Synthesis and Reactions of [Cp* 2Yb] 2 (μ-Me) and [Cp* 2Yb] 2 (μ-Me

Article pubs.acs.org/Organometallics

Synthesis and Reactions of [Cp*2Yb]2(μ-Me) and [Cp*2Yb]2(μ-Me)(Me) and Related Yb2(II, III) and Yb2(III, III) Compounds Marc D. Walter,†,‡ Phillip T. Matsunaga,† Carol J. Burns,† Laurent Maron,§ and Richard A. Andersen*,† †

Department of Chemistry and Chemical Sciences Division of Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, United States ‡ Technische Universität Braunschweig, Institut für Anorganische und Analytische Chemie, Hagenring 30, 38106 Braunschweig, Germany § Université de Toulouse, INSA-UPS-LPCNO and CNRS-LPCNO, 135 Avenue de Rangueil, F-31077 Toulouse, France S Supporting Information *

ABSTRACT: A new type of synthesis, referred to as oxidative methylation, is developed for [Cp* 2 Yb] 2 (μ-X) and [Cp*2Yb]2(μ-X)(X), where X = Me, using MeCu or Cp*2VMe as the methyl transfer reagent and Cp*2Yb. The synthetic methodology is extended to other X derivatives such as the halides and BH4. Reaction of [Cp*2Yb]2(μ-Me)(Me) and H2 yields the mixed-valent hydride [Cp*2Yb]2(μ-H), which eliminates H2 on gentle heating, forming Cp*2Yb. When Cp*2VX is replaced by Cp*2TiX, 1:1 adducts based upon Ti(III,d1) are isolated. The X-ray crystal structure of [Cp*2Yb](μ-Me)[TiCp*2] shows that the methyl group bridges the two different decamethylmetallocene fragments in a near-linear fashion, a geometry that is likely to resemble the transition state of the single-electron-transfer precursor complex. A CASSCF computational study on the mixed-valent hydride [Cp*2Yb]2(μ-H) shows that the ground state is a spin doublet in which the hydride forms a symmetric bridge to both Yb atoms. The three spins forming the ground-state doublet are aligned as Yb(f13(α),(dz2)0)···H···Yb(f13(α),(dz2)1(β)), and the unpaired d electron is delocalized between the dz2 orbitals of the two Yb centers via the hydride bridge using the σ* orbital of the Yb(dz2)−H bond. The first excited state lies 0.09 eV (725 cm−1) higher in energy and is a spin quartet in which the three spins are aligned as Yb(f13(α),(dz2)0)···H···Yb(f13(α),(dz2)1(α)), also giving rise to delocalization of the d electron between the dz2 orbitals of the two Yb centers. The second spin doublet resembles the Lewis structure with an asymmetric μ-H bridge in which the Yb(II) metallocene has a closed-shell electronic configuration and is approximately 0.15 eV (1210 cm−1) higher in energy than the ground-state delocalized open-shell doublet. The electronic structure of the mixed-valent methyl is closely related to that of the hydride, but the methyl group is localized on the Cp*2YbIII fragment. Electronic energies (ΔE) computed at the DFT (B3PW91) level of theory provide insights into the thermochemistry of the formation and decomposition of [Cp*2Yb]2(μ-H). The BDE for Yb−H is ca. 15 kcal/mol stronger than that for the corresponding Yb−Me in the monomeric metallocenes. In contrast, formation of [Cp*2Yb]2(μ-CH3) is ca. 60 kcal/mol more exothermic than the formation of [Cp*2Yb]2(μ-H). This difference is ascribed to enhanced intramolecular steric repulsion between the Cp*2Yb moieties in the linear Yb−H−Yb unit.

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(SiMe3)2 are base-free monomers,2 but the synthesis of basefree methyl derivatives remains a challenge. The dimeric compounds [(C5H5)2M(μ-Me)]2 are known for M = Dy, Ho, Er, Tm, Yb, in which the methyl groups symmetrically bridge the Cp2M units.3 The monosubstituted cyclopentadienyl derivatives [(C5H4R)2M(μ-Me)]2 (R = CMe3, M = Ce4 and R = CMe3, SiMe3, M = Tb, Yb, Lu) are also known in which the methyl groups symmetrically bridge the Cp2M units in M = Ce, Tb.5 In the C5Me5 series the only base-free methyl known

INTRODUCTION

Although many metallocene alkyl compounds of the 4f-block metals are known, the base-free alkyls in general and the methyls in particular are rare.1 The high Lewis acidity and the large radii of these metals results in compounds that bind tenaciously to Lewis bases such as ethers and amines in the solid state or solution, preventing coordination of and therefore reaction with weak donor ligands such as saturated or olefinic hydrocarbons. Several strategies have been developed to minimize or prevent coordination of Lewis bases; these include using cyclopentadienyl ligands substituted with alkyl groups with large steric requirements such as C5Me5 (Cp*) and alkyl groups with bulky substituents. For example, (C5Me5)2MCH© XXXX American Chemical Society

Special Issue: Organometallic Actinide and Lanthanide Chemistry Received: May 23, 2017

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DOI: 10.1021/acs.organomet.7b00384 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

the methyl derivatives Me2Zn and MeCu were chosen to test the redox synthesis strategy. Reactions of Me2Zn and MeCu with Cp*2Yb. Addition of Me2Zn to Cp*2Yb in pentane at 20 °C in a molar ratio of 1.75 to 1.00, forms an orange precipitate that dissolves over a few hours to give a purple solution and a metallic mirror on the walls of the Schlenk tube. The mirror is presumably zinc metal. The minimum net reaction is as illustrated in eq 5 in which Cp*2Yb is oxidized and Me2Zn is reduced.

to us is [Cp*2Lu]2(μ-Me)(Me), whose structure in the solid state contains one terminal methyl group and one that bridges the two lutetium atoms in a linear fashion.6−8 In solution the dimeric lutetium methyl exists in a dimer−monomer equilibrium, for which the thermodynamic constants are ΔH = 12.6 kcal mol−1, ΔS = 32.8 cal mol−1 K−1, and ΔG(298 K) = +2.7 kcal mol−1 in cyclohexane.6,9 The lutetium methyl was the first example of a molecular compound shown to exchange the bound methyl group with 13CH4 by the now-common σ-bond metathesis mechanism.10 The lutetium methyl inserts olefins, the mechanism of which was used to develop a model for the chain growth reaction and its termination by a β-methyl elimination process in polymerization of ethylene.11 However, the rather laborious synthesis of the base-free [Cp*2Lu]2(μMe)(Me) has hindered the extension of these spectacular reactivity patterns to the other 4f-block metals and therefore prevented a comprehensive understanding of the mechanism of the reactions in general and specifically for that of the methane exchange reactions. This article outlines the evolution of a new strategy for the synthesis of the decamethylytterbocene methyls, the 4f13 neighbor of the 4f14 lutetium.

Cp*2 Yb + 1.5Me2Zn → Cp*2 Yb(μ‐Me)2 ZnMe + 0.5Zn (5)

Purple prisms of Cp*2Yb(μ-Me)2ZnMe (Figure 1 and Table 1) are isolated from pentane. The compound melts at 216−218

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RESULTS Strategy. The traditional syntheses of d- and f-block metal methyls involve the exchange of a X group in a [M]-X compound by an alkali or alkaline earth methyl compound.12 The traditional synthesis of [Cp*2Lu]2(μ-Me)Me is outlined in eqs 1−4.6 [Cp*2 LuCl 2]− + 2MeLi → [Cp*2 LuMe2]− + 2LiCl

(1) Figure 1. ORTEP of Cp*2Yb(μ-Me)2ZnMe. Non-hydrogen atoms were refined anisotropically, and the thermal ellipsoids are drawn at the 30% probability level. The hydrogen atoms of the bridging Yb(μMe)2ZnMe groups were located in the difference Fourier map and refined isotropically. Selected bond distances (Å): Yb−C1 2.545(6); Yb−C2 2.529(7); Zn−C1 2.064(7); Zn−C2 2.075(7); Zn−C3 1.960(6); Yb ··· H11 2.35(6); Yb ··· H21 2.31(6).

[Cp*2 LuMe2]− + AlMe3 → Cp*2 Lu(μ‐Me)2 AlMe2 + Me− (2) OEt 2

Cp*2 Lu(μ‐Me)2 AlMe2 ⎯⎯⎯⎯→ Cp*2 LuMe(OEt 2) + Me3Al(OEt 2) (3) NEt3

Cp*2 LuMe(OEt 2) ⎯⎯⎯⎯⎯⎯→ Cp*2 LuMe vacuum

°C and gives a [M − ZnMe3]+ molecular ion in the EI mass spectrum and a single resonance in the 1H NMR spectrum at δH 2.9 ppm in toluene-d8 at 30 °C, presumably due to the Cp* resonance. Lowering the temperature to 4 °C results in the appearance of a very broad resonance at δH −15 ppm in addition to a more intense resonance at 3.2 ppm (ν1/2 = 45 Hz). The chemical shifts change on further cooling until at −70 °C two resonances are visible, whose integration is approximate at 30:3, implying that the bridging methyl resonance is not observed. Examination of the 1H NMR spectrum in the presence of a small amount of added Me2Zn results in the appearance of two resonances at δH 2.92 ppm (ν1/2 = 30 Hz) and a more intense resonance at −0.4 ppm (ν1/2 = 140 Hz) at 30 °C. Lowering the temperature to −10 °C results in the appearance of three resonances at δH −17 (ν1/2 = 140 Hz), 3.3 (ν1/2 = 45 Hz), and a sharp singlet at −0.14 ppm. The last resonance is due to Me2Zn. Cooling to −50 °C results in chemical shifts identical with those at −70 °C in the absence of Me2Zn in addition to a resonance attributed to Me2Zn. These results show that intermolecular exchange between free and bound Me2Zn is rapid at 30 °C but slow at −10 °C. Unfortunately, nothing can be said about the intramolecular exchange rate due to the line width of the Me−Zn resonances at all temperatures. The zincate Cp*2Yb(μ-Me)2ZnMe is crystallized unchanged from Et2O, and an alternative synthesis is addition of Me2Zn to Cp*2Yb(OEt2) in a 1.5:1.0 molar ratio

(4)

This methodology could not be extended to ytterbium, since a weak Lewis base such as Et2O does not displace AlMe3 from Cp*2Yb(μ-Me)2AlMe2.13 The reason is that an equilibrium between [Cp*2Yb]2(μ-Me)(Me) and Cp*2YbMe + AlMe3 is either small or nonexistent in solution, since free AlMe3 does not exchange with coordinated AlMe3 on the NMR time scale. An alternative synthetic methodology was devised that does not depend on Lewis acid−base equilibria in solution but relies on the redox property of decamethylytterbocene and maingroup methyl compounds that are soluble in hydrocarbon solvents or can be used in an ether-free environment. The reduction potentials of Cp*2Yb+ are −1.78 and −1.48 V in acetonitrile and tetrahydrofuran, respectively,14 relative to Cp2Fe+/0, which translate to −1.12 and −0.86 V relative to NHE, respectively, using the value of Cp2Fe+/0 in MeCN relative to NHE as +0.665 V.15 This implies that a metal− methyl, whose reduction potential is less negative, is thermodynamically capable of oxidizing Cp*2Yb; using the reduction potential for the Mn+/0 values in water relative to NHE as a guide suggests Zn2+/0 (−0.76 V), Hg2+/0 (+0.85 V), Cu+/0 (+0.52 V), and Au+/0 (+1.69 V)16 as potential candidates. These values, however, are only a qualitative guide, since electrode potentials and the redox potentials in inert solvents are unknown for the metal−methyl derivatives. Accordingly, B

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Organometallics Table 1. Selected Bond Distances (Å) and Angles (deg) Cp*2Yb(μ-Me)2ZnMe

Cp*2Yb(μ-Ph)2ZnPh

[Cp*2Yb]2(μ-Me)

[Cp*2Yb](μ-Br)[TiCp*2]

[Cp*2Yb](μ-Me)[TiCp*2]

Yb−Cp(C) (range)

2.594(5)−2.642(5)

2.620(5)−2.733(5)

2.653(4)−2.715(3)

2.665(5)−2.708(5)

Yb−Cp(C) (av)

2.615 ± 0.016

2.651 ± 0.041

2.679 ± 0.021

2.681 ± 0.014

Yb−Cpcent (av) Cpcent−Yb−Cpcent Ti−Cp(C) (range) Ti−Cp(C) (av) Ti−Cpcent Cpcent−Ti−Cpcent Yb−X M−X Yb−X−M

2.32 139

2.36 136

Yb(III), 2.551(6)−2.592(6); Yb(II), 2.636(6)−2.704(6) Yb(III), 2.578 ± 0.017; Yb(II), 2.673 ± 0.021 Yb(III): 2.28; Yb(II): 2.39 Yb(III): 143; Yb(II): 145

2.39 142 2.363(3)−2.410(4) 2.383 ± 0.019 2.05 143 2.8986(4) 2.5729(7) 147.13(2)

2.40 144 2.361(5)−2.431(5) 2.397 ± 0.021 2.07 143 2.792(7) 2.193(7) 174.2(3)

Yb(II), 2.755(5) Yb(III), 2.345(5) 169.7(3)

ideal organometallic compound that is stable enough to be isolated at low temperatures and able to function as an oxidative methyl transfer reagent. Addition of a pentane solution of Cp*2Yb to a suspension of MeCu in either a 1:1.7 or 1:0.6 molar ratio gives [Cp*2Yb]2(μ-Me)(Me) or [Cp*2Yb]2(μ-Me), respectively. The Yb2(III,III) dimer crystallizes as red blocks from pentane in high yield (88%).20 It melts at 219−222 °C with decomposition and yields a monomeric M+ signal in the EI mass spectrum. The 1H NMR spectrum in toluene-d8 at 22 °C shows a resonance at δH 16.7 ppm (ν1/2 = 475 Hz) assumed to be due to Cp*. Cooling the sample results in a δH vs T−1 plot (available in the Supporting Information) that is nonlinear until the resonance disappears into the baseline at −30 °C and does not reappear at −70 °C. The nonlinearity implies that a dimer−monomer equilibrium exists in toluene solution, as observed for the lutetium analogue; the resonance for Yb−Me is not observed. The Yb2(II,III) mixedvalent monomethyl crystallizes as red crystals from toluene in 55% yield (see Figure 3 and Table 1). The crystals melt at

in aliphatic hydrocarbons to give the base-free zincate in excellent yield on a 2−3 g scale. Crystallization of the zincate from THF gives Cp*2YbMe(thf) as orange crystals from hexane, which melt at 162−163 °C (see the Experimental Section for details). The 1:1 Et2O adduct of Cp*2YbMe is known; a complete elemental analysis has been reported.11c It is noted that “An ORTEP drawing of YbCp*2Me(OEt2) is provided in the abstract” of ref 28,11c but no such figure is available. Diethyl ether is displaced by THF, and an ORTEP of Cp*2YbMe(thf) is available (Figure 1 in ref 11c) showing Yb− C(Cp) 2.639 Å, Yb−C(Me) 2.357(8) Å, and Yb−O 2.342(6) Å.11c The molecular structure of the η2-Me3Zn unit in lithium (pentamethyldiethylenetriamine)trimethylzincate reveals structural features similar to those found in Figure 1.17 The geometry, in a view down the C1···C2 atoms, is illustrated in the Newman projection in Figure 2. The Yb−H11−C1−C2

Figure 2. Newman projection.

and Yb−H21−C2−C1 torsion angles of 1.32 and 0.95°, respectively, show a crow’s foot orientation with C1−H11 and C2−H21 displaying an eclipsed orientation relative to Yb and C1−H12−H13 and C2−H22−H23 showing an eclipsed orientation relative to each other but staggered relative to Zn. The orientation of the proximal C−H groups relative to Yb results in short Yb···H(av) and Yb···C(av) contact distances of 2.33 and 2.54 Å, respectively, which classify them as threecenter−two-electron Yb···H−C bonds that are referred to as agostic interactions.18 The C−H bonds are σ donors to the Cp*2Yb fragment, and they are strong enough to persist in diethyl ether but not tetrahydrofuran; unfortunately, the ΔG⧧(Tc) value for bridge−terminal exchange cannot be determined, since the μ-Me resonances are not observed. The physical properies of methylcopper are very different from those of dimethylzinc. MeCu is insoluble in hydrocarbons or diethyl ether and presumably has a polymeric structure;19 in contrast, dimethylzinc is a volatile monomer that boils at 44 °C. Methylcopper slowly decomposes at 0 °C to give ethane and a copper mirror. These physical properties seem to make it an

Figure 3. ORTEP of [Cp*2Yb]2(μ-Me). Non-hydrogen atoms were refined anisotropically, and the thermal ellipsoids are drawn at the 30% probability level. The hydrogen atoms of the bridging Me group were located in the difference Fourier map but included in idealized positions using a riding model.

223−225 °C with decomposition and give a [M2 − Cp*]+ molecular ion in the EI mass spectrum. The 1H NMR spectrum in toluene-d8 contains a resonance at δH 5.5 ppm (ν1/2 = 150 Hz). Cooling the sample results in a nonlinear δH vs T−1 plot; the δH vs T−1 plot is linear from 370 to 330 K and from 320 to 265 K, each with different slopes (the plot is available in the Supporting Information). The nonlinear behavior implies that an equilibrium exists in solution between Cp*2YbMe and Cp*2Yb or that there is methyl group site exchange; since the C

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Organometallics

observed at 30 °C. The different behaviors of Ph2Hg and (C6F5)2Hg as oxidative phenyl-transfer reagents have been observed in the reaction of Cp2Yb(dme).22 The lack of reaction with Ph2Hg is rationalized by the higher E1/2 value of −2.6 V for Ph2Hg relative to that of (C6F5)2Hg of −1.15 V, vs SCE in polarographic studies in DME solution.23 It is noteworthy that (PhCC)2Hg (E1/2 = −1.75 V) is a phenylalkynyl transfer reagent in the reaction between Cp 2 Yb(dme) 22 and (TpMe2)2Sm.24 In contrast, Tilley and co-workers demonstrated that for the more reducing Cp*2Sm the reaction with (C6F5)2Hg and Ph2Hg forms Cp*2SmC6F5 and Cp*2SmPh, respectively.25 Hence, the reactions with Cp*2Yb and MeCu are excellent synthetic routes to the [Yb]-Me compounds, provided that attention to detail is observed. However, the heterogeneous nature of the reaction makes mechanistic information difficult and another organometallic methylating reagent that is thermally stable and soluble in hydrocarbons is needed. This reagent is developed next. Reactions of Cp*2VX with Cp*2Yb. The decamethylvanadocene methyl Cp*2VMe seems to be an ideal candidate, since it is soluble in hydrocarbons, as is the reduction product, Cp*2V. The redox reactions of Cp*2V are complex in MeCN, but it is rapidly oxidized by [Cp2Fe][PF6] to [Cp*2V(NCMe)][PF6];26 the E1/2 value of Cp2Fe+/0 in MeCN is 0.66 V relative to NHE,15 and Cp* 2 VMe should be thermodynamically competent for oxidative methylation. The pentamethylcyclopentadienyl vanadium compounds are used, rather than the unsubstituted analogues, to eliminate the possibility of Cp* for Cp ring exchange, since ring exchange does indeed occur with kinetically labile metallocenes.27 Ring exchange also occurs when CpZnMe is added to Cp*2Yb;28 Cp*ZnMe29 and an insoluble light green solid, presumably Cp*YbCp,27 are formed rapidly. Mixing Cp*2Yb with Cp*2VMe in a 2:1 molar ratio in hexane rapidly forms a red precipitate that, on crystallization from toluene, is identified as [Cp*2Yb]2(μ-Me) in good yield. Cp*2V is isolated from the mother liquor. When a 1:1 stoichiometry is used, isolation of [Cp*2Yb]2(μ-Me)(Me) is tedious, since the reactants and products have similar solubility, resulting in a low isolated yield of [Yb]-Me. Therefore, the MeCu reaction is the best available synthesis for [Cp*2Yb]2(μ-Me)(Me). The synthetic methodology developed for the ytterbocene methyls can also be extended to the halides. The use of Cp*2VX as an atom-transfer reagent is an excellent synthetic method for synthesis of base-free [Cp*2Yb]2(μ-X)(X) and [Cp*2Yb]2(μ-X), since the byproducts are soluble and the ytterbocene halides are insoluble in aliphatic hydrocarbons. The mixed-valent compounds are sparingly soluble in aromatic hydrocarbons, but the Cp*2YbX compounds are much less

methyl resonances are not observed, this distinction cannot be made. Addition of Cp*2Yb shifts the resonance upfield to δH 3.3 ppm at 22 °C, consistent with the postulated equilibrium. The mixed-valence methyl derivative is also obtained by mixing Cp*2YbMe and Cp*2Yb in a 1:1 molar ratio in pentane. A red precipitate forms immediately, and [Cp*2Yb]2(μ-Me) crystallizes from toluene in quantitative yield. Reactions of Ph2Zn and (C6F5)2Hg with Cp*2Yb. In order to show the generality of the oxidative methyl transfer reaction, diphenylzinc and bis(pentafluorophenyl)mercury compounds were studied briefly. Addition of Cp*2Yb or Cp*2Yb(OEt2) to a suspension of Ph2Zn in toluene in a 1:1.5 molar ratio results in a dark purple solution along with the appearance of a zinc mirror on the walls of the reaction flask. Purple crystals of Cp*2Yb(μ-Ph)2ZnPh are obtained on crystallization from pentane in good yield. The ORTEP shows that the structure is similar to that of the trimethylzincate (Figure 4). Some bond distances and angles are given in Table

Figure 4. ORTEP of Cp*2Yb(μ-Ph)2ZnPh. Non-hydrogen atoms were refined anisotropically, and the thermal ellipsoids are drawn at the 30% level. Selected bond distances (Å): Yb−C21 2.726(4); Yb−C22 3.186(6); Yb−C27 2.555(4); Zn−C21 2.088(5); Zn−C27 2.182(4).

1 and in the figure caption of Figure 4. In solution, the 1H NMR spectrum is averaged, as only the Cp*, m-H, and p-H resonances are clearly observed and the o-H resonance appears as a very broad feature. The crystal structure of Li(O-nBu2)(Ph3Zn) has been reported; the geometry of the zincate units is similar.21 Although Ph2Hg is recovered unchanged on stirring with Cp*2Yb in toluene for 3 days, (C6F5)2Hg, gives a purple solution in pentane along with mercury metal after a few hours, from which Cp*2 YbC 6 F 5 is isolated on cooling. The pentafluoro compound gives a monomeric molecular ion in the EI mass spectrum. The 1H and 19F NMR chemical shifts are Table 2. Physical Properties of [Cp*2Yb]2(μ-X)

μeffd (Θ) b

X

color

mp

Fa Cl Br H BH4 Me

brown brown brown purple green-brown red

289−295 282 (dec) 252 (dec) 196 294 223−225 (dec)

1

H NMR

c

11.0 (153) 13.6 (150) 7.8 (98) 8.97 (73) 5.46 (150)

5−60 K

100−300 K

4.11 (0) 3.83 (−0.9) 3.68 (−1.1) 4.03 (−2.0) 4.41 (−1.14) 4.56 (−0.45)

4.69 (−25) 4.32 (−26) 4.23 (−30) 4.25 (−30) 4.78 (−12) 4.75 (−6)

Reference 32. bIn units of °C. cCp* resonances in either C6D6 or C7D8 at 20−25 °C, in δ units relative to Me4Si with line width at half height given in parentheses (ν1/2 in Hz). dIn units of μB (θ is given in K). a

D

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Organometallics soluble in that solvent. The low solubility hinders obtaining suitable single crystals for X-ray diffraction, but these compounds are likely to have structures similar to those observed for Cp*4Y2Cl230 and Cp*6Sm3Cl3.31 The mixed-valent [Cp*2Yb]2(μ-X) compounds are likely to be isostructural with the fluoride derivative, in which the fluoride ligand forms a linear bridge between the Cp*2YbIII and the Cp*2YbII units, similar to the bridging methyl atom mentioned above,32 and with [(C5Me4CHMe2)2Sm]2(μ-Cl).33 Some physical properties of the mixed-valent compounds are given in Table 2. The effective magnetic moment as a function of temperature is shown in Figure 5. Synthetic details for all compounds are

Figure 7. ORTEP of [Cp*2Yb](μ-Me)[TiCp*2]. Non-hydrogen atoms were refined anisotropically, and the thermal ellipsoids are drawn at the 30% probability level. The H atoms attached to C1 were located in the Fourier difference map and refined isotropically. Selected bond distances (Å): Yb−H1 2.43(6); Yb−H2 2.63(7); Yb− H3 2.53(7); C1−H1 1.06(7); C1−H2 0.70(7); C1−H3 0.62(7).

Table 1.35 The key feature in these structures is the nearly linear Ti−X−Yb angle. Although it is not surprising that a halide atom can form a linear bridge between two dissimilar metallocenes, a linear methyl group is rare. Examples include Cp*Be(μ-Me)YbCp*2,36 [(C5H4Me)6U2(μ-Me)]−,37 and the Lu and Yb compounds mentioned above. The isolation and structure of these adducts are informative, since they provide a link to the possible geometries of the transition states that precede the X group exchange. Reactivity of Cp*2YbMe. The motivation for developing a useful synthesis of the base-free ytterbium methyl Cp*2YbMe is to compare its reactivity patterns with those of Cp*2LuMe. Both methyls have similar geometric structures in the solid state and both exist in equilibria in solution but differ by one electron: 4f13 vs 4f14. The ytterbium methyl reacts with H2 and D2 to form the purple mixed-valent compounds [Cp*2Yb]2(μH) and [Cp*2Yb]2(μ-D), respectively. The hydride is sparingly soluble in aromatic hydrocarbons and melts at 196−197 °C with decomposition. The mixed-valent formulation follows from the variable-temperature magnetic susceptibility, which is similar to that of [Cp*2Yb]2(μ-F) and the other mixed-valent compounds outlined above (see Table 2 and Figure 5). In solution, the hydride [Cp*2Yb]2(μ-H) eliminates dihydrogen; heating a dilute solution in a sealed NMR tube in C6D6 at 70 °C for 14 days results in complete conversion to Cp*2Yb (see the Supporting Information for details). The dihydrogen elimination reaction proceeds rapidly in an open Schlenk tube at this temperature. This experiment points to differences between two isostructural metallocenes that differ only by one electron (Scheme 1). The lutetium methyl Cp*2LuMe reacts with H2, Me4Si, and PhH to give Cp*2LuH, Cp*2LuCH2SiMe3, Cp*2LuPh, and Cp*2Lu(1,4-C6H4)LuCp*2, respectively, and the hydride reacts with OEt2 to give Cp*2LuOEt.38 The ytterbium methyl does react with H2, but a mixed-valent hydride results and the hydride does not metalate C6D6 or cleave the C−O bond in diethyl ether. The different reactivity patterns continue since Cp*2YbMe does not exchange with 13 CH4 or react with CH or CD bonds in Me4Si or C6D6. Both Cp*2YbMe and Cp*2LuMe give isolable adducts with Et2O and THF. Some reactions of Cp*2YbMe and Cp*2LuMe are shown in Scheme 1. The ytterbium amido Cp*2Yb(NH2)(NH3) is formed on addition of NH3 to Cp*2YbMe, but the coordinated ammonia ligand cannot be removed upon addition of Me3Al;

Figure 5. Solid-state μeff vs T plot for [Cp*2Yb]2(μ-X).

included in the Experimental Section, while selected physical properties of [Cp*2Yb]2(μ-X)(X) and Cp*2VX are summarized in the Supporting Information. Reactions of Cp*2TiX with Cp*2Yb. Reduction of Cp*2TiX compounds is more difficult than for their vanadium analogues, as shown by the ease of preparing vanadocenes relative to titanocenes.34 As a consequence, mixing Cp*2Yb with Cp*2TiX results in formation of 1:1 adducts whose magnetic properties are consistent with Ti(III) (d1) and Cp*2Yb (f14); the synthetic details, variable-temperature susceptibility, and EPR data are available in the Supporting Information. The ORTEP of Cp*2Ti(μ-Br)YbCp*2 and Cp*2Ti(μ-Me)YbCp*2 are shown in Figures 6 and 7, respectively, and bond distances and angles are given in

Figure 6. ORTEP of [Cp*2Yb](μ-Br)[TiCp*2]. Non-hydrogen atoms were refined anisotropically, and the thermal ellipsoids are drawn at the 30% probability level. E

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Organometallics Scheme 1. Selected Reactivity Patterns of Cp*2YbMe and Cp*2LuMe

instead, Cp*2Yb(μ-NH2)2AlMe2 is isolated.39 It is noteworthy that heating a toluene solution of Cp*2Yb(μ-Me)2ZnMe to 100 °C and slowly removing the toluene solution afford Cp*2Yb (see Scheme 1 and the Experimental Section for details). Molecular Orbital (MO) Model for [Cp*2Yb]2(μ-X) (X = H, Me). The most interesting molecules from a molecular electronic structure point of view are the mixed-valent adducts [Cp*2Yb]2(μ-X) (X = H, Me). The mixed-valent hydride results on exposure of Cp*2YbMe to H2, and it eliminates dihydrogen on heating in aromatic solvents. This reactivity pattern is not available to Lu and points to the thermodynamic stability and kinetic accessibility of the Yb(II) electronic configuration. Similar reactivity patterns have been mentioned briefly when either (C5H5)2YbMe or (C5H4Me)2YbMe (or their hydrogenolysis products) is heated; subsequent crystallization from thf gives the thf adducts of the corresponding bivalent ytterbocenes. 4 0 Hydrogenolysis of TpYb(CH2SiMe3)x(thf)y gives a Yb(II) dimeric hydride, a hexameric dihydride, or [Tp2Yb] depending on the substituents on the Tp ligand.41 DFT computations at the B3PW91 level of theory were performed to gain some insights into the thermochemistry of the hydrogenation reactions of Cp* 2 YbMe to give [Cp* 2 Yb] 2 (μ-H) and the thermal decomposition of [Cp*2Yb]2(μ-H) to yield Cp*2YbH. The electronic energies were computed in gas phase for which ΔE ≈ ΔH. We first probed the synthesis of [Cp*2Yb]2(μ-H) starting from Cp*2YbMe and H2:

This reaction is slightly endothermic in the gas phase, but the insolubility of [Cp*2Yb]2(μ-H) should drive the reaction to be exothermic. The translational entropy is approximately zero, and therefore ΔG should be exoergic, which agrees with the experimental reaction in hydrocarbon solvents. The decomposition of [Cp*2Yb]2(μ-H) to Cp*2Yb + 1/2 H2 shown in eqs 10 and 11 gives the net reaction shown in eq 12. The decomposition reaction (eq 12) is strongly exothermic. Since the translational entropy increases, the reaction is also expected to be strongly exoergic. Cp*2 Yb−H−YbCp*2 → Cp*2 Yb + Cp*2 YbH ΔE = −41 kcal/mol

Cp*2 YbH → Cp*2 Yb + 1/2H 2 ΔE = −23 kcal/mol

ΔE = −64 kcal/mol

Cp*2 YbH → Cp*2 Yb + 1/2H 2 (7)

Cp*2 YbH + Cp*2 Yb → [Cp*2 Yb]2 (μ‐H) ΔE = +41 kcal/mol

Cp*2 YbMe + Cp*2 Yb → [Cp*2 Yb]2 (μ‐Me) ΔE = −19 kcal/mol

(8)

Cp*2 YbMe + Cp*2 YbH + H 2 → [Cp*2 Yb]2 (μ‐H) ΔE = +4 kcal/mol

(13)

On first sight, the thermochemistry for the formation [Cp*2Yb]2(μ-H) and [Cp*2Yb]2(μ-Me) appears counterintuitive, since the Yb−H bond is 15 kcal/mol stronger than that of Yb-Me. However, a closer inspection of the computed molecular structures for both mixed-valent dimers reveal

eqs 6-8 can be combined to give the net reaction shown in eq 9.

+ MeH + 1/2H 2

(12)

In addition, since the bond dissociation energies (BDEs) of MeH and H2 are roughly the same, the BDE of Cp*2Yb−H is greater than that of Cp*2Yb-Me by 15 kcal/mol (eq 6). This qualitatively agrees with our experiments and the very few thermochemical values reported in the literature. For example, the BDEs for Cp*2Sm−H and Cp*2Sm−CH(SiMe3)2 are 52 and 46 kcal/mol, respectively,42 whereas the analogous values for Cp*2Lu−H and Cp*2Lu−CH(SiMe3)2 are identical at 67 kcal/mol.43 Furthermore, addition of Cp*2YbMe to Cp*2Yb in hydrocarbons rapidly yields [Cp*2Yb]2(μ-Me). In agreement with the experiment our computations predict that the reaction shown in eq 13 is strongly exothermic (ΔE = −19 kcal/mol) in contrast to the analogous reaction to form [Cp*2Yb]2(μ-H), which is uphill by ΔE = +41 kcal/mol (eq 8).

(6)

ΔE = −22 kcal/mol

(11)

[Cp*2 Yb]2 (μ‐H) → 2Cp*2 Yb + 1/2H 2

Cp*2 YbMe + H 2 → Cp*2 YbH + MeH ΔE = −15 kcal/mol

(10)

(9) F

DOI: 10.1021/acs.organomet.7b00384 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Computed Electronic Structure at the CASSCF Level of Theory for [Cp*2Yb]2(μ-H)

this system but lies 0.15 eV (1210 cm−1) above the ground state. Instead, CASSCF calculations predict a symmetric Yb··· H···Yb bridge between the two Yb atoms (see Figure 8), which

significantly larger steric repulsion within [Cp*2Yb]2(μ-H), for which the closest nonbonded C···C contacts between the methyl groups of the two Cp*2Yb fragments range between 3.54 and 4.05 Å. The van der Waals radii for carbon atoms and methyl groups are 1.7 and 2.0 Å, respectively,44 and therefore steric repulsions between the Cp* 2 Yb fragments in [Cp* 2 Yb] 2 (μ-H) are destabilizing. In contrast, for [Cp*2Yb]2(μ-Me) the shortest nonbonding C···C contacts are significantly longer (4.01−4.10 Å) and therefore are above the van der Waals radii cutoff. The electronic structures of the mixed-valent compounds [Cp*2Yb]2(μ-X) (X = H, Me) have been subsequently investigated by computational studies. Starting from the DFT optimized molecular structures, three different electronic structures have been probed by CASSCF computations. For this purpose, the number of active SCF orbitals and active electrons is varied. However, the size of the molecule required that we have to limit the CASSCF computations to 11 active orbitals and 21 electrons instead of 15 active orbitals (4f orbitals on each Yb and the σ* Yb-R orbital with R = H, CH3) and 27 electrons. Gratifyingly, reduction of the active space confirms the stability of this solution. Furthermore, we demonstrate that only the f±3 orbitals are active, so that the same results are obtained when only 5 active orbitals and 9 electrons are considered: i.e., the electrons were distributed over four 4f orbitals and the σ* Yb-R orbital with R = H, CH3. These computations then give rise to two doublet states and one quartet state (Scheme 2). It also noteworthy that an MP2 correction to the CASSCF results has no pronounced influence on the relative energies of the individual states, since they are of similar magnitude for the doublet (0.0576 au) and quartet states (0.0573 au). Spin−orbit coupling is not explicitly considered, since this would be rather difficult for these large bimetallic species. However, the open-shell doublet (2H) and quartet (4H) states are based on two f13 configurations, for which the atomic spin−orbit operator dominates and therefore similar stabilization is expected, which will be different from that of the mixed-valent f13−f14 configuration (2F doublet). The open-shell doublet 2H will experience some stabilization because its J = 13/2 and J = 11/2 components can interact with the J = 13/2 and J = 11/2 components of the associated quartet (4H), which will slightly stabilize the open-shell doublet 2 H state while the 4H state is slightly destabilized. The mixedvalent doublet state (2F) has J = 7/2 and J = 5/2 components that will not interact with the other two states (2H and 4H). Since the open-shell doublet (2H) is already the ground state at the CASSCF level, the overall energetic order of the different states will remain unchanged. Perhaps surprisingly, the traditional Lewis structure Cp*2Yb−H···YbCp*2, in which the Yb−H interaction is strongly localized on the Cp*2YbIII fragment and the Cp*2YbII is a closed-shell diluent, is not the electronic ground state in

Figure 8. Highest Hartree−Fock SOMO for the ground-state doublet of [Cp*2Yb]2(μ-H).

originates from the promotion of one 4f electron into the 5dz2 orbital, generating a three-center−three-electron MO (see Scheme 2). The unpaired d electron is delocalized between the 5dz2 orbital of the two Yb atoms via the σ* orbital of the Yb−H bond. The three spins can then be aligned either antiferromagnetically or ferromagnetically, which form the ground-state doublet and the quartet excited state, respectively, which is 0.09 eV (725 cm−1) higher in energy (Scheme 2). The quartet state may be thermally populated (at 300 K, kT is 205 cm−1) and contribute to the effective magnetic moment of 4.25 μB at high temperature. The calculated mixed-valent methyl-bridged species [Cp*2Yb]2(μ-Me) features an electronic structure similar to that of the corresponding hydride-bridged complex (Scheme 3). However, the Yb−C bond distances are now significantly asymmetric with one short and one long distance of 2.46 Å (experimental 2.35 Å) and 2.72 Å (experimental 2.76 Å), respectively (Figure 9). The computed Yb−C−Yb angle of 172° agrees with the experimental angle of 170°. The energy separation between the ground-state doublet and the excited state is also significantly reduced, with the quartet state being nearly degenerate with the doublet ground state (161 cm−1) (Scheme 3) and may therefore contribute to the higher effective magnetic moment relative to that of [Cp*2Yb]2(μ-H). The μ-CH3 bridge is asymmetric, in contrast to the μ-H bridge, which is a result of a less efficient delocalization of the unpaired d electron between the two Yb atoms via the σ* orbital of the Yb−H bond.

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DISCUSSION Although the use of MeCu and Cp*2VMe as oxidative methyl transfer reagents are good synthetic methodologies for the G

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Organometallics Scheme 3. Computed Electronic Structure at the CASSCF Level of Theory for [Cp*2Yb]2(μ-Me)

parentage orbital is used to build the 2c-2e Yb−H σ-bonding MO. The empty σ* orbital accepts a dz2-parentage electron from the Cp*2Yb fragment, creating the spin doublet and quartet states that differ in energy by 725 cm−1, the doublet being lower in energy. In the hydride the spins are delocalized over the dz2 orbitals and the 1s1 orbital, but in the methyl the spins are localized and the doublet and quartet states are nearly degenerate, with the doublet being 161 cm−1 lower in energy. Although no thermochemical quantities have been determined experimentally, the calculated values of ΔE, which are related to ΔH, are informative. The computed ΔE values for the formation and decomposition of [Cp*2Yb]2(μ-H) are consistent with the experimental results. Furthermore, although the calculated values of ΔE indicate that Yb−H is ca. 15 kcal/ mol greater than Yb−Me in the monomeric metallocenes (eq 6), the ΔE value for the formation reactions of [Cp*2Yb]2(μH) and [Cp*2Yb]2(μ-CH3) are +41 and −19 kcal/mol, respectively, which imply that the Yb−C bonds in [Cp*2Yb]2(μ-CH3) are ca. 60 kcal/mol stronger than the Yb−H bonds in [Cp*2Yb]2(μ-H). These relative ΔE values also illustrate that the formation of the bridging methyl is more exothermic than the bridging hydride by ca. 60 kcal/mol, a result that is in contrast with the relative values of Yb−H and Yb−Me in the monomers. This reversal is ascribed to greater intramolecular steric repulsion between the Cp*2Yb groups in the linear Yb−H−Yb unit. The original reason for preparing Cp*2YbMe was to extend the pioneering studies of Watson on Cp*2LuMe to its nearest neighbor to the left in the periodic table, Yb. The different outcome of the hydrogenolysis reaction, which results in the formation of the mixed-valent hydride and the lack of C−H bond activation of Cp*2YbMe, demonstrates the different reactivity patterns between the open-shell (Yb(III), f13) and the closed-shell (Lu(III), f14) centers, but the origins of the difference remain a mystery. The computed potential energy surface for methane for the methyl exchange in Cp*2LuMe indicates that quantum mechanical tunneling makes a significant contribution to the transition-state energy in the metathesis transition state.46 Furthermore, Watson has shown that the relative rates of Cp*2LuMe + H−R → Cp*2LuR + MeH roughly parallel the relative acidities of H−R and, more importantly, kH/kD for the reaction of Cp*2LuMe + C6H6/ C6D6 is 6.0 at 70 °C; the large value is consistent with quantum mechanical tunneling in the transition state.47 However, how tunneling may differentiate the reactivity patterns awaits further study.

Figure 9. Highest Hartree−Fock SOMO for the ground-state doublet of [Cp*2Yb]2(μ-CH3).

ytterbocene methyls [Cp*2Yb]2(μ-Me)(Me) and [Cp*2Yb]2(μMe), the reaction mechanism is not obvious. Two possible general mechanistic pathways may be proposed: (i) a free or trapped radical formed by homolytic cleavage of the Cu− or V−Me bond as Cp*2Yb gives up an electron to Cu or V and (ii) a single-electron transfer (SET) from Cp*2Yb to the LUMO of MeCu or Cp*2VMe triggering the methyl anion to switch partners. A somewhat related two-electron switch of partners between (C6F5)2Hg and Yb(metal) has been suggested from a recent DFT computational study.45 Qualitatively the second alternative for the productive exchange of a methyl group from Cp*2VMe to Cp*2Yb can be rationalized by molecular orbital (MO) consideration. The initial event, after adduct formation, is a single-electron transfer of an f- or dparentage electron to the V−Me σ* orbital of Cp*2VMe, a process that is thermodynamically favorable for Cp*2VMe but not for Cp*2TiMe. The population of the V−Me σ* orbital weakens the V−Me σ-bond, lowering the activation energy for the methyl anion to exchange metallocene partners, forming a Yb−Me σ bond, and leaving behind Cp*2V. This qualitative picture is helpful, since it can be used to classify the observed reactivity patterns in the following manner: (i) adduct formation, Cp*2Yb···X−TiCp*2, in which the SET pathway is a high-energy process and no exchange occurs, (ii) productive exchange in the adduct Cp*2Yb···X−VCp*2 or Cp*2Yb···X−M (M = Cu, ZnX), in which the SET pathway is thermodynamically possible, and (iii) degenerate exchange in the adduct Cp*2Yb···X−YbCp*2, a self-exchange that may account for the nonlinear δ vs T−1 plot of the Cp* chemical shifts (see the Supporting Information). Three “active” electrons are involved in this postulated mechanism, in which the energy of the SET step controls the net reaction. This mechanistic proposal is strengthened by the computational CASSCF results outlined above. The CASSCF calculation shows that the ground-state spin doublet and first-excited-state spin quartet of the mixedvalent hydride is developed by promotion of a f-parentage electron into an empty dz2 orbital on Cp*2Yb and the dz2-

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CONCLUSIONS

Although new and potentially useful synthetic routes to the ytterbocene methyls, which may be extended to related metallocene methyl derivatives, have been developed and different reactivity patterns for the open-shell Yb(III,f13) and H

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Organometallics closed-shell Lu(III,f14) have been documented, the origins of these differences remain obscure. More importantly, a CASSCF calculation of the electronic structure of the mixed-valent hydride gives rise to a MO model that is different from the traditional Lewis structural model for the Yb−H(Me) bond. The Lewis structure for Cp*YbH is that the two-center−twoelectron Yb−H bond is formed from an electron in an f orbital and a hydrogen 1s1 electron. The new model indicates that the Yb−H bond results from a dz2 parentage orbital and the hydrogen 1s1 atomic orbital; the dz2 orbital results from promotion of an f electron into the empty bent sandwich MO of a1 symmetry (C2v point group). The f → d promotion48 results in greater orbital overlap that counteracts the enthalpic penalty for hybridization. In the mixed-valent [Cp*2Yb]2(μ-H), the ground state is a doublet with the electronic structure Yb(f13(α))···H···Yb(f13(α),d1(β)) that is lower in energy than the Yb(f13(α))-H···Yb(f14) traditional doublet Lewis structure by approximately 0.15 eV. This f → d hybridization lowers the energy of the quartet state and Yb(f 13 (α))···H···Yb(f13(α),d1(α)) is only slightly higher in energy than the corresponding quartet state in [Cp*2Yb]2(μ-Me). Thus, the bond models for these f-element organometallic compounds continue to evolve as new experimental and computational studies become available.

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remaining hexane was removed under reduced pressure. The product was vacuum-distilled (10−2 mm Hg) with a bath temperature of 50 °C. Approximately 25 mL of a colorless, viscous liquid was obtained. The 1 H NMR spectrum was consistent with that for the previously reported compound.56 1H NMR (C6D6, 24 °C): δ −0.55 (s, 6H), 0.94 (t, 3H, JHH = 7 Hz), 3.40 (q, 2H, JHH = 7 Hz). Synthesis of Methylcopper. This complex was synthesized according to a previously reported procedure.57 The reaction must be performed in an elongated Schlenk tube to avoid splashing the unstable methylcopper on the walls of the reaction vessel. A solution of Me2AlOEt (2.5 mL, 15.3 mmol) in diethyl ether (15 mL) was added dropwise from an addition funnel to a suspension of bis(2,4pentanedionato)copper(II) (1.00 g, 3.82 mmol) in diethyl ether (50 mL) at −40 °C. The stirred mixture was warmed to −10 °C, whereupon the color became lime green. The reaction vessel must be periodically vented to the nitrogen manifold to relieve the buildup of pressure. The mixture was maintained between −10 and −5 °C for an additional 4 h. During this time, the suspension gradually became bright yellow. The yellow solid was allowed to settle, and the subsequent purification was determined by one of two conditions. Need for Elimination of Diethyl Ether from the Subsequent Reaction. The mixture was cooled to −40 °C, and the mother liquor was removed by filtration using a canula with a paper filter attached. Any product trapped on the paper filter was carefully scraped back into the reaction vessel under a flow of nitrogen without decomposition. The residue was washed with diethyl ether, cooled to −40 °C, washed three times with 40 mL portions of hexane, and cooled to −40 °C. The residue was exposed to vacuum for 0.5 h at −30 °C. The product became dark yellow-brown, but it was still reactive. The product was used as a suspension in the appropriate solvent at −40 °C. Stirring the suspension returned the product to its original fluffy, yellow appearance. Subsequent Reaction Unaffected by Coordinating Solvents. The mixture was cooled to −40 °C, and the mother liquor was removed by filtration using a cannula with a paper filter attached. Any product trapped on the paper filter was carefully scraped back into the reaction vessel under a flow of nitrogen without decomposition. The residue was washed twice with 40 mL portions of diethyl ether at −40 °C. The product was used as a suspension in the appropriate solvent at −40 °C. In the reactions with methylcopper, the suspension was maintained below −20 °C as the reacting species was added, since the methylcopper will spontaneously decompose above 0 °C to form metallic copper. Synthesis of (Me5C5)2Yb(μ-Me)2ZnMe. Method 1. From (Me5C5)2Yb. (C5Me5)2Yb (0.25 g, 0.56 mmol) was dissolved in pentane (25 mL) and added to a solution of Me2Zn (0.060 mL, 0.094 g, 0.98 mmol) in pentane (5 mL) with stirring. An orange flocculent precipitate formed rapidly. The solution was stirred for 14 h. After several hours, the orange precipitate had redissolved and the solution had turned purple; the color change was accompanied by the formation of a zinc mirror on the walls of the flask. The solution was filtered, concentrated to 3 mL in volume, and cooled to −78 °C. Dark purple prisms were isolated and dried under reduced pressure. The yield was 0.25 g (81%). This was the only product isolated, even when the reagents were combined in different stoichiometry. Method 2. From (Me5C5)2Yb(OEt2). (C5Me5)2Yb(OEt2) (2.82 g, 5.45 mmol) was suspended in hexane (45 mL), and a solution of Me2Zn (0.49 mL, 0.78 g, 8.16 mmol) in pentane (5 mL) was added to this with stirring. The ytterbium etherate dissolved over a period of hours, giving a small amount of orange precipitate which rapidly redissolved to give a purple solution and zinc metal. The solution was stirred for 16 h and then the metal was allowed to settle and the solution was filtered. The filtrate was concentrated to 30 mL and cooled to −78 °C, producing large purple blocks. A second crop was isolated from the mother liquors by concentrating and cooling. The combined yield was 2.75 g (91%). The compound melted at 216−218 °C. IR (Nujol): ν̅ 2803 m, 2771 m, 2728 m, 2678 sh, 2041 vw br, 1654 vw br, 1490 m, 1440 s, 1312 vw, 1220 w, 1163 vw, 1152 w, 1060 w, 1024 m, 739 sh, 725 w, 689 w, 643 vs, 618 sh, 590 m, 529 s, 403 m, 387 w, 312 vs, 284 sh cm−1. Anal. Calcd for C23H39YbZn: C, 49.84; H,

EXPERIMENTAL SECTION

General Comments. All reactions and product manipulations were carried out under dry nitrogen using standard Schlenk and drybox techniques. Dry, oxygen-free solvents were employed throughout. The elemental analyses were performed by the analytical facility at the University of California at Berkeley. The 1H NMR chemical shifts were given in ppm relative to Me4Si, the 19F NMR chemical shifts referenced relative to C6F6 at δF 0 ppm (δF(CFCl3) = δF(C6F6) − 162.9 ppm). Magnetic measurements were conducted in a 7 T Quantum Design MPMS magnetometer utilizing a superconducting quantum interference device (SQUID). Measurements were performed in KEL-F buckets, and the raw magnetization data were also corrected for the overall diamagnetism of the molecule using Pascal constants.49 Cp*2Yb,50 Me2Zn,51 (C6F5)2Hg,52 and Ph2Hg53 were prepared as previously described. Synthetic details for Cp*2VX and [Cp*2Yb](μ-X)[TiCp*2] are provided in the Supporting Information. Synthesis of Bis(2,4-pentanedionato)copper(II). This complex was synthesized according to a published procedure.54 To a 100 mL solution of aqueous copper(II) nitrate trihydrate (10.0 g, 41.4 mmol) was added 15 mL of concentrated aqueous ammonia. After the mixture was stirred for several minutes, a dark, blue-purple solution was obtained. 2,4-Pentanedione (11.0 mL, 107 mmol) was added dropwise to this solution. A light blue precipitate immediately appeared, and the mixture was stirred at room temperature for an additional 10 min. The solid was collected in a Buchner funnel and washed three times with 50 mL portions of distilled water. The residue was washed with chloroform (20 mL) and then dissolved in chloroform (150 mL), forming a deep blue solution. The solution was filtered, and the filtrate was cooled to −30 °C, affording light blue needles. Additional crops of crystals were obtained by concentrating the mother liquor. The yield was quantitative except for manipulative losses. The IR spectrum was identical with that of the previously reported complex.55 The product was dried under vacuum at 100 °C prior to use. Synthesis of Dimethylaluminum Ethoxide. A solution of trimethylaluminum (40 mL, 0.42 mol) in hexane (200 mL) was cooled to −40 °C. To this was added a solution of ethanol (24 mL, 0.41 mol) in hexane (40 mL) dropwise from an addition funnel over 1.5 h. The ethanol was distilled from magnesium ethoxide immediately prior to use. After complete addition, the solution was concentrated to approximately 80 mL at 0 °C under reduced pressure. The remaining solution was transferred to a microdistillation apparatus, and the I

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Organometallics 7.10. Found: C, 49.91; H, 7.11. 1H NMR (C6D6, 20 °C): δ 2.90 (ν1/2 = 27 Hz); the zinc methyl groups were not visible at this temperature. The EI mass spectrum did not show a molecular ion; the highest observable m/z fragments were due to [(Me5C5)2Yb]+ (444), [(Me5C5)Yb]+ (309), [Yb]+ (174), and ions due to ring fragmentation. Attempts To Remove ZnMe2 from (Me5C5)2Yb(μ-Me)2ZnMe. (Me5C5)2Yb(μ-Me)2ZnMe (2.52 g, 4.55 mmol) was dissolved in toluene (50 mL) to give a dark purple solution. This solution was heated to 100 °C, and the solvent was removed very slowly (over a period of 2−3 h) under reduced pressure, leaving a dark red-brown residue. This process was then repeated with a smaller volume of toluene (50 mL). The residue was extracted with pentane (100 mL), leaving behind a small amount of red-brown powder. The extract was concentrated to 30 mL in volume and cooled to −25 °C. Green crystals were isolated from this solution, which lost solvent and turned brown under vacuum. Examination of the IR spectrum and melting point confirmed that this material was (Me5C5)2Yb. An additional crop of crystals was isolated from the mother liquors by concentrating and cooling. The total amount of the base-free ytterbium complex isolated from solution was 0.52 g (26%). Synthesis of (Me 5C5) 2Yb(μ-Ph)2ZnPh. Method 1. From (Me5C5)2Yb. (C5Me5)2Yb (0.28 g, 0.63 mmol) was dissolved in toluene (20 mL) and added with stirring to a slurry of Ph2Zn (0.34 g, 0.96 mmol) in toluene (15 mL). The solution became dark, but no precipitate formed immediately. The solution was stirred for 24 h, at which time the color had changed to purple, accompanied by the formation of a zinc mirror on the walls of the flask. The solution was filtered, and the toluene was removed under reduced pressure. The purple residue was extracted with pentane (35 mL), filtered, concentrated to a volume of 5 mL, and cooled to −78 °C, producing purple blocks. The isolated yield was 0.32 g (68%). Method 2. From (Me5C5)2Yb(OEt2). (C5Me5)2Yb(OEt2) (0.73 g, 1.4 mmol) and Ph2Zn (0.46 g, 2.1 mmol) were dissolved in toluene (20 mL), and the solution was stirred for 14 h, producing a dark purple solution and a zinc mirror on the walls of the flask. The solution was filtered, and the toluene was removed under reduced pressure. The purple residue was extracted with pentane (40 mL); the extract was filtered, concentrated to 10 mL, and cooled to −78 °C, producing purple crystals. A second crop of crystals was isolated from the mother liquors by concentrating and cooling. The combined yield of the two batches was 0.63 g (60%). The compound melted with decomposition at 111−114 °C. IR (Nujol): ν̅ 3059 m, 3037 m, 2729 w, 2668 vw, 1952 vw, 1882 vw, 1868 vw, 1819w, 1772 vw, 1757 vw, 1671 vw, 1628 vw, 1593 m, 1573 vw, 1558 vw, 1490 s, 1420 s, 1297 m, 1240 m, 1191 w, 1171 sh, 1153 w, 1100 m, 1074 m, 1048 s, 1024 m, 1011 sh, 993 m, 963 vw, 938 sh, 925 vw, 87 5w, 855 sh, 844w, 827 vw, 759 m, 722 vs, 709 vs, 701 vs, 674 w, 654 w, 632 w, 624 w, 619 w, 606 sh, 599 w, 573 vw, 496 wbr, 442 m, 438 m, 435 m, 383 m br, 339 sh, 300 s br cm−1. Anal. Calcd for C38H45YbZn: C, 61.64; H, 6.13. Found: C, 61.58; H, 6.24. 1H NMR (C6D6, 20 °C): δ 12.98 (ν1/2 = 193 Hz, 6H), 7.37 (ν1/2 = 380 Hz, 30H), 5.78 (ν1/2 = 205 Hz, 3H). The ortho protons were visible only as a very broad resonance (ν1/2 ≈ 1500 Hz) at 63 ppm. The El mass spectrum did not show a molecular ion (740); the highest weight m/z fragment was [(Me5C5)2YbPh]+ (521), followed by [(Me5C5)2Yb]+ (444), [(Me5C5)YbPh]+ (385), [(Me5C5)Yb]+ (309), [Yb]+ (174), and ions due to ring fragmentation. Reaction with (Me5C5)2Yb with HgPh2. (C5Me5)2Yb (0.38 g, 0.86 mmol) and Ph2Hg (0.15 g, 0.42 mmol) were dissolved together in toluene (35 mL) and stirred for 3 days. There was no precipitation of mercury or color change. The solution was then refluxed for 6 h. The solution remained red-orange, and no precipitate formed. After it was cooled, the solution was reduced in volume to 10 mL and cooled to −25 °C, producing white crystals. Examination of the IR of the crystals revealed that they were HgPh2. When the reaction was carried out in hexane, the results were the same. Synthesis of (Me5C5)2Yb(C6F5). (C5Me5)2Yb (0.43 g, 0.97 mmol) was dissolved in pentane (20 mL) and was added to a solution of (C6F5)2Hg (0.26 g, 0.49 mmol) dissolved in pentane (10 mL). The solution was stirred for 14 h, which resulted in the precipitation of mercury metal and a color change to purple. The solution was filtered,

concentrated to 10 mL in volume, and cooled to −78 °C and the purple crystalline solid was isolated by filtration. A second crop of crystals was isolated from the mother liquors by concentrating the solution further and cooling to −78 °C. The combined yield was 0.37 g (63%). Mp: 129−132 °C. The material was recrystallized from pentane to remove impurities of (C6F5)2Hg. IR (Nujol): ν̅ 2730 w, 1632 m, 1603 m, 1531 m, 1510 sh, 1495 vs, 1435 vs, 1360 m, 1309 m, 1291 w, 1262 m, 1247 s, 1208 vw, 1164 m, 1151 w, 1099 sh, 1084 s, 1066 s, 1027 vs, 964 m, 914 vs br, 894 sh, 881 sh, 865 vw, 834 sh, 820 sh, 803 s, 746 vw, 725 w, 696 vw, 672 w, 635 vw, 621 vw, 586 w, 573 vw, 551 vw, 475 w, 444 vw, 392 m br, 345 m, 327 vs, 308 sh cm−1. Anal. Calcd for C26H30F5Yb: C, 51.13; H, 4.95. Found: C, 50.89; H, 5.21. 1H NMR (C6D6, 30 °C): δ 12.0 (ν1/2 = 335 Hz). 19F NMR (C6D6, 30 °C): δ 47.0 (ν1/2 = 1400 Hz, 2F), 1.79 (ν1/2 = 29 Hz, 1F), −7.30 (ν1/2 = 73 Hz, 2F). To verify the presence of the C6F5 group, a sample of the compound was hydrolyzed in C6D6. While there was no evidence of C6F5H in the 1H NMR spectrum of the hydrolysate, it was visible in the 19F NMR spectrum: δ 24.86 (m, 2F), 10.01 (t, 1F), 1.76 ( m, 2F). The El mass spectrum showed a molecular ion (611), as well as m/z fragments corresponding to [(Me5C5)Yb(C6F5)]+ (475), [(Me5C5)YbF]+ (463), [(Me5C5)2Yb]+ (444), [(Me5C5)Yb]+ (309), [Yb]+ (174), and ions corresponding to ligand fragmentation. Synthesis of (Me5C5)2YbMe. Methylcopper (0.30 g, 3.8 mmol) was synthesized as described above. The reaction must be performed in an elongated Schlenk tube to avoid splashing the unstable methylcopper on the walls. The mother liquor was removed from the bright yellow product by cannula filtration, and the residue was filtered and washed once with Et2O (60 mL) and three times with 60 mL portions of pentane at −40 °C. The solid was exposed to vacuum for 0.5 h at −30 °C. Pentane (40 mL), cooled to −30 °C, was added to the MeCu to form a suspension. To the suspension was added a solution of (Me5C5)2Yb (1.00 g, 2.25 mmol) in pentane (40 mL). The mixture was then warmed slowly to room temperature over several hours. During this time, the mixture became deep red and a copper mirror formed on the walls of the reaction vessel. The mixture was stirred for an additional 8 h at room temperature. In order to obtain the product cleanly, the finely divided Cu metal formed in the reaction must be allowed to settle thoroughly. The solution was filtered through a glass fiber filter (Whatman GFID), and the filtrate was cooled to −40 °C, affording red blocks. Concentration of the mother liquor provided additional crops of crystals for a total yield of 0.91 g (2.0 mmol, 88%). Mp: 219−221 °C dec. IR (Nujol): ν̅ 2725 m, 2048 vw, 1489w, 1260 m, 1166 w, 1093 s, 1061 m, 1023 s, 949 vw, 885 s, 824 w, 803 m, 721 w, 674 m, 625 w, 613 w, 592 m, 539 w, 516 w, 422 w, 393 s, 323 s, 297 s cm−1. 1H NMR (C7D8, 22 °C): δ 16.7 (ν1/2 = 473 Hz). The protons on the ytterbium-bound methyl group were not observed. The spectrum of a hydrolyzed sample (with D2O) in C6D6 showed resonances for Me5C5D only. EI-MS: m/z 455 (23, 9); 456 (63, 42); 457 (86, 70); 458 (94, 60); 459 (100, 100); 460 (40, 22); 461 (38, 38); 462 (11, 8). There was no evidence for the existence of the dimeric form in the gas phase. Anal. Calcd for C21H33Yb: C, 54.99; H, 7.26. Found: C, 55.03; H, 7.28. Synthesis of (Me5C5)2YbMe(OEt2). A solution of (Me5C5)2Yb(OEt2) (0.50 g, 0.97 mmol) in diethyl ether (15 mL) was added to a suspension of freshly prepared methylcopper (0.14 g, 1.8 mmol) in diethyl ether (50 mL) at −40 °C. Methylcopper was prepared as described above. The reaction must be performed in an elongated Schlenk tube to avoid splashing the unstable methylcopper on the walls. The stirred mixture was warmed to room temperature over several hours and then stirred for an additional 12 h. The red solution was filtered from the metallic Cu as in the base-free preparation, and the filtrate was concentrated to approximately 20 mL and cooled to −20 °C to yield red blocky crystals. Several crops were obtained from the mother liquor for a combined yield of 0.47 g (0.88 mmol, 91%). Mp: 196−200 °C. IR (Nujol): ν̅ 2725 m, 1647 w, 1489 w, 1316 w, 1288 w, 1256 vw, 1187 m, 1146 m, 1108 m, 1093 s, 1045 s, 1023 m, 1001 s, 950 w, 894 s, 828 w, 799 m, 774 s, 724 w, 673 m, 626 w, 617 w, 595 m, 519 m, 478 w, 393 s, 324 m, 295 s cm−1. 1H NMR (C6D6, 23 °C): δ 5.95 (30H, ν1/2 = 387 Hz), 38.90 (6H, ν1/2 = 308 Hz). Neither the methylene protons of diethyl ether nor those of the ytterbium J

DOI: 10.1021/acs.organomet.7b00384 Organometallics XXXX, XXX, XXX−XXX

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Organometallics methyl were observed. The 1H NMR spectrum of a hydrolyzed sample verified the 2:1 stoichiometry of Me5C5H to Et2O in the product. Anal. Calcd for C25H43OYb: C, 56.35; H, 8.14. Found: C, 56.12; H, 8.02. Synthesis of (Me5C5)2YbMe(thf). From Cp*2YbMe. A sample of (Me5C5)2YbMe (0.15 g, 0.33 mmol) was dissolved in tetrahydrofuran (10 mL), forming a red-orange solution. The tetrahydrofuran was then removed under reduced pressure, and the resulting residue was extracted with hexane (15 mL). The solution was filtered, concentrated to approximately 10 mL, and cooled to −80 °C. A mass of red-orange needles was isolated (0.10 g, 0.19 mmol, 58%), whose physical properties were identical with those in the subsection below. From Cp*2Yb(μ-Me)2ZnMe. (Me5C5)2Yb(μ-Me)2ZnMe (0.23 g, 0.42 mmol) was dissolved in THF (20 mL), yielding an orange solution. The solvent was removed under reduced pressure, and the residue was recrystallized from warm hexane to give 0.19 g of orange crystals (86%). The compound melted and bubbled at 162−163 °C. IR (Nujol): ν̅ 2806 w, 2779 w, 2721 w, 2041 vw br, 1607 vw br, 1494 sh, 1440 vs, 1407 sh, 1344 w, 1314 vw, 1296 vw, 1257 w, 1177 sh, 1153 m, 1102 s, 1063 m, 1039 sh, 1021 vs, 959 w, 923 w, 869 s, 842 sh, 737 sh, 725 w, 695 w, 675 m, 624 w, 594 w, 567 vw, 531 w, 489 vw, 429 w br, 397 s, 301 vs br cm−1. Anal. Calcd for C25H41OYb: C, 56.57; H, 7.79. Found: C, 55.23; H, 7.68. 1H NMR (C6D6, 20 °C): δ 1.16 (ν1/2 = 185 Hz). To confirm the presence of THF, a sample of the compound was hydrolyzed in C6D6, and the hydrolysate was examined by 1H NMR spectroscopy. Both THF and C5Me5H were observed in the spectrum in a 1:2 ratio. The El mass spectrum did not show a molecular ion at m/z 531; the highest weight m/z fragments were [(Me5C5)2YbMe]+ (459), [(Me5C5)2Yb]+ (444), [(MeC5)Yb]+ (309), [Yb]+ (174), and ions due to ring fragmentation. Synthesis of (Me5C5)2Yb(μ-Me)AlMe3. Trimethylaluminum (0.19 mL of a 1.15 M hexane solution, 0.22 mmol) was added by syringe to a solution of (Me5C5)2YbMe (0.10 g, 0.22 mmol) in hexane (10 mL). Upon mixing, the solution immediately changed color from red to purple. After the mixture was stirred at room temperature for 0.5 h, a purple precipitate was observed. The volatile materials were completely removed under reduced pressure, and the residue was extracted with toluene (10 mL). The purple solution was filtered, and the filtrate was concentrated to approximately 3 mL. The solution was warmed to redissolve the precipitates. The solution was cooled to −40 °C, affording purple crystals that lost solvent upon exposure to vacuum (0.10 g, 0.19 mmol, 84%). The IR spectrum was identical with that of the previously reported compound.13 Mp: 214−218 °C (lit.13 220− 222 °C). Polymerization of Ethylene by (C5Me5)2YbMe. A solution of (C5Me5)2YbMe (0.05 g, 0.11 mmol) in hexane (20 mL) was placed in a Fischer−Porter thick-walled pressure bottle. The bottle was pressurized with ethylene (7 atm), and the mixture was stirred at room temperature. A colorless solid quickly formed and the pressure returned to approximately 1 atm within 5 min. The mixture had a slightly pinkish tint due to the presence of the ytterbium complex. The mixture was then hydrolyzed with dilute aqueous hydrochloric acid and collected on a filter frit. The colorless solid was then washed with additional dilute hydrochloric acid, distilled water, and acetone and thoroughly air-dried. The yield of polymer was 0.72 g. The polycrystalline polymer melted at 131 °C. Synthesis of [(Me5C5)2Yb]2(μ-H). A solution of (Me5C5)2YbMe (0.12 g, 0.26 mmol) in hexane (20 mL) was placed in a Fischer− Porter thick-walled pressure bottle. The atmosphere was flushed three times with dihydrogen before a static pressure of dihydrogen (10 atm) was applied to the system. Upon stirring, the solution quickly changed color from red to purple. The solution was stirred at room temperature for 3 h, during which time a purple solid appeared. The pressure was then released, and the mixture was transferred to a Schlenk tube. The solid was filtered, and residual solvent was removed under reduced pressure, leaving a purple powder (0.07 g, 0.08 mmol, 60%). Mp: 196−197 °C dec. IR (Nujol): ν̅ 2724 w, 1653 w br, 1486 sh, 1445s, 1395 m, 1315 vw, 1299 vw, 1152 m, 1021 m, 974 vw, 956 w, 940 sh br, 722 w, 675 vw, 638 sh, 620 m, 591w, 550 vw, 516 vw, 486 m br, 382 m br, 324 s, 289 m br, 264 m br cm−1. Anal. Calcd for

C40H61Yb2: C, 54.08; H, 6.93. Found: C, 51.93; H, 7.07. The compound was sparingly soluble in hydrocarbon solvents, but a NMR sample was prepared by heating the compound in C6D6 in a closed system. 1H NMR (C6D6, 30 °C): δ 7.80 (ν1/2 = 98 Hz). A sample of the compound was hydrolyzed (H2O), and the hydrolysate was examined by 1H NNR spectroscopy. Only C5Me5H was observed. A sample of the compound (0.14 g, 0.16 mmol) was slurried in 10 mL of toluene and heated to 90 °C, and the solvent was removed slowly under vacuum, leaving a brown residue. This residue was extracted with hexane (15 mL) and filtered. The filtrate was reduced to 3 mL in volume and cooled to −25 °C; ca. 0.10 g of (Me5C5)2Yb was isolated as green needles which lost solvent and turned brown under vacuum and identified by its 1H NMR spectrum. Synthesis of [(Me5C5)2Yb]2(μ-D). A solution of (Me5C5)2YbMe (0.08 g, 0.2 mmol) in hexane (15 mL) was placed in a Fischer−Porter thick-walled pressure bottle. The atmosphere was flushed three times with D2 before a static pressure of D2 (13 atm) was applied to the system. Upon stirring, the solution quickly changed color from red to purple and a purple precipitate appeared after a few minutes. After it was stirred at room temperature for 5 h, the mixture was transferred to a Schlenk tube and filtered. The yield of the purple powder was 0.05 g (0.06 mmol, 64%). IR (Nujol): ν̅ 2724 m, 2041 vw br, 1653 vw br, 1491 m, 1443 s, 1311 vw, 1208 vw, 1166 w, 1151 w, 1023 s, 975 sh, 929 m br, 872 sh, 741 sh, 724 w, 695 vw, 624 vw, 590 w, 550 vw, 444 m, 384 m, 359 w, 322 s, 262 s cm−1. The IR spectra of the H2 and D2 products are not superimposable; there appears to be a broad feature underneath the region of the H2 product spectrum from 1300 to 1400 cm−1 which moves to 900−1000 cm−1 in the D2 product, but no single sharp band can be identified as ν̅(Yb−H/D). Synthesis of [(Me 5 C 5 ) 2 Yb] 2 (μ-Me). Method 1. From (Me5C5)2YbMe. A solution of (C5Me5)2Yb (0.18 g, 0.41 mmol) in pentane (10 mL) was added to a solution of (C5Me5)2YbMe (0.19 g, 0.41 mmol) in pentane (5 mL). Upon addition, a brick red precipitate immediately formed. After the mixture was stirred at room temperature for 1 h, the product was filtered and residual solvent was removed from the residue under reduced pressure. Recrystallization of the residue from toluene (5 mL) afforded dark red crystals (0.33 g, 0.37 mmol, 90%), whose physical properties were identical with those obtained in the synthesis described in the subsection below. Method 2. From Methylcopper. Methylcopper (0.063 g, 0.80 mmol) was synthesized as described above. The reaction must be performed in an elongated Schlenk tube to avoid splashing the unstable methylcopper on the walls. The bright yellow product was filtered and washed once with Et2O (60 mL) and three times with 60 mL portions of pentane at −40 °C. The solid was exposed to vacuum for 40 min at −30 °C. Pentane (30 mL), cooled to −30 °C, was added to the methylcopper to form a suspension. To the suspension was added a solution of (C5Me5)2Yb (0.57 g, 1.3 mmol) in pentane (30 mL). The mixture was then warmed slowly to room temperature over several hours. During this time, the mixture became deep red and a copper mirror formed on the walls of the reaction vessel. The mixture was stirred for an additional 8 h at room temperature. The solid was filtered, and residual solvent was removed from the residue under reduced pressure. The residue was extracted with toluene (40 mL), and the product was filtered twice using glass fiber filters (Whatman GF/D) to separate it from the finely divided metallic copper. The filtrate was then concentrated to approximately 20 mL and cooled to −40 °C, producing brick red crystals. Concentration of the mother liquor provided additional crops of crystals for a total yield of 0.32 g (0.35 mmol, 55%). Mp: 223−225 °C dec. IR (Nujol): ν̅ 2725 m, 2038 w, 1653 w, 1489 m, 1259 w, 1162 w, 1099 vw, 1061 w, 1023 m, 957s, 803 m, 724 w, 696 w, 661s, 623 m, 588 m, 554 w, 431 m, 380 m, 358 m, 327 s, 302 m, 276 s cm−1. 1H NMR (C7D8, 30 °C): δ 5.46 (ν1/2 = 150 Hz). The protons on the ytterbium-bound methyl group were not observed. The highest isotopic cluster in the EI mass spectrum was centered at m/z 768 [M+ − Me5C5]. Anal. Calcd for C41H63Yb2: C, 54.57; H, 7.04. Found: C, 54.67; H, 7.21. Synthesis of (Me5C5)2YbNH2(NH3). A solution of (Me5C5)2YbMe (0.27 g, 0.59 mmol) in hexane (15 mL) was frozen in a liquid nitrogen bath. An excess of anhydrous ammonia was then vacuum-transferred K

DOI: 10.1021/acs.organomet.7b00384 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

cooled to −20 °C, giving (Me5C5)2V, which was purified by vacuum sublimation and identified by its melting point (mp: 299 °C (lit.26 299−300 °C)). Synthesis of (Me5C5)2YbF(thf). Tetrahydrofuran (5 mL) was added to (Me5C5)2YbF (0.10 g, 0.22 mmol), forming a red-orange solution. After complete dissolution, the tetrahydrofuran was removed under reduced pressure. The residue was extracted with pentane (10 mL) and filtered. The filtrate was concentrated to 5 mL, and cooling to −80 °C afforded red-orange, flaky crystals (0.07 g, 0.1 mmol, 61%). The IR and 1H NMR spectra were identical with those of the previously reported compound.28,58 Mp: 160−163 °C (lit.28 158−162 °C). Synthesis of (Me5C5)2YbCl. A solution of (Me5C5)2Yb (1.57 g, 3.54 mmol) in hexane (50 mL) was added to a solution of (Me5C5)2VCl (1.26 g, 3.53 mmol) in hexane (30 mL). The mixture was stirred at room temperature for 2 h, producing a blue-purple precipitate. The precipitate was filtered, and the residue was washed with pentane (20 mL). Residual solvent was removed from the residue under reduced pressure, leaving a blue-purple powder (1.55 g, 3.24 mmol, 91%). Mp: 280 °C dec. IR (Nujol): ν̅ 2725 w, 1606 w, 1487 w, 1264 w, 1167 w, 1099 w, 1064 w, 1024 m, 954 w, 803 w, 725 w, 630 vw, 614 w, 593 w, 552 vw, 388 m, 331 s, 304 s, 258 s cm−1. 1H NMR (C7D8, 28 °C): δ 19.75 (ν1/2 = 295 Hz). EI-MS: m/z 475 (4.3, 7.1); 476 (30, 34); 477 (58, 59); 478 (64, 60); 479 (100, 100); 480 (30, 33); 481 (57, 57); 482 (7.9, 12); 483 (8.4, 11); 484 (0.5, 2.2). Anal. Calcd for C20H30CIYb: C, 50.13; H, 6.32. Found: C, 53.75; H, 6.81. The filtrate from the reaction mixture was concentrated to 20 mL and cooled to −20 °C, giving (Me5C5)2V, which was purified by vacuum sublimation and identified by its melting point (mp: 299 °C (lit.26 299−300 °C)). Synthesis of (Me5C5)2YbCl(THF). Tetrahydrofuran (10 mL) was added to (Me5C5)2YbCl (0.18 g, 0.38 mmol), forming a magenta solution. After complete dissolution, the tetrahydrofuran was removed under reduced pressure. The residue was extracted with a 2/1 pentane/toluene mixture (30 mL) and filtered. Cooling of the filtrate to −80 °C afforded purple needles (0.08 g, 0.15 mmol, 39%). The IR spectrum was identical with that of the previously reported compound.59 Mp: 218−220 °C (lit.59 221−223 °C). Synthesis of (Me5C5)2YbBr. A solution of (Me5C5)2Yb (0.25 g, 0.56 mmol) in hexane (15 mL) was added to a solution of (Me5C5)2VBr (0.23 g, 0.57 mmol) in hexane (15 mL). The mixture was stirred at room temperature for 1 h, producing a blue precipitate, which was filtered and washed with pentane (15 mL). Residual solvent was removed from the residue under reduced pressure, leaving a blue powder (0.23 g, 0.44 mmol, 78%). Mp: 275 °C dec. IR (Nujol): ν̅ 2725 w, 1596 vw, 1261 vw, 1164 w, 1098 vw, 1064 w, 1022 m, 954 vw, 802 w, 721 w, 614 vw, 590 w, 551 vw, 423 vw, 389 w, 328 s, 305 s cm−1. 1H NMR (C7D8, 30 °C): δ 14.35 (ν1/2 = 316 Hz). EI-MS: m/z 520 (15, 25); 521 (33, 47); 322 (60, 60); 323 (100, 100); 324 (42, 47); 325 (65, 80); 326 (12, 17); 327 (19, 22). Anal. Calcd for C20H30BrYb: C, 45.87; H, 5.78. Found: C, 46.85; H, 5.87. The filtrate from the reaction mixture was concentrated to 10 mL and cooled to −20 °C, giving (Me5C5)2V, which was purified by vacuum sublimation and identified by its melting point (mp: 299 °C (lit.26 299−300 °C)). Synthesis of (Me5C5)2YbI. A solution of (Me5C5)2Yb (0.25 g, 0.56 mmol) in pentane (30 mL) was added to a solution of (Me5C5)2VI (0.25 g, 0.56 mmol) in pentane (10 mL). The mixture was stirred at room temperature for 1 h, during which time a blue precipitate appeared. The solid was filtered and washed with pentane (10 mL). Residual solvent was removed from the residue under reduced pressure, leaving a blue powder (0.25 g, 0.44 mmol, 78%). Mp: 258 °C dec. IR (Nujol): ν̅ 2725 w, 1486 w, 1165 w, 1098 w, 1061 w, 1020 m, 957 w, 805 w, 770 w, 723 w, 614 vw, 597 m, 384 m, 308 s cm−1. EIMS: m/z 567 (8.3, 8.7); 568 (42, 42); 569 (73, 70); 570 (61, 60); 571 (100, 100); 572 (21, 21); 573 (37; 38); 574 (7.4, 8.0). Anal. Calcd for C20H30IYb: C, 42.09; H, 5.30. Found: C, 40.63; H, 5.15. The filtrate from the reaction mixture was concentrated to 10 mL and slowly cooled to −20 °C, giving (Me5C5)2V, which was purified by vacuum sublimation and identified by its melting point (mp: 299 °C (lit.26 299−300 °C)).

into the reaction vessel. The mixture was warmed to room temperature while the system was vented to the nitrogen manifold to release excess ammonia. While the mixture was warmed, an orange solid formed which then redissolved as the mixture reached room temperature. The deep orange solution was stirred for an additional 0.5 h before the hexane was completely removed under reduced pressure. The residue was extracted with pentane (25 mL) and filtered. Slow cooling of the filtrate to −80 °C afforded yellow-orange needles (0.20 g, 0.42 mmol, 71%). The compound did not melt to 330 °C. IR (Nujol): ν̅ 3361 m, 3280 m, 2725 m, 1590 m, 1522 s, 1493 w, 1261 w, 1183 s, 1097 w, 1061 w, 1021 m, 951 w, 819 vw, 803 m, 722 w, 644 m, 628 vw, 620 vw, 593 w, 552 vw, 498 s, 423 s, 385 vw, 301 s cm−1. 1H NMR (C6D6, 25 °C): δ 2.76 (ν1/2 = 30 Hz). Neither the protons of the amide nor the ammonia ligands were observed in the 1H NMR spectrum. The highest isotopic cluster in the EI mass spectrum was centered at m/z 460 amu [M+ − NH3]. Anal. Calcd for C20H35N2Yb: C, 50.39; H, 7.41; N, 5.88. Found: C, 50.05; H, 7.36; N, 5.73. Synthesis of (Me5C5)2YbND2(ND3). A solution of (Me5C5)2YbMe (0.21 g, 0.46 mmol) in hexane (10 mL) was frozen in a liquid nitrogen bath. An excess of anhydrous ammonia-d3 was then vacuumtransferred into the reaction vessel. The mixture was warmed to room temperature while the system was vented to the nitrogen manifold to release excess ammonia-d3. While the mixture was warmed, an orange solid formed which then redissolved as the mixture reached room temperature. The deep orange solution was stirred for an additional 3 h before the volatile materials were removed under reduced pressure. The residue was extracted with pentane (20 mL) and filtered. The filtrate was concentrated to approximately 5 mL with warming to redissolve the solid and slowly cooled to −80 °C to produce yellow-orange needles (0.16 g, 0.33 mmol, 70%). The compound did not melt to 330 °C. IR (Nujol): ν̅ 2725 m, 2504 m, 2449 w, 2408 w, 2380 m, 2311 vw, 2044 vw, 1653 w, 1524 w, 1491 w, 1342 m, 1262 w, 1238 m, 1166 m, 1130 s, 1097 m, 1061 w, 1023 m, 992 s, 915 s, 802 m, 723 m, 645 m, 629 w, 618 w, 593 w, 555 w, 491 m, 469 s, 387 s br, 354 m, 301 s br cm−1. 1H NMR (C6D6, 25 °C): δ 2.78 (ν1/2 = 30 Hz). EI-MS: m/z 478 (37, 9); 479 (63, 42); 480 (92, 70); 481 (90, 60); 482 (100, 100); 483 (82, 22); 484 (75, 38); 485 (50, 8). Synthesis of (Me5C5)2Yb(μ-NH2)2AlMe2. To a solution of (Me5C5)2YbNH2(NH3) (0.12 g, 0.25 mmol) in hexane (10 mL) was added dropwise via syringe 0.22 mL (0.25 mmol) of a 1.15 M hexane solution of AlMe3. Upon mixing, the color immediately changed from bright orange to deep red and gas evolution was observed. After it was stirred at room temperature for 0.5 h, the solution was filtered and the filtrate was cooled to −40 °C to produce deep red crystals (0.10 g, 0.19 mmol, 75%). Mp: 195−197 °C dec. IR (Nujol): ν̅ 3385 m, 3331 m, 2725 m, 1790 vw, 1645 vw, 1586 m, 1489 m, 1422 w, 1262 vw, 1188 s, 1164 vw, 1090 vw, 1059 vw, 1023 m, 949 vw, 867 vw, 800 vw, 773 s, 738 s, 652 m, 624 m, 617 s, 593 m, 522 m, 507 m, 413 m, 382 m, 347 m, 304 s cm−1. 1H NMR (C6D6, 24 °C): δ 4.86 (30H), −22.5 (6H). The protons on the amide ligands were not observed. EI-MS: m/z 529 (9, 8); 530 (41, 41); 531 (69, 70); 532 (61, 61); 533 (100, 100); 534 (24, 24); 535 (36, 38); 536 (9,9). Anal. Calcd for C22H40N2AIYb: C, 49.59; H, 7.57; N, 5.26. Found: C, 48.93; H, 7.63; N, 4.40. Synthesis of (Me5C5)2YbF. A solution of (Me5C5)2Yb (0.40 g, 0.90 mmol) in pentane (20 mL) was added to a solution of (Me5C5)2VF (0.31 g, 0.90 mmol) in pentane (10 mL). Upon mixing, a purple precipitate immediately appeared. The mixture was stirred at room temperature for 1 h and filtered. The solid was washed with pentane (30 mL) and filtered. Residual solvent was removed from the residue under reduced pressure, leaving a purple powder (0.32 g, 0.69 mmol, 77%). Mp: 285 °C dec. lR (Nujol): ν̅ 2725 w, 1646 w, 1490 vw, 1256 w, 1167 w, 1099 w, 1064 w, 1029 m, 954 vw, 803 m, 722 w, 668 vw, 636 vw, 617 w, 593 w, 552 w, 485 m, 412 s, 385 m, 369 m, 320 s, 299 s cm−1. No resonances were observed in the 1H NMR spectrum in C7D8 at room temperature. EI-MS: m/z 460 (43, 42); 461 (67, 70); 462 (57, 60); 463 (100, 100); 464 (21, 21); 465 (38, 38). Anal. Calcd for C20H30FYb: C, 51.92; H, 6.54. Found: C, 52.14; H, 6.63. The filtrate from the reaction mixture was concentrated to 10 mL and L

DOI: 10.1021/acs.organomet.7b00384 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Synthesis of (Me5C5)2YbI(THF). A sample of (Me5C5)2YbI (0.35 g, 0.61 mmol) was dissolved in tetrahydrofuran (10 mL), forming a deep purple solution. The solution was filtered, and the filtrate was concentrated to approximately 5 mL. Cooling to −80 °C produced a mass of purple crystals (0.15 g, 0.23 mmol, 38%). The IR spectrum was identical with that of the previously reported compound.60 Synthesis of (Me5C5)2YbBH4. A solution of (Me5C5)2Yb (0.23 g, 0.52 mmol) in hexane (15 mL) was added to a solution of (Me5C5)2VBH4 (0.17 g, 0.51 mmol) in hexane (15 mL). After the mixture was stirred at room temperature for 0.5 h, the solvent was removed under reduced pressure. The residue was extracted with hexane (10 mL). The purple powder that did not dissolve in hexane was crystallized from toluene (5 mL) at −40 °C, affording purple needles (0.12 g, 0.26 mmol, 50%). Mp: 210−212 °C. IR (Nujol): ν̅ 2725 m, 2337 s, 2198 s, 1606 m, 1493 m, 1263 vw, 1219 m, 1178 m, 1118 w, 1064 w, 1023 m, 957 vw, 803 w, 727 m, 696 m, 626 vw, 614 vw, 595 w, 557 vw, 466 m, 384 m, 330 s, 305 s cm−1. 1H NMR (C7D8, 30 °C): δ 13.4 (ν1/2 = 124 Hz). The protons on the borohydride ligand were not observed. EI-MS: m/z 455 (19, 18); 456 (62, 56); 457 (80, 81); 458 (83, 81); 459 (100, 100); 460 (27, 29); 461 (37, 38). Anal. Calcd for C20H34BYb: C, 52.37; H, 7.48. Found: C, 52.57; H, 7.47. Synthesis of (Me5C5)2YbBD4. A solution of (Me5C5)2Yb (0.16 g, 0.36 mmol) in pentane (5 mL) was added to a solution of (Me5C5)2VBD4 (0.12 g, 0.35 mmol) in pentane (5 mL). The purple mixture was stirred at room temperature for 1 h before the volatile materials were removed under reduced pressure. The residue was extracted with toluene (5 mL) and filtered. The filtrate was concentrated to approximately 2 mL and cooled to −40 °C, producing a purple powder (0.11 g, 0.24 mmol, 66%). Mp: 194−198 °C. IR: 2766 vw, 2725 m, 2367 w, 2176 w, 1952 vw, 1727 s, 1638 m, 1563 m, 1262 m, 1203 vw, 1164 vw, 1098 m, 1064 m, 1023 s, 959 w, 918 m, 868 vw, 804 m, 729 w, 695 w, 590 w, 463 m, 424 w, 388 m, 331 s, 303 s cm−1. Reaction of (Me5C5)2Yb. With (Me5C5)2VN3. A solution of (Me5C5)2Yb (0.12 g, 0.27 mmol) in hexane (10 mL) was added to a solution of (Me5C5)2VN3 (0.10 g, 0.28 mmol) in hexane (10 mL). Upon mixing, a light purple precipitate immediately appeared. After the mixture was stirred at room temperature for 1 h, the solid was filtered and washed with hexane (15 mL). Residual solvent was removed from the residue under reduced pressure, giving 0.12 g of a purple powder. The physical properties of this product were identical with those of the material obtained from the reaction of (Me5C5)2Yb with trimethylsilyl azide described in the subsection below. With Trimethylsilyl Azide. Trimethylsilyl azide (0.14 mL, 1.1 mmol) was added to a solution of (Me5C5)2Yb (0.24 g, 0.54 mmol) in pentane (20 mL). The mixture was stirred for 12 h at room temperature, producing a purple precipitate. The precipitate was filtered and washed with pentane (10 mL). Residual solvent was removed from the residue under reduced pressure, giving 0.14 g of a purple powder. The product did not melt to 330 °C. IR (Nujol): ν̅ 2725 m, 2155 vs br, 2088 s, 1656 vw, 1489 w, 1267 vw, 1165 w, 1098 w, 1061 w, 1022 m, 952 vw, 802 w, 723 w, 635 vw, 601 m, 594 w, 584 m, 566 vw, 552 vw, 434 w, 390 m, 316 s, 298 s cm−1. The highest isotopic cluster in the EI mass spectrum was centered at m/z 444 corresponding to [(Me5C5)2Yb]+. Anal. Calcd for C20H30N3Yb: C, 49.46; H, 6.23; N, 8.66. Found: C, 49.85; H, 6.37; N, 9.63. Synthesis of [(Me5C5)2Yb]2(μ-F). A solution of (Me5C5)2Yb (0.07 g, 0.2 mmol) in pentane (10 mL) was added to a suspension of (Me5C5)2YbF (0.07 g, 0.2 mmol) in pentane (5 mL). The mixture was stirred at room temperature for 1 h, producing a brown solid. The product was filtered, and residual solvent was removed under reduced pressure, leaving a brown powder (0.09 g, 0.1 mmol, 66%). The IR spectrum was identical with that of the previously reported compound.32 Mp: 289−295 °C (lit.32 290 °C). There was no evidence for the formation of the tetrameric complex (Me5C5)6Yb4(μF)4.61 Synthesis of [(Me5C5)2Yb]2(μ-Cl). A solution of (Me5C5)2VCl (0.18 g, 0.50 mmol) in hexane (20 mL) was added to a solution of (Me5C5)2Yb (0.44 g, 0.99 mmol) in hexane (10 mL). The mixture was

stirred at room temperature for 12 h, during which time a brown microcrystalline precipitate appeared. The solid was filtered, and the residue was dissolved in toluene (10 mL). Filtration followed by cooling of the filtrate to −20 °C afforded dark brown blocks (0.41 g, 0.44 mmol, 90%). Mp: 282 °C dec. IR (Nujol): ν̅ 2725 w, 2040 vw, 1796 vw, 1655 w, 1488 w, 1164 w, 1100 vw, 1062 w, 1024 m, 951 w, 803 w, 727 w, 692 w, 658 w, 628 vw, 616 vw, 590 w, 552 w, 464 w, 392 m, 384 m, 362 m, 331 s, 274 s cm−1. 1H NMR (C7D8, 31 °C): δ 11.02 (ν1/2 = 153 Hz). A molecular ion was not observed in the EI mass spectrum. The highest isotopic cluster was centered at m/z 479 corresponding to [(C5Me5)2YbCl]+. Anal. Calcd for C40H60CIYb2: C, 52.06; H, 6.56. Found: C, 52.17; H, 6.54. Synthesis of [(Me5C5)2Yb]2(μ-Br). A solution of (Me5C5)2VBr (0.22 g, 0.55 mmol) in hexane (20 mL) was added to a solution of (Me5C5)2Yb (0.49 g, 1.1 mmol) in hexane (10 mL). Upon mixing, a brown microcrystalline precipitate immediately appeared. After the mixture was stirred at room temperature for 12 h, the solid was filtered and washed with pentane (20 mL). Residual solvent was removed from the residue under reduced pressure and the residue was crystallized from toluene (10 mL) at −20 °C, affording dark brown blocks (0.37 g, 0.38 mmol, 70%). Mp: 252 °C dec. IR (Nujol): ν̅ 2725 m, 2040 w, 1652 w, 1484 w, 1265 vw, 1163 w, 1087 vw, 1063 w, 1024 m, 948 vw, 802 w, 723 w, 633 vw, 617 vw, 587 w, 554 w, 468 vw, 392 w, 362 w, 329 s, 276 s cm−1. 1H NMR (C7D8, 30 °C): δ 13.66 (ν1/2 = 150 Hz). A molecular ion was not observed in the EI mass spectrum. The highest isotopic cluster was centered at m/z 523 corresponding to [(Me5C5)2YbBr]+. Anal. Calcd for C40H60BrYb2: C, 49.57; H, 6.26. Found: C, 49.76; H, 6.23. Synthesis of [(Me5C5)2Yb]2(μ-Me) from (C5Me5)2Yb and (C5Me5)2VMe. A solution of (C5Me5)2VMe62 (0.11 g, 0.33 mmol) in hexane (5 mL) was added to a solution of (C5Me5)2Yb (0.29 g, 0.65 mmol) in hexane (10 mL). Upon addition, a red-brown precipitate immediately formed. After the mixture was stirred at room temperature for 1 h, the solid was filtered and the residue was washed with hexane (5 mL). Crystallization of the residue from toluene (5 mL) at −20 °C afforded brick red crystals (0.21 g, 0.23 mmol, 71%). The IR and 1H NMR spectra were identical with those of the compound obtained from the reaction of (C 5Me5) 2Yb with methylcopper as described above. Synthesis of [(Me5C5)2Yb]2(μ-BH4). A solution of (Me5C5)2VBH4 (0.07 g, 0.2 mmol) in pentane (10 mL) was added to a solution of (Me5C5)2Yb (0.18 g, 0.41 mmol) in pentane (10 mL). Upon mixing, a brown microcrystalline precipitate appeared. After the mixture was stirred for 1 h at room temperature, the product was filtered and the residue was washed with pentane (20 mL). Crystallization of the residue from toluene (3 mL) at −20 °C afforded green-brown crystals (0.13 g, 0.14 mmol, 71%). Mp: 294 °C dec. IR (Nujol): ν̅ 2725 m, 2389 m, 2323 s, 2194 s, 2142 m, 1891 w, 1652 w, 1600 vw, 1484 w, 1229 s, 1163 w, 1099 vw, 1088 vw, 1064 w, 1022 m, 948 w, 802 w, 728 w, 695 vw, 667 vw, 632 vw, 618 vw, 588 w, 552 w, 409 m, 390 m, 357 w, 332 s, 274 s cm−1. 1H NMR (C7D8, 30 °C): δ 8.97 (ν1/2 = 73 Hz). The protons on the borohydride ligand were not observed. The highest isotopic cluster in the EI mass spectrum was centered at m/z 751 [M − Me5C5]+. Anal. Calcd for C40H64BYb2: C, 53.24; H, 7.15. Found: C, 52.94; H, 7.18. Synthesis of [(Me5C5)2Yb]2(μ-BD4). A solution of (Me5C5)2VBD4 (0.10 g, 0.29 mmol) in pentane (5 mL) was added to a solution of (Me5C5)2Yb (0.26 g, 0.59 mmol) in pentane (5 mL). After the mixture was stirred for 1 h at room temperature, the pentane was completely removed under reduced pressure. The residue was extracted with toluene (5 mL) and filtered. Concentration of the filtrate to approximately 3 mL followed by cooling to −40 °C afforded greenbrown crystals (0.14 g, 0.15 mmol, 53%). The product did not melt to 330 °C. IR (Nujol): ν̅ 2725 m, 2389 w, 2328 w, 2149 w, 2037 vw, 1792 w, 1750 sh, 1714 s, 1633 m, 1559 s, 1483 sh, 1261 w, 1163 w, 1099 vw, 1062 w, 1023 m, 969 m, 923 m, 800 w, 730 w, 695 w, 659 w, 632 vw, 598 w, 553 w, 465 vw, 389 m, 359 m, 331 s, 272 s cm−1. Computational Details. All calculations were carried out with the Gaussian 09 suite of programs63 using several methods, including density functional theory (DFT) with the B3PW9164 hybrid M

DOI: 10.1021/acs.organomet.7b00384 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics functional, CASSCF with various active spaces, and CASSCF-MP2. Given the size of the molecules, some restrictions in the basis set were required. The ytterbium center was treated with a small-core relativistic pseudopotential (RECP) ([Ar] + 3d)65 in combination with its adapted basis set (segmented basis set that includes up to g functions), whereas the carbon and hydrogen atoms were treated with an all-electron double-ξ 6-31G(d,p) basis set.66 These basis sets were used in previous studies and allowed us to correctly predict physical properties such as XANES spectra and solid-state magnetism correctly.67 Geometry optimizations were performed without any symmetry constraints at the DFT level. The CASSCF calculations were then carried out on this optimized geometry using the SCF orbitals, varying the number of active orbitals and active electrons. The size of the molecule did not allow us to perform a CASSCF computation with 15 active orbitals (4f orbitals on each Yb and the σ* Yb−R orbital with R = H or CH3) and 27 electrons, but it was possible to use 11 active orbitals and 21 electrons. Reduction of the active space confirmed the stability of this solution. Since only the f±3 orbitals are active, the same results were obtained when only 5 active orbitals and 9 electrons were considered: i.e., the electrons were distributed over four 4f orbitals and the σ* Yb−R orbital with R = H, CH3. Applying the MP2 correction to the CASSCF results had no significant influence on the relative energies of the individual states, since they were of similar magnitude (0.0576 and 0.0573 au for the doublet and quartet states, respectively).

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grateful to the Alexander von Humboldt Foundation for a grant of experienced researcher and the Chinese Academy of Science. CalMip is acknowledged for a generous computational grant.

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(1) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. Rev. 1995, 95, 865−986. (2) (a) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091−8103. (b) Mauermann, H.; Swepston, P. N.; Marks, T. J. Organometallics 1985, 4, 200−202. (c) Heeres, H. J.; Renkema, J.; Booij, M.; Meetsma, A.; Teuben, J. H. Organometallics 1988, 7, 2495−2502. (d) Schumann, H.; Rosenthal, E. C. E.; Kociok-Köhn, G.; Molander, G. A.; Winterfeld, J. J. Organomet. Chem. 1995, 496, 233−240. (3) Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L.; Hunter, W. E. J. Chem. Soc., Dalton Trans. 1979, 54−61. (4) Stults, S. D.; Andersen, R. A.; Zalkin, A. J. Organomet. Chem. 1993, 462, 175−182. (5) Voskoboynikov, A. Z.; Parshina, I. N.; Shestakova, A. K.; Butin, K. P.; Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J. A. K. Organometallics 1997, 16, 4041−4055. (6) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51−6. (7) An ORTEP drawing is shown in Figure 2 of reference 6 along with some key bond distances and angles. The hydrogen atoms were not located, but in a more recent structure the hydrogen atoms on the bridging methyl group were located and refined: (a) Evans, W. J.; Peroth, J. M.; Ziller, J. W. J. Am. Chem. Soc. 2005, 127, 3894−3909. The heavy-atom distances in this structural determination are Lu−C (bridge) 2.756(9) and 2.440(9) Å and Lu−C (terminal) 2.344(12) Å: (b) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51−56. The heavy-atom distances in this structural determination are Lu−C (bridge) 2.737(3) and 2.442(3) Å and Lu−C (terminal) 2.423(3) Å.7a Evans et al. reported the synthesis on a 300 mg scale starting from Cp*2LuBPh4 and MeLi suspended in methylcyclohexane. (8) Evans, W. J.; Perotti, J. M.; Ziller, J. W. J. Am. Chem. Soc. 2005, 127, 3894−3909. (9) Busch, M. A.; Harlow, R.; Watson, P. L. Inorg. Chim. Acta 1987, 140, 15−20. (10) (a) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491−3. (b) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203−219. (c) Balcells, D.; Clot, E.; Eisenstein, O. Chem. Rev. 2010, 110, 749−823. (11) (a) Watson, P. L. J. Am. Chem. Soc. 1982, 104, 337−9. (b) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471−3. (c) Watson, P. L.; Herskovitz, T. ACS Symp. Ser. 1983, 212, 459−79. (12) Coates, G. E.; Green, M. L. H.; Wade, K. Organometallic Compounds, 3rd ed.; Methuen: London, 1967; Vols. 1 and 2. (13) Berg, D. J.; Andersen, R. A. Organometallics 2003, 22, 627−632. (14) (a) Finke, R. G.; Keenan, S. R.; Schiraldi, D. A.; Watson, P. L. Organometallics 1986, 5, 598−601. (b) Da Re, R. E.; Kuehl, C. J.; Brown, M. G.; Rocha, R. C.; Bauer, E. D.; John, K. D.; Morris, D. E.; Shreve, A. P.; Sarrao, J. L. Inorg. Chem. 2003, 42, 5551−5559. (15) Gennett, T.; Milner, D. F.; Weaver, M. J. J. Phys. Chem. 1985, 89, 2787−2794. (16) Huheey, J. E. Inorganic Chemistry, 3rd ed.; Harper & Row: New York, 1983; Appendix F. (17) Merkel, S.; Stern, D.; Henn, J.; Stalke, D. Angew. Chem., Int. Ed. 2009, 48, 6350−6353. (18) (a) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395−408. (b) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem. 1988, 36, 1−124. (c) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6908−6914. (19) Ikariya, T.; Yamamoto, A. J. Organomet. Chem. 1974, 72, 145− 151. (20) Single crystals were obtained from a concentrated pentane solution at −80 °C. The cell parameters determined at −100 °C of a = 10.820(2) Å, b = 26.986(2) Å, c = 14.224(2) Å, and β = 108.85(2)°

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00384. Additional experimental details, solid-state magnetic susceptibility studies, thermal decomposition of [Cp*2Yb]2(μ-H), variable-temperature (VT) 1H NMR studies, and crystallographic data (PDF) Computed molecule Cartesian coordinates of [Cp*2Yb]2(μ-H) and [Cp*2Yb]2(μ-CH3) (MOL) Accession Codes

CCDC 1551343−1551347 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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail for R.A.A.: [email protected]. ORCID

Marc D. Walter: 0000-0002-4682-8749 Laurent Maron: 0000-0003-2653-8557 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences (OBES), of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. We thank Fred Hollander (at CHEXRAY, the UC Berkeley X-ray diffraction facility) for assistance with the crystallography. C.J.B. thanks the Fannie and John Hertz Foundation for a fellowship. R.A.A. acknowledges the Alexander von Humboldt Foundation for a reinvitation grant within the Humboldt Senior Research Award program. L.M. is N

DOI: 10.1021/acs.organomet.7b00384 Organometallics XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.organomet.7b00384 Organometallics XXXX, XXX, XXX−XXX