The work summarized here demonstrates a new concept for exploiting dense phase CO(2), media considered to be "green" solvents, for homogeneous catalytic oxidation reactions. According to this concept, the conventional organic solvent medium used in catalytic chemical reactions is replaced substantially (up to 80 vol %) by CO(2), at moderate pressures (tens of bars), to create a continuum of CO(2)-expanded solvent media. A particular benefit is found for oxidation catalysis; the presence of CO(2) in the mixed medium increases the O(2) solubility by ca. 100 times compared to that in the neat organic solvent while the retained organic solvent serves an essential role by solubilizing the transition metal catalyst. We show that CO(2)-expanded solvents provide optimal properties for maximizing oxidation rates that are typically 1-2 orders of magnitude greater than those obtained with either the neat organic solvent or supercritical CO(2) as the reaction medium. These advantages are demonstrated with examples of homogeneous oxidations of a substituted phenol and of cyclohexene by molecular O(2) using transition metal catalysts, cobalt Schiff-base and iron porphyrin complexes, respectively, in CO(2)-expanded CH(3)CN.

This first study of O(2) oxidation (autoxidation) of substituted phenols catalyzed by a dioxygen carrier in supercritical carbon dioxide (scCO(2)) provides additional insights into the established mechanism of reactions that have been much studied in conventional solvents. As has been long believed, the cobalt(II) dioxygen carriers of the class represented by [[N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato(2-)]cobalt(II)], Co(salen), show both oxidase and oxygenase activities during oxygenation of substituted phenols in scCO(2). The catalytic autoxidation of 2,6-di-tert-butylphenol (DTBP) and 3,5-di-tert-butylphenol (35-DTBP) in scCO(2) was studied by analysis of products in batch reactions with carefully controlled variables, in the presence of a large excess of O(2), at 207 bar of total pressure and a reaction temperature of 70 degrees C. The oxidation of 35-DTBP yielded only traces of products under the same experimental conditions that converted DTBP totally to a mixture of the oxygenation product 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) and the related product of radical coupling 3,5,3',5'-tetra-tert-butyl-4,4'-diphenoquinone (TTDBQ). The effects on conversion of DTBP to products and on selectivity between the two products were studied for variations in temperature and the concentrations of catalyst, oxygen, and methylimidazole. Selectivity in favor of the O-transfer product DTBQ over the self-coupling of the phenoxy radical was observed upon changing the oxygen concentration. In contrast, selectivity remained unaffected over a wide range of temperatures and catalyst concentrations. The oxygen dependence of both the conversion and selectivity showed saturation effects identifying the dioxygen complex as the effective oxidant in both the initial radical formation step and the oxygenation of that radical. No direct reaction is observed between the electrophilic phenoxy radical and O(2).

A redox model study of [NiFe] hydrogenase has examined a series of five polymetallics based on the metalation of the dithiolate complex [1,5-bis(mercaptoethyl)-1,5-diazacyclooctane]Ni(II), Ni-1. Crystal structures of three polymetallics of the series have been reported earlier: [(Ni-1)(2)()Ni]Cl(2)(), [(Ni-1)(2)()FeCl(2)()](2)(), and [(Ni-1)(3)()(ZnCl)(2)()]Cl(2)(). Two are described here: [(Ni-1)(2)()Pd]Cl(2)().2H(2)()Ocrystallizes in the monoclinic system, space group P2(1)/c with cell constants a = 12.212(4) Å, b = 7.642(2) Å, c = 16.625(3) Å, beta = 107.69(2) degrees, V = 1443.230(0) Å(3), Z = 2, R = 0.051, and R(w) = 0.056. [(Ni-1)(2)()CoCl]PF(6)() crystallizes in the triclinic system, space group P&onemacr;, with cell constants a = 8.14(2) Å, b = 13.85(2) Å, c = 15.67(2) Å, alpha = 113.59(10) degrees, beta = 101.84(14) degrees, gamma = 94.0(2) degrees, V = 1561.620(0)Å(3), Z = 2, R = 0.072, and R(w) = 0.077. In all Ni-1 serves as a bidentate metallothiolate ligand with a "hinge" angle in the range 105-118 degrees and Ni-M distances of 2.7- 3.7 Å. The most accessible redox event is shown by EPR and electrochemistry to reside in the N(2)S(2)Ni unit and is the Ni(II/I) couple. Charge neutralization of the thiolate sulfurs by metalation can (dependent on the interacting metal) stabilize the Ni(I) state as efficiently as methylation forming a thioether. The implication of these results for the heterometallic active site of [NiFe]-hydrogenase as structured from Desulfovibrio gigas (Volbeda, A., et al. Nature, 1995, 373, 580), the generality of the Ni(&mgr;-SR)(2)M hinge structure, and a possible explanation for the unusual redox potentials are discussed.

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