Substitution of chloride by nitrosyl ligand in a scorpionate ruthenium(III) compound: A theoretical study

Inorganica Chimica Acta 362 (2009) 4651–4658

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Substitution of chloride by nitrosyl ligand in a scorpionate ruthenium(III) compound: A theoretical study Gabriel Aullón a,*, Santiago Alvarez a, Roberto Cao b, Mayreli Ortiz b, Alicia M. Díaz-García b a b

Departament de Química Inorgànica, Institut de Química Teòrica i Computacional, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain Laboratorio de Bioinorgánica, Facultad de Química, Universidad de La Habana, Zapata y G. Vedado, 10400 La Habana, Cuba

a r t i c l e

i n f o

Article history: Received 22 December 2008 Received in revised form 3 June 2009 Accepted 4 June 2009 Available online 11 June 2009 Dedicated to Swiatoslaw Trofimenko.

a b s t r a c t A theoretical study of the ruthenium(III) complex [RuCl2(pz2CHSO3)(en)] and of its nitrosyl-substituted product [Ru(NO)Cl(pz2CHSO3)(en)]+ is presented, based on density functional calculations. Several isomers of each compound differing in the position of the anionic tail of a bis(3,4-dimethyl-1-yl)methanesulfonate scorpionate ligand, pz2CHSO3 , relative to the monodentate ligands have been optimized. A two-step mechanism is proposed for the ligand substitution reaction that is consistent with the computational results and the weak coordination of the sulfonate group. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Nitric oxide ligand Ruthenium complexes DFT calculations

1. Introduction Nitric oxide plays a fundamental role in several biochemical processes such as blood pressure regulation, neurological function and immune response [1–3]. In this context, a disfunction of nitric oxide has been associated with diseases such as hypertension, epilepsy, arthritis, diabetes and septic shock. The importance of these functions for humans led to the concession of the 1998 Nobel Prize in Medicine to Murad, Furchgott and Ignarro ‘‘for their discoveries concerning nitric oxide as a signalling molecule in the cardiovascular system” [4–6]. Some transition metal compounds have long been known to regulate nitric oxide formation, metabolism and function in biological systems [7], but the chemistry of nitrosyl complexes with transition metals has been of interest to inorganic chemists in a wider sense [8–11]. Recently, Fe and Ru complexes have been developed aiming at clinical applications, among which nitrosylruthenium derivatives hold promise as NO carriers in the photodynamic therapy of tumors and other biological applications [10]. Moreover, [Ru(NO)(salen)Cl] is an effective catalyst for Diels– Alder reactions in various media, including water [12]. The radical nature of the NO molecule, with one unpaired electron in a p* orbital, makes it more reactive toward transition metals that other diatomic molecules such as CO or N2. The M–NO moieties are usually classified by the Enemark–Feltham notation based on electron counting, {M(NO)m}n, where n is * Corresponding author. Tel.: +34 934039759; fax: +34 934907725. E-mail address: [email protected] (G. Aullón). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.06.007

the total number of the electrons in the metal d and p*(NO) orbitals, since assignment of the formal oxidation numbers may lead to ambiguities [13]. For the {Ru(NO)}6 configuration, i.e. a (RuNO)3+ fragment [14], one should expect relatively short Ru–NO and N–O bonds and a nearly linear Ru–N–O bond angle (>160°), consistent with multiple bonding between RuII and NO+ [15]. On the other hand, for the alternative description RuIV–NO , a significantly bent Ru–N–O angle (120–140°) should be expected [16]. We have found that [RuCl2(pz2CHSO3)(en)] (1), in which the pz2CHSO3 ligand acts as bidentate through the N atoms of the pyrazolyl rings (Hpz = 3,4-dimethylpyrazole), reacts with NO in aqueous solution at room temperature to give the [RuCl(NO) (pz2CHSO3)(en)]+ cation (2) in a chloride substitution reaction. In order to understand such behavior we have undertaken a theoretical investigation of the ligand substitution reaction. This was done by means of calculations based on density functional theory (DFT) on the ruthenium complexes trans-[Ru(L)Cl(pz2CHSO3)(en)] (L = Cl, NO). 2. Results 2.1. trans-[RuCl2(j2-pz2CHSO3)(en)] Two isomers of trans-[RuCl2(pz2CHSO3)(en)], endo (1a) and exo (1b) have been optimized. Those isomers differ in the position of the sulfonate group relative to the neighboring chloro ligand. The optimized structures are shown in Fig. 1, their main structural parameters are presented in Table 1 and their atomic coordinates

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O

O

O

O

S

O

H S

C

H

Cl N

O

N

N

N

N

N Ru N

N Cl

endo

exo

1a

1b +

N

HC

Cl N

N Ru

N

+

SO3-

NO

N

N

Cl

SO3HC

Cl N

N Ru

N

N

C

N

N

N

N Ru

N

N

Cl

NO

syn

anti 2

are supplied as Supporting Information. Our calculations show that the exo isomer is more stable than the endo one by 6.9 kcal mol 1. The different character of the two N-donor ligands is reflected in the Ru–N distances, the bonds to ethylenediamine being 0.06 Å longer than those to pyrazolyl rings (averages: Ru–Npz = 2.11 and Ru–Nen = 2.17 Å), while the N–Ru–N bond angles are smaller than 90° due to the chelating nature of the two N-donor ligands (see Table 1). In contrast, the Ru–Cl distances corresponding to the chloro ligand vicinal to the sulfonate group are significantly different in the two isomers: 2.35 and 2.41 Å in the endo and exo forms, respectively. In the endo isomer, an oxygen atom of the sulfonate group is close to the coordinated chloride (2.80 Å, 1a), whereas in the exo isomer the same Cl atom presents a Cl


H contact with the methine group (2.33 Å, 1b), significantly shorter than expected for M– Cl


H–C weak hydrogen bonds (2.80 Å) [17]. It is likely that the stabilisation of the exo isomer provided by such a weak hydrogen

bond, combined with the repulsion between the O and Cl atoms in the endo form, accounts for the higher energy of the latter stereoisomer. Two alternative electronic configurations have been found to yield similar energies and structural parameters for 1a and 1b. Both have the unpaired electron of the d5-RuIII ion in an orbital with p*(Ru–Cl) character, an expected result given the p-donor character of the chloro ligands. Given their similar energy and structural parameters, we will consider only the lowest energy electron configuration from here on. Thus, the unpaired electron is found in the metal-centered p|| (t2g) orbital shown in Fig. 2. 2.2. [RuCl(j3-pz2CHSO3)(en)]+ The structure of the compound resulting from chloride dissociation of the dichloro complex, [RuCl(pz2CHSO3)(en)]+, schemati-

Fig. 1. Optimized molecular structure of the two isomers of [RuCl2(pz2CHSO3)(en)] (1a, left; 1b, right) in their doublet ground state, showing the different orientation of the sulfonate group. Hydrogen atom involved in H


Cl bonding is shown (dashed line).

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Table 1 Main structural parameters for the optimized molecular geometries of two isomers of [RuCl2(pz2CHSO3)(en)] with the electron configuration A shown in Fig. 2. Similar energies and geometries are obtained for the alternative electron configuration B. Parametera

1a (endo)

1b (exo)

Ru–Npz Ru–Nen Ru–Cl(1) Ru–Cl(2) Npz–Ru–Npz Nen–Ru–Nen Cl(1)–Ru–Cl(2) Relative energy Electron configurationb

2.124 2.180 2.352 2.437 84.8 79.0 162.1 +6.9 (p\)2(p||)1

2.100 2.175 2.420 2.429 85.4 79.8 162.8 0.0 (p\)2(p||)1

a b

Distances in Å, angles in degrees, energies in kcal mol Occupation of the t2g orbitals (see Fig. 2).

1

.

cally shown in 3, has also been optimized. The calculated geometry agrees well with that proposed for the analogous reduced complex [RuCl(j3-pz2CHSO3)(en)], that has been recently synthesized and spectroscopically characterized, but whose crystal structure has not been solved [18]. This structure is unexpected since the sulfonate group is a poor ligand and normally acts as a non-coordinating anion [16], although under certain conditions it can be a better ligand than water [19]. We note the short calculated Ru–O distance (2.04 Å), compared to those experimentally found in terminal [20] or bridging [21] sulfonate complexes (2.15–2.17 Å), and consistent with the range observed for Ru–O(R) bond lengths (2.08–2.28 Å, with no significant differences between RuII and RuIII complexes) [22]. Some examples of sulfonate groups bound to a transition metal with longer distances have been reported in the Cambridge Structural Database [23], (e.g. Ta: 2.25 [24]; Co: 2.41 [25]; or Cu: 2.38–2.42 Å [26–28]) and, in some cases, it can even replace one of the pyrazolylic groups of a scorpionate ligand [29].

O

O S

H

O

C N N

N

N

Ru

N

N Cl

3 2.3. [RuCl(NO)(j2-pz2CHSO3)(en)]+ Four isomers of trans-[RuCl(NO)(pz2CHSO3)(en)]+ have been optimized. These differ in the endo or exo position of the sulfonate group and in its vicinal (syn) or distal (anti) position relative to the NO ligand (2). Besides, we have considered for each isomer two spin states, a singlet corresponding to a RuII–NO+ formulation and a triplet corresponding to either the diradical RuIII–NO form or to Ru(IV)–NO . The optimized geometries are shown in Fig. 3 (atomic coordinates supplied as Supporting Information) and their relative energies, as well as their main structural parameters are presented in Table 2. For all four isomers analyzed, the ground state is always a singlet. The most stable isomer is the one with the syn-endo geometry, and the rest of the isomers are 19 kcal mol 1 or more higher in energy. The bond angle of the nitrosyl ligand (Ru–N–O = 148°), intermediate between those found for linear (>160°) and bent (130°)

Fig. 2. Energy diagram for the metal d orbitals in [RuCl2(pz2CHSO3)(en)] (1) with two alternative electron configurations, 1A and 1B. The energies shown correspond to the 1a isomer; similar values are found for 1b.

nitrosyls, is a structural feature that deserves closer inspection and will be discussed below. The other three isomers, in contrast, have nearly linear nitrosyls (M–N–O P 167°). The triplet state of the chloro-nitrosyl complex has an unpaired electron localized at the Ru atom and one upaired electron located in a p*(NO) orbital (calculated spin densities of about 0.8 at both Ru and the NO ligand), as corresponds to a (t2g)5(p*NO)1 electron configuration, consistent with a diradical character formally described as RuIII–NO that shows a bent MNO group. An alternative syn-endo structure with an O-bonded nitrosyl ligand has also been explored, since metastable excited states with such a coordination mode have been proposed by Coppens for [Ru(NO2)4(OH)(NO)]2 and [Fe(CN)5(NO)]2 [30]. The singlet and triplet states of that complex have been found to be 35 and 47 kcal mol 1 higher in energy than the N-bonded analogue, respectively, thus ruling out the existence of this linkage isomer under normal conditions. 3. Discussion The equivalence in the NMR signals of both pyrazolyl rings, and also ethylenediamine, indicates that both chelate ligands lie on the equatorial plane in [RuCl(NO)(pz2CHSO3)(en)]+. We assumed a similar ligand distribution for paramagnetic [RuCl2(pz2CHSO3)(en)] (undefined NMR spectrum), and this was confirmed by optimization of the structure, as represented in 4. The wide and undefined NMR signals observed for [RuCl2(pz2CHSO3)(en)] confirm its paramagnetic nature. Additionally, its EPR spectrum is characteristic of a ruthenium(III) complex (g1 = 2.025, g2 = 1.99, g3 = 1.98).

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Fig. 3. Optimized structures for the isomers of [RuCl(NO)(pz2CHSO3)(en)]+ (2) in the singlet ground state. The lowest energy form is the endo-syn isomer.

The analysis of the optimized geometry of the two isomers of [RuCl2(pz2CHSO3)(en)] tells us that the sulfonate tail of the scorpionate ligand is kept away from the metal atom due to the existence of a weak hydrogen bond to the axially coordinated chloro ligand in the exo isomer, but produces an O


Cl repulsion in the case of the endo isomer, thus making the former 6.9 kcal mol 1 more stable (4.3 in water solution). The interconversion of these two isomers can occur through inversion of the Ru(NN)2C ring, as shown in 4, although a significant barrier for such an inversion can be expected.

shown in 3, is a likely intermediate in the substitution reaction. Since in this case ligand dissociation requires the separation of the positive and negative charges of an ionic pair, this reaction is expected to be energetically unfeasible in the gas phase and can only be achieved in solution. For that reason we have calculated such a species in aqueous solution, where it is seen to be thermodinamically favored by 19.6 kcal mol 1 from endo (1a) isomer (15.4 from exo 1b one). In our optimized structure, the pending sulfonate group occupies the coordination position left vacant by the leaving chloro ligand (alternatively, donor solvents could play a similar role, as re-

O O

H S

O

C

Cl

Cl N N

N

N

N

Ru N

N

N

Cl

H

N N

N Cl

C O

N Ru

S

O

O

4

Due to the high steric hindrance of octahedral [RuCl2(pz2CHSO3)(en)] the reaction with NO should take place through a dissociative mechanism (D). The negatively charged sulfonate group should assist chloride dissociation, which is the rate determining step. The compound resulting [RuCl(pz2CHSO3)(en)]+, schematically

cently proposed) [31]. In this compound, the bonding parameters of the rest of the molecule are similar to those obtained for the dichloro precursor. As for the nitrosyl complex [RuCl(NO)(pz2CHSO3)(en)]+, we discuss only the results of the syn-endo isomer (2a) for simplicity,

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G. Aullón et al. / Inorganica Chimica Acta 362 (2009) 4651–4658 Table 2 Main calculated structural data for the four optimized isomers of [RuCl(NO)(pz2CHSO3)(en)]+ (2 and Fig. 3). Distances in Å, angles in degrees, energies in kcal mol Isomer

endo-syn

exo-syn

endo-anti

1

.

exo-anti

Parameter

S=0

S=1

S=0

S=1

S=0

S=1

S=0

S=1

Ru–Npz Ru–Nen Ru–Cl Ru–N N–O Npz–Ru–Npz Nen–Ru–Nen Cl–Ru–N Ru–N–O Relative energy

2.125 2.190 2.382 1.835 1.151 86.0 78.4 167.1 148.2 0.0

2.091 2.193 2.415 1.991 1.166 86.2 78.0 172.3 127.8 +30.6

2.107 2.196 2.366 1.793 1.151 85.9 79.3 170.1 167.6 +18.7

2.115 2.178 2.427 1.943 1.177 84.7 78.9 170.8 135.5 +23.7

2.115 2.196 2.297 1.800 1.157 85.2 78.3 171.8 168.8 +26.2

2.144 2.182 2.460 1.892 1.184 85.0 78.6 170.4 137.0 +35.0

2.110 2.194 2.374 1.782 1.152 85.5 78.7 172.7 176.8 +26.7

2.129 2.170 2.454 1.914 1.182 83.9 79.3 169.0 135.8 +37.4

since the other three isomers are significantly higher in energy. For such a {RuNO}6 system, the singlet ground state is consistent with its formulation as (RuII–NO+). A puzzling structural feature of this compound is its Ru–N–O bond angle (148°), intermediate between the nearly linear one expected for (RuII–NO+) and the strongly bent angle (120°) expected for a (RuIII–NO) situation (5a) [15]. Nevertheless, it is important to mention that the m(NO) was observed at 1831 cm 1, a relative low value for a linear Ru–N–O bond. These data can be compared with those for well identified {RuNO}6 compounds, such as [Ru(NH3)5(NO)]3+ and trans-[Ru(OH)(NH3)4 (NO)]2+, for which the (RuII–NO+) formulation has been established and which show practically linear bond angles (Ru–N–O = 179°) [32]. Also several structures of isoelectronic (MNO)3+ (M = Ru, Os) and (MNO)2+ (M = Mn, Re) transition metal complexes have been reported in recent years that feature a linear coordination of the nitrosyl [10]. There are, however, some precedents of significantly bent nitrosyl ligands in isoelectronic complexes, such as [(TTP)Ru(NO)(p-C6H4F)] [33] and [(OEP)Ru(NO)(p-C6H4F)] [34], with Ru–N– O bond angles of 152° and 155°, respectively.

O O S H

N

N

O H

Ru

N

N N

Cl

O N

N

Nen

Nen RuII

Npz

Nen

RuIII

Npz

Nen

Npz

Npz Cl

Cl

5a

5b

To understand the unusual Ru–N–O bond angle in the optimized structure, it is instructive to compare the evolution of that angle and of the calculated energy in water along the path for association of the nitrosyl ligand (Fig. 4). It can be seen that the nitrosyl passes through a nearly linear situation at Ru–N  2.75 Å, becomes strongly bent (Ru–N–O = 120° at Ru–N = 2.5 Å) as the NO ligand continues to approach Ru, and finally tends to straighten up at

O O

O

S

O O

N

O

C N

O

O

C

N N N

N

Ru

N

S N N

Cl

H

N

N

H Ru

N

N N

Cl

Fig. 4. Energy (line and circles) of the singlet state of [RuCl(NO)(pz2CHSO3)(en)]+ along the pathway for the substitution of the coordinated sulfonate group by NO, and evolution of the Ru–N–O angle (stars) along the same path. The optimized structure corresponds to Ru–N = 1.835 Å.

O

O O

O

S

N

O

C N

O

C

N N N

N

Ru

N

N N

Cl

Fig. 5. Energy evolution along the reaction path from 3 to 2 in the triplet state (triangles) and in the optimized singlet state (circles).

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the equilibrium distance (1.84 Å), reaching its optimized value of 148°. Notice that the energy reaches its maximum practically for the strongly bent position of the nitrosyl (Ru–N  2.5 Å) and decreases as it tends to linearity. This suggests that NO bending is associated to the barrier for the substitution reaction. The high energy along the path, when the nitrosyl is strongly bent, is associated not only to the cleavage of the Ru–O bond, but also to repulsion between the N lone pair and the leaving sulfonate group (6). On the other hand, the fact that linearization in the final part of the reaction stops at 148° is due to the presence of the neighboring sulfonate group, since a larger angle results in severe repulsion between the O atoms of the nitrosyl and sulfonate groups (7). This explanation is further supported by the fact that in the other isomers of the chloro-nitrosyl complex 2, in which the sulfonate tail of the scorpion is away from the NO, the Ru–N–O bond angle is larger than 167° (Table 2). Moreover, when the sulfonate tail is replaced by a carboxylate one, as in the complex [Ru(NO)(pz2CHCO2)2]+, the oxoanion remains bonded to the ruthenium center due to the more coordinating nature of the carboxylate group [35].

O

O S

H

The substitution of the sulfonate group by NO can be followed by optimizing the structures at decreasing Ru–NO distances, both in the singlet and triplet states. The resulting energy profiles (Fig. 5) show that the triplet state is higher in energy than the singlet for most of the pathway, and in particular at the transition state and for the product of the substitution reaction. The energy barrier is associated to the cleavage of the Ru–sulfonate bond and to a non linear attack of the NO due to the sterical hindering provided by the tail of the scorpion. In this regard, it is nicely seen that the Cl–Ru–NO angle is linearly correlated with the distance between the entering and leaving donor atoms. Furthermore, it must be noted that there are significant differences between the geometries of the two spin states throughout the pathway, being similar only at a particular point (close to Ru–N = 2.25 Å), at which the Ru–NO fragment presents a similar geometry but the tail of the scorpion is much closer to the nitrosyl in the singlet than in the triplet state. Therefore, the slow relaxation to a diamagnetic state observed experimentally is consistent with a mechanism that proceeds through the triplet channel, since the differences in geometry seem

O N

N

N

H

N

N

N

N

O

C N

N

Ru

O

S

O

O

C N

O

N

N

Ru

N

N

Cl

Cl

6

7

Energy/kcal·mol-1 TS 15

10

1a

5

0

2a*

1b

-5

-10

2a 3 -15

[RuIII-Cl]

[RuIII]+ + Cl-

[RuII-NO+]+ + Cl-

Fig. 6. Proposed mechanism for the substitution of a chloride by NO in [RuCl2(pz2CHSO3)(en)], passing through the intermediate 3. The energy values have been calculated taking into account the presence of water as a solvent.

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to preclude an intersystem crossing, and a slow relaxation to the singlet state should then be expected (see Fig. 6). 4. Conclusions The present theoretical study has allowed us to establish the molecular and electronic structures of the [RuCl2(pz2CHSO3)(en)] and [RuCl(NO)(pz2CHSO3)(en)]+ complexes in their different isomeric forms. The results of our calculations are in agreement with the structural data found in the literature for related compounds. The analysis of the nitrosyl complex shows clearly a preference for a nearly linear Ru–N–O group in the ground state, pointing to a NO+ ligand coordinated to a RuII center. The substitution of a chloride ligand by the nitrosyl proceeds through an intermediate complex in which the sulfonate group is coordinated to ruthenium. An energy profile for this reaction in solution has been presented, and the energy curves for the substitution reaction along the triplet and singlet channels suggest that it proceeds through a triplet state and ends up in the final singlet ground state through a slow relaxation process. 5. Experimental 5.1. General procedure All chemicals used were of high quality and purchased from Aldrich. CH3CN was dried and distilled before use. All solutions were purged with N2 to eliminate O2 and manipulated under inert atmosphere. 5.2. Spectroscopy IR spectra were recorded on a Perkin–Elmer 883 FT-IR spectrometer, preparing the samples as KBr tablets. EPR spectra were recorded on a Bruker ESP 300 X-band. 15N NMR spectra were obtained with a Bruker AV 400 (40.53 MHz for 15N). 5.3. Synthesis of [RuCl2(pz2CHSO3)(en)] (1) About 0.0500 g (0.24 mmol) of RuCl3, 0.0700 g (0.24 mmol) of 1 and 16 lL (0.24 mmol) of ethylenediamine were dissolved in 20 ml CH3CN under N2 atmosphere containing 0.0700 g (0.24 mmol) of pz2CHSO3 and stirred for 6 h. The dark brown solution was evaporated and extracted several times with acetone and evaporated to dryness under vacuum. A dark brown hygroscopic solid was obtained. Yield: 0.102 g (87%). Anal. Calc. for C13H23Cl2N6O3RuS (1): C, 30.50; H, 4.82; N, 16.31. Found: C, 30.41; H, 4.85; N, 16.25%. IR (KBr, cm 1): 3235 mas(NH2), 3120 ms(NH2), 1565 m(C=N), 1199 mas (SO3), 1118 ms(SO3). 5.4. Synthesis of [RuCl(NO)(pz2CHSO3)(en)]Cl (2) About 0.1000 g of [RuCl2(pz2CHSO3)(en)] was dissolved in CH3CN and mixed with 1 mL CH3CN solution saturated with NO and evaporated under a flow of N2. A brownish red crystalline solid was obtained. Yield: 0.093 g (92%). Anal. Calc. for C13H23Cl2N7O4RuS (2): C, 28.63; H, 4.25; N 17.98. Found: C, 28.59; H, 4.28; N, 17.91%. IR (KBr, cm 1): 3234 mas(NH2), 3135 ms(NH2), 1831 m(NO), 1557 m(C@N), 1189 mas(SO3), 1116 ms(SO3). 1H NMR (250 MHz, CD3OD, 298 K): d = 2.15 (s, 6 H, Me3), 2.45 (s, 6 H, Me5), 2.25–2.65 [t, 4 H, CH2(en)], 5.60 (s, 2 H, H4), 6.85 (s, 1 H, CH) ppm. 5.5. Reaction between [RuCl2(pz2CHSO3)(en)] (1) and NO Compound [RuCl2(pz2CHSO3)(en)] (1, 6 mg, 11.6 lmol) was dissolved in tert-butanol and deuterium oxide mixture (26:259 lL),

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and an aqueous solution of NO was added (715 lL). A sample of 500 lL was used for 15N NMR measures, and data recollected was finished when signal appears. Final sample was analyzed in situ being stable between 295 and 340 K, and inactive by EPR. 5.6. Computational details Density functional calculations were carried out using the GAUSSpackage [36], in which the hybrid method known as UB3LYP was applied, applying the Becke three parameters exchange functional [37] and the Lee–Yang–Parr correlation functional [38]. The basis set of valence double-f quality and effective core potentials to represent the innermost electrons of the ruthenium atoms known as LANL2DZ were used [39]. Similar descriptions were used for heavy main group elements as Cl and S [40], supplemented with an extra d-polarization functions [41]. The light elements (N, C, O and H) were described by 6-31g* [42,43]. Electronic configurations of each complex have been analyzed by a natural bond order (NBO) analysis [44]. The a and b spin orbitals were obtained from unrestricted calculations and they are drawn at the same energy for simplicity the orbital diagrams. Solvent effects were taken into account by means of the polarizable continuum model calculations [45,46], using standard options of PCM and cavity keywords [36]. Energies were calculated with water (e = 78.39) as solvent, keeping the geometry optimized for the isolated species (single-point calculations). IAN98

Acknowledgments Financial support to this work was provided by the Spanish Dirección General de Investigación (DGI) through grants CTQ200806670-C02-01/BQU and by Comissionat per a Universitats i Recerca of Generalitat de Catalunya through grant 2005SGR-0036. The computing resources at the Centre de Supercomputació de Catalunya (CESCA) were made available in part through a grant of Fundació Catalana per a la Recerca (FCR) and Universitat de Barcelona. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2009.06.007. References [1] J. Lancaster Jr. (Ed.), Nitric Oxide, Principles and Actions, Academic Press, San Diego, 1996. [2] M. Feelish, J.S. Stamler (Eds.), Methods in Nitric Oxide Research, Wiley, Chichester, 1996. [3] L.J. Ignarro (Ed.), Nitric Oxide, Biology and Pathobiology, Academic Press, San Diego, 2000. [4] F. Murad, Angew. Chem., Int. Ed. Engl. 38 (1999) 1857. [5] R.F. Furchgott, Angew. Chem., Int. Ed. Engl. 38 (1999) 1870. [6] L.J. Ignarro, Angew. Chem., Int. Ed. Engl. 38 (1999) 1882. [7] P.G. Wang, M. Xian, X. Tang, X. Wu, Z. Wen, T. Cai, A.J. Janczuk, Chem. Rev. 102 (2002) 1091. [8] G.B. Richter-Addo, P. Legzdins, Metal Nitrosyls, Oxford University Press, New York, 1992. [9] B.L. Westcott, J.H. Enemark, Transition metal nitrosyls, in: E.I. Solomon, A.B.P. Lever (Eds.), Inorganic Electronic Structure and Spectroscopy, II: Applications and Case Studies, Wiley, New York, 1999. [10] T.W. Hayton, P. Legzdins, W.B. Sharp, Chem. Rev. 102 (2002) 935. and references therein. [11] P.C. Ford, I.M. Lorkovic, Chem. Rev. 102 (2002) 993. [12] W. Odenkirk, A.L. Rheingold, B. Bosnich, J. Am. Chem. Soc. 114 (1992) 6392. [13] J.H. Enemark, R.D. Feltham, Coord. Chem. Rev. 13 (1974) 339. [14] A.W. Addison, J. Chem. Educ. 74 (1997) 1354. [15] E. Tfouni, M. Krieger, B.R. McGarvey, D.W. Franco, Coord. Chem. Rev. 236 (2003) 57. [16] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic Chemistry, sixth ed., Wiley, New York, 1999. [17] G. Aullón, D. Bellamy, L. Brammer, E. Bruton, A.G. Orpen, Chem. Commun. (1998) 653.

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