Reactivity of Ln(II) Complexes Supported by (C5H4Me)1– Ligands

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Reactivity of Ln(II) Complexes Supported by (C5H4Me)1− Ligands with THF and PhSiH3: Isolation of Ring-Opened, Bridging Alkoxyalkyl, Hydride, and Silyl Products David H. Woen, Daniel N. Huh, Joseph W. Ziller, and William J. Evans* Department of Chemistry, University of California, Irvine, California 92697-2025, United States

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ABSTRACT: Reduction of CpMe3Ln(THF), 1-Ln (Ln = La and Gd; CpMe = C5H4Me), with KC8 in the presence of 2.2.2-cryptand (crypt) generates dark solutions, 2-Ln, with EPR spectra consistent with Ln(II) complexes: an eight-line hyperfine pattern for La at g = 1.971 and a broad single line for Gd at g = 1.988. The solutions decompose within minutes, and in the La system, a decomposition product was isolated in which a molecule of THF had been reduced by two electrons and ring-opened to generate an alkoxyalkyl-bridged bimetallic La(III) complex, [K(crypt)]2[(CpMe3La)2(μ-OCH2CH2CH2CH2)], 3-La. An analogous Pr complex, 3-Pr, was also crystallographically characterized. Since decomposition products were not readily isolated from the analogous yttrium CpMe3Y(THF)/KC8/crypt reaction, the composition of the solution of 2-Y was probed by addition of the hydrogen delivery reagent, PhSiH3, which had previously been reported to form U(III) hydrides from tris(cyclopentadienyl) U(II) complexes. This generated an Y(III) silyl complex, [K(crypt)][CpMe3Y(SiH2Ph)], 4, in addition to a hydride product, [K(crypt)][(CpMe3Y)2(μ-H)], 5. The retention of three CpMe ligands per metal in 3-Ln, 4, and 5 is consistent with the presence of a (CpMe3Ln)1− species in 2-Ln.

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INTRODUCTION Recent studies of the reductive chemistry of tris(cyclopentadienyl) rare-earth metal complexes, (C5R5)3Ln (R = H, SiMe3, alkyl), have shown that complexes of M(II) ions can be accessed across the lanthanide series1−4 as well as with Y,5 Th,6 U,7,8 Pu,9 and Np.10 Scheme 1 shows examples for C5H3(SiMe3)2 (Cp′′)1,6,8,9,11,12 and C5H4SiMe3 (Cp′)2−5,7 complexes, but examples with C5H3(CMe3)2 (Cptt),13,14 C5H2(CMe3)3 (Cpttt),14,15 and the tris(aryloxide) mesitylene ligand, ((Ad,MeArO)3mes),16−18 are also known. For the rare-earth metals, the stability of the Ln(II) complexes is highly dependent on the specific ligand. As shown in Scheme 1, C5H4SiMe3 (Cp′) allows the isolation of crystalline Ln(II) complexes from La to Lu, as well as Y,2−5,7 but C5H3(SiMe3)2 (Cp″) only provides crystals for La-Nd.1,11 Another previous study showed that the use of the monomethyl-substituted ligand, C5H4Me (CpMe), gave only in situ EPR evidence for an Y(II) complex that decomposed within minutes.19 The nature of the unstable CpMe3Ln/KC8 solutions has been probed, and we report here the isolation of one of the decomposition products in the case of the larger metals, La and Pr, a complex of a ring-opened THF molecule that has been converted to a bidentate, bridging, alkoxyalkyl dianion, (μ-OCH2CH2CH2CH2)2−. Since ring-opened THF decomposition products were not isolable with yttrium, the presence of Y(II) complexes was © XXXX American Chemical Society

probed with the hydride delivery reagent, PhSiH3. Phenylsilane is a reagent commonly used to provide hydrogen to transition metal,20−31 lanthanide,32−39 and actinide complexes.8,32−37,40 Although transition metals readily form H−M−SiH2Ph complexes,20−31 the f-elements typically react with PhSiH3 to form only hydride complexes.32−37 Since PhSiH3 has been used to characterize the reactivity of newly discovered U(II) ions to make isolable U(III) hydride products as shown in Scheme 2,8 it was chosen to probe the presence of Y(II) generated in situ from a CpMe3Y/KC8 solution. This has led to not only an Y(III) hydride product but also the first example of a rare-earth metal−SiH2Ph complex. Although rare-earth metal silyl complexes are well-known,41−52 there have been no previous reports of Ln−SiH2Ph complexes.

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RESULTS Reductions of CpMe3Ln(THF), 1-Ln (Ln = La, Pr, Gd). Reduction studies using the (CpMe3)3− ligand system were originally investigated with Y. Reduction of CpMe3Y(THF) with potassium graphite in THF at −35 °C provided a dark solution with a two-line hyperfine EPR spectrum consistent with Y(II) since 89Y has a nuclear spin of 1/2.19 Since the Received: June 17, 2018

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DOI: 10.1021/acs.organomet.8b00419 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Scheme 1. Reduction of Tris(mono- or di-silylcyclopentadienyl) M(III) Complexes To Generate Crystallographically Characterized M(II) Complexes

Scheme 2. Reactivity of U2+ Complexes with PhSiH3 To Form Hydride Complexes

coupling constants as a function of ligand was found for the reduction products of CpMe3Y, Cp′3Y, and Cp″3Y: 46.9, 36.6, and 36.1 G, respectively.19 The trends for both yttrium and lanthanum suggest that the Me substituent makes the cyclopentadienyl ligand more electron donating than Me3Si in this system. This is consistent with previous studies by Lappert13 as well as Bercaw and Parkin.54 Consequently, CpMe ligands put the most electron density on the metal, followed by Cp′ and Cp″. For 2-La and 2-Pr, pale-colored decomposition products were successfully identified by X-ray crystallography as bimetallic compounds containing alkoxyalkyl bridges derived from the ring opening of THF, [K(crypt)]2[(CpMe3Ln)2(μOCH2CH2CH2CH2)], 3-Ln (Figure 2, Scheme 3). This was the first time that such THF decomposition products were identifiable in these reduction reactions. Complexes 3-La and 3-Pr crystallize in the monoclinic space group P21/c, and their crystal structures are isomorphous. They crystallize as bimetallic species with inversion centers so that there is just one CpMe3Ln unit and one K(crypt) moiety in each unit cell. Refinement of the structures was complicated by the fact that the dianionic bridging ligand is nonsymmetrical.

yttrium solution was unstable and no reaction products were isolated, studies were extended to the larger metals La, Pr, and Gd to determine the generality of the yttrium result. Reductions of 1-La, 1-Pr, and 1-Gd using KC8 in the presence of crypt at −35 °C generated deep dark brown solutions, 2-La, 2-Pr, and 2-Gd, respectively, that decomposed within several minutes at −35 °C to faint yellow, as was previously found for yttrium.5 For lanthanum and gadolinium, EPR spectroscopic measurements were performed on the fresh reduction products since the crystallographically characterized La(II) complexes, [K(crypt)][Cp″3La]1 and [K(crypt)][Cp′3La],2 exhibit EPR spectra with eight-line hyperfine patterns consistent with I = 7/2 of 139La, whereas a solution of the Gd(II) complex, [K(crypt)][Cp′3Gd], 1-Gd, exhibited a broad singlet.4 The spectra obtained on 2-La and 2-Gd, Figure 1, were similar to

Figure 1. X-band EPR spectra of the reduction product of CpMe3La(THF), 2-La (left), and CpMe3Gd(THF), 2-Gd (right), collected at 298 K. Simulated spectra are shown as dotted lines. A small feature in the middle of the spectrum of 2-La is attributed to the presence of an electride impurity, which arises from the reaction of residual crypt with KC8.53

those of the previously characterized Ln(II) complexes, suggesting the presence of Ln(II) in these CpMe-ligated systems. The room temperature EPR spectrum of 2-La displays an eight-line hyperfine pattern at g = 1.971 with A = 195 G, and that of 2-Gd displays a broad singlet at g = 1.988. The 195 G coupling constant for 2-La is larger than the 154 G value for [K(crypt)][Cp′3La],2 which is larger than the 133 G value for [K(crypt)][Cp″3La].1 A similar ordering of

Figure 2. Thermal ellipsoid plot of one bimetallic dianion of [K(crypt)]2[(CpMe3Ln)2(μ-OCH2CH2CH2CH2)], 3-La, drawn at 50% probability level. Disorder about the bridging ligand and a methylcyclopentadienyl ligand of each metal, as well as hydrogen atoms, two THF molecules, and two [K(crypt)]+ countercations are omitted for clarity. 3-Pr is isomorphous. B

DOI: 10.1021/acs.organomet.8b00419 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 3. Decomposition of Ln2+ Products from the Reduction of CpMe3Ln(THF) (Ln = La, Pr), 3-Ln

reaction with PhSiH3 was examined to see if a product analogous to those in Scheme 2 could be isolated. Addition of PhSiH3 in THF at −35 °C to a dark solution freshly generated from the KC8 reduction of a THF solution of CpMe3Y(THF) and crypt, 2-Y, immediately formed a faint yellow solution. Removal of the solvent by evaporation only yielded oily material, but crystalline solids were successfully grown in low quantities from THF by layer diffusion with Et2O. Surprisingly, X-ray crystallographic analysis of the colorless crystalline product revealed not the expected hydride product, but an yttrium-SiH2Ph complex, [K(crypt)][CpMe3Y(SiH2Ph)], 4 (Figure 3, Scheme 4).

This led to disorder in the bridging ligands, since the bridging ligand could be oriented as (μ-OCH2CH2CH2CH2)2− or the reverse, (μ-CH2CH2CH2CH2O)2−. This situation with the inversion symmetry initially gave a refinement in which the bridge appeared to have six atoms that could be modeled by (μ-CH2CH2CH2CH2CH2CH2)2−! The 2.631 and 2.587 Å average Ln−(CpMe ring centroid) distances of 3-La and 3-Pr are both longer than the 2.580 and 2.540 Å average Ln−(Cp ring centroid) distances of Cp3La(THF)55 and Cp3Pr(THF),56 respectively, (Cp = C5H5) which are the closest tris(cyclopentadienyl) ligand complexes available for comparison. This is consistent with the fact that the 2.045(7) and 2.002(5) Å La−O and Pr−O distances in 3-Ln are much shorter than the 2.57(1)55 and 2.551(2) Å56 La−O(THF) and Pr−O(THF) distances in Cp3Ln(THF). The 2.045(7) and 2.002(5) Å Ln−O distances in 3-La and 3-Pr are also much shorter than the Ln−C distance of the bridging ligand, the 2.731(1) Å La−C(43) and 2.730(9) Å Pr−C(43) distances. These differences between the Ln−O and Ln−C distances are much more pronounced than those found between the 2.328(2)−2.330(2) Å Nd−O distances in the hydroxide complex, [(C5H4tBu)2Nd]2(μOH)2,57 and the 2.70(2)−2.90(5) Å Nd−C distances in the methyl analogue, [(C5H4tBu)2Nd]2(μ-Me)2.58 The 1H NMR spectrum of the product of the decomposition of 2-La shows resonances consistent with the X-ray crystallographic data of 3-La. Two sets of CpMe resonances with equal integrations are observed: one at δ 5.58 (m, 6H, C5H4Me), 5.48 (m, 6H, C5H4Me), and 2.16 (s, 9H, C5H4Me) ppm, and another one at δ 5.56 (m, 6H, C5H4Me), 5.45 (m, 6H, C5H4Me), and 2.13 (s, 9H, C5H4Me) ppm. This is consistent with the differing proton environments in one CpMe3La moiety compared to the other within the bimetallic dianion. Additionally, a set of resonances consistent with the bridging ring-opened THF was also observed: δ 3.81 (t, 2H, OCH2CH2CH2CH2), 1.55 (q, 2H, OCH2CH2CH2CH2), 1.39 (q, 2H, OCH2CH2CH2CH2), and −0.22 (q, 2H, OCH2CH2CH2CH2) ppm. Each metal center in 3-Ln retains all three of the anionic cyclopentadienyl ligands. This suggests that the reductions of CpMe3La(THF) and CpMe3Pr(THF) likely generate Ln(II) complexes that still contain three cyclopentadienyl ligands, i.e., that 2-La and 2-Pr are likely to be [CpMe3La]1− and [CpMe3Pr]1−, respectively, with [K(crypt)]1+ as the countercation. PhSiH3 Reactions. In the case of the reduction of CpMe3Y to form 2-Y,19 a decomposition product analogous to 3-Ln was not isolated. The NMR spectrum of the decomposition product had resonances that could be interpreted as 3-Y, but the spectrum was not definitive. To further characterize the solutions formed from the KC8 reduction of CpMe3Y(THF), a

Figure 3. Thermal ellipsoid plot of [K(crypt)][CpMe3Y(SiH2Ph)], 4. Hydrogen atoms are omitted for clarity.

Although transition metal−SiH 2 Ph complexes are known,21,25,28,59−63 there are no reports of rare-earth− SiH2Ph complexes in the literature to our knowledge. However, an alkaline earth metal−SiH2Ph complex has been reported from the reaction of a tetraazacyclododecane Mg hydride complex with PhSiH3.64 The 2.465 Å average Y−CpMe centroid distances and the 117.7° average Cnt−Y−Cnt angles of this 10-coordinate Y3+ complex are similar to the 2.451 Å and 117.1° averages of the 10-coordinate starting material, CpMe3Y(THF). The Y−Si bond length was determined to be 2.953(1) Å, which is similar to the 2.979(3) and 2.961(2) Å bond lengths observed in the six-coordinate complexes, (R3Si)YI2(THF)3 and [(Et)R2Si]YI2(THF) (R = SiMe3), respectively.43 C

DOI: 10.1021/acs.organomet.8b00419 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 4. Reactivity of 2-Y with PhSiH3

distance, and the 4.513 Å Ce···Ce and 4.523 Å (La···La) distances of those previously reported bridging monohydride complexes,70,71 just as the nine-coordinate ionic radius of yttrium is between those of the other metals: 1.216 Å, La; 1.196 Å, Ce; 1.075 Å, Y; 1.032 Å Lu.72

Multiple crystals from different sample batches were identified by X-ray crystallography as 4, but all NMR analyses did not reveal any resonances that would correspond to the −SiH2Ph moiety. It is possible that this species was formed only as a minor product or that it decomposed in the d8-THF solution at room temperature. Interestingly, in addition to weak resonances from small amounts of impurities, we observed only one set of CpMe resonances and a triplet at δ −0.33 ppm with a coupling constant of 34.3 Hz. This coupling constant is within the 1JYH = 27.2−34.6 Hz range found in previous reports of bimetallic yttrium bridging dihydride complexes.65−68 The negative shift is fairly close to the δ −1.03 ppm of the anionic tetrahydride complex, {Li(THF)4)}{[Cp2Y(μ-H)]3(μ3-H)}.69 This is consistent with the X-ray data described in the next paragraph. When the reaction of the dark solution of 2-Y with PhSiH3 was repeated using excess PhSiH3, colorless crystals of 5 were obtained in low quantity which had a different morphology than those of 4. X-ray crystallography suggested that 5 is the bimetallic yttrium complex, [K(crypt)][(CpMe3Y)2(μ-H)] (Figure 4, Scheme 4), but the crystal data of 5 were not of

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DISCUSSION The dark color of the initial solutions generated by potassium graphite reduction of CpMe3Ln(THF) for La, Gd, and Y are consistent with the formation of the Ln(II) ions as was previously observed in the Cp′3Ln reductions that led to crystallographically characterized Ln(II) complexes. The instability of these dark solutions can be understood from the THF decomposition reaction shown in Scheme 3. These Ln(II) ions with CpMe ligands decompose in their THF solvent which could explain why many LnA3/M reactions (A = anion; M = alkali metal) give dark solutions that maintain their color only transiently. Even if reactions are conducted in Et2O, it is common to have THF vapor in gloveboxes that contain coordinating solvents, and hence this could lead to decomposition. It was fortunate in the cases of 3-La and 3-Pr that the alkoxyalkyl decomposition product could be isolated. With the smaller metal yttrium, crystallographic confirmation of this product was elusive. Although THF can be ring-opened in many ways,73−80 the two-electron ring-opening reduction in Scheme 3 is unusual. The formal half-reaction is shown in eq 1. Examples of transition metal complexes containing [O(CH2)4]2− bridging moieties obtained from ring-opened THF have previously been reported: (R3SiO)3Ti[O(CH2)4]Ti(OSiR3)3 (R = tBu3;78 OtBu79) and {(Giso)ZrCl[O(CH2)4]}280 (Giso = [(2,6-iPr2C6H3)N]2C[N(C6H11)2]).

Just as Scheme 3 is unusual, so is Scheme 4 in that the PhSiH3 reagent formed not only the usual target of these silane reactions, a hydride, [K(crypt)][(CpMe3Y)2(μ-H)], 5, but also a silyl product, [K(crypt)][CpMe3Y(SiH2Ph)], 4. This is the first example of a rare-earth-SiH2Ph complex to our knowledge and shows one possible fate of the silyl group after the hydride is delivered. It is interesting to note that the (CpMe)33− ligand system had also previously provided the first rare-earth metal complexes with primary phosphine, CpMe3Ln(PH2Mes) (Ln = Y and Dy, Mes = mesityl),81 and arsenine, CpMe3Ln(AsH2Mes) (Ln = Y82 and Dy83). However, those complexes were isolated as neutral species since the phosphine and arsenine ligands acted as neutral donors rather than anionic ligands as in the case of silicon in 4. Hence, we are still intrigued that the yttrium complexes 4 and 5 retain all three of the CpMe rings of the original CpMe3Y(THF) starting material and are isolated as anions. A more common class of rare-earth metallocenes is the

Figure 4. Thermal ellipsoid plot of [K(crypt)][(CpMe3Y)2(μ-H)], 5, drawn at 50% probability level. No bridging moiety could be located. Hydrogen atoms are omitted for clarity.

high enough quality to allow the identification of the hydride ligand. However, each metal is located ∼0.4 Å out of the plane of its three Cp-ring centroids toward the other metal and the ∼4.3 Å Y···Y distance is too long for a metal−metal bond. Since anionic bridging monohydride complexes [K(THF)6][(Cp3Lu)2(μ-H)]70 and {K(crown)(toluene)2}{[(C5H4SitBuMe2)3Ln]2(μ-H)} (Ln = La, Ce; crown = 18-crown-6)71 are known, it is likely that 5 is analogous to those complexes. The ∼4.3 Å Y···Y distance of 5 is between the 4.18 Å Lu···Lu D

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Organometallics

CRYO500 MHz spectrometers (13C NMR at 125 MHz) at 298 K unless otherwise stated and referenced internally to residual protiosolvent resonances. Electron paramagnetic resonance (EPR) spectra were collected using a Bruker EMX spectrometer equipped with an ER041XG microwave bridge in THF at 298. EPR simulations were performed using EasySpin.87 IR samples were prepared as KBr pellets on a Varian 1000 FT-IR system. Elemental analyses were conducted on a PerkinElmer 2400 Series II CHNS elemental analyzer. CpMe3La(THF), 1-La. Following the procedure for CpMe3Y(THF),19 a pale yellow THF (7 mL) solution of NaCpMe (244 mg, 2.39 mmol) was added to a THF (2 mL) slurry of LaCl3 (196 mg, 0.80 mmol) to yield CpMe3La(THF) (147 mg, 49%) as a pale yellow powder after workup. Colorless crystals were grown from a toluene/ hexane solution at −35 °C. 1H NMR (C6D6): δ 6.00 (m, C5H4Me, 6H), 5.89 (m, C5H4Me, 6H), 3.35 (m, C4H8O, 4H), 2.25 (s, C5H4Me, 9H), 1.16 (m, C4H8O, 4H). 13C{1H} (C6D6): δ 115.18 (C5H4Me), 111.75 (C5H4Me), 72.41 (C4H8O), 25.49 (C4H8O), 15.35 (C5H4Me). IR: 3073m, 2980s, 2957s, 2920s, 2893s, 2856s, 2730w, 1733w, 1715w, 1697w, 1683w, 1652w, 1634w, 1558w, 1540w, 1520w, 1507w, 1490m, 1455m, 1412w, 1378m, 1362w, 1338m, 1311w, 1289w, 1260w, 1237m, 1168m, 1135w, 1109w, 1087m, 1062m, 1043s, 1032s, 1014s, 971w, 929m, 860s, 818s, 751s, 736s, 683w, 673m, 666m, 618m cm−1. Anal. Calcd for C22H29OLa: C, 58.93; H, 6.52. Found: C, 58.52; H, 6.32. CpMe3Pr(THF), 1-Pr. Following the procedure for CpMe3Y(THF),19 a pale yellow THF (7 mL) solution of KCpMe (67 mg, 0.27 mmol) was added to a THF (2 mL) slurry of PrCl3 (96 mg, 0.81 mmol) to yield CpMe3Pr(THF) (60 mg, 49%) as green solids after workup. Colorless crystals were grown from a toluene/hexane solution at −35 °C. IR: 3079m, 2981s, 2961s, 2923s, 2892s, 2859s, 2730m, 1734w, 1717w, 1698w, 1685w, 1653w, 1646w, 1634w, 1622w, 1576w, 1558w, 1540w, 1521w, 1507w, 1490m, 1455s, 1415w, 1378m, 1361w, 1348w, 1338m, 1309w, 1289w, 1262w, 1238m, 1167m, 1131w, 1109w, 1080w, 1064m, 1044s, 1034s, 1015s, 961w, 953w, 929m, 861s, 823s, 760s, 748s, 665m, 645w, 615s cm−1. Anal. Calcd for C22H29OPr: C, 58.67; H, 6.49. Found: C, 58.41; H, 6.38. CpMe3Gd(THF), 1-Gd. Following the procedure for CpMe3Y(THF),19 a pale yellow THF (5 mL) solution of NaCpMe (370 mg, 3.62 mmol) was added to a THF (5 mL) slurry of GdCl3 (318 mg, 1.21 mmol) to yield CpMe3Gd(THF), 1-Gd (327 mg, 58.1%), as a pale yellow powder after workup. Colorless crystals were grown from a toluene/hexane solution at −35 °C. IR: 3089m, 2925w, 2861w, 2733s, 1619w, 1491s, 1454m, 1380s, 1340s, 1239s, 1167s, 1045s, 1012s, 935s, 830s, 775m, 613s cm−1. Anal. Calcd for C22H29OGd: C, 56.62; H, 6.26. Found: C, 56.30; H, 6.07. Reduction of CpMe3Ln(THF), 1-Ln. In an argon-filled glovebox, crypt (8 mg, 0.022 mmol) and 1-La (10 mg, 0.022 mmol) were dissolved in THF (0.5 mL) and placed in the freezer for a few hours. The cold solution was run through a prechilled filter pipet packed with KC8 into a prechilled EPR tube to obtain a deep brown solution, 2-La, which was kept cold until it could be inserted into the EPR spectrometer. Similarly, crypt (99 mg, 0.26 mmol) and 1-Gd (114 mg, 0.244 mmol) solution in THF (5 mL) was passed through a KC8 packed column, and the resulting dark solution of 2-Gd was analyzed by EPR spectroscopy. [K(crypt)]2[(CpMe3La)2(μ-OC3H6CH2)], 3-La. In an argon-filled glovebox, crypt (34 mg, 0.091 mmol) and CpMe3La(THF) (41 mg, 0.091 mmol) were dissolved in THF (1 mL) and placed in the freezer for a few hours. KC8 was added to the solution to generate a black mixture. The mixture was quickly filtered to give a dark brown solution and transferred to the bottom of a prechilled hexane (5 mL) solution for layer diffusion. Before the sample was transferred back into a −35 °C glovebox freezer, it was observed that the dark color was already fading. After 24 h, off-white crystalline solids were obtained. The solids were washed with room temperature Et2O (3×) and dried to give 38 mg of crude product (25%). Recrystallization of this material by THF/hexane layer diffusion at −35 °C gave a very small yield of single crystals of [K(crypt)]2[(CpMe3La)2(μ-OC3H6CH2)], 3-La, suitable for X-ray diffraction. 1H NMR (THF-d8): δ 5.58 (m, 6H, C5H4Me), 5.56 (m, 6H, C5H4Me), 5.48 (m, 6H,

neutral bent complexes of formula (C5R5)2LnX(THF), where X is a monodentate anion. The presence of three (CpMe)1− anions in 4 and 5 as well as in 3-Ln is consistent with, but not definitive, for the presence of tris(cyclopentadienyl) complexes in the reduction solutions, i.e., (CpMe3Ln)1− complexes. In any case, the (CpMe3)3− ligand system did not provide more stable Ln2+ complexes than previously found for the (Cp′3)3− ligand set,2,4 which is still the best ligand system to isolate Ln2+ ions across the entire series. EPR spectroscopy studies provided hyperfine coupling constant data in concordance with previous reports that alkyl substituents cause cyclopentadienyl ligands to be more electron donating, whereas silyl substituents cause them to be less electron donating in these complexes.13,54 Hence, there could be an electronic explanation as to why the (Cp′3)3− ligand set allows the isolation of the “nontraditional” divalent ions that were previously thought to be too reducing to isolate, whereas (CpMe3)3− does not. In addition to electronic factors, it appears that steric effects also play a role in the isolation of Ln(II) ions since the bulkier silyl-cyclopentadienyl ligand system, (Cp″3)3−, did not provide a stable Y(II) complex,19 presumably because of steric oversaturation. In the case with the (CpMe3)3− ligand system, steric undersaturation could be the reason that the Ln(II) complexes are significantly less stable and highly reactive. The steric undersaturation and the consequently more open coordination environment provide the basis for these Ln(II) complexes to readily react with THF to form the Ln(III) decomposition product, [K(crypt)]2[(CpMe3Ln)2(μ-OC3H6CH2)], 3-Ln. This steric undersaturation may make these solutions desirable for reactivity studies of the Ln(II) ions as shown by the isolation of [K(2.2.2-cryptand)][CpMe3Y(SiH2Ph)], 4, and [K(crypt)][(CpMe3Y)2(μ-H)], 5.

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CONCLUSION Reduction of CpMe3Ln in THF for Ln = La, Gd, and Y provides solutions with spectral characteristics consistent with Ln(II) ions and reactivity patterns suggestive of (Cp Me 3Ln) 1− complexes. The instability of these complexes can be understood in terms of coordinative unsaturation and high electron density on the metal center that leads to reductive ring opening of THF to form an alkoxyalkyl dianion. The Ln(II) solutions may have potential for new reactivity patterns as demonstrated by the isolation of both hydride and silyl products from reactions with PhSiH3.

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EXPERIMENTAL SECTION

All manipulations and syntheses described below were conducted with rigorous exclusion of air and water using standard Schlenk line and glovebox techniques under an argon or dinitrogen atmosphere. Solvents were sparged with UHP argon and dried by passage through columns containing Q-5 and molecular sieves prior to use. Deuterated NMR solvents was dried over NaK alloy, degassed by three freeze− pump−thaw cycles, and vacuum transferred before use. Methylcyclopentadiene were dried over molecular sieves and degassed by three freeze−pump−thaw cycles. 2.2.2-Cryptand (crypt) was purchased from Merck and dried under reduced pressure before use. KCpMe and NaCpMe (CpMe = C5H4Me) were synthesized via an adaptation of a literature procedure84 in which the organic dimer was cracked and distilled onto a toluene solution of KN(SiMe3)2 or NaN(SiMe3)2, respectively. The resulting white precipitate was then washed with hexane and dried. KC8,85 anhydrous LnCl3 (Ln = La, Pr, Gd),86 and CpMe3Y(THF),19 1-Y, were prepared according to the literature. 1H NMR spectra were recorded on Bruker GN500 or E

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Organometallics Accession Codes

C5H4Me), 5.45 (m, 6H, C5H4Me), 3.81 (t, 2H, OCH2CH2CH2CH2), 3.57 (crypt), 3.53 (t, 24H, crypt), 2.54 (t, 24H, crypt), 2.16 (s, 9H, C5H4Me), 2.13 (s, 9H, C5H4Me), 1.55 (q, 2H, OCH2CH2CH2CH2), 1.39 (q, 2H, OCH2CH2CH2CH2), and −0.22 (m,2H, OCH2CH2CH2CH2) ppm. 13C{1H} (THF-d8): δ 113.45 (C5H4Me), 110.99 (C5H4Me), 110.94 (C5H4Me), 110.81 (C5H4Me), 110.72 (C5H4Me) ppm. The 13C resonances of the (μ-OCH2CH2CH2CH2)2− bridging ligand were not observed. Each of these one-carbon signals is unique and could not be distinguished from the baseline. IR: 3074w, 2959m, 2884s, 2816m, 2762m, 2660w, 1956w, 1476m, 1459m, 1444m, 1412w, 1353s, 1295m, 1200m, 1134s, 1103vs, 1078s, 1029m, 952s, 932m, 806m, 731s, 621w cm−1. [K(crypt)]2[(CpMe3Pr)2(μ-OC3H6CH2)], 3-Pr. Following the procedure for 3-La, a 1 mL THF solution of crypt (36 mg, 0.095 mmol) and CpMe3Pr(THF) (43 mg, 0.095 mmol) was reacted with KC8 to give an off-white crystalline solid. The solids were washed with room temperature Et2O (3×) and dried to give 35 mg of crude product (22%). Single crystals of [K(crypt)]2[(CpMe3Pr)2(μ-OC3H6CH2)], 3Pr, suitable for X-ray diffraction were grown from THF/hexane layer diffusion at −35 °C. IR: 3075w, 2959m, 2886s, 2812m, 2760m, 2627w, 2670w, 1958w, 1476m, 1459m, 1444m, 1410w, 1356s, 1298m, 1266m, 1237w, 1172w, 1133s, 1105vs, 1079s, 1043w, 1029m, 950s, 931m, 803m, 733s, 619w cm−1. [K(crypt)][CpMe3Y(SiH2Ph)], 4. In an argon-filled glovebox, a THF (5 mL) solution of CpMe3Y(THF) (71 mg, 0.22 mmol) and crypt (82 mg, 0.22 mmol) was prechilled at −35 °C before it was pushed through a prechilled filter pipet packed with KC8 onto a stirring prechilled THF (5 mL) solution containing PhSiH3 (30 mg, 0.28 mmol). The resulting dark solution quickly turned pale yellow, which was then concentrated to ∼3 mL, layered with Et2O, and left at −35 °C. Colorless crystals of 4 suitable for X-ray diffraction were obtained after several days. Several different crystals from different sample batches were identified as 4 by X-ray crystallography, but NMR spectroscopy analysis of these samples showed no resonances corresponding to the −SiH2Ph group. It is possible that complex 4 decomposes at room temperature in THF. Surprisingly, all of the spectra showed the hydride peak discussed below under the experimental section for complex 5. It appears that complex 5 is a coproduct of this reaction. [K(2.2.2-cryptand)][(CpMe3Y)2(μ-H)], 5. In an argon-filled glovebox, a THF (2 mL) solution of CpMe3Y(THF) (71 mg, 0.22 mmol) and crypt (82 mg, 0.22 mmol) was prechilled at −35 °C before it was pushed through a prechilled filter pipet packed with KC8 onto a stirring prechilled THF (1 mL) solution containing PhSiH3 (50 mg, 0.46 mmol). The resulting dark solution quickly turned pale yellow. The solution was transferred using a pipet to the bottom of a prechilled Et2O (15 mL) solution and was left at −35 °C. Colorless crystals of 5 suitable for X-ray diffraction were obtained after several days. 1H NMR analysis of the product in THF-d8 showed a triplet at δ −0.33 (t, μ-H, 1JYH = 34.3 Hz) ppm, which is similar to those of other Y bridging hydride complexes.65−68 In addition, a set of CpMe resonances were observed but with higher integration values. This suggests that there may be other species present in the sample that have indistinguishable CpMe resonances from 5. 1H NMR (THF-d8): δ 5.74 (m, C5H4Me), 5.62 (m, C5H4Me), 3.54 (s, OC2H4Ot), 3.49 (t, NCH2CH2O, 3JHH = 5.0 Hz), 2.51 (t, NCH2CH2O, 3JHH = 5.0 Hz), 2.28 (s, C5H4Me), −0.33 (t, μ-H, 1JYH = 34.3 Hz) ppm. X-ray Data Collection, Structure Determination, and Refinement. Crystallographic details on complexes 3-La, 3-Pr, 4, and 5 are summarized in the Supporting Information.

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CCDC 1849829−1849831 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

David H. Woen: 0000-0002-5764-1453 Daniel N. Huh: 0000-0001-7887-0856 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 for support under CHE-1565776. We also thank Dr. Jason R. Jones and Michael K. Wojnar for assistance with X-ray crystallography as well as the laser facility at UC Irvine and Professor A. S. Borovik for spectroscopic assistance.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00419. Crystallographic details for 3-La, 3-Pr, 4, and 5; references; and definitions (PDF) Cartesian coordinates (XYZ) F

DOI: 10.1021/acs.organomet.8b00419 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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DOI: 10.1021/acs.organomet.8b00419 Organometallics XXXX, XXX, XXX−XXX