Impact of Organoaluminum Compounds on Phenoxyimine Ligands in

Article pubs.acs.org/Organometallics

Impact of Organoaluminum Compounds on Phenoxyimine Ligands in Coordinative Olefin Polymerization. A Theoretical Study Zygmunt Flisak,* Grzegorz P. Spaleniak, and Maria Bremmek Faculty of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland S Supporting Information *

ABSTRACT: The reduction of the phenoxyimine moiety in three individual speciesnamely free ligand, aluminum complex, and titanium complexwith aluminum alkyls and aluminum hydride has been studied by means of DFT. It was demonstrated that the free phenoxyimine ligand in an equimolar mixture with trimethylaluminum does not undergo reduction. Instead, experimentally observed formation of the six-membered cyclic aluminum−phenoxyimine complex, useful in the ring-opening polymerization of lactones, takes place as the kinetically and thermodynamically favored process. However, it is anticipated that a 2-fold excess of the aluminum compound, especially aluminum hydride, acting on the resulting cyclic complex can convert the imine to the aluminum-subsituted amine functionality easily with an energetic barrier of approximately 10 kcal/mol. Finally, the propensity of the imine moiety in the titanium-based precursor of the coordinative olefin polymerization toward reduction with organoaluminum compounds is revealed and the mechanism of this reaction is also suggested.

1. INTRODUCTION Aluminum alkyls and their derivatives have been applied as activators of the coordinative polymerization catalysts, starting from the early TiCl4-based system through heterogeneous catalysts modified with Lewis bases to metallocenes and postmetallocenes. Although methylaluminoxane (MAO) is usually applied to activate titanium and zirconium bis(phenoxyimine) complexes, aluminum alkyls can also be utilized, either in a mixture with MAO1 or by themselves.2 Aluminum alkyls are strong Lewis acids and potent alkylating and reducing agents. Nevertheless, relatively little is known about the processes that take place during catalyst activation with organoaluminum compounds, especially in the case of the classical heterogeneous systems. It is widely accepted that the activation process consists of ligand exchange (alkylation) or halide abstraction, sometimes associated with metal reduction.3 The latter process is less likely when a less effective reducing agent, e.g. MAO, is applied as a cocatalyst.4 Additionallyin heterogeneous systemsthe reduction may be accompanied by possible aggregation of active sites, which is also dependent on the transition-metal content in the system.5 Experimentalists have gathered enough evidence to support the hypothesis stating that the kind of cocatalyst has a profound impact on the performance of an olefin polymerization catalyst. For instance, a bis-phenoxyimine zirconium catalyst treated with methylalumoxane affords a catalyst which displays high activity and produces a polymer of low molecular weight, whereas using the same precursor activated with a mixture of perfluorophenylborate and triisobutylaluminum leads to a © XXXX American Chemical Society

system with significantly lower activity, which in turn produces polymers of a high molecular weight.1 On the other hand, it was demonstrated that for the salan systems, with no CN bonds available, the choice of cocatalyst has little effect on the activity.6 These phenomena can be attributed to possible redox processes that may occur in the polymerization medium. There is strong rationale for the noninnocence of phenoxyimine ligands and their susceptibility to redox reactions during activation with aluminum alkyls reported in the literature. Indirect evidence for the reduction of the imine group incorporated into the ligand in the group 4 transition metal complex with triisobutylaluminum (and its possible contaminant, aluminum hydride) was demonstrated for the phenoxyimine,1,7,8 iminephenoxy,9,10 pyrrolideamine,11 and salen moieties,12,13 although at that time it was not clear whether a free imine group can be reduced at all.11 An example of alkylation of the imine moiety by trimethylaluminum, useful for synthetic purposes, was reported only a few years later.14 However, the mere presence of both a CN group and an organoaluminum compound does not imply reduction in every case. For example, imine group reduction within the titanium salen complex was not observed for trimethylaluminum.12 Furthermore, formation of cyclic, six-membered aluminum complexes having imine CN bonds in the reaction between the free phenoxyimine (also called salicylaldimine or iminophenol) and AlMe315,16 suggests that the acid−base Received: April 17, 2013

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Figure 1. Possible reactions between the phenoxyimine ligand and trimethylaluminum. the B3LYP functional performs relatively well for organic molecules35 and redox processes,36 even those involving transition-metal atoms.37 Together with the 6-31+G(d,p) basis set it has been used as a part of the protocol for calculating redox potentials in organic chemistry.38 The transition states were located using the synchronous transit and quasi-Newtonian method.39,40 The nature of all stationary points, except for those containing the titanium atom, was confirmed by carrying out vibrational analysis. All the values shown in the energetic profiles are electronic energies; Table 1 contains also Gibbs free energies for the selected processes. In all calculations, the Gaussian 09 suite of programs41 was employed with default convergence criteria. For SCF, the density-based criterion was set to 1.0 × 10−8 au. For geometry optimization the following thresholds were chosen: a maximum force of 4.5 × 10−4 au and a maximum displacement of 1.8 × 10−3 au.

reactions involving aluminum alkyls are competitive with the reduction processes and may take priority. From experimental work, it is unknown whether the phenoxyimine ligand in such complexes can undergo subsequent reduction to the amine species. Theoretical works rarely deal with the interactions of the cocatalyst with other components of the catalytic system, monomers and polymers. The first reports on this subject described the processes of alkylation and reduction of titanium chlorides with aluminum alkyls and their derivatives treated at the HF or DFT level.17−19 Later, the scope of the investigation was extended to a larger number of interacting species and possible reactions;20,21 calculations for the reduction of titanium and vanadium chlorides with aluminum alkyls at a higher level of theory were also carried out.22 Even more surprising is the fact that the experimentally widely applied reduction of unsaturated organic compounds, e.g. ketones, aldehydes, and imines, with lithium aluminum hydride or sodium borohydride have received less attention from theoreticians than is deserved. Only recently has there been an attempt to elucidate the mechanisms of ketone reduction with LiAlH4 and NaBH423,24 as well as the reaction of oximes with diisobutylaluminum hydride.25 The aim of the present work is to determine the energetic profiles for certain plausible reactions between the phenoxyimine ligand (both in its free form and bound to aluminum and titanium) with organoaluminum compounds by means of DFT calculations. Emphasis is placed on the reduction processes which are thought to modify the properties of the phenoxyimine-based olefin polymerization catalysts.

3. RESULTS AND DISCUSSION Within this work, we have investigated several possible reactions: formation of a six-membered four-coordinate aluminum phenoxyimine from a free phenoxyimine ligand and trimethylaluminum as reported by Nomura,15 competitive reduction of the CN double bond, further reduction of the phenoxyimine−Al cyclic complex with an excess of organoaluminum compound, and reduction of the phenoxyimine ligand within the titanium complexa precursor of the coordinative olefin polymerization catalyst. 3.1. Reactions of the Ligand with AlMe3. Phenoxyimines, apart from exhibiting an intramolecular hydrogen bond, may exist as two tautomers: phenol-imine (O−H···N) and keto-amine (O···H−N). The former is usually preferred,42 with a relatively small proton transfer barrier influenced by both the nature of the molecule and the solvent.43 According to these experimental findings, the starting point for our calculations will be a phenol-imine form of Nmethylsalicylaldimine (I; see Figure 1). The aluminum alkyl can coordinate either to the oxygen or to the nitrogen atom of the phenoxyimine, forming II or III, respectively. Both processes are exoenergetic and barrierless, but the formation of II is thermodynamically preferred (see Figure 2). This species can then undergo a proton transfer with a very low barrier to yield the O···H−N form IV, whose energy is lower by

2. COMPUTATIONAL DETAILS The B3LYP density functional26,27 was applied in all calculations. To describe the lone electron pairs present in the molecules of ligands and participating in the complex formation reactions, a basis set with a diffuse function had to be used. Taking into account these considerations, and aiming at a reasonable accuracy and acceptable computational cost, especially for species containing several aluminum atoms, we selected the 6-31+G(d,p) basis set28−31 for all atoms except for titanium, where the LANL2TZ basis set32 obtained from the EMSL Basis Set Library33,34 was used. Despite its well-known shortcomings, B

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much as 10 kcal/mol above I and probably can be neither isolated nor detected. Had it existed, it would have had properties different from those of I. Our calculations indicate thatunlike Iits reaction with AlMe3 would preferrably lead to the Al−N complex located 15 kcal/mol below the reactants, whereas only 8 kcal/mol would be released as a result of the Al−O species formation. This example confirms the wellknown fact that the presence of hydrogen bonds can control the course of the chemical reaction.46 Concluding this part of our study, we can state that, independent of the starting materials (I, I′) or intermediate products (II, III), the reaction between the phenoxyimine and AlMe3 always leads to V and the formation of the sixmembered four-coordinated aluminum phenoyximine complex is thermodynamically and kinetically preferred with respect to the competitive process of salicylaldimine reduction. This theoretical result is consistent with the experimental findings.15 3.2. Reducing Strength of Aluminum Compounds. In the following, we envisage and anticipate the possible reactants and conditions that might potentially lead to the reduction of the carbon−nitrogen double bond in the phenoxyimine molecule. The reduction processes with selected aluminum compounds involve the intermediate complexes III, VII, and IX (see Figure 4); it must be mentioned, however, that these are Figure 2. Energetic profiles of the reactions beetween the phenoxyimine ligand and trimethylaluminum. Solid and dashed lines represent the formation of the six-membered aluminum phenoxyimine complex; the dotted line corresponds to phenoxyimine reduction.

almost 7 kcal/mol in comparison with II. The reaction then proceeds through the transition state TSIV-V to the cyclic aluminum complex V with the liberation of methane. If, according to the Boltzmann distribution, we allow the formation of small amounts of III, then there are two parallel reactions possible. One of them leads directly to V with a barrier of only 12 kcal/mol (TSIII-V). The other is the reduction associated with the methyl group transfer,44,45 prevented by the significantly higher barrier of ca. 26 kcal/ mol (TSIII-VI), and yielding the Al-substituted amine species VI. It would be interesting to examine the possible reaction paths starting from the phenoxyimine keto-amine form I′ (see Figure 3). According to our calculations, it is located only 4 kcal/mol

Figure 4. Reduction of phenoxyimine I with AlMe3, AlEt3, and AlH3.

not thermodynamically preferred, in comparison with the compounds containing the aluminum atom attached to oxygen similar to II (see Figures 1 and 2). Furthermore, all reduction products (VI, VIII, X) are stabilized by the Al−O interactions, leading to tetrahedral coordination of the metal. Depending on the kind of aluminum compound used as a reducing agent, there are three possible scenarios of phenoxyimine reduction, resulting in amines with an Alcontaining substituent at the nitrogen atom (see Figure 4). First, if trimethylaluminum is applied, alkyl transfer to the imine carbon atom takes place, leading to VI as discussed in the previous section. Second, using AlEt3 or higher aluminum alkyls, there is the possibility of hydrogen transfer from an alkyl group to the imine carbon atom and the amine VIII is produced with the release of ethylene. This reaction resembles

Figure 3. Phenoxyimine: phenol-imine (I), keto-amine (I′), and nohydrogen-bond (I″) forms.

above the phenol-imine form with an interconversion barrier of only 5.6 kcal/mol. It is not surprising that the main intermediate product, which I′ can form in the reaction with trimethylaluminum, is IV; due to the higher energy of the starting keto-amine form I′ in comparison with I, this product is even more stabilized. Therefore, we think that the tautomeric equilibrium has no impact on the outcome of the reaction between the phenoxyimine and trimethylaluminum. The hydrogen bond in phenoxyimines is relatively strong. It turns out that the I″ conformer (see Figure 3) is located as C

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the β-hydrogen transfer from the growing polymer chain to the olefin, which constitutes the termination process in coordinative olefin polymerization;47−51 the structures of the corresponding transition states are also similar. We believe that the ethyl group transfer, comparable to the III → VI reaction, is also possible in this case, but we expect its barrier to be relatively high. Finally, if aluminum hydride is used, hydrogen transfer from aluminum to the imine carbon atom occurs, yielding the amine X. The energetic profiles of the reactions mentioned above (see Figure 5) indicate that the three aluminum compounds differ in

reducing strength toward the phenoxyimine. The reaction with trimethylaluminum is impeded by a relatively large barrier of 26.4 kcal/mol. The reduction barrier can be lowered by 3 kcal/ mol if one replaces AlMe3 with AlEt3. In this case, the transition state is located slightly lower on the potential energy surface; destabilization of the intermediate VII with respect to III does not exceed the error of the computational method. According to our calculations, aluminum hydride should be the most efficient reducing agent, with a barrier of 20.1 kcal/mol, despite strong stabilization of the intermediate IX in comparison with III and VII. It must be mentioned that in this case the energetic barrier is comparable to that corresponding to the formation of the cyclic phenoxyimine aluminum complex from the phenoxyimine and AlMe3 (I → V). 3.3. Reactions of the Cyclic Al−FI Complex with Excess AlMe3. It should be pointed out that the reaction between trimethylaluminum and phenoxyimine reported in reference 15 was carried out primarily under stoichiometric conditions or with a 2-fold excess of AlMe3. Herewith we will attempt to answer the question whether an excess of organoaluminum compound promotes prospective reduction of the imine carbon−nitrogen double bond. The starting point here is the product of the stoichiometric reaction between the phenoxyimine ligand and trimethylaluminum, treated with an excess of the latter; i.e., we consider the stepwise process leading from I to V and further to other products (see Figures 1 and 6). At the initial stage of these considerations, we wanted to examine the stability of the Al−N coordinative bond. The ring-opening product XI lies ca. 30 kcal/mol above V; it is strongly thermodynamically unfavored and unlikely to be found; therefore, we decided neither to calculate the energy of the corresponding transition state nor to follow any further products derived from it. The stability of V and its resistance to ring opening are dictated not only by the strength of the Al−N coordinative bond but also by the propensity of aluminum toward the formation of fourcoordinated rather than three-coordinated compounds. However, exposing V to the trimethylaluminum molecule yields the

Figure 5. Energetic profiles of the reactions beetween phenoxyimine and AlMe3 (dotted line), AlEt3 (dashed line), and AlH3 (solid line).

Figure 6. Possible reactions involving the cyclic Al−FI complex V. D

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species XII, in which AlMe3 is attached to the oxygen atom and the Al−N bond is weakened, which facilitates the process of ring opening, leading first to XIIIA, in which the nitrogen atom is pointing toward the aluminum, and then, after rotation along the C−C bond, to XIII. The alternative attack of the AlMe3 molecule at the nitrogen atom of V produces no energetic effect, probably due to the lack of an available free electron pair on nitrogen. It should be mentioned that the compound XII was also obtained and characterized by the Nomura group.15 The first step of the reaction of V with AlH3 also involves coordination of the aluminum hydride to the oxygen atom. Another AlH3 molecule has restricted access to XIV, since the nitrogen atom in V is not reactive toward organoaluminum compounds (vide supra), and the vicinity of the oxygen atom is sterically crowded. Indeed, attaching another AlH3 molecule to XIV is only slightly exoenergetic at only −2 kcal/mol. However, the reducing agent can attack the carbon−nitrogen double bond in XIV and the imine species can be reduced to form the corresponding amine derivative XV with a remarkably low barrier of only 9.6 kcal/mol. Hereby we have demonstrated that the reduction of the aluminum cyclic complex V by means of aluminum hydride is thermodynamically and kinetically possible. It should be stressed that proceeding from I to V to XV, i.e. performing the reduction of the phenoxyimine ligand, requires at least a 3fold excess of aluminum with respect to the phenoxyimine. 3.4. Reactions of the Titanium Bis(phenoxyimine) Complex with Organoaluminum Compounds. In the final part of this work we will briefly address the reduction of the phenoxyimine moiety in the octahedral titanium bis(phenoxyimine) complex during catalyst activation process. This complex exhibits isomerism, and in solution several isomers may exist in equilibrium; here we will be dealing with the most stable species,52−54 denoted as XVI. It was demonstrated theoretically that the cleavage of the metal−ligand bond in an example of the octahedral complexes with the ligands containing atoms of different donor numbers takes place easily,55 and we suspect that the titanium bis(phenoxyimine) complex XVI behaves in a similar way. Indeed, the species with the dangling imine group XVII is located only 5.4 kcal/mol higher than the starting material. We believe that this imino group, treated with AlH3, could undergo further reduction to yield the corresponding amine. On the other hand, when XVI is exposed to AlH3, the aluminum hydride attaches preferentially to the nitrogen atom, unlike in both the free phenoxyimine I and the cyclic aluminum complex V, where the attacks at the oxygen are clearly preferred (see Figures 2 and 7). This reaction, proceeding with the simultaneous cleavage of the Ti−N coordinative bond and subsequent rotation along the C−C bond, leads to XVIII, which is located 18.8 kcal/mol below the reactants. Next, the reduction of the imine group can occur with the relatively high, but still surmountable, energetic barrier of 24.5 kcal/mol. The product is located 35.2 kcal/mol below the reactants and exhibits strong O−Al interactions that assist the aluminum atom in regaining its tetrahedral coordination. It must be mentioned that the set of reactions suggested and depicted in Figure 8 is probably far from being complete. For example, one can envisage possible reduction of the imine group mediated by the lower-oxidation-state titanium species that can possibly be formed in the reaction between XVI and the organoaluminum compound. However, the experimental

Figure 7. Energetic profiles of the selected reactions of the cyclic FI− Al complex V.

Figure 8. Possible reactions involving the titanium bis(phenoxyimine) complex XVI.

data on this subject are too vague to support theoretical calculations. 3.5. Effect of Entropy on Selected Processes. It is widely known that the trimethylaluminum dimerization process is highly exoenergetic and DFT methods underestimate its energetic effect.56 Our calculations do not take into account the energy required to dissociate the dimer and start from the monomeric aluminum compounds. One should be aware that the entropic term assists in the dissociation of these species, even the most exoenergetic (AlH3)2. Therefore, we expect that sufficient amounts of monomeric forms can be available to participate in the reduction process, especially at elevated temperatures. On the other hand, entropy counteracts the reaction of the organoaluminum compound with the free phenoxyimine, cyclic aluminum complex, and titanium complex E

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evidence for the feasibility of the bis(phenoxyimine)zirconium complex reduction may now be supported by our calculations for the titanium case. While the phenoxyimine might intuitively seem susceptible to reduction processes, noninnocent behavior of the virtually less reactive tetrahydrofuran ligand in a heterogeneous Zigler− Natta system has been demonstrated very recently,58 even without the participation of the aggressive organoaluminum compound. Thus, on the grounds of our calculations although it was imposible to predict and consider every viable interaction between the reactantswe can claim that phenoxyimine can be considered as being a noninnocent ligand in the coordinative olefin polymerization process, subject to further experimental verification.

(see Table 1); therefore, the entropic term should have little effect on the overall reaction. Table 1. Thermodynamics of Selected Processes Involving Phenoxyimine Ligands and Organoaluminum Compounds at 298 Ka process I→V

I→X

V → XV

XVI → XIX

species

E, kcal/mol

G, kcal/mol

I + AlMe3 II TSII-IV IV TSIV-V V I + AlH3 IX TSIX-X X V + 2AlH3 XIV TSXIV−XV XV XVI + AlH3 XVIII TSXVIII-XIX XIX

0.0 −12.6 −12.6 −19.4 −1.6 −56.6 0.0 −15.1 5.0 −31.1 0.0 −19.5 −9.9 −54.8 0.0 −18.8 5.7 −35.2

0.0 0.2 1.1 −6.5 12.9 −52.7 0.0 −3.7 17.5 −19.3 0.0 7.9 17.5 −27.4

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ASSOCIATED CONTENT

S Supporting Information *

Tables giving Cartesian coordinates of the optimized structures I−XIX. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.F.: [email protected]. Notes

a

The authors declare no competing financial interest.

Electronic energies and Gibbs free energies are relative with respect to substrates.

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ACKNOWLEDGMENTS The Wroclaw Supercomputing and Networking Centre as well as Academic Computer Centre CYFRONET AGH (Grant No. MNiSW/IBMBCHS21/UOpolski/005/2009) are acknowledged for a generous allotment of computer time and software.

It is also important to note that, while the entropic term cancels the negative energy of the AlR3 association to either the phenoxyimine ligand or its complexes, the barriers of the reactions leading to the final products (V, X, and XV) expressed in terms of ΔE and ΔG are approximately equal.

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4. CONCLUSIONS We have investigated the phenoxyimine moiety reduction by organoaluminum compounds in the following three starting materials: the ligand itself, the aluminum phenoxyimine complex, and the titanium bis(phenoxyimine) complex. The calculations were carried out for a selected set of reactions, and we are aware that there might be certain competitive processes that provide lower-energy pathways. We assumed that the reduction process can proceed according to the three possible scenarios, depending on the nature of the organoaluminum compound applied: methyl group transfer and two kinds of hydrogen transfers (either from the alkyl group or from the aluminum atom). The results of our calculations indicate that aluminum hydride is the most potent reducing agent. We have demonstrated that all of the starting materials can be reduced by an excess of aluminum hydride and the most preferred reaction would be the reduction of the aluminum complex. It must be mentioned that certain processes discussed within this paper require more than 1 equiv of organoaluminum compound with respect to the phenoxyimine moiety. Nevertheless, a much higher excess of cocatalyst, well beyond the stoichiometry required by its possible function, is practically always encountered in a real system.3,57 Finally, it should be stressed that our calculations rationalize the existence (and preference over some competitive species) of the two compounds that were characterized experimentally, namely V and XII,15 and predict an easy route to the reduced species XV. Apart from that, strong but indirect experimental

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dx.doi.org/10.1021/om4003347 | Organometallics XXXX, XXX, XXX−XXX