Homoleptic Trivalent Tris(Alkyl) Rare Earth Compounds.

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Homoleptic Trivalent Tris(Alkyl) Rare Earth Compounds Aradhana Pindwal, KaKing Yan, Smita Patnaik, Bradley M Schmidt, Arkady Ellern, Igor I. Slowing, Cheolbeom Bae, and Aaron D. Sadow J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09521 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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Journal of the American Chemical Society

Homoleptic Trivalent Tris(Alkyl) Rare Earth Compounds Aradhana Pindwal,a,b KaKing Yan,a,b Smita Patnaik,a,b Bradley M. Schmidt,a,b Arkady Ellern,b Igor I. Slowing,a,b Cheolbeom Bae,c Aaron D. Sadowa,b,* a

US DOE Ames Laboratory, Iowa State University, Ames, Iowa 50011, United States Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, Iowa 50011, United States c Kumho Petrochemical R&D Center, Daejeon 34044, Republic of Korea b

ABSTRACT: Homoleptic tris(alkyl) rare earth complexes Ln{C(SiHMe2)3}3 (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) are synthesized in high yield from LnI3THFn and 3 equiv. of KC(SiHMe2)3. X-ray diffraction studies reveal 1a-d are isostructural, pseudo-C3symmetric molecules that contain two secondary Ln↼HSi interactions per alkyl ligand (six total). Spectroscopic assignments are supported by comparison with Ln{C(SiDMe2)3}3 and DFT calculations. The Ln↼HSi and terminal SiH exchange rapidly on the NMR timescale at room temperature but the two motifs are resolved at low temperature. Variable temperature NMR studies provide activation parameters for the exchange process in 1a (∆H‡ = 8.2(4) kcal·mol–1; ∆S‡ = –1(2) cal·mol–1K–1) and 1a-d9 (∆H‡ = 7.7(3) kcal·mol–1; ∆S‡ = –4(2) cal·mol–1K–1). Comparisons of lineshapes, rate constants (kH/kD), and slopes of ln(k/T) vs. 1/T plots for 1a and 1a-d9 reveal that an inverse isotope effect dominates at low temperature. DFT calculations identify four low energy intermediates containing five β-Si–H⇀Ln and one γ-C–H⇀Ln and suggest the pathway for Ln↼HSi/SiH exchange involves rotation of a single C(SiHMe2)3 ligand that is coordinated to the Ln center through the La–C bond and one secondary interaction. In addition, these robust organometallic compounds persist in solution and in the solid state up to 80 °C, providing potential for their use in a range of synthetic applications. For example, reactions of Ln{C(SiHMe2)3}3 and ancillary proligands, such as bis-1,1-(4,4dimethyl-2-oxazolinyl)ethane (HMeC(OxMe2)2) give {MeC(OxMe2)2}Ln{C(SiHMe2)3}2, and reactions with disilazanes provide solvent-free lanthanoid tris(disilazides).

the lanthanides,13 and often the compounds of these elements exhibit the greatest activity and selectivity in organometallictype catalytic reactions such as hydrosilylation and hydroamination.14 Thus, new homoleptic complexes are needed to develop lanthanum and cerium starting materials for myriad potential applications. Ligand design strategies have sought to overcome these difficulties (Chart 1). Organolanthanide complexes with multidetatate ligands, such as α-metalated N,Ndimethylbenzylamine or ortho-dimethylaminobenzyl, persist

INTRODUCTION Homoleptic organometallic compounds, which contain only one type of ligand bonded to a metal center,1,2 isolate fundamental features of metal-ligand bonding for study. These species often contain unusual geometric and spectroscopic features, such as trigonal prismatic WMe6 or pyramidal Ln{CH(SiMe3)2}3,2h, 3 challenging our notions about structure, bonding, and secondary metal-ligand interactions.4 Such compounds, including organometallic species with only metalcarbon bonds, are also valuable in synthetic chemistry as reagents or homogeneous precatalysts,5,6,7 well-defined starting materials for single-site grafted catalytic species,8 precursors for materials in chemical vapor deposition or atomic layer deposition processes,9 and for combination with a range of ancillary ligands as entry-points into reactive organometallic compounds.10 Organolanthanide homoleptics are typically coordinated by β-hydrogen-free alkyl ligands, namely CH(SiMe3)2,3a CH2SiMe3,11 and CH2Aryl,12 although compounds of the latter two ligands typically contain additional donors. The applications of such organometallic compounds, particularly those of the abundant early lanthanides (La, Ce, Pr, Nd), are limited by their relatively short lifetimes at room temperature, challenging multistep syntheses, and the difficulty to exclude THF or salt byproducts from the metal center's coordination sphere. For example, La(CH2Ph)3THF3 and Ce(CH2Ph)3THF3 readily extrude toluene at room temperature with half-lives of a few hours. Notably, these elements are the most earth abundant of

Chart 1. Rare earth homoleptic alkyls supported by peripheral coordination or steric bulk. 1

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at room temperature.15,16 A second strategy relies on bulky ligands, such as CH(SiMe3)2 mentioned above, which provides Ln{CH(SiMe3)2}3 (Ln = La, Ce, Y, Lu).3a, 17 The largest lanthanide compounds’ syntheses are challenging, and their applications have been limited. Alternatively, the substantial steric profile of C(SiMe3)3 (trisyl) has thus far restricted its chemistry to divalent dialkyl lanthanides.18 These trimethylsilyl derivatives contain secondary Ln↼Me–Si interactions and adopt distorted solid-state structures. For example, Yb{C(SiMe3)3}2 is bent (C–Yb–C 137°) rather than linear,18a and Ln{CH(SiMe3)2}3 (Ln = La, Lu) are pyramidal (∑CLnC = 324 - 330°) rather than planar.3a, 8f Zwitterionic homoleptic tetramethyl aluminates Ln(AlMe4)3 provide starting materials for accessing novel organolanthanide compounds, including LuMe3 and YMe3.10c, 10g, 19 Organo-rare earth compounds, however, are not necessarily limited to β-hydrogen-free alkyls, and this idea could greatly expand the kinds of ligands available for homoleptic species. For example, isolable [LntBu4]–2f, 20 and Cp2LutBu(THF)21 eliminate isobutylene only under relatively forcing conditions. In catalytic ethylene polymerizations, ultra-high molecular weight products are obtained with rare earth catalysts, partly because β-hydrogen elimination is slow.22 β-agostic CH groups in alkyls are proposed intermediates in β-hydrogen elimination; however, the presence of β-hydrogen may even stabilize reactive alkyl groups, as in Cp*2ScEt23 and other agostic compounds.24 In fact, an alternative approach for stabilizing metal centers in homoleptic compounds could involve β-hydrogencontaining ligands that form bridging Ln↼H–Si interactions. The polarization of the Si–H bond is inverted in comparison to that of C–H bonds, thus we refer to bridging Ln↼H–Si structures as three center-two electron (3c-2e), nonclassical, or agostic-like interactions rather than strictly agostic.25 There are structural and spectroscopic similarities between β-agostic CH and agostic-like SiH; however, the M↼H–Si interaction in silazides of high valent d0 metal centers (including rare earth elements) has been suggested to involve polarization that increases the negative charge on the β-H, resulting in metalligand bonding dominated by electrostatic effects.26 This β-SiH strategy has been utilized primarily in ligands with Ln–N bonds such as tetramethyldisilazide27 and t-butyl dimethylsilazide.28 Early metal and rare earth silazides containing β-SiH often form agostic-like structures evident from low energy Si–H vibrations in IR spectra, low 1JSiH values in 1 H or 29Si NMR spectra, and acute Ln–N–Si angles in solidstate structures.27a, 27c, 29 For example, the crystal structures of Ln{N(SiHMe2)tBu}3 (Ln = Sc, Y, Lu, Er) reveal secondary interactions that are also reflected in characteristic low energy νSiH bands in infrared spectra.28b, 28c The Ln centers are pyramidalized in these silazido compounds, as are the metal centers in Ln{N(SiMe3)2}3 and the alkyl Ln{CH(SiMe3)2}3 compounds described above,30 and all of these species exhibit secondary interactions between rare earth center and the SiMe groups. Very bulky disilazido ligands, such as N(SiMe2tBu)2, however, afford planar Ln{N(SiMe2tBu)2}3 with long Ln–Me– Si distances.31 A few tetramethyldisilazido ansa-lanthanidocene compounds form unusual diagostic-like β-H–Si interactions,32 although this motif is not observed in most homoleptic tetramethyldisilazido rare earth compounds. Instead, additional donors such as THF give Ln{N(SiHMe2)2}3L27a or dimeric species

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[Ln{N(SiHMe2)2}3]2 form.33 In substitution reactions, the amine byproduct of displaced amide ligands may affect subsequent chemistry through its action as a base or ligand, and silazanes may act as silylating agents in reaction mixtures. Instead, an alkyl ligand design strategy based on βdimethylsilyl groups combines the stabilization of 3c-2e Ln↼H–Si, the reactivity of M–C bonds, and inert hydrocarbon byproducts of substitution reactions. Experimental studies and theoretical analysis of corresponding alkyl ligands containing bridging β-SiH groups are limited,34 particularly for structural, spectroscopic, and reactivity comparisons with β-agostic CH groups. In yttrium tris(alkyl) Y{C(SiHMe2)3}3,35 evidence for six Y↼H–Si interactions was provided by low temperature NMR spectroscopy, but crystallographic characterization of the structure was elusive so the coordination geometry, ligand structure, and bonding could not be assessed. Crystallographically characterized bis(alkyl) M{C(SiHMe2)3}2THF2 (M = Ca, Yb)34b, 35-36 also contain nonclassical M↼H–Si interactions, although the fluxionality of these compound hindered solution-phase analysis. Recently, we communicated the synthesis and structure of the cerium compound Ce{C(SiHMe2)3}3 (1b) which supported the spectroscopic assignment of Y{C(SiHMe2)3}3. Compound 1b is a precursor for the zwitterionic complex Ce{C(SiHMe2)3}2HB(C6F5)3; the latter compound catalyzes hydrosilylation of acrylates to give α-silyl esters.37 In addition, we reported an alkyl ligand that combines features of benzyl and the β-SiHMe2, namely C(SiHMe2)2Ph and a series of its homoleptic lanthanide compounds in which the LaC3 core is trigonal planar.38 Here, we report the full synthesis of the lightest and largest homoleptic tris(alkyl) complexes (La, Ce, Pr, and Nd). We sought easily synthesized, solvent-free, homoleptic compounds of the early rare earth elements. Lanthanum has the largest ionic radius of trivalent and highly electropositive 4f metal centers, making synthesis of homoleptic donor-free alkyls particularly challenging. In addition, we sought to compare the spectroscopic properties of Ln{C(SiHMe2)3}3 compounds, experimentally characterize structures and energetics associated with the compounds’ fluxionality, combine those spectroscopic and kinetic studies with a detailed theoretical analysis, and demonstrate the compounds' applications as synthetic precursors.

RESULTS AND DISCUSSION Synthesis and Characterization of Ln{C(SiHMe2)3}3 (Ln = La (1a), Ce (1b), Pr (1c), Nd (1d)). The homoleptic rare earth tris(alkyl) complexes Ln{C(SiHMe2)3}3 are synthesized in high yield by reaction of LnI3(THF)n (n = 4 for La, Ce; x = 3 for Pr, Nd) and 3 equiv. of KC(SiHMe2)3 in benzene for 1.5 12 h at room temperature (eq. 1). Reaction times (e.g., 1.5 h for La and 12 h for Nd) were evaluated based on color changes from red (of KC(SiHMe2)3) to the color of the isolated product.

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Crude La{C(SiHMe2)3}3 (1a) and Ce{C(SiHMe2)3}3 (1b) are obtained as pale yellowish solids and form hexagonshaped, colorless crystals upon recrystallization from pentane. Pr{C(SiHMe2)3}3 (1c) crystallizes as green plate-like crystals from toluene, while Nd{C(SiHMe2)3}3 (1d) forms blue blocky crystals from pentane. In contrast to lanthanide iodide precursors, the combination of anhydrous rare earth halides LaBr3, LaCl3, CeCl3, or NdCl3 and KC(SiHMe2)3 in benzene or THF at room temperature does not provide detectable quantities of the corresponding organometallic compounds. YCl3, however, reacts with KC(SiHMe2)3 to afford Y{C(SiHMe2)3}3.35 The identities of 1a-d are unambiguously established by single crystal X-ray diffraction studies (see below) and elemental analysis. The selectively isotopically-labeled compounds Ln{C(SiDMe2)3}3 (1a-d-d9) are synthesized from KC(SiDMe2)3.34b The labeled species facilitate characterization and study of the fluxional processes of 1a-d. The similar infrared spectra for 1a-d (Figure 1) suggested an isostructural series, and the νSiH region from 1800 - 2200 cm–1 was particularly informative. All four spectra contained a band at ca. 2107 cm–1 assigned to the stretching mode of a 2c2e SiH group. The spectra also contained a broad, lower energy band at ~1830 cm–1 assigned to the SiH stretching mode in the bridging Ln↼H–Si moiety. These assignments were

were not SiH bending modes, and were assigned to CH3 bending modes on the basis of DFT calculations. The 1H NMR spectrum of diamagnetic 1a, acquired at room temperature in benzene-d6, contained doublet and multiplet resonances at 0.41 ppm (54 H, 3JHH = 3.4 Hz) and 4.26 ppm (9 H, 1JSiH = 137 Hz) in a 6:1 integrated ratio. The 1H NMR spectrum of 1a-d9 revealed only a singlet at 0.41 ppm, while the 2H NMR spectrum showed a resonance at 4.26 ppm. The room temperature 1H NMR spectra of paramagnetic 1b-1d contained signals in the diamagnetic region resulting from exchangeaveraged signals, see below. The 1H NMR spectrum of 1b showed two broad peaks at 1.0 (54 H, 89 Hz width at halfheight) and 4.9 ppm (9 H, 169 Hz at half-height) assigned to SiMe2 and SiH, respectively.37 The 1H and 2H NMR spectra of 1b-d9 supported these assignments. Also, the 1H NMR spectra of 1c and 1d each contained two resonances. The SiMe groups in each were assigned to broad 1H NMR signals with similar chemical shifts of 2.2 (540 Hz at half-height) and 1.8 ppm (775 Hz at half-height) for praseodymium and neodymium, respectively. The second peak, attributed to the SiH group, exhibited a greater averaged paramagnetic chemical shift for Nd (27.8 ppm) than Pr (12 ppm), giving the trend Ce (f1) < Pr (f2) < Nd (f3). The room temperature 13C{1H} NMR spectrum of 1a, acquired in benzene-d6, contained resonances at 31.8 and 3.6 ppm assigned to LaC and Me, respectively. The corresponding spectrum of 1b revealed only a single resonance at 11.5 ppm, while signals were not detected in spectra for 1c and 1d. A resonance at –13.1 ppm was observed in the 29Si INEPT NMR spectrum of 1a, while no peaks appeared in the analogous spectra of 1b-d. Single Crystal X-ray Diffraction Studies. Compounds 1ad are highly crystalline in pure form, and X-ray diffraction experiments provide the molecular structures (see Figure 2 for 1a and Supporting Information (SI) for selected distances and crystallographic information files (CIF) for 1a-d). While all four compounds crystallize in the P-1 space group (Z = 2), only 1a-c are isostructural and contain only the Ln{C(SiHMe2)3}3 molecule in the asymmetric unit. Compound 1d, with Nd as the smallest metal center of the series, crystallizes with a similar but not identical molecular structure and a benzene molecule in the unit cell. The difference between La-Pr vs. Nd structures involves the relative positions of nonbridging SiH. Important structural features of the Ln{C(SiHMe2)3}3 compounds include the configuration and symmetry of the molecules, the geometry of the C(SiHMe2)3 ligands, and the six nonclassical Ln↼H–Si motifs. The geometry of the LnC3 core is trigonal planar, as assessed by the ∠C–Ln–C which vary from 119.04(8) to 121.14(8)° and ∑C–Ln–C = 359.9°. The six H atoms involved in Ln↼H–Si, located objectively in the Fourier electron difference map, form an oblique trigonal prismatic structure. Likewise, the Si centers involved in Ln↼H–Si have short La…Si distances, and these six Si centers are also vertices of an oblique trigonal prism that surrounds the Ln center. The C1–C3 atoms cap the quadrilateral faces and are offset from the centroid average of the vertices. These three quadrilateral faces are related by pseudo-C3 operations, as are the C(SiHMe2)3 ligands, while the two end-capping triangles are related by a pseudo-mirror plane that includes the Ln1 and C1C3 atoms, as well as Si3, Si6, and Si9 atoms. For 1a-c, the nonbridging SiHMe2 groups of Si3, Si6, and Si9 are also relat-

Figure 1. Infrared spectra of SiH region for Ln{C(SiHMe2)3}3 (La: 1a; Ce: 1b; Pr: 1c; Nd: 1d).

supported by IR spectra of isotopomers 1a-d-d9 and DFT calculations (see below). Thus, the IR spectra of 1a-d-d9 contained bands grouped at ~1530 and 1324-29 cm–1 while the νSiH signals noted above were not observed (for 1a and 1a-d9, compare Figures S8 and S10). The IR spectrum of 1a also contained a broad, non-Gaussian-shaped signal at 1024 cm–1 which was replaced by a broad signal at 680 cm–1 in the spectrum of 1a-d9. The effect of isotopic substitution on these signals indicates their association with SiH/SiD functionality. On the basis of DFT calculations described below, the latter signals are characterized as modes of La↼H–Si moieties, specifically identified as bending motions of the Si–H along the vectors expected for La–H stretching motion (see Figure 3A and DFT discussion below). Only two other signals are evident in the region from 1100 to 1800 cm–1. These signals, at 1254 and 1418 cm–1, were present in IR spectra of 1a and 1a-d9, clearly 3



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ed by C3 operations, and the three groups are oriented with their SiHs pointing toward one side of the LaC3 plane (rather than remaining in the plane as required for perfect C3h symmetry) to give pseudo-C3 symmetry. In addition to the cocrystallized benzene molecule, the Nd compound 1d is distinct from 1a-c in its conformation of the C(SiHMe2)3 ligands. While Si3, Si6, and Si9 atoms of the SiHMe2 groups are contained in the Nd1, C1, C2, and C3 plane, only two SiH groups are pointed to one side of the plane, while the third points to the other. Thus, 1a-d adopt non-crystallographically imposed geometries that approach C3h symmetry. A C3-symmetric structure was predicted in the gas-phase DFT calculations of Y{C(SiHMe2)3}3 by Lein and Harrison.34a

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Ce{CH(SiMe3)2}3, similar to that given above for the lanthanum analogues, reveals up to 0.2 Å longer Ce–C distances in 1b. In addition, the compounds Ce{CH(SiMe3)SiMe2OMe}317 and Ce(CH2Ph)3THF312c also crystallize with shorter distances (2.598(4) Å and 2.608(2) Å, respectively) than the Ce–C ditances in 1b, although they are not as short as in Ce{CH(SiMe3)2}3. Each C(SiHMe2)3 ligand in 1a also contains two short La…H and two short La…Si distances, and the positions of these atoms generate the oblique trigonal prisms described above. The La…H distances range from 2.42(3) to 2.50(3) Å, and these distances are shorter than the La…H distances of 2.70(3) and 2.66(4) Å in diagostic tetramethyldisilazido lanthanum compound Me2Si(C5Me4)2LaN(SiHMe2)232 but within 3σ error of distances in the corresponding solvent-free dimeric disilazido [La{N(SiHMe2)2}3]2 (2.56(6) Å).33 The La…Si distances in 1a range from 3.2126(7) to 3.2500(8) Å, are only slightly longer than the sum of La and Si covalent radii (3.18 Å),41 and are equivalent to the distances in Me2Si(C5Me4)2LaN(SiHMe2)232 (3.244(1) and 3.246(1) Å). The La…Si distances in the β-SiC agostic Cp*La{CH(SiMe3)2}2 of 3.35(1) and 3.42(1) Å are longer than in 1a.42 Additional structural evidence for a multicenter interaction between La and the C(SiHMe2)3 ligands comes from ∠La–C–Si, which are ≤90° for the SiHMe2 groups with close La…H and La…Si contacts. These groups also have small ∠La–C–Si–H torsion angles, which vary from 0.36 – 10.68°. Distortions of the C(SiHMe2)3 group accompany the secondary interactions with the Ln center. The Cα–Si distances in La↼H–Si structures are generally shorter than those of classical SiHMe2 groups. Across the 1a-d series, the average Cα–Si distance for Ln↼H–Si is 1.828 ± 0.006 Å, whereas the average Cα–Si distance associated with nonbridging SiH groups is 1.849 ± 0.003 Å. In one C(SiHMe2)3 ligand of 1a as an example, the C3–Si7 and C3–Si8 distances of 1.820(1) and 1.832(2) Å for bridging La↼H–Si are shorter than the C3–Si9 distance of 1.854(2) Å in the nonbridging Si. In contrast to 1a-d, previously characterized Yb{C(SiHMe2)3}2THF2 and YbC(SiHMe2)3HB(C6F5)3THF2 do not contain lengthened Si– H bonds or shortened Si–C bonds in their nonclassical groups.36 The neutron structure of Cp*La{CH(SiMe3)2}2 shows statistically longer Si–C bonds for groups involved in secondary interactions.42 The Si–H distances of Ln↼H–Si tend to be longer than those of classical silyl hydrides, however the groups cannot be distinguished within 3σ error. The angles associated with the central carbon in the C(SiHMe2)3 ligands also highlight its distorted geometry. In general, acute or nearly acute ∠Ln–C–Si (averaged over 1a-d: 88.8±0.9°) are associated with the bridging Ln↼H–Si structures, while angles associated with nonbridging Si are obtuse and show greater variation (average over 1a-d: 124±4°). The sum of the ∠Si–C–Si around the central carbon averaged for 1a-d is 350±2°, indicating the central carbon is significantly distorted from an idealized tetrahedral geometry toward planarity. DFT Calculations of Ground State Structure(s). The series of compounds Ln{C(SiHMe2)3}3 were modeled using density functional theory (DFT). The lowest energy calculated structure for each rare earth element contains six Ln↼H–Si arranged in a pseudo-C3 symmetric configuration (species A, see Figures 3A and S37). The calculated La…H distances are longer than crystallographically estimated distances, and these

Figure 2. Thermal ellipsoid plot of La{C(SiHMe2)3}3 (1a) along the C3 axis at 35% probability. H atoms bonded to Si were located objectively in the Fourier difference map and are included in the representation. All other H atoms are not plotted for clarity. Dashed curves highlight short La–H distances.

The trigonal planar geometry of 1a-d contrasts the pyramidal Ln{CH(SiMe3)2}3 and Ln{N(SiMe3)2}3. The C–La–C angles in La{CH(SiMe3)2}3, for example, are ca. 110° (∑ = 330°).3a That structure contains three equivalent and short La– Cα distances (2.515(9) Å) and three La–Me close contacts (~3.12 Å) described as β-agostic La↼Me–Si interactions. The distortion from expected VSEPR geometry is attributed to a second-order Jahn-Teller effect resulting from partial occupation of d orbitals.4d, 39 A similar pyramidal geometry is reported for solid-state structures of Ln{N(SiMe3)2}3, and these also contain β-agostic La↼Me–Si interactions.30, 40 The La–Cα distances (La1–C1, 2.697(2), La1–C2, 2.679(2), La1–C3, 2.684(2) Å) in 1a are long, ca. 0.16 Å longer than the distances in La{CH(SiMe3)2}3.3a Six-coordinate La(CH2Ph)3THF3 (2.648 ± 0.002 Å), La(CH2C6H4-4Me)3THF3 (2.627 ± 0.002 Å),12a La{CH(NMe2)Ph}3 (2.65 ± 0.01 Å)15 and La(CH2C6H4NMe2)3 (2.64 ± 0.01 Å)16 also contain La–C interatomic distances that are shorter than in 1a. The Ln–C distances follow the trends expected on the basis of the ionic radii of the metal center (La > Ce > Pr > Nd). The distances in Ce{C(SiHMe2)3}3 of 2.651(2), 2.659(2), and 2.672(2) Å37 are shorter than 1a. A comparison of 1b and 4


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are grouped into sets based on their location with respect to the LaC3 plane. The La…H distances from one side of the plane are 2.50 Å, while the distances on the other side of the plane are 2.62 Å. Calculated vibrational frequencies of Si–H stretching motions are 2220, 2215, and 2214 (Si–H) and 1951, 1938, 1933, 1925, 1920, and 1916 cm–1 (La↼H–Si). The large range (1951 – 1916 cm–1; mean, 1930 cm–1) for the La↼H–Si, narrow range for the Si–H, and difference between the two means (calc., 286 cm–1) are in good agreement with the experimental data (difference in maxima, 277 cm–1). These calculations validate the assignments of infrared frequencies to Ln↼H–Si structural features observed in the X-ray structures of La, Ce, and Pr compounds. In addition, six frequencies in the Hessian calculation at 1103, 1085, 1076, 1023, 1012, and 1004 cm–1 correspond to vibrations of the La↼H–Si. Vectors of the A-type symmetric breathing mode (1103 cm–1) are shown in Figure 3 for A, in which the groups with longer La…H distances (H88, H91, and H93) have greater intensity. The other A-type mode appeared at 1023 cm–1, corresponding to a greater intensity motion of the shorter La…H (H86, H89, H92). The other modes are asymmetric, and appear to move with E-like displacements (although the modes are not degenerate). Bending modes of nonbridging SiH groups were only observed at lower energy, from 974 – 963 and 907 – 888 cm–1, in DFT-calculated IR spectra of La{C(SiHMe2)3}3, and the modes over this energy range contained SiH bends of both bridging and nonbridging SiH groups mixed with other atomic motion. In X-ray crystallographic analysis, Si–H interatomic distances in terminal and bridged structures were indistinguishable outside of error. However, the distances determined by DFT calculations show that Si–H in La↼H–Si are longer (1.545 Å) than in nonbridging groups (1.491 Å). The effect of La↼H–Si on Cα–Si distances revealed in the X-ray crystallographic studies are duplicated in the DFT calculations, with shorter (1.83 Å) distances for Cα–Si of bridging SiH moieties and longer (1.85 Å) for Cα–Si of nonbridging SiH. The conformation of a second minimum La{C(SiHMe2)3}3B, located only 0.57 kcal/mol (∆G) above A, corresponds to the structure of Nd{C(SiHMe2)3}3 determined by an X-ray diffraction study. The difference between A and B is the rotation of the C61–Si10 bond. As in A, there are long and short La…H distances in each C(SiHMe2)3 ligand in B (2.53 vs 2.59 Å; 2.53 vs 2.55; 2.51 vs 2.59 Å). However, the difference in La…H distances is smaller than in A, and short and long La…H distances are mixed above and below the LaC3 plane. Variable Temperature NMR Studies. In the previously reported yttrium analogue Y{C(SiHMe2)3}3, the room temperature 1H NMR spectrum contained one SiH resonance with a low 1JSiH value (134 Hz).35 The SiH were resolved at –80 °C into two signals that were classified as 2c-2e (4.71 ppm, 1JSiH = 190 Hz) and 3c-2e (3.4 ppm, 1JSiH = 108 Hz) on the basis of chemical shift and coupling constant. The similarly low 1JSiH value of 1a at room temperature (137 Hz) and its solid-state structure (determined at low temperature) suggested inequivalent SiHMe2 groups. Therefore, variable temperature NMR experiments were performed on 1a. As the temperature of a sample in toluene-d8 was lowered, the SiH resonance broadened to the coalescence point at 210 K. At 190 K, two SiH resonances at 4.78 ppm (3 H, 1JSiH = 186 Hz) and 4.04 ppm (6 H, 1JSiH = 114 Hz) and three methyl resonances at 0.55, 0.49, and 0.42 ppm (18 H each) were resolved, yet broad. The

Figure 3. Illustration of the interconversion of two minima of La{C(SiHMe2)3}3, A and B and transition state TS-AB. The nonbridging SiH of A are all on one side of a plane defined by La, C11, C36, and C61, whereas only two of the nonbridging SiH of B are on the same side of that same plane. The nonbridging H bonded to Si are highlighted in the above images.

downfield signal was assigned to a 2c-2e SiH bond on the basis of its similar 1JSiH value to that of HC(SiHMe2)3 (4.33 ppm, 180 Hz), whereas the upfield-shifted peak at 4.04 ppm was assigned to SiH groups participating in secondary interactions. A COSY experiment performed on 1a at 190 K contained correlations from two SiMe2 resonances at 0.49 and 0.55 ppm with the upfield SiH at 4.04 ppm, thus assigning these signals to the SiHMe2 groups involved in agostic-like interactions with the La center. The upfield methyl signal at 0.42 ppm was assigned to the normal SiHMe2 group. 5



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The room temperature chemical shifts of 1b also represent signals averaged by a fast exchange process. The 1H NMR spectrum acquired at 200 K contained sharp, paramagnetically shifted signals at 10.6, 9.3 and –11.6 ppm (18 H each) and broad resonances at 24.5 and –19.7 ppm assigned to classical and nonclassical SiH, respectively. These signals were assigned on the basis of their 1:2 integrated ratio and their absence in corresponding spectra of 1b-d9. Note that the weighted average of these signals resulting from fast exchange placed the two peaks in the diamagnetic region of the room temperature spectrum. Only four resonances were resolved in the low temperature 1H NMR spectra of 1c and 1d rather than the five signals in 1a and 1b, and the missing peak was likely associated with the bridging Ln↼H–Si moiety. Likewise, the 13C{1H} NMR spectrum of 1a acquired at 190 K in toluene-d8 contained three upfield resonances at 4.2, 3.2, and 2.1 ppm and one downfield resonance at 30.0 ppm (assigned as noted above). The latter signal was more shielded than La{CH(SiMe3)2}3 (75.2 ppm),3a but downfield of Y{C(SiHMe2)3}3 (r.t., 17.3 ppm). The two most upfield 13C NMR signals were assigned to Ln↼H–SiMe2 on the basis of 1 H-13C HMQC experiments. The 29Si INEPT NMR room temperature resonance at –13.1 ppm split into two signals at 190 K, with one at –9.3 (assigned to Ln↼H–Si) and the other at –18.5 ppm. Although the two types of SiHs give very different one-bond coupling constants, the 29Si NMR chemical shifts are surprisingly similar and upfield. These values contrast the downfield 29Si NMR signals that might be expected from structures in which the Si–H bond is partly broken from an arrested β-H elimination, either in the form of a partially-formed silene or a partly-formed silylium site.43,44 We also noted upfield 29Si NMR signals for bridging Zr↼H–Si in Cp2ZrN(SiHMe2)2}H (–62.9 ppm) and [Cp2ZrN(SiHMe2)2]+ (–43 ppm).45 However, similarly low 1 JSiH were observed for an isolated SiH/Lewis base adduct.46 To summarize, the low temperature 1H, 13C, and 29Si NMR data indicate that the three C(SiHMe2)3 ligands in 1a-d and Y{C(SiHMe2)3}3 are equivalent, and each ligand contains two types of SiHMe2 groups in a 1:2 ratio. The first type of SiHMe2 group contains a classically-bonded SiH and enantiotopic Me groups. Each alkyl ligand contains two of the second type of SiHMe2 group, which is characterized by bridging Ln↼H– Si and inequivalent methyls. Thus, the spectroscopic data and the X-ray diffraction data provide consistent structural assignments for 1a-d. Kinetics of Ln↼H–Si/H–Si Exchange. The process that exchanges inequivalent methyl groups is also responsible for exchange of inequivalent SiH groups. 13C{1H} NMR spectra for 1a, acquired from 153 K to 297 K in pentane-d12, were simulated using iNMR to obtain rate constants for the exchange process.47 In the aliphatic hydrocarbon solvent, which was used instead of toluene-d8 to extend the temperature range of the study, three methyl signals of 1a coalesce at 174 K. The temperature dependent rate constants over this range, fit to the Eyring equation, provided ∆H‡ of 8.2(4) kcal⋅mol–1 and ∆S‡ of –1(2) cal⋅mol–1K–1 (Table 1). These activation parameters reveal a change from the exchange process for Y{C(SiHMe2)3}3 in toluene-d8, for which the corresponding values were previously measured as 13.4(3) kcal⋅mol–1 and 11(1) cal⋅mol–1K–1.35 The large difference in kinetic parameters between Y and La is unexpected given the similar spectroscopic properties of

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Table 1. Activation parameters for Ln↼H–Si/Si–H exchange in La{C(SiHMe2)3}3 (Ln = La, Y) from lineshape analysis of spectra measured in pentane-d12. Compound

∆H‡ (kcal⋅mol-1)

∆S‡ (cal⋅mol-1⋅K-1)

La{C(SiHMe2)3}3 (1a)

8.2(4)

–1(2)

La{C(SiDMe2)3}3 (1a-d9)

7.7(3)

–4(2)

Y{C(SiHMe2)3}3a

13.4(3)

11(1)

Y{C(SiHMe2)3}3

11.2(3)

3(1)

Y{C(SiDMe2)3}3

10.5(3)

0(1)

a

Measured in toluene-d8. the Ln{C(SiHMe2)3}3 series. However, the coalescence temperature of 1a in toluene-d8 is significantly higher than in pentane. We therefore remeasured the exchange rate for Y{C(SiHMe2)3}3 in pentane-d12 from 180 K to 260 K. Qualitatively, the SiH exchange in La 1a is much faster (coalescence at 174 K) compared to Y{C(SiHMe2)3}3 (coalescence at 228 K). Rate constants from simulated spectra of Y{C(SiHMe2)3}3 provide ∆H‡ = 11.2(2) kcal⋅mol–1 and ∆S‡ = 3(1) cal⋅mol–1K–1, with the larger ∆H‡ and small ∆S‡ values suggesting that the disruption of the Ln↼H–Si is the primary contribution to the activation barrier. Thus, the La↼H–Si in 1a is weaker than Y↼H–Si in Y{C(SiHMe2)3}3. DFT Calculations of Higher Energy Geometries. The one imaginary frequency at the saddle point between species A and B corresponds to rotation around one C–Si bond (TS-AB in Figure 3). The activation barrier (∆G‡) for this process is 6.06 kcal/mol. At this saddle point, the nonbridging H94 of one C(SiHMe2)3 enters the La1, C11, C36, and C61 plane as it moves from the position below the plane in La{C(SiHMe2)3}3A to a position above that plane. In this species, the La1-H94 distance of 3.358 Å is longer than expected for a bonding interaction. Moreover, none of La↼H–Si close contacts are disrupted in this process. Thus, the exchange between A, TS-AB, and B conformations does not result in La↼H–Si/Si–H exchange. Taken together, these results imply that the exchange process involves disruption of one or two La↼H–Si rather than intermediates or transitions states with seven La↼H–Si. On the basis of this idea, C1-symmetric structures with only five Ln↼H–Si were explored as possible intermediates in exchange of bridging and terminal SiH. These structures were optimized from the conformational isomers of A by rotating a C(SiHMe2)3 ligand around its La–C bond and/or rotating Cα– Si bonds of the ligand. Four local minima are only slightly higher energy (∆G, M06) at 2.50 (C, Figures 4 and S40), 2.90 (D, Figures 4 and S43), 4.54 (E, Figure S46), and 4.93 kcal/mol (F, Figure S48) and are likely important configurations. Interestingly, a bridging Ln↼H–Si is replaced by a γagostic CH in each of these species. These isomers were not detected in low temperature NMR or in X-ray structures (only one C3-symmetric species was observed); however, IR spectroscopy provided some support for the population of at least one of these species (see Table S16). The calculated species (C-F) contain four νSiH bands associated with four nonbridging SiH groups (vs. three bands in the Hessian calculation of A). The fourth νSiH in the higher energy states (ν6) absorbs at a lower frequency than ν7 – ν9 in La{C(SiHMe2)3}3-A and B. Interestingly, the experimental IR spectra of 1a-d contained a 6


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red-shifted shoulder on the nonbridging νSiH signal. On the basis of the comparison of experimental and calculated spectra, we attributed the red-shifted νSiH shoulder to low-lying higher energy intermediate states populated at room temperature. Likely, these states are important in the process(es) involved in Ln↼H–Si/H–Si exchange. In all four higher energy intermediates, only one of the three C(SiHMe2)3 ligand’s conformations is substantially affected (atoms were allowed to move in the geometry optimizations, and minor differences in the “ancillary” C(SiHMe2)3 ligands were observed). The geometries of structures C, D, E, and F are distinguished by the specific La↼H–Si that is broken and the La↼H–C that is formed. For C, E and F, the new La↼H– C and the disrupted La↼H–Si form within the same SiHMe2 group. As shown in Figure 4, the γ-agostic H75 of C is part of the SiHMe2 group that contains H93, which had bridged to La1 in ground state A. Ground state species A and intermediate C are related by a rotation of a secondary interactioncontaining SiHMe2 group around its Ccentral–Si bond. In intermediate D, the new La↼H–C (H81) and the disrupted La↼H– Si (H93) involve different SiHMe2 groups.

Transition state geometries corresponding to saddle points between ground state A and higher energy geometries C-F were located and are labelled TS-AC (∆G‡ = 2.91 kcal/mol relative to A; Figures 4 and S41), TS-AD (∆G‡ = 4.14 kcal/mol; Figures 4 and S44), TS-AE (∆G‡ = 8.43 kcal/mol; Figure S47), and TS-AF (∆G‡ = 8.67 kcal/mol; Figure S49). Still, the transition states TS-AC – TS-AF do not correspond to rotation of the C(SiHMe2)3 ligand around an axis along the La–C bond, and the La↼H–Si/SiH exchange is not described by these structures. Instead, transition state geometries TS-CC' (∆G‡ = 12.42 kcal/mol relative to A; Figures 4 and S42) and TS-DD' (∆G‡ = 10.19 kcal/mol; Figures 4 and S45) correspond to rotation of the C(SiHMe2)3 ligand, connecting C and D to their degenerate configurations C' and D', respectively. At both saddle points for the C(SiHMe2)3 ligand rotation, the La1…H92 bridging interaction is maintained, and the agostic La↼H–C in C and in D are broken (La1…H75/H81 > 3 Å). We were unable to locate the transition state for the direct conversion of A to A' that simultaneously exchange La↼H–Si and H–Si.

Figure 4. Calculated possible intermediates in the exchange of SiHMe2 groups in fluxional 1a-d (showing just the ligand undergoing exchange). The transformation of A to D involves a exchange of La↼H–Si by a γ-agostic CH from the (previously) non-interacting SiHMe2; this process occurs while the La1-H92 interaction is maintained. A to C trades secondary interactions within an SiHMe2 group. The γagostic interactions formed in C and D are broken in TS-CC' and TS-DD', but one La↼H–Si is maintained as the C(SiHMe2)3 ligand rotates around the La1–C61 bond. A', C', and D' are mirror images of A, C and D, respectively.

Importantly, there is an unambiguous temperaturedependent isotope effect for the exchange process, which is clearly revealed by qualitative comparison of line shapes for 1a vs. 1a-d9 or Y{C(SiHMe2)3}3 vs. Y{C(SiDMe2)3}3 spectra at equivalent temperatures. For example, comparison of 13 C{1H} NMR spectra of Y{C(SiHMe2)3}3 and Y{C(SiDMe2)3}3 ranging from low to high temperature (see Figure S26-27) shows faster exchange for labeled isotopomer

Isotope Effects on Ln↼H–Si/H–Si Exchange. The temperature-dependent fluxional processes measured for the isotopomers 1a-d9 and Y{C(SiDMe2)3}3 give activation parameters listed in Table 1. The coalescence temperatures for 1a and 1a-d9 (174 K) are essentially identical, as are the coalescence temperatures for Y{C(SiHMe2)3}3 and Y{C(SiDMe2)3}3. Thus kH/kD ~ 1 at the coalescence point. 7



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(wider lines) at low temperature and faster exchange (sharper lines for the natural abundance sample above the coalescence point. To better compare data from ~40 13C{1H} NMR spectra from 150 K to 200 K for 1a (and 180 K to 260 K for Y{C(SiHMe2)3}3), Eyring parameters were used to calculate rate constants (kH/kDEyring) for specific temperatures within this range. A few measurements were performed at the same temperature, and the isotope effect from these data are calculated directly from rate constants determined by simulation (kH/kDsimulation). The data, analyzed in this manner, provide numeric values that are generally consistent with expectations based on inspection of linewidths of spectra. While the absolute values of the isotope effects must be interpreted with considerably caution, the numeric results provide a reliable framework to analyze general trends. For Y{C(SiHMe2)3}3 vs. Y{C(SiDMe2)3}3, at lower temperatures kH/kDEyring-180K = 0.56 whereas above the coalescence point, kH/kDEyring-260K = 1.1. 13C{1H} spectra were acquired at these temperatures, and the isotope effects calculated directly from lineshape analysis are kH/kDsimulation-180K = 0.55 and kH/kDsimulation-260K = 1.14. Note that these are not independent data, but instead the latter data represent the comparison of individual points while rate constants extracted from the Eyring plot data include contributions from all temperatures. The same trend is observed in the case of 1a/1a-d9. The isotope effect for La↼H–Si/H–Si exchange at lower temperature is kH/kDEyring-161K = 0.88 and higher temperature is kH/kDEyring197K = 1.2. There is larger error in this comparison than in the yttrium example, and kH/kDsimulation-197K = 0.95 from direct simulation of the spectra. Nonetheless, the trend from both numeric analysis and inspection of spectra indicates that kH/kD for La↼H–Si/H–Si exchange increases over the temperature range accessible in our measurements. The temperature dependence of isotope effects is generally complex,48 and the analysis should consider a number of points. Kinetic isotope effects (k.i.e.) are normal (kH/kD > 1) for situations in which the E–H vibration is lost in the transition state. In contrast, systems that retain H- or D-based vibrational modes (such as in equilibrium isotope effects, e.i.e.) the value of KH/KD or kH/kD is guided by stabilization of the configuration with deuterium in the higher frequency oscillator. The analysis for exchange in Ln{C(SiHMe2)3}3 compounds is best understood by considering the computational structures. As noted in the IR analysis and vibrational calculations, the νSiH of 3c-2e Ln↼H–Si appear at lower energy than for the nonbridged Si–H. Calculated intermediates C and D contain four nonbridging Si–H with higher energy νSiH modes and five bridging Si–H with lower energy modes. On this basis, the equilibrium between calculated ground state species A and intermediates C and D would be shifted toward the latter species by substitution of Si–H groups with deuterium to give an inverse e.i.e. In TS-AD, the sixth 3c-2e is mostly disrupted, and the (unscaled) νSiH of that group is calculated to be 2114 cm–1 (i.e., it is product-like). Likewise, there are only five low energy νSiH associated with TS-AC, TS-CC', and TS-DD', and there are four νSiH in the region associated with 2c-2e SiH groups. In this situation, the presence of deuterium in the oscillator would result in stabilization of the transition states TS-AC, TS-AD, TS-CC', and TS-DD' with respect to the ground state configuration. These considerations predict an inverse k.i.e. for the

Page 8 of 22

Ln↼H–Si/H–Si exchange between ground states A and A', which is consistent with the observed low temperature NMR measurements. Two additional points likely contribute to the observed temperature dependence of the k.i.e. First, the Ln↼H–Si in A have six intense energy SiH bending modes (1000 – 1100 cm–1, noted above) corresponding to vibrations along the vectors connecting the Ln and H. The only nonbridging SiH bending modes evident in the calculated Hessian occur at lower energy, and no bands from 1100 – 1800 cm–1 could be assigned to SiH groups in the experimental spectrum of 1a (by comparison with the spectrum of 1a-d9). That is, the energetic ordering for bending modes (bridging > nonbridging) and stretching modes (bridging < nonbridging) are reversed. In the calculated higher energy intermediates C and D, there are only five bending modes associated with La↼H–Si. As a result, these bending modes would work to favor the ground state A configuration in the labeled analogue 1-d9 in comparison with the unlabeled species 1. The contributions of the sets of bending and stretching modes to the e.i.e. oppose each other; however, if the contributions have inequivalent response to temperature, then a temperature dependent e.i.e. would be expected. Likewise, the number of bending modes associated with La↼H–Si moieties in transition states TS-AC, TS-AD, TSCC' and TS-DD' are reduced with respect to the number in A. That is, all the higher energy configurations have fewer La↼H–Si-related bending and stretching modes, and the competing effects of these two groups could result in the observed temperature dependent k.i.e for the exchange of Ln↼H–Si/Si– H in 1a and Y{C(SiHMe2)3}3. Second, there are at least five intermediates (B, C, D, E, and F) that are calculated to be below the energy of the transition states for exchange (TS-CC' and TS-DD'). As the temperature of the system is increased, the accessible pathways and population of high energy intermediates is expected to change, and these changes could also influence the observed k.i.e. Ln{C(SiHMe2)3}3 as Synthetic Precursors. Interestingly, 1a-d are thermally robust. The t1/2 for decomposition of 1a to the alkane, HC(SiHMe2)3, is approximately 80 h at 80 °C in benzene-d6. Neither the β-elimination product Me2Si=C(SiHMe2)2 nor its [2+2] disilacyclobutane dimer were observed in 1H NMR spectra of thermalized solutions of 1a-d. We also noticed that the compounds are remarkably air stable (for rare earth alkyls). For example, solutions of 1d in benzene may be exposed to air for a few minutes without discoloration, and the HC(SiHMe2)3 product of hydrolysis forms relatively slowly. The persistence of these compounds in solution at elevated temperature, and their relatively slow decomposition in air, may facilitate their application in ligand substitution reactions, as catalyst or materials precursors. For example, reaction of 1a and bis-1,1-(4,4-dimethyl-2oxazolinyl)ethane (MeHC(OxMe2)2) or bis-1,1-(4,4-dimethyl2-oxazolinyl)phenylmethane (PhHC(OxMe2)2) gives Me2 {MeC(Ox )2}La{C(SiHMe2)3}2 (2a) or {Ph(OxMe2)2}La{C(SiHMe2)3}2 (3a), respectively (eq. 2). R H R O La{C(SiHMe 2)3}3 +

O N

N

1a R = Me, Ph

pentane

O

O N

r.t. or 60 °C, 12 h

N La

– HC(SiHMe 2)3 (Me 2HSi) 3C

C(SiHMe 2)3

R = Me (2a), 72% R = Ph (3a), 45%

8


(2)

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The 1H NMR spectra of compounds 2a and 3a contained multiplets at 4.60 ppm (1JSiH = 163 Hz) and 4.65 ppm (1JSiH = 143 Hz), respectively, assigned to the SiH groups. Compounds 2a and 3a appear to be C2v symmetric at room temperature, based on singlets at 3.52 (4 H) and 1.30 ppm (12 H) for 2a and at 3.37 (4 H) and 1.35 ppm (12 H) for 3a assigned to the bisoxazolinato ligands. The IR spectrum of 2a contained a sharp band at 2113 cm–1 and a broad signal at 1869 cm–1, assigned to νSiH modes of terminal SiH and bridging La↼H–Si moieties. In compound 3a, three signals associated with νSiH were observed at 2107, 1914 and 1876 cm–1 (νSiH). Despite these low energy SiH stretches and unlike 1a, only one SiH signal was observed in low temperature 1H NMR spectra of 3a. That is, the chelated monoanionic bis(oxazolinato) ligands increase the rate of La↼H–Si/H–Si exchange. In addition, these homoleptic tris(alkyl) lanthanides are starting materials for other rare earth compounds through protonolytic alkane elimination. Compounds 1a-d and 3 equiv. of tetramethyldisilazane react at room temperature to yield [Ln{N(SiHMe2)2}3]2 quantitatively in micromolar-scale reactions (Scheme 1). The tris(disilazido)lanthanide compounds are readily isolated from the HC(SiHMe2)3 byproduct by extraction with hexamethyldisiloxane. Scheme 1. Synthesis of [Ln{N(SiHMe2)2}3]2.

theoretical analysis suggests that the pyramidal distortion is driven by a second-order Jahn-Teller effect rather than by the formation of secondary interactions.4d, 39 This effect involves partial d orbital occupation, and as a result, the limitations of VSEPR for prediction of transition metal complexes’ geometries also apply to organolanthanides. The present series of compounds indicates that secondary interactions and planar geometries are able to co-exist in the same molecules, further suggesting that secondary interactions are not the underlying cause for geometric distortions. Moreover, because it is unreasonable that VSEPR should be appropriate for predicting the geometry of some organolanthanides (these compounds) but not others, we suggest that the trigonal planar LnC3 core results from interligand steric interactions rather than charge cloud repulsions. These interactions are sufficient to overcome the electronically favored tendency to form pyramidal structures. Second, the Ln{C(SiHMe2)3}3 system provides an important opportunity to analyze agostic-like secondary interactions with crystallographic, spectroscopic, and kinetic measurements paired with DFT calculations. The six nonclassical Ln↼H–Si interactions are structurally and energetically important, as indicated by X-ray crystallographic and gas-phase DFT calculations. While the three ligands are clearly organized in a pseudo-C3 arrangement dominated by Ln–C and Ln↼H–Si bonding, the experimentally-determined conformations of 1a-c (La-Pr) in comparision to 1d (Nd) as well as the corresponding calculated equivalent energies of A and B indicate that there is some flexibility in the ligands’ and complexes’ assembly. The NMR and IR spectroscopic signatures that report on the Ln↼H–Si interactions, paired with crystallographic analysis, are useful to identify structures of other lanthanide analogues. For example, IR spectroscopy can inform during the synthesis of paramagnetic compounds. Indeed, the molecular structure of neodymium 1d was initially proposed on the basis of its IR spectrum prior to diffraction analysis. In addition, Y{C(SiHMe2)3}3 crystallizes in a habit that has not yielded a solution, in contrast to the diffraction results of 1a-d. Nonetheless, the spectroscopic properties of the yttrium compound clearly indicate that it is isostructural to 1a-d. NMR spectroscopic analyses also suggest that the Ln↼H–Si interactions are stronger for Y than La. We interpret smaller one-bond silicon-hydrogen coupling constants as indicative of stronger Ln…H interactions because 1JSiH values are decreased in Ln↼H–Si vs. nonbridging Si–H. The 1JSiH (108 Hz) for Y{C(SiHMe2)3}3 is lower than in 1a (114 Hz), and thus the secondary interactions in the yttrium species are more stabilized than in the lanthanum analogue. We should note that the IR data is in conflict with this idea because the νSiH for Y{C(SiHMe2)3}3 (1945 cm–1) is higher than 1a (1928 cm–1). This change may reflect solution-phase vs. solid-state measurements or inequivalent packing for Y and La species in the solid state; however, clearly spectroscopic features associated with such bridging structures are an indirect probe and should be interpreted carefully in assessments of the strength of the interaction. Still, the idea that Ln↼H–Si are stronger for yttrium vs. lanthanum is also supported by the NMR analysis of the compounds’ fluxionality. The coalescence temperature is higher for Y than La, the rate of Ln↼H–Si/Si–H exchange is faster for La than Y at equivalent temperatures, and the activation enthalpy for exchange is larger for Y than La. The overall

[La{N(SiHMe2)2}3]233 and [Ce{N(SiHMe2)2}3]227c have been previously prepared by reaction of Ln{N(SiMe3)2}3 and HN(SiHMe2)2. [La{N(SiHMe2)2}3]2 crystallizes as a C1symmetric dimer, while the cerium analogue is not crystallographically characterized. The structures of the series are assigned on the basis of the similarity of their IR spectra with the La congener. The IR spectrum of [Ce{N(SiHMe2)2}3]2 contained bands at 2079 and 1915 cm–1 assigned to nonbridging and bridging Ce↼H–Si groups, respectively. The previously unknown [Pr{N(SiHMe2)2}3]2, remarkably requires more forcing conditions than La, Ce, or Nd analogues, and was characterized by infrared spectroscopy to contain absorption peaks at 2121 and 1929 cm–1. The corresponding signals in [Nd{N(SiHMe2)2}3]2 appear at 2091 and 1922 cm–1 respectively.

CONCLUDING REMARKS The tris(dimethylsilyl)methyl family of organolanthanides contains a number of unusual structural and spectroscopic features that, along with the complexes’ reactivity, provide insight into the metal-ligand bonding. First, the LnC3 core is planar as predicted by VSEPR, and these compounds contain six Ln↼H–Si interactions per molecule. While many homoleptic organo-rare earth compounds contain secondary interactions, pyramidal coordination is typically observed rather than planar geometry predicted by VSEPR.3a, 8f While pyramidal geometries and secondary interactions often occur together, 9



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symmetry constraints and all of the vibrational frequencies of the optimized structures were calculated to characterize whether the obtained structures are local minima. We used quasi-relativistic energyconsistent 4f-in-core pseudopotentials modeling trivalent lanthanides and corresponding valence basis set (7s6p5d)/[5s4p3d] for lanthanides.54 The number of valence electrons is 11 for La, Ce, Pr, and Nd), which were treated explicitly. The numbers of core electrons, i.e., electrons in the 1s–4f shells, included in pseudopotentials are 46, 47, 48, and 49 for trivalent La, Ce, Pr, and Nd, respectively. The Pople split-valence 6-311G(d,p) basis set was used for all other lighter elements.55 La{C(SiHMe2)3}3 (1a). LaI3(THF)4 (0.365 g, 0.452 mmol) and KC(SiHMe2)3 (0.310 g, 1.36 mmol) were stirred in 10 mL of benzene at room temperature for 1.5 h. Over the course of the reaction, the solution color changed from a red to yellow, while a white precipitate was observed throughout. Evaporation of the volatile materials, pentane extraction (3 × 5 mL), and evaporation of the pentane afforded a spectroscopically pure sticky yellow solid (0.269 g, 0.380 mmol, 84.0%). This solid was recrystallized at –30 °C from a minimal amount of pentane to obtain 1a as colorless crystals. 1H NMR (benzene-d6, 600 MHz, 25 °C): δ 4.26 (m, 1JSiH = 137 Hz, 9 H, SiH), 0.41 (d, 3JHH = 3.4 Hz, 54 H, SiMe2). 1H NMR (toluene-d8, 600 MHz, –94 °C): δ 4.78 (3 H, 1JSiH = 186 Hz, SiH), 4.04 (6 H, 1JSiH = 114 Hz, La↼HSiMe2), 0.55 (br, 18 H, La↼HSiMe2), 0.49 (br, 18 H, La↼HSiMe2), 0.42 (br, 18 H, SiMe2). 13C{1H} NMR (benzene-d6, 150 MHz, 25 °C): δ 31.82 (LaC), 3.63 (SiMe2). 13C{1H} NMR (toluene-d8, 150 MHz, –94 °C): δ 30.00 (LaC), 4.2 (SiHMe2), 3.2 (La↼HSiMe2), 2.1 (La↼HSiMe2). 29Si{1H} NMR (benzene-d6, 119 MHz, 25 °C): δ –13.1. 29Si{1H} NMR (toluene-d8, 119 MHz, –94 °C): δ –9.3 (1JSiH = 114 Hz, La↼HSiMe2), –18.5 (1JSiH = 186 Hz, SiHMe2). IR (KBr, cm–1): 2953 s, 2900 m, 2110 s (νSiH), 1829 s br (νSiH), 1418 w, 1254 s, 1025 s br, 951 m, 888 s br, 837 s, 777 m, 690 w, 678 w, 645 w, 630 w. Anal. Calcd for C21H63Si9La: C, 35.66; H, 8.98. Found: C, 36.56; H, 9.24. Mp, 114-117 °C. Pr{C(SiHMe2)3}3 (1c). PrI3(THF)3 (0.162 g, 0.220 mmol) and KC(SiHMe2)3 (0.173 g, 0.760 mmol) react over 6 h (solution changes from red to yellow) to provide the yellow product (0.146 g, 0.206 mmol, 93.7%), which was isolated following the procedure for 1a described above. This material was recrystallized at –30 °C from a minimal amount of toluene to obtain 1c as yellow-green X-ray quality crystals. 1H NMR (benzene-d6, 600 MHz, 25 °C): δ 12 (br, 9 H, SiH), 2.2 (br, 54 H, SiMe2). 1H NMR (toluene-d8, 600 MHz, –74 °C): δ –47 (br, SiH), 20.7 (s, SiMe2), 19.9 (s, SiMe2), –24.9 (s, SiMe2). IR (KBr, cm–1): 2954 s, 2899 s, 2108 s (νSiH), 1830 s br (νSiH), 1700 w, 1598 w, 1493 w, 1418 m, 1253 s, 1061 s br, 952 s br, 907 s, 885 m, 835 s, 777 s, 688 m, 677 m, 544 m, 624 m, 450 m, 388 m. Anal. Calcd. for C21H63Si9Pr: C, 35.55; H, 8.95. Found: C, 35.43; H, 8.83. Mp, 115118 °C. Nd{C(SiHMe2)3}3 (1d). NdI3(THF)3 (0.204 g, 0.275 mmol) and KC(SiHMe2)3 (0.189 g, 0.827 mmol) react over 12 h (solution changes from red to green) to afford the product (0.176 g, 0.247 mmol, 89.7%), which was isolated following the procedure for 1a described above. This solid was recrystallized at –30 °C from a minimal amount of pentane to obtain 1d as blue-green crystals. 1H NMR (benzene-d6, 600 MHz, 25 °C): δ 27.8 (br, SiH), 1.8 (br, SiMe2). IR (KBr, cm–1): 2954 s, 2900 s, 2108 s (νSiH), 1829 s br (νSiH), 1418 w, 1253 s, 1192 s br, 1058 br, 952 s br, 886 s, 835 w, 778 s, 689 s. Anal. Calcd. for C21H63Si9Nd: C, 35.39; H, 8.91. Found: C, 35.48; H, 9.11. Mp, 119122 °C. {MeC(OxMe2)2}La{C(SiHMe2)3}2 (2a). La{C(SiHMe2)3}3 (0.609 g, 0.861 mmol) and MeHC(OxMe2)2 (0.193 g, 0.861 mmol) were stirred in pentane (5 mL) at 25 °C for 12 h. The volatile materials were evaporated, and the residue was extracted with hexamethyldisiloxane (3 × 5 mL). Evaporation of the hexamethyldisiloxane afforded {MeC(OxMe2)2}La{C(SiHMe2)3}2 (0.462 g, 0.622 mmol, 72.3%) in good yield as a sticky solid. 1H NMR (benzene-d6, 600 MHz, 25 °C): δ 4.60 (m, 6 H, SiH), 3.51 (s, 4 H, CNCMe2CH2O), 2.40 (s, 3 H, MeC), 1.30 (s, 12 H, CNCMe2CH2O), 0.42 (d, 36 H, SiMe2). 13C{1H} NMR (benzene-d6, 150 MHz, 25 °C): δ 171.35 (CNCMe2CH2O),

activation barrier for Ln↼H–Si/Si–H exchange (in pentane) originates primarily from the enthalpic term in the Eyring analysis. The lowest enthalpic limit for breaking Ln↼H–Si is ca. 8 kcal/mol for La and 11 kcal/mol for Y. The temperaturedependent isotope effects for Ln↼H–Si/Si–H exchange also indicate that disruption of the Ln…H/D interaction is intimately involved in the activation barrier. Finally, DFT calculations indicate that only one Ln↼H–Si is disrupted during exchange. Together, these data indicate that the enthalpy of activation provides information about the strength of the Ln…H interaction, and that interactions are stronger for Y than La. DFT calculations reveal that γ-agostic CH-containing intermediates play a role in Ln↼H–Si/Si–H exchange; however, the agostic interactions are broken in the transition states as the C(SiHMe2)3 ligand rotates. The second lowest energy calculated species is only 2.5 kcal/mol higher than the six-fold βSiH bridging ground state, suggesting that there is only a minor energetic gain from six Ln↼H–Si in comparison to five Ln↼H–Si and one Ln↼H–C. Experimental evidence for the γagostic CH is indirectly provided by the non-Gaussian-shaped IR signal for the nonbridging νSiH. A second piece of evidence for Ln↼H–C playing a role in the fluxionality of Ln{C(SiHMe2)3}3 is the significant solvent effect on the process. Coalescence temperatures for 1a and Y{C(SiHMe2)3}3 are much higher in toluene-d8 than in pentane-d12. Moreover, the activation parameters reveal a moderately positive (∆S‡ = 11(1) cal·mol–1K–1) for exchange in Y{C(SiHMe2)3}3 dissolved in toluene-d8, indicating an order to the transition state not present in pentane-d12 (∆S‡ = 3(1) cal·mol–1K–1). A reasonable explanation for these observations is that a toluene adduct [Ln{C(SiHMe2)3}3·C6H5Me] is involved in the Ln↼H–Si/Si– H exchange, perhaps replacing the γ-agostic CH-containing intermediate. Finally, the thermally robust nature and substitution chemistry of Ln{C(SiHMe2)3}3 compounds begin to highlight their potential in synthetic applications, particularly in the preparation of catalytically active early lanthanide compounds. These applications are highlighted by the synthesis of homoleptic silazido compounds and (bis-oxazolinato)lanthanide dialkyls and our recently reported catalytic hydrosilylation with 1b/B(C6F5)3.37 We are currently studying Ln{C(SiHMe2)3}3 as convenient precursors for single-site surface-supported catalysts and in polymerization chemistry.

EXPERIMENTAL SECTION General. All manipulations were performed under a dry argon atmosphere using standard Schlenk techniques or under a nitrogen atmosphere in a glovebox unless otherwise indicated. Water and oxygen were removed from benzene and pentane solvents using an IT PureSolv system. Benzene-d6, toluene-d8, and pentane-d12 were heated to reflux over Na/K alloy, vacuum-transferred, and stored over activated molecular sieves. The compounds LaI3(THF)4, CeI3(THF)4, PrI3(THF)3 and NdI3(THF)3,49 KC(SiHMe2)335 and Ce{C(SiHMe2)3}3 (1b)37 were prepared following literature procedures. 1H, 13C{1H}, 11 B, 15N and 29Si{1H} NMR spectra were collected on Bruker DRX400, or Avance III-600 or Agilent MR 400 spectrometers. Infrared spectra were measured on a Bruker Vertex 80 spectrometer. Elemental analyses were performed using a Perkin-Elmer 2400 Series II CHN/S. X-ray diffraction data was collected on a Bruker APEX II diffractometer. Computational Details. All DFT calculations were performed using the program package GAUSSIAN 09.50 Calculations were carried out at the DFT level of theory using M06,51 B3LYP,52 and BLYP functionals.52a, 53 Geometry optimizations were achieved without any

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78.17 (CNCMe2CH2O), 66.30 (CNCMe2CH2O), 30.97 (C(SiHMe2)3), 28.00 (CNCMe2CH2O), 13.58 (MeC), 3.19 (SiMe2). 1H–29Si HMBC NMR (benzene-d6, 119.3 MHz, 25 °C): δ –17.7 (SiHMe2). 15N{1H} NMR (benzene-d6, 61 MHz, 25 °C): δ –165.9 (CNCMe2CH2O). IR (KBr, cm–1): 2960 s, 2895 s, 2112 s (νSiH), 1872 s br (νSiH), 1588 s (νCN), 1572 s (νCN), 1498 s, 1446 w, 1365 s, 1293 s, 1250 s, 1189 s, 1150 s, 1073 s br, 1024 s br, 943 s br, 895 br, 834 w, 768 s, 741 s, 700 s, 678 s, 635 s, 568 s. Anal. Calcd for C26H61LaN2O2Si6: C, 42.13; H, 8.30; N, 3.78. Found: C, 41.98; H, 8.38; N, 3.65. Mp, 110-112 °C. {PhC(OxMe2)2}La{C(SiHMe2)3}2 (3a). La{C(SiHMe2)3}3 (0.224 g, 0.317 mmol) was added to a benzene (3 mL) solution of PhHC(OxMe2)2 (0.091 g, 0.317 mmol). The resulting yellow mixture was heated at 60 °C for 12 h. The solution was evaporated under reduced pressure to give a yellow solid. The residue was washed with pentane (5 × 3 mL), and the volatiles were evaporated to dryness in vacuo to give {PhC(OxMe2)2}La{C(SiHMe2)3}2 (0.115 g, 0.143 mmol, 45.2%) as a pale yellow solid. 1H NMR (benzene-d6, 600 MHz, 25 °C): δ 7.76 (d, 3JHH = 7.1 Hz, 2 H, ortho-C6H5), 7.36 (vt, 3JHH = 7.7 Hz, 2 H, meta-C6H5), 7.15 (m, 1 H, para-C6H5) 4.64 (m 3JHH = 3.3 Hz, 1JSiH = 144 Hz, 6 H, SiH), 3.36 (s, 4 H, CNCMe2CH2O), 1.35 (s, 12 H, CNCMe2CH2O), 0.44 (d, 3JHH = 3.3 Hz, 36 H, SiMe2). 13C{1H} NMR (benzene-d6, 150 MHz, 25 °C): δ 171.00 (CNCMe2CH2O), 138.68 (C6H5), 133.32 (C6H5), 127.68 (C6H5), 125.91 (C6H5), 78.19 {PhC(CNCMe2CH2O)}, 77.79 (CNCMe2CH2O), 65.75 (CNCMe2CH2O), 29.76 (LaC(SiHMe2)3), 27.51 (CNCMe2CH2O), 2.95 (SiMe2). 1H-29Si HMBC NMR (benzene-d6, 119.3 MHz, 25 °C): δ – 17.2 (SiHMe2). 15N{1H} NMR (benzene-d6, 61 MHz, 25 °C): δ – 168.1 (CNCMe2CH2O). IR (KBr, cm–1): 2964 s, 2896 s, 2107 s (νSiH), 1914 (νSiH), 1876 s br (νSiH), 1564 s (νCN), 1478 s, 1436 w, 1376 s, 1299 s, 1251 s, 1187 s, 1165 s, 1044 s br, 943 s br, 919 s br, 897 br, 836 w, 809 s, 775 s, 757 s, 700 s, 679 s, 657 s. Anal. Calcd for C31H63LaN2O2Si6: C, 46.35; H, 7.91; N, 3.49. Found: C, 45.90; H, 7.50; N, 3.68. Mp, 109-111 °C.

2. (a) Zucchini, U.; Albizzati, E.; Giannini, U., J. Organomet. Chem. 1971, 26, 357-372; (b) Kruse, W., J. Organomet. Chem. 1972, 42, C39-C42; (c) Shortland, A. J.; Wilkinson, G., J. Chem. Soc., Dalton Trans. 1973, 872-876; (d) Galyer, A. L.; Wilkinson, G., J. Chem. Soc., Dalton Trans. 1976, 2235-2238; (e) Mertis, K.; Wilkinson, G., J. Chem. Soc., Dalton Trans. 1976, 1488-1492; (f) Wayda, A. L.; Evans, W. J., J. Am. Chem. Soc. 1978, 100, 71197121; (g) Morse, P. M.; Girolami, G. S., J. Am. Chem. Soc. 1989, 111, 4114-4116; (h) Pfennig, V.; Seppelt, K., Science 1996, 271, 626-628; (i) Kleinhenz, S.; Pfennig, V.; Seppelt, K., Chem. Eur. J. 1998, 4, 1687-1691; (j) Seaman, L. A.; Hrobárik, P.; Schettini, M. F.; Fortier, S.; Kaupp, M.; Hayton, T. W., Angew. Chem. Int. Ed. 2013, 52, 3259-3263. 3. (a) Hitchcock, P. B.; Lappert, M. F.; Smith, R. G.; Bartlett, R. A.; Power, P. P., J. Chem. Soc., Chem. Commun. 1988, 1007-1009; (b) Vaid, T. P.; Veige, A. S.; Lobkovsky, E. B.; Glassey, W. V.; Wolczanski, P. T.; Liable-Sands, L. M.; Rheingold, A. L.; Cundari, T. R., J. Am. Chem. Soc. 1998, 120, 10067-10079. 4. (a) Burdett, J. K., Molecular shapes: theoretical models of inorganic stereochemistry. Wiley: New York, 1980; p 287; (b) Kaupp, M., J. Am. Chem. Soc. 1996, 118, 3018-3024; (c) Kaupp, M., Angew. Chem. Int. Ed. 2001, 40, 3534-3565; (d) Perrin, L.; Maron, L.; Eisenstein, O., Faraday Discuss. 2003, 124, 25-39. 5. (a) Seyferth, D., Organometallics 2001, 20, 2940-2955; (b) Seyferth, D., Organometallics 2009, 28, 1598-1605. 6. (a) Kawaoka, A. M.; Douglass, M. R.; Marks, T. J., Organometallics 2003, 22, 4630-4632; (b) Zimmermann, M.; Frøystein, N. Å.; Fischbach, A.; Sirsch, P.; Dietrich, H. M.; Törnroos, K. W.; Herdtweck, E.; Anwander, R., Chem. Eur. J. 2007, 13, 8784-8800; (c) Lühl, A.; Hartenstein, L.; Blechert, S.; Roesky, P. W., Organometallics 2012, 31, 7109-7116. 7. Taube, R.; Windisch, H.; Maiwald, S.; Hemling, H.; Schumann, H., J. Organomet. Chem. 1996, 513, 49-61. 8. (a) Quignard, F.; Choplin, A.; Basset, J.-M., J. Chem. Soc., Chem. Commun. 1991, 1589-1590; (b) Quignard, F.; Lecuyer, C.; Bougault, C.; Lefebvre, F.; Choplin, A.; Olivier, D.; Basset, J. M., Inorg. Chem. 1992, 31, 928-930; (c) Amor Nait Ajjou, J.; Scott, S. L., Organometallics 1997, 16, 86-92; (d) Copéret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J.-M., Angew. Chem. Int. Ed. 2003, 42, 156-181; (e) Samantaray, M. K.; Callens, E.; Abou-Hamad, E.; Rossini, A. J.; Widdifield, C. M.; Dey, R.; Emsley, L.; Basset, J.-M., J. Am. Chem. Soc. 2014, 136, 1054-1061; (f) Conley, M. P.; Lapadula, G.; Sanders, K.; Gajan, D.; Lesage, A.; del Rosal, I.; Maron, L.; Lukens, W. W.; Copéret, C.; Andersen, R. A., J. Am. Chem. Soc. 2016, 138, 3831-3843. 9. (a) Edelmann, F. T., Chem. Soc. Rev. 2009, 38, 2253-2268; (b) Vlaisavljevich, B.; Miró, P.; Koballa, D.; Todorova, T. K.; Daly, S. R.; Girolami, G. S.; Cramer, C. J.; Gagliardi, L., J. Phys. Chem. C 2012, 116, 23194-23200; (c) George, S. M., Chem. Rev. 2010, 110, 111-131. 10. (a) Hultzsch, K. C.; Spaniol, T. P.; Okuda, J., Angew. Chem. Int. Ed. 1999, 38, 227-230; (b) Trifonov, A. A.; Spaniol, T. P.; Okuda, J., Organometallics 2001, 20, 4869-4874; (c) Dietrich, H. M.; Raudaschl-Sieber, G.; Anwander, R., Angew. Chem. Int. Ed. 2005, 44, 5303-5306; (d) Zimmermann, M.; Törnroos, K. W.; Anwander, R., Organometallics 2006, 25, 3593-3598; (e) Zhang, L.; Suzuki, T.; Luo, Y.; Nishiura, M.; Hou, Z., Angew. Chem. Int. Ed. 2007, 46, 1909-1913; (f) Cheng, J.; Saliu, K.; Kiel, G. Y.; Ferguson, M. J.; McDonald, R.; Takats, J., Angew. Chem. Int. Ed. 2008, 47, 4910-4913; (g) Rannveig Litlabø; Zimmermann, M.; Saliu, K.; Takats, J.; Törnroos, K. W.; Anwander, R., Angew. Chem. Int. Ed. 2008, 47, 9560-9564; (h) Kraft, S. J.; Fanwick, P. E.; Bart, S. C., J. Am. Chem. Soc. 2012, 134, 6160-6168. 11. (a) Lappert, M. F.; Pearce, R., J. Chem. Soc., Chem. Commun. 1973, 126-126; (b) Atwood, J. L.; Hunter, W. E.; Rogers, R. D.; Holton, J.; McMeeking, J.; Pearce, R.; Lappert, M. F., J. Chem. Soc., Chem. Commun. 1978, 140-142; (c) Schumann, H.; Freckmann, D. M. M.; Dechert, S., Z. Anorg. Allg. Chem. 2002, 628, 2422-2426. 12. (a) Bambirra, S.; Meetsma, A.; Hessen, B., Organometallics 2006, 25, 3454-3462; (b) Bambirra, S.; van Leusen, D.; Tazelaar,

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures for isotopically substituted 1-d9 and homoleptic disilazido compounds. NMR and IR spectra of new compounds and interatomic distances (PDF) Crystallographic information files (CIF) for La{C(SiHMe2)3}3 (1a), Ce{C(SiHMe2)3}3 (1b), Pr{C(SiHMe2)3}3 (1c), and Nd{C(SiHMe2)3}3 (1d).

AUTHOR INFORMATION Corresponding Author * [email protected]

ACKNOWLEDGMENT This research (La, Ce, Pr) was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. Support for synthesis and characterization of Nd{C(SiHMe2)3}3 and DFT calculations was provided by Korea Kumho Petrochemical Company (KKPC) through the ISU-KKPC Research Initiative Program.

REFERENCES 1. (a) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H., Chem. Rev. 2002, 102, 1851-1896; (b) Zimmermann, M.; Anwander, R., Chem. Rev. 2010, 110, 6194-6259; (c) Schumann, H., Comments Inorg. Chem. 1983, 2, 247-259.

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Chart 1. Rare earth homoleptic alkyls supported by peripheral coordination or steric bulk.

101x99mm (600 x 600 DPI)


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Figure 1. Infrared spectra of SiH region for Ln{C(SiHMe2)3}3 (La: 1a; Ce: 1b; Pr: 1c; Nd: 1d). 130x153mm (288 x 288 DPI)


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Figure 2. Thermal ellipsoid plot of La{C(SiHMe2)3}3 (1a) along the C3 axis at 35% probability. H atoms bonded to Si were located objectively in the Fourier difference map and are included in the representation. All other H atoms are not plotted for clarity. Dashed curves highlight short La–H distances. 829x797mm (150 x 150 DPI)



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Figure 3. Illustration of the interconversion of two minima of La{C(SiHMe2)3}3, A and B and transition state TS-AB. The nonbridging SiH of A are all on one side of a plane defined by La, C11, C36, and C61, whereas only two of the nonbridging SiH of B are on the same side of that same plane. The nonbridging H bonded to Si are highlighted in the above images. 1059x1404mm (72 x 72 DPI)


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Scheme 1. Synthesis of [Ln{N(SiHMe2)2}3]2.

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H94 H92

H81

H75 H81

H94

H92

H94

La1

La1

La1

H93 H93

‡ ΔG = 2.91 kcal/mol

H75

H81

H75 C ΔG = 2.50 kcal/mol

Si–C rotation

H92

C’

H93

TS-CC’ ‡ ΔG = 12.42 kcal/mol

Si–C rotation ‡ ΔG = 2.91 kcal/mol

ΔG = 2.50 kcal/mol

La–C rotation

H81

H92 H94

H94 La1

H75

No Direct Path from A to A’ Could Be Located

H81

H93

La1 H93

H75

A

A’

H81

ΔG = 0 kcal/mol Si–C rotation ‡ ΔG = 4.14 kcal/mol

H93 H75

ΔG = 0 kcal/mol

H92

H93

Si–C rotation

H75 H94


La1

La1

La1 H81

H92 H92

H81

D TS-DD’

ΔG = 2.90 kcal/mol

H94


La–C rotation ACS Paragon Plus Environment

H92

H93

H94

D’

H75

ΔG = 2.90 kcal/mol

Alkaline earth complexes of silylated aminopyridinato ligands: homoleptic compounds and heterobimetallic coordination polymers.

Catalytic hydroacetylenation of carbodiimides with homoleptic alkaline earth hexamethyldisilazides.

Rare earth.

Homoleptic organocobalt(III) compounds with intermediate spin.

Rare earth replacement magnets.

Rare earth magnets and impaction.

Rare-Earth-metal methylidene complexes.

Mechanism of Rare Earth Incorporation and Crystal Growth of Rare Earth Containing Type-I Clathrates.

Earth abundant element compounds in homogeneous catalysis.

Rare earth intensifying screens in veterinary radiology.

Theoretical screening of intermetallic ThMn12-type phases for new hard-magnetic compounds with low rare earth content.

Enhanced negative thermal expansion in La(1-x)Pr(x)Fe10.7Co0.8Si1.5 compounds by doping the magnetic rare-earth element praseodymium.

Influence of the ionic liquid cation on the solvent extraction of trivalent rare-earth ions by mixtures of Cyanex 923 and ionic liquids.

Recovery and separation of rare Earth elements using salmon milt.

Rare earth element recycling from waste nickel-metal hydride batteries.

Rare Earth Ion-Doped Upconversion Nanocrystals: Synthesis and Surface Modification.

Rare-earth magnet ingestion-related injuries among children, 2000-2012.

Restoration of rare earth mine areas: organic amendments and phytoremediation.

Catholuminescence properties of rare earth doped CaSnO3 phosphor.

Contamination in the Rare-Earth Element Orthophosphate Reference Samples.

Heteroatom-assisted olefin polymerization by rare-earth metal catalysts.

Rare Earth Nanoprobes for Functional Biomolecular Imaging and Theranostics.

Homometallic rare-Earth metal phosphinidene clusters: synthesis and reactivity.

Synthesis and thermoluminescence properties of rare earth-doped NaMgBO3 phosphor.