Azo-containing phosphine complexes of palladium(II) and platinum(II) and their effectiveness in the Heck reaction: Crystal and molecular structure of [PdCl2{PPh2{1-(4-MeC6H4N 2)-2-OC(O)Me-C10H5}]·2EtOH

Article abstract of DOI:10.1016/S0022-328X(99)00407-6

Reaction of Na[MCl₄] (M=Pd or Pd) with the azo-containing phosphines Ph₂P{1-(4-RC₆H₄N ₂)-2-OR′-C₁₀H₅} {R=Me (I), NMe₂ (II); R′=C(O)Me} affords the complexes [MCl₂L₂] (1-4) in good yield. Complexes 1-4 have all been fully characterised by elemental analysis, ¹H-, ¹³C{¹H}-, and ³¹P{¹H}-NMR spectroscopy and UV-visible spectroscopy. The use of 1 in the Heck reaction has been investigated and shown to effect up to 1000 turnovers.

Full text of DOI:10.1016/S0022-328X(99)00407-6

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 590 (1999) 123–128
Azo-containing phosphine complexes of palladium(II) and
platinum(II) and their effectiveness in the Heck reaction: crystal
and molecular structure of
[PdCl2{PPh2{1-(4-MeC6H4N2)-2-OC(O)MeꢀC10H5}]·2EtOH
Mark J. Alder, Wendy I. Cross, Kevin R. Flower *, R.G. Pritchard
Department of Chemistry, UMIST, PO Box 88, Manchester, M60 1QD, UK
Received 25 May 1999; accepted 25 June 1999
Abstract
Reaction of Na[MCl4] (M=Pd or Pd) with the azo-containing phosphines Ph2P{1-(4-RC6H4N2)-2-OR%-C10H5} {R=Me (I),
NMe2 (II); R%=C(O)Me} affords the complexes [MCl2L2] (1–4) in good yield. Complexes 1–4 have all been fully characterised
1
by elemental analysis, H-, 13C{1H}-, and 31P{1H}-NMR spectroscopy and UV–visible spectroscopy. The use of 1 in the Heck
reaction has been investigated and shown to effect up to 1000 turnovers. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Azophosphines; Platinum; Palladium; Heck
the reactions are generally carried out with 1–5 mol%
loadings in Pd, which means the reactions are not
1. Introduction
particularly cost effective [2]. We recently reported the
The use of palladium-based compounds to effect,
preparation of azo-containing phosphines [8] and some
catalytically, organic transformations is widespread [1].
coordination compounds of Group 6 metal carbonyls
In particular, the vinylation of aryl halides by the
[9]. Herein we report the preparation of some palladium
so-named Heck reaction [2] is currently subject to much
and platinum complexes and the use of the palladium
interest [3]. Recent studies reported in this area by
various groups have shown that TONs (turnover num-
complexes in the Heck reaction.
bers) close to and exceeding 1 000 000 have been
achieved by palladium complexes that contain chelating
ligands. For example, Herrmann et al. have shown that
2. Results and discussion
complexes based upon cyclometallated trio-tolylphos-
phine afford a TON of up to 40 000 for aryl chlorides
2.1. Complex characterisation
[4]; Shaw et al. have similarly shown that cyclometal-
lated 1-naphthyl phosphine complexes effect TON up
to 1 000 000 [5]; additionally Shaw and Perera have
shown that palladium complexes containing chelating
The azo-containing phosphines I and II, Fig. 1, were
prepared by the literature method [8] and reacted with
Na2[MCl4] to give the complexes trans-[PdCl2L2] 1 and
3 and cis-[PtCl2L2] 2 and 4 in good yield. 1–4 were all
phosphines are equally effective [6], contrary to previ-
characterised by elemental analysis, solvates were con-
ous reports to the contrary; furthermore Bedford and
firmed by repeated analysis on separate samples (see
co-workers have shown that palladium complexes with
1
Table 1), UV–visible spectroscopy (Table 2) and H-,
cyclometallated phosphite ligand sets also give rise to
highly effective Heck catalysts [7]. In general, however,
31P{1H}- and 13C{1H}-NMR (see Tables 3 and 4).
Compound 1·2EtOH was further characterised by a
single-crystal X-ray diffraction study, Table 5. The
results of the Heck reactions carried out using 1 as the
* Corresponding author. Tel.: +44-161-2004538.
E-mail address: k.r.flower@umist.ac.uk (K.R. Flower)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00407-6

124
M.J. Alder et al. / Journal of Organometallic Chemistry 590 (1999) 123–128
Table 2
UV–visible data for complexes 1–4 a
Complex
u
m
1
2
484.5
356.5
488.0
351
423
434.5
3167
18083
3568
10438
17172
18173
3
4
a Spectra recorded in CHCl3, m=mol dm−3 cm−1; umax=nm.
Fig. 1. The azo-containing phosphines I and II.
source of the palladium catalyst are presented in Table
6. The reason for using the ester-functionalised phos-
phines rather than the free azohydroxyphosphines was
that 1-phenylazonaphthalen-2-ols are known to com-
plex to metal centres as OꢀN chelates and we did not
want this to be a competing reaction [10].
2.2. Molecular structure of 1
The molecular structure and numbering scheme for
1·2EtOH can be found in Fig. 3; the disordered ethanol
solvates are omitted for clarity, and only half the atoms
are numbered as half were generated using the symme-
try transformation −x, y+2, −z. The solid-state
structure confirms the spectroscopic data that the com-
plex exists in the expected square planar form with the
phosphines mutually trans to each other. All of the
bond lengths in the complex are within the expected
The UV–visible spectra all display the expected ab-
sorptions for azo-containing species. The intensity of
the absorptions by the azo chromophore is primarily
responsible for the colour in these complexes. The
presence of the dimethylamino group on the phenylazo
ring causes the observed shift in the azo absorbance of
80 nm to longer wavelength in 3 and 4 through a
conjugated interaction with the electron-accepting azo-
chromophore [11].
,
ranges with PdꢀP(1)=2.3235(19) A, PdꢀCl(1)=
,
,
2.792(2) A, and N(1)ꢀN(2)=1.241(9) A.
1
2.3. The Heck reaction
The H-NMR spectra, Table 3, all show the expected
resonances for the phosphines, which are slightly per-
turbed on complexation and show retention of the ester
moiety. The 31P{1H}-NMR spectra for 1–4 all show a
singlet resonance shifted downfield from the free phos-
phine; 2 and 4 each show a singlet resonance straddled
Having prepared 1–4, we carried out a preliminary
investigation into the suitability of these complexes to
effect the Heck vinylation of arenes. The Heck reaction
is well known to be tolerant to many different func-
tional groups [2] and we could only find one example of
a coupling reaction involving precursors that contained
an azo moiety and a loading of 5 mol% Pd was used in
the coupling reaction [14]. The initial reaction was
carried out in THF where iodotoluene was reacted with
methyl acrylate in the presence of 1 in (1 mol% loading)
and triethylamine as the base. The reaction was left for
17 h and after work up was shown to yield methyl
cinnimate and some unreacted starting materials: the
yield of pure material was 50%. We found a better yield
could be obtained if a little free phosphine was added
1
by the expected platinum satellites, JPtꢀP 3700 Hz; the
magnitude of the coupling constant is indicative of a cis
geometry. The 13C{1H}-NMR spectra are all consistent
with the formulation of the complexes 1 and 3 as
trans-bisphosphine complexes; the numbering scheme
for resonance assignment is illustrated in Fig. 2. All of
the resonances were easily assigned using published
substituent effects [12] and the presence of phosphorus
coupling to the carbon nuclei, giving rise to virtual
triplets for the carbon atoms in close proximity to the
phosphorus nuclei [13].
Table 1
Physical and analytical data for compounds 1–4 a
Compound
Colour
Yield (%)
m/z
Mp (°C)
C (%)
H (%)
N (%)
1
Orange
Orange
Red
72
59
56
55
236
222
228
210
63.5 (64.5)
58.2 (58.5)
57.2 (57.1)
55.4 (55.0)
4.3 (4.4)
4.0 (4.0)
4.4 (4.3)
4.0 (4.0)
4.5 (4.9)
3.9 (4.4)
5.8 (6.1)
6.0 (6.1)
2·0.5CH2Cl2
3·2CH2Cl2
4·CHCl3
1207 [MꢀCl]+
1265 [MꢀCl]+
Brown
a Calculated values in parentheses.

M.J. Alder et al. / Journal of Organometallic Chemistry 590 (1999) 123–128
125
Table 3
1H-NMR data a (l) ppm and 31P{1H}-NMR data b (l) ppm for compounds 1–4
Compound
31P (l)
1H (l)
1
2
3
24.5
8.6 (d, JHH 6.5, 2H, ArꢀH); 8.3 (t, JHH 6.0, 2H, ArꢀH); 7.9–7.8 (bm, 16H, ArꢀH); 7.5–7.3
(bm, 18H, ArꢀH); 2.5 (s, 6H, CH3); 2.3 (s, 6H, CH3).
8.2 (d, JHH 8.5, 2H, ArꢀH); 7.8–7.6 (bm, 20H, ArꢀH); 7.3–7.2 (bm, 16H, ArꢀH); 5.29,
(s, 1H, CH2Cl2); 2.5 (s, 6H, CH3); 2.3 (s, 6H, CH3).
8.6 (d, JHH 9.3, 2H, ArꢀH); 8.3 (t, JHH 6.0, 2H, ArꢀH); 7.9–7.8 (bm, 22H, ArꢀH); 7.4
(d, JHH 6.8, 4H, ArꢀH); 7.3 (d, JHH 8.8, 4H, ArꢀH); 6.8 (d, JHH 9.0, 4H, ArꢀH); 5.29,
(s, 4H, CH2Cl2); 3.1 (s, 12H, CH3); 2.3 (s, 6H, CH3).
15.8 [JPtꢀP 3676]
24.5
4
15.7 [JPtꢀP 3678]
8.2 (d, JHH 9.5, 2H, ArꢀH); 7.8–7.6 (bm, 18H, ArꢀH); 7.3–7.2 (bm, 15H, ArꢀH, CHCl3);
6.8 [d, JHH 8.5, 4H, ArꢀH); 3.1 [s, 12H, CH3]; 2.3 (s, 6H, CH3).
a Spectra recorded in CDCl3 and referenced to CHCl3; coupling constants in Hz.
b Spectra recorded in CDCl3 and referenced to 85% H3PO4; coupling constants in Hz.
Table 4
13C{1H}-NMR a data (l) for compounds 1–4
Compound 13C {1H} (l) ppm
1
2
3
4
169.4 [s, CO]; 151.5 [s, C(15)]; 142.4 [s, C(18)]; 138.5 [s, C(1)]; 138.3 [s, C(2)]; 135.7 [vt, J 6.5, C(5)]; 135.1 [vt, J 6.5, C(12)];
131.9 [vt, J 6.4, C(7)]; 131.5 [s, C(4)]; 130.7 [s, C(17)]; 129.9 [s, C(14)]; 129.2 [vt, J 24.7, C(6)];128.2 [vt, J 5.8, C(13)]; 123.7
[vt, J 4.4, C(8)]; 123.3 [s, C(16)]; 122.8 [s, C(3)]; 21.5 [s, CH3]; 20.9 [s, CH3].
169.1 [s, CO], 151.4 [s, C(15)]; 142.6 [s, C(18)]; 138.9 [s, C(1)]; 137.9 [s, C(2)]; 135.9 [vt, J 5.5, C(5)]; 135.0 [vt, J 4.4, C(12)];
132.1 [d, J 9.5, C(7)]; 131.1 [s, C(4)]; 131.0 [s, C(17)]; 130.6 [s, C(9)]; 129.8 [s, C(14)]; 129.1 [vt, J 26.9, C(6)]; 128.0 [vt, J 5.0,
C(13)]; 123.6 [vt, J 5.1, C(8)]; 122.8 [s, C(16)]; 120.1 [s, C(3)]; 21.5 [s, CH3]; 20.9 [s, CH3].
169.4 [s, CO]; 152.8 [s, C(18)]; 144.6 [s, C(15)]; 139.1 [s, C(1)]; 138.2 [s, C(2)]; 136.0 [vt, J 6.5, C(5)]; 135.1 [vt, J 5.8, C(12)];
131.4 [vt, J 9.4, C(7)]; 130.8 [s, C(9)]; 130.6 [s, C(14)]; 129.9 [s, C(4)]; 129.4 [vt, J 24.7, C(6)];128.2 [vt, J 5.1, C(13)]; 124.1
[vt, J 4.4, C(8)]; 123.4 [s, C(16)]; 121.5 [s, C(3)]; 111.4 [s, C(17)]; 40.3 [s, CH3]; 30.0[s, CH3].
169.3 [s, CO]; 152.7 [s, C(18)]; 144.6 [s, C(15)]; 138.6 [s, C(1)]; 138.5 [s, C(2)]; 136.0 [vt, J 6.5, C(5)]; 134.9 [vt, J 5.4, C(12)];
132.0 [vt, J 9.4, C(7)]; 131.0 [s, C(14)]; 130.6 [s, C(9)]; 130.0 [vt, J 5.5, C(8)]; 129.4 [s, C(4)]; 128.0 [vt, J 5.8, C(13)]; 123.9
[s, C(16)]; 123.7 [s, C(3)]; 111.6 [s, C(17)]; 40.4 [s, CH3]; 30.0 [s, CH3].
a Spectra recorded in CDCl3 at 75.5 MHz and referenced to CDCl3 (77.0 ppm); coupling constants in Hz. See Fig. 2 for numbering scheme.
to the reaction mixture. To see if the yield could be
improved by increasing the temperature, we chose to do
the reaction in di-nbutyl ether; carrying out the reaction
in this solvent caused complete conversion to the cinni-
mate, effecting a TON of 100: again a small amount of
additional phosphine was added. The coupling reaction
was then effected between methylacrylate and either
p-iodotoluene p-bromoanisole, 6-bromo-2-methoxy-
naphthalene or p-chlorotoluene with 1 as catalyst pre-
cursor and triethylamine as the base and found the
following. The reaction with p-chlorotoluene did not
proceed to any appreciable extent, but up to 1000
turnovers could be obtained with p-bromoanisole.
While these results are not outstanding by recent ad-
vances [3–6], they show that Pd complexes that contain
azo-functionalised phosphines are capable of catalysing
the Heck reaction. We are encouraged by these results
to attempt to prepare azo-containing phosphines that
have potential to act as PꢀN chelates to the metal
centre, since they should display enhanced reactivity in
the Heck reaction, especially when the results reported
by Shaw and Perera on chelating phosphine complexes
are borne in mind [6]. The problem associated with the
chelating azo-phosphine approach, however, is the lack
of synthetic methodology for their preparation and it is
extremely likely that the synthesis will be non-trivial: we
are currently investigating a range of possible routes.
Table 5
a
,
Selected bond lengths (A) and angles (°) for 1
Pd(1)ꢀCl(1)
P(1)ꢀC(24)
P(1)ꢀC(5)
O(1)ꢀC(30)
N(1)ꢀC(2)
C(1)ꢀC(8)
C(2)ꢀC(10)
C(32)ꢀC(33)
C(30)ꢀC(31)
2.279(2) Pd(1)ꢀP(1)
1.810(8) P(1)ꢀC(18)
1.813(8) O(1)ꢀC(1)
1.503(19) N(1)ꢀN(2)
1.407(10) N(2)ꢀC(11)
1.369(14) C(1)ꢀC(2)
1.425(11) C(3)ꢀC(4)
2.3235(19)
1.812(8)
1.392(11)
1.241(9)
1.433(11)
1.385(13)
1.348(11)
1.223(19)
1.59(2)
1.71(2)
O(2)ꢀC(30)
Cl(1)ꢀPd(1)ꢀCl(1)c1
Cl(1)c1ꢀPd(1)ꢀP(1)
180.0
Cl(1)ꢀPd(1)ꢀP(1)
92.97(7)
87.03(7) Cl(1)ꢀPd(1)ꢀP(1)c1 87.03(7)
Cl(1)c1ꢀPd(1)ꢀP(1)c1 92.97(7) P(1)ꢀPd(1)ꢀP(1)c1 180.00(10)
C(1)ꢀO(1)ꢀC(30)
N(1)ꢀN(2)ꢀC(11)
C(2)ꢀC(1)ꢀO(1)
C(16)ꢀC(11)ꢀN(2)
115.6(10) N(2)ꢀN(1)ꢀC(2)
114.3(8) C(8)ꢀC(1)ꢀC(2)
121.7(8) C(1)ꢀC(2)ꢀN(1)
115.7(9) C(12)ꢀC(11)ꢀN(2)
114.7(7)
121.7(8)
126.5(8)
124.3(8)
a Symmetry transformations used to generate equivalent atoms:
c1 −x, −y+2, −z.

126
M.J. Alder et al. / Journal of Organometallic Chemistry 590 (1999) 123–128
Table 6
Summary of the Heck reactions using 1 as palladium precursor
Aryl Halide
Alkene
Base Catalyst loading Solvent
Time (h)
Yield(%)
TON
Extra phosphine
a
4-Iodotoluene
4-Iodotoluene
4-Iodotoluene
4-Iodotoluene
4-Bromoanisole
4-Bromoanisole
6-Bromo-2-methoxynaph-
thalene
Methylacrylate
Methylacrylate
Methylacrylate
Methylacrylate
Methylacrylate
Methylacrylate
Methylacrylate
Et3N
Et3N
Et3N
Et3N
Et3N
1
1
2
1
1
THF
THF
THF
Bu2O
Bu2O
Bu2O
Bu2O
17
17
17
48
48
72
48
50
55
75
50
55
38
100
no
yes
no
yes
yes
a
a
a
100
100 a/58 b 100/58
Et3N 0.1
100 a/71 b 1000/710 yes
a
Et3N
Et3N
1
100
100
yes
a
4-Chlorotoluene
Methylacrylate
1
Bu2O
72
0
0
yes
a Yield by NMR.
b Isolated yield.
3. Conclusions
spectra were referenced externally to 85% H3PO4. Posi-
tive FAB spectra were obtained on a Kratos MS50TC
spectrometer in a 3-nitrobenzyl alcohol matrix. UV–
visible spectra were recorded on a Shimadzu UV20101
PC spectrometer in CHCl3 (AR) from BDH. Elemental
analyses were performed by the Microanalytical Ser-
vice, Department of Chemistry, UMIST; solvates of
crystallisation were confirmed by NMR data and re-
peated elemental analysis. The syntheses of the metal
complexes were carried out in the open and the Heck
reactions were carried out under an atmosphere of
dinitrogen.
We have shown that a collection of palladium and
platinum complexes containing azo-functionalised
phosphines can be prepared and that the palladium
complexes are reasonable precatalysts for the Heck
reaction.
4. Experimental
4.1. General considerations
All solvents were dried by refluxing over an appropri-
ate drying agent and distilled before use. The azophos-
phines I, II were prepared by the published method [8].
Na2[MCl4] (M=Pd, Pt) were loaned to us by Johnson
Matthey; all other chemicals were purchased from com-
mercial sources and used as received. Melting points
were measured on a Griffin Melting Point Apparatus
and are uncorrected. 1H-NMR (200.2 MHz) and
31P{1H}-NMR (81.3 MHz) spectra were recorded on a
Bruker DPX200 spectrometer; 13C{1H}-NMR (75.5
MHz) spectra were recorded on a Bruker DPX300
4.2. Complex preparation
4.2.1. trans-[Pd(PPh2{1-(4-RC6H4N2)-2-
OC[O]MeꢀC10H5})2Cl2] 1
To Na2[PdCl4] (0.075 g, 0.26 mmol) dissolved in H2O
(1 ml) was added dropwise I (0.25 g, 0.51 mmol) in
THF (5 ml) and the resulting solution was stirred for 2
h. An orange powder was collected by filtration,
washed with water, followed by dry diethyl ether and
dried to give 1 (0.22 g, 72%).
1
spectrometer; H and 13C{1H} spectra were referenced
Complex 3 was prepared in an analogous manner.
to CHCl3 (l 7.26) and CHCl3 (l 77.0) and 31P{1H}
Recrystallisation
from
CH2Cl2–hexane
yielded
3·2CH2Cl2 as a brown powder (0.17 g, 59%).
4.2.2. cis-[Pt(PPh2{1-(4-RC6H4N2)-2-
OC[O]MeꢀC10H5})2Cl2] 2
To Na2[PtCl4] (0.098 g, 0.26 mmol) dissolved in H2O
(1 ml) at 60°C was added dropwise I (0.25 g, 0.51
mmol) in THF (5 ml) and the resulting solution was
stirred for 3 h at 60°C. The solution was cooled and the
THF was removed under reduced pressure. CH2Cl2 (5
ml) was added to the resulting crude material, dried
over anhydrous MgSO4 and filtered. Hexane was added
to the solution resulting in a brown powder which was
collected by filtration and dried to give 2·0.5CH2Cl2
(0.22 g, 72%).
Fig. 2. Numbering scheme used in assigning 13C{1H}-NMR spectra.

M.J. Alder et al. / Journal of Organometallic Chemistry 590 (1999) 123–128
127
Fig. 3. ORTEP representation of 1 showing the atomic numbering scheme.
Complex 4 was prepared in an analogous manner.
Recrystallisation from CHCl3–hexane gave 4·CHCl3 as
a brown powder (0.17 g, 55%).
2|(I)]; R1 0.0751, wR2 0.1724; R (all data) R1=0.1300,
wR2=0.2024. Selected bond lengths and angles can be
found in Table 5.
4.3. Heck reactions
5. Supplementary material
A typical Heck reaction was performed (for a cata-
lyst loading of 1 mol%) as follows:
All crystallographic data have been deposited at
Cambridge Crystallographic Data Centre, CCDC No.
124615. Copies of this information can be obtained free
of charge from: The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (fax: +44-1123-336-033;
e-mail: deposit@ccdc.ac.uk or www: http://www.ccdc.
cam.ac.uk).
To 4-iodotoluene (0.94 g, 4.3 mmol) in di-n-butyl
ether (20 ml) were added methyl acrylate (0.55 g, 6.5
mmol), Et3N (0.8 ml, 5.6 mmol), 3 (0.05 g, 0.043 mmol)
and I (0.05 g, 0.1 mmol) under an atmosphere of
dinitrogen. The mixture was refluxed for 48 h and then
cooled to room temperature. The solvent was removed
in vacuo, the crude product extracted with diethyl ether
and filtered. The diethyl ether was removed under
reduced pressure affording 0.65 g of a solid, a small
portion of which was dissolved in CDCl3 and the
1H-NMR spectrum recorded.
Acknowledgements
M.J.A. and W.I.C. would like to thank the EPSRC
for the award of postgraduate studentships. We also
thank Johnson Matthey for the loan of platinum and
palladium salts.
4.4. Crystallography
An orange tablet (0.2×0.1×0.1 mm) of 1·2EtOH
(C62H56Cl2N4O4P2Pd·2C2H5OH; Mw 1246.44) was
mounted on a Nonius MACH 4-circle diffractometer
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