DFT Study on Mechanism of Olefin ... - ACS Publications

968

Organometallics 2009, 28, 968–977

DFT Study on Mechanism of Olefin Hydroalumination by XAlBui2 in the Presence of Cp2ZrCl2 Catalyst. I. Simulation of Intermediate Formation in Reaction of HAlBui2 with Cp2ZrCl2 Evgeniy Yu. Pankratyev,† Tatyana V. Tyumkina,† Lyudmila V. Parfenova,† Leonard M. Khalilov,*,† Sergey L. Khursan,‡ and Usein M. Dzhemilev† Institute of Petrochemistry and Catalysis of Russian Academy of Sciences, Russia, 450075, Ufa, Prospekt Oktyabrya, 141, and Institute of Organic Chemistry of Ufa Scientific Center of Russian Academy of Sciences, Russia, 450054, Ufa, Prospekt Oktyabrya, 71 ReceiVed May 3, 2008

The mechanism of intermediate formation in the reaction of HAlBui2 with Cp2ZrCl2 has been studied by DFT calculations. Stationary points have been localized on the potential energy surface of the reaction; enthalpy has been calculated for each of them at T ) 203 K. It was shown that the thermodynamically probable way of the process includes formation of a bridge bimetallic complex [Cp2ZrCl2 · HAlBui2], which with dissociation gives Cp2ZrHCl. The subsequent coordination of Cp2ZrHCl with HAlBui2 provides the intermediate [Cp2ZrHCl · HAlBui2], which dimerizes into an experimentally observed complex [Cp2ZrH2 · ClAlBui2]2. The competing stage of [Cp2ZrHCl · HAlBui2] interaction with HAlBui2 yields complex [Cp2ZrH2 · ClAlBui2 · HAlBui2], which shows low activity in the olefin hydrometalation. As a result, the scheme of the intermediates’ formations and transformations in the reaction of HAlBui2 with Cp2ZrCl2 was proposed. Introduction The reaction of thermal olefin hydroalumination was developed in the middle of past century. Application of metal complex catalysts in hydroalumination opened the possibility to carry out this reaction under mild conditions with high regio- and stereoselectivity (see, for example, reviews in refs 2a-c). As a result, alkene hydroalumination became one of the fundamental methods applied in organic and organometallic chemistry for reduction and direct functionalization of unsaturated compounds and for synthesis of the highest organoaluminum compounds (OAC) of different structure. Information concerning mechanistic studies of the olefin’s catalytic hydroalumination is limited. Moreover, all the discussions published earlier are hypothetical.3 According to ref 3, catalytic hydroalumination runs through zirconium hydrides, which hydrometallate olefins. Therefore, we carried out experimental studies on the mechanism of the olefin hydroalumination by alkylalanes (HAlBui2, ClAlBui2, and AlBui3) in the presence of zirconium complexes using dynamic NMR spectroscopy, encounter synthesis, and kinetic studies.1 As a result, we suggested a model (Scheme 1) based on the experimental results, which imply participation of key complexes 4A and 6A. The structure of novel complex 4A was established by means of NMR spectroscopy and cryoscopy. The NMR structure information obtained by us1a for complex 6A was identical to * To whom correspondence should be addressed. E-mail: [email protected]. † Institute of Petrochemistry and Catalysis. ‡ Institute of Organic Chemistry of Ufa Scientific Center. (1) (a) Parfenova, L. V.; Pechatkina, S. V.; Khalilov, L. M.; Dzhemilev, U. M. Russ. Chem. Bull., Int. Ed. 2005, 54, 316. (b) Parfenova, L V.; Balaev, A. V.; Gubaidullin, I. M.; Pechatkina, S. V.; Abzalilova, L. R.; Spivak, S. I.; Khalilov, L. M.; Dzhemilev, U. M. Int. J. Chem. Kinet. 2007, 39, 333. (c) Parfenova, L. V.; Vil’danova, R. F.; Pechatkina, S. V.; Khalilov, L. M.; Dzhemilev, U. M. J. Organomet. Chem. 2007, 692, 3424.

literature data.4 Complex 4A readily interacts with olefins forming alkyl zirconium complex Cp2ZrCl(CH2CH2R) and alkylalane Bui2AlCH2CH2R, whereas complex 6A is relatively inactive. However, NMR method sets restrictions that do not allow one to specify confidently the mechanism of chemical reactions and to establish the structure of unstable Zr,Al complexes that precede complexes 4A and 6A. Moreover, the method does not reveal all the structure features of even stable compounds such as, for example, complexes 4A and 6A. Thus, the aim of the proposed publication series is theoretical quantum chemical mechanism studies of olefin hydroalumination by XAlBui2 (X ) H, Cl, Bui) in the presence of Cp2ZrCl2 catalyst. Our first article is dedicated to the development of a quantum chemical model which describes the intermediates’ formation and further transformations in the system Cp2ZrCl2-HAlBui2 (Scheme 2). We choose this system because of its simplicity due to the absence of β-hydride activation processes, which are observed in the systems containing ClAlBui2 and AlBui3.

Computational Details Geometric parameter optimization for the complexes, vibrational frequency analysis, transition state (TS) search, and scan along intrinsic reaction coordinates (IRC) were carried out using programs Priroda04 and Priroda-06 developed by Laikov.5 The generalized gradient approximation (GGA) for the exchange-correlation functional by (2) (a) Tolstikov, G. A.; Yur’ev, V. P. Aluminiiorganicheskii sintez (Organoaluminium syntheses); Nauka: Moscow, 1979. (b) Dzhemilev, U. M.; Ibragimov, A. G. Russ. Chem. ReV. 2000, 69, 121. (c) Dzhemilev, U. M.; Ibragimov, A. G. In Modern Reduction Methods; Andersson, P. G., Munslow, P. J., Eds.; Wiley-VCH Verlag GmbH&Co.KGaA: Weinheim, Germany, 2008. (3) (a) Sato, F.; Sato, S.; Sato, M. J. Organomet. Chem. 1976, 122, C25. (b) Negishi, E.; Yoshida, T. Tetrahedron Lett. 1980, 21, 1501. (4) (a) Shoer, L. I.; Gell, K. I.; Schwartz, G. J. Organomet. Chem. 1977, 136, 19. (b) Siedle, A. R.; Newmark, R. A.; Schroepfer, J. N.; Lyon, P. A. Organometallics 1991, 10, 400–404.

10.1021/om800393j CCC: $40.75  2009 American Chemical Society Publication on Web 01/29/2009

Mechanism of Olefin Hydroalumination by XAlBui2

Organometallics, Vol. 28, No. 4, 2009 969 Scheme 1

Scheme 2

Perdew, Burke, and Ernzerhof was employed.6 The electronic configurations of the molecular systems were described by the orbital basis sets of contracted Gaussian-type functions of size (5s1p)/[3s1p] for H, (11s6p2d)/[6s3p2d] for C, (15s11p2d)/[10s6p2d] for Al and Cl,

and (20s16p11d)/[14s11p7d] for Zr, which were used in combination with the density-fitting basis sets of uncontracted Gaussian-type functions of size (5s2p) for H, (10s3p3d1f) for C, (14s3p3d1f1g) for Al and Cl, and (22s5p5d4f4g) for Zr. The IRC was calculated for each

Figure 1. Optimized structure of the initial compounds Cp2ZrCl2 and HAlBui2 (in the brackets, we provided the relative energy of the conformer).

970 Organometallics, Vol. 28, No. 4, 2009 obtained TS in order to verify the presence of this TS on the potential energy surface (PES). The main factors, which defined the selection of the quantum chemical method, were the following: (i) the program has been successfully used for the systems containing 4d transition metals,7 (ii) the computational speed, and (iii) accuracy for our systems. Vibrational frequencies and thermodynamic data were calculated in terms of ideal gas approximations. NMR shifts were calculated using the method GIAO,8 which is implemented in the program. All values of enthalpies and activation energies were determined at the 203 K because complexes 4 and 6 were observed simultaneously in the NMR spectra at this temperature.1a

Results and Discussion Structure Optimization of Cp2ZrCl2 and HAlBui2. On the first stage, we optimized the structures of the initial compounds: Cp2ZrCl2 and HAlBui2 (Figure 1). The experimental9 (X-ray analysis) bond length Zr-C is 2.52 Å, which corresponds to the calculated value of 2.55 Å. Moreover, experimental and theoretical values of the Cl-Zr-Cl angle are very close as well (exptl 97.1°, calcd 99.7°). The 1H and 13C NMR spectra of Cp2ZrCl2 show the only signal of Cp rings; that is, five carbon and hydrogen atoms of the π-ligand are magnetically equivalent, which could be due to free rotation of Cp rings in the solution because of insignificant conformational barriers. It is known that HAlBui2 forms trimers [HAlBui2]3 in a solution.10 The computed value of the enthalpy of [HAlBui2]3 dissociation ([HAlBui2]3 T [HAlBui2]2 + HAlBui2) is sufficiently high: ∆DHo ) 19.9 kcal/mol (Table 1). However, we assumed that the dissociation of trimer precedes the interaction of HAlBui2 with Cp2ZrCl2, and all further calculations were carried out with the monomer form of the OAC. It was established that the linear conformation 2A is the most likely structure of HAlBui2 (Figure 1). However, in the calculated Zr,Al complexes, compound 2 exists in conformation 2B, which is different from the others by the rotation of isobutyl groups around the Al-C bond. Interaction of Cp2ZrCl2 with HAlBui2. The initial step (1) of the Cp2ZrCl2 interaction with HAlBui2 can occur in two ways: (i) attachment of HAlBui2 from the outside of two chlorine atoms of Cp2ZrCl2 and (ii) between them.

Cp2ZrCl2 (1) + HAlBui2 (2) f [Cp2ZrCl2·HAlBui2] (7A) (1) Complex 7A (Figure 2) is formed in the case (i), while complex 7B is the result of way (ii). It was established that complex 7A is 3.5 kcal/mol more stable than 7B (Figure 3 and Table 1). Further, complex 7A can dissociate into Cp2ZrHCl (5) and ClAlBui2 according to the reaction 2:

[Cp2ZrCl2·HAlBui2] (7A) f Cp2ZrHCl (5) + ClAlBui2 (3) (2) On the PES of the reaction 2, we localized TS(7A-8) and local minimum 8 (Figure 2). Figure 2 shows that the process 7Af8 is accompanied with gradual increasing of Zr-Cl distance, consequent bond breaking, and formation of ClAlBui2, which was found experimentally.1a The dissociation enthalpy of 7A is 15.1 kcal/mol, while the barrier of Zr-Cl bond breaking is insignificant (2.8 kcal/mol); therefore, the major energetic expenditures are connected with the breaking of the Al-H bond (Figure 3). Totally, the reaction of Cp2ZrCl2 with HAlBui2, which gives Cp2ZrHCl (5) and ClAlBui2, is exothermic at -3.8 kcal/mol. Thus, the formation of Cp2ZrHCl

PankratyeV et al. Table 1. Calculated Thermodynamic Parameters for Reactions 1-14 at T ) 203 K (∆S [cal/mol · K]; ∆H, ∆G [kcal/mol]) reaction

∆S°

∆H°

1 2 7A f 8 8f5+3 3 4 5 6 7 8 10A f 11 11 f 4B 9 10A + 2 f 12 12 f 6B 6B f 6C 10 10A + 2 f 13 13 f 14 14 f 6B + 10A 11 12 13 17A f 17B 14 [HAlBui2]3 f [HAlBui2]2 + HAlBui2 [ClAlBui2]2 f 2 ClAlBui2

-42.7 44.7 7.1 37.6 -50.2 -49.4 -42.6 43.5 -47.8 -45.9 -46.8 1.0 -40.0 -33.4 -6.6 -5.7 5.9 -37.4 -8.6 51.8 -46.8 -40.0 42.5 -4.5 -46.5 40.2

-18.8 15.1 1.9 13.2 -8.2 2.0 -25.4 31.0 -6.2 -6.6 3.4 -10.0 -19.3 -5.0 -14.3 -1.7 -12.7 -8.4 6.3 -10.6 -12.4 -17.7 12.6 1.0 -16.4 19.9

40.5

∆G°

∆S q

-10.2 6.0 0.4 -0.8 5.6 2.0 -40.6 12.0 -16.8 22.1 3.5 2.7 12.9 -38.8 -10.2 -4.4 -11.2 1.8 -13.0 -7.4 -0.5 -8.8 -13.9 -0.8 8.0 -15.5 -21.1 12.7 -2.9 -9.6 4.0 2.0 -7.0 11.8

∆H q

∆G q

2.8

2.9

0.4

8.6

9.6 5.1

17.5 6.0

4.7 0.6

6.2 2.3

8.8 1.8

11.9 -0.8

24.3 16.0

on the first stage is consistent with the hypothesis proposed earlier.3b Moreover, Cp2ZrHCl was isolated in the reaction of Cp2ZrCl2 with HAlBui2 in THF,11 which is known to break Zr,Al complexes. Unfortunately, we did not observe the formation of Cp2ZrHCl in toluene (benzene) solution during the NMR study of Cp2ZrCl2 and HAlBui2 interaction. Furthermore, the reaction of Cp2ZrCl2 with HAlBui2 in toluene in the presence of Et3N (Et2NH)1a,c,4a is a procedure for synthesis of pure Cp2ZrH2. This experimental fact proves that the ligand exchange between Cp2ZrCl2 and HAlBui2 provides Zr complexes containing [Cp2ZrH2] species, which can be formed due to the following fast transformations of 5. Evolution of Cp2ZrHCl. We assumed three ways for the evolution of Cp2ZrHCl: dimerization of 5 into complex 9 (reaction 3, Figure 4), interaction with Cp2ZrCl2 (reaction 4), or interaction with HAlBui2 yielding complex 10 (reaction 5):

2Cp2ZrHCl (5) f [Cp2ZrHCl]2 (9)

(3)

Cp2ZrHCl (5) + Cp2ZrCl2 (1) f [Cp2ZrHCl · Cp2ZrCl2] (9A) (4) Cp2ZrHCl (5) + HAlBui2 (2) f [Cp2ZrH2 · ClAlBui2] (10A) (5) It was established that dimerization (3) goes over an insignificant barrier of 0.4 kcal/mol (TS(5-9), Figure 5); the enthalpy of the reaction is -8.2 kcal/mol. Dimer 9 has symmetric structure, where zirconium atoms are bonded by two hydride bridges that are located in the plane perpendicular to the Cp rings. Chlorine atoms have trans-orientation with respect to the Zr-Zr axis. At catalytic conditions, Cp2ZrHCl concentration is very small; therefore, we did not consider the further interaction of dimer 9 with HAlBui2. The comparison of the reaction 4 enthalpy (2.0 kcal/mol) with the enthalpies of reactions 3 (-8.2 kcal/mol) and 5 (-25.4 kcal/ mol) contributes to low probability of pathway 4. (5) (a) Laikov, D. N. Chem. Phys. Lett. 1997, 281, 151. (b) Laikov, D. N. Ph.D. Dissertation, Moscow State University, 2000. (c) Laikov, D. N.; Ustynyuk, Yu. A. Russ. Chem. Bull., Int. Ed. 2005, 54, 820. (6) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865.


Organometallics, Vol. 28, No. 4, 2009 971

Figure 2. Structures of the complexes which correspond to the state points on the PES of complex 5 formation.

Figure 3. Energetic profile of Cp2ZrHCl formation (reaction enthalpies in kcal/mol are shown in square brackets).

Cp2ZrHCl reacts with HAlBui2 (reaction 5) similarly to process 1; that is, HAlBui2 can link either to the outside of (7) (a) Ustynyuk, Yu. A.; Ustynyuk, L. Yu.; Laikov, D. N.; Lunin, V. V. J. Organomet. Chem. 2000, 597, 182. (b) Nifant’ev, I. E.; Ustynyuk, L. Yu.; Laikov, D. N. Organometallics 2001, 20, 5375. (c) Nifant’ev, I. E.; Ustynyuk, L. Yu.; Besedin, D. V. Organometallics 2003, 22, 2619. (8) Wolf, S. K.; Ziegler, T. J. Chem. Phys. 1998, 109, 895. (9) Green, J. C.; Green, M. L. H.; Prout, C. K. J. Chem. Soc., Chem. Commun. 1972, 6, 421. (10) Mole, T.; Jeffery, E. A. Organoaluminium Compounds; Elsevier: Amsterdam, 1972; 470 pp. (11) Negishi, E.; Huang, Z. Org. Lett. 2006, 8, 3675.

the H-Zr-Cl fragment to give complex 10A or between the chlorine and hydrogen atoms of Cp2ZrHCl, providing isomeric complexes 10B and 10C (Figure 4). Complexes 10A and 10B have the same energy and appear to be 3.0 kcal/ mol more stable than 10C, which contains hydride bridges. The enthalpy of reaction 5 is -25.4 kcal/mol (Figure 5). Figure 5 demonstrates higher probability of reaction 5 in comparison with 3. Complex 10B is formed in a similar way; it undergoes the same reactions; consequently, further calculations were carried out only for 10A. This complex was not observed in

972 Organometallics, Vol. 28, No. 4, 2009

PankratyeV et al.

Figure 4. Structures of the complexes which correspond to the state points on the PES of the formation of complexes 9 and 10.

the time scale of NMR; therefore, it goes through the following rapid transformations. Transformations of [Cp2ZrHCl · HAlBui2] (10A). A priori 10A can react with the compounds, which exist in the reaction media, or run through the monomolecular transformation: [Cp2ZrH2 · ClAlBui2] (10A) f Cp2ZrH2 + ClAlBui2 (3) (6)

[Cp2ZrH2 · ClAlBui2] (10A) + Cp2ZrHCl (5) f [Cl - (Cp2ZrH)2 - Al(Cl)Bui2] (9B) (7) 2[Cp2ZrH2 · ClAlBui2] (10A) f [Cp2ZrH2 · ClAlBui2]2 (4B) (8)

[Cp2ZrH2 · ClAlBui2] (10A) + HAlBui2 (2) f [Cp2ZrH2 · HAlBui2 · ClAlBui2] (6B) (9) Complex 10A dissociation followed by formation of Cp2ZrH2 and ClAlBui2 (reaction 6) is difficult (∆rH° ) 31.0 kcal/mol); this indicates low probability of pure Cp2ZrH2 separation in the process.

Interaction of 10A with Cp2ZrHCl (reaction 7) has activation barrier of 7.2 kcal/mol, ∆rH° ) -3.6 kcal/mol. The excess of HAlBui2 in the hydroalumination system supports low probability of the pathway 7. Nevertheless, minor channels 3 and 7 under appropriate conditions (for example, with deficiency of HAlBui2) could make the contribution to the rate of the whole process since the products of reactions 3 and 7 either dissociate to the initial compounds or react with HAlBui2 and form complex 4B with high probability (see below). The most probable paths of 10A progress, that is, dimerization (8) and interaction with HAlBui2 (9) will be discussed below. I. Formation and Structure of Complex 4. The key complex 4B is a dimerization product of intermediate 10A (reaction 8). Reaction 8 is exothermic at -6.6 kcal/mol (Figure 6). The value of ∆RG° for the reaction is positive (∆RG° ) +2.9 kcal/mol) but has insignificant modulus; therefore, one can assume that the reaction is reversible. Probably, reaction 10A f 4B is possible due to a change in concentration of other complexes and initial reagents that participate in the whole



Figure 5. Energetic profile of alternative pathways of Cp2ZrHCl transformation.

Figure 6. Energetic profile of complexes 10A and 4B transformations.

process because the reaction flow depends on the value and sign of ∆G ) ∆G° + RT ln Π(ci). Reaction 8 proceeds in two steps. PES of the reaction has a local minimum 11; also it has TS(10A-11) and TS(11-4B) (Figure 7). The first step is the coordination of two molecules of 10A via formation of a Zr-H-Zr bridge and simultaneous lengthening of the Zr · · · Cl bond (TS(10A-11)). This bond is completely broken in the local minimum 11. The second step implies further coordination followed by formation of the second hydride bridge Zr-H-Zr and breaking of the other Zr · · · Cl bond via TS(11-4B). As a result, complex 4B is a dimeric zirconocene dihydride [Cp2ZrH2]2, in which two Zr atoms are connected with ClAlBui2 through Zr-H-Al bridge bonds. The activation barriers of the two steps are 9.5 and 5.1 kcal/mol, respectively; these barriers are less than that for the reverse reactions (15.2 and 6.1 kcal/mol). Therefore, the monomer-dimer equilibrium 10A T 4B shifts to the dimer. Previously, we assumed that the structure of complex 4 has an additional Zr-Cl-Al bond. However, the optimization followed by vibrational frequency calculation showed that the structure is characterized by two imaginary vibrational frequencies, where each of them corresponds to breaking of

one of the terminal bridge bonds. Therefore, we proposed that complex 4B undergoes step by step isomerization, which leads to the structures with bridge Al · · · Cl · · · Zr bonds (4C and 4D) (Figure 7). The isomerization runs through TS(4B-4C) and TS(4C-4D). The activation energies for conversions 4B f 4C and 4C f 4D are 17.3 and 15.5 kcal/ mol, correspondingly. Those energies are greater than that for the reverse stages 4C f 4B (4.5 kcal/mol) and 4D f 4C (4.7 kcal/mol). Moreover, the process of the breaking Al · · · H · · · Zr bonds and formation of Al · · · Cl · · · Zr bonds is endothermic. Therefore, it could be assumed that equilibrium 4B T 4C T 4D is shifted to complex 4B. The theoretical conclusions are in a good agreement with NMR data. Thus, the HH COSY NMR spectrum1a of 4 shows cross-peaks between the broadened signals of hydrogens of Zr-H-Zr and Zr-H-Al bridges. Besides the other reasons, such as interaction with quadruple 27Al, the broadening of


PankratyeV et al.

Figure 7. Structures of the complexes which correspond to the state points on the PES of complex 4 formation and isomerization.

the signals could be caused by dynamic equilibrium between monomer-dimeric 10A T 4B. Finally, we calculated 13C chemical shifts of the Cp rings of dimeric isomers 4B, 4C, and 4D (Table 2). The value of calculated chemical shifts for Cp rings of structure 4B was found to be the closest to the experimental one. II. Formation of Inactive Complex 6. Interaction of 10A with HAlBui2 (reaction 9) provides a stable complex 6B, the structure of which was discussed in the literature.1a,4 On the first step of the reaction, Cl atom of 10A binds to Al of the HAlBui2. The local minimum 12 was found on the PES of the reaction (Figure 6), where the Al-Cl distance is 2.63 Å. Reaction 9 is characterized by TS(12-6B) with an activation barrier 4.7 kcal/mol, in which the Zr-Cl distance increases from 2.82 to 3.35 Å with the simultaneous decreasing of the distance between Zr and H atoms of HAlBui2. This rearrangement of electron density leads to formation of new bonds in compound 6B (Figure 8).

Moreover, it is known from the experimental data1a that complex 4B readily reacts with HAlBui2 to give complex 6B. Thus, we considered the direct pathway 4B f 6B (reaction 10):

[Cp2ZrH2 · ClAlBui2]2 (4B) + HAlBui2 (2) f [Cp2ZrH2 · HAlBui2 · ClAlBui2] (6B) + [Cp2ZrH2 · ClAlBui2] (10A) (10) As a result, we found an almost barrier-free way (Figure 6) where Al and H atoms of HAlBui2 attach correspondingly to Cl and Zr atoms of complex 4B with consequent destruction of the dihydride bridge (Figure 9). The process runs through the local minima 13, 14, and TS(13-14), TS(14-6B) with two insignificant barriers of 8.8 and 1.8 kcal/mol, respectively. According to Figure 6, formation of complex 6B is practically irreversible because the energy barriers of both reactions 6B f 10A and 6B f 4B are sufficiently high: 19.0



Table 2. Experimental and Calculated NMR 13C Chemical Shifts of The Cp Rings of Complexes 1, 4B, 4C, 4D, 6B, 6C, 15, 16, 17A, 17B, and 18 (ppm) substance

δcalcd

δexptl

∆δ

Cp2ZrCl2 (1) [Cp2ZrH2 · ClAlBui2]2 (4B) [Cp2ZrH2 · ClAlBui2]2 (4C) [Cp2ZrH2 · ClAlBui2]2 (4D) [Cp2ZrH2 · HAlBui2 · ClAlBui2] (6B) [Cp2ZrH2 · HAlBui2 · ClAlBui2] (6C) [Cp2ZrH2 · HAlBui2 · (ClAlBui2)2] (15) [Cp2ZrH2 · (HAlBui2)2 · ClAlBui2] (16) [Cp2ZrH2 · (HAlBui2)2] (17A) [Cp2ZrH2 · (HAlBui2)2] (17B) [Cp2ZrH2 · (HAlBui2)3] (18)

119.1 111.0 113.2 115.2 104.2 104.6 109.2 108.9 103.0 103.4 107.7

116.0 106.1

3.1 4.9 7.1 9.1 -1.0 -0.6 4.1 3.7 -2.2 -1.8 2.6

105.2

and 12.4 kcal/mol, respectively. This conclusion is also in agreement with experimental results.1a Obviously, reactions 8 and 9 are competing stages. Figure 6 shows that reaction 10A f 6B runs with smaller energetic expenditures than formation of key complex 4B. Moreover, complex 4B could transform into the same complex 6B via pathway 10. Therefore, the predomination of 6B in the reaction media should be expected; this is observed experimentally at temperatures higher than 223 K. In the discussions above, for the simplicity of explanation, and also taking into account literature data,1a,b we assumed complex 6B as being inactive in the olefins hydroalumination. At the same time, it is important to remark that the issue of the inactive complex nature remains questionable in multiple aspects. On the one hand, the structure of complex 6B was determined on the basis of NMR spectroscopy1a,4 and confirmed by cryoscopic measurements.4a On the other hand, the free bond Zr-H in complex 6B is a potentially active center in the reactions with olefins and other organometallic compounds. Due to this reason, we performed an additional study of possible transformations of 6B into other potentially inactive forms. Scanning PES of the system under investigation, we found complex 6C, which is isomeric to 6B (Figure 9) and thermodynamically more stable by 2.1 kcal/mol. Transformation of 6B into 6C runs through TS(6B-6C) with a small activation barrier of 0.6 kcal/mol. The energy profit which is reached due to formation of new Al-H bonds in complex 6C in comparison with 6B, in many aspects, is compensated by tension of tricyclic structure of 6C: the Al-H distances (1.86-1.97 Å) are increased

Figure 8. Energetic profile of complex 6B transformations.

approximately by 0.20-0.25 Å in comparison with that in complex 6B. It is reasonable to assume that association of 6C or 6B with an additional molecule of OAC should give more stable structure with “normal” Al-H bridge bonds. Indeed, the Al-H bond of complex 6B can coordinate ClAlBui2 (reaction 11) with formation of complex 14, which is shown in Figure 9. The reaction proceeds with no barrier, and the enthalpy of the process is -12.4 kcal/mol.

[Cp2ZrH2 · HAlBui2 · ClAlBui2] (6B) + ClAlBui2 f [Cp2ZrH2 · HAlBui2 · (ClAlBui2)2] (15) (11) [Cp2ZrH2 · HAlBui2 · ClAlBui2] (6B) + HAlBui2 f [Cp2ZrH2 · (HAlBui2)2 · ClAlBui2] (16) (12) [Cp2ZrH2 · (HAlBui2)2 · ClAlBui2] (15) f ClAlBui2 + [Cp2ZrH2 · (HAlBui2)2] (17A) (13) [Cp2ZrH2 · (HAlBui2)2] (17A) + HAlBui2 f [Cp2ZrH2 · (HAlBui2)3] (18) (14) Thus, complexes 6B, 6C, 15, 16, 17A, 17B, and 18 appear to be possible candidates for the role of the inactive complex. Table 2 presents calculated and experimental 13C NMR shifts of Cp rings for the complexes. Nevertheless, the results of calculation do not allow one to make a straightforward decision in favor of selection of any structure. From the one side, we observed good correspondence of experimental and calculated NMR shifts for complexes 6B and 6C. From the other side, on the example of complexes 1 and 4B, it was shown that GIAO calculation gives bigger values of NMR shifts by at least 3-5 ppm compared to the experiment. Probably, in the process of the catalytic olefin hydroalumination, a mixture of complexes 6B, 6C, 15, 16, 17A, 17B, and 18 is formed. Their ratio could depend on the reaction conditions. For example, under the condition of ClAlBui2 excess (catalytic olefin hydroalumination by ClAlBui2), the formation of complex 15 is possible (via reaction 11). Also, under the conditions of HAlBui2 excess (catalytic hydroalumination by HAlBui2), the formation of complex 18 (via reaction 14) can be observed. Thus, for the sounder conclusion about the nature of the inactive complex,


PankratyeV et al.

Figure 9. Structures of complexes which correspond to the state points on the PES of complex 6 formation and isomerization. Scheme 3

extra theoretical and experimental NMR studies are necessary; these studies are currently conducted in our laboratory.

Conclusion

dynamically probable pathways providing two key complexes 4B and 6B, which were identified recently by the dynamic NMR spectroscopy. Thus, the following Scheme 3 of the formation and transformation of the complexes was proposed:

The mechanism of intermediate formation in the reaction of HAlBui2 with Cp2ZrCl2 has been studied by DFT calculations. The calculations allowed us to estimate the thermo-

On the first step, the interligand exchange between HAlBui2 and Cp2ZrCl2 runs through the bridge intermediate [Cp2ZrCl2 · HAlBui2] (7A), which further dissociates into Cp2ZrHCl (5) and


ClAlBui2. The next stage implies the subsequent interaction of 5 with a new molecule of HAlBui2 and formation of intermediate [Cp2ZrHCl · HAlBui2] (10A). This complex could transform by two pathways: dimerization into complex [Cp2ZrH2 · ClAlBui2]2 (4B) and reaction of 10A or 4B with HAlBui2 to give trihydride compound [Cp2ZrH2 · ClAlBui2 · HAlBui2] (6B). The formation of complex 6B is reasonably irreversible. The transformations of complex 6B into complexes 6C, 15, 16, 17A, 17B, 18, which are potentially inactive in the reactions with olefins, are proposed.

Acknowledgment. The authors thank the Foundation of the President of Russian Federation (Program for Support


of Leading Scientific Schools, to U.M.D., Grant NSh2349.2008.3, Program for Support of Young Ph.D. Scientists, to L.V.P., Grant MK-4526.2007.3), and the Russian Foundation of Basic Research (Grant No. 08-03-97010) for financial support. Supporting Information Available: Tables of Cartesian coordinates, total energies, and thermodynamic data of all species mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org. OM800393J

DFT Study on Mechanism of Olefin ... - ACS Publications

Recommend Documents