Preparation of a cobalt-containing P,N-bidentate ligand ligated palladium complex: Its applications in amination and Suzuki cross-coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2008.12.046

A cobalt-containing P,N-ligand, alkyne-bridged dicobalt phosphine [(μ-PPh₂CH₂PPh₂)Co₂(CO)₄(μ,η-Me₂NCH₂C{triple bond, long}CP(Cy)₂)] (4c), was prepared from the reaction of Me₂NCH₂C{triple bond, long}CPR₂ (2c) with a dppm-bridged dicobalt complex [Co₂(CO)₆(μ-P,P-PPh₂CH₂PPh₂)] (3). Further reaction of 4c with Pd(COD)Cl₂ gave a 4c-chelated palladium dichloride complex (μ-PPh₂CH₂PPh₂)Co₂(CO)₄(μ,η-Cy₂PC{triple bond, long}CCH₂NMe₂)PdCl₂ (5c). Compound 5c was characterized by spectroscopic methods and crystal structure of 5c was determined by single-crystal X-ray diffraction techniques. It shows that 4c is an authentic cobalt-containing P,N-bidentate ligand. Amination and Suzuki reactions employing either a 4c/palladium salt system or an isolated 5c as the catalytic precursor led to satisfactory results.

Full text of DOI:10.1016/j.jorganchem.2008.12.046

Journal of Organometallic Chemistry 694 (2009) 1473–1481
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Preparation of a cobalt-containing P,N-bidentate ligand ligated palladium complex:
Its applications in amination and Suzuki cross-coupling reactions
*
Wen-Yuan Chiang, Fung-E Hong
Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 40227, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 October 2008
Received in revised form 17 December 2008
Accepted 18 December 2008
Available online 25 December 2008
A
(l,g
cobalt-containing P,N-ligand, alkyne-bridged dicobalt phosphine [(
-Me2NCH2C„CP(Cy)2)] (4c), was prepared from the reaction of Me2NCH2C„CPR2 (2c) with a
dppm-bridged dicobalt complex [Co2(CO)6( -P,P-PPh2CH2PPh2)] (3). Further reaction of 4c with
Pd(COD)Cl2 gave 4c-chelated palladium dichloride complex -PPh2CH2PPh2)Co2(CO)4
-Cy2PC„CCH2NMe2)- PdCl2 (5c). Compound 5c was characterized by spectroscopic methods and
l-PPh2CH2PPh2)Co2(CO)4
l
a
(l
(l,g
crystal structure of 5c was determined by single-crystal X-ray diffraction techniques. It shows that 4c
is an authentic cobalt-containing P,N-bidentate ligand. Amination and Suzuki reactions employing either
a 4c/palladium salt system or an isolated 5c as the catalytic precursor led to satisfactory results.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Bidentate ligand
P,N-ligand
Palladium complex
Cobalt complex
Suzuki reaction
Amination
1. Introduction
nation sites, in the complexation of transition metal has attracted
attention from many researchers [35–43]. The advantages of
Transition metal complexes catalyzed cross-coupling reactions,
assisted by ligands, are doubtless the most utilized means for mak-
ing the C–X (X = C, N, O, S,. . .) bonds in modern synthetic chemistry
[1–7]. As demonstrated repeatedly, a well-chosen ligand is essen-
tial to the success of the coupling reaction [8]. Up to now, there
are five categories of ligands listed as the most frequently used:
(a) bis(diphenylphosphino)alkyl; (b) trialkylphosphine; (c) aryl-
dialkylphosphine; (d) palladacycle; (e) N-heterocyclic carbine.
Among them, phosphines remain as the most commonly employed
ligands in the catalytic reactions mediated by ligand-assisted tran-
sition metals [9–12,8,13–31]. Bidentate phosphines have been
found particularly crucial to the reductive elimination process,
the last step, of a catalytic cycle of the common cross-coupling
reactions. At least there are two advantages of employing biden-
tate phosphine ligand: first, the enforced cis-form of the metal
complex by the bidentate ligand enhances the rate of reductive
elimination process; second, an undesired side reaction, b-hydride
elimination, might be avoided due to less room left for the process.
Two of the most representative homofunctional bidentate phos-
phines are BINAP [32] and DPPF [33]. The former is a typical organ-
ic phosphine; the letter is the most cited metal-containing
phosphine. Recently, the subject of employing hybrid ligand [34]
such as P,N-bidentate ligand, which contains both P- and N-coordi-
employing hybrid ligand are obvious. On the one hand, the P,N-
bidentate ligand is less air-sensitive than di-phosphine; on the
other, the ready formation/breaking of the nitrogen–metal bond
in the metal complex is a profitable feature in catalytic reaction
[44,45]. According to the HSAB theory, the chemical bonding is
thermodynamically unfavorable for the hard base (nitrogen) and
the soft acid (palladium) [46–48].
Our previous works had shown that several cobalt-containing
P,N-ligands, alkyne-bridged dicobalt phosphines [(
Co2(CO)4(
-Me2NCH2C„CPR2)] (4a: R = tBu; 4b: R = Ph; 4c:
R = Cy), could be prepared from the reactions of a dppm-bridged
l-PPh2CH2PPh2)-
l,g
dicobalt complex [Co2(CO)6(l-PPh2CH2PPh2)] 3 with correspond-
ing alkynylphosphines Me2NCH2C„CPR2 (2a: R = tBu; 2b: R = Ph;
2c: R = Cy) (Scheme 1) [49].
An attempt to prepare 4a-chelated palladium dichloride com-
plex (
or
l
-PPh2CH2PPh2)Co2(CO)4(
l
,
g
-Ph2PC„CCH2NMe2)PdCl2 (5a)
-Ph2PC„CCH2NMe2)]2-
trans-[( -PPh2CH2PPh2)Co2(CO)4(l,g
l
PdCl2 (6a), from the newly made P,N-ligand 4a with PdCl2 was
not successful (Scheme 1). Rather, the reaction of 4a with another
palladium source, [(
lated palladium complex ion pair [( -PPh2CH2PPh2)Co2(CO)4
-Ph2PC„CCH2NMe2)Pd( g (7a).
3-C3H5)]+[( 3-C3H5)PdCl2]À
g
3-C3H5)PdCl]2, yielded an unusual 4a-che-
l
(l,g
g
Here, 4a is an authentic cobalt-containing P,N-bidentate ligand
[49]. Satisfactory efficiencies were observed for the amination
reactions of aryl bromides with morpholine employing either a
4a-chelated palladium complex composed in situ or pre-formed
7a as the catalytic precursor [49].
* Corresponding author. Tel.: +886 4 22840411x607; fax: +886 4 22862547.
E-mail address: fehong@dragon.nchu.edu.tw (F.-E. Hong).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.12.046

1474
W.-Y. Chiang, F.-E. Hong / Journal of Organometallic Chemistry 694 (2009) 1473–1481
N
1. n-BuLi, -78oC, 1 hr
2. ClPR2, THF, rt
+
N
Co (CO) (dppm)
3
2
6
H
PR2
toluene, 75oC
1
2
a: R=tBu
b: R=Ph
c: R=Cy
NMe2
R2
P
R2
P
Cl
C
C
(tBu)2
Cl
P
C
Pd
N
C
+ PdCl2 or Pd(COD)Cl2
Pd
+
Cl
Me2
C
C
Cl
5a (Not Observed)
5c
Me
N
(CO)2
2
6a (Not Observed)
6c (Not Observed)
Ph2
P
PR2
Co
C
C
P
NMe2
Co
(CO)2
Ph2
R2
P
R2
P
4a, 4b, 4c
+ [(η3-C3H5)PdCl]2
C
+ xs. KBF4
-
C
C
Pd
Pd
Pd
BF4
(CO)2
Co
Ph2
P
C
N
N
Cl
Cl
C
C
Me2
C
C
Me2
P
Co
(CO)2
7a
7c
7cB
Ph2
Scheme 1.
Table 1
Crystal data of 5c.
The electronic and steric characteristics of a substituent such as
electron-withdrawing/donating capacity and bulkiness of a phos-
phine ligand are two crucial factors that affect the performance
of a catalytic reaction [10,21,11,12,8,13,50,51,15–20]. Two fre-
Compound
Formula
Formula weight
Crystal system
Space group
a (Å)
5c
C47H52Cl2Co2NO5P3Pd
1071.02
Monoclinic
P21
14.449(2)
11.7700(18)
15.824(2)
90.00
t
quently employed electron-donating and bulk groups, À Bu and
–Cy, are among the most efficient substituents. More than often,
the catalytic performance of the former is better than the latter
[52–63]. In comparison with the catalytic efficiency of the 4a-che-
lated palladium complex, the 4c ligated palladium complex is pre-
pared and its capability as an effective catalyst precursor either in
Suzuki or amination reaction is presented.
b (Å)
c (Å)
a
(°)
b (°)
111.942(3)
90.00
c
(°)
V (Å3)
2496.1(6)
2
1.462
Z
2. Results and discussion
Dcalc (Mg/m3)
k (Mo K
(mmÀ1
h Range (°)
Observed reflections (F > 4
Number of refined parameters
aR1 for significant reflections
bwR2 for significant reflections
cGoodness-of-fit
a
)
) (Å)
0.71073
1.260
1.52–26.07
9446
545
0.0487
2.1. Complexation of 4c with palladium sources
l
r(F))
The preparation of cobalt-containing P,N-ligand 4c was demon-
strated elsewhere [49]. The 4c-chelated palladium dichloride com-
plex (l-PPh2CH2PPh2)Co2(CO)4(l,g-Cy2PC„CCH2NMe2)PdCl2 (5c)
0.1003
0.937
can be prepared by the reaction of the newly made P,N-ligand 4c
with PdCl2, a commonly used palladium source, under 60 °C
(Scheme 1). Alternatively, 5c could be obtained easily by the reac-
tion of 4c with Pd(COD)Cl2 at 25 °C for only 10 min. Compound 5c
was characterized by spectroscopic methods as well as single-crys-
tal X-ray diffraction techniques (Table 1). In the 1H NMR spectrum
of 5c, one set of quantet shows up at 3.70 ppm which is corre-
sponding to the methylene protons of the coordinated dppm
ligand. A singlet observed at 3.41 ppm is assigned to the methylene
adjacent to nitrogen. The 31P NMR spectrum displays two singlets
at 38.8 and 49.3 ppm. The former signal is assigned to the 2 equiv.
phosphorus atoms of the coordinated dppm ligand; while the latter
peak is assigned to the palladium-coordinated phosphorus site. It
is a significant shift from the free phosphorus in 4c which appears
at 15.11 ppm. The ORTEP diagram of 5c are depicted in Fig. 1. As
revealed in 5c, 4c behaves as a conventional cobalt-containing
P,N-bidentate ligand. The palladium center is in a square planar
environment. There are methanol molecules encapsulated during
the crystal growing process. The reaction of 4c with a different
a
R1 = |
wR2 = {
GoF = [
R
(|Fo| À |Fc|)/|
RFo||.
b
c
2
R
[w(Fo À Fc2)2/
R
[w(Fo2)2}}1/2; w = 0.0500 for 5c.
w(Fo À Fc2)2/(Nrflns À Nparams)]1/2
.
2
R
palladium source, [(
7c. In the excess KBF4, a unique 4c-chelated palladium complex
g
3-C3H5)PdCl]2, resulted in7a-related compound,
ion pair [(l-dppm)Co2(CO)4(l,g-Ph2PC„CCH2NMe2)Pd(g
3-C3H5)]+-
[BF4]À 7cB was yielded. The two chlorides in 5c are subjected to
the replacement of other suitable substituents.
Selected bond distances (Å) and angles (°): Pd(1)–P(3) 2.241(2);
Pd(1)–N(1) 2.180(7); Pd(1)–Cl(1) 2.296(2); Pd(1)–Cl(2) 2.425(2);
P(3)–C(1) 1.766(8); P(3)–C(12) 1.833(9); P(3)–C(6) 1.840(8);
N(1)–C(5) 1.494(10); N(1)–C(3) 1.494(10); N(1)–C(4) 1.495(10);
C(1)–C(2) 1.766(8); C(2)–C(3) 1.498(12); N(1)-Pd(1)–Cl(1) 170.70(19);
P(3)–Pd(1)–Cl(1) 85.34(9); N(1)–Pd(1)–P(3) 97.51(18); N(1)–
Pd(1)–Cl(2) 89.97(19); P(3)–Pd(1)–Cl(2) 169.25(9); Cl(1)–Pd(1)–
Cl(2) 88.49(9).

W.-Y. Chiang, F.-E. Hong / Journal of Organometallic Chemistry 694 (2009) 1473–1481
1475
Fig. 1. ORTEP drawing of 5c. Hydrogen atoms are omitted for clarity.
Table 2
2.2. Structural comparison of 5c and 7a
Comparison for selected structural parameters of 5c and 7a.
The structures of 5c and 7a reveal that each molecule is con-
sisted of a P,N-ligands and palladium fragment (see Diagram 1).
The structural comparison of 5c and 7a are listed in Table 2. As
shown, the bond lengths of Pd–P and Pd–N are all shorter in 5c
than in 7a. It is also true for the Pd–Cl bonds. The bond angle of
P–Pd–N is narrower in 5c than in 7a. The bond angle of Cl–Pd–Cl
is also much wider in 7a than in 5c.
5c
7a
Bond length(Å)
Pd–P
Pd–N
Pd–Cl(1)
Pd–Cl(2)
2.241(2)
2.180(7)
2.296(2)
2.425(2)
2.3227(14)
2.226(4)
2.3856(16)
2.3693(18)
Bond angle(°)
P–Pd–N
Cl–Pd–Cl
97.51(18)
88.49(9)
100.03(11)
97.61(6)
2.3. Application of 4c in palladium-catalyzed amination reactions
As known, an efficient metal-catalyzed cross-coupling reaction
is regulated by a number of factors such as palladium salt, ligand,
base, solvent, temperature and reaction hour [2]. Here, the general
procedures for the optimization on the reaction conditions for the
amination process was surveyed as the follows (Scheme 2). A prop-
er sized Schlenk tube was charged with a magnetic stirrer then was
placed with 1.0 mmol of bromobenzene, 1.2 equiv. of morpholine,
1.4 equiv. of base, 1.0 mL of solvent, and 1.0 mmol% of 4c/palla-
dium salt. The mixture was stirred while heating for certain time
and then workup followed.
Cy
2
t
(CO)2
Co
( Bu)
P
2
P
Cl
Cl
(CO)2
Co
P
C
C
Ph2
P
Pd
N
C
C
Pd
Ph2
Pd
P
Co
(CO)2
N
Me
Ph2
P
Me
2
Cl
Cl
Co
(CO)2
Ph2
2
According to the generally accepted mechanism, deprotonation
of the coordinated amine is one of the most critical steps in the cat-
alytic cycle of amination reaction. Thereby, a well-chosen base is
5c
7a
Diagram 1. Generalized structure for 5c and 7a.

1476
W.-Y. Chiang, F.-E. Hong / Journal of Organometallic Chemistry 694 (2009) 1473–1481
4c/[Pd], Solvent
N
O
+
HN
O
Br
Base, 80oC
Scheme 2.
essential to the performance of a catalytic reaction. As shown in
Table 3, a rather good yield (>91%) was obtained when the reaction
temperature was raised up from 60 to 80 °C (Entry 2). At the ele-
vated temperature, the best base/solvent combination is NaOtBu/
toluene. Other bases did not show encouraging results. The good
performance of this combination might partly be due to the reduc-
ing capability of NaOtBu in toluene solvent.
molecular structure of 5c. Ligand 4c here acts as a bidentate ligand.
Nevertheless, it seems that the active catalytic species in the ami-
nation reactions are different for using 4a or 4c as ligand.
Subsequently, the impact of various solvents on the reaction
was evaluated. As demonstrated, the reaction rate is greatly af-
fected by the solvent used (Table 6). For instance, the coupling
reactions in 1,4-dioxane, THF and DMF are ineffective (Entries 2–
4). However, the yield was greatly improved when toluene was
used as a solvent (Entry 1). Extensive application of eco-friendly
solvents such as water in chemical reaction is a touching appeal
of Green Chemistry [66,67]. Nevertheless, the reaction employed
water as the solvent turned out to be rather ineffective probably
due to its insolubility of catalyst in water (Entry 5). A mixed sol-
vent system, however, did show better result (Entry 6).
The catalytic performance of a coupling reaction is also greatly
affected by the particular palladium salt employed. Two factors are
among the most concerned. First, the reduction potentials of Pd(II)
to Pd(0) shall be different for various palladium sources used; sec-
ond, the solubility of the potential intermediate(s) shall be differ-
ent for various combinations of ligand/palladium salt. As shown,
an excellent yield was obtained with either Pd(COD)Cl2 or
[(g
3-C3H5)PdCl]2 as palladium source (Table 4, Entries 2 and 4).
Interestingly, the efficiency of the coupling reaction is affected
by the amount of solvent used (Table 7). The best efficiency was
observed while more concentrated solution was present (Entry 1).
Although the latter complex is very efficient, the use of this com-
plex is not practical due to its sensitivity to air. Unexpectedly,
the commonly used Pd(OAc)2 did not show competitive result in
this case (Entry 1). As reported in a palladium free study, CuI alone
plays a major role as the catalyst precursor in the coupling reaction
[64]. It had been tested. Nevertheless, no observable reactivity had
been shown here (Entry 7).
Table 5
Amination reaction under various 4c/[(
g
3-C3H5)PdCl]2 ratios.a
Entry 4c/[(
g
3-C3H5)PdCl]2
4c/Pd atom
(mmol%)
NMR conversion
(%)b,c
(mmol%)
To establish the best ligand-to-metal ratio, the amination reac-
tion was carried out under various 4c/[(g
3-C3H5)PdCl]2 molar ra-
1
2
3
4
5
6
0:0.5
0.5:0.5
1:0.5
2:0.5
1:0.25
1:1
0:1
0.5:1
1:1
2:1
1:0.5
1:2
3.0 (trace)
27.8 (14.6)
91.4 (83.1)
72.3 (52.6)
26.1 (12.7)
40.5 (97.9)
tios and conditions shown in Table 5. The best yield was
achieved with 1:0.5 ratio (Table 5, Entry 3). This observation is
worth noting since it is generally believed that two molar equiv.
of a monodentate phosphine ligand are required for an effective
catalytic Suzuki reaction [65]. Moreover, it is consistent with the
a
Reaction conditions: 1.0 mmol of bromobenzene, 1.2 mmol morpholine, and
1.4 mmol of NaOtBu, 2.0 mL toluene, various ratios of 4c/[( 3-C3H5)PdCl]2, 80 °C,
g
2 h.
b
Average of two runs.
Values in parentheses are taken from Ref. [49]. Amination reactions were car-
c
Table 3
ried out with 4a/[(g
3-C3H5)PdCl]2 as catalytic precursor.
Amination reaction using 4c/[(g
3-C3H5)PdCl]2 and various bases.a
Entry
Base
Temperature (°C)
NMR conversion (%)b
1
2
3
4
5
6
NaOtBu
NaOtBu
KF
K2CO3
K3PO4
NaOH
60
80
80
80
80
80
22.4
91.4
Trace
Trace
Trace
Trace
Table 6
Amination reaction employing 4c/[(
g
3-C3H5)PdCl]2 in various solvents.a
NMR conversion (%)b
Entry
Solvent
Toluene
1
2
3
4
5
6
91.4
16.3
3.8
N.R.
1.2
a
Reaction conditions: 1.0 mmol of bromobenzene, 1.2 mmol morpholine, and
1,4-Dioxane
THF
1.4 mmol of base, 2.0 mL toluene, 1.0 mmol% 4c/[(
g
3-C3H5)PdCl]2, 2 h.
b
Average of two runs.
DMF
H2O
MeOH/H2O (1:1)
42.8
Table 4
a
Amination reaction using various Pd sources.a
Reaction conditions are the same as in the footnote of Table 5 except for the
solvent used.
NMR conversion (%)b
b
Entry
Pd source
Average of two runs.
1
2
3
4
5
6
7
8
9
10
Pd(OAc)2
34.9
67.4
2.2
91.4
27.8
30.9
N.R.
78.0
95.3
85.5
Pd(COD)Cl2
PdCl2(CH3CN)2
Table 7
[(g
3-C3H5)PdCl]2
Dependence of the yield of the amination reaction in amount of solvent used.a
Pd2(dba)2
Pd(acac)2
CuI/4c
5c
7c
7cB
Entry
Toluene (mL)
NMR conversion (%)b
1
2
3
1.0
2.0
3.0
>99.0
91.4
82.0
a
a
Reaction conditions are the same as in the footnote of Table 5 except for the
amount of toluene used.
Reaction conditions: 1.0 mmol of bromobenzene, 1.2 mmol morpholine, and
1.4 mmol of NaOtBu, 2.0 mL toluene, 4c/[Pd] = 1:1, 80 °C, 2 h.
b
b
Average of two runs.
Average of two runs.

W.-Y. Chiang, F.-E. Hong / Journal of Organometallic Chemistry 694 (2009) 1473–1481
1477
It is a common observation that better conversion is achieved
with aryl halides bearing an electron-withdrawing rather than an
electron-donating substituent in a palladium-catalyzed Suzuki
coupling reaction [16]. Indeed, it is basically correct for the reac-
tion with the oxidative addition process as the rate-determining-
step (r.d.s.). Nevertheless, it may not be true for other reactions,
such as Heck reaction, where the r.d.s. might be anywhere but
the oxidative addition process [68,69,63,70]. As shown in Table 8,
excellent performance was observed for substances with elec-
tron-withdrawing group (Entries 9–11). Poor performances were
observed for substances with electron-donating group (Entries 2
and 3). It is consistent with the common observation. The unac-
ceptable performances of the cyano-substituted bromobenzene
might be caused by the coordination of the cyano-substituent to
the palladium center thus to retard the activity of the catalyst
(Entries 6–8).
For comparison, results from the same amination reaction with
similar conditions for using 4a as active ligand (Ref. [49]) were
added to Table 8. In general, 4a is a more efficient ligand than that
of 4c. It confirms the general observation that the efficiencies of the
trialkylphosphines bearing bulky substituent are in the order of
–tBu > –Cy. It seems that the factor of the reductive elimination
process dominates the final reaction rate.
After the extensive survey of the 4c assisted palladium complex
catalyzed amination reaction, the optimized condition was found
to be carried out in toluene at 80 °C with NaOtBu as base and
[(
g
3-C3H5)PdCl]2 as the palladium salt. Nevertheless, this result is
inferior to Buchwald’s work on similar system which stated an
Table 8
Dependence of the yield of the amination reaction using 4c/[(g
3-C3H5)PdCl]2 on substrate.a
Entry
Substrate
Product
Yield (%)b
1
>99.0 (98.0)
Br
N
O
2
3
67.6 (93.5)
58.4 (94.0)
Me
Br
Me
N
O
MeO
MeO
Br
MeO
MeO
N
O
4
17.4 (87.2)
Br
N
O
OMe
Br
OMe
N
5
6
7
Trace (14.5)
12.2 (NA)
O
NC
NC
Br
NC
NC
N
O
26.6 (54.0)
Br
N
O
CN
CN
8
4.3 (NA)
Br
N
O
O
H
O
H
9
>99.0 (98.0c)
88.9 (NA)
Br
N
O
10
O2N
Br
O2N
N
O
11
>99.0 (NA)
Br
N
O
a
Reaction conditions: 1.0 mmol of aryl halide, 1.2 mmol morpholine, and 1.4 mmol of NaOtBu, 1.0 mL toluene, 1 mol% 4c/[(
g
3-C3H5)PdCl]2, 80 °C, 2 h, isolated yields,
average of two runs.
b
Values in parentheses are taken from Ref. [49]. Amination reactions were carried out with 4a/[(g
3-C3H5)PdCl]2 as catalytic precursor.
0.5 h.
c

1478
W.-Y. Chiang, F.-E. Hong / Journal of Organometallic Chemistry 694 (2009) 1473–1481
Table 11
Suzuki reactions employing 4c/[(
excellent performance on amination of arylcholide and morpholine
in mild condition by using Davephos (2-dicylohexylphosphino-2-
N,N-dimethylamnobiphenyl) as the active P,N-ligand [71].
g
3-C3H5)PdCl]2 and various bases.a
Base
Entry
Yield (%)b
1
2
3
4
5
6
NaOtBu
KF
K2CO3
K3PO4
NaOH
-
53.4
24.4
71.0
>99.0
91.3
N.R.
2.4. Applications of 4c in palladium-catalyzed Suzuki reactions
The optimized condition for employing 4c/palladium salt in Su-
zuki reaction was surveyed by varying factors, such as the sources
of palladium salt, base, solvent, temperature, and reaction hour.
The general procedures for the Suzuki coupling reaction were
shown as the follows. The reaction was proceed with 1.0 mmol
4-bromobenoaldehyde, 1.2 mmol phenylboronic acid, 2.0 mmol
base in 1.0 mL solvent, and with 1.0 mol% of 4c/palladium salt, un-
der designated temperature and reaction times depending on the
reactions executed.
To begin with, the efficiencies of the reactions with various pal-
ladium salts were examined. Reactions were carried out using 4c
as a ligand in K3PO4/toluene system (Table 9). The ratio of ligand
to number of palladium atom is 1:1. Reactions were carried out un-
der 85 °C for one hour. Excellent performances were observed for
using Pd(OAc)2, Pd2(dba)3 and Pd(acac)2 as the palladium sources
(Entries 1–3). In order to differentiate the efficiencies of these three
palladium salts, the reaction temperature was lowered to 55 °C and
the solution was stirred for 1.5 h. Pd(OAc)2 was found as a better
palladium source in this case (Entries 7–9).
a
Reaction conditions: 1.0 mmol of aryl halide, 1.2 mmol phenylboronic acid, and
2.0 mmol of base, 2.0 mL toluene, 85 °C, 1 h, 1 mmol% 4c/Pd(OAc)2.
b
Average of two runs, determined by NMR.
Table 12
Dependence of the Suzuki reaction yield on solvent.a
Entry
Solvent
Conversion (%)b
1
2
3
4
5
6
Toluene (1 mL)
Toluene (2 mL)
Toluene (3 mL)
1,4-Dioxane
H2O
>99.0
>99.0
67.6
84.0
Trace
67.2
DMF
a
Reaction conditions: 1.0 mmol of aryl halide, 1.2 mmol phenylboronic acid, and
2.0 mmol of K3PO4, 2.0 mL solvent, 85 °C, 1 h, 1 mmol% 4c/Pd(OAc)2.
b
Average of two runs, determined by NMR.
Compound 4c is a potential bidentate ligand. In principle, it
shall be able to bind to palladium atom in a chelating mode. Suzuki
reactions were carried out in toluene in the presence of K3PO4 un-
der various 4c/Pd(OAc)2 ratios in order to determine the preferred
coordinating manner (Table 10). As expected, the optimum yield
was achieved with 4c/Pd(OAc)2 = 1/1 (Entry 1). It indicates that
4c most likely acts as a bidentate ligand and the conformation of
the active species in this reaction might be similar as 5c.
performance was observed with K3PO4 (Entry 4). Unexpectedly, the
yield was noticeably low when a strong base such as NaOtBu was
employed (Entry 1). No conversion of the reactants was observed
in the absence of base (Entry 6).
The solubility of the reactants and catalyst in the solvent is crit-
ical to the success of the coupling reaction. The concentration of
the reaction is also crucial. Subsequently, the impacts of various
solvents on the reactions were evaluated. As shown in Table 12,
the reaction is greatly affected by the nature of the solvent used.
For instance, the coupling reactions in toluene and 1,4-dioxane
were rather effective (Entries 1–4). However, the performance
was not acceptable for the rest of solvents. Although water is re-
garded as a solvent of ecological friendly, the insolubility of reac-
tants and catalyst in aqueous solution limits it application. The
effect of the concentration of the reaction is also demonstrated.
The efficiency is better for more concentrated solution (Entry 1).
As mentioned, a better conversion is achieved with aryl halides
bearing an electron-withdrawing rather than an electron-donating
substituent in a palladium-catalyzed Suzuki coupling reaction [16].
As shown in Table 13, less efficient performances were observed
for the substances with electron-donating groups (Entries 2 and
3). On the contrast, the catalytic performance was excellent for
the substance with electron-withdrawing group (Entry 7). Never-
theless, the performances were not as expected for substance with
electron-donating group –CN (Entries 4–6). Beside the severe steric
effect plays in Entry 6, the unacceptable performances of the cya-
no-substituted bromobenzene might be caused by the coordina-
tion of cyano- to palladium center thus to retard the activity of
the catalyst (Entries 4 and 5).
As known, the kind of base employed in Suzuki reaction is cru-
cial to the catalytic efficiency [72–74]. The influence of the base
used in this reaction was examined (Table 11). As shown, the best
Table 9
Dependence of the Suzuki reaction yield on the Pd sources.a
Entry
Pd source
Time (h)
Temperature (°C)
Conversion (%)b
1
2
3
4
5
6
7
8
9
Pd(OAc)2
Pd2(dba)3
Pd(acac)2
1.0
1.0
1.0
1.0
1.0
1.0
1.5
1.5
1.5
85
85
85
85
85
85
55
55
55
>99.0
>99.0
>99.0
54.9
27.2
47.2
42.9
37.9
26.2
[(g
3-C3H5)PdCl]2
Pd(COD)Cl2
PdCl2(CH3CN)2
Pd(OAc)2
Pd2(dba)3
Pd(acac)2
a
Reaction conditions: 1.0 mmol of aryl halide, 1.2 mmol phenylboronic acid, and
2.0 mmol of K3PO4, 2.0 mL toluene, 1 mmol% 4c/[Pd], determined by NMR.
b
Average of two runs.
Table 10
Dependence of the Suzuki reaction yield on the 4c/Pd(OAc)2 ratio.a
Entry
4c/Pd(OAc)2 (mmol%)
Conversion (%)b
For comparison, Suzuki reactions as shown in Scheme 3 were
1
2
3
4
5
6
1:1
2:1
1:0.5
1:2
0.5:0.5
1:0
>99.0
54.7
43.4
28.3
19.7
Trace
carried out in toluene employing
a P,N-ligand 4c-chelated
Pd(OAc)2 or a P,P-ligand chelated palladium complex 8 [75]. A sat-
isfying conversion rate, 91.7%, was observed using 4c/Pd(OAc)2 as
the catalyst precursor when the reaction was carried out at 65 °C
for 5 h; it was only 71.6% conversion for 8 while carried out under
the same temperature for 16 h. Obviously, the performance of the
palladium complex with the P,N-ligand surpasses that with the
P,P-ligand in this reaction condition (see Diagram 2).
a
Reaction conditions: 1.0 mmol of aryl halide, 1.2 mmol phenylboronic acid, and
2.0 mmol of K3PO4, 2.0 mL toluene, 85 °C, 1 h, 1 mmol% 4c/Pd(OAc)2.
b
Average of two runs, determined by NMR.

W.-Y. Chiang, F.-E. Hong / Journal of Organometallic Chemistry 694 (2009) 1473–1481
1479
300S spectrometer. In addition, some routine 1H NMR spectra were
recorded either on a Gemini-200 spectrometer at 200.00 MHz or
Varian-400 spectrometer at 400.00 MHz. The chemical shifts are
reported in ppm relative to internal standards CHCl3 (d 7.26),
CH2Cl2 (d 5.30) or CH3C(@O)CH3 (d 2.09). 31P and 13C NMR spectra
were recorded at 121.44 and 75.46 MHz, respectively. The chemi-
cal shifts for the former and the latter are reported in ppm relative
to internal standards H3PO4 (d 0.0) and CHCl3 (d 77) or CH2Cl2 (d
53), respectively. Mass spectra were recorded on JOEL JMS-SX/SX
102A GC/MS/MS spectrometer. Elemental analyses were recorded
on Heraeus CHN-O-S-Rapid.
Table 13
Dependence of the yield of the Suzuki reaction using 4c/Pd(OAc)2 on substrate.a
Entry Substrate
Product
Conversion (%)a
1
>99.0
Br
2
55.0
42.1
78.7
Me
Br
Br
Me
3
MeO
Br MeO
3.2. Synthesis and characterization of 5c
4
NC
NC
NC
A 100 mL round-bottom flask equipped with a magnetic stirrer
was charged with 1.00 mmol of 4c (0.894 g), one molar equiv. of
Pd(COD)Cl2 (1.00 mmol, 0.286 g) and 15 mL toluene. The solution
was stirred at 25 °C for 10 min before the solvent was removed un-
der reduced pressure. The residue was dissolved in 5 mL CH2Cl2
and then further separated by CTLC. A yellowish-brown band
NC
5
80.2
Br
CN
CN
was eluted out by ethyl acetate and was identified as
(l-
6
7
65.2
PPh2CH2PPh2)Co2(CO)4( -Cy2PC„CCH2NMe2)PdCl2 5c. The yield
l,g
Br
was 91.3% (0.913 mmol, 0.978 g).
3.2.1. Selected spectroscopic data for 5c
O
H
O
H
>99.0b
1H NMR(CD2Cl2, d (ppm)): 7.68–7.15 (m, 20H, arene), 3.70 (q,
2H, dppm), 3.41 (s, 2H, CH2), 2.35–2.10 (m, 6H, CH3), 2.07–1.25
(m, 22H, cyclohexyl); 31P NMR(CDCl3, d (ppm)): 49.3 (s, 1P,
P(Cy)2), 38.8 (s, 2P, dppm); 13C NMR (CD2Cl2, d (ppm)): 206.0–
202.2 (s, 4C, CO), 137.1–128.8 (m, 24C, arene), 107.5 (s, 1C, P–
C„C), 67.2 (s, 1C, C–N), 50.7 (s, 6C, N–C) 38.6 (t, 1C, dppm),
28.3–26.2 (m, 12C, cyclohexyl); MS (FAB): m/z = 1016 [MÀ2CO]+;
Anal. Calc.: C, 51.59; H, 4.89; N, 1.31. Found: C, 48.56; H, 5.78; N,
0.93%.
Br
a
Reaction conditions: 1.0 mmol of arylhalide, 1.2 mmol phenylboric acid, and
2.0 mmol of K3PO4, 2.0 mL toluene, 1.0 mmol% 4c/Pd(OAc)2, 85 °C, 2 h, isolated
yields, average of two runs.
b
1 h.
2.5. Summary
3.3. Synthesis and characterization of 7c
We have developed a general route for the preparation of
several fascinating cobalt-containing P,N-bidentate ligands. These
ligands have proven to be fairly efficient in the palladium-cata-
lyzed aryl halide amination reaction. A P,N-chelated palladium
complex 5c was characterized by spectroscopic means as well as
single-crystal X-ray diffraction method. Fair to excellent perfor-
mances of 4c/palladium salt in situ or isolated 5c in amination as
well as Suzuki reactions were resulted in designated reaction con-
ditions. This work also confirms the general observation that the
efficiencies of the trialkyl- or triaryl-phosphines bearing bulky sub-
A 100 mL round-bottom flask equipped with a magnetic stirrer
was charged with 0.01 mmol of 4c (0.0089 g), one molar equiv. of
[(g
3-C3H5)PdCl]2 (0.01 mmol, 0.0036 g) and 5 mL toluene. The
solution was stirred at 25 °C for 30 min before the solvent was re-
moved under reduced pressure. The residue was dissolved in 5 mL
CH2Cl2 and then further separated by CTLC. A dark-red band was
eluted out by ethyl acetate and was identified as [( -PPh2CH2PPh2)-
l
Co2(CO)4( -Cy2PC„CCH2NMe2)Pd( g
l,g
g
3-C3H5)]+ [( 3-C3H5)PdCl2]À
t
(7c). The yield was 99% (0.0099 mmol, 0.011 g).
stituent are in the order of À Bu > ÀCy > ÀPh.
3.3.1. Selected spectroscopic data for 7c
3. Experimental
1H NMR(CD2Cl2, d (ppm)): 7.46–7.12 (m, 20H, arene), 5.59, 4.49
(m, 2H, allylic HHC@C(H)–CHH), 4.01 (d, JH–H = 6.8 Hz, 4H, allylic
3.1. General
All operations were performed in a nitrogen flushed glove box
or in a vacuum system. Freshly distilled solvents were used. All
processes of separations of the products were performed by Cen-
trifugal Thin Layer Chromatography (CTLC, Chromatotron, Harrison
model 8924) or column chromatography. GC analyses were per-
formed on a HP-5890 FID GC with a QUADREX 007-CW fused silica
30 m column, and data were recorded on a HP ChemStation. Most
of the 1H NMR spectra were recorded on a 300 MHz Varian VXR-
Ph2
(CO)2
Co
Ph2
P
P
Cl
Cl
C
C
Pd
P
Co
(CO)2
Ph2
P
Ph2
8
Diagram 2.
O
O
H
4c/[Pd]
+
Br
B(OH)2
solvent, base
H
Scheme 3.

1480
W.-Y. Chiang, F.-E. Hong / Journal of Organometallic Chemistry 694 (2009) 1473–1481
HHC@C(H)–CHH), 2.96 (d, JH–H = 12.4 Hz, 4H, allylic HHC@C(H)–
CHH), 3.72 (m, 2H, dppm), 3.58 (s, 2H, –CH2N(CH3)2), 2.29 (s, 6H,
–CH2N(CH3)2), 2.05–1.34 (m, 22H, cyclohexyl); 31P NMR(CDCl3, d
(ppm)): 47.4 (s, 1P, P(Cy)2), 39.7 (s, 2P, dppm); 13C NMR (CD2Cl2,
d (ppm)): Cation: 206.5–202.8 (s, 4C, CO), 137.6–128.7 (m, 24C,
arene), 120.2 (s, 1C, allylic C@C–C), 106.8 (s, 1C, P–C„C), 70.8 (d,
1C, P–C„C), 66.8 (s, 1C, C–N), 62.2 (m, 2C, allylic C@C–C), 45.0
(s, 6C, N–C), 37.6 (t, 1C, dppm), 30.7 (s, 4C, (Cy)2), 29.2 (s, 4C,
(Cy)2), 27.5, 27.0 (d, JC–P = 25.6 Hz, JC–P = 53.1 Hz, 2C, P(Cy)2), 26.2
(s, 4C, (Cy)2); Anion: 110.8 (s, 1C, allylic C@C–C), 55.4 (2C, allylic
C@C–C); MS (FAB): m/z = 1042 [M]+.
ture was solved by direct methods using a SHELXTL package [76]. All
non-H atoms were located from successive Fourier maps and
hydrogen atoms were refined using a riding model. Anisotropic
thermal parameters were used for all non-H atoms, and fixed iso-
tropic parameters were used for H atoms [77]. Crystallographic
data for compounds 5c are summarized in Table 1.
Acknowledgments
We thank the National Science Council of the ROC (Grant NSC
95-2113-M-005-015-MY3) for financial support.
3.4. Synthesis and characterization of 7cB
Appendix A. Supplementary material
A 100 mL round-bottom flask equipped with a magnetic stirrer
was charged with 0.01 mmol of 4c (0.0089 g), 0.005 mmol of [(g3
-
CCDC 692831 contains the supplementary crystallographic data
for compound 5c. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.jorganchem.2008.12.046.
C3H5)PdCl]2 (0.0018 g), 0.005 mmol of NaBF4 (0.0006 g) and 5 mL
toluene. The solution was stirred at 25 °C for 30 min before the sol-
vent was removed under reduced pressure. The orange-red colored
residue was purified by CTLC and was identified as [(
PPh2)Co2(CO)4( -Me2NCH2C„CP(Cy)2)Pd(
3-C3H5)]+[BF4]À (7cB).
The yield was 99%.
l-PPh2CH2-
l,g
g
References
3.4.1. Selected spectroscopic data for 7cB
1H NMR(CD2Cl2, d (ppm)): 7.43–7.10 (m, 20H, arene), 5.48 (m, 1
H, allylic HHC@C(H)–CHH), 4.47 (m, 1H, dppm) 4.07 (d, 2H
JH–H = 6.8 Hz, 2H, allylic HHC@C(H)–CHH), 3.36 (m, 1H, dppm),
3.01 (d, JH–H = 12.0 Hz, 2H, allylic HHC@C(H)–CHH), 2.64 (s, 2H,
–CH2N(CH3)2), 1.92 (s, 6H, –CH2N(CH3)2), 2.07–1.26 (m, 22 H,
cyclohexyl);
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