Novel cationic dinuclear η3-methallylpalladium complexes containing N,N′-donor bridging ligands: Synthesis, characterization and crystal structure

Article abstract of DOI:10.1016/j.jorganchem.2016.07.022

A series of bifunctional ligands of the type, 1,4-{(2-C₅H₄N)RC?=?N}₂-C₆H₄[R?=?H (L1), Me (L2)], 1,4-{(2-C₅H₄[Formula presented]₂-(C₆H₄)₂[L3] and 1,4-{(2-C₅H₄[Formula presented]₂-C₂H₄[L4] were synthesized and fully characterized by IR,¹H NMR and¹³C NMR spectroscopy. These ligands react with Pd(dba)₂in the presence of the salt methallyloxytris(dimethylamino)phosphonium hexafluorophosphate [C₄H₇OP(NMe₂)₃]⁺PF₆^?to give the novel cationic dinuclear η³-methallylpalladium complexes [Formula presented] in high yields. All new complexes [Formula presented] have been characterized by CHN analyses,¹H,¹³C,³¹P NMR and IR spectroscopy. The X-ray crystal structure of complex C1 has been determined.

Full text of DOI:10.1016/j.jorganchem.2016.07.022

Accepted Manuscript
3
Novel cationic dinuclear η -methallylpalladium complexes containing N,N'-donor
bridging ligands: Synthesis, characterization and crystal structure
Sana Dridi, Ali Mechria, Paolo Sgarbossa, Roberta Bertani, Moncef Msaddek
PII:
S0022-328X(16)30315-1
10.1016/j.jorganchem.2016.07.022
JOM 19573
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 13 January 2016
Revised Date: 20 June 2016
Accepted Date: 29 July 2016
Please cite this article as: S. Dridi, A. Mechria, P. Sgarbossa, R. Bertani, M. Msaddek, Novel cationic
3
dinuclear η -methallylpalladium complexes containing N,N'-donor bridging ligands: Synthesis,
characterization and crystal structure, Journal of Organometallic Chemistry (2016), doi: 10.1016/
j.jorganchem.2016.07.022.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Novel cationic dinuclear η3-methallylpalladium complexes containing
N,N’-donor bridging ligands : Synthesis, characterization and crystal
structure
Sana Dridia, Ali Mechriaa,*, Paolo Sgarbossab, Roberta Bertanib and Moncef Msaddeka
a Laboratoire de Chimie Hétérocyclique, Produits naturels et Réactivité : L.C.H.P.N.R, Faculté des
sciences de Monastir, Bd de l’environnement, 5000, Monastir, Tunisie.
b Department of Industrial Engineering, University of Padua, Via F. Marzolo, 9, 35131 Padova, Italy.
Abstract
A series of bifunctional ligands of the type, 1,4-{(2-C5H4N)RC=N}2-C6H4 [R = H (L1), Me
(L2)], 1,4-{(2-C5H4N)HC=N}2-(C6H4)2 [L3] and 1,4-{(2-C5H4N)HC=N}2-C2H4 [L4] were
1
synthesized and fully characterized by IR, H NMR and 13C NMR spectroscopy. These
ligands
react
with
Pd(dba)2
in
the
presence
of
the
salt
-
methallyloxytris(dimethylamino)phosphonium hexafluorophosphate [C4H7OP(NMe2)3]+PF6
to give the novel cationic dinuclear η3-methallylpalladium complexes C1-C4 in high yields.
All new complexes C1-C4 have been characterized by CHN analyses, 1H, 13C, 31P NMR and
IR spectroscopy. The X-ray crystal structure of complex C1 has been determined.
Keywords
Dinuclear, cationic, methallylpalldium
Introduction
The chemistry of binuclear palladium(II) complexes has attracted considerable interest
over recent years [1]. Isolation of allyl palladium and ortho-palladated complexes in the
1960’s gave new impetus to the development of binuclear palladium complexes owing to their
relevance in organic transformations. The field has become multidisciplinary in nature
encompassing areas such as organic synthesis, catalysis, material-science, photophysical
chemistry, chemotherapy as well as several other areas [2].
Binuclear organometallic compounds are interesting in view of the expectation that
their reactivity in synthesis and catalysis may differ significantly from that of analogous
mononuclear complexes [3]. Additional oxidation states might also be accessible to binuclear
complexes as a result of possible stabilization via metal-metal interactions [4]. Interaction
between nearby metal centers could potentially cause increased reaction rates or yield
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transformation rates not possible with mononuclear species [5]. In particular, the distance
between the two metal centers and their orientation relative to each other may be important
for catalytic performance [6].
Recently, an increased effort has been devoted to the design of ligands and metal
complexes aimed at homogeneous catalytic conversions. The designs have primarily been
focused on the ligands since the reactivity, selectivity and stability of transition metal
compounds in homogeneous catalytic reactions are highly dependent on the ligand
environment. In the perspective, multinuclear group-X metal complexes serve as efficient
catalysts in olefin poly/oligomerizations due to the electronic interactions found in the form of
cooperative effects between metal centers. The same electronic effects cannot be expected in
their mononuclear counterparts [7–10]. Subsequently, there are few works on modified a-
diimine ligand systems, however, only few reports showed the use of bis-a-diimine ligands in
the formation of active group-X metal complexes for ethylene poly/oligomerizations [11-14].
A variety of related dinucleating ligands with two pyridyl-imine binding sites
separated by aromatic spacers have recently been investigated by Hannon et al. for similar
reasons [15].
In the last few years, several new cationic allyl complexes of palladium and nickel
containing α-diimine ligands have been synthesized by our group [16-24]. As part of our
ongoing research in this field, focused in developing novel cationic palladium(II) complexes,
benefiting from cooperative effects between adjacent active metal centers, we report herein
the synthesis of a series of new cationic dinuclear methallyl palladium(II) complexes bearing
α,α’-Diimine ligands containing two pyridyl-imine binding sites separated by aromatic or
aliphatic spacers. The molecular structure of the cationic bimetallic methallyl Pd(II) complex
C1 was determined by single crystal X-ray crystallography.
Results and discussion
Synthesis and characterization
All the dinucleating ligands (L1-L4) were synthesized according to published
procedures by classical condensation reaction of two equivalents of 2-pyridinecarboxaldehyde
or 2-acetylpyridine in ethanol with 1,4-diaminobenzene, benzidine and ethylenediamine,
2

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respectively [25-28]. Variation of the spacer group therefore allows for the facile design of
ligands and subsequent metal complexes with different physical and chemical properties.
The new cationic dinuclear methallyl palladium complexes C1-C4 were prepared via
an oxidative addition of methallyloxyphosphonium salt to the zerovalent compound Pd(dba)2
in the presence of bis(pyridinylimino) ligands (scheme 1). The reaction is carried out in
CH2Cl2 at room temperature and afford, in quantitative yields, the cationic bimetallic
methallyl Pd(II) complexes.
Scheme 1. Synthesis of cationic binuclear bis(methallylPd(II)) complexes C1-C4.
These new cationic dinuclear methallyl palladium complexes (C1-C4) were isolated as
a yellow to orange solids soluble in dichloromethane and acetonitrile but sparingly soluble in
1
diethyl ether and in n-hexane. H, 13C, 31P NMR, IR spectroscopy and elemental analyses
were consistent with those palladium(II) complexes formulation.
The NMR spectra of the complexes (C1-C4) were recorded in CD3CN and the proton
numbering employed is shown in Scheme 1. The NMR spectra and the number of signals
show that all the compounds are symmetrical. The proton signals of the aromatic groups of
the imine system including the pyridine were observed to be shifted downfield by
approximately δ 0.3-0.6 ppm in complexes (C1-C4) in comparison to those of the free ligands
L1-L4, while the 13C NMR peaks of the complexes were shifted to low field by approximately
δ 5-9 ppm as compared with the free ligands. This is a result of the lowering of the electron
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density of the N,N’-ligands on complexation. The most notable effect upon coordination of
the ligand to the metal is the change in the chemical shift of the aldimine (CH=N) and
ketimine (CMe=N) protons which are shifted in all the complexes.
The 1H NMR spectra of C1-C4 show one set of protons corresponding to the pyridine
moieties of the ligand in the rang δ 7.27-8.80 ppm.
The signal of the aliphatic protons of complex C4 is shifted upfield compared to the
alkanediyl protons of the free ligand. The methylene protons, (=NCH2-) bonded to the
azomethine nitrogen appear at δ 4 ppm as a singlet in the ligand spectra. This signal shift to
around 3.35 ppm in the spectrum of the metal complex.
-
The presence of the two non-coordinated PF6 counter-ions in complexes C1-C4 was
confirmed by 31P NMR spectroscopy. In addition, IR spectra of the complexes showed typical
lower absorption bands between 1627 and 1631 cm-1 compared to their respective ligands [25-
28]. All IR spectra of cationic dinuclear palladium(II) complexes present the characteristic
-
band of the PF6 counter-ion between 829 and 838 cm-1.
Molecular structure of complex C1
Single crystals of complex C1 suitable for X-ray analysis were grown by slow diffusion of
diethyl ether into a dichloromethane solution of complex C1 at room temperature.
Crystallographic data and structure refinement parameters are given in Table 1 while selected
bond lengths and angles are listed in Tables 2. The molecular structure of complex C1 shows
an asymmetric unit formed by a half molecule of C1 while the corresponding molecular
structure is generated by symmetry. Molecular structure of complex C1 is shown in Fig 1. The
structure of complex C1 is based on a centro-symmetrical bimetallic palladium-containing
-
dicationic unit and charge balanced by two PF6 counteranions without direct interactions as
appears from the large distance between the metal and the nearest fluorine atom (Pd-F1 = 3.64
Å). The geometry around every palladium atom in the complex C1 could be described as
distorted square planar consisting of two carbon atoms (C1 and C3) of the allyl group and two
nitrogen atoms (N1 and N2) of the ligand. For instance, the relatively small N(1)-Pd(1)-N(2)
bond angle in complex C1 of 78.3(2)° is a result of chelating ligand steric constraints.
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Fig. 1. ORTEP representation of the molecular structure of complex C1 with atom labeling
scheme and anisotropic displacement ellipsoid depicted at 40% probability. Hydrogen atoms
-
and PF6 anions are omitted for clarity.
The bond angles around the Pd atom which sum to 360(3)° are consistent with a planar
geometry. The metal-chelate ring (Pd(1)-N1-C9-C10-N2) in complex C1 is almost flat as
indicated by the torsion angles of -6.9(8)°, 3.0(10)° and 2.6(8)° for Pd(1)-N1-C9-C10 , N2-
C10-C9-N1 and Pd(1)-N2-C10-C9, respectively. The methyl on the allyl group is slightly
tilted out of the allyl plane by approximately 13° as indicated by the torsion angle of
166.7(8)° for C4-C2-C3-C1. The allyl plane makes an angle of 112.5(7)° with the palladium
coordinative plane which is normal for η3-2-methylallyl complexes of palladium [16-24]. The
average Pd-Npyridine bonds lengths in complex C1 [Pd(1)-N1 = 2.122(6) Å] are shorter than the
Pd-Nimine distance [Pd(1)-N2 = 2.133(5) Å] compares well with the bonds lengths of Pd-N1 =
2.082(5) Å and Pd-N2 = 2.089(6) Å observed in related palladium complex [16]. The Pd-C
bond lengths in C1 lie between 2.107(8) and 2.151(7) Å compare well with the Pd-C distances
observed in other related palladium complexes [16, 18-19].
5

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Table 1
Crystal data and structure refinement for complex C1.
Chemical formula
Crystal size (mm3)
Formula weight
Crystal system
Space group
a (Å)
C26H28N4Pd2P2F12
0.06 x 0.04 x 0.02
899.26
monoclinic
C2/c
18.071(2)
16.2390(19)
14.0447(15)
90
b (Å)
c (Å)
α (°)
β (°)
128.057(2)
90
γ (°)
V ( Å3)
3245.3(7)
4
Z
T (K)
293(2)
Dcalc (g cm-3)
Unique reflections
R(int)
1.841
3207
0.0651
θ range (°)
1.903 to 26.390
0.71073
1.301
λ (Å)
ꢀ (mm-1)
F(000)
1768
Goodness-of-fit
R1 [I > 2σ(I)]
wR2
1.046
0.0549
0.1426
6

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Table 2
Experimental bond distances and bond angles of complex C1.
Bond lengths (Å)
Bond angles (°)
N1-Pd1-N2
N1-Pd1-C1
N2-Pd1-C1
N1-Pd1-C3
N2-Pd1-C3
C1-Pd1-C3
N1-Pd1-C2
N2-Pd1-C2
C1-Pd1-C2
C3-Pd1-C2
Pd1-N1
Pd1-N2
Pd1-C1
Pd1-C2
Pd1-C3
N1-C9
2.122(6)
78.3(2)
2.133(5)
2.118(8)
2.151(7)
2.107(8)
1.353(9)
1.340(9)
1.289(8)
1.425(8)
1.458(9)
105.3(3)
175.6(3)
173.1(3)
108.6(3)
67.8(3)
N1-C5
135.7(3)
139.5(3)
38.7(3)
N2-C10
N2-C11
C10-C9
38.3(3)
Conclusion
The oxidative addition of methallyloxytris(dimethylamino)phosphonium hexafluorophosphate
to the zerovalent Pd(dba)2 in the presence of α,α’-Diimine compound produces new cationic
dinuclear methallyl palladium(II) complexes in which the ligands coordinative via the
pyridine and imine nitrogen atoms. Solid-state structure indicates a distorted square planar
arrangement around every palladium centre. The evaluation of those new dinuclear methallyl
palladium(II) complexes in catalytic dimerization of methylacrylates is in progress.
Experimental
Materials and methods
All reactions were carried out under dry argon atmosphere using standard Schlenk techniques.
Dichloromethane and hexane were refluxed and distilled from phosphorus pentoxide (P2O5)
while diethyl ether was dried over sodium wire and benzophenone. The ligands L1, L2, L3
and
L4
were
prepared
according
to
the
literature
methods
[25-28].
Methallyloxytris(dimethylamino)phosphonium hexafluorophosphate [29], Pd(dba)2 [30] were
prepared according to literature methods. NMR spectra were recorded on a Bruker AVANCE-
400 NMR spectrometer. H and C chemical shifts were given in ppm and referenced to the
31
residual solvent resonance relative to TMS Starting materials. P chemical shifts were given
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in ppm relative to an 85% H3PO4 external reference. IR spectra were recorded on a Perkin-
Elmer Spectrum two FT-IR instrument with the Universal ATR Sampling Accessory.
Elemental analyses were performed on a on a Perkin-Elmer 2400 series II CHNS/O analyzer.
Synthesis of methallyloxytris(dimethylamino)phosphonium hexafluorophosphate 2
Tris(dimethylamino)phosphine (2.29 g, 14 mmol) was added drop-wise to a stirred solution of
2-methylprop-2-en-1-ol (1 g, 14 mmol) and CCl4 (2.46 g, 16 mmol) in dry CH2Cl2 (30 mL)
cooled at -30 °C. Then, 15 mL of distilled water was added and the reaction mixture was
allowed to proceed at room temperature for 2 h. Organic phase was decanted and extracted
with water (2 x 40 mL). Aqueous phases were collected, washed with Et2O and decanted to
give an aqueous solution of methallyloxyphosphonium chloride. After the addition of KPF6
(3.68 g, 20 mmol) in 10 mL of water, an immediate precipitate was formed. The mixture was
filtered and the crude product was washed with Et2O and dried under vacuum.
Recrystallization from a mixture of chloroform: Et2O solution afforded salt 2 as a white solid.
1
Yield: 4.15 g (78%). IR [ν (cm-1)]: 836 (PF-6). H NMR (399.96 MHz, CDCl3): 1.78 (s, 3H,
CH3-allyl); 2.76 (d, 18H, (CH3)2N); 4.54 (d, 2H, CH2-O); 5.01 (s, 1H, CH2); 5.08 (s, 1H,
CH2). 13C NMR (100.43 MHz, CDCl3): 18.7 (CH3-allyl); 36.7 (CH3-N); 71.9 (CH2-O); 114.3
(CH2); 137.7 (C).
Synthesis of Pd(II) complexes
Synthesis of complex C1
Pd(dba)2 (150 mg, 0.26 mmol), the ligand L1 (30 mg, 0.13 mmol) and CH2Cl2 (20 mL) were
combined in a Schlenk tube and stirred at room temperature. After 15 min,
methallyloxytris(dimethylamino)phosphonium hexafluorophosphate (98.6 mg, 0.26 mmol)
was added and stirring continue overnight. The resulting solution was filtered through celite,
the solvent was removed under vacuum and the solid residue was washed with Et2O (3 x 20
mL) to give complex C1 as an orange solid in 91% yield. Recrystallization from a mixture of
CH2Cl2: Et2O solution afforded single crystals suitable for X-ray analysis. Anal. Calc. for
C26H28F12N4P2Pd2 (898.84): C, 34.74; H, 3.13; N, 6.23. Found: C, 35.56; H, 2.96; N, 6.43. IR
-
[ν (cm-1)]: 829 (PF6 ); 1627, 1591 (C=N). 1H NMR (399.96 MHz, CD3CN): 2.24 (s, 3H, CH3-
allyl); 3.45 (s, 2H, Hanti); 4.08 (s, 2H, Hsyn); 7.69 (s, 2H, Har); 7.84-7.92 (ddd, 1H, H6py); 8.17-
8

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8.23 (dd, 1H, H8py); 8.31-8.39 (td, 1H, H7py); 8.87 (s, 1H, H-C=N); 8.90-8.94 (dd, 1H, H5py).
13C NMR (100.43 MHz, CD3CN): 23.55 (C4); 63.79 (C1,3); 124.79, 130.72, 131.19, 137.50
(C2); 142.41, 150.94, 154.53, 155.55, 169.26 (C=N). 31P NMR (161.97 MHz, CD3CN): -
-
143.94 (sept, PF6 ).
Complex C2
Complex C2 was prepared in a similar manner to complex C1. Pd(dba)2 (150 mg, 0.26
mmol), the ligand L2 (34 mg, 0.13 mmol) and salt 2 (98.6 mg, 0.26 mmol). Yield : 87% .
Anal. Calc. for C28H32F12N4P2Pd2 (926.84): C, 36.28; H, 3.48; N, 6.04. Found: C, 36.68; H,
-
1
3.58; N, 5.93. IR [ν (cm-1)]: 830 (PF6 ); 1613, 1596 (C=N). H NMR (399.96 MHz MHz,
CD3CN): 2.19 (s, 3H, CH3-C=N); 2.49 (s, 3H, CH3-allyl); 3.27 (s, 2H, Hanti); 3.70 (s, 2H,
Hsyn); 7.31 (s, 2H, Har); 7.84-7.91 (ddd, 1H, H6py); 8.24-8.28 (dd, 1H, H8py); 8.29-8.40 (td, 1H,
13
H7py); 8.91-8.95 (dd, 1H, H5py). C NMR (100.43 MHz, CD3CN): 17.45 (CH3-C=N); 22.74
(C4); 62.11 (C1,3); 122.24, 127.69, 128.95, 129.79, 136.63 (C2); 141.15, 148.29, 154.39,
176.81 (C=N). 31P NMR (161.97 MHz, CD3CN): -144.64 (sept, PF6 ).
-
Complex C3
Complex C3 was prepared in a similar manner to complex C1. Pd(dba)2 (150 mg, 0.26
mmol), the ligand L3 (47 mg, 0.13 mmol) and salt 2 (98.6 mg, 0.26 mmol). Yield : 93%.
Anal. Calc. for C32H32F12N4P2Pd2 (974.84): C, 39.42; H, 3.3; N, 5.74. Found: C, 40.32; H,
-
3.04; N, 5.19. IR [ν (cm-1)]: 831 (PF6 ); 1618, 1593 (C=N). 1H NMR (399.96 MHz, CD3CN):
2.20 (s, 3H, CH3-allyl); 3.41 (s, 2H, Hanti); 4.03 (s, 2H, Hsyn); 7.63-7.64 (d, 2H, Har); 7.81-7.87
(ddd, 1H, H6py); 7.87-7.89 (d, 2H, Har); 8.14-8.20 (dd, 1H, H8py); 8.28-8.31 (td, 1H, H7py);
8.84 (s, 1H, H-C=N); 8.87-8.88 (dd, 1H, H5py). 13C NMR (100.43 MHz, CD3CN): 22.53
(C4); 62.67 (C1,3); 123.20, 128.15, 129.41, 129.92, 136.32 (C2); 140.35, 141.32, 149.23,
-
153.68; 154.44; 167.35 (C=N). 31P NMR (161.97 MHz, CD3CN): -144.61 (sept, PF6 ).
Complex C4
Complex C4 was prepared in a similar manner to complex C1. Pd(dba)2 (150 mg, 0.26
mmol), the ligand L4 (31 mg, 0.13 mmol) and salt 2 (98.6 mg, 0.26 mmol). Yield : 81% .
Anal. Calc. for C22H28F12N4P2Pd2 (851.25): C, 31.04; H, 3.32; N, 6.58. Found: C, 30.32; H,
9

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-
3.53; N, 5.88. IR [ν (cm-1)]: 838 (PF6 ); 1631, 1596 (C=N). 1H NMR (399.96 MHz, CD3CN):
2.24 (s, 3H, CH3-allyl); 3.36 (s, 2H, Haliphatique); 4.19 (s, 2H, Hanti); 4.37 (s, 2H, Hsyn); 7.75-
7.95 (ddd, 1H, H6py); 7.98-7.99 (dd, 1H, H8py); 8.18-8.26 (td, 1H, H7py); 8.59 (s, 1H, H-C=N);
8.81-8.84 (dd, 1H, H5py). 13C NMR (100.43 MHz, CD3CN): 22.59 (C4); 61.24 (Carbon
aliphatique) ; 62.23 (C1,3); 128.63, 129.84, 136.08 (C2); 141.22, 152.95, 154.34, 170.97
-
(C=N). 31P NMR (161.97 MHz, CD3CN): -144.62 (sept, PF6 ).
Crystal structure determination and refinement
X-ray quality crystals of C1 were grown from a mixture of CH2Cl2/n-hexane solution upon
standing at ambient temperature. An orange prism, was chosen by size, habit, and polarized
light microscopy and mounted on a glass fiber. Intensity data were collected at ambient
temperature on Bruker AXS Kappa-CCD Apex II diffractometer equipped with graphite
monochromated Mo Kα radiation. Reflection data were corrected for Lorentz-polarization
effects but not for absorption. The structure was solved by direct methods and subsequent
difference Fourier techniques (SIR-92) [31] and refined with SHELXL-2013 [24]. Non-
hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were
placed in idealized positions on parent atoms in the final refinement. Details of the crystal
parameters, data collection, and structure refinement are given in Table 1.
Acknowledgments
The authors thank USCR, D-RX service at the faculty of sciences of Sfax, Sfax University,
Tunisia for the realization of single crystal DRX analysis.
Supplementary material
CCDC 1057854 contains the crystallographic data for the structural analysis of C1. Copies of
this
information
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/Community/Requestastructure/Pages/DataRequest.aspx or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ; United Kingdom; Fax: +44 (0)1223 336033.
10

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•
•
This work represents a facile way to access cationic bis(methallylpalldium) complexes
bearing dinucleating nitrogen ligands.
The complexes were prepared via an oxidative addition to zerovalent palladium
compound.
•
•
Distorted square-planar arrangement around the palladium center.
Novel cationic bimetallic palladium precursors can be useful as catalysts.