Mössbauer, electron paramagnetic resonance, and density functional theory studies of synthetic S = ¹/₂ Fe^III-O- Fe^IV=O Complexes. Superexchange-mediated spin transition at the Fe^IV=O Site

Previously we have characterized two high-valent complexes [LFe ^IV(μ-O)₂Fe^IIIL], 1, and [LFe ^IV(O)(μ-O)(OH) Fe^IVL], 4. Addition of hydroxide or fluoride to 1 produces two new complexes, 1-OH and 1-F. Electron paramagnetic resonance (EPR) and Mössbauer studies show that both complexes have an S = ¹/₂ ground state which results from antiferromagnetic coupling of the spins of a high-spin (S_a = ⁵/₂) Fe^III and a high-spin (S_b = 2) Fe^IV site. 1-OH can also be obtained by a 1-electron reduction of 4, which has been shown to have an Fe^IV=O site. Radiolytic reduction of 4 at 77 K yields a Mössbauer spectrum identical to that observed for 1-OH, showing that the latter contains an Fe^IV=O. Interestingly, the Fe^IV=O moiety has S_b = 1 in 4 and S_b = 2 in 1-OH and 1-F. From the temperature dependence of the S = ¹/₂ signal we have determined the exchange coupling constant J (ℋ = Ĵ _a·̂_b convention) to be 90 ± 20 cm ^-1 for both 1-OH and 1-F. Broken-symmetry density functional theory (DFT) calculations yield J = 135 cm^-1 for 1-OH and J = 104 cm ^-1 for 1-F, in good agreement with the experiments. DFT analysis shows that the S_b = 1 → S_b = 2 transition of the Fe^IV=O site upon reduction of the Fe^IV-OH site to high-spin Fe^III is driven primarily by the strong antiferromagnetic exchange in the (S_a = ⁵/₂, S_b = 2) couple.