MotW14 – A nonreducible, high-valent Mn(V) complex
Research involving the formation and characterization of high-valent Mn complexes has been slow. They are proposed to be active intermediates in biological and synthetic reactions, such as epoxidations and hydroxylations. They are also important intermediates in NR group transfer reactions, or pericyclic reactions in which a pi bond is transformed into a sigma bond while another sigma bond migrates.
At John Hopkins University, a manganese(V) imido complex [(TBP8Cz)MnV(NMes)] was synthesized from the Mn(III) complex [(TBP8Cz)MnMIII] (Lansky, D.; Kosack, J.; Narducci Sarjeant, A.; Goldberg, D. Inorg. Chem. 2006, 45, 8477-8479). Even with a high-valent MnV center, the complex is very resistant to reduction. The 1H-NMR spectrum showed a diamagnetic molecule, indicating a low-spin MnV (d2) species. The structure was confirmed via X-ray crystallography. The crystal contains two independent molecules, with the imido axial ligands pointing away from each other. A short Mn-N imido distance suggests a stronger pi overlap between the terminal imido ligand and the empty metal dxz/dyz orbitals. The metal ion is 0.55 Angstroms above the average plan of the four pyrrole N atoms.
Reactions of the Mn(V) complex with alkenes were unsuccessful. H-atom abstraction should be able to occur in this molecule, but it was completely unreactive toward even a highly reactive H-atom donor. This lack of reactivity shows that the complex cannot undergo even weak H abstraction. The explanation could be that the rate-determining step is the reduction of MnV to MnIV which is not easily reached, and therefore the thermodynamics do not favor the H abstraction. The complex is also very resistant to reduction, even as a high-valent species, as shown by the electochemical data.