Synthesis of cobalt-containing monodentate phosphine ligand and application toward Suzuki cross-coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2004.11.030

A bulky, dicobalt complexed, mono-dentate phosphine, [(μ-PPh ₂CH₂PPh₂)Co₂(CO)₄](μ, η-PhC≡CPCy₂) (4), was prepared from the reactions of the bis(diphenylphosphino)methylene (dppm)-bridged dicobalt complex Co ₂(CO)₆(μ-Ph₂PCH₂PPh₂) (2) with PhC≡CPCy₂ (3). Combination of 4 and Pd(OAc) ₂ (1:1) gave an active catalyst for the palladium-catalyzed Suzuki coupling of aryl bromides with phenylboronic acid; the catalytic reactions can be performed even under low catalyst loadings (0.1-0.001 mol% 4/Pd(OAc) ₂). Compound 4 has been proved to be an authentic and effective mono-dentate phosphine ligand. Crucial factors such as 4/Pd(OAc)₂ ratio, base being used, solvent volume, temperature, and electronic variation of the aryl bromides in reactions were also investigated and results are reported.

Full text of DOI:10.1016/j.jorganchem.2004.11.030

Journal of Organometallic Chemistry 690 (2005) 1249–1257
www.elsevier.com/locate/jorganchem
Synthesis of cobalt-containing monodentate phosphine ligand
and application toward Suzuki cross-coupling reactions
*
Fung-E Hong , Yi-Jung Ho, Yu-Chang Chang, Yi-Luen Huang
Department of Chemistry, National Chung-Hsing University, Taichung 40227, Taiwan
Received 9 September 2004; accepted 17 November 2004
Abstract
A bulky, dicobalt complexed, mono-dentate phosphine, [(l-PPh2CH2PPh2)Co2(CO)4](l,g-PhC„CPCy2) (4), was prepared from
the reactions of the bis(diphenylphosphino)methylene (dppm)-bridged dicobalt complex Co2(CO)6(l-Ph2PCH2PPh2) (2) with
PhC„CPCy2 (3). Combination of 4 and Pd(OAc)2 (1:1) gave an active catalyst for the palladium-catalyzed Suzuki coupling of aryl
bromides with phenylboronic acid; the catalytic reactions can be performed even under low catalyst loadings (0.1–0.001 mol% 4/
Pd(OAc)2). Compound 4 has been proved to be an authentic and effective mono-dentate phosphine ligand. Crucial factors such
as 4/Pd(OAc)2 ratio, base being used, solvent volume, temperature, and electronic variation of the aryl bromides in reactions were
also investigated and results are reported.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Suzuki reaction; Palladium complex; Bulky phosphine ligand; Cobalt-containing complex; Alkyne-bridged dicobalt complex; Homog-
eneous catalysis
1. Introduction
established mechanistically that a well performed ligand
constitutes species typically with either bulky and/or
electron-rich character [1b,1e,1g,5]. Therefore, bulky
monodentate phosphine ligands, such as tri-tert-butyl-
phosphine [1a,1b,2a,2b,3a,3b,6], biphenyl-2-yl-di-tert-
butyl-phosphine, biphenyl-2-yl-dicyclohexyl-phosphine
[1c,1d,1e,1f,7], and more recently even a ferrocene deriv-
ative mono- or di-alkyl-phosphino-pentaphenyl-ferro-
cene [1g,2c,8], have been widely applied in Suzukiꢀs
palladium-catalyzed reaction. By its nature, the metal-
containing phosphine ligand such as di-phenyl-phosph-
ino-pentaphenyl-ferrocene, dppf, guarantees the bulkiness.
In general, the catalytic performances are satisfactory
while using these types of ligands. One of the most ad-
mired advantages of using dppf-like ligands is the flexi-
bility of the biting angle which is caused by the free
rotation of the two phosphino-containing Cp rings. By
that way, it is possible for the ligand to adjust automat-
ically to a proper biting position toward the targeted
metal and make a firm chelation. However, there are
Many phosphine ligands modified palladium cata-
lysts were known for their notable capabilities in cou-
pling aryl halides with unsaturated organic moieties in
a variety of reactions such as Suzuki [1], Heck [2], Stille
[3], Sonogashira [4], and etc. In recent years, Suzukiꢀs
palladium-catalyzed reaction has received wide accep-
tance in the field of cross-coupling reaction because of
its exceptional capability in making C–C bond from a
sp2 or non-b-hydride-containing electrophile with a
boronic acid derivative. Catalytic reaction employed
by phosphine modified palladium complex is often ben-
efited from its mild reaction conditions and selectivity in
some cases. Consequently, numerous types of phos-
phines have been developed and their capacities as com-
petent ligands have been demonstrated. It was
*
Corresponding author. Tel./fax: +886 4 22862547.
E-mail address: fehong@dragon.nchu.edu.tw (Fung-E Hong).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.11.030

1250
Fung-E Hong et al. / Journal of Organometallic Chemistry 690 (2005) 1249–1257
several annoying side effects accompanied with the
advantages were reported as well. One of them is that
dppf-like ligands might coordinate to more than one
metal centers at the same time thereby causes the decay-
ing of the ligandꢀs reactivity. Therefore, we sought to de-
velop a new type of metal-containing phosphine ligands
that would be bulky and electron-rich in nature [1g,9].
Our previous works had demonstrated that several al-
kyne-bridged, dicobalt complexed, monodentate phos-
phines, [(l-PPh2CH2PPh2)Co2(CO)4][l,g-RC„CPPh2]
(1a: R = Ph; 1b: R = CMe3; 1c: R = SiMe3), acted as po-
tential bulky ligands in the palladium-catalyzed Suzuki
coupling reaction [10].
(CO)4](l,g-PhC„CP(@O)Cy2) (5), was obtained as well
along with 4. Both compounds 4 and 5 were character-
ized by spectroscopic means as well as by X-ray diffrac-
tion methods (Table 1).
In 1H NMR, there are two distinct chemical shifts 3.32
and 4.25 ppm were being observed for the methylene
protons of 4, whereas the 31P NMR spectrum displays
two sets of singlet in the ratio of 2:1 at the chemical shifts
of 38.5 and 16.8 ppm, respectively, for three phospho-
rous atoms. Similar observations of spectroscopic data
are perceived in the case of 5. In 1H NMR, a large down-
field shift for one of the methylene protons at 5.96 ppm is
observed due to the weak intramolecular hydrogen
bonding between the oxide and the adjacent methylene
proton [10]. The 31P NMR spectrum of 5 shows two sing-
lets in the ratio of 1:2 at 50.2 and 36.0 ppm for three
phosphorous atoms and large downfield shifts are ob-
served for the oxidized phosphorous atoms.
The ORTEP diagram for 4 is presented in Fig. 1. As
shown, the phenyl rings of the bridged alkyne are
pointed away from the center of the molecule to prevent
severe steric hindrance. All of the coordinated carbonyl
ligands are situated at the terminal positions. Interest-
ingly, the coordinated dppm ligand and the substituent
–PCy2 are located on the opposite sides of molecule to
minimize the steric effect among the bulky dicyclohexyl
group. Similar structural pattern is observed for 5 as di-
pected in Fig. 2. The presumption of intramolecular
hydrogen bonding between the oxygen atom (of the oxi-
dized phosphorus) and the methylene proton (of the
coordinated dppm) is not validated in this solid state
structure. Nevertheless, a weak intramolecular interac-
tion might be achieved through fluxional motion of
the bridged dppm, which is evidenced by the observation
of a large downfield shift in 1H NMR at 5.96 ppm for 5.
R
Ph2P
C
C
(OC)2Co
Ph2P
Co(CO)2
PPh2
1a: R=Ph
1b: R=CMe3
1c: R=SiMe3
C
H2
It is known that a palladium complex is more air-stable
when it is coordinated by a phosphine with cyclohexyl
substituent than that of tert-butyl one [1e]. The former
complex is also allowed for higher turnover numbers
in catalytic reaction, and run even at low catalyst loading
[1e]. Therefore, a bulky, dicobalt complexed, phosphine
with dicyclohexyl substituents on the phosphorous atom
was designed and synthesized and employed as a
monodentate phosphine ligand in the palladium-cata-
lyzed Suzuki cross-coupling reactions. Herein some
remarkable results are reported.
2. Results and discussion
2.1. Preparation of cobalt-containing monodentate
phosphine ligands
2.2. Suzuki reaction using cobalt-containing phosphine
ligand 4 with Pd complex
Reaction of a dppm-bridged dicobalt compound
[Co2(CO)6(l-P,P-PPh2CH2PPh2)] (2) [11] with one mo-
lar equivalent of alkynyl phosphine PhC„CPCy2 (3)
[12] in toluene at 80 °C afforded alkyne-bridged dicobalt
Palladium-catalyzed Suzukiꢀs coupling reactions
using bromobenzene and phenylboronic acid as reaction
substrates were carried out by employing the newly
made cobalt-containing phosphine ligand 4. First, the
impact on the yield as a function of the amount of
ligand employed was investigated. The catalytic process
under investigation is consists of 1.0 mmol of aryl
compound
[(l-PPh2CH2PPh2)Co2(CO)4][l,g-PhC„
CPCy2] (4). (Scheme 1). During the chromatographic
processes, an oxidized complex, [(l-PPh2CH2PPh2)Co2
O
PCy2
Ph
PCy2
Ph
C
C
C
C
(CO)
3Co
(CO)
3
Co
+
3
PCy2C
CPh
80 oC, 14 h, toluene
+
(CO)
(CO)
2Co
Co
PPh2
Ph2P
2
PPh2
2
(CO) Co
Ph2P
Co(CO)
PPh2
2
2
Ph2P
C
C
H2
4
H2
5
Scheme 1. Synthesis of cobalt-containing monodentate phosphine ligand.

Fung-E Hong et al. / Journal of Organometallic Chemistry 690 (2005) 1249–1257
1251
Table 1
Crystal data of 4 and 5
Formula
C49H49Co2O4P3 Æ CH2Cl2
997.58
Triclinic
C49H49Co2O5P3 Æ CH2Cl2
Formula weight
Crystal systtem
Space group
1013.58
Monoclinic
P2(1)/c
13.092(2)
20.633(4)
19.234(3)
–
ꢀ
P1
˚
a (A)
12.079(2)
16.950(3)
24.432(4)
82.501(4)
83.149(4)
79.855(4)
4858.2(14)
4
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
107.634(4)
–
3
˚
V (A )
4951.5(14)
4
1.360
Z
Dcalc (Mg/m3)
1.364
0.71073
0.934
˚
k(Mo Ka) (A)
0.71073
0.919
l (mmꢀ1
)
h Range (°)
1.84–26.14
3106
1084
1.49–26.01
1773
559
Observed reflections (F > 4r(F))
Number of refined parameters
aR1 for significant reflections
bwR2 for significant reflections
cGoodness-of-fit
0.0956
0.2406
0.972
0.0580
0.1089
0.902
P
P
a
b
c
R1 = | (|Fo| ꢀ |Fc|)/| Fo||.
P
P
2
2
wR2 ¼ f ½wðF 2o ꢀ F c2Þꢁ = ½wðF 2oÞ ꢁg1=2; w ¼ 0:1315 and 0:0500 for 4 and 5.
P
2
1=2
GOF ¼ ½ wðF 2o ꢀ F c2Þ =ðNrflns ꢀ NparamsÞꢁ
.
Fig. 2. ORTEP drawing of 5. Hydrogen atoms are omitted for clarity.
˚
Fig. 1. ORTEP drawing of 4. Hydrogen atoms are omitted for clarity.
˚
Selected bond lengths (A) and bond angles (°): Co(1)–C(1) 1.962(7),
Selected bond lengths (A) and bond angles (°): Co(1)–C(1) 1.935(9),
Co(1)–C(2) 1.948(7), Co(1)–P(2) 2.241(2), Co(1)–Co(2) 2.4759(14),
Co(2)–C(2) 1.952(7), Co(2)–C(1) 1.970(7), Co(2)–P(3) 2.225(2), P(1)–
O(1) 1.487(5), P(1)–C(1) 1.768(7), P(2)–C(49) 1.847(7), P(3)–C(49)
1.819(6), C(1)–C(2) 1.357(8), C(2)–C(3) 1.498(9), C(1)–Co(1)–C(2)
40.6(2), P(2)–Co(1)–Co(2) 93.80(6), C(2)–Co(2)–C(1) 40.5(2), P(3)–
Co(2)–Co(1) 99.18(7), C(49)–P(2)–Co(1) 110.8(2), C(49)–P(3)–Co(2)
109.9(2), C(2)–C(1)–P(1) 138.9(6), Co(1)–C(1)–Co(2) 78.1(3), C(1)–
C(2)–C(3) 136.3(7), Co(2)–C(2)–Co(1) 78.8(3), P(3)–C(49)–P(2)
109.3(3).
Co(1)–C(2) 1.968(10), Co(1)–P(2) 2.216(3), Co(1)–Co(2) 2.477(2),
Co(2)–C(2) 1.936(10), Co(2)–C(1) 1.971(10), Co(2)–P(3) 2.249(3),
P(1)–C(1) 1.826(10), P(2)–C(49) 1.839(10), P(3)–C(49) 1.817(9), C(1)–
C(2) 1.343(12), C(2)–C(3) 1.494(13), C(1)–Co(1)–C(2) 40.2(4), P(2)–
Co(1)–Co(2) 100.31(9), C(2)–Co(2)–C(1) 40.2(4), P(3)–Co(2)–Co(1)
89.89(9), C(49)–P(2)–Co(1) 109.3(3), C(49)–P(3)–Co(2) 110.4(3), C(2)–
C(1)–P(1) 135.5(8), Co(1)–C(1)–Co(2) 78.7(4), C(1)–C(2)–C(3)
137.5(9) Co(2)–C(2)–Co(1) 78.7(4), P(3)–C(49)–P(2) 107.7(5).

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Fung-E Hong et al. / Journal of Organometallic Chemistry 690 (2005) 1249–1257
halide, 1.5 equivalent of phenylboronic acid, 1 mol% of
Pd(OAc)2 (based on aryl halide), 1–2 mol% of 4 (based
on Pd(OAc)2), 1 mL of toluene, and 3.0 equivalent of
KF (based on aryl halide). The reaction mixture was
stirred at 40 °C for a designated time period then work-
up followed. The reactions were catalyzed smoothly by
4/Pd(OAc)2 complexes and in the absence of a phos-
phine complex, the reaction rates were much slower
and only partial conversions are observed (Table 2).
The conversion of bromobenzene against reaction time
is depicted in Fig. 3.
The fact that by increasing the 4/Pd(OAc)2 ratio from
1:1 to 2:1 resulted in decreasing the yields of the biphenyl
is noteworthy since it is generally believed that two mo-
lar equivalents of monodentate phosphine ligands are
required for an effective catalytic Suzuki reaction [13].
This observation indicates that the optimum active cat-
alyst, while 4 being used as ligand, is a low-coordinated
palladium complex; an over-coordinated palladium
complex in fact effectively diminishes its catalytic activ-
ity. Indeed, many experimental observations point to the
same conclusion that the active catalysts are probably
monophosphine complexes [14]. In addition, our previ-
ous works had demonstrated that a 4-like ligand, [(l-
Ph2PCH2PPh2)Co2(CO)4(l,g-PPh2C„CP(@O)Ph2)], is
able to act as chelating ligand through its phosphrous
site and one of the cobalt centers [15]. Preference for
uni-phosphine ligand coordination is due to an extra,
unusual cobalt–palladium direct bonding. Unfortu-
nately, attempts to gain suitable crystallines of the
4-chelated palladium complex for structural determina-
tion were unsuccessful. The crystal growing process al-
ways led the formation of the phosphine oxidized
product 5.
Self-coupling of the aryl boronic acid is an unwanted
side reaction encountered in palladium complexes, phos-
phines mediated, catalyzed coupling reactions [16].
When 4 was employed as the coordinating ligand, small
amounts of biphenyl were observed (<5% by GC) as a
result of aryl–aryl exchange followed by coupling with
an aryl boronic acid even at 100 °C.
Subsequently, the impact of the base used on the
reaction was examined. Among all the bases used, KF
was found to be the most effective while the reaction
was run in toluene (Table 3, entry 1) and the other bases
such as K3PO4 and K2CO3 were substantially less effec-
tive than KF under the similar conditions (Table 3, en-
tries 2 and 3). The bases Na2CO3, Cs2CO3, CsF and
NEt3 were failed to promote the reaction and gave very
low conversions (Table 3, entries 4–7). Using KF or
K3PO4 did not notably affect the reaction rates at 100
°C reaction temperature. However, the yields of the
reactions carried out at room temperature were lowered
when K3PO4 was used. In Suzukiꢀs cross-coupling reac-
tions, it is generally believed that Lewis-base facilitate
the reaction process by binding it to the organoboron re-
agent and forming a more reactive four-coordinate
‘‘ate’’ complex, which then transfers the organic moiety
to the palladium [1b,1h,1i,17]. For example, the treat-
ment of arylboronic acid with excess KF produces
[Ar–BF3]ꢀ, a reactive species which facilitates the aryl
group transformation [18]. Interestingly, decreasing the
amount of KF (2.0 molar equiv.) used in reaction re-
sulted in increasing yield of the biphenyl (Table 2, entry
6 and Table 3, entry 1). This could be caused by the poor
solubility of inorganic base in organic solvent [19].
Table 2
Suzuki coupling reactions employing phosphine 4a
Entry
Pd (mol%)
Ligand (mol%)
Time (h)
Yield (%)
1
2
3
4
5
6
7
a
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
1.0
1.0
1.0
0
3
6
15
17
74
35
48
72
30
24
3
6
24
24
Reaction conditions: 1.0 equiv. aryl bromide, 1.5 equiv. boronic
acid, 3.0 equiv. KF, cat Pd(OAc)2, cat ligand 4, toluene (1 mL/mmol
aryl bromide), 40 °C. Yields (average of two or more experiments) of
products are determined by GC.
100
80
60
40
20
0
4/Pd (1:1)
4/Pd (2:1)
Table 3
Suzuki coupling reactions employing phosphine 4 at various basesa
Entry
Base
Yield (%)
1
2
3
4
5
6
7
a
KF
92
70
71
5
K3PO4
K2CO3
Na2CO3
Cs2CO3
CsF
8
10
5
N(Et)3
0
6
12
Time (h)
18
24
Reaction conditions: 1.0 equiv. aryl bromide, 1.5 equiv. boronic
acid, 2.0 equiv. base, 1 mol% Pd(OAc)2, 1 mol% cat ligand 4, toluene
(1 mL/mmol aryl bromide), 40 °C, 24 h. Yields (average of two or more
experiments) of compounds are determined by GC.
Fig. 3. Conversion of bromobenzene under the conditions listed in
Table 2.

Fung-E Hong et al. / Journal of Organometallic Chemistry 690 (2005) 1249–1257
1253
The reaction rate is also greatly affected by the
amount of solvent used. For example, the coupling reac-
tions were sluggish (<30% conversion) when 3 mL of to-
luene was used for the coupling of phenylboronic acid
with bromobenzene. However, the yields of the biaryl
were greatly improved (>90% conversion) by decreasing
the amount of solvent to 1 mL provided that other fac-
tors are remained the same. With this small amount of
solvent, the reactions are visibly heterogeneous. Using
these optimized conditions, the efficiency of 4/Pd(OAc)2
as catalyst in the reactions were investigated with vari-
ous substituted arylhalides. As shown in Table 4, the
cross-coupling reactions of arylboronic acids with di-
verse substituted bromobenzenes including electron
poor derivatives (Table 4, entries 2–4) were performed
efficiently and all completed at 40 °C. As expected, reac-
tions involving electron-rich aryl bromides were consid-
erably slower, nevertheless, heated to 60 °C allowed high
conversions (Table 4, entries 5 and 6). While a variety of
substrates were coupled with the catalyst system of 4/
Pd(OAc)2, reactions employing aryl bromide-containing
heteroatom such as 2-bromothiophene did not proceed
for completion (Table 4, entry 7). It is possible that
the sulfur donor of the reagent or product might have
displaced 4 in the coordination and resulted in the for-
mation of a thiophene-containing palladium compound.
Several closely related heteroatom-coordinated palla-
dium complexes have been reported with much less reac-
tivities in coupling reactions [20].
The 4/Pd(OAc)2 catalyst system allows for effective
Suzuki coupling reactions at low catalyst loadings and
with high turnover numbers (Table 5). Reactions with
various ratios of substrate versus catalyst loadings were
examined. As high as 99,000 turnover number was
achieved when 0.001 mol% 4/Pd(OAc)2 was employed
as catalyst (Table 5, entry 5).
The palladium catalyst derived from phosphine li-
gand 4 with Cy substituent was noticeably more reactive
than that derived from Ph substituent (less bulky and
electron-rich ligand) reported earlier [10]. As shown
above, both activated and unactivated aryl bromides
can be coupled with arylboronic acids in the presence
of 1% 4/Pd(OAc)2 (1:1) complex. These results support
the argument that the both electronic and steric factors
are important to ligand activity. The reason for the
highly active 4/Pd(OAc)2 catalyst system in Suzuki reac-
tion is not well understood for now. However, we
believe that several structural features of this cobalt-
containing phosphine ligand are of importance. First,
the electron-richness of the phosphine, due to the elec-
tron-donating effect from coordinated dppm, facilitates
the oxidative addition of the aryl bromide and binds
tightly to the metal. Second and the most unique feature
of the ligand is that the enhanced solubility of the
Table 4
Suzuki coupling of aryl bromides employing phosphine 4a
Halide
Product
Entry
Temp. (oC)
40
Time (h)
24
Yeild (%)
92
Br
Br
1
O
O
2
3
HC
HC
40
40
16
16
99
99
O
O
MeC
Br
MeC
O
O
PhC
Br
Br
PhC
40
60
16
16
99
92
4
5
6
Me
Me
Me
Me
Br
Br
60
60
16
24
91
70
S
S
7
a
Reaction conditions: 1.0 equiv. aryl bromide, 1.5 equiv. boronic acid, 2.0 equiv. KF, 1 mol% Pd(OAc) , 1 mol% cat ligand 4, toluene (1 mL/
2
mmol aryl bromide); reaction times have not been minimized. Yields (average of two or more experiments) of compounds are determined by GC
except entries 5–7 are presented in isolated yields.

1254
Fung-E Hong et al. / Journal of Organometallic Chemistry 690 (2005) 1249–1257
Table 5
Suzuki coupling of aryl bromides employing phosphine 4 at various substrates/Pd ratiosa
4/Pd(OAc)2(1:1)
Br
B(OH)2
+
3.0 equiv KF, toluene
Entry
1
Pd (mol%)
0.1
Temp. (°C)
Time (h)
Yield (%)
TON
700
960
40
60
24
24
70
96
2
3
4
0.05
80
80
16
24
85
99
4250
4950
0.01
80
80
16
24
79
94
7900
9400
0.005
0.001
80
100
24
6
80
99
40,000
49,500
5
80
100
24
16
70
99
70,000
99,000
a
Reaction conditions: 1.0 equiv. aryl bromide, 1.5 equiv. boronic acid, 3.0 equiv. KF, cat Pd(OAc)2, cat ligand 4 (1L/Pd), toluene (1 mL/mmol
aryl bromide); reaction times have not been minimized. Yields (average of two or more experiments) of compounds are determined by GC.
cobalt-containing phosphine ligand in organic solvent,
due to several attached organic moieties, which prevents
the 4-coordinated palladium intermediates from precip-
itation in the catalytic cycle. As known, ferrocenyl phos-
phine is not quite soluble in organic solvent because of
its intrinsic ionic character. Third, the bulkiness of the
ligand promotes the process of the reductive elimination
[21].
Chromatography (CTLC, Chromatotron, Harrison
model 8924) or column chromatography. GC analyses
were performed on a HP-5890 FID GC with a QUA-
DREX 007-CW fused silica 30 m column, and data
1
were recorded on a HP ChemStation. H NMR spectra
were recorded on 300 MHz Varian VXR-300S spec-
trometer. The chemical shifts are reported in ppm rel-
ative to internal standard CHCl3 or CH2Cl2. 31P and
13C NMR spectra were recorded at 121.44 and 75.46
1
2.3. Summary
MHz, respectively. Some other routine H NMR spec-
tra were recorded over Gemini-200 spectrometer at
200.00 MHz or Varian-400 spectrometer at 400.00
MHz. IR spectra of samples using KBr were recorded
on a Hitachi 270-30 spectrometer. Mass spectra were
recorded on JOEL JMS-SX/SX 102A GC/MS/MS
spectrometer. Elemental analyses were recorded on
Heraeus CHN-O-S-Rapid.
We have developed a novel and efficient catalyst sys-
tem of organometallic phosphine ligands for the palla-
dium-catalyzed Suzuki coupling of aryl bromides with
phenylboronic acid. The catalytic reactions were per-
formed at ambient condition using low catalyst load-
ings. Indeed, results of Suzuki reactions employing 4
as the ligand were more efficient than that using 1a–
c; no matter what the reaction conditions or the load-
ings of catalyst employed. Further studies concerning
the effectiveness of this class of phosphine ligands for
other palladium-catalyzed reactions, as well as mecha-
nistic studies to determine the factors responsible for
the high activities of these catalysts are currently
underway.
3.2. Synthesis of PhCCPCy2 (3)
Phenylacetylene (1.021 g, 10.000 mmol) and dimeth-
ylether (10 mL) were placed in a 100 ml round-
bottomed flask charged with magnetic stirrer. In a
separate round-bottomed flask, one molar equivalent
of n-butyllithium (2.0 M in cyclohexane) dissolved in
dimethylether (5 mL) was taken. The latter solution
was slowly added to the above mixture at ꢀ78 °C
and remained to stir at that temperature for more than
one hour. Then, one molar equivalent of chloro-
dicyclohexylphosphine (2.327 g, 10.0 mmol) dissolved
in 5 ml dimethylether (5 mL) was slowly added to it.
The reaction mixture was allowed to warm up to room
temperature and contiuned to stir for another 14 h.
The solvent was then removed under reduced pressure
and toluene was added to precipitate the lithium
3. Experimental
3.1. General
All operations were performed in a nitrogen flushed
glove box or in a vacuum system. Freshly distilled sol-
vents were used. All processes of separations of the
products were performed by Centrifugal Thin Layer

Fung-E Hong et al. / Journal of Organometallic Chemistry 690 (2005) 1249–1257
1255
chloride. After filtration, the resulted solution was fur-
ther purified by chromatography. The white solid ob-
tained in needle form was identified as 3. The
isolated yield obtained in the reaction is 75.0% (2.238
g, 7.500 mmol).
Complex 3: 1H NMR (CDCl3, d/ppm): 7.47–7.26 (5H,
arene), 2.00–1.23 (22H, PCy2). 31P NMR (CDCl3, d/
ppm): ꢀ19.8 (s, 1P, PCy2).
halide (1.000 mmol) through a rubber septum. The aryl
halides being solids at room temperature were added
prior to the evacuation/backfill cycle. The flask was
sealed with Teflon screw cap and the solution was stirred
at the required temperature for designated hours. Then,
HCl(aq) (2.3 M, 10 mL) was added to quench the reac-
tion. For entries 2–4 in Table 4, only H2O rather than
HCl(aq) was added. The aqueous layer was extrated
with CH2Cl2 (30 mL). The organic extracts were dried
over anhydrous magnesium sulfate and the solution
was filtered through silica gel. The solvent was removed
on a rotary evaporator. The residue was dissolved in tol-
uene (9 mL), naphthalene (internal standard, 1 M in tol-
uene, 1 mL) was added, and the mixture was analyzed
by GC. Isolation and purification of the coupled prod-
ucts from the representative reactions was done by col-
umn chromatography (silica gel, CH2Cl2/hexane).
3.3. Synthesis of [(l-PPh2CH2PPh2)Co2(CO)4](l,g-
PhCCPCy2) (4), [(l-PPh2CH2PPh2)Co2(CO)4](l,g-
PhCCP(@O)Cy2) (5)
1.000 mmol of dicobalt octacarbonyl, Co2(CO)8
(0.342 g), 1.000 mmol of dppm (0.385 g) and 10 mL of
toluene were taken in a 100 mL round-bottomed flask
charged with magnetic stirrer. The solution was stirred
at 65 °C for 6 h, a yellow-colored compound, Co2-
(CO)6(l-P,P-PPh2CH2PPh2), was yielded. Without sep-
aration, the reaction flask was further charged with
one molar equivalent of 3 (0.298 g) in 5 mL of toluene
and then the solution was allowed to stir at 80 °C for
14 h. Then the solvent was removed under reduced pres-
sure and the resulted dark red-colored residue was sep-
arated by CTLC. A purple band was eluted out by
mixed solvent system (CH2Cl2: hexane = 1:1) and the
compound was identified as 4 with the yield of 77.0%
(0.703 g, 0.770 mmol). A small red band, followed by
4 during the chromatographic process, was eluted out
and the red-colored compound was identified as 5 with
the yield of 9.8% (0.091 g, 0.098 mmol).
3.5. General procedure for the Suzuki coupling reactions
at low catalyst loadings (<0.1 mol% Pd)
An oven-dried sealable Schlenk flask was evacuated
and backfilled with argon and charged with the boronic
acid (0.183 g, 1.500 mmol) and KF (0.174 g, 3.000
mmol). The flask was evacuated and backfilled with ar-
gon, and toluene (1 mL) and the aryl halide (1.000
mmol) were added through a rubber septum. A separate
flask was charged with Pd(OAc)2 (2.200 mg, 0.010
mmol) and 4 (9.127 mg, 0.010 mmol) and purged with
argon. Toluene (10 mL) was added, the mixture was stir-
red for 1 min at room temperature, and then 100 lL of
this solution (0.01 mol% Pd, 0.01 mol% 4) was added to
the Schlenk flask. The flask was sealed with Teflon screw
cap and the solution was stirred at the required temper-
ature for designated hours. Then, HCl(aq) (2.3 M, 10
mL) was added to quench the reaction. The aqueous
layer was extracted with CH2Cl2 (30 mL). The organic
extracts were dried over anhydrous magnesium sulfate
and the solution was filtered through silica gel. The sol-
vent was removed on a rotary evaporator. The residue
was dissolved in toluene (9 mL), naphthalene (internal
standard, 1 M in toluene, 1 mL) was added, and the
mixture was analyzed by GC.
1
Complex 4: H NMR (CDCl3, ppm): 7.77–7.00 (m,
25H, arene), 3.30–3.16 (m, 2H, CH2), 2.12–0.92 (m,
22H, PCy2). 13C NMR (CDCl3, ppm): 132.4–128.1
(30C, arene), 26.2–25.8 (12C, PCy2). 31P NMR (CDCl3,
ppm): 38.5 (s, 2P, dppm), 16.8 (s, 1P, PCy2). IR (KBr,
cmꢀ1): 2015(s), 1989(s), 1964(s) (C@O). MS (FAB): m/
z 913 (M+). Anal. Calc. for C49H49Co2O4P3: C, 64.48;
H, 5.41. Found: C, 63.73; H, 5.86%.
1
Complex 5: H NMR (CDCl3, d/ppm): 7.77–7.26 (m,
25H, arene), 5.96, 3.27 (s, s, 2H, CH2), 2.06–1.11 (m,
22H, PCy2). 13C NMR (CDCl3, d/ppm): 132.4–128.1
(30C, arene), 26.5–25.7 (12C, PCy2). 31P NMR (CDCl3,
d/ppm): 50.2 (s, 1P, PCy2), 36.0 (s, 2P, dppm). IR (KBr,
cmꢀ1): 2024(s), 2000(s), 1970(s) (C@O). MS (FAB): m/z
929 (M+). Anal. Calc. for C49H49Co2O5P3: C, 62.69; H,
5.62. Found: C, 63.37; H, 5.32%.
3.6. X-ray crystallographic studies
Suitable crystals of 4 and 5 were sealed in thin-walled
glass capillaries under nitrogen atmosphere and
mounted on a Bruker AXS SMART 1000 diffractome-
ter. Intensity data were collected in 1350 frames with
increasing x (width of 0.3° per frame). The absorption
correction was based on the symmetry equivalent reflec-
tions using SADABS program. The space group deter-
mination was based on a check of the Laue symmetry
and systematic absences, and was confirmed using the
structure solution. The structure was solved by direct
3.4. General procedure for the Suzuki coupling reactions
The four reactants, Pd(OAc)2 (2.200 mg, 0.010
mmol), phosphine ligand 4 (9.127 mg, 0.010 mmol),
the boronic acid (0.183 g, 1.500 mmol) and KF (0.116
g, 2.000 mmol), were taken into a suitable oven-dried
Schlenk flask. The flask was evacuated and backfilled
with nitrogen before adding toluene (1 mL) and the aryl

1256
Fung-E Hong et al. / Journal of Organometallic Chemistry 690 (2005) 1249–1257
[4] T. Hundertmark, A.F. Littke, S.L. Buchwald, G.C. Fu, Org.
Lett. 2 (2000) 1729.
methods using a SHELXTL package [22]. All non-H
atoms were located from successive Fourier maps and
the hydrogen atoms were refined using a riding model.
Anisotropic thermal parameters were used for all non-
H atoms and fixed isotropic parameters were used for
H atoms [23]. Crystallographic data of 4 and 5 are sum-
marized in Table 1.
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Acknowledgment
(c) M. Kawatsura, J.F. Hartwig, J. Am. Chem. Soc. 121 (1999)
1473;
(d) G. Mann, C. Incarvito, A.L. Rheingold, J.F. Hartwig, J. Am.
Chem. Soc. 121 (1999) 3224;
We are grateful to the National Science Council of
the R.O.C. (Grant NSC 93-2113-M-005-020) for finan-
cially supporting this research.
(e) J.F. Hartwig, M. Kawatsura, S.I. Hauck, K.H. Shaughnessy,
L.M. Alcazar-Roman, J. Org. Chem. 64 (1999) 5575;
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H.W. Turner, T. Uno, W.H. Weinberg, Tetrahedron Lett. 40
(1999) 3855;
Appendix A. Supplementary data
(g) M. Watanabe, M. Nishiyama, Y. Koie, Tetrahedron Lett. 40
(1999) 8837;
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC Nos. 230925 and 232204 for com-
pounds 4 and 5, respectively. Copies of this information
may be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk
or www: http://www.ccdc.cam.ac.uk). Supplementary
data associated with this article can be found, in the on-
line version, at doi:10.1016/j.jorganchem.2004.11.030.
(h) S.R. Chemler, D. Trauner, S.J. Danishefsky, Angew. Chem.,
Int. Ed. 40 (2001) 4544;
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˚
0.96 A.