Monomeric Tetraalkylaluminates of Divalent Ytterbium Stabilized by a

Nov 6, 2009 - Synopsis. Monomeric hydrocarbyl complexes (TptBu,Me)Yb(AlR4) (R = Me, Et) can be selectively synthesized by applying a silylamide ...
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Organometallics 2009, 28, 6750–6754 DOI: 10.1021/om900744k

Monomeric Tetraalkylaluminates of Divalent Ytterbium Stabilized by a Bulky Tris(pyrazolyl)borate Ligand Rannveig Litlabø,† Kuburat Saliu,‡ Michael J. Ferguson,‡ Robert McDonald,‡ Josef Takats,*,‡ and Reiner Anwander*,†,´ †

Department of Chemistry, University of Bergen, All egaten 41, 5007 Bergen, Norway, ‡Department of Chemistry, University of Alberta, Edmonton, AB, Canada T6G 2G2, and ´Institut f€ ur Anorganische Chemie, Universit€ at T€ ubingen, Auf der Morgenstelle 18, 72076 T€ ubingen, Germany Received August 26, 2009

Mononuclear heterobimetallic complexes (TptBu,Me)Yb(AlR4) (R = Me, Et) were obtained in high yields according to a silylamide elimination protocol employing (TptBu,Me)Yb[N(SiMe3)2] and AlR3. The solid-state structures revealed κ3-coordinated TptBu,Me ligands and η2-coordinated [AlR4] moieties, with additional Yb 3 3 3 H agostic interactions from the bridging methyl and methylene groups of the [AlR4] ligands. These agostic interactions however are not strong enough to maintain a rigid structure in solution. 1H NMR spectroscopy showed a highly fluxional ligand environment with only one set of signals for both TptBu,Me and [AlR4] ligands at temperatures down to -80 C. Introduction Bulky scorpionate ligands, such as the monoanionic tris(30 R-5-R0 -pyrazolyl)borate ligand set (TpR,R , Trofimenko’s scorpionates), provide a unique ancillary ligand environment for a rich organo-rare-earth metal chemistry.1-3 Particularly, the superbulky TptBu,Me ligand (cone angle: 243)1a,c facilitated the stabilization and isolation of monoligand rare-earth metal(III) bis(alkyl) derivatives, such as the five-coordinate complexes (TptBu,Me)Y(CH2SiMe3)24 and (TptBu,Me)Y(CH3)2(AlMe3).5 Moreover, this sterically demanding [NNN]- ligand was successfully exploited for the even more challenging synthesis of discrete Ln(II) organometallics, as evidenced by the X-ray structurally authenticated dimeric hydride complex [(TptBu,Me)YbII(μ-H)]26 and monomeric hydrocarbyl derivatives (TptBu,Me)LnII[CH(SiMe3)2] (Ln = Yb, Tm).7,8 Recently, we found that alkylaluminate ligands give access to a variety of thermally stable and nonsolvated heteroleptic hydrocarbyl *To whom correspondence should be addressed. E-mail: joe.takats@ ualberta.ca or [email protected]. Fax: þ49(0)707129-2436. (1) (a) Trofimenko, S. Scorpionates: The Coordination Chemistry of Polypyrazolylborate Ligands; Imperial College Press: London, 1999. (b) Trofimenko, S. Chem. Rev. 1993, 93, 943. (c) Pettinari, C. Scorpionates II: Chelating Borane Ligands; Imperial College Press: London, 2008. (2) Ferrence, G. M.; Takats, J. J. Organomet. Chem. 2002, 647, 84. (3) Marques, N.; Sella, A.; Takats, J. Chem. Rev. 2002, 102, 2137. (4) Cheng, J.; Saliu, K.; Kiel, G. Y.; Ferguson, M. J.; McDonald, R.; Takats, J. Angew. Chem. Int. Ed. 2008, 47, 4910. (5) (a) Zimmermann, M.; Takats, J.; Kiel, G.; T€ ornroos, K. W.; Anwander, R. Chem. Commun. 2008, 612. (b) Litlaboe , R.; Zimmermann, M.; Saliu, K.; Takats, J.; T€ornroos, K. W.; Anwander, R. Angew. Chem. Int. Ed. 2008, 47, 9560. (6) Ferrence, G. M.; McDonald, R.; Takats, J. Angew. Chem. Int. Ed. 1999, 38, 2233. (7) Hasinoff, L.; Takats, J.; Zhang, X. W.; Bond, P. H.; Rogers, R. D. J. Am. Chem. Soc. 1994, 116, 8833. (8) Cheng, J.; Takats, J.; Ferguson, M. J.; McDonald, R. J. Am. Chem. Soc. 2008, 130, 1544. (9) For a review, see: Fischbach, A.; Anwander, R. Adv. Polym. Sci. 2006, 204, 155. pubs.acs.org/Organometallics

Published on Web 11/06/2009

complexes of the trivalent rare-earth metals that are not hampered by ate-complex formation.9 For example, protonolysis and salt metathesis of Ln(AlMe4)3 with HTptBu,Me and KTptBu,Me, respectively, afforded the rare-earth metal complexes (TptBu,Me)Y(CH3)(μ-CH3AlMe3) (Ln = Y, Lu), containing a terminal methyl group, and the Tebbe analogue, (TptBu,Me)La[(CH2)(AlMe3)2].5 Compared to the steadily growing reports on LnIII-AlR4 derivatives, the chemistry involving divalent lanthanides is still in its infancy. Examples are limited to the homoleptic derivatives [Ln(AlR4)2]n (Ln = Sm, Yb; R = Me, Et, iBu),10,11 monomeric adduct complexes Ln(AlR4)2(Do)x [Do = THF, pyridine, x = 2, R = Et, Ln = Sm, Yb; Do = 1,10-phenanthroline, x = 1, R = Me (Yb), Et (Sm, Yb)],11,12 solventseparated ion pairs, such as [η5-(fluorenyl)Yb(THF)4][AlMe4]13 and [(C5Me5)Yb(THF)4][AlMe4],11 and heteroleptic complexes carrying additional aryloxide ligands, such as [Et2Al(OAriPr,H)2Yb(AlEt4)]2.14,15 We have now extended this research to the synthesis and characterization of divalent ytterbium alkylaluminate scorpionate complexes bearing the bulky TptBu,Me as a stabilizing ligand.

Results and Discussion The silylamide route has already been shown as a viable strategy for the synthesis of peralkylated Yb(II) complexes.10,11 Accordingly, treatment of the heteroleptic (10) Klimpel, M. G.; Anwander, R.; Tafipolsky, M.; Scherer, W. Organometallics 2001, 20, 3983. (11) Sommerfeldt, H.-M.; Meermann, C.; Schrems, M. G.; T€ ornroos, K. W.; Froe ystein, N. A.; Miller, R. J.; Scheidt, E. W.; Scherer, W.; Anwander, R. Dalton Trans. 2008, 1899. (12) Schrems, M. G.; Dietrich, H. M.; T€ ornroos, K. W.; Anwander, R. Chem. Commun. 2005, 5922. (13) Nakamura, H.; Nakayama, Y.; Yasuda, H.; Maruo, T.; Kanehisa, N.; Kai, Y. Organometallics 2000, 19, 5392. (14) Sommerfeldt, H.-M.; Meermann, C.; T€ ornroos, K. W.; Anwander, R. Inorg. Chem. 2008, 47, 4696. (15) Korobkov, I.; Gambarotta, S. Organometallics 2009, 28, 4009. r 2009 American Chemical Society

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Scheme 1. Synthesis of (TptBu,Me)Yb(AlR4) (2: R = Me (a), Et (b))

complex (TptBu,Me)Yb[N(SiMe3)2]7 (1) with a small excess (>2 equiv) of AlR3 in pentane gave the (TptBu,Me)Yb(AlR4) (2: R = Me (a), Et (b)) complexes in high yields (Scheme 1).16 Addition of AlR3 to the orange-red solution of 1, at RT, did not result in an immediately visible change; however, during the next 20 min a yellow-orange precipitate formed. After 2 h compounds 2 could be separated from the remaining reaction mixture as bright yellow powdery solid by centrifugation. The products are partly soluble in pentane, with the ethyl derivative 2b being the more soluble, which, however, hampers its quantitative harvesting because of the difficult-to-separate pentane-soluble byproduct {R2Al[N(SiMe3)2]}2. It was gratifying to see that TptBu,Me ligand transfer to aluminum was not complicating the synthesis of complexes 2. Such an outcome could have occurred in view of the well-known affinity of N-chelating ligands to coordinate to the hard Lewis acidic Al(III) center, as exemplified by transmetalation between KTpMe,Me/TlTptBu and AlMe3,17 and ligand transfer observed when N-chelated lanthanide complexes were treated with aluminum reagents.5a,18,19 The RT 1H NMR spectra of complexes 2 showed only one set of signals for the TptBu,Me ligand, with chemical shifts quite comparable to the starting compound, (TptBu,Me)Yb[N(SiMe3)2] (1).7 Two singlets were observed in the alkyl region with an integral ratio of 27:9, accounting for the (16) Attempts to synthesize the corresponding thulium derivatives (TptBu,Me)Tm(AlR4) (R = Me, Et) failed. Separation of light green soluble fractions from dark insoluble materials, in addition to the formation of a thin metal coating on the reaction vials, indicated some TmII to TmIII oxidation and reduction of AlR3 to Al0. (17) Looney, A.; Parkin, G. Polyhedron 1990, 9, 265. (18) Duchateau, R.; vanWee, C. T.; Meetsma, A.; vanDuijnen, P. T.; Teuben, J. H. Organometallics 1996, 15, 2279. (19) Zimmermann, M.; Estler, F.; Herdtweck, E.; T€ ornroos, K. W.; Anwander, R. Organometallics 2007, 26, 6029.

protons of the TptBu,Me ligand’s 3-tert-butyl (1.32 ppm (2a), 1.34 ppm (2b)) and 5-methyl (2.07 ppm (2a), 2.05 ppm (2b)) substituents, respectively. A singlet attributable to the three 4-pyrazolyl protons was observed at 5.62 ppm (2a) and 5.64 ppm (2b). The B-H proton appeared as a broad doublet at 4.7 ppm for both 2a and 2b. The [AlR4] moieties also gave only one set of signals with a narrow singlet at 0.05 ppm for the methyl groups in 2a and a quartet at 0.45 ppm and triplet at 1.68 ppm (3JHH =7.8 Hz) for the CH2 and CH3 groups in 2b, with excellent integral match for the formulas (TptBu,Me)Yb(AlR4). The 13C NMR spectra were also in agreement with this molecular composition and featured only one set of peaks for the TptBu,Me ligand and one for the [AlR4] moieties. The 13C signals of the methyl substituents on the pyrazolyl rings and the CH3 groups of the tetraethylaluminum ligand in complex 2b were initially difficult to distinguish; however, they were finally assigned by an HSQC NMR experiment. The 171Yb NMR spectra showed singlets at 505 (2a) and 470 ppm (2b), which are considerably downfield compared to the homoleptic complex [Yb(AlEt4)2]n (δ = 363 ppm),11 but at much higher field than in the related Tp complexes containing Yb-C σ-bonds, (TptBu,Me)Yb[CH(SiMe3)2] (δ = 865 ppm) and (TptBu,Me)Yb(CH2SiMe3)(THF) (δ = 985 ppm).7 For further comparison, similar 171Yb chemical shifts were detected for six-coordinate complex [Yb(C6F5)2(THF)4] (δ = 463 ppm)20 and (TptBu,Me)Yb(μ-HBEt3)(THF) (δ = 413 ppm), displaying Yb 3 3 3 (H-C) agostic interactions. While the molecular composition of complexes 2 was evident from the NMR spectroscopic results and the elemental analyses, the connectivity of the ligands was still unclear. Routinely, the TptBu,Me ligand coordinates to the lanthanide metal center in a κ3 fashion, through three (20) Deacon, G. B.; Forsyth, C. M. Organometallics 2003, 22, 1349.

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Figure 1. Molecular structure of 2a (ORTEP drawing). Atoms are represented by atomic displacement ellipsoids set at the 50% level. Nonsignificant hydrogen atoms are omitted for clarity.

nitrogen atoms, but other coordination modes have been reported. For example, in the case of the sterically oversaturated complexes (TptBu,Me)2Ln (Ln = Sm, Yb), one of the TptBu,Me ligands coordinates in a normal κ3(N,N,N) fashion, while the other one exhibits a κ2(N,N) coordination with an additional Ln 3 3 3 (H-B) agostic interaction.21 On the other hand, in the sterically unsaturated complex (TptBu,Me)Yb(C5H4SiMe3) two of the pyrazolyls are coordinated in the normal fashion, while the third one coordinates to the ytterbium metal center with both nitrogen atoms.22 [AlR4] ligands are also known to show different coordination behavior depending on the steric environment, featuring either η1, η2, or η3 coordination to the lanthanide metal center.5,9,23,24 Complexes 2 showed only one set of NMR signals at ambient temperature for both the TptBu,Me and [AlR4] ligands, indicating fluxional solution behavior. In order to slow down the rearrangement process, VT NMR studies were performed in toluene-d8; however, no significant line broadening was observed even at -80 C. These findings are in accord with the highly dynamic behavior of the homoleptic derivatives [Ln(AlR4)2]n (Ln = Sm, Yb; R = Et, iBu),10,11 which is due to the low charge and large size of the YbII metal center (ionic radii for seven-coordination: 1.08 A˚)25 and the attendant weak bonding interaction between it and the [AlR4] ligand. The hapticity of the TptBu,Me ligand can be further analyzed by the frequency of the B-H stretching vibration.26 Akita and co-workers showed that RhI-TpiPr complexes display different B-H stretching frequencies for a bidentate (κ2, ∼2470 cm-1) and tridentate bonding mode (κ3, >2530 cm-1), respectively.26 Although the frequencies (21) (a) Zhang, X. W.; McDonald, R.; Takats, J. New J. Chem. 1995, 19, 573. (b) Saliu, K. O.; Takats, J.; Ferguson, M. Acta Crystallogr. 2009, E65, m643. (22) Ferrence, G. M.; McDonald, R.; Morissette, M.; Takats, J. J. Organomet. Chem. 2000, 596, 95. (23) Dietrich, H. M.; Schuster, O.; T€ ornroos, K. W.; Anwander, R. Angew. Chem. Int. Ed. 2006, 45, 4858. (24) Zimmermann, M.; T€ ornroos, K. W.; Anwander, R. Angew. Chem., Int. Ed. 2007, 46, 3126. (25) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (26) Akita, M.; Ohta, K.; Takahashi, Y.; Hikichi, S.; Morooka, Y. Organometallics 1997, 16, 4121.

Litlabø et al.

Figure 2. Molecular structure of 2b (ORTEP drawing). Atoms are represented by atomic displacement ellipsoids set at the 50% level. Nonsignificant hydrogen atoms are omitted for clarity.27

for divalent 2a (2477 cm-1) and 2b (2480 cm-1) suggest a κ2bonding, it is more realistic to draw a comparsion to the trivalent analogues (TptBu,Me)Ln(Me)(AlMe4) (Ln = Y, Lu) and (TptBu,Me)La[(CH2)(AlMe3)2],5 which feature tridentate TptBu,Me ligands. However, conclusive determination of the bonding had to await X-ray structure analysis, especially of the subtleties on the interactions. Single crystals of 2a and 2b suitable for X-ray analysis were obtained from saturated toluene/pentane mixtures. Both complexes crystallize in the triclinic space group P1. Perspective views of complex 2a and 2b are shown in Figures 1 and 2, respectively. Selected bond distances and angles are listed in Table 1 and Table 2 gives crystal data and data collection parameters. Both complexes exhibit the same overall five-coordinate geometry, with a tridentate TptBu,Me ligand and an η2coordinated [AlR4] moiety.27 The molecular geometry is best described as distorted trigonal bipyramidal with N32 and C2(2a)/C3(2b) occupying the axial positions and N12, N22, and C1 the equatorial sites. As typical in related five-coordinate (TptBu,Me)YbX(THF) (X = I, CH2SiMe3, OC6H2Me3-2,4,6, and BH4) complexes,28 the axial Yb-N32 distance is longer than the equatorial bond lengths; otherwise the Yb-N bond lengths are similar to those in the above complexes. The Yb-C bond distances (2a Yb-C (av) 2.700 A˚; 2b Yb-C (av) 2.674 A˚) are slightly longer than those observed in hexacoordinate Yb(AlMe4)2(Phen) (Yb-C (av) 2.611 A˚), but only marginally longer than those observed in hexacoordinate Yb(AlEt4)2(THF)2 (Yb-C (av) 2.663 A˚).11,12 The [AlR4] group coordinates to the ytterbium metal center in a typical planar η2 fashion (2a — C1-Yb-C2-Al -0.34; 2b — C1-Yb-C3-Al -7.73). Additional Yb 3 3 3 (H-C) R-agostic interactions are observed for two of the protons on each of the bridging methyl groups in complex 2a (Yb 3 3 3 H (av) 2.60 A˚) and for both protons in each of the bridging CH2 groups in complex 2b (Yb 3 3 3 H (av) 2.41 A˚). The latter shorter distances are perhaps an indication of a (27) In complex 2b one of the tert-butyl groups and the terminal Et groups of the [AlEt4] ligand are disordered. Only the major orientation of the disordered groups is shown. (28) Saliu, K. O.; Maunder, G.; Ferguson, M.; Sella, A.; Takats, J. Inorg. Chim. Acta 2009, 362, 4616, and references therein.

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Table 1. Selected Interatomic Distances and Angles for (TptBu,Me)Yb(AlR4) (2a,b) 2a (R = Me)

2b (R = Et)

Bond Distances (A˚) Yb-C1 Yb-C2/C3 Yb-N12 Yb-N22 Yb-N32 Yb 3 3 3 H1A Yb 3 3 3 H1B Yb 3 3 3 H2A/H3A Yb 3 3 3 H2B/H3B

2.702(3) 2.698(3) 2.431(2) 2.437(2) 2.487(2) 2.59(3) 2.64(3) 2.59(3) 2.57(3)

2.656(2) 2.691(2) 2.437(2) 2.435(2) 2.473(2) 2.37 2.39 2.47 2.39

Bond Angles (deg) C1-Yb-C2/C3 N32-Yb-C2/C3 Yb-C1-Al/AlA Yb-C2/C3-Al/AlA Yb-C1-H1A Yb-C1-H1B Yb-C2/C3-H2A/H3A Yb-C2/C3-H2B/H3B Yb-C1-C2 Yb-C3-C4

77.67(9) 154.94(8) 85.1(1) 85.4(1) 73(2) 76(2) 73(2) 72(2)

78.97(7) 163.43(7) 89.1(1) 86.8(1) 62.4 63.5 66.4 62.2 166.3(2) 169.6(2)

Table 2. Crystal Data and Data Collection Parameters of Complexes 2a and 2b 2a

2b

)

)

chemical formula C28H52AlBN6Yb C32H60AlBN6Yb 683.59 739.69 Mr cryst dimens (mm) 0.20  0.19  0.11 0.52  0.28  0.25 cryst syst triclinic triclinic P1 (No. 2) space group P1 (No. 2) ˚ a (A) 11.8519(4) 12.2403(5) b (A˚) 12.6876(5) 12.5915(5) c (A˚) 12.7193(5) 13.7790(6) R (deg) 81.4737(4) 81.9634(5) β (deg) 78.9276(4) 81.2313(4) γ (deg) 64.2133(4) 64.3889(4) 1685.57(11) 1886.12(14) V (A˚3) Z 2 2 700 764 F000 T (K) 173(2) 173(2) 1.347 1.302 Fcalcd (g cm-3) 2.825 2.530 μ (mm-1) θ range (deg) 1.64-27.53 1.50-26.40 a 0.0238 0.0175 R1 (obsd) a 0.0556 0.0472 wR2 (all) 1.027 1.036 GOF (all)a P P P a 2 2 2 P R1 = ( Fo| P |Fc )/ |Fo|; wR2 = { [w(Fo - Fc ) ]/ [w(Fo2)2]}1/2; GOF = [ w(Fo2 - Fc2)2]/(n - p)]1/2 (n = number of data; p = number of parameters varied; w = [σ2(Fo2 þ (a0P)2 þ a1P]-1 where P = [Max(Fo2,0) þ 2Fc2]/3; for 2a, a0 = 0.0292, a1 = 0; for 2b, a0 = 0.0286, a1 = 0.5485).

somewhat stronger agostic interaction between Yb and the [AlEt4] ligand, and lending support to this hypothesis are the more acute Yb-C-H angles in compound 2b compared to 2a. The nature of the agostic Yb---(H-CAl) interactions is similar to those found in the neutron diffraction study of [Nd(AlMe4)3 3 (Al2Me6)0.5].29 Although the orientation of the TptBu,Me ligand tBu substituents points some of the hydrogens closer to ytterbium than the sum of the van der Waals radii (C18 and C28), it is not clear whether these can be attributed also to agostic Yb---HC interactions. (29) Klooster, W. T.; Lu, R. S.; Anwander, R.; Evans, W. J.; Koetzle, T. E.; Bau, R. Angew. Chem. Int. Ed. 1998, 37, 1267.

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Complexes 2a and 2b represent the first examples of mononuclear divalent lanthanide complexes containing η2-coordinated [AlR4] ligands without any additional donor stabilization. We have previously investigated the addition of donor molecules to the peralkylated complexes [Ln(AlR4)2]n (Ln = Sm, Yb; R = Me, Et) and observed the formation of stable donor adducts Ln(AlR4)2Dox11,12 or solvent-separated ion pairs of types [(C5Me5)YbII(THF)4][AlMe4]11 and [YbII(thf)6][AlR4]2.30,31 Given their high fluxionality in solution, similar donor addition protocols could be anticipated for complexes 2. Accordingly, we performed NMR-scale reactions of complexes 2a and 2b with a small excess of Et2O or THF in C6D6 solutions. However, the 1H NMR spectra did not indicate any significant shifts for either the [AlR4] signals or the Et2O/THF peaks, even when a large excess of Et2O/THF was added.

Conclusions The silylamide elimination route presents a straightforward approach for the high-yield synthesis of (TptBu,Me)Yb(AlR4) (R = Me, Et) complexes from (TptBu,Me)Yb[N(SiMe3)2] and AlR3. Interestingly, TptBu,Me ligand transfer to Al(III) was not observed. Enhanced steric unsaturation of the large YbII metal centers is revealed by the solid-state structures, which show that, in addition to the anticipated κ3-TptBu,Me and η2-[AlR4] bonding, the [AlR4] fragments are engaged in Yb 3 3 3 (H-C) agostic interactions. In solution these monomeric complexes are highly fluxional with temperature-invariant NMR spectral features down to -80 C. Neither donor-adduct formation nor donor-induced cleavage was observed when the complexes were treated with an excess of diethyl ether or THF.

Experimental Section General Considerations. All operations were performed with rigorous exclusion of air and water, using standard Schlenk, high-vacuum, and glovebox techniques (Vacuum Atmospheres Company glovebox). Solvents were distilled from Na/K alloy/ benzophenone ketyl (toluene, Et2O, THF) or CaH2 (pentane) under nitrogen and degassed by three freeze-pump-thaw cycles prior to use. Deuterated solvents (C6D6 and toluene-d8; Cambridge Isotope Laboratories) were dried over Na/K alloy/ benzophenone ketyl, degassed by three freeze-pump-thaw cycles, and vacuum-transferred prior to use. AlMe3 and AlEt3 were purchased from Aldrich and used as received. (TptBu,Me)Yb[N(SiMe3)2] was prepared according to the published procedure.7 Elemental analyses and IR spectroscopy were performed by the staff of the Analytical and Instrumentation Laboratory, Department of Chemistry, University of Alberta. NMR spectra were recorded on Varian Inova 400, 500, and 600 MHz instruments, with 1H and 13C shifts referenced to internal solvent resonances and reported in parts per million relative to TMS. For determining the chemical shifts of 11B, 27Al, and 171Yb spectra IUPAC’s unified chemical shift scale and standard references were used.32 Synthesis of (TptBu,Me)Yb(AlMe4) (2a). In a glovebox, 3 equiv of AlMe3 (38 μL, 0.40 mmol) were added dropwise to a stirred (30) Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L.; Hunter, W. E. J. Chem. Soc., Dalton Trans. 1979, 54. (31) Michel, O.; Meermann, C.; T€ ornroos, K. W.; Anwander, R. Organometallics 2009, 28, 9783. (32) Harris, R. K.; Becker, E. D.; De Menezes, S. M. C.; Goodfellow, R.; Granger, P. Pure Appl. Chem. 2001, 73, 1795.

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solution of (TptBu,Me)Yb[N(SiMe3)2] (1, 100 mg, 0.13 mmol) in 4 mL of pentane. The reaction mixture was stirred another 2 h at ambient temperature while the formation of a yellow precipitate was observed. The product was separated via centrifugation and washed with 2  2 mL of cold pentane to yield (TptBu,Me)Yb(AlMe4) (2a, 77 mg, 0.11 mmol, 87%) as a bright yellow powdery solid. Crystallization from a toluene/pentane solution at -40 C afforded yellow crystals suitable for X-ray diffraction analysis. 1H NMR (500 MHz, C6D6, 27 C): δ 5.62 (s, 3 H, 4-pz-H), 4.7 (d vbr, 1JBH = 120 Hz, 1 H, BH), 2.07 (s, 9 H, pzCH3), 1.32 (s, 27 H, pz-C(CH3)3), 0.05 (s, 12 H, Al(CH3)4) ppm. 1 H NMR (400 MHz, Tol-d8, -80 C): δ 5.41 (s, 3 H, 4-pz-H), 4.5 (s vbr, 1 H, BH), 1.94 (s, 9 H, pz-CH3), 1.31 (s, 27 H, pz-C(CH3)3), 0.20 (s, 12 H, Al(CH3)4) ppm. 13C NMR (125 MHz, C6D6, 27 C): δ 163.7 (5-pz-C), 146.5 (3-pz-C), 103.3 (4-pz-C), 32.3 (pz-C(CH3)3), 31.4 (pz-C(CH3)3), 13.3 (pz-CH3), -1.0 (s br, Al(CH3)4) ppm. 171Yb{1H} NMR (70 MHz, C6D6, 27 C): δ 505 (s) ppm. 27Al{1H} NMR (104 MHz, C6D6, 27 C): δ 149 (s br) ppm. 11B{1H} NMR (160 MHz, C6D6, 27 C): δ -7.9 (s br) ppm. IR (cm-1): 3116 w, 2960 m, 2866 m, 2477 m (B-H), 1636 w, 1560 w, 1539 s, 1463 m, 1426 s, 1388 w, 1362 s, 1347 s, 1285 w, 1249 w, 1191 s, 1133 w, 1105 m, 1064 w, 1026 w, 1011 w, 983 w, 930 w, 841 m, 781 s, 685 s. Anal. Calcd for C28H52N6AlBYb: C, 49.20; H, 7.67; N, 12.29. Found: C, 49.26; H, 7.59; N, 11.77. Synthesis of (TptBu,Me)Yb(AlEt4) (2b). In a glovebox, 3 equiv of AlEt3 (60 μL, 0.44 mmol) were added dropwise to a stirred solution of (TptBu,Me)Yb[N(SiMe3)2] (1, 111 mg, 0.15 mmol) in 4 mL of pentane. The reaction mixture was stirred another 2 h at ambient temperature while the formation of a yellow precipitate was observed. The product was separated via centrifugation and washed with 2  2 mL of cold pentane to yield (TptBu,Me)Yb(AlEt4) (2b, 84 mg, 0.11 mmol, 77%) as a bright yellow powdery solid. Crystallization from a toluene/pentane solution at -40 C afforded yellow crystals suitable for X-ray diffraction analysis. 1H NMR (600 MHz, C6D6, 27 C): δ 5.64 (s, 3 H, 4-pzH), 4.7 (d vbr, 1JBH = 110 Hz, 1 H, BH), 2.05 (s, 9 H, pz-CH3), 1.68 (t, 12 H, 3JHH 7.8 Hz, Al(CH2CH3)4), 1.34 (s, 27 H, pzC(CH3)3), 0.45 (q, 8 H, 3JHH 7.8 Hz, Al(CH2CH3)4) ppm. 1H NMR (400 MHz, Tol-d8, -60 C): δ 5.48 (s, 3 H, 4-pz-H), 4.6 (s vbr, 1 H, BH), 1.95 (s, 9 H, pz-CH3), 1.82 (s br, 12 H, Al(CH2CH3)4), 1.34 (s, 27 H, pz-C(CH3)3), 0.57 (s br, 8 H, Al(CH2CH3)4) ppm. 13C NMR (100 MHz, C6D6, 27 C): δ 164.0 (5-pz-C), 146.6 (3-pz-C), 103.5 (4-pz-C), 32.2 (pz-C(CH3)3), 31.2 (pz-C(CH3)3), 13.4 (pz-CH3), 12.7 (Al(CH2CH3)4), 8.8 (s br, Al(CH2CH3)4) ppm. 171Yb{1H}

Litlabø et al. NMR (70 MHz, C6D6, 27 C): δ 470 (s) ppm. 27Al{1H} NMR (104 MHz, C6D6, 27 C): δ 148 (s br) ppm. 11B{1H} NMR (160 MHz, C6D6, 27 C): δ -7.9 (s br) ppm. IR (cm-1): 3114 w, 2958 m, 2861 m, 2480 m (B-H), 1563 w, 1539 m, 1462 m, 1425 s, 1361 s, 1347 s, 1286 w, 1242 w, 1191 s, 1105 s, 1058 s, 1026 w, 1012 w, 984 w, 880 w, 842 w, 777 s, 728 w. Anal. Calcd for C32H60N6AlBYb: C, 51.96; H, 8.18; N, 11.36. Found: C, 51.99; H, 8.43; N, 10.23. X-ray Crystallography and Crystal Structure Determination of 2a and 2b. Crystals for X-ray analysis were obtained as described in the preparations. The crystals were manipulated in the glovebox, coated with Paratone-N oil, and transferred to a cold N2 gas stream on the diffractometer. Data collections were performed at -100 C on a Bruker D8/APEX II CCD diffractometer, using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚).33 The structures were solved by using Patterson search/structure expansion (DIRDIF-99)32 (2a) or by direct methods (SHELXS-97)34 (2b). Full-matrix least-squares refinements (on F2) were completed using the program SHELXL97;35 all non-hydrogen atoms were refined with anisotropic displacement parameters. In complex 2b the tert-butyl group attached to the pyrazolyl ring defined by atoms N31, N32, C33, C34, and C35, and the Al atom and the two noncoordinated ethyl groups of the [AlEt4] ligand were disordered. The geometry of the disordered tert-butyl group was restrained to be the same as that of one of the ordered tert-butyl groups by use of the SHELXL SAME instruction. The Al-C5 distances within the disordered tetraethylaluminate were restrained to be equal; the Al-C7 distances were also restrained to be equal.

Acknowledgment. Financial support from the Norwegian Research Council (R.A., Project No. 182547/I30) and NSERC (Discover grant to J.T.) is gratefully acknowledged. Supporting Information Available: CIF files giving full crystallographic data for complexes 2a and 2b. This material is available free of charge via the Internet at http://pubs.acs.org. (33) Programs for diffractometer operation, data collection, data reduction, and absorption correction were those supplied by Bruker. (34) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda, S.; Israel, R.; Gould, R. O.; Smits, J. M. M. The DIRDIF-99 program system; Crystallography Laboratory, University of Nijmegen: The Netherlands, 1999. (35) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.