Expanding the Origin of Stereocontrol in Propene Polymerization

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Expanding the Origin of Stereocontrol in Propene Polymerization Catalysis Claudio De Rosa, Rocco Di Girolamo, and Giovanni Talarico* Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Via Cintia, 80126 Napoli, Italy S Supporting Information *

ABSTRACT: Originally developed on heterogeneous Ziegler−Natta (ZN) catalysts, the model of “chiral growing chain conformation” developed by Corradini to explain the origin of stereocontrol in propene polymerization was extended to all stereoselective polymerization catalysts. The idea that the chiral recognition is performed by the site-chirality through the conformation of growing chain represents the “marker” of ZN toward, for example, asymmetric catalysis in which a chiral host molecule recognizes directly two enantiomeric guest molecules. In this paper, by using DFT calculations, we show that the origin of the stereocontrol for the new generation of Hf(IV)pyridylamido-based catalysts is somewhat different and more similar to the asymmetric catalysis. KEYWORDS: propene polymerization catalysis, stereoselective catalysis, metallocene and postmetallocene systems, Ziegler−Natta catalysts, propene stereocontrol

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combination with a second catalyst, performing novel olefin block copolymers via “chain shuttling”.10−12 The industrial relevance coupled chemical challenges because the orthometalation of the aryl substituent bound to the pyridine increases the number of Hf−C σ-bonds suitable for olefin insertion (a M−CAlkyl and a M−CAryl bonds, respectively, see systems 2−4 in Chart 1, top). Computational methods based on density functional theory (DFT) suggested that the first monomer insertion might occurs at M−CAryl bond, thus modifying the ligand chemical structure in situ13 (see the catalytic species II−IV after activation of cocatalysts in Chart 1, bottom).14 In propene polymerization, different active sites may be formed if insertion into the Hf-CAryl bond is not fully regio- or enantioselective15 (see the formation of a first chiral center *a on systems II−IV) and even more if the ligand precursor shows additional stereogenic center (see *b for system 4 in Chart 1). In this Letter, by using DFT calculations (see Supporting Information for computational details), we will show that this family of catalysts saves additional surprises by ruling out the mechanism of stereocontrol based on growing chain orientation and showing unprecedented until now “chiral recognition”. We first analyzed system 1a reported by Coates et al.16 because after methyl abstraction, the cationic active specie Ia forms, which is a well-defined model for the monoinserted active species II−IV. The more stable transition states (TSs) for primary (or 1,2) propene insertion at each site are reported

he Ziegler−Natta catalysis (ZN) combines a simple catalytic cycle, insertion of a monomer molecule into a M−C σ bond (where M is a coordinatively unsaturated transition metal center) with an amazing propene enantioselectivity. The catalytic species select one of the two enantiofaces of propene in the favored regiochemistry with an isotacticity (usually measured as % of mmmm pentad in the polypropylene 13 C NMR spectra) that might reach ≥99%.1 The mechanism to explain the origin of stereocontrol was proposed in the 80s by Corradini and co-workers using molecular mechanics calculations on a model of heterogeneous active site.2 Since then, any polypropylene microstructure (isotactic, syndiotactic, atactic or hemi-isotactic via primary or secondary propagation) reported later on by the discovery of ansa-metallocenes3 or postmetallocenes4 was easily accommodated within the framework of a such model based on the “chiral growing chain orientation” mechanism of stereocontrol. Although an excellent review can be found in literature,5 for the sake of readability, the Corradini’s model applied on a prototypical ansa-metallocene is reported in Scheme S1 of Supporting Information. The “intuition” that the chiral recognition is performed by the site-chirality through the conformation of growing chain represented a “marker” of ZN toward, for example, asymmetric catalysis in which a chiral host molecule recognize directly two enantiomeric guest molecules.6 Discovered by high-throughput screening (HTS) within Dow/Symyx collaboration,7−9 the Hf(IV)-pyridylamido-based olefin polymerization catalysts (see Chart 1) have emerged immediately because they are producing highly isotactic polypropylenes with high molecular mass with a solution process technology at temperature >100 °C. Furthermore, they undergo reversible trans-alkylation in © XXXX American Chemical Society

Received: March 24, 2016 Revised: May 4, 2016

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the site showing the growing polymer chain sp3 carbon atoms (Figure 1 A) and the olefin sp2 carbon atom (Figure 1 B), trans to the N of the pryridine fragment, respectively. In Table 1, we report the energetic difference (in parentheses the calculated free energies) of the two lowest TSs as ΔE(ΔG)site1/site2. Surprisingly, we found a remarkable preference (more than 6 kcal/mol) for the insertion at site 1 (see first column of Table 1). This implies that the regular chain migration mechanism17 (switching of monomer and growing polymer chain at each insertion step) is almost completely modified by the so-called site epimerization or alternatively the “chain skipping” due to the energetic difference the M center can assume from the pyramidal square-based geometry to the distorted trigonal bipyramidal (see Figure 1A and B, respectively). However, a site epimerization mechanism does not ensure per se a stereoselective process. A more detailed analysis of the propene insertion mode at site 1 was revealing. In Figure 2, we

Chart 1. Catalytic Precursors of This Study (Top) and the Corresponding Cationic Active Species (Bottom) after Cocatalyst Activationa

a

With L are indicated the two vacant sites occupied by a propene molecule and iso-butyl group to simulate the propagating species. With *a is reported the stereogenic center formed by insertion of C3H6 in the Hf−CAryl bond, and with *b is marked the additional stereogenic center bridging the amide and pyridine fragment.

in Figure 1. Due to the C1-symmetry, the active center shows two diasterotopic active sites, and we define site 1 and site 2 as

Figure 2. Preferred 1,2 propene insertion TSs into the growing polymer chain at site 1 with si (A) and re (B) enantioface. For both TSs, the growing polymer chain does not assume a chiral conformation, and the chain−monomer interactions are negligible. The energetic differences are due to steric ligand−monomer interactions (iPr substituents on phenyl ring, see Chart 1) reported with a dashed red arrow in (B). H atoms are omitted for clarity with the exception of the ones bonded to the first C of the growing chain.

report the lowest-energy paths TS corresponding to the 1,2 propene insertion with si (Figure 2A) and re (Figure 2B) enantioface. The energetic difference we found (see ΔE(ΔG)stereo value in Table 1) is in good agreement with the experimental results,16 but importantly, we noted the following: (a) there is no clear preference for agostic interaction (at the opposite of A and A′ of Scheme S1); (b) the conformation of the growing polymer chain is similar in the two cases (differently from B and B′ in Scheme S1); (c) the absence of a chiral orientation of the growing polymer cancel the chain− monomer interaction (responsible of the propene enantioface

Figure 1. Preferred 1,2 propene insertion TSs into the growing polymer chain at site 1 (A) and site 2 (B). The coordination geometry about Hf is best described by a pyramidal square-based (A) and a distorted trigonal bipyramidal (B), respectively. With the dotted lines are remarked the first C sp3 atom of the growing polymer chain and the olefin C sp2 trans to the N atom of pyridine fragment. H atoms are omitted for clarity.

Table 1. Energetic Values (Free Energies) for the TSs Calculated for the Systems I−IV (See Chart 1)a system system system system system system system system

Ia Ib II IIIa IIIb IVa IVb

ΔE(ΔG)site1/site2b 6.9 5.0 5.6 6.9 4.9 7.9 5.2

(5.4) (6.2) (5.4) (6.6) (4.8) (7.6) (5.7)

ΔE(ΔG)stereoc 2.4 −0.4 1.1 2.8 0.8 3.2 0.8

(1.4) (−0.1) (1.4) (2.0) (0.5) (4.4) (0.9)

ΔE(ΔG)regiod 1.6 1.2 1.5 2.1 2.2 3.8 3.1

(1.1) (1.4) (1.1) (2.2) (2.0) (4.4) (3.8)

ΔE(ΔG)enantio2,1e 3.9(3.7) 3.0 (2.6) 5.7 (5.5) 5.8 (4.5) 5.0 (5.0) 5.2 (5.3) 2.1(1.7)

a Numbers (in kcal/mol) are referred to the most stable TS set as reference point. bEnergetic difference between the lowest-energy path TS insertion at site 1 and at site 2. cCalculated stereoselectivity at site 1. dCalculated regioselectivity at site 1. eCalculated 2,1 insertion enantioselectivity at site 1. For definition of site 1 and site 2, see text and Figure 1.

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do not change the above picture (compare in Table 1 systems I−IVa with system I−IVb). As a final remark, we would like to comment about the role of stereogenic centers *a, *b (see Chart 1) and their role on the stereocontrol. From our data, it appears that they reinforce the direct ligand monomer interaction by a double role: (a) they limit the free movement of monomer by changing the angle between M−1Naphthyl; (b) they freeze the rotation of the 2,6-iPr2C6H3 ring making more effective the direct ligand control of monomer enantioface. This could explain the formation of isotactic polypropylenes with a melting point Tm > 150 °C at a process temperature above 100 °C.9 In conclusion, to explain the still unknown mechanism of stereoselectivity promoted by Hf(IV)-pyridylamido-based olefin polymerization catalysts, we found a new origin of stereocontrol because the well-accepted model of “chiral growing chain orientation” was ineffective for this class of catalysts. We are well aware that catalyst tuning based on the model we proposed is more difficult than the “chiral growing chain orientation” one, due to the larger number of variables involved to obtain precise placement in space. Nevertheless, it offers a new perspective for catalysts design and, at the same time, reconciles the ZN to the asymmetric catalysis.

selection, see C and C′ in Scheme S1). These facts seem to contradict the generally accepted mechanism of stereocontrol, and we hypothesized that the selection of the propene enantioface (which justified the energetic difference ΔE(ΔG)stereo) is dictated directly by the active site through the substituents suitably located on the ligand framework (in particular the iPr groups on the phenyl ring, see Figure 2). To verify this hypothesis, we performed calculations on the system Ib which is characterized by the presence of smaller Me group on the phenyl ring (see Chart 1), and accordingly, we found in practice no propene enantioface preference (see ΔE(ΔG)stereo value for the system Ib in Table 1). To the best of our knowledge, this origin of stereocontrol has never been reported in stereoselective olefin polymerization catalysis. The implications on the targeted design of new catalysts are noteworthy, and we decided to perform additional calculations. The “chiral growing chain orientation” model was founded also on the experimental evidence reported by Zambelli et al.18 on the non-enantioselective insertion of propene into the M− CH3 bond, which lacks conformational control of the growing polymer chain. In Figure S1 (see Supporting Information), we report the two TS structures for the propene insertion into a M−CH3 bond for the system Ia. Accordingly, we found a remarkable preference for the same propene enantioface reported in Figure 2A (and at odds with findings related to metallocenes and postmetallocenes reported up to now). In order to verify if this new mechanism of stereocontrol can be extended to the whole class of Hf(IV)-pyridylamido-based olefin polymerization catalysts, we performed calculations on systems 2−4 and report the data in Table 1. The species II−IV are characterized by a seven-member metallacycle with respect to six-member metallacycle of I, and experimentally, the geometry of the expanded metallacycle influences the microstructure producing low isotactic polypropylenes with II19 and higher isotacticity with IIIa19 and IVa.9 Our set of data in Table 1 is again consistent with the experimental data; in particular, the energetic difference reported in the ΔE(ΔG)site1/site2 column establishes a clear preference for the insertion at site 1 (so confirming the site isomerization mechanism for the pyridylamido-based systems reported here), whereas the data of ΔE(ΔG)stereo column show a lower value for II and larger values for IIIa and IVa (on this point, we will discuss later). Anyway, the TS structures for II−IV show the same features reported in Figures 1 and 2. A further look to the regiochemistry of propene insertion allows a final validation of our model In fact, the 13C NMR spectra of polypropylenes synthesized by systems Ia and IVa reported in the literature9,16 clearly distinguish the signal of regiodefects (secondary or 2,1 insertion). Their stereostructure has been attributed to the preference of the catalyst to insert the same propene enantioface irrespective of the regiochemistry9,16,20 (whereas isotactic ansa-metallocenes and postmetallocenes catalysts prefer the opposite propene enantioface).21 Our calculations performed on the systems I−IV show the following: (a) the preferred propene enantioface for the occasional 2,1 insertion is the same of 1,2 insertion (see Figure S2 A on Supporting Information); (b) a decreasing of regiodefects going from II−IV (see the ΔE(ΔG)regio column of Table 1); (c) the enantioselectivity of 2,1 insertion is high for systems I−IV, (see the ΔE(ΔG)enantio2,1 column of Table 1 and Figure S2) and further modifications by replacing iPr with Me

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00863. Figures S1 and S2 and computational details (PDF) Cartesian coordinates of the structures discussed in the text (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to the memory of Prof. Paolo Corradini. The authors wish to thank the Italian Ministry of the Education, University and Research (MIUR) (Project PRIN No. 20085LE7AZ) and Cariplo foundation (“Crystalline Elastomers Project”) for financial support.

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REFERENCES

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