A [Mo₂O₂S₂]-based ring system incorporating tartrate as the bridging ligand: synthesis, structure and catalytic activity of Cs₄[Mo₂O₂(μ-S)₂]₂(μ₄-tart)₂ (tart=[C₄H₂O₆]⁴⁻)

1 Introduction

In the past decade, polyoxothiometalates have emerged as a fascinating class of novel compounds which appear as ring-like clusters based upon the self-condensation of [M₂O₂(μ-S)₂]²⁺ (M=Mo^V, W^V) oxothio cations around anionic templates such as phosphate, metalate or polycarboxylate ions. Their [M₂O₂(μ-S)₂]²⁺ building units are connected through double hydroxo bridges [1], [2], [3]. To date, a large family of cyclic polyoxothiomolybdates built by connecting the [M₂O₂(μ-S)₂]²⁺ cations by oxygen-containing organic ligands have been isolated and structurally characterized, leading to a series of cyclic molecular materials with electrocatalytic properties in the hydrogen evolution reaction process [4], [5]. Since Sécheresse and co-workers first reported a neutral cyclic oxothio cluster [Mo₁₂S₁₂O₁₂(OH)₁₂(H₂O)₆], a series of anionic oxothiomolybdenum wheels of flexible and fluxional host–guest assemblies with nuclearity varying in the Mo₈–Mo₁₈ range have been synthesized [1], [6], [7]. As well known, tartaric acid and metal-tartrato complexes are important tools for the resolution of racemic compounds [8]. Interesting structures involving bridging tart⁴⁻ ligands were found in dinuclear vanadium, tungsten and molybdenum complexes [8], [9], [10], [11], [12]. In the course of our research on the synthesis of polynuclear metaloxysulfide clusters [13], we tried the reaction of [Mo₂S₂O₂(H₂O)₆]²⁺ with racemic tartaric acid (tartH₄) in the presence of alkali halide. A new [Mo₂O₂S₂]-based ring cluster Cs₄[Mo₂O₂(μ-S)₂]₂(μ₄-tart)₂ (1) incorporating tart⁴⁻ as a bridging ligand has been isolated. In this paper, the synthesis, structural characterization and catalytic activity of the title cluster are reported.

2 Experimental section

3 Results and discussion

The reaction involved complete deprotonation to produce the tart⁴⁻ anion as a ligand, and no self-condensation of the [M₂S₂O₂]²⁺ units occurred. Each molybdenum atom is chelated by a pair of oxygen atoms from carboxyl and hydroxyl groups and the ring cluster 1 is formed. The title cluster has been characterized by ¹H NMR, infrared spectroscopy and microanalyses (Figs. S1 and S2). One singlet at around 3.12 ppm was observed in the ¹H NMR spectrum; the other two peaks are attributed to solvent DMSO (2.51 ppm) and water (3.37 ppm). The Mo=O bonds give two strong bands at 946 and 921 cm⁻¹. A medium-intensity vibration at 549 cm⁻¹ is attributable to the Mo–O stretching. The peaks at 1632 and 1375 cm⁻¹ for the typical ν(CO₂⁻) in the infrared spectrum indicates the carboxylate groups of the tart⁴⁻ ligand in the title cluster. The less intense absorptions at 462 and 419 cm⁻¹ for the Mo–S–Mo stretching modes appear as weak and sharp bands in the wavenumber region below 500 cm⁻¹, which is in good agreement with the values previously reported in the literature [11], [13].

The structure of the title cluster was confirmed by an X-ray diffraction study. Crystallographic data and experimental details are shown in Table 1. The new tetranuclear tartrato cluster of molybdenum(V) 1 shows two {Mo₂S₂O₂} units linked by two bridging tart⁴⁻ ligands, as shown in Figs. 1 and 2. Each Mo^V atom is pentacoordinate. The geometry of the coordination polyhedron can be described in terms of a square pyramid distorted toward a trigonal bipyramid. Three of the coordination sites on the Mo^V atom are occupied by a terminal oxygen atom and two sulfur atoms, and the other two by two oxygen atoms from a tart ligand (one from hydroxylate and the other one from carboxylate). The bond lengths of Mo^V–Mo^V [2.8650(8) Å], Mo=O [1.684(3) Å] and Mo–S [2.3102(15), 2.3243(14) Å] are comparable to the related clusters containing {Mo₂S₂O₂} units [7]. The Mo1–O2 bond length is 1.996(4) Å, near to that of Mo1–O3 [2.086(4) Å]. The bond angle of O2–Mo1–O3 is 76.77(14)°, similar to that in the peroxo complex K₄[Mo₂O₂(O₂)₄(C₄H₂O₆)]·4H₂O (75.83(5)°) with a tetradentately bridging tartrate [10]. The Cs1 and Cs2 atoms adopt ten and nine coordination modes, respectively, with Cs–O and Cs–S bond lengths in the ranges of 3.005(4)–3.572(4) Å and 3.654(2)–3.715(2) Å, respectively.

Fig. 1:

Molecular structure of the {[Mo₂O₂(μ-S)₂]₂(μ₄-tart)₂}⁴⁻ anion in 1. Selected distances (Å) and angles (deg): Mo(1)–O(4) 1.684(3), Mo(1)–O(2) 1.996(4), Mo(1)–O(3) 2.086(4), Mo(1)–S(1) 2.3243(14), Mo(1)–S(2) 2.3102(15), Mo(1)–Mo(2) 2.8650(8), C(1)–O(1) 1.219(6), C(1)–O(3) 1.316(6), Mo(2)–O(5) 1.678(4), Mo(2)–O(6) 2.099(4), Mo(2)–O(7) 2.003(4), Mo(2)–S(1) 2.3171(15), Mo(2)–S(2) 2.3301(15), C(4)–O(6) 1.301(6), C(4)–O(8) 1.232(6); O(2)–Mo(1)–O(3) 76.77(14), O(4)–Mo(1)–O(2) 108.37(17), O(4)–Mo(1)–O(3) 102.88(17), O(4)–Mo(1)–S(1) 104.73(13), O(4)–Mo(1)–S(2) 105.58(15), S(2)–Mo(1)–S(1) 101.07(5), S(1)–Mo(2)–S(2) 100.69(5), O(7)–Mo(2)–O(6) 77.24(14).

Fig. 2:

Molecular structure of 1; hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Cs(1)–O(4) 3.128(4), Cs(1)–O(5) 3.304(4), Cs(1)–S(1) 3.7148(16), Cs(2)–O(7) 3.057(4), Cs(2)–S(2) 3.4483(16), Cs(2)–S(1A) 3.6988(16), Cs(2)–O(1A) 3.325(4), Cs(2)–O(3A) 3.202(4); O(4)–Cs(1)–O(5) 65.22(10), O(4)–Cs(1)–S(1) 54.92(7), O(5)–Cs(1)–S(1) 53.36(7), O(3A)–Cs(2)–O(1A) 39.63(9).

Cluster 1 was loaded on the mesoporous silica SBA-15 by the immersion method, and the supported catalyst of 1/SBA-15 was characterized by IR spectroscopy, low-angle X-ray diffraction (LAXRD), transmission electron microscopy (TEM) and low-pressure N₂ sorption measurement. The LAXRD patterns (Fig. S3) and TEM images (Fig. S4) of SBA-15 and 1/SBA-15 indicated that the supported catalyst maintained the pore structure of SBA-15. The energy dispersive X-ray spectrum (Fig. S5) showed the peaks of C, O, Mo, S and Cs, which indicated that the molybdenum sulfide cluster has been loaded on SBA-15. Figure 3a shows the N₂ sorption isotherms and pore distribution of SBA-15 and 1/SBA-15. The Brunauer-Emmett-Teller surface area and pore volume of 1/SBA-15 are lower than those of SBA-15 (Table S1), because the cluster occupies part of the inner surface area and the micropore volume [13]. The reducibility of the 1/SBA-15 catalyst was studied by the temperature-programmed reduction (TPR) method in the temperature range 20–600°C. The TPR curve of 1/SBA-15 (Fig. 3b) indicated hydrogen absorption from 330°C to 362°C and reached a peak at 347°C.

Fig. 3:

(a) N₂ sorption isotherms of SBA-15 (■) and 1/SBA-15 (●); (b) TPR curve of 1/SBA-15. The insets are the corresponding pore size distribution curves.

The catalytic activity of 1/SBA-15 was evaluated in the thiophene-ethanol system. In a stainless autoclave reactor, the catalytic reaction was carried out at 280°C and for different times. From 2 h to 16 h, the conversion rate of thiophene increased with the reaction time. After 16 h, the hydrodesulfurization (HDS) effect was no longer increased (Fig. S6). Subsequently, the thiophene-ethanol system was studied by gas chromatography (GC) and GC-mass spectrometry (GC-MS). The GC result showed that 85.8% of thiophene was desulfurized after 16 h at 280°C. The GC-MS result indicated that the HDS product of thiophene was 1-butanol (Fig. S7). This product was also reported by other oxothiomolybdenum catalytic system containing {Mo₂O₂S₂} building unit [13], while molybdenum hydride complexes could catalyze the cleavage of the C–S bonds of thiophene to liberate but-1-ene [19]. Compared to (Co)MoS₂ catalysts [20], 1/SBA-15 showed relatively higher catalytic activity for the HDS of thiophene.

In summary, a new [Mo₂O₂S₂]-based ring cluster 1 incorporating tart⁴⁻ as a bridging ligand was synthesized and characterized by single-crystal X-ray diffraction along with spectroscopic methods. The cyclic cluster polyanion consists of two dinuclear [Mo₂O₂(μ-S)₂]²⁺ moieties and two bridging tart⁴⁻ ligands. The cyclic polyoxo-thiomolybdate cluster was effectively supported on SBA-15. Catalytic activity of the supported catalyst was tested for the thiophene HDS reaction.

4 Supplementary information

NMR and IR spectra and other supporting data associated with this article can be found in the online version (DOI: https://doi.org/10.1515/znb-2017-0002).