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NMR Chemical Shift Analysis Decodes Olefin Oligo- and Polymerization

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NMR Chemical Shift Analysis Decodes Olefin Oligo- and Polymerization NMR chemical shift analysis decodes olefin oligo- and PNAS PLUS polymerization activity of d0 group 4 metal complexes Christopher P. Gordona, Satoru Shirasea,b, Keishi Yamamotoa, Richard A. Andersenc,1, Odile Eisensteind,e,1, and Christophe Copéreta,1 aDepartment of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland; bDepartment of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, 560-8531 Osaka, Japan; cDepartment of Chemistry, University of California, Berkeley, CA 94720; dInstitut Charles Gerhardt, UMR 5253 CNRS-Université de Montpellier–École Nationale Supérieure de Chimie de Montpellier (UM-ENSCM), 34095 Montpellier, France; and eHylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, University of Oslo, Blindern, 0315 Oslo, Norway Edited by Tobin J. Marks, Northwestern University, Evanston, IL, and approved May 16, 2018 (received for review February 24, 2018) d0 metal-alkyl complexes (M = Ti, Zr, and Hf) show specific activity and in some cases, metallacycloheptanes have been proposed as key and selectivity in olefin polymerization and oligomerization reaction intermediates (Fig. 2A)(36–48). In fact, Cp2M(ethylene) depending on their ligand set and charge. Here, we show by a complexes (M = Ti, Zr, and Hf) are known to react with ethylene 13 combined experimental and computational study that the C to generate the corresponding metallacyclopentane, Cp2M(C4H8), NMR chemical shift tensors of the α-carbon of metal alkyls that in a stoichiometric manner (49–51). The industrially important undergo olefin insertion signal the presence of partial alkylidene ethylene dimerization catalyst that is prepared from (BuO)4Ti and – character in the metal carbon bond, which facilitates this reaction. AlEt3 is thought to involve the corresponding putative isolobal π The alkylidene character is traced back to the -donating interac- (BuO)2Ti(ethylene) and titanacyclopentane complexes (Fig. 2B) tion of a filled orbital on the alkyl group with an empty low-lying (52, 53), while a catalytic trimerizationisobservedwithiso- x metal d-orbital of appropriate symmetry. This molecular orbital electronic cationic Ti species derived from Cp(η -Ar)TiCl3 picture establishes a connection between olefin insertion into a for instance (Fig. 2C)(54–56). The formation of the metal- metal-alkyl bond and olefin metathesis and a close link between lacyclopentane intermediate is often described as an oxidative the Cossee–Arlmann and Green–Rooney polymerization mecha- coupling of two ethylene molecules because of the change of 13 nisms. The C NMR chemical shifts, the α-H agostic interaction, formal oxidation state from a d2 Ti(II) olefin complex to a d0 and the low activation barrier of ethylene insertion are, therefore, Ti(IV) metallacyclopentane complex. Alternatively, this step can the results of the same orbital interactions, thus establishing also be viewed as a nonoxidative ethylene insertion reaction in chemical shift tensors as a descriptor for olefin insertion. the metallacyclopropane, similar to the elementary step involved in formation of the metallacycloheptane from the metallacyclo- olefin polymerization | olefin oligomerization | NMR spectroscopy | pentane (Fig. 2A). chemical shift tensor analysis | frontier molecular orbitals Recent studies on alkene and alkyne metathesis catalysts have shown that combining solid-state NMR spectroscopy and anal- CHEMISTRY ligomerization and polymerization processes are a highly ysis of the chemical shift tensors (CSTs) provides information on Oactive field of research (1–5) and at the heart of the pet- the nature of the frontier molecular orbitals and the localization rochemical industry, producing α-olefins and polyolefins, re- of low-lying empty orbitals in transition metal complexes, giving spectively, that are some of the most important commodity chemicals and polymers (6). In polymerization, the coordination Significance and insertion of an olefin into a metal–carbon bond, referred to – as the Cossee Arlmann mechanism, are the key elementary steps The rational understanding and design of catalysts pose major A – for the polymer-chain growth process (Fig. 1 )(711). To be challenges to chemists. While catalysts are involved in around operative, this mechanism requires electrophilic metal centers, 90% of industrial chemical processes, their discovery and de- typically in a cationic form, with an empty coordination site (12– velopment are usually based on screening and serendipity. 16). Neutral metal-alkyl complexes are typically less efficient at Here, we show through a detailed analysis of the NMR chem- inserting olefins, in sharp contrast to the corresponding metal ical shift that the activity of olefin polymerization and oligo- hydrides (17). In the alternative Green–Rooney mechanism, merization catalysts is directly related to the chemical shift of chain growth has been proposed to involve alkylidene species the carbon atom bound to the metal center. This relation is formed by α-H transfer from a metal-alkyl species. In this case, traced to specific frontier molecular orbitals, which induce the carbon–carbon bond formation takes place via a [2 + 2]- π-character in the metal-alkyl bond, thereby favoring insertion. cycloaddition, yielding a metallacyclobutane intermediate (Fig. This result not only reveals a surprising analogy between olefin 1B) (7, 8, 18, 19). This mechanism is postulated for the formation polymerization and metathesis, but also establishes chemical of polyethylene with Ta(=CHtBu)(H)I2(PMe3)3 (20). However, shift as a predictive descriptor for catalytic activity in these if the alkylidene is viewed as an X2 ligand, this mechanism re- industrially relevant processes. quires a change of the metal formal oxidation state and is, therefore, unlikely to be relevant for polymerization processes Author contributions: C.P.G., R.A.A., O.E., and C.C. designed research; C.P.G., S.S., K.Y., 0 R.A.A., O.E., and C.C. performed research; S.S. and K.Y. contributed new reagents/analytic involving d transition metal catalysts. tools; C.P.G., S.S., K.Y., R.A.A., O.E., and C.C. analyzed data; and C.P.G., R.A.A., O.E., and Later studies have discussed the role of α-C–H agostic inter- C.C. wrote the paper. actions in the ground state and/or in the transition state to assist The authors declare no conflict of interest. the olefin insertion step, favoring the stereoselective insertion of This article is a PNAS Direct Submission. α -olefins (e.g., propylene) into the growing polymer chain (Fig. Published under the PNAS license. C – 1 ) (21 27). A selection of olefin polymerization catalysts is 1To whom correspondence may be addressed. Email: [email protected], odile. shown in Fig. 1D (13, 28–35). [email protected], or [email protected]. In oligomerization processes, such as the selective dimeriza- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. tion and trimerization of ethylene with catalysts based on early 1073/pnas.1803382115/-/DCSupplemental. transition metals, such as Ti, Ta, or Cr, metallacyclopentanes Published online June 11, 2018. www.pnas.org/cgi/doi/10.1073/pnas.1803382115 PNAS | vol. 115 | no. 26 | E5867–E5876 Downloaded by guest on September 26, 2021 and Hf)] as well as the cationic oligomerization intermediates A x + x + x + Cp(η -Ar)Ti(C2H4) ,Cp(η -Ar)Ti(C2H4)2 ,Cp(η -Ar)Ti(C4H8) , x + and Cp(η -Ar)Ti(C6H12) (Fig. 3B). The solid-state NMR spectra of the model compounds [(C5Me5)2Ti(C2H4)] and Cp2Zr(C4H8) B are also measured experimentally. The chemical shifts (δiso, δ11, δ22, δ33) for all analyzed Zr- based compounds related to olefin polymerization are shown in Table 1 along with selected Ti and Hf derivatives. The data for Ti-based compounds related to olefin oligomerization and se- C lected Zr compounds are summarized in Table 2. Good agree- ment between calculated and experimental values is obtained when data are available. A complete list of all calculated com- pounds is found in SI Appendix, Table S1. D Although many computational studies have been carried out on these complexes and their role in oligo- and polymerization reactions (67–83), the electronic structures and relative energies of relevant reactants, intermediates, and transition states are computed with the same method used for optimizing the ge- ometries for NMR calculations for consistency. Development of the CST Along Reaction Pathways. Olefin polymerization. This subsection focuses on Zr-based olefin polymerization catalysts. The CST of analogous Ti and Hf com- pounds follows a similar trend, with the Ti compounds having the most deshielded values (SI Appendix,Figs.S1andS2). The devel- Fig. 1. Proposed mechanisms for olefin insertion involving electrophilic opment of the CST during ethylene polymerization is summarized in metal centers: (A) Cossee–Arlman mechanism, (B) Green–Rooney mecha- Fig. 4A. The most significant changes of the CST are associated with – nism, and (C) modified Green Rooney mechanism. (D) Typical olefin poly- the most deshielded δ11 component, which is oriented perpendicular merization catalysts (M = Ti, Zr, Hf) (13, 28–35). to the σh plane for all of the aforementioned compounds. The δ11 component is moderately deshielded in Cp2ZrEtCl (δ11 = 102 ppm). The calculations show that, on abstraction of information about the nature of the metal–carbon chemical bond − + the Cl ligand, Cp ZrEt can exist in both a β-H agostic struc- and associated reactivity (57–62). We thus reasoned that a chemical 2 ture and an α-H agostic structure. The β-H agostic compound is shift analysis based on a combined experimental and computational − calculated to be preferred by around 4.5 kcal mol 1, in agree- approach would be a valuable tool to probe the electronic structure ment with previous studies (68–71). Since olefin insertion into of reaction intermediates in polymerization and oligomerization – α processes and to identify key requirements that differentiate be- metal carbon bonds has been proposed to be assisted by an -H agostic interaction, in the ground and/or the transition state tween active and inactive catalysts or between dimerization and α β trimerization catalysts.
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