Pd(ii) and Ni(ii) complexes featuring a "phosphasalen" ligand: Synthesis and DFT study

Article abstract of DOI:10.1039/c1dt10906e

A phosphorus analog of salen ligands featuring iminophosphorane functionalities in place of the imine groups was synthesised in 2 steps from o-diphenylphosphinophenol via the preparation of the corresponding bis-aminophosphonium salt. This novel tetradent

Full text of DOI:10.1039/c1dt10906e

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PAPER
Pd(II) and Ni(II) complexes featuring a “phosphasalen” ligand: synthesis and
DFT study†
Thi-Phuong-Anh Cao, Ste´phanie Labouille, Audrey Auffrant,* Yves Jean,* Xavier F. Le Goff and
Pascal Le Floch‡
Received 13th May 2011, Accepted 19th July 2011
DOI: 10.1039/c1dt10906e
A phosphorus analog of salen ligands featuring iminophosphorane functionalities in place of the imine
groups was synthesised in 2 steps from o-diphenylphosphinophenol via the preparation of the
corresponding bis-aminophosphonium salt. This novel tetradentate ligand (1), which we named
phosphasalen, was coordinated to Pd(II) and Ni(II) metal centres affording complexes 6 and 7
respectively, which were characterised by multinuclear NMR, elemental and X-ray diffraction analyses.
Both neutral complexes adopt a nearly square-planar geometry around the metal with coordination of
all iminophosphorane and phenolate moieties. The electronic properties of these new complexes were
investigated by cyclic voltammetry and comparison with known salens was made when possible.
Moreover, the particular behaviour of the phosphasalen nickel complex 7 was further investigated
through magnetic moment measurements and a DFT study.
or heteroatom–X5 (X = C or heteroatom) bond formations. For
those processes, the electronic properties of the metal centre is
Introduction
N,N¢-Bis(salicylidine)ethylenediamines generally named “salen”
are among the most renowned ligands in catalysis (Scheme 1).
Their tremendous popularity can be ascribed to their easy access
(a simple condensation of a diamine with a salicylaldehyde)
allowing large skeleton variation, their ability to stabilise numerous
metals in various oxidation states,1 and the facile introduction
of chirality which has led to the development of a myriad of
very successful asymmetric catalysts,2 including recoverable ones.3
Salen complexes were demonstrated to be active either for C–C4
of most importance. Therefore, replacing the imine of a salen
ligand by an iminophosphorane will open the way to a new
class of complexes whose potential in catalysis would contrast
with that of salen complexes. With this in mind, we targeted
the synthesis of the phosphorus counterparts of salen ligands
featuring two phenolates and two iminophosphoranes (P N)
which we propose to name “phosphasalen” (Scheme 1). We
expected them to exhibit markedly different steric and electronic
properties compared to salen derivatives. Whereas the pyramidal
phosphorus of an iminophosphorane bears two R substituents
(Scheme 1), the carbon of an imine is trigonal planar with only
one R group. In terms of electronics, iminophosphoranes do not
possess any p accepting ability because of the absence of any p-
system and behave as strong s and p donors thanks to the presence
of two lone pairs at the nitrogen atom. All these features explain
why iminophosphorane ligands, though much less investigated
compared to imine derivatives have attracted the attention of
numerous research groups, who have demonstrated their potential
in coordination chemistry6 and developed catalytic applications.7
Synthetically, iminophosphoranes are commonly prepared by
a Staudinger reaction8 involving the condensation of an azide
and a phosphine. This methodology though very clean requires
the handling of azides which are generally difficult to prepare or
hazardous, so that most research groups employ commercially
available derivatives. In order to introduce more functional
diversity at the nitrogen atom, we and others preferred a synthetic
methodology relying on the Kirsanov reaction9 consisting in the
initial formation of an aminophosphonium bromide salt followed
by its deprotonation. In particular, this has allowed us to develop
Scheme 1 Carbon and phosphorus based salen type ligands.
Laboratoire He´te´roe´le´ments et Coordination, Ecole Polytechnique,
CNRS, 91128, Palaiseau Cedex, France. E-mail: audrey.auffrant@
polytechnique.edu, yves.jean@polytechnique.edu
† Electronic supplementary information (ESI) available: Crystallographic
data for 6, and 7 as cif files, complete Gaussian reference, computed
Cartesian coordinates, energies, three lower frequencies of all optimised
structures. CCDC reference numbers 804616 and 804617. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10906e
‡ Deceased on March 17th, 2010
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Scheme 2 Synthesis of phosphasalen ligand 1.
iminophosphorane based tetradentate ligands.10 In this paper
we show how this synthetic methodology can be applied to
the preparation of a phosphorus containing salen derivative. Its
coordination in a tetradentate manner to Pd(II) and Ni(II) centres
is also presented. Cyclovoltammetric studies were conducted to
get a quantitative assessment of the electronic properties of
phosphasalen complexes compared to salen ones. In the case of
the [Ni(phosphasalen)] complex further insight into the electronic
and magnetic properties was gained through a comparative DFT
study.
such an ethylenediamine linker.10a On the contrary the resonance
of the NH proton appears as a multiplet around 6.80 ppm which
is quite shielded compared to the same reference. Then, the
phosphasalen ligand 1 was generated from 5 by deprotonation
using potassium hexamethyldisilazane (KHMDS, 4 equiv.) in
THF at room temperature (Scheme 2). The formation of 1 was
1
monitored by in situ 31P{ H} NMR, the completeness of the
reaction was indicated by the presence of one singlet at d(THF) =
18.6 ppm. Potassium bromide salts formed were filtered off, the
solvent was evaporated and the obtained residue was washed
with petroleum ether to deliver 1 as a white powder in 99%
yield. The structure of 1 was unambiguously established by NMR
spectroscopy, and elemental analysis.
Results and discussion
The coordination behaviour of 1 towards Pd(II) and Ni(II) metal
centres was then evaluated (Scheme 3). As we noted that it was
unnecessary to isolate the free ligand 1, all coordination exper-
iments were conducted from the bis-aminophosphonium salt 5.
Thus, for the synthesis of the palladium complex 6, deprotonation
of 5 was carried out in THF, the potassium bromide salts were
removed and [PdCl2(PhCN)2] was added to the in situ generated
ligand 1. The solution turned immediately yellow and after one
night stirring at room temperature a brown solid precipitated from
the reaction mixture. This solid was dissolved in dichloromethane
to eliminate the insoluble potassium salt. After evaporation of
dichloromethane and washing of the residue with petroleum ether,
Synthesis of ligand 1 and its coordination to group 10 metal centres
The synthesis of the phosphasalen ligand 1 featuring an ethy-
lene diamine link was first attempted from the protected o-
diphenylphosphinophenol 2 (Scheme 2), which was prepared
following a literature procedure.11 The corresponding phospho-
nium bromide salt was reacted with ethylenediamine (0.5 equiv.)
in the presence of triethylamine (1 equiv.) to give the bis-
aminophosphonium salt 3 in 70% yield after aqueous work-up.
Nevertheless, as the following step involving deprotection of the
phenol with anhydrous HCl gas gave irreproducible results, we de-
cided to modify our strategy and performed first the deprotection
of the phenol functionality and then the Kirsanov reaction. O-
diphenylphosphinophenol 4 was obtained from 2 as decribed,11
its bromination followed by reaction with ethylenediamine in
the presence of tributylamine (1 equiv.) gave the expected bis-
aminophosphonium salt 5 in 63% yield (Scheme 2). Tributylamine
was used in place of triethylamine because of the solubility of its
ammonium hydrobromine salt in THF which enabled its easy re-
moval. 5 is only poorly soluble in chloroform but was characterised
1
1
by multinuclear NMR (31P, H, 13C) in CDCl3. In 31P{ H} spec-
troscopy the equivalent phosphorus atoms of 5 appear as a singlet
at d(CDCl3) = 39.3 ppm. In the 1H NMR spectrum, the resonances
of the bridging methylene protons appear as a doublet (3JPH = 10.5
Hz) with a chemical shift (d(CDCl3) = 3.43 ppm) similar to the one
observed for bis(phosphine-aminophosphonium) salts featuring
Scheme 3 Coordination of 1 to Pd(II) and Ni(II).
10030 | Dalton Trans., 2011, 40, 10029–10037
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the palladium complex 6 was obtained in 73% yield as a yellow
[Pd(salen)] (N1–C1–C2–N2: 55.76(29)◦ vs. 34.5◦). Concerning the
crystal packing no peculiarity was observed for 6 whereas the
molecules of the salen complex were axially packed because of
p-stacking interactions between the phenyl rings.
solid. It was characterised by multinuclear NMR spectrocopy and
1
elemental analysis. In 31P{ H} NMR, coordination is indicated by
a significant deshielding of the phosphorus nuclei appearing as
1
a singlet at d(CD2Cl2) = 33.7 ppm. In contrast, in the H NMR
A similar procedure was adopted to prepare the phos-
phasalen nickel complex 7 (Scheme 3). [NiBr2(DME)2] (DME:
dimethoxyethane) was added to an in situ generated potassium
bromide salt free THF solution of 1, inducing a rapid colour
change of the solution from pale yellow to deep purple. After
30 min a purple solid precipitated out of the reaction mixture,
the colour of which faded. This solid was filtered off and a
work-up similar to that described for 6 was carried out. This
allowed the isolation of 7 as a purple solid in 82% yield.
As for 6, no decomposition in air was noticed. This complex
was characterised by multinuclear spectroscopy and elemental
spectrum little change was seen from ligand 1 except the resonance
of the methylene protons which are shifted upfield (2.54 ppm vs.
3.42 ppm in 1).
Note that reactions performed with other palladium precursors
failed; with [Pd(PPh3)2Cl2] no coordination was observed whereas
with [Pd(COD)Cl2] the reaction was very slow. Importantly
whereas the free ligand is sensitive to moisture, the obtained
complex proved to be stable on the bench for weeks as already
shown for other iminophosphorane complexes.10a
Definitive evidence concerning the structure of 6 was provided
by X-ray crystal analysis of the complex. Suitable crystals
were obtained by diffusing petroleum ether into a saturated
dichloromethane solution of 6 at room temperature. A view of one
molecule of 6 is presented in Fig. 1. The complex crystallised with
four molecules of dichloromethane. As expected for a diamagnetic
d8 complex 6 adopts a square planar geometry around the metal
with almost no deviation from planarity (O2–N2–N1–O1 dihedral
angle 0.05(13)◦). The four coordination sites are occupied by the
two iminophosphorane and phenolate groups. The bond lengths
of the Pd–N and Pd–O were measured at 2.014(3), 2,017(3) and
1
analysis. Interestingly, the 31P{ H} NMR spectrum realised in
CD2Cl2 at room temperature exhibited a broad signal around
33 ppm whereas at -80 ◦C the signal sharpened to give a nice singlet
at d(CD2Cl2) = 32.8 ppm, confirming the coordination of the
1
iminophosphorane moieties. 1H and 13C{ H} NMR spectra were
recorded at room temperature on a concentrated CD2Cl2 solution.
The spectra were very similar to those of complex 6, except that
the resolution of peaks was lower. Definitive evidence concerning
the structure of 7 was brought by X-ray diffraction analysis (see
Fig. 2) realised on single crystals obtained by diffusing petroleum
ether into a saturated dichloromethane solution of 7. The complex
crystallised with one molecule of dichloromethane. In the solid
state, complex 7 adopts a nearly square planar geometry with a
small deviation from planarity (N2–N1–O1–O2 dihedral angle
at 3.64(35)◦). The Ni–O bonds are measured at 1.863(4) and
˚
2.029(2), 2.039(2) A respectively. Thus these bonds are longer than
the Pd–N and Pd–O distances (1.952(2), 1.957(2) and 1.9981(16),
˚
2.0087(16) A respectively) measured in the corresponding salen
complex.12 Due to the presence of two phenyl substituents on
each phosphorus atom, the molecule is also much less planar than
related [Pd(salen)]12, as an example the dihedral angle N1–C4–C3–
P1 was measured at 25.64(38)◦ vs. -3.02◦ in the imine derivative.
In the same manner the flexible ethylene diamine chain is also
much more largely deviated from the median plan in 6 than in
˚
˚
1.881(3) A while the Ni–N distances are at 1.888(4) and 1.905(4) A.
These bond lengths are on average longer than those recorded for
the [Ni(salen)] complex13 (1.853 and 1.851 A for the Ni–O bonds
˚
˚
and 1.851 and 1.849 A for the Ni–N bonds). As observed for
Fig. 1 ORTEP view of complex 6. Hydrogen atoms have been omitted for
Fig. 2 ORTEP view of complex 7. Hydrogen atoms have been omitted for
clarity. Thermal ellipsoids ◦are drawn at the 50% probability level. Selected
clarity. Thermal ellipsoids ◦are drawn at the 50% probability level. Selected
˚
˚
distances (A) and angles ( ): Pd1–N1 2.014(3), Pd1–N2 2.017(3), N1–P1
distances (A) and angles ( ): Ni1–N1 1.905(4), Ni1–N2 1.888(4), P1–N1
1.620(3), N2–P2 1.623(3), Pd1–O1 2.029(2), Pd1–O2 2.039(2), N1–Pd1–O1
96.6(1), N1–Pd1–N2 82.6(1), N2–Pd1–O2 96.1(1), O1–Pd1–O2 84.7(2),
N1–P1–C3 107.4(2), N2–P2–C21 106.6(2), N1–C1–C2–N2 55.76(29),
N1–C4–C3–P1 25.64(38), O2–N2–N1–O1 0.05(13).
1.613(4), P2–N2 1.617(4), Ni1–O1 1.881(3), Ni1–O2 1.863(4), O1–Ni1–N1
95.4(2), N2–Ni1–N1 85.4(2), O2–Ni1–N2 95.4(2), O2–Ni1–O1 83.9(2),
N1–P1–C3 108.2(2), N2–P2–C21 108.1(2), N2–C2–C1–N1 -45.00(56),
N1–C4–C3–P1 22.57(68), N2–N1–O1–O2 3.64(35).
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Table 1 Crystal data and structure refinement details for 6 and 7
Complex
6
7
Mol. formula
Mol. weight
Crystal habit
C38H32N2O2P2Pd,4(CH2Cl2)
1056.70
Orange block
C38H32N2NiO2P2,CH2Cl2
754.23
Purple needle
Crystal dimens./mm
Crystal system
Space group
0.28 ¥ 0.18 ¥ 0.16
0.32 ¥ 0.10 ¥ 0.04
Orthorhombic
Monoclinic
Pna21
23.441(1)
10.218(1)
19.401(1)
90.00
90.00
90.00
4646.9(6)
Pc
14.095(1)
9.040(1)
16.233(1)
90.00
122.384(4)
90.00
1746.7(3)
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
4
2
d/g cm-3
1.510
1.434
F(000)
2136
0.965
30.02
780
0.838
26.00
m/cm-1
qmax (deg.)
HKL ranges
No of measd/indep. rflns
No. reflections used
Rint
-33 31; -10 14; -27 20
44198/12183
10068
0.0323
multi-scan; 0.7738 min, 0.8608 max
515
19
0.0432/0.1104
0.38(2)
1.092
1.382(0.082)/-0.874(0.082)
804616
-13 17; -10 11; -19 20
9847/5021
4499
0.0244
multi-scan; 0.7753 min, 0.9672 max
433
10
0.0449/0.1219
0.22(16)
1.044
0.909(0.069)/-0.722(0.069)
804617
Abs. corr.
N◦ params refined
Rflns/param.
R1, wR2 (I > 2sI)
Flack’s parameter
GoF
-3
˚
difference peak/hole/e A
CCDC number
Table 2 Electrochemical data for phosphasalen complexes 6 and 7a
palladium complex 6, the overall geometry of the phosphasalen
complex is much less planar than the salen counterpart, thus
the dihedral angle N1–C4–C3–P2 was measured at 22.57(68)◦ vs.
3.30◦ on average in the imine derivative. Moreover, while molecules
of [Ni(salen)] complexes were axially packed either thanks to p-
stacking interactions between the phenyl rings when the carbon
of the imine function is unsubstituted14 or through weak H-bond
when this carbon bears a phenyl group,15 no particular packing
was seen for 7.
Complexes
7b
6d
Fc
EaP1, V
EcP1, V
0.65
0.51
0.58
1.00c
0.76
0.55
0.43
0.49
1.01
E1
1/2
ipa/ipc
EaP2, V
EcP2, V
1.10
1.03
1.06
1.12e
E1
1/2
ipa/ipc
In order to compare the electronic properties of phosphasalen
and salen complexes the electrochemical properties of 6 and
7 were examined by cyclic voltammetry in a mixture of di-
choloromethane/dimethylformamide (volume ratio 60/40) at
room temperature using a gold disk as a working electrode and a
Pt gauze as a counter electrode. The potentials measured relative
to SCE are reported in Table 2. For the sake of comparison with
literature data, the potential of ferrocene is also indicated.
For nickel complex 7, no reduction was observed within the
potential window (0 V, -2.0 V) considered, and a reversible one
electron oxidation took place at 0.58 V (DEp = 152 mV). Interest-
ingly for the salen complex oxidation takes place at E1/2(DMF) =
0.81 V, in conditions where oxidation of ferrocene occurs at
0.48 V.16 Thus, the oxidation potential of the phosphasalen nickel
complex 7 is only 0.1 V higher than that of ferrocene, whereas that
of the [Ni(salen)] derivative is 0.33 V higher compared to the same
reference. This clearly illustrated the better donating ability of the
phosphasalen ligand which renders the nickel centre more electron
rich and allows an easier oxidation. For the palladium complex
6, electrochemical studies were more difficult due to the more
limited solubility of this complex in presence of DMF, thus lower
a Tetrabutylammonium tetrafluoroborate salt as electrolyte (c = 0.12 mol
L-1); scan rate = 100 mV s-1; b c = 3.8 mmol L-1; c ratio was observed to
change with scan rate with a value of 1.46 at 50 mV s-1; d c = 2.0 mmol L-1;
e ratio was observed to change with scan rate with a value of 1.27 at 50 mV
s-1.
concentrations were employed. Nevertheless, no reduction was
observed within the potential range explored and two oxidation
waves were seen. The first one is irreversible at 0.76 V and the
second one at 1.06 V which is pseudo-reversible (Ipc/Ipa ~ 1.12) in
the presence of a coordinating solvent. Thus, the first oxidation of
the palladium complex 6 occurred at a higher potential compared
to the nickel counterpart 7 which implies a less electron rich
metal centre. Nevertheless a second oxidation can only be seen
in the case of the palladium derivative which may be related
to the increase of the thermodynamic stability of the highest
oxidation states of elements within the row Ni–Pd–Pt.17 Indeed,
for the [Pd(salen)] complex two irreversible oxidation waves were
observed18 at 0.96 and 1.07 V in acetonitrile namely 0.44 and
0.55 V higher than the oxidation potential of ferrocene in the
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Table 3 Comparison of selected geometric parameters coming from X-ray diffraction analysis and DFT calculations
Optimised singlet
structure
Optimised triplet
structure
Optimised singlet
structure
Optimised triplet
structure
X-ray
X-ray
N2–Ni1
N1–Ni1
Ni–O2
Ni–O1
O2–Ni–N2
O1–Ni–N1
O1–Ni–O2
N1–Ni–N2
N1–Ni–N2–O2
N2–Ni–N1–O1
O2–Ni–O1–N1
O1–Ni–O2–N2
1.851(3)
1.849(3)
1.853(3)
1.851(5)
94.1(4)
94.2(4)
85.5(4)
86.3(5)
178.5
1.857
1.856
1.847
1.848
94.31
93.99
85.4
1.986
1.989
1.900
1.899
91.6
91.5
105.2
82.9
153.4
151.4
158.5
156.1
N2–Ni1
N1–Ni1
Ni–O2
Ni–O1
O2–Ni–N2
O1–Ni–N1
O1–Ni–O2
N1–Ni–N2
N2–Ni–N1–O1
N1–Ni–N2–O2
O1–Ni–O2–N2
O2–Ni–O1–N1
1.888(4)
1.905(4)
1.863(4)
1,881(3)
95.4(2)
95.4(2)
83.9(2)
1.897
1.894
1.865
1.883
94.7
95.1
84.4
1.966
2.011
1.914
1.953
97.5
96.3
101.1
85.8
127.2
155.1
130.6
162.0
86.4
85.4(2)
85.8
177.5
177.8
177.5
177.8
176.40(35)
178.68(39)
176.38(42)
178.64(39)
179.4
178.8
179.4
178.8
178.6
178.7
178.5
same conditions (0.518 V). The first oxidation is thus easier
for the phosphasalen derivative (only 0.27 V higher than the
ferrocene oxidation potential) in agreement with the presence of
a better electron-donating ligand. The second oxidation occurs
on the contrary at a comparable potential for the salen derivative
and the phosphasalen one (namely 0.55 and 0.58 V higher than
E11/2(Fc+/Fc)), but as already established this oxidation in the
[Pd(salen)] complex corresponds to the electron transfer from a
molecular orbital localised predominantly on the ligand and not
on the metal centre.18a,19
Contrary to the palladium phosphalen complex which is
undoubtedly diamagnetic giving finely resolved NMR spectra,
the difficulty to obtain nicely resolved spectra for the nickel
derivative at room temperature prompted us to further investigate
this complex. Indeed, we wondered if at room temperature the
phosphasalen nickel complex 7 would exist under two different
spin states (a singlet and a triplet one, the latter perturbing the
NMR measurements). Therefore, we also measured the magnetic
moment of phosphasalen complex 7 by the Evans method.20 At
room temperature a magnetic moment of 1.40 mB was measured.
This value is very low compared to literature values of 3.0–4.0 mB
expected for fully tetrahedral Ni(II) complexes.21 but values around
1.3 mB were reported by Braunstein and coworkers for Ni(II)
complexes22 adopting a distorted tetrahedral geometry in solution.
Therefore the magnetic moment measured in solution for 7 may
suggest a distorted geometry (the rigidity of the phosphasalen
ligand should exclude a tetrahedral geometry) and/or that the
triplet state is partially populated (the NMR is not silent at room
temperature).23
nickel complex 7 with the Gaussian 03 set of programs24 in
combination with the B3PW91 functional25 (see experimental
section for further details). Optimizations of both complexes 7 and
[Ni(salen)] were performed in the singlet and triplet states. Table
3 gathers some selected geometric parameters for each optimised
structure together with the available experimental data from X-ray
diffraction analysis.13 On the whole there is an excellent agreement
between the experimental structure and the theoretical parameters
optimised for the singlet state.
A nearly square planar geometry was found for both complexes
with dih◦edral angles around the metal centre lying between 176
˚
and 179 . Mean deviations of 0.01 A were found for the bond
lengths and 1◦ for the bond angles.
In both complexes, the HOMO involves a d orbital on the metal
centre in p antibonding interaction with the oxygen and nitrogen
ligands (Fig. 3). However the HOMO of the phosphasalen
complex is significantly higher in energy than that of the salen
complex (-4.56 instead of -5.12 eV). As a consequence the
vertical ionisation energy calculated for the phosphasalen complex
(5.73 eV) is lower than that for the salen complex (6.49 eV), a result
in agreement with the easier oxidation found experimentally for
the former.
Two major geometrical changes were found going from the
singlet to the triplet state. First there is a distortion from the
square planar geometry toward a distorted tetrahedral geometry.
This change is illustrated by the dihedral angles given in Table 3
which vary from nearly 180◦ (the value for an ideal square planar
arrangement) in the singlet state to values between 162 and 127◦
(instead of 120◦ for an ideal tetrahedral structure) in the triplet
state. Such a rearrangement was expected for the low spin-high
spin change in complexes of d8-ML4 type. Note however that the
geometrical evolution is larger for the more flexible phosphasalen
than for the salen complex (mean value of 143.7◦ for the dihedral
angles instead of 154.9◦ for the latter). On the other hand, the
metal–ligand bond distances were found to be significantly longer
As such an observation was not made for salen Ni(II) complexes
either we undertook a theoretical study to compare the electronic
properties of the [Ni(salen)] and [Ni(phosphasalen)] complexes.
Theoretical study of Ni(II) phosphasalen and salen complexes
˚
in the triplet than in the singlet state, by 0.094 and 0.076 A in the
Density functional theory (DFT) calculations were performed on
salen and the phosphasalen complexes, respectively (mean values).
(N,N¢-ethylene-bis(salicylideneiminato))nickel and phosphasalen
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Fig. 3 HOMO of [Ni(salen)] (a) and [Ni(phosphasalen)] (b) complexes
given by DFT calculations.
Fig. 4 Antibonding d orbital involved in the singlet triplet excitation for
the salen (a) and phosphasalen (b) complex.
Formation of the triplet state in the square planar singlet state
involves first removing one electron of the HOMO. According to
the metal–ligand p antibonding character of this orbital (see Fig.
3), it should shorten the metal–ligand bond distances. However
the electron left from the HOMO now occupies an orbital which
was vacant in the singlet state, the fifth d orbital of these d8
complexes. Since it is strongly s antibonding between the metal
and the ligands (Fig. 4), its occupation entails a lengthening of the
metal–ligand bonds. So two factors work in opposite directions but
the latter dominates (the s antibonding interactions are stronger
than the p ones). A lengthening of the metal–ligand distances thus
occurs on going to the high spin state, a trend which is preserved
upon distortion toward a pseudo-tetrahedral arrangement (see
Table 3).
Scheme 4 Thermodynamical cycle to determine the singlet–triplet energy
difference (DEST).
From an energetic point of view the singlet state was found to
be 9.0 kcal mol-1 more stable than the triplet state for the salen
complex in agreement with the experimental evidence for a low
spin complex in solution.26 A strikingly different situation was
found for the phosphasalen complex with a triplet state slightly
more stable than the singlet state (DE = -1.4 kcal mol-1). A
triplet ground state in solution as well as a small singlet–triplet
energy difference (which suggests that both spin configurations
could coexist in solution), are in agreement with the experimental
measurement of the magnetic moment of complex 7. In order to
rationalise the evolution of the singlet–triplet energy difference
(DEST), we made use of the following thermodynamical cycle
(Scheme 4): i) starting from the optimised square planar singlet
state (1[SP]), one electron is removed from the HOMO keeping
the geometry of the complex unchanged (→ [SP]+); the energy
associated with this first step (DE1) is the vertical ionisation
energy of the complex; ii) the removed electron is placed in the
antibonding d orbital depicted in Fig. 4 leading to the triplet state
3
in the square planar geometry (→ [SP] with an energy change of
DE2); iii) finally, the geometry is relaxed to reach the equilibrium
geometry for the triplet state (→ distorted tetrahedral structure,
3[“Td”] with an energy change of DE3).
The computed energy changes associated with each step as well
as the total energy change (DEST) are reported in Table 4 for both
salen and phosphasalen complexes. It clearly appears that the
electronic factor responsible for the lowering of the DEST energy
difference between the singlet and the triplet states (by 10.4 kcal
mol-1) is the major lowering (by 17.5 kcal mol-1) of the ionisation
energy in going from the salen to the phosphasalen complex (see
third line in Table 4).
10034 | Dalton Trans., 2011, 40, 10029–10037
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Table 4 Energy decomposition (kcal mol-1), according to Scheme 4, of
(cm-1) using for the intensity the following abbreviations: w (weak),
m (medium), s (strong). Electronic ionization mass spectra (EI-
MS) were recorded with a JEOL GCmate instrument. Elemental
analyses were performed by the elemental analysis service of the
London Metropolitan University (United Kingdom).
the singlet–triplet energy difference for salen and phosphasalen complexes
DE
DE2
DE3
DEST
[Ni(salen)]
[Ni(phosphasalen)]
DE([Ni(salen)]-[Ni(phosphasalen)]) -17.5
149.6
132.1
-129.6 -11.0 +9.0
-122.0 -11.5 -1.4
+7.0
-0.5
-10.4
Synthesis of 5. Br2 (213 mL, 4.14 mmol) was added to a
solution of o-diphenylphosphinophenol 4 (1.153 g, 4.14 mmol)
in dichloromethane (30 mL) at -78 ◦C. The cold bath was
then removed. After one hour stirring at room temperature,
the resulting pale yellow solution was cooled down to -78 ◦C.
Tributylamine (987 mL, 4.14 mmol) and then ethylenediamine
(138.5 mL, 2.07 mmol) were added to this solution. Stirring was
continued for 2 days at room temperature to yield a yellow solution
and a white precipitate. Dichloromethane was evaporated under
vacuum. THF (30 mL) was introduced resulting in the formation
of a heavy white slurry. The white solid was separated by filtration
under N2, washed with THF (4 ¥ 10 mL), and dried under vacuum
(1.060 g, 63%).
Conclusions
In conclusion we have presented the synthesis of a novel tetraden-
tate ligand combining iminophosphoranes and phenolates desig-
nated as phosphasalen since it can be viewed as a phosphorous
analog of a salen derivative. The coordination of this ligand to
Pd(II) and Ni(II) metal centres gave stable neutral nearly square
planar complexes, which were characterised by NMR spectrocopy,
elemental analyses and X-ray diffraction analysis. In order to
shed light on the electronic differences between the salen and
phosphasalen complexes, cyclic voltammetry experiments were
realised. As anticipated considering the better donating ability
of the iminophosphorane based ligands, oxidation was easier
for the phosphorus contained complexes compared to the salen
ones. Moreover as the [Ni(phosphasalen)] complex 7 displayed
a magnetic moment measured at 1.40 mB suggesting a low lying
triplet state, we carried out a comparative DFT study of [Ni(salen)]
and [Ni(phosphasalen)] to shed light on the differences observed
for these two complexes. Calculations confirm a small singlet–
triplet energy difference (1.4 kcal mol-1) in favour of the triplet
state. This divergent behaviour for carbon and phosphorus based
complexes was mainly traced to an easier ionisation of the
phosphasalen complex. We are now pursuing a coordination
study of phosphasalen ligands. They are currently applied in
homogeneous catalysis and results on this topic will be reported
in due course. The synthesis of chiral derivatives is also currently
explored.
1
1
31P{ H} NMR (CDCl3): d 39.3 (s, PV); H NMR (CDCl3): d
7.67–7.59 (12H, m, p-CH(PPh2), o-CH(PPh2)), 7.52–7.46 (12H,
m, m-CH(PPh2), CaH, CbH), 6.86–6.75 (4H, m, CcH, NH), 6.57
(2H, dd, 3JH,H = 8.0 Hz, 3JP,H = 14.5 Hz, CdH), 3.43 (4H, d, 3JP,H
=
¯
10.5 Hz, CH2); 13C{ H} NMR (CDCl3): d 159.7 (d, 2JP,C = 0.5 Hz,
1
OCIV), 136.1 (d, JP,C = 1.0 Hz, CbH), 133.3 (d, JP,C = 11.5 Hz,
4
2
Cd H), 133.1 (d, 4JP,C = 3.0 Hz, p-CH(PPh2)), 131.9 (d, 2JP,C = 11.5
3
Hz, o-CH(PPh2)), 128.5 (d, JP,C = 13.5 Hz, m-CH(PPh2)), 121.4
(d, JP,C = 106.5 Hz, CIV(PPh2)), 119.6 (d, JP,C = 13.5 Hz, CcH),
1
3
117.7 (d, 3JP,C = 7.0 Hz, CaH), 106.8 (d, 2JP,C = 104.0 Hz, PPh2CIV),
43.3 (d, JP,C = 1.5 Hz, CH2). HR-EI-MS: 307.1132 ([M-2Br]2+;
2
C38H36N2O2P22+; calcd 307.1126.
Synthesis of 1. KHMDS (1.165 g, 5.84 mmol) was added to
a suspension of 5 (1.130 g, 1.46 mmol) in THF (50 mL). After
overnight stirring, potassium bromide was removed by filtration,
giving a colorless solution. THF was evaporated in vacuo to yield a
white solid, which was washed twice with petroleum ether (50 mL)
giving 1 (0.992 g, 99%).
Experimental section
1
1
31P{ H} NMR (THF-D8): d 18.6 (s, PV); H NMR (THF-d8):
5
d 7.73–7.67 (8H, m, o-CH(PPh2)), 7.62–7.57 (4H, m, JP,H = 1.5
Synthesis
Hz, p-CH(PPh2)), 7.55–7.49 (8H, m, m-CH(PPh2), 7.09 (2H, ddd,
3JH,H = 8.5 Hz, 3JH,H = 7.0 Hz, 4JH,H = 1.5 Hz, CbH), 6.60 (2H, vt,
3JH,H = 8.5 Hz, 4JP,H = 8.5 Hz, CaH), 6.47 (2H, ddd,3JH,H = 7.0 Hz,
General considerations. All experiments, unless otherwise
stated, were performed under an atmosphere of dry nitro-
gen or argon using standard schlenk and glove box tech-
niques. Solvents were freshly distilled under dry nitrogen
from Na/benzophenone (THF, diethylether, petroleum ether),
from P2O5 (dichloromethane). O-diphenylphosphinophenol 4,11
[NiBr2(DME)]27 and [PdCl2(PhCN)2]28 were prepared according
to the literature. All other reagents and chemicals were obtained
commercially and used without further purification. Nuclear
magnetic resonance spectra were recorded on a Bruker Avance
4
3
3JP,H = 16.0 Hz, JH,H = 1.5 Hz, Cd H), 6.05 (2H, vtd, JH,H = 7.0
Hz, 4JP,H = 3.0 Hz, CcH), 3.42 (4H, dd, 3JP,H = 6.0 Hz, 3JP¢,H = 6.5
1
2
Hz, CH2); 13C{ H} NMR (THF-d8): d 174.3 (d, JP,C = 3.5 Hz,
OCIV), 132.9 (d, JP,C = 13.0 Hz, Cd H), 132.8 (d, JP,C = 77.0 Hz,
2
1
CIV(PPh2)), 132.1 (d, JP,C = 1.5 Hz, CbH), 131.7 (d, JP,C = 8.0
Hz, o-CH(PPh2)), 129.0 (d, 4JP,C = 2.0 Hz, p-CH(PPh2)), 127.0 (d,
3JP,C = 10.5 Hz, m-CH(PPh2)), 121.8 (d, 3JP,C = 9.0 Hz, CaH), 112.5
(d, 1JP,C = 128.5 Hz, PPh2CIV), 107.0 (d, 3JP,C = 15.0 Hz, CcH), 50.4
(dd, 2JP,C = 27.0 Hz, 3JP,C = 8.0 Hz, CH2); IR (cm-1): 1588 (m), 1535
(w), 1469 (s), 1455 (m), 1444 (m), 1349 (m), 1255 (m), 1136 (s),
1104 (m), 1060 (m), 1031 (w), 991 (w), 846 (m), 813 (w), 788 (w),
762 (s), 736 (m), 707 (s), 675 (w). Anal. Calcd for C38H32K2N2O2P2:
C, 66.26; H, 4.68; N, 4.07. Found: C, 66.19; H, 4.73; N, 3.99.
4
2
1
300 spectrometer operating at 300 MHz for H, 75.5 MHz for
13C and 121.5 MHz for 31P. Solvent peaks were used as internal
references for 1H and 13C chemical shifts (ppm). 31P are relative to a
85%H3PO4 external reference. Coupling constants are expressed in
hertz. The following abbreviations are used: br, broad; s, singlet; d,
doublet; dd, doublet of doublets; t, triplet; m, multiplet; v, virtual.
The labeling schemes of the phosphasalen derivatives are given
in Schemes 1 and 2. IR spectra were recorded on a Perkin Elmer
spectrometer and are reported in terms of frequency of absorption
Synthesis of 6. THF (5 mL) was added to a mixture of
KHMDS (94.3 mg, 0.47 mmol) and 5 (91.4 mg, 0.12 mmol).
After 2 h, potassium salt was removed by centrifugation and
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[PdCl2(PhCN)2] (45.3 mg, 0.12 mmol) was added to the obtained
colorless solution, which became immediately yellow. After 16
h, the precipitated brown solid was separated by centrifugation,
dissolved in dichloromethane to remove the insoluble potassium
salt. Then the solvent was evaporated and the obtained residue
was washed with petroleum ether (5 mL) to give 6 as a yellow solid
(63 mg, 73%)
The crystal structure were solved using SIR 9729 and Shelxl-97.30
ORTEP drawings were made using ORTEP III for Windows.31
Electrochemical study
The electrochemical experiments were performed using a Vestastat
potentiostat/galvanostat with a three-electrode cell using a Au
disk as working electrode, a Pt gauze as the counter electrode,
and a saturated calomel electrode as the reference electrode.
Measurements were made in a CH2Cl2/DMF solvent mixture
(60/40) with a concentration of 3.8 mmol L-1 and 2.0 mmol L-1
for the nickel and palladium complexes respectively. Tetrabutylam-
monium tetrafluoroborate salt served as electrolyte (concentration
0.12 mol L-1). Potentials were recorded at 50 mV s-1.
1
31P{ H} NMR (CD2Cl2): d 33.7 (s, PV); 1H NMR (CD2Cl2): d
7.55–7.46 (12H, m, o-CH(PPh2), p-CH(PPh2)), 7.44–7.38 (8H, m,
m-CH(PPh2)), 7.07 (2H, vtdd, 3JH,H = 7.0 Hz, 3JH,H = 8.5 Hz,4JH,H
1.5 Hz, 5JP,H = 0.5 Hz, CbH), 6.78 (2H, ddd, 3JH,H = 8.5 Hz, 4JH,H
1.0 Hz, 4JP,H = 6.5 Hz, CaH), 6.32 (2H, ddd, 3JH,H = 8.0 Hz, 4JH,H
=
=
=
3
3
1.5 Hz, JP,H = 16.0 Hz, CdH), 6.18 (2H, dddd, JH,H = 8.0 Hz,
3JH,H = 7.0 Hz, 4JH,H = 1.0 Hz, 4JP,H = 3.0 Hz, CcH), 2.54 (4H, vt,
1
JP,H = 4.5 Hz, CH2); 13C{ H} NMR (CD2Cl2): d 172.5 (s, OCIV-),
2
Computational details
134.9 (br. s, CbH); 134.2 (d, JP,C = 10.0 Hz, o-CH(PPh2)), 133.3
4
(br. s, JP,C = 1.0 Hz, p-CH(PPh2)), 133.1 (2JP,C = 13.0 Hz, CdH),
DFT calculations were performed with the Gaussian 03 series of
programs24 with the B3PW91 functional.25 Nickel was represented
by the relativistic effective core potential of Hay and Wadt32a
and the associated triple zeta quality basis set,32b augmented by
a f polarization function as proposed by Frenking32c. All non-
metallic atoms were described with the 6-31G* Pople basis set. The
optimised structures were characterised by vibration frequencies
calculations.
129.7 (d, 3JP,C = 12.0 Hz, m-CH(PPh2)), 127.2 (d, 1JP,C = 89.0 Hz,
3
3
CIV(PPh2)), 122.8 (d, JP,C = 8.0 Hz, CaH), 114.0 (d, JP,C = 15.0
Hz, CcH), 110.4 (d, JP,C = 122.0 Hz, PPh2CIV), 53.8 (dd, JP,C
=
1
2
17.5 Hz, 3JP,C = 5.0 Hz, CH2); IR (cm-1): 1586 (m), 1534 (w), 1459
(m, nCC aromatic), 1442 (s), 1327 (m), 1257 (m), 1124 (w), 1110(s),
1084(w), 1069(w), 1020 (w), 1000 (w), 867 (m, b), 800 (m), 769 (w),
747 (s), 722 (w), 710 (w), 693 (s). Anal. Calcd for C38H32N2O2P2Pd:
C, 63.65; H, 4.50; N, 3.91. Found: C, 63.74; H, 4.30; N, 3.94.
Synthesis of 7. THF (5 mL) was added to a mixture of
KHMDS (94.3 mg, 0.47 mmol) and 5 (91.4 mg, 0.12 mmol).
After 2 h, potassium salt was removed by centrifugation and
[NiBr2(DME)] (36.4 mg, 0.12 mmol) was added to the obtained
colorless solution which became immediately deep purple. After
30 min, a purple solid precipitated out from the mixture and
the color of the solution faded. The solid was separated by
centrifugation, and then dissolved in dichloromethane to remove
the insoluble potassium salt. Evaporation of the solvent residue
and washing with petroleum ether (5 mL) gave 7 as a purple solid
(65 mg, 82%).
Acknowledgements
The CNRS, the Ecole Polytechnique and the IDRIS (for computer
time, Project No. 91616) are thanked for supporting this work.
Notes and references
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31P{ H} NMR (CD2Cl2): d 32.8 (s, PV); 1H NMR (CD2Cl2): d
7.76–7.66 (12H, m, o-CH(PPh2), p-CH(PPh2)), 7.62–7.56 (8H, m,
m-CH(PPh2), 7.10 (2H, vtd, 3JH,H = 7.5 Hz, 4JH,H = 1.5 Hz, CbH),
6.69 (2H, dd, 3JH,H = 7.5 Hz, 3JP,H = 6.5 Hz, CaH), 6.49 (2H, ddd,
4
3
3JH,H = 7.5 Hz, JH,H = 1.5 Hz, JP,H = 15.0 Hz, CdH), 6.26 (2H,
vtd, 3JH,H = 7.5 Hz, 4JP,H = 3.0 Hz, CcH), 2.16–2.13 (4H, m, CH2);
13C{ H} NMR (CD2Cl2): d 170.1 (s, OCIV), 133.3 (d, JP,C = 1.0
1
4
Hz, CbH), 133.1 (d, 2JP,C = 10.0 Hz, o-CH(PPh2)), 132.3 (d, 4JP,C
2.0 Hz, p-CH(PPh2)), 131.2 (2JP,C = 12.0 Hz, Cd H), 128.9 (d, 3JP,C
=
=
12.0 Hz, m-CH(PPh2)), 127.4 (d, 1JP,C = 90.5 Hz, CIV(PPh2)), 122.7
(d, 3JP,C = 8.0 Hz, CaH), 113.1 (d, 3JP,C = 14.5 Hz, CcH), 108.0 (d,
1JP,C = 119.6 Hz, PPh2-CIV), 50.0 (dd, JP,C = 16.0 Hz, JP,C = 2.0
Hz, CH2); IR (cm-1): 1586 (m), 1534 (w), 1459 (m), 1440 (s), (1300
m), 1275 (m), 1137 (w), 1116(s), 1098(m), 1081(m), 1027 (m), 1000
(w), 889 (w), 867 (w), 861(w), 811 (m), 778 (w), 752 (s), 727 (w),
710 (w), 697 (s). Anal. Calcd for C38H32N2NiO2P2: C, 68.19; H,
4.82; N, 4.19. Found: C, 68.00; H, 4.74; N, 4.05.
2
3
X-ray crystallography
Data were collected at 150 K on a Kappa APEX II diffractometer
˚
using a Mo-Ka (l = 0.71069 A) X-ray source and a graphite
monochromator. Experimental details are described in Table 1
10036 | Dalton Trans., 2011, 40, 10029–10037
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View Article Online
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