Pulse radiolysis metal complexes

The capacity of cyclic ligands to stabilize less-common oxidation states of a coordinated metal ion has been well-documented. For example, both the high-spin and low-spin Ni(n) complexes of cyclam are oxidized more readily to Ni(m) species than are corresponding open-chain complexes. Chemical, electrochemical, pulse radiolysis and flash photolysis techniques have all been used to effect redox changes in particular complexes (Haines McAuley, 1982) however the major emphasis has been given to electrochemical studies. 

Many transition metal complexes have been considered as synzymes for superoxide anion dismutation and activity as SOD mimics. The stability and toxicity of any metal complex intended for pharmaceutical application is of paramount concern, and the complex must also be determined to be truly catalytic for superoxide ion dismutation. Because the catalytic activity of SOD1, for instance, is essentially diffusion-controlled with rates of 2 x 1 () M 1 s 1, fast analytic techniques must be used to directly measure the decay of superoxide anion in testing complexes as SOD mimics. One needs to distinguish between the uncatalyzed stoichiometric decay of the superoxide anion (second-order kinetic behavior) and true catalytic SOD dismutation (first-order behavior with [O ] [synzyme] and many turnovers of SOD mimic catalytic behavior). Indirect detection methods such as those in which a steady-state concentration of superoxide anion is generated from a xanthine/xanthine oxidase system will not measure catalytic synzyme behavior but instead will evaluate the potential SOD mimic as a stoichiometric superoxide scavenger. Two methodologies, stopped-flow kinetic analysis and pulse radiolysis, are fast methods that will measure SOD mimic catalytic behavior. These methods are briefly described in reference 11 and in Section 3.7.2 of Chapter 3. 

Properties of Transition Metal Complexes with Metal-Carbon Bonds in Aqueous Solutions as Studied by Pulse Radiolysis... 

A number of rate constants for reactions of transients derived from the reduction of metal ions and metal complexes were determined by pulse radiolysis [58]. Because of the shortlived character of atoms and oligomers, the determination of their redox potential is possible only by kinetic methods using pulse radiolysis. In the couple Mj/M , the reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. These reactions obviously occur in competition with the cascade of coalescence processes. The unknown potential °(M /M ) is derived by comparing the action of several reference systems of different potentials. 

When the irradiation of metal ion solutions is performed in the presence of the ligands CO or PPhs, metal reduction, ligandation, and aggregation reactions compete, leading to reduced metal complexes and then to stable molecular clusters, such as Chini clusters Pt3(CO)6] with m = 3-10 (i.e., 9-30 Pt atoms) [97], or other metal clusters [98]. The synthesis is selective and m is controlled by adjusting the dose m decreases at high doses). The mechanism of the reduction has been determined recently by pulse radiolysis [99]. Molecular clusters [Pt3(CO)6]5 have been observed by STM [100]. 

The time-resolved studies of the cluster formation achieved by pulse radiolysis techniques allow one to better understand the main kinetic factors which affect the final cluster size found, not only in the radiolytic method but also in other reduction (chemical or photochemical) techniques. Generally, reducing chemical agents are thermodynamically unable to reduce directly metal ions into atoms (Section 20.4) unless they are complexed or adsorbed on walls or dust particles. Therefore, we explain the higher sizes and the broad dispersity obtained in this case by in situ reduction on fewer sites. A classic... 

PROPERTIES OF TRANSITION METAL COMPLEXES WITH METAL-CARBON BONDS IN AQUEOUS SOLUTIONS AS STUDIED BY PULSE RADIOLYSIS... 

Redox reactions involving the nickel(IV) complex are also subject to divalent metal ion catalysis (170, 171). Oxidations of the two-electron reductant ascorbate (40) and the one-electron reductant [Fe(CN)6]4-(172) have been examined in some detail. Both reactions have as the rate-determining step the transfer of one electron from the reductant to nickel(IV) in an outer-sphere process to give an undetected nickel(III) transient. Spectroscopic properties of the nickel(III) species have been determined by pulse radiolysis (41). 

There is an extensive chemistry of the nickel(I) ion generated by pulse radiolysis which is beyond the scope of this review. Complexes with saturated amines such as 1,2-diaminoethane have been studied by this method and by the y radiolysis of aqueous glasses, but the species formed have no more than a transient existence. The imine ligands phen and bpy offer a more attractive environment for nickel(I) by allowing electron delocalization over the ligand n system (178,179). A number of complexes of these ligands have been reported in y-radiolysis studies. The EPR spectra indicate that reduction is primarily metal centered with a significant orbital contribution. Electrochemical reduction of [NiH(bpy)3]2+ in anhydrous acetonitrile results in [Ni (bpy)3] +, which can be detected by EPR methods. The reduction potential is reported to be —1.55 V but the complex is thermodynamically unstable with... 

Buxton GV, Mulazzani QG, Ross AB (1995) Critical review of rate constants for reactions of transients from metal ions and metal complexes in aqueous solutions. J Phys Ref Data 24 1035-1349 Cheek CH, Swinnerton JW (1964) The radiation-induced chain reaction between nitrous oxide and hydrogen in aqueous solutions. J Phys Chem 68 1429-1432 Christensen H, Sehested K (1980) Pulse radiolysis at high temperatures and high pressures. Radiat Phys Chem 16 183-186... 

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