Neutral Lipophilic Palladium(II) Complexes and their Applications in Electrocatalytic Hydrogen Production and C-C Coupling Reactions

Article abstract of DOI:10.1002/ejic.201901283

Three neutral palladium(II) complexes [Pd(L_n)₂] (n = 1,2,3) containing benzotriazole-phenolate ligands HL₁ = 2-(2H-benzotriazol-2-yl)-6-dodecyl-4-methylphenol; HL₂ = 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol; HL₃ = 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol were synthesized and characterized by several spectroscopic methods (¹H, ¹³C NMR, UV-Visible), ESI mass spectrometry and single-crystal X-ray diffraction. The geometry around the palladium(II) centre for each complex is described as distorted square planar. The complexes were tested as potential catalysts for electrochemical proton reduction in DMF using trifluoroacetic acid (TFA). Gas analysis under electrocatalytic conditions at a boron-doped diamond working electrode confirmed the hydrogen production. After performing similar gas analysis experiments using a working mercury pool electrode, no hydrogen was detected which supports the concept that the active species of the catalytic process are palladium particles formed on the surface of the working electrode. The palladium(II) benzotriazolyl phenolate complexes facilitate the formation of particles for electrocatalytic hydrogen production. Furthermore, the palladium(II) complexes PdL₂ and PdL₃ were able to catalyse microwave-assisted Heck and Sonogashira C–C coupling reactions.

Full text of DOI:10.1002/ejic.201901283

A Journal of
Accepted Article
Title: Neutral Lipophilic Palladium(II) Complexes and their Applications
in Electrocatalytic Hydrogen Production and C-C Coupling
Reactions
Authors: Olesea Cuzan-Munteanu, Dumitru Sirbu, Michel Giorgi,
Sergiu Shova, Elizabeth Gibson, Marius Réglier, Maylis Orio,
Luísa Martins, and Andrew Benniston
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201901283
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European Journal of Inorganic Chemistry
10.1002/ejic.201901283
FULL PAPER
Neutral Lipophilic Palladium(II) Complexes and their Applications
in Electrocatalytic Hydrogen Production and C-C Coupling
Reactions
Olesea Cuzan-Munteanu,[b,c] Dumitru Sirbu, Michel Giorgi, Sergiu Shova, Elizabeth A. Gibson,
[a]
[d]
[e]
[a]
Marius Réglier, Maylis Orio, Luísa M. D. R. S. Martins, and Andrew C. Benniston[a]
[
c]
[c]
[f]
This work is dedicated to the memory of Prof. Constantin Turta for his outstanding contribution to the field of inorganic chemistry.
Abstract: Three neutral palladium(II) complexes [Pd(L
containing benzotriazole-phenolate ligands HL = 2-(2H-benzotriazol-
-yl)-6-dodecyl-4-methylphenol; HL = 2-(2H-benzotriazol-2-yl)-4,6-
di-tert-pentylphenol; HL = 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-
-phenylethyl)phenol were synthesized and characterized by several
n
)
2
] (n = 1,2,3)
complexes facilitate the formation of particles for electrocatalytic
hydrogen production. Furthermore, the palladium(II) complexes PdL
and PdL were able to catalyze microwave-assisted Heck and
1
2
2
2
3
3
Sonogashira C-C coupling reactions.
1
1
13
spectroscopic methods ( H,
C NMR, UV˗Visible), ESI mass
Introduction
spectrometry and single-crystal X-ray diffraction. The geometry
around the palladium(II) center for each complex is described as
distorted square planar. The complexes were tested as potential
catalysts for electrochemical proton reduction in DMF using
trifluoracetic acid (TFA). Gas analysis under electrocatalytic
conditions at a boron-doped diamond working electrode confirmed the
hydrogen production. After performing similar gas analysis
experiments using a working mercury pool electrode, no hydrogen
was detected which supports the concept that the active species of
the catalytic process are palladium particles formed on the surface of
the working electrode. The palladium(II) benzotriazolyl phenolate
Palladium chemistry is a highly investigated field of study and is
the basis for many industrially important processes.[1] The
oxidative activity of palladium-based catalysts is one main
dynamic area of investigation,[2] and several groups are interested
not only in the catalytic activity of palladium complexes, but also
in the reaction mechanism, isolation of intermediates and
characterization of different redox-active species.[ Another
pertinent feature of palladium is its ability when complexed to
perform reductive-based reactions thus making it a truly versatile
metal. Focusing in the first instance on reductive chemistry,
palladium and its complexes are considered to be good catalysts
for hydrogen evolution reaction,[4] though there is still much scope
for improvement especially in the area of electrocatalytic
3]
hydrogen
generation
from
protons.
The
dinuclear
[
5]
palladium(I)1,3-bis[(2-chloro)benzene]triazenide complex, for
instance, was shown to produce hydrogen from acetic acid in
dichloromethane (DCM), based on cyclic voltammetry
experiments, but no detection of the electrolysis product was
[
a]
Prof A C Benniston, Dr E Gibson, Dr D Sirbu,
Chemistry-School of Natural & Environmental Sciences
Newcastle University
Newcastle upon Tyne, NE1 7RU, UK
E-mail: andrew.benniston@ncl.ac.uk
Dr O Cuzan-Munteanu
[6]
performed. The palladium(II) meso-tetraferrocenylporphyrine
[
[
b]
c]
also showed some promising results for electrocatalytic H
2
Institute of Chemistry
3
Academiei Street
production and in this case the product could be identified. A
faradaic efficiency of 70% was obtained for proton reduction in
dimethylformamide (DMF) at -1.5 V vs. saturated calomel
electrode (SCE) at a glassy carbon working electrode. A
mononuclear palladium complex based on a thiosemicarbazone
ligand[7] has shown electrocatalytic behaviour for proton
reduction in DMF in the presence of TFA, however the faradaic
efficiency of this compound was only 34% with a turnover number
of 2 after 4h.
Chisinau MD-2028, Moldova
Prof M Réglier, Dr M Orio, Dr O Cuzan-Munteanu
Aix Marseille Universitꢀ
Centrale Marseille, CNRS, ISM2 UMR 7313,
Marseille 13397, France
Dr M Giorgi
Aix Marseille Universitꢀ
CNRS, Centrale Marseille, FSCM, Spectropole, Marseille, France
Dr S Shova
Institute of Macromolecular Chemistry "Petru Poni", Department of
Inorganic Polymers,
[
[
d]
e]
4
1A Grigore Ghica Voda Alley,
The reductive carbon-carbon bond forming capabilities of in-situ
palladium species is dominated by coupling reactions such as
Suzuki,[8] Sonagashira,[9] Heck[10] and Negishi,[11] relying in many
cases on the production of zero-valent palladium in the first
instance. The coordination sphere of the palladium is provided by
Iasi-700487, Romania
Prof L M D R S Martins
Centro de Química Estrutural,
Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco
Pais, 1049-001
[f]
E-mail: luisammartins@tecnico.ulisboa.pt
ligands generally comprising soft donor atoms such
a
phosphorus (e.g., PPh ), but other donor atoms have found
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201901283
FULL PAPER
application. For example, the 1,2,4-triazole-based palladium
chromatographic purification is one reason for non-quantitative
3
)-N,C,N][10] was successfully
complex PdI[η -(C
6
H
3
2
-2,6-(CH Tz)
2
yields.
tested as catalyst for the Heck coupling reaction. Another
palladium complex derived from 1,2,4-triazolin-5-ylidene
The 1H NMR spectra of the free ligands (see Supporting
a
Information) show the presence of a signal in the region 11.78 –
[
9]
containing ligand which coordinates through the CNC atoms has
shown good catalytic performance in Sonogashira coupling with
a turnover number of 8000. Also noted is the recently reported
series of palladium(II) complexes with 3-(2-pyridyl)-5R-1,2,4-
triazoles, where R = ethyl, n-propyl, i-propyl and t-butyl,[8] which
are capable to catalyse the microwave-assisted Suzuki–Miyaura
cross-coupling reaction in the presence of base.
1
1.07 ppm which is attributed to the OH proton. The OH signal is
not present in the spectra for the complexes supporting that the
metal coordinates to the deprotonated oxygen-atom.
A
1
representative H NMR spectrum for PdL
3
is shown in Figure 2,
highlighting the four signals of the inequivalent methyl groups. It
1
is interesting to note that the H NMR spectrum for HL
3
displays
two signals for the methyl groups. The aromatic region between 6
and 8 ppm is dominated by clear signals attributed to the
benzotriazole: 11 signals are attributed to 32 protons, while in the
aliphatic region 1–3 ppm are observed 5 different proton species:
4 signals are attributed to 24 protons from the methyl groups and
the co-crystallized acetone peak.
As a follow up to our own work,[6,12] the focus was to develop
palladium(II) coordinately unsaturated complexes based on
robust and readily accessible ligands. A compound based upon
the triazole-phenol moiety (Figure 1) offers a bidentate binding
motif, and when deprotonated and complexed to palladium(II) in
a 2:1 ratio forms a neutral complex. To increase solubility in
common organic solvents the phenol group is a suitable unit to
incorporate lipophilic alkyl groups. The three complexes (PdL
1
-
PdL ) which were developed are promising for the electrocatalytic
3
proton reduction into hydrogen and for Sonogashira and Heck C-
C bond formation.
1
Figure 2. A 400 MHz H NMR spectrum for PdL
3
in CDCl
3
showing the four
inequivalent methyl proton signals and the aromatic region. * = acetone.
Single crystal X-ray diffraction analysis
By using careful crystallization methods, a number of the
products afforded crystals suitable for single-crystal X-ray
diffraction analysis. Collected in Table S1 of Supporting
Figure 1. Representation of the palladium(II) complexes discussed in the text
and their nomenclature.
Information are the relevant data for the structures HL
3
,
PdL .H O, PdL and PdL .(CH CO. The ligand HL
2
2
3
,
3
3
)
2
3
crystallized in the monoclinic centrosymmetric space group
P2/c with two independent molecules in the asymmetric unit,
one of them showing some disorder at one phenyl site (two
Results and Discussion
Synthesis and characterization
3
positions). The molecular structure of one molecule of HL is
shown in Figure 3. All bond lengths and angles are within
expected ranges. The hydroxyl group is intramolecular
hydrogen bonded to one of the nitrogen atoms within the
triazole unit to generate a six membered ring. The phenyl
groups are positioned anti with respect to each other, although
this is undoubtedly not the lowest energy arrangement; a basic
The commercially available compounds 2-(2H-benzotriazol-2-yl)-
4
,6-di-tert-pentylphenol (HL
bis(1-methyl-1-phenylethyl)phenol (HL
while 2-(2H-benzotriazol-2-yl)-6-dodecyl-4-methylphenol (HL
2
) and 2-(2H-benzotriazol-2-yl)-4,6-
3
) were used as received,
1
)
was purified by column chromatographically before use. The
three new palladium(II) coordination compounds (Figure 1) were
obtained by the complexation of the ligands with palladium(II)
acetate in very good yields (>80%). The requirement of
+
MM molecular mechanics calculation identified several close
in energy conformations (see Supporting Information). The
3
structure of HL also reveals the presence of intermolecular
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European Journal of Inorganic Chemistry
10.1002/ejic.201901283
FULL PAPER
interactions within the crystal as at least four relevant -
stacking and five CH/ interactions can be reported (see Table
S2 of Supporting Information).
Figure 3. Single crystal X-ray diffraction determined molecular structure for
3
HL drawn at 40% probability level showing the intramolecular hydrogen
bond.
Figure 4. Single crystal X-ray diffraction determined molecular structure for
PdL
of atoms and distances in Å (top) and a view down the N4-Pd1-N1 axis
bottom).
3
drawn at as 40% probability level ellipsoids showing selected labelling
The molecular structure for the palladium complex with two L3
3
ligands is illustrated in Figure 4. Complex PdL crystallized in
(
the P21/c space group. The coordination mode at the
palladium ion is a slightly distorted square planar arrangement,
so that oxygen atoms of the hydroxyl are positioned trans, with
the same arrangement observed for the nitrogen atoms of the
triazole moieties. The N4-Pd1-N1 and O1-Pd1-O2 bond angles
are 170.91(9) and 176.14(8), respectively. The Pd1-N1 and
Pd1-N4 bond lengths of 2.001(2) Å and 2.013(2) Å are as
expected. The Pd1-O1 and Pd1-O2 bond lengths of 1.995(2)
Å and 1.999(2) Å are not within the standard deviation error
different from the Pd-N bond lengths. Unlike the ligand alone,
the complex is stabilized by two pairs of intramolecular -
stacking interactions between the benzotriazoles and two
phenyl groups: the distances between centroids of rings
N1/N2/N3/C60/C55 and C8/C9/C10/C11/C12/C13 and
between centroids of rings N4/N5/N6/C30/C25 and
C38/C39/C40/C41/C42/C43 are equal to 3.5566(18) Å and
Compound PdL
group with two independent molecules in the asymmetric unit
see Supporting Information). The Pd atoms reside on the 2-
fold axis and their coordination sphere is identical to that of
complex PdL . Indeed the geometrical features observed in
PdL .CH CO are comparable to those observed in PdL , with
3 3 2
.(CH ) CO crystallized in the C2/c space
(
3
3
3
3
two pairs of intramolecular -stacking interactions between the
benzotriazoles and two phenyl groups, but the accessible sides
of the Pd planes are now facing each other thus forming a
pseudo dimer with the Pd1-Pd2 distance close to 4.2 Å.
2 2
Compound PdL .H O crystallized in the P21/n space group
with two independent molecules in the asymmetric unit. The
asymmetric unit (see Supporting Information) comprises one
palladium(II) atom surrounded by two molecules of
2
deprotonated L ligand, coordinated to the metal through two
3.572(2) Å, respectively, while the dihedral angles between
nitrogen and two oxygen atoms. The carbon atoms of the
methyl and ethyl groups are partially delocalized and the
structure contains a delocalized co-crystallised half water
molecule.
these pairs of rings are equal to 1.28(16)° and 6.01(18)°,
respectively. Thus, the complex adopts a rooftop conformation
with one side of the Pd plane fully accessible, while the other
side is partially protected by the two phenyl cycles involved in
the -stacking interactions (Figure 4).
Theoretical calculations
To elucidate both their structural and electronic features, the
neutral, monoreduced and monooxidized complexes were
subjected to DFT geometry optimizations (Figure 5A-C).
Comparison of the DFT-optimized structure of PdL
crystallographic one showed a pretty good agreement between
the two sets of data, which confirmed that PdL can be best
described as a diamagnetic square planar Pd(II) complex
PdL , S = 0). Whereas the Highest Occupied Molecular
3
with the
3
(
3
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European Journal of Inorganic Chemistry
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FULL PAPER
Orbital (HOMO) for PdL
3
resides on the ligand, the Lowest
the ligands (Figure 7) is relatively straightforward and similar.
Unoccupied Molecular Orbital (LUMO) is very much centred on
the palladium metal (Figure 6A-B), and is identified as the 4dx2-
y2 orbital which is classically the unoccupied frontier orbital for
The cathodic part of the CV is dominated by a quasi-reversible
0
redox signal with E1/2 = -2.20 V, -2.22 V, -2.15 V for HL
1
, HL
2
,
3
HL , respectively. In the anodic region an irreversible signal is
a,1
a square planar complex. The mono-reduced PdL
3
complex is
observed at E
respectively.
p
1 2 3
= 0.80 V, 0.90 V, 0.95 V for HL , HL , HL ,
best described as a Pd(I) paramagnetic species, S = ½,
because of the elongation of the Pd-N and Pd-O bond lengths
which supports that reduction occurs at the metal centre
consistent with a Single Occupied Molecular Orbital (SOMO)
being mainly distributed over the Pd centre and its coordinating
atoms (Figure 6C). In the case of the mono-oxidized PdL
3
complex it would appear that removal of an electron occurs at
the ligand since the computed C-O and C-C bond lengths of
the phenolate groups are affected. The species produced is
therefore best described as paramagnetic Pd(II)-ligand radical
species (S = ½) which is attested by the composition of the
SOMO of the system (Figure 6D).
1 2 3 2
Figure 7. Cyclic voltammogram recorded for HL , HL and HL in N -purged
+
DMF containing 0.1M TBAPF
electrode.
6
vs Fc /Fc at a glassy carbon working
The redox behaviour of the complexes PdL
Figure 8, Table 1, Table S6) were studied by cyclic
voltammetry in dried DMF at a glassy-carbon electrode using
1 2 3
, PdL and PdL
(
0
.1 M TBAPF
6
as the supporting electrolyte. All potentials are
+
referenced to the Fc /Fe redox couple. There are clear
similarities in the cyclic voltammograms for all three
complexes. Upon oxidative scanning two clear irreversible
waves are observed, which by comparison to the CVs of Figure
3
Figure 5. DFT-optimized structures of the neutral PdL complex (A), the
mono-reduced PdL complex (B) and the one-electron oxidized PdL
3
3
complex (C). Hydrogen atoms were omitted for clarity.
7, are ligand-based oxidations. The presence of two clear
peaks is interpreted as oxidation at one ligand site affects the
oxidation of the second ligand presumably by electronic
communication via the Pd centre. The reduction side of the
voltammograms are more interesting since they display a new
wave at ca. – 1.5 V and are associated with the redox process
2
+
+
at the metal site (Pd /Pd ). By increasing the potential window
an additional irreversible wave is observed at a potential that
is very close to the ligand-based redox process (cf. Figure 7).
There is presumably some degradation process since upon
reversing the scan an additional wave is observed at around -
0.3 V. More breakdown products are evident when the
potential scanning window is increased further, arising from the
additional irreversible wave at ca. -2.9 V.
Figure 6. DFT-computed HOMO (A) and LUMO (B) for the neutral PdL
3
Table 1. CV data containing the half-wave potential of the first reduction
complex, SOMO for the mono-reduced PdL
3
(C) and SOMO for the one-
process for complexes.[a]
electron oxidized PdL (D). Hydrogen atoms were omitted for clarity.
3
2
E
1/2
Cyclic voltammetry
The redox behaviour of the ligands HL
Table 1) were studied by cyclic voltammetry in dried DMF at a
glassy-carbon electrode using 0.1 M tetra-Butylammonium PF
TBAPF ) as the supporting electrolyte. All potentials are
referenced to the Fc /Fc redox couple. The redox behaviour for
PdL
PdL
PdL
1
2
3
-1.57
-1.51
-1.59
1
2 3
, HL , HL (Figure 7,
[
a]
+
6
Data collected in dry DMF (0.1 M TBAPF ) and referenced to Fc /Fc.
6
(
6
+
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European Journal of Inorganic Chemistry
10.1002/ejic.201901283
FULL PAPER
appears at around 390 nm is related to the coordination of the
ligand to the palladium and is of ligand-to metal charge transfer
(
LMCT) character.
The addition of up to 100 equivalents of TFA or acetic acid to
a solution of PdL in DMF did not result in any significant
3
changes to the absorption profile. There is a decrease in
intensity and change in shape of the absorption band located
at 390 nm upon formation of the mono-reduced species as
expected since the LMCT transition is altered slightly. This new
band does not change significantly in the presence of excess
TFA. It would appear that the complex and its mono-reduced
form are stable in the presence of excess acid (see Supporting
Information).
1 2 3 2
Figure 8. Cyclic voltammograms recorded for PdL , PdL and PdL in N -
Electrocatalysis
+
purged DMF containing 0.1M TBAPF
6
vs Fc /Fc at a glassy carbon working
electrode over different potential ranges both in the reduction mode (red,
blue and green lines) and in oxidation mode (purple lines).
Given the behaviour of the complexes upon reduction we were
interested to see if they also presented an electrocatalytic
activity and were able to drive hydrogen production. The
addition of either trifluoroacetic acid or acetic acid as a proton
source to complexes PdL
formation of catalytic waves in the cathodic region of the cyclic
voltammograms (see Supporting Information). typical
experiment is shown in Figure 10 for PdL . By using the “first
derivative technique” the number and potential of the formed
waves was estimated. For complex PdL three distinct waves
1 2 3
, PdL and PdL resulted in the
A
3
3
were observed at -1.1 V, -1.6 V and -1.8 V, respectively. The
current response increased with the addition of acid and the
+
maximum for the peak shifted from -1.5 V to -1.9 V vs. Fc /Fc.
1 2 3
Figure 9. Electronic absorption spectra for ligands HL , HL and HL and for
complexes PdL , PdL and PdL in DMF.
1
2
3
UV-Visible absorption spectroscopy
The room temperature electronic absorption spectra recorded
for HL , HL and HL in DMF show two broad bands with
absorption maxima λmax at 305 and 340 nm for HL and HL
and with λmax at 305 and 345 nm for HL (Figure 9), and the
1
2
3
1
2
3
3
Figure 10. Selected cyclic voltammograms for PdL (1 mM) at a glassy
relevant parameters are summarized in Table S6 of Supporting
Information. In addition to ligand-based electronic transitions
carbon electrode in dry DMF in the presence of increasing quantities of TFA.
Conditions: T = 298 K, scan rate 100 mV/s, supporting electrolyte: 0.1M
the absorption spectra recorded for PdL
DMF also show two broad bands with absorption maxima λmax
at 390 nm for PdL and PdL and with λmax at 395 nm for PdL
Figure 9). For the complexes we also observe a minor shift for
1 2 3
, PdL and PdL in
6
TBAPF .
Gas analysis
1
2
3
A continuous flow-rig with in-line GC analysis was used to
(
detect
the
molecular
hydrogen
produced
during
the first transition from 305 to 310 nm. The second electronic
transition present for the ligands at 340-350 nm is absent for
the case of the palladium complexes. This observation would
suggest that the transition is associated with the hydroxyl
group and likely of n- character. The new band which
electrocatalytic proton reduction using PdL
presence of TFA.[13] Applying a potential of -0.36 V vs NHE to
3
in DMF in the
a
boron-doped diamond working electrode resulted in
detection of molecular hydrogen (Figure 11). The rate of H
produced was determined by calibration with a known flow rate
2
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European Journal of Inorganic Chemistry
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FULL PAPER
Sonogashira and Heck C-C cross couplings
2
of a H /Ar mixture, by checking the proportionality between the
peak area and the volume of injected hydrogen per minute.
When no complex was employed no hydrogen was detected
The industrially important Sonogashira Pd-catalysed C-C
coupling of a sp-hybridized carbon from a terminal alkyne with
during five hours of electrolysis. The presence of 1 mM of PdL
3
2
a sp -hybridized carbon from an aryl halide to afford a
resulted in three orders of magnitude increase of current at the
same applied potential and immediate constant production of
hydrogen as indicated by the GC analysis. Almost seven
turnovers were reached after five hours of electrolysis and no
sign of hydrogen production decrease was observed at the end
of this time. At the same time a low constant faradaic yield of
about 50 % was noticed. The UV-Vis spectrum and the cyclic
voltammogram of the electrolysis solution did not change
significantly during electrolysis. This would suggest that the low
faradaic yield of the detected hydrogen is more likely to be a
result of substrate reduction. Palladium complexes are well
known catalysts for chemical and electrochemical
hydrogenation of organic compounds.[14] As the produced
hydrogen cannot be removed immediately from the solution
hydrogenation of the electrolysis mixture components is quite
likely.
substituted acetylene traditionally requires the presence of a
copper co-catalyst which brings some problems (e.g.,
competitive homocoupling to the diyne) to the catalytic
system.[15,16] To address such issues, copper-free synthetic
strategies are in high demand. Herein, the Cu-free
Sonogashira C-C cross coupling of phenylacetylene and
iodobenzene (as a model reaction) was attempted in the
presence of PdL
Figure 12a). At room temperature no C-C coupling products
were detected, therefore MW irradiation was the chosen
green) heating mode. In order to probe the synthetic scope of
2 3
or PdL , under microwave (MW) irradiation
(
(
this Cu-free Pd-catalysed MW-assisted Sonogashira-type
reaction, experimental conditions were optimized by using
different bases, solvents, as well as MW power and reaction
temperatures, and the results for the best combinations are
presented in Table 2.
Interestingly, after performing similar gas analysis experiments
using a working mercury pool electrode no gaseous hydrogen
formation was observed. This finding does suggest that the
molecular complex is not the active catalyst, and that it is more
likely a precursor for generation of the catalytic species. The
simplest answer is metallic palladium formation upon reduction
of Pd(II) to Pd(0) in the complex accompanied by loss of the
ligands. As most of the Pd complex seems to be still present in
solution after electrolysis, most likely only a small percentage
Figure 12. Microwave (MW) assisted a) Sonogashira C-C cross-coupling
reaction of phenylacetylene and iodobenzene or b) Heck C-C cross-coupling
3
of PdL is initially converted to metallic Pd and is responsible
2 3
between styrene and iodobenzene, catalysed by PdL or PdL .
for the electrocatalysis. The reductive hydrogenation of the free
ligand seems to be the most likely candidate to compete with
hydrogen production.
Table 2. Selected results from the MW-assisted Cu-free cross-coupling of
iodobenzene with phenylacetylene catalysed by PdL
2
or PdL
Time
3
.[a]
Yield
[
Entry
Catalyst
T
Base
Solvent
TON
b]
3 [c
ºC
50
90
70
mins
15
%
10
1
2
3
Et
3
N
DCM
75
79
87
7.5
7.9
8.7
PdL
PdL
2
K
2
CO
3
DMF
30
Et
3
N
MeCN/
15
2
H O
4
5
6
50
90
70
Et
3
N
DCM
15
30
15
73
76
84
7.3
7.6
8.4
3
K
2
CO
3
DMF
Et
3
N
MeCN/
2
H O
7
Pd(OAc)
2
70
3
Et N
MeCN/
15
40
4.0
2
H O
[
a]
Reaction conditions: Pd-catalyst (0.01 mmol), iodobenzene (1 mmol),
phenylacetylene (1.1 mmol), base (1.2 mmol), solvent (3 mL), MW (50 W).
Isolated yield. TON (Turnover number) = moles of product per mol of
Figure 11. Electrocatalytic hydrogen production vs. time and charge vs. time
[
b]
[c]
by applying -0.36 V vs. NHE to a glassy carbon electrode in 0.1 M TBAPF
solution of DMF containing 90 mM TFA and 1 mM PdL complex.
6
catalyst.
3
As seen from the data in Table 2 very similar results for both
compounds were obtained, leading to maximum yields of 87%
2
and 84% of diphenylacetylene for the Pd-complexes PdL and
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European Journal of Inorganic Chemistry
10.1002/ejic.201901283
FULL PAPER
PdL
an acetonitrile/water medium. The catalytic activity of
complexes PdL and PdL was also tested for the versatile and
industrially important[17–19] Csp -Csp coupling of organic
halides with olefins (Heck reaction). The selected halide
substrates were styrene and iodobenzene. The reaction was
performed according to Figure 12b using triphenylphosphine
as the reducing agent, in the presence of triethylamine, in a
sealed reactor and under microwave irradiation, typically at
3
, respectively, after 15 mins of MW irradiation at 70 °C in
All chemicals were purchased from commercial sources and
used as received unless otherwise stated. Basic solvents for
synthesis were dried using literature methods. Solvents for
spectroscopic investigations were of the highest purity available.
2
3
2
2
Electrochemical investigations were carried out on Pd
4 6
incorporated complexes in DMF/n-NBu PF (0.1 M) solution at
ambient temperature using a three-electrode configuration
(
glassy carbon working electrode, Pt counter electrode,
+
Ag/AgNO
3
reference). All potentials are presented vs Fc /Fc
80 °C. In all Heck experiments, the selectivity for trans-stilbene
redox couple. Gaseous H
2
detection was performed using the
was 100% (the only product detected). After 1 h of MW
irradiation (90 W power) at 80 °C under optimized conditions,
similar yields, 91% and 88%, respectively, of trans-stilbene
method described by Summers et al.[ 1 mM solutions of
13]
6
catalyst in DMF containing TFA (90 mM) and TBAPF (0.1 M)
were prepared from thoroughly degassed stock solutions and
stored under argon. Argon gas was continually flowed through
the solution and into a 6-port 2-position switch (VICI) at a
2 3
were obtained in the presence of PdL and PdL .
−
1
It is worth noting that for both Sonogashira and Heck C-C cross
couplings, the catalytic performance of PdL and PdL was
clearly superior to that exhibited by their Pd precursor,
Pd(OAc) As presented in Table (entry 7) for the
Sonogashira reaction, the coupling product maximum yield of
0% was obtained when Pd(OAc) was used as catalyst. In
addition, the Heck reaction carried out in the presence of
Pd(OAc) led only to 54% of trans-stilbene. The coordination
sphere at the palladium metal for PdL and PdL during the
constant flow (10 mL min ), which was maintained using a
mass flow controller (Bronkhorst, E-Flow series). A 200 µL
sample was analysed automatically every 3.2 mins using a gas
chromatograph (Shimadzu GC2014) with a thermal conductivity
detector operating at 50 °C. The sample was initially passed
through a dry ice trap to remove any condensable solvents.
Argon was used as the carrier gas and H was detected on an
2
activated molecular sieve column (Shincarbon ST, Restek).
Integration of a plot of the production rate versus time yielded
2
3
2
.
2
4
2
2
2
3
the total amount of H
an IVIUMSTAT potentiostat, a boron doped diamond working
electrode, platinum mesh counter electrode, Ag/AgNO (0.01 M)
2
produced. A voltage was applied using
catalytic cycle is evidently disposed to facilitate the
stereoselective reaction. Noting that the molecular structure of
3
3
PdL does show that one face of the complex is hindered by
reference electrode were employed and the potentials were
converted to the NHE scale by adding 0.54 V.[21]
the phenyl groups, this feature may play a significant role in
controlling the reductive elimination step.
Elemental analyses were performed using a Thermo Finnigan
EA 1112 instrument. The results were validated by at least two
measurements. UV-Vis spectra were recorded on a Varian Cary
Conclusions
50 spectrophotometer from 200-800 nm with samples in a 1.0
Neutral lipophilic palladium complexes are readily prepared
through the employment of cheap and readily available organic
benzotriazolyl phenolate precursors. The complexes show
limited capabilities as electrocatalytic hydrogen producers,
since it appears that they decompose during repeating cycling,
likely forming metallic palladium particles. Nevertheless, they
are more productive in C-C coupling reactions outperforming
cm pathlength quartz cuvette. FT-IR spectra were recorded in
attenuated total reflection (ATR) mode on a Bruker TENSOR 27
spectrometer equipped with a single-reflection DuraSamplIR
diamond ATR accessory. Intensities are mentioned in the
parenthesis as vs = very strong, s = strong, m = medium, w =
weak, vw = very weak and sh = shoulder. NMR spectra were
recorded on a BruckerAvance III nanobay spectrometer (300 or
4
00 MHz) using TMS as internal reference. ESI-MS analyses
were performed using SYNAPT G2 HDMS (Waters)
spectrometer equipped with pneumatically assisted
2
the simple neutral complex Pd(OAc) in Sonogashira and Heck
a
reactions. The lipophilic nature of the complexes does open up
the opportunity to possibly perform cross-coupling reactions in
more controlled environments such as oil droplets or emulsions
that have application in flow chemistry.[20] In addition, the basic
ligand structure is also set up for the addition of other
a
Atmospheric Pressure Ionization (API) source. The ion-spray
voltage was 2.8 kV, the orifice lens was 20 V, and the nitrogen
−1
flux (nebulization) was 100 L h . The HR mass spectra were
obtained with a time-of-flight (TOF) analyzer. The sample was
placed in a methanol/3mM ammonium acetate.
1 2
functionality. For example, the R and R groups (Figure 1)
could readily contain sites for attachment to surfaces, polymers
and beads to support heterogeneous coupling reactions.
Future work is intended to explore the development of the
ligands along the outlined lines.
Synthesis
methylphenolato]palladium(II) (PdL
To solution of 2-(2H-benzotriazol-2-yl)-6-dodecyl-4-
methylphenol (HL ) (0.25 g, 0.6 mmol) in THF (3 mL) at ambient
temperature was added dropwise a solution of Pd(OAc) (0.07
g, 0.3 mmol) in THF (4 mL). The obtained orange mixture with
of
bis[2-(2H-benzotriazol-2-yl)-6-dodecyl-4-
1
)
a
1
2
Experimental Section
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European Journal of Inorganic Chemistry
10.1002/ejic.201901283
FULL PAPER
the total volume of 7 mL was stirred at 20 °C. The reaction was
monitored by TLC. After 48 h volatile materials were removed
under vacuum. The oily product obtained was purified by flash
chromatography on silica gel using the eluent petroleum
ambient temperature was added dropwise a solution of
Pd(OAc) (0.22 g, 1.0 mmol) in THF (15 mL). The obtained deep
2
red mixture with the total volume of 25 mL was stirred at 20 °C.
The reaction was monitored by TLC. After 48 h volatile materials
were removed under vacuum. The obtained product was
purified by flash chromatography on silica gel using the eluent
ether/methylene chloride (99:1). Compound PdL
as an orange oily product. Yield: 0.2325 g, 83.7%. IR,  (cm )
2954.9 (m), 2922.1 (s), 2852.7 (m), 1608.6 (w), 1572.0 (w),
1
was isolated
-1
=
petroleum ether/methylene chloride (99:1). Compound PdL
3
1
1
1
1
556.5 (w), 1490.9 (m), 1465.9 (s), 1411.9 (w), 1377.1 (w),
352.1 (m), 1342.4 (m), 1307.7 (m), 1288.4 (m), 1244.1 (s),
203.6 (m), 1149.6 (m), 1134.1 (w), 1114.8 (w), 1078.2 (w),
037.7 (w), 987.5 (w), 976.0 (w), 947.0 (w), 906.5 (m), 856.4
was isolated as an orange solid. Yield: 0.9165 g (93.95 %.
Suitable crystals for single crystal X-ray diffraction study of
3
(PdL ) were grown by recrystallization from DMF (orange
hexagons). Elemental Anal.: Calcd.: C, 72.10; H, 5.65; N, 8.41;
(
m), 819.7 (m), 763.8 (m), 734.9 (vs), 680.9 (w), 648.1 (w),
(C60
1
H
56
N
6
O
2
Pd); Found: C, 72.17; H, 5.57; N, 8.28 %. IR,  (cm-
1
636.5 (w), 557.4 (m), 511.1 (w), 443.6 (w). H-NMR (400 MHz,
) = 2964.5 (w), 2933.7 (w), 2868.1 (w), 1597.0 (w), 1492.9 (w),
1462.0 (m), 1440.8 (m), 1408.0 (m), 1384.9 (w), 1354.0 (w),
1311.6 (m), 1288.4 (m), 1251.8 (m), 1230.6 (m), 1220.9 (m),
1188.1 (w), 1145.7 (w), 1128.3 (w), 1107.1 (w), 1072.4 (w),
1030.0 (w), 987.5 (w), 933.5 (w), 896.9 (w), 873.7 (w), 856.4 (w),
815.9 (m), 761.9 (s), 748.4 (s), 715.6 (w), 696.3 (vs), 667.4 (w),
650.0 (m), 636.5 (w), 615.3 (w), 605.6 (m), 578.6 (w), 561.3 (m),
CDCl
.0 Hz, CH), 7.59 (s, 2H, CH), 7.55 (t, 4H, J = 6.5Hz CH,), 6.81
s, 2H, CH), 2.26 (s, 6H, CH, CH -methyl), 1.22 (t, 6H, J = 7.0
3
, ppm): δ 8.48 (broad singlet, 2H, CH), 8.00 (d, 2H, J =
9
(
3
Hz, CH, CH
3
-dodecyl), 0.86 (m, 44H, CH, CH
2
-dodecyl). ESI-
MS (DCM): m/z 891.45 [M+H] . UV-Vis (DMF): λmax, nm (ε, 103
+
−
1
−1
L mol cm ): 310 (22.0), 390 (8.6).
551.6 (m), 528.48 (w), 509.2 (w), 497.6 (m), 472.6 (w), 455.2
Synthesis
pentylphenolato]palladium(II) (PdL
To solution of 2-(2H-benzotriazol-2-yl)-4,6-di-tert-
pentylphenol (HL ) (0.70 g, 2.0 mmol) in THF (10 mL) at
ambient temperature was added dropwise a solution of
Pd(OAc) (0.22 g, 1.0 mmol) in THF (15 mL). The obtained dark
of
bis[2-(2H-Benzotriazol-2-yl)-4,6-di-tert-
(m), 443.6 (m), 410.8 (m), 403.1 (m), 383.8 (w), 366.5 (w), 326.0
1
2
)
(m). H-NMR (400 MHz, CDCl
3
, ppm): δ 7.82 (d, 2H, J = 8.7 Hz
a
CH), 7.67 (d, 2H, J = 8.7 Hz), 7.64 (s, 2H, CH), 7.39 (d, 4H, J =
7.4 Hz CH,), 7.36 (t, 2H, J = 7.6 Hz CH,) 7.29 (s, 2H, CH), 7.27
(d, 6H, J = 8.2 Hz, CH), 7.12 (t, 2H, J = 7.6 Hz, CH,) 6.65 (d, 4H,
J = 7.7 Hz CH), 6.24 (t, 4H, J = 7.4 Hz CH), 6.03 (t, 2H, J = 7.4
2
2
red mixture with the total volume of 25 mL was stirred at 20 °C.
The reaction was monitored by TLC. After 48 h volatile materials
were removed under vacuum. The product obtained was
purified by flash chromatography on silica gel using the eluent
Hz CH), 1.79 (s, 6H, CH
1.18 (s, 6H, CH
3
), 1.76 (s, 6H, CH
3
), 1.51 (s, 6H, CH
3
),
+
3
). ESI-MS (DCM): m/z 999.36 [M+H] . UV-Vis
3
−1
−1
(DMF): λmax, nm (ε, 10 L mol cm ): 310 (35.8), 390 (14.8).
Recrystallization of (PdL ) from acetone gave the compound
(PdL .(CH CO). Orange needles. For
[Pd(C30 O) ·0.5(CH CO]: Elemental Anal.: Calcd.: C,
2.5Pd); Found: C, 71.90; H,
3
petroleum ether/methylene chloride (99:1). Compound PdL
2
3
3 2
)
was isolated as an orange solid. Yield: 0.7309 g, 92.4 %.
Suitable crystals for single crystal X-ray diffraction study of
H
28
N
3
2
3 2
)
71.81; H, 5.78; N, 8.17; (C61.5
5.61; N, 8.10 %.
59 6
H N O
(
2
PdL .H2O) were grown by recrystallization from chloroform.
Elemental Anal.: Calcd.: C, 65.46; H, 6.99; N, 10.41;
(
(
(
1
1
1
C
44
-
H
56
N
6
O
2
Pd); Found: C, 65.66; H, 6.95; N, 10.15 %. IR, 
Single crystal X-ray diffraction analysis
The intensity data for compounds HL
1
cm ) = 2960.7 (m), 2910.5 (w), 2873.9 (w), 1573.9 (w), 1460.1
m), 1440.8 (m), 1404.1 (m), 1381.0 (w), 1352.1 (w), 1328.9 (w),
309.6 (m), 1282.6 (m), 1244.1 (m), 1228.6 (m), 1199.7 (w),
166.9 (w), 1149.6 (w), 1137.97 (w), 1111.0 (w), 1057.0 (w),
006.8 (w), 987.5 (w), 974.0 (vw), 939.3 (w), 896.9 (vw), 871.8
3
,
PdL
3
and
PdL
SuperNova diffractometer and on a Rigaku Oxford Diffraction
Xcalibur, Eos diffractometer for PdL .H O using CuK radiation
(λ = 1.54184 Å) for the ligand HL and MoK radiation (λ =
3 3 2
.(CH ) CO were collected on a Rigaku Oxford Diffraction
2
2
α
3
α
(
(
(
(
w), 840.9 (vw), 825.5 (w), 808.2 (w), 781.2 (w), 763.8 (w), 740.7
vs), 723.3 (m), 690.5 (w), 663.5 (w), 638.4 (w), 592.1 (w), 563.2
0.71073 Å) for the Pd complexes. Data collection, cell
refinement and data reduction were performed with CrysAlisPro
(Rigaku Oxford Diffraction), the structure were solved with
SIR92[22] or SHELXS[23] and SHELXL-2013[23] was used for full
matrix least squares refinement. The hydrogen atoms were
m), 538.1 (w), 516.9 (m), 505.3 (w), 474.5 (w), 441.7 (m), 395.4
1
w), 356.8 (w), 314.4 (m). H-NMR (400 MHz, CDCl
3
): δ 8.34 (d,
2
H, J = 8.3 Hz CH), 8.04 (d, 2H, J = 6.7 Hz CH), 7.78 (s, 2H,
CH), 7.58 (t, 4H, J = 9.3 Hz CH), 7.19 (s, 2H, CH), 1.69 (d, 8H,
J = 8.7 Hz CH -ethyl), 1.34 (s, 12H, CH -methyl), 1.16 (s, 6H,
-methyl), 1.00 (s, 6H, CH -methyl), 0.81 (t, 6H, J = 7.3 Hz
found experimentally for compounds HL
the disordered phenyl for HL were they were introduced at
geometrical positions. For compounds PdL .(CH CO and
PdL .H O the H-atoms were all introduced at geometrical
3 3
and PdL , except for
2
3
3
CH
CH
3
3
3
3 2
)
3
-ethyl), 0.45 (t, 6H, J = 7.9 Hz CH
3
-ethyl). ESI-MS (DCM):
2
2
m/z 807.36 [M+H] . UV-Vis (DMF): λmax, nm (ε, 103 L mol
+
−1
positions. For each structure the hydrogen atoms were refined
as riding atoms with their Uiso parameters fixed to 1.2Ueq
−
1
cm ): 310 (34.2), 390 (13.9).
(
parent atom) for the aromatics carbons and to 1.5Ueq (parent
Synthesis of bis[2-(2H-Benzotriazol-2-yl)-4,6-bis(1-methyl-
atom) for the remaining atoms.
1
-phenylethyl)phenolato]palladium(II) (PdL
To a solution of 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-
phenylethyl)phenol (HL ) (0.90 g, 2.0 mmol) in THF (10 mL) at
3
)
CCDC 1965954 (for HL
3
2 2
), 1966865 (for PdL .H O), 1965953
3
(for PdL ) and 1965955 (for PdL
3
3
.(CH CO) contain the
3 2
)
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European Journal of Inorganic Chemistry
10.1002/ejic.201901283
FULL PAPER
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
and evaporated to dryness under a stream of dinitrogen. Then,
1
the residue was dissolved in CDCl
3
and analysed by H NMR at
Bruker 300 and 400 UltraShield™ spectrometers. The yield of
C-C coupling product (relatively to iodobenzene) was
[
32]
DFT calculations
established using an internal standard. Control experiments
were performed under the same reaction conditions but in the
All theoretical calculations were performed with the ORCA
program package.[24] Full geometry optimizations and signle
point calculations were carried out using the hybrid functional
B3LYP[25,26] in combination with the TZV/P[27] basis set for all
2
presence of Pd(OAc) instead of any complex catalyst. Blank
experiments (without a metal catalyst) were performed and no
C-C coupling products were detected. In addition, a mercury
drop experiment was performed by adding two drops of metallic
Hg to the abovementioned reaction mixture and running the
reaction under the optimized reaction conditions. After the
reaction, the obtained mixture was cooled to room temperature,
filtered, and treated and analysed as above. No significant
difference in conversion was observed.
atoms and by taking advantage of the resolution of the identity
RI) approximation in the Split-RI-J variant[
28]
with the
(
[
29]
appropriate Coulomb fitting sets. Increased integration grids
Grid4 in ORCA convention) and tight SCF convergence criteria
(
were used. Solvent effects were accounted for according to the
experimental conditions. For that purpose, we used the DMF (
=
38.3) solvent within the framework of the conductor like
screening (COSMO) dielectric continuum approach.[30]
Molecular orbitals were generated using the orca plot utility
program and were visualized with the Chemcraft program.[31]
Acknowledgments
Dr. Amélie Kochem is thanked for useful discussions and advice.
This work has been partially supported by the Fundação para a
The microwave-assisted C-C cross coupling reactions were
performed in G10 borosilicate glass tubes (10 mL capacity
reaction tube with a 13 mm internal diameter) in a focused
Anton Paar Monowave 300 reactor fitted with a rotational
system and an IR temperature detector.
Ciência
e
a
Tecnologia (FCT), Portugal; projects
UID/QUI/00100/2019 and PTDC/QEQ-ERQ/1648/2014 are
acknowledged. Newcastle University is thanked for support to
the project.
Sonogashira C-C coupling
The desired amount of catalyst (0.001-0.010 mmol),
phenylacetylene (1.0 mmol), iodobenzene (1.1 mmol), Et
Keywords: Palladium complexes • Electrocatalysis • Heck •
Sonogashira • X-ray structures
3
N
(
1.2-3.0 mmol) and the solvent (3.0 mL) were introduced into
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and the 1,2-diphenylethyne isolated and spectroscopically
characterized. Control experiments were performed under the
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European Journal of Inorganic Chemistry
10.1002/ejic.201901283
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Neutral lipophilic palladium complexes
based on benzotriazole-phenolate
ligands can operate as C-C bond
coupling catalysts (e.g., Heck,
Sonogashira). Alternatively upon
electroreduction in DMF containing
TFA they form palladium particles
which can produce hydrogen gas by
proton reduction.
Palladium Complexes
Olesea Cuzan-Munteanu, Dumitru
Sirbu, Michel Giorgi, Sergiu Shova,
Elizabeth A. Gibson, Marius Réglier,
Maylis Orio, Luísa M. D. R. S. Martins
and Andrew C. Benniston
Page No. – Page No.
Neutral Lipophilic Palladium(II)
Complexes and their Applications in
Electrocatalytic Hydrogen
Reduction and C-C Coupling
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