Catalytic dioxygen activation by (nitro)(meso-tetrakis(2-N-methylpyridyl)porphyrinato)cobalt(III) cation derivatives electrostatically immobilized in Nafion films:: An experimental and DFT investigation

Complexes of the (nitro)(meso-tetrakis(2-/N-methylpyridyl)porphyinato)cobalt(III) cation, [LCoTMpyP(2)(NO(2))](4+), in which L = water or ethanol have been immobilized through ionic attraction within Nafion films (Naf). These immobilized six-coordinate species, [LCoTMPyP(2)(NO(2))/Naf], have been found to catalyze the oxidation of triphenylphosphine in ethanol solution by dioxygen, therefore retaining the capacity to activate dioxygen catalytically without an additional reducing agent as was previously observed in nonaqueous solution for the non-ionic (nitro)cobalt porphyrin analogs. Heating these immobilized six-coordinate species under vacuum conditions results in the formation of the five-coordinate nitro derivatives, [CoTMPyP(2)(NO(2))/Naf] at 85 degrees C and [CoTMPyP(2)/Naf] at 110 degrees C. The catalytic oxidation of gas-phase cyclohexene with O(2) is supported only by the resulting immobilized five-coordinate nitro complex as was previously seen with the corresponding solution-phase catalyst in dichloromethane solution. The simultaneous catalytic oxidation of triphenylphosphine and cyclohexene with O(2) in the presence of the Nafion-bound six-coordinate ethanol nitro complex is also observed; however, this process is not seen for the CoTPP derivative in dichloromethane solution. The oxidation reactions do not occur with unmodified Nafion film or with Nafion-supported [BrCo(III)TmpyPYNaf or [Co(II)TmpyP]/Naf, indicating the necessity for the nitro/nitrosyl ligand in the oxidation mechanism. The existence of a second reactive intermediate is indicated because the two simultaneous oxidation reactions depend on two distinct oxygen atom-transfer steps having different reactivity. The absence of homogeneous cyclohexene oxidation by the six-coordinate (H(2)O)CoTPP(NO(2)) derivatives in the presence of Ph(3)P and O(2) in dichloromethane solution indicates that the second reactive intermediate is lost by an unidentified route only in solution, implying that the immobilization of it in Nafion allows it to react with cyclohexene. Although direct observation of this species has not been achieved, a comparitive DFT study of likely intermediates in several catalytic oxidation mechanisms at the BP 6-31G* level supports the possibility that this intermediate is a peroxynitro species on the basis of relative thermodynamic accessibility. The alternate intermediates evaluated include the reduced cobalt(II) porphyrin, the dioxygen adduct cobalt(Ill)-O(2)(-), the oxidized cobalt(II) pi-cation radical, and the nitrito complex, cobalt(III)-ONO.