Triazenides as suitable ligands in the synthesis of palladium compounds in three different oxidation states: I, II, and III

Article abstract of DOI:10.1021/om500702j

New orthometalated dinuclear triazenide palladium(II) compounds of the general formula Pd₂[(C₆H₄)PPh₂]₂[R-N-N-N-R]₂ (R = C₆H₅, 3a; o-BrC₆H₄, o-3b; o-MeOC₆H₄, o-3c; o-MeC₆H₄, o-3d; p-BrC₆H₄, p-3b; p-MeOC₆H₄, p-3c; p-MeC₆H₄, p-3d) have been synthesized and structurally characterized. The characteristics of these compounds were compared with the isoelectronic formamidinate derivatives. These triazenide compounds have been suitable starting products in the synthesis of new not so common dinuclear palladium(I) compounds and new unusual palladium(III) ones. In the presence of an excess of the triazenide ligand, compounds o-3b and o-3c underwent a reduction process giving dinuclear palladium(I) compounds, Pd₂[R-N-N-N-R]₂ (R = o-BrC₆H₄, o-4b; R = o-MeOC₆H₄, o-4c). DFT calculations verified the importance of the mostly noncovalent Pd···Br or Pd···OMe axial interactions on the stability of these compounds. Under cyclic voltammetric conditions, compounds 3 with the only exception of compound o-3b, were found to undergo a first reversible wave that is assigned as the Pd₂⁵⁺/Pd₂⁴⁺ pair. Compounds p-3b-d also showed at more positive potentials a second reversible wave, Pd₂⁵⁺/Pd₂⁶⁺ pair. When the electrochemical oxidation was performed in the presence of chloride, the Cl-Pd(III)-Pd(III)-Cl species were detected. By chemical oxidation of the palladium(II) complexes with PhICl₂, two new relatively stable dinuclear palladium(III) compounds, Pd₂[(C₆H₄)PPh₂]₂[R-N-N-N-R]₂Cl₂ (R = C₆H₅, 5a; R = p-BrC₆H₄, p-5b), were synthesized and spectroscopically characterized at low temperature. DFT calculations have been performed to study the stability of all the palladium complexes. The isolated palladium(III) and -(II) compounds have been tested as precatalysts at room temperature in the catalytic 2-phenylation of indole with [Ph₂I]PF₆. With 3a the reaction was complete in the shortest reaction time, 7 h, with a 88% isolated yield. The highest yield (99%) but with higher reaction time, 24 h, was obtained with o-3d when CH₂Cl₂ was added to the reaction medium with the aim of solving the palladium complex. These catalytic results were compared with those obtained with other orthometalated palladium compounds with isoelectronic ligands: formamidinate and carboxylates. In the search for a possible first step in this catalytic oxidation process we have also performed DFT calculations exploring the potential Pd(III) intermediate formed by axial Pd-Ph interactions.

Full text of DOI:10.1021/om500702j

Article
pubs.acs.org/Organometallics
Triazenides as Suitable Ligands in the Synthesis of Palladium
Compounds in Three Different Oxidation States: I, II, and III
Susana Ibanez,† Larisa Oresmaa,‡ Francisco Estevan,† Pipsa Hirva,*,‡ Mercedes Sanau,†
́
̃
,†
a
́
and M Angeles Ubeda*
†Departament de Química Inorgan
̀
ica, Universitat de Valen
̀
cia, Dr. Moliner 50, 46100-Burjassot, Valencia, Spain
‡Department of Chemistry, University of Eastern Finland, Joensuu Campus, P.O. Box 111, FI-80101 Joensuu, Finland
S
* Supporting Information
ABSTRACT: New orthometalated dinuclear triazenide
palladium(II) compounds of the general formula Pd2[(C6H4)-
PPh2]2[R−N−N−N−R]2 (R = C6H5, 3a; o-BrC6H4, o-3b; o-
MeOC6H4, o-3c; o-MeC6H4, o-3d ; p-BrC6H4, p-3b; p-
MeOC6H4, p-3c; p-MeC6H4, p-3d) have been synthesized and structurally characterized. The characteristics of these
compounds were compared with the isoelectronic formamidinate derivatives. These triazenide compounds have been suitable
starting products in the synthesis of new not so common dinuclear palladium(I) compounds and new unusual palladium(III)
ones. In the presence of an excess of the triazenide ligand, compounds o-3b and o-3c underwent a reduction process giving
dinuclear palladium(I) compounds, Pd2[R−N−N−N−R]2 (R = o-BrC6H4, o-4b; R = o-MeOC6H4, o-4c). DFT calculations
verified the importance of the mostly noncovalent Pd···Br or Pd···OMe axial interactions on the stability of these compounds.
Under cyclic voltammetric conditions, compounds 3 with the only exception of compound o-3b, were found to undergo a first
4+
reversible wave that is assigned as the Pd25+/Pd2 pair. Compounds p-3b-d also showed at more positive potentials a second
6+
reversible wave, Pd25+/Pd2 pair. When the electrochemical oxidation was performed in the presence of chloride, the Cl−
Pd(III)−Pd(III)−Cl species were detected. By chemical oxidation of the palladium(II) complexes with PhICl2, two new
relatively stable dinuclear palladium(III) compounds, Pd2[(C6H4)PPh2]2[R−N−N−N−R]2Cl2 (R = C6H5, 5a; R = p-BrC6H4, p-
5b), were synthesized and spectroscopically characterized at low temperature. DFT calculations have been performed to study
the stability of all the palladium complexes. The isolated palladium(III) and -(II) compounds have been tested as precatalysts at
room temperature in the catalytic 2-phenylation of indole with [Ph2I]PF6. With 3a the reaction was complete in the shortest
reaction time, 7 h, with a 88% isolated yield. The highest yield (99%) but with higher reaction time, 24 h, was obtained with o-3d
when CH2Cl2 was added to the reaction medium with the aim of solving the palladium complex. These catalytic results were
compared with those obtained with other orthometalated palladium compounds with isoelectronic ligands: formamidinate and
carboxylates. In the search for a possible first step in this catalytic oxidation process we have also performed DFT calculations
exploring the potential Pd(III) intermediate formed by axial Pd−Ph interactions.
INTRODUCTION
■
N,N′ donor ligands such as guanidinates, amidinates, and
triazenides are versatile ligands in coordination chemistry, but
in the case of the triazenes, their use is less developed1 even
though the aryltriazenides have been studied for their
interesting structural, anticancer, and reactivity properties.2
The aryltriazenide anions are “short-bite” three atoms donor
Figure 1. Aryltriazene and the coordination modes of the triazenide
coordination modes (Figure 1).1b,3,4b The chelating mode is
ligands.
ligands that can act in monodentate, chelating, or bridging
less common for these ligands that exhibit a propensity to
coordinate in bridging mode, in part, attributed to the parallel
projection of the two N-donor atom orbitals because of the lack
of substituents in the central atoms that avoids some steric
interferences.1,3
The triazenide ligands, even though they are isoelectronic
with amidinate ligands, are weaker donors and more amenable
to binding to late transition metals.4 Triazenide ligands are also
isoelectronic with carboxylate ones, and the complexes with
these bridging ligands exhibit some M−M interactions.5
The palladium triazenide chemistry is scarce, and few of
these complexes have been structurally characterized. Both
coordination modes, chelating6 and bridging,4,7 were observed
in the palladium(I)4 and -(II)6,7 compounds described in the
literature.
Received: July 7, 2014
© XXXX American Chemical Society
A
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In the last years our research has focused on the
substituted indoles and pyrroles catalyzed by the mononuclear
palladium(II) compound IMesPd(O2CCH3)2 [IMes = 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] using hyperva-
lent iodine salts as arylation agents.15c The reaction proceeded
under very mild conditions, generally at room temperature, and
it was completely air/moisture insensitive. The authors
proposed a Pd(II)/(IV) pathway in these systems.
palladium(III) organometallic chemistry, specifically in the
6+
dinuclear compounds with Pd2 core and M−M bond, being
the first that reported the synthesis in high yield of these
complexes.8 This chemistry is nowadays not well-known, and
only a few mononuclear and dinuclear compounds have been
characterized.9
In a previous paper, we have studied this catalytic reaction
using [Ph2I]PF6 and orthometalated phenolate and acetylacet-
onate palladium(II) and -(III) compounds as precatalyts. Some
results showed that the reaction was complete in short periods
of time with high isolated yields.8d
Following the protocol described in the literature, the
dinuclear metalated triazenide compounds have been tested in
the direct 2-phenylation of the N-unprotected indole at room
temperature obtaining, in some cases, high isolated yields and
short reaction times in the synthesis of 2-phenylindole.
Moreover, in the search for the possible first step in this
catalytic oxidation process via an Pd(III) intermediate, the
interaction of the phenyl groups with the palladium at the axial
sites has been studied by DFT calculations.
Ritter and co-workers have synthesized and characterized not
only other discrete dinuclear palladium(III) complexes10 but
also a transition state analogue for the oxidation of a
palladium(II) compound to a palladium(III) one.11 Further-
more, Mirica and co-workers reported a new cationic dinuclear
compound with a delocalized PdIII−X−PdIII electronic
structure.12
The synthetic accessibility of palladium compounds in high
oxidation states (III and IV) carried out in the last years has
allowed the development of new strategies in which these
compounds are considered as intermediates in oxidation
catalytic processes. Sanford and Hickman described the
advantages of “high valent” organometallic palladium(III) and
-(IV) intermediates over the more common “low valent”
analogues.13 Ritter and co-workers brought forward the first
evidence of dinuclear palladium(III) intermediates in Pd-
catalyzed C−H oxidation reactions.10,14 The advantages of
dinuclear palladium(III) intermediates versus the mononuclear
palladium(IV) ones are based in a possible metal−metal
cooperation that can lower the oxidation barriers for the
oxidation processes.
RESULTS AND DISCUSSION
■
A. Dinuclear Palladium(II) Compounds with Bridging
Triazenide Ligands. Synthesis and Characterization.
Compounds 3 of the general formula Pd2[(C6H4)PPh2]2[R−
N−N−N−R]2 were obtained following two different proce-
dures described in Scheme 1
The triazenides as bridging ligands have allowed us to study
the palladium chemistry on three of its oxidation states: the
most common II, the not so common I, and the unusual III. In
this paper we present the synthesis and characterization of new
orthometalated dinuclear palladium(III) and -(I) compounds
(Figure 2 A, B) with the M−M bond that were obtained
respectively by oxidation or reduction of the new palladium(II)
precursors (Figure 2C).
i. Compounds 3a (R = C6H5), o-3b (R = o-BrC6H4), p-3b (R
= p-BrC6H4), and p-3d (R = p-MeC6H4) were synthesized by
reaction of the tetranuclear palladium(II) compound, 1, with
the corresponding silver triazenide (ratio = 1:4) (Scheme 1a).
ii. Compounds o-3c (R = o-MeOC6H4), o-3d (R = o-
MeC6H4), and p-3c (R = p-MeOC6H4) were also obtained
from 1 via the cationic solvated dinuclear compound 2, by
reaction with potassium or sodium triazenides (ratio = 1:4)
(Scheme 1b).
The 31P NMR spectra of compounds of type 3 showed a
single signal, and their δ (ppm) values are listed in Table 1.
Compounds o-3b−d with ortho-substituted triazenide ligands in
comparison with the para- and unsubstituted ones (p-3b−d
and 3a) show δ values shifted toward high fields. It can be
considered that this fact is due to axial interactions of the ortho-
substituents of the phenyl groups in the triazenide ligands with
the palladium atoms. The interactions X−Pd (X = Br, MeO) in
compounds o-3b and o-3c have been studied by DFT
calculations in Section E.
Figure 2. Dinuclear triazenide palladium(III) (A), palladium(I) (B),
and palladium(II) (C) compounds.
Compounds 3a, o-3b,d, and p-3b−d were structurally
characterized by single-crystal X-ray diffraction methods
(CCDC 996514-996516, 996518-996520). Figure 3 shows
the ORTEP view for compounds o-3b and p-3b. Selected bond
distances and angles for the six compounds are given in Table
2.
The six compounds show similar paddlewheel structures in
which the bimetallic unit is supported by two metalated
phosphines and two triazenide ligands, completing the
palladium square planar coordination mode. The Pd−Pd
distances from 2.6618(9) to 2.7129(9) Å are shorter than the
sum of the van der Waals palladium radii (3.26 Å). We have
included in this paper a charge density analysis at the Pd−Pd
bond critical points (BCPs) that supports a Pd(II)···Pd(II)
interaction with some electrons sharing in all the palladium(II)
compounds, already observed in previous dinuclear palladium-
DFT calculations have been performed for all the palladium-
(III) and -(II) compounds to establish results for the
geometrical and electronic differences. In addition, the relative
stability of the bimetallic Pd(II), Pd(III), and Pd(I) compounds
formed in the synthetic reaction steps has been investigated.
The results are compared with the previously published for
compounds with isoelectronic ligands: formamidinates and
carboxylates.8a,c
Arylated heteroarenes are compounds with a marked interest
because of their optical and electronic properties and their
biomedical applications. In their synthesis the direct C−H
arylation has emerged as a viable alternative for the traditional
cross-coupling reaction, representing an economically and
environmentally attractive strategy.15a,b Sanford and co-workers
reported a new protocol for the direct 2-arylation of free and N-
B
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Scheme 1. a) and b) Synthesis of Palladium(II) Compounds, Pd2[(C6H4)PPh2]2[R−N−N−N−R]2, and c) Synthesis of
Palladium(I) Compounds, Pd2 [R−N−N−N−R]2
Table 1. 31P NMR Spectroscopy Data at 298 and 200 K for
grams are shown in Figures 4, 5, and S1, and the
electrochemical potential data versus SCE are summarized in
Palladium(II) and -(III) Compounds
Table 4.
palladium(II) compounds
palladium(III) compounds
With the exception of o-3b, all the dinuclear compounds 3
showed one first quasireversible (3a, o-3c,d) or reversible (p-
3b−d) wave that can be described in terms of one-electron
δ (ppm)
δ (ppm)
compound
298 K
200 K
compound
δ (ppm) 200 K
−35.5
3a
3.3
4.4
1.0
3.1
1.2
5a
4+
transfer process in the starting Pd2 complexes yielding the
o-3b
o-3c
o-3d
0.7
o-5b
o-5c
o-5d
8,10a,16
5+
mixed-valent Pd2 cationic species.
2.5
−30.8, −31.0
A second reversible wave was observed for compounds with
−0.3
−43.5, −48.9, −56.2,
the para-substituted phenyl triazenide ligands, p-3b−d, that was
−57.1
+5
assigned to the Pd2+6/Pd2 couple.
p-3b
p-3c
p-3d
3.8
3.4
3.4
4.3
4.7
4.6
p-5b
p-5c
p-5d
−34.9
−34.5
−35.3
Irreversible oxidation of the triazenes occurs at potentials
around 1.5 V (Figure S2).
The comparison of the anodic and cathodic potential values
of the first and the second oxidation process among the
triazenide and p-substituted triazenide compounds (3a, p-3b−
d) with the corresponding ones for the equivalent formami-
dinate Pd2[(C6H4)PPh2]2[R−N−C(H)−N−R]2 (R = C6H5, p-
MeOC6H4, p-MeC6H4) compounds showed higher potential
values in all the triazenide derivatives.8c Even though the
triazenide ligands are isoelectronic with amidinate ligands these
results support the fact that the triazenides are weaker donor
ligands.
(II) compounds.8c,d The nature of the interaction is not purely
electrostatic but a typical metallophilic interaction, as can be
seen from the properties of the electron density shown in Table
S2, and further discussed in the computational section.
The short Pd···Br distance of 3.0122(9) Å in compound o-3b
supports an axial interaction which results in an increase of the
Pd(II)−Pd(II) distance. Compound o-3b shows the largest
distance compared with the compounds without an axial
interaction.
Since the stability of the species formed in the oxidation
could be influenced by the presence of ligands which occupy
the axial position, cyclic voltammograms were also performed
in the presence of two mols of chloride from tetraethylammo-
nium chloride (TEACl) per one mol of dinuclear palladium(II)
compound. In p-3b,d, the voltammograms of which showed
two reversible waves, the addition of chloride changed their
electrochemical behavior as it is observed in Figure 5 for
The Pd−Pd distances are also shorter than the observed in
similar formamidinate dinuclear palladium compounds pre-
viously studied and characterized (2.711−2.717 Å) (Table 3).8c
Electrochemical Studies. As a previous step in the synthesis
of palladium(III) compounds we have carried out studies on
the electrochemical behavior of the palladium(II) precursors.
Electrochemical oxidation of type 3 compounds was conducted
in CH2Cl2 solution at room temperature. The cyclic voltammo-
C
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Figure 3. ORTEP view for compounds o-3b and p-3b, with the H atoms and the nonmetalated phenyl groups of the phosphines omitted for clarity.
Ellipsoids drawn at 25% probability.
Table 2. Selected Distances (Å) and Angles (deg) for Compounds 3a, o-3b, p-3b, p-3c, o-3d, and p-3d
compounds
3a
o-3b
p-3b
p-3c
o-3d
p-3d
Pd(1)−Pd(2)
Pd(1)−P(1)
Pd(1)−N(1)
Pd(1)−N(2)
Pd(1)−C(42)
N(1)−Pd(1)−C(42)
N(1)−Pd(1)−N(2)
2.6771(8)
2.2743(12)
2.099(4)
2.7129(9)
2.2537(13)
2.125(4)
2.6623(6)
2.2666(16)
2.116(5)
2.160(5)
2.018(6)
92.3(2)
2.6794(9)
2.279(2)
2.115(6)
2.145(5)
2.007(7)
90.6(3)
2.6696(8)
2.2630(19)
2.115(6)
2.136(5)
2.020(7)
90.5(2)
2.6618(9)
2.266(2)
2.121(7)
2.144(8)
2.031(8)
93.7(3)
2.178(3)
2.158(4)
2.010(4)
2.018(4)
88.77(16)
88.85(15)
89.76(17)
89.02(15)
87.2(2)
88.8(2)
87.1(2)
85.4(3)
additional wave corresponding to the Cl− → Cl2 oxidation was
observed at 1.3 V (Figure S6).
Table 3. Pd−Pd Distances in Formamidinate Palladium(II)
a
Compounds, Pd2[(C6H4)PPh2]2[R−N−C(H)−N−R]2
In the case of compound p-3c the voltammogram registered
in the presence of two equivalents of chloride maintains the
reversible wave assigned to the Pd2+5/Pd2+4 couple, but now the
second oxidation process is irreversible. No new waves that can
be assigned to oxidized species with Cl−Pd interactions were
detected (Figure S7). The Cl− → Cl2 oxidation was also
observed.
This behavior was already observed in other dinuclear
palladium(III) compounds and implies that the oxidation of the
dinuclear palladium(II) compound would be followed by the
addition of chloride in the axial positions to give Cl−Pd(III)−
Pd(III)−Cl species.16
a
Reference 8c.
B. Dinuclear Palladium(I) Compounds with Bridging
Triazenide Ligands. The reaction of 1 with an excess of the
silver triazenides with Lewis basic donor substituents in the
ortho position of their phenyl groups allowed the evolution of o-
3b and o-3c toward the dinuclear palladium(I) compounds
Pd2[R−N−N−N−R]2 (R = o-BrC6H4, o-4b; R = o-MeOC6H4,
o-4c) (Scheme 1c), being quantitative in the case of o-3c. This
procedure involves the Pd(II) → Pd(I) reduction that was also
observed by Walsh and co-workers in the reaction of the
bis(acetonitrile)palladium(II) dichloride with a mixture of
triethylamine and a triazenide with o- MeCO2 or o-MeO-
substituents.4b
Suitable crystals for single-crystal X-ray diffraction methods
were obtained from the reaction medium. Compound o-4b
could be only structurally characterized (Figure 6) (CCDC
996517), whereas o-4c had been previously characterized.4b
The structure of o-4b was consistent with those reported by
Walsh and co-workers4c and Zhan and co-workers4d,e with
compound p-3b. When two equivalents of TEACl were added
the voltammograms showed one single oxidation anodic wave
at a potential slightly lower to the first one recorded for the
parent compound. This wave approaches an irreversible two-
electron oxidation process followed, in the subsequent cathodic
scan, by a reduction wave at 0.35 V and whose anodic
counterpart is entirely absent. The anodic and cathodic waves
of the starting p-3b disappeared completely. The electro-
chemical oxidation involves a two-electron oxidation with
addition of two chlorides giving Cl−Pd(III)−Pd(III)−Cl
species. The voltammogram of the palladium(III) compound
p-5b with axial Pd−Cl bonds (Figure S3) showed the same
anodic wave supporting the formation of this compound by
electrochemical reaction of the corresponding palladium(II)
compound in the presence of the chloride. Compounds o-3c
(Figure S4), 3a (Figure S5), and o-3d with only one reversible
wave showed the same behavior. With excess of TEACl an
D
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Figure 4. Cyclic voltammograms for compounds o-3d and p-3d.
Figure 5. Cyclic voltammogram for compound p-3b (black). Cyclic
voltammogram of p-3b with two equivalents of TEACl (red).
Figure 6. ORTEP view for compound o-4b with the H atoms omitted
for clarity. Important bond distances (Å) and angles (deg) are Pd(1)−
Pd(2), 2.5235(14); Pd(1)−N(1), 2.478(8); Pd(1)−N(2), 2.559(8);
Pd(1)−Br(1), 2.7627(13); N(1)−Pd(1)−Pd(2), 88.97(16); N(1)−
Pd(1)−N(2), 169.4(2).
Table 4. Summary of Electrochemical Potential Data for
a
Compounds 3 versus SCE
b
b
ox1
compound
E1/2 (V)
ΔEp (mV)
E1/2ox2 (V)
ΔEp (mV)
Lewis basic group of the phenyltriazenide ligand, showing a
distorted square planar geometry with N(1)−Pd(1)−N(2),
N(1)−Pd(1)−Br(1), and N(1)−Pd(1)−Pd(1A) angles of
169.4(2), 112.74 (1), and 88.97(16)°, respectively. The
complex has essentially planar bicyclic cores, formed by the
two N3-ligands and the {M2} unit.
The Pd(1)−Pd(2) distance of 2.5235 Å is consistent with a
Pd−Pd bond, and it is longer than that observed for other
dinuclear triazenide palladium(I) compounds (2.4148−2.4309
Å).4b,d The Pd(I)···Br and Pd(I)···OMe interactions have been
studied by DFT calculations.
3a
0.92
0.48
1.15
0.90
1.05
0.61
0.81
125
o-3b
o-3c
o-3d
p-3b
p-3c
p-3d
120
80
70
68
65
1.33
0.89
1.10
82
78
68
a
Electrochemical potentials in V vs SCE in CH2Cl2/0.2 M Bu4NPF6.
b
Scan rate 50 mV s−1. Difference between peak potentials in mV.
different o-phenyl substituted triazenide ligands, Pd2[R−N−
N−N−R′]2 (R = R′ = o-MeOC6H4, o-MeCO2C6H4, or o-
ClC6H4; R = o-MeCO2C6H4 and R′ = C6H5, p-BrC6H4 or p-
MeC6H4). In compound o-4b, each triazenide ligand bridges
the two metal centers and the palladium(I) is surrounded by
one Pd, two N, and the Br from one of the two ortho pendent
The structure of o-4b is similar to those side-by-side
palladium(I) compounds of the type Pd2X2(PP)2 (PP =
bis(diphenylphosphino)methane, X= Cl, Br, C6Cl5,
CF3CO2;17a PP = (R,R)-bis(tert-butylmethylphosphino)-
methane, X = Br;17b PP = aminobis(o-methoxyphenylphos-
phonite), X = Br17c) with anionic ligands in axial positions, and
E
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the cationic [Pd2(PP)(CNtBu)2](BF4)2 (PP = bis-
(diphenylphosphino)methane.17d These compounds have no
planar bicyclic cores, the dihedral angle (Φ) between the two
planes formed by the two metal centers, and two phosphorus
atoms from the ligands (PMMP) has values from 33.0 to 50.5°.
The Pd−Pd distances are longer with values between 2.594 and
2.699 Å.17a,c
a single signal around −35 ppm. No o-5b was obtained because
of the fairly strong Pd···Br interaction that blocks the axial sites
preventing the reaction with chlorine. The less strong Pd···
OMe interaction allows the oxidation of o-3c giving a mixture
of compounds as shown by the 31P NMR spectrum. Some
steric considerations also need to be taken into account,
compound o-3d with o-Me groups gave also a mixture of
oxidized compounds, and the 31P NMR spectrum showed
signals at higher fields suggesting that the product was oxidized
but no Pd−Cl bonds were formed.
Compounds 5a and p-5b were relatively stable at room
temperature, and they have been synthesized and characterized
by 13C, 1H, and 31P NMR spectroscopy at 223 K. Unfortunately
suitable crystals for X-ray diffraction methods could not be
obtained.
D. DFT Studies of Pd(I), -(II), and -(III) Compounds.
Stability of the Palladium(I) Compounds. The preparation of
these palladium(I) compounds is associated with the character-
istics of the o-phenyl substituents in the triazenide ligands that
interact in the axial positions. Walsh and co-workers compared
the behavior of some o-substituted phenyltriazenides, while
only palladium(I) compounds were obtained with MeCO2 and
MeO substituted triazenides, the one with a MeS group gave a
palladium(II) compound. The authors consider that the greater
donating ability of the MeS group results in a more electron
rich palladium center, more stable to the reduction.4b
We have studied computationally the relative stability of the
Pd(I) compounds by calculating the substituent effect on the
relative energies of a model reaction Pd2L2 + 2L′H ⇒ Pd2L′2 +
2LH, where L represents the nonsubstituted triazenide ligand
and L′ represents the ligands substituted with Br, OMe, and Me
substituents. Table 5 lists the reaction energies for the ortho-
and para-substituted complexes relative to the nonsubstituted
one.
In a previous paper18 we have published the synthesis and
structural characterization of the compound Pd2[μ-
C9H8NPPh2]2(NCCH3)2 in which the two palladium(I)
metal centers are bridged by two P,N-ligands obtained by
deprotonation of the indolyl nitrogen in the diphenyl(3-methyl-
2-indolyl) phosphine, C9H8NPPh2. Two acetonitrile molecules
are coordinated in axial positions. The Pd−Pd distance is
2.5459(8) Å, and, in this case, the Φ value (37.05°) is bigger
due to higher steric and orbital interactions minimized by a
higher twisting about the Pd−Pd axis.
C. Dinuclear Palladium(III) Compounds with Bridging
5+
Triazenide Ligands. The reversibility observed for the Pd2
/
Pd2 and Pd26+/Pd2 processes in most of the compounds of
type 3 suggested that they are suitable starting materials to
access the high-valent dinuclear palladium compounds. Further
evidence of their suitability could be obtained by adding
chloride in the axial positions when the electrochemical
oxidation was performed in the presence of TEACl. Addition-
ally, previous DFT calculations verified that the formation of
Pd−Cl bonds can stabilize some dinuclear palladium(III)
compounds.8c,d These observations prompted us to undertake
the synthesis of new palladium(III) compounds by chemical
oxidation with PhICl2.
4+
5+
Chemical Oxidation of Compounds 3. In an attempt to
synthesize new palladium(III) compounds of general formula
Pd2[(C6H4)PPh2]2[R−N−N−N−R]2Cl2 we have monitored
by 31P NMR spectroscopy from 200 to 298 K the reaction of
compounds of type 3 with the oxidant PhICl2 (Scheme 2).
These experiments allowed us to get information about the
stability of the palladium(III) compounds obtained.
Table 5. Effect of the Substitution of the Triazenide Ligands
on the Reaction Energies of the Model Reaction Pd2L2 +
2L′H ⇒ Pd2L′2 + 2LH
Scheme 2. Reaction of 3 with PhICl2 To Give Dinuclear
Palladium(III) Compounds, Pd2[(C6H4)PPh2]2[R−N−N−
N−R]2Cl2 (5)
a
ΔEr [kJ mol‑1]
Pd(I) compound
ortho
para
4a
4b
4c
4d
0
−231
−29
+33
+1
−5
−2
a
Relative to the energy of nonsubstituted structure 4a.
The results show a clear preference of the o-Br phenyl
substituted ligand to form a stable Pd(I) complex. The
enhanced stability has to be due to structural interactions,
since the para-substituted ligands are notably less favorable
than o-4b. Also o-4c is somewhat stabilized compared to 4a.
Figure 7 presents the optimized structures o-4b and o-4c which
have additional intramolecular interactions at the axial sites of
the complexes. The most important properties of the electronic
density at the bond critical points corresponding to these
intramolecular interactions are shown in Figure 7.
Figure 7 verifies the importance of the axial interactions on
the stability of the Pd(I) complexes and explains why the o-Br
and o-MeO phenyl substituted ligands are the preferred ones in
forming the reduced Pd(I) complex. Although the axial
interactions were mostly noncovalent in nature, as can be
PhICl2 was added at 200 K to a solution of the
corresponding dinuclear palladium(II) compound (stoichio-
metric ratio PhICl2/Pd2: 1/1) in CD2Cl2 in a NMR tube, and
immediately the 31P NMR spectrum was recorded and the date
was listed in Table 1. In agreement with the literature, the
appearance of one signal shifted toward high fields around 30
ppm in relation to that observed for the corresponding
compound 3 indicates the formation of a palladium(III)
compound.8 Compounds 5a, p-5b, p-5c, and p-5d with bridging
non-o-phenyl substituted triazenide ligands were obtained as
the only product of the reaction, their 31P NMR spectra showed
F
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ligands, which were substituted at the para-position of the
phenyl groups. On the other hand, the ortho-substituted
complexes o-3b and o-3c showed a rather different behavior of
the energetically most favorable optimized structures (Figure
S8). As already seen in the Pd(I) complexes, both o-Br and o-
OMe phenyl substituents could interact with the palladium
atoms at the axial site. The axial Pd···Br interactions resembled
similar intramolecular interactions found already in the Pd(I)
compound o-4b, but the interaction energy was rather much
weaker in the case of Pd(II) than in the Pd(I) complex (EINT
was −41.4 kJ mol−1, −26.4 kJ mol−1, and −88.4 kJ mol−1 in o-
3b, o-3c, and o-4b, respectively).
The energetically most favorable Pd(II) isomer was in most
cases the para-substituted derivative, except with the ortho-
substituted bromine, where the intramolecular Pd···Br inter-
actions stabilized the structure. Because of this stabilization, the
reaction of compound o-3b with an excess of triazenide ligand
evolved more slowly toward the dinuclear palladium(I)
compound, o-4b, than compound o-3c even though DFT
calculations exhibited greater stability for compound o-4b than
for o-4c.
The geometries of nearly all of the complexes agreed well
with the obtained experimental crystal structures. However,
there was one striking difference in the computational and
experimental structures: compound o-3b with an o-Br
substituted phenyl triazenide ligand showed a notably longer
Pd−Pd distance and a shorter Pd···Br distance than was
measured from the X-ray structure, 2.750 Å versus 2.713 Å and
2.856 Å versus 3.012 Å, respectively. A clear explanation for this
difference can be obtained by inspecting the crystal structure in
detail. The solid state structure consists of dinuclear molecules,
which attach together with intermolecular Br···Br interactions,
thus forming a chain of molecules (Figure S9).
The intermolecular interactions were studied further with a
model of two adjacent molecules cut from the crystal structure.
The QTAIM analysis showed that the intermolecular Br···Br
interactions are weak and purely noncovalent (Table S1,
compound o-3b_dim). However, they have an important effect
on the solid state structure of the compound and also on the
intramolecular interactions, such as the Pd···Pd distance.
Details of the interactions and the QTAIM analysis can be
found in the Supporting Information (Tables S1−S3). Since
the weak intermolecular interactions are not normally present
in solution, the computational results of single molecules will
give reliable information on the relative electronic effects of
different ligands.
Figure 7. Optimized structures of the Pd(I) complexes o-4b and o-4c
with selected intramolecular interactions. The properties of the
electron density according to the QTAIM analysis are shown in the
corresponding boxes for the axial Pd···Br and Pd···O interactions. ρBCP
(Pd−X) = electron density at the bond critical point; |V|/G = ratio
between potential energy density and kinetic energy density; EINT
=
interaction energy at the BCP, Ω(Pd,X) = delocalization index
between atoms Pd and X.
seen in the value of |V|/G, which for pure electrostatic
interactions gets the value of <1, the strength of the interaction
was very large (EINT was −88.4 kJ mol−1 and −65.3 kJ mol−1
for the Pd···Br and Pd···O interactions, respectively). For the
delocalization index, which can be regarded as the bond index,
we obtained values of 0.54 and 0.27, which are rather typical for
noncovalent metal−ligand bonding. In both complexes o-4b
and o-4c, the Pd−Pd bond index was about 1, showing strong
metal−metal bonding with interaction energies of −104.5 kJ
mol−1 and −106.8 kJ mol−1 at the bond critical points,
respectively. The strong bonding between the Pd atoms was
also seen in short Pd−Pd distances (Figure 7).
Structures of the Pd(II) and Pd(III) Triazenide Compounds.
We performed geometry optimization of the different
substitution options, which have been experimentally obtained,
to gain insight on the favorability of the oxidation reactions.
All complexes were fully optimized at the DFT level of
theory, and the geometries and electronic properties were
compared with the previously studied palladium compounds
with the formamidinate ligand.8c The effect of the different
ortho- or para-phenyl substituents on the Pd−Pd distance in
Pd(II) and Pd(III) compounds can be seen in Table 6.
Generally, the substituent effect was rather small, and results
were very similar for the Pd−Pd distances in all triazenide
According to the QTAIM analysis, the Pd(II)···Pd(II)
interaction in compounds type 3 shows some electron sharing,
Table 6. Effect of the Substitution of the Phenyl Triazenide Ligands on the Pd−Pd Distances in Palladium(II) and
Palladium(III) Compounds
a
Pd−Pd distances (Å)
palladium(II) compounds
palladium(III) compounds
compound
ortho
para
compound
ortho
para
3a
3b
3c
2.692 (2.6771)
2.750 (2.7129)
2.705
5a
5b
5c
5d
2.556
2.603
2.592
2.595
2.691(2.6623)
2.693 (2.6794)
2.692 (2.6618)
c
2.557
2.559
2.557
3d
b
2.706 (2.6696)
d
d
2.736 (2.715)
2.593
a
b
c
Experimental distances in brackets. [Pd2[(C6H4)PPh2]2[C6H5N−C(H)−NC6H5]2; Pd2[(C6H4)PPh2]2[C6H5−N−C(H)−N−C6H5]2Cl2;
d
Reference 8c.
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and therefore the interaction is not entirely noncovalent, which
is seen in the value ratio of the potential energy density and the
kinetic energy density, |V|/G > 1. Also the negative and rather
large interaction energy, ranging from −60 kJ mol−1 to −68 kJ
mol−1, indicate substantial interaction between the two
palladium atoms. The bond delocalization index at the
corresponding Pd−Pd bond critical point, which can be
regarded as bond index, gets nonzero values around 0.35.
The substituent effect of the triazenide ligands on the Pd−Pd
bond is small, showing a slight preference for energies of the
compounds with parasubstituted triazenide ligands. This
strengthening of the Pd(II)···Pd(II) interaction is in line with
the decreased metal−metal distance in the parasubstituted
compounds.
Table 7. Effect of the Substitution of the Phenyl Triazenide
Ligands on the Reaction Energies of the Oxidation Model
Reaction
ΔEr [kJ mol‑1]
oxidation reaction with Cl2
ortho
para
3a → 5a
3b → 5b
3c → 5c
3d → 5d
a
−88
+96
−87
−75
−84
−91
−96
−92
a
Pd2[(C6H4)PPh2]2[C6H5N−C(H)-NC6H5]2 → Pd2[(C6H4)-
PPh2]2[C6H5−N−C(H)−N−C6H5]2Cl2.
As already noticed in the previous study of the O,O′-
chelating ligands,8d the metal−metal bond shortens when the
compound is oxidized from Pd24+ to Pd26+. With the triazenide
ligands the shortening was somewhat smaller than with the
chelating ligands but rather similar than with the bridging
carboxylates, about 0.15 Å.8c,d Similar contraction of the Pd−Pd
bond distance in the oxidized Pd(III)−Pd(III) complexes was
also found in the study of Powers and Ritter.10 QTAIM analysis
shows increased electron density and hence increased degree of
covalency and bond delocalization index at the Pd(III)−Pd(III)
BCPs. There is also a small but systematic increase in the
corresponding interaction energies (Table S3).
When the Pd−Pd distance was compared with the previously
studied Pd(II) and Pd(III) compounds with the formamidinate
ligand, the difference in the electronic nature of the ligands
could be seen in the computationally optimized Pd−Pd
distances. It should be noted that the effect of ligand
substitution was studied also in the case of the formamidina-
tes.8c The Pd−Pd distances were shorter in the case of
triazenides than in formamidinate, which results in slight
differences in the properties of the charge density (Tables S2
and S3). However, in both triazenides and amidinates the
trends were the same; the preferred Pd(II) isomer was the
ortho-phenyl substituted one only for bromine substituent,
since similar stabilizing axial Pd···Br interactions were possible.
Stability of the Oxidized Compounds. The stability of the
oxidized compounds was studied by calculating the relative
energies of the chemical oxidation model reaction:
compounds. Otherwise, the reaction energies were very similar
to the corresponding formamidinate ligands, suggesting similar
behavior upon oxidation.
E. Metalated Palladium(II) and -(III) Compounds As
Precatalysts in the Direct Phenylation of Indole.
Compounds 3, 5a, and p-5b have been tested as precatalysts
in the direct 2-phenylation of the N-unprotected indole (6) at
room temperature with [Ph2I]PF6 in acetic acid to give
compound 7 (Scheme 3). The reaction has been followed by
1H NMR spectroscopy. The results are displayed in Table 8.
With compound 3a and under the conditions described in
́
Scheme 3, the 2-phenylation of indole proceeded in only 7 h
giving an isolated yield for 7 of 88% (Table 8, entry 1). In the
same reaction conditions, o-3c and p-3b−d gave high yields
(81−84%) but with higher reactions times (19−96 h) (Table 8,
entries 3−6). Compounds o-3b and p-3b−d showed some
insolubility in acetic acid nevertheless they showed activity
(Table 8 entries 2, 4−6). With the insoluble compounds the
reaction was also performed with addition of CH2Cl2 until their
complete solution and different results were obtained. With the
most insoluble compound o-3d the reaction was completed in
24 h giving the highest isolated yield (99%) observed (Table 8,
entry 10). On the other hand, when p-3b was used, the reaction
was complete in half time (96 → 48 h) increasing the yield for
7 up to 87% (Table 8 entry 11), but in the case of o-3b the time
was significantly reduced (264 → 48 h) although the yield
decreased from 78 to 64% (Table 8, entry 9). With p-3c,d the
reaction was complete in the same reaction time, but in both
cases a significant decrease was observed in the isolated yield
for 7 (Table 8, entries 12 and 13).
Pd2[(C6H4)PPh2]2[R−N−N−N−R]2 + Cl2
⇒ Pd2[(C6H4)PPh2]2[R−N−N−N−R]2Cl2
Results published by Sanford and co-workers showed a yield
of 81% and a longer reaction time (15 h) for this reaction using
[Ph2I]BF4 as oxidant and IMeSPd(OAc)2 (IMes = 1,3-
bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene) as potential cat-
alyst.15
Table 7 lists the obtained reaction energies. Again, the
various triazenide compounds are compared with the
corresponding formamidinate compounds.
As can be expected, the compounds with para-substituted
phenyl groups in the triazenide ligands were more readily
oxidized with axial chloride ions than the ortho-substituted
ones. Furthermore, when the phenyl groups had a Br
substituent at the ortho-position, the fairly strong interaction
with the Pd atoms blocked the axial site inhibiting the reaction
with chloride. This is well in agreement with the experimental
findings, since no palladium(III) compound was obtained by
reaction of o-3b with PhICl2 at 200 K. With the o-OMe
substituted ligand the Pd···O interaction was weak enough to
be overcome with the stronger Pd−Cl bond; therefore, the
oxidized compound was found feasible. The 31P NMR
spectrum of the reaction of o-3c with PhICl2 at 200 K showed
two signals at shift values corresponding to palladium(III)
The isolated palladium(III) compounds have also been
tested in the catalytic reaction. Compounds 5a gave 7 in a
shorter time and a higher yield than p-5b (24 versus 48 h and
76 versus 59%, respectively) (Table 8, entries 7 and 8).
When the reaction was developed without the presence on
palladium complexes, compound 7 was not observed.
At the end of each reaction a reddish solid was obtained. The
electron microscopy spectrum of the isolated solid showed that
it contains mostly fluorine with some palladium. The isolated
reddish solid did not catalyze the phenylation reaction. The
HPF6 obtained by reaction of [Ph2I]PF6 with acetic acid
decomposed at room temperature giving HF, which attacks the
glass Schlenk tubes.19
H
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Scheme 3. Direct Phenylation of Indole
the similar catalytic reaction of room temperature direct 2-
phenylation of 1-methylindole, showing to be excellent
precatalysts giving high isolated yields in 2-phenyl-1-methyl-
indole with short reaction times (93 and 91% and 2 and 6 h,
respectively).8d Applied as a precatalyst in the catalytic reaction
studied in this paper, low yields for 7, 37 and 59% and reaction
times of 15 and 20 h were respectively obtained (Table 9,
entries 6 and 7).
Lower yields for 7 and longer reaction times were obtained
(Table 9, entries 10 and 11) when the orthometalated
mononuclear palladium(II) compounds Pd[(C6H4)PPh2]-
BrPPh3 and Pd2[(C6H4)PPh2]2[C6H4OC(O)CH3]2, in which
redox cooperation is not possible, were used as precatalysts.
Investigation on the 2-Phenylation of Indole. Sanford and
co-workers with mononuclear palladium(II) compounds
proposed that this reaction proceeds via a mononuclear
Pd(IV)-phenyl intermediate.13,15c On the other hand, also
dinuclear palladium(III) complexes have been proposed as
active species for several catalytic reactions.14 Especially,
Sanford, Yates, and co-workers have suggested that a
phenylated dinuclear palladium(III) cation would initiate the
catalysis cycle in the formation of mppPh (mpp = 3-methyl-2-
phenylpyridine).20
In the search for the possible first step in the catalytic
oxidation process in the 2-phenylation of indole with dinuclear
palladium(II) compounds and whereas its oxidation enables the
access to dinuclear palladium(III) compounds, we have
explored the Pd(III) intermediate by performing DFT
calculations on the axial Pd-Ph interactions.
We optimized the compounds with axial phenyl ligands and
compared the relative stability of the products by calculating
the stabilization energies as well as the effect of the interaction
to the nature of the Pd−Pd bonds. Table 10 shows the relative
stability energies (referenced to the most favorable reaction
energy in p-3c) and the Pd−Pd and Pd−C(Ph) bond distances
for the bisphenyl derivatives.
Table 8. 2-Phenylation of Indole with Compounds Type 3,
5a, and p-5b as Precatalyst
reaction time
(h)
7 yield
entry
conditions
Scheme 3
precatalyst
(%)
1
3a
7
88
78
84
84
81
82
76
59
64
99
87
68
66
2
o-3b
o-3c
p-3b
p-3c
p-3d
5a
264
48
96
20
19
24
48
48
24
48
19
19
3
4
5
6
7
8
p-5b
o-3b
o-3d
p-3b
p-3c
p-3d
9
Scheme 3 + addition of
CH2Cl2
10
11
12
13
Since the carboxylate and formamidinate ligands are
isoelectronic with the triazenide ones, we have tested some of
the these orthometalated formamidinate and carboxylate
dinuclear palladium(II) compounds in this catalytic reaction.
The results listed in Table 9 were compared with the previous
obtained with the triazenide derivatives (Table 8).
With the carboxylate derivatives, Pd2[(C6H4)-
PPh2]2[O2CCH3]2 and Pd2[(C6H4)PPh2]2[O2CCF3]2, 7 was
obtained in higher yields and reaction times in intermediate
values in relation to those observed for the triazenide
derivatives (Table 9, entries 1 and 2).
In the case of the formamidinate derivatives, Pd2[(C6H4)-
PPh2]2[(C6H4)NC(H)N(C6H4)]2, Pd2[(C6H4)PPh2]2[(p-
CH3C6H4)NC(H)N(p-CH3C6H4)]2, and Pd2[(C6H4)-
PPh2]2[(p-CH3OC6H4)NC(H)N(p-CH3OC6H4)]2, the yields
were lower with low reaction time for the two first products
(Table 9, entries 3−5).
In a previous paper we have tested the dinuclear palladium-
(II) compounds with [O−O] donor ligands, Pd2[(C6H4)-
PPh2]2[acac]2 and Pd2[(C6H4)PPh2]2[C6H4OC(O)CH3]2, in
Interaction of two phenyls with the palladium atoms at the
axial sites was sterically very demanding because of the bulky
phenyl groups in both the metalated phosphines and the
Table 9. 2-Phenylation of Indole with Dinuclear and Mononuclear Palladium(II) and -(III) Compounds
reaction time
7 yield
entry
conditions
precatalyst
Pd2[(C6H4)PPh2]2[O2CCH3]2
(h)
(%)
1
2
3
4
5
Scheme 3
21
98
98
79
71
66
dinuclear compounds
Pd2[(C6H4)PPh2]2[O2CCF3]2
36
Pd2[(C6H4)PPh2]2[(C6H4)N−C(H)-N(C6H4)]2
Pd2[(C6H4)PPh2]2[(o-MeC6H4)N−C(H)-N(4-CH3C6H4)]2
13
11
Pd2[(C6H4)PPh2]2[(o-MeOC6H4)N−C(H)-N(4-
120
CH3OC6H4)]2
6
Pd2[(C6H4)PPh2]2[acac]2
15
37
59
44
59
86
96
7
Pd2[(C6H4)PPh2]2[C6H4OC(O)CH3]2
20
8
Scheme 3 + addition of CH2Cl2 Pd2[(C6H4)PPh2]2[O2CCH3]2Cl2
Pd2[(C6H4)PPh2]2[O2CCF3]2Cl2
168
168
168
168
9
mononuclear compounds
10
11
Pd[(C6H4)PPh2]BrPPh3
Pd[(C6H4)PPh2]Br[(4-BrC6H4)PPh2]
I
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Table 10. Effect Axial Interaction of Phenyl Groups on the Relative Stability of the Compounds 3a, o-3b−d, and p-3b−d and the
Corresponding Pd−Pd and Pd−C(Ph) Distances
ΔE [kJ mol‑1]
Pd−Pd [Å]
Pd−C(Ph) [Å]
compound
substituent
ortho
para
ortho
para
ortho
para
3a
3b
3c
H
17
195
23
2.745
2.816
2.775
2.807
2.840
2.186
2.170
2.180
2.173
2.168
Br
40
0
2.712
2.790
2.746
2.208
2.147
2.183
OMe
Me
H
3d
a
36
15
11
a
Pd2[(C6H4)PPh2]2[C6H5N−C(H)−NC6H5]2.
triazenide ligands. However, we were able to find optimized
structures for all compounds, even if the relative stability of the
products with ortho-substituted ligands was poor at least in the
case of the o-Br triazenide compound. The weaker stability of
the ortho-bromide substituted compounds was not entirely due
to steric effects, since also the para-substituted derivative
showed somewhat poorer stability. The substituent effect on
the stability trends was also consistent with the experimental
reaction times in the catalytic phenylation of indole, since the
Br-substituted catalysts o-3b and p-3b led to longer reaction
times, thus indicating less stability of the intermediate
compound.
The stability of the complexes could be mostly related to the
differences in the Pd−C(Ph) distances in the products, the
shorter the distance, the better the stability. Furthermore,
shorter Pd−C(Ph) distances led to longer Pd−Pd distances, as
can be seen in Table S4, contrary to the interaction with axial
chlorides, where stronger Pd−Cl interaction shortened the Pd−
Pd distance and led to formation of the Pd−Pd bond with
considerable covalent character. Figure 8 shows the properties
of the electron density at the corresponding bond critical points
for the nonsubstituted complexes 3a and 5a and the
corresponding intermediate 3a+2Ph. The large electron density
at the BCP and the large interaction energy supported the
strong nature of the Pd−C(Ph) bond.
As we have observed earlier,8e interaction with axial chlorides
changes the nature of the Pd···Pd interaction by reducing the
bond distance and increasing the covalent nature of the
interaction, as can be seen in the increased |V|/G ratio from
1.23 to 1.43 for 3a and 5a, respectively. The axial interaction
also slightly increased the strength of the Pd···Pd interaction.
Also axial interaction with phenyl groups in the intermediate
structure had the same effect of increasing the covalent nature
of the bond between the palladium atoms, even though the
distance between palladiums was larger and the strength of the
interaction smaller than in 3a (Figure 8). However, when the
properties of the axial Pd···Cl and Pd···C(Ph) interactions were
compared, the interaction with the axial phenyls proved to be
very strong (EINT = −123.2 kJ mol−1), approaching the strength
of covalent bonding, which is most probably due to larger
electron density at the Pd···C(Ph) bond critical point. The
strong Pd···C(Ph) bonding is the source of enhanced stability
of the intermediate structures and supports also the
experimental suggestion that the axial Pd−Cl interaction can
be replaced with the axial Pd−Ph interaction. We also
optimized a monophenylated Pd(III) complex with one axial
phenyl group and one chloride ligand and performed a QTAIM
analysis for the nature of the interactions. According to the
results shown in Figure S10, there is a strong axial effect from
the phenyl group which reduces the strength of the opposite
Pd−Cl interaction and facilitates the removal of the second
chlorine and formation of the bisphenylated complex.
Therefore, the bisphenylated intermediate is a very feasible
option for the catalytic 2-phenylation of indole. Furthermore,
similar intermediate structures can be effective in the
phenylation process involving both bimetallic Pd(II) and
Pd(III) complexes.
CONCLUSION
■
In a nutshell, the ortho- and para-phenyl substituted triazenides
have shown to be very suitable ligands in the synthesis of new
dinuclear palladium compounds in a wide range of oxidation
states: I, II, and III. Compounds of the general formula
Pd2[(C6H4)PPh2]2[R−N−N−N−R]2 can undergo a reduction
or oxidation process giving respectively Pd2[R−N−N−N−R]2
or Pd2[(C6H4)PPh2]2[R−N−N−N−R]2Cl2 compounds. All
the compounds have been characterized by NMR spectroscopy
and some of them also by X-ray diffraction methods. DFT
calculations verified the importance of the mostly noncovalent
Pd···Br or Pd···OMe axial interactions on the stability of the
palladium(I) compounds, Pd2[R−N−N−N−R]2, versus the
corresponding ones in oxidation state II, Pd2[(C6H4)-
PPh2]2[R−N−N−N−R]2. Under cyclic voltammetric condi-
tions, the triazenide dinuclear palladium(II) compounds type 3
Figure 8. Comparison of the properties of the Pd−Pd and the axial
interactions in complexes type 3a, 5a, and 3a+2Ph. Hydrogens and
free phenyl groups of the phosphines have been omitted for clarity.
The properties of the electron density according to the QTAIM
analysis are shown in the corresponding boxes for the PdPd and Pd···
C(Ph) interactions. ρBCP (Pd−X) = electron density at the bond
critical point; |V|/G = ratio between potential energy density and
kinetic energy density; EINT = interaction energy at the BCP, Ω(Pd,X)
= delocalization index between atoms Pd and X.
J
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electrode was a Pt electrode with a surface of 3.1 mm2, the counter
electrode a Pt wire, and the reference electrode Ag/AgCl (saturated
KCl). The solvent was CH2Cl2−0.2 M Bu4NPF6, the solution was 0.5
mM for compounds, and the scan rate was 50 mV/s. In these
conditions Ea for the couple Fc/Fc+ was 0.550 V.
Synthesis of the Silver Triazenide Salts, Ag[R−N−N−N−R].
The silver salts were prepared as follows. In a 1:1 stoichiometric ratio,
a solution of AgNO3 in water (1 mL) was added under vigorous
stirring to a CH2Cl2 solution (2 mL) of the corresponding triazene
(0.5 mmol) previously deprotonated with a methanolic solution of
NaOMe. Yellow precipitates were obtained in all the cases. Each solid
was filtered, washed with MeOH (2 × 4 mL), and vacuum-dried. They
were used without further purification. Yields: 135.1 mg (87%) R =
C6H5; 226 mg (98%) R = o-BrC6H4; 217 mg (94%) R = p-BrC6H4;
134 mg (81%) R = p-MeC6H4.
were oxidized showing one (3a, o-3c-d) or two reversible (p-
3b−d) waves assigned to Pd25+/Pd24+ and Pd26+/Pd25+ couples.
When the electrochemical oxidation was carried out in the
presence of chloride, the loss of two electrons is followed by the
addition of two chlorides giving the Cl−Pd(III)−Pd(III)−Cl
species that have been detected, showing the accessibility of the
palladium(II) compounds to ones in a high-valent oxidation
state, III. The chemical oxidation of compounds 3 with PhICl2
allowed to isolate two new unusual dinuclear palladium(III)
compounds, Pd2[(C6H4)PPh2]2[R−N−N−N−R]2Cl2 (R =
C6H5, 5a; R = p-BrC6H4, p-5b), that were characterized by
spectroscopic techniques at low temperatures.
The computational studies showed that the energetically
most favorable palladium(II) complexes were the ones with
para-substituted phenyltriazenide, except in the case of the
ortho-substituted bromine derivative where the intramolecular
Pd···Br interaction stabilized its structure. Also the DFT
calculations are in agreement with the experimental finding that
we did not succeed in obtaining the o-Br phenyltriazenide
palladium(III) derivative, because the fairly strong Pd···Br
interaction can block the axial site inhibiting the addition of
chloride.
The isolated palladium(II) and (III) compounds were tested
in the direct phenylation of indole at room temperature with
[Ph2I]PF6 in acetic acid. In the case of 3a the reaction
proceeded with the lowest reaction time, 7 h, and with an 88%
isolated yield, better results than that published by Sanford and
co-workers, 15 h, and 81% with a mononuclear palladium(II)
compound as precatalyst. The highest yield, 99%, was obtained
with o-3d and addition of CH2Cl2 to the reaction medium but
with a higher reaction time, 24 h. DFT calculations have been
executed in order to study the possible first step in this catalytic
reaction. The formation of the Pd(III) intermediate com-
pounds have been explored considering the axial Pd−(C)Ph
interactions that have been proved to be stronger than the Pd−
Cl ones (EINT = −123.2 kJ mol−1 versus EINT = −77.4 kJ
mol−1).
Preparation of Pd2[(C6H4)PPh2]2[R−N−N−N−R]2, R = C6H5
(3a), R = o-BrC6H4 (o-3b), R = p-BrC6H4 (p-3b), R = p-MeC6H4 (p-
3d). Compounds were prepared by reaction of compound 1 with the
corresponding silver triazenide. [Pd((C6H4)PPh2)Br]4 (1) (100 mg,
0.056 mmol) was suspended in CH2Cl2 (10 mL), and the
corresponding silver triazenide (0.224 mmol) was added. The reaction
mixture was allowed to stir for 24 h after which the crude was filtered
through a short plug of Celite. The solution was evaporated to dryness,
and the solid precipitated by the addition of hexane was collected by
filtration and washed with hexane. Orange-yellow microcrystalline
powders were obtained by this method. Yields: 59.6 mg (48%) 3a;
143.6 mg (89%) o-3b; 129.3 mg (80%) p-3b; 101.6 mg (77%) p-3d.
1
Characterization Data for 3a. H NMR (CDCl3, 500 MHz, 298
K) δ 7.69 (m, 4H, ar), 7.52 (m, 2H, ar), 7.41 (m, 4H, ar), 7.30 (m, 4H,
ar), 7.21 (m, 8H, ar), 7.10 (m, 8H, ar), 6.91 (m, 8H, ar), 6.81 (m, 8H,
ar), 6.57 (m, 2H, ar); 31P NMR (CDCl3, 202 MHz, 298 K) δ 3.3 (s);
13C NMR (CDCl3, 125 MHz, 298 K) δ 163.0 (m, C metalated), 151.4
(s, C−N, trans to C), 150.7 (t, C−N, trans to P, 3JP−C = 2 Hz), 141.4−
121.4 (ar). Anal. Calcd for C60H48N6P2Pd2: C, 63.38; H, 4.25; N 7.45.
Found: C, 62.73; H, 3.82; N 7.02.
X-ray Crystal Structure Data for 3a (CCDC 996514). Empirical
formula: C60H48N6P2Pd2·CH2Cl2, crystal system: monoclinic, space
group: P2(1)/n, a = 11.147 (2) Å, b = 25.517 (5) Å, c = 19.740 (4) Å,
β = 103.50 (3)°, V = 5459.6 (19) Å3, Z = 4 crystal dimensions: 0.25 ×
0.25 × 0.23 mm3; MoKα radiation, 273 (2) K; 30002 reflections,
10975 independent; (μ = 0.861 mm−1); refinement (on F2) with
SHELXTL (version 6.1), 659 parameters, 0 restraints, R1= 0.0510 (I >
2σ) and wR2 (all data) = 0.1507, GOF = 1.108, max/min residual
electron density: 1.397/−1.102 e Å−3.
EXPERIMENTAL SECTION
■
Characterization Data for o-3b. 1H NMR (CDCl3, 500 MHz, 298
K) δ 7.42 (m, 2H, ar), 7.37 (m, 8H, ar), 7.25 (m, 2H, ar), 7.10 (m, 4H,
ar), 7.02 (m, 4H, ar), 6.93 (m, 4H, ar), 6.84 (m, 10H, ar), 6.73 (m, 8H,
ar), 6.39 (m, 2H, ar); 31P NMR (CDCl3, 202 MHz, 298 K) δ 0.7 (s);
13C NMR (CDCl3, 125 MHz, 298 K) δ 165.4 (m, C metalated), 151.5
(t, C−N, trans to P, 3JP−C = 2 Hz), 150.1 (s, C−N, trans to C), 142.5−
113.0 (ar). Anal. Calcd for C60H44 Br4N6P2Pd2: C, 49.90; H, 3.05; N
5.82. Found: C, 49.44; H, 2.87; N 5.58.
All the reactions were carried out under a dry nitrogen atmosphere
using Schlenk techniques. Solvents were purified according to standard
procedures. Commercially available reagents were used as purchased.
[Pd((C6H4)PPh2)Br]4,21 1, and iodobenzene dichloride (PhICl2)22
were synthesized according to literature procedures. The triazenes
were prepared by the procedure adapted from the literature involving
the diazotation and coupling of anilines.23 Solvent mixtures are v/v
mixtures. Column chromatography was performed on silica gel (35−
70 mesh).
X-ray Crystal Structure Data for o-3b (CCDC 996515). Empirical
formula: C60H44Br4N6P2Pd2, crystal system: monoclinic, space group:
C2/c, a = 17.970 (4) Å, b = 13.656 (3) Å, c = 22.774 (5) Å, β = 102.97
(3)°, V = 5.446.2 (19) Å3, Z = 4, crystal dimensions: 0.26 × 0.24 ×
0.22 mm3; MoKα radiation, 273 (2) K; 14924 reflections, 5536
independent; (μ = 3.700 mm−1), refinement (on F2) with SHELXTL
(version 6.1), 334 parameters, 0 restraints, R1= 0.0427 (I > 2σ) and
wR2 (all data) = 0.1370, GOF = 1.140, max/min residual electron
density: 0.885/−1.516 e Å−3.
NMR spectra were recorded on Bruker 400 and 500 AMX
spectrometers as solutions in deuterated chloroform or dichloro-
methane at 298 K and low temperature. Chemical shifts are reported
in ppm, using TMS (1H, 13C) and 85% H3PO4 (31P) as references.
The coupling constants (J) are in hertz (Hz).
Elemental analysis was provided by SCSIE of University of Valencia.
X-ray structure determinations were carried out on a Bruker Nonius
Kappa CCD diffractometer (Mo Kα radiation, λ = 0.71073 Å). The
structures were solved by direct methods using SHELXTL software
package.24 The correct positions for the heavy atoms were deduced
from an E map. Subsequent least-squares refinement and difference
Fourier calculations revealed the positions of the remaining non-
hydrogen atoms. Hydrogen atoms were placed in geometrically
generated positions and refined riding on the atom to which they are
attached.
Characterization Data for p-3b. 1H NMR (CDCl3, 500 MHz, 298
K) δ 7.37 (m, 4H, ar), 7.30 (m, 2H, ar), 7.21 (m, 2H, ar), 7.09 (m,
12H, ar), 6.99 (m, 4H, ar), 6.89 (m, 4H, ar), 6.81 (m, 4H, ar), 6.69 (m,
4H, ar), 6.63 (m, 4H, ar), 6.46 (m, 4H, ar); 31P NMR (CDCl3, 202
MHz, 298 K) δ 3.8 (s); 13C NMR (CDCl3, 125 MHz, 298 K) δ 161.6
(m, C metalated), 150.1 (s, C−N, trans to C), 149.3 (t, C−N, trans to
3
P, JP−C = 2 Hz), 141.2−116.3 (ar). Anal. Calcd for C60H44
The electrochemical studies were carried out at room temperature
with a 273 A PAR potentiostat in a three-electrode cell. The working
Br4N6P2Pd2: C, 49.90; H, 3.05; N 5.82. Found: C, 49.50; H, 3.25;
N 5.42.
K
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X-ray Crystal Structure Data for p-3b (CCDC 996518). Empirical
formula: C60H44Br4N6P2Pd2, crystal system: monoclinic, space group:
P2(1)/c, a = 10.9730 (4) Å, b = 23.5120 (5) Å, c = 22.6210 (9) Å, β =
103.2150 (14)°, V = 5681.6 (3) Å3, Z = 4, crystal dimensions: 0.27 ×
0.25 × 0.21 mm3, MoKα radiation, 273 (2) K; 20424 reflections, 12699
independent, (μ = 1.687 mm−1), refinement (on F2) with SHELXTL
(version 6.1), 667 parameters, 0 restraints, R1= 0.0563 (I > 2σ) and
wR2 (all data) = 0.1767, GOF = 1.065, max/min residual electron
density: 1.054/−1.037 e Å−3.
X-ray Crystal Structure Data for p-3c (CCDC 996519). Empirical
formula: C64H56N6P2Pd2O6, crystal system: monoclinic, space group:
P2(1)/c, a = 14.027 (3) Å, b = 18.857 (4) Å, c = 24.459 (5) Å, β =
105.43 (3)°, V = 6236 (2) Å3, Z = 4, crystal dimensions: 0.24 × 0.20 ×
0.18 mm3, MoKα radiation, 273 (2) K; 17083 reflections, 10200
independent, (μ = 1.363 mm−1); refinement (on F2) with SHELXTL
(version 6.1), 725 parameters, 0 restraints, R1= 0.0649 (I > 2σ) and
wR2 (all data) = 0.1731, GOF = 1.125, max/min residual electron
density: 0.935/−0.610e Å−3.
Synthesis of Pd2[(C6H4)PPh2]2[R−N−N−N−R]2, R = o-
MeOC6H4 (o-3c). Sodium triazenide was prepared in dried THF by
reaction of o-MeOPh-NN-NH-o-MeOPh (0.223 mmol) and
sodium hydride (0.223 mmol) and added to [Pd2[(C6H4)-
PPh2]2(NCMe)4][BF4]2 obtained by reaction of a suspension of
[Pd((C6H4)PPh2)Br]4 (1) (100 mg, 0.056 mmol) in 20 mL of
CH2Cl2/NCMe (8/1) with AgBF4 (43 mg, 0.223 mmol). After 5 min
of stirring, the solution was evaporated to dryness. The orange crude
product obtained was extracted with dichloromethane, filtered over a
short plug of silica, and precipitated by addition of hexane to give an
orange microcrystalline powder, which was collected by filtration and
washed with hexane. Yield: 84.4 mg (62%).
Characterization Data for p-3d. 1H NMR (CDCl3, 500 MHz, 298
K) δ 7.43 (m, 4H, ar), 7.34 (m, 2H, ar), 7.16 (m, 2H, ar), 7.11 (m, 4H,
ar), 7.07 (m, 2H, ar), 6.95 (m, 8H, ar), 6.77 (m, 12H, ar), 6.65 (m, 8H,
ar), 6.44 (m, 2H, ar), 2.14 (s, 12H, CH3); 31P NMR (CDCl3, 202
MHz, 298 K) δ 3.4 (s); 13C NMR (CDCl3, 125 MHz, 298 K) δ 163.4
(m, C metalated), 149.2 (s, C−N, trans to C), 148.4 (t, C−N, trans to
3
P, JP−C = 2 Hz), 141.4−121.2 (ar), 20.8 (s, CH3). Anal. Calcd for
C64H56N6P2Pd2: C, 64.93; H, 4.73; N 7.10. Found: C, 64.65; H, 4.98;
N 6.60.
X-ray Crystal Structure Data for p-3d (CCDC 996520). Empirical
formula: C64H56N6P2Pd2, crystal system: monoclinic, space group:
P2(1)/c, a = 10.9050 (3) Å, b = 23.3620 (6) Å, c = 22.4590 (6) Å, β =
102.3250 (14)°, V = 5589.8 (3) Å3, Z = 4, crystal dimensions: 0.28 ×
0.26 × 0.22 mm3, MoKα radiation, 273 (2) K; 20464 reflections, 11201
independent, (μ = 1.407 mm−1), refinement (on F2) with SHELXTL
(version 6.1), 668 parameters, 0 restraints, R1= 0.0819 (I > 2σ) and
wR2 (all data) = 0.2517, GOF = 1.256, max/min residual electron
density: 1.195/−1.473 e Å−3.
Preparation of Pd2[(C6H4)PPh2]2[R−N−N−N−R]2, R = o-
MeC6H4 (o-3d); R = p-MeOC6H4 (p-3c). These compounds were
obtained by reaction of the potassium triazenide with the cationic
[Pd2[(C6H4)PPh2]2(NCMe)4][BF4]2. The potassium triazenide was
prepared by reaction of a CH2Cl2 solution (2 mL) of the triazene
(0.223 mmol) with potassium hydroxide (0.223 mmol) dissolved in
the minimum amount of methanol. Each solution was added to
[Pd2[(C6H4)PPh2]2(NCMe)4][BF4]2 that was prepared by reaction of
a suspension of [Pd((C6H4)PPh2)Br]4 (1) (100 mg, 0.056 mmol) in
20 mL of CH2Cl2/NCMe (8/1) with AgBF4 (43 mg, 0.223 mmol).
After 5 min of stirring, the solution was evaporated to dryness. The
yellow crude product obtained was extracted with dichloromethane,
filtered over a short plug of Celite, and precipitated by addition of
hexane to give an orange-yellow microcrystalline powder, which was
collected by filtration and washed with hexane. Yield: 44.0 mg (31.5%)
o-3d; 84.4 mg (62%) (p-3c).
Characterization Data for o-3c. 1H NMR (CDCl3, 500 MHz, 298
K) δ 7.42 (m, 4H, ar), 7.34 (m, 2H, ar), 7.13 (m, 2H, ar), 7.11 (m, 4H,
ar), 7.07 (m, 2H, ar), 6.95 (m, 8H, ar), 6.77 (m, 12H, ar), 6.63 (m, 8H,
ar), 6.44 (m, 2H, ar), 1.47 (s,12H, CH3); 31P NMR (CDCl3, 202
MHz, 298 K) δ 2.5 (s); 13C NMR (CDCl3, 125 MHz, 298 K) δ 165.9
(m, C metalated), 152.9 (s, C−N), 150.5 (s, C−N), 141.4−121.0 (ar),
20.8 (s, CH3). Anal. Calcd for C64H56N6 O4P2Pd2: C, 61.59; H, 4.81;
N 6.73. Found: C, 61.24; H, 4.28; N 6.42.
Study by 31P NMR Spectroscopy between 200 to 298 K for
the Oxidation Reactions of Compounds 3a−d with PhICl2. A
solution of the corresponding palladium(II) compound (3a, o-3b−d,
and p-3b-d) (0.004 mmol) in CD2Cl2 (0.5 mL) was cooled in a NMR
1
tube at 200 K, and the H and 31P NMR spectra were recorded. A
stoichiometric quantity of PhICl2 (0.004 mmol) was added, and the
color of the solution immediately changed from yellow to red. The 31P
NMR spectra were recorded at different temperatures from 200 to 298
K.
Synthesis of Pd2[(C6H4)PPh2]2[RNNNR]2Cl2 R = C6H5 (5a); R =
p-BrC6H4 (p-5b). To a solution at 223 K of 3a or p-3b (0.044 mmol)
in CH2Cl2 (5 mL) was added iodobenzene dichloride (18 mg, 0.066
mmol). The solution immediately changed from yellow to red. After 5
min of stirring, the solution was evaporated to dryness and hexane was
added. The red microcrystalline precipitate obtained was isolated by
filtration and washed with hexane. Yield: 40.2 mg (76%) 5a; 44.5 mg
(72%) p-5b.
Characterization Data for o-3d. 1H NMR (CDCl3, 500 MHz, 298
K) δ 8.0 (m, 2H, ar), 7.53 (m, 2H, ar), 7.19 (m, 8H, ar), 7.05 (m, 16H,
ar), 6.90 (m, 4H, ar), 6.82 (m, 4H, ar), 6.74 (m, 8H, ar), 1.54 (s, 6H,
CH3) 1.47 (s, 6H, CH3); 31P NMR (CDCl3, 202 MHz, 298 K) δ −0.3
(s); 13C NMR (CDCl3, 125 MHz, 223 K) δ 163.2 (m, C metalated),
149.2 (s, C−N), 148.4 (s, C−N), 141.4−121.0 (ar), 20.8 (s, CH3).
Anal. Calcd for C64H56N6P2Pd2: C, 64.93; H, 4.73; N 7.10. Found: C,
64.22; H, 4.48; N 6.91.
1
Characterization Data for 5a. H NMR (CD2Cl2, 400 MHz, 223
K) δ 7.9 (m, 2H, ar), 7.8 (m, 2H, ar), 7.7 (m, 4H, ar), 7.5 (m, 6H, ar),
7.3 (m, 2H, ar), 7.2 (m, 4H, ar), 7.0 (m, 24H, ar), 6.8 (m, 2H, ar), 6.5
(m, 2H, ar); 31P NMR (CD2Cl2, 162 MHz, 223 K) δ −35.6 (s); 13C
NMR (CD2Cl2, 100 MHz, 223 K) δ 154.6 (m, C metalated), 150.1 (s,
3
C−N, trans to C), 149.3 (d, C−N, trans to P, JP−C = 5 Hz), 148.9−
X-ray Crystal Structure Data for o-3d (CCDC 996516). Empirical
formula: C64H56N6P2Pd2, crystal system: monoclinic, space group:
P2(1), a = 11.358 (2) Å, b = 21.407(4) Å, c = 11.467 (2) Å, β = 105.09
(3)°, V = 2696.0 (9) Å3, Z = 4, crystal dimensions: 0.25 × 0.23 × 0.20
mm3, MoKα radiation, 273 (2) K; 10196 reflections, 10196
independent, (μ = 0.775 mm−1); refinement (on F2) with SHELXTL
(version 6.1), 667 parameters, 1 restraints, R1= 0.0505 (I > 2σ) and
wR2 (all data) = 0.1394, GOF = 1.072, max/min residual electron
density: 0.796/−0.841 e Å−3.
122−2 (ar).
1
Characterization Data for p-5b. H NMR (CD2Cl2, 400 MHz,
223 K) δ 7.9 (m, 4H, ar), 7.8 (m, 2H, ar), 7.7 (m, 4H, ar), 7.5 (m, 4H,
ar), 7.4 (m, 4H, ar), 7.1 (m, 12H, ar), 7.0 (m, 8H, ar), 6.9 (m, 4H, ar),
6.8 (m, 2H, ar); 31P NMR (CD2Cl2, 162 MHz, 223 K) δ −34.7 (s);
13C NMR (CD2Cl2, 100 MHz, 223 K) δ 154.4 (m, C metalated), 148.9
(s, C−N), 148.2 (s, C−N), 141.2−117.4 (ar).
Direct Catalytic Phenylation of Indole. The procedure reported
by Sandford and co-workers has been followed.13 Indole (58.6 mg, 0.5
mmol) and the precatalysts (0.0125 mmol, 2.5 mol % of Pd) were
dissolved in CH3CO2H (5 mL) (in the case of the precatalysts o-3b, p-
3b, p-3c, and p-3d a minimum volume of CH2Cl2 was added to
dissolve), and the solution was stirred at 298 K for 5 min. [Ph2I]PF6
(426.0 mg, 1 mmol) was added, and the resulting solution stirred at
298 K until no indole was observed by 1H NMR. The reaction mixture
was filtered through a plug of siliceous earth and evaporated to
dryness. The resulting oil was dissolved in CH2Cl2 (25 mL) and
extracted with aqueous NaHCO3 (2 × 40 mL). The organic phase was
Characterization Data for p-3c. 1H NMR (CD2Cl2, 400 MHz, 223
K) δ 7.6 (m, 4H, ar), 7.4 (m, 2H, ar), 7.3 (m, 2H, ar), 7.2 (m, 2H, ar),
7.1 (m, 8H, ar), 7.0 (m, 8H, ar), 6.8 (m, 4H, ar), 6.7 (m, 8H, ar), 6.6
(m, 4H, ar), 6.5 (m, 2H, ar), 3.7 (s, 12H, CH3); 31P NMR (CD2Cl2,
162 MHz, 223 K) δ 4.4 (s); 13C NMR (CD2Cl2, 100 MHz, 223 K) δ
163.3 (m, C metalated), 155.6 (s, C−N), 155.5 (s, C−N), 144.6−
112.4 (ar), 55.3 (s, CH3O), 55.3 (s, CH3O). Anal. Calcd for
C64H56N6O4P2Pd2: C, 61.59; H, 4.81; N 6.73. Found: C, 62.05; H,
4.32; N 6.98.
L
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dried with Na2SO4, concentrated, and purified by chromatography on
silica gel using as eluent hexanes/ethyl acetate (96:4).
Professor Pascual Lahuerta on the occasion of his 70th
birthday.
Characterization Data for 7. 1H NMR (CDCl3, 400 MHz) δ 8.31
(br s, 1H), 7.69−7.65 (m, 3H), 7.49−7.39 (m, 3H), 7.356 (t, 1H,
3JH−H = 7.4 Hz), 7.23 (ddd, 1H, 3JH−H = 7.8 Hz, 3JH−H = 7.0 Hz, 3JH−H
= 1.2 Hz), 7.16 (ddd, 1H, 3JH−H = 7.7 Hz, 3JH−H = 7.0 Hz, 3JH−H = 1.0
Hz), 6.86 (s, 1H), 13C NMR (CDCl3, 100 MHz) δ 137.8 (s, C
quaternary), 136.7 (s, C quaternary), 132.2 (s, C quaternary), 129.2 (s,
2 C−H), 129.0 (s, 2 C−H), 127.7 (s, C quaternary), 125.1 (s, C−H),
122.3 (s, C−H), 120.6 (s, C−H), 120.2 (s, C−H), 110.9 (s C−H),
99.9 (s, C−H).
REFERENCES
■
(1) (a) Albertin, G.; Antoniutti, S.; Bedin, M.; Castro, J.; García-
Fontan
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, S. Inorg. Chem. 2006, 45, 3816−3825. (b) Johnson, A. L.;
Willcocks, A. M.; Richards, S. P. Inorg. Chem. 2009, 48, 8613−8622.
(2) Kimbal, D. B.; Herges, R.; Haley, M. M. J. Am. Chem. Soc. 2002,
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(3) (a) Payehghadr, M.; Rofouei, M. K.; Morsali, A.; Shamsipur, M.
Inorg. Chim. Acta 2007, 360, 1792−1798. (b) GuhaRoy, C.; Butcher,
R. J.; Bhattacharya, S. J. Organomet. Chem. 2008, 693, 3923−3931.
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Computational Details. All models were fully optimized with the
Gaussian09 program package25 at the DFT level of theory. A hybrid
density functional B3PW9126,27 was utilized together with the basis set
consisting of the Stuttgart-Dresden effective core potential basis set
with an additional p-polarization function for Pd atoms (SDD(p)) and
the standard all-electron basis set 6-31G(d) for all other atoms.
Frequency calculations with no scaling were performed to ensure
optimization to true minima. None of the optimized structures gave
imaginary frequencies.
To further study the electronic properties of the complexes, we
performed topological charge density analysis with the QTAIM
(Quantum Theory of Atoms in Molecules)28 methods, which allowed
us to access the nature of the bonding via calculating different
properties of the electron density at the bond critical points (BCPs).
The analysis was done with the AIMALL program29 using the wave
functions obtained from the DFT calculations with the computation-
ally optimized structures.
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ASSOCIATED CONTENT
* Supporting Information
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Voltammograms for compounds 3a, o-3b, p-3c, voltammo-
grams for p-bromophenyltriazene, voltammogram for com-
pound p-5b, cyclic voltammogram for compound p-3c with
TEACl, cyclic voltammogram for compound 3a with TEACl,
cyclic voltammogram for TEACl in CH2Cl2, cyclic voltammo-
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ortho-phenyl substituted triazenide complexes o-3b and o-3c,
noncovalent interactions in the crystal structure of the o-3b
triazenide complex, comparison of the properties of the Pd−Pd
and the axial interactions in the monophenylated complex type
3a+Ph+Cl, selected properties of the electron density at the
critical points for selected weak intramolecular interactions for
Pd(II) compounds o-3b, o-3c, and o-3b_dim, selected
properties of the electron density at the bond critical point
for Pd(II) compounds 3a, o-3b−d, and o-3b_dim, selected
properties of the electron density at the Pd−Pd bond critical
point for Pd(III) compounds 5a, o-5b−d, and p-5b-d, the effect
axial interaction of phenyl groups on the Pd−Pd and Pd−
C(Ph) distances in the compounds 3a, o-3b−d, and p-3b−d,
and crystallographic data of compounds 3a, o-3b, o-3d, p-3b, p-
3c, p-3d, and o-4b in CIF format. This material is available free
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: angeles.ubeda@uv.es.
*E-mail: pipsa.hirva@uef.fi.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The authors thank the Universidad de Valencia for financial
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