Proton-Coupled Electron-Transfer Reactivity Controls Iron versus Sulfur Oxidation in Nonheme Iron-Thiolate Complexes.

Reaction of the five-coordinate Fe(NS) complexes, Fe(iPrTACN)(abt) (abt = aminobenzenethiolate, X = H, CF), with a one-electron oxidant and an appropriate base leads to net H atom loss, generating new Fe(iminobenzenethiolate) complexes that were characterized by single-crystal X-ray diffraction (XRD), as well as UV-vis, EPR, and Mössbauer spectroscopies. The spectroscopic data indicate that the iminobenzenethiolate complexes have = 3/2 ground states. In the absence of a base, oxidation of the Fe(abt) complexes leads to disulfide formation instead of oxidation at the metal center. Bracketing studies with separated proton-coupled electron-transfer (PCET) reagents show that the Fe(aminobenzenethiolate) and Fe(iminobenzenethiolate) forms are readily interconvertible by H/e transfer and provide a measure of the bond dissociation free energy (BDFE) for the coordinated N-H bond between 64 and 69 kcal mol. This work shows that coordination to the iron center causes a dramatic weakening of the N-H bond and that Fe- versus S-oxidation in a nonheme iron complex can be controlled by the protonation state of an ancillary amino donor.