Synthesis and Characterization of Rare Earth Siloxide Complexes, M

Dec 5, 2014 - For La, Nd, and Sm, dimeric structures in which the bridging -OSi(OtBu)3 group is κ3-coordinated are formed. When 4 equiv of silanol is...
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Synthesis and Characterization of Rare Earth Siloxide Complexes, M[OSi(OtBu)3]3(L)x where L is HOSi(OtBu)3 and x = 0 or 1 Giuseppe Lapadula,† Matthew P. Conley,† Christophe Copéret,*,† and Richard A. Andersen‡ †

ETH Zürich, Department of Chemistry and Applied Biosciences, Vladimir Prelog Weg 2, CH-8093 Zürich, Switzerland Department of Chemistry, University of California, Berkeley, California 94720, United States

‡

S Supporting Information *

ABSTRACT: The reaction of lanthanide and group 3 amides, M[N(SiMe3)3]3, with 3 equiv of tris(tert-butoxy)silanol, (tBuO)3SiOH, gives M[OSi(OtBu)3]3 complexes. Their solid-state structure depends on the size of the lanthanide metal; for M = Sc, Yb, and Lu, the primary coordination sphere is a square-based pyramid in which two siloxides are κ2coordinated and one is κ1-coordinated. For La, Nd, and Sm, dimeric structures in which the bridging -OSi(OtBu)3 group is κ3-coordinated are formed. When 4 equiv of silanol is used, the resulting molecules crystallize as 4:1 Si:M adducts, in which a neutral silanol remains coordinated, i.e., M[(OSi(OtBu)3)3](HOSi(OtBu)3). For M = Sc, Y, Eu, Yb, and Lu, the solid-state structures are based on five-coordinate M, M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3). For M = Ce, Nd, and Sm, the coordination number of the metal reaches six in molecules with the general formula M(OSi(OtBu)3)2(κ2-OSi(OtBu)3)(κ2-HOSi(OtBu)3). In solution, the 1H nuclear magnetic resonance (NMR) spectra are fluxional, resulting in all tBu groups being chemically equivalent. The solid-state 1H, 13C, and 29Si NMR spectra are consistent with the stereochemistry found in the X-ray crystal structure. [(tBuO)3SiO]xMLn complexes can model surface species,20−22 as in [((tBuO)3SiO)Mo(NAr)(CH-tBu)(CH2-tBu)]23,24 and [Nd(OSi(OtBu)3)(AlMe4)2(AlMe3)],25,26 that are analogues for corresponding silica-supported metathesis and diene polymerization catalysts, respectively. Finally, the (tBuO)3SiOligand has been used to stabilize low-coordination number U(III) or U(IV) complexes27,28 and can be used to tune the lanthanide reactivity with small molecules.29 We are particularly interested in the coordination chemistry of (tBuO)3SiO- because the high inherent flexibility of this ligand can stabilize several coordination geometries that are fluxional (vide supra) and represent the various possible environments in the corresponding surface sites in silica materials. Despite the extensive use of (tBuO)3SiO- in the examples described above, only a few lanthanide derivatives are known.30,31 Lanthanides have large ionic radii and their felectrons participate minimally in chemical bonding. Accordingly they display moderate Lewis acidity, oxophilicity, and high coordination numbers, like their large d0 analogues. Lanthanide ions can accommodate up to 12 small ligands in the first coordination sphere. Low coordination numbers are achieved only with bulky ligands, such as silylamides,32−36 substituted aryloxides,37−41 and alkyls,42−54 a topic exploited by Lappert and his co-workers.55,56

1. INTRODUCTION The tris(tert-butoxy)siloxy group, (tBuO)3SiO-, is a seemingly simple ligand that has a rich metal coordination chemistry. As shown in Scheme 1, the (tBuO)3SiO- ligand can coordinate to Scheme 1. Possible Modes of Metal Ligand Coordination of the Siloxide Ligand

metals as a terminal κ1, terminal κ2, bridging κ1 μ2, and bridging κ2 μ2 ligand.1−10 In [(tBuO)3SiO]xMLn metal complexes, the (tBuO)3SiO- ligand can undergo thermolysis to form mixedmetal oxide materials with a predetermined M:Si stoichiometry, a feature exploited extensively for the d-block metals.4−10 These compounds have been used as precursors for homogeneous11,12 and single-site heterogeneous catalysts6,13−15 as well as supported and non-supported metal particles.16−19 In addition, © XXXX American Chemical Society

Special Issue: Mike Lappert Memorial Issue Received: October 17, 2014

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complexes are shown in Figure 1. Relevant bond lengths and angles are listed in Table 1. These compounds consist of

We recently reported that the addition of 4 equiv of tris(tertbutoxy)silanol, (tBuO)3SiOH, to lanthanide amides M[N(SiMe3)3]3 (M = Y or Yb) gave monomeric neutral complexes with the composition M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3) (M = Y or Yb) that are stabilized by incorporation of an additional neutral silanol.57 Here we report a general gram-scale synthetic route for lanthanide(III) siloxides with two different Si:M ratios, 3:1 and 4:1, and a systematic study of their properties in the solid state. The nuclearity and the first coordination sphere are determined by the metal size and by the number of equivalents of silanol used in the synthetic step. The amides of Sc, Yb, and Lu react with 3 equiv of HOSi(OtBu)3 to give monomeric trisiloxy complexes; with 4 equiv, monomeric compounds of the general formula M(OSi(OtBu)3)3(κ2HOSi(OtBu)3) are obtained for Sc, Y, Ce, Nd, Sm, Eu, Yb, and Lu, except for lanthanum, which forms a dimeric complex.

Table 1. Summary of the Bond Distances and Angles from Xray Crystallography of the M(OSi(OtBu)3)3 Complexes distances (Å) M3+ radius58 M−O1 M−O2 M−O3 M−O4 M−O5 angles (deg) O1−M−O2 O2−M−O3 O3−M−O4 ωa

2. RESULTS 2.1. Synthesis and X-ray Crystal Structures. M[N(SiMe3)2]3 (M = Sc, Yb, Lu, La, Nd, and Sm) reacts with HOSi(OtBu)3 in a 1:3 stoichiometry to give the corresponding siloxide derivatives and silylamine. The resulting solid-state structure depends on the size of the metal; when M is small (Sc, Yb, and Lu), the compounds have monomeric structures (Scheme 2a), while with larger M3+ ions (La, Nd, and Sm), the structures are dimeric (Scheme 2b).

1-Sc

1-Yb

1-Lu

0.89 2.215(2) 2.012(2) 2.203(2) 2.008(3) 1.925(2)

1.01 2.339(2) 2.135(2) 2.317(3) 2.133(3) 2.029(3)

1.00 2.3128(1) 2.1359(1) 2.2930(1) 2.1351(2) 2.0207(1)

69.73(9) 95.19(9) 69.89(9) 58

66.54(9) 100.01(9) 66.62(8) 63

67.23(9) 100.14(9) 67.07(9) 64

ω is the dihedral angle between the O1−M−O2 and O3−M−O4 planes. a

strongly distorted square-based pyramids. The extent of the distortion is estimated by ω, the dihedral angle between the O1−M−O2 and O3−M−O4 planes, whose values are 0° and 90° for an idealized square-based pyramid and trigonal bipyramid, respectively. For 1-Sc, 1-Yb, and 1-Lu, the ω values are 58°, 63°, and 64°, respectively. The negatively charged oxygen atoms (O2, O4, and O5) have O−M distances that are shorter than the neutral O−M distance [Sc−O1 = 2.215(2) Å vs Sc−O2 = 2.012(2) Å; Yb−O1 = 2.339(2) Å vs Yb−O2 = 2.135(2) Å; O1−Lu = 2.3128(1) Å vs O2−Lu = 2.1359(1) Å]. The apical negatively charged oxygen atom forms a bond significantly shorter than those in the basal plane [M−O5 = 1.925(2), 2.029(3), and 2.0207(1) Å for 1-Sc, 1-Yb, and 1-Lu, respectively]. When M is La, Nd, and Sm, dimeric structures are obtained, in which each lanthanide is five-coordinated with κ2-μ-κ1 siloxides bridging two metal centers. 1-Sm, which is the smallest in this series, was reported by Hou.31 1-La was previously reported,59 although the single-crystal structure was not provided. Figure 2 shows the solid-state structure of 1-La. Each lanthanum atom is five-coordinated with κ2-μ-κ1 siloxides bridging two metal centers. The La···La distance is 4.028(9) Å. The siloxides that are κ1-coordinated have O−La distances shorter than those of the κ2-μ-κ1 siloxides [La1−O1 and La1− O3 = 2.206(6) and 2.197(7) Å, respectively, vs La1−O2 and La−O4 = 2.558(6) and 2.431(5) Å, respectively]. Despite

Scheme 2. Reaction of Lanthanide Amides with 3 equiv of HOSi(OtBu)3 To Give (a) M(OSi(OtBu)3)3 and (b) [M(OSi(OtBu)3)3]2

The monomeric 1-Sc, 1-Yb, and 1-Lu complexes are fivecoordinate in which two siloxides are κ2-coordinated and one is κ1-coordinated (Scheme 2a). ORTEP diagrams of the three

Figure 1. ORTEP diagrams of M(OSi(OtBu)3)3: (a) 1-Yb, (b) 1-Sc, and (c) 1-Lu. Ellipsoids at the 50% probability level; hydrogen atoms and methyl groups omitted for the sake of clarity. B

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Figure 2. ORTEP diagram of 1-La. Ellipsoids at the 50% probability level; hydrogen atoms and methyl groups omitted for the sake of clarity.

Figure 3. ORTEP diagrams of M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3): (a) 2-Sc and (b) 2-Eu. Ellipsoids at the 50% probability level; hydrogen atoms and methyl groups omitted for the sake of clarity.

multiple attempts, a single crystal of sufficient quality for 1-Nd could not be obtained. However, the unit cell and the first coordination sphere are similar to those found for the samarium and lanthanum siloxides, suggesting a similar solidstate structure. The addition of 4 equiv of silanol to M[N(SiMe3)2]3 (M = Sc, Y, Eu, Yb, and Lu) gives monomeric neutral complexes that crystallize with an additional molecule of silanol to form either M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3) or M(OSi(OtBu)3)2(κ2OSi(OtBu)3)(κ2-HOSi(OtBu)3) (Scheme 3). The choice

M−O distance follows in general the ionic radius; for example, M−O3 = 1.9323(9), 2.096(3), 2.211(9), and 2.087(4) Å for 2Sc, 2-Y, 2-Eu, and 2-Yb, respectively. When M = Ce, Nd, and Sm, the compounds crystallize as sixcoordinate compounds with the formula M(OSi(OtBu)3)2(κ2OSi(OtBu)3)(κ2-HOSi(OtBu)3); the ORTEP diagrams of the 2-Ce and 2-Nd siloxides are shown in Figure 4. In this composition, the molecule has two of the three anionic ligands binding in a κ1 manner cis to each other, while the remaining siloxide is coordinated in a κ2 manner. Despite multiple attempts to crystallize 2-Sm, a single crystal of suitable quality could not be obtained; however, the unit cell, the first coordination sphere, and the position of the heavy atoms are similar to those found for 2-Nd. In the case of 2-Ce, one molecule is found in the unit cell. The molecule is sixcoordinate, and the neutral oxygen atoms attached to cerium form bonds longer than those of the negatively charged species [Ce1−O1 = 2.554(5) Å, Ce1−O2 = 2.690(4) Å, and in particular the Ce−O5 bond is the most elongated, 2.787(4) Å, vs Ce1−O3 = 2.207(5) Å, Ce1−O4 = 2.273(4) Å, and Ce1− O2 = 2.260(4) Å], with a consequent large distortion of the octahedron. 2-Nd shows the same trend but with shorter distances [Nd1−O1 = 2.504(8) Å, Nd1−O2 = 2.652(8) Å, and Nd1−O5 = 2.61(1) Å for neutral oxygen atoms vs Nd1−O3 = 2.32(1) Å, Nd1−O4 = 2.201(9) Å, and Nd1−O6 = 2.145(9) Å, respectively]. The ORTEP diagrams are shown in Figure 4. 2.2. Nuclear Magnetic Resonance (NMR) Properties. The siloxides were also investigated by solution NMR spectroscopy. The 1H NMR spectrum for 1-Sc shows a single resonance at 1.45 ppm showing that the siloxide ligands are equivalent at room temperature and that the siloxide ligands are fluxional. 1-Lu has a similar feature at 1.55 ppm along with a minor peak (5%) at 1.58 ppm. For 1-Yb, a single broad peak is observed in the 1H NMR spectrum due to the paramagnetic Yb center (all NMR spectra are given in the Supporting Information; solution NMR in Figures S1−S19 and solidstate NMR in Figures S20−S36). The dimers also display a single peak in their 1H NMR spectra, in contrast with literature data, where three resonances were reported for 1-La.59 However, dissolving a single crystal of 1-La in C6D6 gives a sharp single resonance at 1.47 ppm (Figure S6 of the Supporting Information), confirming that all the siloxide ligands are equivalent in solution at room temperature. For the paramagnetic 1-Sm and 1-Nd, the NMR spectra are broad and unresolved. In a C6D6 solution, the 1H NMR spectrum of the diamagnetic M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3) (2-Y, 2-Sc, and 2-Lu) shows two resonances, one major (93−99%) and

Scheme 3. Reaction of Lanthanide Amides with 4 equiv of HOSi(OtBu)3 To Give (a) M(OSi(OtBu)3)3(κ2HOSi(OtBu)3) or (b) M(OSi(OtBu)3)2(κ2-OSi(OtBu)3)(κ2HOSi(OtBu)3)

between these two structural isomers depends on the size of the lanthanide; smaller lanthanides form M(OSi(OtBu)3)3(κ2HOSi(OtBu) 3), while larger lanthanides form M(OSi(OtBu)3)2(κ2-OSi(OtBu)3)(κ2-HOSi(OtBu)3). The reaction of La(N(SiMe3)2)3 with 4 equiv of silanol gives only the dimeric 3:1 Si:La complex 1-La (Scheme 2 and Figure 2). The solid-state structures for 2-Sc and 2-Eu are shown in panels a and b of Figure 3, respectively; the structures of the 2Y and 2-Yb siloxides were previously reported.57 Bond lengths and angles are listed in Table 2. A single-crystal structure of 2Lu could not be obtained, though the unit cell and the position of the heavy atoms are similar to those found for 2-Sc and 2-Eu complexes. These complexes crystallize in a distorted squarebased pyramidal structure (dihedral angles of 56°, 61°, 61°, and 66° for 2-Sc, 2-Y, 2-Eu, and 2-Yb, respectively), and the same trends observed in the 3:1 Si:M compounds are observed. Thus, the three oxygen atoms that are κ1 and are negatively charged (O3−O5) form bonds shorter than those of the chelating neutral silanol oxygen atoms (O1 and O2); for example, Eu−O1 = 2.580(9) Å versus Eu−O5 = 2.139(9) Å, and Sc−O1 = 2.4226(9) Å versus Sc−O5 = 1.9211(7) Å. The C

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Table 2. Summary of the Bond Distances and Angles from X-ray Crystallography of the M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3) Adducts distances (Å) M3+ radius58 M−O1 M−O2 M−O3 M−O4 M−O5 angles (deg) O1−M−O2 O2−M−O3 O3−M−O4 ωa a

2-Sc

2-Y

2-Eu

2-Yb

0.89 2.4226(9) 2.2112(9) 1.9323(9) 1.9345(7) 1.9211(7)

1.04 2.522(5) 2.359(3) 2.096(3) 2.053(4) 2.165(5)

1.09 2.580(9) 2.457(9) 2.211(9) 2.148(9) 2.139(9)

1.01 2.475(3) 2.335(4) 2.087(4) 2.044(4) 2.095(4)

61.05(3) 93.88(3) 102.91(4) 56

58.30(4) 97.18(4) 108.59(5) 61

56.8(5) 97.5(5) 109.2(5) 61

59.6(1) 95.5(1) 108.6(1) 66

ω is the dihedral angle between the O1−M−O2 and O3−M−O4 planes.

Figure 4. ORTEP diagrams of M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3) and M(OSi(OtBu)3)2(κ2-OSi(OtBu)3)(κ2-HOSi(OtBu)3): (a) 2-Ce and (b) 2-Nd. Ellipsoids at the 50% probability level; hydrogen atoms and methyl groups omitted for the sake of clarity.

one minor (1−7%) (Figures S8, S11, and S13 of the Supporting Information). The major peak shows that all the methyl groups in the molecule are equivalent at room temperature. Therefore, it is fluxional on the NMR time scale. To gain further insights into the minor species, which is present in all M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3) molecules, additional spectroscopic experiments are required. Previous studies suggested that an equilibrium is present in solutions of 2-Y,57 as the 1H resonances are concentration-dependent. Moreover, the signal assigned to free HOSi(OtBu)3 appears when the temperature is lowered to −40 °C and increases in intensity when the temperature is reduced to −50 °C, indicating that the minor species is the silanol free 1-Y monomer (Figures S16 of the Supporting Information). This was confirmed by 1H−1H exchange spectroscopy (EXSY). At room temperature, the minor and major species correlate (Figure 5a). At −40 °C, the free silanol resonance (Figure 5b) correlates with the resonance of the major species. This behavior is consistent with a fast exchange between 2-Y and free HOSi(OtBu)3 rather than between 1-Y and free silanol, because of the smaller amount of 1-Y present in solution. Similar equilibria are present for the other lanthanide compounds that are diamagnetic; however, the paramagnetic compounds have only broadened resonances in their 1H NMR spectra. These spectra are available in the Supporting Information (Figures S17 and S18). 2-Y was also investigated by solid-state NMR spectroscopy. The 1H magic angle spinning (MAS) spectrum of the powdered solid shows a single broad peak centered at 1.4

Figure 5. (a) 1H−1H EXSY 1H NMR spectrum of 2-Y in d8-toluene recorded at 400 MHz and 298 K with a contact time of 0.3 s and a recycle time of 1 s. (b) 1H−1H EXSY 1H NMR spectrum of 2-Y in d8toluene recorded at 400 MHz and 223 K with a contact time of 0.3 s and a recycle time of 1 s.

ppm along with a smaller resonance at 11 ppm (Figure S32 of the Supporting Information), assigned to the -OH of the chelating silanol. The 13C cross-polarization magic angle spinning (CPMAS) spectrum shows five resonances in the -CH3 region (33.1, 32.5, 32.0, 31.7, and 30.5 ppm) and six in the tertiary carbon region (82.3, 76.1, 75.7, 72.5, 71.0, and 70.5 ppm) assigned to the OtBu groups (Figure S33 of the Supporting Information). The 29Si CPMAS spectrum of 2-Y D

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shows four peaks at −94.8, −97.5, −98.1, and −99.8 ppm (Figure 6; see Figure S34 of the Supporting Information for the

four ligands, and in the case of 1-Lu and 1-Yb (63° and 64°, respectively), the distortion is more pronounced than in 1-Sc (58°).

4. CONCLUSIONS We have shown that monomeric siloxide complexes can be prepared from all lanthanide amides on gram scales with the exception of lanthanum, which results exclusively in a dimeric complex under the experimental conditions used. It is noteworthy that homoleptic tris-siloxide pentacoordinated complexes can be obtained for the smaller ions (M = Sc, Yb, and Lu) because two of the siloxides adopt κ2 coordination. For larger lanthanides, a fourth equivalent of (HOSi(OtBu)3)3 is necessary to generate a stable pentacoordinated monomeric complex, in which the fourth silanol acts as a weakly bound readily displaceable κ2-stabilizing ligand.57 We are currently exploring the reactivity of these complexes and using them as precursors for the synthesis of new materials.

Figure 6. 29Si cross-polarization magic angle spinning (CP MAS) spectrum of 2-Y recorded at a MAS frequency of 5 kHz; 512 scans were recorded with a cross-polarization time of 7 ms and a recycle time of 1 s.

5. EXPERIMENTAL SECTION 5.1. General Procedure. All the experiments were conducted in a dry, oxygen free argon environment using Schlenk and glovebox techniques. Dichloromethane, pentane, and toluene were purified using MBraun SPS columns, degassed before being used, and stored over molecular sieves. Deuterated solvents C7D8 and C6D6 were dried over and distilled from sodium, and CD2Cl2 was dried over and distilled from from P2O5. The M(N(SiMe3)2)3 precursors were synthesized using a modified literature procedure32 that involved sublimation at 120 °C and 10−5 mbar followed by crystallization from pentane. The 1H, 13C, and 29Si NMR spectra were obtained on Bruker DRX 200, DRX 250, DRX 300, DRX 400, and DRX 500 spectrometers. Solution spectra were recorded in the given solvent at room temperature unless specified otherwise; the 1H, 13C, and 29Si chemical shifts are reported in δ values relative to Me4Si. For the solidstate spectra, a Bruker DRX 700 instrument was used. The MAS frequency was set at 10 kHz for all 1H and 13C spectra and 5 kHz for 29 Si, and the samples were contained in a 4 mm zirconia rotor prepared in the glovebox. Elemental analyses were performed by mikroanalytisches Labor Pascher, Germany. 5.2. X-ray Crystallography. The crystals were placed in paratone and mounted in the beam under a flow of nitrogen at 100 K on a Bruker SMART APEX II diffractometer equipped with a CCD area detector using Mo Kα radiation. An empirical absorption correction was performed with SADABS-2008/1 (Bruker) in all cases. The structures were determined by direct methods or Patterson methods (SHELXS-97) followed by least-squares refinement (SHELXL-97) using the OLEX 2−1.2 suite of programs.60 The non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed at calculated positions; the details are available in the Supporting Information. 5.3. Synthetic Procedures for the Synthesis of Trisiloxy M Complexes. A representative procedure is shown for 1-Sc and 2-Sc; the detailed synthetic procedures for each 1-M and 2-M can be found in the Supporting Information. 5.3.1. Reaction of M(N(SiMe3)2)3 with 3 equiv of HOSi(OtBu)3 (1M) (1-Sc) Sc(OSi(OtBu)3)3. To a solution of 1.00 g of Sc(N(SiMe3)2)3 (1.90 mmol, 1.0 equiv) in 40 mL of CH2Cl2 was added a solution of 1.53 g of HOSi(OtBu)3 (5.70 mmol, 3 equiv) in 40 mL of CH2Cl2. After the mixture had been stirred for 8 h, the solvent was removed under reduced pressure, and the residue was extracted with dichloromethane leaving an insoluble solid. The filtrate was concentrated to half of its initial volume and cooled at −40 °C. After 24 h, large transparent blocks appeared, which were collected by filtration, dried under reduced pressure, and stored under argon: yield 1.2 g (74%); 1H NMR (300 MHz, C6D6) δ 1.46 (99%) along with minor species at δ 1.49 (1%); 13C NMR (75 MHz, C6D6) δ 73.8 (CtBu), 31.5 (CH3tBu); 29Si NMR (60 MHz, C6D6) δ −94.6. Crystals

entire spectrum). All these data are consistent with four distinct -OSi(OtBu)3 groups as observed in the single-crystal structure; these results show the utility of solid-state NMR spectroscopy as a structural tool for these diamagnetic molecules, which are fluxional in solution.

3. DISCUSSION The common theme in the 3:1 and 4:1 structures is that the smaller lanthanides are five-coordinate monomers. In the case of the 3:1 complexes M(OSi(OtBu)3)3 (1-Yb, 1-Lu, and 1-Sc), the trend in M−O distances follows the ionic radius (Table 1), and generally, the neutral M−OtBu distances are longer than the anionic M−OSi distances by approximately 0.2 Å. When the ion becomes too large to accommodate the same coordination sphere, the complexes dimerize, forming structures with two bridging μ2 siloxides in the cases of 1Sm, 1-Nd, and 1-La. In the M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3) adducts, the 3+ M −O distance in the solid state (2-Sc, 2-Y, 2-Eu, and 2Yb) follows the trend of the metal ion radius (Table 2); however, when the ionic radius reaches the critical value of 1.09 Å (M = Sm), the molecule adopts a distorted six-coordinated octahedral geometry in the solid state. This is observed for M = Sm, Nd, and Ce, while for M = La, even the presence of an additional 1 equiv of HOSi(OtBu)3)3 does not yield the adduct; however, the dimer 1-La is isolated. This result suggests that when the ion becomes too large to accommodate the same coordination geometry, the dimeric structures with two μ2 siloxides form. When five- and six-coordinate compounds are isolated, the M−O bond distances also follow the established trend in metal radii for these metals; i.e., they contract across the series. Thus, as the radii become smaller, the coordination numbers decrease to minimize the increasing intramolecular repulsions. In general, the M−O bonds of the siloxides, which are not chelating, are slightly shorter than those of the κ2 siloxides or silanols. Note that the dihedral planes are similar in both the 3:1 and 4:1 ratios, because the distances are slightly shorter for the M(OSi(OtBu)3)3 complexes, which is not surprising because the M3+ ion must accommodate three rather than E

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suitable for X-ray diffraction were grown by slow concentration of the mother liquor and overnight crystallization at −40 °C. Anal. Calcd for C36H91O12ScSi3: C, 51.77%wt; H, 9.77%wt. Found: C, 51.89%wt; H, 9.93%wt. 5.3.2. Reaction of M(N(SiMe3)2)3 with 4 equiv of HOSi(OtBu)3 (2M) (2-Sc) Sc(OSi(OtBu)3)3(κ2-HOSi(OtBu)3). The compound was prepared in 82% yield. To a solution of 1.00 g of Sc(N(SiMe3)2)3 (1.90 mmol, 1.0 equiv) in 20 mL of CH2Cl2 was added a solution of 3.8 g of HOSi(OtBu)3 (7.6 mmol, 4 equiv) in 20 mL of CH2Cl2. After the mixture had been stirred for 8 h, half of the solvent was evaporated under reduced pressure and the solution was cooled at −40 °C. After 1 day, large transparent blocks were obtained, collected by filtration, dried under reduced pressure, and stored under argon. Crystals suitable for single-crystal diffraction were grown by slow concentration of the mother liquor and overnight crystallization at −40 °C: 1H NMR (300 MHz, C6D6) δ 1.44 (99%) along with minor species at δ 1.40 (1%); 13C NMR (75 MHz, C6D6) δ 72.6 (CtBu), 32.2 (CH3tBu); 29Si NMR (60 MHz, C6D6) δ −98.2. Anal. Calcd for C48H109O16ScSi4: C, 52.43%wt; H, 9.99%wt. Found: C, 51.75%wt; H, 10.14%wt.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data and spectra and one CIF file. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Swiss National Foundation for financial support (SNF200021_137691/1) and Florian Allouche for fruitful discussion.

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DEDICATION This article is dedicated to the memory of M. F. Lappert for his contributions to lanthanide organometallic and coordination chemistry.

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