A stable (amino)(phosphino)carbene as bidentate ligand for palladium and nickel complexes: Synthesis, structure, and catalytic activity

Article abstract of DOI:10.1016/j.jorganchem.2005.06.036

Two original complexes featuring an (amino)(phosphino)carbene η²-bonded to the metal have been obtained in 60% and 80% yields, by addition of the corresponding stable carbene to PdCl₂(cod) and NiCl₂(PPh₃)_2<

Full text of DOI:10.1016/j.jorganchem.2005.06.036

Journal of Organometallic Chemistry 690 (2005) 5541–5545
www.elsevier.com/locate/jorganchem
A stable (amino)(phosphino)carbene as bidentate ligand
for palladium and nickel complexes: Synthesis, structure,
and catalytic activity
a
a,b
a
c
´
´
Emmanuelle Teuma , Celine Lyon-Saunier , Heinz Gornitzka , Gerard Mignani ,
a,
a,b,
*
Antoine Baceiredo *, Guy Bertrand
a
Laboratoire He´te´rochimie Fondamentale et Applique´e, UMR 5069, Universite´ Paul Sabatier, 118 route de Narbonne,
F-31062 Toulouse cedex 04, France
UCR-CNRS Joint Research Chemistry Laboratory, UMR 2282, Department of Chemistry, University of California, Riverside, CA 92521-0403, USA
b
c
`
Rhodia Recherches, Centre de Recherches de Lyon, 85 Avenue des Freres Perret BP 62, 69192 Saint-Fons Cedex, France
Received 5 May 2005; accepted 23 June 2005
Available online 8 August 2005
Abstract
Two original complexes featuring an (amino)(phosphino)carbene g2-bonded to the metal have been obtained in 60% and 80%
yields, by addition of the corresponding stable carbene to PdCl2(cod) and NiCl2(PPh3)2, respectively. Both complexes have been
fully characterized including X-ray diffraction studies. The catalytic activity of the palladium complex has been evaluated for aryl
amination reactions.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Carbenes; Bidentate ligands; Palladium complexes; Nickel complexes; Aryl amination reaction
1. Introduction
oretical studies suggested that in contrast with NHCs A,
carbenes B tend to adopt widened valence angles at the
central carbon; as a consequence, conformational
changes to a bent carbene structure are required to allow
metal complexation, and this process is energetically too
costly [4]. These conclusions have recently been corrob-
orated by experimental observations. A considerable
contraction of the bond angle about the carbene center
of (phosphino)(aryl)carbenes C was observed for the
RhCl(nbd)(C) complex (from 162ꢁ for the free carbene
C to 119ꢁ for the complex) [5]. (Amino)(alkyl or
aryl)carbenes D and E feature more acute bond angle
(120–121ꢁ) than their phosphino counterparts C, and
therefore easily undergo complexation with metals [6].
However, it is important to note that although com-
plexes featuring carbenes C–E are thermally stable, they
appear to be much more air sensitive than the corre-
sponding NHC complexes, which partly precludes their
Over the years, the success of homogeneous catalysis
can be attributed largely to the development of a diverse
range of ligand frameworks that have been used to tune
the behavior of a variety of metal-containing systems.
Recently, spectacular achievements have been made in
this area using N-heterocyclic carbenes (NHCs) A as
strong r-donor ligands for transition metals [1] (Scheme
1). In contrast, very little is known on the efficiency of
stable non-heterocyclic carbenes [2] as ligands for transi-
tion metal catalysts. Despite their early discovery, direct
complexation of (phosphino)(silyl)carbenes B [3] has not
yet been reported, and all our attempts have failed. The-
*
Corresponding authors. Tel.: +33 5 61 55 62 86; fax: +33 5 61 55 82
04 (G. Bertrand).
E-mail address: gbertran@chimie.ups-tlse.fr (G. Bertrand).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.06.036

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E. Teuma et al. / Journal of Organometallic Chemistry 690 (2005) 5541–5545
P
Si
P
Ar
N
Alk
N
Ar
N
P
N
N
N
N
116o
101-106o
152-162o
162o
120o
121o
121o
G
A
B
D
E
C
F
Scheme 1.
use in catalysis. Herrmann [8] has recently suggested
that the poor coordination behavior of acyclic diamino
carbenes F [7], compared to cyclic diamino carbenes A
might be due to the larger N–C–N angle (121ꢁ compared
to 101–106ꢁ). Interestingly, (amino)(phosphino)carbenes
G, feature the most acute carbene bond angle of all the
known stable acyclic carbenes: 116ꢁ [9]. Moreover, we
have already shown that carbenes G reacts with
[Rh(CO)2Cl]2 to afford the cationic complex featuring
two (amino)(phosphino)carbenes g2-bonded to the me-
tal [10]. In other words, G can act as 4-electrons donors,
with a strong r-donor carbene next to a relatively labile
phosphine. This very peculiar topology prompted us to
study further the coordination behavior of carbenes G.
Here we report the preparation of two original group
10 metal complexes of a stable carbene of type G, as well
as preliminary results concerning the catalytic activity of
the palladium complex in aryl amination reactions.
Fig. 1. X-ray crystal structure of complex 2 (ellipsoids are drawn at the
50% probability level, and hydrogen atoms are omitted for clarity).
˚
Selected bond lengths [A] and angles [ꢁ]: C(1)–Pd(1) 2.019(7); P(1)–
Pd(1) 2.205(4); P(1)–C(1) 1.791(7); C(1)–N(1) 1.262(9) and P(1)–Pd(1)–
C(1) 49.93(18); P(1)–Pd(1)–Cl(1) 106.15(9); Cl(1)–Pd(1)–Cl(2) 93.38(8);
Cl(2)–Pd(1)–C(1) 110.91(19).
2. Results and discussion
The (amino)(phosphino)carbene 1 reacts at room tem-
perature with one equivalent of PdCl2(cod) complex. The
reaction proceeds very cleanly, and complex 2 was iso-
lated, after recrystallization from a CH2Cl2 solution at
À30 ꢁC, as yellow crystals, in 60% yield. The analogous
nickel complex 3 was prepared similarly by treatment
of a THF solution of the (amino)(phosphino)carbene 1
with one equivalent of NiCl2(PPh3)2. It was isolated as
red crystals, in 80% yield, by slow recrystallisation from
a CDCl3 solution at À30 ꢁC (Scheme 2).
The high field 31P NMR chemical shifts observed (2:
À44.2 ppm; 3: À82.1 ppm) suggested the formation of
three-membered metallocycles. The g2-coordination
mode was unambiguously established by X-ray diffrac-
tion studies (Figs. 1 and 2). In complexes 2 and 3, the
metal is in a distorted square-planar environment (sum
of bond angles around Pd 360.37ꢁ and around Ni
361.17ꢁ). The carbene–palladium bond distance
Fig. 2. X-ray crystal structure of complex 3 (ellipsoids are drawn at the
50% probability level, and hydrogen atoms are omitted for clarity).
˚
Selected bond lengths [A] and angles [ꢁ]: C(1)–Ni(1) 1.841(4); P(1)–
Ni(1) 2.1068(13); P(1)–C(1) 1.755(4); C(1)–N(1) 1.300(5) and P(1)–
Ni(1)–C(1) 52.25(14); P(1)–Ni(1)–Cl(2) 102.21(5); Cl(2)–Ni(1)–Cl(1)
96.00(5); Cl(1)–Ni(1)–C(1) 110.71(14).
˚
(2.019 A) is in the typical range for C–Pd single bonds,
and very similar to that observed for related N-heterocy-
˚
clic carbene palladium complexes (1.95–2.07 A) [11].
˚
(i-Pr)2N
The nickel–carbene bond length of 1.841 A is slightly
PdCl2(cod)
P(t-Bu)2
(i-Pr)2N
P(t-Bu)2
shorter than observed for nickel(II) complexes bearing
Cl and NHC ligands cis to one another [12]. The phos-
phorus–metal bond distances, 2.204 and 2.107 A are in
the typical range for P–Pd and P–Ni single bonds,
respectively. In both complexes, the nitrogen atom
M
or NiCl2(PPh3)2
1
Cl Cl
˚
2: M = Pd; 3: M = Ni
Scheme 2.

E. Teuma et al. / Journal of Organometallic Chemistry 690 (2005) 5541–5545
5543
Table 3
adopts a trigonal planar geometry with a short NC bond
Arylation of morpholine (1.0 equiv.) with diverse aryl halides
(1.0 equiv.) at 140 ꢁC using 0.5 mol% of complex 2, t-BuONa
(1.4 equiv.), and xylene as solvent
˚
˚
distance (2: 1.262 A; 3: 1.300 A), which is very similar to
˚
that of free (amino)(phosphino)carbenes (1.30 A). This
indicates that, the carbene–metal interaction results
from the donation of the carbene lone pair into an
empty metal-based orbital; back-donation from the me-
tal to the carbene center is almost negligible.
Three-membered metallocycles 2 and 3 are very sta-
ble toward air, moisture and heat. This is due to the fact
that the phosphorus is not implicated in the stabilization
of the carbene center, and thus has a lone pair fully
available to complex the metal. We expected that this
peculiar topology would confer original catalytic prop-
erties to complexes 2 and 3. The first results of the cat-
alytic study with complex 2 in aryl amination reactions
[13] are summarized below.
Complex 2 was first tested for the catalytic aryl ami-
nation reaction using 4-bromotoluene and morpholine
as coupling partners (Tables 1 and 2). The use of organic
bases, such as t-BuONa, t-BuOK and t-AmONa, led,
after 5 h at 110 ꢁC, to the formation of the coupling
product in 75%, 55% and 65% conversions, respectively;
increasing the reaction time to 21 h gave 80%, 70% and
70% conversions, respectively (Table 1, entries 1–6). Un-
der similar conditions, inorganic bases, such as Cs2CO3
Entry Solvent
Time (h) Conversion (%)
1
2
3
4
5
6
4-Bromotoluene
4-Bromoanisole
3,5-Dibromotoluene
1-Bromo-4-fluorobenzene
1-Bromo-4-chlororobenzene 20
4-Chlorotoluene
1
20
20
20
90
80
10 (mono), 60 (di)
85
90
20
1
or NaOH, appeared to be inefficient; conversion of 4-
bromotoluene does not exceed 25% after 21 h (Table
1, entries 7–10). Thus t-BuONa appears to be the most
efficient base for this reaction. Then we examined the
influence of the solvent. Xylene gave the best results,
with a conversion reaching 90% in 2 h at 110 ꢁC, and
even 98% after 18 h (Table 2, entries 7 and 8). t-AmOH
could be an alternative choice, since a 95% conversion
was obtained in 18 h (Table 2, entry 4).
Using the optimized conditions (t-BuONa; xylene),
the aryl amination reactions of a variety of aryl halides
with morpholine were studied. As can be seen in Table 3,
electron-rich and electron-poor aryl bromides afforded
the corresponding coupling products in good yields (en-
tries 1–5). Unfortunately, aryl chlorides are not effi-
ciently coupled, since only 20% of conversion occurred
when 4-chlorotoluene was used (entry 6); this explains
the total selectivity (involving the bromide) observed en-
tries 4 and 5.
Table 1
Influence of the base (1.4 equiv.) on the arylation of morpholine
(1.0 equiv.) with 4-bromotoluene (1.0 equiv.) at 110 ꢁC using 1 mol%
of complex 2, and dioxane as solvent
Entry
Base
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
t-BuONa
t-BuONa
t-BuOK
t-BuOK
t-AmONa
t-AmONa
Cs2CO3
Cs2CO3
NaOH
5
21
5
21
5
23
5
21
5
75
80
55
70
65
70
10
20
8
3. Conclusion
Although the results of the catalytic tests are not
impressive, they are extremely stimulating. Indeed, as
mentioned in the introduction, cyclic carbenes are much
more efficient as ligands for transition metal catalysts
than their acyclic variants. Therefore, it is quite reason-
able to believe that cyclic versions of (amino)(phos-
phino)carbenes will have very interesting ligand
properties. The synthesis of such species is under current
investigations.
10
NaOH
21
25
Table 2
Influence of the solvent on the arylation of morpholine (1.0 equiv.)
with 4-bromotoluene (1.0 equiv.) at 110 ꢁC using 0.5 mol% of complex
2, and t-BuONa (1.4 equiv.)
Entry
Solvent
Time (h)
Conversion (%)
4. Experimental
1
2
3
4
5
6
7
8
9
Anisole
Anisole
t-AmOH
t-AmOH
NMP
2
18
2
18
2
18
2
18
2
30
55
55
95
25
55
90
98
40
50
All experiments were carried out under dry argon
using standard Schlenk or dry box techniques. THF
and ether were distilled under argon from sodium/ben-
zophenone. Pentane and CH2Cl2 were distilled under ar-
gon from P2O5 Æ PdCl2(cod) and NiCl2(PPh3)2 were used
as supplied from Strem Chemicals. Compound 1 was
prepared according to the reported procedure [9a].
Commercially available reagents were used without
NMP
Xylene
Xylene
DME
10
DME
18

5544
E. Teuma et al. / Journal of Organometallic Chemistry 690 (2005) 5541–5545
further purification. 31P NMR, 1H NMR and 13C NMR
spectra were recorded on Bruker AMX 250, AMX400
and AMX500 instruments. Chemical shifts are reported
in ppm downfield from Me4Si and were referenced to
solvent peaks (1H, 13C) or external 85% H3PO4 (31P).
Coupling constants are given in Hz.
an oil-coated shock-cooled crystal on a Bruker-AXS
CCD 1000 diffractometer with Mo Ka radiation
˚
(k = 0.71073 A). The structures were solved by direct
methods (SHELXS-97) [14] and all non-hydrogen atoms
were refined anisotropically using the least-squares
method on F2 [15]. Crystal data for 2: C16H34Cl4NPPd,
ꢀ
M = 519.61, triclinic, space group P1 with a =
˚
˚
˚
4.1. Synthesis of the (amino)(phosphino)carbene
palladium complex 2
9.78(2) A, b = 10.07(2) A, c = 12.21(2) A, a = 91.11(3)ꢁ,
3
˚
b = 90.68(3)ꢁ, c = 97.50(3)ꢁ, V = 1192(3) A , Z = 2.
5111 reflections (3325 independent, Rint = 0.0894), larg-
est electron density residue: 0.954 e AÀ3, R1 = 0.0539
˚
A slight excess of freshly prepared (amino)(phos-
phino)carbene 1 in THF (3 mL) was added to a CH2Cl2
solution (10 mL) of PdCl2(cod) complex (1.4 mmol,
0.40 g). After the solution mixture was stirred at room
temperature for 3 h, 31P NMR spectroscopy indicated
the quantitative formation of complex 2. Cooling the
solution to 0 ꢁC gave a yellow precipitate, which was fil-
tered, washed with diethyl ether (2 · 20 mL), and dried
under reduced pressure. After recrystallization from a
CH2Cl2 solution at À30 ꢁC, complex 2 was isolated as
(for I > 2r(I)) and wR2 = 0.1409 (all data). Crystal data
for 3: C16H33Cl5NNiP, M = 506.36, orthorhombic,
˚
space group P212121 with a = 9.710(1) A, b =
3
˚
˚
˚
12.037(1) A, c = 20.624(2) A, V = 2410.6(4) A , Z =
4.11972 reflections (4065 independent, Rint = 0.0596),
largest electron density residue: 0.843 e AÀ3, R1 =
˚
0.0454 (for I > 2r(I)) and wR2 = 0.0890 (all data).
4.4. Typical reaction procedure for aryl amination
reactions
yellow crystals (0.36 g, 60%). 31P NMR (CD2Cl2):
3
À44.2 ppm. 1H NMR (CD2Cl2): 1.55 (d, JP–H
=
3
19.4 Hz, 18H, PCCH3), 1.57 (d, JH–H = 6.7 Hz, 6H,
NCHCH3), 1.91 (d, JH–H = 6.8 Hz, 6H, NCHCH3),
4.02 (sept, JH–H = 6.7 Hz, 1H, NCHCH3), 4.35 (broad
To a solution (6 mL) of aryl halide (6 mmol), amine
(6 mmol) and base (8.4 mmol) was added complex 2.
The reaction mixture was stirred at 110 ꢁC for the time
mentioned in Tables 1–3 and monitored by GC analysis.
3
3
sept, 1H, NCHCH3). 13C NMR (CD2Cl2): 20.7
(NCHCH3), 21.7 (NCHCH3), 30.6 (d, JP–C = 2.9 Hz,
PCCH3), 39.4 (d, JP–C = 10.0 Hz, PCCH3), 57.5
(NCHCH3), 80.4 (d, JP–C = 5.0 Hz, N CHCH3), 202.0
(PdC).
5. Supplementary material
CCDC – 270980 (2), 270981 (3) contain the supple-
mentary crystallographic data for this paper. These data
can be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ UK,
fax: +44 1223 336 033, email: deposit@ccdc.cam.ac.uk
or at www.ccdc.cam.ac.uk/conts/retrieving.html.
4.2. Synthesis of the (amino)(phosphino)carbene nickel
complex 3
A slight excess of freshly prepared (amino)(phos-
phino)carbene 1 in THF (3 mL) was added to a CH2Cl2
solution (10 mL) of NiCl2(PPh3)2 (1.4 mmol, 0.92 g).
The solution was stirred for 3 h at room temperature.
After filtration, 10 mL of pentane was added and a pre-
cipitate was formed. After filtration, the solid was
washed with a THF/pentane mixture (3 · 10 mL) and
dried under reduced pressure. After recrystallization
from a CDCl3 solution at À30 ꢁC, complex 3 was ob-
tained as red crystals (0.43 g, 80%). 31P NMR (CDCl3):
Acknowledgements
Thanks are due the NIH (R01 GM 68825) and RHO-
DIA for financial support of this work.
1
3
À82.06 ppm. H NMR (CDCl3): 1.44 (d, JH–H = 6.5,
References
3
6H, NCHCH3), 1.65 (d, JP–H = 18.5, 18H, PCCH3),
3
3
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