Carlo Bartocci

Photochemistry of iron-porphyrin complexes. Biomimetics and catalysis

Abstract An intramolecular electron transfer can occur in iron porphyrin complexes that are irradiated with light of suitable wavelength corresponding to axial ligand-to-metal charge transfer transitions. This process leads to the reduction of Fe(III) to Fe(II) and to the oxidation of the axial ligand to a radical species. The efficiency of the photoredox process is increased in the presence of species able of trapping the ferrous complex and/or the radical in competition with the back electron transfer process. Photoredox reactions of iron porphyrins and heme-proteins (cytochrome c) are examined in the framework of the fundamental role of electron transfer processes in biological systems. The products of the primary photoprocess can induce reactions leading to reduction or oxidation of various substrates with catalytic efficiency. This biomimetic aspect of the photoredox behavior of iron porphyrins is pointed out examining the results obtained in investigations concerning the iron-porphyrin based photoreduction of CCl4 in homogeneous solution, and photooxygenation of alkanes in both homogeneous and heteterogeneous systems.

Photo-oxidative cyanation of aromatics on semiconductor powder suspensions I: oxidation processes involving radical species

Abstract Illumination of platinized WO 3 and TiO 2 suspensions in CH 3 CN/H 2 O mixed solvents containing dimethoxybenzene (DMB) and cyanide leads to the formation of cyanoanisole together with side products. Laser flash photolysis of the transparent colloidal dispersions shows the formation of DMB +. . The amount of DMB + formed decreases in the presence of CN − , which indicates the competitive oxidation of DMB and Cn − rather than the reaction of DMB + and CN − . Using the electron spin resonance spin trapping technique we report for the first time evidence of CN and CH 2 CN radical formation in illuminated semiconductor suspensions. Use of dry CH 3 CN, DMB and (Bu) 4 + CN − decreases further the yield of cyanoanisole due to formation of a cyanated derivative of tributylamine which, in turn, originates from (Bu) 4 + after cyanide oxidation to CN .

Photoreduction of chlorohemin in pure pyridine

Abstract Chlorohemin (Fe(III)PPCl) undergoes photoreduction when irradiated in pure pyridine solution with 400–450 nm light. A thermal reduction is observed to occur simultaneously with the photochemical one, but after a one hour irradiation about 75% of the reduction product is formed in a photochemical way. Both five and six-coordinated species are observed to be present in solution; however, only the Fe(III)PPpy+ five coordinated complex is photoreducible. A mechanism is proposed whereby the primary photochemical act is an axial pyridine → iron electron transfer process yielding Fe(II)PP and py+ species. The Fe(II)PP moiety gives rise to the formation of the spectrophotometrically detectable Fe(II)(PP(py)2 complex. ESR spin trapping results are consistent with the formation of 2-pyridyl radicals from py+ cation by fast transfer of a proton to a pyridine molecule.

An ESR spin trapping investigation on the photoreduction of chlorohemin in mixed solvents

Abstract Radical scavenging by nitrone compounds is used in conjunction with ESR spectroscopy in order to obtain evidence for radical formation in the photoreduction of chlorohemin in aqueous alcoholic mixed solvents. The formation of adducts between hydrogen atoms and the spin traps appears to be the most important process in aerated and in pyridine-containing solutions. Evidence for the formation of hydroxyethyl adducts is obtained when the alcoholic component of the solvent is ethanol. In deaerated solutions not containing pyridine, no evidence is obtained for radical formation. The results are interpreted in terms of hydrogen abstraction by the nitrone from an alkoxy radical formed together with Fe(II) in the primary photoreduction. The abstraction process strongly competes with cage re-oxydation, as well as with the diffusion of the alkoxy radical in the solvent bulk where it would rapidly react to give hydroxy radicals. The role of pyridine and oxygen may respectively be stabilizing and oxidizing the Fe(II) species before it undergoes cage re-oxidation.

Intramolecular photoredox reactions in iron(III) cytochrome c and its azide derivative

Abstract The irradiation of deaerated solutions of horse heart cytochrome c causes the reduction of Fe(III) to Fe(II). The dependence of the photoreaction quantum yield on pH shows that the photoreactive species is a form of cytochrome c which contains methionine-80 and histidine-18 as heme ligands. The primary photochemical event consists of an electron transfer from the sulphur of methionine- 80 to iron. The re-oxidation of the photochemically obtained Fe(II) protein gives a Fe(III) cytochrome which exhibits a typical low-spin absorption spectrum, lacking the 695-nm band and indicating that a strong field ligand, other than methionine-80, coordinates to the sixth binding site of the heme iron. Spectrophotometric titration of the photochemically modified Fe(III) cytochrome shows that histidine- 18 remains bound in the fifth position. The substitution of methionine-80 with the more oxidizable azide ligand increases the efficiency of the intramolecular electron transfer. Azide radicals, detected by spin-trapping ESR technique, are formed in the primary act. Visible-UV spectral data indicate that histidine-18 and methionine-80 occupy the fifth and sixth position, respectively, in the photoreaction product. All the results obtained correlate well with those previously obtained in investigations concerning the photoredox behavior of iron porphyrin complexes.

Dimerization processes of cis-[Pt(NH3)2(H2O)2]2+ in aqueous solution

Abstract The oligomerization reactions of aqueous solutions of cis-[Pt(NH 3 ) 2 (H 2 O) 2 ] 2+ have been investigated by UV spectrophotometry in the pH 4–5 range. In these conditions, the dimer, di-μ-hydroxobis[diammineplatinum(II)] cation, is the only detectable reaction product. The dependence of the dimerization kinetics on pH and ionic strength indicates that dimer formation comes principally from cis-[Pt(NH 3 ) 2 (H 2 O)(OH)] + , with a minor, though not negligible, contribution from cis-[Pt(NH 3 2 (H 2 O) 2 ] 2+ .

Oxidative addition - reductive elimination sequences in the photochemistry of some bis(phosphine)platinum complexes

The photolysis of [L2Pt(C2H4)] (L = PPh3, P(p-C6H4CH3)3 complexes in halocarbon solvents (CH2Cl2, CH2Br2) gives C2H4 and the coordinatively unsaturated species [L2Pt]. Photolysis of platinum metallacycles [L2Pt(CH2)4] (L = PPh3, P(n-Bu)3) generates alkanes, alkenes and [L2Pt]. The [L2Pt] centers are very reactive, and under prolonged photolysis undergo oxidative addition of CH2Cl2 forming the trans-[L2Pt(CH2Cl)Cl] complexes. Under appropriately controlled conditions the trans complexes isomerize to cis species before bimolecular C2H4 elimination occurs and [L2PtCl2] is formed as the final product. The oxidative addition-reductive elimination mechanism is discussed on the basis of spin-trapping experiments, quantum yield values, and the sensitivity to radical inhibitors and to solvents.

Photochemistry of dimeric and trimeric hydroxo-bridged diammine platinum(II) complexes in aqueous solution

The irradiation of aerated aqueous solutions containing cis-Pt(NH3)2(H2O)2+2 with 254-nm light gives rise to the formation of a blue product identified as a mixed valence (Pt(II)/Pt(IV) = 2) hydroxo bridged trinuclear cationic complex [Pt3(NH3)4(OH)6]2+. The photoreactive species have been identified as the dimeric and trimeric hydroxo bridged platinum compounds that are present in solution and have the maximum concentration at pH 6.5. The reaction is initiated in a OH → Pt charge transfer excited state, to give a primary photoreaction intermediate which undergoes thermal air oxidation to give the final blue product.

Inorganic photophysics in solution. Part 5.—Bis(8-hydroxyquinolinato)platinum(II)—absorption, excited state absorption and emission spectra, kinetics and energetics

The spectroscopy and photophysics of bis(8-hydroxyquinolinato)platinum(II) have been examined from the following viewpoints: (i) the sharp dependence of the ground-state absorption peaks upon solvent (as indicated by a correlation of vmax with the parameter ET), (ii) the solvent-dependence of the luminescence lifetime (which is small), (iii) the temperature-dependence (and hence the energetics) of the non-radiative decay process from the luminescent state and (iv) the discovery of an intense excited-state absorption spectrum (λmax 565, 850 and 930 nm).

Photocatalytic reactions in the 2,3,7,8,12,13,17,18-octaethylporphyrinatoiron(III)–ethanol–carbon tetrachloride system

The photochemistry of ethanol solutions of 2,3,7,8,12,13,17,18-octaethylporphyrinatoiron(III), [Fe(oep)]+, has been investigated in the presence of CCl4 and of different amounts of oxygen. Continuous irradiation with 330–440 nm light induces an ethanol-to-oxygen electron transfer occurring with the assistance of an iron(III) porphyrin ethanolate complex. At a partial pressure of oxygen ranging from 0.1 to 1 Torr, the photoproducts, hydroxyethyl radical and hyperoxide ion O2– give rise to a catalytic process, with > 100 turnovers of [Fe(oep)]+, leading to the reduction of carbon tetrachloride to chloroform and chloride ions and with oxidation of ethanol to acetaldehyde. The rate of the photocatalytic process was observed to depend on the oxygen concentration, excitation wavelength, and the presence of a radical scavenger. A chain mechanism has been formulated involving radical species that can be quenched by oxygen to yield peroxyl radicals.