Accelerating Ni(ii) precatalyst initiation using reactive ligands and its impact on chain-growth polymerizations

Article abstract of DOI:10.1039/c2dt32735j

Nickel(ii) complexes with varying reactive ligands, which were designed to selectively accelerate the initiation rate without influencing the propagation rate in the chain-growth polymerization of π-conjugated monomers, were investigated. Precatalysts with electronically varied reacting groups led to faster initiation rates and narrower molecular weight distributions. Computational studies revealed that the reductive elimination rates are largely modulated by the ability of the two reacting arenes to stabilize the increasing electron density on the catalyst during reductive elimination. Overall, these studies provide insight into a key mechanistic step of cross-coupling reactions (reductive elimination) and highlight the importance of initiation in controlled chain-growth polymerizations.

Full text of DOI:10.1039/c2dt32735j

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Accelerating Ni(II) precatalyst initiation using reactive
ligands and its impact on chain-growth
polymerizations†
Cite this: Dalton Trans., 2013, 42, 4218
Se Ryeon Lee,a Jacob W. G. Bloom,b Steven E. Wheeler*b and Anne J. McNeil*a
Nickel(II) complexes with varying reactive ligands, which were designed to selectively accelerate the
initiation rate without influencing the propagation rate in the chain-growth polymerization of π-conju-
gated monomers, were investigated. Precatalysts with electronically varied reacting groups led to faster
initiation rates and narrower molecular weight distributions. Computational studies revealed that the
reductive elimination rates are largely modulated by the ability of the two reacting arenes to stabilize the
increasing electron density on the catalyst during reductive elimination. Overall, these studies provide
insight into a key mechanistic step of cross-coupling reactions (reductive elimination) and highlight the
importance of initiation in controlled chain-growth polymerizations.
Received 15th November 2012,
Accepted 12th December 2012
DOI: 10.1039/c2dt32735j
www.rsc.org/dalton
and chain-termination processes, all of which impact the
polymer molecular weight, copolymer sequence and poly-
Introduction
Organic π-conjugated polymers are used in energy-related dispersity. As a result, the continued development of new
applications such as photovoltaics (PVs),1 light-emitting catalysts is needed.
diodes (LEDs)2 and field effect transistors (FETs).3 In all of
We13 and others14 have investigated the impact of ancillary
these applications, the device performance can be substan- ligand structure on the Ni-catalyzed chain-growth polymeriz-
tially influenced by the polymer molecular weight and copoly- ations. We recently reported that catalysts ligated by electron-
mer sequence, as well as the molecular weight distribution. donating phosphines led to polymers with lower polydispersity
For example, Kowalewski and coworkers observed improved indexes (PDIs) than catalysts containing electron-withdrawing
charge carrier mobilities with higher molecular weight poly- phosphines.13a These results were attributed to the impact
(3-hexylthiophene) in thin-film FETs.4 In another example, of increased electron-density in promoting the formation
Galvin and co-workers observed higher LED efficiencies when and reactivity of
a key intermediate (i.e., Ni(0)-polymer
poly((2,5-bisoctyloxy)-1,4-phenylene vinylene)s with narrower π-complex).15 During these studies we observed a surprisingly
molecular weight distributions were utilized.5 Because device slow precatalyst initiation relative to propagation (krel ∼ 20),
performance is dependent on molecular weight, copolymer even for the conventional catalyst containing 1,2-bis(diphenyl-
sequence and polydispersity, synthesizing π-conjugated poly- phosphino)ethane (dppe) as the ligand.13a Slow precatalyst
mers with control over these variables is important. The initiation leads to broader sequence and molecular weight dis-
recently developed Ni- and Pd-catalyzed chain-growth polymer- tributions in chain-growth polymerizations. Further studies
ization methods6–8 have enabled unprecedented control over revealed that the turnover-limiting step for both initiation and
polymer molecular weight and copolymer sequence.9–12 Never- propagation is the same. As a consequence, the ancillary
theless, the current methods exhibit sluggish precatalyst ligand cannot be used to selectively accelerate the initiation.
initiation (compared to propagation), as well as chain-transfer An alternative is to use a reactive ligand (e.g., a functionalized
arene) to increase the precatalyst initiation rate without influ-
encing the propagation rate.
We report herein the synthesis of four new Ni precatalysts
with electronically varied reactive ligands and their impact on
the initiation rate. Our precatalyst design was inspired by
Shekhar and Hartwig’s study of reductive elimination rates in
(L–L)Pt(Ar)(Ar′) complexes.16 The fastest rates were observed
when the two reactive arenes were the most electronically
differentiated. These rate differences were attributed to the
aDepartment of Chemistry and Macromolecular Science and Engineering Program,
University of Michigan, 930 North University Avenue, Ann Arbor, Michigan
48109-1055, USA. E-mail: ajmcneil@umich.edu
bDepartment of Chemistry, Texas A&M University, PO Box 30012, College Station,
Texas 77842-3012, USA. E-mail: wheeler@chem.tamu.edu
†Electronic supplementary information (ESI) available: Experimental details;
rate and spectroscopic data; MALDI-TOF MS data; computational details. See
DOI: 10.1039/c2dt32735j
4218 | Dalton Trans., 2013, 42, 4218–4222
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increased electrophilicity and nucleophilicity of the two react- A quenched sample revealed exclusive formation of regio-
ing arenes. We applied the same rationale to design a series of isomer 2, indicating selective metathesis of the C–Br bond
precatalysts with varying reactive ligands. Using this approach, (ESI†).21
we demonstrate herein that precatalyst initiation can be selec-
tively accelerated over propagation, and that faster initiations
produce polymer samples with narrower molecular weight dis-
tributions. Combined, these results highlight the important
role of initiation in chain-growth polymerizations. Because the
fastest rates were obtained when the reactive ligand was the
most electronically differentiated from the monomer, we origi-
ð1Þ
nally attributed this effect to the nucleophilicity/electrophili-
city of the reacting arenes. However, computational studies
revealed no correlation between the free energy barriers and
atomic charges at the reacting carbons. Instead, a strong corre-
lation was observed between the free energy barriers and the
change in charge delocalization onto the reactive ligands
during the turnover-limiting step. As a consequence, the
fastest rates are predicted to occur when the reactive ligand
is substituted with resonance-based electron-withdrawing
d½3ꢀ
¼ ꢁkre½3ꢀ
ð2Þ
groups. Overall, these results demonstrate that a simple
approach for improving chain-growth polymerizations of
π-conjugated monomers is to modify the reactive ligand’s elec-
tronic properties.
dt
Initiation of precatalysts 1a–d with stoichiometric amounts
of Grignard 2 was monitored via 19F NMR spectroscopy at 0 °C
in THF (Fig. 1A and ESI†). Triphenylphosphine (PPh3) was
used to trap the Ni(0) as complex 4 after reductive elimination.
Control experiments revealed that excess PPh3 has no effect on
reaction rates (ESI†). Because transmetalation was too fast to
monitor under these conditions, reductive elimination from
intermediates 3a–d was followed as a function of time (Fig. 1A/B).
The data were fit to the corresponding rate equation (eqn (2)),
providing the rate constants for reductive elimination
(Table 1).22 As anticipated, the reactive ligand had a dramatic
impact on the reductive elimination rate constants. For
example, intermediate 3c (R = F) exhibited a reductive
Results and discussion
To determine the reactive ligand influence on precatalyst
initiation, Ni precatalysts 1a–d were prepared (ESI†). The reac-
tive ligand includes: (i) an ortho-trifluoroethoxy substituent for
facile analysis via 19F NMR spectroscopy and stabilization,17
and (ii) para-substituents with varying electronic properties to
tune the reductive elimination rate. Commercially available
dppe was selected as the ancillary ligand because it mediates
chain-growth polymerizations of several important π-conju-
gated monomers.18
Fig. 1 (A) Representative 19F NMR spectroscopic data for the reaction depicted
in eqn (1) using catalyst 1b. (B) Representative fit of the data to eqn (2) to
obtain the rate constant.
Initiation involves precatalyst 1 undergoing transmetalation
with monomer (i.e., aryl Grignard), followed by reductive elim-
ination (eqn (1)). To prevent polymerization in our model
system, Grignard 2 was used as a substitute for monomer 6.
The key difference is that the 4-Cl substituent (in 2) minimizes
the likelihood of further propagation due to its low reactivity
in oxidative addition reactions.19 Importantly, there should be
minimal rate differences between the model system and the
polymerization because Cl and Br have similar electronic prop-
erties. Grignard 2 was prepared by Grignard metathesis with
i-PrMgCl and 1-bromo-4-chloro-2,5-dimethoxybenzene (ESI†).20
Table 1 Experimental rate constants (kre) and free energy of activation (ΔG‡)
for precatalyst initiation
Precatalyst
kre (×10−3 s−1
)
ΔG‡ (kcal mol−1
)
1a
1b
1c
1d
6.88 0.06
18.6
19.9
21.3
20.5
0.671 0.006
0.0520 0.0001
0.222 0.002
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elimination rate constant that was two orders of magnitude precatalysts, indicating that chain-transfer or chain-termi-
slower than intermediate 3a (R = NMe2). Interestingly, reduc- nation reactions are not occurring under these conditions
tive eliminations from both electron-poor intermediate 3d (R = (ESI†). Thus, the broadening of the PDI observed with these
CF3) and electron-rich intermediate 3b (R = OMe) were similar precatalysts can be largely attributed to the different relative
in magnitude, suggesting that the difference in reactive ligand rates of initiation versus propagation. To illustrate the magni-
electronic properties (compared to the monomer) is the predo- tude of this effect, Fig. 2C/D overlays the estimated concen-
minant factor. The fastest rate constants were obtained when tration of unreacted precatalyst 1a/1c with monomer 6 versus
the reactive ligand contained a strongly resonance donating time for the polymerizations. These plots highlight the signifi-
substituent, consistent with the notion that electrophilicity/ cance and underappreciated impact of a slow initiation on the
nucleophilicity of the reactive ligands contributes to the rates. chain-growth polymerizations. Overall, these studies reveal
Significantly, the fastest precatalyst (1a) underwent initiation that selectively accelerating initiation using reactive ligands is
at a rate similar to propagation (kprop = 9.7 × 10−3 s−1).13a,23 a successful strategy for narrowing the molecular weight distri-
This result suggests that 1a should produce polymer samples butions in chain-growth polymerizations of π-conjugated
with the narrowest molecular weight distributions. Overall, monomers.
these studies revealed that the reactive ligands dramatically
influence the initiation rate.
Monomer 6 was then polymerized with precatalysts 1a–d to
evaluate the impact of initiation rate on the resulting polymer
samples. The polymerization was monitored via in situ IR spec-
ð3Þ
troscopy and aliquots were periodically withdrawn and
quenched to determine the number-average molecular weight
(Mn) and PDI as a function of conversion by gel permeation
chromatography (GPC). As evident in Fig. 2A, the fastest initiat-
To provide a framework for generalizing this approach, the
ing precatalyst (1a) yielded polymers with the lowest PDI free energy barriers for reductive elimination from 3 were com-
whereas the slowest initiating precatalyst (1c) yielded polymers puted for the four substituents examined experimentally, as
with the highest PDI. The GPC data reveals that the broaden- well as R = H and 15 other para-substituents (see ESI† for full
ing of the PDI is due to low molecular weight oligomers list). Computations were performed using Guassian09, and
(Fig. 2B and ESI†). Because PDIs reflect slow initiations as well employed the BP86 DFT functional24 paired with the 6-311+G(d)
as chain-transfer and chain-termination events, MALDI-TOF basis set25 for non-metal atoms and the SDB-cc-pVTZ basis
MS analysis of the end-groups in low molecular weight oligo- set with the small core, fully relativistic effective core poten-
mers was used to distinguish between these pathways. These tial26 for Ni. The trend in predicted activation free energies
studies revealed exclusively Ar/H end-groups for all four (ΔG‡) for 3a–d are in agreement with the experimental data.27
If the reductive elimination rates were purely determined by
arene electrophilicity/nucleophilicity, as initially anticipated,
the computed free energy barriers should correlate with the
differences in atomic charges at the reacting carbons (as pre-
dicted by natural population analyses). Surprisingly, no such
correlation was observed (ESI†). Instead, a modest correlation
(r2 = 0.65) was observed between the free energy barriers and
the Hammett σ− values,28 which characterize the ability of a
para-substituent to stabilize negative charge through direct reso-
nance effects (Fig. 3A). The importance of charge delocaliza-
tion was further supported by a strong correlation (r2 = 0.94)
between the barrier heights and the change in the charge on
the two arenes on going from 3 to the transition state (Fig. 3B).
This correlation indicates that the reductive elimination rates
are largely modulated by the ability of the two arenes to stabil-
ize the increasing electron density on the catalyst during
reductive elimination. Importantly, the strong correlation is
only observed when both rings are considered, indicating that
there is some interplay between the delocalizing effect of the
two arenes in the transition state. These results predict that
the fastest initiation rates with monomer 2 (or 6) will occur
with strongly resonance-based electron-withdrawing substitu-
Fig. 2 (A) Plot of polydispersity index (PDI) versus monomer conversion for the
polymerization of 6 using precatalysts 1a–d. (B) Representative gel permeation
chromatograms at varying percent conversions for the polymerization of 6 with
ents (e.g., R = NO2) on precatalyst 1. These and related reactive
ligands will be the subject of future studies.
precatalyst 1c. (C) and (D) Plot of [monomer] and [precatalyst] versus time for
the polymerization of 6 with precatalysts 1a (C) and 1c (D).
4220 | Dalton Trans., 2013, 42, 4218–4222
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Conclusions
In summary, we have demonstrated that modifying the reactive
ligand on the Ni precatalyst can be used to selectively acceler-
ate initiation over propagation. We have shown that faster
initiations lead to polymer samples with narrower molecular
weight distributions, highlighting the importance of precata-
lyst initiation in these chain-growth polymerizations. Signi-
ficantly, the PDIs obtained with precatalyst 1a are the lowest
reported for poly(2,5-bis(hexyloxy)phenylene) synthesis. Com-
putational studies predict that the lowest activation free
energies can be achieved with precatalysts containing resonance-
based electron-withdrawing substituents, in contrast to our
initial assumption about the nucleophilicity/electrophilicity of
the reactive ligands. Although this work focuses on aryl cross-
coupling reactions, the results are anticipated to be generaliz-
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because the mechanism (including the turnover-limiting step)
was previously shown to be similar.
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We thank the National Science Foundation (CAREER
CHE-0954610) and the ACS Petroleum Research Fund (ACS
PRF 50645-DNI6) for support of this work, as well as the
TAMU Supercomputing Center for providing computational
resources. S. R. L. thanks the National Science Foundation for
a predoctoral fellowship. J. W. G. B. thanks A. Pitts for fruitful
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