Synthesis and Structural Characterization of Tris - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Organometallic Zirconium Compounds in an Oxygen-Rich Coordination Environment: Synthesis and Structural Characterization of Tris(oxoimidazolyl)hydroboratozirconium Compounds Ahmed Al-Harbi, Matthew J. Hammond, and Gerard Parkin* Department of Chemistry, Columbia University, New York, New York 10027, United States S Supporting Information *

ABSTRACT: A series of tris(oxoimidazolyl)hydroborato ligands, which serve as L2X [O3] donors, have been employed to obtain organometallic zirconium compounds in an uncommon oxygen-rich coordination environment. For example, Cp[ToMeBenz]ZrCl2 has been synthesized via the reaction of [ToMeBenz]Na with CpZrCl3 and bears a structural resemblance to the bent metallocene dichloride derivative Cp2ZrCl2. t In addition, the half-sandwich counterparts [ToMeBenz]ZrCl3 and [ToBu ]ZrCl3 have t been obtained by metathesis of ZrCl4 with [ToMeBenz]Na and [ToBu ]Na, respectively. The structurally related zirconium benzyl compounds [ToRBenz]Zr(CH2Ph)3 (R = Me, But, 1-Ad) have also been synthesized via the reactions of [ToRBenz]Tl with Zr(CH2Ph)4, and X-ray diffraction studies demonstrate that the benzyl ligands in these compounds are conformationally flexible and exhibit a large range of Zr−CH2−Ph bond angles (94.7−131.7°). Protolytic cleavage of one of the benzyl ligands of [ToRBenz]Zr(CH2Ph)3 (R = But, 1-Ad) may be achieved by treatment with [PhNHMe2][B(C6F5)4] to generate {[ToRBenz]Zr(CH2Ph)2}[B(C6F5)4], which are catalysts for the t polymerization of ethylene. The molecular structure of the ether adduct, {[ToBu Benz]Zr(CH2Ph)2(OEt2)}[B(C6F5)4], has been determined by X-ray diffraction. In addition to the use of tris(oxoimidazolyl)hydroborato ligands, bis(oxoimidazolyl)hydroborato ligands have also been used to obtain zirconium benzyl compounds in oxygen-rich environments, namely, [BoMeBenz]2Zr(CH2Ph)2 and [BoAdBenz]2Zr(CH2Ph)2.

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INTRODUCTION

RESULTS AND DISCUSSION One approach for incorporating [ToR] ligands into zirconium compounds involves thet metathesis of a Zr−Cl bond with [ToR]Na.15 Thus, [ToBu ]ZrCl3 and [ToMeBenz]ZrCl3 may be t obtained via the respective reactions of ZrCl4 with [ToBu ]Na and MeBenz ]Na (Scheme 1), and their molecular structures have [To been determined by X-ray diffraction, as illustrated in Figures 1 and 2. In addition to metathesis reactions with ZrCl4, CpZrCl3 may be employed to afford the hybrid compound Cp[ToMeBenz]ZrCl2, which contains both Cp and [ToMeBenz] ligands (Scheme 1). The successful synthesis of the zirconocene analogue Cp[ToMeBenz]ZrCl2, which has been structurally characterized by X-ray diffraction (Figure 3), is of significance because the corresponding reaction of CpZrCl3 with [CpCo{P(O)(OEt)2}3]Na does not yield Cp[CpCo{P(O)(OEt)2}3]ZrCl2. Specifically, rather than undergoing metathesis of the Zr−Cl bond, CpZrCl3 reacts with [CpCo{P(O)(OEt)2}3]Na to afford [CpCo{P(O)(OEt)2}3]ZrCl3 due to the preferential substitution of the cyclopentadienyl ligand.16,17 Another illustration of the difficulty of obtaining zirconocene analogues using the [CpCo{P(O)(OEt)2}3] ligand is provided by the fact that the reaction of ZrCl4(THF)2 with [CpCo{P(O)(OEt)2}3]Na forms a dinuclear compound, resulting from the dealkylation of two phosphonate alkyl groups per

Cyclopentadienyl ligands have played an important role in the development of organometallic chemistry, as exemplified by the metallocene systems and their use as olefin polymerization catalysts.1,2 Electronically related to cyclopentadienyl are C3-symmetric L2X3 tripodal ligands that feature a variety of donor arrays,4,5 of which the tris(pyrazolyl)hydroborato [N3] donor ligand system, [TpR,R′], has been studied most extensively.4 By comparison, C3-symmetric [O3] donors are less common6−9 but offer potential for mimicking molecular species that are grafted to oxide surfaces.10−13 With respect to L2X [O3] donors, the most commonly encountered class of ligands is that of the [CpCo{P(O)(OR)2}3] ligand system.6 Despite the formal analogy between [CpCo{P(O)(OR)2}3] and [TpR,R′] ligands, however, an important distinction is that the different placement of the R substituents results in the [O3] ligands being much less sterically demanding than [TpR,R′]. Therefore, since sterically demanding ligands are often advantageous for supporting metal centers with uncommon coordination environments, we recently introduced a new class of L2X [O3] ligands that are based on oxoimidazolyl donors, namely, tris(oxoimidazolyl)hydroborato, [ToR].14 In addition, we have also reported related bis(oxoimidazolyl)hydroborato ligands, [BoR], which provide complementary LX [O2] donor arrays. Here, we describe the application of these bis- and tris(oxoimidazolyl)hydroborato ligands to organozirconium chemistry. © XXXX American Chemical Society

Received: November 6, 2017

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DOI: 10.1021/acs.inorgchem.7b02832 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1

Figure 3. Molecular structure of Cp[ToMeBenz]ZrCl2.

zirconium, rather than [CpCo{P(O)(OEt)2}3]2ZrCl2.16 Moreover, ZrCl4(THF)2 reacts with the ethyl counterpart [CpCo{P(O)Et2}3]Na to afford ionic {[CpCo{P(O)Et)2}3]2Zr}Cl2, in which the chlorides do not coordinate to the zirconium, and the complex possesses a six-coordinate octahedral geometry.16 The use of the [ToMeBenz] ligand, therefore, provides access to a long sought-after zirconocene analogue in which a cyclopentadienyl ligand is replaced with an [O3] donor array. In accord with the bent metallocene analogy, the molecular structure of Cp[ToMeBenz]ZrCl2 (Figure 3) bears a resemblance to that of Cp2ZrCl2,18 as summarized in Table 1. For example, Table 1. Comparison of the Metrical Data for Cp[ToMeBenz]ZrCl2, Cp[TmMe]ZrCl2, and Cp2ZrCl2

Zr−Crange/Å Zr−Cav/Å Zr−Ccent/Å Zr−Cl/Å Cl−Zr−Cl/deg Cpcent−Zr−Y/deg

t

Cp[TmMe]ZrCl2a

Cp2ZrCl2b

Cp[ToMeBenz]ZrCl2

2.51−2.57 2.54 2.26 2.52 97.9 133.5 (Y = B)

2.47−2.52 2.50 2.20 2.45 97.0 129.2 (Y = Cpcent)

2.51−2.57 2.54 2.24 2.48 93.9 130.5 (Y = B)

Figure 1. Molecular structure of [ToBu ]ZrCl3.

a

Figure 2. Molecular structure of [ToMeBenz]ZrCl3.

the Cpcent−Zr−B angle of Cp[ToMeBenz]ZrCl2 (130.5°) is similar to the Cpcent−Zr−Cpcent angle of Cp2ZrCl2 (129.2°),18a while the Cl−Zr−Cl bond angle of Cp[ToMeBenz]ZrCl2 (93.9°) is similar to that of Cp2ZrCl2 (97.0°). The molecular structure of Cp[ToMeBenz]ZrCl2 is also similar to that of the mercapto counterpart Cp[TmMe]ZrCl2,19 but an interesting difference pertains to the 1H NMR spectroscopic properties. Specifically, whereas the methyl groups of the [ToMeBenz] ligand of Cp[ToMeBenz]ZrCl2 adopt a 2:1 pattern,20 those of the [TmMe] ligand of Cp[TmMe]ZrCl2 adopt a 1:1:1 pattern.19 The origin of this difference is associated with the chirality of Cp[ToMeBenz]ZrCl2 and Cp[TmMe]ZrCl2 due to the propeller-like twist21 of the [ToMeBenz] and [TmMe] ligands. Thus, in the absence of enantiomer interconversion, a 1:1:1 pattern of methyl groups is predicted, while a 2:1 pattern is predicted if interconversion is facile and the ligand arm that is trans to the cyclopentadienyl ligand remains in place. The observation that Cp[ToMeBenz]ZrCl2 exhibits a 2:1 pattern, while Cp[TmMe]ZrCl2 retains a 1:1:1 pattern,19 indicates that the barrier to B

Data taken from ref 19. bData taken from ref 18a.


Article

Inorganic Chemistry enantiomer interconversion of Cp[ToMeBenz]ZrCl2 is significantly lower than that of Cp[TmMe]ZrCl2.22 Enantiomer interconversion requires the “inversion” of each arm, and the mechanism could involve either dissociation of the chalcogen23 or direct inversion at the coordinated chalcogen. Assuming that the enantiomer interconversions for Cp[ToMeBenz]ZrCl2 and Cp[TmMe]ZrCl2 occur via a common mechanism, the observation that the process is more facile for Cp[ToMeBenz]ZrCl2 suggests that the mechanism does not involve dissociation of an arm because the Zr−O bonds are expected to be stronger than the corresponding Zr−S bonds.24 Additional support for a nondissociative mechanism is provided by the observation that barriers to inversion are typically lower for second-row rather than third-row elements.25 Calculations on [TmH]ZnCl and [TmH]Mn(CO)3 provide further evidence that enantiomer interconversion of such ligands may occur via a nondissociative mechanism that features a transition state in which both the metal and boron reside in the mercaptoimidazolyl plane.23b The approach to the transition state is accompanied by an increase in the M−S−C bond angle, and since the Zr−O−C angles for Cp[ToMeBenz]ZrCl2 (132.3− 137.1°) are significantly greater than the corresponding Zr−S−C angles for Cp[TmMe]ZrCl2 (107.9−113.8°),19,26,27 it is evident that it would be easier for the oxoimidazolyl system Cp[ToMeBenz]ZrCl2 to access the transition state than would the mercapto system Cp[TmMe]ZrCl2. Thus, the tris(oxoimidazolyl)hydroborato ligands may be considered to be more flexible than their sulfur counterparts in these systems. In addition to the synthesis of chloride derivatives, we also sought benzyl derivatives because such compounds have been used extensively as polymerization catalysts,28 including heterogenized single-site catalysts.29 Although compounds of the class [ToR]Zr(CH2Ph)3 could, in principle, be synthesized via the alkylation of [ToR]ZrCl3, previous studies have noted synthetic difficulties associated with this approach for group 4 metal alkyl compounds of the type [TpR,R′]MRxX3−x.30 An alternative approach for the synthesis of [TpR,R′]M alkyl derivatives, however, involves the direct metathesis of M−R bonds. For example, we previously reported that thallium compounds of the type [TpR,R′]Tl are effective reagents for the synthesis of [TpR,R′]MRx−1 derivatives (e.g., M = Mg, Zn, Al) via reactions with MRx,31,32 because decomposition of the incipient TlR byproduct provides an effective driving force for the reaction.33 Accordingly, the benzyl compounds [ToR]Zr(CH2Ph)3 may be conveniently obtained by the metathesis of Zr(CH2Ph)4 with the corresponding thallium reagent [ToR]Tl (Scheme 2), an approach that has

Scheme 2

Figure 4. Molecular structure of [ToMeBenz]Zr(CH2Ph)3 (modification A).

t

also been employed for [TmBu ]M(CH2Ph)3 (M = Zr, Hf)34 and [TpMs*]Hf(CH2Ph)3.30,35 The molecular structures of [ToR]Zr(CH2Ph)3 have been determined by X-ray diffraction, as illustrated in Figures 4−9. Previous studies have demonstrated that benzyl ligands can adopt a variety of coordination modes, which include η1, η2, η3, η4, and η7 hapticities, and we have recently provided a means to classify these coordination modes.36,37 In this regard, it is interesting to note that the methylbenzimidazolyl derivative, [ToMeBenz]Zr(CH2Ph)3, has been structurally characterized in several modifications that exhibit different conformations of the benzyl ligands, with a range of bond angles. The conformation of the benzyl ligands in [ToR]Zr(CH2Ph)3 can be quantified by the magnitude of the B···Zr−C−Ph torsion angle (Figure 10), with values of |τ| > 90° being classified as “up” (with the phenyl group directed away from the [ToMeBenz]Zr moiety) and values of |τ| < 90° being classified as “down”

Figure 5. Molecular structure of [ToMeBenz]Zr(CH2Ph)3 (modification B).

(with the phenyl group directed towards the [ToMeBenz]Zr moiety). With this perspective, one of the modifications, which has approximately C3 molecular symmetry, possesses a “3 down” conformation, whereas the other modifications possess “1 down:2 up” conformations. Adopting the criteria that Zr−CH2−Ph bond angles of >97° and ≤97° are respectively used to classify η1 and η2 coordination,36 the three benzyl ligands of the “3 down” conformer, with Zr−CH2−Ph bond angles in the range 103.7−111.4°, are appropriately described as η1, which is the most commonly encountered coordination mode for benzyl ligands.38 In contrast, the “1 down:2 up” conformers each possess one benzyl group that approaches an η2 description, with Zr−CH2−Ph bond angles of 94.6° and 99.8° (Table 2). Such variability of the benzyl ligand coordination mode is in accord with our previous study, which demonstrates that the C


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Inorganic Chemistry

Figure 6. Molecular structure of [ToMeBenz]Zr(CH2Ph)3 (modification C). Figure 9. Molecular structure of [ToAdBenz]Zr(CH2Ph)3.

t

Figure 7. Molecular structure of [ToBu Benz]Zr(CH2Ph)3 (modification A).

Figure 8. Molecular structure of [ToBu Benz]Zr(CH2Ph)3 (modification B).

Figure 10. Classification of of the benzyl conformation according to the B···Zr−CH2−Ph torsion angle (τ).

Zr−CH2−Ph bond angle is very flexible and that relatively little energy is required to perturb it.36 For example, the Zr−CH2−Ph bond angles in structurally characterized compounds listed in the Cambridge Structural Database (CSD)27 range from 62.1°39 to

144.4°.40 As an illustration of the flexibility within a single compound, we have reported that the Zr−CH2−Ph bond angles within Zr(CH2Ph)4 range from 81.6(1)° to 106.7(2)°,36 while Arnold has noted that {CyNC[N(SiMe3)2]NCy}Zr(CH2Ph)3

t

D


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Inorganic Chemistry

and adamantyl groups that would destabilize “down” conformations. Another interesting feature of [ToRBenz]Zr(CH2Ph)3 is that the two methylene hydrogen atoms of each benzyl ligand appear as a singlet in the 1H NMR spectrum, even though each molecule possesses a molecular C3 axis, such that the hydrogen atoms are expected to be diastereotopic and, hence, chemically inequivalent.43 The fact that the methylene groups appear as a singlet is, nevertheless, rationalized by facile enantiomer interconversion due to propeller inversion of the [ToRBenz] ligand, as noted above for Cp[ToMeBenz]ZrCl2. In this regard, it is pertinent to note that the methylene hydrogen atoms of the mercaptoimidazolyl t counterpart [TmBu ]Zr(CH2Ph)3 appear as two doublets in the 1 H NMR spectrum.34 As such, the result is in accord with the spectral differences noted above for Cp[ToMeBenz]ZrCl2 and Cp[TmMe]ZrCl2, which also indicate that the oxoimidazolyl system is more flexible than the sulfur counterpart. In addition to incorporating [ToR] ligands via the reactions of Zr(CH2Ph)4 with the thallium reagent, the bis(oxobenzimidazolyl) compounds [BoMeBenz]2Zr(CH2Ph)2 and [BoAdBenz]2Zr(CH2Ph)2 may be synthesized analogously (Scheme 3).

Table 2. Benzyl Ligand Conformations of [ToR]Zr(CH2Ph)3 As Classified by B···Zr−C−Ph Torsion (τ) and Zr−CH2−Ph Bond Angles τ/deg [ToMeBenz]Zr(CH2Ph)3 (modification A)

[ToMeBenz]Zr(CH2Ph)3 (modification B)

[ToMeBenz]Zr(CH2Ph)3 (modification C)

t

[ToBu Benz]Zr(CH2Ph)3 (modification A)

t

[ToBu Benz]Zr(CH2Ph)3 (modification B)

[ToAdBenz]Zr(CH2Ph)3

a

Zr−CH2−Ph/deg classificationa

39.43

121.19

d, η1

114.31 129.81 40.47

115.67 94.62 118.07

u, η1 u, η2 d, η1

117.96 129.58 54.95

116.88 99.84 111.44

u, η1 u, η2 d, η1

57.59 61.81 141.83

103.70 111.30 120.86

d, η1 d, η1 u, η1

141.83 141.83 55.63

120.86 120.86 131.73

u, η1 u, η1 d, η1

81.08 178.37 142.49 142.49 142.50

128.77 122.25 122.75 122.75 122.75

d, η1 u, η1 u, η1 u, η1 u, η1

Scheme 3

d = down; u = up.

exists as two polymorphs with Zr−CH2−Ph bond angles that span the ranges 88.7(4)−123.2(4)° and 104.6(2)−115.9(2)°.41,42 Although density functional theory (DFT) geometry optimization (B3LYP) on several conformations of [ToMeBenz]Zr(CH2Ph)3 indicates that the “3 down” conformer is the more stable (Figure 11), the difference is small, such that crystal packing effects are sufficient to allow the observation of different conformers. t Interestingly, X-ray diffraction studies on [ToBu Benz]Zr(CH2Ph)3 AdBenz (Figures 7 and 8) and [To ]Zr(CH2Ph)3 (Figure 9) reveal conformations that are different to those observed experimentally t for [ToMeBenz]Zr(CH2Ph)3. Of particular note, both [ToBu Benz]Zr(CH2Ph)3 (Figure 7) and [ToAdBenz]Zr(CH2Ph)3 (Figure 9) exhibit “3 up” conformations. By comparison to [ToMeBenz]Zr(CH2Ph)3, the occurrence of the “3 up” conformations for t [ToBu Benz]Zr(CH2Ph)3 and [ToAdBenz]Zr(CH2Ph)3 is undoubtedly facilitated by the greater steric demands of the tert-butyl

The molecular structure of [BoMeBenz]2Zr(CH2Ph)2 (Figure 12) has been determined by X-ray diffraction, thereby demonstrating that coordination of the two [BoRBenz] ligands is supplemented by three-center−two-electron44 Zr···H−B secondary interactions.45 This coordination mode is reproduced well by DFT geometry optimization on [BoMeBenz]2Zr(CH2Ph)2 (Figure 13), as illustrated by a comparison of the experimental (3.33 and 3.45 Å) and calculated (3.36 and 3.54 Å) Zr···B distances.

Figure 11. Relative energies of DFT geometry-optimized conformers of [ToMeBenz]Zr(CH2Ph)3. E


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Inorganic Chemistry

t

Figure 14. Molecular structure of {[ToBu Benz]Zr(CH2Ph)2(OEt2)}[B(C6F5)4] (only the cation is shown for clarity).

Figure 12. Molecular structure of [BoMeBenz]2Zr(CH2Ph)2.

t

{[ToBu Benz]Zr(CH2Ph)2(OEt2)}+ is noteworthy because treatment of [TpMe2]Zr(CH2Ph)3 with [Ph3C][B(C6F5)4] at −60 °C generates {[TpMe2]Zr(CH2Ph)2}+, but decomposition occurs upon warming to 0 °C, resulting in the formation of {[PhCH2BpMe2]Zr(CH2Ph)(pzMe2)}+.40 t Despite its cationic nature, however, {[ToBu Benz]Zr(CH2Ph)2(OEt2)}[B(C6F5)4] is not an effective active catalyst for ethylene t polymerization. Postulating that the inefficiency of {[ToBu Benz]Zr(CH2Ph)2(OEt2)}[B(C6F5)4] as a catalyst is a consequence of the coordinated ether inhibiting ethylene binding, the possibility of obtaining a catalytic system by using a less strongly coordinating solvent was investigated. It is, therefore, significant that treatment of t [ToBu Benz]Zr(CH2Ph)3 with [PhNHMe2][B(C6F5)4] in benzene t

affords {[ToBu Benz]Zr(CH2Ph)2}[B(C6F5)4], which does serve as an effective catalyst, with an activity of 48 kg of polyethylene (PE) [mol of Zr]−1 [h]−1 [atm of C2H4]−1. [ToAdBenz]Zr(CH2Ph)3 activated by [PhNHMe2][B(C6F5)4] behaves similarly, with an activity of 160 kg of PE [mol of Zr]−1 [h]−1 [atm of C2H4]−1.49 In accord with the suggestion that the low activity of t {[ToBu Benz]Zr(CH2Ph)2(OEt2)}+ is a consequence of coordiof ethylene, DFT nated Et2O inhibiting the association t geometry optimization of {[ToBu Benz]Zr(CH2Ph)2(OEt2)}+ and t {[ToBu Benz]Zr(CH2Ph)2}+ (Figure 15) indicates that coordination of Et2O to zirconium is accompanied by an energy change of ΔESCF = −4.43 kcal mol−1.50

Figure 13. DFT geometry-optimized structure of [BoMeBenz]2Zr(CH2Ph)2 (hydrogen atoms on carbon atoms are omitted for clarity).

Generation of Cationic {[ToRBenz]Zr(CH2Ph)2(OEt2)}+ and {[ToRBenz]Zr(CH2Ph)2}+ and Their Use as Polymerization Catalysts. The use of cyclopentadienyl metal complexes, including metallocenes, for the generation of single-site olefin polymerization catalysts is now well established and is of considerable industrial interest.1 A more recent extension of this area has focused on the discovery of noncyclopentadienyl catalysts.46,47 For example, attention has been directed towards the use of the N3 donor [TpR,R′] ligands, which belong to the same L2X covalent bond classification3 as cyclopentadienyl.48 As an illustration, both [TpR,R′]ZrCl348a and [TpR,R′]Zr(CH2Ph)340,48b have been employed as precatalysts for olefin polymerization. Therefore, we have investigated the ability of the [O3] donor [ToRBenz] ligand to provide a catalytic system. Common methods for activating neutral zirconium alkyl compounds for the polymerization of olefins involve (i) alkyl abstraction via reaction with either [Ph3C][B(C6F5)4] or B(C6F5)3 and (ii) protolytic cleavage with [PhNHMe2][B(C6F5)4].1 In t this regard, treatment of [ToBu Benz]Zr(CH2Ph)3 with [PhNHMe2]-

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SUMMARY In summary, bis- and tris(oxoimidazolyl)hydroborato ligands have been employed to obtain zirconium compounds in oxygenrich coordination environments that are uncommon for organometallic derivatives of this element. For example, the synthesis of the zirconocene counterpart Cp[ToMeBenz]ZrCl2 is noteworthy because the corresponding reaction of CpZrCl3 with [CpCo{P(O)(OEt)2}3]Na does not yield the analogous zirconocene species, Cp[CpCo{P(O)(OEt)2}3]ZrCl2, but rather forms [CpCo{P(O)(OEt)2}3]ZrCl3 due to the preferential displacement of the cyclopentadienyl ligand from zirconium. The reactivity of [ToRBenz]Tl (R = Me, But, 1-Ad) towards Zr(CH2Ph)4 provides a means to obtain benzyl compounds [ToRBenz]Zr(CH2Ph)3, which are counterparts to half-sandwich derivatives CpRZr(CH2Ph)3. In accord with this analogy,

t

[B(C6F5)4] in Et2O affords {[ToBu Benz]Zr(CH2Ph)2(OEt2)}[B(C6F5)4] (Scheme 2), which has been structurally characterized by X-ray diffraction (Figure 14). The ability to isolate F


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Inorganic Chemistry

suspension. The mixture was treated with n-hexane (5 mL) to precipitate more material, which was isolated by filtration. The precipitate was washed with n-hexane, dried in vacuo, and then extracted with dichloromethane (5 mL). The volatile components were removed in vacuo to give [ToMeBenz]CpZrCl2 as a white powder (30 mg, 67%). Anal. Calcd for Cp[ToMeBenz]ZrCl2: C, 51.2; H, 4.0; N, 12.3. Found: C, 50.8; H, 3.9; N, 11.3. t

t

Figure 15. DFT geometry-optimized structures of {[ToBu Benz]Zrt

(CH2Ph)2(OEt2)}+ and {[ToBu Benz]Zr(CH2Ph)2}+ (hydrogen atoms on carbon atoms are omitted for clarity).

protolytic cleavage of [ToRBenz]Zr(CH2Ph)3 with [PhNHMe2][B(C6F5)4] affords {[ToRBenz]Zr(CH2Ph)2}+, which can serve as a catalyst for the polymerization of ethylene.

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Synthesis of [ToBu ]ZrCl3. A mixture of [ToBu ]Na (16 mg, 0.04 mmol) and ZrCl4 (12.3 mg, 0.05 mmol) was treated with benzene (1 mL) and heated at 60 °C for 4 h. The mixture was filtered, and the solid was extracted with chloroform (3 × 3 mL). The volatile components were removed in vacuo, and the residue obtained was washed t with hexanes (3 mL), yielding [ToBu ]ZrCl3 as an off-white powder (6 mg, 27%). Synthesis of [ToMeBenz]Zr(CH2Ph)3. A solution of Zr(CH2Ph)4 (27.7 mg, 0.06 mmol) in benzene (6 mL) was added to [ToMeBenz]Tl (40.0 mg, 0.06 mmol). The mixture was stirred at room temperature for 10 min, resulting in the immediate deposition of thallium. The mixture was filtered and the filtrate was treated with pentane (20 mL), thereby precipitating [ToMeBenz]Zr(CH2Ph)3. The supernatant was decanted, and the product was washed with pentane (5 mL) and dried in vacuo to afford [ToMeBenz]Zr(CH2Ph)3 (20.0 mg, 40%) as a yellow powder. Crystals of [ToMeBenz]Zr(CH2Ph)3 suitable for X-ray diffraction were obtained from a solution in CH2Cl2. Anal. Calcd for [ToMeBenz]Zr(CH2Ph)3: C, 64.5; H, 5.5; N, 10.8. Found: C, 64.3; H, 5.4; N, 10.6.

EXPERIMENTAL SECTION

General Considerations. All manipulations were performed using a combination of glovebox, high vacuum, and Schlenk techniques under a nitrogen or argon atmosphere.51 Solvents were purified and degassed by standard procedures. 1H NMR spectra were measured on Bruker 300 DRX, Bruker 400 DRX, Bruker 400 Cyber-enabled Avance III, and Bruker Avance 500 DMX spectrometers. 1H NMR chemical shifts (see the Supporting Information, SI) 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 and 5.32 for CDHCl2).52 13C NMR chemical shifts (see the SI) are reported in ppm relative to SiMe4 (δ = 0) and were referenced internally with respect to the solvent (δ 128.06 for C6D6 and 53.84 for CD2Cl2).52 Coupling constants are given in hertz. IR spectra were recorded on a PerkinElmer Spectrum Two spectrometer and are reported in reciprocal centimeters. Mass spectra were obtained on a Jeol JMS-HX110H tandem double-focusing mass spectrometer with a 10 kV accelerating voltage equipped with a fast-atom-bombardment ion source. CpZrCl3 and Cp2ZrCl2 were obtained from Sigma-Aldrich, while Zr(CH2Ph)436 and [ToR]M (M = Na, Tl)14,53 were prepared by literature methods. Caution! Thallium compounds are toxic and must be handled and disposed of safely. X-ray Structure Determinations. X-ray diffraction data were collected on a Bruker Apex II diffractometer. 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 2014/7).54 Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC 1584045−1584052).55 Computational Details. Calculations were carried out using DFT, as implemented in the Jaguar 8.9 (release 15) suite of ab initio quantum chemistry programs.56 Geometry optimizations were performed with the B3LYP density functional using the LACVP** basis set. Cartesian coordinates, and the energies of the geometry-optimized structures are provided in the SI. Synthesis of [ToMeBenz]ZrCl3. A mixture of [ToMeBenz]Na·diglyme (40 mg, 0.07 mmol) and ZrCl4 (18 mg, 0.08 mmol) was placed in an ampoule, treated with dichloromethane (6 mL), and heated overnight at 50 °C. After this period, the mixture was filtered and the volatile components were removed from the filtrate in vacuo. The solid residue was washed with acetonitrile (3 mL) and hexane (3 mL) to yield [ToMeBenz]ZrCl3 as a white powder (14 mg, 33%). Anal. Calcd for [ToMeBenz]ZrCl3·CH2Cl2: C, 40.8; H, 3.3; N, 11.4. Found: C, 41.0; H, 3.4; N, 11.0. Synthesis of Cp[ToMeBenz]ZrCl2. A mixture of CpZrCl3 (18 mg, 0.07 mmol) and [ToMeBenz]Na·diglyme (40 mg, 0.07 mmol) was placed in an ampoule and treated with benzene (5 mL). The mixture was stirred at room temperature for a period of 2 h, during which it became a

t

Synthesis of [ToBu Benz]Zr(CH2Ph)3. A solution of Zr(CH2Ph)4 t

(23 mg, 0.05 mmol) in toluene (3 mL) was added to [ToBu Benz]Tl (40 mg, 0.05 mmol). The mixture was stirred at room temperature for 10 min, resulting in the deposition of thallium. After this period, the t

mixture was filtered. Yellow crystals of [ToBu Benz]Zr(CH2Ph)3 were obtained by the diffusion of pentane into the filtrate at −15 °C, isolated, washed with pentane, and dried in vacuo (27.0 mg, 57%). Crystals of t [ToBu Benz]Zr(CH2Ph)3 suitable for X-ray diffraction were obtained by t the slow diffusion of pentane into a toluene solution of [ToBu Benz]ZrButBenz (CH2Ph)3 at −15 °C. Anal. Calcd for [To ]Zr(CH2Ph)3: C, 68.7; H, 6.5; N, 8.9. Found: C, 67.5; H, 6.5; N, 8.5. Synthesis of [ToAdBenz]Zr(CH2Ph)3. A solution of Zr(CH2Ph)4 (17.2 mg, 0.038 mmol) in benzene (4 mL) was added to [ToAdBenz]Tl· THF (41.0 mg, 0.038 mmol). The mixture was stirred at room temperature for 10 min, thereby resulting in the immediate deposition of thallium. The mixture was filtered and the filtrate was treated with pentane (15 mL), thereby precipitating [ToAdBenz]Zr(CH2Ph)3. The supernatant was decanted, and the product was washed with pentane (5 mL) and dried in vacuo to afford [ToAdBenz]Zr(CH2Ph)3 (27 mg, 61%) as a yellow powder. Yellow crystals of [ToAdBenz]Zr(CH2Ph)3 suitable for X-ray diffraction were obtained from a solution of [ToAdBenz]Zr(CH2Ph)3 in CH2Cl2. Anal. Calcd for [ToAdBenz]Zr(CH2Ph)3·0.4CH2Cl2: C, 71.7; H, 6.6; N, 6.9. Found: C, 71.7; H, 7.1; N, 6.9. Synthesis of [BoMeBenz]2Zr(CH2Ph)2. A mixture of Zr(CH2Ph)4 (17.8 mg, 0.04 mmol) and [BoMeBenz]Tl (40.0 mg, 0.08 mmol) was treated with benzene (2 mL). The mixture was stirred at room temperature for 5 min, thereby resulting in the immediate deposition of thallium. The mixture was filtered and the filtrate was treated with pentane (20 mL), thereby precipitating [BoMeBenz]2Zr(CH2Ph)2. The supernatant was decanted, and the product was washed with pentane (10 mL) and dried in vacuo to afford [BoMeBenz]2Zr(CH2Ph)2 (18.0 mg, 52%) as a yellow powder. Yellow crystals of [BoMeBenz]2Zr(CH2Ph)2 suitable for X-ray diffraction were obtained from the slow diffusion of pentane into a toluene solution of [BoMeBenz]2Zr(CH2Ph)2 at −15 °C. Synthesis of [BoAdBenz]2Zr(CH2Ph)2. A mixture of Zr(CH2Ph)4 (17.3 mg, 0.04 mmol) and [BoAdBenz]Tl·0.5THF (60 mg, 0.08 mmol) was treated with toluene (4 mL). The mixture was stirred at room temperature for 10 min, thereby resulting in the immediate deposition of thallium. The mixture was filtered and the filtrate was treated with pentane (20 mL), thereby precipitating [BoAdBenz]2Zr(CH2Ph)2. The supernatant was decanted, and the product was washed with G


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pentane (10 mL) and dried in vacuo to afford [BoAdBenz]2Zr(CH2Ph)2 (25.0 mg, 48%) as a yellow powder.

ACKNOWLEDGMENTS We thank the U.S. Department of Energy, Office of Basic Energy Sciences (Grant DE-FG02-93ER14339), for support of this research.57 A.A.-H. thanks the Government of Saudi Arabia for a scholarship.

t

Synthesis of {[ToBu Benz]Zr(CH2Ph)2(OEt2)}[B(C6F5)4]. A mixture t

of [ToBu Benz]Zr(CH2Ph)3 (29.5 mg, 0.03 mmol) and [PhNHMe2][B(C6F5)4] (25.0 mg, 0.03 mmol) was treated with Et2O (4 mL), stirred at room temperature for several minutes, and filtered. The mixture t was filtered, and yellow crystals of {[ToBu Benz]Zr(CH2Ph)2(OEt2)[B(C6F5)4] were obtained by the diffusion of pentane into the filtrate at t −15 °C. The crystals of {[ToBu Benz]Zr(CH2Ph)2(OEt2)[B(C6F5)4] were isolated, washed with pentane (3 mL), and dried (15.0 mg, 30%). t Generation and Ethylene Polymerization Activity of {[ToBu Benz]t Zr(CH2Ph)2}[B(C6F5)4]. A solution of [ToBu Benz]Zr(CH2Ph)3 (10 mg, 0.01 mmol) in C6D6 (2 mL) was added to [PhNHMe2][B(C6F5)4] (8.5 mg, 0.01 mmol), and the combination was mixed for several minutes, filtered, and analyzed by 1H NMR spectroscopy, thereby t demonstrating the formation of a mixture of {[ToBu Benz]Zr(CH2Ph)2}+, Me2NPh, and PhCH3. Benzene (4 mL) was added to the above solution, which was degassed and treated with ethylene (1 atm). The reaction mixture was stirred at room temperature for 10 min while maintaining an ethylene pressure of 1 atm. After this period, the mixture was quenched with methanol (5 mL), followed by dilute HClaq (1 M, 20 mL). The polymer was collected by filtration, washed with methanol (10 mL), and then dried in vacuo to a constant weight (80 mg, corresponding to an activity of 48 kg of PE [mol of Zr]−1 [h]−1 [atm of C2H4]−1). Generation and Ethylene Polymerization Activity of {[ToAdBenz]Zr(CH2Ph)2}[B(C6F5)4]. A solution of [ToAdBenz]Zr(CH2Ph)3 (7 mg, 0.006 mmol) in C6D6 (2 mL) was added to [PhNHMe2][B(C6F5)4] (4.7 mg, 0.006 mmol), and the combination was mixed for several minutes, filtered, and analyzed by 1H NMR spectroscopy, thereby demonstrating the formation of a mixture of {[ToAdBenz]Zr(CH2Ph)2}+, Me2NPh, and PhCH3. Benzene (4 mL) was added to the above solution, which was degassed and treated with ethylene (1 atm). The reaction mixture was stirred at room temperature for 10 min while maintaining an ethylene pressure of 1 atm. After this period, the mixture was quenched with methanol (5 mL), followed by dilute HClaq (1 M, 20 mL). The polymer was collected by filtration, washed with methanol (10 mL), and dried in vacuo to a constant weight (160 mg, corresponding to an activity of 160 kg of PE [mol of Zr]−1 [h]−1 [atm of C2H4]−1).

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REFERENCES

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CCDC 1584045−1584052 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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t

t

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authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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