Paired interacting orbitals (PIO) study on the formation of the active site in Ziegler catalyst system

Journal of Molecular Catalysis, 87 ( 1994 ) 243-262 Eleevier Science B.V., Amsterdam

243

M306

Paired interacting orbitale (PIO)study on the formation of the active site in Ziegler catalyst system AkinobuShiga’**,Hiroshi Kawamura-Kuribayashib,Toshio Sasakib “Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd., 6 Kitahara, Tsukuba 300-32

(Japan) bChiba Research Laboratory, Sumitomo Chemical Co., Ltd., 2-1, Kitasode, Sodegaura-shi, Chiba 299-02 (Japan) (Received July 12,1993; accepted September 9,1993)

Abstract The mechanism of the active site formation in Ziegler polymerization catalysts has been studied using the paired interacting orbitale (PIO) based on an extended Htickel calculation. Deformation of a titanium chloride molecule and partial dissociation of an organoaluminum dimer have been shown to take place by approaching the two species together. A six membered cyclic intermediate complex hae been formed in a reaction plane, involving the Ti atom, the C atom, two Al atoms, the Cl* atom and the bridging atom. An elongation of the C-Al bond, Al-bridging atom bond, the Ti-Cl* bond and shrinkage of the C-Ti bond take place in concert in the transition state region. Key factors of activating the complex are the valency of the titanium and the structure whether or not it has a Cl anion trans to the Cl* atom. An active site is the complex between the alkylated titanium chloride and the aluminum chloride produced in the course of the reaction. The formation of the active site in titanium chlorides/organoaluminum systems has been clarified to become easier in the following order: d’-Sq (equare) pyramidal> do-Sq pyramidal 3 do-hinge like titanium chloride. do-triangle pyramidal titanium chloride is unfavorable for the formation of active sites. Key words: alumini~, Natta catalyst

extended HUckel calculation paired interacting orbit.& titanium, Ziegler-

Introduction One of the most important discoveriesin this century in chemistry and in chemical industries is that of the Ziegler-Natta catalysts for the polymerization of olefins. Highly active catalysts have modernized the polyolefin manufacturing processes and now gas phase processes are becoming the mainstream of new production technologies. Many studies have been reported on the Ziegler polymerization mecha‘corresponding

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A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

244

nism. However there are still unsolved problems, for example, the mechanism of active site formation. Though the proposition that the active site is a metal (titanium)-alkyl bond has widely been accepted, only a limited number of studies on the alkylation of the titanium compounds with organoaluminum compounds have been reported so far [ 11. Minsker et al. [ 21 reported a theoretical study on physical adsorption and succeeding chemisorption of organoaluminum compounds on the crystal surface of transition metal halides. They obtained the structure of the chemisorbed complex and suggested the important role of the alkyl-aluminum compound chemisorbed on the crystal surface of Tic&. However, the successive initiation step is still vague. Recent progress in computational methods with ab initio molecular orbital theory makes it possible to determine the potential energy profile of a full catalytic cycle [ 31. However, from a practical point of view, ab initio calculation is not easy to do for such large catalytic systems as used in industry. Furthermore molecular orbitals are delocalized over the whole molecular frameworks in these systems. It is not easy to see the interactions between two large molecular systems. Fujimoto et al. [ 41. proposed a method for determining unequivocally the orbitals which should play dominant roles in interactions between two systems. The interaction was represented compactly in terms of a few pairs of localized orbitals. In each orbital pair, one orbital belongs to one fragment species and the other orbital to the other fragment species. They called those orbitals “paired interacting orbitals” (PIO). Although this analysis was proposed originally for ab initio calculations, we reported that this approach was also useful in analyzing the results of extended Hiickel calculations [ 51. This method is particularly of use as a conventional tool to gain insight into the reaction mechanisms of large catalytic systems without invoking time-consuming calculations. Here we report a PI0 analysis on the active site formation between titanium chlorides and alkylaluminums.

Method The active site formation is schematically shown by equation (1) . TiCl, + [R,AlClS_,]2

-+RTiC1,_l +R,_1AlCl~_,~R,A1C13_,

(1)

where R = H or alkyl. The model compounds which we employ here are (a) titanium chlorides: do and dl-sq pyramidal- [Tic&] n (where n is - 1 or - 2 ) and do-modified-Td[ TiCld]O and (b) organoaluminum compounds: [ H,Al] 2, [ H,AlCl], and [CH,(H)AlCl.H,AlCl]. The geometries of these compounds are given in the Appendix.

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

245

We study the active site formation by the following procedures: (1) A reaction path has been determined by an ab initio calculation for the simplified molecular system ( [Tic&] -‘/ [ H,Al] 2). (2) We have determined the model structure of the key states of the reaction described above by referring to the result of (1) . (3) We have calculated extended Htickel MOs of these states and obtained PIOs according to the procedure proposed by Fujimoto et al.

Ab initio calculation We used the restricted Hartree-Fock energy gradient technique of GAUS-

SIAN 90 for the geometry optimization [ 61. The basis functions used for Ti were MID14 of Huzinaga et al. [ 71 for the 5F state, which were augmented by two sets of valence p functions (exponent=0.083 and 0.028) with the overall splitvalence contraction (43321/4311/ 31). For C, H, Al and Cl atoms we used the 3-21G [8,9] basis functions.

PI0 analysis Molecular orbitals were calculated by the extended Htickel method [lo]. The extended Hiickel parameters are given in the Appendix. PIOs were obtained by applying the procedure proposed by Fujimoto et al. [ 41. It is summarized as follows: (1) We expand the MOs of a complex in terms of the MOs of two fragment species, by determining the expansion coefficients cid, cm+jdand dkf, d,+lf in Eq. (2)

f=1,2,...,m+n

(2)

where @ are MOs of the complex [C I,# and v/ are the MOs of the fragment [A] and [B ] respectively, m and n indicate the number of occupied MOs of A and of B, respectively, and M and N represent the number of basis functions of A and of B, respectively. (2) We construct an interaction matrix P which represents the interaction between the MOs of the fragment [A] and the MOs of the fragment [B]

(3) in which

246

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

=2 C Ci#h,/

Pi,k

f=l

i=l-m&=1-n i=l-m&=1-N-n

f=l m+n ~2

Pm+j,k

C f=l

Gn+j&kj

j=I-M-m&=1-n

m+n

pm+j,n+l

=2 f=lC Cm+jjdn+ljj=I-M-m,l=l-N-n

(3) We get transformation matrices U* (for A) and Un (for B) by p+pu* = uJ*r u?v=

(4)

(Y”)-‘+P,~u$ r

(v=1,2...N)

(5)

(4) and finally we calculate the PIOs by Eqs. (6) and (7) C;=&&% r

(forA)

(6) (7)

The NxM (IV< M) orbital interactions in complex C can thus be reduced to the interactions of N PIOs, N indicating the smaller of the numbers of MOs of the two fragments, A and B. PI0 calculations were carried out on an LUMMOX system with NEC PC98OlRA [ll].

Results and discussion 1. Determination of a reaction path We employed a simplified model system: [ ClvTiCll] -I/ [ H,Al] 2. Four intermediates (H-14) were obtained by ab initio calculations of the system by assuming the Cs symmetry with respect to the plane composed of the Ti atom, Cl* atom and two Al atoms. The structure of the rest of the titanium chloride has been fixed. The structures obtained are shown in Fig. 1. The intermediate states (11) and (12 ) represent the complexes at an early stage of the reaction in which the H3Al dimer retains its hydrogen bridge structure. Partial dissociation of the HBAl dimer has taken place, standing for the

-iO-

0.0

lo-

AE : Ei - E(12) (i) state

8.7

0.0

Fig. 1. Energy profile of the model system: [ CI’TiCl, ] -‘/ [H,Al j2.

AE

03)

-

248

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

proceeding of the reaction. Then, the intermediate complex (13) which has a six membered ring composed of Ti, H*, All, H, Al2 and Cl*, is formed. The HOMO of the partially dissociated H3Al dimer has a large amplitude on the H* atom and the LUMO of it has a large amplitude on the Al2 atom. A remarkable change in the structure is observed on the stage in going from complex (13) to complex (14). In complex (14) we see the formation of the Ti-H* bond and an elongation of the Ti-Cl* bond. Since the formally dissociated state (5) is very unstable, it is not appropriate to consider the state (5) as a final state. We explore next the route from complex (13) to complex (14) in detail by applying the PI0 analysis. We assume here two routes to produce the active site. Route 1: the fragment H2A11and the fragment molecule H-A12H2 roll out of the Cs symmetry plane toward the x-y plane. Route 2: the fragment H,Al’ moves downward to approach the Cl* atom in the Cs symmetry plane and the fragment molecule H-A12H2 moves also downward in the Cs symmetry plane, keeping the coordination to the Cl* atom, in a cooperative manner. Then, the fragment H,Al’ and the hydrogen atom of H-Al2 roll out of the Cs plane toward the X-Y plane. We assume also several model states along these routes by referring to the results of the ab initio calculation described above. The structure of the model (B) is illustrated in Fig. 2. The bond distances of Cl*-Al2 (2.60 A), A1’r2-H (1.55 A) and the vertical distance between Al’ and the x-y plane (2.30 A) are fixed. The distances between H* and Al’ ( R1), between H* and Ti (R, ) and between Cl* and Ti ( R3) and the rotation angles 0, and 0, are varied. The Hiickel total energies of those models are summarized in Table 1. The total energies of the models B2 and B4 tell us that Route 1 is less favourable than Route 2. A relatively small energy barrier, less than 0.5 eV, is observed along Route 2. As the reaction proceeds, new bonds are formed between the H* atom and

Fig. 2. The structure of the model (B).

249

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262 TABLE I Hiickel total energies of the model (B )

Bl B2 B3 B4

E B6 B7 B8 B9 BlO Bll B12 B13

RI (A)

& (A)

& (A)

1.62 t 1.82

2.00

2.30

t t t t t

t t t t t

t

2.02 2.32 2.69

1.60

t t t t t t

t t

t t

t t

03 (4

2.60 t t

t t t t

(El

0,”

0:

F)

0

0

45

45

0

0

45

45

0 0 0

0 0 0

45 90 180 0 0 180

0 0 0 45 180 180

Total energy (eV) -946.26 - 944 78 Szi4 - 945.08 - 945.97 - 945.68 - 945.17 - 945.65 - 946.06 -946.11 -945.81 -945.85 - 945.92

G) 04

Fig. 3. The division of the model (B) into (a) two fragments D and E corresponding to the reactants, (b) two fragments F and G corresponding to the products.

the Ti atom and between the Al’ atom and the Cl* atom, whereas old bonds are weakened between the H* atom and the Al’ atom and between the Ti atom and the Cl* atom. We divided the model complex B into two parts: one way is dividing complex B into the titanium chloride (fragment D) and the H*A1’H,*H,A12 molecule (fragment E), which correspond to the reactants. The other way is division into the hydrotitanium chloride (fragment F) and the H2A1’Cl.H3A12 molecule (fragment G) which correspond to the products. They are shown in Fig. 3. The geometry of the combined systems D and E and that of F and G are the same as those of the intermediate complex (B ) ( [D + E ] = [F + G ] 3 (B ) ) .

250

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

We follow the progress of the reaction by comparing the contour maps and overlap populations of the PIOs. Here we look at the top two PIOs (PIO-1 and PIO-2) that have much larger contributions to the interaction than the other orbital pairs. The contour maps of the PIO-1 and -2 between the fragment D and the fragment E and those between the fragment F and the fragment G in the intermediate complex Bl and in the intermediate complex B7 are shown in Fig. 4 and in Fig. 5, respectively. As for the definition of models Bl and B7, see Fig. 2 and Table 1. We see in-phase overlaps in the regions between the H* atom and the Ti atom, between the All atom and the Cl* atom and between the Al2 atom and the Cl* atom (see Fig. 4.). These in-phase overlaps are stronger in complex B7 than in complex Bl. This indicates that new bonds are formed between that pair of atoms. We see also in-phase overlaps in the regions between the H* atom and the Al’ atom and between the Ti atom and the Cl* atom (see Fig. 5. ) . The in-phase overlap between the H* atom and the Al’ atom is weaker, however, in complex B7 than in complex Bl. The bond between the H* atom and the Al’ atom has almost disappeared. Thus, we estimate that complex B7 comes out in the neighborhood of the final stage of the reaction. We may confirm this result by comparing the overlap populations and the dissociation energies between the fragments D and E with those of the fragments F and G. These overlap populations and coordination energies are summarized in Table 2.

PIO-1

PIO-2

X

(a)

(b)

Fig. 4. The contour maps of the PIO-1 and PIO-2 between the fragment D and the fragment E, (a) the model Bl, (b) the model B7.

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

251

PIO-1

PIO-2

!-

PX

(a)

(b)

Fig. 5. The contour mapsof the PIO-1 and PIO-2 between fragment F and fragment G, (a) the model Bl, (b) the model B7.

Both the sum of the overlap populations of the PIO-1 and -2 and the coordination energies ( -A&) increase graduallyon going from complex Bl to complex B7 in case of the division of (B ) into D and E. On the contrary,both the overlappopulations and the coordinationenergies ( - dE, ) decreasegradually on going from complex Bl to complex B7, in the case of the division of (B) into F and G. At the transitionstate, ( -LIE,) shouldbe equalto ( -d&J. It is seen, however,that ( -LIE,) is much smallerthan ( -dE,) at the complex state, B7. This means that the structuralchange takes place easily to give the product from complex B7. From the resultsdescribedabove, we conclude that the active site formation proceeds in the following steps : (1) Deformation of a titanium chloride moleculetakes place by interacting with an aluminumhydridedimer, (2) a complex is formed between the partially dissociatedaluminumhydride dimer and the deformedtitanium chloride, (3) the four main structuraldeformations,elongationsof the H*-Al1bond, of the All-H bond and of the Ti-Cl* bond and a decrease in the distance between H* atom and a Ti atom, occur in concert, keeping Cs symmetryin the complex formed above, (4) and then, the fragmentH2A1’and the fragmentmoleculeH-A12H2roll out of the Cs plane downwardto their positions in the final product. A final product (an active site) should be the complex, [HTiCld*H2AlCl HaAl] -. 2. The effect of orgarwaluminum compounds We the compare [Tic&]-‘/[H,AlCl], and [Tic&] -‘/ [ CH3( H)AlC1.H2AlCl] systemswith the [Tic&] -‘/ [ H,Al] 2 system.

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

252 TABLE 2

Overlap populations of PIO-1 and PIO-2 of the model complex (B) and the coordination energy -A& and - AEz. (a) Division of models into two fragments D and E; (b ) division of models into two fragments F and G Model

(a)

(b)

PIO-1

PIO-2

1

-AE; (eV)

PIO-1

B1(Bol)

0.094

B3 B5(B,,5) BW%6) B7

0.109 0.122 0.141 0.189

0.148 0.155 0.164 0.177 0.190

0.242 0.264 0.286 0.318 0.379

0.59 0.80 0.99 1.27 1.94

0.239 0.195 0.144 0.091 0.096

0.109 0.112 0.117 0.102 0.023

0.348 0.307 0.261 0.193 0.119

1.09 1.05 0.78 0.66 0.50

B11 B15 B16 A12

0.085 0.121 0.140

0.148 0.160 0.174

0.233 0.281 0.314

0.85 1.40 1.66

0.234 0.092 0.082 0.077

0.110 0.111 0.060 0.073

0.344 0.203 0.142 0.150

1.28 0.42 0.36 0.29

0.068 0.098 0.102

0.091 0.141 0.185

0.159 0.239 0.287

0.65 0.79 1.31

0.125 0.116 0.050 0.067

0.135 0.114 0.066 0.081

0.260 0.230 0.116 0.148

0.81 0.46 0.06 0.24

0.032’ 0.034 0.035’ 0.049 0.039” 0.051

0.045’ 0.045 0.070” 0.070 0.090” 0.092

0.156

0.58

0.19

0.45

0.228

0.24

0.272

0.96

0.067” 0.068 0.057” 0.057 0.044” 0.033 0.026” 0.040

0.257

0.224

0.060’ 0.062 0.056” 0.058 0.003” 0.025 0.029” 0.034

Bsl B,5 B,6

BJ B,5

1

0.105

-AE; (eV)

-0.17

0.129

0.05

0.080 0.111 0.111

0.091 0.141 0.188

0.171 0.252 0.299

1.10 1.76 2.29

0.120 0.111 0.062 0.067

0.135 0.114 0.058 - 0.003

0.255 0.225 0.120 0.064

1.32 0.72 0.33 0.07

0.068 0.095 0.098

0.090 0.140 0.184

0.158 0.235 0.282

0.70 0.92 1.42

0.133 0.122 0.080 0.098

0.134 0.113 0.055 0.064

0.267 0.235 0.135 0.162

1.89 1.12 0.70 0.82

z Bsl Bc5 BS6 A,2

PIO-2

*AE,= [EB- (ED+EE)]. bAE,= [EB- (EF+&)]. =ff spin.

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

253

TABLE 3 The Hiickel total energies of the model states

i=

AiO

0

1 2 3 4 5

-

-946.17 1190.97 1297.86 1305.76 1147.58 1147.58

Ail

-

- 945.67 1190.41 1296.98 1304.94 1145.62 1146.08

Bil

-

-946.26 1189.70 1295.72 1303.88 1145.06 1145.25

BP -

- 945.97 1189.49 1295.67 1303.93 1145.28 1144.86

Bi6

-

- 945.68 1189.39 1295.98 1304.23 1145.60 1145.15

Ai

-

- 945.92 1189.57 1296.11 1304.79 1145.48 1145.47

We have chosen six states (A$), Ail, Bil, Bi5, B,6 and Ai2) along the reaction path as described in part 1, corresponding respectively to the reactants, the intermediate complex 13, the complexes in the transition region Bl, B5 and B6 and the final complex 14. The suffix i represents the reaction systems: i=O; do-Sq pyramidal i=2; do-Sq [TiCM-‘/W&11~, i=l; do-Sqpyramidal [Tic&]-‘/[H,AlCl],, pyramidal [Tic&] -‘/ [CH,(H)AlCl*H,AlCl], i=3; d’-Sq pyramidal [TiC1,]-2/ [CH,(H)AlCl*H2AlC1], i=4; do-hinge like [TiCld]‘/ do-trigonal pyramidal [ TiCl, ]“/ [CH,(H)AlCl*H,AlCl] and i=5; [ CH3 (H) AlCl*H,AlCl] . Their extended Hiickel energies are summarized in Table 3. [TiC1~-*/[H+41C1]2 system Fig. 6 shows the energy of the intermediate states relative to the reactants (AiO). Most of the intermediates, including Ail and Ai2, are unstable in comparison with the reactants, because their structures have not been optimized. The extended Hiickel calculation may have some limitations to determine the energetics of a process involving bond breaking and formation. It is possible however to estimate whether or not a sharp and large change in energy (more than 1 eV) appears along the reaction path. Comparing the [H,AlCl], system with the [H,Al], system, a relatively large amount of energy is necessary to reach B1l from All. The energy difference is due to the difference in the dissociation energies of the two organoaluminum dimers. Being activated to B1l, the energy profile going from B1l to A12 is flat and similar to that from Bol to &2. From a comparison of Fig. 7 (a) and (b) with Fig. 4 (a) and Fig. 5 (a) respectively, we find that orbital interactions in B1l, expressed by PIO-1 and -2, are very similar to those in Bol. The B1l state is rather closer to the final product state than Bol is. This is shown by the fact that the energy difference, -LIE, - ( -dEl ) , is smaller in B,l than in Bol (see Table 2).

254

A. Shiga et al. /J. Mol. Catd. 87 (1994) 243-262

[B/1,5,61

[A/l1

X

[A121 -

i = 0 : do-Sq pyramidal ~iCl#/

....

i - 1 : do-Sq pyramidal [nC15]-‘/ [HzAICI 12

[HsAI 12

2.00 g Q

....

....

B/5

B/6

*.*.

....

W

l.OO-

-

I

A/l

A/O

Bjl

A/2

Fig. 6. The energy profile of the [TiClJ -‘/ [ H,AlCl ] 2system in the extended Hiickel calculation dE=E(Ain) orE(Bin)-E(A,O).

PIO-1

PIO-2

2 f

L--tX

(4

(W

Fig. 7. The contour maps of the PIO-1 and PIO-2 in B1l, (a) the pair of the fragments D and E, (b) the pair of the fragments F and G.

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

255

[TiCl~-‘/[CH,(H)AZCZ*H~ZC1] Fig. 8 shows the energyprofile of the [ TiCIS]-‘/ [ CH3( H)AlC1*H2A1C1] system. The AJ state is not so stable as &l because of the steric repulsionbetween the methyl group and the Cl atom that is located trans to the Cl* atom. A tilt of the methyl group in the Cs symmetryplane is necessaryfor effective coordination of the methyl groupto the titaniumatom in the neighborhoodof the transition state. Since the dissociation energy of the methylaluminum compound is similar in magnitudeto that of the [ H,AlCl] 2, it requiresadditional energyfor the tilting of the methyl group to reach the transitionregion in the [ TiCl,] -‘/ [ CH3(H)AlCl-H,AlCl] system. The contour maps of the PIO-1 and -2 betweenthe fragment (D) and the fragment (E) and those between the fragment (F) and the fragment (G) for BJ are shown in Fig. 9 (a) and (b), respectively. Orbital interactionsshown in Fig. 9 (a) and (b) are basicallythe same as those shown in Fig. 7 (a) and (b). From the result that the value of -AE,- ( -AE,) is negative and that AE, is very small in the B26 and A22 states (see Table 2), it is suggestedthat the coordination of the aluminum

‘i

**... p _Ti__&__-.-~A&,

/I i_ [Ai 11

[AI 01

&-Cl

Q

[B/ 1,5,61

Cl

[A, 21

i I 2 : do-Sq pyramidal(TiCI& [CH3(H)AICI~H2AICI] .... i = 3 : d’-Sq pyramidal[rCl~~~/ [CH3(H)AICI.H2AICI ]

-

3.00-

2.00 -

....

z w a

....

-

l.OO-

....

....

A/O

Ail

B/ 1

B/6

A12

[Tic&] / [ CH, (H )AlCl~H&lCl] system in the extended orE(Bjn)-E(A,O).

Fig. 8. The energy profile of the do, or d’-

HtickelcalculationdE=E(Ain)

B/ 5

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

256

PIO-1

0

PIO-2

z

L

X

(4

(b)

Fig. 9. The contour maps of the PIO-1 and PIO-2 in BJ, (b) the pair of the fragments F and G.

(a) the pair of the fragments D and E,

compound to the methyltitanium chloride is weaker in this case than in the case of [Tic&] -‘/ [ HzAICl] 2 system. 3. The effect of titanium chlorides We examine next the dl- [Tic&] -“/ [ CH3 (H )AlCl.H,AlCl] system and the do-modified Td- [Tic&]‘/ [ CHB(H) AlCl*H,AlCl] system. We have chosen the same six states along the reaction path as described in part 2. The extended Hiickel energies are summarized in Table 3. The overlap populations, ( -dE, ) and ( -dE,) are given in Table 2. d1-[~Cl~-2/[CH,(H)A1Cl*H~1CI/ system The energy profile of the system is shown in Fig. 8. In comparison with the do-system, it requires relatively less energy to reach the Al state in the d’ system. In the do system, the titanium chloride in A21 is less stable than that in A20, because the Ti-Cl* bond in A21 has a Cl anion trans to Cl*. On the other hand in the d’-system, the SOMO energy ( -8.22 eV) of the titanium chloride in AS1 is more stable than that ( - 7.90 eV) in A30. The instability of the titanium chloride in As1 is somewhat compensated by the stabilization of the SOMO in the dl-system. As shown in Fig. 10, orbital interactions in the d’system are basically the same as those in the do-system shown in Fig. 9, in the a! spin part. The coordination energies ( -dE, or -dE,) of the d’-system are smaller than those of the do-system because of the repulsion between the SOMO of the titanium fragment and the methyl group or the Cl* atom, as is expressed by

257

A. Shiga et al. /J. Mol. Catal. 67 (1994) 243-262

PIO-1

PIO-2

(a)

X

(b)

Fig. 10. The (Yspin contour maps of the PIO-1 and PIO-2 in BJ, (a) the pair of the fragments D and E, (b) the pair of the fragments F and G.

the difference in the overlap population of a! spin electrons and that of /I spin electrons (see Table 2 ) *. The methylation of the [Tic&] is more favorable in the d’ state than in the do state. d”-[~Cl~o/[CH,(H)AICl~H~lC~ system Two structures of titanium chlorides are possible on complex formation, depending on the direction of attack of the aluminum compound. One is a hinge like structure in which a Cl anion is located trans to the TiCl* bond (Eq. 8) and the other is a trigonal pyramid in which no Cl ligand is located trans to the Ti-Cl* bond (Eq. 9).

(8) 48 Cl

Cl

Cl

Cl

4\ Cl

Al compound

Cl

Cl

*In the case of BJ or B&i, the difference is expressed in the PIO-3.

258

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262

Energyprofiles of these systemsare shown in Fig. 11. The A41 state is less stable than A51because of the trans Cl anion effect in the hinge like structureof the titaniumtetrachloridein AJ. In the transition region [B], the methyl group approachesthe Ti atom, whilethe Cl*atom leavesfrom the Ti atom. In the case of Eq. (8)) the approach of the methylgrouptakes place easilybecausethe trans Cl anion is absent. On the contrary, in the case of Eq. (9) the approach of the methyl group is not easy because of the trans Cl anion effect on the incomingmethyl group and of the repulsion between two equatorial Cl anions and the methyl group. The absence of the trans effect on the Cl* atom in the approach (9) gives rise to a relativelylargecoordination energy ( -LIE,) (see Table 2 ) . It is suggested, therefore, that the methylation in the do-[TiC14]‘/ [CH3(H)AlCl*H,AlCl] system occurs along the reaction path in which the titaniumchloride has a hinge like structure. In comparisonwith the do-[Tic&] -’ system,it needs a considerablylarge amount of energy to reach A,1 in the do-[TiC14]0system, because the trans effect in the hinge like structureof the titaniumchloride is strongerthan that in the squarepyramidalstructureof the titaniumchlorideof AJ. Once a tran-

[*i 11

[*iOl

(B/1,5,61

i = 4 : do-hinge like (TiCI,]‘/ [CH3(H)AICI.H2AICI ] i = 5 : do-trigonal pyramidal (TiC14]“/ [CH3(H)Al.H2AICI

]

(A/ 21

..

-

-..

Bi 1

B/5

B/6

....

2.00

Q

Cl

1.00

1

A/O

Ai 1

Ai

Fig. 11. The energy profile of the [TiCl, ] / [ CH3 (H )AlCl~H,AlCl ] system in the extended Hiickel calculation dE=E(Ain) or E(Bin) -E(A,O).

A. Shiga et al, /J. Mol. Catal. 87 (1994) 243-262

259

sition region [B] is reached, no significant difference is observed in the energy profiles.

Conclusion Active site formation of Ziegler catalysts schematically shown by equation (1’ ) proceeds by the following steps: Cl*-TiCl, +R,A1’X,_,*R,A12X,_, +R-TiCl,

+R,_1A11(C1)X3_,.R,A12X3_,

(1’ )

R = H or alkyl, X = H or alkyl or Cl. (1)Deformation of the titanium chloride is initiated by an approach of an organoaluminum dimer, (2) a titanium complex is formed between a partially dissociated organoaluminum dimer and the deformed titanium chloride in a Cs symmetry plane, (3) the structural deformation of the complex, which includes elongation of the bonds between the organ0 group and the Al’ atom, between the Al’ atom and the bridged atom X and between the Ti atom and the Cl* atom and shrinkage of the bond between the organ0 group and the Ti atom, occur in concert, keeping the Cs symmetry, and (4) the complex is led into an active site. The dissociation energy of an organoaluminum dimer and the deformation energy of a titanium chloride are important for the formation of the titanium complex. Key factors controlling the activation of the titanium complex are the valency of the titanium and whether or not the complex has a Cl anion trans to the Cl* atom. As a result, the active site formation in titanium chloride/ organoaluminum systems becomes easier in the following order: trialkylaluminum dimer > dialkylaluminum chloride dimer, and dl-Sq pyramidal > do-Sq pyramidal z do-hinge like titanium chloride. It is natural to assume that the active site of the alkylated titanium compound is forming a complex with the aluminum compound, and the incoming olefin pushes off the weakly complexed aluminum compound to initiate the polymerization of the olefins.

Acknowledgments We thank Prof. H. F’ujimoto for helpful advice and Sumitomo Chemical Co. for permission to publish.

A. Shiga et al. 1 J. Mol. Catal. 87 (1994) 243-262

260

References 1

2

3 4

5

6 7 8 9 10 11

(a) G. Natta, J. Polymer Sci., 34 (1959) 21; (b) L.A.M. Rodriguez, H.M. van Looyand J.A. Gabant, J. Polymer Sci. A-l, 4 (1966) 1905; (c) L.A.M. Rodriguez, H.M. van Looy and J.A. Gabant, J. Polymer Sci. A-l, 4 (1966) 1917; (d) H.M. van Looy, L.A.M. Rodriguez and J.A. Gabant, J. Polymer Sci. A-l, 4 (1966) 1927; (e) L.A.M. Rodriguez and H.M. van Looy, J. Polymer Sci. A-l, 4 (1966) 1951; (f ) L.A.M. Rodriguez and H.M. van Looy, J. Polymer Sci. A-l, 4 (1966) 1971. (a) K.S. Minsker, M.M. Karpasas, Yu.B. Monakov and G.E. Zaikov, Eur. Polymer J. 21 (1985) 973; (b) O.A. Ponomarev, K.S. Minsker, V.M. Pshenichnikov and Yu.A. Sangalov, Vysokomol soyed. A17, (1975) 309. H. Kawamura-Kuribayashi, N. Koga and K. Morokuma, J. Am. Chem. Sot., 114 (1992) 2359. (a) H. Fujimoto, N. Koga and K. Fukui, J. Am. Chem. Sot., 103 (1981) 7452; (b) H. Fujimoto, N. Koga and I. Hataue, J. Phys. Chem., 88 ( 1984) 3539; (c ) H. Fujimoto, T. Yamasaki, H. Mizutani, and N. Koga, J. Am. Chem. Sot., 107 (1985) 6157. (a) A. Shiga, H. Kawamura, T. Ebara, T. Sasaki, and Y. Kikuzono, J. Organomet. Chem., 366 (1989) 95; (b) A. Shiga, H. Kawamura-Kuribayashi and T. Sasaki, J. Mol. Catal., 77 (1992) 135; (c) A. Shiga, H. Kawamura-Kuribayashi andT. Sasaki, J. Mol. Catal., 79 (1993) 95. J.A. Pople et al, GAUSSIAN 90, Gaussian Inc., Pittsburgh, PA, 1990. S. Huzinaga, J. Andzelm, M. Klobukowski, E. Radzio-Andzelm, Y. Sakai and H. Tatewaki, Gaussian Basis Sets For Molecular Calculations, Elsvier, Amsterdam, 1984. J.S. Binkley, J.A. Pople and W.J. Hehere, J. Am. Chem. Sot., 102 (1980) 939. M.S. Gordon, J.S. Binkiey, J.A. Pople, W.J. Pietro and W.J. Hehere, J. Am. Chem. Sot., 104 (1982) 2797. J. Howell, A. Rossi, D. Wallace, K. Haraki and R. Hoffmann, QCPE Program No. 344. H. Katsumi, Y. Kikuzono, M. Yoshida, A. Shiga, and H. Fujimoto, Chem. Info. and Comp. SCl. (Japan) Preprint, 12 (1989) 72.

Appendix Geometric parameters of the models are given in Table 4. Coulomb integrals and orbital exponents are listed in Table 5. TABLE 4

Bond lengths (A) and bond angles ( ’ ) of the models Organoaluminums

Bond lengths Al-H Al-Cl Al-c C-H Ti-Cl

Trigonalbipyremidal [ CPTiCl, ]

Modified tetrahedral [CPTiCl,]

Intermediate complex A,,1

A11

AJd

Ai2’

(A) 1.55 2.30 2.00 1.10 2.15

2.15

1.55 2.30

1.55 2.30

2.15

2.15

1.55 2.30 2.00 1.10 2.15

1.55 2.30 1.10 2.15

261

A. Shiga et al. /J. Mol. Catal. 87 (1994) 243-262 TABLE 4 (Continued) Organoaluminums

Trigonalbipyramidal [ClTiClJ

Modified tetrahedral [Cl’TiCls]

Ti-H* Ti-Cl* Ti-C All-X A12-X A12-Cl*

Intermediate complex &l

A,1 2.00 2.30

2.00 2.30

1.64

Aild

Ai2”

2.71

2.30 3.45 2.41

1.60 2.60 2.30 2.30

2.52

2.43

2.60

Bond angks (“) L&AW LXbAUrbb LA&41 LHCH LHCAI L HCTi L ClTiCl L ClvTiCl L TiH*Al’ L TiCAI’ L H*AllX L CAI’Cl L Al’XAl* L Al’CAP L XAwl’ L clAl*c1* L AwlTi L TiCl*Al’ L cl*Allxz

124.0 90.0 90.0 109.5 109.5 90.0 135.0

90.0 135.0

124.0

124.0

90.0 180.0 152.0

90.0 180.0 180.0

124.0

90.0

112.0

135.0

132.0

95.0

120.0

130.0

124.0

124.0

144.8’ 90.0 180.0

109.5 90.0 180.0

91.4 136.4 145.2 84.5 155.5 90.0 180.0

‘&: terminal H, C, Cl. bXb: bridge H, Cl. ‘Tilt angle of the methyl group. di=3-5. “i= l-5 TABLE 5 Extended Htickel parameters Orbital

Hii

Q

(2

Cl

c2

H 1s c 2s C 2P Al 3s Al 3p Cl 38 Cl 3p Ti 4s Ti 4p Ti 3d

- 13.6 -21.4 -11.4 - 12.3 -6.50 -30.0 - 15.0 -8.97 -5.44 - 10.81

1.30 1.625 1.625 1.167 1.167 2.033 2.033 1.075 0.675 4.55

1.40

0.4206

0.7839