Steric Control of Reactivity of Non-Heme μ-Hydroxo Diiron(II) Complexes with Oxygen:  Isolation of a Strongly Coupled μ-Oxo Fe(II)Fe(III) Dimer

The reactions of [Fe₂(OH)(R₃CCOO)₂(Me₃tacn)₂]⁺ (R = F (1-F), Me (1-Me), Ph (1-Ph)) with dioxygen initially afford a μ-oxo bridged mixed valent diiron(II)(III) complex [Fe₂O(R₃CCOO)₂(Me₃tacn)₂]⁺ (R = Me (2-Me) and Ph (2−Ph)) and subsequently the diiron(III) complexes 3-Me and 3-Ph. The reactions in acetonitrile are fast for 1-F, slower for 1-Me, and slow enough for 1-Ph so that crystals of 2-Ph as the triflate (2-Ph-OTf) and BPh₄ salts could be isolated and their structures determined. The oxidant N-methylmorpholine N-oxide (MMNO), is cleaner in that the byproduct, H₂O, does not further oxidize 2 as does the byproduct, H₂O₂, from the reaction with O₂. The crystal structures indicate that 2 has an oxo bridge with the Fe···Fe distances in 2-Ph-OTf (3.123(1) Å) and 2-Ph-BPh₄ (3.155(1) Å) similar to those of 3. The magnetic susceptibility data are fit for 1-Ph to two equivalent high-spin Fe(II) sites with H = −2JS₁·S₂, D = 2.823, g = 2.00, and J = −12.1 cm^-1, for 2-Ph to an S = ¹/₂ ground state with a high-spin Fe(III) site with D = 1.46, g = 2.00, and a high-spin Fe(II) site with D = 13.93, E/D = 0.285, g = 2.069, and J = −144 cm^-1, and for 2-Me to an S = ¹/₂ ground state with a high-spin Fe(III) site with D = 2.39, g = 2.00, and a high-spin Fe(II) site with D = −11.42, g = 2.041 and J = −119 cm^-1. The EPR spectrum is consistent with the S = ¹/₂ ground state (g = 1.97, 1.93, 1.90 at 10 K) and is observed even at 85 K. The Mössbauer of 2-Ph exhibits a poorly resolved doublet suggesting a Class II mixed valent species (Fe(II) and Fe(III) sites with δ = 1.09, 0.6, ΔE_Q = 2.45, 2.35, and Γ = 0.55, 0.75 mm s^-1, respectively). Electrospray mass spectra in MeCN with ¹⁸O labeled 1-−Ph and 1-Me indicated that they reacted by an outer-sphere process with ¹⁶O₂ or MMN¹⁶O as none of the oxidant is incorporated into 2.