- Mechanistic insights into hydride-transfer and electron-transfer reactions by a manganese(IV)-oxo porphyrin complex
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Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogs to a manganese(IV)-oxo porphyrin complex, (TMP)MnIV(O) [TMP = 5,10,15,20-tetrakis(2,4,6-trimethylphenyl) porphyrin], occurs via disproportionation of (TMP)MnIV(O) to [(TMP)MnIII] + and [(TMP)MnV(O)]+ that acts as the actual hydride acceptor. In contrast, electron transfer from ferrocene derivatives to (TMP)MnIV(O) occurs directly to afford ferricenium ions and (TMP)MnIII(OH) products. The disproportionation rate constant of (TMP)MnIV(O) was determined by the dependence of the observed second-order rate constants on concentrations of NADH analogs to be (8.0 ± 0.6) × 106 M-1 s-1 in acetonitrile at 298 K. The disproportionation rate constant of (TMP)Mn IV(O) in hydride-transfer reactions increases linearly with increasing acid concentration, whereas the rate constant of electron transfer from ferrocene to (TMP)MnIV(O) remains constant irrespective of the acid concentration. The rate constants of electron transfer from a series of ferrocene derivatives to (TMP)MnIV(O) were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energy of electron transfer by the (TMP)MnIV(O) complex.
- Fukuzumi, Shunichi,Fujioka, Naofumi,Kotani, Hiroaki,Ohkubo, Kei,Lee, Yong-Min,Nam, Wonwoo
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p. 17127 - 17134
(2010/03/25)
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- Synthesis, Characterization, and Reactivity of Oxomanganese(IV) Porphyrin Complexes
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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) IIITPM(Cl), 1> with 1,2 equiv of tetramethylammonium hydroxyde (TMA(OH)) and 1.2 equiv of m-chloroperoxybenzoic acid (m-CPBA) in CH2Cl2 produced a second complex formulated as MnIVTPM(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, IVTPM(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 CH2Cl2 solutions 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 ca. 4 and a weak unresolved signal at g ca. 2 consistent with a high-spin (S=3/2) assignment of the MnIV ions.The MnIV=O stretching frequency in 2 was identified at 754 cm-1 by 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 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 H2(18)O, the stereoisomeric epoxides showed a significantly different (18)O content.Further, (18)O was found to reside in the oxidation products when this reaction was carried out in the presence of (18)O2.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.
- Groves, John T.,Stern, Michael K.
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p. 8628 - 8638
(2007/10/02)
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