Organometallic mediated radical polymerization of vinyl acetate using bis(imino)pyridine vanadium trichloride complexes

Article abstract of DOI:10.1039/c3dt32625j

The synthesis and characterization of one novel proligand and six novel vanadium(iii) trichloride complexes is described. The controlled radical polymerization activity towards vinyl acetate of these, and eight other bis(imino)pyridine vanadium trichlorid

Full text of DOI:10.1039/c3dt32625j

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Organometallic mediated radical polymerization of
vinyl acetate using bis(imino)pyridine vanadium
trichloride complexes†‡
Cite this: Dalton Trans., 2013, 42, 9157
Mitchell R. Perry,a Laura E. N. Allan,a,b Andreas Deckenc and Michael P. Shaver*a,b
The synthesis and characterization of one novel proligand and six novel vanadium(III) trichloride com-
plexes is described. The controlled radical polymerization activity towards vinyl acetate of these, and
eight other bis(imino)pyridine vanadium trichloride complexes previously reported, is investigated. Those
complexes possessing variation at the N-aryl para-position with no steric protection offered by ortho-sub-
stituents (4 examples) result in poor control over poly(vinyl acetate) polymerization. Control is improved
with increasing steric bulk at the ortho-position of the N-aryl substituent (4 examples) although attempts
to increase steric bulk past isopropyl were unsuccessful. Synthesizing bis(imino)pyridine vanadium trichlo-
ride complexes with substituted imine backbones restores polymerization control when aliphatic substi-
tuents are used (4 examples) but ceases to make any drastic improvements on catalyst lifetime.
Modification of the polymerization conditions is also investigated, in an attempt to improve the catalyst
lifetime. Expansion of the monomer scope to include other vinyl esters, particularly those derived from
renewable resources, shows promising results.
Received 2nd November 2012,
Accepted 9th January 2013
DOI: 10.1039/c3dt32625j
www.rsc.org/dalton
Introduction
The importance of controlled radical polymerization (CRP) is
growing as the demand for specific polymer architectures with
controlled properties increases.1–3 While several techniques to
realize this goal have been developed,3–6 the central tenet of
CRP aims to minimize the concentration of propagating radi-
cals at any one time. This is achieved by establishing equili-
brium (ka/kd, where ka is the activation rate constant and kd is
the deactivation rate constant) between a dormant, ‘capped’
species and an active species capable of propagation
(Scheme 1), thereby decreasing undesirable side reactions
including radical combination and disproportionation.7 Atom
transfer radical polymerization (ATRP) is probably the most
researched metal-mediated CRP technique and utilizes a
Scheme 1 Activation and deactivation processes in CRP reactions.
halogen spin-trap to generate polymers reversibly terminated
with carbon–halogen bonds.8–13
Much recent research interest has focused on organometal-
lic mediated radical polymerization (OMRP),1–3,13–16 which
uses an organometallic species as the radical trap to impart
control over the polymerization. Cobalt systems are the most
prolific in OMRP, with Co(acac)2 capable of controlling the
polymerization of several monomer families including
acrylates,17–19 acrylonitrile20–23 and vinyl acetate (VAc),24–29
a
notoriously difficult monomer to control due to the difficulty
in both activating this monomer and controlling the equili-
brium between the unstabilized radical and the dormant
species. A long induction period and the use of an inconveni-
ent initiator, V-70, which requires storage at low temperatures,
are negatives of this system. The induction period can be cir-
cumvented with PVAc-Co(acac)2 macroinitiators, however,
laborious synthesis and purification hinder commercialization
of this methodology.28,29 Other alternatives to decrease this
initiation period include utilizing a redox system comprising
the sodium salt of citric acid and lauroyl or benzoyl peroxide,
capable of generating vinyl ester polymers with PDIs as low as
aUniversity of Prince Edward Island, 550 University Avenue, C1A 4P3 Charlottetown,
PE, Canada
bSchool of Chemistry, University of Edinburgh, Joseph Black Building, West Mains
Road, Edinburgh, EH9 3JJ, UK. E-mail: michael.shaver@ed.ac.uk;
Fax: +44 (0) 131650 4743; Tel: +44 (0) 131 650 4726
cUniversity of New Brunswick, P.O. Box 4400, E3B 6E2 Fredericton, NB, Canada
†Celebrating 300 years of Chemistry at Edinburgh.
‡Electronic supplementary information (ESI) available: Additional polymeriz-
ation data, X-ray crystallography data, GPC traces. CCDC 891100 and 891101. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3dt32625j
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Fig. 1 Efficient mediators of vinyl acetate OMRP.
Scheme 2 Synthesis of substituted [BIMPY]VCl3 complexes.
1.2.30,31 Modification of this versatile cobalt system to include
fluorinated acac substituents,17,32 the more electron-donating
tetramethylheptadionato ligands,33 as well as isoelectronic
β-ketoiminate ligands34 (Fig. 1) have expanded the scope of
complexes to show efficacy in vinyl acetate OMRP. However,
aside from cobalt systems, there remains a paucity of metal
systems capable of efficiently controlling the polymerization of
VAc through OMRP or other CRP techniques.
Bis(imino)pyridine [BIMPY] complexes, where [BIMPY] =
2,6-(ArNvCMe)2C5H3N, with a variety of metals have been
developed previously for ethylene polymerization, as documen-
ted in a comprehensive review by Gibson et al.35 and other
noteworthy literature reports.36–46 Vanadium [BIMPY] com-
plexes are active in olefin polymerization,38–40,47–50 and our
group has recently shown its promise in OMRP.51,52 Using a
vanadium(III) [BIMPY] system (Fig. 1), where Ar = 2,6-iPr2C6H3
([BIMPY]dippVCl3), 1, with azobisisobutyronitrile (AIBN) as the
initiator, controlled poly(vinyl acetate) (PVAc) can be syn-
thesized with excellent agreement between theoretical and
experimental molecular weights and narrow PDIs (<1.3).
Control is maintained for approximately 3 hours at 120 °C,
after which time molecular weights deviate from theoretical
values and productive polymerization ceases.51,52 This report
details the synthetic investigation of structurally similar
vanadium(III) complexes for the OMRP of vinyl acetate, to
further investigate this loss of control. Attention is also paid to
expansion of the monomer scope of [BIMPY]dippVCl3 to
include other vinyl ester monomers and optimization of the
polymerization conditions.
Fig. 2 X-ray crystal structure of 12, with hydrogen atoms removed for clarity
and ellipsoids shown at the 50% probability level.
Crystals suitable for X-ray crystallography were obtained for
novel complexes 12 and 14 from the slow evaporation of dilute
acetonitrile solutions. In the X-ray crystal structure of 12 the
vanadium centre lies in a distorted octahedral geometry with
an equatorial plane defined by the three nitrogen atoms of the
[BIMPY] ligand [N(3)–V(1)–N(2) = 75.38(8)°, N(2)–V(1)–N(1) =
75.52(8)°, N(1)–V(1)–N(3) = 150.72(8)°] and one chlorine atom
[N(3)–V(1)–Cl(1) = 103.67(6)°, N(1)–V(1)–Cl(1) = 105.57(6)°,
N(2)–V(1)–Cl(1) = 176.42(6)°] (Fig. 2). The remaining chlorine
atoms (Cl(2) and Cl(3)) form a bent axial vector [Cl(2)–V(1)–
Cl(3) = 168.36(3)°], albeit with a large deviation from 180°,
potentially due to the increased steric repulsion present in this
system. Imine bond distances [C(7)–N(3) = 1.292(3) Å, C(1)–
N(1) = 1.280(3) Å] are in good agreement with similar structures
in the literature.53 V–N(pyridine) π-bonding is suggested by
the shorter vanadium–nitrogen bond distance present between
the metal centre and the pyridine nitrogen [V(1)–N(2) = 2.063(2)
Å] versus the vanadium–nitrogen bonds to imino nitrogen
atoms [V(1)–N(3) = 2.188(2) Å and V(1)–N(1) = 2.213(2) Å].
Selected bond lengths and angles are summarized in Table 1.
The equatorial plane defined by the three nitrogen atoms of
the [BIMPY] ligand [N(3)–V(1)–N(2) = 74.58(17)°, N(2)–V(1)–
N(1) = 74.98(17)°, N(1)–V(1)–N(3) = 149.38(16)°] and one
Results and discussion
Synthesis of vanadium(III) complexes
A variety of proligands were synthesized from the imine con-
densation between the appropriate ketone or aldehyde and the
relevant amine. From these proligands, a family of structurally
similar [BIMPY]VCl3 complexes were obtained, following a pre-
viously reported procedure.52 Complexes with substituents
varying in electronic character and steric bulk at the para- and
ortho-positions of the N-aryl substituent and at the imine
carbon backbone positions were synthesized (Scheme 2).
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Table 1 Selected geometrical parameters for complexes 12 and 14
Table 2 Polymerization data for 1–9a
% Conversionb
Mn
Mn,th
PDI
c
d
Bond distances (Å) and angles (°)
Complex
Complex 12
Complex 14
1
2
3
4
5
26
44
30
29
42
2830
8330
5440
7590
159 210
4350
3190
10 270
7310
10 000
11 010
4520
6350
4310
4300
3030
1.3
1.8
1.6
1.8
1.4
1.4
1.4
2.1
1.7
2.1
2.7
V(1)–Cl(1)
V(1)–Cl(2)
V(1)–Cl(3)
V(1)–N(1)
V(1)–N(2)
V(1)–N(3)
N(1)–C(1)
N(3)–C(7)
N(1)–V(1)–N(2)
N(2)–V(1)–N(3)
N(1)–V(1)–N(3)
N(1)–V(1)–Cl(1)
N(3)–V(1)–Cl(1)
N(2)–V(1)–Cl(1)
Cl(2)–V(1)–Cl(3)
2.2918(13)
2.3339(11)
2.2862(12)
2.213(2)
2.063(2)
2.188(2)
1.280(3)
1.292(3)
75.52(8)
75.38(8)
150.72(8)
105.57(6)
103.67(6)
176.42(6)
168.36(3)
V(1)–Cl(1)
V(1)–Cl(2)
V(1)–Cl(3)
V(1)–N(1)
V(1)–N(2)
V(1)–N(3)
N(1)–C(1)
N(3)–C(7)
N(1)–V(1)–N(2)
N(2)–V(1)–N(3)
N(1)–V(1)–N(3)
N(1)–V(1)–Cl(1)
N(3)–V(1)–Cl(1)
N(2)–V(1)–Cl(1)
Cl(2)–V(1)–Cl(3)
2.3033(15)
2.3393(16)
2.3181(17)
2.153(4)
2.074(4)
2.155(4)
5e
6
7
8
9
22
48
41
48
65
3150
8180
5230
7300
9310
1.305(6)
1.301(6)
74.98(17)
74.58(17)
149.38(16)
104.81(12)
105.75(12)
177.40(14)
173.30(7)
a Reactions carried out in bulk at 120 °C for 3 h using AIBN and
monomer : catalyst : initiator ratios of 100 : 1 : 0.6. b Determined
gravimetrically. c Mn obtained using polystyrene standards and
d
corrected for VAc. Mn,th = [M]0/[I]0 × MW(monomer) × % conv. + MW
(catalyst). e Polymerization run with monomer : catalyst : initiator ratio
of 100 : 0.6 : 3.
is observed, control is poor as indicated by the relatively broad
PDIs (1.4–1.8). Interestingly, the presence of an electron-with-
drawing para-fluoro substituent in the complex, 5, generates
polymers with bimodal GPC traces (Fig. S1, ESI‡). The expected
signal for a polymer of molecular weight ca. 4000 Da is compli-
cated by a further, high molecular weight peak at ca. 159 000 Da,
resulting from rapid, uncontrolled polymerization. Increasing
the concentration of complex in the polymerization reaction
circumvents this problem as more radical trap is present in
the system to deactivate the propagating chains, resulting in
consistently monomodal GPC traces. However, polymeriz-
ations were still less controlled than those utilizing the parent
complex, [BIMPY]dippVCl3, 1.
Fig. 3 X-ray crystal structure of 14, with hydrogen atoms removed for clarity
and ellipsoids shown at the 50% probability level.
Polymerization results obtained using complexes 2–5
suggest that the N-aryl para-substituents are not the predomi-
nant determining factor with respect to imparting control in
the polymerization of vinyl acetate. We felt that the loss of
control was likely due to the removal of steric bulk from the
ortho positions. Therefore, a series of complexes varying in
N-aryl ortho-substitution, 6–8, were screened. Complex 6, pos-
sessing 2,6-dimethyl substituents resulted in poorly controlled
polymerization reactions, with PDIs of 2.1. However, when
increasing the steric bulk from methyl, 6, to ethyl, 7, a signifi-
cant decrease in the PDI to 1.7 is observed. Furthermore, when
moving to isopropyl ortho-substituents, 1, a further decrease in
PDI, to 1.3, is observed. This strongly supports the suggestion
that increased steric bulk at the ortho-positions of the aryl ring
increases the OMRP control which these [BIMPY]VCl3 com-
plexes exert over vinyl acetate. Attempts to increase this bulk
further are ongoing but have so far been unsuccessful, due to
unfavourable condensation reactions. A comparison of data
obtained for 6 and 8, which differ only in the presence of a
methyl group at the para-position illustrates the lack of impor-
tance of the para-substituent: both complexes reach 48% con-
version in 3 h, with PDIs of 2.1, indicating that para-
substitution does not play a significant role in tuning the
polymerization behaviour of these vanadium complexes.
chlorine atom [N(3)–V(1)–Cl(1) = 105.75(12)°, N(1)–V(1)–Cl(1) =
104.81(12)°, N(2)–V(1)–Cl(1) = 177.40(14)°] that makes up the
distorted octahedral geometry of 14 is comparable to that of 12
(Fig. 3). The axial vector defined by the vanadium–chlorine
bonds [Cl(2)–V(1)–Cl(3) = 173.30(7)°] is expectedly less bent
than those in the crystal structure of 12, as 14 possesses no
steric repulsion from aryl ortho substituents. With the
decreased steric bulk at the 2,6-aryl positions, there is a signifi-
cant increase in the Cl(2)–V(1)–Cl(3) bond angles. It is also
evident that the equatorial vanadium–chlorine bonds present
in 14 [V(1)–Cl(1) = 2.3033(15) Å, V(1)–Cl(2) = 2.3393(16) Å] are
consistently longer than those found in 12. Furthermore,
imine bond distances [C(7)–N(3) = 1.301(6) Å, C(1)–N(1) =
1.305(6) Å] are noticeably longer than those of 12 and other
complexes possessing alkyl-substituted imine carbons,53
suggesting a weaker imine bond and a more electrophilic
imine carbon.
Vinyl acetate polymerization data
Variation of the para-substituent at the N-aryl (R2) position
(Scheme 2, complexes 2–5) resulted in interesting polymeriz-
ation activity towards VAc (Table 2). Although reasonable agree-
ment between experimental and theoretical molecular weights
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Interestingly, screening complex 9 (Table 2), possessing an
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The effect of increasing steric bulk on the backbone imine
aliphatic cyclohexyl N-substituent, led to relatively high conver- carbon can be best observed in the series of complexes with
sion (65%) and a large increase in PDI (2.7). It is likely that the 2,6-diisopropyl phenyl N-aryl substituents and varying R1 sub-
disruption in conjugation throughout the ligand backbone is stituents. As bulk is increased from methyl, 1, to ethyl, 12, to
responsible for loss of the ligand non-innocence.52 The rela- isopropyl, 13, control over the polymerization is maintained,
tively high conversions and high PDIs support a metal centre with PDIs of 1.2–1.3 and excellent correlation between exper-
that is more electron-rich and less likely to form a bond with imental and theoretical molecular weights. Higher molecular
propagating radicals, resulting in poor deactivation and an weight polymers were targeted using 12, running the reactions
uncontrolled polymerization.
with an increased concentration of monomer (1 : 200 : 0.6 and
The results presented in Table 2 suggest that steric bulk is 1 : 500 : 0.6). Control was slightly decreased, with PDIs of
necessary at the ortho-positions to impart control over the 1.3–1.4, and increased molecular weights of 3570 and 6680,
polymerization of the reactive vinyl acetate monomer. It is respectively. Conversions decreased to 12.5% in both cases.
likely that increased steric bulk may offer protection from This slight decrease in control is attributed to the increased
attack by incoming radicals, a theory supported by our compu- concentration of monomer in the system, which leads to a
tational investigation.52 To further probe the system,
a
larger number of uncontrolled monomer insertions before the
2-methylpyridyl alkylated [BIMPY]dippV(III) compound was syn- propagating species becomes dormant, thereby increasing
thesized using the method of Gambarotta et al.53 This structu- PDIs.
rally similar compound lacks the conjugation responsible for
The absence of N-aryl ortho substituents in complex 14,
ligand non-innocence in the [BIMPY]VCl3 complexes and is which has phenyl substituents at both the imine carbon and
suggestive of products arising from radical attack at the ligand N-aryl positions, resulted in low conversions, independent of
backbone. Screening this V(III) complex for the polymerization reaction time. Interestingly, in contrast to 11, which has 2,6-
of VAc resulted in no polymer, supporting alkylation via diisopropyl phenyl substituents at the N-aryl position and
radical attack on the ligand framework acting as a deactivation phenyl substituents at the imine carbon, excellent control was
mechanism.
achieved, with molecular weights in good agreement with
One way to mitigate the occurrence of this radical attack is theoretical values and low PDIs of 1.3, albeit at low conver-
to protect the face of the ligand by increasing steric bulk at the sions (16%). This is indicative of radical attack occurring early
imine carbon position (R1). A family of [BIMPY]VCl3 catalysts due to lack of protection from ortho-substituents.
with different substituents at the imine carbon were syn-
thesized, 10–14, with their VAc OMRP screening results sum-
marized in Table 3.
Reaction optimization
As expected, complex 10, with no steric protection at the
imine carbon position, exhibits low polymerization activity
generating only short oligomers that could not be accurately
characterized by GPC. Similar data are obtained after conduct-
ing the polymerization for 5 hours. Complex 11, with a phenyl
imine substituent, gives relatively high conversions (50%) after
3 hours, but displays poor control as indicated by the broad
PDI of 2.0. The presence of aromatic substituents on the back-
bone potentially disrupts the ligand non-innocence and the
OMRP equilibrium. The planar phenyl group does appear to
offer some steric protection; however, the electron-withdrawing
character of this substituent soon trumps the increased steric
bulk by creating a more electrophilic imine carbon, susceptible
to radical attack. Conversions did not increase above 50%
despite reaction times as long as 5 hours.
Optimization of vinyl acetate polymerization was carried out
using the parent [BIMPY]dippVCl3 complex, 1, since it remains
one of the most efficient OMRP mediators in this family of bis
(imino)pyridine complexes, 1–14.
The use of higher initiator concentrations results in
increased conversions, from 26% when 0.6 equivalents of
AIBN are used to 46% with 6 equivalents of AIBN (Table 4).
Molecular weights decrease somewhat (ca. 1000 Da for the
10-fold increase in [AIBN]), but not as much as might be
expected for these elevated radical concentrations. PDIs are
largely unaffected, remaining at ca. 1.3 even when 6 equiva-
lents of AIBN are used, indicating that the radical exchange is
rapid and trapping of the propagating chains by the vanadium
complex is efficient. However, extending reaction times to
6 hours does not result in higher conversions at any of the
Table 3 Polymerization data for 10–14a
Table 4 Effect of AIBN concentration on VAc polymerization using 1a
Complex
% Conv.
Mn
Mn,th
PDI AIBN equiv.
% Conv.
Mn
PDI
10
11
12
13
14
15
50
22
25
16
—
—
—
0.6
1.2
6
28
31
47
4820
2430
1710
1.3
1.3
1.3
9270
2480
3060
2400
7230
2360
3450
2180
2.0
1.2
1.3
1.3
a Reactions carried out in bulk at 120 °C for 3 h using AIBN and
monomer : catalyst ratios of 100 : 1 with other parameters as in
Table 2.
a Reaction and characterization details as provided in Table 2.
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Table 5 Effect of temperature on VAc polymerization using 1a
Table 7 Polymerization data for vinyl ester monomers using 1a
% Conv.b
Mn
Mn,th
PDI
c
d
Temp. (°C)
% Conv.
Mn
Mn,th
PDI Monomer
Time/h
120
110
100
90
28
23
25
29
4820
4070
4370
4970
3830
3390
4230
4770
1.3
1.3
1.4
1.4
VPr
VPr
VPv
VPv
VBz
VBz
VLa
VLa
VLa
VLae
VSt
6
24
6
24
6
24
1
6
64
6
1
3
20
18
36
30
20
19
10
16
16
7
3530
3570
9340
10 550
3960
3280
8760
8380
9040
4460
11 060
9930
12 540
3990
3660
8320
7040
5620
5370
3700
6070
5390
2440
3670
4160
5470
1.3
1.4
1.4
1.4
1.6
1.5
1.5
1.5
1.6
1.4
1.5
1.5
1.7
a Reactions carried out in bulk at variable temperatures for 3 h with
other reaction and characterization parameters as in Table 2.
Table 6 Effect of temperature on VAc polymerization using V-65 and 1a
24
27
51
VSt
VSt
Temp. (°C)
% Conv.
Mn
Mn,th
PDI
6
a Reactions carried out in bulk at 120 °C using AIBN and ratios of
110
100
90
80
70
27
25
23
29
36
4580
4290
4000
4880
5900
5620
5420
5410
7170
7940
1.4
1.4
1.4
1.4
1.5
100 : 1 : 0.6 for monomer : catalyst : initiator. b Determined by 1H NMR
d
spectroscopy. c Mn corrected for VAc. Mn,th
= [M]0/[I]0 × MW
(monomer) × %conversion + MW(catalyst). e Reaction conducted in
1 mL toluene.
a Reactions carried out in bulk for 6 h using V-65 with other reaction
and characterization parameters as in Table 2.
molecular weights and PDIs largely unaffected by longer reac-
tion times. Some deviations between observed and theoretical
molecular weights are to be expected, as the experimental mol-
ecular weights were corrected using the vinyl acetate conver-
sion factor which will not account for the differences in
hydrodynamic volumes between these vinyl ester polymers and
poly(vinyl acetate).
As observed for poly(vinyl acetate), decreasing the tempera-
ture of the polymerization from 120–90 °C has little effect on
prolonging the catalyst lifetime. Similar trends to those
described for vinyl acetate are observed with vinyl propionate,
vinyl pivalate and vinyl benzoate (Tables S5–S7, ESI‡) although
conversions are somewhat improved at lower temperatures for
both vinyl propionate and vinyl pivalate. PDIs again tend to be
broader at these lower temperatures, while the molecular
weights increase as higher conversions are reached.
The study of vinyl esters was further extended to include
long chain vinyl monomers of fatty acids. In particular, vinyl
laurate (VLa) and vinyl stearate (VSt) were chosen due to their
commercial availability and scarcity of reports focusing on
control over these renewable monomers. One very recent litera-
ture report utilizes Co(acac)2 to generate homopolymers and
block co-polymers of VLa and VSt with good control (PDI <
1.3), using a redox initiator.54 Conversions for the screening
reactions of VLa using 1 were limited to 16% conversion at
120 °C, initiated by AIBN, despite reaction times as long as
64 h (Table 7). Control is modest when the polymerizations are
carried out in bulk, with broadened PDIs of 1.5–1.6. Running
the polymerizations in solution offers improved control and
PDIs of 1.4, but a concomitant decrease in conversion is
radical concentrations investigated (Table S2, ESI‡), indicating
that catalyst death occurs irrespective of radical concentration.
We then investigated the effects of temperature on the
polymerization behaviour of our system. Decreasing the temp-
erature of the polymerization from 120–90 °C has little effect
on VAc conversions, which are between 23–29% (Table 5). As
expected, PDIs are broader at the lower temperatures, as a
result of less efficient AIBN initiation at these temperatures
and slower radical exchange between propagating and
dormant species. After 6 hours, the catalyst is essentially in-
active as no increase in conversion is observed when running
the polymerizations for 24 hours (Table S3, ESI‡).
We also explored the efficacy of 1 using another radical
initiator, V-65, which has a lower 10 hour half-life decompo-
sition temperature than AIBN. Although lower temperatures
using V-65 favour higher conversions (Table 6), which decrease
from 36% at 70 °C to 27% at 110 °C, the PDIs increase signifi-
cantly from 1.36 (at 110 °C) to 1.51 (at 70 °C) as the tempera-
ture is decreased. This implies that radical exchange is less
efficient at these lower temperatures, which results in the
increased PDIs but also in a slower rate of catalyst death,
meaning that conversions are also increased. Molecular
weights are slightly higher than theoretical values, with greater
deviation at the lower temperatures. Catalyst death is still sig-
nificant, as illustrated by the conversions at 24 h (Table S4,
ESI‡), which are almost identical to those obtained at 6 h.
Expansion of the monomer scope
[BIMPY]dippVCl3, 1, has previously shown some efficacy in the observed, as the likelihood of radical combination increases.
polymerization of vinyl propionate (VPr), vinyl pivalate (VPv) Increased conversions are observed with VSt, as the inherent
and vinyl benzoate (VBz).52 While control over VPr and VPv is viscosity of the molten monomer hinders termination, a prop-
reasonable, with PDIs of ≤1.4, polymerization of VBz yields erty of the decreased rates of diffusion present in the reac-
polymers with broadened PDIs of 1.5–1.6 (Table 7). No pro- tion.55 It should also be noted that larger deviations between
ductive polymerization occurs after 6 hours, with conversions, theoretical and experimental molecular weights are observed
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for these monomers and are attributed to the different hydro- stretches. DCM and MeCN and liquid monomers were dried
dynamic volumes of the respective polymers, as the data in over CaH2, trap-to-trap distilled and degassed in three freeze–
Table 7 are corrected for VAc.
pump–thaw cycles prior to use. All solvents were degassed
prior to use.
Ligands synthesized according to literature procedures are
listed here: L1 (R1 = Me, R2 = 2,6-iPr2C6H3),36 L2 (R1 = Me, R2 =
Ph),56 L3 (R1 = Me, R2 = 4-MeC6H4),37 L4 (R1 = Me, R2 = 4-
Conclusions
We report the synthesis of several new vanadium trichloride MeOC6H4),37 L5 (R1 = Me, R2 = 4-FC6H4),44 L6 (R1 = Me, R2 =
complexes based on bis(imino)pyridyl ligand frameworks. A 2,6-Me2C6H3),36 L7 (R1 = Me, R2 = 2,6-Et2C6H3),57 L8 (R1 = Me,
family of structurally similar complexes were screened for their R2 = 2,4,6-Me3C6H2),36 L9 (R1 = Me, R2 = C6H11, Cy),57 L10 (R1
polymerization activity towards VAc in bulk. Substituents at the = H, R2 = 2,6-iPr2C6H3),36 L11 (R1 = Ph, R2 = 2,6-iPr2C6H3),58
N-aryl para-position (2–5) have less impact than the ortho-sub- L12 (R1 = Et, R2 = 2,6-iPr2C6H3),43 L13 (R1
= =
iPr, R2
stituents (1, 6–8) on imparting control over the polymerization. 2,6-iPr2C6H3).43 All characterization data for these ligands were
Increasing the steric bulk at the ortho-positions of the N-aryl consistent with the literature.
substituent increases control over the polymerization, with iso-
Synthesis of ligands
L14, C31H23N3. To synthesize L14, with R1 = Ph and R2 = Ph,
propyl substituents giving the best results. Furthermore, sub-
stitution at the imine carbon also significantly affects
polymerization behaviour. No steric bulk, 10, hinders polymer- 2,6-dibenzoylpyridine (4.0 g, 13.9 mmol), an excess of aniline
ization early, as does the presence of a phenyl group on the (4 mL, 4.1 g, 40.0 mmol) and a catalytic amount of p-TsOH
backbone, 11, although this effect can be somewhat mitigated was refluxed in toluene for 4 days. After cooling to room temp-
by removing ortho-substitution at the N-aryl positions, 14. Alky- erature, the mixture was filtered and the solvent removed in
lating the imine carbon substituents restores control over the vacuo to yield brown/yellow oil which was dissolved in CHCl3
polymerization (1, 12, 13), but has so far been ineffective at sig- and filtered to remove any remaining p-toluenesulfonic acid.
nificantly increasing the lifetime of these OMRP catalysts. Recrystallizing the residue from hot MeOH yielded a yellow
1
Attempts to further increase steric bulk at the N-aryl ortho- crystalline solid, 2.1 g, 33%. H NMR spectroscopic analysis at
positions are ongoing. Use of N-alkyl substituents interrupts room temperature showed a region of complex resonances
the delocalization within the ligand framework and results in between 6.7 and 8.5 ppm, due to restricted rotation of the
inferior OMRP behaviour (9).
phenyl rings on the NMR timescale. Resolution of this region
1
The efficacy of 1 with respect to other monomers and via high temperature H NMR spectroscopy gave the following
initiators was also explored, with modest control observed for resonances: 1H NMR (C6D6, 80 °C): 8.31 (br s, 2H, m-Py-H),
vinyl laurate, vinyl stearate and vinyl benzoate. Improved 7.93 (br s, 1H, p-Py-H), 6.73–7.28 (m, 20H, Ar-H). 13C NMR
control over vinyl propionate and vinyl pivalate was observed, (DMSO-d6, 115 °C): 166.81, 165.72, 165.47, 156.08, 155.28,
although catalyst lifetimes were not significantly improved for 154.38, 153.49, 150.42, 150.21, 137.24, 137.16, 136.73, 134.57,
any monomer by lowering the reaction temperatures, changing 131.23, 131.14, 129.56, 129.41, 128.60, 128.49, 128.40, 127.40,
initiators or by increasing the concentration of initiator.
127.11, 125.32, 123.64, 123.53, 123.43, 122.67, 120.51, 120.45,
120.26, 120.15. Anal. calcd for C31H23N3: C, 85.10; H, 5.30; N,
9.60. Found: C, 85.24; H, 5.49; N, 9.38. IR (ATR, νCvN) at
1616 cm−1
.
Experimental
General considerations
Synthesis of complexes
All chemistry with air/moisture sensitive compounds was con- Many of the [BIMPY]VCl3 complexes described in this report
ducted under nitrogen using standard Schlenk and cannula are found elsewhere in the literature, with 9–14 being novel
techniques or performed in an MBraun LABmaster sp glovebox compounds. Characterization of all compounds is included to
equipped with a −35 °C freezer, [O2] and [H2O] analyzers. maintain consistency throughout. Complexes of the form
Elemental analyses were performed at Guelph Chemical Labo- LnVCl3 were prepared using a modified literature procedure,
ratories Ltd. NMR spectroscopy was performed on a Bruker under inert conditions by dissolving the requisite bis(imino)
Avance 300 MHz spectrometer, operating at 300 and 75 MHz pyridyl ligand and a stoichiometric amount of VCl3(THF)3 in
for 1H and 13C nuclei respectively, at 293 K unless otherwise dichloromethane. The dark red/purple solutions were left to
stated. J values are given in Hz. All 13C NMR spectra reported stir overnight. Capricious solubility trends between the com-
are proton decoupled and calibrated relative to the residual plexes were observed as many precipitated as dark solids and
1
proton signals in the deuterated solvent. All paramagnetic H could be observed within a few hours, while others remained
NMR spectroscopy signals are reported as singlets, unless in solution. In all cases, mixtures were filtered. Any precipitate
otherwise stated. Infra-red spectra were obtained either as was washed with DCM and pentane. Filtrates were reduced,
nujol mulls or as solids using transmittance or ATR (Attenu- and precipitated and washed with pentane before drying in
ated Total Reflectance), respectively, on a Bruker Alpha spec- vacuo. If impure, the solids were extracted into acetonitrile, or
trometer, with listed absorbances corresponding to imine another appropriate solvent, to purify the complex. The
9162 | Dalton Trans., 2013, 42, 9157–9165
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1
paramagnetic nature of these compounds made characteriz- 82%). H NMR (CD3CN): 88.30, 6H, NvCCH3, 11.86, 4H, o-Ar-
ation by 1H NMR spectroscopy difficult in many cases, but H, 5.29, 4H, m-Ar-H, −15.82, 2H, m-Py-H, −25.20, 1H, p-Py-H.
characteristic shifts of diamagnetic protons to paramagnetic μeff. = 2.9 B.M. (vs. 2.83). IR (nujol, cm−1): 1582. Anal. calcd for
regions (∼55 and ∼−5 ppm) were helpful in confirming C21H17Cl3F2N3V: C, 49.78; H, 3.38; N, 8.29. Found: C, 50.00; H,
complex formation. A consistent bathochromic IR shift of 3.45; N, 8.08.
imine bonds characteristic of metallation could also be
observed in IR. Evans’ method59 was used to determine the stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL). The
number of unpaired electrons.
purple solid was dried in vacuo (0.68 g, 95%). 1H NMR
C25H27Cl3N3V, 6. As for 1, using L6 (0.60 g, 1.6 mmol), a
C33H43Cl3N3V, 1. In a glovebox, L1 (1.00 g, 2.1 mmol) and (CD3CN): 56.24 (6H, NvCCH3), 7.27 (4H, m-Ar-CH), 6.26 (2H,
VCl3(THF)3 (0.78 g, 2.1 mmol) were combined in a Schlenk p-Ar-CH), 4.58 (12H, Ar-CH3), −1.38 (2H, m-Py-H), −12.60 (1H,
tube and dissolved in CH2Cl2 (50 mL). The solution was p-Py-H). μeff. = 3.0 B.M. (vs. 2.83). IR (nujol, cm−1): 1572. Anal.
allowed to stir overnight, then filtered and the filtrate reduced calcd for C25H27Cl3N3V: C, 57.00; H, 5.17; N, 7.98. Found: C,
to approximately 15 mL. Pentane was added (ca. 30 mL) to pre- 57.05; H, 5.21; N, 7.90.
cipitate a dark solid that was isolated via filtration. The dark
C29H35Cl3N3V, 7. As for 1 using L7 (1.10 g, 2.6 mmol), a stoi-
purple solid was extracted into hot MeCN and any remaining chiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL). The
solids were removed via filtration. The mixture was reduced dark brown solid was dried in vacuo (0.92 g, 67%). 1H NMR
in vacuo, washed with pentane and dried for several hours. A (CD3CN): 46.63 (6H, NvCCH3), 7.50 (m-Ar-H, 4H), 6.09 (2H,
dark purple solid was obtained (1.12 g, 85%). 1H NMR p-Ar-H), 3.99 (8H, Ar-CH2CH3), 1.54 (12H, Ar-CH2CH3), −0.41
(CDCl3): 54.76 (6H, NvCCH3), 8.27 (4H, m-Ar-H), 5.44 (2H, (2H, m-Py-H), −5.57 (1H, p-Py-H). μeff. = 3.1 B.M. (vs. 2.83). IR
p-Ar-H), 4.41 (4H, CH(CH3)2), 2.79 (24H, CH(CH3)2), −0.92 (1H, (nujol, cm−1): 1575. Anal. calcd for C29H35Cl3N3V: C, 59.75; H,
p-Py-H), −5.81 (2H, o-Py-H). μeff. = 3.3 B.M. (vs. 2.83). IR (nujol, 6.05; N, 7.21. Found: C, 59.90; H, 6.16; N, 7.21.
cm−1): 1574. Anal. calcd for C33H43Cl3N3V: C, 62.03; H, 6.78; N,
6.58. Found: C, 62.61; H, 7.01; N, 6.60.
C27H31Cl3N3V, 8. As for 1, using L8 (0.62 g, 1.6 mmol), a
stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL). The
C21H19Cl3N3V, 2. In a glovebox, L2 (1.10 g, 3.5 mmol) and dark brown solid was dried in vacuo (0.60 g, 70%). 1H NMR
VCl3(THF)3 (1.31 g, 3.5 mmol) were combined in a Schlenk tube (CDCl3): 61.34 (6H, NvCCH3), 8.61 (4H, m-Ar-H), 7.20 (12H,
and dissolved in CH2Cl2 (50 mL). The solution was allowed to o-Ar-CH3), 6.4 (6H p-Ar-CH3), −2.3 (2H, m-Py-H), −10.7 (1H, p-
stir overnight, then filtered and the filtrate reduced to approxi- Py-H). μeff. = 2.6 B.M. (vs. 2.83). IR (nujol, cm−1): 1577. Anal.
mately 15 mL. Pentane was added (ca. 30 mL) to precipitate a calcd for C27H31Cl3N3V: C, 58.45; H, 5.63; N, 7.57. Found: C,
dark solid that was isolated via filtration. The dark brown solid 58.40; H, 5.60; N, 7.62.
was extracted into hot MeCN and any remaining solids were
C21H31Cl3N3V, 9. As for 1, using L9 (1.06 g, 3.3 mmol), a
removed via filtration. The mixture was reduced in vacuo, stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL),
washed with pentane and dried for several hours. A dark brown yielding a dark purple solid (1.13 g, 85%). 1H NMR (CDCl3):
1
solid was obtained (1.01 g, 73%). H NMR (THF-d8): 83.56 (6H, 53.28 (6H, NvCCH3), 5.32 (8H, o-Cy-H2), 4.84 (8H, m-Cy-H2),
NvCCH3), 7.61 (2H, p-Ar-H), 7.15 (4H, m-Ar-H), 6.51 (4H, o-Ar- 3.07 (4H, p-Cy-H2), 0.91 (1H, p-Py-H), −4.65 (2H, m-Py-H). μeff.
=
H), −16.07 (2H, m-Py-H), −17.09 (1H, p-Py-H). μeff. = 3.3 B.M. (vs. 3.0 B.M. (vs. 2.83). IR (nujol, cm−1): 1578. Anal. calcd for
2.83). IR (nujol, cm−1): 1582. Anal. calcd for C21H19Cl3N3V: C, C21H31Cl3N3V: C, 52.24; H, 6.47; N, 8.70. Found: C, 52.01; H,
53.59; H, 4.07; N, 8.93. Found: C, 53.61; H, 4.01; N, 9.00.
6.29; N, 8.88.
C23H23Cl3N3V, 3. As for 1, using L3 (0.97 g, 2.8 mmol), a
C31H39Cl3N3V, 10. As for 1, using L10 (1.01 g, 2.2 mmol), a
stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL). A stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL),
1
1
purple solid was isolated and dried in vacuo (1.20 g, 85%). H yielding a dark purple solid (0.66 g, 67%). H NMR (CD3CN):
NMR (CD3CN): 88.99 (6H, NvCCH3), 12.20 (4H, o-Ar-H), 4.73 8.16 (2H, NvCH), 6.88 (4H, m-Ar-H), 5.44 (2H, p-Ar-H), 2.97
(4H, m-Ar-H), −1.14 (6H, Ar-CH3), −16.54 (2H, m-Py-H), −26.77 (4H, CH(CH3)2), 1.25 (24H, CH(CH3)2), −5.70 (1H, p-Py-H),
(1H, p-Py-H). μeff. = 2.5 B.M. (vs. 2.83). IR (nujol, cm−1): 1582. −101.00 (2H, m-Py-H). μeff. = 2.7 B.M. (vs. 2.83) IR (nujol,
Anal. calcd for C23H23Cl3N3V: C, 55.39; H, 4.65; N, 8.43. cm−1): 1570. Anal. calcd for C31H39Cl3N3V: C, 60.94; H, 6.43; N,
Found: C, 55.25; H, 4.38; Cl, 8.70.
6.88. Found: C, 61.19; H, 6.18; N, 7.02.
C23H23Cl3N3O2V, 4. As for 1, using L4 (0.51 g, 1.4 mmol), a
C43H47Cl3N3V, 11. As for 1, using L11 (2.12 g, 3.5 mmol), a
stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL). A stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL),
dark brown solid was recovered and dried in vacuo (0.71 g, yielding a dark purple solid (1.42 g, 58%). 1H NMR (CDCl3):
95%). 1H NMR (CD3CN): 92.29 (6H, NvCCH3), 12.93 (6H, 9.46 (4H, NvC-o-Ar-H), 8.19 (4H, NvC-m-Ar-H), 6.27 (2H,
OCH3), 4.51 (4H, m-Ar-H), 2.16 (4H, o-Ar-H), 1.27 (1H, m-Py-H), NvC-p-Ar-H), 5.30 (4H, CH(CH3)2), 4.69 (4H, CvN-m-Ar-H),
−18.06 (1H, p-Py-H). μeff. = 2.9 B.M. (vs. 2.83). IR (nujol, cm−1): 4.35 (CvN-p-Ar-H), 2.19 (24H, CH(CH3)2), 0.08 (1H, p-Py-H),
1585. Anal. calcd for C23H23Cl3N3O2V: C, 52.05; H, 4.37; N, −8.08 (2H, m-Py-H). μeff. = 2.6 B.M. (vs. 2.83). IR (nujol, cm−1):
7.92. Found: C, 52.01; H, 4.35; N, 7.98.
1567. Anal. calcd for C43H47Cl3N3V: C, 67.67; H, 6.21; N, 5.51.
C21H17Cl3F2N3V, 5. As for 1 using L5 (0.53 g, 1.5 mmol), a Found: C, 67.45; H, 5.95; N, 5.24.
stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL). A
C35H47Cl3N3V, 12. As for 1, using L12 (0.75 g, 1.5 mmol), a
dark brown solid was obtained and dried in vacuo (0.48 g, stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL),
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1
yielding a dark purple solid (0.69 g, 70%). H NMR (CD3CN):
32.08 (4H, NvC-CH2CH3), 8.67 (4H, CvN-m-Ar-H), 5.44 (2H,
Acknowledgements
CvN-p-Ar-H), 4.77 (4H, CHCH3), 1.21 (6H, NvCCH2CH3), 1.02 The authors thank the Natural Sciences and Engineering
(24H, ArCHCH3), 0.48 (1H, p-Py-H), −4.19 (2H, m-Py-H). μeff.
=
Research Council of Canada, the Canada Foundation for Inno-
2.4 B.M. (vs. 2.83). IR (nujol, cm−1): 1574. Anal. calcd for vation, the Atlantic Canada Opportunities Agency, the Univer-
C35H47Cl3N3V: C, 63.02; H, 7.10; N, 6.30. Found: C, 63.55; H, sity of Prince Edward Island and the University of Edinburgh
7;45; N, 5.97. Elemental analysis was inaccurate, likely due to for funding.
decomposition of the sample in transit.
C37H51Cl3N3V, 13. As for 1, using L13 (0.44 g, 0.8 mmol), a
stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL),
Notes and references
yielding a dark purple solid (0.30 g, 53%). μeff. = 2.8 B.M. (vs.
1
2.83). IR (nujol, cm−1): 1565. H NMR could not be accurately
assigned, due to the complexity of the spectrum. Anal. calcd
for C37H51Cl3N3V: C, 63.93; H, 7.40; N, 6.05. Found: 63.57; H,
7.13; N, 5.83.
1 A. Debuigne, R. Poli, C. Jérôme, R. Jérôme and
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C31H23Cl3N3V, 14. As for 1, using L14 (0.78 g, 1.8 mmol), a
stoichiometric amount of VCl3(THF)3 and CH2Cl2 (50 mL),
yielding a dark purple solid (0.92 g, 89%). H NMR (CD3CN):
11.34 (4H, NvC-o-Ar-CH), 10.61 (4H, NvC-m-Ar-CH), 8.79 (2H,
NvC-p-Ar-CH), 5.75 (4H, CvN-o-Ar-CH), 4.11 (2H, CvN-p-Ar-
CH), 2.10 (CvN-m-CH), −17.53 (2H, m-Py-H), −21.43 (1H, p-Py-
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for C31H23Cl3N3V: C, 62.59; H, 3.90; N, 7.06. Found: C, 62.77;
H, 4.12; N, 6.85.
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Polymerization experiments
All polymerization experiments were conducted under inert
atmosphere. Monomer, initiator and catalyst were loaded into
an ampoule or sealed vial along with a magnetic stir-bar with
a desired volume of solvent, if required, prior to being placed
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9164 | Dalton Trans., 2013, 42, 9157–9165
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Dalton Trans., 2013, 42, 9157–9165 | 9165