The first example of photochemical reduction of nitrite into nitrogen monoxide by a dinuclear Ru(ii)-Cu(ii) complex and photoinduced intramolecular electron transfer reaction between Ru(ii) and Cu(ii) moieties

Article abstract of DOI:10.1039/b907484h

The photochemical reduction of nitrite to NO by the dinuclear Ru(ii)-Cu(ii) complex ([Ru(bpy)₂(Mebpy-COOC₃H₆)Me ₂bpaCu(H₂O)(ClO₄)](ClO₄) ·(PF₆)₂·(H₂

Full text of DOI:10.1039/b907484h

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The first example of photochemical reduction of nitrite into nitrogen
monoxide by a dinuclear Ru(II)–Cu(II) complex and photoinduced
intramolecular electron transfer reaction between Ru(II) and Cu(II) moieties†
Naoko Isoda, Yuki Torii, Tomoko Okada, Makoto Misoo, Hiroshi Yokoyama, Noriaki Ikeda, Masaki Nojiri,
Shinnichiro Suzuki and Kazuya Yamaguchi*
Received 15th April 2009, Accepted 24th September 2009
First published as an Advance Article on the web 8th October 2009
DOI: 10.1039/b907484h
The photochemical reduction of nitrite to NO by
the dinuclear Ru(II)–Cu(II) complex ([Ru(bpy)2(Mebpy-
COOC3H6)Me2bpaCu(H2O)(ClO4)](ClO4)·(PF6)2·(H2O) was
observed in the absence of sacrificial electron donor reagents
in CH2Cl2. The reaction rate of the photoreduction of nitrite
depended upon the concentration of the excited Ru(II) moiety
in the complex. The photoinduced intramolecular electron
transfer rate constants between the Ru(II) and Cu(II) moieties
(*Ru(II)–Cu(II) → Ru(III)–Cu(I) and Ru(III)–Cu(I) → Ru(II)–
Cu(II)) in the dinuclear complex were calculated to be 2.3 ¥
109 and 8.3 ¥ 107 s-1, respectively, by laser flash photolysis.
We have also reported the photochemical reduction of nitrite to
NO catalysed by the CuMe2bpa complex with a photosensitiser5
[Ru2+(bpy)3]Cl2 in H2O.6 In this paper, we present the first example
of the photochemical reduction of nitrite to nitrogen monoxide by
the dinuclear Ru(II)–Cu(II) complex (Ru–Cu = [Ru(bpy)2(Mebpy-
COOC3H6)Me2bpaCu(H2O)(ClO4)](ClO4)·(PF6)2·H2O, Chart 1)
in the absence of sacrificial electron donor reagents in CH2Cl2. The
photoinduced intramolecular electron transfer reaction between
the Ru(II) and Cu(II) moieties in the Ru–Cu complex was also
investigated by laser flash photolysis.
Copper-containing nitrite reductase (CuNIR) plays a key role in
biological denitrification,1 which is the dissimilatory reduction of
-
-
nitrate (NO3 ) or nitrite (NO2 ) usually leading to the production
of dinitrogen by prokaryotic organisms, and an environmentally
important process for the inorganic nitrogen cycle. CuNIR, which
-
catalyses the reduction of nitrite to nitrogen monoxide (NO2
+
e- + 2H+ → NO + H2O), has two kinds of Cu centres per subunit.
Type 1 Cu accepts an electron from an external electron carrier,
and type 2 Cu, which accepts an electron from the reduced type
-
1 Cu site, is the reduction centre of NO2 . The type 2 Cu site
Chart 1
shows a distorted tetrahedral geometry and is ligated by three His
imidazolyls and a water molecule. The coordinated water to the
type 2 Cu is displaced by the substrate (NO2 ). Subsequently, the
The synthesis of Ru–Cu is described in the ESI.† The absorption
spectrum of Ru–Cu in CH2Cl2 (Fig. S1†) displays two bands
at 458 (e = 17 000 M-1 cm-1) and 686 nm (e = 160 M-1 cm-1)
in the visible region, which are characteristic of MLCT of
bpy coordinated to the Ru(II) ion and d–d transition of the
Cu(II) complex, respectively. The emission spectrum of Ru–Cu
in acetonitrile (Fig. S2†) displays an emission band at 647 nm,
which originates from the lowest MLCT state of predominantly
triplet character.7 The wavelength maximum is red-shifted by
24 nm and the emission intensity is one third lower than
that of the corresponding Ru–Zn complex ([Ru(bpy)2(Mebpy-
COOC3H6)Me2bpaZnCl2](PF6)2·H2O). The spectroscopic data
show that the Cu(II) moiety partially quenches the excited
state of Ru(II) in Ru–Cu. The redox potentials for Ru3+/Ru2+
and Cu2+/Cu1+ couples of Ru–Cu in CH2Cl2 with 0.1 M
[N(nC4H9)4]ClO4 as the supporting electrolyte were observed at
928 mV and 86 mV vs. Ag/AgNO3, respectively. The difference
in E1/2 values (0.84 eV) is the driving force (DG◦) for the
intramolecular electron transfer in Ru–Cu.
-
reduction of nitrite to NO proceeds with an attack of protons on
the nitrite bound to the Cu site and a concomitant electron transfer
reaction from the type 1 Cu.2 Several model complexes of the type
2 Cu site, which is the CuNIR active site, have been reported.3,4
Recently, we reported that [CuMe2bpa(ClO4)](ClO4) (Me2bpa =
bis(6-methyl-2-pyridylmethyl)amine) catalyses the reduction of
nitrite with acid to produce NO effectively.4 The coordination
modes of the nitrite ligands in CuMe2bpa complexes depends on
the oxidation state of the copper ion; thus the nitrite is coordinated
to Cu(II) through two oxygen atoms (O,O¢-coordination mode)
and to Cu(I) through one nitrogen atom (N-coordination mode).
Department of Chemistry, Graduate School of Science, Osaka University,
1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan. E-mail: kazu@
ch.wani.osaka-u.ac.jp; Fax: +81-6-6850-5785; Tel: +81-6-6850-5768
† Electronic supplementary information (ESI) available: Synthesis of
Ru–Cu, NMR spectra of L, Ru–Cu and Ru–Zn, absorption spectrum
of Ru–Cu, emission spectra of Ru–Cu and Ru–Zn, transient absorption
spectra of Ru–Cu and Ru–Zn, time profiles of Ru(II) ground and excited
states and Ru(III) absorptions in Ru–Cu and Ru–Zn, EPR spectrum of
NO-Fe(DTCS)2 and calibration curve of [NO] vs. EPR peak height. See
DOI: 10.1039/b907484h
Photochemical NO production from deaerated CH2Cl2 solution
including 100 mM Ru–Cu, 15 mM [(Ph3P)2N]NO2, and 6 mM
trifluoroacetic acid was observed at 28 ◦C under irradiation
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with 460 nm light (power = 3.0 mW cm-1) and the continuous
flow of Ar (1 ml/min), where the vent gas was passed into the
aqueous solution including 2.5 mM FeCl3 and 12.5 mM N-
Ru–Cu (eqn (3)). It is interesting to note that the photoreduction
of nitrite to NO proceeds until the amount of NO generated is
equal to ca. 10% of Ru–Cu despite the lack of addition of any
sacrificial reductants, such as EDTA. As the absorption spectrum
of the Ru–Cu solution after the photochemical reaction was very
similar to that before the reaction, the Ru3+–Cu2+ species could
not be confirmed. The Ru3+–Cu2+ species may react with some
compounds (such as CH2Cl2) in the system and return to the
Ru2+–Cu2+ species (eqn (4)), although we could not clarify what
the compound which reacts with the Ru3+–Cu2+ species is. The NO
generation was not observed when H2O or CH3CN was employed
instead of CH2Cl2. We speculate that the organic solvent (CH2Cl2)
may play the role of sacrificial reductant in this system. In the
Rubpy3 and CuMe2bpa unconjugated system in H2O,6 the addition
of a sacrificial reductant (KSeCN) is essential for the catalytic
reduction of nitrite to nitrogen oxide.
◦
(dithiocarboxy)sarcosine (DTCS) at 5 C. The NO generated in
the reaction vessel was transported to the Fe(DTCS)3 solution
by carrier Ar gas and reacted with Fe(DTCS)3 to produce NO-
Fe(DTCS)2 immediately.8 The distinct triplet EPR spectrum (g =
2.040 and AN = 1.27 mT), which is identified as the NO-Fe(DTCS)2
complex, of the NO trapping solution is observed at room
temperature (Fig. S3†). The yield of NO generated was estimated
based on the calibration curve of the amount of NO vs. the peak
height of the EPR spectrum (Fig. S4†). As shown in Fig. 1, the
linear generation of NO from the Ru–Cu solution including nitrite
and acid under visible light irradiation within 30 min was observed,
while the NO formation did not occur when Ru–Zn was employed
instead of Ru–Cu. Therefore, the nitrite reduction site must be the
Cu(II) moiety in Ru–Cu.
Transient absorption spectra of Ru–Cu and Ru–Zn were ob-
served by nanosecond laser flash photolysis (lex = 532 nm, fwhm
4 ns) in deaerated CH3CN at 298 K (Fig. S5 and S6†). The recovery
of the Ru(II) ground state bleach at 460 nm and the decay curves of
absorption at 370 nm due to the excited Ru(II) moiety are a mirror
image in Ru–Zn, with both the recovery and decay having lifetimes
of 1.2 ms (Fig. S7†). Both the time profiles of the Ru(II) ground
state absorption at 460 nm and the 3MLCT state absorption of the
Ru(II) moiety at 370 nm in Ru–Cu show bi-exponential curves
(Fig. S8†). As the lifetime (1.1 ms) of the slow component is
almost the same as those obtained for Ru–Zn, the component
is assigned to the emission process of the Ru(II) moiety in Ru–Cu.
The fast components with lifetimes of 5.1 and 7.9 ns at 370 and
460 nm, respectively, are suggested to be due to the intramolecular
electron transfer from the excited Ru(II) to Cu(II) moieties in
Ru–Cu because another spectral change at 503 nm due to the
Ru(III) moiety was observed in the transient absorption spectra
of Ru–Cu (Fig. S8†).5 Unfortunately, the reduction of the Cu(II)
moiety could not be observed because of the very small extinction
coefficient for the Cu(II) moiety, and there is no absorption band
for the Cu(I) complex in the visible region. To determine the
detailed lifetime of the fast component, the transient absorption
spectra of Ru–Cu and Ru–Zn were observed by subpicosecond
laser flash photolysis (lex = 400 nm, fwhm ca. 300 fs) in deaerated
CH3CN at 298 K (Fig. 2). The spectral change at 503 nm was
clearly observed for Ru–Cu, but not for Ru–Zn. Time profiles
of absorbance at 503 nm due to the Ru(III) moiety in Ru–Cu
are shown in Fig. 3. From the results of the transient absorption
spectra of Ru–Cu, the rate constant of the intramolecular electron
Fig. 1 NO generated upon photolysis (l = 460 nm, light power:
3.0 mW cm-1) of 100 mM Ru–Cu (᭹), 100 mM Ru–Zn (᭺) and 100 mM
[CuMe2bpa(ClO4)](ClO4) (ꢀ) in deaerated CH2Cl2 solution including
15 mM [(Ph3P)2N]NO2 and 6 mM trifluoroacetic acid at 28 ◦C under
Ar atmosphere. Inset: plots of generated NO concentration vs. light power
upon photolysis of 100 mM Ru–Cu in deaerated CH2Cl2 solution including
15 mM [(Ph3P)2N]NO2 and 6 mM trifluoroacetic acid at 28 ◦C under Ar
atmosphere for 30 min.
A plausible mechanism of the photoreduction of nitrite to NO
may be written as follows:
Ru2+–Cu2+ + hn → Ru2+*–Cu2+
Ru2+*–Cu2+ → Ru3+–Cu1+
(1)
(2)
(3)
(4)
-
Ru3+–Cu1+ + NO2 + 2H+ → Ru3+–Cu2+ + NO + H2O
Ru3+–Cu2+ + X(e.g. CH2Cl2) → Ru2+–Cu2+ + X∑(e.g. Cl∑)
As the yield of NO generated was proportional to the light
power (inset of Fig. 1), the photoreduction of the nitrite reaction
rate should be dependant upon the concentration of the excited
Ru(II) moiety in the complex. Eqn (2) is the intermolecular electron
transfer step. The Cu moiety accepts an electron from the excited
Ru(II) moiety, and the reduction of nitrite to NO proceeds with an
attack of protons on the nitrite bound to the reduced Cu moiety in
Fig. 2 Transient absorption spectra after excitation of Ru–Cu (a) and
Ru–Zn (b) at 400 nm (FWHM ca. 300 fs) in deaerated CH3CN at
298 K. Sample concentration is 60 mM.
10176 | Dalton Trans., 2009, 10175–10177
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Ru(II) moiety in the complex. The intramolecular electron transfer
reaction rate was estimated to be ~2 ¥ 109 s-1 by laser flash
photolysis. The photoreduction of nitrite to NO proceeded until
the amount of NO generated was equal to about 10% of the
dinuclear complex despite the lack of any sacrificial reductants.
We thank Prof. Satoshi Fujii (Konan University) for suggesting
the NO trap method.
Notes and references
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23957.
Fig. 3 Time profile of the absorbance at 503 nm due to the Ru(III) state by
laser flash photolysis (lex = 532 nm, FWHM 4 ns) of a deaerated CH3CN
solution of Ru–Cu (60 mM) at 298 K. Inset: time profile of the absorbance
at 503 nm due to the Ru(III) state by laser flash photolysis (lex = 400 nm,
FWHM ca. 300 fs) of a deaerated CH3CN solution of Ru–Cu (60 mM) at
298 K.
3 (a) W. B. Tolman, Inorg. Chem., 1991, 30, 4877; (b) P. P. Paul and
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transfer from the excited Ru(II) to Cu(II) moieties (*Ru(II)–
Cu(II) → Ru(III)–Cu(I)) in Ru–Cu and the back electron transfer
rate constant (Ru(III)–Cu(I) → Ru(II)–Cu(II)) were calculated to be
2.3 ¥ 109 and 8.3 ¥ 107 s-1, respectively. The latter value is somewhat
larger than the reported intramolecular electron transfer rate (1.1 ¥
107 s-1) from Mn(II) to photooxidised Ru(III) moieties in a Ru(II)–
Mn(II) complex (DG◦ = 0.59 eV).7
4 H. Yokoyama, K. Yamaguchi, M. Sugimoto and S. Suzuki, Eur. J. Inorg.
Chem., 2005, 1435.
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Conclusions
6 K. Yamaguchi, T. Okada and S. Suzuki, Inorg. Chem. Commun., 2006,
9, 989.
This is the first example of the photoreduction of nitrite to NO by
a binuclear complex in the absence of any sacrificial electron donor
reagents in CH2Cl2, and the photoinduced intramolecular electron
transfer reaction from the Ru(II) to Cu(II) moieties in the binuclear
Ru(II)–Cu(II) complex. The reaction rate of the photoreduction of
nitrite should be dependent on the concentration of the excited
7 M. L. A. Abrahamsson, H. B. Baudin, A. Tran, C. Philouze, K. E. Berg,
˚
M. K. Raymond-Johansson, L. Sun, B. Akermark, S. Styring and L.
Hammarstro¨m, Inorg. Chem., 2002, 41, 1534.
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Fujii, K. Kobayashi, S. Tagawa and T. Yoshimura, J. Chem. Soc., Dalton
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 10175–10177 | 10177
©