Room temperature synthesis, crystal structure and selected properties of the new compound [Mn₂(bipy)₄SbS₄](ClO₄)

1 Introduction

The thioantimonate chemistry is dominated by Sb(III)S_x (x = 3–5) units which are in most structures further condensed to form larger building units [1–5] like [Sb₃S₆]^3– or [Sb₄S₈]^4– rings [6–8], chains or layers with compositions [Sb₄S₇]^2– [9–17], [Sb₃S₅]^– [18], [Sb₆S₁₀]^2– [19–21] or [Sb₈S₁₃]^2– [17, 20, 22–24] or even three-dimensional networks [9, 25–28]. Compared to thioantimonates(III) the structural chemistry of [Sb(V)S₄]^3– is much less developed and in most compounds the cations and the [Sb(V)S₄]^3– anions are separated [29–35]. There are only few exceptions where bond formation between the cation and [SbS₄]^3– was observed [36, 37]. In these compounds the SbS₄ group mostly acts in a monodentate fashion like in [Mn(dach)₃][Mn(dach)₂(SbS₄)₂] · 6H₂O [38] (dach = 1,2-diaminocyclohexane) or the pseudo-polymorphs [Co(dien)₂][Co(tren)SbS₄]₂ · 4H₂O [36] (dien = diethylenetetraamine, tren = tris(2-aminoethyl-)amine) and [Co(dien)₂][Co(tren)SbS₄]₂ · 0.5H₂O [39]. However, bi- and tridentately acting [SbS₄]^3– anions are observed in very few compounds [40–43]. There are also few examples for mixed-valent Sb(III,V) thioantimonates like [Mn(dien)₂]₂[MnSb₂S₇] [44] or the isostructural compounds [Ni(dien)₂]₂[Sb₄S₉] [45] and [Co(dien)₂]₂[Sb₄S₉] [46]. Among the examples for an incorporation of transition metal cations into thioantimonates Mn²⁺ plays a special role. While for ions like Ni²⁺, Co²⁺, Fe²⁺ or Zn²⁺ synthetic tricks are required to force Sb–S–M²⁺ (M = Ni, Co, Fe, Zn) bond formation, [10, 47–50] Mn²⁺ is easily integrated into thioantimonate networks [51–57].

Recently we reported a new synthesis strategy for the generation of thioantimonates(III) utilizing Na₃SbS₄·9H₂O (Schlippe’s salt) as source. The [SbS₄]^3– anion undergoes a sequence of chemical reactions with water supplying SbS₃^3– besides other antimony compounds. Applying Ni²⁺ or Fe²⁺ complexes, the reactions with Schlippe’s salt afforded crystallization of [Ni(bipy)₃][Sb₆S₁₀], [Fe(bipy)₃][Sb₆S₁₀], and [Ni(dibipy)₃][Sb₆S₁₀] (bipy = 2,2′-bipyridine, dibipy = 4,4′-dimethyl-2,2′-bipyridine) [58]. We now extended the synthetic work to Mn²⁺ complexes using [Mn(bipy)₃](ClO₄)₂ as starting material. By adding water to a mixture of [Mn(bipy)₃](ClO₄)₂ and Na₃SbS₄ · 9H₂O at room temperature a phase pure polycrystalline powder was obtained. Upon applying solvothermal techniques the slurry yielded single crystals of a compound which could then be identified as [Mn₂(bipy)₄SbS₄](ClO₄). The reaction also partially led to a polysulfide, namely [Mn(bipy)₂S₆] and an unknown compound.

The new compound [Mn₂(bipy)₄SbS₄](ClO₄) features a [Mn₂(bipy)₄SbS₄]⁺ cation and a perchlorate anion. Here we report the preparation, crystal structure and selected properties of this compound.

2 Results and discussion

[Mn₂(bipy)₄SbS₄](ClO₄) crystallizes in the monoclinic space group C2/c with two unique S atoms and one Mn atom as well as two bipy ligands on general positions. The perchlorate anion is disordered over two positions around a two-fold axis. The Mn²⁺ ion is in a distorted octahedral environment of four N atoms of the two bipyridine ligands and two S atoms of a [SbS₄]^3– unit (Fig. 1). The Mn–N and Mn–S bond lengths (2.222(4)–2.305(4) Å for Mn–N and 2.5804(13) and 2.6429(13) Å for Mn–S) and angles (Table 1) are in the range known from other thioantimonates [10, 38, 41, 56, 59–63]. The [SbS₄]^3– unit acts as a tetradentate bridging ligand and exhibits a distorted tetrahedral geometry. The Sb–S bond lengths in the [SbS₄]^3– unit are in the normal range (2.3246(11)–2.3342(11) Å) known from other thioantimonates(V) [36, 38, 39, 41, 64]. The S–Sb–S angles (100.53–116.95°) indicate a moderate distortion of the tetrahedral geometry.

Fig. 1:

Structure of the [Mn₂(bipy)₄SbS₄]⁺ cation of the title compound. H atoms are omitted for clarity.

The cations form rods running along [001]. Along [010] the cations and anions alternate in the fashion ···cation-cation-anion-cation-cation···. Between the cations and the anions C–H···O (O···H distances: 2.33–2.63 Å; C–H···O angles: 131.7–155.7°) and C–H···S interactions (H···S distances 2.86–2.89 Å. C–H···S angles 148.0–168.3°) are observed (Table 2).

Off-center parallel interactions of bipyridine ligands of adjacent complexes with distances ranging from 3.19 to 3.54 Å (Fig. 2) lead to a two-dimensional layer-like arrangement in the (010) plane (Fig. 3). The intermolecular distances between stacked bipyridine ligands indicate attractive interactions as was already demonstrated for arenes of different sizes in reference [65]. Recently, the interaction energy for the stacking of phen molecules was calculated for [Mn₂(phen)₄(Sn₂S₆)] [66] exhibiting similar separations of the aromatic rings. The values obtained are around 10 kcal/mol and one can assume that these intermolecular interactions contribute to the arrangements of the complexes and the stability of the crystals [65].

Fig. 2:

Different arrangements of the bipy molecules in the structure of [Mn₂(bipy)₄SbS₄](ClO₄). The dashed red lines show the shortest distances (values in Å).

Fig. 3:

Arrangement of the [Mn₂(bipy)₄SbS₄]⁺ cations in the crystallographic ac plane. H atoms are omitted for clarity and π-π interactions are indicated by dashed lines.

The new compound contains a hitherto unknown [Mn₂(bipy)₄SbS₄]⁺ cation. There is only one example reported in thioantimonate(III) chemistry, where a cationic species containing a thioantimonate anion is observed [67]. In the crystal structure of {[Eu(dien)₂]₂(μ₄-Sb₂S₅)}Cl₂ the [Sb₂S₅]^4– anion connects two [Eu(dien)₂]³⁺ complexes to form the [Eu(dien)₂]₂(μ₄-Sb₂S₅)]²⁺ cation which is charge balanced by two chloride anions [67].

Another interesting feature is the fact that the [SbS₄]^3– anion acts as a tetradentate ligand. The majority of the thioantimonate(V) compounds consist of separated cations and tetrahedral [SbS₄]^3– anions, or of monodentately acting [SbS₄]^3–. Only a few thioantimonates(V) contain [SbS₄]^3– acting as a bidentate or tridentate ligand. Examples for the bidentate mode are [Mn(dien)₂][Mn(dien)SbS₄]₂ [41] and [Ln(en)₃(H₂O)SbS₄ ] (Ln = Sm [37], Pr [68]). The μ₃-bridinging mode is more often observed, especially in rare earth cation containing compounds in which [SbS₄]^3– anions serve as ligands to form one-dimensional chains [43, 64, 69]. The first example for the μ₄-mode of the anion has now been detected in [Mn₂(bipy)₄SbS₄](ClO₄).

The title compound was prepared under similar conditions as used for the synthesis of [Ni(bipy)₃][Sb₆S₁₀], [Fe(bipy)₃][Sb₆S₁₀] and [Ni(DiBipy)₃][Sb₆S₁₀] [58] reported recently. Using the corresponding Mn²⁺ complex in the synthesis leads to the formation of the title compound instead of anionic thioantimonate(III) layers. One reason may be that [Mn₂(bipy)₄SbS₄](ClO₄) crystallized very fast and first crystallites are already observed within a few seconds after mixing the educts in water at room temperature. Hence one can assume that the concentration of the slowly generated [SbS₃]^3– anion is too low to allow formation of the anionic network by condensation of thioantimonate(III) species. This assumption is supported by time dependent synthesis studies where small needles of [Ni(bipy)₃]Sb₆S₁₀ were formed only after 3.5 h. For the title compound, the formation mechanism might be a ligand exchange reaction in which one bipy ligand of a [Mn(bipy)₃]²⁺ complex is replaced by a [SbS₄]^3– unit according to the following equation.

2 [Mn(bipy)3]2++ 4ClO4–+ 3Na++ SbS43–→ [Mn2(bipy)4SbS4](ClO4)↓+ 3Na++ 3ClO4–+ 2bipy

The stability constants of [TM(bipy)₃]²⁺ complexes provide an explanation for this pathway. With log β₃ = 20.1 and 17.5 for Ni²⁺ and Fe²⁺ complexes, the stability constants are about three times larger than for the Mn²⁺ complex (β₃ = 6) [70]. Hence, it can be assumed that [Mn(bipy)₃]²⁺ is labile so that it dissociates and the tetrathioantimonate ion coordinates to Mn²⁺ cations within seconds. The robust Ni²⁺/Fe²⁺ complexes are stable in the presence of the Sb(III)S₃ species generated in time which then condense around the complexes forming the [Sb₆S₁₀]^2– networks. Even though the title compound immediately crystallized as a pure material when the two solutions were mixed, solvothermal conditions were required for the growth of crystals suitable for the structure analysis (Fig. 4). During the solvothermal reaction the title compound was only found as a by-product besides the polysulfide complex [Mn(bipy)₂S₆] [71] and an unknown compound (Fig. 5) containing Sb, S and Mn according to EDX analysis (Mn 17.9, Sb 22.9, S 59.2 at%). The appearance of the polysulfide anion and the sulfur rich unknown compound suggests that redox reactions take place during the synthesis at 140 °C and that the unknown compound might contain Sb(III).

Fig. 4:

Experimental X-ray powder pattern of [Mn₂(bipy)₄SbS₄](ClO₄) obtained by room temperature synthesis, together with the simulated pattern.

Fig. 5:

Simulated X-ray powder patterns of [Mn₂(bipy)₄SbS₄](ClO₄) and [Mn(bipy)₂S₆] and experimental X-ray powder pattern of the product of the solvothermal synthesis.

2.1 UV/Vis spectroscopy

By applying the Kubelka–Munk method, the band gap was estimated to 2.4 eV. In the spectrum further maxima are observed at 4.9 and 4.2 eV, which are also observed for [Mn(bipy)₃]²⁺ complexes and that can be assigned to charge-transfer transitions (Fig. 6) [72].

Fig. 6:

UV/Vis diffuse reflectance spectrum of [Mn₂(bipy)₄SbS₄](ClO₄). The red line indicates the estimation of the band gap.

2.2 Magnetic properties

The inverse magnetic susceptibility of the title compound is shown in Fig. 7. The evolution of χT with decreasing temperature indicates very weak ferromagnetic interactions and below 40 K the curve exhibits a pronounced drop. Evaluation of the data with the Curie–Weiss law yields an effective magnetic moment of 5.54 μ_B per Mn²⁺ ion which is lower than the spin only value of 5.92 μ_B. The Weiss constant of –2.3 K may indicate weak antiferromagnetic exchange interactions between the two Mn²⁺ centers mediated by the thioantimonate ion.

Fig. 7:

Temperature dependence of the product χT of [Mn₂(bipy)₄SbS₄](ClO₄). The insert shows the inverse magnetic susceptibility.

3 Conclusion

A simple room temperature synthetic route afforded the new thioantimonate(V) compound [Mn₂(bipy)₄SbS₄](ClO₄) featuring the novel [Mn₂(bipy)₄SbS₄]⁺ cation. The SbS₄^3– anion acts as a tetradentate bridging ligand, a connection mode observed here for the first time. Under solvothermal conditions the compound is at least partially decomposed and [Mn(bipy)₂S₆] is formed. In further investigations the synthetic potential of the new room temperature approach for the generation of new thioantimonate compounds will be explored.

4 Experimental section