Stabilization of molecular lanthanide polysulfides by bulky scorpionate ligands

Abstract

Well-defined lanthanide polysulfide complexes containing S₄²⁻ and S₅²⁻ ligands, the samarium(III) pentasulfide complex Sm(Tp^iPr₂)(κ¹-3,5-ⁱPr₂Hpz)(S₅) and the tetrasulfide-bridged binuclear ytterbium(III) complex (μ-S₄)[Yb(Tp^iPr₂)(κ¹-3,5-ⁱPr₂Hpz)(κ²-3,5-ⁱPr₂pz)]₂ (Tp^iPr₂ = hydro-tris(3,5-diisopropylpyrazolyl)borate), have been synthesized and structurally characterized by single-crystal X-ray diffraction.

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Introduction

Polysulfides of the rare-earth metals (Ln) have been known for over 100 years.^1,2 The most common types of these materials are LnS₂ ^3,4 and LnS_2−x,^5,6 but well-defined, more complicated species such as Ln₈S_14.9 (Ln = Dy, Ho) have also been described.⁷ The synthetic routes to such lanthanide polysulfides normally require high-temperature and/or high-pressure techniques.^8,9 In sharp contrast, molecular lanthanide complexes containing polysulfide ligands remain little explored, although a number of interesting inorganic^10–17 and organometallic^18–20 sulfido clusters have been reported in recent years. Most of these contain the S₂²⁻ ligand, while (Cp*₂Sm)₂(S₃)(THF) (Cp* = pentamethylcyclo-pentadienyl) appears to be the only lanthanide complex with an S₃²⁻ ligand.¹⁰ We describe here the formation and structural characterization of the first lanthanide polysulfide complexes containing the S₄²⁻ and S₅²⁻ ligands, namely Sm(Tp^iPr₂)(κ¹-3,5-ⁱPr₂Hpz)(S₅) (3) and (μ-S₄)[Yb(Tp^iPr₂)(κ¹-3,5-ⁱPr₂Hpz)(κ²-3,5-ⁱPr₂pz)]₂ (5) (Tp^iPr₂ = hydro-tris(3,5-diisopropylpyrazolyl)borate; 3,5-ⁱPr₂pz = 3,5-diiso-propylpyrazolate; 3,5-ⁱPr₂Hpz = 3,5-diisopropylpyrazole), which are stabilized by bulky scorpionate ligands.

It is well established from the chemistry of the bent metallocenes Cp*₂Ln (Ln = Sm, Eu, Yb) that these strongly reducing lanthanide(II) precursors are easily oxidized by elemental sulfur or diorganodisulfides (RSSR), resulting in the formation of trivalent metallocenes containing S²⁻ or RS⁻ ligands.^10,21,22 Thus, it was of interest to investigate the reactivity of the recently reported “bent sandwich-like” divalent lanthanide scorpionates, Ln(Tp^iPr₂)₂ (Ln = Sm, Eu, Tm, Yb)^23,24 with S₈ with a view to establish whether the presence of bulky scorpionate ligands might lead to the formation of polysulfide-ligated species. Here we report the results of the reactions of Ln(Tp^iPr₂)₂ (Ln = Sm (1), Yb (2)) with sulfur.

Synthesis and characterization

The reaction of Sm(Tp^iPr₂)₂ (1) with sulfur was first carried out in toluene. Addition of one equiv. of S₈ to a toluene solution of 1 at r.t., resulted in a rapid color change from dark green to orange, with some unreacted sulfur remaining at the bottom of the flask. Recovery of the sulfur showed that only half an equivalent of sulfur (S₈) reacted with 1. Solvent removal produced a somewhat sticky yellow solid, the usual form of crude products due to the greasy, lipophilic nature of the Tp^iPr₂ ligand. Curiously, crystallization attempts from various solvent mixtures (Et₂O/THF; Et₂O/hexane) first produced small amounts of yellow crystals, which proved to be elemental sulfur, orthorhombic and monoclinic forms. From further crystallization attempts only a few suitable crystals for X-ray analysis could be isolated, which were shown to be Sm(Tp^iPr₂)(κ¹-3,5-ⁱPr₂Hpz)(S₅) (3), Scheme 1. The observation that the first crystalline materials were elemental sulfur indicated either that the amount of sulfur in solution was more than needed for the reaction or that some intermediate Sm-polysulfide species underwent sulfur extrusion. To answer the question as to the stoichiometric amount of sulfur needed to oxidize the precursor Sm(Tp^iPr₂)₂ compound, a second reaction was carried out which showed that bleaching of the dark green THF solution of 1 at RT occurred after the addition of only 1 equiv. of sulfur (S) per Sm(II). Crystallization of the crude, yellow, sticky solid produced a small amount of white crystals which proved to be Sm(Tp^iPr₂)₂(κ²-3,5-ⁱPr₂pz) (4, Scheme 1). Unfortunately, numerous other crystallization attempts from various mixed solvent system did not yield crystals suitable for X-ray analysis, hence the nature of the sulfur containing species from this second reaction remain unknown.

Scheme 1 Reactions of Ln(Tp^iPr₂)₂ (Ln = Sm, Yb) with elemental sulfur.

The analogous reaction of Yb(Tp^iPr₂)₂ (2) with sulfur was found to be more straightforward. Stirring of equimolar amounts (Yb : S) of 2 and sulfur in toluene for 24 h caused a color change from bright red to amber.

Recrystallization of the reaction product from n-pentane afforded orange, cube-like single crystals of 5·2C₅H₁₂ suitable for X-ray diffraction.

The constitution and molecular structure of compounds 3, 4 and 5 were revealed by single crystal X-ray diffraction studies. The structures of 3 and 5 are shown in Fig. 1 and 2, respectively. The structure of compound 4 can be found in the ESI.‡ The structure of 3 consists of a normal κ³-Tp^iPr₂ ligand, a coordinated, neutral κ¹-3,5-ⁱPr₂Hpz ligand, and the electronic demand of the Sm(III) ion is satisfied by the dianionic S₅²⁻ polysulfide ligand. The most remarkable feature of the compound is the conformation of the SmS₅ six-membered ring. As opposed to the classical chair conformation found in transition-metal complexes containing the MS₅ moiety, such as Cp₂MS₅ (M = Ti, Zr, Hf), [FeS₅(μ-S)]₂²⁻ and [Pt(S₅)₃]²⁻,^25–30 and that in cyclo-S₆,³¹ the SmS₅ six-membered ring adopts the twist-boat conformation. It is noteworthy that a similar ring conformation was also observed in the related actinide f-element compound, (C₅Me₅)₂ThS₅,³² and may signal that the twist-boat conformation is a common feature of An/LnS₅ containing compounds. The similarities between the two structures extend to the M–S distances as well. Interestingly not only the terminal S(alpha) atoms are bonded to Sm (Sm–S1/S5 2.755(1)/2.792(1) Å) but also the two S(beta) atoms (Sm–S2/S4 2.952(1)/3.090(1) Å); the corresponding distances in the C₂ symmetric (C₅Me₅)₂ThS₅ compound are Th–S1/S1′ and Th–S2/S2′, 2.768(4) and 3.036(4), respectively. In the transition-metal MS₅ compounds with the chair conformation only the terminal sulfurs of the chelating S₅²⁻ ligand are coordinated to the metal.

Fig. 1 Molecular structure of 3 in the crystal showing the atom-labeling scheme. Selected bond lengths [Å] and angles [°]: Sm–N12 2.533(2), Sm–N22 2.531(2), Sm–N32 2.509(2), Sm–N41 2.606(2), Sm–S1 2.755(1), Sm–S5 2.792(1), Sm–S2 2.952(1), Sm–S4 3.090(1); Sm–S1–S2 74.66(3), Sm–S5–S4 77.88(3), S1–Sm–S5 113.10(2), S1–S2–S3 106.63(5), S5–S4–S3 108.54(5), S2–S3–S4 101.42(4).

Fig. 2 Molecular structure of 5 in the crystal showing the atom-labeling scheme. Symmetry operator to generate equivalent atoms: ′ 2 − x, y, 1 − z. Selected bond lengths [Å] and angles [°]: Yb–N12 2.423(7), Yb–N22 2.421(7), Yb–N32 2.406(6), Yb–N41 2.304(7), Yb–N42 2.244(7), Yb–N52 2.443(7), Yb–S1 2.703(2), S1–S2 2.072(3), S2–S2′ 2.044(5), Yb–S1–S2 106.0(1), S1–S2–S2′ 106.8(1), S1–S2–S2′–S1′ dihedral angle 69.5(6).

Compound 5 is a dinuclear ytterbium(III) polysulfide complex in which two [Yb(Tp^iPr₂)(κ¹-3,5-ⁱPr₂Hpz)(κ²-3,5-ⁱPr₂pz)] moieties are bridged by a μ-S₄²⁻ ligand. Each formally 7-coordinated ytterbium ion is bonded to an original κ³-Tp^iPr₂ ligand as well as a neutral κ¹-3,5-ⁱPr₂Hpz ligand and an anionic κ²-3,5-ⁱPr₂pz ligand which were formed as a result of ligand fragmentation.³³ Certainly this accounts for the fairly low isolated yield of compound 5 (32%). In accordance with the overall C₂-symmetry of the molecule, the ¹¹B NMR spectrum of 5 shows only a single resonance (δ = −4.4 ppm). The conformation of the bridging μ-S₄²⁻ ligand closely resembles that of the tetrasulfide dianion in crystalline Na₂S₄ ^34,35 with a dihedral angle of 69.5(6)° (97.81° in Na₂S₄). The Yb–S bond length is 2.703(2) Å.

Conclusions

In summarizing the results reported here, although formation of compounds 3, 4 and 5 was accompanied by Tp^iPr₂ ligand fragmentation, their isolation underscores the following points: 1. The fact that Sm(Tp^iPr₂)₂(κ²-3,5-ⁱPr₂pz) (4) contains a κ²-pyrazolide wedged between two (Tp^iPr₂)₂ ligand, holds out the hope that under suitable conditions, highly reducing Ln(Tp^iPr₂)₂ compounds might be able to bind and activate interesting small molecules like CO, CO₂, etc. 2. The very rapid reaction of compound 1 with sulfur and the rather non-selective formation of compound 3 underlines the stronger reducing power of the Sm(II) precursor compared with Yb(II). Thus, the reactivity of Yb(Tp^iPr₂)₂ is significantly reduced and allows for the more straightforward preparation of the tetrasulfide derivative 5. 3. The successful isolation and structural characterization of the first lanthanide(III) polysulfide complexes, Sm(Tp^iPr₂)(κ¹-3,5-ⁱPr₂Hpz)(S₅) (3) and (μ-S₄)[Yb(Tp^iPr₂)(κ¹-3,5-ⁱPr₂Hpz)(κ²-3,5-ⁱPr₂pz)]₂ (5) encourages the quest for more rational synthesis of similar lanthanide polysulfides and the study of their bonding characteristics. Such studies are underway in our laboratories.

This work was financially supported by the Otto-von-Guericke-Universität Magdeburg and the University of Alberta. JT thanks Jackie Kiplinger (Los Alamos National Laboratory) for useful discussions and we thank the reviewers for helpful comments.

Experimental section

Synthesis of complex 3

(i) Sulfur flakes (56.7 mg, 0.22 mmol S₈) were added to a stirred toluene solution of 1 (239 mg, 0.22 mmol) at r.t. After only about a minute the color changed from dark green to orange, with substantial amount of unreacted sulfur (ca. 28 mg) at the bottom of the round-bottom flask. Decantation from the unreacted sulfur, followed by solvent removal gave a slightly sticky, yellow solid. Extraction with n-pentane left behind a small amount of pale yellow solid, shown to be sulfur (orthorhombic) by X-ray diffraction on crystals obtained from Et₂O/THF solvent mixture. Crystallization attempts from n-pentane only produced powdery solids. Crystallization from Et₂O/n-hexane solvent mixtures, two attempts, produced small amounts of yellow crystals which were again shown to be sulfur (orthorhombic and monoclinic forms). Ultimately from THF/hexamethyldisiloxane (HMDSO), a small number of pale yellow crystals were harvested, which proved to be compound 3.

(ii) Small amounts of sulfur flakes were added to a stirred THF solution of 1 (523 mg, 0.484 mmol) at r.t. After the addition of only 16 mg (0.5 mmol of S) the original dark green color changed to orange. Solvent removal gave a slightly sticky, yellow solid. Crystallizations from Et₂O/n-hexane solvent mixtures, twice produced crystals of compound 3. Numerous other crystallization attempts failed to produce crystals suitable for X-ray analysis and failed to give sulfur-containing material.

Synthesis of complex 5

Powdered sulfur (14 mg, 0.44 mmol) was added to a solution of compound 2 (484 mg, 0.44 mmol) in toluene (20 ml). After 24 h stirring at r.t. the color had changed from bright red to amber. Removal of the solvent and recrystallization from n-pentane (15 ml, −5 °C) afforded 152 mg (32%) of 5 as orange, cube-like crystals. Mp. 130 °C. ¹H NMR (toluene-d₈, 400 MHz): δ 10.53 (s br, 6H, pz), 5.40 (s br, 2H, C–HⁱPr), 3.04 (s, 2H, pz), 2.0–2.2 (s br, 2H, B–H), 2.38 (s, 12H, CH₃ⁱPr), 1.23 (m, 12H, CH₂ of n-pentane), 0.87 (t, 12H, CH₃ of n-pentane), −0.20 (s br, 6H, C–H ⁱPr), −0.65 (s, 2H, pz), −0.99 (s, 2H, C–H ⁱPr), −2.33 (s br, 12H, CH₃ⁱPr), −6.22 (s br, 48H, CH₃ⁱPr), −12.00 (s br, 48H, CH₃ⁱPr).¹³C NMR (toluene-d₈, 100 MHz): δ = 173.8 (q-C pz), 150.9 (q-C pz), 102.3 (CH pz) 87.3 (CH pz), 34.4 (CH₂ of n-pentane), 27.3 (CH ⁱPr), 23.5(CH₃ⁱPr), 22.7 (CH₂ of n-pentane), 21.5 (CH₃ⁱPr), 19.5 (C–H ⁱPr), 15.6 (CH₃ⁱPr), 14.1 (CH₃ of n-pentane). ¹¹B NMR (toluene-d₈, 128 MHz): δ = −4.4 ppm. Elemental analysis calcd for C₁₀₀H₁₇₈B₂N₂₀S₄Yb₂ (5·2C₅H₁₂): C 55.69, H 8.32, N 12.99; found C 56.04, H 8.64, N 12.50.

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Footnotes

† Dedicated to Professor R. Dieter Fischer on the occasion of his 80th birthday.

‡ Electronic supplementary information (ESI) available: Full crystallographic data for 3, 4, and 5. CCDC 1472924, 1472923 and 1472811. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt01439a

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