Palladium pincer complexes featuring an unsymmetrical SCN indene-based ligand with a hemilabile pyridine sidearm

Article abstract of DOI:10.1039/c9dt00898e

A new unsymmetrical indene-based pro-ligand featuring thiophosphinoyle and methylpyridine sidearms 2 was prepared. Coordination and cyclometalation in the presence of [PdCl₂(PhCN)₂] and PS-DIEA afforded three well-defined 2-indenyl S

Full text of DOI:10.1039/c9dt00898e

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Palladium pincer complexes featuring an unsymmet
D
r
O
icI:
a
10
l
.10
S
3
C
9/C9DT00898E
N
indene-based ligand with an hemilabile pyridine sidearm
a
a
b
a
a
Paul Brunel, Chloé Lhardy, Sonia Mallet-Ladeira, Julien Monot,* Blanca Martin-Vaca* and Didier
Bourissou*
Received 00th January 20xx,
Accepted 00th January 20xx
a
DOI: 10.1039/x0xx00000x
The new unsymmetrical indene-based pro-ligand featuring thiophosphinoyle and methylpyridine sidearms 2 was prepared.
Coordination and cyclometalation in the presence of [PdCl
pincer complexes 3a-c. The lability of the pyridine moiety has been evidenced upon treatment with triphenylphosphine and
,6-dimethylphenylisocyanide. In addition, reversible C–Pd bond cleavage has been demonstrated under Brönsted acid/base
2 2
(PhCN) ] and PS-DIEA afforded three well-defined 2-indenyl SCN
2
conditions. The indenediide SCN pincer complex 4 was prepared by deprotonation of 3a in the presence of
triphenylphosphine. Preliminary catalytic tests on the cyloisomerization of 4-pentynoic acid have underlined the impact of
the pyridine sidearm on catalytic activity.
the pincer complex B (Chart 1) with migration of the CC double
bond.8
Here, we report the preparation of a new unsymmetrical
indene-based ligand featuring thiophosphinoyle and
Introduction
Over the last decades, metal/ligand cooperativity (MLC) has
become a very active and flourishing domain in catalysis, in
particular thanks to the progress of the chemistry of pincer
methylpyridine sidearms as a fusion of models A and B (Chart
1, compounds C). Coordination to Pd leading to the
1
complexes. Starting from ruthenium pincer complexes, MLC
corresponding indenyl and indenediide pincer complexes is
described. The hemilability of the pyridine moiety and its impact
on the catalytic activity of the indenediide pincer complex are
discussed.
has been developed with different late transitions metals (Ru, Rh,
2
Ir…), and more recently first row transition metals (Fe, Mn,
3
Ni…). Despite the forefront position of Pd in catalysis, scarce
examples of MLC involve Pd pincer complexes so far.4,5 This is
most likely due to the tridentate nature of pincer ligands and to
the square-planar geometry of Pd(II) which result in only one
available “working” site at Pd, limiting thereby the variety of
possible transformations.
To overcome this limitation, the design of a hemilabile pincer
6
ligand is appealing. According to the HSAB theory, the soft
Pd(II) prefers soft donor atoms such as P or S over hard N
2
donors. Accordingly, the incorporation of a N(sp ) lateral donor
site into a tridentate ligand is attractive to achieve hemilability in
7
pincer Pd(II) complexes. Wendt and coll. applied this strategy
for the preparation of a hemilabile unsymmetrical PCN
palladium pincer complex and demonstrated its efficiency in the
catalytic decarboxylative cross-coupling of propiolic acids and
aryl halides.
As part of our ongoing research program on indenediide
pincer complexes (Chart 1, compound A),4
b,c
Chart 1. Bisthiophosphinoyl-indenediide (A), bismethylpyridine-indenyle (B) and the
new thiophosphinoyl-methylpyridine indenyl and indenediide (C) Pd pincer complexes.
we have
demonstrated the high structural modularity of these systems and
improved thereby their catalytic activity in cycloisomerization
reactions. Seeking for further structural modifications, we
reported last year the synthesis of a tridentate indene-based
Results and discussion
ligand with two pyridine sidearms. Coordination to Pd afforded Preparation of the indenyl and indenediide complexes (3a-c,
4
)
8
Starting from compound 1, the pro-ligand 2 was readily
prepared by deprotonation and addition of
chlorodiisopropylphosphine followed by oxidation with an
excess of elemental sulphur (scheme 1). After purification by
flash chromatography, compound 2 was obtained in 76% yield
as a brown viscous oil. The introduction of the thiophosphinoyle
arm is confirmed by the appearance of a sharp singlet at 70 ppm
in 31P NMR spectroscopy. The H NMR signal for the Csp3–H
a. CNRS, Université Paul Sabatier, Laboratoire Hétérochimie Fondamentale et
Appliquée (LHFA, UMR 5069), 118 Route de Narbonne, 31062 Toulouse Cedex 09,
France. Email: monot@chimie.ups-tlse.fr; bmv@chimie.ups-tlse.fr and
dbouriss@chimie.ups-tlse.fr
Université de Toulouse, UPS, Institut de Chimie de Toulouse, FR2599, 118 Route
de Narbonne, F-31062 Toulouse, France.
b.
†Electronic Supplementary Information (ESI) available: Synthetic procedures and
1
characterizations, in-situ reactIR analysis details, relevant NMR spectra, X-ray
crystallographic experimental details. CCDC-1898996 (3b) and CCDC-1898855 (5). bearing the thiophosphinoyle sidearm appears with the expected
For ESI and crystallographic data in CIF See DOI: 10.1039/x0xx00000x
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multiplicity (dd, JHP = 23.5 and JHH = 1.8 Hz) at 4.26 ppm, signals at 81.9 and 82.5 ppm (in a 7 / 3 ratio,VieSwcAhrteicmleeOn2lin)e.
similarly to what was observed for the bis(thiophosphinoyle) Increasing the reaction time to 20 h iDnOIo: r1d0.e1r03t9o/C9pDroTb00e89th8Ee
indene pro-ligand (Chart 1, A).4b
interconversion between them, lead in fact to the formation of a
third compound as apparent from the additional singlet observed
at 84.9 ppm in the P NMR spectrum. These three species could
be separated by flash chromatography with a good overall yield
31
n
1
2
-
BuLi
(76%). Stopping the reaction at 30 min allows to obtain 3a as the
N
i
N
- Pr PCl
2
major product.
3- S8
For the three complexes, the coordination of both the
thiophosphinoyle and pyridine sidearms to the Pd center was
i
Pr P
2
S
1
2
76%
Scheme 1. Preparation of the pro-ligand 2 from 1.
3
confirmed by NMR spectroscopy in CDCl . For each species, the
31P NMR spectra displays a sharp singlet (at 81.9 and 82.5 ppm,
respectively) shifted downfield by at least 10 ppm compared to
that of the pro-ligand (70 ppm). In the H NMR spectrum, the
Then, pro-ligand 2 was reacted with [PdCl
2
(PhCN)
2
] in DCM
1
at room temperature. After 30 min, an important precipitate was
observed. The absence of 31P NMR signal indicated complete
consumption of 2. The high insolubility of the solid obtained
after filtration prevented its characterization. Surmising that this
precipitate results from the protonation of the pyridine sidearm
upon activation of the C1–H bond, the reaction of 2 with
H
3
ortho of the pyridine ring is shifted downfield for all complexes
(9.61, 9.70 and 9.93 ppm) with respect to that of 2 (8.55 ppm).
In the case of compound 3b, the carbon atom connecting the
2
pyridine to the ligand backbone is no longer part of a CH group
but a Csp2–H group, as indicated by the H and 13C NMR signals
at 6.94 and 115.5 ppm, respectively. This indicates migration of
the C=C double bond, as in the case of the bis(pyridine)indenyl
pincer complex B. Consistently, C1 and C2 display typical
1
2 2
[PdCl (PhCN) ] was performed in the presence of polystyrene-
supported diisopropylethyl amine (PS-DIEA) as an external
base. After 30 min in DCM at room temperature, 31P NMR
spectroscopy showed complete consumption of 2 and the
apparition of two well-defined species giving rise to singlet
8
13
1
features for Csp3–H moieties ( C NMR: 53.6 and 37.1 ppm, H
NMR: doublet of doublets at 4.63 and 5.12 ppm).
N
N
30 min
+
Pd Cl
Pd Cl
76%
i
i
Pr P
2
S
Pr P
2
S
PS-DIEA
2
3a
3b
[
PdCl (NCPh) ]
N
2
3a / 3b = 7 / 3
DCM, r. t.
i
Pr P
2
S
N
N
N
2
0 h
+
+
2
Pd Cl
Pd Cl
Pd Cl
77%
i
i
Pr P
2
S
Pr P
2
S
iPr2P
S
3a
3b
3c
3a / 3b / 3c = 4 / 3 / 3
Scheme 2. Synthesis of the SCN Pd indenyle complexes 3.
Figure 1. Ellipsoid drawings (50% probability) of the molecular structure of 3b. For clarity,
lattice solvent molecules and hydrogen atoms are omitted, except H1, H2 and H10.
Crystals were grown by slow diffusion of pentane in a
saturated dichloromethane solution at room temperature. The
structure of 3b was definitely confirmed by X-ray diffraction
analysis. As shown in Figure 1, the ligand is coordinated by N,
C2 and S, and the Pd center in a slightly distorted square-planar
environment. The C3 and C10 atoms are both in trigonal planar
environment and form a CC double bond (dC3-C10 = 1.345(6) Å).
Conversely, C1 and C2 are tetrahedral and the two respective H
atoms (H1 and H2) are syn to each other.8
Assignment of complexes 3a and 3c as 2-indenyl Pd
complexes, without migration of the CC double bond, was based
2
| J. Name., 2012, 00, 1-3
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on NMR spectroscopy. The 13C NMR spectra clearly shows the study was carried out on the indenyl complexes 3.VTiewheArtaicdledOitniloinne
DOI: 10.1039/C9DT00898E
presence of an aliphatic CH
2
signal (at 37.48 and 39.45 ppm, of increasing amounts of triphenylphosphine to a solution of 3a
2
respectively) for the methylpyridine sidearm. For 3c, the CH
2
in d -DCM at room temperature was monitored by NMR
1
protons resonate in H NMR as an AM system (2.74 and 3.46 spectroscopy (See Figure 2).
ppm) due to the proximity of the asymmetric C3 center. The
carbon of the indene directly bonded to P (C1) is quaternary and
appears in the Csp2 zone (135.5 ppm, JCP = 92.0 Hz). For 3a, the
CH signal at 63.77 ppm displays a large coupling to phosphorus
1
(
J
CP = 45.8 Hz), as the associated H NMR signal at 4.78 ppm
HP = 20.6 Hz).
(J
Having the palladium indenyl complexes 3 in hands, the
preparation of the corresponding indenediide complex was
attempted. First, a similar strategy to that applied for the
synthesis of the bis(thiophosphinoyle) indenediide palladium
dimer4b was tested on 3a, using NaHMDS as a base (Scheme 3,
top). No deprotonation occurred but isomerization of 3a into 3b
and 3c was observed.
Figure 2. Stacking of 1H NMR spectra for 3a (region 9.70-8.50 ppm) with sequential
2
additions of triphenylphosphine in d -DCM at 293 K.
N
i
Pd
S
P Pr2
The addition of triphenylphosphine resulted in a broadening
and upfield shift of the H NMR signal attributed to the Hortho
atom of the pyridine group (Figure 2). The 31P NMR signal
iPr2P
S
Pd
1
NaHMDS
N
X
KO Bu
PPh3
80 °C to r.t.
N
related to the thiophosphinoyle sidearm disappeared, whereas a
new broad signal at 80.9 ppm was observed. At the same time,
the singlet signal corresponding to the free triphenylphosphine at
-5.18 ppm significantly broadened and a second large signal
appeared at 28.7 ppm. This is consistent with the coordination of
Pd Cl
t
iPr2P
S
N
DCM,
-
Pd PPh3
3a
iPr2P
S
PPh
3
to Pd with relatively fast exchange between free and
resulted
4
86 %
coordinated phosphine. Adding 50 equivalents of PPh
3
Scheme 3. Preparation of Pd indenediide complex 4.
in a resolved doublet signal for Hortho at 8.50 ppm in the region
of the free ligand (8.55 ppm for pro-ligand 2). All these
observations are consistent with the displacement of the pyridine
At this stage, we decided to target a chloropalladate type
complex using the previously reported procedure.4b
Unfortunately, the addition of a base to 3a in the presence of
ammonium chloride salt did not lead to the desired compound.
3
sidearm at Pd by PPh in the presence of an excess of the
phosphine. However, the isolation and characterization of the
resulting complex was unsuccessful due to the high fluxionality
of the system.
t
Using BuOK as the base in the presence of a more strongly
coordinating co-ligand such as triphenylphosphine proved more
fruitful, leading to a unique well-defined compound (Scheme 3,
down). Complex 4 was isolated as a dark red powder after
precipitation with pentane in an excellent yield (86%). Two
sharp doublet signals are observed in the 31P NMR spectrum at
In order to further substantiate the decoordination of the N
sidearm, we turned to a less hindered partner, namely 2,6-
dimethylphenyl isocyanide (Scheme 4). We hypothesized an
easier displacement of the pyridine and thus, the use of a lower
amount of the external co-ligand.
7
6.6 and 17.8 ppm (JCP = 45.5 Hz) for the thiophosphinoyle
The addition of one equivalent of 2,6-dimethylphenyl
isocyanide to a solution of 3a in DCM at -80°C led to half
conversion into a new compound 5 displaying a singlet at 71.8
sidearm and coordinated triphenylphosphine, respectively.
Moreover, the disappearance of the H atom at C1 is clearly
1
13
apparent from H and C NMR (a Cq signal is observed for C1),
31
ppm in P NMR. Two equivalents of isocyanide were necessary
to convert quantitatively 3a into 5. The structure of 5 was
determined by X-ray diffraction (crystals grown by slow
diffusion of pentane in a saturated DCM solution at room
temperature). A first observation confirms the decoordination of
the pyridine sidearm and the coordination of an isocyanide to the
Pd center. More surprising is the presence of a second unit of
isocyanide inserted into the Pd–Csp2 bond resulting in a 6-
membered metallacycle with chair conformation. The palladium
center is in a slightly distorted square-planar environment. The
confirming the formation of an indenediide moiety. Complex 4
is only moderately stable in solution. Its half-life time was
6
estimated to ~10 h in CDCl
3
and 21 h in d -benzene at room
9
temperature. This poor stability prevented the obtention of
crystals suitable for X-ray diffraction analysis.
Evaluation of the hemilabity of the pyridine moiety.
The lability of the pyridine moiety of the pincer ligand was
investigated next. Due to the low stability of 4 in solution, the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
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formation of 5 most likely results from displacement of the thus the exact nature of the fourth ligand at PdVie(wXArt=icleCOlnlioner
DOI: 10.1039/C9DT00898E
pyridine by an isocyanide, followed by its insertion into the Pd– CF
C2 bond and coordination of a second isocyanide unit. This is
the first time that an insertion process into the central Pd–C bond
is observed with this kind of indenyle pincer complexes, but a
similar process was previously reported by Gebbink and Szabó
with SCsp2S and PCsp2S Pd pincer complexes.10
3
CO
2
) remains unknown.
N
Pd
S
N
H
O2CCF3
TFA
H
Pd Cl
DIEA
X
i
S
i
Pr P
2
Pr P
2
3b
3
X = Cl or CF CO2
6
C
N
N
Scheme 5. Equilibrium between 3b and 6 under Brönsted acid-base conditions
N
DCM
N
Pd Cl
2
N
-
80°C to r.t.
Interestingly, the transformation of 3b into 6 is reversible.
Following the formation of 6 by reacting 3b with 3 equivalents
of TFA, 3 equivalents of a base (DIEA) were added. NMR
showed instantaneous and quantitative reformation of 3b (Figure
Pd
iPr2P
S
iPr2P
S
Cl
3a
5
Scheme 4. Decoordination of the methylpyridine arm and insertion of an isocyanide unit.
4). Thus, the cleavage / formation of the C2–Pd bond is
reversible under Brönsted acid/base conditions. The protonation
of the C2–metalated ligand likely represents the initial stage of
ligand dissociation under strongly acidic conditions.
Conversely, the formation of the C2–Pd bond from 6 is
reminiscent of the formation of the SCS bis(thiophosphinoyl)
methylindenyl Pd pincer complex.12
Despite the low stability of 4 in solution, we were curious
about the impact that the pyridine sidearm may have on catalytic
activity. Therefore, the activity of 4 was evaluated in the
cycloisomerization of 4-pentynoic acid as a model reaction for
which complete conversion was observed after 1 h with the
i
.4b,13 The
palladium indenediide dimer [Ind( Pr
2
P=S)
2
Pd]
2
3
reaction was carried out in CDCl at room temperature, with 5
Figure 3. Ellipsoid drawings (50% probability) of the molecular structure of 5. For mol % of catalyst loading and in the absence of any external base
clarity, lattice solvent molecules and hydrogen atoms are omitted, except H1 and
H10.
(Scheme 6). The reaction proceeded readily and 62% conversion
into the 5-exo lactone was observed after 1 h.14 In order to
Thus, reacting the indenyl complex with a phosphine and an elucidate the potential role of the methylpyridine sidearm, we
isocyanide established the lability of the pyridine. We then performed a few control experiments. First, complex 7 was
studied its behaviour under Brönsted acid/base conditions. While prepared by adding 2 equivalents of triphenylphosphine to the
no reaction was observed upon addition of an excess of acetic corresponding indenediide dimer and engaged in the catalytic
2
acid (> 10 equivalents) to a solution of complex 3b in d -DCM, cycloisomerization reaction. In this case, the catalytic activity
a clean and complete reaction was observed upon addition of was completely shut down (no reaction after 1 h at room
three equivalents of trifluoroacetic acid. Close inspection of the temperature), most likely due to the saturation of the Pd
1H and 13C NMR data revealed the migration of the C=C double coordination sphere by PPh . This control experiment is
3
bond to reform an indene moiety and the cleavage of the C2–Pd consistent with the need of a free coordination site for the π
1
bond (Scheme 5). The H NMR signal at 6.94 ppm associated activation of the C≡C triple bond and thus suggests that the labile
with the –CH= linker of 3b disappeared to give an AM system character of the methylpyridine is involved in the catalytic
integrating for 2 protons (5.1 and 3.83 ppm) and correlating in transformation. Given this, both the indenediide moiety and the
1
3
C NMR with an aliphatic CH
2
signal (40.19 ppm). pyridine moiety may contribute to the deprotonation (and
1
13
Concomitantly, the H and C NMR signals for the central C–H activation) of the carboxyl acid group. A second control
moiety are noticeably deshielded as the result of the change in C experiment was thus performed using the protonated
3
2
hybridization (from sp to sp ): from 5.12 to 6.39 ppm for H2, chloroindenyl complex 3b. In this case, no conversion was
and from 37.05 to 93.71 ppm for C2. In addition, reactIR detected after 1 h, indicating that decoordination of pyridine is
monitoring shows the appearance of bands at 1375 and 1610 cm-1 not enough for the cycloisomerization to occur, confirming
upon formation of 6. These data are diagnostic for κ1 thereby the necessity of the electron-rich indenediide moiety in
coordination of the trifluoroacetate group to the Pd center (see addition to the free coordination site at Pd.
ESI, fig S2).11 Attempts to isolate and crystallize 6 failed so far,
4
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9DT00898E
300c 2018 135 (1D 1H)
300c 2018 133 (1D 1H)
300c 2018 131 (1D 1H)
N
H
Pd Cl
iPr2P
S
3b
3
eq DIEA
N
Pd
S
O2CCF3
H
X
6
CH2
iPr2P
3
eq TFA
N
H2
CH=
Pd Cl
H2
H1
H1
S
3b
P
Pr2
i
ppm
9
8,5
8
7,5
7
6,5
6
5,5
5
4,5
4
3,5
3
2,5
2
1,5
1
0,5
0
-0,5
1
Figure 4. H NMR monitoring of the addition of trifluoroacetic acid (3 equivalents, turquoise spectrum) and then N,N-diisopropylethylamine (3 equivalents, red spectrum).
pincer complexes. We have previously demonstrated that subtle
O
5
% [Pd]
O
structural modulations of the thiophosphinoyle sidearms can
enhance greatly the robustness of indenediide Pd complexes.4b
Future work will aim to increase the stability of such
unsymmetrical indenediide complexes by modification of the
thiophosphinoyle sidearm, with the aim to take advantage of the
hemilabile character of the pyridine moiety in catalysis.
O
HO
CDCl3
1
h, r.t.
Conv.
4
:
62 %
n.r.
7
:
3b: n.r.
iPr2P
S
N
Pd PPh3
S
Pd PPh3
i
iPr2P
Pr P
Conflicts of interest
S
2
4
7
“There are no conflicts to declare”.
Scheme 6. Evaluation of the catalytic performances of indenediide Pd complexes 4, 7
and 3b in the cyclization of 4-pentynoic acid. (Conversions estimated by 1H NMR
spectroscopy)
Acknowledgements
Financial support from the Centre National de la Recherche
Scientifique, the Université de Toulouse and the Agence
Nationale de la Recherche (ANR CE6-CYCLOOP) is
acknowledged. We thank Corinne Routaboul (ICT, Toulouse) for
helpful assistance with the in situ infrared experiment with the
ReactIR.
Conclusions
In summary, the new unsymmetrical indene based pro-ligand
2
featuring thiophosphinoyle and methylpyridine sidearms was
prepared. Coordination to Pd afforded three isomeric complexes
a-c that were separated and fully characterized. Competition
3
experiments in the presence of Lewis bases demonstrate the
lability of the methylpyridine sidearm of 3 while Brönsted
acid/base conditions promotes reversible cleavage of the C2-Pd
bond. A new SCN indenediide complex 4 stabilized by a
triphenylphosphine co-ligand was also prepared. Preliminary
catalytic tests underlined the impact of the methylpyridine
sidearm on catalytic activity.
Notes and references
1
J. R. Khusnutdinova and D. Milstein, Angew. Chem. Int. Ed.,
015, 54, 12236.
2
2
For selected examples, see: (a) C. Gunanathan and D. Milstein,
Acc. Chem. Res., 2011, 44, 588; (b) C. Gunanathan and D.
Milstein, Chem. Rev., 2014, 114, 12024; (c) T. Zweifel, J. V.
Naubron, T. Büttner, T. Ott and H. Grützmacher, Angew.
Chem. Int. Ed., 2008, 47, 3245; (d) T. Zweifel, J. V. Naubron and
These results illustrate the interest and potential that
hemilability may have on catalytic applications of indenediide
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H. Grützmacher, Angew. Chem. Int. Ed., 2009, 48, 559; (e) D.
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Gelman and S. Musa, ACS Catal., 2012, 2, 2456; (f) S. Musa, L.
Ackermann and D. Gelman, Adv. Synth. Catal., 2013, 355,
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3
077; (g) A. V. Polukeev and O. F. Wendt, J. Organomet.
Chem., 2018, 867, 33.
3
For selected examples, see: (a) T. Zell and D. Milstein, Acc.
Chem. Res., 2015, 48, 1979; (b) N. Gorgas and K. Kirchner, Acc.
Chem. Res., 2018, 51, 1558; (c) F. Kallmeier and R. Kempe,
Angew. Chem. Int. Ed., 2018, 57, 46; (d) A. Kumar, T. Janes, N.
A. Espinosa-Jalapa and D. Milstein, Angew. Chem. Int. Ed.,
2
018, 57, 12076; (e) Y. K. Jang, T. Krückel, M. Rueping and O.
El-Sepelgy, Org. Lett., 2018, 20, 7779; (f) C. Erken, A. Kaithal,
S. Sen, T. Weyhermüller, M. Hölscher, C. Werlé and W.
Leitner, Nat. Commun., 2018, 9, 4521; (g) D. Oren, Y. Diskin-
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4 No further conversion was observed after 1 h.
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| J. Name., 2012, 00, 1-3
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Dalton Transactions
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DOI: 10.1039/C9DT00898E
Graphical Abstract
Palladium pincer complexes featuring an unsymmetrical SCN indene-based
ligand with an hemilabile pyridine sidearm
a
a
b
a
a
a
Paul Brunel, Chloé Lhardy, Sonia Mallet-Ladeira, Julien Monot,* Blanca Martin-Vaca* and Didier Bourissou*
“Combining thiophosphinoyle and methylpyridine sidearms to endow hemilabile character to
indenediide palladium pincer complexes”