Palladium diphenyl-2-pyridylphosphine complexes

Article abstract of DOI:10.1039/a805102j

The stoichiometric reaction of diphenyl-2-pyridylphosphine (Ph₂PC₅H₄N-2) and palladium(II) acetate afforded the palladium(I) dimer [Pd₂(μ-Ph₂PC₅H₄N) ₂(OAc)₂] 1. Attempts to isolate the monomer, [Pd(μ-Ph₂PC₅H₄N)₂(OAc) ₂], from 2:1 stoichiometry reactions (Ph₂PC₅H₄N:Pd), resulted only in the isolation of 1. When >3 mole equivalents of Ph₂PC₅H₄N were used the zerovalent palladium complex [Pd(Ph₂PC₅H₄N)₃] was isolated. Other palladium(I) dimers with carboxylic or sulfonic anionic ligands have been prepared by a metathesis reaction of the co-ordinated acetates. The complexes 1 and [Pd₂(μ-Ph₂PC₅H₄N) ₂(O₂CCF₃)₂] 2 have been structurally characterised as head-to-tail dimers. The mononuclear complex [Pd(Ph₂PC₅H₄N)₂(O ₂CCF₃)₂] has been obtained by reaction of Ph₂C₅H₄N with [Pd(CH₃CN)₂(O₂CCF₃)₂]. Complex 1 reacts with dimethyl acetylenedicarboxylate to give [Pd₂(μ-Ph₂PC₅H₄N) ₂-(OAc)₂(μ-MeO₂CC=CCO₂Me)], while attempts to isolate an A-frame carbonyl complex were unsuccessful.

Full text of DOI:10.1039/a805102j

View Article Online / Journal Homepage / Table of Contents for this issue
Palladium diphenyl-2-pyridylphosphine complexes
Athanasia Dervisi,a Peter G. Edwards,*a Paul D. Newman,a Robert P. Tooze,b Simon J. Colesa
and Michael B. Hursthousea
a Department of Chemistry, University of Wales Cardiff, Cardiff, UK CF1 3TB
b ICI Acrylics, Wilton, Middlesbrough, UK TS80 8JE
Received 2nd July 1998, Accepted 30th September 1998
The stoichiometric reaction of diphenyl-2-pyridylphosphine (Ph2PC5H4N-2) and palladium() acetate afforded the
palladium() dimer [Pd2(µ-Ph2PC5H4N)2(OAc)2] 1. Attempts to isolate the monomer, [Pd(µ-Ph2PC5H4N)2(OAc)2],
from 2:1 stoichiometry reactions (Ph2PC5H4N:Pd), resulted only in the isolation of 1. When >3 mole equivalents
of Ph2PC5H4N were used the zerovalent palladium complex [Pd(Ph2PC5H4N)3] was isolated. Other palladium()
dimers with carboxylic or sulfonic anionic ligands have been prepared by a metathesis reaction of the co-ordinated
acetates. The complexes 1 and [Pd2(µ-Ph2PC5H4N)2(O2CCF3)2] 2 have been structurally characterised as head-to-tail
dimers. The mononuclear complex [Pd(Ph2PC5H4N)2(O2CCF3)2] has been obtained by reaction of Ph2C5H4N with
[Pd(CH3CN)2(O2CCF3)2]. Complex 1 reacts with dimethyl acetylenedicarboxylate to give [Pd2(µ-Ph2PC5H4N)2-
(OAc) (µ-MeO CC᎐CCO Me)], while attempts to isolate an A-frame carbonyl complex were unsuccessful.
᎐
2
2
2
Introduction
Both in terms of their high activity and superior selectivity
attained under mild conditions, complexes generated in situ
from mixtures of palladium acetate, diphenyl-2-pyridyl-
phosphine (Ph2PC5H4N-2) and sulfonic acids are highly effi-
cient catalysts for the methoxycarbonylation of propyne to
methyl methacrylate (MMA).1 With this in mind we have
undertaken a study on the co-ordination and reaction chem-
istry of this catalyst system. Broadly speaking two catalytic
mechanisms may be in operation, namely the hydride and the
methoxycarbonyl cycles. Drent et al.2 have presented a compel-
ling argument for the operation of the latter based on selectivity
improvements obtained on increasing the steric bulk of the
phosphine ligand. More recently others have provided evidence
for the existence of the hydride cycle.3
Scheme 1
Results and discussion
Palladium complexes of arylphosphines show enhanced
catalytic performance in carbonylation reactions when the well
established triphenylphosphine ligand is replaced by Ph2PC5-
H4N-2.1 Optimum rates and selectivities are realised when a
protic acid co-catalyst whose conjugate base is weakly co-
ordinating, e.g. trifluoromethanesulfonic acid, is present.4 How-
ever, much of the co-ordination chemistry of Ph2PC5H4N dimers
has been established with halides as terminal donors and the
possible non-innocent involvement of the secondary ligands
has been largely ignored. The disposition of nitrogen and phos-
phorus donors in (Ph2PC5H4N) render it an excellent bridging
ligand for late-transition metals, such that a host of homo- and
hetero-binuclear dimers have been reported.5 The usual syn-
thetic approach to the dimers is through the coupling of two
preformed mononuclear complexes (Scheme 1). A change in the
formal oxidation state of the metals is implicit in such a reac-
tion. The resulting dimers exist in two geometric forms: the
more stable head-to-tail (H-T) isomer has the two like donors
bound to different metals, whereas the head to head (H-H) iso-
mer has the nitrogen and phosphorus donors at the same metal
(Scheme 1). Several examples of the less stable H-H isomers
have been isolated.5a,f,6
Palladium(I) dimers
Phosphorus NMR data for the compounds discussed below are
collected in Table 1. The dimer [Pd2(µ-Ph2PC5H4N)2(OAc)2] 1 is
obtained in high yield from the reaction of palladium() acetate
and Ph2PC5H4N in a 1:1 molar ratio. This contrasts with
analogous compounds containing halides as terminal donors;
dimers are only obtained when two preformed mononuclear
species are combined (Scheme 1). In the present case, the redox
chemistry is not solely metal-based, rather two acetate groups
undergo a reductive elimination to generate a palladium(0)
species which reacts with a palladium() complex to give the
palladium() dimer and the elimination product, diacetyl
peroxide (Scheme 2).
The inability to synthesize Pd(Ph2PC5H4N)2X2 (X = CH3-
CO2Ϫ, CF3CO2Ϫ, CH3SO3Ϫ, p-H3CC6H4SO3 or CF3SO3
Ϫ
Ϫ
)
compounds by simple mixing (see below) is indicative of their
inherent instability toward reductive elimination. It is well
established that triphenylphosphine can reduce palladium() to
palladium(0) when oxygen-bearing anions are present,7 e.g.
[Pd(PPh3)2(OAc)2] is reduced spontaneously (but slowly) to a
palladium(0) species with concomitant production of phos-
phine oxide.8
When a 1:1 mixture of Pd(OAc)2 and Ph2PC5H4N is moni-
tored by 31P NMR spectroscopy the initial spectrum reveals a
number of signals between δ 10 and 20. Complex 1 (δ 4.5) is
present as a minor product 20 min after mixing, with a species
giving a signal at δ 9.4 predominating. The peak at δ 9.4 is
Prompted by the above observations, we report here aspects
of the co-ordination chemistry of Ph2PC5H4N with palladium
in the presence of oxygen-bearing secondary ligands, includ-
ing acetate. Unlike the known systems with halides, the oxy-
ligands promote the formation of palladium() dimers from
solely palladium() precursors.
J. Chem. Soc., Dalton Trans., 1998, 3771–3776
3771

View Article Online
Table 1 The 31P NMR data for the Ph2PC5H4N complexes
[Pd2(Ph2PC5H4N)2I2] were obtained. The methyl iodo complex,
[Pd(Ph2PC5H4N)2(CH3)I], which might be expected from
oxidative addition to a palladium(0) intermediate, was not
observed. The mononuclear diodo complex could arise from
successive additions of MeI to a palladium intermediate with
the liberation of ethane. It is noteworthy that Amatore et al.8
reported the formation of the oxidative addition product [Pd-
(PPh3)2(Ph)I] from the reaction of [Pd(PPh3)2] with iodobenzene.
The 31P NMR spectrum of a 1:2 Pd(OAc)2–Ph2PC5H4N mix-
ture is substantially different from that of the 1:1 mixture. A
very broad signal appears at δ 26.2 (in CH2Cl2) with a second at
δ 16.2 but neither could be assigned. At the end of the reaction
several phosphorus containing species were observed, but only
the dimer 1 was isolated upon crystallisation. Ratios of pyridyl
phosphine to Pd(OAc)2 greater than 3:1 afford the zerovalent
complex [Pd(Ph2PC5H4N)3] which has been isolated and char-
acterised as detailed in the Experimental section.
Compound
31P-{1H} (δ)a
1 [Pd2(µ-Ph2PC5H4N)2(OAc)2]
ϩ4.5
ϩ0.1
Ϫ3.3
Ϫ2.4
Ϫ3.1
ϩ14.8
ϩ23.7
ϩ23.4
ϩ31.6
ϩ21.7
Ϫ6.7
2 [Pd2(µ-Ph2PC5H4N)2(O2CCF3)2]
3 [Pd2(µ-Ph2PC5H4N)2(O3SCH3)2]
4 [Pd2(µ-Ph2PC5H4N)2(O3SCF3)2]
5 [Pd2(µ-Ph2PC5H4N)2(O3SC6H4CH3-p)2]
6 [Pd(Ph2PC5H4N)2(O2CCF3)2]
᎐
7 [Pd(Ph2PC5H4N)2(C᎐CPh)2]
᎐
᎐
8 [Pd(Ph2PC5H4N)2(C᎐CMe)2]
᎐
9 [Pd2(µ-Ph2PC5H4N)2(OAc)2(µ-DMAD)]
10 [Pd(Ph2PC5H4N)3]b
Ph2PC5H4N
a In CDCl3, unless otherwise mentioned. b In d6-benzene.
In contrast to previous observations with triphenylphos-
phine,9 formation of Ph2PC5H4N oxide is not taking place in
the stoichiometric mixture of Ph2PC5H4N–Pd(OAc)2, as evi-
denced by the absence of a resonance at δ 20 in the final
spectrum of the reaction mixture. The possible formation of the
N-oxide is also excluded as all the phosphine ligand in the 1:1
mixture is utilised to form the palladium complex 1. The oper-
ation of an alternative reaction pathway for Ph2PC5H4N with
respect to PPh3 is likely to be a result of the second available
(pyridine) donor in Ph2PC5H4N. Thus, stable dimeric com-
plexes such as 1 form readily when the palladium(0) intermedi-
ate species is generated in situ in the presence of unchanged
Pd(OAc)2; the bridging configuration of the phosphine in the
palladium() dimeric product would no doubt render the ligand
less labile and hence less available for subsequent oxidation
than in related PPh3 systems. In the case of PPh3 such an inter-
mediate is likely to decompose in the absence of any other sub-
strate along with (in the presence of an appropriate oxygen
donor) formation of the phosphine oxide. The detection of
acetic anhydride in the final solution suggests an electron
transfer directly from the acetate to the metal centre without
Ph2PC5H4N involvement and is probably best described as a
reductive elimination in the classical sense.
Scheme 2
assigned to [Pd(Ph2PC5H4N)2(OAc)2] which could not be
isolated; the remaining peaks are presumably due to other
intermediates. As the reaction proceeds the signals due to
[Pd(Ph2PC5H4N)2(OAc)2] and other intermediates decay while
that for 1 grows until it is the major component after 24 h
following which 1 can be isolated in good yield (ca. 80%). Our
failure to isolate the bis(acetate) complex is due to the spon-
taneous elimination of both acetates to form a zerovalent
complex of lower co-ordination number (akin to the PPh3
analogue); evidence for the formation of the unstable diacyl
peroxide species (Scheme 2) appears to account for the fate of
the acetate groups (see below). The unassigned signals are likely
to arise from different isomers of this palladium(0) intermediate
where diphenyl-2-pyridylphosphine may act as a chelating,
bridging or mono co-ordinated ligand and which then reacts
with Pd(OAc)2 or [Pd(Ph2PC5H4N)2(OAc)2] to form the dimer
1. The unstable diacyl peroxide species is likely to decompose to
acetic anhydride and oxygen as indicated in the 13C-{1H} NMR
spectrum of the mixture after the reaction is complete; signals
at δ 21.13 and 165.40 being assigned to acetic anhydride. Acetic
The dimers 2–5 are conveniently prepared by the addition of
one mol equivalent of the protic acid of the appropriate oxy-
anion to the 1:1 mixture of Pd(OAc)2 and Ph2PC5H4N. Anion
exchange proceeds readily as the sulfonic and trifluoroacetic
acids employed herein are appreciably stronger protic acids
than is acetic.
X-Ray quality crystals of [Pd2(Ph2PC5H4N)2(OAc)2]ؒCH2Cl2
1
and [Pd2(Ph2PC5H4N)2(O2CCF3)2]ؒ0.5CH2Cl2
2
were
obtained by slow diffusion of light petroleum into solutions of
the appropriate complex in dichloromethane. There are two
independent molecules of 2 in the asymmetric unit which,
although crystallographically distinct, are structurally similar.
The molecular structures of 1 and one of the independent
molecules of 2 with the adopted numbering scheme are shown
in Figs. 1 and 2. Selected bond lengths and angles are summar-
ised in Table 2.
Although several structures of complexes with bridging
Ph2C5H4N ligands have been reported,5a,f,6b,10 the structures of 1
and 2 are the first to contain an unsupported palladium–oxygen
bond. The Pd–Pd bond lengths of 2.579(1) and 2.561(1) Å,
respectively, are the shortest reported for the palladium()–
Ph2PC5H4N dimeric series {cf. 2.597 Å for [Pd2(Ph2PC5H4N)2I2]
and 2.594 Å for [Pd2(Ph2PC5H4N)2Cl2]5a,11} reflecting the
weaker interaction of oxygen donors with the metal. In both 1
and 2 the two Pd–O bond lengths are quite distinct being
2.189(3), 2.139(4) and 2.183(6), 2.307(8) Å, respectively. There
appears to be a correlation between the Pd–O bond length and
the accompanying P–Pd–O bond angle: the longer terminal
Pd–O bonds in both complexes are associated with the tighter
P–Pd–O angle (Table 2) and it would appear that the steric bulk
1
anhydride is also observed in the H NMR spectrum of this
mixture along with a trace of acetic acid, presumably arising
from trace hydrolysis due to adventitious water; these reson-
ances account for approximately half of the total acetate CH3
resonance intensities. Their identity is confirmed by further
addition of an acetic anhydride–acetic acid mixture to the
NMR sample. Loss of oxygen from the elimination by-product
was further established by the addition of 2,3-dimethylbut-2-ene
to the reaction mixture. Formation of the corresponding
epoxide was observed by GCMS analysis.
When iodomethane was added to a 1:2 mixture of Pd(OAc)2
and Ph2PC5H4N, monomeric [Pd(Ph2PC5H4N)2I2 and dimeric
3772
J. Chem. Soc., Dalton Trans., 1998, 3771–3776

View Article Online
Table 2 Selected bond lengths (Å) and angles (Њ) for [Pd2(Ph2PC5-
H4N)2(OAc)2] 1 and [Pd2(Ph2PC5C4N)2(O2CCF3)2] 2
1
Pd(1)–N(2)
Pd(1)–P(1)
Pd(1)–Pd(2)
Pd(1)–O(1)
2.117(5)
2.197(2)
2.5795(7)
2.189(3)
Pd(2)–O(3)
Pd(2)–N(1)
Pd(2)–P(2)
2.139(4)
2.114(4)
2.201(2)
N(2)–Pd(1)–P(1)
P(2)–Pd(2)–Pd(1)
N(2)–Pd(1)–Pd(2)
O(3)–Pd(2)–Pd(1) 170.3(1)
N(1)–Pd(2)–P(2)
N(2)–Pd(1)–Pd(2)
174.2(1)
80.7(2)
93.7(1)
N(2)–Pd(1)–O(1)
P(2)–Pd(2)–O(3)
P(1)–Pd(1)–Pd(2)
N(1)–Pd(2)–O(3)
O(1)–Pd(1)–P(1)
92.1(2)
98.0(1)
80.61(4)
86.9(2)
93.5(1)
173.9(1)
93.7(1)
O(1)–Pd(1)–Pd(2) 173.0(1)
2
Pd(1)–N(2)
Pd(1)–P(1)
Pd(1)–Pd(2)
Pd(2)–O(1)
2.110(7)
2.197(3)
2.561(1)
2.183(6)
P(1)–C(5)
1.817(8)
2.307(8)
2.114(7)
2.216(2)
Pd(1)–O(3)
Pd(2)–N(1)
Pd(2)–P(2)
Fig. 2 Molecular structure and atom labelling scheme of [Pd2(Ph2-
PC5H4N)2(O2CCF3)2]. Hydrogen atoms are omitted for clarity.
N(2)–Pd(1)–P(1)
N(2)–Pd(1)–O(3)
N(2)–Pd(1)–Pd(2)
O(3)–Pd(1)–Pd(2) 168.4(2)
N(1)–Pd(2)–P(2)
N(1)–Pd(2)–Pd(1)
174.7(2)
92.2(3)
93.6(2)
P(2)–Pd(2)–Pd(1)
P(1)–Pd(1)–O(3)
P(1)–Pd(1)–Pd(2)
N(1)–Pd(2)–O(1)
O(1)–Pd(2)–P(2)
80.62(8)
92.9(2)
81.11(9)
87.9(2)
97.6(2)
ligands are cisoid.5a,12 The palladium centres have planar
co-ordination geometries with the angles between the
palladium–ligand bonds varying from 80.6(4) to 98.0(1)Њ in
both complexes. Unlike [Pd2(Ph2PC5H4N)2I2], the two halves of
the dimer are quite distinct with respect to their L–Pd–LЈ bond
angles. The Pd–N–C–P–Pd five-membered rings are non-
planar, and the eight–membered framework adopts a twisted
boat conformation.
173.6(2)
94.4(2)
O(1)–Pd(2)–Pd(1) 171.9(2)
The phosphorus atoms in the binuclear complexes 1–5 are
equivalent giving a sharp singlet in their 31P-{1H} NMR spec-
tra. It is reasonable to assign the solution species as H-T iso-
mers in accord with the solid-state structures presented above
and the established thermodynamic stability of such a geom-
etry. Palladium dimers of Ph2PC5H4N preferentially adopt a
H-T arrangement as the labile nature of the Pd–P bond allows
this thermodynamically more stable isomer to predominate.
Variable temperature 31P-{1H} NMR studies show no evidence
of fluxionality of the Ph2PC5H4N ligand, precluding the possi-
bility of H-T ↔ H-H isomerisation. The 31P-{1H} NMR data
show that there is a tend in δ(31P) as a function of the anionic
ligand, with the dimers 2–5 containing the less basic trifluoro-
acetate and especially sulfonate groups showing 31P shifts to
high field of those of the parental acetate dimer.
The 1H NMR spectra of the complexes closely resemble
that of uncomplexed Ph2PC5H4N. At least three of the
pyridyl protons give discrete signals, whereas two unresolved
multiplets appear for the phenyl groups. The only signifi-
cant co-ordination shift is for the proton ortho to the
phosphorus centre, H(6), which resonates between 0.4 and
0.7 ppm upfield of its position in the spectrum of free
Ph2PC5H4N.
Fig. 1 Molecular structure and atom labelling scheme of [Pd2(Ph2-
PC5H4N)2(OAc)2]. Hydrogen atoms are omitted for clarity.
of the two phenyl substituents on the phosphorus atoms causes
a lengthening of the Pd–O bond as the P–Pd–O angle contracts.
The Pd–O distances of 2.18 1 Å in 1 and 2 compare well with
those observed in [Pd2(dppm)2(O2CCF3)2].12 The mean of the
Pd–O lengths is considerably shorter for the acetate ligand
reflecting its better donor ability with respect to the more
weakly co-ordinating trifluoro analogue. However, due to the
trans influence of the metal–metal bond13 and the larger radius
of PdI, both are longer than equivalent Pd–O bonds in mono-
nuclear palladium() compounds.
The Pd–P (1, average 2.199; 2, 2.207 and 2.209 Å for the
second independent molecule) and Pd–N (1, average 2.116; 2,
2.112 and 2.128 Å for the second independent molecule) dis-
tances are similar to those for the iodo- (Pd–P, average 2.212
and Pd–N, 2.112 Å) and chloro- (Pd–P, average 2.205 and Pd–
N, 2.114 Å) analogues. The Pd–P distances are shorter than
those found in complexes of bridging diphosphines and this is
attributed to the relative trans influences of the P and N atoms.
The Pd–Pd–O angles deviate from linearity by 9.66/7.1Њ for 1
and 11.6/8.1Њ for 2 and are transoid with respect to the M–M
axis in both complexes as opposed to the dimers [Pd2(dppm)2-
(O2CCF3)2] and [Pd2(Ph2PC5H4N)2I2] where the terminal
Changes in the electronic framework of the aromatic rings on
co-ordination are inferred from the 13C NMR spectra, where
low-field shifts of up to 5 ppm are observed for all carbons in
the pyridine ring with respect to those of Ph2PC5H4N. The
phenyl carbons are largely unaltered except for the ipso-
carbons, C(1), which shift 8 ppm upfield upon co-ordination.
In the solid state IR spectrum of complex 1 the acetates have
ν(C᎐O) and ν(C–O) at 1581 and 1366 cmϪ1, respectively; these
᎐
frequencies suggest that inferred structural information should
be treated with caution in these systems since the former is
lower than that reported for monodentate palladium acetates
and compares to anticipated values for bridging or anionic
acetates, whereas the latter is within the range expected for
monodentate co-ordination.14 The symmetric carbonyl stretch
is assigned at 1681 cmϪ1 for 2, again within the expected range
(1680–1720 cmϪ1) for monodentate binding.12 For the com-
plexes 3–5 broad, strong ν(SO3) stretches are seen between 1260
J. Chem. Soc., Dalton Trans., 1998, 3771–3776
3773

View Article Online
and 1006 cmϪ1 which are split implying bound (monodentate)
sulfonates.15
carbon monoxide in solution although the nature of the species
is not clear since the NMR and IR spectra of the adduct are
otherwise very similar to those of 1.
Despite the reluctance of the dimers to insert carbon mon-
Palladium(II) complexes
oxide into the Pd–Pd bond, [Pd2(Ph2PC5H4N)2(OAc)2] 1 reacts
᎐
The only complex of the type [PdX2(Ph2PC5H4N)2], where X is
an oxyanion, isolated during the present study was cis-
[Pd(Ph2PC5H4N)2(O2CCF3)2] 6. Although as already detailed
for the parent acetate system, such species are believed to be
present during the early stages of reaction, they are unstable
with respect to reductive elimination and give palladium()
dimers as the only isolable compounds. The bis(ligand)bis(tri-
fluoroacetate) complex was synthesized by the reaction of
[Pd(CH3CN)2(O2CCF3)2] with 2 moles of Ph2PC5H4N. Both cis
and trans isomers of [Pd(Ph2PC5H4N)2(O2CCF3)2] are possible,
although the formation of only one is indicated by 31P NMR
spectroscopy where a single phosphorus resonance is observed
(δ 14.8). Inspection of the 13C NMR spectrum confirms the
geometry as cis, where pertinent ligand resonances occur as
doublets showing coupling to only one phosphorus atom. It is
readily with DMAD (MeO C᎐CO Me) to give the expected
᎐
2
2
dimetallated alkene product 9, in accord with the previously
reported chloride analogue. Other unactivated alkynes such as
butyne and terminal alkynes did not afford the corresponding
insertion products.
The two phenyl rings attached to each phosphorus atom
in the co-ordinated Ph2PC5H4N ligands are no longer equivalent
in complex 9. Consequently, two sets of resonances appear in
1
the H and 13C-{1H} NMR spectra, one for the phenyl ring
facing the acetate group and one for the ring facing the alkene
group. In the IR spectrum a strong doublet appears due to the
carbonyl groups of DMAD at 1726.4 and 1697.5 cmϪ1
[ν(C᎐O)]. An absorption due to the C᎐C bond could not be
᎐
᎐
assigned.
3
1
known that JCP is usually of equivalent magnitude to JCP in
trans isomers giving virtual triplets for carbons directly bound
to phosphorus:16 this is clearly not the case here, and the isol-
ated complex is assigned a cis-[Pd(Ph2PC5H4N)2(O2CCF3)2]
stereochemistry.
Only palladium() dimers were isolated when the same pro-
cedure was applied for the attempted preparation of [Pd(Ph2-
Ϫ
PC5H4N)2X2] (where X = CH3SO3Ϫ, CF3SO3 or p-H3CC6H4-
SO3Ϫ). Efforts to procure palladium() complexes from other
starting materials were also unsuccessful: the reaction of
[Pd(Ph2PC5H4N)2Cl2] with two equivalents of CF3SO3Ag
afforded only the dimer 4, re-emphasising the inherent stability
Conclusion
of these dimers.
The alkynyl complexes [Pd(Ph PC H N) (C᎐CPh) ] 7 and
᎐
Equimolar solutions of palladium() acetate and Ph2PC5H4N
are unstable with respect to reductive elimination to give ini-
tially palladium(0) species which appear to conproportionate
with PdII to give ultimately palladium() dimers. No external
reductant is required for this reaction which occurs more
readily than with Ph3P in place of Ph2PC5H4N or than for
palladium halides. These observations may be relevant to the
alkoxycarbonylation of alkynes where palladium diphenyl-2-
pyridylphosphine complexes with oxyanions are known to be
far more effective catalysts than triphenylphosphine analoges
or than with palladium halides.
᎐
2
5
4
2
2
᎐
[Pd(Ph PC H N) (C᎐CMe) ]
8 were obtained from the
᎐
2
5
4
2
2
reaction of palladium acetate with diphenyl-2-pyridyl-
phosphine and a ten-fold excess of the appropriate acetylene.
Both white solids reveal an infrared band of medium
Ϫ1
᎐
intensity characteristic of ν(C᎐C) stretches (2105 cm for 7
᎐
and 2110 cmϪ1 for 8). Alkynylpalladium() complexes can
be obtained by oxidative addition of alkynyl halides to
palladium(0) complexes or via metathesis reactions of halo-
17
᎐
genopalladium() complexes with Li(C᎐CR). It has been
᎐
reported recently that alkynylpalladium() complexes can be
prepared by the direct reaction of a palladium() complex with
a terminal alkyne.18
Experimental
When complex 10 is dissolved in acetonitrile Ph2PC5H4N
remains chelated to the palladium centre without a solvent
molecule replacing the co-ordinated pyridyl nitrogen. Addi-
tionally, variable temperature 31P-{1H} NMR spectroscopy
(CDCl3, ≤60 ЊC) showed a lack of exchange between the mono
co-ordinated and chelated ligand. These results suggest form-
ation of a kinetically rigid four-membered chelate.
Reactions were performed under an atmosphere of nitrogen
using standard Schlenk techniques, solid complexes being
stored in a desiccator under air. All solvents were refluxed
under nitrogen over sodium–benzophenone and were distilled
immediately prior to use with the exception of dichloromethane
which was dried over CaH2 and toluene which was refluxed over
sodium. Light petroleum had boiling point 40–60Њ. The 31P-
{1H} NMR spectra (referenced to 85% H3PO4) were collected
on a JEOL FX90Q spectrometer operating at 36.2 MHz,
1H (400.13) and 13C-{1H} (100 MHz) spectra on a Bruker
DPX400 unless stated otherwise and referenced to SiMe4. The
NMR assignments are with respect to the labelling scheme
shown below. Infrared spectra were recorded as KBr discs on a
Nicolet 510 FT-IR spectrophotometer. The compounds
[PdCl2(PhCN)2], [Pd2(dba)3] (dba = dibenzylideneacetone)21
and [PdCl2(Ph2PC5H4N)2]5a,c were prepared by literature pro-
cedures. The [Pd(CH3CN)nX2] (X = O2CCF3, O3SCH3, O3SCF3,
or O3SC6H4CH3-p) complexes were prepared by a procedure
similar to that reported for [Pd(CH3CN)4(O3SCF3)2].22
Balch and co-workers19 have shown that [Pd2(Ph2PC5H4N)2-
Cl2] reacts with CO to give an unbridged Pd–Pd complex with
terminal carbonyl ligands, as indicated by the presence of new
bands at 2019 and 1994 cmϪ1 in the solution infrared spectrum.
Attempts to isolate the carbonyl complex resulted only in the
recovery of [Pd2(Ph2PC5H4N)2Cl2].
Exposure of a CDCl3 solution of complex 1 to 1 atm of
carbon monoxide causes no change in the 1H or 31P-{1H} NMR
spectra, although new infrared bands appear in solution. A very
strong sharp absorbance arises at 2253 cmϪ1 and although not
assigned is very close to the value of 2248 cmϪ1 reported for
[Pd(CO)4]2ϩ 20 while others at 1820.9, 1793.4 and 1712.9 cmϪ1
,
may indicate the presence of bridging carbonyls. The solid that
precipitates from solution was isolated and its IR spectrum
recorded as a KBr disc. A medium intensity band at 1873
cmϪ1 was observed while the rest of the spectrum remains
unchanged. The available data show an interaction of 1 with
3774
J. Chem. Soc., Dalton Trans., 1998, 3771–3776

View Article Online
Preparation
13C NMR/DEPT δ 134.0 (C2, t, J 5.8), 129.0 (C3, t, J 5.8), 130.9
(C4, s), 167.9 (C5, d, J 71.9), 138.9 (C6, s), 125.9 (C8, s), 153.3
(C9, t, J 5.8 Hz), 141.7/139.3/128.2/26.1 (tosylate).
Ph2PC5H4N. A modified literature procedure23 was followed
for the synthesis of Ph2PC5H4N. Sodium metal (1.06 g, 46
mmol) was dissolved in liquid ammonia (100 ml) at Ϫ80 ЊC and
triphenylphosphine (6.00 g, 23 mmol) added as a solid. The
resultant mixture was stirred for 30 min. A diethyl ether solu-
tion of tert-butyl chloride (10 ml, 23 mmol) was then added to
destroy the phenylsodium formed, followed by a diethyl ether
solution (10 ml) of 2-bromopyridine (2.2 ml, 23 mmol). The
temperature was allowed to rise slowly (8 h) to room tempera-
ture during which time the ammonia evaporated to leave a yel-
low solid. Saturated aqueous ammonium chloride was added to
the solid, and the aqueous phase extracted with CH2Cl2 (3 × 50
ml). The organic extracts were combined and the solvent
removed under reduced pressure. The product was obtained as
white crystals after recrystallisation from hot light petroleum.
Yield 5.0 g, 83%. (Found: C, 77.3; H, 5.4; N, 5.3. Calc. for
cis-[Pd2(Ph2PC5H4N)2(O2CCF3)2] 6. A solution of Ph2PC5-
H4N (0.25 g, 0.96 mmol) in acetonitrile (10 ml) was added slow-
ly to a solution of [Pd(CH3CN)2(O2CCF3)2] (0.2 g, 0.48 mmol)
in acetonitrile (20 ml) to give a pale yellow solution. After stir-
ring for 2 h the solvent was evaporated and the resultant yellow
oil stirred overnight with light petroleum to afford a yellow
powder (Found: C, 53.1; H, 3.2; N, 3.1. Calc. for C38H28F6-
1
N2O4P2Pd: C, 53.13; H, 3.29; N, 3.26%). H NMR (CDCl3, δ):
7.29 (H6, br), 7.78 (H7, br), 7.69 (H8, br) and 8.27 (H9, br). 13
C
NMR/DEPT (CDCl3, δ): 125.2 (C1, d, J 17.4 Hz), 134.2 (C2, d,
J 10.9), 128.8 (C3, d, J 11.8), 132.1 (C4, s), 156 (C5, d), 149.5 (C6,
d, J 16.2), 130.8 (C7, d, 20.7 J Hz), 124.8 (C8, s), 137.8 (C9),
116.1 (CF3, q, J 292) and 160.3 (CO2, q, J 36 Hz).
1
C17H14NP: C, 77.56; H, 5.57; N, 5.32%). H NMR (CDCl3, δ):
᎐
7.30 (H6, d), 7.76 (H7, t), 7.36 (H8, t) and 8.86 (H9, d). 13C
NMR/DEPT (CDCl3, δ): 136.2 (C1, d, J 11.6), 134.2 (C2, d,
J 20.0), 128.7 (C3, d, J 7.4), 129.1 (C4, s), 164.0 (C5, d, J 3), 135.8
(C6, d, J 2.1), 127.9 (C7, d, J 15.2), 122.2 (C8, s), and 150.4 (C9,
d, J 13 Hz).
cis-[Pd (Ph PC H N) (C᎐CR) ] (R ؍ Ph 7 or Me 8). To the
᎐
2
2
5
4
2
2
red solution formed on addition of Ph2PC5H4N (0.48 g, 1.9
mmol) to a suspension of Pd(OAc)2 (0.2 g, 0.9 mmol) in CH2Cl2
(40 ml) was added phenylacetylene (1 ml) or propyne (1 ml) as
᎐
appropriate. The complexes [Pd(Ph PC H N) (C᎐CR) ] pre-
᎐
2
5
4
2
2
cipitated as white solids.
᎐
[Pd(Ph PC H N) (C᎐CPh) ] 7 (Found: C, 71.9; H, 4.4; N, 3.2.
᎐
2
5
4
2
2
Calc. for C50H38N2P2Pd: C, 71.90; H, 4.59; N, 3.35%): 1H NMR
[Pd2(Ph2PC5H4N)2X2] 1–5. A solution of Ph2PC5H4N (0.24 g,
0.9 mmol) in dichloromethane (10 ml) was added slowly to
a suspension of palladium() acetate (0.2 g, 0.9 mmol) in
dichloromethane (20 ml). For complexes 2–5, one equivalent of
the corresponding acid, HX, was subsequently added and the
mixture stirred (3 h). The red solution was filtered through
Celite and evaporated to dryness. The resultant red solid was
washed with Et2O (2 × 30 ml) and dried in vacuo. Red crystals
were obtained by slow diffusion of light petroleum into a
dichloromethane solution. Typical yields >80%. Infrared as
KBr discs, and NMR data in CDCl3.
(CDCl3, δ 8.7 (d, 2H), 8.23 (br, 2H), 8.03 (br, 6H), 7.5 (t, 2H),
7.3 (m, 12H), 7.19 (m, 4H), 6.9 (m, 6H) and 6.34 (d, 4H).
᎐
[Pd(Ph PC H N) (C᎐CMe) ] 8 (Found: C, 68.2; H, 4.9; N, 4.0.
᎐
2
5
4
2
2
Calc. for C40H34N2P2Pd: C, 67.56; H, 4.82; N, 3.94%).
[Pd2(Ph2PC5H4N)2(OAc)2(ì-DMAD)] 9. Dimethyl acetyl-
enedicarboxylate (0.3 ml) was added to a solution of [Pd2-
(Ph2PC5H4N)2(OAc)2] (0.09 g, 0.4 mmol) in toluene (30 ml) and
the red solution stirred for 3 h. Addition of light petroleum (100
ml) precipitated a yellow solid (Found: C, 44.7; H, 3.2; N, 3.2.
Calc. for C40H42N2O8P2Pd2: C, 46.44; H, 3.03; N 3.01%). IR
(KBr disk, cmϪ1): ν(DMAD) 1726.4s, 1697.5s, 1263.5s, 1217.5s,
(OAc) 1583.7s, 1386.9m, 1327.1m, (Ph2PC5H4N) 1435.1s,
[Pd2(µ-Ph2PC5H4N)2(OAc)2] 1 (Found: C, 53.7; H, 4.4; N,
3.7. Calc. for C38H34N2O4P2Pd2: C, 53.23; H, 4.00; N, 3.27%):
IR ν(C᎐O) 1581.0s, ν(C–O) 1365.6vs, 1318.7m cmϪ1; H NMR
1
᎐
δ 6.90 (H6, br s), 7.70 (H7, t, 7.8 J Hz), 8.94 (H9, br s) and 1.44
(OAc); 13C NMR/DEPT δ 133.6 (C2, t, J 5.6), 128.4 (C3, t,
J 4.9), 130.1 (C4, s), 169.4 (C5, dd, J 70.1/5.3), 137.6 (C6, s),
129.4 (C7, d, J 24.1), 125.6 (C8, s), 152.4 (C9, t, J 5.6 Hz), 23.4
(CH3) and 176.1 (CO2Ϫ).
1
1096.6s, 696.35m and 534.32s. H NMR (CDCl3, δ): 7.58 (H6,
m), 9.40 (H9, br), 7.98 (m, 4H), 7.76 (m, 4H), 7.45 (m, 4H), 7.25
(m, 4H), 2.86 (s, 3H, DMAD) and 1.35 (s, OAc). 13C-{1H}
NMR (CDCl3, δ): 133.1 (o-C of Ph, d, J 50), 132.0 (o-C of Ph,
d, J 46), 129.6 (p-C of Ph, s), 129.2 (p-C of Ph, s), 127.5 (m-C
of Ph, d, J 43), (m-CPh, d, J 43 Hz), 161.2 (s, O2CCH3), 14.27
(s, O2CCH3), 176.46 (s, CO2CH3) and 49.88 (s, CO2CH3).
[Pd2(µ-Ph2PC5H4N)2(O2CCF3)2] 2 (Found: C, 47.2; H, 2.9; N,
3.0. Calc. for C38H28F6N2O4P2Pd2: C, 47.28; H, 2.92; N, 3.08%):
ν(C᎐O) 1681.2vs, ν(C–O) 1409.4m, ν(CF ) 1193.7/1131.2 cmϪ1
;
᎐
3
1H NMR δ 6.88 (H6, br s), 7.76 (H7, t, J 7.8 Hz) and 8.77 (H9, br
s); 13C NMR/DEPT δ 128.5 (C1, t, J 27.9), 133.5 (C2, t, J 6.0),
129.0 (C3, t, J 5.3), 130.9 (C4, s), 167.7 (C5, dd, J 70.9/6.6), 138.5
(C6, s), 129.8 (C7, d, J 4.9), 126.2 (C8, s), 151.6 (C9, t, J 6.0),
116.0 (CF3, q, 292.5 Hz) and 160.3 (CO2Ϫ, q, J 35 Hz).
[Pd2(Ph2PC5H4N)2(O3SCH3)2] 3 (Found: C, 46.4; H, 3.7; N,
3.0. Calc. for C36H34N2O6P2Pd2S2: C, 46.52; H, 3.69; N, 3.01%):
ν(SO3) 1249.7vs, 1142.4s, 1097.6s, 1006.3vs (Ph2PC5H4N),
535.55s cmϪ1; 1 H NMR δ 6.70 (H6, br s), 7.70 (H7, t, J 7.8), 7.48
(H8, t, J 7.8 Hz), 8.97 (H9, br s), 2.06 (CH3SO3), 7.2–7.4 (Ph, m);
13C NMR/DEPT δ 128.6 (C1, dd, J 58.3/4.9), 134.3 (C2, t), 129.6
(C3, t), 131.4 (C4, t), 167.9 (C5, dd, J 79/6.3), 139.2 (C6, s), 126.7
(C8, s), 153.4 (C9, t, J 6.3 Hz) and 39.1 (CH3, s).
[Pd2(Ph2PC5H4N)3] 10. A solution of Ph2PC5H4N (1.66 g, 6.3
mmol) in methanol (5 ml) was added to a suspension of
Pd(OAc)2 (0.2 g, 0.9 mmol) in methanol (20 ml). The mixture
was stirred for 5 h during which time a bright yellow solid
precipitated. Diethyl ether (50 ml) was added to complete pre-
cipitation and the resultant yellow solid filtered off, washed with
Et2O (2 × 30 ml) and dried in vacuo. Yellow crystals of complex
10 were obtained by slow diffusion of light petroleum into a
toluene solution of the complex (Found: C, 71.0; H, 4.6; N, 4.7.
Calc. for C51H42N2P2Pd: C, 68.35; H, 4.72; N, 4.69%). 1H NMR
(C6D6, δ): 8.46 (d, 2H), 7.78 (m, 12H), 7.42 (m, 3H), 7.12 (m,
18H), 6.86 (t, 3H) and 6.52 (dd, 3H).
[Pd2(µ-Ph2PC5H4N)2(O3SCF3)2] 4 (Found: C, 41.5; H, 3.5; N,
3.3. Calc. for C36H28F6N2O6P2Pd2S2: C, 41.68; H, 2.72; N,
2.70%): ν(SO3) 1257.8vs, 1180s, 1032.0/1004.7s cmϪ1; 1H NMR
δ 6.80 (H6, br s), 7.80 (H7, t, 7.8 Hz), 8.76 (H9, br s), 7.30–7.75
(Ph, m).
Crystallography
Data for compounds 1 and 2 were recorded on a FAST
TV Area detector diffractometer, with a molybdenum target
[λ(Mo-Kα) = 0.71069 Å], equipped with an Oxford Cryo-
systems cryostat and driven by MADNES software operating
on a MicroVax 3200 computer, following previously described
procedures.24 Crystals of 1 and 2 were mounted on glass fibres
using the oil drop technique and collected at 150 and 120 K
respectively. The structures were solved via heavy atom methods
[Pd2(µ-Ph2PC5H4N)2(O3SC6H4CH3-p)2] 5 (Found: C, 52.4;
H, 3.9; N, 2.5. Calc. for C48H42N2O6P2Pd2S2: C, 53.29; H, 3.91;
N, 2.59%): ν(SO3) 1193.7, 1031.2/1009.0vs cmϪ1 1H NMR
;
δ 6.64 (H6, br s), 7.69 (H7, t, J 7.6 Hz), 8.92 (H9, br s), 7.34 (Ph,
m), 6.93 (tosyl, d, 8 Hz), 6.75 (tosyl, d, 8 Hz) and 2.22 (tosyl, s);
J. Chem. Soc., Dalton Trans., 1998, 3771–3776
3775

View Article Online
Table 3 Crystal data for [Pd2(µ-Ph2PC5H4N)2(OAc)2] 1 and [Pd2(µ-Ph2PC5H4N)2(O2CCF3)2] 2
1
2
Empirical formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
C38H34N2O4P2Pd2ؒCH2Cl2
940.06
Monoclinic
P21/a
12.532(2)
20.159(4)
14.7610(6)
104.24(3)
3614.7(10)
4
1.663
1.201
C38H28F6N2O4P2Pd2
965.08
Monoclinic
P21
12.413(9)
20.703(10)
15.580(9)
108.53(10)
3796(4)
β/Њ
U/Å3
Z
4
Dc/Mg mϪ3
1.762
1.176
θ/mmϪ1
F(000)
1812
1992
Crystal size/mm
θ range/Њ
Index ranges
0.18 × 0.07 × 0.145
1.75 to 25.01
Ϫ10 р h р 14, Ϫ23 р k р 23,
Ϫ17 р l р 16
14885
5391
0.0959
0.2 × 0.11 × 0.2
1.84 to 25.03
Ϫ13 р h р 13, 22 р k р 21,
18 р l р 18
16331
9636
0.0689
Reflections collected
Independent reflections
Rint
Data/restraints/parameters
Goodness of fit on F2
Final R, wR2 [I > 2σ(I)]
(all data)
5383/0/455
1.027
0.0478, 0.1067
0.0630, 0.1143
1.956 and Ϫ0.992
9628/1/1000
1.059
0.0438. 0.1078
0.0472, 0.1112
1.436 and Ϫ0.874
0.02(3)
Largest difference peak and hole/e ÅϪ3
Absolute structure parameter
(SHELXS),25 and then subjected to full matrix least-squares
1984, 23, 4309; (b) J. P. Farr, F. E. Wood and A. L. Balch, Inorg.
Chem., 1983, 22, 3387.
7 L. Malatesta and M. Angoletta, J. Chem. Soc., 1957, 1186.
8 C. Amatore, E. Carre, A. Jutand and M. A. M’Barki, Organo-
metallics, 1995, 14, 1818.
9 C. Amatore, E. Carre, A. Jutand and M. A. M’Barki, Organo-
metallics, 1992, 11, 3009.
10 Z.-Z. Zhang, H.-K. Wang, H.-G. Wang and R.-J. Wang,
J. Organomet. Chem., 1986, 314, 357.
2
refinement on Fo (SHELXL 93).26 Non-hydrogen atoms were
made anisotropic, with hydrogens in calculated positions (C᎐H
0.96 Å, with Uiso tied to Ueq of the parent atoms). The weighting
2
scheme used was w = 1/[(2Fo )]. The disordered solvent mole-
cule present in 1 was freely refined as two components, with the
major part having 75% occupancy. Absorption corrections were
applied using DIFABS27 and diagrams drawn with SNOOPI.28
Further details are given in Table 3.
CCDC reference number 186/1182.
See http://www.rsc.org/suppdata/dt/1998/3771/ for crystallo-
graphic files in .cif format.
11 T. Suzuki, N. Iitaka, S. Kuradi, M. Kita, K. Kashiwava, S. Ohba and
J. Fujita, Bull. Chem. Soc. Jpn., 1992, 65, 1817.
12 T. E. Kraft, C. I. Hejna and J. S. Smith, Inorg. Chem., 1990, 29, 2682.
13 D. J. Wink, Acta Crystallogr., Sect. C, 1990, 46, 56.
14 K. Nakamoto, Infrared and raman spectroscopy of inorganic and
coordination compounds, Wiley, New York, 4th edn., 1986, p. 232.
15 S. P. Gejji, K. Hermansson and J. Lindgren, J. Phys. Chem., 1993,
97, 3712.
Acknowledgements
We are grateful to ICI Acrylics for financial support for this
study (A. D. and P. D. N.) and to EPSRC for support for the
crystallography unit at Cardiff.
16 G. Balimann, H. Motschi and P. S. Pregosin, Inorg. Chim. Acta,
1977, 23, 191.
17 M. Akita, M. Terada, S. Oyama and Y. Moro-oka, Organometallics,
1990, 9, 816; J. H. Nelson, A. W. Verstuyft, J. D. Kelly and H. B.
Jonassen, Inorg. Chem., 1974, 13, 27; H. Hoberg and H. J. Riegel,
J. Organomet. Chem., 1983, 241, 245; H. F. Klein, B. Zettel,
U. Florke and H. J. Haupt, Chem. Ber., 1992, 125, 9.
18 A. Bacchi, M. Carcelli, M. Costa, P. Pelagatti and C. Pelizzi, Gazz.
Chim. Ital., 1994, 124, 429.
References
1. E. Drent, D. Arnoldy and P. H. M. Budzelaar, J. Organomet.
Chem., 1993, 455, 247.
2 E. Drent, W. W. Jager, J. J. Keijsper and F. G. M. Niele, in Applied
homogeneous catalysis with organometallic compounds, eds.
B. Cornils and W. A. Hermann, VCH, Weinheim, 1996, vol. 2,
p. 1119.
3 A. Scrivanti, V. Beghetto, E. Campagna, M. Zanato and U.
Matteoli, Organometallics, 1998, 17, 630.
4 E. Drent, D. Arnoldy and P. H. M. Budzelaar, J. Organomet. Chem.,
1994, 475, 57.
19 A. Maisonnat, J. P. Farr and A. L. Balch, Inorg. Chim. Acta, 1981,
53, L217.
20 M. Bochmann, Organometallics 1: complexes with transition metal
carbon σ-bonds, Oxford chemistry primers, 1996, p. 13.
21 M. F. Rettig and P. M. Maitlis, Inorg. Synth, 1977, 17, 134.
22 E. Drent, J. Organomet. Chem., 1991, 417, 235.
23 P. A. A. Klusener, J. C. L. Sukerbuyk and P. A. Verbrugge, Eur. Pat.
Appl., EP-A-499328I, 1992.
24 S. R. Drake, M. B. Hursthouse, K. M. A. Malik and S. A. S. Miller,
Inorg. Chem., 1993, 32, 4653.
25 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
26 G. M. Sheldrick, SHELXL 93, University of Göttingen, 1993.
27 N. P. C. Walker and D. Stuart, Acta Crystallogr., Sect. A, 1983, 39,
158. Adapted for FAST geometry by A. Karaulov, University of
Wales, Cardiff, 1991.
5 (a) Y. Xie, C. Lee, Y. Yang, S. Rettig and B. R. James, Can. J. Chem.,
1991, 70, 751; (b) J. P. Farr, M. M. Olmstead, C. H. Hunt and
A. L. Balch, Inorg. Chem., 1981, 20, 1182; (c) G. De Munno,
J. Organomet. Chem., 1993, 450, 263; (d) J. P. Farr, M. M. Olmstead
and A. L. Balch, Inorg. Chem., 1983, 22, 1229; (e) M. L. Kullberg
and C. P. Kubiak, Organometallics, 1984, 3, 632; ( f ) J. P. Farr,
M. M. Olmstead, N. M. Rutherford, F. E. Wood and A. L. Balch,
Organometallics, 1983, 2, 1758; (g) C. G. Arena, F. Faraone,
M. Lanfranchi, E. Rotondo and A. Tiripicchio, Inorg. Chem., 1992,
31, 4797.
28 K. Davis and K. Prout, University of Oxford, 1993.
Paper 8/05102J
6 (a) F. E. Wood, J. Hvoslef, H. Hope and A. L. Balch, Inorg. Chem.,
3776
J. Chem. Soc., Dalton Trans., 1998, 3771–3776