Photosensitization via dye coordination: A new strategy to synthesize metal nitrosyls that release NO under visible light

Article abstract of DOI:10.1021/ja070247x

Direct coordination of the dye resorufin (Resf) to the ruthenium center photosensitizes the Ru-NO bond and improves NO photolability of the designed {Ru-NO}⁶ nitrosyl [(Me₂bpb)Ru(NO)(Resf)] under visible light. Copyright

Full text of DOI:10.1021/ja070247x

Published on Web 04/04/2007
Photosensitization via Dye Coordination: A New Strategy to Synthesize Metal
Nitrosyls That Release NO under Visible Light
†
‡
,†
Michael J. Rose, Marilyn M. Olmstead, and Pradip K. Mascharak*
Department of Chemistry and Biochemistry, UniVersity of California, Santa Cruz, California 95064, and
Department of Chemistry, UniVersity of California, DaVis, California 95616
Received January 19, 2007; E-mail: pradip@chemistry.ucsc.edu
The recent discovery of the roles of nitric oxide (NO) in various
biological processes such as blood pressure control, neurotrans-
mission, immune response, and cell apoptosis1 has raised interest
in exogenous NO donors that can deliver NO to specific targets
,2
3
under controlled conditions. NO donors that provide NO upon
exposure to light have drawn special attention because of their utility
4
in photodynamic therapy (PDT) for selected types of cancer. To
date, a number of iron, manganese, and ruthenium complexes of
NO (metal nitrosyls) have been identified that release NO upon
illumination.5 The photosensitivity of these nitrosyls is largely
-8
governed by the location of the d
electronic absorption spectra. For example, Ru-NO complexes such
as [Ru(PaPy )(NO)](BF and [Ru(salen)(NO)(Cl)] exhibit metal-
to-ligand charge transfer (MLCT) bands only in the UV region (λmax
π
(M) f π*(NO) transition in their
3
4 2
)
Figure 1. ORTEP diagram (50% probability level) of [(Me2bpb)Ru(NO)-
Resf)] (1). Selected bond distances (Å) and angles (deg): Ru-N5 )
.7347(16), N5-O3 ) 1.159(2), Ru-N1 ) 2.1250(15), Ru-N2 ) 1.9862-
)
light.
300-450 nm), and hence their photosensitivity is limited to UV
(
1
5c,6b
Nitrosyls such as [(Me
2
bpb)Ru(NO)(Cl)] or [(Me
2
bpb)-
+
Ru(NO)(py)] (where H
2
Me
2
bpb ) 1,2-bis(pyridine-2-carboxa-
(15), Ru-N3 ) 1.9803(15), Ru-N4 ) 2.1384(15), Ru-O4 ) 2.0110(13),
mido)-4,5-dimethyl benzene; H ) dissociable protons), reported
Ru-N5-O3 ) 178.13(15), Ru-O4-C23 ) 128.46(11).
8
by us recently, also release NO when exposed to UV light. Since
one major requirement of a potential NO donor for PDT is
sensitivity toward visible light, further modification(s) of these
nitrosyls is necessary to impart significant absorption in the visible
2
solvato species [(Me bpb)Ru(NO)(MeCN)]. Following removal of
AgCl, the brown-green filtrate was mixed with 1 equiv of the purple
sodium salt of Resf and the mixture was heated to reflux for 12 h.
The resulting bright red solution was then concentrated and stored
at -20 °C. Bright red crystals of 1 were isolated after 24 h (yield:
(g450 nm) region.
To increase the photosensitivity of such ruthenium nitrosyls to
visible light, we decided to utilize strongly colored dye molecules
as ligands to sensitize the metal center. Our intention was to enhance
the photolability of the Ru-NO moiety in the visible range by
energy transfer from a light-absorbing pendant chromophore to the
Ru-NO unit. In this work, we have employed the tricyclic dye
3
65%). Vapor diffusion of pentane into a CHCl solution of 1 at 35
°C afforded dark-red crystals suitable for diffraction study. The
structure of 1 (Figure 1) reveals that Resf is coordinated to the Ru
center through the phenolato-O donor center, trans to the NO
moiety. The planar, tricyclic dye is bent away from the equatorial
plane (Ru-O4-C23 bond angle ) 128.46(11)°). As expected, the
-1
resorufin (Resf) that displays intense absorption (ꢀ ≈ 105 000 M
-
1
2-
cm ) in the visible range (λmax ) 600 nm). This dye also contains
a phenolato-O donor that facilitates direct ligation of this sensitizer
to a metal center. We now report that the coordination of Resf to
Ru-N bond distances associated with the Me bpb ligand are
2
similar to those noted for [(Me bpb)Ru(NO)(Cl)]. The nearly linear
2
Ru-N-O bond angle of 178.13(15)°, short N-O bond (1.1259-
-
1
2
the Ru center of [(Me bpb)Ru(NO)(Cl)] indeed photosensitizes the
(2) Å), ν value of 1843 cm (KBr disk), and clean diamagnetic
NO
1
Ru-NO moiety toward visible light.
H NMR spectrum (see Supporting Information) of 1 are all typical
of {RuNO} nitrosyls. Unlike many other {RuNO} complexes,
is highly stable in aqueous solutions (pH 7) and hence suitable
6
5-8
6
1
for potential biological use.
Complex 1 exhibits an intense absorption band at 500 nm (ꢀ ≈
-
1
-1
1
2
2 000 M cm , Figure 3) and unlike [(Me bpb)Ru(NO)(Cl)],
rapidly releases NO (detected by NO electrode, Figure 3 inset) when
exposed to visible light (g455 nm). The loss of NO generates new
peaks at 360 and 750 nm, with clean isosbestic points at 420 and
5
15 nm (Supporting Information, Figure S1). The paramagnetic
The dye-tethered nitrosyl [(Me
synthesized via replacement of the chloride ligand of [(Me
Ru(NO)(Cl)] with Resf. A slurry of [(Me bpb)Ru(NO)(Cl)] was
in hot MeCN to obtain the
2
bpb)Ru(NO)(Resf)] (1) was
Ru(III) photoproduct (solvato species) exhibits strong EPR signal
with g ) 2.26, 2.22, and 2.04 (Supporting Information, Figure S2).
To unequivocally demonstrate the photosensitizing effect of dye
2
bpb)-
2
first treated with 1 equiv of AgBF
4
coordination in 1, we have also synthesized [(Me
OH)] (2) by heating [(Me bpb)Ru(NO)(MeCN)] in moist MeCN
containing a few drops of aniline (as a base).
2
bpb)Ru(NO)-
(
2
†
University of California, Santa Cruz.
‡
University of California, Davis.
5342
9
J. AM. CHEM. SOC. 2007, 129, 5342-5343
10.1021/ja070247x CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
tons), in which extended quinoline units replace pyridines in the
8
ligand frame. Previously, we reported a ∼50 nm shift in the λmax
2 2
in going from [(Me bpb)Ru(NO)(Cl)] to [(Me bQb)Ru(NO)(Cl)].
In the present work, we have isolated the dye-bound nitrosyl [(Me
2
-
bQb)Ru(NO)(Resf)] (3) from [(Me bQb)Ru(NO)(Cl)] by following
2
1
a synthetic protocol similar to 1 ( H NMR spectrum shown in Figure
S3, Supporting Information). Much to our expectation, 3 exhibits
convergence of the λmax of the nitrosyl moiety with that of the
coordinated dye resulting in a single band near 510 nm (Figure 3)
and releases NO more efficiently (φ500 ) 0.102 ( 0.009). The dye-
sensitized nitrosyls 1 and 3 are significantly more efficient than
other Ru-NO complexes studied in the visible region, such as [Ru-
Figure 2. ORTEP diagram (50% probability level) of [(Me2bpb)Ru(NO)-
(
OH)] (2). Selected bond distances (Å) and angles (deg): Ru-N5 ) 1.7538-
(13), N5-O3 ) 1.1648(18), Ru-N1 ) 2.1408(13), Ru-N2 ) 1.9950(13),
Ru-N3 ) 1.9900(13), Ru-N4 ) 2.1235(13), Ru-O4 ) 1.9457(11), Ru-
N5-O3 ) 172.67(12), Ru-O4-H4a ) 111.2(16).
3 5 2 6 5
NH ) (pz)Ru(bpy) (NO)](PF )
(φ532 ) 0.025).11 Direct coordina-
(
tion of the dye to the metal center appears to be crucial, as other
NO donors with peripheral chromophores (not bonded to the metal
center) such as fluorescein-derivatized Roussin’s salts exhibit much
smaller φ values (φ436 ) 0.0036 ( 0.0005).9 This is further
corroborated by negligible photoactivity of the hydroxide-bound
complex 2 (φ500 < 0.001) under visible light.
In summary, we have shown that ruthenium nitrosyls with
photoactive bands in the UV region can be photosensitized to visible
light by coordination of a strongly absorbing dye molecule. Using
the red dye resorufin, we have substantially increased the photo-
activity of two ruthenium NO donors in the visible region. Aqueous
solutions of these complexes generate rapid bursts of NO upon
exposure to light pulses (Figure 3 inset). This property could have
potential use in biological systems.
Figure 3. Electronic absorption spectra in DMF of the dye-bound complex
Acknowledgment. This research was supported by a grant from
the National Science Foundation (Grant CHE-0553405). Experi-
mental assistance from Christine Beavers is gratefully acknowl-
edged.
[
(
(
(Me2bpb)Ru(NO)(Resf)] (1) (red line, λmax ) 500 nm), [(Me2bpb)Ru(NO)-
OH)] (2) (green line, λmax ) 395 nm) and [(Me2bQb)Ru(NO)(Resf)] (3)
violet line, λmax ) 510 nm). The inset shows the NO amperogram indicating
photorelease of NO from 1 in aqueous solution (pH 7) with short pulses of
visible light (g455 nm) for the indicated time periods.
Supporting Information Available: Changes in electronic absorp-
tion spectrum observed upon photolysis of 1 (Figure S1), EPR spectrum
The structure of 2 (Figure 2) is nearly identical to that of 1
1
of the photoproduct of 1 (Figure S2), H NMR spectra of 1-3 (Figure
5
(including the N O donor set) except for the dye ligand. As a
S3), and X-ray crystallographic data in CIF format for 1 and 2. This
material is available free of charge via the Internet at http://pubs.acs.org.
consequence, the pale orange solution of 2 exhibits negligible
absorbance near 500 nm (Figure 3); the only band observed is near
∼
400 nm, corresponding to the d
π
(Ru) f π*(NO) transition
References
6
commonly observed with {RuNO} nitrosyls. No photorelease of
NO is observed with 2 under illumination with the same visible
light.
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(
(
,10
0.008) also confirms that 1 is an efficient NO donor.
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π
(Ru) f
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3
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bQb2 (H
-
more conjugated ligand Me
2
2
Me
2
bQb ) 1,2-bis(quinal-
dine-2-carboxamido)-4,5-dimethyl benzene; H ) dissociable pro-
JA070247X
J. AM. CHEM. SOC.
9
VOL. 129, NO. 17, 2007 5343