Vjacheslav P. Grivin

New Insight into Photochemistry of Ferrioxalate

Optical spectroscopy and nanosecond flash photolysis (Nd:YAG laser, 355 nm, pulse duration 5 ns, mean energy 5 mJ/pulse) were used to study the photochemistry of Fe(III)(C2O4)3(3-) complex in aqueous solutions. The main photochemical process was found to be intramolecular electron transfer from the ligand to Fe(III) ion with formation of a primary radical complex [(C2O4)2Fe(II)(C2O4(*))](3-). The yield of radical species (i.e., CO2(*-) and C2O4(*-)) was found to be less than 6% of Fe(III)(C2O4)3(3-) disappeared after flash. [(C2O4)2Fe(II)(C2O4(*))](3-) dissociates reversibly into oxalate ion and a secondary radical complex, [(C2O4)Fe(II)(C2O4(*))](-). The latter reacts with the initial complex and dissociates to Fe(II)(C2O4) and oxalate radical. In this framework, the absorption spectra and rate constants of the reactions of all intermediates were determined.

Intermediates formed by laser flash photolysis of [PtCl6]2− in aqueous solutions

The stationary photolysis of [PtCl(6)](2-) in aqueous solutions (10(-5)-10(-4) M) at the region of 313 nm leads to its photoaquation with a quantum yield of 0.19. Laser flash photolysis experiments (308 nm) provided evidence of the formation of Pt(iii) intermediates, namely [PtCl(4)(OH)(H(2)O)](2-) and [PtCl(4)](-), and Cl(2) (-) radical anions. The Pt(iii) complexes formed as a result of an intrasphere electron transfer from Cl(-) ligands to the excited Pt(iv) ion. However, the main ( approximately 90%) photolysis channel was not accompanied by the transfer of Cl atoms to the solvent bulk. The photoaquation of [PtCl(6)](2-) results from the back electron transfer in the secondary geminate pair, [PtCl(5)(H(2)O)](2-)-Cl. The relative yield of Pt(iii) intermediates, recorded after the completion of all processes in the geminate pair, was less than 10% of the number of disappearing initial [PtCl(6)](2-) complexes.

Redox processes in photochemistry of Pt( iv ) hexahaloid complexes

Ultrafast pump–probe spectroscopy (λpump = 405 nm) was applied to study the primary photochemical processes for PtCl62− and PtBr62− complexes in aqueous and alcohol solutions. For PtCl62−, an intermediate with a lifetime of ca. 200 ps was registered and identified as an Adamson radical pair [PtIIICl52−⋯Cl˙]. The transformations of the primary intermediate give rise to successive formation of different Pt(III) species. The reactions of Pt(III) results in chain photoaquation in aqueous solutions and reduction of Pt(IV) to Pt(II) in alcohol solutions. For PtBr62− complex, the previously reported (I. L. Zheldakov, M. N. Ryazantsev and A. N. Tarnovsky, J. Phys. Chem. Lett., 2011, 2, 1540; I. L. Zheldakov, PhD thesis, Bowling Green State University, 2010) formation of active 3PtBr5− intermediate is followed by very fast (15 ps) aquation of Pt(IV) in aqueous solutions and parallel reactions of solvation and reduction of Pt(IV) to Pt(II) in alcohol solutions. All the processes in alcohols are finished within 0.5 ns. The data of ultrafast experiments are supported by nanosecond laser flash photolysis and stationary photolysis.

Optical spectroscopy of perfluorothiophenyl, perfluorothionaphthyl, xanthate and dithiophosphinate radicals

Laser flash photolysis has been used to record the optical spectra of sulfur-containing radicals forming from photodissoci- ation of diphenyl disulfide, perfluorodiphenyl disulfide, perfluoro-2,2 X -dinaphthyl disulfide, diisopropyldixantogene and . bis diisobutylthio-phosphoryl-disulfane . The extinction coefficients of absorption bands were determined from the reaction of S-radicals with a stable nitroxyl radical. The rate constant of this reaction was for all radicals close to 10 9 M y1 s y1 and successfully competes with the reaction of recombination. The presence of a narrow and strong absorption band in the optical spectrum of a nitroxyl radical and the absence of absorption in the reaction products allows one to accurately determine the extinction coefficients of the absorption bands of S-radicals. q 2000 Published by Elsevier Science B.V.

Photochemistry of IrCl62− complex in alcohol solutions

Abstract Both stationary and laser flash photolysis were used to study the photochemistry of IrCl 6 2− in alcohol solutions. The primary photochemical photolysis process at 308 nm was demonstrated to involve electron transfer from the solvent to the excited complex. When the photolysis was performed at 248 nm, the photodissociation of the excited complex, with the elimination of a chlorine atom from the coordination sphere of iridium, was accompanied by electron transfer. At room temperature, the rate constants of the reaction between the primary alcohol radicals and the IrCl 6 2− complex were determined to be (3.2±0.1) × 10 9 M −1 s −1 for hydroxymethyl and (2.3±0.1) × 10 9 M −1 s −1 for α-hydroxyethyl radicals. The dark reactions of the IrCl 6 2− complex arising from photolysis are discussed.

Chain processes in the photochemistry of PtIV halide complexes in aqueous solutions

The mechanisms of the photoaquation of PtCl62− and PtBr62− complexes were compared by the experimental results on stationary photolysis, nanosecond laser flash photolysis, and ultrafast pump-probe spectroscopy. The formation of the photoaquation product of the bromide complex, viz., PtBr5(H2O)−, was shown to proceed via the mechanism of heterolytic cleavage of the Pt-Br bond, and the platinum cation remained tetravalent in the course of the whole process. For the chloride complex, the Pt-Cl bond cleavage was found to be homolytic, and precursors of the photoaquation product, viz., PtCl5(H2O)− complex, are intermediates of trivalent platinum sequentially transforming into each other. The reactions of these intermediates determine the chain character of the photoaquation process.

Mechanism of Br2·− and Cl2·− radical anions formation upon IrCl62− photoreduction in methanol solutions containing free Br− and Cl− ions

Abstract Laser flash photolysis has been used to study the formation of X 2 ·− radical ions (X = Br and Cl) upon photoreduction of IrCl 6 2− complex in methanol solutions, containing free X − ions. It has been shown that two channels of halogen atom formation are possible. These are the reactions Of X − ions with either the primary radical cation CH 3 OH ·+ which results from single electron transfer from a methanol molecule to the excited complex or the intermediate, arising from transformation of primary light-excited IrCl 6 2− complex.

Photoreduction of IrCl62− complex in alcohol solutions and its reaction with hydroxyalkyl radicals

The photochemistry of IrCl62− complex in simple alcohols have been studied using laser-flash photolysis. Single electron transfer from the solvent molecule to the light-excited complex has been shown to be the primary photochemical process. Quantum yields of the photoreduction of IrCl62− complex and the rate constants of its reaction with hydroxyalkyl radicals were determined at 200–330 K. Deviations of the rate constants from Debye–Smoluchowski equation for diffusion-controlled reactions are discussed.

Paramagnetic intermediates in the photolysis of 7-silanorbornadiene studied by means of spin chemistry method

Abstract The influence of an external magnetic field on the yield of photo-decomposition products of 7,7′-dimethyl-7-silanorbornadiene derivative has been detected in laser pulse photolysis experiments. The observations of magnetic field effects alterations in the presence of scavengers, O 2 and PPh 3 , in combination with 1 H-CIDNP data form the basis for the identification of the structure of paramagnetic intermediates involved in the process. It has been shown that magnetic field effects originate in biradical intermediates. These species result from both endocyclic SiC bond cleavage in the initial compound and the reaction of dimethylsilylene (in a singlet or a triplet excited state) with starting 7-silanorbornadiene. To explain the influence of O 2 upon the magnetic field effects, the reversible formation of oxygen complex with biradical species has been suggested.

Photochemistry of dithiocarbamate complexes of Ni(IV). I. Processes of photoreduction of complexes

Abstract Laser flash photolysis and stationary photolysis of liquid solutions and frozen methanol matrices were used to demonstrate that the photochemical transformations of dithiocarbamate complexes of Ni(IV) occur within the coordination sphere. The spectral and kinetic characteristics of the intermediates were established; the kinetic parameters were determined for the primary process of NiS bond cleavage and further stages.