Doctoral Degrees (Chemistry)

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Browsing Doctoral Degrees (Chemistry) by Author "Conradie, Jeanet"

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    Chemical kinetics, electrochemistry and structural aspects of ferrocene-containing b-diketonato complexes of rhodium(I) and iridium(I)
    (University of the Free State, 1999) Conradie, Jeanet
    English: Synthetic routes to prepare new Rh(I)-β-diketonate complexes [Rh(FcCOCHCOR)(CO)2] and [Rh(FcCOCHCOR)(CO)(PPh3)] with Fc = ferrocenyl and R = Fc, C6H5, CH3 and CF3 have been developed and optimized. Optimized synthetic routes to iridium(I) complexes, [Ir(R'COCHCOR)(cod)], with R' = Fc and R = C6H5, CH3 and CF3, or with R' = CF3 and R = CH2CH3, CH(CH3)2 and C(CH3)3 have also been developed. 1H and 31P NMR studies indicated that for complexes of the type [Rh(β-diketonato)(CO)(PPh3)] with an unsymmetrical β-diketonato ligand, at least two main isomers exist in solution. The structure of one isomer of [Rh(fctfa)(CO)(PPh3)], as well as crystal structures of FcCOCH2COCF3, [Rh(fctfa)(CO)2] and [Rh(fctfa)(CO)(PPh3)(CH3)(I)] were solved. The chemical kinetics of the oxidative addition of iodomethane to [Rh(FcCOCHCOR)(CO)(PPh3)] has been studied utilizing IR, UV/visible, 1H NMR and 31P NMR techniques. The NMR studies revealed that the rate of oxidative addition of iodomethane to the different [Rh(FcCOCHCOR)(CO)(PPh3)] isomers was the same. A complete general reaction sequence for the oxidative addition of iodomethane to all [Rh(bidentate ligand)(CO)(PPh3)] complexes is: First set of reactionsSecond set of reactionsThird set of reactions{[Rh(III)-alkyl1] [Rh(III)-acyl1] }K2=k2/k-2Rh(I)+ CH3I k1k-1k3k-3k4k-4[Rh(III)-alkyl2][Rh(III)-acyl2] 1H and 31P NMR studies further showed that all rhodium-containing complexes in the above mentioned reaction scheme, are actually composed of at least two main isomers, that is Rh(I)A the rate of substitution becomes faster when the group electronegativity of the R groups increases. This tendency is, as expected, exactly the opposite to what was observed during oxidative addition. A general reaction mechanism for both Rh and Ir complexes was presented. An additional study on the rate of the β-diketonato substitution with 1,10-phenanthroline in complexes of the type [Ir(CF3COCHCOR)(cod)] with R = CH3, CH2CH3, CH(CH3)2 and C(CH3)3 showed that the size of R does not hamper the rate of substitution. All substitution reactions were independent of a solvent step. The cyclic voltammetry study of all the ferrocene-containing β-diketonato complexes of rhodium(I) and iridium(I) synthesized, exhibited a single electrochemically reversible redox couple corresponding to the formal reduction potential of the ferrocenyl group of the β-diketonato ligand coordinated to the rhodium or iridium complexes, as well as an electrochemically irreversible anodic oxidation peak which corresponds to the oxidation of the metal = Rh or Ir. The 31P NMR study on different six-membered chelate complexes, [Rh(L,L'-BID)(CO)(PPh3)], and related Rh(III) complexes, indicated a general decrease in coupling constants 1J(31P-103Rh) as the Rh-P bond length, determined by X-ray crystallography, increases according to the relationship d(Rh-P) = -0.0014(1) x 1J(31P-103Rh) + 2.49(2). The Rh-P bond lengths, d(Rh-P), varied between 2.23 Å and 2.36 Å. The electron density on the Rh(I) and Ir(I) metal centres was manipulated over a wide range by changing the R group on the coordinated ligand (FcCOCHCOR)- from the highly electron donating Fc group (χFc = 1.87) to C6H5, (χC6H5 = 2.21) to CH3(χCH3 = 2.34) to the strongly electron withdrawing CF3 group (χCF3 = 3.01). The effect of the different R groups on the β-diketonato ligand (FcCOCHCOR)- coordinated to the rhodium(I) and iridium(I) complexes was not only observed in kinetic rate constants, but also in formal reduction potentials of the ferrocenyl group, the oxidation potential of Rh(I) or Ir(I), pKa-values, IR stretching frequencies, and crystallographic bond lengths.