Stereochemistry as a mechanistic probe in photoaquation of chromium(III) complexes




Fernando, Sellapperumage

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The unifying theme of this dissertation is the use of stereochemistry as a probe of mechanism in Cr(lll) photosubstitution reactions. Mechanistic stereochemical change has been shown to be a requirement for axial photoaquation reactions of Cr(lll) complexes but the situation for equatorial ligand loss is less clear. To explore this, trans-[special characters omitted], (2,3,2-tet = N,N’-Bis(2-aminoethyl)-1,3-propanediamine) was prepared and characterized. It was photo-labile with a proton uptake quantum yield of 0.09 ± 0.01 in acidic aqueous solution. No detectable amount of CN- was photoreleased (Φcn- ≤ 0.02) and the major photoproduct was Cr(2,3,2- tetH) [special characters omitted]. In neutral or basic media this product exhibited rapid (t1/2 = 8 min in pH 6 at 25°C) thermal recoordination of the dangling amine ligand giving [special characters omitted]. HPLC study of the stereochemistry of this recoordination and the ligand field analysis of the visible spectrum showed that the photoproduct was trans-[special characters omitted]. Flash photolysis experiments with conductivity detection showed that the reaction goes completely via the doublet state. The thermal and emission properties were also investigated. In the [special characters omitted] lowest energy state of the molecule the ligand motion required for trans attack in the equatorial plane is obstructed by the 2,3,2-tet ligand. If stereochemical change is a requirement for substitution from this state it should therefore be photo-inert . The higher energy [special characters omitted] state could be photoactive but the photoproduct would be expected to be [special characters omitted]. Since the trans product was observed we conclude that mechanistic stereochemical change is not a requirement or photoaquation of [special characters omitted]. This complete stereoretentive photoreaction observed is unusual and is discussed in terms of existing theoretical models. For studies of wavelength dependence of products and their stereochemistry, the compound [special characters omitted], was prepared and characterized (tn = 1,3-diaminopropane). Both cyanide and tn of the molecule were photoaquated. The proton uptake measurements showed that the photolysis behavior was nonlinear owing to quenching of the photoreactant doublet state by photoproducts. The total product quantum yields were therefore based on the zero time slopes of the proton uptake data. Quantum yields are; (irradiation wavelength, nm): Φ(CN-), Φ(tnH+); 435: 0.035 ± 0.004, 0.048 ± 0.005; 456: 0.023 ± 0.004. 0.052 ± 0.004. Loss of cyanide is not predicted by photochemical theory and its occurrence is attributed to the role of ligand interactions in directing photoaquation modes. The quenching by photoproducts, however, made [special characters omitted] unsuitable for the wavelength dependence studies. Wavelength dependence of the product distribution in the prompt photoaquation of [special characters omitted] has previously been reported. The commonly used quenchers in such studies of excited state reactivity are [special characters omitted] and [special characters omitted]. They have not been suitable in all instances due to various problems such as reactive quenching, instability of compounds in base, precipitation of highly charged cationic complexes, significant absorption of visible light or solubility problems in aqueous solutions. We report the synthesis and characterization of [special characters omitted]. This compound showed the necessary thermo (t1/2 = 5 hours in pH 2 at 20°C) and photo (Φ tot = 0.04 ± 0.01) stability and was free from the above problems. The wavelength effect in [special characters omitted] was reinvestigated and the percentage [special characters omitted] product found was 38 ± 1% for the unquenched reaction and 47% for the prompt reaction, whether the quencher was [special characters omitted] or OH-. These percentages differ from the values reported previously and the implications of these new results are discussed.



Stereochemistry, Photochemistry