C O M M U N I C A T I O N S
these two distributions cannot be generated by two different
catalysts since this would require chain extension by four carbons
at a time; a catalyst with two differentiated sites could, however,
account for the observed distribution (vide infra).
third, the propagating metallacycle must be able to “swing” from
one site to the other, at a rate competitive with monomer insertion.
Any relaxation of these constraints leads to substantial deviations
from the observed distribution.
With a view to obtaining an understanding of the mechanism of
chain propagation, a labeling experiment14 was performed whereby
2/MAO was treated with a 50:50 mix of C2H4 and C2D4. This
afforded predominantly 1-olefins comprising even-numbered H/D
isotopomers for both the major (C4n) and minor (C4n+2) products,
consistent with a metallacyclic propagation mechanism.15
Figure 3. Experimental and modeled distributions for the C4n and C4n+2
series of 1-olefins generated using 2/MAO.
These observations suggest that it should be possible to further
control the product distribution by judicious ligand modifications
and through changes to the reaction conditions. Studies are currently
in progress to refine the selectivity profiles for these catalysts.
Acknowledgment. Ineos Technologies is thanked for financial
support; Prof. J. E. Bercaw is thanked for helpful discussions.
Supporting Information Available: Crystallographic data for 4.
Ligand and complex syntheses, procedures for ethylene oligomerization,
GC and GC-MS spectra, and mechanistic model. This material is
Figure 2. Molecular weight distribution of linear R-olefins obtained from
the oligomerization of ethylene using catalyst 2/MAO (expansion: GC-
MS trace of the C12 fraction arising from treatment of 2/MAO with a
50:50 mix of C2H4 and C2D4).
Since the C4n+2 metallacycle products derive directly from the
C4n metallacycles, and vice versa, examination of mol % versus
carbon number plots for each series can provide useful mechanistic
insight (Figure 3). While the major (C4n) series follows a Schulz-
Flory distribution, it can be seen that the minor (C4n+2) series
deviates substantially from Schulz-Flory behavior. The overall
distribution can be modeled using a statistical treatment (see
Supporting Information) in which the metallacycle occupies two
distinct sites. Such differentiated sites could readily arise, and indeed
might be anticipated, from the nonplanarity of the central nitrogen
donor.
References
(1) See: Vogt, D. Oligomerization of ethylene to higher linear R-olefins; in
Applied Homogeneous Catalysis with Organometallic Compounds; Cornils,
B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, Germany, 2000;
Chapter 2, pp 245-258.
(2) Miller, S. A. Chem. Proc. Eng. 1969, 50, 103.
(3) (a) Freitas, E.; Gum, C. Chem. Eng. Prog. 1973, 73. (b) Hirose, K.; Keim,
W. J. Mol. Catal. 1992, 73, 271. (c) Keim, W. Angew. Chem., Int. Ed.
Engl. 1990, 29, 235. (d) Peuckert, M.; Keim, W. Organometallics 1983,
2, 594.
(4) (a) Reagen, W. K. (to Phillips Petroleum Company), EP 0417477, 1991.
(b) Reagen, W. K.; Pettijohn, T. M.; Freeman, J. W. (to Phillips Petroleum
Company), US Patent 5523507, 1996.
(5) (a) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.; Grimm, S.; Wasserscheid,
P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 4337. (b) Kohn, R. D.;
Haufe, M.; Mihan, S.; Lilge, D. Chem. Commun. 2000, 1927.
(6) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass D.
F. Chem. Commun. 2002, 858.
(7) (a) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Hu, C.; Englert, U.;
Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334. (b) Bollmann, A.;
Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H.;
McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto, S.; Overett, M.;
Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am. Chem. Soc.
2004, 126, 14712.
Scheme 2. Model Based on Two (A and B) Propagating Sites
(kP,A/kA ≈ 0.87; kT,A/kA ≈ 0.13; kS,A/kA ≈ 0.0; kP,B/kB ≈ 0.89; kT,B/kB
≈ 0.0; kS,B/kB ≈ 0.11; kX ) kP,A + kT,A + kS,A (X ) A or B);
disfavored pathways are indicated by broken arrows)
(8) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Morgan, D.; Dixon, J.
T.; Bollmann, A.; Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc.
2003, 125, 5272.
(9) Gibson, V. C.; Tomov, A. K. (to BP Chemicals Ltd), WO 2004/083263.
(10) Thompson, L. K.; Ramaswamy, B. S.; Seymour, E. A. Can. J. Chem.
1977, 55, 878.
(11) Carney, M. J.; Robertson, N. J.; Halfen, J. A.; Zakharov, L. N.; Rheingold,
A. L. Organometallics 2004, 23, 6184.
(12) Ceniceros-Gomez, A. E.; Barba-Behrens, N.; Quiroz-Castro, M. E.; Bernes,
S.; Noth, H.; Castillo-Blum, S. E. Polyhedron 2000, 19, 1821.
(13) The products consist of C4-C64+ linear R-olefins (LAO). The fraction,
analyzable by GC, is described as liquid fraction. The solid fraction,
obtained by precipitation with methanol, was analyzed by GPC and
afforded Mn values in the range of 800-900 (PDI 1.5-1.6). The LAO
content of the solid fraction is 94-98 mol % (1H and 13C{1H} NMR).
(14) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2004, 126, 1304.
A good overall fit could only be obtained when limiting
constraints are applied to the model (see Scheme 2 and Figure 3).
First, the initially formed metallacyclopentane must have a strong
preference for one of the two sites; second, elimination preferentially
occurs from one of the two sites (A as shown in Scheme 2), and
(15) Tomov, A. K.; Chirinos, J. J.; Jones, D. J.; Long, R. J.; Gibson, V. C. J.
Am. Chem. Soc. 2005, 127, 10166.
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