Electron transfer reactions with metal-porphyrin

H. Bruhn, J. Westerhausen, J.F. Holzwarth, and J.H. Fuhrop COMBINED STOPPED-FLOW/CONTINUOUS-FLOW ARRANGEMENT FOR KINETIC MEASUREMENTS IN THE SECOND TO MICROSECOND RANGE INVESTIGATION OF ELECTRON-TRANSFER REACTIONS WITH METAL-PORPHYRIN COMPLEXES... 

There are several excellent photosensitizers one of them is [Ru(bpy)3]2+ [6]. There are two optical isomers in this complex one is A [Ru(bpy)3]2+ and the other is A-[Ru(bpy)3]2 +, as shown in Scheme 1. Thus one can expect to perform the stereoselective electron transfer reaction with A- and A-[Ru(bpy)3]2 +. Unfortunately, however, the racemization of [Ru(bpy)3]2+ is induced photochemically [7]. The reasonable way to suppress the photoracemiza-tion of this complex is to introduce the optically active organic functional group into the transition metal complexes, as will be discussed in Sec. II.B. The other photosensitizer that is useful for the photoinduced electron transfer reaction is the copper(I) complexes with 1,10-phenanthroline and their derivatives [8,9]. Zinc(II) porphyrin is also an excellent photosensitizer for photoinduced electron transfer reaction [10]. In these complexes, molecular chirality does not exist, unlike in [Ru(bpy)3]2 +. Thus one must introduce some chiral functional group into these compounds, to use these complexes as chiral photosensitizers. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

Redox equilibrium of Ag(I I [-porphyrin /Ag(III) is characterized with = 0.59 V versus SCE [412]. Evidently, corroles and carbaporphyrins are able to stabilize the Ag(III) oxidation state, presumably due to the presence of 7r-electron donors, which reduce the formal oxidation state of the metal in such complex [396]. It is expected that such complexes have potential practical applications, for example, as the catalysts in the electron-transfer reactions. 

The model of electron transfer in gas-phase metal-molecule reactions can be extended to more complex systems such as the collisions of metastable rare gas atoms with molecules to produce negative molecular ions [306], In surface chemistry the harpoon model describes the forces between the reagents after the electron transfer has been applied to reactions of molecules with metal surfaces [120]. Another domain, involving the reaction of metal ions with complex systems could be interpreted in the framework of electron transfers in the porphyrin site of the heme within hemoglobin, addition of oxygen to the Fe " " results in an electron transfer from the metal to the oxygen. The dynamics of this attachement and of the photo-induced detachment could be viewed in that perspective. 

The CV curves obtained for carbons with preadsorbed copper shown in Figs. 45 (curves b, b, c, c ) and 46 (a-a")) exhibit only slight peaks of the Cu(II)/Cu(I) couple and broad waves due to the redox reaction of surface carbon functionalities (.see Section IV). However, preadsorbed copper enhances the peaks of the redox process in bulk solution (especially the anodic peaks for D—H and D—Ox samples), as can be seen in Fig. 46 (curves c-c"). The low electrochemical activity of samples with preadsorbed copper species observed in neutral solution is the result of partial desorption (ion exchange with Na ) of copper as well as the formation of an imperfect metalic layer (microcrystallites). Deactivation of the carbon electrode as a result of spontaneous reduction of metal ions (silver) was observed earlier [279,280]. The increase in anodic peaks for D—H and D—Ox modified samples with preadsorbed copper suggests that in spite of electrochemical inactivity, the surface copper species facilitate electron transfer reactions between the carbon electrode and the ionic form at the electrode-solution interface. The fact that the electrochemical activity of the D—N sample is lowest indicates the formation of strong complexes between ad.sorbed cations and surface nitrogen-containing functionalities (similar to porphyrin) [281]. Between —0.35 V and -1-0.80 V, copper (II) in the porphyrin complex (carbon electrode modifier) is not reduced, so there can be no reoxidation peak of copper (0) [281]. 

Porphyrin-nitrosyl complexes with six other metal ions are also known, and all but one of which has been electrochemically investigated. These are Ru [69, 73, 94-96], Os [5], Rh[97], Cr [98], Mo [99] and Mn [100]. Some nitrosyl metalloporphyrins can be reversibly reduced or oxidized by one or two electrons without loss of the NO ligand and this generally occurs when the electrode reactions involve the 7T-conjugated macrocycle in the case of a metal-centered reduction or oxidation, however, the electron-transfer reactions will most often be accompanied by a loss of the NO ligand, resulting in an irreversible oxidation as shown in Fig. 7 for the case of (TPP)Cr(NO) and (TPP)Mn(NO) in CH2CI2. 

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