Influence of the second coordination sphere on O₂ activation by a nonheme iron(II) thiolate complex

The synthesis and characterization of a new ligand, 1-(bis(pyridin-2-ylmethyl) amino)-2-methylpropane-2-thiolate (BPA^Me2S^-) and its nonheme iron complex, Fe^II(BPA^Me2S)Br (1), is reported. Reaction of 1 with O₂ at -20 °C generates a high-spin iron(III)-hydroxide complex, [Fe^III(OH)(BPA^Me2S)(Br)] (2), that was characterized by UV-vis, ⁵⁷Fe Mössbauer, and electron paramagnetic resonance (EPR) spectroscopies, and electrospray ionization mass spectrometry (ESI-MS). Density functional theory (DFT) calculations were employed to support the spectroscopic assignments. In a previous report (J. Am. Chem. Soc.2024, 146, 7915-7921), the related iron(II) complex, Fe^II(BNPA^Me2S)Br (BNPA^Me2S^- = (bis((6-(neopentylamino)pyridinyl) methyl)amino)-2-methylpropane-2-thiolate) was reported and shown to react with O₂ at low temperature to give a rare iron(III)-superoxide intermediate, which then converts to an S‑oxygenated sulfinate as seen for the nonheme iron thiol dioxygenases. This complex includes two hydrogen bonding neopentylamino groups in the second coordination sphere. Complex 1 does not include these H-bonding groups, and its reactivity with O₂ does not yield a stabilized Fe/O₂ intermediate or S‑oxygenated products, although the data suggest an inner-sphere mechanism and formation of an iron‑oxygen species that is capable of abstracting hydrogen atoms from solvent or weak CH bond substrates. This study indicates that the H-bond donors are critical for stabilizing the Fe^III(O₂^-•) intermediate with the BNPA^Me2S^- ligand, which in turn leads to S‑oxygenation, as opposed to H-atom abstraction, following O₂ activation by the nonheme iron center_.