Photochemistry oxalate complexes

We focus initially on the photochemical behaviour of complexes of Fe(III) with simple carboxylic acids and give particular attention to oxalic acid. This compound is prevalent in atmospheric aerosols [28], provides a simple example of environmentally important light-mediated ligand-to-metal charge transfer (LMCT) processes which result in ligand decarboxylation [27] and is used to initiate the degradation of contaminants both in the absence and presence of added hydrogen peroxide (via the so-called modified photo-Fenton process [29,30]). In addition, the photochemistry of Fe(III)-oxalate complexes has been studied in detail, as it is the basis of... 

Our experimental results for the enantioselective magnetochiral photochemistry of the Cr(III) tris-oxalate complex are described in Ref. 28. These experiments were done under such conditions that the true magnetochiral effect should dominate the cascaded one. Therefore the first term on the right-hand side of Eq. (29) is expected to dominate. Here we will experimentally verify the validity of this expression. 

Figure 9 illustrates the complicated interacticMis of iron-oxalato complex photochemistry with radical chemistry and the chemistry of organic substances. The main impacts of iron complex photochemistry are ultimately (1) breaking of C-C bonds and thus degradation of the hgand (oxalate) and (2) formation of radicals... 

Photochemistry of Oxalate and Dithiooxalate Complexes of Nickel, Palladium, and Platinum... 

Photooxidation of coordinated oxalate has been known since the earliest studies of transition metal photochemistry (42). In these reactions oxalate ligand is photooxidized to CO2, and up to two metal centers are reduced by one electron (e.g. ferrioxalate). We wondered whether the oxalate ligand could be a two-electron photoreductant, by simultaneous or rapid sequential electron transfer, with metals prone to 2e redox processes. Application of this concept to l6e square planar d complexes, Equation 15, was attractive because it should produce solvated I4e metal complexes that are inorganic analogues of... 

We have chosen hematite oxalate as a model system, since the photochemical properties of colloidal hematite (Stramel and Thomas, 1986) and the photochemistry of iron(III) oxalato complexes in solution (Parker and Hatchard, 1959) have been studied extensively. The experiments presented in this section were carried out as batch experiments with monodispersed suspensions of hematite (diameter of the particles 50 and 100 nm), synthesized according to Penners and Koopal (1986) and checked by electron microscopy and X-ray diffraction. An experimental technique developed for the study of photoredox reactions with colloidal systems (Sulzberger, 1983) has been used. A pH of 3 was chosen to maximize the adsorption of oxalate at the hematite surface. This case study is described in detail by Siffert (1989) and Siffert et al. (manuscript in preparation). 

The two transition metal complexes, [Cr(ox)3]3" and [Cr(bpy)3]3+ (ox=ox-alate, bpy=2,2 -bipyridine) depicted in Fig. 3a are well known chromophores in transition metal photochemistry and photophysics. In the three-dimensional oxalate network structure of composition [Cr(bpy)3][NaCr(ox)3]C104, the two can be combined in an unique manner [16]. The sodium ions, in fact, serve as glue in such a way that each oxalate ligand serves as bridging... 

The most intensive studies of the photolysis of uranyl complexes of carboxylic acids have been conducted on the uranyl-oxalic acid system, particularly because of its use as a chemical actinometer. The overall photochemistry occuring in the uranyl oxalate actinometer solution is summarized by the following equations ... 

Ferrioxalate complexes are thought to hold a major portion of Fe(III) in atmospheric waters [167]. Although such complexes are widely used as chemical actinometers [206] and have been the subject of numerous experimental investigations [207-218], the exact primary step in ferrioxalate photochemistry is stiU controversial. Two different versions of the ferrioxalate reaction mechanism have been proposed following the excitation of the complex [219]. One possibility is an intramolecular electron transfer from the oxalate ligand to the center ion Fe(in) and the formation of a long lived radical complex (19) or the formation of a C204 radical (20) [213, 215] ... 

Figure 14 shows simulated concentration time profiles for the Fe(III) ligands pyruvate and oxalate, which have mostly lower concentrations during the daytime, when the photochemistry as described here is active. Thus, it has to be emphasized that Fe(III) complex photolysis reactions can be a major sink for the carboxylate species besides radical reactions, and it is crucial not to neglect these reactions when the fate of carboxylic acids in the atmospheric aqueous phase is considered. 

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