Mono- And binuclear cyclometallated palladium(II) complexes containing bridging (N,O-) and terminal (N-) imidate ligands: Air stable, thermally robust and recyclable catalysts for cross-coupling processes

Article abstract of DOI:10.1039/b413886d

Novel dinuclear cyclometallated palladium complexes [{Pd(μ-NCO) (CN)}₂], containing asymmetric imidato -NCO- bridging units have been synthesised [CN = 7,8-benzoquinolyl; -NCO- = succinimidate (1c), phthalimidate (2c) or maleimidate (3c)]. The

Full text of DOI:10.1039/b413886d

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F U L L P A P E R
Mono- and binuclear cyclometallated palladium(II) complexes
containing bridging (N,O-) and terminal (N-) imidate ligands: Air
stable, thermally robust and recyclable catalysts for cross-coupling
processes
Ian J. S. Fairlamb,*a Anant R. Kapdi,a Adam F. Lee,a Gregorio Sa´nchez,b Gregorio Lo´pez,b
Jose´ Luis Serrano,c Luis Garc´ıa,c Jose´ Pe´rezc and Eduardo Pe´rezc
a Department of Chemistry, University of York, Heslington, York, UK YO10 5DD.
E-mail: ijsf1@york.ac.uk; Fax: 44 (0)1904 432516; Tel: 44 (0)1904 434091
b Departamento de Qu´ımica Inorga´nica, Universidad de Murcia, 30071, Murcia, Spain
c
´
Departamento de Ingenier´ıa Minera, Geolo´gica y Cartogra´fica. Area de Qu´ımica Inorga´nica,
30203, Cartagena, Spain
Received 9th September 2004, Accepted 13th October 2004
First published as an Advance Article on the web 27th October 2004
Novel dinuclear cyclometallated palladium complexes [{Pd(l-NCO)(C∧N)}2], containing asymmetric imidato
–NCO– bridging units have been synthesised [C∧N = 7,8-benzoquinolyl; –NCO– = succinimidate (1c), phthalimidate
(2c) or maleimidate (3c)]. The reaction of these complexes, and the previously reported analogous imidate precursors
containing a phenylazophenyl (1a–3a) or 2-pyridylphenyl (1b–3b) backbone, with tertiary phosphines provides novel
mononuclear N-bonded imidate derivatives of the general formula [Pd(C∧N)(imidate)(L)] [L = PPh3, P(4-F-C6H4)3
or P(4-MeO-C6H4)3]. The single crystal structures of [Pd(azb)(phthalimidate)(P(4-MeO-C6H4)3)] (9a) and
[Pd(bzq)(phthalimidate)(PPh3)] (7c) have been established. Dinuclear complexes (1a–3a, 1b–3b, 1c–3c) demonstrate
outstanding thermal stability in the solid-state, as shown by thermoanalytical techniques. A marked influence of
bridging imidate groups on the initial decomposition temperature is observed. The dinuclear and mononuclear
derivatives are shown to be active catalysts/precatalysts for the Suzuki–Miyaura cross-coupling reactions of aryl
bromides with aryl boronic acids, and the Sonogashira reactions of aryl halides with phenyl acetylene (in the presence
and absence of Cu(I) salts). The conversions appear to be dependent, to some extent, on the type of imidate ligand,
suggesting a role for these pseudohalides in the catalytic cycle in both cross-coupling processes. Lower catalyst
loadings in ‘copper-free’ Sonogashira cross-couplings favour higher turnover frequencies. We have further
determined that these catalysts may be recycled using a poly(ethylene oxide) (PEO)/methanol solvent medium in
Suzuki–Miyaura cross-coupling. Once the reaction is complete, product extraction into a hexane/diethyl ether
mixture (1 : 1, v/v) gives cross-coupled products in good yields (with purity > 95%). The polar phase can then be
re-used several times without appreciable loss of catalytic activity.
activated aryl chlorides, where water and tetrabutylammonium
Introduction
bromide played a key role.9 Dinuclear C,N-palladacycles and
Over the last twenty-five years great advances in the development
of palladium-catalysed carbon–carbon and carbon–heteroatom
the phosphine adducts, of the type [Pd(C∧N)(PR3)(X)] [C∧N =
imine or amine based orthometallated ligand; X = Cl or trifluo-
roacetate (TFA)], exhibit increased catalytic activity in Suzuki–
Miyaura cross-coupling.10 Notably, Bedford and co-workers
have emphasised the enhanced activity of these palladacycles
in comparison with conventional precursors. Crucially, they
observed the important role played by both the phosphine and
the anionic ligand “X” in the overall performance of catalysts
(catalyst activity and lifetime). Understanding the role played
by the anionic ligand has been of interest to us, particularly with
respect to imidato ligands, such as maleimide and derivatives,
phthalimide and succinimide. Imidato ligands offer, potentially,
monodentate (N-coordination, as in I) or bidentate modes of
coordination (N,O or N,N) (Fig. 1, as in II–IV). The hemilability
associated with O-coordination might offer stabilisation of key
catalytic intermediates.
bond forming processes have been made.1 Key reactions, such as
Sonogashira, Stille and Suzuki–Miyaura coupling,2 have made
significant contributions to synthetic chemistry, not least in the
synthesis of important natural products and materials.3 For
this reason, the design of electron rich palladium complexes,
possessing strong r-donor ligands, that mediate mild cross-
coupling reactions of activated and deactivated organohalide
substrates with appropriate nucleophiles, has attracted contin-
ued attention.4 The use of cyclometallated Pd(II) complexes
as precatalysts, has in part, led the advances seen in the
last decade. Phosphapalladacycles and N-heterocyclic carbene
Pd-complexes were first successfully introduced by Herrmann
and co-workers, as general catalysts for C–C cross-coupling
4a,4c
reactions, which demonstrate broad substrate scope.
Since
then, less complicated systems, as well as “pincer” palladacycles,
containing functional groups bound to Pd by N-, P- or S-
atoms, have been shown to be efficient catalysts for Suzuki–
4d,5
Miyaura cross-coupling, amongst other coupling reactions.
With regard to N-donor palladacycles, Bedford6 and others7
have developed the use of dinuclear chloride bridged orthopal-
ladated N,N-dimethylbenzylamine complexes in cross-coupling
processes utilising aryl bromides. Related dinuclear orthomet-
allated complexes containing imine8 and oxime groups also
exhibit interesting catalytic activity; in the latter case, with de-
Fig. 1 Possible coordination modes of succinimide ligands to metal
centres.
3 9 7 0
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
©

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Indeed, as a part of ongoing studies into Pd- and Pt-imidato
complexes, several synthetic routes to various complexes have
been studied. The catalytic application of some of these new
complexes has been described by our group in Stille and Suzuki–
Miyaura cross-coupling reactions.11,12
The oxidative addition of N-bromosuccinimide (NBS) to
Pd(0) and Pt(0) precursors in the presence of phosphine
ligands affords complexes of the formula, [M(NCOC2H4CO)-
(PPh3)2(Br)].13 The Pd-complex has proved to be an efficient
and selective catalyst for Stille11 and Suzuki12 cross-coupling
reactions, and the succinimidate ligand, a pseudohalide that
has been described as a r-acceptor14 and moderate p-donor
(stronger p-donor than chlorine),15 plays an interesting role
in the catalytic process. We have also synthesised bridged di-
l-imidate palladacyclopentadiene complexes starting from the
appropriate binuclear hydroxo palladium complex [NBu4]2-
[Pd2{C4(COOMe)4}2(l-OH)2],16 by means of a simple acid–base
reaction, which is well established in our group.17 The hemi-
labile behaviour18 of the new di-l-imidate complexes towards
monodentate ligands, such as acetonitrile, phosphines (Ph3P, (p-
C6H4F)3P and n-Bu3P), pyridine, p-toluidine and tetrahydroth-
iophene, has allowed the preparation of mononuclear imidate
palladacyclopentadiene derivatives, which are catalytically ac-
tive in the Stille reaction. A comparison of their catalytic proper-
ties with [1,2,3,4-tetrakis(methoxycarbonyl)-1,3-butadiene-1,4-
11b
diyl]palladium(II) (TCPC), Pd(NCOC2H4CO)(PPh3)2Br and
Pd(NCOC2H4CO)(dppe)Br in the Stille reaction has been
reported, revealing an interesting dependence in both the yields
and reaction times on the presence and type of imidate ligand
bound to Pd. We have also recently described the preparation of
mono- and dinuclear imidato complexes with a classical C∧N-
cyclometallated backbone (C∧N = phenylazophenyl (azb, a) or
2-pyridylphenyl (phpy, b)) employing, as precursors, the well
known complexes with bridging acetate groups.19 The struc-
tural properties of mononuclear derivatives of the type, [Pd(azb)-
(succinimidate)(PR3)], were investigated in a subsequent study.20
In view of the successful catalytic results, vide supra, in
relation to C∧N–palladacyclic complexes and our experience
in this field with other imidato complexes, we report here
the preparation of new dinuclear orthometallated palladium(II)
derivatives, possessing a 7,8-benzoquinolyl (bzq, c) ligand with
bridging succinimidate, maleimidate or phthalimidate ligands;
their use as precursors to mononuclear compounds of the type
[Pd(C∧N)(imidate)(PR3)]; and further the application of the
novel complexes as catalysts for Suzuki–Miyaura coupling of
aryl bromides with aryl boronic acids and the Sonogashira
coupling reactions of aryl bromides with alkynes. The latter
reaction has been studied in the presence and absence of a Cu(I)
co-catalyst.
Scheme 1 Synthesis of dinuclear complexes, 1a–3a, 1b–3b and 1c–3c.
around the parent ion are consistent with the expected natural
isotopic distribution. We have also carried out TG, DTG, and
DSC studies of dinuclear imidato complexes (1a–3a, 1b–3b, 1c–
3c) in a dynamic nitrogen atmosphere. From this perspective, we
have described previously21 the thermal analysis of mononuclear
and halide-bridged ortho-palladated (azb and phpy based)
complexes, and some studies dealing with the systematic thermal
behaviour ofcyclopalladated pseudohalogen-bridgedcomplexes
(dmba and bzan based) have been recently reported.22 The TG
curves of our complexes display a one-stage decomposition pat-
tern that reveals a high thermal stability, which is confirmed by
the corresponding exothermic peaks in DSC (see Experimental
section). A sharp mass loss is observed for succinimidate and
phthalimidate derivatives, independent of the cyclometallated
ligand, whereas maleimidate compounds 3a, 3b and 3c, show a
slow and irregular decomposition. These data indicate the bzq
and phpy derivatives exhibit similar and higher decomposition
temperatures than the corresponding azb compounds.
Results
Synthesis and characterisation
The acetato-bridged cyclometallated dimer [{Pd(l-OOCMe)-
(bzq)}2] reacts with succinimide, phthalimide or maleimide in
refluxing acetone to give dinuclear complexes in which the
imidato ligands replace the bridging acetate group as presented
in Scheme 1.
The new 7,8-benzoquinolyl derivatives are air-stable yellow
solids and their infrared spectra show the characteristic absorp-
tions of the cyclometallated ligand, partially overlapped with
two strong bands attributed to imidato-carbonyl stretching,
in the range 1736–1720 and 1570–1614 cm−1, respectively.
Thermal stability is also influenced by the nature of the
bridging ligand. Here the phthalimidate complexes are more
stable as a series. The poor solubility of 1c–3c in common
deuterated solvents, and the partial dissociation experienced
in coordinating solvents such as CD3CN, thwarted detailed
NMR spectroscopic analysis. However, the expected reaction
of these imidato-bridged complexes (1c–3c), as well as the other
15b
According to previously reported data the appearance of two
bands in this region suggests coordination of one carbonyl group
to the metal as a part of an –NCO– bridging unit. Further
evidence for the dinuclearity of complexes (1c–3c) comes from
the FAB mass spectrometry, as inferred from the m/z values for
the observed fragments. The abundances of the signals found
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1
3 9 7 1

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Table 1 Mononuclear cyclometallated Pd(II) imidate complexes
Suitable crystals of 7c and 9a were grown from CH2Cl2 and
diethyl ether, enabling their molecular structures to be confirmed
by X-ray single crystal diffraction (Figs. 2 and 3, respectively,
and Table 3). The deviation from the planar coordination has
been quantified by the PCNPd and NNCPd improper torsion
angles.19 The values are 2.66(3)◦ (P1C1N2Pd) and 0.28(5)◦
(N3N2C1Pd) in 9a, while they are 0.80(6)◦ (P1C1N1Pd) and
3.67(9)◦ (N2N1C1Pd) in 7c. Thus, the coordination around Pd
is essentially planar in both complexes, the main distortion about
the ideal geometry is the bite angle that involves the C∧N ligand
(Table 4).
C∧N
Imidate
Ligand
Compound
azb
azb
azb
azb
azb
azb
azb
azb
Succinimidate
Succinimidate
Succinimidate
Phthalimidate
Phthalimidate
Phthalimidate
Maleimidate
Maleimidate
Maleimidate
Succinimidate
Succinimidate
Succinimidate
Phthalimidate
Phthalimidate
Phthalimidate
Maleimidate
Maleimidate
Maleimidate
Succinimidate
Succinimidate
Succinimidate
Phthalimidate
Phthalimidate
Phthalimidate
Maleimidate
Maleimidate
Maleimidate
PPh3
4aa
5aa
6aa
7a
P(4-F-C6H4)3
P(4-OMe-C6H4)3
PPh3
—
8a
P(4-OMe-C6H4)3
PPh3
P(4-F-C6H4)3
P(4-OMe-C6H4)3
PPh3
P(4-F-C6H4)3
P(4-OMe-C6H4)3
PPh3
P(4-F-C6H4)3
P(4-OMe-C6H4)3
PPh3
P(4-F-C6H4)3
P(4-OMe-C6H4)3
PPh3
P(4-F-C6H4)3
P(4-OMe-C6H4)3
PPh3
P(4-F-C6H4)3
P(4-OMe-C6H4)3
PPh3
P(4-F-C6H4)3
P(4-OMe-C6H4)3
9a
10a
11a
12a
4ba
5ba
6ba
7b
azb
phpy
phpy
phpy
phpy
phpy
phpy
phpy
phpy
phpy
bzq
bzq
bzq
bzq
bzq
8b
9b
10b
11b
12b
4c
5c
6c
7c
8c
bzq
bzq
bzq
bzq
9c
10c
11c
12c
azb
=
phenylazophenyl; phpy
=
2-pyridylphenyl, bzq
=
7,8-
benzoquinolyl.a Complexes 4a–6a and 4b–6b have been previously
described.19
Fig. 2 X-Ray crystal structure of 7c. Thermal elipsoids are at 50%
probability. Hydrogens omitted for clarity.
In both structures the imidate ligand is approximately perpen-
dicular to the coordination plane, with angles between planes of
82.7(6)◦ and 81.7(10)◦ for 9a and 7c, respectively. Similar values
are found in succinimidate complexes.20 Following the classifi-
cation described by Dance and Scudder23 for PPh3, based on
measures of the M–P–Cipso–C torsion angle, the conformations
of Pd–PPh3 and Pd–P(4-MeO-C6H4)3 groups may be described
as non-rotors. Here, there are two parallel phenyl rings and one
orthogonal phenyl ring. The Pd–N2 bond length in 7c of 2.098(3)
Scheme 2 Synthesis of mononuclear imidate complexes.
˚
˚
A compares with a Pd–N bond length of 2.026(2) A in trans-
bis(phthalimide)Pd(PPh3)2,24 whereas the Pd–N3 bond length in
bridged cyclometallated dimers (2a, 3a, 2b and 3b), with tertiary
phosphines (PPh3, P(4-F-C6H4)3 and P(4-MeO-C6H4)3), gave the
mononuclear N-bonded imidato complexes (Scheme 2, Table 1).
The reactions take place in refluxing CH2Cl2, yielding air
stable solids with negligible molar conductivity. The IR spectra
exhibit the expected bands for the corresponding cyclometal-
lated backbone, together with those attributed to the incom-
ing phosphine ligand and one strong carbonyl band around
˚
9a is 2.087(2) A. Thus it appears that the phthalimidate ligand,
in both 7c and 9a, is a weaker r-acceptor, which is presumably
associated with the greater electron-withdrawing potential of the
Csp2∧Nsp2 cyclometallated backbone in these complexes.
Application of the imidato-Pd-complexes as catalysts/
precatalysts in Suzuki–Miyaura cross-coupling
1
1640 cm−1. The H NMR data of the mononuclear complexes
is detailed in the Experimental section. These complexes exhibit
the expected 1H resonances for cyclometallated and imidate
ligands. The 31P NMR spectra of the complexes all show
characteristic singlet resonances, with chemical shifts in the
expected range for Pd(II) complexes (Table 2).
The Pd-complexes were screened for catalytic activity in the
cross-coupling of three aryl bromides (13a–c) with phenyl
boronic acid to give 14a–c, respectively (Scheme 3). Standard
conditions, recently reported by us for other imidato-Pd(II)
catalyst systems, were employed in these reactions.
Table 2 31P NMR chemical shifts of the novel mononuclear complexes of the type [Pd(C∧N)(imidate)(PR3)]
Imidate
PPh3
P(4-F-C6H4)3
P(4-OMe-C6H4)3
Phenylazophenyl (azb)
2-Pyridylphenyl (phpy)
7,8-Benzoquinolyl (bzq)
phth
mal
phth
mal
succ
phth
mal
41.5, 7a
41.9, 10a
43.1, 7b
43.2, 10b
42.9, 4c
43.0, 7c
42.6, 10c
39.7, 8a
39.6, 11a
40.9, 8b
40.9, 11b
40.8, 5c
40.9, 8c
40.9, 11c
38.7, 9a
38.6, 12a
40.9, 9b
39.1, 12b
39.3, 6c
39.5, 9c
39.0, 12c
3 9 7 2
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1

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Table 3 Crystal data and structure refinement details for 7c and 9a
7c
9a
Empirical formula
Formula weight
T/K
C39H27N2O2PPd
693.00
173(2)
0.71073
C41H34N3O5PPd
786.08
100(2)
k/nm
0.71073
Crystal system
Monoclinic
P21/c
18.5176(16)
10.5769(8)
16.3681(13)
90
111.731(7)
90
2978.0(4)
4
1.546
0.717
Triclinic
P1
9.2124(4)
14.1362(7)
14.3805(7)
111.4360(10)
94.2200(10)
94.7580(10)
1726.26(14)
2
1.512
0.635
¯
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
qcalc./Mg m−3
l/mm−1
F(000)
1408
804
Crystal size/mm
Reflections collected
Independent reflections
Goodness-of-fit on F2
Final R indices [I>2r(I)]
R indices (all data)
0.28 × 0.20 × 0.12
0.28 × 0.19 × 0.13
7371
20063
5236
0.892
7719
0.904
R1 = 0.0363, xR2 = 0.0740
R1 = 0.0301, xR2 = 0.0769
R1 = 0.0612, xR2 = 0.0790
R1 = 0.0323, xR2 = 0.0787
◦
˚
Table 4 Selected bond lengths (A) and angles ( ) for 7c and 9a
7c
9a
Pd(1)–P(1)
Pd(1)–C(1)
Pd(1)–N(2)
Pd(1)–N(1)
2.2519(10)
2.022(3)
2.098(3)
2.098(3)
—
2.2768(15)
2.0048(19)
2.1088(16)
—
2.0872(17)
173.80(7)
—
78.08(7)
—
95.73(6)
—
95.69(6)
90.51(5)
172.78(5)
—
1.265(2)
1.398(3)
1.440(2)
Pd(1)–N(3)
C(1)–Pd(1)–N(3)
C(1)–Pd(1)–N(2)
C(1)–Pd(1)–N(2)
C(1)–Pd(1)–N(1)
N(3)–Pd(1)–N(2)
N(2)–Pd(1)–N(1)
C(1)–Pd(1)–P(1)
N(3)–Pd(1)–P(1)
N(2)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
N(1)–N(2)
—
171.71(14)
—
82.06(13)
—
91.51(12)
95.35(11)
—
91.16(9)
177.19(9)
—
C(6)–N(1)
C(7)–N(2)
—
—
precursor was employed. Percentage conversion to products
were recorded after 2 h for reactions employing 13a and 13b as
substrates, and after 4 h for the reaction employing 13c — a
less reactive substrate (10 mol% hexadecane was used as
the internal standard). Multiple reactions were simultaneously
quenched after identical reaction times to give an insight into
the catalytic activity of each Pd complex, and thus their relative
reactivity with these substrates.
Fig. 3 X-Ray crystal structure of 9a. Thermal elipsoids are at 50%
probability. Hydrogens omitted for clarity.
The dimeric cyclometallated precursors (1c–3c) all demon-
strated reasonable catalytic activity in the three benchmark
reactions, with yields seen between ca. 70 and 84% (Fig. 4).
Given that these Pd sources do not possess a phosphine ligand,
the catalytic activity and yields are promising. These yields
are, however, improved by the incorporation of a phosphine
ligand. In the reaction of 13a→14a, yields of >87% are seen for
complexes 4c–6c, containing a succinimidate ligand. The best
yield is seen for 6c (99%). In this sense, it appears that the more
electron rich P(4-OMe-C6H4)3 ligand increases the reactivity of
the Pd catalyst in 6c. This trend is also apparent in the other
reactions (13b→14b and 13c→14c).
Scheme 3 Benchmark Suzuki–Miyaura cross-coupling reactions.
With a large number of Pd complexes to be screened,
efforts were focused on use of the 7,8-benzoquinolyl (bzq) Pd-
imidato series (1c–12c) and selected others, taken from the
phenylazobenzene (azb) and 2-pyridylphenyl (phpy) series (7a,
7b, 8b, 9b, 10a, 10b, 11b and 12b). The catalyst loading used
for the mononuclear complexes was 1 mol%, with an exception
made for the dimeric complexes (1c–3c), where 0.5 mol% of each
Some trends are apparent upon altering the type of imidate
ligand among Pd complexes 5c, 8c and 11c, which all contain
P(4-F-C6H4)3 as the phosphine ligand. For example, analysis
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1
3 9 7 3

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Fig. 4 Suzuki–Miyaura cross-coupling using Pd complexes 1c–12c in a THF/2M Na2CO3 mixture (1 : 2, v/v, 3 mL total volume). Key: reaction
of 13a→14a; ꢀ reaction of 13b→4b; ꢁ reaction of 13c→14c. T = 90 ◦C. Reaction time = 2 h for 13a→14a and 13b→14b, = 4 h for 13c→14c.
Concentration of aryl halide [13] = 0.15 M (1.1 equiv.). Concentration of phenyl boronic acid = 0.136 M (1.0 equiv.). % Conversion was determined
by GC analysis using hexadecane as an internal standard.
of the reaction 13b→14b reveals an interesting difference. We
observe a 90% conversion to 14b for 5c (containing succinim-
idate), 91% for 8c (containing phthalimidate) and 83% for 11c
(containing maleimidate). We believe the greatest difference, of
8% conversion seen between 8c and 11c, is real. In reactions
that are run in parallel, under identical conditions with multiple
injections (stirring rate, temperature, reagent, solvent and base
sources), we estimate that the conversion error to cross-coupled
products is 3% (Note: greater errors, 5%, are observed on
performing reactions separately). Hence the choice of imidate,
the anionic ligand, does influence the overall conversion to
products. However this effect is also dependent upon the
particular phosphine ligand, and a subtle interplay of electron-
donation and withdrawal could account for these differences.
These effects may also be substrate specific.
We have further noted that a difference in catalytic activity is
seen on changing the type of cyclometallated backbone (Fig. 5).
For example, taking the reaction of 13c→14c, and comparing Pd
complexes containing malelimidate and PPh3 ligands (10a–c), a
notable difference (≈10%) is seen between the 2-pyridylphenyl
backbone (10b, 87%) and both the phenylazabenzene (10a, 97%)
and 7,8-benzoquinolyl backbones (10c, 98%).
and thus the concentration of catalytically active ‘Pd’. The out-
come here is that the cyclometallated backbone, type of imidate
and presence of phosphine ligand all affect this particular cross-
coupling process.
Application of the imidato-Pd-complexes as catalysts/
precatalysts in Sonogashira cross-coupling
To gain further insight into the influence of the various ligands,
we decided to apply our Pd-complexes to the Sonogashira cross-
coupling25 of 4-bromoacetophenone 13c and phenyl acetylene to
give 15 (Scheme 4). Although, 13c is an activated substrate, the
reaction is not as straight-forward as one might first assume.
When the reaction is mediated by (PPh3)2PdCl2 (1 mol%) in
the presence of CuI (0.5 mol%) in an acetonitrile/triethylamine
mixture (2.5 : 1.5, v:v), a reaction temperature of 100 ◦C is
required to achieve even 63% percent conversion after 2 h (c of
13c = 0.102 M).
It is not unexpected that individual changes in the catalyst
structure affect the overall product conversions. Presumably the
cyclometallated backbone will directly affect the rate of catalyst
initiation, assuming a classic Pd(II)→Pd(0) pre-reduction step,
Scheme 4 Benchmark Sonogashira cross-coupling reaction.
Fig. 5 As for Fig. 4, using Pd complexes 7a, 7b, 8b, 9b, 10a, 10b, 11b and 12b.
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1
3 9 7 4

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Fig. 6 Sonogashira cross-coupling of 4-bromoacetophenone 13c with phenylacetylene to give 15 in a Et3N/CH3CN mixture (2 : 3, v/v, 5 mL total
volume). Key: ꢀ in the absence of CuI, % conv. after 2 h; in the absence of CuI, % conv. after 5 h; ꢁ in the presence of CuI, % conv. after 2 h.
T = 100 ◦C. Concentration of aryl halide [13c] = 0.102 M (1.04 equiv.). Concentration of phenyl acetylene = 0.098 M (1.0 equiv.). % Conversion
was determined by GC analysis using hexadecane as an internal standard.
Fig. 7 As for Fig. 6, using Pd complexes 7a, 7b, 8b, 9b, 10a, 10b, 11b and 12b.
It should be noted that the reactant concentration influences
the rate considerably, and in due course we will present our
findings regarding this generic precatalyst. One point to note
here is that, as expected, higher concentrations give higher
reaction rates, but favour side-products such as the homo-
coupled phenyl acetylene dimer (1,4-bis(phenylethynyl)buta-
1,3-diyne). In these reactions, we did not detect hydrode-
brominated acetophenone, which would facilitate reoxidation
of Pd(0) to Pd(II) to promote homo-coupling.26 It is presumed,
although not proven, that adventitious oxygen promotes the
homo-coupling process at higher concentrations. Keeping the
concentration of 13c <0.13 M serves to minimise this to
the level expected from the Pd(II)→Pd(0) pre-reduction step.
For screening purposes, we employed the general conditions
described above for (PPh3)2PdCl2 (Figs. 6 and 7). We have
further performed the reactions under ‘copper-free’27 conditions
to assess the impact of Cu(I) on the rate of reaction and on the
production of side-products.
take 5 h to reach similar conversions achievable in only 2 h
in the presence of CuI. Although the copper-free Sonogashira
reactions did require longer reactions times, we found that the
GC spectra were cleaner in a number of the examples illustrated
in Figs. 6 and 7. As was the case with the Suzuki–Miyaura cross-
coupling reactions, the presence of a phosphine ligand serves to
improve the overall conversion to 15 (compare 1c–3c with 4c–
12c).
In the presence of CuI, the type of imidate again affects the
overall conversion for complexes containing the P(4-F-C6H4)3
ligand and 7,8-benzoquinolyl backbone (5c, 8c and 11c, Fig. 6).
The highest conversion is seen for 5c containing a succimidate
ligand (86%), whereas alteration to a phthalimidate ligand (8c)
results in a drop to 76%. The lowest conversion is seen in 11c,
possessing a maleimidate ligand (69%). The difference between
5c and 11c of 17% is notable. Although such large differences
are not seen for all the phosphine ligands, clearly the imidate
plays a large role in either catalyst initiation or in the catalytic
cycle.
The first major observation that comes out of screening
complexes 1c–12c is that the presence of CuI serves to accelerate
the reaction across the whole series. Thus for these specific Pd-
catalysts/precatalysts, the Cu(I) salt serves to enhance the rate
of reaction and not to inhibit it, as reported in a unique case by
Reduced catalyst loadings. As good conversions were seen
using 1 mol% Pd, lower catalyst loadings were investigated in the
absence of CuI co-catalyst (Fig. 8 and Table 4). Although lower
catalyst loadings were expected to require longer reaction times,
27e
Buchwald and co-workers. In the absence of CuI, reactions
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1
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a mononuclear, catalytic species. A similar observation28 has
been made by de Vries and co-workers using low loadings of
Pd(OAc)2 for the Heck reaction, wherein Pd nanoparticles have
been implicated.29
The highest TOF and TON were seen using 7b at a loading
of 0.001 mol% (TOF = 6166; TON = 37,000). The difference
in the type of cyclometallated backbone is apparent once again
(compare 7b with 7a (TOF = 4666; TON = 28,000) and 7c
(TOF = 5500; TON = 33,000), containing 2-pyridylphenyl,
phenylazobenzene and 7,8-benzoquinolyl backbones, respec-
tively). A comparison of 4c, 7c and 10c allows us to directly
explore the influence of the imidate ligand. The phthalimidate
ligand in 7c is clearly more active, in order, than 10c (TOF =
4833; TON = 29,000), containing a maleimidate ligand, and
4c, containing a succinimdate ligand (TOF = 3500; TON =
21,000). The reaction profiles for 4c, 7c and 10c showed the initial
rates and associated TOF differences were essentially constant
throughout the reaction.
Fig. 8 Sonogashira cross-coupling, in the absence of CuI, of 4-bro-
moacetophenone 13c with phenylacetylene to give 15 at various Pd
catalyst loadings. Reaction time = 6 h, T = 100 ◦C. Reactant
concentrations as for Fig. 6. Key:
(0.1 mol% Pd); ꢀ (0.01 mol%
Pd); (0.001 mol% Pd).% Conversion was determined by GC analysis
using hexadecane as an internal standard.
Catalyst recycling in Suzuki–Miyaura cross-coupling
It is well known that one of the major drawbacks of homoge-
neous palladium catalysed cross-coupling reactions is product
separation from the catalyst (phosphine ligands, and their
oxides, can further complicate purification.30 Lower catalyst
loadings usually give purer cross-coupled products, however
these active catalysts are often even more difficult to recover
for recycling. Given that loadings of 1 mol% were used
in our Suzuki–Miyarua cross-coupling reactions, vide supra,
we decided to investigate whether it was possible to recycle
these catalysts. Our method for catalyst recycling is based
on a simple protocol developed by Monteiro and co-workers
for Pd(OAc)2/PPh3.31 They demonstrated that a poly(ethylene
oxide)32 (PEO)/methanol solvent medium can be used in
Suzuki–Miyaura cross coupling. At the end of the reaction,
the product was extracted into a non-polar phase, with the
Pd-catalyst remaining in the polar phase. Recharging the polar
phase with new reactants allows the Pd to be re-used without
loss of catalytic activity (over several runs). Two reactions
were chosen for the catalyst recycling studies, namely reaction
13a→14a, and 13c→14c. Complexes 4c and 7c were evaluated
for recycling and the results are shown in Figs. 10 and 11.
In both reactions, 13a→14a and 13c→14c, efficient catalyst
recycling is demonstrated, without appreciable loss of catalytic
activity over five consecutive catalytic runs. Complex 7c is more
active than 4c in both reactions, throughout the reaction series.
all reactions were deliberately analysed after 6 h, allowing us
to assess any differences in the turnover frequency (TOF) with
catalyst loading. This experiment uncovered an exciting obser-
vation; the TOF increased for lower catalyst loadings (Fig. 9 and
Table 5). In terms of overall conversion, if one assumes that one
‘reactive Pd(0) species’ is involved in the reaction, then reducing
the catalyst loading from 0.1 mol% to 0.01 mol% should result
in a 10-fold decrease in conversion. However, this is not the case.
Furthermore the difference in conversion after 6 h between 0.01
and 0.001 mol% catalyst loadings is small (≈3–10%). In other
words the turnover numbers actually increased substantially as
the catalyst loading was reduced from 0.1 mol% to 0.001 mol%.
Thus at higher catalyst loadings, the intrinsic catalytic activity
is lower. This implicates the formation of large unreactive Pd(0)
clusters, formed via aggregation, at higher catalyst loadings,
which may serve as a reservoir for the reactive Pd(0), possibly
Discussion and conclusions
Influence of copper in Sonogashira cross-coupling
It is clear that Cu(I) serves as a co-catalyst and accelerates
the rate of Sonogashira cross-coupling in the presence of the
cyclometalled Pd(II) precursors. However, a closer examination
of Figs. 6 and 7 reveals that the magnitude of this enhancement
depends upon the precise structure of the Pd(II) precatalyst.
Consideration of the binuclear imidate bridged Pd(II) precat-
alysts (1c–3c), which possess a common 7,8-benzoquinolinyl
backbone, finds that Cu(I) is not equally efficacious across the
Fig. 9 Graph to show turnover frequency versus Pd loading (TOF =
(mol of coupled product) (mol Pd)−1 h−1). Insert graph is as for Fig. 8.
Table 5 Effect of catalyst loading in Sonogashira cross-coupling of 13c with phenyl acetylene to give 15 using the conditions given in Fig. 8
[Pd] Catalyst
0.1 mol% [Pd]
0.01 mol% [Pd]
0.001 mol% [Pd]
% Conv.
TON
TOF
% Conv.
TON
TOF
% Conv.
TON
TOF
7a
7b
4c
62
69
50
70
57
620
690
500
700
570
103
115
83
116
95
35
46
25
40
33
3500
4600
2500
4000
3300
583
766
416
666
550
28
37
21
33
29
28000
37000
21000
33000
29000
4666
6166
3500
5500
4833
7c
10c
3 9 7 6
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1

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by the fluorene substituent (11c), which in turn is superior to
Ph3P (10c).
Recycling studies
Two of our palladium precatalysts (4c and 7c) can be success-
fully supported and recycled in Suzuki–Miyaura cross-coupling
reactions. In both reactions (13a→14a and 13c→14c), 7c was
always superior to 4c. Catalyst 4c also exhibited an induc-
tion/activation phase as apparent from the low conversions
during the first runs, which may be linked to its intrinsically
poorer performance. The importance of precatalyst activation
is illustrated by our finding that, provided an initial run was
conducted at 90 ◦C, all subsequent reactions using recycled
catalysts could be performed at temperatures as low as 25 ◦C
without loss in activity. The thermal stability of the initial
precatalyst complexes clearly impacts on these reactions.
Fig. 10 Reaction of 4-bromonitrobenzene 13a with phenyl boronic acid
to give 14a. ꢀ Complex 4c; Complex 7c. T = 90 ◦C. Reaction time =
2 h.
Summary of structural effects — imidate, cyclometallated
backbone and phosphine
The body of results collected herein demonstrates that all parts
of the ligand framework play a role in the cross-coupling reac-
tions evaluated. The choice of imidate is clearly a major factor
as illustrated for the reaction 13c→15 wherein succinimidate 5c,
phthalimidate 8c and maleimidate 11c, provide conversions of
86%, 76% and 69% respectively. Phosphine electronics are also
important in both Sonogashira and Suzuki–Miyaura couplings,
affecting conversions by as much as 20% as shown in Fig. 12.
Although changes to the cyclometallated backbone have some
effect on Sonogashira chemistry, their influence is somewhat
less than that exerted by the anionic imidate and phosphine
ligands, as evidenced by the similar conversions for 7a–c seen
in Fig. 8. However, it is difficult to deconstruct the complex,
interdependent interactions within the ligand framework which
undoubtedly control the formation, propagation and stability of
important Pd species within the catalytic cycle.
Fig. 11 Reaction of 4-bromoanisole 13c with phenyl boronic acid to
give 14c. ꢀ Catalyst 4c; Catalyst 7c. T = 90 ◦C. Reaction time = 4 h.
range of imidates. Thus copper promotional magnitude appears
related to the ability of each imidate to delocalise electron
density from the nitrogen lone pair on to the carbonyl oxygen.
Hence, delocalising ability follows the order, maleimide (3c)
> phthalimide (2c) > succinimide (1c), while the Cu(I) rate-
enhancement follows the reverse sequence (Fig. 12).
Influence of Pd loading in copper-free Sonogashira cross coupling
We have uncovered the first evidence that catalyst loading is
crucial in regulating Pd-catalysed Sonogashira cross-couplings.
In the reaction of 13c with phenyl acetylene to give 15, the TOFs
were approximately inversely proportional to the Pd loading,
which were highest at 0.001 mol%. We anticipate that higher
loadings lead to aggregation of the various palladium species
that are present in the reaction medium. It would also be
interesting to study the effect of Cu(I) salts in this case, to see
whether a similar trend was observed.
Experimental
General details
THF was dried over sodium–benzophenone ketyl (distilled prior
to use). All reactions were conducted under an inert atmosphere
of Ar or N2 on a Schlenk line. Melting points were recorded on
an electrothermal IA9000 Digital Melting Point Apparatus and
are uncorrected. TLC analysis was performed on Merck 5554
aluminium backed silica gel plates and compounds visualized
by ultraviolet light (254 nm), phosphomolybdic acid solution
Fig. 12 Relative effect of added Cu(I) salt for different imidate and
phosphine ligands:
% change for binuclear complexes 2c and 3c
1
(5% in EtOH), or 1% ninhydrin in EtOH. H NMR spectra
relative to 1c; ꢀ % change for mononuclear complexes 11c and 12c
relative to 10c.
were recorded at 270 MHz using a JEOL EX270 spectrometer
or at 400 MHz using a JEOL ECX400 spectrometer; 13C
NMR spectra at 67.9 or 100.5 MHz. Chemical shifts are
reported in parts per million (d) downfield from an internal
tetramethylsilane reference. Coupling constants (J values) are
reported in hertz (Hz), and spin multiplicities are indicated by the
following symbols: s (singlet), d (doublet), t (triplet), q (quartet),
qn (quintet), sx (sextet), m (multiplet), br (broad). C, H, and N
analyses were carried out with a Carlo Erba instrument. IR
spectra were recorded on a Perkin-Elmer spectrophotometer
For mononuclear cyclometallated Pd(II) precatalysts, the
influence of Cu(I) on the reaction was also dependent on
the choice of phosphine. For the series of maleimidate based
complexes, possessing the same 7,8-benzoquinolinyl backbone
(10c–12c), the benefit of Cu(I) promotion scaled with the
electron-donor properties of the phosphine ligand (Fig. 12).
Here the strongly-donating methoxy substituent (12c) works in
synergy with copper giving the greatest enhancement, followed
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1
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16F PC FT-IR, using Nujol mulls between polyethylene sheets.
NMR data (1H and 31P) were recorded on Bruker Avance 200,
300 and 400 spectrometers. Mass spectrometric analyses were
performed on a Fisons VG Autospec double-focusing spectrom-
eter, operated in positive mode. Ions were produced by fast atom
bombardment (FAB) with a beam of 25-keV Cs atoms. The mass
spectrometer was operated with an accelerating voltage of 8 kV
and a resolution of at least 1000. Conductance measurements
were performed with a Crison 525 conductimeter (in acetone
solutions, 5 × 10−4M). TG and DTG curves were recorded on
a Mettler TG-50 thermobalance using ◦an atmosphere of pure
nitrogen (50 cm3 min−1; heating rate 5 C min−1). DSC curves
were recorded on a DSC822e Mettler Toledo instrument. All the
solvents were dried by conventional methods.
The cyclometallated precursor [{Pd(l-OOCMe)(bzq)}2]
[bzq = 7,8-benzoquinolyl] was synthesized following the
reported method.33 Dinuclear imidato precursors [{Pd(l-
NCO)(C∧N)}2] [C∧N = phenylazophenyl (azb); –NCO– =
succinimidate (suc) (1a), phthalimidate (2a) (phth) or maleim-
idate (mal) (3a); C∧N = 2-pyridylphenyl (phpy); –NCO– =
succinimidate (1b), phthalimidate (2b) or maleimidate (3b)]
and mononuclear succinimidate derivatives [Pd(C∧N)(suc)(L)]
[C∧N = azb; L = PPh3 (4a), P(4-F-C6H4)3 (5a), P(4-MeO-C6H4)3
(6a); C∧N = phpy; L = PPh3 (4b), P(4-F-C6H4)3 (5b), P(4-MeO-
C6H4)3 (6b)] were prepared as described in a previous paper.18
Commercially available chemicals were purchased from Aldrich
Chemical Co. and were used without further purification.
[{Pd(l-mal)(azb)}2] 3a. TG mass loss: 52.6%. DTGmax
:
:
:
:
201.3 ◦C. DSCmax: 240.4 ◦C.
[{Pd(l-succ)(phpy)}2] 1b. TG mass loss: 44.70%. DTGmax
335.0 ◦C. DSCmax: 328.5 ◦C.
[{Pd(l-phth)(phpy)}2] 2b. TG mass loss: 47.16%. DTGmax
377.2 ◦C. DSCmax: 371.9 ◦C.
[{Pd(l-mal)(phpy)}2] 3b. TG mass loss: 45.38%. DTGmax
239.3 ◦C. DSCmax: 377.3 ◦C.
Preparation of complexes [Pd(C∧N)(imidate)(L)] [C∧N= azb;
imidate = phth; L = PPh3 (7a), P(4-F-C6H4)3 (8a), P(4-MeO-
C6H4)3 (9a); C∧N= azb; imidate = mal; L = PPh3 (10a),
P(4-F-C6H4)3 (11a), P(4-MeO-C6H4)3 (12a); C∧N = phpy;
imidate = phth; L = PPh3 (7b), P(4-F-C6H4)3 (8b), P(4-MeO-
C6H4)3 (9b)); C∧N = phpy; imidate = mal; L = PPh3 (10b),
P(4-F-C6H4)3 (11b), P(4-MeO-C6H4)3 (12b); C∧N= bzq;
imidate = succ; L = PPh3 (4c), P(4-F-C6H4)3 (5c), P(4-MeO-
C6H4)3 (6c); C∧N= bzq; imidate = phth; L = PPh3 (7c), P(4-F-
C6H4)3 (8c), P(4-MeO-C6H4)3 (9c); C∧N= bzq; imidate = mal;
L = PPh3 (10c), P(4-F-C6H4)3 (11c), P(4-MeO-C6H4)3 (12c)]
The complexes were obtained by treating the appropriated pre-
cursor [{Pd(l-NCO)(C∧N)}2] with the corresponding neutral
ligand (molar ratio 1 : 2) in dichloromethane, according to
the following general method. To a dichloromethane (20 mL)
solution of (2a, 3a, 2b, 3b or 1c–3c) (0.07 g) was added the
stoichiometric amount of phosphine ligand. The solution was
refluxed for 2 h, then concentrated until ca. one fifth of the initial
volume. Slow addition of hexane caused the precipitation of the
title complexes, which were isolated by filtration, washed with
hexane, air-dried and recrystallised from acetone–hexane.
Preparation of the complexes [{Pd(l-NCO)(bzq)}2] [bzq = 7,8-
benzoquinolyl; –NCO– = succinimide (succ) (1c), phthalimide
(2c) (phth) or maleimide (mal) (3c)]
The new complexes were obtained by treating [{Pd(l-
OOCMe)(bzq)}2] with the corresponding imide (molar ratio
1 : 2) in acetone, according to the following general method.
To an acetone (20 mL) solution of [{Pd(l-OOCMe)(bzq)}2]
(0.175 g, 0.255 mmol) was added the stoichiometric amount of
ligand. The resulting suspension was refluxed for 2 h, and then
concentrated to one fifth of the initial volume. Slow addition of
diethyl ether completed the precipitation of the title complexes,
which were isolated by filtration, washed with diethyl ether and
air-dried.
[Pd(azb)(phth)(PPh3)] 7a. (0.07 g, 63%). Anal. calc. for
C38H28N3O2PPd: C, 65.6; H, 4.0; N, 6.0. Found: C, 65.7; H,
4.1; N, 6.1%. IR (cm−1): m(C=O) 1644vs, m(azb) 1600s, m(PPh3)
532m, 514m. FAB-MS (positive mode) m/z: 696 [M]+, 549 [M −
phth]+, 262 [PPh3]+, 154. dH (300 MHz, CDCl3): 8.01 (m, 1H;
azb), 7.74 (m, 6H; azb), 7.72 (m, 2H; phth), 7.20 (m, 17H; 2H
phth +15H phos), 6.71 (m, 1H; azb), 6.53 (m, 1H; azb).
[Pd(azb)(phth){P(4-F-C6H4)3}] 8a. (0.087 g, 72%). Anal.
calc. for C38H25F3N3O2PPd: C, 60.8; H, 3.4; N, 5.6. Found: C,
60.5; H, 3.6; N, 5.9%. IR (cm−1): m(C=O) 1641vs, m(azb) 1618s,
m{P(4-F-C6H4)3} 533m, 525m. FAB-MS (positive mode) m/z:
750 [M]+, 603 [M − phth]+, 288, 154. dH (300 MHz, CDCl3):
8.01 (m, 1H; azb), 7.71 (m, 5H; azb), 7.67(m, 2H; phth), 7.26(m,
5H; 2H phth + 3H phos), 7.17 (m, 4H; 1H azb + 3H phos), 6.94
(m, 6H; phos), 6.75 (m, 1H, azb), 6.46 (m, 1H, azb).
[{Pd(l-succ)(bzq)}2] 1c. (0.140 g, 72%). Anal. calc. for
C34H24N4O4Pd2: C, 53.3; H, 3.2; N, 7.3. Found: C, 53.4; H,
3.3; N, 7.3%. IR (cm−1): m(C=O) 1724s, 1598vs, m(bzq) 834s.
FAB-MS (positive mode) m/z: 765 [{Pd(l-suc)(bzq)}2]+, 667
[{Pd2(suc)(bzq)2}]+, 462 [Pd(bzq)2]+, 284 [Pd(bzq)]+. TG mass
loss: 36.24%. DTGmax: 340.1 ◦C. DSCmax: 340.7 ◦C.
[Pd(azb)(phth){P(4-MeO-C6H4)3}] 9a. (0.075 g, 59%). Anal.
calc. for C41H34N3O5PPd: C, 62.6; H, 4.4; N, 5.3. Found: C,
62.8; H, 4.6; N, 5.6%. IR (cm−1): m(C=O) 1601vs, m(azb) 1639s,
m(P{4-MeO-C6H4)3} 523m, 497m. FAB-MS (positive mode)
m/z: 786 [M]+, 639 [M − phth]+, 352 [P(4-MeO-C6H4)3]+, 154.
dH (300 MHz, CDCl3): 8.01 (m, 1H; azb), 7.63 (m, 11H; 6H
phos + 5 azb), 7.26 (m, 8H; 3H phos + 4H phth + 1 azb),
7.16(m, 4H; 1H azb + 3H phos), 7.58 (m, 1H; azb), 3.67 (s, 9H;
MeO).
[{Pd(l-phth)(bzq)}2] 2c. (0.186 g, 85%). Anal. calc. for
C42H24N4O4Pd2: C, 58.5; H, 2.8; N, 6.5. Found: C, 58.7; H,
3.0; N, 6.6%. IR (cm−1): m(C=O) 1736s, 1636vs, m(bzq) 832s.
FAB-MS (positive mode) m/z: 861 [{Pd(l-pht)(bzq)}2]+, 715
[{Pd2(phth)(bzq)2}]+, 462 [Pd(bzq)2]+, 284 [Pd(bzq)]+. TG mass
loss: 40.20%. DTGmax: 375.1 ◦C. DSCmax: 383.4 ◦C.
[{Pd(l-mal)(bzq)}2] 3c. (0.143 g, 74%). Anal. calc. for
C34H20N4O4Pd2: C, 53.6; H, 2.6; N, 7.4. Found: C, 53.8; H,
2.8; N, 7.6%. IR (cm−1): m(C=O) 1720s, 1614vs, m(bzq) 826s.
FAB-MS (positive mode) m/z: 761 [{Pd(l-mal)(bzq)}2]+, 666
[{Pd2(mal)(bzq)2}]+, 462 [Pd(bzq)2]+, 284 [Pd(bzq)]+. TG mass
loss: 39.70%. DTGmax: 309.0 ◦C. DSCmax: 347.6 ◦C.
[Pd(azb)(mal)(PPh3)] 10a. (0.079 g, 67%). Anal. calc. for
C34H26N3O2PPd: C, 63.2; H, 4.1; N, 6.5. Found: C, 63.0; H, 4.4;
N, 6.7%. IR (cm−1): m(C=O) 1640vs, m(azb) 1617s, m(PPh3) 546m,
531m. FAB-MS (positive mode) m/z: 645 [M]+, 549 [M − mal]+,
262 [PPh3]+, 154. dH (300 MHz, CDCl3): 8.01 (m, 1H; azb), 7.74
(m, 6H; azb), 7.68 (m, 2H; phos), 7.33 (m, 12H; phos), 7.12 (m,
1H; azb), 6.69 (m, 1H; azb), 6.54 (m, 1H, azb), 5.86 (s, 2H, mal).
Thermal analysis of dinuclear complexes 1a–3a and 1b–3b
[{Pd(l-succ)(azb)}2] 1a. TG mass loss: 43.67%. DTGmax
:
:
275.6 ◦C. DSCmax: 285.4 ◦C.
[Pd(azb)(mal){P(4-F-C6H4)3}] 11a. (0.092 g, 67%). Anal.
calc. for C34H23F3N3O2PPd: C, 58.3; H, 3.3; N, 6.0. Found: C,
58.1; H, 3.4; N, 6.3%. IR (cm−1): m(C=O) 1641vs, m(azb) 1620s,
m{P(4-F-C6H4)3} 533m, 517m. FAB-MS (positive mode) m/z:
[{Pd(l-phth)(azb)}2] 2a. TG mass loss: 47.51%. DTGmax
291.7 ◦C. DSCmax: 306.2 ◦C.
3 9 7 8
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1

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700 [M]+, 603 [M − mal]+, 154. dH (300 MHz, CDCl3): 8.01 (m,
1H; azb), 7.69 (m, 5H; azb), 7.54 (m, 2H; phos), 7.33 (m, 2H;
phos), 7.26 (m, 3H; phos), 7.09 (m, 1H; azb), 7.06 (m, 5H, phos),
6.74 (m, 1H, azb), 6.42 (m, 1H; azb), 5.93 (s, 2H; mal).
[Pd(bzq)(succ)(PPh3)] 4c. (0.090 g, 79%). Anal. calc. for
C35H27N2O2PPd: C, 65.2; H, 4.2; N, 4.3. Found: C, 65.4; H, 4.3;
N, 4.3%. IR (cm−1): m(C=O) 1614vs, m(bzq) 830s, m(PPh3) 534m,
492m. FAB-MS (positive mode) m/z: 644 [M]+, 546 [M − succ]+,
262 [PPh3]+, 154. 8.49 (m, 1H; bzq), dH (300 MHz, CDCl3): 8.29
(m, 1H; bzq), 8.01 (m, 6H; phos), 7.59 (m, 1H; bzq), 7.46 (m,
1H; bzq), 7.39 (m, 11H; 2H bzq + 9H phos), 6.91 (m, 1H; bzq),
[Pd(azb)(mal){P(4-MeO-C6H4)3}] 12a. (0.086 g, 67%).
Anal. calc. for C37H32N3O5PPd: C, 60.4; H, 4.4; N, 5.7. Found: C,
60.2; H, 4.5; N, 6.0%. IR (cm−1): m(C=O) 1643vs, m(azb) 1612s,
m{P(4-MeO-C6H4)3} 541m, 517m. FAB-MS (positive mode)
m/z: 736 [M]+, 603 [M − mal]+, 352 [P(4-MeO-C6H4)3]+, 154.
dH (300 MHz, CDCl3): 8.01 (m, 1H; azb), 7.58 (m, 7H; 5 azb +
2 H phos), 7.31 (m, 4H; phos), 7.12 (m, 1H; azb), 6.86 (m, 7H,
1 H azb + 6H phos), 6.59 (m, 1H, azb), 5.89 (s, 2H; mal), 3.79
(s, 9H, MeO).
3
2
6.76 (m, 1H; bzq), 2.34 (dd, 2H succ, J = 4.5, J = 19.8 Hz),
1.73 (dd, 2H succ, 3J = 4.5, 2J = 19.8 Hz).
[Pd(bzq)(succ){P(4-F-C6H4)3}] 5c. (0.101 g, 79%). Anal.
calc. for C35H24F3N2O2PPd: C, 60.1; H, 3.5; N, 4.0. Found:
C, 60.1; H, 3.6; N, 4.2%. IR (cm−1): m(C=O) 1609vs, m(bzq)
827s, m{P(4-F-C6H4)3} 534m, 452m. FAB-MS (positive mode)
m/z: 698 [M]+, 600 [M − succ]+, 316 [P(4-F-C6H4)3]+, 154. dH
(300 MHz, CDCl3): 8.46 (m, 1H; bzq), 8.32 (m, 1H; bzq), 7.86
(m, 6H; phos), 7.74 (m, 1H; bzq), 7.61 (m, 1H; bzq), 7.49 (m,
2H; bzq), 7.09 (m, 6H; phos), 6.95 (m, 1H; bzq), 6.66 (m, 1H;
[Pd(phpy)(phth)(PPh3)] 7b. (0.078 g, 68%). Anal. calc. for
C37H27N2O2PPd: C, 66.4; H, 4.1; N, 4.2. Found: C, 66.2; H,
4.3; N, 4.5%. IR (cm−1): m(C=O) 1646vs, m(phpy) 1602s, m(PPh3)
535m, 516m. FAB-MS (positive mode) m/z: 668 [M]+, 522 [M −
phth]+, 262 [PPh3]+, 154. dH (300 MHz, CDCl3): 8.36 (m, 1H;
phpy), 7.80 (m, 7H; 6H phos + 1H phpy), 7.55 (m, 2H; 1H
phos + 1H phpy), 7.34 (m, 4H; 2H phos + 2H phth), 7.25 (m,
7H; 3H phos + 2H phth + 2H phpy), 7.07 (m, 1H; phos), 6.96
(m, 2H, 1H phos + 1 H phth), 6.59 (m, 3H, 1H phos + 2H
phpy).
3
2
bzq), 2.42 (dd, 2H succ, J = 4.2, J = 21.9 Hz), 1.84 (dd, 2H
succ, 3J = 4.2, 2J = 21.9 Hz).
[Pd(bzq)(succ){P(4-MeOC6H4)3}] 6c. (0.121 g, 90%). Anal.
calc. for C38H33N2O5PPd: C, 62.1; H, 4.5; N, 3.8. Found: C,
62.2; H, 4.6; N, 3.9%. IR (cm−1): m(C=O) 1614vs, m(bzq) 832s,
m(P{4-MeO-C6H4)3} 544m, 458m. FAB-MS (positive mode)
m/z: 735 [M]+, 636 [M − succ]+, 352 [P(4-MeO-C6H4)3]+, 154.
dH (300 MHz, CDCl3): 8.49 (m, 1H; bzq), 8.28 (m, 1H; bzq),
7.76 (m, 7H; 6H phos + 1H bzq), 7.57 (m, 1H; bzq), 7.45 (m,
2H; bzq), 6.93 (m, 1H; bzq), 6.65 (m, 1H; bzq), 6.86 (m, 7H; 6H
phos + 1H bzq), 3.75 (s, 9H, MeO), 2.36 (dd, 2H succ, 3J = 4.5,
2J = 21.9 Hz), 1.84 (dd, 2H succ, 3J = 4.5, 2J = 21.9 Hz).
[Pd(bzq)(phth)(PPh3)] 7c. (0.08 g, 71%). Anal. calc. for
C39H27N2O2PPd: C, 67.6; H, 3.9; N, 4.0. Found: C, 67.8; H,
4.1; N, 4.1%. IR (cm−1): m(C=O) 1642vs, m(bzq) 832s, m(PPh3)
534m, 514m. FAB-MS (positive mode) m/z: 692 [M]+, 546 [M −
phth]+, 262 [PPh3]+, 154. dH (300 MHz, CDCl3): 8.62 (m, 1H;
bzq), 8.27 (m, 7H; 6H phos + 1H bzq), 7.73 (m, 1H; bzq), 7.59
(m, 1H; bzq), 7.38 (m, 8H; 6H phos + 2H bzq), 7.26 (m, 7H; 3H
phos + 4H phth), 6.94 (m, 1H; bzq), 6.79 (m, 1H; bzq).
[Pd(bzq)(phth){P(4-F-C6H4)3}] 8c. (0.090 g, 74%). Anal.
calc. for C39H24F3N2O2PPd: C, 62.7; H, 3.2; N, 3.7. Found: C,
62.9; H, 3.3; N, 3.8%. IR (cm−1): m(C=O) 1650vs, m(bzq) 828s,
m{P(4-F-C6H4)3} 534m, 452m. dH (300 MHz, CDCl3): 8.62 (m,
1H; bzq), 8.31 (m, 1H; bzq), 7.65 (m, 7H; 6H phos + 1H bzq),
7.61 (m, 1H; bzq), 7.47 (m, 6H; 4H phth + 2H bzq), 6.93 (m,
7H; 6H phos + 1H bzq), 6.68 (m, 1H; bzq).
[Pd(bzq)(phth){P(4-MeO-C6H4)3}] 9c. (0.099 g, 78%). Anal.
calc. for C42H33N2O5PPd: C, 64.4; H, 4.2; N, 3.6. Found: C, 64.6;
H, 4.4; N, 3.6%. IR (cm−1): m(C=O) 1644vs, m(bzq) 834s, m(P{4-
MeO-C6H4)3} 542m, 432m. dH (300 MHz, CDCl3): 8.63 (m, 1H;
bzq), 8.27 (m, 1H; bzq), 7.77 (m, 7H; 6H phos + 1H bzq), 7.59
(m, 1H; bzq), 7.41 (m, 6H; 4H phth + 2H bzq), 6.98 (m, 1H;
bzq), 6.87 (m, 1H; bzq), 6.72(m, 6H phos), 3.60 (s, 9H, MeO).
[Pd(phpy)(phth){P(4-F-C6H4)3}] 8b. (0.088 g, 71%). Anal.
calc. for C37H24F3N2O2PPd: C, 61.5; H, 3.3; N, 3.9. Found:
C, 61.2; H, 3.5; N, 4.1%. IR (cm−1): m(C=O) 1650vs, m(phpy)
1600s, m{P(4-F-C6H4)3} 530m, 522m. FAB-MS (positive mode)
m/z: 722 [M]+, 576 [M − phth]+, 316 [P(4-F-C6H4)3]+, 154. dH
(300 MHz, CDCl3): 8.35(m, 1H; phpy), 7.81 (m, 7H; 6H phos +
1H phpy), 7.55 (m, H; phpy), 7.40 (m, 4H; 2H phpy + 2H phth),
7.26 (m, 2H; phth), 7.10 (m, 1H; phos), 6.95 (m, 6H; 5H phos +
1 H phth), 6.51 (m, 2H; phpy).
[Pd(phpy)(phth){P(4-MeO-C6H4)3}] 9b. (0.078 g, 60%).
Anal. calc. for C40H33N2O5PPd: C, 63.3; H, 4.4; N, 3.7. Found: C,
63.5; H, 4.5; N, 4.0%. IR (cm−1): m(C=O) 1644vs, m(phpy) 1692s,
m{P(4-MeO-C6H4)3} 540m, 534m. FAB-MS (positive mode)
m/z: 759 [M]+, 612 [M − phth]+, 352 [P(4-MeO-C6H4)3]+, 154.
dH (300 MHz, CDCl3): 8.35 (m, 1H; phpy), 7.75 (m, 8H; 1H
phth + 6H phos + 1H phpy), 7.56 (m, H; phpy), 7.37 (m, 4H;
2H phpy + 2H phth), 6.73 (m, 3H; 2H phth + 1H phpy), 6.63
(m, 7H, 5H phos + 2H phpy), 3.66 (s, 9H; MeO).
[Pd(phpy)(mal)(PPh3)] 10b. (0.084 g, 69%). Anal. calc. for
C33H25N2O2PPd: C, 64.0; H, 4.1; N, 4.5. Found: C, 64.2; H,
4.4; N, 4.6%. IR (cm−1): m(C=O) 1631vs, m(phpy) 1603s, m(PPh3)
533m, 513m. FAB-MS (positive mode) m/z: 618 [M]+, 522 [M −
mal]+, 262 [PPh3]+, 154. dH (300 MHz, CDCl3): 8.25 (m, 1H;
phpy), 7.83 (m, 9H; 8H phos + 1H phpy), 7.44 (m, 1H; phpy),
7.36 (m, 9H; 7H phos + 2H phpy), 7.05 (m, 1H; phpy), 6.59 (m,
2H; phpy), 6.07 (s, 2H, mal).
[Pd(phpy)(mal){P(4-F-C6H4)3}] 11b. (0.099 g, 69%). Anal.
calc. for C33H22F3N2O2PPd: C, 58.9; H, 3.3; N, 4.2. Found: C,
58.7; H, 3.6; N, 4.5%. IR (cm−1): m(C=O) 1638vs, m(phpy) 1602s,
m{P(4-F-C6H4)3} 532m, 522m. FAB-MS (positive mode) m/z:
672 [M]+, 576 [M − mal]+, 154. dH (300 MHz, CDCl3): 8.25 (m,
1H; phpy), 7.79 (m, 8H; 7H phos + 1H phpy), 7.73 (m, 1H;
phpy), 7.05 (m, 8H; 5H phos + 3H phpy), 6.57 (m, 1H; phpy),
6.50 (m, 1H; phpy), 6.12 (s, 2H, mal).
[Pd(bzq)(mal)(PPh3)] 10c. (0.070 g, 59%). Anal. calc. for
C35H25N2O2PPd: C, 65.4; H, 3.9; N, 4.4. Found: C, 65.6; H, 4.1;
N, 4.6%. IR (cm−1): m(C=O) 1632vs, m(bzq) 830s, m(PPh3) 534m,
458m. FAB-MS (positive mode) m/z: 642 [M]+, 546 [M − mal]+,
262 [PPh3]+, 154. dH (300 MHz, CDCl3): 8.31 (m, 1H; bzq), 8.27
(m, 1H; bzq), 7.52 (m, 10H; 9H phos + 1H bzq), 7.36 (m, 1H;
bzq), 7.24 (m, 2H; bzq), 6.74 (m, 1H; bzq), 6.62 (m, 7H; 6H
phos + 1H bzq), 5.92 (s, 2H; mal).
[Pd(bzq)(mal){P(4-F-C6H4)3}] 11c. (0.066 g, 52%). Anal.
calc. for C35H22F3N2O2PPd: C, 60.3; H, 3.2; N, 4.0. Found: C,
60.3; H, 3.3; N, 4.1%. IR (cm−1): m(C=O) 1632vs, m(bzq) 827s,
m{P(4-F-C6H4)3} 534m, 451m. dH (300 MHz, CDCl3): 8.55 (m,
1H; bzq), 8.32 (m, 1H; bzq) 7.81 (m, 7H; 6H phos + 1H bzq),
7.63 (m, 1H; bzq), 7.48 (m, 2H; bzq), 7.06 (m, 6H phos), 6.96
(m, 1H; bzq), 6,66 (m, 1H; bzq), 6.19 (s, 2H mal).
[Pd(phpy)(mal){P(4-MeO-C6H4)3}] 12b. (0.098 g, 71%).
Anal. calc. for C36H31N2O5PPd: C, 61.0; H, 4.4; N, 3.9. Found: C,
61.1; H, 4.6; N, 4.2%. IR (cm−1): m(C=O) 1642vs, m(phpy) 1590s,
m{P(4-MeO-C6H4)3} 544m, 510m. FAB-MS (positive mode)
m/z: 708 [M]+, 612 [M − mal]+, 352 [P(4-MeO-C6H4)3]+, 154. dH
(300 MHz, CDCl3): 8.25 (m, 1H; phpy), 7.75 (m, 8H; 7H phos +
1H phpy), 7.52 (m, 1H; phpy), 6.95 (m, 1H; phpy), 6.85 (m, 7H;
4H phos + 3H phpy), 6.61 (m, 2H; phpy), 6.07 (s, 2H, mal), 3.80
(s, 9H, MeO).
D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1
3 9 7 9

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Pd catalyst (0.01 equiv.), K3PO4 (0.5 mmol, 2 equiv.), PEO
(1 g, Mw = 100,000) and phenylboronic acid (0.375 mmol, 1.5
equiv.). The flask was evacuated and back filled with N2 and then
the aryl halide (0.25 mmol, 1 equiv.) in dry methanol (4 mL)
was added. The reaction mixture was stirred at 90 ◦C for the
desired time (as specified in the text). The cross-coupled product
was extracted under a stream of N2 with hexane/diethyl ether
(9 : 1, v/v) (4 mL ca. four extractions). The combined apolar
phase was washed with aqueous NaOH (1 M, 5 mL), saturated
NaCl (aq.) (2.5 mL), and then dried (MgSO4). Concentration in
vacuo afforded the biaryl product in > 90% purity as judged by
1H NMR spectroscopy and HRGC. For the recycle reactions,
the polar phase was transferred to a an oven-dried carousel tube
under N2. Then, the aryl halide (0.25 mmol), phenylboronic acid
(0.375 mmol) and K3PO4 (0.5 mmol) were added. The reaction
mixture was stirred at 90 ◦C for the desired time and the product
was isolated as described above (note: that the reactions become
very difficult to stir after the third/forth runs, however this does
not appear to affect the yields of the cross-coupled products).
[Pd(bzq)(phth){P(4-MeO-C6H4)3}] 12c. (0.078 g, 58%).
Anal. calc. for C36H31N2O5PPd: C, 62.3; H, 4.3; N, 3.8. Found:
C, 62.3; H, 4.4; N, 4.0%. IR (cm−1): m(C=O) 1642vs, m(bzq) 824s,
m{P(4-MeO-C6H4)3} 544m, 510m. dH (300 MHz, CDCl3): 8.52
(m, 1H; bzq), 8.27 (m, 1H; bzq), 7.74 (m, 7H; 6H phos + 1H
bzq), 7.58 (m, 1H; bzq), 7.63(m, 2H; bzq), 6.86 (m, 8H; 6H
phos + 2H bzq), 6.14 (s, 2H; mal), 3.89 (s, 9H, MeO).
Crystal structure determination of [Pd(azb)(phth)(P(4-MeO-
C6H4)3)] 9a and [Pd(bzq)(phth)(PPh3)] 7c
Crystals suitable for a diffraction study were grown from
dichloromethane/diethyl ether (complexes 7c and 9a). Data
collection for 9a was performed at 100 K on a Bruker Smart
CCD diffractometer with a nominal crystal to detector distance
of 6.2 cm, diffraction data were collected based on a x scan
run. A total of 2524 (9a) were collected at 0.3◦ intervals and
10 s frame−1. The diffraction frames were integrated using the
SAINT package34 and corrected for absorption with SADABS.35
Data collection for 7c was performed at 173 K on a Siemens P4
diffractometer. The structures were solved by Patterson (9a) and
direct (7c) methods36 and refined by full-matrix least-squares
techniques using anisotropic thermal parameters for non-H
atoms 23 (Table 3).
Acknowledgements
I. J. S. F. thanks for the University of York for financial
assistance (innovation and research priming fund grant) and
Johnson Matthey PLC (UK) for a generous loan of palladium
salts. A. F. L. thanks the University of York for laboratory
refurbishment. Financial support of this work by Direccio´n
General de Investigacio´n (coordinated project-BQU2001-0979-
C02-01/02) is gratefully acknowledged. The referees are thanked
for their constructive comments.
CCDC reference numbers 249864 and 249865.
See http://www.rsc.org/suppdata/dt/b4/b413886d/ for cry-
stallographic data in CIF or other electronic format.
Typical Suzuki–Miyaura cross-coupling reaction
All reactions were performed in a Radleys carousel adapted
for rigorous inert atmosphere reactions. The aryl boronic acid
(0.41 mmol), aryl halide (0.45 mmol), Na2CO3 (1 M (aq.), 1 mL),
THF (1.5 mL), hexadecane (0.1 equiv., internal standard) and
Pd catalyst (0.01 equiv.) were degassed via ‘freeze–pump–thaw’
cycles (on a vacuum line, then transferred to the carousel). The
resulting mixture was heated at 90 ◦C for the specified time and
then quenched by addition of water (10 mL). The mixture was
extracted with CH2Cl2 (3 × 10 mL), which was combined and
washed with saturated NaCl (aq.) (1 × 10 mL), dried (MgSO4),
filtered and then concentrated in vacuo.
References and notes
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Typical Sonogashira cross-coupling reaction
All reactions were performed in a Radleys carousel adapted
for rigorous inert atmosphere reactions. To the aryl halide
13c (0.51 mmol, 1.04 equiv.), hexadecane (0.1 equiv., internal
standard) and Pd catalyst (X mol%; X = 1, 0.1, 0.01 or
0.001) was added dry degassed CH3CN (3.0 mL) and Et3N
(2.0 mL). Phenylacetylene (0.49 mmol, 1.0 equiv.) and CuI
(0.05 equiv.) were added (the latter was absent in the copper-
free experiments). The reaction was heated to 100 ◦C and
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purchased from Fluka) packed in a Pasteur pipette with CH2Cl2
(2.5 mL). The resultant fraction was then treated with 1,2-
bis(diphenylphosphino)ethane (dppe), at a final concentration
containing four equivalents of dppe per maximum theoretical
amount of palladium. This latter preparative sampling step
ensures that no further cross-coupling occurs at room temper-
ature (the time dependence of post-quenched samples showed
no further changes using this method).37 The sample was then
analysed by HRGC analysis.
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14 It is excepted that N-donors are generally strong r-donors, and that
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to the two neighbouring carbonyl groups.
Typical procedure for catalyst recycling in Suzuki-Miyaura
cross-coupling reaction
Using identical substrate quantities to those reported, but using
an adapted procedure: all reactions/runs were carried out
in a Radleys carousel adapted for rigorous inert atmosphere
reactions.31 To an oven-dried carousel tube was added the
3 9 8 0
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D a l t o n T r a n s . , 2 0 0 4 , 3 9 7 0 – 3 9 8 1
3 9 8 1