Controlled di-lithiation enabled synthesis of phosphine-sulfonamide ligands and implications in ethylene oligomerization

Article abstract of DOI:10.1039/d1dt00093d

Catalyst design for ethylene oligomerization has attracted significant interest. Herein, we report the synthesis of phosphine-sulfonamide-derived palladium complexes and examine their performance in ethylene oligomerization. Arresting a dilithiation intermediate ofN-(2-bromophenyl)-4-methylbenzenesulfonamide (1) at ?84 °C selectively producedN-(2-(bis(2-methoxyphenyl)phosphanyl)phenyl)-4-methylbenzenesulfonamide (L1A). However, the same reaction at ?41 °C delivered a different ligand; 2-(bis(2-methoxyphenyl)phosphanyl)-4-methyl-N-phenylbenzenesulfonamide (L2A). The generality of our strategy has been demonstrated by preparingN-(2-(diphenylphosphanyl)phenyl)-4-methylbenzenesulfonamide (L1B) and 2-(diphenylphosphanyl)-4-methyl-N-phenylbenzenesulfonamide (L2B). Subsequently,L1AandL1Bwere treated with a palladium precursor to yield 5-membered complexesC1andC2, respectively. In contrast,L2Aupon treatment with palladium produced a 6-membered metal complexC3. Thus, a small library of 7 palladium complexes (C1-C7) were synthesized by varying the donor moiety (pyridine, DMSO, and acetonitrile). The identity of palladium complexes was unambiguously ascertained using a combination of spectroscopic and analytical methods, including single-crystal X-ray diffraction. The performance of the complexesC1-C7was investigated in ethylene oligomerization and almost all of them were found to be active. The resultant ethylene oligomers were characterized using¹H and¹³C NMR, MALDI-ToF-MS, and GPC. Detailed screening of reaction parameters revealed 100 °C and 40 bars ethylene to be optimal conditions. ComplexC5outperformed other complexes and produced ethylene oligomers with a molecular weight of 1000-1900 g mol^?1

Full text of DOI:10.1039/d1dt00093d

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Controlled di-lithiation enabled synthesis of
phosphine-sulfonamide ligands and implications
in ethylene oligomerization†‡
Cite this: Dalton Trans., 2021, 50,
3717
a,b
a,b
a,b
Nilesh R. Mote, Shahaji R. Gaikwad, Kishor V. Khopade,
c
a,b
Rajesh G. Gonnade
and Samir H. Chikkali
*
Catalyst design for ethylene oligomerization has attracted significant interest. Herein, we report
the synthesis of phosphine-sulfonamide-derived palladium complexes and examine their performance in
ethylene oligomerization. Arresting
benzenesulfonamide (1) at −84 °C selectively produced N-(2-(bis(2-methoxyphenyl)phosphanyl)phenyl)-
-methylbenzenesulfonamide (L1A). However, the same reaction at −41 °C delivered a different ligand;
-(bis(2-methoxyphenyl)phosphanyl)-4-methyl-N-phenylbenzenesulfonamide (L2A). The generality of
a dilithiation intermediate of N-(2-bromophenyl)-4-methyl-
4
2
our strategy has been demonstrated by preparing N-(2-(diphenylphosphanyl)phenyl)-4-methyl-
benzenesulfonamide (L1B) and 2-(diphenylphosphanyl)-4-methyl-N-phenylbenzenesulfonamide (L2B).
Subsequently, L1A and L1B were treated with a palladium precursor to yield 5-membered complexes C1
and C2, respectively. In contrast, L2A upon treatment with palladium produced a 6-membered metal
complex C3. Thus, a small library of 7 palladium complexes (C1–C7) were synthesized by varying the
donor moiety (pyridine, DMSO, and acetonitrile). The identity of palladium complexes was unambiguously
ascertained using a combination of spectroscopic and analytical methods, including single-crystal X-ray
diffraction. The performance of the complexes C1–C7 was investigated in ethylene oligomerization and
1
Received 11th January 2021,
Accepted 4th February 2021
almost all of them were found to be active. The resultant ethylene oligomers were characterized using H
13
and C NMR, MALDI-ToF-MS, and GPC. Detailed screening of reaction parameters revealed 100 °C and
DOI: 10.1039/d1dt00093d
4
0 bars ethylene to be optimal conditions. Complex C5 outperformed other complexes and produced
ethylene oligomers with a molecular weight of 1000–1900 g mol−
1
.
rsc.li/dalton
Furthermore, the method can be customized to produce
dimers, trimers, tetramers, oligomers, and polymers. These
Introduction
8
,9
Metal catalyzed insertion polymerization of ethylene is known resultant products tremendously influence our daily life and
for seven decades and has revolutionized the polymer are being used as chemical intermediates, surfactants, and
1
,2
10,11
industry.
The insertion polymerization can be tailored to thermoplastics.
Medium-chain length intermediates (ca.
obtain high to ultrahigh molecular weight polyethylene (PE), C12 and above) are termed oligomers and are being commer-
1
2
highly branched PE, functional PE, PE with renewable mono- cially produced over half a century. Although several com-
3
–6
mers, etc.
Ligand steric and electronic parameters play a mercial processes are known to produce ethylene oligomers
7
crucial role and enable outstanding performance.
[like the Shell Higher Olefin Process (SHOP)], the foundation
1
3–17
of these technologies appears to be the catalyst structure.
Therefore, catalyst design for ethylene oligomerization to
higher olefins has attracted significant interest from academia
a
Polyolefin Laboratory, Polymer Science and Engineering Division, CSIR-National
Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India.
E-mail: s.chikkali@ncl.res.in
1
8–21
as well as industry.
Although different metals have been
b
tested in ethylene oligomerization, the 1st-row transition
Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan,
2
Rafi Marg, New Delhi-110001, India
metals such as Cr, Fe, Co, and Ni appear to be the most inves-
c
9,22–24
Center for Materials Characterization, CSIR-National Chemical Laboratory,
tigated metals.
Recent trends reveal that judiciously tai-
Dr Homi Bhabha Road, Pune-411008, India
25,26
lored Pd-complexes can catalyze ethylene oligomerization.
For example, specific phosphine-sulfonate-derived palladium
complexes are known to produce ethylene oligomers.
th
†
‡
Dedicated to Dr Swaminathan Sivaram on the occasion of his 75 birthday.
Electronic supplementary information (ESI) available. CCDC 1977110–1977114.
2
7–30
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/d1dt00093d
1
Thus, few catalysts have been reported for ethylene oligomeri-
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anticipated that the sulfonamide ligand with a pendent tosyl
(
external electron-withdrawing –SO ) moiety would coordinate
2
to palladium and might initiate ethylene oligomerization.
Herein, we report the controlled di-lithiation enabled syn-
thesis of four phosphine-sulfonamide ligands, a small library
of seven palladium complexes, and the behaviour of these
complexes in ethylene oligomerization.
Results and discussion
Ligand synthesis
Fig. 1 Molecular structure of ligands (L1A) (left) and (L1B) (right);
solvent molecules and H-atoms (except N–H) are omitted for clarity;
thermal ellipsoids are drawn at the 50% probability level.
The precursor 1 [N-(2-bromophenyl)-4-methylbenzenesulfonamide]
was prepared in 81% isolated yield by modifying the literature
3
5,36
protocol.
2 equivalents of n-BuLi (2 M solution, 1.51 mL, 3.03 mmol)
at −41 °C and chlorobis(2-methoxyphenyl)phosphane (0.43 g,
Compound 1 (0.5 g, 1.53 mmol) was treated with
∼
zation and Chart 1 (top) depicts recent phosphine-sulfonamide
derived catalytic systems.
3
1
1.53 mmol) was added. A P NMR spectrum of the reaction
Brookhart and coworkers investigated the performance of
phosphine-sulfonamide-derived nickel complex A (Chart 1).
The neutral nickel complex catalyzed ethylene oligomerization
and a degree of polymerization between 10 and 35 was
mixture displayed single resonance at −30.0 ppm and sub-
sequent detailed analysis revealed the unexpected formation
of 2-(bis(2-methoxyphenyl)phosphanyl)-4-methyl-N-phenylben-
zenesulfonamide (L2A), instead of the anticipated product
N-(2-(bis(2-methoxyphenyl)phosphanyl)phenyl)-4-methyl-
benzenesulfonamide (L1A) (Scheme 1). The existence of the
3
1
reported. Phosphine-sulfonamide ligated Pd complexes of
type B (Chart 1) have been recently reported by Chen and co-
workers. These B type complexes were active in ethylene dimer-
ization and trimerization. Enhanced activity was observed in
3
1
1
ligand (L2A) was unambiguously confirmed by P, H, 1-2D
NMR, and electrospray ionization mass spectrometry (ESI-MS)
3
2
the presence of Lewis acids such as B(C F ) . In 2015,
6
5 3
(
Fig. S6–S9‡). The NMR assignments and mass spectroscopy
Mecking and co-workers reported the phosphine-sulfonamide
data were found to match with literature reports on similar
phosphine sulfonamide ligands.
ligands capable of forming P–O (C) and P–N (D) chelate with
3
6–38
It is proposed that com-
3
3
palladium (Chart 1). The cationic complexes exhibited high
catalytic activities for ethylene dimerization, yielding butenes
with a high selectivity of up to 97.7%, while the neutral com-
plexes displayed low activity. In 2019, Nozaki and co-workers
reported a carbene-sulfonamide (Chart 1, E) ligand framework
3
4
and prepared palladium complexes. The Pd-complexes cata-
lyzed ethylene oligomerization and ethylene/methyl acrylate
co-oligomerization. Inspired by the above investigations, we
Chart 1 Phosphine-sulfonamide ligated metal complexes in ethylene
oligomerization (A–E) and our approach.
Scheme 1 Controlled di-lithiation of 1 to produce phosphine sulfona-
mide ligands L1A and L2A. Subsequent scope to L1B and L2B.
3718 | Dalton Trans., 2021, 50, 3717–3723
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pound 1 after lithiation generates intermediate 2, which under- Synthesis of palladium complexes
goes inter- or intra-molecular lithium exchange (as shown in
The phosphine-sulfonamide ligands bear at least three donor
atoms in their framework. Such ligands might display three
types of coordination modes and Chart 2 depicts different che-
lation modes. Coordination through neutral phosphine and
anionic amine will result in a five-membered ring (PN chela-
tion) (Chart 2). While the negative charge can delocalize over
the NS arm leading to a PSN complex, the formation of seven-
Scheme 1) to produce species 3 in situ. It is most likely that the
formation of L2A proceeds via this in situ generated species 3. A
paradigm shift could be achieved if the unstable di-lithiated
intermediate 2 could be selectively formed and if the lithium
exchange is arrested. In our attempts to address this bottleneck,
we performed di-lithiation of N-(2-bromophenyl)-4-methyl-
benzenesulfonamide (1) with 2 equivalents of n-BuLi at
different temperatures and time intervals. Our optimization dis-
closed that di-lithiation of 1 at −84 °C for 30 minutes followed
by the addition of chlorobis(2-methoxyphenyl)phosphane
membered palladium can be anticipated through PO chelation
39
(
Chart 2, PO).
The phosphine sulfonamide ligand (L1A) (0.100 g,
0.20 mmol) was treated with a palladium precursor [(TMEDA)
(
An
workup and purification delivered the desired phosphine ligand
L1A) in 85% isolated yield (Scheme 1).
2
PCl) at −84 °C selectively produces L1A. Subsequent acidic
PdMe ] (0.051 g, 0.20 mmol) in pyridine (5 mL) (Scheme 2,
2
31
top). The progress of the reaction was monitored by P NMR,
(
which revealed a single resonance at 26.3 ppm after 2 hours,
indicating completion of the reaction. Co-evaporation and
washing produced a solid compound in 91% isolated yield.
31
The appearance of a single P NMR resonance at −47.4 ppm
clearly indicated the formation of L1A. The identity of L1A was
unambiguously established using a combination of spectro-
scopic and analytical methods (Fig. S10–S15‡). Electrospray
ionization mass spectrometric analysis of L1A revealed a mole-
31
The P NMR spectrum of this residue (displayed a single peak
at 26.3 ppm) suggested coordination of phosphine to palla-
1
dium metal (Fig. S22‡). H NMR revealed a characteristic reso-
+
cular ion peak at m/z = 492.1393 [L1A + H] . Crystals suitable for
13
nance at 0.32 ppm, while the C NMR disclosed a resonance
X-ray diffraction were obtained by layering a DCM solution of
L1A with hexane and the layered Schlenk was stored at 0 °C.
Ligand L1A crystallizes in the triclinic unit cell in the P space
1
at −2.7 ppm that can be assigned to the Pd–Me group in C1
31
1
13
(
Fig. S23 and S24‡). The above P, H, and C NMR shifts
closely match with literature reported similar palladium
group. The molecular structure of L1A clearly shows the phos-
phine is covalently bonded ortho to sulfonamide (Fig. 1, left).
Thus, di-lithiation of 1 at −41 °C produces L2A via inter-
mediate 3, while the reaction at −84 °C selectively delivers
L1A. The controlled lithiation strategy provides access to two
ligands starting from the same precursor. Such a strategy is
highly desirable and could be applied to generate a ligand
library. To demonstrate the generality of this strategy, we set
out to prepare two more ligands, L1B by performing the reac-
tion at −84 °C and L2B at −41 °C (Scheme 1). In our attempts,
38,40
complexes.
The NMR findings were corroborated by
ESI-MS and a molecular ion peak was observed at m/z =
+
6
91.0795 [C1 + H] (Fig. S31‡). An unambiguous demon-
2
equivalents of n-BuLi (2 M, 3.06 mL, 6.13 mmol) were added
to a THF solution of compound 1 at −84 °C. After 30 minutes,
chlorodiphenylphosphane (0.56 mL, 3.06 mmol) was added
dropwise to the above reaction mixture at −84 °C and was
stirred for 16 hours. Work up and purification produced L1B
Chart 2 Possible coordination modes of the phosphine-sulfonamide
ligand.
3
1
1
in 74% isolated yield. A P NMR resonance at −26.4 ppm, H,
1
3
C, and 1-2D NMR confirmed the formation of L1B
Fig. S16‡). The NMR findings were further corroborated by
ESI-MS, which disclosed a molecular ion peak at m/z =
(
+
4
32.1175 Da [L1B + H] (Fig. S19‡). The existence of L1B was
unambiguously ascertained from single-crystal X-ray diffrac-
tion analysis. Single crystal X-ray diffraction measurements
revealed that L1B crystallizes in the triclinic space group and a
P–C bond length of 1.83 Å corresponding to covalent P–C
bonds was observed (Fig. 1, right). As anticipated, when com-
pound (1) was treated with n-BuLi and chlorodiphenylpho-
sphane at −41 °C, the formation of L2B was observed. The
identity of L2B was established by 1-2D NMR spectroscopy and
by using analytical methods. Thus, the controlled lithiation
strategy provides access to 4 different ligands starting from
one common precursor (compound 1), in a single step, in very
good to excellent isolated yields.
Scheme 2 Synthesis of palladium complexes C1, C2 and C3.
Dalton Trans., 2021, 50, 3717–3723 | 3719
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stration of the coordination of L1A was revealed by single- C1. Crystals of C1 grew in the monoclinic unit cell in the Pˉ1
crystal X-ray diffraction of C1. The molecular structure dis- space group. The crystal consists of a central palladium atom
closed the formation of a 5-member ring and PN coordination with a distorted square planar geometry and is surrounded by
to yield C1. Fig. 2 depicts the molecular structure of complex bis-chelating (L1A), a methyl group, and a coordinating pyri-
dine molecule per formula unit. The P–Pd bond distance of
2
.20 Å (Fig. 2, top) confirms the formation of a coordinate
4
1
bond. The donor molecule pyridine revealed a Pd–N2 bond
distance of 2.11 Å.
4
2
The pyridine is placed trans to phosphine while the sulfona-
mide nitrogen atom is trans to the methyl group. The methyl
group on palladium and donor pyridine occupy cis-disposition
(
C26–Pd1–N2 = 86.82°), which is a prerequisite for a metal
4
3
complex to be active in insertion polymerization.
Along the same lines, phosphine sulfonamide ligand L1B
was treated with a palladium precursor to produce palladium
3
1
complex C2 in 92% isolated yield. The P NMR spectrum
revealed a single peak at 40.8 ppm, confirming the coordi-
1
nation of phosphine to palladium metal (Fig. S33‡). While H
1
3
NMR displayed a characteristic resonance at 0.42 ppm,
C
NMR revealed a peak at −2.4 ppm, which can be assigned to
the methyl group located on the palladium in C2 (Fig. S34 and
35‡).
The observed ESI-MS further supported the existence of C2
and single-crystal X-ray diffraction unambiguously ascertained
the formation of C2. C2 crystallizes in a triclinic unit cell with
two molecules of the complex per unit cell (Fig. 2, center). The
central palladium shows a distorted square planar geometry
and similar bonding arrangements to that of C1.
Among L2A and L2B, the coordination behavior of L2A was
investigated. Thus, L2A produced complex C3 in 92% isolated
yield (Scheme 2, bottom). The existence of the complex C3 was
3
1
established using spectroscopic and analytical methods.
P
NMR shows a single peak at 22.8 ppm which suggests coordi-
1
nation of phosphine to palladium metal (Fig. S44‡). H NMR
displayed a characteristic peak at 0.04 ppm for the Pd–Me
group which suggests the formation of the desired Pd complex
C3 (Fig. S45‡). The existence of C3 was further authenticated
by single-crystal X-ray diffraction analysis (Fig. 2, bottom).
X-ray diffraction of C3 revealed that the phosphine-sulfona-
mide ligand (L2A) coordinates to the Pd-center and forms a
6
membered ring, in contrast to 5 membered rings in C1 and
C2 (Fig. 2, top). The metal-containing ring displays a P–Pd–N
angle of ∼94° which is larger than the P–Pd–N angle in C1 and
C2.
It is known in the literature that weakly coordinating donor
groups such as DMSO or acetonitrile display better activity in
4
4–47
ethylene insertion polymerization.
Given the literature pre-
cedence and better performance of such donor groups, we set
out to prepare complexes C4–C7 (Scheme 3). The sodium salt
of L1A and L1B was treated with [(COD)PdMeCl]/[(DMSO)
2
PdMeCl] to produce C4–C7 in good to excellent yields.
Treating sodium salt of ligand (L1A) (L1A.Na) with [(COD)
PdMeCl] produces complex C4 in good yield (66%) (Scheme 3,
top). The existence of complex C4 was established using 1-2D
NMR and ESI-MS analysis. Furthermore, the existence of the
complex C4 was authenticated by the appearance of a pseudo-
Fig. 2 Molecular structures of palladium complexes C1 (top), C2
center), and C3 (bottom); solvent molecules and H-atoms are omitted
for clarity; thermal ellipsoids are drawn at the 50% probability level.
(
3720 | Dalton Trans., 2021, 50, 3717–3723
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Table 1 C1–C7 catalyzed ethylene oligomerization
Temp.
Entry Catalysts (°C)
Press.
(bar)
Yield
(mg)
TOF (mol of OE
per mol of Pd h
−
1
)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
C1
C1
C1
C2
C2
C2
C3
C3
C4
C4
C4
C5
C5
C5
C5
C5
C5
C5
C5
C5
C5
C5
C5
C6
C6
C6
C7
C7
C5
C5
80
90
90
80
90
90
90
100
80
90
90
80
90
100
100
100
100
110
110
110
110
120
100
70
80
90
80
90
90
90
10
10
20
10
10
20
10
40
20
10
20
10
10
10
20
30
40
10
20
30
40
40
40
10
10
10
10
10
10
10
Trace
15
19
Trace
10
15
Trace
NO
60
67
74
64
101
99
104
106
141
103
106
102
99
—
8
11
—
5
—
—
—
34
37
42
36
57
56
58
60
80
58
60
58
56
55
63
—
9
a
0
1
2
3
4
Scheme 3 Synthesis of palladium complexes C4, C5, C6, and C7.
15
16
17
18
19
20
+
molecular ion peak at m/z = 612.0586 [M-DMSO + H]
(
Fig. S55‡).
Similarly, metal complexes C5, C6, and C7 were prepared in
good to excellent yields by treating the ligand (L1A/B) with the
21
22
23
24
97
a
111
Trace
17
43
39
53
97
99
palladium precursor (Scheme 3). The identity of C5–C7 was 25
26
27
28
24
22
30
29
14
established using a combination of 1-2D NMR spectroscopy,
ESI-MS, and analytical methods. A characteristic down-field
3
1
b
c
shifted P NMR, the appearance of Pd–M
0
̲
̲
protons between 29
30
1
3
.04 and 0.55, corresponding up-field shifted C NMR reso-
nances, and ESI-MS unambiguously authenticated the for-
mation of complexes C5–C7 (Fig. S56–S71‡).
Reaction conditions: catalyst = 63 μmol in DCM (10 mL), toluene =
100 mL, ethylene = 10–40 bars, time = 1 h, OE = oligoethylene, NO =
not observed, ND = not determined, the reported yield is after
subtracting catalyst quantity from the final weight of solids/semi-
C1–C7 catalyzed ethylene oligomerization
6 5 3
solids, a = B(C F ) was used as an additive to activate the catalysts, b
and c = oligomerization was performed for 2 h and 4 h respectively.
Neutral palladium complexes such as C1–C7 can catalyze ethyl-
ene oligomerization without the aid of any co-catalyst.
Performance of palladium complexes C1–C7 in ethylene oligo-
merization was examined and Table 1 reports the most impor-
In contrast, the acetonitrile coordinated complex C5 pro-
tant results. The pyridine coordinated palladium catalyst C1 duced ca. 100 mg of semi-solid material under identical con-
was found to produce a trace amount of highly viscous liquid at ditions (Table 1, entry 13). The labile coordination of aceto-
8
9
0 °C (Table 1, entry 1). However, increasing the temperature to nitrile in C5 could be responsible for the increased activity.
0 °C produced 15–19 mg of semi-solid material (Table 1, Motivated by the better performance of C5, detailed screening
entries 2 and 3). A similar quantity of semi-solid material was was conducted for this catalyst. The effect of ethylene pressure
produced by catalyst C2 at 80–90 °C (Table 1, entries 4–6). The at 100 °C was evaluated. As evident from Table 1, entry 14–17,
6
-membered palladium complex C3 produced a trace amount of with increasing ethylene pressure the quantity of ethylene oli-
material at 90 °C. But increasing the polymerization tempera- gomers increased. At 100 °C and 40 bars ethylene pressure,
ture to 100 °C and addition of Lewis acid (B (C F ) ) failed to nearly 140 mg of semisolid material was obtained (Table 1,
6
5 3
produce an observable amount of material (Table 1, entry 8). entry 17). Detailed analysis of this material (Table 1, entry 15)
1
3
The reduced activity of C1–C3 can be ascribed to the strong revealed the presence of ethylene oligomers. A C NMR dis-
coordination of pyridine to the palladium center. It is well docu- closed a characteristic methylene repeat unit peak at 29.3 ppm
mented in the literature that the pyridine coordinated palla- (Fig. S73‡), flanked by other low-intensity signals between 10
dium catalysts often display poor activity.
and 37 ppm. These resonances can be ascribed to various
Literature reports suggest that substituting pyridine with (methyl, ethyl, propyl, butyl, etc.) branches on oligomeric ethyl-
4
8
weakly coordinating donors such as DMSO or acetonitrile ene. The proton NMR revealed an absolute number average
−
1
might help in increasing the catalytic activity. Thus, a DMSO molecular weight of 1100 g mol , and GPC measurements
coordinated complex C4 was evaluated in the ethylene oligo- displayed a similar weight average molecular weight of 1100 g
−
1
merization reaction (Table 1, entries 9–11). Indeed, exposing mol . MALDI-ToF-MS analysis of the resultant material
C4 to ethylene led to the production of an increased quantity revealed a repeat unit mass of 28 Da (Fig. S87 and 88‡). Thus,
(
74 mg) of semi-solid material.
complex C5 produced highly branched ethylene oligomers.
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While further increasing the polymerization temperature to
The pyridine-coordinated complexes C1–C3 were found to
1
1
1
10 °C resulted in diminished polymer yield (Table 1, entries be active; however, these complexes revealed a very poor yield
8–21), increasing the polymerization temperature further to in the ethylene oligomerization reaction. The poor perform-
20 °C proved futile (Table 1, entry 22). When the phenyl sub- ance can be ascribed to the stronger coordination of pyridine
stituted complexes, C6 and C7, were evaluated, the formation (to palladium), without leaving any scope for ethylene coordi-
of up to 53 mg of oligomeric semi-solid material was observed. nation. While weakly coordinating DMSO complex C4 revealed
Thus, complexes C1–C3 with pyridine ligation underperform a better performance with 74 mg of ethylene oligomers, the
and display very low or no activity. However, DMSO and aceto- acetonitrile-coordinated complex C5 revealed an improved per-
nitrile coordinated complexes C4–C7 revealed better perform- formance with ca. 100 mg of ethylene oligomers. Optimization
ance. Among these C4–C7, anisole-derived complexes C4 and of reaction parameters using C5 indicated 100 °C at 40 bars as
C5 displayed better performance than their phenyl counter- an optimal reaction condition. Nearly 140 mg of ethylene oli-
parts C6 and C7. Clearly, complex C5 with anisole substituents gomer with a molecular weight of 1100 g mol− was produced
on the phosphine phosphorus and the ACN donor is a under these optimal conditions.
1
superior catalyst for ethylene oligomerization. Analysis of the
resultant semi-solid material indicates formation of highly
branched oligomers. Formation of branched oligomers
suggests that C4–C7 are susceptible to insertion and fast
Conflicts of interest
β-hydride elimination. The microstructure analysis of oligo- There are no conflicts to declare.
1
3
ethylenes displayed C NMR resonances at 10.9, 14.11, 19.7,
and 27.0 ppm. Based on literature precedence, these peaks can
be ascribed to various methyl, ethyl, propyl and butyl
Acknowledgements
1
9
branches. Thus, the observed branching could be a result of
β-hydride elimination (after a few ethylene insertions) and We are indebted to DST-SERB (EMR/2016/005120), CSIR-NCL
1
3
reinsertion of the eliminated olefin. The down-field shifted
C
(HCP0011), India, and AvH Foundation Bonn, Germany for
NMR resonances at 114.0 and 139.2 (Fig. S73‡) can be financial support.
4
9
assigned to chain-end double bonds. Although C5 is the best
among C1–C7, better catalysts for ethylene oligomerization
1
6,19,20,26
have been reported in the literature.
Notes and references
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Conclusions
14726.
In summary, arresting di-lithiation of N-(2-bromophenyl)-4-
methylbenzenesulfonamide (1) provided access to two
different ligands. Controlled lithiation of 1 at −84 °C selec-
tively yields ligand N-(2-(bis(2-methoxyphenyl)phosphanyl)
phenyl)-4-methylbenzenesulfonamide (L1A), while the same
reaction at −41 °C enabled access to 2-(bis(2-methoxyphenyl)
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(L2A).
Subsequently, ligands N-(2-(diphenylphosphanyl)phenyl)-4-
methylbenzenesulfonamide (L1B) and 2-(diphenylphospha-
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3722 | Dalton Trans., 2021, 50, 3717–3723
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