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Article

Tetramethylcyclopentadienyl Samarium(II) Metallocene Chemistry: Isolation of a Bimetallic Sm(II)/Sm(II) Complex

by
Joseph Q. Nguyen
,
Joseph W. Ziller
and
William J. Evans
*
Department of Chemistry, University of California, Irvine, CA 92697-2025, USA
*
Author to whom correspondence should be addressed.
Inorganics 2023, 11(1), 4; https://doi.org/10.3390/inorganics11010004
Submission received: 1 December 2022 / Revised: 16 December 2022 / Accepted: 17 December 2022 / Published: 21 December 2022
(This article belongs to the Special Issue Synthesis, Properties and Applications of Lanthanide and Actinide Molecular Compounds)
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Versions Notes

Abstract

:
The salt metathesis reaction between one equivalent of SmI2(THF)2 and two equivalents of K(C5Me4H) in THF afforded single crystals of the unusual, toluene-soluble, and asymmetric bimetallic Sm(II)/Sm(II) complex, (C5Me4H)2SmII(μ-η3:η5-C5Me4H)SmII(C5Me4H)(THF)2, instead of the expected product, (C5Me4H)2SmII(THF)2. The toluene-insoluble products of this reaction can be worked up in 1,2-dimethoxyethane (DME) to provide X-ray quality crystals of the monomeric Sm(II) metallocene, (C5Me4H)2SmII(DME). (C5Me4H)2SmII(DME) can also be synthesized directly by the reaction between one equivalent of SmI2(THF)2 and two equivalents of K(C5Me4H) in neat DME. The isolation and characterization of the bimetallic Sm(II)/Sm(II) complex provides supporting evidence for the possible oligomerization that may occur during the synthesis of Sm(II) complexes with cyclopentadienyl ligands that are less sterically bulky and less solubilizing than (C5Me5)1−.

1. Introduction

The synthesis and isolation of the first soluble organometallic Sm(II) complexes, (C5Me5)2Sm(THF)2 [1] and (C5Me5)2Sm [2], in 1981 and 1984, respectively, propelled progress in organosamarium(II) chemistry by being suitable complexes for a series of reactivity studies that showcased the strong reducing properties of the Sm(II) ion in an organometallic environment. Some early representative examples included the reductive homologation of CO [3], the coupling of alkynes and alkenes with CO [4,5], the generation of unusual olefin dianions [6,7], and the first example of dinitrogen reduction by an f-element [8]. Subsequently, this system was explored broadly in many areas [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25].
The pentamethylcyclopentadienyl ligand used in these studies was critical for providing the steric crowding required for isolating the monomeric Sm(II) complexes, the solubility of the Sm(II) complexes essential for reactivity, and the crystallinity of the paramagnetic Sm(III) products necessary for characterization using single-crystal X-ray crystallography. The first Sm(II) cyclopentadienyl complex, [(C5H5)2Sm(THF)]n [26], was reported much earlier, in 1969, but since this unsubstituted cyclopentadienyl complex is insoluble in most solvents, even in THF, it did not provide access to unusual Sm(II) reactivity. It also did not form single crystals suitable for X-ray diffraction. Likewise, [(C5MeH4)2Sm(THF)]n [27,28] displays similar behavior. Since then, Sm(II) complexes using various substituted cyclopentadienyl ligands have been reported, including (C5tBu3H2)2Sm [29], (C5iPr5)2Sm [29], [C5(2,5-Ph2)(3,4-p-tol2)H]2Sm(THF) [30], (C5Ph4H)2Sm(THF) [30], (C9H7)2Sm(THF) [31], (C13H9)2Sm(THF)2 [31], (C5Me4iPr)2Sm(THF) [32], [C5(SiMe3)3H2][C5(SiMe3)2H3]Sm(THF) [33], [C5Me4(CH2C10H7)]2Sm(THF)2 [34], [C5Me4(CH2C10H7)]2Sm [34], [(4-nBu-C6H4)5C5)]2Sm [35], [(4-Et-C6H4)5C5)]2Sm [36], [(4-iPr-C6H4)5C5)]2Sm [36], [(C5Me4)SiMe2(CH2CH═CH2)]2Sm [37], (C5iPr4H)2Sm [29,38], (C5Ph5)2Sm [30,39], and [C5(CH2Ph)5]2Sm [40]; see Scheme 1. One commonality between these Sm(II) complexes is the use of sterically bulky and solubilizing substituents on the cyclopentadienyl ligands [41].
Although many of these Sm(II) metallocenes were characterizable by single-crystal X-ray crystallography, the tetramethylcyclopentadienyl Sm(II) complex, (C5Me4H)2Sm(THF)2 [42], closely related to (C5Me5)2Sm(THF)2, proved to be challenging to crystallize. Although spectroscopic data were reported for the product of the salt metathesis reaction between one equivalent of SmI2(THF)2 and two equivalents of Na(C5Me4H), no crystallographic characterization of the isolated material was described, presumably due to the presence of complicating impurities [42]. Given that (C5Me4H)2Yb [43] and (C5Me4H)2Eu(THF)2 [44] were previously crystallographically characterized, we recently revisited the Sm(II) chemistry of this ligand as part of a broader study of (C5Me4H)1− f-element chemistry in our laboratory. Herein, we report two routes to crystallographically characterizable samarocenes involving this ligand.

2. Results

2.1. Synthesis and Structure of (C5Me4H)2Sm(DME), 2

The reaction between two equivalents of K(C5Me4H) and one equivalent of SmI2(THF)2 in THF at room temperature immediately produced a purple mixture and a white precipitate. Afterwards, centrifugation and filtration of the purple mixture to remove white insoluble solids, presumably KI, afforded a purple solution. Subsequently, purple solids were obtained from this purple solution upon removal of solvent from the supernatant under reduced pressure at room temperature. The vacuum was applied until the purple solids were dry enough to be a free-flowing powder. About 50–60% of the purple solids by mass were extracted with toluene to give a purple–green solution discussed later. However, about half of the purple solids remained undissolved despite multiple extractions in toluene, as shown in Scheme 2.
The purple, toluene-insoluble solids, 1, are also insoluble in alkanes, benzene, and diethyl ether. Elemental analysis suggested that compound 1 is [(C5Me4H)2Sm]n. However, incomplete combustion, which is commonly observed in organometallic f-element chemistry [45,46,47,48,49], was observed over multiple runs (see Experimental section). In support of this, 1 is soluble in THF and the 1H NMR spectrum, taken in THF-d8, matches the data reported for (C5Me4H)2Sm(THF)2 in 1995 [42], showing paramagnetically shifted resonances, which is typical for Sm(II) complexes; see Figure S1, Scheme 2 [1,2,29,30,31,32,33,34,35,36,37,38,39,40]. Despite multiple attempts, no X-ray quality crystals of this THF-solvated species could be grown in our hands which is consistent with the literature [42].
The addition of 1,2-dimethoxyethane (DME) to 1 at room temperature afforded a dark green solution which has a color similar to that of (C5Me5)2Sm(DME) [9]. Concentration of the dark green solution and overnight storage at −35 °C generated dark green crystals of (C5Me4H)2Sm(DME), 2, which were suitable for study by X-ray diffraction, Figure 1, Scheme 2.
Complex 2 can also be synthesized directly by the reaction between K(C5Me4H) and SmI2(THF)2 in neat DME at room temperature, Scheme 3. The 1H NMR spectrum of 2, taken in THF-d8, displays resonances corresponding to one type of (C5Me4H)1− environment as well as one molecule of DME per two (C5Me4H)1− ligands, Figure S2. Furthermore, the resonances are paramagnetically shifted, which is typical for a Sm(II) complex [1,2,29,30,31,32,33,34,35,36,37,38,39,40].
Complex 2 crystallizes in the orthorhombic Pbca space group with two crystallographically independent molecules in the asymmetric unit. Selected bond distances and angles for complex 2 are summarized in Table 1. Each molecule adopts a distorted tetrahedral geometry and is coordinated to two cyclopentadienyl ligands and the two oxygen atoms of a single DME molecule. The two molecules have similar metrical parameters; the Sm–C(C5Me4H) distances fall within the range of 2.741(2)–2.867(2) Å and the Cnt–Sm–Cnt (Cnt = ring centroid) angles are 130.3° for both molecules. Furthermore, the 2.591(1)–2.617(1) Å Sm–O(DME) distances is similar to the Sm–O(DME) distances in (C5Me5)2Sm(DME) [9]. However, the average O(DME)–Sm–O(DME) angle in complex 2 is 63.57(4)°, which is narrower than the 67.2(9)° seen in (C5Me5)2Sm(DME).
In each molecule of 2, the hydrogen substituted carbon atoms on the cyclopentadienyl rings are at the back of the wedge where the two rings are closest to each other and have the most steric repulsion. The orientation of the (C5Me4H)1− rings with respect to each other is slightly different in the two molecules, Figure 2. One molecule has a H(C5Me4H)…H(C5Me4H) distance of 3.196 Å while the other molecule has an analogous distance of 3.683 Å. The fact that two different orientations with similar metrical parameters form the low energy structure that crystallizes suggests that there is no single preferred orientation. In contrast, (C5Me5)2Sm(DME) crystallizes as a single molecule in the asymmetric unit along with one molecule of DME in the lattice [9].
The metrical parameters of 2 are compared to those of other structurally characterized Sm(II) metallocenes in Table 2 and are ordered in increasing Cnt–Sm–Cnt angle. The Sm–Cnt distances in 2 fall within the range typically observed for a Sm(II) metallocene and not for a Sm(III) metallocene [1,2,31,32,33,34,35,36,37,38,39,40]. The Cnt–Sm–Cnt angle of 2 is the second smallest observed for a Sm(II) metallocene, behind the 126.4° seen in (C13H9)2Sm(THF)2 [31]. However, the more acute angle seen in (C13H9)2Sm(THF)2 is likely a consequence of (C13H9)2Sm(THF)2 having a noticeably longer Sm–Cnt distance than the other Sm(II) metallocenes in the literature.

2.2. Synthesis and Structure of (C5Me4H)2SmII(μ-η3:η5-C5Me4H)SmII(C5Me4H)(THF)2, 3

The purple–green toluene solution obtained by extraction of the SmI2(THF)2/K(C5Me4H) reaction mixture as described above led to the isolation of a different Sm(II) metallocene than 1 or 2. Concentration and subsequent storage of the solution at −35 °C in the presence of THF generated purple, X-ray-quality crystals of the bimetallic complex, (C5Me4H)2Sm(μ-η3:η5-C5Me4H)Sm(C5Me4H)(THF)2, 3; see Figure 3, Scheme 2. Complete removal of the solvent resulted in the formation of green solids. However, these green solids exhibit low solubility in non-coordinating solvents, and no X-ray-quality single crystals have been isolated.
Complex 3 crystallizes in the monoclinic P21/n space group with a single molecule in the asymmetric unit. Complex 3 is an asymmetric, bimetallic Sm(II)/Sm(II) complex that consists of a desolvated [(η5-C5Me4H)2Sm] metallocene moiety and a disolvated [(μ-η3:η5-C5Me4H)(η5-C5Me4H)Sm(THF)2] metallocene moiety bridged by a (C5Me4H)1− ligand. A cyclopentadienyl-bridged bimetallic samarium complex has been observed previously in the literature. However, it is the mixed-valent Sm(II)/Sm(III) complex, (C5Me5)2SmII(μ-η5:η5-C5H5)SmIII(C5Me5)2 [50], which contains five cyclopentadienyl rings instead of the four in complex 3. Selected metrical parameters for complex 3 are summarized in Table 3 and are compared with other Sm(II) metallocenes in Table 2.
The coordination sphere of Sm(2) in complex 3 with two (η5-C5Me4H)1− rings and two THF molecules is most similar to that of complex 2 and (C5Me5)2Sm(THF)2 [1]. The 130.2° Cnt–Sm(2)–Cnt angle in 3 is equivalent to those in 2 and the 2.537 Å and 2.586 Å Sm(2)–Cnt distances are within a range typical for a Sm(II) metallocene [1,2,31,32,33,34,35,36,37,38,39,40]. Moreover, the Sm(2)–O(THF) distances of 2.577(2) Å and 2.597(2) Å are similar to the Sm–O(THF) distances of other crystallographically characterized Sm(II) metallocenes, as shown in Table 2.
Sm(1) has two (η5-C5Me4H)1− ligands that have Sm(1)–C(C5Me4H) distances ranging from 2.748(2) Å to 2.940(2) Å. The 122.8° Cnt–Sm(1)–Cnt angle for the two (η5-C5Me4H)1− ligands is much more bent than that of complex 2 because Sm(1) also has an (η3-C5Me4H)1− ligand in its coordination sphere. In a sense, the Sm(1) moiety may be considered a Sm(II) analog with (C5Me4H)1− ligands of the Sm(III) complex, (C5Me5)2Sm(η3-CH2CHCH2) [51]. However, the Sm(III) metallocene allyl complex has a Cnt–Sm–Cnt angle of 140.3°, presumably because it has Sm–C bond distances that are much shorter due to the metal center having a higher oxidation state than Sm(1) in complex 3. The average Sm(1)–C(η5-C5Me4H) distance in 3 is 2.847(2) Å, which is longer than the 2.724(30) Å average bond distance observed in (C5Me5)2Sm(η3-CH2CHCH2). Similarly, the 2.842(2)–2.962(2) Å Sm(1)–C(η3-C5Me4H) distances in 3 for the three closest ring carbon atoms are much longer than the 2.630(15)–2.668(18) Å Sm–C(CH2CHCH2) distances in (C5Me5)2Sm(η3-CH2CHCH2). This is consistent with the 0.1–0.2 Å differences typically observed between Sm(II)–C and Sm(III)–C distances in complexes featuring cyclopentadienyl or allyl ligands [1,2,3,4,5,6,7,8,51]. The other Sm(1)–C(η3-C5Me4H) bond lengths in 3 for the two more distant carbon atoms in the cyclopentadienyl ligand are noticeably longer and are 3.153 Å and 3.160 Å.
The relatively small 122.8° Cnt–Sm(1)–Cnt angle in 3 is reminiscent of the 120° Cnt–Ln–Cnt angle observed in the sterically crowded (C5Me5)3Ln complexes that undergo sterically induced reduction [52]. One characteristic of (C5Me5)3Ln complexes that effect sterically induced reduction reactions is that they have at least one methyl substituent that is displaced from the average plane generated by the cyclopentadienyl ring by at least 0.48 Å [52]. However, the methyl displacements shown for complex 3 in Table 4 do not exceed 0.195 Å, and hence it cannot be considered sterically crowded by this criterion even though the Cnt–Sm–Cnt angle is very small [52].
The 1H NMR spectrum of 3 in C6D6 has resonances that can be attributed to more than one type of (C5Me4H)1− ligand, but the paramagnetism of the Sm(II) complex precluded a definitive assignment; see Figure S3.

2.3. Infrared Spectroscopy and UV-Visible Spectroscopy

Infrared spectroscopy was used to characterize the purple, toluene-insoluble solids, 1, obtained from the reaction between SmI2(THF)2 and K(C5Me4H) in THF, and to compare to the infrared spectra of 2 and 3; see Figure 4 and Figures S4–S6.
The infrared spectra of 1, 2, and 3 all display absorptions centered around 3000 cm−1 and 740 cm−1, which is typical for the C–H stretching vibrations and bending vibrations of the cyclopentadienyl ligands [53], respectively. Another band centered around 1440 cm−1 is observed and is characteristic of C–C vibrations [53]. In addition, the infrared spectrum of 2 has two very strong bands at 1060 cm−1 and 860 cm−1, diagnostic of coordination of an O-donor ligand on a metal center [54,55,56], which for complex 2 is DME. This compares well with the reported infrared data for (C5Me5)2Sm(DME), which also has the strong bands associated with coordination of a DME molecule at 1040 cm−1 and 860 cm−1 [9]. The infrared spectrum of 1 looks very similar to the infrared spectrum of 2, except the bands at 1060 cm−1 and 860 cm−1 are not present; see Figure 4. This suggests that these purple, toluene-insoluble solids, 1, are fully desolvated. This notion is further supported by the infrared spectrum of 3, which has a similar spectrum to 2, including the two very strong bands at 1030 cm−1 and 876 cm−1 corresponding to the THF molecules bound to 3. These bands are similar to the ones found at 1040 cm−1 and 895 cm−1 in the infrared spectrum of (C5Me5)2Sm(THF)2, which are absent in (C5Me5)2Sm [1,2].
The UV-visible spectra of complex 1, 2, and 3 were also collected in THF at room temperature and were compared with the UV-visible spectrum of (C5Me5)2Sm(THF)2, as shown in Figures S7–S11. Despite the differences spectroscopically and structurally between 1, 2, and 3, the UV-visible spectra are identical in THF and display absorptions centered around 390 nm and 550 nm. In comparison to (C5Me5)2Sm(THF)2, there are no noticeable differences. Additionally, no differences are observed whether the UV-visible spectrum of 3 is taken in THF or toluene.

3. Discussion

The reaction between SmI2(THF)2 and K(C5Me4H) in DME readily forms single crystals of (C5Me4H)2Sm(DME), 2, amenable to single-crystal X-ray crystallography. This contrasts with the same reaction in THF, where X-ray-quality crystals of the expected product, (C5Me4H)2Sm(THF)2, have proven elusive. This emphasizes the subtleties that are present in f-element organometallic chemistry. Further evidence on this point comes from the isolation of (C5Me4H)2Sm(μ-η3:η5-C5Me4H)Sm(C5Me4H)(THF)2, 3. The isolation of this asymmetric bimetallic complex that is composed of monometallic [(C5Me4H)2Sm] and [(C5Me4H)2Sm(THF)2] moieties is most surprising.
On the basis of the structures of 2 and 3 and the spectroscopic data obtained in this study, it is likely that removal of solvent under reduced pressure from reactions between SmI2(THF)2 and K(C5Me4H) generates an insoluble [(C5Me4H)2Sm]n species that involves bridging (C5Me4H)1− ligands of the type found in 3. If small amounts of THF are present, toluene-soluble compounds such as the bimetallic complex 3 can be isolated. In the presence of an excess of coordinating solvents such as THF or DME, (C5Me4H)2Sm(THF)2 and (C5Me4H)2Sm(DME) are likely to exist as monomeric species in solution.
These results with the (C5Me4H)1− ligand differ significantly from those of the (C5Me5)1− ligand. In the pentamethylcyclopentadienyl case, the unsolvated (C5Me5)2Sm can be isolated and crystallographically characterized. Evidently, the absence of one methyl group on the cyclopentadienyl ring allows bridging structures such as in 3 which may not be as energetically accessible as for (C5Me5)2Sm. The fact that Sm(1) in 3 can achieve a Cnt–Sm–Cnt angle of 122.8°, the smallest observed for a Sm(II) metallocene, is consistent with this.
The structural data of 2 and 3 present some interesting questions with respect to the locations of the hydrogen substituents on the cyclopentadienyl ring. The existence of two different orientations of the hydrogen substituents in 2 is intriguing as well as its possible importance in making 1 crystallizable compared to (C5Me4H)2Sm(THF)2. Furthermore, the location of these hydrogen substituents in the back of the wedge would seem to allow for a more acute Cnt–Sm–Cnt angle. This is supported by the fact that the angle in complex 2 is 130° whereas the angle is 140° in (C5Me5)2Sm(DME). In complex 3, which has the smallest Sm(II) Cnt–Sm–Cnt angle, 122.8°, only one hydrogen substituent is at the back of the wedge of Sm(1) and the same is true for Sm(2) which has a coordination environment closest to complex 2. Based on these results, there are clearly subtleties in the packing of tetramethylcyclopentadienyl complexes and these are likely to affect the solubility and crystallinity based on these results.
Given the results obtained with Sm(II) complexes with cyclopentadienyl ligands with one fewer methyl group than (C5Me5)1−, it seems understandable that the chemistry of Sm(II) complexes of even less substituted cyclopentadienyl ligands is challenging in terms of solubility and crystallinity. Complexes such as [(C5H5)2Sm(THF)]n and [(C5H4Me)2Sm(THF)]n would be expected to oligomerize and could form a variety of bridging motifs.

4. Experimental Details

All manipulations and syntheses described below were conducted with the rigorous exclusion of air and water using standard Schlenk line and glovebox techniques under an argon atmosphere. Solvents were sparged with UHP argon and dried by passage through columns containing Q-5 and molecular sieves and stored over activated molecular sieves prior to use. Deuterated NMR solvents were dried over NaK alloy, degassed by three freeze–pump–thaw cycles, and vacuum transferred before use. 1H NMR spectra were recorded on a Bruker AVANCE600 MHz spectrometer at 298 K unless otherwise stated and referenced internally to residual protio-solvent resonances. UV-visible spectra were collected in THF at 298 K using a Varian Cary 50 Scan UV-visible spectrometer in a 1 mm Schlenk cuvette fitted with a Teflon stopper unless otherwise stated. Infrared spectra were recorded as compressed solids on an Agilent Cary 630 ATR-FTIR. Elemental analyses were conducted on a Thermo Scientific FlashSmart CHNS/O Elemental Analyzer at UC Irvine Materials Research Institute’s TEMPR facility in Irvine, California. K(C5Me4H) [44] and SmI2(THF)2 [57] were synthesized according to literature procedures.

4.1. Synthesis of 1 from SmI2(THF)2 and K(C5Me4H) in THF

In a glovebox containing coordinating solvents, solid K(C5Me4H) (486 mg, 3.04 mmol) was added to a stirred dark blue solution of SmI2(THF)2 (827 mg, 1.51 mmol) in THF (5 mL) at room temperature. The mixture immediately turned dark purple with concomitant formation of white, insoluble precipitate. The mixture was stirred at room temperature overnight. After the white solids, presumably KI, were removed via centrifugation and filtration, solvent was removed under reduced pressure to yield purple solids. The purple solids were then washed with toluene (4 × 20 mL) until the washings were colorless and the toluene extracts and remaining purple solids (237 mg, 0.603 mmol, 40% assuming a [(C5Me4H)2Sm]n product based on SmI2(THF)2) were separated. IR: 2939 w, 2870 m, 2847 s, 2722 w, 1435 m, 1386 m, 1315 w, 1025 w, 742 vs. UV-vis (THF, room temperature) λmax, nm (ε, M–1 cm–1): 395 (692), 549 (424). Anal. Calcd for 1, presumed to be [C18H26Sm]n: C, 55.04; H, 6.67. Found: C, 46.84; H, 5.897. C, 44.30; H, 5.582. C, 46.79; H, 5.876. Low C and H values were found across multiple runs and suggest incomplete combustion [45,46,47,48,49]. The C to H ratios in the analytical data give formulas of C18H27, C18H27, and C18H27, respectively, compared to the calculated value of C18H26.

4.2. Synthesis of (C5Me4H)2SmII(DME), 2, from 1

Compound 1 (237 mg) was dissolved in DME (10 mL), which immediately produced a dark green solution. Solvent was removed under reduced pressure to yield green solids (265 mg, 0.549 mmol, 36% based on SmI2(THF)2). Dark green crystals of 1, suitable for study by X-ray diffraction, can be grown from a concentrated solution of 1 in DME stored at −35 °C. 1H NMR (THF-d8, 600 MHz): δ 15.28 (br s, 2H, C5Me4H), 9.21 (br s, 12H, C5Me4H), 3.38 (s, 4H, CH3OCH2CH2OCH3), 3.28 (s, 6H, CH3OCH2CH2OCH3), −0.32 (br s, 12H, C5Me4H) ppm. IR: 2859s, 2847s, 2715w, 2206w, 1440m, 1239w, 1189w, 1064 vs, 1015m, 853s, 743vs cm−1. UV-vis (THF, room temperature) λmax, nm (ε, M–1 cm–1): 388 (995), 550 (488). Anal. Calcd for 2 C22H36O2Sm: C, 54.72; H, 7.51. Found: C, 33.32; H, 7.51. C, 25.29; H, 3.72. C, 37.86; H, 5.33. Low C and H values were found across multiple runs and suggest incomplete combustion [45,46,47,48,49]. The C to H ratios in the analytical data give formulas of C22H37, C22H38.5, and C22H37, respectively, compared to the calculated value of C22H36.

4.3. Synthesis of (C5Me4H)2SmII(μ-η3:η5-C5Me4H)SmII(C5Me4H)(THF)2, 3

In a glovebox containing coordinating solvents, the dark purple-green toluene extract obtained from the procedure in 4.1 was concentrated under reduced pressure and stored at −35 °C. Over approximately 1 week, purple crystals of 3, suitable for study by X-ray diffraction, were isolated (79 mg, 0.085 mmol, 11% based on SmI2(THF)2). IR: 2991 w, 2926 m, 2846 s, 2717 w, 1435 m, 1321 w, 1031 vs, 876 vs, 743 vs cm−1. UV-vis (THF, room temperature) λmax, nm (ε, M–1 cm–1): 399 (1320), 552 (809). Anal. Calcd for 3 C44H68O2Sm2: C, 56.84; H, 7.37. Found: C, 55.28; H, 7.486. C, 51.53; H, 7.028. Low C and H values were found across multiple runs and suggest incomplete combustion [45,46,47,48,49]. The C to H ratios in the analytical data to give formulas of C44H71 and C44H71, respectively, compared to the calculated value of C44H68.

4.4. Direct Synthesis of (C5Me4H)2Sm(DME), 2

In a glovebox containing coordinating solvents, solid SmI2(THF)2 (160 mg, 0.29 mmol) was added to a stirred suspension of K(C5Me4H) (94 mg, 0.59 mmol) in 1,2-dimethoxyethane (5 mL) at room temperature, and the mixture gradually turned dark green over one minute. The mixture was allowed to stir at room temperature overnight. White, insoluble solids, presumably KI, were removed by centrifugation and filtration, and the resulting dark green solution was dried under reduced pressure to yield dark green solids. The solids were triturated with hexanes (2 mL) before being dried under reduced pressure to yield 1 as a dark green solid (132 mg, 0.273 mmol, 94%). Crystals of 1, suitable for study by X-ray diffraction, were grown from a concentrated solution in DME stored at −35 °C overnight.

4.5. X-ray Data Collection, Structure Determination, and Refinement for (C5Me4H)2Sm(DME), 2

A green crystal of approximate dimensions 0.114 × 0.209 × 0.481 mm was mounted in a cryoloop and transferred to a Bruker SMART APEX II diffractometer system. The APEX2 [58] program package was used to determine the unit-cell parameters and for data collection (30 sec/frame scan time). The raw frame data was processed using SAINT [59] and SADABS [60] to yield the reflection data file. Subsequent calculations were carried out using the SHELXTL [61] program package. The diffraction symmetry was mmm and the systematic absences were consistent with the orthorhombic space group Pbca that was later determined to be correct. The structure was solved by direct methods and refined on F2 by full-matrix least-squares techniques. The analytical scattering factors [62] for neutral atoms were used throughout the analysis. Hydrogen atoms were included using a riding model. There were two molecules of the formula unit present. Least-squares analysis yielded wR2 = 0.0470 and Goof = 1.039 for 469 variables refined against 12,415 data (0.72 Å), R1 = 0.0195 for those 10,938 data with I > 2.0σ(I).

4.6. X-ray Data Collection, Structure Determination, and Refinement for (C5Me4H)2Sm(μ-η3:η5-C5Me4H)Sm(C5Me4H)(THF)2, 3

A purple crystal of approximate dimensions 0.170 × 0.191 × 0.306 mm was mounted in a cryoloop and transferred to a Bruker SMART APEX II diffractometer system. The APEX2 [58] program package was used to determine the unit-cell parameters and for data collection (20 s/frame scan time). The raw frame data was processed using SAINT [59] and SADABS [60] to yield the reflection data file. Subsequent calculations were carried out using the SHELXTL [61] program package. The diffraction symmetry was 2/m and the systematic absences were consistent with the monoclinic space group P21/n that was later determined to be correct. The structure was solved by direct methods and refined on F2 by full-matrix least-squares techniques. The analytical scattering factors [62] for neutral atoms were used throughout the analysis. Hydrogen atoms were included using a riding model. Least-squares analysis yielded wR2 = 0.0536 and Goof = 1.021 for 449 variables refined against 12,542 data (0.70 Å), R1 = 0.0239 for those 10,610 data with I > 2.0σ(I).

4.7. X-ray Crystallographic Data

CCDC 2221542-2221543 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 1 December 2022), or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033. Crystal data, bond lengths and angles tables, and structure refinement information for complexes 2 and 3 can be found in the supporting information.

5. Conclusions

In conclusion, synthetic routes to two new crystallographically characterizable Sm(II) metallocenes have been discovered using K(C5Me4H) and SmI2(THF)2 as starting materials. Although complex 2 adopts the typical monomeric Sm(II) bent metallocene structural motif, complex 3 is a rare example of an asymmetric bimetallic Sm(II)/Sm(II) complex bridged by a cyclopentadienyl ligand. The isolation of 3 provides supporting evidence for the possible oligomerization that may occur during the synthesis of Sm(II) metallocenes with less bulky and less solubilizing cyclopentadienyl ligands. This behavior can interfere with reactivity and can make the isolation and crystallization of reaction products more challenging. Reactivity studies are currently being pursued to investigate unusual differences in reactivity between (C5Me5)2Sm(THF)2 and complexes 2 and 3 as a result of switching from the pentamethylcyclopentadienyl ligand to the smaller and less substituted tetramethylcyclopentadienyl ligand.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics11010004/s1, Figures S1–S3: 1H NMR spectra of compounds, (C5Me4H)2Sm(THF)2, 2, and 3, respectively; Figures S4–S6: Infrared spectra of compounds 1, 2, and 3, respectively; Figures S7–S11: UV-visible spectra of 1 in THF, 2 in THF, 3 in THF, 3 in toluene, and (C5Me5)2Sm(THF)2 in THF, respectively; Tables S1 and S2: Crystal data, bond lengths, and bond angles of 2; Tables S3 and S4: Crystal data, bond lengths, and bond angles of 3.

Author Contributions

Conceptualization, J.Q.N. and W.J.E.; Methodology, J.Q.N. and W.J.E.; Formal analysis, J.Q.N. and J.W.Z.; Investigation, J.Q.N.; Writing—original draft, J.Q.N. and W.J.E.; Writing—review & editing, J.Q.N. and W.J.E.; Supervision, Joseph W. Ziller and W.J.E.; Project administration, W.J.E.; Funding acquisition, W.J.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the U. S. National Science Foundation, grant number CHE-2154255 (to W.J.E.) and the APC was funded by the MDPI.

Data Availability Statement

Spectroscopic data and detailed crystallographic information can be found in the supplementary materials. Crystallographic data are available via the Cambridge Crystallographic Data Centre (CCDC): 2221542-2221543.

Acknowledgments

We thank the U.S. National Science Foundation for support of the experimental parts of this research under CHE-2154255 (to W.J.E) and the MDPI for funding the APC.

Conflicts of Interest

The authors declare no competing financial interests.

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Inorganics 11 00004 sch001 550
Scheme 1. Examples of crystallographically characterized Sm(II) metallocene complexes.
Scheme 1. Examples of crystallographically characterized Sm(II) metallocene complexes.
Inorganics 11 00004 sch001
Inorganics 11 00004 sch002 550
Scheme 2. Synthetic and workup procedure for the different products of the reaction between SmI2(THF)2 and 2 K(C5Me4H) in THF.
Scheme 2. Synthetic and workup procedure for the different products of the reaction between SmI2(THF)2 and 2 K(C5Me4H) in THF.
Inorganics 11 00004 sch002
Inorganics 11 00004 g001 550
Figure 1. ORTEP representation of complex 2 with selective atom labeling. Ellipsoids are drawn at the 50% probability level. For clarity, hydrogen atoms and the second molecule in the asymmetric unit are not shown.
Figure 1. ORTEP representation of complex 2 with selective atom labeling. Ellipsoids are drawn at the 50% probability level. For clarity, hydrogen atoms and the second molecule in the asymmetric unit are not shown.
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Inorganics 11 00004 sch003 550
Scheme 3. Direct synthesis of (C5Me4H)2Sm(DME), 2.
Scheme 3. Direct synthesis of (C5Me4H)2Sm(DME), 2.
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Inorganics 11 00004 g002 550
Figure 2. Top-down view of both molecules of (C5Me4H)2Sm(DME), 2, in the asymmetric unit. For clarity, hydrogen atoms are not shown.
Figure 2. Top-down view of both molecules of (C5Me4H)2Sm(DME), 2, in the asymmetric unit. For clarity, hydrogen atoms are not shown.
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Inorganics 11 00004 g003 550
Figure 3. ORTEP representation of complex 3 with selective atom labeling. Ellipsoids are drawn at the 50% probability level. For clarity, hydrogen atoms are not shown.
Figure 3. ORTEP representation of complex 3 with selective atom labeling. Ellipsoids are drawn at the 50% probability level. For clarity, hydrogen atoms are not shown.
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Inorganics 11 00004 g004 550
Figure 4. Infrared spectra of 1 (red), 2 (blue), and 3 (green).
Figure 4. Infrared spectra of 1 (red), 2 (blue), and 3 (green).
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Table
Table 1. Selected bond distances (Å) and angles (°) for (C5Me4H)2Sm(DME), 2.
Table 1. Selected bond distances (Å) and angles (°) for (C5Me4H)2Sm(DME), 2.
Sm(1)–O(1)2.591(1)Sm(2)–O(3)2.612(1)
Sm(1)–O(2)2.617(1)Sm(2)–O(4)2.601(1)
Sm(1)–C(1)2.813(2)Sm(2)–C(23)2.794(2)
Sm(1)–C(2)2.821(2)Sm(2)–C(24)2.830(2)
Sm(1)–C(3)2.807(2)Sm(2)–C(25)2.824(2)
Sm(1)–C(4)2.796(2)Sm(2)–C(26)2.786(2)
Sm(1)–C(5)2.787(2)Sm(2)–C(27)2.760(2)
Sm(1)–C(10)2.747(2)Sm(1)–C(32)2.810(2)
Sm(1)–C(11)2.821(2)Sm(1)–C(33)2.826(2)
Sm(1)–C(12)2.867(2)Sm(1)–C(34)2.834(2)
Sm(1)–C(13)2.819(2)Sm(1)–C(35)2.819(2)
Sm(1)–C(14)2.741(2)Sm(1)–C(36)2.787(2)
Sm(1)–Cnt(1)2.534Sm(2)–Cnt(3)2.528
Sm(1)–Cnt(2)2.527Sm(2)–Cnt(4)2.544
O(1)–Sm(1)–O(2)63.92(4)O(3)–Sm(2)–O(4)63.21(4)
Cnt(1)–Sm(1)–Cnt(2)130.3Cnt(3)–Sm(2)–Cnt(4)130.3
Table
Table 2. Comparison of Sm–Cnt (Å) and Sm–O (Å) distances and Cnt–Sm–Cnt angles (°) for various crystallographically characterized Sm(II) metallocenes in order of increasing Cnt–Sm–Cnt angle.
Table 2. Comparison of Sm–Cnt (Å) and Sm–O (Å) distances and Cnt–Sm–Cnt angles (°) for various crystallographically characterized Sm(II) metallocenes in order of increasing Cnt–Sm–Cnt angle.
ComplexSm–Cnt (Å)Sm–O (Å)Cnt–Sm–Cnt (°)
(C5Me4H)2Sm(μ-η3:η5-C5Me4H)Sm(C5Me4H)(THF)2, 3 a2.584, 2.581, 2.537, 2.5862.577(2), 2.597(2)122.8, 130.2
(C13H9)2Sm(THF)2 [31]2.633, 2.6292.560(6), 2.540(6)126.4
(C5Me4H)2Sm(DME), 2 a2.534, 2.527, 2.528, 2.5442.591(1), 2.617(1), 2.601(1), 2.612(1)130.3
(C5Me5)2Sm(THF)2 [1]2.602.63(1)136.7
[C5H2(SiMe3)3][C5H3(SiMe3)2]Sm(THF) [33]2.559, 2.5532.547(3)137.0
[C5Me4(CH2C10H7)]2Sm [34]2.533, 2.529---138.05(6)
(C5Me5)2Sm(THF) [33]2.542, 2.5492.569(3)138.5
[C5Me4(CH2C10H7)]2Sm(THF)2 [34]2.576, 2.5822.646(2), 2.590(2)138.97(3)
(C5Me5)2Sm(DME) [9]2.54, 2.572.52(1), 2.61(2)140
(C5Me5)2Sm [2]2.53---140.1
[(C5Me4)SiMe2(CH2CH═CH2)]2Sm [37]2.551---141.2
(C5Me4iPr)2Sm(THF) [32]2.5312.540(5)141.6
[C5(CH2Ph)5]2Sm [40]2.555, 2.565---141.8
(C5iPr4H)2Sm [29,38]2.51---152.0
[(4-Et-C6H4)5C5)]2Sm [36]2.504, 2.521---166.9(1), 168.0(1)
a This work.
Table
Table 3. Selected bond distances (Å) and angles (°) for (C5Me4H)2Sm(μ-η3:η5-C5Me4H)Sm(C5Me4H)(THF)2, 3.
Table 3. Selected bond distances (Å) and angles (°) for (C5Me4H)2Sm(μ-η3:η5-C5Me4H)Sm(C5Me4H)(THF)2, 3.
Sm(1)–C(1)2.819(2)Sm(2)–C(19)2.814(2)
Sm(1)–C(2)2.931(2)Sm(2)–C(20)2.806(2)
Sm(1)–C(3)2.932(2)Sm(2)–C(21)2.804(2)
Sm(1)–C(4)2.810(2)Sm(2)–C(22)2.811(2)
Sm(1)–C(5)2.751(2)Sm(2)–C(23)2.799(2)
Sm(1)–C(10)2.831(2)Sm(2)–C(28)2.835(2)
Sm(1)–C(11)2.940(2)Sm(2)–C(29)2.872(2)
Sm(1)–C(12)2.919(2)Sm(2)–C(30)2.888(2)
Sm(1)–C(13)2.791(2)Sm(2)–C(31)2.864(2)
Sm(1)–C(14)2.748(2)Sm(2)–C(32)2.807(2)
Sm(1)–C(28)2.962(2)Sm(1)–C(31)2.955(2)
Sm(1)–C(29)3.160(2)Sm(1)–C(32)2.842(2)
Sm(1)–C(30)3.153(2)Sm(2)–Cnt(3)2.537
Sm(1)–Cnt(1)2.584Sm(2)–Cnt(4)2.586
Sm(1)–Cnt(2)2.581Cnt(3)–Sm(2)–Cnt(4)130.2
Cnt(1)–Sm(1)–Cnt(2)122.8Sm(2)–O(1)2.577(2)
Sm(2)–O(2)2.597(2)
Table
Table 4. Displacements for the methyl substituents from the average ring carbon plane generated by the (C5Me4H)1− ligand (Å) and the (average ring carbon plane)–C(ring)–C(Me) angles (°) for Sm(1) in 3.
Table 4. Displacements for the methyl substituents from the average ring carbon plane generated by the (C5Me4H)1− ligand (Å) and the (average ring carbon plane)–C(ring)–C(Me) angles (°) for Sm(1) in 3.
Cnt(1)…C(6)0.032 Å0.67°Cnt(2)…C(15)0.041 Å0.86°
Cnt(1)…C(7)0.153 Å3.25°Cnt(2)…C(16)0.146 Å3.11°
Cnt(1)…C(8)0.195 Å4.15°Cnt(2)…C(17)0.144 Å3.04°
Cnt(1)…C(9)0.044 Å0.93°Cnt(2)…C(18)0.055 Å1.16°
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Nguyen, J.Q.; Ziller, J.W.; Evans, W.J. Tetramethylcyclopentadienyl Samarium(II) Metallocene Chemistry: Isolation of a Bimetallic Sm(II)/Sm(II) Complex. Inorganics 2023, 11, 4. https://doi.org/10.3390/inorganics11010004

AMA Style

Nguyen JQ, Ziller JW, Evans WJ. Tetramethylcyclopentadienyl Samarium(II) Metallocene Chemistry: Isolation of a Bimetallic Sm(II)/Sm(II) Complex. Inorganics. 2023; 11(1):4. https://doi.org/10.3390/inorganics11010004

Chicago/Turabian Style

Nguyen, Joseph Q., Joseph W. Ziller, and William J. Evans. 2023. "Tetramethylcyclopentadienyl Samarium(II) Metallocene Chemistry: Isolation of a Bimetallic Sm(II)/Sm(II) Complex" Inorganics 11, no. 1: 4. https://doi.org/10.3390/inorganics11010004

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