UBC Theses and Dissertations
The reactions of various ruthenium octaethylporphyrin complexes with small gas molecules Walker, Sandra Gail
Interaction of metalloporphyrins, particularly of the iron subgroup, with gas molecules such as 0₂ and CO, remains of considerable interest in terms of comparison with natural heme protein systems. This thesis describes studies on ruthenium(II) porphyrin complexes, the second row analogues of the heme systems. Toluene solutions of the bis(acetonitrile) complex RuOEP(CH₃CN) ₂, (OEP = the dianion of octaethylporphyrin) with or without excess acetonitrile present, are irreversibly oxidized by 0₂ at 30°C. However, with CO, the complex undergoes a clean reaction to give RuOEP(CO)(CH₃CN) with several isosbestic points observed in the UV/VIS spectrum. The kinetic dependence on the CO pressure and acetonitrile concentration are consistent with a dissociative mechanism: [Chemical Reaction I] The kinetic rate constants, k-₁, k-₂ and k-₁/k₂ and the overall equilibrium constant were determined at 30°C in toluene. The Ru0EP(P(n-Bu) ₃) ₂ complex, in toluene, is completely unreactive toward 0₂ at 30°C over a period of several days; although again a monocarbonyl is formed under a CO atmosphere. In the presence of excess P(n-Bu) ₃, under CO, an equilibrium mixture of RuOEP(P(n-Bu) ₃) ₂ and RuOEP(CO)(P(n-Bu) ₃) is formed. The equilibrium constant K for reaction (II) and the thermodynamic parameters AH (3.7 Kcal/mole) and ΔS (10.3 e.u.) are determined along with the rate constants for the dissociative mechanism (cf. Equation I). The low ΔH value implies comparable bond [Chemical Reaction II] strengths between ruthenium and the two ligands P(n-Bu) ₃ and CO. The k -₁/k₂ values for the acetonitrile and phosphine systems are thought to relate to the structure of the five-coordinate intermediate, RuOEP(L); the data suggest the ruthenium is probably more in the porphyrin plane than out of plane, al least compared to analogous iron systems. The major difference between the acetonitrile and phosphine systems is in the k₂value which varies by 10⁵ , partially due to the difference in ir-acidity of the two axial ligands. Toluene solutions of RuOEP(CH₃CN) ₂ bind N₂ and C₂H₄ very weakly. However, due to the extreme photo- and oxygen-sensitivity of the products, no consistent kinetic data could be obtained. Solutions of RuOEP(py) ₂ in neat pyridine, formed in situ from the bis(acetonitrile) complex in pyridine, are completely unreactive toward CO. Even toluene solutions with small amounts of pyridine react only partially (~15% in three days at 25°C) to give the monocarbonyl. Upon dissolving the RuOEP(CH₃CN) ₂ complex in DMA, DMF and THF, the species formed seem to be RuOEP(CH₃CN)(solvent). These species react with CO at 30°C in a two step reaction, an instantaneous part followed by a much slower one; the reactions appear to involve the rapid formation of one monocarbonyl followed by decomposition to the expected monocarbonyl, RuOEP(CO)(solvent). Decarbonylation of the amide solvent to give an amine ligand may be involved in the fast reaction. Judging by the reaction with CO (non-first-order in ruthenium) there are possibly two species present when RuOEP(CH₃CN) ₂ is dissolved in pyrrole, RuOEP(pyrrole)₂ and RuOEP(CH₃CN)(pyrrole), which react at different rates.
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