Influence of nitrosyl coordination on the binding mode of quinaldate inselective ruthenium frameworks. Electronic structure and reactivity aspects{

Abhishek Dutta Chowdhury, Prinaka De, Shaikh M. Mobin and Goutam Kumar Lahiri*

Received 24th October 2011, Accepted 18th January 2012

DOI: 10.1039/c2ra00953f

The nitrosyl complexes, [RuII(trpy)(L)(NO+)Cl]BF4, [1]BF4, and [RuII(trpy)(L)(NO+)](BF4)2,

[2](BF4)2, (trpy = 2,29:69,299-terpyridine, L2 = deprotonated form of unsymmetrical quinaldic acid)

have been synthesized. Single crystal X-ray structures of [1]BF4 and [2](BF4)2 reveal that in the

former L2 binds to the ruthenium ion selectively in a monodentate fashion through the O2 donor

whereas the usual bidentate mode of L2 (O2, N donors) has been retained in [2](BF4)2 with the same

meridional configuration of trpy being seen in both. The Ru–NO group in [1]BF4 or [2](BF4)2,

exhibits almost linear (sp-hybridized form of NO+) geometry. The difference in bonding mode of the

unsymmetrical quinaldate in [1]BF4 and [2](BF4)2 has been reflected in their corresponding n(NO)/

n(CLO) frequencies as well as in their NO based two-step reduction processes, {RuII–NO+} A{RuII–NON} and {RuII–NON}A{RuII–NO2}. The close to bent geometry (sp2-hybridized form of

NON) of the one-electron reduced 1 or [2]+ is been reflected in their DFT optimized structures. The

spin density plot of the reduced species reveals that NO is the primary spin-bearing center with slight

delocalization onto the metal ion which has been reflected in its radical EPR spectrum. [1]+

and [2]2+ undergo facile photorelease of NO with significantly different kNO (s21) and t1/2 (s) values

which eventually lead to the concomitant formation of the corresponding solvent species. The

photoreleased NON can be trapped as an Mb–NO adduct. The reduced species 1 selectively reacts with

the molecular oxygen (O2) at pH y 1 to yield the corresponding nitro species,

[RuII(trpy)(L)(NO2)Cl]2.

Introduction

The important role of nitric oxide (NON) in various biological

and physiological processes, such as blood pressure regulation,

inflammatory response, and apoptosis has drawn renewed

research interest on nitrosyl chemistry in recent years.1–7 NON

is generated in biological processes by nitric oxide synthase

(NOS) during the oxidation of L-arginine to citrulline which

plays important roles in neurotransmission, smooth muscle

vasodilation and platelet disaggregation in mammals.6 The

interaction of NO with the metal ions is important from the

broader perspective of inorganic chemistry including in bioinor-

ganic chemistry.4,5 For example, the reactions of NON with

oxygenated heme-protein have considerable biological signifi-

cance.5 Though some organic nitrites, nitrates and S-nitrsothiols

have medicinal application as vasodilators,2 they are not suitable

for site specific NON delivery processes. However, transition

metal–nitrosyl complexes have shown potential application for

site-specific NON-delivery primarily due to photolabile nature of

the metal–nitrosyl bond.8–10 Among them, ruthenium nitrosyl

complexes have been emerged as a promising class of NO-donor

due to their reduced activity towards oxygen and stability in

water.8,9,11 Besides that, the non-innocent feature of NO

facilitates its accessibility in three different redox states, strongly

electrophilic NO+, neutral NON and anionic NO2, in transition

metal complexes depending on the electronic environment

around the metal center which in turn makes the nitrosyl

function as a versatile ligand in co-ordination chemistry.8,11

Further, ruthenium–nitrosyl complexes have shown considerable

applications in pharmaceuticals, catalysis, molecular electronics

and photochemical devices.12

In this context the present article describes the various aspects

of the coordinated nitrosyl function in the newly designed

selective molecular frameworks of [RuII(trpy)(L)(NO)Cl]n, [1]n,

and [RuII(trpy)(L)(NO)]n, [2]n, (trpy = 2,29:69,299-terpyridine, L2

= deprotonated form of unsymmetrical quinaldic acid). Herein

we report the synthesis and structural characterization of [1]BF4

and [2](BF4)2. Furthermore, the effect of structural diversity of

the co-ligand, L2 (monodentate versus bidentate) on the

electronic structures of [1]n and [2]n with special reference to

electrophilicity, redox potential and reactivity of the coordinated

nitosyl function have been investigated by experimental studies

Department of Chemistry, IIT Bombay, Powai, Mumbai, 400076, India.E-mail: lahiri@chem.iitb.ac.in; Fax: 91-022-2572-3480{ Electronic supplementary information (ESI) available: Characterizationdetails of the complexes, crystallographic material and DFT results (TableS1–S7 and Fig. S1–S9). CCDC reference numbers 838835 and 838836. ForESI and crystallographic data in CIF or other electronic format see DOI:10.1039/c2ra00953f

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and DFT calculations.

Results and discussion

Synthesis, characterization and structural aspects

The complexes with Enemark–Feltham notation13 of {Ru(NO)}6

in [RuII(trpy)(L)(NO+)Cl](BF4), [1]BF4, and [RuII(trpy)(L)

(NO+)](BF4)2, [2](BF4)2, have been synthesized via the direct

reaction of NOBF4 with the previously structurally characterized

precursor complex [RuII(trpy)(L)Cl] (A)14 and the reaction of

NOBF4 with the in situ generated [RuII(trpy)(L)(C2H5OH)]+,

respectively, as shown in Scheme 1 (trpy = 2,29:69,299-terpyridine,

L2 = deprotonated form of unsymmetrical quinaldic acid,

HL).15,16

[1]BF4 and [2](BF4)2 exhibit satisfactory microanalytical and mass

spectral data (Fig. S1{) and show 1 : 1 and 1 : 2 molar conductivities

in acetonitrile solution, respectively (see Experimental Section).

The formation of [1]BF4 and [2](BF4)2 have been authenti-

cated by their single crystal X-ray structures (Fig. 1 and Table 1).

The five-membered chelate ring of L2 (through the O12 and N1

donors of L2) in the precursor A has been retained in [2](BF4)2

along with the usual meridional configuration of trpy.14

However, the direct nitrosylation of A (Scheme 1) surprisingly

leads to the concomitant transformation of the bidentate (O2, N

donors) mode of L2 to the monodentate (O2 donor) in [1]BF4

retaining the same meridional configuration of trpy.

The bond distances and bond angles in [1]BF4 and [2](BF4)2

(Table 2) are in good agreement with the reported data of

analogous complexes.8,9,11 The geometrical constraint due to the

meridional mode of trpy has been reflected in the appreciably

smaller trans angles involving the trpy ligand, N2–Ru–N4 of

156.5(3)u and 159.35(12)u in [1]BF4 and [2](BF4)2, respectively.

The central Ru–N3(trpy) bond lengths of 2.015(6) A in [1]BF4

and 1.981(3) A in [2](BF4)2 are significantly shorter than the

corresponding distances involving the terminal pyridine rings of

trpy, Ru–N2(trpy), 2.083(6) A and 2.075(3) A, and Ru–N4(trpy),

2.079(6) A and 2.079(3) A, respectively. The central Ru–N3(trpy)

distance in [1]BF4 is 0.034 A longer than that in [2](BF4)2 due to

the effect of the strongly p-accepting NO+ trans to the Ru–

N3(trpy). The Ru–O2(monodentate L2) bond distance in [1]BF4

(2.044(5) A) is appreciably longer relative to Ru–O2(chelated

L2) in [2](BF4)2 (1.990(2) A) due to the effect of oppositely

directed s/p-donor Cl2 versus the strongly p-accepting NO+

trans to the Ru–O2(L2) bond. The impact of varying coordina-

tion situations in the complexes has been reflected in their trans

angles: O1–Ru–Cl, 171.85(15)u and N3–Ru–N5, 175.6(3)u in

[1]BF4 versus O1–Ru–N5, 176.43(12)u and N(3)–Ru–N(1),

163.12(12)u in [2](BF4)2. The almost linear mode of Ru–N5–O3Scheme 1 The synthetic outline for [1]BF4 and [2](BF4)2.

Fig. 1 ORTEP diagrams of (a) [1]BF4?5H2O and (b) [2](BF4)2. Thermal

ellipsoids are drawn at 50% probability. The solvents of crystallization,

counter anions and hydrogen atoms are omitted for clarity.

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(NO), 173.0(8)u in [1]BF4 and 175.9(3)u [2](BF4)2 as well as the

triple bond feature of N5MO3 (NO) (1.141(9) A in [1]BF4 and

1.130(4) A in [2](BF4)2) establish the sp-hybridized state of the

nitrogen atom of the NO+ group as expected from the

coordinated p-accepting nitrosonium ion. However, the presence

of the p-accepting trpy ligand (N3) trans to Ru–N5–O3 in [1]BF4

makes the Ru–N–O angle (173.0(8)u) relatively more tilted

compared to that in [2](BF4)2 (175.9(3)u) where the Ru–N–O

group is trans to the s-donating O2 of the chelated L2. The

pendant quinaldate ring in [1]BF4 is slightly tilted (about 96u)with respect to the equatorial plane. The Ru–O1–C1 angle of

118.9(5)u implies that the bound oxygen (O1) is close to the sp2-

hybridized state which in turn introduces a resonating feature of

the carboxylate group (O1–C1, 1.276(9) A and C1–O2, 1.242(10)

A) in [1]BF4 (Table 2).

The DFT calculated bond parameters (Table 2) based on the

optimized structures of [1]+ and [2]2+ (Fig. S2{) are in general

agreement with the X-ray data.

The corresponding one-electron reduced species with

{Ru(NO)}7 configuration in [RuII(trpy)(L)(NON)Cl] (1) and

[RuII(trpy)(L)(NON)]+ ([2]+) can be generated electrochemi-

cally.8,11 The DFT calculated structural parameters of [1]+/1

and [2]2+/[2]+ (Fig. 2 and Table S1{) reveal similar structural

differences between the two sets of complexes based on the

electronic environment around the {Ru–NO} groups. The

s-donating ability of the carboxylate oxygen (O1) of L2 makes

the Ru–N5–O3 angle relatively more bent in [2]+ (139.6u) than in

1 (141.85u) where the Ru–NO group is situated trans to the

p-accepting trpy (N3) ligand (Fig. 3). The difference in calculated

Ru–N5–O3 angles between [2]2+ and [2]+ of y35u is reasonably

larger than that between [1]+ and 1 (y29u) implying greater

electron density on N(5) in [2]+ as has also been revealed by the

NBO studies (Table 3). The double bond feature of N5–O3

(NO), 1.176 A and 1.18 A in 1 and [2]+, respectively, suggests the

sp2 character of the nitrogen atom. The lengthening of the Ru–

N5 bond upon NO based reduction, 0.14 A for [1]+/1 and 0.12 A

for [2]2+/[2]+ due to increasing electron–electron repulsion upon

addition of an electron is in agreement with the established

concept of labilization of the Ru–NO bond on reduction.11h

Spectral and redox aspects

The 1H NMR spectra of [1]+ and [2]2+ in (CD3)2SO exhibit the

calculated number of seventeen partially overlapping aromatic

proton resonances in each case (Fig. S3{). The extent of overlap

of the proton signals is appreciably larger in [2]2+ as compared

to [1]+. The maximum downfield shifted signals in [2]2+ and [1]+

appear at 10.2 ppm and 9.2 ppm, respectively. This can be

attributed to the proton trans to the quinoline nitrogen N(1)

of the chelated L2 opposite to the central pyridine ring of

the p-acceptor trpy in [2]2+ whereas the same atom in the

monodentate quinoline ring in [1]+ exists away from the

coordination sphere.

Table 1 Selected crystallographic parameters

[1]BF4?5H2O [2](BF4)2

Empirical formula C25H17B1Cl1F4N5O8Ru C25H17B2F8N5O3RuMr 738.77 710.13Crystal system Triclinic TriclinicSpace group P1 P1a/A 8.5160(3) 8.4478(2)b/A 10.1858(4) 8.9532(2)c/A 16.7736(6) 17.6431(4)a (u) 92.631(3) 94.079(2)b (u) 102.713(3) 91.367(2)c (u) 93.147(3) 98.405(2)V/A3 1414.66(9) 1315.96(5)Z 2 2m/mm21 0.733 0.693T/K 150(2) K 120(2)Dc/g cm23 1.734 1.792F(000) 736 7042h range (u) 6.54 to 50 6.58 to 50data/restraints/parameters

4952/0/434 4622/0/397

R1, wR2 [I . 2s] 0.0698, 0.2092 0.0396, 0.1046R1, wR2 (all data) 0.0795, 0.2136 0.0435, 0.1067GOF 1.136 1.061largest diff. peak,hole/e A23

2.399 and 20.755 1.307 and 20.889

Table 2 Selected bond distances and bond angles in [1]BF4 and[2](BF4)2

Bond distance (A)/Bond angle (u)

[1]+ [2]2+

X-Ray DFT X-Ray DFT

Ru–N1 — — 2.116(3) 2.19Ru–N2 2.083(6) 2.11 2.075(3) 2.14Ru–N3 2.015(6) 2.03 1.981(3) 2.02Ru–N4 2.079(6) 2.11 2.079(3) 2.13Ru–N5 1.754(7) 1.79 1.758(3) 1.80Ru–O1 2.044(5) 2.05 1.990(2) 1.97Ru–Cl 2.361(2) 2.44 — —O(1)–C1 1.276(9) 1.30 1.317(4) 1.34O(2)–C1 1.242(10) 1.24 1.202(5) 1.21N(5)–O3 1.141(9) 1.14 1.130(4) 1.15N1–Ru–N3 — — 163.12(12) 160.82N2–Ru–N4 156.5(3) 156.32 159.35(12) 158.22O1–Ru–N5 98.3(3) 97.80 176.43(12) 175.03N3–Ru–N5 175.6(3) 175.95 94.83(13) 94.98Cl–Ru–O1 171.85(15) 171.59 — —N5–Ru–N2 100.1(3) 100.80 96.49(13) 90.86N5–Ru–N3 175.6(3) 175.95 94.83(13) 94.98N5–Ru–N4 103.4(3) 102.88 91.29(13) 95.83N5–Ru–N1 — — 101.82(12) 104.18N1–Ru–N4 — — 96.52(11) 99.17N3–Ru–O1 85.8(2) 86.11 84.26(11) 81.61Ru–N5–O3 173.0(8) 170.25 175.9(3) 174.02Ru-1-C1 118.9(5) 117.21 116.6(2) 119.15

Fig. 2 The DFT optimized geometry of (a) 1 and (b) [2]+. The hydrogen

atoms are omitted for clarity.

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The MOs of [1]+ and [2]2+ (Tables S2 and S4{) predict that in

both cases the HOMOs are primarily composed of L2 based

orbitals. The LUMO and LUMO + 1 are however dominated by

NO based orbitals which in effect make them susceptible towards

reduction (Tables S2 and S4{). Accordingly, [1]+ and [2]2+ exhibit

two successive reductions, Eu298,V (DEp, mV) at 0.31(70),

20.34(80) and 0.05(60), 20.64(90) in CH3CN versus SCE

corresponding to the {RuII–NO+} A {RuII–NON} (I) and

{RuII–NON} A {RuII–NO2} (II) couples, respectively (Fig. 4).8,11

The selective trans orientation of two p-accepting groups, trpy and

NO+ in [1]+ makes the reduction processes relatively easier than

that in [2]2+ where the NO+ is trans to the electron rich

O2 donor of L2. This has also been reflected in the greater

natural charge on N(5) (NO) in [1]+ than [2]2+ (Table 3). The

significant contribution of trpy based orbitals in higher UMOs

results in trpy based quasi-reversible reductions at further negative

potentials, Eu298,V (DEp, mV) of 20.84(100), 21.65(110) and

20.7(110), 21.15(110) for [1]+ and [2]2+, respectively.8a–c,11a,h–m,14

[1]+ and [2]2+ exhibit moderately intense transitions in the

near-UV region, 326 nm and 348 nm, respectively, followed by

several intense higher energy intraligand transitions in the UV

region in CH3CN (Fig. 5).8,11 The bands in the near-UV region

are assigned on the basis of the TD-DFT calculations on the

optimized structures of [1]+ and [2]2+ as the RuII(dp)/L(p) ANO+(p*) transition (Tables S5 and S6{). The slight difference in

electronic spectra in the complexes can be attributed to the

mixing of chloride ion orbitals in the HOMOs of [1]+ and the

denticity of L2. Upon one-electron reduction to [1] and [2]+ the

near UV region bands shift to the lower energy region at 556 nm

and 493 nm, respectively, which are assigned to the RuII(dp)/

NON(p) A p*(trpy) and RuII(dp)/NON(p) A L(p*) transitions,

respectively, based on the TD-DFT calculations.

The n(CLO) frequency of the coordinated L2 in the precursor

[(RuII(trpy)(L)(Cl)] (A) at 1635 cm21.14 has been appreciably

shifted to 1667 cm21 and 1696 cm21 in [1]BF4 and [2](BF4)2,

respectively (Fig. S4{) due to their different electronic environ-

ments. The characteristic vibrations of the BF42 counter anion

appear near 1600 cm21 and 1080 cm21. The reasonably high

n(NO) frequencies of [1]BF4 and [2](BF4)2 of 1895 cm21 (DFT:

1906 cm21) and 1926 cm21 (DFT: 1939 cm21), respectively,

imply their moderately electrophilic character. Upon one-

electron reduction to 1 and [2]+, the NO bands at 1895 cm21

and 1926 cm21 disappear. However, the n(NON) frequency of the

reduced state in the expected SWIR region of 1600–1700 cm21

did not resolve properly due to the presence of other

characteristic vibrations of the CLO of coordinated L2, BF42

counter anion as well as aryl ring vibrations.

The EPR spectrum of the representative one-electron reduced

paramagnetic [2]+ in frozen CH3CN/0.1 M Bu4N(PF6) solution

yields g-components at 2.012 (g1), 1.988 (g2) and 1.869 (g3) (Fig.

S5{) with a g-anisotropy (g1 2 g3) of 0.143, which is about twice

a large as that observed for the related {Fe(NO)}7 species due to

f(Ru) # 2f(Fe) (f = spin–orbit coupling constant).10b,11h

However, the value of ,g. = [1/3(g12 + g2

2 + g32)]1/2 = 1.957

implies that the spin is primarily localized on the N(5) of

NO.17,18 The overall negative shift of the g value (Dg = 45.0 ppt)

Fig. 3 Schematic representation of 1 and [2]+.

Table 3 The detailed NBO results of {Ru(NO)}n, (n = 6,7)

Total atomic charge Natural charge

Ru N(5) O(3) Cl Ru N(5) O(3) Cl

[1]+ 0.733 0.135 20.068 20.226 0.737 0.383 20.065 20.491[1] 0.617 0.026 20.182 20.349 0.635 0.184 20.194 20.576[2]2+ 1.013 0.054 20.070 — 0.802 0.317 20.072 —[2]+ 0.840 20.022 20.189 — 0.687 0.139 20.196 —

Fig. 4 Cyclic voltammograms of (a) [1]+ and (b) [2]2+ in CH3CN/0.1 M

[Et4N](ClO4) versus SCE, scan rate:100 mV s21.

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with respect to the free electron value of 2.0023 arises due to the

admixture of higher excited states with nonzero angular

momentum (eqn (1)).18 The radical EPR feature of [2]+ (Fig.

S5{) has been reflected in the significant bending of Ru–N(5)–

O(3) angle (139.69u) and lengthening of the N(5)–O(3) bond

(1.181 A).

g~ge{2

3

X

i

X

n

X

kj

SY0 jkLdkj jYnTSYn Lijdj

�� ��Y0TEn{E0

~gezDg (1)

j: spin orbit coupling constant

L: angular momentum operator

E0: energy of the SOMO

The SOMO of [2]+ indicates s-donation from the NO(p*)

orbital to the Ru(dz2) center which reveals {RuII–NON} as the

predominant oxidation state formulation along with consider-

able mixing of the {RuI–NO+} state due to the presence of 23%

metal contribution. The presence of y10% spin on the metal in

spin density analysis also reveals that spin is mainly concentrated

on NO (Fig. 6). Moreover, s-interaction between Ru and NO

results in spin density at the axial position (Fig. 6) which leads to

a significant hyperfine splitting in the EPR spectrum due to the

N(5) of NO.

Electronic features: theoretical insights

Molecular orbital analysis reveals that the LUMOs of [1]+ and

[2]2+ originate from the interaction of the p*-orbital of NO+ with

the metal’s t2g(dxy) and eg(dz2) orbitals, respectively, (Tables S2

and S4{).10b,11c–h,17 This is attributed to stronger back-bonding

in [1]+ between the metal t2g(dp) orbital and the p*-orbital of

trpy in the filled MOs. The presence of 10–25% metal

contribution in the LUMOs lowers the energy of the metal

t2g(dp) orbitals and thus the HOMOs are mainly composed of

the orbitals of the co-ligands (trpy, L or Cl).10b The significant

metal contribution (y58%) in [2]2+ has been predicted in the

HOMO 2 5 state whereas around 40% metal contribution in [1]+

has been detected in HOMO 2 11 which also provides evidence

for the lesser extent of back-bonding in [2]2+. The presence of a

s/p-donating chloride ion plays a crucial role for the stronger

back-bonding in [1]+. This is reflected in a greater natural charge

on the metal ion in [2]2+ than [1]+ (Table 3). The observed more

linearity of the {Ru–N(5)–O(3)} bond in [2]2+ (175.9(3)u) as

compared to [1]+ (171.0(8)u) suggests relatively larger degeneracy

of the p*-orbitals (LUMO and LUMO + 1) in [2]2+.

The addition of one-electron to the p*-orbital of {RuNO}6 in

[1]+ or [2]2+ leads to the formation of a {RuNO}7 species, 1 or [2]+

and hence lifts the said degeneracy of the p*-orbitals (LUMO and

LUMO + 1) (Tables S3–S4{).10b,17a Such a splitting can be viewed

as a Jahn–Teller splitting due to the lowering of symmetry and the

spin–orbit interaction which is reflected in the bending mode of

Ru–NO with Ru–N(5)–O(3) angles of 141.85u and 139.69 in 1 and

[2]+, respectively, as revealed in their DFT optimized structures.

The SOMO of 1 or [2]+ is primarily composed of the 2p(p)-atomic

orbitals of N5 and O3 (p*(NON)) along with a lesser extent of

4dxy(RuII) orbital of 1 and dz2(RuII) orbital of [2]+. The

appreciable extent of metal contribution in the SOMO suggests

{RuII–NON} as the predominant oxidation state along with partial

mixing of the {RuI–NO+} state.11c–i,l The SOMO of 1 suggests the

presence of a 4dxy(RuII) A pp*(NON) back-bonding interaction

while the SOMO of [2]+ reveals the s-charge donation from the

low-lying singly-occupied pp*(NON) to the metal eg-orbitals. This

can be attributed to the different coordination environment of the

NO function in the complexes: NO is trans to the p-accepting trpy

Fig. 5 Electronic spectra in CH3CN of (a) [1]+ (black), 1 (red) and (b)

[2]2+ (black), [2]+ (red).

Fig. 6 Mulliken atomic spin density plot for [2]+ (N(5): 0.514, O(3):

0.329, Ru: 0.096).

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ligand in the equatorial plane in [1]+ whereas in [2]2+ the NO is

trans to the s-donating O2 of L2 in an axial orientation.

The approximate bent geometry of {Ru–NO}7 in 1 and [2]+

introduces the possibility of a pair of conformational isomers

based on the dihedral angle (h) of Ltrans–Ru–N5–O3, with the

idealized eclipsed and staggered angles of h = 0u and h = 45u,respectively.11b–d,h The geometry optimization at the G03/

(U)B3LYP level reveals that the eclipsed conformation is more

stable by 0.01 eV in [2]+. However, neither eclipsed nor staggered

conformation is found to be perfectly suitable for 1 where the

dihedral angle (h) of Ltrans(N3)–Ru–N5–O3 is 28.3u (Fig. S6{).

Such a pseudo-staggered (h = 28.3u) conformation in 1 is more

stable by 0.05 eV and 0.12 eV with respect to the eclipsed (h = 0u)and staggered (h = 45u) forms, respectively. Another theoretically

possible staggered conformation with the dihedral angle of h =

245u is destabilized under the same level of DFT presumably

due to the repulsive interaction between O3 and O2. The

stereoelectronic effect of the chloride and O2 from bulky L in 1

collectively prevent it exhibiting either the eclipsed (h = 0u) or

staggered (h = 45u) conformation, leading to an energetically

favored pseudo-staggered (h = 28.3u) conformation.

Photo-lability of the {Ru–NO} bond

The facile photo-labilization of the {RuII–NON} bond maintain-

ing the integrity of the remaining part of the molecule is believed

to be significant particularly from the perspective of the

biochemically desired target oriented NON delivery process.8,9

In this context both the nitrosyl complexes, [RuII(trpy)(L)

(NO+)Cl]+, [1]+, and [RuII(trpy)(L)(NO+)]2+, [2]2+, are found

to undergo the facile photocleavage of the {Ru–NO} bond under

the exposure of light in CH3CN as evidenced by their spec-

troscopic signatures (Fig. 7).19 The photolabilization of the

{Ru–NO} bond is accompanied by the formation of the corre-

sponding solvent bound ruthenium(II)-photoadducts, [RuII(trpy)

(L)(Cl)(CH3CN)] and [RuII(trpy)(L))(CH3CN)]+, respectively,

and the transformation proceeds through several isosbestic

points (Fig. 7). The formation of an Mb–NO adduct on passing

the liberated ‘‘NO’’ through the aqueous solution of reduced Mb

under deoxygenated conditions has been evidenced by its

characteristic absorption band at lmax = 420 nm (Fig. S7{).

The estimated first-order rate constant (k/s21) and t1/2/s values of

the photocleavage process are 2.6 6 1021, 2.66 and 2.9 6 1022,

23.8 for [1]+ and [2]2+, respectively. The ten-fold faster

photolability of the {RuII–NON} bond in [1]+ as compared to

[2]2+ can be rationalized based on their structural differences

including the trans influence of co-ligands. The cleavage of the

{RuII–NO+} bond via the photo-irradiation process is known to

proceed through the formation of the intermediate excited S = 1

state in {RuIII–NON}* as shown below.8f

[RuII–NO+] + hn A [RuIII–NON]* A [RuII–solvent] + NON

The formation of a solvated Ru(II) species instead of Ru(III)–

solvate as a photoproduct has been attributed to the absence of

the otherwise expected EPR signal of Ru(III) as has also been

established earlier.8 The electronic effect of p-accepting ligands

certainly facilitates the formation of {RuII–CH3CN} in the

ground state of the photo-product, as has also been established

recently by us.8g It should be noted that the selective

photocleavage of the M–NO bond, maintaining the integrity of

the rest of the molecule is biologically significant.9 The

competition between the pp-orbitals of NO and N3(trpy) for

the same dp-orbital of the metal ion (observed in HOMO 2 4)

makes the {Ru–NO} bond in [1]+ more photolabile. On the other

hand, the greater extent of dp(Ru) A pp(NO) back-bonding as

well as trans-influence of the s-donating carboxylate group (O1

of L2) are the likely factors for the relatively slower photo-

dissociation process in [2]2+.

Reactivity of {Ru–NO} towards molecular oxygen: dioxygenase

activity

The reaction of NO with the oxygenated heme-proteins has

immense biological importance.5 Though nitrogen monoxide

plays crucial roles in various physiological processes, over-

production of NO can also lead to several toxicological

processes, such as cell death and DNA damage, primarily due

to the formation of highly reactive peroxynitrite (2OONLO).20

Nitric oxide dioxygenase (NODs) catalyzes the reaction of NO

Fig. 7 Time evolution of the electronic spectra of (a) [1]+, concentra-

tion: 0.57 6 1024 M in CH3CN, time intervals: 2 s, and (b) [2]2+,

concentration: 0.21 6 1025 M in CH3CN, time intervals: 5 s under the

exposure of light (Xe lamp, 350 W). Insets show the absorbance versus

time plots at (a) 326 nm and (b) 482 nm corresponding to the solvent

species in each case.

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with dioxygen to yield the environmentally and physiologically

benign nitrate anion (NO32) in mammals.21 Highly reactive

peroxynitrite (2OONLO or oxoperoxonitrate) is known to be the

powerful oxidant/nitrating agent in nitric oxide biochemistry and

has an influential role in stress injury.21 The selective conversion

of peroxynitrite (2OONLO) to biologically benign nitrate takes

place in oxygenated heme-proteins to control the overproduction

of NON22,23 but in synthetic model systems, peroxynitrite usually

transforms to nitrite.24a–b,d

Although the reduced [2]+ remains inactive towards molecular

oxygen (O2), 1 (in situ generated via the addition of N2H4 in the

CH3CN solution of [1]+ or [2]2+) is found to activate molecular

oxygen at 298 K. Upon bubbling dioxygen (O2) in an acetonitrile

solution of 1 at pH y 1, the intensity of the peaks at 525 nm and

376 nm corresponding to 1 periodically decreases with the

concomitant growth of a new peak at 476 nm (Fig. S8{). The

transformation proceeds through several isosbestic points at

501 nm, 399 nm and 355 nm, implying a clean process with the

selective involvement of two species, [RuII(trpy)(L)(NON)Cl] (1)

and [RuII(trpy)(L)(NO2)Cl]2 ([1a]2). The formation of [1a]2 is

evidenced from its ESI-MS(+) peak at 554.88 corresponding to

{([1a]–Cl) + H+} (calcd. mass: 554.03) (Scheme 2 and Fig. S9{).

The transformation of 1 to [1a]2 is likely to occur via the

intermediate transient peroxynitrite species {Ru–(OONLO)}

([1a9]2). Consequently, the oxygenation reaction of 1 in the

presence of tyrosine-mimic 2,4-di-tert-butylphenol at pH y 1

results in the nitrated product, 2,4-di-tert-butyl-6-nitrophenol

(NO2–DTBP) and oxidative coupling product, 2,29-dihydroxy-

3,39,5,59-tetra-tert-butyl-1,19-biphenyl as depicted in Scheme 2.24

Conclusions

The present article highlights the following points:

- The unprecedented hemilabile feature of the chelated

quinaldate ligand (L2) in the presence of NO+ in the molecular

framework of [1]+ comprising of selective co-ligands, p-acepting

trpy and s/p-donating chloride.

- The electronic aspects of the coordinated nitrosyl function in

[1]BF4 and [2](BF4)2 differ appreciably based on their specific

structural features as evidenced by their n(NO) frequencies and

Eu(RuII–NO+/RuII–NON) potentials.

- The built in electronic structural differences in [1]+ and

[2]2+ have further been reflected in the rate of light induced

{RuII–NO+} bond cleavage.

- Reduced 1 has been selectively transformed to the

corresponding {RuII–NO2} species through contact with mole-

cular oxygen, although the corresponding [2]+ failed to exhibit

any such activity with O2.

Experimental section

Materials

The precursor complexes Ru(trpy)Cl325a and [RuII(trpy)(L)(Cl)]

14 (trpy = 2,29:69,299-terpyridine) were prepared according to

literature procedures. The ligand quinaldic acid (HL) and other

reagents and chemicals were obtained from Aldrich and used as

received. For spectroscopic and electrochemical studies HPLC-

grade solvents were used.

Physical measurements

1H NMR spectra were recorded in (CD3)2SO on a 400 MHz

Bruker spectrometer. Chemical shift data are quoted as d in ppm

and as s, d, dd, t, q and m representing singlet, doublet, doublet of

doublet, triplet, quartet, sextet and multiplet peaks, respectively.

IR and UV-vis spectra were recorded using Thermo Nicolet 320

and Perkin Elmer Lambda 950 spectrophotometers, respectively.

ESI-mass spectra were recorded using micromass Q-TOF. Cyclic

voltammetric studies were carried out using a PAR model 273A

electrochemistry system. Platinum wire working and auxiliary

electrodes and an aqueous SCE were used in a three-electrode

configuration. The supporting electrolyte was 0.1 mol dm23

[NEt4](ClO4), and the solute concentration was y1023 mol dm23.

The half-wave potential Eu298 was set equal to 0.5(Epa + Epc),

where Epa and Epc are the anodic and cathodic cyclic voltammetric

peak potentials, respectively. Elemental analyses were carried out

on Perkin-Elmer 240C elemental analyzer. The 2,4-di-tert-butyl

phenol (DTBP) reactions were monitored by gas chromatographic

technique with a FID detector (Shimadzu GC-2014 gas chroma-

tograph) as well as GCMS (Hewlett-Packard GCD-HP1800A).

The EPR measurement was made in a two-electrode capillary tube

with a X-band Bruker system ESP300.25b

Scheme 2 Dioxygenase activity of 1 at pH y 1.

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Synthesis of [RuII(trpy)(L)(NO+)Cl]BF4 ([1]BF4). 50 mg of

[RuII(trpy)(L)(Cl)] (A, 0.09 mmol) and 4 equivalent of NOBF4

(42 mg, 0.36 mmol) were taken in 20 cm3 dichloromethane. The

mixture was stirred initially at 0 uC for 30 min followed by at

298 K for another 2 h. During the course of the reaction the

initially blue solution of [RuII(trpy)(L)(Cl)] (A) changed to red

and a dark yellow precipitate was formed. The precipitate was

allowed to settle inside the refrigerator for 1 h and then filtered

off under reduced pressure and washed thoroughly with

dichloromethane and diethyl ether and dried in vacuo. The solid

mass was recrystallized from 1 : 1 acetonitrile–benzene solution.

Yield: 45 mg (75%). Anal. calcd. for C25H17N5ClO3BF4Ru (Mol.

wt.: 659.01): C, 45.52; H, 2.60; N, 10.62. Found: C, 45.35; H,

2.58; N, 10.53%. Molar conductivity (LM [V21 cm2 M21],

CH3CN): 105. ESI(+)-MS (m/z, CH3CN): 572.05 ([1]+), 543.04

([1–NO + H]+), 537.08 ([1–NO–Cl]+). 1H NMR (400 MHz,

(CD3)2SO): d (ppm) (J/Hz): 9.25 (1H, d, 8.4), 9.09 (2H, d, 8.4),

9.02 (2H, d, 8.0), 8.97 (1H, d, 7.2), 8.70 (1H, d, 8.8), 8.67 (3H,

m), 8.34 (2H, m), 8.15 (1H, t, 7.84, 7.20), 8.02 (2H, d, 7.3), 7.79

(2H, m). IR (KBr) n(BF42): 1083, 1601; n(NO+):1895; n(CLO):

1667 cm21. l/nm (CH3CN) (e/dm3 mol21 cm21): 455(sh),

349(10 380), 326(15 120), 315(14 120), 283(16 540).

Synthesis of [RuII(trpy)(L)(NO+)](BF4)2 ([2](BF4)2). A mixture

of [RuII(trpy)(L)(Cl)] (50 mg, 0.09 mmol) and AgNO3 (153 mg,

0.9 mmol) were taken in 25 cm3 of ethanol and heated to reflux

with constant stirring for 1 h. The solution was cooled to room-

temperature and filtered through Celite to remove the white

precipitate of AgCl. Nitrosonium tetrafluoroborate (NOBF4,

16 mg, 0.14 mmol) was then added to the above filtrate and the

resulting solution was stirred for 6 h. Partial removal of the

solvent under reduced pressure resulted in dark solid which was

collected through filtration. The solid mass thus obtained was

washed several times with diethyl ether and dried in vacuo. The

product was recrystallized from 1 : 1 benzene–acetonitrile. Yield:

26 mg (55%). Anal. calcd. for C25H17N5O3B2F8Ru (Mol. wt.

711.04): C, 42.19; H, 2.41; N, 9.85. Found: C, 42.25; H, 2.45; N,

9.89%. Molar conductivity (LM [V21 cm2 M21], CH3CN): 190.

ESI(+)-MS (m/z, CH3CN): 624.31 ([2](BF4)2–BF4]+). 1H NMR

(400 MHz, (CD3)2SO): d (ppm) (J/Hz): 10.25 (1H, d, 8.5), 9.18

(2H, m), 8.92 (5H, m), 8.49 (2H, m), 8.28 (1H, d, 8.4), 7.94 (3H,

m), 7.75 (1H, m), 7.61 (2H, m). IR (KBr), n(BF42): 1083, 1599;

n(NO+):1927; n(CLO): 1696 cm21. l/nm (CH3CN) (e/dm3 mol21

cm21): 346(20 920), 325(20 360), 289(24 380), 279(26 770).

Trapping of photoreleased ‘‘NO’’ by myoglobin

3.0 cm3 acetonitrile solution of the nitroso species ([1]+ or [2]2+)

was initially taken in a quartz cuvette of optical path length of

1 cm. The cuvette was sealed with a rubber septum and the

solution was deoxygenated by purging nitrogen gas. The

photolyis was carried out for 10 min using a Xe 350 W lamp.

The photoreleased free ‘‘NO’’ was allowed to pass through the

reduced myoglobin solution in water using a cannula and the

UV-vis. spectrum was recorded.

Dioxygenase activity

The conversion process of [RuII(trpy)(L)(NON)Cl] (1) A[RuII(trpy)(L)(NO2)Cl]2 ([1a]2) in acetonitrile at pH y 1 and

in presence of bubbling O2 was monitored by following the

increase in absorbance of the new band at 476 nm corresponding

to lmax of the nitro species [1a]2 till the changes of intensity

completely levelled off. The above experiment was then repeated

in presence of 2,4-di-tert-butylphenol (DTBP) at 298 K and the

formation of corresponding products, 2,4-di-tert-butyl-6-nitro-

phenol (NO2-DTBP) and oxidative coupling product, 2,29-dihy-

droxy-3,39,5,59-tetra-tert-butyl-1,19-biphenyl were confirmed by

GC and GCMS.

Crystallography

Single crystals of [1]BF4 and [2](BF4)2 were grown by slow

evaporation of their 1 : 1 acetonitrile–benzene solutions. Single

crystal X-ray diffraction data were collected using an OXFORD

XCALIBUR-S CCD single crystal X-ray diffractometer at

150 K. The structures were solved and refined by full-matrix

least-squares techniques on F2 using the SHELX-97 program.26

All data were corrected for Lorentz polarization and absorption

effects, and the non-hydrogen atoms were refined anisotropi-

cally. Hydrogen atoms were included in the refinement process

as per the riding model. The complex [1]BF4 was crystallized

with five water molecules. CCDC-838835 and CCDC-838836

contain the supplementary crystallographic data for [1]+ and

[2]2+, respectively. These data can be obtained free of charge

from The Cambridge Crystallographic Data Centre via

www.ccdc.cam.ac.uk/data_request/cif

Computational details

Full geometry optimizations were carried out at the (R)B3LYP

and (U)B3LYP levels27,28 using density functional theory

method with Gaussian 03 (revision C.02).29 All elements except

ruthenium were assigned the 6-31G(d) basis set during geometry

optimizations and 6-31G(d,p) during single point and property

calculations. The LanL2DZ basis set with effective core potential

was employed for the ruthenium atom.30,31 Vertical electronic

excitations based on (R)/(U)-B3LYP optimized geometries were

computed for the time-dependent density functional theory (TD-

DFT)32 in acetonitrile using the Polarizable continuum model

(PCM) of Tomasi and co-workers,33 specifically, the conductor

like PCM (CPCM) in conjugation with the united atom

topological model (UAO radii, implemented in Gaussian 03)

was applied.33–35 GaussSum was used to calculate the fractional

contributions of various groups to each molecular orbital.36 No

symmetry constraints were imposed during structural optimiza-

tions, and the nature of the optimized structures and energy

minima were defined by subsequent frequency calculations.

Natural bond orbital analyses were performed using the NBO

3.1 module of Gaussian 03 on optimized geometry.37 All the

calculated structures were visualized with ChemCraft.38

Acknowledgements

The financial support received from Department of Science and

Technology (DST), New Delhi and Council of Scientific and

Industrial Research (CSIR), New Delhi (fellowship to A.D.C.)

and University Grant Commission (UGC) (fellowship to P.D.),

New Delhi is gratefully acknowledged. X-Ray and GCMS

studies were carried out at the National Single-Crystal X-ray

3444 | RSC Adv., 2012, 2, 3437–3446 This journal is � The Royal Society of Chemistry 2012

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Diffraction Facility and Sophisticated Analytical

Instrumentation Facilities (SAIF), IIT Bombay, respectively.

Computational facilities from the Department of Chemistry, IIT

Bombay are gratefully acknowledged. We thank Mr. Thomas

Scherer, Institut fur Anorganische Chemie, Universitat Stuttgart

for EPR measurement.

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