Chemistry of Tetradentate [C,N : C,N] Iminophosphorane Palladacycles: Preparation, Reactivity and Theoretical Calculations

Article abstract of DOI:10.1002/open.202000253

A theoretical and experimental study of tetradentate [C,N : C,N] iminophosphorane palladacycles was carried out for the purpose of elucidating their behavior as compared to the parent Schiff base analogues to determine the prospect of encountering new A-frame structures for the iminophosphorane derivatives. The DFT calculations were in agreement with the experimental results regarding the performance of these ligands. New insights into the chemistry of the related dinuclear species have been obtained.

Full text of DOI:10.1002/open.202000253

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Chemistry of Tetradentate [C,N:C,N] Iminophosphorane
Palladacycles: Preparation, Reactivity and Theoretical
Calculations
Paula Munín-Cruz, Francisco Reigosa, Marcos Rúa-Sueiro, Juan M. Ortigueira,
[a]
M. Teresa Pereira, and José M. Vila*
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Dedicated to the late Professor Bernard L. Shaw, The University of Leeds, UK
A theoretical and experimental study of tetradentate [C,N:C,N]
iminophosphorane palladacycles was carried out for the
purpose of elucidating their behavior as compared to the
parent Schiff base analogues to determine the prospect of
encountering new A-frame structures for the iminophosphor-
ane derivatives. The DFT calculations were in agreement with
the experimental results regarding the performance of these
ligands. New insights into the chemistry of the related dinuclear
species have been obtained.
1
. Introduction
phorane ligands, namely, the chelate to bridging shift in
[10]
diphosphane palladacycles and the triphenylphosphine chal-
[1]
[9d]
Palladacycles cover a rather wide range of compounds within
the organometallic family; they have been researched for nearly
cogenide metallacycles, respectively.
We reasoned that by
merging the two concepts we would be able to establish yet
another new performance of these species in the hope of
further extending the scope of this chemistry, i.e., by treatment
of the iminophosphorane tetradentate [C,N:C,N] ligand pallada-
cycles with tertiary diphosphanes, which should render multi-
[
2]
six decades, since the first example was synthesized. One
would not be wrong to say that this ever growing interest in
such compounds could be mainly due to the variety of the
bonds at the metal center, i.e., the palladium atom, which
makes their reactivity one of the most extended allowing by far
a good deal of applications in numerous fields of chemistry. As
well as their use in photochemistry or as liquid crystals two
of their most important usages are as pre-catalysts in cross-
coupling reactions, such as Suzuki-Miyaura, Mizoroki-Heck or
Negishi and as notable powerful anticancer agents Although
a majority of palladacycles are prepared from ligands contain-
ing C=N double bonds, such as Schiff bases, semicarbazones,
thiosemicarbazones and the like, there is growing interest in
[
10]
nuclear complexes with the possibility of A-frame structures
as encountered earlier by us, or alternatively display the more
[3]
[4]
[11]
classical palladacycle behavior to give complexes with only
bridging or chelating diphosphanes, as appropriate (Scheme 1).
The polarization of the P=N double bond and hence its greater
basicity as compared to its C=N counterpart play an important
role in the final result since it produces a stronger metal-donor
atom bond. Herein we give an account of our results addressing
this issue that has been supported, in part, by DFT calculations.
[5]
[6]
[7]
[8]
palladacycles that stem from the iminophosphoranes R P=NR,
3
where the polarized P=N double bond, shows a strong σ-donor
behavior with minor π-acceptor properties which makes them
[9]
very suitable ligands for the cyclopalladation reaction. Along
with our more recent research on palladacycles we have come
upon several new behaviors in their chemistry among which we
may highlight the following two milestones when employing
tetradentate [C,N:C,N] ligands, or terdentate [C,N,O] iminophos-
[
a] Dr. P. Munín-Cruz, F. Reigosa, M. Rúa-Sueiro, Dr. J. M. Ortigueira,
Prof. M. T. Pereira, Prof. J. M. Vila
Departamento de Química Inorgánica
Universidad de Santiago de Compostela
1
5782 Santiago de Compostela Spain
E-mail: josemanuel.vila@usc.es
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.202000253
©
2020 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any med-
ium, provided the original work is properly cited, the use is non-commercial
and no modifications or adaptations are made.
Scheme 1. a) Classical palladacycle with bridging diphosphanes; b) A-frame
structure.
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2
. Results and Discussion
chlorideÀ bridged 3a–f complexes as air-stable solids (see
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Experimental). The IR spectra showed absence of the acetate
stretches and two bands assigned to the υ(PdÀ Cl) vibration
modes for the unsymmetrical Pd Cl moiety, ca. 310 and
For the convenience of the reader the compounds and
reactions are shown in Scheme 2. Preparative details, character-
ising microanalytical data, IR and NMR data are in the
Experimental section. Treatment of the diamines (p-NH C H ) X
2
2
À 1
245 cm for the PdÀ Cl
and PdÀ Cl
bonds, respectively,
transC
transN
along with the differing trans influence of the nitrogen and
phenyl carbon atoms, also respectively.
2
6
4 2
(X=O, SO , CH ) with sodium azide followed by P(RC H ) (R=p-
2
2
6
5
3
MeO, Me) gave the six iminophosphorane ligands a–f. The
reaction of the latter with palladium(II) acetate in toluene
yielded the tetranuclear complexes 1a–f as air-stable solids
which were fully characterized (See Experimental). The IR
In a previous work we described formation of double A-
frame complexes which could be accomplished by choosing an
appropriate flexible bidentate Schiff base ligand, bis(N-2,3,4-
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trimethoxybenzylidene)-4,4’-sulfonyldianiline,
or
-4,4’-
À 1
[10]
spectra show the υ(P=N) band ca. 1220 cm , shifted by ca.
oxydianiline,
as opposed to the phenylenediimine-C,N:C,N
À 100 as compared to the starting ligand, and the υ (COO) and
analogues, where this was not possible, due to the restricted
flexibility of the ligand imposed by the rigidness of the central
phenylene ring itself, hindering opening of the metallacycle
ring needed to provide the extra vacant coordination site on
as
À 1
υ (COO) acetate stretches ca. 1565 and 1410 cm , respectively,
s
the separation of which is consistent with bridging acetate
ligands.
The acetate-bridged complexes were converted into the
halido-bridged analogues by treatment of 1a–f in dichloro-
methane with aqueous sodium chloride to give the
[12]
[
11]
the metal for coordination of the second phosphine ligand.
These species are closely related to the field of
[13]
supramolecular coordination complexes (SCCs), and we found
Scheme 2. Reaction sequence leading to the dinuclear and to the expected A-frame compounds.
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À 1
À 1
they may host small molecules such as chloroform or acetone.
Then, our purpose was to further investigate those systems
showing a similar reactivity in order to delimit the compounds
that present these characteristics and expand the family of
palladacycle SCCs; where this does not seem to be the case
satisfactory explanations would be essential.
ΔH 5.25, ΔG 1.52 kJmol , 3a, and ΔH 3.66, ΔG 3.33 kJmol ,
3f (related to the E conformer); these should be responsible for
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the most intense signals in the P NMR spectra, whilst third
conformer, ΔH 15.74, ΔG 13.56, 3a, ΔH15.49, ΔG9.46 kJmol ,
À 1
3f, would give rise to the weaker signal appearing only
occasionally. The angle between the chelated diphosphine rings
[Pd,P,C,P] in the ground state conformer E was 84.31°, 3a, and
80.76°, 3f, and the palladium coordination planes [C,N,P,P] were
at 72.88°, 3a, 86.58°, 3f, In conformer Z the analogous values
were 58.44° and 54.78°, 3a, and 49.89° and 46.56, 3f,
respectively. These differences were ascribed to the relative
orientations of the [Pd,P,C,P] rings. In conformer E, one ring
We also sought to prepare the dinuclear compounds, which
shall be discussed first; they are the derivatives of the halide-
bridged complexes with Ph PCH PPh (dppm), Ph PN(Me)PPh
2
2
2
2
2
(
dppma), as well as Ph P(CH ) PPh (dppp) included for
2 2 3 2
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comparative purposes. Thus, reaction of 2(a–f) with the
corresponding diphosphane in 1:4 molar ratio followed by
treatment with ammonium hexafluorophosphate gave the
dinuclear palladacycles 3–5(a–f) as pure air-stable 1:2 electro-
lytes, which were fully characterized (see Experimental section).
points out of the spacer angle PhOPh, 3a, PhCH Ph, 3f, and the
2
other one towards the inner side. In Z both rings are orientated
outwards from the spacer angle. In both cases these arrange-
ments minimize the repulsions of the phosphine phenyl rings
1
In the H NMR spectra a doublet of doublets ca. 4.5 and
2
.3 ppm was assigned to the PCH P 3(a–f) and PN(Me)P 5(a–f)
and the Ph P=N rings. Likewise, as further proof torsion angles
were measured referenced to the diamine spacer (Figure 1).
2
2
protons, respectively, whereas the P(CH ) P resonances were a
multiplet. The H5 resonance showed coupling to both
2
3
Thus, the [Pd,N,CH ,N] torsion angles were of 166.47° and
2
4
4
phosphorus nuclei ( JP
H5 ca. 10 Hz, JPcis-NH5 ca. 8 Hz). The
79.23° (conformer E); 146.66° and 148.36° (conformer Z), 3a,
and of 174.40° and 92.06° (conformer E); 138.96° and 157.00°
(conformer Z), 3f; data which are in agreement with the
proposed layouts. The third conformer, which has the highest
energy, displays the phenyl rings from the diphosphine close to
trans-N
31
P NMR spectra showed three doublets of doublets for the P=
N nucleus and the two inequivalent phosphorus nuclei of the
diphosphane in each case. The position of the former resonance
remained fairly unchanged in all the complexes ca. 45–50 ppm.
With respect to this signal those for the dppm and dppp
compounds, ca. À 8.5/À 30 ppm, 3(a–f), 23/1.5 ppm, 5(a–f),
appear moderately and very strongly highfield shifted, respec-
the N=PPh units with steric hindrance that lead to the most
2
unstable arrangement; the corresponding NMR resonance was
3
1
observed only once, in 3f. Low and high temperature P NMR
were run for compound 3f. At 193 K the spectrum showed
both conformers E and Z were still present, albeit the E/Z ratio
was modified from an initial 50:50% to 65:35%, and no signal
attributable to the third conformer was observed. At 313 K the
signals appeared as only one set of resonances for the P=N, and
diphosphane nuclei, with all the possible arrangements merg-
ing into one sole conformer.
3
1
tively, in agreement with the influence of ring size on the
P
[14]
31
chemical shifts.
resonances of the four-membered ring dppma compounds,
(a–f), 59/51 ppm, underwent highfield shifts with respect to
On the contrary, the P diphosphane
4
the free diphosphane resonance (+73.7 ppm) associated with
shielding caused by the quite strongly strained four-membered
ring. In all cases the phosphane resonance at lower frequency
was assigned to the phosphorus nucleus trans to the phenyl
carbon atom, in agreement with the higher trans influence of
the latter with respect to the nitrogen atom. Furthermore, for
some of the compounds close inspection of their spectra
revealed two, at times three, sets of resonances for the three
nuclei, suggesting existence of conformers. The ratio of the
conformers was uneven but reached a 50:50 distribution in
some cases. To ascertain this issue a DFT analysis was
performed on compound 3f using the package of programs
g16. The reported geometry and frequency calculations were
carried out at the B3LYP/LANL2DZ-ECP/6-31G(d) level of theory
for compound 3a and ONIOM B3LYP/LANL2DZ-ECP/6-31G(d):
B3LYP/3-21G for compound 3f and no imaginary frequencies
were found, confirming the stationary points. Three conformers
were proposed and optimized resulting in structures that vary
according to the orientation of the metallated units, the
rotation of which is strongly hindered by the presence of the
iminophosphorane and phosphane phenyl rings. No restrictions
were set for the para-disubstituted phenyl rings bonded to the
The tests carried out in order to prepare tetranuclear A-
[
10]
frame species 7a–f similar to those reported earlier by us,
[15]
from the dinuclear complexes 3a–f (e.g. 3f!7f, see Scheme 3),
where unsuccessful. Thus, upon the negative outcome obtained
in attempts to synthesize the tetranuclear compounds depicted
in Scheme 2, we performed DFT calculations in order to provide
theoretical confirmation of these results. The structure that
would result from the formation of a double A-frame com-
3
1
P
À OÀ and À CH À spacers since it was considered that this did
2
not have a significant contribution to the relative energy of the
conformers. We considered the two most stable conformers,
coined as E and Z, which showed a relative energy difference of
Figure 1. Conformers E and Z for 3a and 3f. Phenyl rings on the phosphorus
atoms have been omitted for clarity.
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Scheme 3. Proposed formation of tetranuclear A-frame complex 7f from
compound 3f.
pound from compounds type 7a–f, namely 7f with the CH2
spacer as depicted in Scheme 3, was studied.
[10]
The crystal structure of a previous work was used as a
base optimized using a B3LYP/3-21G level of theory. The
frequency study was carried out with the same basis as for the
chelated f type compounds (vide supra), using an ONIOM model
studying the close environment of the cyclometalated moiety
at a B3LYP/L2DZ2-ECP/6-31G(d) level and the rest of the
molecule at 3-21G.
Figure 2. Calculated tetranuclear A-frame palladacycle 7f.
phase situation, and endergonic in solution, confirming that
this reaction is not spontaneuous.
The resulting structure shows imaginary frequencies, but
they are coincident with the rotation of the methyl groups
attached to the phenyl rings and are not considered to
significantly affect the energy values used to calculate the
energy of the formation reaction of this compound. To further
investigate the thermochemistry involved in the transformation
of the chelated species into the less common double A-frame
For comparative reasons with the phosphine derivatives, we
prepared the acac [O,O] complexes 6(a–f) containing two stable
sixÀ membered rings at the metal centers, which were charac-
terized accordingly. The IR spectra showed υ(CÀ C) and υ(CÀ O)
1
stretches in the expected ranges, and in the H NMR spectra
two singlet resonances were assigned to the two inequivalent
[10]
1
structures, as we observed in previous work, the energy of
the reaction was calculated with the data obtained for the
chelate structures, the resulting A-frame structure and the
chloride ion optimized using a B3LYP/6-31G(d) level of theory
methyl groups ca. 1.9 and 1.6 ppm. The H NMR spectrum for
6a and 6f did not show additional resonances to those
expected and the low and high temperature NMR spectra did
not provide any variations with respect to the room temper-
ature spectra. This leads us to conclude that it is the presence
of phenyl rings on the chelating ligand at palladium, that
hinders the structural rearrangement/rotation/interconversion
leading to the aforementioned conformers.
(Figure 2).
The enthalpy and Gibbs free energy values for this reaction
were À 18.14 kcal/mol and 87.37 kcal/mol respectively, making
this reaction exothermic, albeit endergonic. This explains why
the spontaneous formation of the A-frame structure that
occurred in the case of double Schiff bases was not observed,
where the study by DFT yielded negative values for enthalpy 3. Conclusions
[10]
and negative reaction free energy of ca. 200 kcal/mol.
After gas phase geometries were optimized, solvation
effects were taken into account in order to further investigate
the thermochemistry of the reaction. The structures were
optimized and frequency calculations were performed at the
same level of theory but using the polarizable continuum
model, acetone as solvent and the ONIOMPCM=x setting,
where the reaction field is computed separately for each level
using the cavity of the real system. The enthalpy and Gibbs free
energy values for this reaction were 189.98 kcal/mol and
Phosphorus NMR spectra of the dinuclear chelated phosphine
compounds showed the presence of several diverse spatial
arrangements assignable to the existence of the conformers
found in the DFT calculations, in terms of the most energetically
stable and of their spatial disposition. High and low temper-
ature NMR were also performed with the chelated phosphine
and the acac derivatives, showing that in the case of the acac
the presence of conformers was not detected. Also, DFT
calculations confirmed the experimental fact that complexes
with the chelated diphosphines are more stable than the
tetranuclear framework; this has to do presumably with the
relative strengths of the C=P!Pd and C=N!Pd bonds in either
2
91.96 kcal/mol respectively. Thus, both positive values show
that the reaction should be endothermic, as opposed to the gas
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case. Thus, in principle the hoped-for tetranuclear A-frame
palladacycles with iminophosphorane ligands could not be
obtained, as occurred with their Schiff base analogues.
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General Comments
[
16]
Solvents were purified by standard methods.
Palladium(II)
[
4] a) M. Marcos, Metallomesogens, VCH, Weinheim, 1996; b) D. Pucci, G.
Barberio, A. Bellusci, A. Crispini, M. Ghedini, J. Organomet. Chem. 2006,
691, 1138–1142.
acetate, Thallium(I) acetylacetonate, (p-NH C H ) SO , (p-NH C H ) O,
1
1
1
1
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1
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2
6
4 2
2
2
6
4 2
(
p-NH C H ) CH , and the phosphines P(pMeOPh)3, P(pMePh)3,
2 6 4 2 2
Ph PCH PPh (dppm) and Ph P(CH ) PPh (dppp) were purchased
2
2
2
2
2 3
2
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2
2
Professor JM Vila. Elemental analyses were performed with a Fisons
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BRUKER FT-MIR model VERTEX 70 V, and Jasco model FT/IR-4600
1
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tubes; chemical shifts, in ppm, are reported downfield relative to
[
[
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1
TMS using the solvent signal (CDCl , δ H=7.26 ppm; DMSO-d ,
3 6
1
1
1
δ H=2.50 ppm; CD Cl2 d6 δ H=5.32 ppm, MeCOMe-d6 δ H=
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31
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o.d. tubes and are reported in ppm relative to external H PO
3
4
(85%). Coupling constants are reported in Hz.
[
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(CESGA) for the use of computational resources.
[
[
Conflict of Interest
The authors declare no conflict of interest.
Keywords: palladium
·
diphosphine
·
azides
·
Manuscript received: September 4, 2020
Revised manuscript received: October 20, 2020
iminophosphorane · density functional theory
ChemistryOpen 2020, 9, 1190–1194
www.chemistryopen.org
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© 2020 The Authors. Published by Wiley-VCH GmbH