Picolinate Anion Research Articles

Enzyme mimicking activity of the Ru(pic)3 complex in the oxidation NADH coenzymes: kinetic and mechanistic studies

Mimicking the enzymatic oxidation of NADH and obtaining mechanistic insight into the electron transfer reaction pertinent to the redox inter-conversion of NADH/NAD+ couple is a long-term challenge in biological and energy-related chemistry. The kinetics and mechanism of the oxidation of NADH by [RuIII(pic)3] (pic– = picolinate anion) has been studied spectrophotometrically and kinetically in aqueous solution at pH 7.0 (MES buffer). The reaction was followed over time at 466 nm (metal to ligand charge transfer band of [RuII(pic)3]–). The rate of NADH oxidation was found to be first-order with respect to Ru(III)-complex concentration. The values of the observed rate constant (k obs) increased linearly with increase in the concentration of NADH. The rate of the reaction was independent of the ionic strength of the reacting solution. Activation parameters (ΔH≠ = 56 ± 1 kJ mol−1 and ΔS≠ = −65 ± 3 J.deg−1 mol−1) were determined from the temperature dependence studies (15-35 °C) of the reaction. Kinetic data and activation parameters are interpreted in terms of a mechanism involving outer-sphere electron transfer.

Kinetics and mechanism of the oxidation of hydrazine with [RuIII(pic)3] in aqueous solution

The kinetics and mechanism of the oxidation of hydrazine by [RuIII(pic)3] (pic- = picolinate anion) were studied spectrophotometrically as a function of hydrazine concentrations at a constant pH of 5.5. The rate of the reaction was followed by monitoring the appearance of the MLCT band of [RuII(pic)3]- at 466 nm. The reaction rate was found to be first-order for Ru(III)-complex concentration. The observed rate constant (k obs) values increased linearly with increase in the hydrazine concentration. The oxidation reaction was studied at varied temperature (20–40 °C). The ΔH≠ (73 ± 3 kJ mol−1 kJ·mol−1) and the negative ΔS≠ (34 ± 7 J·deg−1mol−1) values are consistent with the proposed outer-sphere electron transfer process operative in the oxidation of hydrazine with [RuIII(pic)3]. A working mechanism involving the formation of an active diazene (HN = NH) intermediate followed by its rapid two-electron oxidation by [RuIII(pic)3] in a kinetically inconsequential step producing N2 as sole product is proposed for an overall four-electron oxidation of hydrazine to dinitrogen.

Ionic Liquid-Stabilizing Vanadium Oxo-Cluster Catalysts for One-Step Selective Oxidative Cleavage of β–O–4 Lignin Model Compounds

An easily accessible and efficient catalyst is highly promising for the cleavage of the C–C bond and C–O bond of β–O–4 lignin model compounds. In this work, ionic liquid [Bim][Pic] (Bim = 1-butyl imidazolium, Pic = picolinate anion)-stabilizing vanadium oxo-cluster catalysts were constructed conveniently for selective aerobic oxidation to cleave the β–O–4 linkages into phenols, eaters and acids. The as-synthesized vanadium oxo-cluster catalysts Bim-V-n (n = 1–3) were characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), electron paramagnetic resonance (EPR), mass spectrometry (MS), and elemental analysis. The vanadium in the catalyst exists stably in a mixed-valence state of V4+ and V5+. Molecular oxygen was activated through the mutual transformation of V4+ and V5+ to form superoxide free radicals, which were captured by the EPR spectrum. Especially, Bim-V-2 afforded superior catalytic activity under aerobic conditions (10 h, 120 °C). The highly reversible interconversion of V4+ and V5+ species in vanadium oxo-cluster allowed the coexistence of the mixed-valence vanadium species, which was responsible for oxidizing β–O–4 alcohols to β–O–4 ketones. Moreover, superoxide free radicals can facilitate the cleavage of C–C and C–O bonds in β–O–4 ketones. Especially, 1-butyl imidazolium also played a positive role in the cleavage of C–O bonds by the action of the cation protons. No appreciable loss of catalytic activity was observed, and the catalyst could be recovered and reused up to five times, reflecting the excellent recyclability of the present vanadium oxo-cluster catalysts. The catalytic system can be extended for the cleavage of the C–C bond and C–O bond of β–O–4 of the other lignin model compounds, and the reaction could still be completed efficiently. This provides a new perspective for the selective aerobic oxidation of β–O–4 lignin without co-catalysts.

Bromination and C–C Cross-Coupling Reactions for the C–H Functionalization of Iridium(III) Emitters

The orthometalated phenyl groups of the dimer [Ir(μ-Cl){κ2-C,N-(C6H3Me-py)}2]2 have been selectively brominated, at para-position with regard to the Ir–C bonds, with N-bromosuccinimide. The bromination leads to [Ir(μ-Cl){κ2-C,N-(C6H2MeBr-py)}2]2, which affords the mononuclear derivatives Ir{κ2-C,N-(C6H2MeBr-py)}2{κ2-O,N-[OC(O)-py]}, Ir{κ2-C,N-(C6H2MeBr-py)}2{κ2-O,O-(acac)}, and Ir{κ2-C,N-(C6H2MeBr-py)}2{κ2-C,N-[C6H4-Mepy]} by replacement of the chloride bridges by a picolinate anion, an acac group, and an orthometalated 2-phenyl-5-methylpyridine ligand, respectively. Complexes Ir{κ2-C,N-(C6H2MeBr-py)}2{κ2-O,O-(acac)} and Ir{κ2-C,N-(C6H2MeBr-py)}2{κ2-C,N-[C6H4-Mepy]} have been subsequently postfunctionalized by means of palladium-catalyzed Suzuki–Miyaura cross-coupling to give Ir{κ2-C,N-(C6H2MeR-py)}2{κ2-O,O-(acac)} (R = Me, Ph) and Ir{κ2-C,N-(C6H2Me2-py)}2{κ2-C,N-[C6H4-Mepy]}. These [3b + 3b + 3b′] mononuclear compounds are green-yellow emitters (488–580 nm) upon photoexcitation, in doped poly(methyl methacrylate) (PMMA) film at 5 wt % at room temperature and 2-methyltetrahydrofuran (2-MeTHF) at room temperature and at 77 K. They display lifetimes in the range 1.0–5.0 μs and quantum yields in PMMA films and in 2-MeTHF at room temperature between 0.84 and 0.40.

Synthesis, crystal structures and magnetic properties of picolinate-bridged copper(II) chains

The paper reports the synthesis, crystal structures, and magnetic properties of two copper(II) coordination polymers, Cu2(L1)4 (1) and Cu3(L2)6 (2), based on rigid ethynylene-bridged ditopic ligands containing one picolinate subunit (L1 = 5-(pyridin-3-ylethynyl)picolinate anion; L2 = 5-((4-(methoxycarbonyl)phenyl)ethynyl)picolinate anion). Both 1 and 2 have a chain structure. In 1, the repeating unit in the chain is a doubly bridged picolinate dicopper(II) complex, whereas 2 is built upon trimeric units where the Cu2+ ions are also connected by double picolinate bridges. Both compounds exhibit antiferromagnetic interactions in the solid state. The magnetic properties of 1 and 2 were analyzed in terms of simple dimer (H = –2JSCuSCu) and linear trimer [H = –2J(SCu1SCu2+SCu2SCu1)] models, respectively, neglecting the intermolecular interactions between the polynuclear units. The values of the exchange coupling parameters were J = −0.59 and J = –0.30 cm−1 for 1 and 2, respectively.

Oxidation in vitro of chromium(III) dietary supplements mer-[Cr(pic)3] and trans(S,S)-[Cr(Cys)2]\u2212 by hydrogen peroxide

Biological tests performed using 3T3 fibroblasts indicated low cytotoxicities for the complexes mer-[Cr(pic)3] and trans(S,S)-[Cr(Cys)2]−, where pic = picolinate anion and Cys = cysteine. Oxidation of these complexes by hydrogen peroxide was studied in NaOH and NaHCO3 media. Electronic (UV–Vis) and EPR spectroscopies were used to monitor the reaction course. Hydrogen peroxide oxidizes chromium(III) to both [CrV(O2)4]3− and CrVIO42− anions in alkaline media and practically completely to CrO42− anion in bicarbonate solution. The reactions follow consecutive biphasic or simple first-order kinetics. The first-order decay of [CrV(O2)4]3− anion at pH ≈ 8 was followed by EPR spectroscopy. Based on the obtained kinetic and spectroscopic data, mechanisms for the redox transformations of these chromium(III) complexes are proposed.

Synthesis and detailed characterization of cis-dichloridobispicolinatoruthenate(III) as solid and in solution

Three water-soluble ruthenium(III) compounds, Y[cis-RuCl2(pic)2]⋅nH2O (where pic = picolinate anion, Y = H(Hpic)2+ (1), H2pic+ (2) or K+ (3), n = 2, 1.5, and 2.5, respectively), were synthesized and their X-ray structures determined. Compound 1 was fully characterized both as solid and in aqueous solution by elemental analysis, magnetic susceptibility measurements, EPR, IR and UV-visible spectroscopy, cyclic voltammetry, microwave plasma atomic emission, and mass spectrometry, capillary electrophoresis and chromatographic methods. It was shown that the coordination geometry around the low spin (S = ½) RuIII center is distorted octahedral with the chlorido ligands in a cis configuration. Furthermore, all the measurements showed that the structures in the solid state and solution are absolutely identical with the cis-[RuCl2(pic)2]− anion being inert in aqueous solution. Microbial studies showed that 1 exerts a strong inhibition on the growth and increased mortality level of the bacterial strains – Escherichia coli, Pseudomonas sp., Sarcina sp., Micrococcus sp., Serratia sp., Bacillus cereus, Bacillus subtilis and yeast – Saccharomyces cerevisiae.

A novel near-infrared-emitting cyclometalated platinum (II) complex with donor–acceptor–acceptor chromophores

A novel near-infrared-emitting cyclometalated platinum (II) complex of (TPA-BT-Q)Ptpic with a donor–acceptor–acceptor (D–A–A) chromophores was synthesized and characterized, in which the TPA-BT-Q unit is a cyclometalated ligand of N,N-di(4-octyloxyphenyl)-4-(7-(quinolin-2-yl)-benzo[c][1,2,5]thiadiazol-4-yl)-phenylamine and pic is picolinate anion. Its optophysical, electrochemical and electroluminescent characteristics were primarily studied. An intense UV–vis absorption peak at 540 nm and a strong near-infrared emission peak at 759 nm were observed for (TPA-BT-Q)Ptpic in dichloromethane. Using (TPA-BT-Q)Ptpic as a single dopant and a blend of poly(vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole as a host matrix, the single-emissive-layer polymer light-emitting devices exhibited a near-infrared emission peaked at 760 nm with the maximum external quantum efficiency of 0.12% at 16.6 mA cm−2 and a radiance intensity of 112 μW cm−2 at 11.7 mA cm−2 at the doping concentrations of 2.0 wt%. This work provides an efficient approach to realize near-infrared electrophosphorescent emission with high radiance intensity by employing platinum (II) complex with the D–A–A structure.

(μ2-Chlorido)-(μ2-pyridine-2-carboxylato-1:2κN,O:O)-dichlorido(ethanol-κO)bis[N-hydroxy-1-(pyridin-2-yl)methanimine-κ2N,N′]dicobalt(II)

The dinuclear title compound, [Co2Cl3(C6H4NO2)(C6H6N2O)2(C2H5OH)], contains two six-coordinate CoII atoms with different octa­hedral coordination environments. One CoII atom is coordinated by two N atoms from one pyridine-2-carbaldehyde oxime ligand, by one terminal and one bridging Cl− ion, by one O atom from an ethanol mol­ecule, and by one O atom from a bridging pyridine-2-carboxyl­ate (picolinate) anion. The second CoII atom is coordinated by two N atoms from another pyridine-2-carbaldehyde oxime ligand, one N and one O atom from the bridging picolinate anion, and by one terminal Cl− and one bridging Cl− anion. The structure displays intra­molecular O—H⋯O and O—H⋯Cl hydrogen bonds. Weak C—H⋯Cl hydrogen-bonding inter­actions connect the mol­ecules into a three-dimensional network.

Luminescent Sensor for Carbonate Ion Based on Lanthanide(III) Complexes of 1,4,7,10-Tetraazacyclododecane-1,4,7-Triacetic Acid (DO3A)

Lanthanide(III) complexes of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (H(3)DO3A) are suggested as sensors for sensitive luminescence-based determination of a carbonate anion. Thermodynamic study of association of [Eu(H(2)O)(2)(DO3A)] with bidentate anionic ligands using luminescence spectroscopy reveals an affinity order CO(3)(2-) > oxalate(2-) > picolinate(-) > phthalate(2-) ≈ citrate(3-); presumably as a consequence of an increasing chelate ring size. The ternary [Eu(DO3A)(picolinate)](-) and [Tb(DO3A)(picolinate)](-) complexes show improved photophysical properties due to the antenna effect of the picolinate anion. High quenching effect of carbonate anion and, to a lesser extent also oxalate, enables construction of a linear calibration plot utilizing optimized experimental conditions (e.g. c(LnL) = 0.1 mM, c(picolinate) = 2-5 mM, pH = 7.4, λ(exc) = 286 nm, etc.) for carbonate determination in solution. Both sensors show a comparable sensitivity and the detection limit of about 0.4 mM. In order to improve the photophysical properties of Ln(III) sensor by shift of excitation wavelength about 40 nm to VIS range, the isoquinoline-3-carboxylic acid (IQCA) as antenna ligand was employed instead of picolinic acid. The analysis of commercial samples of European mineral waters was carried out and they were compared to the results obtained by capillary isotachophoresis to confirm there is no inherent (systematic) error to the present analysis. The Ln(III) sensor with IQCA is recommended since it has a better robustness than that with picolinate. The present analytical method is simple and rapid, and it is useful for sensitive determination of bicarbonate/carbonate concentration in water samples under aerobic conditions.

Tetrakis(picolinato-κ2N,O)zirconium(IV) dihydrate

In the title compound, [Zr(C6H4NO2)4]·2H2O, the ZrIV atom is located on a crystallographic fourfold rotoinversion axis () and is coordinated by four picolinate anions with Zr—O and Zr—N distances of 2.120 (2) and 2.393 (2) Å, respectively. An approximate square-anti­prismatic coordination polyhedron of the N,O-coordination ligand atoms is formed, with a distortion towards dodeca­hedral geometry. The crystal packing is stabilized by inter­molecular π–π inter­actions between adjacent picolinate rings [centroid–centroid distances = 3.271 (1) and 3.640 (2) Å], as well as O—H⋯O hydrogen bonds between the solvent mol­ecules and the coordinated ligands, thereby linking the mol­ecules into a supra­molecular three-dimensional network.

2-Amino-5-methylpyridinium picolinate 0.63-hydrate

The asymmetric unit of the title compound, C6H9N2 +·C6H4NO2 −·0.63H2O, contains two crystallographically independent 2-amino-5-methyl­pyridinium cations, a pair of picolinate anions and two water mol­ecules, one with an occupancy of 0.25. Both the 2-amino-5-methyl­pyridine mol­ecules are protonated at the pyridine N atoms. In the crystal structure, the cations, anions and water mol­ecules are linked via N—H⋯O, N—H⋯N and O—H⋯O hydrogen bonds, as well as by C—H⋯O contacts, forming a chain along the b axis. In addition, weak π–π inter­actions are observed between pyridinium rings, with centroid–centroid distances of 3.5306 (13) Å.

Synthesis and characterization of picolinato and nicotinato cobalt(II) complexes containing tris(2-benzimidazolylmethyl)amine

Two new mononuclear cobalt(II) complexes [Co(ntb)(pic)](ClO4) · (CH3OH)2.35 (1) and [Co(ntb)(nic)](ClO4) · CH3OH (2) were synthesized and structurally characterized, where ntb is tris(2-benzimidazolylmethyl)amine, pic is the anion of picolinic acid, and nic is the anion of nicotinic acid. The X-ray analysis indicates that the Co(II) center is six-coordinate in distorted octahedral and five-coordinate in distorted trigonal bipyramidal geometry for 1 and 2, respectively. In 1, the picolinate anion coordinates to Co(II) in a bidentate μ2-N,O chelating mode. In 2, the nicotinate anion coordinates with Co(II) through a monodentate carboxylate oxygen. 1-D chain structures were formed by intermolecular hydrogen bonds in the two complexes and π–π interactions are important for the stabilization of the structures.

Dicyanamido–metal(II) complexes. Part 3: Hydrolysis of the dicyanamide into amidocyanamide during its interaction with Cu(II) perchlorate and 2-pyridylcarbonyl-N,N-bis(2-pyridylmethyl)amine

Abstract The reaction of Cu(ClO4)2, 2-pyridylcarbonyl-N,N-bis(2-pyridylmethyl)amine (DPA-CO-py) and sodium dicyanamide (Nadca) in aqueous medium led to the isolation of three complexes: {[Cu(DPA)(μ1,5-dca)]ClO4}n (1), [Cu2(pic)2(μ1,3-H2NCO–N–CN)2(H2O)2] · 2H2O (2) and Cu(pic)2 · H2O (3). The Cu2+ ion, during its complex formation with DPA-CO-py and dca not only assisted the hydrolysis of DPA-CO-py to the corresponding di(2-methylpyridyl)amine (DPA) and picolinic acid (Hpic) but also resulted in partial hydrolysis of one of the two nitrile groups of the dicyanamide, NC–N–CN− (dca) to amidocynamide, H2NCO–NH–CN. The complexes were structurally characterized and the single X-ray structures for 1 and 2 reveal their identity. Complex 1 forms a 1D polymeric chain with the dca acting as a μ1,5-bridging ligand via the terminal nitrile nitrogen atoms. A distorted SP geometry around the Cu2+ centers was achieved by the three N-atoms of the DPA and by the two nitrile groups of the bridging dca ligands. Complex 2 consists of a dimeric unit with CuN3O2 chromophore in coordination polyhedron that has a distorted SP geometry. The amidocyanamide ion, H2NCO–N–CN− acts as bis(monodentate) μ1,3-bridging ligand via the central amido and terminal nitrile nitrogen atoms whereas the picolinate anion forms a six-membered chelate ring through its pyridyl nitrogen and an oxygen atom of the carboxylate group, and the aqua water occupies the apical position of the structure. Complex 1 exhibits very weak antiferromagnetic coupling (J = −0.31 cm−1) whereas 2 shows a significant antiferromagnetic interaction between the two copper centers (J = −13.6 cm−1).

Hydrogen-bonding Patterns in Pyrimethaminium Picolinate

The title compound was prepared by reacting pyrimethamine [2,4-diamino-5-(p-chlorophenyl)-6-ethyl pyrimidine] and picolinic acid, [C12H14ClN4, C6H4NO2] in methanol. It crystallizes in the monoclinic P21/c space group. The unit-cell parameters are a = 9.5731(2) b = 8.7449(2) c = 21.8582(5)A, β = 99.9590(10)°. The final R-value is 0.0662 for 8517 measured reflections. The asymmetric unit contains a pyrimethamine cation and a picolinate anion. The protonated N1 atom is hydrogen bonded to one of the oxygen atoms via a N-H…O hydrogen bond. The carboxyl oxygen atom bridges the 2-amino group and the 4-amino group on either side in addition to the base pairing to form a DADA array (D = donor & A = acceptor in hydrogen bonding) of quadruple hydrogen bonding. The crystal structure is further stabilized by C-H…O, C-H…N and aromatic stacking interactions.

One-dimensional copper(II) compound with a double out-of-plane carboxylato-bridge – Another polymorphic form of Cu(pyridine-2-carboxylate) 2

The crystal and molecular structure of a blue-violet copper(II) compound of the formula Cu(2-pic) 2 (2-pic = 2-picolinate or pyridine-2-carboxylate ion) was determined. Its structure consists of a copper atom lying in the centre of symmetry, trans square-planar coordinated via two pyridine nitrogens and the carboxylate oxygen atoms of the two picolinate anions. The bonding interaction between the copper and the oxygen of carboxylate atoms of neighboring planes completes the square-planar coordination to a strongly tetragonally distorted octahedral stereochemistry ( CuN 2 O 2 O 2 ′ chromophore), when long axial Cu–O bond distances are included. This intermolecular interaction, with the Cu⋯Cu distance of 5.163 Å, is weak, but permits to define the structure as a one-dimensional chain with a double out-of-plane carboxylate bridge. The magnetic behavior of the compound can be explained as a result of magnetic exchange within the chain through long Cu–O axial bonds ( J = −0.73 cm −1) and a superexchange pathway through the hydrogen bond network between adjacent molecules ( zJ′ = −0.06 cm −1). The magnitude and the nature of the exchange coupling are explained on the basis of the structural data results.

Hydrogen-bonding patterns in trimethoprim picolinate and 2-amino-4,6-dimethylpyrimidinium picolinate hemihydrate

In the title compounds, namely 2,4-diamino-5-[(3,4,5-trimethoxyphenyl)methyl]pyrimidin-1-ium pyridine-2-carboxylate, C14H19N4O3+.C6H4NO2-, (I), and 2-amino-4,6-dimethylpyrimidin-1-ium pyridine-2-carboxylate hemihydrate, C6H10N3+.C6H4NO2-.0.5H2O, (II), the trimethoprim and 2-amino-4,6-dimethylpyrimidin-1-ium cations are protonated at one of the pyrimidine N atoms. In (I), bifurcated hydrogen bonds are observed between a picolinate O atom, the protonated N atom and the 2-amino group; the graph-set designator is R2(1)(6). The pyrimidine moieties of the trimethoprim cations are centrosymmetrically paired through a pair of N-H...N hydrogen bonds. In addition to the base pairing, one of the picolinate O atoms bridges the 2- and 4-amino groups on either side of the paired bases, resulting in a complementary DADA array. In (II), the carboxylate group of the picolinate anion binds with the protonated pyrimidine N atom and the 2-amino group of the pyrimidine moiety through a pair of N-H...O hydrogen bonds, leading to the common ring motif R2(2)(8). The water molecule, which resides on a twofold rotation axis, bridges the carboxylate group of the picolinate anion via O-H...O hydrogen bonds.

Reactions of cyanooxo-molybdates and -tungstates with picolinic acid, 2-pyridinecarboxaldehyde and 2,2′-pyridil: X-ray crystal structure of (PPh 4) 2[Mo(CN) 4O(pic)] · 2.5H 2O

The syntheses of (PPh 4) 2[M(CN) 3O(pic)] · nH 2O (M = Mo, W; n = 2.5, 4; pic = picolinate anion) as well as (PPh 4) 2[Mo(CN) 4O(apic)] · 3H 2O (apic = 2-pyridinecarboxaldehyde) are presented. The salts have been characterised by elemental analysis, IR and UV–Vis spectroscopy, cyclic voltammetry and X-ray crystal structure determination of (PPh 4) 2[Mo(CN) 3O(pic)] · 2.5H 2O. The anion in the latter salt is approximately octahedral with O and N donor atoms of pic situated in the trans and cis position to the MoO bond, respectively. The picolinate complexes have the characteristic MLCT bands in the visible spectra with the absorption maxima linearly dependent on the Reichardt’s E T solvent parameter. These complexes are shown to exhibit the strongest solvatochromic effect of all studied tricyanooxo and hexacyano complexes of Mo(IV) and W(IV). The obtained values of E 1/2 in these salts [0.333 V (Mo) and 0.018 V (W) in 1,2-C 2H 4Cl 2] were found to be the highest of all [M(CN) 3O(LL)] 2− type complexes making these salts show reversibility in a remarkable variety of solvents. The oxidation of coordinated apic to pic has been demonstrated by means of electronic spectroscopy. The isolated salts were used to monitor the reaction of [M(CN) 4O(H 2O)] 2− with 2,2′-pyridil in water–ethanol solution. It was found that 2,2′-pyridil disproportionate to Hpic and apic in the first step and the other decomposition products of 2,2′-pyridil postulated in the literature were not observed.

On the electroreduction mechanism of 2-pyridinecarboxylic (picolinic) acid on mercury electrodes

This paper presents polarographic (direct current, dc, and differential pulse, DP) and voltammetric (linear-sweep cyclic voltammetry) studies on the electroreduction of picolinic acid (2-pyridinecarboxylic acid) on mercury electrodes. The studies were performed in the acidity range 2.25 M H 2SO 4 to pH 11. Above the last pH value no signals were obtained. Kinetic parameters such as Tafel slopes and electrochemical reaction orders have been determined at the potentials corresponding to the foot of the waves. From polarographic, voltammetric and kinetic results, reaction pathways are proposed. The reduction of the species protonated at the heterocyclic nitrogen occurs through a process in which the rate-determining step (r.d.s.), is a protonation placed after two reversible two-electron transfers. In this reaction, proton-donors other than the H + ion can be involved. At pH>1, the recombination of the carboxylate anion with the H + precedes the reduction process. The reduction of picolinate anion, protonated at the heterocyclic nitrogen, consisted of the uptake of one electron and two H + ions, followed by an irreversible one-electron transfer, which is the r.d.s. In both cases, the hydrated aldehyde obtained is reduced only partially, due to the slow rate of the dehydration reaction. Above pH 5.3 the protonation of the pyridine ring precedes the reduction.

A dinuclear oxomolybdenum(V) compound with picolinate ligands

The title compound, di­aqua­di-μ2-oxo-dioxobis­(pyridine-2-carboxyl­ato-O,N)molybdenum(V) monohydrate, [Mo2O2(C6H4NO2)2(H2O)2]·H2O, is a dinuclear complex which consists of two MoV atoms connected together by a metal–metal single bond and two bridging oxo groups. Each MoV atom has a distorted octahedral geometry and is coordinated by three oxo groups, one O atom of the water mol­ecule, and the O and N atoms of one picolinate anion.