Ethylene oligomerisation chromium catalysts with unsymmetrical PCNP ligands

Article abstract of DOI:10.1039/d1dt00287b

Chromium(iii) complexes of chelating diphosphines, with PNP or PCNCP backbones, are excellent catalysts for ethylene tetra- and/or trimerisations. A missing link within this ligand series are unsymmetric chelating diphosphines based on a PCNP scaffold. New bidentate PCNP ligands of the type Ph₂PCH₂N(R)PPh₂(R = 1-naphthyl or 5-quinoline groups,2a-d) have been synthesised and shown to be extremely effective ligands for ethylene tri-/tetramerisations. Three representative tetracarbonyl Cr⁰complexes bearing a single PN(R)P (5), PCN(R)P (6), or PCN(R)CP (7) diphosphine (R = 1-naphthyl) have been prepared from Cr(CO)₄(η⁴-nbd) (nbd = norbornadiene). Furthermore we report a single crystal X-ray diffraction study of these compounds and discuss their structural parameters.

Full text of DOI:10.1039/d1dt00287b

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PAPER
Ethylene oligomerisation chromium catalysts with
unsymmetrical PCNP ligands†
Kevin Blann,a Annette Bollmann,a Gavin M. Brown,b John T. Dixon,a
Mark R. J. Elsegood, b Christopher R. Raw,b Martin B. Smith, *b Kenny Tenza,a
J. Alexander Willemsea and Pumza Zwenia
Cite this: Dalton Trans., 2021, 50,
4345
Chromium(III) complexes of chelating diphosphines, with PNP or PCNCP backbones, are excellent cata-
lysts for ethylene tetra- and/or trimerisations. A missing link within this ligand series are unsymmetric che-
lating diphosphines based on a PCNP scaffold. New bidentate PCNP ligands of the type Ph2PCH2N(R)
PPh2 (R = 1-naphthyl or 5-quinoline groups, 2a–d) have been synthesised and shown to be extremely
effective ligands for ethylene tri-/tetramerisations. Three representative tetracarbonyl Cr0 complexes
bearing a single PN(R)P (5), PCN(R)P (6), or PCN(R)CP (7) diphosphine (R = 1-naphthyl) have been pre-
pared from Cr(CO)4(η4-nbd) (nbd = norbornadiene). Furthermore we report a single crystal X-ray diffrac-
tion study of these compounds and discuss their structural parameters.
Received 27th January 2021,
Accepted 2nd March 2021
DOI: 10.1039/d1dt00287b
rsc.li/dalton
ation of backbone groups including –NN–,14,15 –CC–16,17 and
–NSi– (shown in Chart 1).18
Introduction
There has been considerable interest in developing new homo-
Whilst significant advances in ligand design have aided
geneous catalysts for selective ethylene oligomerisations catalyst performance, there have also been considerable
affording, with high selectivity, linear alkenes such as computational19–22 and mechanistic23–25 efforts to probe the
1-hexene or 1-octene.1 This is largely due to the increase in nature of catalytically important Cr-based intermediates,
demands for commercial products based on polyethylene. and the origin of 1-hexene and 1-octene selectivities. As part of
Small bite angle ligands, based on a bidentate P–N–P scaffold, our studies regarding the synthesis of PNP and PCNCP
have previously been shown to be excellent ligands, in con- ligands,26,27 we explored a missing counterpart to these two
junction with simple CrIII salts, for either selective ethylene tri- classes, namely PCNP bidentate ligands bearing two electroni-
merisation or tetramerisation.2–7 Crucially, such selectivity cally different trivalent phosphorus centres. We report here the
originates from careful tuning of the –PR2 or –NR substituents synthesis of such PCNP ligands and their potential as CrIII-
of the PNP ligands2–7 with the (Ph2P)2NiPr ligand being the based catalysts for ethylene tri-/tetramerisations. To under-
exemplar for ethylene tetramerisation (Chart 1).4
stand the impact of ligand effects on catalyst activity/selectivity
Expanding the ligand scope to include methylene spacers
in the bidentate PNP backbone has been shown to increase
the ligand bite angle around the Cr metal centre.8,9
Furthermore, Le Floch and co-workers were able to switch tri-
and tetramerisation behaviour by R group manipulation of the
phosphine groups of PCNCP-type ligands.10 Inspired by these
findings, chemists have sought to explore the scope of Group
15/16 ligands for Cr-catalysed ethylene oligomerisations11,12
and polymerisations.13 Some examples of P-based ligands
studied, for ethylene tri-/tetramerisations, highlight the vari-
aR & D Division, Sasol Technology (Pty) Ltd., 1 Klasie Havenga Road, Sasolburg,
South Africa
bDepartment of Chemistry, Loughborough University, Loughborough, Leics,
LE11 3TU, UK. E-mail: m.b.smith@lboro.ac.uk
†CCDC 2018019–2018022. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/d1dt00287b
Chart 1 Recent examples of ligands studied for Cr-catalysed ethylene
tri-/tetramerisations.
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we also prepared three simple Cr0 complexes Cr(CO)4{PN(R)P}, the expected structures confirming only single substitution
Cr(CO)4{PCN(R)P} and Cr(CO)4{PCN(R)CP} [R = 1-Naphthyl] had resulted. Hence one resonance in the 31P{1H} NMR
and investigated their X-ray structures. We have attempted to spectra for 1a–d at ca. δ −19 ppm is observed with respect to
correlate Cr–P bond lengths and P–Cr–P ligand bite angles Ph2PCH2OH [δ −10.0 ppm, CDCl3]. In the 1H NMR spectra,
with catalytic activity in order to gain further insight into the
a broad NH resonance was observed in the region of
structure catalyst selectivity/activity relationships.21
δ 3.7–4.5 ppm with the weakly absorbing ν(NH) vibration at
approx. 3300 cm−1 further confirming single P–C bond for-
mation leaving a free NH site for further functionalisation.
The X-ray structure (Fig. 1) of 1a has been determined.
Results and discussion
For the synthesis of unsymmetrical P–C–N–P chelating Unsymmetrical bidentate aminomethylphosphines
ligands,28 we targeted two plausible routes to their preparation
With precursors 1a–d in hand, deprotonation with a slight
(Scheme 1). Both pathways rely on condensation reactions,
excess of LDA, followed by quenching with Ph2PCl and
with elimination of HCl (P–N bond formation) or H2O (P–CH2
workup, gave the bidentate ligands 2a–d in 41–77% isolated
bond formation), the sequence of which varies, depending on
yields (Scheme 2). Ligands 2a–d prepared by this route were
found to be air stable both in the solid state and in solution.
the route chosen. Each initial step involves a single substi-
tution only and it is therefore necessary to prevent double sub-
No reaction occurred between 1a–d, Ph2PCl and either NEt3 (or
stitution, for example PCNCP formation,29 which can result,
nBuLi) following common P–N methodologies used for acces-
depending on the primary amine source.
sing PNP ligands.9,27,30,31
Ligands 2a–d exhibited classic AX patterns in their respect-
Monodentate aminomethylphosphines
ive 31P{1H} NMR spectra due to the inequivalent P nuclei in
Our initial starting point for this work was the realisation that
these compounds. The chemical shifts of these doublets in
R2PN(R1)PR2 and R2 PCN(R1)CPR2 (R1 = iPr) have been shown
2
2
2a–d were all very similar and at approx. δ −21 (PCH2) and δ 68
(PN) ppm10,14 indicating that small changes in the R1 aromatic
substituent have negligible effect on the electronic properties
of the P nuclei. The absence of an NH resonance in the 1H
NMR spectra and of a ν(NH) stretch in the IR spectra con-
firmed that the amine group has successfully been replaced by
a –PPh2 group.
to be excellent ligands, in conjunction with CrIII salts, for
ethylene tetramerisation and trimerisations.2,10 Accordingly,
we began our studies using isopropylamine, with the
attempted syntheses of (i) Ph2PCH2NHiPr and (ii) the reaction
of Ph2PNHiPr with Ph2PCH2OH. In our hands, both routes
were somewhat problematic. During the preparation of
Ph2PCH2NHiPr we often observed large amounts of
(Ph2PCH2)2NiPr (>35% as judged by 31P{1H} NMR). The con- Chromium(0) tetracarbonyl complexes of 2a, 3, and 4
densation of Ph2PNHiPr with Ph2PCH2OH often resulted in
formation of various unidentified phosphorus products,
The unsymmetrical bidentate ligands 2a–d, in addition to the
known PNP (3)30 and PCNCP (4) ligands of 1-naphthylamine
reflecting the instability of the P–N bond with this phosphi-
prepared by reaction with 2 equiv. of either Ph2PCl or
noamine under these conditions. In order to circumvent this,
Ph2PCH2OH respectively, have been evaluated for Cr-catalysed
i
we substituted the more basic PrNH2 for 1-naphthylamine (and
ethylene oligomerisation (vide infra). It was therefore useful to
two substituted analogues) or 5-aminoquinoline and this enabled
briefly ascertain their coordination behaviour and how these
the synthesis of the secondary amines 1a–d (61–82% isolated
ligands react with CrIII. We focused our efforts on the reactivity
yields) to be achieved from Ph2PCH2OH in MeOH (Scheme 2).
The spectroscopic and analytical data are in agreement with
of 2a, 3, and 4 towards Cr(CO)4(η4-nbd) in THF which gave the
corresponding octahedral complexes 5–7 in 56–70% isolated
Fig. 1 Molecular structure of Ph2PCH2{1-N(H)Nap} (1a). All hydrogen
Scheme 1 Generalised synthetic pathways to PCNP ligands.
4346 | Dalton Trans., 2021, 50, 4345–4354
atoms except on N(1) have been omitted for clarity.
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structures are very similar and are also comparable with
known Cr0 complexes bearing PNP32 or PNNP ligands.15 For
complexes 5, 6, and 7·CH2Cl2, the Cr(1)–P(1) and Cr(1)–P(2)
bond lengths are similar, despite the different electronic pro-
perties of the two –PPh2 groups imposed by the additional one
or two methylene groups. As expected, for the Cr0 complexes
there is an increase in P–M–P bite angle and M⋯N distance on
progressing the series from 5 > 6 > 7·CH2Cl2, reflecting an
increase in chelate ring size (Table 1). The 4-membered
chelate ring of 5 is not quite planar as shown by an angle of
164.46(6)° between the Cr(1)–P(1)–P(2) and P(1)–P(2)–N(1)
planes. Compound 7·CH2Cl2 adopts a pseudo-chair confor-
mation with Cr(1) 0.9480(9) Å above and N(1) 0.7380(14) Å
below the P(1)–P(2)–C(6)–C(5) plane. The Cr complex 6 sits in a
twisted envelope conformation with both N(1) and C(5) above
the plane containing Cr(1)–P(1)–P(2) by 0.4501(14) Å and
0.9328(16) Å respectively. Moreover there is an observed differ-
ence in the angle at which the 1-naphthyl substituent resides
with respect to the chelate rings for these three Cr0 complexes.
The torsion angle decreases from 77.612(16)° (5) to 55.74(3)°
(7·CH2Cl2) on going from the 4-membered to 6-membered
chelate rings, whereas 6 displays larger, almost perpendicular,
torsion angles of 88.85(5) and 86.90(5)° for the two indepen-
dent molecules. Finally N(1) in 5 (sum of angles = 360°) and 6
(sum of angles for both molecules = 358°) is essentially planar,
consistent with nitrogen lone pair delocalisation over the four/
Scheme 2 Reagents and conditions: (i) Ph2PCH2OH, MeOH; (ii) LDA,
−78 °C, Ph2PCl.
yields as pale yellow solids (Chart 2). The 31P{1H} NMR spectra five-membered chelates, whereas in 7 the nitrogen atom is pyr-
were in good agreement with structures based on coordinated amidal (sum of angles = 338°).
symmetrical PNP (5, δ 117.5 ppm) and PCNCP (7, δ 41.0 ppm)
Oligomerisation results and discussion
ligands along with an AX spectrum for the unsymmetrical five-
membered PCNP chelate complex 6 [δ 67.9 (PCH2), 142.8 (PN), Despite the mentioned synthetic challenges and the labile
JPP = 32 Hz]. Furthermore the FT–IR spectra of 5–7 reveal nature of the bidentate PCNP diphosphines during purifi-
ligands 2a, 3, and 4 have very similar electronic properties, the cation, four N-naphthyl variants of the PCNP systems (2a–d)
ν(CO) values for 5 and 6 being similar to previously reported with their corresponding PCN(H)- “half molecules” (1a–d) were
complexes with these ligand classes.3a,4,5a,15,31
successfully synthesised which enabled their evaluation as
The geometries of 5, 6, and 7·CH2Cl2 are essentially octa- ligands under ethylene oligomerisation conditions. A few sub-
hedral with respect to the Cr0 centre and the P donor atoms stituents were thus introduced onto the naphthylene moiety to
are in a cis arrangement affording four, five, or six-membered effect variation in the electronic properties of these prospective
chelate rings respectively (Fig. 2). The major features of these oligomerisation ligands. Ligands 1b/2b and 1c/2c were syn-
thesised to explore the impact of the electron withdrawing
chloro and bromo groups onto the amino naphthylene back-
bone whereas the quinoline analogues (1d/2d) contains hetero-
aromatic functionality.
Mixtures of the Ph2PCH2N(R)H ligands (1a–d) and CrIII salt
in solution were activated with MMAO-3A and screened for
ethylene oligomerisation using typical tri- and tetramerisation
conditions. Albeit at comparatively low catalyst activities, these
systems were all active for ethylene oligomerisation upon
MMAO activation (see Table 2). Selectivity towards both
1-hexene and 1-octene were consistently low, resulting in a
broad distribution in α-olefins and consistent high yields of
C10–14 and C16+ olefin. In addition, polyethylene formation was
high, at 70% of the total product generated, whilst catalyst
activity obtained was only around 0.05.
In contrast, the catalyst systems containing the corres-
ponding Ph2PCH2N(R)PPh2 ligands (2a–d) were all highly
Chart 2 Structures of complexes 5, 6, and 7.
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Fig. 2 Molecular structures of 5, 6, and 7·CH2Cl2 from left to right respectively. Only the ipso-Ph and naphthyl carbon atoms of the diphosphines are
shown. All hydrogen atoms and solvent molecules of crystallisation have been omitted for clarity. One of two similar unique molecules of 6 shown.
2b–d resulted in only a minor reduction in 1-octene selectivity.
This is largely in line with observations of Killian et al. which
demonstrated that electronic effects have little bearing on cata-
lyst selectivity of N-aryl substituents for PNP/Cr tetramerisation
catalyst systems.2d On the other hand, steric encumbrance or
bulk substitution on PNP ligand systems in general favours
1-hexene formation over 1-octene,2d,e thus possibly explaining
the lower 1-octene selectivities observed for the 2a–d based cat-
alysts relative to that obtained using ligand 2a.
Table 1 Selected bond lengths (Å) and angles (°) for 5, 6 and 7·CH2Cl2
Compound
5
6a
7·CH2Cl2
P(1)–Cr(1)–P(2)
Cr(1)–P(1)
Cr(1)–P(2)
Cr(1)⋯N(1)
∑ around N(1)
68.787(11)
2.3445(3)
2.3447(3)
3.0093(9)
360
81.322(16)
2.3481(5)
2.3420(5)
3.3195(3)
358
87.144(14)
2.3637(4)
2.3567(4)
3.9569(9)
335
a Two molecules in the asymmetric unit. Values for the second unique
molecule are very similar.
active and selective towards both 1-hexene and 1-octene for-
mation, providing total α-selectivities in excess of 82%, as well
as low polyethylene yields. The Ph2PCH2{1-N(PPh2)Nap}/Cr/
MMAO catalyst system marginally outperformed the halogen
containing and quinoline analogues, yielding a respectable
total α-selectivity of 87% and a 1-C8 to 1-C6 ratio of 1.23.
These catalytic results indicate that the changes in the elec-
tronic and steric encumbrance properties of 2a to yield ligands
Table 2 Catalytic data for 1a–d, 2a–d and known (8–10) ligand systems
Liquid product selectivity (wt %)
Ligand
C6
1-C6
1-C6/ C6
C6 cyclics
C8
1-C8
1-C8/C8
C10–14
C16+
Total α
1C8:1C6
Act.
PE
1
2
3
4
5
6
7
8
1aa
1bb
1cb
1db
2ab
2bb
2cb
2db
8b
14.9
19.0
16.7
29.2
43.2
44.2
45.0
44.0
18.4
27.3
30.4
12.2
14.7
13.8
25.4
39.3
40.1
41.0
40.4
14.1
14.2
25.1
81.9
77.4
82.6
87.0
91.0
90.7
91.1
91.8
76.6
52.0
82.6
-
36.3
28.7
22.0
32.3
48.5
46.2
44.8
42.8
70.1
56.5
62.8
36.3
25.0
17.7
28.9
48.2
46.2
44.5
42.3
69.5
54.6
62.4
99.9
87.1
80.5
89.5
99.4
99.9
98.3
98.8
99.1
96.6
99.4
16.7
15.8
21.3
15.0
7.6
8.5
—
31.1
35.5
38.5
21.7
0.4
0.3
—
48.5
39.7
31.5
54.4
87.4
86.3
85.6
82.6
83.6
68.8
87.5
2.98
1.70
1.29
1.14
1.23
1.15
1.09
1.05
4.93
3.85
2.48
0.047
0.062
0.052
0.042
3.18
2.56
2.70
3.04
1.50
1.59
1.41
74.2
63.2
69.2
69.5
0.5
0.6
0.3
0.4
0.3
0.2
1.4
3.59
1.50
2.88
3.83
3.98
—
—
—
—
9
10
11
4.2
12.2
5.2
9.1
9.1
6.0
2.1
4.8
0.5
9b
10b
a 50 bar, 60 °C, Cr(acac)3 5 µmol, MMAO-3A 480 eq., cyclohexane. b 45 bar, 60 °C, Cr(acac)3 2.5 µmol, MMAO-3A 960 eq., methylcyclohexane.
Activity is given in ton product per g Cr per h.
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Table 3 Effect of reaction pressure on catalyst activity and product selectivity (using 2a)
Liquid product selectivity (wt %)
P
[C2H4] (mol L−1
)
C6
1-C6
1-C6/C6
C6 cyclics
C8
1-C8
1-C8/C8
C10–14
C16+
Total α
1C8:1C6
Act.
PE
30
50
80
3.12
5.22
8.09
46.0
41.1
39.6
42.1
36.7
35.3
91.5
89.3
89.1
3.9
4.3
4.3
44.5
51.4
54.2
44.1
51.0
53.7
99.1
99.2
99.1
8.84
6.38
4.64
0.4
0.5
0.6
86.2
87.7
89.0
1.05
1.39
1.52
2.08
2.72
2.35
0.5
1.2
0.9
Conditions: 300 ml Parr reactor, 2.5 μmol Cr(acac)3, MMAO-3A 960 eq., methylcyclohexane, 60 °C. Activity is given in ton product per g Cr per h.
these PCNP systems, this system was used for further reaction
condition optimisation, focussing on the effects of ethylene
pressure and temperature.
Effect of ethylene pressure
The reaction pressure was varied from 30 to 80 barg at a con-
stant temperature of 60 °C for the entire reaction period.
Increasing the ethylene pressure from 30 to 50 barg resulted in
a 31% increase in catalytic activity, which was not unexpected
as the ethylene concentration in MCH increased 1.7 fold from
3.12 to 5.22 mol L−1 over this pressure range (Table 3).33
However at 80 barg, where the ethylene concentration is
8.09 mol L−1, a reduction in catalyst activity was observed
which could be ascribed to a possible compositional change
Fig. 3 Effect of reaction pressure on product selectivity.
affecting the catalyst solubility. All other trends of decreasing
C6, increasing C8 and total alpha, lower C10–14 and high C8:C6
continue across the pressure range studied here.
In order to assess the performance of these bidentate PCNP
Keeping known mechanistic and kinetic considerations in
based catalyst systems from a broader perspective, given their mind,34,35 the increase in pressure resulted in an expected
high C8 and total alpha selectivities, we wanted to probe the increased C8 selectivity, accompanied by a decrease in 1-C6
structural ligand features with those of known PNP (8), PCCP content within the C6 fraction due to the increase in C6 cyclics
(9), and PNNP (10) based catalyst systems under similar reac- formation (Fig. 3). The increase in 1-octene and C6 cyclics with
tion conditions (see Table 2). Diphosphine 9 was chosen over pressure is indicative of a strong ethylene concentration
the more flexible Ph2PCH2CH2PPh2 (dppe) given the known dependence on these fractions.35 The reduction in C10–14 pro-
lower activity and 1-C6/C8 selectivities.2a While the bidentate ducts formation observed with increasing pressure can be
PCNP based catalysts consistently yielded lower 1-C8 to 1-C6 ascribed to the reduced concentration of the primary 1-C8 and
ratio’s than the catalysts based on ligands 8–10, all four PCNP 1-C6 products present at higher ethylene concentrations,
based systems exhibited considerably higher catalyst activities thereby resulting in an improved total α-selectivity. Based on
than the catalysts containing 8–10. In addition, the total these findings, the effect of reaction temperature at 50 barg
α-selectivity observed for the catalyst system containing ligand ethylene was studied in more detail.
2a was noticeably higher than that obtained using 8 and 9,
mainly as a result of an improved 1-C6 selectivity.
Effect of reaction temperature
Given that the catalyst system containing ligand 2a yielded The reaction temperature was varied from 45 to 80 °C at a con-
the most promising activity and selectivity results amongst stant ethylene pressure (50 barg) over the entire reaction
Table 4 Effect of reaction temperature on catalyst activity and product selectivity (using 2a)
Liquid product selectivity (wt %)
Temp.
[C2H4] (mol L−1
)
C6
1-C6
1-C6/C6
C6 cyclics
C8
1-C8
1-C8/C8
C10–14
C16+
Total α
1C8:1C6
Act.
PE
45
60
80
6.33
5.22
4.24
29.0
41.1
59.5
23.1
36.7
56.8
79.7
89.3
95.5
5.9
4.3
2.7
62.7
51.4
34.4
62.0
51.0
34.1
98.9
99.2
99.1
5.8
6.4
5.7
1.6
0.5
0.2
85.1
87.7
91.0
2.69
1.39
0.60
0.97
2.72
3.11
2.9
1.2
0.6
Conditions: 300 ml Parr reactor, 2.5 μmol Cr(acac)3, MMAO-3A 960 eq., methylcyclohexane, 50 barg C2H4. Activity is given in ton product per g Cr
per h.
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of catalyst selectivity yielding a total α-selectivity as high
as 91%.
Experimental section
General methods
The synthesis of ligands 1a–d, 2a–d, 3,2d,30 4, and complexes
5–7, were carried out using standard Schlenk line techniques
under an inert nitrogen atmosphere. Ph2PCH2OH was pre-
pared according to a known procedure.36 [Cr(CO)4(η4-nbd)]
(nbd = norbornadiene) was prepared according to a known
procedure.37 All other chemicals, including reagent grade
quality solvents, were obtained from commercial sources and
used directly without further purification.
Fig. 4 Effect of reaction temperature on product selectivity at 50 barg.
Infrared spectra were recorded as KBr pellets on
a
PerkinElmer Spectrum 100S (4000–250 cm−1 range) Fourier-
Transform spectrometer. 1H NMR spectra (400 or 500 MHz)
were recorded on a Jeol-ECS-400 FT or Jeol-ECZ-R-500 spectro-
meter with chemical shifts (δ) in ppm to high frequency of Si
(CH3)4 and coupling constants (J) in Hz. 31P{1H} NMR (162 or
202 MHz) spectra were recorded on a Jeol-ECS-400 FT or Jeol-
ECZ-R-500 spectrometer with chemical shifts (δ) in ppm to
high frequency of 85% H3PO4. NMR spectra were measured in
CDCl3 at 298 K. Elemental analyses (PerkinElmer 2400 CHN or
Exeter Analytical, Inc. CE-440 Elemental Analyzers) were per-
formed by the Loughborough University Analytical Service
within the Department of Chemistry.
Preparation of Ph2PCH2{1-N(H)Nap} (1a). Under nitrogen, a
solution of 1-aminonaphthalene (0.574 g, 4.01 mmol) and
Ph2PCH2OH (1.088 g, 4.03 mmol) in freeze–thawed methanol
(30 mL) was stirred for 24 h. The solution was concentrated to
approximately 10 mL under reduced pressure, and the result-
ing solid 1a filtered and dried in vacuo (1.127 g, 82%). 31P
NMR: δ −18.4. 1H NMR: δ 7.9–6.7 (m, arom. H), 4.5 (s, NH),
4.0 ppm (d, J = 4.0 Hz, CH2). FT–IR: ν(NH) 3378 cm−1. Found
C, 80.71; H, 5.69; N, 4.17. C23H20NP requires C, 80.92; H, 5.90;
period. The effect of reaction temperature on catalyst activity
and product selectivity at 50 barg ethylene pressure is illus-
trated by Table 4 and Fig. 4. The catalyst activity improved sig-
nificantly from 0.97 to 3.11 across the temperature range,
despite the concomitant 33% reduction in ethylene concen-
tration associated with the lower ethylene solubility at higher
reaction temperature. This again suggests that the reaction
temperature plays a dominant role in determining the optimal
reaction rate.35
Lower reaction temperature resulted in an increase in
overall C8 selectivity, including an increase in 1-C8 and C6
cyclics, all of which is consistent with the benefit of higher
ethylene concentrations at lower temperatures. The C10–14
selectivity remained constant across the temperature range
evaluated, whilst an increase in both the C16+ and polyethylene
selectivities are observed at lower reaction temperature.
Conclusions
In summary, four unsymmetrical bidentate PCNP ligands have N, 4.10%.
been synthesised in a two-step reaction sequence. Catalytic
Preparation of Ph2PCH2{1-N(H)(4-Cl)Nap} (1b). 1-Amino-4-
testing, in conjunction with CrIII salts, revealed these ligands chloronaphthalene (0.518 g, 2.86 mmol) and Ph2PCH2OH
to generate active catalysts for ethylene tri-/tetramerisation. To (0.651 g, 2.86 mmol) in methanol (20 mL). The solution was
further understand possible origins for this selectivity in terms stirred for 6 d and the solid product isolated (0.716 g, 67%).
of 1-hexene/1-octene formation, Cr0 complexes were prepared 31P NMR: δ −18.7. H NMR data: δ 8.1 (d, J = 8.4 Hz, arom. H),
1
and their single crystal X-ray structures determined. The P–Cr– 7.6–7.1 (m, arom. H), 6.6 (d, J = 8.0 Hz, arom. H), 4.3 (s, NH),
P bite angle was found to increase, within this series, from 3.9 ppm (d, J = 4.0 Hz, CH2). FT–IR: ν(NH) 3442 cm−1. Found
68.787(11)° (Cr–P–N–P) to 81.322(16)/81.524(16)° (Cr–P–N–C– C, 73.30; H, 5.09; N, 3.77. C23H19NPCl requires C, 73.50; H,
P) to 87.144(11)° (Cr–P–C–N–C–P). It should be noted that the 5.10; N, 3.74%.
P–Cr–P bite angle in 6 is ca. 4° smaller than found in the CrIII
Preparation of Ph2PCH2{1-N(H)(4-Br)Nap} (1c). 1-Amino-4-
compound CrCl3(9)(thf) (thf = tetrahydrofuran)16 whilst the bromonaphthalene (1.548 g, 6.76 mmol) and Ph2PCH2OH
equivalent parameter in 7 is comparable with CrCl3{Cy2PCH2N (1.507 g, 6.76 mmol) in methanol (40 mL). The solution was
(iPr)CH2PCy2}(thf) [85.90(5)°] previously reported by Le Floch refluxed, under nitrogen, at 70–80 °C for 6 d. The solid
and co-workers.10 A variety of bidentate PCNP ligands in com- product was isolated (1.720 g, 61%). 31P NMR: δ −18.8. 1H
bination with CrIII and activated with modified methyl alumi- NMR data: δ 8.2 (d, J = 8.8 Hz, arom. H), 7.7–7.3 (m, arom. H),
noxane were found to be highly active for ethylene tri- and tet- 6.7 (d, J = 8.4 Hz, arom. H), 4.5 (s, NH), 4.0 ppm (d, J = 4.0 Hz,
ramerisation. Optimisation of reaction conditions using ethyl- CH2). FT–IR: ν(NH) 3443 cm−1. Found C, 65.44; H, 4.57; N,
ene pressure and temperature furnished further improvement 3.37. C23H19NPBr requires C, 65.73; H, 4.56; N, 3.33%.
4350 | Dalton Trans., 2021, 50, 4345–4354
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Preparation of
Paper
1
Ph2PCH2{5-N(H)Quin}
(1d).
5- (PN), JPP = 8 Hz. H NMR data: δ 8.8 (m, arom. H), 8.3 (d, J =
Aminoquinoline (0.964 g, 6.69 mmol) and Ph2PCH2OH 8.8 Hz, arom. H), 7.8 (d, J = 8.4 Hz, arom. H), 7.6–6.9 (m,
(1.490 g, 6.68 mmol) in methanol (10 mL). The solution was arom. H), 6.7 (d, J = 7.6 Hz, arom. H), 3.9 ppm (m, CH2). FAB
stirred for 11 d. The solid product was isolated (1.753 g, 77%). MS: m/z 527 [M + H]+. Despite numerous attempts, it was not
31P NMR: δ −18.5. 1H NMR data: δ 8.8 (m, arom. H) 7.9–7.1 (m, possible to obtain an analytically pure sample of 2d.
arom. H), 6.7 (d, J = 7.6 Hz, arom. H), 4.4 (s, NH), 3.9 ppm (d,
J = 4.4 Hz, CH2). FT–IR: ν(NH) 3255 cm−1. Found C, 77.15; H, naphthalene (0.186 g, 1.30 mmol) and Ph2PCH2OH (0.584 g,
5.64; N, 8.24. C22H19N2P requires C, 77.18; H, 5.59; N, 8.18%. 2.59 mmol) in MeOH (10 mL) was refluxed for 65 h. The solu-
Preparation of 1-N(CH2PPh2)2Nap (4). A solution of 1-amino-
Preparation of Ph2PCH2{1-N(PPh2)Nap} (2a). Under nitro- tion was concentrated to approx. 5 mL under reduced pressure
gen, a small excess of lithium diisopropylamide (0.70 mL of a and the white solid 4 filtered and dried in vacuo (0.600 g,
2.0 M solution in THF/heptane/ethylbenzene) was added to a 86%). 31P NMR: δ −28.0. 1H NMR data: δ 7.9–7.0 (m, arom. H),
solution of 1a (0.458 g, 1.27 mmol) in freeze–thawed THF 4.3 ppm (d, J = 2.4 Hz, CH2). Found C, 77.58; H, 5.93; N, 2.32.
(25 mL) at −78 °C. The solution was stirred, at −78 °C, for 1 h C36H31NP2·CH3OH requires C, 77.74; H, 6.17; N, 2.45%.
and then warmed to room temperature and stirred for a
Preparation of Cr(CO)4(3) (5). Under N2, a solution of Ph2PN
further 1 h. Ph2PCl (0.23 mL, 1.3 mmol) was added dropwise (1-Nap)PPh2 (3) (0.118 g, 0.189 mmol) and Cr(CO)4(η4-nbd)
at −78 °C and the solution stirred, at room temperature, for (0.049 g, 0.19 mmol) in freeze–thawed THF (20 mL) was heated
1.5 h. The solvent was evaporated to dryness under reduced at 50 °C for 1 h. Upon cooling, the solution was evaporated to
pressure and degassed hexane (10 mL) added. The solution dryness under reduced pressure (0.075 g, 58%). 31P NMR:
1
was stirred at r.t. for 6 h and filtered under nitrogen. The δ 117.5. H NMR data: δ 7.8–6.9 (m, arom. H), 6.8 (d, J = 8.4
solvent was evaporated to dryness under reduced pressure to Hz, arom. H), 6.4 ppm (t, J = 15 Hz, arom. H). FT–IR: ν(CO)
give a yellow solid (0.615 g, 92%). 31P NMR: δ −21.7 (PCH2), 2006, 1917, 1889, 1879 cm−1. Found C, 67.46; H, 4.47; N, 1.81.
67.0 (PN), JPP = 8 Hz. 1H NMR data: δ 8.0–6.7 (m, arom. H), C38H27NP2O4Cr requires C, 67.55; H, 4.04; N, 2.07%. FAB MS:
3.9–3.7 ppm (m, CH2). FAB MS: m/z 340 [M − PPh2]+, 185 m/z 675 [M]+.
[PPh2]+. Despite numerous attempts, it was not possible to
obtain an analytically pure sample of 2a.
Preparation of Cr(CO)4(2a) (6). A solution of 2a (0.140 g,
0.197 mmol) and Cr(CO)4(η4-nbd) (0.051 g, 0.20 mmol) in
Preparation of Ph2PCH2{1-N(PPh2)(4-Cl)Nap} (2b). Lithium freeze–thawed THF (20 mL) was heated, under N2, at 50 °C for
diisopropylamide (0.63 mL of a 2.0 M solution in THF/ 1 h. Upon cooling, the solvent was evaporated to dryness
heptane/ethylbenzene), 1b (0.500 g, 1.14 mmol) and Ph2PCl under reduced pressure (0.096 g, 70%). 31P NMR: δ 67.9
(0.21 mL, 1.2 mmol) in THF (15 mL). The crude solid was puri- (PCH2), 142.8 (PN), JPP = 32 Hz. 1H NMR data: δ 7.8–6.6 (m,
fied by addition of degassed hexane (20 mL) and stirring at r.t. arom. H), 3.7 ppm (m, CH2). FT–IR: ν(CO) 2009, 1921,
for 5 h. The white solid was filtered under nitrogen and dried 1880 cm−1
.
Found C, 62.55; H, 5.12; N, 1.90.
in vacuo (0.386 g, 60%). 31P NMR: δ −21.4 (PCH2), 68.1 (PN), C39H29NP2O4Cr·3H2O requires C, 62.99; H, 4.75; N, 1.88%. FAB
1
JPP = 9 Hz. H NMR data: δ 8.1 (d, J = 8.4 Hz, arom. H), 8.0 (d, MS: m/z 605 [M − 3CO]+, 577 [M − 4CO]+.
J = 8.4 Hz, arom. H), 7.5–6.9 (m, arom. H), 3.9 ppm (m, CH2).
Preparation of Cr(CO)4(4) (7). Ligand
4
(0.138 g,
FAB MS: m/z 560 [M]+. Despite numerous attempts, it was not 0.230 mmol) and Cr(CO)4(η4-nbd) (0.059 g, 0.23 mmol) in
possible to obtain an analytically pure sample of 2b.
freeze–thawed THF (20 mL) was heated, under N2, at 50 °C for
Preparation of Ph2PCH2{1-N(PPh2)(4-Br)Nap} (2c). Lithium 1 h. The solid product was isolated (0.091 g, 56%). 31P NMR:
1
diisopropylamide (0.62 mL of a 2.0 M solution in THF/ δ 41.0. H NMR data: δ 7.8 (d, J = 8.0 Hz, arom. H), 7.7 (d, J =
heptane/ethylbenzene), 1c (0.504 g, 1.12 mmol) and Ph2PCl 8.0 Hz, arom. H), 7.6 (d, J = 8.0 Hz, arom. H), 7.5–7.0 (m,
(0.20 mL, 1.1 mmol) in THF (15 mL). To the crude solid was arom. H), 6.8 (t, J = 15 Hz, arom. H), 6.6 (d, 8.8 Hz, arom. H),
added degassed diethyl ether (20 mL) and the solution stirred 3.7 ppm (m, CH2). FT–IR: ν(CO) 2007, 1923, 1877 cm−1. Found
at room temperature for 5 h. Under nitrogen, the white solid C, 68.24; H, 4.79; N, 2.10. C40H31NP2O4Cr requires C, 68.27; H,
was filtered and dried in vacuo (0.279 g, 41%). 31P NMR: 4.45; N, 1.99%. FAB MS: m/z 591 [M − 4CO]+.
δ −21.4 (PCH2), 68.2 (PN), JPP = 9 Hz. 1H NMR data: δ 8.1 (d, J =
Ethylene oligomerisation catalysis
8.4 Hz, arom. H), 8.0 (d, J = 8.4 Hz, arom. H), 7.6–6.8 (m,
arom. H), 6.6 (d, J = 8.4 Hz, arom. H), 3.9 ppm (m, CH2). FAB
MS: m/z 605 [M + H]+. Despite numerous attempts, it was not species towards moisture and air required all procedures to be
possible to obtain an analytically pure sample of 2c. carried out under dry, inert conditions. This was accomplished
General catalytic techniques. The sensitivity of the catalyst
Preparation of Ph2PCH2{5-N(PPh2)Quin} (2d). Lithium diiso- using either a Braun glove box or using standard Schlenk line
propylamide (0.76 mL of a 2.0 M solution in THF/heptane/ techniques. All catalyst preparations were carried out in oven
ethylbenzene), 1d (0.501 g, 1.38 mmol) and Ph2PCl (0.25 mL, treated glassware. Reagents and solvents were pre-dried using
1.4 mmol) in THF (15 mL). After evaporation to dryness, the techniques described below. Cr(III) acetylacetonate (97%
degassed diethyl ether (20 mL) was added and the solution purity) was obtained from Sigma Aldrich and used without
stirred at r.t. for 5 h. Under nitrogen, the solution was filtered further purification whilst MMAO-3A (7 wt% in heptanes) was
and evaporated to dryness under reduced pressure to give a sourced from AkzoNobel. The Al to Cr ratio used was 960 eq.
yellow solid (0.471 g, 65%). 31P NMR: δ −21.5 (PCH2), 69.3 unless stated otherwise. Ethylene 3.5 (99.95%) purity was
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obtained from Air Liquide or Linde AG. Methylcyclohexane minated after 160 g of ethylene was fed to the reactor by shut-
(99%) (MCH) and cyclohexane (99.5%) were obtained from ting off the ethylene feed followed by immediate cooling the
Sigma Aldrich and purified by percolation through neutral reactor contents, using ice, to around 10 °C. Following slow
alumina. The catalyst concentration solutions employed in release of the excess ethylene from the autoclave, the reaction
reactions were 2.5 μmol Cr and 2.75 μmol ligand in 100 mL mixture was quenched with ethanol and 10% HCl. Nonane
reaction solvent unless otherwise indicated.
was added to the reaction mixture as external standard and
Catalytic runs were carried out in 450 ml Parr autoclaves the liquid phase was analysed by GC-FID. The remainder of
(unless indicated otherwise) fitted with internal cooling coils, the organic layer was filtered to isolate the polymeric material,
baffles and a gas entrainment stirrer. Ethylene uptake during which was dried in an oven at 100 °C overnight and weighed.
catalysis was monitored by Danfoss Massflo (Type Mass 6000) Reaction selectivity data in Tables 2–4 is reported in wt% of
flowmeter. Unless indicated otherwise, all reactions were con- the specific fraction in total liquid products normalised to
ducted under standard conditions at 60 °C and 45 barg ethyl- 100%. Total α-selectivity is defined as the sum of the 1-C6 and
ene pressure in a total volume of catalyst and solvent mixture 1-C8 fractions of total liquid products. Polyethylene (PE)
of 100 mL.
reported is in wt% of total product, activity is given in ton
Catalytic reaction procedure. The reactor was heated to product per g Cr per h and pressure (P) is in barg.
120 °C under vacuum for 1 h and then allowed to cool to room
temperature under nitrogen purge. The pre-weighed reaction
X-ray crystallographic studies
solvent was introduced to the reactor via syringe prior to Suitable crystals of 1a were obtained upon allowing a MeOH
heating the reactor to operating conditions. The ligand was filtrate to stand for several days. Compounds 5, 6, and
dissolved in 25 mL of solvent and an aliquot combined with 7·CH2Cl2 were crystallised by slow diffusion of MeOH into a
the chromium catalyst solution in a Schlenk vessel and stirred CH2Cl2 solution. Details of the data collection parameters and
for ca. 5 min prior to addition of the activator. The resulting crystal data for 1a and 5, 6, and 7·CH2Cl2 are presented in
solution/suspension was transferred to the Parr reactor. The Table 5. All measurements were made on a Bruker AXS SMART
reactor was immediately charged with ethylene to the desired 1000 CCD area-detector diffractometer, at 150 K, using graph-
pressure and the reaction temperature was controlled by circu- ite-monochromated Mo-Ka radiation and narrow frame
lating water through the cooling coils during the catalytic run. exposures (0.3°) in w.38 Cell parameters were refined from the
Ethylene was fed on demand and thorough mixing was observed (w) angles of all strong reflections in each data set.
ensured by stirring at rates of 1200 RPM. The reaction was ter- Intensities were corrected semi-empirically for absorption,
Table 5 Crystallographic data for 1a, 5, 6 and 7·CH2Cl2
Compound
1a
5
6
7·CH2Cl2
Empirical formula
Formula weight
C23H20NP
341.37
C38H27CrNO4P2
675.55
C39H29CrNO4P2
689.57
C41H33Cl2CrNO4P2
788.52
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
Monoclinic
P21/c
Monoclinic
P21/c
a [Å]
b [Å]
c [Å]
α [°]
17.5673(8)
5.5130(3)
18.5133(9)
90
12.1106(4)
19.6906(6)
13.9703(4)
90
22.7979(8)
17.5266(6)
16.9095(6)
90
9.7908(3)
17.7337(6)
21.6935(7)
90
β [°]
93.764(2)
90
1789.12(15)
4
101.0381(4)
90
3269.80(17)
4
96.9792(5)
90
6706.5(4)
8
99.3296(5)
90
3716.8(2)
4
γ [°]
Volume [Å3]
Z
Λ
T [K]
0.71073
150(2)
1.267
0.71073
150(2)
1.372
0.71073
150(2)
1.366
0.71073
150(2)
1.409
Density (calcd.) [Mg m−3
Absorption coeff. [mm−1
Crystal habit and colour
Crystal size [mm3]
θ Range [°]
]
]
0.158
0.490
0.479
0.581
Block, colourless
0.50 × 0.45 × 0.21
2.32–28.79
15117
4328
0.018
3567
230
1.03
0.036, 0.096
0.35, −0.21
Block, yellow
0.70 × 0.43 × 0.20
2.29–30.56
38624
9988
0.021
8721
415
1.04
0.031, 0.087
0.43, −0.50
Block, pale yellow
0.47 × 0.29 × 0.22
2.33–29.50
78337
20341
0.039
14758
847
1.02
0.041, 0.107
0.55, −0.64
Block, pale yellow
0.67 × 0.41 × 0.35
2.23–30.53
40840
11309
0.023
9700
460
1.05
0.039, 0.109
1.95, −1.42
Reflections collected
Independent reflections
Rint
Reflections with F2 > 2σ(F2)
Number of parameters
GOOF
Final Ra, Rw
b
Largest diff peak & hole [eÅ]
a R = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2
.
4352 | Dalton Trans., 2021, 50, 4345–4354
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Paper
based on symmetry-equivalent and repeated reflections.39 The
structures were solved by direct methods (Patterson synthesis
for 5 and 6) and refined on F2 values for all unique data by
full-matrix least-squares.40,41 All non-hydrogen atoms were
refined anisotropically. Carbon-bound hydrogen atoms were
constrained in a riding model with Ueq set to 1.2Ueq of the
carrier atom. Compound 6 contains two very similar molecules
in the asymmetric unit. In 7·CH2Cl2 the solvent molecule of
crystallisation was refined, with geometric and anisotropic dis-
placement parameter restraints, over two sets of positions for
all atoms with major component occupancy 67.8(14)%. CCDC
2018019–2018022† contain the supplementary crystallographic
data for this paper.
(b) A. Carter, S. A. Cohen, N. A. Cooley, A. Murphy, J. Scutt
and D. F. Wass, Chem. Commun., 2002, 858–859.
4 M. F. Haddow, J. Jaltai, M. Hanton, P. G. Pringle,
L. E. Rush, H. A. Sparkes and C. H. Woodall, Dalton Trans.,
2016, 45, 2294–2307.
5 (a) T. Agapie, M. W. Day, L. M. Henling, J. A. Labinger and
J. E. Bercaw, Organometallics, 2006, 25, 2733–2742;
(b) P. R. Elowe, C. McCann, P. G. Pringle, S. K. Spitzmesser
and J. E. Bercaw, Organometallics, 2006, 25, 5255–5260.
6 M. Höhne, N. Peulecke, K. Konieczny, B. H. Müller and
U. Rosenthal, ChemCatChem, 2017, 9, 2467–2472.
7 (a) X. Ji, L. Song, C. Zhang, J. Jiao and J. Zhang, Inorg.
Chim. Acta, 2017, 466, 117–121; (b) Y. Zhou, H. Wu, S. Xu,
X. Zhang, M. Shi and J. Zhang, Dalton Trans., 2015, 44,
9545–9550.
Conflicts of interest
8 (a) D. S. McGuiness, P. Wasserscheid, D. H. Morgan and
J. T. Dixon, Organometallics, 2005, 24, 552–556;
(b) D. S. McGuiness, P. Wasserscheid, W. Keim, C. Hu,
U. Englert, J. T. Dixon and C. Grove, Chem. Commun., 2003,
334–335.
9 Y. Shaikh, K. Albahily, M. Sutcliffe, V. Fomitcheva,
S. Gambarotta, I. Korobkov and R. Duchateau, Angew.
Chem., Int. Ed., 2012, 124, 1366–1369.
10 C. Klemps, E. Payet, L. Magna, L. Saussine, X. F. Le Goff
and P. Le Floch, Chem. – Eur. J., 2009, 15, 8259–8268.
11 (a) S. D. Boelter, D. R. Davies, K. A. Milbrandt,
D. R. Wilson, M. Wiltzius, M. S. Rosen and J. Klosin,
Organometallics, 2020, 39, 967–975; (b) S. D. Boelter,
D. R. Davies, P. Margl, K. A. Milbrandt, D. Mort,
B. A. Vanchura II, D. R. Wilson, M. Wiltzius, M. S. Rosen
and J. Klosin, Organometallics, 2020, 39, 976–987;
(c) C. Zhang, L. Song, H. Wu, X. Ji, J. Jiao and J. Zhang,
Dalton Trans., 2017, 46, 8399–8404.
There are no conflicts of interest to declare.
Acknowledgements
We thank the EPSRC, Sasol Technology (Pty) Ltd, and
Loughborough University for studentships. The EPSRC Mass
Spectrometry Service centre at Swansea are also acknowledged.
Notes and references
1 (a) O. L. Sydora, Organometallics, 2019, 38, 997–1010;
(b) T. Agapie, Coord. Chem. Rev., 2011, 255, 861–880;
(c) P. W. N. M. van Leeuwen, N. D. Clément and M.
J.-L. Tschan, Coord. Chem. Rev., 2011, 255, 1499–1517;
(d) J. T. Dixon, M. J. Green, F. M. Hess and D. H. Morgan,
J. Organomet. Chem., 2004, 689, 3641–3668.
12 L. Zhang, X. Meng, Y. Chen, C. Cao and T. Jiang,
ChemCatChem, 2017, 9, 76–79.
2 (a) A. Bollmann, K. Blann, J. T. Dixon, F. M. Hess,
E. Killian, H. Maumela, D. S. McGuinness, D. H. Morgan, 13 C. Bariashir, C. Huang, G. A. Solan and W.-H. Sun, Coord.
A. Neveling, S. Otto, M. Overett, A. M. Z. Slawin, Chem. Rev., 2019, 385, 208–229.
P. Wasserscheid and S. Kuhlmann, J. Am. Chem. Soc., 2004, 14 B. R. Aluri, N. Peulecke, B. H. Müller, S. Peitz,
126, 14712–14713; (b) K. Blann, A. Bollmann, J. T. Dixon,
A. Spannenberg, M. Hapke and U. Rosenthal,
F. M. Hess, E. Killian, H. Maumela, D. H. Morgan,
Organometallics, 2010, 29, 226–231.
A. Neveling, S. Otto and M. J. Overett, Chem. Commun., 15 L. E. Bowen, M. Charernsuk, T. W. Hey, C. L. McMullin,
2005, 620–621; (c) M. J. Overett, K. Blann, A. Bollmann,
A. G. Orpen and D. F. Wass, Dalton Trans., 2010, 39, 560–
J. T. Dixon, F. Hess, E. Killian, H. Maumela, D. H. Morgan,
567.
A. Neveling and S. Otto, Chem. Commun., 2005, 622–624; 16 M. J. Overett, K. Blann, A. Bollmann, R. de Villiers,
(d) E. Killian, K. Blann, A. Bollmann, J. T. Dixon,
S. Kuhlmann, M. C. Maumela, H. Maumela, D. H. Morgan,
P. Nongodlwana, M. J. Overett, M. Pretorius, K. Höfener
J. T. Dixon, E. Killian, M. C. Maumela, H. Maumela,
D. S. McGuiness, D. H. Morgan, A. Rucklidge and
A. M. Z. Slawin, J. Mol. Catal. A: Chem., 2008, 283, 114–119.
and P. Wasserscheid, J. Mol. Catal. A: Chem., 2007, 270, 17 S.-K. Kim, T.-J. Kim, J.-H. Chung, T.-K. Hahn, S.-S. Chae,
214–218; (e) K. Blann, A. Bollmann, H. de Bod, J. T. Dixon,
E. Killian, P. Nongodlwana, M. C. Maumela, H. Maumela,
H.-S. Lee, M. Cheong and S. O. Kang, Organometallics,
2010, 29, 5805–5811.
A. E. McConnell, D. H. Morgan, M. J. Overett, M. Prétorius, 18 L. Zhang, W. Wei, F. Alam, Y. Chen and T. Jiang, Catal. Sci.
S. Kuhlmann and P. Wasserscheid, J. Catal., 2007, 249,
244–249.
3 (a) T. E. Stennett, T. W. Hey, L. T. Ball, S. R. Flynn,
J. E. Radcliffe, C. L. McMullin, R. L. Wingad and
D. F. Wass, ChemCatChem, 2013, 5, 2946–2954;
Technol., 2017, 7, 5011–5018.
19 (a) S. M. Maley, D.-H. Kwon, N. Rollins, J. C. Stanley,
O. L. Sydora, S. M. Bischof and D. H. Ess, Chem. Sci., 2020,
11, 9665–9674; (b) D.-H. Kwon, S. M. Maley, J. C. Stanley,
O. L. Sydora, S. M. Bischof and D. H. Ess, ACS Catal., 2020,
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 4345–4354 | 4353

View Article Online
Paper
Dalton Transactions
10, 9674–9683; (c) D.-H. Kwon, J. T. Fuller III, U. J. Kilgore, 29 E. Payet, A. Auffrant, X. F. le Goff and P. Le Floch,
O. L. Sydora, S. M. Bischof and D. H. Ess, ACS Catal., 2018,
8, 1138–1142.
J. Organomet. Chem., 2010, 695, 1499–1506.
30 H. T. Al-Masri, Z. Anorg. Allg. Chem., 2012, 638, 1012–1017.
20 M. Gong, Z. Liu, Y. Li, Y. Ma, Q. Sun, J. Zhang and B. Liu, 31 L. M. Broomfield, C. Alonso-Moreno, E. Martin, A. Shafir,
Organometallics, 2016, 35, 972–981.
21 N. Cloete, H. G. Visser, I. Engelbrecht, M. J. Overett,
I. Posadas, V. Ceña and J. A. Castro-Osma, Dalton Trans.,
2017, 46, 16113–16125.
W. F. Gabrielli and A. Roodt, Inorg. Chem., 2013, 52, 2268– 32 L. E. Bowen, M. F. Haddow, A. G. Orpen and D. F. Wass,
2270. Dalton Trans., 2007, 1160–1168.
22 M. J. Overett, K. Blann, A. Bollmann, J. T. Dixon, 33 Calculations performed using Aspen Plus 12.1, 2008, Peng-
D. Haasbroek, E. Killian, H. Maumela, D. S. McGuinness Robinson Equation of State.
and D. H. Morgan, J. Am. Chem. Soc., 2005, 127, 10723– 34 G. J. P. Britovsek, D. S. McGuinness, T. S. Wierenga and
10730. C. T. Young, ACS Catal., 2015, 5, 4152–4166.
23 N. A. Hirscher, J. A. Labinger and T. Agapie, Dalton Trans., 35 R. Walsh, D. H. Morgan, A. Bollmann and J. T. Dixon, Appl.
2019, 48, 40–44. Catal., A, 2006, 306, 184–191.
24 T. Gunasekara, J. Kim, A. Preston, D. K. Steelman, 36 H. Hellman, J. Bader, H. Birkner and O. Schumacher,
G. A. Medvedev, W. N. Delgass, O. L. Sydora, Justus Liebigs Ann. Chem., 1962, 659, 49–56.
J. M. Caruthers and M. M. Abu-Omar, ACS Catal., 2018, 8, 37 M. A. Bennett, L. Pratt and G. Wilkinson, J. Chem. Soc.,
6810–6819. 1961, 2037–2044.
25 A. M. Lifschitz, N. A. Hirscher, H. B. Lee, J. A. Buss and 38 SMART Program for Diffractometer Control, Bruker AXS Inc.,
T. Agapie, Organometallics, 2017, 36, 1640–1648. Madison, WI, 2004.
26 G. M. Brown, M. R. J. Elsegood, A. J. Lake, N. M. Sanchez- 39 (a) SAINT software for CCD diffractometers, Bruker AXS Inc.,
Ballester, M. B. Smith, T. S. Varley and K. Blann,
Eur. J. Inorg. Chem., 2007, 1405–1414.
27 K. G. Gaw, M. B. Smith, J. B. Wright, A. M. Z. Slawin,
Madison, WI, 2004; (b) L. Krause, R. Herbst-Irmer,
G. M. Sheldrick and D. J. Stalke, SADABS software, Appl.
Crystallogr., 2015, 48, 3–10.
S. J. Coles, M. B. Hursthouse and G. J. Tizzard, 40 (a) G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Adv.,
J. Organomet. Chem., 2012, 699, 39–47.
28 H. D. Fokwa, J. F. Vidlak, S. C. Weinberg, I. D. Duplessis,
2015, A71, 3–8; (b) G. M. Sheldrick, Acta Crystallogr., Sect. A:
Found. Adv., 2008, A64, 112–122.
N. D. Schley and M. W. Johnson, Dalton Trans., 2020, 49, 41 G. M. Sheldrick, SHELXTL user manual, version 6.12, Bruker
9957–9960.
AXS Inc., Madison, WI, 2001.
4354 | Dalton Trans., 2021, 50, 4345–4354
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