Palladium iminophosphorane complexes: the pre-cursors to the missing link in triphenylphosphane chalcogenide metallacycles

Article abstract of DOI:10.1039/c8dt03062f

Herein we report on the synthesis, characterization and the ensuing chemistry of iminophosphorane palladacycles. Treatment of Ph₃P═N-(2-OHC₆H₄), 1, with sodium tetrachloropalladate gives 2 with the ligand as terdentate [C,N,O] allowing for only one μ-Cl ligand bonding the metal centers, resulting in a dinuclear complex. Treatment of 2 with PPh₃ gives the mononuclear complex 3, whereas the reaction of 2 with diphosphanes Ph₂P(CH₂)_nPPh₂ in 1?:?2 ratio gives mixtures of 4 and 5 (n = 2) and 6 and 7 (n = 3). From them, the mononuclear complexes 4 and 6, and the dinuclear compounds, 5 and 7, were obtained with the parent ligand as bidentate [C,N]. The former two are of zwitterionic nature void of any counterion, with the phosphane ligand in the chelating mode. In a remarkable case of chemical serendipity, a solution of 2 left to stand produced crystals of complex 8: this is the missing link in the series of triphenylphosphane chalcogenide metallacycles. The experiment is repeatable; however, direct metallation of triphenylphosphane oxide was not possible.

Full text of DOI:10.1039/c8dt03062f

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PAPER
Palladium iminophosphorane complexes: the pre-
cursors to the missing link in triphenylphosphane
chalcogenide metallacycles†
Cite this: Dalton Trans., 2018, 47,
15801
Adolfo Fernández-Figueiras,a Fátima Lucio-Martínez, a Paula Munín-Cruz,a
Paula Polo-Ces,a Francisco Reigosa,a Harry Adams,b M. Teresa Pereira *a and
a
José M. Vila
*
Herein we report on the synthesis, characterization and the ensuing chemistry of iminophosphorane
palladacycles. Treatment of Ph3PvN-(2-OHC6H4), 1, with sodium tetrachloropalladate gives 2 with the
ligand as terdentate [C,N,O] allowing for only one μ-Cl ligand bonding the metal centers, resulting in a
dinuclear complex. Treatment of 2 with PPh3 gives the mononuclear complex 3, whereas the reaction of
2 with diphosphanes Ph2P(CH2)nPPh2 in 1 : 2 ratio gives mixtures of 4 and 5 (n = 2) and 6 and 7 (n = 3).
From them, the mononuclear complexes 4 and 6, and the dinuclear compounds, 5 and 7, were obtained
with the parent ligand as bidentate [C,N]. The former two are of zwitterionic nature void of any counter-
ion, with the phosphane ligand in the chelating mode. In a remarkable case of chemical serendipity, a
solution of 2 left to stand produced crystals of complex 8: this is the missing link in the series of triphenyl-
phosphane chalcogenide metallacycles. The experiment is repeatable; however, direct metallation of tri-
phenylphosphane oxide was not possible.
Received 26th July 2018,
Accepted 11th October 2018
DOI: 10.1039/c8dt03062f
rsc.li/dalton
readily undergo the cyclopalladation reaction.16–20 The biologi-
cal applications and anticancer properties of Pd(II), Pt(II) and
Introduction
Palladacycles are an amply studied branch of organometallics Au(III) cyclometallated iminophosphoranes, and of the analo-
due, to a large extent, to their quite abundant applications: as gous coordination species, have been studied due to the non-
pre-catalysts in cross-coupling reactions, such as Suzuki– toxic character of the ligands.21–23 A further advantage is that
Miyaura,1–3 Mizoroki–Heck,4,5 Negishi6 or Sonogashira7 and as the phosphorus atom in the PR3 fragment can be used as a
rather powerful anticancer agents,8,9 at times comparable to cis- “spectroscopic marker” to study the in vitro stability and the
platin;10 likewise, some exhibit very interesting luminescence oxidation state by 31P–{1H} NMR spectroscopy.24 Moreover,
properties11 or, alternatively, they behave as metallomesogens.12 their luminescence properties make them potentially useful as
A particular case of palladacycles are those that stem from molecular probes in theragnosis,25 and also they serve as pre-
the iminophosphoranes R3PvNR. These are organic sub- catalysts in numerous catalytic processes.26–32
strates which themselves exhibit a wide range of relevant appli-
Following our research on palladacycles more recently we
cations inclusive of their use as superbases,13 as synthetic have come upon new findings when employing terdentate
intermediates, in particular for the Aza-Wittig reaction,14 or as [C,N,S] and tetradentate [C,N:C,N] ligands, namely the auto-
building blocks for P–N back-bone polymers.15 The highly catalysis in the Suzuki–Miyaura reaction,3 the coordination of
polarized PvN double bond, bearing a partial negative charge the bulky potassium cation by the small crown ether rings,33
on the nitrogen atom, shows a strong σ-donor behavior with and the chelate-to-bridging shift in diphosphane pallada-
minor π-acceptor properties enabling them as ligands which cycles,34 all of which have contributed to expanding the
frontiers of this ever growing field. Then, we became interested
in studying related systems with terdentate [C,N,O] ligands
aDepartamento de Química Inorgánica, Universidade de Santiago de Compostela,
Avenida das Ciencias s/n, 15782 Santiago de Compostela, Spain
bDepartment of Chemistry, The University of Sheffield, Sheffield S3 7HF, UK
†Electronic supplementary information (ESI) available: X-ray tables. CCDC
1855707 (8), 1855708 (3), 1855709 (2), 1855710 (7) and 1855711 (4). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt03062f
albeit exchanging the CvN for the PvN double bond, in view
of the interesting properties of the R3PvNR ligands. The
immersion in the chemistry of iminophosphoranes was to
seek new trends in their reactivity and/or structural features, in
order to further extend the scope of this chemistry. The
ensuing results are presented herein.
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To address this study, we have used phosphanes as ancillary
ligands, which readily react with the synthesized terdentate
[C,N,O] iminophosphorane palladacycle in a bridging and
chelating fashion. Herein we give an account of the novel
species encountered, namely two zwitterionic mononuclear
complexes with chelating diphosphane. Furthermore, we also
describe a remarkable case of chemical serendipity that
provided suitable crystals which gave rise to the molecular
structure of the first cyclopalladated triphenylphosphane
oxide, putting an end to the incomplete series of the triphenyl-
phosphane chalcogenide metallacycles. Attempts to make the
latter compound by direct palladation of the phosphane oxide
were to no avail; however the preparation could be repeated by
way of this newly found experiment.
Results and discussion
For the convenience of the reader, the compounds and reac-
tions are shown in Schemes 1 and 2. The compounds
described in this paper were characterised by elemental ana-
Scheme 2 Decomposition reaction of 2 to give 8.
1
lysis (C, H, N), IR and H and 31P–{1H} spectroscopy (see the
the sp2 carbon atoms available for metallation are on the phos-
phorus phenyl rings, giving the endo type species. Thus, treat-
ment of 1 with Na2PdCl4 and NaAcO in refluxing methanol
gave the dinuclear palladacycle 2 as an air-stable yellow solid,
which was fully characterized, in contrast to the tetranuclear
species described by us when using the related terdentate
[C,N,O] with CvN double bonds.36 In the IR spectrum of the
complex, the ν(PvN) stretching band (see the Experimental
section) appeared at lower frequency than the corresponding
one in the free ligand in accordance with nitrogen coordi-
nation to the metal center. In the 1H NMR spectrum, four
additional multiplets appear upon metallation of one of the
phosphorus phenyl rings. The signal for the phosphorus
nucleus in the 31P–{1H} NMR spectrum was downfield shifted
ca. 38 ppm after palladium–nitrogen coordination. Complex 2
consists of two cyclopalladated fragments bonded through a
single bridging chloride ligand. This situation is relatively
unusual in palladacycle chemistry where more often than not
the μ-Cl ligand is accompanied by a bidentate ligand or by
another bridging chloride. Yet however infrequent, a few
examples have been reported in the literature. These are
limited to either palladacycles or to other cyclometallated com-
pounds of cationic37,38 or anionic39 nature, which need the
corresponding counterion for electrical neutrality. Because
palladation proceeds with deprotonation of the OH groups,
and in the absence of any counterion, we tentatively suggest
that the extra negative charge is compensated by H shared
between the two oxygen atoms through hydrogen bonding, to
produce a charge–neutral palladium compound. Further data
regarding this issue are discussed below in the description of
the molecular structure for 2.
Experimental section) and, in part, by X-ray single crystal
diffraction.
Ligand 1 was readily synthesized by the Staudinger reac-
tion35 from triphenylphosphane and 2-hydroxyphenyl azide.
As opposed to the ligands described by Urriolabeitia et al.,18
Suitable crystals of 2 were grown by slowly evaporating an
acetone solution of the compound. Crystal data are given in
the ESI.† The ORTEP illustration of complex 2 is shown in
Scheme 1 Reaction sequence leading to the synthesis of the imino-
phosphorane palladacycles.
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complex.40 Therefore, the unique feature of a dinuclear single-
bridged chloride palladacycle structure is made possible by
the bridging O–H–O unit, whose asymmetry precludes the con-
sideration of two totally equivalent palladacycle centers.
Furthermore, DFT calculations were performed for compound
2, which showed the presence of a strong intramolecular
hydrogen bond between the O(2)–H hydrogen and the O(1)
atom (see the ESI†).
Treatment of 2 with triphenylphosphane gave the neutral
mononuclear compound 3 as a yellow air-stable solid, which
was fully characterized; its reactivity pattern resembles that of
the related dinuclear41 or tetranuclear42 palladacycles.
The IR spectrum showed the shift of the ν(PvN) stretch
from 1313 cm−1 to 1280 cm−1. Two doublets in the 31P–{1H}
NMR spectrum were assigned to the two distinct 31P nuclei at
δ 32.38 (PPh3) and δ 37.91 (PvN) with 3J(PP) 3.9 Hz.
Crystals of compound 3 were obtained by slow evaporation
from an acetone solution. Crystal data are given in the ESI.†
The ORTEP view of 3 is shown in Fig. 3. The compound crys-
talizes with an acetone solvent molecule (omitted for clarity
reasons). The structure is quite similar to that of any one of
the palladated subunits which comprise the structure of com-
Fig. 1 Thermal ellipsoid plot for 2 shown at the 50% probability level.
Hydrogen atoms have been omitted for clarity, except H–O(2). Selected
bond distances (Å) and angles (°) for 2: Pd(1)–C(25) 1.960(3), Pd(1)–N(1)
2.010(3), Pd(1)–O(1) 2.116(2), Pd(1)–Cl(1) 2.3373(9), Pd(2)–C(26) 1.963(3),
Pd(2)–N(2) 1.993(3), Pd(2)–O(2) 2.123(2), Pd(2)–Cl(1) 2.3421(9), P(1)–N(1)
1.619(3), P(2)–N(2) 1.611(3), C(25)–Pd(1)–N(1) 88.71(14), C(25)–Pd(1)–
(O1) 169.85(12), N(1)–Pd(1)–O(1) 81.26(11), C(25)–Pd(1)–Cl(1) 95.12(11),
N(1)–Pd(1)–Cl(1) 176.17(9), O(1)–Pd(1)–Cl(1) 94.91(7), C(26)–Pd(2)–N(2)
88.14(14), C(26)–Pd(2)–O(2) 168.60(13), N(2)–Pd(2)–O(2) 81.36(11),
C(26)–Pd(2)–Cl(1) 95.81(12), N(2)–Pd(2)–Cl(1) 174.31(9), O(2)–Pd(2)–Cl
(1) 94.98(7), Pd(1)–Cl(1)–Pd(2) 93.61(3).
Fig. 1. The crystals consist of discrete molecules separated by pound 2, albeit with a triphenylphosphane ligand bonded to
normal van der Waals distances, bearing two slightly distorted the palladium atom in place of the bridging chloride. All bond
square-planar palladacycle subunits joined by a single chloride lengths and angles are within the expected values, with allow-
bridging ligand with a Pd–Cl–Pd bond angle of 93.61°. Each ance for lengthening of the Pd(1)–N(1) bond, 2.053(2) Å, as
palladium atom is bonded to the ligand through a PvN nitro- compared to complex 2, 1.993(3) and 2.010(3) Å, due to the
gen atom, an aryl carbon atom and a phenoxy oxygen atom. stronger trans influence of the phosphane ligand vs. the brid-
The bond lengths and angles at palladium are fairly the same ging chloride.
for both metallated units. We propose a tautomeric structure
The reaction of 2 with the diphosphanes Ph2P(CH2)nPPh2 (n
where the two oxygen atoms exchange a hydrogen atom, = 2, 3) in 1 : 2 molar ratio in acetone at room temperature gave
necessary to maintain neutrality, as is depicted in Fig. 2. Due mixtures of compounds, 4 and 5 (n = 2) and 6 and 7 (n = 3). In
to the disorder of the center of the molecule, a low theta differ- both cases they could be satisfactorily separated by centrifu-
ence Fourier was used to show where the point of electron ging the corresponding reaction mixture to give solutions con-
density was. This showed the electron density to be closer to taining the mononuclear complexes 4 and 6, with chelating
the O(2) atom. Intramolecular hydrogen bonding is in agree- diphosphane, and solids comprising the dinuclear com-
ment with the short O(1)–O(2) distance of 2.406 Å; the hydro- pounds, 5 and 7, with the bridging diphosphane (Scheme 1).
gen atom has been calculated onto the O(2) atom with a dis-
tance of 0.952 Å and a H–O(1) distance of 1.499 Å; the
O(1)⋯H–O(2) angle is 157.35°. This feature could account for
the Pd–O bond lengths in compound 2, Pd(1)–O(1) 2.116(2)
and Pd(2)–O(2) 2.123(2), as opposed to their analogues in com-
pounds 3, Pd(1)–O(1) 2.0625(18), 7, Pd(1)–O(1) 2.066(2), and 8,
Pd(1)–O(1) 2.053(3). Recently, in a rather similar case T. W. Lee
et al. have described an iridium(III) compound where the
O–H⋯O hydrogen bond contributes to the stability of the
Fig. 3 Thermal ellipsoid plot for 3 shown at the 50% probability level.
Hydrogen atoms and an acetone solvent molecule have been omitted
for clarity. Selected bond distances (Å) and angles (°) for 3: Pd(1)–C(25)
2.013(3), Pd(1)–N(1) 2.052(2), Pd(1)–O(1) 2.0625(18), Pd(1)–P(2) 2.2334
(7), P(1)–N(1) 1.617(2), C(25)–Pd(1)–N(1) 88.02(10), C(25)–Pd(1)–O(1)
169.23(9), N(1)–Pd(1)–O(1) 81.42(8), C(25)–Pd(1)–P(2) 92.02(8), N(1)–Pd
Fig. 2 Tautomeric equilibrium for 2.
(1)–P(2) 178.22(7), O(1)–Pd(1)–P(2) 98.46(6).
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The former two, of zwitterionic nature, are innovative in the
chemistry of iminophosphorane palladacycles.
The molecular structure of 4 consists of discrete molecules
with the palladium atom bonded to two chelating ligands: an
The final products were air-stable solids which were fully iminophosphorane-[C,N] and
a
bis(diphenylphosphino)
characterized. The IR spectra showed the downfield shift of ethane-[P,P] in a slightly distorted square-planar conformation.
the ν(PvN) stretch in all cases appearing ca. 1280 cm−1. The The bond distances are within the expected values with
1H NMR spectra of 5 and 7 showed only one set of signals in differing Pd–P lengths due to the higher trans influence of the
accordance with the symmetrical nature of the complexes. metallated phenyl carbon atom as opposed to the nitrogen
Likewise, in the 31P–{1H} NMR spectra two singlet resonances atom. The palladium–oxygen distance of 4.612 Å precludes any
were assigned to the two sets of equivalent phosphorus nuclei, interaction between them. Likewise, no interaction between
PvN and PPh2. Alternatively, the 31P–{1H} NMR spectra for the P(1) and O(2), which are 3.020 Å apart, was observed. As
complexes 4 and 6 showed doublets and doublets of doublet for complex 7 it may be envisaged as formed by two approxi-
signals, as appropriate, for the three inequivalent 31P nuclei mately square-planar palladium centers bonded through a bis
(see Experimental); with (PPh2 trans-C) at higher field than (diphenylphosphino)propane ligand.
(PPh2 trans-N) in agreement with the greater trans influence of
For compound 7 the X-ray diffraction analysis shows a
the phenyl carbon atom. Fig. 4 and 5 show the ORTEP view of dinuclear molecular structure whereby the two palladated
the molecules of 4 and 7, respectively (see ESI†).
units are linked by a bridging bidentate Ph2P(CH2)3PPh2
ligand, with the metal coordination planes [Pd,C,N,O,P] at an
angle of 66.15°. The structure is crystallographically centro-
symmetric with the inversion center situated at the C(39)
carbon between the two phosphorus atoms. The bond lengths
and angles at palladium are similar to those depicted for the
structures of 3 and 4 (vide supra); along with the structures for
2 and 3 the palladium center is bonded to four different atoms
in a slightly distorted square-planar environment; also, the
Pd–O bond length is shorter than that of its counterpart in
complex 2 (vide supra).
Recently Bhargava et al.43 have reported the cyclometalla-
tion of triphenylphosphane sulfide and selenide Ph3PvX (X =
S, Se) by treatment of the tin derivatives 2-Me3SnC6H4P(X)Ph2
(X = S, Se) with [PdCl2(COD)] that proceeds with formation of
the poorly soluble chloro-bridged dimers [Pd2(μ-Cl)2{κ2-2-
C6H4P(X)Ph2}2] that contain the syn- and anti-isomers; albeit
no crystal structures were reported. However, the analogous
chloro-bridged palladium complex with Ph3PvO yet remained
unknown. Next we give an account of this finding that com-
pletes the missing link in the chalcogenide series of complexes
[Pd2(μ-Cl)2{κ2-2-C6H4P(X)Ph2}2] (X = O, S, Se). When a chloro-
form solution of compound 2 was left to stand in contact with
air at room temperature prior to its use as the starting material
for making the compounds described above we observed the
formation of yellow crystals, which were filtered, conveniently
dried and set aside. The corresponding X-ray crystallographic
analysis proved them to be compound 8 (vide infra), which we
should like to coin as a rather pleasant case of chemical seren-
dipity.44 Regrettably attempts to make compound 8 directly
from triphenylphosphane oxide by standard ortho-palladation
procedures have been unsuccessful so far. Then, our synthetic
strategy consisted of several attempts at again leaving a solu-
tion of 2 to stand at room temperature exposed to air, where-
upon a precipitate was formed, void of any crystals in this
Fig. 4 Thermal ellipsoid plot for 4 shown at the 40% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond distances
(Å) and angles (°) for 4: Pd(1)–N(1) 2.102(2), Pd(1)–P(2) 2.2348(8), P(1)–N
(1) 1.602(2), Pd(1)–P(3) 2.3548(8), C(25)–Pd(1)–N(1) 83.74(11), C(25)–Pd
(1)–P(3) 176.62(8), N(1)–Pd(1)–P(3) 98.54(7), C(25)–Pd(1)–P(2) 92.89(8),
N(1)–Pd(1)–P(2) 173.02(7), P(2)–Pd(1)–P(3) 84.58(3).
Fig. 5 Thermal ellipsoid plot for 7 shown at are drawn at the 50% prob- case, which was filtered and dried to give a yellow powder.
ability level. Hydrogen atoms and a chloroform solvent molecule have
been omitted for clarity. Selected bond distances (Å) and angles (°) for 7:
Pd(1)–C(25) 2.006(3), Pd(1)–N(1) 2.032(3), Pd(1)–O(1) 2.066(2), P(1)–N(1)
1.602(3), Pd(1)–P(2) 2.2384(8), C(25)–Pd(1)–N(1) 87.55(11), C(25)–Pd(1)–
This procedure could be repeated and for the corresponding
powdery solid the spectroscopic evidence seemed to indicate
that compound 8 had been formed. Thus, the 31P–{1H} NMR
spectrum showed a singlet resonance at 35.7 ppm for the two
O(1) 168.60(11), N(1)–Pd(1)–O(1) 81.63(9), C(25)–Pd(1)–P(2) 98.88(9),
N(1)–Pd(1)–P(2) 173.38(7), O(1)–Pd(1)–P(2) 91.85(7).
equivalent 31P nuclei. The IR spectrum showed a band ca.
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Kappa APEX II diffractometer (Mo Kα radiation, λ = 0.71073 Å)
equipped with a graphite monochromator by the method of
the ω and φ scans at 293 K, integrated and corrected for
absorption and solved and refined using routine techniques.
All non-hydrogen atoms were refined anisotropically; hydrogen
atoms were included in calculated positions and refined in the
riding mode.
General procedures
Solvents were used without previous purification. Chemicals
were of reagent grade. The phosphanes PPh3, PPh2(CH2)2PPh2
(dppe) and PPh2(CH2)3PPh2 (dppp) were purchased from
Sigma-Aldrich. Elemental analyses were carried out on a
THERMO FINNIGAN, model FLASH 1112. IR spectra were
recorded with a JASCO FT/IR-4600 spectrometer equipped with
an ATR, model ATR-PRO ONE. The NMR spectra were recorded
on Varian INOVA 400 or Bruker DPX-250 spectrometers.
Fig. 6 Thermal ellipsoid plot for 8 shown at the 40% probability level.
Hydrogen atoms and a second molecule of the compound have been
omitted for clarity. Selected bond distances (Å) and angles (°) for 8: Pd
(1)–C(1) 1.960(4), Pd(1)–Cl(1) 2.3008(14), Pd(1)–O(1) 2.053(3), Pd(1)–Cl
(1#) 2.4383(14), C(1)–Pd(1)–O(1) 86.79(16), C(1)–Pd(1)–Cl(1) 94.58(14),
O(1)–Pd(1)–Cl(1) 178.61(9), C(1)–Pd(1)–Cl(1#) 176.40(14), O(1)–Pd(1)–Cl
(1#) 92.26(9), Cl(1)–Pd(1)–Cl(1#) 86.40(5).
1162 cm−1 assigned to the ν(PvO) stretch, and bands at 310
and 284 cm−1 corresponding to the ν(Pd–Cl) stretches.
Although it is not clear how 8 was formed we suggest that
hydrolysis of the PvN double bond45 produces o-aminophenol
and the PvO bond, which binds to the metal through the
oxygen atom, with the extra chloride ligand stemming as a
residual impurity from the initial metal reagent (Scheme 2).
Furthermore, the hydrolysis of compound 2 in solution was
also attempted. Thus, an NMR tube containing a solution of 2
in wet chloroform was monitored by 31P–{1H} NMR. After 72 h,
the major product showed a resonance at ca. 35 ppm assigned
to the presence of compound 8. It seems likely that triphenyl-
phosphine oxide does not allow metallation of the aryl ring,
whereas starting from 2 the palladium–carbon bond is already
formed. In any case, fine tuning the process is adamant in
order to give a clear picture of the preparation and this study
is presently underway.
Synthesis of 1
2-Hydroxyphenyl azide (300 mg, 2.22 mmol) was added to a
solution of triphenylphosphane (582 mg, 2.22 mmol) in
diethyl ether (20 cm3), and the resulting mixture was stirred at
room temperature for 18 hours. Compound 1 precipitated
from the reaction mixture as a brown powder and was isolated
by centrifugation and dried under vacuum. Yield 534 mg,
65%. C24H20NOP (369.40): calcd: C 78.0, H 5.5, N 3.8; found: C
1
77.9, H 5.7, N 3.8. IR: ν(PvN): 1318 cm−1. H NMR (400 MHz,
CDCl3, δ) 6.45 (m, 2H, H–Ar) 6.65 (m, 1H, H–Ar) 6.89 (m, 1H,
H–Ar) 7.47 (m, 6H, m-PPh3) 7.55 (m, 3H, p-PPh3) 7.72 (m, 6H,
o-PPh3). 13C–{1H} NMR (400 MHz, acetone-d6, δ) 150.6 (C–O)
144.4 (C–N) 139.7 (C–PC6H5) 132.0, 131.9 (Co C6H5P) 128.7,
128.5, 128.3 (Cm, Cp C6H5P) 119.3, 118.5 (Cm C6H4N) 118.2 (Cp
C6H4N) 115.5 (Co C6H4N). 31P–{1H} NMR (400 MHz, CDCl3, δ)
4.83 (s, P).
Crystal data for 8 are given in the ESI† and the ORTEP illus-
tration is shown in Fig. 6 (together with selected bond lengths
and angles). Each palladium atom, adopting a slightly dis-
torted square-planar disposition, is bonded by a chelating
C6H4P(O) group and two bridging chloride ligands. The struc-
ture is of a centrosymmetric dinuclear palladacycle with two
asymmetrically bridging chloride atoms. The phenyl carbon
atom and the oxygen atom are trans to the di-μ-chloride unit
so that these two palladated ligands are in an overall antiparal-
lel arrangement. The asymmetry results from the differing
trans influence of the C-phenyl and oxygen atoms. The Pd2Cl2
unit produces an intramolecular palladium-palladium dis-
tance of 3.456 Å which is longer than in complex 2, 3.411 Å, in
spite that in 8 the two palladium atoms are held together by
two bridging ligands, and excludes the possibility of a metal–
metal bond.
Synthesis of 2
A pressure tube containing Na2PdCl4 (150 mg, 0.51 mmol), 1
(182 mg, 0.51 mmol) and methanol (10 cm3) was sealed under
nitrogen. The resulting mixture was heated at 80 °C and after
4 hours NaAcO (42 mg, 0.51 mmol) was added to the reaction
mixture and stirred at 80 °C for 1 hour. Compound 2 precipi-
tated from the solution and was isolated by centrifugation,
and then was dissolved in dichloromethane and filtered
through silica to remove the black palladium formed. The
solvent was removed under vacuum to give a yellow oil, which
was recrystallized from dichloromethane/hexane to give the
desired product as a yellow solid. Yield 167 mg, 67%.
C48H37ClN2O2P2Pd2 (984.06): calcd: C 58.6, H 3.8, N 2.9;
found: C 58.5, H 3.9, N 2.9. IR ν(OH): 3375 cm−1 ν(PvN):
1
1313 cm−1. H NMR (400 MHz, acetone-d6, δ) 6.31 (m, 1H, H–
Ar) 6.53 (m, 2H, H–Ar) 6.75 (m, 1H, H–Ar) 7.01 (m, 2H, H–Ar)
7.17 (m, 1H, H–Ar) 7.52 (m, 1H, H–Ar) 7.65 (m, 4H, m-PPh2)
4
3
7.75 (m, 2H, p-PPh2) 7.97 (dd, J(HP) 2.6, J(HH) 7.6 Hz, 4H,
o-PPh2). 13C–{1H} NMR (400 MHz, acetone-d6, δ) 156.9 (C–O)
Experimental section
X-ray structure determination. Crystallographic data of the 149.8 (C–Pd) 144.4 (C–N) 138.7 (C–PC6H4) 133.2 (Co C6H5P)
structures described in this work were collected on a Bruker 129.1, 127.3, 126.5 (Cm, Cp C6H5P) 120.0, 119.9 (Cm C6H4N)
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118.6 (Cp C6H4N) 117.8 (Co C6H4N). 31P–{1H} NMR (400 MHz,
acetone-d6, δ) 42.61 (s, P).
Compounds 6 and 7 were obtained following a similar pro-
cedure to that for compounds 4 and 5 using bis(diphenylpho-
sphino)propane instead.
Synthesis of 3
(6): Yellow solid. Yield: 33.2 mg, 45%. C51H44NOP3Pd
Triphenylphosphane (22 mg, 0.08 mmol),
2 (41 mg, (886.24); calcd: C 69.1, H 4.6, N 1.6; found: C 68.9, H 4.8, N
0.04 mmol) and acetone (15 cm3) were added to a flask and 1.4. IR: ν(PvN): 1279 cm−1. H NMR (400 MHz, acetone-d6, δ)
the resulting mixture was stirred for 18 hours; the solvent was 6.37 (m, 1H, H–Ar) 6.61 (m, 1H, H–Ar) 6.79 (m, 2H, H–Ar) 7.04
evaporated to give a yellow oil, which was recrystallized from (m, 2H, H–Ar) 7.27–7.25 (m, 26H, H–Ar) 7.66 (m, 2H, H–Ar)
dichloromethane/hexane to give the desired product as a 7.89 (m, 4H, o-Ar). 13C–{1H} NMR (400 MHz, acetone-d6, δ)
yellow solid. Yield 55.4 mg, 89%. C42H33NOP2Pd (736.09): 153.8 (C–O) 147.3 (C–Pd) 145.3 (C–N) 139.9 (C–PC6H4) 133.3,
calcd: C 68.5, H 4.5, N 1.9; found: C 68.3, H 4.6, N 1.8. IR: 133.0, 132.9 (Co PhP) 129.7, 128.3, 128.0 (Cm, Cp C6H5P) 121.4
ν(PvN): 1280 cm−1. 1H NMR (400 MHz, acetone-d6, δ) 5.93 (m, (Cm C6H4N) 119.7 (Cp C6H4N) 116.8 (Co C6H4N) 31.9 (CH2) 27.6
1H, H–Ar) 6.15 (m, 1H, H–Ar) 6.35 (m, 2H, H–Ar) 6.60 (m, 2H, (CH2) 21.3 (CH2). 31P–{1H} NMR (400 MHz, acetone-d6, δ)
1
3
3
2
3
H–Ar) 6.82 (m, 1H, H–Ar) 6.95 (dd, J(HP) 12.2, J(HH) 7.2 Hz, −2.06 (dd, J(PP) 55.3 Hz, J(PP) 19.0 Hz, PPh2 trans-C) 22.01
1H, H–Ar) 7.44 (m, 6H, m-PPh3) 7.51 (m, 3H, p-PPh3) 7.66 (m, (d, 2J(PP) 55.3 Hz, PPh2 trans-N) 43.97 (d, 3J(PP) 19.0 Hz,
4H, m-PPh2) 7.74 (m, 2H, p-PPh2) 7.79 (m, 6H, o-PPh3) 7.97 (m, PvN).
4H, o-PPh2). 13C–{1H} NMR (400 MHz, acetone-d6, δ) 156.6
(7): Yellow solid. Yield: 18.0 mg, 32%. C75H62N2O2P4Pd2
(C–O) 147.3 (C–Pd) 135.3 (C–PC6H4) 133.2, 133.0, 132.7, 131.1 (1360.04); calcd: C 66.2, H 4.6, N 2.1; found: C 66.2, H 4.7, N
(Co PhP) 129.0, 128.8, 128.0, 127.8 (Cm, Cp C6H5P) 120.8 (Cm 2.1. IR: ν(PvN): 1279 cm−1. 1H NMR (400 MHz, CD2Cl2, δ) 1.98
C6H4N) 117.9 (Cp C6H4N) 117.6 (Co C6H4N). 31P–{1H} NMR (m, 2H, CH2) 2.79 (m, 4H, CH2) 6.08 (m, 2H, H–Ar) 6.31 (m,
3
(400 MHz, acetone-d6, δ) 32.38 (d, 3J(PP) 3.9 Hz, PPh3) 37.91 2H, H–Ar) 6.37 (d, J(HH) 7.8 Hz, 2H, H–Ar) 6.49 (m, 2H, H–
(d, 3J(PP) 3.9 Hz, PvN).
Ar) 6.60 (m, 4H, H–Ar) 6.81 (m, 4H, H–Ar) 7.16 (m, 8H, m-Ar)
7.30 (m, 4H, p-Ar) 7.48 (td, J(HH) 7.7 Hz, J(HP) 2.6 Hz, 8H,
m-PPh2) 7.58 (m, 6H, p-PPh2) 7.76 (m, 8H, o-Ar) 7.88 (dd,
3
4
Synthesis of 4 and 5
Bis(diphenylphosphino)ethane (34 mg, 0.08 mmol) and 2 3J(HP) 11.6 Hz, 3J(HH) 7.7 Hz, 8H, o-PPh2). 13C–{1H} NMR
(41 mg, 0.04 mmol) were added in acetone (15 cm3) and the (400 MHz, acetone-d6, δ) 156.7 (C–O) 147.5 (C–Pd) 145.2 (C–N)
resulting mixture was stirred for 18 hours. Compound 5 preci- 139.4 (C–PC6H4) 123.0 (Cm C6H4N) 120.6 (Cp C6H4N) 117.5 (Co
pitated from the mixture as a yellow powder was isolated by C6H4N) 28.5 (2CH2) 20.6 (CH2). 31P–{1H} NMR (400 MHz,
centrifugation and washed with acetone (10 cm3). Evaporation CD2Cl2, δ) 30.25 (s, PPh2) 31.62 (s, PvN).
of the solvent gave a yellow oil, which was recrystallized from
dichloromethane/hexane to give 4 as a yellow solid.
(4) Yield: 33 mg, 47%. C50H42NOP3Pd (872.22); calcd: C
Conclusions
68.9, H 4.9, N 1.6; found: C 68.7, H 5.0, N 1.5. IR: ν(PvN):
1
1279 cm−1. H NMR (400 MHz, CD2Cl2, δ) 6.07 (m, 1H, H–Ar) We have shown that for the iminophosphorane ligand derived
6.29 (d, 3J(HH) 7.9 Hz, H–Ar) 6.40 (m, 2H, H–Ar) 6.85 (td, from 2-aminophenol, palladacycles with the parent ligand as
4
3J(HH) 7.5 Hz, J(HP)1.8 Hz, H–Ar) 6.93 (m, 2H) 7.11 (m, 1H, terdentate [C,N,O] may be prepared. The pincer-type bonding
H–Ar) 7.25 (m, 4H, m-Ar) 7.38–7.65 (m, 18H, H–Ar) 7.72 (dd, mode of the iminophosphorane promotes a dinuclear species
3
3
3J(HP) 11.9 Hz, J(HH) 7.4 Hz, 4H, o-Ar) 7.86 (dd, J(HP) 12.1 with only one bridging chloride ligand, free of any counterion;
Hz, J(HH) 7.5 Hz, 2H, o-Ar) 8.13 (m, 2H, o-Ar). 13C–{1H} NMR a hydrogen atom between the two oxygen atoms ensures elec-
3
(400 MHz, acetone-d6, δ) 151.8 (C–O) 145.2 (C–N) 139.2 (C– trical neutrality. The ensuing complex may be reacted to give
PC6H4) 134.2, 133.5, 131.8 (Co PhP) 129.0, 128.5, 128.3 (Cm, Cp either typical mononuclear and dinuclear palladacycles, or
C6H5P) 119.7 (Cm C6H4N) 118.8 (Cp C6H4N) 115.7 (Co C6H4N) alternatively by the use of the appropriate diphosphane, new
31.6 (CH2) 28.9 (CH2). 31P–{1H} NMR (400 MHz, CD2Cl2, δ) zwitterionic metallacycles with a chelating phosphorus ligand.
2
3
43.45 (dd, J(PP) 27.7 Hz, J(PP) 15.5 Hz, PPh2 trans-C) 48.44 The most striking feature depicted herein is the serendipitous
(d, 3J(PP) 15.5 Hz, PvN) 59.56 (d, 2J(PP) 27.7 Hz, PPh2 unearthing of a dinuclear chloride-bridged triphenylpho-
trans-N).
sphane oxide palladacycle: the missing link in the series of tri-
(5) yield: 23 mg, 41%. C74H60N2O2P4Pd2 (1346.02): calcd: C phenylphosphane chalcogenide metallacycles, which com-
66.0, H 4.5, N 2.1; found: C 65.8, H 4.6, N 2.2. IR: ν(PvN): pletes the series. Although direct cyclopalladation of the phos-
1
1277 cm−1. H NMR (400 MHz, CDCl3, δ) 6.41 (m, 4H, H–Ar) phane oxide has not yet been possible, attempts to produce
6.53 (m, 2H, H–Ar) 6.63 (m, 2H, H–Ar) 6.73 (m, 2H, H–Ar) 6.91 the complex by the hitherto described method have so far
(m, 4H, H–Ar) 7.35–7.61 (m, 24H, H–PPh2) 7.73–7.89 (m, 18H, been successful.
H–Ar). 13C–{1H} NMR (400 MHz, acetone-d6, δ) 155.7 (C–O)
147.2 (C–Pd) 144.9 (C–N) 138.9 (C–PC6H4) 134.0, 133.7 (Co
PhP) 128.8, 128.4, 127.9 (Cm, Cp C6H5P) 118.1 (Cm C6H4N)
Conflicts of interest
117.8 (Cp C6H4N) 115.4 (Co C6H4N) 29.1 (CH2). 31P–{1H} NMR
(400 MHz, CDCl3, δ) 34.22 (s, PPh2) 36.00 (s, PvN).
There are no conflicts to declare.
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22 M. Carreira, R. Calvo-Sanjuán, M. Sanaú, I. Marzo and
Acknowledgements
M. Contel, Organometallics, 2012, 31, 5772–5781.
This work was made possible thanks to the financial support 23 S. Ramírez-Rave, M. T. Ramírez-Apan, H. Tlahuext,
received from the Xunta de Galicia (Galicia, Spain) under the
Grupos de Referencia Competitiva Programme (Project
D. Morales-Morales, R. A. Toscano and J.-M. Grèvy,
J. Organomet. Chem., 2016, 814, 16–24.
GRC2015/009). F. L.-M. and F. R. thank the Spanish Ministry of 24 N. Shaik, A. Martinez, I. Augustin, H. Giovinazzo, A. Varela-
Education (grants FPU13/05014 and FPU15/07145).
Ramírez, M. Sanaú, R. J. Aguilera and M. Contel, Inorg.
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25 D.-L. Ma, H.-Z. He, K.-H. Leung, D. S.-H. Chan and
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