Photocleavage of coordinated no under visible light from two different classes of organometallic ruthenium nitrosyl complexes: Reversible binding of phenolato function

Article abstract of DOI:10.1021/om101022p

The structurally similar Ru(III) complexes [Ru(L^SB1)(PPh ₃)₂Cl] (1), where L^SB1H₂ = 2-(4-chlorobenzylidineamino)phenol and H = dissociable protons, and [Ru(L ^AZ1)(PPh₃)₂Cl] (2), where L^AZ1H ₂ = 4-methyl-2-(p-tolyldiazenyl)phenol and H = dissociable protons, were reacted with in situ generated NO derived from acidified nitrite solution. These reactivity studies on 1 and 2 afforded the complexes [Ru(L ^SB2H)(PPh₃)₂(NO)Cl](ClO₄) (1a) and [Ru(L^AZ2H)(PPh₃)₂(NO)Cl](ClO₄) (2a), respectively, where L^SB2H = 2-(4-chlorobenzylidineamino)-4- nitrophenol, L^AZ2H = 4-methyl-2-nitro-6-(p-tolyldiazenyl)phenol, and H = dissociable proton. Complexes 1a and 2a were found to be diamagnetic and were characterized by ¹H and ³¹P NMR spectral studies. Both 1a and 2a exhibited v_NO bands near 1800 cm^-1 in their IR spectra. The molecular structures of 1a·CH₂Cl ₂·3H₂O and 2a·2H₂O were determined by X-ray crystallography, and NO coordination as well as ligand nitration were authenticated from the crystal structure. In both of the nitrosyl complexes coordinated NO was found to be photolabile under visible light and photocleaved NO was transferred to reduced myoglobin. The photolability of NO in 1a and 2a afforded the complexes [Ru(L^SB2)(PPh₃) ₂Cl] (1b) and [Ru(L^AZ2)(PPh₃)₂Cl] (2b), respectively, and the molecular structure of 1b·2CH ₂Cl₂ was confirmed by an X-ray crystal structure solution. A novel type of reversible binding of the phenolato function to the metal center was revealed during NO coordination and photocleavage.

Full text of DOI:10.1021/om101022p

ARTICLE
pubs.acs.org/Organometallics
Photocleavage of Coordinated NO under Visible Light from Two
Different Classes of Organometallic Ruthenium Nitrosyl Complexes:
Reversible Binding of Phenolato Function†
Kaushik Ghosh,* Sushil Kumar, Rajan Kumar, Udai P. Singh, and Nidhi Goel
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667 Uttarakhand, India
S Supporting Information
b
ABSTRACT: The structurally similar Ru(III) complexes
[Ru(LSB1)(PPh3)2Cl] (1), where LSB1H2 = 2-(4-chlorobenzyli-
dineamino)phenol and H = dissociable protons, and [Ru(LAZ1)-
(PPh3)2Cl] (2), where LAZ1H2 = 4-methyl-2-(p-tolyldiazenyl)-
phenol and H = dissociable protons, were reacted with in situ
generated NO derived from acidified nitrite solution. These
reactivity studies on 1 and 2 afforded the complexes [Ru(LSB2H)-
(PPh3)2(NO)Cl](ClO4) (1a) and [Ru(LAZ2H)(PPh3)2(NO)Cl]-
(ClO4) (2a), respectively, where LSB2H = 2-(4-chlorobenzyli-
dineamino)-4-nitrophenol, LAZ2H = 4-methyl-2-nitro-6-(p-tolyldia-
zenyl)phenol, and H = dissociable proton. Complexes 1a and 2a
1
were found to be diamagnetic and were characterized by H and 31P NMR spectral studies. Both 1a and 2a exhibited νNO bands near
1800 cmꢀ1 in their IR spectra. The molecular structures of 1a CH2Cl2 3H2O and 2a 2H2O were determined by X-ray crystallography, and
3
3
3
NO coordination as well as ligand nitration were authenticated from the crystal structure. In both of the nitrosyl complexes coordinated NO was
found to be photolabile under visible light and photocleaved NO was transferred to reduced myoglobin. The photolability of NO in 1a and 2a
afforded the complexes [Ru(LSB2)(PPh3)2Cl] (1b) and [Ru(LAZ2)(PPh3)2Cl] (2b), respectively, and the molecular structure of
1b 2CH2Cl2 was confirmed by an X-ray crystal structure solution. A novel type of reversible binding of the phenolato function to the
3
metal center was revealed during NO coordination and photocleavage.
Scheme 1
’ INTRODUCTION
There has been considerable current interest in studies on the
interaction of nitric oxide (NO) with transition-metal complexes.1ꢀ3
Nitric oxide, a diatomic radical species, is an important signaling
molecule in a variety of biological processes such as vasodilatation,
immune response, neurotransmission, apoptosis, etc. in different
cells and tissues.4 In biosystems, NO is synthesized by nitric oxide
synthase (NOS) enzyme and several concentration-dependent
activities of NO were discovered.5 Hence, a particular amount of
and target specific delivery of nitric oxide and scavenging of nitric
oxide6 by transition-metal complexes are important areas in chemical
research. Complexes which could deliver NO upon illumination with
light are important for photodynamic therapy.7 Our interest origi-
nated from the synthesis of novel organometallic ruthenium nitrosyl
complexes which could deliver NO on demand, because these types
of complexes are scarce.8 Recently we have communicated a novel
example of such a complex, where photoinduced NO delivery was
observed under visible light.9 In recent reviews by Mascharak and co-
workers3 and Pfeffer and co-workers,10 ruthenium cyclometalate
complexes were not mentioned for photoinduced delivery of
nitric oxide.
with two different classes of structurally similar organometallic
ruthenium(III) complexes (shown in Scheme 1).12,13
In the present work, we describe the synthesis and character-
ization of the novel σ-aryl ruthenium nitrosyl complexes
[Ru(LSB2H)(PPh3)2(NO)Cl](ClO4) (1a, where LSB2H = 2-
(4-chlorobenzylidineamino)-4-nitrophenol and H = dissociable
proton) and [Ru(LAZ2H)(PPh3)2(NO)Cl](ClO4) (2a, where
As part of our ongoing research on metal nitrosyl complexes,9,11
we have been studying the reactivity of acidified nitrite solution
Received: October 29, 2010
Published: April 11, 2011
r
2011 American Chemical Society
2498
dx.doi.org/10.1021/om101022p Organometallics 2011, 30, 2498–2505
|

Organometallics
ARTICLE
Scheme 2
LAZ2H = 4-methyl-2-nitro-6-(p-tolyldiazenyl)phenol and H =
dissociable proton) derived from [Ru(LSB1)(PPh3)2Cl] (1) and
[Ru(LAZ1)(PPh3)2Cl] (2), respectively, where LSB1H2 = 2-(4-
chlorobenzylidineamino)phenol, LAZ1H2 = 4-methyl-2-(p-tolyl-
diazenyl)phenol, and H = dissociable protons (Schemes 1 and
2). Molecular structures of the resultant complexes 1a CH2Cl2
3
3
3H2O and 2a 2H2O were determined by X-ray crystallography.
3
Ligand nitration in the phenyl ring containing the phenolato
function was also observed in the nitrosyl complexes. Photol-
ability of the coordinated NO in the nitrosyl complexes upon
illumination of visible light was examined, and the liberated
nitric oxide was trapped by reduced myoglobin. We isolated
the complexes [Ru(LSB2)(PPh3)2Cl] (1b) and [Ru(LAZ2)-
(PPh3)2Cl] (2b) obtained after the photorelease of coordinated
NO, and the molecular structure of one of the resultant com-
plexes was authenticated by single-crystal X-ray diffraction. The
dissociation of the rutheniumꢀphenolato bond and re-establish-
ment of the same bond, respectively, during the coordination and
photorelease of NO will be scrutinized in this report.
Figure 1. ORTEP diagram (50% probability level) of the cation of
complex 1a CH2Cl2 3H2O. All hydrogen atoms and solvent molecules
are omitted for clarity.
3
3
’ RESULTS AND DISCUSSION
Synthesis. The precursor complexes [Ru(LSB1)(PPh3)2Cl]
(1) and [Ru(LAZ1)(PPh3)2Cl] (2) were synthesized according
to the procedures reported by Chakravorty and co-workers.12,13
These complexes were reacted with in situ generated NO derived
from acidified NaNO2 solution. The formation of an orange-
yellow color was observed when dichloromethane solutions of 1
(purple) and 2 (green) were treated with acidified NaNO2
solution with continuous stirring for 1 h. We expected the
substitution of Clꢀ with the noninnocent ligand NO, and hence
ꢀ
we added ClO4 ion to stabilize the large cationic ruthenium
complexes [Ru(LSB2H)(PPh3)2(NO)Cl](ClO4) (1a) and [Ru-
(LAZ2H)(PPh3)2(NO)Cl](ClO4) (2a), derived from 1 and 2,
respectively. The nitrosyl complexes 1a and 2a were both
recrystallized from a dichloromethaneꢀmethanol mixture. Com-
plexes 1a and 2a were highly soluble in nonaqueous solvents such
as dichloromethane, acetonitrile, dimethylformamide, and
methanol; however, lower solubility was observed in water.
Photodissociation of coordinated NO from the nitrosyl com-
plexes 1a and 2a afforded two more complexes: [Ru(LSB2)-
(PPh3)2Cl] (1b) and [Ru(LAZ2)(PPh3)2Cl] (2b), respectively.
It has been found that 1a and 2a could be synthesized again from
1b and 2b, respectively, using acid nitrite solution (vide infra).
No reactivity was observed for precursor complexes with acid-
ified water solution without NaNO2.
Figure 2. ORTEP diagram (50% probability level) of the cation of
complex 2a 2H2O. All hydrogen atoms and solvent molecules are
3
omitted for clarity.
are depicted in Figures 1ꢀ3, respectively. The matrix parameters
of these three complexes are described in Table 1, and selected
bond distances and bond angles are given in Table 2.
[Ru(LSB2H)(PPh3)2(NO)Cl]ClO4 (1a CH2Cl2 3H2O) and [Ru-
3
3
(LAZ2H)(PPh3)2(NO)Cl]ClO4 (2a 2H2O). The molecular struc-
3
ture of 1a CH2Cl2 3H2O revealed several interesting aspects of
this resultant complex. Contrary to our expectations, we found
that the Clꢀ ion was still bound to the metal center. The carbanion
3
3
Description of Structures. The molecular structures of
complexes 1a CH2Cl2 3H2O, 2a 2H2O, and 1b 2CH2Cl2
3
3
3
3
2499
dx.doi.org/10.1021/om101022p |Organometallics 2011, 30, 2498–2505

Organometallics
ARTICLE
(C(37)), Cl(1), NO, and imine nitrogen (N(1)) constituted the
equatorial plane, whereas the phosphine groups occupied the trans
positions. Thus, the geometry around the metal center was
distorted octahedral. Interestingly, in 1a CH2Cl2 3H2O, the
3
3
phenolato oxygen was no longer bound to the metal center and
picked up a proton from the reaction mixture, affording a phenolic
ꢀOH function. Hence, the tridentate ligand became bidentate in
the σ-aryl ruthenium nitrosyl complex 1a CH2Cl2 3H2O. The
3
3
phenyl ring containing the phenolic ꢀOH function became tilted
and made an angle of ∼50° with the ligand binding plane. The
RuꢀNNO distance (1.787 Å) was longer than the reported
values,14ꢀ16 which may be due to the trans effect of the carbanion
(C(37)).13 However, this value was smaller than the value
reported by Crutchley and co-workers.17
The molecular structure of 2a 2H2O was found to be similar
3
to the crystal structure of 1a CH2Cl2 3H2O. In 2a 2H2O, the
3
3
3
carbanion (C(37)), Cl1, NO and imine nitrogen (N(1)) con-
stituted the equatorial plane and phosphine groups were oriented
along the axial positions. In 2a 2H2O, the phenolato oxygen
3
became detached from the metal center and the ligand became
bidentate with carbanion and azo-N donation. The phenyl ring
containing the phenolato function made an angle of ∼56° with
the ligand binding plane.
Figure 3. ORTEP diagram (50% probability level) of complex
1b 2CH2Cl2. All hydrogen atoms and solvent molecules are omitted
3
for clarity.
Table 1. Summary of Crystal Data and Data Collection Parameters for 1a CH2Cl2 3H2O, 1b 2CH2Cl2, and 2a 2H2O
3
3
3
3
1a
2a
1b
empirical formula
formula wt
space group
temp (K)
λ(Mo KR) (Å)
cryst syst
a (Å)
C
50H38Cl5N3O11 P2Ru
C50H42Cl2N4O10P2Ru
1092.79
C51H41Cl6N2O3P2Ru
1105.57
1197.09
P1
P1
Cmc21
296(2)
296(2)
293(2)
0.710 73
0.710 73
0.710 73
triclinic
triclinic
orthorhombic
14.854(5)
18.898(5)
17.153(5)
90.000(5)
90.000(5)
90.000(5)
4815(2)
12.637(3)
13.323(3)
17.448(6)
102.666(13)
111.822(9)
98.408(13)
2575.3(13)
2
10.9154(7)
13.5046(8)
18.3960(11)
104.619(2)
90.777(3)
105.312(3)
2521.4(3)
2
b (Å)
c (Å)
R (deg)
γ (deg)
β (deg)
V (Å3)
Z
4
F
calcd (g cmꢀ3
)
1.544
1.439
1.515
cryst size (mm)
F(000)
0.25 ꢁ 0.18 ꢁ 0.14
1212.0
0.32 ꢁ 0.27 ꢁ 0.21
1116.0
0.29 ꢁ 0.23 ꢁ 0.17
2244.0
θ range for data collection
index ranges
1.24ꢀ28.60
ꢀ16 < h < 16
ꢀ16 < k < 17
ꢀ23 < l < 23
1.19ꢀ28.31
ꢀ14 < h < 14
ꢀ17 < k < 14
ꢀ24 < l < 24
1.74ꢀ22.06
ꢀ15 < h < 15
ꢀ19 < k < 19
ꢀ17 < l < 17
refinement method
no. of data/restraints/params
GOFa on F2
full-matrix least squares on F2
12 486/0/650
0.937
11 868/0/625
1.208
2954/1/334
0.945
R1b (I > 2σ(I))
0.0662
0.0587
0.0573
R1 (all data)
0.0869
0.0865
0.0709
wR2c (I > 2σ(I))
0.1872
0.1725
0.1677
wR2 (all data)
2
0.2170
0.1994
0.1968
2
a GOF = [∑[w(Fo ꢀ Fc2)2]/M ꢀ N]1/2 (M = number of reflections, N = number of parameters refined). b R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|. c wR2 = [∑[w(Fo
ꢀ
Fc2)2]/∑[(Fo ) ]]1/2
2 2
.
2500
dx.doi.org/10.1021/om101022p |Organometallics 2011, 30, 2498–2505

Organometallics
ARTICLE
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
1a CH2Cl2 3H2O, 1b 2CH2Cl2, and 2a H2O
derived from crystal structure solution, we found that the only
difference between 1 and 1b was the presence of an ꢀNO2 group
in the ring containing the phenolato function in the tridentate
ligand of complex 1b. Moreover, the bidentate ligands in 1a and
2a retained their tridentate behavior by reestablishing the
3
3
3
3
1a
2a
Bond Lengths (Å)
2.3503(12)
2.4412(12)
2.4309(13)
2.107(5)
RuꢀOPh bond. In the molecular structure of 1b 2CH2Cl2, the
3
Ru(1)ꢀCl(1)
Ru(1)ꢀP(1)
Ru(1)ꢀP(2)
Ru(1)ꢀC(37)
Ru(1)ꢀN(1)
Ru(1)ꢀN(2)
N(2)ꢀO(1)
2.3721(11)
2.4672(12)
2.4529(12)
2.071(4)
carbanion (C(27)), imine nitrogen (N(1)), phenolato oxygen
(Oph), and Clꢀ constituted the equatorial plane, whereas two
PPh3 groups acted as axial ligands. The geometry around the
metal center was found to be distorted octahedral. The RuꢀC
(2.051 Å) distance was consistent with values reported in the
literature.12,13,17 RuꢀN1, RuꢀP, and RuꢀCl bond distances
were also consistent with the values reported earlier.13,20,21
A comparison of the three structures described above revealed
some important points. We have observed that RuꢀP distances
in 2a 2H2O and 1a CH2Cl2 3H2O were slightly greater than
2.129(4)
2.094(3)
1.787(4)
1.805(4)
1.165(5)
1.160(5)
Bond Angles (deg)
172.6(4)
O(1)ꢀN(2)ꢀRu(1)
N(2)ꢀRu(1)ꢀN(1)
N(2)ꢀRu(1)ꢀC(37)
N(1)ꢀRu(1)ꢀC(37)
N(2)ꢀRu(1)ꢀCl(1)
N(1)ꢀRu(1)ꢀCl(1)
C(37)ꢀRu(1)ꢀCl(1)
N(2)ꢀRu(1)ꢀP(1)
N(1)ꢀRu(1)ꢀP(1)
C(37)ꢀRu(1)ꢀP(1)
N(2)ꢀRu(1)ꢀP(2)
N(1)ꢀRu(1)ꢀP(2)
C(37)ꢀRu(1)ꢀP(2)
P(1)ꢀRu(1)ꢀP(2)
170.5(4)
3
3
3
94.15(16)
171.11(18)
77.16(17)
101.06(13)
164.73(11)
87.68(14)
92.86(13)
95.56(11)
86.16(12)
94.25(13)
92.01(11)
88.04(12)
169.20(4)
93.72(15)
169.31(17)
75.63(15)
101.30(12)
164.97(10)
89.34(13)
97.91(12)
94.04(10)
83.90(12)
91.62(12)
93.18(10)
88.18(12)
167.63(4)
the values reported by Chakravorty and co-workers in the
precursor complex13 and by Ramesh and co-workers.20 The
PꢀRuꢀP angle was found to be 174.4° in 2;13 however, in the
nitrosyl complex 2a 2H2O, this angle was 167.6°. A similar
3
observation was also found in our recent report.9 These data
indicated greater distortion of axial bonds in the nitrosyl com-
plexes compared to the precursor complexes. A comparison of
the PꢀRuꢀP angle in 1a CH2Cl2 3H2O and 1b 2CH2Cl2 also
3
3
3
provided similar distortion.
On going from precursor complexes to nitrosyl complexes, it
was observed that in nitrosyl complexes there was approximately
no change in RuꢀCl bond distance; however, there was a slight
increase (0.04ꢀ0.08 Å) in RuꢀN1, RuꢀC,17and RuꢀP bond
lengths. The RuꢀOPh distance was longer than the distances
reported in the literature;15,21 however, it was consistent with
values obtained by Lahiri et al.13 and Kannan et al.20 Comparison
of these data with literature data predicted that the longer
RuꢀOPh bond length was due to the trans effect of the carbanion
donor13 and this may be important for NO coordination.
Spectroscopic Studies. The electronic spectra of complexes
1a and 2a in dichloromethane are displayed in Figure 5. Both
complexes exhibited one strong absorption band with λmax near
310 nm. In addition, a strong absorption band with λmax near
412 nm was found in the UVꢀvis spectrum of complex 2a, while
in the case of 1a a shoulder was found near 400 nm (Figure 5).
The infrared spectra of both 1a and 2a provided an NꢀO
stretching frequency (νNO) near 1800 cmꢀ1, which was expected
for complexes having an {Ru-NO}6 moiety.14ꢀ19 The peaks
around 1090 and 623 cmꢀ1 clearly showed the presence of a
perchlorate ion9 in complexes 1a and 2a. The complexes also
Complex 1b
Bond Lengths (Å)
Ru(1)ꢀCl(1)
Ru(1)ꢀP(1)
Ru(1)ꢀC(27)
2.343(4) Ru(1)ꢀN(1)
2.387(2) Ru(1)ꢀO(1)
2.051(14)
2.053(13)
2.152(9)
Bond Angles (deg)
N(1)ꢀRu(1)ꢀC(27) 77.90(5)
N(1)ꢀRu(1)ꢀCl(1) 179.8(4)
C(27)ꢀRu(1)ꢀCl(1) 101.9(4)
C(27)ꢀRu(1)ꢀO(1) 155.3(5)
O(1)ꢀRu(1)ꢀCl(1) 102.8(3)
O(1)ꢀRu(1)ꢀP(1)
Cl(1)ꢀRu(1)ꢀP(1)
P(1)ꢀRu(1)ꢀP(1)
89.07(5)
88.77(8)
N(1)ꢀRu(1)ꢀP(1)
C(27)ꢀRu(1)ꢀP(1)
91.24(8)
91.46(6)
176.52(15)
Similar to the case for complex 1a CH2Cl2 3H2O, the
3
3
gave rise to a few new bands in the range 1290ꢀ1380 cmꢀ1
,
RuꢀNNO distance (1.805 Å) was found to be longer than the
reported values,14ꢀ16 but it was closer to the value reported by
Crutchley and co-workers.17 The lengthening of this RuꢀNNO
bond may be due to the trans effect of the carbanion.13
which were assigned to probable ring nitration (shown in the
Supporting Information). In all of the complexes, the peaks near
745, 695, and 520 cmꢀ1 confirm the presence of PPh3 groups.20
1H NMR spectra of 1a (shown in Figure 4) and 2a (prepared
and run in the dark) clearly indicated the presence of the
phenolic OꢀH proton at 11.75 and 10.35 ppm, respectively,
along with other protons present in the complexes. The
disappearance of the peak at 11.75 ppm in complex 1a when
the NMR sample solution was shaken with D2O supported the
presence of an exchangeable proton in the ꢀOH functional
group (Figure S13, Supporting Information).22 These data
supported the dissociation of the RuꢀOPh bond during NO
coordination. In both complexes, 31P NMR spectra provided a
single peak at ∼21 ppm, confirming the trans disposition of the
PPh3 groups.23
All these data described above clearly indicated the presence
of a {RuNO}6 moiety (according to Enemark and Feltham
notation18) in both 1a CH2Cl2 3H2O and 2a 2H2O with an
3
3
3
S = 0 ground state. NO stretching frequencies (νNO at
∼1800 cmꢀ1) (vide infra) and NꢀO bond lengths were con-
sistent with reported values.14ꢀ19 RuꢀN and NꢀO distances
and RuꢀNꢀO angles (172.6° for 1a CH2Cl2 3H2O and 170.5°
3
3þ 6
for 2a 2H2O) clearly expressed the {RuII-NO } description of
3
the {RuNO}6 moiety.18,19
[Ru(LSB2)(PPh3)2Cl] (1b 2CH2Cl2). Complex 1b was derived
from 1a after photorelease of coordinated NO upon illumination
3
of visible light (vide infra). On examination of the structure
2501
dx.doi.org/10.1021/om101022p |Organometallics 2011, 30, 2498–2505

Organometallics
ARTICLE
Figure 4. 1H NMR (a) and 31P NMR spectra (b) (500 MHz) of complex 1a in (CD3)2SO at room temperature.
Figure 6. Photodissociation of complex 1a (∼1.8 ꢁ 10ꢀ5 M) in
acetonitrile under illumination with a 60 W tungsten lamp with
isosbestic points at 296 and 369 nm. Inset: electronic absorption
spectrum of complex 1b in dichloromethane.
Figure 5. Electronic absorption spectra of 1a (solid line) and 2a
(broken line) in dichloromethane.
In both 1b and 2b, a charge-transfer band near 425 nm was
observed. Absorption bands near 528 nm for 1b and 650 nm for
2b were also observed. The infrared spectrum of complex 1b
exhibited a band near 1588 cmꢀ1 corresponding to the azo-
methine (νCdN) stretching frequency. This band was similar to
the band reported by Chakravorty and co-workers.12 In the
infrared spectrum of complex 2b, a few characteristic bands in the
(Figure 3). Therefore, these data prompted us to investigate
the flipping of complexes 1a and 1b through NO coordination
and NO dissociation. Hence, we treated complex 1b with
acidified nitrite solution and isolated complex 1a. Then we
studied the photocleavage of NO from complex 1a (shown in
Scheme 3). These data indicated the dissociation of the pheno-
lato ligand during NO interaction probably due to the trans effect
of the carbanion ligand (vide infra). After NO photocleavage, the
open position at the metal center became available for the
phenolato ligand. Retention of phenolato ligand coordination
to the metal center gave rise to the formation of 1b. Hence,
dissociation and re-establishment of RuꢀOPh bond was pre-
dicted during NO coordination and photorelease.
It is important to note here that in many cases of ruthenium
nitrosyl complex syntheses, the Clꢀ ligand was substituted by
NO,3,17 but in this case the RuꢀCl bond was retained. The above
findings are indicative of probable participation of a solvento
species generated due to RuꢀOPh bond dissociation during our
reactivity studies.22,24 However, we did not observe any appreci-
able change in UVꢀvis spectra of 1 and 2 in different solvents
(Figures S14 and S15, Supporting Information). Instead of
solvent, we tried to provide a negatively charged ligand at the
range 1400ꢀ1500 cmꢀ1 indicated the presence of an azo (νNdN
)
stretching frequency.20 In both 1b and 2b, the new bands in the
range 1300ꢀ1380 cmꢀ1 showed the presence of ring nitration.21
Photolysis Experiment of Complex 1a. The dichloromethane
solution of complex 1a can be stored in the dark for weeks.
However, when such a solution was exposed to visible light (60
W tungsten lamp), a rapid change in color from orange-yellow to
red was observed. The photolability of the coordinated NO was
examined by illumination of visible light (60 W tungsten lamp)
through an acetonitrile solution of complex 1a (shown in
Figure 6). No loss of NO was observed when the solution was
kept in the dark.
Interestingly, at the end of the photolability experiment of
complex 1a (Figure 6), we ended up with the new complex 1b.
Spectroscopic characterization and determination of the molec-
ular structure (vide supra) confirmed the formation of 1b
2502
dx.doi.org/10.1021/om101022p |Organometallics 2011, 30, 2498–2505

Organometallics
ARTICLE
Scheme 3
Figure 7. Photodissociation of 2a (∼1.5 ꢁ 10ꢀ5 M) in dichloro-
methane under illumination with a 60 W tungsten lamp with isosbestic
points at 252, 292, 348, 370, and 438 nm. Inset: electronic absorption
spectrum of complex 2b in dichloromethane.
metal center to observe RuꢀOPh bond dissociation or Clꢀ ligand
substitution. We refluxed 1 and 2 with CH3COONa, NaNO2,
NaN3, and NaCN in dimethylformamide, and the formation of
new ruthenium complexes was indicated. This experiment
afforded appreciable changes in UVꢀvis spectra (Figures S16
and S17, Supporting Information); however, simple stirring with
the same reagents described above was not sufficient to cause any
change in 1 and 2. Moreover, from the above reactivity studies we
cannot predict the dissociation of the RuꢀOPh bond by con-
sidering the possibilities of Clꢀ ligand substitution as well as
RuꢀOPh dissociation. Hence, we decided to synthesize 1a from
1b by treating NaNO2 (under refluxing condition) with 1b
followed by acidification. Lahiri and co-workers adopted16 this
procedure for their synthesis of ruthenium nitrosyl complexes,
and the RuꢀNO2 moiety was converted to a RuꢀNO species
under acidic conditions. Synthesis of 1a following the route of
Lahiri and co-workers clearly indicated the dissociation of the
RuꢀOPh bond, which was responsible for the formation of our
nitrosyl complex (details are given in the Supporting Informa-
tion, Scheme S1). However, the yield was very low in this
procedure, and other detailed studies are in progress.
Hence, it is evident from the above data that ligand nitration
and nitrosylation at the metal center are the two events happen-
ing in this reactivity study. In complex 1, ligand nitration was
observed at the para position of the phenolato function. On the
other hand, due to the presence of a methyl group at the para
position in complex 2, ligand nitration was observed at the
position ortho to the phenolato function. This ligand nitration
was probably governed by steric factors. We want to mention
here that in our recent report9 we found oxidative cyclization
along with metal nitrosylation and ligand nitration; however, this
time we did not observe this cyclization, even though complex 1
belongs to the same family. Work is in progress to get a better
insight into the mechanism of oxidative cyclization during this
reactivity study.
Figure 8. Electronic spectra of conversion of reduced Mb to MbꢀNO
adduct upon reaction with 1a in buffer solution on exposure to the light
of a 60 W tungsten lamp: (black line) oxidized Mb (intense band at
409 nm); (red line) reduced Mb (at 434 nm, with excess of sodium
dithionite); (green dotted line) reduced Mb þ 1a solution (∼9 ꢁ 10ꢀ6
M) in the dark for 15 min; (blue line) MbꢀNO adduct (at 420 nm),
obtained by Mb þ 1a solution exposed to 60 W tungsten lamp light
for 5 min.
that the rate of photodissociation of complex 2a was higher
than that of 1a. This may be due to the presence of a higher
absorbance of 2a in the visible region compared to 1a (Figure 5
and Figure S12 (Supporting Information)).15
Transfer of NO to Myoglobin. The photocleavage of the
coordinated NO was also confirmed by trapping the liberated
NO by reduced myoglobin (shown in Figure 8). Electronic
absorption spectra were obtained in phosphate buffer (6.8 pH).
The UVꢀvis spectrum of oxidized myoglobin (Mb) showed an
intense band at 409 nm (Soret band). The UVꢀvis spectrum of
reduced myoglobin at 434 nm was obtained by addition of excess
sodium dithionite to the same cuvette. When an acetonitrile
solution of complex 1a was added to a buffer solution of reduced
myoglobin under dark conditions, no reaction was observed.
However, when the same mixture was exposed to a tungsten
lamp (60 W) for 5 min, its absorption spectrum at 420 nm
showed the formation of an MbꢀNO adduct.14,25 We have also
trapped the photocleaved NO derived from complex 2a by
reduced myoglobin, and the results are deposited in the Support-
ing Information (Figure S11).
Photolysis Experiment of Complex 2a. Illumination of visible
light through a dichloromethane solution of complex 2a afforded
spectral changes along with isosbestic points at 252, 292, 348, 370,
and 438 nm (Figure 7). In this case also we observed a similar type
of NO photodissociation and religation of the phenolato oxygen
and concomitant formation of 2b. It is important to note here
2503
dx.doi.org/10.1021/om101022p |Organometallics 2011, 30, 2498–2505

Organometallics
ARTICLE
’ CONCLUSION
slow evaporation of the dichloromethane/methanol mixture. Yield:
52%. IR (KBr disk, cmꢀ1): 1803 (νNO), 1578 (νCdN), 1383, 1345,
1300 (νNO ), 1089, 623 (νClO ), 745, 696, 521 (νPPh ) cmꢀ1. UVꢀvis
In conclusion, we have prepared two different σ-aryl nitrosyl
ruthenium complexes: [Ru(LSB2H)(PPh3)2(NO)Cl]ClO4 (1a)
and [Ru(LAZ2H)(PPh3)2(NO)CI](ClO4) (2a). The molecular
structures of both complexes were determined by X-ray crystal-
lography. These novel ruthenium cyclometalates were character-
ized by 1H and 31P NMR spectral studies, because the complexes
were diamagnetic with an S = 0 ground state of the metal center.
RuꢀN and NꢀO distances and the RuꢀNꢀO angle from the
crystallographic data along with the NO stretching frequency
(νNO) clearly indicated the presence of {RuNO}6 moiety with a
{RuIINOþ}6 description. Ligand nitration was observed in the
ring bearing the phenolato function. The coordinated NO was
found to be photolabile under visible light. The rate of photo-
dissociation of complex 2a was higher than that of 1a. The
photoreleased NO was trapped by reduced myoglobin. Interest-
ingly, we found out that complexes 1a and 2a gave rise to
complexes 1b and 2b, respectively, after photorelease of NO and
1a and 2a could be regenerated by treating 1b and 2b with acid
nitrite solution. Hence, the meridional tridentate ligand became
bidentate during NO coordination and after photocleavage of
NO the ligand retained its tridentate properties. These data
clearly showed the reversible binding of the phenolato function
to the metal center during photorelease and coordination of NO
at the metal center. To the best of our knowledge, this is the first
example of such types of organometallic NO donors. Modifica-
tion of the ligand and biological application of these novel
complexes are in progress.
2
4
3
(CH3CN; λmax, nm (ε, Mꢀ1 cmꢀ1)): 235 (41 389), 312 (33 445), 392
1
(4889). 31P NMR ((CD3)2SO, 500 MHz): δ 20.92 ppm. H NMR
((CD3)2SO, 500 MHz): δ 11.75 (s, 1H), 8.743 (s, 1H), 8.027 (d, 1H),
7.702 (d, 1H), 7.472ꢀ7.159 (m, 31H), 6.923 (d, 1H), 6.812 (s, 1H),
6.338 (s, 1H).
Interconversion of Complexes [Ru(LSB2H)(PPh3)2(NO)Cl]ClO4
(1a) and[Ru(LSB2)(PPh3)2Cl] (1b). (i) Conversion of Complex 1a into
1b. A yellowish orange dichloromethane solution of complex 1a (0.02 g,
0.019 mmol) was exposed to the light of a tungsten lamp (60 W). Within 0.5
h the solution turned from yellow to red. The solvent was evaporated to
obtain a red-orange solid, and this was washed thoroughly with methanol and
ether. Complex 1b was eluted on an alumina column with a dichloro-
methane/hexane (1/1) mixture. Single crystals of 1b 2CH2Cl2 for
3
X-ray crystallography were obtained within 3 days by slow diffusion of
hexane into a solution of the complex in dichloromethane (0.012 g, 0.013
mmol). Yield: 68%. IR (KBr disk, cmꢀ1): 1630, 1588 (νCdN), 1382, 1340
(νNO ), 1297, 1092, 744, 695, 523 (νPPh ) cmꢀ1. UVꢀvis (CH2Cl2;
λmax, 2nm (ε, Mꢀ1 cmꢀ1)): 431 (6329), 5283(2359).
(ii) Conversion of Complex 1b into 1a. A portion of complex 1b
(0.028 g, 0.03 mmol) was dissolved in 25 mL of dichloromethane in a
100 mL round-bottom flask to give a red solution. Then 20 mL of
acidified distilled water was layered over this solution. Sodium nitrite
(0.2 g, 3 mmol) was added to the bilayer solution, and the mixture was
stirred at room temperature for 45 min to give a yellowish orange
solution of complex 1a. The dichloromethane layer was separated out,
and NaClO4 (in excess) with 5 mL of methanol was added to this
solution. Stirring of this solution was continued for another 1 h. The
solvent was evaporated to give an orange solid of complex 1a (0.015 g,
0.014 mmol). Yield: 47%.
’ EXPERIMENTAL SECTION
Synthesis of [Ru(LAZ1)(PPh3)2Cl] (2). Complex [Ru(LAZ1)-
(PPh3)2Cl] (2) was synthesized from the reaction of [Ru(PPh3)3Cl2]
with the ligand LAZ1H2 (where LAZ1H2 = 4-methyl-2-(p-tolyldiazenyl)-
phenol and H = dissociable protons) in ethanol by following the method
reported earlier.13
Materials. All the solvents used were reagent grade. The analytical
grade reagents sodium nitrite (Sigma Aldrich, Steinheim, Germany),
RuCl3 3H2O and triphenylphosphine (SRL, Mumbai, India), 2-amino-
3
phenol, 2-amino-4-methylphenol, 4-chlorobenzaldehyde, p-toluidine,
and sodium perchlorate hydrate (Himedia Laboratories Pvt. Ltd., Mum-
bai, India), disodium hydrogen phosphate anhydrous (RFCL Ltd., New
Delhi, India), and sodium dihydrogen phosphate (Chemport India Pvt.
Ltd. Mumbai, India) were used as obtained. Double-distilled water was
used in all the experiments. Equine skeletal muscle myoglobin was
obtained from Sigma Aldrich, Steinheim, Germany.
Synthesis of [Ru(LAZ2H)(PPh3)2(NO)Cl]ClO4 (2a). Complex
2a was synthesized using 2 by following the same procedure as for 1a.
Yield: 48%. IR (KBr disk, cmꢀ1): 1805 (νNO), 1623, 1577, 1540, 1479,
1434 (νNdN), 1345, 1305 (νNO ), 1254, 1090, 623 (νClO ), 748, 692,
517 (νPPh ). UVꢀvis (CH2Cl2; λm2 ax, nm (ε, Mꢀ1 cmꢀ1)): 2431 (49 067),
3
312 (30 667), 408 (12 000). 1H NMR (CDCl3, 500 MHz): δ 10.35 (s,
1H), 8.291 (d, 1H), 7.932 (s, 1H), 7.454ꢀ7.175 (m, 31H), 6.825 (s,
1H), 6.084 (s, 1H), 2.187 (s, 3H), 1.813 (s, 3H). 31P NMR (CDCl3, 500
MHz): δ 21.87 ppm.
Caution! Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Only a small amount of material should be
prepared and handled with caution.
Preparation of Complexes. Synthesis of [Ru(LSB1)(PPh3)2Cl]
(1). [Ru(LSB1)(PPh3)2Cl] (1) was synthesized from the reaction of
[Ru(PPh3)3Cl2] with the ligand LSB1H2 (where LSB1H2 = 2-(4-chloroben-
zylideneamino)phenol and H = dissociable protons) in ethanol according
to the method reported earlier.12
Interconversion of Complexes [Ru(LAZ2H)(PPh3)2(NO)Cl]ClO4
(2a) and [Ru(LAZ2)(PPh3)2Cl] (2b). (i) Conversion of Complex 2a
into 2b. A yellowish-orange dichloromethane solution (20 mL) of
complex 2a (0.03 g, 0.028 mmol) was exposed to the light of a
tungsten lamp (60 W). Within 15 min the solution turned from
yellow to light green. The solvent was evaporated, and the residue
was washed thoroughly with methanol and ether to obtain a green
solid of complex 2b (0.018 g, 0.019 mmol). Yield: 67.86%. IR (KBr
disk, cmꢀ1): 1614, 1571, 1547, 1510, 1481, 1434 (νNdN), 1348,
Synthesis of [Ru(LSB2H)(PPh3)2(NO)Cl]ClO4 (1a). A portion of
complex 1 (0.03 g, 0.033 mmol) was dissolved in 30 mL of dichlor-
omethane in a 100 mL round-bottom flask to give a purple solution. To
the above solution was added sodium nitrite (0.3 g, 4.5 mmol) with
acidified distilled water (20 mL), and the mixture was stirred at room
temperature for 1 h to give a yellowish orange solution of complex 1a.
The dichloromethane layer was separated out, and NaClO4 (in excess)
with 5 mL of methanol was added to this solution. Stirring of this
solution was continued for another 1 h. The solvent was evaporated to
give an orange solid. To remove the excess NaClO4, the compound was
further dissolved in dichloromethane and this solution was filtered out.
Complex 1a (0.018 g, 0.017 mmol) was eluted from an alumina column
with a dichloromethane/ methanol (9/1) mixture. Single crystals of the
complex for X-ray crystallography were obtained within 2 days upon
1301 (νNO ), 1256, 1225, 1092, 745, 694, 517 (νPPh ) cmꢀ1. UVꢀvis
2
(CH2Cl2; λmax, nm (ε, Mꢀ1 cmꢀ1)): 270 (25 208),3440 (6042), 650
(4042).
(ii) Conversion of Complex 2b into 2a. A portion of complex 2b
(0.025 g, 0.027 mmol) was dissolved in 20 mL of dichloromethane in a
100 mL round-bottom flask to give a light green solution. Then 25 mL of
acidified distilled water was layered over this solution. Sodium nitrite
(0.2 g, 3 mmol) was added to the bilayer solution, and the mixture was
stirred at room temperature for 45 min to give a yellowish orange
2504
dx.doi.org/10.1021/om101022p |Organometallics 2011, 30, 2498–2505

Organometallics
ARTICLE
solution of complex 2a. The dichloromethane layer was separated out,
and NaClO4 (in excess) with 5 mL of methanol was added to this
solution. Stirring of this solution was continued for another 1 h. The
solvent was evaporated to give an orange solid of complex 2a (0.012 g,
0.011 mmol). Yield: 40.7%.
Physical Measurements. Infrared spectra were obtained as KBr
pellets with a Thermo Nikolet Nexus FT-IR spectrometer, using 16
scans, and are reported in cmꢀ1. Electronic absorption spectra were
recorded in dichloromethane and acetonitrile solvents with a Thermo
Scientific Evolution 600 UVꢀvis spectrophotometer. 1H and 31P NMR
spectra were recorded on a Bruker AVANCE 500.13 MHz spectrometer
in deuterated solvents.
(6) (a) Cameron, B. R.; Darkes, M. C.; Yee, H.; Olsen, M.; Fricker,
S. P.; Skerlj, R. T.; Bridger, G. J.; Davies, N. A.; Wilson, M. T.; Rose, D. J.;
Zubieta, J. Inorg. Chem. 2003, 42, 1868. (b) Fricker, S. P. Platinum Met.
Rev. 1995, 39, 150.
(7) (a) Ford, P. C. Acc. Chem. Res. 2008, 41, 190. (b) Patra, A. K.;
Afshar, R.; Olmstead, M. M.; Mascharak, P. K. Angew. Chem .Int. Ed.
2002, 41, 2512 and references therein.
(8) Holanda, A. K. M.; da Silva, F. O. N.; Sousa, J. R.; Diꢀogenes,
ꢀ
I. C. N.; Carvalho, I. M. M.; Moreira, I. S.; Clarke, M. J.; Lopes, L. G. F.
Inorg. Chim. Acta 2008, 361, 2929.
(9) Ghosh, K.; Kumar, S.; Kumar, R.; Singh, U. P.; Goel, N. Inorg.
Chem. 2010, 49, 7235.
(10) Djukic, J.-P.; Sortais, J.-B.; Barloy, L.; Pfeffer, M. Eur. J. Inorg.
Chem. 2009, 817.
(11) Ghosh, K.; Kumar, S.; Kumar, R. Inorg. Chem. Commun. 2011,
14, 146.
(12) Ghosh, P.; Pramanik, A.; Bag, N.; Lahiri, G. K.; Chakravorty, A.
J. Organomet. Chem. 1993, 454, 237.
X-ray Crystallography. Orange-red crystals of 1a CH2Cl2 3H2O
3
3
and 2a 2H2O were obtained by slow evaporation of solutions of the
3
complexes in CH2Cl2/methanol. The crystal structures showed the
presence of water molecules in the lattice. Crystals of 1b 2CH2Cl2 were
3
obtained by slow diffusion of hexane into the solution of complex 1b in
dichloromethane. The X-ray data collection and processing for the
complexes were performed on a Bruker Kappa Apex-II CCD diffract-
ometer by using graphite-monochromated Mo KR radiation (λ =
0.710 70 Å) at 296 K for 1a 2CH2Cl2 and 2a 2H2O and at 293 K for
(13) Lahiri, G. K.; Bhattacharya, S.; Mukherjee, M.; Mukherjee,
A. K.; Chakravorty, A. Inorg. Chem. 1987, 26, 3359.
(14) Rose, M. J.; Patra, A. K.; Alcid, E. A.; Olmstead, M. M.;
Mascharak, P. K. Inorg. Chem. 2007, 46, 2328.
(15) Rose, M. J.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem.
Soc. 2007, 129, 5342.
(16) Chanda, N.; Paul, D.; Kar, S.; Mobin, S. M.; Datta, A.; Puranik,
V. G.; Rao, K. K.; Lahiri, G. K. Inorg. Chem. 2005, 44, 3499.
(17) Hadadzadeh, H.; DeRosa, M. C.; Yap, G. P. A.; Rezvani, A. R.;
Crutchley, R. J. Inorg. Chem. 2002, 41, 6521.
(18) Enemark, J. H.; Feltham, R. D. Coord. Chem. Rev. 1974, 13, 339.
(19) Fry, N. L.; Rose, M. J.; Rogow, D. L.; Nyitray, C.; Kaur, M.;
Mascharak, P. K. Inorg. Chem. 2010, 49, 1487.
(20) Kannan, S.; Ramesh, R.; Liu, Y. J. Organomet. Chem. 2007,
692, 3380.
(21) Birkmann, B.; Owens, B. T.; Bandyopadhyay, S.; Wu, G.; Ford,
P. C. J. Inorg. Biochem. 2009, 103, 237.
(22) Ghosh, K.; Pattanayak, S.; Chakravorty, A. Organometallics
1998, 17, 1956.
(23) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980,
19, 1404.
(24) (a) Ghosh, K.; Chattopadhyay, S.; Pattanayak, S.; Chakravorty,
A. Organometallics 2001, 20, 1419. (b) Ferstl, W.; Sakodinskaya, I. K.;
Beydoun-Sutter, N.; Borgne, G. L.; Pfeffer, M.; Ryabov, A. D. Organo-
metallics 1997, 16, 411.
3
3
1b CH2Cl2 3H2O. Crystal structures were solved by direct methods.
3
3
Structure solutions, refinement, and data output were carried out with
the SHELXTL program.26,27 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were placed in geometrically calculated
positions and refined using a riding model. Images were created with the
DIAMOND program.28
’ ASSOCIATED CONTENT
S
Supporting Information. Figures giving characteriza-
b
tion data of complexes by IR and NMR spectral studies and
electronic absorption spectra of NO transfer to myoglobin of
complex 2a and CIF files giving X-ray crystallographic data of
complexes 1a CH2Cl2 3H2O, 2a 2H2O, and 1b 2CH2Cl2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
3
3
3
3
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ghoshfcy@iitr.ernet.in. Fax: þ91-1332-273560.
(25) Maji, S.; Sarkar, B.; Patra, M.; Das, A. K.; Mobin, S. M.; Kaim,
W.; Lahiri, G. K. Inorg. Chem. 2008, 47, 3218.
(26) Sheldrick, G. M. Acta. Cryst. A 1990, 46, 467.
(27) Sheldrick, G. M. SHELXTL-NT 2000, version 6.12, Reference
Manual; Pergamon: New York, 1980.
’ ACKNOWLEDGMENT
(28) Klaus, B. DIAMOND, Version 1.2c; University of Bonn, Bonn,
Germany, 1999.
K.G. thanks the CSIR of India for financial assistance (No.
01(2229)/08/EMR-II dated March 6, 2008). S.K. and R.K. thank
the CSIR for financial assistance. U.P.S. thanks the IIT Roorkee
for the single-crystal X-ray facility.
DEDICATION
†Dedicated to Prof. Animesh Chakravorty on his 75th birthday.
’ REFERENCES
(1) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford Uni-
versity Press: New York, 1992.
(2) Richter-Addo, G. B.; Legzdins, P.; Burstyn, J. Chem. Rev. 2002,
102, 857.
(3) Rose, M. J.; Mascharak, P. K. Coord. Chem. Rev. 2008, 252, 2093.
(4) Nitric Oxide: Biology and Pathobiology; Ignarro, L. J., Ed.;
Academic Press: San Diego, CA, 2000.
(5) Rose, M. J.; Mascharak, P. K. Curr. Opin. Chem. Biol. 2008,
12, 238.
2505
dx.doi.org/10.1021/om101022p |Organometallics 2011, 30, 2498–2505