The preparation, isolation, and characterization of two types of oxomanganese(IV) porphyrin complexes are described. The reaction of chloro(5,10,15,20-tetramesitylporphyrinato)manganese(III) [MnIIITMP(Cl), 1] with 1.2 equiv of tetramethylammonium hydroxide (TMA(OH)) and 1.2 equiv of m-chloroperoxybenzoic acid (m-CPBA) in CH2Cl2 produced a second complex formulated as MnIVTMP(O) (2). The reaction of 1 in CH2Cl2 containing excess tetra-n-butylammonium hydroxide (TBA(OH)) at +1.20 V generated a stable oxomanganese(IV) porphyrin complex, [MnIVTMP(O)(OH)] (3). When the reaction stoichiometry was altered, mixtures of complexes 2 and 3 could be prepared. The aerobic reaction of 1 in CHCl3 containing 6 N NaOH and a phase-transfer catalyst resulted in the formation of a similar complex, MnIVTMP(O)(X) (2a). The addition of excess TBA(OH) to CH2Cl2solutions of either 2 or 2a resulted in the quantitative formation of 3. The EPR spectra of 2, 2a, and 3 all displayed a strong broad resonance at g ~ 4 and a weak unresolved signal at g ~ 2 consistent with a high-spin (S = 3/2) assignment of the MnIVions. The MnIV=O stretching frequency in 2 was identified at 754 cm-1by FT-IR spectroscopy. In the case of 3 the MnIV=O stretching frequency was at 712 cm-1. The reaction of 2a with cis-β- methylstyrene under anaerobic conditions produced a mixture of cis- and trans-epoxide in a ratio of 0.17. The reaction of 2 with cis-β-methylstyrene under aerobic conditions produced a different cis-epoxide/trans-epoxide ratio and product distribution than those of the identical reaction run under anaerobic conditions. In the presence of H218O, the stereoisomeric epoxides showed a significantly different 18O content. Further, 18O was found to reside in the oxidation products when this reaction was carried out in the presence of 18O2. Mechanisms for the epoxidation of olefins by 2 under anaerobic and aerobic conditions are discussed, which involve atom transfer from both oxomanganese(V) and oxomanganese(IV) species.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry