Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mn(V) -oxo complex.
Bottom Line: Reaction of the CN(-) complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e(-) -reduced Mn(III) (CN)(-) 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.
Affiliation: Department of Chemistry, The Johns Hopkins University, Baltimore, MD (USA).Show MeSH
Mentions: In an earlier study we showed that addition of excess Bu4N+CN− to MnV(O)(TBP8Cz) in CH2Cl2 led to a large increase in HAT reactivity.[8a] The UV/Vis spectrum of the MnV(O) complex is not sensitive to addition of CN−, but LDI-MS (neg. mode) showed clear evidence for a complex matching the formula [MnV(O)(TBP8Cz)(CN)]−, consistent with formation of a 6-coordinate complex as seen in Scheme 1. However, direct structural information on the cyanide-ligated complex was lacking, in part due to the instability of this complex. We now report the structural characterization of [MnV(O)(TBP8Cz)(CN)]− by XAS. The Mn K-edge XAS spectra of MnV(O)(TBP8Cz) and [MnV(O)(TBP8Cz)(CN)]− (prepared by addition of 10 and 100 equivalents of Bu4N+CN−) are shown in Figure 1 a. All three spectra are consistent with a MnV oxidation state assignment, based on the approximately 6553 eV position of the rising edge. Upon addition of CN−, the edge shifts by about 0.3 eV to lower energy, but not enough to suggest reduction to MnIV. Most notably, upon addition of CN−, the pre-edge remains at high energy (∼6542 eV), but the pre-edge intensity decreases relative to the 5-coordinate MnV–oxo complex. These data are consistent with formation of a 6-coordinate MnV–oxo species. Coordination of a trans axial CN− ligand would move the Mn further into the Cz plane, decreasing the amount of metal 3d–4p mixing, and thus decreasing the observed pre-edge intensity.
Affiliation: Department of Chemistry, The Johns Hopkins University, Baltimore, MD (USA).