Dramatic Influence of an Anionic Donor on the Oxygen-Atom Transfer Reactivity of a Mn(V)–Oxo Complex

Addition of an anionic donor to an Mn(V)(O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [Mn(V)(O)(TBP(8)Cz)(CN)](−) was generated from addition of Bu(4)N(+)CN(−) to the 5-coordinate Mn(V)(O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives Mn–O=1.53 Å, Mn–CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN(−) complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e(−)-reduced Mn(III)(CN)(−) complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives ΔH(≠)=14 kcal mol(−1), ΔS(≠)=−10 cal mol(−1) K(−1). Computational studies fully support the structures, spin states, and relative reactivity of the 5- and 6-coordinate Mn(V)(O) complexes.