New insights into structures, stability, and bonding of μ-allyl ligands coordinated with Pd-Pd and Pd-Pt fragments

Article abstract of DOI:10.1021/om950843c

A series of complexes of μ-allyl ligands coordinated with the Pd-Pd fragment Pd₂(μ-allyl)-(μ-Cl)(PPh₃)₂ were prepared from reactions of the corresponding allylpalladium(II) chlorides with Pd(C₂H₄)(PPh₃)₂. The μ-allyl ligands employed contained both electron-withdrawing (Cl, CN, COOMe, SO₂Ph) and electron-donating (Me, Ph) substituents at either the terminal or central carbon of the allyl framework. The relative ability of a substituted allyl ligand to coordinate to the Pd(I)-Pd(I) fragment vs the mononuclear Pd(II) fragment was determined by means of an allyl ligand exchange equilibrium between the dinuclear and the mononuclear fragments. The allyl ligand bearing the more withdrawing substituent coordinated to the Pd-Pd fragment more strongly than that bearing the less withdrawing substituent. The 1-methyl, 1-phenyl, and 1-chloroallyl ligands on the Pd-Pd bond exist in an anti configuration almost exclusively. X-ray structure determinations of some of the μ-allyl Pd-Pd and Pd-Pt complexes, PdM(μ-allyl)(μ-X)(PPh₃)₂ (11, M = Pt, allyl = CH₂C(COOMe)CH₂, X = Br; 12, M = Pd, allyl = CH₂C(COOMe)CH₂, X = Br; 13, M = Pd, allyl = CH₂CHCH(COOMe), X = SPh) were carried out. The crystal of 11 was isomorphous with that of 12. The results revealed a unique geometrical feature of the bridging allyl ligand; the dihedral angle between the allyl plane and the approximate coordination plane (M-M-X plane) is smaller than 90°. Ab initio MO calculations on the model Pd₂(μ-CH₂CHCH₂)(μ-Br)(PH ₃)₂ were performed to reveal important aspects of the bonding nature; the coordinate bond of the μ-allyl ligand involved not only donation from the allyl nonbonding π orbital to the unoccupied dσ-dσ antibonding orbital of the Pd₂(μ-Br)(PH₃)₂ unit but also back-donation from the occupied dσ-dσ and dπ-dπ bonding combinations of the Pd₂(μ-Br)(PH₃)₂ unit to the allyl π* orbital. The latter is not observed in the mononuclear palladium(II) allyl complex but is found as a characteristic interaction in the dinuclear μ-allyl complex. The above-mentioned unique geometrical and stability features of the dinuclear μ-allyl complexes arise from the back-bonding interaction.

Full text of DOI:10.1021/om950843c

Organometallics 1996, 15, 2089-2097
2089
New In sigh ts in to Str u ctu r es, Sta bility, a n d Bon d in g of
µ-Allyl Liga n d s Coor d in a ted w ith P d -P d a n d P d -P t
F r a gm en ts
Hideo Kurosawa,* Kazuyoshi Hirako, Satoko Natsume, Sensuke Ogoshi,
Nobuko Kanehisa, and Yasushi Kai
Department of Applied Chemistry, Osaka University, Suita, Osaka, J apan
Shigeyoshi Sakaki* and Kaori Takeuchi
Department of Applied Chemistry, Kumamoto University, Kumamoto, J apan
Received October 24, 1995X
A series of complexes of µ-allyl ligands coordinated with the Pd-Pd fragment Pd2(µ-allyl)-
(µ-Cl)(PPh3)2 were prepared from reactions of the corresponding allylpalladium(II) chlorides
with Pd(C2H4)(PPh3)2. The µ-allyl ligands employed contained both electron-withdrawing
(Cl, CN, COOMe, SO2Ph) and electron-donating (Me, Ph) substituents at either the terminal
or central carbon of the allyl framework. The relative ability of a substituted allyl ligand to
coordinate to the Pd(I)-Pd(I) fragment vs the mononuclear Pd(II) fragment was determined
by means of an allyl ligand exchange equilibrium between the dinuclear and the mononuclear
fragments. The allyl ligand bearing the more withdrawing substituent coordinated to the
Pd-Pd fragment more strongly than that bearing the less withdrawing substituent. The
1-methyl, 1-phenyl, and 1-chloroallyl ligands on the Pd-Pd bond exist in an anti configuration
almost exclusively. X-ray structure determinations of some of the µ-allyl Pd-Pd and Pd-
Pt complexes, PdM(µ-allyl)(µ-X)(PPh3)2 (11, M ) Pt, allyl ) CH2C(COOMe)CH2, X ) Br; 12,
M ) Pd, allyl ) CH2C(COOMe)CH2, X ) Br; 13, M ) Pd, allyl ) CH2CHCH(COOMe), X )
SPh) were carried out. The crystal of 11 was isomorphous with that of 12. The results
revealed a unique geometrical feature of the bridging allyl ligand; the dihedral angle between
the allyl plane and the approximate coordination plane (M-M-X plane) is smaller than
90°. Ab initio MO calculations on the model Pd2(µ-CH2CHCH2)(µ-Br)(PH3)2 were performed
to reveal important aspects of the bonding nature; the coordinate bond of the µ-allyl ligand
involved not only donation from the allyl nonbonding π orbital to the unoccupied dσ-dσ
antibonding orbital of the Pd2(µ-Br)(PH3)2 unit but also back-donation from the occupied
dσ-dσ and dπ-dπ bonding combinations of the Pd2(µ-Br)(PH3)2 unit to the allyl π* orbital.
The latter is not observed in the mononuclear palladium(II) allyl complex but is found as a
characteristic interaction in the dinuclear µ-allyl complex. The above-mentioned unique
geometrical and stability features of the dinuclear µ-allyl complexes arise from the back-
bonding interaction.
Increasing attention has been paid to structures and
bonding of hydrocarbon ligands which coordinate to
multinuclear metal centers in a bridging manner be-
cause of their close bearing with metal surface-
hydrocarbon interaction models.1 Several complexes of
µ-allyl ligands bound on metal-metal fragments have
beeen prepared,2,3 and a simple view of their bonding
nature was also presented.2c,e,3d,f However, it seems that
little has been clarified concerning the bonding and
structrual aspects intrinsic to the µ-allyl ligand, espe-
cially when these were compared with the aspects
intrinsic to the extensively studied η3-allyl ligand bound
to mononuclear metal centers.4 By mononuclear com-
plex we mean those which contain no metal-metal
bonds.
Werner and his co-workers have carried out extensive
studies on synthesis and structures of µ-allyl complexes
with Pd-Pd, Pd-Pt and Pt-Pt fragments,3f but the
allyl ligands they employed were mostly restricted to
those of a simple structure type such as unsubstituted
X Abstract published in Advance ACS Abstracts, March 15, 1996.
(1) (a) Beck, W.; Niemer, B.; Wieser, M. Angew. Chem., Int. Ed. Engl.
1993, 32, 923-1110 and references therein. (b) Wadepohl, H. Angew.
Chem., Int. Ed. Engl. 1992, 31, 247-262. (c) Casey, C. P.; Audett, J .
D. Chem. Rev. 1986, 86, 339-352. (d) Stone, F. G. A. Angew. Chem.,
Int. Ed. Engl. 1984, 23, 89-99. (e) Holton, J .; Lappert, M. F.; Pearce,
R.; Yarrow, P. I. W. Chem. Rev. 1983, 83, 135-201.
(2) (a) Chihara, T.; Yamazaki, H. J . Chem. Soc., Dalton Trans. 1995,
1369-1377. (b) J . Organomet. Chem. 1992, 428, 169-186. (c) House-
croft, C. E.; J ohnson, B. F. G.; Lewis, J .; Lunnis, J . A.; Owen, S. M.;
Raithby, P. R. J . Organomet. Chem. 1991, 409, 271-284. (d) Chihara,
T.; Aoki, K.; Yamazaki, H. J . Organomet. Chem. 1990, 383, 367-385.
(e) Chisholm, M. H.; Hampden-Smith, M. J .; Huffman, J . C.; Moodley,
K. G. J . Am. Chem. Soc. 1988, 110, 4070-4071. (f) Bruce, M. I.;
Williams, M. L. J . Organomet. Chem. 1985, 288, C55-C58.
(3) (a) Osakada, K.; Ozawa, Y.; Yamamoto, A. J . Organomet. Chem.
1990, 399, 341-348. (b) Osakada, K.; Chiba, T.; Nakamura, Y.;
Yamamoto, T.; Yamamoto, A. Organometallics 1989, 8, 2602-2605.
(c) Hayashi, Y.; Matsumoto, K.; Nakamura, Y.; Isobe, K. J . Chem. Soc.,
Dalton Trans. 1989, 1519-1525. (d) Zhu, L.; Kostic, N. M. Organo-
metallics 1988, 7, 665-669. (e) Sieler, J .; Helms, M.; Gaube, W.;
Svensson, A.; Lindqvist, O. J . Organomet. Chem. 1987, 320, 129-136.
(f) Werner, H. Adv. Organomet. Chem. 1981, 19, 155-182. (g)
Kobayashi, Y.; Iitaka, Y.; Yamazaki, H. Acta Crystallogr. 1972, B28,
899-906.
(4) (a) Collman, J . P.; Hegedus, L. S.; Norton, J .; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
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M. D.; Eisenstein, O. Organometallics 1984, 3, 887-895.
0276-7333/96/2315-2089$12.00/0 © 1996 American Chemical Society

2090 Organometallics, Vol. 15, No. 8, 1996
Kurosawa et al.
and, at most, 2-alkyl-substituted allyls. Further, the
supporting phosphine ligands in their studies, e.g.
tricyclohexylphosphine, appear to us not suitable, in a
regard to the allylic carbon upon reaction of Pd(C2H4)-
(PPh3)2 with [Pd(η3-allyl)(Cl)]2 (allyl ) trans-(5-(meth-
oxycarbonyl)((1-3-η)-cyclohexenyl) is in marked con-
trast to the inversion of stereochemistry brought about
by the exo attack of Pd(C2H4)(PPh3)2 at the allyl ligand
in the cation [Pd(η3-allyl)(PPh3)2]+.6
1
sense, for a detailed H NMR study through which more
detailed knowledge with regard to the solution proper-
ties can be obtained. In order to gain a deep insight
into the precise bonding nature between the µ-allyl
ligand and the metal-metal fragment, it is necessary
to synthesize a series of µ-allyl complexes which contain
a wide variety of substituents at the allyl carbon and
those supporting ligands that exhibit NMR spectroscopic
features as simple as possible. We wish to describe here
the synthesis of a series of complexes of µ-allyl ligands
coordinated to the Pd-Pd and Pd-Pt fragments with
PPh3 as a supporting ligand and novel aspects of their
solution stabilities and structures, both in the solid state
and in solution. These features will be explained in
terms of the unique nature of bonding between the
µ-allyl ligand and the metal-metal fragment deduced
from MO calculations.
The reaction of an allylplatinum(II) complex contain-
ing a PPh3 ligand with Pd2(dba)3 afforded a µ-allyl
complex with the Pd-Pt bond, 11, in 53% yield (eq 2).
Resu lts
Syn th esis of Com p lexes. According to the general
procedure developed by Werner and co-workers,3f we
synthesized a series of µ-allyl complexes of the Pd-Pd
fragment containing a PPh3 ligand, 1-9, in moderate
yields through reactions between (η3-allyl)palladium-
(II) chlorides and a Pd(0)-ethylene complex (eq 1).
This was considerably stable even if left at room
temperature both in the solid and in solution. Note-
worthy here is that the reaction with the reverse
combination, namely the reaction between the corre-
sponding (2-(methoxycarbonyl)allyl)palladium(II) bro-
mide and Pt(C2H4)(PPh3)2, did not lead to a satisfactory
result; the desired heteronuclear complex was always
contaminated by a comparable amount of the Pd-Pd
analog Pd2[µ-CH2C(COOMe)CH2](µ-Br)(PPh3)2 (12). We
cannot explain why this is so.
X-r a y Str u ctu r e Deter m in a tion . Crystals of 11
suitable for X-ray diffraction analysis were obtained and
subjected to structure determination. In the crystals
of 11, Pd and Pt atoms were disordered in equivalent
occupancy, so that the structure was solved by applying
the atomic scattering factor of Sm, which has an electron
number corresponding to an average of those of Pd and
Pt atoms, for the scattering factor of the metal atoms.
The structures of 12 and of the µ-SPh analog of 4, Pd2[µ-
CH2CHCH(COOMe)](µ-SPh)(PPh3)2 (13), prepared in a
manner similar to eq 1, were also determined by the
X-ray diffraction method. The structures of the three
complexes are shown in Figures 1-3. Table 1 sum-
marizes some relevant bond lengths and angles for these
three complexes and, for comparison, Pd2(µ-CH2CHCH2)-
(µ-Cl)(PPh3)2 (1), determined previously.3e The struc-
tures of the two µ-2-(methoxycarbonyl)allyl complexes
containing the Pd-Pd bond (12) and the Pd-Pt bond
(11) show almost identical geometrical parameters with
regard to the M-allyl bonding. Comparison of the
structure of 12 with the unsubstituted analog 1 also
The complexes containing electron-withdrawing sub-
stituents at the allyl carbon were somewhat easier to
handle than the others, which when kept in solution at
room temperature tended to decompose in a few hours.
The Pd-Pd complexes of µ-allyl ligands containing
electron-donating substituents at the center allyl car-
bon, Pd2(µ-CH2CRCH2)(µ-Cl)(PPh3)2 (R ) Me, OCH2-
PH), were difficult to isolate pure by eq 1 but were
unambiguously characterized by 1H NMR spectroscopy
(see Experimental Section). The 1H NMR spectra of all
the complexes were straightforwardly interpreted; of
particular note was the elucidation of the ratio of syn
and anti isomers of the 1-substituted allyl derivatives
(see later).
The reaction of trans-(5-(methoxycarbonyl)(1-3-η)-
cyclohexenyl)palladium chloride5 with the Pd-ethylene
complex afforded a good yield of the corresponding
dipalladium complex 10, in which the Pd2(µ-Cl)(PPh3)2
moiety lies opposite to the COOMe group with respect
to the cyclohexenyl ring, as comfirmed by NOE experi-
ments. Thus, irradiation of the ortho proton signal of
the P-Ph groups resulted in a 9% increase of the
resonance attributable to the hydrogen geminal to the
COOMe group. This retention of stereochemistry with
(5) Kurosawa, H.; Kajimaru, H.; Ogoshi, S.; Yoneda, H.; Miki, K.;
Kasai, N.; Murai, S.; Ikeda, I. J . Am. Chem. Soc. 1992, 114, 8417-
8424.
(6) (a) Granberg, K. L.; Backvall, J . J . Am. Chem. Soc. 1992, 114,
6858-6863. (b) Kurosawa, H.; Ogoshi, S.; Chatani, N.; Kawasaki, Y.;
Murai, S.; Ideda, I. Chem. Lett. 1990, 1745-1748.

µ-Allyl Ligands Coordinated with Pd-Pd and Pd-Pt
Organometallics, Vol. 15, No. 8, 1996 2091
Ta ble 1. Releva n t Bon d Len gth s (Å) a n d An gles
(d eg)a
11
12
13
1b
MP2 calcdc
M-M
M-Ct
2.675(1) 2.642(1) 2.625(2) 2.623(1) 2.635 (2.636)d
2.05(1)
2.06(1)
2.075(7) 2.07(2) 2.07(1) 2.115 (2.113)
2.088(7) 2.10(2) 2.07(1)
M-Cc
M-X
M-P
2.38(1)
2.55(1)
2.421(7) 2.43(2) 2.45(1) 2.518 (2.515)
2.518(7) 2.47(2) 2.46(1)
2.531(1) 2.5295(9) 2.358(5) 2.433(3) 2.624 (2.624)
2.542(2) 2.537(1) 2.371(5) 2.438(3)
2.235(3) 2.270(2) 2.262(5) 2.285(3) 2.402 (2.398)
2.251(3) 2.284(2) 2.279(4) 2.287(3)
Ct-Cc
1.40(2)
1.46(2)
1.422(9) 1.41(2) 1.37(2) 1.427 (1.427)
1.435(9) 1.41(2) 1.40(2)
Ct-Cc-Ct 126(1)
123.3(8) 124(1) 133.7(9) 126 (126)
F igu r e 1. Molecular structure of 11 with thermal el-
lipsoids at 50% probability levels.
P-M-M 167.05(9) 167.90(6) 160.1(1) 166.5(1) 173 (172)
173.65(9) 173.58(6) 167.5(1) 173.0(1)
M-X-M 63.64(4) 62.88(3) 67.4(1) 65.2(1) 60 (60)
a
Ct terminal carbon of allyl; Cc ) center carbon of allyl.
b Reference 3e. c MP2 calculation for Pd2(µ-CH2CHCH2)(µ-Br)(PH3)2.
d
In parentheses: a triple-ú basis function was used for d electrons
of Pd without any other modification on the other basis functions.
Ta ble 2. Atom ic Devia tion s (Å) fr om th e M-M-X
P la n e a n d Dih ed r a l An gles (d eg) betw een th e
M-M-X a n d Allyl P la n esa
11
12
13
1b
MP2 calcdc
Ct
0.019
-0.136
0.013
-0.187
0.010
0.023
0.042
-0.011
0.022 (0.023)d
Cc
P
0.584
0.585
0.645
0.551
0.669 (0.669)
-0.244
-0.357
-0.251
-0.351
-0.447
-0.539
-0.231
-0.456
-0.31 (-0.31)
θ
84.2
83.7
79.5
82.1
83 (83)
a
Ct ) terminal carbon of allyl; Cc ) center carbon of allyl; θ )
b
F igu r e 2. Molecular structure of 12 with thermal el-
lipsoids at 50% probability levels.
dihedral angle between M-M-X and Ct-Cc-Ct planes. Calcu-
lated by using atomic coordinates reported in ref 3e. c MP2
d
calculation for Pd2(µ-CH2CHCH2)(µ-Br)(PH3)2. See footnote d of
Table 1.
the atomic coordinates reported.3e First, two terminal
carbons (Ct) of the allyl ligand lie almost on the M-M-X
plane, while in the mononuclear complexes they lie off
the coordination plane with the center of gravity of the
allyl triangle being on the coordination plane,7 even
though the vigorous definition of the coordination plane
may be somewhat difficult in the dinuclear complexes.
For example, the P atoms deviate from the M-M-X
plane to some extent, in addition to the downward
bending from the horizontal line connecting two metal
atoms. Apparently these movements of the P atoms are
due to repulsion between the phosphine and the allyl
ligands.
F igu r e 3. Molecular structure of 13 with thermal el-
Second, Table 2 shows that the dihedral angle (θ;
Chart 1) between the allyl plane and the M-M-X plane
is smaller than 90° where the allyl central carbon leans
toward the M-M bond. In contrast, it is well-known
that in mononuclear η3-allyl complexes of Pd and Pt the
allyl plane makes an angle larger than 110° with the
metal coordination plane.7 Thus, the direction of lean-
ing of the allyl plane in the dinuclear µ-allyl complexes
is opposite to that in the mononuclear η3-allyl com-
plexes.
lipsoids at 50% probability levels.
suggests very little geometrical difference between them
except for somewhat longer C-C bonds of the allyl
framework in 12 than those in 1.
We emphasize some unique structure aspects common
to the µ-allyl complexes of the Pd-Pd and Pd-Pt
fragments which have received less attention in the
related, previous structures studies3a,c,e,f,g but which we
believe are key to understanding the metal-allyl bond-
ing nature. These concern the spatial dispositions of
some donor atoms relative to the M-M-X plane (X )
Cl, Br, S), as summarized in Table 2. The data in the
table include those for 1 which we recalculated by using
(7) (a) Maitlis, P. M.; Espinet, P.; Russell, M. J . In Comprehensive
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W.,
Eds.; Pergamon: New York, 1982; Vol. 6, Chapter 38.7. (b) Hartley,
F. R. In ref 7a, Vol. 6, Chapter 39.

2092 Organometallics, Vol. 15, No. 8, 1996
Kurosawa et al.
Ta ble 4. Equ ilibr iu m Con sta n ts (K) of eq 3
Ch a r t 1
R
K
R
K
1-Me
1-Ph
2-OCH2Ph
2-Me
1-COOMe
1.1 × 10-2
1.7 × 10-1
9.2 × 10-3
1.5 × 10-2
1.5 × 10
1-Cl
2-Cl
2-CN
2-COOMe
2-SO2Ph
1.2 × 102
1.4 × 102
2.5 × 102
3.6 × 102
9.8
in eq 3. However, the reaction of each dinuclear
complex with a half equimolar allylpalladium(II) chlo-
ride dimer led to decomposition of the former dinuclear
complex into palladium metal, the remaining metal
moiety being identified as Pd(η3-allyl)(Cl)(PPh3). Equili-
bration in eq 3 was complete within 5 min after mixing
the substrates at 25 °C.
Ta ble 3. Isom er Ra tio of 1-Su bstitu ted Allyl
Com p lexes in CDCl3
1H NMR resonances of the four complexes involved
in the equilibrium can be separately discerned, sug-
gesting that the rate of allyl ligand exchange is slower
than the NMR time scale. Yet spin saturation transfer
experiments on the system R ) H in eq 3 qualitatively
R
X
anti/syn
R
X
anti/syn
Me
Me
Me
Cl
Br
I
97/3
96/4
100/0
Ph
Cl
COOMe
Cl
Cl
Cl
100/0
100/0
21/79
An ti-Syn Isom er ism in Solu tion . The ratios of
the anti and syn isomers of 1-substituted µ-allylic
complexes are summarized in Table 3. Intriguingly,
these complexes showed a strong preference for the anti
configuration, except for the 1-COOMe analog. All the
configurations were unambiguously established on the
basis of spin-spin coupling constants showing a larger
value for the central proton-anti proton coupling than
for the central proton-syn proton coupling. Moreover,
the anti configurations of 2, 3, and 5 were also confirmed
by NOE experiments; irradiation at the methyl reso-
nance of the major isomer of 2 resulted in a 6% increase
of the anti proton resonance at the other allyl end, and
irradiation at the resonance for the proton geminal to
the 1-Ph and 1-Cl substituents led to 7% and 12%
increases of the central proton resonance, respectively.
The anti preference of complexes 2, 3, and 5 is in
marked contrast to trends in mononuclear η3-allyl
systems;8 for example, the corresponding complexes
[Pd(η3-CH2CHCHR)(Cl)]2 (R ) Me, Ph, COOMe) existed
in an almost exclusive syn form and the 1-chloro analog
showed an anti/syn ratio of 11/89. The data in Table 3
correspond to those of thermodynamic origin, since an
NMR monitoring of the reaction between [Pd(η3-CH2-
CHCHCl)(Cl)]2 and Pd(C2H4)(PPh3)2 allowed the syn
isomer of 5 to be detected at the early stages, which
gradually isomerized to the anti isomer. A similar syn
to anti isomerization has been found3b in the µ-1-
methylallyl complex Pd2(µ-CH2CHCHMe)(µ-SPh)(P-
Cy3)2, which likewise showed a high thermodynamic
anti/syn ratio (90/10).
suggested the rate of allyl exchange to be on the order
of 1H spin relaxation time; irradiation at the resonance
frequency for anti-H of 1 (δ 1.48) resulted in a decrease
(ca. 16%) of the signals due to both anti and syn H of
Pd(η3-CH2CHCH2)(Cl)(PPh3) (δ 2.7-4.8). Such satura-
tion transfer from anti to syn protons suggests that
moderately fast syn-anti proton exchange in the com-
plexes involved may also be taking place. Currently we
cannot offer mechanisms of the allyl ligand exchange
shown in eq 3.
Whatever the mechanism is, the substituent effect on
the preferred coordination of an allyl ligand to the Pd-
Pd fragment was deduced by taking advantage of such
allyl group exchange. The equilibrium constant of eq 3
for a given R was determined as an average of more
than three measurements, each measurement employ-
ing a different ratio of composites. The K values for
several R substituents are summarized in Table 4, and
a plot of log K against Hammett’s σ parameter for a
series of 2-substituted allyl complexes is shown in
Figure 4.
Each point in Figure 4 shows no significant deviation
from the least-squares line, suggesting a relatively
insignificant steric effect of the substituent at the
central carbon in determining the relative stability. Of
particular note in Figure 4 is the large F value (4.76).
Moreover, there was observed a closely related trend,
with the 1-substituted analogues showing larger K
values for the complexes having the more electron-
withdrawing substituents, as shown in Table 4. These
results suggest that the allyl ligand containing the
electron-withdrawing substituent at both 1- and 2-posi-
tions of the allyl framework coordinates to the Pd-Pd
fragment more strongly than the allyl ligand containing
the electron-donating substituent does, while the allyl
Allyl Liga n d Exch a n ge betw een Mon o- a n d Di-
n u clea r Com p lexes. When the (µ-allyl)dipalladium
complexes were kept in contact with mononuclear
allylpalladium(II) chloride complexes containing one
PPh3 ligand, facile allyl exchange took place as shown
(8) For a recent strategy to unusual anti complexes of mononuclear
systems, see: (a) Sjogren, M.; Hansson, S.; Norrby, P. O.; Akermark,
B.; Cucciolito, M. E.; Vitagliano, A. Organometallics 1992, 11, 3954-
3964. (b) Ward, Y. D.; Villanueva, L. A.; Allred, G. D.; Payne, S. C.;
Semones, M. A.; Liebeskind, L. S. Organometallics 1995, 14, 4132-
4156.

µ-Allyl Ligands Coordinated with Pd-Pd and Pd-Pt
Organometallics, Vol. 15, No. 8, 1996 2093
Ch a r t 2
F igu r e 4. Plot of log K against Hammett σ constants for
a series of 2-substituted allyl complexes. The σ value for
the OCH2Ph groups is replaced by that for the OCH3 group.
ligand coordination to the mononuclear Pd(II) fragment
might have a much diminished, if any, substituent
dependency of similar trend or might be affected by the
substituent in the reverse order. This situation is
somewhat similar to the greater stability of electron-
withdrawing olefin complexes of M(0) (M ) Ni, Pd, Pt)
with considerable electron flow from M to olefin, while
M(II)-olefin complexes containing donating substitu-
ents at the olefin carbon are more stable than those of
electron-withdrawing olefins.9
allyl ligand is much less negatively charged (-0.07e),
indicating that the dinuclear complex involves weaker
donation from ligand to metal and/or stronger back-
donation from metal to ligand than the mononuclear
complex. This feature would be related to the coordi-
nate bond nature of Pd2(µ-CH2CHCH2)(µ-Br)(PH3)2, as
will be discussed below.
Discu ssion
Ab In itio MO Ca lcu la tion s. The ab initio MO/MP2
calculations were performed on the model complex
Pd2(µ-CH2CHCH2)(µ-Br)(PH3)2. The optimized geom-
etry of this model is also given in Tables 1 and 2.
Although the calculated Pd-Br and Pd-P distances are
longer than the experimental ones,10 the calculated Pd-
Pd, Pd-Ct, Pd-Cc, and Ct-Cc distances agree with
experimental values. The dihedral angle between Pd-
Pd-Br and allyl planes was calculated to be 83°.11 This
value is in good agreement with the experimentally
observed value and is significantly smaller than the
angle observed in mononuclear palladium(II) η3-allyl
complexes. MO calculations also showed that PH3 was
displaced from the Pd-Pd-Br plane by ca. 0.3 Å, like
the experimental structure. From these results, it is
reasonably concluded that the MP2 optimization can
reproduce well the characteristic features of the bonding
interaction between µ-CH2CHCH2 and Pd2(µ-Br)(PH3)2.
It is also worthy of note that there is a clear contrast
in the electron distribution between Pd2(µ-CH2CHCH2)(µ-
Br)(PH3)2 and PdCl(η3-CH2CHCH2)(PH3); in the former
the allyl ligand is considerably negatively charged
(-0.23e), where the allyl radical is defined to have
charge 0. In the mononuclear complex, however, the
The unique structure and solution stability of the
dinuclear Pd µ-allyl complexes clarified in the present
study can be nicely correlated with the nature of the
metal-allyl bonding deduced from the ab initio MO
calculations.
In an analysis of the coordinate bonding nature of the
µ-allyl ligand, the present complex Pd2(µ-CH2CHCH2)-
(µ-Br)(PH3)2 is considered to consist of [Pd2(µ-Br)-
(PH3)2]+ and the allyl anion (Chart 2), as in many
theoretical works.2e,3d,4b The allyl anion possesses the
nonbonding π orbital (nπ) as its HOMO, the π orbital
below the HOMO, and the π* orbital as its LUMO.
These orbitals play important roles in combining the
allyl ligand with the Pd-Pd fragment. In [Pd2(µ-Br)-
(PH3)2]+, the LUMO is the dσ-dσ antibonding orbital,
the next LUMO is the 5s-5s bonding orbital, the HOMO
is the dσ-dσ bonding orbital, and the next HOMO is
the dπ-dπ bonding orbital, into which the Br pz orbital
mixes in an antibonding way. The dπ-dπ antibonding
orbital exists at a lower energy, because the Br pz orbital
cannot mix in it.
Of these orbitals, the dσ-dσ antibonding orbital
(LUMO) overlaps well with the allyl nπ orbital (see
Chart 2), when the Ct atoms lie on the coordination
plane (Pd-Pd-Br plane). The next LUMO, a 5s-5s
bonding orbital, also overlaps with the allyl π orbital,
though to a lesser extent. These two interactions
correspond to the donation from the allyl ligand to the
Pd-Pd fragment. A similar bonding interaction has
been proposed by Zhu and Kostic in the calculation of
Pd2(µ-CH2CHCH2)(µ-Br)(PH3)2.3d The allyl π* orbital
is in the a representation (Cs symmetry) and therefore
overlaps with both the dσ-dσ bonding orbital (HOMO)
(9) Kurosawa, H.; Ikeda, I. J . Organomet. Chem. 1992, 428, 289-
301.
(10) (a) The too long Pd-Br and Pd-P distances have been generally
observed when the effective core potentials of Hay and Wadt were used
with the split valence type basis set on P and halogen. Similar results
were reported previously.10b Addition of the d polarization function
to the ligand atoms considerably improves the optimized bond distance
at the correlated level.10c (b) Low, J . J .; Goddard, W. A. J . Am. Chem.
Soc. 1984, 106, 6928-6937. (c) Sakaki, S.; Satoh, H.; Shono, H.; Ujino,
U. Organometallics, in press.
(11) This angle for the substituted complex Pd2[µ-CH2C(CN)CH2]-
(µ-Br)(PH3)2 was calculated to be 81°.

2094 Organometallics, Vol. 15, No. 8, 1996
Kurosawa et al.
Ch a r t 3
the steric congestion, so that there is not enough room
for the larger Ph substituent.
The origin of the anti preference of the rather small
1-Me and 1-Cl substituents in 2, 5, and their bromide
or iodide analogues does not appear so straightforward.
These substituents cannot necessarily lie on the allylic
plane when located at the anti site, as was demon-
strated in 1-substituted η3-allyl complexes of Mo(II).8b
This might lead us to propose a steric origin also for
the anti preference of 2 or 5, but we must await more
comprehensive synthetic and X-ray structure studies on
these and related complexes before deducing a definitive
explanation including electronic, if any, origins.12
and the dπ-dπ bonding orbital to form a back-donating
interaction from the Pd-Pd fragment to the allyl ligand.
This interaction was neglected in the analysis by Zhu
and Kostic,3d while Chisholm and co-workers proposed
a similar interaction between the allyl π* and the W-W
dπ-dπ bonding orbital in W2(µ-CH2CHCH2)(NH2)4.2e
The geometrical feature of the dinuclear µ-allyl com-
plexes cannot be explained successfully until we take
into account both donating and back-donating interac-
tions (see below). It should be noted that the geometry
characteristic of the mononuclear complexes (θ > 90°)
is due to the occurrence of only the donating interactions
from the allyl ligand to metals.7b
Con clu d in g Rem a r k s
Systematic examinations of structures and solution
stabilities of a series of substituted µ-allyl complexes of
Pd-Pd and Pd-Pt fragments enabled us to clarify the
intrinsic metal-allyl bonding nature in metal-metal-
bonded complexes. The observed structure and stability
trends unique to the µ-allyl metal-metal complexes
include a dihedral angle between the allyl and the
coordination planes which is considerably smaller than
in the mononuclear complexes, preferred coordination
of the allyl ligand having electron-withdrawing substit-
uents to the metal-metal fragment, and unusual anti
configuration preference of the 1-substituted allyl ligand.
These results, together with ab initio MO calculations,
led us to offer an allyl-metal bonding scheme unique
to the metal-metal complexes involving a greater
degree of back-bond interaction from the Pd-Pd bond
to the allyl π* orbital than in mononuclear Pd(II)-allyl
complexes.
The terminal carbon atoms of the allyl group are fixed
on the coordination plane so as to form the allyl nπ to
metal LUMO donating interaction (see above) and to
keep good overlap between the allyl π* and the dσ-dσ
bonding orbitals. Therefore, only one possible geometry
relaxation to yield a good overlap between the allyl π*
orbital and the dπ-dπ bonding orbital is the leaning of
the central carbon atom toward the Pd-Pd bond as
shown in Chart 3. This geometry change decreases the
dihedral angle between the allyl and Pd-Pd-Br planes
to less than 90°. The important role of the back-bonding
interaction is also well reflected in the electron distribu-
tion in the allyl ligand; the allyl ligand is considerably
negatively charged in Pd2(µ-CH2CHCH2)(µ-Br)(PH3)2
but much less negatively charged in PdCl(η3-CH2-
CHCH2)(PH3). The back-donating interaction also ex-
plains the preferred coordination of those allyl ligands
that contain the electron-withdrawing substituent at
either the terminal or central carbon to the Pd-Pd
fragment.
Exp er im en ta l Section
Most of the commercially available reagents were used
without further purification. All manipulations of dinuclear
palladium complexes were carried out under an argon atmo-
sphere by the use of standard vacuum-line techniques. Sol-
vents were dried by standard methods and distilled prior to
use. 1H NMR spectra were obtained on J EOL GSX-270, J EOL
GSX-400, and Bruker AM 600 spectrometers. Chemical shifts
were referenced to internal TMS (1H NMR) or external
P(OMe)3 (31P NMR). Homodecoupling experiments were car-
ried out, when necessary, to help in assigning spin-spin
coupling patterns.
The following complexes were prepared according to the
literature methods: Pd(CH2dCH2)(PPh3)2,13 [Pd(η3-CH2-
CHCHPh)(Cl)]2,14 [Pd(η3-CH2CHCHCOOMe)(Cl)]2,15 [Pd(η3-
CH2CClCH2)(Cl)]2.16
P r ep a r a tion of [P d Cl(η3-CH2CHCHCl)]2. A mixture of
PdCl2 (1.0 g, 5.75 mmol) and LiCl (1.0 g, 23.6 mmol) was
dissolved in 1 mL of water, followed by addition of 20 mL of
THF and 1,3-dichloropropene (2.0 g, 18.0 mmol). Carbon
monoxide was passed slowly under stirring through the
Rationalization of the unusual anti preference of the
1-substituted µ-allyl framework is somewhat difficult.
In the 1-COOMe- and 1-Ph-substituted allyl moiety,
electronically desirable π-conjugation would demand the
substituent group to lie coplanar with the allyl plane,
so that placing these substituents at the anti position
would result in close contact between the anti 3-H of
the allyl group and the oxygen (in COOMe) or ortho
hydrogen (in Ph) atom. The structure of the 1-COOMe
analog 4 or 13 thus can be attributable to the avoidance
of such intraligand repulsions.
(12) (a) Notice that MO calculations of [CH2CHCHCH3]- and
[CH2CHCHF]- suggested the higher stability of the anti configuration
than of the syn counterpart.12b,c (b) Schleyer, P. v. R.; Kaneti, J .; Wu,
Y. D.; Chandrasekhar, J . J . Organomet. Chem. 1992, 426, 143-157
and references therein. (c) Tonachini, G.; Canepa, C. Tetrahedron
1989, 45, 5163-5174.
(13) Visser, A.; ven der Lind, R.; de J ongh, R. O. Inorg. Synth. 1976,
16, 127.
(14) Huttel, R.; Kratzer, J .; Bechter, M. Chem. Ber. 1961, 94, 766-
780.
(15) Tsuji, J .; Imamura, S. Bull. Chem. Soc. J pn. 1967, 40, 197-
201.
(16) Lupin, M. S.; Powell, J .; Shaw, B. L. J . Chem. Soc. A 1966,
1687-1691.
However, the preferred anti configuration of the 1-Ph
analog must be explained by considering interligand
repulsions more seriously. That is, the nearly linear
arrangement of the Ph3P-Pd-Pd-PPh3 framework
cannot accommodate the large 1-Ph substituent lying
at the syn position. Examination of of the structure of
13 indicates that accommodation of the 1-COOMe group
at the syn position is already just marginal in terms of

µ-Allyl Ligands Coordinated with Pd-Pd and Pd-Pt
Organometallics, Vol. 15, No. 8, 1996 2095
reddish brown solution for 3 h. The bright yellow solution
obtained was dried by MgSO4. Filtration of the solution and
evaporation of the solvents under reduced pressure gave yellow
solids which were washed with methanol and water and dried
(929 mg, 74%). 1H NMR (CDCl3): syn isomer, δ 3.02 (d, J )
12.5 Hz, 1H, anti-H), 4.07 (d, J ) 7.3 Hz, 1H, syn-H), 5.16 (d,
J ) 9.1 Hz, 1H, anti-H geminal to Cl), 5.75 (ddd, 1H, center-
H); anti isomer, 4.03 (d, J ) 12.9 Hz, 1H, anti-H), 4.47 (d, J )
7.7 Hz, 1H, syn-H), 5.42 (ddd, 1H, center-H), 5.93 (d, J ) 4.6
Hz, 1H, syn-H geminal to Cl); syn/anti ) 89/11. Mp: 148-
149 °C dec. Anal. Calcd for C3H4Cl2Pd: C, 16.58; H, 1.85.
Found: C, 16.78; H, 1.85.
P r ep a r a tion of [P d (η3-CH2C(COOMe)CH2)(Cl)]2. In an
argon-flushed, 500 mL round-bottomed flask equipped with a
magnetic stirrer, a Dean-Stark trap, and a condenser were
placed R-(bromomethyl)acrylic acid (7.0 g, 42 mmol) and 50
mL of benzene. Approximately 9 mL of a binary azeotrope of
benzene and water was distilled. The Dean-Stark trap was
removed, and 17 mL of absolute methanol and 0.17 mL of
concentrated sulfuric acid were added slowly. The contents
of the flask were boiled in an open atmosphere for 36 h, the
condensate being passed through 15 g of molecular sieves
(Linde 3A) before being returned to the flask. The reaction
mixture was poured into 100 mL of water and neutralized with
solid sodium bicarbonate until CO2 evolution ceased. The
resulting solution was extracted with three 13 mL portions of
ether, and the combined extracts were dried over MgSO4 for
6 h. The ether was removed under reduced pressure in a
rotary evaporator, and the residues were distilled to give a
fraction at 95 °C (0.1 mm) of methyl R-(bromomethyl)acrylate
(2.06 g, 25%). 1H NMR (CDCl3): δ 3.70 (s, 3H), 4.13 (s, 2H),
5.91 (s, 1H), 6.28 (s, 1H). This was converted to [Pd(η3-CH2-
CCOOMeCH2)(Br)]2 using the standard method.17 1H NMR
(CDCl3): δ 3.21 (s, 2H, anti-H), 3.89 (s, 3H, Me), 4.65 (s, 2H,
syn-H). Mp: 167-170 °C dec. Anal. Calcd for C5H7O2BrPd:
C, 21.04; H, 2.47. Found: C, 21.52; H, 2.61.
chloride (0.15-0.25 mmol) was added an equimolar amount
of Pd(CH2dCH2)(PPh3)2 at 0 °C. The reaction mixture im-
mediately turned to a brown solution. After the solution was
stirred for 10 min, the solvent was removed under reduced
pressure at 0 °C. The residue was extracted with CH2Cl2. To
the extract was added hexane, and this solution was kept in
a refrigerator to give yellow solids.
Allyl ) CH2CHCH2:3e yield, 60%; 1H NMR (C6D6) δ 1.62
(d, J H ) 11.6 Hz, 2H, anti-H), 2.93 (m, J H ) 6.8, 11.6 Hz, J P
) 3.6 Hz, 1H, center-H), 3.07 (d of virtual t; J H ) 6.8 Hz, J P
J P′ ) 12 Hz, 2H, syn-H).
+
Allyl ) CH2CHCHMe: yield, 31%. Anti isomer: 1H NMR
(CDCl3) δ 0.40 (dd, J H ) 6.4 Hz, J P ) 4.1 Hz, 3H, anti-Me),
1.75 (dt, J H ) 2.3, 13.0 Hz, J P ) 2.3 Hz, 1H, anti-H), 2.64 (ddd,
J H ) 2.3, 8.6 Hz, J P ) 10.8 Hz, 1H, syn-H), 2.89 (m, 1H, center-
H), 3.57 (m, J H ) 6.4, 7.9 Hz, J P ) 12.1 Hz, 1H, syn-H geminal
to Me); 31P NMR (CDCl3) δ -117.8 (d, J P ) 77 Hz), -116.3
(d). Syn isomer: 1H NMR (CDCl3) δ 1.12 (t, J H ) J P ) 6.2
Hz, 3H, Me); mp 80-83 °C dec. Anal. Calcd for C40H37P2-
ClPd2: C, 58.03; H, 4.50. Found: C, 58.26; H, 4.77.
Allyl ) CH2CHCHP h : yield, 41%; 1H NMR (CDCl3) δ 1.94
(dt, J H ) 2.5, 13.0 Hz, J P ) 2.5 Hz, 1H, anti-H), 2.61 (ddd, J H
) 2.5, 8.4 Hz, J P ) 10.1 Hz, 1H, syn-H), 3.00 (m, 1H, center-
H), 4.25 (dd, J H ) 9.5 Hz, J P ) 10.9 Hz, 1H, syn-H geminal to
Ph); 31P NMR (CDCl3) δ -117.1 (d, J P ) 82 Hz), -113.8 (d);
mp 128-130 °C dec. Anal. Calcd for C45H39P2ClPd2: C, 60.73;
H, 4.42. Found: C, 60.80; H, 4.56.
Allyl ) CH2CHCHCOOMe: yield, 59%. Syn isomer: 1H
NMR (C6D6) δ 1.48 (dt, J H ) 2, 13.2 Hz, J P ) 2 Hz, 1H, anti-
H), 2.81 (s, 3H, Me), 2.88 (ddd, J H ) 2, 8.4 Hz, J P ) 11.1 Hz,
1H, syn-H), 2.99 (dd, J H ) 11.4 Hz, J P ) 2.0 Hz, 1H, anti-H
geminal to COOMe), 3.77 (m, 1H, center-H); 31P NMR (C6D6)
δ -118.0 (d, J P ) 84 Hz), -113.8 (d). Anti isomer: 1H NMR
(C6D6) δ 2.84 (s, 3H, Me), 3.19 (br d, J H ) 13.0 Hz, 1H, anti-
H), 4.01 (t, J H ) J P ) 8.5 Hz, 1H, syn-H geminal to COOMe);
31P NMR (C6D6) δ -116.3 (d, J P ) 93 Hz), -113.7 (d); mp 146-
147 °C dec. Anal. Calcd for C41H37O2P2ClPd2: C, 56.48; H,
4.28. Found: C, 55.68; H, 4.35.
The bromide was converted to the chloride on treatment
with aqueous saturated ammonium chloride solution in CH2-
Cl2 for 24 h at room temperature. 1H NMR (CDCl3): δ 3.20
(s, 2H, anti-H), 3.87 (s, 3H, Me), 4.64 (s, 2H, syn-H). Mp: 157-
158 °C dec. Anal. Calcd for C5H7O2ClPd: C, 24.92; H, 2.93.
Found: C, 24.69; H, 3.01.
P r ep a r a tion of [P d (η3-CH2C(CN)CH2)(Cl)]2. This was
prepared from PdCl2 and 2-cyano-3-chloro-1-propene (prepared
from 2-cyano-3-hydroxy-1-propene18 and SOCl2 in HMPT)
according to the standard method.17 1H NMR (CDCl3): δ 3.34
(s, 2H, anti-H), 4.44 (s, 2H, syn-H). Mp: 112-114 °C dec.
Anal. Calcd for C4H4NClPd: C, 23.10; H, 1.94. Found: C,
23.59; H, 2.32.
1
Allyl ) CH2CHCHCl: yield 72%; H NMR (CDCl3) δ 2.14
(dd, J H ) 12.1 Hz, J P ) 6.6 Hz, 1H, anti-H), 2.65 (t, J H ) J P
) 8.7 Hz, 1H, syn-H), 2.86 (m, 1H, center-H), 4.96 (dd, J H
)
4.9 Hz, J P ) 17.5 Hz, 1H, syn-H geminal to Cl); 31P NMR
(CDCl3) δ -116.8 (d, J P ) 79 Hz), -114.1 (d); mp 140-142 °C
dec. Anal. Calcd for C39H34P2Cl2Pd2: C, 55.21; H, 4.04.
Found: C, 55.43; H, 4.21.
1
Allyl ) CH2CClCH2: yield, 50%; H NMR (CDCl3) δ 1.76
(s, 2H, anti-H), 3.18 (br, 2H, syn-H); mp 138-142 °C dec. Anal.
Calcd for C39H34P2Cl2Pd2: C, 55.21; H, 4.04. Found: C, 55.02;
H, 4.25.
Allyl ) CH2C(COOMe)CH2: yield, 63%; 1H NMR (CDCl3):
δ 1.42 (s, 2H, anti-H), 3.41 (virtual t, J P + J P′ ) 11.6 Hz, 2H,
syn-H), 3.56 (s, 3H, Me); mp 135-136 °C dec. Anal. Calcd
for C41H37O2P2ClPd2: C, 56.48; H, 4.28. Found: C, 56.73; H,
4.59.
P r ep a r a tion of [P d (η3-CH2C(SO2P h )CH2)(Cl)]2. This
was prepared from 3-chloro-2-(phenylsulfonyl)-1-propene19 and
PdCl2 by the standard method.17 1H NMR (CD3CN): δ 3.14
(s, 2H, anti-H), 4.21 (s, 2H, syn-H). Mp: 149-151 °C dec.
Anal. Calcd for C9H9SO2ClPd: C, 33.46; H, 2.81. Found: C,
33.39; H, 2.92.
P r ep a r a tion of [P d (η3-CH2C(OCH2P h )CH2)(Cl)]2. Reac-
tion of Pd2(dba)3‚CHCl3 with a 5-fold excess of 3-chloro-2-
(benzyloxy)propene, prepared by dehydrochlorination of 1,3-
dichloro-2-(benzyloxy)propane20 with potassium tert-butoxide,
in CH2Cl2 afforded pale yellow solids. 1H NMR (CDCl3): δ
2.63 (d, J H ) 2.0 Hz, 2H, anti-H), 3.71 (d, J ) 2.0 Hz, 2H,
syn-H), 4.87 (s, 2H, OCH2). Mp: 178 °C dec. Anal. Calcd for
Allyl ) CH2C(CN)CH2: yield, 67%; 1H NMR (CDCl3) δ 1.41
(s, 2H, anti-H), 2.91 (br, 2H, syn-H); mp 142-146 °C dec. Anal.
Calcd for C40H34NP2ClPd2: C, 57.27; H, 4.08. Found: C, 56.48;
H, 4.23.
1
Allyl ) CH2C(SO2P h )CH2: yield, 75%; H NMR (CDCl3)
δ 1.50 (d, J H ) 4.5 Hz, 2H, anti-H), 3.08 (d of virtual t, J H
)
4.5 Hz, J P + J P′ ) 11.0 Hz, 2H, syn-H); mp 175-179 °C dec.
Anal. Calcd for C45H39SO2P2ClPd2‚1/2CH2Cl2: C, 54.84; H,
4.05. Found: C, 54.88; H, 4.24
C
10H11OClPd: C, 41.56; H, 3.84. Found: C, 41.46; H, 3.83.
The following complexes were prepared in a similar manner.
P r ep a r a tion of P d2(µ-a llyl)(µ-Cl)(P P h 3)2. Gen er a l P r o-
P d 2[µ-CH2C(COOMe)CH2](µ-Br )(P P h 3)2: yield, 55%; 1H
ced u r e. To a THF solution (10 mL) of (η3-allyl)palladium(II)
NMR (CDCl3) δ 1.43 (s, 2H, anti-H), 3.50 (virtual t, J P + J P′
)
10.8 Hz, 2H, syn-H), 3.54 (s, 3H, Me); mp 140-145 °C dec.
Anal. Calcd for C41H37O2P2BrPd2: C, 53.74; H, 4.07. Found:
C, 54.34; H, 3.99.
P d 2[µ-C H C H C H C H 2C H (C O O Me )C H 2](µ-C l)(P P h 3)2
(10): yield, 45%; 1H NMR (CDCl3) δ 1.1-1.3 (m, 4H, CH2),
2.28 (tt, J H ) 3.5, 12.3 Hz, 1H, CH(COOMe)), 2.73 (m, 1H,
(17) Tatsuno, Y.; Yoshida, T.; Otsuka, S. Inorg. Synth. 1979, 19,
220-221.
(18) Villieras, J .; Rambaud, M. Synthesis 1982, 924-926.
(19) Anzeveno, P. B.; Matthews, D. P.; Barney, C. L.; Barbuch, R.
J . J . Org. Chem. 1984, 49, 3134-3138.
(20) Gu, X. P.; Ikeda, I.; Okahara, M. Bull. Chem. Soc. J pn. 1987,
60, 397-398.

2096 Organometallics, Vol. 15, No. 8, 1996
Kurosawa et al.
allyl-center), 3.47 (s, 3H, Me), 3.54 (br, 2H, allyl-terminal); mp
117-119 °C dec. Anal. Calcd for C44H41O2P2ClPd2: C, 57.94;
H, 4.53. Found: C, 56.67; H, 4.57.
H, 4.00. Found: C. 51.54; H, 4.35. To a THF solution (5 mL)
of this material (0.1 mmol, 89.9 mg) was added Pd2(dba)3‚
CHCl3 (0.065 mmol, 67.3 mg). The reaction mixture was
stirred for 20 min and was concentrated. The residue was
purified by silica gel column chromatography with CH2Cl2 as
eluent. The combined CH2Cl2 solution was concentrated and
recrystallized from CH2Cl2/n-hexane to give yellow solids (53
1H NMR Detection of P d 2[µ-CH2CRCH2](µ-Cl)(P P h 3)2
(R ) Me, OCH2P h ). These were formed in CDCl3 according
to eq 1.
R ) Me: 1H NMR (CDCl3) δ 1.34 (t, J P ) 2.6 Hz, 3H, Me),
1.67 (s, 2H, anti-H), 2.98 (virtual t, J P + J P′ ) 12.4 Hz, 2H,
syn-H).
R ) OCH2P h : 1H NMR (CDCl3) δ 1.66 (s 2H, anti-H), 3.07
(br, 2H, syn-H), 4.27 (s, 2H, OCH2).
mg, 53%). 1H NMR (CDCl3): δ 1.17 (br d, J H ) 5 Hz, J Pt
)
80 Hz, 1H, anti-H), 1.57 (s, 1H, anti-H), 2.98 (m, J H ) 5 Hz,
J P ) 11.4 Hz, J Pt ) 88 Hz, 1H, syn-H), 3.52 (br d, J P ) 11.3
Hz, 1H, syn-H), 3.54 (s, 3H, Me). 31P NMR (CDCl3): δ -114.3
(d, J P ) 44.6 Hz, 2J Pt ) 83.8 Hz), -109.5 (d, 1J Pt ) 5928.6 Hz).
Mp: 142-144 °C dec. Anal. Calcd for C41H37O2P2-
BrPdPt‚CH2Cl2: C, 46.28; H, 3.61. Found: C, 46.65; H, 3.67.
1H NMR Mon itor in g of Rea ction betw een [P d Cl(η3-
CH2CHCHCl)]2 a n d P d (CH2dCH2)(P P h 3)2. To a CDCl3
solution (0.5 mL) of [PdCl(η3-CH2CHCHCl)]2 (2.2 mg; 0.01
mmol) in an NMR tube was added 6.9 mg (0.01 mmol) of Pd-
(CH2dCH2)(PPh3)2 at -78 °C. The tube was immediately
placed at an NMR probe precooled to -80 °C. The tempera-
ture of the probe was raised to -40 °C within ca. 20 min, and
1H NMR measurements showed resonances of the anti isomer
of the dinuclear complex (43%) together with those for a
supposedly syn isomer (25%): δ 1.30 (dd, J H ) 0.7, 14.0 Hz,
anti-H), 4.12 (d, J H ) 9.0 Hz, anti-H geminal to Cl). The latter
signal disappeared after the temperature was raised to room
temperature, and instead, the former increased to 66% yield,
with formation of the mononuclear complex Pd(η3-CH2CHCHCl)-
Cl(PPh3) (30%) also having been confirmed at the same time.
P r ep a r a tion of P d 2(µ-CH2CHCHMe)(µ-X)(P P h 3)2 (X )
Br , I). These were prepared by reacting the corresponding
chloride complex with excess LiX in CH2Cl2/acetone solution
at 0 °C and recrystallized from CH2Cl2/n-hexane.
Allyl Liga n d Exch a n ge betw een th e Din u clea r a n d
Mon on u clea r Com p lexes. Substituted µ-allyl complexes
were generated by adding the unsubstituted dinuclear complex
Pd2(µ-CH2CHCH2)(µ-Cl)(PPh3)2 (0.01 mmol) to a CDCl3 solu-
tion (0.5 mL) of a 1- to 5-fold excess amount of (substituted
(η3-allyl)(triphenylphosphine)palladium(II) chloride, which was
prepared from the corresponding (η3-allyl)palladium chloride
dimer and equimolar PPh3. The NMR tube was set in a probe
at 25 °C, and each component involved in eq 3 was integrated
to give K values. Several measurements with different reagent
ratios gave the averaged K values shown in Table 3.
Sin gle-Cr ysta l X-r a y Diffr a ction Stu d y. All data were
obtained on a Rigaku AFC-5R diffractometer with graphite-
monochromated Mo KR radiation. All calculations were car-
ried out with the TEXSAN crystallographic software package
of Molecular Structure Corp. The structure was solved by
direct methods and refined by full-matrix least-squares pro-
cedures, the function minimized being ∑w(|Fo| - |Fc|)2. The
non-hydrogen atoms were refined anisotropically. All the
positions of the hydrogen atoms were calculated by stereo-
chemical considerations.
X ) Br . Anti isomer: 1H NMR (CDCl3) δ 0.31 (dd, J H
6.0 Hz, J P ) 4.5 Hz, 3H, anti-Me), 1.77 (dt, J H ) 2.2, 12.9 Hz,
J P ) 2.2 Hz, 1H, anti-H), 2.73 (ddd, J H ) 2.2, 8.5 Hz, J P
)
)
10.5 Hz, 1H, syn-H), 2.86 (m, 1H, center-H), 3.71 (m, J H ) 6.0,
6.3 Hz, J P ) 12.6 Hz, 1H, syn-H geminal to Me); 31P NMR
(CDCl3) δ -115.0 (d, J P ) 84 Hz), -113.2 (d). Syn isomer: 1H
NMR (CDCl3) δ 1.08 (t, J H ) J P ) 6 Hz, 3H, Me); Mp 103-
105 °C dec. Anal. Calcd for C40H37P2BrPd2: C, 55.07; H, 4.28.
Found: C, 55.09; H, 4.68.
P dP t[µ-CH2C(COOMe)CH2](µ-Br )(P P h 3)2. A yellow crys-
tal (0.3 × 0.3 × 0.3 mm) was obtained from CH2Cl2/n-hexane
at -30 °C and was mounted on a glass fiber with epoxy resin:
C41H37BrO2P2PdPt‚CH2Cl2, Mr ) 1089.33, triclinic, space group
P1h (No. 2), a ) 13.534(2) Å, b ) 14.142(3) Å, c ) 11.676(3) Å,
R ) 111.40(2)°, â ) 96.71(2)°, γ ) 92.34(2)°, V ) 2058.1(9) Å3,
Z ) 2, Dcalc ) 1.758 g/cm3, F(000) ) 1060, µ(Mo KR) ) 40.5
cm-1 by least-squares refinement on diffractometer angles
from automatically centered reflections, 2θ range 27.3-27.5°,
λ ) 0.710 69 Å. The atomic scattering factor of Sm was used
as the scattering factor of metal atoms. The final R and Rw
values were 0.060 and 0.048, respectively, for 5505 reflections
(I > 3.00σ(I)).
P d 2[µ-CH2C(COOMe)CH2](µ-Br )(P P h 3)2. A yellow crys-
tal (0.3 × 0.45 × 0.2 mm) was obtained from CH2Cl2/n-hexane
at -30 °C and was mounted on a glass fiber with epoxy resin:
C41H37BrO2P2Pd2‚CH2Cl2, Mr ) 1001.33, triclinic, space group
P1h (No. 2), a ) 13.509(2) Å, b ) 14.121(2) Å, c ) 11.686(3) Å,
R ) 111.20(1)°, â ) 97.03(2)°, γ ) 92.30(2)°, V ) 2054.3(7) Å3,
Z ) 2, Dcalc ) 1.619 g/cm3, F(000) ) 996, µ(Mo KR) ) 20.9 cm-1
by least-squares refinement on diffractometer angles from
automatically centered reflections, 2θ range 27.3-27.5°, λ )
0.710 69 Å. The final R and Rw values were 0.047 and 0.045,
respectively, for 6826 reflections (I > 3.00σ(I)).
P d 2[µ-CH2CHCHCOOMe](µ-SP h )(P P h 3)2. A yellow crys-
tal (0.3 × 0.2 × 0.2 mm) was obtained from CH2Cl2/n-hexane
at -30 °C and was mounted on a glass fiber with epoxy resin:
C47H42O2P2Pd2S‚H2O, Mr ) 963.67, triclinic, space group P1h
(No. 2), a ) 14.278(2) Å, b ) 15.815(3) Å, c ) 10.281(3) Å, R )
106.12(2)°, â ) 103.12(2)°, γ ) 93.30(1)°, V ) 2154.0(8) Å3, Z
) 2, Dcalc ) 1.486 g/cm3, F(000) ) 976, µ(Mo KR) ) 9.98 cm-1
by least-squares reinement on diffractometer angles from
automatically centered reflections, 2θ range 26.6-27.3°, λ )
0.710 69 Å. The final R and Rw values were 0.081 and 0.061,
respectively, for 4000 reflections (I > 3.00σ(I)).
X ) I: 1H NMR (CDCl3) δ 0.16 (dd, J H ) 6.3 Hz, J P ) 4.6
Hz, 3H, Me), 1.85 (dt, J H ) 2.0, 12.8 Hz, J P ) 2.0 Hz, 1H,
anti-H), 2.86 (m, 1H, center-H), 2.94 (ddd, J H ) 2.0, 8.3 Hz,
J P ) 9.9 Hz, 1H, syn-H), 4.04 (m, J H ) 6.3, 6.3 Hz, J P ) 12.3
Hz, 1H, syn-H geminal to Me); 31P NMR (CDCl3) δ -117.8 (d,
J P ) 77 Hz), -116.3 (d); Mp 114-116 °C dec. Anal. Calcd
for C40H37P2IPd2: C, 52.25; H, 4.06. Found: C, 51.90; H, 4.14.
P r ep a r a t ion of P d 2(µ-CH 2CH CH COOMe)(µ-SP h )(P -
P h 3)2. [Pd(η3-CH2CHCHCOOMe)Cl]2 (0.2 mmol, 48 mg) was
dissolved in 5 mL of CH2Cl2 and cooled to 0 °C. TlSPh (0.22
mmol, 68 mg) was added to the solution. The reaction mixture
was stirred for 4 h at 0 °C and was filtered, and the filtrate
was concentrated to give orange solids (54 mg, 86%) of [Pd-
(η3-CH2CHCHCOOMe)(SPh)]2. To a THF solution (5 mL) of
this material (0.1 mmol, 31.4 mg) cooled to 0 °C was added
Pd(C2H4)(PPh3)2 (0.1 mmol, 65.9 mg). The reaction mixture
was stirred for 1 h at 0 °C and was concentrated. The residue
was recrystallized from CH2Cl2/n-hexane to give yellow solids
(52 mg, 55%). 1H NMR (CDCl3): δ 1.59 (d, J H ) 13.5 Hz, 1H,
anti-H), 2.85 (s, 3H, Me), 3.10 (m, 2H, center-H and syn-H),
4.15 (m, 1H, anti-H geminal to COOMe). 31P NMR (CDCl3):
δ -118.4 (d, J P ) 92.8 Hz), -114.0 (d). Mp: 130-133 °C dec.
Anal. Calcd for C47H42O2P2SPd2‚CH2Cl2: C, 55.94; H, 4.30.
Found: C, 56.02; H, 4.48.
P r ep a r a tion of P d P t[µ-CH2C(COOMe)CH2](µ-Br )(P -
P h 3)2. Methyl R-(bromomethyl)acrylate (0.32 mmol, 224.3 mg)
was added to a CH2Cl2 solution (5 mL) of Pt(CH2dCH2)(PPh3)2
(0.3 mmol, 224.3 mg) cooled to 0 °C. The reaction mixture
was stirred for 2 h at 0 °C. The residue obtained upon
evaporation of the solvent was recrystallized from CH2Cl2/n-
hexane to give white soilds of Pt[CH2C(COOMe)CH2](Br)-
(PPh3)2 (216 mg, 80%). 1H NMR (C6D6): δ 3.5 (br). Mp: 177-
180 °C dec. Anal. Calcd for C41H37O2P2BrPt‚CH2Cl2: C, 51.29;

µ-Allyl Ligands Coordinated with Pd-Pd and Pd-Pt
Organometallics, Vol. 15, No. 8, 1996 2097
MO Ca lcu la tion s. Geometry optimization was carried out
at the MP2 level with the following basis sets.21 Core electrons
of Pd (up to 3d) were replaced with effective core potentials,
and valence electrons were represented with a (5s 5p 4d)/[3s
3p 2d] set.22 For P and Br atoms, split valence basis sets (3s
3p)/[2s 2p]23 were adopted with ECPs used to replace core
electrons. For C and H atoms, MIDI-324 and (4s)/[2s]25 sets
were employed, where one d-polarization function was added
to the C basis set. The geometry of PH3 was taken from the
experimental structure of the free PH3 molecule.26 All the
calculations were performed with the Gaussian 92 program.27
Ack n ow led gm en t. Thanks are due to the Analyti-
cal Center, Faculty of Engineering, Osaka University,
for the use of NMR facilities. Partial support of this
work through Grants-in-aid for Scientific Research from
the Ministry of Education, Science and Culture and
Asahi Glass Foundation is also acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Tables of positional
and thermal parameters and bond lengths and angles, together
with figures giving atom-numbering schemes for 11-13 (29
pages). Ordering information is given on any current mast-
head page.
(21) The HF optimization of Pd2(µ-Br)(PH3)2(µ-C3H5) gives rise to a
somewhat larger dihedral angle (87°), which is larger than the MP2-
optimized value by about 4°. The use of a basis set including no
polarization function also yields a slightly larger dihedral angle (86°)
even at the MP2 level. A method of quality higher than the MP2
optimization with a basis set including a d-polarization function would
yield reasonable dihedral angle. Sakaki, S.; Kurosawa, H. To be
submitted for publication.
(22) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 299-310.
(23) Wadt, W. R.; Hay, P. J . J . Chem. Phys. 1985, 82, 284-298.
(24) Huzinaga, S.; Andzelm, J .; Klobkowski, M.; Radio-Andzelm, E.;
Sakaki, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Calcula-
tions; Elsevier: Amsterdam, 1984.
OM950843C
(26) Herzberg, G. Molecular Spectra and Molecular Structure;
Academic Press: New York, 1974; Vol. 3, p 267.
(27) Frisch, M. J .; Trucks, G. W.; Head-Gordon, M.; Gill, P.; M. W.;
Wong, M. W.; Foresman, J . B.; J ohnson, B. G.; Schlegel, H. B.; Robb,
M. A.; Replogle, E. S.; Gomperts, R.; Andres, J . L.; Raghavachari, K.;
Binkley, J . S.; Gonzalez, C.; Martin, R. L.; Fox, D. J .; DeFrees, D. J .;
Baker, J .; Stewart, J . J . P.; Pople, J . A. Gaussian 92; Guassian Inc.,
Pittsburgh, PA, 1992.
(25) Dunning, T. H.; Hay, P. J . In Methods of Electronic Structure
Theory; Shaefer, H. F., Ed.; Plenum: New York, 1977; p 1.