Nitrosyl Ligand Research Articles

Over or under: hydride attack at the metal versus the coordinated nitrosyl ligand in ferric nitrosyl porphyrins.

Hydride attack at a ferric heme-NO to give an Fe-HNO intermediate is a key step in the global N-cycle. We demonstrate differential reactivity when six- and five-coordinate ferric heme-NO models react with hydride. Although Fe-HNO formation is thermodynamically favored from this reaction, Fe-H formation is kinetically favored for the 5C case.

Mild N-O Bond Cleavage Reactions of a Pyramidalized Nitrosyl Ligand Bridging a Dimolybdenum Center.

Complex [Mo2Cp2(μ-PCy2)(μ-NO)(NO)2] (1) was prepared by reacting [Mo2Cp2(μ-H)(μ-PCy2)(CO)4] with 2 equiv of [NO]BF4 and then treating the resulting product [Mo2Cp2(μ-PCy2)(CO)2(NO)2](BF4) with NaNO2 at 323 K, and it was shown to display a bridging nitrosyl ligand with significant pyramidalization at the N atom, a circumstance related to an unusual behavior concerning degradation of the bridging nitrosyl. Indeed, complex 1 reacts with HBF4·OEt2 to give the nitroxyl-bridged derivative [Mo2Cp2(μ-PCy2)(μ-κ(1):η(2)-HNO)(NO)2](BF4), is reduced by Zn(Hg) in the presence of trace H2O to give the amido complex [Mo2Cp2(μ-PCy2)(μ-NH2)(NO)2], and reacts with excess P(OPh)3 to give the phosphoraniminato-bridged derivative [Mo2Cp2(μ-PCy2){μ-NP(OPh)3}(NO)2].

ChemInform Abstract: A Theoretical Analysis of Ambivalent and Ambiphilic Lewis Acid/Bases with Symmetry Signatures

AbstractReview: 115 refs.

Synthesis, Characterization, and Some Properties of Cp*W(NO)(H)(η(3)-allyl) Complexes.

Sequential treatment at low temperatures of Cp*W(NO)Cl2 in THF with 1 equiv of a binary magnesium allyl reagent, followed by an excess of LiBH4, affords three new Cp*W(NO)(H)(η(3)-allyl) complexes, namely, Cp*W(NO)(H)(η(3)-CH2CHCMe2) (1), Cp*W(NO)(H)(η(3)-CH2CHCHPh) (2), and Cp*W(NO)(H)(η(3)-CH2CHCHMe) (3). Complexes 1-3 are isolable as air-stable, analytically pure yellow solids in good to moderate yields by chromatography or fractional crystallization. In solutions, complex 1 exists as two coordination isomers in an 83:17 ratio differing with respect to the endo/exo orientation of the allyl ligand. In contrast, complexes 2 and 3 each exist as four coordination isomers, all differing by the orientation of their allyl ligands which can have either an endo or an exo orientation with the phenyl or methyl groups being either proximal or distal to the nitrosyl ligand. A DFT computational analysis using the major isomer of Cp*W(NO)(H)(η(3)-CH2CHCHMe) (3a) as the model complex has revealed that its lowest-energy thermal-decomposition pathway involves the intramolecular isomerization of 3a to the 16e η(2)-alkene complex, Cp*W(NO)(η(2)-CH2═CHCH2Me). Such η(2)-alkene complexes are isolable as their 18e PMe3 adducts when compounds 1-3 are thermolyzed in neat PMe3, the other organometallic products formed during these thermolyses being Cp*W(NO)(PMe3)2 (5) and, occasionally, Cp*W(NO)(H)(η(1)-allyl)(PMe3). All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

The effect of nitrido, azide, and nitrosyl ligands on magnetization densities and magnetic properties of iridium PNP pincer-type complexes

The magnetisation density of different iridium PNP pincer-type complexes depends on the chosen quantum chemistry method.

Redox active iron nitrosyl units in proton reduction electrocatalysis.

Base metal, molecular catalysts for the fundamental process of conversion of protons and electrons to dihydrogen, remain a substantial synthetic goal related to a sustainable energy future. Here we report a diiron complex with bridging thiolates in the butterfly shape of the 2Fe2S core of the [FeFe]-hydrogenase active site but with nitrosyl rather than carbonyl or cyanide ligands. This binuclear [(NO)Fe(N2S2)Fe(NO)2](+) complex maintains structural integrity in two redox levels; it consists of a (N2S2)Fe(NO) complex (N2S2=N,N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane) that serves as redox active metallodithiolato bidentate ligand to a redox active dinitrosyl iron unit, Fe(NO)2. Experimental and theoretical methods demonstrate the accommodation of redox levels in both components of the complex, each involving electronically versatile nitrosyl ligands. An interplay of orbital mixing between the Fe(NO) and Fe(NO)2 sites and within the iron nitrosyl bonds in each moiety is revealed, accounting for the interactions that facilitate electron uptake, storage and proton reduction.

Asymmetry within the Fe(NO)2 moiety of dithiolate dinitrosyl iron complexes

Dinitrosyl iron complexes have earned attention due to their applications as nitric oxide donors as well as their interesting electronic structures. Typical Fe–NO electromerism issues are complicated in this case by the presence of a second nitrosyl ligand. Each of the formal oxidation states of iron in such complexes – from Fe(III) to Fe(0) – have been shown to be subject to electromerism. Here, broken-symmetry density functional theory data are shown, that reveal asymmetry within iron dithiolate dinitrosyl models in several formal oxidation states. CASSCF calculations on selected models confirm the multiconfigurational character of the Fe(NO)2 moiety, with the largest contributor at only 44% of the total weight of the wave function. This asymmetry is most noticeable in the excited states; it is further enhanced by solvation and reveals itself even in the ground state in dynamics calculations as well as in reaction pathways connecting the dinitrosyl state to a putative hyponitrite adduct.

From Plates to Spheres: Tuning the Vanadyl Phosphate Morphology by Intercalation of Nitrosyl Oxide

The lamellar matrix VOPO4.2H2O was used to the intercalation of nitrosyl (NO) ligand. The vanadyl phosphate matrix, as well as the prepared intercalated compound (VOPO4.NO.H2O), were characterized by infrared spectroscopy, thermogravimetry, X-ray difractometry, and SEM microscopy. The X-ray diffraction patterns shown that the VOPO4.NO.H2O synthesis occurs with a decrease in the lamellar distance in comparison with VOPO4.2H2O. The SEM micrographs obtained for VOPO4.2H2O and VOPO4.NO.H2O shown that microstructure of vanadyl phosphate is flat square whereas the compound VOPO4.NO.H2O exhibits spheroid grains.

A new member of tetranuclear dinitrosyl iron complexes (DNICs) with 2-mercaptothiazoline ligand: synthesis, structure and properties

A new tetranuclear dinitrosyliron complex [(μ-SC3H4SN)Fe(NO)2]4 (2), each of a Fe center coordinated with two S or two N, was prepared by CO replacement from the reduced precursor (CO)2Fe(NO)2 with 1 equiv of HSC3H4SN (2-mercaptothiazoline) in the presence of O2(g). The structure of 2 is similar to [(Imid-iPr)Fe(NO)2]4 (Imid-iPr = 2-isopropylimidazole) (Hess et al. J Am Chem Soc 133:20426–20434, 2011), and both complexes comprise a quadrilateral plane of irons with corresponding ligands, SC3H4SN− or Imid-iPr−, bridging the edges and two nitrosyl ligands capping the irons at the corners. An additional equiv of SC3H4SN− was added to 2, which results in the mononuclear {Fe(NO)2}9 (SC3H4SN)2Fe(NO)2− (3), in the manner of N bound-[SC3H4SN]. Reaction of (TMEDA)IFe(NO)2 (TMEDA = tetramethylethylenediamine) and complex 3 leads to the formation of complex 2. Dinuclear complex [(μ-C5H7N2)Fe(NO)2]2 (4) can be synthesized by the ligand displacement of SC3H4SN− to C5H7N2− (3,5-dimethylpyrazolate) of 2 (Chong et al. Can J Chem 57:3119–3125, 1979). Complexes 2–4 were characterized by IR and UV–Vis. The molecular structures of 2 and 3 were determined by X-ray single crystal diffraction.

Iron Nitrosyl “Natural” Porphyrinates: Does the Porphyrin Matter?

The synthesis and spectroscopic characterization of three five-coordinate nitrosyliron(II) complexes, [Fe(Porph)(NO)], are reported. These three nitrosyl derivatives, where Porph represents protoporphyrin IX dimethyl ester, mesoporphyrin IX dimethyl ester, or deuteroporphyrin IX dimethyl ester, display notable differences in their properties relative to the symmetrical synthetic porphyrins such as OEP and TPP. The N–O stretching frequencies are in the range of 1651–1660 cm–1, frequencies that are lower than those of synthetic porphyrin derivatives. Mössbauer spectra obtained in both zero and applied magnetic field show that the quadrupole splitting values are slightly larger than those of known synthetic porphyrins. The electronic structures of these naturally occurring porphyrin derivatives are thus seen to be consistently different from those of the synthetic derivatives, the presumed consequence of the asymmetric peripheral substituent pattern. The molecular structure of [Fe(PPIX-DME)(NO)] has been determined by X-ray crystallography. Although disorder of the axial nitrosyl ligand limits the structural quality, this derivative appears to show the same subtle structural features as previously characterized five-coordinate nitrosyls.

Mechanistic Insights into the Formation of N2O by a Nickel Nitrosyl Complex

Reaction of [Ni(NO)(bipy)(Me2phen)][PF6] with 1 equiv of nitric oxide (NO) in CH2Cl2 results in the formation of N2O and [{(Me2phen)Ni(NO)}2(μ-η(1)-N:η(1)-O)-NO2)][PF6] (3), along with the known complex, [Ni(bipy)3][PF6]2 (4). The isolation of complex 3, which contains a nitrite ligand, demonstrates that the reaction of [Ni(NO)(bipy)(Me2phen)][PF6] with exogenous NO results in NO disproportionation and not NO reduction. Complex 3 could also be accessed by reaction of [Ni(NO)(Me2phen)][PF6] (5) with (Me2phen)Ni(NO)(NO2) (7) in good yield. Complexes 3, 5, and 7 were fully characterized, including analysis by X-ray crystallography in the case of 3 and 7. Furthermore, addition of 0.5 equiv of bipy to [Ni(NO)(bipy)][PF6] results in formation of the hyponitrite complex, [{(bipy)Ni(κ(2)-O2N2)}η(1):η(1)-N,N-{Ni(NO)(bipy)}2][PF6]2 (9), in modest yield. Importantly, the hyponitrite ligand in 9 is thought to form via coupling of two NO(-) ligands and not by coupling of a nucleophilic nitrosyl ligand (NO(-)) with an electrophilic nitrosyl ligand (NO(+)). X-ray crystallography reveals that complex 9 features a new binding mode of the cis-hyponitrite ligand.

NO-binding in {Ru(NO)₂}⁸-type [Ru(NO)₂(PR₃)₂X]BF₄ compounds.

Two different structure types were found for a series of mononuclear dinitrosyl complexes of the general formula [RuL2(NO)2X]BF4 (L = monodentate phosphane, X = Cl, Br, I). The {Ru(NO)2}(8)-type target compounds were prepared by the reduction of the respective {RuNO}(6) precursors and subsequent oxidative addition of (NO)BF4. About one half of the new compounds share their molecular structure with the hitherto only representative of this class of dinitrosyls, Pierpont and Eisenberg's [RuCl(NO)2(PPh3)2]PF6·C6H6 (Inorg. Chem., 1972, 11, 1088-1094). The Cs-symmetric cations exhibit both a linear and a bent Ru-N-O fragment, in line with a formal 6 + 2 split of the {Ru(NO)2}(8) electron sum in the sense of a [Ru(II)(NO(+))((1)NO(-))](2+) bonding. The coordination entity's configuration in this subgroup is described by IUPAC's polyhedral symbol SPY-5. Continuous shape measures (CShM) as defined by Alvarez et al. (Coord. Chem. Rev., 2005, 249, 1693-1708) reveal a uniform deviation from the L-M-L angles expected for SPY-5, in a narrower sense, towards a vacant octahedron (vOC-5). DFT calculations confirmed that Enemark and Feltham's analysis (Coord. Chem. Rev., 1974, 13, 339-406) of the electronic situation of the {Ru(NO)2}(8) group remains adequate. The same holds for the second subclass of new compounds the existence of which had been predicted in the same paper by Enemark and Feltham, namely C(2v)-symmetric, TBPY-5-type cations with two almost equally bonded nitrosyl ligands. In agreement with an 8 + 0 distribution of the relevant electrons, the formal [Ru(0)(NO(+))2](2+) entities are found for L/X couples that donate more electron density on the central metal. Two solid compounds (8a/b, 12a/b) were found in both structures including the special case of the P(i)Pr3/Br couple 12a/b, which led to crystals that contained both structure types in the same solid. Conversely, four compounds showed a single form in the solid but both forms in dichloromethane solution in terms of the solutions' IR spectra. The irradiation of crystalline 12 with blue laser light resulted in the photoisomerisation of, mainly, the bent (1)NO(-) ligand in terms of low-temperature IR spectroscopy.

Enthalpies of ligand substitution for [Mo(η5C5H5)(CO)2(NO)] – The role of π-bonding effects in metal–ligand bond strengths

Enthalpies of ligand substitution for [Mo(η5-C5H5)(CO)2(NO)] producing [Mo(η5-C5H5)Mo(CO)(L)(NO)] have been measured by solution calorimetry at 30°C in THF for L=P(OMe)3<PMePh2<SIPr<PMe2Ph<IPr<PMe<PnBu3 (SIPr=1,3-bis(2,6-bis(diisopropylphenyl)imidazolinylidene; IPr=1,3-bis(2,6-bis(diisopropylphenyl)-imidazol-2-ylidene)). The accepting metal fragment [Mo(η5-C5H5)(CO)(NO)] has a vacant site containing strongly π-accepting carbonyl and nitrosyl ligands and this is shown to influence the stability of the product complex. Infrared studies of both νCO and νNO show that metal-to-ligand backbonding increases in the order P(OMe)3<PMe3<SIPr<IPr implying that both steric and electronic factors play a role in determining complex stability. The crystal structures of [Mo(η5-C5H5)(CO)(IPr)(NO)] and [Mo(η5-C5H5)(CO)(SIPr)(NO)] are reported.

Structural Evidence for Lewis Acid Triggered Nitrosyl Bending in Rhenium(–I) Chloro Catalysts for Alkene Hydrogenation Reactions

AbstractThe “catalytic nitrosyl effect” operating in reversible nitrosyl bending governs the performance of NO‐functionalized chlororhenium derivatives as catalysts in 1‐hexene hydrogenation reactions. Structural support for a Lewis acid (LA) induced bending at the NNO atom was provided by terminal attachment of a Lewis acid to one of the nitrosyl ligands in the [ReCl(PR3)2(NO)2] complexes (2: R = iPr a, R = Cy b) to give the products [ReCl(PR3)2(NO)(NO‐LA)][Z] {3: LA = B(C6F5)3; 4: LA = Et+; Z = [B(C6F5)4]–; 5a: LA = SiEt3+, Z = [HB(C6F5)3]–; R = iPr a, Cy b}. X‐ray diffraction studies revealed a moderately bent nitrosyl ligand in the nonderivatized complex 2a [159.3(5)°] and fully bent NO‐LA groups in 3a [131.4(3)°] and 4b [132.7(3)°]. The catalytic activities of the chloro dinitrosyl complexes in the hydrogenation of prototypical 1‐hexene were investigated. The NO‐LA bent complexes 3b and 4b furnished excellent TONs of up to 1.0 × 104 and high TOFs of up to 5.4 × 104 h–1 when the Lewis acids were present as co‐catalysts.

Solid-state photochemistry of molecular photo-switchable species: the role of photocrystallographic techniques

Over the last 30 years, the single-crystal photocrystallographic technique has been developed to determine the three-dimensional crystal and molecular structures of metastable species which have been generated in the crystal photochemically. Transition-metal complexes that have been investigated using this methodology include complexes that contain nitrosyl, dinitrogen, sulfur dioxide and nitrite ligands, all of which form new linkage isomers in the solid state when photoactivated by light of the appropriate wavelength. Both steric and electronic factors determine the level of the conversion from the ground-state structure to the metastable isomeric structure, and both the `reaction cavity' size and the nature of the intermolecular interactions are shown to be among the key factors that influence the percentage conversion.

A Ligand-Assisted Oxygen Reduction Reaction Catalyzed by (Nitro)Cobalt Porphyrins and Phthalocyanines

The redox chemistry of the nitro, nitrosyl, and peroxynitro ligands in cobalt porphyrins and phthalocyanines has been shown to be able to catalytically activate dioxygen in its electrochemical reduction. Two compounds have been focus of this study by cyclic voltammetry and density functional theory computational methods: the cationic porphyrin complex (nitro)(pyridyl)cobalt(III)TMpyP-2 immobilized in a Nafion® film with silver nanoparticles, and (nitro)(dimethylaminopyridyl)cobaltperfluoro-phthalocyanine, (NO2)(Me2Npy)CoF16Pc on a glassy carbon electrode. The porphyrin complex shows catalytic ORR activity that is short-lived, but the phthalocyanine complex appears to be a robust catalyst. DFT computational studies were used to evaluate the possible intermediates of the system and to corroborate the observed behavior.

P38 Mechanism of the reactions of the nitroprusside ion, Fe (CN)5NO2−, with sulfides

The title reaction has been of long–standing interest (”Gmelin” reaction), and is still of main concern, as an example of the chemistry comprising the nitrosyl ligand at the different three redox–states, formally considered as M (NO+), M (NO) and M (NO-) [1] , [2] , [3] . The nitroprusside ion, described as a MII(NO+) species, is widely used in medical experimentation as a “gold–standard” NO–donor drug [4] . On the other hand, the chemistry of H2S/ HS- in bio-relevant situations is receiving an increased attention [5] . Indeed, in both cases, we are dealing with currently accepted hot topics in basic mechanistic chemistry, most relevant to animal– and plant–physiology [6] , [7] . The work has been done at 25.0 °C, pH–range (6.7–12.5), at variable ratios of the reactant concentrations, I = 1 M, and in an anaerobic medium. We provide a deep analysis of the underlying mechanism, previously reported by us [1] , and subsequently questioned by others [7] . We used an experimental set–up comprising time-resolved UV–Vis, FTIR, EPR, stopped–flow and computational techniques, as well as a chemical control of products formation. In the scale of seconds to hours, different species have been characterized, depending on the pH. The first addition–intermediate, containing bound thionitrous acid (nitrososulfane, HSNO) is the precursor of [FeII(CN)5(NOS)]4-, containing the thionitrite ligand, which forms in a few seconds, and evolves rapidly up to the previously described bound nitroxyl (HNO) species, [FeII(CN)5(HNO)]3- [2] . N2O has been detected in the minute–hours time scale (depending on the pH), as well as NH3, with a final production of hexacyanoferrate (II). The production and decay of the different intermediates, and their proper characterization, has a strong mechanistic significance. We provide evidence for the three nitrosyl redox–states appearing as a result of successive reductions of MII(NO+), comprising 1–electron, 2–electron or 6–electron processes. In this context, the studied system shows up noticeable similarities with the behavior of NO 2 - – and NO–reductase enzymes. Key mechanistic features associated with the bioinorganic aspects of bound–nitrosyl chemistry [3] , namely thiolate– and other nucleophile–additions, transnitrosations, trans–labilizations, dinitrosyl–formation, donor abilities and differentiated physiological roles of NO/NO-,HNO, will be addressed. The influence of oxygen and/or light in the reaction course will also be briefly discussed.

OP25 Mechanism of the reactions of the nitroprusside ion [formula omitted] with sulfide

The title reaction has been of long–standing interest (”Gmelin” reaction), and is still of main concern, as an example of the chemistry comprising the nitrosyl ligand at the different three redox–states, formally considered as M (NO+), M (NO) and M (NO-) [1], [2], [3]. The nitroprusside ion, described as a MII(NO+) species, is widely used in medical experimentation as a “gold–standard” NO–donor drug [4]. On the other hand, the chemistry of H2S/ HS- in bio-relevant situations is receiving an increased attention [5]. Indeed, in both cases, we are dealing with currently accepted hot topics in basic mechanistic chemistry, most relevant to animal– and plant–physiology [6], [7]. The work has been done at 25.0 °C, pH–range (6.7–12.5), at variable ratios of the reactant concentrations, I = 1 M, and in an anaerobic medium. We provide a deep analysis of the underlying mechanism, previously reported by us [1], and subsequently questioned by others [7]. We used an experimental set–up comprising time-resolved UV–Vis, FTIR, EPR, stopped–flow and computational techniques, as well as a chemical control of products formation. In the scale of seconds to hours, different species have been characterized, depending on the pH. The first addition–intermediate, containing bound thionitrous acid (nitrososulfane, HSNO) is the precursor of [FeII(CN)5(NOS)]4-, containing the thionitrite ligand, which forms in a few seconds, and evolves rapidly up to the previously described bound nitroxyl (HNO) species, [FeII(CN)5(HNO)]3- [2]. N2O has been detected in the minute–hours time scale (depending on the pH), as well as NH3, with a final production of hexacyanoferrate (II). The production and decay of the different intermediates, and their proper characterization, has a strong mechanistic significance. We provide evidence for the three nitrosyl redox–states appearing as a result of successive reductions of MII(NO+), comprising 1–electron, 2–electron or 6–electron processes. In this context, the studied system shows up noticeable similarities with the behavior of NO2-– and NO–reductase enzymes. Key mechanistic features associated with the bioinorganic aspects of bound–nitrosyl chemistry[3], namely thiolate– and other nucleophile–additions, transnitrosations, trans–labilizations, dinitrosyl–formation, donor abilities and differentiated physiological roles of NO/NO-,HNO, will be addressed. The influence of oxygen and/or light in the reaction course will also be briefly discussed.

Diazald: An entry to diruthenium complexes containing bridging nitrosyl ligands

Reactions of coordinatively unsaturated complexes [Ru2(μ-PtBu2)(μ-H)(μ-PˆP)(CO)4] (PˆP = bisphosphanes, N,N-substituted bis(diphenylphosphanyl)amines) with the nitrosylating reagent diazald (N-methyl-N-nitroso-p-toluenesulfonamide) were investigated. Thus a convenient synthesis affording a series of new diruthenium complexes containing bridging nitrosyl ligands of the general type [Ru2(μ-PtBu2)(μ-NO)(μ-PˆP)(CO)4] was developed and the crystal and molecular structures of five new compounds were determined. Contrary, this reaction principle does not work for the closely related complex [Fe2(μ-PtBu2)(μ-H)(μ-dppm)(CO)4] (dppm = Ph2PCH2PPh2). In this case carbon monoxide was produced in side reactions and the known compound [Fe2(μ-PtBu2)(μ-H)(μ-dppm)(μ-CO)(CO)4] was obtained as the main product. The molecular structure of the latter complex was also determined by X-ray diffraction.

PH-Dependent Picosecond Structural Dynamics in the Distal Pocket of Nitrophorin 4 Investigated by 2D IR Spectroscopy

Nitrophorin 4 (NP4) belongs to a family of pH-sensitive, nitric oxide (NO) transporter proteins that undergo a large structural change from a closed to an open conformation at high pH to allow for NO delivery. Measuring the pH-dependent structural dynamics in NP4-NO around the ligand binding site is crucial for developing a mechanistic understanding of NO binding and release. In this study, we use coherent two-dimensional infrared (2D IR) spectroscopy to measure picosecond structural dynamics sampled by the nitrosyl stretch in NP4-NO as a function of pH at room temperature. Our results show that both the closed and open conformers of the protein are present at low (pD 5.1) and high (pD 7.9) pH conditions. The closed and open conformers are characterized by two frequencies of the nitrosyl stretching vibration labeled A0 and A1, respectively. Analysis of the 2D IR line shapes reveals that at pD 5.1, the closed conformer experiences structural fluctuations arising from solvation dynamics on a ∼3 ps time scale. At pD 7.9, both the open and closed conformers exhibit fluctuations on a ∼1 ps time scale. At both pD conditions, the closed conformers maintain a static distribution of structures within the experimental time window of 100 ps. This is in contrast to the open conformer, which is able to interconvert among its substates on a ∼100 ps time scale. Our results directly measure the time scales of solvation dynamics in the distal pocket, the flexibility of the open conformation at high pH, and the rigidity of the closed conformers at both pH conditions. We discuss how the pH-dependent equilibrium structural fluctuations of the nitrosyl ligand measured in this study are related to the uptake and delivery of nitric oxide in NP4.