Cationic aluminum alkyl complexes incorporating amidinate ligands. Transition-metal-free ethylene polymerization catalysts

Full text of DOI:10.1021/ja971815j

J. Am. Chem. Soc. 1997, 119, 8125-8126
8125
The reaction of {MeC(NiPr)2}AlMe2 (1a) with 1 equiv of
B(C6F5)311 at 23 °C in CD2Cl2 results in the formation of a new
aluminum complex, [2a][MeB(C6F5)3], and consumption of half
of the boron activator (by 11B NMR). The analogous reaction
employing 0.5 equiv of B(C6F5)3 produces [2a][MeB(C6F5)3]
in 83% isolated yield (white solid) with total consumption of
the boron reagent. The reaction of 1a and 1.0 or 0.5 equiv of
[Ph3C][B(C6F5)4]12 under similar conditions yields [2a][B(C6F5)4]
and MeCPh3. The variable temperature 1H and 13C NMR
spectra of [2a][MeB(C6F5)3] and [2a][B(C6F5)4] are identical
except for the anion resonances, and establish that 2a+ is the
dinuclear Me-bridged cation [({MeC(NiPr)2}AlMe)2(µ-Me)]+
Cationic Aluminum Alkyl Complexes Incorporating
Amidinate Ligands. Transition-Metal-Free
Ethylene Polymerization Catalysts
Martyn P. Coles and Richard F. Jordan*
Department of Chemistry, The UniVersity of Iowa
Iowa City, Iowa 52242
ReceiVed June 3, 1997
Cationic aluminum species of the general type AlLX2+ (L )
neutral 2-electron donor, X ) anionic 2-electron donor) and
the corresponding base-stabilized adducts AlLX2(L′)+ (L′ )
labile Lewis base) are of interest for application in catalysis
and synthesis because the combination of an electrophilic
cationic Al center and a potentially reactive Al-X bond should
promote coordination and activation of a range of substrates.1
Known cationic aluminum compounds include 5- to 7-coordinate
1
(Scheme 1). The -20 °C H NMR spectrum of 2a+ contains
two singlets at δ -0.15 and -0.57 in a 2:1 intensity ratio, which
are assigned to the terminal and bridging methyl groups,
respectively. These signals coalesce to a broad singlet (δ -0.38)
at 23 °C, indicating that bridge/terminal methyl exchange is
1
+
rapid under these conditions. The H NMR spectrum of a
(chelate)AlX2 species containing polydentate ether/amine or
tetradentate Schiff-base ligands,2,3 4-coordinate species incor-
porating chelating ligands,4 (amine)2AlX2+ compounds,5 AlX2-
(THF)4+ complexes (X ) H, Cl),6 and (C5R5)2Al+ aluminoce-
nium cations.7 Here we describe a strategy for the design of
reactive 3-coordinate cationic aluminum alkyl compounds which
are capable of polymerizing ethylene.
solution of [2a][MeB(C6F5)3] containing 0.5 equiv of 1a (CD2-
Cl2, 23 °C) contains resonances for each component which are
identical with those observed for separate solutions of the
components, implying that the bridge/terminal exchange of 2a+
is intramolecular. 1H NMR spectra of 2a+ below -60 °C are
more complex and are consistent with the presence of a 1:1
mixture of two slowly exchanging rotamers (anti-Me and
gauche-Me).13
Neutral, d0, group 4 metal LnMR2 complexes (Ln ) ancillary
ligands, e.g. Cp2) can be activated for olefin polymerization and
other reactions by conversion to LnMR(L′)+ or LnMR+ cations.8
We reasoned that (L-X)AlR2 compounds containing a suitable
bidentate, monoanionic ancillary ligand L-X- could be ionized
to (L-X)AlR(L′)+ or (L-X)AlR+ cations using methods
developed for transition metal systems. N,N′-Dialkylamidinates,
RC(NR′)2-, are attractive ligands for this application because
they possess the correct charge and metal binding properties
and can be sterically and electronically tuned by modification
of the R and R′ groups. Several neutral Al compounds
The formation of 2a+ from 1a involves initial generation of
the 3-coordinate {MeC(NiPr)2}AlMe+ cation, which is rapidly
trapped by adduct formation with 1a. Bochmann found that
analogous metallocene species {Cp2M(Me)}2(µ-Me)+ (M ) Ti,
Cp ) indenyl; M ) Zr or Hf, Cp ) C5H5, Me2Si(indenyl)2 or
C2H4(indenyl)2), are formed in a similar manner; however, in
contrast to 2a+, these species are converted to Cp2M(Me)+
cations by reaction with additional activator under mild condi-
tions.14 Recently Marks isolated several dinuclear cationic
metallocenes of this type.15
-
containing MeC(NSiMe3)2-, PhC(NSiMe3)2-, or MeC(NiPr)2
ligands have been described,9 and we have developed general
routes to {RC(NR′)2}AlMe2 compounds (R ) Me, R′ ) Pr,
Replacement of the amidinate methyl substituent of 1a by a
tBu group in 3a,b (Scheme 2) causes an 8° reduction in the
Al-N-R′ bond angles and thus increases the steric congestion
at Al.10 We anticipated that this effect would disfavor the
formation of dinuclear cations. Indeed, the reaction of 3a,b
with 1 equiv of B(C6F5)3 generates the base free ion pairs [{t-
BuC(NR′)2}AlMe][MeB(C6F5)3] (4a,b) in quantitative NMR
yield (Scheme 2). The 1H and 13C NMR spectra of 4a,b contain
resonances for the NR′ groups which are consistent with Cs-
symmetric structures. The 1H NMR spectra (CD2ClCD2Cl, 23
i
t
i
Cy; R ) Bu, R′ ) Pr, Cy, SiMe3).10
(1) (a) Eisch, J. J. In ComprehensiVe Organometallic Chemistry, 2nd
ed.; Housecroft, C. E., Ed.; Pergamon: Oxford, 1995; Vol. 1, pp 431-
502. (b) Eisch, J. J. In ComprehensiVe Organometallic Chemistry, 2nd ed.;
McKillop, A., Ed.; Vol. 11, pp 277-311.
(2) (a) Self, M. F.; Pennington, W. T.; Laske, J. A.; Robinson, G. H.
Organometallics 1991, 10, 36. (b) Richey, H. G., Jr.; BergStresser, G. L.
Organometallics 1988, 7, 1459. (c) Bott, S. G.; Alvanipour, A.; Morley, S.
D.; Atwood, D. A.; Means, C. M.; Coleman, A. W.; Atwood, J. L. Angew.
Chem., Int. Ed. Engl. 1987, 26, 485. (d) Bott, S. G.; Elgamal, H.; Atwood,
J. L. J. Am. Chem. Soc. 1985, 107, 1796. (e) Atwood, J. L.; Bott, S. G.;
May, M. T. J. Coord. Chem. 1991, 23, 313.
(3) (a) Atwood, D. A.; Jegier, J. A.; Rutherford, D. Inorg. Chem. 1996,
35, 63. (b) Atwood, D. A.; Jegier, J. A.; Rutherford, D. J. Am. Chem. Soc.
1995, 117, 6779.
(4) (a) Engelhardt, L. M.; Kynast, U.; Raston, C. L.; White, A. H. Angew.
Chem., Int. Ed. Engl. 1987, 26, 681. (b) Uhl, W.; Wagner, J.; Fenske, D.;
Baum, G. Z. Anorg. Allg. Chem. 1992, 612, 25. (c) Emig, N.; Re´au, R.;
Krautscheid, H.; Fenske, D.; Bertrand, G. J. Am. Chem. Soc. 1996, 118,
5822.
(5) (a) Atwood, D. A.; Jegier, J. A. J. Chem. Soc., Chem. Commun. 1996,
1507. (b) Jegier, J. A.; Atwood, D. A. Inorg. Chem. 1997, 36, 2034.
(6) (a) Knjazhansky, S. Y.; Nomerotsky, I. Y.; Bulychev, B. M.; Belsky,
V. K.; Soloveichik, G. L. Organometallics 1994, 13, 2075. (b) Means, N.
C.; Means, C. M.; Bott, S. G.; Atwood, J. L. Inorg. Chem. 1987, 26, 1466.
(7) (a) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs,
R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1655. (b) Bochmann, M.;
Dawson, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 2226.
(8) (a) Jordan, R. F. AdV. Organomet. Chem. 1991, 32, 325. (b) Guram,
A. S.; Jordan, R. F. In ComprehensiVe Organometallic Chemistry, 2nd ed.;
Lappert, M. F., Ed.; Pergamon: Oxford, 1995; Vol. 4, pp 589-625. (c)
Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255. (d) Marks, T. J.
Acc. Chem. Res. 1992, 25, 57.
-
°C) of 4a and 4b both contain a MeB(C6F5)3 resonance at δ
1.67, which is significantly downfield from the free anion
resonance (δ 0.5).11 Additionally, the 13C and 19F NMR spectra
of 4a,b contain two sets of C6F5 resonances (2:1 ratio in the
19F NMR spectrum). These data for 4a,b are consistent with
structures in which the anion coordinates to Al by a B-Me-
Al bridge and rotation about the B-Me-Al linkage is slow
due to steric crowding. Initial efforts to isolate 4b gave
{tBuC(NCy)2}Al(Me)(C6F5), showing that anion degradation can
occur in these systems. Efforts to isolate 4a,b are continuing.
Lewis base adducts {RC(NR′)2}Al(Me)(L′)+ have been
(10) Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young, U. G., Jr.
Submitted for publication.
(11) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015.
(12) Chien, J. C. W.; Tsai, W.-M.; Rausch, M. D. J. Am. Chem. Soc.
1991, 113, 8570.
(13) The expected statistical ratio of the anti-Me and gauche-Me rotamers
of 2a+ is 1:2. The preference for the anti-Me rotamer results from steric
interactions between the bulky iPr groups.
(9) (a) Lechler, R.; Hausen, H.-D.; Weidlein, J. J. Organomet. Chem.
1989, 359, 1. (b) Ergezinger, C.; Weller, F.; Dehnicke, K. Z. Naturforsch.
1988, 43b, 1621. (c) Kottmair-Maieron, D.; Lechler, R.; Weidlein, J. Z.
Anorg. Allg. Chem. 1991, 593, 111. (d) Duchateau, R.; Meetsma, A.;
Teuben, J. H. J. Chem. Soc., Chem. Commun. 1996, 223.
(14) (a) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1992, 434,
C1. (b) Bochmann, M.; Lancaster, S. J. Angew. Chem., Int. Ed. Engl. 1994,
33, 1634.
(15) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J. J. Am. Chem.
Soc. 1996, 118, 12451.
S0002-7863(97)01815-5 CCC: $14.00 © 1997 American Chemical Society

8126 J. Am. Chem. Soc., Vol. 119, No. 34, 1997
Communications to the Editor
Scheme 1
Scheme 2
strengthening the M-C bond (other factors being equal).19 In
initial studies, we have found that CD2Cl2 solutions of [2a]-
[MeB(C6F5)3] (50 °C) or 4a (23 °C) polymerize ethylene (e1
atm) with low activity. Toluene solutions of 4a polymerize
ethylene (2 atm) to solid polyethylene at 60 °C (activity ) 700
g PE/(mol‚h‚atm); Mw ) 176 100; Mw/Mn ) 2.84; DSC mp
138.2 °C). More active catalysts are generated by activation
of 3a with 1 equiv of [Ph3C][B(C6F5)4] in toluene (2 atm of
ethylene; 60 °C: activity 2480 g PE/(mol‚h‚atm); Mw
)
272 200; Mw/Mn ) 3.30; DSC mp 139.6 °C; 85 °C: activity
3050 g PE/(mol‚h‚atm); Mw ) 184 700; Mw/Mn ) 2.23; DSC
mp 138.2 °C). Our current interpretation of these results is that
3-coordinate {RC(NR′)2}AlR+ cations are the active species in
these polymerizations, and that the activities are strongly
influenced by coordination of {RC(NR′)2}AlMe2 or A- to these
species and, in the MeB(C6F5)3- systems, by anion degradation.
The high molecular weights and narrow molecular weight
distributions are consistent with a single site catalyst with a high
generated in NMR scale reactions. The reactions of 1a or 3a,b
with 1 equiv of [HNMe2Ph][B(C6F5)4]16 yield the corresponding
amine adducts [{RC(NR′)2}Al(Me)(NMe2Ph)][B(C6F5)4] (5a,
k
chain-growth/kchain-transfer ratio.
This work establishes that ({MeC(NiPr)2}AlMe)2(µ-Me)+ and
1
6a,b; Schemes 1 and 2). The H and 13C NMR spectra of 5a
{RC(NR′)2}AlMe(L′)+ cations and [{tBuC(NR′)2}AlMe][MeB-
(C6F5)3] ion pairs are formed by alkyl abstraction from {RC-
(NR′)2}AlMe2, and suggests that {RC(NR′)2}AlR+ cations are
active species in ethylene polymerization. Efforts to isolate base
free [{RC(NR′)2}AlR][A] salts and to develop even more
reactive Al-based catalysts are in progress.
and 6a,b (23 °C) contain amine resonances which are shifted
from those of the free amine, and resonances for the R′ groups
which imply Cs-symmetric structures. These results are con-
sistent with amine coordination to Al. The reaction of [2a]-
[MeB(C6F5)3] with 0.5 equiv of NMe2Ph generates a 1:1 mixture
of 1a and 5a. Similarly, the reaction of [2a][MeB(C6F5)3] or
5a with excess PMe3 generates the phosphine adduct
[{MeC(NiPr)2}AlMe(PMe3)][A-] (7a+, A- ) MeB(C6F5)3- or
B(C6F5)4-), with liberation of 1 equiv of 1a or NMe2Ph. These
ligand exchange reactions show that the order of Lewis basicity
toward {MeC(NiPr)2}AlMe+ is PMe3 > NMe2Ph > 1a > A-.
One potential application of {RC(NR′)2}AlR+ cations is as
transition-metal-free olefin polymerization catalysts. Neutral
aluminum alkyls catalyze the oligomerization of ethylene to C8-
C14 R-olefins at ethylene pressures above 80 atm and temper-
atures of 90-120 °C.17 Monomeric AlR3 species catalyze the
chain growth by repetitive insertion, and the low molecular
weight results from rapid â-H elimination.1a,18 Recently, on
the basis of comparisons of group 4 metal Cp2MR+ and
(C2B9H11)CpMR catalysts, we proposed that a cationic charge
on the active species should inhibit â-H elimination by
Acknowledgment. This work was supported by the Department
of Energy Grant DE-FG02-88ER13935. M.P.C. was supported by a
NATO postdoctoral research fellowship. Gifts of B(C6F5)3 (Boulder
Scientific) and [Ph3C][B(C6F5)4] (Asahi Glass) are gratefully acknowl-
edged.
Supporting Information Available: Synthetic procedures, char-
acterization data for new compounds, and a listing of ethylene
polymerization results (10 pages). See any current masthead page for
ordering and Internet access instructions.
JA971815J
(18) Recently, Martin reported the formation of linear polyethylene (Mw
) ca. 106, activities 1.6 × 10-1 to 2.1 × 10-4 g PE/(mol‚h‚atm)) from the
reaction of Cl2AlCH(Me)AlCl2 or (AlR3)2 with ethylene at 25-50 °C to
reduce the rate of â-H elimination. See: Martin, H.; Bretinger, H. Makromol.
Chem. 1992, 193, 1283.
(19) Kreuder, C.; Jordan, R. F.; Zhang, H. Organometallics 1995, 14,
2993 and references therein.
(16) Turner, H. W. Eur. Pat. Appl. 0 277 004, 1988.
(17) Ziegler, K.; Gellert, H.-G.; Zosel, K.; Holzkamp, E.; Schneider, J.;
So¨ll, M.; Kroll, W.-R. Justus Liebigs Ann. Chem. 1960, 629, 121.