α-iminoenamido ligands: A novel structure for transition-metal activation

Article abstract of DOI:10.1021/om020251b

This communication shows that it is possible to activate nickel complexes containing the α-iminoenamide ligand using B(C₆F₅)₃ or Al(C₆F₅)₃ to generate catalysts that polymerize ethylene. The polymerization of ethylene using these novel compounds is discussed. The novel activation method is confirmed by the X-ray crystallography studies of the methallyl analogs.

Full text of DOI:10.1021/om020251b

3082
Organometallics 2002, 21, 3082-3084
r-Im in oen a m id o Liga n d s: A Novel Str u ctu r e for
Tr a n sition -Meta l Activa tion
Young Heui Kim, Tae Ho Kim, and Bun Yeoul Lee*
Department of Molecular Science and Technology, Ajou University, Suwon 442-749, Korea
David Woodmansee, Xianhui Bu, and Guillermo C. Bazan*
Department of Chemistry, University of California, Santa Barbara, California 93106
Received April 1, 2002
Summary: This communication shows that it is possible
to activate nickel complexes containing the R-iminoena-
mide ligand using B(C6F5)3 or Al(C6F5)3 to generate
catalysts that polymerize ethylene. The polymerization
of ethylene using these novel compounds is discussed.
The novel activation method is confirmed by the X-ray
crystallography studies of the methallyl analogs.
the side opposite from monomer insertion. We give the
structure of {(H3C)C[N(2,6-(CHMe2)2-C6H3)]C[OB(C6F5)3]-
[N(2,6-(CHMe2)2-C6H3)]-κ2N,N}Ni(η3-CH2C6H5) (1), which
The mechanism of precatalyst activation is an es-
sential consideration for the successful design and
further improvement of homogeneous single-site cata-
lysts for olefin polymerization.1 During activation, a
neutral transition-metal complex reacts with a cocata-
lyst to generate a more polarized and electrophilic
species. The best-established methods2 involve hydride
or alkyl abstraction by a neutral Lewis acid, the
protonation of an M-C (or M-H) bond and the use of
aluminoxane-type reagents. A feature that characterizes
the majority of catalytic species is the presence of a
loosely coordinated base, which competes with the
incoming monomer molecule for the vacant site of lowest
energy.3
In work related to the synthesis of branched polyeth-
ylene by tandem catalysis,4 we reported that it is
possible to activate nickel complexes supported by
R-iminocarboxamidato ligands by addition of tris(pen-
tafluorophenyl)borane.5 The key observation is that the
carbonyl functionality of the carboxamide unit coordi-
nates to the Lewis acidic boron and, in doing so, removes
electron density from the nickel atom.6 It is significant
that the partially negative boron atom is situated on
produces polyethylene under mild conditions. For com-
parison, we show the structure of {[(η5-C5Me4)Si-
Me2(η1-NCMe3)]TiMe}{MeB(C6F5)3} (2), a widely used
initiator for the copolymerization of ethylene with
1-alkenes.7 Note how in 2 the {MeB(C6F5)3} anion is
present within the wedge where olefin insertion takes
place. The strength of the Ti-borate interaction is
known to affect polymerization results.8
The well-developed chemistry of enamines is relevant
to the discussion above. It is known that the CR of
enamines9 and of metalloenamines is highly charged
and is capable of reacting with electrophiles.10 The
relationship between the enamide and carboxamide
functionalities is made clear by noting the similarity
between the resonance structures
(1) (a) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem. 1995, 107, 1255; Angew. Chem., Int.
Ed. Engl. 1995, 34, 1143. (b) Transition Metals and Organometallics
as Catalysts for Olefin Polymerization; Kaminsky, W., Sinn, H., Eds.;
Springer-Verlag: Berlin, 1988. (c) Ziegler Catalysts; Fink, G., Mu¨l-
haupt, R., Brintzinger, H. H., Eds.; Springer-Verlag: Berlin, 1995. (d)
Ittel, S. D.; J ohnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169.
(e) Mecking, S. Coord. Chem. Rev. 2000, 203, 325. (f) Metallocenes;
Togni, A., Halterman, R. L., Eds.; Wiley-VCH: New York, 1998. (g)
Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem. 1999,
111, 448; Angew. Chem., Int. Ed. 1999, 38, 429.
(2) Chen, E. Y.-X.; Marks, T. J . Chem. Rev. 2000, 100, 1391.
(3) Considerable mechanistic research continues on the relationship
between the two charged species. For relevant discussion see: Beck,
S.; Lieber, S.; Schaper, F.; Geyer, A.; Brintzinger, H.-H. J . Am. Chem.
Soc. 2001, 123, 1483.
This similarity raises the possibility of activating com-
plexes with R-iminoenamido ligands by creating struc-
tures similar to 1.
Synthetic access into R-iminoenamides with suit-
able steric congestion11 begins with the well-devel-
oped chemistry of R-diimines,12 and in particular
N-aryl-substituted R-diimine ligands.13 Deprotonation
(7) (a) Deck, P. A.; Beswick, C. L.; Marks, T. J . J . Am. Chem. Soc.
1998, 120, 1772. (b) Lanza, G.; Fragala, I. L.; Marks, T. J . J . Am. Chem.
Soc. 1998, 120, 8257. (c) Lai, S.-Y.; Wilson, S. R.; Knight, G. W.;
Stevens, J . C.; Chun, P.-W. S. U.S. Patent 5,272,236, 1993. (d)
McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587.
(8) Chen, Y.-X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1997,
119, 2582.
(4) (a) Komon, Z. J . A.; Bu, X.; Bazan, G. C. J . Am. Chem. Soc. 2000,
122, 1830. (b) Barnhart, R. W.; Bazan, G. C.; Mourney, T. J . Am. Chem.
Soc. 1998, 120, 1082. (c) Komon, Z. J . A.; Bazan, G. C. Macromol. Rapid
Commun. 2001, 22, 467.
(5) Lee, B. Y.; Bazan, G. C.; Vela, J .; Komon, Z. J . A.; Bu, X. J . Am.
Chem. Soc. 2001, 123, 5352.
(9) Hickmott, P. W. Tetrahedron 1982, 38, 1975.
(6) Komon, Z. J . A.; Bu, X.; Bazan, G. C. J . Am. Chem. Soc. 2000,
122, 12379.
(10) March, J . Advanced Organic Chemistry; Wiley: New York,
1985.
10.1021/om020251b CCC: $22.00 © 2002 American Chemical Society
Publication on Web 06/18/2002

Communications
Organometallics, Vol. 21, No. 15, 2002 3083
Sch em e 1a
a
Legend: (i) KH; (ii) B(C6F5)3; (iii) Ni(η3-CH2C6H5)Cl(PMe3); (iv) B(C6F5)3 or Al(C6F5)3; (v) [Ni(η3-CH2CMeCH2)Cl]2.
of {2,6-(CHMe2)2-C6H3}NdC(CH3)C(CH3)dN{2,6-(CH-
Me2)2-C6H3} with KH in THF results in potassium
N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-
1-methylenepropylamide (3) in 92% yield. The vinyl
1
protons (H2CdC) were observed in the H NMR spec-
trum as a pair of doublets at 4.22 and 3.59 ppm (J HH
)
0.8 Hz). When Ni(η3-CH2C6H5)Cl(PMe3) is added to a
C6D6 solution of 3, one observes the disappearance of
these characteristic vinyl peaks. We suspect that the
direct reaction of 3 and Ni(η3-CH2C6H5)Cl(PMe3) leads
to the C-coordinated complex. To block C-Ni bond
formation, compound 3 was first treated with B(C6F5)3
to yield potassium [2,3-bis(2,6-diisopropylphenylimino)-
butyl]tris(pentafluorophenyl)borate (4 in Scheme 1). The
reaction of 4 with Ni(η3-CH2C6H5)Cl(PMe3) results in
the clean formation of [N-(2,6-diisopropylphenyl)-2-(2,6-
diisopropylphenylimino)-1-methylenepropylamido-κ2N,N]-
(η3-benzyl)nickel(II) (5) and insoluble Me3P‚B(C6F5)3.
The NMR data of compound 5 show two interconverting
compounds, which are attributed to pseudorotamers of
the η3-benzyl ligand.
F igu r e 1. ORTEP drawing of 9 drawn at 30% probability.
For clarity, hydrogen atoms are not shown.
signals in C6D6 at 3.61-3.41 and 3.26-3.06 ppm in 9
and at 2.65-2.40 ppm for 10.
Single crystals suitable for X-ray diffraction studies
were obtained for 8-10. In the case of 8, the molecular
connectivity shown in Scheme 1 is confirmed. However,
two statistical orientations are encountered in the lattice
because of a pseudo mirror plane which relates the two
N-aryl groups. This orientational disorder does not allow
for an accurate examination of metrical parameters. The
results for 9 (Figure 1) and 10 (given in the Supporting
Information) show that the two molecules have very
similar overall connectivities. For both, the Ni-N(1)
distance (1.935(2) Å for 9, 1.923(2) Å for 10) is slightly
longer than that observed for Ni-N(2) (1.915(2) Å for
9, 1.916(3) Å for 10). Overall, these observations are
consistent with a substantial positive charge on the
nickel atom in 9 and 10. Comparison of the two
structures shows that the bulky N(1)-bound diisopro-
pylphenyl group in 9 is closer to the methallyl ligand
than in 10. We attribute this difference to steric
interference between the diisopropylphenyl group and
the perfluoroaryl rings. In 10, the longer Al-C(45)
distance (2.053(4) Å) minimizes this interaction in
comparison to the boron counterpart (d[B-C(45)] )
1.725(4) Å).
When 1.0 equiv of B(C6F5)3 is added to a C6D6 solution
1
of 5, one observes by H NMR spectroscopy the appear-
ance of broad peaks at 3.26-3.36 and 3.16-3.26 ppm
(3:1 ratio) corresponding to boron-bound methylene
resonances. Signals from the benzyl ligand are consis-
tent with η3 coordination, and we assign the product as
6 in Scheme 1. Pseudorotamers exist which make
isolation and complete spectral assignment difficult.
14
Similar results were obtained using 5 and Al(C6F5)3
to yield compound 7.
To circumvent the problems associated with the
isomeric preferences of the benzyl ligand, we targeted
the synthesis of isoelectronic complexes supported with
the η3-methallyl fragment. Thus, compound 8 in Scheme
1 was obtained in 80% yield by addition of 3 to [NiCl-
(η3-CH2CMeCH2)]2.15 Addition of B(C6F5)3 or Al(C6F5)3
to 8 gives compounds 9 and 10, respectively. The
diastereotopic B-CH2 protons are observed as broad
(11) (a) J ohnson, L. K.; Mecking, S.; Brookhart, M. J . Am. Chem.
Soc. 1996, 118, 11664. (b) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P.
M.; Ziegler, J . Am. Chem. Soc. 1997, 119, 6177. (c) Svejda, S. A.;
J ohnson, L. K.; Brookhart, J . Am. Chem. Soc. 1999, 121, 10634.
(12) van Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982, 21, 151.
(13) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem. Soc.
1995, 117, 6414.
(14) Extra caution should be exercised when handling Al(C6F5)3 due
to its thermal and shock sensitivity; see: Chen, E. Y.-X.; Kruper, W.
J .; Roof, G.; Wilson, D. R. J . Am. Chem. Soc. 2001, 123, 745.
(15) Heck, R. F. J . Am. Chem. Soc. 1963, 85, 2013.
When ethylene gas was added to an NMR tube
containing a solution of 6 in C6D6, one observes fast
consumption of ethylene with a concomitant precipita-
tion of polymer. However, no polymer is obtained when
the polymerization was conducted in a glass reactor
under high-dilution conditions (10 µmol, 30 mL of

3084 Organometallics, Vol. 21, No. 15, 2002
Communications
Ta ble 1. Eth ylen e Rea ctivitya
the product is considerably higher.5 A range of molec-
ular weight distributions is observed (MWD, 2.4-4.7).
This range is presumably due to the failure to keep
homogeneous conditions because of polymer precipita-
tion and the formation of highly viscous solutions. No
ethylene consumption was observed when the unacti-
vated form of the complex was used (entry 12). Entries
6-11 show that the aluminum activator Al(C6F5)3
serves equally well as B(C6F5)3 for generating a produc-
tive catalyst. Finally, no product is observed when using
the methallyl-containing compounds 9 and 10 (entries
13 and 14). It seems that the methallyl ligand is too
tightly bound to nickel and does not open a coordination
site for ethylene to initiate the reaction.
temp
Mw/
Tm
c
c
entry compd (°C) activityb
Mw
Mn branchesd (°C)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
6
6
6
6
6
7
7
7
7
7e
7
5
9
22
45
60
80
100
22
45
60
80
80
130
490
500
800
510
200
400
530
550
430
560
0
1 330 000 4.7
1 050 000 2.4
708 000 2.4
495 000 2.5
387 000 2.5
1 530 000 3.1
1 120 000 3.0
945 000 2.8
729 000 2.9
574 000 2.5
491 000 2.6
48
65
87
104
100
57
61
74
82
68
41
n.d.f
n.d.
n.d.
87
66
42
37
40
100
60
94
n.d.
60
0
10
60
0
To clarify the role of the additional B(C6F5)3 or Al-
(C6F5)3, additional studies were carried out. No changes
a
Polymerization conditions: 30 mL of toluene, 0.33 mM [Ni],
1
100 psig of ethylene, 10 min, 2.5 equiv of B(C6F5)3 for 6 and 9 or
were observed in H NMR spectra of 6 or 7 in C6D6 by
b
of Al(C6F5)3 for 7 and 10. Activity in kg of product/((mol of Ni)
the addition of 2.5 equiv of B(C6F5)3 or Al(C6F5)3. Adding
a drop of THF-d8 to a C6D6 solution of 6 or 7 produces
5. These observations, coupled with the comparison of
entries 9 and 10 in Table 1, suggest that the additional
B(C6F5)3 or Al(C6F5)3 required to obtain high polymer-
ization activity acts to remove polar impurities from the
reaction mixture.
In summary, attachment of a Lewis acid at the CR
site of R-iminoenamido ligands can be used to activate
ethylene polymerization catalysts. This is a novel ap-
proach to transition-metal activation. The use of the
labile η3-benzyl ligand is important for allowing efficient
conversion to the propagating species.
h). c Determined by GPC in 1,2,4-trichlorobenzene at 140 °C
d
against polystyrene standards. Determined by 1H NMR and
corresponds to the number of branches/1000 carbons. e 1.0 equiv
of Al(C6F5)3. f Not detected.
toluene). Instead, a color change was observed from red
to blue, presumably due to the decomposition of the
catalyst. When an additional 2.5 equiv of B(C6F5)3 was
added in the reactor, we observed fast consumption of
monomer at a monomer pressure of 100 psig. The
results of a series of ethylene polymerization studies are
summarized in Table 1. The activity increases from 22
to 80 °C and then decreases at 100 °C, probably as a
result of catalyst decomposition.16 These activities are
similar to those observed with 1/C2H4 under similar
reaction conditions; however, the molecular weight of
Ack n ow led gm en t. We are grateful to Ajou Univer-
sity for research funds (2001, second semester) and the
Department of Energy for financial assistance.
(16) The overall thermal stability of these catalysts compares
favorably against those of other nickel catalysts reported in the
literature. See: (a) Svejda, S. A.; J ohnson, L. K.; Brookhart, M. J . Am.
Chem. Soc. 1999, 121, 10634. (b) Gates, D. P.; Svejda, S. A.; Onate,
E.; Killian, C. M.; J ohnson, L. K.; White, P. S.; Brookhart, M.
Macromolecules 1999, 121, 10634. (c) Ikeda, S.; Ohhata, F.; Miyoshi,
M.; Tanaka, R.; Minami, T.; Ozawa, F.; Yoshifuji, M. Angew. Chem.,
Int. Ed. 2000, 39, 4512.
Su p p or tin g In for m a tion Ava ila ble: Text giving com-
plete details for the synthesis of all compounds and polymer-
ization procedures and tables giving data from the crystallo-
graphic studies of 8-10. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM020251B