Catalytic Dehydrogenation of Formic Acid Promoted by Triphos-Co Complexes: Two Competing Pathways for H2 Production.

In this study, we reported the synthesis and structural characterization of a triphos-CoII complex [(κ3-triphos)CoII(CH3CN)2]2+ (1) and a triphos-CoI-H complex [(κ2-triphos)HCoI(CO)2] (4). The facile synthetic pathways from 1 to [(κ3-triphos)CoII(κ2-O2CH)]+ (1') and [(κ3-triphos)CoI(CH3CN)]+ (2), respectively, as well as the interconversion between [(κ3-triphos)CoI(CO)2]+ (3) and 4 have been established. The activation energy barrier, associated with the dehydrogenation of a coordinated formate fragment in 1' yielding the corresponding 2 accompanied by the formation of H2 and CO2, was experimentally determined as 23.9 kcal/mol. With 0.01 mol % loading of 1, a maximum TON ∼ 1735 within 18 h and TOF ∼ 483 h-1 for the first 3 h could be achieved. Kinetic isotope effect (KIE) values of 2.25 (kHCOOH/kDCOOH) and 1.36 (kHCOOH/kHCOOD) for the dehydrogenation of formic acid and its deuterated derivatives, respectively, implicate that the H-COOH bond cleavage is likely the rate-determining step. The catalytic mechanism proposed by density functional theory (DFT) calculations coupled with experimental 1H NMR and gas chromatography-mass spectrometry (GC-MS) analysis unveils two competing pathways for H2 production; specifically, deprotonating a HCOO-H bond by a proposed Co-H intermediate C and homolytic cleavage of the CoII-H moiety of C, presumably via a dimeric Co intermediate D containing a [Co2(µ-H)2]2+ core, to yield the corresponding 2 and H2.

Electronic Structure and Transformation of Dinitrosyl Iron Complexes (DNICs) Regulated by Redox Non-Innocent Imino-Substituted Phenoxide Ligand.

The coupled NO-vibrational peaks [IR νNO 1775 s, 1716 vs, 1668 vs cm-1 (THF)] between two adjacent [Fe(NO)2] groups implicate the electron delocalization nature of the singly O-phenoxide-bridged dinuclear dinitrosyliron complex (DNIC) [Fe(NO)2(µ-ON2Me)Fe(NO)2] (1). Electronic interplay between [Fe(NO)2] units and [ON2Me]- ligand in DNIC 1 rationalizes that "hard" O-phenoxide moiety polarizes iron center(s) of [Fe(NO)2] unit(s) to enforce a "constrained" π-conjugation system acting as an electron reservoir to bestow the spin-frustrated {Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2- electron configuration (Stotal = 1/2). This system plays a crucial role in facilitating the ligand-based redox interconversion, working in harmony to control the storage and redox-triggered transport of the [Fe(NO)2]10 unit, while preserving the {Fe(NO)2}9 core in DNICs {Fe(NO)2}9-[·ON2Me]2- [K-18-crown-6-ether)][(ON2Me)Fe(NO)2] (2) and {Fe(NO)2}9-[·ON2Me] [(ON2Me)Fe(NO)2][PF6] (3). Electrochemical studies suggest that the redox interconversion among [{Fe(NO)2}9-[·ON2Me]2-] DNIC 3 ↔ [{Fe(NO)2}9-[ON2Me]-] ↔ [{Fe(NO)2}9-[·ON2Me]] DNIC 2 are kinetically feasible, corroborated by the redox shuttle between O-bridged dimerized [(µ-ONMe)2Fe2(NO)4] (4) and [K-18-crown-6-ether)][(ONMe)Fe(NO)2] (5). In parallel with this finding, the electronic structures of [{Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2-] DNIC 1, [{Fe(NO)2}9-[·ON2Me]2-] DNIC 2, [{Fe(NO)2}9-[·ON2Me]] DNIC 3, [{Fe(NO)2}9-[ONMe]-]2 DNIC 4, and [{Fe(NO)2}9-[·ONMe]2-] DNIC 5 are evidenced by EPR, SQUID, and Fe K-edge pre-edge analyses, respectively.

NO Reduction to N2O Triggered by a Dinuclear Dinitrosyl Iron Complex via the Associated Pathways of Hyponitrite Formation and NO Disproportionation.

In spite of the comprehensive study of the metal-mediated conversion of NO to N2O disclosing the conceivable processes/mechanism in biological and biomimetic studies, in this study, the synthesis cycles and mechanism of NO reduction to N2O triggered by the electronically localized dinuclear {Fe(NO)2}10-{Fe(NO)2}9 dinitrosyl iron complex (DNIC) [Fe(NO)2(µ-bdmap)Fe(NO)2(THF)] (1) (bdmap = 1,3- bis(dimethylamino)-2-propanolate) were investigated in detail. Reductive conversion of NO to N2O triggered by complex 1 in the presence of exogenous ·NO occurs via the simultaneous formation of hyponitrite-bound {[Fe2(NO)4(µ-bdmap)]2(κ4-N2O2)} (2) and [NO2]--bridged [Fe2(NO)4(µ-bdmap)(µ-NO2)] (3) (NO disproportionation yielding N2O and complex 3). EPR/IR spectra, single-crystal X-ray diffraction, and the electrochemical study uncover the reversible redox transformation of {Fe(NO)2}9-{Fe(NO)2}9 [Fe2(NO)4(µ-bdmap)(µ-OC4H8)]+ (7) ↔ {Fe(NO)2}10-{Fe(NO)2}9 1 ↔ {Fe(NO)2}10-{Fe(NO)2}10 [Fe(NO)2(µ-bdmap)Fe(NO)2]- (6) and characterize the formation of complex 1. Also, the synthesis study and DFT computation feature the detailed mechanism of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 DNIC 1 reducing NO to N2O via the associated hyponitrite-formation and NO-disproportionation pathways. Presumably, the THF-bound {Fe(NO)2}9 unit of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 complex 1 served as an electron buffering reservoir for accommodating electron redistribution, and the {Fe(NO)2}10 unit of complex 1 acted as an electron-transfer channel to drive exogeneous ·NO coordination to yield proposed relay intermediate κ2-N,O-[NO]--bridged [Fe2(NO)4(µ-bdmap)(µ-NO)] (A) for NO reduction to N2O.

Photoinduced NO and HNO Production from Mononuclear {FeNO}6 Complex Bearing a Pendant Thiol.

Light triggers the formation of HNO from a metal-nitrosyl species, facilitated by an intramolecular pendant thiol proton. Two {FeNO}6 complexes (the Enemark-Felthan notation), [Fe(NO)(TMSPS2)(TMSPS2H)] (1, TMSPS2H2 = 2,2'-dimercapto-3,3'-bis(trimethylsilyl)diphenyl)phenylphosphine; H is a dissociable proton) with a pendant thiol and [Fe(NO)(TMSPS2)(TMSPS2CH3)] (2) bearing a pendant thioether, are spectroscopically and structurally characterized. Both complexes are highly sensitive to visible light. Upon photolysis, complex 2 undergoes NO dissociation to yield a mononuclear Fe(III) complex, [Fe(TMSPS2)(TMSPS2CH3)] (3). In contrast, the pendant SH of 1 can act as a trap for the departing NO radical upon irradiation, resulting in the formation of an intermediate A with an intramolecular [SH···ON-Fe] interaction. As suggested by computational results (density functional theory), the NO stretching frequency (νNO) is sensitive to the intramolecular interaction between the pendant ligand and the iron-bound NO, and a shift of νNO from 1833 (1) to 1823 cm-1 (A) is observed experimentally. Subsequent photolysis of the intermediate A results in HNO production and a thiyl group that then coordinates to the Fe center for the formation of [Fe(TMSPS2)2] (4). In contrast with the common acid-base coupling pathway, the HNO is not voluntarily yielded from 1 but rather is generated by the photopromoted pathway. The photogenerated HNO can further react with [MnIII(TMSPS3)(DABCO)] (TMSPS3H3 = (2,2'2''-trimercapto-3,3',3''-tris(trimethylsilyl)triphenylphosphine; DABCO = 1,4-diazabicyclo[2.2.2]octane) in organic media to yield anionic [Mn(NO)(TMSPS3)]- (5-) with a {MnNO}6 electronic configuration, whereas [MnIII(TMSPS3)(DABCO)] reacts with NO gas for the formation of a {MnNO}5 species, [Mn(NO)(TMSPS3)] (6). Effective differentiation of the formation of HNO from complex 1 with the pendant SH versus NO from 2 with the pendant SMe is achieved by the employment of [MnIII(TMSPS3)(DABCO)].

NO-to-[N2O2]2--to-N2O Conversion Triggered by {Fe(NO)2}10-{Fe(NO)2}9 Dinuclear Dinitrosyl Iron Complex.

Flavodiiron nitric oxide reductases (FNORs) evolved in some pathogens are known to detoxify NO via two-electron reduction to N2O to mitigate nitrosative stress. In this study, we describe how the electronically localized {Fe(NO)2}10-{Fe(NO)2}9 dinuclear dinitrosyl iron complex (dinuclear DNIC) [(NO)2Fe(µ-bdmap)Fe(NO)2(THF)] (2) (bdmap = 1,3-bis(dimethylamino)-2-propanolate) can induce a reductive coupling of NO to form hyponitrite-coordinated tetranuclear DNIC, which then converts to N2O. Upon the addition of 1 equiv of NO into the dinuclear {Fe(NO)2}10-{Fe(NO)2}9 DNIC 2, the proposed side-on-bound [NO]--bridged [(NO)2Fe(µ-bdmap)(κ2-NO) Fe(NO)2] intermediate may facilitate intermolecular (O)N-N(O) bond coupling to yield the paramagnetic tetranuclear quadridentate trans-hyponitrite-bound {[(NO)2Fe(µ-bdmap)Fe(NO)2]2(κ4-N2O2)} that transforms to [Fe(NO)2(µ-bdmap)]2, along with the release of N2O upon Hbdmap (1,3-bis(dimethylamino)-2-propanol) added.

Electrocatalytic Water Reduction Beginning with a {Fe(NO)2}10-Reduced Dinitrosyliron Complex: Identification of Nitrogen-Doped FeO x(OH) y as a Real Heterogeneous Catalyst.

Electron paramagnetic resonance, IR, single-crystal X-ray diffraction, and density functional theory computation reveal that the electronic structure of α-diimine-coordinated {Fe(NO)2}10-reduced dinitrosyliron complexes (DNICs) may best be described as [{Fe(NO)2}10-L•], with the added electron residing mainly on the α-diimine ligand framework. The combination of electrochemistry, gas chromatography, Fourier transform infrared, X-ray photoelectron spectroscopy, and scanning electron microscopy-energy-dispersive X-ray studies demonstrates that the cathodic potential promotes/triggers the transformation of an α-diimine-coordinated {Fe(NO)2}10-reduced DNIC into a particulate deposit on the electrode, and electrodeposited-film electrodes, CFeO and CFeNO, are kinetically dominant electrocatalysts responsible for hydrogen evolution reaction (HER) from water with quantitative Faradaic efficiency. In comparison with the CFeO electrode reaching a current density of 10 mA/cm2 with an overpotential of 333 mV for HER, the nitrogen-doped iron oxide electrode, CFeNO, requires 147 mV of overpotential to achieve a current density of 10 mA/cm2 in a 1 M NaOH aqueous solution. The CFeNO electrode exhibits higher kinetic efficiency (Tafel slope of 59 mV/dec) than the CFeO electrode (Tafel slope of 122 mV/dec) in alkaline conditions. As opposed to high Rct (74.3 Ω) displayed by the CFeO electrode, the smaller charge-transfer resistance ( Rct) of the CFeNO electrode (34.0 Ω) demonstrated that the better HER catalytic activity may be ascribed to the incorporation of nitrogen into iron oxide architecture, which increases the surface roughness and electroconductivity of the CFeNO electrode (56.9% iron content and nitrogen electron-donating effect) and improves HER catalysis by polarizing the incoming water molecule (acting as a proton tray). This result implicates that a (NH4)2SO4-assisted nitrogen-doping strategy is a direct and effective method to realize synergistic regulation of the reaction dynamics, catalytically active sites and electronic conductivity, endowing this nitrogen-doped material CFeNO electrode as a promising HER electrocatalyst under alkaline conditions.

{Fe(NO)(2)(9) Dinitrosyl Iron Complex Acting as a Vehicle for the NO Radical.

To carry and deliver nitric oxide with a controlled redox state and rate is crucial for its pharmaceutical/medicinal applications. In this study, the capability of cationic {Fe(NO)2}9 dinitrosyl iron complexes (DNICs) [(RDDB)Fe(NO)2]+ (R = Me, Et, Iso; RDDB = N,N'-bis(2,6-dialkylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene) carrying nearly unperturbed nitric oxide radical to form [(RDDB)Fe(NO)2(•NO)]+ was demonstrated and characterized by IR, UV-vis, EPR, NMR, and single-crystal X-ray diffractions. The unique triplet ground state of [(RDDB)Fe(NO)2(•NO)]+ results from the ferromagnetic coupling between two strictly orthogonal orbitals, one from Fe dz2 and the other a π*op orbital of a unique bent axial NO ligand, which is responsible for the growth of a half-field transition (ΔMS = 2) from 70 to 4 K in variable-temperature EPR measurements. Consistent with the NO radical character of coordinated axial NO ligand in complex [(MeDDB)Fe(NO)2(•NO)]+, the simple addition of MeCN/H2O into CH2Cl2 solution of complexes [(RDDB)Fe(NO)2(•NO)]+ at 25 °C released NO as a neutral radical, as demonstrated by the formation of [S5Fe(NO)2]- from [S5Fe(µ-S)2FeS5]2-.

A Structurally Characterized Nonheme Cobalt-Hydroperoxo Complex Derived from Its Superoxo Intermediate via Hydrogen Atom Abstraction.

Bubbling O2 into a THF solution of CoII(BDPP) (1) at -90 °C generates an O2 adduct, Co(BDPP)(O2) (3). The resonance Raman and EPR investigations reveal that 3 contains a low spin cobalt(III) ion bound to a superoxo ligand. Significantly, at -90 °C, 3 can react with 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOH) to form a structurally characterized cobalt(III)-hydroperoxo complex, CoIII(BDPP)(OOH) (4) and TEMPO•. Our findings show that cobalt(III)-superoxo species are capable of performing hydrogen atom abstraction processes. Such a stepwise O2-activating process helps to rationalize cobalt-catalyzed aerobic oxidations and sheds light on the possible mechanism of action for Co-bleomycin.

Dinitrosyl iron complexes (DNICs): from biomimetic synthesis and spectroscopic characterization toward unveiling the biological and catalytic roles of DNICs.

Dinitrosyl iron complexes (DNICs) have been recognized as storage and transport agents of nitric oxide capable of selectively modifying crucial biological targets via its distinct redox forms (NO(+), NO(•) and NO(-)) to initiate the signaling transduction pathways associated with versatile physiological and pathological responses. For decades, the molecular geometry and spectroscopic identification of {Fe(NO)2}(9) DNICs ({Fe(NO)x}(n) where n is the sum of electrons in the Fe 3d orbitals and NO π* orbitals based on Enemark-Feltham notation) in biology were limited to tetrahedral (CN = 4) and EPR g-value ∼2.03, respectively, due to the inadequacy of structurally well-defined biomimetic DNICs as well as the corresponding spectroscopic library accessible in biological environments. The developed synthetic methodologies expand the scope of DNICs into nonclassical square pyramidal and trigonal bipyramidal (CN = 5) and octahedral (CN = 6) {Fe(NO)2}(9) DNICs, as well as two/three accessible redox couples for mononuclear {Fe(NO)2}(9/10) and dinuclear [{Fe(NO)2}(9/10)-{Fe(NO)2}(9/10)] DNICs with biologically relevant S/O/N ligation modes. The unprecedented molecular geometries and electronic states of structurally well-defined DNIC models provide the foundation to construct a spectroscopic library for uncovering the identity of DNICs in biological environments as well as to determine the electronic structures of the {Fe(NO)2} core in qualitative and quantitative fashions by a wide range of spectroscopic methods. On the basis of (15)N NMR, electron paramagnetic resonance (EPR), IR, cyclic voltammetry (CV), superconducting quantum interference device (SQUID) magnetometry, UV-vis, single-crystal X-ray crystallography, and Fe/S K-edge X-ray absorption and Fe Kß X-ray emission spectroscopies, the molecular geometry, ligation modes, nuclearity, and electronic states of the mononuclear {Fe(NO)2}(9/10) and dinuclear [{Fe(NO)2}(9/10)-{Fe(NO)2}(9/10)] DNICs could be characterized and differentiated. In addition, Fe/S K-edge X-ray absorption spectroscopy of tetrahedral DNICs deduced the qualitative assignment of Fe/NO oxidation states of {Fe(NO)2}(9) DNICs as a resonance hybrid of {Fe(II)((•)NO)(NO(-))}(9) and {Fe(III)(NO(-))2}(9) electronic states; the quantitative NO oxidation states of [(PhS)3Fe(NO)](-), [(PhS)2Fe(NO)2](-), and [(PhO)2Fe(NO)2](-) were further achieved by newly developed valence to core Fe Kß X-ray emission spectroscopy as -0.58 ± 0.18, -0.77 ± 0.18, and -0.95 ± 0.18, respectively. The in-depth elaborations of electronic structures provide credible guidance to elucidate (a) the essential roles of DNICs modeling the degradation and repair of [Fe-S] clusters under the presence of NO, (b) transformation of DNIC into S-nitrosothiol (RSNO)/N-nitrosamine (R2NNO) and NO(+)/NO(•)/NO(-), (c) nitrite/nitrate activation producing NO regulated by redox shuttling of {Fe(NO)2}(9) and {Fe(NO)2}(10) DNICs, and (d) DNICs as H2S storage and cellular permeation pathway of DNIC/Roussin's red ester (RRE) for subsequent protein S-nitrosylation. The consolidated efforts on biomimetic synthesis, inorganic spectroscopy, chemical reactivity, and biological functions open avenues to the future designs of DNICs serving as stable inorganic NO(+)/NO(•)/NO(-) donors for pharmaceutical applications.

Axial interactions in the mixed-valent CuA active site and role of the axial methionine in electron transfer.

Within Cu-containing electron transfer active sites, the role of the axial ligand in type 1 sites is well defined, yet its role in the binuclear mixed-valent CuA sites is less clear. Recently, the mutation of the axial Met to Leu in a CuA site engineered into azurin (CuA Az) was found to have a limited effect on E(0) relative to this mutation in blue copper (BC). Detailed low-temperature absorption and magnetic circular dichroism, resonance Raman, and electron paramagnetic resonance studies on CuA Az (WT) and its M123X (X = Q, L, H) axial ligand variants indicated stronger axial ligation in M123L/H. Spectroscopically validated density functional theory calculations show that the smaller ΔE(0) is attributed to H2O coordination to the Cu center in the M123L mutant in CuA but not in the equivalent BC variant. The comparable stabilization energy of the oxidized over the reduced state in CuA and BC (CuA ∼ 180 mV; BC ∼ 250 mV) indicates that the S(Met) influences E(0) similarly in both. Electron delocalization over two Cu centers in CuA was found to minimize the Jahn-Teller distortion induced by the axial Met ligand and lower the inner-sphere reorganization energy. The Cu-S(Met) bond in oxidized CuA is weak (5.2 kcal/mol) but energetically similar to that of BC, which demonstrates that the protein matrix also serves an entatic role in keeping the Met bound to the active site to tune down E(0) while maintaining a low reorganization energy required for rapid electron transfer under physiological conditions.

Spectroscopic definition of the copper active sites in mordenite: selective methane oxidation.

Two distinct [Cu-O-Cu](2+) sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu-O-Cu active site geometry, having a ∠Cu-O-Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu-O-Cu](2+) active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu-O-Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu-O-Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites.

Chelate-Thiolate-Coordinate Ligands Modulating the Configuration and Electrochemical Property of Dinitrosyliron Complexes (DNICs).

As opposed to the reversible redox reaction ({Fe(NO)2 }(10) reduced-form DNIC [(NO)2 Fe(S(CH2 )3 S)](2-) (1)⇌{Fe(NO)2 }(9) oxidized-form [(NO)2 Fe(S(CH2 )3 S)](-) ), the chemical oxidation of the {Fe(NO)2 }(10) DNIC [(NO)2 Fe(S(CH2 )2 S)](2-) (2) generates the dinuclear {Fe(NO)2 }(9) -{Fe(NO)2 }(9) complex [(NO)2 Fe(µ-SC2 H4 S)2 Fe(NO)2 ](2-) (3) bridged by two terminal [SC2 H4 S](2-) ligands. On the basis of the Fe K-edge pre-edge energy and S K-edge XAS, the oxidation of complex 1 yielding [(NO)2 Fe(S(CH2 )3 S)](-) is predominantly a metal-based oxidation. The smaller S1-Fe1-S2 bond angle of 94.1(1)° observed in complex 1 (S1-Fe1-S2 88.6(1)° in complex 2), compared to the bigger bond angle of 100.9(1)° in the {Fe(NO)2 }(9) DNIC [(NO)2 Fe(S(CH2 )3 S)](-) , may be ascribed to the electron-rich {Fe(NO)2 }(10) DNIC preferring a restricted bite angle to alleviate the electronic donation of the chelating thiolate to the electron-rich {Fe(NO)2 }(10) core. The extended transition state and natural orbitals for chemical valence (ETS-NOCV) analysis on the edt-/pdt-chelated {Fe(NO)2 }(9) and {Fe(NO)2 }(10) DNICs demonstrates how two key bonding interactions, that is, a FeS covalent σ bond and thiolate to the Fe d z 2 charge donation, between the chelating thiolate ligand and the {Fe(NO)2 }(9/10) core could be modulated by the backbone lengths of the chelating thiolate ligands to tune the electrochemical redox potential (E1/2 =-1.64 V for complex 1 and E1/2 =-1.33 V for complex 2) and to dictate structural rearrangement/chemical transformations (S-Fe-S bite angle and monomeric vs. dimeric DNICs).

[Cu2O]2+ active site formation in Cu-ZSM-5: geometric and electronic structure requirements for N2O activation.

Understanding the formation mechanism of the [Cu2O](2+) active site in Cu-ZSM-5 is important for the design of efficient catalysts to selectively convert methane to methanol and related value-added chemicals and for N2O decomposition. Spectroscopically validated DFT calculations are used here to evaluate the thermodynamic and kinetic requirements for formation of [Cu2O](2+) active sites from the reaction between binuclear Cu(I) sites and N2O in the 10-membered rings Cu-ZSM-5. Thermodynamically, the most stable Cu(I) center prefers bidentate coordination with a close to linear bite angle. This binuclear Cu(I) site reacts with N2O to generate the experimentally observed [Cu2O](2+) site. Kinetically, the reaction coordinate was evaluated for two representative binuclear Cu(I) sites. When the Cu-Cu distance is sufficiently short (<4.2 Å), N2O can bind in a "bridged" µ-1,1-O fashion and the oxo-transfer reaction is calculated to proceed with a low activation energy barrier (2 kcal/mol). This is in good agreement with the experimental Ea for N2O activation (2.5 ± 0.5 kcal/mol). However, when the Cu-Cu distance is long (>5.0 Å), N2O binds in a "terminal" η(1)-O fashion to a single Cu(I) site of the dimer and the resulting E(a) for N2O activation is significantly higher (16 kcal/mol). Therefore, bridging N2O between two Cu(I) centers is necessary for its efficient two-electron activation in [Cu2O](2+) active site formation. In nature, this N2O reduction reaction is catalyzed by a tetranuclear CuZ cluster that has a higher E(a). The lower E(a) for Cu-ZSM-5 is attributed to the larger thermodynamic driving force resulting from formation of strong Cu(II)-oxo bonds in the ZSM-5 framework.

Spectroscopy and redox chemistry of copper in mordenite.

Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved diffuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu(2+) sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm(-1), and quantified with TPR. Its full description and true location in MOR is currently being investigated.

Tris(imidazolyl) dicopper(I) complex and its reactivity to exert the catalytic oxidation of sterically hindered phenol substrates via a [Cu2O]2+ core.

The Cu ion ligated with histidine residues is a common active site motif of various Cu-containing metalloenzymes exerting versatile catalytic oxidation reactions. Due to the scarcity of structurally characterized biomimetic binuclear Cu(I)-imidazolyl complexes, the bonding interactions between the Cu(I) center and imidazolyl donor ligands as well as their stoichiometric/catalytic oxidation reactivities remain relatively unexplored. In this study, we successfully synthesized a tris(imidazolyl) dicopper(I) complex [CuI(µ-bimeta)3CuI][PF6]2 (1) characterized by single-crystal X-ray diffraction, cyclic voltammetry, and UV-vis absorption spectroscopy. The coordination environment around each Cu(I) center of complex 1 is best described as a regular trigonal geometry and the distance between two Cu(I) centers is ∼3.0521(18) Å. From the O2-/PhIO-titration reactions of complex 1, only 0.5 equiv. of O2(g) and one equiv. of PhIO are required to produce the corresponding oxygenated product complex 1ox, respectively, exhibiting a distinct UV-vis absorption band at ∼640 nm. From the characterization of ESI mass spectrometry, IR, UV-vis spectroscopy, and O2-/PhIO-titration reactions, the molecular identity of complex 1ox is tentatively assigned as [Cu(µ-O)(µ-bimeta)3Cu]2+, which is further corroborated by the spectroscopically calibrated DFT calculations. The oxidation reactivities of complex 1ox were investigated using the well-understood sterically hindered phenol substrates DTBP and TBBP in both stoichiometric and catalytic fashions. In stoichiometric reactions, one equiv. of 1ox is capable of fully converting TBBP into TBOBF in the CH3CN solution at room temperature within 1 h. The optimized selectivity of catalytically oxidizing DTBP into TBOBF (>95% selectivity, ∼100% conversion rate) could be achieved with a 10 mol% loading of complex 1ox using air as the oxidant. From the time-course product distributions of this catalyst system, ∼40% of DTBP was converted into the corresponding TBBP product in the first 5 min. From 10 to 120 min, the oxidation of DTBP to TBBP and the oxidation of TBBP to TBOBF simultaneously proceeded, which was evidenced by the decrease of TBBP and the increase TBOBF.

Nitric oxide generation study of unsymmetrical ß-diketiminato copper(II) nitrite complexes.

ß-Diketiminato copper(II) L1CuCl-L4CuCl and their nitrite complexes L1Cu(O2N) and L2Cu(O2N) have been synthesized and characterized. X-ray analysis of the L1CuCl-L4CuCl complexes clearly reveals their mononuclear structure with a four-coordinated Cu(II) center bound by one chloride and three nitrogen atoms of unsymmetrical ß-diketiminato ligands. Cyclic voltametric analysis of the Cu(II) complexes shows that the length of the pyridyl arm controls the Cu(II)/Cu(I) redox process. DFT and EPR results confirm that the geometry of the Cu(II) complexes is also controlled by the length of the chelating pyridyl arm. The oxygen atom transfer nitrite reduction of the Cu(II) nitrite complexes leads to the formation of copper(I)-PPh3 and OPPh3 which were confirmed by 1H and 31P NMR. The length of the pyridyl arm of the copper(II) nitrite complexes governs the NO-releasing ability. These findings illustrate the important bioinspired behaviour and NO generation from nitrite via oxygen atom transfer of the unsymmetrical ß-diketiminato copper(II) complexes as compared to symmetrical ß-diketiminato copper(II) complexes.

Relative binding affinity of thiolate, imidazolate, phenoxide, and nitrite toward the {Fe(NO)2} motif of dinitrosyl iron complexes (DNICs): the characteristic pre-edge energy of {Fe(NO)2}9 DNICs.

The synthesis, characterization, and transformation of the anionic {Fe(NO)(2)}(9) dinitrosyl iron complexes (DNICs) [(NO)(2)Fe(ONO)(2)](-) (1), [(NO)(2)Fe(OPh)(2)](-) (2), [(NO)(2)Fe(OPh)(C(3)H(3)N(2))](-) (3) (C(3)H(3)N(2) = imidazolate), [(NO)(2)Fe(OPh)(-SC(4)H(3)S)](-) (4), [(NO)(2)Fe(p-OPhF)(2)](-) (5), and [(NO)(2)Fe(SPh)(ONO)](-) (6) were investigated. The binding affinity of ligands ([SPh](-), [-SC(4)H(3)S](-), [C(3)H(3)N(2)](-), [OPh](-), and [NO(2)](-)) toward the {Fe(NO)(2)}(9) motif follows the ligand-displacement series [SPh](-) approximately [-SC(4)H(3)S](-) > [C(3)H(3)N(2)](-) > [OPh](-) > [NO(2)](-). The findings, the pre-edge energy derived from the 1s --> 3d transition in a distorted T(d) environment of the Fe center falling within the range of 7113.4-7113.8 eV for the anionic {Fe(NO)(2)}(9) DNICs, implicate that the iron metal center of DNICs is tailored to minimize the electronic changes accompanying changes in coordinated ligands. Our results bridging the ligand-substitution reaction study and X-ray absorption spectroscopy study of the electronic richness of the {Fe(NO)(2)}(9) core may point the way to understanding the reasons for nature's choice of combinations of cysteine, histidine, and tyrosine in protein-bound DNICs and rationalize that most DNICs characterized/proposed nowadays are bound to the proteins almost through the thiolate groups of cysteinate/glutathione side chains in biological systems.

Half-sandwich Ru(η6-p-cymene) complexes featuring pyrazole appended ligands: Synthesis, DNA binding and in vitro cytotoxicity.

Organometallic Ru(II)-arene complexes have emerged as potential alternatives to platinum appended agents due to their wide range of interesting features such as stability in solution and solid, significant activity, less toxicity and hydrophobic property of arene moiety, etc. Hence, a series of Ru(II)-p-cymene complexes, [(η6-p-cymene)Ru(η2-N,N-L1)Cl]Cl (1), [(η6-p-cymene)Ru(η1-N-L2)Cl2] (2) and [(η6-p-cymene)Ru(η1-N-L3)Cl2] (3) were prepared from pyrazole based ligands [2-(1H-pyrazol-3-yl)pyridine (L1), 3-(furan-2-yl)-1H-pyrazole (L2) and 3-(thiophen-2-yl)-1H-pyrazole (L3)], and [RuCl2-(η6-p-cymene)] dimer. The new Ru(II)-p-cymene complexes were well characterized by elemental analysis, and spectroscopic (FT-IR, UV-Visible, 1H NMR, 13C NMR and mass) and crystallographic methods. The Ru(II)-p-cymene complexes (1-3) were found to adopt their characteristic piano stool geometry around Ru(II) ion. The calf thymus DNA (CT-DNA) binding ability of the new complexes was investigated by electronic absorption spectroscopic titration and viscosity methods. The molecular docking study results showed that complex 1 strongly bound with targeted biomolecules than 2 and 3. Docked poses of bidentate pyrazole based Ru(II)-p-cymene complex 1 revealed that the complex formed a crucial guanine N7 position hydrogen bond with DNA receptor. Complexes 1-3 might hydrolyze under physiological conditions and form aqua complexes 4-8, and docking calculations showed that the aqua complexes bound strongly with the receptors than original complexes. The in vitro cytotoxicity of the Ru(II)-p-cymene complexes and cisplatin was evaluated against triple negative breast cancer (TNBC) MDA-MB-231 cells. Our results showed that the inhibitory effect of bidentate pyrazole based Ru(II)-p-cymene complex 1 on the growth of breast cancer cells was superior to other tested complexes.

Nitric oxide physiological responses and delivery mechanisms probed by water-soluble Roussin's red ester and {Fe(NO)2}10 DNIC.

Dinitrosyl-iron complexes (DNICs) are stable carriers for nitric oxide (NO), an important biological signaling molecule and regulator. However, the insolubility of synthetic DNICs, such as Roussin's red ester (RRE), in water has impaired efforts to unravel their biological functions. Here, we report a water-soluble and structurally well-characterized RRE [Fe(mu-SC2H4COOH)(NO)2]2 (DNIC-1) and a {Fe(NO)2}(10) DNIC [(PPh2(Ph-3-SO3Na))2Fe(NO)2] (DNIC-2), their NO-induced protein regulation, and their cellular uptake mechanism using immortalized vascular endothelial cells as a model. Compared with the most common NO donor, S-nitroso-N-acetyl-penicillamine (SNAP), the in vitro NO release assay showed that both DNICs acted as much slower yet higher stoichiometric NO-release agents with low cytotoxicity (IC50 > 1 mM). Furthermore, L-cysteine facilitated NO release from SNAP and DNIC-1, but not DNIC-2, in a dose- and time-dependent manner. EPR spectroscopic analysis showed, for the first time, that intact DNIC-1 can either diffuse or be transported into cells independently and can transform to either paramagnetic protein bound DNIC in the presence of serum or [DNIC-(Cys)2] with excess L-cysteine under serum-free conditions. Both DNICs subsequently induced NO-dependent upregulation of cellular heat shock protein 70 and in vivo protein S-nitrosylation. We conclude that both novel water-soluble DNICs have potential to release physiologically relevant quantities of NO and can be a good model for deciphering how iron-sulfur-nitrosyl compounds permeate into the cell membrane and for elucidating their physiological significance.

Monoanionic {Mn(NO)}5 and dianionic {Mn(NO)}6 thiolatonitrosylmanganese complexes: [(NO)Mn(L)2]- and [(NO)Mn(L)2]2- (LH2 = 1,2-benzenedithiol and toluene-3,4-dithiol).

The reaction of MnBr(2) and [PPN](2)[S,S-C(6)H(3)-R] (1:2 molar ratio) in THF yielded [(THF)Mn(S,S-C(6)H(3)-R)(2)](-) [R = H (1a), Me (1b); THF = tetrahydrofuran]. Formation of the dimeric [Mn(S,S-C(6)H(3)-R)(2)](2)(2-) [R = H (2a), Me (2b)] was presumed to compensate for the electron-deficient Mn(III) core via two thiolate bridges upon dissolution of complexes 1a and 1b in CH(2)Cl(2). Complex 2a displays antiferromagnetic coupling interaction between two Mn(III) centers (J = -52 cm(-1)), with the effective magnetic moment (mu(eff)) increasing from 0.85 mu(B) at 2.0 K to 4.86 mu(B) at 300 K. The dianionic manganese(II) thiolate complexes [Mn(S,S-C(6)H(3)-R)(2)](2-) [R = H (3a), Me (3b)] were isolated upon the addition of [BH(4)](-) into complexes 1a and 1b or complexes 2a and 2b, respectively. The anionic mononuclear {Mn(NO)}(5) thiolatonitrosylmanganese complexes [(NO)Mn(S,S-C(6)H(3)-R)(2)](-) [R = H (4a), Me (4b)] were obtained from the reaction of NO(g) with the anionic complexes 1a and 1b, respectively, and the subsequent reduction of complexes 4a and 4b yielded the mononuclear {Mn(NO)}(6) [(NO)Mn(S,S-C(6)H(3)-R)(2)](2-) [R = H (5a), Me (5b)]. X-ray structural data, magnetic susceptibility measurement, and magnetic fitting results imply that the electronic structure of complex 4a is best described as a resonance hybrid of [(L)(L)Mn(III)(NO(*))](-) and [(L)(L(*))Mn(III)(NO(-))](-) (L = 1,2-benzenedithiolate) electronic arrangements in a square-pyramidal ligand field. The lower IR v(NO) stretching frequency of complex 5a, compared to that of complex 4a (shifting from 1729 cm(-1) in 4a to 1651 cm(-1) in 5a), supports that one-electron reduction occurs in the {(L)(L(*))Mn(III)} core upon reduction of complex 4a.