Selective Photooxidation of Sulfides Catalyzed by Bis-cyclometalated IrIII Photosensitizers Bearing 2,2′-Dipyridylamine-Based Ligands

Article abstract of DOI:10.1002/chem.201801173

A new family of heteroleptic bis-cyclometalated Ir^III complexes with formula [Ir((Formula presented.))₂((Formula presented.))]Cl ((Formula presented.) =2-phenylpyridinate and (Formula presented.) =2,2′-dipyridylamine or N-benzylated 2,2′-dipyridylamines, were synthesized, characterized, and successfully used as photosensitizers in the catalytic photooxidation of an array of dialkyl, dibenzyl, alkyl aryl, and diaryl sulfides, as well as sulfur-containing amino acids. Furthermore, the reactions proceeded with optimal chemoselectivity, and atom economy under mild conditions. Experimental observations support a dual mechanism in which singlet oxygen and superoxide are the actual oxidants.

Full text of DOI:10.1002/chem.201801173

A Journal of
Accepted Article
Title: Selective Photooxidation of Sulphides catalyzed by
Biscyclometalated IrIII Photosensitizers Bearing 2,2'-
dipyridylamine Based Ligands
Authors: Mónica Vaquero, Alba Ruiz-Riaguas, Marta Martínez-
Alonso, Félix Ángel Jalón, Blanca Rosa Manzano, Ana María
Rodríguez, Gabriel García-Herbosa, Carbayo Aránzazu,
Begoña García, and Gustavo Espino
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201801173
Link to VoR: http://dx.doi.org/10.1002/chem.201801173
Supported by

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FULL PAPER
Selective Photooxidation of Sulphides catalyzed by
III
Biscyclometalated Ir Photosensitizers Bearing 2,2’-
dipyridylamine Based Ligands
[
a]
[a]
[a]
[b]
Mónica Vaquero,* Alba Ruiz-Riaguas, Marta Martínez-Alonso, Félix A. Jalón, Blanca R.
[
b]
[b]
[a]
[a]
[a]
Manzano, Ana M. Rodríguez, Gabriel García-Herbosa, Arancha Carbayo, Begoña García,
[
a]
Gustavo Espino.*
III
Abstract:
complexes with formula [Ir(C^N)
phenylpyridinate and (N^N) = 2,2’-dipyridylamine or N-benzylated-
,2’-dipyridylamine, have been synthesized, characterized and
A
new family of heteroleptic biscyclometalated Ir
the PS triplet excited state; (c) long triplet excited state lifetimes
and good quantum yields in the generation of singlet oxygen
( O ) or alternatively superoxide radical (O ) in the chosen
2 2
solvent; and (d) high solubility and photostability in the solvent
used for the reaction.
2
(N^N)]Cl, where (C^N) 2-
=
1
•
2
successfully used as photosensitizers in the catalytic photooxidation
of an array of dialkyl, dibenzyl, alkylaryl and diaryl sulphides, as well
as sulphur-containing aminoacids. Furthermore, the reactions
proceeded with optimal chemo-selectivity and atom economy under
1
The group of traditional O
2
photosensitizers comprises different
families of organic compounds such as porphyrins,
phthalocyanines, fullerene derivatives and organic dyes (e.g.
Rose Bengal, Methylene Blue), but most of them undergo
degradation upon irradiation and cannot be easily modified in
order to modulate their photophysical properties. On the contrary,
smooth conditions. Experimental observations support
a dual
mechanism where singlet oxygen and superoxide are the actual
oxidants.
II
III
Ru
polypyridyl complexes and Ir
biscyclometallated
1
•
compounds have proved to combine efficient
2 2
O (or O )
Introduction
photosensitizing abilities or photoredox catalytic activity along
with a good photostability and a modular structure which can be
Photocatalysis is becoming a very useful and environmentally
benign strategy in organic synthesis, since it allows transforming
a variety of substrates using mild conditions and light as the
[6,7]
readily modified,
and are emerging as ideal candidates to be
used as PSs in the photocatalytic oxidation of organic substrates
[8]
and biomolecules.
[
1–3]
source of energy instead of heat.
Specifically, aerobic
as the oxidant
photooxidation utilizes plentiful and harmless O
2
Sulfoxides are very important intermediates in organic synthesis,
agent rather than expensive, toxic or corrosive oxidants adopted
in traditional oxidation processes. This approach involves the
use of a photosensitizer (PS) or photocatalyst which is excited
through irradiation with light, in such a way that in the presence
and some of them are commercialized as drugs owing to their
[2,9,10]
pharmacological activity,
e.g. alliin, armodafinil and
[11]
esomeprazole.
Hence, the development of chemo-selective
and efficient methods of synthesis from the respective sulphides
of O
namely singlet oxygen ( O
of an energy transfer process (ET) or an electron transfer
2
, it can generate highly reactive oxygen species (ROS),
[12,13]
is
an
appealing
challenge.
Previously,
aerobic
1
•
2
2
) or superoxide radical (O ) by mean
photooxidation of sulphides has been successfully accomplished
using either metal complexes or organic dyes as PSs. In
particular, it is worth mentioning the systems reported recently
[4]
reaction (eT), respectively. Ultimately, these oxygen species
execute the oxidation of the organic substrates and
biomolecules. The ideal PSs for catalytic photooxidation should
[2,9]
using photosensitizers based on tetra-O-acetyl-riboflavine,
[14]
[15]
II
[16]
II
II
Rose Begal,
complexes,
Thioxanthones,
coenzyme NAD
Pt complexes , Ru -Cu
[
5]
conjugate the following features:
(a) broad overlapping
[17]
+[18]
[19]
and BODIPY dyes.
between the absorption spectra of the PS and the emission
band of the light source (visible light preferably) together with
high molar extinction coefficients; (b) high intersystem crossing
efficiency to guarantee high quantum yields in the formation of
III
However, as far as we know, Ir complexes have not been used
in similar studies and what is more, improvements can still be
made in the field to shorten reaction times, enhance the
photostability and the recyclability of PSs, optimize the selectivity
of the reaction, expand the substrate scope, and avoid organic
solvents and UV light.
[
a]
Dr. M. Vaquero, A. Ruiz-Riaguas, Dr. M. Martínez-Alonso, Dr.
García-Herbosa, Dr. A. Carbayo, Dr. B. García, Dr. G. Espino.
Departamento de Química, Facultad de Ciencias
Universidad de Burgos
In this work, we disclose for the first time the broad applicability
III
of a new family of biscyclometallated Ir complexes with N^N
Plaza Misael Bañuelos s/n, 09001, Burgos, Spain
E-mail: mvaquero@ubu.es
E-mail: gespino@ubu.es
Dr. F A. Jalón, Dr. B.R. Manzano, Dr. A. M. Rodríguez
Departamento de Química Inorgánica, Orgánica y Bioquímica,
Facultad de Químicas
ancillary ligands based on the 2,2’-dipyridylamine scaffold (L1)
as PSs for the selective aerobic photooxidation of organic
sulphides and sulfur containing L-aminoacids. In addition, we
assess the effect of functionalizing the N-H group of L1 with
different benzyl moieties over their photophysical properties,
their photostability and their activity in the above-mentioned
photocatalytic reaction.
[
b]
Universidad de Castilla-La Mancha
Avda. Camilo J. Cela 10, 13071 Ciudad Real, Spain
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
and 2D-NOESY experiments were carried out to assign all the
resonances. Atom labelling, and comparative tables of chemical
shifts are provided in SI (Table SI1 and SI2). The following
general features are deduced from the spectra of the new
complexes:
III
1.- Synthesis and characterization of ligands (L1-L4) and Ir
complexes ([1]Cl-[4]Cl)
Ligands: Ligand L1 (2,2’-dipyridylamine) is commercially
available, and ligands L2-L4 were prepared by reacting L1 with
the appropriate benzyl bromide in the presence of KOH to
deprotonate the N-H group and using DMSO as the solvent at
room temperature (see Scheme 1). This procedure has been
a)
observed for the two C^N ligands due to the symmetric nature of
these complexes (C -symmetry).
b) A single set of 4 resonances is recorded for the two pyridyl
rings of the N^N ligands owing to their symmetry equivalence.
c) Two geminally coupled doublets at around 5.2 and 5.7
A single set of 8 resonances (each integrating as 2 H) is
2
[
20–24]
adapted and optimized from those reported in the literature.
ppm (1 H each) are observed for the diastereotopic protons of
the methylene group in complexes [2]Cl-[4]Cl, whereas a singlet
(2 H) is observed for the homotopic protons of the -CH groups
2
in the respective free ligands. This symptomatic change after
coordination is the result of the chirality inherent to tris-chelate
octahedral complexes.
III
III
Cyclometalated Ir complexes. The new Ir compounds [1]Cl-
4]Cl of general formula [Ir(C^N) (N^N)]Cl (C^N 2-
L1-L4) were synthesized
[
2
=
-
phenylpyridinate (ppy ), N^N
according to the classical procedure,
steps: preparation of the well-known chloro-bridged dimer [Ir(-
Cl)(C^N) , followed by its reaction with the corresponding N^N
ligand in a mixture of CH Cl /MeOH at 50 °C as sketched in
=
[
25]
which involves two
2
]
2
d)
A strongly deshielded singlet is observed at 11.09 ppm for
2
2
the N-H group of complex [1]Cl.
Scheme 2. The desired compounds were obtained in good
yields as the chloride salts of the Ir cationic complexes in the
form of racemic mixtures ( and  enantiomers). Moreover, they
e) In general, the pyridine protons of the N^N ligand undergo a
shift to lower field with respect to the free ligand.
III
1
9
1
The F{ H} NMR spectrum of [4]Cl exhibits a singlet at -60.9
ppm for the –CF group.
are air- and moisture-stable yellow solids, soluble in common
3
+
organic solvents. Complex [1] has been described previously in

[26]
the form of the PF
6

salt,
but the synthesis and
The high resolution HR-ESI(+) mass spectra recorded for the
new complexes [1]Cl-[4]Cl, exhibited in every case a parent
characterization of the Cl salt had not been reported so far.
peak envelope, whose m/z ratio and isotopic pattern are fully
+
compatible with cationic fragments of formulae [Ir(C^N)
2
(N^N)] .
Moreover, the spectrum of [1]Cl features a second peak
+
2
envelope attributable to the fragment [Ir(C^N) ] , owing to the
[
27]
loss of L1 (see Figure SI1).
3. Crystal Structure by X-ray Diffraction
Single crystals of [1]Cl·CH
diffraction analysis were obtained by slow evaporation of a
solution of [1]Cl in dichloromethane. Complex
1]Cl·CH ·0.25H O crystallizes in the monoclinic space group
P2 /c. The ORTEP diagram for ()-[1]Cl is depicted in Figure 1,
2 2 2
Cl ·0.25H O suitable for X-ray
Scheme 1. Reaction protocol for the synthesis of ancillary ligands L2-L4.
[
2
Cl
2
2
1
selected bond lengths and angles with estimated standard
deviations are gathered in Table 1, and crystallographic
refinement parameters are given in the Supporting Information
(
Table SI3). The corresponding unit cell shows two pairs of the
enantiomers () and () stemming from helical chirality implicit
in trischelate octahedral metal complexes. Furthermore, the unit

cell contains four Cl counterions plus four molecules of CH
2
Cl
2
Scheme 2. Reaction protocol for the synthesis of complexes [1]Cl-[4]Cl (Bn =
2 2 2
and one molecule of H O. The CH Cl molecules display
benzyl).
2
positional disorder and the proton atoms of H O have not been
localized. The iridium center displays
octahedral coordination geometry with the expected cis-C,C and
trans-N,N mutual disposition for the C^N ligands.
a slightly distorted
III
2
. Characterization of Ir complexes [1]Cl-[4]Cl in solution
All the compounds synthesized in this work have been fully
characterized by NMR, HR-ESI Mass and IR spectroscopy, as
well as by elemental analysis. In addition, the structure of
complex [1]Cl has been confirmed by X-ray diffraction.
The bond lengths and angles are very similar to those reported

[26]
for the PF6 salt (see Table 1). In particular, the Ir-Cppy and Ir-
Nppy bond distances lie in the expected ranges for 2-
[
26,28–33]
phenylpyridinate ligands, that is, very close to 2 Å.
The
1
The H NMR spectra of the above-mentioned ligands and
bond lengths for the ancillary ligand are significantly longer due
6
complexes were recorded in [D ]DMSO at 25 °C and 2D-COSY
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Chemistry - A European Journal
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to the strong trans influence attributed to the C atoms of the C^N
ligands, Ir(1)-N(3) = 2.177(4) Å and Ir(1)-N(4) = 2.166(4) Å.
In addition, a remarkable motif in this structure is formed by pairs
of complex cations interacting through a parallel four-fold pyridyl
[34,35]
[
38]
embrace which involves an offset  stacking interaction and
two CH contacts (see Figure SI4 and Table SI5). Two pyridine
-
rings of each cation (one of a ppy ligand and another from the
N^N ligand) take part in this embrace. This is a particular case of
the more general term multiple aryl embrace, a “synthon” that is
receiving increasing attention in the field of supramolecular
[
39]
chemistry.
4. Photophysical properties
4.1. Electronic absorption properties. The UV-vis absorption
spectra of ligands L1-L4 (see Figure SI5, and data in Table SI6)
and complexes [1]Cl-[4]Cl (see Figure 2, data in Table 2) were
-5
recorded in acetonitrile solutions (10 M) at 25 °C.
The spectra of complexes [1]Cl-[4]Cl feature two intense
absorption bands between 190 and 330 nm, which are attributed
to singlet spin-allowed ligand centered ( LC) transitions taking
place in both the N^N and the N^C ligands. Very weak and
unstructured bands are also observed from 330 nm to 440 or
Figure 1: ORTEP diagram for ()-[1]Cl forming part of the asymmetric unit of
racemic [1]Cl·CH Cl ·0.25H O. Thermal ellipsoids are shown at the 30%
and H O molecules have been omitted for the
2
2
2
1
probability level. The CH
sake of clarity.
2
Cl
2
2
4
50 nm approximately (see Figure 2 and Table 2). These bands
1
3
are assigned to spin-allowed MLCT and spin-forbidden MLCT
3
Table 1. Selected bond lengths (Å) and angles (°) for the crystal structure of
(
singlet to triplet d(Ir) N^N)), LLCT (singlet to triplet,
[
2
a][26]
[1]Cl·CH
2
Cl
2
·0.25H
2 6
O and [1]PF ·(CH
2
Cl
2
)
3
3
(N^N) *(C^N)) and LC (singlet to triplet 3
[
27]
transitions

Distances
[1]Cl
[1]PF
6
Angles
[1]Cl
[1]PF
6
Ir(1)-N(1)
Ir(1)-N(2)
Ir(1)-N(3)
Ir(1)-N(4)
Ir(1)-C(7)
Ir(1)-C(18)
2.054(4)
2.040(4)
2.177(4)
2.166(4)
2.006(5)
2.004(5)
2.049(6)
2.037(6)
2.173(6)
2.168(6)
2.010(7)
2.008(7)
C(7)-Ir(1)-N(1)
C(18)-Ir(1)-N(2)
N(4)-Ir(1)-N(3)
N(2)-Ir(1)-N(1)
C(7)-Ir(1)-N(4)
C(18)-Ir(1)-N(3)
80.20(19)
80.51(19)
86.44(15)
169.85(16)
176.34(17)
177.73(18)
80.1(4)
80.5(3)
86.0(2)
172.7(2)
176.7(3)
177.0(3)
[
a]
[26]
Data for [1]PF
comparison.
6
have been taken from the literature
and are shown for
-
5
The bite angle for the N^N ligand, N(4)-Ir(1)-N(3) = 86.44(15)°, is
comparable to those previously reported for six-membered N-Ir-
N iridacycles in analogous derivatives, and the C-Ir-N bite angles
Figure 2. UV-vis absorption spectra of [1]Cl-[4]Cl (10 M) in acetonitrile at
5 °C
2
-
Taking into account that most of the photocatalytic experiments
have been done in a mixture of DMSO/H O (3:2) (vide infra), the
for the ppy ligands, C(7)-Ir(1)-N(1) and C(18)-Ir(1)-N(2), are also
[
32,36,37]
2
standard (80.20(19)° and 80.51(19)°, respectively).
-5
absorption spectra of complexes [1]Cl-[4]Cl (10 M) were also
recorded in this solvent system at 25 ºC (see Figure SI7). Thus,
we proved that the absorption spectra of our PSs and the
emission spectrum of the blue light used in the photooxidation
assays partially overlap one to another and hence we conclude
that excitation of our PSs is feasible under this light source ( =
Moreover, the C^N chelate rings are essentially planar, whereas
the N^N chelate ring exhibits a boat conformation. The 3D
crystal packing is hold down by intermolecular hydrogen bonds
(
with the participation of the cations, the solvent molecules and
-
the Cl anion), CH- contacts and offset  stacking interactions.
It is worth remarking the pivotal cohesive role exerted by the Cl
-
460 nm) (see Figure SI8). Furthermore, no important
counterion, that takes part in five hydrogen bonding contacts
-
-
solvatochromic effects were observed when comparing the
spectra in both solvent systems (compare Table 2 and Table
SI7).
(
NH···Cl or CH···Cl ) as an acceptor involving three different
+
2 2
units of [1] and a CH Cl molecule as donors (see Table SI4
and Figure SI2). Columns of  stacking interactions that
extend along c axis are formed involving alternate phenyl and
pyridine rings of the same cyclometalating ligand (see Figure SI3
and Table SI5).
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Chemistry - A European Journal
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FULL PAPER
2 6 2
deoxygenated DMSO/H O (3:2) and [D ]DMSO/D O (3:2) (see
Table 2. Wavelengths (abs) and molar extinction coefficients () at the
absorption maxima and relative maxima in the UV-vis spectra of
complexes [1]Cl-[4]Cl measured in acetonitrile at 25 C.
Figure SI10 and Table 4). Although, solvatochromic effects on
the emission wavelengths were insignificant, the half-life times
(1/2) of [1]Cl and [3]Cl are considerably enhanced in compariso
to those for acetonitrile solutions, suggesting that the presence
of polar protic solvents could stabilize the excited state, in
agreement with a non-negligible contribution of polar charge
ᵒ
3
-1
-1
Complex

abs [nm]
 [10 M cm ]
64.8, 41.7, 3.3
126.8, 63.8, 6.1
117.3, 53.1, 9.2
92.1, 43.9, 7.8
[1]Cl
[2]Cl
[3]Cl
[4]Cl
196, 255, 386
196, 254, 386
196, 254, 342
196, 255, 343
transfer (CT) states to the triplet, T
1
, which is mainly LC in
nature. In particular, the 1/2 values recorded in DMSO/H
and [D ]DMSO/D O were 1365 and 1356 ns, respectively.
[
41]
2
O
6
2
-
5
4.2. Photoluminescence properties
Table 3. Photophysical data for complexes [1]Cl-[4]Cl (10 M) in
deoxygenated acetonitrile at 25 C.
ᵒ
[
a]
[b]
[c]
[d]
Complex
1]Cl
[2]Cl
3]Cl
[4]Cl

em [nm]
 [nm]

PL [%]
1/2 [ns]
The complexes [1]Cl-[4]Cl show photoluminescence in the cyan
hue under irradiation with UV light. The respective emission
[
480, 510
478, 508
478, 508
478, 508
170, 200
168, 198
168, 198
168. 198
45.07
4.46
4.33
4.94
256.4
208.0
264.0
470.1
-5
spectra were recorded for 10
M solutions in dry and
deoxygenated acetonitrile at 25 °C (see Figure 3 and Table 3).
All the spectra are very similar and show a broad band with two
maxima at 478 and 508 nm, and a shoulder around 550 nm,
revealing the negligible effect that substitution of the N-H group
with different benzyl groups exerts on the emitting excited states.
[
[
a]em were recorded with exc = 310 nm [b] : Stokes Shift. [c] Quantum
Yields were recorded with exc = 405 nm [d] 1/2 were recorded with exc
75 nm.
Phosphorescent emission in complexes of the type
=
+
[
Ir(C^N)
2
(N^N)] takes place from the lowest-lying triplet state
3
1
(T ), which usually is made up from a combination of metal-to-
3
3
Regarding the photoluminescence quantum yield ( ), both
complexes, [1]Cl and [3]Cl, exhibited better emission
PL
ligand and ligand-to-ligand charge transfer ( MLCT and LLCT,
3
[40]
respectively) and ligand-centered ( LC) triplet states.
efficiencies in DMSO/H
particularly remarkable in the case of [3]Cl and is likely related
to the higher photostability of this complex in DMSO/H O (vide
infra). In the mixture [D ]DMSO/D O (3:2) the emission is also
efficient and shows PL values above 40%.
2 3
O (3:2) than in CH CN. This is
Nevertheless, the structured form of the bands recorded for our
3
complexes points out to
a
dominant LC nature of the
[
27]
2
transitions.
As for the quantum yields, PL, complex [1]Cl
6
2
exhibits the highest value (45.1 %), whereas the benzyl
derivatives undergo a significant drop in the quantum efficiency
below 5% (4.3-4.9 %) (see Table 3 and Figure SI9).
-
5
Table 4. Photophysical data for complexes [1]Cl and [3]Cl (10 M) in
ᵒ
DMSO/H
2
O (3:2) and [D
6
]DMSO/D
2
O (3:2) at 25 C.
III
Ir PS
(

[nm]
em

[nm]

[%]
PL

1/2
[ns]
[
a]
[b]
[c]
[d]
solvent)
1]Cl (DMSO/H
1]Cl ([D ]DMSO/D
3]Cl (DMSO/H O)
3]Cl ([D ]DMSO/D
[
[
[
[
2
O)
480, 510
480, 511
478, 508
478, 508
170, 200
170, 201
168. 198
168. 198
65.21
45.37
55.84
43.02
1482.74
1821.61
1365.08
1356.08
6
2
2
O)
O)
2
6
[
a]em were recorded with exc = 310 nm [b] : Stokes Shift. [c] Quantum
Yields were recorded with exc = 405 nm [d] 1/2 were recorded with exc
75 nm.
=
3
-
5
Figure 3. Emission spectra of complexes of [1]Cl-[4]Cl in acetonitrile (10 M)
at 25 °C.
5. Electrochemical behavior
Moreover, the emission spectra and photophysical properties
were also recorded for [1]Cl and [3]Cl in the mixtures of
solvents used in the photocatalytic experiments, namely,
The redox potentials of [1]Cl-[4]Cl and thioanisole were
determined in degassed acetonitrile solutions by cyclic
n
voltammetry (CV) using [ Bu
4
N][PF
6
] as supporting electrolyte
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Chemistry - A European Journal
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FULL PAPER
and glassy carbon as the working electrode. Potentials are given
+
with respect to the ferrocene/ferrocenium (Fc/Fc ) couple (Figure
SI11 and Table SI8). Thus, in the anodic region all the
Table 5. Catalyst screening and control experiments in the photooxidation of
2
thioanisole (1a) under an O atmosphere and blue-LED irradiation ( = 460
photosensitizers exhibit an irreversible oxidation peak between
[
a]
-
nm).
+0.61 and +0.65 V, which is attributed to the oxidation of the Cl
anion. In addition, all the PS display a quasi-reversible peak
between +0.87 and + 0.89 V, which is assigned to the oxidation
III
of Ir core. In the cathodic region all the complexes feature an
irreversible peak between -2.28 and -2.41 V ascribed to a ligand
[
26,27]
centered (L1-L4) reduction.
From the redox potentials and
[
b]
the Eem values (obtained from λem) we calculated the redox
potential for the reductive and oxidative quenching in order to
validate a possible redox mechanism for the photocatalytic
activity of [1]Cl-[4]Cl.
Entry
Compound
Conditions
Conv. (%)
1
2
3
4
5
[1]Cl
-
O
2
, Dark
, light
, light
0
O
2
1
6. Photostability of complexes [1]Cl and [3]Cl
[1]Cl
[1]Cl
[1]Cl
N
2
0
[
c]
2
O ,light, NaN
3
<1
30
The photostability of complexes [1]Cl and [3]Cl in solutions of
-2
3 6 2
both CD CN and [D ]DMSO/D O (3:2) (1.4·10 M), was studied
2
O ,light, 1,4-
1
[d]
by H NMR over a period of 24 h under irradiation (LED blue
light, λirr = 460 nm, 24 W) in a NMR tube exposed to
atmospheric air. Complex [1]Cl exhibits an acceptable
photostability in both solvent systems, with decomposition
values of 13 % and 8% respectively, after 24 h. However, the
benzoquinone
6
[1]Cl
2
O ,light, 1,4-
15
[d]
dimethoxybenzene
[
[
f]
f]
7
8
9
[Ir(C^N)
[Ir(C^N)
[1]Cl
2
(DMSO)Cl]
2
O , light
2
O , light
2
O , light
2
O , light
2
O , light
2
O , light
<1
<1
1
species resulting from degration have not been identified (see H
2
(DMSO)Cl] + L3
NMR spectra in Figures SI12 and S13). Complex [3]Cl is also
sufficiently stable in the mixture of solvents [D
though decomposes slowly by dissociation of the N^N ligand
2% and 21% after 3 h and 24 h, respectively; see spectra in
Figure SI15). On the contrary, [3]Cl is highly photosensitive in
CD CN showing 96 % degradation after 24 h (Figure SI14). On
6
]DMSO/D
2
O (3:2),
[e]
59 (80)
[
e]
1
0
1
[2]Cl
64 (92)
(
[e]
1
[3]Cl
67 (>99)
3
[
e]
the other hand, equivalent solutions of [3]Cl are stable in the
12
[4]Cl
66 (82)
dark at least after 2 weeks (Figure SI16).
-
2
[
[
a] Thioanisole (1a) (10 mM), PS (10 mM, 0.1 mol %) in a mixture of
]DMSO/D (0.5 mL, 3:2) at room temperature, under saturated
or N and under irradiation with blue light (LED, λ =
60 nm, 24 W) during 12 h in a septum-capped tube. [b] Conversion yields
D
6
2
O
a
7. Photooxidation of sulfides
atmosphere of either O
4
2
2
1
Renaud et al. have previously reported on the photocatalytic
activity of complex [1]PF in the aza-Henry reaction between N-
phenyl tetrahydroisoquinoline and nitromethane under blue LED
were determined by H NMR analysis of the crude mixture as average values
of two independent experiments. Overoxidation to the corresponding sulfone
product was not detected in any experiment. [c] NaN
Benzoquinone or 1,4 dimetoxybenzene (11 mM) [e] 18 h of irradiation. [f]
Species formed from [Ir(-Cl)(C^N) in DMSO containing solutions.
6
3
(11 mM). [d]
[
42]
[16]
irradiation. Inspired by this work and others, we performed
preliminary catalytic experiments on the photooxidation of
thioanisole (1a) to methyl phenyl sulfoxide (2a) in the presence
2 2
]
Moreover, the reaction is chemoselective, since no traces of
over-oxidation products (sulfones) have been detected. The
2
of [1]Cl at room temperature, under an O atmosphere and with
blue LED irradiation in different solvents (Table SI9). As a result,
we concluded that the reaction was only feasible in protic
control experiments in the dark, in the absence of PS or in a N
2
atmosphere were unproductive (see entries 1-3 in Table 5), and
the tests carried out in the presence of the free ligands as PSs
solvents, such as MeOH and the mixture DMSO/D
we chose the latter as the most beneficial in terms of conversion.
2
O (3:2) and
1
[9]
were also sterile (see Table SI10). Besides, the use of either the
This fact suggests a
catalysts screening, using [1]Cl-[4]Cl (0.1 mol %) as PSs, for
the above-mentioned oxidation in a mixture of [D ]DMSO/D
3:2) under the same conditions. This model transformation
2
O pathway. Next, we carried out a
III
Ir
dimer, [Ir(μ-Cl)(C^N)
2
]
2
(which evolves in situ to
[
Ir(C^N)
2
(DMSO)Cl] in DMSO solutions) or a mixture of this
6
2
O
precursor and L3 as potential PSs only yielded traces of
sulfoxide 2a (entries 7 and 8 in Table 5). Hence, even though
(
III
proceeds smoothly in the presence of the Ir PSs to give
moderate or good yields after 12 h, and close to quantitative
conversions after 18 h (entries 9-12 in Table 5). Interestingly, the
[
3]Cl decomposes slightly into [Ir(C^N)
2
(DMSO)Cl] and L3 in
[
D
6
]DMSO/D O, these species do not contribute to the observed
2
1
photocatalytic activity. In view of the above results, the
catalytic activities determined by H NMR for the different PSs
III
photosensitizing role of the new Ir complexes is clearly
were very similar, though benzylation seems to improve slightly
the oxidation rate.
established. Additional experiments were performed to
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Chemistry - A European Journal
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determine the actual oxidant and formulate
a
plausible
quencher,
intermediates. All in all, the experimental data insinuate a dual
mechanism. See the discussion below and Scheme 3 for more
details.
To scrutinize the substrate scope of this methodology, the most
active PS, [3]Cl, was selected to photocatalyze the oxidation of
1
mechanism. In particular, the addition of NaN
3
( O
2
entry 4 in Table 5) drastically suppressed the photooxidation of
the substrate suggesting that the process is essentially mediated
1
[2],[43],[44]
by O
2
in an energy transfer process (ET)
.
.
different sulphides (1b-1g) under an O
2
atmosphere. The results
Table 6. Photooxidation of sulphides 1 with [3]Cl under an O
and blue-LED irradiation ( = 460 nm).
2
atmosphere
are summarized in Table and the respective control
6
[
a]
experiments are compiled in Table SI11. Photoactivation of
complex [3]Cl (0.1 mol %) catalyzed the oxidation of 4-Cl-
thioanisole (1b), 4-OMe-thioanisole (1c), and dibenzylsulphide
(
(
1e) with moderate conversions after 12 hours, (46 to 72%)
entries 3, 5 and 9 in Table 6, respectively), whereas the dialkyl
sulphide (1g) underwent complete transformation to the
corresponding sulfoxide (entry 13, Table 6). On the contrary,
diphenylsulphide (1f) was less prone to photooxidation (9 %
conv.; entry 11 in Table 6).
[
b]
Entry
Compound
Conditions
, light
Conv. (%)
Moreover, the photosensitized oxidation of all the above-
mentioned substrates was also tested using atmospheric oxygen
(air) instead of pure O in the presence of [3]Cl at the same
2
catalyst loading (0.1 mol %). Under these conditions the
conversion values decreased notably for most of the sulphides
[
c][d]
1
2
3
4
5
6
7
8
9
O
2
67, >99
58, >99
46, >99
1
1
1
1
1
1
1
a
b
c
d
e
f
[
c]
d]
d]
d]
d]
d]
d]
[c]
air, light
, light
[
c]
O
2
(
see entries 2, 4, 6, 10 and 12 in Table 6). Increasing the
[
[c]
catalyst loading to 1 mol %, sulphides 1a, 1c and 1e were
oxidized to the corresponding sulfoxides with excellent
air, light
17, 53
[
c]
2
O , light
53, >99
31, >99
conversions (>99%) under either pure O
2 2
or atmospheric O
(
see entries 1-2, 5-6, and 9-10 in Table 6, respectively). Even
[
[c]
air, light
the oxidation of 1f was enhanced until 44 % conv. with this
[
c]
catalyst loading (see entries 11 and 12 in Table 6). The reluctant
2
O , light
19, 29
1
reaction of diaryl-sulphides with
2
O as compared to arylalkyl-
[
[c]
air, light
, light
14
and dialkyl-sulphides has been previously reported and could be
[
45–47]
related to the electrophilic nature of singlet oxygen.
In other
*), compared to
2
, and hence it reacts more readily with sulphides having
[
c]
1
O
2
72, >99
words,
O
2
possesses a low-energy LUMO (
y
3
O
[
[c]
10
11
12
13
14
air, light
, light
38, >99
electron rich S atoms (alkyl sulphides) to form relatively stable
persulfoxide intermediates (see mechanism in Scheme 3).
[
c]
O
2
9, 44
Substrate 4-nitrothioanisole (1d) is very singular and the results
obtained in the corresponding photooxidation experiments
deserve a detailed discussion. First of all, the yield for this
sulphide in the presence of [3]Cl is very low (see entries 7 and 8
in Table 6). This fact could be attributed to a combination of
different effects reported in the literature: (a) the electron-
withdrawing nature of the nitro group that decreases the
nucleophilic character of the S atom and makes 1d less prone to
[
[c]
air, light
9, 43
[
c]
2
O , light
>99, >99
g
[
[c]
air, light
>99, >99
-
2
[
[
a] Sulphide (10 mM), [3]Cl (10 mM, 0.1 mol %) in
]DMSO/D O (0.5 mL, 3:2) at room temperature, under a saturated
and under irradiation with blue light (LED, λ = 460 nm, 24
a mixture of
D
6
2
1
[9]
atmosphere of O
2
photooxidation by electrophilic
O
2
;
(b) the ability of nitro
W) during 12 h in a septum-capped tube. [b] Conversion yields were
determined by H NMR analysis of the crude mixtures as average values of
two independent experiments. Overoxidation to the corresponding sulfone
product was not detected in any experiment. [c] [3]Cl= 1 mol %. [d]
Reaction carried out in an open test tube.[d] Results previously described in
Table 5 (entry 9) and inserted for comparation.
1
compounds to deactivate excited states of PSs that could lead to
[
48]
quenching of the T
1
state of [3]Cl by 1d;
(c) the ability of
[
2]
trapping radicals attributed to the nitro group, which could
inhibit the eT pathway in the photooxidation of 1d. In the second
place, the yield for 1d in the absence of PS is surprisingly high
•

(15%, entry 7 in Table SI11). Indeed, the self-induced
Other additives such as 1,4-benzoquinone (quencher of O
2
)
photooxidation of 4-nitrophenyl alkyl sulphides has been
previously described and explained as a consequence of their
capability of sensitizing the conversion of O to O .
2 2
and 1,4-dimethoxybenzene (quencher of sulfide radical cations,
•
+ [16]
2
R S ) (entries 5 and 6 in Table 5) were also used as indirect
3
1
[49]
•
•+
probes for O
2
2
and R S . Both scavengers gave rise to a partial
decrease in the production of 2a in agreement with a small
We have done additional photocatalytic assays for 1a with [1]Cl
contribution of an electron transfer (eT) pathway, in which the
•
•+
in DMSO/H O (3:2) and [D ]DMSO/D O (3:2) (See Table SI9).
2
6
2
real oxidant is
O
2
and where
R
2
S
are proposed as
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Thus, the oxidation rate with [1]Cl is higher using deuterated
(LED, = 460 nm, 24 W) in a septum-capped tube. [b] Conversion yields
were determined by H NMR analysis of the crude mixture as average
values of two independent experiments. ESI-MS spectra were recorded to
confirm the identity of products. Methionine sulfoxide was obtained as a 1:1
mixture of diastereoisomers which are epimers at the sulfur atom, that is
1
solvents, in agreement with its longer 1/2 in [D
versus DMSO/H O (3:2) (Table 4), which suggests a main
pathway mediated by
6 2
]DMSO/D O (3:2)
2
1
[2]
O
2
.
We postulate that the presence of
the N-H group in [1]Cl facilitates the interaction between the
excited state of the PS and protic solvents as well as H or D
exchange leading to faster quenching in non-deuterated
S C S C
(S ,S ) and (R ,S ) and Cystine was obtained as the (R,R) diastereomer.
Overoxidation to methionine sulfone or cysteine sulfenic, sulfinic or sulfonic
acids was not detected in any experiment. A turbid suspension attributed to
cystine precipitation was observed after stopping the reaction with L-
cysteine, which was acidified to dissolved it. [c] 18 h of irradiation.
solvents. Analogous experiments with [3]Cl in DMSO/H
2
O (3:2)
(
Table SI12) and [D ]DMSO/D O (3:2) (Table 6) gave similar
6
2
conversion values in line with the comparable 1/2 values
obtained in both solvent systems for [3]Cl, which proves the
absence of isotopic effects for this PS in accordance with the
lack of functional groups able to interact efficiently with protic
solvents.
In order to demonstrate that other sources of light can be used
in this transformation, photocatalytic experiments with a CFL
lamp have been done, using complex [3]Cl as the catalyst (0.1
9
. Mechanistic proposal
On the basis of our experimental observations (see Table 8) and
[
2][50,51]
the bibliographic background,
dual mechanism for the photosensitized oxidation of sulphides
under O atmosphere in the presence of complexes [1]Cl-[4]Cl
as photosensitizers (
Scheme 3): this involves an ET pathway mediated by O
we tentatively propose a
2
1
2
(see
(see
6 2
mol% and 1 mol%) in [D ]DMSO/D O (3:2) (see Table SI13).
•
Scheme 3a) and a concurrent eT pathway mediated by O
Scheme 3b).
2
The results are similar to those obtained with the blue LED light.
8. Photooxidation of sulfur containing L-Aminoacids.
Table 8. Experimental observations with mechanistic implications.
Next, we decided to test the potential of complexes [1]Cl and
3]Cl in the photooxidation of the sulfur-containing aminoacids,
Pathway
Evidences
[
L-cysteine and L-methionine. The results collected in Table 7
demonstrate that the new PSs are active in the chemoselective
oxidation of both L-aminoacids to cystine and methionine
1


Exp. with NaN
3 2
( O ) unproductive (Table 5).
Low conv. for diphenylsulphide (1f) (Table 6).
 Low conv. for 4-nitrothioanisole (1d) (Table 6).
 Protic solvents promote oxidation of 1a (Table SI8).
1
ET ( O
2
)
1
sulfoxide, respectively ( H NMR spectra in Figures SI24 and

Higher conv. for [1]Cl in deuterated solv. (Table SI8).
SI25). Again, the control experiments confirm the active role of
III
the Ir derivatives (see entries 1 and 4 in Table 7).

Low conv. with 1,4-benzoquinone (Table 5).
 Low conv. with 1,4-dimetoxybenzene (Table 5).
eT (O
2
•)

Electrochemical experiments.
Table 7. Photo-oxidation of L-methionine or L-cysteine under an
atmosphere and blue-LED irradiation.
O
2
[
a]
1
•
1
O →O
2
2
Step
 Exp. with NaN
3
( O
2
) unproductive (Table 5).
3
1
2 2
The ET route starts with the photosensitization of O to O in
III
III
three steps: (a) the Ir photosensitizer [Ir ] is excited with visible
light from the ground state (S ) to higher energy singlet states
), which undergo partial relaxation to the lowest energy
0
(S
n
1
III
excited singlet state, S
1
, ( [Ir ]*); (b) next, the Ir-PS may go
state to the
, ( [Ir ]*); (c) subsequently,
through an intersystem crossing process from the S
1
3
III
lowest energy excited triplet state, T
1
3
3
III
O
2
quenches the [Ir ]* via an energy transfer (ET) process to
1
1
1
produce O
2
. The electronic configuration of O
2
( 
g
) bestows an
[
b]
Entry
Compound
Substrate
L-Met
Conditions
Conv. (%)
electrophilic character to this excited state of dioxygen that
[
51]
facilitates the oxidation of nucleophiles, such as sulphides.
1
2
3
4
5
6
-
O
O
O
O
O
O
2
, light
2
, light
2
, light
2
, light
2
, light
2
, light
0
1
1
Therefore, singlet oxygen O
2
( 
g
) reacts with the corresponding
[
[
c]
c]
[1]Cl
[3]Cl
-
L-Met
62, 95
75, 99
44
sulphide generating the persulfoxide intermediate, A, which in
turn reacts with other molecule of sulphide to produce two
L-Met
[
52]
molecules of the respective sulfoxide.
L-Cys
L-Cys
L-Cys
[1]Cl
[3]Cl
>99
>99
-
2
[
[
a] L-methionine/L-cysteine (10 mM), PS (10 mM, 0.1 mol %) in a mixture of
D6]DMSO/D O (0.5 mL, 3:2 or 1:10 respectively) at room temperature,
and under irradiation with blue light
2
under a saturated atmosphere of O
2
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Chemistry - A European Journal
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FULL PAPER
In conclusion, we have synthesised and fully characterized a
III
new family of Ir complexes of general formula [Ir(ppy)
2
(N^N)]Cl,
where N^N stands for either 2,2’-bipyridylamine (L1) or its
benzylated derivatives (L2-L4). The new compounds exhibit
photoluminescent properties and remarkable photocatalytic
activity in the aerobic oxidation of organic sulphides. In particular,
[
2
3]Cl was identified as the most active PS in the O -mediated
photooxidation of thioanisole and exhibited excellent
performances in the selective achievement of different dialkyl,
alkylaryl and diaryl sulfoxides from their respective sulphides.
Likewise, the oxidation of S-containing L-aminoacids was
selectively accomplished under similar conditions. By-products
or over-oxidation products were not detected in any case.
Moreover, the catalytic transformations proceeded in a mixture
of DMSO/H
2
O at room temperature, with low catalyst loading
and in the absence of additives or sacrificial oxidants. Indeed,
1
•
the experimental data suggest that both O
2
and O
2
contribute
as the actual oxidant agents in alternative pathways. To the best
of our knowledge, systematic studies on the photooxidation of
III
sulphides using Ir complexes as PSs have not been described
previously and so, this contribution could open a new line of
work for Ir-based photocatalysis. To conclude, future
developments in this area should focus on the design of new
PSs to improve both their photostability and their absorption
properties.
Experimental Section
Scheme 3. Dual mechanistic proposal for the photocatalytic oxidation of
1
sulphides. a) photooxidation mediated by
2
O (ET pathway); b) photooxidation
•

mediated by O
2
(eT pathway).
General experimental procedure for the synthesis of ligands L2-L4
In a 100 mL Schlenk flask, previously purged with nitrogen, N,N-
dipyridylamine (dpa, L1, 2.34 mmol) was added to a suspension of
powdered KOH (9.45 mmol) in DMSO (4 mL) and the mixture was stirred
for 40 min to facilitate the deprotonation of dpa (L1). Then, the
appropriate benzyl bromide (2.34 mmol) was added, and the reaction
mixture was stirred at room temperature for 24 h. Addition of water (5
mL) followed by fridge cooling (24 h) gave rise to precipitation of the
crude ligand which was collected by filtration, washed with water (3 x 5
mL) and dried under vacuum. The resulting products were recrystallized
in n-hexane to produce the desired pure ligands.
Alternatively, the photooxidation can occur through an eT
pathway. In this case, the highly reductive Ir excited species,
III
III
IV III
[
Ir ]* (Eº(Ir /Ir *) = -1.71 V, see Scheme SI1) can reduce a
•
2 2
molecule of O to generate the superoxide radical (O ) and
concurrently the intermediate [Ir ] through oxidative quenching.
Next, the [Ir ] intermediate (Eº(Ir /Ir ) = + 0.87 V) can oxidize a
2
molecule of sulphide to generate a sulphide radical cation, R S
IV
IV
IV III
•
+
•+
(
Eº(R
2
S /R
2
S)
=
+1.02 V, experimentally measured for
III
thioanisol by CV) and concomitantly regenerate the [Ir ] PS.
This step is plausible, though the process is slightly endergonic
Eº = -0.15 V), in as much as R
quickly in a different species.
superoxide (O ) reacts with R S to produce the corresponding
•+
(
2
S
reacts and transforms
[
53]
General experimental procedure for the synthesis of complexes
Indeed, we propose that the
•
•+
[1]Cl-[4]Cl
2
2
persulfoxide, A, and finally the sulfoxide, upon the intervention of
another molecule of sulphide. However, taking into account the
almost complete inhibition of the reaction in the presence of
In a 100 mL Schlenk flask, previously purged with nitrogen, the N^N
2 2
ligand L1-L4 (0.18 mmol) was added to a solution of [Ir(-Cl)(C^N) ]
NaN
3
(entry 4, Table 5), we speculate that NaN
3
(strong
(0.100 g, 0.09 mmol) in a mixture of dichloromethane (10 mL) and
methanol (5 mL). The resulting solution was stirred at 50 °C for 6 h under
a nitrogen atmosphere. Diethyl ether or n-hexane (20 mL) was added to
precipitate a crude solid, which was filtered and washed with diethyl ether
IV
reducing agent) can interfere with this pathway by reducing [Ir ]
to [Ir ].
III
(2×10 mL). The solid was dried under vacuum.
Conclusions
X-ray Crystallography.
A summary of crystal data collection and refinement parameters for [1]Cl
are given in Table SI3. A single crystal of [1]Cl was coated in high-
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Chemistry - A European Journal
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0
42U16), is gratefully acknowledged. We are also indebted to P.
vacuum grease, mounted on a glass fiber and transferred to a Bruker
SMART APEX CCD-based diffractometer equipped with a graphite
monochromated Mo-Kα radiation source ( = 0.71073 Å). The highly
Castroviejo and M. Mansilla, from PCI of the University of Burgos, for
technical support.
[
54]
redundant datasets were integrated using SAINT
and corrected for
Keywords: Biscyclometalated Iridium(III) Complexes • Oxygen
photosensitizers • Photocatalysis • Singlet Oxygen • Sulphides
photooxidation.
Lorentz and polarization effects. The absorption correction was based on
the function fitting to the empirical transmission surface as sampled by
[
55]
multiple equivalent measurements with the program SADABS.
The
[
56]
software package WINGX was used for space group determination,
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III
Selective
Photosensitizers:
and
Efficient
Ir
Mónica Vaquero*, Alba Ruiz-Riaguas,
Marta Martínez-Alonso, Félix A. Jalón,
Blanca R. Manzano, Ana M. Rodríguez,
Selective
III
biscyclometalated Ir photosensitizers
for the oxidation of sulphides under
visible-light irradiation are described.
Experimental observations support a
dual mechanism where singlet oxygen
and superoxide are the actual
oxidants.
Gabriel
García-Herbosa,
Arancha
Carbayo, Begoña García, Gustavo
Espino*
Page No. – Page No.
Selective Photooxidation of
Sulphides catalyzed by
III
Biscyclometalated Ir
Photosensitizers Bearing 2,2’-
dipyridylamine Based Ligands
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Title
Text for Table of Contents
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