Design, synthesis, application, and DFT investigation of Suzuki-Miyaura reactions of a dicobalt carbonyl-containing phosphine ligand

Article abstract of DOI:10.1021/om050060j

The preparation and characterization of a novel dicobalt-containing monophosphine ligand, 4a, is presented. Palladium-catalyzed Suzuki-Miyuara reactions employing 4a/Pd(OAc)₂ were pursued. The optimized reaction conditions were found to start w

Full text of DOI:10.1021/om050060j

5686
Organometallics 2005, 24, 5686-5695
Design, Synthesis, Application, and DFT Investigation of
Suzuki-Miyaura Reactions of a Dicobalt
Carbonyl-Containing Phosphine Ligand
Yu-Chang Chang, Jian-Cheng Lee, and Fung-E. Hong*
Department of Chemistry, National Chung-Hsing University, Taichung 40227, Taiwan
Received January 30, 2005
The preparation and characterization of a novel dicobalt-containing monophosphine ligand,
4a, is presented. Palladium-catalyzed Suzuki-Miyuara reactions employing 4a/Pd(OAc)2
were pursued. The optimized reaction conditions were found to start with 1 molar equiv of
arylhalides, 1.5-fold of phenylbronic acid, 3-fold of KF in 1 mL of THF, and 1 mol % of 4a/
Pd(OAc)2 as catalytic precursor. The 31P NMR studies reveal moderate reductive capacity of
4a toward Pd(OAc)2. The unique bonding mode of 4a toward Pd ensures that the ratio of
4a/Pd is equal to 1:1. Two plausible active species, I and [I-OAc]-, were proposed as the
catalytically active species in the Suzuki-Miyaura cross-coupling reaction. The validity of
this assumption was examined by 31P NMR spectra and density functional theory (DFT)
means. In addition, we have demonstrated theoretically that the dicobalt moiety of 4a acts
as an effective auxiliary in stabilizing the Pd(0) center during catalytic reaction.
1. Introduction
zuki-Miyaura cross-coupling reactions,5 to our best
knowledge, a systematical investigation of transition-
metal-containing phosphines (TM-phosphines) has yet
remained a relatively uncultivated land.6,7 Three cat-
egories of TM-phosphines, which have been reported
with appreciable efficiencies in Suzuki-Miyaura reac-
tions, are depicted in Figure 1 according to their
organometallic backbones. The common features that
make these phosphines attractive are as follows: (1)
easy to prepare with satisfactory yield; (2) stable toward
air and therefore easy to handle; (3) adjustable bulki-
ness. It is noteworthy that all the metal centers in these
three kinds of phosphines play the role of “spectator”.
This affects the catalytic efficiency only via indirect
means such as electron-withdrawing/donating and steric
effects rather than forming a direct metal-substrate
bond.7a,8 Despite the limitation, these modifiable, bulky
metal-containing substituents are undoubtedly one of
the most unique features of these types of ligands.
The authors of this paper here contribute a new
category of metal-containing phosphine ligands that will
allow us to access straightforwardly a family of phos-
The Suzuki-Miyaura reaction1 is one of the most
versatile catalytic cross-coupling reactions that comprise
C-C, C-H, C-N, C-O, C-S, C-P, or C-M bond
formation.2 Most of the reactions employ phosphine-
assisted palladium complexes as catalytic precursors.
As known, a phosphine with both the characteristics of
bulkiness and electron-richness not only accelerates the
rate of oxidative addition of arylhalide to Pd(0)Ln (L:
phosphine; n ) 1-2) but also speeds up the process of
the reductive elimination of diaryl from the Pd(II)
center.3,4
Although various synthetic methods have been ex-
plored in search of more versatile and efficient organic
phosphines for the palladium/phosphine-catalyzed Su-
* To whom correspondence should be addressed. Fax: (+886) 2-04-
22862547. E-mail: fehong@dragon.nchu.edu.tw.
(1) (a) Suzuki, A. In Metal-Catalyzed Cross-coupling Reactions;
Diederich, F.; Stang, P. J. Eds.; Wiley-VCH: Weinheim, Germany,
1998; Chapter 2. (b) Miyura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457-
2483. (c) Suzuki, A. J. Organomet. Chem. 1999, 576, 147-168.
(2) General reviews, see: (a) Cross-Coupling Reactions-A Practical
Guide. In Topics in Current Chemistry; Houk, K. N.; Sessler, H.; Lehn,
J.-M.; Ley, S. V.; De Meijere, A.; Schreiber, S. L.; Thiem, J.; Trost, B.
M.; Vo¨gtle, F.; Yamamoto, H., Eds.; Springer-Verlag: Heidelberg, 2002;
Vol. 219. (b) Hegedus, L. S. In Organometallics in Organic Synthesis;
Schlosser, M., Eds.; Wiley: New York, 1994; p 383. (c) Palladium
Reagents and Catalysts: Innovations in Organic Synthesis; Fuji, J.,
Eds.; Wiley: Chichester, 1995. (d) Geissler, H. In Transition Metals
for Organic Synthesis; Beller, M.; Bolm, C., Eds.; Wiley-VCH: Wein-
heim, 1998; Vol. 1, pp 158-193. (e) Metal-Catalyzed Cross-coupling
Reactions; Diederich, F.; Stang, P. J., Eds.; Wiley-VCH: Weinheim,
Germany, 1998. (f) Palladium in Heterocyclic Chemistry: A Guide for
the Synthetic Chemist; Li, J. J.; Gribble, G. W., Eds.; Pergamon:
Amsterdam, 2000.
(4) (a) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122,
4020-4028. (b) Zapf, A.; Ehrentraut, A.; Beller, M. Angew. Chem., Int.
Ed. 2000, 39, 4153-4155. (c) Clarke, M. L.; Cole-Hamilton, D. J.;
Woolins, J. D. Dalton Trans. 2001, 2721-2723. (d) Li, G. Y. J. Org.
Chem. 2002, 67, 3643-3650.
(5) Miura, M. Angew. Chem., Int. Ed. 2004, 43, 2201-2203.
(6) General reviews, see: (a) Delacroix, O.; Gladysz, J. A. Chem.
Commun. 2003, 665-675. (b) Salzer, A. Coord. Chem. Rev. 2003, 242,
59-72.
(7) (a) Planas, J. G.; Gladysz, J. A. Inorg. Chem. 2002, 41, 6947-
6949. (b) Hu, Q.-S.; Lu, Y.; Tang, Z.-Y.; Yu, H.-B. J. Am. Chem. Soc.
2003, 125, 2856-2857. (c) Mann, G.; Hartwig, G. F. J. Am. Chem. Soc.
1996, 118, 13109-13110. (d) Pickett, T. E.; Roca, F. X.; Richards, C.
J. J. Org. Chem. 2003, 68, 2592-2599. (e) Kataoka, N.; Shelby, Q.;
Stambuli, J. P.; Hartwig, J. F. J. Org. Chem. 2002, 67, 5553-5566. (f)
Tang, Z.-Y.; Lu, Y.; Hu, Q.-S. Org. Lett. 2003, 5, 297-300. (g) Jesen,
J. F.; Johannsen, M. Org. Lett. 2003, 5, 3025-3028.
(3) (a) Chemler, S. R.; Trauner, D.; Danishefsky, S. J. Angew. Chem.,
Int. Ed. 2001, 40, 4544-4568. (b) Wolfe, J. P.; Tomori, H.; Sadighi, J.
P.; Yin, J.; Buchwald, S. L. J. Org. Chem. 2000, 65, 1158-1174. (c)
Wolfe, J. P.; Buchwald, S. L. Angew. Chem., Int. Ed. 1999, 38, 2413-
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Am. Chem. Soc. 1999, 121, 9550-9561. (e) Bei, X.; Crevier, T.; Guram,
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10.1021/om050060j CCC: $30.25 © 2005 American Chemical Society
Publication on Web 10/12/2005

Dicobalt Carbonyl-Containing Phosphine Ligand
Organometallics, Vol. 24, No. 23, 2005 5687
Figure 1.
Chart 1
phosphine plays the role in reducing the Pd(II) to Pd(0)
species. Hayashi et al. reported the effective reducing
capacity of (R)-BINAP ((R)-2,2′-bis(diphenylphosphino)-
1,1′-binaphthyl) toward Pd(OAc)2. Accompanied with
the reduction of Pd(II) to Pd(0), one of the phosphino
sites of BINAP was oxidized.12 This experimental result
provides direct evidence of the production of a catalyti-
cally active Pd(0) complex from Pd(II) by a bulky
phosphine. As known, the organoborane is also a
capable reducing agent besides phosphine in Suzuki-
Miyaura reactions.13
In this study, the preparation of a TM-phosphine,
[(µ-PPh2CH2PPh2)Co2(CO)4][µ,η-PhCtCP(tBu2)] (4a), an
organometallic version of di-tert-butylphosphine, and its
applications in a phosphine-modified, palladium-cata-
lyzed cross-coupling reactions are presented. A moderate
reducing capacity of 4a toward Pd(II) will also be
demonstrated later. This occurred through stoichiomet-
ric reduction of Pd(II) to Pd(0) by 4a and was monitored
in situ by 31P NMR spectra. Two potential catalytically
active species, I and [I-OAc]-, involved in the catalytic
reaction are proposed. By the way, the anionic species
[I-OAc]- analogue [(PPh3)2Pd0(OAc)]- was studied ex-
tensively by Jutand and Shaik et al.14 To explicate the
structures of I and [I-OAc]-, DFT studies on two
simplified model compounds, II and [II-OAc]-, were
pursued. Structural comparison between the theoreti-
phines with bulkiness and hopefully electron-richness.
Extending from our current interest in alkyne-bridged
dicobalt skeleton (Co2(CO)6(µ-R1CtCR2)) systems,9
a
new series of novel cobalt-containing (mono- or di-) TM-
phosphines has been developed and their catalytic
efficiencies in phosphine-modified, palladium-catalyzed
Suzuki-Miyaura reactions were evaluated (Chart 1).In
recent DFT studies on the PPh3/Pd(OAc)2-catalyzed
cross-coupling reactions,10 a three-coordinate anionic
[(PPh3)2Pd0(OAc)]- was proposed as the catalytically
active species.11 In the studies, they also proposed that
(9) (a) Dickson, R. S. Adv. Organomet. Chem. 1974, 12, 323-377.
(b) Caffyn, A. J. M.; Nicholas, K. M. in Comprehensive Organometallic
Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Elsevier
Science Inc.: New York, 1995; Vol. 12, p 685. (c) Schore, N. E. In
Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F.
G. A.; Wilkinson, G., Eds.; Elsevier Science Inc.: New York, 1995; Vol.
12, p 703. (d) Templeton, J. L. In Advances in Organometallic
Chemistry; Stone, F. G. A.; West, R., Eds.; Academic Press Inc.: New
York, 1989; Vol. 29, p 1. (e) Lukehart, C. M. In Fundamental Transition
Metal Organometallic Chemistry; Monterey, C., Ed.; Brools/Cole, 1985.
(f) Chetcuti, M. J. In Comprehensive Organometallic Chemistry II; Abel,
E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Elsevier Science Inc.: New
York, 1995; Vol. 10, p 23.
(10) (a) Goossen, L. J.; Koley, D.; Hermann, H. L.; Thiel, W.
Organometallics 2004, 23, 2398-2410. (b) Kozuch, S.; Shaik, S.;
Jutand, A.; Amatore, C. Chem. Eur. J. 2004, 10, 3072-3080.
(11) Amatore, C.; Jutand, A.; M’Barki, M. A. Organometallics 1992,
11, 3009-3013.
(12) Ozawa, F.; Kubo, A.; Hayashi, T. Chem. Lett. 1992, 2177-2180.
(13) Mareno-Mana˜s, M.; Pe´rez, M.; Pleixats, R. J. Org. Chem. 1996,
61, 2346-2351.
(14) Kozuch, S.; Amatore, C.; Jutand, A.; Shaik, S. Organometallics
2005, 24, 2319-2330.

5688 Organometallics, Vol. 24, No. 23, 2005
Chang et al.
Chart 2
Scheme 1
Table 1. Crystal Data of 3 and 4a
3
strated the unique character of a number of dicobalt-
containing mono-16 and diphosphine17 ligands in palla-
dium-catalyzed cross-coupling reactions. During the
course of searching for more versatile and efficient
phosphine ligands, we were attracted by two of the most
efficient organic phosphine ligands: 2-(di-tert-butylphos-
phino)biphenyl and tri-tert-butylphosphine. The bulky
substitutes and electron-donating ability of tert-butyl
groups are believed to play a vital role in the processes
of oxidative addition, transmetalation, and reductive
elimination. Consequently, an organometallic version of
4a
formula
fw
C3H22Co2O6P2 C45H45Co2O4P3
670.29
monoclinic
P2(1)/n
10.4249(8)
13.7370(11)
20.5729(15)
92.199(2)
2944.0(4)
4
860.58
cryst syst
space group
a (Å)
monoclinic
P2(1)/n
14.0309(9)
18.7606(12)
16.4738(11)
91.1690(10)
4335.5(5)
4
b (Å)
c (Å)
â (deg)
V (Å3)
Z
Dc (Mg/m3)
λ(Mo KR) (Å)
1.512
0.71073
1.278
1.672
0.71073
0.916
µ (mm-1
)
2θ range (deg)
1.78 to 26.02
1.81 to 26.02
7931
no. of obsd reflns (F > 4σ(F)) 5990
no. of refined params
R1 for significant reflnsa
wR2 for significant reflnsb
GoFc
370
487
0.0319
0.0782
0.966
0.0352
0.0877
0.952
a
b
2
R1 ) |∑(|Fo| - |Fc|)/|∑Fo||. wR2 ) {∑[w(Fo - Fc2)]2/
∑[w(Fo2)2]}1/2; w ) 0.0499 and 0.0561 for 3 and 4a. c GoF ) [∑w(Fo
2
2
- Fc )2/(Nrflns - Nparams)]1/2
.
cally predicted geometries and the experimentally ob-
served structures was described (Chart 2). On the basis
of the previous calculations, the process of the genera-
tion of the monoligated palladium species as the active
catalyst is thereby postulated.15
2. Results and Discussion
2.1. Preparation of a Dicobalt-Containing Phos-
phine Ligand, 4a. Our previous works have demon-
(15) Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44,
366-374.
Figure 2. Molecular structure of (µ-PPh2CH2PPh2)Co2-
(CO)6, 3. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Co(1)-Co(2) 2.4568(5),
Co(1)-P(1) 2.2658(7), Co(2)-P(2) 2.2388(7), Co(1)-C(1)
1.953(3), Co(1)-C(2) 1.946(3), Co(2)-C(1) 1.933(3), Co(2)-
C(2) 1.921(3), P(1)-Co(1)-Co(2) 97.27(2), P(2)-Co(2)-
Co(1) 99.96(2), P(2)-C(31)-P(1) 115.92(13).
(16) (a) Hong, F.-E.; Ho, Y.-J.; Chang, Y.-C.; Lai, Y.-C. Tetrahedron
2004, 60, 2639-2645. (b) Hong, F.-E.; Chang, C.-P.; Chang, Y.-C.
Dalton Trans. 2003, 3892-3897. (c) Hong, F.-E.; Lai, Y.-C.; Ho, Y.-J.;
Chang, Y.-C. J. Organomet. Chem. 2003, 688, 161-167.
(17) (a) Hong, F.-E.; Chang, Y.-C.; Chang, C.-P.; Huang, Y.-L. Dalton
Trans. 2004, 157-165. (b) Hong, F.-E.; Chang, Y.-C.; Chang, R.-E.;
Chen, S.-C.; Ko, B.-T. Organometallics 2002, 21, 961-967.

Dicobalt Carbonyl-Containing Phosphine Ligand
Organometallics, Vol. 24, No. 23, 2005 5689
Table 2. Suzuki-Miyaura Coupling Reactions
Employing Various 4a/Pd(OAc)2 Ratios with KF
entrya 4a/Pd ratio (mol %) ligand base time (h) yield (%)b
1
2
2:1
1:1
4a
4a
4a
4a
4a
4a
4a
4a
5d
5
3 KF
3 KF
3 KF
3 KF
3 KF
3 KF
3 KF
3 KF
3 KF
3 KF
3 KF
24
24
3
37
99
13
30
85
73
34
7
3
2:1
4
1.5:1
1:1
3
5
3
6
0.5:1
0.25:1
0:1
3
7
3
8
3
9
2:1
3
99
83
98c
10
11
1:1
3
2:1
5
3
a Average of two runs. b Determined by gas chromatography.
c Room temperature. See: Wolfe, J. P.; Singer, R. A.; Yang, B. H.;
Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 9550. d 2-(Di-tert-
butylphosphino)biphenyl 5.
Scheme 2 4a-Assisted Suzuki-Miyaura
Cross-Coupling Reaction
P(3), respectively. As shown, 4a can be regarded as a
phosphine consisting of two bulky substituents, tBu, and
one even more bulkier substituent, [(µ-PPh2CH2PPh2)-
Co2(CO)4][µ,η-PhCtC-]. Thereby, 4a provides more
acute steric hindrance than the conventional P(tBu)3.19
2.2. Suzuki-Miyaura Cross-Coupling Reactions
Using Cobalt-Containing Phosphine Ligand 4a
with Pd(OAc)2. Suzuki’s coupling reactions were car-
ried out in situ by employing the newly made cobalt-
containing palladium complex modified by phosphine
ligand 4a (Table 2). Mostly, the reactions started with
1 molar equiv of 2-bromothiophene, 1.5-fold phenyl-
bronic acid, and 3-fold of KF in 1 mL of THF, and with
1 mol % of 4a/Pd(OAc)2 under 40 °C for designated times
(Scheme 2).
A low yield, 37%, was obtained when the reaction was
conducted with 4a/Pd(OAc)2 ) 2:1 after 24 h (Table 2,
entry 1). To our surprise, rather good yields were
observed, even at a low reaction temperature, 40 °C,
when the 4a/Pd(OAc)2 ratio was changed to 1:1 (Table
2, entries 2, 5). By contrast, the best outcome is always
achieved when the 5/Pd(OAc)2 ratio is set close to 2
(Table 2, entries 9, 10). To validate this, the efficiencies
of the working catalyst with various ratios of ligand/
metal were examined. As shown in Table 2, the best
result is achieved when the 4a/Pd(OAc)2 ratio is 1:1.
Therefore, it is concluded that the bonding modes of the
active species for the complexes 5/Pd(OAc)2 and 4a/Pd-
(OAc)2 are different. As proposed, monophosphine-
coordinated complexes Pd(0)L (L ) phosphine) might
Figure 3. Molecular structure of [(µ-PPh2CH2PPh2)Co2-
(CO)4](µ,η-PhCtCP(tBu2), 4a. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg):
Co(1)-Co(2) 2.4906(5), Co(1)-P(2) 2.2322(7), Co(2)-P(3)
2.2343(7), Co(1)-C(1) 2.009(2), Co(1)-C(2) 1.955(2), Co(2)-
C(1) 1.983(2), Co(2)-C(2) 1.965(2), P(1)-C(1) 1.811(2),
P(2)-Co(1)-Co(2) 92.86(2), P(3)-Co(2)-Co(1) 98.75(2),
P(3)-C(45)-P(2) 109.71(12).
2-(di-tert-butylphosphino)biphenyl, TM-phosphine [(µ-
PPh2CH2PPh2)Co2(CO)4][µ,η-PhCtCP(tBu2)] (4a), was
prepared by the procedures as shown (Scheme 1).
First, two alkynyl phosphines, PhCtCP(tBu)2 (1) and
PhCtCP(dO)(tBu)2 (2), were synthesized by the pro-
cedures modified from the literature.18 Further treat-
ment of a DPPM-bridged dicobalt compound [Co2(CO)6-
(µ-P,P-PPh2CH2PPh2)] (3) with 1 molar equiv of alkynyl
phosphines PhCtCP(tBu)2 (1) in toluene at 110 °C
afforded the alkyne-bridged dicobalt compound [(µ-PPh2-
CH2PPh2)Co2(CO)4][µ,η-PhCtCP(tBu)2] (4a) and a small
amount of oxidized [(µ-PPh2CH2PPh2)Co2(CO)4][µ,η-
PhCtCP(dO)(tBu)2] (4b). The latter compound can also
be prepared directly from the reaction of 3 with PhCt
CP(dO)(tBu)2 (2). Both 4a and 4b were characterized
by spectroscopic means, and the crystal structure of 4a
was determined by X-ray diffraction methods (Table 1,
Figure 3). For comparison, the crystal structure of 3 was
determined and is presented (Table 1, Figure 2). The
bond length of Co(1)-Co(2), 2.456 Å, is within the
normal range of typical cobalt-cobalt bonds of this kind.
In addition, the Co-P bond lengths, 2.266 and 2.2399
Å, show that they are genuine phosphine-to-cobalt
dative bonds. The structure of 4a reveals that the bond
distances and angles are within the normal range of
typical 4-like compounds (Table 1, Figure 3).17a,c The
bond distances are 1.8110, 2.4906, 2.2322, and 2.2343
Å, for P(1)-C(1), Co(1)-Co(2), Co(1)-P(2), and Co(2)-
(19) (a) Littke, A. F.; Fu, G. C. J. Org. Chem. 1999, 64, 10-11. (b)
Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989-7000. (c)
Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 1999, 38, 2411-2413.
(d) Littke, A. F.; Schwarz, L.; Fu, G. C. J. Am. Chem. Soc. 2002, 124,
6343-6348. (e) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu,
G. C. Org. Lett. 2000, 2, 1729-1731. (f) Nishiyama, M.; Yamamoto,
T.; Koie, Y. Tetrahedron Lett. 1998, 39, 617-620. (g) Yamamoto, T.;
Nishiyama, M.; Koie, Y. Tetrahedron Lett. 1998, 39, 2367-2370. (h)
Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 1473-
1478. (i) Mann, G.; Incarvito, C.; Rheingold, A. L.; Hartwig, J. F. J.
Am. Chem. Soc. 1999, 121, 3224-3225. (j) Hartwig, J. F.; Kawatsura,
M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-Roman, L. M. J. Org.
Chem. 1999, 64, 5575-5580. (k) Watanabe, M.; Nishiyama, M.; Koie,
Y. Tetrahedron Lett. 1999, 40, 8837-8840. (l) Dai, C.; Fu, G. C. J. Am.
Chem. Soc. 2001, 123, 2719-2724.
(18) O’Connor, T. J.; Patel, H. A. Can. J. Chem. 1971, 49, 2706-
2711.

5690 Organometallics, Vol. 24, No. 23, 2005
Chang et al.
Table 3. Suzuki-Miyaura Coupling Reactions
Table 4. Suzuki-Miyaura Couplings of
Employing 4a and Various Bases
Arylbromides Employing 4a at Various 4a/Pd
Ratios and Reaction Conditionsa
entrya Pd/L ratio ligand
base
time (h) yield (%)b
temp time yield
1
2
3
4
5
6
7
8
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
4a
4a
4a
4a
4a
4a
4a
4a
3 KF
3
3
3
3
3
3
3
3
85
79
27
14
77
3
entryb Pd/L ratioc
(°C)
(h)
(%)d
TON
2 KF
1e
2e
3e
4e
5e
6f
7f
1:1
40
40
60
60
60
60
60
5
5
91
72
99
99
3
24
41
91 (100)g
2 CsF
0.5:0.5
144 (200)g
2 Na2CO3
2 K2CO3
2 Cs2CO3
2 K3PO4
2 NEt3
0.5:0.5
0.1:0.1
0.01:0.01
0.01:0.01
0.01:0.01
5
200 (200)g
5
1000 (1000)g
300 (10 000)g
2400 (10 000)g
4100 (10 000)g
29
7
5
5
25
a Average of two runs. b Determined by gas chromatography.
a Reaction conditions (not optimized): 1.0 equiv of arylhalide,
1.5 equiv of phenylboronic acid, and 3 equiv of KF. b Average of
two runs. c Pd/L: mol % Pd to mol % L. d Determined by gas
chromatography. e Product: 2-phenylthiophene. f Product: biphe-
nyl. g Theoretical TONs.
be the catalytically active species.20 It is believed that
a three-coordinate palladium complex (Ar)(X)Pd(II)/4a
is formed as the active intermediate when a sterically
hindered phosphine such as 4a is employed.21 In the
presence of excess ligand 4a, the concentration of the
monophosphine complex is decreased, thereby reducing
the catalytic efficiency.
As shown experimentally, a well-chosen base/solvent
system is crucial for a good performance in the Suzuki-
Miyaura reaction.22 The importance of choosing an
appropriate base/solvent system was also well studied
theoretically by Ujaque and Maseras et al.23 It was
stated that under the assistance of bases the trans-
metalation process of organoboronic acid with Ar-
Pd(II)-X proceeded more favorably energetically. Table
3 summarizes results from the reactions carried out
employing the optimized ligand/metal ratio (4a:Pd-
(OAc)2 ) 1:1) at 40 °C in 1 mL of THF with a variety of
bases. Satisfactory catalytic efficiencies were found
while using KF as base. Accordingly, the best efficiency
was achieved when the reaction was carried out with
3-fold (rather than 2-fold) KF in THF (entries 1 and 2).
Nevertheless, using a large quantity of base always
leads to serious solubility and stirring problems. There-
fore, 2 equiv of bases was used throughout the rest of
the trials (entries 2-8). Note that the catalytic activity
in the K3PO4/THF system could not match its typical
performance (entry 7). A rather low yield was also
observed in the system where triethylamine was em-
ployed as base (entry 8).
Furthermore, the catalytic reactions operating at low
loading of the 4a-modified palladium complex were
pursued, and the results are shown in Table 4. The
reactions were carried out using 2-bromothiophene and
phenylboronic acid (entries 1-5) or bromobenzene and
phenylboronic acid (entries 6, 7) as substrates and
employing the optimized catalytic system (1 mol % 4a/
Pd(OAc)2 ) 1:1, 3-fold KF, 1 mL of THF). The yield could
reach as high as 91% within 5 h at 40°C (entry 1). On
the other hand, the yield is lowered, 72%, when the
Figure 4. Variable-temperature 31P NMR spectra of 4a
in toluene-d8 from 20 to -90 °C.
amount of acting catalyst was reduced to 0.5 mol %
(entry 2), and an almost complete conversion was
observed when the reaction temperature was raised to
60 °C (entry 3). In addition, high conversion and TONs
could be reached at 60 °C even with 0.1 mol % catalyst
(entry 4). However, only negligible conversion is de-
tected by GC when the ratio of the catalyst loading was
as low as 0.01 mol % (entry 5). The yields were
appreciable when bromobenzene, rather than 2-bro-
mothiophene, was used as the reaction substrate (en-
tries 6, 7).
2.3. 31P NMR Studies on 4a and 4a/Pd(OAc)2.
2.3.1. DPPM Fluxional Behavior of 4a:Variable-
Temperature 31P NMR of 4a in THF-d8. Variable-
temperature 31P NMR experiments of 4a in toluene-d8
were recorded. There was no notable chemical shift
variations of the matching signals, -P(tBu)2 and DPPM,
as the temperature was raised from 20 °C to 60 °C. On
the other hand, when the measuring temperature was
lowered to -60 °C, a broad signal of DPPM was
observed (Figure 4). When the probing temperature was
fixed at -70 °C, a set of doublet signals of DPPM
became apparent. The chemically nonequivalent envi-
ronments of the two phosphorus atoms, as a result, led
to the split of the 31P NMR peaks. Upon further lowering
of the monitoring temperatures to -80 and -90 °C, a
set of quartet signals was observed. The observation of
the low-temperature NMR spectra is in good agreement
(20) (a) Stambuli, J. P.; Bu¨hl, M.; Hartwig, J. F. J. Am. Chem. Soc.
2002, 124, 9346-9347. (b) Beare, N. A.; Hartwig, J. F. J. Org. Chem.
2002, 67, 541-555. (c) Stambuli, J. P.; Stauffer, S. R.; Shaughnessy,
K. H.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 2677-2678. (d)
Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295-4298. (e) Aranoys,
A.; Old, D. W.; Kiyomory, A.; Wolfe, J. P.; Sadighi, J. P.; Buchwald, S.
L. J. Am. Chem. Soc. 1999, 121, 4369-4378.
(21) (a) Walker, S. D.; Bader, T. E.; Martinelli, J. R.; Buchwald, S.
L. Angew. Chem. Int. Ed. 2004, 43, 1871-1876. (b) Yin, J.; Rainka,
M. P.; Zhang, X. X.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124,
1162-1163.
(22) Miyaura, N.; Yamada, K.; Suginome, H.; Suzuki, A. J. Am.
Chem. Soc. 1985, 107, 972-980.
(23) Braga, A. A.; Morgon, N. H.; Ujaque, G.; Maseras, F. J. Am.
Chem. Soc. 2005, 127, 9298-9307.

Dicobalt Carbonyl-Containing Phosphine Ligand
Organometallics, Vol. 24, No. 23, 2005 5691
Figure 5. 31P NMR spectra monitored from the reaction of 4a with Pd(OAc)2 in THF-d8 at 40 °C. P: 4a, PdO: 4b, P*:
unknown. Left: 4a/Pd(OAc)2 ) 1:1; Right: 4a/Pd(OAc)2 ) 2:1.
Scheme 3
with the low symmetry of the compound revealed in the
solid state.
as P*. The composition of P* is proposed as a 4a-
coordinated palladium complex (4a)Pd(0)(OAc)-. Inter-
estingly, the ratio of P*/4b remained 1:1 throughout all
the measurements. After 210 min, one more equivalent
of 4a was introduced into the NMR tube. There was no
obvious change observed for the ratio of P*/4b. It is
noteworthy that the presence of excess 4a is detected
and the integration ratio of P*/4b/4a is 1:1:2. This
explains the observation that all the Pd(OAc)2 was
consumed by the addition of the first batch of 4a in the
earlier stage of the 31P NMR experiment.
2.4. Computational Studies of the Modeled Ac-
tive Species II and [II-OAc-]. Our previous work has
shown that the palladium complex 4c-PdCl2 could be
prepared from the reaction of a 4a-like ligand, 4c, with
(COD)PdCl2 (Scheme 3). The crystal structure of 4c-
PdCl2 reveals a unique bonding mode between Pd and
4c. It leads to the ratio of Pd/ligand ) 1:1.16b Note that
in 4c-PdCl2 a direct Co-Pd covalent bond (r ) 2.6171(5)
Å) was formed that is joined with a PfPd dative bond.
2.3.2. Reductive Ability of 4a toward Pd(OAc)2
in the Absence of Boronic Acid: 31P NMR Spectra
of 4a/Pd(OAc)2 in THF-d8. It is known that organobo-
ronic acid is a capable reducing agent toward Pd(II) in
the Suzuki-Miyaura reaction. Hence, the investigation
of the reducing capacity of 4a toward Pd(OAc)2 was
carried out in the absence of phenylboronic acid. A
sequence of 31P NMR experiments were conducted at
mild temperature, 40 °C, to monitor the conversion of
Pd(OAc)2 as well as to trace the composition of the
plausible acting catalytic species. The spectra were
recorded at preselected time intervals after the in situ
addition of 4a to Pd(OAc)2 (Figure 5). The initial NMR
sample was composed of 0.01 mmol of 4a, Pd(OAc)2, and
0.5 mL of THF-d8. The whole measurement took 210
min, and the corresponding spectra are shown on the
left-hand side of Figure 5 (from the top down). The
formation of the oxidized 4b complex (62.4 ppm) was
observed 3 min after the initial mixing. It is believed
that Pd(II) was reduced to Pd(0) by 4a, and the reduc-
tion was accompanied with the formation of 4b. The
moderate reducing capacity of phosphine to Pd(OAc)2
was demonstrated.24 A new peak was observed at 96.3
ppm. Its identity is unknown and is tentatively assigned
It was proposed that either mono- or diphosphine-
coordinated Pd(0) species, (R3P)nPd(0) (n ) 1 or 2), could
be the major component of the catalytically active
species in the phosphine-modified, palladium-catalyzed
cross-coupling reactions.3b,25 Interestingly, a unique
π-bonding, which is through η1- or η2-arene toward Pd(0)
in an ((aryl)3P)nPd(0) complex, was reported.25 This
(24) Amatore, C.; Jutand, A. Acc. Chem. Rev. 2003, 33, 314-321.

5692 Organometallics, Vol. 24, No. 23, 2005
Chang et al.
Figure 6. B3LYP/631LAN-optimized geometries of compound IIa, TS, and IIb. Energies given are total energies in kcal/
mol. Natural charges (NPA) are in parentheses.
Figure 7. B3LYP/631LAN-optimized geometries of compound II-OAc-, II, and OAc-. The acetate binding energy (D°) of
I-OAc- is -50.1 kcal/mol. For optimized geometry of I-OAc- (in Å): Co1-Co2: 2.507, C-Co2: 1.941, O-Co2: 2.869,
P1-Pd: 2.355, C-Pd: 2.025, O-Pd: 2.725, C1-C2: 1.357, O1-Pd: 2.494, O2-Pd: 2.236.
Table 5. Selected Bond Lengths (Å) of the
exceptional type of bonding is described as a crucial
Geometrically Optimized IIa, TS, and IIb
factor concerning the catalyst’s activity and its endur-
IIa
TS
IIb
ance in the catalytic cycle.26 Therefore, we are interested
in examining the plausible bonding modes of 4a inside
the two potentially active species, I and [I-OAc-], by
theoretical means, since it is difficult to obtain the
structural data experimentally. Seeing that the density
functional theory (DFT) method, which incorporates
electron correlation effects, always provides us with
reliable results in the studies of transition metal
catalytic reactions,27 it was employed here to pursue the
task.
Co1-Co2
Pd‚‚‚Co2
P1-Pd
C-Co2
C-Pd
2.460
3.295
2.345
1.872
2.071
2.481
2.462
2.938
2.406
1.886
2.034
2.686
2.474
2.640
2.422
2.009
1.986
2.933
O-Pd
listed in Table 5. Noteworthy, two stable isomers, IIa
and IIb, with discernible differences in the distances
of Pd‚‚‚Co2 (about 0.655 Å) have been observed. The
optimized structure of IIb shows that there exists a
metal-metal interaction (rPd-Co2 ) 2.640 Å) and a
The computational works employing a simplified
mode of II, with the four phenyl groups of DPPM of 4a
being replaced by hydrogen atoms, were undertaken.
The DFT studies on these two model active species, II
and [II-OAc-], via harmonic vibrational frequency
analysis (Nimag ) 0) have proven them to be local
minima at the B3LYP/631LAN level of theory (Figure
6 and Figure 7). Selected optimized bond lengths are
bridge carbonyl group between Pd and Co2 (rPd-C
)
1.990 Å; rCo2-C ) 1.986 Å; rPd-O ) 2.933 Å). By contrast,
IIa shows no Pd‚‚‚Co2 metal-metal interaction (rPd-Co2
) 3.295 Å) and there exists a dative bond from a linear
µ2-CO to Pd (rPd-C ) 2.071 Å; rPd-O ) 2.481 Å; rCo2-C
)
1.872 Å). The species IIb is slightly more stable than
IIa by only 0.6 kcal/mol. The IIa T IIb isomerization
occurs reversibly, through the transition state TS, with
small energy barrier (1.4 kcal/mol). The proximity of the
Pd metal center can be regarded as a hybridization of
two bonding modes: one is a terminal linear µ2-CO to
Pd dative bond (IIa), and the other is a bridge carbonyl
(µ2-CtO) to Pd-Co bond (IIb). The changes of bond
lengths, lengthening of P1-Pd and O-Pd bonds as well
(25) (a) Faller, J. W.; Sarantopoulos, N. Organometallics 2004, 23,
2008-2014. (b) Reid, S. M.; Boyle, R. C.; Mauge, J. T.; Fink, M. J. J.
Am. Chem. Soc. 2003, 125, 7816-7817. (c) Yin, J.; Rainka, M. P.;
Zhang, X. X.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 1162-
1163. (d) Wolfe, J. P.; Tomori, H.; Sadighi, J. P.; Yin, J.; Buchwald, S.
L. J. Org. Chem. 2000, 65, 1158-1174.
(26) Walker, S. D.; Barder, T. E.; Martinelli, J. R.; Buchwald, S. L.
Angew. Chem., Int. Ed. 2004, 43, 1871-1876.
(27) Null, S.; Hall, M. B. Chem. Rev. 2000, 100, 353-406.

Dicobalt Carbonyl-Containing Phosphine Ligand
Organometallics, Vol. 24, No. 23, 2005 5693
Table 6. Wiberg Bond Index (WBI) of the
Geometrically Optimized IIa, TS, and IIb
IIa
TS
IIb
Co1-Co2
Pd‚‚‚Co2
P1-Pd
0.276
0.048
0.440
0.273
0.347
0.067
0.257
0.264
0.113
as shortening of Pd-Co2 bond from IIa to IIb, support
this view. Here, one of the cobalt atoms, Co2, and its
coordinated carbonyl group in 4a act as electron donors
in II. This moiety donates electron density to the
zerovalent palladium center. The dicobalt fragment of
the metal-containing phosphine 4a here acts not merely
as an irrelevant spectator as in the conventional TM-
phosphines mentioned previously but as an active
participant. The unique characteristic of 4a indeed
opens a new territory of using metal-containing phos-
phines as ligands for cross-coupling reactions.
Figure 8. Localized Kohn-Sham orbital (B3LYP/6311LAN)
of IIb (contour intervals: 0.025 in e‚au-3) and schematic
orbital interactions.
rally force part of the electron density from the Co1-
Co2 bond to the newly formed Pd‚‚‚Co2 bond. In this
way the electron density donated from P1 will certainly
be reduced. A localized molecular orbital (LMO) as
depicted in Figure 8 represents the distribution of
d-electrons in the three-center bond of Co1-Co2‚‚‚Pd.
The observation of the Co1-Co2‚‚‚Pd bond from the
LMO and the variation of bond orders are in accord with
the fact that charges indeed transfer from Co1 to Pd
through Co2. Consequently, the dicobalt fragment in 4a
plays an important role as electron reservoir that
stabilizes the 4a-coordinated palladium catalyst.
The natural charge28 for several selected atoms in
model compounds IIa, TS, and IIb provides rather
interesting insight into the bonding within molecules
(see Figure 6). It reveals that the increase in charges of
Co2 and P1 is accompnaied with the decrease in charges
of Co1 and Pd when the distance of Pd‚‚‚Co2 becomes
closer. This implies that Co1 serves as an electron
reservoir that pulls or pushes electron density from or
toward the Co2. In IIb, the Pd metal receives less from
P1, which is due to the formation of a Pd-Co2 interac-
tion. It also shows that the electron densities are pushed
from Co1 to the Co2-Pd bond. A similar argument was
stated in a theoretical study of the Pauson-Khand
reaction by Nakamura et al.29 One of the cobalt atoms
of 4a provides a Co-Pd interaction, thus stabilizing the
Pd(0)L intermediate and providing additional steric
hindrance for the metal center. In some respects, this
unusual Co-Pd bond mode of IIb operates like the
π-systemofdialkylphenylphosphinesmentionedpreviously.3b
The π-system of the aromatic ring is able to stabilize
the Pd center; nevertheless, it may also lead to cyclo-
metalation that will reduce the lifetime of the active
species.5 This commonly observed drawback of arylphos-
phines should not be a problem in the 4a/Pd catalytic
systems. Moreover, the dissociation process of OA- from
[II-OAc]- to II is unfavorable by 50.1 kcal/mol (Figure
7). This observation echoes Amatore and Jutand’s report
on the coordination of acetate anion onto the catalyti-
cally active Pd(0)PPh3 species.
The analyses of binding modes of Co1-Co2‚‚‚Pd
within the two intermediates, IIa and IIb, are of
interest to us. The bond orders (WBI)30 of Co1-Co2, Pd‚
‚‚Co2, and P1-Pd were examined (Table 6). As expected,
the bond order of Co1-Co2 becomes lower from IIa to
IIb, while the Pd‚‚‚Co2 metal-metal interaction be-
comes stronger (from 0.048 to 0.264). Associated with
the variation of metal-metal interactions, the P1-Pd
bond order is weaker in IIb (0.113) than in IIa (0.440).
This can be realized by the fact that the increasing
interaction of the Pd‚‚‚Co2 in IIb than in IIa is
observed. The increasing Pd‚‚‚Co2 interaction will natu-
3. Concluding Remarks
We have demonstrated the preparation, characteriza-
tion, and reactivity studies of a novel cobalt-containing
phosphine ligand, 4a. Preliminary results show that 4a
is an efficient phosphine ligand in Suzuki-Miyaura
cross-coupling reactions. Based on both NMR and DFT
studies, two 4a-coordinated palladium compounds, I and
[I-OAc]-, are proposed as the active, catalytic species
in Suzuki-Miyaura cross-coupling reactions. The for-
mation of Pd‚‚‚Co interactions and electron density flows
among metals in model active species II provides a vivid
example showing that the dicobalt fragment of the
metal-containing phosphine 4a acts as an electron
reservoir in stabilizing the Pd(0) center in the course of
the catalytic reaction.
4. Experimental Section
General Procedures. All operations were performed in a
nitrogen-flushed glovebox or in a vacuum system. Freshly
distilled solvents were used. All processes for the separation
of the products were performed by centrifugal thin layer
chromatography (CTLC, Chromatotron, Harrison model 8924).
1H NMR spectra were recorded (Varian VXR-300S spectrom-
eter) at 300.00 MHz; chemical shifts are reported in ppm
relative to deuterated solvent peaks. 31P and 13C NMR spectra
were recorded at 121.44 and 75.46 MHz, respectively. 1H NMR
spectra of variable-temperature experiments were recorded by
the same instrument. Routine 1H NMR spectra were recorded
with a Gemini-200 spectrometer at 200.00 MHz or a Varian-
400 spectrometer at 400.00 MHz. IR spectra of the sample
powder in KBr were recorded on a Hitachi 270-30 spectrom-
eter. Mass spectra were recorded on a JEOL JMS-SX/SX 102A
GC/MS/MS spectrometer. Elemental analyses were recorded
on a Heraeus CHN-O-S-Rapid analyzer.
(28) (a) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066-
4073. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys.
1985, 83, 735-746.
4.1. Synthesis of PhCtCP(tBu)2, 1. The preparative
procedures of 1 were modified from the literature.13 Into a 100
cm3 round flask were first placed phenylacetylene (2.05 g,
20.00 mmol) and 30 mL of ether. The solution was stirred at
(29) Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2001, 123,
1703-1708.
(30) Wiberg, K. B. Tetrahedron 1968, B, 1083-1096.

5694 Organometallics, Vol. 24, No. 23, 2005
Chang et al.
-78 °C before 1.1 molar equiv of n-butyllithium (22.0 mmol,
11.0 mL, 2.0 M in cyclohexane) was dropped slowly into the
solution. It remained at -78 °C for 2 h before 1.0 molar equiv
of di-tert-butylchlorophosphine (3.61 g, 20.00 mmol), dissolved
in 4 mL of ether, was added slowly into the solution. The
reaction mixture was then allowed to warm to room temper-
ature and then stirred again for the next 4 days. Later, the
solvent was removed under reduced pressure and then toluene
was added to precipitate the lithium chloride. After filtration,
the resulting solution was further purified through flash
chromatography. A white solid was obtained and identified as
1 in a yield of 80.0% (3.92 g, 15.91 mmol). A similar quantity
of product was obtained as reported previously by the litera-
ture.31
1: 1H NMR (CDCl3, δ/ppm) 7.48-7.45(m, 2H, arene), 7.32-
7.30(m, 3H, arene), 1.30(d, JP-H ) 12.6 Hz, 18H, tBu); 13C NMR
(CDCl3, δ/ppm) 131.4(s, 2C, meta-position of arene), 128.1(s,
2C, ortho-position of arene), 123.6(s, 1C, para-position of
arene), 105.5(d, JP-H ) 2.3 Hz, 1C, ipso-position of arene),
88.1(d, JP-H)20.4 Hz, 1C, PCtC), 32.8(d, JP-H ) 16.1 Hz, 2C,
-C(CH3)3), 29.6(d, JP-H)14.5 Hz, 6C, -C(CH3)3); 31P NMR
(CDCl3, δ/ppm) 12.2(s, 1P, CtCP); IR (KBr, cm-1) 2176(m, Ct
C). Anal. Calcd: C, 78.01; H, 9.41. Found: C, 77.4; H, 9.30.
MS(FAB) m/z ) 246.0(P+).
4.2. Synthesis of PhCtCP(dO)(tBu)2, 2. The title com-
pound was prepared according to the literature procedures.32
Into a 100 cm3 round flask were placed 5.00 mmol of 1 (1.23
g) and 15 mL of THF. At 0 °C, 1.5 mL of 30% H2O2 was added
to the solution drop-by-drop, and then the mixture was stirred
for 30 min. The reaction temperature was raised to 25 °C and
stirred for another 2 h. The mixture was poured into a
separatory funnel with 25 mL of CH2Cl2 and 10 mL of water.
Later, the organic layer was collected, dried with MgSO4,
filtered through silica gel, and concentrated under reduced
pressure. A pale yellow solid was identified as the title
compound and produced in quantitative yield.
another 16 h. The solvent was removed under reduced pres-
sure, and the resulting dark red residue was subjected to
purification by CTLC chromatography. The first band, dark
red in color, was eluted out by mixed solvent (CH2Cl2:hexane
) 1:3) and was identified as 4a in a yield of 72.0% (0.62 g,
0.72 mmol). Then, the second band, red-colored, was eluted
out by mixed solvent (EA:CH2Cl2 ) 1:10) and was identified
as 4b in a yield of 27.0% (0.24 g, 0.27 mmol). Interestingly,
4a exhibits a dark red band in the CTLC plate and blackish
green in solution.
Compound 4b can also be prepared directly from the
reaction of 3 and 2. As mentioned, a mixture containing mostly
3 was prepared according to the procedures shown in section
4.3. Without further separation, the reaction flask was charged
with 1 molar equiv of compound 2 (0.26 g), which was dissolved
in 5 mL of toluene. The mixed solution was then allowed to
react at 85 °C for 20 h before the solvent was removed under
reduced pressure. The resulting red-colored residue was
subjected to purification by CTLC chromatography. The only
red band was eluted out by mixed solvent (EA:CH2Cl2 ) 1:10).
It was identified as 4b and was produced in quantitative yield
(0.88 g, 1.00 mmol).
4a: 1H NMR (CDCl3, δ/ppm) 7.92-6.94(m, 25H, arene),
3.31(m, 1H, DPPM), 3.03(m, 1H, DPPM), 1.60(d, JP-H ) 17.1
Hz, tBu, #1), 1.26(d, JP-H ) 10.8 Hz, tBu, #2); 13C NMR (CDCl3,
δ/ppm) 132.48-128.52(30C, arenes), 30.5(d, JP-C ) 12.1 Hz,
3C, -C(CH3)3), 28.15(s, 6C, -C(CH3)3); 31P NMR (CDCl3,
δ/ppm) 113.9(s, 1P, CtCP, #1), 44.3(s, 1P, CtCP, #2), 37.0(s,
2P, DPPM, #2), 34.8(s, 2P, DPPM,#1); IR (KBr, cm-1) 2020(s),
1994(s), 1966(s) (COs). Anal. Calcd: C, 62.80; H, 5.27. Found:
C, 60.83; H, 5.72. MS(FAB) m/z ) 861.0 (P+).
4b: 1H NMR (CDCl3, δ/ppm) 7.55-7.02(m, 25 H, arene),
3.53(m, 1H, DPPM), 3.24(m, 1H, DPPM), 1.42(d, JP-H ) 13.8
Hz, 18H, tBu); 13C NMR (CDCl3, δ/ppm) 206.26(s, 2C, COs),
201.67(s, 2C, COs), 143.03-125.77(30C, arenes), 110.21(s, 1C,
PCt), 65.79(1C PhCt), 38.11(d, JP-C ) 62.9 Hz, 1C, -C(CH3)3),
33.51(t, JP-C ) 21.4 Hz, 1C, -CH2 of DPPM), 28.11(s, 3C,
-C(CH3)3); 31P NMR (CDCl3, δ/ppm) 59.1(s, 1 P, CtC(PdO)),
36.3(s, 2 P, DPPM); IR (KBr, cm-1) 2025(s), 1998(s), 1968(s)
(COs). Anal. Calcd: C, 73.26; H, 8.84. Found: C, 72.0; H, 8.57.
MS(FAB) m/z ) 876.7 (P+).
4.5. General Procedures for the Suzuki-Miyaura
Cross-Coupling Reaction and Characterization of Prod-
ucts. The Suzuki-Miyaura coupling reaction was performed
according to Buchwald’s procedures.3d Normally, the ratio of
the palladium source, Pd(OAc)2, to the reacting substances is
around 1%, while the ratio of 4a:Pd(OAc)2 ranges from 2:1 to
0:1 depending on the reaction conditions executed. The gen-
eralized procedures are shown as follows. A suitable oven-dried
Schlenk flask, which was previously evacuated and backfilled
with nitrogen, was charged with Pd(OAc)2 (2.20 mg, 0.01
mmol), 2-0-fold 4a, 1.5-fold boronic acid (0.18 g, 1.50 mmol),
and 2- or 3-fold bases. Then, 1 mL of THF and 1.00 mmol of
aryl halide were added. The flask was then sealed with a
Teflon screw cap, and the reaction mixture was heated to 40
or 60 °C depending on the reaction requirements. Subse-
quently, HCl(aq) (2.3 M, 10 mL) was added to the resulting
solution and the reaction was quenched. The organic layer was
extracted with CH2Cl2 (10 mL × 3). Then, the combined
organic layer was dried over anhydrous magnesium sulfate,
filtered, and concentrated with a rotary evaporator. The crude
material was passed quickly through a small column of silica
gel to get rid of solid impurities. Then, the residue was
dissolved in a 10 mL of toluene solution with naphthalene as
the internal standard. The final yield was calibrated and
determined by gas chromatography.
2: 1H NMR (CDCl3, δ/ppm) 7.54-7.52(m, 2H, arene), 7.44-
7.34(m, 3H, arene), 1.38(d, JP-H ) 14.8 Hz, 18H, tBu); 13C NMR
(CDCl3, δ/ppm) 132.3(s, 2C, meta-position of arene), 130.2(s,
2C, ortho-position of arene), 128.5(s, 1C, para-position of
arene), 120.4(d, JP-H ) 3.7 Hz, 1C, ipso-position of arene),
103.3(d, JP-H ) 20.1 Hz, 1C, P(dO)CtC), 81.2(d, JP-H ) 128.8
Hz, 1C, CtCP(dO)), 36.2(d, JP-H ) 72.0 Hz, 2C, -C(CH3)3),
26.3(s, 6C, -C(CH3)3); 31P NMR (CDCl3, δ/ppm) 44.7(s, 1P, Ct
CP); IR (KBr, cm-1) 2180 (m, CtC). Anal. Calcd: C, 73.26; H,
8.84. Found: C, 72.95; H, 8.48. MS(FAB) m/z ) 263.0 (P+
1).
+
4.3. Synthesis of (µ-PPh2CH2PPh2)Co2(CO)6, 3.33A 100
cm3 flask was charged with 1.00 mmol of dicobalt octacarbonyl,
Co2(CO)8 (0.34 g), 1 molar equiv of DPPM (0.39 g), and 10 mL
of toluene. The solution was stirred at 65 °C for 4 h and gave
a major product, a yellow-colored, biphosphino-coordinated (µ-
PPh2CH2PPh2)Co2(CO)6, and a trace amount of green-colored,
monophosphino-coordinated (Ph2CH2PPh2)Co2(CO)7. Without
further separation, the reaction mixture was used in the
succeeding reactions. Also, suitable crystals of 3 were obtained
from the mixture solvent system (CH2Cl2/hexane ) 1:1) at 4
°C, and its structure was determined by the X-ray diffraction
method.
4.4. Synthesis of [(µ-PPh2CH2PPh2)Co2(CO)4][µ,η-PhCt
CP(tBu)2], 4a, and [(µ-PPh2CH2PPh2)Co2(CO)4][µ,η-PhCt
CP(tBu)2], 4b. Compound 3 was prepared by the procedures
as shown in section 4.3. Without further separation, the
reaction flask was charged with 1 molar equiv of 1 (0.246 g),
in 5 mL of toluene, and then the mixture was allowed to stir
at 65 °C for 1 h. This was followed by reacting at 110 °C for
4.6. Experimental Methods for NMR Studies. Variable-
Temperature 31P NMR of 4a in THF-d8. A suitable amount
of 4a was first dissolved in THF-d8 and then was placed in a
sealed NMR tube. The spectra were monitored and recorded
(31) Empsall, H. D.; Hyde, E. M.; Mentzer, E.; Shaw, B. L. J. Chem.
Soc. Dalton Trans. 1977, 2285-2291.
(32) Liu, B.; Wang, K. K.; Peterson, J. L. J. Org. Chem. 1996, 61,
8503-8507.

Dicobalt Carbonyl-Containing Phosphine Ligand
Organometallics, Vol. 24, No. 23, 2005 5695
in a 300 MHz NMR spectrometer over the designated tem-
peratures and time intervals.
631LAN-optimized geometries. The wave functions were ana-
lyzed with MOLDEN40 to plot electron density contours and
visualize the IR eigenvectors.
4.7. Computational Details. Calculations reported in this
study were performed via density functional theory34 using the
Gaussian0335 series of packages. To make the computations
feasible, the size-reduced model compound with four phenyl
groups of DPPM of 4a being replaced by four hydrogen atoms
was employed. Geometries were optimized at the B3LYP/
631LAN level of theory,36 in which the LANL2DZ including
the double-ú basis set for the valence and outermost core
orbitals combined with a pseudopotential was used for Co and
Pd,37 while the 6-31G(d) basis set was used for the other atoms.
All the optimized geometries were minima and verified by the
harmonic vibrational frequency analysis, which was obtained
via analytical energy second derivatives. All the stationary
points found were characterized via harmonic vibrational
frequency analysis as minima and transition states. IRC33
analyses follow the geometry optimization to ensure the
transition structures are smoothly connected by two proximal
minima along the reaction coordinate. Natural charges28 and
Wiberg bond indices30 were evaluated with Weinhold’s NBO
method.38 Localized Kohn-Sham orbitals (LMO)39 were ob-
tained by Boys’ localization procedure from the occupied
B3LYP/631LAN Kohn-Sham orbitals based on the B3LYP/
4.8. X-ray Crystallographic Studies. Suitable crystals of
3 and 4a were sealed in thin-walled glass capillaries under a
nitrogen atmosphere and mounted on a Bruker AXS SMART
1000 diffractometer. Intensity data were collected in 1350
frames with increasing width (0.3° per frame). The absorption
correction was based on the symmetry equivalent reflections
using the SADABS program. The space group determination
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 methods using the SHELXTL
package.41 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 isotropic parameters were used for H atoms.42
Crystallographic data of 3 and 4a are summarized in Table 1.
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center,
CCDC nos. 260502 and 260503 for compounds 3 and 4a,
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).
(33) (a) Hohenberg, P.; Kohn, W. Phys. Rev. 1964, 136, B864. (b)
Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, A1133. (c) Parr, R. G.;
Yang, W. Density-functional theory of atoms and molecules; Oxford
University Press: Oxford, 1989.
Acknowledgment. We are grateful to the National
Science Council of the R.O.C. (Grant NSC 93-2113-M-
005-020) for financial support. Excellent service and
generous computational quota were provided by Na-
tional Center for High-Performance Computing (NCHC).
(34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P.
Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; and Pople, J. A. Gaussian 03, Revision B.04; Gaussian,
Inc.: Wallingford, CT, 2004.
(35) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785-789.
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(b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (c) Hay,
P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
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899-926.
Supporting Information Available: Figures and tables
of the geometry determined from both X-ray crystallographic
determination, 4a, and B3LYP/631LAN calculations, 4A,
tables of energies in hartrees, and optimized structures of
model compounds IIa, TS, IIb, [IIa-OAc-], and 4A are
available.
OM050060J
(39) (a) Boys, S. F.; Quantum Theory of Atoms, Molecules, and Solid
State; Lowdin, P. O., Ed.; Academic Press: New York, 1968; pp 235-
262. (b) Haddon, R. C.; Williams, G. R. J. Chem. Phys. Lett. 1976, 42,
453-455. (c) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, A1133-
1138.
(40) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des.
2000, 14, 123-134.
(41) Sheldrick, G. M. SHELXTL PLUS User’s Manual, Revision 4.1;
Nicolet XRD Corporation: Madison, WI, 1991.
(42) The hydrogen atoms were set to ride on carbon or oxygen atoms
in their idealized positions and held fixed with the C-H distances of
0.96Å.