Palladium(II) and platinum(II) derivatives with N,O ligands

Article abstract of DOI:10.1039/a805842c

The preparation of a series of Pd(II) and Pt(II) derivatives with three N,O hybrid ligands 1-(2-pyridyl)ethyldiphenylphosphine oxide, L¹; (2-pyridyl)methyldiphenylphosphine oxide, L² and 2-pyridyldiphenylphosphine oxide, L³ is described. Depending on the ligand to metal molar ratio, either 1:1 [LMCl₂] {[L¹PdCl₂] 1, [L²PdCl₂] 2, [L³PdCl₂] 3, [L¹PtCl₂] 4} or 2:1 [L₂MCl₂] {[L¹₂PdCl₂] 5, [L²₂PdCl₂] 6, [L¹₂PtCl₂] 7} adducts are obtained. In addition, an ionic species [L¹₂Pd][PF₆]₂ 8, has been isolated by treatment of complex 5 with AgPF₆. The structures in the solid state of compounds 5b·MeOH-H₂O, 6·MeOH and 4·CH₂Cl₂ have been solved by X-ray diffraction. The structures of trans-[L¹₂PdCl₂] 5b (one of the diastereomers of 5) and trans-[L²₂PdCl₂] 6 are rather different, the former being a chiral molecule (R,R enantiomer) with a C₂ axis perpendicular to the plane of the metal, the latter, 6, being a centrosymmetric molecule. In both the species the ligand is bonded through the nitrogen atom. In complex [L¹PtCl₂] 4, the ligand is coordinated through the N and O atoms to form a six-membered ring which adopts a boat-like conformation. The structure reveals a fairly short interaction between one hydrogen atom of the Me group in an axial position and the platinum atom, Pt ... H 2.69 A. Short contacts Pd ... H are also observed in the structures of compounds 5b and 6. Some aspects of the reactivity of the new species are discussed.

Full text of DOI:10.1039/a805842c

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Palladium(II) and platinum(II) derivatives with N,O ligands
Giovanni Minghetti,*a Sergio Stoccoro,a Maria Agostina Cinellu,a Antonio Zucca,a
Mario Manassero*b and Mirella Sansonib
a Dipartimento di Chimica, Università di Sassari, via Vienna 2, I-07100 Sassari, Italy.
E-mail: mingh@ssmain.uniss.it
b Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università di Milano,
Centro CNR, via Venezian 21, I-20133 Milano, Italy. E-mail: m.manassero@csmtbo.mi.cnr.it
Received 27th July 1998, Accepted 27th October 1998
The preparation of a series of Pd() and Pt() derivatives with three N,O hybrid ligands 1-(2-pyridyl)ethyldiphenyl-
phosphine oxide, L1; (2-pyridyl)methyldiphenylphosphine oxide, L2 and 2-pyridyldiphenylphosphine oxide, L3 is
described. Depending on the ligand to metal molar ratio, either 1:1 [LMCl2] {[L1PdCl2] 1, [L2PdCl2] 2, [L3PdCl2] 3,
[L1PtCl2] 4} or 2:1 [L2MCl2] {[L1 PdCl2] 5, [L2 PdCl2] 6, [L1 PtCl2] 7} adducts are obtained. In addition, an ionic
2
2
2
species [L1 Pd][PF6]2 8, has been isolated by treatment of complex 5 with AgPF6. The structures in the solid state
2
of compounds 5bؒMeOHؒH2O, 6ؒMeOH and 4ؒCH2Cl2 have been solved by X-ray diffraction. The structures of
trans-[L1 PdCl2] 5b (one of the diastereomers of 5) and trans-[L2 PdCl2] 6 are rather different, the former being
2
2
a chiral molecule (R,R enantiomer) with a C2 axis perpendicular to the plane of the metal, the latter, 6, being a
centrosymmetric molecule. In both the species the ligand is bonded through the nitrogen atom. In complex [L1PtCl2]
4, the ligand is coordinated through the N and O atoms to form a six-membered ring which adopts a boat-like
conformation. The structure reveals a fairly short interaction between one hydrogen atom of the Me group in an
axial position and the platinum atom, Pt ؒ ؒ ؒ H 2.69 Å. Short contacts Pd ؒ ؒ ؒ H are also observed in the structures
of compounds 5b and 6. Some aspects of the reactivity of the new species are discussed.
L2 and 2-pyridyldiphenylphosphine oxide, L3 have been pre-
Introduction
pared according to established methods (see Experimental
The coordination of ligands containing different donor atoms
section).13
to transition metal ions has been a topic of interest for many
years.1 In recent years much work has been devoted to so-called
H Me
Ph
P
H H
Ph
O
hemilabile hybrid ligands,2 i.e. ligands having one of their donor
sites with little affinity toward the acceptor, so as to be easily
removed from the coordination sphere of the metal. Complexes
with ligands able to produce a vacancy in the coordination of
the metal without complete dissociation should be potential
catalytic precursors in synthetic processes.3 In this context
the interaction of typical soft acceptors such as Rh(), Pd()
or Pt() with ligands having a P,N4 or a P,O5 set of donors
has been extensively studied. Among the P,N ligands, special
attention has been devoted to pyridylphosphines,6 whereas
much less investigated is the behaviour of the corresponding
pyridylphosphine oxides. At present, as far as we know, the
literature concerning the coordination of these ligands is
restricted to the synthesis and structural characterization of
[L3PtBr4]7 [L3 = Ph2P(O)py-2], to a kinetic study on the
dynamic behaviour of the Pd() complexes [L3Pd(C6F5)Br]
and [L3Pd(C6F5)2],8 and to a study on the coordination
behaviour of the same ligand towards Fe()9 and Co().10
In addition, a rhodium–L3 system has been investigated in
hydroformylation reactions.11 Here we present a study on the
reactivity of palladium() and platinum() ions with the
ligands Ph2P(O)CH(Me)py-2 (L1), Ph2P(O)CH2py-2 (L2) and
Ph2P(O)py-2 (L3), which have different chain lengths between
the donor atoms and different substituents at the carbon atom
of the chain.
Ph
Ph
Ph
Ph
P
P
O
O
N
N
N
L1
L2
L3
1:1 Adducts, [LMCl2]
The reaction of L1 and L2 with Na2[PdCl4] or K2[PtCl4] (molar
ratio ligand:metal = 1:1) in methanol solution affords the com-
plexes [L1PdCl2] 1, [L2PdCl2] 2 and [L1PtCl2] 4 in moderate to
good yields. The analytical data and the conductivity measure-
ments are consistent with neutral 1:1 adducts [LMCl2].
Ph
Ph
Cl
Cl
M=Pd, R = Me
M=Pd, R = H
M=Pt, R = Me
1
2
4
O
N
P
H
R
M
Spectroscopic IR and NMR evidences mainly support a
chelating behaviour of the ligands. In the IR spectra the P᎐O
᎐
stretching vibrations are strongly shifted to lower wavenumbers
with respect to the free ligand (see Table 2 and Experimental
A preliminary report of part of this work has been given.12
section) indicating a weakening of the P᎐O bond consistent
᎐
with coordination of the oxygen atom.14 The 31P{1H} NMR
spectra show at room temperature only one resonance, remark-
ably deshielded compared to that of the free ligand (Tables 1
and 2). On the other hand the deshielding of the H(6) proton
Results and discussion
The N,O ligands 1-(2-pyridyl)ethyldiphenylphosphine oxide
(racemic form), L1; (2-pyridyl)methyldiphenylphosphine oxide,
1
in the H NMR spectra and the shift of the pyridine ring
J. Chem. Soc., Dalton Trans., 1998, 4119–4126
4119

Table 1 1H and 31P NMR dataa
δ (1H)
Compound
Solvent
T/ЊC
ϩ20
CH3
CH2
CH
H(6)
Aromatics
δ (31P)
[L1PdCl2] 1
CDCl3
CDCl3
2.46 br
2.86 dd (7.1) [16.6]
1.72 dd (6.9) [17.2]
4.97 br
4.95 m (nr)[nr]
5.72 dq (6.9) [21.4]
9.04 d br
8.97 d
9.06 d
8.91 dd
8.85 dd br
7.22–7.98
7.09–8.29
49.7
50.3
51.1
45.4
Ϫ50
α (2)
β (1)
[L2PdCl2] 2
CD2Cl2
ϩ20
Ϫ85
4.55 d [11.9]
4.09 dd br HA (ca. 16) [nr]
5.12 dd br HB (ca. 16) [ca. 16]
7.06–7.85
7.2–8.0
[L3PdCl2] 3
[L1PtCl2] 4
CD2Cl2
CDCl3
CD2Cl2
ϩ20
ϩ20
Ϫ70
9.34 dd
9.25 dd
7.46–8.09
7.20–8.10
6.99–8.22
60.5
52.9
53.8
55.1
34.3
33.6
34.7
34.0
33.5
33.6
29.3 br
29.7 br
29.1
30.2
29.6
33.6
33.1, 34.1, 36.5
2.11 br
5.11 br
α (1)
β (6)
α (3)
β (2)
α (2)
β (1)
2.56 dd (7.4) [17.0]
1.53 dd (7.3) [17.4]
1.91 dd (7.2) [15.0]
2.11 dd (7.2) [15.0]
1.88 dd (7.2) [15.0]
2.10 dd (7.2) [15.0]
2.0 br
4.10 m br (nr)
5.63 m (7.3) [21.3]
5.40 br
9.06b
b
[L1 PdCl2] 5a
CDCl3
ϩ20
Ϫ20
9.18 br
8.80 br
9.18 dd
8.80 dd
8.8–9.2 br
9.05 dd
8.75–9.10 br
6.97–8.20
6.95–8.16
2
5.80 br
5.36 m (nr)
5.75 m (nr)
5.6 br
ϩ55
ϩ20
ϩ20
7.0–8.2 br
7.16–8.04
7.15–8.10
[L1 PdCl2] 5b
CDCl3
CDCl3
1.74 dd (7.3) [14.7]
5.35 m (7.3) [nr]
[L22PdCl2] 6
5.2 br
2
CDCl3
CDCl3
ϩ55
Ϫ50
5.12 d [13.1]
5.14 d [13.0]
4.96 d [13.2]
8.90 br
8.80
9.00
7.15–8.15
7.20–8.10
α (3)
β (2)
α (3)
[L1 PtCl2] 7
CDCl3
ϩ20
0.46 dd (7.2) [14.5]
0.75 dd (7.3) [14.6]
ca. 1.70d
5.54 (7.2) [7.2]
9.13
6.86–8.18
2
β, γ, δ
5.65 mb
8.91,b 8.93b
9.26
(1:1:1)c
1.90 dd (7.1) [15.1]
2.21 dd (7.2) [17.0]
2.16e
[L1 Pd][PF6]2 8
CD2Cl2
ϩ20
α (5)
β (2)
5.16 (7.2)b
8.72 dd
8.75e
7.40–7.98
57.0 Ϫ143.6 ([PF6]Ϫ)
57.2
2
b
a Chemical shifts in ppm relative to TMS (1H) and external 10% H3PO4 (31P), coupling constants are given in Hz, JHH in parentheses, JHP in square brackets, br = broad, nr = not resolved. b Signals partially overlapping.
c The signals of isomers β, γ, δ, in approximate ratio 1:1:1, are not specifically assigned. d Obscured by the signal of dissolved H2O. e Partially obscured by the signal of isomer α.

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Table 2 1H, 31P NMR and selected IR data for the ligands
NMRa
δ (1H)
IRb
Compound
CH3
CH2
CH
H(6)
Other aromatics
7.00–7.91
δ (31P)
33.9
P᎐O
CN (py)
1583 m
1581 m
1590 m
᎐
L1
L2
L3
1.58 dd
(7.5) [15.8]
3.98 m
(7.5) [7.5]
8.32 d
8.40 d
8.80 d
1183 s
1183 s
1190 s
3.95 d [30]
7.05–7.90
30.4
7.35–8.34
22.0
a Solvent CDCl3, chemical shifts in ppm relative to internal TMS (1H) or external H3PO4 (31P), coupling constants are given in Hz, JHH in parentheses,
JPH in square brackets. b IR spectra in Nujol mulls, νmax in cmϪ1
.
vibrations in the IR spectra (νmax/cmϪ1: 1599, 1; 1596, 2; 1600, 4;
Table 3 Selected bond distances (Å) and angles (Њ) with e.s.d.s in par-
entheses for 4ؒCH2Cl2 (a), 5bؒMeOHؒH2O (b) and 6ؒMeOH (c)
1583, L1; 1581, L2) account for the coordination of the nitrogen
1
atom.15 At room temperature the H NMR spectra indicate a
(a)
non-rigid situation. Compounds 1 and 4 show broad signals
Pt–Cl(1)
Pt–O
P–O
2.267(2)
2.037(5)
1.527(5)
Pt–Cl(2)
Pt–N(1)
Pt ؒ ؒ ؒ H(83)
2.289(2)
2.030(6)
2.69
for the resonances of the Me and CH protons and complex 2
2
one sharp signal for the CH2 protons [δ 4.55 d, J(P–H) = 11.9
1
Hz]. At low temperature (Ϫ85 ЊC) the H NMR spectrum of
complex 2 shows the non-equivalence of the CH2 protons
with a complex pattern consistent with an ABX system (X = 31P).
This behaviour is in agreement with a boat conformation of
the six-membered ring.
At room temperature on the NMR time-scale the rapid
upsetting of the boat results in a single set of signals, whereas at
low temperature the equilibrium is frozen. In the case of the
complexes 1 and 4 two conformers are expected, with a methyl
(α) or a hydrogen (β) pointing toward the metal.
Cl(1)–Pt–Cl(2)
Cl(1)–Pt–N(1)
Cl(2)–Pt–N(1)
91.5(1)
92.2(2)
175.8(2)
Cl(1)–Pt–O
Cl(2)–Pt–O
O–Pt–N(1)
178.6(2)
89.5(2)
86.8(2)
(b)
Pd–Cl
P–O
2.299(1)
1.492(3)
Pd–N(1)
Pd ؒ ؒ ؒ H(7)
2.036(3)
2.66
Cl–Pd–ClЈ
Cl–Pd–N(1Ј)
178.5(1)
89.7(1)
Cl–Pd–N(1)
N(1)–Pd–N(1Ј)
90.3(1)
176.5(1)
H
Me
P
(c)
Me
H
Ph
Ph
Ph
Ph
Cl
Cl
Pd–Cl
P–O
2.298(1)
1.485(2)
Pd–N(1)
Pd ؒ ؒ ؒ H(71)
2.035(2)
2.61
Cl
Cl
P
M
M
O
O
N
N
Cl–Pd–N(1)
89.9(1)
Cl–Pd–N(1Ј)
90.1(1)
α
β
In one of the conformers the signal of the CH3 protons is
strongly deshielded with respect to the free ligand (∆δ = 1.28
ppm) whereas in the other conformer it is the CH proton that
undergoes a remarkable downfield shift (∆δ = 1.74 ppm). Like-
wise in complex 2 one of the two protons (δ 5.12) is strongly
deshielded with respect to the the other (δ = 4.09) and to the free
ligand (δ 3.95).
Note that in the platinum complex 4, no coupling is
observed between 195Pt and 31P, as previously observed for other
P–O–Pt systems in six-membered rings.16
Assignment of the resonance to conformers α and β, respec-
tively, is in agreement with the data reported in relation to weak
interactions between d8 metal ions and C–H protons in axial
position.17
A weak interaction between a hydrogen atom and the metal
has been confirmed, in the solid state, by the resolution of the
structure of complex 4.
Fig. 1 An ORTEP view of compound 4. Ellipsoids are drawn at the
30% probability level.
The crystal structure of 4ؒCH2Cl2 consists of the packing of
[L1PtCl2] and dichloromethane molecules in the molar ratio
1:1, with no unusual van der Waals contacts. Principal bond
parameters are reported in Table 3. An ORTEP18 view of the
[L1PtCl2] molecule is shown in Fig. 1. The Pt atom is in a square
planar coordination with a slight pyramidal distortion, max-
imum distances from the best plane being ϩ0.022(1) Å for
Pt and Ϫ0.016(6) Å for N(1). The Pt–Cl(1) and Pt–Cl(2) bond
lengths are 2.267(2) and 2.289(2) Å, respectively, suggesting
that the trans influence of the oxygen atom is slightly lower than
that of nitrogen. They can be compared with the Pd–Cl(1)
distance, 2.277(1) Å, found in [PdLCl2] 9, where L is a chiral
pyridylphosphine6i and the Cl atom is trans to a pyridinic
nitrogen atom. The Pt–N(1), 2.030(6) Å, and Pt–O, 2.037(5) Å,
are very similar and can be compared with the Pd–N bond
length, 2.039(3) Å, found in 9. The P–O distance, 1.527(5) Å, is
longer than that observed when the oxygen atom is not co-
ordinated [compare for instance 1.492(3) Å in 5bؒMeOHؒH2O
and 1.485(2) Å in 6ؒMeOH]. The six-membered metallacycle is
in a boat conformation, with the methyl group in an axial pos-
ition (see Fig. 1). The methylic hydrogen atom H(83) lies at 2.69
Å from the Pt atom, a distance shorter than the sum of the
corresponding van der Waals radii (1.20–1.45 Å for hydrogen
and 1.70–1.80 Å for platinum).19 In the boat-like six-membered
J. Chem. Soc., Dalton Trans., 1998, 4119–4126
4121

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ring the best plane of atoms P, O, N(1) and C(2) forms dihedral
angles of 43.1(3) and 44.8(4)Њ with the Pt–O–N(1) and P–C(7)–
C(2) planes, respectively.
The third ligand, L3, was used with the aim of comparing
the stability of a five-membered N,O ring with respect to a
six-membered one. By reaction with Na2[PdCl4] or trans-
[(PhCN)2PdCl2], the species [L3PdCl2] 3 was obtained in good
yield. Spectroscopic data are in agreement with a chelating
behaviour of the ligand.
2:1 Adducts, [L2MCl2]
When the reaction between Na2[PdCl4] and L1 or L2 is carried
out with a 2:1 L:Pd ratio, adding the solution of Na2[PdCl4] to
the solution of the ligand (i.e. in the presence of an excess of
ligand), complexes [L1 PdCl2] 5 and [L2 PdCl2] 6 are obtained.
2
2
In the IR spectra no significant shift of the P᎐O stretching
᎐
vibration is observed, suggesting that the oxygen atom is
not involved in the coordination. The absorptions at 348 (5)
and 345 (6) cmϪ1 are indicative of a trans disposition of the
chlorine atoms, as usually observed for [L2PdCl2] derivatives20
(L = pyridine or substituted pyridines).
Fig. 2 An ORTEP view of compound 5b. Primed atoms are related to
unprimed ones by the symmetry operation Ϫx, y Ϫ x, 1/3 Ϫ z. Ellips-
oids as in Fig. 1.
lie on the same side, and the same is true for Pt and H(83)
in [L1PtCl2] 4, with respect to the best plane of atoms Cl(1),
Cl(2), O and N(1), thus suggesting that these metal–hydrogen
interactions are attractive in nature and are responsible for the
pyramidal distortion of the metal coordination.
1
The H NMR spectrum of 5 at room temperature is rather
complex showing three sets of signals. Taking into account that
L1 has a stereogenic centre (asymmetric carbon atom), three
stereoisomers are possible, two isomers are chiral and together
constitute a pair of enantiomers (R,R; S,S). The third isomer is
diastereomeric to the two above mentioned isomers and is a
meso compound.21 In addition, other signals could originate
from hindered rotation of the ligand around the Pd–N bonds,
as previously observed for Pd() derivatives with 2-benzyl-
pyridines.22
The fraction less soluble in MeOH, 5a, has a more complex,
temperature dependent 1H NMR spectrum. At room tem-
perature two sets of broad signals are observed that turn into a
single set at ϩ55 ЊC (CDCl3) and sharpen at Ϫ50 ЊC. Assuming
that 5a is the achiral diastereomer (meso compound) the
presence of two sets of signals can be due to the presence
of conformational isomers (rotamers). A similar behaviour
has been observed previously for trans-[(2-PhCH2py)2PdCl2]
species.22
The two diastereomers 5a and 5b were separated taking
advantage of the different solubility in methanol. The more
1
soluble fraction, 5b, shows in the H NMR spectrum one set
of sharp signals: in particular a double doublet at δ 1.74 (JHH
=
With ligand L2, having no stereogenic centres, [L2 PdCl2] 6,
2
7.3 Hz and JPH = 14.7 Hz) for the Me group and a multiplet at
δ 5.35 (JHH = 7.3 Hz, JPH not resolved) for the CH protons. The
H(6) proton is deshielded (∆δ = 0.73 ppm) as usually observed
for a pyridine coordinated cis to a chlorine atom. The 31P{1H}
NMR spectrum shows only one signal at δ 33.6 confirming that
only one definite species is present.
analogous to 5, is obtained.
1
The H and 31P{1H} NMR spectra of complex 6 are tem-
perature dependent. At Ϫ50 ЊC two species are observed in
molar ratio ca. 3:2; at high temperature (ϩ50 ЊC) coalescence
of the signals is observed. This dynamic behaviour is remi-
niscent of that previously described for the achiral form (meso)
of complex 5.
To provide definitive characterization of diastereomer 5b
and to determine the solid-state stereochemistry, a single-crystal
X-ray diffraction study was undertaken. Compound 5b was
found to be one of the chiral diastereomers (R,R enantiomer).
Crystals of 5bؒMeOHؒH2O consist of the packing of
Ph
Ph
Ph
Ph
O
O
P
P
O
Ph
Ph
P
[L1 PdCl2], methanol and water molecules in the molar ratio
2
Cl
N
Cl
N
N
1:1:1, with no unusual van der Waals contacts. Principal bond
parameters are reported in Table 3. An ORTEP view of the
Pd
Pd
Cl
Cl
N
[L1 PdCl2] molecule is shown in Fig. 2. The molecule possesses
2
crystallographic C2 symmetry, with the two-fold axis per-
pendicular to the best plane of the metal coordination and
passing through the Pd atom. The essentially square-planar
coordination of the metal atom displays a slight pyramidal
distortion, with maximum distances from the best plane of
ϩ0.037(1) and Ϫ0.025(3) Å for Pd and N(1), respectively. The
Pd–Cl, 2.299(1), and Pd–N(1), 2.036(3) Å, bond lengths are in
the range observed for similar systems: see for instance Pd–
Cl(1) 2.297(1) Å in [PdCl2{Me2C(pz)2}] 10 (Hpz = pyrazole),23
where the Cl atom is trans to a pyrazolic nitrogen atom, and Pt–
N 2.013(3) and 2.021(3) Å in C24H28N2O8Pt,24 where two substi-
tuted pyridines are trans to each other. The P–O bond length,
1.492(3) Å, is almost coincident within three e.s.d.s with that
found in free Ph3PO, 1.46(1) Å.25 The C(7) and C(7Ј) asym-
metric carbon atoms display R absolute configurations, and
their H(7) and H(7Ј) hydrogen atoms lie at 2.66 Å from the Pd
atom, a rather short interaction, similar to that found in
[L1PtCl2] 4, and discussed above. With respect to the best plane
of atoms Cl, N(1), ClЈ and N(1Ј) the Pd, H(7) and H(7Ј) atoms
P
Ph
Ph
O
Crystals of 6ؒMeOH consist of the packing of [L2 PdCl2]
2
and methanol molecules in a molar ratio of 1:1 with no
unusual van der Waals contacts. Principal bond parameters are
reported in Table 3. An ORTEP view of the [L2 PdCl2] molecule
2
is shown in Fig. 3. The molecule lies on a crystallographic
inversion centre, so that the coordination around the metal
atom is exactly planar. The Pd–Cl, 2.298(1), Pd–N(1), 2.035(2),
and P–O, 1.485(2) Å, bond lengths are statistically coincident
with those found in [L1 PdCl2] 5b. Also in this case two
2
hydrogen atoms bonded to C(7) and C(7Ј), namely H(71) and
H(71Ј), display rather short contacts with the metal atom, 2.61
Å. If these interactions are taken into account, the coordination
around the Pd atom becomes distorted octahedral.
It is noteworthy that in the case of ligand L3, under com-
parable conditions, no 2:1 adduct was obtained.
4122
J. Chem. Soc., Dalton Trans., 1998, 4119–4126

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The N,O ligand can be removed from complex 3 by reaction
with a strong ligand such as PPh3 (molar ratio 1:1) to give
[(Ph3P)PdCl2]2. Reaction with 3,5-dimethylpyridine (1:1) gives
a complex mixture of unidentified species.
Finally, in order to investigate other aspects of the reactivity
of these N,O ligands, an attempt has been made to synthesize
ionic species [L1 Pd]2ϩ. It has been observed that removal
2
of the chloride ligands from [L1PdCl2] by means of silver()
ions, followed by addition of L1, does not give the expected
product. At variance, a bis-chelate species [L1 Pd][PF6]2 8,
2
was obtained by removing the chloride ions with AgPF6 from
[L1 PdCl2] 5. This preliminary result suggests that the synthesis
2
of these complexes requires that both the molecules of the
ligand are previously bonded to the metal through the nitrogen
atom: in the intermediate, arising from abstraction of the
chloride ions, the coordinative unsaturation is easily filled by
the oxygen atoms. IR and 31P{1H} NMR data support the
endobidentate behaviour of L1 in complex 8.
Experimental
The ligands L1, L2 and L3 were prepared according to literature
methods13 as briefly described below. Na2PdCl4ؒxH2O (Pd
29.15%) was provided by Johnson Matthey and K2PtCl4ؒxH2O
(Pt 43.035%) by Engelhard s.r.l.
Elemental analyses were performed with a Perkin-Elmer
Elemental Analyzer 240B by Mr A. Canu (Dipartimento di
Chimica, Università di Sassari). Conductivities were measured
with a Philips PW 9505 conductimeter. Infrared spectra were
recorded with a Perkin-Elmer 983 or FT-IR Bruker IFS66
spectrophotometer using Nujol mulls or in CH2Cl2 solution.
1H and 31P{1H} NMR spectra were recorded with a Varian
VXR 300 spectrometer operating at 299.9 and 121.4 MHz
respectively, and are collected in Tables 1 and 2. Chemical shifts
are given in ppm relative to internal TMS (1H) and external
85% H3PO4 (31P). Mass spectra (positive ion) were obtained
with a VG 7070EQ, instrument operating under FAB con-
ditions with 3-nitrobenzyl alcohol (NBA) as supporting matrix.
Fig. 3 An ORTEP view of compound 6. Primed atoms are related to
unprimed ones by the symmetry operation 1 Ϫ x, Ϫy, 1 Ϫ z. Ellipsoids
as in Fig. 1.
At variance with the palladium complex 5, in the case of the
analogous platinum derivative, [L1 PtCl2] 7, we were unable to
2
achieve separation of the species observed in solution. The
31P{1H} NMR spectrum of complex 7 shows four resonances
(ca. 1:1:3:1), in a region which rules out endobidentate co-
ordination. In the 1H NMR spectrum, four signals are observed
for the methyl protons in the range δ 0.4–2.0 and a complex
multiplet for the C–H protons. More H(6) resonances (partially
overlapping) around δ 9 (free ligand, δ 8.34) confirm the
coordination of the pyridine ring. A rationale of the presence
of more species is not easy. Tentatively, it can be assumed
that, in addition to two diastereomers, geometric isomers are
formed. Indeed, whereas [L2PdCl2] complexes are almost
exclusively isolated as trans isomers, in the case of the corre-
sponding platinum derivatives, both cis- and trans- isomers
have been reported.
Preparation of ligands
The ligands L1 and L2 were prepared according to a procedure
reported for the synthesis of the corresponding pyridylphos-
phine,13b by using chlorodiphenylphosphine oxide in place of
chlorodiphenylphosphine. L1 was synthesized by dropwise
addition at Ϫ78 ЊC of a solution of BuLi in hexane to a solu-
tion of 2-ethylpyridine in Et2O, followed by treatment with
chlorodiphenylphosphine oxide; L2 is easily accessible by drop-
wise addition at Ϫ78 ЊC of a solution of BuLi in hexane to
a solution of α-picoline in Et2O, followed by reaction with
chlorodiphenylphosphine oxide in Et2O.
To compare the stability of a six-membered N,O ring with
respect to a five-membered one, the reactions of [L1PdCl2] 1
and [L3PdCl2] 3 with L1 and L3, respectively, have been investi-
gated.
The reaction of complex 1 with L1 gives complex 5, i.e. the
2:1 adduct, whereas the reaction of complex 3 with L3 does not
occur.
Ligand L3 was prepared from 2-pyPPh2 by oxidation at 0 ЊC
in acetone with H2O2 (30% in H2O), according to ref. 13(a).
N
O
Cl
Cl
N
N
Cl
N
P
[L1PdCl2] 1
+
O
P
Pd
Pd
O
To a solution of Na2[PdCl4]ؒxH2O (0.365 g, 1.00 mmol) in
MeOH (20 ml) was added a methanolic solution (10 ml) of
L1 (0.307 g, 1.00 mmol). The solution was stirred for 4 h. The
brick-red solution obtained was evaporated to dryness and
crystallized from CH2Cl2–diethyl ether to give the analytical
product as an orange red solid. Yield 57%, mp 230 ЊC
(decomp.) (Found: C, 42.87; H, 3.59; N, 2.49. Calc. for
C19H18NOPPdCl2ؒCH2Cl2: C, 42.18; H, 3.54; N, 2.46%). ΛM
(5 × 10Ϫ4 M, acetone) = 15 ΩϪ1 cm2 molϪ1. FAB mass spectrum:
m/z 448 (M Ϫ Cl), 413 (M Ϫ 2Cl); IR (Nujol) νmax/cmϪ1:
1599m, 1120s, 340m.
P
P
O
Cl
L1
1
5
The latter behaviour is consistent with the unsuccessful
attempts to obtain 2:1 adducts of L3 as previously mentioned.
Reactions of [L1PdCl2] 1 with L3 and of [L3PdCl2] 3 with L1
give a mixture of species, including [L3PdCl2] and [L1 PdCl2],
2
but not the [L1L3PdCl2] complex in the former case, and
[L1 PdCl2], [L1PdCl2] and free L3 in the latter case (1H and 31P
2
NMR criterion). Although an unambiguous conclusion on the
relative stability of the two cycles was not achieved, at least on
the basis of the reaction between the chelate complexes 1 and
3 with L1 and L3, respectively, it seems that coordination of
oxygen is more easily displaced from a six- rather than from
a five-membered ring.
[L2PdCl2] 2
To a solution of Na2[PdCl4] (0.100 g, 0.27 mmol) in water
(10 ml) was added a methanolic solution (10 ml) of L2 (0.080 g,
J. Chem. Soc., Dalton Trans., 1998, 4119–4126
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0.27 mmol). The mixture was stirred for 2 h. The precipitate
formed was filtered, washed with water, EtOH and Et2O to
give the analytical sample as a yellow solid. Yield 89%, mp
252–254 ЊC (Found: C, 45.81; H, 3.64; N, 2.93. Calc. for
C18H16NOPPdCl2: C, 45.94; H, 3.43; N, 2.98%). ΛM (5 × 10Ϫ4
M, acetone) = 13 ΩϪ1 cm2 molϪ1. FAB mass spectrum: m/z 434
(M Ϫ Cl), 399 (M Ϫ 2Cl). IR (Nujol) νmax/cmϪ1: 1596m, 1126s,
325m.
[L12PtCl2] 7
To an ethanolic solution (5 ml) of L1 (307 mg, 1.00 mmol) an
aqueous solution of K2[PtCl4] (207 mg, 0.5 mmol, 15 ml) was
added under vigorous stirring. The mixture was stirred for 3
days. The precipitate formed was filtered and washed with H2O,
EtOH and Et2O and recrystallized from CH2Cl2–Et2O to give
the analytical sample as a pale yellow solid. Yield 80% (Found:
C, 50.04; H, 4.28; N, 3.29. Calc. for C38H36N2O2P2PtCl2ؒH2O:
C, 50.79; H, 4.26; N, 3.12%). IR (Nujol) νmax/cmϪ1: 1600m,
1181s, 350m (br).
[L3PdCl2] 3
(1) To a methanolic solution (30 ml) of Na2[PdCl4] (0.365 g,
1.00 mmol) was added dropwise a solution of L3 (0.279 g,
1.00 mmol) in MeOH (10 ml). The mixture was stirred for
12 h. The mustard yellow precipitate formed was filtered and
recrystallized from CH2Cl2–Et2O to give the analytical sample.
Yield 70%.
[L12Pd][PF6]2 8
To a solution of 5 (0.100 g, 0.126 mmol) in CH2Cl2 (20 ml)
0.064 g (0.253 mmol) of Ag[PF6] in the same solvent were
added. The mixture was stirred for 2.5 h in the dark. The AgCl
formed was filtered: the resulting solution was evaporated to
a small volume and diethyl ether was added. The precipitate
formed was filtered and washed with Et2O to give the analytical
sample as a pale yellow solid. Yield 80%, mp 200 ЊC (Found:
C, 44.91; H, 3.59; N, 2.98. Calc. for C38H36N2O2P4PdF12:
C, 45.15; H, 3.59; N, 2.77%). ΛM (5 × 10Ϫ4 M, CH2Cl2) =
24 ΩϪ1 cm2 molϪ1; (5 × 10Ϫ4 M, acetone) = 220 ΩϪ1 cm2 molϪ1.
FAB mass spectrum: m/z 720 [Mϩ], 413 (M Ϫ L1). IR (Nujol)
νmax/cmϪ1: 1602m, 1122s.
Complex 3 was obtained also with a L3 :Pd 2:1 molar ratio.
The excess ligand L3 was recovered quantitatively from the
methanolic solution.
(2) To a benzenic solution (50 ml) of trans-[(PhCN)2PdCl2]
(0.3815 g, 0.99 mmol) were added under stirring 0.279 g (1.00
mmol) of L3. The mixture was stirred for 8 h. The precipitate
formed was filtered and recrystallized from CH2Cl2–petroleum
ether (bp range 40–60 ЊC) to give the analytical mustard yellow
sample. Yield 92%. Mp 280 ЊC (decomp.) (Found: C, 44.93; H,
3.24; N, 3.22. Calc. for C17H14NOPPdCl2: C, 44.70; H, 3.07; N,
3.07%). FAB mass spectrum: m/z 420 (M Ϫ Cl), 385 (M Ϫ 2Cl).
IR (Nujol) νmax/cmϪ1: 1585w, 1120s, 339s (br).
Reaction of [L1PdCl2] with L1
To a methanolic solution (30 ml) of 1 (0.100 g, 0.206 mmol)
was added under vigorous stirring a solution of L1 (0.0635 g,
0.207 mmol) in MeOH (10 ml). A yellow precipitate was
formed. The mixture was stirred for 24 h and filtered to give the
analytical sample, 5. Yield 74%, mp 273 ЊC (decomp.) (Found:
C, 57.02; H, 4.35; N, 3.50. Calc. for C38H36N2O2P2PdCl2: C,
57.63; H, 4.58; N, 3.54%).
[L1PtCl2] 4
To an ethanolic solution (5 ml) of L1 (111 mg, 0.36 mmol) an
aqueous solution of K2[PtCl4] (150 mg, 0.36 mmol, 10 ml) was
added under vigorous stirring. The mixture was stirred for 6
days, then filtered over celite, evaporated to dryness and crystal-
lized from CH2Cl2 and Et2O to give the analytical sample as a
pale yellow solid. Yield 47.3%, mp 212 ЊC (Found: C, 36.60;
H, 3.06; N, 2.08. Calc. for C19H18NOPPtCl2ؒCH2Cl2: C, 36.47;
H, 3.04; N, 2.13%). IR (Nujol) νmax/cmϪ1: 1600m, 1124s, 345s
(br).
Reaction of [L3PdCl2] with L3
To a methanolic suspension (20 ml) of 3 (0.100 g, 0.22 mmol)
was added under vigorous stirring a solution of L3 (0.0610 g,
0.22 mmol) in MeOH (15 ml). After 24 h, the solid was
collected and found to be unreacted [L3PdCl2].
The reaction was also carried out in solution (NMR tube)
[L1 PdCl2] 5
2
1
using a mixture of solvents [CD2Cl2–CD3OD (5:1)]. The H
To a solution of L1 (0.584 g, 1.90 mmol) in MeOH (10 ml)
was added dropwise a methanolic solution (20 ml) of Na2[Pd-
Cl4]ؒxH2O (0.346 g, 0.95 mmol). The yellow precipitate formed
was filtered to give a first sample 5a (0.376 g). The solution was
evaporated to a small volume and treated with Et2O. The pre-
cipitate formed was filtered to give a second sample 5b (0.230 g).
5a: Yield 49%, mp 274 ЊC (decomp.) (Found: C, 56.62; H,
4.49; N, 3.35. Calc. for C38H36N2O2P2PdCl2ؒH2O: C, 56.35; H,
4.73; N, 3.46%). FAB mass spectrum: m/z 755 (M Ϫ Cl), 720
(M Ϫ 2Cl), 413 (M Ϫ 2Cl Ϫ L1). IR (Nujol) νmax/cmϪ1: 1608m,
1180s, 348m.
and 31P NMR spectra showed only the presence of [L3PdCl2]
and free ligand.
Reaction of [L1PdCl2] with L3
To a methanolic solution (15 ml) of 1 (0.0664 g, 0.137 mmol)
was added under vigorous stirring a solution of L3 (0.0390 g,
0.137 mmol) in MeOH (15 ml). The mixture was stirred for 2 h:
1
the precipitate formed was filtered off. H and 31P NMR spec-
tra (CDCl3) showed a mixture of compounds [L1 PdCl2] and
2
[L3PdCl2].
5b: Yield 29%, mp 240–241ЊC (decomp.) (Found: C, 54.59;
H, 4.79; N, 3.51. Calc. for C38H36N2O2P2PdCl2ؒH2OؒCH3OH:
C, 55.63; H, 5.03; N, 3.33%). IR (Nujol) νmax/cmϪ1: 1608m,
1180s, 348m.
Reaction of [L3PdCl2] with PPh3
To a solution of 3 (0.100 g, 0.22 mmol) in dichloromethane
(30 ml) was added under vigorous stirring a solution of PPh3
(0.0575 g, 0.22 mmol) in the same solvent (20 ml). The colour
of the solution turned immediately to brilliant-orange. The
solution was evaporated to a small volume and diethyl ether
was added. The precipitate formed was filtered off and washed
with Et2O to give the analytical sample {[(Ph3P)PdCl2]2} in
almost quantitative yield.
[L22PdCl2] 6
To a methanolic solution (10 ml) of L2 (0.100 g, 0.34 mmol) was
added dropwise a solution of Na2[PdCl4] (0.63 g, 0.17 mmol)
in H2O (10 ml). The yellow precipitate formed was filtered
and recrystallized from CH2Cl2 and Et2O to give the analytical
sample. Yield 76%, mp 242–243 ЊC (Found: C, 55.42; H, 4.62;
N, 3.39. Calc. for C36H32N2O2P2PdCl2ؒMeOH: C, 55.83; H,
4.56; N, 3.52%). FAB mass spectrum: m/z 727 (M Ϫ Cl),
692 (M Ϫ 2Cl). IR (Nujol) νmax/cmϪ1: 1602m, 1587w, 1189s,
345m.
X-Ray data collections and structure determinations
Crystal data and other experimental details are summarized
in Table 4. A crystal of 4ؒCH2Cl2 was mounted on an Enraf-
Nonius CAD-4 diffractometer at room temperature using
4124
J. Chem. Soc., Dalton Trans., 1998, 4119–4126

View Article Online
Table 4 Crystallographic data
Compound
Formula
M
4ؒCH2Cl2
C20H20Cl4NOPPt
658.3
5bؒMeOHؒH2O
C39H42Cl2N2O4P2Pd
842.0
6ؒMeOH
C37H36Cl2N2O3P2Pd
796.0
Crystal system
Space group
a/Å
b/Å
Monoclinic
P21/c (no. 14)
12.753(3)
14.353(4)
12.863(3)
100.51(2)
2315.0(1.0)
4
Trigonal
P3121 (no. 152)
16.742(2)
Monoclinic
P21/c (no. 14)
10.522(2)
11.090(2)
16.220(4)
108.87(1)
1791.0(7)
2
c/Å
12.798(1)
β/Њ
U/Å3
3106.6(6)
3
Z
T/K
298
66.7
298
6.9
298
7.9
µ(Mo-Kα)/cmϪ1
Measured reflections (total,
4040, 4040
34876, 4893
19704, 4096
independent)
Unique observed reflections
with I > 3σ(I )
2558
2903
3014
Final R and RЈ
0.030, 0.032
0.027, 0.034
0.023, 0.031
Mo-Kα radiation (λ = 0.71073 Å) with a graphite crystal
monochromator in the incident beam. Cell parameters and
orientation matrix were obtained from the least squares refine-
ments of 25 reflections in the range 3 < θ < 23Њ. A periodic
monitoring of three standard reflections revealed a crystal
decay, on X-ray exposure, which was evaluated as about 16.1%
(on intensities) at the end of data collection. The diffracted
intensities were corrected for Lorentz, polarization, decay and
absorption effects (empirical correction).26 Crystals of 5bؒMe-
OHؒH2O and of 6ؒMeOH were mounted on a Siemens SMART
CCD area detector diffractometer at room temperature using
Mo-Kα radiation with a graphite crystal monochromator in the
incident beam. Cell parameters and orientation matrix were
obtained from the least-squares refinement of 105 (for 5bؒMe-
OHؒH2O) and 86 (for 6ؒMeOH) reflections measured in three
different sets of 15 frames each, in the range 3 < θ < 23Њ. At
the end of data collections the first 50 frames, containing
264 reflections (for 5bؒMeOHؒH2O) and 242 reflections
(for 6ؒMeOH), were recollected to have a monitoring of
crystal decay, which was not observed, so that no time-decay
correction was needed. The collected frames were processed
with the software SAINT,27a and an absorption correction was
applied (SADABS27b) to the 34876 collected reflections of
5bؒMeOHؒH2O [4893 of which are unique with Rint = 0.023
refinement of the structure enantiomorph in space group P3221
led to R = 0.031 and RЈ = 0.039. The final Fourier maps showed
maximum residuals of 0.62(9) e ÅϪ3 at 0.33 Å from a chlorine
atom of the solvent molecule in compound 4ؒCH2Cl2, 0.50(6)
e ÅϪ3 at 0.69 Å from the oxygen atom of MeOH in 5bؒMe-
OHؒH2O, and 0.37(5) e ÅϪ3 at 0.82 Å from Cl in 6ؒMeOH.
CCDC reference number 186/1223.
Acknowledgements
Financial support from Ministero dell’Università e della
Ricerca Scientifica e Tecnologica (MURST, 40%) and Con-
siglio Nazionale delle Ricerche (CNR) is gratefully acknow-
ledged.
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