Thin-layer spectroelectrochemical study of tetrakis(4-N-methylpyridyl)porphinecobalt(III)

Cyanide Scavenging by a Cobalt Schiff-Base Macrocycle: A Cost-Effective Alternative to Corrinoids

The complex of cobalt(II) with the ligand 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]heptadeca-1(17)2,11,13,15-pentaene (CoN4[11.3.1]) has been shown to bind two molecules of cyanide in a cooperative fashion with an association constant of 2.7 (±0.2) × 10(5). In vivo, irrespective of whether it is initially administered as the Co(II) or Co(III) cation, EPR spectroscopic measurements on blood samples show that at physiological levels of reductant (principally ascorbate) CoN4[11.3.1] becomes quantitatively reduced to the Co(II) form. However, following addition of sodium cyanide, a dicyano Co(III) species is formed, both in blood and in buffered aqueous solution at neutral pH. In keeping with other cobalt-containing cyanide-scavenging macrocycles like cobinamide and cobalt(III) meso-tetra(4-N-methylpyridyl)porphine, we found that CoN4[11.3.1] exhibits rapid oxygen turnover in the presence of the physiological reductant ascorbate. This behavior could potentially render CoN4[11.3.1] cytotoxic and/or interfere with evaluations of the antidotal capability of the complex toward cyanide through respirometric measurements, particularly since cyanide rapidly inhibits this process, adding further complexity. A sublethal mouse model was used to assess the effectiveness of CoN4[11.3.1] as a potential cyanide antidote. The administration of CoN4[11.3.1] prophylactically to sodium cyanide-intoxicated mice resulted in the time required for the surviving animals to recover from "knockdown" (unconsciousness) being significantly decreased (3 ± 2 min) compared to that of the controls (22 ± 5 min). All observations are consistent with the demonstrated antidotal activity of CoN4[11.3.1] operating through a cyanide-scavenging mechanism, which is associated with a Co(II) → Co(III) oxidation of the cation. To test for postintoxication neuromuscular sequelae, the ability of mice to remain in position on a rotating cylinder (RotaRod test) was assessed during and after recovery. While intoxicated animals given CoN4[11.3.1] did recover ∼30 min more quickly than controls given only toxicant, there were no indications of longer-term problems in either group, as determined by continuing the RotaRod testing up to 24 h after the intoxications and routine behavioral observations for a further week.

Effect of Ascorbate on the Cyanide-Scavenging Capability of Cobalt(III) meso-Tetra(4-N-methylpyridyl)porphine Pentaiodide: Deactivation by Reduction?

The Co(III)-containing water-soluble metalloporphyrin cobalt(III) meso-tetra(4-N-methylpyridyl)porphine pentaiodide (Co(III)TMPyP) is a potential cyanide-scavenging agent. The rate of reduction of Co(III)TMPyP by ascorbate is facile enough that conversion to the Co(II)-containing Co(II)TMPyP should occur within minutes at prevailing in vivo levels of the reductant. It follows that any cyanide-decorporating capability of the metalloporphyrin should depend more on the cyanide-binding characteristics of Co(II)TMPyP than those of the administered form, Co(III)TMPyP. Addition of cyanide to buffered aqueous solutions of Co(II)TMPyP (pH 7.4, 25-37 °C) results in quite rapid (k2 = ∼10(3) M(-1) s(-1)) binding/substitution of cyanide anion in the two available axial positions with high affinity (K'β = 10(10) to 10(11)). Electron paramagnetic resonance spectroscopic measurements and cyclic voltammetry indicate that cyanide induces oxidation to the Co(III)-containing dicyano species. The constraints that these observations put on plausible mechanisms for the reaction of Co(II)TMPyP with cyanide are discussed. Experiments in which Co(III)TMPyP and cyanide were added to freshly drawn mouse blood showed the same sequence of reactions (metalloporphyrin reduction → cyanide binding/substitution → reoxidation) to occur. Therefore, in cyanide-scavenging applications with this metalloporphyrin, we should be taking advantage of both the improved rate of ligand substitution at Co(II) compared to that at Co(III) and the increased affinity of Co(III) for anionic ligands compared to that of Co(II). Finally, using an established sublethal mouse model for cyanide intoxication, Co(III)TMPyP, administered either 5 min before (prophylaxis) or 1 min after the toxicant, is shown to have very significant antidotal capability. Possible explanations for the results of a previous contradictory study, which failed to find any prophylactic effect of Co(III)TMPyP toward cyanide intoxication, are considered.

Hydrogen Peroxide as a Sustainable Energy Carrier: Electrocatalytic Production of Hydrogen Peroxide and the Fuel Cell

This review describes homogeneous and heterogeneous catalytic reduction of dioxygen with metal complexes focusing on the catalytic two-electron reduction of dioxygen to produce hydrogen peroxide. Whether two-electron reduction of dioxygen to produce hydrogen peroxide or four-electron O2-reduction to produce water occurs depends on the types of metals and ligands that are utilized. Those factors controlling the two processes are discussed in terms of metal-oxygen intermediates involved in the catalysis. Metal complexes acting as catalysts for selective two-electron reduction of oxygen can be utilized as metal complex-modified electrodes in the electrocatalytic reduction to produce hydrogen peroxide. Hydrogen peroxide thus produced can be used as a fuel in a hydrogen peroxide fuel cell. A hydrogen peroxide fuel cell can be operated with a one-compartment structure without a membrane, which is certainly more promising for the development of low-cost fuel cells as compared with two compartment hydrogen fuel cells that require membranes. Hydrogen peroxide is regarded as an environmentally benign energy carrier because it can be produced by the electrocatalytic two-electron reduction of O2, which is abundant in air, using solar cells; the hydrogen peroxide thus produced could then be readily stored and then used as needed to generate electricity through the use of hydrogen peroxide fuel cells.

Four quadruple metal–metal bonds lined up: linear nonachromium(ii) metal string complexes

Through a new pyrazine-modulated penta-pyridyl-tetraamine ligand, H(4)N(9)-mpz, linear nonachromium(II) complexes with four quadruple metal-metal bonds were successfully obtained, and their structure, magnetic and electrochemistry properties were studied.

Weak antiferromagnetic coupling for novel linear hexanuclear nickel(II) string complexes (Ni6 12+) and partial metal-metal bonds in their one-electron reduction products (Ni6 11+)

The preparation, crystal structures, magnetic properties and electrochemistry of novel linear hexanuclear nickel string complexes (Ni6(12+)) and their corresponding 1-e(-) reduction products (Ni6(11+)) are reported. In these complexes, the hexanickel chain is in a symmetrical arrangement (approximately D(4) symmetry) and is helically supported by four bpyany(2-) ligands [bpyany(2-) = the dianion of 2,7-bis(alpha-pyridylamino)-1,8-naphthyridine]. The Ni6(12+) complexes show that the two terminal nickel ions have high-spin states (S = 1) and the four inner ones have low-spin states (S = 0). The two terminal nickel ions exhibit weak antiferromagnetic coupling of ca.-5 cm(-1). All of Ni6(12+) complexes display three reversible redox couples at about -0.70, -0.20 and +1.10 V (vs. Ag/AgCl). The first reduction wave at about -0.20 V suggests facility of 1-e(-) reduction for the Ni(6)(12+) compounds. The reaction of Ni(6)(12+) complexes with hydrazine afforded the 1-e(-) reduction products (Ni6(11+)). As far as we are aware, the shortest bond distance of 2.202 A with a partial metal-metal bond was observed in Ni6(11+) compounds. The magnetic results of these Ni6(11+) compounds are in agreement with a localized model, in which the two terminal nickel ions are in a spin state of S = 1 whereas the central Ni3-Ni4 pair in a spin state of S = 1/2. The N6(11+) compounds show relatively strong antiferromagnetic coupling of about 60 cm(-1) between the terminal and the central dinickel ions.

Novel linear hexanuclear cobalt string complexes (Co6(12+)) and one-electron reduction products (Co6(11+)) supported by four bpyany2- ligands

The new ligand, 2,7-bis(alpha-pyridylamino)-1,8-naphthyridine (H2bpyany), was synthesized by the reaction of 2,7-dichloro-1,8-naphthyridine with 2-aminopyridine in the presence of t-BuOK under palladium(0)-catalyzed conditions. The preparation and characterization of novel hexacobalt string complexes, [Co6(mu6-bpyany)4(NCS)2](PF6)n (n=1 (1); n=2 (2)) and [Co6(mu6-bpyany)4(OTf)2](OTf)n (n = 2 (3); n = 1 (4)) are presented. The crystal structures for compounds have been determined by X-ray crystallography. Compounds 1 and 4 have the Co6 11+ configurations and are air-stable. Compounds 2 and 3 with Co6 12+ configurations are structurally similar to 1 and 4, respectively. The electrochemistry of 1 displays four redox couples at E1/2= -0.55, +0.38, +0.91, and +1.18 V (vs. Ag/AgCl). The magnetic data show that compounds 1 and 4 are in a spin state of S = 1/2, and 2 and 3 in a spin state of S = 1. The results of the EHMO calculations on compounds 1 and 2 are in agreement with their magnetic measurements.

Control of magnetic spin states by various mixed anionic ligands in trinickel complexes: synthesis, crystal structures and physical properties

This study provides an opportunity to control the magnetic spin of nickel atoms using various mixed anionic ligands. A series of linear trinickel complexes supported by two kinds of ligands, oligo-alpha-pyridylamido and sulfonyl amido/amido, were synthesized and their structures were determined by X-ray diffraction. The three nickel atoms of [Ni(3)(Lpts)(2)(dpa)(2)] (dpa(-) = dipyridylamido, Lpts(2-) = N,N'-bis(p-toluenesulfonyl)pyridyldiamido) display short Ni-N ( approximately 1.90 Angstrom) bond distances, which are consistent with a low spin state of Ni(II) ions, and exhibit spin states of (0, 0, 0) for the three Ni(II) ions. One of the terminal Ni(II) ions of [Ni(3)(Lms)(2)(dpa)(2)(H(2)O)] (Lms(2-) = N,N'-bis(4-methylsulfonyl)-pyridyldiamido) and [Ni(3)(Lpts)(2)(pepteaH(2))] (pepteaH(2)(2-) = pentapyridyldiamidodiamine) bonded with an axial ligand exhibits a square pyramidal (NiN(4)X) geometry with long Ni-N bond distances ( approximately 2.10 Angstrom) which are consistent with a high spin Ni(II) configuration. The spin states of these trinickel complexes are (1, 0, 0). Complexes interchanged by the removal or addition of an axial water molecule. The structural features of are comparable with those of . Both the terminal Ni(II) ions in [Ni(3)(LAc)(2)(dpa)(2)] (Lac(2-) = N,N'-biacetyl-pyridyldiamido) are in square pyramidal geometry and exhibit high spin. The spin states of the nickel ions in are (1, 0, 1), and the two terminal nickel ions exhibit antiferromagnetic interactions. The molecular structure of [Ni(3)(Lpts)(2)(dpa)(2)](BF(4)), which was obtained by the one-electron oxidation is similar to those of the neutral analogue , except for the presence of a counter anion to compensate for the positive charge on the Ni(3) core. All of the Ni-Ni bond lengths of are slightly shorter (ca. 0.05 Angstrom) than those in the neutral analogues. This is attributed to the formation of partial Ni-Ni bonding.

Synthesis, structures, magnetism and electrochemical properties of triruthenium–acetylide complexes

A series of triruthenium complexes with arylacetylide axial ligands Ru(3)(dpa)(4)(C(2)X)(2)(BF(4))(y)(dpa = dipyridylamido; X = Fc, y= 0 (1); X = Ph, y= 0 (2); X = PhOCH(3), y= 1 (3); X = PhC(5)H(11), y= 1 (4); X = PhCN, y= 0 (5); X = PhNO(2), y= 0 (6)) have been synthesized. The crystal structures show that the Ru-Ru bond lengths (2.3304(9)-2.3572(5)A) of these compounds are longer than those of Ru(3)(dpa)(4)Cl(2)(Ru-Ru=2.2537(1)A). This is ascribed to the formation of the stronger pi-backbonding from metal to axial ligand which weakens the Ru-Ru interactions and the bond order is reduced in the triruthenium unit. Cyclic voltammetry and differential pulse voltammetry show that compound exhibits electronic coupling between the two ferrocenyl units with DeltaE(1/2) close to 100 mV. Compounds 2-6 display three triruthenium-based reversible one-electron redox couples, two oxidations and one reduction, and the electrode potentials shift upon varying the substituents. A linear relationship is observed when the Hammett constants are plotted against the redox potentials.

Alkyl- and aryl-substituted corroles. 4. Solvent effects on the electrochemical and spectral properties of cobalt corroles

Solvent effects on the electrochemistry and spectroscopic properties of alkyl- and aryl-substituted corroles in nonaqueous media are reported. The oxidation and reduction of six compounds containing zero to seven phenyl or substituted phenyl groups on the macrocycle were studied in four different nonaqueous solvents (CH(2)Cl(2), PhCN, THF, and pyridine) containing 0.1 M tetra-n-butylammonium perchlorate. Dimers were formed upon oxidation of all corroles in CH(2)Cl(2), but this was not the case in the other three solvents, where either monomers or dimers were formed upon oxidation depending upon the solvent Gutmann donor number and the number or location of aryl substituents on the macrocycle. The half-wave potentials were analyzed as a function of the number of aryl substituents on the macrocycle as well as the concentration of added pyridine to PhCN solutions of the compound, and these data were combined with data from the spectroelectrochemistry experiments to determine the stoichiometry of the species actually in solution after the first oxidation or first reduction of each compound. The results of these experiments indicate that reduction of the bispyridine adduct (Cor)Co(III)(py)(2) proceeds via the monopyridine complex (Cor)Co(III)(py) to give in each case the unligated cobalt(II) corrole [(Cor)Co(II)](-). In contrast, pyridine remains coordinated after electrooxidation, and the final product was characterized as [(Cor)Co(III)(py)(2)](+).

Metalloporphyrins are potent inhibitors of lipid peroxidation

The objectives of these studies were to determine whether metalloporphyrins could inhibit lipid peroxidation, characterize factors that influence their potency and compare their potency to prototypical antioxidants. Lipid peroxidation was initiated with iron and ascorbate in rat brain homogenates and the formation of thiobarbituric acid reactive species was used as an index of lipid peroxidation. Metalloporphyrins were found to be a novel and potent class of lipid peroxidation inhibitors. Inhibition of lipid peroxidation by metalloporphyrins was dependent on the transition metal ligated to the porphyrin, indicating that metal centered redox chemistry was important to the mechanism of their antioxidant activities. Manganese porphyrins with the highest superoxide dismutase (SOD) activities, MnOBTM-4-PyP and MnTM-2-PyP (charges are omitted throughout text for clarity), were the most potent inhibitors of lipid peroxidation with calculated IC50s of 1.3 and 1.0 microM, respectively. These manganese porphyrins were 2 orders of magnitude more potent than either trolox (IC50 = 204 microM) or rutin (IC50 = 112 microM). The potencies of the manganese porphyrins were related not only to their redox potentials and SOD activities, but also to other factors that may contribute to their ability to act as electron acceptors. The broad array of antioxidant activities possessed by metalloporphyrins make them attractive therapeutic agents in disease states that involve the overproduction of reactive oxygen species.

Reduction reactions of water soluble cyano-cobalt(III)-porphyrins: metal versus ligand centered processes

Reduction reactions of dicyano-cobalt(III)-porphyrins [potential in vivo cyanide scavenger drugs] were studied by radiolytic and electrochemical methods using the water soluble tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP). For [(CN)2CoIIITPPS]-, reduction occurs stepwise to the CoII, CoI, and finally to the phlorin anion. This behavior is similar to that of the cobalt porphyrins in the absence of cyanide, except that the cyanide ligand shifts the reduction potentials to much more negative values. On the other hand, under radiolytic conditions, [(CN)2CoIIITMPyP]- is reduced on the porphyrin macrocycle by one electron to give the CoIII pi-radical anion, which disproportionates into the initial complex and the two-electron ring reduced CoIII phlorin. The radical anion is also formed by intramolecular electron transfer subsequent to the reaction of CoIITMPyP and cyanide. The results are compared with the chemistry of Vitamin B-12.

Chemical properties of water-soluble porphyrins 3. The reaction of superoxide radicals with some metalloporphyrins

The catalytic efficiency of some water soluble metalloporphyrins upon the disproportionation of the superoxide radicals has been determined by observing the rate of decay of the radical at 254 nm. The Fe(III), Mn(III), Co(III), Ni(II), Cu(II) and Zn(II) derivatives of tetrakis-(4-N-methylpyridyl) porphyrin (TMPyP), tetra(4-N,N,N-trimethylanilinium) porphyrin (TAP), and tetra(4-sulphonatophenyl) porphyrin (TPPS4) were studied. The order of catalytic efficiency found was Fe(III)TMPyP much greater than Mn(III)TMPyP greater than Co(III)TMPyP approximately equal to Mn(TAP) greater than Fe(III)TPPS4. Other metalloporphyrins did not show catalytic activity. Reduction potentials of the catalytically active metalloporphyrins determined from cyclic voltamograms indicate the importance of having a second energetically accessible oxidation state available to the metal ion for catalysis. The catalytic action of the above metalloporphyrins is discussed in terms of inner and outer-sphere mechanisms. It is suggested that with the exception of Fe(III)TMPyP which reacts via an inner-sphere mechanism, the others are governed by an outer-sphere mechanism.

Catalysis of the disproportionation of superoxide by metalloporphyrins

The efficiencies of several metalloporphyrin complexes at catalyzing the disproportionation of superoxide have been determined at pH 10 in both carbonate and borate buffer systems. Catalytic rate constants were obtained for the iron(III) and cobalt(III) derivatives of tetrakis(4-N-methylpyridyl) porphine, for tetraphenylporphinesulfonatoferrate(III) and for hemin. In addition, the effects of added bovine serum albumin and imidazole were studied. The order of catalytic efficiency is FeTMpyP greater than FeTMpyP(Im)2 greater than FeTPPS(Im)2 approximately FeTPPS approximately FeTPPS.BSA approximately Fe(EDTA) greater than or approximately CoTMpyP greater than hemin(Im)2 greater than or approximately hemin.