The mechanism of polymerization of butadiene by ligand-free nickel(II) complexes

Article abstract of DOI:10.1021/ja070364s

The polymerization of 1,3-dienes has been well-studied using an array of metal-containing complexes. Although a variety of (π-allyl) Ni(II) catalysts have been shown to be active for diene polymerization, details concerning the mechanism of chain growth are largely speculative and supported primarily by DFT calculations. The observation by low-temperature NMR spectroscopy of the key intermediates and the catalyst resting states in the chain growth are described here and provide a more complete and detailed mechanism of butadiene polymerization. Copyright

Full text of DOI:10.1021/ja070364s

Published on Web 03/20/2007
The Mechanism of Polymerization of Butadiene by “Ligand-Free” Nickel(II)
Complexes
Abby R. O’Connor, Peter S. White, and Maurice Brookhart*
Department of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290
Received January 17, 2007; E-mail: mbrookhart@unc.edu
Scheme 1
The stereoselective polymerization of butadiene (BD) has been
investigated using a variety of transition metal1-4 and lanthanide
catalysts.5 Numerous Ni(II) π-allyl complexes have been used as
initiators,6-9 but the most reactive one identified to date is “ligand-
free” Ni(II) wrap-around complex A (Scheme 1), which yields
polybutadiene (PBD) exhibiting 93% cis-1,4 enchainment.10,11
The chain growth mechanism proposed by Taube and Tobisch,
based largely on DFT calculations,12-15 is shown in Scheme 1. The
most stable form of the propagating species (the catalyst resting
state) is assumed to be syn complex B analogous in structure to
initiator A. To achieve a cis-1,4 enchainment, isomerization to the
anti-allyl species C must occur; C-C coupling is proposed to
proceed from η4-BD complex D formed from C. A key finding,
Scheme 2
based on DFT results, is that insertion is driven by a fifth ligand
provided by the π-bond of the growing chain. Species E is proposed
to account for trans-1,4 enchainment.
We report here low-temperature observation of highly reactive
ligand-free π-allyl complexes [(allyl)Ni][B(ArF)4] (1) and [(2-
methallyl)Ni][B(ArF)4] (2),16 the first observation of η4-BD and η4-
isoprene (IP) (allyl)Ni complexes [(2-methallyl)Ni(η4-BD)][B(ArF)4]
(3) and [(2-methallyl)Ni(η4-IP)][B(ArF)4] (4), and their reactions
with BD, which provide a modified, more complete mechanism of
BD polymerization by ligand-free Ni(II) systems.
As shown in Scheme 2, reaction of B(C6F5)3 with [(allyl)Ni-
(NCR′)2][B(ArF)4] (R′ ) Me, 5, or 3,5-(CF3)2C6H3, 6) or the
analogous 2-methallyl derivatives (7, 8) at -60 °C generates the
ligand-free complex 1 or 2 (in equilibrium with the starting catalyst),
which were characterized by 1H, 13C, and 19F NMR spectroscopy.17
Ni is coordinated to a single aryl ring; separate 1H and 19F
1
complexes 10 and 11.17,18 The H NMR spectrum of 10 is shown
in Figure 1, and the molecular structure was verified by X-ray
crystallography (Figure 2).17 The π-allyl moiety is in an anti
configuration, and all C-C double bonds are cis as expected from
coupling of an anti π-allyl unit with BD. The coordinated π-bonds
(C1-C2, C5-C6) exhibit C-Ni distances between 2.19 and 2.29
Å, while the noncoordinated π-bond shows C-Ni distances of 2.47
and 2.68 Å.
resonances are observed for the coordinated and three noncoordi-
nated rings. 19F VT NMR experiments (-60 to -18 °C) showed
the barriers to intramolecular Ni migration among the rings as ∆Gq
) 11.4 kcal/mol for 1 and 11.8 kcal/mol for 2. Exchange of
coordinated B(ArF)4- with free B(ArF)4- occurs on the NMR time
scale above 0 °C.
Complexes 1 and 2 react at -80 °C with 1-2 equiv of BD or
IP to yield equilibrium quantities of π-allyl η4-diene complexes
[(allyl)Ni(η4-BD)]+ (9), [(2-methallyl)Ni(η4-BD)]+ (3), and [(2-
methallyl)Ni(η4-IP)]+ (4).17 Formation of the π-allyl diene com-
plexes is favored for 2-methallyl as the allyl partner and IP as the
diene partner, thus 4 can be formed nearly quantitatively from 2.17
Warming 4 to -30 °C results in no observable insertion; however,
treatment of 4 (-50 °C) with BD results in rapid insertion and BD
consumption supporting Taube and Tobisch’s contention that a fifth
ligand drives insertion. Treatment of either 1 or 2 with 3 equiv of
BD results in rapid insertion of 3 equiv of diene at temperatures as
low as -110 °C to yield a single species Ni(C15H23)+B(ArF)4- (10)
We expected that species analogous to 10 and 11 would be the
catalyst resting states wherein the growing chain would be attached
at C1. This proved not to be the case. Exposure of 10 to 45 equiv
of BD at -30 °C results in uptake of BD and formation of a new
complex, which contains an anti-π-allyl Ni unit and a coordinated
vinyl group from 1,2 insertion (Figure S4). After ca. 15 turnovers,
only 50% of 10 has been consumed (and after ca. 38 turnovers,
75% consumption), indicating the initial insertion of BD into 10 is
slow relative to subsequent insertions. Reaction of 11 with BD is
more straightforward and clarifies the situation (Scheme 3).
Treatment with BD (42 equiv) at -30 °C results in rapid uptake of
15 equiv of BD and clean quantitative, formation of the vinyl-
coordinated species (12). The hydrogens of the free vinylidene end
group (δ 4.68 and 4.65) integrate for 1.0 H each relative to the
-
and Ni(C16H25)+B(ArF)4 (11),17 respectively. No intermediates
were detected in these reactions.
1
Two-dimensional HMQC and H-1H COSY NMR techniques
established wrap-around structures as shown in Scheme 2 for
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C O M M U N I C A T I O N S
species analogous to 12. NMR studies establish the vinyl-
coordinated species to have the same structures with exception of
the end group17 (Figures S3 and S4).
These experiments show that the true catalyst resting state is a
stable vinyl-coordinated species, 12, formed following a (rare) 1,2
insertion.19 The cis-1,4 insertions may well proceed through a
Taube/Tobisch-type precursor D (Scheme 1), but if so, it seems
likely that species analogous to complexes 10 or 11, rather than C,
would result.
Figure 1. 1H NMR of complex 10 (500 MHz) in CD2Cl2 at -30 °C.17
Polymerization of BD, catalyzed by in situ generated complex
2, was carried out in CH2Cl2 at -30 °C under argon yielding PBD,
which has the following microstructure: 94% cis-1,4, 5% trans-
1,4, and 1% 1,2 enchainment. The Mn values are approximately
8000 with a PDI of 1.5.17
In summary, the identities of key intermediates and catalyst
resting states in the polymerization of BD by ligand-free (allyl)
Ni(II) species have been observed for the first time. Cationic (2-
methallyl)Ni(II) complexes of η4-BD and η4-IP were prepared and
characterized by low-temperature NMR. These highly reactive
species insert 3 equiv of BD or IP at very low temperatures to
yield stable wrap-around complexes of types 10 and 11. Although
these species insert BD, they do not represent the catalyst resting
state(s). The resting states are formed following a 1,2 BD insertion
and exhibit coordination of the resulting vinyl group as shown in
structure 12.
Figure 2. Molecular structure of complex 10. Atomic displacement
ellipsoids are drawn at 50% probability. Selected bond lengths (Å): Ni(1)-
C(1) 2.189(8), Ni(1)-C(2) 2.218(8), Ni(1)-C(5) 2.248(7), Ni(1)-C(6)
2.285(7), Ni(1)-C(9) 2.678, Ni(1)-C(10) 2.465(10), Ni(1)-C(13) 2.087(8),
-
Ni(1)-C(14) 2.051(8), Ni(1)-C(15) 2.116(9). H atoms and B(ArF)4 are
omitted.
Scheme 3
Acknowledgment. We thank the National Science Foundation
(CHE-0615704) for support, and T. Pintauer for useful discussions.
Supporting Information Available: Experimental procedures and
characterization of complexes 1-12. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
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Scheme 4
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coordinated vinyl end group protons (δ 3.95 (d, J ) 16.5 Hz), 3.65
(d, J ) 9.0 Hz)) and the syn (δ 4.46 (d, J ) 7.5 Hz)) and anti (δ
2.94 (d, J ) 13.5 Hz)) π-allyl signals (Figure S3).
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Scheme 4 summarizes a proposed mechanism for PBD chain
growth using catalyst 11. Complex 11 initiates rapidly, and multiple,
fast 1,4 BD insertions occur until a 1,2 insertion takes place to
form a stable vinyl-coordinated species, 12. Subsequent insertions
occur via a slow 1,4 insertion of BD into 12 followed by a sequence
of rapid 1,4 insertions until another 1,2 insertion takes place and
the stable vinyl-coordinated species 12 is regenerated. In the case
of 10, the terminal unsubstituted double bond (C1-C2) is more
strongly coordinated then the 2-Me-substituted double bond in 11;
thus, the first 1,4 insertion of BD is slow relative to subsequent
1,4 insertions, and a considerable amount of 10 remains after
multiple BD insertions along with formation of a vinyl-coordinated
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(16) There is precedence for η6-coordination of bulky arenes to Ni(II)
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(17) See Supporting Information.
(18) Reaction of complex 10 with 2 equiv of NC(C6H3(CF3)2) results in
decoordination of the olefins as evidenced by the downfield shifts in the
1H NMR spectrum. See Supporting Information.
(19) From the presence of ca. 1% vinyl groups in the PBD formed, on average,
100 1,4 insertions must occur prior to a 1,2 insertion. These ratios are
consistent with our NMR observations.
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