Trigonal-Planar versus Pyramidal Geometries in the Tris(ring

Oct 2, 2009 - Department of Chemistry, University of California, Irvine, California 92697- .... Filipp Furche , Gerd Meyer , Joseph W. Ziller and Will...
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Organometallics 2009, 28, 6073–6078 DOI: 10.1021/om9006104

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Trigonal-Planar versus Pyramidal Geometries in the Tris(ring) Heteroleptic Divalent Lanthanide Complexes (C5Me5)Ln(μ-η6:η1-Ph)2BPh2: Crystallographic and Density Functional Theory Analysis William J. Evans,*,† Justin R. Walensky,† Filipp Furche,† Antonio G. DiPasquale,§ and Arnold L. Rheingold§ †

Department of Chemistry, University of California, Irvine, California 92697-2025, and Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, MC 0358 La Jolla, California 92093-0358

§

Received July 13, 2009

To examine the difference between the trigonal-planar structure of the tris(ring) heteroleptic 4f14 Yb2þ complex (C5Me5)Yb(μ-η6:η1-Ph)2BPh2, 1, and the pyramidal geometry of the 4f6 Sm2þ analogue (C5Me5)Sm(μ-η6:η1-Ph)2BPh2, 2, the structure of the half-filled-shell 4f7 Eu2þ complex was of interest. (C5Me5)Eu(μ-η6:η1-Ph)2BPh2, 3, was synthesized by reaction of (C5Me5)2Eu with [Et3NH][BPh4] and characterized by X-ray crystallography. All three complexes were analyzed using density functional theory. In addition, (C5Me5)- exchange reactions were performed in order to examine the preference of the divalent lanthanide ions for ligand sets containing one or two (C5Me5)- groups.

Introduction Crystallographic characterization of the pentamethylcyclopentadienyl tetraphenylborate Sm2þ and Yb2þ complexes (C5Me5)Ln(μ-η6:η1-Ph)2BPh2, synthesized according to eq 1,

revealed structures in which three polyhapto aromatic rings surrounded the divalent lanthanide ions.1 In each case, the tetraphenylborate group functioned as a chelate, providing two η6-phenyl ligands. The optimal geometry for packing three rings around a metal ion is trigonal-planar, and this is the structure found for the closed-shell 4f14 Yb2þ compound (C5Me5)Yb(μ-η6:η1-Ph)2BPh2, 1, Figure 1. However, the open-shell 4f6 Sm2þ complex (C5Me5)Sm(μ-η6:η1-Ph)2BPh2, 2, adopted a structure in which the ring centroids of the *Corresponding author. Fax: 949-824-2210. E-mail: wevans@uci. edu. (1) Evans, W. J.; Champagne, T. M.; Ziller, J. W. Organometallics 2007, 26, 1204. (2) (a) Evans, W. J.; Hughes, L. A.; Hanusa, T. P. J. Am. Chem. Soc. 1984, 106, 4270. (b) Evans, W. J.; Hughes, L. A.; Hanusa, T. P. Organometallics 1986, 5, 1285. (c) Schultz, M.; Burns, C. J.; Schwartz, D. J.; Andersen, R. A. Organometallics 2000, 19, 781. (3) (a) Ortiz, J. V.; Hoffmann, R. Inorg. Chem. 1985, 24, 2095. (b) Andersen, R. A.; Boncella, J. M.; Burns, C. J.; Green, J. C.; Hohl, D.; Roesch, N. J. Chem. Soc., Chem. Commun. 1986, 5, 405. (c) Green, J. C.; Hohl, D.; Roesch, N. Organometallics 1987, 6, 712. (d) Kaupp, M.; Schleyer, P. v. R.; Dolg, M.; Stoll, H. J. Am. Chem. Soc. 1992, 114, 8202. (e) Hollis, T. K.; Burdett, J. K.; Bosnich, B. Organometallics 1993, 12, 3385. (f) Boudreaux, E. A.; Baxter, E. Int. J. Quantum Chem. 1994, 28, 565. (g) Bosnich, B. Chem. Soc. Rev. 1994, 387. (h) Timofeeva, T. V.; Lii, J.-H.; Allinger, N. L. J. Am. Chem. Soc. 1995, 117, 7452. r 2009 American Chemical Society

two η6-phenyl groups of the (BPh4)- ligand and ring centroid of the (C5Me5)- ligand had a pyramidal arrangement, Figure 1.1 If these complexes are viewed as bis(ligand) metal complexes, ML2 (L=ligand), in which one ligand is (BPh4)- and the other is (C5Me5)-, the situation is reminiscent of the bent versus linear structural studies of the metallocenes (C5Me5)2Ln2,3 and the heteroleptic (C5Me5)Ln(C8H8)Ln(C5Me5) complexes.4 In those cases, bent structures analogous to that in samarium complex 2 rather than the more common linear ML2 geometry were observed in all cases. Hence, the “pseudolinear” structure of the ytterbium complex 1 is quite unusual. In the metallocene systems, it was observed that the (ring centroid)-metal-(ring centroid) angle could be correlated with metal size.5 The smaller the metal, the larger the (ring centroid)-metal-(ring centroid) angle; that is, the structure is more linear. Since ytterbium is the smaller metal, complex 1 follows this trend. However, in the lanthanide metallocene series this never led to a linear structure: (C5Me5)2Yb has a 145° (ring centroid)-Yb-(ring centroid) angle,2b and in (C5Me5)Yb(C8H8)Yb(C5Me5) it is 160°.3c The structures of the trivalent Ln[N(SiMe3)2]36 and Ln[CH(SiMe3)2]37 complexes are pyramidal rather than trigonal-planar, but this (4) (a) Evans, W. J.; Shreeve, J. L.; Ziller, J. W. Polyhedron 1995, 14, 2945. (b) Evans, W. J.; Clark, R. D.; Ansari, M. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9555. (c) Evans, W. J.; Johnston, M. A.; Greci, M. A.; Ziller, J. W. Organometallics 1999, 18, 1460. (5) Williams, R. A.; Hanusa, T. P.; Huffman, J. C. Organometallics 1990, 9, 1128. (6) (a) Eu: Ghotra, J. S.; Hursthouse, M. B.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1973, 669. (b) La: Clark, D. L.; Gordon, J. C.; Hay, P. J.; Martin, R. L.; Poli, R. Organometallics 2002, 21, 5000. (c) Sm: Brady, E. D.; Clark, D. L.; Gordon, J. C.; Hay, P. J.; Keogh, D. W.; Poli, R.; Scott, B. L.; Watkin, J. G. Inorg. Chem. 2003, 42, 6682. (d) Yb: Niemeyer, M. Z. Anorg. Allg. Chem. 2002, 628, 647. (7) Hitchcock, P. B.; Lappert, M. F.; Smith, R. G.; Bartlett, R. A.; Power, P. P. J. Chem. Soc., Chem. Commun. 1988, 1007. Published on Web 10/02/2009

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Evans et al. Table 1. X-ray Data Collection Parameters for (C5Me5)Eu(BPh4), 3 empirical formula

Figure 1. Side-on views of (C5Me5)Yb(μ-η6:η1-Ph)2BPh2, 1 (left), and (C5Me5)Sm(μ-η6:η1-Ph)2BPh2, 2 (right), showing trigonal-planar arrangement of three ring centroids in 1 and pyramidal arrangement in 2. Thermal ellipsoids were drawn at the 50% probability level. Hydrogen atoms have been excluded for clarity.

deviation from the optimal packing geometry has been explained by additional agostic interactions with the silylmethyl substituents.6b It seemed possible that the difference in structure between 1 and 2 could be due to electronic factors since the ytterbium complex is closed shell and the samarium complex is not. However, with the f elements and particularly the lanthanides, steric factors rather than electronic factors usually predominate. In this regard, it was of interest to synthesize and structurally characterize the 4f7 Eu2þ complex. Since this compound would have a symmetrical half-filled shell, one could argue that it should be like the ytterbium complex. On the other hand, since europium is closer in size to samarium than ytterbium, steric arguments would suggest that it should have a structure similar to that of the samarium complex. To test these ideas, the europium complex was synthesized and structurally characterized. In addition, all three complexes were examined by density functional theory to search for an electronic basis for the structural differences. Exchange reactions between (C5Me5)Ln(BPh4) and (C5Me5)2Ln0 (Ln, Ln0=Sm, Eu, Yb) were also conducted to determine if the difference in structure influenced reactivity.

Results Synthesis and Structure of (C5Me5)Eu(BPh4), 3. Complex 3 was synthesized in good yield (75%) following the procedure shown in eq 1 used to make 1 and 2,1 i.e., by protonation of a (C5Me5)- ligand in (C5Me5)2Eu with [Et3NH][BPh4]. Due to the paramagnetism of Eu2þ (μ=3.47 μB), the 1H NMR spectrum of 3 showed only a broad resonance from -8 to -16 ppm, 4000 Hz wide. Complex 3 had an IR spectrum analogous to that of 1 and 2.1 Complex 3 was analyzed by X-ray diffraction and found to be isomorphous with the samarium complex 2, as shown in Figure 2. Hence, the europium complex does not adopt the trigonal-planar structure expected for an ML3 complex and found for 1, but crystallizes with the two phenyl ring centroids of the (BPh4)- ligand and the (η5-C5Me5)- ring centroid in a pyramidal arrangement like that in 2. Selected bond distances and angles are given in Table 2. As expected for isomorphous complexes of metals of similar radii, the bond distances in 3 are very similar to those in 2 when the

C34H35BEu, 3

fw temp (K) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) volume (A˚3) Z Fcalcd (Mg/m3) μ (mm-1) R1 [I > 2.0σ(I)]a wR2 (all data)a

606.39 100(2) monoclinic P21/c 14.822(3) 9.8770(18) 18.808(4) 90 107.200(3) 90 2630.4(8) 4 1.531 2.406 0.0276 0.0678 P P a 2 2 2 P 2 2 1/2 Definitions: wR2 = [ [w(Fo - Fc ) ]/ [w(Fo ) ] ] , R1 = ||Fo| P - |Fc||/ |Fo|.

0.02 A˚ difference in ionic radii is considered.8 The (C5Me5 centroid)-Ln-(C6H5 ring centroid) angle in 3 (55°) is also similar to that in 1 (56°).1 The 2.774(3)-2.786(3) A˚ Eu-C(C5Me5) bond distances in 3 are in the range observed for Eu2þ metallocenes2,9 and shorter than the 2.837(9)-2.870(10) A˚ lengths in trivalent [(C5Me5)Eu(CH(SiMe3)2)(C5Me5)K(THF)2]n10 and (C5Me5)Eu(C8H8)Eu(C5Me5).4c Density Functional Calculations. In order to explore the structural differences between the trigonal-planar Yb complex and the pyramidal Sm and Eu analogues, density functional theory (DFT) was used to investigate the molecular and electronic structures in the gas phase. A characteristic feature of all three compounds is extremely low harmonic vibrational frequencies for the B-Ln-(C5Me5) bending mode on the order of 20 cm-1, indicating very shallow bending potentials and flexible structures. This is consistent with an ionic bond between the [Ln(C5Me5)]þ units and the tetraphenylborate anions. When such noncovalent interactions are modeled by gas phase calculations, small perturbations caused by crystal packing or inaccuracies in the computational treatment can have large effects on the structures. The computed Ln-(C5Me5) distances agree well with the X-ray crystallography data, Table 3. On the other hand, the Ln-(C6H5) distances are computed to be almost 0.1 A˚ shorter than measured in the crystal. This significant deviation is likely due to differences between the gas phase and the crystal structures rather than inaccuracies of the present computational treatments: TPSS errors in bond distances for similar compounds are smaller than 0.05 A˚ and the results are stable upon addition of Hartree-Fock exchange, e.g., using the TPSSh hybrid functional.11 This suggests that the significant overestimation of the interaction between the [Ln(C5Me5)]þ unit and the tetraphenylborate anion observed in the calculations is caused by the absence of other ions and the resulting low coordination number in the gas phase. The decrease of ionic radii with decreasing coordination number is well established in solid-state chemistry. (8) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (9) Evans, W. J.; Foster, S. E. J. Organomet. Chem. 1992, 433, 79. (10) Hou, Z.; Zhang, Y.; Nishiura, M.; Wakatsuki, Y. Organometallics 2003, 22, 129. (11) Staroverov, V. N.; Scuseria, G. E.; Tao, J.; Perdew, J. P. J. Chem. Phys. 2003, 119, 12129.

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Figure 2. Two views of (C5Me5)Eu(μ-η6:η1-Ph)2BPh2, 3, with the thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms have been excluded for clarity. Table 2. Selected Bond Distances (A˚) and Angles (deg) for (C5Me5)Yb(BPh4), 1,1 (C5Me5)Sm(BPh4), 2,1 and (C5Me5)Eu(BPh4), 3

Ln(1)-Cnt(C5Me5) Ln(1)-Cnt(C6H5) Ln(1)-C(11) Ln(1)-C(12) Ln(1)-C(13) Ln(1)-C(14) Ln(1)-C(15) Ln(1)-C(16) Ln(1)-C(21) Ln(1)-C(22) Ln(1)-C(23) Ln(1)-C(24) Ln(1)-C(25) Ln(1)-C(26) Cnt(C5Me5)-Ln(1)-Cnt(C6H5) Cnt(C5Me5)-Ln(1)-Cnt(C6H5) Cnt(C6H5)-Ln(1)-Cnt(C6H5) Cnt(C5Me5)-Ln(1)-B(1)

C34H40BYb, 1

C34H40BSm, 2

C34H40BEu, 3

2.381 2.605, 2.639 2.926(4) 2.932(4) 2.977(4) 3.015(5) 2.973(4) 2.911(4) 2.895(4) 2.930(5) 3.055(5) 3.128(5) 3.010(5) 2.884(5) 124.7 125.8 109.5 179.4

2.516 2.700, 2.776 2.789(3) 3.035(3) 3.020(3) 3.029(3) 3.054(3) 3.046(3) 3.080(3) 2.996(3) 3.069(4) 3.168(4) 3.191(4) 3.143(3) 125.0 123.8 103.8 152.4

2.505 2.730, 2.819 3.113(3) 3.257(3) 3.298(3) 3.207(3) 3.036(3) 2.953(3) 3.045(2) 2.999(3) 3.062(3) 3.113(3) 3.117(3) 3.059(3) 121.4 125.0 103.3 146.2

Table 3. Computed TPSS/TZVP Bond Distances (A˚) and Angles (deg) for (C5Me5)Yb(BPh4), 1, (C5Me5)Sm(BPh4), 2, and (C5Me5)Eu(BPh4), 3

Ln(1)-Cnt(C5Me5) Ln(1)-Cnt(C6H5) Cnt(C5Me5)-Ln(1)-B(1)

1

2

3

2.382 2.532, 2.533 177.1

2.492 2.635, 2.645 177.8

2.485 2.679, 2.684 177.3

There are no hints of a significant participation of metal f orbitals in bonding; the computed natural f populations of 13.83 for 1, 5.82 for 2, and 6.86 for 3 are very close to the expected values for Yb2þ, Sm2þ, and Eu2þ, respectively. The most striking difference between the computed and the X-ray structures is the nearly trigonal-planar coordination of the Ln atom predicted by the calculations for all three compounds. Nevertheless, the calculations do show a finite but small deviation of the Cnt(C5Me5)-Ln(1)-B(1) angle from linearity by 2-3°, corresponding to a slight polarization of the metal core; see Table 3. This pattern is reminiscent of ionic compounds of heavier group II metals and lanthanides such as (C5Me5)2Ba or (C5Me5)2Eu, showing bent structures in the crystal but quasi-linear structures and increased (C5Me5)2-metal distances in the gas phase.3d A widely accepted explanation is based on a combination of core polarization effects and soft bending potentials.3d

The present results suggest a similar origin of the pyramidal coordination of 2 and 3 in the crystal: going from the gas phase to the crystal structure, the Ln-C6H5 distance increases by almost 0.1 A˚, which further softens the bending potential and makes the linear structures increasingly unstable. Core polarization and packing effects are thus strong enough to cause significant bending of 2 and 3, as in heavy group II metallocenes. In the Yb compound 1, the Ln-C6H5 distance is shorter than in 2 and 3 by more than 0.1 A˚. Since Yb is smaller and less polarizable than Sm and Eu, it is plausible that 1 essentially retains the gas phase structure even in the crystal. Crystal Packing. The extended packing of each complex was examined in order to further probe the nature of the structural arrangements. Since the closest intermolecular contacts in each of the molecules involve hydrogen atoms that were put into the crystallographic refinement model in fixed positions, the accuracy of these numbers is limited. However, the numbers can be used in a comparative way to evaluate packing in the three cases. The closest intermolecular approach in 3 is estimated to be the 2.35 A˚ distance between H(24B) on a η1:η6-phenyl ring and H(32A) on a phenyl substituent attached only to boron in another molecule of 3 in the unit cell, Figure 3. This distance is near the 2.4 A˚ sum of the van der Waals radii of

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Figure 3. Thermal ellipsoid plot of the crystal packing in 3 shown at the 50% probability level.

two hydrogen atoms.12 The next closest interaction is the 2.49 A˚ distance between the H(6CB) position of a (C5Me5)ligand and the H(42A) atom in a phenyl ring attached only to boron in an adjacent molecule. The closest intramolecular interaction occurs between H(24B) and H(6CB), which is 2.41 A˚, while the corresponding distance on the opposite side involving H(8BB)-H(14B) is 2.49 A˚. The analogous distances in 2, in the same respective order, are 2.29 and 2.34 A˚ (intermolecular) and 2.34 and 2.46 A˚ (intramolecular). In both complexes, the rings are clearly tightly packed both inter- and intramolecularly. In contrast to this situation in 2 and 3, the closest nonbonding interactions between rings in 1 are intramolecular, not intermolecular. There are no intermolecular interactions less than 3 A˚, but there are two intramolecular nonbonding distances at or below the sum of the van der Waals radii. As shown in Figure 4, these are the 2.25 A˚ H(54A)H(43C) and 2.41 A˚ H(48A)-H(41A) distances, each of which involves an η1:η6-phenyl ring hydrogen and a (C5Me5)-. Since the ytterbium ion is smaller than samarium and europium, the three rings around the metal are closer together in 1 than in 2 and 3. Since this makes the coordination environment around ytterbium more crowded, it may require the sterically optimal trigonal-planar arrangement. It can then be rationalized that the shorter intramolecular contacts in 1 are more important than intermolecular interactions in this system. On the other hand, for the larger ions, the intermolecular close contacts may dominate the structure. A pyramidal structure rather than trigonal may optimize the overall packing by minimizing the intermolecular contacts. The structural variation that could occur depending on long-range contacts is consistent with the DFT calculations that show a low activation barrier for rotation of the rings. In addition to these observations, examination of the packing revealed a 2.91 A˚ distance between the europium metal center in 3 and a hydrogen, H(35), on a phenyl ring of another molecule of 3 in the unit cell, Figure 5. The next nearest interaction involving Eu is the 3.58 A˚ distance to H(34) on a phenyl ring of another molecule. The analogous Sm distances in 2 are 2.93 and 3.74 A˚, respectively. The 2.91-2.93 A˚ distances are shorter than the 3.21(2) A˚ observed for agostic interactions in NaEu[N(SiMe3)2]313 and (12) Klein, R. A. Chem. Phys. Lett. 2006, 425, 128. (13) Tilley, T. D.; Andersen, R. A.; Zalkin, A. Inorg. Chem. 1984, 23, 2271.

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Figure 4. Thermal ellipsoid plot of the crystal packing in 1 shown at the 30% probability level.

Figure 5. Thermal ellipsoid plot of extended packing of complex 3 shown at the 50% probability level.

3.32(1)-3.46(1) A˚ in [(Me3Si)2N]2Sm(THF)2.14 As shown in Figure 5, H(34) is oriented toward Eu from the open side of the structure created by the pyramidal arrangement of rings. The figure also shows that these Eu 3 3 3 H distances form pairs of molecules such that along with Eu(1D) 3 3 3 H(34B) there is a Eu(1B) 3 3 3 H(34D). These types of Ln 3 3 3 H interactions are not found, nor are they possible, in trigonalplanar 1. (C5Me5)- versus (BPh4)- Competition Reactions. Exchange reactions between (C5Me5)Ln(BPh4) and (C5Me5)2Ln0 (Ln, Ln0 = Sm, Eu, Yb) were examined to obtain more information about the kinetic and thermodynamic preferences of the divalent lanthanide ions for (C5Me5)- versus (BPh4)- as a function of metal size and electron configuration.15 All combinations were examined, but exchange was only observed for the reactions between (C5Me5)Yb(BPh4), 1, and (C5Me5)2Sm, eq 2, (C5Me5)Yb(BPh4), 1, and (C5Me5)2Eu, eq 3, and (C5Me5)Eu(BPh4), 3, and (C5Me5)2Sm, eq 4. No evidence of starting (14) Evans, W. J.; Drummond, D. K.; Zhang, H.; Atwood, J. L. Inorg. Chem. 1988, 27, 575. (15) The reaction chemistry of (C5Me5)Ln(BPh4), Ln = Sm, Yb, has been found to include ligand redistribution: Evans, W. J.; Walensky, J. R.; Champagne, T. M.; Ziller, J. W.; DiPasquale, A. G.; Rheingold, A. L. J. Organomet. Chem. 2009, 694, 1238.

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material or any other byproducts was found in eqs 2-4; that is, the equilibrium lies far to the right in each case. Consistent with that, combinations of (C5Me5)2Yb with (C5Me5)Sm(BPh4), 2, and with (C5Me5)Eu(BPh4), 3, showed no change. The reaction of (C5Me5)Sm(BPh4), 2, and (C5Me5)2Eu was difficult to monitor due to the high paramagnetism and nuclear spin (I= 7/2) of Eu, but 2 was identified and no (C5Me5)2Sm was observed in the 1H NMR spectrum.

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electrostatic arguments. The three possible Ln/Ln0 combinations each favor the products in which Ln0 is the smaller lanthanide. Hence, the (C5Me5)- ligand, which has a full formal negative charge on the C5 ring, prefers the smaller metal with the higher charge to radius ratio, compared to the (BPh4)- ligand, which has a more delocalized formal negative charge. It is interesting to note that even the small change in radial size from Sm to Eu influences the coordination chemistry of these lanthanide complexes.

Conclusion The importance of the size of the metal in lanthanide chemistry is demonstrated by the structure of (C5Me5)Eu(BPh4), which is pyramidal like its samarium analogue rather than trigonal-planar like that of the smaller ytterbium. Exchange reactions between (C5Me5)Ln(BPh4) and (C5Me5)2Ln0 also are influenced by radial size, with the smaller metal of higher charge to radius ratio preferring the most electron rich ligand, (C5Me5)-.

Experimental Section Discussion Since the pyramidal structure of the 4f7 complex (C5Me5)Eu(BPh4), 3, matches the structure of the 4f6 samarium analogue, 2, rather than the 4f14 ytterbium compound, 1, it appears that there is no correlation between half-filled and filled shell species that differentiates their structures from those of ions with unsymmetrically filled electron configurations. This is consistent with the traditional view that the 4f orbitals are not highly influential due to their limited radial extension.3c The DFT calculations on the molecular species in the gas phase support the idea that the 4fn configuration is not a determining factor in the structure since the same structure is predicted for each complex. Since the DFT calculations predict the sterically optimized (nearly) trigonal-planar structure in each case, this suggests that the deviation from trigonal planar in 1 to pyramidal in 2 and 3 arises from factors that could be intermolecular, i.e., packing forces in the solid state. The trigonal-planar arrangement found for the smallest metal, Yb, is optimal for both electronic (based on the DFT calculations) and steric reasons. Examination of intra- and intermolecular close contacts suggests that due to the shorter bond distances with the smaller ytterbium, close contacts between the three rings require the geometry that provides the most space to each ligand, i.e., trigonal-planar. With the larger metals, intramolecular contacts are less problematic and intermolecular packing may be more influential. This may lead to the bent structures. The bent structures allow the Ln 3 3 3 H agostic contacts in 2 and 3 that may also tip the balance between trigonalplanar and pyramidal in this system. Polarization3d of the comparatively large Sm and Eu cores may be an additional factor responsible for the bent structures of 2 and 3. The structures of the (C5Me5)Ln(BPh4) complexes of Sm, Eu, and Yb are like those of the (C5Me5)2Ln complexes in that the two largest metals are very similar and the smaller metal has a structure that favors more room for the ligands. The exchange reactions between (C5Me5)Ln(BPh4) and (C5Me5)2Ln0 can be rationalized on the basis of simple

General Experimental Procedures. The syntheses and manipulations described were conducted with rigorous exclusion of air and water using Schlenk, vacuum line, and glovebox techniques. All syntheses were conducted in an argon atmosphere glovebox that was free of coordinating solvents. Solvents were sparged with UHP argon and dried by passage through columns containing Q-5 and molecular sieves. Benzene-d6 and toluene-d8 (Cambridge Isotope Laboratories) were dried over NaK alloy and benzophenone, degassed by three freeze-pump-thaw cycles, and vacuum transferred. NMR experiments were conducted with Bruker 400 or 500 MHz spectrometers. Elemental analysis was performed by Desert Analytics (Tucson, AZ). (C5Me5)2Eu,2b (C5Me5)2Sm,1,2a (C5Me5)2Yb,1,2c,16 (C5Me5)Ln(BPh4) (Ln = Sm, Yb),1 and [Et3NH][BPh4]17 were prepared by literature methods. (C5Me5)Eu(BPh4), 3. Et3NHBPh4 (144 mg, 0.338 mmol) was added slowly to a stirred solution of (C5Me5)2Eu (136 mg, 0.322 mmol) in benzene (10 mL). After 5 min, the solvent was removed to yield an orange microcrystalline solid (146 mg, 75%). 1 H NMR (C6D6, 298 K): δ -8 to -16 ppm (br). Single crystals suitable for X-ray diffraction were grown by slow evaporation of C6D6 at 25 °C. Anal. Calcd for C34H35BEu: C, 67.34; H, 5.82; B, 1.78; Eu, 25.06. Found: C, 67.55; H, 5.96; B, 1.49; Eu, 24.70. IR (KBr): 3058m, 3038s, 2958s, 2922s, 2853s, 1478s, 1428s, 1377w, 1262w, 1186w, 1159w, 1030s, 856m, 746s, 709s, 604s cm-1. (C5Me5)- Exchange. (C5Me5)Yb(BPh4) (10 mg, 0.016 mmol) and (C5Me5)2Sm (7 mg, 0.02 mmol) were combined in a scintillation vial in d8-toluene (1 mL), stirred for 10 min, and transferred to an NMR tube. The 1H NMR spectrum revealed (C5Me5)Sm(BPh4) and (C5Me5)2Yb as the only products with no starting material remaining. (C5Me5)Eu(BPh4) (10 mg, 0.016 mmol) and (C5Me5)2Sm (7 mg, 0.02 mmol) were added in d8toluene and transferred to an NMR tube. The 1H NMR spectrum revealed (C5Me5)Sm(BPh4) as a broad resonance from -2.3 to -3.8 ppm. (C5Me5)2Eu could not be seen. (C5Me5)Yb(BPh4) (10 mg, 0.016 mmol) and (C5Me5)2Eu (7 mg, 0.02 mmol) were similarly combined in d8-toluene and transferred to an NMR tube. The 1H NMR spectrum revealed (C5Me5)Eu(BPh4) as a broad resonance from -8 to -16 ppm and a singlet for (C5Me5)2Yb at 1.96 ppm. (16) Burns, C. J.; Andersen, R. A. J. Organomet. Chem. 1987, 325, 31. (17) Evans, W. J.; Johnston, M. A.; Greci, M. A.; Gummersheimer, T. S.; Ziller, J. W. Polyhedron 2003, 22, 119.

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Computational Details. The structures of 1, 2, and 3 were optimized using the Tao, Perdew, Staroverov, and Scuseria meta-GGA functional (TPSS)18 and polarized triple-ζ valence basis sets (def2-TZVP, def-TZVPP for Sm, Eu, Yb)19 along with Stuttgart pseudopotentials20 for the lanthanide atoms. These small-core pseudopotentials use an [Ar]3d10 core and are capable of describing core polarization. The multipole-accelerated (18) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401. (19) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 18, 3297. (20) Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1989, 90, 1730. € (21) (a) Eichkorn, K.; Treulter, O.; Ohm, H.; H€aser, M; Ahlrichs, R. Chem. Phys. Lett. 1995, 242, 652. (b) Sierka, M.; Hogekamp, A.; Ahlrichs, R. J. Chem. Phys. 2003, 118, 9136. (c) Weigend, F. Phys. Chem. Chem. Phys. 2006, 9, 1057. (22) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346. (23) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (24) TURBOMOLE V5-10; Turbomole GmbH: Karlsruhe, 2007, http://www.turbomole.com/.

Evans et al. resolution of the identity (MARI-J) approximation for the Coulomb energy was used throughout.21 The structures investigated here made it necessary to use very fine quadrature grids that were of size m522 and included quadrature weight derivatives in the gradient calculations, and converge structures tightly (maximum gradient norm