Variable coordination behaviour of pyrazole-containing N,P and N,P(O) ligands towards palladium(ii)

Article abstract of DOI:10.1039/b704051b

Three bidentate, mixed-donor ligands based on a triphenylphosphine unit bearing a pyrazole group in the ortho-position of one phenyl ring have been synthesised; the N,P ligand [2-(3-pyrazolyl)phenyl]diphenylphosphine pzphos has been synthesised and transf

Full text of DOI:10.1039/b704051b

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Variable coordination behaviour of pyrazole-containing N,P and N,P(O)
ligands towards palladium(II)†
Steven Kealey,a Nicholas J. Long,*a Philip W. Miller,a Andrew J. P. White,a Peter B. Hitchcockb and
Antony Geea,c
Received 16th March 2007, Accepted 12th April 2007
First published as an Advance Article on the web 2nd May 2007
DOI: 10.1039/b704051b
Three bidentate, mixed-donor ligands based on a triphenylphosphine unit bearing a pyrazole group in
the ortho-position of one phenyl ring have been synthesised; the N,P ligand [2-(3-pyrazolyl)phenyl]-
diphenylphosphine pzphos has been synthesised and transformed into new N,P(O) and N,P(S)
derivatives, [2-(3-pyrazolyl)phenyl]diphenylphosphine oxide pzphos(O) and [2-(3-pyrazolyl)phenyl]-
diphenylphosphine sulfide pzphos(S), respectively. The coordination chemistry of pzphos and pzphos(O)
towards palladium(II) has been investigated. Depending on the ligand to metal molar ratio employed in
the reactions of palladium(II) with pzphos , either the 1 : 1 chelate [Pd(pzphos)Cl2] 1a or the 2 : 1 N,P
chelate [Pd(pzphos)2]Cl2 1b was obtained. 1b contains two six-membered chelate rings in which the
chlorides have been displaced from the inner coordination sphere of palladium. Exchange of the
chloride anions in 1b for perchlorate anions was achieved using AgClO4 to give [Pd(pzphos)2][ClO4]2 1c.
Reaction of pzphos(O) under the same conditions forms the 2 : 1 adduct [Pd(pzphos(O))2Cl2] 2b
regardless of the metal to ligand ratio or the order of addition of reactants. Unlike the N,P chelate 1b,
the N,P(O) ligands in complex 2b bind in a monodentate fashion through the N-donor atoms of the
pyrazole rings. Abstraction of the chloro ligands in compound 2b using AgClO4 gave the 2 : 1 N,P(O)
chelate [Pd{pzphos(O)}2][ClO4]2 2c, in which entropically unfavourable 7-membered chelate rings are
formed. X-Ray diffraction has been used to confirm the solid-state structures of the pzphos(O) ligand
and the complexes 1b, 1c, 2b and 2c.
well as in small-molecule sensing,12 functional materials,3,13,14 and
Introduction
molecular activation.15–21
Over recent years, considerable attention has been drawn towards
Coincidentally, during the course of our research, the ligand, [2-
the coordination chemistry of hybrid ligands i.e. those molecules
(3-pyrazolyl)phenyl]diphenylphosphine (pzphos), which consists
containing significantly different donor groups. Of particular
interest are the metal complexes of hybrid ‘hemilabile’ ligands,1,2
of a triphenylphosphine unit (soft donor) bearing a pyrazole
group (hard donor) in the ortho-position of one phenyl ring, was
independently published,22,23 albeit by a different synthetic route.
Our route is shown in Scheme 1 and we are interested in this
family of pyrazole-containing ligands for a number of reasons,
namely; (i) the potential hemilability of the parent complex; (ii) the
ease of derivatisation by chalcogenation of the phosphine moiety
thus giving rise new to N,P(O), N,P(S) and N,P(Se) ligands; (iii)
the scope for further functionalisation by alkylation of the N–H
moiety. This last feature is particularly appealing as it may enable
modifications of the steric or electronic properties of the ligand,
which could be used to tune the ligand’s coordination behaviour
towards a metal centre.
containing at least one substitutionally inert donor group plus one
or more substitutionally labile donor groups. These ligands, in the
presence of a small molecule substrate, can allow the donor group
with the lowest affinity towards the metal to dissociate to create a
vacant coordination site which is then available to accommodate
the new substrate. The ‘anchor’ effect of the stronger, permanently-
bound donor group means that after the substrate has left the
coordination sphere of the metal, chelation can again occur to
stabilise the metal. These enticing properties of hemilabile ligands
has led them to receive much interest in the field of catalysis,2–11 as
A structurally similar ligand bearing the same triphenylphos-
phine unit, in this case attached to the pyrazolyl ring via the
nitrogen atom (1-position) has recently been reported and has
been shown to be active in the nickel catalysed oligomerisation of
ethylene.24 Furthermore, a bis(pyrazol-1-yl)methane ligand bear-
ing diphenylphosphine groups directly attached to the pyrazole
rings, rather than through a phenyl-linker, has also been described
in the literature,25 thus highlighting the ongoing interest in this field
of chemistry. In addition, the coordination behaviour of a range of
aDepartment of Chemistry, Imperial College London, South Kensington,
London, UK SW7 2AZ. E-mail: n.long@imperial.ac.uk; Tel: +44 (0)20
7594 5781
bDepartment of Chemistry, School of Life Sciences, University of Sussex,
Falmer, Brighton, UK BN1 7QJ
cGSK Clinical Imaging Centre, Imperial College London, Hammersmith
Hospital, Du Cane Road, London, UK W12 0NN
† Electronic supplementary information (ESI) available: Crystallographic
and NMR data. See DOI: 10.1039/b704051b
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Scheme 1 Synthesis of pzphos(O) and pzphos(S). Reagents and condi-
tions: (i) KPPh2, THF, reflux, 1 h; (ii) N,N-dimethylformamide dimethyl
acetal, EtOH, reflux, 12 h; (iii) N2H4.H2O, EtOH, reflux, 3 h; (iv) S8,
CH2Cl2, rt, 1 h; (v) H2O2, CH2Cl2, rt, 1 h.
N,P ligands26–28 is well established in the literature and to a lesser
extent, the chemistry of N,P(O) ligands.29–36 The work presented
herein, describes the formation of two chalcogen derivatives of
pzphos and the coordination chemistry of the N,P and N,P(O)
ligands, pzphos and pzphos(O), towards palladium(II).
Results and discussion
Ligand synthesis
The synthesis of the three ligands is outlined in Scheme 1: Ligand
precursor A was synthesised using a different approach to the
three-step synthesis described recently by Sun et al.22 Instead,
a facile one step synthesis which involved heating a solution of
potassium diphenylphosphide and 2-fluoroacetophenone in THF
was used to synthesise A rapidly and in high yield.37 A was then
converted into the pzphos ligand in good yield in two steps. The
chalcogen derivatives were synthesised by reaction of pzphos with
hydrogen peroxide or sulfur to produce the respective phosphine
oxide and phosphine sulfide ligands, pzphos(O) and pzphos(S). All
three ligands were isolated as air-stable, white solids in good yields
and were fully characterised. As shown in Table 1, the 31P NMR
spectra of the phosphorus(V) species pzphos(O) and pzphos(S)
show characteristic upfield shifted singlets at +35.8 ppm and
+43.3 ppm, respectively, compared to the phosphorus(III) parent
ligand, pzphos at −11.4 ppm.
Slow diffusion of pentane into a chloroform solution of
the ligand produced single crystals suitable for X-ray analysis.
The solid-state structure of pzphos(O) shows that the molecule
dimerises in the solid state via intermolecular hydrogen bonding
between adjacent molecules (Fig. 1). It is interesting to note
that the hydrogen bond from the pyrazole N–H donor is shared
equally between two acceptors on the opposing molecule: the
unprotonated pyrazole nitrogen and the oxygen atom of the P(O)
moiety. This dimerisation is also observed in solution as the N–H
in the 1H NMR spectrum is far downfield at 13.24 ppm, whereas
=
the parent complex pzphos (containing no P O group) shows the
corresponding N–H peak at 10.57 ppm (Table 1). This is likely
to be the result of reduced electron density around the N–H
hydrogen, due to the sharing of its electron density with both
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of the 1 : 1 complex 1a. Complex 1b was isolated as a yellow solid
by precipitation of a dichloromethane solution with hexane.
It was also possible to form 1b by a stepwise process by
first forming and isolating 1a and then reacting this with a
further equivalent of the pzphos ligand (shown in Scheme 2). The
formation of the bis-chelate species 1b is noteworthy as it has been
previously reported that it was only possible to isolate the mono-
chelate species 1a, even when using a large excess of pzphos in the
reaction with Pd(PhCN)2Cl2.22
The 31P{H} NMR spectrum of 1b showed one singlet at
+31.6 ppm (c.f. pzphos ligand; d = −11.4 ppm) indicating the
coordination of the pzphos ligands to the palladium centre in
chemically equivalent environments. The N–H hydrogen of the
pyrazole ring was found to be significantly shifted downfield
Fig. 1 The molecular structure of pzphos(O) (50% probability ellipsoids).
1
in the H NMR spectrum to 14.86 ppm compared to the free
˚
The N–H · · · O hydrogen bonds have N · · · O, H · · · O (A), and N–H · · · O
(◦) of 2.91, 2.54 and 133 respectively; the N–H · · · N hydrogen bonds have
pzphos ligand (d = 10.57 ppm), suggesting coordination of the
adjacent nitrogen atom of the pyrazole ring to the metal centre
(Table 2). The bis-ligated nature of 1b was confirmed by ESI
mass spectrometry which showed a molecular ion peak at 761
m/z corresponding to [Pd(pzphos)2]+ and suitable crystals for X-
ray diffraction were obtained by slow diffusion of pentane into a
chloroform solution of 1b.
◦
˚
N · · · N, H · · · N (A), and N–H · · · N ( ) of 2.94, 2.22 and 141 respectively.
the pyrazole nitrogen and the oxygen atom. A similar effect may
also be observed with the sulfur derivative, pzphos(S), as evidenced
by the downfield position (12.54 ppm) of the corresponding N–H
hydrogen in the 1H NMR spectrum.
The solid state structure of 1b shows the palladium to have
a distorted square planar geometry with the two pzphos ligands
chelating in a cis fashion resulting in a dicationic species in which
two chloride anions balance the charge (Fig. 2, Table 3). The
N–H hydrogen atoms are linked to the chloride anions by N–
H · · · Cl hydrogen bonds (see ESI).† Within each ligand the linked
C6 and C3N2 rings are distinctly twisted with respect to each other,
the torsion angles for the C–C bond between them being ca. 20
and 26◦ for the N(4) and N(2) ligands, respectively. The two
{Pd,P,N} coordination planes are twisted with respect to each
other by ca. 15◦, a distortion that may in part be due to a p–p
stacking interaction between pendant phenyl rings on adjacent
ligands (dashed line in Fig. 2). These interactions are supported
by the ambient temperature 1H NMR spectrum which shows some
significantly broadened peaks in the aromatic region. Following
this observation, variable temperature spectra were recorded upon
cooling the solution to 223 K (aromatic region shown in Fig. 3).
At 223 K, the spectrum consists of sharp peaks and as the
temperature is increased, some of these peaks coalesce forming
broad signals. It is apparent that not all of these aromatic
Coordination chemistry
Reactions of N,P ligand, pzphos. The reactions performed
with the N,P ligand pzphos are outlined in Scheme 2. Addition
of pzphos in dichloromethane to Pd(COD)Cl2 in a 1 : 1 molar
ratio gave a yellow precipitate that could be crystallised by slow
diffusion of pentane into a chloroform solution of the complex.
This complex 1a was analysed by NMR spectroscopy and X-
ray crystallography and was found to be identical to the complex
independently synthesised by Sun et al.22 by the similar reaction of
pzphos with Pd(PhCN)2Cl2. In this complex, a single pzphos ligand
chelates to the palladium via the N and P donor groups with the
remaining two coordination sites occupied by chloro ligands.
Upon increasing the ligand-to-metal ratio from 1 : 1 to 2 : 1,
we found that addition of Pd(COD)Cl2 to pzphos gave a different
product in solution. This was evidenced by the 31P NMR spectrum
of the reaction solution (taken after 1 h of stirring) which contained
a major peak at +31.8 ppm, corresponding to the new species, and
an additional smaller peak at +25.7 ppm corresponding to traces
Scheme 2 Reactions of pzphos with palladium(II).
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Fig. 2 Molecular structure of complex 1b (30% probability ellipsoids).
The p–p stacking interaction (dashed line) has centroid · · · centroid and
˚
mean interplanar separations of ca. 3.57 and 3.18 A respectively, the two
rings being inclined by ca. 7◦.
◦
˚
Table 3 Selected bond lengths (A) and angles ( ) for 1b
Pd–N(2)
Pd–N(4)
N(2)–N(1)
2.072(4)
2.074(4)
1.360(6)
Pd–P(1)
Pd–P(2)
N(4)–N(3)
2.2565(13)
2.2541(13)
1.344(6)
N(2)–Pd–P(1)
N(2)–Pd–P(2)
P(1)–Pd–P(2)
83.86(12)
168.33(12)
101.57(5)
N(4)–Pd–N(2)
P(1)–Pd–N(4)
N(4)–Pd–P(2)
92.77(16)
169.39(12)
83.63(12)
hydrogens are involved in fluxional behaviour as some resonances
are temperature-independent. From the COSY 1H NMR spectrum
recorded at 223 K it has been possible to assign the peaks in
these spectra; the non-fluxional hydrogens have been assigned as
those attached to the pyrazolyl and phenyl rings which comprise
the backbone of the ligand. The remaining hydrogens, which
all display fluxional behaviour, are those which are present in
the diphenylphosphine moiety of the ligand–metal complex. We
ascribe this low temperature phenomenon to hindered rotation
about the P–Ph bonds resulting in hydrogens occupying differ-
ent spatial positions and hence unique chemical environments
for periods of time comparable to the NMR timescale. These
hydrogens are therefore observed as separate resonances while
at higher temperatures, the energy barrier to rotation is overcome
hence these peaks begin to coalescence.
Exchange of the chloride counter-anions in 1b for perchlorate
anions was achieved by addition of two equivalents of silver
perchlorate, with the driving force of this reaction being the
precipitation of silver chloride (Scheme 2). The 31P NMR of the
yellow solid product 1c shows a single peak with chemical shift
+34.1 ppm (cf. 1b; d = +31.6 ppm) and the N–H in the 1H NMR
1
spectrum is found at 13.42 ppm (Table 2). The H NMR shows
broad peaks corresponding to the diphenylphosphine hydrogens.
These broad peaks split into sharp resonances at low temperatures
as seen previously for complex 1b (see ESI).†
Layering of hexane above a dichloromethane solution of 1c
yielded single crystals suitable for X-ray analysis. The solid state
structure of 1c shows the palladium to have a distorted square
planar geometry with the two pzphos ligands chelating in a cis
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Fig. 3 Variable temperature (283 K to 223 K) 1H NMR spectra of complex 1b.
fashion (Fig. 4, Table 4). As seen for complex 1b, a p–p stacking
interaction between pendant phenyl rings on adjacent ligands is
observed (dashed line in Fig. 4), and may be responsible for the
twisting of the two {Pd,P,N} coordination planes with respect to
each other (by ca. 14◦). Within each ligand the linked C6 and C3N2
rings are distinctly twisted with respect to each other, the torsion
angles for the C–C bond between them being ca. 25 and 26◦ for
the N(1) and N(31) ligands respectively. The N–H hydrogen atoms
are linked to the perchlorate anions by N–H · · · O hydrogen bonds
(see ESI).†
Reactions of N,P(O) ligand, pzphos(O). The N,P(O) ligand
pzphos(O) displays different coordination behaviour since the
‘soft’ phosphine group is now replaced by a ‘hard’ phosphine
oxide donor that has a lower affinity towards palladium(II).
Formation of a chelate ring is now entropically disfavoured due
to the increased size of the potential chelate ring, now seven-
membered as opposed to the six-membered complexes of pzphos
described above. The reactions performed between pzphos(O) and
Pd(COD)Cl2 are outlined in Scheme 3.
The reactivity of pzphos(O) was first examined by addition of
a dichloromethane solution of Pd(COD)Cl2 to two equivalents of
pzphos(O). The 31P{H} NMR spectrum of the reaction solution
was recorded after stirring at room temperature for 1 h, with the
chemical shift of the predominant species occurring at +33.0 ppm
(cf. free ligand; d = 35.8 ppm). This indicated that the oxide
group was not coordinated to the palladium as a significant
downfield shift is expected to occur on coordination as a result
of the inductive deshielding effect upon the phosphorus atom.
The product 2b was isolated by addition of hexane to the reaction
solution and the 31P{H} and 1H NMR spectra were recorded of the
resultant yellow solid (Table 2). The downfield position of the N–
H in the 1H NMR spectrum (13.51 ppm) suggests coordination of
the pyrazolyl ring has occurred, in a similar fashion to the pzphos
complexes described above. As such, it was hypothesised that a 2 : 1
ligand-to-metal complex had formed with the two ligands binding
through their pyrazole nitrogens only and with the remaining
Fig. 4 Molecular structure of the cation in 1c (30% probability ellipsoids).
The p–p stacking interaction (dashed line) has centroid · · · centroid and
˚
mean interplanar separations of ca. 3.89 and 3.25 A respectively, the two
rings being inclined by ca. 2◦.
◦
˚
Table 4 Selected bond lengths (A) and angles ( ) for 1c
Pd–N(1)
Pd–N(31)
N(1)–N(2)
2.0738(17)
2.0763(17)
1.357(3)
Pd–P(12)
Pd–P(42)
N(31)–N(32)
2.2614(6)
2.2545(6)
1.352(2)
N(1)–Pd–P(12)
N(1)–Pd–P(42)
P(12)–Pd–P(42)
85.68(5)
170.37(5)
99.39(2)
N(1)–Pd–N(31)
P(12)–Pd–N(31)
N(31)–Pd–P(42)
91.73(7)
169.37(5)
84.71(5)
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Scheme 3 Reactions of pzphos(O) with palladium(II).
coordination sites around the palladium centre being occupied
by two chloro ligands. This stoichiometry was supported by FAB
(+ve) mass spectral data showing a palladium isotope pattern at
869 m/z corresponding to [Pd{pzphos(O)}2Cl2]+. To confirm this
structure, crystals were grown by slow diffusion of pentane into a
chloroform solution of the 2b and were subsequently analysed by
X-ray crystallography.
The solid state structure of 2b shows the palladium to have a
square planar geometry with the two pzphos ligands bound in a
monodentate fashion via Pd–N bonds. In contrast to the pzphos
complex 1b, the two ligands are coordinated in a trans fashion
with two chloro ligands occupying the remaining two sites around
the palladium (Fig. 5, Table 5). Within each ligand the linked
C6 and C3N2 rings are distinctly twisted with respect to each
other, the torsion angle for the C–C bond between them being
ca. 39◦. This distortion may be caused by the molecule adopting a
conformation that facilitates intramolecular N–H · · · O hydrogen
bonding between the N(1)–H donor and the O(1) acceptor.
disfavours chelation, in contrast to the behaviour of the pzphos
ligand. Since the ligands do not chelate, the trans form is observed
as steric interactions between the two ligands are minimised.
This structure further differs from the uncoordinated pzphos(O)
ligand and the pzphos complexes 1a–1c in that the N–H hydrogen
has ‘swapped’ the nitrogen atom to which it is bonded, and
consequently, coordination to palladium now occurs through the
nitrogen in the 1ꢀ-position (Fig. 6).
Fig. 6 Variable bonding modes of pzphos (left) and pzphos(O) (right).
This coordination mode in 2b is presumed to result from the
monodentate behaviour of the pzphos(O) ligand which permits
the fluxional N–H hydrogen to reside on the nitrogen atom
that produces the most energetically favourable configuration in
the resultant complex, i.e. where steric interactions between the
ligand moieties and the metal are minimised. This is achieved
by coordination at the 1ꢀ-position, thus leaving the bulky triph-
enylphosphine oxide moiety on the ‘other side’ of the pyrazolyl
ring, pointing away from the metal. This bonding mode has
previously been observed for a palladium(II) complex of a 3-
substituted pyrazole ligand.38 Further stabilisation for this form
is also achieved by the formation of intramolecular N–H · · · O
hydrogen bonds.
The lack of formation of a seven-membered chelate has
been observed previously with another N,P(O) ligand towards
palladium(II).36 Although formation of a seven-membered chelate
has been observed in the palladium(II) complexes of N,N39 and
N,P ligands,40 it is disfavoured in the case of oxygen donors due
to the lower affinity of these donor groups towards palladium.
In contrast, chelation of six-membered N,P(O) ligands around
palladium has been observed previously,31 although in one case it
required an excess of palladium over ligand and heating at reflux
for several hours.35 In our attempts to synthesise the mono chelate
structure 2a, we found that even when using forcing conditions
(extended reaction times, reflux, excess of palladium), the bis
Fig. 5 Molecular structure of 2b (30% probability ellipsoids). The
intramolecular N–H · · · O hydrogen bonds (dashed lines) have N · · · O,
◦
˚
H · · · O (A), and N–H · · · O ( ) of 2.69, 1.92 and 144 respectively.
The combination of the hard oxide donor on pzphos(O) and the
increased size of the potential chelate ring (now seven-membered)
◦
˚
Table 5 Selected bond lengths (A) and angles ( ) for 2b
Pd–N(2)
N(1)–N(2)
1.999(2)
1.348(3)
Pd–Cl(1)
P(1)–O(1)
2.2995(7)
1.496(2)
N(2)–Pd–Cl(1)
N(2ꢀ)–Pd–Cl(1)
89.19(7)
90.81(7)
N(2)–Pd–N(2ꢀ)
Cl(1)ꢀ–Pd–Cl(1)
180
180
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adduct 2b would form preferentially with no traces of a chelated
species being observed by 31P NMR spectroscopy.
Conclusions
Relatively simple ligand tuning has been employed to demonstrate
the variable coordination behaviour of the N,P ligand pzphos and
the related N,P(O) ligand, pzphos(O) towards palladium(II). This
tuning effect has demonstrated the potential hemilability of these
species, while pzphos favoured chelation in all cases, pzphos(O)
acted as a monodentate N-donor that could be forced into a
bidentate chelating mode by abstraction of the chloro ligands. To
our knowledge, this is the first structurally characterised example
of an N,P(O) hybrid ligand chelating at a palladium(II) centre.41
Investigations are currently underway in our laboratory into the
catalytic properties of these complexes as well as the further
functionalisation of the ligands via substitution at the pyrazolyl
nitrogen. We have found the coordination chemistry of the N,P(S)
ligand pzphos(S) to be less straightforward, and the results of these
investigations will be presented in due course.
It was, however, anticipated that by adding two equivalents of
silver perchlorate to a solution of 2b, coordination of the oxide
group could be encouraged by removal of the chloro ligands.
This reaction was performed accordingly and after stirring the
reaction solution at room temperature for 30 min, the 31P{H}
NMR spectrum revealed a mixture in which the predominant
species occurred at 52.9 ppm, with smaller peaks present at
35.3 and 33.3 ppm. This species was subsequently isolated as a
yellow powder by addition of hexane to the reaction mixture and
recrystallisation from hot MeOH. The 31P{H} NMR spectrum
showed a singlet at 53.8 ppm enabling us to tentatively ascribe
this complex as the bis-chelated species 2c as its 31P{H} chemical
shift closely resembled that of a similarly coordinated N,P(O)
species.31 In order to confirm the structure of 2c, X-ray analysis
was performed on a single crystal grown by slow diffusion of
pentane into a chloroform solution of the complex. Indeed, the
solid state structure shows the palladium to have a square planar
geometry with two pzphos(O) ligands chelating in a trans fashion
(Fig. 7, Table 6). Each of the seven membered C3NP(O)Pd chelate
rings has a distorted boat conformation. As a consequence of
chelation, the linked C6 and C3N2 rings within each ligand are
distinctly twisted with respect to each other, the torsion angle for
the C–C bond between them being ca. 50◦.
Experimental
All reagents, unless stated otherwise were purchased from com-
mercial suppliers without further purification. Elemental analyses
were performed by Stephen Boyer at the London Metropolitan
1
University. H, 13C{H} and 31P{H} NMR spectra were recorded
on Bruker Av-400, DRX-400 or Av-500 spectrometers. Chemical
shifts are reported in ppm using the residual proton impurities in
the solvents. Pseudo-triplets which occur as a result of identical J-
value coupling to two chemically inequivalent protons are assigned
as dd and are recognised by the inclusion of only one J-value. Mass
spectra were recorded on a Micromass Autospec Q spectrometer
by Mr J. Barton and Mr G. Tucker of the Department of
Chemistry, Imperial College, London. Infrared absorption spectra
were collected either as KBr discs or Nujol mulls using a Perkin
Elmer RX FT-IR spectrometer. Ligand precursor A was prepared
as reported previously.37
Ligand syntheses
1-(2ꢀ-Diphenylphosphino)3-dimethylamino-2-propene-1-one B.
Using a variation of a literature procedure,42 2ꢀ-(diphenyl-
phosphino)acetophenone (12.35 g, 40.6 mmol) and N,N-
dimethylformamide dimethylacetal (25 ml, excess) were heated
at 120 ◦C for 12 h over which time the yellow solution turned
deep brown. After cooling to room temperature, the excess N,N-
dimethylformamide dimethyl acetal was removed in vacuo to leave
a dark orange solid which was broken up and stirred in pentane
(100 ml) and filtered to produce an orange solid which was dried
in vacuo (13.48 g, 92%). Analysis was carried out on a sample
Fig. 7 Molecular structure of the cation in 2c (30% probability ellipsoids).
This structure reveals that the N–H hydrogen of the pzphos(O)
ligand has reverted back to the 1-position of the pyrazole ring
(Fig. 7). This makes available the nitrogen in the 2-position for
coordination to the metal and thus allows formation of the seven-
membered chelate. Tautomerisation of this kind is required for
chelation as when the ligand coordinates via the nitrogen in the
1ꢀ-position (as in 2b), the rigidity of the ligand prevents the oxide
group from approaching the palladium centre.
recrystallised from CH2Cl2/hexane. H NMR (+25 ◦C, CDCl3,
1
4
400 MHz), d = 7.64 [dd (3JHH = 6.9 Hz, JHH = 3.8 Hz), 1H,
3
4
3-Ph], 7.38 [ddd (3JHH = 7.7 Hz, JHH = 7.6 Hz, JHH = 1.2 Hz),
1H, 4-Ph], 7.27–7.35 [m, 12H, 5-Ph + o,m,p-Ph + CHNMe2], 7.06
◦
˚
Table 6 Selected bond lengths (A) and angles ( ) for 2c
[dd (3JHH = 7.3 Hz, JHH = 3.5 Hz), 1H, 6-Ph], 5.43 [d (3JHH
=
4
12.6 Hz), 1H, COCH], 2.96 [s, 3H, NCH3], 2.71 [s, 3H, NCH3].
31P{H} NMR (+25 ◦C, CDCl3, 162.05 MHz), d = −10.3. IR (KBr
disc), m/cm−11637(CO). MS(EI), m/z = 359 [M]+. Anal. (%); Calc.
(found) for C23H22NOP: C, 76.9 (76.8); H, 6.17 (6.03); N, 3.90
(3.83).
Pd–N(2)
N(1)–N(2)
2.008(4)
1.339(7)
Pd–O(1)
P(1)–O(1)
2.009(3)
1.526(4)
N(2)–Pd–O(1)
N(2)–Pd–N(2ꢀ)
92.89(16)
180
N(2ꢀ)–Pd–O(1)
O(1)–Pd–O(1ꢀ)
87.11(16)
180
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[2-(3-Pyrazolyl)phenyl]diphenylphosphine pzphos. Hydrazine
hydrate (10 ml, excess) was added dropwise to a solution of B
(12.00 g, 33.4 mmol) in hot ethanol (150 ml) and stirred under
reflux for 3 h. Removal of the solvent gave an orange solid
to which was added water (50 ml) and the solid was collected
by filtration. The aqueous washings were extracted into CHCl3
(200 ml) and the organic fraction was separated and used to
dissolve the solid. Drying (MgSO4), filtration and concentration
followed by recrystallisation from CHCl3/hexane gave a colourless
solid. Yield = 9.33 g (91%). 1H NMR (+25 ◦C, CDCl3, 500 MHz),
pz], 139.3 [s, 5-pz], 133.4 [d (2JCP = 11.2 Hz), 6-Ph], 133.2 [s, i-Ph],
132.6 [s, 1-Ph], 132.4 [d (3JCP = 9.8 Hz), 3-Ph], 132.0 [d (2JCP
=
10.7 Hz), o-Ph], 131.9 [s, 4-Ph], 131.7 [s, p-Ph], 130.7 [s, 2-Ph],
128.4 [d (3JCP = 12.7 Hz), m-Ph], 128.1 [d (3JCP = 12.3 Hz), 5-Ph],
105.9 [s, 4-pz]. IR (KBr disc), m/cm−1 3220 cm−1(NH). MS(EI),
m/z = 360 [M]+. Anal. (%): Calc. (found) for C21H17N2PS: C, 70.0
(70.1); H, 4.75 (4.83); N, 7.77 (7.66).
Complex syntheses
4
d = 10.57 [s, 1H, NH], 7.60 [dd (3JHH = 7.1 Hz, JHH = 3.7 Hz),
Synthesis of palladium perchlorate complexes. Caution! Al-
though we did not observe any explosive behaviour with the
perchlorate compounds described below, all metal perchlorates
must be treated as potentially explosive species and appropriate
safety measures must be taken. See for example reference.43
1H, 3-Ph], 7.48 [d (3JHH = 2.0 Hz), 1H, 5-pz], 7.42 [ddd (3JHH
=
3
4
7.7 Hz, JHH = 7.6 Hz, JHH = 1.3 Hz), 1H, 4-Ph], 7.33–7.37 [m,
6H, m,p-Ph], 7.26–7.31 [m, 4H, o-Ph], 7.03 [ddd (3JHP = 7.8 Hz,
4
3JHH = 4.2 Hz, JHH = 1.1 Hz), 1H, 6-Ph], 6.34 [m, 1H, 4-pz].
31P{H} NMR (+25 ◦C, CDCl3, 162.05 MHz), d = −11.4. 13C{H}
[Pd(pzphos)Cl2] 1a. Pzphos (0.33 g, 1.0 mmol) in CH2Cl2
(10 ml) was added dropwise to a stirred solution of Pd(COD)Cl2
(0.29 g, 1.0 mmol) in CH2Cl2 (20 ml). The orange solution was
stirred for 10 min causing a yellow solid to precipitate. This was
collected by filtration, stirred with hexane (20 ml), filtered and
dried in vacuo to give a bright yellow solid. Yield = 0.46 g (91%).
1H NMR (+25 ◦C, CDCl3, 500 MHz), d = 13.14 [s, 1H, NH] 7.90
[ddd (3JHH = 7.9 Hz, 4JHH = 4.4 Hz, 5JHH = 1.0 Hz), 1H, 3-Ph], 7.70
◦
NMR (+25 C, CDCl3, 500 MHz), d = 146.9 [s, 3-pz], 140.0 [s,
5-pz], 137.1 [d, (1JCP = 10.6 Hz), i-Ph], 136.1 [d (2JCP = 17.3 Hz),
2-Ph], 134.3 [s, 6-Ph], 133.9 [d, (2JCP = 19.7 Hz), o-Ph], 131.8
[s, 1-Ph], 130.0 [d (3JCP = 5.5 Hz), 3-Ph], 128.9 [s, 5-Ph], 128.8
[s, 4-Ph], 128.6 [d (3JCP = 7.6 Hz), m-Ph], 128.3 [s, p-Ph], 106.5
[s, 4-pz]. IR (KBr disc), m/cm−1 3187 cm−1(NH). MS(EI), m/z =
328 [M]+. Anal. (%): Calc. (found) for C21H17N2P: C, 76.8 (77.0);
H, 5.22 (5.00); N, 8.53 (8.35).
[dddd (3JHH = 7.9 Hz, 4JHH = 1.5 Hz), 1H, 4-Ph], 7.65 [dd (3JHH
=
2.8 Hz, 3JHH = 1.8 Hz), 1H, 5-pz], 7.61 [m, 4H, o-Ph], 7.54 [m, 2H,
[2-(3-Pyrazolyl)phenyl]diphenylphosphine oxide pzphos(O).
A
p-Ph], 7.44 [m, 4H, m-Ph], 7.42 [m, 1H, 5-Ph], 7.05 [ddd (3JHP
=
solution of pzphos (0.30 g, 0.91 mmol) and H2O2 (27.5% in H2O,
135 lL, 1.10 mmol) in CH2Cl2 (30 ml) was stirred for 1 h after
which time H2O (40 ml) was added and the organic layer separated,
dried (MgSO4) and concentrated to dryness to give a colourless
solid. Yield = 0.20 g (63%). 1H NMR (+25 ◦C, CDCl3, 500 MHz),
d = 13.24 [s, 1H, NH], 7.78 [ddd (3JHH = 7.8 Hz, 4JHH = 4.2 Hz,
5JHH = 1.0 Hz), 1H, 3-Ph], 7.61 [m, 5H, o-Ph + 4-Ph], 7.50 [m, 2H,
p-Ph], 7.41 [m, 4H, m-Ph], 7.32 [d (3JHH = 1.8 Hz), 1H, 5-pz], 7.30
10.8 Hz, 3JHH = 7.9 Hz, 4JHH = 1.2 Hz), 1H, 6-Ph], 6.86 [m, 1H,
◦
4-pz]. 31P{H} NMR (+25 C, CDCl3, 162.05 MHz): d = +25.6.
IR (KBr disc), m/cm−1 3230 cm−1(NH). MS(EI), m/z = 471 [M–
Cl]+. Anal. (%): Calc. (found) for C21H17Cl2N2PPd: C, 49.9 (49.8);
H, 3.39 (3.33); N, 5.54 (5.42). Yellow needles suitable for X-ray
studies were grown by slow diffusion of pentane in a chloroform
solution of the complex.
[m, 1H, 5-Ph], 7.14 [ddd (3JHP = 14.6 Hz, 3JHH = 7.8 Hz, 4JHH
=
[Pd(pzphos)2]Cl2 1b. Pd(COD)Cl2 (65 mg, 0.23 mmol) in
CH2Cl2 (15 ml) was added dropwise to a solution of pzphos
(150 mg, 0.46 mmol) in CH2Cl2 (15 ml). After stirring overnight,
the volume of the solvent was reduced to 10 ml and hexane (10 ml)
was added causing a yellow solid to precipitate. This was collected
by filtration and dried in vacuo. Yield = 150 mg (78%). 1H NMR
1.1 Hz), 1H, 6-Ph], 6.25 [s, 1H, 4-pz]; 31P{H} NMR (+25 ◦C,
CDCl3, 162.05 MHz), d = +35.8. 13C{H} NMR (+25 ◦C, CDCl3,
500 MHz), d = 141.8 [s, 3-pz], 139.7 [s, 5-pz], 134.9 [d (2JCP
=
8.0 Hz), 2-Ph], 134.1 [d (2JCP = 12.1 Hz), 6-Ph], 132.7 [s, 4-Ph],
132.2 [s, p-Ph], 131.7 [d (2JCP = 9.8 Hz), o-Ph], 131.3 [d (3JCP
=
=
9.6 Hz), 3-Ph], 130.9 [s, i-Ph], 130.0 [s, 1-Ph], 128.6 [d (3JCP
(−50 ◦C, CDCl3, 400 MHz), d = 14.54 [s, 1H, NH], 8.05 [dd
12.4 Hz), m-Ph], 127.3 [d (3JCP = 12.4 Hz), 5-Ph], 105.3 [s, 4-pz].
IR (Nujol), m/cm−1 3186 cm−1(NH); MS(EI), m/z = 344 [M]+.
Anal. (%): Calc. (found) for C21H17N2OP: C, 73.3 (73.1); H, 4.98
(5.01); N, 8.14 (8.02). Crystals suitable for X-ray studies were
grown by slow diffusion of pentane in a chloroform solution of
this complex.
4
(3JHH = 12.8 Hz, JHH = 7.9 Hz), 1H, Ph], 7.96 [dd (3JHH
=
4
7.5 Hz, JHH = 3.4 Hz), 1H, 3-Ph], 7.96 [br s, 2H, Ph], 7.76 [s,
1H, 5-pz], 7.64 [dd, (3JHH = 7.6 Hz), 1H, 4-Ph], 7.42 [m, 3H, Ph],
7.30 [dd (3JHH = 7.7 Hz), 1H, 5-Ph], 7.22 [m, 1H, Ph], 7.06 [dd
(3JHH = 7.4 Hz), 1H, Ph], 6.79 [m, 2H, 6-Ph + Ph], 6.53 [s, 1H, 4-
pz], 5.94 [dd (3JHH = 9.0 Hz), 1H, Ph]. 31P{H} NMR (+25 ◦C,
CDCl3, 162.05 MHz), d = +31.6 (s). IR (KBr disc), m/cm−1
3290 cm−1(NH). MS(ESI), m/z = 761 [Pd(pzphos)2]+. Anal. (%):
Calc. (found) for C42H34Cl2N4P2Pd.2CH2Cl2: C, 52.6 (52.6); H,
3.82 (3.62); N, 5.58 (5.37). Yellow crystals suitable for X-ray studies
were grown by slow diffusion of pentane in a chloroform solution
of this complex.
[2-(3-Pyrazolyl)phenyl]diphenylphosphine sulfide pzphos(S).
A
solution of pzphos (0.30 g, 0.91 mmol) and S8 (44 mg, 0.17 mmol)
in CH2Cl2 (15 ml) was stirred for 1 h and then reduced in volume
to ∼5 ml and hexane added dropwise. The resultant off-white
precipitate was collected by filtration, washed with hexane and
dried in vacuo. Yield = 0.25 g (76%). 1H NMR (+25 ◦C, CDCl3,
500 MHz), d = 12.54 [s, 1H, NH], 7.82 [dd (3JHH = 13.6 Hz, 4JHH
=
Alternative synthesis of 1b. A solution of pzphos (32 mg,
0.10 mmol) in CH2Cl2 (10 ml) was added to a solution of 1a
(50 mg, 0.10 mmol) in CH2Cl2 (10 ml) and stirred overnight. The
solution was concentrated to half the volume and hexane (10 ml)
added dropwise causing a yellow solid to precipitate. The solid was
collected by filtration and dried in vacuo to give 1b as evidenced by
7.1 Hz), 4H, o-Ph], 7.57 [m, 2H, 3-Ph + 4-Ph], 7.44 [m, 2H, p-Ph],
7.37 [m, 4H, m-Ph], 7.37 [m, 1H, 5-Ph], 7.18 [dd (3JHP = 14.9 Hz,
3JHH = 8.0 Hz), 1H, 6-Ph], 7.17 [d (3JHH = 1.8 Hz), 1H, 5-pz], 6.08
◦
[s, 1H, 4-pz]. 31P{H} NMR (+25 C, CDCl3, 162.05 MHz), d =
+43.3. 13C{H} NMR (+25 ◦C, CDCl3, 500 MHz), d = 139.7 [s, 3-
2830 | Dalton Trans., 2007, 2823–2832
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identical 1H and 31P{H} NMR data for both compounds. Yield =
7.7 Hz), 1H, 4-Ph], 7.60–7.85 [m, 10 H, o,m,p-Ph], 7.35–7.50 [m,
2 H, 5,6-Ph], 7.31 [d (3JHH = 2.0 Hz), 1H, 5-pz], 6.02 [m, 1H,
55 mg (70%).
◦
4-pz]. 31P{H} NMR (+25 C, CDCl3, 162.05 MHz), d = +53.9
[Pd(pzphos)2][ClO4]2 1c. Pd(COD)Cl2 (87 mg, 0.30 mmol)
in CH2Cl2 (5 ml) was added dropwise to a solution of pzphos
(200 mg, 0.61 mmol) in CH2Cl2 (20 ml). After stirring for 1 h,
silver perchlorate (114 mg, 0.64 mmol) was added and the flask
stirred overnight in the dark during which time AgCl precipitated.
Filtration through a pad of Celite and concentration of the filtrate
to dryness gave a yellow powder which was stirred with hexane
(20 ml), filtered and recrystallised from hot methanol to give a
yellow solid. Yield = 180 mg (61%). 1H NMR (+25 ◦C, d6-acetone,
(s). IR (CH2Cl2 solution), m/cm−1 3686 cm−1(NH). MS(FAB +ve),
m/z = 900 [Pd{pzphos(O)}2ClO4]+. Anal. (%): Calc. (found) for
C42H34Cl2N4O10P2Pd: C, 50.8 (50.7); H, 3.45 (3.43); N, 5.64 (5.58).
Yellow crystals suitable for X-ray studies were grown by slow
diffusion of pentane in a chloroform solution of this complex.
CCDC reference numbers 640789–640793.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b704051b
400 MHz), d = 13.42 [s, 1H, NH], 8.19 [dd (3JHH = 2.8 Hz, 4JHH
0.7 Hz), 1H, 5-pz], 8.08 [ddd (3JHH = 7.8 Hz, 4JHH = 4.3 Hz, 5JHH
=
=
Acknowledgements
1.2 Hz), 1H, 3-Ph], 7.90 (br m, 2H, Ph), 7.85 [dd (3JHH = 7.8 Hz),
1H, 4-Ph], 7.40–7.70 [br m, 4H, Ph], 7.59 [dd (3JHH = 8.0 Hz),
1H, 5-Ph], 7.08–7.25 [br m, 2H, Ph], 7.05 [ddd (3JHP = 11.1 Hz,
3JHH = 7.9 Hz, 4JHH = 0.8 Hz), 1H, 6-Ph], 6.99 [dd (3JHH = 7.8 Hz,
We gratefully acknowledge financial support from GlaxoSmithK-
line and the Department of Chemistry, Imperial College, London.
◦
3JHH = 1.6 Hz), 1H, 4-pz]. 31P{H} NMR (+25 C, d6-acetone,
References
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for C42H34Cl2N4O8P2Pd: C, 52.4 (52.5); H, 3.56 (3.69); N, 5.82
(5.73). Single crystals for X-ray analysis were grown in an NMR
tube by layering hexane above a CH2Cl2 solution of the product.
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H, 3.96 (4.00); N, 6.47 (6.39). Yellow crystals suitable for X-ray
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solution of this complex.
3
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[Pd{pzphos(O)}2][ClO4]2 2c. AgClO4 (25 mg, excess) was
added to a suspension of 2b (34 mg, 0.04 mmol) in CH2Cl2 (5 ml).
After stirring overnight, half the solvent was evaporated under
a stream of N2 to give a yellow solution to which hexane was
added dropwise (5 ml) causing a yellow precipitate to form. This
was collected by filtration, washed with hexane (5 ml) and dried
1
in vacuo to give 2c as yellow powder. Yield = 24 mg (60%). H
◦
NMR (+25 C, CDCl3, 400 MHz), d = 12.35 [s, 1H, NH], 8.27
4
[dd (3JHH = 7.4 Hz, JHH = 4.4 Hz), 1H, 3-Ph], 8.12 [dd (3JHH
=
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2832 | Dalton Trans., 2007, 2823–2832
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©