Alumina as a Simultaneous Support and Co Catalyst: Cationic

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Alumina as a Simultaneous Support and Co Catalyst: Cationic Hafnium Complex Evidenced by Experimental and DFT Analyses Marco Delgado,† Catherine C. Santini,*,† Françoise Delbecq,*,‡ Raphae¨l Wischert,‡ Boris Le Guennic,‡ Ge´raldine Tosin,† Roger Spitz,† Jean-Marie Basset,† and Philippe Sautet‡ Institut de Chimie de Lyon, UniVersite´ de Lyon, C2P2, UMR 5265 CNRS-ESCPE Lyon, 43 bd du 11 NoVembre 1918, F-69626 Villeurbanne Cedex, France, and Laboratoire de Chimie, UniVersite´ de Lyon, CNRS-ENS Lyon, 46 alleé d’Italie, F-69364 Lyon Cedex 07, France ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: September 10, 2010

Electron-poor transition metal complexes are of high interest in polymerization or oligomerization, but they require the use of a Lewis acid cocatalyst in order to reach the cationic active structure. The structure of the surface complexes obtained by grafting Hf(CH2tBu)4, 1, on γ-alumina has been resolved by a combined experimental (mass balance analysis, labeling, in situ IR, NMR) and theoretical (DFT calculations) study. Thermolysis, oxidation, and hydrogenolysis reactions have unambiguously proved the presence of two kinds of neopentyl-metal bonds: Hf-CH2tBu and Al-CH2tBu. Three coexisting surface complexes have been fully characterized and quantified: a monoaluminoxy [(tAlIVO)Hf(CH2tBu)3], a neutral bis-aluminoxy [(tAlIVO)(AlsO)Hf(CH2tBu)2], and a zwitterionic bis-aluminoxy complex [(tAlIVO)(AlsO)Hf(CH2tBu)2]+[(CH2tBu)Als]- in 40%, 26%, and 34% yield, respectively. In 13C NMR calculations the important effect of spin-orbit coupling has been underlined on the chemical shifts of the carbon atoms directly linked to hafnium. Hence, a large fraction of the grafted complex is in a cationic structure, explaining why this system is active in polymerization (>103 kg of PE/mol of Hf · h · atm) without the need of a cocatalyst, since alumina plays the dual role of solid support and Lewis acid. Introduction Surface organometallic chemistry represents a constructive and rational approach for the preparation of well-defined active sites at solid surfaces, giving the possibility to characterize elementary reaction steps and to develop a fundamental basis for the synthesis of tailormade catalysts. Electron-poor complexes of transition metals of column IV have recently shown high catalytic activity in polymerization or oligomerization reactions, and hence, design of heterogeneous catalysts from these homogeneous complexes is a topic of high interest.1-4 One difficulty is that these complexes are not active in their neutral form but require the presence of a Lewis acid cocatalyst in order to reach the active cationic form.5 The best and most frequently used Lewis cocatalyst, methylaluminoxane (MAO), has a complex and still not completely understood structure.5,6 Constructing a heterogeneous catalyst where the grafted complex7 is already in an active cationic structure, thus avoiding use of the cocatalyst, is hence a topic of general interest.8,9 On a silica surface, grafting reactions of metal alkyl complexes unfortunately only yield neutral surface complexes. This reaction occurs on the surface hydroxyl groups or with siloxane bridges leading, in that second case, to the transfer of an alkyl group to the surface.10-12 For example, grafted tetraalkyl hafnium complexes [i.e., Hf(CH2tBu)4] on silica are inactive in polymerization.13 γ-Alumina, which is ubiquitously used in industry as a catalyst support, has a richer chemistry than silica since it combines a variety of hydroxyl groups with several modes of * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † C2P2. ‡ ENS-Lyon.

coordination and a Lewis acid character with varying strength on aluminum atoms, depending on different low Al coordination from tri- to pentacoordinated. Generally, alkyl complexes of transition metals supported on alumina are more active in catalytic polymerization reactions or alkane metathesis than those supported on silica.14-16 The role of the aluminum Lewis centers for the chemisorptive interaction of the metal-organic complex and more generally the specificity of alumina for this process is still the subject of debate. In some cases the formation of partially or fully cationic surface complexes has been proposed based on NMR spectroscopy10,14,16-27 and on theoretical calculations (density functional theory, DFT).27-30 Consequently, the cationic or neutral character of the surface metal species is a key aspect for its catalytic properties. Advances in software and computational resources now allow modeling extended periodic systems, such as surfaces, and hence provide new tools to investigate the termination of oxide surfaces, in realistic temperatures and hydration levels, and the grafting process of organometallic complexes, thereby giving new insights on catalytic site properties.31-34 The surface alkyl or hydride complexes of zirconium are precursors to several relatively well-defined active heterogeneous Ziegler-Natta catalysts of polymerization13,14,16,35-39 and of depolymerization.38 In contrast to zirconium, data on the characterization and catalytic properties of hafnium surface complexes are very scarce, apart some silica-grafted hafnium complexes.40-42 Nevertheless, in some reports hafnium complexes are described as more active than the zirconium analogues in polymerization of R-olefins,13,43,44 more selective in oligomerization of propene,45,46 and faster in the hydrogenolysis of butane.40 These differences in activity could be related to a better stability of the catalyst from the stronger Hf-C bond [D(Zr-C)

10.1021/jp104999n  2010 American Chemical Society Published on Web 10/14/2010

Alumina as a Simultaneous Support and Co Catalyst

Figure 1. Assignments of OH bands to different hydroxyl groups on the surface of γ-Al2O3-(500°C) by IR spectroscopy.54

) 54 kcal · mol-1, D(Hf-C) ) 58 kcal · mol-1].47 Furthermore, a tendency to undergo β-methyl transfer is reported.44,48 Moreover, as a Hf-O bond is stronger than a Zr-O bond, the risk of metal leaching could be reduced.49 In this paper, we study the reaction of [Hf(CH2tBu)4] (1) with γ-alumina dehydrated at 500 °C through a combined experimental and theoretical approach. We focus on the role of surface hydroxyl groups and Lewis acidic sites for the mechanism of formation and stabilization of neutral and cationic surface alkyl complexes. Their nature and structure have been established by elemental analysis, infrared spectroscopy, 13C solid-state NMR, and density functional theory calculations. We show that while the grafting reaction occurs on a surface OH group, a large fraction of the surface complexes (34%) is in a cationic structure from a transfer of one alkyl ligand toward a nearby Lewis aluminum center, hence explaining the polymerization activity. We therefore clearly establish the dual chemical character of the alumina support. Results and Discussions 1. Analytical Results for the Reaction of [Hf(CH2tBu)4] (1) with γ-Alumina Partially Dehydroxylated at 500 °C. Fumed γ-alumina (Aeroxide AluC) provided by Degussa was used in this study. Before use it was calcined in a flow of dry air at 500 °C for 12 h and then dehydroxylated at 500 °C under dynamic vacuum (10-5 Torr, 15 h). This support was named γ-Al2O3-(500°C). The specific surface area determined by BET experiments is 121 ( 10 m2 g-1, and the density of surface Al-OH groups has been determined by 1H NMR solid-state NMR equal at ca. 2 OH nm-2. The 1H NMR spectrum for γ-Al2O3-(500°C) shows two resonances centered at -0.4 and 1.4 ppm.50 The resolution of these two principal resonances shows several peaks.51 The 27Al solid-state NMR spectra for γ-Al2O3-(500°C) show two peaks, one at 5 ( 5 ppm which is three times the intensity of the one at 66 ( 5 ppm. These peaks represent aluminum atoms in octahedral and tetrahedral coordination, respectively, in agreement with the literature, which states that γ-alumina can more or less be represented by 75% Al atoms in octahedral environments and 25% Al atoms in tetrahedral environments.52,53 The IR spectrum (Figure 1) describes the region of the OH vibrations of the alumina sample used in this work. According to a previous theoretical model,53 the hydroxyl groups (Al-OH) of this spectrum are characterized by different ν(Al-OH)

J. Phys. Chem. C, Vol. 114, No. 43, 2010 18517 stretching modes in the 3800-3600 cm-1 region. The bands at 3785-3800, 3760-3780, and 3730-3735 cm-1 are assigned, respectively, to µ1(HO)-AlIV, µ1(HO)-AlVI, and µ1(HO)-AlV terminal hydroxyl groups. The bands at 3710-3690 and 3590-3650 cm-1 are assigned, respectively, to bridging µ2(HO)(AlVI)2 and µ3(HO)(AlVI)3 hydroxyls groups. The IR spectrum of γ-Al2O3-(500°C) shows five broad ν(Al-OH) bands centered at 3792, 3774, 3727, 3684, and 3631 cm-1. They are assigned to terminal µ1(HO)-AlIV (A), µ1(HO)-AlVI (B), and µ1(HO)-AlV (C) groups and bridging µ2(HO)(AlVI)2 (D) and µ3(HO)(AlVI)3 (E) groups, respectively (Figure 1).54 1.1. In Situ IR Study. The preparation of 1 and Hf(13CH2tBu)4 1* was already reported in the literature.41 The in situ IR spectrum for reaction of an excess of 1 on a γ-Al2O3-(500°C) disk at 25 °C shows a near total consumption of the ν(Al-OH) bands above 3700 cm-1, assigned to AlIV-µ1(OH) and AlVI-µ1(OH), as already observed in the case of [Zr(CH2tBu)4].27 Meanwhile, the bands below 3700 cm-1 are consumed to a lesser extent (Figure 2B and 2C). Combined with the theoretical assignment of the OH bands, this indicates that grafting primarily occurs on terminal OH groups and not on bridge or hollow ones. Moreover, characteristic bands in the 3000-2700 and 1500-1300 cm-1 regions, associated with ν(CH) and δ(CH) vibrations of the neopentyl ligands on Hf, have appeared (Figure 2A). 1.2. Mass Balance Analyses. Reaction of 1 with γAl2O3-(500°C) was performed using the impregnation workup previously reported41 using 400 mg of 1 (866 µmol) and 1.5 g of γ-Al2O3-(500°C) (603 µmol of tAlO-H). After contact of the support at room temperature for 2 h with 1 dissolved in pentane, a white solid was obtained. The evolution of 450 µmol of neopentane, tBuCH3, detected and quantified by gas chromatography, as the only gaseous product, corresponds to 1.6 ( 0.1 tBuCH3 per grafted Hf (Hfg), i.e., 8 C per Hfg. The resulting white surface organometallic species Hf(CH2tBu)4/γAl2O3-(500°C), referred to as 1-γ-Al2O3-(500°C), obtained after washing the excess of 1 and drying under high vacuum at 25 °C, contains 3.2 ( 0.2 wt % of Hf, which corresponds to 180 µmol of Hfg per gram of catalyst. Elemental analysis of 1-γAl2O3-(500°C) indicates the presence of 13 ( 2 C per Hfg (3.2 ( 0.2 wt % Hf, 2.8 ( 0.2 wt % C) leading to a total amount of carbon per Hfg of 21 ( 2 (the value is 20 in the initial complex 1). 1.3. Hydrogenolysis and Hydrolysis Reactions. Hydrolysis of 1-γ-Al2O3-(500°C) at 25 °C produces 1.3 ( 0.2 equivs of tBuCH3 per Hfg, instead of 2.4, which does not complete the mass balance as in the case of the silica surface.41 It is worth noting that hydrogenolysis of 1-γ-Al2O3-(500°C) in batch conditions has to be performed up to 400 °C in order to obtain the correct carbon mass balance (Table 1). At 400 °C, the total amount of evolved methane and ethane corresponds to 9.3 ( 0.2 C/Hfg, and for this step, elemental analysis of this residual solid indicates 5.8 ( 0.2 C per Hfg [% wt C ) 1.1 ( 0.2, % wt Hf ) 3 ( 0.2]. Due to the error on elemental analysis of C traces at the end of the reaction, the carbon mass balance shows a slight discrepancy between the theoretical value: ΣC/Hfg ) 24 ( 2 and the experimental one 20. If the hydrogenolysis reaction is performed in a continuous flow reactor, at 150 °C 1.84 equivs of tBuCH3/Hfg and at 250 °C 0.56 equivs of tBuCH3/Hfg are evolved. These results indicate the presence of two kinds of metal neopentyl bonds on the surface. First, a Hf-CH2tBu bond is hydrogenolyzed at 150 °C as found for 1-SiO2(800°C),42 and second, probably an Al-CH2tBu bond is hydrogenolized above 250 °C, as expected

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Figure 2. IR spectra: (A) (a) γ-Al2O3-(500°C) and (b) sample after sublimation of Hf(CH2tBu)4 at 70 °C followed by vacuum treatment (10-5 Torr) at 30 °C to give 1-γ-Al2O3-(500°C). (B) Zoomed in view of the ν(Al-OH) region [3850-3500 cm-1] of (a) γ-Al2O3-(500°C) and (b) 1-γ-Al2O3-(500°C). (C) Zoomed in view of the ν(Al-OH) region [3850-3500 cm-1] of subtraction of the spectra b - a.

TABLE 1: Analytical Data of the Modified Surface, 1-γ-Al2O3-(500°C), Resulting from Reaction at Room Temperature of [Hf(CH2tBu)4 ], 1, onto γ-Al2O3-(500°C) (≡AlOH) Hf Hf/ nm-2 nm-2 (≡AlOH) ( 0.1 ( 0.1 ( 0.1 2

1

0.5

tBuCH3/ Hfg grafting ( 0.1 1.6 Σ C ) 8

ΣC a wt % fter Hfg wt % C/Hfg grafting ( 0.2 C ( 0.2 ( 2 E.A. ( 2 3.2

2.8

13

21

due to the different bond strength (D(Hf-C) ) 58 kcal · mol-1; D(Al-C) ) 77 kcal · mol-1) (Figure 3).55 1.4. Solid-State NMR Spectroscopy. While no specific feature is observed in the 1H NMR spectrum (an asymmetric broad band centered at 0.9 ppm), the 13C CP-MAS solid-state NMR spectrum of 1-γ-Al2O3-(500°C) shows a very broad resonance centered at around 82 ppm and peaks at 34, 30, and 24

Figure 3. Instantaneous number of Cn evolved per mole of grafted hafnium as a function of time during hydrogenolysis of 1-γAl2O3-(500°C) from 25 to 400 °C in a continuous flow reactor.

ppm (Figure 4a). In the spectrum of the corresponding 13C compound, labeled at the R-position Hf(13CH2tBu)4/Al2O3-(500°C), 1*-γ-Al2O3-(500°C), the resonance centered at 80 ppm increases drastically and the intensity of the peak at 30 ppm is also enhanced. This is consistent with their attribution to methylene groups of 13CH2tBu fragments, as observed in the case of [Zr(13CH2tBu)4]/γ-Al2O3-(500°C).27 The resonances centered at 100, 91, 82, and 73 ppm (Figure 4b) can be attributed to several types of methylene carbon13CH2tBu, probably linked to Hf. The resonance at 30 ppm was assigned to methylene carbon 13 CH2tBu linked to Al by comparison with the 13C CP-MAS solid-state NMR spectrum of γ-Al2O3-(500°C) after reaction with trisneopentylaluminum, Al(CH2tBu)3 [δCH2Al ) 30 ppm] (Figure 5A). Finally, the small peak at 25 ppm is attributed to traces of oxidized [M-O-(CH2C(CH3)3] bonds.41 However, as observed in Figures 4c, 4d, and 5B, the peak at 25 ppm is much higher than the peaks at 74 (AlOCH2) and 82 ppm (Hf-OCH2), respectively (Figures 5B and 4d), which cannot explain the intensity of the peaks observed at the same chemical shift range in spectrum of Figure 4b. 1.5. Reaction of Dry Oxygen. To confirm the presence of two kinds of metal neopentyl bonds on the surface, sample 1-γAl2O3-(500°C) has been treated under dry oxygen. Indeed, reaction of 1-SiO2-(800°C) with dry oxygen at 25 °C leads to a unique species (tSiO)Hf(OCH2tBu)3. Its 13C CP-MAS NMR spectrum has three peaks at 25, 33, and 82 ppm, which can be readily assigned to the methyl groups CH3, the quaternary carbon C(CH)3, and the carbon of the CH2 group bonded to the oxygen, respectively (Figure 4d). In the case of (tSiO)Zr(CH2tBu)3 under oxygen or neopentanol, similar chemical shifts were observed at 25, 30, and 82 ppm.56 On the other hand, γ-Al2O3(500°C) modified with Al(CH2tBu)3 and partially oxidized under dry oxygen gives rise to new resonances at 74 and 25 ppm in the 13C CP-MAS NMR spectrum, assigned to neopentoxy ligands linked to the Al atom (Figure 5B).57


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Figure 4. 13C CP-MAS solid state NMR spectra of (a) 1-γ-Al2O3-(500°C), (b) 1*-γ-Al2O3-(500°C), (c) 1-γ-Al2O3-(500°C) after reaction under dry oxygen at 25 °C, and (d) 1-SiO2-(800°C) after reaction under dry oxygen at 25 °C.

Figure 5. 13C CP-MAS solid state NMR spectrum of the (A) γAl2O3-(500°C) surface after reaction with Al(CH2tBu) and (B) part A + dry oxygen.

When 1-γ-Al2O3-(500°C) was treated with dry oxygen at room temperature, the 13C CP-MAS NMR spectrum of the resulting material displayed five resonances at 80, 72, 33, 30, and 25 ppm (Figure 4c). By comparison with the 13C CP-MAS NMR spectrum of 1-SiO2-(800°C) under dry oxygen (Figure 4d), the peaks at 80, 33, and 25 ppm were assigned to the carbon atoms of the OCH2tBu ligand linked to the Hf atom. Moreover, the resonances at 72 and 30 ppm were unambiguously assigned to alkoxy ligands linked to Al atoms.57 Consequently, the mass balance, the amount of neopentane evolved during the grafting reaction, the NMR spectral data,

and in particular the result of the reaction with oxygen, imply that reaction of Hf(CH2tBu)4 with γ-Al2O3-(500°C) yields several surface complexes in which the CH2tBu ligand is linked to either Hf or Al atoms. In order to support our experimental findings and improve the characterization of the different surface complexes, we performed DFT calculations on 1-γ-Al2O3-(500°C), as already done for Zr(CH2tBu)4/Al2O3-(500°C).27 2. Computional Study of the Reaction of Hf(CH2tBu)4 with γ-Al2O3-(500°C). The (110) surface of γ-Al2O3, which is the majority surface on nanoparticles and is partially hydrated under preparation conditions, has been selected. The model of this partially hydrated γ-alumina surface has been previously developed by some of us.27,53 For our model in the present study the degree of hydration corresponds to three water molecules per unit cell, yielding a hydroxyl density of 8.8 OH nm-2 on that surface. Previous studies have demonstrated that OHµ1-AlIV is the most reactive hydroxyl group on a hydrated surface. The grafting reaction is thus favored on HO-µ1-AlIV over HO-µ2-(AlV,AlV) and H2O-µ1-AlV.30,33,53,54 The optimized bond lengths for Hf(CH2tBu)4 (1) [Hf-C ) 2.20 Å] are in good agreement with EXAFS data for 1 in benzene [Hf-C ) 2.19 Å].41 This distance is also consistent with the corresponding bond distances reported for analogous molecular complexes, such as [Cp*Hf(Me)2[N(Et)C(Me)N(tBu)]] (2.24 and 2.25 Å)58 and [[MesNON]Hf(CH2tBu)2 ] (2.21 and 2.22 Å).59 Since the experimental results strongly suggest formation of several species during grafting of 1 on γ-Al2O3-(500°C), the stability of a selected set of possible grafted products was considered and calculated: a neutral monoaluminoxy (2), a cationic monoaluminoxy (3), a neutral bis-aluminoxy (4), and a cationic bis-aluminoxy (5) surface complex, following our previous results with Zr(CH2tBu)4/γ-Al2O3.30 The first grafting step takes place on µ1(HO)-AlIV, the most reactive hydroxyl group, and leads to formation of the monoaluminoxy surface complex (tAlIVO)Hf(CH2tBu)3, 2, with the evolution of one neopentane molecule (Scheme 1). This reaction is exothermic by 220 kJ/mol. In the optimized structure of 2

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SCHEME 1: Possible Reaction Pathways and Stable Intermediates for the Grafting of Hf(CH2tBu)4 (1) on γ-Al2O3-(500°C)a

a

The reaction energies ∆E are obtained by DFT calculations.

Figure 6. Model structure of the surface monoaluminoxy complex (tAlIVO)Hf(CH2tBu)3 (2). Blue atoms correspond to hafnium, gray atoms to carbon, white atoms to hydrogen, red atoms to oxygen, and pink atoms to aluminum. (A) side view and (B) top view of (tAlIVO)Hf(CH2tBu)3. On the top view, the green circle corresponds to the grafting site and blue circles to remaining OH groups.

(Figure 6, Table 2), the Hf-O and Al-O bond lengths are similar to those obtained by X-ray diffraction studies for the molecular complexes LAlIV(Me)(µ-O)Hf(Me)Cp2 [L ) CH(N)(Ar)(CMe))2, Ar ) 2,6-iPr2C6H3Hf-C] (Hf-O 1.919 and Al-O 1.71 Å).60 The three Hf-C bond distances are very similar to those we found by EXAFS for 1 in benzene (2.19 Å),41 although Hf-C3 is slightly longer (2.24 Å). However, the values of the three O-Hf-CH2 angles, R, β, and γ, are different, with R being significantly smaller (104.6°). This suggests an interaction between this neopentyl group and the surface. We subsequently studied reactions of 2 with adjacent Lewis acidic sites and surface hydroxyl groups. The transfer of one neopentyl ligand onto an adjacent Al Lewis acidic site can generate a cationic complex 3 [(tAlIVO)Hf(CH2tBu)2]+[(CH2tBu)Als]-. Such a structure is found 26 kJ/mol less stable than 2, even if 3 is further stabilized by the oxygen atom of a surface aluminoxane bridge (Table 2). As the reaction leading

to 3 is endoenergetic, this species is probably not formed. It is worth noting that in the case of the methyl model compound M(CH3)4 (M ) Zr27 and Hf [this work]), the partially cationic complex 3 [[(tAlIVO)M(CH3)2][(CH3)Als] is 98 and 103 kJ mol-1, respectively, more stable than the stable neutral monoaluminoxy (tAlIVO)M(CH3)3 complex. In homogeneous complexes the alkyl abstraction chemistry in toluene of compounds of the type Cp2MHfMeR (R ) alkyl) with various electrophiles (E) undergoes methyl abstraction to form [Cp2HfR]+[EMe]-.61-63 It is hence of crucial importance to consider the complete ligand in the simulation and not a CH3 model ligand. Starting from 2, a second elimination of neopentane can occur by reaction with an adjacent AlsOH. This leads to a neutral bisaluminoxy surface species (tAlIVO)(AlsO)Hf(CH2tBu)2 (4). In fact, two possible conformations 4a and 4b have been found for 4 by rotation around the Hf-C1 bond, the neopentyl fragment (CH2tBu) being either vertical or oriented toward the surface.

(Als)2OHfC1 ) 84.2 OAlC1 ) 90.2 OAlC1 ) 111.2 OAlC1 ) 96.3 HOHfC3 ) 88.4

(Als)2OHfC3 ) 157.7 HfO ) 2.24

HfC1 ) 2.26 HfC1 ) 2.37 Hf · · · C1 ) 2.80 Hf · · · C1 ) 5.56 Zr · · · C1 ) 3.04 HfO ) 2.31 HfO ) 2.17 HfO ) 2.19 ZrO ) 2.18

HfO ) 2.41

HfO ) 2.36 HfO ) 1.92 HfO ) 1.92 HfO ) 1.91 HfO ) 2.00 ZrO ) 1.94 -400 -445 -446 -318

The reference for ∆E is the alumina surface and complex 1 in the gas phase. R ) OHfC3; β ) OHfC1; γ ) OHfC2. a

AlC1 ) 2.95 AlC1 ) 2.08 AlC1 ) 2.01 AlC1 ) 2.09

AlC1 ) 2.08

R ) 108.5 R ) 106.7 R ) 93.5 R ) 104.9

β ) 121.2 β ) 91.2

γ ) 113.7

angles (deg)

β ) 119.5 R ) 104.6

HfC3 ) 2.24 HfC3 ) 2.32 HfC3 ) 2.24 HfC3 ) 2.29 HfC3 ) 2.29 HfC3 ) 2.22 ZrC3 ) 2.3 HfC2 ) 2.21 HfC2 ) 2.25 HfC1 ) 2.20

intramolecular distances (Å)

HfO ) 1.96 HfO ) 1.90 HfO ) 1.95 HfO ) 2.04 HfO ) 1.94 HfO ) 1.98 ZrO ) 1.98 -220

2 3 4a 4b 5a 5b Zr+

∆E, kJ mol-1

TABLE 2: Energetic and Geometric DFT Results for the Surface Complexes, (tAlIVO)Hf(CH2tBu)3 (2), [(tAlIVO)Hf(CH2tBu)2]+[(CH2tBu)Alv]- (3), (AlsO)(tAlIVO)Hf(CH2tBu)2 (4a and 4b), [(tAlIVO)(AlsO)(AlsOAls)Hf(CH2tBu)]+[(CH2tBu)Alv]- (5a), [(tAlIVO)(AlsO)(AlsOAls)Hf(CH2tBu)]+[(CH2tBu)AlIII]- (5b), and Zr+ Represents the Cationic Complex [(tAlIVO)(AlsO) (AlsOAls)Zr(CH2tBu)]+[(CH2tBu)Als]- 27 a


J. Phys. Chem. C, Vol. 114, No. 43, 2010 18521 In 4a and 4b the tetracoordinate hafnium atom is linked covalently to a tetrahedral aluminum atom (tAlIV-O-Hf ) 1.95 and 2.04 Å, respectively) and a pentahedral aluminum atom (AlV-O-Hf ) 1.92 Å) (Table 2 and Figures 7 and 8). In 4b, there is an additional longer coordination bond with a surface aluminoxane bridge [Hf-O ) 2.31 Å]. The Hf-C bond lengths are larger than in 2, as generally observed when more donor atoms such as O or N, are coordinated to the hafnium atom.59 In 4a, angle R is larger than in 2 and the corresponding neopentyl ligand is farther from the surface (Table 2 and Figure 7). Formation of 4a is exoenergetic (∆E ) -180 kJ · mol-1), and the additional interactions in 4b stabilize this form by an extra 45 kJ · mol-1. 4a and 4b can further react with adjacent Lewis acidic sites (AlV) to form the zwitterionic bis-aluminoxy surface complex [(tAlIVO)(AlsO)Hf(CH2tBu)]+[(CH2tBu)Als]- (5a). This reaction is exoenergetic from 4a (∆E ) -46 kJ mol-1) and isoenergetic from 4b. In 5a, the hafnium atom is tetracoordinated but is linked to the surface by two short covalent bonds (tAlIV-O-Hf, AlV-O-Hf) of 1.94 and 1.91 Å, respectively, and by a longer dative interaction [AlO-Hf] of 2.17 Å (Figures 9 and 10). No additional interaction is found between the surface and the neopentyl group, as in 2 and 4. The Hf-C bond has the same length as in 4b, 2.29 Å, owing to the presence of three O atoms linked to Hf. In contrast, all Hf-O aluminoxy bond lengths are smaller than in 4, particularly the dative [AlO]-Hf bond which in this case acquires a partial covalent character. The second neopentyl group is linked to the Al atom by a covalent bond (2.08 Å). If the geometries of 4b and 5a are compared, there is a double energy minimum for the C1 ligand, one 4b with a main interaction with Hf (2.37 Å), a weak interaction with Al (2.95 Å), and a second 5a with a formally cationic Hf in interaction with the counteranion (LAlS)- (Hf-C 2.8 Å, Al-C 2.08 Å). In 5a, the Hf · · · C1 distance in the ion pair is 2.8 Å, shorter than in the case of the similar species found for Zr, [(AlsO)2Zr(CH2tBu)]+[(tBuCH2)(Als)], 3.04 Å. The same tendency is observed with the C1AlO angle value, 90.2° for 5a and 96.3° for the Zr analogue. Consequently and conversely to the Zr complex, the methylene C1H2 group in 5a could be considered as having a bridging potentiality between the Hf and the Al atoms, which induces a less cationic character for 5a. The transformation of 4a into either 4b or 5a is thermodynamically favorable. 5a can also be obtained from 4b by breaking the Hf-C1 bond and further rotating the neopentyl group. Although these transformations have not yet been studied from a kinetic point of view, we will see in the following that the presence of both 4 and 5a is strongly suggested by the NMR spectrum. It has been shown in the case of the Zr methyl complexes that the energy of the grafting reaction does not depend much on the aluminum type, the grafting on OH-µ1-AlIV being only favored by 10 kJ · mol-1.27 In the same paper, the 13C NMR peak around 30 ppm has been assigned to a CH2 group linked to a non-hydroxylated tricoordinated AlIII defect. Moreover, in the grafting of ZrCp2CH3, migration of the methyl group on an AlIII defect has also been observed.14 The existence of such defects on γ-alumina has been proved previously.30 Hence, another zwitterionic structure 5b has been built on a surface containing a dehydroxylated aluminum atom, with a cationic bigrafted [Hf(CH2tBu)2]+ complex on two AlIV (giving two AlV) and an anionic [Al-CH2tBu]- on this surface defect AlIII. The grafting energy of this system is ca. -318 kJ · mol-1 relative to the bare surface with the defect. The structure of 5b is described

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Figure 7. Model structure of the surface bis-aluminoxy complex, (AlsO)(tAlIVO)(AlvO)Hf(CH2tBu)2 (4a): blue atoms correspond to hafnium, gray atoms to carbon, white atoms to hydrogen, red atoms to oxygen, and pink atoms to aluminum. (A) Side view and (B) top view of 4a. On the top view, the green circle corresponds to the grafting site and blue circles to remaining OH groups.

Figure 8. Model structure of the surface bisaluminoxy complex, (AlsO)(tAlIVO)Hf(CH2tBu)2 (4b): blue atoms correspond to hafnium, gray atoms to carbon, white atoms to hydrogen, red atoms to oxygen. and pink atoms to aluminum. (A) Side view and (B) top view of 4b. On the top view, the green circle corresponds to the grafting site and blue circles to remaining OH groups.

in Figure 11 and Table 2. The Al-C1 distance in 5b, 2.01 Å, is shorter than in 5a, 2.08 Å, which reflects the stronger Lewis acidity of AlIII. The hafnium atom is linked to four surface oxygen atoms by two covalent bonds and two dative bonds. In comparison, the reaction of Zr(CH2tBu)4 with the γ-Al2O3-(500°C) surface mainly leads to a cationic complex [(AlsO)2Zr(CH2tBu)]+[(tBuCH2)(Als)]- (equivalent to 5). The different reactivity of Zr and Hf complexes in the grafting process could be related to the stronger Hf-C bond compared to Zr-C, which renders Hf-C cleavage more difficult. Moreover, shorter Hf-O-Al (1.94 Å) than Zr-O-Al bonds (1.98 Å) imply that in the Hf complexes the neopentyl ligands are closer to the alumina surface than in Zr complexes. Therefore, the monoaluminoxy complex 2 is present in the case of Hf and not in the case of Zr. Another effect of the different bond strengths is the partial bridging character of the neopentyl ligand between Al and Hf in the case of 5a, in contrast to the Zr analogue. Comparison and Interpretation of Analytical and Computational Data for the Surface Hf Complexes. Combined use of DFT calculations and 13C CP-MAS solid-state NMR

showed that reaction of Zr(CH2tBu)4 with γ-Al2O3-(500°C) yields the cationic bis-aluminoxy complex [(AlsO)2Zr(CH2tBu)]+[(tBuCH2)(Als)]-, δCH2Zr ) 99 ppm).27 In the case of the reaction of 1 with γ-Al2O3-(500°C), combined use of mass balance analysis and experimental NMR data suggests that the surface complexes (tAlIVO)Hf(CH2tBu)3 (2), (tAlIVO)(AlsO)Hf(CH2tBu)2 (4), and [(tAlIVO)(AlsO)(Al-O-Al)]Hf(CH2tBu)]+[tBuCH2AlV]- (5) are present on the surface. To further refine the comparison between theory and experiment, the 13C NMR chemical shifts of the major surface species 2, 4a,b, and 5a,b have been calculated. First, nonrelativistic calculations were performed in the periodic framework (CASTEP) for the free complex 1 (see Computational Methods). The chemical shifts of the quaternary carbons (42-44 ppm) and the carbons of the methyl groups (37-41 ppm) are in the usually expected range. However, the calculated δ13CH2 was found to be 93 ppm, up shielded by 24 ppm compared to the experimental data (δ13CH2Exp ) 117 ppm). To elucidate such discrepancy with experiment, additional tests were done at the molecular level using the Gaussian code (see Computational Methods). The



Figure 9. Model structure of the surface cationic bisaluminoxy complex [(tAlIVO)(AlsO)Hf (CH2tBu)]+[(CH2tBu)Als]- (5a): blue atoms correspond to hafnium, gray atoms to carbon, white atoms to hydrogen, red atoms to oxygen, and pink atoms to aluminum. (A) Side view and (B) top view of 5a. On the top view, the green circles correspond to the grafting site and blue circle to remaining OH groups.

Figure 10. Magnification of the surface cationic bisaluminoxy complex [(tAlIVO)(AlsO)Hf (CH2tBu)]+[(CH2tBu)Als]- (5a).

Figure 11. Model structure of the cationic bis-aluminoxy complex [(tAlvO)(AlsO)Hf (CH2tBu)]+[(CH2tBu)AlIII]- (5b): blue atom corresponds to hafnium, gray atoms to carbon, white atoms to hydrogen, red atoms to oxygen, and pink atoms to aluminum.

calculated chemical shifts are similar to the ones obtained by the periodic calculations. Again, the calculated δ13CH2 for 1 are up shielded (by 28 ppm) compared to the experimental data, whatever the basis set and the DFT functional we chose. Addition of one f function on hafnium did not have any significant effect on the calculated chemical shifts. Stuttgart/ Dresden ECPs have also been compared without any improvement. From these calculations on the free complex, it appears

that the chemical shift δ13C of the CH2 group directly linked to Hf is not correctly reproduced at this level of calculation, in contrast to the case of the Zr complex.27 Since hafnium is a transition metal in the third line in the periodic table, the relativistic effects are likely to be important and have to be treated with great care. If relativistic pseudopotentials are sufficient for describing the structures and energies, they have limitations for treating the inner-shell properties like NMR. In those cases, explicit spin-orbit coupling (SO) has to be introduced.64,65 Effectively, when the spin-orbit coupling is introduced in the NMR calculations through the two-component ZORA approximation (see Computational Methods), the value obtained in 1 for δ 13CH2 agrees well with the experimental one (113 vs 117 ppm). As expected, the chemical shifts of the other carbon atoms, not coordinated to Hf, are not affected. Hence, a good description of the whole NMR spectra of the hafnium complexes requires taking into account the relativistic effects. For the calculations of the 13C NMR chemical shifts of the grafted complexes two difficulties arose: (i) Calculations including spin-orbit coupling can only be performed on a model cluster of the surface since the relativistic effects cannot be taken into account in the periodic calculations and (ii) we observed that the results strongly depend on the size of the used cluster. Moreover, in some cases the calculations did not converge. To circumvent these problems, we combined the two approaches by extracting from the molecular relativistic SO calculations a spin-orbit correction that we applied to the periodically calculated value of the NMR shift of the carbon linked to Hf (24 ppm), while the results for all other carbon atoms of the grafted complexes were not modified (see Table 3). An interesting part of the NMR spectrum is between 70 and 110 ppm (see Figure 4). Hence, we report in Table 3 the δ 13C values for the methylene CH2 carbons of the grafted complexes 2-5 obtained with the periodic approach. A spin-orbit correction has been applied to the CH2 carbon atoms linked to Hf (+ 24 ppm) but not to those linked to an Al atom. For 2, three different δ values are obtained for the three CH2 carbons. Indeed, we have shown before than an interaction exists in 2 between the neopentyl group corresponding to C3 and the surface. δ 13C3 is indeed smaller than the shifts for C1 or C2. However, the calculations are performed at 0 K, and the

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TABLE 3: Chemical Shifts δ13C (in ppm) for the Methylene Group CH2 in the Grafted Complexesa 2 CASTEP corrected shifted

4a

4b

5a

5b

C1

C2

C3

average

C1

C3

average

C1

C3

C1

C3

C1

C3

92 116

86 110

74 98

84 108 100

63 87 79

66 90 82

65 89 81

74 98 90

65 89 81

57 57 49

57 81 73

39 39 31

73 97 89

a The first line corresponds to the calculated values, the second one to the values corrected from spin-orbit effects (+24 ppm), and the third one to a shift of the whole spectrum based on the highest peak.

Figure 12. Deconvolution of the 13C CP-MAS solid-state NMR spectra of 1-γ-Al2O3(500°C) in the range 50-130 ppm.

experimental spectrum is measured at room temperature. That means that in the experimental conditions the small interaction cited above probably no longer exists and there is free rotation. δ13C3 is then calculated as the average of the three values (108 ppm). In 4a, Hf is in a tetrahedral environment and the two neopentyl groups have almost the same chemical shifts (∼89 ppm). The situation is different for 4b in which the neopentyl group corresponding to C1 is directed toward the surface. Hence, two values have to be considered (98 and 89 ppm). An interesting case is the cationic complex 5a. With CASTEP, the same value is obtained for both CH2 groups whatever the atom on which the neopentyl is attached, Hf or Al. After spin-orbit correction, δ13C3 is shifted to a higher value since C3 is linked to Hf and δ13C1 is unmodified. The periodic NMR calculations for 5b give two peaks at 39 and 73 for δ13C1 and δ13C3, respectively, which, after correction, correspond to 39 and 97 ppm. With these considerations in mind, the experimental spectrum can be tentatively assigned. The calculated chemical shifts are found at 108, 98, 89, 81, and 57 ppm. The experimental spectrum can be deconvoluted into four main peaks centered at 100, 91, 82, and 73 ppm (Figure 12). A good agreement with most of the calculated values immediately appears if these values are shifted down by 8 ppm, an acceptable error margin in such NMR calculations. This allows the assignment of the experimental spectrum in the following manner: the peak at 100 ppm is assigned to the monoaluminoxy complex 2. In this complex, rotation around the O-Hf bond is free and its NMR spectrum at room temperature presents a unique peak as does the free complex 1. The peak at 91 ppm corresponds to 4b and 5b and the one at 82 ppm to a mixture of 4a and 4b. Finally, the experimental peak at 73 ppm can be assigned to δ 13C3 of the carbon bound to Hf in the cationic complexes 5a. However, the second peak corresponding to 5a at 49 ppm has no

correspondence in the experimental spectrum, where no peak exists between 40 and 65 ppm. Let us consider now the low part of the experimental spectrum. With the periodic approach, the chemical shifts for the carbons of the methyl groups are all in the range 36-45 ppm (28-37 ppm with the shift of 8 ppm) and correspond to the high peak at 34 ppm in the experimental spectrum (Figure 4). The chemical shifts for the quaternary carbons are in the range 42-45 ppm (34-37 ppm) and are not distinguishable from those of the methyl groups. An important feature is the assignment of the peak at 30 ppm to the [AlIII-CH2tBu]- part of 5b (calculated 31 ppm), in perfect agreement with previous work,27 and with the calculations on a periodic model of the chemical shift for the [O-AlIV-CH2tBu] species on a totally dehydrated surface (31 ppm for a tetrahedral AlIV). Hence, the NMR simulations indicate that one neopentyl ligand departs from the grafted Hf complex and migrates to a neighboring AlIII defect, forming a Al-CH2tBu unit and a tetrahedral Al center. The 13C NMR spectrum is very sensitive to the carbon coordination. It not only proves that the ligand is interacting with a surface Al but also that it is well separated from the Hf metal center. Indeed, in the calculated structure 5a, where the ligand migrates to a surface Al but remains in the proximity of the Hf (2.80 Å), the C chemical shift is calculated at ∼50 ppm after corrections, a value absent from the experimental spectrum. In addition, the comparison with simulations shows that the Hf moiety must be grafted on a AlIII-AlIV pair to give the lowest carbon chemical shift δ 13CH2 (73 ppm) for the ligand remaining on the Hf center. Such a zwitterionic structure with Hf on a AlIII-AlIV pair and a Np ligand on a defect AlIII has been calculated. This gives the highest grafting energy (-555 kJ · mol-1). The corresponding δ13CH2 are, after corrections, 74 and 29 ppm for Np on Hf and Np on Al, respectively, in agreement with experimental values.



TABLE 4: Initial Activity of 1-γ-Al2O3-(500°C) and 1-SiO2(800°C) in Olefin Polymerization

catalyst

initial activity (kg of PE/ (mol of Hf · h · atm)) ethylene

initial activity (kg of PP/ (mol of Hf · h · atm)) propylene

initial activity (kg of PI/ (mol of Hf · h · atm)) isobutylene

1-γ-Al2O3-(500°C) 1-SiO2(800 °C)13

1287 0

1278 0

1411

The cationic nature of the Hf-CH2tBu moiety has also been proved by a Bader’s analysis of the charge density.66 The single ligand Hf-CH2tBu fragment in complex 5b shows a more positive charge than the Hf-(CH2tBu)2 fragment in complex 4a (the charge difference is +0.7). For 5a, where the CH2 group is closer to Hf (2.80 Å), the charge difference is a little smaller (+0.6). These results justify the term “cationic” applied to complexes 5. From a rough estimate of the peak surfaces, it appears that the bigrafted species [either neutral 4 or cationic 5] are in a larger amount than the monografted species 2, in fair agreement with the quantity of alkanes (neopentane and methane/ethane, see section 1.2) evolved during the grafting reaction and hydrogenolysis reaction, allowing determination of the amount of 2, 4, and 5: 40%, 26%, and 34% respectively (see Experimental Section). The tetrahedral aluminum atoms represent only 25% of the total surface aluminum atoms. This favors the separation of the ion pair, which could hence be the origin of the high polymerization activity of Hf alkyl complexes grafted on γ-alumina, as we will see in the next section. Indeed in the case of a homogeneous complex, activation is achieved with a Lewis acid, forming an ion pair. It was shown that the two ions remain coordinated and that the rate-limiting step for the polymerization reaction is the partial displacement of the ion to allow olefin insertion on the cation.67 Hence, the longer the distance between the cation and the counteranion, the easier this displacement and the more active the catalyst. The constraint introduced by the surface results in a better charge separation than in the homogeneous phase. 4. Preliminary Polymerization Tests. All of the above results are in favor of the presence of surface hafnium complexes having a cationic character. As it is admitted that the cationic or neutral character of the surface metal species is a key aspect for its catalytic properties,10-12,14,16 the catalytic activity of 1-γAl2O3-(500°C) was tested in ethylene, propylene, and isobutylene polymerization in the absence of any cocatalyst. The results are summarized in Table 4. As expected, the presence of the ionic character of 1-γAl2O3(500°C) induces a quite significant polymerization catalytic activity (>103 kg of PE/(mol of Hf · h · atm) in the absence of any cocatalyst,67 contrary to the inactive silica surface neopentyl-hafnium complexes, 1-SiO2(800°C).13 It is worth noting that the activity is similar to homogeneous analogues needing the presence of a cocatalyst (MAO).6 These very promising results confirm the presence of hafnium cationic surface complexes and the dual role of alumina as support and cocatalyst. Conclusion With the aim of systematically investigating the catalytic activity of group IV metal complexes supported on oxides, we tackled the study of the tetraneopentyl hafnium complex. Previous data showed that this complex grafted on silica is

inactive in ethylene polymerization, in contrast with our present results that prove that the same complex grafted on γ-alumina is very active. Combination of many experimental techniques (mass balance analysis, in situ IR, 13C solid-state NMR, hydrogenolysis, and oxidation) and DFT calculations evidenced that reaction of Hf(CH2tBu)4 (1) with the γ-Al2O3-(500°C) surface affords several surface complexes, mainly the monoaluminoxy (tAlIVO)Hf(CH2tBu)3 (2), bis-aluminoxy neutral (tAlIVO)(AlVO)Hf(CH2tBu)2 (4), or cationic [(tAlIVO)(AlVO)(Al-O-Al)]Hf(CH2tBu)]+[(CH2tBu)AlV]- (5) in relative amounts of 40%, 26%, and 34%, respectively. The existence of cationic species explains the observed high activity in polymerization of this catalyst. Hence γ-alumina plays the dual role of grafting support and Lewis cocatalyst, avoiding the use of an external cocatalyst. Experimental Section Computational Methods and Systems. The calculations were performed in the framework of density functional theory (DFT) using a periodic description of the system as implemented in the VASP code.68,69 The generalized gradient approximation was used in the formulation of Perdew and Wang PW91.70 Atomic cores were described with the projected augmented wave method (PAW), which is equivalent to an all-electron frozen core approach.71,72 The one-electron wave functions are developed in a basis set of plane waves. With the selected PAW potentials, a cutoff energy of 275 eV is adequate. For the bulk model of alumina the calculations were done at the Γ point of the Brillouin zone.73 With these parameters a volume of 380 Å3 has been obtained in agreement with the literature.74 The trihydrated (110) surface of γ-Al2O3, corresponding to γ-Al2O3-(500°C), was represented by a four-layer slab. Since we worked with the bulky neopentyl ligands, we had to use a large unit cell with lattice parameters of 16.19 × 16.83 × 23 Å3. This corresponds to a vacuum zone of 17.8 Å. Due to the large size of the surface unit cell, a Γ-point approach is sufficient. During the optimizations, the two lowest layers are kept fixed in the positions of the bulk and the two uppermost layers are allowed to relax together with the grafted complex. For the 13C chemical shift calculations we used two different approaches, i.e., molecular and periodic. First, molecular calculations were performed with the GIAO method75 implemented in the Gaussian03 code76 with the hybrid functional B3LYP.77-80 The IGLO-II basis set was used for carbon and hydrogen atoms.81 For other atoms (Al, O, Hf), the Hay and Wadt effective core potentials82-84 were used with the adapted LANL2DZ basis set. Test calculations were also performed with the B3PW91 functional and the 6-311+G** basis set for carbon and hydrogen and then by adding a f polarization function to Hf (ζ ) 0.784).85 In a second step, relativistic calculations including spin-orbit (SO) coupling have been performed using the ADF program package.86,87 It incorporates a modified version88 of the code developed by Wolff et al.89 for the twocomponent relativistic computation of nuclear shielding constants, based on the zeroth-order regular approximation (ZORA) Hamiltonian.90,91 These relativistic calculations are done with the BP86 functional and an all-electron Slater-type TZP basis set for all atoms. For the grafted complexes, these calculations were done on selected clusters extracted from the periodic slabs and containing the first and second neighbors of the atoms involved in the grafting. The dangling bonds on the oxygen atoms were replaced by H or H2O in order to ensure neutrality. Finally, periodic calculations with the method implemented in CASTEP92,93 have also been done with the PBE functional and a cutoff of 489 eV, the geometries of the systems being those

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obtained with VASP. The NMR calculations are performed using the gauge including projector augmented wave approach (GIPAW).93 For the free complex, the calculations are done in a 16 × 16 × 23 Å3 box. In each case, the reference for calculating the chemical shifts is TMS (tetramethyl silane), for which the shielding constants of C and H atoms are calculated with the corresponding method (δA ) σTMS - σA). A charge analysis has been done following Bader’s method,66 with the code developed in ref 94 and compatible with VASP. General Procedure. All experiments were carried out using standard air-free methodology in an argon-filled Vacuum Atmospheres glovebox on a Schlenk line or in a Schlenk-type apparatus interfaced to a high-vacuum line (10-5 Torr). Solvents were purified and dried according to standard procedures. tBuCH2MgCl was prepared from tBuCH2Cl (SAFC 99%) and Mg turnings (99%, Lancaster). The synthesis of Hf(CH2tBu)4 and Hf(13CH2tBu)4 was realized as already reported.41 HfCl4 (Cezus, 270 ppm of Zr) was used as received. Elemental analysis was performed at the CNRS Central Analysis Service of Solaize (Hf) and at the University of Bourgogne (C, H). Gas-phase analyses were performed on a Hewlett-Packard 5890 series II gas chromatograph equipped with a flame ionization detector and KCl/Al2O3 on a fused silica column (50 m × 0.32 mm). Infrared spectra were recorded on an Nicolet 5700 FT-IR using an infrared cell equipped with CaF2 windows, allowing in situ studies. Typically 16 scans were accumulated for each spectrum (resolution, 4 cm-1). BET experiments were performed on Micromeritics ASAP 2020. The nitrogen adsorption and desorption isotherms were measured at 77 K using Micromeritics automated gas adsorption analyzer. Before the adsorption measurements, the samples were degassed under vacuum at 200 °C for 12 h. The surface area was calculated by Brunauer-Emmett-Telleller (BET) mode. All solid-state NMR spectra were recorded under MAS (υR ) 10 kHz) on a Bru¨ker Avance 500 spectrometer equipped with a standard 4-mm double-bearing probe head and operating at 500.13 and 125.73 MHz for 1H and 13C, respectively. The freshly prepared samples were immediately introduced in the 4-mm zirconia rotor in a glovebox and tightly closed. Compressed air was used for both bearing and driving the rotors. Chemical shifts are reported in ppm downfield from SiMe4 ((0.1 and 1 ppm for 1H and 13C NMR spectra, respectively). The typical crosspolarization sequence was used for the 13C CP MAS NMR spectra: 90° proton pulse, cross-polarization step to carbon spins, and detection of the carbon magnetization under proton decoupling TPPM-15.95 For the CP step, a ramp radio frequency (rf) field centered at νCP ) 60 kHz was applied on protons, while the carbon rf field was matched to obtain optimal signal. The contact time for CP was set to 5 ms. An exponential line broadening of 80 Hz was applied before Fourier transform. All other details are given in the captions to the figures. Preparation of Partially Dehydroxylated Alumina at 500 °C, γ-Al2O3. γ-Al2O3 from Degussa C Aerosil (100 m2/g) was calcined at 515 °C under N2/O2 flow overnight and then partially dehydroxylated at 500 °C under a high vacuum (10-5 Torr) for 15 h to give a white solid. γ-Al2O3-(500°C) from Degussa C Aerosil has a specific surface area, calculated by BET experiment, of 121 ( 10 m2 g-1 and containing ca. 2.0 OH/nm2 (0.33 mmol/ g). 1H MAS NMR: 0.4 [µ2(OH),µ3(OH)] and 1.4 ppm [µ1(OH)]. 27 Al CP MAS NMR: δAl ) 4 (3 Al, Oh) and 62 ppm (1 Al, Td). Grafting of 1 for IR Spectroscopy. γ-Alumina (70 ( 2 mg) was pressed into a 18 mm self-supporting disk, adjusted in a

Delgado et al. sample holder, and introduced into a glass reactor equipped with CaF2 windows. The supports were calcined under air at 510 °C and partially dehydroxylated at 500 °C. The complex [Hf(CH2tBu)4] was sublimed at 70 °C under dynamic vacuum (10-5 Torr) onto the alumina disk. The solid was homogenolyzed at 50 °C for 2 h, and then the excess complex was desorbed off at 70 °C and recondensed into the break-seal equipment cooled by liquid nitrogen, which was then sealed off using a torch. An infrared spectrum was recorded at every relevant step. Preparation of Hf(CH2tBu)4/γ-Al2O3-(500°C) by Impregnation of 1 on γ-Al2O3-(500°C). A mixture of [Hf(CH2tBu)4], 1 (400 mg, 865 µmol, 1equiv), and γ-Al2O3-500 (1.5 g, 603 µmol OH, 0.7equiv) in pentane (20 mL) was stirred at 25 °C for 2 h. After filtration, the solid was washed three times with pentane, and all volatile compounds were condensed into another reactor (of known volume) so as to quantify neopentane evolved during the grafting. The resulting white powder was dried under vacuum (10-5 Torr) to yield 1.53 g of Hf(CH2tBu)4/γ-Al2O3-(500°C). Gas analyses by chromatography indicate formation of 450 ( 30 µmol of neopentane during the grafting (1.6 ( 0.1 CH3tBu/Hf, 0.7 ( 0.1 CH3tBu/(AlOH), 0.5 ( 0.1 Hf/(AlOH)). Elemental analysis of Hf(CH2tBu)4/Al2O3-(500°C): Hf 3.2 wt %; C 2.8 wt % (13 ( 2 C/Hf). Solid-state MAS 1H NMR 500 MHz: δ 0.9 ppm. CP/MAS 13C NMR: δ 80, 34, 30, and 24 ppm. Continuous Flow Hydrogenolysis of 1-γ-Al2O3-(500°C) in a Tubular Reactor. 1-γ-Al2O3-(500°C) (ca. 300 mg) was loaded under argon in a continuous tubular reactor. The sample was flushed with hydrogen (flow rate of 3 mL/min) at room temperature and then heated at a rate of 50 °C · h-1 from room temperature to 400 °C. The increase of temperature from room temperature to 400 °C is done in 450 min; then it remains stable until 1100 min. The temperature was carefully recorded as a function of time, and the composition of evolved gas was determined online by gaseous chromatography. Calculation of Relative Quantities of the Species 2, 4, and 5 Resulting from the Grafting of Hf(CH2tBu)4 on γ-Al2O3-(500°C). Grafting of Hf(CH2tBu)4 on γ-Al2O3-(500°C) releases 1.6 tBuCH3 in the gas phase corresponding to 40% of 2 and 60% of (4 + 5). During the hydrogenolysis reaction at 150 °C, 1.84 neopentane (tBuCH3) are evolved corresponding to cleavage of all Hf-CH2tBu bonds in surface complexes 2, 4, and 5. 2 afforded three tBuCH3/Hf, 4 two tBuCH3/Hf, and 5 one tBuCH3/Hf. The hydrogenolysis reaction above 250 °C releases the remaining 0.56 tBuCH3, which can be attributed to hydrogenolysis of tBuCH2-Als bond. This value corresponds to the quantity of 5.

[(≡AlIVO)Hf(CH2tBu)3] + [(≡AlIVO)(AlsO)Hf(CH2tBu)2] + [(≡AlIVO)Hf(CH2tBu)2]+[(CH2tBu)Als] ) 2.4tBuCH3 If R is the quantity of 4 and β that of 5, we can calculate R and β from the equation

0.4 × 3 + 0.6 × 2(R + β) ) 2.4 which leads to (R + β) ) 1 As β is equal to 0.56, R is found equal to 0.44. Knowing R and β, we can determine starting from the equation that 2 is present at 40%, 4 at 26%, and 5 at 34%. Preparation of Hf(13CH2tBu)4/γ-Al2O3-(500°C) by Impregnation of 1* on Al2O3-(500°C). A mixture of [Hf(13CH2tBu)4], 1 (147 mg, 320 µmol, 1.2 equiv), and γ-Al2O3-(500°C) (0.4 g, 161 µmol

Alumina as a Simultaneous Support and Co Catalyst OH) in pentane (10 mL) was stirred at 25 °C for 2 h. After filtration, the solid was washed three times with pentane and all volatile compounds were condensed into another reactor (of known volume) so as to quantify neopentane evolved during the grafting. The resulting white powder was dried under vacuum (10-5 Torr) to yield 0.4 g of Hf(13CH2tBu)4/γAl2O3-(500°C). Gas analyses by chromatography indicate formation of 115 ( 10 µmol of neopentane during the grafting (2.2 ( 0.1 CH3tBu/Hf, 0.7 ( 0.1 CH3tBu/(AlOH), 0.3 ( 0.05 Hf/(AlOH)). Elemental analysis of Hf(13CH2tBu)4/Al2O3-(500°C): Hf 2.3 wt %. Solid-state MAS 1H NMR 300 MHz: δ 0.8 ppm. CP/MAS 13C NMR: δ 100, 90, 82, 74, 34, 30, and 24 ppm. Gas-Phase Stopped Flow Polymerization.96 The reactor cell where the polymerization reaction occurs consists of a metal chamber closed with a frittered metal cartridge. In a glovebox, the reactor is filled with a 30-35 mg of supported catalyst (2.7-3.3 wt % of Hf) diluted in 700-750 mg of alumina. It is then plunged into a water bath heated at 70 °C to ensure that the inlet gas of the reactor is at the same temperature as the catalytic medium before starting the polymerization. Thermocouples at the inlet and outlet of the reactor allow us to record the temperature rise of the gas phase as it flows through the bed. Temperature measurements are received each 0.5 s by a data acquisition unit (Hewlet Packard 34970A) and are analyzed in a computer using an Agilent Bench Link Data loger program. After purging the feed lines, the reactor was attached to the system and the thermocouples connected to the data acquisition unit. Argon was passed through the feed lines until the reactor temperature remains constant and equal to the temperature of the heating bath. Then, the argon feed valve was closed, the metering valve on the ethylene feed line was set to the desired level, the solenoid valves on the feed line were set at the desired level, and the pressure of the ethylene feed was also set. The solenoid is programmed to open for a given time. Finally, the reactor is quenched by carbon dioxide, stopping the reaction. The activity is determined from the weight of the obtained polymer during a given time from the following equation,

activity )

mass of polymer 1 1 t nHF P

where t ) time in h, nHf ) moles of Hf, P ) monomer pressure in atm, and mass of polymer ) polymer formed in a given time (kg). Acknowledgment. The authors thank PSMN (Poˆle Scientifique de Mode´lisation Nume´rique) at ENS Lyon for CPU time, Dr. Se´bastien Norsic for the flow reactor experiments, and Anne Baudouin for solid-state NMR experiments. References and Notes (1) Takeuchi, D. Dalton Trans. 2010, 39, 311–328. (2) Lamberti, M.; Mazzeo, M.; Pellecchia, C. Dalton Trans. 2009, 8831–8837. (3) Kaminsky, W.; Funck, A.; Haehnsen, H. Dalton Trans. 2009, 8803– 8810. (4) Golisz, S. R.; Bercaw, J. E. Macromolecules 2009, 42, 8751–8762. (5) Severn, J. R.; Chadwick, J. C. Tailor-Made Polymers Via Immobilization of Alpha-Olefin Polymerization Catalysts; Wiley-VCH: Weinheim, 2008. (6) Scheirs, J.; Kaminsky, W. Metallocene Based Polyolefins; Wiley & Sons: Chichester, 2000. (7) Hlatky, G. G. Chem. ReV. 2000, 100, 1347–1376. (b) Fink, G.; Steinmetz, B.; Zechlin, J.; Przybyla, C.; Tesche, B. Chem. ReV. 2000, 100, 1377–1390. (c) Hlatky, G. G.; Upton, D. J.; Turner, H. W. PCT Int. WO 91/09882, 1991, Exxon Chemical Co. (d) Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840–842. (e) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1992, 434, C1. (f) Hlatky, G. G.; Eckman, R. R.;

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