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CO2-expanded acetonitrile and methylene chloride have been used in this first detailed study of catalytic O2oxidations in these remarkably effective reaction media. The autoxidation of 2,6-di-tert-butylphenol DTBP with the cobalt Schiff-base Cosalen in these so-called CO2-expanded liquids CXLs has been extensively studied using precisely controlled and monitored batch reactions. The dependence of conversion, selectivity and turn-over-frequency on various reaction parameters, including temperature, O2, catalyst, and solvent composition has been evaluated. The rates of O2-oxidation in CXLs are typically 1–2 orders of magnitude greater than those obtained with either the neat organic solvent or supercritical CO2as reaction media. In keeping with the proposed mechanism, the dependence of both the selectivity and conversion on O2concentration and catalyst concentration indicates that the O2adduct, and not free O2, serves as oxidant in two critical steps in these systems. The increase in conversion with increasing temperature supports formation of the phenoxy radical as the rate determining step. In contrast, the temperature independence of selectivity is as expected for two competing radical coupling reactions. The balance between O2solubility and mixed-solvent dielectric constant determines some of the benefits of the CXLs. Because of the greatly increased solubility of O2in CXLs, the conversion in those media is substantially greater than that in either scCO2or the neat organic solvent. However, conversion eventually decreases with increasing CO2content of the solvent because of the decreasing dielectric constant of the medium. The solubilities of O2and Cosalen have been determined in CXLs based on methylene chloride.

The crystal structures are reported of trans‐dioxocyclam dihydrate, C10H20N4O2·2H2O, a structural isomer of the well known cis‐dioxocyclam, and of its novel Ni complex, (1,4,8,11‐tetra­aza­cyclo­tetra­decane‐2,9‐dionato‐κ4N)­nickel(II) dihydrate, [Ni(C10H18N4O2)]·2H2O, the first example of a trans­ition metal complex of this ligand. Both mol­ecules lie on crystallographic centres of inversion. The free ligand has two of its N atoms turned outwards from the ring and hydrogen bonded to water mol­ecules. A major conformational change takes place in the complex in which the ligand binds in a transtetradentate fashion, as suggested by the electronic spectrum. The nickel(II) ion is low spin, although the electronic spectrum of the complex in water indicates an equilibrium mixture of low‐spin and high‐spin species. The irreversible electrochemical oxidation of [NiL1] (L1is deprotonated trans‐dioxocyclam, C10H18N4O2) in water occurs at a potential of 0.964 V [versusSHE (standard hydrogen electrode)], which is very similar to that for the Ni–cis‐dioxocyclam complex.