Dramatic Influence of an Anionic Donor on the Oxygen-Atom Transfer Reactivity of a MnV-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 Bu4N+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 MnO=1.53 angstrom, MnCN=2.21 angstrom. 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 24000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives H=14kcalmol(-1), S=-10calmol(-1)K(-1). Computational studies fully support the structures, spin states, and relative reactivity of the 5- and 6-coordinate Mn-V(O) complexes.