Crystal Structure of Cationic η3-Methallylpalladium Complexes Bearing Aliphatic Iminopyridine Ligands

Article abstract of DOI:10.1134/S0022476619070126

Three aliphatic ligands are synthesized and fully characterized by IR, ¹H and ¹³C NMR spectroscopy. These ligands react with a zerovalent compound Pd(dba)₂ in the presence of methallyloxytris(dimethylamino) phosphonium hexafluorophosphate salt [C₄H₇OP(NMe₂)₃]⁺PF6? as an allylating agent for the synthesis of cationic η₃-methallylpalladium complexes to give three cationic mononuclear η₃3-methallylpalladium complexes in high yields. These formed complexes are characterized by IR, ¹H NMR and ¹³C NMR spectroscopy. One of them is characterized by X-ray diffraction. A DFT-optimized structure is also discussed.

Full text of DOI:10.1134/S0022476619070126

ISSN 0022-4766, Journal of Structural Chemistry, 2019, Vol. 60, No. 7, pp. 1110-1118. © Pleiades Publishing, Inc., 2019.
Text © The Author(s), 2019, published in Zhurnal Strukturnoi Khimii, 2019, Vol. 60, No. 7, pp. 1157-1164.
CRYSTAL STRUCTURE OF CATIONIC
3
η -METHALLYLPALLADIUM COMPLEXES
BEARING ALIPHATIC IMINOPYRIDINE LIGANDS
1
S. Dridi , A. Mechria *, and M. Msaddek
1
1
1 13
Three aliphatic ligands are synthesized and fully characterized by IR, H and C NMR spectroscopy. These
2
ligands react with a zerovalent compound Pd(dba) in the presence of methallyloxytris(dimethylamino)
+
] PF as an allylating agent for the synthesis of
−
phosphonium hexafluorophosphate salt [C
4
H
7 2 3
OP(NMe )
6
3 3
cationic η -methallylpalladium complexes to give three cationic mononuclear η -methallylpalladium
1
13
complexes in high yields. These formed complexes are characterized by IR, H NMR and C NMR
spectroscopy. One of them is characterized by X-ray diffraction. A DFT-optimized structure is also
discussed.
DOI: 10.1134/S0022476619070126
Keywords: methallylpalladium, cationic, aliphatic iminopyridine, crystal structure.
INTRODUCTION
Schiff bases with an imine moiety such as α- and β-diimines [1-8], 2,6-bis(imino)pyridines [9-16], pyridylimines,
and N-substituted 2-iminoalkylpyridines have significant importance in chemistry, especially in the development of Schiff
base complexes [17-43]. Many Schiff base complexes show an excellent catalytic activity in various reactions such as olefin
polymerisation [2-4, 6-15, 31, 40], hydrogenation and hydrosilylation [44], olefin epoxidation [45], organic transformations
[5, 16, 24, 25, 36, 37, 43], and aerobic oxidation reactions [46-48]. In addition, several complexes were described with
various metal centers and different ligand structures, and many studies have reported the effects of ligand substitution
patterns on the catalytic activity [49-53]. These structural variations include the increased presence of N-aryl substituted
2-iminomethylpyridine in these transition metal complexes. However, upon varying the steric and electronic properties of the
ligands, no substantial change has been observed in the properties of the complexes [54, 55]. On the other hand, N-alkyl
substituents in the iminopyridine ligands may adjust the steric and electronic interactions and tune the physical and chemical
properties of iminopyridine complexes [56-60].
Despite our previous reports on cationic methallyl palladium(II) complexes with N-aryl substituted pyridylimine
derivatives, little is known regarding cationic palladium complexes with bidentate N-substituted 2-iminoalkylpyridines
ligands. As part of our ongoing research, we report the synthesis of new cationic methallyl palladium(II) complexes
containing aliphatic iminopyridine ligands. The molecular structure of one of them was determined by X-ray crystallography.
1
Laboratoire de Chimie Hétérocyclique, Produits Naturels et Réactivité : L.C.H.P.N.R, Faculté des Sciences de
Monastir, Bd de l'Environnement, Monastir, Tunisia; *ali.mechria@fsm.rnu.tn. Original article submitted September 10,
2018; revised November 19, 2018; accepted November 28, 2018.
1110
0022-4766/19/6007-1110 © 2019 by Pleiades Publishing, Ltd.

EXPERIMENTAL
Materials and instruments. All reactions were carried out under the dry argon atmosphere using standard Schlenk
techniques. Solvents were refluxed over an appropriate drying agent and distilled prior to use. Heptylamine, isopentylamine,
cyclohexylamine, and pyridine-2-carboxaldehyde were purchased from Sigma-Aldrich. Pyridylimine aliphatic ligands [61],
Pd(dba)
2
[62], and methallyloxytris(dimethylamino)phosphonium hexafluorophosphate [63] were prepared according to the
literatures methods.
1
13
H NMR (600 MHz) and C NMR (150.91 MHz) spectra were recorded on a Bruker AMX 600 spectrometers using
CD CN as a solvent. H and C chemical shifts were given in ppm and referenced to the residual solvent resonance relative to
3
TMS. Infrared (IR) spectra were measured on a Perkin-Elmer Spectrum two FT-IR instruments with the Universal ATR
Sampling Accessory, while the elemental analyses (C, H, N) of the prepared complexes were performed on a Perkin-Elmer
2400 seriesII CHNS/O analyzer.
2 2 2
Synthesis of complex C1. Pd(dba) (100 mg, 0.173 mmol), the ligand L1 (30 mg, 0.17 mmol) and CH Cl (20 mL)
were combined in
dimethylamino)phosphonium hexafluorophosphate (64 mg, 0.17 mmol) was added and the stirring process was continued
overnight. The resulting solution was then filtered through celite. The solvent was removed under vacuum and the solid
a
Schlenk tube and stirred at room temperature. After 15 min, methallyloxytris
(
residue was washed with Et
2 2 2 2
O (3×20 mL) to give yellow solid complex C1. Recrystallization from a CH Cl :Et O mixture
afforded single crystals suitable for the X-ray analysis. Yield: 71 mg (85%). Anal. calc. for C H F N PPd (482.74):
15 23 6 2
–
1
−
1
C 37.32, H 4.80, N 5.80. Found: C 37.31, H 4.76, N 5.80. IR (ν, cm ): 824 (PF ); 1599, 1634 (C=N). H NMR
6
(600.13 MHz, CD CN): 0.82 (d, 6H, 2CH ); 1.50-1.53 (td, 2H, CH ); 1.78 (m, 1H, CH); 2.00 (s, 3H, CH -allyl); 3.11 (s, 2H,
3 3 2 3
2
Hanti); 3.76-3.78 (td, 2H, CH ); 3.91 (s, 2H, Hsyn); 7.57-7.59 (dd, 1H, H2py); 7.79-7.80 (d, 1H, H4py); 8.04-8.07 (td, 1H, H3py);
13
3
8.42 (s, 1H, HC=N); 8.62-8.63 (d, 1H, H1py). C NMR (150.91 MHz, CD CN): 22.77, 23.69 (C15); 26.59, 39.89, 61.83
(C
12,14); 62.73, 128.92, 130.31, 136.55 (C13), 142.21, 154.79, 155.32; 168.91 (C=N).
Synthesis of complex C2. Complex C2 was prepared in a similar manner as used for the synthesis of complex C1
2
with Pd(dba) (100 mg, 0.173 mmol), the ligand L2 (35 mg, 0.17 mmol), and salt 2 (64 mg, 0.17 mmol). Yield: 65 mg (74%).
–
1
−
6
Anal. calc. for C17
27 6 2
H F N PPd (510.79): C 39.97, H 5.32, N 5.48. Found: C 39.08, H 5.87, N 5.55. IR (ν, cm ): 828 (PF );
1
599, 1639 (C=N). H NMR (600.13 MHz, CD CN): 0.87-0.92 (t, 3H, CH ); 1.31-1.37 (ddd, 8H, 4(CH )); 1.76-1.81 (t, 2H,
1
3
3
2
CH
.94-7.97 (d, 1H, H4py); 8.19-8.22 (td, 1H, H3py); 8.56 (s, 1H, HC=N); 8.78-8.79 (d, 1H, H1py). C NMR (150.91 MHz,
CD CN): 13.04 (CH ), 22.01 (C15); 22.35, 25.92, 28.33, 29.60, 31.14, 60.52, 63.11 (C12,14); 127.62, 129.01, 135.27, 140.90,
2 3 2
); 2.16 (s, 3H, CH -allyl); 3.27 (s, 2H, Hanti); 3.89-3.94 (td, 2H, CH ); 4.08 (s, 2H, Hsyn); 7.72-7.76 (ddd, 1H, H2py);
13
7
3
3
153.49 (C13), 154.01; 167.57 (C=N).
Synthesis of complex C3. Complex C3 was prepared in a similar manner as used for the synthesis of complex C1
2
with Pd(dba) (100 mg, 0.173 mmol), the ligand L3 (32 mg, 0.17 mmol) and salt 2 (64 mg, 0.17 mmol). Yield: 59 mg (69%).
–
1
−
6
Anal. calc. for C16
H
23 6
F N
2
PPd (494.75): C 38.84, H 4.68, N 5.66. Found: C 38.13, H 4.34, N 5.42. IR (ν, cm ): 827 (PF );
1
596, 1631 (C=N). H NMR (600.13 MHz, CD CN): 1.07-1.15 (qt, 2H, amine ring); 1.24-1.32 (qt, 3H, amine ring); 1.33-
3
1
3
1.40 (qt,3H, amine ring); 1.56-1.59 (dq, 2H, amine ring); 2.00 (s, 3H, CH -allyl); 3.14 (s, 2H, Hanti); 3.49-3.54 (tt, 1H, CH,
amine ring); 4.01 (s, 2H, Hsyn); 7.56-7.58 (ddd, 1H, H2py); 7.79-7.80 (d, 1H, H4py); 8.04-8.07 (td, 1H, H3py); 8.48 (s, 1H,
13
HC=N); 8.62-8.64 (d, 1H, H1py). C NMR (150.91 MHz, CD
29.05, 130.23, 135.65, 142.23, 154.86 (C13), 155.21; 166.90 (C=N).
Crystal structure determination and refinement. X-ray quality crystals of C1 were grown from a CH
solution which was kept standing at ambient temperature. Data were collected on a Bruker-Nonius Kappa-CCD
diffractometer using graphite monochromated MoK radiation (λ == 0.71073 Å) at ambient temperature. Absorption
3
CN): 23.57 (C15); 25.23, 25.90, 33.98, 62.52 (C12,14); 71.32,
1
2 2 2
Cl /Et O
α
correction was applied using the multi-scan method [64]. The structure was solved by direct methods [65] and refined by the
2
full matrix least squares method [66] on F against all reflections, using anisotropic displacement parameters for non-H
1111

TABLE 1. Crystal, Collection, and Refinement Data
Chemical formula
Formula weight
T, K
15 23 6 2
C H F N P Pd
482.72
293(2)
Triclinic
P-1
Crystal system
Space group
a, b, c, Å
10.2530(9), 10.4100(10), 11.3460(17)
α, β, γ, deg.
101.418(9), 104.697(11), 117.269(8)
3
V, Å
969.18(19)
2
Z
3
Density, g/cm
1.654
–
1
μ, mm
1.095
Refl. coll.
Max. θ, deg.
Indep. refl.
9594
27.5
4315
R
int
0.0265
Data / restraints / param.
R, wR (I > 2σ(I ))
R, wR (all data)
4315 / 101 / 336
0.0387, 0.0974
0.0508, 0.1109
0.800, –0.679
3
Max. peak and hole, e/Å
atoms, with the aid of the WinGX program [67]. H atoms of phenyl, alkyl, and imino groups were determined
stereochemically and refined by employing the riding model. H atoms of the allyl group were found in Fourier difference
maps and their coordinates were refined. For all H atoms, Uiso was fixed at 1.2 times Ueq of the carrier atom (1.5 in the case of
H atoms of the methyl group). On analyzing the Fourier maps, a static disorder was found in the alkyl tail and the
hexafluorophosphate ion. The two split positions were refined with some restraints on bond lengths and thermal parameters.
A summary of crystal, collection, and refinement data is shown in Table 1. This analysis of the crystal packing was
performed using the Mercury program [68].
Density functional theory (DFT) details. Calculations using the DFT method were carried out to obtain
an overview of the electronic structures and bonding properties of complex C1. The DFT [69-70] calculations were
performed with the long-range corrected hybrid density meta-GGA functional wB97XD [71-73] with dispersion corrections.
In this study, the double-zeta Pople-type 6-31G(d,p) basis set [74] was chosen for all atoms except palladium which was
described by the widely used double zeta Los Alamos National Laboratory 2 basis set LANL2DZ [75-77] along with the
corresponding effective core potentials (ECPs). This mixed basis set was created through the use of the GEN keyword in
Gaussian 09.
RESULTS AND DISCUSSION
The synthesis routes of ligands and cationic methallyl Pd(II) complexes are shown in Scheme 1. The ligands were
obtained in good yields of 98% (L1), 96% (L2), 98% (L3) from the condensation reaction between pyridine-2-
carboxaldehyde and appropriate X-amine (X = isopentyl, heptyl, cyclohexyl) in dichloromethane [61]. These ligands
containing N-alkyl imine groups showed a higher solubility than N-aryl imine groups. New cationic methallyl complexes
C1–C3 of palladium(II) were obtained in high yields by the addition of methallyloxyphosphonium salt to zerovalent Pd(dba)
2
in the presence of the iminopyridine aliphatic ligand. The reaction was carried out in methylene chloride at room temperature
Scheme 1).
(
1112

Scheme 1. Synthesis of pyridylimine aliphatic ligands and Pd(II) complexes.
These diamagnetic Pd(II) complexes C1–C3 were isolated as yellow solids. The solubility of these new aliphatic
pyridinylimine complexes has grown considerably in common solvents compared to N-aryl substituted ones.
The variation of the steric and electronic proprieties of iminopyridine ligands was insured by changing the aliphatic
substituent at the imine N atom.
–1
The FT-IR spectra of C1–C3 complexes showed typical lower absorption bands between 1631 cm and 1639 cm
–1
compared to their respective pyridylimine aliphatic ligands [61]. IR spectra of cationic complexes showed the characteristic
−
–1
–1
3
band of the PF counterion between 824 cm and 828 cm . However, the NMR spectra of C1–C3 were recorded in CD CN;
6
1
the proton numbering is shown in Scheme 1. The H NMR spectra of all complexes showed similar trends, especially in the
allyl groups, which were observed as singlet signals found to be around 4 ppm assigned to Hsyn and another singlet found to
be around 3.14 ppm assigned to Hanti, while the methyl protons resonatedat 2 ppm.
In addition to the corresponding, the aromatic region of the pyridine ring (δ range 7.56-8.63 ppm) shifted downfield
by approximately δ 0.3-0.6 ppm in complexes (C1–C3) in comparison to those of the free ligands L1–L3. The change in the
chemical shift of the aldimine (CH=N) protons in all the complexes confirmed the coordination of the ligand to the metal.
13
The C NMR peaks of the cationic palladium complexes were shifted to a low field by approximately δ 6-8 ppm
compared to their free ligands.
The IR and NMR spectroscopy confirmed the coordination of iminopyridine aliphatic and methallyl ligands to the
zerovalent compound Pd(dba)2.
Crystal growth. A single crystal of the C1 complex (Fig. 1) suitable for the X-ray analysis was grown by slow
evaporation of a dichloromethane/diethyl ether solution at room temperature. The single crystal X-ray structural analysis of
complex C1 showed a distorted square planar geometry around the palladium atom consisting of two N atoms (N1 and N2) of
the ligand and two carbon atoms (C12 and C14) of the allyl group.
Crystallographic data and structural refinement parameters are shown in Table 1 while selected bond lengths and
angles are presented in Tables 2 and 3. Complex C1 showed a structure consisting of two entities of a freely associated
3
+
−
6
[
4 7
(η -C H )Pd(L1)] cation and a PF counter anion without direct interactions, which is evident from the large distance
between metal and the nearby fluorine atom (Pd–F3a = 4.395 Å).
In complex C1, the Pd(1)–Nimine bond length (Pd(1)–N2 = 2.091(3) Å) is shorter than that of the Pd–Npyridine bond
length (Pd(1)–N1 = 2.100(3) Å) due to different basicities of the imine and pyridine groups, as compared with the bond
lengths Pd–N1 = 2.020(7) Å and Pd–N2 = 2.044(6) Å observed in the related palladium complex [78]. The Pd–C bond length
ranged within 2.100(5)–2.139(4) Å compared to that of the Pd–C bond length observed in other related palladium complexes
[79-83].
1113

Fig. 1. ORTEP representation of the molecular structure of C1 with the atom labelling
scheme and anisotropic displacement ellipsoids depicted at a 30% probability.
Hydrogen atoms are omitted for clarity.
3
+
−
4 7
TABLE 2. Experimental and Theoretical Bond Distances (Å) of [(η -C H )Pd(L1)] PF6
Bond length
Experimental
Theoretical
Pd–N1
Pd–N2
Pd–C12
Pd–C13
Pd–C14
N1–C1
N1–C5
N2–C6
N2–C7
2.100(3)
2.091(3)
2.100(5)
2.139(4)
2.120(5)
1.332(5)
1.339(5)
1.248(6)
1.489(7)
2.170
2.165
2.165
2.216
2.163
1.338
1.358
1.282
1.468
3
+
−
4 7
TABLE 3. Experimental and Theoretical Bond Angles (deg.) of [(η -C H )Pd(L1)] PF6
Bond length
Experimental
Theoretical
N1–Pd–N2
N1–Pd–C12
N2–Pd–C14
N1–Pd–C13
N2–Pd–C13
C12–Pd–C14
N1–Pd–C14
N2–Pd–C12
C12–Pd–C13
C13–Pd–C14
78.39(13)
105.60(19)
108.26(19)
137.86(16)
139.83(17)
67.4(2)
77.75
107.18
108.17
138.16
139.06
66.77
172.01(17)
173.70(19)
38.6(2)
173.45
174.38
37.72
37.9(2)
37.77
The N(1)–Pd(1)–N(2) bond angle of 78.39(13)° in C1 is relatively small, as a result of chelating ligand steric
constraints. The metal-chelate ring (Pd(1)–N1–C5–C6–N2) in complex C1 is almost flat as indicated by the torsion angles of
2.3(5)°, –2.5(4)°, –0.7(4)° for N1–C5–C6–N2, Pd(1)–N(1)–C5–C6, and Pd(1)–N(2)–C6–C5, respectively. The C15–C13–
C12–C14 torsion angle of 167.44(8)° indicated that the methyl on the allyl group is slightly inclined out of the allyl plane by
1114

3
Fig. 2. DFT-optimized structure of the [(η -C H )Pd(L1)]
+
4
7
cation with the same atom labelling scheme as in the
ORTEP representation. Hydrogen atoms are omitted for
clarity.
approximately 12°. The allyl plane form an angle of 112.1(4)° with the palladium coordinative plane, that is normal for
3
η -2-methylallyl complexes of palladium [79-83].
3
+
4 7
DFT calculations. The geometry around the Pd atom of the [(η -C H )Pd(L1)] cation reveals a distorted square
planar arrangement (Fig. 2). The formal charge of palladium is 2+ in complex C1. The calculated charge on the Pd atom
slated from the natural population analysis is 0.190. This result may be obtained from a charge donation from the ligand to
the metal center and also showed a strong σ-donor character of the ligand.
The experimental data are well replicated in the calculations, as one can see from the data given in Tables 2 and 3. In
general, the predicted bond lengths and angles are in good agreement with the values of the X-ray crystal structure data. It
can be seen from the data collected in Tables 2 and 3 that the bond angles changed maximum by 2° while the bond lengths
are maxim elongated by 0.07 Å in the calculated gas phase structure. Experimental and theoretical geometric parameters are
summarized in Tables 2 and 3.
CONCLUSIONS
The oxidative addition of methallyloxytris(dimethylamino)phosphonium hexafluorophosphate to zerovalent
2
Pd(dba) in the presence of pyridylimine aliphatic compounds produced new monometallic cationic methallyl palladium(II)
complexes in which the ligands coordinated via the pyridine and imine nitrogen atoms. The coordination geometry around the
Pd(II) centre in cationic pyridylimine Pd(II) complex C1 was distorted square planar. The DFT calculation using the
wB97XD functional indicated that the general trends observed in the experimental data were well replicated in the
calculations, as one could see from the data given in Tables 2 and 3. Catalytic properties of complexes C1–C3 are being
investigated.
ACKNOWLEDGMENTS
The authors thank USCR, D-RX service at the Department of Chemical Sciences, University of Naples Federico II,
Naples, Italy for the performance of the single crystal XRD analysis.
1115

ADDITIONAL INFORMATION
Supplementary material. CCDC 1557908 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.
CONFLICT OF INTERESTS
The authors declare that they have no conflict of interests.
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