Non-emissive RuII Polypyridyl Complexes as Efficient and Selective Photosensitizers for the Photooxidation of Benzylamines

Article abstract of DOI:10.1002/chem.202001460

Five new Ru^II polypyridyl complexes bearing N-(arylsulfonyl)-8-amidoquinolate ligands and three of their biscyclometalated Ir^III congeners have been prepared and employed as photocatalysts (PCs) in the photooxidation of benzylamines with O₂. In particular, the new Ru^II complexes do not exhibit photoluminescence, rather they harvest visible light efficiently and are very stable in solution under irradiation with blue light. Their non-emissive behavior has been related to the low electrochemical energy gaps and rationalized on the basis of theoretical calculations (DFT analysis) that predict low S₀←T₁ energy values. Moreover, the Ru^II complexes, despite being non-emissive, display excellent activities in the selective photocatalytic transformation of benzylamines into the corresponding imines. The presence of an electron-withdrawing group (-CF3) on the arene ring of the N-(arylsulfonyl)-8-amidoquinolate ligand improves the photocatalytic activity of the corresponding photocatalyst. Furthermore, all the experimental evidence, including transient absorption spectroscopy measurements suggest that singlet oxygen is the actual oxidant. The Ir^III analogues are considerably more photosensitive and consequently less efficient photosensitizers (PSs).

Full text of DOI:10.1002/chem.202001460

Accepted Article
Accepted Article
Title: Non-emissive Ru(II) polypyridyl complexes as efficient and
selective photosensitizers for the photooxidation of benzylamines
Authors: Gustavo Espino, Cristina Yagüe, Igor Echevarría, Mónica
Vaquero, Jairo Fidalgo, Arancha Carbayo, Félix A. Jalón,
Joao Lima, Artur Moro, and Blanca R. Manzano
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
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.202001460
Link to VoR: https://doi.org/10.1002/chem.202001460
01/2020

10.1002/chem.202001460
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Non-emissive Ru(II) polypyridyl complexes as efficient and
selective photosensitizers for the photooxidation of
benzylamines
Cristina Yagüe,[a] Igor Echevarría,[a] Mónica Vaquero,*[a] Jairo Fidalgo,[a] Arancha Carbayo,[a] Félix A.
Jalón,[b] Joao Lima,[c] Artur Moro,[c] Blanca R. Manzano[b] and Gustavo Espino*[a]
[a]
C. Yagüe, I. Echevarría, Dr. M. Vaquero, J. Fidalgo, Dr. A. Carbayo and Dr. G. Espino.
Departamento de Química, Facultad de Ciencias
Universidad de Burgos.
Plaza Misael Bañuelos s/n, 09001, Burgos, Spain.
E-mail: mvaquero@ubu.es, gespino@ubu.es
[b]
[c]
Dr. F. A. Jalón and Dr. B. R. Manzano.
Departamento de Química Inorgánica, Orgánica y Bioquímica. Facultad de Químicas.
Universidad de Castilla-La Mancha, Avda. Camilo J. Cela 10, 13071 Ciudad Real, Spain.
Dr. J.C. Lima and Dr. Artur Moro
LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia,
Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal.
Supporting information for this article is given via a link at the end of the document.
Abstract: Five new Ru(II) polypyridyl complexes bearing N-
(arylsulfonyl)-8-amidoquinolate ligands and three of their
biscyclometalated Ir(III) congeners have been prepared and
employed as photocatalysts (PCs) in the photooxidation of
benzylamines with O2. In particular, the new Ru(II) complexes do not
exhibit photoluminescence but they harvest visible light efficiently and
are very stable in solution under irradiation with blue light. Their non-
emissive behavior has been related to the low electrochemical energy
gaps and rationalized on the basis of theoretical calculations (DFT
analysis), which predict low S0  T1 energy values. Moreover, the
Ru(II) complexes, despite being non-emissive, display excellent
activities in the selective photocatalytic transformation of
benzylamines into the corresponding imines. The presence of an
electron withdrawing group (–CF3) on the arene ring of the N-
(arylsulfonyl)-8-amidoquinolate ligand improves the photocatalytic
activity of the corresponding photocatalyst. Furthermore, all the
into chemical energy through the generation of unique reactive
species.[2] Although organic dyes do not produce heavy-metal
waste, transition-metal photosensitizers (PSs) typically feature
longer excited-state lifetimes, broader redox windows and require
lower catalyst loadings.[3] In particular, Ru(II) and Ir(III) polypyridyl
complexes of the types [Ru(bpy)3]2+ (bpy = 2,2’-bipyridine) and
fac-[Ir(ppy)3] (ppy
=
2-phenylpyridinate) are paradigmatic
photosensitizers widely used in photocatalysis.[4]
On the other hand, imines are important precursors or
intermediates in the synthesis of nitrogen-containing
compounds.[5,6] Traditionally, they have been used as
electrophiles towards different nucleophiles affording amines,[7,8]
-amino nitriles[9] and N-containing heterocycles.[10]
Several methods are known for the preparation of imines: (a)
condensation of amines and aldehydes; (b) oxidation of amines
to imines using stoichiometric oxidants;[11] (c) catalytic oxidation
of primary and secondary amines;[12,13] (d) photocatalytic
oxidation of primary and secondary amines;[14,15] and (e)
photooxidative cross-coupling between amines and benzyl
halides.[16] Nevertheless, application of method (a) is sometimes
limited by the formation of H2O and the sensitivity of imines versus
hydrolysis. Method (b) involves the use of corrosive oxidants and
produces undesirable waste. Moreover, methods (a), (b) and (c)
require high temperatures. On the other hand, the photocatalytic
methods (d) and (e) are typically performed at room temperature
and use O2 as a clean and cheap oxidant source.[17]
During the last years different homogeneous and heterogeneous
photocatalytic systems have been described to obtain imines from
benzylamines or dibenzylamines. It is worth mentioning the
protocols based on Rh(I),[18] Ir(III),[19] Os(II)[20] and Au(III)[21]
complexes, porphycene derivatives,[22] porphyrin and Ru(II)
porphyrin complexes,[15] phenothiazine dyes[17] and BODIPY
derivatives[23] as homogeneous photocatalysts. Remarkable
heterogeneous photosensitizers are those based on carbon
Quantum Dots,[24] dye-sensitized TiO2,[25] Nb2O5,[26] carbon
nitride,[27] WS2 nanosheets,[28] ZnIn2S4,[29] carbazolic conjugated
polymers[30,31] and Ru-sensitized metal-organic frameworks.[32]
Nonetheless, some of these systems employ high temperatures
experimental
evidences,
including
Transient
Absorption
Spectroscopy measurements suggest that singlet oxygen is the actual
oxidant. The Ir(III) analogues are considerably more photosensitive
and consequently less efficient photosensitizers (PSs).
Introduction
Visible-light photocatalysis is a very attractive synthetic tool in as
much as it uses visible light as the energy source for the
construction of new C-C and C-heteroatom bonds in a green,
efficient and selective manner. The mild conditions required in this
type of methodology (room temperature and visible light) prevent
side reactions from taking place and make it compatible with
diverse functional groups present in the substrates. Besides,
photocatalysis involves electronic excitation steps, which
significantly change the reactivity of chemical compounds,
allowing unique reaction pathways that afford non-conventional
chemical products.[1] In other words, photocatalysis relies on the
ability of some heterogeneous semiconductors, organometallic
complexes and organic dyes to harvest visible light and convert it
1
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and O2 pressures,[24,27] some others use undesirable solvents
(e.g.: benzene),[18,26] or exhibit moderate selectivity.[25,29,30]
Polypyridyl Ru(II) complexes of the type [Ru(bpy)3]2+ have been
successfully employed as photocatalysts in multiple organic
transformations.[33] More specifically, they have also been used in
aerial photooxidation reactions, in which they are efficient
photosensitizers due to their ability to produce singlet oxygen
(1O2), or radical anion superoxide (O2•−) from the ground state of
molecular oxygen (3O2). However, as far as we know the use of
monocationic Ru(II) polypyridyl complexes of general formula
[Ru(bpy)2(N^N)]+ (N^N: monoanionic ligand) as photocatalysts
has not been explored, even though they could exhibit high
potential as light-harvesting antennas.[34]
pro-ligand in the presence of an excess of NaHCO3 in
CH2Cl2/MeOH (Scheme 1). These photosensitizers were used in
the photocatalytic tests for comparison purposes. Complex [IrL1]
and other related derivatives had been previously reported by
Aoki.[41] Moreover, the reaction of rac-[Ir(-Cl)(ppy)2]2 with ligand
HL6 gave a mixture of the expected product along with the
respective methanolysis derivative in which the F atom in para
has been substituted by a –OMe group (see Table SI2 and
Scheme SI2).[34,42,43] The Ir(III) derivatives also exhibit C1-
symmetry and were isolated as racemic mixtures in the form of
air- and moisture-stable yellow solids.
In this paper, we report on the design of new bis-heteroleptic
Ru(II) and Ir(III) photosensitizers bearing N-(arylsulfonyl)-8-
amidoquinolate ligands and on the successful evaluation of their
photocatalytic properties in the smooth and selective oxidation of
primary and secondary benzylamines using O2 as the oxidant.
Interestingly, we found that the new Ru(II) complexes display
excellent photocatalytic activity, despite being non-emissive. An
in-depth study of their photophysical and electrochemical
properties supported by theoretical calculations is also provided.
Results and Discussion
Synthesis and characterization of pro-ligands (HL1-HL6)
and their Ru(II) and Ir(III) complexes
Scheme 1. Synthesis of the pro-ligands HL1-HL6, the new Ru(II) polypyridyl
complexes [RuL1]Cl-[RuL5]Cl and the Ir(III) derivatives [IrL1], [IrL4] and [IrL5].
All the complexes are obtained as racemic mixtures, although only the
 enantiomers are shown.
Pro-ligands HL1-HL6: The N-(quinolin-8-yl)arenenesulfonamide
pro-ligands HL1-HL3, HL5 and HL6 were prepared following a
procedure adapted from those reported previously in the
literature.[35,36] This protocol consists in reacting 8-aminoquinoline
with the appropriate sulfonyl chloride in the presence of
triethylamine, in dichloromethane, at room temperature (Scheme
1). However, pro-ligand HL4 was prepared by stirring 8-
aminoquinoline in the presence of the corresponding sulfonyl
chloride in pyridine at 0 C and then at room temperature.[37] Some
of these pro-ligands have been previously used to prepare Ru(II)
complexes of general formula [Ru(p-cym)Cl(N^N)] (N^N = L1, L2
and L3)[38] and [Ru(bpy)(SCN)(N^N)] (N^N = L1, L2).[39]
Characterization of the new complexes
The composition and structure of these compounds were
determined by multinuclear and 2D NMR spectroscopy, together
with high resolution mass spectrometry (HR-ESI), IR
spectroscopy and elemental analysis (See SI). The 1H NMR
spectra of the Ru(II) complexes were recorded in CDCl3 at 25 C
and exhibit the following distinctive general features: (a) The N-H
peaks observed as broad singlets above 9.2 ppm for pro-ligands
HL1-HL5 are missing in the spectra of complexes [RuL1]Cl-
[RuL5]Cl; (b) The resonances for both the bipyridyl and the N-
(arylsulfonyl)-8-amidoquinolate ligands are displaced with regard
to those of rac-cis-[Ru(bpy)2Cl2] and the free pro-ligands,
respectively; (c) Four sets of peaks are identified for the four
inequivalent pyridyl rings in agreement with the asymmetric
nature of these derivatives. The HR-ESI(+) spectra of complexes
[RuL1]Cl-[RuL5]Cl displayed envelope peaks compatible with
the expected monocationic species of formulae [Ru(bpy)2(N^N)]+
(see the spectrum for [RuL4]Cl in Figure SI1). The
characterization data of the Ir(III) complexes are gathered in the
supporting information (see the HR-ESI(+) mass spectrum for
[IrL5] in Figure SI2).
Synthesis of the ruthenium complexes. The new Ru(II)
complexes [RuL1]Cl-[RuL5]Cl of general formula rac-
[Ru(bpy)2(N^N)]Cl (N^N = L1–L5, i.e.: deprotonated HL1–HL5)
were prepared by refluxing an ethanol solution of the starting
material rac-cis-[Ru(bpy)2Cl2]·2H2O and the pro-ligands HL1–
HL5 in the presence of NaHCO3 (Scheme 1). We failed to obtain
the expected complex with L6 as a pure product. Instead, we
obtained a mixture of the desired product and the side-product
derived from ethanolysis of the F atom in the para position
(replacement of the F atom with a –OEt group, Table SI1 and
Scheme SI1).[40] All the Ru(II) derivatives exhibit C1-symmetry
and were isolated as racemic mixtures in the form of dark red
solids which are air- and moisture-stable.
Synthesis of the iridium complexes. The neutral Ir(III)
complexes of general formula rac-[Ir(ppy)2(N^N)] (N^N = L1, L4
and L5 for [IrL1], [IrL4] and [IrL5], respectively) were synthesized
by refluxing a mixture of rac-[Ir(-Cl)(ppy)2]2 and the appropriate
Crystal Structure. Attempts to get single crystals of the chloride
salts of the Ru complexes were fruitless. Conversely, we obtained
single crystals of [RuL4]PF6 suitable for X-Ray diffraction by slow
diffusion of an aqueous solution of NH4PF6 into a solution of
2
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[RuL4]Cl in methanol. The ORTEP diagram for cation complex of
()-[RuL4]PF6 is shown in
aryl substituent on the sulphonamide group influences
considerably the photostability of these derivatives. The Ir(III)
complexes are markedly less stable under light exposure. Indeed,
complexes [IrL1] and [IrL5] decompose completely after 24 and
Figure 1, whereas selected bond distances and angles and
crystallographic refinement parameters are collected in Tables
SI3 and SI4. The unit cell consists of two pairs of enantiomeric
cation complexes ( and ) plus four PF6− counteranions.
6
h respectively, while complex [IrL4] exhibit a 26 %
photodegradation after 24 h (see Figures SI10-SI12).
Electronic absorption properties
The UV-Vis absorption spectra of the above-mentioned Ru and Ir
complexes (see Figure 2 and data in Table 1) were recorded in
acetonitrile solutions (5·10-5 M) at 25 C. In particular, the
absorption spectra of complexes [RuL1]Cl-[RuL5]Cl are virtually
identical and show two intense absorption bands centred at about
247 and 293 nm, which can be attributed to singlet spin-allowed
ligand centred (1LC) transitions involving both bipyridyl ligands
and the ancillary L1-L5 ligands. Two weak, unstructured and
broad bands are also observed in the intervals 320-380 nm and
381-600 nm, approximately (see Figure 2 and Table 1). These
bands are assigned to spin-allowed 1MLCT, 1LLCT and spin-
forbidden 3MLCT (singlet to triplet d(Ru) → (N^N)), 3LLCT
3
(singlet to triplet, 3(N^N) → *(C^N)) and LC (singlet to triplet
3 → ) transitions.
−
Figure 1. ORTEP diagram for ()-[RuL4]PF6. The PF6 counteranion and
hydrogen atoms have been removed for the sake of clarity. Thermal ellipsoids
are shown at the 30% probability level.
The coordination polyhedron exhibits
a slightly distorted
octahedral geometry and it is formed by the three expected (N^N)-
chelate ligands: two bpy and the formally anionic L4. The Ru-Npy
bond distances are standard (2.045-2.058 Å) and shorter than the
Ru-N bond lengths determined for L4. More specifically, the latter
(Ru1-N5, 2.062 and Ru1-N6, 2.124 Å) are similar to those
reported for a related complex with a monoanionic ligand of the
type N-(carboxyaryl)amidoquinolate.[34] Moreover, the N-Ru-N
chelate angles compare well with those reported for similar
derivatives (78.30°-80.00°).[34] The 3D crystal network displays
dimeric entities formed by pairs of enantiomers (,), which
interact one to another through several hydrogen bonds: C1H1---
O3; C10SH10S---O2, C6SH6S---O2 (Figure SI3).
Figure 2. UV-Vis absorption spectra of [RuL1]Cl-[RuL5]Cl and [1]Cl2 (5·10-5
M) in acetonitrile at 25 °C and emission spectrum of the blue LED light used in
photocatalysis.
Table 1. Wavelengths (abs) and molar extinction coefficients () at the
absorption maxima and relative maxima in the UV-Vis spectra of complexes
[RuL1]Cl-[RuL5]Cl and [1]Cl2 measured in acetonitrile at 25 ᵒC.
Photostability of the photosensitizers
Photostability is a highly desirable property in the context of
photocatalytic reactions. In fact, the overall photocatalytic
performance of metallo-organic complexes is primarily limited by
their photolysis, which is thought to occur through the
photoinduced solvolysis of the metal-(N^N) bonds.[44] Hence, the
stability of complexes [RuL1]Cl-[RuL5]Cl and [Ru(bpy)3]Cl2
(henceforth denoted as [1]Cl2 or [1]2+) in CD3CN (1.4·10-2 M), first
Complex
[RuL1]Cl

abs [nm]
 [M-1 cm-1]
0.471, 0.505, 0.088,[a]
0.115[a]
247, 292, 340,[a] 483[a]
248, 293, 344,[a] 479[a]
0.398, 0.466, 0.080,[a]
0.103[a]
[RuL2]Cl
[RuL3]Cl
[RuL4]Cl
248, 292, 342,[a] 482[a]
242, 287, 333,[a] 460[a]
0.345, 0.389, 0.070,[a] 0.088[a]
0.471, 0.481, 0.085,[a] 0.107[a]
in the dark, and then under irradiation with blue light (LED, λirr
=
1
460 nm, 24 W) was monitored by H NMR over a period of 24 h
in a NMR tube capped with a septum under atmospheric air (see
Figures SI4-SI9). All the compounds are utterly stable in the dark.
On the other hand, complexes [RuL1]Cl-[RuL3]Cl and [1]Cl2
exhibited moderate photostability in deuterated acetonitrile, with
decomposition values of 17 %, 28 %, 16 % and 19 % after 24 h,
respectively (see Figures SI4-SI6 and SI9). Moreover, [RuL4]Cl
showed no symptoms of photodegradation and [RuL5]Cl
displayed only 2 % of decomposition after 24 h under irradiation
(see Figures SI7 and SI8, respectively). Thus, it seems that the
0.709, 0456, 0.476, 0.09,[a]
0.108[a]
[RuL5]Cl
[1]Cl2
221, 242, 287, 334,[a] 456[a]
238, 282, 445
0.275, 0.838, 0.152
[a] Midpoint of broad absorption band.
In comparison with the reference compound [1]2+, the
replacement of one bpy ligand on the former with the anionic
ligands L1-L5 broadens and shifts to the red the absorption profile
3
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Fc+/Fc, which is ascribed again to the RuII-(N^N) region (N^N =
L1-L5), and produces species of the type [RuIII-(N^N)•]3+ (vide
infra and see Scheme 2). In the cathodic region, all the new
complexes feature two common reversible peaks: the first one
between −1.83 and −1.92 V is imputed to the reduction of one
bipyridyl ligand ([RuII(bpy)2(N^N−)]+ → [RuII(bpy−)(bpy)(N^N−)])
and the second one, between −2.07 and −2.24 V, is attributed to
the reduction of the other bipyridyl ligand ([RuII(bpy−)(bpy)(N^N−)]
→ [RuII(bpy−)(bpy−)(N^N−)]−)[48] in agreement with the topology of
the LUMO level that is based on bpy(*) orbitals (See theoretical
calculations below). In the voltammogram of [RuL4]Cl, there are
extra peaks assigned to the typical reduction of the –NO2
group.[49] The electrochemical band gaps (E1/2) calculated from
the first reversible oxidation potential and the first reversible
reduction potential for the new PCs (2.07-2.22 V) are lower than
for [1]2+ (2.61 V) and anticipate a lower S0  T1 energy for the
new Ru complexes relative to [1]2+ (Table 2). Cyclic
voltammograms for [IrL1], [IrL4] and [IrL5] are also given in
Figure SI4 and Table SI5.
of complexes [RuL1]Cl-[RuL5]Cl and consequently improves
visible light harvesting.[34] The emission spectrum of the blue light
used in the photooxidation assays is also included in Figure 2 to
show the excellent overlapping with the absorption spectra of our
PSs. In other words, we conclude that excitation of our PSs is
optimal under this light source ( = 460 nm).
Photoluminescence spectroscopy
Attempts to record the emission spectra for the new Ru(II)
complexes in deoxygenated acetonitrile or DMSO solutions (10-5
M) at different excitation wavelengths and room temperature
failed. Therefore, these compounds are practically non-emissive
at room temperature in contrast to the reference complex
[1](PF6)2, which features moderate PLQY (Photoluminescence
Quantum Yield)[45] and long excited-state lifetime.[3] For this
reason, photoluminescence quantum yields and excited state
lifetimes could not be determined for the Ru-PCs. The iridium
derivatives [IrL1], [IrL4] and [IrL5] exhibit weak emission
properties but PLQY and excited state lifetimes could neither be
experimentally determined precisely due to the poor photostability
of these derivatives. Non-emissive complexes at room
temperature have scarcely been used as photosensitizers, likely
due to the widely accepted premise that a high PLQY and a long-
excited state lifetime ensure good performance to a particular
photocatalyst (PC). By contrast, we postulate that our non-
emissive Ru and Ir complexes could exhibit photocatalytic activity,
assuming efficient intersystem crossing typical of d6-Ru(II)
polypyridyl and d6-Ir(III) biscyclometalated complexes, and
provided that the photosensitizing process is kinetically faster
than the non-radiative decay. More precisely, in the context of O2-
mediated photooxidation reactions, the preliminary condition for a
PC should be that the short-lived excited state of the PC (PC*)
undergoes deactivation through interaction with O2 faster than
non-radiative decay.
•
Figure 3. Cyclic Voltammograms of complexes [RuL3]Cl and [1]Cl2 in
acetonitrile solution (10−3 M), using 0.1 M [nBu4N][PF6] as supporting electrolyte
and recorded with scan rate of 0.10 V·s−1. The starting and final potentials (Ei,
Ef) are indicated by (•) and the sense of the scan (clockwise) is indicated by the
arrow.
Electrochemical behaviour
The redox potentials of [RuL1]Cl-[RuL5]Cl and [1]Cl2 were
determined in degassed acetonitrile solutions by cyclic
voltammetry (CV) using [nBu4N][PF6] as supporting electrolyte
and glassy carbon as the working electrode (see cyclic
voltammograms in Figure SI13 and SI14, respectively).[46]
Potentials are given with respect to the ferrocene/ferrocenium
(Fc+/Fc) couple. Figure 3 illustrates the cyclic voltammogram of
[RuL3]Cl as a representative example of all the new Ru dyes and
that of [1]Cl2 for comparison purposes. Thus, in the anodic region
all the photosensitizers exhibit an irreversible oxidation peak
between +0.54 and +0.75 V, which is attributed to the oxidation of
the chloride counteranion (Cl− → Cl2). In addition, all the PSs
display a reversible redox peak between +0.22 and +0.30 V
versus Fc+/Fc, which is assigned to the RuII-(N^N) (N^N = L1-L5)
environment (see the hybrid topology of the HOMO level in DFT
calculations below,
Table 2. Cyclic voltammetry data referenced to Fc+/Fc in acetonitrile solution
(10−3 M).[a]
Complex
[1]Cl2
Eox1/2 (V)
Ered1/2 (V)
E1/2 (V)
-2.17 (r)
-1.82 (r)
-1.72 (r)
+0.89 (r)
2.61
+0.67 (ir)[b]
+1.28 (ir)
+0.56 (ir)[b]
+0.24 (r)
-2.07 (r)
-1.83 (r)
[RuL1]Cl
[RuL2]Cl
[RuL3]Cl
2.07
2.09
2.15
-2.19 (r)
-1.87 (r)
+1.26 (ir)
+0.65 (ir)[b]
+0.22 (r)
+1.30 (ir)
+0.58 (ir)[b]
+0.28 (r)
-2.16 (r)
-1.87 (r)
-2.24 (r)
-1.92 (r)
+1.36 (ir)
+0.75 (ir)[b]
+0.30 (r)
[RuL4]Cl
[RuL5]Cl
2.22
2.09
-1.38 (ir)[c]
-1.20 (ir)[c]
Figure 4 and Figure SI15). This involves a strong cathodic shift
relative to the reference compound [1]2+ which, on the other hand,
exhibits a localized metal-centred RuIII/RuII redox couple. This
shift is reasonable taking into account the anionic nature and
strong electron donating character of ligands L1 to L5.[34],[47]
Contrary to [1]Cl2, all the new dyes exhibit an extra irreversible
oxidation peak in the range between +1.26 and +1.36 V versus
+1.28 (ir)
+0.59 (ir)[b]
+0.23 (r)
-2.17 (r)
-1.86 (r)
[a] Measured using 0.1 M [nBu4N][PF6] as supporting electrolyte and a scan rate
of 0.10 V·s−1 (r = reversible, ir = irreversible). [b]Oxidation of Cl−. [c]Reduction of
the –NO2 group. E1/2 = Eox1/2 – Ered1/2 = first reversible oxidation potential – first
reversible reduction potential.
4
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LUMO+2
LUMO+2
LUMO+1
LUMO+1
LUMO
-1.86
-2.21
-2.37
LUMO
HOMO
LUMO
HOMO
-2.48
-2.58
LUMO
HOMO
2.89
3.56
-5.26
-5.76
• •
• •
HOMO
-6.14
• •
• • • •
-6.30
• •
-6.31
HOMO-2
HOMO-2
HOMO-1
HOMO-1
HOMO-3
Figure 4. Schematic representation showing the energies and the isovalue contour plots calculated for the frontier molecular orbitals of [1]2+ and [RuL3]+. HOMO-
3 is shown instead of HOMO-2 for [RuL3]+.
+
Scheme 2. Illustration of the oxidation processes for the new Ru(II) complexes. [RuIIIL3]2 and [RuIIL3•]2+ are the canonical forms proposed for the first oxidation
event, while [RuIIIL3•]3+ is the species formed upon the second electron oxidation event.
.
topologies imply a good covalent Ru(d)-L3() molecular orbital
overlap, as suggested by Rochford et al., for isostructural Ru(II)
complexes with N-carboxyamidoquinolate ligands.[34] Moreover,
the HOMO of [RuL3]+ is greatly destabilized (0.88 eV) compared
to the corresponding orbital of [1]2+, which can be imputed to the
higher electron donor ability of the formally anionic L3 ligand
Theoretical calculations
Density functional theory (DFT) and time-dependent DFT (TD-
DFT) calculations were performed on the cations of complexes
[RuL1]+-[RuL3]+ and also for the reference complex [1]2+ to get a
better understanding of the electrochemical and photophysical
properties discussed above. Calculations were carried out at the
B3LYP/(6-31GDP+LANL2DZ) level including solvent effects
(CH3CN) (see the Experimental Section for details). Calculations
predict a near-octahedral structure for cations [RuL1]+-[RuL3]+
and [1]2+ in their ground electronic state (S0), in good agreement
with the crystal structure of [RuL4]PF6.
versus bpy in [1]2+. This matches well with the experimental Eox
1/2
values (Table 2), since the oxidation of [RuL3]+ is easier than that
of [1]2+. Furthermore, the first oxidized form of [RuL3]+ is better
described by the combination of two canonical structures: the first
one involving a RuIII centre, [RuIIIL3]2+ and the second one a
radical L3• ligand, [RuIIL3•]2+ (see Scheme 2).[34] Rationally, the
second oxidation event experimentally observed for [RuL3]+, and
its analogues, likely leads to the formation of species of the type
[RuIIIL3•]3+ (see Scheme 2). In addition, the contribution of the aryl
groups (-Ar) to the frontier molecular orbitals (MO) and, in
particular to HOMO-3, responsible for the (Ru-L) bonding
overlapping, is negligible, revealing an ineffective conjugation
between the aryl groups and the fluorogenic backbone. This, in
turn, is consistent with the weak effect experimentally observed
on the photophysical and electrochemical properties of the new
ruthenium compounds exerted by the different aryl groups.
Figure 4 features the isovalue contour plots calculated for the
molecular orbitals (MOs) of [1]2+ and [RuL3]+ (as an example) at
their electronic ground state (S0). The HOMO and degenerate
HOMO-1 and HOMO-2 of [1]2+ are filled, mainly located in the
metal centre and exhibit a non-bonding character, whereas the
empty LUMO and degenerate LUMO+1 and LUMO+2 are located
in the bpy ligands and display a non-bonding character as well.
Hence, the Ru-N interactions in [1]2+ exhibit mainly a -bonding
character with negligible -bonding nature. By contrast, the
HOMO and HOMO-3 of [RuL3]+ consist of a mixture of d orbitals
of Ru(II) and  orbitals of the amidoquinolate fragment of L3 with
-antibonding (HOMO) and -bonding (HOMO-3) character at the
Ru-Namido interface, respectively (see Figure SI15). These
The LUMO of [RuL3]+ is fully located over a bpy ligand and is also
destabilized relative to the LUMO of [1]2+, but to a lesser extend
(0.21 eV), in agreement with the experimental Ered1/2 values (Table
2). Consequently, the HOMO-LUMO band gap is narrower for
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[RuL3]+ than for [1]2+ (2.89 vs 3.56 eV) in harmony with the
electrochemical band gaps (see E1/2 in Table 2). Moreover, the
low lying excited states of complexes [RuL1]+-[RuL3]+ and [1]2+
were calculated at the optimized geometries of the ground state
(S0) by mean of the time-dependent DFT (TD-DFT) approach.
Table SI6 summarizes the vertical excitation energies calculated
for the first three singlets and triplets, together with their molecular
orbital description and electronic nature. A summary of these
results is graphically shown in Figure 5. The three first triplet
states of [1]2+ are originated from HOMO → LUMO+1, HOMO →
LUMO+2 and HOMO → LUMO excitations, respectively, and all
of them display 3MLCT character. On the contrary, the three first
triplet states of [RuL1]+-[RuL3]+ essentially stem from HOMO →
LUMO, HOMO → LUMO+2 and HOMO → LUMO+1 excitations,
respectively, and are very close in energy. In particular, the
HOMO → LUMO excitation for the new dyes involves the
promotion of one electron from the Ru-(N^N) ((N^N) = L1, L2, L3)
(HOMO) environment to a bpy ligand (LUMO), and hence the T1
conversion of 1a into the imine 2a (entry 1, Table 3). In addition,
the transformation took place with excellent selectivity towards
the oxidative coupling product (2a), since neither overoxidation
products (e.g.: benzonitrile, benzamide, N-benzylhydroxylamine,
benzaldehyde oxime,[52] N-benzylidene-benzylamine N-oxide
(nitrone), or N-benzylbenzamide), nor hydrolysis products
(benzaldehyde) were detected (see Scheme SI3).
Table 3. Photocatalytic oxidative coupling of benzylamine 1a with [RuL1]Cl
and control experiments.[a]
Entry
Conditions
Yield (%)
1
2
3
4
5
6
[RuL1]Cl, O2, light
> 99
7
3
excited state exhibits a mixed MLCT/3LLCT character. Besides,
the first three triplet states for [RuL1]+-[RuL3]+ display lower E0
values (S0  T1) than those of [1]2+ in agreement with the strong
destabilization of the HOMO when a bpy ligand is replaced with a
formally anionic N-(arylsulfonyl)-8-amidoquinolate ligand (Figure
5). These calculations support the energy-gap law [knr  exp(−E0)]
as the main hypothesis for the non-emissive behaviour of the new
dyes.[34,50] According to this empirical law, the rate constant for
[RuL1]Cl, O2, no light
No PC, O2, light
5
[RuL1]Cl, N2, light
12
96
5
[RuL1]Cl, O2, light, TEMPO b
[RuL1]Cl, O2, light, DABCO c
non-radiative decay (knr) increases exponentially with
a
[a] Reaction conditions: Benzylamine 1a (10 mM), [RuL1]Cl (10-2 mM, 0.1
mol %), CH3CN (0.5 mL) at room temperature, under saturated
decreasing S0  T1 energy (E0).
a
atmosphere of either O2 or N2 (1 atm) and under irradiation with blue light
(LED,  = 460 nm, 24 W) during 14 h in a septum-capped tube. Conversion
yields were determined by 1H NMR analysis of the crude mixture as average
values of at least two independent experiments. Overoxidation and/or
hydrolysis products were not detected in any case. [b] TEMPO (11 mM). [c]
DABCO (11 mM).
T3
T2
2.59
2.54
T1: H → L+1 (3MLCT)
T2: H → L+2 (3MLCT)
T3: H → L (3MLCT)
T1: H → L (3MLCT/3LLCT)
T2: H → L+2 (3MLCT/3LC)
T3: H → L+1 (3MLCT/3LLCT)
T1
T3
2.20
2.12
T3
2.17
T3
2.15
T2
T1
T2
2.10
2.05
T2
T1
2.08
2.02
2.07
T1
Control experiments verified the participation of O2 and the
photocatalytic nature of this oxidation process, since oxygen, light
irradiation and the photosensitizer are all essential to accomplish
the transformation of 1a into 2a (entries 2, 3 and 4, Table 3). In
addition, an experiment in the presence of 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO, entry 5, Table 3), a O2•− scavenger,[53]
gave a yield of 96%. On the contrary, an experiment in the
presence of 1,4-diazabicyclo[2.2.2]octane (DABCO, entry 6,
Table 3), a singlet oxygen quencher,[8] supressed almost
quantitatively the oxidative coupling. These results suggest that
1O2 is the actual oxidant agent and rule out the participation of
superoxide in the mechanism of the reaction.
S0
S0
S0
S0
[1]2+
[RuL1]+
[RuL2]+
[RuL3]+
Figure 5. Energy diagram showing the energy values calculated for the lowest-
energy triplet excited states (Tn) of complexes [RuL1]+-[RuL3]+ and [1]2+
Different colours are used to display the different electronic nature of the Tn
states according to the chromatic key in the boxes.
.
Photocatalytic Oxidation of Benzylamines
Then, the new Ru(II) complexes and their Ir(III) counterparts were
tested as photocatalysts in the oxidative coupling of benzylamine
(1a) using the above-mentioned conditions. Complex [1]Cl2 was
also included in this screening with comparative purposes. Close
to quantitative transformations to the imine 2a were observed
after 14 hours for [RuL1]Cl-[RuL3]Cl, [RuL5]Cl and [1]Cl2 (see
entries 1-3, 5 and 9, Table 4). On the other hand, for [IrL1] and
[IrL5] moderate yields of 70 % and 59 % were determined (entries
6 and 8, Table 4). The worse performance of these Ir(III) PCs
compared to the Ru(II) analogues could be related to the lower
photostability of the formers in comparison to the Ru PCs.
Next, we decided to evaluate the photosensitizing properties of
the new Ru(II) and Ir(III) complexes in the photooxidation of
benzylamines, using CH3CN as solvent as previously reported in
the literature.[23,51] First of all, we chose benzylamine (1a) as
model substrate and [RuL1]Cl as the photosensitizer (0.1 % mol,
S/C = 1000) and irradiated a solution of both in acetonitrile with
blue light ( = 460 nm) under an O2 atmosphere (1 atm) at room
temperature during 14 hours. To our delight, we observed full
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solvents.[58],[59] Conclusively, complexes [RuL3]Cl and [RuL5]Cl
were selected for additional experiments due to their good
performance ([RuL3]Cl) and a combination of good photostability
and good activity ([RuL5]Cl).
Table 4. Catalyst screening in the photocatalytic oxidative self-coupling of
benzylamine 1a.[a]
To demonstrate the validity of this photocatalytic protocol, the
optimized conditions were applied to different primary and
secondary benzylamines (1b-1m) in the presence of either
[RuL3]Cl or [RuL5]Cl (Table 5). Thus, primary benzylamines 1b-
1f with both electron-donor or electron-withdrawing groups in the
para or ortho positions of the phenyl ring were oxidized to the
corresponding imines with excellent, 99 or >99 % (2b, 2c, 2d in
Table 5), or good yields, 95 % (2e) and 73 % (2f) (entries 2-6,
Table 5) and the respective functional groups (methoxy, methyl,
fluor or acetal) were well tolerated under the afore-mentioned
reaction conditions. Heterocyclic methylamines, such as 1g and
1h (entries 7-8, Table 5), were also efficiently transformed into the
expected self-coupled imines (90-89 %), even though 1h is known
to poison metal catalysts.[28] Moreover, the oxidation of non-
active[30] secondary benzylamines such as dibenzylamine (1i) and
1,2,3,4-tetrahydroisoquinoline (1j) proceeded satisfactorily to give
the corresponding imines 2a and 2j with high yields, >99 and 93 %,
respectively (entries 9-10, Table 5). Indeed, the oxidation of the
Entry
Photocatalyst
Time (h)
Yield (%)
1
[RuL1]Cl
[RuL2]Cl
[RuL3]Cl
[RuL4]Cl
[RuL5]Cl
[IrL1]
14
14
14
14
14
14
14
14
14
6
99
2
94
3
> 99
5
4
5
> 99
70
6
7
[IrL4]
3
8
[IrL5]
59
9
[1]Cl2
> 99
78
10
11
12
13
14
15
16
[RuL1]Cl
[RuL2]Cl
[RuL3]Cl
[RuL5]Cl
[RuL3]Cl
[RuL3]Cl
[RuL3]Cl
unsymmetrical secondary amine (1j) took place in
a
regioselective manner to give the conjugated N-benzylidene
product 2j (entry 10, Table 5). In additional experiments, full
conversion to the desired products was obtained for substrates
1e, 1f, 1g and 1h, when the photocatalyst loading was increased
to 1 mol % (see entries 5-8 in Table 5). By contrast, the application
of this protocol to other challenging substrates such as -
methylbenzylamine (1k), n-hexylamine (1l) or 1,2,3,4-
tetrahydroquinoline (1m) failed to produce the expected imines in
agreement with the results reported for other catalytic systems
(entries 11,[32],[60] 12[61],[62],[63],[64] and 13[26],[65] in Table 5).
Consequently, the presence of either steric hindrance on the -C
as in 1k, or nonactivated (non-benzylic) -H, as in 1l and 1m, is
strongly detrimental for this oxidation. In other words, the lower
reactivity of 1l and 1m can be rationalized due to the lower acidity
of the -H in these substrates. The regioselectivity in the oxidation
of substrate 1j further supports this interpretation. Nevertheless,
the new photocatalysts afford a wide range of secondary
aldimines from benzylamines with different functional groups in
the aromatic ring in an efficient and smooth manner.
6
75
6
91
6
66
1
69[b]
97[b]
96[b],[c]
2
1
[a] Reaction conditions: benzylamine 1a (10 mM), PC (10-2 mM, 0.1 mol %),
CH3CN (0.5 mL) at room temperature, under a saturated atmosphere (1 atm)
of O2 and under irradiation with blue light (LED,  = 460 nm, 24 W) during
the indicated time in a septum-capped tube. Conversion yields were
determined by 1H NMR analysis of the crude mixture as average values of
at least two independent experiments. Overoxidation and/or hydrolysis
products were not detected in any case. [b] [RuL3]Cl (1 mol %). [c] Reaction
performed in CD3CN.
Moreover, the two complexes with a nitro group in the
sulphonamide ligand, [RuL4]Cl and [IrL4], provided very low
yields (entries 4 and 7 in Table 4), suggesting that the presence
of this functional group deactivates the excited state of the
respective PC as previously reported.[54–57] Further experiments
were carried out for the active Ru(II) PCs decreasing the reaction
time until 6 h to identify the most efficient PC and to establish
possible structure-activity relationships (entries 10-13 in Table 4).
Thus, similar results were obtained for [RuL1]Cl, [RuL2]Cl and
[RuL5]Cl, whereas for [RuL3]Cl the yield was remarkably higher
which suggests that the presence of the electron-withdrawing
group –CF3 is beneficial for the reaction rate in some way. Then,
the catalyst loading of [RuL3]Cl was increased up to 1 mol % to
show that high yields of 2a can also be obtained at low reaction
times (entries 14 and 15 in Table 4). Besides, we demonstrated
that the yield of 2a is higher when the reaction is performed in
CD3CN relative to CH3CN (compared entries 14 and 16 in Table
Proposal of a reaction mechanism
We have formulated a detailed mechanistic proposal for the Ru-
promoted photooxidative coupling of benzylamines based on both
our experimental results (Table 5) and the literature
background[66] (see Scheme 3). This proposal consists of three
different processes: (a) photosensitization of 3O2 to 1O2; (b)
oxidation of benzylamine to the respective primary aldimine and
(c) coupling between the primary aldimine and a second molecule
of benzylamine.
1
4), which is consistent with the participation of O2 as the actual
oxidant since the lifetime of this species is longer in deuterated
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Table 5. Photooxidation experiments on amines 1a-1m with [RuL3]Cl or [RuL5]Cl under an O2 atmosphere and blue-LED irradiation ( = 460 nm).[a]
Entry
1
Substrate
PC
Product
Time (h)
Yield (%)
[RuL3]Cl
> 99
14
2
14
14
> 99
> 99
[RuL3]Cl
3
4
[RuL3]Cl
[RuL5]Cl
14
14
99
5
[RuL3]Cl
95 (>99)[b]
6
[RuL3]Cl
[RuL3]Cl
[RuL5]Cl
[RuL3]Cl
[RuL5]Cl
[RuL3]Cl
[RuL3]Cl
[RuL3]Cl
14
14
14
14
14
14
14
14
73 (> 99)[b]
7
90 (> 99)[b]
8
89 (>99)[b]
9
> 99
93
0
10
11
12
0
0
13
[a] Reaction conditions: Amines 1a-1m (10 mM), [RuL3]Cl or [RuL5]Cl (10-2 mM, 0.1 mol %), CH3CN (0.5 mL) at room temperature, under a saturated
atmosphere of O2 and under irradiation with blue light (LED,  = 460 nm, 24 W) during 14 h in a septum-capped tube. Conversion yields were determined by 1H
NMR analysis of the crude mixture as average values of two independent experiments. Overoxidation and/or hydrolysis products were not detected in any case.
[b] [RuL3]Cl or [RuL5]Cl (1 mol %).
8
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substrates 1l and 1m and by the regioselective oxidation of 1j.
Seeberger et al. postulated the formation of superoxide as an
intermediate in the mechanism for the photosensitized oxidation
of dibenzylamines mediated by 1O2 in the presence of Ir(III)
PCs.[66] However, our experimental results rule out the formation
of this species under the conditions of our protocol.
(a) Photosensitization. We postulate that the prototypical Ru(II)
photocatalyst, [RuII], absorbs photons under irradiation with blue
light to reach an excited singlet state, 1[RuII]*, which then
undergoes intersystem crossing to the lowest-energy triplet
excited state, T1
=
3[RuII]*. Afterwards, 3O2 quenches 3[RuII]*
1
through an energy transfer process to generate O2 (Scheme 3)
as revealed by experimental evidences (see Table 7).
(c) Amine-imine coupling. Next, we propose the activation of the
primary aldimine by in situ formed acidic H2O2,[70] which generates
intermediate A. Consequently, the electrophilic carbon atom of
the protonated aldimine A undergoes an attack by the nucleophilic
N atom of a second molecule of benzylamine to give the amine-
ammonium intermediate B. After that, we suggest an internal
(b) Oxidation. As detailed in Scheme 4, we propose that the
oxidation of benzylamine by 1O2, first, involves the formation of a
charge-transfer exciplex[66] between singlet oxygen and the amine
based on the electrophilic nature of singlet oxygen[67–69] and the
nucleophilic character of the amine substrate. This step is
followed by two sequential single electron transfer (SET) steps
and two internal proton transfer (PT) processes and gives rise to
the primary aldimine and one molecule of H2O2. The acidity of the
-H in intermediate III when using benzylamines (vs. alkyl- or aryl-
amines) is crucial as proved by the negative results obtained with
+
proton transfer process that forms species C where -NH3 is a
very good leaving group. Consequently, the release of NH3 is
strongly favoured affording the final product (secondary aldimine)
and regenerating the acidic catalyst H2O2 (see Scheme 3).
Scheme 3. Mechanism for the photocatalytic oxidative self-coupling of benzylamine to give the corresponding N-benzylimine in the presence of the Ru(II)
photosensitizers and mediated by 1O2 and global chemical equation for the same process.
Scheme 4. Mechanism for the electron transfer step, i.e. oxidation step of benzylamine to the primary aldimine.
9
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In consequence, three key features must be highlighted in this
mechanism: the photosensitizing ability of the Ru(II) complexes
that promotes the production of 1O2; the electrophilic nature of 1O2
that facilitates its reaction with nucleophilic amines and the weak
acidic character of the in situ generated H2O2 that assists in the
catalytic activation of the primary aldimine.
0.0
-0.1
-0.2
320 nm
435 nm
Transient Absorption Spectroscopy Measurements.
In order to investigate the quenching process in which the triplet
excited states of complexes [RuL3]Cl and [RuL5]Cl are involved,
their transient absorption spectra were acquired in the absence
and in the presence of O2 and are presented in Figure SI26 (in
orange). The spectral range above 365 nm reveals a negative
optical density variation (ΔOD). This bleach means that the
ground state extinction coefficient is higher than the excited state
extinction coefficient along that range. On the other hand, at 320
nm, the transient spectra exhibit a positive ΔOD, because there is
an absorption band of the excited state with a stronger absorption
coefficient than the ground state absorption band (Figure SI26).
UV-Vis absorption spectra for [RuL3]Cl and [RuL5]Cl remained
unchanged before and after Transient Absorption Spectroscopy
experiments, indicating that no photodegradation occurred
throughout the full set of experiments (Figure SI27).
0
20
40
60
80
100
time (ns)
0.05
0.00
320 nm
435 nm
Figure 6 features the kinetic traces corresponding to the excited
states of [RuL3]Cl and [RuL5]Cl at 320 nm (absorption decay)
and 435 nm (bleach recovery) in deaerated acetonitrile solutions.
Both traces go back to zero with the same rate constant for each
complex (see Table 6). Moreover, the respective kinetic traces in
the presence of oxygen return fully to baseline, as well (Figure
SI28). The total recovery of the observed transients to the ground
state in the very short time scale of the measurements makes us
confident to assign the quenching by oxygen to energy transfer.
The ground state recovery furthermore is fitted with a single
exponential law. The absence of any observable intermediate
species and the total recovery of traces (final ΔOD is zero)
suggests that photoinduced electron transfer to produce
superoxide either does not exist or does not lead to the escape of
superoxide due to fast recombination to the same initial ground
state reactants.
-0.05
0
20
40
60
80
100
time (ns)
Figure 6. Time-resolved transients at 320 nm (absorption decay) and 435 nm
(bleach recovery) recorded (λex = 355 nm) for deaerated acetonitrile solutions
of [RuL3]Cl (up) and [RuL5]Cl (Bottom).
Table 6. Summary of recorded decay-times of the transients observed at 320
nm in acetonitrile (A (λex = 355 nm) ~ 0.2) for [RuL3]Cl and [RuL5]Cl and
calculated quenching rate constants.
Lifetimes at 320 nm (ns)
Compound
kq (L·mol-1·s-1)
without O2
with O2
6.0
[RuL3]Cl
[RuL5]Cl
7.9
4.6
1.7  1010
1.6  1010
Recorded decay rates at 320 nm for both compounds, on aerated
and degassed conditions, are presented in Table 6. Although the
lifetimes are very short (close to the laser pulse width) which
introduces an error in the absolute lifetime values obtained
without deconvolution of the laser pulse, there is a reproducible
difference between the lifetimes of the transient at 320 nm in the
presence and absence of oxygen, compatible with quenching by
oxygen. Using a simple Stern-Volmer analysis (eq. 1):
3.9
Detection of H2O2. H2O2 was detected by 1H NMR (CD3CN) as a
broad peak between 8.3 and 8.7 ppm in the reaction crude
mixtures obtained for the photocatalytic oxidation of substrates
1a-1j (except 1g) in close to an equimolar ratio with respect to
each imine product (Figures SI16-SI24).[17] This experimental
evidence further supports the mechanism detailed in Scheme 3
andScheme 4. Alternative pathways have been described in the
literature for the photocatalytic oxidation of benzylamines in the
presence of O2, but either they do not involve the formation of
hydrogen peroxide[29] or propose that the in situ generated H2O2
acts as a secondary oxidant and therefore is consumed.[27]
0/ = 1 + kq·0·[O2]
(eq. 1)
where 0, is the lifetime in degassed solutions; , is the lifetime in
aerated condition and [O2] is the concentration of O2 found in air
equilibrated acetonitrile solutions ([O2] = 2.34  10-3 M at room
temperature),[71] it is possible to calculate the rate constant for the
quenching by oxygen process, kq (Table 6). The quenching rate
constants obtained in this way are of the same order of magnitude
as the value known for oxygen diffusion in acetonitrile (1.9  1010
L·mol-1·s-1) at room temperature.[71]
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(2.423 mmol) was dissolved in dichloromethane (12 mL), cooled with an
ice bath and stirred for a few minutes. Then, this solution was added slowly
at 0 C over the first one, and the resulting solution was stirred at room
temperature for 22 h under nitrogen atmosphere. The evolution of the
reaction was monitored by TLC. Subsequently, an aqueous saturated
solution of NaHCO3 (3 X 20 mL) was added to extract the inorganic salts.
The organic phase was separated and dried over MgSO4. After that, the
suspension was filtered, and the filtrate was concentrated under reduced
pressure. Finally, the crude mixture was purified by column
chromatography on silica gel (CH2Cl2/AcOEt, (10:1)) to obtain the desired
pure ligands.
Table 7. Experimental evidences for the participation of 1O2 in the
photooxidation of benzylamines promoted by the new Ru(II) PCs.
Entry
Evidences
Experiment
Low conversion in the
presence of DABCO.
1
Table 3 (entry 6)
Higher conversion in the
presence of deuterated
acetonitrile.
Total decay of [RuL3]Cl
transients in the presence
of O2.
Table 4 (compare entries
14 and 16)
2
3
TAS experiments
General experimental procedure for the synthesis of complexes
[RuL1]Cl-[RuL5]Cl
In
a 100 mL Schlenk flask, previously purged with nitrogen, the
Conclusion
corresponding pro-ligand (0.170 mmol) and rac-cis-[Ru(bpy)2Cl2]·2H2O
(0.170 mmol) were dissolved in ethanol (19 mL). Then, NaHCO3 (0.849
mmol) was added and the suspension was stirred under reflux at 90 °C for
20 h under nitrogen atmosphere. The resulting mixture was cooled at room
temperature and filtered to remove the excess of NaHCO3. After that, its
volume was reduced by evaporating the solvent under reduced pressure.
The product was precipitated with diethyl ether (10 mL) and filtered. The
dark red solid crude was washed with water (5 mL), diethyl ether (5 mL),
filtered and dried under vacuum.
In summary, we have designed and prepared five new Ru(II)
polypyridyl complexes and three related Ir(III) biscyclometalated
derivatives bearing N-(arylsulfonyl)-8-amidoquinolate ligands, as
potential photocatalysts. The Ru(II) PSs exhibit moderate or
excellent photostability under blue light irradiation, whereas their
Ir(III) counterparts are very light-sensitive. The Ru(II) complexes
harvest visible light efficiently but are non-emissive, which could
be related to the low S0  T1 energy gap as shown by DFT
calculations and electrochemical data. Furthermore, we have
demonstrated that the Ru(II) PCs display excellent photocatalytic
activity in the O2-mediated oxidation of primary or secondary
benzylamines, despite being non-emissive. The Ir(III) derivatives
are less efficient photosensitizers likely due to their
photosensitivity. More specifically, the protocol for the preparation
of the corresponding secondary aldimines is green, safe, efficient
and highly selective, given that it requires low catalyst loadings,
low O2 pressure and works smoothly at room temperature,
providing excellent yields of the imine products in short times. In
addition, it can be applied to a wide variety of substrates with
different functional groups, while no over-oxidation or hydrolysis
side-products are formed in any case. Finally, for this
transformation, we propose a detailed mechanism mediated by
1O2 as the actual oxidant in view of the experimental results (TAS
measurements and others). As a logical corollary of this work we
have demonstrated that the use of monoanionic N-(arylsulfonyl)-
8-amidoquinolate ligands in the design of polypyridyl Ru(II)
complexes shortens their S0  T1 energy gap relative to
[Ru(bpy)3]2+, which in turn, improves their visible light harvesting
ability at the expense of a non-emissive behaviour. Nonetheless,
the resulting complexes are still active photocatalysts, likely owing
to the fact that the photosensitizing process is faster than the non-
radiative decay. Conclusively, we believe that this work could
pave the way for the study of the photocatalytic activity of similar
monocationic [Ru(bpy)2(N^N)]+ complexes or even other non-
emissive dyes.
General experimental procedure for the synthesis of complexes
[IrL1], [IrL4] and [IrL5]
In
a 100 mL Schlenk flask, previously purged with nitrogen, the
corresponding pro-ligand (0.186 mmol) and rac-[Ir(-Cl)(ppy)2]2 (0.094
mmol) were dissolved in 18 mL of a mixture of CH2Cl2/MeOH (8:10). An
excess of NaHCO3 (0.932 mmol, 1:5 with respect to the respective pro-
ligand) was added, and the suspension was stirred and refluxed at 60 °C
for 24 h under nitrogen atmosphere. The resulting mixture was cooled at
room temperature and the solvents were evaporated under reduced
pressure. Then, the crude solid was washed with water (2 X 3 mL) and n-
hexane (2 X 3 mL) and dried under vacuum for 4 h.
X-ray crystallography
Data collection and refinement parameters for [RuL4]PF6 are summarized
in Table SI3 of the Supporting Information. A single crystal of the complex
was coated with high-vacuum grease, mounted on a glass fiber, and
transferred to a Bruker SMART APEX CCD-based diffractometer equipped
with a graphite-monochromated MoK radiation source (=0.71073 a).
The highly redundant datasets were integrated with SAINT[72] and
corrected for Lorentzian and polarization effects. The absorption correction
was based on the function fitting to the empirical transmission surface as
sampled by multiple equivalent measurements with the program
SADABS.[73] The software package WingX[74] was used for space-group
determination, structure solution, and OLEX 2 1.2.10[75] was used for
refinement by full-matrix least-squares methods based on F2. A successful
solution by direct methods provided most non-hydrogen atoms from the E
map. The remaining non-hydrogen atoms were located in an alternating
series of least squares cycles and difference Fourier maps. All non-
hydrogen atoms were refined with anisotropic displacement coefficients.
Hydrogen atoms were placed by using a riding model and included in the
refinement at calculated positions.
Experimental Section
CCDC 1970014 [RuL4]PF6 contains the supplementary crystallographic
data for this paper. These data are provided free of charge by The
Cambridge Crystallographic Data Centre.
General experimental procedure for the synthesis of ligands HL1-HL6
In a 100 mL Schlenk flask, 8-aminoquinoline (1.615 mmol) was dissolved
in dichloromethane (12 mL). Then, triethylamine (2.418 mmol) was added
to the solution at 0 °C and the mixture was stirred for 10 minutes. After that,
in a different 100 mL Schlenk flask, the appropriate arylsulfonyl chloride
11
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10.1002/chem.202001460
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Theoretical calculations
setup composed of a LKS 60 ns laser photolysis spectrometer from
Applied Photophysics, with a Brilliant Q-Switch Nd:YAG laser from Quantel,
using the third harmonics (λex = 355 nm, laser pulse half-width equal to 4
ns). Measurements were repeated in degassed and aerated samples.
Density functional theory (DFT) calculations were carried out with the D.01
revision of the Gaussian 09 package,[76] using the Becke’s three-
parameter B3LYP exchange-correlation functional,[77,78] together with the
6-31G(d,p) basis set for H, C, N, O, F, and S,[79,80] and the “double-”
quality LANL2DZ basis set for the Ru element.[81] The geometries of the
singlet ground state (S0) and the lowest-energy triplet state (T1) were fully
optimized without imposing any symmetry restriction. The geometries of
the triplet states were calculated at the spin-unrestricted UB3LYP level
with a spin multiplicity of 3. All the calculations were performed in the
presence of the solvent (acetonitrile). Solvent effects were considered
within the self-consistent reaction field (SCRF) theory using the SMD
keyword that performs a polarized continuum model (PCM)[82] calculation
using the solvatation model of Thrular et al.[83] Time-dependent DFT (TD-
DFT) calculations of the lowest-lying 15 singlets and triplets were
performed in the presence of the solvent at the minimum-energy geometry
optimized for the ground state (S0).
Acknowledgements
We gratefully acknowledge the financial support provided by the
Spanish Ministerio de Ciencia, Innovación y Universidades
(RTI2018-100709-B-C21 and CTQ (QMC)- RED2018-102471-T)
and Junta de Castilla y León (BU087G19). M. V. is grateful for the
financial support received from the Consejería de Educación-
Junta de Castilla y León-FEDER (BU042U16-BU305P18).
We are also indebted to Pilar Castroviejo and Marta Mansilla (PCI
of the Universidad de Burgos) and Javier Pérez (Photophysics
Unit of Institute of Chemical Research of Catalonia (ICIQ)) for the
technical support and Dr. José Vicente Cuevas Vicario for his
advices regarding theoretical calculations.
Electrochemical measurements
Electrochemical measurements were performed using
a portable
Keywords: Ru(II) polypyridyl complexes • Sulfonamides •
Photooxidation of amines •Photocatalysis • Singlet oxygen.
Bipotentiostat STAT 300 (DropSens) equipment controlled by DropView
(DropSens). All experiments were carried out using a three-electrode cell,
namely: a glassy-carbon disk with a diameter of 3 mm as the working
electrode, a platinum wire as the auxiliary electrode, and a RE-5B Ag/AgCl
(BASi) reference electrode. Oxygen was removed from the solution by
bubbling argon for 10 min and keeping the argon atmosphere along the
whole experiment. The measurements were recorded for acetonitrile
solutions of the complexes (10−3 M) in the presence of [nBu4N][PF6] (0.1
M) as the supporting electrolyte by cyclic voltammetry (CV) at a scan rate
of 100 mV s−1 in a clockwise direction. Ferrocene was added at the end of
all the experiments as the internal reference in order to refer the potentials
to the redox pair ferrocenium/ferrocene (Fc+/Fc) under the conditions of
our experiments. The potential experimentally determined for the redox
couple Fc+/Fc was E1/2 = 0.46  0.005 V vs. Ag/AgCl. Therefore, the
experimental redox potentials were calculated from the corresponding
voltammograms as: Eº (vs AgCl/Ag) = (Eap + Ecp)/2, for reversible peaks
where Eap and Ecp stand for anodic and cathodic peak potentials,
respectively. However, for irreversible peaks, the potentials were
calculated as either the Eap maximum or Ecp minimum. Eº (vs Fc+/Fc) = Eº
(vs AgCl/Ag) – 0.443, for potential values reported in reference to the
(Fc+/Fc) redox couple.
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13
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Entry for the Table of Contents
A family of monocationic non-emissive Ru(II) polypyridyl complexes exhibit efficient and selective photocatalytic activity in the
photooxidation of benzylamines to imines. The presence of the formally anionic N-(arylsulfonyl)-8-amidoquinolate ligands enable these
dyes to harvest visible light effectively, while their non-emissive behavior is related to their low energy gaps. Experimental evidences
suggest that 1O2 is the actual oxidant.
Institute and/or researcher Twitter usernames: @MVaqueroG; @UBUinvestiga; @UBUEstudiantes
14
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