Synthesis and electrospray mass spectrometry of palladium(II) diphosphine complexes from oxidative addition of 2-bromopyridine to Pd0

Article abstract of DOI:10.1039/a803779e

Oxidative addition reactions of palladium(0) phosphine complexes with 2-bromopyridine gave rise to a series of structurally distinctive complexes, namely trans-(N,P)-[Pd₂Br₂(PPh₃) ₂(μ-C₅H₄N-C²,N)₂] 1, [Pd₂(μ-C₅H₄N-C²,N) ₂-(μ-dppm)₂]Br₂ 2, [Pd₂(η¹-dppp)₂-(μ-C₅H ₄N-C²,N)₂(μ-dppp)]Br₂ 3, trans-[{PdBr(η¹-C₅H₄N-C ²)(μ-dppb)}_n] 4, trans-(N,P)-[Pd₂Br₂(μ-C₅H ₄N-C²,N)₂(μ-dppb)] 5 and cis-[PdBr(η¹-C₅H₄NH-C ²)(η²-dppf)]Br 6 [Ph₂P(CH₂)_nPPh₂, n = 1(dppm), 3(dppp) or 4(dppb); dppf= Fe(Ph₂PC₅H₄)₂]. Similarly, trans-(N,P)-[Pd₂Cl₂(μ-C₉H ₆N-C²,N)₂(μ-dppb)] 7 has been obtained from 2-chloroquinoline and [Pd(dppb)₂]. An array of structural possibilities is envisaged based on the different co-ordination modes of the pyridine (C or/and N bonded; terminal or bridging; pyridyl, pyridine or pyridinium), phosphine (terminal, bridging or chelating) bromide (terminal or ionised) ligands. Complexes 2 and 3, but not the others, can be obtained from phosphine exchange reactions of 1. Complexes 5 and 6 were analysed by X-ray single-crystal crystallographic methods. The former reveals a dinuclear structure with a dppb ligand bridging diagonally two metals that are juxtaposed by two syn-bridging pyridyl groups. It represents an unusual dinuclear core stabilised by two types of bridging ligands of contrasting steric and geometric demands. The latter shows a cationic and mononuclear square planar palladium(II) complex containing a chelating dppf, terminal bromide and an unusual C-bonded pyridyl group with the N-site protonated. The fragmentation of these complexes was investigated by electrospray mass spectrometry under different cone voltages. Breakdown of the dinuclear framework is facilitated by addition of H-Br to the N-Pd bonds of the bridging pyridyl group.

Full text of DOI:10.1039/a803779e

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Synthesis and electrospray mass spectrometry of palladium(II)
diphosphine complexes from oxidative addition of 2-bromopyridine
to Pd0
Clavius C. H. Chin,a Jeremy S. L. Yeo,a Z. H. Loh,a J. J. Vittal,a W. Hendersonb and
T. S. Andy Hor*a
a Department of Chemistry, Faculty of Science, National University of Singapore, Kent Ridge,
119260 Singapore
b Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton, New Zealand
Received 19th May 1998, Accepted 22nd September 1998
Oxidative addition reactions of palladium(0) phosphine complexes with 2-bromopyridine gave rise to a series of
structurally distinctive complexes, namely trans-(N,P)-[Pd2Br2(PPh3)2(µ-C5H4N-C 2,N)2] 1, [Pd2(µ-C5H4N-C 2,N)2-
(µ-dppm)2]Br2 2, [Pd2(η1-dppp)2(µ-C5H4N-C 2,N)2(µ-dppp)]Br2 3, trans-[{PdBr(η1-C5H4N-C 2)(µ-dppb)}n] 4, trans-
(N,P)-[Pd2Br2(µ-C5H4N-C 2,N)2(µ-dppb)] 5 and cis-[PdBr(η1-C5H4NH-C 2)(η2-dppf)]Br 6 [Ph2P(CH2)nPPh2,
n = 1(dppm), 3(dppp) or 4(dppb); dppf = Fe(Ph2PC5H4)2]. Similarly, trans-(N,P)-[Pd2Cl2(µ-C9H6N-C 2,N)2(µ-dppb)]
7 has been obtained from 2-chloroquinoline and [Pd(dppb)2]. An array of structural possibilities is envisaged based
on the different co-ordination modes of the pyridine (C or/and N bonded; terminal or bridging; pyridyl, pyridine or
pyridinium), phosphine (terminal, bridging or chelating) bromide (terminal or ionised) ligands. Complexes 2 and 3,
but not the others, can be obtained from phosphine exchange reactions of 1. Complexes 5 and 6 were analysed by
X-ray single-crystal crystallographic methods. The former reveals a dinuclear structure with a dppb ligand bridging
diagonally two metals that are juxtaposed by two syn-bridging pyridyl groups. It represents an unusual dinuclear core
stabilised by two types of bridging ligands of contrasting steric and geometric demands. The latter shows a cationic
and mononuclear square planar palladium() complex containing a chelating dppf, terminal bromide and an unusual
C-bonded pyridyl group with the N-site protonated. The fragmentation of these complexes was investigated by
electrospray mass spectrometry under different cone voltages. Breakdown of the dinuclear framework is facilitated by
addition of H–Br to the N–Pd bonds of the bridging pyridyl group.
break down to mononuclear complexes upon ligand ex-
Introduction
change.14a The involvement of dinuclear palladium() com-
Since the turn of the century the chemistry of polymers has
plexes in the catalytic cycle is generally neglected despite their
expanded enormously.1 Organic bifunctional polymers contain-
first synthesis back in 198012 and subsequent works.14 Accord-
ing heterocyclic rings have wide applications as they exhibit
ingly, the current understanding of the mononuclear catalytic
properties such as optical non-linearity2 and anisotropic elec-
intermediates is inadequate for PdCl2(P–P)15 which contain
trical conductivity.3 Hitherto, these polymers have been used as
bidentate phosphines (P–P) such as dppe, dppp [Ph2P(CH2)3-
PPh2] and dppf as supporting ligands. The literature is poorly
electrochemical energy storage4 and electrochromic devices,5 as
molecular catalysts6 and in molecular electronics.7 A conveni-
developed on the structures of these catalytic intermediates and
ent synthetic pathway for these polymers is the polymerisation
the co-ordination and structural roles of diphosphine on a
of their respective monomers. The future applications of these
dinuclear framework. In this paper, using 2-bromopyridine as a
polymers hence hinge on our ability chemically to manipulate
their monomeric or oligomeric building blocks. Among the
model, we examine the stabilising effects and structural influ-
ence of different diphosphines on a dinuclear skeleton. The
most successful methodologies for the current synthesis of
complexes formed are characterised by X-ray single-crystal
these precursors are metal-catalysed hetero-coupling reactions,
diffractometry and electrospray mass spectrometry (ESMS).
e.g. Grignard,8 Stille9 and Suzuki10 couplings, in which organo-
metallic catalyst precursors, especially palladium(0) and pal-
ladium() phosphine complexes, are inevitably involved. The
common use of PPh3 as supporting ligands leads to the syn-
thesis of a range of monomeric complexes that are catalytically
related. For example, oxidative addition reaction of 3- or 4-
bromopyridine to [Pd(PPh3)4] results in trans-[PdBr(η1-C5H4N-
C n)(PPh3)2] (n = 3 or 4),11,12 which readily undergoes metathesis
Results and discussion
Synthesis
Oxidative addition of 2-bromopyridine to [Pd(PPh3)4] occurs
readily at room temperature (r.t.) to give a near-quantitative
yield of trans-(N,P)-[Pd2Br2(PPh3)2(µ-C5H4N-C 2,N)2] 1. The
same complex was reported to be obtainable only under reflux
conditions.13 Its identity is substantiated by NMR spectroscopy
and ESMS (see below). Similar reaction occurs between
[Pd2(dppm)3] and 2-bromopyridine in refluxing toluene to give
[Pd2(µ-C5H4N-C 2,N)2(µ-dppm)2]Br2 2. The 31P NMR analysis
shows two sets of triplets indicating two pairs of phosphines
which are chemically distinct. The parent molecular ion was
detected by ESMS analysis (m/z ca. 569). Complex 2 is one of
the few palladium dinuclear complexes with no terminal but
Ϫ
Ϫ
Ϫ
with anions such as N3 , NCOϪ, NCSϪ, NCSeϪ, ClO4 , BF4
Ϫ 13
and PF6 . These mononuclear complexes are generally con-
sidered as catalytically active. By a careful choice of incoming
nucleophile (Nuc), one would obtain [Pd(Nuc)(η1-C5H4N-
C n)(PPh3)2] (n = 3 or 4) which could reductively eliminate
NH4C5-Nuc as a hetero-coupled product and regenerate
[Pd(PPh3)2] as the active catalyst. Even in the cases when
dinuclear complexes are formed, e.g. 2-bromopyridine giving
trans-(N,P)-[Pd2Br2(PPh3)2(µ-C5H4N-C 2,N)2] 1,13 they could
J. Chem. Soc., Dalton Trans., 1998, 3777–3784
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four bridging ligands, namely two C- and N-bonded pyridyl
and two dppm groups. It can also be obtained from the
stoichiometric phosphine-exchange reaction of 1 with dppm.
Similar reaction of [Pd(dppp)2] with 2-bromopyridine yields a
third structural type of dinuclear complex [Pd2(η1-dppp)2-
(µ-C5H4N-C 2,N)2(µ-dppp)]Br2 3. It contains three dppp
ligands, two of which are pendant and one is in a diagonal-
bridging mode; this is consistent with the three discrete reson-
ances in the 31P NMR spectrum. The ESMS spectrum gives a
molecular peak at m/z = 803.4, which agrees with the dicationic
configuration. Similar to 2, phosphine exchange reaction of 1
with an excess of dppp yields 3 as the major product. Reaction
between 2-bromopyridine and [Pd(dppb)2] [dppb = Ph2-
P(CH2)4PPh2] gives different products depending on the reac-
tion temperatures. At r.t. a complex analysed to be trans-
[{PdBr(η1-C5H4N-C 2)(µ-dppb)}n] 4 resulted. Under toluene
reflux the major product is trans-(N,P)-[Pd2Br2(C5H4N-
C 2,N)2(µ-dppb)] 5. Although the nuclearity of complex 4 is
presently unclear, its role as a precursor of 5 based on a tri-
nuclear structural model can be proposed.† Conversion is
achieved by basic attack of the free pyridyl nitrogen on the
neighbouring metal followed by elimination of the phosphine
(Scheme 1). The by-product in this conversion, cis-[PdBr(η1-
Fig. 1 An ORTEP drawing (50% probability level) of the molecular
structure of cis-[PdBr(η1-C5H4NH-C 2)(η2-dppf)]Br 6.
ports the formulation of 6. Complex trans-(N,P)-[Pd2Cl2(µ-
C9H6N-C 2,N)2(µ-dppb)] 7, the chloroquinolyl analogue of 5, is
isolated from a r.t. reaction between [Pd(dppb)2] and 2-
chloroquinoline.
Structures
The isolated complexes illustrate a wide range of structural
variations. All six complexes 1–6 are structurally distinct
whereas 5 and 7 are isostructural. Complexes 1–3 and 5 are
dinuclear with two bridging pyridyl ligands and none to two
bridging phosphine ligands. Single-crystal X-ray diffraction
analysis of 6 reveals a mononuclear d8 cation with a chelating
dppf and an unusual C-co-ordinated pyridyl with the nitrogen
site protonated giving a ligand best described as a neutral
pyridinium (Fig. 1, Table 1).
Scheme 1
The close contact of N(1)H ؒ ؒ ؒ Br(2) [N(1)–H(11) 0.860,
Br(2)–H(11) 2.367(4) Å; N(1)–H(11)–Br(2) 177.61(9)Њ] strongly
supports the protonated model. The established higher trans
influence of the Pd–C bond weakens significantly the opposite
Pd–P bond [2.3863(8) Å] which faces Br. A similar phenom-
enon has been observed in a cis-[PdBr(aryl)(dppe)] complex.16
The P–Pd–P chelate angle [102.08(3)Њ] is comparable to those
found in other square-planar complexes [97.5(1)–104.4(2)Њ].17
The staggered conformation of the ferrocenyl rings [τ 40.8(6)Њ]
is consistent with those in other square-planar chelates.18 The
pyridyl ligand is found primarily in the forms of C-bonded
pyridyl and pyridinium and C,N-bonded pyridyl. It is a ter-
minal ligand C-bonded in both 4 and 6, with 6 having the nitro-
gen end protonated. The phosphine is bridging in most cases (2,
3, 4, 5 and 7) but chelating in 6 and unidentate in 3. In 5, the
bridging phosphine is in the diagonal-bridging mode trans to
the nitrogen end of pyridyl, while for 2 the diphosphine displays
the syn-bridging mode and is necessarily trans to both C and N
ends. In 3 the bridging dppp is trans to the N-donors whilst the
terminal ones are opposite the carbon ends. These structural
variations are attributed to both electronic and steric factors.
The high trans-directing phosphine causes the N of the bridg-
ing pyridyl to adopt the trans-P position. This governs the
diagonal-bridging mode for dppb in 5 and accounts for the
chemically equivalent phosphine sites on a hetero-bridged sys-
tem. In 3 the distinction is less obvious as there are phosphines
C5H4N-C 2)(η2-dppb)], can add to the yield of 5 by a simple
dimerisation process with the elimination of phosphine. Similar
reaction between 2-bromopyridine and [Pd(dppf)2] in refluxing
toluene gives a dark red-brown complex characterised as cis-
[PdBr(η1-C5H4NH-C 2)(η2-dppf)]Br 6. The 31P NMR spectrum
shows two sets of doublets expected for a mononuclear struc-
ture with inequivalent phosphine sites. There is no evidence for
the expected product cis-[PdBr(η1-C5H4N-C 2)(η2-dppf)]. The
free pyridyl nitrogen is probably very prone to protonation by
a trace of HBr in the 2-bromopyridine.‡ The fact that 6 cannot be
obtained from the exchange reaction of 1 with dppf also sup-
† The complex cannot be monomeric since the phosphine sites are
chemically equivalent (31P NMR). A dimeric model is possible but
not likely as it would entail an unusual [Pd2(µ-dppb)2] moiety that is
geometrically constrained. A trimeric model is possible as it has a
reasonable mechanistic path for the generation of a monomeric and
dimeric (5) pyridyl complexes under ESMS conditions (described later)
although there is no definitive experimental support. The ESMS spec-
trum at 5 V only reveals dppbO2, M ϩ Hϩ and 2M ϩ Hϩ. Addition of
the pyridine only breaks up the “oligomer” and gives monomeric
species.
‡ One alternative is that oxidation gives rise to cis-[Pd(η2-C5H4-
N-C 2,N)(dppf)]Br which rapidly undergoes H–Br addition across the
N–Pd bond (in a manner similar to the formation of 2a from 2
described later) to relieve the stress on the strained chelate.
3778
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Table 1 Selected bond lengths (Å) and angles (Њ) for (a) trans-(N,P)-
both palladium planes) also reflect such parity. It is notable that
[Pd2Br2(µ-C5H4N-C 2,N)2(µ-dppb)]ؒMeCN
C5H4NH-C 2)(η2-dppf)]Brؒ0.5MeOH 6
5
and (b) cis-[PdBr(η1-
the Pd–Br [2.5197(5) Å] bond is significantly longer than that in
6 [2.4695(4) Å]. This is indicative of the higher trans influence
of the pyridyl carbon in 5 (compared to the phosphino group
in 6) and suggests that the terminal bromide is susceptible to
ligand exchanges. This is verified in the formation of 2 and 3
from 1 and is an important consideration in the design of
these Pd2 species as catalysts for hetero-couplings. Comparison
of the crystal structures of 1 and 5 shows that the diphosphine
ligand increases the torsional twist of Br–Pd ؒ ؒ ؒ Pd–Br. The
compounds dppm and dppf, on the other hand, are too short
and crowded respectively to fit into the diagonal-bridge ‘cavity’
and hence their equivalents of 5 cannot be obtained. For
entropic reasons it is not favourable for 5 to take up two
additional dangling dppb ligands like that in 3. The pin-wheel-
like structure of 2 is stabilised by four bridging ligands of
comparable steric requirements. There is little driving force
for such a structure to disrupt and give way to any structure
with dangling phosphines. The steric demand of dppf favours
a chelating mode that provides an impetus for 6 to be
mononuclear.
(a)
Pd(1)–C(6)
Pd(1)–P(2)
Pd(2)–N(2)
Pd(2)–Br(2)
N(1)–C(5)
N(2)–C(10)
1.993(4)
2.2609(10)
2.096(3)
2.5196(5)
1.352(5)
1.348(5)
Pd(1)–N(1)
Pd(1)–Br(1)
Pd(2)–C(1)
Pd(2)–P(1)
N(1)–C(1)
N(2)–C(6)
2.093(3)
2.5197(5)
2.003(4)
2.2678(11)
1.352(5)
1.356(5)
C(6)–Pd(1)–N(1)
N(1)–Pd(1)–P(1)
N(1)–Pd(1)–Br(1)
C(6)–Pd(1)–Pd(2)
P(1)–Pd(1)–Pd(2)
C(1)–Pd(2)–N(2)
N(2)–Pd(2)–P(2)
N(2)–Pd(2)–Br(2)
C(1)–Pd(2)–Pd(1)
P(2)–Pd(2)–Pd(1)
87.4(2)
169.31(9)
91.12(9)
64.29(12)
108.83(3)
87.10(14)
170.06(10)
90.62(9)
64.30(12)
109.68(3)
C(6)–Pd(1)–P(2)
C(6)–Pd(1)–Br(1)
P(1)–Pd(1)–Br(1)
N(1)–Pd(1)–Pd(2)
92.52(12)
173.78(11)
90.95(3)
61.63(9)
Br(1)–Pd(1)–Pd(2) 119.28(2)
C(1)–Pd(2)–P(2)
C(1)–Pd(2)–Br(2)
P(2)–Pd(2)–Br(2)
N(2)–Pd(2)–Pd(1)
93.29(11)
172.35(11)
90.22(3)
61.64(10)
Br(2)–Pd(2)–Pd(1) 120.72(2)
(b)
Although we have successfully assembled a quinolyl deriv-
ative of a tribridged system, viz. 7, it is worth noting that
refluxing 2-chloroquinoline with [Pd2(dppm)3] in toluene, con-
trary to what is found with 2 or 7, does not result in the isolation
of any quinolyl complex but an unexpected product, [Pd2Cl2-
(µ-dppm)2]. One possible explanation is that, although similar
oxidative addition can occur, a quinolyl-bridged structure
analogous to that of 2 or 7 could break down under forcing
conditions (Scheme 2). Chloride attack would open the bridge
Pd(1)–C(11)
Pd(1)–P(1)
P(1)–C(1)
N(1)–C(11)
N(1)–H(11)
2.037(3)
2.3863(8)
1.812(3)
1.341(5)
0.860
Pd(1)–P(2)
Pd(1)–Br(1)
P(2)–C(6)
N(1)–C(15)
Br(2)–H(11)
2.2837(8)
2.4695(4)
1.804(3)
1.353(5)
2.367(4)
C(11)–Pd(1)–P(2)
P(1)–Pd(1)–P(2)
P(2)–Pd(1)–Br(1)
87.50(9)
102.08(3)
172.00(2)
C(11)–Pd(1)–P(1)
C(11)–Pd(1)–Br(1)
P(1)–Pd(1)–Br(1)
169.77(9)
84.50(9)
85.91(2)
N(1)–H(11)–Br(2) 177.61(9)
Fig. 2 An ORTEP drawing (50% probability level) of the molecular
structure of trans-(N,P)-[Pd2Br2(µ-C5H4N-C 2,N)2(µ-dppb)] 5.
trans to both carbon and nitrogen ends. The two bridging
pyridyl rings juxtapose the metal atoms and determine the
Pd–Pd separation, thereby restricting the type of diphosphine
ligands that are suitable for the diagonal-bridging mode. The
long and flexible carbon skeletal chains of dppp and dppb do
not favour a syn-bridge conformation but permit the diagonal-
bridge formation.
These ideas are illustrated in the single-crystal X-ray molecu-
lar structure of 5 (Fig. 2). It is an unusual dinuclear structure
showing how two sterically contrasting bridging ligands can be
accommodated within a dinuclear framework. With the two
square-planar palladium atoms locked by two pyridyl bridges
in a syn conformation [dihedral angle between the Pd(1)–C(6)–
N(2)–Pd(2) and Pd(1)–N(1)–C(1)–Pd(2) planes is 77.06(7)Њ],
the dppb ligand takes advantage of its long alkyl chain by
traversing in a diagonal-bridging mode. This forces the Br(1)–
Pd(1) ؒ ؒ ؒ Pd(2)–Br(2) torsional angle to twist from an ideal 0 to
88.11(4)Њ. As a result the phosphines fit into the Pd2 pocket and
are chemically equivalent, which is consistent with the solution
NMR data. The similar Pd–P bond lengths [2.2609(10) and
2.2678 (11) Å] (and similar Pd–Br and Pd–N bond lengths on
Scheme 2
by converting a C,N-bonded to a C-bonded quinolyl group.
Intramolecular reductive elimination would follow and yield
2,2Ј-biquinoline and the observed product, [Pd2Cl2(µ-dppm)2].
Similar PdI–PdI complexes of the dppp, dppb and dppf
analogues are rare and their synthesis from this path is a subject
of our current investigation.
These results suggest that a slight change in the skeletal prop-
erty of the diphosphine would have a significant influence on
the dinuclear framework. This intricate influence is manifested
in different bonding modes of the phosphines, which result in
different structures. For a diphosphine that favours a chelating
mode the dinuclear unit can break down to a mononuclear
complex, as witnessed in 6. Attempts to synthesize the dppe
analogue of 5 by oxidative addition have so far been unsuccess-
ful. This is reminiscent of the sensitivity of the dinuclear struc-
ture to its diphosphine skeletal residue. In the phosphine
exchange reactions, dppm and dppp react smoothly with 1 to
give 2 and 3 respectively. However, dppb, dppf and dppe, whose
co-ordinating ability is not known to be weaker, do not show
J. Chem. Soc., Dalton Trans., 1998, 3777–3784
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any activity. These differences emphasise the stability bridging
of dppm (e.g. in A-frame type complexes) and possible isolation
of the unidentate mode of dppp due to its optimum chain
length. The similar ease of replacement of PPh3 or BrϪ in 1
makes structural prediction of any ligand-exchanged products
difficult.
Among all the complexes isolated, only 6 is mononuclear.
Preliminary results indicate that metathesis (and deproton-
ation) of 1 with Liϩ(C4H3S)Ϫ (C4H3S = 2-thienyl) gives trans-
(N,P)-[Pd2(η1-C4H3S)2(PPh3)2(µ-C5H4N-C 2,N)2] which under-
goes thermolysis in toluene to give 2-thienylpyridine.19 This
suggests that at least some of the many known Pd-catalysed
hetero-coupling reactions do not proceed through mononuclear
intermediates, especially when the phosphines used are biden-
tate, and that the dinuclear complexes isolated in this work are
catalytically significant.
Electrospray mass spectrometry
The emerging importance of ESMS as a soft ionisation mass
spectrometry for organometallic complexes is evident.20 In this
work it is used to investigate the dinuclear-to-mononuclear
breakdown of the Pd2 species and unveil the various fragmen-
tation possibilities of a dinuclear core. The information thus
obtained would help to understand the Pd-catalysed coupling
mechanisms and design of new catalysts using diphosphines as
ligand support. Similar use of ESMS to observe catalytic
intermediates in the Pd0-catalysed Suzuki coupling has been
reported.21 At a very low cone voltage of 5 V, both mono-
cationic trans-(N,P)-[Pd2Br(MeCN)(PPh3)2(µ-C5H4N-C 2,N)2]ϩ
1a (m/z = 1013.9) and dicationic species trans-(N,P)-[Pd2-
(MeCN)2(PPh3)2(µ-C5H4N-C 2,N)2]2ϩ 1b, (m/z = 487.9), from
MeCN displacement of Br in 1, are observed with the latter as
the dominant species (Fig. 3). The displacement of halide and
subsequent co-ordination of MeCN to the parent fragment is a
Fig. 3 Positive-ion electrospray mass spectra of complex 1 in MeCN–
water solvent (1:1 v/v) at cone voltages of (a) 5, (b) 20, (c) 30 and (d) 40
V. The spectra show the formation of the bis(acetonitrile) dication 1b at
low cone voltages, which loses MeCN ligands (giving 1d and 1c) and
fragments to the monomeric species 1f and 1g as the cone voltage is
raised.
Ϫ
common phenomenon in ESMS.22 The BF4 salt of 1b was
independently synthesized and its ESMS spectra analysed.
They generally agree with those obtained from 1 and support
that 1b is an essential fragmentation product of 1. At 20 V
ligand fragmentation begins to occur, giving rise to [Pd2-
(PPh3)2(µ-C5H4N-C 2,N)2]2ϩ 1c (m/z = 446.3) and [Pd2(MeCN)-
(PPh3)2(µ-C5H4N-C 2,N)2]2ϩ 1d (m/z = 467.4). The preferred dis-
sociation of BrϪ and MeCN suggests that the bond strength of
Pd–Y decreases in the order Y = PPh3 > MeCN > BrϪ. At 30 V
the amount of 1c and 1d increases and 1c becomes the domin-
ant species. A species 1e (m/z = 340.4) is formed whose isotope
pattern suggests a phosphonium cation [Ph3PC5H4N]ϩ, formed
by dissociative coupling between the phosphine and pyridyl lig-
ands. At 40 V fragmentation to monopalladium species occurs.
The peak of 1b is overshadowed by the monomeric ion
[Pd(MeCN)(C5H4N)(PPh3)]ϩ 1f, having the same m/z value but
peaks separated by 1 mass unit, and 1c by peaks due to
[Pd(C5H4N)(PPh3)]ϩ 1g. The pyridyl ligand remains intact
even at 40 V, which supports C-bonded13,14b,23 rather than the
generally weaker N-bonded pyridine. Another new species,
[Pd2Br(C5H4N)2(PPh3)2]ϩ 1h, at m/z = 972.4 also surfaces at this
high voltage. These results are consistent with the general
observation that increasing the cone voltage results in ions of
lower charge. These experiments provide unequivocal support
that the dinuclear structure is maintained in solution. Disso-
ciation of BrϪ and MeCN occurs prior to the monomeris-
ation through cleavage of the bridging pyridyl groups. Bromide
dissociation is most facile and is probably assisted by the
MeCN present. Similar results have been observed for other
metal phosphine–halide complexes.24 To substantiate the latter
point, the ESMS spectra of 1 were recorded with the sample
solution doped with a few drops of pyridine. As expected,
species arising from bromide/acetonitrile replacement by pyri-
dine,viz.trans-(N,P)-[Pd2Br(η1-C5H5N-N)(PPh3)2(µ-C5H4N-C 2,
N)2]ϩ 1i (m/z = 1051.5) {and its MeCN substitution complex,
Scheme 3
trans-(N,P)-[Pd2(MeCN)(η1-C5H5N-N)(PPh3)2(µ-C5H4N-C 2,
N)2]2ϩ 1k (m/z = 507.0)} and trans-(N,P)-[Pd2(η1-C5H5N-
N)2(PPh3)2(µ-C5H4N-C 2,N)2]2ϩ 1j (m/z = 525.5) were detected
(Scheme 3). These results suggest a rich ligand replacement
chemistry in 1 which can be developed in line with the catalytic
initiatives.
The parent ion of complex 2 (m/z = 569) was detected as a
3780
J. Chem. Soc., Dalton Trans., 1998, 3777–3784

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minor species at low cone voltage (5 V). The occurrence of two
major peaks (m/z = 609.1 and 1217.7) merits special comment.
The former peak is attributed to [Pd2H(Br)(C5H4N)2(dppm)2]2ϩ
2a, which is formed from protonation of 2, while the latter
is most likely an ion pair of 2 ϩ BrϪ. The parent ion of 3
(m/z = 803.4) was detected at 5 V. The presence of three differ-
ent types of ligands, viz. bridging pyridyl, bridging phosphine
and unidentate phosphine, creates a number of possibilities for
ligand transformation and molecular fragmentation. A large
number of molecular peaks observed at such a low cone voltage
supports this projection. Similar to 2, complex 3 readily adds
HϩBrϪ to give [Pd2H(Br)(C5H4N)2(dppp)3]2ϩ 3a (m/z = 843.3),
[Pd2H2Br2(C5H4N)2(dppp)3]2ϩ 3b (m/z = 884.5), [PdH(Br)-
(C5H4N)(dppp)]ϩ 3c (m/z = 677.5) and [PdH(Br)(C5H4N)-
(dppp)2]2ϩ 3d (m/z = 1089.9). Chelation of one of the dangling
phosphines in 3 could lead to an immediate rupture of the
phosphine bridge which triggers the splitting of 3 to two mono-
nuclear fragments: [Pd(C5H4N)(dppp)2]2ϩ 3f (m/z = 1007.8) and
[Pd(C5H4N)(dppp)]2ϩ 3g (m/z = 595.6). In the presence of trace
HϩBrϪ, and especially at higher voltages (e.g. 40 V), the amount
of 3d, 3f and 3g would decrease while that of 3c is observed to
increase relatively. Species 3c is thus a possible sink in a series of
acid-promoted fragmentations. As it is structurally identical to
6, detection of 3c together with its precursors under ESMS
conditions provides a mechanistic mapping for the formation
of 6 under synthetic conditions. Ligand displacement of dppp
in 3 by BrϪ giving [Pd2Br(C5H4N)2(dppp)2]ϩ 3e (m/z = 1433.8) is
also evident.
recorded at 121.39 MHz with 85% H3PO4 as external reference.
Elemental analyses were performed by the Microanalytical
Laboratory staff of the Department of Chemistry in the
National University of Singapore. The facile solvation of many
dppf (other diphosphines) complexes precludes an accurate
microanalysis of some complexes.25 All solvents were of
analytical grade and freshly distilled before use. All chemical
reagents were purchased commercially and used as received.
The complexes [Pd(PPh3)4], [Pd2(dppm)3], [Pd(dppp)2], [Pd-
(dppb)2] and [Pd(dppf)2] were prepared by the literature
methods with slight modification.26
Electrospray mass spectrometry
Electrospray mass spectra were recorded using a VG Platform
II instrument, using MeCN–water (1:1) or MeOH as the
mobile phase solvent. Cone voltages were typically varied from
5 to 80 V in order to investigate the effect of higher voltages on
fragmentation pathways of the various compounds observed.
The peaks in the ESMS are identified by the most intense m/z
value within the isotopic mass distribution. Further details of
the experimental set-up used are available elsewhere.27 Isotope
patterns were recorded under high-resolution conditions for all
major ions and compared with theoretical patterns obtained
using the Isotope program.28 In all cases there was good agree-
ment between the experimental and calculated isotopic mass
distributions.
Syntheses
At 5 V, HϩBrϪ elimination from complex 6 generates [Pd-
(C5H4N)(dppf)]ϩ 6a (m/z = 737.4); dimeric species correspond-
ing to formulae [Pd2HBr2(C5H4N)2(dppf)2]ϩ 6b (m/z = 1637.7)
and [Pd2H2Br3(C5H4N)2(dppf)2]ϩ 6c (m/z = 1718.0) are also
observed. The protonation site and hence the structures of
these species are uncertain. At 20 V a fresh species [Pd(C5H4-
NH)(dppf)]2ϩ 6d (m/z = 369.5) is detected, which can either be
formed from 6 through a bromide dissociation or 6a through
protonation. As the cone voltage is increased to 80 V, 6d
disappears and 6a emerges as the dominant species.
trans-(N,P)-[Pd2Br2(PPh3)2(ì-C5H4N-C 2,N)2] 1. Freshly pre-
pared [Pd(PPh3)4]26 [obtained in situ from PdCl2 (1.000 g, 5.64
mmol)] was dissolved in toluene and 2-bromopyridine (1.620 g,
10.25 mmol) added slowly. The resulting mixture was stirred for
2 h, resulting in the formation of a yellow-green suspension.
The product was filtered off and washed thoroughly with an
excess of diethyl ether before recrystallisation from chloro-
form–hexane (2.91 g, 98%) (Found: C, 52.6; H, 3.8; Br, 13.5; N,
2.8; P, 5.8; Pd, 19.8. C23H19BrNPPd requires C, 52.5; H, 3.7; Br,
15.2; N, 2.7; P, 5.9; Pd, 20.2%). δP(CDCl3) 29.8 (s).
[Pd2(ì-C5H4N-C 2,N)2(ì-dppm)2]Br2 2. 2-Bromopyridine
(0.724 g, 4.58 mmol) was added to a toluene solution of
[Pd2(dppm)3]26 [obtained in situ from PdCl2 (0.310 g, 1.75
mmol)]. The resulting mixture was refluxed for ca. 1 d after
which light yellow crystals were formed. The mixture was fil-
tered and the crystals washed with an excess of toluene and
diethyl ether and recrystallised from methanol (0.96 g, 85%)
(Found: C, 54.1; H, 4.1; Br, 15.5; N, 2.1; P, 9.3; Pd, 13.0.
C30H26BrNP2Pd requires C, 55.5; H, 4.0; Br, 12.3; N, 2.2; P, 9.6;
Pd, 16.4%). δC(CD3OD) 152.6 (C2). δP(CD3OD) 11.8 (t) and 3.5
(t) [J(P–P) = 30 Hz].
Conclusion
Our work demonstrated that the structures of the palladium()
pyridyl complexes are highly sensitive to the supporting phos-
phines. Although their structures are not easily predictable, they
usually represent a compromise of conflicting demands of the
diphosphine and pyridyl ligands. In some cases the resultant
complexes arise from a series of molecular rearrangements
whose mechanisms are complex and unknown. Based on the
products formed under ESMS conditions, one can gain some
valuable insight on the ligand transformation and co-
ordination mode changes during the molecular rearrangement
process. Although the ESMS data alone cannot provide defini-
tive structural proof of various transient species detected, a
meaningful discussion on the structural changes can be pre-
sented when supported by synthetic and crystallographic
data. Collectively, this information provides a platform for us
to understand the formation mechanism of the complexes
concerned.
[Pd2(ç1-dppp)2(ì-C5H4N-C 2,N)2(ì-dppp)]Br2 3. Freshly pre-
pared [Pd(dppp)2]26 [obtained in situ from PdCl2 (0.182 g, 1.02
mmol)] was dissolved in toluene and 2-bromopyridine (0.178 g,
1.13 mmol) added slowly. The resulting mixture was refluxed for
2 h, resulting in the formation of off-white crystals. The crystals
were filtered off, washed with an excess of toluene and diethyl
ether and recrystallised from methanol (0.90 g, 99%) (Found: C,
62.2; H, 5.2; Br, 9.5; N, 1.7; P, 11.3; Pd, 10.2. C91H86Br2N2P6Pd2
requires C, 61.9; H, 4.9; Br, 9.1; N, 1.6; P, 10.5; Pd, 12.1%).
δC(CD3OD) 183.5 (C2). δP(CD3OD) 12.9 (dd) [2J(P–P) 330,
Experimental
General procedures
3
3J(P–P) 35], 3.4 (dd) [2J(P–P) 330, J(P–P) 54 Hz] and Ϫ4.5
All reactions involving light-sensitive compounds were per-
formed shielded from light. Unless stated otherwise, all reac-
tions were performed under a positive pressure of purified
argon. All 1H NMR spectra were recorded at 300 MHz at 25 ЊC
on a Bruker ACF 300 spectrometer. Chemical shifts are
reported in ppm to high frequency with Me4Si as internal
(unresolved multiplets).
trans-[{PdBr(ç1-C5H4N-C 2)(ì-dppb)}n]ؒC6H5Me 4. 2-Bromo-
pyridine (0.604 g, 3.82 mmol) was added to a toluene solution
of [Pd(dppb)2]26 [obtained in situ from PdCl2 (0.184 g, 1.04
mmol)]. The resulting mixture was stirred at r.t. until bright
yellow crystals were precipitated. After filtration the crystals
1
standard for H NMR spectra. The 31P NMR spectra were
J. Chem. Soc., Dalton Trans., 1998, 3777–3784
3781

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were washed with an excess of toluene and diethyl ether and
recrystallised from methanol–diethyl ether (0.37 g, 75%)
(Found: C, 56.5; H, 5.0; Br, 11.5; N, 1.9; P, 7.9; Pd, 14.8.
C33H32BrNP2Pd requires C, 57.4; H, 4.7; Br, 11.6; N, 2.0; P, 9.0;
Pd, 15.4%). δC(CDCl3) 131.7 (C2). δP(CDCl3) 32.8 (s).
were collected by filtration, washed with an excess of toluene
and diethyl ether and recrystallised from methanol (0.30 g,
47 %) (Found: C, 57.9; H, 4.2; Cl, 6.7; P, 8.9; Pd, 15.1.
C25H22ClP2Pd requires C, 57.1; H, 4.2; Cl, 6.7; P, 11.7; Pd,
20.2%). δP(CDCl3) Ϫ3.1 (s).
trans-(N,P)-[Pd2Br2(ì-C5H4N-C 2,N)2(ì-dppb)] 5. 2-Bromo-
pyridine (0.604 g, 3.82 mmol) was added to a toluene solution
of [Pd(dppb)2]26 [obtained in situ from PdCl2 (0.184 g, 1.04
mmol)]. The resulting mixture was refluxed until light yellow
crystals were precipitated. After filtration the crystals were
washed with an excess of toluene, methanol and diethyl ether.
The product was recrystallised from dichloromethane–pentane
(0.37 g, 75%) (Found: C, 47.1; H, 3.9; Br, 23.0; N, 3.3; P, 6.4; Pd,
18.7. C19H18BrNPPd requires C, 47.9; H, 3.8; Br, 16.7; N, 3.0;
P, 6.5; Pd, 22.3%). δP(CDCl3) 25.2 (s).
trans-(N,P)-[Pd2(MeCN)2(PPh3)2(ì-C5H4N-C 2,N)2][BF4]2 1b.
The salt AgBF4 (0.342 g, 1.76 mmol) was added to a fresh
acetonitrile solution of complex 1 (0.853 g, 0.81 mmol). The
resulting mixture was stirred at r.t. for ca. 2 h. The mixture was
filtered and diethyl ether was added to the filtrate for precipi-
tation (yield 0.76 g, 85%) (Found: C, 53.9; H, 4.1; B, 2.0; F, 13.8;
N, 4.6; P, 5.0; Pd, 18.1. C25H22BF4N2PPd requires C, 54.2; H,
4.0; B, 1.9; F, 13.3; N, 5.1; P, 5.6; Pd, 19.2%). δP(CD3CN)
29.5 (s).
X-Ray crystallography
cis-[PdBr(ç1-C5H4NH-C 2)(ç2-dppf)]Brؒ0.5H2O 6. 2-Bromo-
pyridine (0.433 g, 2.74 mmol) was added to a toluene solution
of [Pd(dppf)2]26 [obtained in situ from PdCl2 (0.405 g, 2.28
mmol)]. The resulting mixture was refluxed until deep red crys-
tals were precipitated from an indigo solution (about 2–3 d).
After filtration, the crystals were washed with an excess of tolu-
ene and diethyl ether and recrystallised from methanol–diethyl
ether (0.14 g, 14%) (Found: C, 51.5; H, 3.7; Br, 14.7; N, 1.6;
P, 7.5; Fe, 6.6; Pd, 12.3. C39H33Br2FeNP2Pdؒ0.5H2O requires
C, 51.5; H, 3.8; Br, 17.6; Fe, 6.2; N, 1.5; P, 6.8; Pd, 11.7%).
δC(CDCl3) 140.1 (C2). δP(CDCl3) 29.4 (d) and 14.2 (d) [2J(P–P)
19 Hz].
Single-crystal X-ray diffraction experiments were carried out on
a Siemens SMART CCD diffractometer with a sealed tube at
23 ЊC and Mo-Kα radiation (λ 0.71073 Å). Preliminary cell
constants were obtained from 45 frames (width of 0.3Њ in Ω)
data. Final cell parameters were obtained by global refinements
of reflections obtained from integration of all the frame data.
The data were collected with a frame width of 0.3Њ in Ω and a
counting time of 20 s per frame at a crystal-to-detector distance
of 5.027 cm. The software SMART29 was used for collecting
frames of data, indexing reflections and determination of
lattice parameters, SAINT17 for integration of intensity of
reflections and scaling, SADABS30 for absorption correction
and SHELXTL31 for space group and structure determination,
refinements, graphics and structure reporting.
Light yellow crystals of complex 5 were grown from a sample
solution in acetonitrile and dichloromethane by slow evapor-
ation at r.t. A suitable crystal of dimensions 0.35 × 0.25 × 0.2
mm was selected, wedged inside a glass capillary tube and
flame-sealed. A total of 12548 reflections were collected in the θ
range 1.74–29.04Њ (Ϫ12 < h < 13, Ϫ16 < k < 17, Ϫ22 < l < 22).
trans-(N,P)-[Pd2Cl2(ì-C9H6N-C 2,N)2(ì-dppb)] 7. 2-Chloro-
quinoline (0.210 g, 1.28 mmol) was added to a toluene solution
of [Pd(dppb)2]26 [obtained in situ from PdCl2 (0.227 g, 1.04
mmol)]. The resulting mixture was stirred at r.t. for ca. 2 d until
off-white crystals were precipitated. After filtration the crystals
were washed with an excess of toluene and diethyl ether (0.05 g,
17%) (Found: C, 57.6; H, 4.3; Cl, 6.4; N, 2.5; P, 4.9; Pd, 23.3.
C23H20ClNPPd requires C, 57.2; H, 4.2; Cl, 7.3; N, 2.9; P, 6.4;
Pd, 22.0%). δP(CDCl3) 25.7 (s).
¯
For Z = 2, the space group P1 was chosen in the triclinic sys-
tem. All the non-hydrogen atoms in the neutral molecule were
refined anisotropically. One disordered molecule of acetonitrile
was located in the Fourier-difference synthesis (occupancy 0.5/
0.5). Ideal bond distances and angles were imposed for these
acetonitrile solvates and common isotropic thermal parameters
were refined for each model. A riding model was used to place
the hydrogen atoms in their idealised positions. In the final
least-squares refinement cycles on F2 the model converged to
R = 0.0394, RЈ = 0.0956, and S = 1.042 for 7322 reflections with
Fo > 4σ(Fo) and 437 parameters, and R = 0.0529 and RЈ =
0.1033 for all 8929 data. In the final Fourier-difference synthesis
the electron density fluctuated in the range 0.77 to Ϫ0.84 e ÅϪ3.
The top peak was associated with N(1s) at a distance of 0.48 Å.
There was no shift in the final cycles. An extinction correction
was refined to 0.0009(2). There is a non-crystallographic two-
fold symmetry in the molecule. However, MISSYM32 did not
reveal any additional symmetry.
Reddish brown rectangular crystals of complex 6 were grown
by a layering method using methanol–diethyl ether. A suitable
crystal of dimensions 0.50 × 0.23 × 0.13 mm was mounted at
the end of a glass fibre. A total of 12 093 reflections were col-
lected in the θ range 2.07–29.34Њ (Ϫ12 < h < 13, Ϫ10 < k < 14,
Ϫ25 < l < 25). All the non-hydrogen atoms were refined aniso-
tropically. One disordered molecule of water was located in the
Fourier-difference synthesis (occupancy 0.25/0.25). A riding
model was used to place the hydrogen atoms in their idealised
positions except for the water molecule. In the final least-
squares refinement cycles on F2 the model converged to
R = 0.0395, RЈ = 0.1060, and S = 1.036 for 7000 reflections with
Fo > 4σ(Fo) and 434 parameters, and R = 0.0510 and RЈ =
0.1126 for all 8582 data. In the final Fourier-difference synthesis
Complex 2 from phosphine exchange reaction between 1 and
dppm in 1:1 ratio. Complex 1 (0.020 g, 1.73 × 10Ϫ2 mmol) and
dppm (0.007 g, 1.73 × 10Ϫ2 mmol) were suspended in methanol
and stirred at r.t. for 3 d. The resultant yellow solution was
filtered, concentrated and diethyl ether was added for precipi-
tation. The light yellow solid (complex 2) was collected by
filtration, washed with an excess of diethyl ether and dried in
vacuo (0.02 g, 62%) (Found: C, 53.8; H, 4.6; Br, 12.3; N, 2.1;
P, 8.4; Pd, 15.4. C60H52Br2N2P4Pd2 requires C, 55.5; H, 4.0; Br,
12.3; N, 2.2; P, 9.6; Pd, 16.4%). δP(CD3OD) 11.8 (t) and 3.5 (t)
[2J(P–P) 30 Hz].
Complex 3 from phosphine exchange reaction between 1 and an
excess of dppp. Complex 1 (0.050 g, 4.33 × 10Ϫ5 mmol) and an
excess of dppp (0.098 g, 1.73 × 10Ϫ4 mmol) were dissolved in
toluene and stirred at r.t. for 2 d. The resultant off-white sus-
pension was filtered, and the residue (complex 3) was washed
with an excess of toluene and diethyl ether (0.01 g, 85%)
(Found: C, 61.8; H, 4.5; Br, 7.0; N, 1.3; P, 10.6; Pd, 12.6.
C91H86Br2N2P6Pd2 requires C, 61.9; H, 4.9; Br, 9.1; N, 1.6; P,
3
10.5; Pd, 12.1%). δP(CD3OD) 12.9 (dd) [2J(P–P) 330, J(P–P)
35], 3.4 (dd) [2J(P–P) 330, 3J(P–P) 54 Hz] and Ϫ4.5
(unresolved multiplets).
Reaction of [Pd2(dppm)3] with 2-chloroquinoline. 2-Chloro-
quinoline (0.198 g, 1.21 mmol) was added to a fresh toluene
solution of [Pd2(dppm)3]26 [obtained in situ from PdCl2 (0.212
g, 1.20 mmol)]. The resulting mixture was refluxed for ca. 2 d to
give bright orange crystals of [Pd2Cl2(µ-dppm)2]. The crystals
3782
J. Chem. Soc., Dalton Trans., 1998, 3777–3784

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Table
2
Crystallographic data for trans-(N,P)-[Pd2Br2(µ-C5H4N-C 2,N)2(µ-dppb)]ؒMeCN and cis-[PdBr(η1-C5H4NH-C 2)(η2-dppf)]Brؒ
5
0.5MeOH 6
5ؒMeCN
C38H36Br2N2P2Pd2ؒMeCN
996.30
6ؒ0.5MeOH
C39H33Br2FeNP2Pdؒ0.5MeOH
908.68
Chemical formula
M
Colour and habit
Crystal system
Space group
Light yellow block
Triclinic
P1
Brownish red block
Triclinic
P1
¯
¯
a/Å
b/Å
c/Å
α/Њ
10.4392(2)
12.6975(2)
16.8905(3)
70.198(1)
75.766(1)
73.886(1)
1994.96(6)
2
9.7095(1)
10.6396(1)
18.9842(1)
89.378(1)
85.003(1)
67.925(1)
1809.98(3)
2
β/Њ
γ/Њ
U/Å3
Z
F(000)
984
1.659
902
1.667
Dc/g cmϪ3
Rint
0.0201
3.016
0.0256
3.224
µ/mmϪ1
Reflections collected
Independent reflections
Observed reflections, n
Residual electron densities/e ÅϪ3
R
12548
8929
7322
ϩ0.770 to Ϫ0.844
0.0394
12093
8582
7000
ϩ1.092 to Ϫ1.151
0.0395
RЈ
0.0956
0.1060
Goodness of fit, S
1.042
1.036
7 T. Kauffman, E. Wienhöfer and A. Woltermann, Angew. Chem., Int.
Ed. Engl., 1971, 10, 741.
8 M. Ishikura, M. Kamada and M. Terashima, Synthesis, 1984, 11,
96; K. Kahmann, H. Sigel and H. Erlenmeyer, Helv. Chim. Acta,
1964, 47, 1754; H. Wynberg, T. J. van Bergen and R. M. Kellogg,
J. Org. Chem., 1969, 34, 3175.
9 S. Gronowitz, J. Malm and A.-B. Hörnfeldt, J. Organomet. Chem.,
1993, 460, 127.
10 A. Suzuki, Acc. Chem. Res., 1982, 15, 178; N. Miyaura and
A. Suzuki, Chem. Rev., 1995, 95, 2457.
the electron density fluctuated in the range 1.092 to Ϫ1.151 e
ÅϪ3. The top four peaks were associated with Pd(1). There
was no shift in the final cycles. Although we were not able to
locate the pyridinium proton directly, all the data are more con-
sistent with a C- rather than an N-bonded model. The former
gives a lower R and RЈ factors. The difference in the isotopic
thermal parameters between the atoms in question is less in
the C-bonded model. The residual electron density in the
N-bonded case also vanishes in the C-bonded model. Further
crystallographic details are given in Table 2.
11 K. Isobe, E. Kai, Y. Nakamura, K. Nichisuke, T. Miwa,
S. Kawaguchi, K. Kinoshita and K. Nakatsu, J. Am. Chem. Soc.,
1980, 102, 2475.
CCDC reference number 186/1171.
12 K. Nakatsu, K. Kinoshita, H. Kanda, K. Isobe, Y. Nakamura,
S. Kawaguchi and U. Nishinomiya, Chem. Lett., 1980, 913.
13 K. Isobe and S. Kawaguchi, Heterocycles, 1981, 16, 1603.
14 (a) K. Nakatsu, K. Kafuku and H. Yamaoka, Inorg. Chim. Acta,
1981, 54, L69; H. Tanaka, K. Isobe, S. Kawaguchi and S. Okeya,
Bull. Chem. Soc. Jpn., 1984, 57, 456; K. Isobe, K. Nanjo,
Y. Nakamura and S. Kawaguchi, Bull. Chem. Soc. Jpn., 1986, 59,
2141; K. Isobe, Y. Nakamura, T. Miwa and S. Kawaguchi, Bull.
Chem. Soc. Jpn., 1987, 60, 149; A. Mantovani, J. Organomet. Chem.,
1983, 255, 385; (b) K. Isobe, Y. Nakamura and S. Kawaguchi, Bull.
Chem. Soc. Jpn., 1989, 62, 1802; (c) L. Canovese, F. Visentin,
P. Uguagliati, F. Dibianca, A. Fontana and B. Crociani, J.
Organomet. Chem., 1996, 525, 43.
Acknowledgements
The authors acknowledge the National University of Singapore
(NUS) (Grant RP950695) for financial support and the tech-
nical staff for supporting services. We thank Y. Xie for some
preliminary work and Y. P. Leong for assistance in the prepar-
ation of this manuscript. C. C. H. C. acknowledges National
University of Singapore for a scholarship award. W. H. thanks
the University of Waikato and the New Zealand Lottery
Grants Board for financial assistance, and W. Jackson for
technical support.
15 K.-S. Gan and T. S. A. Hor, in Ferrocenes – Homogeneous Catalysis,
Organic Synthesis, Materials Science, eds. A. Togni and T. Hayashi,
VCH, Weinheim, 1995, ch. 1, pp. 3–104, and refs. therein.
16 H. Trauner, P. Lefloch, J. M. Lefour, L. Ricard and F. Mathey,
Synthesis-Stuttgart, 1995, 717.
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