Ligand Coordination Driven by Monomer and Polymer Chain: The

Jul 11, 2017 - Salalen–Ti systems for propene polymerization catalysis were originally developed under the assumption of a “directional site epime...
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Ligand Coordination Driven by Monomer and Polymer Chain: The Intriguing Case of Salalen−Ti Catalyst for Propene Polymerization Giovanni Talarico*,† and Peter H. M. Budzelaar‡ †

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Via Cintia, 80126 Napoli, Italy Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

‡

S Supporting Information *

ABSTRACT: Salalen−Ti systems for propene polymerization catalysis were originally developed under the assumption of a “directional site epimerization promoted electronically” strategy and allowed the synthesis of polypropylene samples with extraordinarily high isotacticities. Our DFT calculations revealed an alternative and more intriguing interpretation of polymerization behavior of such systems. Ligand coordination around the metal center driven by the monomer and the polymer chain dictates the formation of active species different from the ones expected from X-ray structures of precursors. The “synergic” role of R1 and R3 substituents is responsible of the extraordinarily high isotacticities reported in propene polymerization.

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INTRODUCTION The discovery of well-defined homogeneous catalysts for αolefin polymerization based on metallocene1 and non-metallocene2 families has led to successful design strategies for catalysts producing fine-tuned polymer microstructures. The origin of enantioselectivity is generally accepted to be a twostage process first proposed by Corradini:3 the metal environment orients the growing chain, which in turn selects one particular enantioface of the monomer. Stereoerrors can be due to either chain misorientation or monomer misorientation relative to the chain, with the former typically dominant.4 This stereoregulation mechanism is steric in nature, although electronic factors obviously play a role in defining the geometry of the catalytic site. In this context, one recent approach to catalytic tuning deserves mention. Kol et al.5 proposed the use of catalysts with inequivalent coordination sites, where electronic asymmetry would enforce a back-skip of the growing chain after each insertion.6 To achieve this, they used salalen type ligands (Scheme 1, ligands L1−L3) which are hybrids between more commonly used salan7 and salen ligands. Salalen ligands were reported to coordinate in fac-mer (FM) mode (see Chart 1), and this was confirmed by the X-ray structure of L1Ti(O-iPr)2.5 FM coordination could indeed lead to a significant asymmetry between the two coordination sites involved in polymerization: one is trans to Nimine and the other trans to a phenoxy group. In the alternative fac-fac (FF) coordination mode (more commonly observed with salan ligands7,8) the two © XXXX American Chemical Society

Scheme 1. Ligands Studied (Le and La Are Minimal Models Included for Comparison)

Received: April 25, 2017 Revised: June 19, 2017

A

DOI: 10.1021/acs.macromol.7b00846 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Chart 1. Coordination Modes fac-mer (FM) and fac-fac (FF) for Salalen Catalyst Precursors

Table 1. Calculated Electronic Energy (Free Energy) Preference (kcal/mol) for FM over FF Coordination ligand Le L1 L2 L3 La a

X = O-iPr 4.8 2.6 3.2 1.2 −5.5

(3.3) (3.4) (2.6) (1.0) (−6.9)

X = Cl

X = Me

7.2 (6.8) 7.9 (7.7) 7.8 (7.5) 4.9(5.1) −4.6 (−4.6)

1.4 (−0.8) 1.8 (−0.2) 1.3 (−0.4) −1.2(−3.4) −7.5 (−8.9)

X1 = Cl, X2 = Me 6.2 6.7 6.6 3.0 −4.8

X1 = Me, X2 = Cl 3.5 (1.9), 1.2 (2.5)a 3.6 (2.5), 0.1 (0.0)a 3.8 (2.3), 0.4 (1.2)a 0.5(−1.4), 0.4 (1.9)a −7.3 (−8.7)

(6.0) (6.4) (5.7) (4.0) (−4.8)

Difference between the two FM structures with the Me and Cl trans to Nimine and to O, respectively.

Table 2. iBu/Propene Insertion Transition States: Calculated Electronic Energy (Free Energy) Preference (kcal/mol) for FM over FF Coordination, for Chain Position, and for Stereoselectivity ligand Le L1 L2 L3 L4

FM/FFa −5.8 −5.4 −5.8 −8.4 −7.3

(−5.2) (−5.3) (−5.6) (−6.7) (−7.2)

FF 1/2b 0.7 0.7 0.8 0.3 0.4

(0.6) (0.5) (0.9) (−0.7) (0.2)

FM 1/2b 1.0 0.7 2.1 1.0 0.6

FF stereoc site 1

(1.6) (0.6) (2.2) (0.0) (1.3)

0.1 4.4 0.9 3.5 2.5

(0.2) (4.4) (1.6) (3.3) (2.7)

FF stereoc site 2 0.0 5.8 1.9 4.9 3.8

(−0.6) (7.1) (2.5) (5.4) (3.8)

FM stereod 0.4 2.1 0.9 1.5 2.2

(0.6) (2.3) (1.0) (2.1) (2.6)

a

Energetic differences (free energies) for propene TSs comparing FM and FF structures (negative values indicate a preference for FF insertion). Energetic differences (free energies) for propene TSs at the site 1 and site 2 for FF (third column) and FM (fourth column) species. cCalculated stereoselectivity at site 1 and site 2 for FF structures. For definition of site 1 and site 2 see text and Figure 2. dCalculated stereoselectivity at site 1 for FM structures. For definition of site 1 and site 2 see text and Figure S4. b

sites differ only in the hybridization of the N atoms trans to them (Nimine, sp2 vs Namine, sp3) and enforced back-skip9,10 after each insertion would not be expected. In the present paper, we report on a density functional (DFT) study (computational details are reported in ref 11 and the Supporting Information) of the coordination preference of salalen ligands, showing that the above-mentioned elegant picture of catalyst tuning is likely to play a secondary role, whereas an unprecedented active role of monomer and polymer chain in the formation of active species is revealed.

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in the absence of steric effects. Negative values indicate a preference for the FF coordination mode. Inspection of the Table 1 shows that for salalen complexes bearing O-iPr groups at Ti the FM conformation is clearly preferred (∼3 kcal/mol), in agreement with the X-ray structure of L1Ti(O-iPr)2 (major details are reported in the Supporting Information; see Figure S1). The preference is even higher for the dichlorides (∼7 kcal/mol). However, for the dimethyl complexes the two coordination modes are nearly equal in energy. Comparison with the mixed (chloro)(methyl) complexes shows that most of the destabilization of the FM mode is due to the Me group trans to a phenoxy oxygen, as would be expected based on trans influence. Preferences for real ligands L1 and L2 are very similar to those for model ligand Le, indicating that they are not due to the bulky substituents in the ligands used by Kol. Buried volume analyses11,12 also indicate that FM and FF arrangements occupy similar amounts of space around the metal center, although subtle differences can be noted (see Supporting Information and Figure S2). Finally, the salan ligand La consistently has a preference for FF coordination, but the trends in dependency on X groups follow those observed with salalens.

RESULTS AND DISCUSSION

We first discuss coordination preference as a function of the X groups at the metal center and then investigate the implications for stereoselectivity of olefin insertion. The energy differences (free energies in parentheses) between FM and FF coordination modes for various catalyst precursors (X = O-iPr, Cl, Me) derived from ligands L1−L3 studied by Kol5 are collected in Table 1. Also included are data for one unsubstituted model salalen ligand (Le) and the corresponding salan ligand (La) to illustrate intrinsic preference B

DOI: 10.1021/acs.macromol.7b00846 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Propene insertion TSs into L1Ti(iBu)+ chain with FF (A) and FM (B) conformations. In structure A the α-agostic interaction is evident (∠M−C−C = 130°) whereas in B it is less pronounced (∠M−C−C = 122°).

Figure 2. Propene insertion TSs into L1Ti(iBu)+ with FF coordination mode at the site 1 (A) and the site 2 (B). The structures show the growing polymer chain (A) and the incoming propene molecule (B) trans to the N sp2 imine atoms. H atoms are omitted for clarity.

Figure 3. Propene TS stereoerrors into FF L1Ti(iBu)+ (site 1) due to monomer misorientation relative to the chain (A) and chain misorientation (B). The TS energy (free energy) of A is 2.1 (3.2) kcal/mol lower than B. R1 and R3 (here Ad and I, respectively) both act sterically (with arrows) to increase the TS energies with respect to the right insertion (reported in Figure 2A). H atoms are omitted for clarity.

necessitated the calculation of 12 TSs per ligand.14 For both FM and FF geometries, we have labeled as “site 1” the arrangement with the olefin in the position of X1 and the chain in the position of X2 (see Chart 1). Surprisingly, the results reported in Table 2 show a consistent and significant (∼5 kcal/ mol) preference for insertion in the FF geometry: the change on going from LTiCl2 to LTi(iBu···propene)+ is even larger than from LTiCl2 to LTiMe2. Examples of the propene insertion TSs derived from L1Ti FF and FM species are reported in Figure 1. Salan skeletons are known to rearrange easily under polymerization conditions,15 so it seems likely that also for salalen complexes the path actually followed will be the one corresponding to the lowest energy insertion TS, i.e., with an

The sensitivity of the FM/FF energy difference to the nature of the groups X at the metal center throws some doubt on the straightforward assumption that under polymerization conditions the FM coordination mode is preserved. For model ligand Le we therefore explored a number of further variations of the X groups, which only confirmed this sensitivity, with value spanning a range of 1.5 (2.1) kcal/mol for −CH2CH CHCH2− to −1.6 (−2.7) kcal/mol for −(CH2)3− (see Figure S3). In view of this dependence it is important to check the coordination preference under polymerization conditions, i.e., at the relevant olefin insertion transition states (TS) for both stereoselective and stereoerror paths4,13 with ligands Le and L1−L3. The uncertainty about preferred coordination mode and the existence of inequivalent reactive sites at the metal C

DOI: 10.1021/acs.macromol.7b00846 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. Propene TS stereoerrors into FM L1Ti(iBu)+ due to chain misorientation (A) and monomer misorientation relative to the chain (B). The steric effect of R1, with arrow, increases the TS energy with respect to right insertion (reported in Figure 2B), whereas the effect of R3 is lost. H atoms are omitted for clarity.

A second issue is back-skip between site 1 and site 2. We have no way of knowing whether this will be fast relative to propagation. Since a β-agostic interaction has to be broken, the barrier might well be substantial. In view of the small energy differences between site 1 and site 2, both sites would contribute in any case, either alternatingly or in a more random fashion. Since both sites are highly stereoselective and they prefer the same propene enantioface, the rate of back-skip does not affect the outcome much.

FF arrangement rather than the FM mode assumed in the literature.5 Interestingly, for each of the two ligand coordination modes, the two reactive sites 1 and 2 are close in energy (1−2 kcal/mol, see Table 2). This is expected for the FF coordination (see e.g. Figure 2), but one might have expected a larger difference for the FM due to the trans influence16 (see Figure S4). Looking at the predicted stereoselectivity for the FF and FM species, we note that at the preferred site 1 stereoselectivity is high for both FM and FF coordination modes for L1 and L3 and lower for L2, which nicely agrees with the experimental results.5 A similar trend is obtained also for site 2 (see Table 2 and Figure S5) which, incidentally, prefers the same propene enantioface as site 1 with even higher stereoselectivity values (see Figure 2). The higher stereoselectivities calculated for FF with respect to FM species deserve further investigation considering that L1 and L3 systems are known to produce polypropylene with extraordinarily high isotacticities.5 For these systems we find that at site 1 the dominant source of stereoerrors is monomer misorientation relative to the chain (see Figure 3A) instead of the more common chain misorientation (see Figure 3B). It appears that in the present system stereoselectivity is determined by a synergic combination of ligand−chain interaction and direct ligand−monomer interaction. The bulky R1 group is responsible for the chain orientation (destabilizing the TS in Figure 3B) while at the same time the presence of the R3 group destabilizes si propene coordination directly for both Figure 3A and Figure 3B. To prove this “synergic effect”, we replaced I with H in the R3 position for system L1 (see system L4 in Table 2), obtaining a FF stereoselectivity value of only 2.5 kcal/mol (see Figure S6) instead of 4.4 kcal/mol (see Table 1). Analysis of the stereoerrors expected for FM species shows that the R1 substituent still have their steric effect on the chain misorientation, but the “synergic” effect of R3 directly interacting with the monomer is lost (see Figure 4), leading to a lower stereoselectivity with respect to FF (see Table 1). As expected, replacing I with H in the R3 position does not change the stereoselectivity value of the FM species (see L4 results in Table 2 and Figure S7). What are the implications of an FF/FM equilibrium for catalysis? For salan catalysts, isomerization is relatively fast.15 For salalen catalysts, even if isomerization would require the time equivalent of several complete polymer chains, the catalyst would on average stay in the energetically preferred FF form, and hence, regardless of where/when isomerization happened, propagation would mostly involve the FF isomer.

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CONCLUSIONS Salalen−Ti catalysts for propene polymerizaton have been developed on the basis of a “directional site epimerization promoted electronically” strategy.5 This approach was claimed to be a successful way to modify the stereoselectivity of propene polymerization by using electronic asymmetry to enforce a back-skip of the growing chain after each insertion. Our DFT calculations paint a somewhat different but more intriguing picture where the diastereoisomeric equilibrium between FF and FM species depends of the chemical nature of the remaining X groups at the metal center, at least in part through their trans influence.17 The formation of (different) active sites by diastereoisomeric wrapping around the metal center driven by monomer and growing chain is unprecedented in polymerization catalysis.18 For the only reported case to date, a Hf(IV)-pyridylamido-based system,19 the active species originates from in situ chemical ligand modification by the monomer.20−24 Nevertheless, the present exception to the consolidated relationships between cationic active species/ neutral precursors developed for metallocene25,26 and nonmetallocene systems may open new routes to stereoselective catalysis.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00846. Figures S1−S7 and computational details (PDF) Cartesian coordinates structures Table 1 (XYZ) Cartesian coordinates structures Table 2 (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.T.). ORCID

Giovanni Talarico: 0000-0002-4861-0444 D

DOI: 10.1021/acs.macromol.7b00846 Macromolecules XXXX, XXX, XXX−XXX

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(13) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Selectivity in Propene Polymerization with Metallocene Catalysts. Chem. Rev. 2000, 100, 1253−1345. (14) As a matter of fact, the presence of two different conformations for the FM species double the number of TS FM calculations; see Supporting Information for details. (15) Ciancaleoni, G.; Fraldi, N.; Cipullo, R.; Busico, V.; Macchioni, A.; Budzelaar, P. H. M. Structure/Properties Relationship for Bis(phenoxyamine)Zr(IV)-Based Olefin Polymerization Catalysts: A Simple DFT Model To Predict Catalytic Activity. Macromolecules 2012, 45, 4046−4053. (16) Coe, B. J.; Glenwright, S. J. Trans-effects in octahedral transition metal complexes. Coord. Chem. Rev. 2000, 203, 5−80. (17) The large set of variables involved in the FF versus FM equilibrium (e.g., central metal atom as well as the right combination of electronic and steric effects by R1 and R3 substituents or the chemical nature of heteroatoms) led us to predict that FM active species may be formed in solution by suitable ligand framework. (18) It has been reported that the chirality of the growing chain may stabilize the Δ or Λ chirality of fluxional octahedral systems explaining the syndiotacticity obtained with C2-symmetric octahedral Ti− (bisphenoxyimine) complexes in propene polymerization (see e.g.: Milano, G.; Cavallo, L.; Guerra, G. Site Chirality as a Messenger in Chain-End Stereocontrolled Propene Polymerization. J. Am. Chem. Soc. 2002, 124, 13368−13369 andref 3). However,in our paper we are discussing of diasteroisomeric wrapping modedriven by both monomer and the growing chain and not of stereoisomers. (19) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M. K.; Murphy, V.; Shoemaker, J. A. W.; Turner, H.; Rosen, R. K.; Stevens, J. C.; Alfano, F.; Busico, V.; Cipullo, R.; Talarico, G. Nonconventional Catalysts for Isotactic Propene Polymerization in Solution Developed by Using High-ThroughputScreening Technologies. Angew. Chem., Int. Ed. 2006, 45, 3278−3283. (20) Froese, R. D. J.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Mechanism of Activation of a Hafnium Pyridyl-Amide Olefin Polymerization Catalyst: Ligand Modification by Monomer. J. Am. Chem. Soc. 2007, 129, 7831−7840. (21) Domski, G. J.; Edson, J. B.; Keresztes, I.; Lobkovsky, E. B.; Coates, G. W. Synthesis of a new olefin polymerization catalyst supported by an sp3-C donor via insertion of a ligand-appended alkene into the Hf−C bond of a neutral pyridylamidohafnium trimethyl complex. Chem. Commun. 2008, 6137−6139. (22) Zuccaccia, C.; Macchioni, A.; Busico, V.; Cipullo, R.; Talarico, G.; Alfano, F.; Boone, H. W.; Frazier, K. A.; Hustad, P. D.; Stevens, J. C.; Vosejpka, P. C.; Abboud, K. A. Intra- and Intermolecular NMR Studies on the Activation of Arylcyclometallated Hafnium PyridylAmido Olefin Polymerization Precatalysts. J. Am. Chem. Soc. 2008, 130, 10354−10368. (23) Zuccaccia, C.; Busico, V.; Cipullo, R.; Talarico, G.; Froese, R. D. J.; Vosejpka, P. C.; Hustad, P. D.; Macchioni, A. On the First Insertion of α-Olefins in Hafnium Pyridyl-Amido Polymerization Catalysts. Organometallics 2009, 28, 5445−5458. (24) De Rosa, C.; Di Girolamo, R.; Talarico, G. Expanding the Origin of Stereocontrol in Propene Polymerization Catalysis. ACS Catal. 2016, 6, 3767−3770. (25) Farina, M. Old and new problems in polymer stereochemistry. Macromol. Symp. 1995, 89, 489−498. (26) Ewen, J. A. Symmetry rules and reaction mechanisms of ZieglerNatta catalysts. J. Mol. Catal. A: Chem. 1998, 128, 103−109.

Peter H. M. Budzelaar: 0000-0003-0039-4479 Present Address

P.H.M.B.: Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Via Cintia, 80126 Napoli, Italy. Funding

This research was supported by the University of Naples Federico II (Ricerca di Ateneo 2017 of University of Naples Federico II, DR_409_2017). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank the Italian Ministry of the Education, University and Research (MIUR) (Project PRIN No. 20085LE7AZ) and Cariplo foundation (“Crystalline Elastomers Project”) for financial support.

(1) Gladysz, J. A. Frontiers in Metal-Catalyzed Polymerization. Chem. Rev. 2011, 111 (3), 1167. (2) For a recent review on post-metallocene see: Baier, M. C.; Zuideveld, M. A.; Mecking, S. Post-Metallocenes in the Industrial Production of Polyolefins. Angew. Chem., Int. Ed. 2014, 53, 9722− 9744. (3) Corradini, P.; Guerra, G.; Cavallo, L. Do New Century Catalysts Unravel the Mechanism of Stereocontrol of Old Ziegler-Natta Catalysts? Acc. Chem. Res. 2004, 37, 231−241. and references therein. (4) Talarico, G.; Budzelaar, P. H. M. Analysis of Stereochemistry Control in Homogeneous Olefin Polymerization Catalysis. Organometallics 2014, 33, 5974−5982. (5) Press, K.; Cohen, A.; Goldberg, I.; Venditto, V.; Mazzeo, M.; Kol, M. Salalen Titanium Complexes in the Highly Isospecific Polymerization of 1-Hexene and Propylene. Angew. Chem., Int. Ed. 2011, 50, 3529−3532. (6) Bochmann, M. The Chemistry of Catalyst Activation: The Case of Group 4 Polymerization Catalysts. Organometallics 2010, 29, 4711− 4740. (7) Tshuva, E. Y.; Goldberg, I.; Kol, M. Isospecific living polymerization of 1-hexene by a readily avaible nonmetallocene C2symmetrical zirconium catalyst. J. Am. Chem. Soc. 2000, 122, 10706− 10707. (8) For propene polymerization promoted by salan ligand see: Busico, V.; Cipullo, R.; Pellecchia, R.; Ronca, S.; Roviello, G.; Talarico, G. Design of stereoselective Ziegler-Natta propene polymerization catalysts. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15321−15326. and references therein. (9) Ewen, J. A.; Elder, M. J.; Jones, R. L.; Haspeslagh, L.; Atwood, J. L.; Bott, S. G.; Robinson, K. Metallocene/polypropylene structure relationships: implications on polymerization and stereochemical control mechanisms. Makromol. Chem., Macromol. Symp. 1991, 48− 49, 253−295. (10) Tomasi, S.; Razavi, A.; Ziegler, T. Stereoregularity, Regioselectivity, and Dormancy in Polymerizations Catalyzed by C1Symmetric Fluorenyl-Based Metallocenes. A Theoretical Study Based on Density Functional Theory. Organometallics 2009, 28, 2609−2618. (11) Falivene, L.; Cavallo, L.; Talarico, G. Buried Volume Analysis for Propene Polymerization Catalysis Promoted by Group 4 Metals: A Tool for Molecular Mass Prediction. ACS Catal. 2015, 5, 6815−6822. (12) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L. SambVca 2. A Web Tool for Analyzing Catalytic Pockets with Topographic Steric Maps. Organometallics 2016, 35, 2286−2293 and references therein. See also the https://www.molnac.unisa.it/OMtools/sambvca2.0/index. html.10.1021/acs.organomet.6b00371. E

DOI: 10.1021/acs.macromol.7b00846 Macromolecules XXXX, XXX, XXX−XXX