A palladium(II) complex of a new iminophosphine ligand derived from diethylenetriamine and 2-(diphenylphosphino)benzaldehyde

Article abstract of DOI:10.1016/S0020-1693(00)00181-X

The mononuclear complexes [Pd(oPdien)]X₂ {oPdien = N-(2-diphenylphosphinobenzylidene)diethylenetriamine: (4a) X₂ = [ZnCl₄]^2-; (4b) X = [PF₆]^-; (4c) X = Cl^-} are described. Complex 4a formed in high yield when [Pd(oPCHO)₂Cl₂] [oPCHO = 2-(diphenylphosphino)benzaldehyde] (1) and [Zn(dien)Cl₂] (2) were reacted together in an unsuccessful attempt to template formation of a bimetallic iminophosphine-bridged complex. The X-ray crystal structure of 4b has been determined. The ligand oPdien is the first iminophosphine derived from dien and has been isolated and used to prepare 4c. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00181-X

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Inorganica Chimica Acta 307 (2000) 134–138
Note
A palladium(II) complex of a new iminophosphine ligand derived
from diethylenetriamine and 2-(diphenylphosphino)benzaldehyde
Scott E. Watkins, Donald C. Craig, Stephen B. Colbran *
School of Chemistry, Uni6ersity of New South Wales, Sydney, NSW 2052, Australia
Received 4 February 2000; accepted 22 May 2000
Abstract
The mononuclear complexes [Pd(oPdien)]X2 {oPdien=N-(2-diphenylphosphinobenzylidene)diethylenetriamine: (4a) X2=
[ZnCl4]2−; (4b) X=[PF6]−; (4c) X=Cl−} are described. Complex 4a formed in high yield when [Pd(oPCHO)2Cl2] [oPCHO=2-
(diphenylphosphino)benzaldehyde] (1) and [Zn(dien)Cl2] (2) were reacted together in an unsuccessful attempt to template
formation of a bimetallic iminophosphine-bridged complex. The X-ray crystal structure of 4b has been determined. The ligand
oPdien is the first iminophosphine derived from dien and has been isolated and used to prepare 4c. © 2000 Elsevier Science S.A.
All rights reserved.
Keywords: Crystal structures; Palladium complexes; Iminophosphine complexes
imine-type ligands and has been employed extensively
in the synthesis of metal complexes which show impor-
tant catalytic and dioxygen binding properties [14–17].
In this report we describe the reaction between a pre-
formed Pd(II) complex of oPCHO and a Zn(II) com-
plex of dien in an attempt to template the preparation
of an iminophosphine-bridged Pd, Zn dimer. The reac-
tion resulted in the adventitious formation of a
mononuclear Pd(II) complex of the iminophosphine
derived from dien and oPCHO, the synthesis and char-
acterisation of which is discussed. The ligand is the first
example of an iminophosphine prepared from dien.
1. Introduction
Our recent work on the functionalisation of phosphi-
nes with N-donor groups [1] has led us to investigate
the synthesis of potentially multinucleating iminophos-
phine ligands. The presence of both soft and hard base
donor atoms in these types of ligand has already
prompted reports of their use in preparing heteronu-
clear complexes [2]. Mononuclear complexes of these
same polydentate iminophosphine ligands have also
attracted much recent interest due to the role they play
in facilitating catalytic reactions by stabilising metal
centres in a variety of oxidation states [3–6].
The use of 2-(diphenylphosphino)benzaldehyde (oP-
CHO) as a building block for iminophosphines was first
reported in the Schiff-base condensation with ethylene-
diamine [7,8] and has since been extended to a wide
variety of other amines [9–13]. Diethylenetriamine
(dien) is another key component in the synthesis of
2. Experimental
2.1. Materials
2-(Diphenylphosphino)benzaldehyde was synthesised
by the method of Rachfuss [18], bis(benzoni-
trile)palladium(II) dichloride by the method of Ander-
son [19] and diethylenetriaminezinc(II) dichloride by
the method of Curtis [20].
* Corresponding author. Tel.: +61-2-9385 4737; fax: +61-2-9385
6141.
E-mail address: s.colbran@unsw.edu.au (S.B. Colbran).
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00181-X

S.E. Watkins et al. / Inorganica Chimica Acta 307 (2000) 134–138
135
2.2. Synthesis of [PdIICl2(oPCHO)2] (1)
2.4. Synthesis of [PdII(oPdien)][PF6]2 (4b)
A solution of 2-(diphenylphosphino)benzaldehyde
(0.974 g, 3.36 mmol) in dichloromethane (20 cm3) was
added dropwise to an ice cooled stirred solution of
[Pd(PhCN)2Cl2] (0.645 g, 1.68 mmol) in di-
chloromethane (45 cm3). The solution was allowed to
warm to room temperature gradually over 2 h and the
orange solid that formed was collected by filtration.
Additional compound was obtained by adding diethyl
ether to the initial filtrate. The complex was recrys-
tallised from dichloromethane–diethyl ether to give
orange crystals of [Pd(PCHO)2Cl2] (1.02 g, 80%).
Found: C, 59.84; H, 4.06. C38H30Cl2O2PdP2 requires: C,
60.22; H, 3.99%. P NMR (CDCl3): l 20.81 (s). H NMR
(CDCl3): l 10.47 (1 H, s), 8.0 (1 H, d), 7.99 (1 H, d),
7.81 (4 H, q), 7.57 (1 H, t), 7.43 (6 H, m), 7.03 (1 H, q).
A suspension of [Pd(oPdien)][ZnCl4] (0.498 g, 0.722
mmol) in acetonitrile (50 cm3) was treated with
[NH4][PF6] until the complex completely dissolved. The
solution was filtered while still hot, the volume reduced
and water added until the first signs of cloudiness were
observed. The white solid which formed on standing in
a freezer for several hours was collected by filtration
and recrystallised from acetonitrile–isopropanol to give
colourless plates of [Pd(oPdien)][PF6]2 (0.376 g, 94%).
Found: C, 35.07; H, 3.52; N, 5.31. C23H26F12N3PdP3
requires: C, 35.79; H, 3.39; N, 5.45%. P NMR
((CD3)2SO): l 27.51 (s), −143.11 [sept, J(PF) 707 Hz].
H NMR ((CD3)2SO): l 8.85 (1 H, s), 8.11 (1 H, t), 7.93
(2 H, m), 7.80–7.40 (10 H, m), 7.22 (1 H, br s), 5.66 (1
H, br s), 4.91 (1 H, br s), 4.44 (1 H, m), 4.26 (1 H, m)
and 3.07 (6 H, m). ES MS m/z: 240 {[Pd(oPdien)]2+}.
A small portion of the sample was recrystallised from
1-butanol–methanol to yield colourless crystals suitable
for X-ray analysis.
2.3. Synthesis of [PdII(oPdien)][ZnIICl4] (4a)
A suspension of [Zn(dien)Cl2] (0.415 g, 1.74 mmol)
and [Pd(PCHO)2Cl2] (1.32 g, 1.74 mmol) in ethanol
(100 cm3) was heated at reflux for 2 h with the yellow
colour gradually fading. The cream solid was collected
by filtration and recrystallised from methanol–diethyl
ether to give white flowers of [Pd(oPdien)][ZnCl4] (0.973
g, 81%). Found: C, 40.13; H, 4.18; N, 5.34.
C23H26Cl4N3PdPZn·2(CH3OH) requires: C, 39.87; H,
4.55; N, 5.58%. P NMR ((CD3)2SO): l 27.35 (s). H
NMR ((CD3)2SO): l 8.89 (1 H, s), 8.12 (1 H, t), 7.93 (2
H, m), 7.80–7.50 (10 H, m), 7.43 (1 H, t), 4.44 (1 H,
m), 4.33 (1 H, m), 3.08 (6 H, m). ES MS m/z: 240
{[Pd(oPdien)]2+}.
2.5. Synthesis of [PdII(oPdien)]Cl2 (4c)
A solution of oPCHO (1.30 g, 4.48 mmol) in toluene
(60 cm3) was added slowly over about 15 min via a
cannula to a stirred solution of dien (1.85 g, 17.9 mmol)
at reflux in toluene (20 cm3) in a Dean–Stark appara-
tus. Heating of the solution at reflux was continued for
20 h followed by cooling to room temperature and
removal of the solvent under vacuum to give an orange
oil (oPdien) which was used without further purifica-
tion. P NMR (CDCl3): l −15.48 (s), −16.63 (s). ES
MS m/z: 376 {[H(oPdien)]+}. A stirred solution of this
crude oil (0.46 g, 1.2 mmol) in dichloromethane (12
cm3) was cooled in an ice bath and treated with a
solution of [Pd(PhCN)2Cl2] (0.42 g, 1.1 mmol) in
dichloromethane (10 cm3) resulting in the immediate
formation of a white precipitate. Stirring was continued
for 4 h allowing the solution to gradually warm to
room temperature. The white precipitate was collected
by filtration and washed with diethyl ether to give
[PdII(oPdien)]Cl2 (0.509 g, 75%). P NMR ((CD3)2SO): l
27.06 (s). H NMR ((CD3)2SO): l 8.91 (1 H, s), 8.11 (1
H, t), 7.92 (2 H, m), 7.80–7.50 (10 H, m), 7.39 (1 H,
m), 5.55 (1 H, br s), 4.42 (2 H, m) and 3.10 (6 H, m).
ES MS m/z: 240 {[Pd(oPdien)]2+}.
3. Results and discussion
Our objective was to synthesise a bimetallic complex
bridged by an iminophosphine ligand derived from
dien, e.g. 3 in Scheme 1. Molecular models suggest that
the ligand derived from the condensation of two
molecules of oPCHO with dien is ideally set-up to
Scheme 1. (i) EtOH reflux; (ii) [NH4][PF6], MeCN; (iii) toluene reflux;
(iv) [PdCl2(PhCN)2], CH2Cl2.

136
S.E. Watkins et al. / Inorganica Chimica Acta 307 (2000) 134–138
Table 1
of the required stoichiometry were reacted together
should provide a feasible route to bimetallic complexes
such as 3. It was envisaged that by initially conferring
the required geometry onto the metal ions, the forma-
tion of the targeted complex should be facilitated.
To test these ideas a reaction between a palladium(II)
complex of oPCHO, [Pd(oPCHO)2Cl2] (1), and a
zinc(II) complex of dien, [Zn(dien)Cl2] (2) [20], was
carried out. The palladium(II) complex, 1, was chosen
to give the required transoid geometry and the diamag-
netism of the metal centres in 1 and 2 enabled conve-
Numerical crystal and refinement data for 4b
Formula
C23H26F12N3P3Pd
771.8
Formula mass
Crystal size
Space group
0.30×0.19×0.06
P21/c
8.841(4)
18.606(4)
17.505(9)
90
91.74(2)
90
21(1)
4
1.78
Enraf–Nonius
CAD-4, q/2q
Mo Ka 0.71073
8.96
,
a (A)
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
T (°C)
Z
1
nient monitoring of the reaction by H and 31P NMR
Dcalc (g cm−3
)
spectroscopy. First, 1, a new complex, was prepared in
80% yield from oPCHO [18] and [Pd(PhCN)2Cl2] [19] in
dichloromethane. Then a suspension of 1 and 2 was
heated at reflux in ethanol for several hours with the
colour changing from yellow to cream. The suspension
was then cooled and the cream precipitate, 4a, collected
by filtration. A metathesis reaction of 4a with excess
[NH4][PF6] gave colourless plates of 4b.
Diffractometer; scan mode
,
Radiation (u, A)
v (cm−1
F(000)
)
1536.0
0.91, 0.77
25
5238
0.011 (178)
3479
Max., min. transmission factors
2qmax
Number of intensity measurements
Rmerge for number of equivelent reflections
Number of observed reflections in final
refinement [I/|(I)\3]
Number of variables in final refinement
R
Rw
Elemental analyses identify 4a,b as complexes of the
new iminophosphine, N-(2-diphenylphosphinobenzylid-
ene)diethylenetriamine (oPdien), formed from the con-
densation of dien and one equivalent of oPCHO.
Whilst the elemental and ICP analyses reveal 4a to be a
1:1 Pd–Zn complex, the absence of zinc in the analyses
of 4b confirms the presence of the tetrachlorozincate
anion in 4a which is replaced by the hexafluorophos-
phate anion in 4b. Complexes 4a and 4b give identical
spectroscopic data with a characteristic imine resonance
237
0.039
0.054
1.84
Goodness of fit
Max., min. peaks in final difference map
0.84, −0.88
−3
,
(e A
)
Table 2
1
at about l 8.9 in the H NMR spectra, a singlet at
,
Selected bond lengths (A) and angles (°) for 4b
about l 27 in the 31P NMR spectra and a single peak at
m/z 240 in the ES mass spectra corresponding to the
[Pd(oPdien)]2+ molecular ion.
PdꢀP
2.232(1)
1.978(4)
2.064(4)
2.043(5)
1.283(7)
PdꢀN1
PdꢀN2
PdꢀN3
N1ꢀC7
1
The complicated H NMR spectra of both 4a and 4b
precluded unequivocal assignment of the structure of
the cation and, therefore, an X-ray crystallographic
study of crystals of 4b obtained from methanol–1-bu-
tanol was undertaken. Numerical crystal and refine-
ment data for 4b are summarised in Table 1, and key
bond lengths and angles are listed in Table 2. The
molecular structure of the cation of 4b (Fig. 1) confirms
the N3P-coordination of oPdien to palladium. The sum
of the bond angles about palladium is 704° which
compares to 720° for a perfect square planar arrange-
ment. A slight ‘puckering’ of the complex about the P
to N2 vector causes the deviation from perfect square
planar geometry and results in the N1ꢀPdꢀN3 bond
angle of 166.0(2)° compared to the PꢀPdꢀN2 bond
angle of 178.0(1)°. The PdꢀN(amine) and PdꢀP bond
lengths are comparable to those in a similar palladium-
PꢀPdꢀN1
PꢀPdꢀN2
PꢀPdꢀN3
93.5(1)
178.0(1)
98.7(1)
84.6(2)
166.0(2)
83.3(2)
132.4(4)
109.4(3)
117.6(5)
106.0(3)
106.3(3)
119.3(5)
109.8(4)
N1ꢀPdꢀN2
N1ꢀPdꢀN3
N2ꢀPdꢀN3
PdꢀN1ꢀC7
PdꢀN1ꢀC8
C7ꢀN1ꢀC8
PdꢀN2ꢀC9
PdꢀN2ꢀC10
C9ꢀN2ꢀC10
PdꢀN3ꢀC11
(II) complex with
box)(PPh3)][BF4]2
zoline-2%-yl}pyridine], a complex which also has two
a
N3P-donor ligand, [Pd(py-
coordinate to one metal by the two phosphine groups
and to a second metal centre via the N3-donor set of
the dien moiety. We thought that a metal template
‘type’ reaction where two pre-formed metal complexes
[pybox=2,6-bis{4%-(S)-benzyloxa-
five-membered rings about the palladium centre [21].

S.E. Watkins et al. / Inorganica Chimica Acta 307 (2000) 134–138
137
Fig. 1. Drawing of the complex cation 4b showing the 10% thermal ellipsoids at 294 K.
Another palladium(II) complex with a N2P-donor lig-
and, [Pd(PNN)CH3]+ [PNN=N-(2-diphenylphosphi-
nobenzylidene)-2-(2-pyridyl)ethylamine] [22], exhibits a
dien in toluene to give an oil which gave a peak at m/z
376 in its ES mass spectrum corresponding to the
protonated free ligand, [H(oPdien)]+. The 31P NMR
spectrum of the ligand shows peaks at l −16.63 and
−15.48 for free oPdien and the isomer resulting from
addition of the secondary amine group to the imine
bond, as is observed in similar dien-based ligands [14].
These isomers equilibrate in solution [14], and the crude
oil was treated with [Pd(PhCN)2Cl2] to give a complex
4c in 75% yield (Scheme 1). Complex 4c gave identical
spectroscopic data to 4a and 4b confirming that it
contains the [Pd(oPdien)]2+ cation. This demonstrates
that the free ligand, oPdien, can be prepared directly
from its organic constituents and that using pre-formed
oPdien is the best route to 4.
,
PdꢀP bond length of 2.2007(16) A compared to
,
2.232(1) A in 4b whilst the PdꢀN(imine) bond in
[Pd(PNN)CH3]+ at 2.131(5) A is slightly longer than
that observed in 4b [1.978(4) A], differences attributable
to the formation of three chelate rings in 4b.
,
,
The formation of mononuclear 4 from the stable
bis(phosphino)palladium(II) complex
1 indicates a
strong driving force for the coordination of the N
donor groups to palladium. The preference for chelate
ring formation realised in the formation of two five
membered and a six membered ring about the palla-
dium centre is the most likely explanation for the
rearrangements observed. An alternative coordination
mode for the N3P-donor group of oPdien is found in
the complex, [Ag3{(PN)3N}2]3+
[(PN)3N={(o-
Ph2PC6H4)CHꢁNCH2CH2}3N] (5) prepared by the Ag+
templated reaction of N,N,N-tris(2-aminoethyl)amine
and oPCHO [23]. In the resulting complex (5) the
five-coordinate silver ions were bound by two
iminophosphine arms and the amine with the remaining
iminophosphine arm bridging to the adjacent four-co-
ordinate Ag+ ion, Fig. 2. Notably, it was reported that
attempts at direct synthesis of [(PN)3N] were unsuccess-
ful and the complex could only be synthesised in a
metal template reaction. This lead us to investigate
whether or not free oPdien could be prepared.
Therefore, oPCHO was reacted with an excess of
Fig. 2. Drawing of [Ag3{(PN)3N}2]3+ (5) [23].

138
S.E. Watkins et al. / Inorganica Chimica Acta 307 (2000) 134–138
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The synthesis of the first iminophosphine ligand
based on dien through the Schiff-base condensation
with 2-(diphenylphosphino)benzaldehyde is described.
The new ligand, oPdien, is unique amongst iminophos-
phines in that it has the potential to coordinate either
as a tetradentate P,N,N,N-donor as in 4b; as a triden-
tate N,N,N-donor like dien in a complex with an
uncoordinated, dangling phosphine group; as a biden-
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sizes closely resembles the demonstrated ‘intrinsic flexi-
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oxidation states leads to the observed catalytic activity
of a palladium complex of it [4], a result suggesting that
complexes of oPdien might display equally novel
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