Activated Niobium and Tantalum Imido Complexes: From Tuneable Polymerization to Selective Ethylene Dimerization Systems

Article abstract of DOI:10.1002/cctc.201801849

The niobium and tantalum imido complexes [CpMCl₂(NDipp)], [MCl₃(NR)(dme)] (R=^tBu, Ph, 2,6-ⁱPr₂C₆H₃ (Dipp), and Mes), and [TaCl₃(NDipp)(tmeda)] were tested in combination with EtAlCl₂ for the dimerization of ethylene. The niobium systems afforded dimers or polymers, depending on the nature of the imido ligand, with overall productivities in the range 720 to 13,720 (mol C₂H₄)(mol Nb)^?1. The nature of the polyethylene produced (LDPE or HDPE) depended on the imido ligand and the niobium concentration at which catalysis was run. In contrast, the tantalum/dme systems all mediated ethylene dimerization with productivities of up to 4,503 (mol C₂H₄)(mol Ta)^?1, with overall selectivities to butenes of between 73–81 wt %; selectivity within the dimer fraction to 1-butene was in the range 72 to 100 %. The productivity of [TaCl₃(NDipp)(tmeda)] was six times higher than that of its dme-bearing counterpart, but at the cost of selectivity to 1-butene. For the tantalum imido-mediated ethylene dimerization the composition of the product slate formed is indicative of a metallacyclic mechanism being operative.

Full text of DOI:10.1002/cctc.201801849

Accepted Article
Title: Activated niobium and tantalum imido complexes: from tuneable
polymerization to selective ethylene dimerization systems
Authors: Antonis Messinis, Andrei Batsanov, Judith Howard, Martin
Hanton, and Philip William Dyer
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10.1002/cctc.201801849
ChemCatChem
FULL PAPER
Activated niobium and tantalum imido complexes: from tuneable
polymerization to selective ethylene dimerization systems
Antonis M. Messinis,[a],[b] Andrei S. Batsanov,[b] Judith A. K. Howard,[b] Martin J. Hanton,*[c],[d] and Philip
W. Dyer *[a],[b]
Abstract: The niobium and tantalum imido complexes
[CpMCl2(NDipp)], [MCl3(NR)(dme)] (R = tBu, Ph, 2,6-iPr2C6H3 (Dipp),
and Mes), and [TaCl3(NDipp)(tmeda)] were tested in combination with
EtAlCl2 for the dimerization of ethylene. The niobium systems afforded
dimers or polymers, depending on the nature of the imido ligand, with
overall productivities in the range 720 to 13,720 (mol C2H4)(mol Nb)-1.
The nature of the polyethylene produced (LDPE or HDPE) depended
on the imido ligand and the niobium concentration at which catalysis
was run. In contrast, the tantalum/dme systems all mediated ethylene
dimerization with productivities of up to 4,503 (mol C2H4)(mol Ta)–1,
with overall selectivities to butenes of between 73 - 81 wt%; selectivity
within the dimer fraction to 1-butene was in the range 72 to 100%. The
productivity of [TaCl3(NDipp)(tmeda)] was six times higher than that
of its dme-bearing counterpart, but at the cost of selectivity to 1-
butene. For the tantalum imido-mediated ethylene dimerization the
composition of the product slate formed is indicative of a metallacyclic
mechanism being operative.
intermediates used in the manufacture of flavours, perfumes, and
pharmaceuticals.[3]
Due to the diverse, high-demand applications of LAOs, their
efficient and economic manufacture is, and is likely to remain, an
area of intense research. Indeed, to date a variety of ethylene
dimerization and oligomerization systems have been developed
employing mainly homogeneous, but also a limited number of
heterogeneous catalysts,[4] which together encompass a range of
metals including nickel,[5] cobalt,[6] iron,[6a] palladium,[7] platinum,[8]
chromium,[9] molybdenum,[10] and vanadium.[11] Currently, the
most industrially relevant ethylene dimerization system is that
employed in Axens’ AlphaButol process. This comprises a soluble
catalyst derived from [Ti(OiPr)4]/triethylaluminium, operating at
50-55 °C and 20-27 bar ethylene pressure.[12] The dimerization
activities obtained in the AlphaButol process are moderate, lying
in the range 170 to 16,550 (mol C2H4)(mol Ti)-1h-1. Nevertheless,
the lack of high catalytic activity is offset by the high selectivity of
the AlphaButol system towards 1-butene (93-95%), which is
accompanied by only low levels of polymer formation (<0.5
wt%),[12a, 12c-j] a factor that is significant since it helps minimize
reactor fouling and maximises atom economy.[13]
Introduction
Following our study of the olefin dimerization performance
of various molybdenum imido-derived catalytic systems,[10] we
reported recently the use of a series of analogous tungsten imido
pre-catalysts for the conversion of ethylene to 1-butene.
Compared to their lighter group 6 congeners these tungsten
systems display much higher activity (up to 409,410 (mol
C2H4)(mol W)-1h-1) and selectivity (up to 85.5 wt% in 1-butene).[14]
With these results in hand, we wanted to explore if this
periodic performance trend was evident for other transition metal
imido dimerization catalyst systems. In this context, Nomura and
co-workers have extensively described the application of
vanadium imido pre-catalysts.[11] However, to the best of our
knowledge, there are no reports of niobium-containing
dimerization catalysts, and only scarce examples of tantalum-
based systems.[15] Given that versatile syntheses of both niobium
and tantalum imido complexes are well established,[16] herein we
explore the applicability of such niobium and tantalum imido
complexes in catalytic ethylene dimerization in combination with
EtAlCl2. In particular, the influence of the readily varied
organoimido substituent is explored.
The industrial importance of linear alpha olefins (LAOs) is well
established. LAOs are used in the production of a wide range of
everyday products such as synthetic lubricants, plasticizer and
detergent alcohols, synthetic fatty acids, and polymers.[1]
Consequently, the annual production of LAOs is significant at
around 5 million metric tons, with a market worth of USD 14 billion
in 2014.[1d] Of the various LAOs produced on an industrial scale,
1-butene is the most widely used, primarily as a co-monomer in
the production of polyethylene,[2] but also in the production of
methyl ethyl ketone and valeraldehyde, which are key
[a]
Dr Antonis M. Messinis and Dr Philip W. Dyer
Center for Sustainable Chemical Processes,
Department of Chemistry
Durham University
South Road, Durham, DH1 3LE, UK
E-mail: p.w.dyer@durham.ac.uk
Dr Andrei S. Batsanov, Prof. Judith A. K. Howard
Department of Chemistry
[b]
Durham University
South Road, Durham, DH1 3LE, UK
Dr Martin J. Hanton
Sasol UK Ltd
Purdie Building, North Haugh, St Andrews, Fife, KY16 9ST, U.K.
Current address: TÜV SÜD Ltd, East Kilbride, Glasgow, G75 0QF,
UK
[c]
[d]
Results and Discussion
Synthesis and characterization of complexes
E-mail: Martin.Hanton@tuv-sud.co.uk
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201801849
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FULL PAPER
The niobium and tantalum imido complexes 1-4 were synthesized
according to literature procedures (or adaptations thereof) from
the respective pentahalides (Scheme 1).[16] Previously unreported
complexes 2c and 4c were structurally characterized by single-
crystal X-ray diffraction (Figure 1), together with 2b, 4a and two
polymorphs of 4b (see SI).[16a]
compared to the clearly less bulky NMes group in complexes 2c
and 4c) due to the lack of symmetry of the Dipp moiety, which
results in the isopropyl methyl groups being located away from
the metal in the solid state molecular structures used to
calculate %VBur. In solution, rotation of the Dipp motif will result in
increased steric demands around the metal centre. Analysis of
the computed topographic steric maps (Table S1),[18a] indicated
that the organoimido substituents R are remote from the metal
centre (negative scale in the steric maps, Figure S14).
Consequently, the direct steric pressure of the imido ligands on
the other ligands bound at the metal will be minimal, although the
organic substituents clearly occupy different regions of space
within the metals’ wider coordination sphere.
i. CpSn(nBu)3
1: M = Nb
3: M = Ta
M
MCl5
DippN
Cl
ii. DippNHTMS
Cl
Cl
RNH2, 2 ZnCl2, 2 py
dme
O
2: M = Nb
4: M = Ta
Table 1. Analysis of the steric impact of organoimido ligands in complexes
2a-d and 4a-d using the %VBur approach, estimation of the trans influence,
ΔdMO, and the degree of distortion away from true octahedral geometry,
RN M O
Cl
Cl
R = tBu (a)
Ph (b)
δ(M).
[a]
Complex
%VBur
18.7
17.0
21.1
δ(M), Å [b]
ΔdMO, Å [d]
0.202(8)
0.132(2)
Ref.
Mes (c)
Dipp (d)
[NbCl3(NtBu)(dme)] (2a)
[NbCl3(NPh)(dme)] (2b)
[NbCl3(NMes)(dme)] (2c)
0.329 [c]
[5]
0.327
this work
0.325 [c]
0.328
0.153(2) [c] this work
Scheme 1. Synthesis of niobium and tantalum imido complexes 1–4, using
[NbCl3(NDipp)(dme)] (2d) 21.8
0.120(5)
0.222(3)
0.160(5) [c] this work
[6]
previously reported procedures or modifications thereof.[16]
[TaCl3(NtBu)(dme)] (4a)
[TaCl3(NPh)(dme)] (4b)
[TaCl3(NMes)(dme)] (4c)
[TaCl3(NDipp)(dme)] (4d)
19.1
16.8
21.0
21.6
0.375
0.335 [c]
this work
0.347
0.191(6)
0.128(8)
this work
[6]
The complexes 2a-d and 4a-d (Tables S5, S6) were found
to all have distorted octahedral coordination about the metal atom,
with a mer arrangement of the Cl ligands. Quantitatively, the
distortion was described by the displacement of the metal atom
from the Cl(1)Cl(2)Cl(3)O(2) plane towards the N atom (0.33-0.38
Å), Table 1. The metal-nitrogen distances are all typical of pseudo
triple bonds, lying between 1.72 and 1.77 Å.[17] The shortest M-N
distances were found for the tBu imido complexes 2a (1.722(2) Å)
and 4a (1.747(3) Å), with those of the aryl-substituted derivatives
being longer due to conjugation between the aromatic ring and
M≡N π-systems (evidenced by short N-C bonds).
0.340
[a] Values calculated using the SambVca software [19] and the parameters
defined in the SI. [b] Displacement of the M atom towards N from the plane
of four cis-ligands, Lcis = Cl(1), Cl(2), Cl(3) and O(2) in the present work. [c]
Average of independent molecules. [d] degree of lengthening (ΔdMO) of the
trans M-O(1) versus the cis M-O(2) bonds (Tables S5, S6).
The relative differences in the trans influence of a wide
range of ligands, including imido groups, have been used
extensively to assess ligand electronic and, to an extent, steric
demands.[17, 20] Consequently, the trans influence of the imido
motifs in the two series of structurally analogous complexes 2a-d
and 4a-d were assessed. Given the deviation from ideal
octahedral geometry for these mer-[MCl3(NR)(dme)] complexes,
in particular the deviation from linearity of the trans-N-M-O bond
angle (~167-171°), the trans influences were estimated from the
degree of lengthening (ΔdMO) of the trans M-O(1) versus the cis
M-O(2) bonds (Tables S5, S6). The statistically significant values
of ΔdMO for both the niobium (2a-d) and tantalum (4a-d)
complexes vary from 0.12 to 0.22 Å, with values for both series
increasing in the order NDipp < NPh < NMes < NtBu. This
experimentally-determined trans influence ordering exactly
parallels the electronic characteristics of the four different
organoimido motifs as assessed from the pKa values of the parent
In assessing the catalytic dimerization performance of the
various imido complexes we have tested here, we sought to
establish
a
correlation with the steric and electronic
characteristics of the various organoimido (NR) motifs. The steric
demands of each of the imido substituents was assessed using
the percentage buried volume (%VBur) and topographic steric map
approaches approach developed by Cavallo and co-workers.[18]
This methodology imbeds the ligand of interest within a sphere of
set radius from the metal centre. From the van der Waals radius
of each atom within this sphere, the percentage volume occupied
by the ligand (%VBur) was calculated from the experimentally-
determined X-ray crystallographic data using the free online
software, SambVca.[19] The parameters used to calculate
the %VBur for the various organoimido fragments using SambVca
are described in the SI.
+
aminimum ions (RNH3 ): R = Dipp (4.38), Ph (4.64), Mes (4.65),
and tBu (10.71).[21] Although it has been reported previously that
the origins of the trans influence of imido ligands in an octahedral
coordination manifold are complex (including π-electronic effects
and electronic repulsion),[22] our observations are consistent with
electronic factors being the principal component contributing to
The %VBur analysis revealed the steric demands of the
various imido substituents to lie in the order NPh < tBuN < NMes
< NDipp for both series of complexes 2a-d and 4a-d, albeit with
only modest differences (Table 1). However, it must be noted that
the %VBur values calculated for the NDipp imido complexes 2d
and 4d are likely to be underestimated (especially when
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10.1002/cctc.201801849
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Figure 1. X-Ray molecular structures of [NbCl3(NMes)(dme)] (2c, top, one of
the independent molecules) and [TaCl3(NMes)(dme)] (4c). Thermal ellipsoids
are shown at the 50% probability level.
the variation in trans influence of imido ligands of complexes 2a-
d and 4a-d.[23]
Catalyst testing
The catalytic ethylene oligomerization performance of the niobium
and tantalum complexes
1 to 4 was screened using 15
equivalents of EtAlCl2 as activator at 60 °C, 40 barg ethylene
pressure, and with chlorobenzene as solvent (Table 2). The
reaction conditions employed here mirror those previously
optimised for related industrially-competitive tungsten imido-
based dimerization systems, thus allowing direct comparison of
14, 24]
the performance of the Nb and Ta imido complexes.[10,
Utilisation of chlorobenzene as reaction medium for the catalytic
testing was adopted since we have previously demonstrated that
this was the optimal solvent for ethylene dimerisation mediated by
tungsten imido pre-catalysts and hence its choice facilitates a
direct comparison of the performance of the group 5 and 6
catalyst systems; note, catalyst optimisation was not a focus of
24]
this current study.[14,
Use of EtAlCl2 rather than less well-
defined aluminoxane-type activators (e.g. MAO or MMAO) was
chosen to avoid differences in performance arising from well-
documented batch-to-batch variations for the latter.[25] Each
Table 2. Results from catalytic ethylene dimerization tests conducted at 40 barg ethylene pressure and 60 °C with niobium (1, 2a-d) and tantalum (3, 4a-d)
complexes and EtAlCl2 as activator.[a]
Selectivity within the liquid fraction[d, e]
Time,
min
Product
mass, g
PE,
wt%
Run #
Pre-catalyst
TON[b]
Activity[c]
Linear
C6 in
C6, %
C4, wt%
(1-C4 in C4, %)
C8+,
wt%
C6, wt%
(1-C6 in C6, %)
1-C4,
wt%
[CpNbCl2(NDipp)] (1)
[NbCl3(NtBu)(dme)] (2a)
[NbCl3(NPh)(dme)] (2b)
[NbCl3(NMes)(dme)] (2c)
[NbCl3(NDipp)(dme)] (2d)
[NbCl3(NDipp)(dme)] (2d) [f]
[CpTaCl3(NDipp)] (3)
1
2
10
3.4
7.5
2.3
7.9
7.8
10
720
4,600
8,600
2,290
13,720
9,450
300
4,340
81,190
68,610
59,830
104,200
72,380
1,790
0.406
2.581
4.825
1.287
7.698
1.325
0.167
2.123
1.573
1.349
2.527
14.839
1.1
62.7
5.2
97.9 (97.9)
75.8 (97.8)
92.9 (85.6)
93.5 (94.7)
8.8 (98.6)
62.7 (98.0)
73.0 (100)
79.8 (100)
80.7 (100)
77.6 (99.8)
79.3 (100)
79.6 (72.4)
95.8
74.1
79.5
88.5
8.7
2.1 (0.00)
7.4 (100)
-
0.0
16.8
0.9
92.8
85.1
90.7
85.0
100.0
85.1
72.4
81.7
80.7
68.9
79.5
3
6.2 (35.6)
4.4 (69.6)
4.2 (92.0)
5.3 (88.1)
15.4 (74.0)
19.3 (67.4)
17.9 (77.8)
21.2 (75.6)
17.2 (64.1)
15.1 (33.2)
4
25.1
70.7
60.2
6.0
2.1
5
87.0
32.0
11.6
0.9
6
61.4
73.0
79.8
80.7
77.4
79.3
57.6
7
[TaCl3(NtBu)(dme)] (4a)
[TaCl3(NPh)(dme)] (4b)
[TaCl3(NMes)(dme)] (4c)
[TaCl3(NDipp)(dme)] (4d)
[TaCl3(NDipp)(tmeda)] (5)
8
6.5
5.6
5
3,780
2,800
2,410
4,500
26,450
34,970
29,940
28,590
35,710
21,160
2.7
9
2.2
1.4
10
11
12
7.0
1.2
7.6
75
8.8
3.5
0.0
5.3
[a] Conditions: 20 μmol metal complex; 300 μmol EtAlCl2; PhCl 74 mL; 60 °C; ethylene pressure (40 barg); stirrer speed 1000 rpm; nonane standard (1.000 mL);
catalytic runs were performed until consumption of C2H4 dropped below 0.2 g min−1 or until the reactor was filled. [b] TON (productivity) is reported in (mol C2H4)(mol
M)−1. [c] Activity is reported in (mol C2H4)(mol M)−1 h−1. [d] Selectivity to dimers, trimers, and oligomers within the total soluble product fraction expressed in wt%.
The % content of the linear terminal olefin within the dimer and trimer fractions are in parentheses. [e] Investigation of catalytic reproducibility and error analyses
are detailed in Ref. [14b]. [f] 5.0 μmol of complex and 75.0 μmol of EtAlCl2 were used.
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10.1002/cctc.201801849
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FULL PAPER
catalyst test was allowed to run until ethylene uptake dropped
below the detection limit (0.2 g min−1), to gain information on the
lifetime of the catalytic system. Only reporting activities can be
misleading in terms of overall catalytic performance, with very
high activities being achieved by stopping the reaction just after
the most productive phase of catalysis.
Since the niobium complex 2d afforded significant amounts
of polyethylene, something that has a direct impact on the
viscosity of the reaction medium, the influence of mass transfer
effects on the organic product distribution was probed by
repeating the test described in Run 5 (Table 2) at a higher dilution
(Run 6). This reduction in niobium concentration led to a decrease
in the overall catalytic activity and productivity (TON) from
104,199 to 72,375 (mol C2H4)(mol Nb)−1 h−1 and 13,720 to 9,449
(mol C2H4)(mol Nb)−1, respectively, instead of the anticipated
increase due to the improved mass transfer of ethylene. This drop
in performance in Run 6 was accompanied by a slight decrease
in the selectivity towards polyethylene (from 70.7 to 60.2 wt%),
while the amount of butene within the liquid fraction dramatically
increased from 8.7 to 61.4 wt% at the expense of C8+ oligomers.
Ethylene dimerization with niobium imido complexes
The organic products produced using each of the niobium pre-
catalysts 1,2a-d were polyethylene, butenes, and small amounts
of hexenes in ratios heavily dependent on the metal’s substitution
pattern (Table 2, Runs 1-6). Nevertheless, the selectivity towards
1-butene within the C4 fraction was high in all cases, ranging
between 85.6% and 98.6%, indicating that little 1-butene
isomerization occurred during catalysis. Complex 1 exhibited the
lowest activity and productivity within the series of niobium imido
complexes screened, 4,342 (mol C2H4)(mol Nb)−1 h−1 and 724
(mol C2H4)(mol Nb)−1, respectively (Run 1). Like complex 1, the
structurally similar complex [CpNbCl2(N-2-tBu-C6H4)] was also a
poor ethylene polymerization catalyst when activated with 30
equivalents of Et2AlCl.[26] Similar low polymerization activity was
seen with other CpNb-based catalysts.[15f, 27] Consideration of the
computed topographic steric maps for complexes 1 and 2d (Table
S1) very clearly demonstrated the geometric and steric
constraints imposed on the incoming substrate olefin for 1
consistent with its low activity. The poor activity achieved using
pre-catalyst 1 is consistent with its high selectivity towards 1-
butene; the low concentration of 1-butene present limited re-
incorporation/oligomerization.[28] Although steric constraints are
likely to significantly influence the catalytic performance of 1,
electronic effects from the Cp ligand cannot however be excluded.
The catalytic behaviour of pre-catalysts 2 was dependent on
the nature of the imido substituent: 2b produced mainly butenes
(Table 2, Run 3), 2a and 2d showed selectivity to polymer over
butenes, (Table 2 Runs 2 and 5), while 2c covered the middle
ground between dimerization and polymerization with 25.1 wt% of
the products comprising polyethylene (Table 2, Run 4). The
polymerization activities observed with pre-catalysts 2 were
comparable to those of previously reported niobium-based
ethylene polymerization systems.[27, 29]
For pre-catalysts 2b-d (Figure 2) as the steric demands of
the imido substituents were increased (NPh < NMes < NDipp; see
Table 1), so the extent of oligomer (>C4) and polymer formation
increased (Figure 2). This trend is attributed mainly to steric
effects since not only are the structures of complexes 2b-d
identical (vide supra), but the electronic characteristics of the
NDipp, NMes, and NPh groups can be regarded as near-
equivalent based on the pKa values (in water) of the
corresponding anilines (PhNH2, MesNH2, and DippNH2), which lie
in the range 4.0 and 4.6.[21] In contrast, although the organoimido
moiety of the pre-catalyst [NbCl3(NtBu)(dme)] (2a) occupies a
slightly greater volume than that of the imido unit in the
phenylimido derivative 2b, %VBur = 18.7 (tBu) and 17.0 (Ph),
something that might be anticipated to favour polymerization (vide
infra), the tBuN moiety (pKa tBuNH2 = 10.7) is significantly more
electron-rich than the PhN unit and hence the origins of the
product selectivity could not be attributed definitively (c.f. Runs 2
and 3).
Figure 2. Comparison of catalytic activity towards polyethylene, butenes,
hexenes, and oligomers as a function of organoimido ligand substituent of the
niobium pre-catalysts 2a-d.
The properties of the insoluble polyethylenes produced using
each of the pre-catalysts/conditions were probed by DSC
(Differential Scanning Calorimetry) analysis (Table S2). The
materials isolated from each of the Runs 2-4 (pre-catalysts 2a-c)
were characteristic of HDPE.[30] In contrast, the nature of the
polyethylene produced using complex 2d (Table S2, Run 5) was
consistent with it being a blend of HDPE rich in LDPE.[31] However,
when the catalysis test was repeated at a lower concentration of
complex 2d (Table 2, Run 6) HDPE is produced with the highest
peak melting temperature, density, and crystallinity observed
within the series (Table S2). Thus, the catalytic behaviour of the
Nb imido complexes 2a-d with EtAlCl2 as activator can be tuned
not only in terms of the product selectivity (1-butene versus
polyethylene), but also in terms of the nature of the polyethylene
produced.
From a mechanistic perspective it is not possible for a single
catalytically-active species to simultaneously selectively produce
dimers and polymers under the same operating conditions (e.g.
2c). Consequently, the variations in product distributions
achieved using pre-catalysts 2a-d were attributed to the formation
of dimerization-, oligomerization-, and polymerization-active
species in ratios that are governed by the nature of the
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10.1002/cctc.201801849
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organoimido substituent. This helps explain the decreased
formation of LDPE, which occured on lowering the pre-catalyst
2d/15 EtAlCl2 concentration (Table 2, Runs 5 and 6). It is
speculated that the ethylene polymerization catalyst formed
during Run 5 is able to co-polymerize ethylene with the 1-butene
and oligomers produced in situ from the ethylene oligomerization
catalyst that forms simultaneously upon activation (Scheme 2,
top). This type of in situ co-polymerization (also known as
concurrent tandem catalysis) is well documented.[32] At a lower
niobium concentration (Table 2, Run 6) less active
oligomerization and polymerization catalysts (lower overall
catalytic activity) were formed, which being less capable of
incorporating α-olefins lead to selective formation of 1-butene and
HDPE (low levels of branching), respectively (Scheme 2, bottom).
Note, the differences in composition of the polymeric materials
produced using 2d at different concentrations could also be
attributed to chain walking at higher concentration, leading to
formation of the observed LDPE. Although this would be
consistent with a number of reports in which late transition metal-
based polymerisation catalysts have been shown to afford LDPE
via such a chain walking mechanism (Scheme 3), very few
R
P
P
Co-polymer
R
ethylene +
co-monomer
LnM
P
Co-polymerization
R
R
R
P
LnM
Co-monomer
insertion
P
P
LnM
Propagating polymer
H
examples of such behaviour are known for early transition metal
β-H elimination
LnM
34]
systems and is hence unlikely here.[33,
Furthermore, the
niobium systems 2 also gave reasonable selectivity to 1-butene
and linear hexenes (e.g. 35.6% 1-hexene selectivity, Table 1,
entry 3), whose formation is at odds with chain walking. To help
discriminate between a co-polymerisation or a chain walking
pathway, or indeed the possibility that both types of mechanism
were operative, the structure of the resulting LDPE should be
re-insertion
P
Chain Walking
LnM
1
studied by H/13C NMR spectroscopy. If the LDPE product only
contains ethyl and butyl branches, a chain walking mechanism
β-H elimination/
re-insertion cycles
34]
can be eliminated.[33,
However, attempts to undertake
spectroscopic analysis of the polymeric materials obtained was
precluded by their incomplete solubility in either 1,1,2,2-
tetrachlorethane (TCE) or trichlorobenzene even at elevated
temperatures.
P
P
P
Scheme 3. Comparison of co-polymerization and chain walking pathways for
the formation of LDPE.
Ethylene dimerization with tantalum imido complexes
In contrast to the niobium imido pre-catalysts, their corresponding
tantalum analogues 3 and 4 performed as dedicated ethylene
dimerization systems (Table 2, Runs 7-12) upon activation with
15 equivalents of EtAlCl2. Depending on the imido substituent,
activities between 1,786 and 35,709 (mol C2H4)(mol Ta)−1 h−1
were achieved. However, these values are less than half those
achieved using the analogous niobium pre-catalysts. This
parallels reports in the literature that identify the same trends in
relative performance between analogous Nb and Ta
polymerization systems.[30b, 35]
The selectivities towards butenes in the liquid fraction
achieved using pre-catalysts 3, 4a-d in combination with EtAlCl2
were found to be between 77.6 and 80.7 wt%, with low levels of
polymer formation (2.2 - 8.8 wt% of the total products fraction).
Scheme 2. Proposed concentration-dependent behaviour of pre-catalysts 2d:
at 20 μmol (top) production of LDPE along with oligomers; at 5 μmol (bottom)
formation of 1-butene and HDPE.
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10.1002/cctc.201801849
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However, the most distinctive feature of these tantalum imido
systems is their very high selectivity towards butenes, with 1-
butene being formed in most cases (Table 2, Runs 7-11); here the
predominant by-products are hexenes. This is of key importance
considering that 1-butene of high purity is necessary for polymer
applications and that separation of C4 components is costly,
whereas separation of C4 and C6+ fractions is comparatively
attributable to both complexes 1 and 3 having very congested
coordination spheres as identified from the topographical steric
maps (Table S1). In contrast to the behaviour determined for the
niobium-based pre-catalysts 2a-d, changing the nature of the
organoimido ligands of the analogous tantalum systems 4a-d has
a less pronounced effect on catalytic performance.
The catalytic performance of the dme complex (4d) versus
straightforward, hence economically/industrially viable.[12f] Indeed, that of its tmeda (5) counterpart were markedly different (Table 2:
this type of situation is clearly exemplified by Axens’ Alphabutol
process where achieving high selectivity rather than high activity
is considered more important from an economic standpoint (vide
infra).
Run 11 vs Run 12). Although catalysis using pre-catalyst 5 led to
a slight decrease in catalytic activity compared with that achieved
using 4d, the productivity increased six-fold with no polymer
produced, while the selectivity towards the C4 and C6 fractions
remained practically unchanged. Unfortunately, the above
improvements in catalytic performance derived from the use of
tmeda- rather than dme-substituted pre-catalysts came at the cost
of selectivity towards 1-butene, which dropped from 100% to
72.4%. This difference in catalytic performance between pre-
catalysts 4d and 5 in combination with EtAlCl2 demonstrate that
the Lewis base that stabilizes the tantalum imido complexes (i.e.
dme and tmeda) can significantly impact catalysis and is not
simply displaced from the metal centre and immediately
sequestered by the aluminium activator (as observed previously
for related molybdenum bis(imido)-derived oligomerization
catalysts[10]), hinting at the complexity of the underlying
mechanism. However, it should be kept in mind that M-Calkyl bond
strengths increase significantly down a group something that will
render migratory insertion and reductive elimination processes
less favourable for the tantalum systems over the niobium
counterparts and hence likely reduce catalytic activity and the
preference for polymerisation, consistent with the observed
performance (Table 2).[36]
Figure 3. Comparison of catalytic activity and productivity as a function of imido
ligand substituent for the tantalum pre-catalysts 4a-d.
In contrast to the niobium imido pre-catalysts, their tantalum
counterparts favour highly selective dimerization. This makes it
easier to draw some preliminary conclusions on their mode of
operation. When exploring the mechanism of any ethylene
oligomerization system the principle pathways to be considered
are either a metallacyclic[30a, 37] or a step-wise addition process.[38]
From an analysis of the organic products generated by the
tantalum imido-based catalysts examined herein (4a-d, 5), the
As observed for [CpNbCl2(NDipp)] (1) the analogous
tantalum complex [CpTaCl2(NDipp)] (3) exhibits significantly
lower activity and productivity compared to that delivered by its
cyclopentadienyl-free counterparts 4a-d (Table 2). This is
Table 3. Comparison of the expected and experimentally-observed products
produced from various oligomerization mechanisms.[a]
principal dimerization mechanism is consistent with
a
metallacyclic pathway. Firstly, the complete selectivity of the
tantalum imido systems towards 1-butene coupled with the high
selectivity towards 1-hexene in the trimer fraction is entirely
consistent with a metallacycle mechanism, which is generally
considered to be more selective than the alternative step-wise
addition pathway.[38, 39] Secondly, the identities of the di- and tri-
meric organic products obtained using the tantalum imido pre-
catalysts match well with the products that are predicted
theoretically to be produced from a metallacyclic pathway rather
than those from a step-wise addition process (Table 3). Finally,
the synthesis and isolation of tantallacyclopentane derivatives
has been reported, something exemplifying their potential
formation here under our catalytic test conditions.[40] Nevertheless,
it must be noted that a full mechanistic study of this system would
be necessary in order to definitively determine the mechanism
behind the tantalum imido-mediated dimerization of ethylene,
something outside the scope of this preliminary study.[41]
Int.
C6
3 Me-
1-C5
3 Me-
2-C5
2 Et-
1-C4
1-C4
2-C4
1-C6
Step-wise
addition with
chain walking
✔
✔
✔
✔
✔
✔
✔
✔
✔
Step-wise
addition
without chain
walking
✔
✔
Metallacycle
Observed
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
[a] See Scheme S1 and S2 for an explanation of how each pathway leads to
production of the different products. 1-C4 = 1-butene, 2-C4 = 2-butene, 1-C6 =
1-hexene, Int. C6 = internal hexenes, 3 Me-1-C5 = 3 methyl-1-pentene, 3 Me-
2-C5 = 3 methyl-2-pentene, 2 Et-1-C4 = 2 ethyl-1-butene.
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10.1002/cctc.201801849
ChemCatChem
FULL PAPER
shift of the solvent (13C) or to external 85% H3PO4 aqueous solution
(
Conclusions
31P).[42] 1H and 13C NMR spectra were assigned with the aid of COSY,
HSQC and HMBC experiments. Chemical shifts are reported in ppm and
coupling constants in Hz. GC-FID analyses were conducted on a Perkin
Elmer Clarus 400 GC or an Agilent Technologies 6890N GC instrument,
both equipped with a PONA column (50 m × 0.20 mm × 0.50 µm) and
supplied with H2 as a carrier gas. Hydrogenative GC-FID analysis was
performed using an Agilent Technologies 6890N or a Perkin Elmer Clarus
400 GC System equipped with an inlet liner packed with hydrogenating
catalyst (Pt on Chromosorb W at 200 oC) and PONA column (50 m × 0.20
mm × 0.50 µm).[43] DSC analysis of polyethylene was performed on a TA
Instruments DSC Q1000. The samples were heated from room
temperature to 200 °C with a heating rate of 10 °C/min. The temperature
of each sample was held constant at 200 °C for five minutes in order to
erase its thermal history and then cooled down to 10 °C with a cooling rate
of 10 °C/min. The samples were then kept at 10 °C for five minutes
followed by re-heating at 10 °C/min to 200 °C. The peak melting
temperature and the crystallinity were determined from the final heating
cycle,[30a] while the density of each resin was calculated from the peak
melting temperature according to literature procedures.[44] Traditional GPC
analysis of the polyethylene samples was not possible since samples were
either insoluble or, at best, only partially soluble in 1,2-dichlorobenzene or
in 1,1,2,2-tetrachloroethane (even at 150 °C). Catalysis experiments were
performed in a batch fashion in 250 mL volume Buchi Miniclaves or in a
1.2 L volume Premex Pinto autoclave equipped with stainless steel vessels
Despite their structural similarity, niobium and tantalum imido pre-
catalysts of the type MCl3(NR)(dme) mediate ethylene
oligomerization in strikingly different ways upon activation with
EtAlCl2. Although their overall catalytic activity is only moderate,
these systems offer unusual product selectivities. Depending on
the nature of the organoimido substituent the niobium complexes
2a-d either instigate dimerization or polymerization of ethylene,
with greater steric bulk favouring the latter. Additionally, the nature
of the polyethylene produced could be changed from LDPE to
HDPE simply by performing the catalysis at a lower niobium
concentration. In contrast, the analogous tantalum complexes
(4a-d, 5) function as selective ethylene dimerization pre-catalysts
with 100% selectivity towards formation of 1-butene, essentially
irrespective of the nature of the organoimido substituent, albeit
with lower overall activities compared to their niobium
counterparts. Lastly, based on the identity of the ethylene
dimerization and trimerization products formed, it is concluded
that the tantalum imido-mediated ethylene dimerization described
herein occurs via a metallacyclic rather than a step-wise addition
mechanism. Together these results exemplify not only the
electronic and steric impact of the organoimido substituent on
with integral thermal-fluid jackets (connected to
a Huber 405 W
ethylene oligomerization, which provides
a
straightforward
thermostatic bath), internal cooling coils (tap water), and mechanical
mixing via customized gas-entraining stirrers (1000 rpm). Dried and
degassed solvent was added via a gas-tight syringe under Ar (Grade 4.5,
supplied by Linde). Ethylene (Grade 4.5) was supplied by Linde and
passed through oxygen and moisture scrubbing columns prior to use;
ethylene flow was measured using a Siemens Sitrans F C Massflo system
(Mass 6000-Mass 2100) and the data logged. All catalytic tests were
allowed to run until ethylene uptake had dropped below 0.2 g min–1 or the
reactor had become filled with product.
method of reaction tuning, but also that the catalytic performance
of such systems is intimately influenced by the metal, a trend
mirrored across both group 5 and 6 systems. Indeed, these
results reveal that very subtle changes in pre-catalyst structure
can lead to dramatic differences in catalytic performance. Further
investigation is intended to further elucidate the specific function
of the imido ligands.
Synthesis of reagents and complexes
Experimental Section
The following compounds were all prepared according to previously
reported literature procedures or slight modifications thereof:
[CpSnnBu3],[45] DippNHTMS,[46] [CpNbCl4],[47] [CpNbCl2(NDipp)] (1),[16d]
General experimental procedures
All operations were conducted under an atmosphere of dry nitrogen using
standard Schlenk and cannula techniques, or in a Saffron Scientific
nitrogen-filled glove box, unless otherwise stated. Nitrogen gas was
passed through a drying column (silica/CaCO3/P2O5). Bulk solvents were
purified using an Innovative Technologies SPS facility, except for
chlorobenzene, which was dried by distillation from calcium hydride, and
for dme that was distilled under nitrogen from sodium/benzophenone. All
solvents were degassed prior to use using standard methods. CD2Cl2 and
CD3COCD3 were purchased from Goss Scientific and C6D6 from Apollo
Scientific. Each deuterated solvent was distilled from CaH2, degassed and
stored under nitrogen. EtAlCl2, TaCl5, NbCl3(dme), and nBuLi solution
(2.5M in hexanes) were purchased from Sigma-Aldrich and used as
received. Bu3SnCl was purchased from Sigma-Aldrich and degassed prior
to use. 2,6-iPr2-C6H3-NH2 (DippNH2) and tBuNH2 were purchased from
Sigma-Aldrich, distilled from CaH2 and degassed prior to use.
Cyclopentadiene was obtained via reactive distillation from
dicyclopentadiene, which was purchased from Sigma-Aldrich. NbCl5 and
ZnCl2 were purchased from Alfa-Aesar and used as received. Nonane,
MesNH2, PhNH2, and pyridine were purchased from Alfa-Aesar, dried over
calcium hydride, distilled and degassed prior to use. Solution phase NMR
spectra were collected on a Varian Mercury 400 or 200, a Varian Inova
500, a Varian VNMRS-700 or 600 and a Bruker Advance 400 at ambient
probe temperatures (290 K) unless otherwise stated. Chemical shifts were
[NbCl3(NtBu)(dme)]
[NbCl3(NDipp)(dme)] (2d),[16a] [CpTaCl4],[47] [CpTaCl2(NDipp)] (3),[16b]
[TaCl3(NtBu)(dme)] [TaCl3(NPh)(dme)]
(4a),[16a] (4b),[16a]
(2a),[16a]
[NbCl3(NPh)(dme)]
(2b),[16a]
[TaCl3(NDipp)(dme)] (4d),[16a] and [TaCl3(NDipp)(tmeda)] (5).[16c]
Successful synthesis was confirmed by comparison of the 1H and 13C NMR
spectra of the obtained product with the spectra reported in the literature.
[NbCl3(NMes)(dme)] (2c). An adaptation of the method described by
Korolev et al. was used:[16a] a Schlenk was loaded with NbCl5 (5.00 g, 18.5
mmol) and 1,2-dimethoxyethane (3.30 mL), with the resulting mixture
being stirred until the solids dissolved. ZnCl2 (5.02 g, 36.8 mmol) was
added to the solution to give a white precipitate. The mixture was then
cooled to –40 °C and a solution of MesNH2 (2.60 mL, 18.5 mmol) and
pyridine (2.98 mL, 36.8 mmol) in CH2Cl2 (20 mL) was added drop-wise.
After addition was complete, the white precipitate dissolved and the dark
purple solution generated was stirred at ambient temperature. After 17
hours of stirring a white precipitate formed, which was removed by filtration.
The filtrate was evaporated to dryness to give a purple solid, which was
extracted with hot CH2Cl2 until the extracts were lightly coloured. The
extracts were combined, condensed, and stored at –35 °C. After 17 hours
purple crystals formed, which were isolated, ground and dried under
reduced pressure to give 2c in the form of a purple powder in 63% yield
(4.93 g). Crystals of 2c suitable for an X-ray crystallographic analysis were
referenced to residual proton impurities in the deuterated solvent (1H), 13
C
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10.1002/cctc.201801849
ChemCatChem
FULL PAPER
obtained by layering a DCM solution of the compound with pentane. 1H
NMR (700 MHz, CDCl3) δ = 6.76 (s, 2H, Hm), 4.14 (m, 2H, OCH2A), 4.08(m,
2H, OCH2B), 3.98 (s, 3H, OCH3A), 3.95 (s, 3H, OCH3B), 2.79 (s, 6H, CoCH3),
2.30 (s, 3H, CpCH3); 13C NMR (176 MHz, CDCl3) δ = 151.15 (s, Cipso),
138.21 (s, Co), 137.04 (s, Cp), 128.15 (s, Cm), 75.67 (s, OCH2A), 70.77 (s,
OCH2B), 69.06 (s, OCH3A), 62.53 (s, OCH3B), 21.10 (s, CpCH3), 19.36 (s,
CoCH3); elemental analysis calcd (%) for C13H21Cl3NNbO2 (422.57): C
36.95, H 5.01, N 3.31; found: C 37.05, H 4.89, N 3.33.
Carswell (Durham University) are thanked for fruitful discussions
and for performing the DSC analyses, respectively.
Keywords: dimerization • ethylene • niobium • tantalum • imido
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ray crystallographic analysis were obtained by layering a concentrated
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= 6.85 (s, 2H, Hm), 4.22 (m, 2H, OCH2A), 4.12 (s, 3H, OCH3A), 4.08 (m, 2H,
OCH2B), 4.01 (s, 3H, OCH3B), 2.79 (s, 6H, CoCH3), 2.40 (s, 3H, CpCH3);
13C NMR (176 MHz, CDCl3) δ = 148.85 (s, Cipso), 138.52 (s, Co), 135.45 (s,
Cp), 127.51 (s, Cm), 76.12 (s, OCH2A), 71.00 (s, OCH3A), 70.31 (s, OCH2A),
63.00 (s, OCH3B), 20.71 (s, CpCH3), 19.14 (s, CoCH3); elemental analysis
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Dilute stock solutions (PhCl) of each of the pre-catalysts were prepared
and stored in glass ampoules fitted with J Youngs’ taps using standard
Schlenk techniques. The rigorously cleaned autoclave was heated (130
oC) under vacuum for 60 mins, then cooled to reaction temperature (60 °C)
and back-filled with ethylene (10 barg), which was then vented to 0 barg
via a septum to purge the inlet valve. Dried and degassed solvent (PhCl,
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sequentially via syringe. The autoclave was pressurized with ethylene to
10 barg and vented. The appropriate quantity (0.60 mL, 300 μmol) of a
stock solution of the activator EtAlCl2 (0.50 M in PhCl) was added to initiate
catalysis and the vessel was immediately pressurised with ethylene (40
barg). The pressure was kept constant throughout the reaction by the
continuous addition of ethylene, which was controlled and monitored via
the flow meter. Heating and cooling were controlled to maintain a stable
reaction temperature. Once ethylene uptake had dropped below 0.2 g
min−1, or the autoclave was filled by product, the gas supply was closed
and the reactor was cooled to and maintained at 5 °C or lower using an ice
bath. The reactor was then slowly and carefully vented with a portion of
the vent gas being directly fed to a GC-FID instrument equipped with gas-
sampling loop; where butenes were observed here by GC, the amount lost
was ≤5% of the total butenes content of the liquid fraction. The reactor
contents were treated with 1000 µL of nonane (GC internal standard) and
10% HCl (aq) and a sample of the cold reaction mixture was collected with
a cold (–30 °C) pipette and extracted into cold (-30 °C) toluene. A sample
of the organic phase was immediately taken for GC-FID analysis as well
as hydrogenative GC-FID analysis for determination of the branching
selectivity in the trimers fraction. Any precipitated polyethylene formed was
collected by filtration, washed first with 10% HCl (aq) and then repeatedly
with EtOH to remove all metal-containing salts, then finally washed with
acetone before drying to constant weight overnight at 100 °C and weighed.
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Acknowledgements
The authors thank Sasol Group Technology, the EPSRC, and
Durham University for funding and permission to publish this work.
Dr David Smith (Sasol Group Technology) and Mr Douglas
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10.1002/cctc.201801849
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FULL PAPER
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This article is protected by copyright. All rights reserved.

10.1002/cctc.201801849
ChemCatChem
FULL PAPER
Entry for the Table of Contents
FULL PAPER
A. M. Messinis, A. S. Batsanov, J. A. K.
Howard, M. J. Hanton,* P. W. Dyer*
M = Nb
Cl
Polyethylene +
+ C6
(+ C6)
O
RN M O
Page No. – Page No.
EtAlCl2
Cl
Cl
Activated niobium and tantalum imido
complexes: from tuneable
M = Ta
polymerization to selective ethylene
dimerization systems
Tuning catalysis: Activation of niobium mono(imido) complexes with EtAlCl2
affords catalysts that produce dimers or polyethylene depending on the nature of
the imido substituent. In contrast, the analogous tantalum systems, irrespective of
the organoimido motif, mediate predominantly ethylene di- and tri-merization with
overall selectivities to butenes of between 73.0 - 80.7 wt% and selectivity within the
dimer fraction to 1-butene in the range 72.4 to 100%.
This article is protected by copyright. All rights reserved.