Nitrosyl Ligand Research Articles

A New Contribution Confirming the Analogy Between NO+ and Related Aryldiazo Ligands

AbstractThe reaction of the cyclometalated five‐coordinate 16 electron iridium(III) complex [IrCl(H){P(tBu)2C6H4–κ2P,C}{P(tBu)2Ph}] (1) with the diazonium salt [p‐BrC6H4N2]BF4 in dichloromethane at room temperature was investigated. By spontaneous reductive elimination the iridium(I) complex salt trans‐[IrCl(N2C6H4Br‐p){(P(tBu)2)Ph}2]BF4 (2) was obtained in good yield. Therefore, this observed reaction behavior exhibits the diazonium unit as a strong π‐acceptor group and confirms furthermore the analogy between nitrosyl and aryldiazonium ligands. The new compound 2 has been characterized by standard spectroscopic methods and the molecular structure of 2 was determined by a single‐crystal X‐ray diffraction analysis (monoclinic, space group Cc, Z=4).

Trinuclear and Tetranuclear Ruthenium Carbonyl Nitrosyls: Oxidation of a Carbonyl Ligand by an Adjacent Nitrosyl Ligand.

Trinuclear and tetranuclear ruthenium carbonyls of the types Ru3(CO)n(NO)2, Ru3(N)(CO)n(NO), Ru3(N)2(CO)n, Ru3(N)(CO)n(NCO), Ru3(CO)n(NCO)(NO), Ru4(N)(CO)n(NO), Ru4(N)(CO)n(NCO), and Ru4(N)2(CO)n related to species observed experimentally in the chemistry of Ru3(CO)10(µ-NO)2 have been investigated using density functional theory. In all cases, the experimentally observed structures have been found to be low-energy structures. The low-energy trinuclear structures typically have a central strongly bent Ru-Ru-Ru chain with terminal CO groups and bridging nitrosyl, isocyanate, and/or nitride ligands across the end of the chain. The low-energy tetranuclear structures typically have a central Ru4N unit with terminal CO groups and a non-bonded pair of ruthenium atoms bridged by a nitrosyl or isocyanate group.

Electronic structure of light-induced nitrosyl linkage isomers revealed by X-ray absorption spectroscopy at Ru L3,2-edges

X-ray absorption spectroscopy (XAS) is a powerful tool for examining changes of the electronic and molecular structure following light-induced excitation of a molecule. Specifically, this method can be applied to investigate the ground (GS, RuNO) and metastable states (MS1, RuON and MS2, Ruη2(NO)) of the nitrosyl ligand (NO), which differ in their coordination mode to the metal. In this work, we report for the first time experimental and theoretical (DFT) Ru L3,2-edge XA spectra for the octahedral complex trans-[RuNOPy4F](ClO4)2 (1, Py = pyridine) in both ground and metastable states. The transition from GS to MS1 using 420 nm light excitation leads to a significant downshift of the 2p → LUMO(+1) peaks by about 0.5–0.8 eV, attributed to the destabilisation of 2p orbitals and stabilization of LUMO(+1). Subsequent irradiation of MS1 at 920 nm produces isomer MS2, for which even greater stabilization of LUMO occurs, though without a significant change in 2p energy. The change in 2p energy is attributed to a variation in the charge on the Ru atom after NO isomerization, while LUMO(+1) stabilization is related to changes in the Ru(NO) bond length and the composition of this orbital.

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.

C≡N and N≡O Bond Cleavages of Acetonitrile and Nitrosyl Ligands at a Dimolybdenum Center to Render Ethylidyne and Acetamidinate Ligands.

Extended reduction of [Mo2Cp2(μ-Cl)(μ-PtBu2)(NO)2] (1) with Na(Hg) in acetonitrile (MeCN) at room temperature resulted in an unprecedented full cleavage of the C≡N bond of a coordinated MeCN molecule to yield the vinylidene derivative Na[Mo2Cp2(μ-PtBu2)(μ-CCH2)(NO)2], which upon protonation with (NH4)PF6 gave the ethylidyne complex [Mo2Cp2(μ-PtBu2)(μ-CMe)(NO)2] [Mo1-Mo2 = 2.9218(2) Å] in a selective and reversible way. Controlled reduction of 1 at 273 K yielded instead, after protonation, the 30-electron acetamidinate complex [Mo2Cp2(μ-PtBu2)(μ-κN:κN'-HNCMeNH)(μ-NO)]PF6 [Mo1-Mo2 = 2.603(2) Å], in a process thought to stem from the paramagnetic MeCN-bridged intermediate [Mo2Cp2(μ-PtBu2)(μ-NCMe)(NO)2], followed by a complex sequence of elementary steps including cleavage of the N≡O bond of a nitrosyl ligand.

CW or Pulsed laser – That is the Question: Comparative Steady‐state Photocrystallographic Analysis of Metal Nitrosyl Linkage Isomers

AbstractThe investigation of photo‐induced effects is profoundly influenced by the characteristics of photo‐excitation sources. In this study, we present a comprehensive analysis of the structures of two photoinduced linkage isomers (PLI) in a ruthenium nitrosyl complex, trans‐[Ru(py)4F(NO)](ClO4)2, following irradiation with both pulsed and continuous wave (CW) light sources under low temperature conditions. The X‐ray (photo)diffraction analysis shows that the resulting PLI generated from the two types of irradiation sources, an isonitrosyl configuration of the nitrosyl ligand in the so‐called metastable state MS1, and a side‐on configuration of the nitrosyl ligand in the metastable state MS2, are identical. In‐situ optical absorption spectroscopy was employed during CW and pulsed irradiation, enabling the monitoring of the population process of these PLI. The results obtained from the infrared spectroscopic analysis after pulsed irradiation give insight into the population mechanism illustrating that the generation of the isonitrosyl MS1 occurs through a two‐step process, via the second PLI, the side‐on configuration MS2.

Multi-stimuli responsive behaviors in a new chiral hybrid nitroprusside salt (R-3-hydroxypyrrolidinium)2[Fe(CN)5(NO)]

Multi-stimuli responsive materials controlled and coupled by two or more channels have a broad range of applications in the field of switches, memories, and molecular machines. The exploration of the material is currently focused on the pure organic system, which limits the development of such materials greatly. In this work, we present a new chiral organic-inorganic hybrid salt, (R-3-hydroxypyrrolidinium)2[Fe(CN)5(NO)] (1), which exhibits rare multi-stimuli responsive behaviors in thermal, mechanical and optical channels. In detail, 1 undergoes a C2-P21221 phase transition deriving from the thermal motion of organic cations with the increase of temperature, but the reverse transition can only be induced by mechanical pressure. Moreover, polycrystalline hybrid salt showed photo-responsive performance, i.e., the ground-state N-bound nitrosyl ligand adopts two configurations in excited states caused by light in 532 nm irradiation, accompanying with a photo-induced structural transformation of the anionic framework. Namely, the thermal motion characteristics of organic cations, the photoresponse characteristics of anionic inorganic skeleton and the pressure characteristics from hydrogen bonds are simultaneously integrated in 1. This unprecedented coupling mechanism of multi-stimuli responses makes 1 a potential candidate for future multichannel data storage applications.

Local force constants and charges of the nitrosyl ligand in photoinduced NO linkage isomers in a prototypical ruthenium nitrosyl complex.

Photoinduced linkage isomers (PLI) of the NO ligand in transition-metal nitrosyl compounds can be identified by vibrational spectroscopy due to the large shifts of the (NO) stretching vibration. We present a detailed experimental and theoretical study of the prototypical compound K2[RuCl5NO], where (NO) shifts by ≈150 cm-1 when going from the N-bound (κN) ground state (GS) to the oxygen-bound (κO) metastable linkage isomer MS1, and by ≈360 cm-1 when going to the side-on (κ2N,O) metastable linkage isomer MS2. We show that the experimentally observed N-O stretching modes of the GS, MS1, and MS2 exhibit strong coupling with the Ru-N and Ru-O stretching modes, which can be decoupled using the local mode vibrational theory formalism. From the resulting decoupled pure two-atomic harmonic oscillators the local force constants are determined, which all follow the same quadratic behavior on the wavenumber. A Bader charge analysis shows that the total charge on the NO ligand is not correlated to the observed frequency shift of (NO).

Anti-bacterial, anti-biofilm and synergistic effects of phenazine-based ruthenium(II) complexes.

Antimicrobial resistance has become a global threat to human health, which is coupled with the lack of novel drugs. Metallocompounds have emerged as promising diverse scaffolds for the development of new antibiotics. Herein, we prepared some metal compounds mainly focusing on cis-[Ru(bpy)(dppz)(SO3)(NO)](PF6) (PR02, bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine), in which phenazinic and nitric oxide ligands along with sulfite conferred some key properties. This compound exhibited a redox potential for bound NO+/0 of -0.252 V (vs. Ag|AgCl) and a high pH for nitrosyl-nitro conversion of 9.16, making the nitrosyl ligand the major species. These compounds were still able to bind to DNA structures. Interestingly, reduced glutathione (GSH) was unable to promote significant NO/HNO release, an uncommon feature of many similar systems. However, this reducing agent was essential to generate superoxide radicals. Antimicrobial studies were carried out using six bacterial strains, where none or very low activity was observed for Gram-negative bacteria. However, PR02 and PR (cis-[Ru(bpy)(dppz)Cl2]) showed high antibacterial activity in some Gram-positive strains (MBC for S. aureus up to 4.9 μmol L-1), where the activity of PR02 was similar to or at least 4-fold better than that of PR. Besides, PR02 showed capacity to inhibit bacterial biofilm formation, a major health issue leading to bacterial tolerance to antibiotics. Interestingly, we also showed that PR02 can function in synergism with the known antibiotic ampicillin, improving their action up to 4-fold even against resistant strains. Altogether, these results showed that PR02 is a promising antimicrobial nitrosyl ruthenium compound combining features beyond its killing action, which deserves further biological studies.

Performance Tests of the Second-Order Approximate Internally Contracted Multireference Coupled-Cluster Singles and Doubles Method icMRCC2.

Benchmark results are presented for the second-order approximation of the internally contracted multireference coupled-cluster method with single and double excitations, icMRCC2 [Köhn, Bargholz, J. Chem. Phys. 2019, 151, 041106], which was designed as a multireference analogue of the single-reference second-order approximate coupled-cluster method CC2 [Christiansen, Koch, Jørgensen, Chem. Phys. Lett. 1995, 243, 409-418]. Vertical excitation energies of various small to medium-sized organic molecules are investigated based on established test sets from the literature. Additionally, the spectroscopic constants of ground and excited states of diatomics and the geometric parameters of excited triatomic molecules were determined and compared to the experimental data. The results show that the method clearly extends the applicability of single-reference CC2, including doubly excited states, and also artifacts of CC2 like too low Rydberg excitations and too weak multiple bonds are eliminated. The method is computationally more demanding than standard multireference second-order perturbation theories but improves significantly in accuracy, as shown by the benchmark results. In addition, it is demonstrated that small active spaces are often sufficient to obtain accurate energies with icMRCC2. Example applications like the automerization of cyclobutadiene, the deactivation pathway of ethylene, and the excited states of an iron complex with a noninnocent nitrosyl ligand demonstrate the potential of icMRCC2 in cases with strong multireference character.

Gelatin Carbon Dots Interaction with Nitrosyl Ruthenium Complex: Fluorescence Quenching and Chemiluminescence Mechanisms.

Carbon dots (CDs) exhibit luminescence, biocompatibility, and higher water solubility. This material has been developed for biological applications, specifically in bioimaging. In this work, the gelatin carbon dots (CDg) was obtained from commercial gelatin using a hydrothermal method in domestic microwave, and the suppression fluorescent mechanism were enhanced by the addition of the [RuII(bdq)(NO)(tpy)]3+ (Rubdq-NO+) complex ion. After purification through a dialysis bag, the resulting CDs (CDg) exhibit fluorescent emission at 400nm and maintained fluorescence stability in an aqueous solution (pH = 7) for 30days under 5 ◦C. Fluorescence quenching studies revealed an electrostatic interaction between the negative charge from CDg (δ = - 20mV) and the positively charged nitrosyl (NO+) ligand of the ruthenium complex (Rubdq-NO+), resulting in quenching of the CDg fluorescence due to the inner filter effects (IFE). The chemiluminescence reaction of CDg and Rubdq-NO-CDg in presence of norepinephrine (NOR) were evaluated. NOR in PBS are liable to undergo spontaneous oxidation to quinone form (NOR-quinone). CDg are believed interact with NOR-quinone and an electron transfer occur obtained CDg+ accompanied to green emission fluorescence (520nm). While for Rubdq-NO-CDg in presence of NOR, the green emission occurs accompanied by NO0 release using DAF-2 probe.

Molecular Design of a Metal-Nitrosyl Ferroelectric with Reversible Photoisomerization.

The development of photo-responsive ferroelectrics whose polarization may be remotely controlled by optical means is of fundamental importance for basic research and technological applications. Herein, we report the design and synthesis of a new metal-nitrosyl ferroelectric crystal (DMA)(PIP)[Fe(CN)5(NO)] (1) (DMA = dimethylammonium, PIP = piperidinium) with potential phototunable polarization via a dual-organic-cation molecular design strategy. Compared to the parent non-ferroelectric (MA)2[Fe(CN)5(NO)] (MA = methylammonium) material with a phase transition at 207 K, the introduction of larger dual organic cations both lowers the crystal symmetry affording robust ferroelectricity and increases the energy barrier of molecular motions, endowing 1 with a large polarization of up to 7.6 μC cm-2 and a high Curie temperature (Tc) of 316 K. Infrared spectroscopy shows that the reversible photoisomerization of the nitrosyl ligand is accomplished by light irradiation. Specifically, the ground state with the N-bound nitrosyl ligand conformation can be reversibly switched to both the metastable state I (MSI) with isonitrosyl conformation and the metastable state II (MSII) with side-on nitrosyl conformation. Quantum chemistry calculations suggest that the photoisomerization significantly changes the dipole moment of the [Fe(CN)5(NO)]2- anion, thus leading to three ferroelectric states with different values of macroscopic polarization. Such optical accessibility and controllability of different ferroelectric states via photoinduced nitrosyl linkage isomerization open up a new and attractive route to optically controllable macroscopic polarization.

Dinitrosyl iron complexes with thiol-containing ligands as sources of universal cytotoxins - nitrosonium cations

It was shown that the release of a half of nitrosyl ligands from dinitrosyl iron units in the form of nitrosonium cation (NO+) from binuclear dinitrosyl iron complexes with thiol-containing ligands (B-DNIC) during DNIC decomposition in acid solutions is increased with the decrease of the stability of these complexes and completely blocked with the increase of the concentrations of free thiols (non-included into B-DNIC) up to the level that was two times and more than that of dinitrosyl iron units. It was demonstrated that the less stable B-DNIC with mercaptosuccinate degrade in an acidic environment at ambient temperature, while the decomposition of more stable B-DNIC with glutathione was only marked when the solution was heated at 80°C. The inhibition of NO+ release from B-DNIC in the presence of elevated free thiol level in the solution was due to the ability of free thiols to induce the reduction of NO+ to NO.

Bonding in PdCl(NO) and Related Nitrosylmetal Species of the Enemark–Feltham {MNO}10Type

AbstractThe crystal‐structure analysis of the coordination polymer PdCl(NO) (1) by Evers et al. shows a two‐dimensional PdCl network of four‐coordinate Pd atoms and triply bridging chlorido ligands. The fourth ligand at Pd is a terminally bonded nitrosyl ligand in a bent PdNO function (Pd−N−O=129°). In terms of the Enemark–Feltham notation,1is an {MNO}10compound which leaves a choice regarding the assignment of the ten electrons as d10‐Pd0Cl−(NO+), d9‐PdICl−(NO) or d8‐PdIICl−(NO−) with 0, 1 and 2 electrons allocated at the nitrosyl ligand, respectively. The quantum‐chemical treatment by DFT methods and subsequent determination of the oxidation states (OSs) by Salvador's Effective‐OS (EOS) method shows highly covalent Pd−NO bonds which narrowly end up on the Pd0Cl−(NO+) side in the course of the OS assignment's heterolytic bond‐cleavage procedure. Related halonitrosyls are included to examine the obvious suitability of both linear and bent MNO units as ‘ground‐state canditates’ of {MNO}10systems. Local‐mode analysis confirmed the recently formulated weakening of both the metal−nitrosyl and the N−O bond on bending a linear MNO function.

Front Cover: Bond Strength of a Diatomic Acceptor Ligand: A Reliable Measure of Its Antibond Occupation and Its Charge? (Eur. J. Inorg. Chem. 32/2022)

The Front Cover shows, pictorially as the collapse of a house of cards, the repeal of the customary correlation of a nitrosyl ligand's charge and the strength of its N−O bond. Isomeric couples comprising both a linear and a bent MNO moiety show, in fact, a weaker bond of the bent form in terms of a markedly red-shifted NO stretch. However, the charge of the NO group is not a function of the bond strength. Hence, the assignment of an 1NO− to a bent-bonded nitrosyl and NO+ to a linear-bonded one is unjustified as unraveled by local-mode and QTAIM analysis. More information can be found in the Research Article by J. Popp and P. Klüfers.

X‐ray Absorption Spectroscopy for the Characterization of Photoinduced Linkage Isomers in Single Crystals: Experimental Setup and XANES Results**

AbstractPhotoinduced linkage isomers (PLI) of the nitrosyl ligand in single crystals of Na2[Fe(CN)5NO] ⋅ 2H2O (SNP, sodium nitroprusside) have been studied by X‐ray absorption spectroscopy (XAS) at low temperatures, using a cryostat allowing in‐situ polarized LASER irradiation and XAS measurements. In the XANES (X‐ray absorption near edge structure) region a significant modification of the pre‐edge peak intensities is observed after irradiation of the ground state (GS) nitrosyl NO‐κN isomer with light in the blue and infrared spectral range, used for the generation of the isonitrosyl NO‐κO isomer (MS1) and the side‐on bound κ2N,O isomer (MS2), respectively. These observations are interpreted with respect to the changes in the nature of the frontier (unoccupied) molecular orbitals of the different linkage isomer configurations, expected from earlier calculations.

Pas de Deux of an NO Couple: Synchronous Photoswitching from a Double-Linear to a Double-Bent Ru(NO)2 Core under Nitrosyl Charge Conservation.

The {Ru(NO)2 }10 dinitrosylruthenium complex [Ru(NO)2 (PPh3 )2 ] (1) shows photo-induced linkage isomerism (PLI) of a special kind: the two NO ligands switch, on photo-excitation, synchronously from the ground state (GS) with two almost linear RuNO functions to a metastable state (MS) which persists up to 230 K and can be populated to ≈50 %. The MS was experimentally characterised by photo-crystallography, IR spectroscopy and DS-calorimetry as a double-bent variant of the double-linear GS. The experimental results are confirmed by computation which unravels the GS/MS transition as a disrotatory synchronous 50° turn of the two nitrosyl ligands. Although 1 shows the usual redshift of the N-O stretch on bending the MNO unit, there is no increased charge transfer from Ru to NO along the GS-to-MS path. In terms of the effective-oxidation-state (EOS) method, both isomers of 1 and the transition state are Ru-II (NO+ )2 species.

Bond Strength of a Diatomic Acceptor Ligand: A Reliable Measure of Its Antibond Occupation and Its Charge?

AbstractA nitrosyl ligand is bonded to a central metal mainly via a mostly covalent normal bond and a coordinative metal‐to‐NO π‐backbond. A recent analysis had unravelled similar bonding characteristics of both linear and bent CoNO moieties in terms of ligand charge and antibond occupation. Thus, there should be no justification for the usual assignment of an NO+ ligand to a linear MNO unit and a singlet‐NO− ligand to a bent one. This claim seems to contradict that bending an MNO unit weakens the N−O bond with a marked red‐shift of the N−O stretch as one indicator. In this work, the failure of Dewar‐Chatt‐Duncanson‐derived conclusions is demonstrated for linear/bent isomer couples by the analysis of M−N and N−O bond strengths. Instead of DCD behavior, lateral electrostatic influence on NO and other diatomic ligands modulates the intraligand bond strength in a similar way as has been shown in former work for polar interaction of a charge with CO in the ‘non‐classical’ carbonyls. Methodologically, local‐mode analysis is used to determine bond strengths. Oxidation states are determined by the effective‐oxidation‐state (EOS) method.

An Integrated View of Nitrogen Oxyanion Deoxygenation in Solution Chemistry and Electrospray Ion Production.

There has been an increasing interest in chemistry involving nitrogen oxyanions, largely due to the environmental hazards associated with increased concentrations of these anions leading to eutrophication and aquatic "dead zones". Herein, we report the synthesis and characterization of a suite of MNOx complexes (M = Co, Zn: x = 2, 3). Reductive deoxygenation of cobalt bis(nitrite) complexes with bis(boryl)pyrazine is faster for cobalt than previously reported nickel, and pendant O-bound nitrito ligand is still readily deoxygenated, despite potential implication of an isonitrosyl primary product. Deoxygenation of zinc oxyanion complexes is also facile, despite zinc being unable to stabilize a nitrosyl ligand, with liberation of nitric oxide and nitrous oxide, indicating N-N bond formation. X-ray photoelectron spectroscopy is effective for discriminating the types of nitrogen in these molecules. ESI mass spectrometry of a suite of M(NOx)y (x = 2, 3 and y = 1, 2) shows that the primary form of ionization is loss of an oxyanion ligand, which can be alleviated via the addition of tetrabutylammonium (TBA) as a nonintuitive cation pair for the neutral oxyanion complexes. We have shown these complexes to be subject to deoxygenation, and there is evidence for nitrogen oxyanion reduction in several cases in the ESI plume. The attractive force between cation and neutral is explored experimentally and computationally and attributed to hydrogen bonding of the nitrogen oxyanion ligands with ammonium α-CH2 protons. One example of ESI-induced reductive dimerization is mimicked by bulk solution synthesis, and that product is characterized by X-ray diffraction to contain two Co(NO)2+ groups linked by a highly conjugated diazapolyene.

Bent and Linear {CoNO}8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls

Among the isoelectronic ligands CN-, CO, and NO+, an oblique bonding to the metal is well-established for the nitrosyl ligand, with M-N-O angles down to ≈120°. In the last decades, the nitrosyl community got into the habit of addressing a bent-bonded nitrosyl ligand as 1NO-. Thus, because various redox forms of a nitrosyl ligand seem to exist, the ligand is considered to be "noninnocent" because of the obvious ambiguity of an oxidation state (OS) assignment of the ligand and metal. Among the bent-bonded species, the low-spin {CoNO}8 class is prototypic. From this class, some 20 new nitrosyl compounds, the X-ray structure determinations of which comply with strict quality criteria, were analyzed with respect to the OS issue. As a result, the effective OS method shows a low-spin d8 CoI-NO+ couple instead of a negative OS of the ligand at the BP86/def2-TZVP (+D3, +CPCM with infinite permittivity) level of theory. The same holds for some new members of the linear subclass of {CoNO}8 compounds. For all compounds, a largely invariable "real" charge of ≈ -0.3 e was obtained from population analyses. All of these electron-rich d8 species strive to manage Pauli repulsion between the metal electrons and the lone pair at the nitrosyl's nitrogen atom, with the bending of the CoNO unit as the most frequent escape.