Intramolecular light induced activation of a Salen-MnIII complex by a ruthenium photosensitizer

Article abstract of DOI:10.1039/c0cc01710h

We have designed a molecular system consisting of a heteroleptic [Ru(bpy)₂L]²⁺ chromophore covalently linked to a Mn ^III-Salen unit. We demonstrate the light induced oxidation of the Mn^III center in this putativ

Full text of DOI:10.1039/c0cc01710h

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Intramolecular light induced activation of a Salen–MnIII complex
by a ruthenium photosensitizerw
Christian Herrero,*a Joseph L. Hughes,a Annamaria Quaranta,a Nicholas Cox,b
A. William Rutherford,a Winfried Leibla and Ally Aukaulooac
Received 2nd June 2010, Accepted 19th August 2010
DOI: 10.1039/c0cc01710h
We have designed a molecular system consisting of a hetero-
leptic [Ru(bpy)2L]2+ chromophore covalently linked to a
MnIII–Salen unit. We demonstrate the light induced oxidation
of the MnIII center in this putative photo-catalyst assembly to a
MnIV high spin intermediate. Both oxidation states have been
characterized by transient absorption and EPR techniques.
The design of photoactive molecules for light-induced catalysis
has attracted significant interest in recent years.1–3 The
challenge is to develop robust systems where light absorption
can initiate electron transfer events that lead to the activation
of a catalytic center and subsequently product formation in
high yields. Light-driven catalytic reductive processes have
been reported for decades,4–6 however, photo-induced
oxidation reactions have been difficult to attain.7,8 Such
Scheme 1 Synthetic pathway for the formation of Ru–Salen–Mn.
achievements would find application in the photochemical
aspects of solar energy conversion9 and in the realm of green
chemistry.10 Moreover, such an approach may also offer
potential new ways for studying catalytic reaction mechanisms.
In this communication we report the synthesis of a ruthenium(II)
tris-bipyridine analogue [Ru(bpy)2L]2+ (photosensitizer) that is
covalently linked to a Salen–Mn unit, which is commonly known
as the Jacobsen catalyst (Scheme 1). When activated by chemical
oxidation, MnIII coordinated in the salen cavity is capable of
promoting the catalytic epoxidation of olefins.11 The mechanism
of this two-electron reaction is not yet fully understood, but it has
been hypothesized to involve a MnV–oxo species.12,13 Our aim is to
access such high-valent Mn states by exploiting the oxidation
potential generated at the photosensitizer (B1.30 V vs. SCE) by its
excitation with visible light (420–530 nm) in the presence of an
electron acceptor (Scheme 2).14 The Salen–Mn system was
selected due to its known chemical reactivity and the available
published spectroscopic data on the intermediates generated
when activated by oxidants.15
Conditions: (i) CH3COOH, p-hydroxybenzaldehyde, NH4OAc, 3 h,
100 1C, 68% yield; (ii) CF3COOH, HMTA, 3 days, 100 1C, 56% yield;
(iii) MeOH, mol sieves, TEA, 18 h, 67% yield; (iv) MeOH,
Ru(bpy)2Cl2, 60% yield; (v) AcN, Mn(OAc)2, air, NaCl, 57% yield.
salicylaldehyde analogue 3 was performed via a modified Duff
reaction. Formation of the dissymmetric salen complex 5 was
achieved by condensation of 3 with the protected mono-imino
synthon 4,17 which had been obtained by reaction of salicylal-
dehyde with the mono chloride salt of 1,2-diaminocyclohexane
(mixture of cis and trans isomers). Synthesis of 6 was performed
by coordination of Ru(bpy)2Cl218 to the phenanthroline end of
5. Target compound 7 (abbreviated as RuII–Salen–MnIII) was
obtained by insertion of manganese (II) acetate in the salen
cavity. MnII was oxidized to its MnIII state by air oxidation and
subjected to anion exchange with NaCl.
UV/vis absorption of RuII–Salen–MnIII (Fig. 1, trace a) in
acetonitrile shows bands at 290 nm, 330 nm, and a double-
humped MLCT band peaking at 460 nm, all of which
are attributable to electronic transitions characteristic of
[Ru(bpy)3]2+. The change to a heteroleptic system containing
the Salen–MnIII moiety does not significantly alter either the
steady-state absorption characteristics of the chromophore
compared to those of the parent complex [Ru(bpy)3]2+ or
the electronic configuration of the MnIII ion coordinated in
either the salen complex 8 (trace e) or the bi-metallic complex 7
(trace f), as it is shown by parallel mode EPR experiments
The synthetic pathway leading to the target compound 7 is
summarized in Scheme 1 (see also ESIw). 1,10-Phenanthroline-
5,6-dione 116 was reacted with p-hydroxybenzaldehyde via a
Steck–Day reaction in order to form the phenol precursor 2.
Mono-formylation of this compound in order to obtain the
a
´
CEA, iBiTec-S, Service de Bioenergetique Biologie Structurale et
Mecanismes (SB SM), F-91191 Gif-sur-Yvette, France.
´
2
´
E-mail: christian.herrero@cea.fr
b Max Planck Institute fur Bioanorganische Chemie,
¨
Stiftstrasse 34-36, D-45470, Mulheim an der Ruhr, Germany
¨
Institut de Chimie Moleculaire et des Materiaux d’Orsay,
c
´
´
UMR-CNRS 8182, Universite de Paris-Sud XI, F-91405 Orsay, France
´
Scheme
2
Light-induced reactions of the RuII–Salen–MnIII
w Electronic supplementary information (ESI) available: Experimental
procedures for synthesis, EPR, flash photolysis and light-activation
studies. See DOI: 10.1039/c0cc01710h
photosensitizer–catalyst in the presence of irreversible electron
acceptor, NBD.
c
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Chem. Commun., 2010, 46, 7605–7607 7605

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acetonitrile solution at 201C, both fast (t1/2 o 50 ns) and slow
(t1/2 E 110 ꢀ 20 ms, Fig. 2 inset) recovery phases of the 1MLCT
were found in the presence of B40 mM 4-nitrobenzenedi-
azonium tetrafluoroborate (NBD) as irreversible electron acceptor.
The fast recovery phase is attributed to the oxidative quench-
ing of the photo-generated [RuIII–Lꢁꢂ]* excited state by the
NBD electron acceptor, forming [RuIII–L] (eqn 2). This phase
correlated with the appearance (t1/2 o 50 ns) of an absorption
peaking near 800 nm that was stable for >10 ms. Absorption
control experiments on RuII–Salen complex (6) showed forma-
tion of a similar band, while EPR experiments on the same
complex indicated the formation of an organic radical species
(ESIw, Fig. S2 and S3). We therefore tentatively attribute this
absorption to Mn-free centers (B10–15% according to absorp-
tion; see ESIw, Fig. S2) in the nominally RuII–Salen–MnIII
sample (7),20 where reduction of RuIII by the ligand would lead
to subsequent generation of a phenoxyl radical species
ꢁ
[Ru –L]–[Salen] - [Ru –L]–[Salen +], although this absorp-
tion spectrum appears to be substantially narrower and blue-
shifted compared to published established values closer to
900–1000 nm.15
III
II
The slow phase of 1MLCT recovery was matched by the
appearance of an absorption peaking at 680 nm that was
stable for seconds in acetonitrile and is shown at t = 100 ms in
Fig. 2 as trace (c). The inset in Fig. 2 shows kinetic data for the
absorption changes at 460 nm (triangles) and 680 nm (squares)
from 3 ms to 700 ms. The data can be fit by single exponentials
(solid traces) with t1/2 = 110 ꢀ 20 and 70 ꢀ 20 ms, respectively.
The slow phase of RuII–polypyridyl 1MLCT recovery is
attributed to an intramolecular electron transfer from MnIII
Fig.
1 Room temperature absorption spectra in acetonitrile of
RuII–Salen–MnIII (7), RuII–Salen (6), Salen–MnIII (8), and Salen ligand,
traces (a), (b), (c), and (d) respectively. Inset: X-band parallel mode
EPR spectra of Salen–MnIII (e) and RuII–Salen–MnIII (f) complexes in
4 : 5 (v/v) propionitrile/butyronitrile. EPR measurement conditions:
temperature 5 K, microwave frequency 9.34 GHz, microwave power
32 mW, modulation amplitude 8 G, time constant 163.8 ms.
(see Fig. 1 inset). Salen–MnIII complex 8 exhibits a very weak
absorption at wavelengths longer than 440 nm (trace c),
compared to the RuII–polypyridyl MLCT transitions
(trace b). Thus, excitation of the RuII–Salen–MnIII complex
1
at 450–530 nm leads to the formation of Ru MLCT.
In the absence of electron acceptor, transient absorption
measurements at 20 1C on the RuII–Salen–MnIII complex (7)
showed that the recovery of the RuII–polypyridyl ground state
absorption following a B10 mJ, 7 ns flash at 460 nm exhibited
biphasic kinetics, with fast (t1/2 o 50 ns) and slow (t1/2
=
800 ꢀ 50 ns) components, matching the 3MLCT emission
(see ESIw, Fig. S1). This is in contrast to the parent complex 6,
where only one lifetime in acetonitrile was observed (t1/2
850 ꢀ 50 ns), typical for RuII–polypyridyl complexes.19
=
The fast (t1/2 o 50 ns) transient in 7 is attributed to quenching
of the 3MLCT* RuII–polypyridyl state by energy transfer to
low-lying excited states associated with the MnIII. The slow
component in the RuII–Salen–MnIII complex is attributed to
Mn-free centers. The decrease in the emission yield for the
nominally RuII–Salen–MnIII complex compared to 6 indicated
that there were typically B10–15% of Mn-free centers.
Cyclic voltammetry of RuII–Salen–MnIII in acetonitrile
shows two quasireversible waves at 1.39 V and 0.86 V
(vs. SCE) that were attributed to the RuII/RuIII and MnIII/MnIV
couples, respectively. The potential of the MnIII/MnIV couple in
the Ru–Salen–Mn complex is not significantly different from the
reference Salen–Mn, complex 8, at 0.79 V vs. SCE (see ESIw).
These electrochemical results show that upon light-induced
formation of RuIII there is over half a volt of driving force to
oxidize the MnIII ion. Following an B7 ns flash at 532 nm in
Fig. 2 Transient absorption spectra of RuII–Salen–MnIII (B0.16 mM)
in acetonitrile in the presence of an irreversible electron acceptor
(B40 mM NBD) following an B7 ns, B30 mJ flash at 532 nm. Spectra
shown at 3 ms, 500 ms and 100 ms as traces (a), (b) and (c), respectively.
Kinetic data, normalized and overlaid for comparison, are shown for
the slow phase (t1/2 E 100 ms) of RuII–polypyridyl 1MLCT recovery at
460 nm (inset, triangles) and the matching formation of phenolate–
MnIV LMCT absorption at 680 nm (inset, squares).
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Continuous irradiation of the oxidized sample in the
presence of irreversible electron acceptor NBD leads to the
disappearance of the MnIV EPR signal. We attribute this
decay to either dark reactions where the MnIV ion is reduced
back to its original MnIII state or to light reactions in which
the MnIV state is further oxidized to MnV. Preliminary results
indicate that the decay of the MnIV signal is dependent on
several factors such as solvent and axial ligation to the Mn
metal center.
The results presented here demonstrate that in our Ru–Salen–
Mn complex, we can induce the intramolecular oxidation of
MnIII to MnIV by visible light absorption of the photosensitizer
component, and subsequent electron transfer from the Salen–Mn
moiety. This work opens the way for controlled light-induced
activation of the Jacobsen catalyst23 as well as providing a tool
by which identification of intermediates could be achieved. Work
in this direction is currently ongoing in our laboratory.
This work was supported by ANR-HYPHO, the EU/
Energy SOLAR-H2 project (FP7 contract 212508), and by
Fig. 3 X-Band EPR spectra of Ru–Salen–Mn showing light-induced
conversion from MnIII to MnIV (see text for details). EPR measure-
ment conditions: temperature 4 K, microwave frequency 9.61 GHz
(perpendicular mode), 9.34 GHz (parallel mode), microwave power
32 mW, modulation amplitude 11.37 G, modulation frequency
100 kHz, time constant 163 ms, sampling time 82 ms, gain 40 dB.
the Conseil Gene
´ ´
ral de l0Essonne. JLH was supported in
part by a Human Frontiers Science Program Fellowship,
LT00710/2008.
(eqn 3), as it matches the appearance of the 680 nm absorption,
and therefore excludes the formation of a ligand centered
radical as an intermediate. This absorption is well established
as a phenolate–MnIV LMCT transition.15
Notes and references
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Dual mode X-band EPR spectroscopy was used to
characterize the Ru–Salen–Mn complex (7) and to verify the
identity of the oxidized component of the complex following
light activation in the presence of an irreversible electron
acceptor. MnIII is a d4 (S = 2) ion. Typically, species for this
type do not display Kramers degeneracy at zero-field. As a
consequence MnIII is usually spectroscopically silent in the
conventional EPR experiment (X-band, 9 GHz). However, as
the MnIII is in an axial ligand environment (E/D E 0),
transitions can be observed using parallel polarization
EPR.21 Fig.
3 (top left trace, before hn) shows the
10 A. C. Benniston and A. Harriman, Mater. Today (Oxford, UK),
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parallel mode EPR spectrum of RuII–Salen–MnIII in
acetonitrile:methanol (4 : 1, v/v), which exhibits six well-
resolved hyperfine lines centered at g E 8.1 that are split by
43 G. The line-shape and field position of these signals are
consistent with earlier published work on Salen–MnIII
complexes.22 The same analysis performed in a pure aceto-
nitrile glass did not yield a well-resolved MnIII hyperfine six-
line signal. Oxidation of the MnIII center was achieved by
excitation of RuII–Salen–MnIII (0.38 mM in acetonitrile:-
methanol 4 : 1, v/v) at ꢂ12 1C with B10 mW cmꢂ2 of
514.5 nm light from an Ar+ laser for 5 minutes in the presence
of 40 mM of NBD as electron acceptor. Following this illumina-
tion, no MnIII signal was observed by parallel mode EPR.
Concomitant with the loss of the MnIII signal was the emergence
of a new, broad perpendicular mode signal with a positive peak
at g E 4.6 (Fig. 3). This signal is characteristic of a spin S = 3/2
species i.e. MnIV,15,22 in which the zero-field splitting (D) of the
MnIV is of the order of the microwave quantum (0.3 cmꢂ1 at
X-band). These results are consistent with experiments involving
the oxidation of RuII–Salen–MnIII with chemical agents where
similar results were obtained (ESIw, Fig. S4).
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of electron acceptor. EPR experiments after illumination on the
Ru–Salen–Mn complex did not show the presence of an organic
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Chem. Commun., 2010, 46, 7605–7607 7607