Stabilization of molecular lanthanide polysul fi des by bulky scorpionate ligands † ‡

Polysulfides of the rare-earth metals (Ln) have been known for over 100 years. The most common types of these materials are LnS2 3,4 and LnS2−x, 5,6 but well-defined, more complicated species such as Ln8S14.9 (Ln = Dy, Ho) have also been described. The synthetic routes to such lanthanide polysulfides normally require high-temperature and/or high-pressure techniques. In sharp contrast, molecular lanthanide complexes containing polysulfide ligands remain little explored, although a number of interesting inorganic and organometallic sulfido clusters have been reported in recent years. Most of these contain the S2 2− ligand, while (Cp*2Sm)2(S3)(THF) (Cp* = pentamethylcyclo-pentadienyl) appears to be the only lanthanide complex with an S3 2−

It is well established from the chemistry of the bent metallocenes Cp* 2 Ln (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 S 2− or RS − ligands. 10,21,22hus, 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 S 8 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 iPr 2 ) 2 (Ln = Sm (1), Yb (2)) with sulfur.

Synthesis and characterization
The reaction of Sm(Tp iPr 2 ) 2 (1) with sulfur was first carried out in toluene.Addition of one equiv.of S 8 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 (S 8 ) 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 (Et 2 O/THF; Et 2 O/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 ) (κ 1 -3,5-i Pr 2 Hpz)(S 5 ) (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-polysul-fide 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 (κ 2 -3,5-i Pr 2 pz) (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.
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•2C 5 H 12 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 κ 3 -Tp iPr 2 ligand, a coordinated, neutral κ 1 -3,5-i Pr 2 Hpz ligand, and the electronic demand of the Sm(III) ion is satisfied by the dianionic S 5 2− polysulfide ligand.The most remarkable feature of the compound is the conformation of the SmS 5 six-membered ring.As opposed to the classical chair conformation found in transition-metal complexes containing the MS 5 moiety, such as Cp 2 MS 5 (M = Ti, Zr, Hf ), [FeS 5 (µ-S)] 2 2− and [Pt(S 5 ) 3 ] 2− , [25][26][27][28][29][30] and that in cyclo-S 6 , 31 the SmS 5 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, (C 5 Me 5 ) 2 ThS 5 , 32 and may signal that the twist-boat conformation is a common feature of An/LnS 5 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

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 iPr 2 ) 2 (κ 2 -3,5-i Pr 2 pz) (4) contains a κ 2 -pyrazolide wedged between two (Tp iPr2 ) 2 ligand, holds out the hope that under suitable conditions, highly reducing Ln(Tp iPr 2 ) 2 compounds might be able to bind and activate interesting small molecules like CO, CO 2 , 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 iPr 2 )(κ 1 -3,5-i Pr 2 Hpz)(S 5 ) ( 3) and (µ-S 4 )[Yb(Tp iPr 2 )-(κ 1 -3,5-i Pr 2 Hpz)(κ 2 -3,5-i Pr 2 pz)] 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.

Synthesis of complex 3
(i) Sulfur flakes (56.7 mg, 0.22 mmol S 8 ) 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 Et 2 O/THF solvent mixture.Crystallization attempts from n-pentane only produced powdery solids.Crystallization from Et 2 O/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 Et 2 O/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.