Synthesis, Structural Characterization, and Cyclometalation Chemistry

May 11, 2015 - The κ1-m-terphenyl complex of tantalum, [ArTol2]Ta(NMe2)3Cl ([ArTol2] = 2,6-di-p-tolylphenyl), has been synthesized by the reaction of...
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Synthesis, Structural Characterization, and Cyclometalation Chemistry of Tantalum Terphenyl Compounds Aaron Sattler and Gerard Parkin* Department of Chemistry, Columbia University, 3000 Broadway, MC 3115, New York, New York 10027, United States S Supporting Information *

ABSTRACT: The κ1-m-terphenyl complex of tantalum, [ArTol2]Ta(NMe2)3Cl ([ArTol2] = 2,6-di-p-tolylphenyl), has been synthesized by the reaction of [Ta(NMe2)3Cl2]2 with two equivalents of [ArTol2]Li. [ArTol2]Ta(NMe2)3Cl provides access to a variety of monoalkyl compounds, [ArTol2]Ta(NMe2)3R (R = Me, Et, Prn, Bun, and Np; Np = CH2But), via the reactions with the corresponding RLi. In addition, the reaction of [Ta(NMe2)3Cl2]2 with excess [ArTol2]Li affords the bis(terphenyl) complex, [ArTol2]2Ta(NMe2)3, while the reaction of [ArTol2]Ta(NMe2)3Cl with LiBH4 gives the borohydride complex, [ArTol2]Ta(NMe2)3(κ2-BH4). The dichloride compound, [ArTol2]Ta(NMe2)2Cl2, which is obtained via the reaction of [ArTol2]Ta(NMe2)3Cl with Me3SiCl, provides access to a series of dialkyl derivatives, [ArTol2]Ta(NMe2)2R2 (R = Me, Et, Prn, Bun, and Np), via the reactions with the corresponding RLi. The κ1-mterphenyl ligands in these complexes are susceptible to metalation. Thus, [ArTol2]Ta(NMe2)3R eliminates RH to afford [κ2ArTol,Tol′]Ta(NMe2)3 (Tol′ = C6H3Me), while [ArTol2]Ta(NMe2)2Np2 eliminates NpH to form [κ2-ArTol,Tol′]Ta(NMe2)2Np.

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([ArTol2]) to synthesize [ArTol2]TaMe3Cl and [ArTol2]TaMe2Cl2 represents, to the best of our knowledge, the first application of terphenyl ligands to tantalum chemistry.6

INTRODUCTION

Ancillary ligands play a critical role in organometallic chemistry by allowing tunable properties of a metal center.1 One such class that has received considerable attention is that of the mterphenyl ligand ([ArAr2]) (Figure 1), which provides significant steric protection by virtue of the outer aryl rings. Two common examples are [ArMes2] and [ArDipp2] (Figure 1).2 Indeed,

Figure 2. Elements for which κ1-m-terphenyl compounds have been structurally characterized by X-ray diffraction (blue), and those for which there are no structurally characterized examples listed in the Cambridge Structural Database (red). Figure 1. Examples of [κ1-m-terphenyl] ligands.

While many of the applications of m-terphenyl ligands have focused on the use of steric protection to allow for the isolation of uncommon and low coordination complexes,2 our interest pertains to the potential of using such ligands to construct multidentate variants via cyclometalation. As an illustration, we recently employed this approach to obtain the first examples of transition metal compounds that feature a [CCC] X3-donor pincer ligand,36 namely [κ3-ArTol′2]Ta(PMe3)2MeCl, [κ3-

structurally characterized [κ1-ArAr2]M compounds are known for a substantial number of metals, including transition metals (Sc,3 Y,3,4 Ti,5 Ta,6 Cr,7 Mo,8 Mn,7d,9 Fe,7d,9a−f,10 Co,7d,9b−d,11 Cu,12 Ag,12a,13 Au14), main group metals (Li,15 Na,16 Be,17 Mg,6,11a Ca,18 Zn,19 Cd,19a,20 Hg,16,19a Al,21 Ga,15d,22 In,23 Tl,24 Ge,25 Sn,26 Pb,27 As,28d,29 Sb,28 Bi,28d,30), and lanthanides (Sm,31,32b Eu,32d Er,33 Tm,4,34 Yb,3,4,32 Lu35). However, despite this interest in the use of terphenyl ligands, there are still many elements that have received little attention (Figure 2). In this regard, our recent use of the 2,6-di-p-tolylphenyl ligand © 2015 American Chemical Society

Received: November 17, 2014 Published: May 11, 2015 1828

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Organometallics ArTol′2]Ta(PMe3)2Me2, [κ3-ArTol′2]Ta(PMe3)2Cl2, and [κ3ArTol′2]Ta(PMe3)2(η6-C6H6) (Tol′ = C6H3Me).6 For example, the pincer complex, [κ3-ArTol′2]Ta(PMe3)2MeCl was obtained by both (i) addition of PMe3 to the terphenyl complex, [ArTol2]TaMe3Cl, and (ii) treatment of Ta(PMe3)2Me3Cl2 with [ArTol2]Li. Such compounds provide valuable complements to other complexes that feature X3-donor pincer ligands37,38 and also those that feature LX239 and L2X40 [CCC] donors. Remarkably, even though hundreds of metal−terphenyl complexes have been synthesized, there were no examples of structurally characterized cyclometalated transition metal derivatives listed in the Cambridge Structural Database41,42 prior to our report of the use of cyclometalation to form a [CCC] pincer ligand.6 Thus, while cyclometalation43 is a widely studied and well-known subset of C−H bond activation,44 it is essentially absent with respect to terphenyl chemistry. Therefore, we report here the synthesis of other terphenyl tantalum compounds and their subsequent C−H bond cleavage cyclometalation reactions.

Scheme 2

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RESULTS AND DISCUSSION Synthesis of [ArTol2]Ta(NMe2)3X (X = Cl, R, BH4). In addition to utilizing [ArTol2]TaMe3Cl to provide access to pincer complexes via formal hydrogen atom abstraction from an aryl group,6 we have investigated related compounds that have the potential to undergo similar transformations. Specifically, since Me2N ligands are capable of hydrogen atom abstraction reactions via elimination of Me2NH,45 we decided to synthesize the dimethylamido counterpart of [ArTol2]TaMe3Cl, namely [ArTol2]Ta(NMe2)3Cl (1−Cl). Indeed, the latter compound can be obtained via the reaction of [Ta(NMe2)3Cl2]246,47 with two equivalents of [Ar Tol 2 ]Li, as illustrated in Scheme 1. Furthermore, the bis(terphenyl) complex, [ArTol2]2Ta(NMe2)3 (1−Ar Tol 2 ), can be obtained upon treatment of [Ta-

Figure 3. Molecular structure of [ArTol2]Ta(NMe2)3Cl.

Scheme 1

(NMe2)3Cl2]2 with excess [ArTol2]Li or by reaction of [ArTol2]Ta(NMe2)3Cl with [ArTol2]Li (Scheme 2). The molecular structures of [ArTol2]Ta(NMe2)3Cl and [ArTol2]2Ta(NMe2)3 have been determined by X-ray diffraction, as illustrated respectively in Figures 3 and 4. The geometries of five-coordinate compounds are often expressed in terms of the τ5 five-coordinate geometry index, which corresponds to (β − α)/60, where β − α is the difference between the two largest angles.48 In this regard, the τ5 values (Table 1) indicate that both [ArTol2]Ta(NMe2)3Cl (0.37) and [ArTol2]2Ta(NMe2)3 (0.36) adopt geometries that are significantly distorted from trigonal bipyramidal (with an idealized value of 1.00) towards a square pyramidal geometry (with an idealized value of 0.00). Nevertheless, from the perspective of the trigonal bipyramidal description, the axial sites of [ArTol2]Ta(NMe2)3Cl are occupied by NMe2 and Cl ligands [with N−Ta−Cl = 162.1(1)°], while

Figure 4. Molecular structure of [ArTol2]2Ta(NMe2)3.

the axial sites of [ArTol2]2Ta(NMe2)3 are occupied by two NMe2 ligands [with N−Ta−N = 160.8(2)°]. Another interesting feature of the structures of [ArTol2]Ta(NMe2)3Cl and [ArTol2]2Ta(NMe2)3 is that the tantalum atoms are displaced from the plane of the terphenyl ligands (i.e., the plane of the central aryl group), as measured by their Ta− Cipso−Cpara angles (Table 1). Thus, the displacement of the tantalum atom of [ArTol2]Ta(NMe2)3Cl is characterized by a Ta−Cipso−Cpara angle of 162.8°, while the two terphenyl ligands of [ArTol2]2Ta(NMe2)3 exhibit displacements of 154.4° and 177.3°,49 despite the fact that the Ta−C bond lengths are 1829

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Organometallics Table 1. Selected Metrical Data Pertaining to {[ArTol2]Ta} Derivatives τ5

Ta−Cipso−Cpara (deg)

d(Ta−CAr) (Å)

[ArTol2]Ta(NMe2)3Cl [ArTol2]2Ta(NMe2)3

0.366 0.359

[ArTol2]Ta(NMe2)3Me [ArTol2]Ta(NMe2)3Et [ArTol2]Ta(NMe2)3Prn [ArTol2]Ta(NMe2)3Bun [ArTol2]Ta(NMe2)3Np [ArTol2]Ta(NMe2)3(κ2-BH4) [ArTol2]Ta(NMe2)2Cl2b [ArTol2]Ta(NMe2)2Me2 [ArTol2]Ta(NMe2)2Et2

0.342 0.729 0.578 0.419 0.489 0.366a 0.433 0.477 0.491 0.823

162.8 177.3 154.4 174.5 161.0 160.7 176.9 179.3 174.9 166.5c 168.0c 180.0 169.2

2.248(3) 2.296(6) 2.307(6) 2.255(3) 2.240(4) 2.246(4) 2.268(2) 2.274(2) 2.260(2) 2.208(5) 2.197(5) 2.221(2) 2.198(3)

[ArTol2]Ta(NMe2)2Prn2

0.537

178.3

2.229(4)

[ArTol2]Ta(NMe2)2Bun2

0.517d

176.3

2.221(2)

[ArTol2]Ta(NMe2)2Np2

0.781

177.2

2.225(3)

[ArTol2]TaMe3Cle

0.629

145.1

2.116(3)

[ArTol2]TaMe2Cl2e

0.843

157.1

2.141(4)

[κ2-ArTol,Tol′]Ta(NMe2)3

0.640

171.9

[κ2-ArTol,Tol′]Ta(NMe2)2Np

0.348

162.3

2.297(3) 2.204(3) 2.264(5) 2.219(5)

d(Ta−CR) (Å)

Δ (Å)

2.162(3) 2.196(4) 2.210(4) 2.161(3) 2.174(2)

0.093 0.044 0.036 0.107 0.100

2.167(2) 2.170(4) 2.175(4) 2.177(4) 2.173(5) 2.164(3) 2.184(4)f 2.189(7)f 2.170(3) 2.172(3) 2.134(4) 2.150(4) 2.190(4) 2.145(5) 2.159(4)

0.054 0.028 0.023 0.052 0.056 0.057 0.037 0.032 0.055 0.053 0.018 0.034 0.074 0.004 0.018

2.200(5)

0.064 0.019

The listed τ5 value assumes that the borohydride ligand occupies a single coordination site that is represented by boron. bValues for two crystallographically independent molecules. cThe values correspond to an in-plane, rather than out-of-plane distortion. dValue does not include a minor component of a disordered n-butyl group. eData taken from ref 6. fTwo components of a disordered butyl group. a

As observed for [ArTol2]Ta(NMe2)3Cl, the tantalum coordination geometries in the alkyl compounds [ArTol2]Ta(NMe2)3R are distorted from trigonal bipyramidal towards a square pyramidal geometry (Table 1). However, in contrast to [ArTol2]Ta(NMe2)3Cl, in which the chloride ligand is located in an axial site with respect to the perspective of a trigonal bipyramidal geometry, the alkyl ligands of [ArTol2]Ta(NMe2)3R are located in equatorial sites, similar to the aryl ligands of [ArTol2]2Ta(NMe2)3 (Figure 4). This site preference is in accord with previous studies which predict that, for d0 metal complexes with a trigonal bipyramidal geometry, the more electronegative ligands favor the axial sites.51 On this basis, the alkyl groups would be expected to reside in the equatorial site, although other factors, such as steric interactions and π-donor effects, could also play a role. Similar to [ArTol2]Ta(NMe2)3Cl and [ArTol2]2Ta(NMe2)3, the tantalum atoms of [ArTol2]Ta(NMe2)3R are also displaced from the aryl plane (Table 1). Thus, the Ta−Cipso−Cpara angles of [ArTol2]Ta(NMe2)3R range from 160.7° to 179.3° but, interestingly, there is no discernible trend based on steric factors. For example, the neopentyl compound [ArTol2]Ta(NMe2)3Np (179.3°) exhibits the smallest displacement, while the ethyl derivative [ArTol2]Ta(NMe2)3Et (161.0°) exhibits a substantial displacement. Moreover, in contrast to the ethyl complex, the methyl derivative [ArTol2]Ta(NMe2)3Me (174.5°) shows relatively little displacement. Despite the substantial

similar [2.296(6) Å and 2.307(6) Å]. The variable displacements of the metal from the plane indicate that the κ1-terphenyl ligand is rather flexible in this system and can adjust its position to accommodate the steric demands of other ligands. κ1Terphenyl compounds do not typically exhibit displacements of this magnitude,50 although substantial distortions have been observed in compounds that feature two terphenyl ligands,32a,d as illustrated by the observation that [ArPh2]2Eu(THF)2 possesses Eu−Cipso−Cpara angles of 145.4° and 159.9°.32d It is also pertinent to note that not only do κ1-terphenyl ligands exhibit out-of-plane distortions, but in-plane distortions have also been observed. For example, the chromium complex i {[ArPr 2]Cr(μ-Cl)}2 exhibits Cr−Cipso−Cortho angles of 99.9° and 142.0°,9d which deviate considerably from the average value of 122.5° for structurally characterized M−aryl compounds listed in the Cambridge Structural Database.41 In addition to metathesis of the chloride ligand of [ArTol2]Ta(NMe2)3Cl with [ArTol2]Li providing access to the bis(terphenyl) complex [ArTol2 ]2 Ta(NMe 2 ) 3 , [Ar Tol 2]Ta(NMe2)3Cl also provides a means to obtain a series of alkyl derivatives, [ArTol2]Ta(NMe2)3R (1−R: R = Me, Et, Prn, Bun, or Np; Np = CH2But), via reaction with RLi (Scheme 2). The molecular structures of each of the tantalum alkyl complexes has been determined by X-ray diffraction, as illustrated in Figures 5−9. 1830

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Organometallics

Figure 8. Molecular structure of [ArTol2]Ta(NMe2)3Bun.

Figure 5. Molecular structure of [ArTol2]Ta(NMe2)3Me.

Figure 6. Molecular structure of [ArTol2]Ta(NMe2)3Et.

Figure 9. Molecular structure of [ArTol2]Ta(NMe2)3Np.

Figure 7. Molecular structure of [ArTol2]Ta(NMe2)3Prn. Figure 10. Molecular structure of [ArTol2]Ta(NMe2)3(κ2-BH4).

range of Ta−Cipso−Cpara angles, however, the Ta−[ArTol2] bonds in [ArTol2]Ta(NMe2)3X fall in the narrow range of 2.240(4)−2.307(6) Å (i.e., 0.067 Å) and are comparable to the mean bond length of 2.22 Å for structurally characterized tantalum aryl compounds listed in the Cambridge Structural Database.41 Furthermore, the Ta−R bond lengths also have a narrow range of 2.161(3)−2.210(4) Å (i.e., 0.05 Å), all of which are slightly shorter than the Ta−[ArTol2] bonds by 0.04− 0.11 Å (Table 1). In addition to affording a series of alkyl compounds, [ArTol2]Ta(NMe2)3R, the borohydride compound [ArTol2]Ta(NMe2)3(κ2-BH4) has also been obtained by treatment of [ArTol2]Ta(NMe2)3Cl with LiBH4 (Scheme 2) and the molecular structure has been determined by X-ray diffraction

(Figure 10). Tantalum borohydride compounds are not common, with there being only one structurally characterized example, namely Ta2(μ-BH3)(μ-dmpm)3(κ2-BH4)2,52 listed in the Cambridge Structural Database.41,53 The Ta···B distance within [ArTol2]Ta(NMe2)3(κ2-BH4) [2.570(2) Å] is comparable to those in Ta2(μ-BH3)(μ-dmpm)3(κ2-BH4)2 (2.54−2.59 Å),52 and the bonding in both complexes may be viewed as comprising two 3-center-2-electron Ta−H−B bonding interactions.54 In solution, the terphenyl compounds [ArTol2]Ta(NMe2)3R exhibit limited stability and undergo cyclometalation at elevated temperatures (100 °C), with concomitant elimination of RH, to form [κ2-ArTol,Tol′]Ta(NMe2)3 (2), as illustrated in Scheme 3. 1831

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Organometallics Scheme 3

Table 2. Ta−C Bond Lengths Pertaining to Coordination of [κ2-ArTol,Tol′] and [κ3-ArTol′2] Ligands

[κ2-ArTol,Tol′]Ta(NMe2)3 [κ2-ArTol,Tol′]Ta(NMe2)2Np [κ3-ArTol′2]Ta(PMe3)2Me2a [κ3-ArTol′2]Ta(PMe3)2Cl2a [κ3-ArTol′2]Ta(PMe3)2MeCla [κ3-ArTol′2]Ta(PMe3)2(η6-C6H6)a a

Ta−Ccent (Å)

Ta−Clatt#1 (Å)

Ta−Clatt#2 (Å)

2.297(3) 2.264(5) 2.200(5) 2.227(4) 2.190(3) 2.243(2)

2.204(3) 2.219(5) 2.230(5) 2.207(3) 2.230(3) 2.356(2)

2.243(5) 2.207(3) 2.227(3) 2.362(2)

Data taken from ref 6.

The borohydride compound, [ArTol2]Ta(NMe2)3(κ2-BH4) (1−BH4), is stable at room temperature but may be induced to undergo cyclometalation by addition of PMe3. It is postulated that the cyclometalation of [ArTol2]Ta(NMe2)3(κ2-BH4) occurs via elimination of H2 from a hydride intermediate, [[ArTol2]Ta(NMe2)3H], which is generated by release of Me3P→BH356 upon addition of PMe3. Despite the fact that [ArTol2]2Ta(NMe2)3, [ArTol2]Ta(NMe2)3R, and [ArTol2]Ta(NMe2)3(κ2BH4) undergo cyclometalation with different facility, a common feature is that the transformations do not involve elimination of Me2NH. As such, this observation indicates that, in this system, the Me2N ligands are less prone than either [ArTol2], R, or H to abstract a hydrogen from an [ArTol2] ligand. This result could be a reflection of (i) Ta−N bonds (accompanied by π-donation) being stronger than Ta−H and Ta−C bonds57,58 and (ii) C−H and H−H bonds being stronger than N−H bonds,59 both of

The bis(terphenyl) compound, [ArTol2]2Ta(NMe2)3, also undergoes cyclometalation to form [κ2-ArTol,Tol′]Ta(NMe2)3 and [ArTol2]H. However, the transformation is much more facile for [ArTol2]2Ta(NMe2)3, occurring over a period of 1 day at room temperature. Spectroscopically, the formation of [κ2-ArTol,Tol′]Ta(NMe2)3 is indicated by a loss of symmetry associated with the terphenyl ligand. Thus, the [κ2-ArTol,Tol′] moiety is characterized by the observation of (i) two inequivalent methyl groups in both the 1 H [δ(1H) = 2.19 and 2.38 ppm] and 13C [δ(13C{1H}) = 21.2 and 21.9 ppm] NMR spectra, (ii) eight aromatic CH signals, rather than the four observed for [ArTol2]Ta(NMe2)3X complexes, and (iii) two 13C signals with chemical shifts of 197.7 and 199.6 ppm that are distinctive for aryl carbons bonded to tantalum.55 The molecular structure of [κ2-ArTol,Tol′]Ta(NMe2)3 has also been determined by X-ray diffraction (Figure 11) and possesses

Scheme 4

Figure 11. Molecular structure of [κ2-ArTol,Tol′]Ta(NMe2)3.

a τ5 value of 0.64, which indicates a significant distortion from a trigonal bipyramidal geometry. With respect to the κ2coordination mode, it is pertinent to note that although the Ta−C bond lengths of 2.204(3) Å and 2.297(3) Å (Table 2) for [κ2-ArTol,Tol′]Ta(NMe2)3 are comparable to those of the κ1terphenyl compounds (Table 1), the Ta−C bond involving the lateral aryl group is distinctly shorter (by 0.093 Å) than that for the central aryl group. This observation also contrasts with the fact that the central Ta−C bond is shorter than the lateral Ta−C bond in the majority of the κ3-pincer complexes (Table 2). 1832

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Figure 15. Molecular structure of [ArTol2]Ta(NMe2)2Prn2. Figure 12. Molecular structure of [ArTol2]Ta(NMe2)2Cl2.

Figure 16. Molecular structure of [ArTol2]Ta(NMe2)2Bun2.

Figure 13. Molecular structure of [ArTol2]Ta(NMe2)2Me2.

Figure 17. Molecular structure of [ArTol2]Ta(NMe2)2Np2.

Figure 14. Molecular structure of [ArTol2]Ta(NMe2)2Et2.

(NMe2)2Cl2 and [ArTol2]Ta(NMe2)2R2 (R = Me, Et, Prn, Bun, and Np) have been determined by X-ray diffraction, as illustrated in Figures 12−17. An interesting feature of the structures of the dialkyl compounds, [ArTol2]Ta(NMe2)2R2, is that, by comparison to the monoalkyls, [ArTol2]Ta(NMe2)3R, the terphenyl ligands do not display any significant distortions and the tantalum lies close to the aryl plane.60 For example, the Ta−Cipso−Cpara angles are in the range of 169.2−180.0° (Table 1). Thus, the dialkyls have molecular geometries that have approximate C2 symmetry, with the alkyl groups located in the equatorial plane

which would provide a driving force for elimination of RH or H2 rather than Me2NH. Synthesis of [ArTol2]Ta(NMe2)2X2 (X = Cl, R). In addition to synthesizing the monoalkyl compounds, [Ar Tol2]Ta(NMe2)3R, we have also synthesized dialkyl compounds, [ArTol2]Ta(NMe2)2R2 (3-R), by the sequence illustrated in Scheme 4. Specifically, the [ArTol2]Ta(NMe2)2R2 complexes are obtained via the dichloride complex [ArTol2]Ta(NMe2)2Cl2 (3Cl), which is formed by treatment of [ArTol2]Ta(NMe2)3Cl with Me3SiCl. The molecular structures of [ArTol2]Ta1833

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Organometallics Scheme 5

Scheme 6 Figure 18. Molecular structure of [κ2-ArTol,Tol′]Ta(NMe2)2Np.

pendent molecules of [ArTol2]Ta(NMe2)2Cl2 exhibit Ta−C−C angles of (i) 111.1(3)° and 131.0(4)° and (ii) 113.4(3)° and 128.0(4)°. However, while significant, i these distortions are not as extreme as noted above for {[ArPr 2]Cr(μ-Cl)}2 (99.9° and 142.0°).9d Although the dialkyls [ArTol2]Ta(NMe2)2R2 (R = Me, Et, Prn, Bun) are stable in solution at room temperature,61 the dineopentyl complex, [ArTol2]Ta(NMe2)2Np2, is unstable and cyclometalates to give [κ2-ArTol,Tol′]Ta(NMe2)2Np (4), as illustrated in Scheme 5.62 As observed for the monoalkyls, [Ar T ol 2 ]Ta(NMe 2 ) 3 R, cyclometalation of [Ar T ol 2 ]Ta(NMe2)2Np2 also involves abstraction of the aryl hydrogen by a Np ligand rather than by a Me2N ligand. The molecular structure of [κ2-ArTol,Tol′]Ta(NMe2)2Np has been determined by X-ray diffraction and is illustrated in Figure 18. As described for [κ2-ArTol,Tol′]Ta(NMe2)3, the lateral Ta−C bond of [κ2ArTol,Tol′]Ta(NMe2)2Np is shorter than the central Ta−C bond

which, as noted above, is in accord with the prediction that electronegative substituents favor the axial sites.51 In contrast to the alkyl compounds, the dichloride complex, [ArTol2]Ta(NMe2)2Cl2, exhibits an in-plane distortion of the aryl group, as indicated by a significant deviation of the Ta−C−C aryl bond angles from the average value of 122.5° for M−aryl compounds.41 Specifically, the two crystallographically indeScheme 7

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Organometallics (Table 2), although the difference (0.045 Å) is less significant than for the former compound. The kinetics of cyclometalation of [ArTol2]Ta(NMe2)2Np2 to form [κ2-ArTol,Tol′]Ta(NMe2)2Np have been measured by using 1 H NMR spectroscopy. As illustrated in Figure 19, the conversion is characterized by first-order kinetics with a rate constant of 3.7(1) × 10−4 s−1, corresponding to a free energy of activation (ΔG⧧) of 23.8(1) kcal mol−1 at 47 °C. Interestingly, rather than [κ2-ArTol,Tol′]Ta(NMe2)2Np undergoing subsequent cyclometalation and eliminating NpH at elevated temperatures, [κ2-ArTol,Tol′]Ta(NMe2)2Np undergoes a transformation to form a compound that is proposed to be an isomer, [κ2-Ar*Tol,Tol′]Ta(NMe2)2Np (4*), in which the tantalum is attached to a different position on the central ring, as indicated by the “*” (Scheme 6).63,64 In addition to the NMR spectroscopic characterization of [κ2-Ar*Tol,Tol′]Ta(NMe2)2Np, additional information pertaining to the rearrangement is provided by analysis of the isotopomer of [ArTol 2 ]H that is released upon addition of CD 3 OD. Specifically, a combination of 1H and 2H NMR spectroscopy demonstrates that deuterium of the d2-[ArTol2]H released is not located on a central ring position that is ortho to both tolyl groups; rather, it is located on a position that is ortho to one tolyl group and para to the other, as illustrated in Scheme 7. In contrast, addition of CD3OD to [κ2-ArTol,Tol′]Ta(NMe2)2Np releases d2-[ArTol2]H which is enriched with deuterium in the site that is mutually ortho to both tolyl groups, an assignment that is confirmed by analysis of the [ArTol2]D that is released upon reaction of [ArTol2]Li with CD3OD. The kinetics of the isomerization have been measured by using 1H NMR spectroscopy (Figure 20), which indicates that the reaction is first-order (Figure 21), with a rate constant of 1.2(1) × 10−4 s−1, corresponding to a free energy of activation

Figure 20. Sequential conversion of [ArTol2]Ta(NMe2)2Np2 to [κ2ArTol,Tol′]Ta(NMe2)2Np and [κ2-Ar*Tol,Tol′]Ta(NMe2)2Np at 67 °C as measured by 1H NMR spectroscopy. Data points were acquired at 1.14 min intervals.

Figure 21. First-order plot for the conversion of [κ2-ArTol,Tol′]Ta(NMe2)2Np to [κ2-Ar*Tol,Tol′]Ta(NMe2)2Np at 67 °C using the data in Figure 20. The data starts at 20 min, which is the point at which [ArTol2]Ta(NMe2)2Np2 was effectively consumed.

Two possibilities for the mechanism of the transformation involve (i) α-H abstraction of a methylene hydrogen of the neopentyl ligand (Scheme 8) and (ii) β-H abstraction of a methyl hydrogen of the Me2N ligand (Scheme 9). For example, an α-H abstraction reaction involving the neopentyl group would cleave the Ta−C bond (forming a neopentylidene intermediate and a new C−H bond) and subsequent rotation of the terphenyl ligand about its C−C bond would allow for the formation of a new Ta−C bond via the formal reverse of the cleavage reaction, i.e., 1,2-addition across the TaC double bond. The alternative mechanistic scenario in which cleavage of the Ta−C bond is accompanied by a β-H abstraction proceeds similarly, with the principal difference being the fact that the intermediate possesses a metallaaziridine moiety (Scheme 9). Evidence to distinguish between these two possibilities has been provided by examining the isomerization of the d2isotopologue [κ2-ArTol,Tol′]Ta(NMe2)2(CD2But).66 Specifically, isomerization via α-H abstraction would be accompanied by incorporation of hydrogen into the methylene moiety of the neopentyl ligand and deuterium into the arene, whereas no scrambling would be observed for a mechanism involving β-H

Figure 19. First-order plot for the conversion of [ArTol2]Ta(NMe2)2Np2 to [κ2-ArTol,Tol′]Ta(NMe2)2Np at 47 °C (data points were acquired at 0.66 min intervals).

(ΔG⧧) of 26.1(1) kcal mol−1 at 67 °C. The mechanism of formation of [κ2-Ar*Tol,Tol′]Ta(NMe2)2Np is of interest since it requires several bond cleavage and formation reactions. In this regard, a mechanism that involves simple cleavage of a Ta−C bond can be discounted because it is expected to be a high energy process that is considerably more endoergic than the measured value of ΔG⧧ [26.1(1) kcal mol−1].65 As such, it is evident that the Ta−C bond cleavage step must be coupled with C−H bond formation. 1835

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complexes is susceptible to intramolecular C−H bond cleavage reactions, thereby affording unprecedented examples of structurally characterized cyclometalated transition metal terphenyl derivatives. For example, the monoalkyl compounds [ArTol2]Ta(NMe2)3R undergo cyclometalation to form [κ2ArTol,Tol′]Ta(NMe2)3 via elimination of RH. Likewise, the dineopentyl compound [ArTol2]Ta(NMe2)2Np2 eliminates NpH to form [κ2-ArTol,Tol′]Ta(NMe2)2Np. A common feature of both of these transformations is that the abstraction of the hydrogen from the terphenyl ligand is achieved by an alkyl group rather than by a dimethylamido ligand.

Scheme 8

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EXPERIMENTAL SECTION

General Considerations. All manipulations were performed using a combination of glovebox, high vacuum, and Schlenk techniques under an argon atmosphere unless otherwise specified.69 Solvents were purified and degassed by using standard procedures. 1H NMR spectra were measured on Bruker 300 DRX, Bruker 300 DPX, Bruker 400 Avance III, Bruker 400 Cyber-enabled Avance III, and Bruker 500 DMX spectrometers. 1H chemical shifts are reported in ppm relative to SiMe4 (δ = 0) and were referenced internally with respect to the protio solvent impurity (δ 7.16 for C6D5H).70 13C NMR spectra are reported in ppm relative to SiMe4 (δ = 0) and were referenced internally with respect to the solvent (δ 128.06 for C6D6).70 11B NMR are reported in ppm relative to BF3·Et2O (δ = 0) and were referenced electronically by using the solvent 1H frequency.71 Coupling constants are given in hertz. MeLi, EtLi, and BunLi were obtained commercially from Aldrich, while PrnLi,72 NpLi,73 and [ArTol2]Li,6 were prepared by literature methods. In each case, solvent was removed from the solution of RLi prior to use. ButCD2Li74 was prepared from ButCD2I.75,76 Me3SiCl was obtained from Aldrich, dried over CaH2 prior to use, and vacuum transferred directly into the reaction vessel. [Ta(NMe2)3Cl2]246 was prepared by the literature method. X-ray Structure Determinations. X-ray diffraction data were collected on a Bruker Apex II diffractometer. Crystal data, data collection, and refinement parameters are summarized in Table S1 in the Supporting Information. The structures were solved using direct methods and standard difference map techniques and were refined by full-matrix least-squares procedures on F2 with SHELXTL (version 2008/4).77 Synthesis of [ArTol2]Ta(NMe2)3Cl. A solution of [ArTol2]Li (760 mg, 2.88 mmol) in Et2O (10 mL) was added to a stirred suspension of [Ta(NMe2)3Cl2]2 (1.0 g, 1.30 mmol) in Et2O (15 mL) over a period of 5 min. The mixture was then stirred for an additional 30 min, after which period the volatile components were removed in vacuo. The resulting solid was first washed with pentane (15 mL) and then extracted into pentane (100 mL). The volatile components were removed from the extract in vacuo to give a golden-yellow/brown powder. The powder was washed twice with pentane (ca. 5 and 3 mL) and dried in vacuo to afford [ArTol2]Ta(NMe2)3Cl (427 mg, 28% yield). X-ray quality crystals were obtained from a solution in pentane at −15 °C. Anal. Calcd: C, 51.5%, H, 5.8%, N, 6.9%. Found: C, 51.4%, H, 5.9%, N, 6.8%. 1H NMR (C6D6): 2.17 [s, 6H of Me of ArTol2], 2.86 [s, 18H of (NMe2)3], 7.05 [d, 3JH−H = 8, 4H of ArTol2], 7.19 [dd, 3JH−H = 8, 3JH−H = 7, 1H of ArTol2], 7.30 [d, 3JH−H = 8, 2H of ArTol2], 7.74 [br s, 4H of ArTol2]. 13C{1H} NMR (C6D6): 21.1 [s, 2C of Me of ArTol2], 46.9 [s, 6C of (NMe2)3], 126.0 [s, 1C of ArTol2], 128.8 [s, 4C of ArTol2], 130.4 [s, 2C of ArTol2], 130.9 [br s, 4C of ArTol2], 136.0 [s, 2C of ArTol2], 144.7 [s, 2C of ArTol2], 146.5 [s, 2C of ArTol2], 202.5 [s, 1C of ArTol2]. Synthesis of [ArTol2]Ta(NMe2)3Me. A solution of [ArTol2]Ta(NMe2)3Cl (30 mg, 0.05 mmol) in d6-benzene (ca. 0.7 mL) in an NMR tube equipped with a J. Young valve was treated with MeLi (10 mg, 0.45 mmol) in Et2O (ca. 0.1 mL). The mixture was analyzed by 1 H NMR spectroscopy, thereby demonstrating conversion to [ArTol2]Ta(NMe2)3Me. The mixture was lyophilized and extracted into pentane (ca. 1 mL) and filtered through Celite. The extract was placed at −15 °C, thereby depositing light-yellow crystals of

Scheme 9

abstraction from NMe 2 . Significantly, isomerization of [κ2‑ArTol,Tol′]Ta(NMe2)2(CD2But) yields [κ2-Ar*Tol,Tol′]Ta(NMe2)2(CD2But), with no scrambling of deuterium,67 thereby indicating that a mechanism involving α-H abstraction does not occur. Thus, a mechanism involving β-H abstraction is the more likely possibility. Further evidence in support of this suggestion is provided by the fact that cyclometalation of Me2N ligands on tantalum has been previously observed.68

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SUMMARY In conclusion, a series of κ1-m-terphenyl [ArTol2] tantalum complexes that feature dimethylamido ligands, namely [ArTol2]Ta(NMe2)3X (X = Me, Et, Prn, Bun, Np, [ArTol2], and BH4) and [ArTol2]Ta(NMe2)2R2 (R = Me, Et, Prn, Bun, and Np) have been synthesized. Together with [ArTol2]TaMe3Cl and [ArTol2]TaMe2Cl2, these complexes constitute the first examples of structurally characterized κ1-m-terphenyl [ArTol2] tantalum compounds. Interestingly, the terphenyl ligand in these 1836

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Organometallics [ArTol2]Ta(NMe2)3Me over a period of 1 day. The mother liquor was removed using a pipet, and the crystals were washed with cold (−15 °C) pentane (2 × 1 mL), and dried in vacuo, giving [ArTol2]Ta(NMe2)3Me (15 mg, 52% yield). X-ray quality crystals were obtained from a solution in pentane at −15 °C. Anal. Calcd: C, 55.4%, H, 6.5%, N, 7.2%. Found: C, 55.4%, H, 6.6%, N, 7.0%. 1H NMR (C6D6): 0.25 [s, 3H of TaMe], 2.20 [s, 6H of Me of ArTol2], 3.01 [br s, 18H of (NMe2)3], 7.08 [d, 3JH−H = 8, 4H of ArTol2], 7.20 [t, 3JH−H = 8, 1H of ArTol2], 7.35 [d, 3JH−H = 8, 2H of ArTol2], 7.46 [d, 3JH−H = 8, 4H of ArTol2]. 13C{1H} NMR (C6D6): 21.2 [s, 2C of Me of ArTol2], 46.6 [s, 6C of (NMe2)3], 53.3 [s, 1C of TaMe], 125.2 [s, 1C of ArTol2], 129.0 [br s, 4C of ArTol2], 129.8 [s, 4C of ArTol2], 130.4 [s, 2C of ArTol2], 135.8 [very br, 2C of ArTol2], 145.9 [very br, 4C of ArTol2], 200.3 [s, 1C of ArTol2]. Synthesis of [ArTol2]Ta(NMe2)3Et. EtLi (2 mg, 0.05 mmol) was added to [ArTol2]Ta(NMe2)3Cl (30 mg, 0.05 mmol) in d6-benzene (ca. 0.7 mL) in a small vial. The suspension was mixed for 1 min, filtered through Celite, and transferred to an NMR tube equipped with a J. Young valve. The pale-yellow to colorless solution was analyzed by 1H NMR spectroscopy, thereby demonstrating conversion to [ArTol2]Ta(NMe2)3Et. The mixture was then lyophilized and extracted into pentane (ca. 1 mL). The extract was allowed to evaporate slowly at room temperature to give [ArTol2]Ta(NMe2)3Et as light-yellow crystals. The crystals were isolated, washed with cold (−15 °C) pentane (1 mL), and dried in vacuo to give [ArTol2]Ta(NMe2)3Et (8 mg, 27% yield). X-ray quality crystals were obtained from a solution in pentane at −15 °C. 1H NMR (C6D6): 0.42 [q, 3JH−H = 8, 2H of TaCH2CH3], 1.77 [t, 3JH−H = 8, 3H of TaCH2CH3], 2.20 [s, 6H of Me of ArTol2], 3.00 [br s, 18H of (NMe2)3], 7.08 [d, 3JH−H = 8, 4H of ArTol2], 7.20 [t, 3JH−H = 8, 1H of ArTol2], 7.34 [d, 3JH−H = 8, 2H of ArTol2], 7.46 [d, 3JH−H = 8, 4H of ArTol2]. 13C{1H} NMR (C6D6): 15.2 [s, 1C of TaCH2CH3], 21.2 [s, 2C of Me of ArTol2], 46.8 [s, 6C of (NMe2)3], 67.2 [s, 1C of TaCH2CH3], 125.1 [s, 1C of ArTol2], 128.9 [br s, 4C of ArTol2], 130.0 [br s, 4C of ArTol2], 130.4 [s, 2C of ArTol2], 136.2 [very br, 2C of ArTol2], 146.1 [very br, 4C of ArTol2], 200.5 [s, 1C of ArTol2]. Synthesis of [ArTol2]Ta(NMe2)3Prn. PrnLi (5 mg, 0.10 mmol) was added to [ArTol2]Ta(NMe2)3Cl (30 mg, 0.05 mmol) in d6-benzene (ca. 0.7 mL) in a small vial. The suspension was mixed for 1 min, filtered through Celite, and transferred to an NMR tube equipped with a J. Young valve. The pale-yellow to colorless solution was analyzed by 1H NMR spectroscopy, thereby demonstrating conversion to [ArTol2]Ta(NMe2)3Prn. The mixture was lyophilized and extracted into pentane (ca. 1 mL). The extract was placed at −15 °C, thereby depositing lightyellow crystals of [ArTol2]Ta(NMe2)3Prn over a period of ca. 1 week. The mother liquor was removed using a pipet, and the crystals were washed with cold (−15 °C) pentane (1 mL) and dried in vacuo, giving [ArTol2]Ta(NMe2)3Prn (6 mg, 20% yield). X-ray quality crystals were obtained from a solution in pentane at −15 °C. Anal. Calcd: C, 56.8%, H, 6.9%, N, 6.8%. Found: C, 56.0%, H, 6.7%, N, 5.8%. 1H NMR (C6D6): 0.28 [m, 2H of TaCH2CH2CH3], 0.93 [t, 3JH−H = 7, 3H of TaCH2CH2CH3], 1.80 [m, 2H of TaCH2CH2CH3], 2.22 [s, 6H of Me of ArTol2], 3.00 [br s, 18H of (NMe2)3], 7.08 [d, 3JH−H = 8, 4H of ArTol2], 7.20 [t, 3JH−H = 8, 1H of ArTol2], 7.35 [d, 3JH−H = 8, 2H of ArTol2], 7.46 [d, 3JH−H = 8, 4H of ArTol2]. 13C{1H} NMR (C6D6): 21.2 [s, 2C of Me of ArTol2], 22.3 [s, 1C of TaCH2CH2CH3], 24.6 [s, 1C of TaCH2CH2CH3], 46.8 [s, 6C of (NMe2)3], 79.6 [s, 1C of TaCH2CH2CH3], 125.1 [s, 1C of ArTol2], 128.9 [br s, 4C of ArTol2], 130.0 [br s, 4C of ArTol2], 130.3 [s, 2C of ArTol2], 136.2 [very br, 2C of ArTol2], 146.1 [very br, 4C of ArTol2], 200.9 [s, 1C of ArTol2]. Synthesis of [ArTol2]Ta(NMe2)3Bun. A solution of BunLi (3 mg, 0.05 mmol) in d6-benzene (0.2 mL) was added to [ArTol2]Ta(NMe2)3Cl (30 mg, 0.05 mmol) in d6-benzene (ca. 1 mL) in a small vial. The suspension was mixed for 1 min, filtered through Celite, and transferred to an NMR tube equipped with a J. Young valve. The paleyellow to colorless solution was analyzed by 1H NMR spectroscopy, thereby demonstrating conversion to [ArTol2]Ta(NMe2)3Bun. The mixture was then lyophilized and extracted into pentane (ca. 1 mL). The extract was placed at −15 °C and allowed to evaporate slowly, thereby forming light-yellow crystals of [ArTol2]Ta(NMe2)3Bun over

the period of ca. 1 week. The crystals were dried in vacuo, giving [ArTol2]Ta(NMe2)3Bun (9 mg, 29% yield). X-ray quality crystals were obtained from a solution in pentane at −15 °C. 1H NMR (C6D6): 0.25 [m, 2H of TaCH2CH2CH2CH3], 0.91 [t, 3JH−H = 7, 3H of TaCH2CH2CH2CH3], 1.23 [m, 2H of TaCH2CH2CH2CH3], 1.76 [m, 2H of TaCH2CH2CH2CH3], 2.22 [s, 6H of Me of ArTol2], 3.01 [br s, 18H of (NMe2)3], 7.08 [d, 3JH−H = 8, 4H of ArTol2], 7.20 [t, 3JH−H = 8, 1H of ArTol2], 7.35 [d, 3JH−H = 8, 2H of ArTol2], 7.47 [d, 3JH−H = 8, 4H of Ar T o l 2 ]. 1 3 C{ 1 H} NMR (C 6 D 6 ): 13.7 [s, 1C of TaCH2CH2CH2CH3], 21.2 [s, 2C of Me of ArTol2], 30.4 [s, 1C of TaCH2CH2CH2CH3], 33.1 [s, 1C of TaCH2CH2CH2CH3], 46.8 [s, 6C of (NMe2)3], 79.3 [s, 1C of TaCH2CH2CH2CH3], 125.1 [s, 1C of ArTol2], 128.9 [br s, 4C of ArTol2], 130.1 [br s, 4C of ArTol2], 130.3 [s, 2C of ArTol2], 136.1 [very br, 2C of ArTol2], 146.0 [very br, 4C of ArTol2], 200.8 [s, 1C of ArTol2]. Synthesis of [ArTol2]Ta(NMe2)3Np. NpLi (5 mg, 0.06 mmol) was added to [ArTol2]Ta(NMe2)3Cl (30 mg, 0.05 mmol) in d6-benzene (ca. 0.7 mL) in a small vial. The suspension was mixed for 1 min, filtered through Celite, and transferred to an NMR tube equipped with a J. Young valve. The pale-yellow to colorless solution was analyzed by 1H NMR spectroscopy, thereby demonstrating conversion to [ArTol2]Ta(NMe2)3Np. The mixture was then lyophilized and extracted into pentane (ca. 1 mL). The extract was placed at −15 °C, thereby depositing light-yellow crystals of [ArTol2]Ta(NMe2)3Np over the period of ca. 1 week. The mother liquor was removed by using a pipet, and the crystals were washed with cold (−15 °C) pentane (2 × 1 mL) and dried in vacuo, giving [ArTol2]Ta(NMe2)3Np (10 mg, 31% yield). X-ray quality crystals were obtained from a solution in pentane at −15 °C. Anal. Calcd: C, 58.0%, H, 7.2%, N, 6.6%. Found: C, 57.5%, H, 6.7%, N, 6.2%. 1H NMR (C6D6): 0.04 [s, 2H of TaCH2CMe3], 0.98 [s, 9H of TaCH2CMe3], 2.23 [s, 6H of Me of ArTol2], 3.05 [br s, 18H of (NMe2)3], 7.07 [br, 4H of ArTol2], 7.21 [t, 3JH−H = 8, 1H of ArTol2], 7.40 [br, 4H of ArTol2], 7.52 [br, 2H of ArTol2]. 13C{1H} NMR (C6D6): 21.2 [s, 2C of Me of ArTol2], 34.3 [s, 1C of TaCH2CMe3], 37.7 [s, 1C of TaCH2CMe3], 46.8 [s, 6C of (NMe2)3], 98.7 [s, 1C of TaCH2CMe3], 124.7 [s, 1C of ArTol2], 129.2 [br s, 4C of ArTol2], 129.8 [br s, 4C of ArTol2], 130.8 [br s, 2C of ArTol2], 135.3 [br s, 1C of ArTol2], 136.7 [br s, 1C of ArTol2], 143.8 [br s, 1C of ArTol2], 144.8 [br s, 1C of ArTol2], 145.6 [br s, 1C of ArTol2], 146.8 [br s, 1C of ArTol2], 201.3 [s, 1C of ArTol2]. Synthesis of [ArTol2]2Ta(NMe2)3. [ArTol2]Li (15 mg, 0.06 mmol) was added to a solution of [ArTol2]Ta(NMe2)3Cl (25 mg, 0.04 mmol) in d6-benzene (ca. 0.7 mL) in a small vial. The suspension was mixed for 1 min, filtered through Celite, and transferred to an NMR tube equipped with a J. Young valve. The sample was analyzed by 1H NMR spectroscopy, thereby demonstrating conversion to [ArTol2]2Ta(NMe2)3. The sample was then lyophilized and extracted with pentane (ca. 1 mL). The extract was placed at −15 °C for 1 week, thereby depositing yellow crystals of [ArTol2]2Ta(NMe2)3·0.5(C5H12) over a period of ca. 1 week. The mother liquor was removed by using a pipet, and the crystals were washed with cold (−15 °C) pentane (2 × 1 mL), giving [ArTol2]2Ta(NMe2)3·0.5(C5H12) (14 mg, 39% yield). Xray quality crystals were obtained from a solution in pentane at −15 °C. Anal. Calcd: C, 66.7%, H, 6.3%, N, 5.1%. Found: C, 65.6%, H, 6.3%, N, 4.5%. 1H NMR (C6D6): 2.19 [s, 12H of Me of ArTol2], 2.51 [s, 18H of (NMe2)3], 7.08 [d, 3JH−H = 7, 8H of ArTol2], 7.10 [t, 3JH−H = 8, 2H of ArTol2], 7.24 [d, 3JH−H = 7, 4H of ArTol2], 7.40 [d, 3JH−H = 7, 8H of ArTol2]. 13C{1H} NMR (C6D6): 21.1 [s, 4C of Me of ArTol2], 47.0 [s, 6C of (NMe2)3], 125.4 [s, 2C of ArTol2], 128.4 [s, 8C of ArTol2], 130.1 [s, 8C of ArTol2], 131.3 [s, 4C of ArTol2], 134.9 [s, 4C of ArTol2], 147.8 [s, 4C of ArTol2], 152.2 [s, 4C of ArTol2], 199.2 [s, 2C of ArTol2]. Synthesis of [ArTol2]Ta(NMe2)3(κ2-BH4). A mixture of [ArTol2]Ta(NMe2)3Cl (50 mg, 0.08 mmol) and LiBH4 (3 mg, 0.14 mmol) was treated with Et2O (ca. 0.5 mL). The volatile components were removed in vacuo after 30 min, and the off-white solid obtained was extracted with pentane (2 mL). The solution was placed at −15 °C, thereby depositing colorless crystals of [ArTol2]Ta(NMe2)3(κ2-BH4) over a period of 1 day. The mother liquor was removed using a pipet, and the crystals were washed with cold (−15 °C) pentane (2 × 1 mL) 1837

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ArTol,Tol′]Ta(NMe2)3, Me3PBH3 (δ 0.61 [d, 2JPH = 10, 9H, PMe3], 1.33 [d, 2JPH = 16, 1:1:1:1 q, 1JBH = 96, 3H, BH3]) and H2. Synthesis of [ArTol2]Ta(NMe2)2Cl2. [ArTol2]Ta(NMe2)3Cl (100 mg, 0.17 mmol) in d6-benzene (ca. 1 mL) in an NMR tube equipped with a J. Young valve was treated with excess Me3SiCl (ca. 0.7 mL) via vapor transfer. The mixture was shaken for 1 min and allowed to stand at room temperature for 10 min. The sample was analyzed by 1H NMR spectroscopy, thereby demonstrating conversion to [ArTol2]Ta(NMe2)2Cl2 and Me3SiNMe2. The volatile components were removed in vacuo, leaving a yellow oily residue, which was extracted into pentane (5 × 1 mL), with each extract being placed in a small vial. The extracts were placed at −15 °C, thereby depositing yellow crystals of [ArTol2]Ta(NMe2)2Cl2. The mother liquor was removed from each vial using a pipet, and the crystals were washed with cold (−15 °C) pentane (1 mL each), combined, and dried in vacuo to give [ArTol2]Ta(NMe2)2Cl2 (30 mg). Additionally, the yellow solid remaining after the pentane extractions was extracted into benzene and lyophilized to give an additional batch of [ArTol2]Ta(NMe2)2Cl2 (23 mg). The total yield of [ArTol2]Ta(NMe2)2Cl2 is 53 mg (54%). Xray quality crystals were obtained by evaporation of a solution in benzene. Anal. Calcd: C, 48.3%, H, 4.9%, N, 4.7%. Found: C, 49.1%, H, 4.5%, N, 4.2%. 1H NMR (C6D6): 2.18 [s, 6H of Me of ArTol2], 2.94 [s, 12H of (NMe2)2], 7.07 [d, 3JH−H = 8, 4H of ArTol2], 7.16 [m, 1H of ArTol2], 7.29 [d, 3JH−H = 7, 2H of ArTol2], 7.72 [br d, 3JH−H = 7, 4H of ArTol2]. 13C{1H} NMR (C6D6): 21.3 [s, 2C of Me of ArTol2], 47.7 [s, 4C of (NMe2)2], 128.3 [s, 1C of ArTol2, under C6D6 signal, located by HSQC], 129.3 [s, 4C of ArTol2], 130.4 [s, 2C of ArTol2], 131.4 [s, 4C of ArTol2], 137.5 [s, 2C of ArTol2], 139.4 [br s, 2C of ArTol2], 144.4 [s, 2C of ArTol2], 201.4 [s, 1C of ArTol2]. Synthesis of [ArTol2]Ta(NMe2)2Me2. A solution of [ArTol2]Ta(NMe2)2Cl2 (20 mg, 0.03 mmol) in d6-benzene (ca. 0.7 mL) in an NMR tube equipped with a J. Young valve was treated with MeLi (10 mg, 0.45 mmol) in Et2O (ca. 0.2 mL), and mixture was allowed to stand for ca. 1 h. The sample was analyzed by 1H NMR spectroscopy, thereby demonstrating conversion to [ArTol2]Ta(NMe2)2Me2. The sample was lyophilized, extracted into d6-benzene, and analyzed by 1H NMR spectroscopy, thereby demonstrating that the sample consisted of [ArTol2]Ta(NMe2)2Me2 as the major product, in addition to [ArTol2] H (