Chromium complexes supported by bidentate thioether-imine [N,S] ligands: synthesis and ethylene oligomerization studies

Article abstract of DOI:10.1039/d0nj04642f

A series of new Cr(iii) complexes of general formula [Cr{N,S}(THF)Cl₃] (2a, N,S = PhS-2-PhCH = N-^tBu;2b, N,S = PhS-2-PhCH = N-C₆H₄-4-OMe;2c, N,S = PhS-2-PhCH = N-C₆H₅;2d, N,S =^tB

Full text of DOI:10.1039/d0nj04642f

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Chromium Complexes Supported by Bidentate Thioether-Imine
[N,S] Ligands: Synthesis and Ethylene Oligomerization Studies
J. L. S. Milani,a A. F.P. Biajoli,a F. I. Batista,b R. S. Oliboni,b and O. L. Casagrande Jr,*a
Received 00th January 20xx,
Accepted 00th January 20xx
A series of new Cr(III) complexes of general formula [Cr{N,S}(THF)Cl3] (2a, N,S = PhS-2-PhCH=N-tBu; 2b, N,S = PhS-2-PhCH=N-
C6H4-4-OMe; 2c, N,S = PhS-2-PhCH=N-C6H5; 2d, N,S = tBuS-2-PhCH=N-C6H5) were prepared and characterized by elemental
analysis, IR spectroscopy and ESI-HRMS. DFT calculations confirm that mer isomers are more stable by 23.9-34.3 kcalꢀmol−1
than the fac isomers. Upon activation with methylaluminoxane (MAO), chromium precatalysts 2a-2d showed moderate
activity in ethylene oligomerization (TOF = 3.426.6 × 103 (mol ethylene)(mol Cr)1h1 at 80 °C) with Schultz-Flory distribution
of oligomers (KC4-C12  0.90) and production of polymer varying from 1.2 to 9.8 wt.%. The amount of 1-butene is the largest
component in the liquid fraction suggesting that these precatalysts operate via a Cossee-Arlman mechanism. The catalytic
activity is mainly controlled by electronic effects at the thioether/imine groups. In general, the precatalysts bearing electron-
donating substituents (tert-butyl, p-C6H4OMe) (2a, 2b and 2d) showed higher activities in comparison to 2c containing an
electron-withdrawing (i.e., phenyl) substituent. Notably, the MAO loading has strong influence on the activity of 2a. Thus,
the use of 2000 equiv. led to a remarkable increase in the catalytic activity [TOF = 971,100 (mol ethylene) (mol Cr)−1 h−1] with
high selectivity to formation of oligomers (98.8 wt.%).
DOI: 10.1039/x0xx00000x
840 g/g of Cr·h-1), with a selectivity toward 1-C6 of 98.1 wt.% and
production of a very small amount of polymer (0.16 wt.%).5b
1. Introduction
Linear -olefins (LAO) are important petrochemicals precursors
for preparing linear low-density polyethylene (LLDPE),
plasticizers, surfactants for detergents, base stock, additives for
synthetic lubricants, and alcohols for plasticizers.1 Several
commercial processes (Shell, Ineos, IFP/Sabic Alphabutol,
Sabic/Linde Alpha-SABLIN and Chevron-Phillips) produce a
wide distribution of LAO carbon chain lengths from C4 to C20.2
Furthermore, several companies (e. g. CPChem and Sasol) have
successfully commercialized a tri/tetramerization technology
for the production of 1-hexene and 1-octene.3 The last decades
have witnessed the development of several class of chromium
complexes containing
a large variety of bidentate and
tridentate ligands.4 While, the vast majority of these ligands
comprise primarily of phosphorus and/or nitrogen as donor
atoms, just a few examples of S-based ligands have been used
in the synthesis of active chromium species.5 McGuinness,
Wasserscheid and co-workers reported a highly efficient new
series of chromium catalysts for the trimerization of ethylene
based bis-(2-alkylsulphanyl-ethyl)-amine (SNS) ligands. For
instance, the activation of [bis-(2-ethylsulphanyl-ethyl)-
amine]CrCl3 (A, Chart 1) with 280 equiv. of MAO yield an
extremely high active catalytic system (TOF = 298 900 h-1; 160
Chart 1. Examples of chromium complexes for oligomerization
catalysis based on S-based ligands.
In 2005, the same research group reported a detailed study
of this system evaluating the effect of central donor atom and
chelate ring size.5c For instance, the replacement of NH central
donor group by phenylphosphino (PPh) unit [(EtSCH2CH2)2PPh
(SPS)] did not lead to selective trimerization, but rather a Schulz-
Flory distribution of olefins along with production of higher
a. Laboratory of Molecular Catalysis, Instituto de Química, Universidade Federal do
Rio Grande do Sul, Avenida Bento Gonçalves, 9500, 90501-970, Porto Alegre, RS
(Brazil). E-mail: osvaldo.casagrande@ufrgs.br
amount
of
polymer.
Complex
b. Centro de Ciências Químicas, Farmacêuticas e de Alimentos – CCQFA, 96010-900,
Pelotas, RS, (Brazil)
CrCl3[(EtSCH2CH2CH2)N(H)(CH2CH2SEt)] (B, Chart 1) which
contains an expansion of the chelate ring on one side of the
† Electronic Supplementary Information (ESI) available: Tables S1-S6 and Figures S1-
S10. See DOI: 10.1039/x0xx00000x
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ligand rise to an active trimerization catalyst. However, the and THF. The identity of 2a-2d was established on the basis of
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activity of this complex is less than half that of its symmetrical elemental analysis, IR spectroscopy, andDhOiIg: h10-.r1e0s3o9l/uDt0iNoJn04m64a2sFs
analogue A, and selectivity toward C6 is likewise lower.
spectrometry (HRMS) for 2a and 2b. All attempts to recrystallize
Gambarotta, Duchateau and co-workers have shown that the precatalysts 2a–2d using various crystallization procedures
activation of Cr(III) complex bearing 2,6-bis(RSCH2)pyridine [R= failed resulting in amorphous materials, unfortunately not
Ph, Cy] with MAO in toluene led to a highly selective catalysts suitable for a single- crystal X-ray diffraction analysis. Thus, we
for the production of 1-C6.5d For instance, [(2,6-(CySCH2)2- decided to perform DFT calculations to estimate ground state
py)CrCl3] (C, Chart 1) when treated with 500 equiv. of MAO optimized structures of chromium precatalysts 2a-2d aiming to
exhibited moderate activity (5177 g/gCr·h-1) producing almost get more insight about their geometrical preferences.
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exclusively 1-hexene (99.6 mol %).
Initially the DFT calculations were carried out in order to
More recently, Makume and co-workers described a assess the relative stability of the fac- and mer-isomers of the
systematic process condition optimization study using P,S- Cr complexes. The relative free Gibbs energy values for the
bidentate ligand/Cr(acac)3 catalyst system.5d Depending on the isomers are summarized in Table S1. In all cases, the mer
reactions conditions the amount of solids was dramatically isomers are more stable by 23.9-34.3 kcalꢀmol−1 than the fac
reduced, while maintaining reasonable activities and very high isomers. Optimized mer-structures for the ground state
liquid product selectivity. Under optimized conditions (2 equiv. obtained at the BP86/def2-TZVP level of theory are shown in
of ligand, 960 equiv. MMAO, 50 °C, 100 mL MCH, 45 bar Fig. 1. All complexes exhibited a slightly distorted octahedral
ethylene, 30 min.), the 2-(cyclohexylthio)phenyl](2- geometries with cis bidentate S,N coordination of thioether-
isopropylphenyl)(phenyl)phosphine/Cr(acac)3 catalyst system imine ligand, three chloride ligands, and the THF molecule
(D, Chart 1) showed high catalytic activity (575 691 g/gCr/h) completing the coordination sphere. DFT computed bond
together with relatively low solids (5.2%), 61.2% 1-octene distances and bond angle values are listed in Table 1. The Cr-S
selectivity and 75.6% 1-octene + 1-hexene combined in the [2.483-2.521 Å], Cr-N [2.069-2.126 Å], and Cr-Clav [2.297-2.304
liquid oligomer fraction.
Å] bond distances are comparable in magnitude with related
As part of our continuing interest in developing catalytic chromium structures reported in the literature.5b,6
oligomerization systems, the present contribution focuses on
the synthesis of Cr(III) complexes bearing bidentate thioether-
Imine [N,S] (E, chart 1) aiming to evaluate the catalytic potential
of them in ethylene oligomerization. Under optimized
oligomerization conditions this class of Cr catalysts display high
activities with a Schulz−Flory olefin distribution. This paper will
discuss the oligomerization results, evaluating the role of the
ligand, and the experimental parameters on the activity,
selectivity, and product distribution.
2a
2b
2. Results and discussion
2.1. Synthesis and characterization of chromium complexes
bearing thioether-imine bidentate ligands
The thioether-imine ligands (1a-1d) were synthesized by
Schiff base condensations between the primary amines and the
corresponding thioether benzaldehyde in methanol. Treatment
of 1a-1d with CrCl3(THF)3 in THF yielded after workup the
corresponding chromium complexes 2a-2d, which were
isolated as brown solids in good yields (62-91%) as presented in
Scheme 1.
2c
2d
Fig.
1
Ground state optimized structures of mer-
[Cr{N,S}(THF)Cl3] (2a-2d) at the BP86/def2-TZVP level of theory.
Table 1 Selected geometrical parameters for complexes 2a-2d.
2a
2.126
2b
2.074
2c
2.069
2d
2.075
Cr-N
Cr-S
Cr-O
Cr-Clav
N-Cr-O
2.483
2.127
2.304
171.34
2.480
2.106
2.301
172.66
2.480
2.093
2.300
173.86
2.521
2.131
2.297
178.24
Scheme 1. Synthesis of chromium complexes 2a-2d.
These complexes, which are air- and moisture-sensitive,
show moderate solubility at room temperature in acetonitrile
2 | J. Name., 2012, 00, 1-3
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S-Cr-Cl
Cl-Cr-Cl 169.68
171.43
174.87
170.04
175.25
169.61
173.89
169.81
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2.2. Ethylene Oligomerization Results
Initial catalytic screening of 2a-2d was performed in toluene
at 80 °C, 20 bar constant ethylene pressure, and using MAO
containing 20 wt.% TMA as cocatalyst ([Al]/[Cr] = 300). The most
relevant oligomerization results are listed in Table 2. The
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catalyst systems 2a-2d/MAO afforded
a
nonselective
distribution of α-olefins with moderate activity in ethylene
oligomerization (TOF’s = 3,400-26,600 mol(ethylene)·mol(Cr)-
1·h-1) over 15-minute runs. Quite different results have been
found for similar chromium complexes containing a pendant
donor [Cr(MeS-2-PhCH=N-(CH2)2Y)Cl3] (Y = PPh2; OMe, NMe2)
0
2a
2b
2c
2d
where upon activation with MAO led to
a primarily
Fig. 2 Influence of the nature of chromium precatalyst 2a-2d
on TOF and selectivity for oligomers (T = 80 °C, 20 bar, time =
15 min, [Al]/[Cr] = 300).
polymerization systems; in this case, the amounts of oligomers
in the liquid fraction attained only 2.0–19.0 wt% of the total of
products.7 These contrasting results might be associate to the
presence of third donor group to the thioether-imine backbone
that hinders the formation of reduced chromium species, which
is assumed to be responsible for the nonselective
oligomerization behavior.4
As presented in Fig. 2, the substituents on the thioether
and/or imine groups of the ligand play an important role on the
catalytic performance of the precatalysts. In particular, the
highest activity among the four precatalysts screened was
reached with 2b (TOF = 26,600 mol(ethylene)·mol(Cr)-1·h-1).
In order to investigate the steric effect of the thioether-
imine ligands on the activity, the topographic steric maps of 2a-
2a
2b
%VBur = 44.5
%VBur = 42.2
2d
2c
2d were evaluated from the percent buried volume (%Vbur
)
using the SambVca 2.1 software,8 and metrical data from the
optimized structures (Fig. 3). The results are compiled assuming
a 3.50 Å for the sphere radius, omitted hydrogen atoms and
scaled bond radii by 1.17, as recommended. The %Vbur values
ranging from 42.1 to 44.5 imply that the substituents at
thioether or imine group did not cause a significant influence on
the steric properties of the chromium complexes. Thus, we can
speculate that the catalytic activity is mainly controlled by
electronic and not by steric effects of the substituent at the
thioether/imine groups.
%VBur = 42.8
%VBur = 42.1
Fig. 3. Topographic steric maps of the thioether-imine ligands in
2a-2d.
All chromium complexes 2a-2d produce oligomers ranging
from C4 to C14+ with a high selectivity for -olefins. As illustrated
in Fig. 4, the selectivities within the liquid fraction are very
similar indicating that the R1 and R2 have no significant influence
on the product distribution. The distribution of oligomers
obtained by the 2a−2d/MAO systems follows Schulz–Flory rules
with similar K values (KC4-C12  0.90) as presented in Fig. 5.
In general, the precatalysts bearing electron-donating
substituents (tert-butyl, p-C6H4OMe) (2a, 2b and 2d) showed
higher activities in comparison to 2c containing an electron-
withdrawing (i.e., phenyl) substituents. Furthermore, this
positive effect on the catalytic performance is more
pronounced for chromium complexes containing electron-
donating substituents at the imine group (2a and 2b). Hence,
precatalyst 2a with a tert-butyl group in the imine unit is ca. 2.7
times more active than 2d that contains the same group bound
to the thioether moiety (compare entries 1 and 4). Moreover,
by comparing 2c and 2d, it was observed that the presence of
phenyl group at the thioether unit led to a substantial decrease
in the TOF. These results possibly arise from the poor
coordination of the S-based ligands to Cr atom and thus
destabilizing the Cr active species.5e-f
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