Organometallics 1996,14, 964-974
964
Molecular Orbital Study on Acetylene Insertion into a Zr-R 0 Bond (R = H, CH3) in Cationic Zirconocene Complexes Isabella Hyla-Kryspin, Shuqiang Niu, and Rolf Gleiter" Organisch-Chemisches Institut der Universitat Heidelberg, I m Neuenheimer Feld 270, D 69120 Heidelberg, Germany Received July 19, 1994@ All-electron RHF and RMP2 ab initio calculations with split valence basis sets of doubleand triple-6 quality have been used to study reaction paths for the reaction of acetylene (1) with the model compounds C12ZrH+ (2) and C12ZrCH3+ (3). At the RMPB//RHF level the insertion process is calculated to have a barrier of 0.2 (2) and 5.1 kcaUmol(3) with respect to complexed acetylene and to be 86.9 (2) and 53.3 kcaUmol(3) exothermic with respect to free acetylene. The transition states (TS) are four membered cycles. The products and TS of ClZZrCHs(C2Hz)+are stabilized through an agostic interaction. The calculated energetics of the reaction path are compared with available theoretical as well as experimental data for insertion of unsaturated hydrocarbons into metal-R (R = H, CH3) u bonds. On the basis of extended Hiickel calculations a MO picture of the insertion path is provided for the reaction of acetylene with CpZZrH+ and CpZZrCHs+.
Introduction The insertion reactions of olefins and acetylenes into transition-metal hydrogen and carbon bonds are the fundamental steps in industrially important catalytic processes such as hydrogenation, hydroformylation, isomerization,and polymerization as well as in stoichiometric transformation of organic systems.l In the present paper we will report the results of ab initio and extended Huckel MO calculations concerning the insertion reactions of acetylene, which are closely related to Ziegler-Natta olefin polymerizations.2 During the past half-century many studies with various experimental3 and theoretical methods4have been carried out in order to understand the basic principles of this polymerization and to identify the active catalyst of the catalytic systems. Although initial efforts by Dyachkovskii et aL5 to identify the catalytically active species were unsucAbstract published in Advance ACS Abstracts, December 1, 1994. (1)(a) Parshall, G. W. Homogenous Catalysis; Wiley: New York, 1980.(b) Wilkinson, G., Stone, F. G. A,, Abel, E. W. Eds. Comprehensive Organometallic Chemistry; Pergamon Press: New York, 1982.(c) Dotz, K. H., Hoffmann, R. W., Eds. Organic Synthesis uia Organometallics; Vieweg: Braunschweig, Germany, 1980. (2)(a)Natta, G.; Pino, P.; Mazzanti, G.; Giannini, U. J.Inorg. Nucl. Chem. 1958,8,612. (b) Breslow, D. S.; Newburg, N. R. J.A m . Chem. SOC.1959,81,81.(c) Ziegler, K.;Gellert, H. G.; Zosel, K.; Holzkamp, E.; Schneider, J.; So11, M.; Kroll, W. R. Justus Liebigs Ann. Chem. 1960, 629,121.(d) Sinn, H.;Kolk, E.; J . Organomet. Chem. 1968, 6, 373. (3)Reviews on Ziegler-Natta polymerization: (a) Boor, J., Jr. Ziegler-Natta Catalysts and Polymerization; Academic Press: New York, 1979.(b) Pino, P.; Mulhaupt, R. Angew. Chem., Int. Ed. Engl. 1980, 19, 857. (c) Eisch, J . J.; Galle, J. E.; Piotrowski, A. M. In Transition Metal Catalyzed Polymerization; Alkenes and Dienes; Quirk, R. P., Ed.; Honvood: New York, 1983.(d) Eisch, J. J.; Boleslawski, M. P.; Piotrowski, A. M. In Transition Metals and Organometallics as Catalysts for Olefin Polymerization; Kaminsky, W., Linn, M., Eds.; Springer-Verlag: Berlin, Heidelberg, 1988. (e) Allen, G. B. Comprehensiue Polymer Science; Pergamon Press: Oxford, U.K., 1989. (4)(a) Armstrong, D. R.; Perkin, S. P. G.; Stewart, J . J . P. J.Chem. Soc., Dalton Trans. 1972,1972.(b) Sakaki, S.;Kato, H.; Kanai, H.; (c) Fukui, K.; Inagaki, Tarama, K. Bull. Chem. SOC.Jpn. 1975,48,813. S. J.Am. Chem. SOC.1975,97,4445. (d) Novaro, 0.; Blaisten-Barojas, E.; Clementi, E.; Giunchi, G.; Ruiz-Vizcaya, M. E. J. Chem. Phys. 1978, 68,2337.(e) Balazs, A. C.; Johnson, K. H. J . Chem. Phys. 1982,77, 3148. (0Fujimoto, H.; Yamasaki, T.; Mizutani, H.; Koga, N. J . Am. Chem. SOC.1985,107,6157.
cessful, they suggested that highly electrophilic, cationic metallocene alkyl complexes CpMR+ participate in the polymerization process. The recent experimental work of many groups6 provides compelling support for the identification of the Cp2MR+ species (M = group 4 metals) as active catalysts as well as for the widely accepted Cossee mechanism7 of the polymerization process. According to the Cossee mechanism the propagation step in the polymerization of olefins occurs via a prior coordination of the n bond to the vacant coordination site of the active catalyst, followed by olefin complexation through a four-membered transition state in a 2n 20 reaction involving the C-C n bond and metal-alkyl u bond. This recreates a vacant coordination site on the active catalyst, and the process continues. Recent theoretical investigationsssupport the view that the Cossee mechanism is indeed a reliable mechanism for homogeneous Ziegler-Natta polymerization. The insertion and a-bond metathesis reaction of single
+
@
0276-7333/95/2314-0964$09.00/0
( 5 ) Dyachkovskii, F. S.; Shilova, A. K.; Shilov, A. E. J . Polym. Sci., Part C 1967,16,2333. (6)For reviews see: (a)Rosenthal, M. R. J . Chem. Educ. 1973,50, (c) Beck, W.; Sunkel, 331.(b) Lawrance, G. A.; Chem. Rev. 1986,86,17. K. Chem. Rev. 1988,88,1405.(d) Lawrance, G. A. Adu. Inorg. Chem. 1989,34,145.(e) Bochmann, M.Angew. Chem., Int. Ed. Engl. 1992, 31, 1181.(0Straws, S. H. Chem. Reu. 1993,93,927. For a related example see: (g) Yang, X.; Stern, C. L.; Marks, T. J . J. Am. Chem. SOC.1991,113,3623.Angew. Chem., Int. Ed. Engl. 1992,31,1375. (h) Alelyunas, Y. W.; Jordan, R. F.; Echols, S. F.; Borkowsky, S. L.; Bradley, P. K. Organometallics 1991, 10, 1406. (i) Crowther, D. J.; Borkowsky, S. L.; Swenson, D.; Meyer, T. Y.; Jordan, R. F. Organometallics 1993,12, 2897.(i) Eisch, J. J.; Caldwell, K. R.; Werner, S.; Kruger, C. Organometallics 1991,10,3417. (7)Cossee, P. J.Catal. 1964,3,80.(b) Adman, E. J.; Cossee, P. J. Catal. 1984,3,99. (8)(a) Jolly, C. A.; Marynick, D. S. J.Am. Chem. SOC.1989,111, 7968. (b) Castonguay, L. A.; Rapp6, A. K. J. Am. Chem. SOC.1992, 114,5832.(c) Kawamura-Kuribayashi, H.; Koga, N.; Morokuma, K. J . A m . Chem. SOC.1992,114,2359. (d) Kawamura-Kuribayashi, H.; Koga, N.; Morokuma, K. J.Am. Chen. Soc. 1992,114,8687.(e) Rappe, A. K. Organometallics 1990,9,466.(0Ziegler, T.;Folga, E.; Berces, A. J . Am. Chem. SOC.1993,115,636.(g) Siegbahn, P. E. M. J.Am. Chem. SOC.1993,115,5803.(h) Woo, T. K.; Fan, L.; Ziegler, T. Organometallics 1994,13,432,2252. (i) Bierwagen, E. P.; Bercaw, J. E.; Goddard, W. A., 111. J.Am. Chem. SOC.1994,116,1481.(i) Axe, F. U.; Coffin, J. M. J . Phys. Chem. 1994,98,2567.
0 1995 American Chemical Society
Acetylene Znsertion into a Zr-R
0 Bond
Organometallics, Vol. 14, No. 2, 1995 965 Scheme 1
Reactants
rr-complex
4a 5a 7a 9a
Transition s t a t e
1
Chart 1
la: M=Cp2Ti'
I l a : M'=Cp2Ti.Cp2Zr.Cp2Hf: M'=B.AI,Ga
I b : M=rac-C2H,(indenyi)(Zr'
IIb: M'=Cp2Zr': M2=Cp2Zr
and multiple alkynes with an electrophilic do active species CpzMRn+ ( n = 0, 1) have been reported, and evidence for single- and multiple-insertion products of relevance to alkyne oligomerization and polymerization has been provided as well.9 Fourteen valence-electron complexes with general formula Ia,b have been found and characterized as the products of the insertion reaction of bulky alkyne with CpzMCHs+ (M = Ti, Zr).lo It is interesting to note that the alkyne insertion reaction with CpzZrR+ lla as well as r2-alkynetransition metal speciesllb-' has been postulated for the primary step in the reactions leading to the products IIa,b, in which one vinylic carbon atom has an unusually planar tetracoordinate geometry. Prompted by the experimental achievements in the field of alkyne insertion reactions and continuing our previous study on stabilizing factors responsible for the structure of complexes I and 1112we decided to investigate in more detail the model reactions displayed in Scheme 1. In contrast to olefin insertion reactions, theoretical investigations on alkyne insertion reactions are scarce.se,f To the best of our knowledge, no theoretical investigations on the reactions from Scheme 1 have been reported so far. (9)(a) McDade, C.; Bercaw, J. E. J . Organomet. Chem. 1985,279, 281. (b) Thompson, M.E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. SOC.1987,109,203. ( c ) Burger, B. J.;Thompson, M. E.; Cotter, (d) Christ, C. W. D.; Bercaw, J. E. J . Am. Chem. Soc. 1990,112,1566. S., Jr.; Eyler, J. R.; Richardson, D. E. J. Am. Chem. SOC.1990,112, 596.(e) Horton, A. D.; Orpen, A. G.; Organometallics 1992,11, 8. (10)(a) Eisch, J. J.; Piotrowski, A. M.; Brownstein, S. K.; Gabe, E. J.; Lee, F. L. J . A m . Chem. SOC.1985,107, 7219. (b) Horton, A. D.; Orpen, A. G. Organometallics 1991,10, 3910. (11)(a) Horton, A. D.; Orpen, A. G. Angew. Chem., Znt. Ed. Engl. 1992,31,876.(b) Erker, G.; Zwettler, R.; Kriiger, C.; Noe, R.; Werner, S. J. Am. Chem. SOC.1990,112, 9620. (c) Erker, G.; Albrecht, M.; Kriiger, C.; Werner, S. organometallics 1991,10,3791.(d) Erker, G. Nachr. Chem. Tech. Lab. 1992,40,1099. Comments Inorg. Chem. 1992, 13,111. (e) Erker, G.; Albrecht, M.; Werner, S.; Nolte, M.; Kriiger, C. Chem. Ber. 1992,125,1953.(0 Erker, G.; Albrecht, M.; Krtiger, C.; (g) Erker, G.; Albrecht, Werner, S. J.Am. Chem. SOC.1992,114,8531. M.; Kriiger, C.; Werner, S.; Binger, P.; Langhauser, F. Organometallics 1992,11, 3517.(h) Albrecht, M.;Erker, G.; Nolte, M.; Kriiger, C. J . Organomet. Chem. 1992,427,C21. (i) Erker, G.; Rottger, D. Angew. Chem., Int. Ed. Engl. 1993,105,1623. (12)(a) Gleiter, R.; Hyla-Kryspin, I.; Niu, S.-Q.; Erker, G. Organometallics 1993,12, 3828;1994,13,744. (b) Angew. Chem., Int. Ed. Engl. 1993,32,754.
Product
J
4b 5b 7b 9b
4c
CI
H:
5c
CI
CHj:
7c 9c
Cp
H:
CP
CH,:
Calculations Ab initio calculation^^^ were carried out using CartesianGaussian basis sets. A single basis set was adapted throughout the paper. For Zr we selected a (14,9,7) basis set obtained by adding a p-type orbital exponent (0.12) to the optimized (14,8, 7) set from ref 14. The choice of the additional p exponent guarantees a comparable distribution of the radial density function of the 5p and 5s orbitals. The contraction is [6, 4,41,corresponding t o a single-5 description for the inner shells and the 5p shell, double-E for 5s, and triple-5 for 4d. Basis sets of respective size (10, 6), (9,51, and (4)were used for chlorine, carbon and hydrogen and contracted to split ~a1ence.l~ The geometry optimizations were carried out using the energy gradient technique at the restricted Hartree-Fock (RHF) level. For an estimation of correlation effects and better energetics, restricted second-order M~ller-Plesset perturbation (RMP2) calculations16"were carried out with geometries optimized at the RHF level (RMPS//RHF). It was argued in the literature that in the case of the second-row transitionmetal compounds it is reasonable to determine the energetics at SCF-optimized geometries since the geometries optimized at SCF and correlated levels give similar relative energies.lsbSc Extended Hiickel calculations were carried out with standard parameters for all atoms.17
Results and Discussion Ab Initio Calculationsfor Insertion of Acetylene into Zr-H and Zr-CHs 0 Bonds. In this section we report the optimized structures of reactants (1-31, intermediates (4a, 5a), transition states (4b, 5b),and products (4c, 5c) and discuss the energetics for the reaction displayed in Scheme 1. In our ab initio calculations we replaced the Cp groups of the real molecules by chlorine ligands, which has been shown (13)We used the Gaussian 86 program by M. J. Frisch, J. S. Binkley, H. B. Schlegel, K. Raghavachari, C. F. Melius, R. L. Martin, J. J. P. Stewart, F. W. Bobrowicz, C. M. Rohlfing, L. R. Kahn, D. J. Defrees, R. Seeger, R. A. Whiteside, D. J. Fox, E. M. Fleuder, and J. A. Pople (Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984)and Gaussian 92 (Revision A) by M. J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W. Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees, J . Baker, J. J . P. Stewart, and J . A. Pople (Gaussian, Inc., Pittsburgh, PA, 1992). (14)Hyla-Kryspin, I.;Demuynck, J.; Strich, A,; BBnard, M. J . Chem. Phys. 1981,75,3954. (15)(a) Roos, B.; Siegbahn, P. E. M. Theor. Chim. Acta 1970,17, 209. (b) Dunning, T. H., J r . J . Chem. Phys. 1970, 53, 2823. ( c ) Huzinaga, S. J . Chem. Phys. 1965,42,1293. (16)(a) Mgller, C.; Plesset, M. S. Phys. Reu. 1934,46, 618. (b) Siegbahn, P. E. M. Chem. Phys. Lett. 1993,205, 290. ( c ) See also: Weiss, H.; Ehrig, M; Ahlrichs, R. J . Am. Chem. SOC.1994,116, 4919. (17)(a) Summerville, R. H.; Hoffmann, R. J . A m . Chem. SOC.1976, 98,7240.(b) Hofmann, P.; Stauffert, P.; Schore, N. E. Chem. Ber. 1982, 115, 2153.
966 Organometallics,Vol.14,No.2, 1995
- 1.202 ~
0
Ep(a.u.) EF2”RHF (a.u.)
Hyla-Kryspin et al.
-
-1.055
-76.79666 -76.96855
-4448.5779 1 -4448.92525
-4487.64958 4488.09468
-4487.65160 -4488.09797
3‘
2 3 Figure 1. Optimized geometries of the reactants 1-3 and 3’at the RHF level. 1
0 *0.326
-
0.928 -0.299 w
-0.326 V
* O 055 U
*0.286
2
1
+0.289
3
3’ +0.415
*0.411 *0.41 5
-0 016 + O 270
+0.273
4a Sa 5a‘ Figure 2. Atomic charge distributions and bond overlap populations of the reactants 1-3 and 3‘ and intermediate n complexes 4a, 5a, and Sa’, obtained with Mulliken population analysis at the RHF level. in other studies to provide a good theoretical substitute for the actual bent metallocene system.8a-c,e,iJ8 A. Reactants: C2H2 (11, C12ZrH+(21, C4ZrC&+ (3). The fully optimized geometries of 2,3and 3’ under C, symmetry constraint, together with those of 1, are shown in Figure 1. Structure 3’ differs from 3 by rotating the CH3 group around the Zr-C bond. For 3 two CH u bonds are staggered with respect to the ZrC1 bonds; in 3 they are eclipsed. Although in 3’one of the ZrCH angles is smaller than the other ones, pointing to the possibly agostic interaction, structure 3’ is 1.3 kcal/mol (RHF) or 2.1 kcal/mol (RMP2//RHF)less stable than 3. Similar properties were found for the model compound cl~TiCH3+.~ The Mulliken population analysis shown in Figure 2 reveals that the Zr-C bond of 3 and 3‘ is more polarized than the Zr-H bond of 2. The calculated overlap populations (Figure 2) agree with the experimentally determined order of the bond strength D(Zr-H) > D(Zr-C) found in the gas-phase studies for CpzZrH+ and CpzZrCH3.lg (18)(a) Rapp6, A.K.; Goddard, W. A,, 111. J.Am. Chem. SOC.1982, 104,297. (b) Upton, T. H.; Rapp6, A. K. J.Am. Chem. SOC.1985,107, 1206. ( c ) Rapp6, A. K. Organometallics 1987, 6, 354. (d) Koga, N.; Morokuma, K. J. Am. Chem. SOC.1988, 110, 108;Chem. Rev. 1991, 91, 823.
B. Intermediate Acetylene Complexes: C12ZrH(C2H2)+ (4a) and C12ZrC&(C2H2)+ (Sa, Sa’). The optimized structural parameters of the intermediate acetylene n complexes 4a, Sa, and 5a‘ are shown in Figure 3. For the hydride complex the geometry optimizations were performed without any symmetry constraint. The final structures converged to 4a with C, symmetry. To simplify the analysis the structures 5a and 5a’ were optimized under C, symmetry constraint. Similar theoretical investigations on the insertion of ethylene into the M-C (M = Ti, Zr) u bond showed that sometimes more stable equilibrium structures can be found with C1 symmetry, whose energy, however, is only 0.5-1.0 kcal/mol lower than those of the appropriate C, structures.8c~dJ8d In 5a and 5a’ the Zr-CH3 bond is longer by 0.02 A with respect t o 3 and 3. The elongation of the Zr-H bond in 4a is insignificant. In 4a, 5a, and 5a’ the C(l)-C(2) bond is 0.01-0.025 A longer than in free acetylene. The coordination of acet lene is not symmetric. The distance Zr-C(l) is 0.456 (4a),0.466 A (Sa),and 0.433 A (Sa’)shorter than Zr-C(2). From the Mulliken population analysis shown in Figure 2, it
1y ~~
(19)Christ, C. S.; Eyler, J. R.; Richardson, D. E. J.Am. Chem. SOC. 1988,110, 4038.
Organometallics, Vol. 14, No. 2, 1995 967
Acetylene Insertion into a Zr-R u Bond
EF(a.u.) EY‘RHF(u.u,)
-4525.44446 -4525.97605
-4564.50911 -4565.13267
4a
5a
-4564.50493 -4565.12808
Sa’
Figure 3. Optimized structural parameters of the x complexes 4a, 5a, and Sa‘ at the RHF level.
EF((a.u.) -4525.44417 EFz”RHF(u.u.) -4525.97578 4b
-4564.49298 -4565.12445
5b
5b’
EF(a.u.) -4525.50092 EF2”RHF(a.u.) -4526.03231 4c
-4564.53637 -4565.15138
-4564.533 19 -4565.15033
5C
5C’
-4564.4901 1 -4565.12178
Figure 4. Optimized geometries of the transition states 4b, Sb,and 5 b and of the products 4c, Sc, and 5c’ at the RHF level.
C. Transition States ClzZr(H)(C2Hz)+(4b) and follows that coordinated acetylene is polarized as a Cl&-(CI&)(C&)+ (Sb,5b’) and Products C12zrC&+ result of charge transfer from the x system to the (4c) and ClsZrCzH2C&+ (5c, 5c’). The optimized positively charged and coordinatively unsaturated zirstructural parameters of the transition states (TS) 4b, conium center. A detailed MO analysis will be pre5b, and 5 b and of the products 4c, 5c, and Sc‘ are sented in the next section. At the RMPW/RHF level the shown in Figure 4. Figures 5 and 6 display the relative acetylene binding energy amounts to 51.6kcaVmol(4a), potential energy profiles for the insertion reaction paths. 41.5kcal/mol(5a), and 40.7kcdmol (Sa’). These values The differences of the overlap populations (AOP) and can be compared with ethylene binding energies of 33of-the 53 kcaVmo1 calculated for Ti and Zr ~ o m p l e x e s . ~ ~ ~ ~ ~ ~ , gelectron densities ( A q ) between the TS and the intermediate x complexes as well as between the In the case Of 4a, the acetylene binding energy is about products and the TS are collected in Table 1. In Table 31 kcaVmol greater than that calculated for C12ScH2 we present a comparison of the energetics for the ( C ~ H Z as ) ~ “a result of stronger bonding interaction of reaction of olefin with Sc and Zr complexes, calculated the diffuse valence shell of the Zr center with the n a t comparable levels of theory. Similar t o the case for system.
Hyla-Kryspinet al.
968 Organometallics, Vol. 14, No. 2, 1995 (kcal/mol) reactant 0.0
n-complex
TS
Table 1. Differences in the Overlap Populations (AOP) and in the Electron Densities (Aq). between Fully Optimized Transition States and K Complexes and between Fully Optimized Products and Transition States
product
staggered
4b-4a
-
-51 6
-
RHF RMPZ//RHF
’
-86.9
RC
Figure 5. Potential energy profiles of acetylene insertion into the Zr-H (T bond of ClzZrH+ at the RHF and RMP2/ /RHF levels. E(kcal/mol)
t
reactant
n-- c o m p l e x
TS
AOPz,-cl 0.019 AOPz,-ci 0.008 AOPzr-~3/c3 -0.021 AOPZ,-H h 0 P c 1 - c ~ -0.030 AOPCLH~,C? 0.008 AoPc3-~ A9-a +0.012 Aqcl -0.002 Aqc2 -0.001 A ~ H ~ / c ? +0.002 AqH
+ C2H4 SiH2CpzZrCH3+ + C& ClzScH + CzHz C12ZrH+ + C2H2 C12ZrCH3’ + C2H2
C12ZrCH3’
-
staggered
........ ... .
eclipsed
L
0.112 -0.066 -0.249
0.051 0.043 -0.024 0.032 -0.220 -0.120 0.281 0.036 -0.060 -0.105 +0.185 -0.01 1 +0.038 +0.273 -0.065 -0.194 -0.160 f0.071
eclisped
5c-5b
5b‘-5a’
5c‘-5b‘
0.106 -0.093 -0.191 0.052 -0.259 0.211 -0.020 -0.160 +0.094 +0.200 -0.160 f0.071
0.049 0.024 -0.024 0.009 -0.114 0.023 -0.029 +0.141 f0.045 -0.120 -0.063 +0.017
0.105 -0.084 -0.185 0.028 -0.282 0.211 -0.001 -0.169 +0.084 f0.152 -0.146 f0.035
Table 2. Calculated n-Bonding Energies (A&),Reaction Barriers (A&), and Overall Exothermicity (AE)for the Reaction of Olefm with Sc and Zr Complexe54 reaction
\-%
5b-5a
A positive sign means an increase in the electron density.
product
-26.2
4c-4b
calculation method
AElb
AE,c
AEb
ref
RHF GVB-CI//RHF RHF RMP2//RHF GVB-CI//RHF RHF RMP2//RHF RHF RMP2//RHF
-37.0 -33.0 -19.1 -33.4 -20.2 -43.9 -51.6 -38.2 -41.5
22.0 24.0 16.7 6.0 6.9 0.2 0.2 10.1 5.1
-26.0 -20.0 -26.8 -40.9 -45.2 -79.3 -86.9 -55.3 -53.3
8b
8d 8e d d
Calculated at comparable levels of theory. All energy values are given in kcallmol. Energy is given relative to the free molecules from column 1. Energy is given relative to the intermediate R complex. This work
-55.3
(1
RC
reactant
n-complex
TS
product
2 1
l -
staggered
-53.3
RC
Figure 6. Potential energy profiles of acetylene insertion into the Zr-C u bond of ClzZrCH3+ with staggered and eclipsed structures at the RHF (top) and RMP2//RMF (bottom) levels.
the insertion reaction of ethylene,s the TS 4b, 5b, and 5 b are four-membered cycles (Figure 4). For the TS 4b we find the saddle point to be “early”;the C(l)-C(2) bond has only lengthened by 0.011 A, the Zr-H bond has stretched by 0.024 A, and the newly formed C-H bond has a distance of 2.02 A. Regardless of the calculation level, the activation energy is only 0.2 kcaV mol (Figure 5), perhaps as a result of small changes in bond populations and charge reorganizations between 4a and 4b (Table 1). The product 4c is 35 kcallmol more stable than the n complex 4a. 4c shows typical features
of a P-CH agostic structure. Its geometrical and electronic structure has been discussed elsewhere.12* Referenced to free acetylene, the exothermicity of the reaction is 86.9 kcaVmo1 (Figure 5). The small activation barrier and large exothermicity suggest that the insertion reaction of acetylene into the Zr-H bond of C12ZrH+ should be irreversible. In the TS 5b and 5b’the geometry of the CH3 group deviates from the normal sp3 structure. The CH bonds directed toward the Zr atom are elongated t o 1.119 A in 5b and 1.101A in 5b;the correspondingZrCH angles are lower than the sp3value (Figure 4). Such structural deformations suggest that 5b and 5 b can be stabilized through an a-agostic interaction. The TS 5b is 1.8kcall mol (RHF) or 1.7 kcaVmol (RMPBIIRHF) more stable than 5b. Note that the a-CH agostic interaction has been proposed to assist the mechanism of olefin insertion reactions on the basis of either experimenta120or theoretical investigations.8,21 In the early stage of the TS), the sp3 hybrid insertion reaction (ncomplex orbital of the CH3 group originally involved in the ZrC(3) o bond rotates from being bound to Zr to being slightly bound to C(2) as shown in Chart 2. We note that the angles H-C(3)- “new sp3 hybrid direction” do not much deviate from the sp3 value, and thus in this case, only the elongation of the C-H bond can be
-
(20) (a) Brookhart, M.; Green, M. L. H. J. Orgunomet. Chem. 1983, 250,395.(b) Brookhart, M.;Green, M. L. H.; Pardy, R. B. A. J.Chem. SOC.,Chem. Commun. l98S,691.(c) Schmidt, G. F.; Brookhart, M. J . Am. Chem. Soc. 1986,107, 1443.(d) Brookhart, M.;Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem. 1988,36,1. (e) Piers, W. E.; Bercaw, J. E. J . Am. Chem. SOC.1990,112, 9406. (0 Roell, W.;Brintzinger, H.-H.; Rieger, B.; Zolk, R. Angew. Chem., Int. Ed. Engl. 1990,29,279.
Acetylene Znsertion into a Zr-R
Organometallics, Vol. 14, No. 2, 1995 969
0 Bond
r(ev)
4a'
40'
-10.0
2a'
2a'
a' 1a'
I
-15.0
7a
7c
7b
Figure 8. Walsh diagram for the insertion reaction CpzZrH+ CzH2 from the x complex 7a to the product 7c.
+
Scheme 2 H
3a'
7a
6
20'
1
Figure 7. Simplified interaction diagram for the interaction between CpzZrH+ (6) and acetylene (1)to give the x complex 7a. Chart 2 ti
\
30' H
ni
n;
bond formation is more advanced than for the C(3)C(2)bond, which is still very long. The most important electronic features of the early stage of the insertion reaction (ncomplex TS) concern the charge reorganization on the Zr center and the C(3) atom (Table 1). We observe an electron density transfer from the C(3) atom t o the Zr center. This suggests that electrostatic interactions, for example with solvent molecules, can influence the geometry of the TS as well as the energy barrier. The calculated RHF activation energies are 10.1 and 9.3 kcaVmo1 for 5b and 5b, respectively (Figure 6). At the RMPBIIRHF level they are lowered to 5.1 kcaVmol(5b) and 4 kcaVmol(5b'). To the best of our knowledge, no experimental data for an activation energy of the insertion reaction of unsatured hydrocarbons into the Zr-C bonds have been reported so far. However, our results can be related to the experimentally determined enthalpy of the activation of 9.7 kcaV mol for the reaction of Cp*ScCH3 with 2-butynegcor to the calculated value of 6 kcaVmo1 for the reaction of (SiHzCpz)ZrCH3+with ethylene.8d It is interesting to note that the experimental estimate of the energy barrier for the propagation step in the polymerization reactions of olefins ranges from 6 to 12 kcal/mo1.22 In the products 5c and 5c', the angle Zr-C(l),-C(2)8 deviates greatly from the expected sp2 value (120") for nondistorted vinyl complexes. The C-H bonds directed towards the Zr atom are now shorter than in the TS,
-
n-complex
TS
considered as a theoretical verification for an agostic interaction. For the TS 5b and 5 b we find that the C(l)-C(2) bond is lengthened by 0.044 A (5b) and 0.042 A (5b'); the Zr-C(1)-C(2) bond angle is reduced by 8.9" (5b) and 5.2" (5b) with respect to the n complexes 5a and 5a'. The Zr-C(3) bond is stretched by 0.087 A (5b)and 0.041 A (Sb). The newly formed C(3)-C(2) bond has a distance of 2.194 (5b) and 2.317 A (5b). These results, together with the changes of bond overlap populations (Table 11, not only suggest that the TS 5b and 5 b are "early" but also allow us to suppose that steric effects are operative at the TS. The influence of steric effects on the geometry of the TS was mentioned in theoretical investigations of the reaction of CpzTiCHS+ with ethylene.8a2d The new bond formation at the TS is asynchronous in the sense that the Zr-C(l) (21) Prosenc, M.-H.; Janiak, C.; Brintzinger, H.-H. Organometallics 1992,11, 4036.
4a'
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970 Organometallics, Vol. 14, No. 2, 1995
x-complex(7a)
2a'" .,,
TS(7b)
I
Product(7c)
I
2"
Figure 9. Contour plots of the molecular orbitals involved in the insertion reaction of the n complex (7a), TS (7b),and product (7c). The values of the contour lines are f0.02, f0.04, f0.06, fO.10, f0.14, 10.18, f0.24, f0.30, f0.36, and f0.42.
1a'
20'
20'
1a '
4a'
20'
Figure 10. Intermixing patterns of the la'-5a' MOs of CpzZr(H)(CzHd+.
but they are still longer than in the jz complexes (Figures 3 and 4). The C(2)g-C(3), bond is stretched by 3% in 5c and 5% in 5c' with respect to the normal (22) (a) Natta, G.; Pasquon, I. Adu. Catal. 1959,11, 1. (b)Chien, J. C. W. J . Am. Chem. Soc. 1959, 81, 86. ( c ) Machon, J.; Herman, R.; Houteaux, J. P. J. Polym. Sei., Polym. Symp. 1975,52, 107. (d) Chien, J. C. W.; Razavi, A. J . Polym. Sei.,Part A: Polym. Chem. 1988, 26, 2369.
C-C bond. Such features are indicative of Cg-Cy and Cy-H agostic interactions, with the former being stronger in 5c'. In 5c the Cy-H bond is longer and the CgCy bond is shorter with respect to 5c'. At the RMP2// RHF level structure 5c is 0.7 kcal/mol more stable than 5c', showing that one in-plane Cy-H bond of 5c contributes more to the stabilizing interactions than two such out-of-plane bonds in 5c'. This property has an electronic origin12*and was experimentally documented in the case of rac-C2H4(indenyl)zZrSiMe&=CMezf.lOb In the late stage of the insertion reaction (TS product) the Zr-C(3) bond is breaking and the two new B bonds Zr-C(Oa and C(2)g-C(3), are formed; the multiple bond transforms to the vinylic double bond (Figure 4, Table 1). In this stage of the reaction one can observe large density reorganization on the vinyl ligand (Table 1).The calculated RMP2//RHF overall exothermicity of the insertion reaction of acetylene into the Zr-C bond amounts to 53.3 kcaYmol for the staggered and 52.6 kcaVmol for the eclipsed structure. It is about 13 kcaV
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Organometallics, Vol. 14, No. 2, 1995 971
Acetylene Znsertion into a Zr-R u Bond
40’
20’ 2a’
9b
Sa
9c
Figure 11. Walsh diagram of the occupied MOs for the insertion reaction CpzZrCHS+ + C2Hz from the x complex 9a to the product 9c. mol greater than the calculated exothermicity for the reaction of (SiH2Cpz)ZrCH3+with ethylene.8d To finish this section, we stress that the early stage of the insertion reaction (ncomplex TS) is characterized through an electron density reorganization at the Zr center and the R(3) atom (R = H, C). The late stage of the reaction (TS product) is characterized through a new bond formation and subsequent electron density reorganization in the vinyl ligand. The calculated energy barriers are smaller and the overall exothermicity greater than in the case of ethylene insertions into Zr-C u bonds or acetylene insertion into the Sc-H u bond (Table 2). Molecular Orbital Picture of the Insertion Reaction Based on Extended Hiickel calculations. In this section we present a qualitative MO picture for the insertion reaction of acetylene into Zr-H and Zr-C u bonds in the zirconocene complexes CpzZrH+ (6) and CpzZrCHs+ (8). Since the Cp units do not participate directly in the insertion reaction, a t each stage of the reaction the geometry of the CpzZr fragment has been kept fixed at CzUsymmetry with bond distances and angles taken from available experimental data for zirconocene complexes.23 The geometrical parameters of the ligands (H, CH3, C2H2) in the intermediate x complexes (7a, 9a), TS (7b, 9b), and products (7c, 9c) have been taken from the ab initio optimized structures described in the previous section. A. Insertion of Acetylene into the Zr-H (r Bond of Cp2ZrH(C2H2)+. To characterize the electronic structure of the intermediate x complex Cp2ZrH(C2H2)’ (7a),we adopt Hoffmann’s fragment MO approach,24
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(23) (a) Hyla-Kryspin, I.; Gleiter, R.; Kriiger, C.; Zwettler, R.; Erker, G. Organometallics 1990,9, 517. (b) Erker, G.; Zwettler, R.; Kriiger, C.; Hyla-Kryspin, I.; Gleiter, R. Organometallics 1990,9,524. ( c ) Erker, G.; Zwettler, R.; Kriiger, C.; Schlund, R.; Hyla-Kryspin, I.; Gleiter, R. J. Organomet. Chem. 1988,346, C15. (24)Albright, T. A,; Burdett, J. K.; Whangbo, M. H. In Orbital Interactions in Chemistry; Wiley: New York, 1985.
constructing interactions between the valence MOs of acetylene (1) with those of the CpzZrH+fragment. The valence MOs of both fragments are described in detail in the l i t e r a t ~ r e .In ~ ~Figure 7 we display only those valence MOs that are involved in the stabilizing interactions. In the localized picture the la’ MO of 6 describes the Zr-H u bond and the two lowest unoccupied MOs 2a’ and 3a’ describe the acceptor levels in the symmetry plane of the metallocene fragment. They are shown on the left side of Figure 7. The acetylene xi+ orbital presents a donor function to the metal center; it interacts with la’ and 3a’ of 6 t o produce the three MOs la‘, 2a‘, and 3a’ of 7a. This four-electron-three-’ orbital interaction is further stabilized by the bonding admixture of the LUMO (xi-) of 1. The admixture of xi- is crucial because it leads t o a partial relief of the Pauli repulsion between the C2H2 xi+ and CpzZrH+ la’ MOs as well as redistributes the electron density in the 2a’ MO and changes the shape of the 4a’ MO of 7a, as shown in Scheme 2. We notice that the 2a‘ and 4a’ MOs of 7a represent the bonding and antibonding counterparts for the interaction of 6 with the x system of 1. The LUMO (3a’) is not involved in stabilizing interactions. In Figure 8 we display a Walsh diagram along the reaction path, going from the n complex (7a)through the TS (7b)to the product (7c). The corresponding MO plots are shown in Figure 9. The phase relationships of the occupied MOs in 7a and 7c (Figure 9) suggest a smooth transformation of the Zr-H u bond (la’ MO of 7a) to the C(2)-H u bond (la’ MO of 7c) as well as of the Zrn-acetylene bond (2a’ MO of 7a) to the Zr-C(l) u bond (2a’ MO of 7c). The insertion reaction under study is actually a 1,2 addition and is inherently connected with an avoided crossing of occupied and empty level^.^^,^^ Four orbitals undergo strongly avoided crossings, and substantial intermixing is operating during the reaction path. The most important intermixing patterns are shown in Figure 10. The bonding admixture of 2a’ and 5a’ to la’ diminishes the metal character of la’ and increases the p component on C(2). It is clear that such a transformation facilitates the Zr-H bond-breaking process as well as the C(2)-H bond formation. Similarly, the admixture of la’ and 4a’ to 2a’ weakens the Zr-n-acetylene bond and facilitates the Zr-C(l) bond formation. One can suppose that the intermixing between the MOs will be different a t different stages of the insertion reaction. It seems that in the early stage of the reaction (xcomplex TS) the intermixing among occupied orbitals la’ and 2a’ and the empty 4a’ is dominant, leading to charge reorganization on the Zr and H atoms and in the late stage of the reaction (TS product) the large admixture of the empty 5a’ MO controls smooth new bond formation. In accord with the general analysis given by Thorn and Hoffmann,z6 our MO picture reveals that the Zr center plays a dual role as electron acceptor and electron donator. In the early stage of the reaction it is able to accept electrons from the xi+ orbital of 1 and in the late stage of the reaction it induces electron back-donation t o the vinylic ligand due to the strong admixture of the empty 5a’b~i-I MO to the occupied la’ and 2a’ levels. Both features,
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(25)Bickelhaupt, F. M.; Baerends, E. J.; Nibbering, N. M. M.; Ziegler, T. J . A m . Chem. SOC.1993, 115, 9160.
972 Organometallics, Vol.14,No. 2, 1995 I
mcomplex(9a)
I
Hyla-Kryspin et al.
Product(9c)
TS(9b) I
I
..
. .
I
Figure 12. Contour plots of the molecular orbitals la’-4a’ in the n complex (gal,TS (9b),and product (9c).The values of the contour lines are as in Figure 9. efficient electron donation and back-donation, will lower the energy barrier of the insertion reaction.26 B. Insertion of Acetylene into the Zr-C (ibond of Cp2ZrC&+. Figure 11 displays the correlation diagram of the important occupied orbitals of the n-complex (9a) with those of the TS (9b) and product ( 9 ~ )The . MOs 4a‘ and 3a‘ of 9a represent the initial Zr-C(3) and Zr-n-acetylene bonds, respectively. The MOs 2a‘ and la’ of 9a represent the in-plane C(3)-H and C(l)-C(2) u bonds. In the early stage of the insertion reaction they are stabilized by an a-agostic interaction. During the reaction the MOs la’ and 2a’ mix with each other as well as with 3a’ and smoothly transform to the la’ and 2a’ MOs of 9c. In 9c, the two MOs la’ and 2a’ still have an agostic interaction. On (26) Thorn, D.L.;Hoffmann, R. J.Am. Chem. SOC.1978,100,2079.
going from the n complex 9a to the product 9c, the ZrC(3) u bond (4a’ MO of 9a) and the Zr-n-acetylene bond (3a’ MO of 9a) smoothly transform to the Zr-C(l) and C(2)-C(3) u bonds of 9c, as a result of substantial admixture of the occupied la’, 2a’ and empty 6a’, 7a‘ levels. The MO plots of the la’-4a’ levels of the n complex (gal, TS (9b),and product (9c) are shown in Figure 12. Although the predicted energy barrier for the insertion reaction of acetylene into the Zr-C u bond is greater than for the Zr-H u bond, the MO pictures of both insertion reactions are essentially similar. The low energy barrier of the latter reaction can be traced back t o the absence of steric repulsion during the hydrogen migration from the Zr to the C(2) atom. In Figure 13 we present the three-dimensional MO plots of the MOs representing the C(3)-H u bonds that can
Organometallics, Vol. 14, No. 2, 1995 973
Acetylene Insertion into a Zr-R u Bond
Product(9c)
x-complex(9a)
I
I
I
1
TS(9b’)
x-complex(9a’) la‘
/
1a’
I
Product(9c’) /
1a’
I
1
I
Figure 13. Three-dimensional MO plots of the MOs representing the in-plane and out-of-plane C-H bonds in the x complex, TS, and product of Cp2Zr(CH3)(C2H2)+with staggered (top)and eclipsed structures (bottom).The intervals of the contour lines are f0.04.
be involved in the agostic interaction in the case of either the staggered (9a-9b-9c) or eclipsed structures (9a’-9b’-9c’) of Cp2ZrCH3(C2H2)+. In the n complexes 9a and 9a’ agostic interactions are not present. The TS (9b)and product (9c)are stabilized through one inplane Zr-H-C(3) agostic interaction. The la” MOs of structures 9 and 9’ do not contribute to the agostic interaction, due to the absence of overlap between the two out-of-plane C(3)-H u bonds and the high lying, empty a” orbital of the Zr center. However, some positive overlap with the in-plane accepting orbital of the Zr center is present in the la’ MO of 9b’ and 9c‘. The Zr-H-C(3) bonding interaction in the TS lowers the energy barrier of the insertion reaction.
Conclusions We have studied with the ab initio MO method the optimized structures and energy profiles of acetylene insertion into the Zr-H and Zr-C u bonds of C12ZrR+ (R = H, CH3). The transition states are four-membered cycles. For the alkyl complex we have shown that the transition state is influenced by electronic and steric effects. The calculated energy barrier is smaller and the acetylene binding energy as well as the overall exothermicity are greater than the theoretical values obtained for similar ethylene insertion reactions into the Zr-C u bond.8d The products and transition states of the alkyl complex are stabilized through an agostic interaction. The acetylene insertion into the Zr-H bond
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974 Organometallics, Vol. 14, No. 2, 1995
of C12ZrH+ has a lower barrier and is more exothermic than the insertion into the Zr-C bond of ClzZrCH3+. The calculated small activation barrier (0.2 kcal/mol) and large exothermicity (86.9 kcdmol) suggest that the insertion of acetylene into the Zr-H bond of C12ZrH+ should be irreversible. The acetylene insertion into the Zr-R bond (R = H, CH3) has a lower barrier than the insertion into S C - R , ~due ~ ~t o~ ~the better donoracceptor interactions of the diffuse valence shell of the Zr center with the hydrocarbon JC system. The MO analysis carried out on the basis of extended Huckel calculations with CpzZrR+ (R = H, CH3) revealed that the valence MOs of the metallocene system undergo strongly avoided crossing and a substantial intermixing is operating during the insertion reaction path. The
intermixing between the valence MOs facilitates both the Zr-R bond breaking and R-C(2) bond formation. During the reaction path the Zr center plays an electron acceptor and electron donator dual role, with the latter more important at the final stage of the insertion reaction.
Acknowledgment. We are grateful t o the Bundesministerium fur Forschung und Technologie (FKZ: Grant No. 03D0005B3), the Deutsche Forschungsgemeinschaft (Grant No. SFB 247), and the Fonds der Chemischen Industrie for financial support. We thank Prof. G. Erker for helpful discussions. OM940572Q
