Dipalladium(I) complexes of ortho- and para-functionalized 1,3-bis(aryl)triazenide ligands: Synthesis, structure and catalytic activity

Article abstract of DOI:10.1016/j.ica.2019.03.004

The synthesis and characterization of a series dipalladium(I) complexes of formulae [Pd{1-[2′-(methoxycarbonyl)phenyl]-3-[4′-X-phenyl]triazenide}]₂ [X = F (4), Cl (5), Br (6)] are reported. The crystal structure of complex 4 was determined by X-ray diffraction studies. The previously reported dipalladium(I) 1–3 as well as the new complexes 4–6 were used as catalysts in the Heck and Suzuki couplings of different p-substituted bromobenzenes.

Full text of DOI:10.1016/j.ica.2019.03.004

Accepted Manuscript
Research paper
Dipalladium(I) complexes of ortho- and para-functionalized 1,3-Bis(aryl)tria-
zenide ligands: synthesis, structure and catalytic activity
Erick Correa-Ayala, Carlos Campos-Alvarado, Daniel Chávez, Simón
Hernández-Ortega, David Morales-Morales, Valentín Miranda-Soto, Miguel
Parra-Hake
PII:
S0020-1693(18)31844-9
https://doi.org/10.1016/j.ica.2019.03.004
ICA 18809
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 December 2018
1 March 2019
4 March 2019
Please cite this article as: E. Correa-Ayala, C. Campos-Alvarado, D. Chávez, S. Hernández-Ortega, D. Morales-
Morales, V. Miranda-Soto, M. Parra-Hake, Dipalladium(I) complexes of ortho- and para-functionalized 1,3-
Bis(aryl)triazenide ligands: synthesis, structure and catalytic activity, Inorganica Chimica Acta (2019), doi: https://
doi.org/10.1016/j.ica.2019.03.004
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Dipalladium(I) complexes of ortho- and para-functionalized 1,3-Bis(aryl)triazenide
ligands: synthesis, structure and catalytic activity
Erick Correa-Ayalaa, Carlos Campos-Alvaradoa, Daniel Cháveza, Simón Hernández-Ortegab,
David Morales-Moralesb, Valentín Miranda-Sotoa,*, Miguel Parra-Hakea,*
aTecnológico Nacional de México/Instituto Tecnológico de Tijuana, Centro de Graduados e
Investigación en Química, Apartado Postal 1166, Tijuana, B.C. 22000, México
bInstituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior. Cd.
Universitaria Coyoacán, México D.F. 04510, México
Abstract
The synthesis and characterization of a series dipalladium(I) complexes of formulae
[Pd{1-[2'-(methoxycarbonyl)phenyl]-3-[4'-X-phenyl]triazenide}]2 [X = F (4), Cl (5), Br (6)] are
reported. The crystal structure of complex 4 was determined by X-ray diffraction studies. The
previously reported dipalladium(I) 1 to 3 as well as the new complexes 4 to 6 were used as
catalysts in the Heck and Suzuki couplings of different p-substituted bromobenzenes.
Keywords: Triazenide ligands, Pd(I) complexes, Catalysis, Cross coupling reactions, Heck
couplings, Suzuki-Miyaura couplings.
1

Highlights



Triazenide Pd(I) dinuclear complexes were obtained
The Pd(I) dinuclear complexes are efficient catalysts in Heck and Suzuki couplings
Substituents in the triazenide ligands are relevant in their catalytic activity
2

Graphical abstract
The series of dipalladium(I) complexes [Pd{1-[2'-(methoxycarbonyl)phenyl]-3-[4'-X-
phenyl]triazenide}]2 [(X = F (4), Cl (5), Br (6)] were synthesized and used as efficient catalysts
in Heck and Suzuki-Miyaura couplings of different p-substituted bromobenzenes.
3

1. Introduction
Catalysis using dinuclear transition metal complexes has recently received considerable
attention [1-5] since cooperation of two metal centers can improve the selectivity and efficiency
of the catalyst and promote reactions that are not possible using a single metal center. In this
context, dinuclear complexes with the relatively rare PdI oxidation state have emerged as a
promising class of catalysts for cross-coupling reactions [6-24].
The challenge in designing dipalladium(I) catalysts lays in the selection of ligands
capable of stabilizing the PdI and to adequately respond to geometric and electronic
reorganization of the bimetallic core during the course of the catalytic cycle. 1,3-
bis(aryl)triazenides are a promising choice due to their propensity to coordinate in a bridging
fashion stabilizing bimetallic cores in a variety of oxidation states [25-29]. The traditional 1,3-
bis(aryl)triazenide ligands stabilize the common PdII oxidation state [30-33] but are not capable
of stabilizing the PdI-PdI core. However, our research group has reported the first
dipalladium(I)-triazenide complexes by using ortho-functionalized 1,3-bis(aryl)triazenide
ligands (Figure 1) [34]. It was found that the PdI-PdI core was stabilized when hard weakly
coordinating groups in the triazenide ligand were used (1-3). This and other reports of our
research group have shown that the ortho-substituents in the 1,3-bis(aryl)triazenide ligands can
have a dramatic impact on the chemistry [35-37] and catalytic activity [38-40] of their
complexes.
Figure 1. Dipalladium-triazenide complexes (1-3) previously reported [34].
4

More recently, a couple of dipalladium(I) complexes supported by ortho-functionalized
1,3-bis(aryl)triazenide ligands have been reported by Zhan and co-workers [41,42].
Furthermore, Úbeda and co-workers reported the synthesis, characterization and DFT
calculations of some dipalladium(I)-triazenide complexes, including the previously reported
complex 3 [43]. However, noteworthy the fact that there are not previous reports of
dipalladium(I)-triazenide complexes used as catalyst in cross-coupling reactions.
Thus, we report here the synthesis and characterization of three new dipalladium(I)-
triazenide
complexes
of
formula
[Pd{1-[2'-(methoxycarbonyl)phenyl]-3-[4'-X-
phenyl]triazenide}]2 [X = F (4), Cl (5), Br (6)]. The objective is to determine the catalytic
activity of these three new complexes and the previously reported dipalladium(I) complexes 1-3
[34] in C-C cross-coupling reactions. It was expected that the relative catalytic efficiency to be
dependent on the ortho-substituent in complexes 1-3 and the para-substituent in complexes 4-6.
2. Experimental Section
2.1. General comments
All synthetic work was carried out in air or under argon using standard Schlenk
techniques or an inert atmosphere dry-box, where indicated. All solvents were dried over
appropriate drying agents. PdCl2 (Aldrich) and other reagents are commercially available and
were used as received. [Pd(CH3CN)2(Cl)2] [44], 1-[2'-(methoxycarbonyl)phenyl]-3-[4'-(fluoro)
phenyl]triazene (L1) [40], 1-[2'-(methoxycarbonyl)phenyl]-3-[4'-(chloro)phenyl]triazene (L2)
[40], 1-[2'-(methoxycarbonyl)phenyl]-3-[4'-(bromo)phenyl]triazene (L3) [40], [Pd{1,3-bis[2'-
(methoxycarbonyl)phenyl]triazenide}]2 (1) [34], [Pd{1-[2'-(methoxycarbonyl)phenyl]-3-[4'-
methylphenyl]triazenide}]2 (2) [34] and [Pd{1,3-bis[2'-(methoxy)phenyl]triazenide}]2 (3) [34]
were synthesized according to previously reported procedures. NMR spectra were recorded at
400 MHz with a Bruker Avance III spectrometer at 30 C unless otherwise specified. 1H and 13C
NMR chemical shifts are reported in ppm referenced to residual solvent resonances (1H NMR:
13
7.16 for C6HD5 in C6D6, 1.94 for CHD2CN in CD3CN, 2.05 for acetone-d5 in acetone-d6. C
NMR: 128.39, 1.39, and 29.92 for benzene-d6, acetonitrile-d3 and acetone-d6, respectively).
Coupling constants J are given in Hertz (Hz). IR spectra were recorded on a Perkin-Elmer FT-
5

IR 1605 spectrophotometer. Melting points were measured in an Electrothermal GAC 88629
apparatus. Mass spectra were obtained by direct insertion on an Agilent Technologies 59756
Instrument. High Resolution Mass Spectrometry (HRMS) data were obtained in a micrOTOF-Q
III MS instrument with electrospray ionization using sodium formate as calibrant.
2.2. Synthesis of complexes
2.2.1. [Pd{1-[2'-(methoxycarbonyl)phenyl]-3-[4'-fluorophenyl]triazenide}]2 (4)
Triazene L1 (52.65 mg, 0.1927 mmol, 1.0 eq) was dissolved in CH3CN (5 mL), to this
solution triethylamine (38.94 mg, 0.3854 mmol, 2 eq) was added with stirring. To the resulting
reaction mixture, a solution of [Pd(CH3CN)2(Cl)2] (50 mg, 0.1927 mmol, 1.0 eq) in CH3CN (5
mL) was slowly added, and the resulting mixture was stirred for 30 min at room temperature,
during which time a red precipitate formed. The reaction mixture was filtered to obtain a reddish
microcrystalline solid, which was recrystallized by vapor diffusion of pentane into a
concentrated solution of the product in CH2Cl2 at room temperature to give red crystals, suitable
for X-ray diffraction analysis (53.17 mg, 0.0703 mmol, 73%). MP = 198–201 ºC. IR(KBr):
1
3014, 2957, 1652, 1595, 1557, 1499, 1428, 1368, 1251, 1187, 1084, 826, 748 cm–1. H NMR
[CDCl3, 400 MHz]: δ 7.85 (dd, J = 8.4, 1.6 Hz, 2H, ArA3), 7.54 (dd, J = 8.4, 1.2 Hz, 2H, ArA6),
7.37 (dd, J = 9.2, 5.2 Hz, 4H, ArB2,6), 7.33 (td, J = 7.2, 1.6 Hz, 2H, ArA5), 6.98 (t, J = 8.8 Hz,
4H, ArB3,5), 6.93 (td, J = 8.4, 1.2 Hz, 2H, ArA4), 3.62 (s, 6H, OCH3(A7)). 13C {1H} NMR [CDCl3,
100 MHz]: δ 169.8 (CA7), 161.1 (d, J = 243.0 Hz, CB4), 151.8 (CA1), 146.9 (d, J = 2.0 Hz, CB1),
134.3 (CA5), 131.9 (CA3), 124.7 (d, J = 8.0 Hz, CB2,6), 122.2 (CA4), 120.0 (CA6), 116.9 (CA2),
114.7 (d, J = 22.0 Hz, CB3,5), 53.8 (CA8). HRMS (ESI-TOF) m/z: [M + Na] Calc. for
C28H22N6O4F2Pd2Na 780.9649; Found 780.9647. Anal. Calc. for C28H22N6O4F2Pd2 (757.36): C,
44.40; H, 2.93; N, 11.10. Found, C, 44.19; H, 2.85; N, 11.04.
6

2.2.2. [Pd{1-[2'-(methoxycarbonyl)phenyl]-3-[4'-chlorophenyl]triazenide}]2 (5)
Complex 5 was prepared in the same manner as 4 using triazene L2 (55.82 mg, 0.1927
mmol, 1.0 eq). Red crystals (49.34 mg, 0.0626 mmol, 65%). MP = 222–224 ºC. IR(KBr): 3023,
1
2937, 1651, 1557, 1476, 1433, 1358, 1194, 1157, 1085, 819, 752 cm–1. H NMR [CDCl3, 400
MHz]: δ 7.86 (dd, J = 8.4, 1.6 Hz, 2H, ArA3), 7.53 (dd, J = 8.4, 0.8 Hz, 2H, ArA6), 7.37 (d, J =
8.8 Hz, 4H, ArB2,6), 7.35 (td, J = 8.4, 1.6 Hz, 2H, ArA5), 7.23 (d, J = 8.8 Hz, 4H, ArB3,5), 6.97 (td,
13
J = 8.0, 1.2 Hz, 2H, ArA4), 3.66 (s, 6H, OCH3(A8)). C {1H} NMR [CDCl3, 100 MHz]: δ 169.8
(CA7), 151.6 (CA1), 149.3 (CB1), 134.3 (CA5), 131.9 (CA3), 131.1 (CB4), 128.1 (CB3,5), 124.4
(CB2,6), 122.5 (CA4), 120.2 (CA6), 117.2 (CA2), 53.9 (CA8). HRMS (ESI-TOF) m/z: [M + Na]
Calc. for C28H22N6O4Cl2Pd2Na 812.9046; Found 812.9042. Anal. Calc. for C28H22N6O4Cl2Pd2
(790.26): C, 42.56; H, 2.81; N, 10.63. Found, C, 42.58; H, 2.78; N, 10.52.
2.2.3. [Pd{1-[2'-(methoxycarbonyl)phenyl]-3-[4'-bromophenyl]triazenide}]2 (6)
Complex 6 was prepared in the same manner as 4 using triazene L3 (64.39 mg, 0.1927
mmol, 1.0 eq). Red crystals (38.87 mg, 0.0443 mmol, 46%). MP = 234–236 ºC. IR(KBr): 3023,
2937, 1650, 1560, 1476, 1355, 1193, 1157, 1080, 818, 753 cm–1. 1H NMR [CDCl3, 400 MHz]: δ
7.86 (dd, J = 8.4, 1.6 Hz, 2H, ArA3), 7.53 (dd, J = 8.4, 0.8 Hz, 2H, ArA6), 7.38 (d, J = 8.8 Hz,
4H, ArB3,5), 7.35 (m, 2H, ArA5), 7.32 (d, J = 8.8 Hz, 4H, ArB2,6), 6.97 (td, J = 8.4, 1.2 Hz, 2H,
13
ArA4), 3.66 (s, 6H, OCH3(A8)). C {1H} NMR [CDCl3, 100 MHz]: δ 169.8 (CA7), 151.6 (CA1),
149.7 (CB1), 134.3 (CA5), 131.9 (CA3), 131.1 (CB3,5), 124.8 (CB2,6), 122.6 (CA4), 120.2 (CA6),
119.0 (CB4), 117.2 (CA2), 53.9 (CA8). HRMS (ESI-TOF) m/z: [M + Na] Calc. for
7

C28H22N6O4Br2Pd2Na 902.8026; Found 902.8023. Anal. Calc. for C28H22N6O4Br2Pd2 (879.17):
C, 38.25; H, 2.52; N, 9.56; Found, C, 38.10; H, 2.47; N, 9.49.
2.3. X-ray crystallography
A yellow prism crystal of compound 4, was grown from CH2Cl2/diethyl ether and
mounted on a glass fiber, then placed on a Bruker Smart Apex II diffractometer with a Mo-
target X-ray source ( = 0.71073Å). The detector was placed at a distance of 5.0 cm from the
crystal and frames were collected with a scan width of 0.5 in  and exposure time of 10 s/frame.
9883 reflections were collected and integrated with the Bruker SAINT software package using a
narrow-frame integration algorithm. Systematic absences and intensity statistics were used in
monoclinic P2/c space group. The structure was solved using Patterson methods using
SHELXS-2014/7 program [45]. The remaining atoms were located via a few cycles of least
squares refinements and difference Fourier maps. Hydrogen atoms were input at calculated
positions and allowed to ride on the atoms to which they are attached. Thermal parameters were
refined for hydrogen atoms on the phenyl groups using a Ueq = 1.2 Å and 1.5 for methyl
groups. The final cycle of refinement was carried out on all non-zero data using SHELXL-
2014/7 [45]. Absorption correction was applied using SADABS program. The structure was
solved as a half of molecule, and symmetry plane is needed to generate the whole molecule.
Crystallographic data and refinement parameters are shown in Table S1.
2.4. General procedure for Heck couplings
In open air, a Schlenk tube equipped with a magnetic stirring bar was charged with
DMF (3 mL), aryl halide (6.369 mmol), alkene (6.369 mmol), 1.2 eq of base and the catalyst
(3.582 × 10−3 mmol). The tube was sealed and fully immersed in a 120 ºC silicon oil bath. After
the prescribed reaction time, the mixture was cooled to room temperature and the organic phase
was analyzed by gas chromatography-mass spectrometry (GC-MS).
2.5. General procedure for Suzuki couplings
In open air, a Schlenk tube equipped with a magnetic stirring bar was charged with
DMF (3 mL), aryl halide (6.369 mmol), boronic acid (6.369 mmol), 1.2 eq of base and the
8

catalyst (3.582 × 10−3 mmol). The tube was sealed and fully immersed in a 100 ºC silicon oil
bath. After the prescribed reaction time, the mixture was cooled to room temperature and the
organic phase was analyzed by gas chromatography-mass spectrometry (GC-MS).
3. Results and discussion
3.1. Synthesis and general characterization
3.1.1. Dipalladium(I) complexes
The synthesis and characterization of triazenes L1-L3 [40] and complexes 1-3 [34] have
been described previously. Complexes 4-6 were prepared by combination of bis(acetonitrile)-
palladium(II) dichloride in acetonitrile with a mixture of the corresponding triazene (L1-L3)
and triethylamine at room temperature (Eq. 1). The air- and moisture-stable red complexes were
purified after recrystallization (dichloromethane/pentane) with yields in the range of 73-46%.
As is the case for complexes 1-3 (Figure 1) analytical and spectroscopic data (IR, NMR
and HRMS), and X-ray crystal structure determination revealed dipalladium(I) complexes (Eq.
1). It is assumed that the d9 Pd(I) centers are spin coupled through a Pd-Pd bond, since the d9
configuration is expected to exhibit paramagnetic behavior and compounds 4-6 are diamagnetic
and amenable to NMR analysis. This behavior has been previously observed in other
dipalladium(I) complexes [46-54]. Reduction of Pd(II) to Pd(I) is attributed to the triethylamine
used for the deprotonation of the triazene to form the triazenide ligand, as reported by Walsh for
the synthesis of complexes 1-3 [34]. Similarly, Lu and Peters found that a related
bis(phosphine)Pd(II) complex was reduced to Pd(I) in good yield in the presence of an excess of
a tertiary amine [55].
9

The 1H and 13C {1H} NMR data of complexes 4-6 are summarized in Tables S3 and S4,
respectively. The integration of sixteen hydrogens in the aromatic region (7.86−6.93 ppm) of
each complex corresponds to the presence of two equivalent triazenide ligands. The methoxy
13
group appears as a singlet at ~3.64 ppm and integrates for six hydrogens. The C {1H} NMR
spectra of complexes 4-6 show a single carbonyl signal at 169.8 ppm, slightly downfield from
the unbound ligands (167.7 ppm) [40]. The spectra of complexes 5 and 6 showed 10 signals for
the carbon atoms in the aromatic region, while that of complex 4, exhibits the carbons in the
aromatic ring B as doublets, due to the coupling with the p-F (Table S4). Interestingly, as a
result of the σ-acceptor and π-donor effects of the para-halogen substituents [56], for complexes
4 (p-F, 161.1 ppm), 5 (p-Cl, 131.1 ppm) and 6 (p-Br, 119.0 ppm), the signals due to the B4
carbons are shifted downfield, as the para-halogen substituent becomes more electronegative.
3.2. Description of the crystal structure
X-ray quality crystals were obtained from vapor phase diffusion of pentane into a
concentrated solution of complex 4 in dichloromethane. The ORTEP diagram of 4 is shown in
Figure 2.
The molecular structure of 4 consists of two directly bonded palladium(I) atoms bridged
by two mutually trans triazenide ligands. The structure has a C2 symmetry axis rendering the
atoms equivalent by symmetry. The oxygen atom of the o-methoxycarbonyl substituent
occupies the site trans to the Pd1-Pd1 axis.
The structure shows a slightly distorted square planar geometry with N3-Pd1-O1, O1-Pd1-
N1, N1-Pd1-Pd1 and N3-Pd1-Pd1 angles of 86.58(15), 101.66(15), 84.27(11) and 87.47(11)°,
respectively. The Pd-N distances (ca. 2.02 Å), the N-N bonds nearly equivalent (~1.31 Å) and
the N1-N2-N3 angle of 115.30(4)° indicate a pronounced π delocalization around the metal-
triazenide system.
10

Figure 2. Molecular structure of 4 (H atoms are omitted for clarity; thermal ellipsoid set at 30%
probability). Bond lengths (Å): Pd1-Pd1, 2.3999(8); Pd1-N1, 2.031(4); Pd1-N3, 2.016(4); Pd1-O1,
2.198(4); N1-N2, 1.311(6); N2-N3, 1.315(6); C4-F1, 1.369(6); C13-O1, 1.219(6). Bond angles
(°):N1-N2-N3, 115.30(4); Pd1-Pd1-O1, 174.00(10); Pd1-Pd1-N1, 84.27(11); Pd1-Pd1-N3, 87.47(11);
N1-Pd1-N3, 171.69(16); N1-Pd1-O1, 101.66(15); N3-Pd1-O1, 86.58(15); O1-C13-O2, 119.70(5).
Although the structural values are consistent with the closely related complexes 1 and 2
[34] (Figure 1), there are some important differences. For instance, complexes 4 (p-
fluorophenyl, 15.52o) and 2 (p-tolyl, 53.81°) differ in the dihedral angle (Ф) formed between the
NNN plane. The smaller dihedral angle in 4 suggests a stronger π delocalization of the
triazenide system into the p-fluorophenyl ring as a result of the electronic effect induced by the
p-F [57].
Also, in contrast to the structure of complexes 1-3, and other related dipalladium(I)-
triazenide complexes [41-43], in complex 4 the eight-membered Pd2(NNN)2 ring, is not planar.
The molecule adopts a "twist boat configuration", with a torsion angle N1-Pd1-Pd1-N3 (τ) of
19.7° (Figure S3). It has been reported that an increase on the torsion angle minimizes repulsive
overlap of the out-of-plane metal dπ orbitals on adjacent palladium(I) centers [58-61] (which are
minimal at 45°). The twist conformation also allows the ligands to span a shorter PdI-PdI bond
[58-61]. In this context, complex 4 exhibits the shortest PdI-PdI distance [2.3998(8) Å] of the
dipalladium(I)-triazenide complexes reported [34,41-43]. Besides, to the best of our knowledge,
the related dipalladium(I)-naphthyridine complex (a) (Figure 3) reported by Bera and co-
11

workers [24] is the only other dipalladium(I) complex with a shorter PdI-PdI distance than 4, by
0.0046 Å. Thus, for comparison a selected list of N^N-bridging-dipalladium(I) complexes
along with their PdI-PdI distances are collected in Table 1.
Table 1. Comparison of PdI-PdI bond lengths in selected N^N-bridging-dipalladium(I) complexes.a
complex
dPd-Pd (Å)
2.3952(8)
Ref.
[24]
[46]
[47]
[47]
[41]
[42]
[43]
[34]
[34]
a [Pd2(μ-acfNP)2](BF4)2
b [Pd2(μ-NPN)2](BF4)2
c [Pd2(μ-dpfam)(η3-allyl)]
d [Pd2(μ-qaam)(η3-allyl)]
2.5489(7)
2.6073(3)
2.5260(2)
2.4277(5)
2.4228(3)
2.5235(4)
2.4309(3)
2.4202(3)
2.4158(4)
2.3998(8)
e [Pd2(Ar-NNN-Ar')2] (Ar = o-CO2Me, Ar' = o-Cl)
f [Pd2(Ar-NNN-Ar')2] (Ar = Ar' = o-Cl)
g [Pd2(Ar-NNN-Ar')2] (Ar = Ar' = o-Br)
1
2
3
4
[34]
b
aAbbreviations: NPN= 2,2'-((phenylphosphanediyl)bis(methylene))bis(4,5-dihydrooxazole), dpfam= N,N'-
bis[2-(diphenylphosphino)phenyl]formamidinate, qaam= N,N'-di-8-quinolylacetamidinate, acfNP=
[{(5,7-dimethyl-1,8-naphthyridin-2-yl)amino}carbonyl]ferrocene. bThis work.
Figure 3. Dipalladium(I) complexes a-d.
The short PdI-PdI distances in these complexes can be related to a constrained ligand
bite, weak axial coordination [24,34] and the torsion angle previously discussed. This behavior
can be better observed in Figure 4, which shows that for complexes 1 [2.4309(3) Å], 2
[2.4202(3) Å] and 4 [2.3998(8) Å] the PdI-PdI distance decreases as the torsion angle (τ)
increases [1 (0.4°), 2 (3.2°) and 4 (19.7°)]. However, complex 3 with a PdI-PdI distance of
12

2.4158(4) Å and torsion angle (τ) of 0.87°, lays out this trend due to the different ortho-
substituent coordinated to the PdI-PdI axis (o-methoxy). The influence of the axial coordination
on the PdI-PdI distances, has also been observed in dipalladium(I)-phosphine [61] and
dipalladium(I)-amidinate complexes [47].
30
a
25
20
15
10
5
4
y = -748.77x + 1818.3
R² = 0.9547
2
1
0
2.39
-5
2.4
2.41
2.42
2.43
2.44
PdI-PdI distance (Å)
Figure 4. Plot of torsion angles [N1-Pd-Pd-N3 (τ)] vs PdI-PdI bond distances for dipalladium(I)-
triazenide complexes 1, 2 and 4, and the dipalladium(I)-naphthyridine complex a.
3.3. Catalysis
Suzuki [17-23] and Heck couplings [24] as well as other cross-coupling reactions [8-16]
mediated by dipalladium(I) complexes have been recently reported. In some cases their catalytic
activity has been attributed to the dissociation of the PdI-PdI complex into PdII or Pd0 catalytic
active monomers that start the catalytic cycles [6,16-18]. However, for more robust
dipalladium(I) complexes a bimetallic mechanism involving PdI-PdI/PdII···PdII redox cycle has
been proposed [10-12,24].
In this context, dipalladium-triazenide complexes 1-6 can be relevant due to the stability
beneficial effect of the ortho-substituents of the triazenide ligands, which in addition to
modulate the oxidation state of the dimetal core [34,38], may confer hemilabile properties to the
ligands, whose role would be to provide a coordination site on the metal center and facilitate the
binding of the substrates [62-65]. Moreover, the ortho-substituted 1,3-bis(aryl)triazenide ligand
13

as bridging-chelate could suppress the cleavage of the PdI-PdI core [24,47], allowing a
bimetallic mechanism as proposed by Bera and co-workers [24].
Thus, with the complexes on hand we started the catalytic evaluation of the series of
complexes 1-3 in Heck couplings of styrene and bromobenzene, using 0.056% mol of the
complexes, DMF as solvent and Na2CO3 as base, varying time and reaction temperatures,
preparing the samples in the open air. With these experiments (3 h at 150 °C) it was found that
complexes 1 and 2 are the best with yields of 67 and 63%, respectively. Noteworthy, raising the
reaction temperature above 150 °C, lead to the formation of palladium black. Thus, in order to
avoid this, the reaction temperature was reduced to 120 °C. At this temperature, the best
catalytic activity was produced by complex 1 (34% at 4 h and 78% at 24 h) albeit reducing the
yields and increasing the reaction time. In addition, other organic (DMAP [66,67], NEt3) and
inorganic (Cs2CO3) bases were examined, showing NEt3 to be the best base for the process. In
order to further explore the catalytic potential of this complex, Heck couplings were carried out
with chlorobenzene, however no conversion was observed with this substrate, probably due to
the higher energy required to activate the C-Cl bond [68-70]. Thus, with the best catalyst
defined and the reactions conditions optimized a series of Heck couplings with different para-
substituted bromobenzenes using complex 1 were performed, in order to evaluate the electronic
effect of the different substituents of the bromobenzene on the production of stilbenes (Table 2).
The results in Table 2 clearly show, as expected, that bromobenzenes containing the
more electron-withdrawing substituents afford the higher yields of the series. These results are
in agreement with Hammett parameter values [71]. This behavior can be better observed in
Figure 5 where a fairly linear trend is obtained [66,67]. The catalytic activity of complex 1, in
regard to turnover numbers (TON), is better than that obtained by Bera and co-workers with a
dipalladium(I)-naphthyridine complex, although they attained higher yields in shorter reaction
times [24].
14

Table 2. Heck couplings of para-substituted bromobenzenes with styrene catalyzed by 1.a
Entry
R
-OCH3
-CH3
-H
Hammett parameter
TON
480
Yield (%)b
E/Z/Gem
89/11/0
90/10/0
92/8/0
1
2
3
4
5
6
−0.27
−0.17
0
27
33
60
59
97
99
587
1066
1049
1725
1760
-Cl
0.23
0.42
0.5
92/8/0
-CHO
97/3/0
-COCH3
96/4/0
aConditions: 6.369 mmol of bromobenzene, 6.369 mmol of styrene, 7.640 mmol of Et3N, 3.582 × 10−3
b
mmol of complex 1 and 3 mL of DMF. Yields obtained by GC are based on remaining bromobenzene
and are the average of two runs.
120
-COCH3
-CHO
100
80
-H
60
-Cl
-CH3
40
y = 94.806x + 50.448
y = 93.616x + 51.422
20
-OCH3
R² = 0.9326
R2 = 0.9471
0
-0.55
-0.35
-0.15
0.05
0.25
0.45
0.65
Hammett parameter (σ)
Figure 5. Graphic of Hammett parameter (σ) of the para-substituted bromobenzenes vs yield of
stilbenes.
Similar experiments using para-substituted bromobenzenes with α-methyl styrene,
under the same reaction conditions using 1 as catalysts, were also successful to produce
trisubstituted olefins, however the yields were lower (52-16%) (Table 3), probably due to the
15

steric effect of the α-methylstyrene [66]. The results attained exhibit a similar trend to that
observed for styrene where a linear behavior can be clearly noted (Figure 6).
Table 3. Heck reactions of para-substituted bromobenzenes with α-methylstyrene catalyzed by 1.a
Entry
R
-OCH3
-CH3
-H
Hammett parameter
TON
284
302
569
764
818
523
Yield (%)b
E/Z/Gem
89/11/0
90/10/0
92/8/0
1
2
3
4
5
6
−0.27
−0.17
0
16
17
32
43
46
52
-Cl
0.23
0.42
0.5
92/8/0
-CHO
-COCH3
97/3/0
96/4/0
aConditions: 6.369 mmol of bromobenzene, 6.369 mmol of styrene, 7.640 mmol of Et3N, 3.582 × 10−3
mmol of catalyst 1 and 3 mL of DMF. bYields obtained by GC are based on remaining bromobenzene and
are the average of two runs.
60
-COCH3
50
-Cl
40
30
20
10
0
-CHO
-H
y = 47.524x + 28.71
R² = 0.9634
-OCH3
-CH3
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
Hammett parameter (σ)
Figure 6. Graphic of Hammett parameter (σ) of the para-substituted bromobenzenes vs yield of
olefins.
16

Further, it was important to explore whether the series of complexes 4-6 may exhibit a
better performance than compound 1 in similar Heck couplings, as a function of the different
para-halogen substituents in the 1,3-bis(aryl)triazenide ligands of the complexes, under the
same reaction conditions (DMF, NEt3, 120 °C, 4 h). The results (Table 4) show yields
comparable (4, 59%; 5, 60% and 6, 63%) to those obtained with 1 (60%). Thus, indicating that
the electronic effect of the para-halogen substituents plays no significant role in the catalytic
activities of this series of complexes. However, the presence of a less electronegative para-
halogen substituent in the 1,3-bis(aryl)triazenide ligand slightly enhances the activity of the
catalyst.
Table 4. Catalytic activity of complexes 1, and 4-6 in Heck couplings.a
Entry
Compound
TON
1066
1049
1066
1120
Yield (%)b
E/Z/Gem
92/8/0
1
2
3
4
60
59
60
63
1
4
5
6
92/7/1
89/10/1
90/10/0
aConditions: 6.369 mmol of bromobenzene, 6.369 mmol of styrene, 7.640 mmol of Et3N, 3.582 × 10−3
mmol of catalyst (1, 4-6) and 3 mL of DMF. bYields obtained by GC are based on remaining
bromobenzene and are the average of two runs.
Suzuki couplings
With the good results obtained in the Heck couplings it was decided to expand the
catalytic scope of the series of complexes 1-3 in Suzuki-Miyaura couplings of bromobenzene
and phenylboronic acid for the production of biphenyl. Initially, reactions were performed using
the optimized conditions found for the Heck couplings (0.056% mol catalyst, DMF and
Na2CO3) while the reaction time and temperatures were varied. The results showed complex 3
to be the best catalyst at different reaction temperatures (80, 100 and 120 °C), from which 100
°C, was chosen as the optimal, attaining 63% yield of biphenyl after 6 h of reaction. Noteworthy
17

the fact that in the Heck couplings, complex 1 was the best catalysts and complex 3 was one of
the less active. This may lead to infer that is in fact the nature of the functional groups at the
ortho-position of the 1,3-bis(aryl)triazenide ligands affecting and thus it can be modulated in
order to improve the catalytic properties of their complexes [38-40].
Further, given the fundamental importance that the base plays in Suzuki-Miyaura
couplings, other alkaline carbonates were used at 100 °C, 8 h (reaction time was increased from
6 to 8 h to have better yields with bases such as Li2CO3 that exhibited very low yields at the
above chose reaction time of 6 h) of reaction and complex 3 as catalyst (0.056% mol). The best
yields were obtained using Na2CO3 (71 %) and Cs2CO3 (73%), while other bases employed
afforded reduced conversions (Li2CO3 41%; K2CO3 63%; Rb2CO3 67%). Based on the slight
yield differences observed for Cs2CO3 and Na2CO3 and on the prices of these chemicals we
decided to continue our studies using Na2CO3 as the selected base.
Thus, under these optimized conditions (0.056% mol of catalyst, Na2CO3, DMF, 100
°C, 8 h) the catalytic activity of complex 3 towards a series of para-substituted bromobenzenes
was explored [72-76] (Table 5). Once again, as expected, the results showed that
bromobenzenes having electron-withdrawing para-substituents were more active than those
with electron-donating groups. The conversion increases as a direct function of the Hammett
parameter [71] of the para-substituents, reaching the highest conversion of 76% for the p-F
bromobenzene. However, by increasing the Hammett parameter to 0.23 (p-Cl-bromobenzene)
and 0.5 (p-COCH3-bromobenzene) a slightly decrease in conversion was observed (74%)
(Figure 7). As suggested by Scheck [77] and Norrby [78], a concave-downwards Hammett plot,
may suggest a change in the rate-determining step resulting from the different electronic
contributions of the substituents.
18

Table 5. Suzuki couplings of para-substituted bromomenzenes with phenylboronic acid catalyzed by 3.a
Entry
R
-OCH3
-CH3
-H
Hammett parameter
TON
925
Yield (%)b
1
2
3
4
5
6
−0.27
−0.17
0
52
65
72
76
74
74
1156
1280
1351
1316
1316
-F
0.06
0.23
0.5
-Cl
-COCH3
aConditions: 6.369 mmol of bromobenzene, 6.369 mmol of phenylboronic acid, 7.640 mmol of Na2CO3,
b
3.582 × 10−3 mmol of catalyst 3 and 3 mL of DMF. Yields obtained by GC are based on remaining
bromobenzene and are the average of two runs.
-F
80
70
60
50
40
30
20
10
0
-COCH3
-CH3
-Cl
-H
-OCH3
y = 70.572x + 73.351
R² = 0.901
y = 0.7975x + 73.842
R² = 0.012
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
Hammett parameter (σ)
Figure 7. Graphic of Hammett parameter of the para-substituent in bromobenzene vs yield of
biphenyl.
Complexes 4-6 were also used as catalysts for Suzuki couplings under optimized
conditions (Table 6). Complex 4 with the more electronegative para-halogen substituent (p-F)
shows the best activity with a yield of 76%, whereas, 5 (p-Cl) and 6 (p-Br) show yields of 75
and 73%, respectively. This trend contrasts with the results obtained in the Heck reaction (Table
19

4) where the presence of a less electronegative para-halogen substituent enhances slightly the
activity of the catalyst.
Table 6. Catalytic activity of complexes 4-6 in the Suzuki reaction.a
Entry
complexes
TON
1351
1333
1298
Yield (%)b
1
2
3
76
75
73
4
5
6
aConditions: 6.369 mmol of bromobenzene, 6.369 mmol of phenylboronic acid, 7.640 mmol of Na2CO3,
3.582 × 10−3 mmol of catalyst (4-6) and 3 mL of DMF. bYields obtained by GC are based on remaining
bromobenzene and are the average of two runs.
4. Conclusions
A series of dipalladium(I)-triazenide complexes [Pd{1-(2'-methoxycarbonylphenyl)-3-
(4'-X-pnenyl)triazenide}]2 [X = F (4); Cl (5); Br (6)] have been synthesized and fully
characterized by IR, NMR and HRMS. The crystal structure of 4 determined by X-ray
diffraction studies show some common features with other dipalladium(I)-triazenide complexes,
but also some significant differences, since the eight-membered Pd2(NNN)2 ring adopts a "twist
boat configuration" with the higher torsion angle [N1-Pd1-Pd1-N3 (τ), 19.7°] and the shortest PdI-
PdI distance [2.3998(8) Å] among dipalladium(I)-triazenide complexes. Pd(I) complexes have
the advantage of being used as catalysts under air. Therefore, the series of dipalladium(I)-
triazenide complexes 1-3 were tested as catalysts in Heck and Suzuki-Miyaura couplings. Under
optimized conditions, complex 1 exhibited the highest activity in the Heck reaction, while
complex 3 showed to be the best in the Suzuki coupling, suggesting their efficiencies to be
dependent on the ortho-substituent in the triazenide ligand. Complex 1 is also an efficient
catalyst for the olefinic coupling of different para-substituted bromobenzenes with both styrene
and α-methylstyrene, producing better yields with bromobenzenes containing electron-
20

withdrawing substituents. Complex 3 was efficient in the coupling of phenylboronic acid with a
series of para-bromobenzenes. Additionally, complexes 4-6 were tested as catalysts, showing
comparable yields with complex 1 in the Heck reaction, and comparable yields with compound
3 in the Suzuki reaction, indicating the para-halogen substituent in the triazenide ligands to play
no significant role in the catalytic activity of these complexes.
Acknowledgements
This work was supported by Consejo Nacional de Ciencia y Tecnología (CONACyT)
Grant 60467 and Dirección General de Educación Tecnológica (DGEST) Grant 5150.13-P.
E.C.-A (349527) and C.C.-A (183618) thank CONACyT for graduate fellowships. We thank
CONACyT for ITT NMR and HRMS facilities (Grants INFR-2011-3-173395 and INFR-2012-
01-187686). E.C.-A. thanks Dr. A. Zajac of ESET, LLC for complementary financial support.
Supplementary material
CCDC 1859275 (4), contains the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in the online version, at
http://xxxxxxxxxxx
21

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