Stereoselective preparation, structures, and reactivities of phosphine-bridged mixed-metal trinuclear and pentanuclear complexes with tris[2-(diphenylphosphino)ethyl]phosphine

Article abstract of DOI:10.1021/ic060277l

The phosphine-bridged linear trinuclear and pentanuclear complexes with Pd(II)-Pt(II)-Pd(II), Ni(II)-Pt(II)-Ni(II), and Rh(III)-Pd(II)-Pt(II)-Pd(II)- Rh(III) metal-ion sequences were almost quantitatively formed by the stepwise phosphine-bridging reaction of the terminal phosphino groups of tris[2-(diphenylphosphino)ethyl]phosphine (pp₃), which is the tetradentate bound ligand of the starting Pd(II) and Ni(II) complexes. The solid-state structures of the trinuclear complexes were determined by X-ray structural analyses, and the structures of the polynuclear complexes in solution were characterized by NMR spectroscopy. The trans and cis isomers of the trinuclear and pentanuclear complexes, which arise from the geometry around the Pt(II) center, were selectively obtained simply by changing the counteranion of the starting complexes: the tetrafluoroborate salts, [MX(pp₃)] (BF₄) [M = Pd(II) or Ni(II), X = Cl^- or 4-chlorothiophenolate (4-Cltp^-)], gave only the trans isomers, and the chloride salt, [PdCl(pp₃)]CI, gave only the cis isomers. The formation of the trinuclear complex with the 4-Cltp^- and chloro ligands, trans-[Pt(4-Cltp)₂-{PdCl(pp₃)} ₂](BF₄)₂, proceeded with exchange between the thiolato ligand in the starting Pd(II) complex, [Pd(4-CltP)(pp ₃)](BF₄), and the chloro ligands in the starting Pt(II) complex, trans-[PtCl₂(NCC₆H₅)₂], retaining the trans geometry around the Pt(II) center. In contrast, the formation reaction between [PdCl(pp₃)]Cl and trans-[PtCl ₂(NCC₆H₅)₂] was accompanied by the trans-to-cis geometrical change on the Pt(II) center to give the trinuclear complex, cis-[PtCl₂{PdCl(pp₃)}₂]Cl₂. The mechanisms of these structural conversions during the formation reactions were elucidated by the ³¹P NMR and absorption spectral changes. The differences in the catalytic activity for the Heck reaction were discussed in connection with the bridging structures of the polynuclear complexes in the catalytic cycle.

Full text of DOI:10.1021/ic060277l

Inorg. Chem. 2006, 45, 4859−4866
Stereoselective Preparation, Structures, and Reactivities of
Phosphine-Bridged Mixed-Metal Trinuclear and Pentanuclear Complexes
with Tris[2-(diphenylphosphino)ethyl]phosphine
Sen-ichi Aizawa,*,† Kenji Saito,† Tatsuya Kawamoto,‡ and Emi Matsumoto†
Faculty of Engineering, Toyama UniVersity, 3190 Gofuku, Toyama 930-8555, Japan, and
Department of Chemistry, Graduate School of Science, Osaka UniVersity, 1-16 Machikaneyama,
Toyonaka, Osaka 560-0043, Japan
Received February 17, 2006
The phosphine-bridged linear trinuclear and pentanuclear complexes with Pd(II)−Pt(II)−Pd(II), Ni(II)−Pt(II)−Ni(II),
and Rh(III) Pd(II) Pt(II) Pd(II) Rh(III) metal-ion sequences were almost quantitatively formed by the stepwise
−
−
−
−
phosphine-bridging reaction of the terminal phosphino groups of tris[2-(diphenylphosphino)ethyl]phosphine (pp3),
which is the tetradentate bound ligand of the starting Pd(II) and Ni(II) complexes. The solid-state structures of the
trinuclear complexes were determined by X-ray structural analyses, and the structures of the polynuclear complexes
in solution were characterized by NMR spectroscopy. The trans and cis isomers of the trinuclear and pentanuclear
complexes, which arise from the geometry around the Pt(II) center, were selectively obtained simply by changing
the counteranion of the starting complexes: the tetrafluoroborate salts, [MX(pp3)](BF4) [M
) Pd(II) or Ni(II), X )
Cl- or 4-chlorothiophenolate (4-Cltp-)], gave only the trans isomers, and the chloride salt, [PdCl(pp3)]Cl, gave only
the cis isomers. The formation of the trinuclear complex with the 4-Cltp- and chloro ligands, trans-[Pt(4-Cltp)2-
{PdCl(pp3)}2](BF4)2, proceeded with exchange between the thiolato ligand in the starting Pd(II) complex, [Pd(4-
Cltp)(pp3)](BF4), and the chloro ligands in the starting Pt(II) complex, trans-[PtCl2(NCC6H5)2], retaining the trans
geometry around the Pt(II) center. In contrast, the formation reaction between [PdCl(pp3)]Cl and trans-[PtCl2(NCC6H5)2]
was accompanied by the trans-to-cis geometrical change on the Pt(II) center to give the trinuclear complex, cis-
[PtCl2{PdCl(pp3)}2]Cl2. The mechanisms of these structural conversions during the formation reactions were elucidated
by the 31P NMR and absorption spectral changes. The differences in the catalytic activity for the Heck reaction
were discussed in connection with the bridging structures of the polynuclear complexes in the catalytic cycle.
Introduction
be investigated straightforward and systematically. Previ-
ously, we have found that one terminal phosphino group of
the five-coordinate trigonal-bipyramidal Pd(II) complex, [Pd-
(4-Cltp)(pp3)]+ [1; 4-Cltp ) 4-chlorothiophenolate, pp3 )
tris[2-(diphenylphosphino)ethyl]phosphine], is selectively
oxidized by photolysis and further oxidation of another
terminal phosphino group proceeds quantitatively by reaction
with more than an equimolar amount of 4-Cltp- (Scheme
1).1 Expecting that intermediates with a dissociated terminal
phosphino group (I1 and I2) are formed, we have designed
the stepwise phosphine-bridging reaction with different metal
ions instead of stepwise oxidation. Though a few phosphine-
bridged complexes with two kinds of metal ions have been
Cooperative effects of different metal complexes are
attractive for the design of new chemical reactions. Because
various phosphine complexes have been successfully used
for a variety of catalytic reactions recently, deriving new
cooperative catalytic effects can be one of the next goals.
However, in the case where different catalysts exist inde-
pendently in the reaction solution, the cooperative catalytic
action may be entropically unfavorable. Therefore, if one
can prepare the mixed-metal polynuclear complexes with an
intended metal-ion sequence, the cooperative effect of the
different metal centers of the catalysts and cocatalysts can
* To whom correspondence should be addressed. E-mail: saizawa@
eng.toyama-u.ac.jp (S.-i.A.). Phone and fax: (+81) 76 445 6980.
† Toyama University.
(1) Aizawa, S.; Kawamoto, T.; Saito, K. Inorg. Chim. Acta 2004, 357,
‡ Osaka University.
2191.
10.1021/ic060277l CCC: $33.50
Published on Web 05/13/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 12, 2006 4859

Aizawa et al.
[PdCl(p3)]Cl (3Cl)5 were prepared by the reported procedures. 2Cl
was converted to 2(BF4) by using 1 equiv of AgBF4 in chloroform.
[PdCl2(p2)] (4) was prepared by a procedure similar to that for 3Cl
by the reaction of an aqueous solution of K2[PdCl4] and a
chloroform solution of p2 followed by concentration of the
chloroform layer.
Scheme 1. Stepwise Oxidation Reaction of the pp3 Ligand in 1
[NiCl(pp3)]Cl (5Cl). 5Cl was prepared by the reported procedure
with some modification.6 To a solution containing NiCl2 (0.115 g,
0.883 mmol) in water (20 cm3) were added a solution containing
pp3 (0.592 g, 0.883 mmol) in chloroform (20 cm3) and then ethanol
(30 cm3). The resultant purple solution was stirred at 50 °C for 2
h. After the addition of water (200 cm3), the purple complex was
extracted with chloroform (150 cm3). The extract was concentrated
to ca. 10 cm3 and kept in a refrigerator to give purple needle crystals.
Yield: 89%. Anal. Calcd for C42H42Cl2NiP4‚2H2O: C, 60.32; H,
5.54; N, 0.00. Found: C, 59.87; H, 5.57; N, 0.00.
[NiCl(pp3)](BF4) [5(BF4)]. 5(BF4) was prepared by a procedure
similar to that reported for 5(AsF6).7 To a solution containing NiCl2
(0.090 g, 0.697 mmol) and Bu4NBF4 (0.673 g, 2.04 mmol) in
ethanol (15 cm3) were added a solution containing pp3 (0.467 g,
0.697 mmol) in chloroform (5 cm3) and then ethanol (30 cm3). The
resultant reddish-purple solution was heated at 50 °C for 2 h and
then boiled under reflux for 6 h. After the solution was concentrated
to half the original volume, the resultant dark-purple crystals were
collected by filtration, washed with ethanol and ether, and dried in
vacuo. Yield: 66%. Anal. Calcd for C42H42BClF4NiP4: C, 59.23;
H, 4.97; N, 0.00. Found: C, 59.18; H, 5.10; N, 0.00.
trans-[Pt(4-Cltp)2{PdCl(pp3)}2](BF4)2 [6(BF4)2]. 1(BF4) (0.103
g, 0.103 mmol) and trans-[PtCl2(NCC6H5)2] (0.025 g, 0.052 mmol)
were dissolved in acetonitrile (1 cm3). The reaction solution was
concentrated by slow evaporation of the solvent to give yellow
crystals. The crystals were collected by filtration and washed with
ethanol. Yield: 76%. Anal. Calcd for C96H92B2Cl4F8P8Pd2PtS2‚
H2O: C, 50.15; H, 4.12; N, 0.00. Found: C, 49.77; H, 4.11; N,
0.00. 195Pt{1H} NMR (CHCl3): δ 38.64 (t, 1JPt-P ) 2767 Hz). The
single crystals were obtained by recrystallization from chloroform.
trans-[PtCl2{PdCl(pp3)}2](BF4)2 [7a(BF4)2]. To a solution of
2(BF4) (0.249 g, 0.277 mmol) in chloroform (10 cm3) was added
trans-[PtCl2(NCC6H5)2] (0.064 g, 0.136 mmol) followed by filtra-
tion. Pale-yellow crystals were obtained by the addition of ethanol
and water to the filtrate. Yield: 66%. Anal. Calcd for C84H84B2-
Cl4F8P8Pd2Pt‚H2O: C, 48.44; H, 4.07; N, 0.00. Found: C, 48.40;
H, 4.14; N, 0.00. The single crystals were obtained by recrystal-
lization from chloroform.
reported recently,2 we have pursued synthetic studies of
phosphine-bridged polynuclear complexes for the quantitative
preparation of trinuclear and pentanuclear mixed-metal
complexes with intended metal sequences, taking advantage
of the formation of Pd(II) intermediates with a dissociated
terminal phosphino group. The selective preparation of their
isomers and some structural conversions during the formation
reactions are also reported in this paper. Furthermore, because
phosphinepalladium(II) complexes have been widely used
for the C-C coupling reactions as the catalysts,3 we
investigated the Heck reaction3b,c by using the polynuclear
and mononuclear complexes and discussed their structures
in the catalytic reactions.
Experimental Section
Materials. Chloroform (Wako, infinitely pure) was dried over
activated 4A molecular sieves and used as a solvent for UV-vis
absorption spectral measurements. Tris[2-(diphenylphosphino)ethyl]-
phosphine (pp3, Aldrich), bis[2-(diphenylphosphino)ethyl]phen-
ylphosphine (p3, Strem), 1,2-bis(diphenylphosphino)ethane (p2,
Kanto Chemical), 4-chlorothiophenol (H-4-Cltp, Wako), tetrakis-
(acetonitrile)palladium(II) tetrafluoroborate ([Pd(NCCH3)4](BF4)2,
Aldrich), potassium tetrachloropalladate(II) (K2[PdCl4], Aldrich),
trans-bis(benzonitrile)dichloroplatinum(II) (trans-[PtCl2(NCC6H5)2],
Strem), dichlorodi-µ-chlorobis(pentamethylcyclopentadienyl)di-
rhodium(III)4 ([Rh2Cl2(µ-Cl)2Cp*2], Aldrich), silver tetrafluorobo-
rate (AgBF4, Wako), nickel chloride (Wako), tetra-n-butylammo-
nium tetrafluoroborate (Bu4NBF4, Aldrich), tetra-n-butylammonium
chloride (Bu4NCl, Wako), iodobenzene (Kanto Chemical), styrene
(Wako), tributylamine (Wako), and bis(2-butoxyethyl) ether (Wako)
were used for the preparation or catalytic reactions without further
purification.
cis-[PtCl2{PdCl(pp3)}2]Cl2 (7bCl2). 2Cl (0.194 g, 0.229 mmol)
and trans-[PtCl2(NCC6H5)2] (0.054 g, 0.114 mmol) were dissolved
in chloroform (1 cm3). The reaction mixture was allowed to stand
at room temperature for 24 h to give pale-yellow crystals. The
crystals were filtered and recrystallized from acetonitrile. Yield:
90%. Anal. Calcd for C84H84Cl6P8Pd2Pt‚CH3CN‚H2O: C, 51.11;
H, 4.44; N, 0.69. Found: C, 50.52; H, 4.40; N, 0.62. 195Pt{1H}
NMR (CHCl3): δ 289.29 (t, 1JPt-P ) 3690 Hz). The single crystals
were obtained by recrystallization from acetonitrile.
Preparation of Complexes. Mononuclear Palladium(II) Com-
plexes. [Pd(4-Cltp)(pp3)](BF4) [1(BF4)],1 [PdCl(pp3)]Cl (2Cl),5 and
trans-[PtCl2{NiCl(pp3)}2](BF4)2 [8(BF4)2]. Dark-brown crystals
were obtained by a procedure similar to that for 6(BF4)2 using 0.099
g (0.12 mmol) of 5(BF4), 0.030 g (0.064 mmol) of trans-[PtCl2-
(NCC6H5)2], and 1.4 cm3 of acetonitrile. Yield: 60%. Anal. Calcd
for C84H84B2Cl4F8P8Ni2Pt‚H2O: C, 50.77; H, 4.36; N, 0.00.
Found: C, 50.65; H, 4.36; N, 0.00.
(2) (a) Nair, P.; Anderson, G. K.; Rath, N. P. Inorg. Chem. Commun.
2003, 6, 1307. (b) Wachtler, H.; Schuh, W.; Ongania, K.-H.; Kopacka,
H.; Wurst, K.; Peringer, P. J. Chem. Soc., Dalton Trans. 2002, 2532.
(c) Annibale, G.; Bergamini, P.; Bertolasi, V.; Besco, E.; Cattabriga,
M.; Rossi, R. Inorg. Chim. Acta 2002, 333, 116.
(3) (a) Tsuji, J. Palladium Reagents and Catalysts; Wiley: Chichester,
U.K., 1995. (b) Heck, R. F. Palladium Reagents in Organic Synthesis;
VCH: Weinheim, Germany, 1996. (c) Beletskaya, I. P.; Cheprakov,
A. V. Chem. ReV. 2000, 100, 3009.
(4) Churchill, M. R.; Julis, S. A.; Rotella, F. J. Inorg. Chem. 1977, 16,
1137.
(6) King, R. B.; Kapoor, R. N.; Saran, M. S.; Kapoor, P. N. Inorg. Chem.
1971, 10, 1851.
(7) Hohman, W. H.; Kountz, D. J.; Meek, D. W. Inorg. Chem. 1986, 25,
(5) Aizawa, S.; Iida, T.; Funahashi, S. Inorg. Chem. 1996, 35, 5163.
616.
4860 Inorganic Chemistry, Vol. 45, No. 12, 2006

Phosphine-Bridged Mixed-Metal Polynuclear Complexes
Table 1. Crystallographic Data for 6(BF4)2‚2CHCl3, 7a(BF4)2‚H2O‚CHCl3, and 7bCl2‚H2O‚CH3CN
6(BF4)2‚2CHCl3
C98H94B2Cl10F8P8Pd2PtS2
2519.76
triclinic
7a(BF4)2‚H2O‚CHCl3
C85H87B2Cl7F8OP8Pd2Pt
2202.08
orthorhombic
Pbca
28.276(4)
23.610(4)
13.933(3)
7bCl2‚H2O‚CH3CN
C92H98Cl6N4OP8Pd2Pt
2144.21
monoclinic
P21/c
20.121(6)
27.80(1)
18.758(5)
formula
fw
cryst syst
space group
a, Å
P1h
15.510(2)
15.854(3)
22.923(3)
97.53(1)
102.72(1)
96.58(1)
5390(1)
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
114.06(2)
9301(2)
4
1.572
2.275
0.075
0.117
9579(5)
4
1.487
2.169
0.052
0.102
2
D
calcd, g cm-3
1.552
2.082
0.050
0.086
µ, cm-1
R1a
wR2b
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2, where w ) 1/σ2(Fo)2.
2
2
2
trans-[Pt(4-Cltp)2{PdCl2(pp3)RhCl2Cp*}2] (9). To a suspension
containing 6(BF4) (0.084 g, 0.037 mmol) and [Rh2Cl2(µ-Cl)2Cp*2]
(0.042 g, 0.068 mmol) in chloroform was gradually added a
chloroform solution of Bu4NCl (0.037 g, 0.133 mmol). This mixture
was stirred at 30 °C for several minutes and then filtered. The filtrate
was concentrated to ca. 3 cm3 followed by the addition of ethanol
and a small amount of water to give pale-red crystals. Yield: 29%.
Anal. Calcd for C116H122Cl10P8Pd2PtRh2S2‚2CHCl3: C, 46.70; H,
4.12; N, 0.00. Found: C, 46.80; H, 4.34; N, 0.00.
trans-[PtCl2{PdCl2(pp3)RhCl2Cp*}2] (10a). Pale-red crystals
of 10a were obtained by a procedure similar to that of 9 using
0.084 g (0.040 mmol) of 7a(BF4)2, 0.045 (0.073 mmol) of [Rh2-
Cl2(µ-Cl)2Cp*2], and 0.041 g (0.15 mmol) of Bu4NCl. Yield: 55%.
Anal. Calcd for C104H114Cl10P8Pd2PtRh2‚H2O: C, 48.08; H, 4.50;
N, 0.00. Found: C, 47.99; H, 4.48; N, 0.00.
cis-[PtCl2{PdCl(pp3)RhCl2Cp*}2] (10b). 7bCl2 (0.029 g, 0.015
mmol) and [Rh2Cl2(µ-Cl)2Cp*2] (0.010 g, 0.017 mmol) were
dissolved in chloroform (10 cm3) at 30 °C. To the resultant red
solution was added diethyl ether, and the mixture was kept in a
refrigerator to give pale-red crystals. Yield: 55%. Anal. Calcd for
Figure 1. ORTEP diagram of a complex cation of 6.
1
Measurements. 195Pt, 31P, and H NMR spectra were recorded
on a JEOL JNM-A400 FT-NMR spectrometer operating at 85.48,
160.70, and 399.65 MHz, respectively. To determine the chemical
shifts of 195Pt and 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 containing deuterated water as a lock solvent and
K2[Pt(CN)4] and phosphoric acid as references, respectively. The
C
104H114Cl10P8Pd2PtRh2‚2H2O: C, 47.86; H, 4.56; N, 0.00.
Found: C, 47.48; H, 4.36; N, 0.00.
1
crystallizing solvents in the crystals were confirmed by H NMR.
X-ray Structural Determinations. The crystal and molecular
structures of 6(BF4)2, 7a(BF4)2, and 7bCl2 have been determined
by X-ray diffraction methods (Table 1). Each crystal was sealed in
a Lindemann glass-capillary tube with the mother liquor. Intensity
data of 6(BF4)2 and 7a(BF4)2 were collected on a Rigaku AFC-5R
diffractometer using Mo KR radiation (0.710 73 Å) in ω-2θ and
ω scan modes, respectively, and those of 7bCl2 were collected on
a Rigaku AFC-7R diffractometer in the ω scan mode. All data were
corrected for Lorentz and polarization effects, and an absorption
correction based on ψ scans was applied for 7a(BF4)2 and 7bCl2.
The structures were solved using direct methods8 and refined by
full-matrix least-squares procedures on F2 using the CrystalStructure
program.9-11 All non-hydrogen atoms were refined anisotropically,
and all of the hydrogen atoms were refined using a riding method.
UV-vis absorption spectra were recorded on a Perkin-Elmer
Lambda 19 spectrophotometer.
General Procedure for the C-C Coupling Reaction. Reactions
of iodobenzene (28.2 mmol) with styrene (42.2 mmol) in N,N-
dimethylformamide (3 cm3) were carried out under nitrogen at 125
°C in the presence of tributylamine (75 mmol) as a base and the
mononuclear or polynuclear complexes (0.035 mmol for 2Cl, 3Cl,
and 4; 0.017 mmol for 7a(BF4)2, 7bCl2, 10a, and 10b) as catalysts.
1
The yields were calculated by the H NMR intensity of the ortho
protons of trans-stilbene formed on the basis of the intensity of
the ethylene protons of bis(2-butoxyethyl) ether contained as an
internal reference and followed as a function of time.
Results and Discussion
(8) Sheldrick, G. M. SHELXS-86; University of Go¨ttingen: Go¨ttingen,
Germany, 1986. Sheldrick, G. M. SHELXS-97; University of Go¨ttin-
gen: Go¨ttingen, Germany, 1997.
(9) CrystalStructure 3.5.1; Crystal Structure Analysis Package, Rigaku
and Rigaku/MSC, 2000-2003.
(10) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.
CRYSTALS, Issue 10; Chemical Crystallography Laboratory: Oxford,
U.K., 1996.
Structure of Complexes. The linear trinuclear structures
with alternate metal arrangements of Pd(II)-Pt(II)-Pd(II)
for 6(BF4)2, 7a(BF4)2, and 7bCl2 were confirmed by X-ray
analyses (Figures 1-3). The crystal structures revealed that
the thiolato ligand in [Pd(4-Cltp)(pp3)](BF4) and chloro
ligands in trans-[PtCl2(NCC6H5)2] were exchanged in the
trinuclear complex 6 and the geometry of trans-[PtCl2-
(NCC6H5)2] was changed to cis geometry on the Pt(II) center
(11) Though full-matrix least-squares refinements for 7a were carried out
considering the disorder of the phenyl group, the result was not
improved with any occupancy factors.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4861

Aizawa et al.
appreciably, while the Pt-Pt-Pt angles for 6 and 7a and
the Pt-Pt-Cl (trans) angles for 7b are within the normal
range (172.8-180.0°) (Table 2). Such a distortion by the
pp3 ligand, which acts as a tridentate ligand in the Pd(II)
terminals, was similarly observed for the square-planar Pd-
12
(II) complexes with the linear tridentate p3 and pp3O1
ligands.
The 31P NMR spectra of the trinuclear complexes 6, 7a,
and 7b in chloroform are consistent with their crystal
structures. By comparison with the 31P NMR spectra of [Pd-
(4-Clpt)(pp3O)]1 (Table 3), the signals in the ranges of
114.4-118.4 ppm and 46.0-47.2 ppm coupled with each
other (3JP-P ) 7.3-7.6 Hz) are reasonably assigned to those
for the central and two terminal P atoms of pp3 coordinated
to the square-planar Pd(II) ion, respectively. The other signals
(12.2-14.3 ppm) with a pair of satellite peaks due to 195Pt
(1JP-Pt ) 2580-3690 Hz) coupled with the central P atom
(3JP-P ) 44-55 Hz) are assigned to the terminal P atoms
bridging to the Pt(II) center. The triplets of 195Pt NMR for 6
and 7b (see the Experimental Section), which show the
Figure 2. ORTEP diagram of a complex cation of 7a.
1
coupling constants equal to the respective JP-Pt values
obtained by 31P NMR, indicate that the two equivalent
terminal P atoms are coordinated to the Pt(II) center. Because
the AA′XX′ spin system is made up of the two chemically
equivalent terminal P atoms (A and A′) bridging to the Pt-
(II) center and the two central P atoms (X and X′) in the
Pd(II) moieties, the virtual 2JP-P coupling through the Pt(II)
center can be observed. In fact, the 31P NMR spectral
simulations13 for the bridging P atoms (Figure 4) revealed
Figure 3. ORTEP diagram of a complex cation of 7b.
Table 2. Average Bond Distances (Å) and Angles (deg) for the
Trinuclear Complexes
6
7a
7b
2
large differences in the JP-P values attributed to the
Pt-Pta
Pt-S
2.300
2.344
2.329
2.252
difference in geometry around the Pt(II) center; that is, the
2
Pt-Cl
Pd-Pc
2.286
2.210
2.341
2.363
2.350
2.216
2.320
2.357
considerably large JP-P values were observed for the trans
a
2.223
2.328
2.368
isomers, 6 (500 Hz) and 7a (400 Hz), and the relatively small
values for the cis isomer, 7b (13 Hz) (Figure 4 and Table
3). Furthermore, the relatively large 1JP-Pt value for 7b (3690
Hz) compared with those for 6 (2767 Hz) and 7a (2580 Hz)
corresponds to the shorter Pt-P bond distances for 7b than
those for 6 and 7a observed in the crystal structures. The
31P NMR signals for 8, which was prepared by the reaction
of trans-[PtCl2(NCC6H5)2] and the trigonal-bipyramidal Ni-
(II) complex 5(BF4) instead of 2(BF4), are consistent with
the phosphine-bridged trinuclear structure with the terminal
and central P atoms on the square-planar Ni(II) ions and the
other terminal P atoms bridging to the Pt(II) center. The
spectral pattern showing the strong coupling through the Pt-
(II) center (2JP-P ) 580 Hz) and the relatively small value
Pd-Pta
Pd-Cl
Pt-Pt-Pta
Pt-Pt-Cla
S-Pt-S
Cl-Pt-Cl
Pt-Pd-Pta
Pc-Pd-Cla
174.6
180.0
98.0
172.8
175.4
180.0
166.4
176.8
86.8
155.6
174.5
159.3
175.3
a Pt and Pc denote the terminal and central P atoms of the pp3 ligand.
in 7b, while the trans geometry was retained in 7a. As shown
in Table 2, the structures of the terminal Pd(II) moieties are
almost the same, having similar average bond distances for
the central and terminal phosphino groups of the pp3 ligand
(Pd-Pc ) 2.210-2.223 Å and Pd-Pt ) 2.320-2.341 Å,
respectively) and the chloro ligands (Pd-Cl ) 2.357-2.368
Å), whereas the bond distances around the Pt(II) center
change appreciably depending on its geometry; namely, the
average Pt-P distances for 6 (2.300 Å) and 7a (2.329 Å)
are obviously longer than that for 7b (2.252 Å), and the
average Pt-Cl distance (2.350 Å) for 7b is elongated
compared with 7a (2.286 Å) because of the trans influence
of the P atoms. Furthermore, it is apparent that the square-
planar structure of the Pd(II) moieties is highly distorted by
the chelate strain of the pp3 ligand compared with that of
the Pt(II) center; that is, the average Pt-Pd-Pt bond angles
for 6, 7a, and 7b (155.6-166.4°) deviate from 180°
1
of JP-Pt (2555 Hz) similar to those for 6 and 7a indicates
the retention of the trans geometry around the Pt(II) center.
The 31P NMR spectrum of 9 in chloroform exhibits four
signals corresponding to the four P atoms of the pp3 ligand,
as shown in Figure 5. Multiplets at 88.6 and 89.5 ppm and
doublets at 68.1 and 68.5 ppm coupled with each other (3JP-P
) 12 Hz) can be assigned to the central and terminal P atoms
on the square-planar Pd(II) ions by comparison with the 31
P
NMR spectrum of [Pd(4-Cltp)2(pp3O2)].1 Signals at 14.3 and
(12) Aizawa, S.; Sone, Y.; Kawamoto, T.; Yamada, S.; Nakamura, M. Inorg.
Chim. Acta, 2002, 338, 235.
(13) gNMR, version 5.0; Adept Scientific plc: Herts, U.K., 2003.
4862 Inorganic Chemistry, Vol. 45, No. 12, 2006

Phosphine-Bridged Mixed-Metal Polynuclear Complexes
Table 3. 31P NMR Data for Trinuclear and Pentanuclear Complexes
[Pd(4-Cltp)-
(pp3O)]+ a
6
7a
7b
8
9
10a
10b
Pt-Ptb
δ
14.3 (t, 2P)
2767
500
14.0 (t, 2P)
2580
400
12.2 (dt, 2P) 14.0 (t, 2P)
14.3, 14.7 (t, 2P)
13.4, 13.7 (t, 2P) 9.8, 10.1 (dt, 2P)
1JP-Pt
2JP-P
3JP-P
δ
3690
13
48
2555
580
45
2647
550
49
2531
365
44
3657
13
52
55
44
PdOc
32.3 (d, P)
3JP-P 33
Pt-Pdd
δ
47.6 (d, 2P) 47.2 (d, 4P)
46.7 (d, 4P)
7.4
46.0 (d, 4P)
7.6
47.7 (d, 4P)
44.3
68.1, 68.5 (d, 2P) 68.4, 68.5 (d, 2P) 68.8, 69.1 (d, 2P)
12
3JP-P 16.5
7.3
12
14
Pc-Pde
Pt-Rhf
δ
107.0 (dt, P) 118.4 (m, 2P) 115.6 (m, 2P) 114.4 (m, 2P) 115.9 (m, 2P) 88.6, 89.5 (m, 2P) 89.3 (m, 2P)
27.6, 27.7 (dd, 2P) 27.8 (dd, 2P)
88.8 (m, 2P)
27.9 (dd, 2P)
144
δ
1JP-Rh
144
54
145
55
3JP-P
52
a Reference 1. b Terminal P atom bridging to the Pt(II) center. c Oxidized P atom. d Terminal P atom in the Pd(II) moiety. e Central P atom in the Pd(II)
moiety. f Terminal P atom bridging to the Rh(III) terminal.
Figure 5. 31P NMR spectrum of 9 in chloroform. Asterisks and ref denote
satellite peaks due to 195Pt and the signal for D3PO4 in the outer D2O,
respectively.
Figure 4. Observed (left) and simulated (right) 31P NMR spectra for the
(III) terminals by comparison with the reported 31P NMR
bridging P atoms of 6 (a), 7a (b), and 7b (c). The simulated spectra are
depicted by using JP-P ) 500 Hz and JP-P ) 55 Hz for 6, JP-P ) 400
chemical shifts and 1JP-Rh values for the Rh(III) complexes
having [RhCl2Cp*L] (L ) diphenylphosphino group) moi-
eties with a three-legged piano-stool structure.14 The 1H NMR
chemical shift (1.34 ppm) and 2JH-Rh value (3.2 Hz) for the
bound Cp* are also in good agreement with those for the
[RhCl2Cp*L] moieties.14 Two sets of the four 31P NMR
signals correspond to formation of the meso and racemic
isomers by combination of the two chiral central P atoms in
the two Pd(II) moieties. The above NMR spectral behaviors
unquestionably indicate that 9 has the Rh(III)-Pd(II)-Pt-
(II)-Pd(II)-Rh(III) pentanuclear structure with trans ge-
ometry of the Pt(II) center and the three-legged piano-stool
structure of the terminal Rh(III) moieties linked by the pp3
ligands on the square-planar Pd(II) ions. The structural
assignments for 10a and 10b were carried out similarly by
the 31P NMR spectra though the 31P NMR signals for meso
2
3
2
3
2
3
Hz and JP-P ) 44 Hz for 7a, and JP-P ) 13 Hz and JP-P ) 48 Hz for
7b.
14.7 ppm showing coupling with 195Pt (1JP-Pt ) 2647 Hz)
and the central P atom (3JP-P ) 49 Hz) are assigned to the
terminal P atoms bridging to the Pt(II) center. The NMR
spectral simulation13 determined the large coupling constant
(2JP-P ) 550 Hz) between the two equivalent terminal P
atoms through the Pt(II) center as obtained for 6, consistent
with the trans geometry around the Pt(II) square plane.
Agreements of the 31P NMR chemical shifts for the terminal
P atoms on the Pt(II) center and the AB splitting pattern of
1H NMR for the bound 4-Cltp- ligands between 6 (δ 6.28
and 6.67, 3JH-H ) 9 Hz) and 9 (δ 6.37 and 6.55, 3JH-H ) 9
Hz) indicate the same trans thiolato structure of the Pt(II)
centers. The other signals at 27.6 and 27.7 ppm coupled with
103Rh (1JP-Rh ) 144 Hz) and the central P atom (3JP-P ) 54
Hz) are assigned to the terminal P atoms bound to the Rh-
(14) Stoppioni, P.; Vaira, M. D.; Maitlis, P. M. J. Chem. Soc., Dalton Trans.
1982, 1147.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4863

Aizawa et al.
and racemic isomers are overlapped in the regions of the
central P atoms in the Pd(II) moieties and the terminal P
atoms in the terminal Rh(III) moieties. The trans and cis
geometries of the Pt(II) centers for 10a and 10b were
demonstrated by the large difference in the 31P NMR
coupling through the Pt(II) center (2JP-P ) 365 Hz for 10a
1
and 13 Hz for 10b) and the difference in the JP-Pt value
(2531 Hz for 10a and 3657 Hz for 10b) similarly to the
geometries for 7a and 7b. These facts indicate that the
bridging reactions of the trinuclear Pd(II)-Pt(II)-Pd(II)
complexes with [Rh2Cl2(µ-Cl)2Cp*2] successfully proceed
to give the pentanuclear structure retaining the geometries
around the Pt(II) center and the three-legged piano-stool Rh-
(III) structure.
Bridging Reaction. We have previously reported the
selective oxidation of the terminal P atom of the pp3 ligand
in the equatorial position of 1(BF4) in acetonitrile and
chloroform and have expected the formation of the square-
planar intermediate with one dissociated pendant phosphino
group even in the inert solvent (I1 in Scheme 1).1 From the
fact that further oxidation of another terminal P atom
proceeds quantitatively by the addition of excess 4-Cltp-, it
is probable that the terminal P atom of [Pd(4-Cltp)(pp3O)]+
is dissociated by the substitution reaction with 4-Cltp- to
form the intermediate I2 in Scheme 1, while the P atoms of
the bidentate pp3O2 ligand in [Pd(4-Cltp)2(pp3O2)] are hardly
dissociated.1 In the present work, we have attempted to
employ the dissociated phosphino groups in the respective
intermediates as linking groups for the stepwise bridging
reactions to the Pt(II) and Rh(III) ions, expecting that the
chloride counteranion of 7b and the chloride anion added in
the preparations for 9 and 10a play the same role as 4-Cltp-
used for the second oxidation step. In fact, the reaction
solutions for the preparation of the mixed-metal polynuclear
complexes, 6(BF4)2, 7a(BF4)2, 7bCl2, 8(BF4)2, 9, 10a, and
10b, exhibit 31P NMR spectra coinciding with those of the
corresponding isolated complexes with little impurity. Fur-
thermore, the 31P NMR spectra of the isolated complexes in
chloroform were not changed at room temperature at least
for several days. These facts indicate that the above poly-
nuclear complexes with intended metal-ion sequences were
formed quantitatively and the isolated complexes are stable
in the inert solvent. In other words, the enthalpy loss by the
Pd(II)-P bond dissociation and the entropic energy loss by
the phosphine bridging are sufficiently compensated for by
the enthalpy gain for the Pt(II)-P or Rh(III)-P bond
formation.
Figure 6. Absorption spectra of a chloroform solution containing 1 (2.3
× 10-5 mol kg-1) and trans-[PtCl2(NCC6H5)2] (1.0 × 10-5 mol kg-1) at
30 s (a), 3600 s (b), and 32400 s (c) after the reaction started. The direction
of the spectral change is denoted by arrows, and the inset shows the spectral
change in the d-d absorption region for the five-coordinate trigonal-
bipyramidal Pd(II) complexes.
chloride ion, the Ni(II)-P bonds were broken by coordination
of the chloride ion. Such a difference in reactivity between
Pd(II) and Ni(II) is attributed to a difference in the relative
bond strength for the coordinated P atoms.
Structural Conversion. Considering the structures of the
final products, additional transformations besides the phos-
phine bridging were required for the formation of 6 and 7b
where the exchange between the thiolato ligand on Pd(II)
and the chloro ligands on Pt(II) proceeded for 6 and the trans-
to-cis geometrical change on the Pt(II) center proceeded for
7b.
The absorption spectral change during the formation
reaction of 6 indicated two-step successive reactions where
the isosbestic points shifted from 262, 325, and 343 nm to
260, 271, 320, and 357 nm. The lower energy d-d absorption
1
band (1A1′ f E′) at around 550 nm characteristic of the
five-coordinate trigonal-bipyramidal Pd(II) complexes5,16
disappeared in the first step and the shoulder at around 440
nm assignable to the d-d absorption band for the square-
planar [palladium(thiolato)(phosphine)3] chromophore12 dis-
appeared in the second step (Figure 6). This spectral behavior
suggests that the bridging of the terminal P of the pp3 ligand
in 1 proceeded initially and the monodentate ligands, 4-Cltp-
on the Pd(II) ion and Cl- on the Pt(II) ion, were exchanged
subsequently. This mechanism was confirmed by a change
The cis isomer of the Ni(II)-Pt(II)-Ni(II) trinuclear
complex and the Rh(III)-Ni(II)-Pt(II)-Ni(II)-Rh(III) pen-
tanuclear complex could not be obtained by a procedure
similar to that for 7bCl2 by using 5Cl instead of 2Cl and
that for 10a by using 8(BF4)2 instead of 7a(BF4)2, respec-
tively, because the phosphine transfer reaction from Ni(II)
onto Pt(II) proceeded to give [PtCl(pp3)]Cl.15 Considering
that the trinuclear complex 8(BF4)2 was formed quantitatively
and was stable in chloroform in the absence of the free
1
in the 31P and H NMR spectra where the NMR signals of
the bound pp3 and 4-Cltp- ligands for the initially formed
trinuclear complex17 decreased and those for the final product
6 increased gradually. Benzonitrile dissociated from Pt(II)
by the initial phosphine-bridging reaction probably acts as
(15) Ferna´ndez, D.; Seijo, M. I. G.; Ke´gl, T.; Peto˜cz, G.; Kolla´r, L.;
(16) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier:
Ferna´ndez, M. E. G. Inorg. Chem. 2002, 41, 4435.
Amsterdam, The Netherlands, 1984; Chapter 6.
4864 Inorganic Chemistry, Vol. 45, No. 12, 2006

Phosphine-Bridged Mixed-Metal Polynuclear Complexes
Scheme 2. Formation Reaction of 6 from 1
Scheme 3. Formation Reaction of 7b via 7a
the attacking ligand that is required for the associative
substitution reaction of the four-coordinate square-planar d8
metal complexes18 (Scheme 2). Furthermore, this fact justi-
fied the two-term rate law, k ) k1 + k2[entering ligand],
with the solvolysis path k1, where k2 is the rate constant for
the direct ligand substitution path, which has been proposed
on the basis of the kinetic results for the ligand substitution
of the square-planar Pd(II) and Pt(II) complexes.18,19 From
a thermodynamic point of view, this exchange reaction
demonstrates that the difference in the affinities for the
thiolato S atom between Pt(II) and Pd(II) is much greater
than that for the chloro ligand. The retention of the trans
geometry around Pt(II) during the ligand exchange indicates
that the total trans influence between the S and P atoms on
the square-planar Pt(II) ion exceeds the total of the trans
influence between the S atoms and that between the P atoms
in 6, considering that little difference in the steric crowding
can be found between the trans and cis geometries.
isomerization proceeded gradually to give 7b quantitatively
in several hours. The isomerization of 7a to 7b was also
completed by the addition of 1 equiv of Bu4NCl to a
chloroform solution of 7a(BF4)2. In contrast, the trans-to-
cis isomerizations of the pentanuclear complex 10a did not
proceed in the presence of excess Bu4NCl (see the Experi-
mental Section). Such a difference in the isomerization
behavior between the trinuclear and pentanuclear complexes
indicates that the geometrical change is initiated by an attack
of the outer Cl- onto the terminal Pd(II) moiety followed
by the dissociation of the terminal phosphino group of the
pp3 ligand acting as tridentate in the Pd(II) terminal. The
terminal phosphino group of the pp3 ligand acting as
bidentate in the Pd(II) moieties in the pentanuclear complex
10b is hardly dissociated by the outer attacking ligands,
considering that the further oxidation of [Pd(4-Cltp)(pp3O2)]
(Scheme 1) did not proceed in the presence of excess
4-Cltp-.1 Such a high reactivity of the tridentate pp3 ligand
on Pd(II) compared with the bidentate pp3 one is attributed
to the trans effect between the terminal P atoms and the
chelate strain of the tridentate pp3 ligand as observed in the
crystal structures in 6, 7a, 7b, and [Pd(4-Cltp)(pp3O)]+.1 The
dissociated terminal phosphino group of the pp3 ligand can
replace the bridging terminal phosphino group of the same
pp3 ligand on the Pt(II) center with the geometrical change
(Scheme 3). The above isomerization behavior of the
trinuclear complex demonstrates that kinetically stable 7a
The reaction solution for the preparation of 7b by using
2Cl exhibited the 31P NMR signals of 7a initially, and the
1
2
(17) 31P{1H} NMR (CHCl3): δ 13.7 (t, 2P, JP-Pt ) 2570 Hz, JP-P
)
450 Hz, 3JP-P ) 44 Hz), 47.8 (d, 4P, 3JP-P ) 16.7 Hz), 109.8 (tt, 2P).
1H NMR (CHCl3): δ 6.72, 6.51 (AB system, JH-H ) 8 Hz).
3
(18) Wilkins, R. W. Kinetics and Mechanism of Reaction of Transition
Metal Complexes, 2nd ed.; VCH: Weinheim, Germany, 1991; Chapter
4. (b) Cross, R. J.; Twigg, M. V. Mechanisms of Inorganic and
Organometallic Reactions; Plenum: New York, 1988; Chapter 5. (c)
Lin, Z.; Hall, M. B. Inorg. Chem. 1991, 30, 646.
(19) Gray, H. B. J. Am. Chem. Soc. 1962, 84, 1548.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4865

Aizawa et al.
bidentate in 10b. Shortening of the induction period and
acceleration of the reaction were the following order:
mononuclear 2Cl < mononuclear 3Cl < trinuclear 7a(BF4)2
and 7bCl2 < pentanuclear 9b and mononuclear 4 (Figure
7), depending on the number of bound P atoms on the Pd-
(II) ions (four for 2, three for 3, 7a, and 7b, and two for 4
and 9b) but not on their polynuclear structures, geometries
around Pt(II), or counterions. Considering that the ease of
the prereduction and the subsequent oxidative addition of
the substrates depend on the coordination environment
around Pd(II),3 it is probable that the phosphine bridging to
the relatively inert metal ions, Pt(II) and Rh(III), is not broken
in the catalytic reaction, maintaining the numbers of the
phosphino groups that can be coordinated to Pd(II). On the
other hand, better retention of the catalytic activity for the
trinuclear complexes 7a(BF4)2 and 7bCl2 compared with the
mononuclear complex 3Cl may be attributed to the effect of
the neighboring Pt(II) ion. Recently, we reported the rapid
equilibrium of the Pt(II) complex with the pp3 ligand between
the five-coordinate trigonal-bipyramidal geometry and the
five-coordinate square-pyramidal one with an iodide ion in
the apical position, [Pt(pt)(pp3)]+ + I- h [PtI(pt)(pp3)] (pt
) 1-propanethiolate).20 This result suggests that the five-
coordinate structures of Pt(II) complexes are considerably
stabilized by the soft P donors. Because at least one of the
three bound P atoms on the Pd(II) ion in 3Cl, 7a(BF4)2, and
7bCl2 needs to be dissociated during the process from the
oxidative addition of aryl halide to the reductive elimination
of the coupled product,3 it is possible that the Pt(II) center
in the trinuclear complexes attaches the dissociated phos-
phino group in the apical position in the catalytic cycle. Such
an interaction between the Pt(II) ion and the P atom may
assist the acceleration of the catalytic reaction and protect
the dissociated phosphino group against oxidation not to form
phosphine oxide. This can be regarded as one of the
cooperative effects of the bridged metal ion.
Figure 7. Changes in the yield of stilbene with time in Heck reactions
catalyzed by 2Cl, 3Cl, 4, 7a(BF4)2, 7bCl2, and 10b.
in the absence of the entering ligand was converted into the
thermodynamically more stable isomer 7b, avoiding the trans
influence between the P atoms on the Pt(II) center. It is
noteworthy that the trans and cis isomers of the trinuclear
and pentanuclear complexes are formed quantitatively and
prepared selectively simply by changing the counteranion
of the starting Pd(II) complexes.
C-C Coupling Reaction. It is important to confirm
whether the bridging structures of the mixed-metal complexes
are maintained in the employed reaction system in order to
investigate cooperative effects of the different metal ions.
We adopted the Heck-type C-C coupling reaction3b,c that
is one of the most widespread Pd-catalyzed reactions.
Because it is difficult to observe the transient structures of
catalytically active species under the usual conditions of the
catalytic reactions, we intended to obtain some structural
information about the active species from differences in
catalytic activity.
The catalytic activities of the chloro mononuclear (2Cl),
trinuclear [7a(BF4)2 and 7bCl2], and pentanuclear (10b)
complexes were compared to one another using iodobenzene
and styrene as the substrates under the same conditions in
which the concentrations of the catalysts were normalized
by using the concentrations of Pd(II) (see the Experimental
Section). Furthermore, the catalytic activities of the mono-
nuclear complexes with the tridentate p3 ligand (3Cl) and
the bidentate p2 ligand (4) are also compared with those of
7a(BF4)2, 7bCl2, and 10b because the pp3 ligands in the Pd-
(II) moieties act as tridentate in 7a(BF4)2 and 7bCl2 and
Supporting Information Available: X-ray crystallographic
information including final atomic coordinates, anisotropic thermal
parameters, and full bond lengths and angles in CIF format. This
material is available free of charge via the Internet at http://
pubs.ac.org.
IC060277L
(20) Aizawa, S.; Kobayashi. T.; Kawamoto, T. Inorg. Chim. Acta 2005,
358, 2319.
4866 Inorganic Chemistry, Vol. 45, No. 12, 2006