Donor Complexes on the Surface

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Article 4

Spectroscopic Evidences for TiCl-Donor Complexes at the Surface of MgCl-Supported Ziegler-Natta Catalysts 2

Alessandro Piovano, Maddalena D'Amore, * Thushara K S, and Elena Groppo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00903 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The Journal of Physical Chemistry

Spectroscopic Evidences for TiCl4-Donor Complexes at the Surface of MgCl2-Supported Ziegler-Natta Catalysts Alessandro Piovano, Maddalena D’Amore,* K. S. Thushara, Elena Groppo* Department of Chemistry, INSTM and NIS Centre, University of Torino, via Giuria 7, 10125 Torino (Italy). ABSTRACT: ABSTRACT: In this work we addressed the question of the location of the electron donors (we chose ethylbenzoate, EB, for practical reasons) at the surface of MgCl2-supported Ziegler-Natta catalysts, by synergistically coupling: a) an in situ investigation of the EB adsorption process by FT-IR spectroscopy; b) an evaluation of the MgCl2 surfaces available for the adsorption of CO as a molecular probe at each step of the pre-catalyst synthesis; c) an accurate quantum mechanical DFT-D study of a few TiCl4-EB complexes. Our experimental data indicate that homogeneous-like TiCl4-EB complexes are formed, loosely bonded to the MgCl2 surfaces, as a consequence of a rather high mobility of the adsorbed EB and TiCl4. Our DFT-D computational results demonstrate that monomeric TiCl4(EB) and TiCl4(EB)2 complexes might indeed exist on different catalytically relevant MgCl2 surfaces (although steric repulsion do not allow reaching a full surface coverage), whose computed IR spectra are highly compatible with the experimental ones. The whole set of data converge to a final scenario whereby the internal donor has the function to induce a certain mobility for TiCl4 simultaneously acting as a “surfactant”, in certain cases providing a particularly exposed TiCl4 species readily available for further reaction with the aluminium alkyl activator. The relevant role of the MgCl2 support in assisting the self-organization of the catalyst components clearly emerges.

1. INTRODUCTION The addition of third components to improve the catalyst performances (in terms of activity, selectivity, life-time and so on) is a very diffuse practice in heterogeneous catalysis. These substances (often simple molecules) are called dopants, promoters, modifiers, or even poisons, depending on the type of catalyst. In most of the cases, their use in the industrial practice is the result of an empiric optimization of the catalyst preparation, but the exact location of these third substances with respect to the other catalyst components (typically, the support and the active phase) is often unclear. This is particularly true in the field of MgCl2-supported ZieglerNatta catalysts for olefin polymerization, where the introduction in the catalyst composition of the so called electron donors (Lewis bases molecules) dates back to the 1960s, and led to significant improvements both in the catalyst productivity and stereospecificity.14 Today, thanks to the large choice of electron donors, ZieglerNatta catalysts are highly versatile systems which have not only the monopoly of the industrial production of isotactic polypropylene, but also cover many other sectors in the polyolefin market. The electron donors, that can be added during the catalyst preparation (internal donors, ID) and/or together with the cocatalyst and the monomer during the olefin polymerization (external donors, ED), significantly modify the catalysts’ functioning. Not only they do increase the stereoselectivity, but also influence the regioselectivity, the hydrogen response (and hence the molecular weight distribution of the polymers), the catalytic active sites distribution and the morphology of the catalyst.2,5-24 It turns out that the electron donors are remarkably powerful instruments to tune the performances of Ziegler-Natta catalysts and the properties of the produced polyolefins. This is the reason why in the last decades most of the industrial research in this field was directed to the screening of new electron donors, and to the optimization of new ID/ED couples.23,24 Remarkably well-performing formulations have been achieved, but nevertheless the final structure of the catalyst

and in particular the location of the electron donors and their relationship with the other catalyst components remain elusive. A lot of experimental25-44 and computational20,44-60 work has been done to shed light on the structure of Ziegler-Natta catalysts at a molecular level, with particular emphasis on the localization of the electron donors and on their influence on the structure of the active sites. Unambiguous interpretation of the results is difficult due to the extreme complexity of the catalyst formulation. It is known that the electron donors exert multiple interaction with all the other catalyst components, including the support (MgCl2), the active phase (TiCl4) and the co-catalyst (usually an aluminium alkyl or an aluminium alkyl chloride), the extent of these interactions heavily depending on the catalyst preparation.52 The picture is further complicated if the dynamic behaviour of these catalysts is considered. Indeed, there are evidences that the electron donors are mobile on the MgCl2 surface, and their mobility affects the structure and the performances of the catalyst.52,55,61 As far as the role and the states of the IDs is concerned, several contrasting opinions have been formulated since the early history of these catalysts. According to some authors, the ID coordinates directly on a vacancy of the titanium sites, thus creating new active (and stereo-specific) centres.9,56,62-67 This hypothesis is in agreement with many experimental observations on the produced polymer, the direct detection of TiCl4/ester complexes on MgCl2 by EXAFS spectroscopy,68 and further corroborated by the fact that homogeneous complexes are formed between TiCl4 and many Lewis bases molecules. However, it is weakened by the finding that the IDs react with the aluminium alkyls and they can also be extracted from the catalyst in different amount.24 Hence, their direct role in influencing the catalyst selectivity can be questioned. At the other extreme, it was proposed that the ID only interacts with the MgCl2 support, indirectly controlling the location and the distribution of the TiCl4 species on the MgCl2 surfaces and preventing the formation of aspecific sites. This model, proposed first by Corradini and accepted by most of the scientific community for many dec-

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ades,27,69-71 is now starting to decline on the basis of the new and always more accurate computational studies, which indicate that the interaction of TiCl4 with the MgCl2 surfaces is very weak.46,72 Between these two hypothesis, a third one (indicated as “coadsorption model”) implies that the ID coordinates to the MgCl2 surface in the near proximity of -and with non-bonding interaction with- the titanium active sites, influencing their steric and electronic properties.24,46,47,73,74 According to this model, the different coordination modes and the flexibility of the ID molecule might be responsible of the large variety of stereospecific active sites. Although at present the latter hypothesis seems to be the most reliable model to explain the role of IDs in stereo-regulation, there is not a consensus in the scientific community. In this work, we focus our attention on the adsorption of ethyl benzoate (EB) on MgCl2 and MgCl2/TiCl4 pre-catalyst and we address the question of the location of EB by synergistically coupling an experimental and a computational approach.75 Although nowadays EB is not the most popular ID, it was chosen here for practical reasons (e.g., high vapour pressure) and for its simplicity (i.e., just one ester binding group). Moreover, the number of experimental works performed on EB largely exceeds those performed on any other electron donor, allowing an easy comparison with the results in the literature. Our experimental strategy consists in the in situ preparation of a highly disordered MgCl2 by controlled dealcoholation of the MgCl2·6MeOH adduct (MeOH = methanol), mimicking the routinely adopted methods to synthesize industrial heterogeneous Ziegler-Natta catalysts,76,77 followed by adsorption of EB from the vapor phase (prior or after TiCl4 chemisorption). The whole process is followed in situ by FT-IR spectroscopy as a function of the EB coverage. FT-IR spectroscopy has long been used as a powerful tool for investigating a variety of surface species in catalysis,78-82 and it turns out to be particularly efficient in the study of the adsorption of electron donors on Ziegler-Natta catalysts. 27,31-33,36,38,46,83,84 Indeed, most of the electron donors have chemical groups (such as the carbonyl group) with intense characteristic absorption bands, that have been traditionally used as markers of complexation. Complementary information on the location of EB on MgCl2 are obtained by means of FT-IR spectroscopy of CO adsorbed at 100 K. We have recently demonstrated that this is a powerful method to experimentally probe the exposed MgCl2 surfaces and to determine the relative proportion of the pentacoordinated and tetra-coordinated ones.75,85 Indeed, the (CO) values for adsorbed CO are sensitive indicators of the coordinative unsaturation and polarizing ability of the Mg2+ cations, and hence of their Lewis acid strength. The experimental work was complemented with an accurate quantum mechanical DFT-D study of a few TiCl4-EB complexes at the MgCl2 surfaces, chosen on the basis of previous knowledge and of the new experimental results, aimed at determining their stability and vibrational properties. We wish to underline that our combined experimental and computational approach presents several elements of novelty in this field. 1) Although FT-IR spectroscopy has been largely used in the past to characterize several electron donors on MgCl2supported Ziegler-Natta catalysts, the majority of works report exsitu collected FT-IR spectra,27,31-33,36,38,46,83,84 while only a few of them concern an in situ investigation of the electron donor adsorption process.40,65,86 There are no other works in the literature reporting the use of FT-IR spectroscopy of CO adsorbed at 100 K on MgCl2 and MgCl2/TiCl4 before and after the adsorption of electron donors. On the basis of the whole set of experimental and computational data reported in this work it might be worth of reconsidering the hypothesis formulated in the past on the location of IDs on MgCl2 and on their interaction with the TiCl4 species. Although the

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results are limited here to the case of EB, the same approach can be easily extended to more industrially relevant electron donor molecules, and also the effect of the aluminium alkyl on the MgCl2/EB/TiCl4 system will be investigated in the next future.

2. EXPERIMENTAL SECTION 2.1 Materials MgCl2 powder mesh, anhydrous methanol, hexane, ethyl benzoate (EB), TiCl4, and toluene (all from Sigma – Aldrich) were used as received. All the synthesis were performed directly into the cell used for the characterization, in order to avoid poisoning and to control each step by FT-IR spectroscopy. Synthesis of the support (MgCl2) The MgCl2·6MeOH adduct was synthesized following a distillation azeotropic method as described elsewhere.87 The activated MgCl2 was obtained through a controlled de-alcoholation of the MgCl2·6MeOH adduct in dynamic vacuum (5 10-4 mbar) at 473 K for prolonged time, as reported in our previous work.75,85 The resulting MgCl2 is constituted by small and highly disordered nanocrystals and has a surface area of 100 m2/g. 2.1 Synthesis of the binary (MgCl2/EB) and ternary (MgCl2/EB/TiCl4) systems. EB was dosed step-by-step on the MgCl2 support directly from the gas phase at room temperature, up to saturate the MgCl2 surface but not in excess (i.e. avoiding the formation of a liquid layer of EB). The so obtained MgCl2/EB system was successively heated in dynamic vacuum at 373 K, in order to remove eventual residues of physisorbed EB. Then, the binary system was titanated in the presence of TiCl4 vapors at room temperature to give the ternary MgCl2/EB/TiCl4 system, which was again degassed at 373 K to remove all the species not strongly bound to the surface. Synthesis of the pre-catalyst (MgCl2/TiCl4) and of the ternary (MgCl2/TiCl4/EB) systems The MgCl2/TiCl4 pre-catalyst was prepared by titanating the MgCl2 support in the presence of TiCl4 vapours at 353 K, followed by degassing at the same temperature, resulting in a final titanium loading close to 1.0 wt% with respect to MgCl2.75 The pre-catalyst was successively interacted with EB from the gas phase at room temperature as described above, resulting in the MgCl2/TiCl4/EB ternary system. Synthesis of the (TiCl4-EB)2 complex. A few millilitres of pure TiCl4 and ethyl benzoate were added simultaneously dropwise into a flask containing anhydrous hexane at room temperature in inert atmosphere, following a wellestablished general procedure for the formation of coordination complexes of titanium tetrahalides.88 Precipitation of a yellow powder occurred from the very beginning of the addition. The powder was filtered, washed several times with hexane, and dried under dynamic vacuum at room temperature. A FT-IR spectrum in ATR mode was collected at the end of the synthesis to check the stoichiometry of the complex, with reference to the literature data.89 2.2 Characterization methods FT-IR spectra were recorded in transmission mode on a Bruker Vertex70 spectrophotometer equipped with a MCT detector, at a resolution of 2 cm-1. The samples were measured as self-supporting pellets inside a quartz cell equipped with KBr windows, allowing to perform thermal treatments in high vacuum or in the presence of gases. The standard procedure was as follows. A thin self-


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supporting pellet of the MgCl2·6MeOH adduct is prepared in the glove box and placed inside the measurement cell, where it is gradually de-alcoholated as described in Ref. 75, and successively treated in the presence of EB and/or TiCl4 as described above. EB and/or TiCl4 are dosed step-by-step, and FT-IR spectra are collected at increasing EB and/or TiCl4 coverage. All the FT-IR spectra are reported after subtraction of the spectrum of neat MgCl2, which contains only a few absorption bands due to small amount of methanol residues (or its decomposition products),75 likely occupying the defects of the nano-crystals. The surface properties of MgCl2, MgCl2/EB, MgCl2/EB/TiCl4 and MgCl2/TiCl4/EB samples were successively investigated by FT-IR spectroscopy of CO adsorbed at 100 K. The procedure is as follows. CO is dosed at room temperature (equilibrium pressure PCO = 50 mbar), and the temperature is gradually decreased down to 100 K. FT-IR spectra are collected at 100 K at different degrees of CO coverage (θCO), starting from CO full coverage and gradually decreasing the equilibrium pressure through controlled expansions. All these FT-IR spectra are shown after subtraction of the spectrum of the same sample collected before the CO dosage.

3. COMPUTATIONAL METHODS All calculations were performed using the periodic ab-initio CRYSTAL17 code90 based on atom-centred Gaussian basis set, which allows the accurate and easy evaluation of the exact HartreeFock (HF) exchange, and hence is particularly well suited to treat hybrid functionals with extremely high efficiency. The simulations were done resorting to density functional approximation (DFT) methods, adopting the hybrid Becke, three parameter, Lee−Yang−Parr (B3LYP) functional,91 which provides excellent simulation of the structure and vibrations for crystalline materials and adsorbed molecules. An intrinsic drawback of all common Generalized Gradient Approximation (GGA) functionals, including hybrids, is that they cannot describe long-range electron correlations that are responsible for van der Waals (dispersive) forces. The overwhelming role of dispersion interactions in adsorption processes has been widely identified.92 For this reason, an empirical dispersion correction to the energy and gradients was applied, according to the DFT-D method proposed by Grimme.93 The parameters modified by Civalleri and co-workers for crystalline systems, often reported as B3LYP-D*,94 were adopted instead of the Grimme’s standard set of parameters. The same computational strategy was successfully adopted by some of us in a number of recent papers on the systems under investigation.54,75,85,95 For the Mg and Cl atoms we adopted the “customized” TZVP quality basis set.95 For C and O atoms we used the Ahlrichs TZV plus polarization bases,96 whereas the VTZP97 functions have been adopted for H atoms. The Gauss−Legendre quadrature and Lebedev schemes were used to generate angular and radial points of a pruned grid consisting of 99 radial points and a maximum number of 1454 angular points in the region of chemical interest over which electron density and its gradient were integrated.98 Tolerances that control the Coulomb and exchange series in periodical systems were set to values 7 7 7 7 20. All the bi-electronic integrals, Coloumb and exchange, were evaluated exactly. For all the B3LYP-D calculations, 10 K points were adopted. For our 2D periodic systems internal coordinates were optimized using the analytical gradient method to optimize the atomic positions. Calculations were performed on two of the thermodynamically most stable surfaces in the environmental conditions of operation, as determined in our previous calculations,75,85,95 namely the (110) and the (107) surfaces. Different possible complexes of TiCl4 with one or two molecules of EB have been considered. Adsorption of

these complexes on the α-MgCl2 surfaces were simulated by resorting to the slab approach. The adsorption energies were calculated as ΔEads(hkl) = EMgCl2(hkl)+complex – Ecomplex – EMgCl2(hkl), where EMgCl2(hkl)+complex is the energy of the complex adsorbed on each hkl surface of α-MgCl2 as obtained after a careful slab optimizations, Ecomplex is the energy of the isolated TiCl4-EB complex in gas phase, and EMgCl2(hkl) is the energy of the corresponding naked α-MgCl2 surface.75,85,95 In particular, a super-cell 2x2 was built to model a degree of coverage θ = 0.25 on the (110) surface, whereas a 3x1 super-cell was built to model a degree of coverage θ = 0.67 for the (107) surfaces, corresponding in both cases to the maximum coverage still allowing the chemisorption of the TiCl4-EB complexes. Moreover, in order to achieve well converged surface energies and consequently reliable results, it was necessary to build very thick MgCl2 slabs, in particular for the highly unsaturated (110) planes. Taken together, these two requirements brought to very large models consisting in hundreds atoms and a huge number of electrons. CRYSTAL17 code in its massive parallel version (MPP), that has been specifically tailored to achieve an excellent scalability in terms of both speed-up and memory usage,99 was mandatory to investigate such large systems. To inspect the molecular vibrations of the TiCl4-EB complexes at the MgCl2 surfaces, the geometry optimization was followed by harmonic vibrational frequency calculations. Harmonic frequencies were computed with the same CRYSTAL17 code at Γ point, whereas the intensity for each normal mode was estimated by calculation of dipole moment variation along the mode through the Berry phase method.100 For the simulation of the IR spectra, only a fragment around the metal-organic adduct was considered for constructing the reduced Hessian matrix. This computational method has been extensively validated for accurate prediction of IR spectroscopic features of adducts on surfaces.

4. RESULTS AND DISCUSSION 4.1. Interaction of EB with MgCl2 Figure 1 shows the FT-IR spectra of liquid EB (Figure 1a) in comparison with the spectra collected upon EB adsorption on activated MgCl2 (Figure 1b) and the successive reaction of the MgCl2/EB sample with TiCl4 at room temperature (Figure 1c).The characteristic FT-IR absorption bands of ethyl benzoate are located in the 1700 – 1000 cm-1 spectral region, as shown in Figure 1a. In particular, the band at 1713 cm-1 is assigned to (C=O), while that at 1270 cm-1 is due to the (C-O-C) vibrational mode. Most of the other weak bands in the spectrum are related to the vibrations of the phenyl ring, except for a few of them which are associated to the vibrational modes of the ethyl group (e.g. the intense band at 1103 cm-1 is associated to the wagging of CH3). It is widely documented that the positions of the (C=O) and (C-O-C) absorption bands substantially change upon EB complexation, while the others are much less sensitive. Figure 1b reports the sequence of FT-IR spectra collected during the progressive adsorption at room temperature of EB on activated MgCl2 as a function of the EB coverage (from spectrum 1 to spectrum 5), while spectrum 6 was collected after degassing the sample at 373 K. It is worth noticing that the EB dosage was limited in order to avoid the presence of loosely coordinated EB. Upon EB adsorption on activated MgCl2 the above mentioned (C=O) and (C-O-C) absorption bands shift in position and become more complex with respect to liquid EB. In particular, the (C=O) band downward shifts to about 1700 cm-1, with the appearance of a



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that proposed by Potapov et al.31 The results are summarized in Table 1 and Figure S1. Three Gaussian bands, located around 1700, 1675 and 1650 cm-1, were sufficient to satisfactorily fit the broad (C=O) band. Absorption bands in very similar position were previously assigned to EB coordinated to penta-, tetra- and tricoordinated Mg2+ cations, respectively.31 The relative intensity of the three components was found to change from sample to sample, depending on the synthesis procedure. Table 1. Integrated band intensity as determined by the deconvoludeconvolution of of the FTFT-IR spectra, for MgCl2/EB, MgCl2/EB/TiCl4 and MgCl2/TiCl4/EB samples. Sample

16801680-1675 cm-1 Strong acid Mg2+

1650 cm-1 Defect Mg2+

12.7

41.0

2.0

MgCl2/EB/TiCl4

0.0

37.7

2.1

MgCl2/TiCl4/EB

10.2

22.0

1.6

MgCl2/EB

Figure 1. Part a): FT-IR spectrum of liquid EB in the 1700 – 1000 cm-1 spectral region. Part b): sequence of background subtracted FT-IR spectra collected during adsorption of EB on activated MgCl2 as a function of the EB coverage (from spectrum 1 to spectrum 5). Spectrum 6 was collected after degassing at 150 °C, and corresponds to the MgCl2/EB sample. Part c): sequence of background subtracted FT-IR spectra collected during adsorption of TiCl4 at room temperature on the MgCl2/EB sample (from spectrum 6 to spectrum 12), 12 resulting in the MgCl2/EB/TiCl4 pre-catalyst.

pronounced shoulder at lower frequency. The (C-O-C) is split in two bands at 1327 – 1307 cm-1. The whole FT-IR spectrum is barely affected by the degassing step at 373 K (spectrum 6), indicating that EB is strongly adsorbed at the MgCl2 surface, as reported in the literature.27 Nevertheless, a few interesting changes are observed: 1) the (C=O) band shifts to 1685 cm-1 and becomes more symmetric; 2) the shoulder around 1290 cm-1 in the (C-O-C) region disappears. These changes are ascribed to the removal of a tiny amount of physisorbed EB. Analogous shifts of the (C=O) and (C-O-C) bands are commonly observed when EB is coordinated to Lewis acids through the C=O group, and reflect a weakening of the C=O double bond accompanied by a strengthening of the C-O single bond. The broadness of the (C=O) band (reported also for other adsorbed IDs) indicates the presence of several low-coordinated Mg2+ cations at the MgCl2 surface, characterized by different Lewis acidic properties. The less coordinated sites (e.g. Mg2+ cations at the edges of the MgCl2 nano-crystals) display more pronounced Lewis acidic properties than the more coordinated ones (Mg2+ cations at the MgCl2 surfaces). As a consequence, an EB molecule adsorbed on a penta-coordinated Mg2+ cation is characterized by a (C=O) band at higher frequency than the same molecule adsorbed on a tricoordinated Mg2+ site. Traditionally, the relative amount of each individual EB-Mg2+ complex has been determined by a deconvolution of the (C=O) band in separate components, based on the assumption that each carbonyl band has the same molar extinction coefficient. Herein, we adopted a deconvolution model similar to

Integrated band intensity (a.u.) 1700 cm-1 Weak acid Mg2+

These results were compared with the picture from the FT-IR spectra of CO adsorbed at 100 K on the neat MgCl2 (Figure 2a), on the MgCl2/EB sample (Figure 2b) and on the MgCl2/EB/TiCl4 pre-catalyst (Figure 2c). The starting point, i.e. the sequence of spectra of CO adsorbed on the neat MgCl2 shown in Figure 2a, was deeply discussed in our previous work.75,85 Briefly, the most intense absorption band, located at 2182 cm-1 for maximum θCO and shifting to 2194 cm-1 for θCO → 0, was interpreted as the superposition of three contributions, namely CO adsorbed on the (110), (015) and (012) surfaces. On these surfaces the Mg2+ cations are either tetracoordinated (on the (110) surface) or penta-coordinated but with a high polarizing ability due to the proximity of chlorine ligands belonging to a neighbouring layer. This indicates that the position of the (C=O) band depends not only on the coordination number of the Mg2+ cation, but also on the local field, as already demonstrated by Chizallet et al.101,102 for CO adsorption on amorphous SiO2-Al2O3. Hereafter we will refer to these Mg2+ sites with the name of “strongly acidic Mg2+”. The second absorption band, located at 2163 cm-1 at maximum θCO and much less sensitive to the decrease of coverage, was assigned to CO adsorbed on the (104) penta-coordinated families of surfaces. We will call these sites as “weakly acidic Mg2+ sites”. No absorption bands due to CO adsorbed on defects (expected around 2200 cm-1) are observed. We will adopt the same terminology to assign the (C=O) absorption bands of adsorbed EB. Hence, the component around 1700 cm-1 is due to EB adsorbed on weakly acidic Mg2+ sites, that around 1680 cm-1 to EB coordinated to strongly acidic Mg2+ cations, and that around 1650 cm-1 to EB adsorbed on defects. This assignment overcomes the concept of crystal surfaces, which is sometimes criticized when dealing with MgCl2 crystals of nanometric dimension and highly disordered, and focus the attention on the Lewis acidic properties of the exposed Mg2+ sites, which are those governing the whole adsorption phenomenon and the surface reactivity. Hence, the FT-IR spectrum of ethyl benzoate adsorbed at room temperature on MgCl2 (Table 1) and the FT-IR spectra of CO adsorbed at 100 K on the same neat MgCl2, agree in that our activated MgCl2 displays a much larger population of strongly acidic Mg2+ sites than poorly acidic Mg2+ sites. This is in fair agreement with the morphological analysis presented in our previous work, where we provided experimental and theoretical proofs that the (110), (015) and (012) surfaces are highly stabilized when MgCl2 s


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Figure 2. Background subtracted FT-IR spectra (in the (CO) region) of CO adsorbed at 100 K on activated MgCl2 (part a), MgCl2/EB (part b) and MgCl2/EB/TiCl4 (part c), as a function of the CO coverage (maximum θCO in bold).

nano-crystals are shaped in the presence of methanol at high temperature.75 In contrast, the amount of defective Mg2+ sites is negligible, probably because they are still covered by methanol residues. The convergence of the two experimental methods (FT-IR spectroscopy of adsorbed EB and of adsorbed CO) indicates that EB can be considered as a reliable probe for the uncoordinated Mg2+ sites at the MgCl2 surfaces. CO adsorption at 100 K was repeated on the MgCl2/EB sample, and the corresponding FT-IR spectra are shown in Figure 2b. These spectra are substantially different from those collected on neat MgCl2 (Figure 2a). At first, the absorption band at higher wavenumbers is no more observed, indicating that all the strongly acidic Mg2+ sites are covered by the adsorbed EB and hence they are no more available for CO adsorption. As a consequence, the integrated absorbance of the (C=O) band around 1680 cm-1 reported in Table 1 (41.0 a.u.) represents the maximum coverage of EB on strongly acidic Mg2+ cations. In contrast, the absorption band at lower wavenumbers is still observed, but lower in intensity, much broader, and slightly shifted at lower values (2160 cm-1). This means that EB is also adsorbed on the weakly acidic Mg2+ sites, but full coverage is not reached in this case. This observation agrees with other studies on different electron donors, which demonstrate that complete coverage of the MgCl2 surface is difficult, especially for bulkier donors.51,52,55 Due to the incomplete EB coverage, the CO molecules can adsorb on the still available weakly acidic Mg2+ sites, but they are influenced by the presence of the EB molecules adsorbed nearby. This explains the slight shift and the broadness of the corresponding absorption band. Finally, a third weak band is observed around 2138 cm-1, very easily reversible upon degassing, which is assigned to physisorbed CO. This band indicates the presence of additional very weak adsorption sites for CO. We propose that the presence of adsorbed EB molecules define “organic channels” on the MgCl2 surface, where the small CO molecules can easily physisorb. Summarizing, the whole set of FT-IR data shown in Figure 1b and Figure 2a-b demonstrate that EB is equally adsorbed on all the available surfaces of activated MgCl2, in agreement with the general idea that flexible and spatially compact donors (such as monoesters, but also phthalates and succinates) show no clear preference for a MgCl2 surface, because they can adopt various orientations.46,52,103,104 The relative intensities of the three components constituting the broad (C=O) band in the FT-IR spectrum

of MgCl2/EB reflect the relative proportion of the exposed Mg2+ cations with different Lewis acidity. 4.2. Interaction of TiCl4 with MgCl2/EB The MgCl2/EB sample was successively titanated with TiCl4 vapors at room temperature. Figure 1c shows the sequence of FTIR spectra collected during the progressive adsorption/reaction of TiCl4 as a function of the TiCl4 coverage (from spectrum 6 to spectrum 12). 12 Contrarily to what observed on neat MgCl2, the titanation of MgCl2/EB is effective even at room temperature, testifying that EB helps in retaining the TiCl4 in the pre-catalyst.105 Moreover, a degassing step at 373 K does not cause any change in the FT-IR spectrum, indicating that the reaction products are strongly adsorbed on the MgCl2 surface. TiCl4 fast reacts with the MgCl2/EB system, as evidenced by the rapid transformation of the FT-IR spectra in the (C=O) region. In particular, the broad band centred around 1680 cm-1 shifts to 1670 cm-1 and slightly decreases in intensity, and a new intense absorption appears in the 1600 – 1500 cm-1 region. A magnification of spectrum 12 is reported in Figure 3 (MgCl2/EB/TiCl4 spectrum) and compared to that of the MgCl2/EB system. The shift and the slight decrease in intensity of the broad (C=O) band assigned to EB adsorbed on MgCl2 indicate that the speciation of the EB-Mg2+ complexes changes after interaction with TiCl4. This is demonstrated by the results of the de-convolution process summarized in Table 1 (and Figure S1b). In particular: 1) the component at 1700 cm-1, attributed to EB adsorbed on weakly acidic Mg2+ sites, completely disappears; 2) the band centred around 1680 cm-1, due to EB adsorbed on strongly acidic Mg2+ cations, slightly decreases in intensity (of about 10%); 3) the band at 1650 cm-1, assigned to EB adsorbed on defect sites, remains almost unchanged. At the same time, two strong bands rapidly appear at 1592 and 1550 cm-1. A couple of strong absorption bands in this spectral region is characteristic of complexes between TiCl4 and EB. As a comparison, the FT-IR spectrum of the (TiCl4-EB)2 complex is shown in the bottom part of Figure 3. This spectrum shows two strong bands at 1591 and 1563 cm-1, separated by a dip at 1580 cm1 . Both bands owe a major part of their intensity to the (C=O) vibrational mode, although formally the band at 1591 cm-1 is assigned to a phenyl vibration. The dip at 1580 cm-1 is originated by the Fermi resonance between the phenyl mode at this frequency which couples with the (C=O) mode.106,107 Similar spectra were reported in the literature for other TiCl4-EB complexes.89,108



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surface through a few bridging chlorine atoms. Additional details will be added in the following sections.

4.3. Reversing the order: interaction of EB with the MgCl2/TiCl4 pre-catalyst

Figure 3. FT-IR spectra, in the (C=O) and (C-O-C) vibrational regions, MgCl2/EB, MgCl2/EB/TiCl4 and MgCl2/TiCl4/EB samples, compared to those of liquid EB and (TiCl4-EB)2 complex, as references. The position of the main absorption bands related to TiCl4-EB complexes is reported. Vertical dotted lines identify the position of the two main absorption bands for liquid EB. The spectra are vertically translated for clarity.

Although the two strong (C=O) bands occur at slightly different wavenumbers depending on the position of EB (e.g. trans to a bridged or to a terminal chlorine), all the TiCl4-EB complexes do show a couple of strong absorption bands centred around 1550 cm1 . In the case of MgCl2/EB/TiCl4, most of the EB involved in the formation of the new complexes was previously adsorbed on weakly acidic Mg2+ cations, whereas only a minor fraction of the EB adsorbed on the strongly acidic Mg2+ cations is removed by TiCl4 (Table 1). These results are in agreement with previous observations of a rather high mobility of the electron donors on the (104) surface, and of a relative high stiffness of the same donors on the (110) surface. 51,52,55 The homogeneous-like TiCl4-EB complexes are stable also after a degassing step at 373 K. In order to shed light on their location on the MgCl2 surface, we performed FT-IR spectroscopy of CO adsorbed at 100 K. Figure 2c shows the FT-IR spectra of CO adsorbed at 100 K on the MgCl2/EB/TiCl4 sample as a function of the CO coverage. At a first glance, it is evident that the total intensity of the spectra is higher than before TiCl4 adsorption, i.e. more CO is adsorbed. This observation is important because it signifies that TiCl4 does not additionally poison the MgCl2 surface, rather it partially cleans it. Hence, a simple co-adsorption model (i.e. TiCl4 occupies the available sites nearby an adsorbed EB molecule) cannot be exhaustive to explain our experimental data. In such a case we would have expected a decrease of the total amount of adsorbed CO. More in details, with respect to the experiment performed before TiCl4 reaction (Figure 2b), the following changes are observed: 1) the band at higher wavenumbers (CO on strongly acidic Mg2+ cations) reappears again, although its intensity is only 20% of that observed for CO adsorbed on neat MgCl2 (Figure 2a); 2) the band around 2160 cm-1 (CO adsorbed on weakly acidic Mg2+ sites) becomes sharper, and approaches that observed for CO adsorbed on neat MgCl2; 3) the band around 2138 cm-1 assigned to physisorbed CO is un-modified. This experiment clearly demonstrates that the homogeneous-like TiCl4-EB complexes do not poison the MgCl2 surface. A model that may reconcile all these experimental observation involves the aggregation of TiCl4 and EB molecules to form homogeneous-like complexes loosely bound to the MgCl2

The experiment was repeated by reversing the order of addition of the components: first the MgCl2/TiCl4 pre-catalyst was prepared as described in the Experimental Section, and then EB was added from the vapour phase (and monitored in situ by FT-IR spectroscopy, as shown in Figure 4). The FT-IR spectra of CO adsorbed at 100 K on MgCl2 before and after titanation by TiCl4 are shown in Figure 5a and Figure 5b, and were deeply commented in our previous work.75 After reaction with TiCl4, the IR absorption band assigned to CO adsorbed on strongly acidic Mg2+ sites greatly decreases in intensity, whereas the band assigned to CO adsorbed on weakly acidic Mg2+ cations is almost unaffected. These data provide an evidence that TiCl4 is preferentially adsorbed on the strongly acidic Mg2+ sites, although a full coverage is not reached (i.e. there are still Mg2+ cations available for CO adsorption).

Figure 4. Sequence of background subtracted FT-IR spectra collected during adsorption of EB on the MgCl2/TiCl4 pre-catalyst, as a function of the EB coverage (from spectrum 1 to spectrum 4).

Figure 4 shows the sequence of FT-IR spectra collected during the progressive adsorption/reaction of EB on the MgCl2/TiCl4 precatalyst at room temperature as a function of the EB coverage (from spectrum 1 to spectrum 4). Since the very beginning a complex series of absorption bands appear in the ν(C=O) spectral region, testifying the co-presence of EB adsorbed on Mg2+ cations and of TiCl4-EB complexes. All the absorption bands increase in intensity at the same rate, indicating that the corresponding species are formed simultaneously. Also in this case, the amount of adsorbed EB was limited to avoid the presence of loosely bound species. Spectrum 4 is magnified in Figure 3 (MgCl2/TiCl4/EB spectrum). This spectrum is clearly different with respect to those of MgCl2/EB and MgCl2/EB/TiCl4. Starting the discussion from the absorption band associated with EB-Mg2+ complexes, it is constituted by the overlap of three components (Figure S1c), whose integrated absorbance is summarized in Table 1. The amount of EB adsorbed on the weakly acidic Mg2+ sites (band around 1700 cm-1) and on the defective Mg2+ sites (band around 1650 cm-1) are almost equal to those found on the MgCl2/EB system. In contrast, much less EB is adsorbed on the strongly acidic Mg2+ sites, which are clearly occupied by something else and surely by chemisorbed TiCl4 as previously demonstrated.75 Concerning the ν(C=O) region characteristic of the TiCl4-EB complexes, three main bands are observed around 1628, 1602 and 1576 cm-1, i.e. slightly different from those observed in the spectrum of MgCl2/EB/TiCl4 system. By looking at the deconvolution in Figure S1c, the most important difference of this spectrum respect to those previously discussed is the presence of a new band around 1630 cm-1. Hence, by


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Figure 5. Background subtracted FT-IR spectra (in the (CO) region) of CO adsorbed at 100 K on activated MgCl2 (part a), MgCl2/TiCl4 (part b) and MgCl2/TiCl4/EB (part c), as a function of the CO coverage (maximum θCO in bold).

reversing the order of addition of TiCl4 and EB, different TiCl4-EB homogeneous-like complexes are formed on the MgCl2 surface. Even more interesting, the FT-IR spectra of CO adsorbed at 100 K on the MgCl2/TiCl4/EB system (Figure 5c) reveal that EB does not act as a poisoning but rather it makes the MgCl2 surface more available for CO adsorption with respect to the MgCl2/TiCl4 pre-catalyst (Figure 5b). Indeed, the absorption band assigned to CO adsorbed on strongly acidic Mg2+ cations (around 2180 cm-1) gains again intensity, although it does not reach the value initially observed in the spectra of CO adsorbed on neat MgCl2. Since a fraction of the strongly acidic Mg2+ sites are involved in the interaction with EB (absorption band around 1670 cm-1), these data imply that the formation of the TiCl4-EB homogeneous-like complexes occurs through a detach of most of the TiCl4 chemisorbed on the strongly acidic Mg2+ sites and that they are only weakly bonded to the MgCl2 surface through a few chlorine bridges.

4.4.Computational investigation complexes on MgCl2

of

TiCl4-EB

Parallel to the experimental characterization of the TiCl4-EB complexes at the MgCl2 surfaces, we carried out a series of DFT-D calculations aimed at evaluating the structure, the relative stability and the vibrational properties of a few TiCl4-EB complexes that could be representative of those detected experimentally. In particular, we selected two surfaces of MgCl2, the (110) and the (107), because they are among the most stable ones in the presence of solvent molecules acting as donors,85 and are representative of more acidic (exposing 4-fold coordinated Mg2+ cations) and less acidic (exposing 5-fold coordinated Mg2+ cations) surfaces, respectively. On the (110) surface we have modelled two adducts characterized by a different Ti:EB ratio, namely TiCl4EB and TiCl4(EB)2. On the (107) surface we started modelling the Ti2Cl8EB complex, since historically this type of surfaces were considered capable of stabilizing Ti chloride dimers, that for long time were pointed out as the stereo-selective active species for polypropylene.69 Although recent theoretical calculations demonstrated that adsorption of Ti2Cl8 dimers on MgCl2 surfaces is not feasible,95 the presence of the donor might alter the energetic of the process, giving a new chance for the dimers formation. To avoid steric repulsion between the adsorbed adducts, we adopted a coverage of θ = 1/4 and θ = 2/3 for the complexes at the (110) and (107) surfaces, respectively. At low θ values the computations become very demanding, since they require the construction of super-cells of increasing dimensions. Also

for this reason it was not possible to cover all possible TiCl4-EB adducts and binding modes. In particular, adsorption of TiCl4-EB adducts in special defective sites, such as the step defects on the (104) surface recently suggested by Cavallo and co-workers,53 are cases that remain to be examined. Figure 6 shows the investigated models, both before (left side) and after (right side) the geometry optimization, while Figure S2 shows the same models from another perspective. On the (110) surface, the starting TiCl4-EB adducts were constructed by partially detaching TiCl4 from the equilibrium position as determined in Ref. 72, where Ti is octahedrally coordinated in a very similar way to the qualitative models early proposed by Corradini in the assumption of a strong epitaxial chemisorption.71 The initial detachment was necessary to allow for the creation of some adsorption vacancies around titanium where EB can insert. The same was done for the starting MgCl2(107)/Ti2Cl8EB model, where the Ti2Cl8 dimer was partially displaced from the (107) surface. Curiously, the geometry optimization process followed different paths in the 3 cases. • The optimized MgCl2(110)/TiCl4(EB)2 model (Figure 6b’) is the only one that retains its original structure (Figure 6b). TiCl4 remains in interaction with a 4-fold coordinated Mg2+ cation through a bridging Cl ligand (d(Mg⋅⋅⋅Cl) = 2.56 Å) and maintains a 6-fold coordination, with the two EB molecules bonded through the carboxyl oxygen (d(Ti⋅⋅⋅O=C) = 2.15 Å). • In contrast, the optimized MgCl2(110)/TiCl4EB model (Figure 6a’) appears very different from the initial guess (Figure 6a). TiCl4 is completely displaced from the MgCl2(110) surface and the TiCl4EB adduct is now bonded to MgCl2 through the oxygen of the ester group of EB (d(Mg⋅⋅⋅(R)O-C) = 2.25 Å). Interestingly, Ti remains partially uncoordinated, having only five ligands (4 Cl and the C=O group of EB) in its coordination sphere. Locally, the structure of the TiCl4 moiety is quite peculiar, in that three chlorine ligands stay approximately in the same plane, while the other two ligands (1 Cl and C=O of EB) point in the perpendicular direction. This structure might led to an unusual reactivity towards the Al-alkyl activator. • Finally, the MgCl2(107)/Ti2Cl8EB model (Figure 6c) resulted to be not stable. During the geometry optimization, the Ti2Cl8 dimer was broken, with the consequent release of a TiCl4 molecule. The calculation was restarted for the MgCl2(107)/TiCl4EB model, resulting in a 6-fold coordinated monomeric TiCl4 site, bonded to the MgCl2(107) surface



Figure 6. Models of different TiCl4-EB complexes adsorbed at different MgCl2 surfaces, before (left) and after (right) the geometry optimization. A magnification of the adduct is shown for clarity. Figure S2 shows the same models from a different perspective.

Table 2. Calculated adsorption energies (ΔEads ads, in kJ/mol) for the TiCl4EB and TiCl4(EB)2 complexes on the (110) and (107) MgCl2 surfaces. Models

ΔEads (kJ/mol)

MgCl2(110)/TiCl4EB

-98.9

MgCl2(110)/TiCl4EB2

-83.8

MgCl2(107)/TiCl4EBa

-120.5

a

Evaluated as the product →MgCl2(107)/TiCl4EB+TiCl4)

of

reaction

MgCl2(107)+Ti2Cl8EB

2) By going from MgCl2(110)/TiCl4 to the corresponding complexes with EB, half of the (110) surface previously occupied by TiCl4 is made available for adsorption of probe molecules, such as CO. This result is in good agreement with the experimental observation that a larger portion of the surface is available for CO adsorption when EB is dosed on MgCl2/TiCl4 or when TiCl4 is dosed on MgCl2/EB. Successively, the vibrational frequencies of the TiCl4-EB complexes on the MgCl2 surfaces have been calculated in the harmonic approximation. Table 3 summarizes the predicted (C=O) values for the investigated models, compared to those calculated for the TiCl4(EB)x (x=1, 2) complexes and for EB in the gas phase. For the EB molecule, the predicted (C=O) value is 68 cm-1 higher than the experimental one for liquid EB, the discrepancy being mostly due to the anharmonicity of the vibrational mode. The (C=O) stretching mode of EB is strongly affected by the interaction with TiCl4, resulting in a split of the band and in a relevant bathochromic shift with respect to pure EB ( ). The values, summarized in Table 3, are more important for the TiCl4(EB) complex than for the TiCl4(EB)2 one. Even larger values are predicted for the three TiCl4-EB complexes on the MgCl2 surfaces. These values are in very well agreement with those observed experimentally ( (C=O) in Table 3). Hence, our computational results strongly support the hypothesis that TiCl4(EB)x (x = 1,2) complexes are formed in the co-presence of TiCl4 and EB on MgCl2, and are stable on both tetra- and penta-coordinated MgCl2 surfaces. Table 3. 3. Calculated normal modes, modes, (C=O), for the TiCl4EB and TiCl4(EB)2 complexes on the (110) and (107) MgCl2 surfaces, and corresponding shifts with respect to pure EB, (C=O). For comparison, also the main absorption bands observed in the FTFT-IR spectra of liquid EB, MgCl2/EB/TiCl4 and MgCl2/TiCl4/EB, (C=O), are reported, as well as the correcorresponding shifts with respect to pure EB, (C=O). Models EB

through two bridging Cl ligands and with the EB coordinated to the Ti center through the C=O group (Figure 6c’). Table 2 summarizes the adsorption energies for the three investigated models. The following considerations can be made. 1) Stable TiCl4EB and TiCl4(EB)2 complexes can exist on the (110) surfaces. Even in the absence of the Gibbs free energies (not available because too computationally demanding), our experience and previous works in this field75,85,95 indicate that adsorption energies lower than about -80 kJ/mol correspond to stable adducts. In contrast, the dimeric Ti2Cl8EB complex not only does not adsorb on MgCl2, but even more it is disrupted to give again a monomeric TiCl4EB adduct. These results give an additional clue towards the non-existence of Ti2Cl8 dimeric species in MgCl2-based Ziegler-Natta catalysts, even in the presence of the electron donors. At the same time, limited to ∆Eads values (i.e. total electron energy variations) the monomeric TiCl4EB adduct on the (107) surface stands as the most stable complex among those investigated in this work. It is worth noticing that the structure of these adducts is perfectly compatible with the Ti-Cl and Ti-ester average distances as determines by EXAFS in an early work.68

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(C=O) -1

(C=O)a

(cm )

(cm-1)

1781

-

TiCl4EB

1676, 1642

-105, -139

TiCl4EB2

1732, 1685

-49, -96

MgCl2(110)/TiCl4EB

1686

-95

MgCl2(110)/TiCl4EB2

1666, 1643

-115, -138

MgCl2(107)/TiCl4EB

1640, 1619

-141, -163

(C=O)

(C=O)b

Samples

(cm )

(cm-1)

EB liquid

1713

-

MgCl2/EB/TiCl4

1592, 1550

-121, -163

MgCl2/TiCl4/EB

1628, 1602, 1576

-85, -111, -137

a

-1

b

,

5. CONCLUSIONS The formation of homogeneous-like complexes between EB (or other electron donors) and TiCl4 is well known since long time, although their role in the Ziegler-Natta catalysis is still questioned. Our data provide, to the best of our knowledge for the first time, some insights into the formation of these complexes and their location on the MgCl2 surface. This was made possible by coupling a


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specific synthetic protocol, that allowed the step-by-step surface characterization, with DFT-D calculation on a few TiCl4-EB complexes representative of those detected experimentally. In our experimental conditions, homogeneous-like TiCl4-EB complexes are formed on the MgCl2 surface whichever is the order of addition (first EB, then TiCl4, or the reverse). At the same time, our FT-IR data of CO adsorbed at 100 K clearly indicate that, even in the presence of these homogeneous-like complexes, a large fraction of the MgCl2 surface is still available for CO adsorption, larger than that available in the presence of EB only, or of TiCl4 only. This implies that the formation of these complexes occurs through the partial detachment of the EB or TiCl4 strongly chemisorbed at the MgCl2 surfaces and indicates a rather high mobility of the adsorbed EB and TiCl4, in agreement with many other observations in the literature. Our DFT-D computational results demonstrate that monomeric TiCl4EB and TiCl4EB2 complexes might indeed exist at the MgCl2 surfaces, while dimeric Ti2Cl8EB complexes are unstable and easily broken to give back TiCl4EB and a TiCl4 molecule. These complexes are bonded to the MgCl2 surface either through bridging chlorine ligands, or alternatively through the ester oxygen of the EB molecule, in the latter case creating unexpected opportunities for the reaction with the aluminium alkyl activator. In all the cases, steric repulsions do not allow a full surface coverage, leaving a fraction of surface Mg2+ cations available for the adsorption of additional incoming molecules, such as CO. The computed IR spectra for these complexes are highly compatible with the experimental ones. We are aware that our work has some limits. Our experimental approach is not the same as the industrial preparation of ZieglerNatta catalysts (which involve for example the use of solvents), while on the computational side we have explored only a limited number of models, that are the most reasonable complexation reactions and binding modes on the basis of common chemical sense and literature suggestions. Nevertheless, the combination of the two approaches finally leads to a reliable and consistent scenario. According to our results, one of the roles of the IDs in ZieglerNatta catalysis is that of inducing a certain mobility for TiCl4, simultaneously acting as a “surfactants” to maintain a high dispersion of the active phase. Moreover, our results could explain also the reason why the IDs have an influence in the selectivity of the propene polymerization, even though they are largely removed by the co-catalyst. Indeed, the structure of the TiCl4-ID complexes is initially templated by the ID and it is clearly different if the aluminium alkyl finds in its way isolated weakly chemisorbed TiCl4 species (as in the absence of ID), or highly dispersed TiCl4-ID complexes. Our observations do not exclude the presence of EB and TiCl4 independently adsorbed on the MgCl2 surface in close proximity, neither we intend to claim that the surface-bonded TiCl4-EB complexes play a dominant role in the Ziegler-Natta catalysis. Still, we do believe that these results should induce to reconsider the hypothesis on the location of IDs on MgCl2 and on their interaction with TiCl4. So far the literature has proposed only surface or defective positions with both electron donors and AlEt3 bound in proximity of the Ti centre, but never considering the possibility that they directly bind to the Ti species to form complexes on the surfaces (whose existence is well known in organometallic chemistry). The existence of these complexes has been proved by our spectroscopic measures on pre-catalysts, although their likely influence on the stereospecific behavior of the Ti sites after the reaction with AlR3 is still to be further investigated, also in consideration of the contemporary presence of donors directly bound to surfaces in different ways that cannot be excluded.

Finally, as a general comment, our results suggest a relevant role of the support in assisting the self-organization of the catalyst components, more than being a mere physical support. The support paves the route to not trivial reactions and may open easier mechanistic paths towards the products. The former case of MgCl2(110)/TiCl4EB, where the TiCl4EB adduct rearranges with the EB directly binding the surface and the TiCl4 moiety strongly exposed to the external environment, and the latter case of MgCl2(107)/Ti2Cl8EB ending in the split of the dimeric Ti2Cl8EB complex into TiCl4 and bound TiCl4EB, are clear examples of that active role of the support.

ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge via the Internet on the ACS Publications website. Deconvolution of the FT-IR absorption bands in the ν(C=O) vibrational region for MgCl2/EB, MgCl2/EB/TiCl4 and MgCl2/TiCl4/EB samples. Models of different TiCl4-EB complexes at different MgCl2 surfaces, from a different perspective than in Figure 6.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We are grateful to Adriano Zecchina, Silvia Bordiga, Gabriele Ricchiardi and Mimmo Civalleri for useful discussion. We acknowledge the CINECA award under the ISCRA initiative, for the availability of high performance computing resources and support. The present computational study, in fact, has been possible thanks to POLCAT (Modeling heterogeneous Ziegler-Natta catalysts for olefin conversion) and SASP (Simulation of Active sites of Polimerization) ISCRA C projects. Finally, the whole work has been economically supported by the Progetto di Ateneo/CSP 2014 (Torino_call2014_L1_73).

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Donor Complexes on the Surface

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