Electrophilic metal precursors and a β-diimine ligand for nickel(II)- and palladium(II)-catalyzed ethylene polymerization

Article abstract of DOI:10.1021/om960968x

The α-diimine ligand ArN=C(Me)C(Me)=NAr (Ar = 2,6-C₆H₃-ⁱPr₂) reacts with Pd(OAc)₂ in the presence of HBF₄·Et₂O, or [Pd(MeCN)₄](BF₄)₂ in the absence of acid, to give ethylene polymerization catalysts. The reactions of a related β-diimine ligand with [Pd(MeCN)₄]-(BF₄)₂ and (1,2-dimethoxyethane)NiBr₂, and the polymerization activity of catalysts derived from it, are discussed.

Full text of DOI:10.1021/om960968x

1514
Organometallics 1997, 16, 1514-1516
Electr op h ilic Meta l P r ecu r sor s a n d a â-Diim in e Liga n d
for Nick el(II)- a n d P a lla d iu m (II)-Ca ta lyzed Eth ylen e
P olym er iza tion †
J erald Feldman,* Stephan J . McLain, Anju Parthasarathy, William J . Marshall,
J oseph C. Calabrese, and Samuel D. Arthur
Central Research & Development, Experimental Station, DuPont Company,
Wilmington, Delaware 19880-0328
Received November 14, 1996X
Summary: The R-diimine ligand ArNdC(Me)C(Me)dNAr
(Ar ) 2,6-C6H3-iPr2) reacts with Pd(OAc)2 in the presence
of HBF4‚Et2O, or [Pd(MeCN)4](BF4)2 in the absence of
acid, to give ethylene polymerization catalysts. The
reactions of a related â-diimine ligand with [Pd(MeCN)4]-
(BF4)2 and (1,2-dimethoxyethane)NiBr2, and the polym-
erization activity of catalysts derived from it, are dis-
cussed.
The discovery of a new class of Ni(II) and Pd(II)
catalysts for the polymerization of ethylene and R-olefins
was reported recently by Brookhart and co-workers.1
Complex 1 is an example of such a catalyst; 1 incorpo-
rates a sterically hindered R-diimine ligand, which
allows it to polymerize ethylene and R-olefins to high
molecular weight. In the course of our studies on these
of magnitude lower than those typically observed with
1, a reflection of the different anions present {BF4 vs
-
B[(3,5-C6H3-(CF3)2]4-}.4 An active ethylene polymeri-
zation catalyst can also be generated from Pd(OAc)2 plus
HBF4‚Et2O in the presence of ArNdC(Me)C(Me)dNAr.3
Again, highly branched polyethylene is produced (1200
turnovers in 1.5 h).5 The above reactions also proceed
in methylene chloride6 and in aromatic solvents.
We synthesized {[η2-ArNdC(Me)C(Me)dNAr]Pd-
(MeCN)2}(BF4)2 (2) by the reaction shown in eq 2.7
systems, we have found some new precursors and
ligands for diimine-based Ni(II)- and Pd(II)-catalyzed
ethylene polymerization. Our results are detailed here.
We have found that catalytically active Pd species can
be generated in situ from simple precursors, e.g., weakly
2
ligated Pd(II) dications. Reaction of [Pd(MeCN)4](BF4)2
with ArNdC(Me)C(Me)dNAr (Ar ) 2,6-C6H3-iPr2) un-
der 300 psi of ethylene in chloroform results in forma-
tion of amorphous polyethylene (eq 1).3 The polymer is
highly branched, and the branching level and distribu-
tion are identical to those obtained when the well-
defined initiator 1 is used as catalyst (103 methyl-ended
branches per 1000 methylene units).1 The total turnover
number based on Pd is 7700 in 3.0 h, which is compa-
rable to the activity exhibited by 1 and suggests that
most of the Pd present eventually forms active catalyst.
Molecular weight determination by GPC (trichloroben-
zene vs linear polyethylene) gave values of Mn ) 10 800,
Mw ) 21 200, and Mw/Mn ) 2.0; these are about an order
Complex 2 is also a catalyst for ethylene polymerization,
but it is much less active than the system formed in
situ in eq 1 (1100 turnovers in 9 h at 400 psi of C2H4).3,8
A plausible explanation is that 2 reacts less rapidly to
give an initiating species than [Pd(MeCN)4](BF4)2.
(4) With well-defined catalysts analogous to 1, polymer molecular
weight is inversely related to the coordinating ability of the anion:
-
McLain, S. J .; Feldman, J . unpublished results. BF4 is a relatively
coordinating anion: Beck, W.; Su¨nkel, K. Chem. Rev. 1988, 88, 1405.
(5) Mn
) 30 500 and Mw ) 43 000 (Mw/Mn ) 1.4) vs linear
polyethylene; polymer contained 100 methyl-ended branches per 1000
methylene units.
(6) An experiment identical to that shown in eq 1,3 but run in CH2-
Cl2, gave 6970 turnovers in 3.0 h (Mn ) 12 300; Mw ) 23 500 (Mw/Mn
) 1.9) vs linear polyethylene in trichlorobenzene.
† Contribution no. 7507.
X Abstract published in Advance ACS Abstracts, March 15, 1997.
(1) (a) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem.
Soc. 1995, 117, 6414. (b) J ohnson, L. K.; Mecking, S.; Brookhart, M.
J . Am. Chem. Soc. 1996, 118, 267.
(7) The X-ray crystal structure of 2 is included as Supporting
Information. Selected data for 2: 1H NMR (CD2Cl2, 25 °C) δ 7.51 (t,
2H, Hp), 7.34 (d, 4H, Hm), 3.22 (sept, 4H, CHMe2), 2.52 (s, 6H, NdCMe),
1.95 (s, 6H, NtCMe), 1.49 (d, 12H, CHMeMe′), 1.31 (d, 12H, CHMeMe′).
Analytical and X-ray samples were crystallized by slow diffusion of
pentane into an acetonitrile solution of 2, giving the bis(MeCN) solvate.
Anal. Calcd for C36H52B2F8N6Pd: C, 50.94; H, 6.17; N, 9.90. Found:
C, 50.52; H, 6.17; N, 9.59.
(2) (a) Schramm, R. F.; Wayland, B. B. Chem. Commun. 1968, 898.
(b) Sen, A.; Lai, T.; Thomas, R. R. J . Organomet. Chem. 1988, 358,
567.
(3) Experimental details are given in the Supporting Information.
S0276-7333(96)00968-5 CCC: $14.00 © 1997 American Chemical Society

Communications
Organometallics, Vol. 16, No. 8, 1997 1515
The mechanism by which the above systems initiate
is presently unclear. Sen and others have demonstrated
that electrophilic precursors such as [Pd(MeCN)4](BF4)2
and related Pd(II) complexes [e.g., (dppe)Pd(BF4)2] are
catalysts for the addition polymerization of norbornene
and styrene and for the oligomerization of ethylene and
R-olefins.2b,9 Propagation via cationic mechanisms has
often been suggested for these reactions.2b,9a-f However,
Risse et al. have shown that norbornene polymerization
catalyzed by [Pd(MeCN)4](BF4)2 almost certainly pro-
ceeds by an insertion mechanism.10 Although the
mechanism for catalyst initiation in norbornene polym-
erization is still uncertain,9j water appears to be neces-
sary in the [Pd(MeCN)4](BF4)2-catalyzed polymerization
of 2,3-diester derivatives of norbornadiene in aceto-
nitrile, suggesting initiation by Wacker-type nucleo-
philic attack by water on a coordinated olefin to give a
Pd σ-alkyl complex.11 We have not ruled out initiation
by adventitious nucleophile in our system. However,
we note that the polymerization reaction shown in eq 1
gives essentially identical yields in ethanol-stabilized,
reagent grade chloroform and in amylene-stabilized
chloroform dried by distillation from P2O5.12
Regardless of the actual mechanism for catalyst
initiation, the ethylene polymerizations described here
clearly proceed by an insertion mechanism. We propose
that, in oligomerizations of ethylene catalyzed by [Pd-
(MeCN)4](BF4)2 and other dicationic Pd(II) complexes,
propagation also proceeds by an insertion mechanism;
the catalysts described here polymerize ethylene because
they incorporate a sterically hindered R-diimine ligand.13
We prepared an analogous, sterically hindered â-di-
imine ligand and examined its reactions with Pd(II) and
Ni(II) catalyst precursors (eqs 3 and 4). NMR data for
3 are consistent with the symmetrical, hydrogen-bridged
“â-iminoamine” structure shown.14 Although complexes
incorporating anionic ligands of this type have been
reported,14-16 examples in which the ligand is bound as
a neutral donor appear to be rare.16 Reaction between
3 and [Pd(MeCN)4](BF4)2 in acetonitrile at 25 °C gives
1
3‚HBF4 (identified by H NMR spectroscopy) and com-
plex 4 (eq 3). Despite repeated attempts, we were
unable to obtain crystals of 4 in pure form free from
coprecipitated 3‚HBF4. Nevertheless, we were able to
identify 4 on the basis of X-ray crystallography;17 the
structure of the trication and selected distances and
bond angles are shown in Figure 1. The complex, which
crystallizes as the tris(acetonitrile) solvate, has an
unusual structure: the central carbon atom of the
â-diimine ligand is σ-bound to a Pd(MeCN)3+ fragment,
2+
while the nitrogen atoms are bound to a Pd(MeCN)2
fragment. Bond distances within the six-membered
chelate ring are consistent with the localized diimine
structure drawn in eq 4; the ring itself adopts a boat
conformation. Most likely, 4 arises from initial forma-
tion of 3•HBF4 and [(η2-ArNC(Me)dCHC(Me)dNAr)Pd-
(MeCN)2]+, followed by electrophilic attack on Câ of the
(8) Mn
) 24 000 and Mw ) 43 100 (Mw/Mn ) 1.8) vs linear
polyethylene; polymer contained 94 methyl-ended branches per 1000
methylene units.
(9) (a) Sen, A.; Lai, T. J . Am. Chem. Soc. 1981, 103, 4627. (b) Sen,
A.; Lai, T. Organometallics 1982, 1, 415. (c) Lai, T.; Sen, A. Organo-
metallics 1984, 3, 866. (d) Sen, A. Acc. Chem. Res. 1988, 21, 421. (e)
Sen, A.; J iang, Z. Organometallics 1993, 12, 1406. (f) Drent, E. Pure
Appl. Chem. 1990, 62, 661. (g) Mehler, C.; Risse, W. Makromol. Chem.,
Rapid Commun. 1991, 12, 255. (h) Mehler, C.; Risse, W. Makromol.
Chem., Rapid Commun. 1992, 13, 455. (i) Breunig, S.; Risse, W.
Makromol. Chem. 1992, 193, 2915. (j) Mehler, C.; Risse, W. Macro-
molecules 1992, 25, 4226.
(10) An insertion mechanism is indicated by the regioselectivity of
the polymerization.9g,j The recent preparation of well-defined Pd(II)
alkyl and allyl catalysts for norbornene/norbornadiene polymerizations
also suggests an insertion type mechanism: (a) Safir, A. L.; Novak, B.
M. Macromolecules 1995, 28, 5396. (b) Safir, A. L.; Novak, B. M. Polym.
Prepr., Am. Chem. Soc. Div. Polym. Chem. 1994, 35, 901. (c) Goodall,
B. L.; Benedikt, G. M.; McIntosh, L. H.; Barnes, D. A. U.S. Patent
5,468,819, Nov 21, 1995. (d) Reinmuth, A.; Mathew, J . P.; Melia, J .;
Risse, W. Makromol. Chem., Rapid Commun. 1996, 17, 173.
(11) Novak, B. M.; Safir, A. L. Polym. Prepr., Am. Chem. Soc. Div.
Polym. Chem. 1996, 37, 335.
latter by a second equivalent of [Pd(MeCN)4]2+
.
Unlike the analogous reaction of R-diimines, reaction
between 3 and [Pd(MeCN)4](BF4)2 under 1000 psi of
ethylene at 25 °C in CDCl3 resulted in primarily the
formation of butenes, and at most a trace of polymer.
Whether this is due to the sensitivity of the â-CH2 group
toward activation or an intrinsic problem with this
nonrigid chelate ring system is not yet known. At-
tempts to prepare well-defined Pd catalysts (cf. complex
1) from ligand 3 have thus far been unsuccessful.
A â-diimine complex of Ni(II) was prepared as shown
in eq 4. Purple complex 5 is paramagnetic and displays
(12) Addition of 100 µL of water to the reaction shown in eq 1 causes
catalyst activity to decrease (2625 turnovers in 3 h) and dramatically
reduces polymer molecular weight (Mn ) 670). Thus, water appears
to serve as a chain transfer agent. We further conclude that, if
adventitious water is playing a role in catalyst initiation, it need be
present in only trace amounts.
(15) (a) Holm, R. H; O’Connor, M. J . Prog. Inorg. Chem. 1971, 14,
241. (b) Healy, P. C.; Bendall, M. R.; Doddrell, D. M.; Skelton, B. W.;
White, A. H. Aust. J . Chem. 1979, 32, 727.
(16) Honeybourne, C. L.; Webb, G. A. Chem. Commun. 1968, 739.
(17) Crystal data for 4‚(MeCN)3: Pd2F12N10C45B3H65, triclinic, P1
(No. 2); a ) 11.853(2), b ) 13.734(4), and c ) 18.883(5) Å; R ) 100.75-
(2), â ) 92.92(2), and γ ) 114.33(2)° from 25 reflections; T ) -70 °C,
V ) 2723.5 Å3, Z ) 2, Dc ) 1.487 g/cm3. A yellow wedge, ∼0.32 mm ×
0.27 mm × 0.34 mm, obtained by slow diffusion of petroleum ether
into MeCN at 25 °C, was used for the data collection; 10 320 reflections
were collected in the range 2.2° e 2θ e 50.0°, with scan width 1.20-
2.30°ω and scan speed 1.70-6.70 deg/min. Final R ) 0.040, Rw ) 0.037,
error of fit ) 1.15, max ∆/σ ) 0.24.
(13) When less sterically hindered R-diimine ligands are employed,
oligomers result; e.g., reaction of [Pd(MeCN)4](BF4)2 with (2-R-
C6H4)NdCMeCMedN(2-R-C6H4) under 300 psi of C2H4 in CDCl3 gave
t
branched oligomers (R ) Bu, Mn ) 203; R ) Ph, Mn ) 128).
(14) Full details are given in the Supporting Information. Ligands
of this type have been prepared previously: Parks, J . E.; Holm, R. H.
Inorg. Chem. 1968, 7, 1408.

1516 Organometallics, Vol. 16, No. 8, 1997
Communications
a
b
F igu r e 1. Molecular structure of the trication in 4.
Acetonitriles of crystallization are omitted. Selected bond
lengths (Å) and angles (deg): Pd1-N1 ) 2.006(4), Pd1-
N4 ) 2.010(4), Pd1-N2 ) 2.009(4), Pd1-N3 ) 1.997(4),
C21-N2 ) 1.274(5), C23-N3 ) 1.294(5), C21-C22 )
1.477(6), C22-C23 ) 1.469(6), Pd2-C22 ) 2.084(4), N2-
Pd1-N3 ) 92.1(2), Pd2-N7-C9 ) 158.5(4).
a contact-shifted 1H NMR spectrum with relatively
narrow line widths at room temperature.18 The X-ray
crystal structure of 5 is shown in Figure 2a.19 The
Ni(II) atom adopts a pseudotetrahedral coordination
geometry; the ligand is bound as the â-diimine tau-
tomer. As in complex 4, the six-membered chelate ring
sits in a boat conformation (Figure 2b).
F igu r e 2. (a) Molecular structure of 5. Methylene chloride
of crystallization is omitted. Selected bond lengths (Å) and
angles (deg): Ni-Br1 ) 2.376(1), Ni-Br2 ) 2.352(1), Ni-
N1 ) 2.010(5), Ni-N2 ) 2.022(5), N1-C3 ) 1.280(8), N2-
C5 ) 1.269(8), C3-C4 ) 1.513(9), C4-C5 ) 1.500(9), Br1-
Ni-Br2 ) 118.54(5), Br1-Ni-N1 ) 102.0(2), Br1-Ni-N2
) 105.6(1), Br2-Ni-N2 ) 112.1(2), Br2-Ni-N1
)
121.3(2), Ni1-Ni-N2 ) 93.7(2). (b) View of the chelate ring
in 5. The Ar rings are omitted for clarity.
Complex 5 is a precursor for ethylene polymerization
(eq 5).3 The polymerization reaction differs from that
ring has been noted with SHOP catalysts for ethylene
oligomerization.21
In conclusion, our results suggest that new catalyst
precursors and ligands for diimine-based Ni(II)- and
Pd(II)-catalyzed olefin polymerization await discovery.
Our ongoing efforts in this area will be reported in due
course.
catalyzed by the analogous R-diimine complex [ArNdC-
(Me)C(Me)dNAr]NiBr2 in two significant ways: the
polyethylene produced is more crystalline (linear)20 and
complex 5 is a less active catalyst precursor (1800
turnovers in 3.5 h).1a This may be related to the larger
chelate ring size in complex 5; a similar decrease in
activity in going from a five- to a six-membered chelate
Ackn owledgm en t. We thank Dr. Lynda K. Johnson,
Dr. Chris M. Killian, and Prof. Maurice Brookhart for
sharing their results prior to publication, Matt Schiff-
hauer, Alicia Glatfelter, and Pete Butera for skilled
technical assistance, and Dr. Steven D. Ittel for helpful
discussions.
(18) Selected data for 5: 1H NMR (CDCl3, 25 °C) δ 22.3 (s, 4H, Hm),
9.2 (br, 4H, CHMe2), 3.5 (s, 12H, CHMeMe′), 2.4 (s, 12H, CHMeMe′),
1.1 (mult, 2H, â-CH2), -16.3 (s, 2H, Hp), -21.8 (s, 6H, R-Me). Anal.
Calcd for C29H42Br2N2Ni: C, 54.67; H, 6.64; N, 4.40. Found: C, 54.38;
H, 6.74; N, 4.29.
Su p p or tin g In for m a tion Ava ila ble: Text giving details
of the synthesis of 2, 3, and 5 and 1H NMR characterization
of the products formed in the reaction between [Pd(MeCN)4]-
(BF4)2 and 3 and tables giving data for the X-ray analyses of
2, 4, and 5 (31 pages). Ordering information is given on any
current masthead page.
(19) Crystal data for 5‚CH2Cl2: Br2NiCl2N2C30H44, monoclinic, P21/n
(No. 14); a ) 19.347(3), b ) 8.018(1), and c ) 21.532(1) Å, â ) 101.10-
(1)° from 50 reflections; T ) -100 °C, V ) 3277.6 Å3, Z ) 4, Dc ) 1.463
g/cm3. A purple rod, ∼0.26 mm × 0.24 mm × 0.53 mm, obtained from
CH2Cl2/pentane at -40 °C, was used for the data collection; 6665
reflections were collected in the range 4.0° e 2θ e 50.0° with scan
width 2.80°ω and scan speed 2.90-19.50 deg/min. Final R ) 0.050,
Rw ) 0.038, error of fit ) 1.06, max ∆/σ ) 0.00.
OM960968X
(20) Tm ) 120.2 °C; ∆Hf ) 155 J /g.
(21) Keim, W.; Schulz, R. P. J . Mol. Catal. 1994, 92, 21.