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Effects of the Grafting of Lanthanum Complexes on a Silica Surface on the Reactivity: Influence on Ethylene, Propylene, and 1,3Butadiene Homopolymerization Iker Del Rosal,* Ahmed Yahia, and Laurent Maron* †

Université de Toulouse, INSA, UPS, LPCNO, IRSAMC, 135 avenue de Rangueil, F-31077 Toulouse, France CNRS, UMR 5215, IRSAMC, F-31077 Toulouse, France

‡

S Supporting Information *

ABSTRACT: In this contribution, we report full details of the ethylene, 1,3butadiene, and propylene homopolymerization processes mediated by alkylated bis(trimethyl)silylamide lanthanide-grafted complexes using a density functional theory (DFT) study of the initiation and first propagation steps. These systems allows us (i) to examine the role of the grafting mode on the kinetics and thermodynamics of the three processes considered, (ii) to confirm the catalytic behavior of these grafted complexes in ethylene polymerization, (iii) to rationalize the experimental preference for 1,4-cis polymerization of 1,3-butadiene, and (iv) to provide unprecedented information on the catalytic activity of the lanthanide-grafted complex as a propylene hompolymerization catalyst.

1. INTRODUCTION Polyolefin and poly(diene)s are important industrial products for many applications in plastics, automotives, textiles, etc. For example, because of their low cost and good mechanical and physical properties, polypropylene and polyethylene can be tailored in various applications such as food packaging, carpets that replace natural fibers, biomedical applications, etc.1 These two polymers are also two of the basic components of twothirds of the amount of plastic resins widely used in the industrial production of plastic products. In the same way, the polymerization of conjugated dienes, such as butadienes, is one of the everlasting topics in industrial chemistry.2 Polybutadiene (PB) is, among others, used in golf ball cores because of its outstanding resiliency but also in tires because of its excellent abrasion resistance and low rolling resistance. The current importance of these polymers is intrinsically related to the rapid development of this field since the original discoveries, in the early 1950s, of Ziegler and Natta (Ziegler−Natta catalysts3) and Hogan and Banks (Phillips catalysts4). From an industrial point of view, heterogeneous catalysts, such as Ziegler−Natta systems, are generally preferred because in many cases the catalyst can be more easily recovered, which leads to lower costs for production and purification. Moreover, the easier solid−liquid separation reduces the solvent consumption, which is in agreement with the constant demand for greener chemical processes. In the case of homogeneous catalysts, such as metallocene and late-transition-metal catalysts, every entity can act as a single active site. Thus, these catalysts, based on sophisticated organometallic molecules, can be tailored to control the polymerization in an unprecedented © XXXX American Chemical Society

fashion by variation of the organic ligand and thus the steric and electronic environment of the metal center.5 This makes homogeneous catalysts intrinsically more active and selective compared to heterogeneous catalysts. Therefore, a catalytic system, which takes the advantages of both homogeneous and heterogeneous catalysis, would greatly enhance the interest for industrial applications. The concept of surface organometallic chemistry (or the heterogenization of homogeneous catalysts) has been developed as a possible answer to this problem.6 Its main objective is to afford organometallic surface moieties by the formation of polar covalent surface−metal bonds through protonolysis of a metal−ligand bond by surface hydroxyl groups. The grafting reaction is generally performed on silica, alumina, titania, or magnesia. However, in most cases, the support was silica because of the large surface area and good mechanical and thermal properties of this oxide and also because SiO2 does not generate parallel reactions. Moreover, silica is a highly versatile material because one can choose between, e.g., crystalline, mesoporous, and amorphous silica as well as different thermal treatments of the surface (ranging from 200 to 750 °C). The former would change the specific area of the surface, whereas the latter would lead to a difference of surface silanols, inducing a diversity of grafted complexes at the surface. In this context, the use of supported rare-earth compounds for olefin and conjugated diene homopolymerizaSpecial Issue: New Trends and Applications for Lanthanides Received: May 21, 2016

A

DOI: 10.1021/acs.inorgchem.6b01238 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Schematic representation of the possible surface species obtained by the grafting reaction of lanthanide silylamides on a dehydroxylated silica surface at 250 °C8b,c,e (Ln = Sc, Y, La, Nd, Sm, Gd, Dy).

(DFT) study of the initiation and propagation steps of ethylene and 1,3-butadiene polymerization promoted by a supported lanthanide complex on a silica surface. Special attention was paid to the influence of the grafting mode, i.e., the effect of the dehydroxylation temperature. In the same way, and prospectively, the use of lanthanum-supported systems as catalysts for propylene polymerization was also investigated. For the sake of clarity, the study of the polymerization reaction of ethylene, 1,3-butadiene, and propylene promoted by a silica-supported lanthanum bis(trimethyl)silylamide complex is analyzed separately in the following subsections.

tion is a matter of ongoing interest because of the high industrial economic impact of the polymers formed, their low toxicity, and their moderate cost. The experimental grafting reaction of lanthanide amide complexes, of the type Ln[N(R)2]3 (Ln = Sc, Y, La, Nd, Sm, Gd, Dy), has already been described by Anwander et al.7 among others8 (see Figure 1). In the same way, the grafting reaction of lanthanide chloride,9 borohydride,10 or comporting aluminate ligands11 has also been studied by different experimental groups. Polyolefin and poly(diene) polymerization by rare-earth complexes is well-known in homogeneous chemistry.12 However, one of the main problems with homogeneous rareearth complexes is the relatively low activity in α-olefin polymerization due to deactivation of the catalyst and also in some cases stereoselectivity issues. In that sense, immobilization of the catalyst on a surface might be a way to solve these problems. To our knowledge, very little work has been reported on the use of supported lanthanide complexes as polyolefin or poly(diene) polymerization catalysts. Bochmann et al.8b evaluated, in 2005, the catalytic activity of seven Ln[N(SiMe3)2]3 (Ln = Sc, Y, La, Nd, Sm, Gd, Dy) complexes grafted onto a silica surface dehydroxylated at 500 °C. This study showed that, in conjunction with an alkyl aluminum activator (AliBu3, TIBA), all seven grafted complexes were found to be active for the homo- and copolymerization of ethylene and 1,3butadiene. In the case of butadiene, all of the different lanthanides tested achieved very similar polymerization results, in terms of both activity and stereochemistry. More precisely, the seven grafted catalysts considered showed good activity for the polymerization of 1,3-butadiene and produced predominantly 1,4-cis-polybutadiene. In the same way, in 2006, Mortreux et al.8e reported the catalytic activity of four Ln[N(SiMe3)2]3 (Ln = Y, La, Nd, Sm) complexes grafted onto a silica surface dehydroxylated at 250, 500, or 750 °C. In agreement with the previous work of Bochmann et al., the four complexes are active as ethylene polymerization catalysts, upon mixing of the silica-supported catalyst and the alkylating agent (TIBA). This study also underlined a different ethylene polymerization activity according to both the metal center and the dehydroxylation temperature: (i) neodymium catalysts are generally more active than the lanthanum ones, with the exception of the studies realized with a silica surface dehydroxylated at 250 °C. In this case, lanthanum is slightly more efficient than neodymium; (ii) a similar catalytic activity is obtained for lanthanum compounds grafted onto silica dehydroxylated at 250 or 700 °C, whereas a significant effect is observed for neodymium-based systems, where the activity increases with the pretreatment temperature. In this contribution, we report full details of the ethylene and 1,3-butadiene polymerization processes in order to give a suitable explanation of the aforementioned experimental findings. Thus, we have undertaken a density functional theory

2. COMPUTATIONAL DETAILS All DFT calculations were performed with Gaussian 03.13 Calculations were carried out at the DFT level of theory using the hybrid functional B3PW91.14 Geometry optimizations were achieved without any symmetry restriction. Calculations of the vibrational frequencies were systematically done in order to characterize the nature of the stationary points. Stuttgart effective core potentials15 and their associated basis sets were used for silicon and lanthanum. The basis sets were augmented by a set of polarization functions (ζd = 0.284 for Si and ζf = 1.000 for La). H, N, C, and O atoms were treated with 631G(d,p) double- ζ basis sets.16 The electron density and partial charge distribution were examined in terms of localized electron-pair bonding units using the NBO program.17 Through this method, the input atomic orbital basis set is transformed via natural atomic orbitals and natural hybrid orbitals into natural bond orbitals (NBOs), which correspond to the localized one-center (“lone-pair”) and two-center (“bond”) elements of the Lewis structure. All possible interactions between “filled” (donor) Lewis-type NBO and “empty” (acceptor) non-Lewis NBO orbitals, together with their energetic quantification (stabilization energy), have been obtained by a second-order perturbation theory analysis of the Fock matrix. Only a stabilization energy higher than 10 kcal mol−1 has been considered. In this work, ΔH was calculated instead of ΔG because of the well-known erroneous computational description of variation of the entropy for a reaction where two reactants give only one product.18

3. RESULTS AND DISCUSSION 3.1. Lanthanide-Grafted Complexes Considered. In the present study, in order to study the catalytic activity of a supported lanthanum complex on a silica surface dehydroxylated either at 200 °C (SiO2−200) or at 700 °C (SiO2−700), we consider as surface models our previously defined five cagelike polyoligosilsesquioxane derivatives labeled as c, ac, b, abc, and bc (Figure 2).19 These models has been found to be useful to confirm and further refine the understanding of the grafting reaction of several organometallic complexes onto silica dehydroxylated at 700 °C20 or 200 °C.19,21 In previous works,21 the grafting of a lanthanide bis(trimethyl)silylamide complex on a dehydroxylated silica surface at 700 or 200 °C was reported. In the aforementioned studies, we have also shown that the grafted complex can be safely simplified for DFT computations, replacing the B


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according to the dehydroxylation temperature, the key question is to determine which grafting mode leads to the lowest energy barrier for ethylene insertion. The enthalpy profile of the two first steps of the homopolymerization of ethylene, i.e., the initiation and first propagation steps, is depicted in Figure 4. A view of the different transition states and intermediates is available in Figures S2 and S3 in the Supporting Information. For comparison, we have also provided (see Table 1) the results for the same polymerization steps mediated by a homogeneous Cp2LaMe complex. In all cases, the reaction begins with the formation of an exothermic adduct with a stabilization energy of −1.9 kcal mol−1 (Aetb‑1), −4.6 kcal mol−1 (Aetc‑1), and −8.1 kcal mol−1 (Aetc‑2), with respect to the separated reactants, corresponding to the coordination of ethylene to the metal center. The formation of this adduct is followed by the insertion of ethylene into the La−Me bond of the different grafted compounds through a four-centered transition state (π bond−metathesis transition state22). For the three grafted species, the energy barriers for this first insertion were found to be accessible with similar activation barriers between +10.2 and +13.1 kcal mol−1 with respect to the respective previous adducts. The geometry of the corresponding transition state is also quite similar and reveals, as anticipated, a marked elongation of the La−CMe and Cet1− Cet2 bond distances. It is also interesting to note that, in the three cases, the La−O3 and La−O4 distances increase with respect to the initial reactants (2.751 and 3.413 Å vs 2.705 and 2.812 Å for [La]@c-1, 2.895 and 2.938 Å vs 2.895 and 3.137 Å for [La]@c-2, and 2.846 and 2.855 Å vs 2.716 and 2.722 Å for [La]@b-1). For the monografted complexes, although the active site is not as accessible as that for bigrafted ones, monocoordination to the surface allows the La atom to modulate the electronic assistance of the surface through interaction (or not) with the neighboring O atoms of the siloxane bridges. This ability to adjust the assistance of the surface at each step makes the three grafted complexes chemically competitive. Thermodynamically, the formation of insertion products (propyl products) is an exothermic reaction, with respect to the separated reactants, by around −20.4 kcal mol−1 regardless of the grafting mode considered. The second ethylene insertion, corresponding to the first propagation step, has also been computed in order to gain deeper insight into the polymerization process. For this second insertion, we considered front-side “migratory” versus back-side “stationary” ethylene insertions (Figure 5) from the previous

Figure 2. Representation of the five systems, with different silanol groups, used as models of a silica surface dehydroxylated at 200 °C.

trimethylsilyl groups by H atoms. However, as experimentally observed, the successful polymerization reaction of ethylene and 1,3-butadiene mediated by grafted bis(trimethyl)silylamide complexes/TIBA originates from the in situ formation of an active alkyl species. Thus, in the following discussion, we considered as catalytically active species the alkyl complexes in which the bis(trimethyl)silylamide groups were replaced by methyl groups and the hexamethyldisilazane molecule by NH3. According to these previous works and in order to shed light on the role of the grafting mode on the kinetic and thermodynamic parameters of the polymerization reaction of ethylene, 1,3butadiene, and propylene, three different grafting modes were considered (see Figure 3). 3.2. Polymerization Reactions. 3.2.1. Ethylene Polymerization. Considering the coexistence of different grafting modes

Figure 3. Optimized structures of the grafted complexes considered to study the polymerization reaction of ethylene, 1,3-butadiene, and propylene, called [La]@c-1 (a), [La]@c-2 (b), and [La]@b-1 (c). C


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Figure 4. Reaction enthalpy profile of the initiation and first propagation steps of the homopolymerization of ethylene. [La] refers to fragment Si14O21H20La(NH3) for [La]@b-1 and fragment Si13O20H17La for [La]@c-1 and [La]@c-2.

Table 1. Energy Data (Enthalpy, in kcal mol−1) of the Initiation and First Propagation Steps of the Ethylene Polymerization Mediated by Cp2LaMea

The first propagation step begins, as for the initiation step, by the coordination of ethylene to the metal center. In this second step, as for the initiation step, a stability difference is observed according to the steric hindrance around the metal center. Indeed, the higher the steric hindrance, the lower the stabilization energy, with respect to the separated reactants, i.e., propyl product + ethylene. More precisely, for the most hindered system (Cetb‑1), the coordination of ethylene is endothermic by 0.9 kcal mol−1, whereas for the less hindered one (Cetc‑2), the coordination of ethylene is exothermic by 4.2 kcal mol−1. From these intermediates, the second insertion reaction takes place via a low-energy process, with an activation barrier calculated between 2.7 and 7.9 kcal mol−1. The formation of the final products is a highly exothermic process (roughly −25.2 kcal mol−1 with respect to the initiation step products), regardless of the grafting mode considered. Upon analysis of the preferred kinetic pathway in more detail, it is worth noting that the rate-limiting step corresponds to the initiation reaction. In this step, the three transition states are quite similar. Thus, the three grafting modes are competitive from a thermodynamic and kinetic point of view, which is in agreement with the aforementioned experimental observations, i.e., a similar catalytic activity for lanthanum compounds grafted onto silica dehydroxylated at 250 and 700 °C. It is also worth noting that the catalytic activity of grafted complexes is similar to that of the homogeneous Cp2LaMe system with an activation barrier of 7.4 kcal mol−1 for the initiation step and 6.1 kcal mol−1 for the first propagation step. The relative stabilities of the products are also similar, −21.1 kcal mol−1 for the initiation step and −23.3 kcal mol−1 for the first propagation step. From Betc‑1, the insertion of a second ethylene monomer can also be achieved through the formation of a second growing

Initiation Step AetCp2

TS(AetCp2→BetCp2)

BetCp2

−7.6

−0.2 First Propagation Step

−21.1

CetCp2

TS(CetCp2→DetCp2)

DetCp2

−29.5

−23.4

−46.4

a

The labels A−D refer to the intermediates and transition states in Figure 4.

Figure 5. Schematic representation of the transition states involved in the front-side and back-side insertions. [La] refers to fragments Si14O21H20La(NH3) for [La]@b-1, and Si13O20H17La for [La]@c-1 and [La]@c-2.

propyl products. In all cases, the formation of the final insertion products is predicted to be thermodynamically exothermic and kinetically accessible. Thus, for the sake of clarity, because of the similarity between the relative enthalpies of the intermediates and the activation barriers, the following discussion on the first propagation step has been limited to the front-side insertions. D


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different intermediates and transition states are depicted in Figure S4 in the Supporting Information. As aforementioned, from Betc‑1, it is possible to form a second growing arm by the insertion of an ethylene on the La−Me bond. The energy data for this pathway as well as the structure of the intermediates and transition states are similar to that of the first insertion (initiation step, Figure 4). Hence, the coordination of a second ethylene monomer to Betc‑1 leads to the formation of an exothermic complex, C′etc‑1, with an enthalpy of −4.4 kcal mol−1 with respect to Betc‑1. Subsequently, from C′etc‑1, the insertion of the second ethylene takes places through the transition state TS(C′etc‑1 → D′etc‑1), yielding the final product D′etc‑1. The activation energy for this step is calculated to be 10.4 kcal mol−1 above C′etc‑1. The formation of D′etc‑1 is predicted to be exothermic (−20.5 kcal mol−1 with respect to Betc‑1), leading to an overall thermodynamically favorable and kinetically accessible reaction. However, the activation barrier leading to the formation of a second growing arm is, because of the geometrical constraint imposed by the propyl ligand, higher than that calculated for the insertion in the same arm (10.4 vs 4.6 kcal mol−1, with respect to C′etc‑1 and Cetc‑1, respectively). In this case, and considering the activation barrier difference between both pathways, it is likely that ethylene polymerization takes place only on one arm. 3.2.2. 1,3-Butadiene Polymerization. An enthalpy profile has also been computed for the homopolymerization of 1,3butadiene, i.e., 1,4-trans and 1,4-cis insertions of butadiene into the [La]−alkyl bond of the three grafted complexes considered. The initiation step of the 1,3-butadiene polymerization is first analyzed (see Figure 7). A view of the different transition states and intermediates is available in Figures S5 and S6 in the Supporting Information. As for ethylene insertion, the 1,4-trans and 1,4-cis insertions into the alkyl−lanthanum bond of the three grafted complexes considered are thermodynamically favorable, between 30.1 and 40.1 kcal mol−1 with respect to the

Figure 6. Calculated enthalpy profile of the second monomer insertion in the polymerization of ethylene mediated by Betc‑1.

Figure 7. Calculated enthalpy profile of 1,3-butadiene homopolymerization mediated by [La]@c-1, [La]@c-2, and [La]@b-1. [La] refers to fragment Si14O21H20La(NH3) for [La]@b-1 and fragment Si13O20H17La for [La]@c-1 and [La]@c-2. E


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Table 2. Energy Data (Enthalpy, in kcal mol−1) of the First Propagation Step of the 1,3-Butadiene Polymerization Mediated by [La]@c-1, [La]@c-2, and [La]@b-1 [La]@c-1

ΔrH

bu,cis−cis TS(A b‑1→B b‑1) bu,cis−cis B b‑1 Abu,cis−transb‑1 TS(Abu,cis−transb‑1→Bbu,cis−transb‑1) Bbu,cis−transb‑1 Abu,trans−cisb‑1 TS(Abu, trans−cisb‑1→Bbu, trans−cisb‑1) Bbu, trans−cisb‑1 Abu,trans−transb‑1 TS(Abu, trans−transb‑1→Bbu, trans−transb‑1) Bbu, trans−transb‑1

−35.7 −28.0 −53.0 −38.0 −24.1 −55.6 −36.6 −29.6 −54.3 −38.5 −27.4 −57.2

Abu,cis−cisb‑1 bu,cis−cis

[La]@c-2

ΔrH

A b‑1 TS(Abu,cis−cisb‑1→Bbu,cis−cisb‑1) Bbu,cis−cisb‑1 Abu,cis−transb‑1 TS(Abu,cis−transb‑1→Bbu,cis−transb‑1) Bbu,cis−transb‑1 Abu,trans−cisb‑1 TS(Abu, trans−cisb‑1→Bbu, trans−cisb‑1) Bbu, trans−cisb‑1 Abu,trans−transb‑1 TS(Abu, trans−transb‑1→Bbu, trans−transb‑1) Bbu, trans−transb‑1

−32.4 −25.3 −57.5 −39.6 −23.4 −54.9 −34.7 −27.2 −56.3 −36.9 −23.0 −58.6

bu,cis−cis

separated reactants. This stability is due to the formation of an allylic group in striking difference with the alkyl product formed by the ethylene insertion into the same alkyl−lanthanum bond. The activation barriers for both insertion reactions, located between 3.3 and 11.3 kcal mol−1 for the 1,4-cis insertion and between 6.9 and 16.1 kcal mol−1 for the 1,4-trans insertion depending on the grafting mode, correspond to kinetically accessible processes. However, the activation barrier is calculated to be slightly lower for the 1,4-cis insertion than for the 1,4-trans insertion by roughly 4.0 kcal mol−1. This is in good agreement with the experimentally observed predominant formation of 1,4-cis-polybutadiene.8b This is an interesting result because the lanthanide-based homogeneous catalysts mainly lead to 1,4-trans-polybutadiene.12a,23 In a second step, the propagation and its stereoselectivity have been investigated (see Table 2). Thus, the formation of cis−cis, cis−trans, trans− cis, and trans−trans sequences has been considered. Thermodynamically, regardless of the grafting mode, all of the insertion products are exothermic. Nevertheless, the 1,4trans insertions are slightly more favorable thermodynamically than the 1,4-cis ones by 3.1 kcal mol−1. Kinetically, the activation barrier for the 1,4-cis insertions is still calculated to be lower than that for 1,4-trans insertions by roughly 6.0 kcal mol−1. The preference for the 1,4-cis insertion is then associated with the less sterically demanding cis insertion over the trans one. Thus, in line with literature interpretations,24 the 1,4-cis-polybutadiene sequence corresponds to the kinetic product, whereas the 1,4-trans-polybutadiene sequence corresponds to the thermodynamic one. 3.2.3. Propylene Polymerization. It was of interest to extend our calculations to study α-olefin polymerization mediated by lanthanum-grafted complexes. This is of interest because the lanthanide homogeneous complexes are not efficient catalysts for α-olefin (propylene) polymerization because they mainly lead to deactivation products by allylic activation. Moreover, no data were found in the literature concerning the use of lanthanide-grafted complexes as propylene polymerization catalysts. From a theoretical point of view, this is probably due to the fact that propylene insertion is a complicated process. It involves, in principle, the study of four different paths: two main types of insertions, the primary or 1,2 insertion and the secondary or 2,1 insertion, and two different C−H bond activations or hydrogen-transfer reactions, leading to an allylic or vinylic product (Figure 8). Moreover, a prochiral olefin such as propylene may coordinate and insert into a La−C carbon bond via either the re or si enantioface, which

[La]@b-1

ΔrH

A b‑1 TS(Abu,cis−cisb‑1→Bbu,cis−cisb‑1) Bbu,cis−cisb‑1 Abu,cis−transb‑1 TS(Abu,cis−transb‑1→Bbu,cis−transb‑1) Bbu,cis−transb‑1 Abu,trans−cisb‑1 TS(Abu, trans−cisb‑1→Bbu, trans−cisb‑1) Bbu, trans−cisb‑1 Abu,trans−transb‑1 TS(Abu, trans−transb‑1→Bbu, trans−transb‑1) Bbu, trans−transb‑1

−31.7 −14.6 −49.4 −35.4 −14.1 −52.9 −32.0 −18.7 −48.4 −34.7 −18.4 −52.7

bu,cis−cis

Figure 8. Elementary steps explored in the homopolymerization of propylene mediated by lanthanide-grafted complexes.

considerably increases the complexity of these kinds of studies. The aim of this work is to evaluate the catalytic activity of lanthanum-grafted complexes as propylene polymerization catalysts. Thus, the initiation and first propagation steps of the homopolymerization of propylene mediated by [La]@c-1, [La]@c-2, and [La]@b-1 have been considered. In the present study, only the regioselectivity (discrimination between the two insertions and the two possible C−H bond activation reactions) of the polymerization reaction has been studied. Consequently, for the second insertion, in which R or S chiral centers can be generated in the final product, only coordination on the si enantioface has been considered. The enthalpy reaction profiles for the initiation step of the homopolymerization of propylene mediated by [La]@c-1, [La] @c-2, and [La]@b-1 are depicted in Figures 9 and 10. A view of the different transition states and intermediates is available in Figures S7−S9 in the Supporting Information. Concerning the insertion reactions, in all cases, the reaction begins by the formation of an exothermic π adduct in which the propylene is coordinated with the si enantioface to the metal center. It is worth noting that, as for previous monomers, a stability difference is observed according to the steric hindrance around the metal center. Indeed, the higher the steric hindrance, the lower the stabilization energy, with respect to the separated F


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Figure 9. Reaction enthalpy profile of the initiation step of the homopolymerization of propylene mediated by lanthanum complexes mono- and bigrafted onto a SiO2−700 silica surface. [La] refers to fragment Si13O20H17La for [La]@c-1 and [La]@c-2.

preferred regardless of the grafting mode considered. Indeed, the 1,2-insertion barriers are lower, in comparison to those of 2,1 insertion by 3.0, 3.1, and 4.2 kcal mol−1 for [La]@c-1, [La] @c-2, and [La]@b-1, respectively. This difference it is partially due to the higher steric repulsion between the methyl group and the silica surface in the secondary insertion than in the primary one but also from the well-known lower stability of the secondary alkyl anion with respect to the primary ones. To illustrate this point, the transition state structures for the two insertion reactions, for [La]@b-1, are shown in Figure 11. Thus, in the optimized transition states of the primary insertion, there is no particular steric repulsion between the methyl group of the propylene molecule and the silica surface. However, for the secondary insertion, the methyl group points toward the silica surface, which leads to a steric destabilization of the transition state. In the same way, not only are the 1,2insertion barriers lower than those from 2,1 insertion but stabilization of the 1,2-insertion products is also larger by 3.6, 1.5, and 4.0 kcal mol−1 for [La]@c-1, [La]@c-2, and [La]@b-1, respectively. We subsequently investigated two common25 processes leading to deactivation of the catalysts: (i) the vinylic C−H bond activation in which a H atom from CH2 of an incoming propylene is transferred to the coordinated alkyl group (CH3),

Figure 10. Reaction enthalpy profile of the initiation step of the homopolymerization of propylene mediated by a lanthanum complex bigrafted onto a SiO2−200 silica surface. [La] refers to fragment Si14O21H20La(NH3).

reactants. From these adducts, both from kinetic and thermodynamic points of view, 1,2 insertion is strongly

Figure 11. Optimized structures of the transition states for (a) 1,2- and (b) 2,1-propylene insertions into the La−Me bond of [La]@b-1. [La] refers to fragment Si14O21H20La(NH3). G


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Figure 12. Reaction enthalpy profile of the first propagation step of the homopolymerization of propylene mediated by Bpr,1−2c‑1, Bpr,1−2c‑2, and Bpr,1−2b‑1. The label B refers to the final products in Figures 9 and 10

lanthanum complex bigrafted onto a silica surface dehydroxylated at 200 °C, i.e., [La]@b-1, the activation barrier of the allylic C−H bond activation is 4.0 kcal mol−1 higher than those found for the 1,2 insertion. On the other hand, the π−allylic product corresponds to the thermodynamic product regardless of the grafting mode. The second propylene insertion, corresponding to the first propagation step, has also been computed in order to gain deeper insight into the use of grafted complexes as propylene homopolymerization catalysts. For the first propagation step, as for the initiation step, the vinylic C−H bond activation is not competitive because it exhibits higher activation barriers and affords less stable products than the allylic C−H bond activation. In the same way, the 2,1 insertion is also not competitive with respect to the 1,2 insertion. Thus, for the sake of clarity, in the following discussion only the 1,2 insertion and the allylic C−H bond activation has been considered. The enthalpy reaction profiles for the first propagation step of the homopolymerization of propylene mediated by [La]@c-1, [La] @c-2, and [La]@b-1 are depicted in Figure 12. A view of the different transition states and intermediates is available in Figures S10−S12 in the Supporting Information. It is worth noting that, as we can see in Figure 12, the first propagation step follows the same trend as that observed on the initiation step. For this second step, the coordination adducts showed stabilization energies comparable to those obtained for the initiation step. In the same way, all of the reactions are exothermic regardless of the grafting mode. For [La]@c-1 and [La]@c-2, the allylic C−H bond activation of propylene takes place with a low activation barrier, around 12 kcal mol−1 with respect to the π adduct. However, the activation barrier for the 1,2 insertion, around 13.9 kcal mol−1, is close to that of the allylic activation. For [La]@b-1, the activation barrier of the allylic C−H bond activation is 3.1 kcal mol−1 higher than that found for the 1,2 insertion. Thus, in a prospective way, the present calculations show that the lanthanum complexes grafted onto a silica surface dehydroxylated at 700 °C are probably

leading to the formation of a vinyl complex with the concomitant release of a methane molecule; (ii) the allylic C−H bond activation in which a H atom from CH3 of an incoming propylene is transferred to the coordinated alkyl group (CH3), leading to the formation of a η3 complex with the concomitant release of a methane molecule. As for the insertion reactions, these activation reactions begin with the formation of an exothermic π adduct (around −5.6 kcal mol−1 for [La]@c-1, −9.8 kcal mol−1 for [La]@c-2, and −1.7 kcal mol−1 for [La]@ b-1 with respect to the separated reactants). The second-order perturbation NBO analysis reveals, in all cases, a significant electronic delocalization from the double bond of propylene to empty d orbitals of the metal center. From these adducts, it is worth noting that, regardless of the grafting mode, the vinylic C−H bond activation is not competitive because it exhibits high activation barriers and yields less stable products than the two insertion reactions and the allylic C−H bond activation. The vinylic C−H bond activation is less kinetically accessible than the other reactions considered because of the high energy penalty needed for breaking the π interaction between the propylene monomer and metal center. However, concerning the allylic C−H bond activation, the transition states for [La]@ c-1 and [La]@c-2 were located respectively at +14.0 and +14.7 kcal mol−1, above the corresponding π adducts. These barriers are similar to those found for the 1,2-insertion reactions, 14.0 versus 14.9 kcal mol−1 for [La]@c-1 and 14.7 versus 14.5 kcal mol−1 for [La]@c-2. Thus, the allylic C−H bond activation is, from a kinetic point of view, a competitive reaction pathway for lanthanum complexes grafted onto a silica surface dehydroxylated at 700 °C. The low barrier observed for the allylic C−H bond activation can be explained by the stable formation of an allylic-type interaction between the propylene monomer and metal center. However, for [La]@b-1, the high steric hindrance around the metal center induced by the bigrafting mode, leading to a partial insertion into the silica surface, induces a decrease of the allyl−metal interaction strength and, consequently, destabilization of this transition state. Thus, for a H


Inorganic Chemistry

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poor catalysts for the homopolymerization of propylene because a large amount of catalyst should be deactivated by the allylic C−H bond activation of propylene. However, a high catalytic activity is expected by using a lanthanum complex bigrafted onto a silica surface dehydroxylated at 200 °C as the propylene homopolymerization catalyst, indicating the crucial influence of the grafting mode on the selectivity and activity of the catalyst. Therefore, an in-depth study of the stereochemistry of the obtained polymer with [La]@b-1 has to be carried out to finally evaluate the potential of these grafted catalysts.

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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.inorgchem.6b01238. Optimized structures (PDF) Cartesian coordinates (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

4. CONCLUSION

Notes

The authors declare no competing financial interest.

In this study, we have theoretically explored the homopolymerization of ethylene, propylene, and 1,3-butadiene mediated by lanthanum complexes grafted onto a silica surface dehydroxylated at 700 or 200 °C. We have specifically examined the role of the grafting mode, i.e., the impact of the podality of supported lanthanide alkylated complexes on the kinetics and thermodynamics of the ethylene, 1,3-butadiene, and propylene homopolymerization reaction pathways. Therefore, we considered three grafting modes: monografted ([La]@c-1), bigrafted ([La]@b-1), or bigrafted after breaking of a Si−O−Si bridge ([La]@c-2). Our study highlights, in agreement with the experimental results, that alkylated lanthanum-grafted complexes are successful in ethylene polymerization. The ethylene polymerization mediated by lanthanum-grafted complexes are predicted to be kinetically accessible and thermodynamically favorable, regardless of the grafting mode. We have also highlighted that the high flexibility of the monografting coordination allows the La atom to modulate the electronic assistance of the surface through interaction (or not) with the neighboring O atoms of the siloxane bridges. This ability to adjust the assistance of the surface at each step makes the three grafted complexes comparable from a reactivity point of view. The experimental preference for 1,4-cis polymerization of 1,3-butadiene has also been investigated and rationalized. Thermodynamically, regardless of the grafting mode, all of the butadiene insertion products are exothermic. Nevertheless, 1,4trans insertions are slightly more favorable thermodynamically than the 1,4-cis ones but, kinetically, the activation barrier for the 1,4-cis insertions is calculated to be lower than that for the 1,4-trans insertions. The preference for the 1,4-cis insertion is then associated with the less sterically demanding cis insertion over the trans insertion. This study also provides unprecedented information on the catalytic activity of the lanthanide-grafted complex as propylene hompolymerization catalysts. The present calculations show that the lanthanum complexes grafted onto a silica surface dehydroxylated at 700 °C are probably poor catalysts for the homopolymerization of propylene because a large amount of catalyst should be deactivated by the allylic C−H bond activation of propylene. However, catalytic activity is expected using a lanthanum complex bigrafted onto a silica surface dehydroxylated at 200 °C as the propylene homopolymerization catalyst. In this case, the active site is not as accessible as that for complexes grafted onto a SiO2−700 surface. Thus, the transition state leading to the formation of the high stable allylic product is destabilized, which promotes the formation of the 1,2-insertion product.

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ACKNOWLEDGMENTS This work was performed using HPC resources from CALMIPEOS (Grant p0833). REFERENCES

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