General route to halide-bridged organopalladium A-frame complexes and studies of reductive elimination from these bimetallic systems

Article abstract of DOI:10.1021/om010626d

Reactions of [Pd₂Cl₂(μ-dppm)₂] with RMgX (R = Me, Et, Bu, Ph, C₆H₄Me-4) at low temperature, followed by addition of CBr₄ and excess NH₄PF₆ or 1 equiv of TIPF₆, provided halide-bridged organopalladium A-frame complexes of the form [Pd₂R₂(μ-X)(μ-dppm)₂]PF₆. Mixed metal complexes were obtained similarly starting from [PdCtCl₂(μ-dppm)₂]. Unsymmetrical A-frames of the type [Pd₂(C₆H₂Me₃-2,4,6)R(μ-Cl)(μ-d ppm)₂]⁺ were generated reaction of [Pd(C₆H₂Me₃-2,4,6) (dppm)₂]⁺ (obtained by treatment of [PdCl₂ (cod)] w mesitylmagnesium bromide at low temperature, followed by 2 equiv of dppm) with [Pd₂R₂(μ-Cl)₂ (AsPh₃)₂]. The organopalladium A-frames did not react readily with CO, but the corresponding acyl derivatives [Pd₂(COR)₂(μ-Cl)(μ-dppm)₂]PF₆ were produced by carbonylation of [Pd₂R₂(μ-Cl)₂(AsPh₃)₂] followed by addition of dppm (R = Me, Et, Bn). Thermal decomposition of [Pd₂(CH₂Ph)₂(μ-Cl)(μ-dppm)₂]Cl was found to be first order in A-frame and resulted in quantitative formation of [Pd₂Cl₂(μ-dppm)₂] and 1,2-diphenylethane. The methyl and aryl complexes underwent both reductive elimination and hydrogen abstraction reactions. [Pd₂Et₂(μ-Br)(μ-ddpm)₂]PF₆ decomposed by β-hydride elimination and subsequent reductive elimination to yield ethene and ethane, whereas the butyl derivative gave both 1- and 2-butene. Acetic acid was formed when [Pd₂(COMe)₂(μ-Cl)(μ-dppm)₂]PF₆ was heated in dmso-d₆ solution, but decarbonylation was the predominant process in dioxane. The molecular structures of [Pd₂(CH₂Ph)₂(μ-Br)(μ-dppm)₂] PF₆·H₂O, 2C₆H₆ and [Pd₂Cl₂(μ-Cl)(μ-dppm)₂]OH· 0.5(CH₃)₂CO are also described.

Full text of DOI:10.1021/om010626d

5212
Organometallics 2001, 20, 5212-5219
Gen er a l Rou te to Ha lid e-Br id ged Or ga n op a lla d iu m
A-F r a m e Com p lexes a n d Stu d ies of Red u ctive
Elim in a tion fr om Th ese Bim eta llic System s
Robert A. Stockland, J r.,† Mesfin J anka, Gretchen R. Hoel, Nigam P. Rath, and
Gordon K. Anderson*
Department of Chemistry and Biochemistry, University of Missouri-St. Louis,
8001 Natural Bridge Road, St. Louis, Missouri 63121
Received J uly 16, 2001
Reactions of [Pd2Cl2(µ-dppm)2] with RMgX (R ) Me, Et, Bu, Ph, C6H4Me-4) at low
temperature, followed by addition of CBr4 and excess NH4PF6 or 1 equiv of TlPF6, provided
halide-bridged organopalladium A-frame complexes of the form [Pd2R2(µ-X)(µ-dppm)2]PF6.
Mixed metal complexes were obtained similarly starting from [PdPtCl2(µ-dppm)2]. Unsym-
metrical A-frames of the type [Pd2(C6H2Me3-2,4,6)R(µ-Cl)(µ-dppm)2]+ were generated by
reaction of [Pd(C6H2Me3-2,4,6)(dppm)2]+ (obtained by treatment of [PdCl2(cod)] with mesi-
tylmagnesium bromide at low temperature, followed by 2 equiv of dppm) with [Pd2R2(µ-
Cl)2(AsPh3)2]. The organopalladium A-frames did not react readily with CO, but the
corresponding acyl derivatives [Pd2(COR)2(µ-Cl)(µ-dppm)2]PF6 were produced by carbony-
lation of [Pd2R2(µ-Cl)2(AsPh3)2] followed by addition of dppm (R ) Me, Et, Bn). Thermal
decomposition of [Pd2(CH2Ph)2(µ-Cl)(µ-dppm)2]Cl was found to be first order in A-frame and
resulted in quantitative formation of [Pd2Cl2(µ-dppm)2] and 1,2-diphenylethane. The methyl
and aryl complexes underwent both reductive elimination and hydrogen abstraction reactions.
[Pd2Et2(µ-Br)(µ-dppm)2]PF6 decomposed by â-hydride elimination and subsequent reductive
elimination to yield ethene and ethane, whereas the butyl derivative gave both 1- and
2-butene. Acetic acid was formed when [Pd2(COMe)2(µ-Cl)(µ-dppm)2]PF6 was heated in dmso-
d6 solution, but decarbonylation was the predominant process in dioxane. The molecular
structures of [Pd2(CH2Ph)2(µ-Br)(µ-dppm)2]PF6‚H2O,2C6H6 and [Pd2Cl2(µ-Cl)(µ-dppm)2]OH‚
0.5(CH3)2CO are also described.
In tr od u ction
by insertion of unsaturated molecules into the Pd-Pd
bond of [Pd2X(C6Y5)(µ-dppm)2] (Y ) F, Cl),9 by addition
of MeI to [Pd2(µ-dppm)3],5 or by dimerization of [PtClR-
(dppm)].6,7 Organorhodium A-frames have been pre-
pared by reactions of [Rh2Cl2(CO)2(µ-dmpm)2] with
PhMgCl10 or of [Rh2Cl2(CO)2(µ-dppm)2] with BnMgCl,11
and we have shown that reactions of [PdX2(dppm)] with
alkyl Grignard reagents yield complexes of the form
[Pd2R2(µ-X)(µ-dppm)2]+.12 An acylrhodium A-frame has
been prepared by reaction of [Rh2(CO)3(µ-dppm)2] with
MeOSO2CF3, but it undergoes decarbonylation at 70 °C
to give [Rh2Me(µ-CO)(CO)(µ-dppm)2]+.13
The term A-frame was coined by Kubiak and Eisen-
berg in 1977 when they prepared [Rh2(µ-S)(CO)2(µ-
dppm)2] by reaction of [Rh2Cl2(CO)2(µ-dppm)2] with
Na2S.1 Since then, many A-frame complexes bridged by
dppm or related ligands, particularly of the group 9 and
10 metals, have been prepared and by a number of
different methods. These methods include insertion of
a small molecule, such as CO, SO2, or CNR, into the
metal-metal bond of a side-by-side dimer,2-4 oxidative
addition to a low-valent dimer,5 or dimerization of a
monomeric precursor.6,7 Group 10 A-frames containing
terminal organic groups, in particular, have been gener-
ated by treating [Pt2Cl2(µ-dppm)2] with Hg(CtCAr)2,8
In this paper we describe a general synthetic route
to organopalladium halide-bridged A-frame complexes,
[Pd2R2(µ-X)(µ-dppm)2]PF6, starting from the readily
available palladium(I) dimer, [Pd2Cl2(µ-dppm)2].14 The
* To whom correspondence should be addressed. Tel: 314-516-5311.
Fax: 314-516-5342. E-mail: ganderson@umsl.edu.
† Current address: Department of Chemistry, Bucknell University,
Lewisburg, PA 17837.
(1) Kubiak, C. P.; Eisenberg, R. J . Am. Chem. Soc. 1977, 99, 6129.
(2) Benner, L. S.; Olmstead, M. M.; Hope, H.; Balch, A. L. J .
Organomet. Chem. 1978, 153, C31.
(3) Benner, L. S.; Balch, A. L. J . Am. Chem. Soc. 1978, 100, 6099.
(4) Lee, C.-L.; J ames, B. R.; Nelson, D. A.; Hallen, R. T. Organo-
metallics 1984, 3, 1360.
(8) Yam, V. W.-W.; Yeung, P. K-Y.; Chan, L.-P.; Kwok, W.-M.;
Phillips, D. L.; Yu, K.-L.; Wong, R. W-K.; Yan, H.; Meng, Q.-J .
Organometallics 1998, 17, 2590.
(9) Espinet, P.; Fornie´s, J .; Fortun˜o, C.; Martinez, F.; Tomas, M.;
Welch, A. J . Organomet. Chem. 1986, 317, 105.
(10) Anderson, D. J .; Kramarz, K. W.; Eisenberg, R. Inorg. Chem.
1996, 35, 2688.
(5) Balch, A. L.; Hunt, C. T.; Lee, C.-L.; Olmstead, M. M.; Farr, J .
P. J . Am. Chem. Soc. 1981, 103, 3764.
(6) Cooper, S. J .; Brown, M. P.; Puddephatt, R. J . Inorg. Chem. 1981,
20, 1374.
(7) Anderson, G. K.; Clark, H. C.; Davies, J . A. J . Organomet. Chem.
1981, 210, l35.
(11) Kramarz, K. W.; Eisenberg, R. Organometallics 1992, 11, 1997.
(12) Stockland, R. A., J r.; Anderson, G. K.; Rath, N. P. Organome-
tallics 1997, 16, 5096.
(13) Shafiq, F.; Kramarz, K. W.; Eisenberg, R. Inorg. Chim. Acta
1993, 213, 111.
(14) Hunt, C. T.; Balch, A. L. Inorg. Chem. 1982, 21, 1641.
10.1021/om010626d CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/01/2001

Halide-Bridged Organopalladium Complexes
Organometallics, Vol. 20, No. 24, 2001 5213
Sch em e 1
decomposition reactions of these complexes, as well as
the analogous acylpalladium derivatives, are also de-
scribed.
Resu lts a n d Discu ssion
Syn t h esis of H a lid e-Br id ged A-F r a m e Com -
p lexes. We have reported previously that halide-
bridged organopalladium A-frame complexes may be
prepared by reaction of [PdClR(cod)] (R ) Me, Bn) or
[Pd2R2(µ-Cl)2(AsPh3)2] (R ) Me, Et, Bn, Ph) with dppm15
or by treatment of [PdX2(dppm)] with RMgX (R ) Me,
Et, Bu, Bn),16 but neither of these approaches is suitable
in all cases. The cyclooctadiene precursors are readily
available only for the methyl and benzyl cases,17,18 and
the reactions of [PdX2(dppm)] with aryl Grignard re-
agents result in reductive elimination of the diaryl from
[PdR2(dppm)] formed in the course of the reaction. With
bulky organic groups (R ) CH2SiMe3, C6H2Me3-2,4,6)
the diorganopalladium complexes are again formed, but
they are resistant to reductive elimination and may be
isolated in pure form.16 In contrast to the above ex-
amples, we have found that treatment of the dppm-
bridged palladium(I) dimer with a Grignard reagent in
CH2Cl2 solution at low temperature, followed by addi-
tion of CBr4 and TlPF6, allows complexes of the type
[Pd2R2(µ-Br)(µ-dppm)2]PF6 to be prepared for a wide
variety of R groups.
We noticed that when [PdX2(dppm)] (X ) Cl, Br) was
treated with 1 equiv of ArMgX, reductive elimination
of diaryl occurred and the palladium(I) complex [Pd2X2-
(µ-dppm)2] was formed. When excess Grignard reagent
was used, however, a species consistent with the for-
mulation [Pd2Ar2(µ-dppm)2] was generated,16 and we
surmised that addition to the Pd-Pd bond to produce
an A-frame complex might take place with suitable
substrates, such as a halogen source or H+. Thus, when
a solution of [Pd2Cl2(µ-dppm)2] (δ(P) -2.5) in CH2Cl2
solution at -78 °C was treated with MeMgBr, the
original resonance was replaced by one at 19.1 ppm,
which we assign to [Pd2Me2(µ-dppm)2].19 When the
solution was allowed to warm to ambient temperature,
the complex decomposed rapidly. If it was maintained
at -78 °C, however, it was stable for several hours.
Addition of 1 equiv of Br2 resulted in a 1:4 ratio of [Pd2-
Me2(µ-Br)(µ-dppm)2]+ and [PdBr2(dppm)] (-52.4 ppm).
When excess Br2 was employed, [PdBr2(dppm)] was the
sole product. With 1 equiv of N-bromosuccinimide a 1:1
mixture of the two products was formed, and [PdBr2-
(dppm)] was again the only product when excess NBS
was added.
and bromide-bridged A-frames was formed because the
methyl groups were introduced using MeMgBr. The
mechanism of halogenation of the metal-metal bond is
unclear, but the reaction with CBr4 produces [Pd2Me2-
(µ-Br)(µ-dppm)2]Br quantitatively. This method is toler-
ant of a variety of alkyl and aryl groups, including
methyl, ethyl, n-butyl, benzyl, phenyl, and 4-tolyl. The
exceptions are bulky groups, such as CH2SiMe3 and
mesityl, where only one organic group is incorporated,
giving unsymmetrical complexes of the form [Pd2BrR-
(µ-dppm)2] and [Pd2BrR(µ-Br)(µ-dppm)2]+. Although the
A-frame complexes are quite stable as their bromide
salts, they are not isolated easily, but metathesis with
excess NH4PF6 or 1 equiv of TlPF6 allows their isolation
as the hexafluorophosphate salts, [Pd2R2(µ-Br)(µ-dppm)2]-
PF6.
This method may be extended to heterobimetallic
A-frames of the form [PdPtAr2(µ-Br)(µ-dppm)2]PF6.
Treatment of a CH2Cl2 solution of [PdPtCl2(µ-dppm)2]20
with PhMgBr or 4-MeC6H4MgBr at -78 °C, followed by
addition of CBr4 and 1 equiv of TlPF6, generated
[PdPtAr2(µ-Br)(µ-dppm)2]PF6 in high yield. This ap-
proach to mixed metal A-frames appears to be limited
to aryl substituents, however. When [PdPtCl2(µ-dppm)2]
was treated with MeMgBr at -78 °C, followed by CBr4
and TlPF6, analysis of the reaction mixture by 31P NMR
spectroscopy revealed the presence of a number of
products that included [PdPtMe2(µ-Br)(µ-dppm)2]+, [PdPt-
Me2(µ-dppm)2], and the unsymmetrically substituted
[PdPtMeBr(µ-Br)(µ-dppm)2]+. A similar mixture of prod-
ucts was obtained if the reaction was performed at
ambient temperature. The observation of the side-by-
side dimer [PdPtMe2(µ-dppm)2], even at 25 °C, suggests
that the Pd-Pt bond is less reactive toward CBr4. The
lower reactivity of the Pt-Cl bond, compared with the
Pd-Cl bond, may be responsible for the detection of the
unsymmetrical species [PdPtMeBr(µ-Br)(µ-dppm)2]+.
We have shown that unsymmetrically substituted
diplatinum complexes of the type [Pt2RR′(µ-Cl)(µ-dppm)2]-
PF6 (R, R′ ) Me, Et, Ph) can be obtained by treatment
of [PtClR(cod)] with 2 equiv of dppm to produce [PtR-
(dppm-PP)(dppm-P)]+, followed by addition of [PtClR′-
(cod)].21 Such an approach to unsymmetrical A-frames
is generally unavailable for palladium. We had noted,
however, that with sterically demanding organic groups
Use of CBr4 proved more successful. Addition of 3-4
equiv of CBr4 to a solution of [Pd2Me2(µ-dppm)2] resulted
in formation of the A-frame complex [Pd2Me2(µ-Br)(µ-
dppm)2]+ as the sole palladium-containing product
(Scheme 1). When CCl4 was used, a mixture of chloride-
(15) Stockland, R. A. J r.; Anderson, G. K.; Rath, N. P. Inorg. Chim.
Acta 1997, 259, 173.
(16) Stockland, R. A., J r.; Anderson, G. K.; Rath, N. P. Organome-
tallics 1997, 16, 5096.
(17) Rulke, R. E.; Ernsting, J . M.; Spek, A. L.; Elsevier, C. J .; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
(18) Stockland, R. A., J r.; Anderson, G. K.; Rath, N. P.; Braddock-
Wilking, J .; Ellegood, J . C. Can. J . Chem. 1996, 74, 1990.
(19) Young, S. J .; Kellenberger, B.; Reibenspies, J . H.; Himmel, S.
E.; Manning, M.; Anderson, O. P.; Stille, J . K. J . Am. Chem. Soc. 1988,
110, 5744.
(20) Pringle, P. G.; Shaw, B. L. J . Chem. Soc., Dalton Trans. 1983,
889.
(21) Fallis, K. A.; Xu, C.; Anderson, G. K. Organometallics 1993,
12, 2243.

5214 Organometallics, Vol. 20, No. 24, 2001
Stockland et al.
Sch em e 2
Sch em e 3
ride, in the form of (Ph3P)2N+Cl- or Et4NCl, had no
effect. Since acylpalladium derivatives were not readily
available by this method, we sought an alternative
approach.
the reaction of [PdCl2(dppm)] with RMgX generated
[PdR2(dppm)], and further treatment with HCl gave the
A-frames [Pd2R2(µ-Cl)(µ-dppm)2]+. This presumably pro-
ceeds via [PdClR(dppm)], and we thought it might be
possible to isolate such species at low temperature.
Reaction of [PdR2(dppm)] (R ) CH2SiMe3, mesityl) with
1 equiv of HCl at -78 °C, however, gave an intractable
mixture of products. We have found, though, that
treatment of [PdCl2(cod)] with 1.5 equiv of mesitylmag-
nesium bromide in CH2Cl2 solution at low temperature
gives [PdCl(C6H2Me3-2,4,6)(cod)] quite cleanly. Addition
of 2 equiv of dppm results in formation of the cationic
complex [Pd(C6H2Me3-2,4,6)(dppm-PP)(dppm-P)]+. At
low temperature, the 31P{1H} NMR spectrum of this
species exhibits four resonances. The uncoordinated P
atom resonates at -28.7 ppm and appears as a broad
doublet (2J PP ) 67 Hz). The signal due to the coordi-
nated P atom of the monodentate ligand is observed at
11.9 ppm and shows coupling to all of the other P atoms
Previously we found that treatment of [PdClMe(cod)]
with CO (1 atm) resulted in carbonylation to give [PdCl-
(COMe)(cod)].24 Since [PdClMe(cod)] reacts with dppm
to produce an A-frame complex, it seemed likely that
[PdCl(COMe)(cod)] would be a suitable precursor to
[Pd2(COMe)2(µ-Cl)(µ-dppm)2]+. Indeed, treatment of
[PdClMe(cod)] with CO, followed by 1 equiv of dppm,
generated the acetylpalladium A-frame, which was
isolated as its PF6- salt. Since [PdCl(CH2Ph)(cod)] is the
only other compound of this type that is readily avail-
able, we chose to investigate the reactions of [Pd2R2(µ-
Cl)2(AsPh3)2] (R ) Me, Et, CH2Ph, Ph) with carbon
monoxide (since reactions of [Pd2R2(µ-Cl)2(AsPh3)2] with
dppm also produce A-frames). When CO was bubbled
through a CH2Cl2 solution of [Pd2Me2(µ-Cl)2(AsPh3)2],
there was no observable color change, but carbonylation
did occur. Further treatment with 2 equiv of dppm and
1 equiv of TlPF6 allowed isolation of the A-frame
complex [Pd2(COMe)2(µ-Cl)(µ-dppm)2]PF6 in high yield.
The corresponding reactions where R ) Et or CH2Ph
proceeded similarly (Scheme 3).
Treatment of [Pd2Ph2(µ-Cl)2(AsPh3)2] with CO and
dppm, however, resulted in two species. One of these
appeared to be consistent with [Pd2(COPh)2(µ-Cl)(µ-
dppm)2]+, whereas the other was an unsymmetrical
species that seemed to have one benzoyl and one phenyl
substituent. These two compounds could not be sepa-
rated. They were formed in an approximately 1:1 ratio
regardless of the length of time for which CO was passed
through the solution (5 min, 20 min, or 5 h). There was
also no change in the ratio of the two species after
standing at ambient temperature for 12 h, i.e., loss of
CO did not occur spontaneously. When [Pd2Ph2(µ-Cl)2-
(AsPh3)2] was allowed to react with 13CO, two peaks
were observed in the 13C{1H} NMR spectrum at 213.6
and 214.5 ppm. These chemical shifts are consistent
with benzoyl rather than terminal carbonyl groups.
These could be due to the cis and trans forms of [Pd2-
(COPh)2(µ-Cl)2(AsPh3)2], or perhaps to one isomer of
[Pd2(COPh)2(µ-Cl)2(AsPh3)2] (or to both if the cis and
trans forms are indistinguishable by 13C NMR spectros-
copy) and the partially carbonylated [Pd2Ph(COPh)(µ-
Cl)2(AsPh3)2]. Addition of dppm caused the 13C reso-
2
(ddd, J PP ) 391, 67, and 29 Hz). The two P atoms of
the chelated dppm ligand give rise to resonances at
-34.9 and -35.9, each being observed as a doublet of
doublets. The coupling between these two P atoms is
45 Hz. Warming the solution to ambient temperature
results in formation of [Pd2(C6H2Me3-2,4,6)2(µ-Br)(µ-
dppm)2]+ and free dppm. If the solution is kept at -78
°C, however, and treated with 0.5 equiv of [Pd2R2(µ-Cl)2-
(AsPh3)2] (R ) Me, Et, Ph), subsequent warming to
ambient temperature affords the unsymmetrical A-
frame [Pd2(C6H2Me3-2,4,6)R(µ-X)(µ-dppm)2]+ as a mix-
ture of chloride- and bromide-bridged species (about 5%
of the symmetrical A-frames were formed also) (Scheme
2). We would later convert these to hydride-bridged
derivatives,22 so it was unnecessary to attempt to
separate the chloride- and bromide-containing materi-
als.
Balch has reported that [Pd2Me2(µ-X)(µ-dppm)2]X
reacts with carbon monoxide to produce the correspond-
ing acetylpalladium dimers, but only at elevated CO
pressures. The rate of CO insertion was found to depend
on the nature of the halide (I > Br > Cl).23 We found
that no reaction occurred when CO was bubbled through
solutions of [Pd2R2(µ-X)(µ-dppm)2]PF6 (R ) Me, Ph,
C6H4Me-4) for extended periods. Addition of free chlo-
(22) Stockland, R. A., J r.; Anderson G. K.; Rath N. P. J . Am. Chem.
Soc. 1999, 121, 7945.
(23) Lee, C.-L.; Hunt, C. T.; Balch, A. L. Organometallics 1982, 1,
824.
(24) Ladipo, F. T.; Anderson, G. K. Organometallics 1994, 13, 303.

Halide-Bridged Organopalladium Complexes
Organometallics, Vol. 20, No. 24, 2001 5215
nances to shift to higher frequency, but two resonances
were still observed (217.8 and 218.7 ppm). Both signals
were broad, but with Gaussian enhancement the high-
frequency resonance could be resolved into a triplet (2J PC
) 4 Hz), consistent with a C atom lying cis to two
equivalent P atoms.
If the reaction of [Pd2Ph2(µ-Cl)2(AsPh3)2] with CO was
simply slow, then longer reaction times should result
in greater conversion to [Pd2(COPh)2(µ-Cl)2(AsPh3)2],
and hence [Pd2(COPh)2(µ-Cl)(µ-dppm)2]+, but the ratio
of products did not appear to change with reaction time.
If an equilibrium exists between [Pd2(COPh)2(µ-Cl)2-
(AsPh3)2], [Pd2(COPh)Ph(µ-Cl)2(AsPh3)2], and free CO,
such that the ratio of palladium complexes is ap-
proximately 1:1, then dppm addition would generate the
symmetrical and unsymmetrical A-frames in the same
ratio. Since the reaction with dppm is rapid, this would
be consistent with the same ratio of products being
formed, irrespective of the carbonylation time.
F igu r e 1. Projection view of the molecular structure of
the [Pd2(CH2Ph)2(µ-Br)(µ-dppm)2]+ cation using 50% prob-
ability ellipsoids, showing the atom-labeling scheme.
The halide-bridged A-frame complexes have been
characterized by elemental analysis, by 1H and 31P NMR
spectroscopy, and, in the case of [Pd2(CH2Ph)2(µ-Br)(µ-
dppm)2]PF6, by X-ray crystallography. The symmetrical
dipalladium species give rise to a single 31P resonance,
whereas the unsymmetrical derivatives [Pd2(C6H2Me3-
2,4,6)R(µ-X)(µ-dppm)2]+ and [PdPtR2(µ-X)(µ-dppm)2]+
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for th e [P d 2(CH2P h )2(µ-Br )(µ-d p p m )2]+ Ca tion
Pd(1)-Br
2.5461(7)
2.3344(13)
2.3180(13)
2.095(5)
Pd(2)-Br
2.5480(7)
2.3342(13)
2.3179(13)
2.087(5)
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-C(3)
Pd(2)-P(3)
Pd(2)-P(4)
Pd(2)-C(10)
1
exhibit the expected two signals in each case. Their H
NMR spectra contain the expected resonances for the
organic substituents on the metals. The methylene
hydrogens of the dppm ligands are nonequivalent,
consistent with a rigid A-frame structure in which one
hydrogen is disposed toward the bridging halide and the
other is directed toward the opposite face of the dimeric
unit. In the presence of free halide ion these two signals
coalesce into one broad resonance, indicative of a
fluxional process involving halide attack on one metal
center and bridge opening to generate the face-to-face
species [Pd2R2X2(µ-dppm)2]. In the benzyl derivative,
[Pd2(CH2Ph)2(µ-Br)(µ-dppm)2]+, the aromatic signals
associated with the benzyl group appear at unexpectedly
low frequencies as a doublet at 5.17 ppm due to the
o-hydrogens, and triplets at 6.35 and 6.65 ppm due to
the m- and p-hydrogens, respectively.
Pd(1)-Br-Pd(2)
P(1)-Pd(1)-Br
P(2)-Pd(1)-Br
Br-Pd(1)-C(3)
P(3)-Pd(2)-Br
P(4)-Pd(2)-Br
Br-Pd(2)-C(10)
82.42(2)
84.44(3)
93.27(4)
165.6(2)
84.45(4)
93.28(4)
165.6(2)
P(1)-Pd(1)-P(2)
P(1)-Pd(1)-C(3)
P(2)-Pd(1)-C(3)
P(3)-Pd(2)-P(4)
P(3)-Pd(2)-C(10)
P(4)-Pd(2)-C(10)
172.35(5)
94.7(2)
89.3(2)
172.36(5)
94.9(2)
89.1(2)
bromide, with C-Pd-Br angles of 165.6(2)°. The P-Pd-
Pd-P torsion angles are 21.2°, demonstrating that there
is significant twisting of the P-Pd-P units about the
Pd‚‚‚Pd axis.
One of the o-hydrogens of each benzyl aromatic ring
points directly toward the centroid of a dppm phenyl
ring. The angle about the hydrogen is 171.6°, with an
H-centroid distance of 2.57 Å. This hydrogen should
experience an increase in shielding due to the ring
current effect. Rotation of the aromatic ring will occur
in solution, averaging the environments of the o-
Molecu la r Str u ctu r e of [P d 2(CH2P h )2(µ-Br )(µ-
d p p m )2]P F 6.H2O,2C6H6. Slow evaporation of a CH2-
Cl2/C6H6 solution of [Pd2(CH2Ph)2(µ-Br)(µ-dppm)2]PF6,
obtained from [Pd2Cl2(µ-dppm)2] and PhCH2MgBr, dppm,
and NH4PF6, produced orange cuboids. The compound
crystallizes in the monoclinic space group Pm. In
1
hydrogens. The H NMR resonance due to these hydro-
gens is shifted to low frequency (5.17 ppm), however,
suggesting that a similar conformation is maintained
in solution.12 The m- and p-hydrogens experience a ring
current also, but the resonances for those hydrogens are
affected to a lesser extent (6.35 and 6.65 ppm).
-
addition to two cations and two PF6 anions, the unit
cell contains two water molecules and four molecules
of benzene. The molecular structure of the cation is
shown in Figure 1, and selected bond distances and
angles are given in Table 1.
Elim in a tion Rea ction s. These halide-bridged A-
frames represent suitable examples in which to study
elimination reactions involving a dinuclear system.
When a dmso-d6 solution of [Pd2(CH2Ph)2(µ-Cl)(µ-dppm)2]-
Cl was heated to 97 °C and monitored by 31P NMR
spectroscopy, the original resonance at 12.0 ppm gradu-
ally decreased in intensity and a new signal appeared
at -2.8 ppm. The new signal was identified as being
due to [Pd2Cl2(µ-dppm)2]. Changes were also noted in
the 1H NMR spectrum. The broad signals due to the
CH2 groups of the benzyl moiety (2.89 ppm) and dppm
(3.90 ppm) gradually decreased, along with the aromatic
signals at 5.20, 6.27, and 6.65 ppm. These were replaced
The cation contains two PdP2CBr square planes
inclined toward each other at an angle of 82.42(2)°. The
Pd-Pd distance is 3.356 Å, indicating the lack of any
metal-metal bonding. The Pd-P distances are normal,
and the P-Pd-P angles are 172.35(5)°. The Pd2(µ-
dppm)2 unit forms an elongated boat, with the CH2
groups lying on the same side as the bridging bromide.
The molecule exhibits classic A-frame geometry, with
the bromide and benzyl groups located on opposite sides
of the Pd2P4 plane. The benzyl groups lie trans to the

5216 Organometallics, Vol. 20, No. 24, 2001
Stockland et al.
Sch em e 4
To try to distinguish between these two possibilities,
we carried out the elimination from [Pd2(CH2Ph)2(µ-Cl)-
(µ-dppm)2]Cl in the presence of 4-MeC6H4CH2Cl. The
reaction was monitored by 31P NMR spectroscopy, and
[Pd2Cl2(µ-dppm)2] was the only Pd-containing product
formed. Analysis of the organic products by GC-MS
revealed the presence of 4-MeC6H4CH2Cl and PhCH2-
CH2Ph only; that is, no crossover product was formed.
If a mechanism involving elimination and readdition of
PhCH2Cl was operative, we would expect incorporation
of 4-MeC6H4CH2Cl to form 4-MeC6H4CH2CH2Ph. Thus,
if a nonradical process is involved, it appears most likely
that the benzyl group is transferred intramolecularly.
When a dmso-d6 solution of [Pd2Me2(µ-Cl)(µ-dppm)2]-
Cl was heated to 107 °C for 2 h, a 5:1 ratio of methane
and ethane was produced. When such a solution was
heated rapidly to 127 °C, a 1:1 ratio was formed. [Pd2-
Cl2(µ-dppm)2] was produced in less than 50% yield (by
NMR) in either case. Ethane is the expected product of
reductive elimination, whereas hydrogen abstraction
must take place in order to produce methane. Hydrogen
abstraction could occur from an organic fragment within
the complex or from dmso or adventitious water. When
D2O was deliberately added to the dmso-d6 solution, no
change in the rate of decomposition of [Pd2Me2(µ-Cl)-
(µ-dppm)2]Cl at 127 °C was detected. The temperature
dependence of the product ratio implies that hydrogen
abstraction is a lower energy process, and reductive
elimination competes effectively only when the reaction
is performed at higher temperatures.
by a sharp singlet at 1.89 ppm and a quintet at 4.35
ppm (2J PH ) 4.0 Hz). The quintet is due to the dppm
CH2 groups in [Pd2Cl2(µ-dppm)2], whereas the singlet
is assigned to the CH2 groups of 1,2-diphenylethane
(Scheme 4). Subsequent analysis by GC-MS confirmed
the presence of the latter. When a solution of [Pd2(CH2-
Ph)2(µ-Cl)(µ-dppm)2]PF6 was treated similarly, 1,2-
diphenylethane was formed, but [Pd2Cl2(µ-dppm)2] was
formed in less than 40% yield (by NMR) and extensive
decomposition was observed. This is consistent with
there being only one chloride for every two palladium
atoms in the starting complex.
The disappearance of [Pd2(CH2Ph)2(µ-Cl)(µ-dppm)2]Cl
1
was monitored by H and 31P NMR spectroscopy. Plots
of log[complex] vs time were found to be linear, and the
rate of elimination was unchanged when the reactant
concentration was varied by a factor of 4. Both observa-
tions are consistent with the elimination being first
order in A-frame complex. Rate constants were deter-
mined at several temperatures (82-112 °C), and the
activation parameters for the process were determined
to be ∆Gq ) 47 ( 1 kcal/mol, ∆Hq ) 28 ( 1 kcal/mol,
and ∆Sq ) 50 ( 5 cal/mol‚K. These parameters are
consistent with a significant degree of bond breaking
in the transition state. The nature of the transition state
is difficult to establish, however, since the exact nature
of the starting material is uncertain. The starting
material is fluxional, consisting of an unknown ratio of
[Pd2(CH2Ph)2(µ-Cl)(µ-dppm)2]+ (as its chloride salt) and
the bridge-opened form [Pd2(CH2Ph)2Cl2(µ-dppm)2]. Even
when a CDCl3 solution of [Pd2(CH2Ph)2(µ-Cl)(µ-dppm)2]-
Cl was cooled to -60 °C, the signals remained broad.
The lack of free chloride in [Pd2(CH2Ph)2(µ-Cl)(µ-dppm)2]-
PF6 results in spectra that are indicative of a static
system in CDCl3 solution, but broad signals are ob-
served in dmso-d6 solution, where the solvent is appar-
ently sufficiently nucleophilic to reversibly cleave the
chloride bridge.
Elimination of 1,2-diphenylethane could take place
through a 1,1-elimination where both organic groups
must first become bonded to the same palladium center,
by means of a 1,2-elimination involving a four-centered
intermediate or transition state, or by homolytic cleav-
age of the two Pd-C bonds. We believe the first is most
likely, although a radical process certainly cannot be
ruled out.11 Transfer of one benzyl group to the adjacent
metal could occur by elimination of benzyl chloride from
one palladium, promoted by Cl- attack, followed by
oxidative addition to the other. Alternatively, direct
transfer could take place via a transient bridging benzyl
group, with or without the involvement of free Cl- ions.
Similar results were obtained for the arylpalladium
derivatives [Pd2Ar2(µ-Br)(µ-dppm)2]PF6 (Ar ) Ph, 4-Me-
C6H4). When a dmso-d6 solution of [Pd2Ph2(µ-Br)(µ-
dppm)2]PF6 was heated to 127 °C for 1 h, the 31P NMR
signal due to the A-frame complex gradually disap-
peared, and GC-MS analysis confirmed the formation
of benzene and diphenyl in an approximately 1:1 ratio.
Toluene and 4,4′-dimethyldiphenyl were formed in a 1:1
ratio when [Pd2(4-MeC6H4)2(µ-Br)(µ-dppm)2]PF6 was
heated to 97 °C, but at 127 °C the latter was produced
almost exclusively. In neither case was the palladium(I)
dimer observed. If 1 equiv of (Ph3P)2N+Cl- was added
to a solution of [Pd2(4-MeC6H4)2(µ-Br)(µ-dppm)2]PF6,
there was no change in the ratio of organic products
formed at 97 °C, but a signal due to [Pd2Cl2(µ-dppm)2]
was observed in the 31P NMR spectrum. Again, the
source of hydrogen in forming benzene or toluene is
uncertain. Heating a dmso-d6 solution of [Pd2(4-MeC6H4)2-
(µ-Br)(µ-dppm)2]PF6, to which excess D2O had been
added, to 97 °C for 1 h produced C6H5CH3 and (4-
CH3C6H4)2. GC-MS analysis provided no evidence for
deuterium incorporation. This suggests that the hydro-
gen is abstracted from another aryl group or from dppm.
Elimination from the ethyl derivative [Pd2Et2(µ-Br)-
(µ-dppm)2]PF6 took place more readily. When a dmso-
d6 solution was heated to 67 °C for 1 h, the 31P NMR
signal at 16.0 ppm gradually diminished in intensity
and was replaced by several resonances in the range
20-30 ppm. The 1H signals due to the ethyl groups also
disappeared, and they were replaced by two sharp
singlets at 0.80 and 4.13 ppm, in an approximately 1:1
ratio, which were assigned to ethane and ethene,
respectively. Ethene is likely to be formed by â-hydride
elimination, although the palladium hydride thus formed

Halide-Bridged Organopalladium Complexes
Organometallics, Vol. 20, No. 24, 2001 5217
Sch em e 5
Sch em e 6
Reductive elimination from an acyl complex of the
type [Pd2(COR)2(µ-Cl)(µ-dppm)2]+ could generate R,â-
diketones. Indeed, treatment of [Rh2Me2(µ-CO)(µ-dppm)2]
with CO (25 °C, 100 atm) gave 2,3-butanedione as the
major organic product.26 Heating a dmso-d6 solution of
[Pd2(COMe)2(µ-Cl)(µ-dppm)2]PF6 to 97 °C in the absence
of free CO, however, resulted in formation of acetic acid
and an unidentified palladium-containing complex (δ(P)
-9.2). If dioxane was employed as solvent, acetic acid
was not formed. Instead, [Pd2(COMe)2(µ-Cl)(µ-dppm)2]+
(δ(P) 7.2) underwent decarbonylation to yield the cor-
responding methylpalladium derivative (δ(P) 17.6).
When the reaction was monitored by 31P NMR spec-
troscopy, the partially decarbonylated intermediate [Pd2-
Me(COMe)(µ-Cl)(µ-dppm)2]+ could be detected (δ(P) 9.3
and 15.0) (Scheme 6). Similar decarbonylations have
been reported by Balch.23 Formation of acetic acid was
unexpected, however. It seemed possible that trace
amounts of moisture present in the dmso-d6 could
promote this reaction. When water was added deliber-
ately to a dioxane solution of [Pd2(COMe)2(µ-Cl)(µ-
dppm)2]PF6, however, heating to 97 °C produced only a
small amount of acetic acid. Formation of acetic acid
seemed to depend not only on the presence of water,
but also on the use of dmso as the solvent.
Molecu la r Str u ctu r e of [P d 2Cl2(µ-Cl)(µ-d p p m )2]-
OH‚0.5(CH3)2CO. The complex crystallizes as orange-
red plates in the tetragonal space group P412121. In
addition to four molecules of the A-frame cation and
associated hydroxide anions, the unit cell contains two
acetone molecules. The molecular structure of the cation
is shown in Figure 2, and selected bond distances and
angles are presented in Table 2. The structure of the
cation is similar to that of [Hg2Cl2(µ-Cl)(µ-dppm)2]+,
formed unexpectedly in the reaction of [Pd2Cl2(µ-
dppm)2] with elemental mercury.26
1
could not be detected by H NMR spectroscopy. Ethane
would result from reductive elimination of the hydride
and the remaining ethyl group.
A dmso-d6 solution of [Pd2Et2(µ-Br)(µ-dppm)2]PF6, to
which excess D2O had been added, was maintained at
67 °C and monitored for several hours by 1H NMR
spectroscopy. The resonance due to ethene grew rapidly,
but after 30 min it began to decrease in intensity. The
signal for ethane grew in more slowly, but its intensity
did not diminish with time. This may be explained as
follows. Ethene would be formed first by â-elimination,
leaving a Pd-H species. If ethane elimination were
rapid, then C2H6 would be formed. If not, exchange with
D2O could occur to generate a Pd-D species, from which
elimination would yield C2H5D. Also, â-hydride elimina-
tion may well be reversible, and reinsertion of C2H4 into
the Pd-D bond would give a Pd-CH2CH2D fragment.
A second â-elimination could produce C2H3D and Pd-
H. Repeated H-D exchange with D2O and insertion/â-
elimination would generate a series of partially deuter-
ated ethenes, which could account for the decrease in
intensity of the signal at 4.13 ppm.
When a sample of [Pd2Et2(µ-Cl)(µ-dppm)2]Cl in CDCl3/
acetone-d6 solution was kept at -40 °C for a prolonged
period, orange crystals deposited on the sides of the
NMR tube. These were identified by X-ray crystal-
lography as [Pd2Cl2(µ-Cl)(µ-dppm)2]OH (vide infra),
apparently formed through a series of bond-breaking
and bond-forming reactions, presumably involving the
chlorinated solvent and adventitious water.
The butylpalladium A-frame, [Pd2Bu2(µ-Br)(µ-dppm)2]-
PF6, also undergoes â-hydride elimination. This complex
is more soluble in common organic solvents, so the
elimination could be monitored in toluene solution.
When a toluene-d8 solution of the complex was main-
tained at 67 °C, the resonances due to the butyl groups
gradually diminished in intensity over 1 h. Simulta-
The cation consists of two palladium centers linked
by two dppm ligands and a chloride. The geometry about
each palladium is approximately square planar, and the
two planes are inclined toward each other at an angle
of 87.5° through the bridging chloride. The Pd-Pd
distance is 3.170 Å, too long for the existence of any
significant metal-metal interaction. The unique Pd-P
distances are 2.340(2) and 2.304(2) Å, and the P-Pd-P
angles are 169.91(8)°. The terminal Pd-Cl distances are
2.371(2) Å, and the bridging Pd-Cl bond lengths are
shorter at 2.293(2) Å. The Cl-Pd-Cl angles are
2
neously, signals due to 1-butene (δ(H) 0.89 (t, J HH ) 8
Hz), 1.92 (m), 4.92 (m), 5.75 (m)) gradually appeared.
Additional multiplets at 1.45 and 5.35 ppm were also
observed, and these were identified as being associated
with 2-butene. While 1-butene is the expected â-elimi-
nation product, reinsertion into the resulting Pd-H
bond could form either an n-butyl- or sec-butylpalladium
complex, and â-elimination from the latter would gener-
ate 2-butene (Scheme 5). In contrast to the ethyl case,
no alkane was observed on thermolysis of [Pd2Bu2(µ-
Br)(µ-dppm)2]PF6.
(25) Kramarz, K. W.; Eisenschmid, T. C.; Deutsch, D. A.; Eisenberg,
R. J . Am. Chem. Soc. 1991, 113, 5090.
(26) Harvey, P. D.; Aye, K. T.; Hierso, K.; Isabel, E.; Lognot, I.;
Mugnier, Y.; Rochon, F. D. Inorg. Chem. 1994, 33, 5982.

5218 Organometallics, Vol. 20, No. 24, 2001
Stockland et al.
tracted with CH2Cl2 and passed through a short column of
neutral alumina, eluting with CH2Cl2 (50 mL). The solvent
was removed and the crude material was crystallized from
CH2Cl2/Et2O. Due to incorporation of ether, which could not
be removed by pumping, the crystals were dissolved in CH2-
Cl2 and the solvent was removed rapidly. The residue was
dried in vacuo, leaving a pale yellow solid (0.12 g, 92%). Anal.
Calcd for C62H54BrF6P5Pd2: C, 54.71; H, 3.97. Found: C, 54.24;
1
2
H, 4.04. H NMR (acetone-d6): δ(H) 4.44 (dq, 2H, J HH ) 14.2
Hz, 2J PH ) 3.7 Hz, PCH2P); 4.61 (dq, 2H, 2J HH ) 14.2 Hz, 2J PH
) 5.2 Hz, PCH2P); 6.54-6.58 (m, 10H, PdC6H5); 7.05-7.9 (m,
40H, PC6H5). 31P{1H} NMR (acetone-d6): δ(P) 11.5 (s).
[P d 2(C6H4Me-4)2(µ-Br )(µ-d p p m )2]P F 6 was prepared simi-
larly and isolated as a pale yellow solid in 83% yield. Anal.
Calcd for C64H58BrF6P5Pd2: C, 55.34; H, 4.18. Found: C, 55.15;
H, 4.26. 1H NMR (CDCl3): δ(H) 1.97 (s, 6H, CH3) 3.71 (dq,
2H, 2J HH ) 13.9 Hz, 2J PH ) 3.4 Hz, PCH2P); 4.38 (dq, 2H, 2J HH
2
) 13.9 Hz, J PH ) 5.5 Hz, PCH2P); 6.35-6.40 (m, 8H, C6H4-
Me); 6.98-7.67 (m, 40H, PC6H5). 31P{1H} NMR (CDCl3): δ(P)
10.5 (s).
The methyl, ethyl, n-butyl, and benzyl derivatives, prepared
previously from [PdBr2(dppm)] and the appropriate Grignards
reagent,12 were also prepared in good yield by the above
method.
F igu r e 2. Projection view of the molecular structure of
the [Pd2Cl2(µ-Cl)(µ-dppm)2]+ cation using 50% probability
ellipsoids, showing the atom-labeling scheme.
[P d P t(C6H4Me-4)2(µ-Br )(µ-d p p m )2]P F 6. To a stirred CH2-
Cl2 solution (30 mL) of [PdPtCl2(µ-dppm)2] (0.10 g, 0.088 mmol)
at -78 °C was added 4-MeC6H4MgBr (1.0 mL of a 1.0 M
solution). The solution darkened immediately, and it was
allowed to stir for 14 h. Methanol (1.0 mL), CBr4 (0.10 g, 0.30
mmol), and TlPF6 (0.031 g, 0.089 mmol) were introduced, and
the mixture was allowed to warm to ambient temperature
while the solvents were removed under reduced pressure. The
product was isolated in a manner similar to that described
for the dipalladium case. It was obtained as a pale yellow solid
in 85% yield. Anal. Calcd for C64H58BrF6P5Pd2: C, 52.01; H,
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for th e [P d 2Cl2(µ-Cl)(µ-d p p m )2]+ Ca tion
Pd(1)-Cl(1)
Pd(1)-P(1)
2.293(2)
2.340(2)
Pd(1)-Cl(2)
Pd(1)-P(2)
2.371(2)
2.304(2)
Pd(1)-Cl(1)-Pd(1)′
P(1)-Pd(1)-Cl(1)
Cl(1)-Pd(1)-Cl(2)
P(2)-Pd(1)-Cl(2)
87.46(10) P(1)-Pd(1)-P(2) 169.91(8)
87.55(5)
173.27(8)
93.20(7)
P(1)-Pd(1)-Cl(2)
P(2)-Pd(1)-Cl(1)
95.48(7)
83.29(5)
173.27(8)°. The P-Pd-Pd-P torsion angles are 9.0°,
indicating that there is only a slight twisting of the
structure. As in the benzyl case, the dppm ligands are
oriented to give an elongated boat configuration, with
the methylene moieties on the same side as the bridging
chloride. This is typical of group 10 A-frames, exceptions
being [Pd2Cl2(µ-CO)(µ-dppm)2], which adopts an elon-
gated chair conformation,27 and [Pd2(C6H2Me3-2,4,6)2-
(µ-Br)(µ-dppm)2]+, which was found in a boat confor-
mation but with the CH2 groups on the opposite face
from the bridging bromide.16
3.93. Found: C, 51.77; H, 3.96. 1H NMR (CDCl3): δ(H) 1.91
2
(s, 3H, C6H4CH3); 1.98 (s, 3H, C6H4CH3); 4.03 (dq, 2H, J HH
)
14.1 Hz, 2J PH ) 3.8 Hz, PCH2P); 4.44 (dq, 2H, 2J HH ) 14.1 Hz,
2J PH ) 5.9 Hz, PCH2P); 6.2-6.6 (m, 8H, C6H4Me); 7.0-7.8 (m,
40H, PC6H5). 31P{1H} NMR (CDCl3): δ(P) 8.3 (m, PdP2); 10.7
1
(m, J PtP ) 2955 Hz, PtP2).
[P d P t(C6H5)2(µ-Br )(µ-d p p m )2]P F 6 was prepared similarly
and isolated as a pale yellow solid in 85% yield. Anal. Calcd
for C62H54BrF6P5Pd2: C, 51.34; H, 3.72. Found: C, 51.20; H,
3.87.
[P d 2(COMe)2(µ-Cl)(µ-d p p m )2]P F 6. Meth od 1. Carbon
monoxide was bubbled through a stirred CH2Cl2 solution (30
mL) of [PdClMe(cod)] (0.10 g, 0.38 mmol) for 1 h, then dppm
(0.15 g, 0.38 mmol), methanol (1.0 mL) and TlPF6 (0.13 g, 0.38
mmol) were added. The reaction mixture was stirred for a
further 30 min, then the solvents were removed. The residue
was washed with ether and pentane, then extracted with CH2-
Cl2 (30 mL). The resulting dark solution was passed through
a short, neutral alumina column (5 cm), eluting with CH2Cl2
(30 mL). The solvent was removed, and the crude material
was precipitated from CH2Cl2/Et2O as a yellow solid. The solid
was dissolved in CH2Cl2 and the solvent was removed rapidly.
The residue was dried in vacuo, leaving a pale yellow solid
(0.21 g, 90%). Anal. Calcd for C54H50ClF6O2P5Pd2: C, 51.94;
H, 4.01. Found: C, 51.67; H, 4.04. 1H NMR (CDCl3): δ(H) 1.67
Exp er im en ta l Section
All reactions were carried out under an atmosphere of argon.
The complexes [PdClMe(cod)],17 [Pd2Cl2(µ-dppm)2],14 and
[PdPtCl2(µ-dppm)2]20 were prepared by reported methods.
1
Grignard reagents were purchased from Aldrich. H and 31P-
{1H} NMR spectra were recorded on a Varian Unity plus 300
or Bruker ARX-500 spectrometer. Chemical shifts are relative
to the residual solvent resonance or external H3PO4, respec-
tively, positive shifts representing deshielding. GC-MS data
were obtained on a Hewlett-Packard 5988 instrument. Mi-
croanalyses were performed by Atlantic Microlab, Inc, Nor-
cross, GA.
2
2
[P d 2(C6H5)2(µ-Br )(µ-d p p m )2]P F 6. To a stirred CH2Cl2
solution (30 mL) of [Pd2Cl2(µ-dppm)2] (0.10 g, 0.095 mmol) at
-78 °C was added an ether solution of PhMgBr (0.5 mL of a
3.0 M solution). The solution turned dark red immediately and
was allowed to stir for 4 h. Methanol (1.0 mL), CBr4 (0.10 g,
0.30 mmol), and TlPF6 (0.034 g, 0.097 mmol) were added, and
the mixture was allowed to warm to ambient temperature,
while the solvents were removed under reduced pressure. After
washing with ether and hexane, the solid residue was ex-
(s, 6H, COCH3); 3.39 (dq, 2H, J HH ) 13.7 Hz, J PH ) 4.6 Hz,
PCH2P); 4.18 (dq, 2H, 2J HH ) 13.7 Hz, 2J PH ) 5.3 Hz, PCH2P);
7.20-7.70 (m, 40H, PC6H5). 31P{1H} NMR (CDCl3): δ(P) 7.3
(s).
Meth od 2. To a suspension of [Pd2Cl2(µ-Cl)2(AsPh3)2] (0.10
g, 0.10 mmol) in CH2Cl2 (30 mL) was added Me4Sn (0.042 mL,
0.31 mmol). The reaction mixture was stirred for 3 h to
generate [Pd2Me2(µ-Cl)2(AsPh3)2] in situ. After bubbling CO
through the solution for 1 h, dppm (0.079 g, 0.21 mmol),
methanol (1.0 mL), and TlPF6 (0.072 g, 0.21 mmol) were added,
and the contents of the flask were stirred for an additional 30
(27) Kullberg, M. L.; Kubiak, C. P. Inorg. Chem. 1986, 25, 26.

Halide-Bridged Organopalladium Complexes
Organometallics, Vol. 20, No. 24, 2001 5219
Ta ble 3. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
P a r a m eter s for
min before the solvents were removed. The resulting solid was
purified as in method 1, and the product was obtained in 87%
yield.
[P d 2(CH2P h )2(µ-Br )(µ-d p p m )2]P F 6‚H2O,2C6H6 a n d
[P d 2Cl2(µ-Cl)(µ-d p p m )2]OH‚0.5(CH3)2CO
[P d 2(COEt)2(µ-Cl)(µ-d p p m )2]P F 6. This complex was pre-
pared by method 2 and isolated in 87% yield. Anal. Calcd for
[Pd2(CH2Ph)2(µ-Br)(µ- [Pd2Cl2(µ-Cl)(µ-dppm)2]-
dppm)2]PF6‚H2O,2C6H6
OH‚0.5(CH3)2CO
C
56H54ClF6O2P5Pd2: C, 52.69; H, 4.23. Found: C, 52.40; H,
4.31. 1H NMR (CDCl3): δ(H) 0.16 (t, 6H, 3J HH ) 7.0 Hz, CH3);
2.00 (q, 4H, 3J HH ) 7.0 Hz, COCH2); 3.41 (dq, 2H, 2J HH ) 13.5
Hz, 2J PH ) 4.4 Hz, PCH2P); 4.13 (dq, 2H, 2J HH ) 13.7 Hz, 2J PH
) 5.3 Hz, PCH2P); 7.20-7.90 (m, 40H, PC6H5). 31P{1H} NMR
(CDCl3): δ(P) 7.4 (s).
cryst syst
space group,
Z
a (Å)
b (Å)
monoclinic
Pm, 2
tetragonal
P412121, 4
12.6462(14)
27.937(3)
12.662(2)
90
116.544(6)
90
14.8997(9)
14.8997(9)
26.308(2)
90
90
90
c (Å)
[P d 2(COCH2P h )2(µ-Cl)(µ-d p p m )2]P F 6. This complex was
prepared by method 2 and isolated in 90% yield. Anal. Calcd
for C66H58ClF6O2P5Pd2: C, 56.60; H, 4.15. Found: C, 56.15;
H, 4.26. 1H NMR (CDCl3): δ(H) 3.30 (s, 4H, COCH2); 3.47 (dq,
2H, 2J HH ) 13.8 Hz, 2J PH ) 4.5 Hz, PCH2P); 4.24 (dq, 2H, 2J HH
) 13.8 Hz, 2J PH ) 5.5 Hz, PCH2P); 5.87 (d, 4H, 3J HH ) 7.5 Hz,
R (deg)
â (deg)
γ (deg)
V (Å3)
density
4001.9(8)
1.427
5840.4(7)
1.389
(g/cm-3
)
3
temp (K)
θ range (deg)
no. of ind
223(2)
1.80 to 27.50
18 773 (0.0000)
223(2)
1.93 to 24.99
5141 (0.2)
CH2C6H5-2,6); 6.88 (t, 4H, J HH ) 7.5 Hz, CH2C6H5-3,5); 6.98
(t, 2H, 3J HH ) 7.5 Hz, CH2C6H5-4); 7.20-7.60 (m, 40H, PC6H5).
31P{1H} NMR (CDCl3): δ(P) 7.4 (s).
reflns (Rint
R(F), Rw(F
)
Elim in a tion Rea ction s. In a typical experiment, the
A-frame complex (0.005 g) was dissolved in dmso-d6 (0.5 mL)
in an NMR tube along with Ph2BnPO (0.0025 g) as an internal
standard. The tube was evacuated and filled with argon. The
sample was placed in the NMR probe and heated rapidly to
the appropriate temperature. The progress of the reaction was
monitored by 1H and 31P{1H} (gated decoupling) NMR spec-
troscopy, the CH2 and 31P resonances being integrated relative
to the corresponding signals from Ph2BnPO. To ensure that
the desired temperature had been reached, data obtained
within the first 5 min were not used. The rate of disappearance
of the A-frame was determined by plotting ln[A] vs time.
X-r a y Cr ysta llogr a p h y. In each case, a crystal was
mounted on a glass fiber in random orientation. Preliminary
examination and data collection were performed using a
Siemens SMART CCD detector system single-crystal X-ray
diffractometer, using graphite-monochromated Mo KR radia-
tion (λ ) 0.71073 Å), equipped with a sealed tube X-ray source
(50 kV × 40 mA). Preliminary unit cell constants were
determined with a set of 45 narrow frame (0.3° in ωj) scans.
The collected frames were integrated using an orientation
matrix determined from the narrow frame scans. The SMART
software package28 was used for data collection as well as
frame integration. Final cell constants were determined by a
global refinement of xyz centroids of 8192 reflections (θ <
19.0°). An empirical absorption correction was applied using
SADABS.29 Structure solution and refinement were carried out
using the SHELXTL-PLUS (5.03) software package.30 The
structure was solved by Patterson methods and refined suc-
2
)
0.0450, 0.1082
0.0549, 0.1147
1.167
0.0532, 0.1175
0.0838, 0.1297
1.079
(F 2>2.0σ(F 2))
2
R(F), Rw(F
(all data)
)
goodness of
2
fit on F
cessfully. Full matrix least-squares refinement was carried out
by minimizing ∑w(Fo - Fc2)2. The non-hydrogen atoms were
refined anisotropically to convergence. The hydrogen atoms
were treated using appropriate riding models (AFIX m3).
Crystal data and structure refinement parameters are given
in Table 3.
2
Ack n ow led gm en t. Thanks are expressed to the
National Science Foundation (grant number CHE-
9508228) for support of this work, and to NSF (grant
numbers CHE-9318696 and 9309690), the U.S. Depart-
ment of Energy (grant number DE-FG02-92CH10499),
and the University of Missouri Research Board for
instrumentation grants.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, atomic coordinates, and anisotropic displacement coef-
ficients for the non-hydrogen atoms, positional and isotropic
displacement coefficients for the hydrogen atoms, and a
complete list of bond distances and angles for [Pd2(CH2Ph)2-
(µ-Br)(µ-dppm)2]PF6‚H2O,2C6H6 and [Pd2Cl2(µ-Cl)(µ-dppm)2]-
OH‚0.5(CH3)2CO. This material is available free of charge via
the Internet at http://pubs.acs.org.
(28) SMART; Siemens Analytical X-ray Division: Madison, WI,
1994.
(29) Blessing, R. H. Acta Crystallogr. A 1995, 51, 33.
(30) Sheldrick, G. M. SHELXTL-PLUS (5.03); Siemens Analytical
X-ray Division: Madison, WI, 1995.
OM010626D