Dye-tethered ruthenium nitrosyls containing planar dicarboxamide tetradentate N4 ligands: Effects of in-plane ligand twist on NO photolability

Article abstract of DOI:10.1021/ic1019873

To examine the steric effects of the in-plane ligands in dye-sensitized {RuNO}⁶ nitrosyls on their NO photolability, two new ligands, namely, 1,2-Bis(pyridine-2-carboxamido)-4,5-dimethoxybenzene (H₂(OMe) ₂bpb) and 1,2-Bis(

Full text of DOI:10.1021/ic1019873

Inorg. Chem. 2011, 50, 317–324 317
DOI: 10.1021/ic1019873
Dye-Tethered Ruthenium Nitrosyls Containing Planar Dicarboxamide Tetradentate
N4 Ligands: Effects of In-Plane Ligand Twist on NO Photolability
Nicole L. Fry, Brandon J. Heilman, and Pradip K. Mascharak*
Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064,
United States
Received September 28, 2010
To examine the steric effects of the in-plane ligands in dye-sensitized {RuNO}6 nitrosyls on their NO photolability, two
new ligands, namely, 1,2-Bis(pyridine-2-carboxamido)-4,5-dimethoxybenzene (H2(OMe)2bpb) and 1,2-Bis(Isoquinoline-
1-carboxamido)-4,5-dimethoxybenzene (H2(OMe)2IQ1, H’s are dissociable carboxamide protons) have been des-
igned and synthesized. The syntheses and spectroscopic properties of {RuNO}6 nitrosyls derived from these two
ligands, namely, [((OMe)2bpb)Ru(NO)(Cl)] (4-Cl), [((OMe)2IQ1)Ru(NO)(Cl)] (5-Cl), [((OMe)2bpb)Ru(NO)(Resf)]
(4-Resf), and [((OMe)2IQ1)Ru(NO)(Resf)] (5-Resf), are reported. The structures of 5-Cl, 4-Resf, and 5-Resf have
been determined by X-ray crystallography. Removal of the in-plane ligand twist in the quinoline-based R2bQb2- ligand
frame (because of steric interactions between the extended quinoline ring systems) in both R2bpb2- and R2IQ12-
(pyridine and 1-isoquinoline rings, respectively, instead of quinoline rings in the equatorial plane) results in enhanced
solution stability, as well as higher quantum yield values for NO photorelease upon exposure to 500 nm light. Both dye-
tethered {RuNO}6 nitrosyls 4-Resf and 5-Resf exhibit greater sensitivity to visible light compared to the chloro-bound
species 4-Cl and 5-Cl. In addition, the dye-tethered nitrosyls are fluorescent and hence can be used as trackable NO
donors in cellular studies.
Introduction
(RONO), nitrates (RONO2), S-nitrosothiols (RSNO), and
diazeniumdiolates (NONOates) have all been shown to deliver
NO to cellular targets.10,11 Recently, there has been increased
interest in the deleterious role of NO and its ability to induce
apoptosis in both healthy and malignant cells.12,13 Develop-
ment of site-specific NO donors could therefore be beneficial
in selective killing of cancer cells.14
Phototriggered NO release from metal nitrosyls provides
one such strategy for targeted delivery of NO to specific
malignant sites. To this end, our group has synthesized
several photoactive nitrosyls containing iron,15 manganese,16
Nitric oxide (NO) plays an important role in many bio-
logical signaling processes including blood pressure regulation,
neurotransmission, inflammatory response, and programmed
cell death (apoptosis).1-6 The regulatory role of NO in such
processes has spurred the development of several exogenous
NO donors as potential drugs.7-9 For example, organic nitrites
*To whom correspondence should be addressed. E-mail: pradip@
chemistry.ucsc.edu.
(1) Ignarro, L. J. Nitric Oxide: Biology and Pathobiology; Academic Press:
San Diego, 2000.
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(3) Fukuto, J. M.; Wink, D. A. Met. Ions Biol. Syst. 1999, 36, 547–595.
(4) Ko, G. Y.; Fang, F. C. Nitric Oxide and Infection; Kluwer Academic/
Plenum Publishers: New York, 1999.
(5) Lincoln, J.; Burnstock,G. Nitric Oxide in Health and Disease; Cam-
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(6) Nitric Oxide Research; Feelisch, M., Stamler, J. S., Eds.; John Wiley and
Sons: Chichester, U.K., 1996.
(7) Wang, P. G.; Cai, T. B.; Taniguchi, N. Nitric Oxide Donors for
Pharmaceutical and Biological Applications; Wiley-VCH: Weinheim, Germany,
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Ferro, A. Rev. Med. Chem. 2005, 5, 247–254. (c) Butler, A. R.; Megson, I. L.
Chem. Rev. 2002, 102, 1155–1165.
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J. A.; Keefer, L. K. Chem. Rev. 2002, 102, 1135–1154.
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J.-P.; Wang, P. G. Curr. Med. Chem. 2000, 7, 821–834.
(12) (a) Mocellin, S.; Bronte, V.; Nitti, D. Med. Res. Rev. 2007, 27, 317–
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2007, 12, 1597–1610. (c) Fukumura, D.; Kashiwagi, S.; Jain, R. K. Nat. Rev.
Cancer 2006, 6, 521–534.
(13) (a) Hirst, D.; Robson, T. J. Pharm. Pharmacol. 2007, 59, 3–13. (b)
Bonavida, B.; Khineche, S.; Huerta-Yepez, S.; Garban, H. Drug Resist. Updates
2006, 9, 157.
(14) (a) Eroy-Reveles, A. A.; Mascharak, P. K. Future Med. Chem. 2009, 1
(8), 1497–1507. (b) Rose, M. J.; Mascharak, P. K. Curr. Opin. Chem. Biol. 2008,
12, 238–244. (c) Ford, P. C.; Bourassa, J.; Miranda, K.; Lee, B.; Lorkovic, I.;
Boggs, S.; Kudo, S.; Laverman, L. Coord. Chem. Rev. 1998, 171, 185–202.
(15) (a) Afshar, R. K.; Patra, A. K.; Olmstead, M. M.; Mascharak, P. K.
Inorg. Chem. 2004, 43, 5736–5743. (b) Patra, A. K.; Rowland, J. M.; Marlin,
D. S.; Bill, E.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 2003, 42, 6812–
6823. (c) Patra, A. K.; Afshar, R. K.; Olmstead, M. M.; Mascharak, P. K. Angew.
Chem., Int. Ed. 2002, 41, 2512–2515.
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A. J. Chem. Rev. 2002, 102, 1091–1134. (c) Wang, K.; Zhang, W.; Xian, M.;
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r
2010 American Chemical Society
Published on Web 11/29/2010
pubs.acs.org/IC

318 Inorganic Chemistry, Vol. 50, No. 1, 2011
Fry et al.
or ruthenium17 metal centers. One benefit of ruthenium
nitrosyls over those containing first row transition metals is
their reduced reactivity toward oxygen and stability in water,
both crucial for biological studies. However, these nitrosyls
typically require ultraviolet (UV) light for photorelease of
NO. To alleviate problems related to exposure of biological
tissues to UV light, we have focused on sensitizing ruthenium
nitrosyls to visible light.18,19 A series {RuNO}6 nitrosyls with
various dicarboxamide tetradentate ligands, namely, [(Me2bpb)-
Ru(NO)(Cl)] (1-Cl), [(Me2bQb)Ru(NO)(Cl)] (2-Cl), [((OMe)2-
bQb)Ru(NO)(Cl)] (3-Cl), have been successfully sensitized to
500 nm visible light via substitution of the chloride ligand
with the conjugated dye resorufin (Resf ).
Figure 1. X-ray structure of [((OMe)2bQb)Ru(NO)(Resf)] from front
angle (left) and side angle (right) displaying twist of the in plane
tetradentate ligand. H atoms are omitted for the sake of clarity.
Figure 2. Design of the new ligand (OMe)2IQ1, intended for relieving
twist in the equatorial plane of the Resf-tethered {RuNO}6 nitrosyls.
stability in aqueous solutions compared to 1-Resf which
comprises pyridine donors. Close examination of the struc-
tural features of these nitrosyls revealed that the ligand
frames containing the quinoline donors (H2R2bQb) bind
the {RuNO}6 center in a twisted fashion because of steric
interactions between the two quinoline moieties in the equa-
torial plane (Figure 1).18 To probe the effect(s) of this twist on
the stability and NO photolability of the resorufin-bound
{RuNO}6 nitrosyls, we have now designed two new dicar-
boxamide ligands containing methoxy-substituted phenyle-
nediamine rings combined with pyridine (H2(OMe)2bpb) or
1-isoquinoline (H2(OMe)2IQ1) donors. Both new ligands bind
the {RuNO}6 center in a planar fashion (with no apparent
twist, Figure 2) in the resulting chloride and resorufin-bound
nitrosyls, namely, [((OMe)2bpb)Ru(NO)(Cl)] (4-Cl), [((OMe)2-
IQ1)Ru(NO)(Cl)] (5-Cl), [((OMe)2bpb)Ru(NO)(Resf)] (4-Re-
sf ), and[((OMe)2IQ1)Ru(NO)(Resf)] (5-Resf). In this account,
we report the syntheses and spectroscopic properties of 4-Cl,
5-Cl, 4-Resf, and 5-Resf in addition to the structures of 5-Cl,
4-Resf, and 5-Resf. The stabilities of these nitrosyls in solutions
and their NO-releasing capacities have also been examined
critically to determine the effects of different substituents on
the in-plane ligand frames and the resulting distortions
arising from such substitutions. Much like 1-Resf, 2-Resf,
and 3-Resf, the present set of nitrosyls exhibit strong fluo-
rescence and adds to the repertoire of “trackable NO donors”.18b
While the resulting resorufin-bound nitrosyls all exhibited
absorption bands with λmax of 500 nm, their efficiencies of
NO photorelease upon exposure to 500 nm light can be
modulated by rational substitutions in the in-plane ligand
frame. For example, substitution of the pyridine donors in
the ligand frame of [(Me2bpb)Ru(NO)(Resf)] (1-Resf) with
quinoline donors in [(Me2bQb)Ru(NO)(Resf)] (2-Resf) and
substitution of the methyl groups in (2-Resf) with more
electron donating methoxy groups in [((OMe)2bQb)Ru(NO)-
(Resf)] (3-Resf) both result in higher quantum yield values at
500 nm (φ500 ≈ 0.05, 0.10, and 0.20, respectively).18 The dif-
ferent ligand frames also lead to differences in the stabilities
of the resorufin-bound nitrosyls in solution. For example,
both quinoline containing 2-Resf and 3-Resf have reduced
(16) (a) Hoffman-Luca, G. C.; Eroy-Reveles, A. A.; Alvarenga, J.;
Mascharak, P. K. Inorg. Chem. 2009, 48, 9104–9111. (b) Eroy-Reveles,
A. A.; Leung, Y.; Beavers, C. M.; Olmstead, M. M.; Mascharak, P. K. J. Am.
Chem. Soc. 2008, 130, 4447–4458. (c) Eroy-Reveles, A. A.; Leung, Y.;
Mascharak, P. K. J. Am. Chem. Soc. 2006, 128, 7166–7167.
(17) (a) Fry, N. L.; Rose, M. J.; Rogow, D. L.; Nyitray, C.; Kaur, M.;
Mascharak, P. K. Inorg. Chem. 2010, 49, 1487–1495. (b) Rose, M. J.;
Mascharak, P. K. Coord. Chem. Rev. 2008, 252, 2093–2114. (c) Rose, M. J.;
Patra, A. K.; Alcid, E. A.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 2007,
46, 2328–2338. (d) Patra, A. K.; Rose, M. J.; Murphy, K. A.; Olmstead, M. M.;
Mascharak, P. K. Inorg. Chem. 2004, 46, 4487–4495.
Experimental Section
Materials. NO gas was purchased from Spectra Gases Inc.
and was purified by passing through a long KOH column prior
to use. RuCl3 xH2O (Aldrich Chemical Co.) was treated several
times with concentrated HCl to prepare the starting metal salt,
3
(18) (a) Rose, M. J.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem.
Soc. 2007, 129, 5342–5343. (b) Rose, M. J.; Fry, N. L.; Marlow, R.; Hinck, L.;
Mascharak, P. K. J. Am. Chem. Soc. 2008, 130, 8834–8846.
RuCl3 3H2O. The solvents were dried by standard techniques
3
and distilled. 1-Isoquinoline was purchased from Wako Chemicals.
1,2-Dimethoxy-4,5-diaminobenzene was synthesized according to
(19) Rose, M. J.; Mascharak, P. K. Inorg. Chem. 2009, 48, 6904–6917.

Article
Inorganic Chemistry, Vol. 50, No. 1, 2011 319
the published procedure.20 All other chemicals were purchased
from Aldrich Chemical Co. and used without further purification.
Synthesis of Ligands. 1,2-Bis(pyridine-2-carboxamido)-4,5-di-
methoxybenzene (H2(OMe)2bpb, H’s Are Dissociable Carboxa-
mide Protons). A mixture of 1,2-dimethoxy-4,5-diaminobenzene
(0.51 g, 3.0 mmol), 2 equiv of picolinic acid (0.74 g, 6.0 mmol), and
2 equiv of P(OPh)3 (1.87 g, 6.0 mmol) was dissolved in 20 mL of
pyridine. The resulting green/brown solution turned amber after
refluxing for 4 h. The solution was cooled and kept at 4 °C
overnight. The resulting tan precipitate was filtered and washed
with EtOH and Et2O and dried in vacuo. Yield: 1.04 g (55%).
Selected IR bands (KBr disk, in cm-1): 3335 (w, νNH), 3285 (w),
1676 (s νCdO), 1662 (s), 1609 (w), 1527 (s), 1504 (s), 1480 (m), 1350
(m), 1210 (vs), 1093 (m), 997 (m), 751 (m), 671 (m). 1H NMR in
CDCl3, δ from TMS: 10.32 (s 2H), 8.57 (d 2H), 8.32 (d 2H), 7.91 (t
2H), 7.47 (m 4H), 3.94 (s 6H).
493 (m). UV/vis in MeCN, λ in nm (ε in M-1 cm-1): 402 (14 600),
495 (29 000). 1H NMR in CDCl3, δ from TMS: 8.75 (d 2H), 8.52 (d
2H), 8.32 (s 2H), 8.13 (t 2H), 7.81 (t 2H), 7.56 (t 2H), 7.30 (d 1H),
7.12 (2 1H), 6.72 (d 1H), 6.11 (s 1H), 6.08 (d 1H), 5.67 (s 1H), 4.00
(s 6H).
[((OMe)2IQ1)Ru(NO)(Cl)] (5-Cl). A yellow solution of H2-
(OMe)2IQ1 (0.15 g, 0.31 mmol) in 20 mL of DMF was treated
with 2.1 equiv of NaH (0.02 g, 0.67 mmol) to generate a red
solution and to it was added a batch of 0.08 g (0.31 mmol) of
RuCl3 3H2O. The dark brown mixture was then heated to
3
reflux temperature for 20 h. Next, the green-brown solution
was cooled to room temperature and filtered to remove NaCl.
The filtrate was degassed, and NO gas was bubbled through it at
reflux temperature for 2 h. The resulting red/maroon solution
was cooled, and the solvent was removed in vacuo. The oily
residue was triturated several times with MeCN to afford a red-
brown solid. This solid was then stirred in hot MeCN and
filtered to remove impurities. The dark maroon solid was finally
washed with Et2O and dried in vacuo. Yield: 0.16 g (80%). Anal.
Calcd. for C28H20ClN5O5Ru: C 52.34, H 3.12, N 10.90; Found:
C 52.12, H 3.22, N 10.53. Selected IR frequencies (KBr disk, in
cm-1): 2923 (w), 1832 (m νNO), 1614 (vs), 1585 (s), 1492 (s), 1253
(s), 1091 (s). 1H NMR in CDCl3, δ from TMS: 10.45 (d 2H), 8.72
(d 2H), 8.61 (s H), 8.06 (d 2H), 7.90 (d 2H), 7.93 (t 2H), 7.90 (t
2H), 4.05 (s 6H). UV/vis in DMF, λ in nm (ε in M-1 cm-1): 290
(27 000), 320 sh (22 300), 475 (8 700).
1,2-Bis(Isoquinoline-1-carboxamido)-4,5-dimethoxybenzene
(H2(OMe)2IQ1, H’s Are Dissociable Carboxamide Protons). A
solution containing 1 equiv of 1,2-dimethoxy-4,5-diaminoben-
zene (1.27 g, 7.56 mmol), 2 equiv of 1-Isoquinolinecarboxylic
acid (2.62 g, 15.1 mmol), and 2 equiv of P(OPh)3 (4.69 g, 15.10
mmol) was prepared in 40 mL of pyridine. The green solution
turned bright red upon heating at 100 °C for 4 h. The solution
was partially condensed and stored at 4 °C overnight. The
resulting bright yellow-orange precipitate was filtered and
washed several times with EtOH and Et2O and dried in vacuo.
Yield: 2.17 g (60%). Selected IR frequencies (KBr disk, in cm-1):
3296 (w, νNH), 1682 (s, νCdO), 1526 (vs), 1478 (s), 1334 (w), 1205
(s), 1108 (w). 1H NMR in CDCl3, δ from TMS: 10.46 (s 2H), 9.72
(d 2H), 8.40 (d 2H), 7.88 (d 2H), 7.82 (d 2H), 7.73 (t 4H), 7.53 (s H).
Syntheses of Metal Complexes. [((OMe)2bpb)Ru(NO)(Cl)]
(4-Cl). A pale yellow solution of H2(OMe)2bpb (0.20 g, 0.53
mmol) in 20 mL of N,N-dimethylformamide (DMF) was treated
with 2.1 equiv of NaH (0.03 g, 1.10 mmol) to generate a bright
[((OMe)2IQ1)Ru(NO)(Resf)] (5-Resf). A solution of [((OMe)2-
IQ1)Ru(NO)(Cl)] (0.13 g, 0.20 mmol) in 20 mL of MeCN was
treated with AgBF4 (0.04 g, 0.20 mmol) and heated to reflux temp-
erature for 3 h. Next, a batch of 0.05 g (0.20 mmol) of Resorufin
dye (as Na salt) was added to the hot maroon solution and
continued to reflux for 4 h. The resulting bright red solution was
cooled to 4 °C, and the dark red precipitate was filtered. The dried
precipitate was then redissolved in CH2Cl2 and filtered to remove
AgCl and NaBF4. The filtrate was concentrated and loaded on a
silica gel column. A CH2Cl2/THF gradient was used to elute the
final product (40% THF). Yield: 0.08 g (45%). Anal. Calcd. for
C40H26N6O8Ru: C 58.61, H 3.17, N 10.26; Found: C 58.45, H 3.31,
N 10.37. Selected IR frequencies (KBr disk, in cm-1): 1827 (vs
orange solution. A batch of 0.14 g (0.53 mmol) of RuCl3 3H2O
3
was then added to the solution of deprotonated ligand, and the
green mixture was heated to reflux temperature for 16 h. Next, it
was cooled to room temperature and filtered to remove solid
NaCl. The dark green filtrate was degassed and NO gas was
bubbled through the solution at reflux temperature for 1 h. The
resulting red/orange solution was cooled, and the solvent was
removed in vacuo. The oily residue was triturated several times
with MeCN to remove DMF. The solid was finally washed with
portions of MeCN (2 ꢀ 5 mL) and tetrahydrofuran (THF, 2 ꢀ
5 mL) to afford an orange-brown compound which was dried in
vacuo. Yield: 0.21 g (72%). Anal. Calcd. for C20H16ClN5O5Ru:
C 44.28, H 2.95, N 12.92; Found: C 44.53, H 3.01, N 12.89.
Selected IR bands (KBr disk, in cm-1): 1845 (vs, νNO), 1631 (vs,
ν
NO), 1617 (s), 1580 (m), 1487 (s), 1323 (m), 1270 (s), 1266 (s), 1199
(m), 1095 (m), 849 (w), 491 (w) . 1H NMR in CDCl3, δ from TMS:
10.27 (d 2H), 8.78 (d 2H), 8.55 (s 2H), 8.14 (d 2H), 8.01 (d 2H), 7.92
(t 2H), 7.83 (t 2H), 7.24 (s 1H), 7.03 (d 1H), 6.72 (d 1H), 5.99 (s 1H),
5.97(d 1H), 5.36 (d 1H), 4.09 (s 6H). UV/vis in MeCN, λ in nm (ε in
M
-1 cm-1): 330 (15 000), 495 (30 000).
Physical Measurements. The 1H NMR spectra were recorded
at 298 K on a Varian Inova 500 MHz instrument. A Perkin-
Elmer Spectrum-One FT-IR spectrometer was used to monitor
the IR spectra of the complexes. The electronic absorption
spectra were obtained with a scanning Cary 50 spectropho-
tometer (Varian Associates). Fluorescence spectra were recorded
with a Perkin-Elmer LS50B Fluorescence/Luminescence Spec-
trometer. Release of NO in aqueous solution upon illumination
was monitored by using the inNO Nitric Oxide Monitoring
System (Innovative Instruments, Inc.) fitted with the ami-NO
2008 electrode. The NO amperograms were recorded using
stirred solutions contained in open vials.
νCdO), 1596 (m), 1566 (w), 1497 (s), 1402 (m), 1368 (m), 1278 (w),
1254 (m), 1080 (w), 988 (w), 872 (w), 756 (w), 489 (w). UV/vis in
1
DMF, λ in nm (ε in M-1 cm-1): 319 (12 500), 420 (7 800). H
NMR in CDCl3, δ from TMS: 8.74 (d 2H), 8.49 (s 2H), 8.39 (d
2H), 8.24 (t 2H), 7.74 (t 2H), 3.98 (s 6H).
[((OMe)2bpb)Ru(NO)(Resf)] (4-Resf). A mixture of [((OMe)2-
bpb)Ru(NO)(Cl)] (0.10 g, 0.18 mmol) and 1 equiv of AgBF4 (0.04
g, 0.18 mmol) in 30 mL of MeCN was heated to reflux for 15 min
and then a batch of 0.04 g (0.18 mmol) of Resorufin (sodium salt)
was added. The resulting red solution was heated to reflux for an
additional 16 h. The red/orange mixture thus obtained was filtered
and concentrated under vacuum and then stored at 4 °C for 12 h.
The red precipitate thus obtained was filtered and washed with
ether (2 ꢀ 5 mL). Yield: 0.04 mg (27%). Anal. Calcd. for C32H22-
N6O8Ru: C 53.41, H 3.06, N 11.68; Found: C 53.37, H 3.12, N
11.89. Selected IR bands (KBr disk, in cm-1): 1831 (s, νNO), 1630
(vs, νCdO), 1592 (vs), 1485 (vs), 1403 (m), 1370 (m), 1322 (w), 1272
(s), 1205 (m), 1102 (m), 1082 (m), 863 (m), 758(w), 683 (w), 588 (w),
X-ray Crystallography. Vapor diffusion of diethyl ether into a
solution of 4-Resf in CHCl3 at 4 °C afforded red crystalline
blades suitable for X-ray studies. Red crystals of 5-Cl, suitable
for diffraction study, were obtained via vapor diffusion of pentane
into a solution of 5-Cl in CHCl3 at room temperature. Vapor
diffusion of diethyl ether into a CH2Cl2/THF solution of 5-Resf
at room temperature afforded red crystals used for the structural
studies. Diffraction data for 4-Resf 2CHCl3 (0.5)Et2O, 5-
3
3
Cl 2CHCl3, and 5-Resf were collected at 150 K on a Bruker
3
APEX-II instrument using monochromated Mo-KR radiation
˚
(λ = 0.71073 A). All diffraction data were corrected for absorp-
tion, and calculations were performed using the SHELXTL
(20) Rosa, D. T.; Reynolds, R. A.; Malinak, S. M.; Coucouvanis, D.
Inorg. Synth. 2002, 33, 112–119.

320 Inorganic Chemistry, Vol. 50, No. 1, 2011
Fry et al.
Table 1. Summary of Crystal Data, Intensity Collection and Refinement Parameters for [((OMe)2bpb)Ru(NO)(Resf)] (CHCl3)2(OEt2)0.5 (4-Resf 2CHCl3 (0.5)Et2O),
3
3
3
[((OMe)2IQ1)Ru(NO)(Cl)] (CHCl3)2 (5-Cl 2CHCl3), [((OMe)2IQ1)Ru(NO)(Resf)] (5-Resf )
3
3
4-Resf
5-Cl
5-Resf
empirical formula
formula weight
crystal color
C36H29Cl6N6O8.5Ru
995.42
red blades
0.12 ꢀ 0.08 ꢀ 0.07
150(2)
0.71073
triclinic
P1
11.1266(11)
12.1021(12)
15.7732(16)
74.0320(10)
77.7320(10)
85.4830(10)
1994.9(3)
2
1.657
0.856
1.033
R1 = 0.0454
wR2 = 0.1042
R1 = 0.0570
wR2 = 0.1114
C30H22Cl7N5O5Ru
881.75
red blades
0.11 ꢀ 0.02 ꢀ 0.02
150(2)
0.71073
monoclinic
P21/c
12.2656(11)
16.9131(15)
20.0497(13)
90
127.317(4)
90
3307.9(5)
4
1.771
1.089
1.012
R1 = 0.0343
wR2 = 0.0691
R1 = 0.0559
wR2 = 0.0769
C40H26N6O8Ru
819.74
red blades
0.090 ꢀ 0.050 ꢀ 0.010
150(2)
0.71073
triclinic
P1
8.765(10)
11.534(13)
16.827(18)
94.414(12)
91.102(11)
97.487(12)
1681(3)
2
1.620
0.535
1.025
R1 = 0.0392
wR2 = 0.0806
R1 = 0.0552
wR2 = 0.0865
crystal size (mm3)
temperature (K)
˚
wavelength (A)
crystal system
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
dcal (g/cm3)
μ (mm-1
)
GOFa on F2
final R indices
[I > 2σ(I )]
R indicesb
All datac
P
P
P
P
a GOF = [ w(Fo2 - Fc2)2/(No - Nv)]1/2 (No = number of observations, Nv = number of variables). b R1 = ||Fo|- |Fc||/ |Fo|. c wR2 =[ w(Fo2 - Fc2)2/
P
w|Fo|2)]1/2
.
˚
Table 2. Selected Bond Distances (A) and Angles (deg) of 4-Resf, 5-Cl and 5-Resf
more spread-out 1-isoquinoline groups in the designed ligand
H2(OMe)2IQ1 in the present work. Such a change did not
affect the extended conjugation in the ligand frame. Since the
methoxy groups were retained, the electron donating ability
of the H2(OMe)2IQ1 ligand was hardly altered. The overall
design therefore ensured removal of the twist in the coordi-
nated ligand in the equatorial plane of the nitrosyls with
minimum changes in other parameters (Figure 2). To com-
plete the investigation on the effect(s) of the twist, we also
designed the methoxy substituted pyridine-based ligand,
H2(OMe)2bpb. When coordinated, the pyridine rings of this
ligand remain far apart in the equatorial plane of the nitrosyls
and hence no steric interaction between them is expected
(Figure 2).
Reaction of RuCl3 with the deprotonated (with NaH)
ligands in DMF followed by the addition of NO(g) produces
the chloride bond species [((OMe)2bpb)Ru(NO)(Cl)] (4-Cl)
and [((OMe)2IQ1)Ru(NO)(Cl)] (5-Cl). Further reaction of
these chloro complexes with AgBF4 in MeCN results in the
formation of the MeCN-bound intermediates which afford
4-Resf and 5-Resf upon reaction with the sodium salt of the
resorufin dye. In the final products, either a Cl- or a Resf -
ligand is bound trans to NO.
All four nitrosyls are soluble in solvents like CHCl3,
MeCN, DMF, and dimethylsulfoxide (DMSO). When dis-
solved in weakly coordinating aprotic solvents like CHCl3
and MeCN, the nitrosyls are indefinitely stable and do not
lose the bound resorufin dye. However, in solvents like
DMSO or in aqueous solutions (pH 5-7), the resorufin-
bound nitrosyls slowly lose the bound dye over the course of
several hours. Compared to the {RuNO}6 nitrosyls contain-
ing twisted quinoline ligands, namely, 2-Resf and 3-Resf,
which lose ∼20%bounddye in1 h, 5-Resf loses ∼10% bound
dye within the same time. The two pyridine-containing nitrosyls
1-Resf and 4-Resf both lose less than 5% bound dye in 1 h.
The dye bound nitrosyls derived from planar ligand frames
with no twist thus exhibit increased stability in H2O. While
4-Resf
Bond Distances
5-Cl
5-Resf
Ru-Ν5
N5-O5
Ru-N1
Ru-N2
Ru-N3
Ru-N4
Ru-Cl1
Ru-O6
1.729(4)
1.169(4)
2.131(3)
1.982(3)
1.980(3)
2.133(3)
1.735(3)
1.153(3)
2.132(3)
1.984(3)
1.970(3)
2.105(3)
2.361(1)
1.732(3)
1.167(3)
2.115(3)
1.975(3)
1.977(3)
2.128(3)
2.004(3)
2.022(2)
Bond Angles
Ru-N5-O5
N5-Ru- Cl1
N5-Ru- O6
169.3(3)
172.5(1)
175.9(2)
174.8(1)
176.5(3)
170.7(1)
(1995-99) software package (Bruker Analytical X-ray Systems
Inc.) for structure solution and refinement. Additional refine-
ment details are contained in the CIF files (Supporting Infor-
mation). Instrument parameters, crystal data, and data collec-
tion parameters for all the complexes are summarized in Table 1.
Selected bond distances and bond angles for 4- Resf 2CHCl3
3
3
(0.5)Et2O, 5-Cl 2CHCl3, and 5-Resf are listed in Table 2.
3
Photolysis Experiments. The quantum yield (φ) values were
obtained using a tunable Apex Illuminator (150W xenon lamp)
equipped with a Cornerstone 130 1/8 M monochromator
(measured intensity of ∼10 mW). Actinochrome N (475/610)
was used to determine the quantum yield values at 500 nm (φ500).
Solutions of 4-Cl, 4-Resf, 5-Cl, and 5-Resf were prepared and
placed in 2 ꢀ 10 mm quartz cuvettes, 1 cm away from the light
source. All solutions were prepared to ensure sufficient absor-
bance (>90%) at the irradiation wavelength, and changes
in electronic spectrum at 788, 650, 630, and 660 nm for 4-Cl,
4-Resf, 5-Cl, and 5-Resf, respectively (<10% photolysis), were
used to determine the extent of photorelease of NO.
Results and Discussion
To remove the twist of the bQb2- ligand frame in the equa-
torial planes of 2-Cl, 2-Resf, 3-Cl, and 3-Resf, we included the

Article
Inorganic Chemistry, Vol. 50, No. 1, 2011 321
Figure 4. Thermal ellipsoid (probability level 50%) plot of [((OMe)2-
IQ1)Ru(NO)(Cl)] (5-Cl) with select atom-labeling. H atoms are omitted
for the sake of clarity.
Figure 3. Thermal ellipsoid (probability level 50%) plot of [((OMe)2bpb)-
Ru(NO)(Resf)] (4-Resf) with select atom-labeling. H atoms are omitted
for the sake of clarity.
the stability of 5-Resf is improved compared to that of the
{RuNO}6 nitrosyls with twisted quinoline ligand frames,
5-Resf is still slightly less stable than the pyridine-containing
nitrosyls 1-Resf and 4-Resf. Collectively, the results suggest
that the use of planar pyridine-based ligand frames provides
the most stability in these designed dye-tethered {RuNO}6
nitrosyls.
Figure 5. X-ray structure of [((OMe)2bQb)Ru(NO)(Cl)] (left) and
[((OMe)2IQ1)Ru(NO)(Cl)] (right) displaying twist of the in-plane tetra-
dentate ligand. H atoms are omitted for the sake of clarity.
Structure of the Complexes. [((OMe)2bpb)Ru(NO)(Resf )]
(4-Resf ). The tetradentate (OMe)2bpb2- ligand frame of
4-Resf is bound to the ruthenium center in a planar fashion
occupying the equatorial plane (Figure 3). The pseudo-
octahedral structure is completed with the phenolato-O
bound resorufin dye trans to NO at the axial positions.
The Ru-N5-O5 bond angle is slightly bent at 169.3(3)°
(Table 2). The substitution of methoxy groups in 4-Resf
compared to methyl groups in 1-Resf does little to alter
the overall geometry of the two structures. In both cases,
the resorufin dye is tilted away from the equatorial plane
o
with ∼120 Ru-O6-C(Resf) bond angles (1-Resf =
128.46(11)°,18b 4-Resf = 126.0(3)°) implying that the
bound oxygen is more sp2 than sp3 hybridized. The dye is
positioned in such a way that it is directly below one of the
pyridine rings of the equatorial ligand and presumably
provides favorable π-π interactions that stabilize the struc-
tures of both 1-Resf and 4-Resf. As a result, this pyridine
ring is very slightly off the plane of the rest of the ligand
Figure 6. Thermal ellipsoid (probability level 50%) plot of [((OMe)2-
IQ1)Ru(NO)(Resf)] (5-Resf) with selected atom-labeling. H atoms are
omitted for the sake of clarity.
˚
˚
frame. The Ru-N5 (1.729(4) A) and N5-O5 (1.169(4) A)
bonds lengths of 4-Resf are also similar to those of 1-Resf
of 5-Cl are very similar to those of 3-Cl (1.7457(18) and
(1.7347(16)18b and 1.159(2) A respectively).
˚
1.148(2) A19 respectively). Both nitrosyls contain almost
˚
19
˚
[((OMe)2IQ1)Ru(NO)(Cl)] (5-Cl). The presence of
linear Ru-N5-O5 bond angles (3-Cl = 175.73(19) A;
6
1-isoquinoline moieties in the tetradentate (OMe)2IQ12-
5-Cl = 175.9(2) A), typical of {RuNO} nitrosyls.
˚
ligand instead of regular quinoline (as in (OMe)2bQb2-
)
[((OMe)2IQ1)Ru(NO)(Resf)] (5-Resf). The (OMe)2IQ12-
ligand frame of 5-Resf is bound to the ruthenium center in the
equatorial plane while the resorufin dye is trans to NO at the
axial positions (Figure 6). Here also, the resorufin dye is
bound through the phenolato-O and a similar tilt of the
bound dye (Ru-O6-C(Resf ) angle of 126.3(2)°) is observed.
The position of the bound resorufin is such that the rings of
the dye are aligned parallel to the rings of one of the 1-iso-
quinoline moieties in the (OMe)2IQ12- ligand frame of
5-Resf.Thisπ-πinteraction causes the 1-isoquinoline moiety
of (OMe)2IQ12- to tilt slightly out of the plane of the rest of
allows the ligand to remain strictly planar when bound to
the ruthenium center in the equatorial plane of 5-Cl
(Figure 4). This is in contrast to the steric interaction of
the extended quinoline groups in the (OMe)2bQb2- ligand
of 3-Cl that led to the twisting of the ligand (∼35°) as
noted in our previous work (Figure 5).19 A chloride ligand
is bound trans to NO in the axial positions generating a
nearly octahedral geometry in the structure of 5-Cl. Exami-
nation of bond lengths in Table 2 reveals that the Ru-
˚
˚
N5(O) (1.735(3) A) and N5-O5 (1.153(3) A) bond lengths

322 Inorganic Chemistry, Vol. 50, No. 1, 2011
Fry et al.
Figure 7. Electronic absorption spectra of 1-Cl17d and 4-Cl (top left), 2-Cl,17d 3-Cl,19 and 5-Cl (top right) in DMF. Electronic absorption spectra of
1-Resf18b and 4-Resf (bottom left), and 2-Resf,18b 3-Resf,18b and 4-Resf (bottom right) in CHCl3.
the tetradentate ligand frame. Despite this tilt of the ligand in
the equatorial plane, 5-Resf is significantly more planar than
that of 3-Resf which contains the regular quinoline ligand,
(OMe)2bQb2-. The Ru-NO unit in 5-Resf comprises Ru-
electron donating groups of increasing strength (Me <
OMe) lowers the energy of the absorption band λmax
further to 490 nm (in the case of [((OMe)2bQb)Ru-
(NO)(Cl)] (3-Cl)).18b Along the same line, 4-Cl displays
a red shift in λmax (395 to 420 nm) because of the
replacement of the Me substituents (in 1-Cl) with OMe
groups on the R2bpb2- ligand frame (Figure 7, top panel).
Interestingly, the absorption band of 4-Cl is more intense
(ε = 7 800 M-1 cm-1) than the corresponding absorption
bands of 1-Cl (ε = 5300 M-1 cm-1),17d 2-Cl (ε = 3 200
˚
N5(O) and N5-O5 bond lengths of 1.732(3) A and
˚
1.167(3) A, respectively, which fall within the ranges observed
˚
in the other resorufin nitrosyls (1.729 -1.7425 A and 1.154-
1.169 A, respectively).18b In addition, the Ru-N5-O5 bond
˚
angle of 5-Resf is almost linear (176.5(3)°) as expected for a
{RuNO}6 nitrosyl.
Spectroscopic Properties. The carbonyl stretching fre-
M
-1 cm-1),17d and 3-Cl (ε = 3750 M-1 cm-1).18b
quency (νCdO) of the free ligand H2(OMe)2bpb (1676 cm-1
)
The electronic absorption spectrum of 5-Cl has an
intense absorption band with λmax of 475 nm (ε = 8700
M
shifts to lower energy upon formation of complexes 4-Cl
(1631 cm-1) and 4-Resf (1630 cm-1) (Supporting Infor-
mation, Figure S1-S2). Similar lowering of νCdO value of
H2(OMe)2IQ1 (1682 cm-1) is observed in 5-Cl (1614
cm-1) and 5-Resf (1617 cm-1) (Supporting Information,
Figure S3-S4). The NO stretching frequencies (νNO) of
4-Cl (1845 cm-1), 4-Resf (1831 cm-1), 5-Cl (1832 cm-1),
and 5-Resf (1827 cm-1) all fall within the range typical for
neutral {RuNO}6 nitrosyl containing dicarboxamide tetra-
dentate ligands.17 All four reported complexes are dia-
magnetic (as expected for{Ru-NO}6 nitrosyls) and display
-1 cm-1). Replacement of the quinoline moiety of 3-Cl
with 1-isoquinoline in 5-Cl, results in a 15 nm blue shift in
its λmax value. However, there is an increase in the
intensity of the absorption band of 5-Cl. Clearly, elimina-
tion of the twist in the equatorial plane (Figure 5) accen-
tuates the light-absorbing capacity of the nitrosyl in this
set. Comparison of the intensities of the visible bands of
1-Cl and 4-Cl (both devoid of twist in the equatorial plane)
indicates that replacement of Me with OMe groups on the
phenylenediamine moiety of the ligand frame also improves
the light harvesting capacity of the resulting nitrosyls.
To further enhance the sensitivity of 4-Cl and 5-Cl
toward visible light, we have replaced the chloride ligands
(trans to NO) with resorufin dye forming 4-Resf and
5-Resf. The absorption spectra of both Resf-conjugates
contain additional intense absorption bands at 500 nm
(ε = 29000 M-1 cm-1 and 31000 M-1 cm-1 respectively
in CHCl3). Interestingly, the deprotonated form of the
unbound dye (Resf-) has an intense absorption band at
590 nm (ε = 105000 M-1 cm-1 in DMF). Upon coordi-
nation to the positively charged metal center, a blue shift
of the dye band (with concomitant reduction in extinction
1
narrow-width peaks in their H NMR spectra in CDCl3
(Supporting Information, Figure S5-S8).
We have previously shown that variation in constituents
on the ligand frame can modulate the electronic absorp-
tion spectrum of the resulting ruthenium nitrosyl.17d For
example, a red shift of the λmax of the absorption band
from 395 nm (in the case of [(Me2bpb)Ru(NO)(Cl)]
(1-Cl)) to 455 nm (in the case of [(Me2bQb)Ru(NO)(Cl)]
(2-Cl)) is accomplished by extending conjugation of the
pyridine moiety by replacement with a quinoline donor
(Figure 7, top panel).17d In addition, substitution of
the phenylenediamine ring of R2bQb2- type ligands with

Article
Inorganic Chemistry, Vol. 50, No. 1, 2011 323
solvent bound Ru(III) photoproduct (eq 1), which typi-
cally exhibits a low-energy absorption band in its electro-
nic spectrum. For example, photolysis of 4-Cl in DMF
under visible light (λirr > 465 nm) generates a low-energy
absorption peak at 800 nm due to the Ru(III) photoprod-
uct (Supporting Information, Figure S9). This transi-
tion has previously been assigned to a ligand-to-metal
charge transfer (LMCT) from the negatively charged
ligand (in this case (OMe)2bpb2-) to the Ru(III) center.21
In addition, two other new absorption peaks at 270 and
375 nm along with clean isosbestic points at 280 and 325
nm are also generated following exposure of 4-Cl to
visible light. Under similar conditions, illumination of
5-Cl also causes changes in its electronic absorption spec-
trum (Supporting Information, Figure S11). New broad
absorption features at 425 and 640 nm are observed, and
isosbestic points are noted at 310 and 335 nm.
Figure 8. Fluorescence emission spectrum of 5-Resf and 1-Resf in
MeCN (λex= 490 nm). Samples with same absorbance values at 490
nm were used.
ðLÞRu-NO þ hν þ solv f ðLÞRuðIIIÞ-solv þ NO• ð1Þ
Comparison of the quantum yield (φ) values of NO
photorelease (in DMF) upon exposure to monochro-
matic 500 nm light of these chloride bound nitrosyls
indicate that 5-Cl (0.035 ( 0.005) is a more efficient NO
donor than 4-Cl (0.010 ( 0.005). Comparison of these
values (Table 3) with those measured under similar con-
ditions for 1-Cl, 2-Cl, and 3-Cl reveals a trend in the
efficiency of NO photorelease of 1-Cl < 2-Cl ∼ 4-Cl < 3-
Cl < 5-Cl. This result confirms that alterations in the
equatorial ligand frame do lead to increased sensitivity to
visible light in ruthenium nitrosyls. In particular, removal
of the twist in the equatorial ligand frame leads to
increased NO photolability as exemplified by the trend
3-Cl < 5-Cl.
[((OMe)2bpb)Ru(NO)(Resf)] (4-Resf) and [((OMe)2-
IQ1)Ru(NO)(Resf)] (5-Resf). A significant increase in
the photosensitivity (to visible light) is noted with 4-Resf
and 5-Resf in which the resorufin dye is directly coordi-
nated to the Ru center. For instance, photolysis of 4-Resf
in MeCN under visible light (λirr > 465 nm) produces new
absorption peaks at 390 and 650 nm in its absorption
spectrum because of rapid photorelease of NO and con-
comitant formation of solvent bound Ru(III) photoprod-
uct. (Supporting Information, Figure S10). Clean iso-
sbestic points at 435 and 520 nm are also observed. In the
case of 5-Resf, illumination of a MeCN solution with
visible light (λirr > 465 nm) causes the main dye-bound
absorption band at 500 nm to blue shift incrementally to
470 nm and a new absorption feature at 675 nm to grow in
(Figure 9). Clean isosbestic points at 330, 475, and 545 nm
are also noted. Quantum yield (φ) values of these dye
bound nitrosyls in DMF were measured to quantitatively
compare the extent of photosensitization to visible light.
Under illumination with monochromatic 500 nm light,
4-Resf exhibits a φ500 value of 0.120 ( 0.008 which is
much greater than that of the parent nitrosyls, 4-Cl. The
quantum yield of 5-Resf (0.271 ( 0.008) is also higher
than that of 5-Cl, clearly showing the beneficial effect of
direct dye conjugation. When all five resorufin bound
coefficient) occurs in the nitrosyl-dye conjugates. We have
previously assigned such changes to the loss of delocali-
zation of negative charge over the heterocyclic π-system
and the loss of overall symmetry of the dye upon coordi-
nation. Indeed, simple protonation of Resf- also brings
about similar changes in its electronic absorption spec-
trum (λmax = 470 nm, ε = 20 000 M-1 cm-1 in DMF).19
A comparison of the five Ru-Resf complexes (1-Resf
through 5-Resf) reveals a uniform shift in the λmax of the
dye absorption band to 500 nm upon complexation.
However, the ε-values of these complexes are greatly varied.
For example, 1-Resf and 2-Resf both have ε-values
around 12 000 M-1 cm-118b while 3-Resf, 4-Resf, and
5-Resf have ε-values around 30 000 M-1 cm-1. The most
apparent difference between the two sets of complexes is
the presence of Me versus OMe substituents on the tetra-
dentate ligand frame. It therefore appears that in these
nitrosyl-dye conjugates, the substituents on the ligand
frame have more pronounced effect on the overall capac-
ity of light absorption compared to the twist of the frame
in the equatorial plane.
In addition to enhanced capacity of visible light ab-
sorption, the nitrosyl-dye conjugates also exhibit fluo-
rescence arising from the bound dye moiety. Solutions of
free resorufin in its deprotonated form are highly fluo-
rescent. Upon direct coordination of the dye to the {RuNO}6
center, the fluorescence is significantly quenched presum-
ably because of energy transfer from the dye to the {RuNO}6
unit. However, solutions of both 4-Resf and 5-Resf in
MeCN still exhibit broad red fluorescence emission bands
of moderate intensity at 593 nm (Figure 8). Similar fluo-
rescence properties have previously been observed with
1-Resf-3-Resf.18b In the case of 1-Resf, the residual fluo-
rescence was enough to monitor the location of complex
during targeted NO delivery in a cellular environment.18b
We anticipate similar use of these nitrosyl-dye conjugates
as “trackable” NO donors in our future experiments.
NO Photolability. [((OMe)2bpb)Ru(NO)(Cl)] (4-Cl)
and [((OMe)2IQ1)Ru(NO) (Cl)] (5-Cl). Exposure of solu-
tions of the parent nitrosyls 4-Cl and 5-Cl in DMF to
visible light promotes rapid release of NO (as evidenced
by the responses of an NO-sensitive electrode). The photo-
dissociation of NO is accompanied by the formation of a
(21) (a) Bordini, J.; Hughes, D. L.; Da Motta Neto, J. D.; da Cunha, J. C.
€
Inorg. Chem. 2002, 41, 5410–5416. (b) Sellmann, D.; Hauβinger, D.; Gottschalk-
Gaudig, T.; Heinemann, F. W. Z. Naturforsch. 2000, 55b, 723–729.

324 Inorganic Chemistry, Vol. 50, No. 1, 2011
Fry et al.
Table 3. Summary of Absorption Parameters (λmax, ε) and Quantum Yield (φ) Values of Selected Nitrosyls
complex
quantum yield φ (λirr, nm)
solvent
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
MeCN
H2O
CHCl3
CHCl3
MeCN/H2O
λ
max, nm (ε, M-1 cm-1
)
[(Me bpb)Ru(NO)(Resf)] (1-Resf)a
[(Me2bQb)Ru(NO)(Resf)] (2-Resf)a
[((OMe)2bQb)Ru(NO)(Resf)] (3-Resf)a
[(OMe2bpb)Ru(NO)(Resf)] (4-Resf)
[(OMe2IQ1)Ru(NO)(Resf)] (5-Resf)
[(Me2bpb)Ru(NO)(Cl)](1-Cl)
0.052 ( 0.008 (500)
0.102 ( 0.009 (500)
0.206 ( 0.008 (500)
0.120 ( 0.008 (500)
0.271 ( 0.008 (500)
0.0008 ( 0.0002 (500)
0.010 ( 0.003 (500)
0.025 ( 0.003 (500)
0.010 ( 0.005 (500)
0.035 ( 0.005 (500)
0.050 ( 0.004 (532)
0.025 ( 0.002 (532)
0.00019 ( 0.00005(546)
0.00025 ( 0.00005(546)
0.0036 ( 0.0005(436)
500 (11 920)
500 (12 300)
500 (27 100)
500 (29 000)
500 (31 000)
395 (5 300)
455 (3 200)
490 (3750)
420 (7 800)
475 (8 700)
530 sh (400)
530 (9 550)
364 (8 500)
538 (10 200)
506 (72 200)
[(Me2bQb)Ru(NO)(Cl)] (2-Cl)
[((OMe)2bQb)Ru(NO)(Cl)] (3-Cl)
[((OMe)2bpb)Ru(NO)(Cl)](4-Cl)
[((OMe)2IQ1)Ru(NO)(Cl)] (5-Cl)
b
[(Py3P)Ru(NO)]BF4
[Ru(NH3)5(pz)Ru(bpy)2(NO)](PF6)5
c
Roussin’s Salt Ester (RSE)d
PPIX-RSEe
Fluor-RSEf
a Ref 18b. b Ref 17c. c Ref 22. d Ref 23. e Ref 24. f Ref 25.
of these nitrosyls. The structures of 4-Resf, 5-Cl
and 5-Resf have been determined by X-ray crystal-
lography.
(b) Unlike the previously reported 2-Cl, 3-Cl, 2-Resf,
and 3-Resf nitrosyls derived from the R2bQb2-
ligand frame that induces ligand twist in their
equatorial plane, the equatorial ligand frames in
the present nitrosyls are planar. Such relief in the
steric strain increases the stability of the nitrosyls
in strongly coordinating solvents including water.
(c) In addition to stability in aqueous solution, re-
moval of ligand twist enhances the sensitivity of
the nitrosyls to visible light. As measured by the
extinction coefficients and quantum yield values
at 500 nm (φ500), addition of OMe substituents on
the ligand frame and removal of steric interac-
tions between the quinoline moieties of the ligand
both improve the sensitivity of the nitrosyls to-
ward visible light. Thus 5-Resf exhibits the high-
est sensitivity to 500 nm light.
Figure 9. Changes in the electronic absorption spectrum upon photo-
lysis of 5-Resf in MeCN following illumination with visible light. Inset:
NO amperogram of 5-Resf in MeCN/H2O upon illumination with visible
light for time periods (in seconds) as indicated.
(d) Direct coordination of the Resf dye results in
significant quenching of its fluorescence in these
dye-tethered nitrosyls. The residual fluorescence
of the Resf-bound nitrosyls however is sufficient
for tracking these photoactive NO donors in
cellular targets. For example, the fluorescence
intensity of 5-Resf (present study) is similar to
that of 1-Resf (Figure 8) which has already been
used as trackable NO donor in cellular studies.18b
ruthenium nitrosyls are compared (1-Resf through
5-Resf, Table 3), those with OMe-substituted ligand
frames exhibit higher φ500 values than the Me-bearing
ligands. Increased visible light absorption by the OMe-
substituted complexes is presumably responsible for such
enhancement. Removal of ligand twist enhances the φ500
value of 5-Resf further to the maximum of 27%.
Summary and Conclusions
Acknowledgment. Support from a grant from the Na-
tional Science Foundation (CHE-0957251) is gratefully
acknowledged. N.F. was partially supported by a COR-
SRG grant from UCSC. We thank Dr. David Rogow for
help in the crystallographic measurements. The X-ray
facility at UCSC is supported by a NSF Major Research
Instrumentation (MRI) grant (CHE-0521569).
The following are the summary and conclusions of this
investigation.
(a) Two new sets of chloride-bound (4-Cl and 5-Cl)
and Resf-bound (4-Resf and 5-Resf) {RuNO}6
nitrosyls have been synthesized from two designed
tetradentate ligands that allowed assessment of
the effects of ligand twists in the equatorial plane
(22) Sauaia, M. G.; de Lima, R. G.; Tedesco, A. C.; da Silva, R. S. J. Am.
Chem. Soc. 2003, 125, 14718–14719.
(23) Conrado, C.; Bourassa, J. L.; Egler, C.; Wecksler, S.; Ford, P. C.
Inorg. Chem. 2003, 42, 2288–2293.
(24) Conrado, C. L.; Wecksler, S.; Egler, C.; Magde, D.; Ford, P. C.
Inorg. Chem. 2004, 43, 5543–5549.
(25) Wecksler, S. R.; Hutchinson, J.; Ford, P. C. Inorg. Chem. 2006, 45,
1192–1200.
Supporting Information Available: FTIR spectra (Figure S1-S4)
and 1H NMR spectra (S5-S8) for 4-Cl, 4-Resf, 5-Cl and
5-Resf, X-ray crystallographic data (in CIF format) for com-
plexes 4-Resf, 5-Cl, and 5-Resf, and changes in the electronic
absorption spectrum upon photolysis of 4-Cl, 4-Resf, 5-Cl
(Figure S9-S11). This material is available free of charge via
the Internet at http://pubs.acs.org.