Chiral Group 4 Cyclopentadienyl Complexes and Their Use in

Jul 17, 2014 - Zoë R. Turner, Jean-Charles Buffet, and Dermot O'Hare*. Chemistry Research .... Nicholas C. BoydeGrant W. SteelmanTimothy P. Hanusa...
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Chiral Group 4 Cyclopentadienyl Complexes and Their Use in Polymerization of Lactide Monomers Zoë R. Turner, Jean-Charles Buffet, and Dermot O’Hare* Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, United Kingdom S Supporting Information *

ABSTRACT: A family of group 4 alkoxide and aryloxide complexes of a chiral cyclopentadienyl-derived (hydro)permethylpentalenyl ligand (C8Me6H; Pn*(H)) have been prepared and fully characterized. Both racemic and enantiopure complexes of all group 4 congeners were prepared with a wide variety of alkoxide and aryloxide ligands. The complexes Pn*(H)Ti(OtBu)3 (1), Pn*(H)Ti(O-2,6-Me-C6H3)3 (2), Pn*(H)Zr(OtBu)3 (3), Pn*(H)Zr(OCH2Ph)3 (4), Pn*(H)Zr(S-OCH{CH3}C6H5)3 (5), Pn*(H)Zr(rac-OCH{CH3}C6H5)3 (6), Pn*(H)Zr(O-2,6-Me-C6H3)3 (7), Pn*(H)Zr(O2,6-iPr-C6H3)3 (8), Pn*(H)ZrCl2(O-2,6-tBu-C6H3) (9), Pn*(H)Hf(O-2,6-Me-C6H3)3 (10), Pn*(H)HfCl(O-2,6-iPr-C6H3)2, (11), and Pn*(H)HfCl2(O-2,6-tBu-C6H3) (12) were prepared by the reaction of Pn*(H)MCl3 complexes with the corresponding potassium alkoxides and aryloxides. Single-crystal X-ray diffraction studies implied that, despite multiple diastereomers being possible for each complex, the diastereomers isolated are limited to configurations in which the methyl group at the chiral center is always oriented anti to the metal center in order to minimize steric hindrance (R,RP and S,SP). The complexes were investigated as initiators for the ring-opening polymerization of L- and rac-lactide in order to ascertain if these mixtures of diastereomers could exert any stereocontrol on the resulting polymerization. Kinetic studies were completed to explore the effects of the metal cation, chiral (hydro)permethylpentalenyl ligand, ancillary ligands, initiator concentration and temperature. Both Pn*(H)Zr(S−OCH{CH3}C6H5)3 and Pn*(H)Zr(rac-OCH{CH3}C6H5)3 demonstrated very high rates of propagation for L- and rac-lactide (1.885 < kobs < 3.442 h−1) at 100 °C. The observed propagation rates using Pn*(H)Zr(racOCH{CH3}C6H5)3 are around 70% faster for L-lactide and rac-lactide in comparison to those using Pn*(H)Zr(SOCH{CH3}C6H5)3. The polymers were characterized by NMR spectroscopy, GPC, and MALDI-ToF mass spectrometry in order to investigate the tacticities and polydispersities of the polymerizations.

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INTRODUCTION

Polylactide (PLA) is a biorenewable, biocompatible, and biodegradable polyester produced by ring-opening polymerization (ROP) of lactide. Polylactide possesses versatile physical properties and has been widely used in biomedical applications such as media for the controlled release of drugs.1−4 The ROP of lactide by single-site catalysts is the most efficient route to PLAs with predictable molecular weight and narrow molecular weight distribution. The past two decades have witnessed the rapid development of initiators for the improved control of polymer stereochemistry, which is one of the critical factors in determining the physical and mechanical properties of a polymeric material.5−8 Various initiators with good stereocontrol during the propagation step have been introduced.9−14 The two stereogenic centers in one lactide molecule results in three distinct configurational isomers: (S,S)-LA (L-LA), (R,R)-LA (D-LA), and (R,S)-LA (meso-LA). The 1:1 mixture of (S,S)-LA and (R,R)-LA is referred to as rac-LA (Figure 1). The use of racemic mixtures of chiral metal complexes for stereoselective transformations have been previously reported;12a,c Arnold et al. described the spontaneous resolution of the © 2014 American Chemical Society

Figure 1. Lactide monomers.

C3-symmetric rac-Y(tBu2P(O)CH2CHtBu)3 and its use to polymerize rac-lactide into separate isotactic chains (Pi = 0.81, 98% conversion, Mw/Mn = 1.23).12c This has important implications for the potential synthesis of stereocomplex polylactide, which is highly crystalline and high melting (Tm up to 230 °C),15 using a single system. Despite group 4 cyclopentadienyl and metallocene complexes being used extensively in a range of catalytic applications such as enantioselective hydrogenation,16 carbonyl olefination,17 alkene isomerization and oligomerization,18 hydroReceived: June 15, 2014 Published: July 17, 2014 3891

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silylation,19 and sterocontrolled olefin polymerization,20 there is a paucity of examples for lactide polymerization.21 Chen and co-workers reported the first example of group 4 metallocenes for the polymerization of L-lactide.22 They found that the neutral Cs-symmetric bis(ester enolate) Ph2C(Cp)(Flu)Zr[OC(OiPr)CMe2]2 produced isotactic PLA with narrow molecular weight distributions (1.05−1.33). C2- and C2v-symmetric zirconocenes proved only marginally active. Pitsikalis and co-workers prepared the half-sandwich titanium complex CpTiCl2(OEt), which was used for the polymerization of L-lactide but proved to be a slow initiator (full conversion after 48 h at 130 °C) without the addition of Lewis acidic CpTiCl3.23a This family has been extended to Cp*TiCl2(OEt) and IndTiCl2(OEt) derivatives. However, none of this family of initiators is well-defined in nature.23b Surprisingly, given the robust nature of cyclopentadienyl ligands, there are relatively few examples of half-sandwich metal complexes bearing chiral cyclopentadienyl ligands which do not contain pendant coordinating groups.24 Of these examples, few have been exploited in catalysis.25 Cramer and co-workers described the use of chiral Rh(I) complexes Rh(CpX*)(C2H4)2 bearing chiral 1,2-substituted cyclopentadienyl ligands.25c In situ oxidation afforded Rh(III) catalysts for the C−H functionalization of benzhydroxamic acid which was high-yielding and enantioselective. Concurrently, Rovis and co-workers reported the biotinylated Rh(III) complexes [Rh(Cp*biotin)Cl2]2, which worked in concert with an artificial metalloenzyme.25d Aware of the small range of reported chiral half-sandwich complexes used in catalysis, we have chosen group 4 alkoxide complexes of a chiral cyclopentadienyl-derived (hydro)permethylpentalenyl ligand (C8Me6H; Pn*(H))26 to investigate the polymerization of lactide. By targeting Pn*(H)ML3 complexes, we can systematically vary the metal center, the stereoelectronic properties of the ancillary ligands (including the introduction of chiral elements), and potentially, the stereoconfiguration of the (hydro)permethylpentalenyl ligand (Figure 2). All of these factors are expected to affect lactide polymerization kinetics and stereocontrol.

Figure 3. Four diastereomers of Pn*(H)ML3 complexes when L does not contain a chiral center.

the effects of ligand chirality and the nature of the metal center on the polymerization activity and stereocontrol.

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RESULTS AND DISCUSSION Synthesis and Characterization of Pn*(H)MCl3. The ligand transfer agent Pn*(H)SnMe3 was prepared by reaction of 1 equiv of Pn*(H)Li with 1 equiv of SnMe3Cl at −78 °C in pentane (Scheme 1). The reaction mixture was warmed to room temperature and stirred for 3 h before removal of the volatiles in vacuo afforded Pn*(H)SnMe3 as an orange oil in high yield (97%). Scheme 1. Synthesis of the Ligand Transfer Agent Pn*(H)SnMe3

There are four possible diastereomers of Pn*(H)SnMe3, and the 1H NMR spectrum shows a 50:50 mixture of isomers. It is most likely that R,R, S,S, R,S, and S,R isomers are all present in the mixture. Ten overlapping singlets define the methyl groups attached to C3, C4, C5, C7, and C8 (CH3-Pn*(H); see Figure 4a for the numbering scheme). There are two sets of doublets and quartets (which display scalar coupling in the 1H−1H COSY spectrum) that are diagnostic of the methyl group (1CH3-Pn*(H)) and hydrogen (Pn*(H)) attached to the sp3hybridized C1 center. A pair of singlets at −0.01 and −0.03 ppm define the SnMe3 protons (2J1H−119Sn = 25.2 Hz, 2J1H−117Sn = 24.2 Hz and 2J1H−119Sn = 25.3 Hz, 2J1H−117Sn = 24.3 Hz, respectively). Group 4 metal trichloride complexes containing the Pn*(H) ligand were prepared by the treatment of 1 equiv of MCl4(thf)x (M = Ti, x = 2; M = Zr, Hf, x = 0) with 1 equiv of Pn*(H)SnMe3 in benzene at 80 °C (Scheme 2). After workup, Pn*(H)TiCl3, [Pn*(H)ZrCl3]2, and [Pn*(H)HfCl3]2 were afforded in 74% (Ti), 87% (Zr), and 78% yields (Hf), respectively. The 1 H NMR spectra of all Pn*(H)MCl3 complexes contained the same diagnostic resonances. For example, in Pn*(H)TiCl3, five singlets between 1.63 and 2.10 ppm define the CH3-Pn*(H) protons and a doublet at 0.93

Figure 2. Three variables in Pn*(H)ML3 complexes for modification.

The monoanionic (hydro)permethylpentalenyl ligand (Pn*(H)) possesses a single chiral center and so exists in two enantiomeric forms, R-Pn*(H) and S-Pn*(H). When the ligand is bound to a metal center, in a piano-stool complex of the form Pn*(H)ML3, planar chirality is a new variable and four diastereomers are possible (R,RP, R,SP, S,RP, and S,SP) when L is not a chiral element (Figure 3). Up to eight diastereomers are possible when L contains a single chiral center. Herein, we describe the preparation and characterization of Pn*(H)ML3 alkoxide complexes of Ti, Zr, and Hf. We report the first examples of the polymerization of L- and rac-lactide with well-defined group 4 half-sandwich complexes and explore 3892

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Scheme 2. Synthesis of Pn*(H)MCl3 (M = Ti, Zr, Hf)a

rac-Pn*(H)SnMe3 to these diasteromeric complexes is not clear. All three complexes have been structurally characterized by single-crystal X-ray diffraction studies. The molecular structures of Pn*(H)TiCl3, [Pn*(H)ZrCl3]2, and [Pn*(H)HfCl3]2 are shown in Figure 4, and selected metrical data are collated in Table 1. Table 1. Selected Bond Lengths (Å) and Angles (deg) of Pn*(H)TiCl3, [Pn*(H)ZrCl3]2, and [Pn*(H)HfCl3]2 M1−centroid M1a−centroid C1−C2 C1−C8 C2−C6 C6−C7 C7−C8 M1−Cl1 M1−Cl3 C2−C1−C8 Cl1−M1−Cl3 Cl1−M1−Cl3a Cl1−M1−centroid

a

Structures shown are representative of the single-crystal X-ray diffraction studies.

ppm (3JHH = 7.4 Hz) and quartet at 3.42 ppm (3JHH = 7.4 Hz) are diagnostic of the 1-CH3-Pn*(H) methyl group and the Pn*(H) proton, respectively. Interestingly, Erker and coworkers reported the only other examples of group 4 complexes supported by a (hydro)pentalenyl ligand;27 on the basis of NMR spectroscopic studies, isolated (1,1-Me,H-3-NMe2C8H4)CpZrCl2, was tentatively assigned to a 2:1 ratio of diastereomers of the relative configurations R,RP and S,RP (Figure 3). In our permethylated system, in all cases, the methyl group attached to C1 is anti to the metal center (as confirmed by single-crystal X-ray diffraction studies). It is reasonable to propose that the steric hindrance associated with the methyl groups in the plane of the (hydro)permethylpentalenyl ligand is such that the configurations where the methyl group attached to the sp3-hybridized carbon are syn to the metal center are disfavored. The stereoconfigurations of these complexes were further resolved by their solid-state structures; crystallization afforded (R,RP)-Pn*(H)TiCl3 exclusively and the dimeric (R,RP,S,SP)- or (S,SP,R,RP)-[Pn*(H)MCl3]2 (M = Zr, Hf) (vide infra). However, the nature of the transformation from

Pn*(H)TiCl3

[Pn*(H)ZrCl3]2

[Pn*(H)HfCl3]2

2.0137(10)

2.1757(19) 2.1811(19) 1.521(5) 1.523(6) 1.408(5) 1.483(6) 1.340(6) 2.4082(11) 2.6027(10) 100.9(3) 141.58(4) 83.06(4) 110.37

2.163(5) 2.155(5) 1.544(18) 1.533(17) 1.396(19) 1.475(17) 1.309(18) 2.376(4) 2.580(3) 99.5(10) 139.06(11) 83.70(11) 110.2

1.497(3) 1.525(3) 1.418(3) 1.471(3) 1.346(3) 2.2427(6) 2.2534(6) 100.64(16) 105.09(3) 115.39

Purple single crystals of (R,RP)-Pn*(H)TiCl3 were grown from a saturated solution of diethyl ether at −30 °C. The molecular structure is monomeric in the solid state; the coordination geometry at the titanium cation is a distortedtetrahedral environment. While there are no comparable (hydro)pentalenyl titanium complexes that have been structurally characterized, the Ti−ring centroid distance (2.0137(10) Å) is comparable to that of other CpRTiCl3 complexes where CpR is a peralkylated cyclopentadienyl ring (2.021 Å in Cp*TiCl3,28a 2.020 Å in CpMe4EtTiCl3).28b The titanium cation is coordinated in a symmetrical way to the (hydro)pentalenyl ligand with a ring slippage parameter of 0.028 Å. The average bond length of the C2−C3−C4−C5−C6 coordinated ring is 1.4230(13) Å, which is consistent with previously reported values. The bond lengths in the second C1−C2−C6−C7−C8 ring are consistent with a localized double bond (C7−C8 = 1.346(3) Å). The chirality about C1 is R, in accordance with the Cahn−Ingold−Prelog rules,29 and the absolute structure R,RP has been determined crystallographically to be that shown

Figure 4. Thermal displacement ellipsoid drawings (50% probability ellipsoids) of (a) Pn*(H)TiCl3, (b) [Pn*(H)ZrCl3]2, and (c) [Pn*(H)HfCl3]2. All hydrogen atoms, apart from those on the chiral centers (C1 or C1a), have been omitted for clarity. 3893

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Titanium tris(aryloxide) and tris(alkoxide) complexes were prepared by the addition of 3 equiv of KOR (where R = tBu, 2,6-Me-C6H3, S-OCH{CH3}C6H5, rac-OCH{CH3}C6H5) at room temperature in benzene or benzene-d6 (Scheme 3). The

in Figure 4. Olive green and pale yellow crystals of [Pn*(H)ZrCl3]2 and [Pn*(H)HfCl3]2, respectively, were grown at room temperature in benzene. The complexes are dimeric in the solid state and are isostructural. The Zr−centroid distances (2.1757(19) and 2.1811(19) Å) are consistent with those in reported Cp R ZrCl 3 complexes (2.176 Å in [Cp*ZrCl3]230a and 2.221 Å in CpPhZrCl3).30b In the (hydro)pentalenyl zirconocene dichloride complexes reported by Erker and co-workers,27 (1,1-Me,H-3-NMe2-Pn)CpZrCl2 is shown to crystallize in the R,RP configuration. There is asymmetric coordination of the C5H3 moiety to the metal center, but this is not observed in [Pn*(H)ZrCl3]2, with the Zr1−C2 (2.510(4) Å), Zr1−C3 (2.510(5) Å), Zr1−C4 (2.452(4) Å), Zr1−C5 (2.471(4) Å), and Zr−C6 (2.505(4) Å) bond lengths all being relatively short. The zirconium centers are not related by symmetry, but the metrical parameters are comparable in both cases. The absolute structure could not be determined crystallographically in this case (and for the Hf analogue), but the crystal packing in the unit cell demonstrates that the methyl group attached the C1(a) is always oriented anti to the metal center; two diastereomeric monomeric units are present in each dimer (R,RP,S,SP and S,SP,R,RP). The changes in M1−centroid distances across all halide complexes are appropriate to the differences in ionic radii (TiIV(4 C.N.) = 0.42 Å, ZrIV(4 C.N.) = 0.59 Å, HfIV(4 C.N.) = 0.58 Å).31 To examine the structure and bonding in the Pn*(H)MCl3 complexes, calculations were performed at the B3LYP level of DFT. The computational results successfully reproduced the established metrical parameters from the single-crystal X-ray diffraction studies (see the Supporting Information). Illustrations of the DFT-computed HOMO and LUMO for Pn*(H)TiCl3 are presented in Figure 5. The HOMO of

Scheme 3. Synthesis of Pn*(H)Ti(OR)3

reactions were instantaneous, and upon workup, Pn*(H)Ti(OtBu)3 (1) and Pn*(H)Ti(O-2,6-Me-C6H3)3 (2) were afforded as dark red and pale yellow solids, respectively, in moderate yields. The benzyl alkoxide derivatives Pn*(H)Ti(SOCH{CH3}C6H5)3 and Pn*(H)Ti(rac-OCH{CH3}C6H5)3 were prepared analogously in C6D6. When less than 3 equiv of KOR was added to Pn*(H)TiCl3, mixtures of mono, bis, and tris (alkoxides) or (aryloxides) were observed and could not be separated. This mixture could be cleanly converted to the Pn*(H)Ti(OR)3 complex by adding a stoichiometric or excess amount of KOR. Zirconium tris(alkoxide) and tris(aryloxide) complexes were made by a salt elimination reaction between [Pn*(H)ZrCl3]2 and 6 equiv of KOR (where OR = OtBu, OCH2C6H5, SOCH{CH3}C6H5, rac-OCH{CH3}C6H5, O-2,6-Me-C6H3, O2,6-iPr-C6H3) at room temperature in C6H6 (Scheme 4) to afford Pn*(H)Zr(OtBu)3, (3), Pn*(H)Zr(OCH2C6H5)3 (4), Pn*(H)Zr(S-OCH{CH3}C6H5)3 (5) Pn*(H)Zr(rac-OCH{CH3}C6H5)3, (6) Pn*(H)Zr(O-2,6-Me-C6H3)3 (7), and Pn*(H)Zr(O-2,6-iPr-C6H3)3 (8), respectively, as pale yellow solids in good yield. When the more sterically demanding KOScheme 4. Synthesis of Pn*(H)Zr(OR)3 and Pn*(H)ZrCl2(OR)

Figure 5. Illustration of the DFT-computed LUMO and HOMO of Pn*(H)TiCl3.

Pn*(H)TiCl3 shows a nonbonding interaction between a metal-based orbital comprising dxy and dx2−y2 character and ligand-based pz2 orbitals. The localized π bond between C7 and C8 is clearly visible and is consistent with the experimental metrical data. The LUMO is principally Ti dxy based. Synthesis and Characterization of Alkoxide and Aryloxide Complexes Pn*(H)MCl3−x(OR)x (x = 1−3). 3894

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2,6-tBu2C6H3 was employed, the mono(alkoxide) Pn*(H)ZrCl2(O-2,6-tBu-C6H3) (9) was the sole organometallic product and was isolated as a pale yellow solid regardless of the reaction stoichiometry (Scheme 4). The mono, bis, and tris(aryloxides) of hafnium, Pn*(H)HfCl3−x(OR)x (x = 1−3), were prepared by reaction of [Pn*(H)HfCl3]2 with 2, 4, or 6 equiv of KOR (where OR = O2,6-Me-C6H3, O-2,6-iPr-C6H3, O-2,6-tBu-C6H3), respectively, at room temperature in benzene (Scheme 5). Upon workup,

With these data in hand, we hoped to observe differing stereocontrol of the polymerization of rac-lactide using the diasteromeric mixtures of (R,RP)- or (S,SP)-Pn*(H)M(OR)3.12c In all cases, the polymerization of L-lactide was also examined as a control reaction in order identify any deleterious epimerization. Polymerization of L- and rac-Lactide. Pseudo-first-order kinetic data of the polymerization of lactide monomers (shown in Figure 6) were recorded with a monomer to initiator ratio of 50 at 100 °C in chloroform-d in order to study the effect of varying the aryloxide substituent.

Scheme 5. Synthesis of Pn*(H)HfCl3−x(OR)x (x = 1−3)

Figure 6. L- and D-lactide monomers.

The results are shown in Table 2 and illustrated in Figure 7. The observed propagation rates (kobs) were determined by analysis of a semilogarithmic plot of ln([LA]0/[LA]t) vs time, where [LA]0 = 0.50 mol/L. Table 2. L-Lactide Polymerization: Variation of the Aryloxide Substituenta

Pn*(H)Hf(O-2,6-Me-C6H3)3 (10), Pn*(H)HfCl(O-2,6-iPr− C6H3)2 (11), and Pn*(H)HfCl2(O-2,6-tBu2C6H3) (12) were afforded as yellow-green solids in moderate to high yield (85− 91%). These reactions proved to be particularly sensitive to the steric demands of the aryloxide ligand. The 1H NMR spectra of the group 4 alkoxide and aryloxide complexes contained the same diagnostic resonances for the (hydro)permethylpentalenyl ligand as the halide complexes; five singlets (∼1.5−2.5 ppm) define the methyl groups in the ligand plane, and a doublet (∼0.8−1.1 ppm) and quartet (∼2.8−3.2 ppm) account for the methyl group and proton attached to the sp3-hybridized carbon, respectively. When the alkoxide substituent contained a chiral center (i.e., racOCH{CH3}C6H5 and S-OCH{CH3}C6H5)3), the spectra revealed more information about the configuration of the complex. For example, the 1H NMR spectrum of Pn*(H)Zr(SOCH{CH3}C6H5)3 indicated two diastereomers that could be distinguished by this technique. For Pn*(H)Zr(rac-OCH{CH3}C6H5)3, four diastereomers could be identified. On the basis of X-ray crystallographic data (see Figure 4 and the Supporting Information), we propose that these diastereomers remain those in which the C9 methyl group is oriented anti to the metal cations, i.e. (R,RP)- or (S,SP)-Pn*(H)M(OR)3, (Figure 3). Single crystals suitable for an X-ray diffraction study have also been obtained for Pn*(H)Ti(OtBu)3 (8) and Pn*(H)Hf(O-2,6-Me-C6H3) (10) (see the Supporting Information).

initiator

T (°C)

2 7 8 9 10 11 12

100 100 100 100 100 100 100

kobs (h−1) 0.113 0.479 0.391 0.043 0.364 0.463 0.086

± ± ± ± ± ± ±

0.014 0.032 0.022 0.003 0.027 0.029 0.020

Mnb

Mw/Mnb

7547

1.74

9176 11680 7669

1.45 1.60 1.53

a Polymerization conditions: 100 °C, [LA]0/[M]0 = 50, [LA]0 = 0.5 M, chloroform-d. bMeasured by GPC, calibrated with PS standards in THF.14

As can be seen in Figure 7, the four complexes bearing the 2,6-Me-C6H3 and 2,6-iPr-C6H3 aromatic groups with zirconium and hafnium metal centers (7, 8, 10, and 11) led to the highest constants of propagation (0.364 < kobs < 0.479 h−1). Both complexes containing the bulky 2,6-tBu-C6H3 substituent (9 and 12) demonstrated the lowest rates of polymerization (with kobs = 0.043 h−1 for Pn*(H)ZrCl2(O-2,6-tBu-C6H3) and kobs = 0.086 h−1 for Pn*(H)HfCl2(O-2,6-tBu-C6H3)). When only the metal is varied (2 (Ti), 7 (Zr), 10 (Hf)), the rates of propagation follow the order [Zr] > [Hf] > [Ti] (kobs = 0.479, 0.391, and 0.113 h−1, respectively). These rates are within the literature range.32 Jones and co-workers reported kobs value of 0.018 and 0.192 h−1 for their hafnium and zirconium salalen complexes under similar polymerization conditions.32c,i Gibson and co-workers reported that Ti(salen)(OiPr)2 demonstrated a kobs value of 0.222 h−1.32f The polydispersities are relatively high (1.45 < Mw/Mn [Hf] > [Ti]. The benzyl alkoxide derived complexes Pn*(H)Zr(S-OCH{CH3}C6H5)3 (5) and Pn*(H)Zr(rac-OCH{CH3}C6H5)3 (6) demonstrated rates of polymerization comparable to those of many of the fastest initiators in the literature. At 60 °C, the polydispersities were low (1.14−1.15), demonstrating controlled polymerization. Complex 6 was faster than 5 for the polymerization of both racand L-lactide (with the rates being similar for both monomers). Disappointingly, we observed no stereocontrol attributed to the (hydro)permethylpentalenyl ligand or the alkoxide substituent, obtaining atatic polymers from polymerization of rac-lactide (no epimerization was observed in the polymerization of Llactide). With regard to the Pn*(H) ligand, the chiral center may be too remote and the substituents at this center too small to exert any effect. Work is in progress to develop other planar chiral cyclopentadienyl complexes of early transition metals in order to achieve rapid, stereocontrolled polymerization of polar monomers.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out using standard Schlenk line or drybox techniques under an atmosphere of 3899

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Organometallics

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heated to 80 °C for 2 h to afford an orange solution. The volatiles were removed in vacuo to yield [Pn*(H)HfCl3]2 as a pale yellow solid. Yield: 0.242 g (71%). Single crystals were grown from a saturated benzene solution at room temperature. 1H NMR (benzene-d6, 23 °C): δ 3.42 (q, 3JHH = 7.4 Hz, Pn*(H)), 1.62, 1.84, 2.01, 2.01, 2.10 (s, 3H each, CH3-Pn*(H)), 0.93 (d, 3H, 3JHH = 7.4 Hz, 1-CH3-Pn*(H)). 13 C{1H} NMR (benzene-d6, 23 °C): δ 150.0, 142.3, 137.2, 123.9, 117.4 (q-Pn*(H)), 45.5 (1-Pn*(H)), 15.9 (1-CH3-Pn*(H)) 12.2, 12.1, 12.0, 11.9, 11.6 (CH3-Pn*(H)). Synthesis of Pn*(H)Ti(OtBu)3 (1). Pn*(H)TiCl3 (0.020 g, 0.0590 mmol) and KOtBu (0.020 g, 0.180 mmol) were combined in C6H6 (0.5 mL) and stirred for 5 min to afford a pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Ti(OtBu)3 as a pale yellow, gluey solid. Crystalline material was precipitated from toluene at −30 °C. Yield: 0.015 g (55%). 1H NMR (benzene-d6, 23 °C): δ 3.35 (q, 3JHH = 7.2 Hz, Pn*(H)), 2.27, 2.24, 2.12, 2.08, 1.84 (s, 3H each, CH3-Pn*(H)), 1.30 (s, 27H, OC(CH3)3), 1.25 (d, 3H, 3JHH = 7.3 Hz, 1-CH3Pn*(H)). 1H NMR (benzene-d6, 23 °C): δ 3.35 (q, 28.6, 121.0, 115.6, 110.5 (q-Pn*(H)), 75.4 (OC(CH3)3), 43.7 (1-Pn*(H)), 33.4 (OC(CH3)3), 16.2 (1-CH3-Pn*(H)), 12.7, 12.6, 12.1, 12.1, 11.4 (CH3-Pn*(H)). Synthesis of Pn*(H)Ti(O-2,6-Me-C6H3)3 (2). Pn*(H)TiCl3 (0.115 g, 0.335 mmol) and KO-2,6-Me-C6H3 (0.161 g, 1.01 mmol) were combined in C6H6 (0.5 mL) and stirred for 5 min to afford a red solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Ti(O-2,6-Me-C6H3)3 as a red-orange solid. Yield: 0.182 g (91%). 1H NMR (benzene-d6, 23 °C): δ 6.90 (d, 6H, 3JHH = 7.2 Hz, 3,5-C6H5), 6.73 (t, 3H, 3JHH = 7.3 Hz, 4-C6H5), 3.59 (q, 1H, 3JHH = 7.7 Hz, Pn*(H)), 2.28 (s, 18H, O-2,6-CH3‑C6H3) 2.21, 2.14, 1.96, 1.58, 1.53 (s, 3H each, CH3-Pn*(H)), 1.08 (d, 3H, 3 JHH = 7.3 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 164.0 (1-C6H3), 147.3, 144.1, 138.8, 130.7 (q-Pn*(H)) 129.1 (Ar), 128.9 (q-Pn*(H)), 127.4 (Ar), 122.5 (q-Pn*(H)), 120.8 (Ar), 116.9 (q-Pn*(H)), 44.4 (1-Pn*(H)), 18.1 (1-CH3-Pn*(H)), 15.3, 13.3, 12.4, 11.9, 11.7, 11.5 (CH3-Pn*(H)). Synthesis of Pn*(H)Zr(OtBu)3 (3). [Pn*(H)ZrCl3]2 (0.028 g, 0.036 mmol) and KOtBu (0.024 g, 0.22 mmol) were combined in C6D6 (0.5 mL) and sonicated for 5 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Zr(OtBu)3 as a pale yellow powder. Yield: 0.022 g (60%). 1H NMR (benzene-d6, 23 °C): δ 3.33 (q, 3JHH = 6.9 Hz, Pn*(H)), 2.25, 2.22, 2.09, 2.05, 1.83 (s, 3H each, CH3Pn*(H)), 1.35 (s, 27H, OC(CH3)3), 1.23 (d, 3H, 3JHH = 6.9 Hz, 1CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 142.8, 137.5, 134.4, 129.3, 123.4, 117.8, 112.9 (q-Pn*(H)), 80.2 (OC(CH3)3), 43.8 (1-Pn*(H)), 33.1 (OC(CH3)3), 15.9 (1-CH3-Pn*(H)), 13.7, 12.8, 12.7, 12.3, 12.2 (CH3-Pn*(H)). Synthesis of Pn*(H)Zr(OCH2C6H5)3 (4). [Pn*(H)ZrCl3]2 (0.100 g, 0.131 mmol) and KOCH2C6H5 (0.115 g, 0.786 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Zr(OCH2C6H5)3 as a pale yellow oily solid at room temperature. Yield: 0.139 g (89%). Anal. Calcd for C35H40O3Zr: C, 70.07; H, 6.72. Found: C, 69.89; H, 6.80. 1H NMR (benzene-d6, 23 °C): δ 7.37−7.03 (overlapping m, 15H, CH2C6H5), 5.10 (s, 6H, CH2C6H5), 3.12 (q, 1H, 3JHH = 7.3 Hz, Pn*(H)), 2.13, 2.09, 1.99, 1.90, 1.68 (s, 3H each, CH3-Pn*(H)), 1.13 (d, 3H, 3JHH = 7.3 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 143.9 (CH2-1-C6H5), 143.0, 135.4, 133.2 (q-Pn*(H)), 128.5 (CH2-2,3,4-C6H5), 127.1 (q-Pn*(H)), 126.9 126.4 (CH2-2,3,4-C6H5), 122.0, 116.5, 111.3 (q-Pn*(H)), 71.7 (CH2C6H5), 43.2 (1-Pn*(H)), 16.2 (1-CH3-Pn*(H)), 12.3, 11.8, 11.5, 11.0, 10.4 (CH3-Pn*(H)). Synthesis of Pn*(H)Zr(S-OCH{CH3}C6H5)3 (5). [Pn*(H)ZrCl3]2 (0.154 g, 0.200 mmol) and S-KOCH{CH3}C6H5 (0.192 g, 1.203 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Zr(SOCH{CH3}C6H5)3 as a pale yellow, oily solid at room temperature. Yield: 0.223 g (87%). Anal. Calcd for C38H46O3Zr: C, 71.09; H, 7.22.

performed using an Enraf-Nonius FR590 KappaCCD diffractometer, utilizing graphite-monochromated Mo Kα X-ray radiation (λ = 0.71073 Å). Intensity data were processed using the DENZO-SMN package36 and corrected for absorption using SORTAV.37 The structures were solved using direct methods (SIR-92)38 or a charge flipping algorithm (SUPERFLIP)39 and refined by full-matrix leastsquares procedures. Polymerization Procedure. The lactide monomer (40 mg) and the complex were introduced into an NMR tube following the desired monomer to initiator ratio. Then 0.57 mL of chloroform-d was added to the compounds, leading to an initial monomer concentration of [LA]0 = 0.5 M. The solution was monitored by 1H NMR spectroscopy. The conversion was determined by comparing the integration of the methine resonance of the polymer to the monomer. Synthesis of Pn*(H)SnMe3. To a slurry of Pn*(H)Li (2.09 g, 10.7 mmol) in pentane (20 mL) at −78 °C was added a solution of SnMe3Cl (2.14 g, 10.7 mmol) in pentane (10 mL). The reaction mixture was warmed to room temperature and stirred for 3 h to afford an orange solution and colorless precipitate of LiCl. This was filtered, and the volatiles were removed in vacuo to afford Pn*(H)SnMe3 (50:50 mixture of diastereomers as judged by 1H NMR spectroscopy) as an orange oil. Yield: 3.56 g (97%). 1H NMR (benzene-d6, 23 °C): δ 2.98 (q, 1H, 3JHH = 7.2 Hz, Pn*(H)), 2.98 (br q, 1H, Pn*(H)), 2.09, 2.05, 2.00 (s, 3H each, CH3-Pn*(H)), 1.95 (overlapping s, 3H each, CH3-Pn*(H)), 1.93, 1.83 (s, 3H each, CH3-Pn*(H)), 1.70 (overlapping s, 3H each, CH3-Pn*(H)), 1.59 (s, 3H, CH3-Pn*(H)), 1.18 (d, 3H, 3JHH = 7.2 Hz, 1-CH3-Pn*(H)), 0.94 (d, 3H, 3JHH = 6.9 Hz, 1CH3-Pn*(H)), −0.01 (s, 9H, 2J1H−119Sn = 25.2 Hz, 2J1H−117Sn = 24.2 Hz, 5-SnMe3-Pn*(H)), −0.03 (s, 9H, 2J1H‑119Sn = 25.3 Hz, 2J1H−117Sn = 24.3 Hz, 5-SnMe3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): 150.2, 147.2, 144.6, 144.4, 129.8, 129.4, 121.3, 119.6 (6× overlapping resonances, q-Pn*(H)), 44.3, 41.8 (1-Pn*(H)), 13.5, 13.4, 12.8, 12.4, 12.3, 12.2, 12.1, 12.0 (2× overlapping resonances, CH3-Pn*(H)), 18.2 17.6 (1-CH3-Pn*(H)), −8.8 (5-SnMe3-Pn*(H), 2J1H‑119Sn = 153 Hz, 2 1 117 J H− Sn = 148 Hz), −9.2 (5-SnMe3-Pn*(H), 2J1H‑119Sn = 157 Hz, 2 1 117 J H− Sn = 150 Hz). Synthesis of Pn*(H)TiCl3. To a slurry of TiCl4(thf)2 (0.408 g, 1.44 mmol) in benzene (2 mL) was added a solution of Pn*(H)SnMe3 (0.505 g, 1.44 mmol) in benzene (2 mL) to afford a dark purple solution. The reaction mixture was heated to 80 °C for 4 h. The volatiles were removed in vacuo to afford Pn*(H)TiCl3 as a purple powder. Yield: 0.363 g (74%). Single crystals suitable for an X-ray diffraction study were grown from a saturated Et2O solution at −35 °C. Anal. Calcd for C14H19Cl3Ti: C, 49.23; H, 5.61. Found: C, 49.18; H, 5.67. 1H NMR (benzene-d6, 23 °C): δ 3.80 (q, 1H, 3JHH = 7.5 Hz, Pn*(H)), 1.57 1.89, 2.02, 2.03, 2.14 (s, 3H each, CH3-Pn*(H)), 0.85 (d, 3H, 3JHH = 7.5 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 153.6, 152.7, 147.4, 140.6, 133.3, 131.3, 127.4 (q-Pn*(H)), 46.8 (1-Pn*(H)), 15.4 (1-CH3-Pn*(H)), 14.5, 14.4, 14.2, 12.2 11.6 (CH3-Pn*(H)). Synthesis of [Pn*(H)ZrCl3]2. To a slurry of ZrCl4 (0.995 g, 4.27 mmol) in benzene (5 mL) was added a solution of Pn*(H)SnMe3 (1.50 g, 4.27 mmol) in benzene (5 mL). The reaction mixture was heated to 80 °C for 72 h to afford a dark green solution. The volatiles were removed in vacuo to yield a green solid. To this was added pentane (15 mL), and the reaction mixture was sonicated for 15 min to afford a fine olive green powder and a pale yellow solution. The reaction mixture was filtered, and the filtrand was dried under reduced pressure to afford [Pn*(H)ZrCl3]2 as an olive green powder. Yield: 1.42 g (87%). Single crystals were grown from a saturated benzene solution at 23 °C. Anal. Calcd for C28H38Cl6Zr2: C, 43.69; H, 4.98. Found: C, 43.57; H, 5.07. 1H NMR (benzene-d6, 23 °C): δ, 3.50 (q, 3 JHH = 7.5 Hz, Pn*(H)), 1.81, 2.01, 2.06 2.17, 2.19 (s, 3H each, CH3Pn*(H)), 0.92 (d, 3H, 3JHH = 7.5 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 151.0, 147.0, 142.1, 133.5, 129.5, 127.4, 120.7 (q-Pn*(H)), 46.0 (1-Pn*(H)), 15.6 (1-CH3-Pn*(H)), 14.2, 13.6, 13.5, 12.3, 12.2 (CH3-Pn*(H)). Synthesis of [Pn*(H)HfCl3]2. To a slurry of HfCl4 (0.164 g, 0.512 mmol) in benzene (2 mL) was added a solution of Pn*(H)SnMe3 (0.180 g, 0.512 mmol) in benzene (2 mL). The reaction mixture was 3900

dx.doi.org/10.1021/om500634a | Organometallics 2014, 33, 3891−3903

Organometallics

Article

Found: C, 70.82; H, 7.30. 1H NMR (benzene-d6, 23 °C): two diastereomers, δ 7.40 (d, 6H, 3JHH = 7.3 Hz, 2,6-C6H5), 7.23 (t, 6H, 3 JHH = 7.3 Hz, 3,5-C6H5), 7.11 (m, 3H, 4-C6H5), 5.30 (q, 3H, 3JHH = 6.1 Hz, CHMe), 3.14, 3.09 (q, 3JHH = 6.9 Hz, Pn*(H)), 2.11, 2.11, 2.08, 2.05, 1.97, 1.97, 1.89, 1.88, 1.70, 1.65 (overlapping s, 3H each, CH3-Pn*(H)), 1.44 (d, 9H, 3JHH = 6.1 Hz, CHMe), 1.12 (d, 3H, 3JHH = 6.9 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): diastereomer 1, δ 148.7, 142.7, 135.5, 133.3 (q-Pn*(H)), 128.6 (3,5C6H5), 126.9 (2,6-C6H5), 125.7 (q-Pn*(H)), 125.7 (4-C6H5), 122.0, 116.4, 111.4 (q-Pn*(H)), 77.0 (CHMe), 43.4 (1-Pn*(H)), 28.4 (CHMe), 16.1 (1-CH3-Pn*(H)), 12.2, 11.8, 11.8, 11.2, 10.6 (CH3Pn*(H)); diastereomer 2, 148.7, 142.7, 135.3, 133.2 (q-Pn*(H)), 128.5 (3,5-C6H5), 126.9 (2,6-C6H5), 125.9 (4-C6H5), 125.5 (q-Pn*(H)), 121.8, 116.4, 111.1 (q-Pn*(H)), 77.0 (CHMe), 43.3 (1-Pn*(H)), 28.4 (CHMe), 16.1 (1-CH3-Pn*(H)), 12.2, 11.8, 11.7, 11.2, 10.6 (CH3Pn*(H)). Synthesis of Pn*(H)Zr(rac-OCH{CH3}C6H5)3 (6). [Pn*(H)ZrCl3]2 (0.159 g, 0.206 mmol) and rac-KOCH{CH3}C6H5 (0.198 g, 1.24 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Zr(racOCH{CH3}C6H5)3 as a pale yellow oily solid at room temperature. Yield: 0.215 g (81%). Anal. Calcd for C38H46O3Zr: C, 71.09; H, 7.22. Found: C, 70.87; H, 7.35. 1H NMR (benzene-d6, 23 °C): mixture of diastereomers, δ 7.48−7.10 (overlapping m, 15H, C6H5), 5.24 (overlapping q, 3H, CHMe), 3.06 (q, 1H, 3JHH = 7.3 Hz, Pn*(H)), 2.12, 2.12, 2.08, 2.07, 1.98, 1.98, 1.90, 1.69, 1.67 (overlapping s, 3H each, CH3-Pn*(H)), 1.50, 1.45, 1.44, 1.41 (overlapping d, 9H, 3JHH = 6.4 Hz, CHMe), 1.12 (d, 3H, 3JHH = 7.3 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): mixture of diastereomers, δ 148.7, 142.7, 135.5, 133.3 (q-Pn*(H)), 128.4 (3,5-C6H5), 127.3 (q-Pn*(H)) 126.9 (2,6-C6H5), 125.6 (4-C6H5), 122.0, 116.4, 111.4 (q-Pn*(H)), 77.0 (CHMe), 43.3 (1-Pn*(H)), 28.4 (CHMe), 16.1 (1-CH3-Pn*(H)), 12.2, 11.8, 11.7, 11.2, 10.6 (CH3-Pn*(H)). Values reported in the 13 C{1H} NMR spectrum are the central values of the multiple overlapping resonances observed. Synthesis of Pn*(H)Zr(O-2,6-Me-C6H3)3 (7). [Pn*(H)ZrCl3]2 (0.152 g, 0.197 mmol) and KO-2,6-Me-C6H3 (0.189 g, 1.18 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Zr(O-2,6-Me-C6H3)3 as a pale yellow powder. Yield: 0.248 g (98%). Anal. Calcd for C38H46O3Zr: C, 71.09; H, 7.22. Found: C, 71.08; H, 7.25. 1H NMR (benzene-d6, 23 °C): δ 6.93 (d, 6H, 3JHH = 7.4 Hz, 3,5-C6H3), 6.75 (t, 3H, 3JHH = 7.3 Hz, 4-C6H3), 3.31 (q, 3JHH = 7.4 Hz, Pn*(H)), 2.25 (s, 18H, O-2,6-CH3-C6H3), 2.20, 2.12, 1.96, 1.69, 1.54 (s, 3H each, CH3Pn*(H)), 1.09 (d, 3H, 3JHH = 7.4 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 159.7 (1-C6H3), 145.3, 139.7, 135.6 (qPn*(H)), 128.9 (3,5-C6H3), 128.0 (q-Pn*(H)), 126.6 (2,6-C6H3), 125.7 (q-Pn*(H)), 120.4 (4-C6H3), 119.0, 113.7 (q-Pn*(H)), 43.9 (1Pn*(H)), 17.9 (O-2,6-CH3‑C6H3), 15.7 (1-CH3-Pn*(H)), 12.5, 11.8, 11.7, 11.5, 11.0 (CH3-Pn*(H)). Synthesis of Pn*(H)Zr(O-2,6-iPr-C6H3)3 (8). [Pn*(H)ZrCl3]2 (0.174 g, 0.225 mmol) and KO-2,6-iPr-C6H3 (0.293 g, 1.35 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Zr(O-2,6-iPr-C6H3)3 as a yellow-green powder. Yield: 0.261 g (74%). Anal. Calcd for C50H70O3Zr: C, 74.11; H, 8.71. Found: C, 73.81; H, 8.62. 1H NMR (benzene-d6, 23 °C): δ 7.11−7.03 (overlapping d, 6H, 3,5-C6H3), 6.96 (app t, 3H, 3JHH = 6.7 Hz, 4-C6H3), 3.52 (overlapping sept, 6H 3JHH = 6.7 Hz, CH(CH3)2), 3.11 (q, 1H, 3JHH = 7.4 Hz, Pn*(H)), 2.17, 2.13, 1.93, 1.85, 1.44 (s, 3H each, CH3-Pn*(H)), 1.32 (d, 12H, 3JHH = 6.7 Hz, CH(CH3)2), 1.27, 1.26 (overlapping d, 12H each, 3JHH = 6.7 Hz, CH(CH3)2), 1.01 (d, 3H, 3JHH = 7.4 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 155.9, 155.8 (1-C6H3), 146.8 (qPn*(H)), 140.6 (q-Pn*(H)), 137.9, 137.7 (2,6-C6H3), 136.4, 127.4 (qPn*(H)), 123.6, 123.5 (3,5-C6H3), 122.1, 122.1 (4-C6H3) 120.5, 116.2 (q-Pn*(H)), 43.3 (1-Pn*(H)), 27.3, 27.2 (CH(CH3)2), 24.9, 24.8, 24.0, 23.9 (CH(CH3)2), 15.6 (1-CH3-Pn*(H)), 12.1, 12.0, 12.0, 11.8,

10.9 (CH3-Pn*(H)). One quaternary resonance accounting for a Pn*(H) carbon was overlapping with the residual protio solvent resonance. Synthesis of Pn*(H)ZrCl2(O-2,6-tBu-C6H3) (9). [Pn*(H)ZrCl3]2 (0.239 g, 0.310 mmol) and KO-2,6-tBu-C6H3 (0.152 g, 0.62 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)ZrCl2(O-2,6-tBuC6H3)2 as a yellow-green powder. Yield: 0.072 g (42%). 1H NMR (benzene-d6, 23 °C): δ 7.13 (d, 2H, 3JHH = 7.7 Hz, 3,5-C6H3), 6.78 (t, 1H, 3JHH = 7.7 Hz, 4-C6H3), 2.96 (q, 1H, 3JHH = 7.4 Hz, Pn*(H)), 2.20, 2.05, 1.98, 1.69 (s, 3H each, CH3-Pn*(H)), 1.41 (br s, 18H, C(CH3)3), 1.32 (s, 3H, CH3-Pn*(H)), 0.86 (d, 6H, 3JHH = 7.4 Hz, CH(CH3)2). 13C{1H} NMR (benzene-d6, 23 °C): δ 161.0 (1-C6H3), 148.2, 145.9, 134.4, 132.3 (q-Pn*(H)), 125.4 (3,5-C6H3), 125.2 (qPn*(H)), 121.4 (4-C6H3), 119.6 (q-Pn*(H)), 44.7 (1-Pn*(H)), 35.6 (C(CH3)3), 32.1 (C(CH3)3), 15.5 (1-CH3-Pn*(H)), 14.0, 12.9, 12.4, 12.4, 11.6 (CH3-Pn*(H)). Two quaternary resonances accounting for the 2,6-C6H3 and a Pn*(H) carbon were overlapping with the residual protio solvent resonance. Synthesis of Pn*(H)Hf(O-2,6-Me-C6H3)3 (10). [Pn*(H)HfCl3]2 (0.194 g, 0.205 mmol) and KO-2,6-Me-C6H3 (0.198 g, 1.23 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Hf(O-2,6-MeC6H3)3 as a pale yellow powder. Yield: 0.255 g (85%). Anal. Calcd for C38H46HfO3: C, 62.58; H, 6.36. Found: C, 62.49; H, 6.38. 1H NMR (benzene-d6, 23 °C): δ 6.93 (d, 6H, 3JHH = 7.4 Hz, 3,5-C6H3), 6.74 (t, 3H, 3JHH = 7.4 Hz, 4-C6H3), 3.27 (q, 1H, 3JHH = 7.4 Hz, Pn*(H)), 2.24 (s, 18H, O-2,6-CH3-C6H3), 2.24, 2.16, 1.99, 1.68, 1.56 (s, 3H each, CH3-Pn*(H)), 1.10 (d, 3H, 3JHH = 7.4 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 159.4 (1-C6H3), 145.1, 138.7, 134.0 (qPn*(H)), 129.0 (3,5-C6H3), 127.8 (q-Pn*(H)), 126.9 (2,6-C6H3), 124.1 (q-Pn*(H)), 120.4 (4-C6H3), 117.6, 112.1 (q-Pn*(H)), 43.9 (1Pn*(H)), 17.8 (O-2,6-CH3-C6H3), 15.5 (1-CH3-Pn*(H)), 12.4, 11.8, 11.6, 11.4, 10.9 (CH3-Pn*(H)). Synthesis of Pn*(H)HfCl(O-2,6-iPr-C6H3)2, (11). [Pn*(H)HfCl3]2 (0.153 g, 0.162 mmol) and KO-2,6-iPr-C6H3 (0.141 g, 0.648 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Hf(O2,6-iPr-C6H3)2Cl as a pale green powder. Yield: 0.214 g (87%). Anal. Calcd for C38H53ClHfO2: C, 60.39; H, 7.07. Found: C, 60.30; H, 7.19. 1 H NMR (benzene-d6, 23 °C): δ 7.10−6.86 (overlapping m, 6H, 3,4,5C6H3), 3.52 (overlapping sept, 4H, CH(CH3)2), 3.11 (q, 1H, 3JHH = 7.4 Hz, Pn*(H)), 2.25, 2.20, 1.92, 1.92, 1.49 (s, 3H each, CH3Pn*(H)), 1.30 (d, 12H, 3JHH = 6.5 Hz, CH(CH3)2), 1.24, 1.23 (overlapping d, 6H each, 3JHH = 6.5 Hz, CH(CH3)2) 1.05 (d, 3H, 3JHH = 7.4 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 155.7, 155.6 (1-C6H3), 146.3, 139.1 (q-Pn*(H)), 137.9, 137.8 (2,6C6H3), 134.7, 127.6, 125.6 (q-Pn*(H)), 123.9, 123.5 (3,5-C6H3), 122.1, 122.0 (4-C6H3), 118.5, 114.5 (q-Pn*(H)), 43.5 (1-Pn*(H)), 27.1, 27.0 ((CH(CH3)2), 24.9, 24.9, 24.1, 24.0 (CH(CH3)2), 15.5 (1CH3-Pn*(H)), 11.9, 11.8, 10.8 (CH3-Pn*(H)). Synthesis of Pn*(H)HfCl2(O-2,6-tBu-C6H3) (12). [Pn*(H)HfCl3]2 (0.162 g, 0.172 mmol) and KO-2,6-tBu-C6H3 (0.084 g, 0.344 mmol) were combined in C6H6 (5 mL) and stirred for 10 min to afford a clear, pale yellow solution and colorless precipitate. Filtration followed by drying of the filtrate in vacuo afforded Pn*(H)Hf(O2,6-tBu-C6H3)Cl2 as a pale green powder. Yield: 0.200 g (91%). 1H NMR (benzene-d6, 23 °C): δ 7.19 (d, 2H, 3JHH = 7.7 Hz, 3,5-C6H3), 6.80 (t, 1H, 3JHH = 7.7 Hz, 4-C6H3), 2.95 (q, 1H, 3JHH = 7.4 Hz, Pn*(H)), 2.29, 2.16, 2.06, 1.71 (s, 3H each, CH3-Pn*(H)), 1.42, 1.44 (br overlapping s, 18H, C(CH3)3), 1.36 (s, 3H, CH3-Pn*(H)), 0.91 (d, 3H, 3JHH = 7.4 Hz, 1-CH3-Pn*(H)). 13C{1H} NMR (benzene-d6, 23 °C): δ 160.7 (1-C6H3), 147.5, 143.3, 133.0, 129.6, 127.6 (q-Pn*(H) or 2,6-C6H3), 125.4 (3,5-C6H3), 123.0 (q-Pn*(H) or 2,6-C6H3), 121.2 (4-C6H3), 117.7 (q-Pn*(H) or 2,6-C6H3), 44.5 (1-Pn*(H)), 35.6 (C(CH3)3), 32.1 (C(CH3)3), 15.5 (1-CH3-Pn*(H)), 13.7 12.6, 12.5, 12.0, 11.4 (CH3-Pn*(H)). One quaternary resonance accounting for 3901

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either the 2,6-C6H3 or a Pn*(H) carbon was overlapping with the residual solvent resonance.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving general experimental details, additional synthetic details, representative NMR spectra, crystallographic data for Pn*(H)TiCl3, [Pn*(H)ZrCl3]2, and [Pn*(H)HfCl3]2, polymerization data, and data for DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*D.O.: tel, +44(0) 1865 272686; e-mail, dermot.ohare@chem. ox.ac.uk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.R.T. and J.-C.B. thank SCG Chemicals of Thailand for funding. Dr. Nick Rees is thanked for help in NMR spectroscopy. Thanks are given to Chemical Crystallography (University of Oxford) for use of the diffractometers and Prof. P. Mountford (University of Oxford) for the use of the GPC.

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REFERENCES

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dx.doi.org/10.1021/om500634a | Organometallics 2014, 33, 3891−3903