Synthetic Aspects of (C5H4SiMe3)3Ln Rare-Earth Chemistry

Apr 22, 2013 - New analogues of Cp′3Ln (Cp′ = C5H4SiMe3, Ln = rare earth) complexes have been synthesized with the largest and smallest lanthanide...
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Synthetic Aspects of (C5H4SiMe3)3Ln Rare-Earth Chemistry: Formation of (C5H4SiMe3)3Lu via [(C5H4SiMe3)2Ln]+ Metallocene Precursors Jeffrey K. Peterson, Matthew R. MacDonald, Joseph W. Ziller, and William J. Evans* Department of Chemistry, University of California, Irvine, California 92697-2025, United States S Supporting Information *

ABSTRACT: New analogues of Cp′3Ln (Cp′ = C5H4SiMe3, Ln = rare earth) complexes have been synthesized with the largest and smallest lanthanides, La and Lu. Due to the smaller size of Lu, the reaction of LuCl3 with 3 equiv of KCp′ produced only traces of Cp′3Lu, yielding [Cp′2Lu(μ-Cl)]2 as the major product. An alternative route was developed in which [Cp′2Lu(μ-Cl)]2 reacts with (C3H5)MgCl to form the tetrameric allyl complex [Cp′2Lu(μ-η1:η1-C3H5)]4. The allyl complex reacts with [HNEt3][BPh4] to generate [Cp′2Lu(THF)2][BPh4], which reacts with KCp′ to produce Cp′3Lu in good yield. The reaction of LaCl3 with 3 equiv of KCp′ in Et2O gave the unsolvated Cp′3La, while the same reaction in rigorously dry THF gave solvated Cp′3La(THF). The reaction of LaCl3 and KCp′ in improperly dried THF gave the mixed-ligand complex Cp′2CpLa(THF) (Cp = C5H5), rather than the expected hydrolysis product, [Cp′2La(μ-OH)]2.

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INTRODUCTION Recent studies of the reductive chemistry of the rare-earth metals have shown that trimethylsilylcyclopentadienyl complexes are excellent starting materials for the synthesis of complexes of metals in novel low oxidation states.1,2 Lappert and co-workers showed that the bis(trimethylsilyl)cyclopentadienyl complexes Cp″3Ln (Cp″ = 1,3-(Me3Si)2C5H3; Ln = La, Ce) can be reduced to form crystalline La2+ and Ce2+ molecular complexes, as shown in eq 1. More recently, the

unknown whether these complexes could be obtained with all the rare earth metals, as the steric requirements for thermal stability can often differ across this series. For example, due to steric unsaturation, the unsolvated [(C5Me5)2LnMe]n complexes of the larger rare-earth metals are less stable than those of the smaller metals.7 Conversely, due to steric crowding, (C5Me5)3Ln and (C5Me4H)3Ln complexes of the smaller rare earths are much more reactive and difficult to access than those of the larger metals.8−11 We report here the synthesis of the Cp′3Ln complexes of the largest and smallest lanthanides, La and Lu. The synthesis of the lanthanum complex from LaCl3 and 3 equiv of KCp′ is straightforward, but an alternative synthesis was developed for lutetium, since the reaction between LuCl3 and 3 equiv of KCp′ gives primarily [Cp′2Lu(μ-Cl)]2.

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monosubstituted trimethylsilylcyclopentadienyl ligand C5H4SiMe3 (Cp′) was useful with the smaller rare earths in providing the first crystallographic data on molecular complexes of Y2+, Ho2+, and Er2+, [(18-crown-6)K][Cp′3Ln], using Cp′3Ln as precursors (eq 2).3,4

In order to attempt the synthesis of other Ln2+ ions along similar lines, the synthesis of previously unknown tris(trimethylsilylcyclopentadienyl) complexes was necessary. Although Cp′3Ln (Ln = Ce,5 Nd,6 Y,4 Ho,3 Er3) complexes can be readily prepared from LnCl3 and 3 equiv of KCp′, it was © 2013 American Chemical Society

EXPERIMENTAL SECTION

All syntheses and manipulations described below were conducted under argon or nitrogen with rigorous exclusion of air and water using glovebox, Schlenk, and vacuum line techniques, except where noted. Solvents were sparged with UHP grade argon (Airgas) and passed through columns containing Q-5 and molecular sieves before use. NMR solvents (Cambridge Isotope Laboratories) were dried over sodium−potassium alloy, degassed by three freeze−pump−thaw cycles, and vacuum-transferred prior to use. Trimethylsilyl chloride (Alfa Aesar) was dried over CaH2 and vacuum-transferred before use. Potassium bis(trimethylsilyl)amide (Sigma-Aldrich) was purified by dissolving in minimal toluene, centrifuging to remove insoluble material, and removing solvent from the supernatant. Allylmagnesium chloride (2.0 M solution in THF, Sigma-Aldrich) was used as received. KCp (Cp = C5H5) was made from potassium bis(trimethylsilyl)amide and freshly cracked C5H6. [HNEt3][BPh4] was synthesized by treatment of NEt3·HCl with NaBPh4 in water, followed by filtration Received: February 8, 2013 Published: April 22, 2013 2625

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and drying under high vacuum (10−5 Torr) for 48 h. Anhydrous LaCl3 and LuCl312 and [Cp′2Lu(μ-Cl)]213 were prepared as previously described. 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded at 25 °C with a Bruker GN500 or CRYO500 spectrometer. Infrared spectra were recorded as KBr pellets on a Varian 1000 FT-IR spectrometer. Elemental analyses were performed on a Perkin-Elmer 2400 Series II CHNS analyzer. KCp′. Following an adaptation of the procedure previously reported,14 in an argon-filled glovebox, trimethylsilyl chloride (5.87 g, 54.0 mmol) was added dropwise to a stirred solution of KCp (5.63 g, 54.0 mmol) in Et2O (300 mL) in a 500 mL round-bottom flask. The resulting mixture was stirred for 4 h. Hexane (200 mL) was added, the mixture was filtered with a medium frit to remove KCl, and the filtrate was transferred to a new 1 L round-bottom flask. Potassium bis(trimethylsilyl)amide (10.8 g, 54.1 mmol) was dissolved in Et2O (50 mL), the solution was added dropwise to the stirred filtrate, and the mixture was stirred for 4 h. Solvent was removed from the colorless solution under vacuum, and the resulting white solids were stirred in hexane (100 mL) overnight. The white insoluble material was collected on a medium frit, washed with hexane, and dried under vacuum to yield a white solid determined to be KCp′ by 1H NMR spectroscopy15 (7.02 g, 74%). 1H NMR (THF-d8): δ 5.78 (m, C5H4SiMe3, 2H), 5.67 (m, C5H4SiMe3, 2H), 0.06 (s, C5H4SiMe3, 9H). [Cp′2Lu(μ-η1:η1-C3H5)]4 (1). In a nitrogen-filled glovebox, a 100 mL round-bottom flask was charged with a solution of [Cp′2Lu(μCl)]2 (1.62 g, 1.67 mmol) in toluene (50 mL). Allylmagnesium chloride (2.0 M solution in THF, 1.80 mL, 3.60 mmol) was added dropwise via syringe to the stirred solution. The reaction mixture was stirred for 2 h, during which time the colorless mixture became yellow. The solvent was removed under vacuum, hexane (40 mL) and 1,4dioxane (1 mL) were added, and the reaction mixture was stirred overnight. The yellow mixture was centrifuged to remove white solids, and the supernatant was filtered and solvent removed under vacuum to give 1 as a light yellow powder (1.40 g, 85%). Yellow crystals of 1 suitable for X-ray diffraction were grown via slow evaporation from C6D6 at room temperature over several weeks. 1H NMR (C6D6): δ 7.52 (quint, 3JHH = 10.0 Hz, C3H5, 1H), 6.68 (m, C5H4SiMe3, 4H), 6.15 (m, C5H4SiMe3, 4H), 3.06 (d, 3JHH = 10.0 Hz, C3H5, 4H), 0.05 (s, C5H4SiMe3, 18H). 13C NMR (C6D6): δ 159.1 (C3H5), 120.8 (C5H4SiMe3), 119.1 (C5H4SiMe3), 113.2 (C5H4SiMe3), 63.0 (C3H5), −0.43 (C5H4SiMe3). IR: 3068 w, 2952 m, 2894 m, 1543 m, 1485 m, 1443 m, 1403 m, 1364 m, 1310 w, 1247 s, 1180 s, 1042 s, 909 m, 834 m, 785 m, 752 m, 687 m, 645 m, 632 m cm−1. Anal. Calcd for C19H31Si2Lu: C, 46.52; H, 6.37. Found: C, 46.78; H, 6.31. [Cp′2Lu(THF)2][BPh4] (2). In a nitrogen-filled glovebox, a suspension of [HNEt3][BPh4] (860 mg, 2.04 mmol) in toluene (10 mL) was added to a stirred solution of 1 (1.10 g, 2.24 mmol) in toluene (40 mL). THF (2 mL) was added, and the resulting pale yellow mixture was stirred overnight, during which time the color changed to white. Hexane (20 mL) was added, and the resulting slurry was centrifuged to collect the insoluble material. The solids were washed with hexane and dried under vacuum to give 2 as a white powder (1.85 g, 99%). Colorless crystals of 2 suitable for X-ray diffraction were obtained by gas-phase diffusion of hexane into a saturated solution of 2 in THF at room temperature. 1H NMR (THFd8): δ 7.30 (m, BPh4, 8H), 6.88 (t, 3JHH = 7.5 Hz, BPh4, 8H), 6.74 (t, 3 JHH = 7.0 Hz, BPh4, 4H), 6.72 (m, C5H4SiMe3, 4H), 6.26 (m, C5H4SiMe3, 4H), 0.31 (s, C5H4SiMe3, 18H). 13C NMR (THF-d8): δ 165.2 (BPh4), 137.1 (BPh4), 125.8 (BPh4), 123.1 (C5H4SiMe3), 122.2 (C5H4SiMe3), 121.9 (BPh4), 116.0 (C5H4SiMe3), 0.3 (C5H4SiMe3). The 1H and 13C resonances of the THF ligands were not observed because of exchange with THF-d8. IR: 3053 m, 2999 m, 2952 m, 2902 m, 1943 w, 1880 w, 1817 w, 1581 m, 1496 w, 1481 m, 1444 m, 1427 m, 1365 m, 1345 w, 1312 w, 1250 s, 1176 s, 1067 w, 1043 s, 992 m, 907 m, 836 s, 804 m, 732 m, 707 m, 631 w, 605 w cm−1. Anal. Calcd for C48H62BO2Si2Lu: C, 63.15; H, 6.85. Found: C, 63.31; H, 6.49. Cp′3Lu (3). In a nitrogen-filled glovebox, a solution of KCp′ (335 mg, 1.90 mmol) in Et2O (20 mL) was added to a stirred slurry of 2 (1.72 g, 1.88 mmol) in Et2O (40 mL). The pale yellow mixture was sealed in a 100 mL side arm Schlenk flask fitted with a greaseless

stopcock and heated to reflux for 2 h. The solvent was removed under vacuum, and the resulting solids were stirred in hexane (40 mL) for 1 h and then heated to reflux in the sealed flask for an additional 2 h. The solvent was removed under vacuum, and the resulting solids were transferred to a glovebox free of coordinating solvents. The solids were extracted with hexane (40 mL) and the extracts filtered to remove white insoluble material, presumably KBPh4 and excess KCp′. The solvent was removed from the yellow filtrate under vacuum, and the resulting solids were extracted with pentane. Removal of pentane under vacuum yielded 3 as a yellow microcrystalline solid (0.950 g, 86%). Pale yellow crystals of 3 suitable for X-ray diffraction were grown from a concentrated pentane solution at −35 °C. 1H NMR (C6D6): δ 6.60 (m, C5H4SiMe3, 6H), 6.18 (m, C5H4SiMe3, 6H), 0.21 (s, C5H4SiMe3, 27H). 13C NMR (C6D6): δ 125.5 (C5H4SiMe3), 119.2 (C5H4SiMe3), 114.4 (C5H4SiMe3), 0.45 (C5H4SiMe3). IR: 3069 w, 2953 m, 2895 m, 2081 w, 1932 w, 1763 w, 1662 w, 1559 w, 1443 m, 1415 m, 1367 m, 1314 m, 1243 s, 1199 s, 1177 s, 1063 m, 1042 s, 906 s, 836 s, 791 s, 779 s, 752 m, 686 m, 632 m cm−1. Anal. Calcd for C24H39Si3Lu: C, 49.12; H, 6.70. Found: C, 48.73; H, 6.59. Cp′3La (4). In a nitrogen-filled glovebox, a solution of KCp′ (970 mg, 5.50 mmol) in Et2O (20 mL) was added to a stirred slurry of LaCl3 (440 mg, 1.79 mmol) in Et2O (20 mL). After it was stirred overnight, the colorless mixture was sealed in a 100 mL side arm Schlenk flask fitted with a greaseless stopcock and heated to reflux for 3 h. The solvent was removed from the mixture under vacuum, and the resulting solids were stirred in hexane (30 mL) while heating to reflux for 3 h, after which the mixture was stirred at room temperature overnight. The solvent was removed under vacuum, and the resulting solids were transferred to a glovebox free of coordinating solvents. The solids were extracted with hexane (30 mL) and filtered to remove white insoluble material, presumably KCl and excess KCp′. The solvent was removed from the colorless filtrate to give 4 as a white powder (530 mg, 54%). Colorless crystals of 4 suitable for X-ray diffraction were grown from a concentrated pentane solution at −35 °C. 1H NMR (C6D6): δ 6.48 (m, C5H4SiMe3, 6H), 6.29 (m, C5H4SiMe3, 6H), 0.21 (s, C5H4SiMe3, 27H). 13C NMR (C6D6): δ 124.8 (C5H4SiMe3), 123.5 (C5H4SiMe3), 118.1 (C5H4SiMe3), 0.00 (C5H4SiMe3). IR: 3061 w, 2954 m, 2894 m, 2077 w, 1931 w, 1735 w, 1640 w, 1544 w, 1441 m, 1412 m, 1362 m, 1311 m, 1243 s, 1193 m, 1177 s, 1058 m, 1040 s, 902 s, 833 s, 769 s, 684 m, 629 m, 530 w cm−1. Anal. Calcd for C24H39Si3La: C, 52.34; H, 7.14. Found: C, 51.98; H, 7.15. Cp′3La(THF) (5). Complex 5 was synthesized as described for 4, with the exception that THF was used instead of Et2O; KCp′ (1.78 g, 10.1 mmol) and LaCl3 (0.798 g, 3.25 mmol) were combined to produce 5 as a white powder (1.70 g, 84%). Colorless crystals of 5 suitable for X-ray diffraction were grown from a concentrated pentane solution at −35 °C. 1H NMR (C6D6): δ 6.50 (m, C5H4SiMe3, 6H), 6.23 (m, C5H4SiMe3, 6H), 3.48 (m, THF, 4H), 1.31 (m, THF, 4H), 0.33 (s, C5H4SiMe3, 27H). 13C NMR (C6D6): δ 123.3 (C5H4SiMe3), 120.6 (C5H4SiMe3), 116.1 (C5H4SiMe3), 71.5 (THF), 26.0 (THF), 1.4 (C5H4SiMe3). IR: 3076 w, 2952 m, 2894 m, 2712 w, 2360 w, 2084 w, 1931 w, 1872 w, 1710 w, 1601 w, 1528 w, 1443 m, 1403 m, 1363 m, 1340 m, 1309 m, 1295 w, 1247 s, 1178 s, 1139 w, 1065 m, 1039 s, 1018 s, 924 m, 904 s, 841 s, 765 m, 687 m, 628 m, 526 w cm−1. Anal. Calcd for C28H47OSi3La: C, 54.00; H, 7.61. Found: C, 53.78; H, 7.40. Cp′2CpLa(THF) (6). An initial attempt to synthesize 4 from LaCl3 and 3 equiv of KCp′ using THF containing up to 50 ppm of water led to the isolation of colorless X-ray-quality single crystals of 6 from pentane at −35 °C. 1H NMR (C6D6): δ 6.49 (m, C5H4SiMe3, 4H), 6.14 (s, C5H5, 5H), 6.13 (m, C5H4SiMe3, 4H), 3.42 (m, THF, 4H), 1.26 (m, THF, 4H), 0.39 (s, C5H4SiMe3, 18H). A more direct approach for the synthesis of 6 was subsequently attempted by reacting KCp′ (400 mg, 2.27 mmol) and KCp (117 mg, 1.12 mmol) with LaCl3 (278 mg, 1.13 mmol) in dry THF (20 mL). After 12 h, the solvent was removed from the white mixture under vacuum and the resulting solids were stirred in hexane (10 mL) for 1 h. The mixture was filtered and the colorless filtrate stored at −35 °C overnight. This produced colorless crystals (68 mg) that were found to contain both 6 and Cp′3La(THF) (5) in a 14:1 ratio as well as other 2626

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(C5Me4H)3Lu9 in good yield. A key step in Scheme 1 is the use of the tetraphenylborate complex [(C5Me4H)2Ln][(μPh)2BPh2]. This is a better precursor than the chloride analogue since the (BPh4)− ion is more easily substituted than chloride from the metallocene cation. Although the synthetic approach in Scheme 1 is well established for pentamethylcyclopentadienyl and tetramethylcyclopentadienyl complexes,8−10,19−23 it has not been extensively used for other types of cyclopentadienyl ligands until now. We describe the use of this approach to Cp′3Lu and the characterization of the first examples of Cp′ analogues of the allyl and tetraphenylborate complexes in Scheme 1, including the structural characterization of an unusual tetrametallic allyl complex containing μ-η1:η1-allyl bridges.24−31 [Cp′2Lu(μ-Cl)]2 reacts with the allyl Grignard reagent (C3H5)MgCl in toluene to form a pale yellow solid that was identified by X-ray diffraction as the tetrameric allyl complex [Cp′2Lu(μ-η1:η1-C3H5)]4 (1) (Scheme 2). Complex 1 was

unidentified products by 1H NMR spectroscopy. Unfortunately, an analytically pure sample of 6 could not be obtained due to similar solubilities of the products. Hydrolysis Studies. An NMR tube capped with a rubber septum was charged with a solution of 5 (40 mg, 0.064 mmol) in THF-d8. A solution of deionized water in THF-d8 (1.3 M) was sparged with argon for 5 min. An aliquot of this solution (50 μL, 0.065 mmol) was added via syringe to the NMR tube, which was immediately shaken. Cp′H (6.51, 6.40, 2.99, −0.03 ppm) and “[Cp′2La(μ-OH)]n” (6.29, 6.09, 5.30, 0.26 ppm) were the only species observed in the 1H NMR spectrum. When the same procedure was followed to add 1 equiv of water (3.5 μL, 0.20 mmol) to a solution of KCp′ (35 mg, 0.20 mmol) in THF-d8, resonances for Cp′H, KOH (4.75 ppm), KCp (5.67 ppm), and Me3SiOSiMe3 (0.07 ppm) were observed in the 1H NMR spectrum. The 1H NMR resonances of these products were assigned according to the literature 14,16,17 and verified by collecting their spectra independently in THF-d8.

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RESULTS AND DISCUSSION Synthesis of Cp′3Lu. In the case of the smallest lanthanide, lutetium, the reaction of LuCl3 with 3 equiv of KCp′ produces primarily the bis(trimethylsilylcyclopentadienyl) chloride dimer [Cp′2Lu(μ-Cl)]2 (eq 3).

Scheme 2. Synthetic Route to Analytically Pure Cp′3Lu (3)

Trace amounts of Cp′3Lu are formed in this reaction, but they are not easily separated from [Cp′2Lu(μ-Cl)]2, and addition of excess KCp′ to the chloride complex has not proven to be an effective route to Cp′3Lu. A similar situation was observed with the (C5Me4H)3Ln series: the reactions of ScCl3 and LuCl3 with 3 equiv of KC5Me4H form primarily (C5Me4H)2ScCl(THF)18 and (C5Me4H)2LuCl2K(THF)2,9 respectively, and the (C5Me4H)3Ln complexes are not generated in significant yields. One successful strategy for forming tris(cyclopentadienyl) complexes of the small rare earths is to use the synthetic route shown in Scheme 1, involving allyl and tetraphenylborate intermediates. This provided (C 5 Me 4 H) 3 Sc 1 8 and

isolated in 85% yield and has a solid-state structure (Figure 1) that differs significantly from the typical monomeric rare-earth allyl metallocene complexes in which the allyl ligands bind in a η3 fashion to a single metal.32 In 1, the allyl ligands bridge two Lu centers in a trans bis-η1 mode to form a 16-membered ring.

Scheme 1. Multistep Synthesis of Sterically Crowded (C5Me4R)3Ln Complexes (R = Me, H)

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but not equivalent at each metal center (Lu1−C17 = 2.508(3) Å and Lu1−C20 = 2.482(3) Å; Lu2−C19 = 2.485(3) Å and Lu2−C22 = 2.509(3) Å). The allylic carbons that have more alkene character (C17 and C22) have the numerically longer distances to Lu than the allylic carbons that have more alkyl character (C20 and C19). The 1H NMR spectrum of 1 shows a single set of (Cp′)− and (C3H5)− resonances, with a quintet observed at 7.52 ppm for the methine CH proton due to equivalent coupling to the four allylic protons. Hence, the allyl ligands are equivalent by NMR in solution. Variable-temperature 1H NMR studies showed only one species over a temperature range of −75 to +100 °C (spectra are given in the Supporting Information). The allyl complex 1 reacts with [HNEt3][BPh4] in toluene/ THF to produce the tetraphenylborate salt [Cp′2Lu(THF)2][BPh4] (2) in nearly quantitative yield (Scheme 2). The structure of 2 (Figure 2a) was determined by single-crystal Xray diffraction to confirm the presence of two THF ligands coordinated to lutetium. The low quality of the crystal data, however, does not allow a detailed analysis of metrical parameters. The tetraphenylborate complex 2 proved to be a much better precursor than the chloride complex [Cp′2Lu(μ-Cl)]2 for a salt metathesis reaction with KCp′. Complex 2 reacts with 1 equiv of KCp′ to produce analytically pure Cp′3Lu (3) as a microcrystalline yellow solid in 86% yield, (Scheme 2). It should be noted that, in contrast to the more sterically crowded (C5Me5)3Ln complexes which can ring-open THF,8,38−40 rigorous exclusion of THF is not necessary for isolation of Cp′3Lu. Hence, there is no need to ensure full desolvation of 2 in order to obtain the THF-free analogue of 2 prior to synthesizing 3. The molecular structure of 3, crystallized at −35 °C from pentane (Figure 2b), was determined to be isomorphous with the La analogue 4 (see below), as well as with the Ce, Nd, Y, Ho, and Er analogues previously reported.3,4,6,41 The average metal−(Cp′ centroid) distance of 2.361 Å in 3 is similar to that in 4 when the difference in ionic radii of nine-coordinate Lu3+ and La3+ are considered. The fact that 3 contains the smallest lanthanide, Lu, suggests that the rest of the lanthanide analogues should be accessible, whether by simple reaction of LnCl 3 with 3 equiv of KCp′ or via the allyl and tetraphenylborate intermediates, as shown in Scheme 2.

Figure 1. Molecular structure of [Cp′2Lu(μ-η1:η1-C3H5)]4 (1) with thermal ellipsoids drawn at the 50% probability level and hydrogen atoms omitted for clarity. Only one component of a disordered trimethylsilyl group is shown.

The μ-η1:η1-allyl ligand has been observed previously in a handful of transition-metal, 24−28,33 alkali/alkaline-earth metal,29−31 and main-group34,35 complexes, but to our knowledge, the closest example to the bridging allyl structure in 1 in a rare-earth complex is the μ-η1:η1-cyclohexadienyl ligand in the polyyttrium compound (C 5 Me 4 SiMe 3 ) 4 Y4H7(C6H9).36 The tetrameric 1 sits about an inversion center such that there are two crystallographically unique Cp′2Lu(μ-η1:η1-C3H5) units in the structure. In each allyl ligand, there is a small difference in carbon−carbon bond lengths (C17−C18 = 1.355(4) Å and C18−C19 = 1.406(4) Å; C20−C21 = 1.400(4) Å and C21−C22 = 1.386(4) Å). This suggests that there is only a slight localization of the allyl π electrons in the solid state, as these bond distances are still intermediate between the typical values for C(sp3)−C(sp2) single bonds and C(sp2)C(sp2) double bonds: 1.51 and 1.32 Å, respectively.37 Consistent with this, the Lu−C(allyl) bond lengths are similar

Figure 2. Molecular structures of (a) [Cp′2Lu(THF)2][BPh4] (2) and (b) Cp′3Lu (3), with thermal ellipsoids drawn at the 50% probability level and hydrogen atoms omitted for clarity. For 2, only one of the two independent molecules in the asymmetric unit is shown. 2628

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Synthesis of Cp′3La. The unsolvated complex Cp′3La (4) can be isolated from LaCl3 and 3 equiv of KCp′ in 54% yield when Et2O is used as a solvent. The solvated complex Cp′3La(THF) (5) can be obtained similarly but in THF in 84% yield. Complex 4 converts quantitatively to 5 with the addition of THF (Scheme 3). Complexes 4 and 5 were both characterized by X-ray crystallography (Figure 3), but disorder in 5 prevented any detailed structural discussion.

Initial attempts to synthesize Cp′3La (4) from LaCl3 and 3 equiv of KCp′ in THF obtained from a drying column that was no longer active in removing water led to the isolation of Cp′2CpLa(THF) (6; Cp = C5H5) which was identified by Xray crystallography. The structure of 6 is very similar to that of 5 (Figure 3). Complex 6 has an average La−(Cp′ centroid) distance of 2.592 Å that is 0.033 Å longer than that in 4, as might be expected for 10-coordinate vs 9-coordinate La3+.42 An additional difference is that the La metal center in 6 is displaced 0.415 Å out of the plane defined by the three ring centroids, while the La metal center in 4 lies within 0.127 Å of this plane. This is consistent with the addition of THF that moves the geometry from trigonal in 4 toward tetrahedral in 6. Addition of 2 equiv of KCp′ and 1 equiv of KCp to a mixture of LaCl3 in anhydrous THF gave NMR data consistent with the formation of 6, but it was formed as a mixture with other products, including 5. Hydrolysis. The serendipitous formation of a Cp− ligand in the synthesis of 6 in inadequately dried THF was surprising, because hydrolysis of rare-earth metallocenes more commonly cleaves a cyclopentadienyl ligand rather than a SiMe3 group. Formation of the [Cp′2La(μ-OH)]n lanthanum analogue of the known [Cp′2Lu(μ-OH)]213 would have been expected. Although organometallic rare-earth complexes typically react with water to give protonated ligands and metal oxides or hydroxides, there have been exceptions where cyclopentadienyl ligands remain intact. For example, the reactions of Cp3Ln (Ln = Y, Ho) and (C5H4Me)3Ho with water produce the aquo complexes Cp3Ln(H2O) and (C5H4Me)3Ho(H2O), respectively.43 The mixed-ligand complex Y(OAr)2(bpzcp) (OAr = OC6H3Me2-2,6; bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-

Scheme 3. Synthetic Routes to Complexes 4−6

Figure 3. Molecular structures of (a) Cp′3La (4), (b) Cp′3La(THF) (5), and (c) Cp′2CpLa(THF) (6), with thermal ellipsoids drawn at the 50% probability level and hydrogen atoms omitted for clarity. For 5 and 6, only one of the two independent molecules in the asymmetric unit is shown. 2629

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[Cp′2Lu(THF)2][BPh4] intermediates. The latter synthesis demonstrates that this chloride/allyl/tetraphenylborate route that has been so successful with the larger cyclopentadienyl ligands, (C5Me5)− and (C5Me4H)−, can also be used with monosubstituted (C5H4SiMe3)−. The unusual bis-η1 allyl bridging mode found in the [Cp′2Lu(μ-η1:η1-C3H5)]4 tetramer demonstrates how the metallocenes of the (C5H4SiMe3)− ligand can differ from those of the more substituted analogues.

diphenylethylcyclopentadienyl) forms [{Y(bpzcp)}(μOH)2(μ3-OH){Y(OAr)2}]2 in the presence of 1.5 equiv of water.44 It was of interest to determine if Cp′3La (4) was initially formed and underwent hydrolysis at the C−SiMe3 bond in one of the Cp′ rings to form 6. However, when a solution of Cp′3La(THF) (5) in THF-d8 was treated with 1 equiv of water, Cp′H16 was observed by 1H NMR spectroscopy as well as resonances consistent with a [Cp′2La(μ-OH)]n complex.13 In contrast, when a solution of KCp′ in THF-d8 was treated with 1 equiv of water, a colorless crystalline precipitate formed and the 1 H NMR spectrum of the remaining solution contained resonances for Cp′H,16 KCp,45 and Me3SiOSiMe317 as well as a broad resonance at 4.75 ppm that matched that observed for KOH in wet THF-d8. When the isolated precipitate was dissolved in additional THF-d8, it was found to be KCp by 1H NMR spectroscopy.45 These products suggest that KCp′ reacts with water in two ways: (1) protonation of the ligand to form Cp′H and metal hydroxide, as seen with Cp′3La(THF) (5) above, and (2) C− SiMe3 cleavage to form KCp and Me3SiOH, which most likely undergoes a condensation reaction46 with another 1 equiv of Me3SiOH to produce Me3SiOSiMe3 and H2O (Scheme 4).

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Figures, tables, and CIF files giving NMR spectra, crystallographic data collection, structure solution, and refinement details, and X-ray diffraction details of compounds 1−6 (CCDC Nos. 923589−923594). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail for W.J.E.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. National Science Foundation (Grant No. CHE-1010002) for support of this research and Jordan F. Corbey for assistance with X-ray crystallography.

Scheme 4. Possible Formation of Cp′2CpLa(THF) (6) via Hydrolysis

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REFERENCES

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Therefore, the most likely explanation for the isolation of the mixed-ligand complex 6 is that water present in the solvent at the time reacted with KCp′ to produce a mixture of KCp′ and KCp, which then reacted with LaCl3. It cannot be ruled out that there was residual KCp in the KCp′ reagent, but 6 was only observed when wet THF was used. The formation of 6 is reminiscent of the isolation of Cp‴Cp″Sm(THF) from SmI2(THF)2 and KCp‴ (Cp‴ = 1,2,4-(Me3Si)3C5H2, Cp″ = 1,3-(Me3Si)2C5H3) (eq 4).47 Formation of this product was

attributed to incomplete conversion of Cp″ to Cp‴ during the ligand synthesis, but in light of the isolation of 6, it is possible that this result was instead due to a single hydrolysis of one of the three SiMe3 groups in KCp‴ preceding the reaction with SmI2(THF)2.

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CONCLUSION Cp′3Ln complexes can be synthesized in good yield for both the largest and smallest lanthanides, La and Lu, although the latter metal requires [Cp′2Lu(μ-Cl)]2, [Cp′2Lu(C3H5)]4, and 2630

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Organometallics

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