Neutral and Cationic N-Heterocyclic Carbene Zirconium and Hafnium

Apr 26, 2013 - Various zirconium and hafnium amido, chloro, and benzyl complexes supported by a tridentate N-heterocyclic carbene bis-phenolate dianio...
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Neutral and Cationic N‑Heterocyclic Carbene Zirconium and Hafnium Benzyl Complexes: Highly Regioselective Oligomerization of 1‑Hexene with a Preference for Trimer Formation Samuel Dagorne,*,† Stéphane Bellemin-Laponnaz,*,‡ and Charles Romain† †

Institut de Chimie de Strasbourg, CNRS UMR 7177, Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg, France Institut de Physique et Chimie des Matériaux de Strasbourg, CNRS UMR 7504, Université de Strasbourg, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France

‡

S Supporting Information *

ABSTRACT: Various zirconium and hafnium amido, chloro, and benzyl complexes supported by a tridentate N-heterocyclic carbene bis-phenolate dianionic ligand ((OCO)2−) have been synthesized and structurally characterized. The alcohol elimination reaction of the protio ligand N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)-4,5-dihydroimidazolium chloride (1) and the metal alkoxide precursors M(OiPr)4(HOiPr) (M = Zr, Hf) and a subsequent alkoxide/chloride exchange reaction (upon addition of trimethylsilyl chloride, TMSCl) afforded the corresponding Zr and Hf carbene dichloro complexes as THF adducts: (OCO)MCl2(THF) (2a-THF, M = Zr; 2b-THF, M = Hf). As determined by single-crystal X-ray crystallographic studies, the molecular structure of the Hf derivative 2b-THF confirmed the proposed formulation and the effective formation of a (OCO)Hf chelate. In the case of Zr, an amine elimination reaction between protio ligand 1 and Zr(NMe2)4 yielded the corresponding Zr amido THF adduct (OCO)Zr(NMe2)(Cl)(THF) (3a-THF) when carried in THF as a solvent, while the Zr−NHMe2 adduct (OCO)Zr(NMe2)(NHMe2)(THF) (3a-NHMe2) was isolated using CH2Cl2 as the reaction solvent. 3a-THF may be readily and quantitatively converted to the dichloro derivative 2a-THF upon addition of TMSCl. The toluene elimination reaction of protio ligand 1 and M(CH2Ph)4 (M = Zr, Hf) followed by a salt metathesis with 1 equiv of PhCH2MgCl afforded the corresponding Zr and Hf carbene dibenzyl complexes (OCO)M(CH2Ph)2 (4a, M = Zr; 4b, M = Hf), whose solid-state structures were confirmed by X-ray crystallography. 4a and 4b each feature a five-coordinate metal center with both benzyl moieties binding in a η2 fashion. The protonolysis reaction between species 4a (or 4b) and [HNMe2Ph][B(C6F5)4] afforded the clean and quantitative formation of the corresponding Zr (or Hf) anilinium benzyl cation 5a+ (or 5b+). Remarkably, the cation 5a+ catalyzes the highly regioselective oligomerization of 1-hexene with a marked preference for trimer formation.

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INTRODUCTION

oxophilic transition-metal complexes bearing NHC ligands is much less developed, and the suitability of NHC-supported oxophilic metal species remains little developed.2 Thus far, only a limited number of NHC-containing oxophilic metal species have found applications in catalysis and include the use of NHC-bearing group 4 complexes in ethylene, isoprene, or raclactide polymerization as well as hydroamination catalysis.3−5

N-heterocyclic carbenes (NHCs) now constitute a wellestablished class of privileged ancillary ligands for coordination to transition-metal complexes and application of the derived complexes in homogeneous catalysis.1 In comparison to their phosphine analogues, NHC-containing metal complexes usually exhibit an inert carbene−metal bond, providing an enhanced stability. This has led to the development of numerous NHCincorporating late-transition-metal catalysts that frequently feature improved activity and/or selectivity in catalysis. In contrast, the use in catalysis of high-oxidation-state and © XXXX American Chemical Society

Received: March 5, 2013

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Scheme 1

involving side reactions and favoring the generation of viable/ stable group 4 alkyl cations, the actual polymerization catalysts.13 The excellent robustness of NHC-incorporating bis(phenolate) group 4 alkoxide complexes synthesized thus far prompted us to extend our studies toward the preparation of (OCO)MR2 (R = alkyl, benzyl) dialkyl/dibenzyl analogues and derived cationic species for subsequent use in α-olefin polymerization/oligomerization. As part of these studies, we here report on the synthesis and characterization of neutral and cationic NHC-bearing zirconium and hafnium benzyl complexes and on their use in 1-hexene oligomerization catalysis. The synthesis and structural characterization of closely related NHC-bearing Zr and Hf chloride and amido derivatives are also included in the present contribution.

We earlier reported on a novel family of tridentate bis(aryloxide)-NHC chelating ligands (OCO), featuring a central NHC donor flanked on each side by an orthosubstituted phenolate moiety, for coordination to Mn(III), V(V), and group 4 metal centers.5−7 In all these complexes, the OCO2− ligand coordinates in a mer fashion to form extremely robust κ3-OCO chelates resulting in air-stable complexes in the solid state, thereby illustrating the suitability of the ligand framework for such metal centers. With respect to their potential usefulness in catalysis, most notably, a heteroleptic Zr alkoxide derivative of the type (tBuOCO)Zr(OiPr)(Cl)(THF) acts as an effective initiator for the controlled, immortal, and highly stereoselective ROP of rac-lactide to produce heterotactic poly(lactic acid) (PLA), the first instance that a NHCbased oxophilic metal catalyst may be competitive with the thus far best reported systems in the area.5a Over the past 10−15 years, non-metallocene group 4 complexes supported by bis(phenolate)-type chelating ligands have attracted considerable attention for use in α-olefin polymerization catalysis in order to access polymers with tailored microstructures.8 In particular, remarkable advances with regard to catalytic activity, stereoselectivity, and catalyst structure/polymer microstructure relationships have been achieved through the use of group 4 species supported by various tetradentate diamino/dithio bis(phenolate) ligands, which may mediate the living and stereospecific polymerization of α-olefins (primarily 1-propene, 1-hexene, and styrene) in a highly effective manner.8d,9 In contrast, the use of tridentate bis(phenolate) ligands bearing a central L donor ligand have met mitigated success. For instance, unlike their tetradentate diamino bis(phenolate) analogues, Zr complexes supported by a tridentate amino bis(phenolate) ligand rapidly deactivate under 1-hexene polymerization conditions to only afford traces of oligomers, highlighting the key role of an extra-donor L ligand in these systems.10 Titanium complexes supported by tridentate thio bis(phenolate) ligands coordinating in a fac fashion efficiently catalyze the syndiotactic polymerization of styrene.11 More recently, various tridentate bis(phenolate) ligand platforms with a central heteroatom L donor were shown to coordinate in a mer fashion to group 4 metal centers, with the resulting complexes being highly active in ethylene polymerization and/or displaying a reasonable activity in propene polymerization and 1-hexene oligomerization.12 Though factors governing polymerization/oligomerization parameters in these tridentate ligand supported systems are rather complicated, the robustness and stability of the metal chelate in such fairly electron-deficient five-coordinate metal species certainly play a crucial role both in avoiding ligand-

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RESULTS AND DISCUSSION Synthesis and Structure of Zr and Hf Amido and Chloro Derivatives Supported by the NHC-Bearing Tridentate Dianionic Ligand tBu(OCO)2−. Access to the desired dibenzyl Zr and Hf complexes [tBu(OCO)]M(CH2Ph)2 (M = Zr, Hf) was first envisaged through a salt metathesis route involving the reaction of the corresponding dichloro derivatives with 2 equiv of PhCH2MgCl, thus first requiring the preparation of dichloro precursors of the type [tBu(OCO)]MCl2. Earlier studies showed that the Ti and Zr chloro alkoxide species [tBu(OCO)]M(Cl)(OiPr)(THF) (M = Ti, Zr) may be prepared in high yield via an alcohol elimination reaction between the protio ligand N,N′-bis(2-hydroxy-3,5-di-tertbutylphenyl)-4,5-dihydroimidazolium chloride (1) and Ti(OiPr)4 or Zr(OiPr)4·HOiPr.5 This approach was extended to Hf chemistry, and the in situ generated species [tBu(OCO)]M(Cl)(OiPr)(THF) (M = Zr, Hf) were each reacted with 1 equiv of trimethylsilyl chloride (TMSCl) to afford the corresponding dichloro compounds [tBu(OCO)]M(Cl)2(THF) as THF adducts (M = Zr, 2a-THF; M = Hf, 2b-THF; Scheme 1) in overall good yields, thus allowing access to 2a-THF and 2b-THF in a one-pot procedure from protio ligand 1 (see the Experimental Section). For instance, the reaction of 1 with Hf(OiPr)4·HOiPr (THF, room temperature, 15 h) afforded crude [tBu(OCO)]Hf(Cl)(OiPr)(THF) (as deduced from 1H NMR analysis), which was subsequently reacted with 1 equiv of TMSCl (toluene, 110 °C, 3 days) to yield 2b-THF as an analytically pure pale yellow solid (67% overall yield). Notably, despite the reaction conditions, the conversion of crude [tBu(OCO)]Hf(Cl)(OiPr)(THF) to 2b-THF proceeded cleanly and quantitatively (>90% conversion, as monitored by 1H NMR) with no observable decomposition products, further B

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first popularized by Jordan et al. in the mid-1990s for the high yield and stereoselective preparation of ansa-zirconocene dichloro derivatives and was also subsequently applied to non-metallocene group 4 chemistry by Schrock et al.14 In the present case, the reaction of protio ligand 1 with 1 equiv of Zr(NMe2)4 (THF, −78 °C to room temperature, 15 h) allowed the formation of the corresponding Zr amido and THF adduct compound [ tBu (OCO)]Zr(Cl)(NMe 2 )(THF) (3a-THF; Scheme 2), which was isolated as a yellow solid in 53% yield.

illustrating the excellent stability and robustness of these (κ3OCO)M chelate compounds. As might have been expected, the formation of the Zr analogue 2a-THF from [tBu(OCO)]Zr(Cl)(OiPr)(THF) and TMSCl occurred under milder conditions (toluene, 50 °C, 15 h). As a comparison, in the case of the Zr derivative, the salt metathesis route involving the in situ prepared (OCO)2− bis(phenolate) NHC, generated via reaction of compound 1 and 3 equiv of nBuLi (THF, −78 °C), and ZrCl4(THF)2 gave the dichloro Zr species 2a-THF in low yield (18% yield).6b The NMR data for the Hf complex 2b-THF are closely related to those already reported of its Zr analogue 2a-THF.6b They are consistent with the effective formation of a (κ3-OCO) Hf chelate and the coordination of a THF molecule onto the Hf(IV) metal center. Under the studied conditions (CD2Cl2, room temperature), 2b-THF exhibits a pseudo-C2v-symmetric structure in solution, as indicated by the 1H NMR resonance assigned to the NCH2CH2N moiety (broadened singlet), presumably due to a relatively fast decoordination/coordination of THF on the NMR time scale. The 13C NMR spectrum of 2b-THF contains a characteristic downfield signal (δ 200.5 ppm) assigned to the Ccarbene−Hf carbon.3e,4b The molecular structure of 2b-THF was confirmed by singlecrystal X-ray crystallographic studies, as depicted in Figure 1,

Scheme 2

The proposed formulation was deduced from NMR data, all consistent with the presence of a (κ3-OCO)Zr chelate, a ZrNMe2 group, a coordinated THF molecule and an overall Cssymmetric structure for 3a-THF under the studied conditions (CD2Cl2, room temperature). The 13C NMR spectrum of 3aTHF includes a characteristic Ccarbene−Zr signal (δ 202.9). Unsurprisingly, carrying out the amine elimination reaction of protio ligand 1 and Zr(NMe2)4 in a noncoordinative solvent such as CH2Cl2 (−78 °C to room temperature, 15 h) led to the formation of the Zr amido and HNMe2 adduct compound [tBu(OCO)]Zr(Cl)(NMe2)(HNMe2) (3a-HNMe2; Scheme 2), isolated as a yellow solid in 59% yield. In comparison to that of 3a-THF, the 1H NMR spectrum 3a-HNMe2 (C6D6) features no THF signals but contains an extra Zr-HNMe2 resonance (δ 1.70 ppm, 6H, br s). The NMR scale amido/chloride exchange reaction between 3a-THF and 1 equiv of TMSCl allowed the clean and quantitative formation of the dichloro complex 2a-THF, as monitored by 1H NMR spectroscopy (CD2Cl2, room temperature, 2 h), thereby validating the amine elimination pathway to access (OCO)ZrCl2-type species. All attempts to access the sought-after (OCO)M(CH2Ph)2 derivatives via a salt metathesis reaction (performed under various conditions) between either 2a-THF or 2b-THF and 2 equiv of PhCH2MgCl remain unsuccessful with, instead, the formation of unusual rearrangement products to be reported in due course.15

Figure 1. Molecular structure of the Hf complex 2b-THF. The hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Hf(1)−C(15) = 2.333(6), Hf(1)−O(1) = 1.956(4), Hf(1)−O(2) = 1.963(4), Hf(1)−O(3) = 2.268(4), Hf(1)−Cl(1) = 2.451(2), Hf(1)−Cl(2) = 2.409(2); O(1)−Hf(1)−O(2) = 154.7(2), Cl(1)−Hf(1)−C(15) = 166.2(1), C(15)−Hf(1)−Cl(2) = 96.8(1), O(3)−Hf(1)−Cl(2) = 179.1(1).

along with selected bonding and geometrical parameters. A summary of crystal and refinement data for 2b-THF is included in Table S1 (Supporting Information). The hafnium atom in compound 2b-THF adopts a slightly distorted octahedral geometry as a result of the mer coordination of the tridentate NHC ligand, with an O−Zr−O bite angle of 154.7(2)°. As earlier observed for the NHC−Zr dichloro species 2a-THF,6b the (κ3-OCO)M chelate of 2b-THF is also distorted from planarity with C(27)−O(2)−O(1)−C(5) = 26.2(8)°, thus reflecting geometrical constraints imposed by the formation of the (OCO)Hf chelate. The Hf−Ccarbene bond distance (2.333(6) Å) is comparable to that reported for other NHC−Hf species.3e,4b In the case of Zr, access to the dichloro compound 2a-THF via an amine elimination−amido/chloride exchange sequential reaction, involving protio ligand 1, Zr(NMe2)4, and TMSCl as reagents, was also investigated. Such a synthetic approach was C

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Synthesis and Structure of NHC-Supported Zr and Hf Dibenzyl Complexes (OCO)M(CH2Ph)2. The toluene elimination route with precursors 1 and M(CH2Ph)4 and a subsequent salt metathesis reaction (with PhCH2MgCl) was found to be most appropriate to access the derivatives [tBu(OCO)]M(CH2Ph)2 (M = Zr, Hf). Thus, the reaction of ligand 1 with M(CH2Ph)4 (M = Zr, Hf) (toluene, −35 °C to room temperature, 15 h) cleanly generated the corresponding chloro benzyl species [tBu(OCO)]M(Cl)(CH2Ph) (M = Zr, Hf) along with 3 equiv of toluene (as deduced from 1H NMR analysis). The latter Zr and Hf species were not isolated but directly reacted with 1 equiv of PhCH2MgCl (toluene, −35 °C to room temperature, 15 h) to afford the dibenzyl complexes [tBu(OCO)]M(CH2Ph)2 (4a, M = Zr; 4b, M = Hf; Scheme 3), which were both isolated in good yields as analytically pure yellow solids.

Figure 2. Molecular structure of the Zr−NHC dibenzyl complex 4a. The hydrogen atoms and the tBu groups are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zr(1)−C(15) = 2.333(1), Zr(1)−O(1) = 1.987(1), Zr(1)−O(2) = 1.982(1), Zr(1)− C(39) = 2.309(2), Zr(1)−C(32) = 2.292(2), Zr(1)−C(33) = 2.409(2), Zr(1)....C(40) = 2.656(2), Zr(1)....C(33) = 2.866(1); Zr(1)−C(39)−C(40) = 86.3(1), Zr(1)−C(32)−C(33) = 96.6(1).

Scheme 3

The NMR data for compounds 4a,b agree with an effective C2v-symmetric structure (CD2Cl2, room temperature and −15 °C) with, in particular, the presence of a singlet resonance for the CH2 benzyl group (δ 1.95 and 2.06 ppm for 4a,b, respectively) and a characteristic 13C resonance (δ 205.8 and 212.6 ppm for 4a,b, respectively) assigned to the Ccarbene carbon. These data are also consistent with a η2 coordination of the benzyl ligands and thus with fairly electron-deficient Zr(IV) and Hf(IV) metal centers in these compounds. An indication of η2 coordination benzyl interactions includes a rather large JCH value for the M−CH2 groups (1JCH = 135 Hz for both 4a and 4b) and an upfield resonance of the Cipso benzyl carbons (δ 139.1 and 138.0 ppm for 4a,b, respectively).16,17 The molecular structures of both 4a and 4b were unambiguously confirmed by X-ray crystallography, and these are depicted in Figures 2 and 3, respectively, along with selected bond and geometrical parameters. A summary of crystal and refinement data for 4a,b is included in Tables S1 and S2 (Supporting Information). As expected, compounds 4a,b exhibit closely related structural features, and these will only be discussed in the case of 4a. The solid-state structure of the Zr complex 4a features a pentacoordinate Zr metal center adopting a trigonal bipyramidal geometry with a nearly planar [tBu(OCO)]Zr chelate (C(18)−O(2)−O(1)−C(1) = 8.2(2)°) and with the Zr(1), C(15), C(32), and C(39) atoms being coplanar, as reflected by the sum of the C(15)−Zr(1)−C(39), C(39)−Zr(1)−C(32), and C(32)−Zr(1)−C(15) bond angles, which is exactly equal to 360°. Importantly, both benzyl groups are distorted, though to a different extent, with small Zr(1)− C(39)−C(40) and Zr(1)−C(32)−C(33) bond angles (86.3(1) and 96.6(1)°, respectively). This, together with rather short

Figure 3. Molecular structure of the Hf−NHC dibenzyl complex 4b. The hydrogen atoms and the tBu groups are omitted for clarity. Selected bond lengths (Å) and angles (deg): Hf(1)−C(15) = 2.308(3), Hf(1)−C(32) = 2.279(3), Hf(1)−C(39) = 2.261(3), Hf(1)−O(2) = 1.976(2), Hf(1)−O(1) = 1.979(2), Hf(1)....C(33) = 2.695(3), Hf(1)....C(40) = 2.845(3); Hf(1)−C(39)−C(40) = 96.9(2), Hf(1)−C(32)−C(33) = 89.2(2).

Zr−Cipso‑Bn distances (Zr(1)−C(40) = 2.656(2) Å and Zr(1)− C(33) = 2.866(1) Å) substantiates η2 coordination of the benzyl for the two Zr−CH2Ph groups, with the C(39)−C(40) benzyl group featuring a stronger interaction with the Zr center.16,17 In fact, the severe distortion of the C(39)−C(40) benzyl group is similar to that observed in cationic complexes such as Cp2Zr(CH2Ph)(MeCN)+ (Zr−C−Cipso = 84.9(4)° and Zr−Cipso = 2.648(6) Å), reflecting the electrophilic nature of the metal center in 4a.18 Another noteworthy structural observation of the solid state of 4a is that the Ph ring of a Zr−Bn group is nearly parallel to the N-heterocyclic ring with C(33)−C(15) and C(33)−C(15) distances (3.219(2) and 3.206(2) Å, respectively) lying a bit below the sum of the van der Waals (vdW) radii (3.4 Å), which indicates some vdW-type close contacts between the Ph ring and Ccarbene atom. The bonding parameters are unaffected by these contacts. Similar interactions are also observed for the Hf analogue 4b (Figure 3). Preparation of NHC-Supported Zr and Hf Benzyl Cations and Use in the Regioselective Oligomerization D

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of 1-Hexene. Discrete cationic group 4 alkyl/benzyl complexes, which may act as olefin polymerization catalysts and/or mediate various organic transformations, may typically be generated via reaction of the corresponding dialkyl/dibenzyl derivatives with activators B(C6F5)3, [Ph3C][B(C6F5)4], and [HNMe2Ph][B(C6F5)4].13 As initial ionization studies, the Zr and Hf dibenzyl species 4a,b were reacted with 1 equiv of B(C6F5)3 or [Ph3C][B(C6F5)4] (CD2Cl2, −15 °C or room temperature, 10 min). As deduced from 1H NMR monitoring experiments, these reactions were not clean, and all yielded complicated mixtures of products. In contrast, the use of the anilinium salt [HNMe2Ph][B(C6F5)4] as an ionizing agent for species 4a,b (CD2Cl2, −15 °C or room temperature, 10 min) led to the clean and quantitative generation of the corresponding Zr−NMe2Ph and Hf−NMe2Ph benzyl cations [tBu(OCO)]M(CH2Ph)(NMe2Ph)+ (5a+, M = Zr; 5b+, M = Hf; Scheme 4) as weakly associated B(C6F5)4− salts under the studied conditions, along with the formation of 1 equiv of toluene (Figure S7, Supporting Information).

conditions, though the p-C6F5 resonance is unaffected (δ −163.2, t, J = 19 Hz). This may reflect weak cation/anion interactions through the o-C6F5 and m-C6F5 fluorine atoms.21 In this regard, coordination of perfluorophenylborates via oC6F5 and m-C6F5 close contacts has been evidenced in the solid state for the salt [Cp*2ZrH][HB(C6F5)3].22 The salt species [5a,b][B(C6F5)4] as well as 1/1 4a,b/ [Ph3C][B(C6F5)4] and 4a,b/B(C6F5)3 mixtures were all tested for 1-hexene polymerization/oligomerization. While the activation of either 4a or 4b with B(C6F5)3 led to no reaction with 1-hexene (C6H5Br, room temperature, 250 equiv of 1hexene), the 1/1 4a/[Ph3C][B(C6F5)4] mixture reacts with 1hexene at room temperature with the total consumption of 250 equiv of 1-hexene within 1 h to yield oligo(1-hexenes), as deduced from SEC data (Mn = 650 g mol−1, Mw/Mn = 1.22; Figure S1, Supporting Information). The 1H NMR spectra of these oligomers agree with no end group regioselectivity, as deduced by the presence of both vinylene and vinylidene signals in a 1/1 ratio (Figure S2, SI). GC-MS analysis indicated an oligomeric distribution in the following percent ratio (going from dimers to heptamers): 5/22/25/28/16/4 (Figure S3, Supporting Information). A similar outcome was observed with the Hf analogue, but the oligomerization of 1-hexene is much slower. In contrast, the Zr anilinium cation 5a+ oligomerizes 1hexene to produce oligomers with vinylene end groups in a highly regioselective manner, as deduced by 1H NMR and SEC data (250 equiv of 1-hexene, room temperature, 5 h, quantitative conversion, Mn = 385 g mol−1, Mw/Mn = 1.07, >99% regioselectivity by GC-MS; Figures S4−S6, Supporting Information): i.e., these species thus arise from a β-elimination reaction from a 2,1-enchained Zr cation. Under these reaction conditions, the Hf anilinium cation 5b+ barely reacts with 1hexene with the production of only a trace amount of oligomers after a prolonged reaction time (250 equiv of 1-hexene, room temperature, 30 h, 10% conversion). Interestingly, GS-MS data for the oligo(hexenes) produced with the Zr cation 5a+ are consistent with the formation of oligo(1-hexenes) with three different chain lengths (trimer/tetramer/pentamer percent ratio 77/16/7; Figure S6, Supporting Information); thus, there is a strong preference for trimer formation (77% relative to all formed oligomers) along with a tetramer/pentamer mixture as a minor component. Hence, going from [Ph3C][B(C6F5)4] to [HNMe2Ph][B(C6F5)4] for ionization of compound 4a dramatically improved the regioselectivity and narrowed the polydispersity of the produced oligo(1-hexenes), both factors being crucial for a controlled α-olefin oligomerization process. On that matter, it may be noted that very few metal catalysts have been reported for the highly regioselective α-olefin oligomerization.12a,23 In the present system, the Zrcoordinated NMe2Ph moiety in cation 5a+ certainly stabilizes the formed cation from an electronic point of view. It also enhances steric hindrance at Zr vs that of “base-free” Zr benzyl cation (presumably generated upon reaction of 4a with [Ph3C][B(C6F5)4], though not characterized), which possibly rationalizes the strong preference for a 2,1-insertion of 1-hexene and the derived high regioselectivity of the oligomerization process.

Scheme 4

Though no sign of decomposition was detectable after 15 h in CD2Cl2 at −15 °C, [5a,b][B(C6F5)4] both rapidly decompose in CD2Cl2 at room temperature (t1/2 = 30 min) to unknown species. This limited stability precluded their isolation in a pure form, and their formulation is based on thorough 1D 1H, 13C, and 19F NMR and 2D NMR (HSQC, HMBC and NOESY) analysis. Given the similar NMR patterns of the Zr and Hf cations, only those for [5a][B(C6F5)4] are discussed here. All NMR data on the cationic NHC-Zr complex 5a+ agree with a Cssymmetric structure (CD2Cl2, −15 °C) with the presence of a 1 H NMR singlet resonance for the CH2 benzyl group (δ 2.66 ppm) and a typical 13C NMR signal assigned to the Zr−Ccarbene (δ 203.0 ppm). The upfield resonance of the benzyl Cipso (δ 131.8 ppm) and the large 1JCH coupling constant of the benzyl CH2 group (1JCH = 140 Hz) indicate that the benzyl group binds in a η2 mode to Zr.17−19 Though the 1H NMR chemical shift of the NMe2Ph methyl groups is nearly identical with that of free NMe2Ph (δ 2.93 vs 2.90 ppm, respectively), the Cipso− NMe2Ph resonance is shifted upfield (δ 139.7 vs 150.7 ppm for free NMe2Ph) and the NMe2Ph 1H NMR resonances are shifted downfield relative to those of free NMe2Ph. These data strongly suggest that NMe2Ph effectively coordinates the Zr center in cation 5a+.20 This is further substantiated by the presence of a NOE correlation between the NMe2Ph moiety and a tBu group of the [tBu(OCO)]Zr chelate. The 19F NMR spectrum of [5a][B(C6F5)4] (CD2Cl2, −15 °C) contains the expected resonances for the B(C6F5)4− anion, but both the oC6F5 and m-C6F5 signals are slightly broadened (causing a disappearance of the fine coupling) under the studied

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SUMMARY AND CONCLUSION Various Zr− and Hf−NHC amido and chloro complexes supported by a tridentate NHC-incorporating bis(phenolate) ligand have been synthesized and structurally characterized. Such compounds may be prepared in good yield from the E

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Organometallics

Article

dryness to give a yellow residue. Washing with pentane of the latter solid and drying in vacuo afforded the Hf dichloro derivative 2b-THF as an analytically pure pale yellow powder (259.4 mg, 67% overall yield). Anal. Calcd: C, 59.13; H, 7.37; N, 3.94. Found: C, 59.26; H, 7.49; N, 3.78. 1H NMR (300 MHz, CD2Cl2): δ 7.30 (d, J = 2 Hz, 2H, aryl-H), 7.05 (d, J = 2 Hz, 2H, aryl-H), 4.41 (br s, 4H, NCH2), 3.82− 3.70 (m, 4H, THF), 1.82−1.75 (m, 4H, THF), 1.58 (s, 18H, tBu), 1.37 (s, 18H, tBu). 13C NMR (75 MHz, CD2Cl2): δ 200.5 (NCN), 147.7 (Cipso, aryl), 141.9 (Cquat, aryl), 139.3 (Cquat, aryl), 131.0 (Cquat, aryl), 120.3 (CH, aryl), 112.8 (CH, aryl), 70.9 (CH2, THF), 48.1 (CH2, NCH2), 35.9 (Cquat, tBu), 35.0 (Cquat, tBu), 31.8 (CH3, tBu), 30.3 (CH3, tBu), 25.8 (CH2, THF). [tBu(OCO)]Zr(Cl)(NMe2)(THF) (3a-THF). A precooled THF solution (25 mL, −78 °C) of the imidazolinium chloride salt 1 (250.0 mg, 0.485 mmol) was added dropwise via a syringe to a THF solution (5 mL) of Zr(NMe2)4 (129.8 mg, 0.485 mmol) precooled to −78 °C. The initial colorless solution slowly turned yellow-green after addition of the zirconium reagent and then was warmed to room temperature and stirred overnight. Evaporation to dryness yielded a yellow-brown residue, which, after recrystallization in THF/pentane (1/5), gave [tBu(OCO)]Zr(Cl)(NMe2)(THF) as an analytically pure yellow powder (144.0 mg, 53% yield). Anal. Calcd: C, 61.76; H, 8.12; N, 5.84. Found: C, 61.87; H, 8.42; N, 5.34. 1H NMR (300 MHz, CD2Cl2): δ 7.26 (d, J = 2 Hz, 2H, aryl-H), 7.03 (d, J = 2 Hz, 2H, arylH), 4.52−4.24 (m, 4H, CH2), 3.66−3.52 (m, 4H, THF), 2.82 (s, 6H, N-Me), 1.68−1.62 (m, 4H, THF), 1.60 (s, 18H, tBu), 1.37 (s, 18H, t Bu). 13C NMR (75 MHz, CD2Cl2): δ 202.9 (NCN), 148.2 (Cipso-O), 140.4 (Cquat, aryl), 138.4 (Cquat, aryl), 131.3 (Cquat, aryl), 119.9 (C-H, aryl), 112.9 (C-H, aryl), 70.6 (CH2, THF), 48.2 (N-CH2), 45.4 (NMe), 36.0 (Cquat, tBu), 34.9 (Cquat, tBu), 31.9 (Me, tBu), 30.5 (Me, t Bu), 25.6 (CH2, THF). [tBu(OCO)]Zr(Cl)(NMe2)(HNMe2) (3a-HNMe2). A precooled CH2Cl2 solution (25 mL, −78 °C) of the imidazolinium chloride salt 1 (250.0 mg, 0.485 mmol) was added dropwise via a cannula to a CH2Cl2 solution (5 mL) of Zr(NMe2)4 (129.8 mg, 0.485 mmol) precooled to −78 °C. The initial colorless solution slowly turned yellow-green after addition of the zirconium reagent and then was warmed to room temperature and stirred overnight. Evaporation to dryness yielded a greenish residue, which, after recrystallization in THF/pentane (1/5), gave [tBu(OCO)]Zr(Cl)(NMe2)(HNMe2) as a yellow solid in pure form on the basis of 1H NMR analysis and elemental analysis (186.4 mg, 59% yield). Anal. Calcd: C, 60.70; H, 8.30; N, 8.09. Found: C, 60.98; H, 8.52; N, 8.35. 1H NMR (300 MHz, C6D6): δ 7.53 (d, J = 2 Hz, 2H, aryl-H), 6.81 (d, J = 2 Hz, 2H, aryl-H), 3.33−3.18 (m, 4H, CH2), 3.14 (s, 6H, Zr−NMe2), 1.88 (s, 18H, tBu), 1.70 (br s, 6H, HNMe2), 1.43 (s, 18H, tBu). [tBu(OCO)]Zr(CH2Ph)2 (4a). A precooled toluene solution (10 mL, −35 °C) of Zr(Bn)4 (316.7 mg, 0.695 mmol) was added dropwise via a pipet to a toluene suspension (40 mL) of the imidazolinium chloride salt 1 (358.0 mg, 0.695 mmol) precooled to −35 °C. The initial colorless suspension slowly turned yellow after addition of the zirconium reagent and then was warmed to room temperature and stirred overnight. Evaporation to dryness yielded a yellow residue, assumed to be [tBu(OCO)]Zr(Cl)(CH2Ph) and used as is. The latter was dissolved in toluene (15 mL) and cooled to −35 °C, and 1 equiv of PhCH2MgBr (2.0 M solution in Et2O, 348.0 μL, 0.695 mmol) was added via a microsyringe. The reaction mixture was warmed to room temperature and stirred overnight to yield a yellow mixture. Filtration of the resulting suspension through Celite on a glass frit and evaporation to dryness yielded a crude yellow solid. Recrystallization of the latter from a concentrated Et2O solution stored at −35 °C yielded pure 4a as an analytically pure yellow solid (396.0 mg, 76% overall yield). Anal. Calcd: C, 72.05; H, 7.79; N, 3.73. Found: C, 72.36; H, 7.69; N, 3.57. 1H NMR (300 MHz, CD2Cl2): δ 7.27 (d, 2H, J = 2 Hz, aryl-H), 6.92 (d, 2H, J = 2 Hz, aryl), 6.83−6.74 (m, 4H, aryl), 6.72−6.64 (m, 2H, aryl), 6.62−6.56 (m, 4H, aryl), 4.01 (s, 4H, NCH2), 1.95 (s, 4H, Ph-CH2), 1.67 (s, 18H, tBu), 1.39 (s, 18H, tBu). 13 C NMR (75 MHz, CD2Cl2): δ 205.8 (Cquat, NCN), 148.0 (Cquat, aryl), 140.9 (Cquat, aryl), 139.1 (Cquat, Bn), 137.3 (Cquat, aryl), 131.9 (Cquat, aryl), 129.3 (CH, Ar-Bn), 129.0 (CH, Ar-Bn), 122.2 (CH, Ar-

corresponding imidazolinium bis(phenol) protio ligand 1 and simple metal precursors via amine and alcohol elimination routes and, when necessary, subsequent derivatization. In contrast, the dibenzyl derivatives 4a,b were only found to be accessible through a toluene elimination reaction of ligand 1 and M(CH2Ph)4 (M = Zr, Hf) and a subsequent salt metathesis reaction with PhCH2MgCl. The protonolysis reaction between species 4a (or 4b) and [HNMe2Ph][B(C6F5)4] afforded the clean and quantitative formation of the corresponding Zr (or Hf) anilinium benzyl cation 5a+ (or 5b+). Remarkably, cation 5a+ catalyzes the highly regioselective oligomerization of 1hexene with a marked preference for trimer formation.

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EXPERIMENTAL SECTION

General Procedures. All experiments were carried out under N2 using standard Schlenk techniques or in a Mbraun Unilab glovebox. THF, dichloromethane, and pentane were first dried through a solvent purification system (MBraun SPS) and stored for at least a couple of days over activated molecular sieves (4 Å) in a glovebox prior to use. CD2Cl2 and C6D6 were purchased from Eurisotop (CEA, Saclay, France), degassed under a N2 flow, and stored over activated molecular sieves (4 Å) in a glovebox prior to use. All other chemicals were used as received. NMR spectra were recorded on Bruker AC 300 and 400 MHz NMR spectrometers, in Teflon-valved J. Young NMR tubes at ambient temperature, unless otherwise indicated. 1H and 13C chemical shifts are reported vs SiMe4 and were determined by reference to the residual 1H and 13C solvent peaks. Mass spectra were recorded by the “service de masse” of the Strasbourg chemistry department. Elemental analyses for all compounds were performed at the Service de Microanalyse of the Université de Strasbourg (Strasbourg, France). GPC analyses were performed on a system equipped with a Shimadzu RID10A refractometer detector using HPLC-grade THF as an eluant. Molecular weights and polydispersity indices (PDIs) were calculated using polystyrene standards. For GCMS, an Agilent 5975C mass spectrometer system equipped with a Varian VF-35 ms column (30 m × 0.25 mm, 0.25 μm) was used. MALDI-TOF mass spectroscopic analyses were performed at the Service de Spectrométrie de Masse de l’Institut de Chimie de Strasbourg and run in a positive mode: samples were prepared by mixing a solution of the polymers in CH2Cl2 with a 0.5 mg/100 mL concentration; dithranol was used as the matrix in a 5/1 volume ratio. In addition to 1H and 13C NMR data, all compounds were analyzed via multinuclear 2D (1H−1H NOESY, 1H−13C HSQC, and HMBC) NMR spectroscopic experiments, allowing unambiguous assignments of characteristic resonances. The protio ligand N,N′-bis(2-hydroxy-3,5di-tert-butylphenyl)-4,5-dihydroimidazolium chloride (1) was synthesized according to literature procedures.6a Preparation of [tBu(OCO)]ZrCl2(THF) (2a-THF; [tBu(OCO)]2− = [κ3-O,C,O-{(3,5-t-Bu2-C6H2O)2N2C3H4}]2−) from [tBu(OCO)]Zr(Cl)(OiPr)(THF). TMSCl (46 μL, 0.430 mmol, 1.1 equiv) was added at room temperature via a syringe to a THF solution (20 mL) of [tBu(OCO)]Zr(Cl)(OiPr)(THF) (250.0 mg, 0.341 mmol), and the resulting mixture was heated overnight at 50 °C with stirring. The solution was evaporated to dryness to give a yellow residue, which was recrystallized from THF/pentane (1/5) to give the Zr dichloro species 2a-THF (115.8 mg, 57% yield), as deduced from NMR data that matched those reported in the literature.6 [tBu(OCO)]HfCl2(THF) (2b-THF). In a drybox, a THF solution (4 mL) of Hf(OiPr)4·iPrOH (230.5 mg, 0.485 mol) was added at room temperature via a pipet to a stirred THF solution (20 mL) of the imidazolinium chloride salt 1 (250.0 mg, 0.485 mmol). The initial colorless solution slowly turned yellow-green within minutes after addition of the hafnium reagent. The reaction mixture was stirred overnight at room temperature and evaporated to dryness to give a yellow-green solid residue, assumed to be crude [tBu(OCO)]Hf(Cl)(OiPr)(THF). The latter crude product was dissolved in toluene (20 mL) and 1.3 equiv of TMSCl (65.5 μL, 0.533 mmol). The resulting solution was stirred for 3 days at 110 °C and then evaporated to F

dx.doi.org/10.1021/om400182d | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

6.98 (d, 2H, J = 2 Hz, aryl), 6.66 (t, J = 8 Hz, 1H, Bn), 6.39 (dd, J = 8 Hz, J = 8 Hz, 2H, Bn), 6.15 (d, J = 8 Hz, 2H, Bn), 4.30−4.20 (m, 4H, NCH2), 3.12 (s, 6H, N-Me), 2.49 (s, 2H, Bn), 1.48 (s, 18H, tBu), 1.36 (s, 18H, tBu). 13C NMR (125 MHz, CD2Cl2, 258 K): δ 207.5 (NCN), 148.1 (Cquat, d, 1JCF = 240 Hz, C6F5), 147.0 (Cquat, aryl), 143.7 (Cquat, aryl), 139.2 (Cquat, aryl), 138.3 (Cquat, d, 1JCF = 240 Hz, C6F5), 137.7 (Cquat, aryl), 136.3 (Cquat, d, 1JCF = 240 Hz, C6F5), 134.9 (CH, aryl), 132.2 (CH, Bn), 131.9 (Cquat, Bn), 130.8 (CH, aryl), 130.2 (Cquat, aryl), 129.0 (CH, Bn), 126.2 (CH, Bn), 124.0 (Cipso-C6F5), 121.4 (CH, aryl), 119.5 (CH, aryl), 113.3 (CH, aryl), 65.2 (CH2, 1JCH = 137 Hz, Bn), 49.2 (N-Me), 49.1 (NCH2), 35.9 (Cquat, tBu), 35.1 (Cquat, t Bu), 31.7 (CH3, tBu), 31.2 (CH3, tBu). 19F NMR (564 MHz, CD2Cl2, 258 K): δ −133.3 (br, 2F, o-C6F5), −163.2 (t, J = 19 Hz, 1F, p-C6F5), −167.1 (br, 2F, m-C6F5). Typical Procedure for 1-Hexene Oligomerization. In a drybox, compound 4a or 4b (0.00633 mmol) was weighed into a sample vial (equipped with a magnetic stirring bar) along with a stoichiometric amount of the activator. Bromobenzene (0.6 mL) was added via a syringe and the resulting solution stirred for 1 min, after which 250 equiv of 1-hexene (1.58 mmol, 196 μL) was syringed into the catalyst mixture. The reaction was monitored by 1H NMR and was quenched with MeOH/HCl after quantitative conversion to oligo(1-hexenes) had been reached. The resulting oligomer was washed several times with MeOH, dried in vacuo until constant weight, and subsequently analyzed by 1H NMR, SEC, and GC-MS.

Bn), 119.4 (Cquat, aryl), 112.7 (Cquat, aryl), 55.0 (CH2-Bn), 48.4 (CH2, NCH2), 36.1 (Cquat, tBu), 35.0 (Cquat, tBu), 31.9 (CH3, tBu), 30.5 (CH3, tBu). [tBu(OCO)]Hf(CH2Ph)2 (4b). Compound 4b was synthesized following a procedure identical with that used for its Zr analogue 4a but using Hf(Bn)4 (210.8 mg, 0.388 mmol) as the metal precursor. [tBu(OCO)]Hf(CH2Ph)2 was isolated as a pale yellow powder (182.0 mg, 56% overall yield). In addition to 1H and 13C NMR data at room temperature, NMR data for complex 4b were also recorded at 258 K for comparison with the data for the derived cation 5b+. Anal. Calcd: C, 64.54; H, 6.98; N, 3.35. Found: C, 64.90; H, 6.99; N, 3.18. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 7.30 (d, 2H, J = 2 Hz, aryl-H), 6.84 (d, 2H, J = 2 Hz, aryl-H), 6.63 (dd, J = 7 Hz, J = 7 Hz, 4H, Bn), 6.58 (tt, J = 7 Hz, J = 2 Hz, 2H, Bn), 6.53 (dd, J = 7 Hz, J = 2 Hz, 4H, Bn), 3.99 (s, 4H, NCH2), 2.06 (s, 4H, CH2-Bn), 1.72 (s, 18H, tBu), 1.39 (s, 18H, tBu). 1H NMR (500 MHz, CD2Cl2, 258 K): δ 7.26 (d, 2H, J = 2 Hz, aryl-H), 6.83 (d, 2H, J = 2 Hz, aryl-H), 6.67−6.57 (m, 6H, Bn), 6.51 (d, J = 7 Hz, 4H, Bn), 3.99 (s, 4H, NCH2), 1.94 (s, 4H, Bn), 1.69 (s, 18H, tBu), 1.36 (s, 18H, tBu). 13C NMR (75 MHz, CD2Cl2, 25 °C): δ 212.6 (NCN), 148.3 (Cquat, aryl), 140.8 (Cquat, aryl), 138.0 (Cipso-Bn), 137.7 (Cquat, aryl), 131.8 (Cquat, aryl), 129.4 (CH, Bn), 128.4 (CH, Bn), 122.1 (CH, Bn), 119.6 (Cquat, aryl), 112.5 (Cquat, aryl), 63.0 (CH2, Bn), 48.6 (NCH2), 36.0 (Cquat, tBu), 35.0 (Cquat, tBu), 31.9 (CH3, tBu), 30.6 (CH3, tBu). 13C NMR (75 MHz, CD2Cl2, 258 K): δ 211.7 (Cquat, NCN), 147.9 (Cquat, aryl), 140.3 (Cquat, aryl), 137.4 (Cquat, Bz), 137.3 (Cquat, aryl), 131.4 (Cquat, aryl), 129.1 (CH, Bn), 128.2 (CH, Bn), 121.9 (CH, Bn), 119.2 (Cquat, aryl), 112.3 (Cquat, aryl), 61.7 (CH2, Bn), 48.3 (CH2, NCH2), 35.7 (Cquat, t Bu), 34.6 (Cquat, tBu), 31.6 (CH3, tBu), 30.1 (CH3, tBu). Generation of [{tBu(OCO)}Zr(CH2Ph)(NMe2Ph)][B(C6F5)4] ([5a][B(C6F5)4]). In a sample vial, 1 equiv of dimethylanilinium tetrakis(pentafluorophenyl)borate ([HNMe2Ph][B(C6F5)4]; 32.0 mg, 0.039 mmol) was quickly added to a precooled CD2Cl2 (0.6 mL) solution (−35 °C) of the Zr dibenzyl derivative 4a (30.0 mg, 0.039 mmol) with vigorous stirring. The resulting orange solution was transferred to a J. Young NMR tube, and NMR analysis (1D and 2D) was immediately carried out (at room temperature or at −15 °C), showing the clean and quantitative formation of the Zr−NMe2Ph cation 5a+ as a weakly associated B(C6F5)4− salt along with 1 equiv of toluene, consistent with a reaction proceeding via a protonolysis reaction between a Zr− CBn bond of 4a and HNMe2Ph+. The limited stability of the salt species [5a][B(C6F5)4] precluded its isolation in pure form. NMR data for [5a][B(C6F5)4] are as follows. 1H NMR (500 MHz, CD2Cl2, 258 K): δ 8.05 (dd, J = 8 Hz, J = 8 Hz, 2H, N-Ph), 7.79 (t, J = 7 Hz, 1H, N-Ph), 7.62 (d, J = 8 Hz, 2H, N-Ph), 7.32 (d, J = 2 Hz, 2H, aryl), 7.01 (d, 2H, J = 2 Hz, aryl), 6.66 (t, J = 8 Hz, 1H, Bn), 6.39 (dd, J = 8 Hz, J = 8 Hz, 2H, Bn), 6.12 (d, J = 8 Hz, 2H, Bn), 4.33−4.24 (m, 4H, NCH2), 2.93 (s, 6H, N-Me), 2.67 (s, 2H, Bn), 1.48 (s, 18H, tBu), 1.36 (s, 18H, tBu). 13C NMR (125 MHz, CD2Cl2, 258 K): δ 203.0 (NCN), 148.1 (Cquat, d, 1JCF = 240 Hz, C6F5), 146.2 (Cquat, aryl), 143.4 (Cquat, aryl), 139.3 (Cquat, aryl), 138.3 (Cquat, d, 1JCF = 240 Hz, C6F5), 137.3 (Cquat, aryl), 136.3 (Cquat, d, 1JCF = 240 Hz, C6F5), 135.3 (CH, aryl), 132.3 (CH, aryl), 131.8 (Cquat, aryl), 130.2 (CH, aryl), 129.7 (Cquat, aryl), 128.9 (CH, aryl), 126.1 (CH, aryl), 124.0 (Cipso, br, C6F5), 120.8 (CH, aryl), 119.1 (CH, aryl), 113.1 (CH, aryl), 63.0 (CH2-Bn), 48.7 (NMe), 48.5 (NCH2), 35.6 (Cquat, tBu), 34.8 (Cquat, tBu), 31.3 (Me, t Bu), 30.8 (Me, tBu). 19F NMR (564 MHz, CD2Cl2, 258 K): δ −133.3 (br, 2F, o-C6F5), −163.2 (t, J = 19 Hz, 1F, p-C6F5), −167.1 (br, 2F, mC6F5). Generation of [{tBu(OCO)}Hf(CH2Ph)(NMe2Ph)][B(C6F5)4] ([5b][B(C6F5)4]). The salt species [5b][B(C6F5)4] was generated using a procedure identical with that for its Zr analogue [5a][B(C6F5)4]. Thus, the clean and quantitative formation of the Hf−NMe2Ph cation 5b+ was deduced from 1D and 2D NMR data within a few minutes (CD2Cl2, room temperature or −15 °C) upon mixing a stoichiometric amount of the neutral Hf dibenzyl precursor 4b (30.0 mg, 0.036 mmol) and [HNMe2Ph][B(C6F5)4] (28.7 mg, 0.036 mmol). NMR data for [5b][B(C6F5)4] are as follows. 1H NMR (500 MHz, CD2Cl2, 258 K): δ 7.93 (dd, J = 8 Hz, J = 8 Hz, 2H, N-Ph), 7.74 (t, J = 7 Hz, 1H, N-Ph), 7.62 (d, J = 8 Hz, 2H, N-Ph), 7.35 (d, J = 2 Hz, 2H, aryl),

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ASSOCIATED CONTENT

* Supporting Information S

Tables, figures, and CIF files giving characterization data for oligomers, including NMR, SEC, and GC-MS data, the 1H NMR spectrum of [5a][B(C6F5)4], and crystal and refinement data for compounds 2b-THF and 4a,b. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.D.); [email protected] (S.B.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the CNRS and The University of Strasbourg for financial support. Lydia Brelot and Corinne Bailly (Service de Crystallographie, Institut de Chimie de Strasbourg) are gratefully acknowledged for the X-ray analysis of all complexes. C.R. acknowledges the MESR (French Ministry of Research) for a Ph.D. fellowship.

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REFERENCES

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dx.doi.org/10.1021/om400182d | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Organometallics 1993, 12, 1936. (d) Tsukahara, T.; Swenson, D. C.; Jordan, R. F. Organometallics 1997, 16, 3303. (e) Ziniuk, Z.; Goldberg, I.; Kol, M. Inorg. Chem. Commun. 1999, 2, 549. (f) Giesbrecht, G. R.; Whitener, G. D.; Arnold, J. Organometallics 2000, 19, 2809. (g) Wiecko, M.; Girnt, D.; Rastatter, M.; Panda, T. K.; Roesky, P. W. Dalton Trans. 2005, 2147. (17) For a recent analysis of the benzyl ligand coordination modes, see: Rong, Y.; Al-Harbi, A.; Parkin, G. Organometallics 2012, 31, 8208. (18) Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett, R. J. Am. Chem. Soc. 1987, 109, 4113. (19) For selected cationic group 4 complexes featuring metal (η2benzyl) interactions, see also: (a) Bochmann, M.; Lancaster, S. J. Organometallics 1993, 12, 633. (b) Crowther, D. J.; Borkowcky, S. L.; Swenson, D.; Meyer, T. Y.; Jordan, R. F. Organometallics 1993, 12, 2897. (c) Bochmann, M.; Lancaster, S. J. Organometallics 1994, 13, 2235. (d) Pellechia, C.; Immirzi, A.; Pappalardo, D.; Peluso, A. Organometallics 1994, 13, 3773. (e) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg, H. Organometallics 1996, 15, 2672. (f) Bei, X.; Swenson, D. C.; Jordan, R. F. Organometallics 1997, 16, 3282. (g) Bouwkamp, M.; van Leusen, D.; Meetsma, A.; Hessen, B. Organometallics 1998, 17, 3645. (20) (a) Wilson, P. A.; Wright, J. A.; Organesyan, V. S.; Lancaster, S. J.; Bochmann, M. Organometallics 2008, 27, 6371. (b) Hollink, E.; Wei, P.; Stephan, D. W. Organometallics 2004, 23, 1562. (21) For a thorough study on ion aggregation and pairing in solution of group 4 alkyl cation/perfluorophenylborate anion salt species, see: Song, F.; Lancaster, S. J.; Cannon, R. D.; Schormann, M.; Humphrey, S. M.; Zuccaccia, C.; Macchioni, A.; Bochmann, M. Organometallics 2005, 24, 1315. (22) Yang, X.; Stern, C. L.; Marks, T. J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1375. (23) (a) Lian, B.; Beckerle, K.; Spaniol, T. P.; Okuda, J. Eur. J. Inorg. Chem. 2009, 311. (b) Lian, B.; Beckerle, K.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2007, 46, 8507. (c) Makio, H.; Tohi, Y.; Saito, J.; Onda, M.; Fujita, T. Macromol. Rapid Commun. 2003, 24, 894. (d) Tsurugi, H.; Mashima, K. Organometallics 2006, 25, 5210.

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dx.doi.org/10.1021/om400182d | Organometallics XXXX, XXX, XXX−XXX