Ru-NO Bond Research Articles

Unveiling the electronic and molecular structure of a trinuclear ruthenium cluster containing one nitrosyl ligand

The understanding of the Ru-NO bond in Ru nitrosyl systems is of great interest due to the non-innocent nature of the NO ligand, and because of the ability of such compounds to release nitric oxide, an essential physiological regulator. The Ru-NO bond description can be even more complicated when it comes to polynuclear systems such as the μ-oxo clusters of general formula [Ru3O(CH3COO)6(L)3]n, which can be electronically localized or delocalized themselves depending on the nature of the L ligands, or present unpaired electrons depending on the Ru ions oxidation state. Herein, we present a detailed electronic- and molecular-structure description of the [Ru3O(CH3COO)6(py)2NO]PF6 (py = pyridine) cluster using multiconfigurational approaches and X-ray diffraction analysis. The X-ray data unveil a linear Ru–NO moiety with a Ru-NO angle of 180°. Although most linear {RuNO}6 complexes have been largely described as RuII–NO+, our valence bond-type analysis based on a CASSCF ground state wavefunction, with an active space comprised of 18 electrons and 16 orbitals, showed a predominance of the RuIII–NO° configuration, with minor contributions from RuIV–NO– and RuII–NO+. These findings provide a more precise framework to rationalize the electronic properties of trinuclear ruthenium nitrosyls, aiding the planning of new NO releasers based on those clusters.

Photoinduced cytotoxic activity of a rare ruthenium nitrosyl phenanthroline complex showing NO generation in human cells.

A new nitro-nitrosyl complex [RuNO(Phen)(NO2)2OH] (1) was synthesized and characterized by X-ray diffraction, where Phen = 1,10-phenanthroline. The complex was crystallized in two different modifications without (1) and with a solvent molecule of DMF (1a). The photolysis process together with the determination of the quantum yield of NO release was investigated in acetonitrile solution using a special flow-through system for the simultaneous registration of infrared (IR) and optical absorption (UV-vis) spectra under irradiation with 450 nm light. The quantum yield of photoinduced NO release was 4.0 ± 0.2%. DFT calculations showed that the main contribution to the absorption band at 450 nm is made by the HOMO/HOMO-1 → LUMO transitions, which are represented by the transfer of electron density from the -OH and -NO2 ligands to the orbitals located on the Ru-NO bond. The dark and photoinduced cytotoxicity of the complex was studied against the human breast adenocarcinoma (MCF-7) and lung carcinoma (A549) cell lines and human non-tumor lung fibroblasts (MRC5). The complex shows a low cytotoxicity on MCF-7 cells (ICdark50 = 90.6 ± 6.2 μM and ICirr.50 = 95.3 ± 11.4 μM) and a moderate dark cytotoxicity on A549 and MRC5 cells (ICdark50 = 33.4 ± 2.6 μM and ICdark50 = 62.6 ± 3.1 μM, respectively), which slightly increases after irradiation (ICirr.50 = 21.2 ± 3.3 μM and ICirr.50 = 47.2 ± 2.3 μM, respectively).

Effect of Electron-donating Group on NO Photolysis of {RuNO}6 Ruthenium Nitrosyl Complexes with N2 O2 Lgands Bearing π-Extended Rings.

In this study, we introduced the electron-donating group (-OH) to the aromatic rings of Ru(salophen)(NO)Cl (0) (salophenH2 =N,N'-(1,2-phenylene)bis(salicylideneimine)) to investigate the influence of the substitution on NO photolysis and NO-releasing dynamics. Three derivative complexes, Ru((o-OH)2 -salophen)(NO)Cl (1), Ru((m-OH)2 -salophen)(NO)Cl (2), and Ru((p-OH)2 -salophen)(NO)Cl (3) were developed and their NO photolysis was monitored by using UV/Vis, EPR, NMR, and IR spectroscopies under white room light. Spectroscopic results indicated that the complexes were diamagnetic Ru(II)-NO+ species which were converted to low-spin Ru(III) species (d5 , S=1/2) and released NO radicals by photons. The conversion was also confirmed by determining the single-crystal structure of the photoproduct of 1. The photochemical quantum yields (ΦNO s) of the photolysis were determined to be 0>1, 2, 3 at both the visible and UV excitations. Femtosecond (fs) time-resolved mid-IR spectroscopy was employed for studying NO-releasing dynamics. The geminate rebinding (GR) rates of the photoreleased NO to the photolyzed complexes were estimated to be 0≃1, 2, 3. DFT and TDDFT computations found that the introduction of the hydroxyl groups elevated the ligand π-bonding orbitals (π (salophen)), resulting in decrease of the HOMO-LUMO gaps in 1-3. The theoretical calculations suggested that the Ru-NNO bond dissociations of the complexes were mostly initiated by the ligand-to-ligand charge transfer (LLCT) of π(salophen)→π*(Ru-NO) with both the visible and UV excitations and the decreasing ΦNO s could be explained by the changes of the electronic structures in which the photoactivable bands of 1-3 have relatively less contribution of transitions related with Ru-NO bond than those of 0.

Ruthenium-nitrosyl complexes as NO-releasing molecules, potential anticancer drugs, and photoswitches based on linkage isomerism.

The synthesis of new types of mono- and polynuclear ruthenium nitrosyl complexes is driving progress in the field of NO generation for a variety of applications. Light-induced Ru-NO bond dissociation in solution may involve transient linkage isomers MS1 (Ru-ON) and MS2 (Ru-η2-NO), which can be detected spectroscopically and analyzed computationally. The investigation of photoisomerization processes in the solid state may be useful for potential application of such complexes for data storage, photochromic or photomagnetic materials or even non-linear optics. Herein we describe the major developments in the synthesis of ruthenium nitrosyl complexes, their photoinduced linkage isomerization (PLI) processes, their NO release both in the solid state and in solution, and their application as potential anticancer drugs. Illustrative examples of such innovations made mainly in the last decade are provided.

A new photoactivable NO-releasing {Ru-NO}6 ruthenium nitrosyl complex with a tetradentate ligand containing aniline and pyridine moieties.

A new type of photoactivable NO-releasing ruthenium nitrosyl complex, [Ru(EPBP)Cl(NO)], with a tetradentate ligand, N,N'-(ethane-1,2-diyldi-o-phenylene)-bis(pyridine-2-carboxamide) (= H2 EPBP) was synthesized. Single crystal X-ray crystallography revealed that the complex has a distorted octahedral coordination geometry and NO is positioned at cis to Cl- ion. NO-photolysis was observed under a white room light. The photodissociation of Ru-NO bond was identified by various techniques including X-ray crystallography, IR, UV/Vis absorption, electron paramagnetic resonance (EPR), and NMR spectroscopies. Quantum yields for the NO-photolysis of the complex in CH3 OH, CHCl3 , DMSO, CH3 CN, and CH3 NO2 were measured to be 0.19-0.36 with 400 (±5) nm excitation. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to understand the details of the photodissociation of the complex. The calculations suggest that the NO photolysis is most likely initiated by the electronic transition from the aniline moiety π MOs (π (aniline)) of the EPBP2- chelating ligand to the π-antibonding MO of Ru-NO (π*(Ru-NO)). Experimental and theoretical investigations indicate that the EPBP2- ligand provides an effective platform forming ruthenium nitrosyl complexes useful for NO-photoreleasing.

Nature of the bond, reduction potential, and solvation properties of ruthenium nitrosyl complexes of the type trans‐[Ru(NH3)4(L)(NO)]2+/3+ and [Ru(salen)(L)(NO)]2+/3+ in different charge and spin states

AbstractIn this work, we present a systematic study of the nature of the RuNO bond, reduction potential, and behavior in the solution of several ruthenium nitrosyl complexes of the type trans‐[Ru(NH3)4(L)(NO)]2+/3+ (L = Cl, H2O, NH3, OH, py = pyridine, POEt3 = triethylphosphite, TVP = trivinyl phosphite) and [Ru(salen)(L)(NO)]2+/3+ (L = Cl, H2O) in different charge and spin states. We employed density functional theory, complete active space self‐consistent field, the quantum theory of atoms in molecules, charge decomposition analysis, and Monte Carlo statistical simulation in aqueous solution to investigate how the different ligands on the coordination sphere of the ruthenium atom can affect the nature of the RuNO bond and, therefore, its reactivity, reduction potential, and solvation properties. The nature of the RuNO bond varies significantly between the singlet, doublet, and triplet electronic states of these complexes, in part due to the change in the coordination mode of the NO ligand and also due to the electron correlation effects along the RuNO bond that changes drastically. For all complexes studied, the RuNO bond shows some intriguing characteristics, such as large kinetic energy density and considerable amount of electron localization between the atoms, besides high multiconfigurational character, with a significant contribution from the RuIIINO0 configuration. In addition, for all the complexes investigated, the NO ligand does not interact with the solvent through hydrogen bonds, making this group accessible for chemical reactions in solution. Another important finding was that some of the RuIIINO compounds can be reduced by biological reducing agents in the singlet ground state, while all of them have high reduction potentials in the triplet excited state, making them suitable candidates for the photorelease of nitric oxide.

A DFT study of cis-[Ru(NO)(NO2)bpy(dye)2]n+ complexes as NO donors

The synthesis and characterization of a new complex, cis-[Ru(NO)(NO2)bpy(AO)2](PF6)2, was reported, and the release of NO from ruthenium-nitrosyl complexes with dye ligands was investigated using DFT and TDDFT methods. The complexes are cis-[Ru(NO)(NO2)bpy(dye)]n+, where the dye is either proflavine (1), acridine orange (2), neutral red (3), resorufin (4), thionol (5), or methylene violet (Bernthsen) (6). The optimized structures are in agreement with the experimental data of analogue complex from the literature. Simulated UV–Visible spectrum of complex 2 showed good agreement with the experimental results. The calculated electron affinities (EA) indicated that the N-coordinated complexes 1–3 should be more easily reduced and that this reduction weakens the Ru–NO bond, thus facilitating the NO release. Reduced complexes (Ru-NO0) presented lower bond dissociation energies (BDE) than the complexes (Ru-NO+). Complexes 1–3 showed the lowest BDE. This suggests a lower energy cost to release NO, and therefore, the N-coordinated complexes are the most promising NO donors among the investigated complexes.

Ruthenium nitrosyl complexes with the molecular framework [RuII(dmdptz)(bpy)(NO)]n+ (dmdptz: N,N-dimethyl-4,6-di(pyridin-2-yl)-1,3,5-triazin-2-amine and bpy: 2,2′-bipyridine). Electronic structure, reactivity aspects, photorelease, and scavenging of NO

Two ruthenium nitrosyl complexes have been stabilized both in {Ru–NO}6 and {Ru–NO}7 configurations which show facile photocleavage of Ru–NO bond on exposure to visible light. The photo liberated NO is captured by reduced myoglobin.

Formation, reactivity, photorelease, and scavenging of NO in ruthenium nitrosyl complexes

The two newly designed nitrosyl complexes with Enemark–Feltham notation {RuNO}6 and {RuNO}7 configurations have been isolated as the perchlorate salts in the molecular framework [RuII(dmdptz)(phen)(NO)]n+ (dmdptz: N,N-dimethyl-4,6-di(pyridin-2-yl)-1,3,5-triazin-2-amine and phen: 1,10-phenanthroline) [RuII(dmdptz)(phen)(NO+)](ClO4)3 [4](ClO4)3 and [RuII(dmdptz)(phen)(NO)](ClO4)2 [5](ClO4)2 respectively. The single crystal X-ray structures of complexes [RuII(dmdptz)(phen)Cl](ClO4) [1](ClO4), [RuII(dmdptz)(phen)(NO2)](ClO4) [3](ClO4) and [4](ClO4)3 have been determined. The π– acceptance of the NO+ moiety in [4](ClO4)3 is reflected from the triple bond characteristic bond length 1.131(5) Å with simultaneous trans angle of 175.3(4)° as a proof of true linear coordination mode. A sizable shift in ν (NO) frequency (Δν = 361 cm−1) on moving from [4](ClO4)3 to [5](ClO4)2 are in good agreement for largely NO centered reduction with the changes in bonding {RuNO}6 [4](ClO4)3 to {RuNO}7 [5](ClO4)2. The redox properties of [4](ClO4)3 along with the precursor complexes, have been investigated. On exposure to visible light in the deoxygenated acetonitrile solution at room temperature both [4](ClO4)3 and [5](ClO4)2 spontaneously transform to their corresponding solvated derivative [RuII(dmdptz)(phen)(CH3CN)](ClO4)2 [2](ClO4)2 via the facile photocleavage of Ru–NO bond with KNO 9.26 × 10−3 min−1 (t1/2 = 74 min) and 4.03 × 10−2 min−1 (t1/2 = 17 min) respectively. The photoreleased “NO” can be scavenged by biologically relevant target molecule myoglobin as an MbNO adduct.

Photoinduced inhibition of DNA repair enzymes and the possible mechanism of photochemical transformations of the ruthenium nitrosyl complex [RuNO(β-Pic)2(NO2)2OH

In this work we have demonstrated that the ruthenium nitrosyl complex [RuNO(β-Pic)2(NO2)2OH] is suitable for investigation of the inactivation of DNA repair enzymes in vitro. Photoinduced inhibition of DNA glycosylases such as E. coli Endo III, plant NtROS1, mammalian mNEIL1 and hNEIL2 occurs to an extent of ≥90% after irradiation with the ruthenium complex. The photophysical and photochemical processes of [RuNO(β-Pic)2(NO2)2OH] were investigated using stationary and time-resolved spectroscopy, and mass spectrometry. A possible mechanism of the photo-processes was proposed from the combined spectroscopic study and DTF calculations, which reveal that the photolysis is multistage. The primary and secondary photolysis stages are the photo-induced cleavage of the Ru-NO bond with the formation of a free nitric oxide and RuIII complex followed by ligand exchange with solvent. For E. coli Endo III, covalent interaction with the photolysis product was confirmed by UV-vis and mass spectrometric methods.

How does the total charge and isomerism influence the Ru-NO ammine complexes?

Nitric oxide plays an important role in several physiological processes. This study investigates model ruthenium ammine coordination compounds to control NO bioavailability: cis-[RuCl(NO)(NH3)4]+ (1+), cis-[RuCl(NO)(NH3)4]2+ (12+), cis-[RuCl(NO)(NH3)4]3+ (13+), trans-[RuCl(NO)(NH3)4]+ (2+), trans-[RuCl(NO)(NH3)4]2+ (22+), trans-[RuCl(NO)(NH3)4]3+ (23+), [Ru(NO)(NH3)5]+ (3+), [Ru(NO)(NH3)5]2+ (32+), and [Ru(NO)(NH3)5]3+ (33+). We employed natural population analysis (NPA) atomic charges (qNPA) and the LUMO to identify the main reduction sites in the complexes 1, 2 and 3. For example, in the transformations 12+ → 1+, 22+ → 2+, and 33+ → 32+, the main reduction site was a NO π* orbital, which accounted for the lower electron density of the Ru-NO bond critical point (BCP) in 1+, 2+, and 32+ than 12+, 22+, and 33+, respectively, as shown by the quantum theory of atoms in molecules (QTAIM). The QTAIM method indicated that the electron density was larger in Ru-NO BCP due to the Cl negative cis- and trans-influence in 12+ and 22+, respectively, as compared with the NH3 influence in 33+. Compared to trans-Cl-Ru-NO in 22+, the interacting quantum atoms method demonstrated that cis-Cl-Ru-NO in 12+ displayed (i) a larger repulsive electrostatic energy, which agreed with qNPA, and (ii) a less negative exchange-correlation energy between Ru and the NO nitrogen atom, which agreed with topological analyses performed by the QTAIM method. Thus, the combination of topological and energy decomposition analyses allowed the mechanism behind the Ru-NO bond to be revealed regarding the influence of the total charge and the relative position of the ligands.

Reactive nitrogen/oxygen species production by nitro/nitrosyl supramolecular ruthenium porphyrin complexes

Abstract This manuscript reports on new nitro/nitrosyl Ru-based complexes, which were synthesized with the purpose of using them as precursors to obtain supramolecular ruthenium porphyrin species ({TPyP[Ru(NO2)(5,5′-Mebipy)2]4}(PF6)4) and ({TPyP[Ru(NO)(5,5′-Mebipy)2]4}(PF6)12). The photochemical and photophysical properties of these porphyrin species were investigated. Results show that the complex containing nitrite is able to produce NO by homolytic O—NO cleavage ( Φ N O P P h = 0.05) while the {TPyP[Ru(NO)(5,5′-Mebipy)2]4}(PF6)12 does it by direct labilization ( Φ N O P P h = 0.53) of the Ru NO bond. Furthermore, a triplet quantum yield of 0.09 and 0.27 was observed for complexes containing nitrite and nitric oxide, respectively. The reactive oxygen species quantum yield for the complex {TPyP[Ru(NO)(5,5′-Mebipy)2]4}(PF6)12 (0.78) is consistent with the sum of quantum yields NO release (0.53) and triplet state (0.27), which suggests that both processes participate in the formation of the reactive species. Our results show that combining these characteristics, NO production and triplet states, on the same platform could induce a synergic effect, leading to a considerable improvement in the photodynamic action of these complexes.

Ruthenium nitrosyl complexes containing pyridine-functionalized carbenes – A theoretical insight

The Ru–NO bonding situation in a set of ruthenium(II) nitrosyl complexes containing pyridine-functionalized carbenes as bidentate ligands is presented. Cheng's complex [(L)Ru(NO)Cl3], where L = 3-tert-butyl-1-(2-pyridyl)imidazol-2-ylidene, 1a, was used as a model structure and the effect of different families of pyridine-functionalized carbene ligands on the Ru–NO bond strength was explored, including imidazolylidenes, triazolylidenes, oxazolylidenes, thiazolylidenes, P-heterocyclic carbenes, imidazolidinone, triazolidinone, among others. The results reveal that the NO+ group binds more strongly to the Ru(II), than carbene carbon or pyridine nitrogen atoms. The EDA-NOCV results show that the nature of the carbene has a direct influence on the lability of the Ru–NO, since it changes the electronic environment around the metallic centre. EDA-NOCV results point out that the nature of the Ru−NO+ interactions (1a–16b) presents a very preponderant covalent character (circa 70%), while the electrostatic character covers circa 30% of the total interaction energy. The energy decomposition still reveals that Ru−NO+ bonds are strengthen in complexes 1a–16a, than in 1b–16c. The weakest Ru−NO+ interactions are observed for complexes containing P-heterocyclic ligands (PHCs), specially for complexes where the NO+ is coordinated trans to the carbene carbon atoms. The metal→ ligand π-back-donation is more intense towards PHC than towards NO+.

Orbital entanglement and CASSCF analysis of the Ru-NO bond in a Ruthenium nitrosyl complex.

Complete active space self-consistent field (CASSCF) wavefunctions and an orbital entanglement analysis obtained from a density-matrix renormalisation group (DMRG) calculation are used to understand the electronic structure, and, in particular, the Ru-NO bond of a Ru nitrosyl complex. Based on the configurations and orbital occupation numbers obtained for the CASSCF wavefunction and on the orbital entropy measurements evaluated for the DMRG wavefunction, we unravel electron correlation effects in the Ru coordination sphere of the complex. It is shown that Ru-NO π bonds show static and dynamic correlation, while other Ru-ligand bonds feature predominantly dynamic correlation. The presence of static correlation requires the use of multiconfigurational methods to describe the Ru-NO bond. Subsequently, the CASSCF wavefunction is analysed in terms of configuration state functions based on localised orbitals. The analysis of the wavefunctions in the electronic singlet ground state and the first triplet state provides a picture of the Ru-NO moiety beyond the standard representation based on formal oxidation states. A distinct description of the Ru and NO fragments is advocated. The electron configuration of Ru is an equally weighted superposition of Ru(II) and Ru(III) configurations, with the Ru(III) configuration originating from charge donation mostly from Cl ligands. However, and contrary to what is typically assumed, the electronic configuration of the NO ligand is best described as electroneutral.

The nature of the M–NO bond in [M(Imidazole)(PPIX)(L)]q complexes (M = Fe2+, Ru2+; L = NO+, NO[rad] and NO−; PPIX = Protoporphyrin IX)

Aiming at understanding the effect of the replacement of iron by ruthenium in the Protoporphyrin IX (PPIX) system of the heme group, in this work density functional theory calculations were performed to investigate the structures, nature of the M–NO bond and electronic spectra of the complexes [M(Im)(PPIX)(L)]q, where M=Fe2+ and Ru2+, Im=Imidazole ring, PPIX=Protoporphyrin IX of the heme group and L=NO+, NO and NO−. Energetic and structural results obtained at the B3LYP/SBKJC/6-31G(d) level of theory indicates that the NO coordinates preferentially through the nitrogen atom and its oxidized form NO+ produces more stable complexes. Charge and energy decomposition analysis revealed that, independent of the coordination mode, the interaction of NO species with the ruthenium fragment is stronger than with iron. TD-DFT calculations predict that the transition that can lead to photodissociation of the Ru–NO bond occurs at 419nm and if the NO coordinates to ruthenium in its oxidized form this transition is blue-shifted to 384nm. The results obtained in this work also gives support to conclude that ruthenium complexes with the protoporphyrin IX (PPIX) system can strongly bind NO, which can be used to capture NO species, and releasing it upon irradiation with visible light.

Theoretical investigation on ruthenium tetraazaporphyrin as potential nitric oxide carrier in biological systems

Nitric oxide (NO) is an important chemical compound involved in many physiological and pathological processes in living organisms. However, nitric oxide is a very reactive radical that needs to be carried through organisms to reach the desired biological target. With the aim of developing new compounds that can be used as biomedical NO carrier agents we carried out a theoretical investigation at B3LYP/6-31+G(d)/LANL2DZ level on the interaction of NO with RuTAP (Ruthenium tetraazaporphyrin) and Ru(L)TAP, where L=Cl-, NH₃, and Pyridine (Py)) and the oxidation state of Ru ranging from +1 to +3. The theoretical calculation results show that the geometric and electronic parameters of the Ru-NO bond are highly dependent on the oxidation state of Ru and of the chemical nature of ligand L at axial position. The results also show clearly that RuTAP and Ru(L)TAP are good potential candidates to be used as NO carriers in living organisms.

Synthesis, spectroscopic analysis and photolabilization of water-soluble ruthenium(iii)–nitrosyl complexes

In this paper, the synthesis, structural and spectroscopic characterization of a series of new Ru(III)-nitrosyls of {RuNO}(6) type with the coligand TPA (tris(2-pyridylmethyl)amine) are presented. The complex [Ru(TPA)Cl(2)(NO)]ClO(4) (2) was prepared from the Ru(III) precursor [Ru(TPA)Cl(2)]ClO(4) (1) by simple reaction with NO gas. This led to the surprising displacement of one of the pyridine (py) arms of TPA by NO (instead of the substitution of a chloride anion by NO), as confirmed by X-ray crystallography. NO complexes where TPA serves as a tetradentate ligand were obtained by reacting the new Ru(II) precursor [Ru(TPA)(NO(2))(2)] (3) with a strong acid. This leads to the dehydration of nitrite to NO(+), and the formation of the {RuNO}(6) complex [Ru(TPA)(ONO)(NO)](PF(6))(2) (4), which was also structurally characterized. Derivatives of 4 where nitrite is replaced by urea (5) or water (6) were also obtained. The nitrosyl complexes obtained this way were then further investigated using IR and FT-Raman spectroscopy. Complex 2 with the two anionic chloride coligands shows the lowest N-O and highest Ru-NO stretching frequencies of 1903 and 619 cm(-1) of all the complexes investigated here. Complexes 5 and 6 where TPA serves as a tetradentate ligand show ν(N-O) at higher energy, 1930 and 1917 cm(-1), respectively, and ν(Ru-NO) at lower energy, 577 and 579 cm(-1), respectively, compared to 2. These vibrational energies, as well as the inverse correlation of ν(N-O) and ν(Ru-NO) observed along this series of complexes, again support the Ru(II)-NO(+) type electronic structure previously proposed for {RuNO}(6) complexes. Finally, we investigated the photolability of the Ru-NO bond upon irradiation with UV light to determine the quantum yields (φ) for NO photorelease in complexes 2, 4, 5, and additional water-soluble complexes [Ru(H(2)edta)(Cl)(NO)] (7) and [Ru(Hedta)(NO)] (8). Although {RuNO}(6) complexes are frequently proposed as NO delivery agents in vivo, studies that investigate how φ is affected by the solvent water are lacking. Our results indicate that neutral water is not a solvent that promotes the photodissociation of NO, which would present a major obstacle to the goal of designing {RuNO}(6) complexes as photolabile NO delivery agents in vivo.

Ruthenium nitrosyl complexes with 1,4,7-trithiacyclononane and 2,2′-bipyridine (bpy) or 2-phenylazopyridine (pap) coligands. Electronic structure and reactivity aspects

The present article describes ruthenium nitrosyl complexes with the {RuNO}(6) and {RuNO}(7) notations in the selective molecular frameworks of [Ru(II)([9]aneS(3))(bpy)(NO(+))](3+) (4(3+)), [Ru(II)([9]aneS(3))(pap) (NO(+))](3+) (8(3+)) and [Ru(II)([9]aneS(3))(bpy)(NO˙)](2+) (4(2+)), [Ru(II)([9]aneS(3))(pap)(NO˙)](2+) (8(2+)) ([9]aneS(3) = 1,4,7-trithiacyclononane, bpy = 2,2'-bipyridine, pap = 2-phenylazopyridine), respectively. The nitrosyl complexes have been synthesized by following a stepwise synthetic procedure: {Ru(II)-Cl} → {Ru(II)-CH(3)CN} → {Ru(II)-NO(2)} → {Ru(II)-NO(+)} → {Ru(II)-NO˙}. The single-crystal X-ray structure of 4(3+) and DFT optimised structures of 4(3+), 8(3+) and 4(2+), 8(2+) establish the localised linear and bent geometries for {Ru-NO(+)} and {Ru-NO˙} complexes, respectively. The crystal structures and (1)H/(13)C NMR suggest the [333] conformation of the coordinated macrocyclic ligand ([9]aneS(3)) in the complexes. The difference in π-accepting strength of the co-ligands, bpy in 4(3+) and pap in 8(3+) (bpy < pap) has been reflected in the ν(NO) frequencies of 1945 cm(-1) (DFT: 1943 cm(-1)) and 1964 cm(-1) (DFT: 1966 cm(-1)) and E°({Ru(II)-NO(+)}/{Ru(II)-NO˙}) of 0.49 and 0.67 V versus SCE, respectively. The ν(NO) frequency of the reduced {Ru-NO˙} state in 4(2+) or 8(2+) however decreases to 1632 cm(-1) (DFT: 1637 cm(-1)) or 1634 cm(-1) (DFT: 1632 cm(-1)), respectively, with the change of the linear {Ru(II)-NO(+)} geometry in 4(3+), 8(3+) to bent {Ru(II)-NO˙} geometry in 4(2+), 8(2+). The preferential stabilisation of the eclipsed conformation of the bent NO in 4(2+) and 8(2+) has been supported by the DFT calculations. The reduced {Ru(II)-NO˙} exhibits free-radical EPR with partial metal contribution revealing the resonance formulation of {Ru(II)-NO˙}(major)↔{Ru(I)-NO(+)}(minor). The electronic transitions of the complexes have been assigned based on the TD-DFT calculations on their DFT optimised structures. The estimated second-order rate constant (k, M(-1) s(-1)) of the reaction of the nucleophile, OH(-) with the electrophilic {Ru(II)-NO(+)} for the bpy derivative (4(3+)) of 1.39 × 10(-1) is half of that determined for the pap derivative (8(3+)), 2.84 × 10(-1) in CH(3)CN at 298 K. The Ru-NO bond in 4(3+) or 8(3+) undergoes facile photolytic cleavage to form the corresponding solvent species {Ru(II)-CH(3)CN}, 2(2+) or 6(2+) with widely varying rate constant values, (k(NO), s(-1)) of 1.12 × 10(-1) (t(1/2) = 6.2 s) and 7.67 × 10(-3) (t(1/2) = 90.3 s), respectively. The photo-released NO can bind to the reduced myoglobin to yield the Mb-NO adduct.

Reactivity, photolability, and computational studies of the ruthenium nitrosyl complex with a substituted cyclam fac-[Ru(NO)Cl2(κ3N4,N8,N11(1-carboxypropyl)cyclam)]Cl·H2O

Chemical reactivity, photolability, and computational studies of the ruthenium nitrosyl complex with a substituted cyclam, fac-[Ru(NO)Cl(2)(κ(3)N(4),N(8),N(11)(1-carboxypropyl)cyclam)]Cl·H(2)O ((1-carboxypropyl)cyclam = 3-(1,4,8,11-tetraazacyclotetradecan-1-yl)propionic acid)), (I) are described. Chloride ligands do not undergo aquation reactions (at 25 °C, pH 3). The rate of nitric oxide (NO) dissociation (k(obs-NO)) upon reduction of I is 2.8 s(-1) at 25 ± 1 °C (in 0.5 mol L(-1) HCl), which is close to the highest value found for related complexes. The uncoordinated carboxyl of I has a pK(a) of ∼3.3, which is close to that of the carboxyl of the non coordinated (1-carboxypropyl)cyclam (pK(a) = 3.4). Two additional pK(a) values were found for I at ∼8.0 and ∼11.5. Upon electrochemical reduction or under irradiation with light (λ(irr) = 350 or 520 nm; pH 7.4), I releases NO in aqueous solution. The cyclam ring N bound to the carboxypropyl group is not coordinated, resulting in a fac configuration that affects the properties and chemical reactivities of I, especially as NO donor, compared with analogous trans complexes. Among the computational models tested, the B3LYP/ECP28MDF, cc-pVDZ resulted in smaller errors for the geometry of I. The computational data helped clarify the experimental acid-base equilibria and indicated the most favourable site for the second deprotonation, which follows that of the carboxyl group. Furthermore, it showed that by changing the pH it is possible to modulate the electron density of I with deprotonation. The calculated NO bond length and the Ru/NO charge ratio indicated that the predominant canonical structure is [Ru(III)NO], but the Ru-NO bond angles and bond index (b.i.) values were less clear; the angles suggested that [Ru(II)NO(+)] could contribute to the electronic structure of I and b.i. values indicated a contribution from [Ru(IV)NO(-)]. Considering that some experimental data are consistent with a [Ru(II)NO(+)] description, while others are in agreement with [Ru(III)NO], the best description for I would be a linear combination of the three canonical forms, with a higher weight for [Ru(II)NO(+)] and [Ru(III)NO].

{Ru–NO} 6 and {Ru–NO} 7 configurations in [Ru(trpy)(tmp)(NO)] n+ (trpy = 2,2 ′:6′,2′′-terpyridine, tmp = 3,4,7,8-tetramethyl-1,10-phenanthroline): An experimental and theoretical investigation

The ruthenium–nitrosyl complexes [Ru II(trpy)(tmp)(NO +)](ClO 4) 3 ([ 4](ClO 4) 3) and [Ru II(trpy)(tmp)(NO )](ClO 4) 2 ([ 5](ClO 4) 2) with {Ru–NO} 6 and {Ru–NO} 7 configurations, respectively (trpy = 2,2′:6′,2′′-terpyridine, tmp = 3,4,7,8-tetramethyl-1,10-phenanthroline) have been isotaled. The nitrosyl complexes [ 4] 3+ and [ 5] 2+ have been generated by following a stepwise synthetic procedure: [Ru II(trpy)(tmp)(X)] n , X/ n = Cl/+ ( 1 +) → CH 3CN/2+ ( 2 2+) → NO 2/+ ( 3 +) → NO +/3+ ( 4 3+) → NO /2+ ( 5 2+). The single-crystal X-ray structures of two precursor complexes [ 1]ClO 4 and [ 3]ClO 4 have been determined. The DFT optimized structures of 4 3+ and 5 2+ suggest that the Ru–N–O geometries in the complexes are linear (177.9°) and bent (141.4°), respectively. The nitrosyl complexes with linear ( 4 3+) and bent ( 5 2+) geometries exhibit ν(NO) frequencies at 1935 cm −1 (DFT: 1993 cm −1) and 1635 cm −1 (DFT: 1684 cm −1), respectively. Complex 4 3+ undergoes two successive reductions at 0.25 V (reversible) and −0.48 V (irreversible) versus SCE involving the redox active NO function, Ru II–NO + ⇄ Ru II–NO and Ru II–NO → Ru II–NO −, respectively, besides the reductions of trpy and tmp at more negative potentials. The DFT calculations on the optimized 4 3+ suggest that LUMO and LUMO+1 are dominated by NO + based orbitals of around 65% contribution along with partial metal contribution of ∼25% due to (dπ)Ru II → π∗(NO +) back-bonding. The lowest energy transitions in 4 3+ and 5 2+ at 360 nm and 467 nm in CH 3CN (TD-DFT: 364 and 459 nm) have been attributed to mixed MLLCT transitions of tmp(π) → NO +(π∗), Ru(dπ)/tmp(π) → NO +(π ∗) and Ru(dπ)/NO (π) → trpy(π ∗), respectively. The paramagnetic reduced species 5 2+ exhibits an anisotropic EPR spectrum with g 1 = 2.018, g 2 = 1.994, g 3 = 1.880 (〈 g〉 = 1.965 and Δ g = 0.138) in CH 3CN, along with 14N (I = 1) hyperfine coupling constant, A2 = 35 G at 110 K due to partial metal contribution in the singly occupied molecular orbital (DFT:SOMO:Ru (34%) and NO (53%)). Consequently, Mulliken spin distributions in 5 2+ are calculated as 0.115 for Ru and 0.855 for NO (N, 0.527; O, 0.328). The reaction of moderately electrophilic nitrosyl center in 4 3+ with the nucleophile, OH − yields the nitro precursor, 3 + with the second-order rate constant value of 1.7 × 10 −1 M −1 s −1 at 298 K in CH 3CN–H 2O (10:1). On exposure to light (Xenon 350 W lamp) both the nitrosyl species, 4 3+ ({Ru II–NO +}) and 5 2+ ({Ru II–NO }) undergo photolytic Ru–NO bond cleavage process but with a widely varying k NO, s −1 ( t 1/2, s) of 1.56 × 10 −1(4.4) and 0.011 × 10 −1(630), respectively.