Manganese porphyrin dimers

Naruta Y, Sasayama M, Sasaki T. Oxygen evolution by oxidation of water with manganese porphyrin dimers. Angew Chem Int Ed. 1994 33(18) 1839—841. 

Similar manganese porphyrin dimers linked by different spacer molecules had catalase activity [196]. The activity was highest when the Mn-Mn separation was ca 4 A, in agreement with the Mn-Mn separation in the hypothetical Mn -O-O-Mn complex [196]. The high-valent Mn porphyrin dimer has been prepared by the oxidation of the Mn dimer with wCPBA. The Mn dimer thus formed was stable for hours at temperatures ranging from —78 to -20 °C and it was detected by ESI MS (electrospray ionization mass spectroscopy) [197]. Cyclooctene was oxidized by mCPBA (1.1 equivalent for each Mn ion) with the Mn dimer to... 

In contrast with the stability of the alkyl-metalloporphyrins discussed above, pulse radiolytic studies on nickel and manganese porphyrins " indicated that reactions of alkyl radicals with these porphyrins yield very unstable species. Both Ni P and Ni P react very rapidly wi4 alkyl radicals to form Ni-C bonds. The only Ni-C bond that was found to be stable was that of CFjNi P. Other RNi P decayed with half lives of the order of seconds to yield Ni P. RNi P decayed even more rapidly, within milliseconds, also forming Ni P. It was suggested that the reaction between R- and Ni P is an equilibrium reaction forming RNi" P and that the decay of this species is through the dimerization of R- + R -. The reaction of alkyl radicals with Mn P is also rapid and probably occurs via addition to the metal, but the adduct immediately decomposes to yield Mn" P. These wide variations in the stability of the metal-carbon bonds in the various alkyl-metalloporphyrins have been rationalized in terms of the radius of the metal ion relative to the size of the porphyrin cavity and in terms of the number of d electrons in the metal center. "... 

Recently, Hulsken et al. have employed STM to probe the mechanism by which manganese porphyrin complexes achieve epoxidation of alkenes with dioxygen [69]. Importantly, this study demonstrates the positive role of solid-support immobilization in precluding the formation of inactive oxido-bridged catalyst dimers (a common deactivation pathway). 

The oxidation of ethylbenzene using iron-haloporphyrins in a solvent-free system under molecular oxygen at 70-110°C gives mixture of a-phenylethylhydroperoxide, methylphenylcarbinole, and acetophenone (1 1 1). The catalyst is (TPFPP=5,10,15,20-tetrakis (pentafluorophenyl) porphyrin). Ethylbenzene conversion does not more than 5%. The oxidation occurs via radical pathway [3 9]. The products of ethylbenzene oxidation with air under mild condition (T > 60°C, atmospheric pressure), catalyzed by [TPPFeJ O or [TPPMnJ O ( 0,-oxo dimeric metalloporphyrins, a,-oxo-bis(tetraphenylporphyrinato)iron (manganese)) without any additive are acetophenone and methylphenylcarbinole. The ethylbenzene oxidation is radical chain oxidation in this case also. The ketone/alcohol (mol/ mol) rations are 3.76 ([TPPMnJ O, ethylbenzene conversion - 8.08%), 2.74 ([TPPFe]20, ethylbenzene conversion - 3.73%) [40]. 

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