Trimethylsilyl versus Bis(trimethylsilyl) Substitution in Tris

Mar 7, 2018 - Trimethylsilyl versus Bis(trimethylsilyl) Substitution in Tris(cyclopentadienyl) Complexes of La, Ce, and Pr: Comparison of Structure, M...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Trimethylsilyl versus Bis(trimethylsilyl) Substitution in Tris(cyclopentadienyl) Complexes of La, Ce, and Pr: Comparison of Structure, Magnetic Properties, and Reactivity Chad T. Palumbo,† Lucy E. Darago,‡ Cory J. Windorff,† Joseph W. Ziller,† and William J. Evans*,† †

Department of Chemistry, University of California, Irvine, California 92697, United States Department of Chemistry, University of California, Berkeley, California 94720, United States

‡

S Supporting Information *

ABSTRACT: To evaluate the effect of cyclopentadienyl ligand substitution in complexes of new +2 ions of the lanthanides, comparisons in reactivity and spectroscopic and magnetic properties have been made between [K(crypt)][Cp′3Ln], 1-Ln (Cp′ = C5H4SiMe3; crypt = 2.2.2-cryptand; Ln = La, Ce, Pr, and Nd), and [K(crypt)][Cp′′3Ln], 2-Ln [Cp′′ = C5H3(SiMe3)2]. The 2-Ln complexes (Ce, Pr, and Nd) were synthesized by reduction of Cp′′3Ln with potassium graphite in the presence of crypt and crystallographically characterized. The structures and UV−visible spectra of 2-Ln are similar to those of 1-Ln, as expected, but greater thermal stability for 2-Ln, expected from comparisons of 2-U and 1-U, was not observed. The magnetic susceptibilities of 2-Ce and 2-Pr were investigated because those of 1-Ce and 1-Pr did not match simple coupling models for 4fn5d1 electron configurations. The magnetic data of the 2-Ln complexes are similar to those of 1-Ln, which suggests that Ce2+ and Pr2+ complexes with 4fn5d1 electron configurations may have more complex electronic structures compared to nontraditional divalent complexes of the later lanthanides. Reactivity studies of isolated samples of 1-Ln and 2-Ln with 1,2-dimethoxyethane (DME) were conducted to determine if methoxide products, found in previous in situ studies of the synthesis of 2-Ln by Lappert and co-workers, would form. Methoxide products were not observed, which shows that the chemistry of the isolated complexes differs from that of the in situ reduction reactions.

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INTRODUCTION For many years, only six of the lanthanide metals were thought to be able to form complexes with the metal in the +2 oxidation state: Eu, Yb, Sm, Tm, Dy, and Nd.1−3 This belief was supported by extensive solid state and solution studies,1−5 as well as calculations of the reduction potentials.6−10 However, recent studies of rare earth reduction chemistry have shown that the divalent oxidation state is accessible for all the lanthanides (except radioactive promethium) by reduction of tris(cyclopentadienyl) metal complexes with potassium or potassium graphite in the presence of a chelate.11−15 This was originally shown with La and Ce by reduction of Cp′′3Ln precursors [Cp′′ = C5H3(SiMe3)2],11 and subsequently for all the lanthanides, using Cp′3Ln precursors, (Cp′ = C5H4SiMe3).4,5,12−14 These reactions, as well as results with Th,16 U,17,18 and Pu,19 are summarized in eq 1, where R = SiMe3 and M = La, Th, U, and Pu or R = H and M = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu, and U. The complexes of the new Ln2+ ions of Y, La, Ce, Pr, Gd, Tb, Ho, Er, and Lu differ from those traditionally known for Nd, Sm, Eu, Dy, Tm, and Yb in several ways. They display only small increases in their M−(Cp′ ring centroid) bond distances compared to their +3 analogs and show much more intense absorptions in the visible region of their electronic absorption spectra compared to complexes of the traditional ions. © XXXX American Chemical Society

These results have been rationalized by density functional theory (DFT) calculations that indicate that reductions of 4fn Ln3+ ions make 4fn5d1, not 4fn+1, ions for these new +2 complexes. This electron configuration is consistent with the crystal field splitting diagram of the tris(cyclopentadienyl) coordination environment which has a single orbital, dz2, as the lowest Received: December 10, 2017

A

DOI: 10.1021/acs.organomet.7b00881 Organometallics XXXX, XXX, XXX−XXX

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THF/Et2O solutions containing 2-Ln held at −35 °C in the glovebox freezer (Figure 1; Table 1).

in energy. It was found that Nd2+ and Dy2+ also adopted 4fn5d1 configurations in this tris(cyclopentadienyl) environment. Although the entire series of (Cp′3Ln)1− has been synthesized and isolated (except radioactive Pm), in the case of the Cp′′ ligand, the only 2-Ln complex isolated in pure form is 2-La. The cerium analog was isolated as a component in a crystal of [K(18-crown-6)(OEt2)2][Cp′′3Ce][Cp′′3Ce], which was determined to contain one Ce2+ ion and one Ce3+ ion.11 Attempts to make (Cp′′3Y)1− with the smaller, lanthanide-like yttrium were unsuccessful.20 The (Cp′′3)3− coordination environment also allowed the isolation of the first molecular examples of Th2+16 and Pu2+,19 eq 1. In these actinide complexes, along with the U2+ analogs,17 it appears that the (Cp′′3)3− coordination environment enables the isolation of more stable divalent complexes when compared to the (Cp′3)3− ligand set. This current study was initiated to compare these two coordination geometries with the large early lanthanides, which are most likely to accommodate three large Cp′′ ligands. Accordingly, we have synthesized pure samples of 2-Ce, 2-Pr, and 2-Nd for the first time and reisolated 2-La for comparison with the 1-Ln analogs. In terms of reactivity, the relative thermal stability of 1-Ln versus 2-Ln is reported, as well as reactivity with dimethoxyethane (DME). The initial attempts by Lappert et al. to make Ln2+ complexes according to eq 1 were conducted in DME.21,22 However, these DME reactions only yielded mixtures of methoxide products, such as [Cp′′2Ln(μ-OMe)]2 (Ln = La,22 Ce,21 and Nd),21 eq 2, and [Cp′′2Nd(μ-OMe)2Li(DME)],21 along with KCp′′ and unidentified polymetallic methoxide products. Since eq 1 is successful in THF and Et2O, but not in DME, it is presumed that the Ln2+ complexes react with DME. However, this had not been directly tested until this study. Finally, in terms of physical properties, it was of great interest to obtain data on additional examples of Ce2+ and Pr2+, since the measured magnetic susceptibilities of 1-Ce and 1-Pr did not match the two most likely electronic configuration models for 4f15d1 and 4f25d1 complexes.23

Figure 1. Thermal ellipsoid plot of [K(2.2.2-cryptand)][Cp′′3Pr], 2-Pr, drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.

Compounds 2-Ce and 2-Pr crystallize in the P1̅ space group and are isomorphous with 2-La, previously reported by Lappert et al.11 Complex 2-Nd has a similar coordination geometry and space group, but instead is isomorphous with the actinide analogs, [K(2.2.2-cryptand)][Cp′′3An] (An = Th and Pu). The anions of 2-Ln contain formally 9-coordinate Ln2+ ions with three cyclopentadienyl ligands coordinated in a trigonal planar fashion. The average Ln−(Cp′′ ring centroid) distances for 2-Ln decrease from La to Nd, consistent with the decreasing ionic radii of the Ln3+ ions with increasing atomic number.24 The Ln−C(Cp′′) average distances in both 2-Ce and 2-Nd are 0.03 Å larger than those in their Cp′′3Ln precursors. These match the 0.032 Å difference of 2-La and Cp′′3La, reported by Lappert.11 Although Cp′′3Pr was crystallized to get metrical data to compare with 2-Pr, the structure was disordered and bond distances were not obtainable. However, by interpolation in the Cp′′3Ln series, the Pr−C(Cp′′) average distance is about 0.03 Å larger, as with the other compounds studied, and consistent with the differences found between 1-Ln and Cp′3Ln. A comparison of the Ln−(Cp′ ring centroid)avg distances in 1-La, 2.59(1) Å, 1-Ce, 2.57(1) Å, 1-Pr, 2.53(1) Å, and 1-Nd, 2.52(1) Å,15 reveals that the Ln−(Cp′′ ring centroid)avg distances in 2-Ln are within experimental error, but the Ln−(Cp′′ ring centroid)avg distances are 0.02−0.03 Å larger. This is similar to the difference in Ln−(Cp′ ring centroid)avg distances between Cp′3Ln and Cp′′3Ln, where the latter distances are 0.02−0.03 Å longer.14,15 The UV−visible spectra of 2-Ln are similar to their [K(2.2.2cryptand)][Cp′3Ln], 1-Ln, analogues, Figure 2. The molar extinction coefficients ε = 2000−6000 M−1 cm−1 of the 2-Ln series are consistent with those of the 4fn5d1 Ln2+ ions found in 1-Ln.4,5,14,15 Interestingly, the absorptions of 2-Ln are red-shifted with respect to 1-Ln by ca. 700 cm−1 in the high energy region and 800 cm−1 in the low energy region. The data on 2-Nd constitute another example of Nd2+ being a configurational crossover ion.4,5,15 Magnetic Susceptibility. Two models have been proposed to describe the interaction of f and d electrons in 4fn5d1 lanthanide complexes.23,27 In the coupled 4fn5d1 model, the spin of the 4fn core couples with the spin of the 5d electron. This new total spin, S, then couples with the orbital angular momentum, L, of the 4fn core to produce a new J value, from which the anticipated magnetic susceptibility times temperature product, χMT, may be calculated. In the uncoupled 4fn5d1 model, the spin of the 5d electron does not couple with the spin from the 4fn electrons.

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RESULTS AND DISCUSSION The Ln2+ complexes [K(2.2.2-cryptand)][Cp′′3Ln], 2-Ln (Ln = Ce, Pr, and Nd), were prepared by the same method used to synthesize the [K(2.2.2-cryptand)][Cp′3Ln], 1-Ln, complexes in eq 1. Potassium graphite was added to prechilled THF solutions (−35 °C) containing Cp′′3Ln and 2.2.2-cryptand. Dark crystals of 2-Ln characterizable by X-ray diffraction were grown upon diffusion of hexane layered onto darkly colored B

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Organometallics Table 1. Selected Bond Lengths (Å) for [K(2.2.2-cryptand)][Cp′′3Ln], 2-Ln, and Their Cp′′3Ln Precursors parameter Ln−(Cp′′ ring centroid) Ln−(Cp′′ ring centroid)avg Ln−C(Cp′′)avg

Cp′′3La25

2-La11

Cp′′3Ce26

2-Ce

2-Pr

Cp′′3Nd25

2-Nd

2.615 2.586 2.605 2.60(1) 2.852(8)

2.606(3) 2.612(3) 2.642(3) 2.62(1) 2.89(2)

2.542 2.579 2.575 2.58(3) 2.83(4)

2.579 2.574 2.609 2.59(2) 2.86(4)

2.558 2.552 2.588 2.57(2) 2.84(3)

2.468 2.558 2.548 2.52(4) 2.789(2)

2.530 2.559 2.543 2.54(1) 2.82(3)

1-Pr, the measured χMT products observed at 300 K were in between those expected from the coupled and uncoupled 4fn5d1 models (Table 2).23 Hence, data on additional samples were sought. Magnetic susceptibility data collected for 2-Ce and 2-Pr are shown in Figure 3. As presented in Table 2, the χMT value of 0.84 emu·K/mol (μeff = 2.59 μB) for 2-Ce at 300 K and 0.1 T is close to the χMT of 0.86 emu·K/mol (μeff = 2.62 μB) found for 1-Ce.23 Similarly, for 2-Pr, the room temperature χMT of 1.37 emu·K/mol (μeff = 3.31 μB) at 0.1 T, reduced to 1.17 emu·K/mol (μeff = 3.06 μB) under an applied magnetic field of 1 T, is close to the χMT of 1.07 emu·K/mol (μeff = 2.93 μB) measured for 1-Pr.23 Temperature-independent paramagnetism is observed for both 2-Ce and 2-Pr, as exhibited by the roughly linear increase of χMT with increasing temperature. Consequently, comparison with the aforementioned electron configuration models and their predicted χMT values, which assume an isolated ground state multiplet that is thermally populated at 300 K, should be limited. As shown in the table, the χMT products of 2-Ce and 2-Pr do not match the calculated values for 4f2 and 4f3 configurations, consistent with a 4fn5d1 assignment. These experimental values fall in between the predicted χMT products for the coupled and uncoupled 4fn5d1 configuration models, but are not close to either. As for 1-Ce and 1-Pr, mixing with excited J states or a HOMO with mixed d and f character are possible sources of the deviation from simple magnetic models observed for 2-Ce and 2-Pr. Reactivity. The thermal stability of the 2-Ln series was evaluated following the protocols used for 1-Ln. The concentration of Ln2+ was monitored at λmax = 540 nm for 1-Ce, 561 nm for 2-Ce, 517 nm for 2-Pr, 417 nm for 1-Nd, and 457 nm for 2-Nd. The decomposition of the 2-Ln series was best modeled with second-order kinetics, as with the 1-Ln complexes.14 Table 3 summarizes kinetic data for 1-Ln and 2-Ln as well as for the corresponding uranium complexes, 1-U and 2-U. In contrast to the pair of uranium complexes, where 2-U is significantly more stable than 1-U, all of the 2-Ln complexes decompose slightly faster than their 1-Ln analogs. Hence, the (Cp′′3)3− coordination environment appears to have more of an effect on the stability of +2 complexes of the actinides than the lanthanides. This is reinforced by the fact that the Th2+ complex [K(18-crown-6)(THF)2][Cp′′3Th],16 shows only 8% decomposition after 8 days at room temperature and a Pu2+ complex, [K(2.2.2-cryptand)][Cp′′3Pu], could be isolated

Figure 2. Experimental UV−vis spectra of 3 mM solutions of (a) [K(2.2.2cryptand)][Cp′3Ce], 1-Ce, and [K(2.2.2-cryptand)][Cp′′3Ce], 2-Ce, (b) [K(2.2.2-cryptand)][Cp′3Pr], 1-Pr, and [K(2.2.2-cryptand)][Cp′′3Pr], 2-Pr, and (c) [K(2.2.2-cryptand)][Cp′3Nd], 1-Nd, and [K(2.2.2cryptand)][Cp′′3Nd], 2-Nd, in THF at 298 K.

Instead, the χMT product resulting from this spin is added on to that from the original J value of the 4fn core. For both 1-Ce and

Table 2. Experimental and Predicted χMT Values for [K(2.2.2-cryptand)][Cp′3Ln], 1-Ln, and [K(2.2.2-cryptand)][Cp′′3Ln], 2-Ln, Collected under a Field of 0.1 T complex

n ( f electrons)

exp. χMTa

χMT (4fn5d1) coupleda

χMT (4fn5d1) uncoupleda

χMT (4fn+1)a

1-Ce 2-Ce 1-Pr

1 1 2 2

0.86 0.84 1.07 1.37 (1.17 at 1 T)

0.33 0.33 0.875 0.875

1.18 1.18 1.98 1.98

1.6 1.6 1.64 1.64

2-Pr a

All χMT are reported in units of emu·K/mol. C

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Figure 3. Variable-field χMT vs T plots for [K(2.2.2-cryptand)][Cp′′3Ln], 2-Ce (left) and 2-Pr (right).

Table 3. Summary of Kinetic Data of the Decomposition of 3 mM Solutions of [K(2.2.2-cryptand)][Cp′3Ln], 1-Ln, and [K(2.2.2-cryptand)][Cp′′3Ln], 2-Ln in THF at 298 Ka

a

complex

λmax (nm)

reaction order

kobs

1-Ce 2-Ce 1-Pr14 2-Pr 1-Nd 2-Nd 1-U18 2-U19

540 561 465 517 417 457 412 470

2 2 2 2 2 2 1 1

6.2(8) × 10−4 M−1 s−1 1.2(3) × 10−3 M−1 s−1 1.9(8) × 10−4 M−1 s−1 1.8(4) × 10−3 M−1 s−1 7.2(5) × 10−4 M−1 s−1 1.5(1) × 10−3 M−1 s−1 1.3(9) × 10−4 s−1 9.8(3) × 10−6 s−1

1-U and 2-U have been added for comparison.

with this ligand set,19 whereas attempts to isolate a crystalline Np2+ complex with Cp′ were unsuccessful.28 Reactions of 1-Ln and 2-Ln with dimethoxyethane (DME) were examined to determine if the isolated Ln2+ complexes would react to form the methoxides [Cp′′2Ln(μ-OMe)]2 (Ln = La,22 Ce,21 and Nd),21 as observed by Lappert in reductions of Cp′′3Ln and (C5H3tBu2)3Ln with Li or K in DME (eq 2). To ensure that the Cp′ methoxides could be identified in these reactions, the [Cp′2Ln(μ-OMe)]2 complexes (Ln = La and Ce), 3-Ln, were independently prepared and crystallographically characterized. The reaction of Cp′3Ln with methanol yields colorless and yellow solutions for La and Ce (see Supporting Information for the UV−visible spectrum of 3-Ce), respectively, from which single crystals of 3-Ln were isolated (eq 3, Figure 4). The metrical

Figure 4. Thermal ellipsoid plot of [Cp′2La(μ-OMe)]2, 3-La, drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. 3-Ce is isomorphous. Selected bond lengths in 2-La: La1−Cnt(Cp′), 2.562, 2.572 Å; La1−C(Cp′)avg, 2.84(2) Å; La1−O1, 2.375(1) Å; La1−O1′, 2.413(1) Å; O1−La1−O1′, 72.69(4)°; La1−O1−La1′, 107.31(4)°. Selected bond lengths in 2-Ce: Ce1−Cnt(Cp′), 2.531, 2.539 Å; Ce1−C(Cp′)avg, 2.81(2) Å; Ce1−O, 2.350(2) Å; Ce1′−O, 2.387(2) Å; O1−Ce1−O1′, 72.16(8)°; Ce1−O1−Ce1′, 107.84(8)°.

[(C5H3tBu2)2Ce(μ-OMe)]2,21 and the hydroxide complexes, [Cp′2Ln(μ-OH)]2 (Ln = Yb30 and Lu).31 A summary of the metrical parameters can be found in Table S4. Over 3−4 days, the dark purple DME solutions of 2-La, 2-Ce, and 2-Pr and the dark red/brown DME solution of 2-Nd slowly turned colorless for La, yellow for Ce, pale yellow for Pr, and blue for Nd. The colors of the 1-Ln reactions were similar except Ce gave an almost colorless solution and the Nd reaction turned yellow. Unfortunately, only intractable oils were recovered from these reactions. Although the 1H NMR spectrum of the diamagnetic product mixture from the 2-La reaction was complicated, Cp′′3La was observed as the major component. The reaction with 1-La similarly gave a complicated spectrum, but with no identifiable resonances. Neither the previously characterized [Cp′′2La(μ-OMe)]222 nor the [Cp′2Ln(μ-OMe)]2, 3-Ln, complexes synthesized for this study were observed despite multiple reactions. The only crystalline product definitively identified was the potassium metallocene, [K(2.2.2-cryptand)][Cp′′2K], which was characterized by X-ray crystallography (see Supporting Information). These DME reactions represent another example of the difference in reactivity observed for isolated Ln2+ complexes with

parameters of 3-La and 3-Ce are consistent with those of previously characterized methoxide complexes, 3-Y29 and D

DOI: 10.1021/acs.organomet.7b00881 Organometallics XXXX, XXX, XXX−XXX

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The absorbance at λmax was used to monitor changes in concentrations over time and the experiments were done in duplicate. Magnetic Measurements. Samples were prepared by adding crystalline powder of 2-Ce and 2-Pr (43.9 and 33.5 mg) to a 5 mm inner diameter quartz tube containing a raised quartz platform. Solid eicosane was added to cover the sample to prevent crystallite torqueing and provide good thermal contact between the sample and the cryostat. The tubes were fitted with Teflon sealable adapters, evacuated on a Schlenk line, and flame-sealed under static vacuum. Following flame sealing, the solid eicosane was melted in a water bath held at 40 °C. Magnetic susceptibility measurements were performed using a Quantum Design MPMS-XL SQUID magnetometer. The dc magnetic susceptibility measurements were collected in the temperature range 2−300 K. Diamagnetic corrections were applied to the data using Pascal’s constants to give χD = −0.00071821 emu/mol (2-Ce), χD = −0.00071821 emu/mol (2-Pr), and χD = −0.00024036 emu/mol (eicosane).34 [K(2.2.2-cryptand)][Cp′′3Ce], 2-Ce. In an argon-filled glovebox, Cp′′3Ce (107 mg, 0.139 mmol) and 2.2.2-cryptand (52 mg, 0.14 mmol) were combined, dissolved in THF (2 mL), and the resultant blue solution chilled to −35 °C in the glovebox freezer. KC8 (28 mg, 0.21 mmol) was quickly added to the stirred blue solution and the reaction mixture immediately turned purple. After 2 min of stirring, the mixture was centrifuged using a prechilled centrifuge tube (−35 °C) to remove the graphite. The supernatant was collected and Et2O (2 mL) was added. The resultant dark-purple solution was layered with hexanes (15 mL) and stored at −35 °C for 3 days to yield 2-Ce as black/purple singlecrystals suitable for X-ray diffraction (110 mg, 67%). IR: 3033w, 2949s, 2884s, 2811m, 1477m, 1456w, 1443m, 1434m, 1396w, 1384w, 1361s, 1355s, 1313w, 1299m, 1258s, 1243s, 1204m, 1134m, 1107s, 1077s, 992w, 951m, 934w, 921m, 828s, 799m, 746s, 679w, 638w, 620w cm−1. UV−vis (THF) λmax nm (ε, M−1 cm−1): 250 (8200), 314 (5100), 400 (3200 shoulder), 565 (4300), 668 (3900). Anal. Calcd for C51H99CeKN2O6Si6: C, 51.73; H, 8.43; N, 2.37. Found: C, 51.77; H, 8.86; N, 2.76. [K(2.2.2-cryptand)][Cp′′3Pr], 2-Pr. As described for 2-Ce, a yellow-green solution of Cp′′3Pr (200 mg, 0.260 mmol) and 2.2.2cryptand (98 mg, 0.26 mmol) in THF (2 mL) was combined with KC8 (53 mg, 0.39 mmol) to produce 2-Pr as black/maroon single crystals suitable for X-ray diffraction (175 mg, 57%). IR: 3043w, 2963s, 2949s, 2888s, 2813m, 1477m, 1457w, 1445m, 1436m, 1395w, 1383w, 1361s, 1354s, 1314w, 1294m, 1260s, 1240s, 1205m, 1136s, 1108s, 1077s, 994w, 952m, 933w, 921m, 833s, 801m, 795m, 744s, 678w, 634w, 622w cm−1. UV−vis (THF) λmax nm (ε, M−1 cm−1): 248 (12100), 310 (4700), 450 (3400 shoulder), 517 (3600), 715 (1100). Anal. Calcd for C51H99KN2O6PrSi6: C, 51.69; H, 8.44; N, 2.36. Found: C, 51.56; H, 8.57; N, 2.42. [K(2.2.2-cryptand)][Cp′′3Nd], 2-Nd. As described for 2-Ce, a green colored solution of Cp′′3Nd (239 mg, 0.309 mmol) and 2.2.2cryptand (116 mg, 0.309 mmol) in THF (2 mL) was combined with KC8 (63 mg, 0.46 mmol) to produce 2-Nd as black/maroon single crystals suitable for X-ray diffraction (325 mg, 89%). IR: 3057w, 3034w, 2949s, 2886s, 2812m, 2760w, 2729w, 1478m, 1456m, 1445m, 1435m, 1397w, 1383w, 1361s, 1356s, 1314w, 1300m, 1260s, 1244s, 1206s, 1175w, 1136s, 1107s, 1078s, 1032w, 994w, 951s, 935m, 922s, 831s, 802s, 746s, 677m, 637m, 621m, 569w, 525w, 480w cm−1. UV−vis (THF) λmax nm (ε, M−1 cm−1): 246 (13000), 308 (5800), 420 (4400 shoulder), 525 (4300 shoulder), 650 (1700 shoulder). Anal. Calcd for C51H99NdKN2O6Si6: C, 51.55; H, 8.40; N, 2.36. Found: C, 51.61; H, 8.62; N, 2.43. [(Cp′2La(μ-OMe)]2, 3-La. In an argon-filled glovebox, methanol (12.8 μL, 0.360 mmol) was added to 10 mL of Et2O and stirred. The methanol solution was then added dropwise to a colorless solution of Cp′3La(THF) (200 mg, 0.360 mmol) in Et2O (10 mL); no color change was observed. The mixture was stirred for 2 h before the volatiles were removed under reduced pressure. The resultant colorless solids were characterized as analytically pure 3-La (129 mg, 91%). Clear colorless single crystals of 3-La characterizable by X-ray diffraction were obtained by cooling a concentrated hexane solution to −35 °C. 1H NMR (C6D6): δ 6.67 (t, JHH = 2.4 Hz, C5H4SiMe3, 8H), 6.43 (t, JHH = 2.4 Hz, C5H4SiMe3, 8H), 3.22 (s, OCH3, 6H), 0.36 (s, C5H4SiMe3, 36H). 13C NMR (C6D6): δ 124.9 (C5H4SiMe3), 121.3 (C5H4SiMe3), 118.2 (C5H4SiMe3), 52.7

their alkali metal countercation sequestered in 2.2.2-cryptand, versus in situ LnA3/M reactions (A = anion; M = alkali metal) that in theory should generate the same species in solution.12 Apparently, the cryptand-stabilized salts, 1-Ln and 2-Ln, have quite different reaction patterns compared to the Ln 2+ intermediates generated in situ from reductions of Cp′3Ln and Cp′′3Ln with Li or K in the absence of a chelating agent.

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CONCLUSION Crystallographic characterization of the [K(2.2.2-cryptand)][Cp′′3Ln], 2-Ln, complexes for Ce, Pr, and Nd shows that additional members of this series can be synthesized in pure form beyond 2-La.11 The results indicate that this (Cp′′3)3− ligand environment is as viable as the (Cp′3)3− ligand set in providing Ln2+ complexes of the early lanthanides; the two series of complexes are analogous in their structural and physical properties.4,5,12−14 The crystal structures and UV−vis spectra of all three new Ln2+ complexes are consistent with the presence of 4fn5d1 ions as found for 1-Ln. The magnetic susceptibility data on 2-Ce and 2-Pr are similar to those of 1-Ce and 1-Pr in that the data neither match those expected for 4fn+1 ions nor fit simple models for the magnetism of 4fn5d1 ions. The thermal stability of the [K(2.2.2-cryptand)][Cp′′3Ln], 2-Ln (Ln = Ce, Pr, and Nd), complexes was found to be slightly less than that of their [K(2.2.2-cryptand)][Cp′3Ln], 1-Ln, analogs. This contrasts with the greater stability of (Cp′′3U)1− versus (Cp′3U)1− and the successful isolation of (Cp′′3Th)1− and (Cp′′3Pu)1− using bis(trimethylsilyl)cyclopentadienyl ligands. Reactions between DME and the isolated Ln2+ complexes, 1-Ln and 2-Ln, did not readily form the methoxide complexes previously observed in Cp′′3Ln/M reductions in DME. This provides additional examples of the differences between the reactivity of isolated complexes of Ln2+ and the dark solutions formed by reducing analogous Ln3+ complexes in the absence of a chelating agent.

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EXPERIMENTAL DETAILS

The syntheses and manipulations described below were conducted under argon with rigorous exclusion of air and water using glovebox, vacuum line, and Schlenk techniques. Solvents were sparged with UHP grade argon (Praxair) and passed through columns containing Q-5 and molecular sieves before use. NMR solvents (Cambridge Isotope Laboratories) were dried over NaK, degassed by three freeze−pump− thaw cycles, and vacuum-transferred before use. Methanol and 1,2-dimethoxyethane were dried with molecular sieves, degassed by three freeze−pump−thaw cycles, and vacuum-transferred before use. KC8,32 Cp′′3Ln (Ln = La,33 Ce,21 Pr,33 and Nd21), and [K(2.2.2-cryptand)][Cp′′3La],11 were prepared according to the literature. 2.2.2-Cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8,8,8]hexacosane, VWR), was placed under vacuum (10−3 Torr) for 12 h before use. 18-Crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane, Aldrich) was sublimed before use. Cyclooctatetraene (Aldrich) was placed over molecular sieves and degassed by three freeze−pump−thaw cycles before use. 1H (500 MHz) and 13C (125 MHz) NMR spectra were obtained on a Bruker GN500 or CRYO500 MHz spectrometer at 298 K. IR samples were prepared as KBr pellets and the spectra were obtained on a Jasco FT/IR-4700 spectrometer. Elemental analyses were performed on a PerkinElmer 2400 Series II CHNS elemental analyzer. UV−vis spectra were collected in THF at 298 K using a Varian Cary 50 Scan UV−vis spectrophotometer. Kinetics experiments were conducted by quickly dissolving 14−20 mg of the analyte in 5.00 mL of THF (∼3 mM), transferring the solution into a quartz cuvette (1 mm path length) equipped with a greaseless stopcock, and immediately cooling the sample to 0 °C until measurement. The sample was quickly warmed to room temperature in a water bath and the spectra were collected at 298 K at intervals of 30 min for complexes 1-Ln (Ln = La, Ce, and Nd) and 2-Ln (Ln = La, Ce, Pr, and Nd). E

DOI: 10.1021/acs.organomet.7b00881 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics (OCH3), 0.62 (C5H4SiMe3). IR: 3084m, 3071m, 3057m, 2951s, 2926m, 2893m, 2855m, 2814m, 2708w, 2664w, 2616w, 2081w, 1932w, 1871w, 1707w, 1638w, 1599w, 1541w, 1441m, 1402m, 1360m, 1310w, 1244s, 1179s, 1042s, 905s, 831s, 772s, 754s, 689m, 637m, 629m, 419m cm−1. Anal. Calcd for C34H58La2O2Si4: C, 45.94; H, 6.58. Found: C, 46.22; H, 6.42. [(Cp′2Ce(μ-OMe)]2, 3-Ce. As described for 3-La, methanol (3.7 μL, 0.091 mmol) in 5 mL of Et2O was added dropwise to a royal blue solution of Cp′3Ce (50 mg, 0.091 mmol) in Et2O (5 mL) which caused an immediate color change to bright yellow. Removal of the solvent and crystallization of the resultant yellow solids from hexane yielded single crystals of 3-Ce characterizable by X-ray diffraction (16 mg, 40%). 1 H NMR (C6D6): δ 24.7 (br, C5H4SiMe3,, 8H), 3.47 (br, C5H4SiMe3, 8H), −2.72 (s, C5H4SiMe3, 36H), −36.7 (br, μ-OMe, 6H). IR: 3086m, 3073m, 3059m, 2951s, 2914m, 2893m, 2860m, 2760w, 2710w, 2139w, 2083w, 1935w, 1707w, 1641w, 1599w, 1543w, 1441m, 1402m, 1360m, 1310w, 1244s, 1177s, 1042s, 905s, 833s, 773s, 754s, 689m, 637m, 629m cm−1. UV−vis (toluene) λmax nm (ε, M−1 cm−1): 290 (5900), 330 (330 shoulder), 402 (100 shoulder), 475 (610). Anal. Calcd for C34H58Ce2O2Si4: C, 45.81; H, 6.56. Found: C, 45.58; H, 6.26. X-ray Data Collection, Structure Determination, and Refinement. Crystallographic details for compounds 2-Ln (Ln = Ce, Pr, and Nd) and 3-Ln (La and Ce) are summarized in the Supporting Information, along with data on [K(2.2.2-cryptand)][Cp″2K].

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(3) Meyer, G. The Divalent State in Solid Rare Earth Metal Halides. In The Rare Earth Elements: Fundamentals and Applications; Atwood, D. A., Ed.; Wiley: Hoboken, NJ, 2012. (4) Evans, W. J. Organometallics 2016, 35, 3088−3100. (5) Woen, D. H.; Evans, W. J. In Handbook on the Physics and Chemistry of Rare Earths; Elsevier: Amsterdam, 2016; Vol. 50, pp 337−394. (6) Morss, L. R. Chem. Rev. 1976, 76, 827−841. (7) Morss, L. R. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Eyring, L., Choppin, G. R., Lander, G. H., Eds.; Elsevier: Amsterdam, 1994; Vol.18, pp 239−291. (8) Kamenskaya, A. N. Russ. J. Inorg. Chem. 1984, 29, 251−258. (9) Mikheev, N. B. Inorg. Chim. Acta 1984, 94, 241−248. (10) Mikheev, N. B.; Kamenskaya, A. N. Coord. Chem. Rev. 1991, 109, 1−59. (11) Hitchcock, P. B.; Lappert, M. F.; Maron, L.; Protchenko, A. V. Angew. Chem., Int. Ed. 2008, 47, 1488−1491. (12) MacDonald, M. R.; Ziller, J. W.; Evans, W. J. J. Am. Chem. Soc. 2011, 133, 15914−159147. (13) MacDonald, M. R.; Bates, J. E.; Fieser, M. E.; Ziller, J. W.; Furche, F.; Evans, W. J. J. Am. Chem. Soc. 2012, 134, 8420−8423. (14) MacDonald, M. R.; Bates, J. E.; Ziller, J. W.; Furche, F.; Evans, W. J. J. Am. Chem. Soc. 2013, 135, 9857−9868. (15) Fieser, M. E.; MacDonald, M. R.; Krull, B. T.; Bates, J. E.; Ziller, J. W.; Furche, F.; Evans, W. J. J. Am. Chem. Soc. 2015, 137, 369−382. (16) Langeslay, R. R.; Fieser, M. E.; Ziller, J. W.; Furche, F.; Evans, W. J. Chem. Sci. 2015, 6, 517−521. (17) Windorff, C. J.; MacDonald, M. R.; Meihaus, M. R.; Ziller, J. W.; Long, J. R.; Evans, W. J. Chem. - Eur. J. 2016, 22, 772−782. (18) MacDonald, M. R.; Fieser, M. E.; Bates, J. E.; Ziller, J. W.; Furche, F.; Evans, W. J. J. Am. Chem. Soc. 2013, 135, 13310−13313. (19) Windorff, C. J.; Chen, G. P.; Cross, J. N.; Evans, W. J.; Furche, F.; Gaunt, A. J.; Janicke, M. T.; Kozimor, S. A.; Scott, B. L. J. Am. Chem. Soc. 2017, 139, 3970−3973. (20) Coles, M. P.; Hitchcock, P. B.; Lappert, M. F.; Protchenko, A. V. Organometallics 2012, 31, 2682−2690. (21) Gun’ko, Y. K.; Hitchcock, P. B.; Lappert, M. F. J. Organomet. Chem. 1995, 499, 213−219. (22) Cassani, M. C.; Lappert, M. F.; Laschi, F. Chem. Commun. 1997, 1563−1564. (23) Meihaus, M. R.; Fieser, M. E.; Corbey, J. F.; Evans, W. J.; Long, J. R. J. Am. Chem. Soc. 2015, 137, 9855−9860. (24) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (25) Xie, Z.; Chui, K.; Liu, Z.; Xue, F.; Zhang, Z.; Mak, T. C. W.; Sun, J. J. Organomet. Chem. 1997, 549, 239−244. (26) Stults, S. D.; Andersen, R. A.; Zalkin, A. Organometallics 1990, 9, 115−122. (27) Anderson, D. M.; Cloke, F. G. N.; Cox, P. A.; Edelstein, N.; Green, J. C.; Pang, T.; Sameh, A. A.; Shalimoff, G. J. Chem. Soc., Chem. Commun. 1989, 53−55. (28) Dutkiewicz, M. S.; Apostolidis, C.; Walter, O.; Arnold, P. L. Chem. Sci. 2017, 8, 2553−2561. (29) Evans, W. J.; Sollberger, M. S.; Shreeve, J. L.; Olofson, J. M.; Hain, J. H., Jr.; Ziller, J. W. Inorg. Chem. 1992, 31, 2492−2501. (30) Hitchcock, P. B.; Lappert, M. F.; Prashar, S. J. Organomet. Chem. 1991, 413, 79−90. (31) Schumann, H.; Keitsch, M. R.; Demtschuk, J.; Molander, G. A. J. Organomet. Chem. 1999, 582, 70−82. (32) Bergbreiter, D. E.; Killough, J. M. J. Am. Chem. Soc. 1978, 100, 2126−2134. (33) Cassani, M. C.; Gun’ko, Y. K.; Hitchcock, P. B.; Lappert, M. F.; Laschi, F. Organometallics 1999, 18, 5539−5547. (34) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532−536.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00881. additional spectroscopic data for 1-Ln (Ce, Nd), 2-Ln (Ce, Pr, Nd), and 3-Ln (La, Ce) (PDF) Accession Codes

CCDC 1585114−1585119 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, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chad T. Palumbo: 0000-0001-6436-4602 William J. Evans: 0000-0002-0651-418X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. National Science Foundation through grants CHE-1565776 (W.J.E.) and CHE-1464841 (Professor Jeffrey R. Long) for support of this research. We thank Professor Long for providing the facilities in which the magnetic data were collected. We thank Dr. Jordan F. Corbey and Dr. Jason R. Jones for crystallographic assistance. We also thank NSF for providing graduate fellowship support for C.T.P. and L.E.D.

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REFERENCES

(1) Bochkarev, M. N. Coord. Chem. Rev. 2004, 248, 835−851. (2) Nief, F. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Bünzli, J.-C. G., Pecharsky, V. K., Eds.; Elsevier: Amsterdam, 2010; Vol. 40, p 241−300. F

DOI: 10.1021/acs.organomet.7b00881 Organometallics XXXX, XXX, XXX−XXX