Synthesis, Characterization, and Reactivity of Oxomanganese(IV) Porphyrin Complexes

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) [Mn^IIITMP(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 Mn^IVTMP(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, [Mn^IVTMP(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, Mn^IVTMP(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 Mn^IVions. The Mn^IV=O stretching frequency in 2 was identified at 754 cm^-1by FT-IR spectroscopy. In the case of 3 the Mn^IV=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.