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The coordination chemistry of ruthenium porphyrin complexes

University of British Columbia

This thesis work reports developments in the coordination chemistry of ruthenium porphyrin complexes, both in terms of the synthesis and chemistry of new compounds, as well as the study of the solution chemistry of some previously reported complexes. The synthesis, characterization and chemistry of ten new Ru(porp) coordination complexes in the oxidation states Ru[superscript]Ⅲ and Ru[superscript]Ⅳ containing halide (Br, CI) and other axial ligands (pyridine, CH₃CN, NH₃ and SbF₆) are described in this thesis. Some additional ten Ru(porp) complexes have been studied in situ. Measurement of the rate constants for forward and reverse reactions and the corresponding equilibrium constant by 'H NMR and UV/visible spectroscopy for the dissociation of PPh₃ ligand from Ru(OEP)L(PPh₃) (OEP is the octaethylporphyrinato dianion; L = CO, PPh₃) in C₇D₈ to generate the previously reported five-coordinate Ru(OEP)L complexes allowed for an estimation of the Ru-P bond strength (64 ± 9 kJ mol⁻¹) in these complexes. A study of PPh₃ dissociation from Ru(OEP)CO(PPh₃) in C₇D₈ and in CDC1₃ indicates that solvation effects play a major role, with CDC1₃ being more capable than C₇D₈ of solvating the Ru(OEP)CO complex. The presence of trace H₂0 in these systems was a major problem, and the coordination of H₂0 to Ru(OEP)L complexes to generate the in situ Ru(OEP)L(H₂0) complexes (L = CO, PPh₃) is described. The formation of Ru(OEP)L(H₂0) and the observed difference in the solvation of Ru(OEP)CO by C₇H₈ and CHC1₃ indicate that truly Five-coordinate species may not exist in solution. The outer-sphere oxidation of Ru [superscript]Ⅳ(OEP)PPh₃ by 0₂ to give [Ru [superscript]Ⅳ(OEP)OH]₂0 was shown to occur only in the presence of H₂0. Mechanistic studies on the previously reported reaction of HCI with [Ru(OEP)]₂ to generate Ru^(OEP)Cl₂ (C. Sishta, M.Sc.Thesis, University of British Columbia, 1986) show that solvent plays a major role in directing this oxidation reaction. A reaction stoichiometry of 4:1 between HCI and [Ru(OEP)]₂ in C₆D₆ or C₇D₈ showed that HCI itself was the oxidant and not trace Cl₂ in HCI, as thought previously. A range of HX acids having pK[subscript]a, values in the range 38 to less than -10 (HX = H₂, MeOH, H₂0, H₂S, CH₃COOH, C₆H₅COOH, HF, CF₃COOH, HN0₃, HBF₄, HCI. HBr, and HSbF₆) were tested for reactivity with [Ru(OEP)]₂in C₆D₆; the data showed that a strong acid (pK[subscript]a < ca. 0) was necessary to initiate reactivity. The complex Ru[superscript]Ⅳ(OEP)(SbF₆)₂ was generated in situ by reacting HSbF₆ with [Ru(OEP)]₂. In CH₂C1₂, a 1:1 stoichiometric reaction between HCI and [Ru(OEP)]₂ was observed, instantly fanning a mixture of products, tentatively formulated as Rura(OEP)H and [Ru[superscript]Ⅲ(OEP)]₂CHCl₂ based on spectroscopic data. The species proved impossible to separate. These same products were formed slowly by the reaction of [Ru(OEP)]₂ with CH₂C1₂ in the absence of HCI, and kinetic studies suggest that a direct reaction of [Ru(OEP)]₂ with CH₂C1₂ is likely, rather than reaction of [Ru(OEP)]₂ with impurities in CH₂C1₂. The product mixture generated Ru(OEP)Cl₂ upon further reaction with HCI, both in the absence and in the presence of air. The complex Ru[superscript]Ⅳ(OEP)(BF₄)₂ was generated in situ by an analogous reaction of aqueous HBF₄ with the product mixture. The required hydrogen-containing co-product from the reaction of HX (X = Br, CI) with [Ru(OEP)|₂ in C₇D₈ or CH₂C1₂ was not detected, but was shown not to be H₂. Oxidation of Ru(porp)(CH₃CN)₂ and Ru(OEP)py₂ (py = pyridine; porp = OEP, TMP (the dianion of tetramesitylporphyrin)) by gaseous HX (X = Br, CI) in the absence of air yielded Ru[superscript]Ⅳ(porp)X₂ complexes. The new compound Ru(TMP)Br₂ was synthesized by this method using the bis(acetonitrile) precursor, and was characterized by spectroscopy; the chloride analogue Ru(TMP)Cl₂ was generated in situ. The magnetic properties (susceptibility and moment) of Ru(OEP)Br₂ from 6 to 300 K are unlike those reported for ruthenium(IV) non-porphyrin complexes, and reveal a significant contribution from temperature-independent paramagnetism. The reaction of Ru(OEP)X₂ (X = Br, CI) with NH₃ gave the complexes Ru[superscript]Ⅲ(OEP)X(NH₃), which upon acidification under an inert atmosphere yielded the Rum(OEP)X compounds. These Ru111 complexes were characterized by spectroscopic techniques, and the solution chemistry of the five-coordinate species Ru(OEP)X was developed: the Ru[superscript]Ⅲ(OEP)X(CH₃CN) species were also characterized. Solvation of the five-coordinate species Ru(OEP)X (X = Br, CI) was observed in coordinating solvents to form the six-coordinate species Ru(OEP)X(solvent) (solvent = py, CH₃CN and MeOH). Estimates of the equilibrium constants for the association of these ligands to Ru(OEP)X were obtained from UV/visible titration experiments in CH₂C1₂. Similarly, the equilibrium constant for the association of Br to Ru(OEP)Br to generate in situ (n-Bu)₄N⁺[Ru[superscript]Ⅲ(OEP)Br⁺₂]", was measured. Disappointingly, the complexes Ru(OEP)X were shown not to catalyze the oxidation of organic substrates such as cyclohexene. Electrochemical and spectroelectrochemical studies of the complexes Ru(OEP)X₂ and Ru(OEP)X (X = Br, CI) showed that the Ru[superscript]Ⅳ/Ru[superscript]Ⅲ couple occurred at 480-460 mV and 950-870 mV vs. NHE, respectively, and that the probable reductant for the reaction of Ru(OEP)X₂ with NH₃ was NH₃ itself. A facile reduction of Ru(OEP)(SbF₆)₂ gave the complex Ru[superscript]Ⅲ(OEP)SbF₆, by a probable homolysis of the Ru-F bond. The outer-sphere oxidation of Ru(OEP)py₂ by air in the presence of HX acids gave the isolated or in situ characterized complexes [Ruin(OEP)py₂]+ X" (X = CI, Br, F, BF₄). Similar oxidation of Ru(OEP)(CH₃CN)₂ formed [Ru(OEP)(CH₃CN)₂]+ Br-. Electrochenucal studies showed that 0₂ in acidic media was capable of oxidizing the Ru(OEP)(solvent)₂ complexes (solvent = py, CH₃CN) to the Ru[superscript]Ⅲ complexes, presumably generating H0₂ .

Text

2011-01-24

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http://hdl.handle.net/2429/30790

Chemistry

University of British Columbia

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