Organometallics 1996, 14, 5005-5007
5005
Intrinsic Ancillary Ligand Effects in Cationic Zirconium Polymerization Catalysts: Reactions of [L2ZrCHs]+ Cations with H2 and C2& N. George Alameddin,? Matthew F. Ryan,? John R. Eyler,? Allen R. Siedle,* and David E. Richardson"?? Department of Chemistry, University of Florida, Gainesville, Florida, 3261 1-7200, and 3M Corporate Research Laboratories, St. Paul, Minnesota 55144 Received June 28, 1995@ Summary: Fourier transform ion cyclotron resonance mass spectrometry was used to study ion /molecule reactions of five cationic methyl metallocenes: [Cp2ZrCHd+ (1;Cp = cyclopentadienyl), [(Ind)(Cp)ZrCHd+ (2; Ind = r/5-indenyl), [IndSrCHd+ (3),[(CHdzSi(Cfl4)2ZrCHd+ (4), and [Flu2ZrCHd+ (5; Flu = v5-fluorenyl). The rate constants and products from reactions with H2 and C& are presented. From the kinetic data, the chelating ligand (CH&Si(Cfld2 increases the intrinsic electrophilicity of the metal complex relative to Cp while substitution of Ind or Flu for Cp leads to a less electrophilic metal center. Electrophilic do group 4 metallocenes, such as CpzZr(CH3)z (Cp = v5-cyclopentadienyl),are precursors of alkene polymerization catalysts.lI2 The active species in these catalysts, now generally accepted to be [LzZrCH31' (L = Cp or related ligands),2 is formed via reaction of the neutral precursor with cocatalysts such as methalumoxane (MAO) and other strong Lewis acids.3 In previous work, we reported gas-phase ion/ molecule reactions of [CpzZrCH31+ (1) with a number of substrates including dihydrogen, nitriles, and unsaturated hydrocarbon^.^ It was demonstrated that the intrinsic reactivity of these methylzirconium cations could be determined for critical reaction pathways such as o-bond metathesis, P-H shift, and alkene insertion. Improvements over the prototypical L = Cp polymerization catalyst have largely involved the modification of the Cp ligands to alter reactivity and stereoselectivity via combinations of electronic and steric effects.l In studies of polymerization activity as a function of the ancillary ligands, it has been shown that electron-rich ligands such as v5-indenyl (Ind) and y5-pentamethylcyclopentadienyl (Cp*) often lead to increased activity relative to the Cp prototype catalyst. These observations seem counterintuitive because the electron density at the metal center would increase for L = Ind or Cp* thereby reducing the electrophilic character of the metal and, presumably, the polymerization activity. In light of this contradiction and the widespread interest in University of Florida.
* 3M Laboratories. +
e Abstract published in Advance ACS Abstracts, October 15, 1995. (1)Mohring, P.C.; Coville, N. J. J. Organomet. Chem. 1994,479,1.
(2)Jordan, R. F. Adu. Organomet. Chem. 1991,32,325. (3)(a) Yang, X.;Stern, C. L.; Marks, T. J. J . Am. Chem. Soc. 1994, 116, 10015 and references therein. (b) Marks, T. J. ACC.Chem. Res. 1992,25,57. (4)(a) Christ, C. S.; Eyler, J. R.; Richardson, D. E. J . Am. Chem. SOC.1988,110,4038.(b) Christ, C.S.; Eyler, J. R.; Richardson, D. E. J. Am. Chem. SOC.l f M , 112, 596. (c) Christ, C. S.; Eyler, J. R.; Richardson, D. E. J . Am. Chem. SOC.1990,112,4778.
zirconocenes with modified ancillary ligands, we have expanded the scope of our original study to include reactions of H2 and C2H4 with the following ions: [(Ind)(Cp)ZrCH31+ (21, [Ind2ZrCH31+ (3), [(CH3)2Si(C5H412ZrCH31+ (41, and [FluzZrCH31+ (5; Flu = $-fluorenyl). It was expected that the rates of these reactions would correlate primarily with the intrinsic electronic effects of the ancillary ligands on the metal center (especially for the sterically undemanding reaction with Hz).The kinetic influence of solvation and ion-pairing, which can significantly affect the reactivity of the cations in the condensed phase, will not be a factor in these experiments. In this way, we can isolate one critical feature of ancillary ligand effects and improve o u r understanding of these complex condensed-phase catalysts. (5) (a)Richardson, D. E. In OrganometallicIon Chemistry;Freiser, B. S., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands,1995;in press. (b) Richardson, D. E.; Ryan, M. F.; Khan, Md. N. I.; Maxwell, K. A. J. Am. Chem. SOC.1992,114,10482.(c) Ryan, M. F.; Eyler, J. R.; Richardson, D. E. J . Am. Chem. SOC.1992,114,8611. (d) Ryan, M. F.; Richardson, D. E.; Lichtenberger, D. L.; Gruhn, N. Organometallics 1994,13, 1190.(e) Ryan, M.F.; Siedle, A. R.; Burk, We proposed5e M. J.;Richardson, D. E. Organometallics 1992,11,4231. the y parameter scale on the basis of gas-phase electron-transfer equilibria (ETE) measurements of the free energy of ionization for a series of substituted ruthenocenes and other metallocenes. The scale is anchored by assigning the arbitrary parameters ycP = 0 and yep* = -1 (Cp* = pentamethylcyclopentadienyl). The parameter y~ is a measure of the electronic influence of the substituted ligand on the metal relative to Cp (a ligand with a negative y value is more electrondonating than Cp). (6)The Fourier transform mass spectrometer used in this work has been described elsewhere (Sharpe, P.; Richardson, D. E. Coord. Chem. Rev. 1989,93,59). Substrate gases were introduced via precision leak Torr, and the precursor valves to pressures of 1 x 10-6-5 x dimethylzirconocene samples (Samuel, E.; Alt, H.; Hrncir, D. C.; Rausch, M. D. J. Organomet. Chem. 1976, 113, 331. Samuel, E.; 1973,95,6263)were sublimed from Rausch, M. D. J . Am. Chem. SOC. a solids probe inserted so that the sample is -50 cm from the cell. The probe tip was heated to maintain a constant sample pressure on Torr (for the dimethyl precursors of 1, the order of 3 x 10-*-3 x 10-20 "C; 2, 60-70 "C; 3, 80-90 "C; 4, 50-60 "C; and 5, 100-120 "C). Electron impact on the dimethyl compounds by 10-12 eV electrons produces ions resulting from the loss of one and two methyl ligands, and monomethyl cations were isolated by resonantly ejecting all other ions formed during the beam event. The methyl cations 1-4 were allowed to thermalize via '50 collisions with the background gas for 0.1-1.5 s (depending on total pressure) prior to obtaining kinetic data. In addition to the reaction with the substrate to produce the desired product ion, reaction of the methyl cation with background water Torr) forms the metallocene hydroxide ion ([LzZrOHI+),and reaction of various cations with the neutral dimethyl parent compound produces dimer ions (i.e., binuclear Zr complex ions). These alternate pathways were incorporated into the full kinetic model used to fit the data. Details of the pressure corrections relevant to this experimental apparatus are discussed by Bruce and Eyler (Bruce, J. E.; Eyler, J. R. J . Am. SOC. Mass Spectrom. 1992,3,727).ks an approximation, one can multiply the rate constants by the ion gauge sensitivity factor (0.4 for Hz and 2.3 for CzH4) and divide by 2 to correct for the difference between the ion gauge and the cell. The errors on the rate constants in Table 1 represent experimental variance and are for comparison to ranges for other rate constants for the same reactant gas. Because of errors in pressure calibration, the absolute rate constants may have larger errors (up to -50%).
0276-733319512314-5005$09.00/0 0 1995 American Chemical Society
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5006 Organometallics, Vol. 14, No. 11, 1995 13 1
1
I
0.2
0
0.5
1 Time ( 5 )
1.5
2
-
Figure 1. Plot of normalized ion intensity vs time for the H ~ ] + products. reaction [ ( C H ~ ) Z S ~ ( C ~ H ~ ) Z Z ~ - CCzH4 Legend: = methyl cation, 'I= hydroxide cation, 0 = v3allyl complex ion, open stacked triangles = binuclear Zr complex ions. Solid lines represent the fit to the kinetic model used to describe the system, including the rate constant given in Table 1for the reaction in eq 2.
+
Table 1. Rate Constants for the Reaction of 1-4 with Ha (kd and C2& (kd cation Zy kl (M-' s-lPsb k2 (M-' s-lPJ 1 2 3 4 5
O.Od -0.41d -0.82d (0.16p -1.3d
(3.9 i 1.2) x (2.6 f 0.7) x (4.8 f 4.4) x (6.9 f 0.3) x 5107
10'0 lo8 lo7 1Olo
(3.4 f 0.9) x 10'0 (2.1 f 1.4) x lo9 (3.5 i 0.4) x lo8 (9.6 i 1.8) x 1O'O 5 107
Error limits on rate constants are quoted with f l u from multiple experiments. Relative errors among the rate constants for each substrate shown in this table are smaller t h a n the absolute errors (up to -50%) since relative rates do not depend on pressure calibration for the reactant gas. Reaction with H2 (eq 1);product ion LZZrH+. Reaction with ethylene (eq 2);product ion L2ZrCSH5+. Reference 5d. e Derived from kinetic data-see text.
We have also applied our previous parametrization of the electronic effects of Cp derivatives in metallocene ionization energetics ( y parameters5) to correlate the new kinetic data with the tendency of the Cp derivatives to act as electron-donating( y < 0) or electron-withdrawing ligands ( y > 0) relative to Cp ( y = 0) itself. This study represents the first application of these parameters to the prediction of intrinsic reactivity for electrophilic metallocenes. The experimental methods for obtaining kinetic data by using Fourier transform ion cyclotron resonance mass spectrometry were similar to those described previ~usly.~ The ? ~ observed iodmolecule reaction pathways were modeled as a series of pseudo-first-order elementary steps. The resulting set of coupled differential equations were solved t o yield an analytical solution describing the time dependence of the intensity for each product ion and reactant ion, and the time dependence of ion intensities was fit to the model by optimizing the rate constants simultaneously. A typical nonlinear least squares fit is shown in Figure 1. Second-orderrate constants are obtained from kobdpaub, where psub is the pressure of the substrate. The rate constants for the Hz and CzH4 reactions are collected in Table 1 and are plotted vs summed y parameters in Figure 2. An effective value of C y for 4 was chosen (+0.16)to give the best fit t o the lines derived from fits to the Ind and Cp complexes.
-1.o
-1.5
-0.5
0.0
ZY Figure 2. Plot of log@)vs Zy for kl (0)and k2 (W). The solid lines represent the best fit to the available y parameters. The dashed lines are the Langevin collisional limits for the second-order rate constants (upper line for CzH4, lower for Hz). Open points are the predicted values of log(k)for the reaction of 5 with H2 (lower) and CZ& (upper). Reaction of H2 with 1-4 forms a hydride complex with loss of C& (eq 1). Sole loss of CH3D in reactions with [L2Zr-CH31+
kl + H, [L,Zr-HI+ + CH,
(1)
D2 is consistent with a four-center a-bond metathesis transition statea4 The rate of reaction 1 will be influenced by the energy of this transition state relative to the reactants, and decreased electron density at the metal would be expected t o lower the energy of an activated complex that involves ligand binding. In the reaction of C2& with 1-4, the substrate inserts into the metal-methyl bond, and the resulting propyl complex dehydrogenates to give the final product, presumably an q3-allyl complex (eq 2h4*pb The rate[L,Zr-CH3]+
+ C,H,
k2
[L,Zr-C3H51f
+ H,
(2)
determining step for this reaction is not certain, but the energies of key intermediates and activated complexes will be affected by the electron deficiency at the metal center as in the simple hydrogenolysis reaction. Attempts to study reactions of the [FluzZrCH3]+ (5) cation were unsuccessful due to the slow rates of reaction. Predicted values of the rates for H2 and C2H4 from the Cy correlation for L = Cp and Ind (Figure 2) suggest that the rate constants are lower than the dynamic range of the experimental method (in favorable cases, 4-5 orders of magnitude below the collisional limited rate constants indicated by the dashed lines in Figure 2). The rate constants for the reactions in eqs 1 and 2 decrease in the order 4 > 1 > 2 > 3 > 5. The hydrogenolysis reaction (eq 1) has a stronger dependence on Cy than the insertioddehydrogenation pathway, and both reactions are significantly retarded by increasing the electron-donating character of the ancillary ligands. We conclude that 4 is the most electrophilic ion of the group by a small margin and that replacement of Cp by Ind or Flu significantly reduces the electrophilicity of the metal center. Fluorenyl (YFlu = -0.65) and indenyl (YInd = -0.41) ligands are strongly
Communications
Organometallics, Vol. 14, No. 11, 1995 5007
electron-donating ligand^^!^ and are expected to decrease indirect predictors of the effect of the (CH3)2Si(C5H4)2 the electrophilicity of the zirconium(rV)center relative ancillary ligand on the reactivity of zirconium(n7) complexes for they do not directly address the question to the bis-Cp complex, as observed. X-ray photoelectron spectroscopy (XPS)studies of LzZrC12 3d5/2 binding of relative electrophilicity of the [ ( C H B ) ~ S ~ ( C ~ H ~ ) ~ Z ~ R I + energies (BE) confirm that Flu and Ind are electronion, which is the critical form of the complex in catalytic donating relative t o Cp (relative to L = Cp a t 182.2 eV, systems. ABE = -0.1 eV for L = Ind (182.1 eV), ABE = -0.3 eV Our results show that the effect of the (CH3)2Si(C5H4)2 for L = Flu (181.9 eV)). ancillary ligand in the 16electron methylzirconium cation is to increase the metal complex electrophilicity It has been argued8 that the properties of the (CH3)3relative to the bis-Cp environment. XPS studies of Si substituent would require that chelating (CHJ2Sidichloride complexes have also shown that the dimeth(C5H4)Z be electron-donating in a zirconium(IV)complex ylsilyl bridge leads to an increase in the Zr 3d5n binding relative t o Cp. This conclusion was supported by XPS energy relative to unbridged FluzZrC12,12 suggesting studies,8which showed that the (CH3)3SiC& ligand is that the general effect of (CH&Si bridging in these electron-donating relative t o Cp. ETE measurements zirconium(IV)metallocenesis to increase electrophilicity on metallocenes have also confirmed the electrona t the metal center by reducing the effective Lewis donating character of (CH3)3SiC5H4 ( Y T M S C ~ = -0.24 basicity of the carbocyclic ligands. The lower basicity based on ferrocene derivative data5a). However, other of the dimethylsilyl-bridgedligands presumably results reports have suggested that (CH3)2Si(C5&)2 is electronfrom structural distortion of the ligand positions away withdrawing relative to Cp in zirconium(IV)chemistry. from their “preferred” binding angles and distances.1° From density functional theory studies, Ziegler and coThe observed trends in the rate constants for Cp- and workersg found a higher positive charge on the Zr in Ind-substituted complexes, 1 > 2 > 3, are consistent [(CH3)2Si(C5H4)2ZrCH31+compared to [CpzZrRI+(based with the strong electron-donating effect expected for on Mulliken population analyses). Electrochemical Ind.5 As noted above, the usual observation has been studies reported by Petersen and co-workers1° show that that the bis-indenyl substitution increases the polymthe electrode potential for the C(CH~)~S~(C~H~)~Z~C~ZI~’erization activity in homogeneous zirconocene(n7) catacouple is -200 mV less negative than that of the lysts when compared t o bis-Cp anal0gs.l The increased [Cp2ZrCl2l0/-couple. XPS studies by Siedle et al.ll show polymerization rates for these IndzZrX2-based catalysts a increased Zr 3 d ~ binding 2 energy in (CH3)2Si(C5H4)2are likely a consequence of decreased termination rates ZrCl2 (+2.4 eV) relative to Cp~ZrCl2,suggesting lower and/or increased initiation rates, because the lower electron density at the metal center in the former case. electrophilicityresulting from replacement of Cp by Ind Such physical measurements involving electrochemical should decrease the propagation rate for monomer reduction to Zr(II1) and core ionization energies are in~erti0n.l~ Rates of termination by /3-R elimination (R = H, alkyl) and of initiation (by separation of an ion (7) Gassman, P. G.; Winter, C. H. J.Am. Chem. Soc. 1988,110,6130. (8) Gassman, P. G.; Deck, P. A.; Winter, C. H.; Dobbs, D. A,; Cao, pair) could both be affected in the required direction by D. H. Organometallics 1992, 11, 959. the increased steric bulk and higher electron-donating (9)Woo, T. K.; Fan, L.; Ziegler, T. Organometallics 1994,13, 2252. tendency of bis-indenyl ancillary ligands. (10)Bajgur, C. S.; Tikkanen, W. R.; Petersen, J. L. Inorg. Chem. 1985,24,2539. (11)Siedle, A. R.; Newmark, R. A.; Lamanna, W. M.; Schroepfer, J. N. Polyhedron 1990, 9, 301. (12)Siedle, A. R. Unpublished work. The Zr(3dsn) binding energies of FluzZrClz and (MeZSiFlu2)ZrClzare 181.9 i~0.1 and 183.2 i~0.1 eV, respectively (C(ls) 285.0 eV). (13)Richardson, D. E.; Alameddin, N. G.; Ryan, M. F.; Siedle, A. R. Manuscript in preparation.
Acknowledgment. This work was supported in part by a grant from the National Science Foundation to D.E.R. and J.R.E. (CHE9311614). XPS studies were done by Dr. L. Zazzera (3M). OM950505B
