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Interactions of axial ligands and dioxygen with cobalt (II) macrocycle complexes Smith, David W.

Abstract

Studies on the interactions of axial ligands and molecular oxygen with Co(II) porphyrin and Schiff base complexes have revealed a number of interesting and significant features regarding the nature of such interactions. The binding of a first axial ligand to cobalt(II) octamethyltetra-benzoporphyrin (CoOMBP), [Chemical Equation] (I) occurs with greater ligand affinity than is observed for other Co(II) porphyrin (CoP) complexes. Also, LCoOMBP is shown by visible spectroscopy to completely bind a second axial ligand, [Chemical Equation] (II) at high ligand concentrations in several cases, while other LCoP complexes do not under similar conditions. These binding properties are attributed to the very weak σ-basicity of OMBP causing approaching axial ligands to see a metal centre with more positive charge. In contrast, molecular oxygen binds more strongly to LCoOMBP complexes, [Chemical Equation] (III) than would be predicted from the σ-basicity properties of OMBP. This observation is explained in terms of the strong π-donating abilities of OMBP enhancing the affinity of the Co(II) complex for dioxygen; thus good evidence is obtained for coordinated dioxygen being able to accept Tr-electron density. The π-donating abilities of OMBP are also invoked to explain the observed similarities in ΔH values associated with K₁ and K₂; it is suggested that the π -interactions inhibit metal centre displacement from the porphyrin plane upon the formation of a five-coordinate adduct. Evidence for π -interactions between porphyrin and axial ligand is provided by an observed enhanced ΔH value for the π -accepting ligand PPh₃. The formation of dioxygen adducts of LCoOMBP is shown to be enhanced in more polar solvents, and a linear correlation between K[sub O₂] and dielectric constant is observed in varying mixtures of the binary toluene-DMF solvent system. Ligand binding is demonstrated to be slightly inhibited in more polar solvent systems. These effects are considered to be due to differences in relative polarity of the products and the reactants of the systems studied. Studies on ligand binding to a variety of CoP complexes show that the compounds with stronger σ-base porphyrins bind ligands less strongly than those with weaker σ -base porphyrins. This trend does not appear to hold when DMF is the axial ligand being studied; the strong IT-donor ability of this ligand is proposed as a possible explanation of the apparent anomalous behavior of DMF. The π-donor effects of DMF are also invoked to explain the enhanced ΔH associated with dioxygen binding to DMFCoOMBP, compared to when predominantly σ -donating-ligand-complexes of CoOMBP are oxygenated. A wide variety of LCoP complexes were monitored for oxygenation, and a good correlation is found between the σ -basicity of the porphyrin and the oxygen affinity of the LCoP complex when L is kept constant; complexes of stronger porphyrin σ -basicity are found to bind dioxygen more strongly than those of weaker σ -basic porphyrins. This is explained in terms of the stronger σ -base putting more electron density on the metal centre, so that transfer of electron density to dioxygen can occur more readily and the cobalt-dioxygen bond (Co(III)-O₂⁻) can be easily formed. Irreversible oxidation of LCoP complexes, 2 LCoP + O₂ → LCoP(O₂)CoPL, (IV) is also examined. In the course of these experiments, strong evidence is obtained for the formation of aggregates of CoOMBP complexes in nonaqueous media at concentrations as low as 10⁻⁵M, the first such CoP aggregates to be observed under these conditions. Oxidation of a variety of LCoP complexes is shown to proceed by a first order process in cobalt, suggestive of an "activated intermediate" in which coordinated dioxygen may more closely resemble the peroxide-like dioxygen in the product than the superoxide-like moiety present in LCoP(O₂). Oxidation of CoOMBP complexes is substantially enhanced when Im is the axial ligand, so much so that formation of the 1:1 Co:dioxygen adduct could not be independently studied, even at lower temperatures where oxidation is normally inhibited. This enhancement is thought to be due to the ability of Im to hydrogen bond to the 1:1 Co: dioxygen adduct, and to thus enhance the formation of the "activated intermediate" and to permit a more facile oxidation of the cobalt porphyrin system. If other non-hydrogen bonding compounds are used as ligands, the LCoOMBP complexes sometimes take days to oxidize in toluene. If a more polar solvent system is used, then the rate of oxidation is increased. This effect is especially manifested when the oxidation of (CH₃-Im)₂CoOMBP is studied in media containing necessarily substantial amounts of polar CH₃-Im; the reaction is now measurable on a stopped-flow kinetics time scale, in spite of the six-coordinate nature of the CoOMBP species present under these conditions. In the Schiff base system, N,N-bis(salicylaldehyde)ethylenediiminato-cobalt(II) (Co(salen)), ligand adduct formation is found to be much weaker and dioxygen adduct formation stronger than in the CoP systems, and this is thought to be due to the greater electron density present on the metal centre on the Co(salen) system. The Co(salen) system is shown to be so dioxygen sensitive that four-coordinate Co(salen) is essentially able to completely oxygenate at low temperatures, although the rate of oxygenation of the four-coordinate complex is many orders of magnitude less than that of the five-coordinate LCo(salen) systems. Compounds with unsaturated carbon-carbon bonds are shown to interact with the cobalt macrocycle complexes. Styrene and 3,4-dichlorobutene under one atmosphere of oxygen cause reversible visible spectral changes to occur with Co(salen) over a period of several days. Maleic anhydride and tetracyanoethylene bind to CoOMBP, but not to other CoP complexes, thus again showing the importance of the π-donating abilities of OMBP.

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