Kinetic studies on thiolato-ligand substitution reactions with halide ions of square-planar palladium(II) complex with bis(2-(diphenylphosphino)ethyl)phenylphosphine

Article abstract of DOI:10.1016/S0020-1693(02)00981-7

The square-planar thiolato palladium(II) complex with a terdentate phosphine bound ligand, [Pd(pt)(p₃)](BF₄) (p₃=bis(2-(diphenylphosphino)ethyl)phenylphosphine, pt=1-propanethiolate) has been synthesized and characterized

Full text of DOI:10.1016/S0020-1693(02)00981-7

Inorganica Chimica Acta 338 (2002) 235ꢄ239
/
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Note
Kinetic studies on thiolato-ligand substitution reactions with halide
ions of square-planar palladium(II) complex with bis(2-
(diphenylphosphino)ethyl)phenylphosphine
Sen-ichi Aizawa a,*, Yasushi Sone b, Tatsuya Kawamoto c, Shinkichi Yamada b,
Motoshi Nakamura b
a Faculty of Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555, Japan
b Faculty of Engineering, Shizuoka University, Johoku, Hamamatsu 432-8561, Japan
c Department of Chemistry, Graduate School of Science, Osaka University, 1-16 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
Received 11 January 2002; accepted 5 April 2002
Abstract
The square-planar thiolato palladium(II) complex with a terdentate phosphine bound ligand, [Pd(pt)(p3)](BF4) (p3ꢀ
(diphenylphosphino)ethyl)phenylphosphine, ptꢀ
1-propanethiolate) has been synthesized and characterized by 31P NMR spectro-
scopy. The solid state structure of [PdI(p3)]I was determined by an X-ray crystal structure analysis. The kinetic parameters for the
thiolato-ligand substitution reactions with halide ions, [Pd(pt)(p3)]ꢁꢁ ꢂ 0[PdX(p3)]ꢁꢁptꢂ (Xꢂ ꢀClꢂ, Brꢂ, Iꢂ), in chloro-
form were obtained as follows: k298ꢀ 10ꢂ4 sꢂ1, DH%ꢀ 3 kJ molꢂ1 and DS%ꢀ
7.22, 1.68 and 10.1ꢃ 6792, 5692 and 639 8097,
12896 and ꢂ929
11 J molꢂ1 Kꢂ1, respectively. These kinetic results are compared with those for the corresponding reactions of
the trigonal-bipyramidal complex with an analogous phosphine bound ligand, [Pd(pt)(pp3)](BF4) (pp3ꢀtris(2-(diphenylpho-
/bis(2-
/
/X
/
/
/
/
/
/
/
/
/
/
ꢂ
/
/
ꢂ
/
/
/
/
/
sphino)ethyl)phosphine).
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Square-planar palladium(II) complexes; Thiolato complexes; Substitution reaction; Reaction mechanism
1. Introduction
nethiolate) [2], the kinetic studies on the corresponding
reactions of the square-planar analog [Pd(pt)(p3)](BF4)
Recently, we have performed the mechanistic studies
on five-coordinate trigonal-bipyramidal palladium(II)
complexes [1,2] to compare the reaction mechanism with
that for four-coordinate square-planar ones which had
been extensively studied so far [3]. However, in order to
make a precise comparison of the reaction mechanism
for both geometries, comparable reaction system includ-
ing the same entering and leaving ligands, coordinated
atoms or groups in the bound ligands, solvent and
counter ion should be investigated. Since we previously
reported the kinetics for thiolato-ligand substitution
reactions with halide ions in chloroform by using
(p3ꢀbis(2-(diphenylphosphino)ethyl)phenylphosphine)
have been carried out.
/
2. Experimental
2.1. Reagents
Chloroform (Wako, ꢀ pure) and deuterated chloro-
form (Wako, 100%) were dried over activated 4A
Molecular Sieves. Tetrabuthylammonium halides
trigonal-bipyramidal [Pd(pt)(pp3)](BF4) (pp3ꢀ
/tris(2-
(Wako), Bu4NX (Xꢂ ꢀClꢂ, Brꢂ, and Iꢂ), were dried
/
(diphenylphosphino)ethyl)phosphine, ptꢀ1-propa-
/
under a vacuum. Bis(2-(diphenylphosphino)ethyl)phe-
nylphosphine (p3, Strem, 97%) and 1-propanethiol (Hpt,
Wako) were used to prepare palladium(II) complexes
without further purification.
* Corresponding author. Tel./fax: ꢁ
/
81-76-445 6980
E-mail address: saizawa@eng.toyama-u.ac.jp (S.-i. Aizawa).
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 9 8 1 - 7

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S. Aizawa et al. / Inorganica Chimica Acta 338 (2002) 235ꢄ239
/
2.2. Preparation of complexes
Table 1
Crystallographic data for [PdI(p3)]I
Empirical formula
Formula weight
Temperature (K)
Crystal habit
C34H33I2P3Pd
894.79
298
2.2.1. [Pd(pt)(p3)](BF4)
To a solution of [Pd(p3)(CH3CN)](BF4)2 [4] (0.32 g,
0.37 mmol) in acetonitrile (30 cm3) was added dropwise
1-propanethiol (Hpt, 0.045 g, 0.59 mmol), and then
ethanol (5 cm3) and 0.1 M aqueous NaOH (1 cm3). The
resultant yellow solution was filtered, and water (ca. 15
cm3) was added to the filtrate to give yellow crystals.
The crystals were collected by filtration and recrystal-
lized from dichloromethane by adding ethanol and
polyhedron
0.20ꢃ0.20ꢃ0.20
tetragonal
I41md
Crystal size (mm)
Crystal system
Space group
˚
a (A)
24.23(1)
12.430(3)
7298(5)
˚
c (A)
3
V (A )
˚
Z
8
1.629
Dcalc (g cmꢂ3
)
water. Yield: 0.20
g
(67%). Anal. Calc. for
[Pd(p3)(pt)](BF4): C, 55.35; H, 5.02; N, 0.00. Found:
m (cmꢂ1
)
b
2.326
0.118, 0.140
R
a, Rw
C, 55.62; H, 4.98; N, 0.00%. 31P NMR (in CHCl3): d
a
(relative to D3PO4 in external D2O)ꢀ
inal), 101.59 (t, central); JPꢀP 20.8 Hz.
/
48.93 (d, term-
RꢀajjFojꢂjFcjj/ajFoj.
b
2 ꢂ1
Rwꢀ[a w(jFojꢂjFcj)2/a wjFoj2]1/2. wꢀ(s2(Fo)ꢁ0.001Fo
)
.
ꢀ
/
2.4. Measurements
2.2.2. [PdX(p3)]X (Xꢂ ꢀClꢂ, Brꢂ, Iꢂ)
/
The halo complexes were prepared by the procedures
described previously [1]. Single crystals of the iodo
complex were obtained by slow evaporation of the
dichloromethane solution containing a small amount
of ethanol and water. 31P NMR (in CHCl3) for the
chloro, bromo and iodo complexes: d (relative to D3PO4
The kinetic measurements for the substitution reac-
tions of the pt complex with halide ions in chloroform
were carried out under pseudo-first-order conditions
where the concentrations of Bu4NX (Xꢂ ꢀClꢂ, Brꢂ,
/
Iꢂ) were in large excess over those of the complex
(Tables S5 and S6, Section 4). The sample preparation
and kinetic measurements were performed under a
nitrogen atmosphere. The absorption spectral changes
were recorded on a JASCO V-570 spectrophotometer.
The temperature of the reaction solution was controlled
in external D2O)ꢀ45.87, 46.78 and 47.20 (d, terminal),
/
111.20, 112.77 and 111.62 (t, central); JPꢀP
and 7.3 Hz, respectively.
ꢀ10.0, 8.3
/
within 9
0.1 K. The 31P NMR spectra were recorded on
/
2.3. X-ray structure analysis
a JEOL JNM-A400 FT-NMR spectrometer operating
at 160.7 MHz. In order to determine the chemical shift
of the 31P NMR, a 3 mm o.d. NMR tube containing the
sample solution was coaxially mounted in a 5 mm o.d.
NMR tube containing deuterated water and phosphoric
acid as a lock solvent and a reference, respectively.
X-ray diffraction measurements for the iodo complex
were performed on a Mac Science MXC3 diffractometer
with Mo Ka radiation. The unit-cell parameters and
orientation matrix were determined by a least-squares
refinement of 22 independent reflections collected in the
range of 3.08B
formed with the u ꢄ
/
2u B
/
35.08. Data collection was per-
2u scan mode with three standard
/
3. Results and discussion
reflections measured after every 100 scans, which
showed no significant decay. An empirical absorption
(c scan) correction was applied. The solution and
refinement were carried out using CRYSTAN-GM (version
6.3.3) [5]. The structure was solved by direct methods
using the programs of SIR 92 [6]. All non-hydrogen
atoms were refined anisotropically and hydrogen atoms
3.1. Characterization
A perspective view of the iodo complex cation is
displayed in Fig. 1 along with the atomic labeling. The
complex cation has a square-planar geometry with a
mirror plane bisecting the p3 ligand. The coordination
bond distance for the central phosphine (P(2)) is
considerably shorter than those for the terminal ones
(P(1)), probably due to the weak trans influence of the
iodo ligand compared with that of the diphenylpho-
sphino group and the chelate strain of the two five-
membered chelate rings in the terdentate p3 ligand
were included in the calculated positions (Cꢀ
/
Hꢀ
A). The atomic scattering factors were taken from ref.
[7]. With Zꢀ8, the complex cation and counter ion sit
on the crystallographic mirror plane 1/2, y, z; 15 atoms
have such coordinates (Pd(1), I(1), I(2), P(2), C(15)ꢀ
C(20) and H(16)ꢀH(20)).
/
0.96
˚
/
/
/
The crystallographic data are summarized in Table 1.
The crystallographic details, positional and thermal
parameters for all atoms, and bond distances and angles
for non-hydrogen atoms are provided as Section 4.
(P(1)ꢀ
/
Pd(1)ꢀ
/
P(2)ꢀ
/
84.38
and
P(1)ꢀ
/
Pd(1)ꢀ
/
I(1)ꢀ
/
96.18). In Table 2, the coordination bond distances of
the iodo complex are compared with those of the

S. Aizawa et al. / Inorganica Chimica Acta 338 (2002) 235ꢄ
/239
237
geometry in spite of the larger ionic radius of the five-
coordinate ion. This fact indicates the relatively strong s
interactions of the axial coordination bonds for the
trigonal-bipyramidal geometry.
The pt and halo complexes exhibit two 31P NMR
signals (triplet and doublet) corresponding to the central
and equivalent terminal phosphorus atoms, respectively
(see Section 2). Such a spectral pattern is consistent with
the square-planar geometry with C2v symmetry. In the
case of the free p3 ligand, the 31P NMR signal of the
central phosphorus atom shows an upfield shift com-
pared with that of the terminal phosphorus atoms in the
diphenylphosphino groups [9]. However, the downfield
shift for the central phosphorus atom is much larger
than that of the terminal phosphorus atoms in the
present complexes. This is explained by the greater
deshielding (diamagnetic contribution) and configura-
tional ununiformity of the valence electrons (paramag-
netic contribution) on the phosphorus atoms caused by
the stronger donation of the central phosphorus atom,
Fig. 1. ORTEP diagram of [PdI(p3)]ꢁ
.
Table 2
Coordination bond distances (A) of [PdI(p3)]ꢁ, [Pd(CF3SO3)(p3)]ꢁ
and [PdI(pp3)]ꢁ
˚
which is observed in the Pdꢀ
/
P bond distances [10,11].
[PdI(p3 )]ꢁ
The upfield shift for the central phosphorus atom of the
pt complex compared with the halo complexes is
attributed to the stronger trans influence of the thiolato
sulfur atom.
Pd(1)ꢀI(1)
Pd(1)ꢀP(1)
Pd(1)ꢀP(2)
2.672(10)
2.311(14)
2.24(3)
[Pd(CF3 SO3 )(p3 )]ꢁ
PdꢀO
PdꢀPt
PdꢀPc
[PdI(pp3 )]ꢁ
2.127(7)
e
3.2. Kinetics
a
2.344
2.192(2)
b
The 31P NMR spectra of solutions containing the pt
complex and a large excess of Bu4NX (Xꢂ ꢀClꢂ, Brꢂ,
/
e
PdꢀI
PdꢀPeq
PdꢀPax
2.687
Iꢂ) in chloroform gradually changed to those of the
corresponding halo complexes. The absorption spectra
c
e
2.42
2.23
d
e
of the diluted solutions (4.15ꢃ
/
10ꢂ5ꢄ
/
4.66ꢃ
/
10ꢂ5 mol
10ꢂ1
a
Terminal phosphorus.
Central phosphorus.
kgꢂ1 of the pt complex and 2.60ꢃ
/
10ꢂ2ꢄ
/
4.58ꢃ
/
b
mol kgꢂ1 of Bu4NX in chloroform) changed with
isosbestic points and finally exhibited the spectra of
the corresponding halo complexes (Fig. S1). The ob-
served pseudo-first-order rate constants of the pt ligand
substitution reactions with halide ions (k) were deter-
mined by a nonlinear least-squares analysis for the
exponential time course of the absorbance (Fig. S1).
The values of k were independent of the halide-ion
concentration (Table S5). Since the limiting dissociative
mechanism is not consistent with the fact that the
present kinetic parameters depend on the entering halide
ions and the values of DS% are considerably negative
(vide infra), it is probable that ion pairs of the positively
charged pt complex with halide ions existing in large
excess are quantitatively formed in chloroform as
usually observed in weakly-polar solvents [12]. Such a
ion-pair mechanism was also observed in the case of the
corresponding reactions of the trigonal-bipyramidal pt
complex, [Pd(pt)(pp3)]ꢁ [2], and was closely reported
for the ligand substitution reactions of the square-planar
platinum(II) complex [13]. Each temperature depen-
dence of the first-order rate constant was fitted to the
c
Equatorial phosphorus.
Axial phosphorus.
Average value.
d
e
corresponding square-planar trifluoromethanesulfonato
complex, [Pd(CF3SO3)(p3)]ꢁ [8] and the trigonal-bipyr-
amidal iodo complex with a tripodal tetradentate
phosphine ligand, [PdI(pp3)]ꢁ [1]. The bond distance
for the central phosphorus atom of [PdI(p3)]ꢁ is a little
longer than that for [Pd(CF3SO3)(p3)]ꢁ. This suggests
that the trans influence of the iodo ligand is stronger
than that of the sulfonato oxygen. The bond distance for
the terminal phosphorus atom of [PdI(p3)]ꢁ is consider-
ably shorter than the average bond distance for those of
[PdI(pp3)]ꢁ. This is due to strong s interactions in the
square plane for the former compared with those in the
equatorial plane for the latter although it is possible that
p interactions are stronger in the latter case. On the
other hand, the axial coordination bond distances for
the central phosphorus atom and iodo ligand in the
trigonal-bipyramidal geometry are comparable to the
corresponding bond distances in the square-planar

238
S. Aizawa et al. / Inorganica Chimica Acta 338 (2002) 235ꢄ239
/
Fig. 2. Temperature dependence of the rate constants for the thiolato-
ligand substitution reactions with Clꢂ (j), Brꢂ (') and Iꢂ (m).
Eyring equation as shown in Fig. 2 to give the values of
DH% and DS% (Table S6). The activation parameters
obtained are summarized in Table 3 together with those
for the corresponding reactions of the trigonal-bipyr-
amidal pt complex.
It is apparent that the mode of activation for the
square-planar pt complex becomes more associative
than that for the trigonal-bipyramidal one showing the
smaller positive values of DH% and larger negative values
of DS% (Table 3). The difference in the reaction
mechanism is explained by the difference in the electro-
nic repulsion in the transition state, because, the four-
coordinate square-planar ground state with the 16-
electron system can easily accept the coming-ligand
orbital compared with the five-coordinate trigonal-
bipyramidal one with the 18-electron system.
Another point worth mentioning is the mechanistic
change caused by the difference in the coming halide
ions. In the case of the trigonal-bipyramidal complex,
the enhancement of the dissociative character from Clꢂ
to Iꢂ is ascribed to an increase in the steric hindrance of
the crowded substitution site. On the other hand, even
Fig. 3. Two reaction mechanisms for the square-planar palladium(II)
complexes. G, T and I denote the ground state, transition state and
intermediate, respectively.
So far, an associative mechanism via a trigonal-bipyr-
amidal transition state or intermediate has been pro-
posed for substitution reactions of square-planar
palladium(II) and platinum(II) complexes (Fig. 3)
[3,14]. If the intermediate is formed, the observed
activation parameters correspond to the energy level of
the preceding transition state in which the coming ligand
is less bound than in the intermediate (Mechanism 2 in
Fig. 3). Furthermore, the transition state in Mechanism
2 (Fig. 3) can be more dissociative compared with the
trigonal-bipyramidal transition state in Mechanism 1
(Fig. 3). Considering that the intermediate is likely to be
formed in systems with p-accepting ligands such as
phosphine and Iꢂ [15,16], the deviation in the order of
the activation parameters for [Pd(pt)(p3)]ꢁ may be
attributed to the formation of the intermediate in the
case of the reaction with Iꢂ.
considering the electron-accepting ability, Iꢂ ꢁ
/
Brꢂ ꢁ
/
Clꢂ, we can not explain by the steric or electronic factor
why the activation mode of the square-planar complex is
the most associative in the case of the reaction with Brꢂ.
Table 3
Activation parameters for substitution reactions of [Pd(p3 or
pp3 a)(pt)]ꢁ with halide ions
4. Supplementary material
Bound Halide k298
DH%
DS%
ligand
ion
(10ꢂ4 sꢂ1
)
(kJ molꢂ1
)
(J molꢂ1 Kꢂ1
)
An X-ray crystallographic data (Table S1), atomic
coordinates and isotropic thermal parameters (Table
S2), anisotropic thermal parameters (Table S3), bond
distances and angles (Table S4), entering-ligand con-
centration (Table S5) and temperature (Table S6)
dependences of rate constants, absorption spectral
changes for substitution reactions (Fig. S1) (in total
nine pages) are available from the author upon request.
p3
p3
Clꢂ
Brꢂ
Iꢂ
Clꢂ
Brꢂ
Iꢂ
7.22
1.68
10.1
3.55
2.76
2.14
6792
5692
6393
7092
8292
8991
ꢂ8097
ꢂ12896
ꢂ92911
ꢂ7795
ꢂ3797
ꢂ1692
p3
pp3
pp3
pp3
a
Ref. [2].

S. Aizawa et al. / Inorganica Chimica Acta 338 (2002) 235ꢄ
/239
239
[5] Programs for X-ray Crystal Structure Determination, MAC
Science, Yokohama, 1994.
Acknowledgements
[6] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J.
Appl. Crystallogr. 26 (1993) 343.
This research was supported by Grant-in-Aid for
Scientific Research (No. 12640583) from the Ministry of
Education, Culture, Sports, Science and Technology of
Japan.
[7] International Tables for X-ray Crystallography, vol. IV, Kynoch,
Birmingham, AL, 1974.
[8] S. Aizawa, T. Yagyu, K. Kato, S. Funahashi, Anal. Sci. (1995)
557.
[9] 31P NMR (in CHCl3): d (relative to D3PO4 in D2O)ꢀ
/
ꢂ16.71 (t,
/
central), ꢂ
/
13.00 (d, terminal); JPꢀP
ꢀ28.9 Hz.
/
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/