Significant zirconium-alkyl group effects on ion pair formation thermodynamics and structural reorganization dynamics in zirconocenium alkyls

Article abstract of DOI:10.1021/om990274z

Substantial Zr-alkyl group (R) effects on ion pair thermodynamic stability as well as on ion pair solution structure and structural dynamics are reported in the [(1,2-Me₂Cp)₂ZrR]⁺[CH₃B(C ₆F₅

Full text of DOI:10.1021/om990274z

2410
Organometallics 1999, 18, 2410-2412
Sign ifica n t Zir con iu m -Alk yl Gr ou p Effects on Ion P a ir
F or m a tion Th er m od yn a m ics a n d Str u ctu r a l
Reor ga n iza tion Dyn a m ics in Zir con ocen iu m Alk yls
Colin L. Beswick and Tobin J . Marks*
Department of Chemistry, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208-3113
Received April 20, 1999
Summary: Substantial Zr-alkyl group (R) effects on ion
pair thermodynamic stability as well as on ion pair
solution structure and structural dynamics are reported
in the [(1,2-Me2Cp)2ZrR]+[CH3B(C6F5)3]- series, where
R ) CH3, CH2C(CH3)3, CH2Si(CH3)3, and CH[Si(CH3)3]2.
These quantitative results underscore the effects such R
moieties are likely to play in group 4 metallocene-
mediated olefin polymerization catalysis.
nium ion pair thermodynamic stability with respect to
the neutral precursors, solution phase structure, and
stereochemical dynamics. We communicate here the
first thermochemical and dynamic NMR study which
indicates that R, R′ effects can be very large.3
Using the well-characterized and spectroscopically
informative bis(1,2-dimethylcyclopentadienyl)zirconocene
framework and B(C6F5)3 as a prototypical abstractor/
cocatalyst,2,4,5 a series of metallocenium alkyl meth-
ylborate ion pairs (2) was synthesized from the corre-
sponding methylalkyls (1)6 (eq 2) and characterized by
A growing database implicates metallocene ancillary
ligation (L, L′), metal identity (M), and the nature of
the abstractor/cocatalyst (A; eq 1) as key factors govern-
ing the thermodynamics and kinetics of single-site
metallocenium catalyst activation as well as the stereo-
chemical dynamics of the tight ion pairing.1,2 The
standard spectroscopic and analytical techniques.7 The
interplay of these variables influences catalyst activity,
thermodynamic stability, chain transfer characteristics,
and regio- and stereochemical aspects of monomer
enchainment in ways that are not yet entirely under-
stood. In actual polymerization catalytic systems, the
identity of alkyls R and R′ can be quite variable, and
little is known quantitatively about how alkyl group
steric and electronic characteristics affect metalloce-
(3) Communicated in part: Beswick, C. L.; Marks, T. J . Abstracts
of Papers; 216th National Meeting of the American Chemical Society,
Boston, MA, Aug 1998; American Chemical Society: Washington, DC,
1998; INOR 141.
(4) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015-10031.
(5) (a) Sun, Y.; Spence, R. E. V. H.; Piers, W. E.; Parvez, M.; Yap,
G. P. A. J . Am. Chem. Soc. 1997, 119, 5132-5143. (b) Wang, Q.; Gillis,
D. J .; Quyoum, R.; J eremic, D.; Tudoret, M.-J .; Baird, M. C. J .
Organomet. Chem. 1997, 527, 7-14. (c) Bochmann, M.; Lancaster, S.
J .; Hursthouse, M. B.; Malik, K. M. A. Organometallics 1994, 13, 2235-
2243.
(1) For leading recent reviews of single-site olefin polymerization,
see: (a) J . Mol. Catal. 1998, 128, 1-337 (special issue on metallocene
and single site olefin catalysts; J ordan, R. F., Ed.). (b) Kaminsky, W.;
Arndt, M. Adv. Polym. Sci. 1997, 127, 144-187. (c) Bochmann, M. J .
Chem. Soc., Dalton Trans. 1996, 255-270. (d) Brintzinger, H. H.;
Fisher, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1143-1170. (e) Soga, K., Terano, M., Eds.
Catalyst Design for Tailor-Made Polyolefins; Elsevier: Tokyo, 1994.
(f) Mo¨hring, P. C.; Coville, N. J . J . Organomet. Chem. 1994, 479, 1-29.
(2) (a) Lanza, G.; Fragala`, I. L.; Marks, T. J . J . Am. Chem. Soc.
1998, 120, 8257-8258. (b) Chen, Y.-X.; Metz, M. V.; Li, L.; Stern, C.
L.; Marks, T. J . J . Am. Chem. Soc. 1998, 120, 6287-6305. (c) Deck,
P. A.; Beswick, C. L.; Marks, T. J . J . Am. Chem. Soc. 1998, 120,
1772-1784. (d) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J .
Am. Chem. Soc. 1998, 120, 4049-4050. (e) J ia, L.; Yang, X.; Stern, C.
L.; Marks, T. J . Organometallics 1997, 16, 842-857. (f) Michael, D.
B.; Coates, G. W.; Hauptman, F.; Waymouth, R. M.; Ziller, J . W. J .
Am. Chem. Soc. 1997, 119, 11174-11182. (g) Shiomura, T.; Asanuma,
T.; Inoue, N. Macromol. Rapid Commun. 1996, 17, 9-14. (h)
Herzog, T. A.; Zubris, D. L.; Bercaw, J . E. J . Am. Chem. Soc. 1996,
118, 11988-11989. (i) Eisch, J . J .; Pombrik, S. I.; Gurtzgen, S.;
Rieger, R.; Vzick, W. In ref 1e, pp 221-235. (j) Giardello, M. A.; Eisen,
M. S.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1995, 117, 12114-
12129.
(6) Methylalkylzirconocenes were prepared via the addition of 1
equiv of the corresponding lithium alkyl to the dichlorozirconocene
followed by treatment with methyllithium or, alternatively, by addition
of the lithium alkyl to a mixed chloromethylzirconocene. See the
Supporting Information for details.
(7) For example, 2d was synthesized when 1d (0.21 g, 0.46 mmol)
and B(C6F5)3 (0.24 g, 0.48 mmol) were added to a vacuum reaction/
filtration flask. Toluene (25 mL) was vacuum-transferred in at -78
°C, and the resulting solution was warmed to 25 °C and stirred for 30
min. The mixture was then removed under reduced pressure, leaving
a viscous orange oil. Pentane (20 mL) was then vacuum-transferred
in, and after stirring, the oil solidified. The solution was filtered and
the solvent removed. The solid product was washed a second time with
pentane, after which bright yellow, analytically pure product (0.39 g,
0.41 mmol) was recovered. Yield: 92%. 1H NMR (CD2Cl2): δ 6.43 (t,
3.12 Hz, 1 H), 6.26 (d, 3.12 Hz, 2 H), 6.16 (d, 3.12 Hz, 2 H), 5.96 (t,
3.12 Hz, 1 H), 3.72 (s, 1 H), 2.15 (s, 6 H), 2.05 (s, 6 H), 0.40 (br,
1
3 H), 0.07 (s, 18 H). 13C NMR (CD2Cl2): ∆ 147.2 (d, J CF ) 241
1
1
Hz), 137.1 (d, J CF ) 244 Hz), 136.0 (d, J CF ) 249 Hz), 129.8 (CCH3),
127.1 (CCH3), 116.2 (CH), 114.0 (CH), 113.1 (CH), 110.3 (CH), 80.7
(ZrCH), 13.7 (CCH3), 13.4 (CCH3), 3.8 (SiCH3). Anal. Calcd for
C
40H40BF15Si2Zr: C, 49.83; H, 4.19. Found: C, 49.39; H, 4.19. See the
Supporting Information for other synthetic details.
10.1021/om990274z CCC: $18.00 © 1999 American Chemical Society
Publication on Web 05/29/1999

Communications
Organometallics, Vol. 18, No. 13, 1999 2411
Ta ble 1. Ion P a ir F or m a tion Th er m od yn a m ics a n d Ion P a ir Reor ga n iza tion Dyn a m ics Da ta for
Meta llocen e Alk yl Ion P a ir s h a vin g th e F or m u la [(1,2-Me2Cp )2Zr R]+[CHB(C6F 5)3]- in Tolu en e Solu tion
a
b,c
b
b
entry
no.
∆Hformation
∆Gq
∆Hq
∆Sq
coalescence temp
(°C in tol-d8)
reorganization
reorganization
reorganization
R
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
2a
2b
2c
2d
CH3
-24.6(8)d
â-Me elim
-22.6(1.0)
-59.2(1.4)
17.6(2)d
13.8(2)e
13.8(2)
8.0(4)g
22(1)d
18(1)
17(1)
9(2)g
13(2)d
15(2)
10(3)
7(4)g
110d,f
CH2C(CH3)3
CH2Si(CH3)3
CH[Si(CH3)3]2
â-Me elim
20f
-100g,h
Determined calorimetrically. Determined via VT NMR experiments. c At the indicated coalescence temperature. See also ref 2c.
a
b
d
e At 0 °C. f Coalescence of diastereotopic CpMe signals. In CDCl2F. Coalescence of diastereotopic TMS groups.
g
h
abstraction process is, within NMR detection limits,
completely selective for the methyl anion.8 The enthal-
pies of eq 2 (∆Hform) were measured in toluene solution
by reaction titration calorimetry using the rigorously
anaerobic methodology and instrumentation described
previously.2c,9 ∆Hform for complex 2b could not be
measured because of facile â-CH3 elimination above 0
°C.10 The thermochemical results (Table 1) indicate that
alkyl effects on ∆Hform, and hence stabilization of the
cationic catalyst, can be rather large. Thus, while the
exothermicity of eq 2 for R ) CH2Si(CH3)3 is comparable
to or slightly less than that for R ) CH3, that for R )
CH[Si(CH3)3]2 is more than twice as large.
groups, and close CHTMS2 methine proton proximity
(as judged by NOE measurements) to only one 1,2-Me2-
Cp ring. Interestingly, at the lowest accessible temper-
atures, restricted rotation about one HC-TMS bond is
observed, suggestive of cation-anion congestion and/
or TMS group donor interactions toward the metal
center (e.g., I), for which there is ample precedent in
Some insight into the driving force(s) for eq 2 and the
sensitivity of ion pair structural mobility on the R group
is provided by variable-temperature NMR spectros-
copy.11 First, the neutral precursor complex 1d 12 is found
to be sterically congested, as indicated by restricted
rotation of the CH[Si(CH3)3]2 group (∆Gq ) 17.4 kcal/
mol at 85 °C).13 The limited solubility of ion pair 2d in
toluene is suggestive of weak ion pairing,2e,14 while the
CH3B(C6F5)3- 1H chemical shift (δ 0.40 in CD2Cl2, 1.24
in toluene-d8) is similar to that of the free anion10b,14
rather than to that in tightly ion paired complexes,2c,4,5
i.e., those having strong Zr+‚‚‚H3CB(C6F5)3- interactions
as assessed by X-ray diffraction. Variable-temperature
NMR spectroscopy of 2d in CDCl2F reveals an instan-
taneous structure at -126 °C in accord with eq 3, as
neutral lanthanocene anologues.15 Steric destabilization
of 1d , together with interaction I and any â-cation
stabilizing effects16 of the CHTMS2 group, likely account
for the large negative ∆Hform. One consequence of the
sterically encumbering/cation-stabilizing zirconocenium
environment of 2d is rather weak ion pairing, as
evidenced by ∆Gq
) 8.0(4) kcal/mol, ∆Hq
) 9(2)
reorg
reorg
kcal/mol, and ∆Sq
) 7(4) cal/(K mol) for eq 3 in
reorg
CDCl2F. Although the slow exchange limit could not be
reached in toluene-d8, the similar line shape changes
are consistent with similar activation parameters.
In contrast to the large R ) CHTMS2 effects on ∆Hform
in 2d , the thermochemical results (Table 1) for the R )
(10) 1H NMR features of 1a and isobutene are observed. For related
examples of such processes, see: (a) Shaffer, T. D.; Canich, J . A. M.;
Squire, K. R. Macromolecules 1998, 31, 5145-5147. (b) Horton, A. D.
Organometallics 1996, 15, 2675-2677. (c) Guo, Z.; Swenson, D. C.;
J ordan, R. F. Organometallics 1994, 13, 1424-1432. (d) Kesti, M. R.;
Waymouth, R. M. J . Am. Chem. Soc. 1992, 114, 3565-3567.
(11) Solution concentrations for variable-temperature NMR line
shape measurements were precisely known and were typically within
5-10 mM.
(12) Spectroscopic and analytical details for 1d : 1H NMR (C6D6) δ
6.49 (“t”, 1 H), 6.46 (“t”, 1 H), 5.96 (“t”, 1 H), 5.27 (“t”, 1 H), 5.19 (“t”,
1 H), 4.58 (“t”, 1 H), 2.08 (s, 3 H), 1.70 (s, 3 H), 1.67 (s, 3 H), 1.40 (s,
3 H), 1.32 (s, 1 H), 0.21 (s, 9 H), 0.20 (s, 9 H), -0.29 (s, 3 H); 13C NMR
(C6D6): δ 125.0 (CCH3), 122.3 (CCH3), 122.2 (CCH3), 119.0 (CH), 118.2
(CCH3), 115.5 (CH), 107.3 (CH), 107.2 (CH), 107.0 (CH), 102.1 (CH),
42.9 (ZrCH), 40.7 (ZrCH3), 13.2 (CCH3), 13.1 (CCH3), 13.0 (CCH3), 12.8
(CCH3), 6.0 (SiCH3), 5.3 (SiCH3). Anal. Calcd for C22H40Si2Zr: C, 58.45;
H, 8.94. Found: C, 58.28, 58.22; H, 8.83, 8.77.
(13) For related systems see: J effery, J .; Lappert, M. F.; Luong-
Thi, N. T.; Webb, M.; Atwood, J . L.; Hunter, W. F. J . Chem. Soc., Dalton
Trans. 1981, 1593-1605.
(14) (a) Beck, S.; Prosenc, M. H.; Brintzinger, H. H. J . Mol. Catal.
A: Chem. 1998, 128, 41-52. (b) Lee, R. A.; Lachicotte, R. J .; Bazan,
G. C. J . Am. Chem. Soc. 1998, 120, 6037-6046. (c) Desjardins, S. Y.;
Way, A. A.; Murray, M. C.; Adirim, D.; Baird, M. C. Organometallics
1998, 17, 2382-2384. (d) Pellecchia, C.; Immirzi, A.; Grassi, A.;
Zambelli, A. Organometallics 1993, 12, 4473-4478.
(15) (a) Klooster, W. T.; Brammer, L.; Schaverien, C. J .; Budzelaar,
P. H. M. J . Am. Chem. Soc. 1999, 121, 1381-1382. (b) Koga, N.;
Morokuma, K. J . Am. Chem. Soc. 1988, 110, 108-112. (c) Di Bella, S.;
Lanza, G.; Fragala`, I. L.; Marks, T. J . Organometallics 1996, 15, 205-
208. (d) Haar, C. M.; Stern, C. L.; Marks, T. J . Organometallics 1996,
15, 1765-1784. (e) Giardello, M. A.; Conticello, V. P.; Brard, L.; Sabat,
M.; Rheingold, A. L.; Stern, C. L. J . Am. Chem. Soc. 1994, 116, 10212-
10240. (f) Di Bella, S.; Gulino, A.; Lanza, G.; Fragala`, I. L.; Stern, D.;
Marks, T. J . Organometallics 1994, 13, 3810-3815.
indicated by four magnetically nonequivalent ring CH3
groups,twomagneticallydistinct TMS (TMS )-Si(CH3)3)
(8) For examples of alkyl groups, other than and not in competition
with methyl, abstracted by B(C6F5)3 see ref 5c and: (a) Gielens, E. E.
C. G.; Tiesnitsch, J . Y.; Hessen, B.; Teuben, J . H. Organometallics 1998,
17, 1652-1654. (b) Horton, A. D.; de With, J . Organometallics 1997,
16, 5424-5436. (c) Pellecchia, C.; Immirzi, A.; Grassi, A.; Zambelli, A.
Organometallics 1993, 12, 4473-4478.
(9) (a) Schock, L. E. Ph.D. Thesis, Northwestern University, J une
1988. (b) Barthel, J . Thermometric Titrations; Wiley: New York, 1975;
Vol. 45, pp 56-76.

2412 Organometallics, Vol. 18, No. 13, 1999
Communications
CH2TMS ion pair (2c) evidence little overall silyl or
alkyl stabilization of the cationic fragment, and ∆Hform
is comparable to that in the R ) CH3 cation (2a ) (-22.6-
(1.0) vs -24.6(8) kcal/mol, respectively). With regard to
ion pair structure and structural dynamics, the R )
CH2TMS (2c) and CH2C(CH3)3 (2b) cations differ mark-
edly from both 2d and 2a . Unlike 2d , the toluene
solubility and CH3B(C6F5)3- 1H chemical shifts in these
complexes (δ ) 0.06 in 2b and -0.03 in 2c) are con-
sistent with tighter ion pairing (stronger Zr+‚‚‚H3CB-
(C6F5)3 interactions).2c,4,5 In accord with this observation,
the enthalpic barriers to ion pair reorganization (eq 4)
F igu r e 1. Enthalpic profiles for [(1,2-Me2Cp)2ZrR]+[CH3B-
(C6F5)3]- ion pair formation from the neutral precursors
and symmetrizing ion pair structural reorganization pro-
cesses as a function of the R group in toluene solution: (---)
R ) CH3; (‚‚‚) R ) CH2TMS; (s) R ) CHTMS2.
times slower reorganization rate at 25 °C), again il-
lustrating significant alkyl group effects on the depth
of the potential wells such structures occupy.
The results of this study are summarized in the
comparative energy surfaces of Figure 1. While the exact
interplay of ancillary ligands, counteranion, metal, and
alkyl substituent/growing polymer chain in the olefin
activation and insertion process is not yet completely
quantified, the present results indicate that variation
in alkyl substituent can modulate the enthalpy of ion
pair (catalyst) stabilization versus the neutral precur-
sors (eq 1) by as much as ∼37 kcal/mol (a factor of ∼3
× 1025 times in equilibrium constant at 25 °C using
∆Sform from related systems2c). Furthermore, variation
in the alkyl group can modulate the barrier to ion pair
reorganization (eq 1) by as much as ∼10-13 kcal/mol
(a factor of ∼2 × 108 times in reorganization rate).
are now ∼8-9 kcal/mol higher than in 2d (∼2 × 105
times slower reorganization rate at 25 °C), with limiting
low-temperature spectra in accord with Cs-symmetric
1
structures.5,7 The small magnitude of J C-H in the 2c
Zr-CH2TMS group (107 Hz) may indicate a weak
R-agostic interaction.17 Magnetic equivalence of the
TMS methyl groups down to the lowest accessible
temperatures argues that interactions as in I must also
be weak.15 These results on ion pairing with bulky
primary alkyls can be compared with those for the
crystallographically characterized R ) CH3 ion pair
(2a ).4 While the ∆Hform value is comparable to that in
Ack n ow led gm en t. We thank Dr. C. G. Fry of the
University of Wisconsin for assistance and expertise
with NMR experiments below -100 °C, the Dow Chemi-
cal Co. for donation of B(C6F5)3, and the DOE (Grant
DE-FG02-86ER1351) for financial support.
-
2c (Table 1) and δ CH3B(C6F5)3 differs marginally (δ
0.02), the barrier to ion pair reorganization (eq 4, R′′ )
H) is substantially larger (∼4-5 kcal/mol; ∼1 × 103
(16) (a) Bassindale, A. R.; Taylor, P. G. In The Chemistry of Organic
Silicon Compounds; Patai, S, Rappoport, Z., Eds.; Wiley: Chichester,
U.K., 1989; Chapter 14, pp 893-956. (b) Fleming, I. In Comprehensive
Organic Chemistry; J ones, D. N., Ed.; Pergamon Press: Oxford, U.K.,
1979; Chapter 13, pp 541-671.
(17) (a) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29,
85-93. (b) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg.
Chem. 1988, 36, 1-124.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures and spectral and analytical data for all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM990274Z