], has yet to be observed for H₃tg.

As an extension of our previous study on the development of diphosphine-bridged digold(I) complexes with two terminal thiolates as the metalloligand [25], we synthesized the title compound, [Au₂(dppe)(H₂tg-S)₂] (1; dppe = 1,2-bis(diphenylphosphino)ethane), in this work. Single-crystal X-ray analysis of 1 revealed that two deprotonated H₂tg⁻ ligands are spanned by a linear {Au₂(dppe)}²⁺ moiety, and each H₂tg⁻ ligand binds to the Au^I center in a non-bridging monodentate-κS mode. The dinuclear complex 1 was connected by intermolecular aurophilic interactions [26,27], forming a 1D chain structure in the crystal lattice. Complex 1 was fully characterized by powder X-ray diffraction (PXRD) analysis, elemental analysis, and various spectroscopic measurements, including X-ray fluorescence, IR, ¹H NMR, ³¹P NMR, diffuse reflection, ESI-mass, and emission spectra.

2. Results and Discussion

2.1. Synthesis and Characterization of 1

The 1:2:2 reaction of [Au₂Cl₂(dppe)] [28], H₃tg, and KOH in EtOH/H₂O gave a pale-yellow powder of 1 in a moderate yield (47 %) (Figure 1). The elemental analysis data of 1 agreed with the formula of a 1:2 adduct of {Au₂(dppe)}²⁺ and H₂tg⁻ without any counter ions or solvent molecules. The bulk purity of 1 was also confirmed by the PXRD study, which was well matched with the simulated pattern of the single-crystal X-ray diffraction data (Figure S2). The X-ray fluorescence spectrum of 1 showed the signals due to Au, S, and P, while no signal due to Cl was detected (Figure S3). The IR spectrum of 1 showed the ν_P–C bands due to dppe and the ν_O–H, and ν_C-O bands due to H₂tg⁻ (Figure S4) [29]. The ¹H NMR spectrum of 1 in CDCl₃ showed a multiplet signal at δ 7.66–7.46 ppm and a doublet signal at δ 2.69 ppm due to phenyl and methylene groups of dppe, respectively (Figure S5). Moreover, the ¹H NMR spectrum showed a multiplet signal at δ 3.80–3.64 ppm, three double-doublet signals at δ 3.66, 3.21, and 3.12 ppm, and two broad singlet signals at δ 3.50 and 2.35 ppm. The multiplet and double-doublet signals were due to the methylene and methine proton signals of H₂tg, and the broad singlet signals are assignable to the two hydroxy groups of H₂tg. The OH proton signals were slightly weak (1.6 or 1.5 H) compared to the expected value (2.0 H), which is probably because of the partial H/D exchange with CDCl₃ [30]. The integral intensity ratio of all signals implies the presence of dppe and H₂tg⁻ in a 1:2 ratio in 1. The ³¹P{¹H} NMR spectrum of 1 in CDCl₃ showed a singlet signal at δ 36.5 (Figure S6). Thus, 1 is characterized as a C₂ or C_s symmetrical linear digold(I) structure, as illustrated in Figure 1. Considering the chirality of two terminal H₂tg⁻ ligands (R or S), two diastereomers, the racemic (RR/SS) and the meso (RS) forms, are possible for 1. However, the ¹H and ³¹P NMR spectroscopy showed only a single set of signals for 1, which implies that the selective formation of one of the two isomers or the rapid structural conversion between the two isomers occurs in solution.

The diffuse reflectance spectrum of 1 showed a broad band at 330 nm in the solid state (Figure S7). While 1 was non-emissive at room temperature, 1 showed a photoluminescence centered at 496 nm at 77 K in the solid state (Figure S8). The emission quantum yield at 77 K was evaluated to be 8.3% using an integration sphere. The origin of the emission band was tentatively assigned to phosphorescence arising from a charge transfer transition from S to Au perturbed by aurophilic interaction (³LMMCT), considering the assignment for related luminescent gold(I) complex with phosphine and thiolate ligands [31,32,33,34,35,36,37,38].

2.2. Crystal Structure of 1

X-ray-quality single crystals of 1 were obtained from the filtrate after standing at room temperature for 1 day. Single-crystal X-ray analysis of 1 revealed that the asymmetric unit contains two Au^I, two halves of dppe, and two H₂tg⁻. The symmetry expansion operation generated two crystallographically independent, but essentially the same, digold(I) complexes (molecules A and B) (Figure 2). Two gold(I) ions adopt an almost linear geometry by coordinating one S atom from H₂tg⁻ and one P atom from dppe. The dppe ligand bridges two Au^I ions in a μ₂-κ²P,P’ mode, while the H₂tg⁻ ligand binds to one Au^I center in a monodentate-κS mode. The Au-S (2.3104(12), 2.304(10), and 2.293(15) Å) and Au-P (2.2652(9), 2.298(2), and 2.219(4) Å) bond distances and S-Au-P angles (175.81(4), 178.3(2), and 173.6(4) °) were within the range of related [Au₂(dppe)(thiolate-S)₂]-type dinuclear complexes [39,40,41,42,43]. The R and S isomers of H₂tg were disordered in a 0.65:0.35 ratio in both molecules A and B (Figure S9). Thus, discriminating between the racemic or meso isomers of 1 was difficult. In the crystal packing, molecules A and B were connected by the short Au···Au contact of 3.1237(19) and 3.132(3) Å, which is shorter than the sum of the VdW radii (3.32 Å) and suggestive of the presence of aurophilic interaction [26,27] (Figure S10). The ESI-mass spectrum of 1 in CH₂Cl₂/MeOH (v/v = 1/1) showed a dominant monovalent signal corresponding to the digold(I) complex molecule {[Au₂(dppe)(H₂tg)₂] + CH₂Cl₂ + Na}⁺ at m/z = 1115, while no signal due to the polymeric species was detected (Figure S11). This observation means that the 1D polymer structure in 1 was dissociated in the solution.

3. Materials and Methods

3.1. Materials

1-thioglycerol was kindly donated by Asahi Chemical Co., Ltd. (Osaka, Japan). [Au₂(dppe)Cl₂] was synthesized based on a literature method [28].

3.2. Physical Measurements

The IR spectrum was recorded with a JASCO FT/IR-4100 infrared spectrophotometer using the ATR method at room temperature. Elemental analyses (C, H, N) were performed at Osaka University using a Yanaco CHN Corder MT-5. High-quality powder X-ray diffraction (PXRD) was performed at 100 K in transmission mode (synchrotron radiation, λ = 0.8 Å; 2θ range = 2–78°; step width = 0.006°; data collection time 1 min) on a diffractometer equipped with an MYTHEN microstrip X-ray detector (Dectris Ltd., Baden, Switzerland) at the SPring-8 BL02B2 beamline [44]. The ground sample was placed in 0.3 mm glass capillary tubes. The samples were rotated during the measurements. ¹H NMR spectra were recorded with a JEOL ECA500 (500 MHz) spectrometer in CDCl₃ with tetramethylsilane (TMS) as an internal standard. ³¹P{¹H} NMR spectra were recorded with a JEOL ECA500 (500 MHz) spectrometer in CDCl₃, and chemical shifts (δ in ppm) were reported with reference to H₃PO₄. X-ray fluorescence spectrometry was performed on a SHIMADZU EDX-7000 spectrometer. The diffuse reflection spectrum was measured with a JASCO V-670 UV/Vis/NIR spectrometer. The photoluminescence spectra were recorded with a JASCO FP-8500 spectrometer at room temperature and 77 K. The internal emission quantum yield (Φ) was obtained via the absolute measuring method using an integrating sphere unit (JASCO ILFC-847), the internal surface of which was coated with highly reflective Spectralon. The ESC-842 calibrated light source (WI) and the ESC-843 calibrated light source (D2) were used to calibrate the emission intensities to measure the absolute quantum yields. The ESI-mass spectrum was measured in a positive mode using a Bruker micrOTOF II instrument.

3.3. X-ray Crystal Structure Determination

Diffraction data for the title compound 1 were recorded on a Rigaku XtaLAB Synergy Custom X-ray diffractometer equipped with a Hypix-6000HE hybrid photon-counting detector and a Rigaku VariMax rotating-anode X-ray source with a Mo target. The intensity data were processed using the CrysAlisPro program (version 1.171.41.122a) and collected using the ω-scan technique. The structures were solved by direct methods using SHELXS [45], and refined by SHELXL [45] using the Olex2 program [46]. H atoms were placed at the calculated positions and refined isotropically, while non-hydrogen atoms were refined anisotropically. The H₂tg ligands, dppe ligands, and gold atoms were positionally disordered over two positions, whose occupancy factors were refined to 0.648 and 0.352. Several DFIX, ISOR, DANG, and RIGU instructions were used to model the disordered parts. The crystallographic data and geometrical parameters of 1 were summarized in Figures S1 and S2.

3.4. Synthesis of [Au₂(H₂tg)₂(dppe)] (1)

[Au₂Cl₂(dppe)] was prepared by the reported method [28]. To a white suspension containing 500 mg of [Au₂Cl₂(dppe)] (0.577 mmol) in EtOH (75 mL), 125 mg of 1-thioglycerol (1.16 mmol) in 11.3 mL of 0.2 M aqueous KOH solution was added. The smoky white mixture was stirred at room temperature in the dark for 3 h, and the resulting pale-yellow precipitate of 1 was collected by filtration. Yield: 277 mg (47%). IR spectrum, ν, cm⁻¹: 3376 (ν_O–H), 1482, 1435 (ν_P–CH2–), 1409, 1103 (ν_P–Ph), 1067 (ν_C–O), 1027 (ν_C–O), 731 (ν_P–CH2–), and 691 (ν_P–Ph). ¹H NMR spectrum (CDCl₃, 500 MHz), δ: 7.66–7.46 (20H, m), 3.80–3.64 (4H, m), 3.66 (2H, dd, J = 11.2, 5.9 Hz), 3.50 (1.6H, s), 3.21 (2H, dd, J = 13.0, 4.2 Hz), 3.12 (2H, dd, J = 13.0, 7.9 Hz), 2.69 (4H, d, J = 1.7 Hz), and 2.35 (1.5H, s). ³¹P{¹H} NMR spectrum (CDCl₃, 202 MHz), δ: 36.5 (s). Anal. Calcd. for [Au₂(H₂tg)₂(dppe)] = C₃₂H₃₈Au₂O₄P₂S₂ (Mw = 1006.66): C, 38.18%; H, 3.80%. Found: C, 38.14%; H, 3.86%.

4. Conclusions

In conclusion, we synthesized a new dinuclear Au^I complex 1 from the reaction of [Au₂Cl₂(dppe)] with H₃tg/KOH in a 1:2 ratio. Single-crystal X-ray analysis revealed that two H₂tg⁻ ligands bind to the linear [Au^I₂(dppe)]²⁺ unit through the Au-S coordination bonds in 1. In the crystal packing, the digold(I) complex 1 is infinitely connected through the intermolecular aurophilic interactions to form a 1D polymeric supramolecular structure. Due to the presence of aurophilic interactions, the solid sample of 1 showed a photoluminescence at 77 K. It should be noted that 1 is the first example of the metal complex having H₂tg⁻ ligands in a non-bridging κS coordination mode. It has been known that the non-bridging thiolato groups can bind to secondary metal ions to form S-bridged multinuclear complexes [25,47,48]. Thus, the reactivity of complex 1 with transition metal ions is undergoing in our group.