Rapid geometrical equilibrium of palladium(II) complexes with tris[2-(diphenylphosphino)ethyl]phosphine disulfide and diselenide and their catalytic activity for Suzuki coupling reaction

Article abstract of DOI:10.1016/j.jorganchem.2006.10.020

Palladium(II) complexes with a tetradentate pseudo-tripodal ligand having two phosphino groups and two phosphine sulfide or selenide groups, pp₃X₂ (pp₃ = tris[2-(diphenylphosphino)ethyl]phosphine, X = S (1) or Se (2)), were prepared from [PdCl(pp₃)]Cl. Both of these phosphine chalcogenide complexes 1 and 2 showed rapid equilibrium between the five-coordinate [PdCl(pp₃X₂)]Cl with two bound phosphine chalcogenide groups and four-coordinate [PdCl₂(pp₃X₂)] with two dissociated pendant ones in chloroform. The thermodynamic parameters for the reaction, [PdCl(pp₃X₂)]⁺ + Cl^-?[PdCl₂(pp₃X₂)], were obtained by low-temperature ³¹P NMR as follows: K²⁹⁸ = 3.7 × 10³ and 5.4 × 10² mol^-1, ΔH^{{ring operator}} = 11.3 ± 0.3 and 13.4 ± 0.4 kJ mol^-1, and ΔS^{{ring operator}} = 106 ± 2 and 97 ± 2 J mol^-1 K^-1 for 1 and 2, respectively. The rate for the geometrical change at 246.7 K for 1 was appreciably faster than that for 2. These thermodynamic and kinetic results indicate that the phosphine selenide Se atoms can stabilize the five-coordinate structure by effective π-back donation from Pd(II) compared with the phosphine sulfide S atoms. Difference in retention of the catalytic activity for Suzuki coupling, 2 > 1 > [PdCl(pp₃ or p₃)]Cl, was explained by difference in the π-accepting ability that stabilizes the catalytically active Pd(0) species. Considering the rapid dissociation-coordination equilibrium of the phosphine chalcogenide groups on Pd(II), it is probable that the oxidative addition and the subsequent transmetallation of the Pd(II) species are hardly blocked by the phosphine chalcogenide groups.

Full text of DOI:10.1016/j.jorganchem.2006.10.020

Journal of Organometallic Chemistry 692 (2007) 813–818
www.elsevier.com/locate/jorganchem
Rapid geometrical equilibrium of palladium(II) complexes with
tris[2-(diphenylphosphino)ethyl]phosphine disulfide and diselenide
and their catalytic activity for Suzuki coupling reaction
*
Sen-ichi Aizawa , Takashi Hase, Tsuyoshi Wada
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
Received 31 August 2006; received in revised form 9 October 2006; accepted 10 October 2006
Available online 18 October 2006
Abstract
Palladium(II) complexes with a tetradentate pseudo-tripodal ligand having two phosphino groups and two phosphine sulfide or sel-
enide groups, pp3X2 (pp3 = tris[2-(diphenylphosphino)ethyl]phosphine, X = S (1) or Se (2)), were prepared from [PdCl(pp3)]Cl. Both of
these phosphine chalcogenide complexes 1 and 2 showed rapid equilibrium between the five-coordinate [PdCl(pp3X2)]Cl with two bound
phosphine chalcogenide groups and four-coordinate [PdCl2(pp3X2)] with two dissociated pendant ones in chloroform. The thermody-
namic parameters for the reaction, [PdCl(pp3X2)]+ + Clꢀ¢[PdCl2(pp3X2)], were obtained by low-temperature 31P NMR as follows:
K
298 = 3.7 · 103 and 5.4 · 102 molꢀ1, DHꢁ = 11.3 0.3 and 13.4 0.4 kJ molꢀ1, and DSꢁ = 106 2 and 97 2 J molꢀ1 Kꢀ1 for 1
and 2, respectively. The rate for the geometrical change at 246.7 K for 1 was appreciably faster than that for 2. These thermodynamic
and kinetic results indicate that the phosphine selenide Se atoms can stabilize the five-coordinate structure by effective p-back donation
from Pd(II) compared with the phosphine sulfide S atoms. Difference in retention of the catalytic activity for Suzuki coupling,
2 > 1 > [PdCl(pp3 or p3)]Cl, was explained by difference in the p-accepting ability that stabilizes the catalytically active Pd(0) species.
Considering the rapid dissociation-coordination equilibrium of the phosphine chalcogenide groups on Pd(II), it is probable that the oxi-
dative addition and the subsequent transmetallation of the Pd(II) species are hardly blocked by the phosphine chalcogenide groups.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Phosphine sulfide; Phosphine selenide; Geometrical equilibrium; Suzuki coupling; p-accepting ability
1. Introduction
form the stable Pd(II) complexes, can not be usually
good C–C coupling catalysts because of difficulty in pre-
reduction into the Pd(0) species and blocking the forma-
tion and subsequent reactions of the Pd(II) substrate
adducts. On the other hand, the weakly ligated systems
are liable to consume the catalysts because the Pd(0) spe-
cies formed after the reductive elimination and prior to
the next oxidative addition in the catalytic cycle are eas-
ily decomposed into inactive ‘‘palladium black’’. In the
conservative phosphine-assisted method, oxidation of
phosphine may contribute to the prereduction of Pd(II)
and the phosphine remaining intact has been considered
as the efficient ligand to stabilize the oxidation state of
Pd(0). However, the resultant phosphine oxide can par-
ticipate in coordination no longer, and furthermore, the
Palladium catalysts are now indispensable in organic
syntheses since they can be used in various useful C–C
coupling reactions [1]. For participation of Pd(II) com-
plexes in the catalytic reactions, reduction of the Pd(II)
complexes to the catalytically active Pd(0) species is
essential for their entry into the catalytic cycle followed
by their oxidative addition with substrates and subse-
quent reactions of the Pd(II) substrate adducts. There-
fore, the fully ligated complexes, in which strongly
coordinated ligands occupy all the coordination sites to
*
Corresponding author. Tel./fax: +81 76 445 6980.
E-mail address: saizawa@u-eng.toyama.ac.jp (S.-i. Aizawa).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.10.020

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S.-i. Aizawa et al. / Journal of Organometallic Chemistry 692 (2007) 813–818
remaining phosphine is susceptible to oxidation and
addition of excess phosphine inhibits the catalytic activity
because of the strong r donation blocking the reactions
of the Pd(II) substrate adducts. Consequently, we need
a new ligand system that can electronically stabilize the
Pd(0) species and do not block the successive reactions
of the Pd(II) substrate adducts as well.
ylboronic acid (Sigma–Aldrich), and bis(2-butoxyethyl)
ether (Wako) were used without further purification.
2.2. Preparation of complexes
2.2.1. [PdCl2(pp3S2] Æ 2CHCl (1)
3
Sulfur (0.0389 g, 1.21 mmol) and [PdCl(pp3)]Cl [5]
(0.505 g, 0.596 mmol) were dissolved in chloroform
(12 cm3) and reacted at 40 ꢁC for 4 days and then filtered.
The filtrate was concentrated to ca. 4 cm3 and ethanol
was added dropwise. The mixture was kept in a freezer
and the resultant colorless crystals were filtered and
air-dried. Yield 0.462 g (67%). Anal. Calc. for
C42H42Cl2P4PdS2 Æ 2CHCl3: C, 45.80; H, 3.84; N, 0.00.
Found: C, 45.34; H, 3.84; N, 0.00%. 31P{1H} NMR
Recently, some phosphine chalcogenides such as phos-
phine sulfides and selenides have been employed as
monodentate and bidentate ligands for some metal ions
[2]. However, very few applications of these ligands to
the palladium-catalyzed C–C coupling reactions have
been reported so far [3]. Considering relatively long
P@S and P@Se double bonds in phosphine chalcogenides
[2], the p bonding orbitals between the P and S or Se
atoms are not so stabilized and the p antibonding orbi-
tals are not so activated either. The moderately low p*
orbitals on the phosphine chalcogenide S or Se atoms
are expected to be good accepting orbitals for the elec-
tron-rich Pd(0) center in the catalytically active species.
Furthermore, because the S and Se atoms in phosphine
chalcogenides are weak r donors for Pd(II) compared
with phosphine P atoms, the phosphine chalcogenide
groups are not likely to block the formation and subse-
quent reactions of the Pd(II) substrate adducts compared
with the phosphino groups.
(CH3CN/H2O 10:1 v/v): d 45.2 (dd, 2P, sulfide), 70.0 (m,
3
1P, terminal), 75.8 (td, 1P, center); JP(sulfide)-P(center)
=
=
3
3
35.4 Hz, JP(sulfide)-P(terminal) = 4.2 Hz, JP(center)-P(terminal)
11.2 Hz. 13C NMR (CD3CN/D2O 10:1 v/v): d 20.5 (d,
PCH2CH2P(S)Ph2,
1JC-P = 27.3 Hz),
24.2
(dd,
1
2
PCH2CH2PPh2, JC-P = 33.5 Hz, JC-P = 12.8 Hz), 27.0
(d, PCH2CH2P(S)Ph2, 1JC-P = 53.6 Hz), 29.8 (dd,
PCH2CH2PPh2, JC-P = 35.5 Hz, JC-P = 11.5 Hz). 1H
NMR (CD3CN/D2O 10:1 v/v): d 2.0–3.4 (m, CH2), d 7.4–
8.1 (m, Ph).
1
2
Previously, we reported stepwise oxidation of five-
coordinatetrigonal-bipyramidal[Pd(4-Cltp)(pp3)]+ (4-Cltpꢀ =
4-chlorothiophenolate, pp3 = tris[2-(diphenylphosphino)-
ethyl]phosphine) to give square–planar [Pd(4-Cltp)(pp3O)]+
and [Pd(4-Cltp)2(pp3O2)] [4], where pp3O and pp3O2 act
entirely as terdentate and bidentate ligands, respectively,
because the phosphine oxide groups do not participate in
the complexation at all (Scheme 1). In the present work, we
have designed the new pp3S2 and pp3Se2 ligand systems
considering the catalytic application.
2.2.2. [PdCl2(pp3Se2)] (2)
Selenium (0.163 g, 2.06 mmol) was suspended in a solu-
tion containing [PdCl(pp3)]Cl [5] (0.501 g, 0.591 mmol) in
chloroform (12 cm3) at 40 ꢁC for 10 days. The remaining
selenium powder was filtered off and the filtrate was con-
centrated to ca. 3 cm3. To this was added ethanol drop-
wise, and the mixture was kept in freezer. The resultant
colorless crystals were filtered and air-dried. Yield 0.48 g
(81%). Anal. Calc. for C42H42Cl2P4PdSe2: C, 50.15; H,
4.21; N, 0.00. Found: C, 49.56; H, 4.26; N, 0.00%.
31P{1H} NMR (acetonitrile/H2O 10:1 v/v): r 35.2 (dd,
2P, selenide), 69.0 (m, 1P, terminal), 73.8 (td, 1P, center);
2. Experimental
3JP(selenide)-P(center) = 35.4 Hz, JP(selenide)-P(terminal) = 5.2 Hz,
3
2.1. Reagents
3JP(center)-P(terminal) = 12.5 Hz. 13C NMR (CD3CN/D2O
1
10:1 v/v):
d
21.7 (d, PCH2CH2P(Se)Ph2, JC-P
=
1
Deuterated chloroform (CDCl3, Aldrich) was dried over
activated 4A Molecular Sieves and used as a solvent for
equilibrium and kinetic measurements. Sulfur (Wako),
selenium (Wako), tetra-n-butylammonium chloride
(Bu4NCl, Wako), iodobenzene (Kanto Chemical), phen-
28.9 Hz), 25.2 (dd, PCH2CH2PPh2, JC-P = 33.4 Hz,
2JC-P = 13.7 Hz), 27.2 (d, PCH2CH2P(Se)Ph2, JC-P
=
1
1
48.7 Hz), 29.2 (dd, PCH2CH2PPh2, JC-P = 34.6 Hz,
2JC-P = 10.7 Hz). 1H NMR (CD3CN/D2O 10:1 v/v): d
2.0–3.5 (m, CH2), d 7.4–8.1 (m, Ph).
O
O
+
+
P
P
P
1/2O2
O
1/2O2
P
P
P
4-Cltp-
Pd
P
P
P
P
P
Pd
Pd
4-Cltp
4-Cltp
P
4-Cltp
4-Cltp
[Pd(4-Cltp)(pp3)]+
[Pd(4-Cltp)(pp3O)]+
[Pd(4-Cltp)2(pp3O2)]
Scheme 1.

S.-i. Aizawa et al. / Journal of Organometallic Chemistry 692 (2007) 813–818
815
2.3. Measurements
S
P
7.3 Hz
+
31P, 13C, and 1H NMR spectra were recorded on a
JEOL JNM-A400 FT-NMR spectrometer operating at
160.70, 100.4, and 399.65 MHz, respectively. In order
to determine the chemical shifts of 31P NMR signals, a
3 mm o.d. NMR tube containing the sample solution
was coaxially mounted in a 5 mm o.d. NMR tube con-
taining deuterated water and phosphoric acid as a lock
solvent and a reference, respectively. The NMR samples
for equilibrium and kinetic experiments for 1 and 2 were
prepared by vacuum distillation of dried CDCl3 into
NMR tubes containing the completely dried sample that
were then flame-sealed after degassing. The equilibrium
constants were evaluated from the initial concentrations
37.5 Hz
7.3 Hz
37.5 Hz
P
t
P
t
P
c
Pd
Cl
P
t
37.5 Hz
7.3 Hz
ref
P=S
P
c
of 1 (1.4 · 10ꢀ3 mol kgꢀ1) and 2 (1.8 · 10ꢀ3 mol kgꢀ1
)
and the relative intensities of the non-decoupled 31P
NMR signals for the equilibrated species obtained by
taking a sufficiently long acquisition time in the temper-
ature ranges from 237.0 to 305.8 K for 1 and from 246.7
to 272.5 K for 2. The chemical exchange rate constants
were given by increase in the transverse relaxation rates
obtained from the 31P NMR spectral simulation [6] of
the line-broadening data for 1 (1.4 · 10ꢀ3 mol kgꢀ1) and
2 (1.8 · 10ꢀ3 mol kgꢀ1) at 246.7 K, changing concentra-
tion of tetra-n-butylammonium chloride (0–5.0 · 10ꢀ2
mol kgꢀ1 for 1 and 0–6.9 · 10ꢀ2 mol kgꢀ1 for 2). The
temperatures of the sample solutions in the NMR probe
were determined by direct measurements of the solvent
temperature in an NMR tube set in the probe using a
Technol Seven D617 digital thermister and were con-
trolled within 0.1 K.
50
100
0
ppm
Fig. 1. 31P NMR spectrum of [PdCl(pp3S)]+ in chloroform. Reference
denotes the signal for D3PO4 in outer D2O.
two kinds of coupling, are reasonably assigned to the pen-
dant phosphine sulfide, two terminal, and central P atoms,
respectively, by comparison with the 31P NMR spectrum of
square–planar [Pd(4-Clpt)(pp3O)]+ (31P{1H} NMR
(CHCl3): d 32.3 (d, 1P, oxide), 47.6 (d, 2P, terminal),
107.0 (dt, 1P center); 3JP(oxide)-P(center) = 33.1 Hz,
3JP(terminal)-P(center) = 16.5 Hz) [4]. While the reaction solu-
tion for the preparation of 1 using excess sulfur initially
showed the 31P NMR signals for the square–planar mono-
sulfide complex, [PdCl(pp3S)]+, the final product 1 exhib-
ited the signals for the two phosphine sulfide P atoms
(45.2 ppm) coupled with the central and terminal P atoms
(3JP-P = 35.4 and 4.2 Hz, respectively) and the central and
terminal P atoms (75.8 and 70.0 ppm, respectively) coupled
with each other (3JP-P = 11.2 Hz) in acetonitrile/H2O (10:1
v/v) (Fig. 2). The weak coupling between the terminal and
sulfide P atoms indicates that both phosphine sulfide
groups also participate in the complexation. The pseudo-
tripodal pp3S2 ligand can hardly take the usual four-
coordinate square–planar geometry and the soft donor
atoms in the tetradentate tripodal ligands such as pp3 [7],
2.4. General procedure for the C–C coupling reaction
Reactions of iodobenzene (21 mmol) with phenylbo-
ronic acid (27 mmol) in DMF (2.5 cm3) degassed with
nitrogen were carried out at 125 ꢁC in the presence of
K2CO3 (25 mmol) as a base and 1, 2, [PdCl(pp3)]Cl, or
[PdCl(p3)]Cl (3.6 · 10ꢀ3 mmol) as catalysts. The yields
were calculated by the 1H NMR intensity of the ortho
protons of biphenyl formed on the basis of the intensity
of the ethylene protons of bis(2-butoxyethyl) ether con-
tained as an internal reference and followed as a function
of time.
3. Results and discussion
tris[2-(diphenylphosphino)ethyl]amine
[7c,7d,7e,7h,7i,8]
3.1. Structure of complexes in solution
and the arsenic analogues [8a,9], and the sulfur- or sele-
nium-containing analogues [10] tend to form the five-coor-
dinate trigonal-bipyramidal d8 metal complexes with an
axial monodentate ligand. Therefore, it is obvious that 1
takes the five-coordinate pseudo-trigonal-bipyramidal
structure as in Fig. 2. From the 31P NMR spectrum, 2 in
wet acetonitrile can be also assigned to the five-coordinate
structure similar to 1 with two coordinated phosphine sel-
enide groups instead of sulfide ones (see Section 2).
The reaction solution of [PdCl(pp3)]Cl [5] with 1 equiv
of sulfur in chloroform showed the 31P NMR spectrum
assignable to that of the square–planar complex with a
pendant phosphine sulfide group, [PdCl(pp3S)]+ (Fig. 1),
where the doublet at 45.1 ppm with JP-P = 37.5 Hz, the
doubly intense doublet at 45.7 ppm with JP-P = 7.3 Hz,
3
3
and the doublet of broad triplet at 113.3 ppm with the

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S.-i. Aizawa et al. / Journal of Organometallic Chemistry 692 (2007) 813–818
S
P
+
+
35.4 Hz
0
P
c
P
35.4 Hz
4.2 Hz
11.2 Hz
P
c
S
P
S
P
Pd
Cl
P
P
t
P
Cl
t
S
P
4.2 Hz S
P
S
-
c
Pd
Cl
P
Pd
t
Cl
[PdCl(pp3S2)]+ (5)
[PdCl2(pp3S2)] (4)
Cl
11.2 Hz
4.2 Hz
ref
+
[PdCl(pp S )]
P
3
2
P=S
t
5
54
4
x 3
P
c
P=S
P
5
c
4
5
4
35.4 Hz
11.2 Hz
237.0 K
287.5 K
305.8 K
P
t
P
c
100
50
0
ppm
Fig. 2. 31P NMR spectrum of 1 in acetonitrile/H2O (10:1 v/v). Reference
denotes the signal for D3PO4 in outer D2O.
3.2. Geometrical equilibrium
326.6 K
The 31P NMR of 1 in dried chloroform at room temper-
ature exhibited the broadened signal for the terminal P
atoms and the almost flattened signal for the central P
atom. As the temperature was lowered, signals for two
kinds of complexes were clearly observed (Fig. 3): one
was the five-coordinate complex with the two coordinated
phosphine sulfide groups, [PdCl(pp3S2)]+, as observed in
wet acetonitrile and the other was the square–planar com-
plex with the two pendant phosphine sulfide groups,
[PdCl2(pp3S2)], in which the pp3 ligand acts as bidentate.
The latter structure was reasonably assigned from the 31P
NMR spectrum of the pp3 pentanuclear complexes having
the same square–planar dichloropalladium(II) moieties
with the bidentate pp3 ligand (68.4–68.1 and 88.8–
89.3 ppm for terminal and central P atoms, respectively)
[11], in which the other two terminal phosphino groups
bridge to Pt(II) and Rh(III) instead of forming sulfide.
The 31P NMR spectra for the above two complexes were
completely coalesced at ca 50 ꢁC (Fig. 3). The above revers-
ible NMR spectral change with temperature revealed rapid
equilibrium between the five-coordinate and four-coordi-
nate geometries in chloroform. In wet acetonitrile solution,
the solvation energy for the complex cation and chloride
anion is enough to release the chloride ion into the solvent,
and consequently, all the potentially binding groups of the
pp3S2 ligand can be coordinated to the Pd(II) center to
form the five-coordinate structure. On the other hand,
the rapid equilibrium indicates that weakly polar solvents
such as chloroform give rise to the competitive coordina-
tion between the chloride ion and the phosphine sulfide
groups because the solvation of non-charged species is pre-
45
65
ppm
44
85
80
75
70
Fig. 3. 31P NMR spectra of 1 in chloroform at various temperatures. 4
and
denote five-coordinate [PdCl(pp3S2)]+ and four coordinate
5
[PdCl2(pp3S2)], respectively.
ferred. The selenide analogue 2 showed almost the same
geometrical equilibrium in dried chloroform though the
equilibrium lay far to the five-coordinate complex com-
pared with that for 1. The colorless crystals of 1 and 2
obtained from the chloroform solution (see Section 2)
may contain the square–planar [PdCl2(pp3X2)] (X = S,
Se) because the crystals give light yellow chloroform solu-
tions at room temperature and yellow acetonitrile solutions
attributable to formation of the five-coordinate complexes,
[PdCl(pp3X2)]+ (X = S, Se), with longer-wavelength shift
of the d–d absorption bands [10].
The equilibrium constants for the geometrical change
from the five-coordinate complex to the four-coordinate
one, [PdCl(pp3S2)]+ + Clꢀ¢[PdCl2(pp3S2)], were obtained
from the reversible 31P NMR spectral change in dried deu-
terated chloroform at low temperatures using the ratios of
the signal intensities for the central P atoms of the two spe-
cies. The equilibrium constants at 298 K (K298) and the
enthalpy and entropy changes for the geometrical change
(DHꢁ and DSꢁ, respectively) listed in Table 1 were deter-
mined by using each temperature dependence of K as
shown in Fig. 4. The positive values of DHꢁ and DSꢁ are
attributed to the decrease in coordination number. A little

S.-i. Aizawa et al. / Journal of Organometallic Chemistry 692 (2007) 813–818
817
Table 1
Thermodynamic parameters for geometrical equilibrium of 1 and 2
298/molꢀ1
DHꢁ/kJ molꢀ1
DSꢁ/J molꢀ1 Kꢀ1
150
100
50
Complex
K
1
2
3.7 · 103
5.4 · 102
11.3 0.3
13.4 0.4
106
97
2
3
k
9
8
7
6
5
4
0
0
0.02
0.04
0.06
0.08
[Cl-] / mol kg-1
K
Fig. 5. Dependence of k (246.7 K) for the geometrical change form five- to
four-coordinate structure of 1 (d) and 2 (j) on chloride ion concentration
in chloroform.
+
+
0.0032 0.0034 0.0036 0.0038
-1 / K-1
0.004
0.0042
X
P
P
Pt
X
P
Pt
Cl
X-,
K'
k
'
T
P
P
P
P
+ Cl
P
P
P
P
Pt
Cl
Fig. 4. Temperature dependence of lnK for the geometrical change from
five- to four-coordinate structure of 1 (d) and 2 (j) in chloroform.
P
[PtCl(pp3)]+
[PtX(pp3)]+
K
'
k' [X-]
'[X-] + 1
larger DHꢁ value for 2 than that for 1 indicates that the Se
atoms can enthalpically stabilize the five-coordinate struc-
ture compared with the S atoms by effective p-back
donation from the occupied non-bonding d orbitals on
the five-coordinate Pd(II) into the p* orbital on the selenide
Se atoms. A little smaller DSꢁ value for 2 than that for 1
probably due to longer Pd–Se bond distances than Pd–S
ones, which reduces entropic stabilization accompanied
by the Pd–Se bond breaking. Such electronic and steric dif-
ferences between the Pd–Se and Pd–S bonds in the five-
coordinate complexes cause the difference in preference
for the five-coordinate geometry between 1 and 2.
(X = Br-, I-)
kobs
=
K
Scheme 2.
substitution reaction of the five-coordinate trigonal-bipyra-
midal [PtCl(pp3)]+ in chloroform as shown in Scheme 2
[12]. It may be possible that the present geometrical change
for the five-coordinate Pd(II) complexes take a similar
reaction mechanism (Scheme 3) where the pre-equilibrium
constant (K0) is small enough (K0[Clꢀ] ꢃ 1) to give the lin-
ear concentration dependence, k = K0k0[Clꢀ], as shown in
Fig. 5. As above described, the thermodynamic study re-
vealed that the selenide coordination of 2 stabilizes the
five-coordinate species [PdCl(pp3Se2)]+ compared with
the sulfide coordination of 1. This fact is consistent with
the smaller K0, and consequently, the smaller k of 2.
The observed rate constants for the geometrical change
from the five-coordinate complex to the four-coordinate
one, k, were obtained from the low-temperature 31P
NMR line-broadening data: the values of k for 1 were
given by increases in the transverse relaxation rate ðDT2ꢀ1
Þ
of the phosphine sulfide P atoms in the four-coordinate
complex using Eq. (1),
k ¼ DTꢀ2 1½4 ꢀ coordinateꢂ=½5 ꢀ coordinateꢂ
ð1Þ
X
P
X
P
+
Cl
P
Pd
Cl
where [4-coordinate] and [5-coordinate] are the concentra-
tions of the four- and five-coordinate complexes, respec-
tively, and the k values for 2 were directly obtained from
DT ꢀ2 1 for the terminal P atom in the five coordinate com-
plex (Supplementary material). As shown in Fig. 5, each
observed rate constant is proportional to the chloride ion
concentration. Recently, from the kinetic results, we have
proposed the preassociation mechanism for the haro-ligand
P
X
Cl-
K'
k'
X
X
P
P
P
P
P
P
P
Pd
Pd
Cl
Cl
Cl
X
P
K'k'[Cl-]
K'[Cl-] + 1
If K'[Cl-] << 1, k = K'k'[Cl-].
(X = S, Se)
k =
Scheme 3.

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S.-i. Aizawa et al. / Journal of Organometallic Chemistry 692 (2007) 813–818
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Belsky, V.N. Spevak, N.K. Skvortsov, Inorg. Chim. Acta 359 (2006)
1031;
0
2000
4000
6000
t / min
Fig. 6. Change in the yield of biphenyl with time in Suzuki coupling
reaction catalyzed by 1 (d), 2 (j), [PdCl(pp3)]Cl (s), and [PdCl(p3)]Cl (h).
(f) P. Sekar, J.A. Ibers, Inorg. Chim. Acta 359 (2006) 2751.
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3.3. C–C coupling reaction
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In order to clarify the electronic effect of the phosphine
chalcogenides on the Pd-catalyzed C–C coupling reactions,
we adopted Suzuki coupling that is one of the most wide-
spread coupling reactions. The catalytic activities of 1
and 2 were compared to those of the phosphine complexes,
[PdCl(pp3)]Cl and [PdCl(p3)]Cl, using iodobenzene and
phenylboronic acid as the substrates under the same
condition.
(b) C.A. Ghilardi, S. Midollini, L. Sacconi, Inorg. Chem. 14 (1975)
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¨
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Organometallics 9 (1990) 2514;
Palladium-catalyzed C–C coupling reactions usually
start from formation of the catalytically active Pd(0) spe-
cies by prereduction of Pd(II). Because the Pd(0) species
is formed again after the reductive elimination of substrates
and prior to the next oxidative addition in the catalytic
cycle, stability of the Pd(0) species is very important for
retention of the catalytic activity. As shown in Fig. 6,
difference in the retentive ability, 2 > 1 > [PdCl(pp3 or
p3)]Cl, were observed in the present Suzuki coupling reac-
tion. Considering the p accepting ability of the selenide Se
atom superior to the sulfide S atom, as mentioned above,
and strong r donating ability of the phosphine P atom,
the retentive ability is consistent with the electron-accept-
ing ability of the ligands, which stabilizes the electron-rich
Pd(0) center. Furthermore, it is probable that the forma-
tion of the substrate adduct and subsequent transmetalla-
tion of the Pd(II) species are not blocked by coordination
of the phosphine chalcogenide groups because they can
be easily dissociated from the Pd(II) center to open the
coordination sites as observed in the rapid geometrical
equilibrium.
(k) M.C. Vaira, M. Peruzzini, P. Stoppioni, Inorg. Chem. 30 (1991)
1001;
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´
M.E.G. Fernandez, Inorg. Chem. 41 (2002) 4435.
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718;
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A. Orlandini, Inorg. Chem. 23 (1984) 922;
(c) F. Cecconi, C.A. Ghilardi, S. Midollini, S. Moneti, A. Orlandini,
G. Scapacci, Inorg. Chim. Acta 189 (1991) 105.
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1544;
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Trans. (1983) 2263.
Appendix A. Supplementary data
[10] A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed., Else-
vier, Amsterdam, 1984 (Chapter 6).
[11] S. Aizawa, K. Saito, T. Kawamoto, E. Matsumoto, Inorg. Chem. 45
(2006) 4859.
[12] S. Aizawa, T. Kobayashi, T. Kawamoto, Inorg. Chim. Acta 358
(2005) 2319.
The thermodynamic and kinetic data for 1 and 2 are
available from the author upon request. Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.jorganchem.2006.10.020.