Vanadium(III) phenoxyimine complexes for ethylene or ε-caprolactone polymerization: Mononuclear versus binuclear pre-catalysts

Article abstract of DOI:10.1039/c2cy20571h

The mononuclear {[C₆H₄NCH(ArO)]₂VCl(THF)} (Ar = 2,4-t-Bu₂C₆H₂ (1), Ar = C ₆H₄ (2)), {O[C₆H₄NCH(ArO)] ₂}VCl(THF) (Ar = 2,4-t-Bu₂C₆H₂ (3), Ar = C₆H₄ (4)) and the binuclear vanadium(iii) complexes {[C₆H₄NCH(ArO)]VCl₂(THF)₂} ₂(μ-CH₂CH₂) (Ar = 2,4-t-Bu₂C ₆H₂ (5), Ar = C₆H₄ (6)), have been synthesized and fully characterized. The compounds [C₆H ₅NCH(ArO)]VCl₂(THF)₂ (Ar = 2,4-t-Bu ₂C₆H₂ (7), Ar = C₆H₄ (8)), [2,4,6-Me₃-C₆H₂NCH(ArO)]VCl₂ (Ar = 2,4-t-Bu₂C₆H₂ (9), Ar = C ₆H₄ (10)) and [2,6-i-Pr₂-C₆H ₃NCH(ArO)]VCl₂(THF)₂ (Ar = 2,4-t-Bu ₂C₆H₂ (11), Ar = C₆H₄ (12)), {μ-CH₂CH₂[NCH(C₆H₄O)] ₂VCl(THF)} (14) and {C₆H₄[NCH(C ₆H₄O)]₂VCl(THF)} (15) were synthesized for comparative polymerization studies. The dizwitterionic compound [2,6-i-Pr ₂-C₆H₃N⁺(H)CH(C₆H ₄O)]₂VCl₂O (13) was also isolated, and presumably formed via a fortuitous hydrolysis reaction. The complexes 2, 5 and 13 have been structurally characterized; the molecular structure of the parent ligand (L5) in 5 is also reported. All complexes have been screened for ethylene as well as ε-caprolactone polymerization, and results are compared against those for known related mono- and bi-nuclear counterparts to evaluate for possible cooperative effects. The compounds 10 and 12 have been supported on modified SiO₂, analysed by XPS and subjected to homo-polymerization (ethylene) and co-polymerization (1-hexene and ethylene) studies. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20571h

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PAPER
Vanadium(III) phenoxyimine complexes for ethylene or
e-caprolactone polymerization: mononuclear versus
binuclear pre-catalysts†
Cite this: Catal. Sci. Technol.,
2013, 3, 152
Lucy Clowes,a Mark Walton,a Carl Redshaw,z*a Yimin Chao,a Alex Walton,b
Pertti Elo,c Victor Sumerinc and David L. Hughesa
The mononuclear {[C6H4NQCH(ArO)]2VCl(THF)} (Ar
=
2,4-t-Bu2C6H2 (1), Ar
=
C6H4 (2)),
{O[C6H4NQCH(ArO)]2}VCl(THF) (Ar = 2,4-t-Bu2C6H2 (3), Ar = C6H4 (4)) and the binuclear vanadium(III)
complexes {[C6H4NQCH(ArO)]VCl2(THF)2}2(m-CH2CH2) (Ar = 2,4-t-Bu2C6H2 (5), Ar = C6H4 (6)), have been
synthesized and fully characterized. The compounds [C6H5NQCH(ArO)]VCl2(THF)2 (Ar = 2,4-t-Bu2C6H2 (7),
Ar = C6H4 (8)), [2,4,6-Me3–C6H2NQCH(ArO)]VCl2 (Ar = 2,4-t-Bu2C6H2 (9), Ar = C6H4 (10)) and [2,6-i-Pr2-
C6H3NQCH(ArO)]VCl2(THF)2 (Ar = 2,4-t-Bu2C6H2 (11), Ar = C6H4 (12)), {m-CH2CH2[NQCH(C6H4O)]2VCl(THF)}
(14) and {C6H4[NQCH(C6H4O)]2VCl(THF)} (15) were synthesized for comparative polymerization studies.
The dizwitterionic compound [2,6-i-Pr2-C6H3N+(H)QCH(C6H4O)]2VCl2O (13) was also isolated, and
presumably formed via a fortuitous hydrolysis reaction. The complexes 2, 5 and 13 have been structurally
characterized; the molecular structure of the parent ligand (L5) in 5 is also reported. All complexes have
been screened for ethylene as well as e-caprolactone polymerization, and results are compared against
those for known related mono- and bi-nuclear counterparts to evaluate for possible cooperative effects.
The compounds 10 and 12 have been supported on modified SiO2, analysed by XPS and subjected to
homo-polymerization (ethylene) and co-polymerization (1-hexene and ethylene) studies.
Received 14th August 2012,
Accepted 1st September 2012
DOI: 10.1039/c2cy20571h
www.rsc.org/catalysis
Introduction
Vanadium catalysts have previously been employed for the
production of high molecular weight polyethylene,1 ethylene/
propylene co-polymers or ethylene/propylene/diene elastomers.2
However, despite their early commercial importance, it is only in
the last few years that highly active a-olefin polymerization
catalysts of group V have appeared in the academic literature.3
Of particular note has been the emergence of salicylaldiminato
or so-called phenoxyimine-type (FI) ligands as one of the ligand
sets of choice for early transition metal catalytic systems. Reports
by Fujita et al. described highly active (65.1 kg-PE (mmol V h)ꢀ1
at 1 bar ethylene pressure) mononuclear pre-catalysts bearing
two phenoxyimines at vanadium (see I, Scheme 1).4 Subsequent
Scheme 1 Known vanadium pre-catalysts I–V.
a School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, UK
b School of Physics & Astronomy, University of Leeds, Leeds, LS2 9JT, UK
c Borealis Polymers Oy, P. O. Box 330, Porvoo, FI-06101, Finland
† Electronic supplementary information (ESI) available: PPR polymerisation data
from Homo-/Co-polymerization runs (Tables S1–S3). CCDC 837291–837293 and
895725. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cy20571h
studies by Nomura, and by Li et al., led to pre-catalysts of the type
II and III, respectively, both of which also proved to be highly active
for ethylene polymerization.5 Our laboratory has also recently
reported phenoxyimine complexes, where the chelate is either
derived from an NO3 tripodal imine ligand set (IV),6 or vanadium
complexes possessing N2O2S2-based ligands (V).7 A general feature
‡ Present address: E-mail: C.Redshaw@hull.ac.uk; Tel: +44 (0)1482-465219.
c
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Scheme 2 Vanadium pre-catalysts screened in this study.
of this chemistry is the adoption of mononuclear pre-catalysts. syntheses of the mononuclear phenoxyimine complexes 7, 8,
However, in a number of other polymerization systems, beneficial 11 and 12.5b Complex 13 was synthesized using the same
cooperative effects are witnessed, leading to either improved method as for compound 12 and was presumably formed via
catalytic performance or differing polymer properties.8 With this a fortuitous hydrolysis reaction. The known salicylaldehyde
in mind, and given the current interest in vanadium, we have bridged phenoxyimine complexes 14 and 15 were synthesised
embarked upon a program to develop new multi-metallic as benchmark catalytic systems.13
vanadium-based pre-catalysts. Furthermore, encouraged by
recent work on zinc,9 we herein report the use of the ligand
Mononuclear vanadium(III) phenoxyimines
Complexes 1 and 2 were prepared by treatment of L1 or L214
family [C6H4NQCH(ArO)]2(m-CH2CH2) (Ar = 2,4-t-Bu2C6H2 L5,
C6H4 L6), which has previously been shown to act in a tetra-
with excess of VCl3(THF)3 in tetrahydrofuran, in the presence of
base (triethylamine), to afford, following work up, the dark red
complexes {[C6H4NQCH(2,4-t-Bu2C6H2O)]2VCl(THF)} (1) and
{[C6H4NQCH(C6H4O)]2VCl(THF)} (2) in yields of 47 and 51%,
respectively. Crystals of 2 suitable for X-ray crystallography were
grown by slow diffusion of the dark red tetrahydrofuran
solution into hexane. Compound 2 crystallises with two inde-
dentate fashion with either zirconium or zinc centres.9,10
Recently, related group IV and V complexes of tetradentate
and bidentate Schiff bases have been screened for their ability
to polymerise various a-olefins.11 We also note that a number of
bimetallic nickel systems have shown increased activities versus
their mononuclear counterparts.12 Herein, polymerization
screening results for the bimetallic vanadium pre-catalysts are
pendent V-complex molecules (together with THF solvent) in
compared under similar conditions with known and novel related
mononuclear related vanadium(III) complexes (Scheme 2).
the cell. The structure of one of the independent molecules of 2
is depicted in Fig. 1, with selected bond lengths and angles
given in the caption; crystallographic data (with those of the
other structures reported in this paper) are collated in Table 3.
Results and discussion
The two molecules in 2 are very similar in conformation, each
The ligands L1–L15 required for each of the corresponding with a six-coordinate vanadium atom with an octahedral coordina-
vanadium(III) complexes 1–15 (Scheme 2) were obtained in good tion pattern. Three of the N2O2 donor atoms of each L22ꢀ ligand
yields via standard condensation of the requisite aniline with are arranged in a meridional mode with the remaining O atom
one or two equivalents of either salicylaldehyde or 3,5-di-tert- bonding normal to the meridional plane. The two remaining sites
butyl-2-hydroxybenzaldehyde. Complexes 1–6, 9–10 were prepared are filled by a chloride ion (trans to the fourth donor atom) and a
following methods employed by Li and co-workers in their THF ligand (trans to an imido N atom). In the folding of the L22ꢀ
c
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Fig. 1 View of one of the two very similar molecules in crystals of
{[C6H4NQCH(C6H4O)]2VCl(THF)} 2 indicating the atom numbering scheme.
Hydrogen atoms and solvent molecules have been omitted for clarity. Thermal
ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and
angles (1): V(1)–O(1) 1.926(4), V(1)–N(2) 2.107(5), V(1)–N(3) 2.092(5), V(1)–O(4)
1.917(5), V(1)–O(5) 2.127(4), V(1)–Cl(1) 2.353(2); O(1)–V(1)–N(2) 83.8(2),
O(1)–V(1)–O(4) 96.41(19), O(1)–V(1)–Cl(1) 169.19(15), N(2)–V(1)–N(3) 89.7(2),
N(2)–V(1)–O(4) 198.8(2), N(3)–V(1)–O(4) 90.2(2), N(3)–V(1)–O(5) 176.12(19),
O(5)–V(1)–Cl(1) 88.77(13).
Fig. 2 View of a molecule of the dizwitterionic complex 13,
[2,6-i-Pr2-C6H3N+(H)QCH(C6H4O)]2VCl2O, indicating the atom numbering
scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond
lengths (Å) and angles (1): V–O(1) 1.987(3), V–O(2) 1.978(3), V–Cl(3) 2.3594(14),
V–Cl(4) 2.3265(12), V–O(5) 1.573(3), O(2)–V–O(1) 155.91(14), O(1)–V–Cl(3)
82.08(10), O(1)–V–Cl(4) 90.33(9), O(5)–V–O(1) 101.11(15), O(2)–V–Cl(3) 81.64(9),
O(2)–V–Cl(4) 90.26(9), O(5)–V–O(2) 101.43(15), Cl(4)–V–Cl(3) 138.69(5),
O(5)–V–Cl(3) 112.20(12), O(5)–V–Cl(4) 109.12(12), C(7)–N(8) 1.312(6),
C(27)–N(28) 1.291(6), H(8)ꢁ ꢁ ꢁO(1) 1.97, N(8)–H(8)ꢁ ꢁ ꢁO(1) 130.0, H(8)ꢁ ꢁ ꢁCl(3) 2.59,
N(8)–H(8)ꢁ ꢁ ꢁCl(3) 149.1, H(28)ꢁ ꢁ ꢁO(2) 1.95, N(28)–H(28)ꢁ ꢁ ꢁO(2) 132.6,
H(28)ꢁ ꢁ ꢁCl(3) 2.68, N(28)–H(28)ꢁ ꢁ ꢁCl(3) 153.2.
ligand around the metal centre, there is twisting of ca 71.71 about
the N(2)–C(21) bond (and equivalent N(7)–C(71) bond in the second
molecule) and ca 54.51 about the N(3)–C(31) and its equivalent
bond; the rotation between the two phenyl groups, as about C(22)–
C(32), is ca 551. In both molecules, the THF ligand rings have
‘twisted’ conformations.
X-ray quality crystals of the ligand L5 by crystallization from
chloroform/methanol. The molecular structure is shown in
Fig. 3, with selected bond lengths and angles in the caption.
The molecule lies about a centre of symmetry at the mid-point of
the C(27)–C(270) bond, in an anti, staggered conformation, and
thus has an ‘S-shape’ rather than curved in a ‘C-shape’ as it
would when enfolding a metal centre. The phenolic hydrogen
atom (in each half of the molecule) was refined freely and is
directed towards the imino nitrogen forming a hydrogen bond,
with H(1o)ꢁ ꢁ ꢁN(20) 1.63 Å.
Compound 12 was synthesized using the reported literature
method.5b However, the use of ‘wet’ ligand (L12 that had been
dried insufficiently; ligand used for formation of compound 12
had been dried in vacuo in excess of 16 hours) resulted in the
formation of a dizwitterionic complex 13 in moderate yield
(ca 37%). Single crystals of 13 were grown from a saturated
acetonitrile solution on prolonged standing at ambient
temperature. The vanadium atom in complex 13 (Fig. 2) is
five-coordinate with an approximately square-pyramidal pattern;
the vanadyl oxygen, O(5), is at the apex, with the pairs of chloride
ligands and phenolate oxygen atoms in opposite corners of the
square base. The two phenolate ligands are arranged with
approximate mirror symmetry about the central VOCl2 plane.
Both NH groups are donors to bifurcated hydrogen bonds; each
is linked to the phenolate O atom of its own ligand and to Cl(3).
There are also two solvent (acetonitrile) molecules in the asym-
metric unit of the cell.
Complexes 5 and 6 were prepared in the same manner as
complex 2. Treatment of L5 or L6 with excess of VCl3(THF)3 in
tetrahydrofuran afforded, following work up, the dark red com-
plexes {[C6H4NQCH(2,4-t-Bu2C6H2O)]VCl2(THF)2}2(m-CH2CH2) (5)
and {[C6H4NQCH(C6H4O)]VCl2(THF)2}2(m-CH2CH2) (6) in yields of
43 and 44%, respectively.
Crystals of 5 suitable for X-ray crystallography were grown by
slow diffusion of n-hexane into a THF solution of 5 at ambient
temperature. The molecular structure is depicted in Fig. 4, with
selected bond lengths and angles given in the caption. Regard-
less of whether the ligands L5 or L6 were treated with one or two
Binuclear vanadium(III) phenoxyimines
The ethylene-bridged ligands, L5 and L6 were prepared in good equivalents of VCl3(THF)3, the di-nuclear species was always
yields using the literature method.10 It proved possible to obtain obtained and in yields of about 40–45% and 65%, respectively.
c
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Fig. 5 X-ray photoelectron survey spectrum of S12. The vanadium V-2p energy
window 525–510 eV is shown inset.
an octahedral pattern, is achieved by the ligation of two chloride
ligands (mutually trans, Cl(3)–V–Cl(4) 176.20(4)1) and two cis-
THF molecules (opposite the iminophenolate N and O atoms,
O(5)–V–O(6) 85.68(9)1) at each V centre.
Fig. 3 View of molecule [C6H4NQCH(2,4-t-Bu2C6H2OH)]2(m-CH2CH2), L5,
indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths (Å) and angles (1): O(1)–C(1) 1.357(2),
O(1)–H(1o) 1.00(3), C(1)–C(2) 1.412(3), C(20)–N(20) 1.283(2), N(20)–C(21)
1.423(2), C(1)–O(1)–H(1o) 105.8(16), O(1)–C(1)–C(6) 119.80(18), C(6)–C(1)–C(2)
120.78(18), N(20)–C(20)–C(2) 122.17(19).
Silica immobilisation
Compounds 10 and 12 were immobilised on pre-treated silica
by refluxing in toluene to give supported structures S10 and
S12; the SiO2 had been heated to 350 1C under dynamic vacuum
for 48 h. X-ray photoelectron spectroscopy (XPS) analysis of the
silica surface confirmed the percentage coverage of the metal
species. Analysis of the resulting photoelectron spectrum of S12
(Fig. 5) showed a V-2p3/2 peak with a single component at 517.0
ꢂ 0.02 eV. The resulting concentration of vanadium was shown
to be 0.054% of the bulk sample. The C-1s peak was used to
calibrate the spectrum; however the calculated carbon concen-
tration in the sample is much higher than expected, now
assigned to the presence of adventitious carbon. Confirmation
that the ligand has not been leached from the system was
ascertained from the identification of N-1s and Cl-2p peaks at
binding energies of 401.0 eV (0.089%) and 199.7 eV (0.162%).
Ethylene polymerization screening
Complexes 1–10 have been evaluated as pre-catalysts for the
polymerization of ethylene, employing DMAC (dimethylalumi-
nium chloride) as co-catalyst, and ETA (ethyltrichloroacetate) as
a re-activator.15 Polymerizations were carried out in toluene
(100 mL) using 4000 molar equiv. of co-catalyst at 25 1C over
30 minutes.
In the presence of DMAC and ETA, the new vanadium(III)
bridged phenoxyimine complexes 1–6 were highly active for the
polymerization of ethylene (Table 1, runs 1–6). However, the
activities (o10 000 g mmolꢀ1 hꢀ1 per bar) were disappointing
in comparison to other V-based pre-catalyst–DMAC–ETA sys-
tems recently reported.11,16 The general trend observed in
Fig. 4 View of molecule {[C6H4NQCH(2,4-t-Bu2C6H2O)]VCl2(THF)2}2(m-CH2CH2) 5
indicating the atom numbering scheme. Hydrogen atoms and the minor disorder
component (in one of the THF ligands) have been omitted for clarity. Thermal
ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and
angles (1): V–O(1) 1.870(2), V–N(2) 2.087(2), V–Cl(3) 2.3546(9), V–Cl(4) 2.3581(9),
V–O(5) 2.107, V–O(6) 2.117(2), O(1)–V–N(2) 89.56(9), O(1)–V–O(5) 92.12(9), O(1)–
V–O(6) 177.31(9), N(2)–V–O(5) 177.87(9), N(2)–V–O(6) 92.60(9), Cl(3)–V–Cl(4)
176.20(4).
Interestingly here, the ligand rather than acting in tetradentate Table 1 is that the tert-butyl containing pre-catalysts showed
fashion as observed previously,10 prefers to act as a bis-bidentate higher activities than did their salicylaldehyde counterparts
ligand, binding to a vanadium centre at either end of the ligand. (e.g. runs 1 vs 2); the exception was 3 cf. 4, whose activities
The centrosymmetry is retained about the C(27)–C(270) bond were of the same magnitude. The enhanced performance
midpoint and there is rotation about the C(21)–N(2) bond to suggests that the steric hindrance of the tert-butyl group did
allow the coordination of the V centres. Six-fold coordination, in not hinder the approach of the monomer, or this was possibly
c
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Table 1 Ethylene polymerization resultsa
Table 2 e-Caprolactone screening results for complexes 1–15a
Pre-catalyst/run Conversionb Mn calculated Mn measuredc PDI
Pre-catalyst/runb Polymerc Activityd Mw
Mn
PDIg Mp
e
f
h
1
2
3
4
5
6
7
8
2.1
8400
2560
1360
1680
9520
4800
3920
5240
2210
3190
1920
2850
1640
4560
1000
217 000 49 300 4.4
143.3
141.9
144.2
145.6
140.6
143.1
143.7
142.6
—
—
—
—
—
1
2
3
4
5
6
7
8
57
47
24
33
48
70
72
100
87
93
97
19
93
94
3
26 024
21 458
10 960
15 070
21 900
31 960
32 870
45 660
39 720
42 460
44 290
8670
10 751
8640
7977
1.4
1.4
1.2
1.2
1.4
2.3
1.2
1.1
1.3
1.1
1.1
—
0.64
0.34
0.42
2.38
1.2
0.98
1.31
0.55
0.80
0.48
0.71
0.41
1.14
0.25
—
—
—
536 000 136 000 3.9
404 000 288 000 1.4
445 000 181 000 2.5
170 000 30 500 5.6
273 000 47 600 5.7
168 000 39 200 4.3
9249
10 080
11 010
4973
4060
9
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
9
13 670
14 260
21 340
—
13 290
4790
10
11
12
13
14
15
10
11
12
13
14
15
42 460
42 920
1370
1.2
1.1
1.4
—
—
4560
a
a
Conditions: 1 bar ethylene; toluene (100 mL); ETA (0.1 mL); reaction
Conditions: monomer/metal = 400; 72 h; 80 1C; 20 mL of toluene;
quenched with dilute HCl, washed with methanol (20 mL) and dried for 2.5 mL of e-caprolactone; 1 equivalent of benzyl alcohol (from a 0.97 M
b
c
b
1
c
12 h in a vacuum oven at 80 1C. 0.5 mmol. Grams of polymer.
solution in toluene). Calculated by H NMR. Mn measured = 0.58 ꢃ
Grams of polymer mmolꢀ1 hꢀ1 per bar. Weight average molecular
weight. Number average molecular weight. Polydispersity index.
1C determined by DSC. All runs carried out at 25 1C, 30 min and Al/V
Mn GPC.17
d
e
f
g
h
(molar ratio) 4000.
e-Caprolactone screening
Complexes 1–15 were also screened, in the presence of benzyl
alcohol, for their ability to polymerize e-caprolactone. Complex
6 was subjected to time, temperature and co-initiator screening
to obtain the optimum conditions. The subsequent e-caprolactone
screening of complexes 1–15 were completed using these condi-
tions (Table 2). All complexes, except 6, formed only oligomers
with low polydispersity (r1.4), which suggested that these ‘poly-
merizations’ occurred without side reactions and, interestingly,
only low molecular weight oligomers were obtained under these
conditions. Complex 6 deviated, in relation to polydispersity, from
the other pre-catalysts screened herein and as such was subjected
to lifetime studies which showed an almost linear increase in
conversion over time.
The above results might be described as disappointing;
however, they should be put into context by comparison with
recent work carried out by Mahha et al.18 who noted that the
heteropolyacids HPA-2 produced lower activities under an inert
atmosphere; this was attributed to the reduction of the metal
centres to V(IV) and/or Mo(V). However, when dioxygen was
added, the reduced species were re-oxidized to the active
vanadium(V) and Mo(VI). Unlike these heteropolyacid systems,
our phenoxyimine complexes show similar conversion rates
without the need to re-oxidise the deactivated species.
simply due to the improved solubility of the tert-butylated
pre-catalyst.
Only in the case of 1 (run 1) does the tetra-dentate nature of
the ligand appear to be beneficial compared with its bi-dentate
counterpart 7 (run 7), though the situation is reversed for the
non-tert-butylated species (runs 2 vs 8). These tetra-dentate
systems must undergo a rearrangement of the trans disposed
chlorines to afford a system which is more catalytically
favorable. Bialek et al. have observed enhanced activity in tert-
butylated tetra-dentate vanadium(IV) systems compared to their
bi-dentate counterparts.11a
In terms of polymer properties, the presence of an oxygen
bridge in 3 and 4 led to increased molecular weight (Mw), and in
the case of the non-tert-butylated complex 4, better control
(Mw/Mn = 1.4, run 4).
When comparing the bi-metallic (5–6) versus the mono-
metallic (1–4 and 7–15) vanadium complexes in Table 1, it
was evident that in the case of the tetra-dentate ligand sets,
activities for the bi-metallic systems were somewhat higher,
whilst the molecular weight (Mw) of the polyethylene obtained
from 6 was at the lower end of those observed herein.
On comparing complexes 9 and 10 with the bi-metallic
complexes 5 and 6, it is evident that for the tert-butyl derivatives
(5 and 9), there is evidence of a co-operative effect in 5;
observed activities for 11 and 12 suggest that this is not simply
a steric effect. In the case of RQH (6 and 10), the beneficial
cooperative effect in 6 is far less pronounced.
Comparison with the bi-dentate systems 7 and 8 reveals
increased activity and molecular weight (Mw) only in the case of
the tert-butylated derivative 5 (run 5); the values for 6 closely
resemble those of 8. In the case of complexes 14 and 15, the
presence of the phenylene backbone was found to be detrimental
to activity. Bialek and Liboska have previously observed enhanced
activity when using cyclohexylene bridged ligand sets.11c
Table 3 Homogenous PPR ethylene polymerization resultsa
Run
Catalystb
Temp.c
Yieldd
Activitye
1
2
3
4
5
6
7
5 (0.10)
5 (0.15)
5 (0.20)
5 (0.25)
5 (0.10)
5 (0.20)
5 (0.25)
60
60
60
60
80
80
80
0.0436
0.0382
0.0408
0.0455
0.0363
0.0475
0.0461
65
38
31
27
54
36
28
a
Conditions: 6.67 bar ethylene, 1 h reaction time, Al/V (molar ratio)
4000. mmol. 1C. Grams of polymer. g mmolꢀ1 hꢀ1 per bar.
b
c
d
e
c
156 Catal. Sci. Technol., 2013, 3, 152--160
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Generally, the salicylaldiminato complexes 2, 4, 6, 8 and 10 the two pre-catalysts to be inactive for the polymerization of
slightly out-performed their tert-butylated counterparts; excep- ethylene using TIBA (Table S1, ESI† runs 1–6); the resulting
tionally, pre-catalyst 11 outperformed complex 12. We note that uptake of ethylene (psi) was due to saturation of heptane rather
pre-catalysts 8, 11 and 14 exhibited the best conversion rates than polymerisation. Co-polymerization was more successful,
and provide excellent control of the molecular weight distribu- where S12 outperformed S10 with peak activities of 1744 and
tion (PDI = 1.1) of all those reported herein.
757 g mmolꢀ1 hꢀ1 barꢀ1 respectively (Table S3, ESI† runs
25 and 19), using EADC as co-catalyst and the addition of
ETA. Use of ETA indeed gave higher activities on a number of
runs, but also led to a large divergence of resulting activities
(Tables S2 and S3, ESI† runs 7–28).
The problems with repeatability seem to stem from the rapid
deactivation of the metal centres; the consumption profiles
when using EDAC and ETA show that the active site was dead
soon after the polymerization run has commenced (Fig. 6).
Co-polymerization using TIBA as co-catalyst was detrimental,
lowering the activity by 60–90% (Chart 1), although the active
site remained active for a much longer period.
Homogenous parallel pressure reactor ethylene screening
Homogenous screening of pre-catalyst 5, in combination with
methylaluminoxane (MAO), produced moderate catalysts for
the polymerization of ethylene (Table 3). Increasing the amount
of catalyst loading was found to be detrimental to the activity.
The varying activity in co-catalyst was explored by Nomura
et al.,19 who showed that the size of MAO forces the catalyst
to form discrete ions, whereas DMAC can form a chloro-bridged
species.
Heterogenous parallel pressure reactor ethylene screening
S10 and S12 were subjected to homo-polymerization (ethylene)
and co-polymerization (1-hexene and ethylene), over a one hour
period. Triisobutyl aluminium (TIBA) and ethylaluminium
dichloride (EADC) were used separately as co-catalysts, with
the addition of ethyl trichloroacetate (ETA) where required. The
metal content contained in the bulk sample was determined by
XPS and this value was used to determine the catalytic activity
of the supported catalysts (Tables S1–S3).
Conclusions
In conclusion, a series of new vanadium(III) complexes bearing
bridged phenoxyimines (1–6) have been synthesized and shown
to be highly active (o10 000 g mmolꢀ1 hꢀ1 per bar) for poly-
ethylene production when used in combination with ETA
and DMAC.
For ethylene polymerization, observed activity values for the
tert-butylated systems 5 and 7 were suggestive of a cooperative
effect operating between the two metal centres in 5; no such effect
was observed for the de-tert-butylated systems (runs 6 and 8).
Comparatively high activity was also observed when employing
the tert-butylated tetra-dentate mono-metallic pre-catalyst 1. The
presence of the tert-butyl groups in such systems was thought to
play a crucial role by enhancing solubility. Mounting vanadium
complexes 10 and 12 upon silica (forming S10 and S12) was
detrimental to the polymerization activity.
The results of the polymerization experiments and condi-
tions are shown in the ESI.† Homo-polymerization has shown
Screening for e-caprolactone polymerization revealed that
the mononuclear species (complexes 8–14) generally exhibited
higher conversions ( Z 87%) than did the binuclear species
(5 and 6) (r70%), with compounds 7 and 15 exceptions to
this rule. Extending the bridge between ligand binding sites in
tetra-dentate compounds led to decreased conversion rates
(compounds 1–4, 14 and 15). A relationship between the size
of the peripheral substituent and percentage conversion was
observed, indicating a need for consideration of steric effects
when designing pre-catalysts for e-caprolactone polymerization.
Fig. 6 Consumption profiles of S10 and S12.
Experimental
All manipulations involving vanadium were carried out under
an atmosphere of nitrogen using standard Schlenk and cannula
techniques or in a conventional nitrogen-filled glove-box.
Solvents were refluxed over an appropriate drying agent, and
distilled and degassed prior to use. Elemental analyses were
performed by the microanalytical services of the London
Metropolitan University. NMR spectra were recorded on a
Varian VXR 400 S spectrometer at 400 MHz or a Gemini at
Chart 1 TIBA vs EADC as co-catalyst for co-polymerization.
c
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300 MHz (1H); chemical shifts are referenced to the residual Complex 4 was obtained as a dark brown powder (0.43 g, 63%).
protio impurity of the deuterated solvent; IR spectra (nujol IR/cmꢀ1 1606(s) (CQN). Elemental analysis for VClC30H26N2O4
mulls, KBr windows) were recorded on Perkin-Elmer 577 and found (calculated): %C = 63.37 (63.78), H = 4.85 (4.64), N =
457 grating spectrophotometers. Solution state magnetic 4.83 (4.96). MS (EI, m/z): 492 [M-THF]+. Magnetic moment m =
susceptibility was measured by Evans’ method on a Varian 1.79mB.
VXR 400 S spectrometer using a 5% d12-cyclohexane–95%
d2-dichloromethane solution. Polymer melting point measure-
Pre-catalyst 55b
ments were carried out using a TA Instruments DSC 2920.
Procedure as described for complex 1 using L5 (0.25 g,
0.6 mmol), VCl3(THF)3 (0.33 g, 0.9 mmol) and Et3N (0.12 ml,
0.9 mmol). Complex 5 was obtained as dark red crystals (0.57 g,
Synthesis of ligands
Ligands L1–L15 were synthesised by the standard condensation 65%). Mp: 110 1C. IR/cmꢀ1 1609(s) (CQN). Elemental analysis
of the corresponding amine with one or two equivalents as for V2Cl4C60H86N2O6 found (calculated): %C = 61.19 (61.33), H =
required of either salicylaldehyde or 3,5-di-tert-butyl-2-hydroxy- 7.23 (7.38), N = 2.38 (2.38). MS (EI, m/z): 693 [M-V-4Cl-4THF]+.
benzaldehyde in ethanol.
Magnetic moment m = 1.90mB.
Synthesis of vanadium complexes
Pre-catalyst 65b
Complexes 1–6 and 9–10, were synthesised following analogous Procedure as described for complex 1 using L6 (0.25 g,
methods to that utilised by Li et al. to synthesise known 0.6 mmol), VCl3(THF)3 (0.49 g, 1.3 mmol) and Et3N (0.18 ml,
complexes 7 and 8.5b Complexes 11, 12, 14 and 15 were 1.3 mmol). Complex 6 was obtained as a red powder (0.34 g,
synthesised by the literature procedures.11 Complex 13 was 61%). IR/cmꢀ1 1608(s) (CQN). Elemental analysis for
synthesized using the same method as that for compound 12, V2Cl4C44H54N2O6 found (calculated): %C = 55.65 (55.59), H =
however use of ‘wet’ ligand (L12, which had been dried insuffi- 5.60 (5.73), N = 3.07 (2.95). MS (EI, m/z): 468 [M-V-4Cl-4THF]+.
ciently) led to a fortuitous hydrolysis reaction.
Magnetic moment m = 2.18mB.
Pre-catalyst 1
Pre-catalyst 9
To a solution of L1 (0.5 g, 0.8 mmol) in THF (20 ml), VCl3(THF)3 To a solution of L9 (0.5 g, 1.4 mmol) in THF (20 ml), VCl3(THF)3
(0.67 g, 1.8 mmol) was added and this solution was stirred for (0.57 g, 1.54 mmol) was added and this solution was stirred for
10 minutes before adding Et3N (0.25 ml, 1.8 mmol). The 10 minutes before adding Et3N (0.21 ml, 1.54 mmol). The
solution was stirred at room temperature for 4 hours and then solution was stirred at room temperature for 4 hours and then
concentrated to approximately 15 mL. The mixture was filtered concentrated to approximately 15 mL. The mixture was filtered
and recrystallised by diffusion of n-hexane (10 mL) to yield and recrystallised by diffusion of n-hexane (10 mL) to yield
complex 1 as a brown powder (0.42 g, 47%). IR/cmꢀ1 1610 (s) complex 9 as a brown powder (0.38 g, 57%). IR/cmꢀ1 1612 (s)
(CQN). Elemental analysis for VClC46H58N2O3 found (calcu- (CQN). Elemental analysis for VCl2C24H32NO found (calculated):
lated): %C = 71.39 (71.44), H = 7.34 (7.56), N = 3.52 (3.62). MS %C = 61.15 (61.02), H = 6.70 (6.83), N = 3.08 (2.97). MS (EI, m/z):
(EI, m/z): 758 [M-Me]+. Magnetic moment m = 2.70mB.
452.2 [M-Cl-tBu+H]+. Magnetic moment m = 1.69mB.
Pre-catalyst 2
Pre-catalyst 10
Procedure as described for complex 1 using L2 (0.5 g, 1.2 mmol), To a solution of L10 (0.5 g, 1.4 mmol) in THF (20 ml),
VCl3(THF)3 (0.97 g, 2.6 mmol) and Et3N (0.36 ml, 2.6 mmol). VCl3(THF)3 (0.57 g, 1.54 mmol) was added and this solution
Complex 2 was obtained as maroon crystals (0.56 g, 51%). was stirred for 10 minutes before adding Et3N (0.21 ml,
IR/cmꢀ1 1605(s) (CQN). Elemental analysis for VClC26H18N2O2 1.54 mmol). The solution was stirred at room temperature for
(bake dried 24 h –THF) found (calculated): %C = 65.40 (65.49), 4 hours and then concentrated to approximately 15 mL. The
H = 3.87 (3.80), N = 5.78 (5.87). MS (EI, m/z): 476 [M-THF]+. mixture was filtered and recrystallised by diffusion of n-hexane
Magnetic moment m = 2.23mB.
(10 mL) to yield complex 10 as a red powder (0.29 g, 36%). IR/cmꢀ1
1631 (s) (CQN). Elemental analysis for VCl2C16H16NO (sample
dried in vacuo for 12 h, loss of THF) found (calculated): %C =
Pre-catalyst 3
Procedure as described for complex 1 using L3 (0.5 g, 0.8 mmol), 61.15 (61.02), H = 6.70 (6.83), N = 3.08 (2.97). MS (EI, m/z): 359 [M]+.
VCl3(THF)3 (0.34 g, 0.9 mmol) and Et3N (0.13 ml, 0.9 mmol). Magnetic moment m = 2.05mB.
Complex 3 was obtained as a dark brown powder (0.34 g, 54%).
IR/cmꢀ1 1609(s) (CQN). Elemental analysis for VClC46H58N2O4
Pre-catalyst 13
found (calculated): %C = 69.81 (69.99), H = 7.55 (7.41), N = 3.35 To a solution of L12 (0.5 g, 1.7 mmol), which had been held
(3.55). MS (EI, m/z): 716 [M-THF]+. Magnetic moment m =1.91mB. under dynamic vacuum for 5 hours, in toluene (40 ml),
VCl3(THF)3 (0.29 g, 1.0 mmol) was added and this solution was
Pre-catalyst 4
refluxed for 12 hours. The solvents were removed in vacuo and the
Procedure as described for complex 1 using L4 (0.5 g, 1.2 mmol), residue was extracted into hot acetonitrile (35 mL) affording dark
VCl3(THF)3 (0.97 g, 2.6 mmol) and Et3N (0.36 ml, 2.6 mmol). orange needles on cooling. (0.49 g, 37%). IR/cmꢀ1 1605 (s) (CQN),
c
158 Catal. Sci. Technol., 2013, 3, 152--160
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927 (VQO). Elemental analysis for VCl2C38H46N2O3 (sample Typical parallel pressure reactor polymerization run
dried in vacuo for 12 h, loss of MeCN) found (calculated): %C =
A pre-weighed glass vial with stirring paddles was sealed and
purged with ethylene. 5 mmol of co-catalyst from a 100 mM
heptane solution was added along with co-monomer (if required).
Heptane was then added to reach a volume of 4000 mL in the
65.00 (65.14), H = 6.70 (6.62), N = 4.12 (4.00). MS (EI, m/z,
sample dried in vacuo 12 h, loss of MeCN): 646.2 [M-MeH-HCl]+
627.3 [M ꢀ 2HCl]+. Magnetic moment m = 0.982mB.
reaction vessel and heated to 80 1C. The ethylene pressure was set
Typical homogenous ethylene polymerization procedure
to 92 psi (6.34 bar) and the catalyst (along with ETA) was added as a
heptane slurry. The run was left stirring for 60 minutes and
quenched with CO2 (35% in N2). The glass vial was dried by
vacuum centrifuge and weighed.
A flame dried (250 mL) pyrex flask was purged several times
with ethylene gas at 1 bar pressure. This pressure was main-
tained throughout the polymerization run. Dry, degassed
toluene (100 mL) and ethyltrichloroacetate (ETA, 0.1 mL,
0.72 mmol) were added and stirred for 10 minutes. The
required temperature was achieved via a water bath, and the
co-catalyst was added; the pre-catalyst was then injected as a
toluene solution. The polymerization time was recorded from
injection of pre-catalyst and left for the required time. The
polymerization solution was quenched by addition of methanol
and acidified water. The solid polyethylene was then filtered
and dried.
X-ray photoelectron spectroscopy analysis
A small amount (Bmg) of powdered sample was pressed onto
adhesive carbon tape. High resolution XPS core level measure-
ments were performed with a VG Escalab 250 in LENNF, Leeds
University, equipped with a conventional hemispherical sector
analyser and controlled by a VGX900 data system. XPS experi-
ments were carried out using a high intensity monochromated
Al-Ka source (1486.6 eV) operated at 15 kV and 20 mA. V-2p,
C-1s, Cl-2p and N-1s spectra were recorded using a pass energy
of 20 eV. The energy scale of the spectrometer was calibrated to
the Ag-3d5/2 peak at 368.3 eV. The binding energy scale was
calibrated to the C-1s at 284.5 eV. High-resolution peak fitting
was performed using CasaXPS software (version 2.3.15).
Typical e-caprolactone ROP procedure
A Schlenk flask (250 mL) previously baked at 160 1C overnight
was charged with the required quantity of pre-catalyst under
glove box conditions. Dry, degassed toluene (20 mL) was added,
the required temperature was achieved, and one equivalent of
benzyl alcohol was then added. The polymerization was
Crystallographic analyses
initiated by addition of the e-caprolactone and was stirred for Single-crystal X-ray diffraction intensities for compound 2 were
the allotted time. Conversion of monomer was determined by measured at 120 K on a Bruker-Nonius Apex II Kappa-CCD
1H NMR, and the polymerization was quenched by addition of diffractometer (at the EPSRC National Crystallography Service),
methanol.
for L5 and 5 at 140 K on an Oxford Diffraction Xcalibur-3 CCD
Table 4 Summary of the crystal data and structure determination results for compounds 2, L5, 5 and 13
Compound
2
L5
5
13
Formula
Formula weight
Crystal system
Space group
2(C30H26ClN2O3V), 1.5THF
C44H56N2O2
644.9
Monoclinic
P21/a
C60H86Cl4N2O6V2
1175.0
Orthorhombic
Pbca
C38H46Cl2N2O3V, 2 MeCN
782.72
Orthorhombic
P21nb
1205.99
Monoclinic
P21/a
Unit cell dimensions
a (Å)
b (Å)
c (Å)
17.8598(14)
10.7750(8)
31.054(2)
90
9.6297(3)
12.1792(4)
16.2276(6)
90
15.4851(3)
12.3449(2)
33.0323(6)
90
11.0873(5)
17.9157(8)
20.6832(14)
90
a (1)
b (1)
101.730(4)
90
5851.2(7)
4
100.851(4)
90
1869.18(11)
2
90
90
6314.5(2)
4
90
90
4108.4(4)
4
g (1)
V (Å3)
Z
Temperature (K)
120(2)
1.369
0.470
140(1)
1.146
0.069
140(1)
1.236
0.512
100
1.262
0.414
Dcalcd (Mg mꢀ3
)
Absorption coefficient, m (mmꢀ1
)
Crystal size (mm3)
0.2 ꢃ 0.16 ꢃ 0.08
0.41 ꢃ 0.30 ꢃ 0.09
0.43 ꢃ 0.41 ꢃ 0.09
0.22 ꢃ 0.01 ꢃ 0.01
2ymax (1)
20
25
25
25
Reflections measured
Unique reflections, Rint
Reflections with F2 > 2s(F2)
27 495
5438, 0.111
3866
0.746, 0.604
737
0.065, 0.116
0.101, 0.127
19 826
3295, 0.106
1981
1.118, 0.904
221
0.050, 0.083
0.106, 0.096
104 961
5557, 0.072
4572
1.020, 0.982
339
0.061, 0.105
0.087, 0.113
9744
5871, 0.048
4516
1.000, 0.480
471
0.051, 0.090
0.078, 0.098
Transmission factors (max, min.)
Number of parameters
R1, wR2 [F2 > 2s(F2)]
R1, wR2 (all data)
Largest difference peak and hole
(e Åꢀ3
)
0.39 and ꢀ0.30
0.16 and ꢀ0.15
0.39 and ꢀ0.31
0.31 and ꢀ0.29
c
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3 For recent vanadium reviews see: (a) K. Nomura, In New Develop-
ments in Catalysis Research , ed. L. P. Bevy, NOVA Science Publish-
ers, New York, USA, 2005, p. 199; (b) C. Redshaw, Dalton Trans.,
2010, 39, 5595; (c) K. Nomura and S. Zhang, Chem. Rev., 2011,
111, 2342; (d) C. Redshaw, in Olefin Upgrading Catalysis by Nitrogen-
based Metal Complexes I, Catalysis by Metal Complexes, 35, ed.
diffractometer, and for 13 at 100 K on a Rigaku AFC12 Kappa-3-
circle, Saturn724 + CCD diffractometer (at the EPSRC National
Crystallography Service); all machines were equipped with
Mo-Ka radiation and graphite monochromator. Intensity data
were measured by thin-slice o- and j-scans.
´
G. Giambastiani and J. Campora, Springer Science + Business Media
The Apex data for 2 were processed with the DENZO/
SCALEPACK20 programs and SADABS,21 the Xcalibur data were
processed (including correcting for absorption) using the
CrysAlis-CCD and -RED programs,22 and the Rigaku data were
processed with CrystalClear-SM.23 Structures were determined
by the direct methods routines in the SHELXS program24 and
refined by full-matrix least-squares methods, on F2’s, in
SHELXL.24
In compound 2, there is disorder in one of the THF ligands,
and one of the THF solvent molecules lies disordered about a
centre of symmetry. The non-hydrogen atoms (except the
minor component C atom in the disordered ligand, and the
half-atoms in the disordered THF molecule) were refined with
anisotropic thermal parameters. The phenolic hydrogen atom
in L5 was located in a difference map and was refined freely.
In 5, there is disorder of a methylene group in one of the
THF ligands; the minor occupancy carbon atom was refined
isotropically. Except for the phenolic hydrogen atom in L5, all
hydrogen atoms were included in idealised positions and their
Uiso values were set to ride on the Ueq values of the parent
carbon atoms. Scattering factors for neutral atoms in all
samples were taken from reference.25 Computer programs used
in this analysis have been noted above, and were run through
WinGX26 on a Dell Precision 370 PC at the University of East
Anglia.
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Crystal data and refinement results for the four complexes
are collated in Table 4. CCDC 837291–837293 and 895725.†
Acknowledgements
16 (a) C. Redshaw, L. Warford, S. H. Dale and M. R. J. Elsegood, Chem.
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The EPSRC is thanked for financial support. The EPSRC Mass
Spectrometry Service (Swansea, UK) and Rapra Smithers Ltd
(GPC on polyethylene samples) are thanked for data collection.
EPSRC National Crystallography Service (Southampton) is
thanked for diffraction data (complexes 2 and 13).
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