PCP pincer palladium complexes and their catalytic properties: Synthesis via the electrophilic ligand introduction route

Article abstract of DOI:10.1021/om060506b

A variety of PCP pincer palladium complexes having phosphinito groups [2,6-(R₂PO)₂C₆H₃PdI (R = alkyl, aryl and Et₂N)] were synthesized via the electrophilic ligand introduction route with high efficiency. Thus, trans-(2,6-dihydroxyphenyl) iodobis(triphenylphosphine)palladium was prepared by the reaction of 2-iodoresorcinol with tetrakis(triphenylphosphine)palladium, and the palladium complex was converted to the PCP pincer palladium complexes by treatment with chlorophosphines in the presence of triethylamine in 82-95% yield. X-ray molecular structure analyses of the pincer complexes revealed that the molecules adopted distorted square-planar geometries. The Suzuki coupling reaction of 4-bromoacetophenone with phenylboronic acid took place in the presence of 0.1-0.01 mol% of the pincer complexes to give 4-acetylbiphenyl with turnover numbers of 328-5790.

Full text of DOI:10.1021/om060506b

Organometallics 2006, 25, 4883-4887
4883
PCP Pincer Palladium Complexes and Their Catalytic Properties:
Synthesis via the Electrophilic Ligand Introduction Route
Tsutomu Kimura and Yasuhiro Uozumi*
Institute for Molecular Science (IMS) and CREST, Higashiyama 5-1, Myodaiji, Okazaki 444-8787, Japan
ReceiVed June 7, 2006
A variety of PCP pincer palladium complexes having phosphinito groups [2,6-(R2PO)2C6H3PdI (R )
alkyl, aryl and Et2N)] were synthesized via the electrophilic ligand introduction route with high efficiency.
Thus, trans-(2,6-dihydroxyphenyl)iodobis(triphenylphosphine)palladium was prepared by the reaction
of 2-iodoresorcinol with tetrakis(triphenylphosphine)palladium, and the palladium complex was converted
to the PCP pincer palladium complexes by treatment with chlorophosphines in the presence of triethylamine
in 82-95% yield. X-ray molecular structure analyses of the pincer complexes revealed that the molecules
adopted distorted square-planar geometries. The Suzuki coupling reaction of 4-bromoacetophenone with
phenylboronic acid took place in the presence of 0.1-0.01 mol % of the pincer complexes to give
4-acetylbiphenyl with turnover numbers of 328-5790.
Scheme 1. Synthetic Strategies for Pincer Complexes
Introduction
Pincer complexes containing terdentate monoanionic ligands
composed of an anionic aryl carbon atom and two mutually
trans-chelating donor sites at the 2,6-positions of the aromatic
ring have attracted much attention not only as highly active
catalysts in synthetic organic chemistry1 but also as organome-
tallic materials in the field of materials science.2 Consequently,
considerable effort has been devoted toward the development
of synthetic methods for pincer complexes over the past
decade.1d,2 Conventional methods use metal introduction routes,
in which the ligand frameworks are initially constructed,
followed by metalation of the ligands to give the pincer
complexes (Scheme 1 (1-a)).3
primary amines reacted with the electrophilic formyl groups at
the 2,6-positions of the metalated aromatic ring (Scheme 2
(2-a)).
However, these methods are not suitable for the synthesis of
pincer complexes having bulky and/or chemically unstable
ligand units. Recently, we developed a new synthetic strategy,
the ligand introduction route, in which the metal is introduced
to the aromatic ring prior to the construction of the ligand units
(Scheme 1 (1-b)). With this synthetic strategy, a novel class of
NCN pincer palladium complexes 1 and 2, having bulky
pyrroloimidazolone groups and moisture-sensitive imino groups,
respectively, which were difficult to synthesize via conventional
metal introduction routes, was successfully synthesized with high
efficiency.4 In these cases, nucleophilic proline anilides and
The electrophilic ligand introduction route, in which the
coordinative electrophiles are introduced to the nucleophilic
functional groups at the 2,6-positions of the aromatic ring, is
an alternative method for the synthesis of the pincer complexes
(Scheme 2 (2-b)). If this concept was applied to the synthesis
of pincer complexes via an electrophilic ligand introduction
route, a wide variety of pincer complexes could be obtained by
the combination of the coordinative electrophiles and the
nucleophilic functional groups. We report here the synthesis of
PCP pincer palladium complexes having phosphinito groups via
the electrophilic ligand introduction route, their structures, and
their catalytic activities in the Suzuki coupling reaction.
* To whom correspondence should be addressed. E-mail: uo@ims.ac.jp.
Tel: +81-564-59-5571. Fax: +81-564-59-5574.
(1) Reviews: (a) van der Boom, M. E.; Milstein, D. Chem. ReV. 2003,
103, 1759. (b) Singleton, J. T. Tetrahedron 2003, 59, 1837. (c) Bedford,
R. B. Chem. Commun. 2003, 1787. (d) Dupont, J.; Consorti, C. S.; Spencer,
J. Chem. ReV. 2005, 105, 2527. (e) Szabo´, K. J. Synlett 2006, 811.
(2) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750.
(3) For the preparation of PCP pincer complexes having phosphinoxy
groups via metal introduction routes, see: (a) Miyazaki, F.; Yamaguchi,
K.; Shibasaki, M. Tetrahedron Lett. 1999, 40, 7379. (b) Bedford, R. B.;
Draper, S. M.; Scully, P. N.; Welch, S. L. New J. Chem. 2000, 24, 745. (c)
Morales-Morales, D.; Grause, C.; Kasaoka, K.; Redo´n, R.; Cramer, R. E.;
Jensen, C. M. Inorg. Chim. Acta 2000, 300-302, 958. (d) Wallner, O. A.;
Szabo´, K. J. Org. Lett. 2004, 6, 1829. (e) Wallner, O. A.; Olsson, V. J.;
Eriksson, L.; Szabo´, K. J. Inorg. Chim. Acta 2006, 359, 1767.
Results and Discussion
Synthesis of PCP Pincer Palladium Complexes. As a key
precursor, we designed trans-(2,6-dihydroxyphenyl)iodobis-
(triphenylphosphine)palladium, 4, in which two nucleophilic
hydroxy groups were attached to the 2,6-positions of the benzene
ring (Scheme 2). The electrophilic introduction of the R2P-
groups to the hydroxy groups of the palladium complex 4 would
lead to the formation of the PCP pincer palladium complexes
having the phosphinito groups 5 along with the dissociation of
triphenylphosphine. The palladium complex 4 was readily
prepared by the oxidative addition of 2-iodoresorcinol (3) to
(4) (a) Takenaka, K.; Uozumi, Y. Org. Lett. 2004, 6, 1833. (b) Takenaka,
K.; Uozumi, Y. AdV. Synth. Catal. 2004, 346, 1693. (c) Takenaka, K.;
Minakawa, M.; Uozumi, Y. J. Am. Chem. Soc. 2005, 127, 12273.
10.1021/om060506b CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/25/2006

4884 Organometallics, Vol. 25, No. 20, 2006
Kimura and Uozumi
Scheme 2. Synthetic Strategies for Pincer Complexes via
Ligand Introduction Routes
Figure 1. (a) ORTEP drawing of the palladium complex 4‚CH2-
Cl2 with a thermal ellipsoid plot (50% probability). (b) ORTEP
drawing of 4‚CH2Cl2 along the I-Pd bond. Hydrogen atoms and
solvated CH2Cl2 are omitted for clarity. Selected bond lengths (Å)
and angles (deg): I-Pd ) 2.7014(8), Pd-P1 ) 2.343(3), Pd-P2
) 2.322(3), Pd-C1 ) 2.051(9), P1-Pd-P2 ) 173.42(9), P1-
Pd-C1 ) 87.4(3), P1-Pd-I ) 93.99(6), P2-Pd-C1 ) 87.4(3),
P2-Pd-I ) 91.48(6), C1-Pd-I ) 176.8(2), P1-Pd-C1-C2 )
-86.6(8), P2-Pd-C1-C6 ) -83.9(8).
Table 1. Synthesis of Pincer Complexes 5 via Electrophilic
Ligand Introduction Routea
entry
R
5
time (h)
yield (%)
1
2
3
4
5
6
C6H5
2-CH3C6H4
CH3CH2
(CH3)2CH
cyclo-C6H11
(CH3CH2)2N
5a
5b
5c
5d
5e
5f
1
92
92
86
95
82
94
24
0.5
1
3
1
a The reaction was carried out with the palladium complex 4 (0.25 mmol),
chlorophosphine (0.55 mmol), and triethylamine (0.55 mmol) in toluene
(2.5 mL) at 25 °C.
readily oxidized in air; therefore, the synthesis of PCP pincer
complexes having dialkylphosphinito groups via the conven-
tional metal introduction routes is laborious. Nevertheless, the
pincer complexes having diethyl- 5c, diisopropyl- 5d, and
dicyclohexylphosphinito groups 5e were readily synthesized by
the reaction of the palladium complex 4 with the corresponding
dialkylchlorophosphines in 86%, 95%, and 82% yield, respec-
tively (entries 3-5). The reaction of the palladium complex 4
with bis(diethylamino)chlorophosphine gave the pincer complex
having the bis(diethylamino)phosphinito groups 5f in 94% yield
(entry 6). The resulting pincer complexes 5 are stable in air
and can be purified by column chromatography on silica gel.
Molecular Structures of PCP Pincer Palladium Com-
plexes. The structures of the pincer complexes 5a-f were
unambiguously determined by X-ray molecular structure analy-
ses (Figure 2, Table 2).3a-c,5 In all cases, the pincer complexes
5 adopt distorted square-planer geometries at the palladium atom
with two phosphorus atoms of the phosphinito groups, the iodine
atom, and the carbon atom of the benzene ring. The palladium-
carbon bond lengths of the pincer complexes 5 (1.98-2.02 Å)
are slightly shorter than that of the palladium complex 4 (2.05
Å) by 0.03-0.07 Å because of the intramolecular coordination
of the phosphinito ligands. The I-Pd-C1 angles of the pincer
complexes 5 (175.0-179.3°) are almost linear, whereas the P1-
Pd-P2 angles (159.9-160.8°) are bent due to the constraints
imposed by the atoms forming the two fused five-membered
rings.
Scheme 3. Synthesis of Palladium Complex 4
tetrakis (triphenylphosphine) palladium in 80% yield (Scheme
3). The structure of the palladium complex 4 was unequivocally
determined by X-ray molecular structure analysis (Figure 1).
The palladium complex 4 adopts a square-planar structure
in which two phosphorus atoms of triphenylphosphines, the
iodine atom, and the carbon atom of the benzene ring are
attached to the central palladium atom, and two phosphorus
atoms are located trans to each other. The dihydroxyphenyl
group is oriented almost perpendicularly with respect to the
square plane.
With the key precursor 4 in hand, we examined the synthesis
of PCP pincer palladium complexes 5 (Table 1). As expected,
the palladium complex 4 reacted with 2.2 equiv of chlorodiphe-
nylphosphine in the presence of triethylamine in toluene at 25
°C for 1 h to give the PCP pincer palladium complex having
diphenylphosphinito groups 5a in 92% yield (entry 1). Similarly,
the pincer complex having the bis(o-tolyl)phosphinito groups
5b was obtained in 92% yield (entry 2). Encouraged by these
results, we turned our attention to the synthesis of the pincer
complexes having dialkylphosphinito groups. In general, triva-
lent organophosphorus compounds having P-alkyl groups are
(5) (a) Wang, Z.; Eberhard, M. R.; Jensen, C. M.; Matsukawa, S.;
Yamamoto, Y. J. Organomet. Chem. 2003, 681, 189. (b) Ogo, S.; Takebe,
Y.; Uehara, K.; Yamazaki, T.; Nakai, H.; Watanabe, Y.; Fukuzumi, S.
Organometallics 2006, 25, 331.

Pincer Pd Complexes Via Ligand Introduction Route
Organometallics, Vol. 25, No. 20, 2006 4885
Table 3. Suzuki Coupling Reaction of 4-Bromoacetophenone
with Phenylboronic Acid Catalyzed by Pincer Complexes 5a
entry
5
5 (mol %)
yield (%)
TON
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
0.01
0.1
0.01
0.1
0.01
0.1
58
60
41
33
20
60
5790
599
4050
328
2040
603
a The reaction was carried out with 4-bromoacetophenone (6) (1.0 mmol),
phenylboronic acid (7) (1.5 mmol), K2CO3 (2.0 mmol), and pincer complex
5 (0.1-0.01 mol %) under reflux in toluene (3.0 mL) for 24 h.
numbers (TONs) of the pincer complexes 5 were 328-579010
and were highly dependent on the substituents at the phosphorus
atom. The diphenyl derivative 5a showed the highest catalytic
activity of the pincer complexes 5.
Conclusion
Figure 2. ORTEP drawings of PCP pincer palladium complexes
5 with a thermal ellipsoid plot (50% probability): (a) 2,6-[(C6H5)2-
PO]2C6H3PdI, 5a, (b) 2,6-[(2-CH3C6H4)2PO]2C6H3PdI, 5b, (c) 2,6-
[(CH3CH2)2PO]2C6H3PdI, 5c, (d) 2,6-{[(CH3)2CH]2PO}2C6H3PdI,
5d, (e) 2,6-[(c-C6H11)2PO]2C6H3PdI, 5e, and (f) 2,6-{[(CH3CH2)2N]2-
PO}2C6H3PdI, 5f. Hydrogen atoms are omitted for clarity. Two
independent molecules were present in one asymmetric unit of 5c
and 5d. One is shown.
In summary, we have developed a new efficient synthetic
route for the PCP pincer palladium complexes that is comple-
mentary to the nucleophilic ligand introduction routes. A variety
of pincer complexes were successfully synthesized via the
electrophilic ligand introduction route, in which the palladium-
carbon bond was initially formed, and the electrophilic ligand
moieties were introduced to the nucleophilic hydroxy groups.
X-ray molecular structure analyses of the pincer complexes
showed that they adopted square-planar geometries. The pincer
complexes were active catalysts for the Suzuki coupling reaction,
and their catalytic activities were dependent on the substituents
at the phosphorus atom. Further applications of the ligand
introduction route to the synthesis of other types of pincer
complexes are underway and will be reported in due course.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
PCP Pincer Palladium Complexes 5
bond lengths (Å)
Pd-P1 Pd-P2
angles (deg)
5
Pd-I
Pd-C1 P1-Pd-P2 I-Pd-C1
5a 2.6402(2) 2.2642(7) 2.2672(8) 1.992(3) 160.30(2) 179.34(7)
5b 2.6515(7) 2.294(2) 2.279(2) 2.021(7) 159.90(7) 175.0(2)
5c 2.6450(4) 2.279(1) 2.2609(9) 1.988(3) 160.20(3) 176.05(9)
5d 2.6605(3) 2.2778(8) 2.2733(8) 1.999(3) 160.02(3) 176.56(8)
5e 2.6408(4) 2.286(1) 2.273(1) 1.981(4) 160.77(4) 175.7(1)
5f 2.6624(6) 2.284(2) 2.277(2) 1.990(6) 160.84(6) 179.0(2)
Experimental Section
General Procedures. All manipulations were performed under
a nitrogen atmosphere. Nitrogen gas was dried by passage through
P2O5. NMR spectra were recorded on a JEOL JNM-A500 spec-
Catalytic Activities of Pincer Palladium Complexes for
the Suzuki Coupling Reaction. As mentioned above, a variety
of pincer palladium complexes are emerging as a new class of
efficient catalysts for C-C bond forming reactions.1 For
example, the PCP pincer palladium complexes with phosphinito
groups have been used as catalysts in the Heck reaction,3a,c,5b,6
Suzuki coupling,3b,5b Stille coupling,5b Sonogashira coupling,7
allylic alkylation,5a allylic stannylation,3d allylation of aldehydes
and imines,3e,8 and R-arylation of ketones.9 To test the catalytic
activities of the pincer complexes 5, the Suzuki coupling reaction
was carried out (Table 3).
1
trometer (500 MHz for H, 125 MHz for 13C, 202 MHz for 31P).
Chemical shifts are reported in δ ppm referenced to an internal
1
tetramethylsilane standard for H NMR. Chemical shifts of 13C
NMR are given relative to CDCl3 as an internal standard (δ 77.0).
The 31P NMR data are reported relative to external 85% H3PO4.
1H, 13C, and 31P NMR spectra were recorded in CDCl3 at 25 °C.
ESI mass spectra were recorded on a JEOL JMS-T100LC spec-
trometer. Melting points were determined using a Yanaco MP-J3
micro melting point apparatus and are uncorrected. The IR spectrum
was obtained using a JASCO FT/IR-460plus spectrophotometer in
ATR mode. Dehydrated dichloromethane and toluene were pur-
chased. Silica gel 60 from a commercial supplier was used in
column chromatography. Commercially available reagents were
used without purification. 2-Iodoresorcinol11 and tetrakis(tri-
The reaction of 4-bromoacetophenone (6) with phenylboronic
acid (7) took place in the presence of 0.1-0.01 mol % of the
pincer complexes 5 to give 4-acetylbiphenyl (8). The turnover
(6) Morales-Morales, D.; Redo´n, R.; Yung, C.; Jensen, C. M. Chem.
Commun. 2000, 1619.
(7) Eberhard, M. R.; Wang, Z.; Jensen, C. M. Chem. Commun. 2002,
818.
(8) (a) Solin, N.; Kjellgren, J.; Szabo´, K. J. J. Am. Chem. Soc. 2004,
126, 7026. (b) Solin, N.; Wallner, O. A.; Szabo´, K. J. Org. Lett. 2005, 7,
689.
(9) Churruca, F.; SanMartin, R.; Tellitu, I.; Dom´ınguez, E. Tetrahedron
Lett. 2006, 47, 3233.
(10) The orthopalladated phosphinite complex, which shows extremely
high activity (TON: 4.8 × 108) in the Suzuki coupling, has been reported,
see: Bedford, R. B.; Hazelwood, S. L.; Horton, P. N.; Hursthouse, M. B.
Dalton Trans. 2003, 4164.
(11) Hamura, T.; Hosoya, T.; Yamaguchi, H.; Kuriyama, Y.; Tanabe,
M.; Miyamoto, M.; Yasui, Y.; Matsumoto, T.; Suzuki, K. HelV. Chim. Acta
2002, 85, 3589.

4886 Organometallics, Vol. 25, No. 20, 2006
Kimura and Uozumi
phenylphosphine)palladium12 were prepared according to proce-
dures in the literature.
Synthesis of trans-(2,6-Dihydroxyphenyl)iodobis(triphenyl-
phosphine)palladium (4). To a suspension of tetrakis(triph-
enylphosphine)palladium (5.78 g, 5.00 mmol) in dichloromethane
(100 mL) was added 2-iodoresorcinol (3) (1.19 g, 5.03 mmol) at
25 °C, and the mixture was stirred at that temperature for 2 h. After
the solvent was removed, the residue was purified by column
chromatography on silica gel using hexane/ethyl acetate as an eluent
to give trans-(2,6-dihydroxyphenyl)iodobis(triphenylphosphine)-
palladium (4) (3.48 g, 4.01 mmol, 80%) as a pale yellow solid.
Mp: 148-150 °C (dec). MS (ESI): m/z 739 ([M - I]+). 1H NMR
(CDCl3): δ 4.71 (s, 2H, OH), 5.49 (d, J ) 7.9 Hz, 2H, Ar-H),
6.27 (t, J ) 7.9 Hz, 1H, Ar-H), 7.25 (t, J ) 6.9 Hz, 12H, Ar-H),
7.33 (t, J ) 6.9 Hz, 6H, Ar-H), 7.56 (q, J ) 6.9 Hz, 12H, Ar-
H). 13C{1H} NMR: δ 106.8 (Ar), 126.9 (Ar), 127.8 (virtual t, J )
5.2 z, Ar), 130.0 (Ar), 131.7 (virtual t, J ) 24.8 Hz, Ar), 134.6
(virtual t, J ) 6.2 Hz, Ar), 155.6 (Ar). 31P{1H}NMR: δ 18.8. Anal.
Calcd for C42H35IO2P2Pd‚CH3CO2C2H5 (955.10): C, 57.85; H, 4.54.
Found: C, 57.77; H, 4.42. IR (ATR mode): ν(OH) 3371 cm-1
.
Synthesis of 2,6-Bis(diphenylphosphinoxy)phenyliodopalladium
(5a). To a suspension of trans-(2,6-dihydroxyphenyl)iodobis-
(triphenylphosphine)palladium (4) (217 mg, 0.250 mmol) in toluene
(2.5 mL) was added triethylamine (55.7 mg, 0.550 mmol) at 0 °C.
To the mixture was added chlorodiphenylphosphine (121 mg, 0.550
mmol) at 0 °C, and the mixture was stirred at 25 °C for 1 h. The
reaction mixture was poured into water (10 mL) and extracted with
chloroform (10 mL). The organic layer was dried over Na2SO4,
filtered, and concentrated under reduced pressure. The residue was
purified by column chromatography on silica gel to give 2,6-bis-
(diphenylphosphinoxy)phenyliodopalladium (5a) (163 mg, 0.229
mmol, 92%) as a pale yellow solid. Mp: >300 °C. MS (ESI): m/z
1
583 ([M - I]+). H NMR (CDCl3): δ 6.79 (d, J ) 7.9 Hz, 2H,
Ar-H), 7.12 (t, J ) 7.9 Hz, 1H, Ar-H), 7.45-7.52 (m, 12H, Ar-
H), 8.00 (q, J ) 6.3 Hz, 8H, Ar-H). 13C{1H} NMR: δ 106.9
(virtual t, J ) 7.8 Hz, Ar), 128.8 (virtual t, J ) 5.7 Hz, Ar), 129.0
(Ar), 132.0 (Ar), 132.4 (virtual t, J ) 8.3 Hz, Ar), 133.4 (virtual t,
J ) 25.9 Hz, Ar), 137.7 (Ar), 164.2 (virtual t, J ) 7.8 Hz, Ar).
31P{1H} NMR: δ 144.0. Anal. Calcd for C30H23IO2P2Pd (710.77):
C, 50.69; H, 3.26. Found: C, 50.40; H, 3.30.
2,6-Bis[bis(2-methylphenyl)phosphinoxy]phenyliodo-
palladium (5b). Yield: 92% (176 mg, 0.229 mmol), pale yellow
solid. Mp: 294-296 °C. MS (ESI): m/z 639 ([M - I]+). 1H NMR
(CDCl3): δ 2.48 (s, 12H, CH3), 6.68 (d, J ) 7.9 Hz, 2H, Ar-H),
7.08 (t, J ) 7.9 Hz, 1H, Ar-H), 7.24-7.29 (t, J ) 7.4 Hz, 8H,
Ar-H), 7.42 (t, J ) 7.4 Hz, 4H, Ar-H), 7.95 (q, J ) 7.4 Hz, 4H,
Ar-H). 13C{1H} NMR: δ 22.5 (CH3), 106.9 (virtual t, J ) 7.8
Hz, Ar), 125.9 (virtual t, J ) 6.7 Hz, Ar), 128.9 (Ar), 130.4 (virtual
t, J ) 25.3 Hz, Ar), 131.8 (virtual t, J ) 3.1 Hz, Ar), 132.4 (Ar),
135.2 (virtual t, J ) 11.9 Hz, Ar), 138.9 (Ar), 141.9 (virtual t, J )
4.7 Hz, Ar), 163.6 (virtual t, J ) 7.2 Hz, Ar). 31P{1H} NMR: δ
153.4. Anal. Calcd for C34H31IO2P2Pd‚H2O (784.90): C, 52.03; H,
4.24. Found: C, 52.30; H, 4.03.
2,6-Bis(diethylphosphinoxy)phenyliodopalladium
(5c).
Yield: 86% (112 mg, 0.216 mmol), colorless solid. Mp: 102-
105 °C. MS (ESI): m/z 424 ([M - I + MeOH]+). 1H NMR
(CDCl3): δ 1.26-1.33 (td, J ) 8.5, 17.1 Hz, 12H, CH3), 2.21-
2.31 (m, 8H, CH2), 6.59 (d, J ) 8.2 Hz, 2H, Ar-H), 7.03 (t, J )
8.2 Hz, 1H, Ar-H). 13C{1H} NMR: δ 8.0 (CH3), 24.4 (virtual t,
J ) 14.0 Hz, CH2), 106.1 (virtual t, J ) 7.8 Hz, Ar), 128.5 (Ar),
136.2 (Ar), 165.3 (virtual t, J ) 7.2 Hz, Ar). 31P{1H} NMR: δ
176.9. Anal. Calcd for C14H23IO2P2Pd (518.60): C, 32.42; H, 4.47.
Found: C, 32.34; H, 4.45.
2,6-Bis[bis(1-methylethyl)phosphinoxy]phenyliodo-
palladium (5d). Yield: 95% (137 mg, 0.237 mmol), colorless solid.
Mp: 148-150 °C. MS (ESI): m/z 447 ([M - I]+). 1H NMR
(12) Coulson, D. R. Inorg. Synth. 1972, 13, 121.

Pincer Pd Complexes Via Ligand Introduction Route
Organometallics, Vol. 25, No. 20, 2006 4887
(CDCl3): δ 1.28 (q, J ) 7.7 Hz, 12H, CH3), 1.39 (q, J ) 7.7 Hz,
12H, CH3), 2.50 (heptet, J ) 7.7 Hz, 4H, CH), 6.59 (d, J ) 8.4
Hz, 2H, Ar-H), 7.00 (t, J ) 8.4 Hz, 1H, Ar-H). 13C{1H} NMR:
δ 16.7 (CH3), 18.1 (virtual t, J ) 3.1 Hz, CH3), 29.3 (virtual t, J
) 11.9 Hz, CH), 105.7 (virtual t, J ) 7.2 Hz, Ar), 128.2 (Ar),
135.2 (Ar), 165.9 (virtual t, J ) 6.2 Hz, Ar). 31P{1H} NMR: δ
187.7. Anal. Calcd for C18H31IO2P2Pd (574.71): C, 37.62; H, 5.44.
Found: C, 37.46; H, 5.41.
%). The mixture was stirred under reflux in toluene for 24 h and
allowed to cool to 25 °C. The reaction mixture was poured into
aqueous HCl (2 M, 10 mL) and extracted with chloroform (2 × 10
mL). The organic layer was dried over Na2SO4, filtered, and
concentrated under reduced pressure. The residue was purified by
column chromatography on silica gel to give 4-acetylbiphenyl (8).
X-ray Structure Determination. Single crystals of 4 and 5a-c
suitable for an X-ray diffraction study were grown by slow diffusion
of hexane into the dichloromethane solution of 4, tert-butyl methyl
ether into the chloroform solution of 5a, ethyl acetate into the
chloroform solution of 5b, and hexane into the ethyl acetate solution
of 5c, respectively. Single crystals of 5d and 5f were obtained by
slow evaporation of ethyl acetate solution of 5d and 5f. Single
crystals of 5e were obtained by the recrystallization of 5e from hot
ethyl acetate. X-ray data for single crystals were collected on a
Rigaku Saturn CCD area detector at -100 °C using graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å). The structures
were solved by heavy-atom Patterson methods (PATTY for 4) or
direct methods (SIR 92 for 5a-e, SIR 97 for 5f) and expanded
using Fourier techniques (DIRDIF99). All non-hydrogen atoms
except for C20 and C22 in 5f were refined anisotropically, and the
hydrogen atoms were refined using the riding model. All calcula-
tions were performed using the CrystalStructure crystallographic
software package. Crystallographic data and details of structure
refinement are summarized in Table 4.
2,6-Bis(dicyclohexylphosphinoxy)phenyliodopalladium (5e).
Yield: 82% (149 mg, 0.203 mmol), colorless solid. Mp: 201-
1
203 °C. MS (ESI): m/z 607 ([M - I]+). H NMR (CDCl3): δ
1.21-1.38 (m, 12H, CH2), 1.55 (dq, J ) 2.9, 12.4 Hz, 4H, CH2),
1.69-1.75 (m, 8H, CH2), 1.82 (d, J ) 11.6 Hz, 8H, CH2), 1.90 (d,
J ) 12.8 Hz, 4H, CH2), 1.99 (d, J ) 12.8 Hz, 4H, CH2), 2.29 (tt,
J ) 2.9, 12.5 Hz, 4H, CH), 6.56 (d, J ) 8.1 Hz, 2H), 6.98 (t, J )
8.1 Hz, 1H). 13C{1H} NMR: δ 25.8 (CH2), 26.3 (virtual t, J ) 5.2
Hz, CH2), 26.5 (virtual t, J ) 7.2 Hz, CH2), 26.7 (CH2), 27.7 (CH2),
38.3 (virtual t, J ) 11.9 Hz, CH), 105.6 (virtual t, J ) 7.2 Hz, Ar),
128.1 (Ar), 135.2 (Ar), 165.8 (virtual t, J ) 6.2 Hz, Ar). 31P{1H}
NMR: δ 180.7. Anal. Calcd for C30H47IO2P2Pd (734.96): C, 49.03;
H, 6.45. Found: C, 48.85; H, 6.44.
2,6-Bis[bis(diethylamino)phosphinoxy]phenyliodopalladium
(5f). Yield: 94% (162 mg, 0.234 mmol), colorless solid. Mp: 94-
96 °C. MS (ESI): m/z 563 ([M - I]+). 1H NMR (CDCl3): δ 1.13
(t, J ) 6.8 Hz, 24H, CH3), 3.21 (octet, J ) 6.8 Hz, 8H, CH2), 3.32
(octet, J ) 6.8 Hz, 8H, CH2), 6.57 (d, J ) 7.8 Hz, 2H, Ar-H),
7.06 (t, J ) 7.8 Hz, 1H, Ar-H). 13C{1H} NMR: δ 14.2 (CH3),
40.3 (virtual t, J ) 5.7 Hz, CH2), 105.8 (virtual t, J ) 8.8 Hz, Ar),
128.7 (Ar), 133.9 (virtual t, J ) 2.6 Hz, Ar), 159.6 (virtual t, J )
9.8 Hz, Ar). 31P{1H} NMR: δ 146.0. Anal. Calcd for C22H43-
IN4O2P2Pd (690.87): C, 38.25; H, 6.27; N, 8.11. Found: C, 38.15;
H, 6.20; N, 8.07.
Acknowledgment. This work was supported by the CREST
program, sponsored by the JST. We also thank the JSPS
(GRANT-in-AID for Scientific Research, No.15205015) and the
MEXT (Scientific Research on Priority Areas, No. 420) for
partial financial support of this work.
General Procedure for the Suzuki Coupling Reaction. To a
suspension of potassium carbonate (2.0 mmol) in toluene (3.0 mL)
was added 4-bromoacetophenone (6) (1.0 mmol), phenylboronic
acid (7) (1.5 mmol), and the pincer complex 5 (0.01 M solution in
toluene, 10-100 µL, 1 × 10-3 to 1 × 10-4 mmol, 0.1-0.01 mol
Supporting Information Available: CIF files giving crystal-
lographic data for 4 and 5a-f. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM060506B