Synthesis of Symmetric and Nonsymmetric NiII Thiophosphinito PECSP (E = S, O) Pincer Complexes and Their Applications in Kumada Coupling under Mild Conditions

Article abstract of DOI:10.1002/ejic.201701442

The reaction of 1,3-dimercaptobenzene with iPr₂PCl in the presence of the base NaH, furnishes the symmetric ligand [C₆H₄-1,3-(SPiPr₂)₂] (1a). The direct reaction of this ligand as well as of the literature-known nonsymmetric ligand [C₆H₄-1-(SPiPr₂)-3-(OPiPr₂)] (1b) with NiCl₂ affords the symmetric bis(thiophosphinito) PSCSP pincer complex [{C₆H₃-2,6-(SPiPr₂)₂}NiCl] (2a) as well as the phosphinito–thiophosphinito POCSP pincer complex [{C₆H₃-2-(SPiPr₂)-6-(OPiPr₂)}NiCl] (2b). Both complexes were fully characterised and their catalytic performance in Kumada coupling of aryl halides and p-tolylmagnesium bromide under mild conditions was evaluated.

Full text of DOI:10.1002/ejic.201701442

A Journal of
Accepted Article
Title: Synthesis of symmetric and non-symmetric Ni(II) thiophosphinito
PECSP (E = S, O) pincer complexes and applications in Kumada
coupling under mild conditions
Authors: Patrick Hasche, Markus Joksch, Georgia Vlachopoulou,
Hemlata Agarwala, Anke Spannenberg, and Torsten
Beweries
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201701442
Link to VoR: http://dx.doi.org/10.1002/ejic.201701442

10.1002/ejic.201701442
European Journal of Inorganic Chemistry
COMMUNICATION
Synthesis of symmetric and non-symmetric Ni(II) thiophosphinito PECSP
(E = S, O) pincer complexes and applications in Kumada coupling under
mild conditions
Patrick Hasche,[a] Markus Joksch,[a] Georgia Vlachopoulou,[a] Hemlata Agarwala,[a,b] Anke
Spannenberg,[a] and Torsten Beweries*[a]
Abstract: The reaction of 1,3-dimercaptobenzene with iPr2PCl in the
presence of the base NaH, furnishes the symmetric ligand [C6H4-1,3-
(SPiPr2)2] (1a). The direct reaction of this ligand as well as of the
literature-known non-symmetric ligand [C6H4-1-(SPiPr2)-3-(OPiPr2)]
(1b) with NiCl2 affords the symmetric bis(thiophosphinito) PSCSP
pincer complex [{C6H3-2,6-(SPiPr2)2}NiCl] (2a) as well as the
phosphinito–thiophosphinito POCSP pincer complex [{C6H3-2-
(SPiPr2)-6-(OPiPr2)}NiCl] (2b). Both complexes were fully
characterized and their catalytic performance in Kumada coupling of
aryl halides and p-tolyl magnesium bromide under mild conditions
was evaluated.
PCP Ir complexes (PCP = 3-1,3-(CH2PR2)2-C6H3).7 As an
extension of this concept, later Huang and co-workers reported
on the catalytic dehydrogenation of n-alkanes and heterocycles
using a related PSCOP pincer Ir catalyst [(iPrPOCSPiPr)IrHCl]
(Figure 1) under relatively mild conditions.8 Comparison of
catalytic activity with the related [(tBuPOCOPtBu)IrHCl] showed
significantly better performance of the POCSP complex, yet it
should be noted that direct comparison of these complexes is
difficult due to subtle differences in the phosphine donor sites.
Later, the same authors presented
a thorough study of
COA/TBE transfer dehydrogenation using isostructural
tBuPOCOPtBu and tBuPOCSPtBu Ir complexes and found lower
activity for the latter catalysts.9 However, hemilability of one of
the side arms of the pincer ligands might be of importance in this
case as it was discussed before for asymmetric pincer
complexes on several occasions.10 Apart from these Ir
complexes, only a handful of other RPOCSPR transition metal
complexes are known (Figure 1). Huang et al. presented a study
Introduction
Transition metal pincer complexes represent
a group of
compounds with very unique and interesting properties, among
which their high thermal stability and robustness as well as the
readily tunable nature of the tridentate ligand should be the most
of acceptorless COA dehydrogenation using
a series of
RPOCSPR Ru catalysts11 and the group of Morales-Morales
reported on a PhPOCSPPh Pd chlorido complex that was tested
for Suzuki-Miyaura cross coupling reaction of bromobenzene
and phenylboronic acid. Also in this case the asymmetric
complex was found to be a better, faster catalyst than the
symmetric PhPOCOPPh analogue under identical reaction
conditions.12
highlighted.1 As
a consequence, pincer compounds have
attracted considerable attention of the chemistry community for
a variety of applications, particularly in homogeneous catalysis.2
Depending on the field of interest, organometallic chemists have
developed different types of pincer complexes, containing a wide
range of donor/acceptor groups, backbone structures, or side-
arm moieties that allow for a rational control of steric and
electronic
properties
of
the
catalysts.3
Moreover,
enantiomerically pure complexes have been employed in
asymmetric catalysis and enantioselective synthesis.4
As one of the most notable variations of pincer compounds,
Jensen5 and Brookhart6 presented bisphosphinite Ir complexes
of the type [(RPOCOPR)IrHX] (RPOCOPR = 3-1,3-(OPR2)2-C6H3;
R = tBu; X = H, Cl) that showed by orders of magnitude better
performance in cycloctane (COA)/t-butyl ethylene (TBE),
transfer dehydrogenation reactions than the parent, oxygen free
Figure 1. Hitherto known RPOCSPR pincer complexes.
[a]
[b]
P. Hasche, M. Joksch, Dr. G. Vlachopoulou, Dr. H. Agarwala, Dr. A.
Spannenberg, PD Dr. T. Beweries
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-
Einstein-Str. 29a, 18059 Rostock, Germany
E-mail: torsten.beweries@catalysis.de, www.catalysis.de
Dr. H. Agarwala
Laboratoire de Chimie des Processus Biologiques, Collège de
France, 11 Place Marcelin Berthelot, 75005 Paris, France
The absence of systematic studies of these asymmetric,
potentially hemilabile ligands along with promising catalytic
effects reported prompted us to further investigate RPOCSPR
complexes of other transition metals. In recent studies, we had
reported on the synthesis of isostructural group 10 tBuPOCOPtBu
halido complexes.13 For this study, we decided to focus on a
comparison of Ni complexes that possess ligands with no, one,
or two S atoms. It should be noted that RPSCSPR ligands or
complexes thereof are unknown to date.
Supporting information for this article is given via a link at the end of
the document. CCDC 1584392 and 1584393 contain the
supplementary crystallographic data for this paper. These data are
provided free of charge by The Cambridge Crystallographic Data
Centre.
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10.1002/ejic.201701442
European Journal of Inorganic Chemistry
COMMUNICATION
Results and Discussion
The reaction of 1,3-dimercaptobenzene with two equivalents of
iPr2PCl in the presence of the base NaH in THF affords the
ligand [C6H4-1,3-(SPiPr2)2] (1a) as a colourless clear viscous oil
with yields >90% (Scheme 1).
Scheme 3. Synthesis of complex 2b.
Investigation of these complexes by 1H NMR spectroscopy in
toluene-d8 shows characteristic signals for the aromatic ligand
backbone along with CH and CH3 resonances due to the PiPr2
groups. 31P{1H} NMR analysis of both complexes 2a and 2b in
toluene-d8 shows that coordination of the ligands to the Ni centre
results in a significant downfield shift of the PiPr2 resonances
(1a: 66.2; 2a: 98.9 ppm and 1b: SPiPr2 65.2, OPiPr2 148.7; 2b:
SPiPr2 105.0, OPiPr2 179.7 ppm). Compared to the analogous
bisphosphinite Ni complex [(iPrPOCOPiPr)NiCl] (2c) which was
described by Zargarian et al., the value for OPiPr2 for 2b is in the
same range (cf. 2c: 185.5 ppm).15 Notably, the substitution of O
for the less electron-withdrawing S in the side arms of the pincer
ligands results in a shielding of the P nuclei and thus gives
resonances for SPiPr2 in the upfield region of the 31P NMR
spectra. Mass spectrometric analysis of both complexes 2a and
2b using chemical ionisation in positive mode shows the
molecular ion peaks along with characteristic signals due to the
loss of the chloride ligands.
Scheme 1. Synthesis of ligand 1a.
Analysis of this ligand by 1H NMR spectroscopy (benzene-d6)
shows the aromatic fragment as well as characteristic signals
due to the isopropyl group at 1.03, 1.14, and 1.82 ppm. 31P{1H}
NMR spectra in benzene-d6 show the presence of a sharp
singlet at 66.2 ppm. Comparison with the -SPR2 31P NMR
resonance of Huang’s iPrPOCSPiPr ligand 1b (cf. 68.7 ppm in
CDCl3) shows that the value for 1a is in the same range despite
the absence of the electron withdrawing -OPR2 moiety. Based
on quantitative 31P NMR spectra, ligand 1a was found to be pure
enough to be directly used in the following metallation step.
For complexation of ligands 1a and 1b to Ni, we have evaluated
different protocols that were previously described for the
synthesis of related complexes of the type [(RPOCOPR)NiCl].14
Reaction of ligand 1a with anhydrous NiCl2 in refluxing toluene in
the presence of dimethylaminopyridine (DMAP) to assist C-H
activation and scavenge the HCl by-product did not give the
desired product 2a. However, when DMAP was omitted and the
Single crystals of both complexes suitable for X-ray analysis
were obtained from saturated toluene (2a) or THF/n-hexane (2b)
solutions at -78 °C. The molecular structures are depicted in
Figure 2.
solvent
was
changed
to
MeCN,
the
symmetric
bis(thiophosphinito) complex 2a was obtained as a yellow,
microcrystalline solid in low yields (around 20%) due to its good
solubility in non-polar solvents (Scheme 2).
Scheme 2. Synthesis of complex 2a.
Figure 2. Molecular structures of complexes 2a and 2b. Thermal ellipsoids
correspond to 30% probability. Hydrogen atoms as well as the second position
of the disordered ligand (for 2b) are omitted for clarity.
Alternatively, this complex can be accessed in a more atom-
efficient and convenient way starting from Ni powder,
diisopropylchlorophosphine, and 1,3-dimercaptobenzene, thus
avoiding previous synthesis and isolation of ligand 1a (Scheme
2). The latter one-pot procedure was presented recently by
Zargarian and co-workers for related [(RPOCOPR)NiCl]
complexes.14c
The synthesis of the related non-symmetric complex
[(iPrPOCSPiPr)NiCl] (2b) can be performed in yields of up to 40%
using the aforementioned protocol that uses DMAP as a base
(Scheme 3). However, also in this case, direct reaction of the
Both complexes display distorted square planar coordination
geometries around the Ni centre.16 Bond lengths are in the
expected range and resemble those found before for complex
2c (Table 1). Density Functional Theory (DFT) calculations were
performed on these complexes (see Experimental Section for
details). The bond distances and angles of the optimised
structures agree well with the values obtained from X-ray
analysis (Table 1 and S2). Interestingly, introduction of the two S
atoms results in a significant change in the coordination
geometry. For 2c coordination around the Ni centre is best
described as distorted square planar, whereas for 2a only very
minor deviation from that coordination geometry is observed,
ligand
precursors,
diisopropylchlorophosphine
and
3-
mercaptophenol, with Ni powder affords the desired complex as
yellow crystals in similar yields (Scheme 3).
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10.1002/ejic.201701442
European Journal of Inorganic Chemistry
COMMUNICATION
Table 1. Selected bond lengths [Å] and angles [°] for complexes 2a, 2b, and 2c.
2a 2b 2c
X-ray
analysis
DFTa
X-ray
analysis
DFTa
X-ray
DFTa
analysisc,d
b
Ni-C
1.9190(15)
1.927
2.255
2.177
2.175
90.30
89.48
90.01
90.21
1.907
2.239
2.145
2.185
83.68
92.82
93.59
89.91
1.879(2)
1.875(2)
1.887
2.225
2.158
2.157
82.50
97.66
97.30
82.54
Ni-Cl
2.2134(4)
2.1671(4)
2.1589(4)
89.82(6)
90.73(2)
89.65(2)
89.80(5)
2.1945(4)
2.1395(4)
2.1944(6)
2.1887(6)
Ni-P1
2.1582(6)
2.1533(6)
Ni-P2
2.1686(4)
2.1603(6)
2.1379(6)
b
C-Ni-P1
P1-Ni-Cl
Cl-Ni-P2
P2-Ni-C
82.20(6)
82.80(6)
93.00(2)
97.70(2)
98.94(2)
93.83(2)
97.88(2)
96.16(2)
b
82.33(6)
82.12(6)
a M06/6-31G*(for C,H,O,S,P,Cl)/LanL2DZ(for Ni)/vacuum. b Values are not given due to strongly disordered POCSP ligand. c Data from Ref. 15. d Two molecules
are present in the asymmetric unit.
most likely due to the larger size of S that releases ring strain in
the five-membered metallacyclic units formed by each of the
side arms of the pincer ligand (Table 1). As a consequence, for
complex 2a, P donor atoms are located above (0.34 Å) and
below (-0.33 Å) the plane formed by the Ni centre, S and the C
atoms of the aryl backbone.17
example, in the reaction of p-Me-C6H4MgBr and PhCl, formation
of 3 was observed in 62% yield. Formation of the homocoupling
product 4 was however a problem in all experiments. In general,
formation of these side products is often observed at higher
reaction temperatures. It is interesting to note, however, that the
selectivity towards the heterocoupled product 3 is higher with 2c
than 2a or 2b, as can be seen from the ratio of 3/4 (Table 2).
Unfortunately, when reactions were performed at 0 °C, formation
of the product of homocoupling 4 could not be suppressed.
However, especially complex 2a still shows promising activity
and good conversion of most aryl halide starting materials
(Table 2, entries 10-12). In contrast to reactions at 25 °C, where
the yield was mainly only determined by the used catalyst,
reactions at 0 °C also revealed PhI as the most useful aryl halide.
In previous studies21d, independent reactions of the catalyst with
both coupling partners were performed to evaluate the possibility
of a radical pathway that includes single electron transfer
between coupling partners and subsequent reduction of the
Ni(II) centre. Therefore, we investigated the reaction of Ni
complexes 2a, 2b, and 2c with 2 equivalents of p-Me-C6H4MgBr
in THF at room temperature and the 31P NMR spectra showed
significant differences with slow formation of the corresponding
aryl complex for 2b and 2c. In case of 2b, this species slowly
converts into an unknown species. For 2a, we did not observe
the aryl complex, instead rapid formation of the aforementioned
unknown species takes place. No reaction occurs between PhI
and any of the catalyst complexes. As we see significant
reactivity of the Ni catalysts with the Grignard component, we
believe that in our case interaction of the Ni(II) halide with the
substrate is a key step as it was reported before.22
For an initial evaluation of the catalytic performance of
complexes 2a and 2b compared to the known 2c, we tested
these as pre-catalysts for Kumada coupling reaction between
different aryl halides and p-tolyl magnesium bromide. Since the
pioneering work in the 1970s by Kumada and Curriu18, group 10
metal catalysed C–C coupling between aryl halides and
Grignard reagents were extensively investigated in the past and
widely used in modern synthetic organic chemistry. Exploration
of catalytic systems for activation of unactivated electrophilic
substrates such as aryl chlorides is still of significant interest.19
A
number of different types of nickel complexes have been
reported to catalyse the Aryl-Alkyl and Alkyl-Alkyl Kumada
coupling effectively.20,21 Each of the complexes 2a-2c was
capable of catalysing the cross coupling reactions in THF at
room temperature within 24 hours to yield the desired biaryl 3
along with considerable amounts of the Grignard homocoupling
product 4 (Table 2) and protio-quenched material (i.e. toluene).
Among the catalysts tested, the Ni bis(phosphinito) POCOP
pincer complex 2c showed relatively low catalytic activity and
only very low conversion of the starting materials (Table 2,
entries 1-3), while 2b (Table 2, entries 4-6), and especially 2a
showed moderate to good conversion after 24 hours at room
temperature and promising catalytic activity with respect to
formation of the coupling product 3 (Table 2, entries 7-9). For
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10.1002/ejic.201701442
European Journal of Inorganic Chemistry
COMMUNICATION
it is observed that the aryl group of the Grignard reagent binds to
Ni, with considerably varying rates of reactivity for the three
complexes 2a-c. Hence, it is not just the electronic situation of
the Ni centre which affects the kinetics of the catalysis, but some
other factors seem to be operating. A detailed explanation of the
mechanistic aspects of the catalysis involved herein as well as
of the further potential of complexes 2a and 2b needs additional
investigation, which is currently being undergone in our lab, and
will be revealed in the upcoming future.
Table 2. Catalytic coupling of p-Me-C6H4MgBr and aryl halides by complexes
2a-2c.a
entry
aryl
catalyst
T
conversion
yield
yield
ratio
halide
[°C]
3 [%]
3 [%]
4 [%]
3/4
Conclusions
1
2
PhCl
PhBr
PhI
2c
2c
2c
2b
2b
2b
2a
2a
2a
2a
2a
2a
25
25
25
25
25
25
25
25
25
0
13
6
14
6
1
1
14.00
6.00
8.00
9.50
2.22
1.95
8.86
3.22
4.06
7.00
3.33
5.45
In summary, we have presented the synthesis and
characterisation of two Ni(II) chloride complexes that display
bis(thiophosphinito)
thiophosphinito POCSP (2b) pincer ligands. Both complexes
were tested in Kumada cross coupling reactions of aryl halides
and p-tolyl magnesium bromide under mild conditions and
showed significantly better activity than the known
bis(phosphinito) POCOP complex 2c.
3
7
8
1
PSCSP
(2a)
and
phosphinito–
4
PhCl
PhBr
PhI
18
41
32
56
48
57
35
35
78
19
51
41
62
58
69
28
30
60
2
5
23
21
7
6
7
PhCl
PhBr
PhI
8
18
17
4
Acknowledgements
9
We thank our technical and analytical staff, in particular
Benjamin Andres, for assistance. Support by LIKAT is gratefully
acknowledged.
10
11
12
PhCl
PhBr
PhI
0
9
0
11
Keywords: nickel • pincer complexes • homogeneous catalysis •
coupling reactions • X-ray analysis
a Reaction conditions: 3.5 mol% catalyst, 0.6 mmol Grignard reagent,
0.5 mmol aryl halide, THF, 24 h. 50 μL of n-dodecane were used as an
internal standard. Yields were determined with GC.
1
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complexes 2a, 2b, and 2c reveals substantial differences that
might explain the differences in reactivity. Looking into the
contribution of different molecular fragments in each of the
complexes, to the molecular orbitals (Tables S3-S5), it can be
noted that for complex 2c, Ni gives the largest contribution
(44%) to the HOMO (Table S5). On the contrary, in the case of
2a (Table S3) and 2b (Table S4), the most significant portion of
the HOMO is made up by the S atoms in the side arm of the
pincer ligands (MO contribution from S: 23% (2a, averaged);
37% (2b)). However, the higher contributions of Ni to the
HOMO−n (n=1-5) orbitals in 2a and 2b should be kept in mind.
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halide catalysed Kumada coupling reactions, oxidation of the Ni
centre is often discussed to be an important step of the catalytic
cycle.20 Therefore, as is apparent from the MO contributions, the
Ni(II) centre in 2c is most likely to undergo oxidation compared
to the other probable sites in the catalyst, during the catalytic
cycle. Alternatively, if the coupling proceeds via reduction of the
precatalyst, it can be predicted from the fragment contributions
(Tables S3-S5) to the LUMO, that all the three complexes would
show similar reactivity towards reduction, which would principally
2
3
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10-11%; Tables S3-S5). However, from preliminary experiments
:
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COMMUNICATION
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Details are depicted in the Supporting Information.
In the literature-known complex 2c, P donor atoms are located above
and below the plane formed by Ni, O and C, however this is much less
pronounced (0.02-0.17 Å).
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Entry for the Table of Contents
COMMUNICATION
Pincer Complexes
Patrick Hasche, Markus Joksch, Georgia
Vlacholouplou, Hemlata Agarwala, Anke
Spannenberg, and Torsten Beweries*
Page No. – Page No.
Nickel POCSP and PSCSP pincer complexes have been prepared for the first time
using two different protocols: metalation of the pincer ligand precursor, as well as
direct one-pot synthesis starting from Ni powder and the arene and phosphine
components. Both complexes were studied in an Aryl-Aryl Kumada coupling,
showing better activity than a similar POCOP complex.
Synthesis of symmetric and non-
symmetric Ni(II) thiophosphinito
PECSP (E = S, O) pincer complexes
and applications in Kumada coupling
under mild conditions
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