Bis(oxamato)palladate(II) complexes: synthesis, crystal structure and application to catalytic Suzuki reaction

Article abstract of DOI:10.1080/00958972.2014.975697

New bis(oxamato)palladate(II) complexes, [Pd(H₂O)₄][Pd(2,6-Me₂pma)₂]·2H₂O (1), (n-Bu₄N)₂[Pd(2,6-Me₂pma)₂]·2H₂O (2a), and (n-Bu₄N)₂[Pd(2,6-Me₂pma)₂]·2CHCl₃ (2b) (2,6-Me₂pma = N-2,6-dimethylphenyoxamate and n-Bu₄N⁺ = tetra-n-butylammonium), have been synthesized and the structures of 1 and 2b characterized by single-crystal X-ray diffraction. Complex 1 is a double salt constituted by tetraaquapalladium(II) cations and bis(oxamato)palladate(II) anions interlinked by hydrogen bonds. The palladium(II) ions in 1 are four-coordinate with two oxygens and two nitrogens from two fully deprotonated oxamate ligands (anion), and four water molecules (cation) building centrosymmetric square-planar surroundings. Centrosymmetric bis(oxamato)palladate(II) anions occur in 2b as in 1, the charge balance in this compound being ensured by the bulky n-Bu₄N⁺. The catalytic role of 1 and 2a for the Suzuki reaction has been investigated by using a series of aryl iodide/bromide derivatives in the conventional organic medium dimethylformamide. The tetraaquapalladium(II) unit in 1 appears to be active in the catalytic Suzuki cross-coupling reactions, but it readily decomposes to inactive palladium black.

Full text of DOI:10.1080/00958972.2014.975697

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Bis(oxamato)palladate(II) complexes:
synthesis, crystal structure and
application to catalytic Suzuki reaction
Francisco Ramón Fortea-Péreza, Donatella Armentanob, Miguel
Julvea, Giovanni De Munnob & Salah-Eddine Stiribaa
a Instituto de Ciencia Molecular (ICMol), Universidad de Valencia,
Valencia, Spain
b CAL, Departimento di Chimica i Tecnologie Chimiche, Università
della Calabria, Cosenza, Italy
Accepted author version posted online: 13 Oct 2014.Published
online: 10 Nov 2014.
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To cite this article: Francisco Ramón Fortea-Pérez, Donatella Armentano, Miguel Julve, Giovanni
De Munno & Salah-Eddine Stiriba (2014): Bis(oxamato)palladate(II) complexes: synthesis, crystal
structure and application to catalytic Suzuki reaction, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2014.975697
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Journal of Coordination Chemistry, 2014
http://dx.doi.org/10.1080/00958972.2014.975697
Bis(oxamato)palladate(II) complexes: synthesis, crystal
structure and application to catalytic Suzuki reaction
FRANCISCO RAMÓN FORTEA-PÉREZ†, DONATELLA ARMENTANO*‡,
MIGUEL JULVE*†, GIOVANNI DE MUNNO‡ and SALAH-EDDINE STIRIBA*†
†Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Valencia, Spain
‡CAL, Departimento di Chimica i Tecnologie Chimiche, Università della Calabria, Cosenza, Italy
(Received 16 September 2014; accepted 29 September 2014)
New bis(oxamato)palladate(II) complexes, [Pd(H2O)4][Pd(2,6-Me2pma)2]·2H2O (1), (n-Bu4N)2[Pd
(2,6-Me2pma)2]·2H2O (2a), and (n-Bu4N)2[Pd(2,6-Me2pma)2]·2CHCl3 (2b) (2,6-Me2pma = N-2,6-
dimethylphenyoxamate and n-Bu4N+ = tetra-n-butylammonium), have been synthesized and the
structures of 1 and 2b characterized by single-crystal X-ray diffraction. Complex 1 is a double salt
constituted by tetraaquapalladium(II) cations and bis(oxamato)palladate(II) anions interlinked by
hydrogen bonds. The palladium(II) ions in 1 are four-coordinate with two oxygens and two nitro-
gens from two fully deprotonated oxamate ligands (anion), and four water molecules (cation) build-
ing centrosymmetric square-planar surroundings. Centrosymmetric bis(oxamato)palladate(II) anions
occur in 2b as in 1, the charge balance in this compound being ensured by the bulky n-Bu4N+. The
catalytic role of 1 and 2a for the Suzuki reaction has been investigated by using a series of aryl
iodide/bromide derivatives in the conventional organic medium dimethylformamide. The tetraaqua-
palladium(II) unit in 1 appears to be active in the catalytic Suzuki cross-coupling reactions, but it
readily decomposes to inactive palladium black.
Keywords: Palladium(II); Oxamate; Crystal structure; Double salt; Suzuki reaction
1. Introduction
N,N′-substituted oxamate or N-substituted oxamidate ligands form very stable complexes
with copper(II) ions in aqueous solution [1]. Understanding the species formed in this
*Corresponding authors. Email: donatella.armentano@unical.it (D. Armentano); miguel.julve@uv.es (M. Julve);
salah.stiriba@uv.es (S.-E. Stiriba)
© 2014 Taylor & Francis

2
F.R. Fortea-Pérez et al.
medium has allowed the rational preparation of a great variety of homo and heterometallic
species [2, 3]. This stability as well as the remarkable ability of oxamate and oxamidate to
mediate magnetic interactions between paramagnetic metal ions bridged by them [4]
accounts for their use in molecular magnetism being for instance at the origin of a rational
approach to molecule-based magnets [5], the introduction of the concept of irregular spin
state [6], and to the extension of the spin polarization mechanism to coordination com-
pounds [7]. More recent efforts with specifically substituted oxamate ligands have been
directed towards the achievement of molecule-based multifunctional magnetic materials [8],
illustrative examples being the preparation and characterization of single ion magnets [9],
single molecule magnets [10], chiral single-chain magnets [11], porous/luminescent magnets
[12], magnetic sensors of small guest molecules [13], pH-controlled switches for reversible
formation of emulsions [14], and chiral 3-D metal–organic polymers [15].
Very recently, diamagnetic square-planar oxamate-containing palladium(II) complexes
have been found to exhibit cytotoxic activity against leukemia cells [16] and also to act as
very stable and highly efficient catalysts in palladium-catalyzed Suzuki and Heck reactions
using active and less reactive aryl halides [17]. These results give an added value to the
coordination chemistry of the oxamate ligands from both biological and organic catalysis
viewpoints.
In the context of our research program with oxamate complexes covering these interest-
ing Pd(II) systems, we have isolated the complexes [Pd(H2O)4][Pd(2,6-Me2pma)2]·2H2O
(1), (n-Bu4N)2[Pd(2,6-Me2pma)2]·2H2O (2a), and (n-Bu4 N)2[Pd(2,6-Me2pma)2]·2CHCl3
(2b) (2,6-Me2pma = N-2,6-dimethylphenyoxamate). Their preparation, spectroscopic and
structural characterization, and the study of their catalytic activity in the palladium-
catalyzed Suzuki cross-coupling reaction of aryl halides and phenylboronic acid are subject
of the present work.
2. Experimental
2.1. Materials and methods
NMR spectra (1H and 13C) were recorded on an Avance DRX 300 Bruker instrument at
room temperature and referenced to residual protons in the solvent (1H) or the solvent 13C
signal (13C). Elemental analyses (C, H, and N) were carried out on a EuroEA3000 analyzer
by the Servei Central d’Instrumentació Científica at the University of Jaume I. FTIR spectra
(4000–450 cm-1) were recorded with a Nicolet 5700 FTIR instrument. GC–MS spectra were
performed with an Agilent 5973N mass spectrometer equipped with capillary columns
(split/splitless, pulsed split, and pulsed splitless) at the Servicio Central de Soporte a la
Investigación Experimental of the University of Valencia. The SEM-Edax microscopy anal-
ysis was performed on a Philips mod. XL 30 ESEM with microanalysis EDAX mod. PV
9760 and HOT STAGE to 1000 °C.
2.2. Synthesis of the N-2,6-dimethylphenyloxamate proligand (EtH-2,6-Me2pma)
The ethyl ester derivative of N-2,6-dimethylphenyloxamic acid was prepared using the
following synthetic procedure: 2,6-dimethylphenylaniline (83 mM) was dissolved in THF
(250 mL), under a dinitrogen atmosphere in a two-necked round flask equipped with a

Bis(oxamato)palladate(II)
3
condenser and subsequently treated with ethyl chlorooxoacetate (9.3 mL, 83 mM) in the
presence of triethylamine (12 mL, 83 mM), at room temperature under continuous stirring
for 30 min. The resulting solution was filtered and the solvent was removed under vacuum
to afford an oily colorless crude, which quickly becomes solid. The white solid was sus-
pended into water and filtered off, then washed with a small amount of diethyl ether, and
dried under vacuum. Yield: 96%. IR (KBr/cm−1): 3244 (N–H), 3027, 2977, 2923 (C–H),
1
1727, 1677 (C=O). H NMR (CDCl3) δ(ppm): 1.42–1.45 (t, 3H, CH3), 2.25 (s, 6H, CH3),
4.42–4.46 (q, 2H, CH2), 7.11–7.13 (m, 3H, Haromatic), 8.46 (s, 1H, NH); 13C NMR (CDCl3)
δ(ppm): 14.4, 18.8, 64.0, 128.3, 128.6, 132.5, 135.4, 155, 161.3. Anal. Calcd for
C12H15NO3 (221 g M−1): C, 65.14; H, 6.83; N, 6.33. Found: C, 65.31; H, 6.73; N, 6.40%.
2.3. Synthesis of [Pd(H2O)4][Pd(2,6-Me2pma)2]·2H2O (1)
An aqueous solution of KOH (15 mL, 1.2 mM) was poured into a two-necked flask con-
taining suspension of the EtH-N-2,6-Me2pma proligand (0.6 mM) in 10 mL of CH3CN.
Then, an aqueous solution of K2[PdCl4] (0.2 g, 0.6 mM) was added dropwise under contin-
uous stirring and the mixture was heated at 60 °C overnight. The resulting yellow solution
afforded yellow parallelepipeds within 2–3 days, upon very slow evaporation at room tem-
perature. Yield: 88%. IR (KBr/cm−1): 3423 3327, 3246, 3155 (O–H), 2911 (C–H), 1670,
1638, 1630, 1577 (C=O). Anal. Calcd for C20H30N2O12Pd2 (1, 702 g M−1): C, 34.16; H,
4.30; N, 3.98. Found: C, 34.01; H, 4.22; N, 4.05%.
2.4. Synthesis of (n-Bu4 N)2[Pd(2,6-Me2pma)2]·2H2O (2a) and (n-Bu4 N)2[Pd(2,6-
Me2pma)2]·2CH2Cl3 (2b)
A methanolic solution of 1.0 M n-Bu4NOH (1.5 mL, 1.2 mM) was added directly to a
suspension of the proligand (0.6 mM, 10 mL of CH3CN) in a two-neck round flask under
continuous stirring. Then, an aqueous solution of K2PdCl4 (0.1, 0.30 mM) was added
dropwise to the resulting solution and the reaction mixture was heated at 60 °C for 10 h.
The resulting mixture was filtered and the volume reduced under vacuum. The remaining
concentrated solution was treated three times with dichloromethane to extract the complex
from the aqueous solution. The addition of n-hexane to the dichloromethane solution affor-
ded 2a, a pale yellow powder which was collected by fltration and dried in the open air.
Yield: 89%. X-ray quality yellow prisms of 2b were grown by slow recystallization of 2a
in a chloroform/n-hexane mixture. The formula of 2b was determined by X-ray diffraction
on single cystals. Spectroscopic data for 2a are given as follows: IR (KBr/cm−1): 3433
1
(O–H), 2961, 2936, 2873 (C–H), 1670, 1638, 1604, 1582 (C=O). H NMR (DMSO-d6)
δ(ppm): 0.90–0.94 (t, 12H, n-Bu4N+), 1.30–1.32 (m, 16H, TBA), 1.55–1.58 (m, 16H,
n-Bu4N+), 2.24 (s, 24H, CH3), 3.13–3.16 (m, 16H, TBA), 6.80–6.87 (m, 6H, Haromatic).
Anal. Calcd for C52H94N4O8Pd (2a, 1008 g M−1): C, 61.85; H, 9.38; N, 5.55. Found: C,
62.05; H, 9.19; N, 5.48%.
2.5. General procedure for the Suzuki reaction
A test tube with screw cap and valve was charged with a magnetic stir bar, the pre-catalyst
(1 or 2a; 1 M% Pd with respect to the aryl halide), the aryl halide (0.50 mM), Et3N
(1.00 mM), the aryl boronic acid (0.75 mM), and dimethylformamide (DMF) (4 mL). The

4
F.R. Fortea-Pérez et al.
mixture was heated under continuous strirring at 80 °C in the open air. The reaction was
monitored using liquid chromatography on silica gel. The reaction mixture was cooled,
diluted with water (3 mL), and extracted three times (3 × 5 mL) with n-pentane. The
extracts were combined and dried over magnesium(II) sulfate. The yield was determined by
GC–MS analysis using perfluorotributylamine (PFTBA).
2.6. X-ray crystallography
Single-crystal X-ray diffraction data for 1 and 2b were collected at 296 K on a Bruker-Nonius
X8-APEXII CCD area detector system using graphite-monochromated Mo Kα radiation
(λ = 0.71073 Å) and processed through the SAINT [18] reduction and SADABS [19] absorp-
tion software. Due to the poor crystal quality, a lower θmax of diffraction was obtained for 2b,
even if all possible steps were undertaken to ensure that the experiment was able to extract
the best diffracting power from the sample. However, since the solution and refinement
parameters are reasonable, we are confident that the crystal structure found is consistent. The
structures were solved by direct methods and subsequently completed by Fourier recycling
using SHELXTL-2013 [20] software package, then refined by full-matrix least-squares
refinements based on F2 with all observed reflections. All non-hydrogen atoms were refined
anisotropically in the case of 1, while they were refined isotropically for 2b in order to reduce
the number of parameters. The hydrogens from the 2,6-dimethylphenyl substituent on the
oxamate (1 and 2b) and those from the tetra-n-butylammonium cations and chloroform lattice
molecules (2b) were included at geometrically calculated positions and refined using a riding
Table 1. Crystallographic data and structure refinement for 1 and 2b.
Compound
1
2b
Empirical formula
Formula weight
Temperature
C
20H30N2O12Pd2
C54H92Cl6N4O6Pd1
1212.42
296(2) K
703.26
296(2) K
Wavelength
0.71073 Å
0.71073 Å
Crystal system
Space group
Monoclinic
C2/c
Monoclinic
P21/n
Unit cell dimensions
a = 22.620(2) Å
b = 7.7498(7) Å
c = 15.5036(15) Å
108.005(7)°
a = 11.940(4) Å
b = 18.197(7) Å
c = 14.892(5) Å
92.922(10)°
β
Volume
2584.7(4) Å3
3231(2) Å3
Z
4
2
Density (calculated)
Absorption coefficient
F (0 0 0)
1.807 Mg m−3
1.246 Mg m−3
1.454 mm−1
0.580 mm−1
1408
1268
Crystal size
0.08 × 0.12 × 0.10 mm3
1.893–27.000°
21312
0.04 × 0.08 × 0.06 mm3
2.134–12.995°
10301
Theta range for data collection
Reflections collected
Independent reflections
Refinement method
Data/restraints/parameters
Final R indices [I > 2σ(I)]
R indices (all data)
cGoodness-of-fit on F2
Largest diff. peak and hole
2820 [R(int) = 0.0252]
Full-matrix least-squares on F2
2820/9/185
846 [R(int) = 0.0633]
Full-matrix least-squares on F2
846/0/136
aR1 = 0.0449, bwR2 = 0.0929
R1 = 0.0519, wR2 = 0.0977
1.108
R1 = 0.0799, wR2 = 0.2094
R1 = 0.0841, wR2 = 0.2128
1.185
3.137 and −2.201 e Å−3
0.434 and −0.312 e Å−3
aR1 = ∑(|Fo| − |Fc|)/∑|Fo|.
bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2
.
cS = [∑w(|Fo| − |Fc|)2/(No − Np)]1/2
.

Bis(oxamato)palladate(II)
5
model. The hydrogens on water molecules in 1 were located on the ΔF map and refined with
restraints on O–H distances and H–O–H angles. The high residual maxima and minima in the
final Fourier difference maps in 1 (3.137 and −2.201 e Å−3, respectively) were near Pd(2).
Crystal data and refinement conditions (1 and 2b), selected bond lengths (1 and 2b) bond
angles (1 and 2b) and hydrogen bonds (1) are listed in tables 1–4.
Table 2. Selected bond lengths (Å) for 1 and 2b.
Compound 1
Pd(1)–O(1)
Pd(1)–N(1)
Pd(2)–O(2W)
Pd(2)–O(1W)
O(1)–C(2)
O(2)–C(1)
O(3)–C(2)
N(1)–C(1)
N(1)–C(3)
2.020(3)
2.023(3)
2.045(5)
2.051(5)
1.291(5)
1.258(5)
1.237(5)
1.329(5)
1.439(5)
C(1)–C(2)
C(3)–C(4)
C(3)–C(8)
C(4)–C(5)
C(4)–C(10)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
1.546(6)
1.402(6)
1.405(5)
1.401(6)
1.519(6)
1.389(7)
1.389(7)
1.409(6)
1.511(6)
Compound 2b
Pd(1)–O(1)
Pd(1)–N(1)
O(3)–C(2)
O(2)–C(1)
O(1)–C(1)
1.97(2)
2.03(2)
1.23(3)
1.20(4)
1.25(4)
N(1)–C(2)
N(1)–C(3)
C(1)–C(2)
C(4)–C(10)
C(8)–C(9)
1.29(3)
1.42(2)
1.62(4)
1.52(3)
1.49(3)
Table 3. Selected bond angles (°) for 1 and 2b.*&
Compound 1
O(1)–Pd(1)–O(1a)
O(1)–Pd(1)–N(1a)
O(1)–Pd(1)–N(1)
N(1a)–Pd(1)–N(1)
O(2Wb)–Pd(2)–O(1W)
O(2W)–Pd(2)–O(1W)
O(1Wb)–Pd(2)–O(1W)
C(2)–O(1)–Pd(1)
C(1)–N(1)–Pd(1)
C(3)–N(1)–Pd(1)
C(1)–N(1)–C(3)
180.0(1)
98.4(1)
81.6(1)
180.0
O(3)–C(2)–C(1)
O(1)–C(2)–C(1)
C(4)–C(3)–C(8)
C(4)–C(3)–N(1)
C(6)–C(5)–C(4)
C(5)–C(6)–C(7)
C(6)–C(7)–C(8)
C(8)–C(3)–N(1)
C(5)–C(4)–C(10)
C(3)–C(4)–C(10)
C(5)–C(4)–C(3)
C(3)–C(8)–C(7)
C(3)–C(8)–C(9)
C(7)–C(8)–C(9)
119.1(4)
116.8(3)
121.9(4)
117.4(3)
120.7(4)
119.8(4)
121.6(4)
120.7(4)
121.1(4)
120.2(4)
118.7(4)
117.4(4)
121.3(4)
121.3(4)
91.5(3)
177.3(2)
88.6(3)
114.0(2)
114.9(3)
123.7(2)
121.2(3)
127.6(4)
119.8(4)
112.5(3)
124.1(4)
O(2)–C(1)–N(1)
O(2)–C(1)–C(2)
N(1)–C(1)–C(2)
O(3)–C(2)–O(1)
Compound 2b
O(1)–Pd(1)–O(1a)
O(1)–Pd(1)–N(1)
O(1)–Pd(1)–N(1a)
C(1)–O(1)–Pd(1)
C(2)–N(1)–C(3)
C(2)–N(1)–Pd(1)
C(3)–N(1)–Pd(1)
O(2)–C(1)–O(1)
O(2)–C(1)–C(2)
O(1)–C(1)–C(2)
180.0(11)
80.7(9)
99.3(9)
118(3)
122(3)
116(3)
121.9(18)
132(4)
114(4)
O(3)–C(2)–N(1)
O(3)–C(2)–C(1)
N(1)–C(2)–C(1)
C(4)–C(3)–N(1)
C(8)–C(3)–N(1)
C(3)–C(4)–C(10)
C(5)–C(4)–C(10)
C(7)–C(8)–C(9)
C(3)–C(8)–C(9)
129(4)
120(4)
111(4)
119(2)
121(2)
121(2)
119(2)
121(2)
119(2)
114(4)
*Symmetry transformations used to generate equivalent atoms: (a) −x + 3/2, −y + 7/2, −z + 1. (b) −x + 1, y, −z + 1/2.
&Symmetry transformations used to generate equivalent atoms: (a) −x + 1, −y + 2, −z.

6
F.R. Fortea-Pérez et al.
Table 4. Hydrogen bonds for 1.*
D–H⋯A
d(D–H), Å
d(H⋯A), Å
d(D⋯A), Å
<(D–H⋯A), °
O(1W)–H(1W1)⋯O(2c)
O(1W)–H(1W1)⋯O(3c)
O(2W)–H(2W2)⋯O(2)
O(3W)–H(3W1)⋯O(3)
O(3W)–H(3W2)⋯O(3d)
0.96(3)
0.96(3)
0.99(3)
0.95(3)
0.96(3)
2.31(4)
2.24(3)
1.97(3)
2.17(4)
2.02(3)
3.095(6)
3.083(6)
2.958(7)
3.005(6)
2.945(6)
139(4)
146(4)
175(4)
146(5)
163(5)
*Symmetry transformations used to generate equivalent atoms: (c) −x + 1, y − 1, −z + 1/2; (d) −x + 1, −y + 4, −z + 1.
3. Results and discussion
3.1. Syntheses and spectroscopic characterization
The preparations of 1 and 2a were done by following the same synthetic methodology that
has been employed for other mononuclear bis(oxamato)palladate(II) species [17]. So, the
reaction of K2[PdCl4] with the N-2,6-dimethylphenyloxamate dianion [produced in situ by
the treatment of its monoester derivative with potassium hydroxide (Pd : oxamate proli-
gand : OH− 1 : 1 : 2 molar ratio) in a mixture of water/acetonitrile (1.5:1, v/v)] at 60 °C
overnight led to the formation of a yellow solution that was allowed to slowly evaporate at
room temperature. Crystals of the double salt [Pd(H2O)4][Pd(2,6-Me2pma)2]·2H2O (1) were
grown within a couple of days (see scheme 1).
The use of n-Bu4NOH as base (Pd : oxamate proligand : OH− 1 : 2 : 4 molar ratio) led to
the formation of the usual anionic bis(oxamato)palladium(II) with the tetra-n-butylammo-
nium countercation (2a). This compound was prepared for comparative purposes of the
reaction conditions as shown in scheme 1. The reaction in water afforded, as previously
reported, the supramolecular complex {K4(H2O)3]cis-[PdII(2,6-Me2pma)2]2}n [17(b)].
Consequently, it appears that the nature of the cation from the base and the solvent of the
2-
N
O
O
O
O
N
O
O
Pd
[Pd(OH2)4]2+
1) 2KOH / CH3CN/H2O
2) K2[PdCl4] /60ºC/overnight
NH
O
O
O
1) 2n-Bu4NOH / CH3CN/H2O
2-
2) 1/2[K2PdCl4]/60ºC/overnight
EtH-2,6-Me2pma
N
O
O
O
+
O
N
O
O
2n-Bu4N
Pd
Scheme 1. Synthetic route for 1 and 2a.

Bis(oxamato)palladate(II)
7
reaction strongly affects the formation, stability, and structural features of the oxamate-
containing palladium(II) compounds.
The high-frequency region of the IR spectrum of 1 exhibits a strong band at 3422 cm−1
due to O–H stretch together with the characteristic C–H vibrations (2961, 2933 and
2873 cm−1) of the phenyl ring of the N-2,6-dimethylphenyloxamate ligand. Additionally,
two strong absorptions are at 1616 and 1641 cm−1 which are attributed to C=O vibrations,
supporting the coordination of the 2,6-Me2pma dianion to palladium(II). Identical peaks
concerning the aromatic C–H and C=O stretching vibrations are observed in the IR spec-
1
trum of 2a, supporting also the occurrence of coordinated 2,6-Me2pma. H NMR experi-
ments support the formation of the 2,6-Me2pma dianion, as indicated by the disappearance
of the peaks of both ethyl and NH groups of the proligand, and its coordination to palla-
dium(II) to yield 2a with n-Bu4N+ as countercation. These spectroscopic features have been
confirmed by X-ray diffraction on single crystals (vide infra).
3.2. Crystal structures of [Pd(H2O)4][Pd(2,6-Me2pma)2]·2H2O (1) and (n-Bu4 N)2[Pd
(2,6-Me2pma)2]·2CHCl3 (2b)
Compounds 1 and 2b crystallize in the monoclinic space groups C2/c and P21/n, respec-
tively, with half a molecule in the asymmetric unit, the second half being generated by the
crystallographic inversion center. Each structure consists of trans-[Pd(2,6-Me2pma)2]2− and
[Pd(H2O)4]2+ (1) or n-Bu4 N+ (2b) counterions plus crystallization water (1) and chloroform
(2b) (figures 1 and 2). Anions and cations in 1 are assembled by an extended network of
hydrogen bonds into a 3-D supramolecular structure (figure 3), while the bis(oxamate)palla-
date(II) species in 2b are well separated by the bulky n-Bu4N+ cations.
The structures of 1 and 2b have in common the presence of the trans-[Pd(2,6-Me2pma]2−
mononuclear unit where the palladium(II) ion is four-coordinate with two amidate nitrogens
[N(1) and N(1a)] and two carboxylate oxygens [O(1) and O(1a)], building a slightly distorted
square-planar coordination. The reduced bite angle of the chelating oxamate [81.6(1)° (1)
and 80.9(9)° (2b)] accounts for deviations from the ideal geometry; in fact, the Pd(II) and the
atoms building its surrounding lie exactly on a plane for symmetry reasons. The Pd–N/Pd–O
Figure 1. ORTEP drawing of the anionic complex structure of 1 showing the atom numbering scheme and the
hydrogen bonds (broken lines) interconnecting the oxamate and aqua ligands.

8
F.R. Fortea-Pérez et al.
Figure 2. Perspective view of the anionic complex structure of 2b showing the atom numbering scheme.
bond lengths of 2.023(3)/2.04(2) (1) and 2.020(3)/1.97(2) Å (2b) compare well with those
found in the oxamato-containing palladium(II) complexes {[Na(H2O)]2trans-[PdII(2,6-Me2p-
ma)2]}n, (n-Bu4N)2trans-[Pd(2-Mepma)2]·4H2O [2-Mepma = N-2-methylphenyloxamate],
(n-Bu4N)2trans-[Pd(4-Mepma)2]·4H2O [4-Mepma = N-4-methylphenyloxamate], Na[Pd
(Hpba)]·2H2O [H4pba = 1,3-propylenebis(oxamic acid)], and K2[Pd(opba)]·2H2O
[H4opba = N,N′-1,2-phenylenebis(oxamic acid)] [average Pd–N/Pd–O distances of 2.020(1)/
2.009(1), 2.021(1)/2.038(1), 2.021(1)/2.021(1), 1.970/2.040, and 1.928/2.055 Å, respec-
tively] [17, 21, 22].
The plane at the Pd(II) ion and the mean plane of the oxamate group are almost coplanar
[the values of the dihedral angle between these two planes (φ) are 2.20(4) (1) and 1.3(4)°
(2b)]. Focusing on the phenyloxamate ligand, the dihedral angle between the 2,6-substituted
phenyl ring and the mean plane of the oxamate group (/) are 81.52(3) (1) and 80.0(4)°
(2b), much closer to orthogonality than those observed in {[Na(H2O)]2trans-[PdII(2,6-
Me2pma)2]}n, (n-Bu4N)2trans-[Pd(2-Mepma)2]·4H2O, and (n-Bu4N)2trans-[Pd(4-Me-
pma)2]·4H2O [/ = 75.79(4), 51.1(1), and 52.2(1)°, respectively] [17].
A tetraaquapalladium(II) cation is present in 1, reminiscent of the situation found in the
double salt [Pd(NH3]4[Pd(opba)] where the tetramminepalladium(II) is a counterion of the
oxamato dianion [Pd(opba)]2− [16]. The palladium(II) ion in this tetraaqua complex is also
four-coordinate in a slightly distorted O(1w)O(2w)O(1wa)O(2wa) square-planar geometry.
The values of the O(1w)–Pd(2)–O(2w), O(1w)–Pd(2)–O(1wa), and O(1w)–Pd(2)–O(2wa)
bond angles are 177.3(2), 88.6(3), and 91.5(3)°, respectively, and the average Pd–Ow length
is 2.048(5) Å.
Focusing on the crystal packing of 1, hydrogen bond interactions involving coordinated
and crystallization waters give layers of hyper-hydrated Pd(2) ions extending on the crystal-
lographic bc plane (figure 3). The outer oxamate oxygens [O(2) and O(3)] of the anionic
complex are anchored to these sheets through further hydrogen bonds, leading to an overall
supramolecular 3-D motif (see table 4). The values of the shortest interionic metal–metal
separations are 7.750(3) and 6.922(3) [Pd(1)⋯Pd(1e) and Pd(2)⋯Pd(1e) Å, respectively;
symmetry code: (e) = x, −y + 1, z].
The oxamate complex anions in the crystal packing of 2b are well separated from each other
most likely because of the presence of bulky n-Bu4N+ cations; the shortest intermetallic

Bis(oxamato)palladate(II)
9
Figure 3. (a) View along the crystallographic c and (b) b axes of fragments of the crystal packing in 1.
Pd(1)⋯Pd(1b) separation is 11.94(3) Å [symmetry code: (b)=−x, y, z] (figure S1, see online
supplemental material at http://dx.doi.org/10.1080/00958972.2014.975697). Chloroform
crystallization molecules are intercalated in the free voids by weak Cl⋯H–C interactions.
3.3. Catalytic studies
Palladium complexes are widely used as catalysts for carbon–carbon bond-forming reactions
[23]. These reactions are key steps in many syntheses of organic chemicals, natural products,

10
F.R. Fortea-Pérez et al.
as well as in a variety of industrial processes [24]. A successful example of this type of
catalysis is the Suzuki cross-coupling reaction that works reliably, tolerating most functional
groups both on the substrate and the products [25]. The catalytic activity of 1 has been tested
in the Suzuki cross-coupling of phenylboronic acid as the standard boronic acid substrate
and different aryl halides as the coupling partners (see table 5). The catalytic activity of 1
was compared to that of 2a. With the new double salt 1 composed of anionic bis(oxamato)
palladate(II) and cationic tetraaquapalladate(II) entities (1 M% Pd, 0.00025 mM of 1),
Table 5. Suzuki cross-coupling reaction of aryl halides phenylboronic acid.a
1% mol Pd / Et3N
R
R
+
X
(HO)2B
DMF, 80ºC
X = I, Br
Yield of biphenyl
product (%) (catalyst
used)b
Entry
1
Aryl halide
Time
3 h
30
35
I
(1)
(2a)
OMe
2
3
3 h
55
(1)
56(2a)
I
I
COMe
18 h
28
(1)
30
(2a)
Me
4
5
18 h
39
(1)
48
(2a)
I
I
CO2Me
30 min
50
(1)
60
(2a)
H
H
6
7
30 min
30 min
23(1)
26 (2)
Br
Br
17
(1)
28
(2a)
OMe
F
8
9
30 min
3 h
35
(1)
30
(2a)
Br
Br
29
(1)
29
(2a)
OMe
aReaction conditions: aryl halide (1.0 mM), PhB(OH)2 (1.2 mM), Et3N (2.0 mM), 1% M of catalyst rel-
ative to arylhalide, DMF (3 mL), T = 80 °C.
bDetermined by GC–MS spectroscopy using PFTBA as internal standard.

Bis(oxamato)palladate(II)
11
the use of 1.0 equivalent of aryl halide, 1.2 equivalents phenylboronic acid and 2.0 equiva-
lents Et3N as base in DMF at 80 °C results in moderate yields of the symmetrical and
unsymmetrical biaryl products (17–55%), employing the less reactive bromo aryls and the
very reactive iodo aryls. Applying the same catalytic conditions, bis(oxamato) palladated(II)
complex 2a featuring one palladium metal center per molecule was tested (1 M% Pd,
0.005 mM of 2a), resulting in approximately the same moderate yields (25–60%) like 1.
The catalytic activity observed for 2a featuring a discrete structure is lower compared to the
activity shown by the same complex assembled through univalent alkaline counterions such
as Na+, K+, Cs+, and Rb+. This is likely due to the bulky n-Bu4N+ present in 2a [17(b)].
The values of the yield of the catalytic biaryl products obtained by using the new double salt
1 were found to be similar or slightly lower than those obtained with the mononuclear com-
plex (n-Bu4N)2[Pd(2,6-Me2pma)2]·2H2O (2a). The same value of TON considering % M of
PdII (twice in 1 than in 2b) in both reactions indicates that the PdII entities are equally
activated because the identical number of mM of PdII were used for 1 and 2b. However,
ready decomposition of the tetraaquapalladium(II) unit in 1 to metal palladium black occurs
leading to inactive catalytic species at high temperatures.
4. Conclusion
A double salt composed of the anionic bis(N-2,6-dimethylphenyloxamato)palladate(II) and
cationic tetraaquapalladium(II), assembled by water as ancillary group, has been obtained.
bis(N-2,6-dimethylphenyloxamato)palladate(II) mononuclear species was also prepared fea-
tured by the organic cation tetra-n-butylammonium. Both compounds were easily prepared
by control of the mixture of reaction solvents. Both catalyze the Suzuki cross-coupling of
aryl halides and phenylboronic acid into biaryl products in moderate yields. The presence
of the tetraaquapalladium(II) unit in a palladium(II) catalyst appears to readily decompose
to palladium(0) black, a feature that shows the relevance of the coordinating ligand or pro-
tecting shell for the formation of active and stable palladium species during the catalytic
cycle of palladium-catalyzed carbon–carbon coupling reactions.
Supplementary material
Crystallographic data for the structural analysis have been deposited with the
Cambridge Crystallographic Data Center; CCDC reference numbers are 1024052 (1)
and 1024053 (2). Copies of this information may be obtained free of charge on appli-
cation to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223 336 033;
E-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk). Supplementary
data associated with this article: Crystal packing of 2 (figure S1).
Acknowledgements
We are grateful to the Spanish Ministerios de Ciencia e Innovación (project CTQ2010-
153645) and de Economía y Competividad (Project CTQ2013-44844P), the Generalitat Val-
enciana (projects ISIC/2012/002and PROMETEO II 2014-070), and the Italian Ministerio

12
F.R. Fortea-Pérez et al.
dell’Istruzione dell Università e della Ricerca Scientifica (MiUR) for their supports.
F.R. F.-P. thanks the MCIIN for a predoctoral FPU fellowship.
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