| Research Abstract |
2,2'-Bis [di (2-pyridyl) methyl] -6,6'-ethylenedipyridine (L^1) and 3,3-di-2-pyridylpropanoic acid (L^2) have been synthesized. These ligands form spontaneously mu-oxo-di-m-acetatodiiron (III) complexes, [Fe_2 (O) (OAc) _2 (L^1)] (X) _2 (x=ClO_4 1a and CF_3SO_3 1b) and [Fe_2 (O) (OAc) _2 (L^2) _2] (2), by the reaction with iron (III) ion and sodium acetate.
These diiron complexes funciton as a model of soluble methane monooxygenase (sMMO) which catalyzes the hydroxylation of methane to methanol in methanotroph. When m-chloroperbenzoic acid (m-CPBA) is used as oxidant, 1 catalyzes the oxygenation of cyclohexane, methylcyclohexane, adamantane, n-alkanes, and ethylbenzene. The catalytic turnover number and the turnover frequency for the oxygenaiton of cyclohexane with 1 are larger than 1000 and 100 cycles/min, respectively. The turnover number of 1 is ten-fold larger than that of a cyclohexane/ (tetraphenyl-porphynato) iron (III) /m-CPBA system. The compound 2 catalyzes the same reaction.
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The reaction ctatlyzed by 2 is much faster than that of 1. The coordination of carboxylate ion makes 2 electrically neutral and enhances the solubility of 2 in non-polar solvent, leading to the rapid and efficient alkane functionalization.
A pale brown solution of 1b in CH_3CN/cyclohexane turns purple by the addition of H_2O_2. The electronic absorption spectrum of the purple solution shows two bands at ca. 500 (epsilon : 1000) and 600 (epsilon : 1000 M^<-1>cm^<-1>) nm, suggesting the formation of a peroxodiiron (III) complex [Fe_2 (O) (OAc) (O_2) (L^1)] (CF_3SO_3) (3). A half life time of 3 in CH_3CN/cyclohexane is 79 min, showing that 3 is thermally quite stable. In order to isolate 3, the purple solution is concentrated to dryness. FAB Mass spectrum of the isolated 3 shows relatively strong parent peaks at m/z : 739,707, and 679, which are assignable to [Fe_2 (O) (OAc) (O_2) (L^1)] ^+, [Fe _2 (O) (OAc) (L^1)] ^+, and [Fe_2 (O) (O_2) (L^1)] ^+, respectively. These data reasonably explain the formation of the peroxodiiron (III) complex 3. Less
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