Donor-Solvent-Dependent Cluster Formation of (C5Me5)SmI2(THF)x

Oct 25, 2016 - In this study, we demonstrate the structural variety of the Cp*SmIy system with respect to the amount of donor molecules (THF) present...
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Donor-Solvent-Dependent Cluster Formation of (C5Me5)SmI2(THF)x‑Type Half-Sandwich Complexes

André M. Bienfait,† Benjamin M. Wolf,‡ Karl W. Törnroos,† and Reiner Anwander*,‡ †

Department of Chemistry, University of Bergen, Allégaten 41, N-5007, Bergen, Norway Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, D-72076, Tübingen, Germany

‡

S Supporting Information *

ABSTRACT: In this study, we demonstrate the structural variety of the Cp*SmIy system with respect to the amount of donor molecules (THF) present. The reaction of SmI2(THF)2 with 0.5 equiv of bis(pentamethylcyclopentadienyl)lead(II), PbCp*2, in THF gives the highly solvated monomeric complex Cp*SmI2(THF)3, when crystallized from THF. The same reaction performed in n-hexane followed by crystallization from toluene gave the partially solvated trimetallic complex [Cp*3Sm3I(μ2-I)3(μ3-I)2(THF)2]. Recrystallization of the Sm3 cluster from either C6D6 at ambient temperature or toluene at −40 °C produced the tetrametallic complex [Cp*4Sm4I(μ2-I)4(μ3-I)2(μ4I)(THF)]. Reaction of SmI2, obtained via treatment of SmI2(THF)2 with excess AlEt3 in n-pentane, with 0.5 equiv of PbCp*2 in toluene led to the donor-free mixed-valent pentametallic complex [Cp*5Sm5(μ2-I)4(μ3-I)4(μ5-I)]. All compounds were characterized by single-crystal X-ray structure analysis.

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INTRODUCTION The discovery of the mild preparation of SmI2(THF)2 and YbI2(THF)2 from 1,2-diiodoethane and the respective metal in THF by Kagan et al. in 1977 marked an efficient entry into divalent organolanthanide chemistry.1 Since then, a large number of complexes have been made accessible from LnI2(THF)2 (Ln = Sm, Eu, Yb; EuI2(THF)2 can be synthesized analogously)2 mainly by salt metathesis reactions.3 Sm(II) complexes, and in particular SmI2(THF)2 (Kagan’s reagent), which crystallizes from THF as the penta(THF) adduct SmI2(THF)5,4 are popular 1-e−-reducing agents in organic synthesis.5 It is only natural that the vast majority of such SmI2mediated organic syntheses have been aiming at the identification and isolation of the targeted reduced organic product, while the concomitantly formed samarium(III) complex was discarded.5 In contrast, the Evans group as well as others have comprehensively investigated the reaction products of Sm(II) organometallics and reducible substrates such as ketones or imines.3,6 There are few examples where oxidation protocols have been employed to transform divalent samarium precursors into Sm(III) reagents. Most of these examples were investigated by the Evans group3 dealing mainly with organosamarium(II) precursors (Scheme 1).7−17 The type of ancillary ligand affects markedly the reaction outcome in terms of ligand redistribution (metallocene versus halfsandwich versus homoleptic complex). For example, the halogenated species Cp*2SmX(THF) (X = Cl, I) were elegantly obtained from Cp*2Sm(THF)2 and 1,2-diiodoethane and tert-butyl chloride, respectively.7 Furthermore, homoleptic SmCp*3 was obtained, along with Cp*Sm(C8H8), from the reaction of unsolvated SmCp*2 with 1,3,5,7-C8H8 in toluene in a donor-solvent-free environment.18 © XXXX American Chemical Society

Later, treatment of Cp*2Sm(OEt2) with 0.5 equiv of PbCp*2 in toluene was described as an improved method for preparing SmCp*3.19 Also the latter synthesis required THF to be rigorously excluded, since it would react with SmCp*3 to form the ring-opened product Cp*2Sm[O(CH2)4C5Me5](THF).19 Lappert et al. used a similar approach to generate the sterically crowded Yb[C5H3(SiMe3)2-1,3]3 from Yb[C5H3(SiMe3)2-1,3]2 and Pb[C5H3(SiMe3)2-1,3]2.20 More recently, Jaroschick et al. employed 1,1′-diphosphaplumbocenes Pb(Dtp)2 (Dtp = 2,5-ditert-butyl-3,4-dimethylphospholyl) and Pb(Dsp)2 (Dsp = 2,5bis(trimethylsilyl)-3,4-dimethylphospholyl) in an oxidative ligand transfer reaction with divalent thulocene complexes.21 All these PbCpx2-related studies deal with the preparation of homoleptic or sterically crowded Ln(III) compounds. We extended this Ln(II)/Pb(II)−Ln(III)/Pb(0) redox approach to rare-earth-metal half-sandwich complexes employing a series of divalent amide and alkylaluminate precursors.22 Accordingly, trivalent complexes Cp*Ln[N(SiMe3)2]2 and Cp*Ln[N(SiHMe2)2]2(THF) (Ln = Sm, Yb), Cp*Eu(pztBu,Me)2(THF)2, and [Cp*2Ln(μ-AlMe4)]2 (Ln = Sm, Yb) could be isolated in moderate to excellent yields.22 Herein, we describe a simple and highly efficient route toward previously unknown Cp*SmI2(THF)3 and its agglomeration behavior upon successive displacement of the THF donor ligands. Desolvation is achieved by either vacuum treatment or addition of the strong Lewis acid AlEt3 in npentane. Received: September 1, 2016

A

DOI: 10.1021/acs.organomet.6b00695 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Representative Heteroleptic Sm(III) Reagents Obtained via Oxidation of Sm(II) Precursors

Scheme 2. Reaction of SmI2(THF)2 with 0.5 equiv of PbCp*2 under Varying (Crystallization) Conditions

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RESULTS AND DISCUSSION Even though on first sight the Sm(II)/Pb(II)−Sm(III)/Pb(0) redox approach toward Cp*SmI2(THF)3 (1) appears somewhat sophisticated, the synthesis of the cerium derivative Cp*CeI2(THF)3 via a salt metathesis approach by Bruno et al. is more elaborate.23 This simplification lies mainly in the availability of the iodide precursors: while the generation of LnI3(THF)x (Ln = La, Ce) demands refluxing the metal with 1,2-diiodoethane for 24 h, followed by Soxhlet extraction for a total of 5 days,23,24 SmI2(THF)2 is conveniently produced in an overnight reaction following Kagan’s protocol.1 Moreover, PbCp*2 is easily prepared by salt metathesis utilizing PbCl2 and 2 equiv of LiCp* in THF.25 According to Scheme 2, the reaction of SmI2(THF)2 with 0.5 equiv of PbCp*2 in THF gives virtually quantitative conversion to the desired product Cp*SmI2(THF)3 (1). Since the reaction proceeds rather fast, as monitored by rapid precipitation of elemental lead, exclusion of light, which is often a prerequisite for transformations with light-sensitive Pb(II) compounds, was not essential. However, the reaction was performed as an overnight reaction, ensuring in large part light exclusion as well as sufficient reaction time. X-ray diffraction analysis of orange single crystals, harvested from a saturated THF solution at −40 °C, revealed complex 1 to be isostructural with the cerium derivative Cp*CeI2(THF)3 (Figure 1, Table S1).23 Similar monomeric structures have been found previously for (C5H4tBu)SmI2(THF)3,26 (C5H5)SmI2(THF)3,11 (C5H4R)SmI2(THF)3 (R = (+)-neomenthyl,27 (S)-CH2CH(CH3OCH2Ph),28 and the nitrogen-donor adducts Cp*NdI2(py)3,29 (C5H4tBu)LnI2(py)3 (Ln = La, Ce, Nd),30 (C5H2(SiMe3)3-1,2,4)LaI2(py)3,31 (C5H3(CH2CH2NMe2)21,2)LaI2(THF),32 (C5H3(CH2CH2NMe2)2-1,3)LaI2(THF),32 and (η5-fluorenyl)LaI2(py)3 (Ln = La, Ce, Nd).33 Interestingly, indenyl complex (C9H7)SmI2(THF)2 was analyzed as a m o n o m e r i c b i s ( T H F ) a d d u c t . 1 0 L ik e t h e o t h e r

Cp′LnI2(donor)3 derivatives, complex 1 features a pseudooctahedral coordination sphere, with the center of the cyclopentadienyl ring and one THF molecule in the apical positions, while the three donor molecules (THF in our case) build up a mer-configuration. The Sm−I distances of 3.137 Å (av) (Table 1) are comparable to those found in (C5H5)SmI2(THF)3 (3.1681(7) and 3.1428(6) Å)11 and (C5H4R)SmI2(THF)3 (R = (+)-neomenthyl: 3.139(1) and 3.140(1) Å).27 It is noteworthy that complexes Cp*SmI2(THF)3 (1) and Cp*CeI2(THF)3 show distinct crystal packing (trigonal space group R3̅ versus monoclinic space group P21/n; Figure 2). The hexagonal arrangement of the molelcules in 1 is such that large channels are formed, as previously observed for Fe[N(SiMe3)2]334 and Ln[N(SiMe3)2]3 (trigonal space group R3̅1c).35 The channels in 1 are up to 9.5 Å wide and very likely contain THF solvent molecules that do not conform to the crystallographic symmetry. Conducting the reaction of SmI2(THF)2 with 0.5 equiv of PbCp*2 in n-hexane (instead of THF, Scheme 2), a routine workup procedure, which included evaporation to dryness under reduced pressure (ca. 5 × 10−2 mbar) and recrystallizaB

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Organometallics

Figure 1. Solid-state structure of Cp*SmI2(THF)3 (1). Hydrogen atoms are omitted for clarity. Atoms are represented by atomic displacement ellipsoids at the 50% level, with the exception of the Cp* ligand, the carbon atoms of which are represented by arbitrary radii for improved visualization. Atom I1 shows a heavy disorder over two positions, with I1 and I1′ accounting for 94% and 6% occupancy each (Figure S1, Supporting Information). Selected interatomic angles (deg): I1−Sm−I2 149.41(4), O1−Sm−O2 152.6(2), I1−Sm−O3 73.9(8), I2−Sm−O3 76(1), I2−Sm−Ct 105.75, O3−Sm−Ct 177.9.

tion from toluene, did not yield putative bis(THF) adduct [Cp*SmI2(THF)2]x, comparable to indenyl complex (C9H7)SmI2(THF)2.10 Instead the trimeric THF-depleted complex [Cp*3Sm3I(μ2-I)3(μ3-I)2(THF)2] (2), with only 2/3 molar equiv of THF left, could be identified by X-ray crystallography (Figure 3, Table 1, Table S1). Complex 2 consists of two Cp*Sm(THF) and one Cp*SmI subunit, which are linked by iodo ligands. Each metal center is six-coordinate. Two coordination sites are occupied by μ2bridging iodo ligands, another two are occupied by μ3-bridging iodo ligands, and the fifth position is filled by the η5coordinated Cp* ligand. The remaining sixth site is occupied either by a terminal iodo ligand or by one of the two THF molecules, which are arranged trans to each other. Overall, the samarium atoms span the base of a trigonal bipyramid, with the two μ3-bridging iodo ligands representing the vertices of the pyramids and the three μ2-bridging iodo ligands engaging in an almost planar six-membered metallacycle. Such trimetallic units [Ln3(μ2-X)3(μ3-X)2] are quite common in rare-earth-metal coordination chemistry,36−38 and the most relevant examples comprise [Cp3Yb3(μ3-Cl)2(μ-Cl)3(THF)3][Cp6Yb6(μ6-Cl)(μCl)12],39 [Y3(OtBu)3(X)(μ2-OtBu)3(μ3-OtBu)(μ3-Cl)(THF)2] (X = Cl, OtBu),40,41 [Er3(OArOMe)4(μ2-F)3(μ3-F)2(THF)4],42 [L3Gd3Cl5][B(C6F5)4]43 (with L being a β-diketiminato ligand), and the very similar samarium complex [{Cp*Sm-

Figure 2. Crystal packing of Cp*SmI2(THF)3 (1) trigonal, top, and of Cp*CeI2(THF)3 monoclinic, bottom,23 seen down the respective caxis. The calculated density is 1.648 and 1.844 g/cm3, respectively. The channels of 1 run along the 3-fold roto-inversion axes, and the maximum residual electron density trace in them is 1.1 e−/Å3. The coordinated THF in 1 facing into the channel is highly disordered. The structure of Cp*CeI2(THF)3 is fully ordered.

(THF)}3(μ-Cl)5][BPh4],15 which was obtained by an oxidation reaction involving Cp*Sm[BPh4] and tert-butyl chloride (Scheme 1). The crystallographic data of compound 2, having two complexes in the asymmetric unit, were of low quality, which is mainly caused by heavy disorder of the Cp* ligands, the coordinated THF molecules, and the three cocrystallized toluene solvent molecules, two of which had to be analytically eliminated from the data set (cf., crystallographic data). In order to improve the crystal quality, a sample of 2 was evaporated to dryness under reduced pressure and the solvent

Table 1. Selected Bond Lengths (Å) for Complexes 1, 2, 3a, 3b, and 4 4c b

Sm−Cta Sm−C Sm−Iterm. Sm−μ2-I Sm−μ3-I Sm−μ4-I Sm−μ5-I Sm−O a

c

d

1

2

3a/3b

2.452 2.709(8)−2.743(8) 3.1068(7), 3.167(2)

2.39−2.42 2.63(3)−2.78(3) 3.010(2)−3.027(2) 3.143(2)−3.195(2) 3.145(2)−3.427(2)

2.384−2.391 2.646(9)−2.702(9) 2.960(7)−3.015(7) 3.092(1)−3.196(1) 3.187(1)−3.349(1) 3.124(1)−3.338(1)

2.40(2)−2.41(2)

2.378(7)−2.383(7)

2.432(5)−2.57(1)

Sm1−Sm4

Sm5

2.343−2.387 2.62(2)−2.70(2)

2.306 2.58(2)−2.62(2)

3.049(1)−3.191(1) 3.159(1)−3.351(2)

3.253(1), 3.303(1) 3.603(1), 3.664(2)

3.281(1)−3.377(1)

3.329(1)

Cp* ring centroid. bTrigonal space group R3̅. cMonoclinic space group P21/c. dMonoclinic space group P21/n. C

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Cp*Sm, one Cp*Sm(THF), and one Cp*SmI subunit, which are linked by iodo ligands. As in compound 2, each metal center is six-coordinate in a quasi-octahedral fashion. Again, two coordination sites of each Sm(III) center are saturated by μ2-bridging iodo ligands. The Cp*Sm subunits have two more coordination sites occupied by μ3-bridging iodo ligands, whereas the Cp*Sm(THF) and Cp*SmI subunits accommodate only one μ3-bridging iodo ligand and either a terminal iodo ligand or THF donor, respectively. The fifth position of all Sm(III) centers is occupied by a μ4-bridging iodo ligand, and the remaining sixth by η 5 -coordinated pentamethylcyclopentadienyl ancillary ligands. The four samarium atoms show a butterfly arrangement rather than a tetrahedral geometry (Figure 4). The μ3-bridging iodo ligands are capping the trigonal faces spanned by the samarium atoms of the two Cp*Sm subunits and the Cp*Sm(THF) and Cp*SmI subunit, respectively. The four μ2-bridging iodo ligands are aligned with the Sm−Sm edges. The μ4-bridging iodo ligand is lying almost perfectly along the Sm−Sm axis of the Cp*Sm(THF) and Cp*SmI subunits. Other tetrametallic rare-earth-metal clusters include halfsandwich complexes Cp*4Sc4I8,45 (C5Me4SiMe3)4Y4H8,46 the oxo-centered (C5H4Me)4Yb4(μ2-Cl)6(μ3-Cl)2(μ4-O)(THF)3,47 and the mixed-valent complex Cp* 6 Yb 4 F 4 . 4 8 The [Y4(OtBu)4(μ2-OtBu)4(μ3-OtBu)2(μ4-O)] subunit of octametallic {[Y 4 (OtBu) 4 (μ 2 -OtBu) 4 (μ 3 -OtBu) 2 (μ 4 -O)(μ2 -Cl) 2 ][Li4(μ2-OtBu)2]}2 shows a structural motif similar to complex 3.41 Given the proneness of similar tri- and tetrametallic subunits to assemble as hexa- and octametallic clusters via halo bridges,36,40,41,49,50 the ultimate removal of the last remaining THF molecule in complex 3 might lead to an octametallic complex, with the formerly terminal iodo ligands now acting as μ2 -bridging ones. Instead of completely desolvating a compound of the composition [Cp*SmI2(THF)x]y (1−3) by heating it under high vacuum,51 we applied a different protocol. This approach featured chemical desolvation of the precursor SmI2(THF)2 by treatment with excess AlEt3 in n-pentane.52 We assumed that the trialkylaluminum reagent might act as a strong Lewis acid, displacing the THF from the samarium by forming the AlEt3(THF) adduct. The latter could be easily removed together with the solvent from putative SmI2 by centrifugation. In fact this process could be readily pursued inside a glovebox, affording unsolvated SmI2 quantitatively as an insoluble green powder. Such SmI2 was reacted with 0.5 equiv of PbCp*2 in toluene overnight. Crystallization of the sparingly soluble purple product from toluene at −40 °C gave a small amount of purple crystals, analyzed as mixed-valent THF-free pentametallic complex [Cp*5Sm5(μ2-I)4(μ3-I)4(μ5-I)] (4) by X-ray structure analysis (Figure 5, Table 1, Table S1). Complex 4 features five Cp*Sm subunits interconnected by a total of nine iodo ligands. As in compounds 2 and 3, each samarium atom is six-coordinate in a quasi-octahedral fashion. The samarium atoms adopt a tetragonal pyramid with the basal Cp*Sm subunits displaying the same coordination environment each involving two μ2- and μ3-bridging iodo ligands. Consequently, the apical samarium atom has four of its coordination sites occupied by μ3-bridging iodo ligands. The coordination sphere of all Sm atoms is completed by a μ5-bridging iodo ligand and a η5-coordinated pentamethylcyclopentadienyl ancillary ligand. The four μ3-bridging iodo ligands are capping the trigonal faces of the Sm5 pyramid, while the four μ2-bridging iodo ligands are located on almost equivalent positions below the Sm4 base. The

Figure 3. Solid-state structure of molecule A of the asymmetric unit of [Cp*SmI](μ2-I)3(μ3-I)2[Cp*Sm(THF)]2 (2). Hydrogen atoms and lattice solvent molecules are omitted for clarity. Heavy atoms are represented by atomic displacement ellipsoids at the 50% level, with the exception of the carbon atoms, which are represented by arbitrary radii for improved visualization. Selected interatomic angles (deg): Sm−Sm−Sm 58.71(2)−60.71(2), Sm−μ2-I−Sm 89.86(5)−93.0(4), Sm−μ3-I−Sm 85.4(4)−90.0(4), O1−Sm1−Ct 104.6, I6−Sm3−Ct 106.3, I4−Sm1−Ct 177.4. The asymmetric unit is shown in the Supporting Information (Figure S2).

was switched to deuterated benzene (C6D6). Crystallization at ambient temperature by slow evaporation of the solvent and subsequent X-ray structure analysis revealed further loss of THF (2/3 to 1/3 molar equiv), implying formation of the tetrametallic cluster Cp*4Sm4I(μ2-I)4(μ3-I)2(μ4-I)(THF) (3a; Figure 4, Table 1, Table S1). Recrystallization of 2 from toluene at −40 °C gave crystals of the same overall structure (3b, Figure S3), containing 2.5 toluene molecules instead of 2.5 C6D6 (3a) in the asymmetric unit. This indicated that in fact the vacuum treatment and not the crystallization conditions caused the loss of THF.44 Tetrametallic 3 consists of two

Figure 4. Solid-state structure of [Cp*SmI](μ2-I)4(μ3-I)2(μ4-I)[Cp*Sm]2[Cp*Sm(THF)] (3a). Hydrogen atoms and lattice solvent molecules (C6D6) are omitted for clarity. Atoms are represented by atomic displacement ellipsoids at the 50% level, with the exception of the Cp* ligand, the carbon atoms of which are represented by arbitrary radii for improved visualization. Selected interatomic angles (deg): Sm1−Sm2,4−Sm3 av 90.67, Sm2−Sm1,3−Sm4 av 60.25, Ct−Sm−I7 176.3−178.4, I7−Sm1−O 71.6(2), I7−Sm3−I8 78.6(1), Sm−μ2I−Sm 91.23(2)−92.84(2), Sm−μ3I−Sm 87.10(2)−90.59(2), Sm1,3−μ4I7− Sm2,4 86.68(2)−88.77(2), Sm1−I7−Sm3 173.10(3). D

DOI: 10.1021/acs.organomet.6b00695 Organometallics XXXX, XXX, XXX−XXX

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3.517(1)−3.5566(6) Å). Given the tetragonal-pyramidal arrangement of the samarium atoms in 4, it seemed plausible to assume that the apical site Sm2 accommodates the only divalent metal center. Sm2 is also the only metal center featuring μ3- and μ5-bridging iodo ligands exclusively. However, the Sm−C(Cp*) distances for Sm1−5 are all comparable to each other within the error limits (2.58(2)−2.70(2) Å; Table 1) while similar to those in, for example, {[Cp*Sm]6Se11}.61 Surprisingly, it is the basal Sm5 atom that shows significantly longer Sm−I distances (Table 1) and therewith should be assigned to the divalent state. The Sm(III)−μ2-I distances observed for complexes 2−4 (Table 1) are significantly shorter than those in divalent complex [Cp*Sm(μ 2-I)(THF) 2]2 (3.356(2)−3.459(2) Å).62 For further comparison, the average Yb(II)−μ2-I distances in anionic cluster [C5Me4SiMe2tBu)6Yb6(μ3-I)8][Li(THF)4] are in the range 3.130(1)−3.193(1) Å.63 Furthermore, in trivalent SmI3(pyridine)3 the Sm−I distances range from 3.0507(3) to 3.1246(3) Å.64 The formation of incompletely oxidized complex 4 probably reflects (persisting) competing reaction pathways in the absence of a donor solvent, involving not only the formation of species [Cp*SmI2] via oxidation of SmI2 but also metathetical reactions generating the constituent fragment [Cp*SmI].

Figure 5. Solid-state structure of [Cp*Sm]5(μ2-I)4(μ3-I)4(μ5-I) (4). Hydrogen atoms and lattice toluene are omitted for clarity. Atoms are represented by atomic displacement ellipsoids at the 50% level, with the exception of the Cp* ligand, the carbon atoms of which are represented by arbitrary radii for improved visualization. Selected interatomic angles (deg): Sm1,3−I2−Sm4,5 87.9983)−92.58(3), Sm2−I2−Sm4 90.13(3), Sm2−I2−Sm5 92.07(3), Ct−Sm−I2 176.2−178.7, μ3-I−Sm2−μ3-I 80.78(3)−89.68(5), Ct−Sm2−μ3-I 106.31−109.50, I2−Sm2−μ3-I 71.35(3)−72.84(3).

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μ5-bridging iodo ligand (I2) is located in-plane with the Sm base, which might raise doubts whether it really is μ5 or merely μ4-bridging. However, the fairly small difference between the Sm2−I2 3.3761(15) Å and the average Sm(base)−I2 bond lengths of 3.2983 Å does suggest a μ5-bridging mode. Even though, due to its elemental composition, the samarium−iodine cage structure of complex 4 is unique, the structural motif is not that uncommon. It is has been detected in alkoxide derivatives [Ln 5 (μ 5 -O)(μ 3 -OiPr) 4 (μ 2 -OiPr)4(OiPr)5] (Ln = Nd,53 Sm,54 Eu,55 Gd,53 Tb,54 Ho,54 Er55) and [Nd5(μ5-O)(μ3-OtBu)4(μ2-OtBu)4(OtBu)556 including the half-sandwich cluster [Cp 5 Y 5 (μ-OCH 3 ) 4 (μ 3 OCH3)4(μ5-O)].57 Structurally related are also the partly alkylated half-sandwich chloro cluster [Cp*5Nd5(AlMe4)(μ2Cl)6(μ3-Cl)2(μ4-Cl)]58 as well as the mixed half-sandwich/ metallocene halo clusters [Cp*6Yb5(μ2-F)6(μ3-F)2(μ4-F)]59 and [Cp*6Sm5(μ2-Cl)6(μ3-Cl2)(μ4-Cl)].36 The latter Sm5 cluster has been isolated from a multistep synthesis of [Cp*2Sm5(μ2-Me)2AlMe3]n (n = 1, 2) involving the reaction sequence Cp*2SmCl(THF) + KO2CPh + AlMe3 + toluene.35 The pentametallic Yb(III) cluster was obtained by treating a green solution of Cp*2Yb(OEt2) in toluene with an equimolar amount of C9F18 (= perfluoro-2,4-dimethyl-3-ethylpent-2-ene) over a period of 9 weeks at ambient temperatue.59 It is noteworthy that the tetrametallic mixed-valent complex {[Cp*Yb(μ2-F)]2[Cp*2Yb(μ2-F)]2}, reported even earlier, was obtained repeatedly by oxidation of YbCp*2 with different molar ratios of AgF.48 It was also pointed out that the tetrametallic mixed-valent complex {[Cp*Yb(μ2-F)]2[Cp*2Yb(μ2-F)]2} might be viewed as an intermediate in the oxidation of YbCp*2 to [Cp*6Yb5(μ2-F)6(μ3-F)2(μ4-F)].59 In contrast to 4, the mixed-valent complex {[Cp*Yb(μ2-F)]2[Cp*2Yb(μ2F)]2} revealed well-defined Yb(II) and Yb(III) subunits.48 Saltmetathetical reactions of SmI2(THF)2 with a dipyrrolyl dianion gave pentametallic complexes [{[μ-Ph2C(C4H3N)2]Sm}5(μ5I)] − [{[μ-Ph 2 C(C 4 H 3 N) 2 ]Sm(THF)} 3 (μ 3 -I)] + and [{[μMePhC(C4H3N)2]Sm}5(μ5-I)]−[K(THF)6]+,60 respectively, where the samarium atoms are pentagonally arranged around a coplanar iodide ion (Sm−(μ5-I), 3.434(1)−3.634(1);

CONCLUSIONS The monomeric half-sandwich complex Cp*SmI2(THF)3 is easily and efficiently accessible according to an oxidation protocol employing SmI2(THF)2 and PbCp*2 in THF. Conducting the same reaction in toluene did not produce putative [Cp*SmI2(THF)2]x but led to the crystallization of THF-depleted tri- and tetrametallic clusters [Cp*3Sm3I(μ2I)3(μ3-I)2(THF)2] and [Cp*4Sm4I(μ2-I)4(μ3-I)2(μ4-I)(THF)], respectively. Lewis-acid (AlEt3)-promoted desolvation of SmI2(THF)2 prior to reaction with the lead oxidant afforded donor-free mixed-valent pentametallic complex [Cp*5Sm5(μ2I)4(μ3-I)4(μ5-I)]. The incomplete oxidation of isolable SmI2 to putative [Cp*SmI2] hints at the limitations of any Sm(II)/ Pb(II)−Sm(III)/Pb(0) redox approach. Choice of the solvent (coordinating donor molecules) and solubility issues not only affect the redox potentials but also facilitate alternative reaction pathways such as nonoxidative ligand exchange and redistribution. Overall, the Sm3, Sm4, and Sm5 species represent rare examples of half-sandwich iodo clusters. We are currently investigating the applicability of other organolead(II) oxidants and alternative pathways to donor-free half-sandwich dihalo complexes [Cp*LnX2].

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EXPERIMENTAL SECTION

General Considerations. All manipulations were performed under rigorous exclusion of air and moisture, using glovebox techniques (MB Braun MB200B;