DOI: 10.1002/chem.201101432 Biomimetic Oxidation Reactions of a Naked Manganese(V)–Oxo Porphyrin Complex Francesco Lanucara [a, b] and Maria Elisa Crestoni* [a] Introduction High-valent metal–oxo complexes are involved as key inter- mediates in useful synthetic and biological oxidation pro- cesses. [1] Among biomimetic oxidants developed to unveil the mechanistic routes activated by metalloenzymes, iron(IV)-oxo porphyrin p-cation radical species (Cpd I) have been extensively investigated for their role as the com- petent catalysts of heme based monooxygenases, including cytochromes P450. [2] Conversely, although endowed with higher reactivity [3] and versatility in various oxidation reactions and implicated in the production of dioxygen at the core of photo- ACHTUNGTRENNUNGsystem II, [4] the analogous manganese–oxo species has eluded detection until Groves first reported the synthesis of water-soluble Mn V -oxo porphyrin complexes and their char- acterization as low-spin (S = 0) ground-state species. [5] Since then, other high-valent manganese–oxo intermedi- ates based on corrole, [6] salen, [7] corrolazine, [8] and porphyrin ligands [5, 9] have been prepared in aqueous or organic sol- vents and numerous experimental and theoretical studies have explored their electronic structures and reactivity in di- verse processes, including olefin epoxidation, C  H bond hy- droxylation, hydride-transfer and oxidation of sulfides, amines, halides, and phosphines. [10, 11] Notably, only recently compelling spectroscopic evidence has been able to characterize Mn V -oxo porphyrin complexes [Mn V (O)X] + prepared in basic solvents as a singlet trans-di- oxomanganese(V) anions (X = O 2 ), unreactive with al- kenes, but capable of abstracting hydride from NADH ana- logues. [12] DFT calculations suggest that the remarkable oxi- dation reactivity of the oxo–hydroxo (X = OH  ) and oxo– aqua (X = H 2 O) complexes that are favored at low pH may [a] Dr. F. Lanucara, Prof. M. E. Crestoni Dipartimento di Chimica e Tecnologie del Farmaco Università di Roma “La Sapienza” P.le A. Moro 5, 00185, Roma (Italy) Fax: (+ 39) 06-4991-3602 E-mail : mariaelisa.crestoni@uniroma1.it [b] Dr. F. Lanucara Present address: School of Chemistry, University of Manchester Manchester Interdisciplinary Biocentre 131 Princess Street, Manchester M1 7ND (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101432. Abstract: The intrinsic reactivity of a manganese(V)-oxo porphyrin complex, a typically fleeting intermediate in cat- alytic oxidation reactions in solution, has been elucidated in a study focused on its gas-phase ion-chemistry. The naked high-valent Mn V -oxo porphyrin intermediate 1 ([(tpfpp)Mn V O] + ; tpfpp = meso-tetrakis(pentafluorophe- nyl)porphinato dianion), has been ob- tained by controlled treatment of [(tpfpp)Mn III ]Cl (2-Cl) with iodosyl- benzene in methanol, delivered in the gas phase by electrospray ionization and assayed by FT-ICR mass spectrom- etry. A direct kinetic study of the re- ACHTUNGTRENNUNGaction with selected substrates, each containing a heteroatom X (X = S, N, P) including amines, sulfides, and phos- phites, was thus performed. Ionic prod- ucts arising from electron transfer (ET), hydride transfer (HT), oxygen- atom transfer (OAT), and formal addi- tion (Add) may be observed, with a predominance of two-electron process- es, whereas the product of hydrogen- atom transfer (HAT), [(tpfpp)Mn IV OH] + , is never detected. A thermochemical threshold for the formation of the product radical cation allows an evaluation of the electron- transfer ability of a Mn V -oxo complex, yielding a lower limit of 7.85 eV for the ionization energy of gaseous [(tpfpp)Mn IV O]. Linear free-energy analyses of the reactions of para-substi- tuted N,N-dimethylanilines and thio- ACHTUNGTRENNUNGanisoles indicate that a considerable amount of positive charge is developed on the heteroatom in the oxidation transition state. Substrates endowed with different heteroatoms, but similar ionization energy display a comparable reaction efficiency, consistent with a mechanism initiated by ET. For the first time, the kinetic acidity of putative hydroxo intermediates playing a role in catalytic oxidations, [(tpfpp)Fe IV OH] + and [(tpfpp)Mn IV OH] + , has been in- vestigated with selected reference bases, revealing a comparatively higher basicity for the ferryl, [(tpfpp)Fe IV O], with respect to the manganyl, [(tpfpp)Mn IV O], unit. Finally, the neat association reaction of 2 has been stud- ied with various ligands showing that harder ligands are more strongly bound. Keywords: bioinorganic chemistry · gas-phase reactions · high-valent metal–oxo complexes · manganese · mass spectrometry  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 12092 – 12100 12092