Axial Ligand Substituted Nonheme Fe~Ⅳ=O Complexes: Observation of Near-UV LMCT Bands and Fe=O Raman Vibrations

摘要

Treatment of 2-NCS and 2-N_3 with PhIO yields the 5=1 Fe~Ⅳ=O species 1-NCS and 1-N_3, respectively, which have ΔE_Q parameters and electronic absorption spectra quite different from those of parent complex 1. The unique near-UV absorption features of 1-NCS and 1-N_3 have allowed us to make the first observation of v(Fe=O) vibrations of S = 1 mononuclear nonheme oxoiron(Ⅳ) complexes by RR spectroscopy. Additionally, we have demonstrated that the reactivity of nonheme oxoiron(Ⅳ) intermediates is markedly influenced by the axial ligands. Given the paucity of either S = 1 or S = 2 nonheme oxoiron(Ⅳ) complexes, it is not well established which chemical properties are governed by the Fe~Ⅳ spin state. It is thus noteworthy that 1-NCS and 1-N_3, which have S = 1 Fe~Ⅳ centers, exhibit O~(2-) → Fe~Ⅳ CT transition energies and v(Fe = O) frequencies quite similar to those of the S = 2 oxoiron(Ⅳ) intermediate of TauD. Therefore, these spectroscopic signatures, which are largely governed by the Fe~Ⅳ = O bonding interactions, may be more sensitive to the ligand environment than the Fe~Ⅳ spin state, in agreement with a theoretically based proposal that Fe~Ⅳ = O bonding is largely unaffected by changes in spin state.