A cyclic mixed-valence Mo^VI/Mo^V polyoxothiomolybdate cluster anion stabilized by a [(μ-I){Ag(PPh₃)₃}₂]⁺ complex cation. Preparation and structure of [(μ-I){Ag(PPh₃)₃}₂]₂ [Mo₈O₈(μ-OH)₂(μ-OEt)₆(μ-S)₈(μ₃-O)₄{Mo(DMF)₂}] · 2EtOH

1 Introduction

Transition metal sulfide systems are prominent as both biological [1] and industrial catalysts [2] because sulfur-coordinated transition metals are involved in facile electron and proton transfer processes, which are responsible for high active-site turnover. The long-standing interest in the chemistry of thiometalates, especially that of molybdate and tungstate, has been strongly driven by the petroleum industry because of the central role of MoS₂ and WS₂ in hydrotreating catalysis [3], including hydrodesulfurization, hydrodenitrogenation and hydrodearomatization catalysts, which has resulted in vigorous studies of M-S (M = Mo or W) coordination reactions [4]. As is well documented, reactions of tetrathiometalates with coinage-metal ions, such as Cu⁺, Ag⁺ or Au⁺, result in the formation of many heterobimetallic clusters with different structural types [5–8]. Addition of an electrophile, such as a proton, elemental sulfur or organic polysulfide to tetrathiometalates leads to the formation of reduced M^V or M^IV (M = Mo or W) species [9] with low nuclearity, rarely exceeding four metal centers. However, the strategy based on transition metal oxy-sulfide clusters with {M₂S₂O₂} (M = Mo or W) building unit needs to be further developed [10, 11] because polyoxothiometalates, relative to polyoxo-metalates, emerge as a fascinating class of novel compounds that provide transition metal ring-like clusters based on the self-condensation of [M₂S₂O₂]²⁺ (M = Mo or W) oxothiocations. Since Cadot et al. [10, 11] firstly reported a neutral cyclic oxothio cluster [Mo₁₂S₁₂O₁₂(OH)₁₂(H₂O)₆], a series of anionic polyoxothiometalates in flexible and fluxional host-guest assemblies with the nuclearity varying in the Mo₈–Mo₁₆ range have been successfully synthesized [12, 13]. In the course of our research on the synthesis of poly-nuclear metal oxysulfide clusters, we have become interested in the interactions between [Mo₂S₂O₂]²⁺ and coinage-metal ions in the presence of organic ligands. A cyclic mixed-valence Mo^VI/Mo^V polyoxothiomolybdate cluster [(μ-I){Ag(PPh₃)₃}₂]₂[Mo₈O₈(μ-OH)₂(μ-OEt)₆(μ-S)₈ (μ₃-O)₄{Mo(DMF)₂}] · 2EtOH has now been isolated. In this paper, the synthesis and structural characterization of the title cluster are described.

2 Experimental section

3 Results and discussion

Treatment of (NMe₄)₂[Mo₂O₂S₆] and Ag(PPh₃)₃I in the presence of NaOEt in DMF/EtOH resulted in the formation of a novel Mo^VI/Mo^V polyoxothiomolybdate cluster [(μ-I){Ag(PPh₃)₃}₂]₂[Mo₈O₈(μ-OH)₂(μ-OEt)₆(μ-S)₈(μ₃-O)₄ {Mo(DMF)₂}] · 2EtOH, isolated in 43 % yield. The cyclic cluster polyanion consists of a ring octamer {Mo₈O₈ (μ-OH)₂(μ-OEt)₆(μ-S)₈(μ₃-O)₄} that encapsulates a central {Mo^VI(DMF)₂(μ₃-O)₄} octahedron. Obviously, the reaction involves a self-condensation of the [Mo₂S₂O₂]²⁺ precursor and a partial substitution of OH⁻ and H₂O by EtO⁻ and DMF, respectively. The Ag⁺ ion is not ligated to sulfur or to oxygen atoms of the cluster anion but acts as a counterion in the form of [(μ-I){Ag(PPh₃)₃}₂]⁺. It has been noted that a similar polyoxothiomolybdate [N(CH₃)₄]₂[Mo₉S₈O₁₂(OH)₈(H₂O)₂] · 5H₂O was previously reported by Dolbecq et al. [20].

The structure of the title cluster was confirmed by an X-ray diffraction study. The unit cell of the title cluster consists of one well-separated polyoxo-thiomolybdate anionic [Mo₈O₈(μ-OH)₂(μ-OEt)₆μ-S)₈(μ₃-O)₄{Mo(DMF)₂}]²⁻ and two independent cationic [(μ-I){Ag(PPh₃)₃}₂]⁺ moieties (shown in Fig. 1) along with two EtOH solvent molecules, a finding supported by satisfactory microanalytical results. The peak at 1647 cm⁻¹ for ν(C=O) in the IR spectrum indicates the presence of DMF in the title cluster [13]. The characteristic intense bands at 972 cm⁻¹ for ν(MoS) and 538 cm⁻¹ for ν(Mo–μ-OH–Mo) are also observed in the IR spectrum [10, 11]. The less intense absorptions at 491, 409 and 352 cm⁻¹ for the Mo–S–Mo stretching modes [10, 11] can be identified since they appear as weak and sharp bands in the low-wavenumber region below 500 cm⁻¹. The UV/Vis spectra of the title cluster in THF, H₂O and DMF solution showed intense absorption bands at 311, 313 and 325 nm, respectively (see Fig. 2), which may be attributed to an intraligand transition of the PPh₃ ligands.

Fig. 1:

Molecular structures of the polyoxothioanion [Mo₈O₈ (μ-OH)₂(μ-OEt)₆(μ-S)₈(μ₃-O)₄{Mo(DMF)₂}]²⁻ (a) and the cation [(μ-I){Ag(PPh₃)₃}₂]⁺ (b), showing 30 % displacement ellipsoids. All hydrogen atoms were omitted for clarity.

Fig. 2:

UV/Vis spectra of the title cluster measured in THF (a), H₂O (b) and DMF (c).

The anion has an imposed invasion symmetry at the Mo^VI atom (Mo1). It can be described as an eight-membered ring encapsulating a Mo^VIO₆ octahedron. The central Mo^VI atom is connected to four bridging oxygen atoms and two oxygen atoms of DMF. Four {Mo₂S₂O₂} units formed an eight-membered ring. The Mo^V atoms are the centers of slightly distorted octahedra connected also to a terminal oxygen atom. In addition, four of the Mo^V atoms are connected to a hydroxo and an ethoxyl group and to two sulfido bridging ligands; the other four Mo^V atoms are connected to two ethoxyl groups and two sulfido bridging ligands. The octahedral coordination sphere of the Mo centers is completed by a bridging oxygen atom bound to the central Mo^VI atom [20]. The octahedra within the {Mo₂S₂O₂} building blocks share edges, but they are also connected by faces, as shown in Fig. 1. Short (ca. 2.8 Å) Mo^V–Mo^V bonds alternate with longer (ca. 3.2 Å) Mo–Mo bonds within the eight-membered ring. The Mo^VI atom exhibits a distorted octahedral coordination [Mo–O =1.662(4)–2.417(4) Å]. In this mixed-valent compound, the electrons are thus strictly localized, as in the so-called molybdenum blue species [21, 22]. In the cation [(μ-I){Ag(PPh₃)₃}₂]⁺, two distorted tetrahedral AgP₃I share an iodid ion. The Ag–P and Ag–I bond lengths are 2.5590(17)–2.5670(17) and 2.9241(7)–2.9244(7) Å, respectively. The Ag–I–Ag bond angle of 178.79(2)° is nearly linear.