Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mnv-Oxo complex

Abstract

Addition of an anionic donor to an MnV(O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [MnV(O)(TBP8 Cz)(CN)]- was generated from addition of Bu4N + CN- to the 5-coordinate MnV(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 MnIII(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 kcalmol-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 MnV(O) complexes.